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Integration of ferroelectric materials on flexible substrates for microwave applications

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INTEGRATION OF FERROELECTRIC MATERIALS ON FLEXIBLE SUBSTRATES
FOR MICROWAVE APPLICATIONS
APPROVED BY SUPERVISING COMMITTEE:
________________________________________
Arturo A Ayon, Ph.D., Chair
________________________________________
Mehdi Shadaram, Ph.D.
________________________________________
C.L.Philip Chen, Ph.D.
________________________________________
Chonglin Chen, Ph.D.
Accepted: _________________________________________
Dean, Graduate School
Copyright 2009 Ramakrishna Kotha
All Rights Reserved
DEDICATION
This dissertation is dedicated to my parents Kesava Rao Kotha and Nagavenu Kotha for their
love and for the sacrifices they made during the course of my education.
INTEGRATION OF FERROELECTRIC MATERIALS ON FLEXIBLE SUBSTRATES
FOR MICROWAVE APPLICATIONS
by
RAMAKRISHNA KOTHA, M.S.
DISSERTATION
Presented to the Graduate Faculty of
The University of Texas at San Antonio
In partial Fulfillment
Of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY IN ELECTRICAL ENGINEERING
THE UNIVERSITY OF TEXAS AT SAN ANTONIO
College of Engineering
Department of Electrical and Computer Engineering
December 2009
UMI Number: 3387656
All rights reserved
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ACKNOWLEDGEMENTS
Firstly, I wish to express my sincere thanks to my advisor Dr.Arturo A Ayon for his
motivation, and guidance throughout the course of this work. I thank him for keeping me on
track all times in my research and guided me in the right direction everytime. Every technical
meeting with him was a great learning experience. Especially, the course “Introduction to Micro
and Nanotechnology “ offered by him, has created a great interest in me towards the field of
Micro-fabrication.
I also thank Dr.Chonglin Chen, for his valuable suggestions and comments for this work.
Thin film course taught by him was beneficial in my research. I would also like to extend my
gratitude to my committee members, Dr.C.L.Philip Chen and Dr.Mehdi Shadram for their help
throughout the course of this work. I am also thankful to Dr.Andrey Chabanov and Dr.Carlos
Garcia for their help.
I thank University of Texas San Antonio, the Department of Electrical and Computer
Engineering, the Department of Physics and Astronomy, and the US Army Research office for
the financial assistance throughout the course of my program.
I would like to extend my gratitude my colleagues, David Elam and Greg Collins, who
were very helpful at crucial times. Also, I would like to thank my friends Justin Ficklen, Ross
Hackworth, Julie Hoy, Renan Moreira,
Dr.Hongbing Ji, Dr.Dhireesha Kudithipudi and
Dr.Ashlesh Murthy.
iv
I would like to thank my parents, Kotha. Kesava Rao and Kotha Nagavenu for their love,
inspiration and enormous support throughout my life. Long conversations with my father had
given me a lot of strength, I needed at critical times of my study. Finally, I thank God, for his
blessings.
December 2009
v
INTEGRATION OF FERROELECTRIC MATERIALS ON FLEXIBLE SUSBTRATES
FOR MICROWAVE APPLICATIONS
Ramakrishna Kotha, Ph.D.
The University of Texas at San Antonio, 2009
Supervising Professor: Arturo A Ayon, Ph.D.
While most of the electronic components are fabricated on widely employed hard
substrates such as silicon, the performance of silicon technology is not sufficient to satisfy the
requirements of novel technologies like flexible electronics where device flexing plays a vital
factor for its functionality. Therefore, novel materials which are compatible with present day
technologies are being constantly explored to meet the ever increasing demands for civilian and
military applications. One solution involves flexible substrates which have numerous advantages
compared to its silicon or GaAs counterparts, including lower cost, light weight, microfabrication compatibility and above all the flexing advantage. Flexible electronics have shown a
great potential in the field of semiconductor technology in fabrication of devices such as sensors,
displays, and transducers and research is being actively conducted in other areas including radio
frequency communications, energy harvesting, textiles.
This dissertation is expounded around three phases, namely,
Phase 1:
Selection of Flexible substrate, thin film materials and fabrication processes.
Phase 2:
Thin film characterization of the selected materials.
Phase 3:
Fabrication, characterization and testing of the phase shifting performance of the
Micro-flex circuits.
vi
TABLE OF CONTENTS
Acknowledgments.......................................................................................................................... iv
Abstract...........................................................................................................................................vi
List of Tables…………………………………………………………………………………...…x
List of Figures………………………………………………………………………………….....xi
Chapter 1: Introduction ....................................................................................................................1
1.1 Introduction …………………………………………………………………………1
1.2 Dissertation Objective ………………………………………………………………3
1.3 Dissertation Organization ………………………………………………………..…6
Chapter 2: Thin Film Characterization………………………………………...............................7
2.1 Introduction ………………………………………………………………………..7
2.2 Thin Film Preparation ……………………………………………………………11
2.3 Thin Film Analysis ………………………………………………………………13
2.3.1 Film Thickness ……………………………………...…………….........13
2.3.2 Film Stress ………………………………………………………………16
2.3.3 Surface Roughness ………………………..……………………………18
2.3.4 Contact Angle …………………………...………………………………19
2.3.5 Thin Film Annealing ……………………………………………………20
2.3.5.1 Surface Roughness ……………………………………....20
2.3.5.2 Thin Film Stress …………………………………….……21
2.3.5.3 Contact Angle …………………………………….………22
2.3.5.4
Micro-Structure Inspection ……….…………...………23
2.4 Conclusion ……………………………………………………………….………27
vii
Chapter 3: PVDF Film Characterization........................................................................................28
3.1 Introduction ………………………………………………………………………28
3.2 PVDF/TrFe …………………………………………………………………….…30
3.2.1
PVDF/TrFe Solution Preparation …………………………………..…31
3.3 Test Set-up Fabrication and Polarization Measurements ………………………31
3.3.1 Kapton Characterization Process ………………………………………31
3.3.2 Mylar Test Structure Preparation …………………………………....…33
3.3.3 Mylar Polarization Measurements …………………………………...…36
3.4 Conclusion ………………………………………………………………………38
Chapter 4: Thin Film Test for Deformation Tolerance ………………………………..…..……39
4.1. Introduction …………………………………………………………………..…39
4.2. Stepper Motor Set-up ……………………………………………………………39
4.3. Results and Discussions …………………………………………………………42
4.3.1. ALD ZnO ………………………………………………………….…42
4.3.2. ALD Al2O3 …………………………………………………………...43
4.3.3. PVDF/TrFe ………………………………………………………...…44
Chapter 5: Microwave Phase Shift measurements on Flexible Substrates ……………………...46
5.1. Introduction …………………………………………………………………..…46
5.2. Microwave Testing and Device Test Set-up …………………………………..46
5.3. Fabrication of Microwave Test Structures …………………………………..…48
5.4. Microwave Measurements ……………………………………………….…...…50
5.4.1.
CPW measurements without Phase Shifting materials ………….…51
5.4.2.
ZnO on Flexible Substrates …………………………………….......55
viii
5.4.2.1. ZnO on Kapton Substrate ………………………....…55
5.4.2.2.
ZnO on Mylar Substrate ………………………………57
5.4.3. Unpoled PVDF/TrFe Films …………………………………………..…58
5.4.3.1.
