close

Вход

Забыли?

вход по аккаунту

?

AFM compatible microfabricated near -field microwave probes for scanning microscopy

код для вставкиСкачать
AFM COMPATIBLE MICROFABRICATED NEAR-FIELD MICROWAVE PROBES
FOR SCANNING MICROSCOPY
by
YAQIANG WANG
Submitted in partial fulfillment of the requirements
For the degree of Doctor of Philosophy
Dissertation Advisor: Dr. Massood Tabib-Azar
Department of Electrical Engineering and Computer Science
CASE WESTERN RESERVE UNIVERSITY
May, 2003
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI Num ber: 3092035
UMI
UMI Microform 3092035
Copyright 2003 by ProQuest Information and Learning Company.
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.
ProQuest Information and Learning Company
300 North Zeeb Road
P.O. Box 1346
Ann Arbor, Ml 48106-1346
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
'fA Q iM ft
W M f r ______________________
_
candidate for th e ___________ 1 h-________________________ degree
(signed)_
(Chair of the Committee)
(Date)
3 /* » y / o 3
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I grant to Case Western Reserve University the right to use this work,
irrespective of any copyright, for the University’s own purposes without cost to
the University or to its students, agents and employees. I further agree that the
University may reproduce and provide single copies of the work, in any format
other than in or from microforms, to the public for the cost of reproduction.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
To my family, for their love and encouragement.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table of Contents
Title........................................................................................................................... i
Dedication.............................................................................................................. iv
Table of Contents...................................................................................................v
List of Figures..................................................................................................... viii
Acknowledgements.............................................................................................. xi
Abstract................................................................................................................. xii
1
Introduction..................................................................................................1
1.1
Motivation and Objective............................................................................................ 1
1.2
B ackground of SNM M ............................................................................................... 3
1.2.1
Concept of Near-Field Microscopy.....................................................................3
1.2.2
Status of Current SNMM Research.....................................................................5
1.3
Outline of Thesis.........................................................................................................12
References............................................................................................................................. 13
2
Design of AFM Compatible SNMM Probes........................................... 18
2.1
Mechanical Design......................................................................................................18
2.2
Microwave Design...........................................
2.3
Mirofabrication Process Design............................................................................... 28
2.4
Microfabrication Process Flow................................................................................. 35
2.5
SNMM Probe Layout D esign................................................................................... 38
21
References.............................................................................................................................39
3
Microfabrication of SNMM Probes..........................................................43
V
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.1 Introduction................................................................................................................. 43
3.2 Microfabrication Process Details and Results.........................................................44
3.2.1
RCA Clean.......................................................................................................... 44
3.2.2
Thermal Oxidation..............................................................................................45
3.2.3
Tip formation....................................................................................................... 47
3.2.4
Thick Photoresist Coating and Patterning.........................................................54
3.2.5
Ion implantation.................................................................................................. 56
3.2.6
Waveguides Formation...................................................................................... 57
3.2.7
Co-axial Tip Formation...................................................................................... 62
3.2.8
Define Beam and Beam Release....................................................................... 64
3.3 Discussion................................................................................................................... 66
References.............................................................................................................................67
4
Microwave Probe Characterization and A pplication..............................69
4.1
AFM Platform for SNMM Probe...............................................................................69
4.2
Mechanical Characterization.....................................................................................71
4.3
Electrical Characterization......................................................................................... 79
4.3.1
DC Measurements................................
79
4.3.2
Microwave Measurements.................................................................................81
4.4
Scanning Microwave Microscopy Application....................................................... 90
4.5
Discussion................................................................................................................... 95
References.............................................................................................................................96
5
Conclusions and Future W ork...................................................................97
5.1
Summary of the Contributions...................................................................................97
vi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5.2
Future W ork................................................................................................................99
Appendix
A: Introduction of Scanning Probe M icroscopes.................. 102
A.l Scanning Tunneling Microscope............................................................................ 102
A.2 Atomic Force Microscope........................................................................................104
A.3 Scanning Capacitance Microscope......................................................................... 106
A.4 Magnetic Force M icroscope....................................................................................110
A.5 Scanning Thermal Microscope.............................................................................. 113
A.6
Near-field Scanning Optical Mciroscope...............................................................115
References...........................................................................................................................118
Appendix
B: Run Card................................................................................120
Bibliography....................................................................................................... 125
vii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
List of Figures
Figure 1-1. Frequency spectrum for different scanning techniques..................................... 2
Figure 1-2. Concept of near-field illumination....................................................................... 4
Figure 1-3. Stripline resonator and EMP probe.......................................................................6
Figure 1-4. An EMP probe experimental setup for microwave measurement..................... 7
Figure 1-5. An SNMM probe based on open-ended transmission line................................. 9
Figure 1-6. An SNMM probe with a shielded coaxial resonator......................................... 11
Figure 1-7. A microfabricated SNMM probe: (a) schematic of the probe structure; (b)
probe interface for instrumentation..................................................................................12
Figure 2-1. SEM photo of a commercial AFM probe........................................................... 19
Figure 2-2. Overview of microprobe dimension................................................................... 21
Figure 2-3. Schematic of the proposed AFM compatible co-axial SNMM probe.............22
Figure 2-4. Co-axial tip and sample interaction.................................................................... 22
Figure 2-5. Simulated electric field intensity patterns (a) without an aperture; (b) with an
aperture.............................................................................................................................. 24
Figure 2-6. Lumped circuit model of the waveguide............................................................25
Figure 2-7. Lumped circuit model of the waveguide, the tip-sample coupling, and
samples: (a) a dielectric sample; (b) aconductivesample...............................................26
Figure 2-8. Cross-section view of the proposed AFM compatible SNMM probe.............28
Figure 2-9. Silicon tip plasma etching procedure................................................................. 29
Figure 2-10. Schematic of low temperature oxidation sharpening......................................30
Figure 2-11. SEM photo of a silicon pattern etched by SF6 in the MFL............................ 31
Figure 2-12. Tip exposure method to form co-axial tip ....................................................... 33
Figure 2-13. M4L™ oxygen plasma etching curve............................................................. 34
Figure 2-14. Linear fit curve shows uniform etch rate in the 10 minutes’ intervals..........35
Figure 2-15. Process flows of SNMM probes....................................................................... 37
Figure 2-16. Overview of the layout of SNMM probes on a 4-inch wafer....................... 38
Figure 2-17. Probe cell layouts with W= 50 pm and different beam lengths.....................39
Figure 3-1. Three-dimensional schematic view of a microfabricated SNMM probe........ 43
Figure 3-2. Optical micrographs of a tip formation progress after plasma etching for..... 49
Figure 3-3. SEM photos of a tip in plasma etching: (a) skew view at 45 ° angle; (b) top
view.................................................................................................................................... 50
Figure 3-4. SEM photo of a silicon tip after oxidation sharpening at 1050 °C: (a)
overview; (b) close view.................................................................................................. 52
Figure 3-5. SEM photos of silicon tips before and after oxidation sharpening at 950 °C
with view angle of 45 °: (a) a silicon tip before oxidation sharpening; (b) a silicon tip
after oxidation sharpening................................................................................................53
Figure 3-6. SEM photos of cross-section view of silicon tips before and after oxidation
sharpening at 950 °C: (a) a silicon tip before oxidation sharpening; (b) a silicon tip
after oxidation sharpening................................................................................................54
Figure 3-7. Optical micrograph of a patterned ion implantation window.......................... 56
Figure 3-8. Micrograph of an aluminum waveguide pattern after photolithography step
No. 3: (a) A1 waveguide from beam to chip body region, (b) A1 waveguide on the tip
region................................................................................................................................. 58
Figure 3-9. Micrograph of a waveguide patternafter LTO deposition..................................59
viii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3-10. Micrograph of a metal shield pattern after photolithography step No. 4......60
Figure 3-11. Micrographs of patterns after photolithography step No. 5 and BOE etch: (a)
micrograph of a device pattern including cantilever beam region and partial chip
body; (b) close view of the pattern at the end of cantilever beam region.................... 61
Figure 3-12. SEM photos of microfabricated co-axially shielded silicon tips using “tip
exposure” technique..........................................................................................................63
Figure 3-13. SEM photo of a V-shaped cantilever beam structure..................................... 64
Figure 3-14. SEM photos of microfabricated SNMM probes: (a) overview of an SNMM
probe; (b) close view of a released cantilever beam...................................................... 65
Figure 3-15. A microfabricated device wafer........................................................................66
Figure 4-1. Thermomicroscope Explorer™ AFM system................................................... 70
Figure 4-2. Schematic of Explorer™ scanner head.............................................................. 71
Figure 4-3. Probe mounting.................................................................................................... 72
Figure 4-4. Explorer™ scanner head and sample holder..................................................... 73
Figure 4-5. Schematic of beam alignment............................................................................. 74
Figure 4-6. Contact mode topography scanning image by an SNMM probe.....................74
Figure 4-7. Contact mode topography scanning image by a commercial probe................ 75
Figure 4-8. Oscillation spectrum of an SNMM probe with f0=170.92 KHz...................... 76
Figure 4-9. Oscillation spectrum of an SNMM probe with fo=50.016 KHz...................... 77
Figure 4-10. Oscillation spectrum of an SNMM probe with f0= l 1.777 KHz.................... 77
Figure 4-11. Normalized resonance spectra of an SNMM probe and a commercial AFM
probe.................................................................................................................................. 78
Figure 4-12.1-V curve measurement between a tip and shield........................................... 79
Figure 4-13. Ohmic contact resistance measurement between a waveguide and tip
80
Figure 4-14.1-V curve measurement between a tip and sample..........................................81
Figure 4-15. Network analyzer and Explorer™ system for microwave characterization. 82
Figure 4-16. Microwave interface on the Explorer™ h ead .................................................82
Figure 4-17. Impedance measurement for a co-axial cable line.......................................... 83
Figure 4-18. Co-axial cable line impedance magnitude in 1-20 GHz range...................... 84
Figure 4-19. Co-axial cable line impedance phase in 1-20 GHz range...............................85
Figure 4-20. An SNMM probe mounted on a tip holder...................................................... 86
Figure 4-21. Electrical connection to a mounted SNMM probe..........................................86
Figure 4-22. Reflection spectra of a tip in air and in contact with a copper sample..........87
Figure 4-23. Reflection spectra in air and over a dielectric sample.................................... 88
Figure 4-24. Reflection spectra in air and over a copper sample with probe region and tip
apex was located over the sample....................................................................................89
Figure 4-25. Refection spectra in air and over a dielectric sample with probe region and
tip apex was located over the sample.............................................................................. 89
Figure 4-26. Reflection (|Sn|) spectra of a tip in air and over a gold sample with a large
sensitivity around 5 GHz..................................................................................................91
Figure 4-27. Schematic of the circuit for scanning microwave microscopy..................... 92
Figure 4-28. Contact mode AFM image by an SNMM probe............................................. 94
Figure 4-29. Simultaneous microwave image by the SNMM probe...................................95
Figure 5-1. Partial layout of next generation probe with shielded waveguide................. 100
Figure 5-2. Recess in the SNMM probe body..................................................................... 101
Figure A -l. STM system configuration............................................................................... 103
ix
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A-2. Schematic of AFM system configuration........................................................105
Figure A-3. An SCM added to an AFM...............................................................................107
Figure A-4. Schematic for dopant profile using SCM........................................................ 109
Figure A-5. SCM measurement of the cross section of a p-channel, Sibulk MOSFET. 110
Figure A-6. Lift-mode operation of MFM probes...............................................................I l l
Figure A-7. Block diagram of a scanning Lorentz force microscopesystem...................113
Figure A-8. SEM photos of an SthM probe........................................................................114
Figure A-9. An SThM probe with electrical feed-through................................................115
Figure A-10. Fabrication of a NSOM probe and the application system........................ 117
Figure A - ll. The setup of a NSOM system in reflection mode........................................ 118
X
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Acknowledgements
I would like to express my great gratitude to my advisor, Prof. Massood TabibAzar, for his guidance, encouragement, and patience throughout this research. Prof.
Massood Tabib-Azar’s broad knowledge and expertise have provided me this opportunity
to enter this interesting and promising MEMS application area.
Many thanks go to my group members for their cooperation and continuous help.
I particular, I would like to thank: Adina Scott for AFM system measurements tutorial
and help; Tao Zhang for his help in microwave measurements; Liang You for his
discussion in microfabrication and cooperation in this project.
Special thanks go to John Sears for SEM training and valuable discussions, and to
MEL staffs for their technical support. Administrative assistance from Elizabeth Fuller
and Cheryl Lange is greatly appreciated.
This work was supported by National Institute of Standards and Technology
(Contract Number: 50-DKNB-1-SB081).
xi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
AFM Compatible Microfabricated Near-Field Microwave Probes for
Scanning Microscopy
Abstract
by
YAQIANG WANG
Scanning local probe microscopy (SLPM) tools such as scanning tunneling
microscope (STM), atomic force microscope (AFM), magnetic force microscope (MFM),
scanning capacitance microscope (SCM), and near-field scanning optical microscope
(NSOM), have become important tools in imaging of materials with near atomic
resolution. These probes, however, either operate with low frequency signals (< 1 GHz)
or operate in the optical regime.
In this thesis, the design and microfabrication of silicon co-axial scanning near­
field microwave probes compatible with AFM imaging were reported. Scanning near­
field microwave microscope (SNMM) imaging is suitable for nondestructive surface and
subsurface characterization of materials over a wide frequency range between 0.1 GHz
and 140 GHz. This novel SLPM tool can bridge the frequency gap that exists among
other commercial SLPM probes as mentioned above.
The microfabricated SNMM probe consists of a silicon V-shaped cantilever
beam, a co-axial tip, and aluminum co-planar waveguides. The V-shaped cantilever
beams have spring constants in the range of 0.1 N/m - 10 N/m, mechanical resonant
frequencies in the range of 10 KHz - 200 KHz, comparable with those of commercial
AFM probes. The tip itself is oxidation-sharpened heavily doped silicon surrounded by
xii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
an oxide layer that acts as insulator and covered with an aluminum co-axial shield layer.
The co-axial tip structure enables the silicon tip to protrude through an aperture in the
aluminum shield layer. The aperture confines the electromagnetic field in the exposed tip
region to perform microwave measurement.
More than 300 SNMM probe chips were fabricated on a 4-inch SOI wafer in a
batch process consisting of seven photolithography steps. Co-axial tip with height ~ 10
pm, apex radius ~ 50 A, and aperture radii in the range between sub-micron and micron
(500 nm - several microns) have been achieved.
Mechanical and electrical characterizations of the SNMM probes were discussed.
The first simultaneous contact AFM and SNMM surface imaging were presented.
xiii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 Introduction
1.1 Motivation and Objective
Scanning local probe microscopy (SLPM) tools, such as the scanning tunneling
microscope (STM) [1-5], atomic force microscope (AFM) [6-10], scanning capacitance
microscope (SCM) [10-15], magnetic force microscope (MFM) [16-20], scanning
thermal microscope (SThM) [21-25], and near-field scanning optical microscope (NSOM)
etc. [26-30], have been extensively used in the laboratory environment to image a variety
of materials with very high spatial resolution; and atomic resolutions in some cases.
These measurements map the local mechanical, electrical, magnetic, and optical
properties of materials over a wide range of frequencies. NSOM, usually which operates
in the optical regime 400 nm-1000 nm to characterize the optical properties, has practical
spatial resolution about 50 nm. AFM probes are widely used to determine topography and
micro-mechanical properties. The typical working frequency domain for AFM probes is
from DC to ~100KHz, with the resolution 1-10 nm. STM, SCM, MFM, and SThM
probes usually work below 1 GHz. However, between the DC and low frequency
measurements offered by STM, AFM and related techniques and optical frequency
offered by NSOM, there is a very large frequency gap (figure 1-1) that is not covered by
existing SLPM. Scanning near-field microwave microscope (SNMM), on the other hand,
has the unique ability o f providing direct im ages o f sub-surface structures with nearly
atomic resolutions owing to the penetration and possible resonant absorption of its
electromagnetic probing signal inside materials. SNMM can be viewed as a local probe
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2
microscope that bridges the frequency gap that exists among current SPLM tools and it
can be viewed as a direct extension of the operation frequencies of these local probes.
Scanning Near-Field Microwave Microscope
DC or low frequency
Optical
AFM, STM , M FM , SCM , SThM
I
NSO M
I
Frequency
1-----------------------------------------
Frequency gap between
local probe
Figure 1-1. Frequency spectrum for different scanning techniques [31].
Micro Electro Mechanical Systems (MEMS) technology has been playing an
important role in increasing the popularity of SLPM [32-36], Almost all the
aforementioned local probes are fabricated with MEMS technology. Microfabricated
cantilevers provide excellent characteristics in terms of small spring constants and high
resonant frequencies. Sharp tips can be integrally fabricated on the cantilevers and batch
fabrication processes are very reproducible. The advantages of MEMS technologies also
lie in miniature structure size, batch fabrication to form large amounts of device die
simultaneously, and easy integration of SLPM microsystems. For example, the
micromachined tips result in extraordinary resolution of AFM probes [37, 38]. The
integration of AFM probes with microfabricated piezoresistors enables the direct read-out
of surface imaging information, furthermore minimizing the AFM systems [39, 40]. The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3
integrated microfabricated actuator on the cantilever beam achieves high-speed surface
scanning while maintaining the high-resolution imaging [41, 42].
The ultimate objective of this project is to take advantage of the existing AFM
system and incorporate appropriate microfabricated tips to perform nondestructive near­
field microwave surface imaging to achieve atomic resolution in the frequency range of
0.1 -140 GHz, bridging the frequency gap in the existing commercial SLPM family.
This thesis presents initial steps toward this goal. The work includes: 1. Design of the
AFM compatible SNMM probe structures. 2. Microfabrication of the proposed SNMM
probes using MEMS technology. 3. Characterization of the microfabricated SNMM
probe devices and apply them for scanning microscopy in the frequency range of 1 - 20
GHz.
1.2 Background of SNMM
1.2.1
Concept o f Near-Field M icroscopy
Near-field, or evanescent field means the electromagnetic field region where the
light source (or microwave signal source) has a distance to the sample much less than the
wavelength of itself, as shown in figure 1-2. When the source signal is traveling through
an aperture with size a « X, the near-field will be created in the distance region d « X.
The near-field is confined in the sub-wavelength distance d, and its field intensity decays
exponentially with the distance d. When a sample is placed within the near-field region,
the near-field is disturbed by the local properties of the sample. As a result, the sample
information will be observed by detecting the perturbed signal in far-field, which is the
field free to travel. The near-field scanning microscopy concept was proposed in 1928 by
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4
E.H. Synge [43]. His idea was to use a small aperture to image a sample surface with sub­
wavelength resolution using optical light to overcome the Abbe limitation, which said
feature size less than }J2 could not be detected by optical signal. The aperture was placed
within near-field distance range of sample. The aperture size was also in the sub­
wavelength range. In 1972 Ash experimentally demonstrated this idea in the microwave
region with centimeter radiation and A./60 resolution [44]. The widespread application of
near-field microscopy became true with the invention of scanning probe techniques in
1980s. The main streams of scanning probe techniques will be reviewed in Appendix A
to provide more background information.
Incident source
M etal sh ielcl^
Aperture^a<<7t
N ear-field /
\
intensity
d
s am pi e
Far-field
-4----------------------------------------+
T
Z
%
Figure 1-2. Concept of near-field illumination.
Scanning near-field microwave microscopy is a relatively new technique in which
very tiny features can be resolved by using suitably fine probe tips (the same function of
aperture shown in figure 1-2). Because the SNMM operates at a relatively low frequency
range compared to those of optical signals, the penetration depth is greater than that of
optical microscopes. Therefore it is appropriate to use SNMM to characterize the bulk
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5
properties of relatively thick semiconductor, conducting, and superconducting films, or
other novel semiconductor materials. Owing to the penetration and possible resonant
absorption of its electromagnetic probing signal inside materials, SNMM has the unique
ability of providing direct images of sub-surface structures with nearly atomic resolutions.
