Chemical Vapor Deposition of Conjugated Polymeric
Thin Films for Photonic and Electronic Applications
by
John Patrick Lock
Master of Science, Chemical Engineering Practice
Massachusetts Institute of Technology, Cambridge, Massachusetts, 2005
Bachelor of Science, Chemical Engineering
University of Colorado at Boulder, 1998
Subitted to the Department of Chemical Engineering
in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY IN CHEMICAL ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2005
© 2005 Massachusetts Institute of Technology. All rights reserved.
(n '
ii
Signature of Author:
.....
Department of Chemical Engineering
May 12, 2005
?
Certified by:
Karen K Gleason
Professor of Chemical Engineering
Thesis Advisor
Accepted by:
MASSACHUSETTS INS11TUTE
OF TECHNOLOGY
Daniel Blankschtein
Professor of Chemical Engineering
Chairman, Committee for Graduate Students
SEP 1 2 205
414?C11ViEs
......
LIBRARIES
7
i
Chemical Vapor Deposition of Conjugated Polymeric
Thin Films for Photonic and Electronic Applications
by
John Patrick Lock
Submitted to the Department of Chemical Engineering
on May 13, 2005 in Partial Fulfillment
of the Requirements for the Degree of
Doctor of Philosophy in Chemical Engineering
Abstract
Conjugated polymers have delocalized electrons along the backbone, facilitating
electrical conductivity. As thin films, they are integral to organic semiconductor devices
emerging in the marketplace, such as flexible displays and plastic solar cells, as well as
next-generation microphotonic chips. A major processing challenge is that these materials are
relatively insoluble. Chemical vapor deposition (CVD) is presented as a synthesis technique for
conjugated polymers as an alternative to electrochemical and liquid dispersion methods. CVD
will continue to be an essential component of the materials toolset for manufacturers of
semiconductor devices.
Polysilanes, with a backbone consisting of silicon atoms instead of carbon, have
delocalized electrons due to the presence of d-orbitals. Plasma-enhanced CVD (PECVD) of
polysilane films was achieved, but they did not exhibit electrical conductivity. Branching
resulting from the energetic plasma process quenches the conjugation. However, photo
oxidation was used to convert Si-Si bonds into Si-O-Si, reducing the refractive index up to 5%.
This led to the direct patterning of tunable waveguides in PECVD hexamethyldisilane (6M2S).
Other essential devices for microphotonics are microring resonators used for filtering an
individual wavelength from "multicolor" light. Photo oxidation of 6M2S, deposited as the
cladding material on ring resonators, allows one to shift the resonant wavelength an order of
magnitude more than conventional thermal trimming techniques. Microphotonics will ultimately
increase computing speeds with chips that operate using light instead of electricity.
A CVD technique was also developed for poly-3,4-ethylenedioxythiophene or PEDOT.
Among conducting polymers, PEDOT has superior conductivity (up to 300 S/cm) and excellent
stability. CVD PEDOT has a conductivity of about 5 S/cm, while 1 S/cm is the figure-of-merit
for a good conducting polymer film. As a charge-injecting layer in organic light-emitting diodes
(OLEDs), PEDOT increases the overall power efficiency 30-50%. CVD can further enhance this
efficiency gain in organic devices by more conformally coating PEDOT on high-area surfaces.
CVD PEDOT films also exhibit reversible electrochromic behavior changing color from their asdeposited sky blue color to a darker blue when they are reduced with an applied volatage. A
50-nm film had a contrast of 16.5% with a switching speed of 27 ms.
Thesis Supervisors:
Title:
Karen K Gleason
Professor Chemical Engineering
2
To Andrea,
Mom, and Loreen
3
Acknowledgments
I can't thank enough all the people who have enabled my thesis research during my time
at MIT. I have had the great fortune of being supported along the way by the most generous
family, friends, mentors, and advisors.
I'd like to thank my advisor, Karen, a truly extraordinary scientist. More often than not,
her outlook concerning the likelihood of success for a given experiment has been right on.
While allowing me the freedom to explore all of my ideas, she is responsible for providing an
overall direction to my project, which I feel has optimized the impact it has achieved.
Professors Vladimir Bulovi6, Paula Hammond, and Bill Green were the members of my
thesis committee. I thank them for their time, encouragement, and advice throughout the years.
It was great having access to such a store of knowledge in their areas of expertise. In addition, I
would like to thank Professor Kimerling and Jurgen Michel for all of their ideas.
The materials side of my project happened mainly in the Gleason Lab alongside a very
stimulating group of people to share office and lab space with. Thank you to all of the Gleason
Group members.
However, the majority of my results have been the demonstration of my
materials in a variety of functional devices, all of which were made in collaboration with a
number of other labs across campus. For their help, I would like to thank the following: Dan
Sparacin and Jessica Sandland in the Kimerling Lab, Jodie Lutkenhaus and Nicole Zacharia in
the Hammond Lab, Jen Yu and Jonathon Tischler in the Bulovi6 Lab, Angela Chen and Rachel
Pytel in the Swager and Hunter Labs, and Tetsuo Sato in the Yokoyama Lab and the University
of Osaka.
4
In addition to all of the students at MIT that I have collaborated with for my research, I
have worked closely with many more through all of the extracurricular activities that have
shaped my overall graduate school experience. First I'd like to thank the members of Advanced
Conductors for the great run in the $50K and the Ignite Clean Energy Competition. Thanks Karl,
Sam, Steve, and Pete - maybe next year!
I'd also like to say cheers to all of the Muddy
bartenders. Finally I'd like to acknowledge all the fellow members of the GSC, TechLink, and
TinyTech.
The companionship of my friends provided me with so much happiness and many
unforgettable experiences - thanks especially to Steve, Kelvin, April, and all of the winos.
Thanks also to the KFJ group for all of the lunchtime conversations.
I really appreciate the outpouring of support that has always been a constant in my life
from Mom, Loreen, my grandparents, and all of my aunts, uncles, and cousins.
Finally, I love you, Andrea. Thank you for being a constant source of support and for
providing such a fine example whenever I need someone to emulate. You've motivated me to
run marathons, be Catholic, and now finish my PhD. I look forward to our life together and I'll
try to be as much an inspiration for you as you have been for me.
5
TABLE OF CONTENTS
Abstract
Dedication
3
Acknowledgments
List of Figures
List of Tables
4
9
13
List of Notations
14
CHAPTER ONE
17
Introduction
1.1 Motivation
18
1.2 c-Conjugated Polymers: Polysilanes
1.2.1
Electrical Properties of Polysilanes
1.2.2
Photolability of Polysilanes
1.2.3
Synthesis Techniques
1.3 v-Conjugated Polymers: PEDOT
1.3.1
Synthesis Techniques
1.4 Outlook:
1.5 Thesis Framework
19
20
22
24
25
27
28
28
CHAPTER TWO
36
Tunable Waveguides via Photo Oxidation of Plasma Polymerized Organosilicon Films
Abstract
37
2.1 Introduction
38
2.2 Experiment
2.3 Results and Discussion
2.3.1
UV Irradiation of PECVD Organosilicon Films
2.3.2 Coupling and Tuning of Slab Mode Waveguides
42
44
44
46
References
51
6
CHAPTER THREE
53
Trimming of Microring Resonators Using Photo-Oxidation of a Plasma-Polymerized
OrganosilaneCladding Material
Abstract
3.1 Introduction
3.2 Experiment
54
55
57
3.3 Discussion
59
3.3.1 Ring Theory
3.3.2 Characterization of CVD Films
3.4 Results
3.5 Conclusions
References
59
59
62
66
67
69
CHAPTER FOUR
Chemical Vapor Deposition of Thin Films of Electrically Conducting PEDOT
Abstract
70
4.1 Introduction
71
4.2 Background
4.2.1 Mechanism for the Oxidative Polymerization of PEDOT
4.3 Experiment
4.4 Results and Discussion
4.5 Conclusion
References
73
73
77
79
83
85
CHAPTER FIVE
89
Electrochemical Investigation of PEDOT Thin Films Deposited Using CVD as a
Candidate Material for Organic Memory and Electrochemical Applications
Abstract
90
5.1 Introduction
91
5.2 Experiment
5.3 Discussion and Results
5.3.1
Cyclic Voltammetry
5.3.2
UV/Vis Spectroscopy
5.3.3 Chrono Amperommetry
5.4 Conclusions
References
93
95
95
97
97
104
106
7
109
CHAPTER SIX
Conclusions
112
CHAPTER SEVEN
Future Directions
116
APPENDIX A
Structural Differences Between CVD and Spin-On Polysilane Films
Objective
A.1 Introduction
A.2 Synthesis of Polysilane Films
A.2.1 Chemical Vapor Deposition
A.2.2 Spin-On Deposition
A.3 CVD and Spin Coating Processing Considerations
A.4 Chemical Composition
A.5 Stability
A.6 UV/Vis Absorption
A.7 Photo Oxidation
A.8 Thermochromism
A.9 Proposed Structure
A. 10 Proposed Applications
8
117
118
118
118
119
120
122
123
125
126
128
130
132
List of Figures
CHAPTER ONE
Figure 1-1:
Molecular repeat unit of a polysilane.
Figure 1-2:
Schematic showing the geminal and vicinal interactions between sp3 orbitals in a
ac-conjugated linear chain of catenated Si atoms.
Figure 1-3:
Conjugated polymers have delocalized electrons that split discreet molecular orbital
energies into bands that are analogous to the conducting and valence bands of
semiconductors.
Figure 1-4:
E xposure to UV light converts a polysilane network into a polysiloxane material.
Figure 1-5:
Direct irradiation of polysilanes with UV light can be used to define low index regions in
the material, which can be useful for patterning optical devices.
Figure 1-6:
Common
conducting
polyaniline,
(d)
polymers
include
(a) polyacetylene,
polyphenylenevinylene,
poly-3,4-diethylenedioxythiophene
(e)
(b)
polypyrrole,
polythiophene,
and
(c)
(f)
(PEDOT).
CHAPTER TWO
Figure 2-1:
Photo oxidation occurs via an insertion reaction when an Si-Si bond is irradiated with UV
light.
This decreases the molecular density of the material and reduces the refractive
index. R and R2 are organic substituents (ie methyl, phenyl, etc).
Figure 2-2:
Schematic of the prism coupling technique for measuring optical properties of light
guiding films.
Figure 2-3:
Contrast curve for a plasma polymerized dimethylsilane film irradiated with 193 nm
light. A maximum refractive index contrast of 0.05 or 3% was achieved with a dosage of
900 mJ/cm 2.
Figure 2-4:
Contrast curve for a plasma polymerized hexamethyldisilane film irradiated with a Hg arc
lamp.
Figure 2-5:
a) Two modes of 633 nm light are coupled into this 0.79 [tm thick plasma polymerized
6M2S film. b) In the same sample, only one mode of 1550 nm light is supported.
9
Figure 2-6:
a) Two modes can be supported by a 0.79 ptm film of 6M2S. b) One mode of 1550 nm
light can be coupled to this film.
Figure 2-7:
For the 6M2S sample with 0.79 [um thickness, the refractive index would have to be
decreased slightly by 0.01 or 1% to have single mode performance at both 633 nm (a)
and 1550 nm (b).
Figure 2-8:
Prism coupling measurements after UV irradiation confirm tunability of 6M2S film. The
6 M2S now has single-mode performance for both 633 nm'(a) and 1550 nm (b).
CHAPTER THREE
Figure 3-1:
Schematic of a ring resonator device.
Figure 3-2:
.JV irradiation causes scission of Si-Si bonds allowing oxygen incorporation, which
lowers the refractive index of the material.
Figure 3-3:
Fitting the Cauchy-Urbach model to ellipsometry data yields the thickness and optical
constants of plasma polymerized 6M2S films.
Figure 3-4:
Reasonable agreement is seen between refractive index contrast results at 1550 nm
collected using an ellipsometer operating in the visible range (450 to 720 nm) and an
ellipsometer operating in the near-IR (800 to 1750 nm).
Figure 3-5:
The refractive index of PECVD 6M2S cladding material decreases with UV irradiation as
a result of photo-oxidation.
Figure 3-6:
1-M mode spectral measurements of a 100 jlm Si3N4 ring resonator (.,1=1564.5 nm)
after 300 and 420 seconds of UV irradiation at 1.7
Figure 3-7:
[tW/cm
2
.
The experimental resonance shifts for TE and TM polarizations are compared with
modeled results.
CHAPTER FOUR
Figure 4-1:
Diaz mechanism for oxidative polymerization.
Figure 4-2:
Neutral PEDOT is oxidized to form a conducting polycation that is charge balanced with
dopant anions.
Figure 4-3:
Acid-initiated coupling promotes chain growth.
Figure 4-4:
Acid initiation can progress to the formation of trimers with broken conjugation.
10
Figure 4-5:
Schematic of CVD reactor for depositing PEDOT films.
Figure 4-6:
FTIR spectra and conductivity values for CVD PEDOT and standard PEDOT films.
Figure 4-7:
Comparison
of PEDOT polymerized in the presence of pyridine before and after
methanol rinse.
Figure 4-8:
Pyridine readily reacts with HCI to form a pyridinium salt.
CHAPTER FIVE
Figure 5-1:
Neutral PEDOT is oxidized to form a conducting polycation that is charge balanced with
dopant anions. Oxidized PEDOT has a transparent light blue color that turns dark purple
upon reduction.
Figure 5-2:
A schematic of a CVD process for the deposition of PEDOT
Figure 5-3:
Cyclic voltammetry indicates that PEDOT is reduced gradually, but oxidizes more
suddenly.
This stems from the conductivity of oxidized PEDOT as opposed to the
non-conducting reduced form.
Figure 5-4:
UIV/Vis spectroscopy indicates that CVD PEDOT has a maximum color contrast of
16.5% at a wavelength of 585 nm.
Figure 5-5:
A square wave form with a step time of 500 msec and potential limits of 400 mV and
-600 mV was chosen for chrono amperommetry measurements.
Figure 5-6:
A CVD PEDOT film 50 nm thick has a swiching speed of about 50 msec for a 90%
change and is as low as 27 msec for an 80% response.
Figure 5-7:
The charge response
of a CVD PEDOT film is proportional
to t
2
indicating a
diffusion-controlled process.
Figure 5-8:
Chrono amperommetry data is condensed into an Anson plot that is useful for calculating
diffusion constants for charge transfer processes.
CVD PEDOT has a diffusion
coefficient on the order of 10-1 cm2/s indicating that the process is controlled by ion
diffusion in the film
11
APPENDIX A
Figure A-I:
Schematic diagram of PECVD reactor.
RF energy introduced to the top electrode
induces a plasma between the two capacitively coupled electrodes.
Figure A-2:
FTIR spectra of CVD polydimethylsilane films are compared with a CVD organosilicon
film and a commercially produced polydimethylsilane powder. Oxygen has not yet been
eliminated,
but there
is a progressive
decline
in the oxygen content of CVD
polydimethylsilane films.
Figure A-3:
FTIR CVD polysilane materials are stable to oxidation over time in normal laboratory
conditions.
This sample was stored for over two weeks in atmosphere under room
lighting.
Figure A-4:
CVD polysilane films show good chemical stability compared to spin-on polysilanes.
Figure A-5:
.JV/Vis spectra show absorption due to ,c-conjugation for spin-on PMPS films at 333 nm.
No corresponding peak is seen for analogous CVD films.
Figure A-6:
Photo oxidation occurs via an insertion reaction when an Si-Si bond is irradiated with UV
light.
This decreases the molecular density of the material and reduces the refractive
index.
Figure A-7:
An increase in the Si-O peak in the FTIR spectrum for a plasma polymerized
climethylsilane film demonstrated photo oxidation of Si-Si bonds with UV irradiation.
Figure A-8:
Absorption of NIR light transforms polysilane chains to a random helix conformation.
1-his interrupts
-conjugation, which decreases the refractive index. This is a reversible
transformation.
Figure A-9:
Thermochromism in spin-on polysilane films evident by swelling and a reduction in the
refractive index is largely absent in analogous CVD materials.
Figure A-10:
A comparison of spin-on and CVD polysilane films indicates a more conjugated
backbone for the spin-on material. Branching and unsaturated silicon atoms are among
the characteristics expected for the amorphous CVD films.
12
List of Tables
CHAPTER FOUR
Table 4-1:
Ring Bands in cm- ' for Monosubstituted Thiophenes
13
List of Acronyms, Abbreviations, and Symbols
Roman Acronyms and Abbreviations
eV
Dimethylsilane
Diethylsilane
Hexamethyldisilane
Area
Cauchy Coefficients
Bayer Chemicals Product (Catalyst)
Bayer Chemicals Product (Monomer)
Bayer Chemicals Product (Polymer)
Concentration of Reacting Species in Film
Wavelengths Between 1530 and 1565 nm
Chrono Amperommetry
Cyclo Voltammetry
Chemical Vapor Deposition
PEDOT Films Deposited with CVD
Thermo-Optic Coefficient of Materials
Hexamethylcyclotrisiloxane
Diameter of Microring
Diffusion Constant
Direct Current
Direct Current Source
3,4-ethylenedioxythiophene
Voltage Potential
Bounding Potentials of Cyclovoltammogram Oxidation Peak
Electron Volt
F
Faraday's Constant
FSR
FTIR
HOMO
HPLC
Free Spectral Range
Fourier Transform Infrared Spectroscopy
Highest Occupied Molecular Orbital
High Pressure Liquid Chromatography
Current
Infrared
Indium Tin Oxide
Boltzmann Constant
Wavelengths Between 1565 and 1610 nm
Liquid Crystal Display
Light-Emitting Diodes
Low Pressure Chemical Vapor Deposition
Lowest Unoccupied Molecular Orbital
Resonant Mode Number
Micro Electro Mechanical Systems
Methylphenylsilane
2MS
2EthS
6M2S
A
An-Cn
BAYTRON C
BAYTRON M
BAYTRON P
C,*
C-Band
CA
C:V
CVD
CVD 1-4
dn/dT
1)3
I)o
I)C
I:)CS
EDOT
E,
EI 1-2
I
IR
ITO
k
L-Band
LCD
LEDs
LPCVD
LUMO
mn
MEMS
NIPS
14
n
ln
ln
NIR
OLEDS
PANI
-PDA
PDHS
PDMS
PECVD
PEDOT
PMPS
PPV
PSS
Q
R
rf
S
S/cm
sccm
SCE
SMF
TE
THF
1TM
TMAH
to
Tg
UJV
UV/Vis
VASE
W
Refractive Index
Effective Refractive Index of Microring Resonator
Charge Difference Between Reduced and Oxidized Species
Near Infrared
Organic Light-Emitting Diodes
Polyaniline
Personal Digital Assistant
Poly-dihexylsilane
Poly-dimethylsilane
Plasma-Enhanced Chemical Vapor Deposition
Poly-3,4-ethylenedioxythiophene
Poly-methylphenylsilane
Poly-phenylenevinylene
Poly-styrene Sulfonic Acid
Charge
Organic Sidegroup Constituent (ie Methyl, Phenyl, etc)
Radio Frequency
Slope From the Anson Plot
Siemens/cm (Units of Bulk Conductivity)
Standard Cubic Centimeters per Minute
Standard Calomel Electrode
Single Mode Fiber
Transverse Electric Polarization of Light
Tetrahydrofuran
Transverse Magnetic Polarization of Light
Tetramethylammonium Hydroxide
Experiment Start Time
Glass Transition Temperature
Ultraviolet Light
Ultraviolet and Visible Light
Variable Angle Spectroscopic Ellipsometry
Film Thickness
15
Greek Symbols
C
0(Film
;t
3.0
ko,2
(Air
(Substrate
,!X
13/k
v
:
cr
Film Thickness
Sigma Bond
Incident Angle of Light Inside Film
Free Space Wavelength of Light
Free Space Wavelength of Resonant Light
Free Space Wavelength of Resonant Light Before Irradiation
Free Space Wavelength of Resonant Light After Irradiation
Goos-Haenchen Shift at Air/Film Interface
Goos-Haenchen Shift at Substrate/Film Interface
Shift in Resonant Wavelength
Speed of Light in Vacuum / Speed of Light Propagation in Film
Scan Rate of Cyclovoltammetry Experiment
Pi Bond
Time of Charge Reversal (CA Experiments)
16
Chapter One
INTRODUCTION
17
1.1
MOTIVATION
In 1977, the field of conducting polymeric materials, also known as synthetic metals,
began with the discovery that polyacetylene conducts electricity l, earning the Nobel Prize in
Chemistry in 2000 for Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa. Recent reviews
examine numerous efforts to incorporate conducting polymers into an increasing number of
electronic devices including light-emitting diodes (LEDs) 2 ' 3, electrochromic materials and
structures4 , microelectronics5 6, portable and large-area displays7 , and photovoltaics 8 . Just as,
"everything that can go digital will go digital", traditional semiconductor devices that we use
every day including computers, cell phones, PDAs, and solar cells, will transition into less
expensive and more disposable organic or plastic forms. Not to be confused with biological
materials, organic simply refers to carbon-based materials as opposed to traditional inorganic
semiconductors.