Unpoled PVDF/TrFe and Mylar Substrates ……….…58
5.4.4. Poled PVDF/TrFe Films on Mylar ………………………………….….62
5.5. Conclusion ………………………………………………………………………67
Chapter 6: Conclusion and Future Work .......................................................................................68
Appendix – A …………………………………………………………………….……………...69
Bibliography ..................................................................................................................................73
Vita
ix
LIST OF TABLES
Table 1.1 Flexible substrate selection table ……………………………………………….…….5
Table 2.1 Typical ALD Program ……………………………………………………………....12
Table 2.2 ALD Film Deposition Conditions …………………………………………………...12
Table 2.3 ALD measured Parameters and tools …………………………………………….…13
Table 2.4 Measured data categorization ………………………………………………………16
Table 4.1 Linear Actuator Specifications …………………………………………...…………40
Table 4.2 Tested Substrates and Thin Films ………………………………...…………………42
x
LIST OF FIGURES
Fig. 1.1
Research survey market report for Silicon semiconductor and Printed
Electronics …………………………………………………………………………….1
Fig. 1.2
Flexible Electronics applications ……………………………………………………2
Fig. 1.3
Research report on Flex electronics market share …………………………………….2
Fig. 2.1
Step Coverage and Deposition rate for different deposition techniques ……………...8
Fig. 2.2
Atomic Layer Deposition technique acceptable temperature window ……………......8
Fig. 2.3
Complete ALD cycle of Al2O3 ………………………………………………………..9
Fig. 2.4
Silicon wafer arrangement in ALD reactor chamber …………………………….......11
Fig. 2.5
ALD Film characterization tools : (a) Ellipsometer, (b) Thin Film Stress
mesasurement tool, (c) Atomic Force Microscope, (d) Contact Angle
Measurement Tool, (e) Scanning Electron Microscope …………………………….13
Fig. 2.6
Thickness measurement locations on a silicon wafer ………………………………..14
Fig. 2.7
Film Thickness as a function of Platen location for Al2O3 of thickness 0.13µm …...14
Fig. 2.8
Film Thickness as a function of Platen location for Al2O3 of thickness 0.29µm ……15
Fig. 2.9
Film Thickness as a function of Platen location for ZnO films of
thickness 0.11µm ……………………………………………………………………15
Fig. 2.10 Film Thickness as a function of Platen location for ZnO of thickness 0.26µm …..…15
Fig. 2.11 Film stress of as-deposited Al2O3 films of thicknesses 0.13 µm and 0.29 µm ….…..17
Fig. 2.12 Film stress of as-deposited ZnO films of thicknesses 0.11 µm and 0.26 µm ….…….17
Fig. 2.13 Surface Roughness of as-deposited Al2O3 thin films …………….…………………18
Fig. 2.14 Surface Roughness measurements of as-deposited ZnO thin films .…………………19
Fig. 2.15 Contact angle measurements of as-deposited Al2O3 thin films ……………………..19
xi
Fig. 2.16
Contact angle measurements of as-deposited ZnO thin films ………………………20
Fig. 2.17 Roughness variation of ZnO films as a function of annealing temperature ………...20
Fig. 2.18
Thin Film Stress with varying temperatures of Al2O3 thin films ……………………21
Fig. 2.19
Thin Film Stress with varying temperatures of ZnO thin films ……………………..22
Fig. 2.20
Contact angle measurements with varying temperatures of Al2O3 thin films ………23
Fig. 2.21
Contact angle measurements with varying temperatures of ZnO thin films ………..23
Fig. 2.22
Top view SEM Micrographs of annealed 0.11 µm ZnO films ……………………...24
Fig. 2.23
Cross section view Micrographs of annealed 0.11 µm ZnO films ………………….25
Fig. 2.24
Top view SEM Micrographs of annealed 0.26 µm ZnO films ……………………...26
Fig. 2.25
Cross section view Micrographs of annealed 0.26 µm ZnO films ………………….27
Fig. 3.1
Ferroelectric material (a) Unpoled sample, (b) Poled sample ………………………28
Fig. 3.2
Polarization Response of a linear dielectric material, and the hysteresis of a
ferroelectric material ……………………………………………………………...…29
Fig. 3.3
PVDF molecular structure …………………………………..………………………30
Fig. 3.4A Flexible Kapton Substrate ……………………….…………………..………………32
Fig. 3.4B Bottom Electrode deposition …………………….…………………..………………32
Fig. 3.4C Spin Coat PVDF/TrFe …………………….………………………...………………32
Fig. 3.4D Top Electrode deposition VDF/TrFeBottom Electrode deposition …………………33
Fig. 3.5
Top view of an annealed PVDF film on a kapton substrate ………………………...33
Fig. 3.6A Mylar Substrate with double side metal layer ……………………………………….34
Fig. 3.6B Substrate after Back side metal removal …………………………………………..…34
Fig. 3.6C Spin coated PVDF/TrFe ……………………………………………………….……..35
xii
Fig. 3.6D Top Electrode deposition ……………………………………………………….……35
Fig. 3.7
Top view of annealed PVDF film on Mylar …………………………………………35
Fig. 3.8
Top view of annealed PVDF film on Mylar substrate …………………….…………36
Fig. 3.9
Polarization measurements of PVDF on Mylar- Spin coated at 3000 rpm .…………37
Fig. 3.10 Polarization measurements of PVDF on Mylar- Spin coated at 2000 rpm ….………37
Fig. 4.1
Linear Actuator ………………………………………………………………………40
Fig. 4.2
Wiring Diagram of Driver/Pulse generator ………………………………………….41
Fig. 4.3
ZnO film tested on Mylar substrate …………………………………………….……42
Fig. 4.4
ZnO film tested on Kapton substrate ………………………………………….……..42
Fig. 4.5
Al2O3 film tested on Mylar substrate ..............……………………………………….43
Fig. 4.6
Al2O3 film tested on Kapton substrate ………………………………………….........43
Fig. 4.7
PVDF film tested on Mylar substrate ………………………………………..……....44
Fig. 4.8
PVDF film tested on Kapton Substrate …………….…………………….……….....44
Fig. 5.1
Microwave measurement equipment: Vector Network Analyzer (VNA)
and Probe station…………………………………………………………………..…47
Fig. 5.2
Fabrication of Microwave Test Structure with ZnO …………………………..……48
Fig. 5.3
Fabrication of Microwave Test Structure with PVDF/TrFe …………………...…….49
Fig. 5.4
Optical Images of fabricated Coplanar waveguides with (a) Zinc Oxide, and (b)
PVDF/TrFe …………………………………………………………………………..50
Fig. 5.5
Fabricated Coplanar waveguides on (a) Kapton and , (b) Mylar ……………………51
Fig. 5.6
Propagation constant of coplanar waveguide on Mylar …………………………… .52
Fig. 5.7 Characteristic Impedance of coplanar waveguide on Mylar …………………........…52
Fig. 5.8 Propagation constant of coplanar waveguide on Kapton …………………………….53
xiii
Fig. 5.9
Characteristic Impedance of coplanar waveguide on Kapton ……………………..53
Fig. 5.10 Phase, S21 of Characteristic Impedance of coplanar waveguide on Kapton ………..54
Fig. 5.11 Phase, S21 of Characteristic Impedance of coplanar waveguide on Kapton ………..54
Fig. 5.12 Coplanar waveguide with ZnO on Kapton …………………………………………55
Fig. 5.13 Phase S21, of ZnO on Kapton substrate …………………………………………….56
Fig. 5.14 Magnitude S21, of ZnO on Kapton substrate ……………………………………….56
Fig. 5.15 Magnitude S21, of ZnO on Mylar substrate ………………………………………..57
Fig. 5.16 Phase S21, of ZnO on Mylar substrate ……………………………………………...57
Fig. 5.17 Phase S21, Unpoled PVDF on Myalr, Annealed 50°C, 30 min …………………….59
Fig 5.18 Magnitude S21, Unpoled PVDF on Myalr, Annealed 50°C, 30 min ………………..59
Fig 5.19 Phase S21, Unpoled PVDF on Mylar, Annealed 50°C, 60 min ……………………..60
Fig. 5.20 Magnitude S21, Unpoled PVDF on Myalr, Annealed 50°C, 60 min ………………….60
Fig. 5.21 Phase S21, Unpoled PVDF on Mylar, Annealed 70°C, 30 min ……………………….61
Fig. 5.22 Magnitude S21, Unpoled PVDF on Myalr, Annealed 70°C, 30 min ………………….61
Fig. 5.23 Poling set-up for Mylar substrates ……………………………………………………62
Fig. 5.24 Phase S21, Poled PVDF on Mylar @ 70°C, 30 min, 25V/µm ………………………..64
Fig 5.25 Magnitude S21, Poled PVDF on Mylar @ 70°C, 30 min, 25V/µm …………………64
Fig 5.26 Phase S21, Poled PVDF on Mylar @ 80°C, 30 min, 25V/µm …………………..……65
Fig. 5.27 Magnitude S21, Poled PVDF on Mylar @ 80°C, 30 min, 25V/µm ………………...65
Fig. 5.28 Phase S21, Poled PVDF on Mylar @ 90°C, 30 min, 25V/µm …………………..….66
Fig. 5.29 Magnitude S21, Poled PVDF on Mylar @ 90°C, 30 min, 25V/µm ………………...66
xiv
CHAPTER ONE: INTRODUCTION
1.1 Introduction
While most of the electronic components are fabricated on widely employed hard
substrates such as silicon, the performance of silicon technology is not sufficient to satisfy the
requirements of novel technologies like flexible electronics where device flexing plays a vital
factor for its functionality. Therefore, novel materials which are compatible with present day
technologies are being constantly explored to meet the ever increasing demands for civilian and
military applications. One solution involves flexible substrates which have numerous advantages
compared to its silicon or GaAs counterparts, including lower cost, light weight, microfabrication compatibility and above all the flexing advantage [1-5]. Flexible electronics have
shown a great potential in the field of semiconductor technology in fabrication of devices such as
sensors [6-7], displays [8-9], and transducers [10] and research is being actively conducted in
other areas including radio frequency communications[11], energy harvesting [12] [13], and
textiles [14], to name a few.
As shown in Figure 1.1, a research survey by IDTechEX [15] indicates that printed
Fig 1.1 Research survey market report for Silicon semiconductor and Printed Electronics [15]
15
electronics will represent a larger market than silicon technology in the upcoming years. Figures
1.2 and 1.3 shows the potential areas in which flexible electronics can be employed and also the
market share predictions for the near future.