The importance of SNMM also lies in:
•
The detection of defects and variations of carrier concentration in semiconductors
as the devices are scaled down.
•
The detection of defects and variations in resistivity in conducting, and
superconducting thin films.
•
The in-situ fault detection in VLSI chips.
•
Dielectric thin film research to map the permittivity and tunability of dielectrics.
•
Imaging the moisture and interpreting subsurface information of biological tissue
for diagnostic purpose.
In biomaterials and cells, the resistivity is directly determined by the water and ionic
content while permittivity increases with density. In cases where resonant absorption of a
molecule falls in the operation frequency range of SNNM, information regarding bonding
and other local characteristic can also be imaged.
1.2.2 Status o f Current SNM M Research
There are some research groups that are developing SNMM or have contributed to
its development in the past. Dr. M. Tabib-Azar’s group at Case Western Reserve
University has done systematic research work in SNMM [45-51]. He was the first
researcher in SNMM community to report a novel evanescent microwave probe (EMP)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
with a 0.4 |im resolution at 1 GHz [46]. In the following texts, the EMP has the same
meaning as the SNMM. The EMP consisted of a stripline resonator, a tapered stainless
wire as the probe tip, and a short feed line coupled to the resonator by a capacitor. The
sample detection was achieved by the resonant frequency shift of the resonator with the
tip approaching the sample. Figure 1-3 shows the configuration of the EMP probe.
insutstor (Ouroid)
^ cen ter M ane of Stripline R esonator
ii) Side View of the Stripline Resonator
Figure 1-3. Stripline resonator and EMP probe [46].
Figure 1-4 shows the experimental set-up schematic for the microwave
measurement [45]. A signal generator was connected to port 3 of a circulator to excite the
resonator that was connect to port 1 of the circulator. A crystal detector was connected to
port 2 of the circulator to produce a DC voltage proportional to the magnitude of the
reflected microwave signal. The active probe components were amounted to a vertical
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
support to control the tip-sample distance with the accuracy of one micron. The sample
platform was controlled by X and Y positioners for horizontal movements and vibrated in
vertical direction by a solenoid at approximately 90 Hz to increase the signal-to-noise
ratio by a lock-in amplifier.
Corpus*
T><"*or $
*>uh 2
'
iP "
l,T ur
L x i - a i .-A, iiwi
If
RF 5igts.fi!.
a05 -'L05GH»
Circuiaio*
S M A tx»wri«2sss5»r
towsttif wxii
Probe
Mtspio Base
DtrttC.* IATik&gg.
M<.nm # 1
Figure 1-4. An EMP probe experimental setup for microwave measurement [45].
EMP probes based on microstripline resonator were used to image dielectrics,
semiconductors, m etals, and biological specim ens [47]. The electron-hole pair
recombination and generation lifetimes of silicon were measured by illuminating the
microstripline resonator with a pulsed light [48], providing a way to determine activation
energies and various parameters of deep levels of semiconductor samples. They
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
demonstrated for the first time the application of EMP probes to perform transient and
steady-state thermography of semiconductor materials and devices [49]. The EMP probe
was capable of mapping temperature distribution with around 1 pm spatial resolution.
Temperature sensitivity better than 0.1 V/K has been achieved with a response time faster
than 1 ps. EMP probes were furthermore used to detect deflections in a Pd-coated
cantilever and to quantify the amount of stress and the resistivity change in the Pd film as
a function of hydrogen concentration [50], Another application of EMP probes was to
detect and image depletion regions in solar cell p-n junctions in real time, providing a
way to develop EMP probes a quantitative way to evaluate the shallow junctions in
semiconductor devices. The steady-state and transient status of the p-n junction’s
depletion region under DC and pulsed forward/reverse bias were mapped [51]. This
experimental data, together with the conductivity calibration of the EMP, was utilized to
calculate doping concentrations, diffusion length, and carrier recombination lifetimes in
the junction.
Groups of Dr. F. C. Wellstood and Dr. S. M. Anlage [52] at University of
Maryland, College Park, described a technique for quantitative topographic imaging
using a near-field microwave microscope. Their SNMM probe was based on an openended transmission line resonator. One end of the transmission line was an open-ended
coaxial probe and the other end was weakly coupled to a microwave source through a
capacitor. Near-field was generated from the exposed tip of the probe center conductor to
couple to the sample. As the SNMM probe scanned over the sample, the resonant
frequency and quality factor of the open transmission line shifted depending on the
surface properties of the region of the sample closest to the probe’s center conductor. The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
9
reflected microwave power was picked up by a diode detector and a sample image was
generated by using a frequency-following feedback circuit as shown in figure 1-5. The
spatial resolution was determined by the diameter of the inner conductor, in their case
was 480 pm. Best surface resistance sensitivity in the range from about 20 to 200 Q/D
has been demonstrated by this setup. Later on they used similar probe with sharper tip at
the end of inner conductor (tip radius of 1 pm) to image domain structure and
quantitatively measure dielectric permittivity and nonlinearity in ferroelectric crystals at
8.1 GHz with a spatial resolution of 1 pm [53].
\t ii'niu,nt
Frequency Control
S<<udl
Directional
Output
Coupler
Diode Detector
Integrator
Ij >.A-in
Amplifier
... "w
*
Decoupler
Oscillator
3 Ml/
Voltage Adder
Probe center
conductor
Probe
Translation stage
Figure 1-5. An SNMM probe based on open-ended transmission line [52].
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
Dr. X.-D. Xiang’s group [54, 55] at Lawrence Berkeley National Laboratory,
California, developed a scanning evanescent microwave microscope that was capable of a
quantitative microscopy of local complex dielectric constant profiles for dielectric
materials. As shown in figure 1-6, the system consists of a high Q A/4 coaxial resonator,
and a sharpened metal tip (with diameter of 50 -100 pm) mounted on the center
conductor of the resonator, protruding out from an aperture on the shield plate. A
sapphire disk with a center hole of size close to the tip wire and a metal layer coating on
the outside surface was used to shield the far-field components. The resonator was both
field emitter and detector. A change of the properties of the sample materials in the
vicinity of the metal tip would cause the change the resonant frequency and Q value of
the resonator. The signal was detected from the power measurement around the resonant
frequency. A spatial resolution of 100 nm had been achieved using this configuration.
The detection sensitivity of 5s/e was analyzed to be ~1 x 10'5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11
Pru&t
mtta
VCO
detector
Integrmof
Coupling
A/D
Convener*
|0(>]>S
/\
Dkxk"
defector
A m n iilie r
C^isiaii >J4
re^onatoi
!Ssmi>le
X-Y-Z siage
Motion
e o n u o llc r
Cortipiik’sr
Sapphire
Sample
/
Meial ^
Coating
Figure 1-6. An SNMM probe with a shielded coaxial resonator [55].
However, the SNMM probes introduced above did not take advantage of MEMS
technology to be able to further improve the resolution of SNMM techniques by forming
sharp tips at sub-micron apex radius. Daniel W. van der Weide’s group at University of
Wisconsin, Madison, is the first to design and microfabricate SNMM probes with a
microscopic coaxial tip and a coplanar waveguide (CPW) on a silicon substrate [56, 57].
Electrical connections have been carefully designed to avoid interference with the tip
scanning along the sample surface since the metallization was on the sample side. Their
SNMM probe is shown in figure 1-7.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
12
Figure 1-7. A microfabricated SNMM probe [56]:
(a) schematic of the probe structure; (b) probe interface for instrumentation.
1.3 Outline of Thesis
In the following chapters, the approaches to achieve the objectives of this work
will be present. In Chapter 2, the design of a novel SNMM probe based on MEMS
technology will be described. The design includes mechanical, electrical, and
microfabrication process considerations. In Chapter 3, the microfabrication process
details of each step and results of SNMM probes will be presented. In Chapter 4, the
mechanical and electrical characterizations of the microfabricated SNMM probes will be
performed. The application of the SNMM probes in scanning microwave imaging will be
conducted. In Chapter 5, the issues of the SNMM probes in microfabrication and
application will be discussed. Ideas of possible future work will be proposed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
13
References
1. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, “7 x 7 reconstruction on Si
(111) resolved in real space,” Phys. Rev. Lett. 50, 120 (1983).
2. A. Kleiner and S. Eggert, “Curvature, hybridization, and STM images of carbon
nanotubes,” Phys. Rev. B 64, 113402 (2001).
3. N. Nilius, N. Ernst, and H.-J. Freund, “Tip influence on plasmon excitations in
single gold particles in an STM,” Phys. Rev. B 65, 115421 (2001).
4. J. Nieminen, S. Lahti, S. Paavilainen, and K. Morgenstem, “Contrast changes in
STM images and relations between different tunneling models,” Phys. Rev. B 66,
165421 (2002).
5. S. Urazhdin, S. H. Tessmer, and R. C. Ashoori, “A simple low-dissipation
amplifier for cryogenic STM,” Rev. Sci. Instrum. 73, 310 (2002).
6. G. Binnig, C. F. Quate, and Ch. Gerber, “Atomic force microscope,” Phys. Rev.
Lett. 56, 930 (1986).
7. T. R. Albrecht and C. F. Quate, “Atomic resolution imaging of a nonconductor by
atomic force microscopy,” J. Appl. Phys. 62, 2599 (1987).
8. M. D. Kirk, T. R. Albrecht, and C. F. Quate, “Low-temperature atomic force
microscopy,” Rev. Sci. Instrum. 59, 833 (1988).
9. A. Vinckier, F. Hennau, K. Kjoller, and L. Hellemans, “Low-cost modification of
a contact atomic force microscope (AFM) into a sound-activated tapping mode
AFM for use in air and liquids,” Rev. Sci. Instrum. 67, 387 (1996).
10. T. Akiyama, S. Gautsch, N. F. de Rooij, U. Staufer, Ph. Niedermann, L. Howald,
D. Muller, A. Tonin, H.-R. Hidber, W. T. Pike, M. H. Hecht, “Atomic force
microscope for planetary applications,” Sensors and Actuators A 91, 321 (2001).
11. J. R. Matey and J. Blanc, “Scanning capacitance microscopy,” J. Appl. Phys. 57
1437 (1985).
12. C. C. Williams, W. P. Hough, and S. A. Rishton, “Scanning capacitance
microscopy on a 25 nm scale,” Appl. Phys. Lett. 55, 203 (1989).
13. R. C. Barrett and C. F. Quate, “Charge storage in a nitride-oxide-silicon medium
by scanning capacitance microscopy,” J. Appl. Phys. 70, 2725 (1991).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
14
14. Y. Huang, C. C. Williams, and J. Slinkman, “Quantitative two-dimensional
dopant profile measurement and inverse modeling by scanning capacitance
microscopy,” Appl. Phys. Lett. 66, 344 (1995).
15. T. Tran, D. R. Oliver, D. J. Thomson, and G. E. Bridges, “"Zeptofarad" (10'21 F)
resolution capacitance sensor for scanning capacitance microscopy,” Rev. Sci.
Instrum. 72, 2618 (2001).
16. U. Hartmann, “Magnetic force microscopy: Some remarks from the
micromagnetic point of view,” J. Appl. Phys. 64, 1561 (1988).
17. H. J. Mamin, D. Rugar, S. E. Lambert, L. P. Franco, I. L. Sanders, T. Yogi, and T.
Beaulieu, “Magnetic force microscopy of recording media,” J. Appl. Phys. 67,
5953 (1990).
18. A. DiCarlo, M. R. Scheinfein, and R. V. Chamberlin, “Magnetic force
microscopy utilizing an ultrasensitive vertical cantilever geometry,” Appl. Phys.
Lett. 61, 2108 (1992).
19. G. D. Skidmore and E. D. Dahlberg, “Improved spatial resolution in magnetic
force microscopy,” Appl. Phys. Lett. 71, 3293 (1997).
20. T. G. Sorop, C. Untiedt, F. Luis, M. Kroll, M. Ra§a, and L. J. de Jongh,
“Magnetization reversal of ferromagnetic nanowires studied by magnetic force
microscopy,” Phys. Rev. B 67, 014402 (2003).
21 .0 . Nakabeppu, M. Chandrachood, Y. Wu, J. Lai, and A. Majumdar, “Scanning
thermal imaging microscopy using composite cantilever probes,” Appl. Phys. Lett.
66, 694 (1995).
22. E. Oesterschulze, M. Stopka, L. Ackermann, W. Scholz, and S. Werner, “Thermal
imaging of thin films by scanning thermal microscope,” J. Vac. Sci. Technol. B 14,
832 (1996).
23. K. Luo, R. W. Herrick, A. Majumdar, and P. Petroff, “Scanning thermal
microscopy of a vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 71, 1604
(1997).
24. G. Mills, H. Zhou, A. Midha, L. Donaldson, and J. M. R. Weaver, “Scanning
thermal microscopy using batch fabricated thermocouple probes,” Appl. Phys.
Lett. 72, 2900 (1998).
25. L. Shi, S. Plyasunov, A. Bachtold, P. L. McEuen, and A. Majumdar, “Scanning
thermal microscopy of carbon nanotubes using batch-fabricated probes,” Appl.
Phys. Lett. 77, 4295 (2000).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15
26. A. Harootunian, E. Betzig, M. Isaacson, and A. Lewis, “Super-resolution
fluorescence near-field scanning optical microscopy,” Appl. Phys. Lett. 49, 674
(1986).
27. M. Isaacson, J. A. Cline, and H. Barshatzky, “Near-field scanning optical
microscopy II,” J. Vac. Sci. Technol. B 9, 3103 (1991).
28. W. M. Duncan, “Near-field scanning optical microscope for microelectronic
materials and devices,” J. Vac. Sci. Technol. A 14, 1914 (1996).
29. M. H. Gray and J. W. P. Hsu, “A variable cryogenic temperature near-field
scanning optical microscope,” Rev. Sci. Instrum. 70, 3355 (1999).
30. P. N. Minh, T. Ono, and M. Esashi, “High throughput aperture near-field
scanning optical microscopy,” Rev. Sci. Instrum. 71, 3111 (2000).
31. Y. Wang and M. Tabib-Azar, “Microfabricated Near-field Scanning Microwave
Probes,” Electron Devices Meeting, IEDM '02. Digest. International, IEEE, 905
( 2002).
32. S. Akamine, T. R. Albrecht, M. J. Zdeblick, and C. F. Quate, “Microfabricated
scanning tunneling microscope,” IEEE Electron Device Letters 10, 490 (1989).
33. T. S. Ravi, B. Marcus, and D. Liu, “Oxidation sharpening of silicon tips, ” J. Vac.
Sci. Technol. B 9, 2733 (1991).
34. D. W. van der Weide and P. Neuzil, “The Nanoscilloscope: Combined
Topography and AC Field Probing with Micromachined Tip,” J. Vac. Sci.
Technol. B 14, 4144 (1996).
35. J. Thaysen, A. Boisen, O. Hansen, and S. Bouwstra, “Atomic force microscopy
probe with piezoresistive read-out and a highly symmetrical Wheatstone bridge
arrangement.” Sensors and Actuators A 83, 47 (2000).
36. D. W. Lee, T. Ono, and M. Esashi, “Cantilerver with integrated resonator for
application of scanning probe microscope.” Sensors and Actuators A 83, 11
( 2001 ).
37. T. R. Albrecht and C. F. Quate, “Atomic resolution with the atomic force
microscope on conductors and nonconductors,” J. Vac. Sci. Technol. A 6, 271
(1988).
38. T. R. Albrecht, S. Akamine, T. E. Carver, and C. F. Quate, “Microfabrication of
cantilever styli for the atomic force microscope,” J. Vac. Sci. Technol. A 8, 3386
(1990).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
16
39. M. Tortonese, R. C. Barrett, and C. F. Quate, “Atomic resolution with an atomic
force microscope using piezoresistive detection,” Appl. Phys. Lett. 62, 834 (1993).
40. J. Brugger, M. Despont, C. Rossel, H. Rothuizen, P. Vettiger, M. Willemin,
“Microfabricated ultrasensitive piezoresistive cantilevers for torque
magnetometry,” Sensors and Actuators A 73, 235 (1999).
41. S. R. Manalis, S. C. Minne, and C. F. Quate, “Atomic force microscopy for high
speed imaging using cantilevers with an integrated actuator and sensor,” Appl.
Phys. Lett. 68, 871 (1996).
42. S. R. Manalis, S. C. Minne, A. Atalar, and C. F. Quate, “High-speed atomic force
microscopy using an integrated actuator and optical lever detection,” Rev. Sci.
Instrum. 67, 3294 (1996).
43. E. H. Synge, “A suggested model for extending microscopic resolution into the
ultra-microscopic region,” Phil. Mag. 6, 356 (1928).
44. E. A. Ash and G. Nichols, "Super-resolution aperture scanning microscope",
Nature 237, 510 (1972).
45. M. Tabib-Azar, N. Shoemaker, and S. Harris, “Non-destructive characterization
of materials by evanescent microwaves.” Measurement Science and Technology 4,
583 (1993).
46. M. Tabib-Azar, D.-P. Su, A. Pohar, S. R. LeClair, and G. Ponchak, “0.4 pm
spatial resolution with 1 GHz (L=30cm) evanescent microwave probe,” Rev. Sci.
Instrum. 70, 1725 (1999).
47. M. Tabib-Azar, P. S. Pathak, G. Ponchak, and S. LeClair, “Nondestructive
superresolution imaging of defects and nonuniformities in metals,
semiconductors, dielectrics, composites, and plants using evanescent
microwaves,” Rev. Sci. Instrum. 70, 2783 (1999).
48. M. Tabib-Azar, D. Akinwande, G. E. Ponchak, and S. R. LeClair, “Evanescent
microwave probes on high-resistivity silicon and its application in
characterization of semiconductors,” Rev. Sci. Instrum. 70, 3083 (1999).
49. M. Tabib-Azar and B. Sutapun, “Novel hydrogen sensors using evanescent
microwave probes,” Rev. Sci. Instrum. 70, 3707 (1999).
50. M. Tabib-Azar, D. Akinwande, G. Ponchak, and S. R. LeClair, “Novel physical
sensors using evanescent microwave probes,” Rev. Sci. Instrum. 70, 3381 (1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
17
51. M. Tabib-Azar and D. Akinwande, “Real-time imaging of semiconductor spacecharge regions using high-spatial resolution evanescent microwave microscope,”
Rev. Sci. Instrum. 71, 1460 (2000).
52. D. E. Steinhauer, C. P. Vlahacos, S. K. Dutta, F. C. Wellstood, and S. M. Anlage,
“Surface resistance imaging with a scanning near-field microwave microscope,”
Appl. Phys. Lett. 71, 1736 (1997).
53. D. E. Steinhauer and S. M. Anlage, “Microwave frequency ferroelectric domain
imaging of deuterated triglycine sulfate crystals,” J. Appl. Phys. 89, 2314 (2001).
54. C. Gao, T. Wei, F. Duewer, Y. Lu, and X.-D. Xiang, “High spatial resolution
quantitative microwave impedance microscopy by a scanning tip microwave near­
field microscope,” Appl. Phys. Lett. 71,1872 (1997).
55. C. Gao and X.-D. Xiang, “Quantitative microwave near-field microscopy of
dielectric properties,” Rev. Sci. Instrum. 69, 3846 (1998).
56. B. T. Rosner and D. W. van der Weide, “High-frequency near-field microscopy,”
Rev. Sci. Instrum.lZ, 2505 (2002).