Benfits of this transition will be realized over time: new and flexible device
forms, thin and light-weight components, and energy efficiency gains amounting to about 10%
of the current US electricity demand.
The advent of organic electronic devices and the
propensity for everything to shrink to the nano scale will facilitate a future of increased
convenience and capability with the evolution of technology.
Conjugated polymers
will become increasingly important
as active materials in
next-generation electronic devices. Conjugated polymers have delocalized electrons along their
backbones enabling charge conduction. They include polyphenylene, polyaniline, polythiophene,
polypyrrole, polycarbazole, and polysilane, among others. Each of these families of polymers
can be substituted with a variety of functional groups to achieve different properties, and new
derivatives continue to be synthesized and studied9 ' 10
18
1.2
a-CONJUGATED
POLYMERS: POLYSILANES
Polysilanes are polymers composed of catenated silicon atoms that form a linear chain".
An example of a polysilane molecular repeat unit is shown in Figure 1-1:
Si
Figure 1-1: Molecular repeat unit of a polysilane.
where R1 and R2 are carbon-based ligands, such as methyl or phenyl groups. Excellent thermal
and mechanical
properties
of these
characteristics and photolability,
materials,
coupled
with their
unusual electronic
have led to many applications including their use as a
photoresist, as a nonlinear optical material for electro optic applications, and as an active
conducting, photoconducting, or charge transporting layer in electronic devices 2 . The first of
these materials was probably made in the early 1920's 3 '
14.
However, the highly crystalline
material attracted little scientific interest because it was generally insoluble and intractable. In
the past 30 years, polysilanes have been rediscovered and modem techniques have been applied
to their characterization.
It seems likely that the future will bring new breakthroughs in the
understanding and development of polysilanes; and, new synthetic procedures remain an
important priority in this field' 2 .
19
1.2.1
ELECTRICAL PROPERTIES OF POLYSILANES
Instead of having conjugated, aromatic, ni-bonding like the majority of conducting
polymers, polysilanes depend on delocalized a-bonding".
The exact mechanism of electron
delocalization in a-conjugated systems is unclear. Overlapping d orbitals are available to silicon
atoms and likely play a role although the antibonding C bonds in silicon polymers are thought to
be of a low enough energy to contribute to delocalization as well. Figure 1-2 depicts a polysilane
segment in its all trans zigzag conformation.
Figure 1-2: Schematic showing the geminal and vicinal interactions between sp 3 orbitals in a
c-conjigated linear chain of catenated Si atoms.
The interaction of vicinal orbitals produces a splitting into bonding HOMO (highest
occupied molecular orbitals) and antibonding LUMO (lowest unoccupied molecular orbitals)".
If there were only vicinal orbital interactions and no geminal orbital interactions, the electrons
would be completely localized between the Si atoms and the HOMO and LUMO energy levels
would be very sharp and distinct. However, with increasing geminal orbital interactions, there is
delocalization of the electrons and a splitting of each of the polymeric HOMO and LUMO levels
20
to yield bands. An optical energy gap between 1.5 and 3eV exists between the HOMO and
LUMO energy levels of polysilanes' 5 .
Many polsilanes are very transmissive in the visible
region and absorb primarily UV light16 .
Al
-
t--I
LUMO <
LUMO
"I
Eg
A,
HOMO
<
1
Atomic
Molecular
sp 3 Orbitals
Orbitals
HOMO
Polymeric
Molecular
Orbitals
Orbitals
Figure 1-3: Conjigated polymers have delocalized electrons that split discreet molecular
orbital energies into bands that are analogous to the conducting and valence bands of
semiconductors.
Many different segments of the polymer chain will have different energy gaps depending
on the number of Si atoms that share the delocalization of charge . Short chain segments where
electrons are delocalized across just a few Si atoms will tends to have larger optical energy gaps
and the light that they absorb is shifted towards the blue part of the spectrum.
delocalized chain segments will tend to have smaller energy gaps.
Longer
Complete delocalization
happens when an electron is delocalized across about 25 atoms17 . Each chain segment with its
accompanying energy gap corresponds to an individual chromophore and the absorption of bulk
polysilane material is like the combined absorption of many different chromophores in
solution 12 .
Interestingly, the degree of delocalization in -conjugated systems is very dependent on
the conformation of the backbone.
Rotations in the chain continually introduce and remove
nodes that change the degree of delocalization along the chain. Delocalization is possible along
21
polysilane chain segments that have trans conformation while the gauche conformation creates a
node that localizes electron interaction 12 . This effect is not as apparent in nr-conjugated systems,
because their double-bonded character requires more energy for chain rotations.
Polysilanes
only require about 1.5kcal/mol for rotation about the Si-Si bond' 8 . In general, ligands increase
the delocalization of the electrons for polysilanes' 9 . This is probably due to increased steric
interactions among the side groups that hinder rotations along the chain.
Therefore,
delocalization and the absorption energy of the material can be manipulated by controlling the
size of the side group components and the degree of substitution of the polysilane chain.
The electrical conductivity of polysilanes is also strongly influenced by the substituents
on the polymer chain, since charge transfer occurs along the delocalized main chain and by
hopping between chains. The dominant carriers in polysilanes are holes. Electron Spectroscopic
Resonance experiments suggest that holes are delocalized on side chains as well as on the main
chain whereas electrons are delocalized only on the main chain2 0. The addition of simple ionic
dopants in the bulk material can also greatly enhance charge transport increasing the
conductivity by orders of magnitude.
1.2.2
PHOTOLABILITY OF POLYSILANES
Polysilanes have been examined as photoresists for 248 and 193nm photolithography 21 '
22,
This application was identified after recognizing that polysilanes undergo very efficient photo
oxidation reactions. Upon exposure to UV light with wavelengths shorter than about 300 nm,
polysilanes undergo an insertion reaction of oxygen into Si-Si and Si-H bonds 23' 24. Figure 1-4
depicts this conversion for an amorphous polysilane network.
22
C
CH
3
3
Si
-- Si
C
3
CH3
hv
CAP
CH
Si
Si-
CH 3
CH3
Si-O-Si-CH
0
°
H
I
CH3
Si--O
CH 3
3
H
I
Si
CH 3
Polysiloxane
Polysilane
Figure 1-4: Exposure to UV light converts a polysilane network into a polysiloxane material.
Oxygen incorporation into the network decreases the molecular density and causes the
refractive index to fall.
This leads to many optical applications like the ability to define
waveguides or gratings in polysilane films using a simple patterning step shown in Figure 1-5. It
is interesting to note that UV patterning of polysilane materials is direct and does not require the
use of an additional photoresist material. As the upper surface of polysilane is exposed to UV
light and converted to polysiloxane, it becomes transmissive to the light. This self-bleaching
mechanism allows for the entire thickness of the film to be irradiated.
23
UV Light Source
Mask
High Index
Polysilane
Low Index
Polysiloxane
Figure 1-5: Direct irradiation of polysilanes with UV light can be used to define low index
regions in the material, which can be useful for patterning optical devices.
1.2.3
SYNTHESIS TECHNIQUES
The first polysilanes were probably synthesized by Kipping13' 14 in the 1920s
using the condensation of dichlorodiphenylsilane with sodium metal.
Despite efforts to find
alternative methods, this modified Wurtz coupling of dichlorosilanes (Eq 1-1) is still the
predominant method of preparating high molecular weight, linear polysilane derivatives12 .
R'R 2 SiC12 + 2Na -> (R1R2Si), + 2NaCI
Many polysilanes are unobtainable with the Wurtz method1 2.
(1-1)
High temperatures are
needed and some side chain substituents other than alkyl and aryl groups are not able to tolerate
the reaction conditions. The Wurtz coupling mechanism is a condensation reaction in solution
and the product is in the form of a precipitate.
Some of these precipitates are completely
insoluble. Polysilanes also decompose before they melt. In fact, after the first polysilanes were
synthesized via Wurtz coupling in the 1920s, they were considered insoluble and intractable. For
this reason, they elicited very little scientific interest until 50 years later. The Wurtz procedure is
24
hazardous, difficult to scale up, and becomes costly on a large scale.
Although it has been
instrumental in synthesizing many of the polysilane materials that are available today for use and
study, there is a need to develop new methods of synthesizing high molecular weight, linear,
polysilanes.
Some chemical vapor deposition processes have been demonstrated to provide a synthetic
route to polysilane materials including the physical evaporation of solid polymer under
vacuum , plasma-enhanced CVD of silane gases2 2
24, 26,
and photo-enhanced CVD of silane
precursors.27, 28 Amorphous films with random polymer orientation and thicknesses ranging
from 100nm to 300nm have been produced with good uniformity2 2 and the microstructure of
some films was found to closely resemble that obtained with the traditional Wurtz condensation
synthesis.2 6
Overcoming the challenge of synthesizing conjugated polysilane materials with
good electrical properties would enable the incorporation of these otherwise difficult-to-process
films in a host of innovative polymeric devices.
1.3
n-CONJUGATED POLYMERS: PEDOT
Most conducting polymers have a -conjugation as opposed to cr-conjugation and
electron delocalization occurs from resonance resulting from alternating single and double
bonds l°. Figure 1-6 shows the structures of the most widely used conducting polymers.
25
(a)
(b)
H
n
(c)
(d)
I
I
n
ri
M
IagM
it,,-/
·n~)
rC/S<
11S ),(f)
0
0
n
Figure 1-6: Common conducting polymers include (a) polyacetylene, (b) polypyrrole, (c)
polyaniline,
(d)
polyphenylenevinylene,
(e)
polythiophene,
and
(f)
poly-3,4-diethylenedioxythiophene (PEDOT).
Perhaps
the
most
poly-3,4-ethylenedioxythiophene
promising
conducting
polymer
so
far
is
(PEDOT) developed by scientists at Bayer AG in Germany2 9 '
31. It was initially designed to block the e-positions on the thiophene ring to prevent undesirable
side reactions. The strategy worked and the ethylene bridge on the molecule also proved to be a
good charge donor to the It-conjugated backbone, giving rise to an unusually high conductivity
of 300 S/cm3 2 . In addition, PEDOT films in their oxidized state were observed to be extremely
stable for conducting polymers and nearly transparent3 3 .
However, like other conjugated
polymers that have a very rigid conformation in order to maintain electron orbital overlap along
the backbone, PEDOT was found to be insoluble9 . Bayer circumvented this problem, though, by
26
using a water soluble polyanion, polystyrene sulfornic acid (PSS), during polymerization as the
charge-balancing dopant. The PEDOT:PSS system is now marketed as BAYTRON PTMand has
good film forming capabilities, retains a conductivity of 10 S/cm, and has good transparency and
extremely good stability.
In fact, the films can be heated in air over 100° C for more than
1000 hours with no major decline in conductivity. Bayer's first major customer for BATRON P
was Agfa who used the material as an anti-static coating on photographic film3 4 -36 . Any spark
generated by static electricity can expose film showing up as a bright spot on developed photos.
Bayer has since enjoyed wide utilization of BAYTRON P as an electrode material in capacitors
and a material for through-hole plating of printed circuit boards3 7 -4 1. BAYTRON P has also been
found to be suitable as a hole-injecting layer in LEDs and photovoltaics, increasing device
efficiency by up to 50%42, 43
1.3.1
SYNTHESIS TECHNIQUES
Most conducting polymer materials are formed via oxidative polymerization of aniline,
pyrrole, thiophene, and their derivatives4 4 . It has not been feasible to process bulk material of
these polymers into thin films since they are insoluble and non-melting, but coating techniques
have been developed on substrates including plastic, glass, metal, fabric and micro- and nanoparticles.
So far, there are four main approaches to form conducting polymeric coatings via
oxidative polymerization
on various materials4 4 : electropolymerization
of monomers at
electrodes, casting a solution of monomer and oxidant on a surface and allowing the solvent to
evaporate, immersing a substrate in a solution of monomer and oxidant during polymerization,
and chemical oxidation of a monomer directly on a substrate surface that has been enriched with
an oxidant.
A few other techniques have been attempted for synthesizing these materials
including physical evaporation4
5s4 7,
plasma-enhanced CVD48 -52 , and thermally initiated CVD53-58,
but the resulting conductivities have been low.
27
1.4
OUTLOOK
Few processing techniques are available for the deposition of thin conducting polymeric
films because they tend to be insoluble and do not melt, precluding subsequent processing9 .
Synthesis techniques that do exist are mostly solution-based and are not compatible with some
substrates like paper or as a top coating on mechanically fragile devices that would not survive
the convential spin-coating technique. The development of a robust vapor-deposition technique
for conducting polymers that preserves their high conductivity and is compatible with moisturesensitive, temperature-sensitive, and mechanically fragile surfaces is needed to broaden their
utilization enabling improved efficiencies in existing devices and development of new devices on
unconventional substrates.
Vapor process typically yield more conformal coatings on rough
surfaces including fibers, micropores, and even microparticles.
The ability to evenly apply
conducting polymers on these surfaces would increase the effective surface area of organic
devices and enable efficiencies that are much better than what is currently available. As the
operating efficiency and production cost of organic electronics improve, the steady conversion of
traditional semiconductors to this new materials set will become apparent in the marketplace.
1.5
THESIS FRAMEWORK
This thesis has been divided into two main materials sets: the deposition and application
of plasma-polymerized polysilane films and the chemical vapor deposition and characterization
of oxidatively polymerized PEDOT.
CHAPTER TWO reports the deposition of a plasma-polymerized polysilane and
its performance as a waveguide. The waveguide was compatible with visible and near-infrared
light and had a refractive index contrast that enables the propagation of multiple modes of light.
A tuning process using UV irradiation reduced the index contrast and converted the waveguides
to single-mode performance.
This demonstrates the ability to improve coupling efficiency for
28
microphotonic chips using polymer waveguides that better match multi-mode long-haul fibers on
one end and single-mode chip level waveguides on the other.
CHAPTER THREE reports the use of plasma-polymerized hexamethyldisilane
applied as the top cladding layer on microring resonator devices. The layer can be tuned using
irradiation with UV light to alter the effective refractive index of the whole device. This presents
a post-production method of trimming the resonant wavelength of microring resonators, which is
becoming a larger issue as device sizes shrink and exact replication of microrings across a wafer
becomes increasingly difficult.
CHAPTER FOUR reports a chemical vapor deposition process for the application
of thin films of PEDOT that is compatible with a range of substrate materials. The CVD films
demonstrate
electrical properties
and spectroscopic
absorptions
that are comparable to
commercially available solution-based PEDOT materials. CVD has coating characteristics that
may improve the hole-injecting abilities of PEDOT in organic devices where they already enable
50% efficiency gains and longer product lifetimes.
CHAPTER FIVE reports the electrochemical characterization of CVD PEDOT
films. Cyclic voltammetry and chrono amperommetry were used to quantify the electrochromic
response of the CVD PEDOT material. Aside from PEDOT's use as a hole-injecting layer, the
ability to quickly switch the films from a transparent light blue color to an opaque purple makes
CVD PEDOT a good candidate material for use in dynamically tinting window glass or as the
funtional material in some organic displays and large-area "electronic paper" applications.
APPENDIX A provides a comprehensive comparison between CVD polysilane
materials and conventional
spin-on films derived from the Wurtz reaction.
Thorough
characterization of the materials support the conclusion that spin-on polysilane films are the
more appropriate choice for electrical applications compared to plasma polymerized films, which
perform better in optical applications.
Each chapter begins with motivation for the specific topic and includes a survey
of the literature describing work relevant to the chapter's focus. The technical chapters then
29
have a description of the experimental methods employed and a discussion of the measurement
results before ending with a brief conclusion.
Each of the chapters has been formatted as a
technical journal paper, so they may be read independently although the chapters are ordered to
build continuity for the entire thesis. Following the four technical chapters, the thesis concludes
with a summary and thoughts on possible future directions based on the work that has been
presented.
30
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T. L. Porter, K. Caple, and G. Caple, "Structure of Chemically Prepared Freestanding and
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"Characterization of low dielectric constant polyaniline thin film synthesized by ac plasma
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the transparency and stability of the conductivity of plasma polymerised thiophene layers,"
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M. Murashima, K. Tanaka, and T. Yamabe, "Electrical-Conductivity
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34
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T. Sorita, H. Fujioka, M. Inoue, and H. Nakajima, "Formation of Polymerized Thiophene Films
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H. Meng, D. F. Perepichka, M. Bendikov, F. Wudl, G. Z. Pan, W. J. Yu, W. J. Dong, and S.
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35
Chapter Two
TUNABLE WAVEGUIDES VIA PHOTO
OXIDATION OF PLASMA POLYMERIZED
ORGANOSILICON FILMS
Lock JP and Gleason KK. Applied Optics 44(9), 1691-1697 (2005).
36
ABSTRACT
Plasma-enhanced chemical vapor deposition of dimethylsilane and hexamethyldisilane
produced thin films with a refractive index of 1.56 ± 0.01 at 633 nm. A decrease in the
refractive index of about 3% was observed after irradiation with UV light using an ArF
laser operating at 193 nm. Lower intensity UV light from a Hg arc lamp induced a slower
and controllable decrease in the refractive index. Top-side prism coupling showed the asdeposited organosilicon films to be multi mode at 633 nm and single mode at 1550 nm. A
model predicted that 30 seconds of UV irradiation with the Hg arc lamp would decrease
the refractive index of the light-guiding film by about 0.01 converting the waveguide to
single-mode operation across the spectrum of essential wavelengths for microphotonics.
Irradiation followed by further coupling experiments confirmed this tunability. Trimming
the refractive index of patternable organosilicon polymeric films presents a method of
optimizing the coupling performance of PECVD microphotonic interconnect layers post
deposition.
Acknowledgements.
