i
Fabrication of Structured Polymer Films Using Vapor
Deposition Techniques
by
Xichong Chen
Submitted in Partial Fulfillment
of the
Requirements for the Degree
Doctor of Philosophy
Supervised by
Professor Mitchell Anthamatten
Department of Mechanical Engineering
Materials Science Program
Arts, Sciences and Engineering
Hajim School of Engineering and Applied Sciences
University of Rochester
Rochester, New York
2009
ii
CURRICULUM VITAE
The author was born in Urumqi, China on August 4, 1977. He attended
Fudan University from 1995 to 1999 and matriculated with a Bachelor of Science
degree in 1999. He then continued his education at Fudan University and
graduated with a Master of Science degree in 2002. He came to the University of
Rochester in the Fall of 2003 and began graduate studies in the Materials Science
Program at Department of Chemical Engineering. He received a Master of
Science degree in Materials Science in 2005. He then pursued his doctoral
research in structured vapor-deposited polymer films under the direction of
Professor Mitchell Anthamatten.
iii
ACKNOWLEDGEMENTS
It is difficult to overstate my gratitude to my Ph.D. research advisor,
Professor Mitchell Anthamatten for his support and guidance of my studies. With
his enthusiasm, his inspiration, and his great efforts to explain things clearly and
simply, he helped to make research fun for me. Throughout my thesis-writing
period, he provided encouragement, sound advice, good teaching and lots of good
ideas. I would have been lost without him.
I am grateful to the other members of the Anthamatten group. I thank
Jiahui Li, Supacharee Roddecha, Alex Papastrat, Kavya Ramachandra, Shuai Zhu
for providing a simulating and fun environment in which to learn and grow. I
would like to thank my officemate Michelle Wrue for taking charge of lab safety
and making my lab experience more enjoyable. I also thank my lab partner
Zachary Green, a former member of the Anthamatten group, for his
encouragement and accompany in the past two years.
I would like to thank Mark Bonino for setup of PBO deposition system,
Christine Pratt of the Department of Mechanical Engineering for assistance with
nano-indentation and WAXS, Debra Blondell at Eastman Kodak for the vacuum
TGA experiments, Julie Smyder of Department of Chemistry and Huanjun Ding
of Department of Physics and Astronomy for the AFM measurements. I also
appreciate the financial support in the form of a Frank J. Horton Research
Fellowship from the Laboratory of Laser Energetics.
iv
Lastly, I would like to thank my family. The encouragement and support
from my beloved wife Lijun Zou and our son, Eric is a powerful source of
inspiration and energy. And most importantly, I wish to thank my parents,
Suoxian Zhang and Jinfeng Chen. They raised me, supported me, taught me, and
loved me. To them I dedicate this thesis.
v
ABSTRACT
New techniques to fabricate structured polymer films using chemical
vapor deposition were developed and studied. Two different vapor deposition
approaches,
one
using
step-growth
polymerization
and
another
using
chain-growth polymerization, were employed. The primary objective of this thesis
was to determine the feasibility of controlling morphology of vapor-deposited
polymer films by introducing non-reactive, immiscible components into the vapor
deposition process.
Poly(amic acid)s, condensation polymers and precursors to rigid-rod
polybenzoxazoles (PBO), formed upon co-deposition of 3, 3’-dihydroxybenzidine
(DHB) and pyromellitic dianhydride (PMDA). Deposited coatings were cured
under inert gas conditions and resulted in the conversion to semi-aromatic
polybenzoxazoles at around 550 °C. Physical and chemical changes occurring
during the curing process were studied with FT-IR, TGA and nanoindentation
experiments. Successful fabrication of PBO films provided a platform to study
simultaneous film growth and phase separation in vapor-deposited condensation
polymers. Control of morphology of vapor-deposited condensation polymer films
was achieved by the fabrication of polyimide/CuPc composite films, which were
made from the co-evaporation of 4,4’-oxydianiline (ODA) and 3,3’,4,4’-biphenyl
tetracarboxylic dianhydride (BPDA) in the presence of non-reactive, third-component
CuPc. Spectroscopy experiments confirm the formation of polyimide segments and
suggest that embedded CuPc molecules have less mobility compared to pure CuPc
films. Electron microscopy and XRD studies show evidence of embedded CuPc
vi
particles at the surface and in the bulk of fine lateral structure with a length scale
of about 100 nm.
Fabrication of poly (methyl methacrylate) (PMMA) films using initiated
chemical vapor deposition (iCVD) provided a platform to investigate film growth and
phase separation for a classical chain-growth polymer. An axisymmetric,
multi-component iCVD apparatus was designed to study the vapor-phase growth of
glassy PMMA films. Key reactor operating parameters, including the hot-zone
temperature, reactor base-pressure, substrate temperature, and the monomer/initiator
molar feed ratio were systematically varied to understand film growth kinetics. The
non-reactive solvent-vapor, t-butanol, was then introduced into the deposition process
to promote polymer film dewetting. When solvent-vapor is used, non-equilibrium
dewetted structures comprising of randomly distributed polymer droplets were
observed. The length-scale of observed topographies, determined using power
spectral density (PSD) analysis, ranges from 5 to 100 microns and can be
influenced by deposition conditions, especially the carrier gas and solvent-vapor
flowrates. Control over lateral length-scale is demonstrated by preparation of
hierarchal “bump-on-bump” topographies. Autophobic dewetting of PMMA from
SiOx/Si substrate during iCVD process is attributed to a thin film instability driven
by both long range van der Waals forces and short range polar interactions.
vii
TABLE OF CONTENTS
Chapter 1.
1.1 1.2 1.3 1.4 1.5 Introduction and Background ..............................................................1 Chemical vapor deposition (CVD) .....................................................1 Condensation chemical vapor deposition ...........................................4 Initiated chemical vapor deposition (iCVD).......................................7 Film morphology ..............................................................................11 Objective and outline ........................................................................17
Chapter 2.
2.1 2.1.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.4 Vapor Deposition and Curing of Polybenzoxazole Precursors .......20 Introduction.......................................................................................20 Polybenzoxazoles (PBOs).................................................................20 Experimental .....................................................................................24 Materials ...........................................................................................24 Vapor deposition polymerization of PBO precursor 3 .....................24 Solution synthesis of PBO precursor 3 .............................................25 Curing of PBO precursors.................................................................26 Characterization ................................................................................26 Results and discussion ......................................................................27 Sublimation of PMDA and DHB monomers ....................................27 Change of chemical structure upon curing .......................................29 Thermal properties ............................................................................33 Mechanical properties of PBO..........................................................34 Summary ...........................................................................................39
Chapter 3.
Morphology of Vapor Deposited Polyimides Containing Copper
Phthalocyanine .....................................................................................41 Introduction.......................................................................................41 Polyimides.........................................................................................41 Copper phthalocyanine .....................................................................42 Organic photovoltaics (OPV) ...........................................................44 Experimental .....................................................................................46 Materials ...........................................................................................46 Substrate preparation ........................................................................47 Vapor deposition of polyimide/CuPc composites.............................47 Characterization ................................................................................49 Results and discussion ......................................................................50 VDP of polyimides ...........................................................................50 VDP of CuPc/polyimide hybrid films...............................................56 Morphology of composite films........................................................60 Summary ...........................................................................................68 3.1 3.1.1 3.1.2 3.1.3 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.3 3.3.1 3.3.2 3.3.3 3.4 viii
Chapter 4.
4.1 4.1.1 4.1.2 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.4 Chapter 5.
5.1 5.1.1 5.2 5.2.1 5.2.2 5.2.3 5.3 5.3.1 5.3.2 5.4 Multi-component Vapor Deposition Polymerization of
Poly(Methyl Methacrylate) in an Axisymmetric Vacuum
Reactor ..................................................................................................70 Introduction.......................................................................................70 Materials and application..................................................................70 iCVD reactor design .........................................................................71 Experimental .....................................................................................77 Materials ...........................................................................................77 Deposition of PMMA films ..............................................................78 Characterization of iCVD PMMA films...........................................80 Results and discussion ......................................................................81 iCVD of PMMA films using base-case conditions...........................81 Hot zone configuration and temperature...........................................86 Reactor base pressure........................................................................90 Substrate temperature........................................................................94 Monomer/initiator molar feed ratio ..................................................95 Reactor modifications .......................................................................97 Summary ...........................................................................................99
Solvent-Assisted Dewetting During Chemical Vapor Deposition
Process.................................................................................................101 Introduction.....................................................................................101 Dewetting and solvent-induced dewetting......................................101 Experimental ...................................................................................106 Materials .........................................................................................106 Initiated chemical vapor deposition of PMMA ..............................107 Characterization of iCVD films ......................................................107 Results and discussion ....................................................................109 Dependence of film morphology on deposition conditions............109 Proposed Dewetting Mechanism ....................................................122 Summary .........................................................................................130
Chapter 6. Summary and Future Work .............................................................132 6.1 Summary .........................................................................................132 6.2 Future work.....................................................................................135 References................................................................................................................138 ix
LIST OF TABLES
Table 2.1 Molar loss ratio at different operating conditions..................................29 Table 2.2 FT-IR peak assignments for vapor-deposited poly (amic acid),
polyimide and PBO. ..............................................................................31
Table 3.1 Summary of typical VDP attempts to grow polyimide films. ...............52 Table 3.2 FT-IR peak assignments for CuPc and vapor-deposited BPDA-ODA
polyimide...............................................................................................56
Table 4.1 Deposition conditions of iCVD experiments.........................................79
Table 5.1 Parameters from fitting k-correlation model for PSD plot of dewetted
and stable films....................................................................................119 x
LIST OF FIGURES
Figure 1.1 Close-up of chemical vapor deposition (CVD) reactor..........................2 Figure 1.2 Schematic drawing of VDP processes with different polymerization
mechanisms. a) Step polymerization; b) chain polymerization; c)
activated monomer polymerization; d) activated substrate
polymerization. ......................................................................................3 Figure 1.3 General two-step imidization reaction....................................................5 Figure 1.4 Schematic drawing of initiated chemical vapor deposition (iCVD)
technique. ...............................................................................................8 Figure 1.5 Schematic drawing of iCVD processes using different dissociate
methods. .................................................................................................9 Figure 1.6 Scheme for free-radical polymerization in iCVD process. ..................10 Figure 1.7 Diagram of growth effects including diffusion, shadowing and
reemission which may affect surface morphology during thin film
deposition.............................................................................................12 Figure 1.8 Principle of polymerization-induced phase separation.........................16
Figure 2.1 Structural comparison of fully-aromatic (1) and partially-aromatic
PBO(2). ................................................................................................21 Figure 2.2 Synthesis of semi-aromatic PBO involving thermal rearrangement of
polyimide precursor. ............................................................................23 Figure 2.3 Schematic diagram of the deposition system inside the vacuum
chamber................................................................................................25 Figure 2.4 Vapor pressure curves of PMDA and DHB from vacuum TGA
measurement. .......................................................................................28 Figure 2.5 FT-IR Spectra of (a) as-deposited film (b) film cured at 200 °C for 5
hours, and (c) film cured at 550 °C for 1 hour. The spectrum in (c) was
taken using reflective mode. ................................................................32 Figure 2.6 TGA thermogram of as-deposited poly (amic acid) film. ....................34 Figure 2.7 (a) Modulus and (b) hardness of vapor-deposited and solution-cast
films at different stages of curing corresponding to poly(amic acid)
(PAA), polyimide (PI), and polybenzoxazole (PBO). .........................36 Figure 2.8 Effect of curing rate on the mechanical properties of cured polyimide:
(a) modulus and (b) hardness...............................................................38
Figure 3.1 Molecular structure of copper phthalocyanine (CuPc).........................43 Figure 3.2 Two types of OPV devices: (a) Schottky-type junction and (b)
pn-heterojunction. ................................................................................45 Figure 3.3 Three-component VDP reactor for depositing polyimide/CuPc
composite. ............................................................................................48 xi
Figure 3.4 Vapor pressure curves of BPDA, ODA and PMDA, obtained by
vacuum TGA measurement. ................................................................51 Figure 3.5 Reaction of BPDA and ODA to form poly(amic acid) and subsequent
curing to yield polyimide. ....................................................................54 Figure 3.6 FT-IR extinction coefficients of a) cured BPDA-ODA polyimide; b)
heat-treated CuPc; and c) a composite polyimide/CuPc film. .............55 Figure 3.7 Optical absorption spectra of: a) pure BPDA-ODA polyimide and CuPc
films; b) Q-band of CuPc and composite polyimide/CuPc films. .......58 Figure 3.8 TEM images of vapor deposited BPDA-ODA poly(amic acid) and
cured polyimide films containing about 30 and 60 vol % CuPc..........61 Figure 3.9 Atomic force micrographs of a) BPDA-ODA polyimide, b) 30 % CuPc
in polyimide, c) 60 % CuPc in polyimide, and d) pure CuPc films.
Film thicknesses for a-d) were measured to be 400, 264, 80, and 141
nm, respectively. ..................................................................................65 Figure 3.10 Temperature dependent X-ray diffraction spectra of vapor-deposited
films: a) pure CuPc, and b) 60 vol % CuPc in BPDA-ODA polyimide.
............................................................................................................67
Figure 4.1 Molecular structure of poly (methyl methacrylate) (PMMA)..............70 Figure 4.2 A schematic drawing of the pancake-shape iCVD reactor setup for
coating on flat substrates. This figure was reproduced from reference
124........................................................................................................73 Figure 4.3 Schematic drawing of axisymmetric iCVD setup. ...............................74 Figure 4.4 (a) Top-view of 11-inch aluminum base-plate; (b) schematic of
custom-built, axisymmetric iCVD apparatus.......................................76 Figure 4.5 Hot-zone configurations employed in iCVD experiments. ..................80 Figure 4.6 Thickness profile of as-deposited PMMA film (8-hour deposition) on a
3-inch-diameter silicon wafer (unit: nanometer). ................................83 Figure 4.7 SEM image of as-deposited PMMA film.............................................83 Figure 4.8 FT-IR spectrum of PMMA deposited using axisymmetric iCVD
reactor. .................................................................................................84 Figure 4.9 Three steps in iCVD of PMMA process. 1) Thermal initiation; 2) free
radical and monomer transport; 3) condensation and reaction. ...........86 Figure 4.10 PMMA deposition rates for different reactor hot-zones in
axisymmetric iCVD reactor operating under base-case conditions. ..87 Figure 4.11 Logarithmic plot of PMMA deposition rate versus hot-zone
temperature.........................................................................................89 Figure 4.12 Experimental results (○) and model fit (dashed line) illustrating the
relationship between deposition rate and reactor base pressure.........90 Figure 4.13 Simplified plug-flow reactor diagram involving one reactor for
thermal initiation and a second reactor for recombination.................91 Figure 4.14 Plot showing deposition rate dependence on substrate temperature. .94 Figure 4.15 Size exclusion chromatograms of vapor deposited PMMA: a) effect of
monomer/initiator molar feed ratio, and b) effect of non-reactive third
component, t-butanol..........................................................................96 xii
Figure 4.16 A hot-zone nozzle was added to improve deposition rate. a) Original
hot-zone design; b) reactor modification with hot-zone nozzle. ........98 Figure 4.17 The dependence of deposition rate on solvent flow rate in base-case
condition.............................................................................................99
Figure 5.1 Schematic drawing of a modulated surface for liquid film on solid
substrate. ............................................................................................103 Figure 5.2 Different stages for a typical dewetting process.The length of the bar is
100 µm. This figure was reproduced from reference 133..................104 Figure 5.3 Interferometry images of as-deposited PMMA films: (a) deposition
without solvent; (b) deposition with t-butanol solvent (S10-N5); and
(c) the same film, following annealing at 120 °C for 2 hours. ..........110 Figure 5.4 Dependence of average mass deposition rate on solvent and carrier gas
flow rate. The introduction of carrier gas independently increases
deposition rate. Solvent-vapor flow increases deposition rate, and
lowers viscosity at the surface of the growing film. ..........................113 Figure 5.5 As-deposited film morphology at different solvent-vapor and carrier
gas flowrates. .....................................................................................115 Figure 5.6 Power spectra density of films deposited with: a) different carrier gas
(N2) and b) different solvent-vapor flowrates. Dashed lines are
least-squares fits to the k-correlation model. .....................................118 Figure 5.7 Process diagram indicating whether dewetting was observed in
solvent-vapor assisted iCVD growth of glassy PMMA films.
Depositions were performed on a) plasma-treated silicon wafer
substrates and b) hydrophobically modified substrates. The symbols ×
and ○ indicate dewetted and wetted films respectively. ..................120 Figure 5.8 Proposed dewetting mechanism. Initially a liquid layer (yellow)
adsorbs on the surface consisting of predominately methyl
methacrylate and t-butanol. As PMMA is polymerized, a strongly
anchored layer of PMMA (blue) coats the substrate. The
solvent-monomer mixture forms an unstable film on top of the
anchored PMMA. (a) If deposition is slow, or if the viscosity is low
enough, then surface fluctuations reach the substrate and dewetting
occurs, leaving glassy PMMA droplet-like features (green). (b) If
deposition is fast enough or the viscosity is high enough, then the
time-scale of dewetting is too slow and a uniform coating, which may
possess small surface undulations, results. ........................................123 Figure 5.9 Topography (left) and phase (right) images for (A, a) dewetted PMMA
film on Si/SiOx substrate; (B, b) dip-coated (and partially dewetted)
PMMA on bare silicon wafer.............................................................125 Figure 5.10 Topography of a “bump-on-bump” structure measured using
interferometry. ..................................................................................130 xiii
LIST OF SYMBOLS AND ABBREVIATIONS
SYMBOLS

Poisson’s ratio
E
Young’s modulus parallel to the surface
E
Young’s modulus perpendicular to the surface
λ
Wavelength
θ
Bragg diffraction angle
Number average molecular weight
Mn
Mw
Weight average molecular weight
ki
Initiation rate constant
Ci
Gas concentration of initiator
LHZ
Hot-zone length
Initial inlet initiator concentration
Ci0
v0,HZ
Total volumetric flow rate of the hot-zone
ax
Reactor cross-sectional area
Xi
Degree of initiator conversion
kr
Recombination rate constant
Concentration of free radical species
Ci
LRZ
Length of the recombination zone
Degree of free radical recombination
Xr
Tsub
Substrate temperature
Thot
Hot-zone temperature
Re
Reynolds number
ρ
Gas density
Gas velocity
vz
D
Diameter of reactor
µ
Dynamic viscosity
Aeff
Effective Hamaker constant
S
The spreading coefficient
h
Film thickness
u
Amplitude of undulation

q
Wavevector
R
Fluctuation growth rate
      Liquid viscosity
Molecular length scale
a2
 Surface tension
Pi      Gas-phase pressure
xiv
ABBREVIATIONS
CVD
VDP
PMDA
ODA
UHV
iCVD
HF-CVD
PE-CVD
piCVD
gCVD
AFM
TEM
SEM
PIPS
PMMA
PBO
CuPc
TBPO
PPA
DHB
FT-IR
TGA
DMAc
KBr
PAA
PI
MLR
6FDA
NTDA
DMDA
OPV
OLED
BPDA
XDA
PTDA
UV/Vis
QCM
RMS
WAXS
TIPS
CFD
CW
Chemical vapor deposition
Vapor deposition polymerization
Pryomellitic dianhydride
4,4’-Oxydianiline
Ultra high vacuum
Initiated chemical vapor deposition
Hot-filament chemical vapor deposition
Plasma-enhanced chemical vapor deposition
Photoinitiated chemical vapor deposition
Grafting chemical vapor deposition
Atomic force microscopy
Transmission electron microscopy
Scanning electron microscopy
Polymerization-induced phase separation
Poly(methyl methacrylate)
Polybenzoxazole
Copper phthalocyanine
tert-Butyl peroxide
Poly(phosphoric acid)
3,3’-Dihydroxy-benzidine
Fourier transform infrared spectroscopy
Thermogravimetry analysis
N, N-Dimethylacetamide
Potassium bromide
Poly(amic acid)
Polyimide
Molar-loss ratio
2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane
dianhydride
Naphthalene tetracarboxylic dianhydride
Decamethylene diamine
Organic photovoltaics
Organic light-emitting diodes
Biphenyl tetracarboxylic dianhydride
Xylene diamine
Perylene tetracarboxylic dianhydride
Ultraviolet/visible
Quartz crystal monitor
Average root-mean-square
Wide-angle X-ray scattering
Thermal-induced phase separation
Computational fluid dynamics
Cooling water
xv
NMP
HPLC
GPC
THF
PDI
TEA
HZ
RZ
MMA
PSD
N-Methylpyrrolidone
High pressure liquid chromatography
Gel permeation chromatography
Tetrahydrofuran
Polydispersity index
Triethylamine
Hot-zone
Recombination zone
Methyl Methacrylate
Power spectral density
1
Chapter 1. Introduction and Background
1.1
Chemical vapor deposition (CVD)
Chemical vapor deposition (CVD) is a versatile chemical process used in the
production of coatings1, powders2, fibers3 and monolithic components4. This method
is usually employed to form dense structural parts or coatings using the reaction or
decomposition of gases with relatively high vapor pressure. In a typical CVD process,
the substrate is exposed to one or more volatile precursors usually in the gas phase,
which react and/or decompose on the substrate to produce the desired deposit. CVD is
a complicated process and Figure 1.1 shows a close-up of a typical CVD reactor.