57. B. T. Rosner, “Near-field microscopy from the microwave regime to the visible,”
Ph.D. Thesis, Department of Electrical Engineering, University of Delaware
( 2002 ).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2 Design of AFM Compatible SNMM Probes
2.1 Mechanical Design
The proposed microwave near-field probes have dimensions compatible with
commercial AFM systems. The mechanical design of the probes has two parts: a
cantilever beam and an integrated tip section. V-shaped cantilever beam structure was
hosen becasue its stability of mechanical vibration. We designed the SNMM probes to
have the same footprint as commercial AFM probes. The elastic properties of
microfabricated cantilever beams are well documented. The resonant frequency of AFM
probes is typically between 10-100 KHz and their effective Hooke’s or spring constants
is typically between 0.1-10 N/m. Silicon and associated materials, such as silicon nitride,
oxide, and metals are extensively used in AFM cantilever probes [1-5]. Figure 2-1 shows
the Scanning Electron Microscope (SEM) photo of a commercial AFM probe with a
SisN4 tip integrated on a V-shaped cantilever beam. It has beam length of 200 pm, beam
width 18 pm, beam thickness of 0.6 pm, and tip height of 5 pm. It has a fundamental
resonant frequency of 17 KHz, and a spring constant of 0.03 N/m. In this work, the
SNMM probes were designed to operate with resonant frequencies 10-200 KHz, the
spring constants 0.1-10 N/m, and tip height 10 pm, comparable values to those of the
commercial AFM probes.
18
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
Figure 2-1. SEM photo of a commercial AFM probe.
It is well known [6] that the spring constant K for V-shaped cantilever beam is
K =
6EI
EW ( O 3
(2- 1)
The mechanical resonant frequency fo is expressed by
352 t
fo =
n
E
L 2 \ 12p
(2 - 2 )
Where W is the beam width, L is its length, and t is its thickness. The Young’s modulus
for (100) orientation silicon is E = 130 GPa [7], mass density p = 2330 kg/m3, the beam
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
width was designed to be 50 (am, the beam thickness 3 - 5 |im, and the beam length 300
pm - 1000 pm. The spring constant and resonant frequency calculation results are listed
in Table 2-1.
3
II
cn
1
L= 450 pm
II
i
II
Resonant frequency
(KHz)
cn
Spring constant
(N/m)
L =300 pm
t = 5 pm
C"F
W = 50 pm
L=1000 pm
t = 3 pm t = 5 pm
3.3
25
0.96
4.5
0.09
0.6
80.5
134.2
35.8
59.7
7.2
12.1
Table 2-1. Spring constant and fundamental resonant frequency calculations for
different beam dimensions.
The second part of the probes is the tip section integrated with the cantilever beam.
The tip position was designed to be as close as possible to the free end of the cantilever.
The area of the free end of the cantilever should be as large as possible to facilitate laser
beam alignment on the AFM scanner head. The chip body that mechanically supports the
beams should be comparable in dimension to commercial probes to allow use in
commercially available AFM systems. The SNMM probe characterization and
application will be carried out on a Thermomicroscopes Explorer™ (now Veeco
Instruments Inc.) AFM scanning system. The introduction of this system will be
described in Chapter 4. It is shown in figure 2-2 that a commercial AFM probe chip body
has a dimension of 3.6 mm long and 1.6 mm wide from the data sheet provided from the
manufacturer of the Explorer™ AFM scanner [8]. Therefore, each probe chip was
designed to be 3.6 mm long and 1.6 mm wide, easily exchangeable with commercial
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
21
AFM probes to be mounted on the scanner for both topology and microwave
measurements.
A
A
B
A_
CO
<6
w—
—v
C
D
1.6 mm
Chip
Figure 2-2. Overview of microprobe dimension [8].
2.2 Microwave Design
From the microwave point of view, the probe has three main sections, as shown in
figure 2-3. The first section is the waveguide that guides the microwave signal from the
generator to the probe tip. The second section is the tip region that confines the fields by
its conical shape and co-axial geometry. The third section is the tip-sample interaction
section.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
22
■
Shield
11_____________
Waveguide
Co-axial tip
(Section 2)
Section 3
Section 1
Figure 2-3. Schematic of the proposed AFM compatible co-axial SNMM probe.
Figure 2-4 illustrates the interaction between a co-axial tip and sample within a
near-field distance. The tip-sample interaction can be represented by a coupling
capacitance Cc. There is also an intrinsic capacitance Cp existing between the sharp tip
and shield layer in the co-axial structure.
Highly
conductive
silicon
Silicon
Insulator
Shield
Sample
Figure 2-4. Co-axial tip and sample interaction.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
23
Figure 2-5 shows the simulation of electromagnetic fields near the tip and the
effect of an aperture (the outer layer of the co-axial tip can be viewed as an aperture) [9].
Figure 2-5a shows the equipotential contours between a tip and a sample without the
existence of shielded aperture. Figure 2-5b shows the equipotential contours between a
tip and a sample with the presence of an aperture. It is clearly shown that the
Electromagnetic field (EM) lines, which are perpendicular to the simulated equipotential
contours, are more confined with the presence of a shielded aperture. The field lines can
be further confined by an appropriate aperture size. Besides the advantage of confining
the EM filed intensity to reduce the effective electrical area A ejf that illuminates the
sample to be measured, the co-axial waveguide structure is also capable of non-cutoff
transverse electromagnetic (TEM) mode wave propagation. Together with the waveguide
structures on the chip body, the proposed SNMM probe can operate at very high
frequencies.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
24
Vo 11: age
1.0000*+003
/ .A U Jk/
9 . 0 0 0 0 e+ 002
8.0000«+00£
7
.
0000*+002
6 .0 0 0 0 e+ 0 0 2
S . 0000*+002
4 . 0000*+002
3 . 0000*+002
2 . 0000«+002
1. 0 0 0 0e+ 00 2
0 . 0 0 00*+000
*
V
*
V
V
1
a
> ■ - ..
A
Uc-ltige
1 l.OO0Ot+OO3
V
V
's
iS
Si /
/ ,
3 . 00004+002
8 . 00004+002
7 . 00004+002
6 . 00004+002
5 . 00004+002
4 . 00004+002
3 . 00004+002
2 . 00004+002
1 . 00004+002
O.OOOOe+OOO
A A
A
A
A
Figure 2-5. Simulated electric field intensity patterns (a) without an aperture [9];
(b) with an aperture [9].
The electrical model for the waveguide section is shown in figure 2-6 [10-12].
The waveguide section is modeled by the well-known lumped inductor and capacitor (LC)
circuit and by its resulting characteristic impedance. The characteristic impedance may be
complex when appropriate leakage and dissipation resistors are added to the LC circuit.
So that R0, Lq, and C0 in series can model the waveguide circuit parameters.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
25
Ro
o
Z in
VW
►
o ------------------------------1
Figure 2-6. Lumped circuit model of the waveguide.
The tip section can also be modeled by a lumped LC circuit, however, since the
tip section is much smaller than the smallest wavelength in our design range (f=l-20 GHz
with A,free=30-1.5 cm), we can simply model it by an additional capacitance (Cp) as shown
in figure 2-7. The tip-sample interaction region can be modeled by the coupling
capacitance Cc in non-contact mode, and by a resistor Rc in the contact mode. For the
microwave measurement to be sensitive, Cp should be as small as possible. Moreover,
semiconducting and insulating samples can be modeled by a resistance (Rs) and a
capacitance (Cs), as shown in figure 2-7a [13]. Metallic samples can be modeled by a
surface resistance (Rs) and an inductance (Ls), as shown in figure 2-7b [13].
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ro
Lo
o-
Zin
O
T
(b)
Figure 2-7. Lumped circuit model of the waveguide, the tip-sample coupling, and
samples: (a) a dielectric sample; (b) a conductive sample.
The sensitivity of a resonant microwave probe in detecting a change in the
sample’s conductance in non-contact mode is given by [10]:
2 riVnm rs 2 ^ ( C n 1 Cc 'f U c0 o0 J vl + C c / Cn)
2 cr.y<5
0/
A ct s
os
~
V.in
( v1 - 2 C J0 C ) cQ
(2-3)
where Vnrms is the root-mean-square (RMS) value of noise voltage, Vjn is the RMS value
of input microwave voltage, C0 and Lo are respectively the capacitance and inductance
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
per unit length of the waveguide, too is the microwave radial frequency, a s is the
sample’s surface conductivity, 8 is the skin depth, and Q is the electrical quality factor of
the waveguide. Similarly, the sensitivity of the resonant probe in detecting a change in
the sample’s permittivity is given by [10]:
Ae,
£S
47iVnmrs C„d e,
Vin
le f f ^ 0
(2-4)
where “d” is the distance between the probe tip and the sample, and A eff is the effective
electrical area of the tip. From the above expressions it can be seen that to increase A a /a s,
Cc should be increased (=£o Aejf/d) and Cp (~Co near the tip) should be reduced compared
to Cs (proportional to es). To achieve very high spatial resolution, both “<i” and Aejj
should be made as small as possible. Aejf is directly determined by the physical size of the
tip and by the extension of the electromagnetic fields. To enable AeJf to be very close to
the physical size of the tip (~ 5-10 nm), the tip should be electromagnetically shielded by
a co-axial metallic layer thicker than the skin depth (l/(7ifpa)1/2) [14] at the operation
frequency. From processing point of view, the thinner film layer is much easier to be
deposited and patterned. This clearly requires that the outer layer of the co-axial tip
should be metal material with high conductivity.
Figure 2-8 shows the cross-section view of the proposed SNMM probe. It consists
of a silicon V-shaped cantilever beam and a coaxially shielded tip on the free end of the
cantilever beam. A metal waveguide is defined on the beam, forming an ohmic contact
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28
with the highly doped co-axially shielded tip region where a sharp silicon tip protrudes
through an aperture opened in the shielding metal layer and the isolation layer.
Co-axially Shielded Tip
^
Metal (waveguide)
Silicon (high-resistivity)
(shield)
Insulator
Heavily-doped Si
Figure 2-8. Cross-section view of the proposed AFM compatible SNMM probe.
2.3 Mirofabrication Process Design
The microfabrication process design is the key part of this work. The possible
microfabrication methods and tools will be introduced, and final approach will be
determined with the consideration of efficiency and availability of facilities. Most of the
process will be conducted in the Microfabrication Lab (MFL) at CWRU.
Microfabrication of the proposed SNMM probes has three main parts. The first
part is the formation of the sharp silicon tip. The silicon tip is first formed by an etched
precursor. The precursor provides the initial shape of the silicon tip, which is realized by
under-etching of a mask that defines the tip using wet etch or dry etch methods, as
illustrated in figure 2-9.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
29
V77A
(a)
(C)
Figure 2-9. Silicon tip plasma etching procedure.
(a) Defining tip mask, (b) Under-etching mask, (c) Forming tip precursor.
A well-known technique to form very sharp silicon tips based on the precursors is
the low temperature oxidation sharpening method [15-16] that relies on an anomaly in
silicon oxidation rate near the apex (high stress region) of the tip. The low temperature
thermal oxidation results in lower oxidation rate near the convex tip (high stress region)
compared to the body of the tip that becomes more concave after subsequent
oxidation/etch steps, as illustrated in figure 2-10. Thus, thermal oxidation and oxide
removal can be performed a few times to sharpen the tip furthermore. Tips with
curvatures less than 50 A can be formed this way.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30
Figure 2-10. Schematic of low temperature oxidation sharpening.
In this work, SF6 plasma etching method with thermal oxide as circular mask was
chosen to form the silicon tip precursor. The lateral etch ratio LR was experimentally
determined to design appropriate mask size to get desired tip height. LR is defined as the
ratio of the lateral etch rate to the vertical etch rate. If LR=1, it is said be isotropic etching.
Otherwise, it is anisotropic etching. Figure 2-11 shows an SEM photo of cross-section
view of an SF6 etched silicon pattern. The lateral etch ratio can be calculated from the
measured lateral etch size and the vertical etch depth. In this SF6 plasma etching
environment, the lateral etch ratio was X
A . This means the circular oxide mask diameter
should be the same dimension as the desired tip height. In this work, it was 10 pm for the
tip height of 10 pm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
5 . 0
k V
x 1 . 0 0 K
30.
0 ^ m
Figure 2-11. SEM photo of a silicon pattern etched by SF6 in the MFL.
For SNMM applications, especially for resonant SNMM probes, a metallic
coating is necessary to reduce the surface resistance of the waveguides. In these
applications, the tip can be high conductivity silicon but a metallic waveguide is required
to guide the signal to and from the tip region. High conductive silicon tip can be formed
by ion implantation method. We chose aluminum as the metal material to be used in this
work because of its high conductivity (p = 2.7 pQ- cm) and easy wet etching with the
commercial available etchant in the MFL at CWRU.
The second part is the fabrication and formation of the cantilever beam itself. This
is well understood and has been carried out by many researchers [17-21]. There are
several existing microfabrication approaches to realize the cantilevers integrated with tips
[22-25]. In our design, we chose Silicon On Insulator (SOI) substrates and Reactive Ion
Etch (RIE) method to realize the cantilevers with tips. The intermediate oxide layer
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
32
serves as an etch stop and allows us to achieve a homogeneous etch depth over a full 4inch wafer during cantilever release processing.
The third part of the SNMM probe is the formation of the coaxial tip. This
involves coating the sharpened tip with an insulation layer followed by an aluminum
shield layer. Considering aluminum waveguide layer should be deposited and patterned
before the insulation layer being patterned, low temperature oxide chemical vapor
deposition method (CVD) was chosen instead of thermal oxidation to form the insulator.
Metallization of the shield will result in deposition of the metallic layer over the tip apex
as well. Removing the metallic and oxide layer from the apex of the tip in a controlled
manner is the most challenging part of the proposed work.
We took advantage of the smart process techniques developed by other
researchers [26-32] in NSOM, SNMM probes microfabrication and combined with the
available facilities at CWRU to form our method to realize the co-axial tips in this work.
The method we developed is the combination of thick photoresist (PR) coating and
oxygen plasma etching. We called the method “tip exposure”. This method is illustrated
in figure 2-12. First the silicon tip is coated with thick PR after the auluminum shield
layer being deposited and patterned (figure 2-12a). After prebaking, the thick PR is
etched by oxygen plasma in a controlled time expose the tip apex with a small opening
(figure 2-12b). At this time, the tip apex is still covered by aluminum shield and oxide
layer. Then aluminum wet etch is used to remove the metal shield not covered by the
thick PR, exposing the oxide insulation layer (figure 2-12c). After that, the oxide layer is
wet etched to expose the silicon tip apex, resulting the desired co-axial structure.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
33
□
Si
(a)
(b)
(c)
(d)
m
O xide
m
PR
c~
A1
Figure 2-12. Tip exposure method to form co-axial tip
The thick PR coating technique was developed in the MFL at CWRU by the
former researchers in the MEMS group [33]. This coating technique can achieve uniform
PR coating on the wafer up to 20 pm without concern of cracking problems. The oxygen
plasma etching is the critical step. We used an oxygen plasma-etching machine with the
model name of M4L™, which is available in the MFL at CWRU, to achieve the co­
axially shielded tip structure. The RF generator of M4L™ has built in 600-watt power at
frequency 13.56 MFIz with automatic impedance matching network. The RF generator is
the source of energy used to generate the plasma. It converts standard 60 Hz AC line
power to 13.65 MHz. The chosen working parameters are 500 seem O2 at pressure of
1000 mT, with RF power density of 0.66 W/cm2. Figure 2-13 shows a plasma-etching
curve of PR thickness on a 4-inch wafer versus etching time. Figure 2-14 shows the linear
fit curve of the data points in equal 10-minute etch intervals. This experiment shows that
the PR etch is not linearly dependent on the etch time, but in the same etch time period,
the etch rate is almost constant, such as the etch rate of 277 A/min in a 10-minute etch
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
34
(shown in figure 2-14). We can utilize this property to achieve different aperture
exposure size in a controlled manner.
70000
PR thickness (A)
60000
50000
40000
30000
20000
10000
0
0
15
30
45
60
75
90
Etch time (min)
Figure 2-13. M4L™ oxygen plasma etching curve.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
35
45000
40000
^
35000
°S 30000
cn
v:
<D 25000
20000
y = -276.76x + 43822
c* 15000
10000
5000
0
15
30
45
60
75
90
Etch time (min)
Figure 2-14. Linear fit curve shows uniform etch rate in the 10 minutes’ intervals.
2.4 Microfabrication Process Flow
Figure 2-15 illustrates the designed process flow according to the sequential
photolithography steps. Totally seven photolithography masks are used in this work. The
approach of the microfabrication process is to form the silicon tip first, then pattern the
waveguides and shields in a sequential manner. After that, the co-axially shielded tips
will be fabricated using the “tip exposure” technique. Finally two steps of RTF etch will
be used to define the V-shaped beams and release the SNMM probes. The key process
steps figure 2-15 are listed below. The cross-section view is along the AA’ direction.
(a) Thermal oxidation, photolithography step No.l, oxide etching, SF6 etching tip.
(b) Tip oxidation sharpening, photolithography step No.2, oxide etching, P+ ion
implantation in the tip region.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
36
(c) A1 alloy deposition, photolithography step No.3, A1 etching to form
waveguide, sintering to form ohmic contact.
(d) Low temperature oxide (LTO) deposition.
(e) A1 deposition, photolithography step No.4, A1 etching to form shield layer,
photolithography step No.5, LTO etching.
(f) Top view after step (e).
(g) Thick PR coating, oxygen plasma etching to expose tip, A1 and LTO etching
to form coaxial geometry.
(h) Photolithography step No. 6, topside silicon RTE etching.
(i) Photolithography step No.7, backside Si DRIE etching, buried oxide layer
etching.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
37
X
/7//77/T/S/ / / / / / / / / / / / / .
(b)
(a)
SZZZ2
r?/r7-r
TT7
(c)
'///////////////////////,
(e)
/ / / / / / / / / / / / /
a
/ / / / / / / / / / / / / / / / ?/ / Y7J/ 71\
(h)
□
/ /// // / // / // / /.
Si
Oxide
P+ diffusion
A1
(i)
Figure 2-15. Process flows of SNMM probes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38
2.5 SNMM Probe Layout Design
The photo mask layout was drawn according to the process flow design, as shown
in figure 2-16. More than 300 chips can be fabricated on a 4-inch wafer with wafer-stage
processing.
n m o i
□□DOCK
0
0
D
nm nm c
nm nm t
m m i
□ m nm r
Figure 2-16. Overview of the layout of SNMM probes on a 4-inch wafer.
Figure 2-17 shows layouts of three probe cells with the beam width 50 pm, the
beam length 300 pm (figure 2-17 a), 450 pm (figure 2-17 b), and 1000 pm (figure 2-17 c).
There were four beams on each chip to connect the probe to the wafer frame. They were
designed to be easily broken with sharp tweezers to pick the individual probe out from
the wafer after the microfabrication was completed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2-17. Probe cell layouts with W= 50 pm and different beam lengths.
References
1. T. R. Albrecht, S. Akamine, T. E. Carver, and C. F. Quate, “Microfabrication of
cantilever styli for the atomic force microscope,” J. Vac. Sci. Technol. A 8, 3386
(1990).
2. A. G. T. Ruiter, M. H. P. Moers, N. F. van Hulst, and M. de Boer,
“Microfabrication of near-field optical probes,” J. Vac. Sci. Technol. B 14, 597
(1996).
3. A. Folch, M. S. Wrighton, and M. A. Schmidt, “Microfabrication of oxidationsharpened silicon tips on silicon nitride cantilevers for atomic force microscopy,”
Journal o f Microelectromechanical Systems 6, 303 (1997).
4. Y. Su, A. G. R. Evans, A. Brunnschweiler, G. Ensell, M. Koch, “Fabrication of
improved piezoresistive silicon cantilever probes for the atomic force
microscope,” Sensors and Actuators A 60, 163 (1997).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
40
5. A. Chand, M. B. Viani, T. E. Schaffer, P. K. Hansma, “Microfabricated small
metal cantilevers with silicon tip for atomic force microscopy,” Journal o f
Microelectromechanical Systems 9, 112 (2000).
6. C. Liu and R. Gamble, “Mass-producible monolithic silicon probes for scanning
probe microscopes,” Sensors and Actuators A 71, 233 (1998).
7. K. E. Peterson, “Silicon as a mechanical material,” Proc. IEEE, 70, 420 (1982).
8. Data sheet from ThermoMicroscopes Inc, (2001).
9.
M. Tabib-Azar, NIST Project Proposal, (2001).
10. M. Tabib-Azar, Evanescent Microwave Microscopy for High-Speed and HighResolution Material Characterizations, Kluwer Academic Publishers, Boston
(2000).