We thank Professor Lionel Kimerling at MIT for access to the Metricon prism
coupler in his lab and Jessica Sandland for training on the machine. This research was supported by, or
supported in part by, the U.S. Army through the Institute for Soldier Nanotechnologies, under Contract
DAAD-19-02-D-0002 with the U.S. Army Research Office. The content does not necessarily reflect the
position of the Government, and no official endorsement should be inferred.
37
2.1
INTRODUCTION
Polymeric waveguide devices are a rapidly developing area of broadband communication
and photonics due to their ease in processibility and integration as compared to conventional
inorganic materials.
Organosilicon polymers deposited using plasma enhanced chemical vapor
deposition (PECVD) are among the materials that meet many of the criteria important in
choosing a light guiding medium.
PECVD presents an all-dry, scalable process that is
compatible with conventional microfabrication techniques2 , having reasonable deposition rates,
low surface roughness, and good uniformity across the wafer.
The highly crosslinked
organosilicon polymeric PECVD films are chemically inert in organic solvents. Organosilicon
polymers are also relatively transparent across the spectrum of communications wavelengths.
The most important communications wavelengths include 840 nm, 1300 nm, and 1550 nm3' 4 and
some microphotonic applications could employ 633 nm light.5 Hornak et al. characterized the
loss of spin-on organosilicon polymers to be as low as 0.5 dB/cm at 633 nm6 and Tien et al
deposited similar materials using PECVD that had losses lower than 0.04 dB/cm at 633 nm.7
The most distinctive advantage of using PECVD organosilicon polymers for waveguide
applications is the ability to change the oxygen content in the film through controlled photo
oxidation.8 Irradiation with UV light shorter than about 300 nm causes a scission of Si-Si and
Si-H bonds in the material followed by the incorporation of oxygen from the atmosphere.9 This
converts the polysilane-like material to one that more resembles polysiloxane.
This photo
oxidation mechanism is depicted in Figure 2-1. The incorporated oxygen causes the molecular
density of the material to decrease, which is accompanied by a decrease in the refractive index.
38
The resulting index contrast can be as much as 15% and has been exploited for the direct
patterning of waveguides 6 and other features as small as 0.5 jtm.l l- 13
R2
2
2
Si
Si --
Si
Rl
R1
R1
hvj
Si- O- Si-O-Si
R1
R1
RI
Figure 2-1: Photo oxidation occurs via an insertion reaction when an Si-Si bond is
irradiated with UV light. This decreases the molecular density of the material and reduces
the refractive index. R1 and R 2 are organic substituents (ie methyl, phenyl, etc).
In addition to enabling directly patternable waveguides, the photo oxidation mechanism
allows the refractive index of organosilicon films to be tuned. It has long been a challenge to
make polymeric waveguides with precise control of the refractive index and current techniques
are time consuming and costly or involve laser writing ribbed features on the surface of
waveguides.' 4 '
15
By irradiating organosilicon polymers with low-intensity UV light sources,
precise trimming and tuning at a controllable rate can take place using a simple Hg arc lamp in
order to correct manufacturing discrepancies of photonic devices or to optimize optical filter or
coupler performance.
16
Tuning the refractive index of organosilicon polymeric films also
39
enables the conversion of multi-mode waveguides into single mode waveguides over the entire
communication spectrum from 633 nm up to 1550 nm. PECVD can easily deposit between 0.6 1 tm of the organosilicon material used for the waveguide core layer and many options exist for
low index materials that can be used for cladding layers in different thicknesses. The resulting
ability to produce waveguides with a variety of cross sectional dimensions and numerical
apertures and toggle from multi-mode to single-mode can be useful for the efficient coupling of
long-haul light from single-mode glass fibers into multi-mode waveguides of photonic devices.14
A number of methods for tuning polymeric waveguides and other optical devices have
been proposed.
Some systems have recently been developed for the reversible tuning of
photonic structures using thermal means.3 '
17
A previous study with PECVD organosilicon
materials looked at irreversible tunability using oxidative annealing. 7 This is an effective tuning
method, but operates on the timescale of minutes rather than seconds.
The same study also
investigated the ability to deposit films with a predetermined refractive index by using a variable
mixture of precursor gases in order to make a polymeric blend of high refractive index material
and low refractive index material. Another method that alters a polymer's refractive index by
charging an electrode patterned using microcontact printing could also address tunability for
polymeric waveguides.'
8
Photobleaching has been used to tune spin-on electro-optical polymers
in devices like filters and waveguide junctions. 9'
20
Applying photobleaching to PECVD
organosilicon material has proven to be effective as well.
UV irradiation of PECVD
organosilicons is a good option when an all-dry process and short conversion times are needed
for the tuning of light guiding films with variable thicknesses.
40
Prism coupling is a useful technique for characterizing the optical properties of light
guiding films.21,22 Prism coupling introduces light into a waveguide through the top surface as
shown in Figure 2-2.
Laser Light
Photo Detector
Film
Substrate
Coupling Head
Figure 2-2: Schematic of the prism coupling technique for measuring optical properties of
light guiding films.
Light couples into a waveguiding material at incident angles such that the phase of a
plane wave in the film is exactly reproduced each time the light reflects .off the bottom interface
of the film and then off the top interface. Any destructive interference quickly quenches light
propagation.
Constructive interference happens at the conditions prescribed by Equation 2-1.
For constructive interference to occur, the total change in the phase of a plane wave after two
passes through the film and a reflection at each of the interfaces must equal 2m7rwhere m = 1, 2,
3,..., is the order of the mode. W is the thickness of the film, n is the refractive index, and Filmis
the incident angle of light inside the film, which can be found based on the angle of light internal
to the prism.
-20Substrate,
The reflections at the top and bottom cladding are described by -20Air and
which are Goos-Haenchen shifts according to Born and Wolf.23
2knW cosO - 2
Air -
2
Substrate
41
2m/IT
(2-1)
Given the refractive index and thickness of a film and the wavelength of coupled light, (2-1 can
be used to predict the number of modes that should be supported by the light guiding medium.
Varying thicknesses of organosilicon polymeric films can be grown using PECVD and
thus waveguides with a range of dimensions can be patterned using existing techniques. In this
paper, the tunability of the refractive index of organosilicons is demonstrated for the conversion
of a waveguide film from multi mode to single mode across a broad range of wavelengths. This
demonstrates high versatility of PECVD organosilicon polymeric light-guiding films for use as a
microphotonics interconnect layer.
2.2
EXPERIMENT
Film depositions were carried out in a custom-built vacuum chamber that has been
described elsewhere.2 4'
25
Quartz slides and silicon wafers were used for substrates.
The
chamber pressure was controlled by a butterfly valve connected to an MKS model 252-A exhaust
valve controller and was maintained at approximately 300 mTorr. A 13.56 MHz rf source and
attached matching network provided capacitively coupled plasma excitation.
A shower head
used for even inlet gas distribution acted as the powered upper electrode and the substrate stage
doubled as the grounded lower electrode. The continuous plasma power was held constant at
50 W. The deposition time was 25 minutes or longer to achieve films that were over 1 pImthick.
For best deposition rate and film uniformity, the stage temperature was maintained at about
50 ° C.
Hexamethyldisilane (Gelest), 6M2S, was used as the organosilicon precursor without
further purification. The monomer vaporized at room temperature and was introduced into the
reactor through the shower head assembly. A flow rate near 10 sccm was maintained with a
needle valve. The reactor and monomer vessel were purged of air to minimize residual oxygen
by pumping the chamber down to 60 mTorr and then filling the chamber nearly to atmospheric
42
pressure with dry nitrogen (BOC, 99.999%). This cycle was performed at least 5 times before
each deposition.
Films deposited using dimethysilane (Gelest), 2MS, as the organosilicon precursor were
used to collect some of the initial contrast curves in this study. The deposition conditions for the
plasma polymerized 2MS films were the same as those described above. The flow rate of the
monomer gas was regulated at 10 sccm using a mass-flow controller.
Contrast curves were collected by irradiating the organosilicon films with varying
intensities of UV light and measuring the effect of the light on the material using variable angle
spectroscopic ellipsometry (VASE).
Plasma polymerized 2MS films were irradiated with
193 nm light using an ArF laser at MIT Lincoln Laboratories. The laser has a spot size of 4 mm
and delivers 50 pulses of light per second. An individual pulse has a fluence of 1 mJ/cm2 . A dry
nitrogen purge stream between the laser and the sample is used to avoid absorption of the
radiation by air. Low-energy contrast curves were obtained for plasma polymerized 6M2S films
using a 350 W Hg arc lamp obtained from Spectra-Physics. The total power density incident on
the sample is 0.5 mW/cm 2 as measured using an Orion PD handheld power meter manufactured
by Ophir Optronics. Ellipsometry was performed using a J.A. Woolam M-2000 spectroscopic
ellipsometer, employing a xenon light source. Data were acquired at three angles (65° , 70° , and
75°) and 225 wavelengths. The Cauchy-Urbach model was used to fit the resulting data yielding
film thickness and the film refractive index at 633 nm.
Top-side prism coupling experiments were conducted using a Model 2010 Prism Coupler
supplied by Metricon Corporation. The prism coupler uses 633 nm and 1550 nm light sources.
43
2.3
RESULTS AND DISCUSSION
2.3.1
UV IRRADIATION OF PECVD
ORGANOSILICON FILMS
The contrast curve for a PECVD grown film from 2MS in response to 193 nm irradiation
is shown in Figure 2-3.
ArF Laser ExposureTime (sec)
0
10
20
30
I
I
1000
1500
40
50
60
2000
2500
3000
0.1
E
CO
C,
0.01 -
o
x
-,
Q)
.t5
I
0.001
I
co
C.,
5,
n nnnœ
0
500
193 nm Dosage (mJ/cm2)
Figure 2-3: Contrast cur ve for a plasma polymerized dimethylsilane film irradiated with
193 nm light. A maximu m refractive index contrast of 0.05 or 3% was achieved with a
dosage of 900 mJ/cm2 .
At a fairly modest dosage of 900 mJ/cm2 , a maximum refractive index contrast of more
than 3% was seen as the refractive index dropped by about 0.05 from an initial value of 1.51
measured at 633 nm. Ellipsometry determined that the irradiated film was uniformly oxidized
throughout its depth of about 1250 A at low exposure dosages of 300 mJ/cm 2 and above. To test
for preferential oxidation at the surface, the film was modeled in VASE as two distinct Cauchy
layers. The refractive index of the bottom layer was assigned the original refractive index of
44
1.51. The thicknesses of the two layers and the refractive index of the top layer were varied to
achieve the best fit between the model and VASE data. In most cases, the combined thickness of
the two layers were within 1% of the total film thickness and for samples irradiated with more
than 300 mJ/cm2 of 193nm light, a thickness of 0 A for the bottom layer resulted in the best fit.
As organosilicon polymeric films are oxidized, they begin to resemble polysiloxane materials
and become increasingly transparent to the incoming UV radiation.
Therefore, underlying
material is exposed as the film bleaches and the refractive index of the film becomes even
throughout. VASE also verified that the film thickness remained constant with exposure to UV
light. Even though oxidation decreases the molecular density, the cross-linked morphology of
the plasma polymerized organosilicon film prevents expansion from occurring.
For a much more gradual and controllable decrease in the refractive index, a Hg arc
lamp is a better irradiation tool as compared to the ArF laser. Its power flux of UV light under
300 nm is about 2 orders of magnitude less than the 193 nm laser. A Hg arc lamp is more
feasible for implementation in a production setting and large surfaces can be evenly irradiated
without stepping. Figure 2-4 shows a contrast curve for plasma polymerized 6M2S irradiated
with a Hg Arc lamp.
45
Hg Arc Lamp ExposureTime (sec)
0
0.1
50
100
150
I
I
I
a*
E
200
250
300
!
I
X
C
Cco
v
0.01
'aC
cu
0.001
I
oa)
rY
0.0001
0
10
20
30
40
2)
UV Under 300 nm Dosage (mJ/cm
Figure 2-4: Contrast curve for a plasma polymerized hexamethyldisilane
with a Hg arc lamp.
2.3.2
film irradiated
COUPLING AND TUNING OF SLAB MODE WAVEGUIDES
Light guiding in a film of plasma polymerized 6M2S was analyzed using prism coupling.
Two distinct modes of 633 nm light were supported in the film as shown by the sharp decreases
in the measured intensity in Figure 2-5a. Only one mode was seen in the sample for 1550 nm
light as seen in Figure 2-5b. The steady decrease in intensity below 48 ° corresponds to light that
coupled into the quartz substrate of the sample when the incident angle inside the prism was too
small for internal reflection to occur in the film.
46
(a)
511D I
I
*-
I
Con,\
I
(b)
I
--
i
Uoo
I
-
I
Ii1
_~~-
I-c~7r
i
400
3cc
(
t~~~~~~~~~~~~~~n
_
I
I,
300 ...
o
300
r~~~~~~~~~~~~~~~~.
~~~~~~~I
'n
100
L_
200
100
100
n
- L
52
51
50
- I
-
49
?
I
-
48
47
46
52
51
50
49
,~~~~~~~~~~~~~~~~~~~~
48
47
46
PrismInternalAngle(deg)
PrismInternalAngle(deg)
Figure 2-5: a) Two modes of 633 nm light are coupled into this 0.79 Am thick plasma
polymerized 6M2S film. b) In the same sample, only one mode of 1550 nm light is
supported.
Since two modes were present at 633 nm, the Metricon device was able to report
independent values for the refractive index and the thickness of the film. The refractive index
was 1.5662 and the film was 0.79 pimthick. With only one mode at 1550 nm, the film thickness
was specified to be 0.79 [tm in order to calculate a refractive index of 1.58. Generally, the
refractive index of a polymer is expected to decrease with increasing wavelength.
The
exaggerated refractive index at 1550 nm is indicative of the specified film thickness being too
low. The effective thickness of the waveguide might in fact be higher at 1550 nm due to a
greater extent of evanescent coupling at the longer wavelength.
However, by assuming a
constant film thickness throughout the experiment, reported contrasts or absolute differences in
the refractive index should have a relatively low error since the crosslinked material is not
expected to change thickness due to swelling or other effects.
47
With the experimental values of refractive index and thickness, the 6M2S film was then
modeled using Equation 2-1 in order to verify the number of modes observed with those
expected in theory. The results are shown in Figure 2-6 for 633 nm (a) and 1550 nm (b):
(a)
(b)
0.9
0.8
0.8
E
i0
u)
E
0.6
0
0.7
C
c
E
0.6
0.4
E
.
0.5
0.2
0.4
0.3
n
1.46
1.48
1.5
1.52
1.54
1.5'
l/k
1.46
1.48
1.5
1.52
1.54
1.56
1/k
Figure 2-6: a) Two modes can be supported by a 0.79 gam film of 6M2S. b) One mode of
1550 nm light can be coupled to this film.
In each figure, a solid black line represents a mode. The bottom one corresponds to the first
mode and the next one corresponds to the second mode. The modes are plotted in terms of fl/k vs
the film thickness.
/k is a ratio of the speed of light in vacuum to the speed of light propagation
in the film and is equal to nsinOFilm.If a curve representing a mode intersects the film thickness,
represented by a dashed line, the model predicts that the mode will couple into the waveguide
film.
The model was then used to determine if the 6M2S sample could have a refractive index
that would give the waveguide single-mode performance for both 633 nm and 1550 nm,
spanning the entire range of critical communications wavelengths. By decreasing the refractive
48
index by only 0.01 or less than 1% and keeping the thickness constant, the model predicted that
only one mode would be present for 633 nm without adversely affecting the single-mode
performance already present at 1550 nm. These predictions are shown in Figure 2-7.
(a)
(b)
0.9
0.9
0.8
E
E
e
cn
0.7
.,
0.6
)
E
(n
a)
C
0.5
E
iT_
0.8
0.7
0.6
0.4
0.5
C0.3
Cl?
1.46
1.48
1.5
1.52
1.54
1.5f
lk
0.4
1.46
1.48
1.5
1.52
1.54
1.56
lk
Figure 2-7: For the 6M2S sample with 0.79 m thickness, the refractive index would have to
be decreased slightly by 0.01 or 1% to have single mode performance at both 633 nm (a) and
1550 nm (b).
Based on the model results, the 6M2S sample was irradiated with the Hg Arc lamp for 30
sec. The contrast curve from Figure 2-4 was used to select an exposure time that would decrease
the refractive index of the sample without overexposing it. After the Hg Arc lamp irradiation,
the sample was again characterized using the prism coupler and these results are shown in Figure
2-8. Indeed the sample continued to host the single mode for 1550 nm light. The first mode for
633 nm light also remains, but the second mode has been practically eliminated. It now overlaps
the coupling region for the quartz substrate. Although not measured, the model predicts that this
film will be single mode for 840 nm and 1310 nm, too.
The refractive index of the film
decreased by about 0.01 from 1.566 to 1.557 and by 0.06 from 1.580 to 1.574 for 633 nm light
49
and 1550 nm light, respectively.
A lack of swelling in crosslinked PECVD organosilicon
material suggests that the waveguide dimensions remain essentially constant with irradiation.
(b)
(a)
500
500
4)0
400
Zn
C
6)
A.Ak
(6
2C4oC)
300
~3
(n
C:
7--% .,,
300
-o
M,
6)
0)
u,
. ....................... ........................ .............................. .....................i..........................I.... ............. .....
200
3)
200
-
E
100
..
100
0
0
52
51
50
49
48
47
46
PrismInternalAngle(deg)
52
I
I
51
50
49
48
47
PrismInternalAngle(deg)
Figure 2-8: Prism coupling measurements after UV irradiation confirm tunability of 6M2S
film. The 6M2S now has single-mode performance for both 633 nm (a) and 1550 nm (b).
Organosilicon
polymers
deposited with PECVD are extremely
interconnect material for microphotonic applications.
versatile as an
Other polymeric materials also have
inexpensive methods of synthesis, but only PECVD organosilicon polymers have a tunable
refractive index that can be used to convert multi-mode waveguides into single-mode operation
over the entire visible and near infrared spectrum of communications wavelengths most useful
for microphotonic devices.
50
46
2.4
REFERENCES
1.
H. Ma, A. K. Y. Jen, and L. R. Dalton, "Polymer-based optical waveguides: Materials,
processing, and devices," Advanced Materials 14(19), 1339-1365 (2002).
2.
A. Grill, L. Perraud, V. Patel, C. Jahnes, and S. Cohen, "Low dielectric constant SiCOH films as
potential candidates for interconnect dielectrics," Mater Res Soc Symp Proc 565, 107-116 (1999).
3.
L. Eldada, "Polymer microphotonics," Proc SPIE 5225, 49-60 (2003).
4.
L. Eldada, "Polymer integrated optics: Promise vs. practicality," Proc SPIE 4642, 11-22 (2002).
5.
K. Wada, H.-C. Luan, D. R. C. Lim, and L. C. Kimerling, "On-chip interconnection beyond
semiconductor roadmap. Silicon microphotonics," Proc SPIE 4870, 365-371 (2002).
6.
L. A. Hornak, T. W. Wedman, and E. W. Kwock, "Polyalkylsilyne photodefined thin-film optical
waveguides," J Appl Phys 67(5), 2235-2239 (1990).
7.
P. K. Tien, G. Smolinsky, and R. J. Martin, "Thin organosilicon films for integrated optics,"
Applied Optics 11(3), 637-642 (1972).
