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Synthesis and characterization of nanostructured BSTO thin-films for microwave applications

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SYNTHESIS AND CHARACTERIZATION OF NANOSTRUCTURED BSTO
THIN^FILMS FOR MICROWAVE APPLICATIONS
Dissertation
Submitted to
The School of Engineering a t the
UNIVERSITY OF DAYTON
In portia! fulfillment of the requirements for
The Degree
Doctor of Philosophy in Electrical Engineering
By
Bonnie D. Riehl
UNIVERSITY OF DAYTON
Dayton, Ohio
May 2004
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UMI N um ber: 3127133
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SYNTHESIS AND CHARACTERIZATION OF NANOSTRUCTURED BSTO
THIN-FILMS FOR MICROWAVE APPLICATIONS
APPROVED BY;
Guru Subramanyam, PhD
Advisory Committee Chairman
Associate Professor, Department of
Electrical and Computer Engineering
Malcolm W. Daniels, PhD
Committee Member
Assistant Professor, Department of
Electrical and Computer Engineering
Paul T. Murray, PhD
Committee Member
Professor, Department of Materials
Science, Senior Scientist, UDRI
Rand Biggers, PhD
Conmmittee Member
Senior Research Physicist,
Air Force Research Laboratory,
WPAFB
I
Saliba, PhD
Dean, School of Engineering
Donald L. Moon, PhD
Associate Dean
Graduate Engineering Programs
and Research, School of Engineering
11
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TABLE OF CONTENTS
ABSTRACT...........................................................................V
ACKNOWLEDGEMENTS.........................................................vli
LIST OF FIGURES...................................................................viii
LIST OF TABLES.....................................................................xi!
L iNTRODUCTION.................................,...............................'l
1.1 M otivation................................................................. 1
1.2 Research Overview....................................................3
1.3 Ferroelectric M aterial..................................................4
1.4 Size Effects................................................................ 10
1.5 S ynthesis,...................................................................! 1
1.6 M aterial................... ...................................................14
1.7 M ateriai/Device Characterization................................ 16
1.8 Outline of Chapters..................................................... 16
2. LITERATURE SURVEY............................................................. 17
2.1 Ferroelectrics............................................................... 17
2.2 Nano-phase Ferroelectrics...........................................20
3. SYNTHESIS AND CHARACTERIZATION TECHNIQUES..................21
111
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3.1 Synthesis................................................................. .21
3.2 AFM/SPM Characterization...................................... .24
3.3 Surface Potential Im aging..........................................25
3.4 Microwave Characterization.....................................26
4. EXPERIMENTAL SETUP AND RESULTS.....................................31
4.1 Film Fabrication.........................................................31
4.2 M icrowave Set-up.....................................................36
5. CHARACTERIZATION.........................................................4 0
5.1 XRD Characterization.
........
.40
5.2 SEM Characterization................................................44
5.3 AFM Characterization,...............................................46
5.4 EFM (Surface Potential) Characterization.....................50
5.5 Microwave Characterization......................................61
6. ANALYSIS AND VALIDATION.,............................................66
6.1 Overview..................................................................6 6
6.2 Future Directions........................................................71
6.3 Conclusions...............................................................7 2
BIBUOGiAPHY.....................................................................74
APPENDIX A .........................................................................79
IV
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SYNTHESIS AND CHARACTERIZATION OF NANOSTiUCTURED BSTO THIN-FILMS
FOR MICROWAVE APPLICATIONS
Abstract
Nanophase synthesis of ferroelectric thin-films of Bao,6Sro.4Ti03 (BSTO) was
studied systematicaliy for applications in tunable m icrowave components.
Synthesis of nanostructured BSTO was perform ed using a pulsed-laser
deposition system with real-time in-situ process control.
The main
research goal was to utilize the pulsed laser deposition parameters to
control the grain growth for low m icrowave loss nanostructured BSTO thinfilms on crystalline substrates such as LaAIOs. These parameters include
the energy density of the laser pulses, wavelength, oxygen partial
pressure, distance between the target and the substrate, and the
substrate
temperature.
The
nanostructural
performed using XRD, SEM and AFM.
characterization
was
M icrowave characterization was
done using coplanar w aveguide lines to characterize the frequency
dependent dielectric properties (& and tanS). BSTO films were grown a t
the same measured tem perature and energy density but in different
oxygen am bient pressures from 19 mTorr through 300 mTorr. Using c o n ta c t
m ode AFM, the grain size was found to decrease as the oxygen am bient
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pressure was reduced from 150 mTorr to 38 mlorr.
The growth process
cha ng ed when the pressure was increased above 150 mlorr. Nanocluster
structures rather than nanoparticies were found a t 225 mTorr. Average
groin sizes less than 100 nm were obtained for oxygen pressures below 75
mTorr.
The XRD spectra Indicate the highly crystalline nature of the film.
M icrowave measurements, performed between 9-18 GHz, suggest the
nano-structured BSTO thin-films on LaAIOs (LAO) substrates are highly
tunable (up to 25%).
Surface potential imaging indicated that the
surface potential of polarized areas to be the sum of contributions from
the surface charge and from the vertical part of the oriented dipoles In
the areas.
VI
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LIST OF FIGURES
Figure 1, Dipole chains for a ferroelectric material,
Figure 2, Ferroelectric cubic, ferroelectric tetragonal,
Figure 3. Ferroelectric dom ain regions in response to spontaneous
polarization. The dork circles indicate the dipoles, which align in the
direction of the electric field.
Figure 4. Hysteresis curve for a ferroelectric material under an applied
electric field.
Figure 5. Flow diagram for the fabrication of sol-gel films.
Figure 6, Relative dielectric constant and loss tangent os a function of
electric field bias for STO.
Figure 7.
Autom ated in-situ, real-time, process-control pulsed-laserdeposition system. Real-time control based on fe e d b a ck from emission
(ES) sensors. Laser energy, cham ber ambient, an d/o r beam footprint can
be adjusted to maintain a TOF set-point based on ES feedback, (Courtesy
of AFRL/MLP, Wright-Patterson APB).
Figure 8. Hierarchial process m odel identifying process control variables
and their com plex Interactions in a pulsed laser deposition system.
Figure 9. Cross-section of the m odified ferroelectric tunable CPW
configuration. The G-C-G refers to Ground-Conductor-Ground.
Figure 10. On-wofer Probe Station a t NASA for M icrowave Measurements.
Figure 11. Mask used on BSTO samples for evaluating microwave properties.
Figure 12. Plot of O 2 (mlorr) pressure versus TOF (ps) from Table 2.
Vlll
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Figure 13, The emission signal from the Ba* com ponent in plume
generated when the materia! passed the monitored position for the PLD
deposition of BSTO on LAO substrate for 150 mTorr O 2 pressure.
Figure 14. Visible wavelengths images of the PLD plume as cham ber
pressure O 2 was increased while holding a constant beam footprint energy
density for PLD deposition of BSTO on LAO substrate.
Figure 15. Contour plots of the intensity Image of PLD plume a t varying
cham ber pressures for deposition of BSTO on LAO substrate a t varying
cham ber pressures, a. 19 mTorr, b. 75 mTorr, c. 300 mTorr.
Figure 16. Film thickness versus cham ber oxygen pressure a t a constant
energy for PLD deposition of BSTO on LAO substrate.
Figure 17. X-ray diffraction pattern for a BSTO film deposited on LAO a t 19
mTorr Oxygen pressure, radiation=20(CuKa), Lam bda=l .5478.
Figure 18. X-ray diffraction pattern for a BSTO film deposited on LAO a t 38
mTorr Oxygen pressure, radiation=20(CuKa), Lam bda=l .5478.
Figure 19. X-ray diffraction pattern for a BSTO film deposited on LAO a t
225 mTorr oxygen pressure, radiat!on=20(CuKa), Lam bda=l .5478.
Figure 20 a. SEM Image of PLD deposited BSTO on LAO substrate a t 300
mTorr oxygen pressure.
Figure 20 b. AFM image of PLD deposition of BSTO on LAO at 300 m lorr
oxygen pressure.
Figure 20 c, SEM image of PLD deposition of BSTO on LAO substrate at 300
mTorr oxygen pressure.
Figure 21, AFM 3-dimensional images (deposition pressure, average
particle height and surface RMS roughness) for PLD deposition of BSTO on
LAO substrate: a. 38 mTorr, 1.5 nm, & 11 nm; b. 75 mTorr, 2.4 nm, & 20nm;
0 . 150 mTorr, 3.8 nm, & 5.1 nm; and d. 225 mTorr, 4.5 nm, & 8.7 nm, e. 300
mTorr.
Figure 22. Plot of O 2 pressure versus particle diameter, d =
where d is
particle diameter, P is O 2 pressure for BSTO deposited on LAO substrate.
IX
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Figure 23. Particle height (nm) and Suiface Roughness (nm) for the AFM
images In Figure 22.
Figure 24 , AFM im age of BSTO film deposited on LAO substrate a t a. 75
m lorr oxygen pressure, b. 150 m lorr oxygen pressure, c. 300 mlorr oxygen
pressure.
Figure 25. Schematic of poling ferroelectric thin films by an AFM
conductive tip, A positive bias applied to the tip with respect to the
substrate aligns dipoies downward. A negative bias applied to the tip
with respect to the substrate aligns dipoles upward.
Figure 26. Platinized high resistivity Si substrate for MMIC applications.
Figure 27 a. Surface potential of BSTO on platinized high resistivity Si a t 38
m lorr O 2 pressure, 5 V probe bias.
Figure 27 b. Surface potential of BSTO on platinized high resistivity SI a t 38
mTorr O 2 pressure, -5 V probe bias.
Figure 27 c. 3-D surface potential for BSTO on platinized high resistivity SI a t
38 m lorr O 2 pressure, -5 V probe bias.
Figure 28. 3-D AFM im age of PLD deposition of BSTO on platinized high
resistivity SI a t 160 m lorr oxygen pressure.
Figure 29. 3-D AFM im age of PLD deposition of BSTO on ptatinized high
resistivity SI a t 300 mTorr oxygen pressure.
Figure 30. Surface potential im age of PLD deposition of BSTO on platinized
high resistivity 81 a t 600 mTorr oxygen pressure.
Figure 31 a, Surface potential PLD deposited BSTO on LAO substrate a t 38
mTorr O 2 pressure, 5 V probe bias.
Figure 31 b. Surface potential BSTO on LAO, 38 mTorr O 2 pressure, -5 V
probe bias.
Figure 32 a. 3-D surface potential a t 75 mTorr O 2 pressure of PLD
deposited BSTO on LAO substrate , -5V probe bias.
Figure 32 b. 3-D surface potential a t 75 mTorr O 2 pressure of PLD
deposited BSTO on LAO substrate, 5V probe bias.
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Figure 33 a, Surface potential of PLD deposited BSTO on LAO substrate a t
300 m lorr O 2 pressure, -5V probe bias.
Figure 33 b, Surface potential of PLD deposited BSTO on LAO substrate a t
300 mTorr O 2 pressure, 5V probe bias.
Figure 33 c. AFM of PLD deposited BSTO on LAO substrate a t 300 m lorr O 2
pressure.
Figure 34. AFM Image of PLD deposition of BSTO on LAO substrate a t 1 Torr
O 2 pressure.
Figure 35. This shows S21 vs frequency for a 38 m lorr test sample sent to
NASA Glenn. The 821 improves as the voltage Is applied from 0 V to 300 V in
step of 50 V. You can also see the phase of 821 tunable in the bottom
picture.
Figure 36. C a pa citan ce versus DC bios for 38 mTorr BSTO on LAO.
Figure 37. Tunabillty of a coplanar ca p a cito r with 5 |im gap as a function
of frequency, showing the highly tunable nature of the nano-structured
ferroelectric thin-films up to 50 GHz. This measurement was performed at
Chalmers University.
Figure 38. Change in dielectric constant with frequency for 38 mTorr sample
of BSTO on LAO.
Figure 39 a. The dipoles ore oriented in the same direction (simple
domain), b. The dipoles while in the same direction are disjoint, c. The
dipoles ore anti-parallel (walls are a t 180°)- d. The dipole walls are a t 90°.
XI
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LIST OF TABLES
Table 1. Appendix A, List of BSTO Applications
Table 2. Appendix A, Synthesis of BSTO by PLD and Sol-gel
Table 3. TOFs recorded for each of the oxygen pressures used.
XU
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CHAPTER 1
INTRODUCTION
1.1
Motivation
There is considerable global interest In ferroelectric thin films for use
in G variety of applications.
m icrowave
engineering
Examples of applications in the area of
include field-dependent varactors,
tunable
resonators, phase shifters, frequency-agile filters, variable-pow er dividers
and varlable-frequency oscillators (1-4).
being
investigated
for
non-linear
Ferroelectric materials are also
applications
such
as
harmonic
generators, pulse shaping, mixing and param etric am plification
(5).
Electronically
tunable
be
constructed
and
filters
applied
using
to
ferroelectric
interference
materials
suppression,
can
secure
communications, dynam ic channel allocation, signal Jamming and
satellite
and
ground-based
applicGtions include
com munications
Other
nonvolatile memories with high-speed access and
unlimited endurance (6), as weil as
sensors (7).
switching.
ferroelectric-sem iconductor UV
The interest in ferroelectrics arises because a ferroelectric
material exhibits spontaneous polarization. A ferroelectric crystal consists
of positive and negative ions.
For a certain tem perature range, the
1
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positive and negative ions becom e displaced from their cubic unit cel!
position,
This results in a net dipole moment.
The orientation can be
reversed with the appilcation of an electric field.
Entry into the
spontaneous polarization tem perature range requires the ferroelectric
crystals to undergo a phase transition, which involves structural changes.
The two phases, with and without o spontaneous dipole m om e nt ore
referred to as the ferroelectric and paraelectric respectively (8). As the
tem perature decreases from above the Curie temperature, Tc, a structural
phase change occurs and the crystal changes from paraelectric to
ferroelectric. Since the potential energy in the ferroelectric phase of the
two polarization orientations has the same energetic minimum, both
polarizations are equally favored.