Fig 1.2 Flexible Electronics applications
Fig 1.3 Research report on Flexible electronics market share [15].
Flexible electronics employ multiple techniques for device fabrication, namely, (i) Ink Jet
printing: which employs a typical inkjet printer with print heads, in which ink is sprayed through
16
the nozzles on the substrates. It is a versatile and affordable technique, that is compatible with
almost any kind of flexible substrate. (ii) Screen printing which is a technique that employs a
porous woven mesh to support stencils that blocks the ink. Openings on the stencil allow the ink
to be transferred to the substrate. A variety of substrates can be employed, and its main limitation
is that very thin films cannot be processed by this technique. (iii) Nanoimprint Lithography:
Nanoscale patterns can be fabricated with low cost, high throughput and high resolution. The
patterns are created by the mechanical deformation of imprint resist and other processes. (iv)
Conventional microfabrication methods such as thin film deposition, photolithography could be
integrated to fabricate devices on flexible substrates for which an extensive characterization has
to be performed in the selection of substrates, thin film materials, and fabrication processes.
Although there are many potential applications for Flexible electronics, relatively little work
has been done with respect to Microwave applications on flexible substrates. Microwave devices
like radars, for instance, would benefit the military and could also have commercial applications
due to the aforementioned advantages. However, for radars to be fabricated, the principle of
phase shifting has to be verified on these substrates. The following section describe the
dissertation objectives for the fabrication of a microwave test structure on Flexible substrate for
Phase shift performance.
1.2 Dissertation Objective
The main objective of this research is to integrate a ferroelectric material on a flexible
substrate to fabricate a microwave device which can be tested for phase shifting performance.
Therefore, the work described in this report is divided in three phases:
Phase 1:
Selection of Flexible substrate, thin film materials and fabrication processes.
17
Phase 2:
Thin film characterization of the selected materials.
Phase 3:
Fabrication, characterization and testing of the phase shifting performance of the
Micro-flex circuits.
Phase 1:
Three parameters were considered in the selection of Flexible substrates, namely,
cleanroom processing compatibility, cost and dielectric properties of the substrate. As seen in
Table 1.1, of the mentioned substrates, Mylar satisfies all of the above conditions, while Kapton,
under some processing conditions could also be an acceptable option. Thus, during this exercise
Kapton was mostly employed to define processing conditions which were subsequently
transferred to Mylar substrates.
Additionally, the following materials were selected for producing the coplanar
waveguides and associated circuitry: (i) sputtered gold thin films for the fabrication of the
transmission lines, (ii) atomic layer deposition (ALD) of Aluminum oxide (Al2O3 ) to be used as
an insulator or even as an encapsulation layer, (iii) ALD Zinc oxide (ZnO) for self-polarization
options, and (iv) spin-casting of PVDF/TrFe (Polyvinylidene Fluoride/Trifluoroethylene) as a
ferroelectric material.
Zinc oxide is a piezoelectric material, therefore, a combination of ZnO and a ferroelectric
layer on substrate subjected to flexing or deformation could be useful in the demonstration of
self-polarizing circuit. However, this was solely considered as an alternative during the duration
of this exercise.
18
Table 1.1
Flexible substrate selection table
Phase 2:
An extensive thin film characterization was performed on the as-grown and annealed
atomic layer deposited films: ZnO and Al2O3; several parameters were measured and reported
herein, including film thickness, surface roughness, film stress, surface energy, and also film
morphology inspection employing scanning electron microscopy (SEM).
Phase 3:
The final phase included the fabrication, testing and characterization of Micro-Flex circuits
for phase shifting performance. The testing included poled and unpoled films. All circuits were
tested with a vector network analyzer (VNA) within the frequency range of 0.4 MHz to 40 GHz.
19
The phase shifting was measured by applying a floating potential to the ferroelectric films while
the coplanar waveguides were being frequency-swept in the VNA.
1.3 Dissertation Organization
The remaining of this dissertation is organized as follows:
Chapter 2 outlines the characterization of atomic layer deposited films of Aluminum Oxide
(Al2O3) and Zinc Oxide (ZnO); Chapter 3 contains the study of a ferroelectric polymer PVDF
which includes thin film preparation, and fabrication of the test device; Chapter 4 discusses the
deformation tolerance of three different thin films on flexible substrates; Chapter 5 describes the
fabrication and microwave performance of flexible microwave circuits; the conclusions and
future work are discussed in Chapter 6.
20
CHAPTER TWO: THIN FILM CHARACTERIZATION
2.1 Introduction
The ever increasing demand for portable devices has resulted in the downscaling of
electronic circuits and devices to the nano-scale regime which, in turn, has demanded advances
in materials science and fabrication equipment. Nano devices have a strong dependence on novel
materials and novel deposition methods, in fact, most devices currently available in the consumer
electronics market were not technologically feasible a few years ago. Case in point, circuits in
the nano-regime employ gate dielectrics like SiO2 with thickness ranging from 10 to 20Å,
however, such dimensions place stringent demands in terms of controlling both the uniformity
and the dielectric properties of the films involved. It is frequently reported that their performance
drops significantly by higher leakage currents due to electron tunneling [16-17]. However, the
aforementioned thicknesses are nowadays possible with the utilization of deposition techniques
such as atomic layer deposition which will be briefly described later in this chapter.
Aluminum oxide (Al2O3) is counted among the films that can be deposited employing
atomic layer deposition. Aluminum oxide is a potential high-k material which can be employed
for a variety of semiconductor devices such as a dielectric layer in thin film capacitors and
DRAM trench structures. In this exercise we investigated various properties including film
thickness, index of refraction, film stress (before and after annealing), surface energy and
microstructure evolution as a function of annealing temperature.
Atomic layer deposition is a technique capable of depositing film monolayers at
relatively lower temperatures. ALD deposited films have been described as having great
potential for RF-MEMS devices to enhance reliability by reducing stiction and dielectric
charging problems [18], for microelectronic devices [19] , gate dielectrics transition to high k
21
dielectric materials [20], DRAM capacitors [21], MOSFETs [22], flat panels [23], solar energy
devices [24] and magnetic read heads [25].
ALD has numerous advantages over other more frequently employed thin film deposition
techniques, such as Physical vapor deposition (PVD) or chemical vapor deposition (CVD), in
terms of large area uniformity, conformality over high aspect ratio structures, thickness control
in the atomic range and film depositions at relatively low temperatures. A succinct comparison
of these techniques with respect to step coverage and deposition rate is shown in Figure 2.1.
Fig. 2.1 Step Coverage and Deposition rate for different deposition techniques
Fig. 2.2 Atomic Layer Deposition technique acceptable temperature window [1]
22
Additionally, ALD has the advantage of enabling the deposition of a variety of oxide,
nitride and metallic films [26-27] at lower temperatures ranging from 400C to as little as 50C.
The typical ALD deposition cycle of Al2O3 is shown in Figure 2.3. ALD reactions are selfterminating reactions that take place in a reactor at pressure ranging from 0.1 to 1 Torr. Source
gases known as precursors are the heart of the ALD process. They must be volatile and thermally
stable with relatively high vapor pressures for efficient transport of the reactants inside the
reactor to deposit one monolayer at a time. Precursors employed to deposit Al2O3 films are
Tetramethylaluminum (TMA) and water vapor in the temperature window of 45C-200C [2832]. A single cycle consisting of four steps, typically suffice to deposit a monolayer of ~1.1Å.
Below is the detailed description of one deposition cycle of Al2O3 :
Fig. 2.3 Complete ALD cycle of Al2O3
STEP 1:
Trimethyl Aluminum (TMA) is pulsed for 1.5 msec in the reactor onto a hydrated
silicon substrate, that is, a surface coated with hydroxyl radicals:
23
Si-O-H. The hydroxyl radicals react with aluminum-mehtyl radicals (Al(CH3)2),
which chemisorb on the surface producing methane molecules (CH4) as a reaction
byproduct. This reaction lasts until the surface is completely passivated.
Al (CH 3 ) 3( g )  : Si  O  H : Si  O  Al (CH 3 ) 2 ( s )  CH 4
STEP 2:
Subsequently the reaction chamber is purged with Nitrogen gas which is pulsed
for 1.5 msec. At this point the reaction byproducts (CH4) as well as any unreacted
TMA molecules are removed from the reaction chamber.
STEP 3:
Water vapor is then pulsed into the reactor for 1.5 msec. The water molecules
react with the dangling aluminum-methyl groups forming Aluminum-oxygen (AlO) bridges and additional reaction byproducts (CH4) are desrbed.
2H2O : Si  O  Al(CH3 )2( s) : Si  O  Al(OH)2( s)  2CH4
STEP 4:
Finally, the reaction chamber is purged with Nitrogen gas which is pulsed for 1.5
msec removing all reaction byproducts and finalizing the cycle for one complete
monolayer.