Mass transport occurs first after precursor gas is introduced into the reactor. The mass
transport could be gas flow which follows streamline through the entire reactor, or the
mass diffusion driven by the concentration gradient. Although CVD is generally
conducted in a low pressure, vacuum environment, gas phase chemistry may still
happen between precursor molecules or electrons and ions from plasma. The most
important part in a CVD process is surface chemistry which includes the formation of
the product films, removal of byproducts after reaction and ion bombardment if
plasma is involved. Vapor processing technique is well suited to modify surface
properties of bulk materials, for example scratch-resistant and antimicrobial
coatings1, or to prepare thin film devices such as thin film transistors5, 6, photovoltaic
cells7, 8, and gas separation membranes9.
2
Figure 1.1 Close-up of chemical vapor deposition (CVD) reactor.
A key advantage of the CVD process lies in the fact that the reactants used are
gases. Therefore, vapor phase processing is differentiated from solution-based
techniques in many ways. These include: 1) film growth is generally slow (~ µm/hr),
enabling films to be grown to precisely desired thicknesses; 2) film properties can be
tuned by adjusting the molar flux ratios of gaseous precursor components; 3)
polymers can be coated onto rough substrates or even onto complex 3D geometries
such as spheres10,
11
; and 4) vapor-derived processes are “solventless” and are
attractive from an environmental standpoint.12
CVD processes have also been widely used to prepare functional polymer
coatings. Vapor deposition polymerization (VDP) of both condensation and addition
polymers have been performed in the absence of plasma and resulted in polymer
chains that are chemically similar to their solution analogs. Generally, VDP can be
3
briefly summarized into four categories depending on the polymerization reaction
mechanism.13
They are (1) step polymerization; (2) chain polymerization; (3)
activated monomer polymerization; and (4) activated substrate polymerization. In
step-growth VDP, two or more monomers with reactive groups are vaporized and
transport to target substrate. Then condensation polymerization between these
monomers occurs to form polymer film (Figure 1.2 a). In chain-growth VDP, an
initiator is used and dissociated in the gas phase. The activated species and monomer
transport to the substrate where chain growth polymerization happens (Figure 1.2 b).
For the activated monomer polymerization, monomer itself is activated by thermal or
electromagnetic energy to generate activated species. These species undergo chain
linking or crosslinking reaction to form polymer (Figure 1.2 c). Alternatively, the
target substrate can be activated before the exposure of monomer vapor. Then the
reaction between activated substrate and reactive monomer results in single
monolayer of polymer film (Figure 1.2 d).
a)
c)
b)
MA
MB
M
d)
M
I
I
I
*
M
M
M
*
***********
Figure 1.2 Schematic drawing of VDP processes with different polymerization
mechanisms. a) Step polymerization; b) chain polymerization; c) activated monomer
polymerization; d) activated substrate polymerization.
4
In this study, condensation chemical vapor deposition using step-growth
mechanism and initiated chemical vapor deposition using chain-growth mechanism
are used to deposit polymer films and will be introduced in detail in the next two
sections, respectively.
1.2 Condensation chemical vapor deposition
Condensation chemical vapor deposition usually involved vaporizing two
different polyfunctional (a polyfunctional monomer is one with two or more
functional groups per molecules) monomers in which each monomer possesses only
one type of functional group. The monomer molecules in the gas phase transport to
the target substrate, then the step-growth polymerization reaction on the substrate can
be represented in a general manner by the equation:
n A A nB B 
 ( A  A  B  B) n
[1.1]
where A and B are the two different types of functional groups.
For a typical vapor deposition system, it consists of a vacuum chamber
equipped with two separately heatable monomer sources, a system to control the
fluxes from the sources, a shutter system and a sample holder. Depending on the
operating system pressure, two different types of source cells have been used.
Resistively or radiatively heated quartz boats or tubes are often used under a system
pressure of P ≥ 10-4 Pa (7.5×10-7 Torr) and are designed for high deposition rates. For
an ultra-high-vacuum (UHV) environment, Knudsen cells are used to allow low
5
deposition rates and to minimize the contamination of the UHV system with organic
materials.14
Condensation
polymers
including
polyamides15,
polyimides16-19,
and
particularly in our group’s laboratory, polybenzoxazoles20, have been successfully
fabricated using VDP techniques. However, the majority of these studies have been
focused on polyimides. Aromatic polyimide thin films were first vapor-deposited
independently by Iijima et al
17
and Salem et al
18
in 1985 and 1986, respectively. It
involves the co-evaporation of two reactive monomers, Pryomellitic dianhydride
(PMDA) and 4,4’-oxydianiline (ODA) onto a substrate under vacuum conditions,
followed by a curing step ( >200°C ) (Figure 1.3).
O
O
O
O
+
O
O
H2N
NH2
O
RT
O
O
HO
O
NH
H
N
OH
O
O
-2nH2O
O
O
N
N
O
n
200°C
O
O
n
Figure 1.3 General two-step imidization reaction.
6
Condensation CVD process is usually performed at pressures below 10-6 Torr.
Due to the large mean free path (>10 m) at this pressure, the gas molecules can be
transported from source to substrate without hitting each other which preclude
gas-phase reaction between feed components. The flux from each monomer source
has to be balanced to fabricate high quality film. This can be accomplished by
thermal or rate control of evaporate sources. In addition, the monomer involved in the
condensation CVD process requires high reactivity, higher than solution or melt
step-growth polymerization reactions.
Compared with solution-casting films, polymer films from condensation CVD
process can have different properties which strongly depend on processing
conditions. For example, VDP polyimides usually have higher density, stiffness but
less hydroscopic then solution-cast polyimides.13 Polymer films from condensation
vapor deposition are usually glassy, nearly isotropic with less orientation than is
typically observed in solution processed films.21
One of the limitations for condensation CVD is the resulting macromolecules
are lower in molecular weight than their solution analogs.22 VDP involves a
solid-state polymerization that, in the absence of solvent, is kinetically hindered due
to low monomer and oligomer mobility.16
In order to deposit relatively high
molecular weight polymer, different approach, i.e. initiated chemical vapor deposition
can be used by introducing highly reactive free radical species. This technique will be
discussed in the next section.
7
1.3 Initiated chemical vapor deposition (iCVD)
Unlike condensation chemical vapor deposition, initiated chemical vapor
deposition (iCVD) is a relatively new deposition technique to make polymer thin
films using chain-growth mechanism.23 iCVD method could be regarded as a subset
of a common technique called hot filament chemical vapor deposition (HF-CVD).
The central part of the HF-CVD setup is a metal filament (typical tantalum or
tungsten). Thermal decomposition is achieved by flowing the gas species past
resistively-heated filament wires, and the activated species generated by this process
undergo polymerization reactions to form a film on the cooled substrate. The
deposition is usually performed at reduced pressure and the substrate is placed within
a small distance (2~3 cm) from the filament. Because there are fewer reaction
pathways in HF-CVD than in the plasma-enhanced CVD (PE-CVD) process,
HF-CVD can provide more well defined chemical structures of the as-deposited films
and minimize generation of structural damage in the films.24, 25
iCVD technique was developed by Gleason et al of Massachusetts Institute of
Technology in late 1990’s.25 It differs from conventional HF-CVD on one main
aspect: an initiator in addition to the monomer is introduced into the vacuum
chamber. Mao and Gleason have demonstrated that iCVD can produce a variety of
functional
polymer
such
as
poly
(tetrafluoroethylene)26,
poly
(glycidyl
methacrylate)27, poly (2-hydroxyethyl methacrylate)28, poly (methyl methacrylate)29
and poly (perfluoroalkylethyl methacrylate)30. In a typical iCVD process, monomer
species and initiator are vaporized and fed at moderately low pressure (~0.1-10 Torr)
8
over array of resistively-heated Nichrome filament (~250-400°C) and across a cooled
substrate. Monomer molecules absorb onto the hot filament and then reemit without
reaction. Initiator molecules undergo hemolytic bond cleavage and desorb as free
radical species. Both monomer and initiators diffuse and adhere to the substrate. Then
solid state free-radical polymerization occurs to form linear chains (Figure 1.4).
Figure 1.4 Schematic drawing of initiated chemical vapor deposition (iCVD)
technique.
Several CVD techniques have been developed based on the principles of
iCVD, as illustrated in Figure 1.5. Photoinitiated chemical vapor deposition (piCVD)
uses a volatile photoinitiator to initiate free-radical polymerization of gaseous
monomers under UV irradiation.31 This technique simplifies the reactor design and
eliminates any heat radiation from hot filament. Grafting Chemical vapor deposition
(gCVD) uses type II photoinitiator to graft polymer chains to the substrate surface.32
9
Resulting grafted polymer layer is resistant to abrasion and is stable against virtually
any solvent. Our laboratory uses a packed hot-zone as an alternative to the hot
filament to dissociate free radical initiator.33 This design enables poly(methyl
methacrylate) (PMMA) to be deposited at relatively low dissociate temperature (~200
°C) compared with hot filament design (>450 °C)29.
Figure 1.5 Schematic drawing of iCVD processes using different dissociate methods.
The mechanism of iCVD polymerization is believed to occur through three
major steps, (1) decomposition of an initiator in the vapor phase to form primary free
radicals, (2) diffusion and adsorption of primary radicals and monomer from the
vapor phase onto a cooled surface, and (3) polymerization of monomer on the surface
to form a continuous polymer coating. The primary radicals first initiate chain
polymerization on the substrate surface by adding to first one monomer unit
(initiation), then these active radical centers propagate by adding more and more
10
monomer units to form longer polymer chain (propagation). These polymer chains
could terminate by coupling or disproportionating with other polymer chain or
reactive radical species (termination)
34
(Figure 1.6). Because of the presence of
free-radical initiator, deposition rate of the iCVD process can excess 10 µm/hr. Due
to the high reactivity of free radical species, resulting polymers have molecular
weights range from 15 K to 50 K Daltons, which could be hardly achieved in the
condensation VDP process.
Radical Generation (filament/UV)
I 2 
2I 
kd
Initiation
I   M 
IM 
ki
Propagation
IM n   M 
IM n 1 
kp
Termination
IM n   IM p  
IM n p I
kt
IM n   IM p  
IM n  IM p
kt
Figure 1.6 Scheme for free-radical polymerization in iCVD process.
The reaction kinetics of iCVD process has been investigated and it is found
that it can be succinctly considered as a free radical polymerization in the bulk yet
without the presence of a liquid phase.35
However, it is distinct in that, unlike bulk
thermal polymerization, iCVD effectively separates the initiator decomposition from
the polymerization events (propagation and termination). This additional degree of
freedom should, in principle, allow a greater degree of control over processing and
11
resulting coating properties. It also provides greater processing flexibility because a
lower substrate temperature could be employed to coat heat-sensitive materials such
as plastics, fabrics, and pharmaceutics.
1.4 Film morphology
Thin film deposition including chemical vapor deposition is the most
ubiquitous and critical part of the processes used to manufacture high-tech devices
such as microprocessors36, memories37, solar cell38, microelectromechanical systems
(MEMS)39, lasers40, solid-state lighting41, and photovoltaics42. The morphology and
microstructure of these thin films directly controls their optical43, magnetic44, and
electrical properties44, which are often significantly different from bulk material
properties. Precise control of morphology and microstructure during thin film growth
is paramount to producing the desired film quality for specific applications. To date,
many thin film deposition techniques have been employed for manufacturing films,
including thermal evaporation45, sputtering deposition46 and chemical vapor
deposition47.
Many deposition factors contribute to the formation of complex morphology
on the surface of a film. First, during deposition process, there is always random
noise that exists naturally because atoms do not arrive at the surface uniformly. Noise
competes with surface smoothing process, such as surface diffusion, to form a rough
morphology if the deposition is performed at a high growth rate. In addition, growth
12
roughness can also be increased by some growth processes such as geometrical
shadowing.48 Shadowing is a result of deposition by non-normal incident flux.49 In
both sputtering deposition and chemical vapor deposition, atoms do not always
approach the surface in parallel and they arrive at the surface with a distribution of
trajectories very often. Another important effect to consider is the sticking coefficient
which is defined as the probability that a particle will stick to the surface when it
strikes.50 The sticking coefficient may not be equal to unity which means particle is
allowed to reemit from the surface upon impact. The particle may then deposit on the
different location which leads to a smoothing effect. Common growth effects which
contribute to the overall film morphology are summarized in Figure 1.7.
Figure 1.7 Diagram of growth effects including diffusion, shadowing and reemission
which may affect surface morphology during thin film deposition.
There are two classes of measurement techniques which allow for a collection
of quantitative information about film morphology: real-space imaging techniques,
13
and diffraction techniques.51 Real-Space imaging techniques include atomic force
microscopy (AFM), transmission electron microscopy (TEM), scanning electron
microscopy (SEM), stylus profilometry, and white light interferometry which is
commonly used in our study. Real-space imaging techniques can provide a direct
visual interpretation of the surface morphology. Diffraction techniques include
high-resolution low-energy electron diffraction (HRLEED), reflection high-energy
electron diffraction (RHEED), X-ray diffraction, and light scattering. For these
techniques, surface morphology information including roughness could be obtained
from the angular distribution of the diffracted pattern. They also provide non-contact
measurement and are capable of getting surface information over a larger area in a
short time compared with real-space imaging techniques.
Condensation chemical vapor deposition20, initiated chemical vapor
deposition33, plasma-enhanced chemical vapor deposition (PE-CVD)52 and other
forms of VDP typically result in smooth, homogeneous films. Although iCVD
technique is capable of fabricating nano-scale pattern within the vapor-deposited
films10, and can be used to deposit conformal polymer coatings directly on
micron-sized particles and carbon nanotubes11,
these fabrication processes and
iCVD in general, offer little control over film morphology and spatial heterogeneity.
The introduction of non-reactive, immiscible species into a vapor deposition
process has been proved effective to leverage phase separation and induce film
heterogeneity.
Porous
or
micro-structured
polymers
including
high-glass-transition-temperature (Tg), rigid-rod polyimide has been fabricated by
14
introducing immiscible nano-sized particles. Porous films53, 54 can be fabricated by
inducing thermal decomposition in these particles (porogens) that are homogeneously
dispersed in a polymeric matrix via vapor deposition. The porogen molecules
decompose to give small volatile fragments which either leave the polymeric matrix
to form pores, or are trapped and may affect transport through chemical interaction.
Various N-boc amino acids including N-boc dodecanoic acid provide potential
candidates for porogens. They are stable at the evaporation temperature, but
decompose while the polymer is still stable and under the Tg of the polymer matrix.
The porosity and pore size are related to the weight percentage of porogen and
characteristics of the polymer including molecular weight. Alternatively, nano-metal
particles can be co-evaporated with polymer precursors, and polymer films with a very
large variety of nanostructures can be fabricated through combination of two vacuum
deposition techniques.55 Thin polymer films are deposited using plasma
polymerization or traditional CVD approach, and metal particles are embedded
during deposition of polymer film, mainly by simultaneous metal evaporation or
metal sputtering. Metal particle distribution, size, shape and orientation can be
controlled by adjusting deposition conditions including metal particle type and
plasma density55, which in turn change the electrical and optical properties of
deposited films. As an example, our laboratory recently co-deposited copper
phthalocyanine (CuPc), a well-studied p-type organic semiconductor, with vacuum
deposited polyimide to influence film morphology. Resulting films show evidence of
15
embedded CuPc particles at the surface and in the bulk of fine lateral structure with a
length scale of about 100 nm.8
A similar approach has been attempted in our laboratory to deposite structured
or even porous polymer films using the techniques combining iCVD and
polymerization-induced phase separation (PIPS). PIPS has also been widely used to
produce and control porosity in thermosetting resins.56 This method is unique because
here phase separation (nucleation and coarsening) and polymerization occur
simultaneously. This approach consists of polymerizing a mixture of the polymer
precursors and a non-reactive solvent, termed the porogen, which will template the
porosity. The porogen must be a solvent for the monomer precursor but a non-solvent
for the polymer. Thus, owing to the substantial decrease in the entropy of mixing on
polymerization, the initially homogeneous solution separates into a solvent-rich phase
and polymer-rich phase. When polymerization and phase separation are complete, the
solvent may be removed by freeze drying, leaving a porous structure,57 as illustrated
in Figure 1.8. The key properties of porous materials including pore size and porosity
will depend on molecular weight and crosslink density of polymer, porogen species
as well as weight percentage of porogen, temperature and the amount of time the
system resides at this stage.
16
Liquid one-phase system
Solid-liquid two phase
system
Solid-gas two phase
system
Figure 1.8 Principle of polymerization-induced phase separation.
Therefore, during the deposition run, the gas mixture containing monomer,
initiator and non-reactive solvent (porogen) flows through the packed hot-zone, and
initiator molecules are thermally dissociated to free radicals. The free radicals will
diffuse then condense on the cool substrate and undergo free radical polymerization.
During early stage of polymerization, the free energy of mixing remains favorable
and whole system is homogenous. However, as the polymer chain grows, mixing
becomes
unfavorable.
polymerization-induced
It
is
expected
that
at
phase
separation
occurs,
a
critical
leaving
chain
length,
polymer-rich
and
porogen-rich domains. As polymerization continues, polymer will become glassy and
the phase-separated morphology will be “arrested”. The porogen then will be
removed after the PIPS completes, leaving porous morphology. In this process, molar
flow rate of each component can be easily adjusted. In principle, molecular weight,
crosslink density of poly (methyl methacrylate), weight percentage of porogen can be
changed as the film grows, leading a one-dimensional, precise control of film
morphology in the direction of film growth.
17
1.5 Objective and outline
The objective of this thesis is to determine the feasibility of controlling
morphology of vapor-deposited polymer films using vapor deposition polymerization
(VDP) techniques. Although several techniques have been developed successfully to
make porous or structured polymer materials, however, no approach has been
unanimously successful in enabling a simple, environmentally friendly and cost
effective process to tune film morphology. To our knowledge, precise spatial control
over polymer morphology along one dimension has not been demonstrated. Chemical
vapor deposition has the ability to adjust the molar flux ratios of gaseous precursor
components during deposition run therefore provides an easy and promising way to
fabricate materials with precise control of morphology.
In this thesis, the effect of process conditions on film morphology is explored
using both step-growth and chain-growth chemical vapor deposition techniques.
Non-reactive, immiscible species are introduced in the vapor deposition process to
induce heterogeneity. In the following chapters, the fabrication of condensation and
addition polymers, and the control of film morphology using chemical vapor
deposition techniques will be discussed in detail:
Rigid-rod Polybenzoxazoles (PBO) films were made using condensation
chemical vapor deposition (Chapter 2). PBO precursors were vapor-deposited then
thermally cured under inert gas condition. At high curing temperature (500~550°C),
precursors undergo decarboxylation to form partially aromatic PBO. The successful
fabrication of PBO precursors illustrates the versatility of CVD process, and provides
18
hands-on experience to achieve morphology control in the condensation polymer
films.
The effects of non-reactive component, i.e. copper phthalocyanine (CuPc), on
morphology of vapor-deposited polyimide film were studied (Chapter 3). A new
three-source vapor deposition system was custom-built to study the structure
development in polyimides. Various dianhydride/diamine reactant pairs were
co-evaporated and CuPc molecules were embedded into the polyimide matrix via
thermal evaporation. The polyimide morphology can be adjusted by changing
processing conditions including annealing temperature.
An axisymmetric iCVD vacuum reactor was designed and custom built
(Chapter 4). Using this reactor, smooth and homogeneous poly (methyl methcrylate)
(PMMA) films can be grown from vapor feeds of methyl methacrylate (MMA)
monomer and t-butyl peroxide (TBPO) initiator. We were trying to determine how
this axisymmetric iCVD reactor is suited to explore depositions involving multiple
components. Several key reactor operating parameters, were systematically varied to
understand film growth kinetics. These include the hot-zone temperature, reactor
base-pressure, substrate temperature, and the monomer/initiator molar feed ratio.
A non-reactive solvent (t-butanol) vapor was then introduced into the
deposition of PMMA to induce morphology change, i.e. dewetting of polymer films
from substrate to form polymer droplets (Chapter 5). We found that different
deposition conditions influenced whether dewetting occurred.
Moreover, control
over lateral length-scale was demonstrated through systematic adjustment of solvent
19
and carrier gas flowrates. An autophobic dewetting mechanism was proposed to
describe the dewetting mechanism during the vapor deposition process.
The next chapter begins with a discussion of vapor deposition of micron-thick
PBO films and will provide a platform to study simultaneous film growth and phase
separation during deposition—the topic of Chapter 3.