8.
R. D. Miller and J. Michl, "Polysilane high polymers," American Chemical Society 89(6), 13591410 (1989).
9.
F. C. Shilling, T. W. Weidman, and A. M. Joshi, "Solid-state characterization of polysilanes
containing the SiH bond," Macromolecular Symposia 86, 131-143 (1994).
10.
A. Watanabe, T. Komatsubara, O. Ito, and M. Matsuda, "SiC/SiO2 micropatterning by ultraviolet
irradiation and heat treatment of a poly(phenylsilyne) film," J Appl Phys 77(6), 2796-2800
(1995).
11.
C. Monget and 0. Joubert, "Plasm polymerized methylsilane II.
performance for 248 nm
lithography," J Vac Sci Technol B 18(2), 785-792 (2000).
12.
0. Joubert, D. Fuard, C. Monget, and T. Weidman, "Plasma polymerized methylsilane III process
optimization for 193 nm lithography applications," J Vac Sci Technol B 18(2), 793-798 (2000).
13.
R. R. Kunz, M. Rothschild, D. J. Ehrlich, S. P. Sawan, and Y.G.Tsai, "Controlled-ambient
photolithography of polysilane resists at 193 nm," J Vac Sci Technol B 7(6), 1629-1633 (1989).
51
14.
L. Eldada, C. Xu, K. M. T. Stengel, L. W. Shacklette, and J. T. Yardley, "Laser-fabricated lowloss single-mode raised-rib waveguiding devices in polymers," Journal of Lightwave Technology
14(7), 1704-1713 (1996).
15.
R. Moosburger and K. Petermann, "4 x 4 digital optical matrix switch using polymeric oversized
rib waveguides," IEEE Photonics Technology Letters 10(5), 684-686 (1998).
16.
K. Yasuo, S. Shinya, B. Gokon, S. Seitoku, E. Soichi, S. Shuichi, . Takashi, K. Keiji, and
S.Shinichiro, "Central wavelength adjustment method for asymmetric directional coupler type
wavelength filter and asymmetric directional coupler type wavelength filter," JP2000075151
(2000).
17.
M. R. Kostrzewa C, Fischbeck G, Schuppert B, and Petermann K, "Tunable polymer optical
add/drop filter for multiwavelength networks," IEEE Photonics Technology Letters 9(11), 14871489 (1997).
18.
D. B. Wolfe, J. C. Love, B. D. Gate, and G. M. Whitesides, "Fabrication of planar optical
waveguides by electrical microcontact printing," Appl Phys Lett 84(10), 1623-1625 (2004).
19.
K. J.-J. Hwang W-Y, Zyung T, Oh M-C, and Shin S-Y, "Postphotobleaching
method for the
control of coupling constant in an electro-optic polymer directional coupler switch," Appl Phys
Lett 67(6), 763-765 (1995).
20.
C. V. Chen A, Marti-Carrera FI, Garner G, Steier WH, and R. Y. Mao SSH, Dalton LR, and Shi
Y, "Trimming of Polymer Waveguide Y-Junction by Rapid Photobleaching for Tuning the Power
Splitting Ratio," IEEE Photonics Technology Letters 9(11), 1499-1501 (1997).
21.
K. W. Beeson, K. A. Horn, M. McFarland, and J. T. Yardley, "Photochemical laser writing of
polymeric optical waveguides," Appl Phys Lett 58(18), 1955-1957 (1991).
22.
P. K. Tien, R. Ulrich, and R. J. Martin, "Modes of propagating light waves in thin deposited
semiconductor films," Appl Phys Lett 14, 291 (1969).
23.
M. Born and E. Wolf, Principles of Optics (Pergamon, New York, 1970).
24.
D. D. Burkey and K. K. Gleason, "Structure and mechanical properties of thin films deposited
from 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane
and water," J Appl Phys 93(9), 5143-5150
(2003).
25.
H. G.
Pryce-Lewis,
D. J.
hexamethylcyclotrisiloxane
Edell,
and K. K.
Gleason,
"Pulsed-PECVD
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from
for use as insulating biomaterials," Chem Mater 12, 3488-3494
(2000).
52
Chapter Three
TRIMMING OF MICRORING
RESONATORS USING PHOTO-OXIDATION OF
A PLASMA-POLYMERIZED ORGANOSILANE
CLADDING MATERIAL
Lock JP, Sparacin DK, Hong C, Michel J, Kimerling LC, and Gleason KK. Applied Optics
(2005). In Press.
53
ABSTRACT
As the complexity of microphotonic devices grows, the ability to precisely trim microring
resonators becomes increasingly important. Photo-oxidation trimming uses UV irradiation
to oxidize a cladding
layer composed of polymerized
hexamethyldisilane
(6M2S)
deposited with plasma-enhanced chemical vapor deposition (PECVD). PECVD 6M2S has
optical properties that are compatible with microring devices and its high crosslinking
renders it insoluble. Photo-oxidation decreases the refractive index of PECVD 6M2S by
nearly 4%, from n=1.52 to n=1.46, enabling large, localized resonance shifts that are not
feasible with thermal trimming techniques.
Resonance shifts from single-mode, 100-pm
diameter Si3N 4 (n=2.2) rings were as large as 12.8 nm for the TE mode and 23.5 nm for the
TM mode.
Experimental results were compared with shifts predicted by theory.
As a
quick and localized technique to produce large and precise resonance shifts, photooxidation trimming provides an attractive alternative to conventional trimming methods.
Acknowledgements.
We would like to thank Gilles Benoit for his help using the Sopra GES 5
spectroscopic ellipsometer and Professor Yoel Fink for access to the equipment.
This work was
supported in part by the MRSEC Program of the National Science Foundation under Contract No. DMR
02-13282 and the U.S. Army through the Institute for Soldier Nanotechnologies, under Contract DAAD19-02-D-0002 with the U.S. Army Research Office. The content does not necessarily reflect the position
of the Government, and no official endorsement should be inferred.
54
3.1
INTRODUCTION
Microring resonators are a basic building block of photonic circuits, enabling complex
functionality for optical systems.
Ring resonators can serve as filters for multiplexing and
demultiplexing broadband optical signals l ' 2, dispersion compensators for accurately controlling
phase3 , lasers4 , and ultrafast all-optical switches5. Figure 3-1 shows the components of a basic
ring resonator device. The ring resonance condition is satisfied when the circumference of the
ring is an integer multiple of the wavelength. For the case of resonance, light coupled to the
input port propagates through the bus waveguide, evanescently couples into the ring and exits the
drop port. Otherwise, light continues in the bus waveguide and exits at the through port. Ring
resonators can be theoretically designed to have ideal channel dropping characteristics: a broad,
steeply sloped, flat-topped spectral response with 100% efficiency 2 .
[22111112,;i1
DropPort
Input Port
ThroughPort
BusWaveguide
Figure 3-1: Schematic of a ring resonator device.
The resonance condition for a microring is described by Equation 3-1 where D is the
diameter of the ring, Xois the free space wavelength of resonant light, m is an integer indicating
the resonator mode number, and n is the effective index of the ring:
55
(m
(3-1)
Precise control over the frequency and wavelength of the resonance condition in each
ring is critical for microphotonics integration. As ring diameters shrink to less than 10 gm, nondeterministic pattern transfer errors limit dimensional precision and preclude the fabrication of
identical devices across an entire wafer.
Even small deviations in film thickness, index of
refraction, and etch processes across a wafer can typically shift the resonant wavelength by
several nanometers.
To compensate for this inherent variability, a post-production trimming
process to precisely define the resonant condition is essential. The basic strategy of trimming is
to change the optical path length of the ring by modifying its effective index. The effective
index depends on the geometry and dimensions of the device, the refractive index of the
waveguide, and the refractive index of the cladding. Trimming usually utilizes resistive micro
heater to induce a thermo-optic response of materials 2 ' 6, where (dn/dT) is typically 10-5-10-4 K- l
for dielectrics and negative 1-4x10-4 K- 1 for polymers. However, the small magnitude of these
thermo-optic coefficients corresponds to a feasible trimming range of only a couple nanometers.
Also, thermal trimming is not capable of localized index trimming on separate discrete areas of
filters that have multiple rings.
An alternative trimming method uses an organosilicon polymer film as the cladding
material and its refractive index is adjusted via photo-oxidation when irradiated with ultraviolet
light. This effect has been demonstrated using a dip-coating technique to coat ring resonators
with a polysilane material 7. Having a refractive index similar to SiO 2, which is the predominant
material of choice for cladding layers, organosilicon polymers can easily be integrated with Si,
SiON, and Si3N4 high index-contrast microring resonators. The material is transmissive over a
broad range of visible and near-IR light8 ' 9 useful for microphotonic applications l° . However,
polymers deposited from solution using dip coating or spin coating can often be redissolved
making the film incompatible with subsequent rinse steps in the microfabrication process. The
56
low degree of crosslinking in solution-based polymers can also make them prone to swelling
when irradiated or contacted with chemical solvents.
As a cladding material, this change in
thickness can inadvertently affect the resonant condition of a microring device.
Using a plasma process to deposit polymerized 6M2S directly onto ring resonators yields
an amorphous and highly cross-linked top cladding layer. PECVD 6M2S is insoluble, does not
swell in organic solvents, and demonstrates good stability in ambient light, atmosphere, and
temperature.
Photo-oxidation trimming was tested on single-mode Si3N 4 ring resonators with
PECVD 6M2S top cladding and resulting resonance shifts were compared to a theoretical model.
3.2
EXPERIMENT
Si3N4 waveguides, designed for single mode operation at X=1550 nm, were fabricated
from a 0.4 glm Si3N4 film deposited in a vertical LPCVD system at 675°C onto 3 lm of oxide as
a bottom cladding layer on (100) Si. A 0.15 lpm polysilicon hard mask layer was deposited in a
LPCVD system at 6250 C. A pattern defining the waveguides and rings was transferred to the
hard mask using a stepper with a 365 nm light source. The features were then defined with a dry
etch process have the following conditions: 96 sccm of Cl 2, 134 sccm of He, a pressure of 400
mtorr, and an RF power of 200 W. The photoresist was removed using an oxygen plasma and
then the waveguide and ring pattern was transferred to the underlying Si3N4 using a dry etch
process with 30 sccm of CHF 3 , a pressure of 25 mtorr, 500 W of RF power, and 90 gauss bias.
A quick dip in HI-Fsolution removed the native oxide layer followed by an immersion in TMAH
solution at 80°C to remove the polysilicon hard mask. The microrings have a diameter of 100
gm and the Si3 N4 waveguides have cross-sectional dimensions of 400 x 750 nm2 .
The top cladding material was deposited directly onto ring resonator devices in a custombuilt vacuum chamber with a 13.56MHz radio frequency plasma source that has been described
elsewhere.1 1 ' 12
The Si3N4 ring resonators were initially cleaned by exposing them to an oxygen
plasma etch for 30 minutes at 100 mTorr, 10 sccm of 02, and 100 W of power. Before the
57
cladding material was deposited, the reactor and monomer vessel were purged to minimize
residual oxygen by pumping the chamber down to 60 mTorr and then filling the chamber nearly
to atmospheric pressure with dry nitrogen (BOC, 99.999%). This cycle was performed 5 times.
6M2S (Gelest), was used as the precursor for the cladding layer. The monomer vaporized at
room temperature and was introduced into the reactor through a shower head assembly. A flow
rate near 10 sccm was maintained with a needle valve. The chamber pressure was controlled at
300 mTorr and a continuous plasma power was held constant at 50 W. The deposition time was
30 minutes achieving films that were approximately 1 gm thick. For the best deposition rate and
film uniformity, the stage temperature was maintained at about 50° C.
Once coated with the organosilane film, waveguide
measurement by cleaving the wafer into individual chips.
samples were prepared for
Spectral characterization of the
samples was done in both TE and TM polarizations by a C+L band, JDS Uniphase swept
wavelength system (tunable laser and broadband photodetector) used in conjunction with a
Newport Auto-Align System. Light was coupled to and from the chip facets by way of SMF-28
fiber with index matching fluid.
Prior to each spectral measurement, the waveguides were
aligned using a separate 1540 nm diode laser and integrating sphere for detection.
MINERALIGHT® handheld lamp (model UVGL-25) emitting
A
=254 nm light positioned above
the sample holder was used as the UV source. The power density incident on the sample was
1.7 tW/cm2 as measured using an Orion PD handheld power meter manufactured by Ophir
Optronics. Spectral measurements were taken after each of a series of exposures to the UV light.
Resonance shifts were calculated by comparing spectral data before and after each UV
irradiation.
The magnitude of the shift was also estimated from in-situ power measurements
from the alignment laser during the UV exposure.
The same UV irradiation process was repeated on a chip from the same wafer and the
refractive index of the 6M2S film was measured after each dosage using a Woolam M-2000
variable angle spectroscopic ellipsometer (VASE). Data were acquired at three angles (65 °, 70 °,
and 75 °) over a range of wavelengths from 450 nm to 740 nm. A Sopra GES5 ellipsometer
58
capable of operating between 800 nm to 1750 nm was used to measure a limited number of
samples to validate results from the Woolam ellipsometer.
3.3
DiSCUSSON
3.3.1
RING THEORY
A model was derived to predict resonance shifts for a given change in effective index of
the microring resonator assuming single-mode operation. In Equation 3-2, AX is defined as the
resonance shift resulting from a trimming process and Ao,1 and 4,2 are resonant free-space
wavelengths where the subscripts 1 and 2 refer to before and after UV irradiation, respectively.
,
i,m2
0 ) lnj
(3-2)
Since the resonant wavelength shift is continuous, the mode number remains fixed (ml
= m2)
throughout the shift, yielding:
2=
(?n, 1)
(3-3)
The effective index of the ring resonator system can be solved numerically by Apollo Mode
Solver software after providing values for layer thicknesses, cross-sectional dimensions, and
material refractive indexes. This provides a link between the refractive index of the cladding
material and the expected shift in the resonant wavelength.
3.3.2
CHARACTERIZATION OF CVD FILMS
The refractive index of organosilicon polymers can be reduced by a controllable amount
using the photo oxidation mechanism illustrated in Figure 3-2. Irradiation with high energy UV
light, having a wavelength less than 315 nm, causes chain scission in the polymeric material and
59
subsequent oxidation converts Si-Si bonds into Si-O-Si bonds.
This reduces the molecular
density of the material leading to the decrease in the refractive index' 3 .
R2
l
-SiR,
R2
R2
Si
Si --
RI
R,
1.53
(A
hv
oD
R2
2
1R2
3
0.
0
Si-O-SiRI
RI
Figure 3-2:
O-Si
RI
1.46
UV irradiation causes scission of Si-Si bonds allowing oxygen incorporation,
which lowers the refractive index of the material.
VASE can be used to monitor the thickness and refractive index of plasma polymerized
6M2S films. The Cauchy-Urbach model 14 is fit to ellipsometry experimental data to determine
the film thickness and optical constants. The model is valid for organosilicon polymers since
they are amorphous and transmissive in the visible and near-IR region.
Figure 3-3 shows
experimental ellipsometry data for one measurement and a fit of the data to the Cauchy-Urbach
model.
60
11nn
-,-
I
I
I
'
I
I
8
S
6
elFit
E 65°
E 70°
E 75°
co
-4
2
300
400
500
600
700
800
Wavelength (nm)
Figure 3-3: Fitting the Cauchy-Urbach model to ellipsometry data yields the thickness and
optical constants of plasma polymerized 6M2S films.
The microring resonators in this experiment were characterized using 1550 nm light and
it was necessary to determine the refractive index of the CVD 6M2S material at the same
wavelength.
However, the Woolam ellipsometer, which has a significantly quicker collection
time than the Sopra GES5 employs a xenon light source and optics that limit its range to
wavelengths between 450 nm and 720 nm. Equation 3-4 was used to calculate the refractive
index of the film at 1550 nm based on the Cauchy-Urbach model.
n(2) = A, + 2,n +,4
l
j4
(3-4)
Extrapolated refractive index values at 1550 nm were validated by repeating a few
measurements with the Sopra GES5 ellipsometer, which operates between 800 and 1750 nm.
Figure 3-4 shows reasonable agreement between refractive index contrast measurements that
were collected with each of the ellipsometers.
The refractive index contrast is collected by
measuring the refractive index of the as-deposited CVD 6M2S film, irradiating it with UV light
61
and then measuring the new refractive index. The refractive index contrast is defined as the
decrease in refractive index (the initial value minus the final value).
0.04
n 1550 nm IR
0.03
n e 633 nm VIS
C
co
0.02
x
rr
C
A
WE
0.01
t"
0
I
200
400
600
800
1000
1200
-0.01
2
UV Flux (tLJ/cm )
Figure 3-4: Reasonable agreement is seen between refractive index contrast results at
1550 nm collected using an ellipsometer operating in the visible range (450 to 720 nm) and
an ellipsometer operating in the near-IR (800 to 1750 nm).
3.4
RESULTS
Figure 3-5 shows the controllable decrease in the refractive index of a 6M2S layer
achieved using photo-oxidation induced by UV irradiation. The overall decrease is - 4%, from
1.52 to 1.46 at X=1550 nm.
The overall index change is about 50% greater than the response
observed with dip-coated polysilane material.
62
1.531.52- :',
E
c 1.510
Uo
EllipsometryData
Exponential Fit
·
,
......
-
..t
1.50U.
x 1.49-
"0
C
c- 1.48-
'I.~~~~~~I
Cu
> 1.47-
*SU^^~~~
0
1.461.45-
,I'1 1
,'
I,
,
.
·
·
·
,
I
·
100 200 300 400 500 600 700 800 900 1000 1100
UV Flux (pJ/cm2)
Figure 3-5: The refractive index of PECVD 6M2S cladding material decreases with UV
irradiation as a result of photo-oxidation.
Output power from microring resonators coated with plasma polymerized 6M2S as a
cladding layer was monitored in situ during UV trimming. Figure 3-6 displays the results for two
specific exposure times where minima occur at the resonant wavelengths. The resonant
wavelengths were observed to shift continuously with irradiation of the 6M2S cladding. The
magnitude of the shift sometimes exceeded the free spectral range (FSR) of the TM polarization
(4.5 nm). Similar results were obtained for the TE polarization (3.9 nm). The overall resonance
shifts, after a UV flux of 1000 J/cm2 , were 12.8 nm for the TE mode and 23.5 nm for the TM
mode (Figure 3-4).
63
-J
1550
1552
1554
1556
1558
1560
1562
1564
Wavelength (nm)
Figure 3-6: TM mode spectral measurements of a 100 Am Si3 N4 ring resonator (o, 1=1564.5
nm) after 300 and 420 seconds of UV irradiation at 1.7 aW/cm 2 .
To compare experiment with theory, Apollo Mode Solver software was used to correlate
the effective index of the ring resonator and the refractive index data of the cladding material for
each polarization. The exponential fit in Figure 3-5 gave an empirical relation between the flux
of UV irradiation and the refractive index of the cladding. Using Equation 3-3, resonance shifts
were calculated for each polarization and the predicted shifts agree fairly well with the
experimental data in terms of functional form and magnitude for both polarizations as seen in
Figure 3-7. Deviations are likely caused by uncertainties in the cladding thickness, which can
lead to errors in the refractive index values reported by ellipsometry to generate the contrast
curve. Although swelling of plasma polymerized 6M2S with oxidation is very minor due to
crosslinking, the thickness of the top cladding layer varies by about 1% from the beginning to the
end of the trimming process.