Ferroelectrics have a high relative
dielectric constant and can be used In dynam ic random access
memories (DRAM). Its ca p a city for being polarized in opposite directions
makes it a
can dida te
for non-volatile
random
access
memories.
Ferroelectric thin films are also frequency agile due to the nonlinear d c
electric-field depe nd en ce of their relative dielectric constant.
This
characteristic can be used for frequency and phase agile m icrowave
circuits. Two popular ferroelectric thin films for frequency agile circuits are
strontium tltanate (SrTiOs) and barium strontium titanate (BaSrTiOs).
The
current trend In ferroelectric device fabrication is to miniaturize
the
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device as much as possible. This leads to the study of particle size effects
in ferroelectric material used for these devices.
One aspect of studying particle size effects in ferroelectrics, is to
determine the ultimate level to which a device fabricated from these
materials
can
be
miniaturized
without
characteristics of the material (9-12),
degrading
the
desirable
The experimental study of size
effects in ferroelectrics has gained momentum because of the advances
in synthesis and material characterization techniques (13-20),
advances
provide
the
means
for
the
study
of
size
These
effects
in
nanostructured ferroelectrics.
1.2
Research Overview
The main reseorch goal was to fabricate low m icrowave loss
nanostructured BSTO thin-films on crystalline substrates such as LaAlOa
and determine the effect the resulting nanostructure had on material
characteristics and device parameters. The objective of this research was
to consider the following questions:
1. Can the nanostructure of the BSTO thin film be varied by changing the
basic deposition parameter, the am bient gas pressure? Oxygen partial
pressure was used as the only variable In this study. The beam energy
density, poise repetition rote, and the substrate temperature were all kept
constant. In-situ process control was used in this research. Our approach
was to monitor the Ba653nm emission signal, and keep the tim e of flight
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(TOF) as a set-point.
A PC based real-time process control system
autom aticaily monitored the TOF setpoint every 3 ms, and adjusted the
other process parameters to maintain the TOF the same for each oxygen
2. Is there a correlation between the film microstructure and microwove
properties? The microsfructure of the BST thin-films was studied for each of
the am bient gas pressure.
Our approach here was to deposit films for
doubling oxygen partial pressures. Samples were obtained for 19 m l, 38
mT, 76 m l, 150 m l, 300 m l, 600 mT etc. The microsfructure of the samples
were studied using Atom ic Force M icroscopy (AFM), Scanning Electron
Mlcroscopy(SEM),
measurements
and
were
X-ray
performed
diiraction(XRD).
using
coplanar
The
m icrowave
waveguide
(CPW)
transmission line and capacitor test structures.
1.3
Ferroelectric Material
A dielectric/ferroelectric material is one th a t is electrically insulating
(nonmetallic) and exhibits, or can be m ade to exhibit an electric dipole
structure. There is a separation of positive and negative ions a t either a
molecular or atom ic level. Dielectric polarization can be visualized as the
action of dipole chains, which form under the influence of an applied
field. The countercharges are bound by their free ends when term inated
on metal surfaces (21-25).
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4-
+
+
V I/
V I/
r v
Bound Charge
Dipole
Free Charge
Figure 1. Dipole chains for a ferroelectric material.
Some polar materials, called pyroelectric, display an electric dipole
m om ent even in the absence of an external electric field.
The
poiarizatlon associated with a spontaneously form ed dipole m om ent is
called spontaneous polarization, Ps.
Piezoelectric materials can be
polarized by the application of an electric field as weil as by the addition
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of a m echanical stress. Ferroeiectrlc materials are both pyroelectric and
piezoelectric (8),
Ferroelectric materials are polar and possess a t least two equilibrium
orientations of the spontaneous polarization vector In the absence of an
applied electric field.
The spontaneous polarization vector may be
switched between these orientations by an applied electric field.
Most
ferroelectrics
high
undergo
a
structural
phase
transition
from
a
tem perature non-ferroelectric (called paraelectric) phase to a lowtem perature ferroelectric phase.
tem perature of the phase transition,
Tc, the Curie temperature, is the
A t temperatures above To the
dieiectric permittivity falls off with tem perature and follows the Curie-Weiss
law (8,26), For the ferroelectrics (example, BoTlOa) that undergo several
phase transitions into successive ferroelectric phases, only the first
ferroelectric phase is called the Curie point.
transition causes anomalies in the dielectric,
properties.
The ferroelectric phase
elastic, and
thermal
The transition is also acco m pa nied with changes in the
dimensions of the crystal unit cell. There is a spontaneous strain, Xs, th a t Is
associated with the phase transition. It is responsible for the difference in
the dimensions of the ferroelectric and paraelectric unit cells (27,28). A
change that can occu r in a ferroelectric material is the transformation
from a paraelectric cubic Into a ferroelectric tetragonal phase.
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ct
Ps
Ps = 0
•
0
Ba
o
*
Ti
at
cubic paradediic fiBse
foicieiectricrtese
Ps
Figure 2. Ferroelectric cubic, ferroelectric tetragonal.
In the ferroelectric phase, a crystal is spontaneously strained with
Qt< Gc < Ct where at and Oc are the o-axes of the tetragonal and cubic
unit cell. Spontaneous polarization can arise along any of these axes with
equal probability when the crystal Is cooled through the ferroelectric
phase transition temperature. Polarization will develop in directions
dependent upon the electrical and m echanical boundary conditions
imposed on the material. Regions of uniformly oriented spontaneous
polarization are colled ferroelectric domains.
Regions betw een tw o
domains are called dom ain walls. Domain walls oriented differently in
response to the spontaneous polarization vector are called ferroelectric
domain walls, and those which differ in orientation to the spontaneous
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strain tensor are called ferroelastic dom ain walls. For a detailed discussion
see (8).
1
i
i
i
i
1
i
i
•
•
• m
•
P
•
• m
•
P
•
•
H
m
•
•
j
i
i
1
I
1
-P
i
I
!
i
i
Domain wall region
Figure 3. Ferroelectric dom ain regions in response to spontaneous
polarization. The dark circles indicate the dipoles, which align in the
direction of the electric field.
A consequence of the domain-wali switching in a ferroelectric
material is the ferroelectric hysteresis loop, as shown in Figure 4. A t small
values of the applied electric field, the polarization increases linearly.
In
this region the field Is not strong enough to switch domains with the
unfavorable direction of polarization.
As the field is increased, the
polarization of domains with an unfavorable direction of polarization will
start to switch In the direction of the field. Zero polarization requires
reversing the field.
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Figure 4. Hysteresis curve for a ferroelectric material under on applied
electric field,
Further increasing of the field in the negative direction will continue the
realignment of dipoles until saturation.
The field is then reduced to zero
and reversed to com plete the cycle (8), The volue of polarization a t E = 0
Is called the remnant polarlzatioa Pr. Ec is called the coercive field, where
the polarization is zero, The spontaneous polarization Ps is the intercept of
the polarization axis.
The coercive field, spontaneous and remnant
polarization and shape of the hysteresis loop m ay be a ffe cte d by the film
thickness,
the
presence
of charged
defects,
m echanical stresses,
preparation conditions, and thermal treatm ent (8,20). The slope of the P
vs E characteristics Is a direct measure of & of the ferroelectric material.
The nonlinear P-E
characteristics
results in
nonlinear electric
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field
d e pe nd en ce of the er.
When a periodicaily varying electric field E is
applied to a ferroelectric material a hysteresis loop is obtained.
This
dynam ic hysteresis Is the representative characteristic of a ferroelectric,
The area is the characteristic param eter used to scale the dynamics of
nonequilibrium first-order phase transitions In ferroelectrics.
The kinetic
nucieotion and growth of new domains can be evaluated from the
details of the hysteresis loop. The remnant polarization Pr and coercive
field Ec can be determ ined from the hysteresis loop (8,21).
1.4
Size Effects
Nanostructured materials have dimensions th a t ore measured on
the length scale in terms of the nanom eter (one billionth of a meter) and
have special properties due to quantum size and surface effects (29,30).
The motivation behind nanostructure engineering is to develop materials
with novel properties through the controlled synthesis and assembly a t
the nano scale.
com m on
No m atter w hat synthesis approach is used there Is a
premise for the
fabrication
of
nonostructured
materials.
Fabrication of nanostructured materials allows the control and potential
enhancem ent of the properties of the m aterial for example, sr, dielectric
loss (tan§), and Curie temperature. To. Nanostructure engineering allows
miniaturization of devices to a criticai size, below which, the material loses
its desired characteristics.
10
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In this research BSTO thin fiims were synthesized with PLD.
material properties were studied for different synthesis conditions.
The
The
overall objective was to obtain a reproducible process for low loss tunable
BSTO films for microwave applications.
1.5
Synthesis
There are a variety of approaches that can be used In the synthesis
of nanostructures. One approach uses ad van ced chem ical techniques
to mix precursor materials.
Processing then allows growth of the material
for the desired nanostructure, an exam ple is sol-gel (31-37).
A flow diagram for the sol-gel fabrication of (Ba,Sr)Ti03 films is given
in Figure 5. Precursor solutions are prepared in a nitrogen dry box a t room
temperature.
Barium diethoxide (Ba(OC 2 H5) 2) and strontium diethoxide
(Sr(OC2 H5) 2)
is
dissolved
in
dehydrated
2-methoxyethanoi
(CH3OCH2CH2OH) and stirred. Titanium tetra-l-propoxide (Ti(OCH(CH3) 2)4 )
will be a d d e d to the barium and strontium solutions (31-38). Crystallization
is achieved with heat treatment.
The reaction represents the synthesis for t > 0° C
Ba(0H)2 ‘ H2O -h Ti(OR)4 ^ BaTiOs
Sr(OH)2 * H2O + Ti(OR)4 ^
SrTiOs
11
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—
1 • Ji £
■'
S«r
£
Heat treat
Multiple depositions
Heat treat
Figure 5. Flow diagram for the fabrication of sol-ge! films.
12
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In another approach, for exam ple Pulsed Laser Deposition (PLD),
the nanostructured materia! Is deposited on a substrate by the ablation of
material from a target with a laser pulse. PLD was chosen as the synthesis
process in this research because it Is recognized as a versatile and
powerful technique for the fabrication of nanostructures (39-45).
The
principle of PLD utilizes an intense laser pulse passing through the optica!
window of a vacuum cham ber, The laser beam is directed onto a solid or
liquid target surface where it is partially absorbed and partially reflected.
Above a threshold power density, target material Is ablated and forms a
highly energetic and directed
plume.
The power density needed to
produce the plume depends on the targ et material and its m orphology
as well as the laser pulse wavelength and duration. Film growth occurs o t
the substrate where energetic adatom s from plume com ponents can
reproduce an epitaxial film with the same stoichiometry as the target (4547).
A reactive gas or Ion source m ay be a d d e d to aid the growth
process. The PLD plume can be a mixture of macro-particles, m olecular
species, atom ic species and ionic species.
Pulsed loser deposition Is
widely used In the deposition of oxide thin films because of the
demonstrated deposition of high quality thin films and heterostructures
(48-69). The fabrication of thin films by PLD depends on several deposition
parameters a variety of deposition conditions, for example: deposition
rote, substrate temperature, am bient gases, torget-substrate geom etry, all
13
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of these influence film nucleatlon and growth mechanisms that were
essential to the growth of nanostructures for this research.
1.6
Material
Ferroelectric thin films were chosen as the material for the research
project because they are frequency agile due to the nonlinear do
electric-field dependence of their relative dielectric constant.
This
characteristic can be used for frequency and phase agile m icrowave
circuits (70-75).
Controiling the grain/particle size and the dom ain state of a
ferroelectric material is useful in obtaining desired m icrowave properties of
the materia! (26,29,62-67, 76-83). The desired result In this research was to
Investigate materia! characteristics for use in frequency and phase agile
m icrowave devices and also gain an understanding of the physics
involved in fabricating
nanostructured
materials.
Barium strontium
titanate was chosen as the ferroelectric material in this study because of
the research already con du cted on the material for m icrowave devices,
which gives a basis for comparison.
Bai-xSrxTiOs (BSTO) thin films have a
high dielectric permittivity th a t can be tuned by a bias electric field
(variable tunabllity with an electric field). Figure 6 illustrates the change in
dielectric constant and loss tangent as a function of electric field bias for
another ferroelectric material strontium titanate (STO).
14
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2000
FVK
T ~ ~ T "—
r
Pt elect,. q=10 nm, W=10 am. L=1 mm
500 nm STO
T = 77 K
d
cd
m
+ -
tanfi
f =
100 kHz
0.04
fi
1500
0
u
o
•fH
U
o 1000
0
0
«eH
Q
0
> 500
0.05
d
0.03 So
//
//
-
d
cd
H
\
■
0.02 m
"'I;,
0
J
t ^
0
0
K
A
/
I— I
V\+
0.01
\
.A*--
^ '
-2
I___I
j
0
1
I
I
4* •
i
2
0.00
Electric Field (V//xm)
Figure 6. Relative dielectric constant and loss tangent as a function of
electric field bias for STO.
Table 2, Appendix A, is provided os a summary of important works
on BSTO bulk as well as thin-films. In particular, the Bao.6Sro,4TI03 formula for
the BSTO was selected as the composition for the research since it
provides the most voltage tunabllity a t room tem perature.
The BSTO
(60/40) thin-film ferroelectric, with a Tc of 290K, is a promising ferroelectric
for room tem perature tunable devices.
15
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1.7 Material/Device Characterization
Characterization of the BSTO thin films was performed using XRD,
SEM and AFM,
Microwave characterization was done using coplanar
w aveguide lines to characterize the frequency dependent dielectric
properties
(er
and tan§). Electrostatic force microscopy (EFM) was used to
characterize the surface potential of the films to gain insight into the
dipole kinetics of the material fabricated and the e ffe ct of changing the
nanostructure of the material had on dipole switching,
1.8 Outline of Chapters
Chapter tw o presents a survey of related in ferroelectric thin films
and nanostructures.