24
2.2 Thin Film Preparation
Prior to deposition, the silicon wafers were cleaned thoroughly and hydrated by the
following procedure:
 Piranha Clean: The wafers were cleaned in a mixture of 2:1 parts of Sulphuric acid
(H2SO4) and Hydrogen Peroxide (H2O2) for a duration of 8 minutes to remove any
organic contaminants on the surface. The wafers were subsequently rinsed employing
de-ionized water (DI water).
 Native Oxide removal: The native oxide was removed by immersing the wafers in a
mixture of 50:1 parts of Hydrogen Fluoride (HF) and De-ionized (DI) water for 10
seconds. Once more, the wafers were subsequently rinsed employing de-ionized water
(DI water).
Upon wafer preparation, for each run four 3” silicon wafers were placed in the ALD reactor
chamber (see Figure 2.4).
Fig. 2.4 Silicon wafer arrangement in ALD reactor chamber.
ALD films of Al2O3 with thicknesses of 0.13μm and 0.29μm, as well ZnO films with
thicknesses of 0.12μm and 0.26μm were deposited at 100ºC. Diethyl Zinc (DEZ) was the
precursor employed for ZnO and TMA for Al2O3 deposition. Table 2.1 shows a typical ALD
program for film deposition, in which the ‘Goto’ command defines the number of deposition
25
cycles and, therefore, controls the film thickness to be deposited. Table 2.2 shows the film
deposition conditions for Al2O3 and ZnO films. The observed growth rate for Al2O3 films was
0.97Å/cycle and 1.45 Å/cycle for ZnO films.
Table 2.1
Typical ALD Program
Typical ALD Program
0
Wait
600s
1
Precursor :
TMA DEZ or Water vapor
0
2
Wait
5s
3
Purge :Nitrogen
1s
4
Wait
5s
5
Goto
1s
6
Flow
0
7
Heater
100C
8
Heater
100C
0.0015s
0.0015s
801s
Table 2.2
ALD Film Deposition Conditions
Film Deposition Conditions
Temp
# of Cycles
Precursors
Run # 1
100 C
1375
TMA + water vapor
Run # 2
100 C
2925
TMA + water vapor
Run # 3
100 C
801
DEZ + water vapor
Run # 4
100 C
2040
DEZ + water vapor
26
2.3 Thin Film Analysis
Upon film deposition, the characterization effort was undertaken collecting the measurements
indicated in Table 2.3 and employing the metrology tools shown in Figure 2.5.
Table 2.3
ALD measured Parameters and tools
Parameter
Film Thickness
Film Stress
Surface Roughness
Contact Angle
Micrographs
(a)
Variable
T
σStress
SRoughness
Contact Angle
Tool
Ellipsometer
Thin Film Stress Measurement Tool
Atomic Force Microscope
Contact Angle Measurement Tool
Scanning Electron Microscope
(b)
(c)
(e)
(d)
Fig. 2.5 ALD Film characterization tools : (a) Ellipsometer, (b) Thin Film Stress Measurement
tool, (c) Atomic Force Microscope, (d) Contact Angle Measurement Tool, (e) Scanning Electron
Microscope
2.3.1
Film Thickness
The thickness and refractive index of ALD films on silicon wafers were measured by
ellipsometry, which is a non-destructive optical technique. The measurements were taken at five
different locations on each wafer (see Figure 2.6), to assess film uniformity.
27
1
4
5
2
3
Fig. 2.6 Thickness measurement locations on a silicon wafer.
The deposited films were observed to have a non-uniformity variation of no more than
4.5%, for all depositions. Additionally, it was found that wafers placed towards the outlet valve
of the chamber exhibited better uniformity when compared to those placed closer to the inlet
valve for both films. This effect is thought to be due to the expansion at the inlet with the
concomitant cooling effect, and the compression at the outlet with the associated heating effect
of the precursors as they are pulsed into the deposition chamber. This molecular energy variation
is thought to improve the mobility of the physisorbed species closer to the outlet and, therefore,
improving film uniformity. The collected film thickness data is shown in Figures 2.7 through
2.10 and the average thickness of the deposited films is shown in Table 2.4.
Fig. 2.7 Film Thickness as a function of Platen location for Al2O3 of thickness 0.13µm
28
Fig. 2.8 Film Thickness as a function of Platen location for Al2O3 of thickness 0.29µm
Fig. 2.9 Film Thickness as a function of Platen location for ZnO of thickness 0.11µm
Fig. 2.10 Film Thickness as a function of Platen location for ZnO of thickness 0.26µm
29
Table 2.4
Measured data categorization
2.3.2
Run # 1
Al2O3 : 0.13 µm
Run # 2
Al2O3 : 0.29 µm
Run # 3
ZnO : 0.11 µm
Run # 4
ZnO : 0.26 µm
Film Stress
Film stress was measured employing a Flexus 2320 tool. This is also a non-destructive
metrology technique that employs dual lasers to measure the change in the radius of curvature
of a substrate due to the presence of a thin film. For this purpose, pre-deposition and postdeposition measurements were taken before and after film deposition. Film stress is calculated
from the following equation:

E
 h

  
(
1

v
)

 6 Rt
where,
σ
= Film stress (Pa)
E/(1- ν) = Biaxial elastic modulus of the substrate (1.805x 1011Pa for 100 Si)
h
= Substrate thickness (in microns)
t
= Film thickness (in microns)
R
= Substrate radius of the curvature (in microns)
30
ALD Al2O3 and ZnO thin films deposited at 100 °C displayed a tensile stress that did not
exceed +400 MPa and +230 MPa respectively (see Figures 2.11 and 2.12). The critical value for
the onset of delamination is known to be +600MPa, thus, film delamination was not anticipated
to be a concern for microfabrication purposes.
Fig. 2.11 Film stress of as-deposited Al2O3 films of thicknesses 0.13 µm and 0.29 µm.
Fig. 2.12 Film stress of as-deposited ZnO films of thicknesses 0.11 µm and 0.26 µm.
31
2.3.3
Surface Roughness
Surface roughness may interfere with microwave measurements in two ways, namely, it
may preclude good contact of the three-pronged vector network analyzer (VNA) probes and it
can promote electromagnetic interference with the applied signal. However, the measured
surface roughness of Al2O3 films (see Figure 2.13) did not exceed 5 Å (which is comparable to
the surface roughness of silicon wafers), while the collected values for ZnO films (see Figure
2.14) did not exceed 19 nm. These values were considered to be sufficient low not to preclude
the correct utilization of the microwave probes, nor interfere with the frequencies of the
electromagnetic signals employed.
Fig. 2.13 Surface Roughness of as-deposited Al2O3 thin films.
32
Fig.2.14. Surface Roughness measurements of as-deposited ZnO thin films.
2.3.4
Contact Angle
Contact angle measurements are relevant because hydrophobic surfaces recurrently
interfere with spin-casting photolithography processes. To this end, a VCA Optima tool was
employed to measure the contact angle of de-ionized water on the ALD film’s surface. An
approach comparable to that utilized for film thickness was utilized, namely, contact angle
measurements were made at five different locations on each wafer. Both films displayed contact
angles that did not exceed 63 (see Figures 2.13 and 2.14) and, therefore, were considered to be
suitable for cleanroom processing without further concerns.
Fig. 2.15 Contact angle measurements of as-deposited Al2O3 thin films.
33
Fig. 2.16 Contact angle measurements of as-deposited ZnO thin films.
2.3.5
Thin Film Annealing:
The thermal treatment of the produced films was carried out by employing a Branstead
Furnace to anneal the ALD films to 900 ºC in steps of 100 ºC in an air ambient. The annealing
was carried out during 60 minutes, and subsequently the samples were analyzed following
protocols comparable to those employed for the as-deposited samples.
2.3.5.1 Surface Roughness
Fig. 2.17 Roughness variation of ZnO films as a function of annealing temperature.
34
While annealed ZnO films of thickness of 0.11 µm did not show a significant difference
in the surface roughness (see Figure 2.17), the roughness of ZnO films of thickness 0.26 µm,
decreased from ~20 nm (as-deposited) to ~12 nm after annealing at 900ºC. We believe that this
variation is due to the noticeable difference in grain size between the films which was observed
during SEM inspection. Additional X-Ray Diffraction analysis would help understand the nature
of this behavior, however, at the time of this dissertation the XRD tool at UTSA was not
available.
2.3.5.2 Thin Film Stress
The Flexus 2320 tool has the capability of measuring real-time stress of thin films for
temperatures up to 400ºC. The thermal energy provided during the measurement is sufficient in
many cases to relax films stress, however, the main concern in this exercise was not to reach or
exceed a tensile stress of +600MPa to preclude delamination. The observations indicated that
film stress after the thermal treatment did not exceed +350 MPa for Al2O3 films and +250 for
ZnO films, therefore, process flows involving these films may incorporate thermal treatments.
Fig. 2.18 Thin Film Stress with varying temperatures of Al2O3 thin films.