Additionally, deposition uniformity of the 6M2S thickness can
vary by another 1 or 2% from sample to sample.
The larger deviation between theory and
experiment for the TM mode, when compared to the TE mode, is most likely caused by these
thickness variations.
The effective index of the TE mode is primarily sensitive to in-plane
64
variations of the core/cladding thickness and or index, such as sidewall roughness, which is a
major source of loss for waveguide devices. Analogously, the effective index of the TM mode is
sensitive to variations out of plane (cladding thickness), which are relatively large considering
how thin the top cladding layer is relative to the evanescent tail length.
25
E
c 20
Q)
8 15
Cu
C
a)
a: 10
0o
()
0
5
0
UV Flux (pJ/cm2)
Figure 3-7: The experimental resonance shifts for TE and TM polarizations are compared
with modeled results.
The rate of the resonance wavelength shift, (d(AX)/dt), is an indicator of the shift
resolution.
This value ranged from about 0.13 nm/s during the initial steep portion of the
exponential curve to as low as 0.01 nm/s in the shallow region. A lower power UV source can
reduce this shift rate and presumably increase the precision if needed. Trimmed ring resonators
were measured over a range of temperatures from 25 to 70°C and the thermal-trimming
coefficient of the resonance shift was found to be -0.10 nm/K corresponding to a thermo-optic
coefficient (dn/clT) for the system of -1.3x104 K'.
This value is on par with other polymer
based ring resonator devices15 . Many polymers used as waveguide cladding materials undergo
densification over a span of weeks or months causing drifts in the refractive index on the order of
10-4 16 corresponding to a AX of -0.1 nm. Although this effect has not yet been quantified for
65
PECVD 6M2S, long-term variances in X0 could be counteracted with a global heater, assuming
uniform aging of the locally trimmed film.
During this experiment, multiple rings were exposed by irradiating the entire chip.
However, the precise nature of this trimming method, coupled with localization of the UV
exposure, can be used to preserve the spectral response of higher order filters that require
multiple rings. This setup would be possible with the use of a microscope and focusing optics
made of quartz or another material transparent to the high energy UV light. Differences between
ring-to-ring and ring-to-bus coupling require localized index trimming on separate areas of the
filter to keep all rings in resonance with each other.
Reducing the refractive index of plasma-polymerized 6M2S is an irreversible process due
to the nature of the photo-oxidation reaction. However, the process allows for some error. For
ring resonators that have small FSRs compared to the maximum desired resonant wavelength
shift, as was the case with the rings used in this experiment, overexposing a ring can be remedied
by simply using more UV light and shifting the resonance one more FSR length.
3.5
CONCLUSIONS
Trimming microring resonators with UV irradiation of a photosensitive top cladding
material allows large resonance shifts compared to conventional trimming techniques and the
ability to localize trimming to very small features of a ring resonator. PECVD 6M2S is a highly
crosslinked and insoluble polymeric material with a refractive index that can decrease through
photo-oxidation by nearly 4%. This index change allows for resonance shifts as large as AX=
23.5 nm with Si3N4 rings. Such a large response is an order of magnitude greater than can be
realized with thermal trimming of Si3N4 ring resonators and is estimated to be 50% more than
what has been shown using a dip-coated polysilane cladding layer. Drifts in the resonance due to
temperature changes or polymer aging are small enough to be maintained long-term with
conventional thermal trimming methods.
66
3.6
REFERENCES
1.
T. Barwicz, M. A. Popovic,
P. T. Rakich, M. R. Watts, H. A. Haus, E. P. Ippen, and H. I. Smith,
"Microring-resonator-based
add-drop filters in SiN: fabrication and analysis," Optics Express
12(7), 1437-1442 (2004).
2.
B. E. Little, S. T. Chu, H. A. Haus, J. Foresi, and J. P. Laine, "Microring resonator channel
dropping filters," Journal of Lightwave Technology 15(6), 998-1005 (1997).
3.
C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo, L. T. Gomez, T. N. Nielsen, and I. Brener,
"Multistage dispersion compensator using ring resonators," Optics Letters 24(22), 1555-1557
(1999).
4.
S. J. Choi, Z. Peng, Q. Yang, S. J. Choi, and P. D. Dapkus, "Eight-channel microdisk CW laser
arrays vertically coupled to common output bus waveguides," Ieee Photonics Technology Letters
16(2), 356-358 (2004).
5.
V. R. Almhneida,C. A. Barrios, R. R. Panepucci, and M. Lipson, "All-optical control of light on a
silicon chip," Nature 431(7012), 1081-1084 (2004).
6.
P. Heimala, P. Katila, J. Aarnio, and A. Heinamaki, "Thermally tunable integrated optical ring
resonator with poly-Si thermistor," Journal of Lightwave Technology 14(10), 2260-2267 (1996).
7.
S. T. Chu, W. G. Pan, S. Sato, T. Kaneko, B. E. Little, and Y. Kokubun, "Wavelength trimming
of a microring resonator filter by means of a UV sensitive polymer overlay," Ieee Photonics
Technology Letters 11(6), 688-690 (1999).
8.
L. A. Hornak, T. W. Weidman, and E. W. Kwock, "Polyalkylsilyne Photodefined Thin-Film
Optical Wave-Guides," Journal of Applied Physics 67(5), 2235-2239 (1990).
9.
P. K. Tien, G. Smolinsky, and R. J. Martin, "Thin organosilicon films for integrated optics,"
Applied Optics 11(3), 637-642 (1972).
10.
L. Eldada, "Polymer microphotonics," Proc SPIE 5225, 49-60 (2003).
11.
D. D. Burkey and K. K. Gleason, "Structure and mechanical properties of thin films deposited
from 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane
(2003).
67
and water," J Appl Phys 93(9), 5143-5150
12.
H.
G. Pryce-Lewis,
D.
hexamethylcyclotrisiloxane
J. Edell,
and
K. K.
Gleason,
"Pulsed-PECVD
films
from
for use as insulating biomaterials," Chem Mater 12, 3488-3494
(2000).
13.
R. D. Miller and J. Michl, "Polysilane High Polymers," Chemical Reviews 89(6), 1359-1410
(1989).
14.
H. G. Tomkins and W. A. McGahan, Spectroscopic Ellipsometry and Reflectometry (WileyInterscience, New York, 1999).
15.
P. Rabiei and W. H. Steier, "Tunable polymer double micro-ring filters," Ieee Photonics
Technology Letters 15(9), 1255-1257 (2003).
16.
C. G. Robertson and G. L. Wilkes, "Refractive index: a probe for monitoring volume relaxation
during physical aging of glassy polymers," Polymer 39(11), 2129-2133 (1998).
68
Chapter Four
CHEMICAL VAPOR DEPOSITION OF THIN
FILMS OF ELECTRICALLY CONDUCTING
PEDOT
Lock JP and Gleason KK. Manuscript in preparation for submission.
69
ABSTRACT
A general transition is underway to use organic materials to produce electronic devices like
OLED displays and plastic solar cells that can be produced at a fraction of the cost of traditional
inorganic semiconductors and offer good operating efficiencies and thinner, more flexible form
factors. Chemical vapor deposition is compatible with a wide variety of substrate materials and
can form thin films of electrically conducting polymers on high surface-area features leading to
the possibility of enhanced device performance without the need for solution-based fabrication
steps. A CVD process has been demonstrated for the deposition of PEDOT with a conductivity
of 4.37 S/cm that is spectroscopically comparable to commercial material. Multiple variations of
the technique are presented that aim to promote the polymerization of conjugated PEDOT while
reducing unwanted side reactions that can inhibit the electrical performance of the material.
Extendable to other oxidatively polymerized organic conductors like polypyrrole, polyaniline,
polythiophene, and their derivatives, CVD offers an all-dry route for incorporating electronic
polymers into organic semiconductor devices.
Acknowledgements. This research was supported by, or supported in part by, the U.S. Army
through the Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-D-0002 with
the U.S. Army Research Office. The content does not necessarily reflect the position of the
Government, and no official endorsement should be inferred.
70
4.1
INTRODUCTION
In 1977, the field of conducting polymeric materials, also known as synthetic metals,
began with the discovery that polyacetylene conducts electricity'.
Recent reviews examine
numerous efforts to incorporate conducting polymers into an increasing number of electronic
devices including light-emitting diodes (LEDs)2' 3, electrochromic materials and structures4 ,
microelectronics 5 6, portable and large-area displays7 , and photovoltaics 8. Perhaps the most
promising conducting polymer so far is poly-3,4-ethylenedioxythiophene
by scientists at Bayer AG9 -1 1.
(PEDOT) developed
PEDOT is extremely stable, nearly transparent, and has an
exceptionally high conductivity of 300 S/cm12 ' 13
Electropolymerization of EDOT has traditionally been the most common deposition
technique for PEDOT and other conducting polymers.
Electrode coatings and free-standing' 4
PEDOT films with conductivities around 300 S/cm are possible, which is an order of magnitude
higher than the conductivity of polypyrrole films deposited using the same method' 5 ' 8 .
Chemical oxidative polymerization of EDOT in a solution containing oxidants like Fe(III)C13 or
Fe(III) tosylate yields PEDOT material with similar conductivities. The reaction mixture can be
cast on a surface leaving a polymerized film as the solvent evaporates 19 22' and films can deposit
on substrates that are immersed in the polymerizing reaction mixture.
However, like other conjugated polymers that have a very rigid conformation in order to
maintain electron orbital overlap along the backbone, PEDOT is insoluble and does not melt,
precluding subsequent processing2 3 . Bayer circumvented this problem by using a water soluble
polyanion, polystyrene sulfornic acid (PSS), during polymerization as the charge-balancing
dopant. The aqueous PEDOT:PSS system, now marketed as BAYTRON
PTM,
has a good shelf
life and film forming capabilities while maintaining its transparency, stability, and a conductivity
of 10 S/cm. BAYTRON P is currently applied as an anti-static coating on photographic film24 '26
for preventing sparks that can appear as a bright spot on developed photos, as an electrode
71
material in capacitors, and for through-hole plating of printed circuit boards2 7 3 1. BAYTRON P
has also been found to be suitable as a hole-injecting layer in LEDs and photovoltaics, increasing
device efficiency by up to 50%32, 33. However, the PSS dopant incorporates a non-conducting
matrix material and unused anions can cause corrosion in devices. Also, the liquid has different
film-forming characteristics depending on whether the substrate is glass, plastic, or other active
layers in a device.
Finally, some devices simply are not compatible with wet processing
techniques.
The development of a robust vapor-deposition technique for PEDOT films can simplify
the coating process on a variety of organic and inorganic materials since it does not depend on
evenly wetting the substrate surface. Vapor-phase deposition can also provide uniform coatings
on substrates with high surface areas due to roughness and fibrous or porous morphologies.
Increasing the effective surface area of devices will improve operating efficiencies and coating
unconventional surfaces like paper, fabric, and small particles can lead to the innovation of new
devices. Recent experiments have advanced the development of vapor-phase techniques using
oxidant-enriched substrates that have been coated with solutions of Fe(III) tosylate and left to
dry. Exposing the treated surface to EDOT vapors results in the polymerization of films with
conductivities reported to be as high as 1000 S/cm3 4' 35. However, no one has yet achieved a
truly all-dry process for the deposition of PEDOT thin films. We propose a chemical vapor
deposition (CVD) technique for polymerizing EDOT using FeC13 as an oxidant to form a
conducting film. We have eliminated the solution-based oxidant-enriching step and designed a
process that promotes the mechanism for the formation of conducting material and reduces
unwanted side reactions.
72
4.2
BACKGROUND
4.2.1
MECHANISM FOR THE OXIDATIVE POLYMERIZATION OF PEDOT
The oxidation of 3,4-ethylenedioxythiophene (EDOT) to form PEDOT is analogous to
the oxidative polymerization of pyrrole, which has been described with a mechanism proposed
by Diaz3 6 ' 37 and is shown in Figure 4-1.
The first step is the oxidation of EDOT, which
generates a radical cation that has several resonance forms. The combination of two of these
radicals and subsequent deprotonation form a neutral dimer.
thiophene ring at the 3,4-positions blocks flcoupling,
Substitution of the EDOT
allowing new bonds only at the 2,5-
positions. The alternating single and double bonds of the dimer give -conjugated or delocalized
electrons, making it easier to remove an electron from the dimer relative to the monomer. With a
lower oxidation potential, the dimer reacts more readily to form other positively charged radicals
that undergo subsequent coupling and deprotonation steps. Eventually, chains of neutral PEDOT
with alternating single and double bonds are formed.
73
H
S
-e-
0
Oxidant
/
H
I
O
O
\/
2
- 2H*
S
O
H
O
'-2H
S
S
Ox.
H
H
Figure
mechanism
4-1:Diaz for oxidative polymerization.
n
Figure 4-1: Diaz mechanism for oxidative polymerization.
The neutral PEDOT polymer is further oxidized to create a positive charge along the
backbone every three or four chain segments. A "dopant" anion ionically binds to the polymer
and balances the charge. The oxidized form of PEDOT shown in Figure 4-2 is the conducting
form of the polymer. Neutral PEDOT has a dark blue/purple color and the doped form is very
light blue.
74
r
0
0
0
S
0
0
S
0
\
//
0
\
0
Ox.
Figure 4-2: Neutral PEDOT is oxidized to form a conducting polycation that is charge
balanced with dopant anions.
The acidic strength of the reaction environment is one aspect of the mechanism to be
considered, because it can have a number of effects on the polymerization conditions, including
the oxidation potential of the oxidant, which can be decreased with the addition of a base3 5' 38
However, acidification of the reaction mixture generally speeds the rate of polymerization
through a competing acid-initiated coupling mechanism, shown in Figure 4-3, which contributes
to additional chain growth.
75
SH
\
\
H H
S
O
/ \
\ 2
H
Sc+H
/
\ 2
\ /
\
O
Figure 4-3: Acid-initiated coupling promotes chain growth.
One caveat of acid-initiated coupling is that the resulting polymer is not conjugated and
will not be electrically conducting without subsequent oxidation.
If the acidic strength is too
high, it is even possible to saturate the 3,4-positions of a reacting EDOT radical resulting in a
trimer with broken conjugation, shown in Figure 4-4, quenching electrical conductivity3 6
H
H
Figure 4-4: Acid initiation can progress to the formation of trimers with broken
conjugation.
76
39
Evidence has also been presented that very acidic conditions can break the dioxy bridge
on the EDOT ring leading to imperfections that reduce conductivity3 5 . Adding pyridine as a base
to the system reduces the acidic strength enough to avoid bond cleavage in the monomer, while
maintaining a high enough oxidation strength for the reaction to proceed, yielding PEDOT films
with conductivities reported to be as high as 1000 S/cm.
4.3
EXPERIMENT
PEDOT depositions were carried out in a custom-built vacuum chamber that has been
described elsewhere4 0 4 1 and is depicted in Figure 4-5. Glass slides and silicon wafers were used
for substrates. The stage is regulated with cooling water and is normally kept at 340 C. A stage
heater is available when stage temperatures greater than 80° C are desired. The stage can also be
biased with a DC Voltage using a Sorenson DCS 600-1.7 power supply. The chamber pressure
was controlled by a butterfly valve connected to an MKS model 252-A exhaust valve controller
and was maintained at approximately 300 mTorr. Fe(III)CI 3 (97%, Aldrich) was selected as the
oxidant. The powder was loaded in a porous crucible with a nominal pore size of 7 im and
positioned above the stage. The crucible was heated to a temperature of about 2400 C where
sublimation of the oxidant begins to occur. Argon (Grade 5.0, BOC Gases) was delivered into
the crucible as a carrier gas for the Fe(III)CI3 vapors. An argon flow rate of 2 sccm was set using
an MKS mass flow controller with a range of 50 sccm N2. Once a yellow film of Fe(III)C13 was
observed on the substrate, the crucible temperature was reduced to end sublimation.
monomer (3,4-ethylenedioxythiophene,
EDOT
Aldrich) heated to 1000 C is then introduced into the
reactor through heated lines and using an MKS 1153 mass flow controller set at 950 C. The
EDOT flow rate is normally 10 sccm. Pyridine (99%, Aldrich) at room temperature can be
evaporated into the reactor using a needle valve to control the flow rate. A deposition time of
30 minutes was used for all of the films.
77
Ar
EDOT
II_
I
E
I
; ..
.
I
I
,..:
..
s,
10
I
a-
T
::"
Pvr
I
Pump
In t
Cooling Water
I Out
Figure 4-5: Schematic of CVD reactor for depositing PEDOT films.
After deposition, the films were dried for at least 2 hours in a vacuum oven heated to
800 C at a gauge pressure of -15 in. Hg. The thickness of the films deposited on glass were
measured on a Tencor P-10 profilometer and conductivity measurements were done with a
four-point probe (Model MWP-6, Jandel Engineering, Ltd). Films on silicon substrates were
measured with FTIR (Nexus 870, Thermo Electron Corporation) for information on chemical
composition. Deposited films were sometimes rinsed in methanol (HPLC Grade, J.T. Baker) or
in a 5.0 mMol dopant solution of nitrosonium hexafluorophosphate, NOPF 6, (96%, Alfa Aesar)
in acetonitrile (ACS Grade, EMT). The rinse step is intended to remove any unreacted monomer
or oxidant in the films as well as short oligomers and reacted oxidant in the form of Fe(II)C12.
After rinsing, the films changed from a cloudy light yellow color to a sky blue hue.
A Bayer formula for the in-situ polymerization of BAYTRON M was used to make a
standard PEDOT film. One mL of EDOT monomer and 39 mL BAYTRON C were combined
and allowed to mix for ten minutes.
The solution was then spun onto silicon and glass at
3000 rpm for 40 seconds using a spincoater (Model P6700, Specialty Coating Systems). The
78
films were heated to 800 C under vacuum for ten minutes, rinsed in methanol, and allowed to
dry.
FTIR spectra and conductivity measurements were collected for the standard PEDOT
material.
4.4
RESULTS AND DISCUSSION
FTIR spectra showing the bonding characteristics of a succession of CVD PEDOT films
are shown in Figure 4-6. For reference, the bottom spectrum is the EDOT monomer and the top
spectrum is a standard PEDOT film made using a Bayer solution-based formula. All of the films
in Figure 4-6 were rinsed in methanol after deposition.
C (S/cm)
300
4.37
0,
dl
U
C
0.79
f0
M
.0
0.02
10-3
Monomer
I
3000
'
·
,· ·
I
1500
. I.
1000
.
500
Wavenumber(cm-')
Figure 4-6: FTIR spectra and conductivity values for CVD PEDOT and standard PEDOT
films.
79
PEDOT deposited with CVD using FeC13 as the oxidant exhibited conductivities ranging
between 10-2 and 101 S/cm. Although 300 S/cm is possible using in-situ polymerization of
BAYTRON M, the CVD films display absorption characteristics of the oxidized, r-conjugated
PEDOT material. Coupling of the monomers at the 2,5-positions of the thiophene ring to form
the polymer corresponds to a disappearance of the C-H stretch4 2 at 3100 cm - l, which happens for
the PEDOT standard and FeCl 3 CVD films. The stretches between 2800 and 3000 cm -' are from
the C-H bonds on the ethylene dioxy bridge and should be retained. However, those stretches
are obscured in the polymerized films and completely hidden in the standard sample. Electron
delocalization also broadens the ring stretches between 500 and 1500 cm -1. This is due to an
averaging effect of resonant bonds and a distribution of multiple oligomer lengths incorporated
in the film. GPC tests to quantify the distribution of oligomers that are present were not possible
since the films are insoluble in multiple solvents, even with sonication.