20
Chapter 2. Vapor Deposition and Curing of Polybenzoxazole
Precursors
2.1 Introduction
2.1.1
Polybenzoxazoles (PBOs)
Throughout the 1960s several new heterocyclic and aromatic polymers were
developed to address the demands of the aerospace industry.58 Rigid-rod
macromolecules with high glass transition temperatures and high thermal stability
were developed including polyimides59, polybenzimidazoles60, polybenzoxazoles61
(PBOs) and polyquinoxalines62. PBOs in particular have been investigated over the
past few decades because of their unusually high modulus and tensile strength in the
fiber form, their excellent thermal stability, flame resistance, and their good
hydrolytic and solvent resistance.63-67 Fully aromatic PBO fibers (with commercial
name
Zylon®)
can
be
obtained
by
the
condensation
reaction
of
4,6-diamino-1,3-benzenediol dihydrochloride with terephthalic acid (TA) in the
presence of poly(phosphoric acid) (PPA) followed by dry-jet, wet-spinning process.63
In the dry-jet, wet-spinning process, the material is placed under high levels of
tension while it is coagulated. The resulting fibers exhibit an impressive tensile
strength of 5.8 GPa, an elastic modulus of 270 GPa,65 and have a characteristic
degradation temperature as high as 680 °C.68
21
PBO’s properties depend on molecular weight, the degree of axial chain
orientation between polymer backbones, and the level of non-covalent interactions
between chains.69 Not surprisingly, there are enormous differences between the
properties of fully-aromatic PBO, i.e. poly (p-phenylenebenzo-bis-benzoxazole) (1),
and those of partially-aromatic PBO, such as 2 (Figure 2.1). PBO 1 is a rigid-rod
macromolecule with a large perisistence length.70,
71
For PBO 1, there are few
opportunities for kinks in the polymer backbone, and free torsion rotation about C-C
bonds has little effect on the overall polymer configuration. On the other hand, the
bonding geometry of 2 permits several kinks in the backbone due to the various
rotational isomeric states about C-C bonds. These rotations improve the solubility and
processibility of PBO 2 at the expense mechanical properties.
N
N
N
O
O
O
n
O
N
n
(1)
(2)
Figure 2.1 Structural comparison of fully-aromatic (1) and partially-aromatic PBO(2).
Laboratory synthesis of semi-aromatic PBOs, such as 2, has been developed
and involves the use of heterocyclic precursor polymers.72, 73 This synthesis strategy
enables the precursor polyamide to be processed, and it does not require aggressive
solvents such as methanesulfonic acid or poly (phosphoric acid). For example, the
reaction between pyromellitic dianhydride (PMDA) and 3,3’-dihydroxy-benzidine
22
(DHB) (Figure 2.2) results in polyhydroxyamide 3 which can be thermally
cyclodehydrated to form a polyimide.
Upon further thermal treatment to ~440 °C,
biphenyllic hydroxyl groups react with the imide carbonyls to form new five-member,
benzoxazole rings. At even higher temperature (500~550 °C) the material undergoes
decarboxylation and releases carbon dioxide to form partially aromatic PBO 2.74
Attempts to achieve fully aromatic PBOs 1 using this strategy have not succeeded.
Rather, direct reaction of 4,6-diaminoresorcinol with PMDA only yields the
polyimide precursor and does not rearrange to form 1. In this case, thermal
conversion is difficult due to the rigid structure of polyimide precursor.
An alternative technique, vapor deposition polymerization, was explored as an
approach to obtaining new high performance films. Over the past decade, full density,
high performance polyimides17,
18, 75
, polyquinoxalines76, and poly(oxadiazoles)77
have been prepared using VDP techniques. VDP is a solventless process whereby
film thickness is easily modulated, conformal coatings can be achieved down to the
nanometer length scale, and resulting coatings are free of defects associated with
solvent evaporation. This study is to investigate the suitability of VDP to process
traditionally intractable PBOs. DHB and PMDA were co-deposited onto flat
substrates to form poly (amic acid) films. Reactant fluxes from each source must be
well balanced to achieve quality films. Coatings were cured under inert gas
conditions, resulting in the conversion to polyimides (150-250 °C), and, at higher
temperatures (550-550 °C) to semi-aromatic polybenzoxazoles. Physical and
chemical changes occurring during the curing process were then studied with Fourier
23
transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA) and
nanoindentation experiments. Vapor-growth of polyimides and curing to form PBO
films results in smooth, homogeneous coatings, and is the first step to achieve
morphology control in vapor-deposited polyimides.
HO
H2N
O
O
OH
NH2
+
O
O
O
O
HO
O
O
OH
H
N
HO
H
N
3
OH
O
n
O
150-200°C
-2nH2O
O
HO
O
OH
N
N
O
O
-2nCO2
n
500-550°C
O
O
N
N
n
Figure 2.2 Synthesis of semi-aromatic PBO involving thermal rearrangement of
polyimide precursor.
24
2.2 Experimental
2.2.1
Materials
Pyromellitic dianhydride (PMDA) was purchased from Lancaster Chemicals
(97% purity) and 3, 3’-dihydroxybenzidine (DHB) was obtained from TCI America
(99% purity). Prior to coating runs, PMDA and DHB were vacuum-dried overnight at
100°C. For solution synthesis, PMDA was recrystallized twice before use according
to
reference78
and
DHB
was
used
without
further
purification.
N,
N-dimethylacetamide (DMAc) was purchased from Alfa Aesar and was used as
received. Potassium bromide (KBr) disks (13 mm in diameter and 1 mm in thickness)
were purchased from International Crystal Laboratories for FT-IR studies.
2.2.2
Vapor deposition polymerization of PBO precursor 3
All attempts to vapor-coat PBO precursors were made using a custom-built
vacuum deposition chamber. This chamber has been extensively used in studies of
polyimide materials and is described in detail elsewhere.79 A simplified drawing of
the apparatus is shown in Figure 2.3. The chamber consists of two
temperature-controlled monomer evaporators that are separated from the target
substrate by 5.0 cm. The individual evaporation rates of PMDA and DHB were
obtained by measuring the mass before and after deposition, to an accuracy of ± 0.1 mg
using a balance. The monomer evaporation rates were measured for different
evaporation temperature to determine the conditions that yield equal-molar deposition
of the monomers. A retractable shutter (3× 5 cm2) is used to shield the substrate from
25
deposition until the monomer sources reach their steady-state evaporation rates. For
each run, monomers were preheated to their deposition temperatures for 30 minutes
prior to opening the shutter. The substrate was rotated at 20 rpm to improve film
uniformity. The chamber pressure during deposition was observed to be between
210-5 and 510-5 Torr. These conditions resulted in polymer deposition rates of about
3 m / hr. For this study films were vapor-coated to about 1 m.
Figure 2.3 Schematic diagram of the deposition system inside the vacuum chamber.
2.2.3
Solution synthesis of PBO precursor 3
Poly (amic acid) (PAA) 3 was synthesized from the low-temperature reaction
of PMDA with DHB in the presence of N, N-dimethylacetamide. DHB (2.5 g, 11.57
mmol) and DMAc (25 g, 26.60 ml) were placed in a 100 ml three-necked flask and
26
stirred at 0 °C for 30 minutes under nitrogen purge. In a separate flask PMDA (2.52 g,
11.57 mmol) was dissolved in DMAc (25.2 g, 26.84 ml), and this solution was added to
the stirring DHB solution through an additional funnel. The reaction mixture was
stirred vigorously at 0 °C for 1 hour, then at room temperature for 24 hours, yielding a
10 wt % solution of poly (amic acid) 3. The resulting solution was spun-cast (3000
rpm) onto KBr optics and glass substrates and the solvent was evaporated in vacuo at
50°C for 6 hours.
2.2.4
Curing of PBO precursors
As-deposited and solution-cast films were thermally cured to different
temperatures (200, and 550 °C) using various heating rates (10, 1, and 0.1 °C / min)
under inert gas condition (Ar).
During the high-temperature cure, even small
amounts of gaseous impurities (oxygen) were found to initiate thermal decomposition
of films and were scrupulously avoided.
2.2.5
Characterization
The thickness of VDP films was measured using a film measurement device
(Filmmetrics, model: F20). FT-IR spectra were recorded using a Nicolet 20SXC
infrared spectrometer. Vapor-coated films were examined using polarized optical
microscopy.
Thermal Gravimetric Analysis (PerkinElmer Pyris) was performed on
as-deposited films at a heating rate of 10 °C/min. Mechanical properties (elastic
modulus and hardness) were measured using nano-indentation techniques (MTS,
Nanoindenter III).
Loads were applied to the sample using a Berkovitch pyramidal
27
diamond indenter until a specified displacement (~50 nm) was reached. The sample
thickness was always at least a factor of ten larger than the indenter displacement.
Raw data gathered included load versus displacement curves. The hardness and
elastic modulus were calculated and averaged from six different indents.
For these
calculations, the Poisson’s ratio  was taken to be 0.41 for poly(amic acid)80, 0.33 for
polyimide80 and 0.30 for poly (p-phenylenebenzo-bis-benzoxazole)81.
2.3 Results and discussion
2.3.1
Sublimation of PMDA and DHB monomers
Vacuum thermogravimetric analysis (vacuum TGA) was performed to find
desired evaporation temperatures for potential precursor monomers. This technique
allows the vapor pressure of each tested materials to be calculated as a function of
temperature. This makes it much easier to achieve stiochiometric balance of monomer
fluxes during co-depositions. Figure 2.4 shows the linear relationship between
logarithm of vapor pressure (log P) and the inverse of temperature (1/T) for PMDA
and DHB, respectively. The vapor pressure can be calculated from this figure which
allows the relative evaporation rates for any temperature to be predicted. These
results can be used as a good starting point for determining evaporation temperatures
of monomer sources.
28
-0.1
Log P (Pa)
-0.2
-0.3
-0.4
-0.5
-0.6
DHB
PMDA
-0.7
2.1
2.2
2.3
1/T (1/K)
2.4
-3
2.5x10
Figure 2.4 Vapor pressure curves of PMDA and DHB from vacuum TGA
measurement.
Based on the preliminary results from vacuum TGA, several depositions were
run to achieve a stiochiometric balance of monomer fluxes leaving the two
evaporators and results are summarized in Table 2.1. The molar-loss ratio (MLR) is
defined as the ratio of moles (PMDA to DHB) leaving the evaporators during a
5-hour coating run.
Because molecular transport to the film depends on several
factors, including reactor geometry, this ratio does not correspond directly to the
molar composition of the as-deposited film.82
Nevertheless, the molar-loss ratio is a
useful quantity—it is simple to measure experimentally, and it serves as a good
indicator of molar flux balance. In order to achieve a MLR near 1:1, temperatures
were adjusted to 213 °C and 153 °C for PMDA and DHB, respectively.
29
Table 2.1 Molar loss ratio at different operating conditions.
Evaporator Temp. (°C)
Molar loss ratio
Run
PMDA
DHB
(PMDA:DHB)
1
153
163
8.2
2
153
203
4.0
3
158
213
1.5
4-9
153
213
1.14 ± 0.03a
a) Reported error is indicative of reproducibility and was
calculated as the standard deviation of six runs.
As expected, films grown under these conditions were of higher quality—they
were light-yellow in color, pinhole-free, transparent, and adhered well to glass
substrates. Films grown with much more PMDA (MLR>2) showed small white
crystals on their surface (birefringence under polarized microscopy) and cannot stand
high temperature. Upon curing, these films have more pinholes and defects than those
deposited near stoichiometric balance.
2.3.2
Change of chemical structure upon curing
Selected FT-IR spectra obtained before and after thermal treatment are shown
in Figure 2.5. The vibration modes corresponding to relevant peaks are listed in Table
2.2 and are based on peak assignments made by others.16, 74, 83 It appears that the
major component of as-deposited films is the PBO precursor, the o-hydroxy poly
(amic acid), as evidenced, for example, by the intense and broad absorption band at
1650 cm-1 due to the amide C=O stretch.
However, FT-IR data indicate that
30
unreacted monomers are also present.
The sharp absorption at 1853 cm-1 is assigned
to the symmetric carbonyl stretching mode of the dianhydride monomer, and the peak
at 1503 cm-1 is partly attributed to the aromatic ring mode of free DHB. The presence
of unreacted monomer was not surprising; in our prior study16 of polyimide
precursors, we also identified unreacted monomers in as-deposited films. Increasing
the substrate temperature during the deposition run may be an effective approach to
eliminate unreacted monomers in as-deposited films. Figure 2.5 b shows an FT-IR
spectrum of a PBO-precursor film following heat-treatment to 200 °C. The anhydride
monomer absorptions were no longer present, and two new imide absorptions were
present: at 1780 cm-1, due to the symmetric C=O imide stretch, and at 1380 cm-1, due
to the imide C-N stretch. The last spectrum, shown in Figure 2.5 c, was taken of a
film heated to 550 °C and held there for one hour.
Medium intensity absorption
bands present at 1600 cm-1 and 1460 cm-1 confirm the formation of benzoxazole
rings.74, 83 The complete disappearance of imide peaks following the high-temperature
curing suggests the thermal conversion from polyimide to PBO is complete.
31
Table 2.2 FT-IR peak assignments for vapor-deposited poly (amic acid), polyimide
and PBO.
No.
Wavenumbers (cm-1)
Vibrational Mode
1
1853
Symmetric carbonyl stretching of dianhydride
2
1790
Asymmetric carbonyl stretching of dianhydride
3
1780
Symmetric C=O imide stretch
4
1650
Amide C=O stretch
5
1600
Benzoxazole ring mode
6
1503
DHB aromatic ring mode
7
1460
Benzoxazole ring mode
8
1380
Imide C-N stretch
9
1241
Ether C-O-C and anhydride C-O-C stretching
32
a)
0.8
Dianhydride
carbonyl stretching
T
0.6
DHB
ring mode
0.4
0.2
Amide C=O
stretch
b)
T
0.6
Imide C-N
stretch
0.4
0.2
Imide C=O
stretch
c)
Benzoxazole
ring mode
T
0.9
0.8
2000
1800
1600
1400
-1
Wavenumber [cm ]
1200
Figure 2.5 FT-IR Spectra of (a) as-deposited film (b) film cured at 200 °C for 5 hours,
and (c) film cured at 550 °C for 1 hour. The spectrum in (c) was taken using reflective
mode.
33
2.3.3
Thermal properties
A TGA scan of an as-deposited film (monomer loss ratio: 1.2) acquired under
N2 atmosphere is shown in Figure 2.6. The sample continuously loses mass over the
entire heating range, even as high as 600 °C. The continuous mass loss is believed to
be associated with the evaporation of unreacted monomers. In addition to this
continuous mass loss, discrete weight reductions were also observed and are
associated with chemical transformations. A mass loss of about 14 % was observed
between 180-220 °C. This is a little larger than the theoretical value of 8 % due to the
loss of water molecules as amide linkages are converted into imide linkages. The
polymer also showed significant mass loss at temperature greater than 400 °C. Part of
this mass loss is attributed to decarboxylation. The fully cured PBO had a residue at
650 °C that was about 50 % of its original mass. This temperature is slightly lower
than the characteristic half-decomposition temperature of commercial PBO fibers
(670.7°C) in nitrogen.68 However, the thermal stability of the vapor-deposited PBO
studied here far exceeds many common thermoplastics; for example, polystyrene
decomposes at around 450 °C and polyimide, also considered a high-performance
material, begins to decomposes at 560 °C.84 This suggests that the hydroxyamide
precursor was transformed to higher char yield structures upon curing.
34
Imidization
0.9
Weight [%]
0.8
Decarboxylation
0.7
0.6
Thermal
Decomposition
0.5
0.4
200
400
600
Temperature [°C]
800
Figure 2.6 TGA thermogram of as-deposited poly (amic acid) film.
2.3.4
Mechanical properties of PBO
Mechanical properties are the hallmark of PBOs. Nanoindentation is ideally
suited for mechanical measurement of micron-thick films because it involves
extremely small volume displacements. Nanoindentation results on vapor-deposited,
solution-cast, and thermally cured films are compared in Figure 2.7. Compared to
solution-cast films, as-deposited and cured VDP films always exhibit higher or
comparable Young’s modulus and hardness values.
This result is a little unexpected
since VDP films are believed to be of lower molecular weight than solution-cast
films. VDP involves a solid-state polymerization that, in the absence of solvent, is
kinetically hindered due to low monomer and oligomer mobility.16 However
vapor-deposited films show unique properties since VDP can be regarded as a
layer-by-layer process. Prior X-ray studies of vapor-deposited polyimides confirmed
35
that chain axes are preferentially oriented parallel to the surface.19 In this study,
differences in Young’s modulus between VDP and solution-cast films are attributed
to an overall higher degree of chain ordering and packing in the VDP form. It is
difficult to assess, quantitatively, how chain anisotropy affects the Young’s modulus
parallel (E) and perpendicular (E) to the surface. While nanoindentation is suitable
for thin films, the measured modulus is a weighted average of E and E.
All heat-treated polymers show higher Young’s modulus than as-deposited or
as-cast polymers. The modulus of as-deposited films increased significantly upon
converting to the polyimide, and then increased slightly upon achieving PBO. The
modulus improvement is attributed to the loss of chain flexibility as hydroxyamide
chains are converted to more rigid, fully-cyclized structures. PI and PBO films also
exhibit higher values of measured hardness than their PAA precursors—regardless of
whether they were solution-cast or vapor-deposited.
36
a)
b)
Figure 2.7 (a) Modulus and (b) hardness of vapor-deposited and solution-cast films at
different stages of curing corresponding to poly(amic acid) (PAA), polyimide (PI), and
polybenzoxazole (PBO).
37
Mechanical properties also depend on curing rate (Figure 2.8). Since
vapor-processed PBO films usually cracked after curing at high temperatures for long
times (>1 hour), we choose to focus on the properties of vapor-processed polyimide
to understand the influence of curing rate on mechanical properties. VDP samples
imidized at a slower heating rate of 0.1°C/min (longer annealing time) exhibit higher
modulus values than those imidized at 1 °C/min and 10 °C/min. Films imidized at the
slower rate even exhibited higher modulus than corresponding PBOs cured at 10 °C/
min. Slow curing may lead to chemical crosslinking and different mechanical
properties as suggested by other authors.79 Our results agree well with the study by
Chang et al. on the curing reactions of solution-cast polyamide PBO-precursors.85
They found that mechanical properties strongly depend on the curing routine. In their
study, the values of the tensile strength and modulus of the precursor films were
enhanced remarkably by increasing the annealing temperature and time.
38
a)
b)
Figure 2.8 Effect of curing rate on the mechanical properties of cured polyimide: (a)
modulus and (b) hardness.
As mentioned in the introduction, mechanical properties of fully and partially
aromatic PBO should differ due to differences in bonding geometries. Indeed, the
measured modulus of partially aromatic vapor-processed PBOs (12.5 GPa) is over
twenty times lower than that of fully-aromatic Zylon® fibers (270 GPa). But the
reported mechanical properties are still much better than most commodity polymers,
and they exhibit outstanding thermal stability. For Zylon fibers, the alignment of
molecules along the PBO fiber axis is extraordinarily high, and polymer chains
become even more aligned when cured under a high stress field.64, 69 Clearly, chain
39
alignment and ordering plays a crucial role in determining the mechanical properties
of PBO. Mechanical processing of PBO such as heat-treatment under uniaxial or
biaxial stresses may be a strategy to improve mechanical properties of
vapor-deposited PBO films.
2.4 Summary
The successful fabrication of PBO shows the versatility of CVD process. We
have demonstrated that VDP techniques can be applied to fabricate micron-thick PBO
films. Co-deposition of PMDA and DHB monomers results in poly (amic acid)
precursor, curing of as-deposited poly (amic acid) under inert gas atmosphere leads to
the formation of polyimides at about 200°C and subsequently to partially-aromatic
polybenzoxazoles at about 550°C.
FT-IR studies confirmed the existence of
unreacted monomer and the formation of polyimide and PBO upon curing. TGA
studies revealed mass losses due to imidization and decarboxylation, as well as a
continuous mass loss of unreacted monomer. Nanoindentation studies showed that the
deposited film’s Young’s modulus and hardness can be improved by a factor of three
after the conversion from poly (amic acid) to PBO, and that the curing rate is an
important process parameter. Most importantly, this study demonstrates, for the first
time, that high performance PBO films can be synthesized using a solventless, VDP
approach. Moreover, the fabrication of homogeneous polyimide and polybenzoxazole
films provides hands-on experience and is a first step toward exploring morphology
40
control in vapor-deposited polyimide films which will be discussed in the next
chapter.
41
Chapter 3. Morphology of Vapor Deposited Polyimides
Containing Copper Phthalocyanine
3.1 Introduction
3.1.1
Polyimides
Polyimides are an important group of polymers because of their many
desirable properties including excellent mechanical properties, low dielectric
constant, low relative permittivity, high breakdown voltage, good processability, wear
resistance, radiation resistance, inertness to solvents and long-term stability.14
Because of these characteristics, polyimides have been used in a wide range of
applications including dielectric, high-temperature adhesive, photoresist, nonlinear
optical material and membranes. Polyimides have also been viewed as a potential
n-type semiconducting materials. Nephthalene
86
86, 87
, perylene
87
and biphenyl
-derived polyimides all exhibit n-type properties in recent study, which make them
perfect candidate in organic electronic devices including organic field effect
transistors88, light-emitting diodes89 and photovoltaics90. The excellent mechanical,
thermal and chemical properties of polyimides provide a significant advantage, since
many semiconducting polymers show relatively poor stability.