Chapter four outlines the experimental setup used
and results obtained from fabrication and characterization of the films. In
Chapter five the results are analyzed and the goals of the proposal are
validated.
16
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CHAPTER 2
LITERATURE SURVEY
2.1
Ferroelectrics
The
potential
ferroelectric,
for
tuning
barium titanate
the
(BoIiOa),
m icrowave
has been
properties
of
the
dem onstrated
by
McNeal, Jang, and Newnham (44). Through the control of grain/particle
size and dom ain state, relaxation frequencies were shown to increase and
loss
tangents
decreased
with
decreasing
graln/particle
size,
A
polycrystalline ceram ic ferroelectric, BaTiOs , having grain sizes of 14.4,
2.14, and 0.26 pm and powder-polym er matrix composites of sizes 1.33
pm, 0.19 pm, and -66 nm, respectively, was used. Through the control of
grain/partlcle size and dom ain state, relaxation frequencies were shown
to increase and loss tangents decreased with decreasing grain/partlcle
size.
Zhang,
ferroelectric
Zhong,
solid
and
solution
Wang
studied
BoxSri-xIiOs
finlte-size
(45).
Their
effects
samples
in the
were
characterized and analyzed by SEM, JEM, and X-ray diffraction. The Curie
temperature decreased with a decrease of grain size.
The Curie peak
becam e broader and eventually disoppeared. Three ferroelectric critical
17
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grain sizes of 317 nm, 246 nm and 176 nm for x=0,3, 0.5, and 0.7
respectively, were evaluated ,
Jiang and Bursill have proposed a new phenom enological model to
study size effects on the first-order phase transition of lead titanate
ferroelectric particles.
The proposed model would try to explain the
experimental results that show the Curie temperature, tetragonality (c /d )
ratio, latent heat, and soft-mode frequency all decrease with decreasing
particle size for PbliOa, BoIiOs and PbZrOs (46).
Wang, Xin, X. Wang, and Zhong report on the size effects on
spherical ferroelectric particles described by the transverse Ising model
taking into consideration the long-range Interactions (47).
In this report
the authors studied the size depe nd en ce of the mean polarization, the
Curie temperature, the m ean susceptibility of the particle, and the critical
size of the particle. The Hamiltonian of the transverse Ising model has the
form:
H = -QI(S/> - V2lJij(SiXSjy -2^EE(Si f
Q is the transverse field, (Si y and
operator a t site /.
(1)
(Sj y
ore com ponents of a spin-t^
Jij is the exchange strength betw een sites / and j. /j is
the effective dipole m om ent of site / and E Is the externa! electric field.
The authors summarize their results: with a decreasing particle size, the
mean polarization and Curie tem perature of the particle decrease to zero
o t a criticQi size of the particle. Here there is size-driven phase transition
18
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from the parelectric phase to the ferroelectric phase.
Long-range
interactions are one of the dom inant contributlors to ferroelectricity.
Jun,
Kim,
and
Lee demonstrated
strain-induced
properties of BSTO thin films fabricated by PLD (48),
ferroelectric
Metai-ferroelectric-
insulator-semlconductor field-effect transistor (MFIS-FET) were fabricated.
The laser source used was a KrF gas laser.
During the deposition laser
energy density, 2,3 J/cm2 ,was m aintained while substrate tem perature
was varied from 600 to 800°C. As the tem perature increased, BSTO films
becam e well-oriented above 750°C while BSTO films grown at 600°C were
randomly oriented polycrystalline. ESI films b e cam e well-oriented when
grown above 750°C,
Saha and Krupanidhi demonstrated the microstructure related
influence on the electrical properties of PLD fabricated BSTO thin films (50),
The films were grown using PLD in the tem perature range of 300°C to
600°C inducing changes in grain size, structure, and morphology.
grown
at
300°C
were
polycrystalline
In nature
with
Films
multigrained
microstructure. Films grown a t 600°C were oriented along the (100) crystal
plane and densely packed. Table 1 Appendix A lists the im portant works
in BSTO thin-films for m icrowave applications.
Table 2 Appendix A lists
important works in the synthesis ob BSTO thin films using PLD and Sol-gel.
19
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2.2
Nano-Phase Ferroelectrics
Ferroelectric
materials ore
typically
multigrained
as a
large
depolarization field (due to buildup of surface charges) forces a single
crystal to divide into several domains.
At m icrowave and optical
frequencies, these multigrained materials exhibit a
relaxation characterized
by a
decreasing er and
large dielectric
increasing tan§.
Mechanisms attributed to dielectric relaxation include piezoelectric
resonance of grains and domains, dom ain wall vibration and shear waves
from dom ain walls contributing to high frequency dispersion of & (er' -j er")
and high tanS ( ! & " / er'|) (82-84).
The dielectric relaxation, which Is
intrinsically dependent upon dom ain state. Is in port de pe nd en t upon the
microstructure.
Previous studies in size effects have shown that: (i)
Relaxation frequencies increased and tan§ decreased with decreasing
grain/particle size (84), (I!) Tc decreases with decreasing grain/particle size
(35),
(iii)
The peak value of £r decreases (84), (iv) Lattice constants
change such that the unit cel! becom es more symmetric (84), (v) There
exists a critical dom ain size below which the ferroelectric behavior
vanishes (64),
(vi)
Single dom ain ferroelectrics can be obtained for a
groin size below 200nm (84), (vii) Domain switching Is quicker in smaller
domains com pared to larger domains, but requires higher electric
fields(84),
20
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CHAPTER 3
SYNTHESIS AND CHARACTERIZATION TECHNIQUES
3.1 Synthesis
The PLD system, seen in Figure 8, was used to deposit BSTO films on
LAO and
BSTO | Pt | TI02/TI layer | Si O 2 1Si substrates. The system is located
a t the Materials Directorate, Air Force Research Laboratory, WrlghtPatterson Air Force Base, Ohio.
The system consists of a plume monitoring and control system which
uses G Lambda Physik 305i excimer laser (KrF, X = 248 nm) with a 17 ns
pulse, a variable-focus optical train, a beam scanning mirror, a Neocera
vacuum cham ber with m ulti-target holder and substrate heater, see
Figure 7. Two optical emission sensor systems are a tta ch e d to either side
of the vacuum cham ber to enable simultaneous monitoring of the same
or different sections of the plume. The first maintains a fixed !lne-of-sight of
a ~1 cm w ide vertical slice of the passing plume a t a manually set 38 mm
from the target. The second system is similar but can be autom atically
positioned anywhere along the plum e centerline from 11 mm aw ay from
the target all the w ay to the substrate heater.
21
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."
oiipowi noN
iiilB M i®
Htti rf-R’WffH
i
3
I
'•
■s®m
■ liiiW iiiP lil
'■ -W
,''.V9 ijtSfflBrTfss
jlH B ^ ^ B liiS ilJ i^
’■.- . ri
;•
Figure 7.
A utom ated in-situ, real-time, process-control pulsed-laserdeposition system. Real-time control based on fe e d b a ck from emission
(ES) sensors. Laser energy, cham ber ambient, an d /o r beam footprint can
be adjusted to maintain a TOF set-point based on ES feedback, (Courtesy
of AFRL/MLP, Wright-Patterson AFB).
The two optical sensors in each system consist of photomultiplier tubes
with inline narrow bandpass filters and slit arrangements to limit the
viewed section of the plume.
plume
emissions
components.
to
The filters are used to restrict m onitored
wavelengths
associated
with
critical
plum e
The monitored emissions generate a time-of-flight (TOF)
22
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versus intensity signal.
The time difference between the start o f the
ablation process (the fireball) ond the maximum intensity from the passing
emitting plume com ponent (main plume), the TOF set point, is sent to the
com puter. This passed TOF provides the fe e db ack for the com puter to
actu ate from one to three deposition control variables (laser excitation
voltage, cham ber am bient pressure, and or beam footprint size, process
control variables). Regulation of these deposition parameters once every
tw o seconds to maintain the TOF setpoint, provides the in-situ real-time
process controi. Also, linked through the com puter is the control of the
scanning mirror, laser repetition rate, and substrate tem perature and
recording of 27 deposition related variables, visible plume images and
control variable settings (64,65,67).
Figure 8 shows the com plex interaction of process variables in a PLD
system th a t must be controlled and monitored to ensure quality and
repeatability In thin film fabrication as well as the ability to m anipulate the
nanostructure of the film.
23
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H ' a i i EB
I
ftacesi C«ntr®l'l
O ' ViSsi&itirfnjjusiailfyAej'^rS:?^
«=v
«
, , «
'
.
S k £ ^ & i^ W 4
^
".s^^
t
'
c
STibsti^:s
Mcr/frfe
',;■' :xT
''■*5,—
Figure 8. Hierarchial process m odel Identifying process control variables
and their com plex interactions in a puised laser deposition system.
3.2 AFM/SPM Characterization
C ontact m ode atom ic force m icroscope images of the BSTO thin
films deposited onto LAO and heterostructure substrates with the PLD
system described above were obtained using a Digital Instruments
MultiMode SPM with a NanoScope lllo controller in air a t am bient
temperatures (located a t AFRL/MLP, WrIght-Patterson, AFB, OH).
Scan
sizes for the images range from 500 x 500 nm to 167 x 167 |jm.
Scan
speeds for the images were betw een 0.5 and 3 Hz. images were obtained
from multiple locations on each BSTO thin film sample. Ail analyses of the
nanostructure dimensions and surface roughness measurements were
24
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perform ed on flattened images. Multiple 500 x 500 nm areas from several
1 X 1 |im images were used to determine nanostructure widths and heights
for each thin film (85-87),
A V eeco NanoProbe was used with the
following specifications:
Cantilever: C
1.3-3.0 N/m (spring constant)
Fo
65-85 Hz ( resonant frequency)
L
222 pm (cantilever length)
Probe:
Material Silicon
Coating
resistivity: 0.01-0.025 ohm-cm
Ptlrs
Diameter 40 nm
3,3 Surface Potenflal Imaging
Surface potential Imaging measures the effective surface voltage
of the sample by adjusting the voltage on the tip, (using AFM described in
Section 3.2), so that it feels a minimum electric force from the sample.
Samples for surface potential measurements need to have a surface
voltage betw een -10 and -i-lO volts.
Samples m ay include both
conducting and nonconducting areas.
Surface potential detection is
acquired in a two-pass procedure where the surface topography is
obtained by the standard TappingM ode In the first pass and the surface
potential is acquired on the second pass.
then displayed,
The tw o measurements are
in the first pass, the cantilever Is m echanically vibrated
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
close to its resonant frequency (using a piezoelectric element).
At the
second pass, the tapping m ode is disabled and an oscillating voltage is
applied to the probe tip. When there Is a DC voltage difference between
the tip and sample an oscillating electric force will develop on the
cantilever a t resonant frequency.
The cantilever vibrates and an
am plitude can be detected. This technique exploits the most im portant
characteristic of ferroelectric materials, the reversal of polarization by an
electric field,
3.4 M icrowave Characterization
Determining the dielectric properties of the ferroelectric thin-films a t
m icrowave frequencies is critical for developing tunable com ponents
such as phase shifters, and filters (. To determine the relative dielectric
constant (er) and the loss-tangent (tan§) of the ferroelectric thin-films, c o ­
planar w aveguide (CPW) transmission lines, and resonators (loaded as
well
as
unloaded)
have
been
designed
for
on-wofer
probing.
Measurements of scattering parameters on the CPW lines and resonators
In conjunction with theoretical quasi-static conform al m apping analysis,
one can determine the dielectric properties of the ferroelectric thin-films,
including the frequency de pe nd en t tanS,
The scattering parameters (S-
parameters) are useful for m icrowave frequencies as they are based on
traveling waves concept.
The S-porameters can be determ ined based
on appropriate im pedance m atching a t both the source and load ends.
26
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The S”param eter S21 is the ratio of the output power to the input power in
dB, and is also called the insertion loss of the circuit under measurement.
Sn is the ratio of the reflected power to the Input pow er in dB. The Sparameters are com plex quantities with a m agnitude and phase. When
we measure S-parameters for a CPW line, the m agnitude of S21 is an
indication of the ioss in a line, and the phase of S21 is an indication of the
eiectrica! length of the line. For more information on S-parameters, please
refer to a modern text book on m icrowave engineering (69).
O nce the ferroelectric thin-films were deposited on the substrates,
test structures were either photollthographically defined or samples were
co a te d with a silver or gold conductor by sputtering, thermal evaporation,
or, in this research, PLD through the shadow mask. The minimum thickness
of the metal layer required is approxim ately three times the skin-depth of
the metal a t the lowest frequency of measurement.
thickness of 1.5 |xm was used,
A conductor
Both silver as well as gold has been used as
conductors.
To understand the e ffe ct of inserting the ferroelectric thin-film in the
CPW structure, tunable com ponents were m odeled such as a coplanar
w oveguide (CPW) transmission line and CPW linear resonators.
section
of
the
m odified
CPW
structure,
consisting
The crossof
a
conductor/ferroelectric/dielectrlc configuration is shown in Figure 9. The
structure consists of a dielectric substrate (typically lanthanum oluminate
27
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(LAO) of 254 |im thick), a BSTO ferroelectric thin-film layer (thickness 't'
typically 300 nm), and a 2 fim
thick A u/A g thin-film for the conductor
(center line) and ground lines (adjacent parallel lines).
The critical design parameters for the tunable circuits are the
characteristic im pedance (Zo), and the effective dielectric constant (eetf)
which are both a function of the electric field dependence £rPE of the
ferroeiectric thin-film.
Im portant geom etric parameters controlling the
above parameters are the width of the center conductor (W), spacing
betw een the ground lines (S), thickness of the ferroelectric thin-film (t), and
the thickness of the substrate (h), The characteristic im pedance of the
line is inversely proportional to the ratio of W/S. The dielectric properties of
the ferroelectric thin-film, and the thickness of the ferroelectric film ore
expected to determine the overall insertion loss of the circuit. The
transmission lines were designed for characteristic im pedance of 50 Q in
the absence of the ferroelectric layer.