35
Fig. 2.19 Thin Film Stress with varying temperatures of ZnO thin films.
2.3.5.3 Contact Angle
There were important contact angle observations made on annealed films. In the case of Al2O3
ALD films, thermal treatments up to 300ºC of the thicker films (0.29 m) increased the
hydrophobicity of the surface to values closer to 105 that is predicted to make spin-casting
difficult while thinner films (0.13 m) will not interfere with such microfabrication processes. A
similar but more pronounced behavior was observed on ZnO films for which the hydrophobicity
was observed to exceed 120 for thicker films (0.26 m) annealed around 400ºC and relaxing to
80º for an annealing temperature of 900ºC. Thus, unless special precautions are taken, ZnO films
annealed at 400ºC hamper spin-casting photolithographic processes.
36
Fig. 2.20Contact angle measurements with varying temperatures of Al2O3 thin films.
Fig. 2.21 Contact angle measurements with varying temperatures of ZnO thin films.
2.3.5.4 Micro-Structure Inspection
The top-view SEM micrographs of the as-deposited films indicate that the grain size has a strong
dependence on films thickness (see Figures 2.22A and 2.24A), thus, the rice-like grains are
relatively smaller for thinner films. Due to the relatively low temperature employed during
deposition the expectation was to observe fully amorphous films. However, the columnar nature
of the deposited films can be ascertained in the cross-section micrographs (see Figures 2.23A
37
and 2.25A), whereas the boundaries are not discernible in those micrographs. This seems to
indicate that the bulk of the films is mostly amorphous.
`
(A) As Dep.
(B) 100 °C
(C) 200 °C
(D) 300 °C
(E) 400 °C
(F) 500 °C
(G) 600 °C
(H) 700 °C
(I) 800 °C
(J) 900 °C
Fig. 2.22 Top view SEM Micrographs of annealed 0.11 µm ZnO films.
38
During the thermal treatment there were two important variations observed in the microstructure,
namely, the grains increased in size and lost the rice-like appearance (see Figures 2.22J and
2.24J) becoming more polyhedra-like. We anticipate that a subsequent X-Ray Diffraction
analysis will help understand the evolution of these films when subjected to the thermal
treatment described herein.
(A) As Dep.
(B) 100 °C
(C) 200 °C
(D) 300 °C
(E) 400 °C
(F) 500 °C
(G) 600 °C
(H) 700 °C
(I) 800 °C
(J) 900 °C
Fig. 2.23 Cross section view Micrographs of annealed 0.11 µm ZnO films.
39
(A) As Dep.
(B) 100 °C
(C) 200 °C
(D) 300 °C
(E) 400 °C
(F) 500 °C
(G) 600 °C
(H) 700 °C
(I) 800 °C
(J) 900 °C
Fig. 2.24 Top view SEM Micrographs of annealed 0.26 µm ZnO films.
40
(A) As Dep.
(B) 100 °C
(C) 200 °C
(D) 300 °C
(E) 400 °C
(F) 500 °C
(G) 600 °C
(H) 700 °C
(I) 800 °C
(J) 900 °C
Fig. 2.25 Cross section view Micrographs of annealed 0.26 µm ZnO films
2.4 Conclusion
Thus, the characterization of atomic layer deposited Al2O3 and ZnO has been carried out, their
microfabrication compatibility has been assessed and the collected information will be employed
in subsequent chapters.
41
CHAPTER THREE: PVDF FILM CHARACTERIZATION
3.1 Introduction
Ferroelectricity is the spontaneous electric polarization of a material that can be reversed
by an application of an external electric field. Most ferroelectric materials are available in the
form of ceramics such as PbTiO3 and BaTiO3 based on titania compounds with a perovskite
structure. These materials encounter a phase transition at a critical temperature from a
centrosymmetric non-polar lattice to a non-centrosymmetric polar lattice. The polarization of
these materials can be made more homogenous by performing a procedure called poling, which
is the application of a DC electric field for a certain period of time and normally supplying heat
to give the molecules energy to reorient themselves (see Figure 3.1).
Fig. 3.1 Ferroelectric material (a) Unpoled sample, (b) Poled sample
Ferroelectric materials are also known as non-linear dielectrics because upon the
application of an electric field to the material, the resulting charge response is non-linear in
nature. In the case of a non-ferroelectric, but dielectric material the polarization is solely present
when an electric field is applied and vanishes when the electric field is removed (see Figure 3.2
42
for an example of a linear dielectric). In contrast, the behavior of a ferroelectric material exhibits
a well-defined hysteresis loop, namely, the application of an electric field promotes a molecular
alignment such that the material remains polarized upon the removal of the electric field (see
Figure 3.2, first quadrant). The polarization can then be canceled or reversed upon the
application of a reversed electric field as can be observed in quadrants II and III in Figure 3.2.
Several important reference points in the hysteresis loop include,
Polarization Saturation, PSat: It is a point at which there is no change in Polarization with
increase in electric field.
Remnant Polarization, PRem: Left over polarization in the material when no electric field is
applied
Coercive Field, EC
: Required electric field to achieve the state of zero polarization.
Fig. 3.2 Polarization Response of a linear dielectric material, and the hysteresis of a ferroelectric
material.
43
3.2 PVDF/TrFe
Several research groups have reported on a variety of brittle ferroelectric materials such
as ceramics for various device applications [33] and, more recently polymeric ferroelectric films
[34-36]. However brittle materials may not be compatible with flexible substrates. A
combination of a polymer [37] with good mechanical properties on flexible substrates would be
an optimal solution for reliable flexing devices. Thus, a polymer with a co-polymer such as
PVDF/TrFe (Polyvinylidene fluoride/Trifluroethylene) which is known have ferroelectric
properties was considered a viable option for this exercise. PVDF/TrFe results in a crystallinity
of around 80-90% after annealing. It has been reported that if the TrFe content is greater than
25%, then PVDF/TrFE copolymer chain formation would be of β-PVDF resulting in higher
piezoelectric response due to higher crystallinity [1] (see the Molecular structure of PVDF/TrFe
in Figure 3.3). Therefore, PVDF/TrFe consisting of 30% TrFe by weight was purchased from
Solvay Solexis, and employed in all subsequent experiments.
Fig. 3.3 PVDF molecular structure
44
3.2.1
PVDF/TrFe Solution Preparation
3.75 grams of P(VDF-TrFe) powder were mixed with Methyl-Ethyl-Ketone (MEK) solvent,
and stirred for a duration of 24 hours using magnetic stirring [38]. To increase the dissolution
of the polymer granules, the viscous solution was heated in a furnace at 70ºC during 30
minutes. Sufficient solution was prepared so that it could be used multiple times.
Subsequently the solution was spin-cast at various speeds, first on kapton to characterize the
process which was then transferred to mylar substrates.
3.3 Test Set-up Fabrication and Polarization Measurements
3.3.1
Kapton Characterization Process
Polarization test structures were fabricated on kapton in order to verify the polarization of
PVDF thin films. As it was previously mentioned, the process was characterized on kapton (see
Figure 3.4) and then transferred to mylar substrates (see Figure 3.6).
(i.) The fabrication process flow started with the sputtering of a gold film with a thickness
of 3000 Å to serve as the bottom electrode. The sputtering was performed employing a
Cressington Table top tool (see Figure 3.4B).
(ii.) This step was followed by the spin-casting of the PVDF/TrFe solution using spin
speeds of 2000 rpm and 3000 rpm respectively obtaining thicknesses of 1.2 µm and 0.8 µm. The
films were then annealed in a furnace at 70ºC for duration of 30 minutes (see Figure 3.4C) and
SEM inspected (see Figure 3.5). The annealed PVDF film surface is porous with a discernible
pore size of ~5µm.
(iii.) The top electrode of gold with a thickness of 3000 Å was also sputtered employing a
shadow mask with an opening of 1mm x 1mm. In the final step, silver wires were connected to
45
the bottom and top electrodes with silver paint, and allowed to dry for 30 minutes (see Figure
3.4D). The silver wires were needed to connect the sample to the CV-plotter to collect the
polarization data.
Upon the successful preparation of the samples, the process was then transferred to mylar
substrates.
Fabrication Process Flow : KAPTON
Kapton
Fig. 3.4A Flexible Kapton Substrate
Kapton
Fig. 3.4B Bottom Electrode deposition
Kapton
Fig. 3.4C Spin Coat PVDF/TrFe
46
Kapton
Fig. 3.4D Top Electrode deposition
Kapton
Gold
PVDF/TrFe
Silver
Fig. 3.5 Top view of an annealed PVDF film on a kapton substrate.
3.3.2
Mylar Test Structure Preparation.
The Mylar substrates were received with a double sided silver coating (see Figure 3.6A).
Thus, an additional step was introduced to chemically remove the metallization on one side of
the substrate. This was accomplished by immersing the sample in a 6:1 Buffered Oxide solution,
NH4OH:HF for a duration of 6 seconds (see Figure 3.6B). During this step, the opposite
47
metallized side was protected with a thin positive photoresist layer which was subsequently
removed with acetone.