The synthesis of aromatic polyimide was first reported in 1908 but it was not
until late 1950s that DuPont developed a successful commercial route to achieve
high-molecular-weight polyimides.59 Its Kapton® brand polyimide type NH films are
42
prepared by reacting 4,4’-oxydianiline (ODA) with pyromellitic dianhydride (PMDA)
in solution. The resulting poly (amic acid) precursor solution is then cast and cured at
300 °C to form the insoluble polyimide (Figure 1.3). Vapor deposition polymerization
(VDP) is an alternative to the industrially important spin-coating (SC) technique to
fabricate polyimide films. It was developed independently in the mid 1980s by
Iijima75 et al. and Salem18 et al. respectively. Heated dianhydride and diamine
monomers are co-deposited onto a solid substrate in vacuum then followed by
imidization reaction. In most of the following investigation of polyimide films
prepared by VDP process, PMDA and ODA were used as dianhydride and diamine
components, respectively. Other diandydride and diamine precursors, such as
2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)91, naphthalene
tetracarboxylic dianhydride (NTDA)8, 92, decamethylene diamine (DMDA)93, were
also used recently. Because this process eliminates solvent, this can be advantageous
in some applications, where the use of solvents can have deleterious effects or where
contaminations have to be strictly avoided. It is also capable of depositing on
complex nonplanar geometries such as spheres.82
3.1.2
Copper phthalocyanine
Copper (II) phthalocyanine (CuPc) is a commercially available macrocyclic
metal complex that possesses nonlinear optical and semiconducting properties (Figure
3.1). Together with other phthalocyanine derivatives, the chemically and thermally
stable CuPc has wide applications in dye processing, spectral sensitization, chemical
sensors, and optical data storage.94
43
Figure 3.1 Molecular structure of copper phthalocyanine (CuPc).
The semiconducting (p-type) behavior of metal phthalocyanines was first
observed in 194895 and they have since attracted great interest in advancement of
prototype organic semiconductors. It is typically spun-cast or co-deposited together
with a polymer onto a substrate. With nano-composites composed of mixture of
non-covalently bound CuPc and polymer, one can achieve an attractive blend of the
optoelectronic properties of CuPc materials with the processability of polymers. As
such they are of major importance in the fabrication of photoconductive films and
photovoltaic devices.55,
96
Polymer hosts are valuable platforms in these hybrid
nanocomposites. Relatively polar polymers, such as poly (vinyl fluoride) and
polyacrylonitrile, has already been shown to have a very significant effect on the
performance of the CuPc particles by enhancing the photogeneration of charge
carriers.
It has been shown that the electron mobility of CuPc increases five times
when it changes from granular to rodlike crystal structure in polymer matrix.97
44
Therefore, for applications including photovoltaics and organic light-emitting diodes
(OLED) devices, control of thin film morphology is crucial.
3.1.3
Organic photovoltaics (OPV)
Organic photovoltaic (OPV) devices have drawn great attention in the past
decade and several reviews on OPV have been published.98,
99
There are many
reasons for the interest in OPVs when comparing with the silicon-based photovoltaics
(PV). The OPVs offer low cost, low thermal budget, solution processing, flexible
substrates and a very high speed of processing. However, photovoltaic devices
incorporating organic materials display low power conversions (less than 5%)
compared with silicon-based PV (which are over 20 %).
In OPV, excitons, which could be regarded as electron-hole pairs, are created
via photoexcitation, then excitons must be dissociated in order to take advantage of
the excited electrons. The earliest OPVs are Schottky-type devices in which a contact
is formed at one of the organic-electrode interfaces in a metal-organic-metal
sandwich (Figure 3.2 a). Such devices are inefficient as charge photogeneration only
takes place in a thin layer near metal-organic interface, limiting the quantum yield of
charge
photogeneration.
Organic
pn-heterojunction
is
an
improvement
to
Schottky-type devices (Figure 3.2 b). It is based on charge generation at an interface
between electron donating (n-type semiconductor) and electron accepting (p-type
semiconductor) materials. Then electron will travel through the n-type semiconductor
and external circuit before recombining with holes at the anode.
45
a)
b)
Figure 3.2 Two types of OPV devices: (a) Schottky-type junction and (b)
pn-heterojunction.
The performance of a bilayer OPV is determined by the efficiency of charge
generation and transport. One of the limitations for a two-layer pn-heterojunction
OPV is that charge photogeneration only takes place in a thin layer near the organic
heterojunction. Quantum efficiency of the devices will be limited because only part of
incident light will be absorbed in this region. One method to increase the width of
photogeneration region is to co-deposit two different organic semiconductor materials
to form a bulk pn-heterojunction. In this case, electrons and holes are generated in
expanded mixed layer and are swept to the transport layers by built-in chemical and
electric potentials.
The ability to vapor-deposit polyimide films will now be extended to fabricate
bulk pn-heterojunctions by co-deposition of n-type vapor-deposited polyimide and
p-type CuPc will be studied in this chapter. The effect of CuPc particles on
polyimide/CuPc composite morphology will be investigated in detail. Factors
including surface energy all affect morphology, making it difficult to control the bulk
46
interface geometry.100 The VDP process is shown as a viable approach toward
quenching phase-separated morphologies of polyimide films containing copper
phthalocyanine, therefore will be used as an ideal technique to fabricate bulk
pn-heterojunction. This study builds on earlier studies involving co-deposition of
CuPc with vapor grown poly(p-phenylene)101 and polyimdes102. The growth of CuPc
into ordered, thin film is an important aspect of charge transport and device efficiency.
CuPc was selected for this study because its crystal habit and morphology are very
sensitive to processing conditions including CuPc purity, substrate type, and
annealing conditions.97, 103-106
3.2 Experimental
3.2.1
Materials
For polyimide coatings, pyromellitic dianhydride (PMDA, 97% purity),
biphenyl tetracarboxylic dianhydride (BPDA, 97%), and xylene diamine (XDA, 99%),
were purchased from Sigma Aldrich and used as received. Naphthalene
tetracarboxylic dianhydride (NTDA, 95%) was purchased from Sigma Aldrich and
purified by sublimation. Perylene tetracarboxylic dianhydride (PTDA, 97%) was
purchased from Fluka and used as received. Oxydianiline (ODA, 98%) was
purchased from Fluka and was ground with a mortar and pestle before use. All
reagents were stored in a dessicator when not in use. CuPc (97%, Sigma Aldrich) was
47
used as received for morphology studies. For photovoltaic device performance studies
zone-refined (>99% pure) CuPc was obtained from Eastman Kodak.
3.2.2
Substrate preparation
Various substrates including glass and quartz microscope slides, polished KBr
disks, and pre-patterned ITO-coated glass slides were used for film deposition.
Standard 3”× 1” glass slides substrates were cleaned by soaking in KOH/ iso-propyl
alcohol basebath solution for two hours, followed by rinsing and drying at 110 °C.
For UV/Vis studies, 1”× 1” quartz substrates were cleaned by immersing in a 70 °C
solution of one part H O , one part NH OH, and five parts water followed by rinsing
2
2
4
and drying at 110 °C for 1 hour. Polished KBr optics (13 mm diameter ×1 mm thick)
were purchased from International Crystal Laboratories for FT-IR studies. 1.5” × 1.5”
pre-patterned ITO-coated glass was cleaned using methanol followed by an air
plasma treatment for three minutes.
3.2.3
Vapor deposition of polyimide/CuPc composites
A shuttered vapor deposition system shown was designed and assembled for
this work (Figure 3.3). The system consists of a 45-cm aluminum cube as the reactor.
-6
A base pressure of 10 Torr was obtained using an Alcatel turbomolecular pump
backed with a mechanical roughing pump. The deposition system contains two low
temperature effusion sources for polyimide reagents, and one high temperature source
for CuPc. The distance between substrate and sources is kept at 30 centimeters. The
low temperature copper evaporators were machined to contain removable ceramic
48
P10-6 Torr
Figure 3.3 Three-component VDP reactor for depositing polyimide/CuPc composite.
49
crucibles that could be weighed before and after each deposition. Using this
procedure, the molar evaporation rate of polyimide reagents was roughly balanced
-6
prior to depositing reported films. At 10 Torr, the mean free path of gas phase
monomer species is >10 meters. Therefore intermolecular collisions are infrequent,
and transport of monomers from sources to the target substrate likely occurs as
individual species. The high temperature source comprises of a tantalum box source
mounted on a movable platform. This source was powered by a Sorenson DCS 8-125
DC power supply and was rate-controlled using a quartz crystal monitor (QCM).
Prior to film deposition, all evaporators were held at steady-state conditions for
several minutes. The shutter was then opened to temporarily expose the substrate to
reagent fluxes. Following deposition, all polyimide films were cured at 200 °C for
one hour in a tube furnace under ambient air.
3.2.4
Characterization
Thickness measurements of as-deposited films were conducted using a Zygo
New View 100 white light interferometer using a 20 μm bipolar scan with a 20x
objective. Fourier Transform Infrared (FT-IR) absorption studies (Brüker IFS/66)
were performed on films deposited on KBr optics. Raw data were collected in percent
transmittance %T and were converted to absorbance A using the relationship A[λ] =
-ln(%T[λ]). A PerkinElmer Lambda 900 was used to collect UV/Vis spectra for films
deposited on quartz substrate. X-ray diffraction data were collected using a Philips
(X'Pert PRO MPD) X-ray diffractometer equipped with an Anton Paar TTK 450 low
50
temperature camera. For elevated temperature X-ray diffraction, the sample was
mounted in a custom-built hot-stage, and samples were held at set-point temperature
for two hours prior to collecting data. Transmission electron microscopy was
performed using a JEOL 2000 EX instrument. Samples were prepared by depositing
material onto a KBr optic, followed by careful dissolution of optic with deionized
water. Isolated film fragments were then floated (< 40 nm thick) onto copper TEM
grids. A TopoMetrix atomic force microscope (AFM) operated in tapping mode was
employed to study surface topography of deposited films.
3.3 Results and discussion
3.3.1
VDP of polyimides
Studies of polyimides and diimides based on naphthalene86, 87, perylene87, 107,
108
and biphenyl86 dianhydrides show promising n-type organic semiconducting
behavior. Several dianhydride/diamine reactant pairs were co-deposited in an attempt
to vapor-coat polyimides. In order to achieve stoichiometric balance between
dianhydride and diamine monomer, evaporation temperatures of several monomers,
including BPDA, PMDA and ODA were first estimated from vacuum TGA results
(Figure 3.4), then confirmed by the readings from QCMs.
51
LogP (Pa)
0.0
-0.5
ODA
PMDA
BPDA
-1.0
2.0
2.1
2.2
2.3
1/T (1/K)
2.4
-3
2.5x10
Figure 3.4 Vapor pressure curves of BPDA, ODA and PMDA, obtained by vacuum
TGA measurement.
As mentioned in the chapter 1, the monomers involved in the condensation
chemical vapor deposition process require high reactivity due to the absence of
solvent. Selected examples of these attempts are summarized in Table 3.1.
Subsequent inspection and FT-IR analysis indicated that most dianhydride-diamine
combinations resulted in little, if any, condensation reaction. Only those dianhydrides
containing strained five-membered rings such as PMDA and BPDA were reactive
enough to undergo polymerization. While conjugated dianhydrides containing
six-membered rings such as NTDA and PTDA are commonly synthesized in solution
at elevated temperatures, the mobility and reactivity of these reagents is limited in the
52
Table 3.1 Summary of typical VDP attempts to grow polyimide films.
dianhydride a
evap.
temp.
[°C]
evaporator
molar loss
rate
[mol / hr]
PMDA
NTDA
NTDA
PTDA
BPDA
173
210
200
340
255
9.72 × 10-5
2.4 × 10-6
1.3 × 10-6
2.1 × 10-6
9.1 × 10-5
polymer
growth rate
[nm / hr]
ODA
XDA
ODA
XDA
ODA
123
35
113
35
140
1.93 × 10-5
6 × 10-6
2.04 × 10-6
25.3 × 10-6
9.0 × 10-5
59
no film
no film
no film
90
O
O
O
a) PMDA = O
O
O
O
O
; NTDA = O
O
O
O
O
PTDA = O
O ;
O;
O
O
O
O
O.
BPDA = O
H2 N
evaporator
molar loss
rate
[mol / hr]
O
O
b) ODA =
diamine b
evap.
temp.
[°C]
O
NH2
; XDA = H2N
NH2 .
solid state. Attempts were made to promote NTDA’s reactivity by increasing the
substrate temperature; however higher substrate temperature resulted in desorption of
monomer species, very low sticking coefficient and no film deposition. Careful
consideration of monomer flux, stoichiometry, monomer purity, and substrate
53
temperature may enable these less reactive, six-membered ring dianhydrides to be
successfully deposited, and this is the subject of a future study.
A
new
vapor-deposited
polyimide,
poly(4,4-oxydiphenylene-biphenyl
tetracarboxylic imide) was successfully synthesized by co-depositing BPDA and
ODA precursor monomers followed by curing in air at 200 °C. The vapor deposition
polymerization to form poly (amic acid) and subsequent curing to form a polyimide is
shown in Figure 3.5. After several attempts, evaporator temperatures were balanced,
aiming to produce homogeneous polymer films containing little to no unreacted
monomer. BPDA and ODA were typically evaporated at 246°C and 126 °C,
respectively, yielding a (cured) polyimide deposition rate of about 130 nm/hour as
measured using white light interferometry. Resulting films were transparent and pale
yellow in color. The remaining study will focus on this polyimide as a host for CuPc
particles.
54
O
H2N
O
NH2
O
O
+
O
O
O
O
O
HO
NH
NH
OH
O
n
O
150-200°C
-2nH2O
O
O
N
N
O
O
n
Figure 3.5 Reaction of BPDA and ODA to form poly(amic acid) and subsequent
curing to yield polyimide.
Vapor-deposited BPDA-ODA polyimide films were studied using FT-IR
spectroscopy. BPDA and ODA were simultaneously deposited and cured on KBr
optics and glass slides, so that both IR spectra and film thickness could be obtained.
The BDPA-ODA polyimide spectrum is shown in figure 3.6a with peaks assignments
made according to prior study. 16, 109 Data confirm the formation of imide group. The
-1
presence of absorption peaks at 1720 and 1380 cm correspond to the imide carbonyl
stretch and the imide ring mode, respectively. Different film thicknesses (0.49, 0.46,
and 1.35 μm) were measured, and the samples’ absorption coefficient, α, defined as
55
the absorbance peak height per unit thickness, was found to be nearly independent of
film thickness.
a)
a)
BPDA-ODA polyimide
-1
m ]
2.0
imide C=Ostr.
imide ring mode
1.0
0.0
b)
b)
-1
m ]
0.8
-1
m ]
c)
c)
CuPc
pyrole ring
str.
C=N-C
C=N-C
bridge
bridge
0.4
0.0
1.5
polyimide + 30%CuPc
PI
1.0
PI
CuPc
CuPc
0.5
0.0
1800
1700
1600
1500
1400
-1
1300
1200
wavenumbers (cm )
Figure 3.6 FT-IR extinction coefficients of a) cured BPDA-ODA polyimide; b)
heat-treated CuPc; and c) a composite polyimide/CuPc film.
56
Table 3.2 FT-IR peak assignments for CuPc and vapor-deposited BPDA-ODA
polyimide.
No.
Wavenumbers (cm-1)
Vibrational Mode
1
1790
Asymmetric carbonyl stretching of dianhydride
2
1720
Symmetric C=O imide stretch
3
1508
C=C vibration of CuPc ring
4
1503
ODA aromatic ring mode
5
1421
Pyrrole ring’s C-C stretching
6
1380
Imide C-N stretch
7
1332
Isoindoline ring’s C-C stretching
8
1286
Isoindoline ring’s C-N vibration
9
1241
Ether C-O-C and anhydride C-O-C stretching
3.3.2
VDP of CuPc/polyimide hybrid films
A series of pure CuPc films with different film thickness were deposited and
studied by FT-IR. As with the BPDA-ODA polyimides, CuPc films were studied to
obtain a relationship between FT-IR spectra and film thickness (figure 3.6 b). The
-1
CuPc absorbance peaks used for this analysis (1286, 1332 and 1421 cm ) were
selected because they did not coincide with the polyimide absorbance spectrum. The
absorption coefficient of these peaks is also nearly independent of film thickness.
Hybrid films were synthesized by co-evaporating CuPc together with
balanced BPDA / ODA precursor monomers, then the composition of
polyimide/CuPc composite was quantitatively analyzed using FT-IR. Neglecting any
57
excess volume on mixing, the bulk volume fraction of CuPc and polyimide can be
independently estimated from FT-IR spectra alone using absorption coefficients of
pure polyimide and CuPc films we obtained above. Based on absorption coefficient
calculation, the FT-IR spectrum in Figure 3.6 c corresponds to 30% (volume fraction)
CuPc embedded in a polyimide host which is in roughly agreement with the
controlled readings from QCMs. However, this analysis neglects interference from
neighboring peaks, and it assumes stoichiometrically balanced BPDA and ODA
deposition rates. The outcome also depends on which absorption bands are selected to
represent pure components. Therefore, throughout the remainder of the discussion,
the composition of hybrid samples will always be referred to according to the molar
flux method.
UV/Vis characterization of pure CuPc, BPDA-ODA polyimide and
polyimide/CuPc composite was performed to understand how optical absorption of
CuPc is affected when it is doped into a host polyimide film. Figure 3.7 a compares
the absorption coefficient α of pure CuPc to that of cured BPDA-ODA polyimide. It
shows that polyimide absorbs strongly in the UV region (< 400 nm), while CuPc
dominates the optical absorption spectrum at wavelengths above 350 nm. The
absorption band of CuPc at 320 nm corresponds to B-band and the doublet absorption
band in the visible region is Q-band. Gouterman’s four-orbital model theoretically
describes the molecular orbitals in the phthalocyanines. It is based on the top two
occupied molecular orbitals (a1u and a2u) and lowest unoccupied orbital (eg) to explain
the allowed transitions in the visible-UV region. In four-orbital model, the Q-band
58
*
and B-band arise from transitions (π to π transition) out of a1u and a2u into the same
eg orbital. The characteristic splitting of Q-band is due to in-phase and out-of-phase
coupling of transition dipoles and is referred to as Davydov splitting.104, 110
a)
a)
-1
 [m ]
20
BPDA-ODA
polyi mide
15
10
CuPc
5
0
200
b)
b)
400
600
 [nm]
800
16
ma x =
696 nm
14
622
12
-1
 [m ]
10
CuPc
616
8
688 nm
6
6 0% CuPc in PI
30% CuPc
in PI
4
2
614
0
500
600
690
700
 [nm]
800
900
Figure 3.7 Optical absorption spectra of: a) pure BPDA-ODA polyimide and CuPc
films; b) Q-band of CuPc and composite polyimide/CuPc films.
59
The Q-band of pure CuPc and two composite films is compared in Figure 3.7
b. The intensity of the higher energy maximum is greater than that of the lower
energy maxima. This is a typical feature of the α phase of CuPc.103 In the pure CuPc
film, the absorption band around 700 nm is broad and tails toward the IR region. In
co-deposited films, there is less absorption in this region. This suggests there are
fewer CuPc-CuPc interactions in the polymer matrix compared to the pure CuPc.
Additionally, the location of the doublet peaks varies between the three films.
Co-deposition causes a slight hypsochromic (blue) shift of these peaks. This indicates
that lower energy photons are more readily absorbed by pure CuPc than by the hybrid
materials.
Effect of thermal annealing on the optical spectra of polyimide film
containing 60% CuPc has also been studied. Although the relative intensity of Q-band
peaks changes slightly between cured and uncured films, there is little variation in the
λmax positions. This indicates curing has little effect on the shape and position of the
Q-band features (data not shown). Other study has shows the peak locations changed
dramatically after annealing for pure CuPc films104 which is contrary to our
observation. The absence of clear peak shifts upon curing indicates that CuPc
particles displays lower mobility when they are embedded into the growing polyimide
matrix. Therefore, the morphology and crystal phase behavior of polyimide/CuPc
composite films will also be less sensitive to heat-treatment than pure CuPc film.
60
3.3.3
Morphology of composite films
The film morphology is correlated with device efficiency therefore is the
focus of this study. Surface and bulk morphology of CuPc, BPDA-ODA polyimide
and polyimide/CuPc composite film was studied using white light interferometry,
atomic formce microscopy (AFM) and transmission electron microscopy (TEM).
To study the bulk morphology, TEM micrographs of films containing 30%
and 60% (volume fraction) CuPc in polyimide, both uncured and cured, are included
in Figure 3.8. The contrast in the images arises from the presence of CuPc which
appears dark in the micrographs. CuPc concentrated regions are clearly visible in all
images. In uncured films (Figure 3.8 a, 3.8 c), CuPc-rich regions are much less
defined, and CuPc appears to be spherical and to be concentrated in amorphous
regions. In both films, concentration fluctuations are observed with a length scale of
less than 100 nm. This length scale is significantly smaller than analogous
CuPc/poly(benzobisimidazobenzophen-anthroline) blends prepared using solution
techniques.111
61
Figure 3.8 TEM images of vapor deposited BPDA-ODA poly(amic acid) and cured
polyimide films containing about 30 and 60 vol % CuPc.