The m ethodology in determining er and tanS for the ferroelectric
thin-films a t m icrowave frequencies Is (I) Determine the effective dielectric
constant of the CPW structure using the phase of the S21 . £eff= (( Total
Phase of S21 in degrees)* Ao/(360*l))2 where Xo is the wavelength in free
space, and I is the length of the CPW line, (ii) Using conform al m apping
technique based on quasi-static analysis, determ ine the £r of the
ferroelectric thin-film (38). Another m ethod Is to use simulation tools such
28
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as Sonnet®f^ and plot the effective dielectric constant versus the relative
dielectric constant of the ferroelectric thin-film, for the frequency of
Interest, and for the specific film thickness(39). O nce w e obtain the sett,
we can extract the value of & from such a plot,
(iii) By measuring the
quality factor of the CPW resonator circuit (3 dB bandw idth/center
frequency), w e can obtain the tanS of the ferroelectric thin-film, as the
unloaded Q -factor will be inversely proportional to the tanS, I.e., tanS
- l / Q u ( 71 - 75 ).
Figure 9. Cross-section of the m odified ferroelectric tunable CPW
configuration, The G-C-G refers to Ground-Conductor-Ground.
29
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■
Figure 10. On-wafer Probe Station a t NASA for M icrowave Measurements.
D IE L E C T R IG E E R ffi
CPWl :•a=7- Hills; -13=315;-tnils; >
0
GPlMira-7:iBis-:bp=3,5-'im
:^:.W=^10 mils;. L^23P;.niils; ■.r >'; i
:E1=62 'nals;' s=3'.;5iib1s :;:': f f 7: t ’
GPW2:- a=7' mis; b=315; -iiiils; • ;
-IQ: Boils;:t2=S09 :i3als5:■: 7: ■
SI:
aiiis •:IA2S0; mils;:
:■:MS2; 'W^IQ-inils;' iAsOO; mils;:;
MSRl: W=10 nii!s;;S#.'5:imIs
:Ll'==#2™ls;l?=250:iMls;::"
MSI, MS2, maMSRIsfeould be:
■.apart - . ' . - . ..-..-,. .
.'From ea ch - o th e r .on- al. sides -by :aflcast
5 mm.
:-7'
Figure 11. Mask used on BSTO samples for evaluating microwave properties.
30
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CHAPTER 4
EXPERIMENTAL
4.1 Film Fabrication
Pulsed laser deposition, see the system in Figure 7, was used to
deposit -300 nm of Bao.6SrOo.4TI03 on LaAIOs substrates. Changing the
am bient gas pressure from deposition to deposition while maintaining a
constant beam energy density, 1.3 x 10® W/cm2, on the target varied the
microstructure of the film. The substrate heater tem perature was held a t
~750°C. A TOF of 4.50 ps a t a distance of 38.1 mm from the target, which in
prior BSTO depositions (64-67) gave good results, was used as a starting TOF
set-point for the following depositions at different am bient O 2 pressures. TOF
set-points for various pressures from 19 to 300 mTorr were generated by
attaining a TOF of 4,5 ps In 150mTorr O 2 and then changing the pressure
from 19 to 300 mTorr while recording the average TOF a t each pressure, and
holding a constant loser energy. Noting that 4.5 ps was recorded a t 150
mTorr as the pressures were increased from 19 mlorr, suggests the desired
energy density had been maintained.
Table 2 summarizes the TOF
associated for each pressure.
31
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No.
1
2
3
4
5
6
O2
pressure
TOF
(m T o ix }
(fis )
19
38
75
150
225
300
3.6
3.66
3.95
4.5
4.95
5.45
Table 3. TOFs recorded for each of the oxygen pressures used in deposition
of BSTO on LAO substrate.
5 ,5 -
5 .0 -
3
U_
4 .5 -
O
H
4 .0 -
3.5 -
50
—r100
-1— — r
150.
200
I
I
250
300
02 pressure (m l)
Figure 12. Plot of O 2 (mTorr) pressure versus TOF (ijs) from Table 2.
The monitored Bo (553 nm) emissions from the plum e generated a
time-of-flight (TOF) versus Intensity signal os seen in Figure 12. emissions
generate a time-of-flight (TOF) versus intensity signal as seen in Figure 12.
32
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The tim e difference between the start of the oblation process (the fireball)
and the maximum intensity from the passing emitting plume com ponent
(main plume), the TOF set point, is sent to the computer.
Inliai Firebal al tg
B a^ 553m a
\
ftein Hum®
/
ijisef lugger
Figure 13. The emission signal from the Ba* com ponent in plume
generated when the material passed the monitored position for the PLD
deposition of BSTO on LAO substrate for 150 mTorr O 2 pressure.
There is a sharp spike when the beam hits the target (fireball). This provides
a timing mark for the start of the deposition process. The feedback, TOF
set-point, is the time betw een the fireball and the maximum intensity of
the Ba* signal.
Plumes from the deposition of BSTO on LAO substrates using the
automated PLD system described above are shown In Figure 14.
33
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CKAMF-RR Oj FRESSFRES CONTROI, BS'I’O PL.l; ME,lJYN AMICS
C ’ S!iviiJE.AR P.EAM ENERGY DEEiSiTUYS
B S f 0 ~ iS 'ia T o r if ^
E iT O '^ ii OsjTos:
5ajXoi‘Sr»'BO:<m
t #>?5ri5
Figure 14. Visible wavelengths images of the PLD plume as cham ber
pressure O 2 was increased while holding a constant beam footprint energy
density for PLD deposition of BSTO on LAO substrate.
As the am bient pressure Is increased from 19 m lorr to 300 mTorr, the plume
compacts and shortens which indicates more material Is being deposited
per pulse and at lesser adatom energies.
Plume confinement aided the
developm ent of nanoclusters on the substrate os will be seen in the section
on AFM/EFM measurements. Deceleration was aided by the background
gas since this decreases the kinetic energy of the ejected species caused
34
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by collisions with the gas species, The mobility of the atoms arriving a t the
substrate surface
a t higher pressures Is decreased,
enabling
the
environment for grain growth.
The foiiowing images de pict plume intensity from target to substrate
with varying O 2 pressure in the PLD chamber.
As seen, the overail intensity
in the plume increases with pressure.
a. 19 mlorr
b. 75 m lorr
c. 300 mlorr
Figure 15. Contour plots of the intensity im age of PLD piume a t varying
cham ber pressures for deposition of BSTO on LAO substrate a t varying
cham ber pressures, a, 19 mTorr, b. 75 mlorr, c. 300 mTorr.
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BSTO Film Thickness versus Deposition Pressure
at a Constart Energy
2000 I
^
:
1500 :
tn
1000 :
u
f—
500 :
5— !— 5
i—“f
0
!
100
I
f
I
I
!
200
fT
I
|
S I
I
300T;
!
p T “T
4 00
I ""T
500
Oxygen Pressure (mlorr)
Figure 16. Film thickness versus cham ber oxygen pressure a t a constant
energy for PLD deposition of BSTO on LAO substrate.
As the plume com pacts and shortens. Figure 16 indicates the trend of
more materia! is being deposited per pulse, the same effect Is seen in XRD
characterization In the foiiowing section.
4.2 Microwave set-up
For the m icrowave measurements, the foiiowing test procedure was
followed:
Step 1: Fabricate CPWl and CPW2 on substrates with no BSTO layer,
Measure the two-port S-parameters for the
m agnitude and phase.
test structures, 821
C hange the phase to “total phase/reiatlve
phase" format.
021 =pi. 360/(2jc) degrees
36
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step 2; Determine the phase constant p (Rads/m).
Step 3: Determine the inductance per unit length (L -0.7 nH/mm for
quick estimation).
Plot C vs frequency based on p=coV(LC).
Step4: From 811, obtain the characteristic im pedance the line.
Step 5: From the m agnitude of 821, determ ine a (Np/m), and hence R.
821 (dB)=8.686 al;
a = R/(2Zc)+ G.Zc/2; G -0 for a perfect dielectric substratae
Now w e hove the reference sample.
Step 6: For the test sample with BST, fabricate three test structures,
CPW l, CPW2, and the resonator.
Ytest/yref = V((Gt+j(oCt)/Oo)Cr)) eqn.A
From the resonator measurements, w e can obtain tan8 for the BST films;
Qu=QL(l +k), where k=S21 (fO )/(l-821 (fO))
Tan6 = 1/Qu;
Step 6; For the test sample with BST, fabricate three test structures,
CPW l, CPW2, and the resonator,
ytest/yref = V((Gt+j®Ct)/(j©Cr)) eqn.A
From the resonator measurements, w e can obtain tanS for the BST films;
Qu=QL(l +k), where k=S21 (fO)/(l -821 (fO))
tan§ = 1/Qu;
Step 7: For the test structures, CPWl, CPW2, measure S parameters.
37
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
O btain a t pt vs frequency os before
Substitute Gt = co.Ct.tan8 in eqn A, and solve for Ct.
Ct=Cfilm+Cref;
Cfilm
«
eO(efilm-esub).
2t/s
conforma!
m apping
technique for the cose of a thin BSTO film, thickness t<0.01 s. (70-74).
The m icrowave measurements, performed between 9-18 GHz,
suggest the nano-structured BSTO thin-films on LaAIOs (LAO) substrates,
deposited a t 38 mTorr oxygen pressure, are highly tunable (up to 25%).
These m icrowave measurements, were also performed
a t Chalmers
University of Technology, were perform ed in the following manner. Test
structures consisting of coplanar capacitors were fabricated on the BST
thin-films using gold electrodes. Test structures with the g a p size (between
the center conductor and the ground line) varying from 5 jum to 50 fxm
were fabricated and tested. The low frequency c a p a c lto n c e and the
loss tangent values were measured a t room tem perature as a function of
applied electric field a t 1.0 MHz using HP 4285 LRC-meter. For high
frequency measurement the devices were co n ta cte d using 200 pm pitch
size mlcroprobes and m icrowave reflection measurements (Sn) were
performed using a HP8510C vector network analyzer.
The DC bias is
applied to the capacitors via network analyzer's Interna! bias tees.
Al!
measurements were carried out a t room tem perature in a frequency
range of 45 MHz to 50 GHz. Ferroelectric's hysteresis measurements were
performed using TF2000 polarization analyzer a t a frequency of 1 kHz a t
38
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
room temperature, Measurements a t NASA Glenn were performed using
an 851OC vector network analyzer with a CPW on-wafer probing station.
High voltage bias tees were used to apply voltages up to 400V.
Measurements were performed with samples under vacuum due to the
high voltages applied.
The ca p a cita n ce and the loss tangent values were measured a t
room tem perature as a function of applied electric field a t 1.0 MHz using
HP 4285 LRC-meter. For high frequency measurement the devices were
co n ta cte d using 200 pm pitch size microprobes and m icrowave reflection
measurements (Sn) are performed using a HP8510C vector network
analyzer. The DC bias was applied to the capacitors via network
analyzer's internal bias tees. All measurements were carried out a t room
tem perature in a frequency range of 45 MHz to 50 GHz. Ferroelectric's
hysteresis measurements were
performed
using TF2000 polarization
analyzer a t a frequency of 1 kHz a t room temperature. Measurements a t
NASA Glenn were performed using an 851 DC vector network analyzer with
CPW on-wafer probing station. High voltage bias tees were used to apply
voltages up to 400V, Measurements were perform ed with samples under
vacuum due to the high voltages applied.
39
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CHAPTER 5
CHARACTERIZATION
6.1 XRD Characterization
X-Ray diffraction (XRD) is a primary tool for finding crystal structure
type and lattice parameters.
Diffraction occurs when wavelength
encounters regular spaced obstacles, which can scatter the Incoming
electrom agnetic
w ave
and
have
spacing
com parable
to
the
wavelength. The diffraction angle depends on the wavelength of the xroys and the distance betw een a d ja ce n t planes.
The series of XRD patterns of
Figures 22, 23, and 23 show the
change in the intensities of BSTO films' (200) peaks, for films deposited at
19 mTorr, 38 mTorr, and 225 mTorr.
40
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19 mTorr
2 orooo
LAO
(200)
BST
(200)
150000
100000 -
50000■
BST
(100 )
~T"
“T "
10
20
LAO
(100)
30
40
50
60
2-Theta (deg)
Figure 17. X-ray diffraction pattern for a BSTO film deposited on LAO a t 19
mTorr Oxygen pressure, radiation=20(CuKa), Lam bda=l .5478.
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38 m Torr
200000
^
BST
LAO
(200 )
(200)
m
c
3
o
O
w
©
150000'
1000Q0■
LAO
(100)
50000 -
i
10
20
30
, 40
50
60
2-Theta (deg)
Figure 18. X-ray diffraction pattern for a BSTO film deposited on LAO a t 38
m lorr Oxygen pressure, radiation=20(CuKa), Lam bda=l .6478.
42
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50000■
«
c
3
O
^
225 mTorr
40000■
BST
30000-
Lao
(200)
(200)
Cfl
c
<D
c
20000■
10000
BST
LAO
(100)
(100)
u “j—
~T ~
10
30
20
,
,
50
40
60
2-Theta' (deg)
Figure 19. X-ray diffraction pattern for a BSTO film deposited on LAO at
225 m lorr oxygen pressure, radiation=20(CuKa), Lam bda=1.5478,
Figures 17, 18 and 19. display X-ray diffraction patterns for the BSTO films
deposited
on
LAO
radiatlon=20(CuKa),
in
19,
38,
and
225
mTorr
of
O2
pressure,
The XRD spectra show th a t as the substrate
coverage increases, the intensity of the (200) reflection for BSTO and the
intensity for the (200) reflection of LAO reverse in relative magnitudes. The
(200) reflection for BSTO is now greater in m agnitude than the (200)
reflection for LAO perhaps Indicating a better surface coverage of the
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
substrate. The XRD spectra also show very distinct crystallinity.