11. M. Tabib-Azar, Micro-Actuators, Kluwer Academic Publishers, Boston (1998).
12. S. Ramo, J. R. Whinner, and T. Van Duzer, Fields and Waves in Communication
Electronics, John Wiley and Sons, Inc., New York (1984).
13. M. Tabib-Azar, D.-P. Su, A. Pohar, S. R. LeClair, and G. Ponchak, “0.4 pm
spatial resolution with 1 GHz (X=30cm) evanescent microwave probe,” Rev. Sci.
Instrum. 70, 1725 (1999).
14. J. D. Kraus, Electromagnetics with applications. McGraw-Hill Companies, Inc.
Boston (1999).
15. T. S. Ravi, B. Marcus, and D. Liu, “Oxidation sharpening of silicon tips,” J. Vac.
Sci. Technol. B 9, 2733 (1991).
16. N. E. McGruer, K. Warner, P. Singhal, J. J. Gu, C. Chan, “Oxidation-sharpened
gate field emitter array process,” IEEE Transaction on Electron Devices 38, 2389
(1991).
17. S. R. Manalis, S. C. Minne, and C. F. Quate, “Atomic force microscopy for high
speed imaging using cantilevers with an integrated actuator and sensor,” Appl.
Phys. Lett. 68, 871 (1996).
18. A. Boisen, O. Hansen and S. Bouwstra, “AFM probes with directly fabricated
tips.” J. Micromech. Microeng. 6, 58 (1996).
19. J. Brugger, M. Despont, C. Rossel, H. Rothuizen, P. Vettiger, M. Willemin,
“Microfabricated ultrasensitive piezoresistive cantilevers for torque
magnetometry,” Sensors and Actuators A 73, 235 (1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
41
20. G. Schurmann, P. F. Indermiihle, U. Staufer, N. F. de Rooij, “Micromachined
SPM probes with sub-100 nm features at tip apex,” Suface and Interface Analysis
27, 299 (1999).
21. D. W. Lee, T. Ono, and M. Esashi, “Cantilerver with integrated resonator for
application of scanning probe microscope.” Sensors and Actuators A 83, 11
( 2001).
22. M. Radmacher, P. Hillner, and P. Hansma, “Scanning nearfield optical
microscope using mcirofabricated probes,” Rev. Sci. Instrum. 65, 2737 (1994).
23. H. U. Danzebrink, O. Ohlsson, and G. Wilkening, “Fabrication and
characterization of optoelectronic near-field oribes based on an SFM cantilever
design,” Ultramciroscopy, 61, 131 (1995).
24. T. Leinhos, M. Stopka, and E. Oesterschulze, “ Micromachined fabrication Si
cantilevers with Schottky diode integrated in tip,” Appl. Phys. A 66, S65 (1998).
25. G. Schurmann, W. Noell, U. Staufer, N. F. de Rooij, “Microfabrication of a
combined AFM-SNOM sensor,” Ultramciroscopy 82, 33 (2000).
26. R. C. Davis, C. C. Williams, and P. Neuzil, “Micromachined submicrometer
photodiode for scanning probe microscopy,” Appl. Phys. Lett. 66, 2309 (1995).
27. Y. Zhang, Y. Zhang, J. Blaser, T. S. Sriram, A. Enver, and R. B. Marcus, “A
thermal microprobe fabricated with wafer-stage processing,” Rev. Sci. Instrum. 69,
2081 (1998).
28. L. Shi, O. Kwon, A. C. Miner, A. Majumdar, “Design and batch fabrication of
probes for sub-100 nm scanning thermal microscopy,” Journal o f
Microelectromechanical Systems 1, 370 (2001).
29. D. W. van der Weide, “Localized picosecond resolution with a near-field
microwave/scanning-force microscope,” Appl. Phys. Lett. 70, 677 (1996).
30. D. W. van der Weide and P. Neuzil, “The nanoscilloscope: Combined topography
and AC field probing with a micromachined tip,” J. Vac. Sci. Technol. B 14, 4144
(1996).
31. T. Bork, V. Agrawal, B. Rosner, P. Gustafson, and D. van der W eide, “Shielded-
tip/cantilever process and interface for multifunctional scanning probe
microscopy,” Solid-State Sensor and Actuator Workshop, Hilton Head Island,
South Carolina, 271 (2000).
32. B. T. Rosner, “Near-field microscopy from the microwave regime to the visible,”
Ph.D. Thesis, Department of Electrical Engineering, University of Delaware
(2002).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
33. H. Niyajima and M. Meheregany, “High-Aspect-Ratio Photolithography for
MEMS Applications.” Journal o f Microelectromechanical Systems 4, 220 (1995).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3 Microfabrication of SNMM Probes
3.1 Introduction
In this Chapter, the details of the SNMM probes microfabrication process are
presented. The final device should be explained before the detailed process steps are
described. Figure 3-1 shows the three-dimensional schematic view of an SNMM probe
after microfabrication. The SNMM probe will be fabricated on the device layer of an SOI
substrate. The handle layer provides mechanical support for the cantilever structure. The
buried oxide layer acts as etching stop layer during the fabrication. An aluminum (Al)
waveguide is patterned on the left arm of the V-shaped cantilever beam, and an Al
shielding layer on the right arm of the V-shaped cantilever beam. A co-axially shielded
tip lies on the free end of the cantilever beam.
n
Si
□
Si
Oxide
Al
Figure 3-1. Three-dimensional schematic view of a microfabricated SNMM probe.
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
44
The starting substrate was a 4-inch double-side polished (DSP) SOI wafer with
device layer 15 pm, buried oxide layer 1 pm, and handle wafer thickness 400 pm. The
device layer of the SOI wafers, i.e., topside of the wafers, consisted of a p-type (100)
orientation silicon layer that was used to form the cantilever beams integrated with tips.
The resistivity of this device layer was 10 Q- cm. The buried oxide layer underneath the
device layer served as etch stop layer both for cantilever beam formation during device
layer RTF, etch and for handle wafer Deep Reactive Ion Etch (DREE) to release the
cantilever beam. This took advantage of the fact that the selectivity between silicon and
oxide is pretty high, e.g., 150, during DRTE etching. The thickness of the buried oxide
layer was chosen to be 1 pm to provide sufficient etch step buffer.
3.2 Microfabrication Process Details and Results
3.2.1
RCA Clean
The process began with wafer cleaning by using RCA method. Bare silicon wafer
should be chemically cleaned prior to a furnace step by RCA method [1]. This technique
first removed organic film contamination on the wafer by immersing the wafer together
with the wafer cassette in the solution of RCA1 for 15 minutes at 80 °C. The RCA1
solution was a fresh mixture of H2O-NH4OH-H2O2 (2700 ml: 500 ml: 500 ml). After
RCA1 clean, the wafer was rinsed in deionized (DI) water with resistivity of 10- 18 MQcm for 3 minutes. Then the wafer was immersed into a mixture of HF (49%)-H20 (100
ml: 5000 ml) for 30 seconds to strip the hydrous oxide film formed during RCA1 step.
Following the immersion in HF, the wafers were rinsed in DI water for 1 minute. The
wafer was then transferred to RCA 2 solution for 15 minutes at 80 °C to remove
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
45
remaining atomic and ionic contaminates. RCA 2 solution was a fresh mixture of H 2 OHCI-H2 O 2 (2700 ml: 500 ml: 500 ml). After RCA 2 clean, the wafer was rinsed in DI
water for 3 minutes. The last step of RCA clean was to put the wafer in a spin-dryer that
used DI water to rinse, and heated N 2 to dry the wafer.
3.2.2 Therm al Oxidation
Immediately following the RCA clean, 1 pm silicon dioxide was thermally grown
at 1100 °C by wet oxidation method on both sides of the wafer in an oxidation furnace.
The oxidation mechanism is based on that rapid silicon oxide is grown when
silicon is exposed to an oxidizing ambient at elevated temperature. The mechanism for
the oxide formation are given
Si (solid) + O2 (vapor) = SiC>2 (solid) Dry oxidation
(3-1)
Si (solid) + 2 H 2 O (vapor) = SiC>2 (solid) + 2H2 Wet oxidation
(3-2)
Dry oxidation happens when silicon is exposed to a stream of dry oxygen, and wet
oxidation occurs when silicon is exposed to stream of oxygen and water vapor. The
reaction occurs at the Si/SiC>2 interface, and silicon is consumed when the interface
moves into silicon during the oxidation. The silicon consumption is 44 % of the final
oxide thickness. Deal and Grove [1] developed a linear parabolic model for an accurate
calculation of the thickness of grown oxide in the 300 A - 2 pm range with temperatures
700 °C - 1300 °C. The model can be expressed by the equation:
x 2 + Ax = B(t + r)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(3-3)
46
(3-4)
B =^
N
B
(3-5)
(3-6)
Where x is the growing oxide thickness; t is the oxidation time; x represents the time
needed to account for a initial oxide layer thickness x; when t = 0; D is diffusion
coefficient of oxidant, a function of temperature and oxidant; ks is the silicon oxidation
rate constant at the Si/SiC>2 interface; h is gas phase mass transfer coefficient; C is the
equilibrium concentration of the oxidant in the bulk of oxide; N is the number of oxidant
molecules in a unit volume of oxide; B is the parabolic rate constant, B/A is the linear
oxidation rate constant.
Equation (3-3) can be simplified according to the oxidation time t. When (t + x)
« A2/4B, the oxide thickness can be calculated by
A
(3-7)
For very long oxidation times, t » x, equation (3.3) becomes
x 2 —Bt
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(3-8)
47
The Dean-Grove model clearly explains thermal oxidation on planar silicon.
However, this model is not suitable for the thermal oxidation process occurs on a nonplanar silicon structure.
3.2.3 Tip form ation
After thermal oxidation the wafer was vapor primed with hexamethyldisilazane
(HMDS) in the HMDS oven. This step is necessary because moisture in the atmosphere
can be absorbed by substrate surfaces so as to reduce the adhesion for the later
photoresist processing [1]. Usually a dehydration bake step is performed. Following the
dehydration step, the wafer is normally primed with a pre-resist coating of a material
designed to improve adhesion furthermore. The most widely used priming substance is
HMDS. One end of HMDS molecule reacts with wafer oxide surfaces to tie up molecular
water, the other end forms a bond with the resist. Therefore, HMDS behaves like
adhesion promoter.
Next the wafer was coated with positive photoresist Shipley 1813 using Solitec™
photoresist spin coater. The coating procedure worked as following: 1. The wafer was
held by a vacuum chuck in the spin coater. 2. A small volume of photoresist was
automatically dispensed on the center of the wafer. 3. The instrument precisely span the
wafer to uniformly spread out a coating of photoresist by spinning the wafer and chuck at
a speed of 4000 rpm (rotations per minute) for 30 seconds. A uniform photoresist layer
with 1.35 pm thickness was coated in this way.
Then the wafer was softbaked at 95 °C in a convection oven for 30 minutes. The
softbake was necessary to evaporate solvent from the spun-on photoresist, improve the
adhesion of the photoresist to the wafer, and anneal the stresses caused in the spinning
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
48
process. After softbake the photoresist thickness became 1.2 pm as a result of the solvent
loss during softbake. Photolithography step No.l was performed to define the tip-etching
mask using a Karl Suss MA6 contact aligner, which had intensity of 25 mW/cm with Hline (wavelength 405 nm) near-UV source. Following exposure, the wafer underwent
development with immersion in Microposit® 351 developer for 60 seconds to form the
tip mask. The tip mask was circular pattern with diameter of 12 pm in order to etch
silicon tip with 10 pm height. The wafer was hardbaked at 115 °C in the convection oven
for 30 minutes before the Buffered Oxide Etch (BOE) was used to pattern the oxide layer.
The BOE etchant has a thermal oxide etch rate of ~ 650 A/min. The purposes of hardbake
are to remove residual solvent in the photoresist, to improve the adhesion, and to enhance
the etch resistance of the photoresist. The dimension of the oxide mask changed to be 10
pm in diameter after BOE due to the isotropic nature of wet etch.
RTE (also refers to plasma etching) was chosen to form the silicon conical tip
structures in this work. The Lam™ is such a system with parallel plate electrodes
configuration. Its parameters for RIE are listed as follows. The etching gas was SF6
(Sulfur hexafluoride) with flow rate of 150 seem, carrier gas He 150 seem, reaction
chamber pressure 300 mTorr, RF power 200 W, the electrodes gap of 1.14 cm. The etch
rates of this recipe were ~ 6500 A/min for silicon, ~ 500 A/min for thermal oxide, and ~
2000 A/min for photoresist, respectively. Figure 3-2 illustrates the tip precursor
formation progress. Figure 3-2a is an optical micrograph of a circular oxide mask after 3
minutes’ plasma etching. Two rings can be clearly seen from this photo. The smaller ring
designates that the tip apex began to form. The larger ring represents the tip base
dimension, which is constant during plasma etching. Similarly, figure 3-2b and figure 3-
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
49
2c show the tip precursor photos after 9 minutes’ and 12 minutes’ etch. It can be seen that
the tip apex size shrank at a steady rate.
_Tip base
-Tip apex
(a)
#
(b)
(c)
Figure 3-2. Optical micrographs of a tip formation progress after plasma etching for
(a) 3minutes. (b) 9 minutes, (c) 12 minutes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
The plasma etching was stopped when the flat apex underneath the mask had a
diameter in the range of 0.5 pm - 1 pm. Figure 3-3a and figure 3-3b show the Scanning
Electron Microscope (SEM) photos of a silicon tip in this stage. Figure 3-3a is the skew
view of the tip in plasma etching at 45 ° angle, in which the photoresist and oxide mask
can be clearly seen. Figure 3-3b is the top view of the same tip.
(b)
Figure 3-3. SEM photos of a tip in plasma etching: (a) skew view at 45 ° angle; (b)
top view.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
51
The final sharp silicon tip can be formed by low temperature oxidation sharpening
method. Two functions can be achieved by this step at the same time. First, it prevents
the early-finished tips from being blunted. Second, the oxidation sharpening is utilized to
sharpen the tips furthermore [2-4]. This is based on the principle of known oxidation
inhibition at regions of high curvature, where the stress buildup suppresses the interfacial
of reaction during oxidation of silicon. The stress is due to the high viscosity of the oxide
at temperature less than 1050 °C, that causing the specific volume difference of oxide
with respect to the silicon. More sharpening can be performed by stripping the oxide with
BOE and oxidizing again at the same temperature. The procedure can be repeated as
necessary until achieving the desired tip sharpness. In this work, we tried oxidation
sharpening at 2 different temperatures by growing and etching 5000
A wet thermal oxide
at temperatures 1050 °C and 950 °C, respectively. Figure 3-4a and figure 3-4b are SEM
photos of a silicon tip after oxidation sharpening at 1050 °C with a view angle of 45 °.
The tip apex radius is around 50 A.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
52
(a)
(b)
Figure 3-4. SEM photo of a silicon tip after oxidation sharpening at 1050 °C:
(a) overview; (b) close view.
Figure 3-5 shows SEM photos of silicon tips comparison before and after
oxidation sharpening at 950 °C with a view angle of 45
It can be seen that the tip
became sharper and the apex radius became smaller after oxidation sharpening. An Al
patch can also be seen near the tip in figure 3-5b, which was intentionally patterned to
facilitate to find the tip when taking SEM photos.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
53
(b)
Figure 3-5. SEM photos of silicon tips before and after oxidation sharpening at 950
°C with view angle of 45 °: (a) a silicon tip before oxidation sharpening; (b) a silicon
tip after oxidation sharpening.
Figure 3-6 shows two SEM photos of the cross-section view of silicon tips before
and after oxidation sharpening at 950 °C. In figure 3-6a, a silicon tip has a half angle of
25° before oxidation sharpening. In figure 3-8b, a silicon tip shows a half angle of 15 °
after oxidation sharpening at 950 °C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
54
5 . 0
kV
x 6. 0 0 K
5.00pm
x S . 0 0 K
5.00pm
(a)
5 . 0
kV
(b)
Figure 3-6. SEM photos of cross-section view of silicon tips before and after
oxidation sharpening at 950 °C: (a) a silicon tip before oxidation sharpening; (b) a
silicon tip after oxidation sharpening.
3.2.4 Thick Photoresist Coating and Patterning
In this process, the 5000 A silicon oxide served not only for oxidation sharpening
but also acting as ion implantation mask in the photolithography step No.2. The thick
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
55
photoresist coating is used here to cover the tip height of 10 pm. This photolithography
step was performed with positive photoresist AZ 9260 and Karl Suss MA6 contact
aligner. Coating process was developed to obtain thick photoresist up to 20 pm while
keeping the uniform photoresist surface [5].
After the HMDS vapor priming in an HMDS oven, the photoresist was spun cast
to the wafer in the following procedure:
1. Flooded center of wafer with 5 ml AZ 9260.
2. Span at 500 rpm for 9 seconds.
3. Changed instantly the spin speed to 1250 rpm for 60 seconds.
4. Changed instantly the spin speed to 6500 rpm for 8 seconds.
This sequence can obtain film thickness 12 pm. Step 2 was intended to distribute the
photoresist evenly across the wafer. The spin speed in step 3 can be flexibly selected to
get the desired thickness. Most of the solvent in the photoresist was evaporated and a
relatively solid film is formed after this step. Step 4 accelerated the evaporation of
solvent and prevent edge bead formation [2], Softbake 60 minutes was performed in a
convection oven at temperature 95 °C. Then photolithography step No.2 was carried out
with Karl Suss MA6 aligner. The exposure time for 12 pm thickness AZ 9260 photoresist
was 15 seconds. Immersion development was preformed with a potassium-based alkaline
developer AZ 400K (diluted by 1: 4 in DI water) for 8 minutes. The exposed silicon
oxide was etched away using BOE. Figure 3-7 is an optical micrograph of a patterned ion
implantation window. Thus, the ion implantation window was opened, centering at the tip
region.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
Figure 3-7. Optical micrograph of a patterned ion implantation window.
3.2.5 Ion im plantation
Ion implantation is a process that introduces energetic impurity atoms into a
single-crystal substrate in order to change the electronic properties of the substrate.
Implantation is usually performed within ion energy range 50 - 500 keV [6]. Ion
implantation is a better method than diffusion since ion implantation can deliver doses to
the target in the range of 1011 - 1017 ions/cm2 and control the dose accuracy to be within
1 % over this range. Diffusion system can only control the impurity concentration to be
5-10 % at high concentration. Ion implantation is a room temperature operation, which
enables a wide variety of masks, such as silicon oxide, silicon nitride, polymer, and metal
films can be used for ion implantation. But ion implantation results in damage to the
target wafer. Thus, annealing at elevated temperatures is required to heal the damages.
Fortunately, rapid thermal annealing (RTA) techniques have avoided the problem of
significant impurities motion during annealing.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
57
In this process, we chose Boron dose of 5 x 1016 ion/cm2 with the implantation
energy of 60 keV. RTA was carried out at 1150 °C for 20 seconds with the temperature
ramp rate of 40 °C/second. This process was performed through outside vendor,
Implantation Sciences Corporation. The 5000 A thermal oxide was thick enough to mask
the energetic ions in this ion implantation recipe, and the carrier concentration in the tip
region was about 1020 cm'3. This concentration level is promising to form ohmic contact
between aluminum waveguide and tip region.
3.2.6 W aveguides Formation
The next step was using BOE to remove the thermal oxide mask for ion
implantation. After that, the aluminum alloy sputtering was performed to deposit 5000 A
thickness film for waveguide layer. The aluminum alloy was chosen to avoid spiking
phenomenon in the interface of silicon of aluminum layer [7]. The alloy composition was
98 % aluminum, 1% silicon, and 1% copper. The chamber pressure for aluminum alloy
sputtering was around 10'6Torr. RF power was 750W, with the deposition rate of 9
A
/second. Photolithography step No.3 defined the dimensions of aluminum waveguides for
microwave signal transportation. Wet aluminum etch was followed to pattern the
waveguide. The etchant consists of 80% phosphoric acid, 10% H 2 O, 5% acetic acid, and
5% nitric acid. Etch rate of aluminum was ~ 7 A /second at room temperature. The
optical micrographs of patterns after aluminum etch are shown in figure 3-8. Figure 3-8a
is an optical micrograph of a waveguide pattern. Figure 3-10b is a micrograph showing
the connection of waveguide to the ion implanted tip region.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
58
--------------
?0 m icron
(b)
Figure 3-8. Micrograph of an aluminum waveguide pattern after photolithography
step No. 3: (a) Al waveguide from beam to chip body region; (b) Al waveguide on
the tip region.