62
After composite films were cured at 200 °C, many of the CuPc molecules
were ordered into rod-like structures 60-250 nm in length. Solid-state molecular
rearrangement from poly (amic acid) to polyimide appears to have enabled partial
crystallization to occur. It is well known that phthalocyanine materials can exist in
several crystalline polymorphs, such as α, β, χ and γ polymorphs.112 The polymorphs
differ based on the tilt angle of molecules within the columns and the mutual
arrangement of the columns. The stable phase is β phase, while the pure CuPc film
deposited at room temperature typically consist of crystallites in α phase whose shape
is similar to rods.113 Transition from α to β phase can be achieved by heating the
α-CuPc films above 250 °C. In vapor-deposited composite films, low mobility of
CuPc particles within the polyimide matrix results in the amorphous phase at room
temperature and partial crystallization at elevated temperature. This observation
indicates polymer matrix is effective to quench crystallization of CuPc particles. In
addition, careful inspection of micrographs shows that most of CuPc crystals are
oriented parallel to the imaging plane, with a small fraction oriented perpendicularly.
The shape of these features is consistent with observations of pure CuPc films,
however, these crystals exhibit a slightly smaller length scale.103,
104
Lacking of
mobility for CuPc particles may attribute to this difference. An alternate explanation
for the smaller crystal size is that crystal growth is constrained by the extremely thin
film geometry (about 35 nm for TEM measurement).
As expected, cured films containing 60% CuPc show significantly more of
rod-like features than films containing 30% CuPc. Dark, amorphous regions that look
63
very similar to the uncured films are still present in the cured images. The presence of
these non-crystalline CuPc domains after curing suggest that much of the CuPc
remains dispersed throughout the polymer. This supports the idea that CuPc has a
lower mobility when supported within a polyimide host.
Surface morphology of BPDA-ODA polyimide, hybrid films, and pure CuPc
was characterized using atomic force microscopy (AFM) and white light
interferometry. The surface of cured BPDA-ODA polyimide is smooth and
featureless, with average root-mean-square (RMS) roughness around 1 nm which is
less than one percent of the film thickness (Figure 3.9 a). Water is produced as a
byproduct during curing, and leaves through diffusion, and this does not appear to
perturb the surface. Composite film containing 30% of CuPc was also studied using
AFM (Figure 3.9 b). Although it appears featureless under white light interferometry,
hemispherical bump-like features with size approximately 150 nm in diameter were
observed in AFM micrographs. Film containing 60% of CuPc has similar morphology
with features about 50~100 nm (Figure 3.9 c). This rough morphology leads to RMS
roughness increase to 22 and 14 nm for composite films containing 30 % and 60 %
CuPc, respectively. The bump length scale appears to increase with film thickness,
which may reflect that surface roughening is nucleated by the subsurface formation of
crystalline CuPc domains. Note that the surface morphology of composite films is
qualitatively different from that of pure, vapor-deposited CuPc (Figure 3.9 d).
64
a)
b)
c)
65
d)
Figure 3.9 Atomic force micrographs of a) BPDA-ODA polyimide, b) 30 % CuPc in
polyimide, c) 60 % CuPc in polyimide, and d) pure CuPc films. Film thicknesses for
a-d) were measured to be 400, 264, 80, and 141 nm, respectively.
Temperature dependent wide-angle X-ray scattering (WAXS) measurement
was conducted to further investigate how polyimide matrix affects the stability and
polymorphic behavior of CuPc (Figure 3.10). It is showed that crystal structure of
vacuum-evaporated CuPc is monoclinic and CuPc macrocycles lie parallel to the
substrate plane114, 115 which is in good agreement with our observations from TEM.
As-deposited poly(amic-acid)/CuPc composites show diffraction with very weak
scattered intensity which indicates CuPc remains in amorphous region in as-deposited
films (data not shown). However, both pure CuPc and polyimide-CuPc composites
exhibit Bragg diffraction peaks around 2θ = 6.8° (1.29 nm spacing) after curing,
corresponding to the α-phase of CuPc.103, 105, 106 For composite films, the intensity of
this diffraction peak is disproportionally weaker which suggests that the co-deposition
66
of polyimide quenches the crystallization of CuPc particles, therefore, there is still a
significant amount of CuPc remains dispersed throughout the polyimide matrix in
non-crystalline form. This explanation is in qualitative agreement with TEM images
(Figure 3.8) that show a lower volume fraction of CuPc crystals than one would
expect based on the materials’ composition.
Upon heat treatment, the scattered intensity of pure CuPc peak begins to
decrease around 300°C. However, the scattered intensity from the CuPc polyimide
composite remains nearly constant and scattering is observed even to 350 °C. In
deposited CuPc films, the α-phase is known to be metastable103, 106, 116, and, above
250 °C, the crystal transforms to the β-phase. Hence, the data presented here indicate
that vapor deposited polyimide can restrain the polymorphic change and stabilizes the
α-phase of CuPc to temperatures above 300 °C.
67
a)
a)
Pure CuPc
Intensity [a.u.]
300 °C
250 °C
200 °C
6
7
b)
b)
2
8
9
Intensity [a.u.]
60% CuPc in BPDA-ODA
350 °C
300 °C
250 °C
200 °C
6
7
2
8
9
Figure 3.10 Temperature dependent X-ray diffraction spectra of vapor-deposited
films: a) pure CuPc, and b) 60 vol % CuPc in BPDA-ODA polyimide.
68
3.4 Summary
In this chapter, vapor deposition polymerization (VDP) was coupled with
phase separation and crystallization to grow novel polyimide/CuPc composite
films. Various combinations of dianhydride/diamine reactant pairs were
evaporated. It was discovered that monomer reactivity is important in solid-state
reaction and BPDA and ODA were chosen as polyimide precursor monomers.
Molar flux of each monomer was then adjusted to achieve the stoichiometric
balance. FT-IR and molar flux analysis were used to independently estimate film
composition, and they roughly agree.
Most importantly, we have demonstrated that condensation VDP can be
used to change morphology of as-deposited films effectively. Third-component,
non-reactive CuPc particles were co-deposited with polyimide precursor
monomers. Resulting films show evidence at the surface and in the bulk of fine
lateral structure with a length scale of about 100 nm from AFM and TEM
measurements, respectively. VDP technique is also effective to quench
crystallization of CuPc particles. Embedded CuPc particles initially appears
amorphous, as evidenced by the less-defined CuPc regions in TEM micrographs,
and undergoes partial crystallization upon thermal treatment from poly (amic acid)
to polyimide at 200 °C. Crystallized CuPc particles inside polyimide matrix
display rod-like morphology which is the characteristic of α-phase CuPc
crystallites. The morphology can be adjusted by changing CuPc volume fraction
and curing temperature. Future studies will examine whether the stable
morphology and high level of interfacial surface area observed in composite films
69
may be useful in developing these or other n-type polyimides as materials for
photovoltaic or OLED devices.
In this study, the length scale of CuPc domain is in the nanometer range. In
the next two chapters, we will introduce an approach to change and control
as-deposited film morphology over a larger lateral length scale. A non-reactive,
immiscible solvent will be used to drive phase separation during initiated
chemical vapor deposition (iCVD) of chain-growth polymers.
70
Chapter 4. Multi-component Vapor Deposition
Polymerization of Poly (Methyl Methacrylate)
in an Axisymmetric Vacuum Reactor
4.1 Introduction
4.1.1
Materials and application
Poly(methyl methacrylate) (PMMA) (Figure 4.1)
is a kind of widely
studied, optically clear, glassy polymer with several thin film applications
including protective coatings117, bonding adhesives117 and low  dielectric
layers118, 119. In its neat form, PMMA exhibits a glass transition temperature (Tg)
around 105°C, well above room temperature, but when it is solvated the glass
transition temperature drops to around room temperature. Generally, it is
synthesized by solution free radical polymerization between methyl methacrylate
monomer and free radical initiator such as t-butyl peroxide, but emulsion
polymerization and bulk polymerization are also routinely used in industry and
their reaction kinetics are well known.
H2 CH3
C C
n
C O
O
CH3
Figure 4.1 Molecular structure of poly (methyl methacrylate) (PMMA).
71
Another important application for PMMA is to fabricate porous
membranes. Torkelson’s work in 1990 demonstrated that asymmetric porous
membranes could be made from PMMA (with weight-average molecular weight
93K Daltons)/porogen system using thermal-induced phase separation (TIPS)
technique.120,
121
TIPS involves rapid quenching of a polymer solution to a
temperature where the solvent is transformed into a non-solvent, then spinodal
decomposition occurs, resulting in glassy, two-phase microstructure. Sulfolane,
t-butanol or xylene have been used as porogen to introduce porosity into PMMA
films. The pore size is in the micron range and can be controlled by adjusting the
weight
percentage
of
porogen,
quenching
temperature
and
time.
Polymerization-induced phase separation (PIPS) is another common strategy to
form two-phase, structured polymers that have been used to create
interpenetrating networks122 and toughened amorphous glasses123.
4.1.2
iCVD reactor design
Geometry of iCVD reactor is important to produce desirable polymer thin
films. Figure 4.2 shows the setup of a traditional pancake-shape initiated chemical
vapor deposition (iCVD) reactor which was custom-designed by Gleason’s group
in late 1990’s.124 It has internal dimensions of 25.4 cm diameter and 3.2 cm
height, used for deposition on 4-inch-diameter silicon substrates. It involved
horizontal flow of monomer and initiator gases across a heated-filament array
which is spaced 1.5 cm apart and positioned horizontally over a cooled target
substrate at a distance of 2.5 cm. Thermal decomposition of initiator molecules
into free radicals occurs near the hot-filament array. Generated free radicals and
72
unreacted monomer molecules adsorb onto the cooled substrate and irreversibly
undergo free radical polymerization. Recent studies indicate that polymerization
inside this iCVD reactor is limited by monomer adsorption and, within the
adsorbed layer, reaction kinetics are analogous to bulk-phase free radical
polymerization.35,
124
The horizontal flow of the initiator vapor eliminates the
“shadowing” fact from filament array during the deposition, and at the same time,
continuously provides free radicals along gas flow direction to ensure the
formation of uniform film. Because of low thermal mass of filaments, high
temperature (> 400 °C) can be easily achieved with very low power input (< 3 W)
which minimizes the heat radiation to the cooled substrate. However, it is
relatively difficult to measure the filament temperature accurately using
thermocouples with comparable thermal mass, hot spot may exist and thermal
decomposition of monomer or initiator species could occur during deposition
process. In addition, our goal is to control the morphology of vapor-deposited
polymer films by introducing non-reactive, immiscible component. However,
multi-component deposition has not been reported so far using this pancake-shape
design. Therefore, new reactor configuration needs to be developed to explore
depositions involving multiple components.
73
Figure 4.2 A schematic drawing of the pancake-shape iCVD reactor setup for
coating on flat substrates. This figure was reproduced from reference 124.
In this study, we employ an alternate reactor configuration to perform
initiated chemical vapor deposition process that avoids filament array altogether
(Figure 4.3). First, gas precursors are passed vertically through a tubular
packed-bed hot-zone which has big thermal mass. The purpose of the hot-zone is
to activate free radical initiator molecules. The hot-zone is isothermal, eliminating
hot-spots that could result in undesirable thermal degradation of initiator or
monomer species. Following the hot-zone, the gas mixture is perpendicularly
directed at a cooled target substrate, and reactants condense and polymerize on the
substrate, analogous to the hot-filament process. Unreacted gases follow
axisymmetric streamlines around substrate and then exit the reactor to the vacuum
pump.
74
Initiator (I) Crosslinker (X)
Monomer (M)
Heated Hot-zone
(~200°C)
Solvent (S)
Controllable pressure
(1-20 Torr)
Back-cooled substrate
(-15°C)
Figure 4.3 Schematic drawing of axisymmetric iCVD setup.
Figure 4.4 is a drawing of the axisymmetric iCVD reactor we
custom-designed and used in this study. The reactor consists of a 27.90 cm
diameter aluminum base-plate that supports a glass cylinder, 27.90 cm in length,
with an aluminum top. Vacuum is achieved using a mechanical pump (BOC
Edwards RV8) through a pump-out port on the reactor, positioned opposite to the
feed port. Reactor pressure is controlled using a downstream throttle valve (MKS
Instruments) together with a Baratron capacitance manometer (MKS Instruments).
Up to four different reagent gases can be supplied from separate bottles on a
manifold on the up-stream side of the reactor. Reagent gases can be individually
heated to ensure that supply-side vapor pressures are high enough to sustain
steady flow. Reagent flowrates are regulated, prior to mixing, using separate
mass-flow controllers or calibrated needle valves. After mixing the reagents
75
outside of the reactor, they are fed into the reactor through a heated transfer line
(Heaters Controller and Sensors, Ontario, Canada). Upon entering the reactor,
reagent gases are fed into a hot-zone consisting of an aluminum cylinder tube (ID
= 11.43 cm).
A band heater, combined with a temperature controller and
read-out (Omega CN 9000A), was used to regulate the hot-zone temperature. The
distance between substrate and hot-zone was kept at 7.62 cm in this study. The
hot-zone can be packed with packing material to improve heat transfer to the
supply gases. A porous aluminum gas diffuser, placed at the end of the hot-zone,
is used to improve the outlet gas-flow uniformity.
The distance between the
hot-zone and the substrate is adjustable. The substrate can be glass slides or
wafers up to 18 cm in diameter. The substrate temperature is controlled by
backside cooling using a recirculating chiller/heater unit (Neslab RTE 740).
The
substrate temperature is also influenced by radiant heat transfer from the hot-zone
and, to a lesser extent, by heat transfer from reagent gases. During operation, the
substrate temperature, measured using a hand-held IR thermometer, was found to
be about 5 °C warmer than the chiller temperature.
The finite volume (FV) computational fluid dynamics (CFD) software
Fluent was used to simulate the transport of gas species in current iCVD reactor
design (Data not shown). The results indicate the gas molecules can be transported
to cooled substrate uniformly. Therefore, this flow configuration was chosen to
minimize mass transfer resistance to the substrate, to further decouple initiation
and polymerization, and to improve film uniformity.
76
a)
b)
Figure 4.4 (a) Top-view of 11-inch aluminum base-plate; (b) schematic of
custom-built, axisymmetric iCVD apparatus.
77
As a model system, glassy films of poly (methyl methacrylate) (PMMA)
are grown using this new reactor configuration. The aim of the present study is to
understand how reactor operating parameters are related to film growth rates and
polymerization
characteristics.
Key
reactor
operating
parameters
were
systematically varied. These include the hot-zone temperature, reactor
base-pressure, substrate temperature and the monomer/initiator molar feed ratio. A
second objective is to demonstrate how this reactor design is suited to explore
depositions
involving
multiple
components.
Multi-component
iCVD
is
demonstrated by co-condensing t-butanol solvent vapor during the growth of
PMMA films. Since t-butanol is non-reactive and has a relatively high vapor
pressure, it can be easily removed following deposition. Here we discuss how the
presence of t-butanol affects the molecular weight of as-deposited PMMA films.
This result is the first step toward developing iCVD as a technique to survey
porous and structured polymer thin films with tunable morphologies.
4.2 Experimental
4.2.1
Materials
Methyl methacrylate (MMA) monomer (Alpha Aesar, 99% purity), t-butyl
peroxide (Sigma Aldrich, 97% purity), and t-butanol (J. T. Baker, 99.99 % purity)
were all used as received, without further purification. 3” Silicon wafers (7.62 cm
diameter) or glass slides were used as the substrates for deposition runs. Wafers
(Silicon International) were used as received. Glass slides were cleaned by
78
soaking in a base-bath and then were vacuum-dried for overnight before each
deposition.
4.2.2
Deposition of PMMA films
During a typical deposition, the system was first pumped down to the
operating pressure (e.g. 7 Torr) while the hot-zone was heated to the required
temperature (200 °C). Next, the substrate was cooled and held at a lower
temperature Tsub (-15 °C). After allowing an hour for the system to approach
thermal equilibrium, monomer and initiator were introduced into the chamber at
set flow rates. For all experiments, the monomer flow rate was fixed at 4 sccm and
monomer/initiator feed molar ratio was adjusted by changing the flow rate of
initiator. Methyl methacrylate (MMA) monomer and t-butyl peroxide (TBPO)
initiator are both volatile and have relatively high vapor pressure (40.02 Torr for
MMA125 and 31.34 Torr for TBPO126 @ 25°C, respectively), therefore, no heating
is needed to vaporize monomer and initiator species. After the deposition was
complete, the chiller and mass flow controllers were turned off.
The reactor
chamber was pumped down for an additional half-hour to ensure complete
removal of unreacted monomers.
79
Table 4.1 Deposition conditions of iCVD experiments. a
Hot-zone
Reactor
Substrate
temperature pressure temperature
Thot [°C]
[Torr]
Tsub [°C]
220
7
-15
Flow rate
of TBPO
initiator
[sccm]
1
Experimental
run sets
base-case
Hot-zone
configurationb
A2
A
A1, A2, A3
220
7
-15
1
B
A2
110, 150,
7
-15
1
0.1, 0.5,
-15
1
-15, -10, -5,
1
180, 200,
220
C
A2
220
1, 3, 5,
7, 10,
15
D
A2
220
7
0, 5, 10, 15
E
A2
220
7
-15
1, 2
a) Flow rate of monomer is kept at 4 sccm, the distance between hot-zone and substrate is 7.62 cm;
b) See Figure 4.5.
80
11.40 cm
A1
1.27 cm
11.40 cm
A2
1.27 cm
11.40 cm
A3
5.08 cm
Figure 4.5 Hot-zone configurations employed in iCVD experiments.
Table 4.1 lists the experimental conditions of 21 different experimental
runs, and Figure 4.5 indicates the different types of reactor hot-zones used.
Hot-zones differ in cylinder height; A2 and A3 were packed with glass beads (6
mm diameter) to promote gas mixing and heat transfer. Other than base-case
conditions, every experiment was run only once and in the order listed in Table
4.1. Each set of experiments was designed to study different process variables:
hot-zone configuration, hot-zone temperature, reactor base-pressure, substrate
temperature and initiator flowrate.
4.2.3
Characterization of iCVD PMMA films
Fourier transform infrared spectroscopy (FT-IR, Brüker IFS/66) was used to
confirm the structure and composition of as-deposited films. Spectra were acquired
81
at 4 cm-1 resolution over the range of 650-4000 cm-1 and averaged over 16 scans.
Thermal Analysis (PerkinElmer, Pyris) was performed on as-deposited films at a
heating rate of 10 °C/min under nitrogen purge. White light interferometry (Zygo
New View 100) was used to observe the film surface morphology and to measure
film thickness and deposition rates. The thickness was averaged from
measurements of three different substrate positions. Film thickness results from
interferometry measurement were confirmed by an optical film measurement
device (Filmmetrics, model: F20). The molecular weight of as-deposited films was
determined by using an Agilent 1100 HPLC system equipped with a Viscotek triple
detector array. For each sample, the film was dissolved from the substrate using
N-Methylpyrrolidone (NMP, Sigma Aldrich). Polymer solutions (1 mg/ml) were
prepared and filtered prior to injecting 100µl of the solution into the GPC system,
containing three columns (two Viscotek Mixed Bed Low MW i-series and one
mixed bed medium MW i-series column) maintained at 60 °C. The software
(OmniSEC v.4.0, Viscotek) calculated the number average molecular weight Mn
and weight-average molecular weight Mw by integration area of GPC traces.
Results were calibrated using a set of narrow poly (methyl methacrylate) standards
of known molecular weight and molecular weight distribution.
4.3 Results and discussion
4.3.1
iCVD of PMMA films using base-case conditions
Several PMMA films were successfully deposited from methyl
methacrylate (MMA) monomer and tert-butyl peroxide (TBPO) as an initiator
82
using the base-case conditions listed in Table 4.1. Typical thicknesses of
as-deposited films were about 3 µm after 8-hour depositions. The resulting film
showed consistent morphology and thickness (thickness deviation of five
as-deposited films is 8%) which illustrates the reproducibility of current setup.
Film thickness measurements on a 3-inch-diameter silicon wafer show good
uniformity with deviation only 2 % over 28 different positions (Figure 4.6).
Resulting films exhibit relatively smooth, featureless surfaces from scanning
electron microscopy (SEM) measurements (Figure 4.7). The RMS roughness, as
determined by white light interferometry, is around 20 nm (data not shown) which
is an order of magnitude larger than the resolution of the instrument. Films are
easily dissolved from substrate using a good solvent for PMMA such as
tetrahydrofuran (THF). From thermal analysis, the as-deposited polymer shows a
glass transition temperature (Tg) of 105 °C and undergoes thermal decomposition
at around 275 °C (data not shown). These thermal characteristics agree well with
PMMA standards.127
83
Figure 4.6 Thickness profile of as-deposited PMMA film (8-hour deposition) on a
3-inch-diameter silicon wafer (unit: nanometer).
Figure 4.7 SEM image of as-deposited PMMA film.