The PVDF/TrFe film was then spin-cast, annealed (see Figure 3.6C), SEM inspected (see
Figures 3.7 and 3.8); subsequently the top gold electrode was sputtered and the silver wires
attached (see Figure 3.6D).
Upon the completion of the test structure preparation, the PVDF/TrFe films were poled
[38-39] by applying a DC voltage of 250 kV/cm at a temperature of 95 °C for 30 minutes. After
the poling step, the top and bottom electrodes were short circuited for 12 hours to drain out any
residual charges.
Once the above procedures were completed, the structures were tested while capturing
the response employing a CV-plotter, from which it is possible to ascertain the polarization
measurements of the ferroelectric material.
Fabrication Process Flow: MYLAR
Mylar
Fig. 3.6A Mylar Substrate with double side metal layer
Mylar
Fig. 3.6B Substrate after Back side metal removal
48
Mylar
Fig. 3.6C Spin coated PVDF/TrFe
Mylar
Fig. 3.6D Top Electrode deposition
Fig. 3.7 Top view of annealed PVDF film on Mylar
49
Fig. 3.8 Top view of annealed PVDF film on Mylar substrate
3.3.3 Mylar Polarization Measurements
Measurements were taken for PVDF films spin coated at 2000 rpm and 3000 rpm
respectively, which produced films of thicknesses 1.2 µm and 0.8 µm.
The values of the remnant polarization (PRem) were extracted from the measured data for
the PVDF thicknesses of interest. The measured Remnant polarization data was 20.5 µC/cm2 for
the thinner PVDF films spin cast at 3000 rpm, and 40.4 µC/cm2 for those spin cast at 2000 rpm.
Higher values of remnant polarization, PRem, may be due in part to the porous films obtained
after annealing, which tend to release the tensile stress[40-42] of the films leading to higher PRem
values.
50
Fig. 3.9 Polarization measurements of PVDF on Mylar- Spin coated at 3000 rpm
Fig. 3.10 Polarization measurements of PVDF on Mylar- Spin coated at 2000 rpm
51
3.4 Conclusion
Ferroelectric measurements of PVDF/TrFe polymer have been performed and verified on
mylar. The observed remnant polarization values of 40.4 µC/cm2 for films of thickness 1.2 µm
are considered a strong indication that phase shifting on mylar can be demonstrated.
52
CHAPTER FOUR: THIN FILM TEST FOR DEFORMATION
TOLERANCE
4.1. Introduction
With an ever increasing demand for light weight and compact devices, there has been
great interest shown in the area of flexible substrates. Some of the advantages of these substrates
are light weight, affordability compared to silicon substrates, compatibility with a number of
standard micro-fabrication techniques, and the possibility to take them to mass production. Some
of these substrates are currently employed in various commercial applications including RFID
tags [43], e-paper [44], Displays [45-46], health care [47] and Solar cells [48] among others.
Flexible devices may comprise various thin films such as metals, dielectrics, polymers in the
micro and nano scale regime, and can be exposed to multiple degrees of movements. Thus, it is
important to have an indication of the tolerance of these films to the deformation normally
encountered when flexing such substrates.
Thus, a study of mechanical reliability of the films was carried out employing a linear
actuator stepper motor which in conjunction with a controller was employed for substrate
flexing. The flexing tolerance was corroborated employing optical inspection.
4.2. Stepper Motor Set-up
A stepper motor is a transducer that converts electrical pulses into mechanical motion,
and has an accurate precise positioning and high reliability. Moreover, it has a wide range of
rotational speeds, is relatively inexpensive, and simple to operate as it works on an Open Loop
operation, where no feedback information about the position is required. With the aforementioned
53
advantages in mind, a stepper motor with a linear actuator was adopted to verify the thin film
repetitive deformation tolerance.
A linear Actuator (model TSMCA42 Series, see Figure 4.1) was purchased from Anaheim
Automation Inc. This permanent-magnet stepper motor internally converts the rotary motion to a
linear motion via a rotating nut and lead screw, and is interfaced to a programmable pulse
generator (MBC25P11). The technical specifications of the linear actuator are listed in Table 4.1.
Table 4.1
Linear Actuator Specifications
Max Pull-in Rate (Steps/sec)
Power Consumption
Insulation Resistance
Bearings
Weight
Operating Temperature Range
Storage Temperature Range
Linear Travel Per Step (in)
Maximum Travel (in)
Bipolar Inductance Per Phase (mH)
Bipolar Series Current (A)
300
10 Watts
20 M-ohms
Radial Ball
5.5 oz (156
gm)
-20 ºC to 70 ºC
-40 ºC to 85 ºC
0.004
0.95
14.8
70
Fig. 4.1 Linear Actuator
The wiring diagram of the driver/pulse generator is shown in Figure 4.2. , where the wire
leads were soldered onto each pin and a breadboard was used to interface to the MBC25P11 and
the power with the motor. The input voltage is rated from 12V to 35V and a DC power supply
from Intertex Electronics provided a 12V input to the pin 2 in the connector P2. The pin 4 or the
“Direction In” pin of connector P2 was wired to be controlled manually by SW9, and the lead is
connected to a 5V square wave from a function generator at any desired frequency. The on-board
potentiometer R-29 is set to a minimum of 0.5A of output current, and the reduced output current
54
option was programmed in SMPG10WIN software allowing only 70% of the set current to be
output. These low current options were employed because a high torque was not
Fig. 4.2 Wiring Diagram of Driver/Pulse generator.
critical to deform the device. In fact, even with all the low current options in effect, the motor
becomes notably hot after 10 minutes of consecutive motion, and therefore a fan or heat sink
would be beneficial for extended periods of operation. The table below shows the various
flexible substrates and thin films that were tested.
Table 4.2
Tested Substrates and Thin Films
Flexible Substrates
Thin Films
Mylar
Kapton
ALD ZnO
ALD Al2O3
PVDF
ALD films of 300nm thickness were deposited on flexible substrates at a temperature of
100ºC, and PVDF polymer was spin coated at 2000 rpm on mylar and 1000 rpm on kapton
substrates, respectively. The motor was operated from SMPG10WIN software, and each sample
was tested continuously for five minutes at three different linear displacements, 0.25 in, 0.50 in,
and 0.75 in, respectively.
55
4.3. Results and Discussions
4.3.1.
ALD ZnO
0.25 in
As Deposited
0.75 in
0.50 in
Fig. 4.3 ZnO film tested on Mylar substrate
As Deposited
0.25 in
0.75 in
0.50 in
Fig. 4.4 ZnO film tested on Kapton substrate
56
4.3.2. ALD Al2O3
As Deposited
0.25 in
0.50 in
0.75 in
Fig. 4.5 Al2O3 film tested on Mylar substrate
As Deposited
0.50 in
Fig. 4.6 Al2O3 film tested on Kapton substrate
57
0.25 in
0.75 in
4.3.3.
PVDF/TrFe
0.25 in
As Deposited
0.50 in
0.75 in
Fig. 4.7 PVDF/TrFe film tested on Mylar substrate
0.25 in
As Deposited
0.50 in
0.75 in
Fig. 4.8 PVDF/TrFe film tested on Kapton Substrate
58
The following conclusions emanate from the deformation tolerance test data collected:
-
ZnO films only show cracking for the largest linear displacement of 0.75 inches on
mylar substrates, but not on kapton.
-
In comparison, Al2O3 films on mylar substrates crack even for linear displacements as
small as 0.25 inches (see Figure 4.5), and the severity of the cracking increased with
larger linear actuator displacements. However, the films deposited on kapton
substrates did not crack.
-
Spin coated PVDF films on mylar and kapton substrates have not shown any cracking
when samples were flexed for various linear displacements.
The observations indicate that films are more susceptible to cracking on mylar than on
kapton. This effect is thought to be due to the mechanical properties of these two materials. The
Young’s modulus, for instance, is twice as large in mylar than in kapton, thus, kapton is thought
to be more able to tolerate deformations compared to mylar. In general, smaller deformations are
preferred to preclude cracking.
59
CHAPTER FIVE: MICROWAVE PHASE SHIFT MEASUREMENTS ON
FLEXIBLE SUBSTRATES
5.1. Introduction
Phase shifting performance on flexible substrates was measured by employing phase
shift/ferroelectric materials patterned above the signal electrode on a transmission line. The
coplanar waveguides (CPW) employed are transmission lines that consist of three electrodes,
namely, ground signal and ground (GSG). They were fabricated on flexible dielectric
substrates. The microwave structures were tested in the frequency range of 0.4MHz to 40
GHz employing a Vector network analyzer with on-wafer measurement capabilities. This
chapter is being expounded in three parts, that is, (i) fabrication of the test structures, (ii)
device testing procedures, and (iii) microwave measured performance.
5.2.