Further, the
velocity of the plume components arriving a t the substrate also decreases
as pressure Increases (for Bo*, from -7.4 km/s at 19 m lorr to -3.4 km/s at
225 mlorr), This is about a factor of 4.6 reduction in arrival energy.
5.2 SEM Characterization
The scanning electron microscopy was perform ed on a Hitachi
S5200 ultra-high resolution scanning electron microscope. All samples
were fractured and mounted in cross-section. The m ounted samples were
co a te d with 2 nm W (tungsten) to help alleviate charging artifacts.
BSTO
illl
iK \
LAO stibsfxate
Figure' 20 a. SEM im age of PLD deposited BSTO on LAO substrate a t 300
mTorr oxygen pressure.
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 20 b. AFM image of PLD
deposition of BSTO on LAO at 300 m lorr oxygen pressure.
o
o
r'^'^idM lrC 9\kJu
■wja
sUA*f“ 4 v
^
*
■*
f
W-jk'*Y,*viV^-
iiL
.;sv rT%->
"^ r
i
-
-* >A
, * i t “ -*“S>3!5iSr'><^ «H A P ^ » *1 !
- ‘^;V
v -'■' M
^ t
' V a'*^v
V * V ^'. /
wffiaWW
Figure 20 c, SEM image of PLD deposition of BSTO on LAO substrate at 300
mTorr oxygen pressure.
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
,V‘
Similar columnar growth shown above was seen in all depositions at
all O 2 pressures considered, this growth pattern is also seen in AFM two
dimensional and three dimensional topography images presented later.
The Aims varied in thickness from close to 200 nm a t 19 m lorr to a b ou t 350
nm a t 300 mlorr.
The films exhibited colum nar growth with about the
same diam eter a t the bottom and the top. As O 2 pressure was doubled in
steps from 38 mTorr to 300 mTorr, the deposited films generally becom e
thicker and rougher. The average colum nar grain size ranged from 35 to
140 nm in the BSTO films.
5.3 AFM Characterization
The frictional AFM images in Figure 26 show the particle/groin size
Increasing with higher O 2 am bient pressures from 38 mTorr to 150 mTorr.
Above 150 mTorr, clustering or coalescing of the groins was observed
starting a t 225 mTorr.
Below 38 mTorr the trend is not clear.
The
preliminary d a ta fits a power equation, d = p-wow ^ where d is the diam eter
of a particle and P Is oxygen pressure for points below 225 mTorr and a t
and above 38 mTorr, see Figure 27 for the d a ta plot.
46
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a.
b. 75 mTorr
38 mTorr
v 't l
Ysp;r\k ’kS:s!^3
iOO iim
100 mil.
d. 225 m Torr
c. 150 m Torr
100 lira
100 n m
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
e. 300 mTorr
iiiu nm
Figure 21. AFM 3-dimensional images (deposition pressure, average
particle height and surface RMS roughness) for PLD deposition of BSTO on
LAO substrate: a. 38 mTorr, 1.5 nm, & 11 nm; b. 76 mTorr, 2.4 nm, & 20nm;
c. 150 mTorr, 3.8 nm, & 5,1 nm; and d. 225 mTorr, 4.5 nm, & 8,7 nm, e. 300
mTorr.
150-,
140-
110 £
100-
02PressumjtmTorr)
Figure 22, Plot of O 2 pressure versus particle diameter, d =
where d is
particle diameter, P is O 2 pressure for BSTO deposited on LAO substrate.
48
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38
36
1.5
n
75
55.4
2.4
20
150
137
3.8
5.1
4 .5
8.7
Not
225
applicable
Figure 23. Particle height (nm) and Surface Roughness (nm) for the AFM
images in Figure 22.
Seen from the AFM Images, there is definitely a com piex grain
structure appearing a t 150 mTorr O 2 pressure and above.
Surface
roughness and particle height increase from 38 m lorr O 2 pressure to
beyond 225 m Torr O 2 deposition pressures. A t the lower pressures 38 and
75 m lorr the grains are more unlformiy distributed and more uniform in
nature.
At lower oxygen pressures, the film growth favors smaller grain
size, which may result in almost single dom ain ferroelectric film.
1
,,,
a. Grain size average < 90 nm.
49
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■
. 250 nm
c. Complex grain structure.
b. Grain size
< 200 nm.
Figure 24 . AFM image of BSTO film deposited on LAO substrate a t a. 75
m lorr oxygen pressure, b. 150 m lorr oxygen pressure, c, 300 m lorr oxygen
pressure,
5,4 EFM (Surface Potential) Characterization
Surface potential imaging measures the effective surface voltage
of the sample by adjusting the voltage on the tip so th a t it feels a
minimum electric force from the sample.
In the figures that follow light
regions correspond to positive domains (polarization is tow ard the bottom
electrode) while dark regions correspond to negative domains with the
polarization vector oriented upward. The schem atic below illustrates the
effects of poling a ferroelectric material (88).
Surtace charge
Sample
Dipoles
Substrate
50
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© 0
Surface
Sanple
Substrate
Figure 25. Schematic of poling ferroelectric thin films by an AFM
conductive tip. A positive bios applied to the tip with respect to the
substrate aligns dipoles downward. A negative bios applied to the tip
with respect to the substrate aligns dipoles upward.
LAO and platinized high resistivity Si substrates with the deposited
BSTO thin film were used and poled for characterization of the surface
potential, which provides information a b ou t the dipole kinetics of
ferroelectric materials. This leads to the link betw een the dom ain structure
of the material and device performance.
The first substrate was LAO,
already mentioned, and the second substrate was BST0/Pt/Ti02/Ti layer/SI
02
/Slllcon, see Figure 26.
51
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BSTO
Pt electrcKie
Ti02/Ti iaver
Si02
Silicon
I
Figure 26. Platinized high resistivity Si substrate for MMiC applications.
Interest in the LAO substrate comes from m icrowave applications.
The LAO offers the advantage of good lattice m atching with the
perovskites.
The high resistivity Si substrate is attractive for monolithic
m icrowave integrated circuits(MMiC). Integration of the ferroelectric thinfilms on high resistivity Si could yield new applications for frequency and
phase agile circuits (88
The samples were poled using an X configuration.
The images
below present surface potentials poled a t +5, -5, +10, -10 volts for thin films
of BSTO on LAO and the high resistivity SI substrate.
52
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Figure 27 a. Surface potential of BSTO on platinized high resistivity SI a t 38
m lorr O 2 pressure, 5 V probe bios.
Figure 27 b. Surface potential of BSTO on platinized high resistivity 81 a t 38
mTorr O2 pressure, -5 V probe bias.
In Figure 32 a and 32 b the switching characteristics for the BSTO film
on the heterostructure show many domains that can no t be switched by
+5 volts bias even a t the smaller particle size. A 3-D view is seen in Figure
27c. of surface potential for the BSTO film deposited a t 38 mTorr oxygen
pressure.
53
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Figure 27 c. 3-D surface potentioi for BSTO on platinized high resistivity SI a t
38 m lorr O 2 pressure, -5 V probe bias,____________
Figure 28. 3-D AFM im age o f PLD deposition of BSTO on platinized high
resistivity SI a t 150 mTorr oxygen pressure.
54
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.
a I.
L
Figure 29. 3-D AFM im age of PLD deposition of BSTO on platinized high
resistivity SI a t 300 m lorr oxygen pressure,
Figure 27, Figure 28, and Figure 29 show the same trend indicated in
BSTO Aims deposited on LAO substrates, grain slze,complex grain structure,
and increasing surface roughness a t higher oxygen pressures. The film
grain structure even a t lower deposition pressures does not possess the
switching quality of BSTO films on the LAO substrate. This is probably due
to poor lattice m atch of the BSTO film with the multi-layered substrate (this
is on area of research that could be initiated in future studies). The e ffe ct
of larger, com plex grains on switching is seen in Figure 30, the white
particles seen in the legs of the poled X indicate grains th a t could not be
switched with -5 voits bias on the EFM probe.
In contrast Figure 31a, Figure 31b, Figure 32a, and Figure 32b show
the excellent switching properties of BSTO films on
LAO substrate
produced a t 38 mTorr and 75 mTorr oxygen pressures.
55
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Figure 30. Surface potential Image of PLD deposition o f BSTO on platinized
high resistivity Si a t 600 m lorr oxygen pressure.
Figure 31 a. Surface potential PLD deposited BSTO on LAO substrate a t 38
mTorr O2 pressure, 5 V probe bias.
56
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Figure 31 b. Surface potential BSTO on LAO, 38 m lorr O 2 pressure, -5 V
)robe bias,
Figure 32 a. 3-D surface potential a t 75 m lorr O 2 pressure of PLD
deposited BSTO on LAO substrate, -5V probe bias.
57
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Figure 32 b. 3-D surface potential a t 75 mTorr O 2 pressure of PLD
deposited BSTO on LAO substrate, 5V probe bias.
Figure 31 a. Figure 31 b. Figure 32 a, and Figure 32 b show that the
com bination of AFM/EFM techniques and high quality ferroelectric thin
films allows manipulation of ferroelectric dom ain as smali as 36 nm
(diameter) with a high degree of precision.
58
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Figure 33 a. Surface potential of PLD deposited BSTO on LAO substrate at
300 m lorr O 2 pressure, -5V probe bias._____
Figure 33 b. Surface potential of PLD deposited BSTO on LAO substrate a t
300 mTorr O 2 pressure, 5V probe bias.
Figure 33 c. AFM of PLD deposited BSTO on LAO
substrate a t 300 mTorr O 2 pressure.
59
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Figure 34. AFM Image of PLD deposition of BSTO on LAO substrate a t 1 Torr
O 2 pressure.
The BSTO film deposited on the LAO substrate and the platinized
high resistivity Si substrate a t 300 and 600 mTorr oxygen pressure samples
has degraded switching properties seen in the images above. This shows
strong dom ain pinning (Inabiiity to switch polarization) which would
indicate the presence of com plex domains, domains separated by 180°
or 90° walls form ed in the clustering process.
Both the 300 and 600 mTorr
oxygen pressure samples on the LAO and platinized high resistivity Si
substrate required a higher voltage, +10 V, to produce a semi-defined
poled areas, also indicating dom ain pinning.
60
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5.6 Microwave Characterization
For m icrowave applications, nanostructured ferroelectric films need
to have a low dleiectric constant (25<£r<1000), a low loss-tangent (or
dissipation factor) (0.001 <tan5<0.005), and a high dielectric tunabillty ((ero£rv)/ ero)>10% wheFG £ro IS the £r o t zero-blos, £rv is the £r a t a bias voltage
V).
Microwave measurements m ade from 9 to 18 GHz, indicate the
nano-structured BSTO thln-films Indicate a tunabillty of up to 25%.
A
sample deposited In 38 mlorr of O 2 and tested by on-wafer cryo-probe,
exhibited the following insertion loss, S21 vs frequency, as seen In Figure 34.
61
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I It iri
.
i
{t.M.iiOifiti{ / vt
J.n \F,ad\r«?1H„.2l «t
.
i
-
/ ,
- - — -^ r
,
^
'5 -^
J-
^
-Z ;2 s < i.te
^
^
'—h
f—~— . T-w,.„ M M t t ja d j
j
^ld03ro£-9
^IsClME-g' '
ii
4-
.'-< ii5 _ '-,
^
-------
s
I
I-
* *«i'l:i
■»»»
>•‘#'1^4i / ^ •*
4— ,______ '■
L^_J ^ 4 » - '
.■■!,)
I
W « l^ w
Figure 35. This shows S21 vs frequency for a 38 m lorr test sample sent to
NASA Glenn. The 821 improves as the voltage is applied from 0 V to 300 V in
step of 50 V. You con also see the phase of 821 tunable in the bottom
picture.
The results of microwave measurements performed a t Chalmers University of
Technology on another 38 mTorr sample, are depicted in Figures 41 and 42.
62
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
-Q-factor
-Capacitance (pF)
I
cvbstol
500
0,55
=
s
O
1200
0,5
400
0,45
300
0,4
200
0,35
100
0,3
-50
0
50
DC BIAS ( V)
Figure 36. C a pa citan ce versus DC bios for 38 mTorr BSTO film on LAO, a t 1
MHz and room temperature.
Figure 36 shows the effect of applied electric field on the BSTO film
deposited on the LAO substrate a t 38 mTorr oxygen pressure.
63
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-C a p a c if^ c e ® OV
“ Capacttance® 40V
C"fO‘4QV SSTO Smscron gap
0,3
0,2
0,1
20
30
Frequency (GHz)
Figure 37. Tunability of a coplanar ca p a cito r with 5 jim g a p as a function
of frequency, showing the highly tunable nature of the nano-structured
ferroelectric thin-films up to 50 GHz. This measurement was performed a t
Chalmers University,
Figure 37 shows the tunability of a 5 fxm ga p ferroelectric varactor over the
frequency range of measurements 1 MHz - 50 GHz.
64
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
•
PermSSvty I
BSTO-S micron gap
1400
1200
1000
800
600
400
0,001
0.01
0,1
1
10
100
Frequency (GHz)
Figure 38. Change in dielectric constant with frequency for 38 mTorr sample
of BSTO on LAO.
Figure 38 shows the com puted relative dielectric permittivity versus
frequency for the 38 m l BST sample. Note that the parasitic inductance
associated with the cap acito r may have caused the drop in permittivity
a t higher frequency.
65
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CHAPTER 6
ANALYSIS AND VALIDATION
6. Discussion:
6.1 Overview
The main research goo! was to utilize the pulsed laser deposition
parameters
to
control
the
grain
growth for low
m icrowave
loss
nanostructured BSTO thin-films on crystalline substrates such as LaAIOs
and
determine the effect on
material characteristics and
device
parameters.
The objectives of this research were to:
1) Change the nanosfrucfyre of the BSTO thin film form ed during PLD by
controlling the basic deposition parameters: beam energy density, pulse
repetition rate, substrate temperature, and am bient gas pressure.