84
3.0
2.5
C=O
stretch
C-H
bend
C-O
stretch
A
2.0
1.5
C-H
stretch
1.0
0.5
0.0
3000
2500
2000
1500
Wavenumbers (cm
1000
-1
)
Figure 4.8 FT-IR spectrum of PMMA deposited using axisymmetric iCVD
reactor.
Figure 4.8 shows an FT-IR spectrum of as-deposited film. The film
exhibits characteristic absorption bands expected for PMMA: C-O stretching
(1240 cm-1), C-H bending (1452 cm-1), C=O stretching (1730 cm-1) and C-H
stretching (2990 cm-1).
The spectrum shows no evidence of alkene groups at
1646 cm-1 indicating that all condensed monomers either reacted to form PMMA
film or evaporated and were removed from the film following deposition.
Using base-case operating conditions, the number-average molecular
weight (Mn) of as-deposited polymer was measured to be 25,000 Daltons. This
molecular weight is comparable to the molecular weight of as-deposited polymer
using the hot-filament reactor designs.27 The polydispersity index (PDI) is around
1.4 which is relatively low compared to conventional radical polymerization in
85
solution.128 The low polydispersity in iCVD process is attributed to bulk free
radical polymerization in the absence of solvent, which limits chain-transfer. In
addition, the mobility of polymer chains decreases with increasing molecular
weight. As a result, short polymer chains grow faster than long polymer chains,
leading to a slightly lower molecular weight distribution.
The growth of glassy PMMA films reported here can be compared to a
recent study by Chan and Gleason resulting in hot-filament iCVD growth of
PMMA. In Chan’s study, polymer films could not be grown using tert-butyl
peroxide (TBPO) as the initiator. It is possible that low molecular weight polymer
was deposited, but no material remained after vacuum drying. However, high
molar mass PMMA films were grown (~ 20 nm/min) using triethylamine (TEA)
as an initiator.29 TEA requires much higher temperatures for dissociation into free
radicals, and, to achieve this, the hot filament in the iCVD reactor was heated to
550 °C. The authors deduced that a higher filament temperature is necessary to
support the propagation kinetics of the monomer. In the current study, high
molecular weight PMMA thin films can be grown using TBPO and a relatively
low hot-zone temperature of 220 °C. This finding suggests that either the hot-zone
is more effective in generating free radicals, or that generated radicals are more
efficiently delivered to the target substrate due to the reactor geometry and the low
substrate temperature.
iCVD of PMMA using the axisymmetric reactor can be broken into three
steps: 1) thermal initiation inside the hot-zone; 2) monomer and free radical mass
transport from the hot-zone to the substrate; and 3) reagent condensation and
polymerization that occurs on the substrate (Figure 4.9). Experiments (sets A-E in
86
Table 4.1) were designed to study each of these steps. The results led to the
development of simple physical models that describe, qualitatively, the physics
occurring in each reactor region.
1)
2)
3)
Figure 4.9 Three steps in iCVD of PMMA process. 1) Thermal initiation; 2) free
radical and monomer transport; 3) condensation and reaction.
4.3.2
Hot zone configuration and temperature
To better understand the efficiency of the hot-zone in activating
free-radical initiator molecules, three different hot-zone designs (Figure 4.5) were
implemented as shown in Figure 4.10. Comparing deposition rates from an
unpacked (A1) to a packed (A2) hot-zone shows that packing with glass beads
significantly increased the deposition rate, by over a factor of two. Although the
residence time is significantly lower for the packed hot-zone A2 (0.25 s)
compared to hollow hot-zone A1 (0.7 s), the glass spheres greatly increase the
87
thermal contact area, promoting free-radical formation. The highest deposition
rate of 570 nm/hr was achieved for configuration A3 involving a packed hot-zone
with an extended length. In comparing A2 to A3, the reactor length was extended
by a factor of four, but the deposition rate increased less than a factor of two. This
suggests that configuration A3 is approaching the equilibrium conversion of
initiator to free radicals at the outlet of the hot-zone.
Deposition rate (nm/hr)
600
500
400
300
200
100
0
A1
A2
A3
Figure 4.10 PMMA deposition rates for different reactor hot-zones in
axisymmetric iCVD reactor operating under base-case conditions.
To further study thermal initiation in the iCVD reactor, PMMA coatings
were deposited at different hot-zone temperatures using reactor configuration A2.
Other process conditions were held constant and are summarized in Table 4.1. As
shown in Figure 4.11, the measured film deposition rate increases about an order
of magnitude with increasing hot-zone temperature. Data can be fitted using an
Arrhenius relationship, and the resulting slope indicates an apparent activation
energy of 97 kJ/mol. This value is significantly less than the literature reported
tert-butyl peroxide activation energy of 153 kJ/mol.129 This difference is attributed
88
to three factors: 1) initiator molecules never achieve thermal equilibrium within
the hot-zone, 2) some recombination of free radicals may occur in the volume
between the hot-zone and the substrate, and 3) additional recombination of
primary radicals may occur on the target substrate.
To summarize, the measured deposition rate is indirectly related to the rate
of initiator dissociation. While homolytic cleavage of the TBPO peroxy bonds
occurs within the hot-zone, only a fraction of the resulting primary free radicals
adsorb on the target substrate and contribute to film growth.
It is not surprising
that many generated free radicals are lost. Compared to solution polymerization,
the initiator-to-monomer molar feed ratio (1:4) in iCVD process is very high, and
consequently, some recombination of gas and surface primary free radicals is
expected. Inasmuch as primary radicals can undergo recombination, they can
also terminate growing polymer chains. These events can influence the
polymerization kinetics in iCVD films.124
89
Thot
220
200
(°C)
180
160
2.2
2.3
140
120
3
Deposition rate (nm/hr)
2
100
9
8
7
6
5
4
2.0
2.1
2.4
3
2.5
2.6
-1
(10 K )
Figure 4.11 Logarithmic plot of PMMA deposition rate versus hot-zone
temperature.
1/Thot
This study introduces the packed hot-zone as an alternative to the
resistively-heated hot-filament free radical source traditionally used in iCVD
processing. Thermal control of the hot-zone is simple and precise. Compared to
the hot-filament, the packed hot-zone has a far greater thermal mass and
eliminates temperature gradients that are suspected around filament edges. The
elimination of hot spots may be an important factor when considering undesirable
high temperature side-reactions. For the experimental conditions considered here,
the kinetics of initiator dissociation within the hot-zone appear to be a key
rate-limiting step for PMMA deposition. Interestingly, this observation is in
contrast to a recent hot-filament iCVD study of poly(alkyl acrylates)35, 124 where
90
the deposition rate was found to depend weakly on hot-filament temperature, with
activation energy of only ~ 12 kJ/mol.
4.3.3
Reactor base pressure
Figure 4.12 shows the effect of reactor base pressure on measured
deposition rates. The data show a maximum deposition rate (325nm/hr) at
pressures about 7 Torr. The maximum can be explained on the basis of the reactor
residence time, which is directly proportional to pressure. At low pressures, gases
quickly pass through the hot-zone, and the extent of primary radical generation is
low. At high pressures, gases pass slowly through the hot-zone, leading to higher
levels of initiation. However, due to lower gas velocities, recombination of free
radicals becomes significant in the volume between the hot-zone and the substrate.
350
Deposition rate (nm/hr)
300
250
200
150
100
50
0
0
5
10
Pressure (Torr)
15
20
Figure 4.12 Experimental results (○) and model fit (dashed line) illustrating the
relationship between deposition rate and reactor base pressure.
91
A crude and simplified plug flow model illustrates these effects and
explains, qualitatively, the rise and fall in deposition rate. This model was used to
generate the least-squares fit shown in Figure 4.12. In this model, flow through the
iCVD system is divided into two isothermal tubular reactors: a hot-zone (HZ) and
a recombination-zone (RZ) reactor (Figure 4.13).
A gas stream containing
initiator molecules (I2) is fed to the hot-zone (HZ) where thermal initiation occurs.
Recombination is only permitted in the recombination-zone (RZ), and the polymer
deposition rate is taken as proportional to the number of generated free radicals
that pass through the RZ without undergoing recombination.
v0, CA0
aX
HZ
LHZ
RZ
LRZ
Cdiss
Figure 4.13 Simplified plug-flow reactor diagram involving one reactor for
thermal initiation and a second reactor for recombination.
In the hot-zone, initiator molecules are thermally activated, leading to the
formation of primary free radicals:
92
I2  2I ·,
[4.1]
where I2 refers to initiator molecules, and I denotes primary free radicals. The
reverse reaction occurs in the recombination zone. Since initiation is a
unimolecular process, the reaction is taken as first order
ri   k i Ci ,
[4.2]
where Ci is the gas concentration of I2, and ki is the initiation rate constant. The
plug flow design equation relating the hot-zone length LHZ to the degree of
initiator conversion Xi is
LHZ  
Ci 0 v0, HZ
ax
XI
dX i
,
ri
0

[4.3]
where Ci0 is the initial inlet initiator concentration, v0,HZ is the total volumetric
flow rate to the hot-zone, and ax is the reactor cross-sectional area.
Since
initiation creates additional gas molecules, the concentration of I2 decreases with
conversion as
1 X i
C i  C i 0 
1 X i

 .

[4.4]
Combining equations [4.2] – [4.4], and integrating, results in
LHZ 
v0, HZ
ki a x
[ X i  2 ln(1  X i )] .
[4.5]
This equation relates the hot-zone reactor length to the chemical conversion. A
similar procedure is repeated for the recombination reaction which is described
using a second-order rate law:
ri  k r Ci2
[4.6]
93
where kr is the recombination rate constant, and Ci is the concentration of free
radical species. For the recombination zone, if the inlet concentration of free
radicals Ci,in is taken as the product, Ci0Xi, this results in
LRZ 
v0, HZ 1  X i   1

 ln(1  X r )  1 / 4 X r  X r


X
4
(
1

)
r 
k r a x Ci 0  2
[4.7]
relating the length of the recombination zone, LRZ , to the fraction of generated
free radicals that undergo recombination, Xr.
Since all experiments were conducted at a fixed molar feed rate, Ci0 can be
calculated using the ideal gas law. Equations [4.5] and [4.7] then relate operating
pressure to the outlet concentration of primary free radicals CI·, out ~ Ci0 Xi (1-Xr).
Again, this quantity is assumed to be proportional to the polymer deposition rate.
The dashed line in Figure 4.12 represents a least squares fit to
experimental data using this simplified plug-flow model. Parameters include
two reaction constants (ki, kr), three reactor dimensions (LHZ, LRZ, and ax) and a
proportionality constant,  that relates the deposition rate to the outlet
concentration of primary free radicals. The reactor dimensions are known (LHZ =
1.26 cm, LRZ = 6.0 cm, ax = 102.6 cm2), the initial initiator concentration is
calculated to be Ci0 = × mol/cm3, and ki is around 10-5 s-1 which was
estimated from the literature130, leaving only two unknowns (kr and ×)
to be fitted. Note that the fitted value of kr (7.03  108 L/mols) is in rough
agreement with measurements of small radical recombination rates.131 The quality
of fit is reasonable and suggests that, at low pressure, deposition rate is
94
determined by thermal initiation, and, at high pressure, recombination cannot be
neglected.
4.3.4 Substrate temperature
The measured polymer deposition rate also depends on the substrate
temperature Tsub. A semi-logarithmic plot of deposition rate versus 1/Tsub is
shown in Figure 4.14.
The data are nearly linear, and a least-squares fit
corresponds to an activation energy of -12 kJ/mol.
On the one hand, a negative
activation energy is expected because lower substrate temperatures lead to higher
levels
of
monomer
adsorption,
therefore
faster
polymerization.
Such
adsorption-limited kinetics were observed in analogous hot-filament reactor
studies35; though, a much higher activation energy was reported (-79.4 kJ/mol).
285
280
Tsub(K)
275
270
265
260
Deposition rate( nm/hr)
4
3
2x10
2
3.5
3.6
1/Tsub
3.7
3
-1
10 (K )
3.8
Figure 4.14 Plot showing deposition rate dependence on substrate temperature.
95
In the present study, diffusive transport of primary free radicals is also
believed to limit deposition. In all experiments inertial forces are on the same
scale as viscous forces.
For example, the dimensionless Reynolds number,
reflecting the ratio of inertial forces to viscous forces, is about 0.7 for the
base-case condition (calculated using the Kinetic Theory of Gases at a
temperature of 400 K). This suggests that diffusive phenomena play important
roles in mass, momentum, and thermal transport and that there are no boundary
layers. Furthermore, the Kinetic Theory of Gases provides scaling expressions for
diffusion coefficients. In particular, the mass diffusion coefficient scales as T3/2.
A lower substrate temperature can therefore reduce the gas phase temperature and
the diffusive flux of free radicals to the substrate. Opposite to this effect, a lower
substrate temperature increases monomer adsorption, promoting polymerization
and film deposition.124 A balance of these two effects determines the observed
deposition rate and explains the reduced sensitivity to substrate temperature
reported here.
4.3.5 Monomer/initiator molar feed ratio
Size exclusion chromatography was used to assess the molecular weight of
several deposited films. Typically films exhibit molecular weights of about 20
kg/mol. The molecular weight of as-deposited PMMA polymer could be adjusted
using two methods. By changing the monomer/initiator feed molar ratio from 2: 1
to 4:1, the number average molecular weight increased from 20 kg/mol to 25
kg/mol (Fig. 4.15 a). A high concentration of free radicals may terminate growing
polymer chains, resulting in the low molecular weight polymer. The effects of
96
introducing a non-reactive, third component into the feed gases is shown in Figure
4.15 b. By introducing the t-butanol into the system, the molecular weight of
as-deposited film increases from 20 to 32 kg/mol. Since iCVD reaction involves
bulk polymerization, as chains grow longer, they lose mobility. The introduction
of t-butanol is believed to improve the mobility of free radical end-groups, acting
as a solvent.
a)
Refractive Index
[M]:[I] = 4:1
Mn : ~25 Kg/mol
[M]:[I] = 2:1
Mn : ~20 Kg/mol
16
18
20
22
Retention Volume (ml)
24
b)
[M]:[I]:[P]=2:1:0
Mn : ~20 Kg/mol
Refractive Index
[M]:[I]:[P]=2:1:10
Mn : ~32 Kg/mol
12
14
16
18
20
22
Retention Volume (ml)
24
Figure 4.15 Size exclusion chromatograms of vapor deposited PMMA: a) effect of
monomer/initiator molar feed ratio, and b) effect of non-reactive third component,
t-butanol.
97
4.3.6 Reactor modifications
In thin film deposition, one of the most important features is the film
thickness. As such, it is very important to know the rate that the material, in this
case PMMA, is being applied to the target substrate. However, the highest
deposition rate we can achieved using our hot-zone design is 560 nm/hr which is
only about half of the deposition rate obtained from hot-filament reactor.29 In our
investigation of substrate temperature, we have demonstrated that diffusive
transport of free radical molecules is one of the limiting steps in our iCVD
process. Reynolds number Re, which reflects the ratio of inertial forces to viscous
forces, is defined as:
Re 
v z D

[4.8]
where ρ is the density of the gas, vz is the gas velocity, D is the diameter and µ is
the dynamic viscosity. High dimensionless Reynolds number indicates greater
inertial forces which allow the free radicals to flow closer to the substrate,
decrease the diffusion length and resistance between the gas feed streamline and
the substrate. Therefore, increasing Reynolds number and promoting the diffusive
transport of free radicals to target substrate will be an efficient way to overcome
the limiting step in iCVD process and achieve high deposition rate.
Carrier gas N2 was used in iCVD process to obtain high deposition rate.
Gas velocity vz as well as Reynolds number can be increased after the introduction
of carrier gas. For example, when 10 sccm N2 was introduced into the base-case
condition, vz increased from 2.1 ×10-3 m/s to 4.3 ×10-3 m/s, Re doubled from 0.7 to
1.56 and corresponding deposition rate increased from 325 to 725 nm/hr. This
98
increase in deposition rate could also be attributed to low concentration of primary
free radicals which decreases the free radical recombination.
A hot-zone nozzle was incorporated into our setup to increase deposition
rate (Figure 4.16). It is found this modification is very efficient to achieve high
deposition rate. Reynolds number increased by more than 20 times with hot-zone
nozzle and deposition rate increased from 325 to 1098 nm/hr using the base-case
condition. However, the deposition rate increased at the expense of film
uniformity. Film thickness measurement showed as-deposited film is only uniform
in the center region. Uniformity deviation on a 3-inch-diameter silicon wafer
increased to 35% over 28 different positions.
a)
b)
Figure 4.16 A hot-zone nozzle was added to improve deposition rate. a) Original
hot-zone design; b) reactor modification with hot-zone nozzle.
Introduction of non-reactive solvent-vapor not only results in high
molecular weight polymer but also leads to high deposition rate. Figure 4.17
shows how polymer deposition rate depends on flowrate of t-butanol
solvent-vapor. Deposition rate increased from 325 to 575 nm/hr after 10 sccm
solvent-vapor was introduced into the base-case condition. Deposition rate starts
to reach a plateau after solvent-vapor flowrate increased even more. Solvent leads
99
to fast polymerization and higher molecular weight polymers. However, in the
deposition with high solvent-vapor flow, the film growth is limited by the
concentration of primary free radicals. The condensation or trapping of small
solvent molecules into as-deposited polymer films may be another reason to
achieve high deposition rate.
Deposition rate (nm/hr)
600
Mn:37 Kg/mol
550
Mn:32 Kg/mol
500
450
400
350
0
10
20
30
40
Porogen flow rate (sccm)
50
Figure 4.17 The dependence of deposition rate on solvent flow rate in base-case
condition.
4.4 Summary
This study demonstrates that poly (methyl methacylate) films can be
deposited at relatively low dissociation temperature using an axisymmetric iCVD
reactor. Mixtures of monomer, methyl methacrylate and initiator, t-butyl peroxide
were fed into a vacuum reactor containing a hot-zone and back-cooled substrate.
100
The resulting as-deposited films have smooth surfaces and exhibit molecular
weights ranging from 20-32 kDa. Several reactor design parameters were studied
to understand film growth limitations. An Arrhenius relationship (with slope 97
kJ/mol) between the deposition rate and the hot-zone temperature indicates that
polymer growth is limited by thermal initiation of free radicals. High deposition
rates require good thermal contact between the hot-zone and feed gases. Upon
increasing the reactor pressure, the rate of film growth rate exhibits a maximum
that is explained using a simple tubular reactor model. At low pressures,
free-radical initiation is inefficient in the hot zone, and at high pressures,
recombination of free radicals above the substrate limits film growth rate. Higher
film growth rates were also observed upon decreasing the substrate temperature.
These data suggest that deposition is limited by two effects: monomer adsorption
on the substrate and free radical diffusion to the substrate. As expected, increasing
the monomer-to-initiator molar feed ratio increased molecular weight. Similar
effects were observed by introducing a third, non-reactive component (t-butanol).
Several efforts have been made to increase the deposition rate.
Introduction of carrier gas increases volumetric flow rate, Reynolds number and
deposition rate effectively. Modification to the hot-zone design, i.e. incorporation
of hot-zone nozzle increases deposition rate at the expense of film uniformity.
Solvent not only leads to longer molecular chain length but also higher deposition
rate. Introduction of solvent-vapor also changes the topography of as-deposited
films. Under certain circumstances, solvent-vapor promotes the dewetting which
will be discussed in detail in the next chapter.
101
Chapter 5. Solvent-Assisted Dewetting During Chemical
Vapor Deposition Process
5.1 Introduction
5.1.1 Dewetting and solvent-induced dewetting
The film stability and its morphology have been studied extensively for its
applications in coatings, adhesives, and dielectric layers and for its fundamental
scientific interests. In particular, the dewetting phenomenon of thin polymer films
has received much attention. Using spin coating or dip coating, smooth polymer
films can be prepared even on nonwettable surface. However, such films are not
stable. When they are heated above glass transition temperature (Tg), they will
dewet the substrate and are transformed into polymer droplets. It is believed that
thick films (greater than microns) may be stable or metastable due to gravity, for
thinner films intermolecular forces such as van der Waals forces dominant system
and lead to an amplification of fluctuations of the film thickness.132, 133
A classical model of thin film rupture was proposed by Vrij in late 60’s.134
It indicates that thin film rupture by thermal fluctuation is analogous to spindonal
decomposition of polymer blends or block copolymer, where height (thickness)
fluctuations in this model correspond to composition fluctuations in the polymer
blends. However, it has to be mentioned that the surface energy rather than the
fluid interfacial energy is the driving force for the film rupture process so this is
only a mathematically analog between these two processes.
102
Dewetting is a consequence of how the total excess free energy (G)
depends on intermolecular interactions and film thickness h. An expression often
used to explain this dependence is:
G 
 Aeff
12h
2
 S P exp(
h
)
l
[5.1]
where Aeff is the effective Hamaker constant, SP is the polar component of the
spreading coefficient, and l is a polar decay length.132, 133 Spreading coefficient,
which indicates the energy difference between bare and wet substrate, is defined
as:
S   SO  (   SL )
[5.2]
where γSO, γSL and γ are the solid/air, solid/liquid, and liquid/air interfacial
tensions, respectively. If  2 G / h 2  0 then surface fluctuations lower the total
excess free energy, and the film spontaneously dewets in accord with spinodal
decomposition.