Microwave Testing and Device Test Set-up:
Microwave measurements of coplanar waveguides were realized using an Anritsu
37000D lightning series Vector Network Analyzer (VNA). The scattering parameters (S11
and S21), were collected for frequencies from 0.4MHz to 40 GHz. Coaxial cables with a
nominal operating frequency range of 50 GHz were employed to connect Infinity Coplanar
Probes to the VNA. The probes employed are nominally 50 Ω impedance coplanar probes
with a ground/signal/ground (GSG) configuration and a pitch size of 150 µm. The test setup
permitted the extraction of the S-parameters from the Device-under-test (DUT). The probes
are attached to manual positioners, that can be navigated to any location on the DUT on the
platform of a Cascade Microtech probe station. A vacuum pump was employed to hold the
60
samples being tested firmly on the platform. The probe station and the VNA set-up are
shown in Figure 5.4.
Fig 5. 1: Microwave measurement equipment: Vector Network Analyzer (VNA)
and Probe station
Cascade’s WinCal software was employed to operate the VNA from a desktop
computer. The PC and the VNA are interfaced by a National Instruments GPIB USB cable.
Prior to taking measurements, the VNA had to be calibrated. VNA calibration is a method to
measure DUTs with known characteristics, and subsequently employ these measurements to
establish the measurement reference planes. Additionally, imperfections in the measurements
that originate from the cables, panel connectors, and probes are corrected. Thus, calibration is
critical. The calibration is performed using a Cascade Impedance Standard Substrate (ISS)
with known microwave characteristics. The ISS consists of short circuited lines, 50 Ω loads
and thru lines. In addition, the ISS includes coplanar waveguides of different lengths for
further verification. On-wafer Line-Reflect-Reflect-Match (LRRM) calibration has been
employed prior to taking microwave measurements. This include testing of Open, Short,
Load and Thru standards respectively. After the calibration is performed, the error
61
corrections are sent to the VNA, and at that point the setup is considered ready for taking
measurement of an arbitrary DUT.
5.3. Fabrication of Microwave Test Structures
The Microwave test structures were fabricated with two different phase shift materials
such as Zinc Oxide (ZnO) and PVDF/TrFe on coplanar waveguide transmission lines.
Coplanar waveguides of different lengths were employed on the substrates to match the
modeled characteristic impedance. The fabrication process flow of the test structures on
flexible substrates is shown in Figures 5.1 and 5.2.
Fabrication Process Flow - ZnO:
Mylar
Kapton
Step 1
Mylar
Kapton
Step 2
Mylar
Kapton
Step 3
Mylar
Silver
Kapton
Gold
ALD ZnO
Fig 5.2: Fabrication of Microwave Test Structure with ZnO
62
Step1: The backside metallization on mylar substrates was chemically etched (Appendix A)
with a buffered oxide etch (6:1) solution for a duration of 6 sec.
Step 2: The Coplanar waveguides (CPWs) on mylar were photolithographically patterned
and defined with a chemical etch. The CPWs on Kapton were fabricated by a lift-off
procedure (see Appendix A), using 300nm sputtered gold thin films.
Step 3: ALD Zinc Oxide (ZnO) thin film of thickness 300nm was patterned on the signal
electrode by employing a Lift-off procedure (Appendix A).
Fabrication Process Flow – PVDF/TrFe:
Mylar
Kapton
Step 1
Mylar
Kapton
Step 2
Mylar
Kapton
Step 3
Mylar
Kapton
Step 4
Mylar
Kapton
Gold
PVDF/TrFe
Fig 5.3: Fabrication of Microwave Test Structure with PVDF/TrFe
63
Step 1: Metal film on Mylar substrate was chemically removed employing BOE (6:1) for
duration of 6 sec.
Step 2: Lift-off process was employed to fabricate 300 nm fold coplanar waveguides on both
Mylar and Kapton substrates.
Step 3: PVDF/TrFe polymer was spin casted (Appendix A) at 2000 rpm for 30 sec. Then,
the samples were annealed at various temperatures and poled at different
temperatures (see section 5.4.4).
Step 4: Acetone solvent was employed to pattern the PVDF/TrFe films on CPW.
a
b
Fig 5.4 Optical Images of fabricated Coplanar waveguides with (a) Zinc Oxide, and (b)
PVDF/TrFe
5.4. Microwave Measurements
The Microwave measurements are described in 4 sections:
i) CPW measurements without Phase shifting materials.
ii) ZnO on Flexible Substrates.
iii) Unpoled PVDF/TrFe films on Flexible Substrates.
iv) Poled PVDF/TrFe films on Mylar.
64
5.4.1.
CPW measurements without Phase Shifting materials
As seen in the Figure 5.4 for the transmitted signal, Phase S21 and Magnitude S21 (Sparameters), measurements up to 40 GHz frequencies were measured for the coplanar
waveguides on Kapton and Mylar substrates.
a
b
Fig 5.5: Fabricated Coplanar waveguides on (a) Kapton and , (b) Mylar
From the extracted S-parameters, the characteristic impedance and propagation constant
are calculated from the following equations [Thesis ref]
Propagation constant is given by:
( S112  S 212  1) 2  (2S11 ) 2
K
(2S11 ) 2
…………………(5.1)
and the characteristic impedance is given by,
Z Z
2
0
(1  S 11 ) 2  S 212
(1  S 112 ) 2  S 212
…………………….........(5.2)
Figures 5.6 to 5.9 display the characteristic impedance and propagation constant of the
coplanar waveguides on the flexible substrates. Difference in the modeled and measured
65
CPW’s characteristic impedance, is due to variation in the CPW dimensions that were
fabricated.
Fig 5.6: Propagation constant of coplanar waveguide on Mylar
Fig 5.7: Characteristic Impedance of coplanar waveguide on Mylar
66
Fig 5.8: Propagation constant of coplanar waveguide on Kapton
Fig 5.9: Characteristic Impedance of coplanar waveguide on Kapton
67
The measured Phase, S21 and Magnitude, S21 of the coplanar waveguides on flexible
substrates and on the Impedance Standard Substrate (ISS) are shown In Figures 5.10 and
5.11. Compared to kapton, mylar substrate displayed relatively higher but tolerable losses.
Fig 5.10: Phase, S21 of Characteristic Impedance of coplanar waveguide on Kapton
Fig 5.11: Phase, S21 of Characteristic Impedance of coplanar waveguide on Kapton
68
5.4.2.
ZnO on Flexible Substrates:
5.4.2.1. ZnO on Kapton Substrate
ZnO is a piezoelectric material that is employed in energy harvesting schemes, among
other applications. The testing protocol that included fabrication and measurement set-up,
was verified as shown in Figure 5.12 for ZnO on a kapton substrate. A microwave signal
was launched employing the previously described probes while a DC voltage of up to 10V
was applied to the signal electrode of the transmission line seeking phase shifting
behavior.
Fig 5.12: Coplanar waveguide with ZnO on Kapton
However, as ZnO is not a ferroelectric material, no Phase shift has been observed (see
Figures 5.13 and 5.14).
69
Fig 5.13: Phase S21, of ZnO on Kapton substrate
Fig 5.14 Magnitude S21, of ZnO on Kapton substrate
70
5.4.2.2. ZnO on Mylar Substrate
A similar test was carried out on mylar substrates with ZnO thin film. However, in
this case, there was a vanishing phase shift observed, as can be seen in Figures 5.15 and
5.16.
Fig 5.15: Magnitude S21, of ZnO on Mylar substrate
Fig 5.16: Phase S21, of ZnO on Mylar substrate
71
5.4.3. Unpoled PVDF/TrFe Films
5.4.3.1. Unpoled PVDF/TrFe and Mylar Substrates:
PVDF/TrFe polymer was prepared using Methyl-Ethyl-Ketone (MEK) solvent, which
has the boiling point around 80°C. The prepared solution was employed for spin casting
experiments. Both substrates with coplanar waveguides (Figure 5.3) were spin coated
with the polymer solution at 2000 rpm for 30 seconds. In order to drive out the solvent
present in the solution, the films were annealed in a furnace. Finally, the performance
dependence on thermal treatments of PVDF/TrFe films was obtained from the
microwave measurements taken on mylar substrates.
Run 1: Films were annealed in a furnace at a temperature of 50°C, for duration of 30
minutes. Then PVDF/TrFe was patterned on the CPW as shown in Figure 5.4 (b).
Microwave measurements were carried out, with and without applied DC
voltages.
Run 2: Annealing time was increased from 30 min to 60 min, at 50°C. Similar test was
performed to measure the microwave performance after PVDF/TrFe polymer has
been patterned.
Run 3: Annealing conditions were changed, and films were annealed at 70°C, for
duration of 30 min. Phase S21, and Magnitude S21 were measured up to 40
GHz.
Annealing characterization runs with three different conditions were performed on
Mylar substrate, followed by measuring microwave phase shift performance. As observed
on Kapton, there was no phase shift observed 50°C annealing conditions(Figures 5.23 –
72
5.26) . As expected, there was a slight phase variation for samples annealed at 70°C for 30
minutes (Figure 5.27 and Figure 5.28).