The am bient gas pressure (O 2) was varied during the deposition
process whiie beam energy density, pulse repetition rote, and substrate
temperature were held constant. By varying the oxygen pressure from 19
mTorr to 1 Torr the nanostructure of the film changed for each of the
substrates LAO and the platinized high resistivity Si, seen In Section 4.4. It
was found that particle diameter, surface height and surface roughness
66
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increased with Increasing oxygen pressure on both of the substrates
chosen. Particles at the lower pressures 19 mTorr, 38 mlorr, and 75 mlorr
were more uniform and appeared to be single domains rather than the
com plex domains, illustrated by clustering, that developed a t oxygen
pressures a t and above 150 mlorr.
2) Develop a repeatable and controllable process for nanostructured film
fabrication.
PLD Is widely
recognized
os a
process that can
produce
nanostructured thin films (33-34). The PLD system a t the Air Force Research
Laboratory, Wright-Patterson AFB, that was used for the film deposition,
has the a d d e d capability of being com puter controlled and monitored.
Figure 3, section 3.1, shows the com plex Interaction of process variables In
a PLD system that must be controlled and m onitored to ensure quality and
repeatability in thin film fabrication as well as the ability to m anipulate the
nanostructure of the film.
Of particular interest In the study were the
parameters of the fabrication process that Influenced grain formation.
The process was validated by the consistent nanostructure developm ent
on two different substrates from single grains to com plex grain formation
by
changing
oxygen
pressures while
keeping
other
PLD process
parameters constant.
3) Determine grain size effect on the m icrowave properties of the film.
67
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Construction of a m icrowave device, the CPW, to test engineering
of the nanostructure of the film, was used to establish a realistic
application of the research. The BSTO film produced a t 38 mTorr oxygen
pressure on the LAO substrate hod the best m icrowave properties. This
film had the smallest groin size and surface roughness and best uniformity
of structure, with a device tunability of up to 25% .
For microwave
applications, nanostructured ferroelectric films need to have a low
dielectric constant (2 5< £ r< 1 0 0 0 ), a low loss-tangent (or dissipation factor)
(0.001 <tan5<0.005), and a high dielectric tunability where ero is the Er a t
zero-bias, & v is the & at a bias voltage V). The material, see Section 4.6,
was well within parameters for low loss and low dielectric constant. The
films fabricated a t higher pressures were poor candidates for m icrowave
applications because of the com plex dom ain structures degrading
polarization switching.
4) Characterize the nanostructured films using XRD, SEM, and AFM.
MD
The XRD spectra show th a t as the substrate coverage increases, the
intensity of the (200) reflection for BSTO and the intensity for the (200)
reflection of LAO reverse In relative magnitudes, The intensity of the (200)
reflection peak for BSTO was now greater in m agnitude than the (200)
reflection for LAO Indicating a better surface coverage of the substrate.
The XRD spectra also showed very distinct crystallinity.
68
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The d a ta
validated the SEM film thickness measurements based on changing
oxygen pressure and the results viewed fabrication process. As oxygen
pressure was increased plume intensity increased and plume geom etry
was com p acte d, producing thicker surface coverage.
SEM
SEM Images gave the cross-sectional view of the films which
corroborated the colum nar growth on the substrate seen In the AFM
images, see Section 4.3.
AFM/EFM (Surface Potential)
AFM/Surface Potential are the characterization methods th a t can
be used to describe the dipole kinetics associated with ferroelectric
materials.
The capability to sustain polarization switching is the limiting
factor in nanostructure engineering of ferroelectric materiai.
Direct
imaging of dom ain structures In thin films, using AFM, and Investigation of
their behavior, using EFM for surface potential, under the applied electric
field provided information of switching phenom ena based on material
structure of the thin BSTO films deposited on LAO and the platinized high
resistivity Si.
AFM images indicated the grain size and surface roughness, both
Increasing in relation to Increasing oxygen pressure.
surface potential techniques
The
AFM /EFM
allow ed the m anipulation of ferroelectric
69
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domains as small as 36 nm with a high degree of precision on the BSTO
films deposited a t 38 m lorr on the LAO substrate.
A t lower oxygen pressures the grains are smaller and more easily
controlled in BSTO deposited on both substrates. Higher pressures create
more com plex grain structures seen in the schem atic below.
These
degrade ferroelectric dom ain switching seen by the surface potential
images produced.
A
ii
A
li
li
Figure 39 a. The dipoles are oriented In the same direction (simple
domain), b. The dipoles while In the same direction are disjoint c. The
dipoles are anti-parallel (walls are a t 180°), d. The dipole walls are a t 90°.
70
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6.2 Future Directions
The arrangem ent of the dipoles and switching characteristics
strongly affects the m acroscopic properties of ferroelectric materials. The
nanoscale behavior of ferroelectric material isn't well understood and
little has been reported on the behavior of surface charge (42-52). The
next step, beyond this study, would be to characterize substrate effects
on single dom ain structures in ferroelectrics and surface properties, design
l-V and C-V studies on simple domains and com plex domains in thin film
ferroelectric materials.
The operation of most of the devices BSTO and other ferroelectrics
rely heavily on are the surface and interface properties of ferroelectric
materials, Details of the polarization and charge distribution in the surface
and interface layers of ferroelectrics and their relationship to the physical
properties of materials has been studied but are still largely unresolved. A
closely related issue is the dom ain structure a t surfaces and Interfaces
which was investigated in this research (53-61).
For the first tim e an effort was m ade to fabricate BSTO films a t gas
pressures below 0.2-10 Torr where nanoclustering has been reported to
occur
(41),
parameters
and
Investigate
material
characteristics
as well as the device perform ance.
and
process
Two substrates were
used, LAO and a mulit-layered BST/Pt electrode/T102/Tl layer/Si02/Siiicon.
The samples fabricated on LAO were of interest for comparison in
71
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m icrowave applications,
The samples fabricated on the mulitlayered
substrate were used for comparison for possible Integration into Into SI
MMICs.
In
particular,
the
polarization
switching
characteristic
in
ferroelectric thin films was of particular interest. The formation of
unswitchable polarization within the nanostructure of the ferroelectric
BSTO was for the first time experimentally shown as o function of size and
complexity of the ferroelectric domains.
6.3. Conclusions
The BSTO piume components velocities were controlled to desired
setpoints by emission spectroscopy. These setpoints were established a t
selected cham ber O 2 pressures and a t a constant laser beam energy
density on the target. The same number of pulses, heater temperature,
target to substrate distance, substrate type, etc. was held constant for this
study. The intention was to only change one of the 17 process control
variables during these depositions, The films varied in thickness from close
to 200 nm a t 19 mTorr to about 350 nm a t 300 mTorr. The films exhibited
columnar growth with about the same diam eter a t the bottom and the
top. As O 2 pressure was doubled in steps from 19 mTorr to 300 mlorr, the
deposited films generally be cam e thicker and rougher.
The overage
columnar grain size ranged from 35 to 140 nm in the BSTO films.
BSTO films average grain size and structure deviated from nomina!
trend of lower pressure producing smaller grains both below 38 mTorr and
72
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above 150 mTorr.
At and above 225 mlorr the grains coalesced to larger
intermediate structures (complex grains). Tunability from 1MHz to 50 GHz a t
40V has been demonstrated in films grown at 38 mlorr (-37 nm width
groins).
Utilizing a
repeatable
and
controllable
process to
synthesize
nanocluster and nanostructured materials is essential for understanding the
fundam ental physics involved in the nanoscience of materials and the
evolution of the science into nanotechnology.
PLD provides, with
estabiished process control a deposition technique required for the
synthesis of high quoilty multicomponent thin films.
This initial single
adjustable parameter study, had to be com pleted before further tailoring of
the
microstructure can
be
enabled
by adjusting
other deposition
parameters individually or combinatorially. Careful examination of the listed
process control variables and their general affect on the critical growth
dynamics would enable fabrication of films with the desired characteristics.
Engineering the groin/particle size and the domain state of a
ferroelectric material is clearly useful to a ffe ct desired microwave properties
of the material.
The desired result Is to produce optimum materia!
characteristics for use in frequency and phase agile microwave devices
and also gain an understanding of the physics involved in fabricating
nanostructured materials,
73
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BIBLIOGRAPHY
1. F.A. M iranda G.Subramanyam; F.W.Van Keuls; R.R.Romanofsky; J.D.
Warner; H.Mueller, IEEE Trans. M icrowave Theory a n d Tech., 48 (7), pp.
1181-89, (2000).
2. R.R. Romonofsky; J.Bernhard; G.Washington; F.W.Van Keuls; F.A.
Miranda; C.L.Canedy, IEEE MTT international M icrowave Symposium
Digest, 2, 1351-54, (2000)
3. G.Subramanyam; F. Van Keuls; F.A. Miranda; R.Romanofsky;
C.Canedy; S. Aggorwal; T.Venkotesan; R. Ramesh, IEEE M icrowave
an d G uided wave Letters, 10, (4), April (2000).
4. R.R.Romanofsky; F.W. Van Keuls; F.A. Miranda, J. Phys. IV. France, B, pp
171-174, (1998),
5. C.H.Mueller, F.A.Miranda, Ferroelectric a n d Acoustic Devices. Eds:
D.Taylor, M.Francombe, A ca de m ic Press, (2000),
6. J.G. Scott and C.A. Araujo, Science 246, 1400 (1989)).
7. S. Li, J.A. Eastman, 2. Li, C.M. Foster, R.E. Newnham, L.E. Cross,
Physics Letters A 212(1996) 341 -346.
8. J. Grindlay, introduction to the Phenomenoloaical Theory of
Ferroelectricitv, Pergamon Press Ltd., London (1970).
9. S. Li, J.A. Eastman, Z. Li, C.M. Foster, R.E. Newnham, L.E. Cross,
Physics Letters A 212(1996) 341 -346.
10. J. Fitz-Geraid, S. Pennycook, H. Gao, R.K. Singh, NanoStrucf. Mater.
12, pp. 1167-1171, (1999).9. H. Shiiboshi, H. Matsuda, M. Kuwabara, J.
Sol-GelSci. an d Techn., 16,129-134, (1999).
11. C.M. Gilmore, J.A. Sprague, NanoStrucf. Mater., 9, 643-650, (1997).
73
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
12. Ayyub; S. C hattopadhyay; K. Sheshadri; R. Lahirl NonoStrucfured
Materials, 12, 713-718, (1999)
13. D.L Mills, Phys. Rev., B 3(11): 3 887 (1971)
14. D.R. Tilley, B. Zeks, Solid State Comrnun., 49(8): 823, (1984).
15. R. Castro-Rodriguez, V.E. Quadreili, F. Calderon, F. Leccobue, B.E.
Watts, Mater. Lett. 34 326-331, (1998)
16. Y.G. Wang, W.L. Zhong, P.L. Zhang, Solid State Commun., 90, 329
(1994).
17. K. Okuwada, J. o f Sol-Gel Science a n d Technology, 16, 77-81, (1999).
18. A. Gruverman, O. Auciello, H. Tokumoto, Integ. Ferr. 19, n 1-4, p 49-83,
(1988)
19. J. Fitz-Geraid, S. Pennycook, H. Goo, R.K. Singh, NanoStruct Mater. 12,
pp. 1167-1171, (1999).9. H. Shiibashi, H. Matsuda, M. Kuwabara, J. SolGel Sol. an d Techn., 16,129-134, (1999).
20. C.M, Gilmore, J.A, Sprague, NanoStruct. Mater., 9, 643-650, (1997).
21. A. von Hippel, (Ed.), Dielectric Materials and Applications, Artech
House, Boston, (1995).
22. R. Ramesh (Ed.), Thin-fiim ferroelectric materials and devices. Kluwer
Academ ic, Norwell, MA,(1997).
23. C.H.Mueller, F.A.Miranda, Ferroelectric a n d Acoustic Devices. Eds:
D.Taylor, M.Francombe, A ca de m ic Press, (2000).
24. K.S.Grabowski, J.S. Horwitz,,D.B.Chrisey, Ferroelectrics. 116, 19, (1991).
25. H. Wu and F.Barnes, Integ. Ferroelectrics., 22(1), 811-825, (1998).
26. Gopinath, Losses in CPW lines, IEEE Trans on M icrowave Theory
and Techniques, vol.30, p p 1101-04, 1982
27. A. Eutenburg, E.J. Romans, Y.C. Fan, C.M. Pegrum, Physica C 3] 2
(1999)91-104.
74
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28. P. Ayyub; S. Chattopadhyay; K. Sheshadri; R. Lahiri, Nanostructured
Materials, 12, 713-718, (1999)
29. M. Eng, Nanotechnology, 10, 405-411, (1999).
30. W., SenguptG, L., J. A p p l Phys., 92(7), 3941, (2002)
31. H. Shiibashi H. Matsuda, M. Kuwabara, J, o f Sol-Gel Science and
Technology, 16, 129-134 (1999)
32. H. Shiibashi; H. Matsuda; Kuwabara, J. Sol-gel Science and
Technology 16, 129-134(1999)
33. H.K. Schmidt; E. Geiter; M, Mennig; H. Krug; C. Becker; T.P. Winkler, J. o f
Sol- Gel Science a n d Technology 13 397-404 (1998),
34. J. Zorzycki, J. o f Sol-Ge! Science a n d Technology. 8 17-22 (1997).
35. R.M. Almeida, J. o f Sol-Gel Science an d Technology 13 51-59 (1998).
36. K. Sulz and K Nagayama, Appl. Phys. A 69 (SuppI:), S235-S238 (1999)
37. J, Zarzycki. J. o f Sol-Gel Sol. a n d Techn. 8, 17-22 (1997).
38. H. Shiibashi, H. Matsuda, M. Kuwabara, J. o f Sol-Gel Science and
Technology, 16, 129-134(1999)
39. D.B.Chrisey, and G.K. Hubier, 1994, Eds., Pulsed Laser Deposition of
Thin Films (Wilev, New York).
40. W., Sengupta, L., J. Appl. Phys., 92(7), 3941, (2002)
41. JBL Rao, D.P, Patel, and L.Sengupto, Integrated Ferroelectrics, 22,
827, (1998).