LPCVD (Low Pressure Chemical Vapor Depostion) was followed to deposit a
o
3000 A layer of Low Temperature Oxide (LTO) at 400 °C in the furnace at the pressure
of 350 mTorr. The 3000
A LTO layer acts as an isolation layer between the aluminum
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
waveguide and the aluminum shield layer in the tip region. Figure 3-9 shows a
microscope photo of an LTO layer deposited on an aluminum waveguide pattern.
*r „
'&
L J S /i
% ♦
*
.3 5 #
r ja L i*
3 t m
---------------- l i b mi cr o n
Figure 3-9. Micrograph of a waveguide pattern after LTO deposition.
Another layer of 1pm aluminum was sputtered and patterned by photolithography
step No. 4 to form the shield layer. Figure 3-10 is an optical micrograph of a device
pattern after photolithography step No. 4. The aluminum shield layer lies on the left side
of the photo, and overlaps the waveguide layer with LTO film in between the two metal
layers.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
mNBKm11
Shield
Waveguide
covered by
LTO
magm
115 micron
Figure 3-10. Micrograph of a metal shield pattern after photolithography step No. 4.
Photolithography step No. 5 was used to pattern the LTO layer after aluminum
wet etch to expose the underneath metal waveguide. Figure 3-11 shows micrographs of
device patterns after photolithography step No. 5. Figure 3-1 la is an optical micrograph
of a probe chip structure after BOE etch of the patterned LTO layer; and figure 3-1 lb is a
close view of the cantilever beam region.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
—
0.1 m m
(a)
MS
m
20 m icron
(b)
Figure 3-11. Micrographs of patterns after photolithography step No. 5 and BOE
etch: (a) micrograph of a device pattern including cantilever beam region and
partial chip body; (b) close view of the pattern at the end of cantilever beam region.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62
3.2.7 Co-axial Tip Formation
A special processing technique called “tip exposure” was implemented to realize
the co-axially shielded tip structure that confined the electromagnetic field in the exposed
tip region to perform microwave measurement. First, thick photoresist AZ 9260 was spun
to achieve uniform coating on the wafer. Then a photoresist plasma stripper M4L™ was
utilized to etch photoresist to the extent that the aluminum-covering tip was barely
exposed. Tuning the parameters of the plasma photoresist stripper can control the
exposure extension. The aperture in the aluminum shield layer was formed by aluminum
wet etch. Finally the conductive silicon tip was exposed by the following LTO etch to
realize the co-axially shielded tip structure. Immersing the wafer into a tank filled with
acetone for 20 minutes then cleaned the photoresist. Figure 3-12 shows SEM photos of
co-axially shielded tips with different dimensions. Figure 3-12a shows a co-axial tip with
aperture radius of ~ 600 nm. Figure 3-12b is a co-axial tip with aperture radius of ~ 500
nm. Figure 3-12c is a co-axial tip with aperture size of ~ 400 nm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
(a)
(c)
Figure 3-12. SEM photos of microfabricated co-axially shielded silicon tips using
“tip exposure” technique.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
3.2.8 Define Beam and Beam Release
Silicon anisotropic RIE was done after photolithography step No. 6 to form a Vshaped cantilever beam structure with thickness of 3 - 5 pm. This shallow dry etch can be
accomplished with Tegal system. SF6 (50 seem) and oxygen (20 seem) were the etching
gas, and He (70 seem) was the carrier gas. The etch rate was 0.3 pm/minute at RF power
of 150 W. Figure 3-13 shows an SEM photo of a cantilever beam structyre etched in this
way.
Figure 3-13. SEM photo of a V-shaped cantilever beam structure.
Next, thick photoresist AZ 9260 was spun to protect the front side of the device
structure before the backside of the SOI handle layer underwent the last photolithography
step. Double side alignment with Karl Suss MA6 was used to define the backside DRIE
region for the release o f the V-shaped cantilever beam. The exposed backside silicon
regions were etched in a DRIE system from STS, Gwent, UK, until reaching the buried
oxide layer. The system provides etching selectivity between Si and Si02 of 150:1, and
selectivity between Si and photoresist of 75:1. This process step was performed through
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
MEMS-Exchange® outside of campus. Finally, buried oxide layer etching and
photoresist stripping in an acetone tank released the SNMM probe. SEM photos of the
microfabricated SNMM probes are shown in figure 3-14.
5 . 0
kV
x 10 0
300pm
(b)
Figure 3-14. SEM photos of microfabricated SNMM probes: (a) overview of an
SNMM probe; (b) close view of a released cantilever beam.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
66
Figure 3-15 shows a photo of an SOI device wafer when all the microfabrication
processes were finished. The void rectangles in the wafer were formed after four SNMM
probes were picked out for characterization.
HIB
Figure 3-15. A microfabricated device wafer.
3.3 Discussion
The designed microfabrication process was implemented and successfully
realized the proposed SNMM probes. The “tip exposure” method was justified to be
practical and efficient. This is a sophisticated process since it took more than 2 months to
finish a run lot provided that all the microfabrication tools were maintained in good
condition.
Moreover, the process yield in this work was steadily improved to be high enough
for future device commercialization. Three process runs were performed in this work.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
67
The process yields for the first and second process run was around 10 % and 25 %,
respectively. The loss came from the tip etch and the “tip exposure” steps. In the tip
plasma-etching step, the tip etching speed was not distributed uniformly across the
substrate. It was observed that the tip etch rates in the middle part of the substrate were
slower those at the edge of the wafer. SEM inspection showed the typical tip height on
the middle part (a circle region with diameter of 20 mm) of the substrate was 9 pm, while
the tip height on the edge region of the substrate was 6 pm due to over etch. When the tip
exposure technique was used to form the co-axial tip structure, the thick photoresist
coating was performed according to cover 9 pm high tip. The oxygen plasma etching
using M4L™ was also designed to expose the 9 pm tips with a small aperture. Therefore,
the tips on the edge of the substrate were still covered by thick photoresist. The co-axial
structures cannot be formed on the edge region of the wafer. The problem caused by the
tip plasma etching and tip exposure can be solved by putting the device layout as close to
the center of the substrate as possible and define the oxygen plasma etch time accordingly
to the minimum tip height. Using the improved process strategy, process yield in the third
run has been accomplished to be more than 50 %.
References
1. S. Wolf and R. N. Tauber, Silicon Processing. Lattice Press, (2000).
2. T. S. Ravi, B. Marcus, and D. Liu, “Oxidation sharpening of silicon tips,” J. Vac.
Sci. Technol. B 9, 2733 (1991).
3. N. E. McGruer, K. Warner, P. Singhal, J. J. Gu, C. Chan, “Oxidation-sharpened
gate field emitter array process,” IEEE Transaction on Electron Devices 38, 2389
(1991).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
68
4. S. Akamine and C. F. Quate, “Low temperature thermal oxidation sharpening of
microcast tips,” J. Vac. Sci. Technol. B 10, 2307 (1992).
5. H. Niyajima and M. Meheregany, “High-Aspect-Ratio Photolithography for
MEMS Applications,” Journal o f Microelectromechanical Systems 4, 220 (1995).
6. S. K. Ghandhi, VLSI Fabrication Principles. John Wiley & Sons, Inc., New York
(1994).
7. D. W. Lee, T. Ono, T. Abe, M. Esashi, “Fabrication of microprobe array with
sub-lOOnm nano-heater for nanometer thermal imaging and data storage,” The
14th IEEE International Conference on Micro Electro Mechanical Systems, 204
( 2001 ).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4 Microwave Probe Characterization and Application
4.1 AFM Platform for SNMM Probe
The characterization for the microfabricated microwave probe consists of two
parts: mechanical characterization and electrical characterization. A commercially
available AFM platform, ThermoMicroscopes Explore™, was used for the measurement
of dynamic properties for the microfabricated microwave probe. The system incorporates
TopoMetrix’ Scanning Tip Technology™ (STT) for rastering the scanning tip over the
sample [1], An integral CCD video camera was included for viewing the tip and sample
at a 45° angle. Imaging was performed at room temperature and in atmospheric
conditions for this work. A personal computer was used in the Explorer microscope
system for all data acquisition, data processing and data display functions. An advanced
electronic control unit, i.e. the ECU-plus™, provided 16-bit scanner control in three
dimensions. The entire system is shown in figure 4-1. At the rear of the ECU there are 16
I/O lines to customize the instrumentation configuration. In this work we used the ECU
I/O ports to input the microwave signals to the AFM system to generate microwave
images.
69
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
Figure 4-1. Thermomicroscope Explorer™ AFM system.
The Explorer Scanning Probe Microscopy (SPM) head in our lab uses tripod
scanners that are constructed from separate X, Y, and Z piezoelectric components. The X
and Y piezos are mounted horizontally, together with the laser tower. The Z piezo is
mounted separately at the bottom of the laser detector assembly. The cantilever with
scanning tip is mounted directly under the Z-scanner. This tripod scanner has the
scanning range of 100 pm x 100 pm x 10 pm in the X, Y, and Z directions, respectively.
Figure 4-2 schematically illustrates the scanner head.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
71
TOWER H A N D L E -
-X SCANNER
Y SCANNER
LASER
LASER
ALIGNMENT
PHOTODETECTOR J
ALIGNMENT S C R E W 'S
PHOTODETECTOR'
■Z SCANNER
Figure 4-2. Schematic of Explorer™ scanner head [1].
4.2 Mechanical Characterization
To characterize the mechanical properties of the microfabricated SNMM probe,
the probe needs to be mounted on a half-moon shaped metallic tip holder by epoxy or
double-sided tape, as shown in figure 4-3.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
CANTILEVERS
POSITIONING
HOLE
MOUNTING MAGNETS
Figure 4-3. Probe mounting [1].
Then the tip holder together with the microwave probe can be mounted to the seat
on the scanner head due to the magnetic force between the seat of the scanner head and
the tip holder. The sample was mounted on the sample holder of the X, Y translator base,
which contains magnets for mounting metallic sample, as shown in figure 4-4. Nonmetallic samples can be mounted with double-sided tape. After that, the scanner head was
mounted on the translator to start the operation of AFM image scanning. The sample can
be translated by turning the X and Y knobs. The sample’s relative position to the probe
cantilever can be monitored by the video monitor.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
73
Scanner head
Sample holder
Figure 4-4. Explorer™ scanner head and sample holder.
The next step was laser beam alignment for proper feedback. Three components
are involved in this step: the laser, mirror, and photodetector. After being turned on, the
laser beam was focused on the backside of the cantilever and bounced off to the
adjustable mirror, then reflected onto the center of the four-quadrant photodetector. A
good alignment was achieved by maximizing the signal generated by the beam in the
photodetector and ensuring each quadrant of the photodetector has nearly equal amounts
of light, as shown in figure 4-5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
LASER
PHOTODETECTOR
MIRROR
CANTILEVER
SAMPLE
PHOTODETECTOR
INTEGRA TED
T IP
Figure 4-5. Schematic of beam alignment [1].
Figure 4-6 is a scanning image of a standard S i0 2 grid obtained with an SNMM
probe working in contact mode. Considering the 1 pm thick aluminum shield layer of the
co-axial tip structure may decrease the effective tip sharpness during the contact image
scanning, this image is pretty good compared to the image scanned with a commercial
Si3N4 AFM probe, as shown in figure 4-7.
0 nm
25 pm
50 pm
Figure 4-6. Contact mode topography scanning image by an SNMM probe.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
50 nm
130.70 nm
0.00 nm
25 nm
Figure 4-7. Contact mode topography scanning image by a commercial probe.
The resonant frequency and quality factor Q of V-shaped beams of the SNMM
probes were measured with SPMLab software in non-contact AFM acquisition. The
quality factor of a resonator is a measure of the energy stored in the resonator divided by
the energy lost per cycle at the resonant frequency. It is given by
(energy stored)
(energy loss/cycle)
(4-1)
Practically Q can be calculated from the oscillation spectrum using
where fo is the resonant frequency, and fi and f2 are the half-power levels or -3dB from
the magnitude at the resonant frequency.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76
Figure 4-8 shows a mechanical oscillation spectrum of a microwave probe in air,
indicating a resonant frequency of 170.92 KHz, the Q factor can be calculated from
Eq.(4-2) to be 317. This is close to the design value fo= 134.2 KHz with L = 300 pm, W
= 50 pm, and t =5 pm. The discrepancy can be accounted for the process deviation of
beam dimensions.
10V
170.17
170.65
171.12
171.6 kHz
Figure 4-8. Oscillation spectrum of an SNMM probe with fo=170.92 KHz.
Figure 4-9 shows an SNMM probe with resonant frequency of 50.016 KHz and Q
factor of 116 in air. The resonant frequency agrees well with the design value of fo= 59.7
KHZ with L = 450 pm, W = 50 pm, and t =5 pm, considering the process deviation of
beam dimensions.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
77
7.1 V
0.6
49.725
49.983
50.242
50.5 kHz
Figure 4-9. Oscillation spectrum of an SNMM probe with fo=50.016 KHz.
Figure 4-10 shows the resonant spectrum of an SNMM probe with resonant
frequency of 11.777 KHz and calculated Q factor value of 67 in air, good agreement with
design value of fo= 12.1 KHz (L = 1000 pm, W = 50 pm, and t =5 pm).
9.2 V
11.457
11.671
11.866
12.1 kHz
Figure 4-10. Oscillation spectrum of an SNMM probe with f<>=11.777 KHz.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
78
Figure 4-11 shows the normalized mechanical oscillation spectrum of an SNMM
probe in air with fo=94.186 KHz and Q factor value of 259. For comparison, we have also
included the oscillation spectrum of a commercially available non-contact metallic tip.
The quality factor of our resonator is 3 times greater than the commercial tip [2].
1.1
Microfabricated
SNMM probe
Commercial
non-contact
conducting
AFM probe
0 .9
T3
3
tI 0.8
o
Z 0 .7
0.6
-
0 .5
0 .9 6
0 .9 7
0 .9 8
0 .9 9
1
1.01
1.02
1 .0 3
1 .0 4
Normalized Frequency
Figure 4-11. Normalized resonance spectra of an SNMM probe and a commercial
AFM probe.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
79
4.3
Electrical Characterization
4.3.1 DC M easurem ents
We measured the DC current versus voltage characteristics of the SNNM probes
to check the leakage between the shielding electrode and the tip. Explorer™ scanner head
and semiconductor parameter analyzer HP 4155B were used to perform this measurement
The results are shown in figure 4-12. The leakage between a tip and a co-axial shield was
around 0.6 pA at 1 V, resulting in a resistive load of around 1.6 MO that is quite
acceptable.
7.00E-07
6.00E-07
5.00E-07
4.00E-07
3.00E-07
2.00E-07
1.00E-07
0.00E+00
-1.00E-07
-0.5
-0.25
0
0.25
0.5
0.75
1
Voltage (V)
Figure 4-12.1-V curve measurement between a tip and shield.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
80
The ohmic contact resistance between an aluminum waveguide and the highly
doped tip was measured by HP 4155B semiconductor parameter analyzer. Figure 4-13
shows the contact resistance is 37 Ohm. This value is pretty good considering the small
contact area of the waveguide and the ion implanted tip.
0.08
R = 37 Ohm
0.06
0.04
< 0.02
C
s
u
-
0.02
-0.04
-0.06
-0.08
-
0.1
■3
■2
1
0
1
2
3
Voltage (V)
Figure 4-13. Ohmic contact resistance measurement between a waveguide and tip.
The contact resistance between a tip and a gold sample was measured using AFM
system and HP 4155 B. Figure 4-14 shows the resistance is around 50 Ohm. This is
impressive since the tip is very sharp. These two measurements in figure 4-13 and figur414 verified that the ion-implanted tip was highly conductive.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
81
0.025
0.02
R = 50 Ohm
0.015
0.01
0.005
0
-0.005
-
0.01
-0.015
-
0.02
-0.025
1
-0.5
0
0.5
1
Voltage (V)
Figure 4-14.1-V curve measurement between a tip and sample.
4.3.2 M icrow ave M easurem ents
We performed the microwave characterization for SNMM probes using HP 8720
C network analyzer and ThermoMicroscopes Explorer™ system. The experimental setup
is shown in figure 4-15.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
82
Network
analyzer
Figure 4-15. Network analyzer and Explorer™ system for microwave
characterization.
The network analyzer operates in the frequency range from 50 MHz to 20 GHz.
The scanner head was processed to add an SMA connector as an interface for microwave
measurements, as shown in figure 4-16.
Figure 4-16. Microwave interface on the Explorer™ head .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
A 0.5 mm diameter co-axial cable (inner cable diameter of 60 (am) was used to
realize the electrical connection from the SMA to the SNMM probe seating on the half­
moon tip holder. The co-axial cable was first characterized to make sure the line
impedance is around 50 Ohm to provide impedance match between microwave signal
source and the SNMM probe. The measurement setup is shown in figure 4-17.
Figure 4-17. Impedance measurement for a co-axial cable line.
The S-parameters were measured using an HP 8720C network analyzer. The line
impedance was calculated using Matlab program according to the following equation [3]:
z 2 =z;
(l + Sn )2 - S
21
(l-V r-s 2
(4-3)
Z is the unknown line impedance, Z0 is the characteristic impedance of the network
analyzer cable that is 50 Ohm, Sn is the measured input reflection coefficient, S21
transmission coefficient.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
84
The magnitude and phase of the calculated line impedance are shown in figure 418 and 19, respectively. Despite two spikes around 8.5 GHz (745 Ohm) and 9.5 GHz
(1350 Ohm), the line impedance values were close to 50 Ohm in the 1-20 GHz frequency
range. So it is acceptable to use this co-axial cable to convey the microwave power to and
from the SNMM probe.
1400
1200
£*
1000
JZ
o
D
T<
33
co>
800
re
E
®
oc
re
a>
600
E
400
T3
3>
200
0
2
4
6
8
10
12
14
16
18
20
F r e q u e n c y (G Hz )
Figure 4-18. Co-axial cable line impedance magnitude in 1-20 GHz range.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
2
- 2 I__________ i________ i____________ i____________ i_________i__________ i__________ i__________ i__________ i___________
0
2
4
6
8
10
12
14
16
18
20
F r e q u e n c y (GHz)
Figure 4-19. Co-axial cable line impedance phase in 1-20 GHz range.
Figure 4-20 shows the mounted AFM-compatible SNMM probe on a half-moon
tip holder. Figure 4-21 shows a half-moon tip holder containing the probe mounted on the
ThermoMicroscopes Explorer™ scanner head. The calibrated 0.5 mm diameter co-axial
cable was connected to the probe using silver print. Care was taken to have only two
signal junctions between the microwave probe and the network analyzer.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
86
Figure 4-20. An SNMM probe mounted on a tip holder.
Figure 4-21. Electrical connection to a mounted SNMM probe.
Figure 4-22 shows the Si\ magnitude spectra of an SNMM probe tip in air and in
soft contact with a copper sample. Only the tip region (not the whole cantilever beam)
was located over the sample. The change in the Sn spectra was related to the interaction
between the near-fields of the tip apex region and the copper sample.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
o
-5
-10
-13
_ -20
SB
■O
~ -25
ifl
— -,30
-35
metal
-40
-45
-SO
0.62
0.71
0.79
0.88
0.97
1.06
1.14
F re q u en cy ( G if /)
Figure 4-22. Reflection spectra of a tip in air and in contact with a copper sample.