Fig 5.17: Phase S21, Unpoled PVDF on Myalr, Annealed 50°C, 30 min
Fig 5.18: Magnitude S21, Unpoled PVDF on Myalr, Annealed 50°C, 30 min
73
Fig 5.19: Phase S21, Unpoled PVDF on Mylar, Annealed 50°C, 60 min
Fig 5.20: Magnitude S21, Unpoled PVDF on Myalr, Annealed 50°C, 60 min
74
Fig 5.21: Phase S21, Unpoled PVDF on Mylar, Annealed 70°C, 30 min
Fig 5.22: Magnitude S21, Unpoled PVDF on Myalr, Annealed 70°C, 30 min
75
5.4.4. Poled PVDF/TrFe Films on Mylar
As mentioned in Chapter 3.1, poling is a technique in which a DC electric field is
applied to a material to align the ferroelectric domains. Prior to poling, i.e. unpoled case, the
domains are randomly aligned, resulting in a vanishing non-linear effect. However, upon the
application of the poling electric field for an extended period of time, the domains can be
aligned in a common direction resulting in a discernable non-linear response. The alignment
of the ferroelectric domains in the material being poled can be improved by supplying
thermal energy during the process.
All Poling experiments were performed for PVDF/TrFe on Mylar substrate under three
different conditions, followed by the measurement of the phase shift variation. The electric
field applied for PVDF/TrFe film was 25 V/µm. Figure 5.29 describes the poling set-up
overview. The interelectrode separation was 46.4 µ.
Metal plates
Mylar Substrate
+ 25 V/µm
Gold Thin Film
PVDF/TrFe Thin Film
Fig 5.23: Poling set-up for Mylar substrates
76
Run 1: Mylar samples were poled at a temperature of 70 °C for duration of 30 min.
PVDF/TrFe polymer was patterned, followed by the measurement of the microwave
performance. Asshown in Figure 5.29, a significant phase shift was observed when
an external DC voltage of 1 V was applied.
Run 2:
Poling temperature was increased to 80°C, while other parameters were maintained
constant.As seen in Figure 5.31, Phase variation decreased compared to the
previously analyzed sample.
Run 3: The poling temperature was further increased to 90°C, and the phase variation was
measured.
The observations indicate that by increasing the poling temperature, the phase variation of
the microwave signal becomes smaller. It is thought that higher temperatures during poling
improve the alignment of the film molecules in a direction orthogonal to the plane of the
wafer, in such a manner that they have a diminished interaction with the applied microwave
signal. Thus, lower values for the poling electric might be preferred to achieve a more
favorable alignment direction, led to a higher energy to align the domains, and might have
polarized directly to the electric field. This effect was observed clearly in Figures 5.24 –
5.29.
77
Fig 5.24: Phase S21, Poled PVDF on Mylar @ 70°C, 30 min, 25V/µm
Fig 5.25: Magnitude S21, Poled PVDF on Mylar @ 70°C, 30 min, 25V/µm
78
Fig 5.26: Phase S21, Poled PVDF on Mylar @ 80°C, 30 min, 25V/µm
Fig 5.27: Magnitude S21, Poled PVDF on Mylar @ 80°C, 30 min, 25V/µm
79
Fig 5.28: Phase S21, Poled PVDF on Mylar @ 90°C, 30 min, 25V/µm
Fig 5.29: Magnitude S21, Poled PVDF on Mylar @ 90°C, 30 min, 25V/µm
80
5.5. Conclusion
The collected microwave measurements corroborate that the gold and PVDF/TrFe structures
fabricated on flexible, mylar substrates in a process flow that integrated a ferroelectric
material exhibit a discernable phase variation when a CMOS-compatible voltage is applied
between the signal and ground electrodes of a coplanar waveguide. The results described
herein will enable the future demonstration of radar-like devices on mylar and other flexible
substrates.
81
CHAPTER SIX: CONCLUSION AND FUTURE WORK
6.1. Conclusion
Firstly, different flexible substrates were considered as candidates for the fabricatation of
microwave devices. The analysis indicated that mylar was the most solid option to carry out this
exercise. The analysis further indicated that it would be beneficial to characterize the fabrication
processes on kapton, another flexible substrate, and then have the process transferred to mylar.
Secondly, ZnO and PVDF/TrFe were employed and extensively characterized studying
various parameters such as thickness, thin film stress, surface energy and surface roughness of
as-grown and annealed ALD films. PVDF/TrFe, known for its ferroelectric properties, was
characterized under different annealing and poling conditions. Conventional microfabrication
techniques such as: photolithography and thin film depositions were employed in the production
of polarization test structures and ultimately the coplanar waveguides. Finally, On-wafer
microwave measurements were carried out employing a Vector Network Analyzer (VNA) to
extract microwave data and verify the phase shifting performance of PVDF/TrFe on mylar
substrates with frequencies up to 40 GHz. The integration of a processing flow involving
ferroelectric films and flexible substrates has, therefore, been demonstrated.
6.2. Future Work
The phase shift variation has been confirmed with PVDF/TrFe on mylar, the processing
approach described herein can be employed to fabricate radar devices on flexible substrates.
Additionally, novel and affordable techniques such as ink jet printing could be incorporated to
82
print the coplanar waveguide transmission lines on flexible substrate, which could decreases the
cost and the time involved in device fabrication.
83
APPENDIX - A
A. Fabrication Process Recipes:
This section lists all process recipes used in fabrication of test structures on mylar and kapton
substrates.
A.1 Metal removal on Mylar substrate:
Commercially obtained mylar has a thin silver film coated on both sides of the substrates. For the
polarization test structures, the metallic film on one side has to be removed. Therefore, the
following processing conditions were employed.
STEP 1: The top side of the substrate was protected with a positive photoresist (Shipley
S-1813), which was spin cast on the side of interest.
a. Spin coat S-1813 resist
:
3000 rpm, 30 sec
b. Pre Bake
:
1 min, 110 °C
c. UV Exposure
:
6 sec
d. Post Bake
:
1 min, 110 °C
(Hot Plate)
STEP 2: The unprotected metallic film on the mylar substrate, was chemically etched
with Buffered Oxide Etch, BOE (6:1) for a duration of 6 seconds, and rinsed thoroughly
with de-ionised water.
STEP3:
Finally, the photoresist on the top side of the substrate was removed with
acetone, followed by a thorugh rinsing with de-ionized water.
84
A.2 Lift-off process on Mylar and Kapton Substrates:
Coplanar waveguide transmission lines are fabricated by a lift-off procedure, which is a well
established process in the field of micro-fabrication. The employed processing conditions are
described below.
The Lift-off procedure consists of three critical steps:
i.
ii.
Photolithography:
a. Spin coat AZ-5214E resist
:
3000 rpm, 30sec
b. Pre Bake ( Hot Plate)
:
90 °C, 80 sec
c. UV exposure (with photomask)
:
6 sec
d. Thermal treatment
:
90 °C, 110 sec
e. Flood Exposure (without photomask)
:
50 sec
f. Development (MIF 726)
:
20 sec
g. Post Bake (Hot Plate)
:
90 °C, 80 sec
Thin Film deposition:
Gold thin film of 300 nm thick was sputtered with a Cressington tool.
iii.
Resist removal:
Acteone solvent was employed to remove the photoresist, followed by a thorough
de-ionized Resist rinsing, and Nitrogen blow dry.
The same processing conditions were also employed to pattern Zinc Oxide (ZnO) on the signal
electrode of the fabricated Coplanar waveguides. Atomic layer deposition was employed for
85
Zinc Oxide film deposition. We have verified that Lift-off process worked for ALD ZnO films
(see Figure 5.4 (a)).
A.3 PVDF/TrFe patterning
As mentioned in Chapter 3 PVDF/TrFe solution was prepared employing MEK solvent, and
later employed for spin casting procedures. Chapter 5 discusses the annealing characterization
and poling of polymer films. Acetone solvent was employed in combination with a shadow mask
to pattern the PVDF/TrFe on top of the signal electrode. (Figure 5.4 (b) ).
86
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90
VITA
Ramakrishna Kotha was born in Guntur, Andhra Pradesh, India on June 27, 1981. He
received the degree of Bachelor of Engineering in Electronics and Communication Engineering
in 2002 from Chaitanya Bharathi Institute of Technology, Hyderabad, Andhra Pradesh, India. He
received his Masters degree in Electrical Engineering from the University of Texas at San
Antonio in 2005, after which he joined the Ph.D. program in Electrical Engineering at the
University of Texas at San Antonio. He is affiliated with MEMS Research Laboratory at UTSA.
His research interests include Material science, Radio frequency communications, novel Micro
and Nanofabrication techniques. He is a member of Eta Kappa Nu Electrical Engineering Honor
Society and a student member of the IEEE.
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