42. P.R. Willmott, J.R. Juber, Reviews o f Modern Physics. 72 (1), January
(2000)
43. M. Ozegowski; K. Meteva; S. Metev; G. Sepold, A p p lie d Surface
Science. 138, p. 68-74 (1999)
44. M.P. McNeal, S-J. Jang, R.E. Newnham, J. o f A p plied Physics, 83 (6),
pp. 3288-7, 15 Mar (1998).
45. L. Zhang, W-L. Zhong, C-L Wong, P-L Zhang, Y-G. Wang, J. o f Physics D:
75
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
AppL Phys., 32(7), pp. 546-551, Mar. 7, (1999),
46. B. Jiang, A. Bursill Physical Review B, 60(14), pp 9978-9982, 1 Oct.,
(1999).
47. C.L. Wong, Y. Xin, X.S. Wang, W.L. Zhong, Physical Review B, 62 (17), 11
424-427, 1 Nov. (2000-1)
48. S. Jun, Y. Kim, J. Lee, Surface a n d Coatings Technology 13] (2000)
553-557.
50. S. Saha, S. Krupanidhi, Journal o f A pplied Physics. Vol. 88, No. 6,
15 Sept. 2000, 3506-3513.
51. S. Chattopadhyay, Nanostructured Materials. 9, pp 551-554, (1997).
52. Y.Wong; W.Zhong; P.Zhang, Science In China (Series A), 38, no.6, pp
724-729, (1995)
53. L.Sengupto, and S. Sengupta, Material Research Innovations, 12, (5),
pp 278-82, April (1999)
54. F. Stork, S.J.P. Laube, JMEPEG 2(5), 721 (1993)
55. R. K. Singh, J. Non-Cryst. Solids 178, 199-209 (1994)
56. H. Haberland (Ed.), Clusters of Atoms and Molecules. Springer-Verlag,
1994
57. W. Marine, L Patrone, B. Luk'yanchuk, M, Sentis, Appl. Surface
Science, 154-155, 345-352 (2000)
58. R. K. Singh, J. Non-Cryst. Solids 178, 199-209 (1994)
59. H. Haberland (Ed.), Clusters of Atoms and Molecules, Springer-Verlag,
1994
60. T. Yoshida, Y.S. Tokeyama, Y. Yamada, K. Mutoh, Appl. Phys. Lett 68
1772 (1996)
61. R. Castro-Rodriguez, V.E. Quadreili, F. Calderon, F. Leccobue, B.E.
Watts, Mater. Lett. 34 326-331, (1998)
76
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62. J. Takasu, J. o f Elecfroceramics, 4:2/3, 327-338, (2000).
63. Y. Tarui; T. Hirai; K. Teramoto; H. Koike; K. Nagashim a Applied Surface
Science. 113/114 656-663, (1997).
64. R, Biggers, G. Kozlowski, J. Jones, D. Dempsey, R. Kleismit,!. Maartense,
J. Busbee, T. Peterson, Integrated Ferroelectfics, 28(1-4),pp. 201-211
INVITED (2000)
65. R. Biggers, G. Norton, 1. M aartense, Private comm unication.
66. D. B. G eohegan, A.A. Puretzky, Appl. Phys. Left. 67, 197, (1995).
67. R. R. Biggers, J. G, Jones, I. Maartense, J, D, Busbee, D. Dempsey, D.
Liptok, D. Lubbers, C. Varanasi, and D. Mast, Eng. Appl. O f Artificial
Infeiligence.U, 627 (1998)
68. Booth, et al., MRSSym. Proc., voi.603, pp.253-264.
69. F. Gordiol, Introduction to Microwaves, Artech House, Dedham,
M g , 1984.
70. G.Subramanyam, F.W. Van Keuls; F.A.Miranda, IEEE M icrowave a n d
G uidedwave Lett., 8, (2), 78-80, (1998).
71. F.A. Miranda, G.Subramanyam; F.W.Van Keuls; R.R.Romanofsky; J.D.
Warner; H.Mueller, IEEE Trans. M icrowave Theory a n d Tech.. 48 (7), pp.
1181-89, (2000).
72. R.R. Romonofsky; J.Bernhard; G.Washington; F.W.Van Keuls; F.A.
Miranda; C.L.Canedy, IEEE MTTinfernafionol M icrowave Symposium
Digest 2, 1351-54, (2000)
73. G.Subramanyam; F. Van Keuls; F.A. Miranda; R.Romanofsky;
C.Canedy; S. Aggarwal; T.Venkotesan; R. Ramesh, IEEE M icrowave
a n d G uided wave Letters, 10, (4), April (2000),
74. R.R.Romanofsky; F.W. Van Keuls; F.A. Miranda, J. Phys. IV. France. 8, pp
171-174, (1998).
75. E. Carlsson; S. Gevorgian, IEEE Transactions On M icrowave Theory And
Techniques, 47: (8) 1544-1552 AUG (1999)
77
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76. S. Gevorgian S, T. Martinsson, A. Deieniv, lEE P-MICROW ANTEN P 144:
(2) 145-148 APR (1997)
78, F.W. Van Keuls, C. Chevalier, F.A.Miranda, C.M.Carlson, T.V.RivkIn,
P.A.Pariila, J.D.Perkins, D.S.GInley, M icrowave and Optica! Technology
Lett., 29(1) 34-37, April 2001
79. - G.Subramanyam; F.W.Van Keuls; F.A.Miranda; C.L.Canedy;
S.Aggarwal; T.Venkotesan; R.Ramesh, Integrated ferroelectrics, 24,
p p 273-285, (1999).
8 0 .1.N. Germanenko; S. Li; S.J. Silvers; M.S. El-Shall, Nanostructured
Materials, 12, 731-736, (1999),
81. K. Murakami; T. Suzuki; T. Mokimura; M. Tamura, App. Phys. A: Materials
Science & Processing, 69 (SuppI.), S13-S15 (1999).
82. L. Patrone; D. Nelson; V. Safarov; M. Sentis;W. Marine, J. o f
Luminescence, 80, pp. 217-221 (1999)
83. L. Patrone; D. Nelson; V.l. Safarov; S. Giorgio; M. Sentis; W. Marine,
A p plied Physics A: Materials Science & Processing, 69(7), pp. S217S221, Dec. (1999).
84. S.Joshi and M.W.Cole, Appl. Phys. Lett., 77, (2), pp 289-291, 10 Juiy
(2000)
85. http://w w w .veeco.com /htm l/D roduct bvmarket.asp. Application
Note from Digital Instruments website: Scanning Probe/Atom ic Force
Microscopy: Technology Overview and Update.
86. h tto ://w w w ,cm m p.ucl.ac.uk/~ata/w ork/stm .htm l, The STM - an
introduction
87. h ttp ://w w w .vee co.co m /h tm l/p rod uct bvmarket.asp. Application
Note from Digital Instruments website: SEM and AFM: Com plem entary
Techniques for High Resolution Surface investigations
88. Multi-Mode SPM Manual version 4.31 c e Digital Instruments Veeco
Metrology group
78
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APPENDIX A
TABLE 1
Review o f Significant works in characterization of bulk and thin-film BST:
Topic
Ferroeiectric
memories
Temperature
coefficient of
dielectric
constontO'CK)
Giant effective
pyroelectric
coefficients in
graded ferroelectrics
Authors
T.Eimori et
al.
R.J.Cava et
al.
Publication
lEDM Digest
1993, pp.631
Appl, Phys.Lett.,
vol.67, no,25,
1995, pp. 3813-15
F.Jin et al.
Appl. Phys, Lett.,
vol.73, no. 19,
1999, pp. 1838-40.
4
Effect of stress on
dielectric properties
of BST thin-films
T.M. Shaw
et al.
Appl. Phys. Lett.,
vol.75, no. 14,
1999, pp.2129-31.
5
Dependence of
dielectric properties
on internal stresses in
BST thin-films
H.Li et al.
Appl.Phys.Lett.,
vol.78, no. 16,
2001, pp.2354-56.
6
Microstructure and
dielectric properties
of BST films on LAO
Y.Gim et ol.
Appl.Phys.Lett.,
vol.77, no.8,
2000, pp. 120002,
7
Infrared optical
properties of BST thinfilms
Z.Huong et
al.
Appl.Phys.Lett.,
vol.77, no.22,
2000, pp.3651-53.
No.
1
2
3
Description
High K BST
capacitors for DRAM
Adding small
quantities of BSN
decreases the TCK
dramatically
Pyroelectric
coefficients ~
5ixC/cm2°C
obtained from
compositionolly
graded BST thin-films
on Si
Stress free
capacitance found
to be 23% higher
than the
capacitance under
residual stress,
BST thin-films on
MgO substrates, with
thickness of 14 to
500nm were
investigated.
Dielectric constant
drops from 2350 for
highly stressed films
to 1700 in relaxed
thicker films.
Bal-xSrxTi03 thinfilms with x=0.1-0.9
were studied. At
room temperature,
the dielectric
constant and
tunobiiity are high
for x<0,4, and
decreases rapidly
for os X increases.
Ba0.8Sr0.2TiO3 thinfilms grown on
Pt/Ti/Si02/Si
substrates were
studied in the
79
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8
Pyroelectric
properties of sol-gel
derived BST thin-films
J-G.Cheng
et al.
Appl. Phys.Lett.,
vol.77, no.7,
2000, pp. 1035-37.
9
Dielectric tunability
and harmonic
generation in BST
thin-films on LAO
J.G. Booth
et al.
Appl.Phys.Lett.,
vol.81, no.4,
2002, pp.718-720.
No.
Topic
Authors
Publication
10
Flexo-electric
polarization of BST
W.Ma et al.
Appl.Phys.Lett,,
vol.81, no. 18,
2002, pp.3440-42.
11
BST thin-films on Si
with Pt electrodes
L.Kinder et
al.
J.Vac.Sci.Tech.,
A17(4), 1999,
pp.2148-50,
spectral range of
2.5-12.6 pm. The
refractive index
decreases and the
extinction
coefficient increases
as the wavelength
increases.
Pyroelectric
coefficient of
Ba0.8Sr0.2TiO3 thinfilms on Pt/Ti/Si02/Si
substrates.
Measured
pyroelectric
coefficient >3.1x10-4
C/m^K from 10-26°C.
Dielectric tuning is
investigated at
nanosecond time
scale in
Ba0.3Sr0.7TiO3 thinfilms at 3 GHz. Full
dielectric tuning can
be expected at
nanosecond time
scales.
Description
Strain-gradient
induced polarization
(flexoelectric effect)
is studied in
Ba0.67Sr0.33TiO3
bulk ceramic in the
paraelectric state.
The flexoelectric
coefficient (pi2) is
roughly proportional
to dielectric
pemittlvity.
Ba0.6Sr0.4TiO3 thinfilms grown by PLD
on Pt/Si02/Si
substrates.
Measured dielectric
constant for a
170nm BST film was
400 with a losstangent of 0.01 -0.03
80
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at 10 kHz.
12
13
14
Effect of annealing
on the structure and
diefectric properties
Composites of BST
and other
noneiectrically
active ceramics
Doped BST for
microwave
applications at room
temperature
Appl. Phys. Lett.,
vo!.69, no.l,
pp.25-27, 1996.
Materials
Research
Innovations,
vol.2, no,5,
pp.278-282, 1999
Integ,
Ferroelectrics.,
vol.22, p p .s n 826, 1998.
L.K.Knouss
et ol.
L.C.Sengup
to et al.
H. Wu and
F, Barnes
Reduced dieiectric
constant and losstangent for low-loss
tunable dielectric
materials
Dielectric properties
of laser ablated Mndoped and
undoped
Ba0.6Sr0.4TI03 thinfilms ore compared
at low frequencies
and at microwave
frequencies, 1% Mn
doped films hod
large tunability of
66% at 40 kV/cm
and a loss-tangent
of 0,006 at zero bias
and 10 GHz,
81
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Review of Significant works in Applications of BST ferroelectrics for Microwave
Components:
No.
Topic
Authors
Publication
1
Oxide
Superconductors and
Ferroelectrics
A.M.Hermann et
ai.
J. Supercon., voi.7,
no.2, 1994, pp 463469.
2
Ceramic phase
shifters for
eiectronicolly
steerobie antennas
Effect of annealing
on the structure and
dieiectric properties
Composites of BST
and other
noneiectrlcaliy active
ceramics
V.K. Varadan et
ai.
Microwave J., 1995,
pp.244-253.
L.K.Knauss et ai.
Appi. Phys, Lett.,
vol.69, no.l, pp.25-27,
1996.
Materials Research
innovations, vol.2,
no.5, pp.278-282,
1999
Doped BST for
microwave
applications at room
temperature
Microwave power
handling
H. Wu and
F.Barnes
integ. Ferroelectrics.,
vol.22, pp.811-825,
1998.
D.Gait, eta!.
Proceedings of MRS
vol.493, pp.341-346,
1998.
Coupled Microstrip
Phase shifters
Van Keuls et ai.
3
4
5
6
7
L.C.Sengupta et
ai.
82
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Descriptio
n
BST
varactor
with MTS
resonator,
for
tunable
devices (1
]
Reduced
dieiectric
constant
and iosstangent
for iowioss
tunable
dieiectric
materials
8
Tunable filters
G.Subramanyam
etai.
Integrated
Ferroelectrics,
9
10
Phased Array
Distributed Analog
Phase shifters
Tunable couplers
R.Romanofsky
A.Nogro eta!.
MRS
IEEE Irons MTT
G.Subramanyam
et al.
IEEE Microwave and
Guded wave Lett.
11
12
83
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
2 pole
filters with
5%
frequency
tenability
at room
temperot
ure
demonstr
oted.
Lange
couplers
with
tunable
coupling
at K-band
frequenci
es
demonstr
ated.
Appendix A (Synthesis of BSTO by PLD and Sol-gel)
Ferroelectricity/
Films
TABLE 2
Solid State Commun. 1984 Landau theory for phase
Journal ofSol-Gel Science
and Technology (1999)
Nanostructured Materials.