An initial CVD film using FeC13to oxidize EDOT is labeled CVD1. Its spectrum shows
a small carbonyl peak at 1700 cm' l, which suggests the existence of broken bonds in the
monomer ether bridge.
This has been reported to occur when the oxidation potential of the
oxidant is too high or when the acid strength of the polymerization environment is too strong3 5 .
As an extreme case, the spectrum of a CVD film using SnC14 as the oxidizing agent is included
in Figure 4-6 to illustrate a sharp peak at 1700 cm 'l indicating an abundance of carbonyl groups.
Sn4+ has a standard reduction potential of 0.154 V versus 0.769 V for Fe3 +43.
The films
exhibiting the carbonyl peak also lack many of the spectral characteristics between 1500 and
500 S/cm-' that are associated with conjugated rings and their conductivities are correspondingly
low.
Winther-Jenkins,
et al reported
the use of pyridine
to achieve
base-inhibited
polymerization of PEDOT films with electrical conductivities as high as 1000 S/cm4 4 . Following
this example, pyridine vapors were fed into the CVD chamber with the monomer after subliming
the oxidizing layer onto the substrate.
The carbonyl stretch is absent from the resulting FTIR
spectrum labeled CVD2 in Figure 4-6.
Also, the ring stretches more closely resemble the
80
conjugated standard PEDOT film and the conductivity improved an order of magnitude to
0.7 S/cm. In Figure 4-7, a comparison of FTIR spectra for CVD2 after deposition with pyridine,
and then after rinsing the film in methanol, provides some insight into the base-inhibited
mechanism.
a (S/cm)
0.79
C
C
.0
0
Cn
M
0.10
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-')
Figure 4-7: Comparison of PEDOT polymerizedin the presence of pyridine before and after
methanol rinse.
After polymerization of EDOT on the substrate, pyridine remains in the film, but washes
out completely in methanol presenting the possibility that it has reacted and is present as a
non-volatile pyridinium salt. Pyridine is sometimes used by organic chemists as a trapping agent
for HCI due to the favorable formation of pyridinium salt, as shown in Figure 4-8. Acid has not
been explicitly added to the deposition of CVD2, but it is reasonable to assume that some HCI is
produced due to the deprotonation steps during PEDOT polymerization and the presence of Clions from the reacted oxidant.
Introducing pyridine to the system could eliminate HC1 in the
system that may have contributed to carbonyl formation in CVD 1. Removal of strong acid from
81
the system as it is formed might also push the deprotonation of coupled oligomers, increasing the
likelihood that chains will become fully conjugated, leading to better film conductivities.
Reaction of pyridine and HCl would also generate more Cl- ions available for doping. Whether
the pyridine acts to reduce the acidic strength of the reaction mixture resulting in a lower
oxidation potential of the oxidant, as suggested by Winther-Jenkens et al, or serves as a quencher
for HCl byproduct, both functions should promote the formation of conducting PEDOT.
Although the CVD2 film was washed in methanol, the unrinsed sample was also electrically
+cl
H-CI
active with a bulk conductivity of 0.1 S/cm.
H+
+
-
Keq 1012
Figure 4-8: Pyridine readily reacts with HCI to form a pyridinium salt.
Heating the sample stage during PEDOT deposition also yielded a film (CVD3) with
spectral characteristics in good agreement with the standard PEDOT and led to an enhanced
conductivity over 4 S/cm. Heating may promote conjugated material by volatilizing more HCl
as it is formed or by contributing energy and accelerating the polymerization reactions.
Biasing
the sample stage with about +3 V during polymerization of the EDOT also resulted in a film
(CVD4) with spectral features most indicative of conducting PEDOT. The conductivity was not
actually measured since biasing required the use of conducting substrates. Biasing likely adds
directionality to the growing chains, producing a more ordered film.
One small feature that remains different between the CVD PEDOT films (CVD2, CVD3,
and CVD4 in Figure 4-6) compared to the standard is a slightly more distinct shoulder at
1400 cm ' . A closer look reveals that the broad absorption overlapping this shoulder peaks at a
slightly higher wavenumbers for the CVD films. Based on the peak assignments in Table 4-142,
82
these two distinctions indicate a slight prevalence of 3-substituted thiophene moieties in the
CVD films compared to the standard. The standard film is expected to have 3-substitution as
well since that is where the diether bridge is attached to the monomer, but a stronger absorption
in this region could indicate that the CVD films have less 2-substitution than the standard film
owing to shorter polymer chains. This could help explain the lower conductivities measured thus
far for the CVD PEDOT films compared to standard PEDOT material.
Table 4-1: Ring Bands in cm-' for Monosubstituted Thiophenes
Out-of-Phase
C=C Stretch
In-Phase
C=C Stretch
C-S-C In-Phase
Stn9tch + C-C Contract
2-Substituted Thiophenes
1535-1514
1454-1430
1361-1347
3-Substituted Thiophenes
1542-1492
1410-1380
1376-1362
Rings
4.5
CONCLUSION
A CVD process has been proposed that forms thin films of electrically active
polymers. The technique has been demonstrated to make PEDOT that has a conductivity over
4 S/cm and is spectroscopically comparable to commercial product deposited from the solution
phase. This technique should also be applicable for other oxidatively polymerized conducting
materials like polypyrrole, polyaniline, polythiophene, and their substituted derivatives.
Side
reactions stemming from acid generation during oxidative polymerization can lead to bond
breakage in the monomer and the formation of unconjugated oligomers that result in films with
low conductivities. These unwanted reactions have been minimized for the CVD technique with
three different methods: introducing pyridine as a base, heating the substrate, and applying a bias
to the sample stage.
83
CVD offers an all-dry process for depositing thin films of conducting polymers, which
are currently available on the market only as solution-based materials.
Commercial PEDOT
films deposited onto anodes from solution facilitate hole injection and have already resulted in
significant efficiency gains on the order of 50% for organic LED and photovoltaic devices.
Using CVD to provide uniform PEDOT coatings on rough electrode surfaces can lead to further
improvements in efficiency by increasing the effective surface area while avoiding sharp
electrode features from protruding through the PEDOT film and shorting the device. Elimination
of the PSS matrix necessary for solution-based PEDOT processing may also reduce corrosion of
neighboring layers, which leads to early device failure. Moderate stage temperatures and vapor
phase coating makes CVD capable of depositing PEDOT on a wide range of unconventional
organic and inorganic high-area surfaces including paper, fabric, and small particles. CVD will
be a significant tool for organic semiconductor manufacturers developing fabrication processes
for next-generation devices.
84
4.6
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A. Aleshin, R. Kiebooms, R. Menon, F. Wudl, and A. J. Heeger, "Metallic conductivity at low
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F. Tran-Van, S. Garreau, G. Louarn, G. Froyer, and C. Chevrot, "Fully undoped and soluble
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22.
D. Hohnholz, A. G. MacDiarmid, D. M. Sarno, and W. E. Jones, "Uniform thin films of poly-3,4ethylenedioxythiophene
(PEDOT) prepared by in-situ deposition," Chemical Communications
(23), 2444-2445 (2001).
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J. L. Bredas and R. J. Silbey, Conjugated polymers: the novel science and technology of highly
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Boston, 1991), pp. xviii, 624 p.
86
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F. Jonas, W. Krafft, and B. Muys, "Poly(3,4-Ethylenedioxythiophene)
- Conductive Coatings,
Technical Applications and Properties," Macromolecular Symposia 100, 169-173 (1995).
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26.
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technical applications and properties," Synthetic Metals 85(1-3), 1397-1398 (1997).
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T. M. Brown, J. S. Kim, R. H. Friend, F. Cacialli, R. Daik, and W. J. Feast, "Built-in field
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hole injection layer," Applied Physics Letters 75(12), 1679-1681
(1999).
33.
G. Greczynski, T. Kugler, M. Keil, W. Osikowicz, M. Fahlman, and W. R. Salaneck,
"Photoelectron spectroscopy of thin films of PEDOT-PSS conjugated polymer blend: a minireview and some new results," Journal of Electron Spectroscopy and Related Phenomena 121(13), 1-17 (2001).
34.
J. Kim, E. Kim, Y. Won, H. Lee, and K. Suh, "The preparation and characteristics of conductive
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thin film by vapor-phase polymerization," Synthetic Metals
139(2), 485-489 (2003).
35.
B. Winther-Jensen and K. West, "Vapor-phase polymerization of 3,4-ethylenedioxythiophene: A
route to highly conducting polymer surface layers," Macromolecules 37(12), 4538-4543 (2004).
36.
S. Sadki, P. Schottland,
N. Brodie, and G. Sabouraud,
"The mechanisms
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37.
E. M. Genies, G. Bidan, and A. F. Diaz, "Spectroelectrochemical
Study of Polypyrrole Films,"
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S. Kirchmeyer and K. Reuter, "Scientific importance, properties and growing applications of
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87
39.
T. F. Otero and J. Rodriguez, "Parallel Kinetic-Studies of the Electrogeneration of Conducting
Polymers - Mixed Materials, Composition and Properties Control," Electrochimica Acta 39(2),
245-253 (1994).
40.
D. D. Burkey and K. K. Gleason, "Structure and mechanical properties of thin films deposited
from 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane
and water," J Appl Phys 93(9), 5143-5150
(2003).
41.
H.
G. Pryce-Lewis,
D.
hexamethylcyclotrisiloxane
J. Edell,
and
K. K. Gleason,
"Pulsed-PECVD
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(2000).
42.
D. Lin-Vien, The Handbook of infrared and raman characteristic frequencies of organic
molecules (Academic Press, Boston, 1991), pp. xvi, 503 p.
43.
A. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, 2nd ed.
(John Wiley, New York, 2001), pp. xxi, 833 p.
44.
B. Winther-Jensen, J. Chen, K. West, and G. Wallace, "Vapor phase polymerization of pyrrole
and thiophene using iron(III) sulfonates as oxidizing agents," Macromolecules 37(16), 5930-5935
(2004).
88
Chapter Five
ELECTROCHEMICAL INVESTIGATION OF
PEDOT
THIN FILMS DEPOSITED USING
CVD AS A CANDIDATE MATERIAL FOR
ORGANIC MEMORY AND
ELECTROCHROMIC APPLICATIONS
Lock JP, Lutkenhaus JL, Zacharia NS, Hammond PT, and Gleason KK. Manuscript in
preparation for submission.
89
ABSTRACT
Conducting polymers are being introduced into a broad range of organic devices that are
flexible, inexpensive, thin, and light-weight compared to their traditional inorganic
semiconductor analogs.
PEDOT is particularly useful as a stable conductor or as a
hole-injecting material that enhances the lifetime of organic devices and increases
operating efficiencies by 30 to 50%. Its electronic structure can also be controlled with
voltage resulting in the ability to switch the conductivity and optical properties.
The
electrochromic behavior of PEDOT is being developed in new devices like organic flash
memory, small active matrix displays, and large-area "electronic paper" prototypes.
A
recently innovated process for making PEDOT from the vapor phase using CVD enables
conformal coatings of the material on high-area surface features like fibers and pores and
the technique is compatible with unconventional substrate materials like paper and fabric.
Electrochemical measurements of CVD PEDOT indicate that the 50-nm films have a
switching speed of 27 msec and a color contrast of 16.5%. A diffusion constant for the
rate of charge transfer in CVD PEDOT was determined using a potential step test and
dimensional analysis indicates that films 10 nm thick would have a response time around
I msec. CVD offers a technique for quickly depositing conformal layers of electrically
conducting PEDOT that exhibits electrochromic behavior.
Acknowledgements. This research was supported by, or supported in part by, the U.S. Army
through the Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-D-0002 with
the U.S. Army Research Office. The content does not necessarily reflect the position of the
Government, and no official endorsement should be inferred.
90
5.1
INTRODUCTION
Conducting polymer materials have been applied in a number of emerging applications
including
light-emitting
diodes
(LEDs)'
2 for
portable
and
large-area
displays3 ,
microelectronics 4 5, and photovoltaics 6 . Perhaps the most promising conducting polymer so far
is a polythiopene substituted with an electron-rich diether bridge on the 3,4-positions that
contributes to the delocalization
poly-3,4-ethylenedioxythiophene
of electrons along the backbone. The polymer is called
(PEDOT) and was developed by scientists at Bayer AG7 9 .
PEDOT is extremely stable, nearly transparent, and has an exceptionally high conductivity of
300 S/cm' °' l. PEDOT is generally used as a hole-injecting layer coated onto device electrodes
and has been shown to increase the efficiency of organic photovoltaics and organic LEDs
(OLEDs) by up to 50%2' 13
In its neutral form, PEDOT has a r-conjugated electronic structure. PEDOT can
be oxidized and converted into a polycation, which is stabilized by dopant anions in the vicinity.
This oxidized state is the conducting form of PEDOT.
The neutral and oxidized forms of
PEDOT are shown in Figure 5-1.
In addition to the applications PEDOT in its conducting form, the ability to oxidize or
reduce the material and cycle it between its conducting and non-conducting states have made it a
candidate for use as the functional material in polymeric memory devices' 4 . Switching speeds of
2 jts have been achieved with the thinnest PEDOT films having a thickness around 25 nm. A
joint venture between Advanced Micro Devices (AMD) and Fujitsu called Spansion is
developing similar technology in an effort to produce organic flash memory.
91
0
0
0
S
0
0
S
0
\/
0
\/
Red.|
0
n
| Ox.
Figure 5-1: Neutral PEDOT is oxidized to form a conducting polycation that is charge
balanced with dopant anions. Oxidized PEDOT has a transparent light blue color that turns
dark purple upon reduction.
PEDOT also exhibits a change in its optical properties as it is reduced and oxidized. The
neutral reduced form of PEDOT absorbs strongly in the visible region due to low absorption
energies of electrons that are delocalized along the conjugated PEDOT backbone. Oxidation of
neutral PEDOT causes a spectral shift of absorption towards the infrared leaving PEDOT films
with a transparent light blue color. PEDOT is capable of a maximum contrast of 54% and other
derivatives of PEDOT have been synthesized including one called ProDOT-Et 2, which is capable
of a 75% contrast change centered at 580 nm 5 .
The devices that operate based on the electrochromic shift of PEDOT depend on the
switching speed that can be attained with the material. Active matrix displays have a refresh
time dictated by how quickly individual pixels can be updated. Displays incorporating PEDOT
as an active electrochromic material have been built that have refresh times as low as 0.1 sec for
small areas on the order of 1 mm 2 . However, larger cells with an area of 15x30 mm2 have been
92
demonstrated that have longer refresh times of 5 sec16. These cells can tiled to form a large-area
display that would be suitable for dynamic signs or electronic paper applications.
Polymer
electrochromic films capable of switching speeds on the order of seconds can also be used to
make "smart glass" for windows with dynamic tinting capabilities 17 . Other OLED devices that
incorporate PEDOT as the hole-injecting layer instead of the active switching material have been
built and are quickly approaching operating efficiencies and lifetimes that are poised to compete
with LCD displays' 8 .
OLED displays have the advantage of very large viewing angles, the
ability to operate in freezing temperatures, new thin and flexible form factors, and increased
efficiency by emitting light as needed instead of filtering a constantly lit backlight.
Currently, PEDOT is primarily available as BAYTRON PTM in an aqueous dispersion of
the polymer stabilized by a soluble polystyrenesulfonate matrix. However, recent advances in a
chemical vapor deposition (CVD) process for PEDOT presents the possibility of more uniform
coatings on substrates with high surface areas due to roughness and fibrous or porous
morphologies. Increasing
the effective surface area of devices
will improve operating
efficiencies and coating unconventional surfaces like paper, fabric, and small particles can lead
to the innovation of new devices.
The ability to apply conformal PEDOT films on patterned
features with short deposition times is another advantage of the CVD process for PEDOT. CVD
PEDOT materials have been characterized
in an electrochemical
cell to quantify its
electrochromic behavior and charge-transfer kinetics.
5.2
EXPERIMENT
PEDOT depositions were carried out in a custom-built vacuum chamber that has been
described elsewhere 9 ' 20 and is depicted in Figure 5-2. Glass slides coated with ITO were used
for substrates. 'The stage is regulated with cooling water and is normally kept at 340 C. The
stage was biased with 3 V using a Sorenson DCS 600-1.7 power supply. The chamber pressure
was controlled by a butterfly valve connected to an MKS model 252-A exhaust valve controller
93
and was maintained at approximately 300 mTorr.
oxidant.
Fe(III)C13 (97%, Aldrich) was used as the
The powder was loaded in a porous crucible with a nominal pore size of 7 gim and
positioned above the stage. The crucible was heated to a temperature of about 240 ° C where
sublimation of the oxidant begins to occur. Argon (Grade 5.0, BOC Gases) was delivered into
the crucible as a carrier gas for the Fe(III)C13 vapors. An argon flow rate of 2 sccm was set using
an MKS mass flow controller with a range of 50 sccm N 2. Once a yellow film of Fe(III)C
observed on the substrate, the crucible temperature was reduced to end sublimation.
monomer (3,4-ethylenedioxythiophene,
3
was
EDOT
Aldrich) heated to 100° C is then introduced into the
reactor through heated lines and using an MKS 1153 mass flow controller set at 950 C. The
EDOT flow rate was 10 sccm. A deposition time of 30 minutes was used for all of the films.
EDOT
1Ar
1
I
To
-
Pump
In t
Cooling Water
-
Pyr
I Out
Figure 5-2: A schematic of a CVD process for the deposition of PEDOT
After deposition, the films were dried for at least 2 hours in a vacuum oven heated to
80 ° C at a gauge pressure of -15 in. Hg. The thickness of the films were measured on a Tencor
P-10 profilometer.
The dried films were rinsed in methanol and a 5 mMol dopant solution of
nitrosonium hexafluorophosphate, NOPF 6, (96%, Alfa Aesar) in acetonitrile (ACS Grade, EMT).
94
The rinse step removes any unreacted monomer or oxidant in the films as well as short oligomers
and reacted oxidant remaining in the form of Fe(II)C12. After rinsing, the films changed from a
cloudy light yellow color to a sky blue hue. The rinsed films were returned to the vacuum oven
and dried for another two hours.
Electrochemical testing took place in an aqueous 0.1 M solution of sulfuric acid (VWR).
The CVD PEDOT film on ITO was the working electrode, platinized copper was the counter
electrode, and a saturated calomel electrode (SCE) was used as the standard.
A potentiostat
(EG&G Printon Applied Research Model 263A) scanned from 0.4 V to -0.6 V based on
preliminary cyclovoltammograms. In-situ UV/VIS spectroscopy was conducted using an optical
fiber to couple light from a StellarNet SL1 light source with a tungsten krypton bulb emitting
from 350 to 1700 nm. The spectrometer was a StellarNet EPP 2000 having a detector with a
range spanning 190 to 2200 nm.
5.3
DISCUSSION AND RESULTS
5.3.1
CYCLIC VOLTAMMETRY
Cyclic voltammograms for a CVD PEDOT film are shown in Figure 5-3 for three
different sweep rates.