Figure 4-23 shows the magnitude Sn spectra of an SNMM probe in air and over a
dielectric sample with relative permittivity sr = 2.2. We were careful to situate only the
tip region over the sample to prevent capacitive coupling between the sample and
unshielded wavegudies region on the V-shaped cantilever beam and chip body.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2*2 dielectric
16.19
K J4
M .49
16.64
16.79
16.94
17.W
Frequency (01 In
Figure 4-23. Reflection spectra in air and over a dielectric sample.
When the coupling region was also located over the sample, the change that
occurs in Sn spectrum was much larger as can be seen in figures 4-24 and 4-25. Figure 424 shows the S n spectra with the probe in air and approaching to a copper sample
(without contact with the sample). The spectra magnitude change is much larger than
those shown in figure 4-22. This is due to the capacitive coupling between the probe
waveguide region and the sample. Figure 4-25 shows the Sn spectra with the probe in air
and over a dielectric sample. Again the spectra magnitude change is much larger than
those shown in figure 4-23.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
89
0
-5
-.10
— -1 5
i1
-20
§i
-2 5
-3 0
-35
16.19
16.34
16.49
16,64
16 ” 9
1 6 .9 4
17.09
Frequency (CHz)
Figure 4-24. Reflection spectra in air and over a copper sample with probe region
and tip apex was located over the sample.
0
-5
-10
23
Xt
~ -\S
■pm
-20
Air
2.2 Dielectric
-2 5
30
16 05
16 1
16.35
16 *
Freiiuiiu\
16.65
F- ™ "-I 11
16.8
16.95
{(ill/)
Figure 4-25. Refection spectra in air and over a dielectric sample with probe region
and tip apex was located over the sample.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
90
We feel that the coupling region on the waveguide layer should also be shielded
in our future generation SNMM probes. These large interactions between the sample and
the probe’s coupling region (waveguide) may become smaller as we increase the tip
height, but the contribution is quite large and may still overshadow the apex interaction
with the sample especially at very high frequencies.
4.4 Scanning Microwave Microscopy Application
We applied the SNMM probe for scanning microwave microscopy applications.
In this procedure, network analyzer HP 8720C was used to determine the operation
frequency point. Figure 4-26 shows the Sn magnitude spectra measurements in air and
over a gold sample in the frequency range from 50 MHz to 20 GHz [4], The top curve in
figure 4-26 shows (|Sn| + 50) dB in air. The middle curve is the values of (|Sn| + 25 dB)
measured when approaching the SNMM probe to a gold sample in the same frequency
range. After subtracting the |Sn| values in air from the |Sn| values when approaching the
gold sample, the bottom curve of figure 4-28 illustrates some interesting points useful for
microwave microscopy applications. For example, it can be seen that there is a peakvalley pair around 1 GHz in the bottom curve of figure 4-26. That means a resonant peak
in the air around 1 GHz shifts to a higher frequency after approaching the tip to the
sample. A peak in about 3 GHz shows that there is a resonant peak arising after
approaching the tip to the sample whereas there is no resonant peak at the same
frequency in the air. The peak-valley pair around 5 GHz indicates that a resonant peak in
air around 5 GHz shifts to a lower frequency after approaching the sample to the tip.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
91
;S 111+50 dB in air
|S 111+25 dB approaching sampl
------- 'V—
SI 1 difference
-10
-20
0
5
10
15
20
Frequency (GHz)
Figure 4-26. Reflection (|Sn|) spectra of a tip in air and over a gold sample with a
large sensitivity around 5 GHz.
We chose the most sensitive frequency point determined by HP 8720C network
analyzer in the S 11 spectra measurements as the excitation and detection frequency point
in the scanning microwave microscopy to get simultaneous topography and microwave
images of sample. Figure 4-27 is the schematic of the circuit used for scanning
microwave microscopy. The RF source in this experiment is an HP 8341B synthesized
sweeper with operation range 10 MHz - 20 GHz. The Lock-in Amplifier (LIA) is EG&G
model 5110, and it provides the amplitude modulation signal to the RF source. We
utilized the I/O signal processing channels from the ECU-plus™ of the
Thermomicroscope Explorer™ system to input the microwave signal to generate
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
92
microwave image. An SR560 low-noise preamplifier from Standford Research Systems
performs both band pass filter (BPF) and pre-amplifier functions in the experiment. The
crystal detector model is HP 8742B with operation range 10 MHz -18 GHz and
sensitivity larger than 50 mV/ pW.
Circulator
SN M M probe
Crystal detector
RF Source
Pre-amplifier
Sample
holder
BPF
Lock-inA m plifier
(AM modulation
source') ________
DAQ
Figure 4-27. Schematic of the circuit for scanning microwave microscopy.
The crystal detector is a microwave diode to act as nonlinear device to realize AM
demodulation. The small-signal approximation of the diode current is [5]
/( ^ ) = / 0 +i = / 0 + v G , + y G ;
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(4-4)
93
Io is the DC bias current, Gd is the dynamic conductance of the diode, Gd’ is the second
derivative of I (V) at V=Vo (DC bias voltage).
For a AM modulated RF signal, the expression of diode voltage can be described as
v(t) = v0(l +mcos comt) cos co0t
(4-5)
where oomis the modulation frequency, coo is the RF carrier frequency that is much larger
than (Dm, m is the amplitude modulation index with the magnitude in the range of 0 - 1.
Then Eq (4.4) can be expressed as
i(t) = voGd (1 + mcos(Omt)cosco0t + ^ - G d (l + mcos(Omt) 2 cos2 co0t
ffl
171
- voG d cos 0)0t + — sin(tu0 + com)t + — sin(m0 - 1om)t
ffl 2
1+ - j - + 2m cos comt +
+ —
4
Gd
d
ffl 2
cos 2comt + cos 2co0t +
m sin(2tu0 + (Om)t + m sin(2(U0 -com)t +
+
ffl 2
(4-6)
ffl 2
cos2Q)0t
ffl2
sin2(a>0 +(om)t + - j - sin2(o)0 - a>m)t
V
1
v2
The desired demodulation output is - j - Gdmcos(Omt , which can be separated
rv
from other signal components by the BPF.
The RF source generated a signal at power level of -10 dBm, which was AMmodulated by the internal oscillator of the LIA (3 KHz), to go to the port 1 of a circulator
that had a pass band containing the working frequency to reach port 2 without loss. The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
94
transmitted RF signal was guided by the waveguide of the SNMM probe to the co-axial
tip region where it interacted with the sample. Variances in the microwave properties
(conductivity, permittivity or permeability) of the sample changed the amplitude and
phase of the reflected RF signal, which traveled back along the waveguide through port 2
to reach port 3 of the circulator. The crystal detector detected the changed envelope of the
AM-modulated RF signal, delivering it to the BPF and preamplifier. The filtered and
amplified envelope signal had the same frequency as the reference signal of LIA. The
output signal of LIA represented the microwave properties of the sample and it was
passed to the ECU to generate the microwave image.
Figure 4-28 and 29 show simultaneous contact mode topography and SNMM
images on a grid sample at 1 GHz. The SNMM image shows some subsurface features
that are absent in the AFM image. Although the spatial resolution is not very high, this is
the first reported SNMM image achieved by a microfabricated SNMM image.
100 p.m
219.03 nm
I
0.00 nm
0 p.m
50 nm
100 nm
Figure 4-28. Contact mode AFM image by an SNMM probe.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.672 V
50 p.m
0.642 V
0 nm
0 nm
50 nm
100 nm
Figure 4-29. Simultaneous microwave image by the SNMM probe.
4.5 Discussion
The microfabricated SNMM probes underwent mechanical characterization using
ThermoMicroscope Explorer™ AFM system, electrical characterization using
ThermoMicroscope Explorer™ AFM system, HP 4155B semiconductor parameter
analyzer, and HP 8720C network analyzer. The SNMM probes demonstrated good
sensitivity over metal and semiconductor samples. We have applied the microfabricated
SNMM probes for scanning microwave microscopy and achieved simultaneous AFM and
microwave images. Although the parasitic coupling between the waveguide and sample
limited the furthermore scanning microwave microscopy applications, it can be solved in
the future generation SNMM probes by correct shielding.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
96
References
1. ThermoMicroscopes Explorer™ Instrument Operation Manual (2000).
2. Y. Wang and M. Tabib-Azar, “Microfabricated Near-field Scanning Microwave
Probes,” Electron Devices Meeting, IEDM '02. Digest. International, IEEE, 905
( 2002 ).
3. Y. Eo and W. R. Eisenstadt, “High-Speed VLSI interconnect modeling based on
S-parameter measurement,” IEEE Transactins on Components, Hybridsm and
Manufacturing Technology, 16 (1993) 555.
4. Y. Wang and M. Tabib-Azar, “Fabrication and Characterization of Evanescent
Microwave Probes Compatible with Atomic Force Microscope For Scanning
Near-Field Microscopy,” Proceedings o f 2002 ASME Inter. Mech. Eng. Congress
& Expo., New Orleans, Louisiana, November 17-22 (2002).
5. D. M. Pozar, Microwave Engineering, Addison Wesley Publishing Company, Inc.
(1990).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5 Conclusions and Future Work
5.1
Summary of the Contributions
A novel scanning near-field microwave microscope probe, that bridges the
frequency gap among other scanning local probe microscopy methods between 1 GHz
and 140 GHz, was designed, microfabricated, and characterized in this work. The SNMM
probe consists of a silicon V-shaped cantilever beam, a co-axially shielded heavily doped
silicon tip, and modified aluminum co-planar waveguides. The co-axial tip structure was
formed using a thick photoresist process combined with an oxygen plasma etching
technique that enabled the silicon tip to protrude through an aperture in the aluminum
shield layer. The microfabrication process utilized seven photolithography masks to form
this novel SNMM probe. We reported, for the first time, the simultaneous AFM
topography image and scanning near-field microwave image at 1 GHz generated by the
SNMM probe. The ability to perform microwave and AFM measurements
simultaneously is quite valuable and it enables AFM topographical images to provide a
reference landscape to understand and interpret the microwave images.
The original contributions in this work are summarized in the following:
1. The structure and microfabrication process of the proposed SNMM probe were
designed. The V-shaped cantilever beams have spring constants in the range of
0.1 N/m - 10 N/m, and mechanical resonant frequency in the range of 10 KHz 200 KHz, compatible with those of commercial AFM systems for contact and
non-contact mode operation. The silicon tip is electrically connected to a strip of
aluminum that forms the active part of the waveguide. The co-axial tip is
97
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
98
surrounded by a CVD oxide layer to insulate it from an aluminum shield layer.
The co-axial tip structure enables the silicon tip to protrude through an aperture in
the aluminum shield layer, which confines the electromagnetic field in the
exposed tip region to perform microwave measurements.
2. The SNMM probes were microfabricated with MEMS technology. More than 300
SNMM probe chips can be fabricated on a 4-inch wafer in a batch process.
Combining dry etching, low temperature oxidation sharpening, and selective ion
implantation, silicon tips were fabricated and integrated on the V-shaped
cantilever beams. Co-axial tip with height ~ 10 pm, apex radius ~ 50 A, and
aperture radii in the range between sub-micron and micron (500 nm - several
micron) have been achieved. The co-axial tip structure was formed by “tip
exposure” technique using a thick photoresist process combined with an oxygen
plasma etching method. The aperture size can be formed in a controlled manner
by tuning the oxygen plasma etching time.
3. Mechanical and electrical characterizations of the SNMM probes were performed
using a ThermoMicrocopes Explorer™ AFM system. The mechanical resonant
frequencies of the SNMM probes have good agreement with the design values,
and the Q factors of the SNMM probes are as high as 300 in air. The contact
topography AFM imaging using the SNMM probe demonstrated satisfactory
mechanical performance. A convenient SMA connector interface was added to
the ThermoMicroscopes Explorer™ scanner head to facilitate the electrical
characterization. The DC electrical characteristics of the SNMM probe were
measured using HP 4155 B semiconductor parameter analyzer. Using DC
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
99
meaurements, the ion-implanted tip region was verified to be highly conductive.
The microwave characterization was performed by using 8720 C network
analyzer to measure the input reflection coefficient Sn with and without metallic
and dielectric samples. The experiments show that the Snsprectra changes of the
SNMM probe in the presence of sample are related to the microwave properties of
the sample, and the SNMM probe demonstrate good sensitivity in detecting the
samples.
4. The simultaneous AFM contact mode topography imaging and near-field
microwave imaging were accomplished at 1 GHz with an AM-modulation and
demodulation RF measurement circuit. Using the microwave measurement along
with AFM opens up a new window to see inside the materials and sets the stage
for hyperspectral imaging of organelles of biological objects as well as electronic
devices and structural materials.
5.2 Future Work
This thesis work has demonstrated that applying the probes for nondestructive
microwave imaging in the frequency range of 1-20 GHz is promising. Some issues
remain in this work, however, and they should be well addressed for the design and
fabrication of the future generation SNMM probes. They include:
1. Shilelding of waveguide on the cantilever beam should be done to reduce the
capacitive coupling between the waveguide and sample. Noise level from
waveguide on the cantilever beam other than co-axial tip region is high as we
mentioned in Chapter 4. Better shielding on waveguide is needed to achieve
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100
correct measurements and higher resolution. When both tip and waveguide are
adequately shielded, the coupling effect between waveguide and sample will
decrease. This will also increase the signal-to-noise ratio and increase spatial
resolution. Figure 5-1 shows a partial layout design of the waveguide and shield
on the beam region for the next generation SNMM probe.
'avegu
s hi e l d
Figure 5-1. Partial layout of next generation probe with shielded waveguide.
2. Satisfactory feed-through for the microwave signal source to the SNMM probe is
needed. Using the silver print to connect the co-axial cable to the SNMM probe
chip body has several disadvantages. The microwave properties of the silver print
are unknown and hard to characterize although it provides good DC connection
between co-axial cable and the probe waveguides. The volume of the droplet that
joins the co-axial cable and the waveguide on the chip body is hard to control by
hand operation. The drying process for the silver print to make the adhesion stable
is time consuming. Moreover, the height of the joint region may counteract the
co-axial tip height, enhancing the parasitic coupling effect between the waveguide
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
101
and sample. Better methods should be considered to make the SNMM probe to
interact smoothly to the outside microwave circuitry. One possible solution is to
make a recess on the probe chip body to accommodate the 0.5 mm co-axial cable
that connects to the outside microwave signal source, minimizing the parasitic
coupling between the co-axial cable in the silver print joint region and sample.
The schematic is shown in figure 5-2. Another possible solution is to use etchthrough technique to transfer the waveguide on the chip body to the backside of
the silicon wafer by a via hole etched through the wafer. Then the interface for the
probe to the outside microwave circuits can be easily realized using traditional
wire-bonding technique. Furthermore, the possible capacitive coupling from the
chip body to the sample will be eliminated.
F igu re 5-2. R ecess in th e SNMM p rob e b od y.
3. Further development work on the process yield can promote the SNMM probe for
future application in the laboratory and industry environment as an additional
metrology tool.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix A: Introduction of Scanning Probe Microscopes
A scanning probe microscope (SPM) generally consists of a sensing probe
(optically sensing or electrically sensing), piezoelectric ceramics (or other type of
actuators) for moving the probe, an electronic control unit, and a computer for controlling
the scan parameters as well as generating images.
A .l Scanning Tunneling Microscope
The scanning tunneling microscope (STM), invented in 1982 by G. Binnig et al.
[1], brought the surface roughness measurements down to the atomic scale for the first
time. From then on, STMs were widely used in the research of surface properties, and
many surface science techniques were developed. The STM operates by mechanically
driving three-dimensional piezoelectric translators to scan a tip over the sample surface,
as shown in figure A-l [2],
102
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
103
X-piczo
X-Y
-piezo
in te g ra to r
translator
Z-piezo
/sensor
electronics
/
/
r
differential
am plifier
sample holder
ample
p otential
bias lead
monitor
Figure A -l. STM system configuration [2].
A feedback control loop is connected to the piezoelectric translators to tightly
control the distance between tip and sample in order to obtain a height profile of the
sample. This is realized by measuring the tunneling current between tip and sample. First
a bias voltage is applied between the tip and sample. Electrons can tunnel across the tipsample gap when the tip is brought very close to the sample. The rate of tunneling
electrons depends exponentially on the tip-sample distance. Therefore the tip will track
the sample surface accordingly when the tunneling current is kept constant by a suitable
feedback loop to adjust the movement of the Z-direction piezoelectric translator, from
which the output voltage is recorded as surface topography information. STM images can
reflect both the topographic and electronic structure of the surface.
The operation principle shows that STM is only suitable for conducting or
semiconductor samples investigation. Vibration isolation is necessary for high-resolution
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
104
STM imaging, and low temperature operation depresses thermal noise and thermal
diffusions to achieve superior resolution.
A.2 Atomic Force Microscope
Atomic force microscope (AFM) can be considered as the most powerful tool
among the SPM family. The AFM was developed to overcome the aforementioned basic
drawback with STM. G. Binnig, C. F. Quate, and Ch. Gerber [3] invented AFM by
showing that fine sample topography can be imaged by monitoring the force between a
sharp tip and sample. Figure A-2 shows a schematic of a typical AFM system. Its
advantage lies in that AFM can image almost any type of surface, including polymers,
ceramics, composites, glass, and biological samples etc. An atomic force microscope is
an instrument that measures the topology of a surface by bringing a cantilever beam into
contact with (or close to) a sample and measuring the deflection of the cantilever as its tip
is scanned over the surface.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
105
la se r diode
f o c u s in g le n s
4 - p a in t
p h o to d e te ct or
cantilever
sample
sample holder
CPU
and monitor
integrator
X-Y tran slato r
Figure A-2. Schematic of AFM system configuration [2].
A topographic map of the sample surface is generated by plotting the cantilever
deflection versus position. A feedback control loop can be used to maintain a small
constant contact force between the tip and sample in contact mode, or a constant spacing
between the tip and sample in the non-contact mode. Typical AFM systems operate in air
environment compared to STM, no need for cumbersome vacuum chambers, and
simplifying the measurement systems. The AFM is capable to achieve high spatial
resolution with nondestructive measurement, endowing it to be a natural choice for
semiconductor metrology. The AFM is able to provide three-dimensional topographical
images, which has been utilized by the semiconductor industry to measure
microroughness. For instance, this feature has been used for characterization of chemical
mechanical polish (CMP) processes and for determination of the surface roughness of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
106
deposited layers. Fabrication sequences may be optimized by repeating the AFM
measurements, avoiding the disadvantages of Scanning Electron Microscopy (SEM) or
Transmission Electron Microscopy (TEM). Both SEM and TEM are expensive,
destructive, and time-consuming cross section inspections tools compare to AFM. The
AFM also has useful applications in semiconductor metrology such as wafer inspection,
mask inspection, defect imaging and analysis, and etch depth measurements [4], This
class of microscope has been proven to be an excellent tool for surface analysis with
atomic resolution. The requirements for accurate force measurements are low force
constants and high resonant frequency of the cantilever beam. Most AFM systems use a
laser beam deflection system first introduced by G. Meyer [5]. As shown in figure A-2, a
laser is bounced back from the back of the reflective cantilever and onto a positionsensitive detector. Typical AFM tips and cantilevers are microfabricated from Si or Si3N4
with tip radius of a few nm to 10s of nm.
Since the advent of AFM, a lot research work has been conducted to improve the
functionality and exploit it to varied application areas. A wide range of other probe
technologies has also been developed based on AFM. Those scanning probes provide
information other than topography, or in addition to topography information.
A.3 Scanning Capacitance Microscope
Scanning capacitance microscope (SCM) was introduced following the advent of
the AFM. The key idea of SCM is to measure the capacitance between a sharp tip and the
sample. Modem semiconductor devices relate their functions greatly to the surface effect.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
107
For example, surface charge in the thermal oxide layer alters the conductivity of the
underlying conducting channel for MOS transistors. SCM can be an ideal tool to perform
semiconductor surface effect measurements, and to measure electrical properties of
dielectric films and their underlying substrates. C. C. Williams et al. [6] first reported a
SCM on 25 nm-scale in 1989. R. C. Barrett and C. F. Quate [7] reported an SCM
application based on AFM in 1991. The SCM probe consisted of a cantilever beam
integrated with a conducting tip. It was scanned against a conducting substrate coated
with a dielectric film. A capacitance sensor was then used to measure the tip-sample
capacitance as a function of lateral position. The deflection of the cantilever can also be
used to measure the surface topography independently. They applied this microscope to
the study of the nitride-oxide-silicon (NOS) system. The experimental system of their
work is shown in figure A-3.
jiMtcnvaensrtivft
cgrrtiisw- and tip
ir.r.