The dynamics of surface fluctuation amplification that leads to dewetting
has been theoretically worked out by Wyart and Dalliant.135 Figure 5.1 describe
the early stage of dewetting, the amplification of height fluctuation in thin films
on a solid substrate.
103
Figure 5.1 Schematic drawing of a modulated surface for liquid film on solid
substrate.
Accordingly, the local film thickness z depends on both position x and
time t, and surface undulations grow according to

z ( x, t )  h  u exp(iqx) exp( Rt )
[5.3]

where h is the original film thickness, u is the amplitude of undulations, q is the
corresponding wavevector, and R is the fluctuation growth rate.
The fastest
growing wavevector qM and its growth rate RM scale with film thickness h and
liquid viscosity according to:

qM  3 / 2 a / h 2

,
RM  3a 4 / 4h 5
[5.4]
where a 2  Aeff / 6 defines a molecular length scale,  is the surface tension.
Dewetting occurs if a growing fluctuation reaches the substrate surface (z = 0).
Poly(methyl methacrylate) (PMMA) and polystyrene (PS) are the most
common system used to study dewetting phenomenon. Experiment was performed
104
by spin-coating a thin layer of PMMA, usually less than 100 nm, onto a PS
substrate, annealing it above the glass transition temperature (Tg) of PMMA then
observe the morphology change under the microscope.133, 136 Figure 5.2 shows
different stages for a typical dewetting process. It usually starts from randomly
distributed holes (Figure 5.2 a) then these holes grow in size and touch each other
(Figure 5.2 b) to form unstable polygon (Figure 5.2 c). After that, polygon breaks,
the materials of polygon are collected into small uniformly distributed droplets
(Figure 5.2 d). The final dewetting morphology depends on annealing
temperature, time and most importantly, film thickness.
(a)
(b)
(c)
(d)
Figure 5.2 Different stages for a typical dewetting process.The length of the bar is
100 µm. This figure was reproduced from reference 133.
105
Compared with well-studied thermal-induced dewetting, solvent-induced
dewetting has received very little attention. Studies on solutal dewetting have been
focused on the experiments where evaporation plays a major role. For example,
the polymer solution was spin coated then dewetting occurs as the solvent
evaporates.137 Generally speaking, a film can be destabilized by long-range forces,
polar interaction and molecular forces in the film having a structure difference.132
However, the main difference between thermal and solvent-driven dewetting is
that the cause of instability is the long-range force of van der Waals interactions in
the thermal dewetting whereas it is the short-range force of polar interactions in
solvent-driven dewetting. In fact, the long-range force in the solvent-driven
dewetting becomes a stabilizing rather than a destabilizing factor due to the
present of solvent. The presence of these two opposing forces, which enables one
to manipulate these forces, is a distinct advantage over the thermal dewetting in
the investigation of dewetting behavior.
In the solvent-driven dewetting process, the spreading coefficient S could
be changed by an order of magnitude with a simple choice of solvent due to the
polar interaction. This could be regarded as another advantage of solvent-driven
dewetting over the thermal dewetting.
To study solvent-induced dewetting, most of the works reported so far use
spin coating or dip coating for making polymer films.132, 138, 139 In this study, a
different approach, initiated chemical vapor deposition was performed in the
presence of solvent-vapor and to understand how solvent-vapor influences polymer
film surface topography. As mentioned in the last chapter, solvent-vapor promotes
the formation of higher molecular weight polymer, and increases the deposition rate.
106
Depositions performed without solvent-vapor always lead to smooth and
featureless films. Introduction of solvent-vapor (t-butanol) during iCVD results in
non-equilibrium, thin-film structures reminiscent of dewetted polymer films. In the
present study, deposition conditions including solvent-vapor and carrier gas
flowrate are systematically adjusted to relate process conditions to the surface
topography and droplet size. Observed droplet features arise from a thin-film
instability that occurs during early stages of film growth. An autophobic dewetting
mechanism is then proposed to describe the topographic development during
deposition. This study develops iCVD as a technique to fabricate structured
polymer films with tunable surface finish. The ability to control surface
morphology of polymer films, in particular poly(methyl methacrylate) may be
useful in general as a surface modification tool, and specifically within the
semiconductor industry118, 119, for use as biosensors140 and to engineer hydrophobic
surfaces141.
5.2 Experimental
5.2.1 Materials
Methyl methacrylate (MMA) (Alpha Aesar, 99% purity), t-butyl peroxide
(Sigma Aldrich, 97% purity), t-butanol (J. T. Baker, 99.99 % purity) were all used
as-received. Three-inch silicon wafers or glass slides were used as deposition
substrates. Prior to deposition, wafers (Silicon International) were soaked into a
liquid mixture containing 7-parts H2SO4 and 3-parts H2O2 for one hour at 120°C,
followed by a 3-minute air-plasma treatment. Alkyl groups were grafted onto
107
cleaned silicon wafers according to a frequently employed procedure.142 Glass
slides were cleaned by soaking in a base-bath and then were vacuum-dried before
each deposition.
5.2.2 Initiated chemical vapor deposition of PMMA
PMMA films were deposited using an axisymmetric vacuum deposition
chamber that is described in last chapter. During a typical experiment, the system
was first pumped down to a base pressure of 7 Torr, and the hot-zone was heated
to the required temperature at 200°C. Then the substrate was cooled and kept at a
fixed temperature (-15°C). After the system was maintained at thermal
equilibrium for half an hour, monomer, initiator, solvent and nitrogen carrier
gasses were fed at set flowrates. In all experiments, the monomer and initiator
flowrates were fixed at 4 sccm and 2 sccm, respectively. Solvent and carrier gas
flowrates were adjusted between 0 to 20 sccm to study the effects of these two
deposition conditions on film topography. In this paper, S and N will be used to
describe the flowrates of solvent and nitrogen carrier gas, respectively. For
example, S5-N5 indicates the deposition with 5 sccm solvent and 5 sccm N2. After
the deposition was complete, the chiller and mass flow controllers were turned off.
The reactor chamber was pumped down for an additional half-hour to remove
unreacted monomers.
5.2.3 Characterization of iCVD films
White light interferometry (Zygo NewView 100) was used to observe the
film surface topography and to measure film thickness and deposition rates. The
thickness was averaged from measurements of three different substrate positions.
108
Film thickness results from interferometry were confirmed by an optical film
measurement device (Filmmetrics, model: F20). To examine the frequency
dependence of the feature length scale in the dewetted structure, 1-D power
spectral density (PSD) were calculated from the raw interferometry data. This was
done by squaring the Fourier transform coefficients of each of the 640 data rows
(480 pixels long). All resulting 1-D power spectra were then averaged to
determine a representative power spectrum. The averaged PSD is calculated by
squaring the averaged power spectra then divided by total length L.143 It is plotted
as a function of frequency for the 352 × 264 µm patch. For each set of data, power
spectra density calculations were also performed on each of 480 data columns
(640 pixels long) and they were in rough agreement. Contact angle measurements
were conducted using a VCA Optima XE system. The surface tension of cleaned
silicon wafer and silicon wafer grafted with alkyl group were calculated using the
Harmonic methods with water, xylene and formamide. Tapping mode AFM was
performed with a Digital Instruments (DI) Nanoscope IIIa scanning probe
microscope. Topographic (height) and phase contrast images were recorded
simultaneously at ambient conditions. Commercial silicon cantilever probe was
oscillated at its fundamental resonance frequencies, which ranged between 60 and
90 kHz. The level of tapping force used during imaging is related to the set-point
ratio, which is defined as the set-point amplitude to the free-oscillation amplitude.
The force level corresponding to set-point ratios of 0.65 (moderate tapping) was
investigated in this study.
109
5.3 Results and discussion
5.3.1 Dependence of film morphology on deposition conditions
Figure 5.3 shows interference microscope images of polymer films grown
without and with a solvent-vapor feed. As-deposited films fabricated without
solvent-vapor show relatively smooth and featureless surfaces (Figure 5.3 a).
Resulting films have thickness deviation only 3% over 20 measurements on a
3-inch silicon wafer substrate, consistent with our study in the last chapter. The
average root-mean-square (RMS) roughness of the control experiment (without
solvent) is about 20 nm which is less than one percent of the film thickness.
It is well know that PMMA wets SiOx/Si substrates. Deposited PMMA
films are thermodynamically stable due to the interfacial energy difference
between SiOx/Si
144
(surface free energy: 72 × 10−3 J m−2) and PMMA145 (surface
free energy: 32.0 × 10−3 J m−2). However, the introduction of a tert-butanol
solvent-vapor feed during the iCVD process significantly influences polymer film
topography. T-butanol was selected because it is stable at the hot-zone
dissociation temperature (200 °C), and, more importantly, t-butanol solubilizes
methyl methacrylate monomer and oligomers, but is insoluble with the
corresponding polymer (PMMA). This feature has been leveraged to fabricate
asymmetric porous membranes using thermal-induced phase separation (TIPS).120,
121
The resulting morphology of a typical film fabricated using a solvent-vapor
flowrate of 10 sccm and a nitrogen (carrier gas) flowrate of 5 sccm (denoted
S10-N5) is shown in Figure 5.3 b. Polymer droplet-like features are randomly
distributed with average domain size around 20 µm. The spacing between
110
a)
200
-3
100
x10
30
25
20
15
10
5
0
m
0
0
100
200
300
m
b)
0.20
0.15
200
0.10
0.05
100
0.00
m
0
0
100
200
300
m
c)
0.20
0.15
200
0.10
0.05
100
0.00
m
0
0
100
200
300
m
Figure 5.3 Interferometry images of as-deposited PMMA films: (a) deposition
without solvent; (b) deposition with t-butanol solvent (S10-N5); and (c) the same
film, following annealing at 120 °C for 2 hours.
111
individual droplets is the in range of 20 to 40 µm. The height of these features
(z-scale) is about 200 nm which is two orders of magnitude larger than the
resolution of the instrument. Remarkably, there is little or no film observed
between droplets, i.e. droplets appear to sit directly on the SiOx/Si substrate.
The film shown in Figure 5.3 b was thermally annealed above the glass
transition of PMMA film to evaluate the stability of the formed features.
Following annealing at 120 °C for 2 hours, the features became more spherical as
shown in Figure 5.3c. This indicates that during vapor-deposition non-equilibrium
features are formed. During annealing, softening of the polymer phase permits the
surface tension to act on the droplet-like features. Resulting features are similar to
thermal-induced dewetted structures that have been widely studied over the past
few decades.133, 136, 146, 147 However, considering the tendency of PMMA to wet
silicon dioxide, it is unlikely that the annealed structures are in equilibrium. The
spreading of PMMA on substrate is a slow process—on a time-scale much longer
than annealing time.139
Qualitatively, observed topologies in Figure 5.3 b may be explained by
thin film dewetting that occurs during film growth. Thin film dewetting is of great
interest and has been studied experimentally133,
149-153
136, 147, 148
and theoretically135,
. Dewetting results when the intermolecular force field is such that the
disjoining pressure exceeds the Laplace pressure arising from surface undulations.
Dewetting can also occur through other mechanisms including surface
nucleation154,
155
, density variation156, and residual stresses157.
Autophobic
dewetting is dewetting of a fluid on top of a film made of identical molecules.
112
Autophobic dewetting of PMMA films from SiOx / Si can be induced by exposure
to a good solvent132, 139.
As mentioned in the introduction section, thickness fluctuation of the thin
film can be affected by the original film thickness, molecular length scale and
surface tension. However, the growth rate of surface instabilities may also be
suppressed by the deposition / condensation of new material.158 If the film grows
fast enough there may not be enough time for surface instabilities to grow and
reach the surface. From the scaling of qM and RM in Equation 5.4, one expects that
faster deposition rates and higher film viscosities should suppress thin film
dewetting.
In the iCVD process, the deposition rate and the thin film viscosity can be
adjusted by changing carrier gas and the solvent-vapor flowrates. Figure 5.4
shows how the average mass deposition rate depends on solvent-vapor and
nitrogen flowrates. The average mass deposition rate was calculated from
interferometry raw data by averaging the film height of all points and multiplying
the result by the density of PMMA (1.16 g/cm3).159 This enables the average
mass of both wetted and dewetted film to be measured.
113
2
10
-5
Mass Deposition Rate (10 g / cm s)
12
8
: no solvent
: 5 sccm solvent
: 10 sccm solvent
6
4
2
0
0
5
10
15
20
N2 Flow Rate (sccm)
Figure 5.4 Dependence of average mass deposition rate on solvent and carrier gas
flow rate. The introduction of carrier gas independently increases deposition rate.
Solvent-vapor flow increases deposition rate, and lowers viscosity at the surface
of the growing film.
The use of nitrogen carrier gas increased the deposition rate regardless of
the presence of solvent-vapor. The carrier gas increases the volumetric flow rate
and enables inertial forces to transport active free radicals closer to the substrate,
reducing diffusive resistance. A similar effect was mentioned in the last chapter
where the reactor base-pressure was shown to influence the gas residence time and
velocity through the hot-zone. The observed increase in deposition rate may also
be attributed to lower concentration of primary free radicals, due to dilution with
the carrier gas, which decreases the recombination rate.
Solvent-vapor also increases the average mass deposition rate, and this
increase is primarily attributed to reduced mass transport resistance at the
substrate. If the total mass deposition rate is plotted against total (solvent +
114
carrier gas) flowrate, then the data in Figure 5.4 nearly fall onto a single line
suggesting the mechanism of accelerated film growth is the same. However, the
solvent also interacts with the substrate. Assuming the solvent-vapor obeys
Henry’s law, then the partial pressure of solvent-vapor establishes equilibrium
with a surface liquid layer on the substrate. The presence of solvent within the
film lowers the film’s viscosity and increases the polymer chain mobility. While
the amount of solvent within the growing film can not be quantified, the presence
of solvent has been shown to lead to faster polymerization and higher molecular
weight polymers.
If the features in Figure 5.3 originate from thin-film dewetting, then,
according to Equation 5.4, both the deposition rate and surface viscosity should
affect the length-scale of surface fluctuations that lead to dewetting. The carrier
gas flowrate can be used to independently vary the film deposition rate, however
solvent-vapor increases film growth rate while lowering surface viscosity.
Figure 5.5 indicates how film topography depends on solvent-vapor and
nitrogen flowrates. All images are 352  264 m and all z-scale are the same and
abide by the color legend in the upper right. All films grown without
solvent-vapor are relatively smooth with measured surface roughness less than 20
nm. Films grown using solvent-vapor feed and a low rate of carrier gas supply
(<10 sccm N2) show evidence of dewetting. The length scale of resulting features
is greater for films deposited with higher solvent flowrate. However, as the
carrier gas flowrate increases (> 10 sccm) as-deposited films tend to be smooth
again. These overall trends are qualitatively consistent with Equation 5.4 and the
premise that high deposition rates and higher viscosity may suppress dewetting.
115
Figure 5.5 As-deposited film morphology at different solvent-vapor and carrier
gas flowrates.
116
Smaller surface fluctuations which are not clearly visible in Figures 5.3
and 5.5 were observed in some of the stable films (S10-N10 and S5-N10), though
the amplitudes (z-scale) of these features are less than 10% of those in dewetted
films. It is not clear what causes these feature, though they may be due to surface
fluctuations at early times of film growth that were dampened by continued
deposition.
1D Power spectra of vapor deposited films were calculated to quantify the
observed roughness and to determine the spatial periodicity of dewetted
features.160 Resulting power spectral density (PSD) curves are plotted against
the surface frequency f in Figure 5.6. Dewetted films have PSDs that exhibit a
plateau at low frequencies and power-law decay at higher frequencies. The
measured PSD were fit to the k-correlation model:
PSD 
A
(1  B f 2 ) ( C 1) / 2
2
where A, B, and C are function parameters.
[5.5]
Fitted parameter values are
summarized in Table 5.1. The fitted value of A is related to the low frequency
power (the plateau) and the peak-to-valley heights of dewetted structures. The
parameter B (m) is related to the transition from the plateau region to the
downward-sloping linear region. The frequency of this “knee” indicates the
average in-plane correlation length between PMMA features. The parameter C
corresponds to the inverse slope at high frequencies and is related to the growth
mechanism of the thin film.
117
Figure 5.6 shows the deposited films’ power spectral density dependence
on carrier gas and solvent-vapor flows. The smoothest film prepared without any
solvent, S0-N0, has no plateau, and its PSD decays nearly linearly- indicative of
uncorrelated fractal growth. Dewetted films, on the other hand, all exhibit a clear
plateau level (A) greater than 0.1 µm3. For these films, the plateau level generally
decreases with carrier gas and increases with solvent flow. The wetted film,
S5-N20, exhibits a plateau even though interference microscopy revealed a
continuous film. This sample exhibits a correlation length (B) similar to that
obtained from dewetted films S5-N0 and S5-N5. While S5-N20 did not ultimately
dewet, the same surface fluctuations may be present that drive dewetting. S5-N10
appears to have a similar crossover point however PSD fitting was not successful
due to a topographic defect in the image.
118
a)
2
10
S5-N0
1
3
Power Spectral Density (m )
10
0
10
-1
10
-2
S5-N5
10
-3
10
S5-N20
-4
10
-5
10
-6
10
-7
10
S5-N10
-8
10
3 4 5 6
0.01
2
3 4 5 6
0.1
2
3 4 5 6
-1
Spatial Frequency (m )
b)
2
10
S5-N0
1
10
0
3
Power Spectral Density (m )
10
-1
10
-2
10
-3
10
S10-N0
-4
10
-5
10
-6
10
-7
10
S0-N0
-8
10
-9
10
-10
10
3 4 5 6
0.01
2
3 4 5 6
0.1
2
3
4 5 6
-1
Spatial Frequency (m )
Figure 5.6 Power spectra density of films deposited with: a) different carrier gas
(N2) and b) different solvent-vapor flowrates. Dashed lines are least-squares fits to
the k-correlation model.
119
Table 5.1 Parameters from fitting k-correlation model for PSD plot of dewetted
and stable films.
iCVD Film
S5-N0
S5-N5
S10-N0
S5-N20
A (µm3)
22.840
0.405
63.096
7.830×10-5
B (µm)
7.500
7.271
100.995
6.378
C
3.413
1.756
2.500
1.580
Dewetted features on hydrophobic surfaces are also observed when
PMMA is deposited in the presence of t-butanol solvent-vapor. For these
experiments the silicon substrate was modified by treating with the
silane-coupling agent tert-butyl trichlorosilane. The presence of a hydrophobic
surface was verified by measuring the water contact angle. Before treatment, the
water contact angle was measured to be 11.3 ± 5°, and after treatment, it was
measured to be 112.3 ± 5°.
Figure 5.7 shows two process diagrams that indicate whether dewetting or
wetting was observed—one diagram is for hydrophobically-modified substrates
and one is for unmodified SiOx/Si substrates. In constructing the diagrams,
dewetted films were easily differentiated from wetted films. Dewetted films all
contain features with z-scale (height) in the range of 0.1~0.2 µm, which is about a
factor of ten higher than features observed in the smooth, wetted films. In both
diagrams, dewetting is observed at low carrier gas flow (lower deposition rates)
and higher solvent-vapor contents. However, the dewetted region is larger for
films deposited onto the hydrophobic surface. On the hydrophobic surface,
experiments focused on trying to locate boundary between the wetting and
120
a)
Solvent Flow Rate (sccm)
10
5
0
0
5
10
15
N2 Flow Rate (sccm)
20
0
5
10
15
N2 Flow Rate (sccm)
20
b)
Solvent Flow Rate (sccm)
10
8
6
4
2
0
Figure 5.7 Process diagram indicating whether dewetting was observed in
solvent-vapor assisted iCVD growth of glassy PMMA films. Depositions were
performed on a) plasma-treated silicon wafer substrates and b) hydrophobically
modified substrates. The symbols × and ○ indicate dewetted and wetted films
respectively.
121
dewetting regions. The correlation length of dewetted features was slightly larger
for films grown on the modified surface (data not shown).
Water contact angles of vapor deposited films were studied to examine
how the textured surfaces described affect wettability. These experiments were
only performed on coatings deposited onto SiOx/Si substrates. As a control, the
water contact angle on plasma-cleaned SiOx/Si was measured to be 11.6 ± 5°.
Vapor-coated PMMA, without solvent, exhibited a water contact angle of 63.3 ±
5° which is somewhat lower than the accepted value of 68°. The dewetted
coatings S10-N0 and S5-N5 are more hydrophobic and exhibit higher contact
angles of 70.0° and 101.3°, respectively. The water contact angle on S10-N0 is
very close to that on smooth PMMA, suggesting either that the vapor coating
completely covered the wafer surface, or that the texture of the PMMA features
can prevent water from contacting the area between the droplets. If, the latter
were true, one would expect a much higher contact angle due to air-water contacts
on the underside of the water droplet. This may occur for S5-N5—a film with a
finer dewetted structure and a much greater water contact angle. Using Cassie’s
law, the fraction of PMMA surface in contact with water can be deduced. From
the contact angle data for S5-N5, this fraction is 59% which is in rough agreement
with the area of polymer surface observed from interferometry shown in Figure
5.5. These contact angle measurements indicate that simultaneous vapor
deposition and dewetting my offer a simple approach to create more hydrophobic
surfaces.