For the 40 mV/s scan, the reduction peak appears at -460 mV and the
oxidation peak is at 60 mV. Therefore, a potential of about 0.5 V is needed to fully drive the
reaction between the oxidized and reduced forms of PEDOT, which is comparable to what has
1
. A peak separation of 0.5 V indicates
been observed with electrochemically deposited films2 '24
that CVD PEDOT is a quasireversible system meaning that the electrochemical reaction in the
material has charge-transfer kinetic limitations. Fully reversible systems have a peak separation
of only 60 mV2 5.
However, a potential of 0.5 V should be within the range of many electronic
devices of interest.
Cyclic voltammetry can offer qualitative information for the oxidation and reduction
mechanisms. Here, our resulting voltammogram shows a broad reduction peak with a shoulder
95
and a sharp oxidation peak. This broad peak and shoulder are indicative of a mechanism more
complicated than just a simple Faradaic reaction.
The initial shoulder is attributed to the
reduction of the initial layer of PEDOT just touching the ITO surface. This layer, once reduced,
no longer conducts electrons and acts as a barrier for further reduction in the film. Consequently,
the outlying film cannot be reduced until the potential is further increased. As more and more
"layers" are reduced, the non-conducting barrier of neutral PEDOT increases, broadening the
reduction peak. Finally, all the PEDOT is reduced and the potential is swept in the opposite
direction.
Oxidation gives a sharp peak and swift response because the first bit of PEDOT
oxidized, that closest to the ITO, oxidizes to its conducting form which facilitates subsequent
oxidation and acts as a viable conduit for electron transfer. This effect has also been observed in
electrochemically deposited PEDOT films2 6 .
150
100
40 mV/s
E
o
20 mV/s
10 mV/s
-50
-100
-150
-150
400
200
0
-200
-400
-600
Voltage (mV)
Figure 5-3: Cyclic voltammetry indicates that PEDOT is reduced gradually, but oxidizes
more suddenly. This stems from the conductivity of oxidized PEDOT as opposed to the
non-conducting reduced form.
96
5.3.2
UVNIs SPECTROSCOPY
UV/Vis spectra were collected for the same CVD PEDOT film to quantify its color
change through the redox cycle. Figure 5-4A shows the progression of the UV/Vis absorbance
as the potential was stepped from 400 mV to -600 mV. The PEDOT film in its initial oxidized
state is transparent and has a light sky blue color. After reduction at -600 mV, the film absorbs
more strongly across the visible spectrum and takes on an opaque dark purple color.
The
maximum contrast change is 16.5%, which occurs at 585 cm. As the film is reoxidized by
stepping the voltage in increments back to 400 mV (Figure 5-4B), the absorbance spectrum
returns to its original value. No visible transmission is lost in the film after the course of the
redox cycle. Additional experiments are needed to determine the number of times the redox
cycle can be repeated without deterioration in the contrast.
5.3.3
CHRONO AMPEROMMETRY
Chrono amperommetry is a potential step experiment that offers information on the
kinetics of redox systems2 5 . Chrono amperommetry was conducted on a CVD PEDOT film to
gain knowledge about the reaction rates of the electrochromic response. A PEDOT film 50 nm
thick on ITO was charged according to the double-step wave form shown in Figure 5-5. The
potentials for the first and second steps were chosen to be 400 mV and -600 mV, respectively, in
order to take the film through a complete redox cycle according to the cyclic voltammogram in
Figure 5-3.
97
A
0.8
400 mV
300 mV
200 mV
- 0.6
--100mV
-- 0mV
--- 100mV
--- 200 mV
-- 300 mV
- -400 mV
U
c 0.4
o
,
0.2
- -500 mV
- -600 mV
-0.2
370
420
470
520
570
620
670
720
770
820
870
Wavelength(nm)
0.8
-- 600 mV
-
- -500 mV
- -400 mV
- -300 mV
0.6
- -200 mV
= 0.4
- -100 mV
-- 0 mV
-- 100 mV
(
0.2
200 mV
300 mV
400 mV
0
-0.2
370
420
470
520
570
620
670
720
770
820
870
Wavelength(nm)
Figure 5-4: UV/Vis spectroscopy indicates that CVD PEDOT has a maximum color contrast
of 16.5% at a wavelength of 585 nm.
98
600
400-
0-
-200-
-400I
0
200
I
400
I
600
I
800
I
1000
Time (msec)
Figure 5-5: A square wave form with a step time of 500 msec and potential limits of 400 mV
and -600 mV was chosen for chrono amperommetry measurements.
Each step was held for 500 msec and the current passing into the film was measured.
Data were collected every millisecond and are plotted versus time in Figure 5-6. The CVD
PEDOT film had a switching speed upon reduction of 49 msec for a 90% change and 31 msec
for an 80% change. The film was able to oxidize slightly quicker and showed a switching speed
of 39 msec for a 90% change and 27 msec for an 80% change. The switching times follow the
trend seen in the cyclovoltammograms of a quicker oxidation process relative to reduction.
99
154-
10-
E
5-
j
0)
C
-5-10-
-15I
0
I
200
400
I
600
I
800
I
1000
Time (msec)
Figure 5-6: A CVD PEDOT film 50 nm thick has a swiching speed of about 50 msec for a
90% change and is as low as 27 msec for an 80% response.
The charge response of materials can also be evaluated with data collected from a
potential step experiment 2 5' 27. The charge transfer in the film can be calculated by integrating
the current over time according to Equation 5-1:
Q(t) = Idt
(5-1)
to
where Q is charge, I is current, and t is the starting time for the experiment.
The charge
response of the CVD PEDOT film is shown in Figure 5-7. The charge is proportional to t /2
according to Equation 5-2, which is an integrated form of the Cottrell Equation2 5:
2nFAD'/2Ct" 2
Qd
(5-2)
2. 1/2
(2)
100
where n is the charge difference between the reduced and oxidized species, F is Faraday's
constant (96,487 C/mol), A is the area of the film, C,* is the concentration of reacting species in
the film, and D,, is the diffusion constant describing the flux of charge in the material.
Qd
does
not include the charging of reactive species adsorbed on the film surface and charging in the
background electrolyte solution. However, these effects happen very quickly compared to the
slow accumulation of charge from the diffusional component and are only seen at small values of
t1/2
t
4.0x10 4 3.5x10 4
E
3.)x10 -
.2
>
2.5x10-
)
2.,Dx10 -
4
-4
4
1.5ix10 -
1.[)Xl04 5
5.C)x10'-
0.0
1
0
t=t
·
200
·
·
i
400
I
I
600
_· ,.
,
800
7
1
1000
Time (msec)
Figure 5-7: The charge response of a CVD PEDOT film is proportional to tn2 indicating a
diffusion-controlled process.
Qd
for values of t 11/ 2 between 0 and T1/2represents charge accumulation for the reduction
process and charge accumulation for the oxidation process will happen in the reverse direction
beginning at t=X.
Qd
for both reduction and oxidation can be combined in an Anson plot as
shown in Figure 5-8, which can be used to calculate the diffusion constant for charged species in
101
the film, Do. For the Anson plot, the charge response of the reversed oxidation process is
corrected for starting at to according to Equation 5-3:
Qd(t > r)=
2nFADJJ42 C*t1
12
1/2 +(t
1/22
_ r)1/2
t1/2
(5-3)
D.UUE-04
4.00E-04'
3.00E-04
5E-04
E
j2 2.00E-04
Reduction
eC 1.00E-04
o0
E
.ooE+00
-1.00E-04*
Oxidation
y =-2.05E-06x- 2.00E-04
-2.00E-04.
…
T
-3.00E-04
0
5
10
15
20
25
Timel" (ms) m2
Figure 5-8: Chrono amperommetry data is condensed into an Anson plot that is useful for
calculating diffusion constants for charge transfer processes. CVD PEDOT has a diffusion
coefficient on the order of 10-1 cm 2/s indicating that the process is controlled by ion diffusion
in the film.
Linear regression is used to collect the slope of the charge response for the reduction and
oxidation of the film. The deviation from linearity at short times is due to the inability of the
potentiostat to instantaneously change the applied potential. Therefore, the first 20% of the data
points are discarded when calculating the slope27 . By rearranging Equation 5-2, one can solve
for Do as follows:
102
'Q
Do=
(5-4)
where S is the slope from the Anson Plot and 0 is the thickness of the film. Do was calculated
for the reduced and oxidized forms of CVD PEDOT using the two slopes in Figure 5-8. Three
methods were used to find Q, all of which gave a result having the same order of magnitude.
Equation 5-1 was used to integrate current response data collected during each step of the
experiment. Q was also calculated by integrating the oxidation peak in the cyclic voltammogram
of the sample based on Equation 5-5:
1
E2
Q=
I dE
(5-5)
V E
E1 and E2 are the potentials on either side of the oxidation peak and v is scan rate of the cyclic
voltammetry experiment. Finally, Q was approximated according to the Faraday's Law for bulk
electrolysis 2 5 :
Q = FCoAO
(5-6)
The concentration of reactive species in the CVD PEDOT film, Co, was calculated based
on a density of 1 g/cm3 for the film and using the monomer formula weight of 142 g/mol. An
assumption was made that only one out of every three monomer units in the PEDOT chain reacts
to the oxidized form. The charge accumulation in the CVD PEDOT film is on the order of
10-4 C/cm2 . The diffusion coefficient governing the flux of charges in reduced PEDOT is about
2x10l0 cm 2 /s, which is between one and two orders of magnitude larger than the diffusion
coefficient of the oxidized material, 8x10-12cm2 /s. These values are on the order of diffusion
coefficients for ion transport in polymers2 8 . Therefore, the switching time of PEDOT is limited
103
by the mobility of ions needed for charge balancing as opposed to the conduction of electrons or
holes in the material.
The dimensionless variable, Dt/
, was about 0.02 for the 50-nm thick CVD PEDOT film
used in this experiment based on the switching time of about 50 msec. Dt/¢ 2 is considerably less
than one confirming that we were operating in a thin-layer regime where the thickness of the film
is significantly smaller than the diffusion layer next to the electrode where concentrations differ
from those in the bulk phase.
Therefore, we can be confident that we are measuring
diffusion-limited effects. Based on the same value of Dt/ 2=0.02, CVD PEDOT films with a
rate-limiting diffusion coefficient of 8x10-12 cm 2/s and a slightly thinner thickness of 10 nm are
capable of switching speeds around 2.5 msec.
5.4 CONCLUSIONS
CVD PEDOT is electrochromic and can be switched from a transparent light blue color
to dark purple by applying a voltage.
response being at 585 nm.
The maximum contrast is 16.5% with the strongest
The contrast can be increased by adding additional conjugated
polymers that absorb in other regions of the visible spectrum. For example, layer-by-layer films
of PEDOT and PANI have shown electrochromic contrasts as high as 82.1%29. PANI has a very
light green color when it is oxidized and transforms to being nearly opaque in its reduced state.
PANI absorbs blue light, so a film containing both PANI and PEDOT is virtually black when it
is reduced. The CVD process should be compatible with the oxidative polymerization of PANI
and could incorporate the material into PEDOT films as a separate layer or potentially as a
copolymer.
The contrast could also be increased with the use of thicker PEDOT films. The
response is repeatable and the films return to their original optical properties after a complete
redox cycle.
Results from potential step tests shows that a 50 nm film of CVD PEDOT has a
switching speed of 27 msec for an 80% response during the oxidation reaction (dark to light) and
104
31.3 msec for reduction (light to dark). Dimensional analysis indicates that CVD PEDOT films
10 nm thick would be capable of switching speeds on the order of 1 msec. This is certainly fast
enough for "electronic paper" applications and is reaching speeds that are needed from active
components in organic displays for portable appliances like PDAs.
PEDOT has proven to be an extremely useful material for organic semiconductor
devices. The ability to deposit PEDOT from the vapor phase will expand its utility in devices
that already contain it and potentially enable the development of new organic electronics
including ones that are fabricated on unconventional
substrates
like paper and fabric.
Electrochromic responses of PEDOT can be applied in dynamically tinting window glass or as
an organic memory device. Future experiments are needed to characterize the lifetime of the
CVD PEDOT materials with respect to time and redox cycling.
devices should also be fabricated and tested.
105
Solid state electrochromic
5.5
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T. M. Brown, J. S. Kim, R. H. Friend, F. Cacialli, R. Daik, and W. J. Feast, "Built-in field
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C. L. Gaupp, D. M. Welsh, and J. R. Reynolds, "Poly(ProDOT-Et-2): A high-contrast, highcoloration efficiency electrochromic polymer," Macromolecular Rapid Communications 23(15),
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107
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C. Kvarnstrom, H. Neugebauer, S. Blomquist, H. J. Ahonen, J. Kankare, and A. Ivaska, "In situ
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108
Chapter Six
CONCLUSIONS
109
This thesis has demonstrated the chemical vapor deposition of two classes of polymers
inspired by the emergence of organic photonic and electronic devices: polysilanes and a
derivative of polythiophene.
Both sets of materials are difficult to process in the solution phase,
but possess properties that would be useful in thin film form. They are transparent materials that
conduct charge, potentially enabling new organic semiconductor devices including solar cells
and full-color displays that are thin, transparent, and flexible.
Polysilane materials deposited via plasma polymerization proved to be highly crosslinked
and amorphous..
While the crosslink density enabled the films to withstand chemical contact
and aging in normal atmospheric conditions, the same effect disrupts
-conjugation, which is
essential for the electrical characteristics of the polymer. However, another remarkable aspect of
polysilanes is the ability to controllably decrease the refractive index through photo oxidation
with UV light.
CVD polysilane films were able to undergo photo oxidation without an
accompanying increase in film thickness, which is normally observed with spin-on polysilanes
that have no crosslink density. The amorphous nature of the film and its transparency across the
visible and infrared regions of the spectrum make CVD polysilanes an excellent optical material
that can be directly patterned with areas of low refractive index. Patterned waveguides were
fabricated and CVD polysilane films were used as a tunable cladding layer of microring
resonators.
PEDOT was chosen as a n-conjugated material to deposit in the vapor phase, because it is
renowned as being the most electrically conducting (up to 300 S/cm) and the most stable of
conjugated polymers. PEDOT was successfully deposited using CVD and a conductivity on the
order of 5 S/cm was achieved.
FTIR spectra of the films are nearly identical to commercial
PEDOT. The stable and insoluble CVD films have demonstrated electrochromic responses to
110
applied voltages, allowing a color change from transparent light blue to opaque purple.
Prototype electrochromic films and organic LEDs have been built. Further improvements to the
process hardware allowing one to control the composition and thickness of the films will result
in a robust vapor phase process for PEDOT. Conformal coatings possible with vapor processing
on unconventional surfaces like paper, fabric, and even small particles will provide devices with
a new level of efficiency while enabling their disposable and low-cost production.
This thesis hopefully highlighted the prospect of vapor-phase processing of conjugated
polymers that will become increasingly useful as organic and microphotonic devices replace
their traditional semiconductor counterparts.
A mechanistic approach to the formation of
photonic and electronic thin films provides an understanding of the capabilities of CVD
polysilane and PEDOT, allowing an informed selection of the applications most suitable for
these materials.
111
Chapter Seven
FUTURE DIRECTIONS
112
As organic materials become more prevalent in microphotonics and semiconductors,
device manufacturers will require a complete toolset for the deposition of conjugated polymers
in a wide range of processing conditions. Chemical vapor deposition has been a vital component
to the fabrication of traditional semiconductors and it is reasonable to assume that this deposition
technology will be as important for next-generation devices. Developing CVD techniques for
conjugated polymers, establishing a broader portfolio of polymer chemistries obtainable by
CVD, and integrating these materials into functional devices is a promising field of future
research.
The existing reactor configuration has primarily been optimized for the deposition of the
polysilanes.
Further design is needed to extend the capability of the reactor for oxidatively
polymerized conducting polymers.
Unlike radical polymerization, which has a chain-reaction
mechanism, oxidative polymerization is step-wise and the extent of film formation depends on
the oxidant available to react. Currently, there is no flow control of oxidant into the reactor and
innovating a way to introduce known amounts of oxidant will provide the means to specify the
film deposition rate and final film thickness.
There is also currently no way of changing the
extent with which the oxidant and monomer are mixed before they adsorb on the surface. Better
mixing of the two precursors may facilitate a more complete reaction.
Characterization techniques that were performed on the conducting polymeric films
during this thesis research
Incorporating
were primarily
in-situ measurement
obtained
after depositions
were complete.
techniques in the reactor including a quartz crystal
microbalance or a mass spectrometer may offer insight into details of the polymerization
mechanism. For example, it has been proposed that HCI generation during deposition may have
an adverse effect on the conductivity of the finished film. However, there is currently no means
113
of quantifying the HCI that is generated or determining the fate of HCl with the introduction of
pyridine into the system, which has been shown to yield higher conductivities.
The current process also leaves reacted (and unreacted) oxidant in the polymer film,
which can be rinsed away using a number of solvents like methanol or acetonitrile. However,
the rinse introduces a wet step to the process that can not be done under vacuum, eliminating
many advantages of an all-dry process. During the rinse, the oxidant dissolves and diffuses out
of the film. This is acceptable when solid substrates are used like glass or plastic. However, for
fabric or paper, the dissolved oxidant leaves a stain in the substrate material. Conducting films
that have been deposited on fabric perform without rinsing, but they can not be washed or
otherwise contact liquids without staining.
The solution to this problem may be finding an
oxidant that is volatile enough to be removed from the film with annealing as opposed to rinsing.
The
second
half
3,4-ethylenedioxythiophene.
of
this
thesis
focused
on
the
CVD
polymerization
of
Although PEDOT has proven to be a very promising material, it is
only one of a growing number of conducting polymers as new derivatives continue to be
reported in the literature. Extending the existing CVD method to other oxidatively polymerized
conductors like polyaniline, polypyrrole, polythiophene, and their various derivatives should be
relatively straightforward.
Many interesting materials may result by overlapping these films or
copolymerizing them to make new materials that have not yet been synthesized in solution or
electrochemically.
Combining different light absorption properties of these materials in a
copolymer may enable the tuning of optical responses that are seen in electrochromic films.
Finally, CVD should be integrated into the production process of light-emitting diodes,
photovoltaics, and other organic semiconductor devices to ascertain potential efficiency gains
over the use of conducting polymers deposited from the liquid phase. PEDOT is already used as
114
a charge-injecting layer in organic LEDs. As a coating between the ITO anode and the organic
light-emitting layer, PEDOT bridges the disparate energy levels of the two materials and allows
for more effective charge injection into the device. Adding this PEDOT layer to OLEDs has
increased operating efficiencies by about 30 - 50%. A prototype OLED has been built using
CVD to deposit this layer and preliminary measurements indicate that more light emission does
result and the quantum efficiency of the device appears to be higher compared to OLEDs with
conventional PEDOT films. This is a very exciting result that needs to be quantified more fully.
Measurements should also be made to understand differences between the morphology of
conventional PEDOT films and CVD PEDOT. Surface roughness of CVD PEDOT may play an
important role in facilitating better light scattering out of OLED devices.
Once an OLED
structure is optimized with CVD PEDOT, devices can be designed that benefit most from the
conformal nature of CVD coatings compared to spin-on films. For example, photovoltaics are
already being built that have extremely high-area porous and fibrous substrates. Retaining the
maximum amount of this area using CVD techniques to deposit conducting polymer layers may
significantly improve the efficiency of plastic solar cells. Today, LEDs are predominantly made
on planar surfaces, but reliably making these OLEDs on high-area substrates may offer
efficiencies and light-scattering behavior beyond what has been achieved so far.
115
ApndiA
STRUCTURAL DIFFERENCES BETWEEN
CVD AND SPIN-ON POLYSILANE FILMS
John Patrick Lock
NSF/MEXT Summer in Japan Program Final Report
Osaka University
Summer 2002
116
OBJECTIVE
*
Measure and compare the properties and performance of polysilane films deposited
by plasma-enhanced chemical vapor deposition (PECVD) and spin coating.
*
Elucidate the structural and chemical characteristics of CVD and spin-on films that
cause differences in electrical and optical behavior.
Acknowledgements.
I would like to thank Professor Masaaki Yokoyama for hosting me in his lab during the
summer of 2002 and contributing his expertise in spin-on polysilane materials.
I especially thank Tetsuo Sato for
collaborating with me during this time and contributing results from his own research on polysilanes.
was funded by the NSF/MEXT Summer in Japan Program.
117
This work
A. 1
INTRODUCTION
Polysilanes are silicon containing polymers that have unusual optoelectronic
properties primarily due to a-conjugation along the backbone. Excellent thermal and mechanical
properties
of these materials, coupled
with their special electronic characteristics
and
photolability, have led to many potential applications including photovoltaics, light emitting
devices, waveguides,
breakthroughs
and microlenses.
It seems likely that the future will bring new
in the understanding and development of polysilanes; and, new synthetic
procedures remain an important priority in this field.
Chemical vapor deposition and spin
coating are two methods under investigation for producing polysilane films. These processes
differ at every level, from substrate selection to the resulting molecular structure.
These
distinctions need to be better understood in order to capitalize on the advantages of each
deposition technique and optimize the application of polysilanes in future devices.
A.2
SYNTHESIS OF POLYSILANE FILMS
A.2.1
CHEMICAL VAPOR DEPOSITION
Plasma enhanced CVD (PECVD) uses continuous excitation of plasma to breakdown a
precursor gas into many products including ions, excited species, neutral radicals, and electrons.
The neutral radicals diffuse to the substrate where they adsorb and polymerize on the surface.
Fixing the substrate temperature influences both adsorption and surface reaction kinetics, which
can provide control over of the growth mechanism of a film.
Other adjustable deposition
parameters include plasma power, chamber pressure, chemical structure of the precursor,
precursor flow rates, and the presence of other chemical species to increase the plasma density or
enhance the polymerization chemistry. The plasma is induced with radiofrequency (RF) that is
118
applied to two capacitively coupled electrodes.
The stage serves as one electrode, which is
grounded. A schematic of a typical PECVD reactor is shown in Figure A-1.
-- I
Figure A-l: Schematic diagram of PPECVD reactor. RF energy introduced to the top
electrode induces a plasma between the two capacitively coupled electrodes.
A.2.2
SPIN-ON DEPOSITION
Spin coating produces thin polysilane films on a flat surface by using high angular
velocities to evenly distribute a dissolved polymer on a spinning substrate.
Rotation rate and
solution viscosity heavily influence the resulting thickness of the film. Viscosity can be altered
by changing the precursor concentration in a solvent or by changing the temperature. Polysilane
precursors are often generated using the Wurtz coupling synthesis to produce straight-chain
polymers that are soluble in organic solvents like toluene and THF. Typical molecular weights
range between 5,000 and 10,000. The Wurtz reaction is shown in Equation A-1.
R 1R 2SiCl 2 + 2Na
->
(RIR2Si), + 2NaCI
119
(A-l)
A.3
CVD AND SPIN COATING PROCESSING CONSIDERATIONS
Many of the distinctions between CVD and spin on polysilane films stem from operating
limitations of the two processes:
·
Precursor Selection: CVD requires vapor phase processing. Silicon-based materials
tend to have high molecular weights and low volatility, which limits CVD precursors
to those with a boiling or sublimation point below about 2000 C at latm.
Some
silanes that have been used to date include dimethylsilane (2MS), diethylsilane
(2EthS), and polymethylphenylsilane (MPS).
Spin-on precursors must be soluble,
which eliminates some like polydimethylsilane (PDMS). Also, the organic sidegroup
substituents must be robust to the relatively energetic Wurtz reaction conditions.
Polymethylphenylsilane
(PMPS) and polydihexylmethylsilane
(PDHS) are among
those that have successfully yielded films.
*
Film Imperfections:
Spin coating is more or less a direct transfer of polymer in
solution to a substrate, so many precursor features can often be retained. CVD tends
to fragment precursor molecules into various radicals creating many possible
chemical pathways resulting in films that can be amorphous. Very energetic particles
in the plasma like ions and other excited species can collide with the film with high
velocities and cause imperfections such as dangling bonds, which are prone to
oxidation.
*
Contamination:
Polysilanes are extremely sensitive to some impurities including
oxygen. Despite the vacuum conditions of CVD and good purity control of spin-on
solutions, it seems to be unrealistic to completely eliminate oxygen content from
either environment.
*
Substrate:
CVD and spin coating are capable of low temperature depositions, so
films can be made at or near room temperature if desired.
120
Substrate choices are
somewhat limited for spin-on processes since high rates of rotation are needed.
Flexible, absorbing, and large area substrates are impractical for spin-on.
Plasma
processing can accommodate flexible, low-temperature substrates like fabric, paper,
and thin plastic sheets as well as large formed surfaces like automobile panels.
*
Integration: Both methods can be interfaced with current semiconductor processing
techniques that may be necessary for interfacing with microelectronic or MEMS
devices.
*
Capital: Vacuum equipment is expensive making CVD a larger capital investment
than spin-on processing.
*
Scalability: CVD is directly scalable and can be applied in roll-to-roll manufacturing
schemes.
*
Environment and Safety: CVD is a solventless technique that operates under low
pressure as uses a small fraction of precursor material that is necessary for spin
coating. This can also be an advantage when very expensive precursors are needed.
Unfortunately, many silicon containing gases are toxic, so their emission has to be
closely monitored.
Aside from the features listed above that distinguish CVD and spin coating processes, a
more mechanistic comparison must also be made.
Many of the applications that are envisioned
for polysilane materials depend on the efficient conversion or interaction of optical, electrical,
and thermal energy and their effects on the material itself.
The mechanisms involved in these
processes happen on the atomic scale of the material. As a result, differences in the molecular
structure of CVD and spin-on polysilanes and their impact on the optoelectronic and
thermochromic behavior of the material need to be better understood.
121
A.4
CHEMICAL COMPOSITION
CVD films have been analyzed using Fourier Transform Infrared Spectroscopy (FTIR) to
confirm that they contain the fundamental components of polysilane materials.
Figure A-2
compares spectra of CVD polydimethylsilane films with a CVD organosilicon film and a
reference
polydimethylsilane
powder
purchased
commercially
precipitated from solution using the Wurtz reaction.
from Gelest
and likely
In addition to the requisite chemical
moieties, there are two notabls impurities in the CVD polysilane films. At about 2150 cm' l,
there is a Si-H stretch due to incomplete polymerization of the precursor. Si-H groups generally
promote instability since they are easily oxidized, especially when irradiated UV light. Between
1000 and 1100 cm' , there is a strong Si-O-Si stretch.
Oxygen incorporated in the silicon
backbone of the material interrupts the r-conjugation and quenches optoelectronic behavior like
semiconductivity and luminescence.
Oxygen is very difficult to completely avoid since even
small amounts present in the ambient preferentially attach to Si during deposition. Si-Si bonds
undergo irreversible oxidation via an insertion mechanism when the film is irradiated with high
energy
UV
light.
Figure
A-2 shows
a progressive
decrease
polydimethylsilane films although it has not been eliminated.
polysiloxane film deposited
from hexamethylcyclotrisiloxane
comparison.
122
of oxygen
in CVD
The top spectrum is a CVD
(D3) and is included for
-CH 3
Si-H
r X ,
SiMe 2
Si-O
Si-Me
ri
r-L-n
2
CVD D3
ganosilicon
I
CVD
2MS
Films
Reference
4000
3500
3000
2500
Wavenumber
2000
1500
1000
500
(cm '1 )
Figure A-2: FTIR spectra of CVD polydimethylsilane films are compared with a CVD
organosilicon film and a commercially produced polydimethylsilane powder. Oxygen has
not yet been eliminated, but there is a progressive decline in the oxygen content of CVD
polydimethylsilane films.
A.5
STABILITY
Although Figure A-2 indicates that there is the presence of Si-H bonds and some oxygen
content compared to commercially available powders, CVD films have proven to be quite stable
after deposition. In Figure A-3, successive FTIR spectra of a CVD PDMS films deposited using
hexamethyldisilane (6M2S) as a precursor indicate no degradation of the sample after two weeks
of exposure to atmosphere and room lighting.
123
1 Hour
I Day
2 Weeks
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm' l )
Figure A-3: FTIR CVD polysilane materials are stable to oxidation over time in normal
laboratory conditions. This sample was stored for over 2 weeks in atmosphere under room
lighting.
CVD 6M2S also results in films that are insoluble in common organic solvents. Figure
A-4 shows a comparison of the CVD polysilane film and a spin-on polysilane films that are
dipped in toluene for 1 minute.
The spin-on film rinses off, while the CVD film remains
unchanged. Ellipsometry measurements have confirmed that there is no thickness change in the
CVD 6M2S films after solvent rinses. Chemical stability of the CVD polysilane films indicates
a crosslinked structure.
124
Spin-On
CVD
Spin-On
CVD
1 Min
Toluene Dip
Figure A-4:
polysilanes.
A.6
CVD polysilane
films show good chemical stability compared to spin-on
UVN/is ABSORPTION
In order to observe
-conjugation,
uninterrupted chains containing about 10 silicon
atoms oriented in the trans configuration must exist.
Oxidation, crosslinking, and other
imperfections in polysilane backbones introduce traps that quench o-conjugation.
A typical
CVD PMPS UV/Vis spectrum shown in Figure A-5 exhibits transparency in the entire visible
portion of the spectrum and becomes increasingly absorptive in the UV region. By comparison,
an analogous spin coated film has well defined absorption peaks at 333 and 280 nm.
333 nm peak has been assigned to Si-Si a-conjugation.
The
A n-component of the conjugation
arising from Si-phenyl bonds is suspected to be the origin of the 280 nm peak. The relative lack
of o-conjugation in CVD polysilane films supports an amorphous, cross-linked structure whereas
spin-on films seem to retain straight, well ordered chains accommodating orbital overlap.
125
-CVD
PMPS -Spin-On
PMPS
'1
0.9
0.8
5 0.7
0.6
'"'
U
.Q
o 0.4
.0
0.2
C.1
0
250
300
350
Wavelength
400
450
500
(nm)
Figure A-5: UV/Vis spectra show absorption due to (s-conjugation for spin-on PMPS films
at 333nm. No corresponding peak is seen for analogous CVD films.
A.7
PHOTO OXIDATION
The refractive index of polysilane materials decreases irreversibly when they are
irradiated with UV light. This has been observed for both the CVD and spin-on films. The
reduction of the refractive index is a consequence of a photo oxidation reaction. UV light with a
wavelength lower than about 315 nm is energetic enough to cleave a Si-Si bond, which has a
bond strength of about 200 kJ mol['. Even minute amounts of oxygen in the ambient will readily
insert between the Si atoms to form an extremely stable Si-O-Si bond as shown in Figure A-6.
When present, Si-H bonds are even more susceptible to photo oxidation. Figure A-7 compares
the FTIR spectrum of a CVD polysilane film before and after irradiation to UV light. A growth
in the Si-O peak corresponds to the photo oxidation of the film.
126
R2
R2
R2
Si---Si-
Si
R1
R1
R1
R2
2
2
hv
Si-O-Si--O-Si
R1
R1
R1
Figure A-6: Photo oxidation occurs via an insertion reaction when an Si-Si bond is
irradiated with UV light. This decreases the molecular density of the material and reduces
the refractive index.
As oxygen incorporates in the polysilane matrix, the molecular density of the material
decreases and this causes a reduction in the refractive index. A 5% reduction in the refractive
index of CVD 6M2S films has been measured compared to about 7% that has been realized with
the spin-on films. The starting refractive index of spin-on PDHS films is somewhat higher at
about 1.7 compared to 1.55 (at X=633 nm) for the CVD 6M2S films. This difference is attributed
to better ordering and less oxygen content of the as-deposited
spin-on polysilane films.
Networking in the CVD polysilane films also prevents significant thickness changes to the film
with photo oxidation whereas the spin-on polysilane sample showing a 7% decrease in the
refractive index exhibited a corresponding thickness increase of 4%.
As polysilanes are
oxidized, they resemble polysiloxane materials and become transparent to the incoming UV
radiation. Therefore, underlying polysilane material is also exposed as the polysilane bleaches.
Ellipsometry testing determined that the irradiated film was uniformly oxidized throughout its
depth of about 1250A at low exposure dosages of 300 mJ'cm '2 and above.
127
Before
Exposure
nS1
After
Exposure
iiS
--
1500 1400 1300 1200 1100 1000 900
Wavenumber
(cm
800
700
600
500
l')
Figure A-7: An increase in the Si-O peak in the FTIR spectrum for a plasma polymerized
dimethylsilane film demonstrated photo oxidation of Si-Si bonds with UV irradiation.
A.8
THERMOCHROMISM
Spin coated polysilane films show a reduction in their refractive index due to variations
in temperature
as well.
This thermochromic
effect
has been
tested
primarily
on
polydihexylsilane (PDHS) samples. Two effects are believed to contribute to this decline in the
refractive index. Straight conjugated polysilane chains in the trans configuration are suspected
of undergoing a chain transformation with the absorption of NIR light inducing a random helix
structure.
As the trans configuration is lost,
causes a decline in the refractive index.
-conjugation of chains is interrupted and this
Cooling returns the chains to their trans structure
restoring the conjugation. The mechanism is depicted in Figure A-8.
128
RI
R2
R
R2
RI
R2
trans
Rl
R2
R1
R2
R1
Si
R1
R2
f
Heatingl
R
R2
Cooling
R2
R
Si
R
l
Si
/-J \iSi
Random
Helix
Figure A-8: Absorption of NIR light transforms polysilane chains to a random helix
conformation. This interrupts a-conjugation, which decreases the refractive index. This is a
reversible transformation.
The second effect contributing to the thermochromism of the material is crystallinity
changes aroung the glass transition temperature Tg. Below this temperature, the polymer has a
crystalline structure.
Above this temperature, the individual chains begin to move and the
crystallinity is reduced causing an abrupt swelling of the material.
This decrease in density
contributes to a reduction in the refractive index. As the polymer cools to temperatures less than
Tg, the material recrystallizes, so the effect is reversible. Figure A-9 presents experimental data
of a PDHS film undergoing heating. The sharp change in refractive index and crystallinity is
attributed to the glass transition temperature of 41°C for PDHS. In contrast, CVD 6M2S films
show very little thermochromism and do not swell significantly with temperature changes, either.
Again, it is expected that crosslinking in the CVD films prevents significant change in the
molecular density.
129
Spin On
CVD
1750
I
W
U)
0
i
1700
'
1650
.
1600
6000
,
1550 ·*ii
5800
I I
5400
C.M*$
*.
·
**.,*'s*4 e*
*0
5200
1500
20
z
30
40
50
60
70
20
80
30
40
Temperature
(C)
50
60
70
80
60
70
80
Temperature
(C)
1.65
1.65
_
1.6
1.6
·
IM
a 1.55
U
1.55
I
U)
I
1.5
1.5
20
30
40
50
60
70
Temperature(C)
20
30
40
50
Temperature
(C)
Figure A-9: Thermochromism in spin-on polysilane films evident by swelling and a
reduction in the refractive index is largely absent in analogous CVD materials.
A.9
PROPOSED STRUCTURE
The presented evidence supports the conclusion that while spin-on polysilane films retain
a linear and ordered structure enabling
crosslinking that quenches
-conjugation, CVD polysilane films have extensive
-conjugation and forms an amorphous film that is resistant to
solvents and swelling when exposed to heat or light. A representation of the proposed structure
for polysilane materials produced from spin coating and CVD techniques is shown in Figure
A-10.
A brief overview of the conclusions leading to the proposed structural differences
follows:
*
Chemical composition - Spin-on films contain some oxygen impurities, but their
composition is more or less directly transferred from the dissolved polymer deposited
on the substrate. CVD films have FTIR spectra that retain many bond types expected
130
in a polysilane film as well as Si-H and Si-O content. Si-O bonds are capable of
crosslinking.
*
Stability - CVD polysilane films are insoluble unlike their spin-on counterparts. This
is a strong indication of crosslinked CVD films and straight-chain spin-on films.
* UV/Vis Absorption - Spin-on polysilane films show two main areas of absorption in
the blue and UV region of the spectrum that corresponds to energy levels of
delocalized electrons. This delocalization requires a linear Si-Si chain at least a few
units long with an all-trans conformation. UV/Vis absorption is practically absent in
the CVD polysilane films.
*
Photo oxidation - Both forms of polysilane film oxidize when irradiated with UV
light indicating a presence of Si-Si bonds in the as-deposited material. However, a
smaller decrease in the refractive index of CVD polysilanes indicates a lower initial
concentration of Si-Si bonds. Swelling in the spin-on films that is not evident in the
CVD films further supports a difference in crosslink density.
*
Thermochromism - Heating evinces a change in the refractive index and thickness of
spin-on polysilane films, corresponding to a decrease in the molecular density of the
material. This indicates a transition from an ordered conformation to a more random
structure. CVD polysilane films are relatively stable to heat.
131
R
R2
R
Si
R2
R
Si-,
0
Si
R2
R
R2
Si
R2
2
Si
Si
/
o
R,
Si
Spin On
R2
:
Si
Si
R2
SiS
i
RI
R2
R
Si
S
Ri
R1
R2
Si
CVD
R
Si
Si
RI
/N
RI
Si
R2
R1
Figure A-10:
A comparison
R2
of spin-on and CVD polysilane
films indicates a more
conjugated backbone for the spin-on material. Branchingand unsaturated silicon atoms are
among the characteristics expected for the amorphous CVD films.
A. 10 PROPOSED APPLICATIONS
Conjugated polysilane films have been demonstrated with the spin-on deposition
technique. Therefore, spin-on polysilane films have potential applications in electronic devices
like LEDs, solar cells, etc. However, the operational stability of polysilanes has presented a
particular challege to the integrating these materials in active devices. Thermochromic responses
of spin-on polysilane materials has developed into useful applications, though. For example,
localized heating with the use of an IR laser has enabled the reversible formation of microlenses
for some optical applications.
CVD polysilanes exhibit good stability with exposure to heat and
temperature making them more readily integrated into manufacturing process with rinse steps
and end applications exposed to variable conditions.
The amorphous nature of the CVD
polysilanes and their transparency to visible and IR light also make the material an interesting
candidate for many light guiding applications in communication devices.
132
For example, the
ability to photo oxidize CVD polysilane films has been developed into a process for directly
patterning waveguides. Inherent processing differences should also be taken into account when
considering the most viable opportunities for CVD and spin-on polysilane materials.
For
instance, flexible, low-temperature substrates like fabric and thin plastic sheets are compatible
with CVD, whereas spin-on processing is limited primarily to planar surfaces.
133