*. r
'
0'4' df• 44« d4
C . v w / ' / r y / / <ry. / 5*
Figure A-3, An SCM added to an AFM [7],
K. Kobayashi et al. [8] presented an SCM system based on an AFM probe capable
of mapping dopant profile without an external capacitance sensor. An AC bias voltage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
108
signal with frequency © was applied between the tip and sample to induce an electrostatic
force oscillating at its third harmonic frequency 3© to be detected by a lock-in amplifier.
This 3© signal represented the information of differential capacitance information of the
semiconductor sample. The dopant profiling images for a silicon test sample were
obtained by SCM using both contact mode and non-contact mode AFM scanning. The
experimental setup is shown for this work is shown in figure A-4. The apparatuses in
dashed line frame are additional ones used in the experiment for non-contact AFM
operation. The operation frequency of non-contact AFM was 29.55 KHz in their report.
In non-contact mode AFM, a signal generator SGI was used to excite the cantilever beam
to vibrate at a fixed frequency slightly higher than its first free resonance frequency. An
RMS-to-DC converter measured the vibration amplitude and fed it to the feedback
controller to maintain the output voltage of a differential amplifier constant. Another
signal generator SG2 was used to set DC bias voltage and AC modulation voltage. The
frequency converter obtained the third harmonic frequency square wave for a reference
signal of the lock-in amplifier that was in-phase to the bias modulation signal.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
109
Photo Diodes
t
/
/ Actuator
\ /
wi
Generator j
G a n iliw
4
RMS to DC:
Converter»
SG1
Feedbacki
Controller |
11
Scanner
DrffererUiltJ
<a
Signal § « Frequency 1
G ew etor I p C&rwartw " 1
SG2
U cM o
n
UT
xm
Figure A-4. Schematic for dopant profile using SCM [8].
SCM is also used to image the local electrical properties in operating MOSFETs,
providing the attractive capability to measure the electrically active devices. C. Y.
Nakakura et al. [9] reported an SCM to map of the carrier density profile inside the
operating device to study carrier movement in the active region of MOSFETs. The
schematically SCM configuration is shown in figure A-5. A commercial SCM was
modified to allow AC bias voltage to be applied directly to the tip, while independent DC
bias to a bulk MOSFET sample. An HP 4145 semiconductor parameter analyzer was
used to bias the four separate regions: well, source, drain, and gate to switch on or off
corresponding active regions during SCM operation to realize active device carrier
concentration and carrier type measurement.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
110
HP 4145
sou
Si MtosiMlt
SCM! Cantilever
Function
Generator
(ac+dc tip b ias)
Lock-in Amo ii'iet
Figure A-5. SCM measurement of the cross section of a p-channel, Si bulk MOSFET
[9].
A.4 Magnetic Force Microscope
Magnetic force microscope (MFM) brings the power of SPM to a convenient and
cost-effective imaging tool that is ideal for many data storage device applications. It has
been widely used to study magnetism on the nanoscale for a variety of systems ranging
from thin-film surfaces to biological samples [10]. By scanning a tiny ferromagnetic
probe over a sample, MFM maps the stray magnetic fields close to a sample surface.
MFM probes are typically microfabricated silicon probes coated with a magnetic thin
film material. The tip is scanned several tens or hundreds of nanometers above the
sample, avoiding contact. Magnetic field gradients exert a force on the tip's magnetic
moment, and monitoring the tip/cantilever response gives a magnetic force image. To
enhance sensitivity, most MFM instruments oscillate the cantilever near its resonant
frequency (around 100 kHz) with a piezoelectric element. Gradients in the magnetic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ill
forces on the tip shift the resonant frequency of the cantilever. Monitoring this shift, or
related changes in oscillation amplitude or phase, produces a magnetic force image.
MFM probes operate in lift-mode to separate the magnetic image from the topography.
In this technique, each line in the raster scan pattern is passed over twice, as shown in
figure A-6. On the first pass, topographical information is recorded using tapping mode
(in which the oscillating cantilever lightly taps the surface). An image of the topography
is obtained by using the oscillation amplitude as a feedback signal for the tip-sample
spacing. Magnetic force data is acquired during a second pass, for which the tip is raised
to a user-selected "lift height." The lift height (typically 20-200 nm) is added point-bypoint to the stored topographical data, thus keeping the tip-sample separation constant
and preventing the tip from interacting with the surface. These two-pass measurements
are taken for every scan line to produce separate topographic and magnetic force images
of the same area.
'
Second pass
Sample
Figure A-6. Lift-mode operation of MFM probes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
112
A. Okuda et al. [11] reported a scanning magnetic force microscopy technique to
study magnetic image of the sample by detecting Lorentz force acting on the tip. They
used a conducting AFM metal tip, not a ferromagnetic tip. They demonstrated a Lorentz
force image due to a stray magnetic flux density from a magnetic hard disk, as shown in
figure A-7. The tip was contacted to the sample and AC bias voltage was applied to the
conductive cantilever against the sample, AC current i(t) flowed in the tip. Because of the
current i(t) and the stray magnetic flux density B from the sample, Lorentz force F was
exerted to the tip. As the direction of the Lorentz force was parallel to the sample, the
Lorentz force twists the cantilever. The optical lever deflection was detected by using the
quadrant photo sensitive diodes (PSD). A-B and C-D signals represented the deflection
and torsion of the cantilever. By applying ac bias voltage at 165 KHz (the resonant
frequency of the cantilever beam) in the ultrahigh vacuum environment, they achieved
simultaneous AFM topography image and Lorentz force image of tracks recorded in a
magnetic hard disk. Although scanning Lorentz microscopy provides in situ measurement
with high spatial resolution, the authors mentioned the sample should be as thin as
electron beam can transmit.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
113
L o r en tz F o r c e Im age
A~B: AFM Signal
C-D; Lateral Force
A-B
2-Phase
C-D
Quadrant
PSD
_
L o c k -In .AMP. —
—
f?
Laser
Function
Diode
Synthesizer
a.c. Tip Voltage
Cantilever
feedback
Circuit
Topographic image
Scanner
Figure A -7 . Block diagram of a scanning Lorentz force microscope system [11].
A.5 Scanning Thermal Microscope
Scanning thermal microscope (SThM) is capable of investigating the thermal
physical phenomena and properties of micro devices and structures [12]. The idea of
SThM technique is to scan a temperature-sensitive tip across sample surface to map the
surface temperature distribution. Tip-sample heat transfer changes the tip temperature,
therefore to determine the sample temperature. SThM has superior spatial resolution than
other thermal imaging techniques. For example, thermal imaging techniques based on
far-field optical method, such as infrared and laser reflectance techniques, can only
achieve spatial resolution on the order of wavelength due to diffraction limitation. L. Shi
reported an SThM probe capable of spatial resolution of sub-100 nm in thermal imaging
[13]. The key element of his SThM work was the thermal probe. Silicon dioxide and
silicon nitride were chosen for tip and cantilever materials according to theoretical model.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
114
Platinum and chrome were chosen for the thermocouple materials for their high thermo
power difference and low thermal conductivity. Figure A-8 shows the SEM photos of an
SThM probe, which contains a thermocouple junction at the tip end. The tip contained a
Pt-Cr thermocouple junction. Pt line and Cr line were patterned on two different beams
along a V-shaped cantilever. A thermally isolated laser reflector was also fabricated to
apply the SThM probe in a commercial AFM system to perform thermal imaging.
'unction
Figure A-8. SEM photos of an SthM probe [13].
D. -W. Lee et al. [14] also fabricated an SThM probe for nanometric thermal
imaging, and developed probe arrays for probe-based data storage. The probe was
fabricated by patterning a small metal wire for a nano-heater at the apex of a pyramidal
Si02 tip, following by Pt/Cr (or Au/Cr) deposition to fill the metal into the etch-pit. After
that, another metal layer (Ni) was deposited on the small wire to form a thermocouple
junction for temperature sensing at the tip end. Metal feed-through were formed on a
Pyrex glass substrate, which was anodic bonded to a probe array to enable the
transmission of a high-speed signal to a signal processing circuit and increase the probe
array density. Figure A-9 shows the schematic diagrams of the thermal probe that also
formed the probe array for application of high-density data storage.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
115
Pulse signal for
A etaatiitg signal
w riling/eras tag
Ni clectrsp iatlB g
jr ® lIN fo r electrical
U olatian and ac tu a tlo
/
▼
Glass
\
SiOj Ni a n d TIC
T iherm ai-electrical model of
l i e Nat»o*wlre fo r le a fin g
Figure A-9. An SThM probe with electrical feed-through [14].
A.6 Near-field Scanning Optical Mciroscope
With the invention of near-field scanning optical microscope (NSOM), it is
practical to achieve resolution in the 50-100 nm range using visible or near infrared light.
NSOM probe can measure local optical and optoelectronic properties, such as
luminescent spectra and optical index of refraction, with sub-diffraction limit resolution
[15]. The resolution of NSOM does not depend on the wavelength of the light, so that
visible and near infrared light can be used. NSOM was first demonstrated by D. W. Pohl
et al [16].
In a typical NSOM system, the near optical field located near the aperture of a
NSOM probe is used to illum inate the surface of sample. The optical transmission or
reflection from surface of the sample is detected by a photodetector. NSOM has been
applied to imaging biological surfaces, single molecule detection, chemical sensing, and
for semiconductor cryogenic spectroscopy. In early cases tapered fibers were used as
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
near-field probes where the fiber was metal coated at an oblique angle to form a
miniaturized aperture at its tip. Although the fiber optical probes for NSOM are currently
commercial available, they suffer the difficulty of batch process. And the small opening
angle of the fiber tip is small, so that the efficiency of forming near-field at the aperture is
low since most of the light is absorbed by the metal-coated fiber. Recent researches in
NSOM field mostly utilize microfabrication methods based on MEMS technology and
combine the probe with a scanning force microscope.
C. Mihalcea et al. [17] described the fabrication of hollow metal tips integrated on
silicon cantilevers as NSOM probes. Figure AlOa shows a schematic of the fabrication
process of the NSOM probe. The measurement setup is illustrated in figure AlOb. The
position of the cantilever was fixed during operation as in a typical AFM system. An
objective focused a He/Ne laser beam into the backside of the hollow metal tip. The
transmitted light was detected by a photomultiplier (PMT) tube to get the signal to be
processed by a DSP. Simultaneous AFM and optical images were recorded for periodic
trenches in an 80 nm thick Cr layer on a Si02 substrate.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
L
O x id e
d
Chromium'-.
&
beam deflection system
cantilever
detector
(b)
Figure A-10. Fabrication of a NSOM probe and the application system [17].
P. N. Minh et al. [15] reported a novel batch fabrication of a miniature aperture at
the apex of S i0 2 tip on a Si cantilever for NSOM. The probe consisted of a Si cantilever
with a tiny aperture at the apex of the tip and an opposed parallel electrode with a narrow
gap for capacitance detection or electrostatic actuation. The NSOM setup is shown in
figure Al l . The fabricated probe was combined with a commercial AFM system. The
near-field was generated by illumination from a laser diode through the aperture. The
reflected evanescent photons are collected by a long working distance lens and detected
by a photomultiplier (PMT). A CCD camera was also used to observe the probe
approaching to the sample. The AFM in contact mode and NSOM images of the Au
grating patterns on Si were simultaneously observed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
Lens
XYZ scanner
Figure A -ll. The setup of a NSOM system in reflection mode [15].
References
1. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, “7 x 7 reconstruction on Si
(111) resolved in real space,” Phys. Rev. Lett. 50, 120 (1983).
2. J. R. Smith, S. A. Campbell, and G.A. Mills, “Probing atoms,” Educ. Chem. 34,
107 (1997).
3. G. Binnig, C. F. Quate, and Ch. Gerber, “Atomic force microscope,” Phys. Rev.
Lett. 56, 930 (1986).
4. H. T. Soh, K. W. Guarini, C. F. Quate, Scanning probe lithography, Kluwer
Academic Publishers (2001)
5. G. Meyer and N. M. Amer, “Novel optical approach to atomic force microscopy,”
Appl. Phys. Lett. 53, 1045 (1988).
6. C. C. Williams, W. P. Hough, and S. A. Rishton, “Scanning capacitance
microscopy on a 25 nm scale,” Appl. Phys. Lett. 55, 203 (1989).
7. R. C. Barrett and C. F. Quate, “Charge storage in a nitride-oxide-silicon medium
by scanning capacitance microscopy,” J. Appl. Phys. 70, 2725 (1991).
8. K. Kobayahsi, “Dopant profiling on semiconducting sample by scanning
capacitance force microscopy,” Appl. Phys. Lett. 81, 2629 (2002).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
119
9. C. Y. Nakakura, P. Tangyunyong, D. L. Hetherington, and M. R. Shaneyfelt,
“Method for the study of semiconductor device operation using scanning
capacitance microscopy,” Rev. Sci. Instrum. 74, 127 (2003).
10. U. Hartmann, “Magnetic force microscopy: Some remarks from the
micromagnetic point of view,” J. Appl. Phys. 64, 1561 (1988).
11. A. Okuda, J. Ichihara, and Y. Majima, “Scanning Lorentz force microscopy,”
Appl. Phys. Lett. 81, 2872 (2002).
12. O. Nakabeppu, M. Chandrachood, Y. Wu, J. Lai, and A. Majumdar, “Scanning
thermal imaging microscopy using composite cantilever probes,” Appl. Phys. Lett.
66, 694 (1995).
13. L. Shi, O. Kwon, A. C. Miner, A. Majumdar, “Design and batch fabrication of
probes for sub-100 nm scanning thermal microscopy,” Journal o f
Microelectromechanical Systems 1, 370 (2001).
14. D. W. Lee, T. Ono, T. Abe, M. Esashi, “Fabrication of mciroprobe array with
sub-lOOnm nano-heater for nanometer thermal imaging and data storage,” The
14th IEEE International Conference on Micro Electro Mechanical Systems,
MEMS 2001, 204 (2001).
15. P. N. Minh, T. Ono, M, Esashi, “Microfabrication of miniature aperture at the
apex of Si02 tip on silicon cantilever for near-field scanning optical microscopy,”
Sensors and Actuators A 80, 163 (2000).
16. D. W. Pohl, W. Denk, and M. Lanz, “Optical stethoscopy: Image recording with
resolution A/20, "Appl. Phys. Lett. 44, 651 (1984)
17. C. Minhalcea, W. Scholz, S. Werner, S. Munster, E. Oesterschulze, and R.
Kassing, “Multipurpose sensor tips for scanning near-field microscopy,” Appl.
Phys. Lett. 68, 3531 (1996).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix B: Run Card
Step
Process Description
1
l.J
1.2
1.3
1.4
Initial I.Oum Thermal Oxide
Record wafer lot, number and scribe ID by diamond cut
RCA Clean
Grow 1.0 pm thermal oxide: 322-2, recipe: #144
Oxide thickness measurement by Nanospec
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
Photolithography of “Tip Formation”
Include all monitor wafers
HMDS: Program #3 and #0
Spin PR: standard 1.3 pm thick Shipleyl 813 PR on front side
Softbake: 30 min at 95 °C
Align and Expose: 1.9 sec, 30 mW, standard contact
Develop: 60 seconds, diluted 351
Hardbake: 30 min at 115 °C
Measure PR thickness: Dektek profiler
3
3.1
3.2
3.3
Ftch thermal oxide for hard mask of tip
Include all monitor wafers
BOE 1.0 pm thermal oxide at etch rate: ~650 A/min
Evaluate oxide removal and tip mask dimension
4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
KIF to form 10 pm high tip
Include all monitor wafers
RIE to form tip by timed control: recipe # 001, SF6 at LAM
Evaluate tip height and shape by SEM using monitor
Finish all wafers
Strip PR: Piranha, 30 min
5
5.1
5.2
5.3
Oxidation for tip sharpening
Include monitor wafers
RCA Clean
Grow thermal oxide by 322-l(#155) or 322-2(#125): 0.5 pm
BO E to rem ove oxide mask at tip area: 30% overetch
Evaluate oxide removal
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
121
5.4
Evaluate oxide thickness: Nanospec
6
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
Photolithography of “Ion Implantation”
Include monitor wafers
HMDS: Program #3 and #0
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60 sec
Bake: 60 min at 95 °C
Align and Expose: 20 sec, 30mW, standard contact
Develop: 8 min, diluted AZ 400K
Evaluate pattern definition and PR removal at openings
Measure PR thickness: Dektek and Nanospec
7
7.1
7.2
7.3
7.4
7.5
Ktch oxide to open windows for ion-implantation
Include monitor wafers
Descum PR by M4L asher,
BOE 0.5 pm thermal oxide
Evaluate oxide removal at windows
Strip PR: Piranha, 30 min
8
8.1
8.2
8.3
8.4
Ion implantation and anneal (drive-in)
Send wafers out for ion-implantation : B l l , 5*10el6, 60 keV
Anneal by vendor and inspect: Optical microscopy
Receive wafers from vendor
Clean wafers: Piranha, 30 min
9
9.1
9.2
Strip thermal oxide layer
BOE 0.5 pm thermal oxide
Evaluate oxide removal everywhere
10
10.1
10.2
10.3
10.4
Al (IO \lctal) coating and sintering
Include monitor wafer
Sputter 0.5 pm Al/Cu/Si at 75OW
Measure Al thickness: Four point resistance, RS32
Al sintering, 322-4, recipe # 802, 450 °C for 20min.