122
5.3.2 Proposed Dewetting Mechanism
During iCVD the film thickness is continuously growing. The growth rate
is limited by transport of initiator to the surface and the reaction kinetics of
polymerization.33 This situation is fundamentally different from the majority of
polymer dewetting studies. Typically constant-thickness polymer films are
thermally annealed or exposed to solvent-vapors, and the entire dewetting process
is followed with time-dependent imaging. In situ studies are not feasible with the
current iCVD setup. While the proposed mechanism shown in Figure 5.8 offers an
explanation for observed topographies, it also draws from several studies of
thermal and solutal dewetting on uniformly-thick polymer films.
Monomer, initiator and solvent first condense on the cooled substrate to
form a physisorbed liquid monolayer. To a rough approximation, the
concentration of each component is assumed to be proportional to the ratio Pi/Pi,sat
where Pi is the gas-phase pressure and Pi,sat is the component’s saturated vapor
pressure.124 Using pure-component vapor pressures at 0 °C the composition of the
physisorbed layer can be estimated.126,
161
If this approximation is made on
S10-N10, for example, one finds the layer is mostly t-butanol (68%), methyl
methacrylate (22%), and the balance is di-tert-butyl-peroxide. This liquid layer is
less than 2 nm thick and may interact with the substrate to form a stable layer.151
When PMMA chains begin to form within the thin liquid layer, they
adhere to the SiOx / Si substrate.150 A layer of polymer forms and is firmly
anchored to the SiOx/Si substrate due to favorable interactions between PMMA
and surface hydroxyl groups. As polymerization proceeds, the underlying SiOx/Si
substrate becomes saturated with PMMA. More polymer chains form, additional
123
monomer and solvent are deposited, and the liquid layer begins to grow in
thickness. At some thickness, the liquid layer becomes unstable and autophobic
dewetting occurs from the anchored PMMA layer.
Figure 5.8 Proposed dewetting mechanism. Initially a liquid layer (yellow)
adsorbs on the surface consisting of predominately methyl methacrylate and
t-butanol. As PMMA is polymerized, a strongly anchored layer of PMMA (blue)
coats the substrate. The solvent-monomer mixture forms an unstable film on top
of the anchored PMMA. (a) If deposition is slow, or if the viscosity is low enough,
then surface fluctuations reach the substrate and dewetting occurs, leaving glassy
PMMA droplet-like features (green). (b) If deposition is fast enough or the
viscosity is high enough, then the time-scale of dewetting is too slow and a
uniform coating, which may possess small surface undulations, results.
Xue et al. have recently studied solvent-vapor induced dewetting of thin
PMMA films on a silicon wafer with a native oxide layer.139 In their study, upon
swelling with a good solvent, solvated polymer chains gain mobility and become
124
entropically distinct from tightly-anchored PMMA. This difference in entropy
establishes a new interface and drives autophobic dewetting akin to polystyrene
thin films dewetting from polystyrene grafted brushes.162, 163
The presence of a tightly-bounded layer is difficult to unambiguously
prove, however, there is indirect evidence. The contact angle measurement of
water on a dewetted structure (S10-N0) shows a water contact angle of 70° which
is much closer to PMMA (63°) than to the bare silicon surface (12°). If bare
silicon surface were exposed, the contact angle would be between 12 and 63°.
Phase-contrast tapping mode AFM was performed on dewetted PMMA samples
and a silicon wafer dip-coated with PMMA, as a control (Figure 5.9). Figure 5.9 a
shows the AFM phase-contrast image of as-deposited dewetted PMMA on Si/SiOx
substrate. The dewetted polymer droplets show little phase contrast (<5 °) in
comparison with the region between them. The phase contrast is determined by
many factors including ambient conditions, instrument operating parameters and
mostly importantly, the tip-sample interaction. The similarity of phase contrast in
the dewetted film indicates that possibility of tightly-bounded PMMA layer
between glassy PMMA droplets. In order to evaluate the phase angle difference
between PMMA and bare Si/SiOx, Figures 5.9 B and 5.9 b show the height and
phase-contrast images of dip-coated PMMA on bare Si/SiOx substrate. The dark
and light areas of Figure 5.9 B are presumed to be bare wafer (dewetted regions)
and PMMA film. The image shows much higher phase contrast (~50 °) between
these two regions. Attempts were also made to use FT-IR microscopy to confirm
the presence of a tightly-bound layer. Unfortunately, the instrument did not have
sufficient resolution to draw conclusions.
125
Figure 5.9 Topography (left) and phase (right) images for (A, a) dewetted PMMA
film on Si/SiOx substrate; (B, b) dip-coated (and partially dewetted) PMMA on
bare silicon wafer.
Several other dewetting mechanisms may be proposed to explain the
observed dewetting during iCVD. Dust particles or compositional variation of
the SiOx/Si substrate may lead to microscale wettability contrast and
heterogeneous dewetting involving a nucleation growth mechanism.154,
155
Alternatively, density in the thin liquid film may exhibit in-plane spatial variation
resulting in van der Waals forces which promote dewetting.156 These mechanisms,
however, do not explain how the dewetting length scale depends on solvent and
126
nitrogen flowrate.
The rapid formation of residual stress may also induce
dewetting during iCVD.
As the polymer chains grow, their conformational
freedom and free volume change remarkably. Polymers may become trapped in
a frozen, non-equilibrium state with significant residual stress stored within the
films. These stresses can result in topographic changes, even at temperatures
well beneath the glass transition temperature.157 However, the smooth edges of the
dewetted structures observed here, and the dependence of the dewetted
length-scale on deposition rate, are most consistent with the proposed autophobic
dewetting mechanism.
Since the solvent-swollen liquid layer is estimated to contain about 60%
t-butanol, the presence of solvent in the liquid layer may promote dewetting from
the PMMA-anchored layer. The further consider this possibility, contact angle
measurements of t-butanol onto SiOx/Si and PMMA-coated substrates were
measured to be 9.2 ± 5.0 and 9.7 ± 5.0 ° respectively. The total surface spreading
coefficient St containing both dispersive (Sd) and polar (Sp) component is related to
the contact angle by:
S t  S d  S P   LV (cos   1)
[5.6]
where  LV is the liquid-vapor surface tension of t-butanol (19.6 dynes/cm)164.
The effective Hamaker constant can be estimated from:
Aeff  ( Asubstrate  Aliquid )( Aair  Aliquid )
[5.7]
where Asubstrate, Aliquid, Aair are the pure component Hamaker constants for different
phases. Estimates for pure component Hamaker constants165, 166 are: ASiOx = 6.6
10-20 J, APMMA = 7.1 10-20 J, Aair = 0, and the pure Hamaker constant for
127
t-butanol was estimated based on surface tension to be 5.9 10-20 J. The effective
Hamaker constants for t-butanol onto SiOx/Si and PMMA are calculated using
equation 5.7 to be -0.34 10-20 J and -0.57 10-20 J respectively. By definition Sd
has an opposite sign of Aeff, therefore t-butanol’s non-polar interactions promote
spreading. Considering this, juxtapose to the measured contact angles, Equation
5.6 suggests that the total spreading coefficient is negative, and polar component
of the spreading coefficient from t-butanol destabilizes the thin film.
After dewetting, higher molecular weight PMMA forms within the
dewetted droplets. These chains are no longer miscible with t-butanol solvent.121
As the polymer phase rejects the solvent it obtains higher viscosity, and eventually
a glassy state is formed between the anchored PMMA layer and the
solvent-monomer liquid layer. This may explain why solvent-induced dewetting
during iCVD results in irregular-shaped (non-equilibrium) polymer features.
Also after dewetting, a new surface of anchored PMMA chains is exposed
between dewetted droplets. Monomer and solvent deposit on the new surface,
chains begin to grow, and the layer thickness grows until it is unstable and
dewetting again occurs.
This cycle repeats itself over and over—each time
additional polymer material is deposited onto the growing polymer droplets.
Autophobic dewetting qualitatively explains how film topologies vary with
carrier gas and solvent-vapor flowrate.
If the film growth rate exceeds the
surface instability growth rate then dewetting can be suppressed. In other words,
dewetting will not occur if the film growth rate ( h / t ) exceeds the
time-derivative of equation 5.3:
128
h
 Ru exp(i qx) exp( Rt )
t
[5.8]
However R and qM are sensitive to film thickness and scale with h-5 and h-2,
respectively, and therefore one would expect sharp boundaries between wetted
and dewetted regions in Figure 5.7. However the boundaries not strictly defined
by this inequality (Equation 5.8) because higher solvent-vapor flowrates also
reduce the liquid film viscosity and accelerate dewetting.
This proposed explanation of dewetting (Figure 5.8) suggests that the
lateral length-scale of dewetting, defined by the fasted growing surface fluctuation
qM, is independent of time. While time-dependent studies of the dewetting
phenomena have not been extensively carried out, there is reason to believe that
the lateral length-scale should grow with time. At some point during film growth,
the PMMA reaches an insoluble, glassy state. Before this occurs, if the polymer
droplets are swollen with solvent and are mobile enough, they may coalesce to
form larger droplets.
For experiments conducted on the hydrophobically modified surfaces, the
surface does not provide hydroxyl groups that can anchor a thin PMMA layer.
Therefore, dewetting from hydrophobically modified surfaces may not be driven
by short-range polar interactions, but may be due only to long-range apolar
interactions, i.e. Aeff > 0. However long-range interactions are difficult to quantify
since, neglecting a formed polymer layer, the system contains at least four layers:
thin film of monomer and solvent / butyl monolayer / SiOx / Si. Nevertheless, the
experimental data indicate dewetting does occur, and, importantly, the
129
length-scale of dewetting is not significantly different from dewetting on SiOx / Si
surfaces.
“Bump-on-bump” structures shown in Figure 5.10 can also be fabricated
and highlight the versatility of solvent-assisted iCVD dewetting. The surface
texture was produced by first producing a dewetted film on a clean silicon wafer
using conditions (S10-N5) that lead to large periodicity (~30 μm). This surface
was characterized and subsequently processed using a different set of conditions
(S5-N5) that lead to much smaller periodicity (~5 μm). Both length scales are
clearly observed in Figure 5.10. comparing Figure 5.10 to the topography of
S10-N5 in Figure 5.5, there appears to be little, if any, late stage ripening of the
larger-scale periodicity. White pixels in Figure 5.10 are the smaller dewetted
features that formed on top of the coarse features. Interestingly, the smaller
features are more frequently observed in the center of the larger features, or
situated directly on the substrate between the larger features. If an unstable film
forms near a large droplet edge, upon dewetting, the liquid material appears to
transport to the larger droplet. The “bump-on-bump” structures are also consistent
with the idea of autophobic dewetting. During the second deposition the unstable
liquid films dewetted directly from glassy PMMA surfaces. Dewetting from
anchored PMMA also occurs in the areas between larger features.
130
0.25
400
200
0.20
0.15
µm
0.10
200
100
0.05
0.00
00
0
200
100
m
400
200
600
300
Figure 5.10 Topography of a “bump-on-bump” structure measured using
interferometry.
5.4 Summary
This study demonstrates that the iCVD technique can be used to fabricate
polymer films with periodic lateral length-scales. When PMMA films are grown
on SiOx/Si substrates in the presence of a non-reactive solvent-vapor (t-butanol),
droplet-like glassy PMMA features form. Our experiments show that the feature
length-scale can be tuned between 5 and 100 µm by adjusting two process
parameters: the nitrogen carrier gas flowrate and the solvent-vapor flowrate. The
carrier gas affects the overall mass flux at the substrate (deposition rate) and the
solvent-vapor also increases the deposition rate, but, more importantly, lowers the
viscosity of the liquid surface layer. The observation of droplet-like structures is
consistent with an autophobic dewetting of a solvent-monomer mixture, likely
containing lower molecular weight PMMA, from a tightly anchored PMMA layer.
131
Dewetting of as-deposited films from hydrophobically modified SiOx/Si surfaces
was also observed, indicating that structures can be grown on hydrophobic
surfaces as well. In this case, dewetting is attributed to long range dispersion
forces between the film and the butyl/SiOx/Si layered substrate. “Bump-on-bump”
structures were fabricated through two successive depositions and illustrate how
dewetting can be used to create hierarchal surfaces. Dewetting in solvent-assisted,
vapor-deposited polymer films represents a simple approach to yielding textured
surfaces on a variety of substrates, and may facilitate the development of
microporous membranes, super-hydrophobic surfaces, and microfluidic devices.
132
Chapter 6.
Summary and Future Work
6.1 Summary
Fabrication of structured polymer thin films using vapor deposition
polymerization (VDP) was explored. VDP technique is a versatile chemical
process to grow macromolecular species directly from gas-phase feeds. It differs
from traditional solution-based techniques in many desirable ways because the
reactants used are gases. Depending on the polymerization reaction mechanism,
VDP can usually be summarized into four categories. Among them, step-growth
VDP and chain-growth VDP have attracted a great deal of attention and have been
used to deposit polymer films in this thesis.
Precise control of morphology and microstructure during thin film
deposition is important to produce the desired film quality and is the primary
focus of current study. Step-growth VDP, chain-growth VDP and other forms of
VDP typically results in smooth, homogeneous films. However, the introduction
of non-reactive, immiscible species into vapor deposition process has been proved
effective to leverage phase separation and introduce heterogeneity.
Solventless VDP approach was first used to fabricate micro-thick
polybenzoxazole films in Chapter 2. Co-deposition of pyromellitic dianhydride
and 3, 3’-dihydroxybenzidine results in poly (amic acid) precursor, evaporation
temperature for each component has to be carefully chosen to achieve
stiochiometric balance and high quality films. Curing of as-deposited poly (amic
acid) under argon purge leads to the formation of polyimides at about 200 °C and
133
subsequently to partially aromatic polybenzoxazoles at 550 °C. The existence of
unreacted monomer in as-deposited poly (amic acid) films and the formation of
polyimide and PBO were confirmed by FT-IR studies. TGA studies showed the
mass losses due to imidization and decarboxylation, as well as a continuous mass
loss which could be attributed to unreacted monomer. Nanoindentation studies
showed as-deposited and cured VDP films surprisingly exhibit higher or
comparable Young’s modulus and hardness compared with solution-cast films.
After the thermal conversion from poly (amic acid) to PBO, modulus and
hardness can be improved by a factor of three. The successful fabrication of PBO
provides a platform to study simultaneous film growth and phase separation in
vapor-deposited condensation polymers including polyimides.
Non-reactive, immiscible copper phthalocyanine was then used to grow
novel polyimide/CuPc composite films and to change the morphology of
as-deposited films in Chapter 3. CuPc particles were evaporated with biphenyl
tetracarboxylic dianhydride (BPDA) and oxydianiline (ODA) polyimide precursor
monomers. FT-IR and UV/Vis of resulting films showed CuPc particles were
embedded in the polyimide matrix. Fine lateral structures at the surface and in the
bulk with a length scale of about 100 nm were observed using AFM and TEM,
respectively. Embedded CuPc particles initially appear amorphous and undergo
partial crystallization upon thermal treatment from poly (amic acid) to polyimide
at 200 °C. Crystallized CuPc particles display rod-like morphology which is the
characteristic of α-phase CuPc crystallites. The morphology can be adjusted by
changing CuPc volume fraction and curing temperature. Stable morphology and
high level of interfacial surface area observed in composite films may be useful in
134
developing these n-type polyimides as materials for photovoltaic or OLED
devices.
Different vapor deposition approach, initiated chemical vapor deposition
(iCVD) uses chain-growth mechanism to grow poly (methyl methacrylate) films
in Chapter 4. This provides a platform to investigate simultaneous film growth
and
phase
separation
in
addition
polymer
films.
An
axisymmetric,
multi-component iCVD reactor was designed and used. Mixture of monomer,
methyl methacrylate and initiator, t-butyl peroxide were fed into this reactor
containing a hot-zone and back-cooled substrate. As-deposited films show smooth,
featureless surfaces and have molecular weight ranging from 20-32 kDa. Several
reactor design parameters including hot-zone temperature, reactor base pressure,
substrate temperature and monomer/initiator feed ratio were systematically
verified to understand film growth limitations. The results from these studies
suggest that deposition in this axisymmetric reactor is limited by thermal initiation
of free radicals, monomer adsorption on the substrate and diffusion transport of
primary free radicals to substrate. Deposition rate increases dramatically when
deposition conditions were adjusted to overcome these limitations. The molecular
weight
of
as-deposited
polymer
can
be
increased
by
changing
the
monomer-to-initiator molar feed ratio or introducing a third, non-reactive
component (t-butanol).
Introduction of non-reactive, immiscible t-butanol solvent-vapor into
deposition process also changes the morphology of as-deposited PMMA films.
Under certain circumstances, solvent-vapor promotes dewetting and randomly
distributed polymer droplets were observed. In Chapter 5, we demonstrate that the
135
iCVD technique is capable of changing periodic length-scale by more than 20
times by adjusting two process parameters: the nitrogen carrier gas flowrate and
the solvent-vapor flowrate. The carrier gas increases the overall mass flux towards
the substrate, and solvent-vapor increases the deposition rate as well as lowers the
viscosity of the liquid surface layer. The morphology of polymer droplet is
consistent with an autophobic dewetting of a solvent-monomer mixture from a
tightly anchored PMMA layer. Hierarchal, “bump-on-bump” structure can be
fabricated through two successive depositions. Most importantly, solvent-assisted
dewetting in vapor-deposited polymer films illustrates a simple way to fabricate
textured surface on a variety of substrates, and may be used to develop
microporous membranes, super-hydrophobic surfaces, and microfluidic devices.
6.2 Future work
As mentioned in Chapter 4, our hot-zone iCVD reactor has several
advantages over traditional hot-filament design, but it is far from perfect. Not all
the fittings and tubing are heated, therefore, liquid condensation could occur
between monomer, initiator sources and reactor chamber due to the temperature
difference. This will decrease the deposition efficiency dramatically. Packed
hot-zone has bigger thermal mass and its temperature can be accurately controlled
which eliminates the hot spots. However, big thermal mass leads to high thermal
radiation which increases the substrate temperature undesirably. High substrate
temperature can result in desorption of monomer and free radical species, low
sticking coefficient and low deposition rate. In addition, vacuum grease was used
136
in the deposition process. This improves the thermal contact between silicon wafer
substrate and back-cooled stage, but may contaminate the vapor-deposited PMMA
films. Several modifications are necessary to solve these problems. Heated
aluminum block has been custom-designed to provide uniform heating not only to
the monomer and initiator sources, but also to the fittings and tubings between
sources and reactor chamber. Instead of messy vacuum grease, small metal clip
will be used to secure silicon wafer substrate and improve the thermal contact. We
are also trying to build vapor deposition system without vacuum. Carrier gas
containing monomer and initiator flows through a heated nozzle where initiator is
thermally dissociated. The nozzle is positioned closely and pointed directly to the
substrate. Then the free-radical polymerization occurs on the cooled substrate.
This design will simplify the reactor, minimize the thermal radiation and increase
the deposition rate.
The ultimate goal of our study is to fabricate the porous polymer films
combining initiated chemical vapor deposition and polymerization-induced phase
separation techniques. However, based on our study, it seems vapor-deposited
PMMA is not an appropriate candidate to achieve this goal. Pores are reported to
be in the micron range if polymerization-induced phase separation is used to make
porous materials. This length scale is comparable to the film thickness we
deposited in our study. However, high deposition rate is difficult to achieve in
iCVD of PMMA films. Highest deposition rates using hot-zone reactor and
hot-filament reactor are both around 1 µm/hr. This relatively low deposition rate is
due to the high vapor pressure of MMA monomer and molecules tend to desorb
from the substrate. Low deposition rate could also be attributed to the rate of
137
propagation of radical polymerization of MMA which does not favor the
production of long PMMA chains.29 Low-vapor-pressure methacrylates, such as
glycidyl methacrylate (GMA) or 2-hydroxyethyl methycrylate (HEMA) might be
good candidates for making porous films using iCVD technique. Careful
consideration of monomer evaporation temperature, type of initiator and porogen
species, and substrate temperature may enable porous polymer films to be
successfully deposited.
The introduction of a forth component, a difunctional monomer as
crosslinker, is our next step to help arrest morphologies. The presence of
crosslinker minimized the gel particle formation where PIPS is present.167
Likewise, the use of crosslinker in the vapor processing of thin film is expected to
facilitate the formation of various morphologies during the coarsening process.
Chan and Gleason have recently demonstrated that crosslinked thin films of poly
(2-hydroxyethyl methacrylate) could be formed using iCVD technique.28 The
resulting hydrogels were prepared to different crosslink densities by adjusting the
flow rate of crosslinker and they exhibited swelling behavior that reflects the
cross-link density. In a similar fashion, crosslinkers will be utilized to arrest
morphologies more rapidly, resulting in structures with smaller, more uniform,
periodic length scale.
138
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