1999
Nanostructured Materials
1997
Journal o f
Superconductivity ]994.
NASA/Tm-1998-206641
Integrated Ferroeiectrics.
1999
A pplie d Physics Letters
April 03, 2000
lEEETransactionson
Microwave Theory and
Techniques April 2000
Journalof
Superconductivity, 1999
InteagratedFerroeiectrlcs,
(1997)
Microwave and O ptical
Technology Letters Feb5
2000
transitions in thick films
Effects of Baking and Annealing
Processes on SrBl2Ta209 Film by
Soi-Gei M ethod
Synthesis and properties of
nanofunctionallized particulate
materials.
C om puter m odeling the
deposition of nanoscale
thin films,
Oxide Superconductors and
Ferroelectrics—Materials for a
new G eneration of Tunable
Microwave Devices
Influence of the Biasing Scheme
on the Performance of
Au/SrTiOa/LaAIOs Thin Film
Conductor/Ferroelecatric
Tunable Ring Resonators
Correlation o f Electric Field and
Critical Design
Parameters for Ferroelectric
Tunable Microwave Filters
Large dielectric constant (>6000)
Bao.4Sro.6Ti03 thin films for highperform ance m icrowave phase
shifters
A K-Band-Frequency
AgileMicrostrip Bandpass
FilterUsingaThinFilmHTSFerroelectric Dielectric
Multilayer Configuration,
Ferroeiectric-HTSC Microwave
Structures;
Fundamental Limitations
Thin film multiplayer
conductor/ferroelectric
tunablem icrow avecom ponentsf
or com m unication applications
Electrically Tunable
Superconducting Quasiiumped
Element Resonator Using Thin-Film
82
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Ferroelectrics
1993 /EEf Trans. Appl,
Supercond.
IEEE Trans. Microwave
Theory Tech. 1969,
IEEE Trans. Appi.
Supercond. 1997
J. Appl. Phys. 1967
IEEE MTT-S Inf. Microwave
Sympl995
!EEE MTT-S IMS 1998
IEEE MTT-S IMS 1998
1997. IEEE Trans
M icrowave Theory Tech
1998. !EE MTT-S Digest
A pplie d Physics Letters.
January 10, 2000,
M icroelectronic Eng.
December, 1995
Journal o f
Elecfroceramics 2000
Sem iconductor Science
a n d Technology. April
1995
M icroelectronic
Engineering
(1995)
Pulsed laser deposition of novel
HTS multilayers for passive and
active device applications
Coplanar w aveguide; A surface
strip transmission line suitable for
nonreciprocal gyrom agnetic
device applications
HTS/ferroelectric devices for
microwave
applications."
Ferroelectric harmonic
generator and iorge-signal
m icrowave characteristics of a
ferroelectric ceram ic
Model for a novel CPW phase
shifter.
Phased array antennas based on
bulk phase
shifting with ferroelectrics.
Ferroelectric phase shifters for
phased array radar
applications
Planar m icrow ave integrated
phase-shifter design with high
purity ferroelectric material
Tunable superconducting
bandstop filters
Nonvolatile program m able twoterminal diodes using a
ferroelectric sem iconductor
Integrated ferroelectric structures
for information
storage and CCD
microprocessor devices
The Ferroelectric Memory and its
Applications
Ferroeiectric thin film technology
for sem iconductor memory
Device processing and
Integration of
ferroelectric thin films for memory
applications
83
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Applied Surface Science
(1997)
A pplication of the ferroelectric
Materials to ULS! memories
J. Supercond. 1992
Monolithic HTS microwave phase
shifter and other devices
Supercond. Sd. Technol.
Thin-film ferroelectric microwave
devices
(1998)
IEEE Cat#
94CH3416-5, 1998
Applied Surface Science
1997
Pulsed laser deposition of
ferroelectric thin films in
conjunction with
superconducting oxides
Application of the ferroelectric
materials to ULSI memories
Ferroelectrics Review 1998 The Physics of Ferroelectric
Sol-Gel Science and
Technology (1997)
C eram ic Thin Films for Memory
Applications
Pulsed Laser Ablation-Synthesis
and Characterization of
Ferroelectric Thin Films and
Heterostructures,
Past and Present of Sol-Gel
Science and Technology
Journal of Sol-Gel
Science and Technology
Spectroscopy an d Structure of
Sol-Gel Systems
Ferroelectric Ttiin Films;
Synthesis and Basic
Prooerties 1996
(1998)
Rep. Prog. Phys. (1998)
Ferroelectric Thin Films:
Synthesis and Basic
Prooerties 1996
Journal of Sol-Gel Science
and Technology (1999)
Applied Surface Science.
(1999)
Journal of Applied Physics.
Nov, 16, 1998,
Applied Physics Letters.
June 23, 1997.
Ferroelectric, dielectric and
piezoelectric properties of
ferroelectric thin films and
ceramics
Metal Alkoxides-Precursors for
Ferroelectric
Materials
Ferroelectric Properties of New
C hem ical Solution Derived SBT
Thin Films for Non-Volatile
Memory Devices
Pulsed loser deposition of
m ulticom ponent metal and
oxide films
Ferroelectric films prepared by
laser ablation directly on SiC
substrates
Stoichiometry and thickness
variation o f YBaaCusOz-x in pulsed
84
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Ferroelectrics/
size effects
laser deposition with a shadow
mask
Thin Solid Films (2000)
Pulsed laser deposition of
organic thin films
Applied Surface Science Pulsed loser deposition of metal
(1995)
and metal multipiayer films
Applied Physics A:
Interaction betw een laser beam
Materials Scieance &
and target In pulsed loser
deposition: loser fluence and
Processing. (1999).
am bient gas effects
Maferiais Letters 34 (1998)
Effects of particle size on the
phase transition in Pb(Zr, Ti)03
grown by the Sol-gel technique
Soiid State Commun., 1994 Size-driven phase transition in
ferroeiectric particles.
Nanostructured Materials
Effect of High
1999
Hydrostatic Pressure on the
Ferroeiectric Properties of
Epitaxiai
Journal o f Vacuum
Science & Technology B:
Microelectronics
Processing and
Phenomena.. May-Jun
1995
Nanostructured Materials,
1999
A cta mater. 1999.
Nanostructured
Materials, 1999
Integrated Ferroelectrics.
1998
Applied Physics A:
Materials Science &
Processing. (1999).
Physical Review B. Vol. 60,
1 Oct. 1999
Nb:Pb(Zro.52Tio,48)03/YBa2Cu307-x
Nanostructures.
Domain structure and
polarization reversal In
ferroelectrics studied by atom ic
force
microscopy
Surface Tension of Uitrafine
Particles
Ion Im plantation-Induced
Nanoscale Particle
Formation in AI2O 3 and Si02 Via
Reduction
Characterization of
Silicon Nonocrystais and
Photoluminescence Quenching
in Solution
Scanning force microscopy:
A pplication to nonoscale studies
of ferroelectric domains
Si nonocrystailites in S1O 2 with
intense visible
photoium inescence synthesized
from SiOx films by loser ablation.
Phenomenoiogicol theory o f size
effects In uitrafine ferroelectric
85
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Science in China (Series
A). June 1995
1992 M icrowave J.
1998 Innovafions in
Materials Congerence
Proceedings of MRS 1995
Journo! o f A pplied
Physics, 15 Mar 1998
Solid State
Communications
1996
NanoStructured Materials
1999
Journal ofSoi-Gel Science
a n d Technology (1999)
A pplied Physics A:
Materials Science &
Prosessing 1999
NanoStructured Materials
1999
M icroelectronic
Engineering (1995)
NanoStructured Materials
1999
Journal o f Sol-Gel Science
a n d Technology (1998).
Adv. Mater. O p t Electron.
(2000)
NanoStructured Materials
1997
particles of lead titanate
Size e ffe ct of ferroelectric
porticies
Ceramic phase shifters for
electronically sterroble antenna
systems
Breakthrough advances in low
loss, tunable
dielectric materials
Thin fiims of novel ferroelectric
composites
The e ffe ct of grain and particle
size on the microwave
properties of barium titanate
(BaTlOa)
Size effects on the quantum
paroeiectric SrliOs
nanocrystals
Single Sep Chemical Synthesis of
Lead Based Relaxor Ferroelectric
Niobate Fine Powders
Effects of Baking and Annealing
Processes on SrBl2Ta209 Film by
Sol-Gel M ethod
Comparison of Pt/Ti02
nanocom posite films prepared
by sputtering and pulsed laser
deposition
Ferroelectric Phase Transitions in
Materials Embedded In Porous
Media
Structural and Electrical
Characterization of SputterDeposited SrTiOs Thin Films
Ion Im plantation-induced
Nanoscale Particle Formation in
AI2O 3 and Si02 Via Reduction
The Sol-Gel Process for NanoTechnoiogies: New
Nanocomposites with interesting
O ptical and M echanical
Properties
Hole Mobilities in Sol-gei Materials
Preparation and Ferroelectric
Phase Transition
86
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Journal of Applied Physics,
Jul 15, 2000
Applied Physics Letters.
August 21, 2000.
PhysicaE: LowDimensional Systems and
Nanostructures 1999
Japanese Journal of
Applied Physics, Part 1:
Regular
Papers and Short Notes
and Review Papers. 1999
Ulframicroscopy 2000
BSTO/STO
Journal o f Sol-Gel Science
and Technology (1999)
Appi. Phys. Left]999
Journal of Sol-gei
Science and Technology
(1999)
Journal of Applied Physics,
Jan 01, 1999
IEEE MTJ-S Digest 1998
IEEE MTT-S Digest 1998
IEEE MJT-S IMS Digest 1998
Studies o f Nonocrystaliine
BoTiOa
Laser-piasmo interocations in 532
nm oblation of Si
Microstructure and dielectric
properties of Boi-xBrxTiOs films
grown on LoAlOa substrates
Dielectric and optical properties
of BoTiOa
mesocrystols
Scanning nonlinear
dielectricmicroscopy with
c o n ta c t sensing mechanism for
observation of nanom eter sized
ferroelectric domains
Investigations into local
ferroelectric properties by
atom ic force microscopy
Low-temperature preparation of
(Ba,Sr)TiOs perovskite phase by
sol-gel m ethod
Tunable and adaptive bandpass
filter using non-linear dielectric
thin film of SrTiOs
Low-Temperature
Preparation of (Ba,Sr)Ti03
Perovskite Phase by Sol-Gel
Method
Fatigue in sol-gel
derived barium titanate films
A Novel K-Band Tunable
Microstrip Bandpass Filter Using a
Thin Film
HTS/Ferroelectric/Dieiectric
Multiplayer Configuration
A Novel K-Band Tunable
Microstrip Bandpass Filter Using a
Thin Film
HTS/Ferroelectric/Dleiectric
Multiplayer Configuration.
Ferroelectric fiims: nonlinear
properties and
applications in m icrowave
devices
87
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Applied Surface Science.
1998
NASA/TM-1998-206964
EEE Microwave and
Guided Wave Letters,
April 2000,
1993 IEEE MTT-S Digst
A pplied Surface Science
A pr 2000
Materials Science and
Engineering 1996
Applied Surface Science
1997
1972, J. AppL Phys
1976 Ferroelectrics
A pplie d Surface Science.
(1998)
United States P ate nts 538
941, 23 July 1996.
1995 AppL Phys. Lett.
IEEE Trans.
Appl.Supercond
1997
Journal o f A pplie d Physics
Nov 2000
Pulsed loser ablation of
ferroelectric composites for
phased array antenna
applications
Several Microstrip-Based
Conductor/Thin Film Ferroelectric
Phase Shifter Designs
Performance of a K-Bond
Voltoge-Controiled Lange
Coupler Using a Ferroelectric
Tunable Microstrip Configuration
Tunable High Temperature
Superconductor Microstrip
Resonators
Nonlinear response and power
handling capability of
ferroelectric
BoxSri-xTiOa film capacitors and
tunable m icrowave devices
A proposal of epitaxial oxide thin
film structures for future oxide
electronics
Functionality o f the
ferroelectric/oxide
sem iconductor interface
Permitivity o f strontium titanate
Dielectric nonlinearity o f the
displacive ferroelectric a t
UHF
Pulsed loser oblation of
ferroelectric composites for
phased array antenna
applications
Superconductor/Insulator Meta!
Oxide hetero Structure for
Electric Field Tunable M icrowave
Device
Electrically tunable coplanar
transmission line resonators using
YBa2Cu307-x/SrTi03 bllayers
Superconductor/non-linear
dielectric bilayers for tunable
and ad aptive m icrow ave
devices
Ferroelectric phase transition and
maximum dielectric permittivity
of displacem ent type
88
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
ferroelectrics (BOxSn-xTiOs)
Journo! o f A pplie d Physics,
15 Jul 1998
1997 J. AppL Phys
Microwave losses in incipient
ferroeiectrics as functions of the
tem perature and biasing field
High tunabiiity of the permittivity
of
Y B a 2 C u 3 0 7 -5 /S rT i0 3
Ferroelectnc Thin Films:
Synthesis a n d Basic 1996
Mater. Lett. 1993
IEEE Cat#94CH3416-5
1998
heterostructures on sapphire
substrates
Soi-Gei Science and PE-MOCVD
o f Dielectric Perovskite Films.
The e ffe ct o f various dopants on
the dielectric properties of
barium strontium titanate
Analysis of ferroelectric thin fiims
deposited by the pulsed laser
deposition m ethod on oxide and
fluoride substrates
Ferroelectric Thin Fiims:
Synthesis and Basic
Properties 1996
A pplie d Physics Letters,
July 19,1999,
D efect Chemistry, Conduction,
and Breakdown
Mechanism of PerovskiteStructure Titonates
Epitaxial
ferroelectric Bao.sSro.sTiOs ghin
films for room -tem perature
tunable element appliations
Journal o f A pplie d Physics
15 Sep 2000
Dielectric properties of puisedlaser deposited SrTiOs fiims at
microwave frequency ranges
89
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