11
11.1
11.2
11.3
11.4
11.5
11.6
Photolithography of “IO Metal”
Include monitor wafers
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60 sec
Bake: 60 min at 95 °C
Align and Expose: 20 sec, 30mW, standard contact
Develop: 16 min, diluted AZ
Evaluate pattern definition, PR thickness and residue
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
122
12
12.1
12.2
12.3
12.4
12.5
Al (IO Metal) Etching
Include monitor wafer
Wet etch Al by Aluminum etchant
Evaluate Al removal: Optical microscopy
Strip PR: Acetone soaking 20 min and Asher 30 min
Evaluate PR removal
13
13.1
13.2
13.3
LPCVD LTO for Insulator between metals
Include monitor wafers
Deposit lm LTO: 321-4, recipe # 400 at -130 A/min
Oxide thickness measurement by Nanospec
14
14.1
14.2
14.3
14.4
Al (Shield Metal) coating and sintering
Include monitor wafer
Sputter 1 pm Al at 750W
Measure Al thickness: Four point resistance, RS32
Al sintering, 322-4, recipe # 805, 300 °C for 60 min
15
15.1
15.2
15.3
15.4
15.5
15.6
Photolithography of “Shield"
Include monitor wafers
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60 sec
Bake: 60 min at 95C
Align and Expose: 20 sec, 30mW, standard contact
Develop: 16 min, diluted AZ
Evaluate pattern definition, PR thickness and residue
16
16.1
16.2
16.3
16.4
16.5
16.6
Al (Shield Metal) Etching
Include monitor wafer
Descum PR by Asher
Wet etch Al by Aluminum etchant
Evaluate Al removal: Optical microscopy
Strip photoresist: Acetone soaking 20 min and Asher 30 min
Evaluate PR removal
17
17.1
17.2
17.3
17.4
17.5
17.6
17.7
Photolithography of “LTO Etch"
Include monitor wafers
HMDS: Program #3 and #0
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60 sec
Bake: 60 min at 95 °C
Align and Expose: 20 sec, 30mW, standard contact
Develop: 8 min, diluted AZ 400K
Evaluate pattern definition, PR thickness and residue
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
123
18
1S.1
18.2
18.3
18.4
18.5
18.6
Etch LTO for beam definition and metal pad
Include monitor wafers
Descum PR by Asher
RIE remove lum LTO: Tegal
Evaluate oxide removal at windows
Strip PR: Acetone soaking 20 min and Asher 30 min
Evaluate PR removal
19
19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.7
19.8
19.9
Tip exposure
Include monitor wafers
HMDS: Program #3 and #0
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60sec
Bake: 60 min at 95 °C
Asher to exposure tip
Wet etch Al by Aluminum etchant
BOE etching LTO
Evaluate tip exposure: Optical microscopy
Strip photoresist: Acetone soaking 20 min and Asher 30 min
Evaluate PR removal
20
20.1
20.2
20.3
20.4
20.5
20.6
20.7
Photolithography of “Define Beam”
Include monitor wafers
HMDS: Program #3 and #0
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60sec
Bake: 60 min at 95 °C
Align and Expose: 20 sec, 30mW, standard contact
Develop: 8 min, diluted AZ 400K
Evaluate pattern definition, PR thickness and residue
21
21.1
21.2
21.3
21.4
21.5
21.6
Etch ~5um silicon till reaching buried oxide layer
Include monitor wafers
Descum PR by Asher
RIE remove ~5 pm device silicon layer: Tegal
Evaluate oxide exposed at opening
Strip photoresist: Acetone soaking 20 min and Asher 30 min
Evaluate PR removal
22
22.1
22.2
22.3
22.4
Protect device side before double-sided alignment
Include monitor wafers
HMDS: Program #3 and #0
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60 sec
Bake: 60 min at 95 °C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
23
23.1
23.2
23.3
23.4
23.5
23.6
23.7
Backside Photolithography of “Beam Release'’
Include monitor wafers
Spin acetone on backside to clean PR residue during step 22
Spin thick PR: AZ9260, full spread, 1350rpm, 60sec
Bake: 60 min at 95 °C
Align and Expose: with IR, 20 sec, 30mW, standard contact
Develop: 8min, diluted AZ 400K
Evaluate pattern definition, PR thickness and residue
24
24.1
24.5
24.6
Backside etch silicon and buried oxide layer
Include monitor wafers
Spin thick PR: AZ 9260, full spread, 1350 rpm, 60 sec on a DRIE
handle wafer
PR bonding SOI wafer to the DRIE handle wafer
Bake: 60 min at 95 °C
DRIE remove ~ 400um silicon layer: Outside foundry through
MEMS -Exchange®
Evaluate BOX exposed at opening
25
25.1
25.2
25.3
25.4
25.5
Release SNMJY1 probes
Descum by Asher 20 min
RIE remove 1pm BOX layer: Tegal
Strip photoresist: Acetone soaking overnight and Asher
Evaluate PR removal
Pick single SNMM probes by tweezer for characterization
24.2
24.3
24.4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Bibliography
Akamine, S.; Albrecht, T. R.; Zdeblick, M. J.; Quate, C. F.; “Microfabricated
scanning tunneling microscope,” IEEE Electron Device Letters 10, 490 (1989).
Akamine, S.; Quate, C. F.; “Low temperature thermal oxidation sharpening of
microcast tips,” J. Vac. Sci. Technol. B 10, 2307 (1992).
Akiyama, T.; Gautsch, S.; de Rooij, N. F.; Staufer, U.; Niedermann, Ph.; Howald, L.;
Muller, D.; Tonin, A.; Hidber, H.-R.; Pike, W. T.; Hecht, M. H.; “Atomic force
microscope for planetary applications,” Sensors and Actuators A 91, 321 (2001).
Albrecht, T. R.; Akamine, S.; Carver, T. E.; Quate, C. F.; “Microfabrication of
cantilever styli for the atomic force microscope,” J. Vac. Sci. Technol. A 8, 3386
(1990).
Albrecht, T. R.; Quate, C. F.; “Atomic resolution imaging of a nonconductor by
atomic force microscopy,” J. Appl. Phys. 62, 2599 (1987).
Albrecht, T. R.; Quate, C. F.; “Atomic resolution with the atomic force microscope
on conductors and nonconductors,” J. Vac. Sci. Technol. A 6, 271 (1988).
Ash, E. A.; Nichols, G.; "Super-resolution aperture scanning microscope", Nature
237,510(1972).
Barrett, R. C.; Quate, C. F.; “Charge storage in a nitride-oxide-silicon medium by
scanning capacitance microscopy,” J. Appl. Phys. 70, 2725 (1991).
Binnig, G.; Quate, C. F.; Gerber, Ch.; “Atomic force microscope,” Phys. Rev. Lett. 56,
930 (1986).
Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E.; “7 x 7 reconstruction on Si (111)
resolved in real space,” Phys. Rev. Lett. 50, 120 (1983).
Boisen, A.; Hansen, O.; Bouwstra, S.; “AFM probes with directly fabricated tips.” J.
Micromech. Micro eng. 6, 58 (1996).
Bork, T.; Agrawal, V.; Rosner, B.; Gustafson, P.; van der Weide, D. W.; “Shieldedtip/cantilever process and interface for multifunctional scanning probe microscopy,”
Solid-State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, 271
( 2000).
Brugger, J.; Despont, M.; Rossel, C.; Rothuizen, H.; Vettiger, P.; Willemin, M.;
“Microfabricated ultrasensitive piezoresistive cantilevers for torque magnetometry,”
Sensors and Actuators A 73, 235 (1999).
125
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
126
Chand, A.; Viani, M. B.; Schaffer, T. E.; Hansma, P. K.; “Microfabricated small
metal cantilevers with silicon tip for atomic force microscopy,” Journal o f
Microelectromechanical Systems 9, 112 (2000).
Danzebrink, H. U.; Ohlsson, O.; Wilkening, G.; “Fabrication and characterization of
optoelectronic near-field oribes based on an SFM cantilever design,”
Ultramciroscopy, 61, 131 (1995).
Davis, R. C.; Williams, C. C.; Neuzil, P.; “Micromachined submicrometer photodiode
for scanning probe microscopy,” Appl. Phys. Lett. 66, 2309 (1995).
DiCarlo, A.; Scheinfein, M. R.; Chamberlin, R. V.; “Magnetic force microscopy
utilizing an ultrasensitive vertical cantilever geometry,” Appl. Phys. Lett. 61, 2108
(1992).
Duncan, W. M.; “Near-field scanning optical microscope for microelectronic
materials and devices,” J. Vac. Sci. Technol. A 14, 1914 (1996).
Folch, A.; Wrighton, M. S.; Schmidt, M. A.; “Microfabrication of oxidationsharpened silicon tips on silicon nitride cantilevers for atomic force microscopy,”
Journal o f Microelectromechanical Systems 6, 303 (1997).
Gao, C.; Wei, T.; Duewer, F.; Lu, Y.; Xiang, X.-D.; “High spatial resolution
quantitative microwave impedance microscopy by a scanning tip microwave near­
field microscope,” Appl. Phys. Lett. 71, 1872 (1997).
Gao, C.; Xiang, X.-D. “Quantitative microwave near-field microscopy of dielectric
properties,” Rev. Sci. Instrum. 69, 3846 (1998).
Ghandhi, S. K.; VLSI Fabrication Principles. John Wiley & Sons, Inc., New York
(1994).
Gray, M. H.; Hsu, J. W. P.; “A variable cryogenic temperature near-field scanning
optical microscope,” Rev. Sci. Instrum. 70, 3355 (1999).
Harootunian, A.; Betzig, E.; Isaacson, M.; Lewis, A.; “Super-resolution fluorescence
near-field scanning optical microscopy,” Appl. Phys. Lett. 49, 674 (1986).
Hartmann, U.; “M agnetic force microscopy: Som e remarks from the m icromagnetic
point of view,” J. Appl. Phys. 64, 1561 (1988).
Huang, Y.; Williams, C. C.; Slinkman, J.; “Quantitative two-dimensional dopant
profile measurement and inverse modeling by scanning capacitance microscopy,”
Appl. Phys. Lett. 66, 344 (1995).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
127
Isaacson, M.; Cline, J. A.; Barshatzky, H.; “Near-field scanning optical microscopy
H,” J. Vac. Sci. Technol. B 9, 3103 (1991).
Kirk, M. D.; Albrecht, T. R.; Quate, C. F.; “Low-temperature atomic force
microscopy,” Rev. Sci. Instrum. 59, 833 (1988).
Kleiner, A.; Eggert, S.; “Curvature, hybridization, and STM images of carbon
nanotubes,” Phys. Rev. B 64, 113402 (2001).
Kraus, J. D.; Electromagnetics with applications. McGraw-Hill Companies, Inc.
Boston (1999).
Lee, D. W.; Ono, T.; Abe, T.; Esashi, M.; “Fabrication of microprobe array with sublOOnm nano-heater for nanometer thermal imaging and data storage,” The 14th IEEE
International Conference on Micro Electro Mechanical Systems, 204 (2001).
Lee, D. W.; Ono, T.; Esashi, M.; “Cantilerver with integrated resonator for
application of scanning probe microscope.” Sensors and Actuators A 83, 11 (2001).
Leinhos, T.; Stopka, M.; Oesterschulze, E.; “ Micromachined fabrication Si
cantilevers with Schottky diode integrated in tip,” Appl. Phys. A 66, S65 (1998).
Liu, C.; Gamble, R.; “Mass-producible monolithic silicon probes for scanning probe
microscopes,” Sensors and Actuators A 71, 233 (1998).
Luo, K.; Herrick, R. W.; Majumdar, A.; Petroff, P.; “Scanning thermal microscopy of
a vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 71, 1604 (1997).
Mamin, H. J.; Rugar, D.; Lambert, S. E.; Franco, L. P.; Sanders, I. L.; Yogi, T.;
Beaulieu, T.; “Magnetic force microscopy of recording media,” J. Appl. Phys. 67,
5953 (1990).
Manalis, S. R.; Minne, S. C.; Atalar, A.; Quate, C. F.; “High-speed atomic force
microscopy using an integrated actuator and optical lever detection,” Rev. Sci.
Instrum. 67, 3294 (1996).
Manalis, S. R.; Minne, S. C.; Quate, C. F.; “Atomic force microscopy for high speed
imaging using cantilevers with an integrated actuator and sensor,” Appl. Phys. Lett.
68, 871 (1996).
Matey, J. R.; Blanc, J.; “Scanning capacitance microscopy,” J. Appl. Phys. 57 1437
(1985).
McGruer, N. E.; Warner, K.; Singhal, P.; Gu, J. J.; Chan, C.; “Oxidation-sharpened
gate field emitter array process,” IEEE Transaction on Electron Devices 38, 2389
(1991).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
128
Mills, G.; Zhou, H.; Midha, A.; Donaldson, L.; Weaver, J. M. R.; “Scanning thermal
microscopy using batch fabricated thermocouple probes,” Appl. Phys. Lett. 72, 2900
(1998).
Minh, P. N.; Ono, T.; Esashi, M.; “High throughput aperture near-field scanning
optical microscopy,” Rev. Sci. Instrum. 71, 3111 (2000).
Nakabeppu, O.; Chandrachood, M.; Wu, Y.; Lai, J.; Majumdar, A.; “Scanning
thermal imaging microscopy using composite cantilever probes,” Appl. Phys. Lett. 66,
694 (1995).
Nieminen, J.; Lahti, S.; Paavilainen, S.; Morgenstem, K.; “Contrast changes in STM
images and relations between different tunneling models,” Phys. Rev. B 66, 165421
( 2002).
Nilius, N.; Ernst, N.; Freund, H.-J.; “Tip influence on plasmon excitations in single
gold particles in an STM,” Phys. Rev. B 65, 115421 (2001).
Niyajima, H.; Meheregany, M.; “High-Aspect-Ratio Photolithography for MEMS
Applications,” Journal o f Microelectromechanical Systems 4, 220 (1995).
Oesterschulze, E.; Stopka, M.; Ackermann, L.; Scholz, W.; Werner, S.; “Thermal
imaging of thin films by scanning thermal microscope,” J. Vac. Sci. Technol. B 14,
832 (1996).
Peterson, K. E.; “Silicon as a mechanical material,” Proc. IEEE, 70, 420 (1982).
Radmacher, M.; Hillner, P.; Hansma, P.; “Scanning nearfield optical microscope
using mcirofabricated probes,” Rev. Sci. Instrum. 65, 2737 (1994).
Ramo, S.; Whinner, J. R.; Van Duzer, T.; Fields and Waves in Communication
Electronics, John Wiley and Sons, Inc., New York (1984).
Ravi, T. S.; Marcus, B.; Liu, D.; “Oxidation sharpening of silicon tips,” J. Vac. Sci.
Technol. B 9, 2733 (1991).
Rosner, B. T.; “Near-field microscopy from the microwave regime to the visible,”
Ph.D. Thesis, Department of Electrical Engineering, University of Delaware (2002).
Rosner, B. T.; van der W eide, D. W.; “H igh-frequency near-field m icroscopy,” Rev.
Sci. Instrum.13, 2505 (2002).
Ruiter, A. G. T.; Moers, M. H. P.; van Hulst, N. F.; de Boer, M.; “Microfabrication of
near-field optical probes,” J. Vac. Sci. Technol. B 14, 597 (1996).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
129
Schtirmann, G.; Indermiihle, P. F.; Staufer, U.; de Rooij, N. F.; “Micromachined SPM
probes with sub-100 nm features at tip apex,” Suface and Interface Analysis 27, 299
(1999).
Schtirmann, G.; Noell, W.; Staufer, U.; de Rooij, N. F.; “Microfabrication of a
combined AFM-SNOM sensor,” Ultramciroscopy 82, 33 (2000).
Shi, L.; Kwon, O.; Miner, A. C.; Majumdar, A.; “Design and batch fabrication of
probes for sub-100 nm scanning thermal microscopy,” Journal o f
Microelectromechanical Systems 1, 370 (2001).
Shi, L.; Plyasunov, S.; Bachtold, A.; McEuen,P. L.; Majumdar, A.; “Scanning
thermal microscopy of carbon nanotubes using batch-fabricated probes,” Appl. Phys.
Lett. 77, 4295 (2000).
Skidmore, G. D.; Dahlberg, E. D.; “Improved spatial resolution in magnetic force
microscopy,” Appl. Phys. Lett. 71, 3293 (1997).
Sorop, T. G.; Untiedt, C.; Luis, F.; Kroll, M.; Ra§a, M.; de Jongh, L. J.;
“Magnetization reversal of ferromagnetic nanowires studied by magnetic force
microscopy,” Phys. Rev. B 67, 014402 (2003).
Steinhauer, D. E.; Anlage, S. M.; “Microwave frequency ferroelectric domain
imaging of deuterated triglycine sulfate crystals,” J. Appl. Phys. 89, 2314 (2001).
Steinhauer, D. E.; Vlahacos, C. P.; Dutta, S. K.; Wellstood, F. C.; Anlage, S. M.;
“Surface resistance imaging with a scanning near-field microwave microscope,” Appl.
Phys. Lett. 71, 1736 (1997).
Su, Y.; Evans, A. G. R.; Brunnschweiler, A.; Ensell, G.; Koch, M.; “Fabrication of
improved piezoresistive silicon cantilever probes for the atomic force microscope,”
Sensors and Actuators A 60, 163 (1997).
Synge, E. H.; “A suggested model for extending microscopic resolution into the ultramicroscopic region,” Phil. Mag. 6, 356 (1928).
Tabib-Azar, M.; Akinwande, D.; “Real-time imaging of semiconductor space-charge
regions using high-spatial resolution evanescent microwave microscope,” Rev. Sci.
Instrum. 71, 1460 (2000).
Tabib-Azar, M.; Akinwande, D.; Ponchak, G. E.; LeClair, S. R.; “Evanescent
microwave probes on high-resistivity silicon and its application in characterization of
semiconductors,” Rev. Sci. Instrum. 70, 3083 (1999).
Tabib-Azar, M.; Akinwande, D.; Ponchak, G.; LeClair, S. R.; “Novel physical
sensors using evanescent microwave probes,” Rev. Sci. Instrum. 70, 3381 (1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
130
Tabib-Azar, M.; Evanescent Microwave Microscopy for High-Speed and HighResolution Material Characterizations, Kluwer Academic Publishers, Boston (2000).
Tabib-Azar, M.; Micro-Actuators, Kluwer Academic Publishers, Boston (1998).
Tabib-Azar, M.; NIST Project Proposal, (2001).
Tabib-Azar, M.; Pathak, P. S.; Ponchak, G.; LeClair, S.; “Nondestructive
superresolution imaging of defects and nonuniformities in metals, semiconductors,
dielectrics, composites, and plants using evanescent microwaves,” Rev. Sci. Instrum.
70, 2783 (1999).
Tabib-Azar, M.; Shoemaker, N.; Harris, S.; “Non-destructive characterization of
materials by evanescent microwaves.” Measurement Science and Technology 4, 583
(1993).
Tabib-Azar, M.; Su, D.-P.; Pohar, A.; LeClair, S. R.; Ponchak, G.; “0.4 pm spatial
resolution with 1 GHz (k=30cm) evanescent microwave probe,” Rev. Sci. Instrum. 70,
1725 (1999).
Tabib-Azar, M.; Sutapun, B.; “Novel hydrogen sensors using evanescent microwave
probes,” Rev. Sci. Instrum. 70, 3707 (1999).
Thaysen, J.; Boisen, A.; Hansen, O.; Bouwstra, S.; “Atomic force microscopy probe
with piezoresistive read-out and a highly symmetrical Wheatstone bridge
arrangement.” Sensors and Actuators A 83, 47 (2000).
Tortonese, M.; Barrett, R. C.; Quate, C. F.; “Atomic resolution with an atomic force
microscope using piezoresistive detection,” Appl. Phys. Lett. 62, 834 (1993).
Tran, T.; Oliver, D. R.; Thomson, D. J.; Bridges, G. E.; “"Zeptofarad" (10'21 F)
resolution capacitance sensor for scanning capacitance microscopy,” Rev. Sci.
Instrum. 72, 2618 (2001).
Urazhdin, S.; Tessmer, S. H.; Ashoori, R. C.; “A simple low-dissipation amplifier for
cryogenic STM,” Rev. Sci. Instrum. 73, 310 (2002).
van der Weide, D. W.; “Localized picosecond resolution with a near-field
microwave/scanning-force microscope,” Appl. Phys. Lett. 70, 677 (1996).
van der Weide, D. W.; Neuzil, P.; “The Nanoscilloscope: Combined Topography and
AC Field Probing with Micromachined Tip,” J. Vac. Sci. Technol. B 14, 4144 (1996).
van der Weide, D. W.; Neuzil, P.; “The nanoscilloscope: Combined topography and
AC field probing with a micromachined tip,” J. Vac. Sci. Technol. B 14, 4144 (1996).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
131
Vinckier, A.; Hennau, F.; Kjoller, K.; Hellemans, L.; “Low-cost modification of a
contact atomic force microscope (AFM) into a sound-activated tapping mode AFM
for use in air and liquids,” Rev. Sci. Instrum. 67, 387 (1996).
Wang, Y.; Tabib-Azar, M.; “Microfabricated Near-field Scanning Microwave
Probes,” Electron Devices Meeting, IEDM '02. Digest. International, IEEE, 905
( 2002).
Wang, Y.; Tabib-Azar, “Fabrication and Characterization of Evanescent Microwave
Probes Compatible with Atomic Force Microscope For Scanning Near-Field
Microscopy,” Proceedings o f 2002 ASME Inter. Mech. Eng. Congress & Expo., New
Orleans, Louisiana, November 17-22 (2002).
Williams, C. C.; Hough, W. P.; Rishton, S. A.; “Scanning capacitance microscopy on
a 25 nm scale,” Appl. Phys. Lett. 55, 203 (1989).
Wolf, S.; Tauber, R. N.; Silicon Processing, Lattice Press (2000).
Zhang, Y.; Zhang, Y.; Blaser, J.; Sriram, T. S.; Enver, A.; Marcus, R. B.; “A thermal
microprobe fabricated with wafer-stage processing,” Rev. Sci. Instrum. 69, 2081
(1998).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Документ
Категория
Без категории
Просмотров
0
Размер файла
11 100 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа