MICROSCOPY AND SPECTROSCOPY OF
WATER UPTAKEIN POLYMER PHOTORESISTSbyLAURIE A. McDONOUGH
A.B., Bowdoin College, 1998

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy
Department of Chemistry and Biochemistry2004

This thesis entitled:
Microscopy and Spectroscopy of Water Uptake in Polymer Photoresists
written by Laurie A. McDonoughHas been approved for the
Department of Chemistry and Biochemistry

Stephen R. Leone

W. Carl Lineberger

Date

The final copy of this thesis has been examined by the signatories, and wefind that both the content and the form meet acceptable presentation standardsof scholarly work in the above mentioned discipline.

ii

McDonough, Laurie A. (Ph.D., Physical Chemistry)
Microscopy and Spectroscopy of Water Uptake in Polymer Photoresists
Thesis Directed by Professor Stephen R. Leone

Water uptake in polymer photoresists is investigated with a combination of
methods. An infrared near-field scanning optical microscope (IR-NSOM) is
incorporated into an environmental chamber for the measurement of samples
composed of alternating lines of poly(t-butylmethacrylate) (PTBMA) and thephotochemically modified poly(methacrylic acid) (PMAA) in both low and highwater vapor environments. The degree of water vapor sorption is measured using theinfrared transmission of 2.85 µm light on a small spatial scale (< 500 nm). The
accompanying topographic swelling of the samples is measured using a shear-force
feedback loop. Distortion of the topographic structure and variation in transmission
contrast indicate that the PMAA zones absorb more water than the PTBMA regionsin the water vapor environment.
Atomic Force Microscopy (AFM) is used to measure the same
PTBMA/PMAA photoresist along with another sample consisting of poly(tbutylcarbonylstyrene) (tBOC) and its modified counterpart, polyhydroxystyrene(PHOST). Distance sweep measurements show that the tip indents further into
PHOST than tBOC regions. The tip-sample interactions do not change when exposed
to humidity, thus allowing the measurement of differential swelling. The
PTBMA/PMAA photoresist is found to display more differential swelling than thetBOC/PHOST photoresist.

iii

Fourier-Transform Infrared (FTIR) absorption spectroscopy is implemented tomeasure the infrared spectrum of water absorbed by the ketal-protectedpoly(hydroxystyrene) (KRS-XE) and tBOC photoresists. The shape and intensity of
the OH stretching band of the water spectrum is monitored in a variety of humidity
conditions in order to obtain information on the hydrogen-bonding interactions
between the water and the polymer chains. The band is deconvoluted into four subbands, which represent four types of water molecule environment. A large portion of
the sorbed water molecules is believed to be strongly bound to the polar sites of thepolymer. The ratios of each type of water are found to be dependent on the humidty
conditions to which the sample was exposed. These findings are used to explain thehumidity dependence of the deprotection reaction rates, since certain types of water
may slow transport of reactive species within the polymer network.

iv

ACKNOWLEDGEMENTS

The completion of this thesis would not have been possible without thecontributions of many people. I would like to thank my advisor, Steve Leone, for his
moral, financial and scientific support during the course of my graduate school career.
He has also contributed by creating a culturally and scientifically diverse group ofpeople with whom I have had the privilege of working. Each member of the Leonegroup has impacted my life in some way. I can’t list them all, but I must mention afew. Tim Barckholtz and Bogdan Dragnea were early mentors who got me started in
the lab. Bogdan and Jan Preusser helped tremendously with the conception andrealization of the IR-NSOM. I also have to thank the members of the Jila machineand electronic shops for their help in building the apparatus. Postdocs John Husband,
Chuck Blackledge and Zee Hwan Kim advised me through some rough patches.
Romain Planques helped with the AFM measurements. Juergen Plenge has been asource of wisdom during the writing of this thesis. Fellow graduate students Jodi
Szarko (my graduate school twin) and Josh Ballard helped me get through all thehighs and lows of graduate school with their support, advice and friendship.
My family has also contributed more support than I ever could have asked for.
They have both believed in me and been my inspiration. I wouldn’t have even madeit to the start of graduate school without my parents, Bill and Sandy McDonough, andtheir love and care kept me going to the end. My grandparents, Renee and Harold
Bonnyman, made sure to teach me that I can do anything I want to in my life. My
Auntie Linda Marshall, and my brother Tim and sister Erin have been there for mevand can always make me laugh. I also have amazing friends back on the east coast
who have kept in touch, visited me, and cheered me up on many occasions- especiallyCyndy Falwell and Caitlin O’Connor. My friends and teammates in Boulder andBerkeley have all helped make graduate school an enjoyable experience. Thanks toeveryone, I am very lucky to have all of you in my life.

vi

CONTENTS

I. Introduction
1.1

Introduction

1

1.2

Polymer Properties

1

1.3

Water in Polymers

2

1.4

Water Diffusion in Polymers

4

1.5

General Applications of Water Sorption Studies

7

1.6

Chemically Amplified Photoresists

10

1.7

Water in Chemically Amplified Photoresists

14

1.8

Experimental Methods

16

References for Chapter I

18

II. Experimental Design of an Infrared Near-field Scanning Optical Microscope(IR-NSOM) in an environmental chamber
2.1

Introduction

23

2.2

Bell Jar Apparatus

25

2.3

Water Introduction

27

2.4

Quartz Crystal Microbalance Uptake Measurements

27

2.5

Infrared Laser

32

2.6

Infrared Fiber Tips

34

2.7

Shear-force Microscopy

41

vii

2.8

Visible NSOM

53

2.9

Conclusions

56

References for Chapter II

58

III. Water Vapor Uptake in Photolithographic Polymers Observed by Infrared
Near-field Scanning Optical Microscopy in a Controlled Environment
3.1

Introduction

60

3.2

Experimental Materials

63

3.3

Experimental Apparatus

64

3.4

Experimental Procedures

66

3.5

Two Micron Features

67

3.6

Four Micron Features

76

3.7

Conclusions

79

References for Chapter III

80

IV. Atomic Force Microscopy Measurements of Humidity Effects on Polymer
Films
4.1

Introduction

82

4.2

Materials

83

4.3

Instrument

84

4.4

Distance Sweep Measurements

84

viii

4.5

Differential Swelling Measurements

89

4.6

Humidity Effects on Distance Sweep Curves

95

4.7

Conclusions

99

References for Chapter IV

100

V. Fourier Transform Infrared Spectroscopy Studies of Water-Polymer
Interactions in Chemically Amplified Photoresists
5.1

Introduction

101

5.2

Materials

104

5.3

FTIR Instrument

104

5.4

Results and Discussion – tBOC system

106

5.5

Results and Discussion – KRS-XE

112

5.6

Comparison of Photoresists

114

5.7

Conclusions

117

References for Chapter V

119

Conclusions

121

References for Conclusions

123

Bibliography

124

ix

List of Tables

Table 3.1

Height and transmission data in various experimental conditions.

Table 4.1

Differential Swelling in patterned, TBOC photoresists.

xList of Figures

Figure 1.1

Different classes of diffusion. a) Fickian b) Two-stage c) Sigmoidald) Case II.

Figure 1.2

Schematic diagram of chemically amplified photoresist processing.

Figure 2.1

Schematic drawing of experimental apparatus.

Figure 2.2

Water vapor reservoir.

Figure 2.3

Saturated vapor pressure at various temperatures for water.

Figure 2.4

Water vapor introduction into bell jar chamber and accompanying
mass uptake by polymer. a) Pressure of water vapor (blue) and massof polymer-absorbed polymer (black) vs. time. b) Enlargement of first250 seconds of the same vapor introduction shown in part a.

Figure 2.5

Lasing cycle for color-center laser.

Figure 2.6

Graph of Laser power vs. Frequency for two different lasers. The FCL
laser has more noise and fluctuation than an infrared diode laser.

Figure 2.7

A schematic diagram of the fiber pulling apparatus.

Figure 2.8

Examples of Scanning Electron Microscope (SEM) images of pulled
infrared fiber tips. a) The first, longer taper. b) The second, shorter
taper. c) The sharp tip end. d) a tip with a longer taper than optimalfor maximum transmittance. e) A tip with a curved “hook.” f) A tip
that has been etched to remove a hook.

Figure 2.9

Schematic diagram of coating apparatus.

xi

Figure 2.10

Schematic diagram of shear-force system.

Figure 2.11

A 12 µm x 12 µm shear-force image of a patterned aluminum on gold
sample. The outline of circle five is superimposed on all the circles todemonstrate the hysteresis of the scanner.

Figure 2.12

Shear force image of patterned polymer photoresist. a) image
measured in ambient conditions. B) image measured in 10-1 Torr.

Figure 2.13

Frequency vs. Amplitude for the tuning fork. The bold trace wasmeasured in 250 mTorr. The thin traces were taken every 5 minutes
after the addition of 18 Torr water vapor. The arrow points in the
direction of passing time.

Figure 2.14

Linescans of polymer photoresist vs. time. a) Linescan measured inbell jar chamber at a pressure of 200 mTorr. B) Linescan measured
with 18 Torr water vapor introduced to the chamber.

Figure 2.15

Characterization experiment with visible NSOM. a) Corresponding
topography and optical transmission linescans. The microscope
scanned in the x direction, the y direction shows time. b) Cartoon ofswelling scenario where the polymer film swells vertically andhorizontally to reveal a feature on the sample. c) Cartoon of swellingscenario where one feature on the film swells in all directions. Both band c are possible explanations for the change in the linescan images
shown in a.

xii

Figure 3.1

Infrared near-field and topographic images (upper panels, 12.5 µm
x12.5 µm) of deep ultraviolet patterned and post-exposure bakedpoly(t-butylmethacrylate) (PTBMA), and corresponding spectralchanges (lower panel) as acquired by an FTIR on non-patterned
exposed and unexposed samples. The chemical specificity of the setup is demonstrated by the difference in contrast when using (a) 2.85
µm wavelength or (b) 2.94 µm. Topographic changes visible in theshear-force image (c) occur due to shrinkage of the exposed areas of
the film.

Figure 3.2

FTIR spectra of unexposed polymer (black) exposed polymer (green),water absorbed by the exposed polymer (red); and a line denoting thewavenumber to which the FCL laser is tuned for the IR-NSOM
experiment.

Figure 3.3

Topographic images of photolithographic polymers with 2 µm widelines. Each image represents a single line scanned 256 times. (a)Topographic linescan of sample in vacuum. (b) Topographic linescanof sample in 16 Torr water vapor environment.

Figure 3.4a) Average line profiles. The solid line is an average profile of Figure
3.3a and the dashed line is an average profile of Figure 3.3b. The redline portions represent PTBMA and the black lines represent PMAA.
b) A schematic drawing of the tip following the sample surface. The
black line is a profile of the sample’s topography, and the red linexiii

shows how the tip follows that surface to produce the topography
image.
Figure 3.5

Transmission images of 2.85 µm light through photolithographic
polymers with 2 µm wide lines. Each image represents a single line
scanned 256 times. a) Transmission linescan recorded in vacuum
conditions. Darker regions represent smaller transmission intensity.
b) Transmission linescan recorded in 16 Torr water vapor
environment. c) Average line profiles of images a and b. The solidline represents the average of the image in a, the dashed line represents
the average of b.

Figure 3.6

Transmission of 2.85 µm laser light through a polymer photoresist
sample. The laser is emitting from a large aperture fiber tip and is far
from the sample, therefore a large spot size of light hits the sample. A
valve is opened slightly to introduce water at a time indicated by the
number 1. The partial pressure of water rises slowly and is indicatedon the graph until it reaches a pressure of 16 Torr and remains
constant.

Figure 3.7

Corresponding topography and transmission linescans in 16 Torr water
vapor environment. The topography line (black) is the same as thedotted line shown in Figure 3.4a. The transmission line (red) is thesame as the dotted line in Figure 3.5c.

Figure 3.8

Average linescans in transmission and topography for 4 micronfeatures of a polymer photoresist in 18 Torr water vapor.
xivFigure 4.1

Distance sweep measurements. a) Schematic drawing of a distancesweep experiment. b) Curves from measurements on a silicon wafer(black lines) and the modified photoresist polymer PHOST (red lines).
The solid lines indicate the approach of the tip to the sample, thedashed lines represent the retraction of the tip from the sample.

Figure 4.2

Distance sweeps measured for PHOST (red lines) and TBOC (bluelines). Solid lines are approach curves, dashed lines are retraction
curves. a) Measured with nitrogen purge in an atmosphere of 11%
RH. b) Measured at 80% humidity.

Figure 4.3

Differential swelling measurements of a TBOC/PHOST photoresist.
a) Tapping mode AFM image measured at 23% RH. b) Profile ofheights vs. distance for one line across the photoresist. c) Histogram of
number of data points for each height represented in the image fittedwith Gaussian curves.

Figure 4.4

Height histograms of 15 x 15 µm tapping mode AFM images
measured on PTBMA/PMAA photoresists. a) Histogram for image
measured under nitrogen purge (13% RH). b) Histogram for image
measured in 77% RH humid environment. c) Profiles of one
horizontal line in the image. Solid lines represent PTBMA regions,
dashed lines represent PMAA regions.

Figure 4.5

Three different theoretical amplitude vs. distance curves.

Figure 4.6

Distance sweep curves on TBOC measured at a variety of A0 values.

xv

Figure 5.1

Deprotection reactions of polymer systems studied: a) Poly(tbutoyxylcarbonyloxystyrene) (tBOC) b) Ketal-protected
Poly(hydroxystyrene) (KRS-XE)

Figure 5.2

Schematic diagram of FTIR spectrometer apparatus.

Figure 5.3

Spectra of water absorbed by the modified tBOC system. a) Blacklines show spectra taken in modified polymer at 10, 21, 37, 67 and
79% RH. The arrow points in the direction of increasing humidity. b)
Deconvoluted spectrum of OH stretching band at 67% RH. Four subbands, which represent four different water environments, are shownby dotted lines. The sub-band with a peak at 2953 cm-1 represents
strongly-bound water molecules, 3257 cm-1 is medium-bound, 3513
cm-1 is weakly-bound, and 3623 cm-1 shows free water molecules.

Figure 5.4

Fraction of total OH stretching band vs. relative humidity for the four
sub-bands representing strongly-bound (), medium-bound (),weakly-bound (▲) and free (●) water molecules in the modified tBOC
polymer sample of Figure 5.3.

Figure 5.5

Results of KRS-XE experiments. a) Spectra of absorbed water taken
at 12%, 21%, 41% and 51% relative humidity. Solid lines show
spectra of the unmodified polymer; dotted lines show the modified
polymer. The arrow points in the direction of increasing humidity. b)
Fraction of total OH stretching band vs. relative humidity for the four
sub-bands representing strongly-bound (), medium-bound (),weakly-bound (▲) and free (●) water molecules for the unmodified

xvi

polymer. c) Fraction of total OH stretching band vs. relative humidity
for the modified polymer.
Figure 5.6

FTIR spectra of water sorbed in three different polymers taken at 68%
humidity.

Figure 5.7

Structures of three polymer photoresists.

xvii

Chapter I:
Introduction

1.1 Introduction
The semiconductor industry has a constant drive to make faster and more
powerful devices. Manufacturers strive to keep up with Moore’s law, which predicts
the number of transistors on a chip will double every 18 months. In order to maintainthis pace, the processing of the polymer photoresists that provide the patterns forthese devices must be understood and controlled with great detail. One factor toconsider is the conditions under which the photoresists are processed, because
contamination due to exposure to water in the air can affect the fabrication.1-8
Because of the small size of the features of these patterns, powerful microscopes suchas the infrared near-field scanning optical microscope (IR-NSOM) and atomic force
microscope (AFM) are used to study water uptake and swelling in the polymers.
Additionally, fourier-transform infrared (FTIR) spectroscopy is used to gather
information on the interactions between water molecules and polymer chains in thephotoresists. Investigations will hopefully provide new insights which can lead to
improvements in polymer photoresists that benefit the microelectronics industry.

1.2 Polymer Properties
Polymers are high molecular weight molecules composed of repeating
subunits that link together to form long chains. They can be naturally occurring, suchas proteins and cellulose, or synthetic, such as polystyrene and Teflon. Molecular1

structure and the way a polymer’s chains are arranged in a solid are important factors
in determining their properties. Polymers are ideal materials for many industrial
applications because they have desirable properties such as durability, processability,transparency and electrical and thermal resistance.9-12 As scientific progresscontinues, polymers are introduced into every aspect of life, from medicine to foodpackaging to computers.
When polymers encounter water, the most ubiquitous substance on earth,their properties and functions are affected. Water in liquid or vapor form is absorbed
into the polymer; water molecules fill the voids that are formed between polymer
chains and then induce relaxation, or swelling of the polymer.9,11,13-15 The effect issimilar to what happens when you wet a sponge. Because of the small size of watermolecules, their ease of condensation, and ability to hydrogen bond, water affects
polymers in ways that are unique in comparison to any other substance.14,16
The presence of water in a polymer film has been shown to affect a variety ofphysical properties including tensile strength, hardness, net dielectric constant andglass transition temperature.1,16,17 The swelling of polymer films causes strain andstress, and can induce cracking, degradation and other signs of physical aging.11,18-21
Plasticization, the softening and increase of flexibility of a polymer material, canoccur. In many cases, the sorption of water and its effects are reversible.13,22

1.3 Water In Polymers
The amount of water absorbed by a polymer depends on several physical andchemical factors. Water molecules are attracted to a polymer’s polar groups. The

2

number and position of those groups on a polymer chain can affect water sorption.9,13
Chemical modification by adding particular groups to a polymer can be done to tailor
a material’s sensitivity. 9,11
The available volume of film affects the amount of water that can beabsorbed. The film’s volume can be broken into three elements.23

The occupied

volume is the volume of material without any free space. The interstitial volume isfree volume that stems from the vibrational energy of the polymer bonds, and the hole
free volume arises from voids formed by polymer chain relaxation. The two types offree volume represent pores that are accessible for water sorption and transport. The
pore size and density of pores are major factors in determining water sorption. 18 The
orientation of the polymer chains determines whether a polymer is amorphous or
crystalline and helps determine the pore size and density. Film thickness is anotherfactor related to volume that affects water uptake. Thin films are shown to behave
very differently than bulk materials.13
Polymers not only have strong bonds between their monomers to form achain, but they also have bonds between their chains called cross-links. Hydrogels
are three-dimensional polymer networks that are able to absorb extremely large
amounts of water. However, they remain insoluble because of their chemical andphysical cross-links. The degree of cross-linking determines the hygroscopic
properties of the polymer.14,22 A useful hydrogel is both sensitive and resistant towater. If a polymer is soluble in water, dissolution problems can be overcome bycross-linking or blending with a hydrophobic polymer.93Temperature affects the amount of water that is absorbed.18 In order tounderstand this effect, one must understand the principle of glass transition
temperature, Tg. This is a characteristic temperature above which the polymer chainsare relaxed, mobile and called “rubbery.” As the temperature of a polymer drops
below Tg, it behaves in an increasingly brittle manner, and can be referred to as“glassy.” The glass transition temperature is a property that varies for each individual
polymer. Water diffusion above Tg is normally faster than in films that are below thecutoff. Temperatures can be controlled for optimal water uptake, depending on thepolymer and the application in use.18
In many applications, additives are included in polymer films to produce
desired properties. These additives, such as rheology modifiers, pigments, anddispersion agents can influence water absorption.18,23 This is mainly because of theiraffect on the available free volume.24 Processing conditions such as anneal sequencealso play a role.13 Although it is known that many of these factors affect water
absorption, it is difficult to determine exactly how the system is affected. Therefore,
it is important to study each unique set of conditions in a process to determine theeffects of water uptake.

1.4 Water Diffusion in Polymers
A few definitions are pertinent before discussion of water transport inpolymers begins. Absorption is the uptake of liquids or vapors by a material.
Sorption is usually a synonym for absorption, but may be used in reference to4equilibrium uptake. Desorption is the loss of liquids or vapors by a material.
Permeation is the transport of liquids or vapors through a material.
Diffusion of water through polymers can follow Fickian or non-Fickian
kinetics.18,24 Fickian transport often occurs in rubbery polymers that possess
sufficient chain mobility to allow water penetration.25 It is an ideal case in which
there are no interference of effects such as polymer chain rearrangement. This isbecause the rate of diffusion of the water is much less than that of the relaxation. It is
described by:

dC d dC =  D(C ) ,dt dx 
dx 

(1.1)

where C is the water concentration, t is time, x is the position in the film in the
direction of diffusion, and D is the diffusion coefficient or diffusivity of the water inthe polymer. A curve of the mass uptake, M, versus t1/2 is linear. The absorption
curve eventually smoothly levels off to a saturation level, M∞. Figure 1.1a shows atheoretical Fickian plot of M(t)/ M∞ as a function of t1/2.
Fickian diffusion is a general description of diffusion through a medium.
However, in polymer networks many of the assumptions inherent in the equations
break down. The main one is that a polymer exposed to water is often not at
equilibrium because its polymer chains are relaxing.24 This generally occurs in glassy
polymers.24,25
A case is usually referred to as non-fickian when anomolous plots are
obtained. Two-stage, sigmoidal and Case II sorption are well known behaviors also
shown in Figure 1.1. Examples of each type of diffusion can be found in the

5

1

1

8

M(t)/M

M(t)/M

80

Fickian

a

b

0

t1/2

1

t

1/2

1

8

sigmoidal

M(t)/M

M(t)/M

80

Two-stagecCase II

d

0

1/2

t

t

Figure 1.1: Different classes of diffusion. a) Fickian b) Two-stage c) Sigmoidal d) Case II

6literature.18,24 Two-stage diffusion (Figure 1.1b) is characterized by a fast, Fickian
absorption followed by slow, non-Fickian behavior after relaxations occur in thepolymer matrix. Sigmoidal sorption, shown in Figure 1.1c has an S-shaped curvewith an inflection point. In this case, swelling of the polymer creates a strain, which
is transient and affects the diffusion.24 It is also referred to as stress-dependent
diffusion.
Case II absorption, shown in Figure 1.1d, is an entirely different type of nonFickian kinetics. The uptake of water is proportional to time rather than the squareroot of time, as in other cases. In this case the rate of the polymer chain relaxation ismuch slower than the rate of diffusion. This creates sharp diffusion fronts or “steptype” concentration profiles in the polymer film.17,24

1.5 General Applications of Water Sorption Studies

The study of water sorption in polymers is important for many applications.
For example, polymers are used in the coatings industry. Coatings have two main
purposes: decoration or aesthetics and protective ability. Polymers can be used ascoatings on metal surfaces to protect the metals from the corrosion that occurs whensubjected to ambient humidity.18,23 A hydrophilic coating may be needed to enhanceadhesion or wettability of the metal surface. A hydrophobic coating can be used toprevent permeation of water.
Coatings are also important in the food industry.10,26,27 Water is probably thesingle most important factor in governing microbial spoilage in foods. Some foods
are produced with reduced moisture; these are nonequilibrium systems and can be7very sensitive to changes in moisture content. Therefore polymers that are excellent
barriers for water are needed for packaging. Because the cost of packaging material
represents an average of 17% of the total cost of goods produced, research on water
sorption properties of polymers can be very valuable.
Another area that can gain from knowledge of water uptake in polymers is
sensors. Humidity sensors are useful for detection for purposes from comfort in thehome to quality control and processing in industries such as food, paper andelectronics.9,12,28 An “electronic nose” is an array of sensors used to characterize
complex samples.29 It consists of various polymers, each sensitive to different
chemical vapors. It can perform a complex analysis of chemical vapors, with morereliability than a human nose. Each sensor responds to a particular chemical in a
selective way such as a color change, resistance change, production of heat, or byswelling.11,12,30 There are many industries that require humidity control, such as
electronics, food, agriculture and cigars; therefore improved humidity sensors are in
great need.
Another major area of research related to water and polymers is drug delivery.
A macromolecular drug delivery system includes a drug that is attached to or
encapsulated in a carrier molecule such as a synthetic polymer. It is an improvement
over oral drugs, which must travel through the entire body before reaching theintended site. The ideal material would be biochemically inert and non-toxic, while
protecting the drug until it releases it at the desired site of action.31
In the early stages of drug delivery, goals were to achieve a delivery profilethat would yield a constant drug level over a long period of time.32 With traditional

8

tablets or injections, drug levels rise sharply then tail off over time. The drug is only
present in an effective concentration for a short period of time. In a controlled
delivery system the drug level in the blood remains constant, at a level between theminimum effective dose and the lethal dose.
Today’s drug delivery research is more sophisticated. Scientists look forsystems that are inert, biocompatible, mechanically strong, comfortable for thepatient, safe from accidental release, simple to administer and remove, and responsive
to environmental changes.32 An effective system would be ideal for treating diabetes.
A polymer capsule would deliver insulin upon detection of glucose in the
bloodstream. It would also have the ability to stop the delivery when an acceptablelevel was reached.
Anti-cancer drugs are often problematic because they can cause significant
problems in non-diseased areas of the body, leading to non-desirable side-effects forthe patient. If a drug were incapsulated in a polymer nanoparticle, it could travel
through the blood stream to the tumor, where it would be the correct size to exit the
blood stream and enter the tumor. Once inside, the environment in the tumor would
cause the polymer to break apart and release the drug. 31,32
There are three primary mechanisms by which active agents can be released
from a polymer drug delivery system. The drug molecules can diffuse out of thepolymer pores; the polymer can degrade and fall apart; or the polymer can swell,increase pore size, and then allow diffusion. Because the body is comprised mainlyof water, a swelling or dissolving response to water is an important polymer property
to understand. An additional mechanism is employed by high molecular weight9

polyethylene oxides (PEOs), which swell and form a gel layer upon contact withwater.33 The gel layer erodes slowly, thus modulating the drug delivery rate.
There seems to be an ever-growing number of applications for polymer
systems for which understanding water sorption properties is valuable. In the evergrowing weight loss industry, superporous hydrogels could be swallowed by a patient
and then swollen to occupy stomach space so there is less room for food and theappetite is suppressed.34 Water management is a critical problem and design issue forpolymer electrolyte fuel cells.35 In the cosmetics industry, hairsprays with polymers
as active ingredients must have stiffness that stands up to humidity.36
However, the most important application for this thesis is use of polymers as
photoresists for design and patterning of microelectronics such as integrated circuits.
The rate of progress in microelectronics is controlled by advances in
microlithographic processesing.37 Therefore, there is abundant research on therelevant tools, methods and materials used. Chemically amplified photoresists are
just one class of materials that continues to extend the limits of feature dimensions tosqueeze more elements onto a smaller piece of semiconductor material. Details onthese photoresists and the importance of understanding their reaction to water
exposure will be discussed below.

1.6 Chemically Amplified Photoresists

Chemically Amplified (CA) photoresists were first conceived by Frechet,Willson and Ito.38,39 They consist of a polymer film with an incorporated photoactive
catalyst such as an onium salt photoacid generator (PAG). Figure 1.2 shows a10Figure 1.2: Schematic diagram of chemically amplified photoresist processing.

11

schematic drawing of the processing. The image of light transmitted through a mask
is precisely focused onto a resist-coated wafer. A catalytic amount of acid isgenerated in the regions that are exposed. The acid then participates in a deprotection
reaction, often activated by the introduction of heat in a post-exposure bake. The acidfragments the acid labile pendent groups of the polymer and is regenerated. In certain
photoresist systems, it has been estimated that each acid molecule cleaves 1000
groups.2 The result is a latent image of the mask pattern. In a positive tone resist, theexposed polymer is converted from hydrophobic to hydrophilic, so it is soluble inbasic developers. (If the radiation renders the polymer less soluble it is called
negative tone.) The modified polymer is dissolved, leaving the polymer in the mask
pattern on the substrate. Etching, followed by removal of the remaining polymer,
provides the desired micropatterned substrate.
There are several factors that affect the resolution and quality of the latentimage and therefore the final pattern. These include the mask/illumination system,
diffusion, photoacid parameters, volatile products, and airborne contaminants such as
water.2,37,40-43
The resolution limit of an optical lithography system is set by:

w=

kλ
,
NA

(1.2)

where w is the minimum resolvable feature, k is an empirical constant determinedby the individual resist process used, λ is the exposure wavelength, and NA is thenumerical aperture of the exposure tool.2,37 The wavelengths used for exposure havebeen becoming smaller and smaller over the years. The unfortunate aspect of thismethod of reducing feature size is that with each new wavelength, a new photoresist

12system must be designed. Currently 193 and 157 nm resist materials are at theforefront, and smaller wavelengths, such as 13 nm extreme ultraviolet wavelengths,
are being investigated.2,44
The light transmitted through the mask is focused onto the resist using acomplex system of lenses. Improved resolution may be achieved by increasing theNA of the lens system. However, this results in a decrease in the depth of focus(DOF), represented by:
DOF = k2 λ /(NA)2 ,(1.3)

where k2 is another process-defined variable and the other variables are the same asabove. Because a large DOF is required for best photoresist performance, a balancemust be struck between the optimal NA and DOF for achieving the highest resolution.
The photoacid generator is one factor that contributes to the value of k.
Widely used onium salts have high quantum yields of acid production, goodabsorption properties, and good solubility and thermal stability in polymer films.2,45
The mechanism for photochemical acid generation is complex and currently not fully
understood, although it is known that the structure of the PAG influences thegenerating efficiency. 2,3
Kinetics of acid transport through the film must also be considered. The acidstrength, acid size and acid volatility significantly influence the relationship between
deprotection chemistry and diffusion. For example, larger, bulkier photoacids havebeen employed because they diffuse slower, thus limiting the scope of the diffusion
reaction.2,45 While some diffusion is necessary for catalysis of the photoresist
chemistry, the resolution will suffer if the photoacid travels too far into the unexposed13

regions. Another way to combat this problem is to employ basic additives to quench
the acid if it diffuses out of the desired areas.2 Also, because the acid diffusion rateincreases with heat, post exposure bake temperatures can be tweaked to optimize aciddiffusion.
The deprotection reaction causes the evolution of volatile products, which
increases the amount of free volume in the polymer film. With more available space
to move around, the photoacid becomes more mobile. The rapid loss of volatile
products is then followed by a relaxation of the chains in the polymer matrix, causing
an effective “shrinking” of the film. The extra free volume is eliminated as the film
becomes denser. There is less room for acid transport and the diffusivity decreases.42
Small molecule products are continuously formed and they can undergo secondary
reactions with other components of the film, including the polymer and the acid.46
Both the reactivity and the diffusivity are constantly changing in the exposed areas ofthe polymer. This series of events shows a characteristic complexity that makes
complete understanding of the acid diffusion very challenging. Investigation into this
topic is vigorous and ongoing.47-51

1.7 Water in Chemically Amplified Photoresists

Yet another factor that influences the diffusion of the acid in the photoresist
and affects the quality and resolution of the latent image is contamination by airborne
water. This is an important factor because the humidity conditions present during
processing affect the outcome of the latent image. Many studies have found thatwater influences photoresist processing by decreasing the sensitivity, meaning the14

acid molecules are involved in fewer bond cleavages, and therefore the chemical
amplification is not as great.1-3,5-8,52
For some resist systems, water’s presence in the film can alter the absorbancecharacteristics. Water absorbs much of the 157 nm light, therefore fewer photoacid
molecules are generated when water is present in the film. 1,6
The presence of water causes swelling, which can distort the patterns formedby the photoresist processing. For example, one study found that microbridges
formed between adjacent features of the pattern.53 It was attributed to swelling of thepolymer, which caused the features to grow together.
In addition, water can compete with the polymer for the photogenerated acid.
The water molecules can react with the acid and create a weaker acid, which is not as
effective in the deprotection.7,8,43 Understanding of the interactions between waterand the photoresist is therefore important in order to produce optimal systems.
There is an increasing need for alternatives to photoresists that are currently
processed with organic solvents.54 The use of these solvents is environmentallyunfriendly and can lead to problems and increased costs for fabrication. In response,water-soluble resists are being investigated. Additionally, newly-developed
immersion lithography uses water as an index-matching medium, to fill up the air
space between the last element of the imaging lens and the surface of the photoresists.
Because both of these resist systems use water in their processing, there is a greaterinterest in the affects of water sorption by polymers.

15

1.8 Experimental Methods

Techniques used by other researchers to study water sorption in polymers
include Fourier Transform Infrared (FTIR) Spectroscopy,55-57 Quartz Crystal
Microbalance (QCM) measurements58-60 and ellipsometry.58,61-64 The highly masssensitive QCM is ideal for studying extremely thin films. However, the techniquerequires spinning the polymer films onto quartz crystals. These substrates arenormally rough in comparison to the silicon and sapphire substrates used forphotoresists. Because each step of the processing affects the outcome of the film,
results on quartz crystal substrates may not be relevant. Ellipsometry is very sensitive
to both the thickness and the refractive index of a film. It is an important tool instudying polymer swelling by small solvents.
The above mentioned methods lack the ability to study water sorption withhigh spatial resolution. Because the chemically amplified photoresists are patternedwith sub-micron features, a powerful microscope instrument is helpful in elucidating
the details of the water uptake. One method that will satisfy this requirement is theInfrared Near-field Scanning Microscope (IR-NSOM). This technique has alreadybeen applied to study latent image formation in CA photoresists.48,65,66 It combines
chemical specificity with high spatial resolution. It can be used to map out the water
uptake of the photoresist on a sub-micron scale, and simultaneously measure
topographical changes due to swelling of the polymer. Modifications to the
instrument are needed in order to add water-vapor measurement capabilities. The
new IR-NSOM in an environmental chamber is described in detail in Chapter II. The

16

results of experiments exploring water vapor uptake in CA photoresists using this
instrument are discussed in Chapter III.
Another powerful tool for spatially resolved studies on polymer films is
Atomic Force Micrscopy (AFM). An AFM provides the ability to measure changes
in topography as well interaction forces between a polymer film and a scanning probethat can provide information on vapor uptake. Chapter IV will explore the limitationsof this method.
In order to find out how the sorbed water molecules interact with polymer
chains in the photoresist, we turned not to a more powerful microscope, but instead tospectroscopy. An FTIR, even without spatial resolution, can teach us about how thewater molecules are absorbed by the polymer matrix by looking at the vibrationalfrequencies of the O-H stretching bands of water and how they are affected byhydrogen bonding. The results of this study will be explained in Chapter V.
The combination of these three methods provides much insight on water vapor
uptake in CA photoresists. The data obtained and the discussion presented will help
analyze results from previous work on these polymers and eventually help to optimize
processing methods.

17References for Chapter I(1)Berger, C. M.; Henderson, C. L. Polymer 2003, 44, 2101.

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22

Chapter II:
Experimental Design of an Infrared Near-field Scanning Optical
Microscope (NSOM) in an Environmental Chamber

2.1 Introduction

Classical microscopy has long been a valuable part of scientific research. The
technique is non-destructive and easy to use, produces images that can be interpretedby the human eye, and is applicable to samples in their natural environment.
However, conventional microscopy is limited by the size of the spot that a light beam
can be focused to with normal lens optics. This “diffraction limit” is known to beequal to approximately λ/2, which is around 250-300 nm for visible light. It is
becoming increasingly important to have the ability to image samples with evengreater spatial resolution, especially in the biological and materials sciences.1-4
The near-field scanning optical microscope (NSOM) is a powerful tool thatcombines high spatial resolution with the specificity and convenience of optical
microscopy. It can be used to simultaneously study the shape and size of samples
along with chemical composition, structure and chemical dynamics. The defining
characteristic is that it overcomes the diffraction limit by using a sub-wavelength
aperture to produce a tiny spot of light on a sample. The obtainable resolution is afunction of the probe size and its proximity to the sample.
The accompanying theory was first proposed by Synge in 1928 in a
revolutionary series of papers.5 However, the principle was not realized until 1972,
when Ash and Nichols demonstrated imaging using a sub-wavelength aperture for23microwave radiation.6 This achievement renewed interest in the topic, and in 1984 a
microscope like the one proposed by Synge was demonstrated by Pohl’s laboratory at
IBM Zurich.7,8 Once several technical difficulties were worked through, such as
aperture fabrication and distance regulation, more rapid development of the techniquebegan in the early 1990’s.1,9,10 The NSOM is now a commercially available
instrument.
The NSOM technique coupled with visible light allows for high spatial
resolution measurement of various processes. Fluorescence imaging of biologicalsamples1,11, single molecule detection11-13 and thin polymer film analysis1,4 are someexamples of successful work using this technique.
Coupling an NSOM with infrared light provides a powerful tool to be used inmeasurement of samples such as polymers and living cells. Vibrational spectroscopy
in the “fingerprint region” can be employed as a contrast mechanism. Because of thelonger wavelengths involved, infrared microscopy can benefit even more than thevisible by overcoming the diffraction limit. However, there are additional technicalobstacles that must be hurdled. Finding an optical fiber material for infrared light is
one challenge, procuring a tunable, bright IR source is another. The absorptions in
the IR are typically weaker, and the sensitivity of IR detectors is usually orders ofmagnitude smaller than visible counterparts. Despite these challenges, several groups
have had success in developing the IR-NSOM.2,14-17
The instrument described here is the first example of an IR-NSOM coupledwith an environmental chamber. This allows for measurement of the effects of asample’s environment on its topographic and optical properties. In particular, the24absorption of water vapor by patterned chemically amplified photoresist samples canbe explored. However, following with the history of the development of the NSOM,
there are many technical challenges to be faced. A description of these issues
follows.

2.2 Bell Jar Apparatus

A schematic drawing of the bell jar apparatus developed for the experiments isshown in Figure 2.1. A krypton ion laser pumps a color-center laser to produce
infrared light tunable from 2.6 – 3.2 µm. This light enters the bell-jar through a
calcium fluoride window in the chamber’s baseplate. The infrared light is coupled
into an optical fiber that ends in a tapered, coated tip. The light emerges from a 200
nm aperture, which is kept close to the sample with a shear-force feedback loop. The
transmitted light is measured by a large area PbS detector which sits directly below
the sample and is coupled to a lock-in amplifier. Both the sample and the detector
rest on a piezo tube scanner, which can move in the x, y and z directions to create 3–
dimensional images. The scanning, the feedback loop, and the detected IR signal are
all controlled and recorded by a commercial scanning probe microscopy controller.
Optical transmission and complementary topographic images are simultaneously
measured. The chamber also includes inlets for introduction of gases such as water
vapor, and pumping with a mechanical roughing pump. Detailed descriptions ofmany components of this instrument are given in the following sections.

25

tuning fork
tip
fiber
couplersamplePbS
detectorpiezo tube
scannercourse xyz
scannercolor-center laser
chopper
Figure 2.2: Schematic drawing of experimental apparatus

26+

Kr Laser2.3 Water Introduction

Water is prepared in a glass reservoir as shown in Figure 2.2. Distilled water
is used to fill the 0.5 L bottom bulb of the glass apparatus approximately 1/3 full. A
dewar filled with liquid nitrogen is placed around the bulb so that the water freezes.
Once the water is frozen, the valve is opened to the system and the reservoir ispumped down to a pressure of approximately 2 x 10-1 Torr. The valve is closed, andthe dewar is removed so the ice melts. This cycle is repeated at least three times, sothat air trapped in the ice during rapid freezing is minimized. The end result is 25 Lof water vapor available for experiments to be performed in a 60 L bell jar. When thewater remains at room temperature for use in experiments, the vapor pressure is 18
Torr. The pressure of water vapor obtainable can be adjusted by changing thetemperature of the liquid water. This can be done by placing a bath of cold water or
ice water around the bottom of the glass bulb as is done with the liquid nitrogen
dewar. Figure 2.3 shows the different vapor pressures of water obtainable vs. thetemperature used.28 Using warm water could produce a partial pressure of water
greater than 18 Torr. However, this is undesirable because the bell jar is at room
temperature and an excess of water vapor would cause condensation in theexperimental chamber.

2.4 Quartz Crystal Microbalance Uptake Measurements

A quartz crystal microbalance is used to measure the kinetics of water uptake
in an unpatterned thin film in order to characterize the experimental chamber. Films
were prepared using a solution of Poly(methylmethacrylate) polymer purchased from

27

H20
Vapor

Valve
H20

To
Bell-jar

LiquidNitrogen
Dewar

Figure 2.2: Water Vapor Reservoir.

28

Pressure (Torr)

60

Saturated Vapor Pressure for Water

40

20

0

-10

0

10

20

30

Temperature ( 0C)

Figure 2.3: Saturated vapor pressure at various temperatures for water.

29

40

Aldrich and dissolved in chloroform at a concentration of 10 mg/ml. The solution
was spun onto a quartz crystal substrate. A commercial spin coater was used to spin
the film for 30 seconds at 5,000 rpm. The result is an approximately one micron thickfilm on a quartz crystal. Excess polymer is removed from the edges of the film withan acetone-soaked cotton swab in order to ensure good contact with the microbalance.
The polymer-coated quartz crystal is loaded into the quartz crystal
microbalance (QCM) apparatus, which is installed in the bell jar. The crystal
oscillates at a certain frequency, and when the polymer absorbs water, mass is gained
and the oscillation frequency changes.29 The change in frequency can be converted tochange in mass, and the mass of water absorbed by the polymer can be determinedusing the following equation:∆f = −Cf • ∆m,(2.1)

where ∆f is the observed frequency change, ∆m is the change in mass per unit area,
and Cf is the sensitivity constant for the crystal used.
Figure 2.4 shows what happens during the introduction of water vapor into thechamber. The clock is started and at 15 seconds the valve to the water reservoir is
opened, and vapor is allowed into the chamber. The pressure is recorded once every
second using a MKS Baratron gauge, and the mass gained by the polymer film ismeasured simultaneously by the QCM. Figure 2.4 shows the normalized pressure andmass vs. time curves; 2.4a shows the evolution over one hour, while figure 2.4bshows just the first 250 seconds.
The pressure in the chamber rises very quickly over the first 50 seconds, thencontinues to increase at a slower rate over the entire hour. The first, fast part occurs

30

12

a

10

20

8
6
Mass
Pressure

4

10

2

Pressure (Torr)

Mass (micrograms)

30

0

0
0

500

1000 1500 2000 2500 3000 3500 4000

Time (seconds)

15

b
10

4

2

Mass
Pressure

5

Pressure (Torr)

Mass (micrograms)

6

0

0
0

50

100

150

200

250

Time (seconds)

Figure 2.4: Water vapor introduction into bell jar chamber and accompanying mass uptake bypolymer. a) Pressure of water vapor (blue) and mass of polymer-absorbed water (black) vs.
time. b) Enlargement of first 250 seconds of the same vapor introduction shown in part a.

31

when the vapor available in the water reservoir flows into the bell jar. The second,slower part occurs as more vapor is created by the liquid water at the bottom of thereservoir. The mass taken up by the polymer follows the same pattern. Figure 2.4bshows that in the first 40 seconds, the slope of the pressure and mass uptake curves
are the same. This means that the mass uptake may be limited by available water
vapor, and therefore, kinetic studies in this chamber are probably not possible. A
smaller chamber would fill more quickly, and therefore may give a better indicationof the rate of water uptake by the polymer. This factor should be considered in futurereal-time experiments.

2.5 Infrared Laser

The laser used to produce infrared light is a Burleigh FCL color-center laser
pumped by a Coherent Krypton ion laser. This type of laser is a four-level systemthat operates with a solid-state crystal. The alkali halide crystal is doped with
impurities during growth to create anionic vacancies filled with electrons called Fcenters. Processing of the crystal with white light at temperatures near -20 °C makes
the F-centers capable of producing stimulated emission. As shown in Figure 2.5, theelectrons absorb the light from the pump laser and are transferred to an excited level.
They undergo fast, radiationless transitions, as well as transitions that produce
emission of infrared light. The crystal will not lase at room temperature, however. It
must be constantly cooled with liquid nitrogen because the fluorescence quantum
efficiency decreases with increasing temperatures.

32

4
3
+

Kr
pump
laser

radiationlesstransitions

1
Figure 2.5: Lasing cycle for color-center laser.

33

2

Infraredemission

The result is a continuously tunable, narrow linewidth, high power output inthe 2.6 – 3.2 µm region. The FCL has advantages over other infrared sources because
it has a narrow linewidth. It produces a higher output than diode lasers, and is easier
to use than an OPO.
One drawback of the FCL laser is the higher level of intensity fluctuations,due to the pump laser. The noise in the FCL is compared to that of an infrared diodelaser in Figure 2.6. A fast-fourier transform (FFT) signal analyzer was used to show
the power of each laser at different frequencies. The FCL has a large amount of noise
at a variety of frequencies, while the laser diode shows a relatively small amount ofnoise. This can limit the experiment if the signal intensity is small, because thesignal-to-noise ratio will not be high enough to measure a clear image. Use of a lockin amplifier to measure signal at a specific chopped frequency can eliminate much,
but not all, of the noise.

2.6 Infrared Fiber Tips

The microscopic aperture needed for overcoming the diffraction limit in the
NSOM experiment is provided by the creation of a coated, tapered fiber tip. The
infrared fiber used for these experiments is SG fiber purchased from Infrared Fiber
Systems, Inc. with an optimal transmission range of 0.45 – 5.0 µm. The fiber has a
200 µm diameter core made up of 53% ZrF4, 20% BaF2, 4%LaF3, 3% AlF3 and 20%
NaF. The core is surrounded by a glass clad, and then covered with a polymericbuffer coating. The polymer coating keeps the brittle core from breaking upon
bending.

34

0.010Laser Power (mW)

0.008

Laser DiodeFCL Laser

0.006

0.004

0.002

0.000

-0.002
0

250

500

750

1000

1250

1500

1750

Frequency(Hz)Figure 2.6: Graph of Laser power vs. Frequency for two different infrared lasers. The FCL
laser has more noise and fluctuation than an infrared diode laser.

35

Approximately 5 cm of the coating and cladding is removed from end of thefiber, so that the glass core is completely exposed. The fiber is dipped in a heatedsolvent, such as 1-methyl-2-pyrrolidinone, for one minute; then dipped in methanol
for several minutes and wiped with a tissue to remove any remaining coating. It is
mounted on a commercial micro-pipette puller.
The apparatus used to pull the fibers is shown in figure 2.7. A standard
procedure is followed.18 When the shutter is opened, the fiber is heated by a CO2 laser
which is focused in the plane of the fiber to a diameter of ~ 1 mm, giving a power
density of 10 W/cm2. The micropipette puller uses a set program of controlled force
and velocity to pull the fiber into a tapered tip while providing control over taper
length and tip size. A constant, weak force is applied to the fiber during heating until
a certain velocity is reached, and then a stronger force is applied until the fiber
breaks. This two step-process produces the best shaped tip for maximum
throughput.15,18 The slow pull results in a taper to a diameter slightly larger than the
wavelength of light, and the faster pull allows for a sharp tip.
Examples of Scanning Electron Microscope (SEM) images of aluminum
coated tips produced by this pulling method are shown in Figure 2.8. Part (a) shows
the long taper, part (b) shows the short taper, and part (c) shows the tip. A tip withthis shape should have a transmittance of around 10-4 in comparison to a cleaved end
fiber.15 It is necessary to use the SEM to check for proper taper length and shapebecause the commercial fibers purchased do not always have a consistentcomposition. A slight change in the material makeup can require a different laser
power. If the heat is too high for a particular fiber, the taper produced can be too

36

Lens
Shutter

CO2 Laser
Micropipette puller
Figure 2.7: A schematic diagram of the fiber pulling apparatus.

37

Infrared
Fiber

38

35,000x5,000x140x

25,000x

Figure 2.8 Examples of Scanning Electron Microscopy (SEM) images of pulled infrared fiber tips. a) The first, longer taper. b) The second,
shorter taper. c) The sharp tip end. d) A tip with a longer taper than optimal for maximum transmittance. e) A tip with a curved "hook."f) A tip that has been etched to remove a hook.

140x

200x

long, and transmittance is decreased. An example of this is shown in Figure 2.8d. On
the other hand, if the laser power is too low, and the fiber is not heated enough,it can break in a jagged fashion, without creating a sharp tip.
Another parameter to monitor is the alignment of the focused laser beam onto
the fiber. If the laser is not aligned properly, the fiber can be heated non-uniformly,
and this can cause the tip to “hook” when pulled. An example of this type of tip isshown in Figure 2.8e. However, because some of the fiber purchased has its own
non-uniformity, it is sometimes not possible to eliminate the hook. In this case, it ispossible to remove the hook via a variation on an etching technique described byMichaels, et al. A 0.6 M HCl and 0.04 M ZrOCl2 etchant solution is prepared andplaced in a small vial. Then 2,2,4-trimethylpentane is dropped over the etchantsolution until a distinct layer can be seen. The hooked tip is lowered so that the endof the tip just penetrates the lower level (by the human eye). After 10 – 20 seconds
the tip is removed and rinsed with acetone for 3-5 minutes. An example of a resulting
tip is shown in Figure 2.8f. Although the hook is removed, the taper is a little longer
than ideal, so the preference is to pull tips that do not have a hook.
Once the pulling parameters are optimized, the pulled fibers are coated in an
evaporative bell jar coater, shown in figure 2.9. The fibers are mounted in the bell jar
on chucks which are rotated via an external stepper motor. A chromium rod
and a tungsten boat holding balled up aluminum wire are mounted on electrodes. The
bell jar chamber is pumped down to a pressure on the order of 10-6 Torr. The metals
are resistively heated and evaporate. When the shutter is open, the evaporated metal

39

Bell Jar
Quartz Crystal
Microbalance
Infrared fibers
Rotating chuck
Shutter
Chromium rod
Tungsten Boat
with Aluminum

Current Supply
Motor

Figure 2.9: Schematic diagram of coating apparatus.

40

coats the infrared fibers. Due to the 40° angle at which they are mounted, the fibers
are coated everywhere except at the tip, creating an aperture with a 200 nm diameter.
(See Figure 2.8c) A 30 nm thin layer of chromium covers the fibers first to improve
the adhesion of the aluminum, then a 120 nm layer of aluminum is coated over thechromium. The thicknesses are measured by an upside-down mounted quartz crystal
microbalance inside the bell jar. A thinner coating could cause light to leak throughthe coating and diminish the resolution of the microscope, while a thicker coating
may make the probe too large to image small topographic changes or prevent lightfrom emerging from the aperture. With this aperture, a resolution of 300 nm is
obtainable.2 The “diffraction limit” of 3 µm light is approximately 1.5 µm, so the
NSOM configuration provides a large improvement.
One major difficulty in the pulling and coating process is the brittle nature of
the infrared fiber. If the tip should touch any surface it will break instantly. If the tip
is only slightly broken, it may still be used for imaging, but will provide a poorer
resolution. However, a large break will cause too large an aperture to image any of
the sample features. Also, if the tip is broken before coating, the coating process may
not form a proper aperture. It is impossible to see with the human eye, or even aclassical microscope, if the tip has been broken. The SEM can be used to check forbreakage and reproducibility, however, it is not convenient or practical to check everytip.

2.7 Shear-Force Microscopy

41

The high resolution of an NSOM is derived not only from a small subwavelength aperture, but also from the tip-sample distance regulation that is provided
by shear-force microscopy. Keeping the tip close to the sample allows for a small
spot size of light on the sample. Maintaining a constant gap between tip and sample
eliminates optical artifacts and allows for measurement of the topography of asample. It also reduces accidental tip-sample contact and therefore decreases tip
breakage. The main components of the shear-force feedback system are a probe, atuning fork, a piezoelectric tube scanner, and a feedback system. They are displayed
in a schematic diagram in figure 2.10.
The optical fiber described in section 2.6 is glued along the side of one prongof a quartz crystal tuning fork with a resonant frequency of around 32 kHz. The
tapered, coated fiber tip is aligned so that it is hanging just below the end of the fork.
The fork is glued to a macor fiber holder. A piezoelectric transducer is also mounted
to the macor and is used to excite the mechanical resonance of the fork. A signal issent to the piezo from the driver circuit, which vibrates the fork and causes the tip todither parallel to the sample surface. The prongs of the fork are electrically coupled,which allows for amplitude measurement with a lock-in amplifier.
The sample is placed on a home-built scanner, which is mounted on a basewith a picomotor controller. This allows the sample to be moved in the z direction
with a remote control. When the sample and tip come within near-field range of eachother, the amplitude of the tip dither is damped by shear forces. A feedback loop,
controlled by a scanning probe microscope controller, allows the tip to be locked in at42

piezoelectrictransducer

Macor tip holder
InfraredFiber

To driving
circuitTo feedback
circuittuning fork
Coated tip
Ruby ball

Quarteredpiezo tube

Spring

Base
Southern quadrant
bundleFigure 2.10: Schematic diagram of shear-force system.

43

a constant gap of approximately 10 – 20 nm above the surface. The controller signals
the scanner to retract or extend its legs to maintain the constant gap.
The scanner is made from an aluminum base, to which four ceramicpiezoelectric tube “legs” have been glued. The piezotubes, purchased from Boston
Piezo-Optics Inc. are made of PZT-5H material, and coated with electroless nickel
electrodes. The outside coating of the tubes is divided into four quadrants and theinside of the tube is negatively poled.

Wires are soldered to each electrode and arecollected into bundles of north-facing quadrants, southern quadrants, eastern
quadrants, western quadrants and insides. These wire bundles are connected to theshear-force feedback loop and allow the scanner to scan in the xy-direction andmaintain a constant gap in the z-direction. The scanner head rests on ruby spheres,
which are placed on the tops of the piezo legs, to allow for smooth scanning.
The feedback loop must be set to a certain dithering amplitude in order to start
the scan. First the tip is dithered while the tip is far from the surface of the sample.
Then the sample is moved up towards the tip using a picomotor controller. This has
remote operation capabilities so that it may be used while the system is undervacuum. Once the tip gets into the near-field range, the amplitude is damped. The
amplitude is set to a value of approximately 70 percent of its free oscillation for the
feedback loop. The picomotor contoller may also be used to make adjustments
during a scan when the feedback loop cannot keep up with topographic changes due
to limitations of the piezo tube scanner.

44

A sample with known features is imaged to test the performance of thescanner. The result is shown in Figure 2.11. The aluminum-on-glass sample wascreated with electron beam lithography and contains aligned lines, circles andtriangles. The image taken with the home built scanner shows curved lines,
indicating a hysteresis. The circles look flattened and are not as uniform as theyshould look. The outline of circle number 5, which is the roundest-looking of thefeatures, is superimposed onto all the circles to clearly demonstrate the variety in theappearance of the shapes. This test indicates that topography measured with thisapparatus should not be taken as absolute. A high performance, commercial scannershould be purchased for better analysis of sample topography. However, most of theexperiments performed with this apparatus are done on simple, well known patterns.
For example, chemically- amplified photoresists contain uniform stripes. Anotherpoint to consider is that most commercially available scanners can not be guaranteed
for use in water vapor environments.
The shear forces that damp the tip dithering have been measured but are still
not well understood.1,19-23 Depending on several factors, the forces are comprised ofsome mixture of electrostatic, van der Waals, and capillary forces.24,25 The
environment surrounding the microscope, including the pressure, humidity andtemperature, can affect the behavior of the shear-force feedback system. Experiments
were performed in the bell jar apparatus to quantify and demonstrate theenvironmental influences.
Figure 2.12 shows two shear-force images of a patterned polymer photoresist.
Figure 2.12a was measured in ambient conditions. Arrows point out horizontal lines,45

10.97 µm

05Figure 2.11: A 12 µm x 12 µm shear-force image of a patterned aluminum on gold sample. The
outline of circle five is superimposed on all the circles to demonstrate the hysteresis of thescanner.

46

a)

b)

Figure 2.12: Shear force image of patterned polymer photoresist. a) image measured in ambientconditions. b) image measured in 10-1 Torr.

47

or “streaks,” which are not part of the topography of the sample. These types of lines
are often present in topography-type scanning probe microscope images, including
shear-force and atomic force microscope (AFM) images. Figure 2.12b shows animage of the same sample, taken on roughly the same spot on the sample as Figure
2.12a, measured in the bell jar at a pressure of 10-1 Torr. This image does not show
any of the streak artifacts present in the ambient measurement. This comparison of
two images in different atmospheric conditions shows a common result.
Because the streaks are present in the ambient image but not the vacuum
image, it is possible that they are caused by the thin water layer present on all
surfaces. This water layer strongly affects the interaction between the tip andsample.4,24,26,27 As the tip approaches the sample, not only is the damping perturbed,
but a “liquid neck” can form.26 The neck can be stretched as the tip and sample pullapart, and the neck can also break and re-form. The formation and/or breakage of theliquid neck is one possible cause of the streaks in the image.
Performing the imaging experiments in vacuum can provide an impressive
improvement in the data quality. Not only are streaks eliminated, but higher spatial
resolution can also be obtained.23 The feedback system is also far more stable invacuum when water is not present. Various experiments were performed to try tounderstand the instability that is introduced when water is present in the chamber.
The results of one such experiment are shown in Figure 2.13. The graphshows amplitude vs. frequency for the tuning fork with a tip mounted on the macor.
Amplitude is measured by a FFT signal analyzer. The tip is far away from the sample
surface, therefore, humidity effects are not a result of tip-sample interactions. The

48

0.5

Amplitude (mV)

0.4

0.3

0.2

0.1

31.5

32.0

32.5

33.0

33.5

34.0

Frequency (kHz)

Figure 2.13: Frequency vs. amplitude for the tuning fork. The bold trace was measured in 250
mTorr. The thin traces were taken every 5 minutes after the addition of 18 Torr water vapor.
The arrow points in direction of progressing time.

49

bold trace was measured at a vacuum pressure of 250 mTorr. The peak amplitude isfound at 32.88 kHz, which is considered the resonant frequency of the fork. Water
vapor was introduced into the chamber to a pressure of 18 Torr. New traces, taken
every five minutes, are represented by the thin lines. The arrow points in the
direction of passing time. As the system is exposed to humidity for a longer period oftime, both the resonant frequency and the amplitude at the resonant frequency
decrease.
The exact cause of this is unknown. One hypothesis is that a film of water on
the tuning fork formed by condensation affects the resonance. However, the growthof such a film would be fast at these water vapor pressures, and the effect in question
evolves over 20 minutes or more. Another theory is that the glue holding the tip on
the macor holder absorbs water. Different types of glue were tried, but were either
unsuccessful in remedying the problem or did not dry quickly enough to hold the fork
in place. This change in resonance causes instability in the shear force measurements
and makes it difficult to measure “real-time” topography changes induced by the
introduction of water vapor.
An attempt to make a real time measurement of topography is shown in
Figure 2.14. This experiment shows linescans, which are images created by a single
line scanned back and forth repeatedly. The x axis represents distance, because thetip moves back and forth over the same 10 µm long line. The y axis represents time,so the image shows any changes that occur in the topography over the time it takes torecord the data. The z axis, represented by the color bar to the right of the image,represents height. Figure 2.14a was recorded in the bell jar chamber at a vacuum

50

a)

Time (min)Height (nm)

25

170

20
15
10
5
0
0

b)

2

4

6

8

10

Time (min)0Distance (µm)

Height (nm)

25

900

20
15
10
5
0
0

2

4

6

8

10

Distance (µm)

Figure 2.14: Linescans of polymer photoresist versus time. a) Linescan measured in bell jar
chamber at a pressure of 200 mTorr. b) Linescan measured with 18 Torr water vapor
introduced to chamber.

51

pressure of 200 mTorr. The sample studied is a patterned polymer photoresist. The
darker areas represent a thinner polymer material which is hydrophilic. The lighter
areas are made of a thicker, more hydrophobic material. There were no changes in theatmosphere, as the same pressure was held during the scan, and therefore no changes
in the topography. As a result, the image looks uniform over time.
Figure 2.14b is a linescan measured in the belljar chamber after 18 Torr ofwater vapor is introduced and already allowed to equilibrate for 45 minutes. The
equilibration is necessary to reliably set the initial tip-sample distance. Otherwise, thechanges in tip behavior and topography are too rapid and great. The resonance andamplitude of the dithering are changing initially as shown in Figure 2.13, therefore
the feedback loop is not reliable enough to measure topography. At the same time,
the polymer is swelling. Both of these changes not only make measurements
difficult, but can also cause the tip to be broken by contact with the sample. The
equilibration period helps overcome these issues. Over the first five minutes it
appears that the thinner, more hydrophilic polymer swells more than the thickpolymer. Between five and 10 minutes it appears that the hydrophilic polymer swells
even further, to the point that it is now the thicker polymer. From ten minutes on, it
appears that this hydrophilic polymer is swelling in the x direction as well as the z
direction. Around 14 minutes the piezotubes in the scanner reached their maximum
retraction and needed to be re-set, causing a distruption in the image. The picomotor
controller allowed for external control of the z-stage over distances not obtainable
solely with the piezo-tube scanner.

52

Although it would be a very interesting result, to be able to watch a polymer
swelling using a shear force microscope, it seems that this image may be due toartifacts. One reason being, at time zero the polymer looks very similar to thevacuum linescan shown in Figure 2.14a. Yet the sample had time to equilibrate after
the introduction of water, so it seems unlikely that there would be almost no change
for 45 minutes, and then it would undergo such a drastic change in 25 minutes.
The change in the appearance of the linescan in time could be due to ongoingchanges in tip-sample interaction. The interaction distance between the tip andsample may be increased when the tip is positioned over a very wet surface. Thiseffect may be exaggerated by the confined nature of the polymer pattern. More work
is needed to understand the factors contributing to this result.

2.8 Visible NSOM

Images were first measured using a 690 nm visible laser diode in order to test
and optimize the apparatus. Figure 2.15 shows examples of complementary
topography and optical linescans of a one micron thick, uniformpoly(methylmethacrylate) (PMMA) film. In order to create this image, a single line
was scanned repeatedly in the x-direction. Therefore, the y-axis represents time.
After the linescan is started the valve to the water was opened slightly and water wasallowed in at a very slow rate, to a partial pressure of 10 Torr. This point is denoted
in figure 2.15a by the line corresponding to “water in.” After approximately 8
minutes the valve to the water vapor supply was closed and the chamber was opened

53

water in0time (min)awater out
20

topographybraw
opticalsmoothed
opticalcFigure 2.15: Characterization experiment with visible NSOM. a) Corresponding Topography
and Optical transmission linescans. The microscope scanned in the x direction, the y direction
shows time. b) Cartoon of swelling scenario where the polymer film swells vertically andhorizontally to reveal a feature on the sample. c) Cartoon of swelling scenario where one featureon the film swells in all directions. Both b and c are possible explanations for the change in thelinescan images shown in a.

54

to the vacuum pump. This point is denoted, “water out.” The chamber pressure at theend of the scan was 0.8 Torr. Because of the very slow rate of water introduction andremoval, it was possible to maintain the feedback loop. If the tip was about to crash
in to the sample, it was possible to use the picomotor controller to adjust the distancefurther than the piezotube scanner could.
The topography image shows a change in the sample occurring with humidity.
A feature appears to be growing in on the right side of the image, shown by the whitecolor, which indicates an increase in height. There are at least two possible scenarios
for this change, shown in figures 2.15b and 2.15c. Figure 2.15b shows the entire filmswelling upward and outward. The feature being imaged, perhaps a grain of polymer,
or even a grain of dust, is out of range at the beginning of the scan. The arrow in the
figure points to the location of the tip as it scans. As the film swells laterally, thefeature comes into view of the microscope. The tip starts to scan more and more of
the feature with each line. When the water is removed, the process is reversed.
Figure 2.15c shows a single feature on the sample swelling. The feature starts out ofview of the microscope and a larger area of it gets scanned as it swells.
The corresponding transmission optical image is taken simultaneously withthe topographic image. Water has a very small overtone absorption at 690nm, so thecontrast could either be due to the presence of water or simply to a change in sample
thickness, and therefore a change in transmission of light. It is very difficult to obtain
any information from the raw optical image. However, with the imaging programs
available, the data can be processed to help with the analysis. In this case, the opticalimage was smoothed by the adjacent averaging of the nearest 10 points. Now we can55see a darker area in transmission corresponding to the bright area in topography.
Where the sample is thicker, less light is allowed through. Perhaps the more swollen
area contains more water, and therefore allows less transmission of 690 nm light.

2.9 Conclusions

An IR-NSOM with environmental chamber has been achieved. The
microscope incorporates a color-center laser, which has a narrow linewidth optimalfor spectroscopic use. However, performance of the NSOM is limited by the low
signal-to-noise ratio produced when a noisy laser is emitted from a tiny aperture. A
ratio of at least 3 to 1 is necessary to obtain recognizable images; this is possible
when all components of the NSOM system are optimized.
The laser light is coupled to an optical fiber with a tapered, coated tip.
Fabrication of the tip is challenging, and special care must be taken not to break thetip during set-up of the experiment. It is estimated that only 1 in 10 tips produced is
not broken during production.30
The tip-sample distance is regulated with a shear-force microscopy feedback
loop. This is based on a home-built piezo tube scanner and is controlled by acommercial controller. It allows for the production of 3-dimensional images. The
scanner has a hysteresis, therefore the topographic data must be used to recognize
shapes and changes in shapes, but not to quantify an absolute measurement.
The microscope is incorporated into an environmental bell jar chamber.
Images can be measured in vacuum, which improves their quality. Water vapor canbe introduced to the experimental chamber. The water affects the shear-forces, and56

care must be taken in analysis of these images. Due to the large volume of the belljar, the vapor may be absorbed by the polymer faster than it fills the chamber, so
adjustments are necessary for true kinetic studies.
Despite the challenges presented by IR-NSOM in an environmental chamber,it successfully produces images that provide information on the properties andfunctions of polymer samples. Chapter 3 provides a detailed account of experiments
on water vapor uptake by chemically amplified photoresists.

57

References for Chapter II(1)Dunn, R. C. Chem. Rev. 1999, 99, 2891.

(2)

Dragnea, B.; Leone, S. R. Int. Rev. Phys. Chem. 2001, 20, 59.

(3)

Hecht, B.; Sick, B.; Wild, U. P.; Deckert, V.; Zenobi, R.; Martin, O. J. F.;

Pohl, D. W. J. Chem. Phys 2000, 112, 7761.
(4)

Barbara, P. F.; Adams, D. M.; O'Connor, D. B. Annu. Rev. Mater. Sci. 1999,

29, 433.
(5)

Synge, E. Philos. Mag. 1928, 6, 356.

(6)

Ash, E. A.; Nicholls, G. Nature 1972, 237, 510.

(7)

Durig, U.; Pohl, D. W.; Rohner, F. J. J. Appl. Phys. 1986, 59, 3318.

(8)

Pohl, D. W.; Denk, W.; Lanz, M.; Rohner, F. J. J. Appl. Phys. 1984, 44, 651.

(9)

Betzig, E.; Trautman, J. K.; Harris, T. D. Science 1991, 251, 1468.

(10)Isaacson, M.; Cline, J. A.; Barshatzky, H. J. Vac. Sci. Technol. B 1991, 9,

3103.
(11)

van Hulst, N. F.; Veerman, J.-A.; Garcìa-Parajò, M. F.; Kulpers, L. K. J.

Chem. Phys 2000, 112, 7799.
(12)Garcìa-Parajò, M. F.; Veerman, J.-A.; van Noort, S. J. T.; de Grooth, B. G.;

van Hulst, N. F. Bioimaging 1998, 6, 43.
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Betzig, E.; Chichester, R. J. Science 1993, 262, 1422.

(14)

Michaels, C. A.; Gu, X.; Chase, B.; Stranick, S. J. Appl. Spectrosc. 2004, 58,

257.
(15)

Dragnea, B.; Preusser, J.; Schade, W.; Leone, S. R. J. Appl. Phys. 1999, 86,

2795.

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(16)

Keilmann, F. Vibrational Spectroscopy 2002, 29, 109.

(17)

Schaller, R. D.; Johnson, J. C.; Wilson, K. R.; Lee, L. F.; Haber, L. H.;

Saykally, R. J. J. Phys. Chem. B 2002, 106.
(18)

Valaskovic, G. A.; Holton, M.; Morrison, G. H. Applied Optics 1995, 34,

1215.
(19)

Karrai, K.; Grober, R. D. Appl. Phys. Lett. 1995, 66, 1842.

(20)

Burns, A. R.; Carpick, R. W. Appl. Phys. Lett. 2001, 78, 317.

(21)

Brunner, R.; Marti, O.; Hollricher, O. J. Appl. Phys. 1999, 86, 7100.

(22)

Chuang, Y.-H.; Wang, C.-J.; Pan, C.-L. Appl. Phys. Lett. 1996, 69, 3312.

(23)

Polonski, V. V.; Yamamoto, Y.; White, J. D.; Kourogi, M.; Ohtsu, M. Jpn. J.

Appl. Phys. 1999, 38, L826.
(24)

Davy, S.; Spajer, M.; Courjon, D. Appl. Phys. Lett. 1998, 73, 2594.

(25)

Lapshin, D. A.; Letokhov, V. S.; Shubeita, G. T.; Sekatskii, S. K.; Dietler, G.

Ultramicroscopy 2004, 99.
(26)

Colchero, J.; Storch, A.; Luna, M.; Gòmez Herrero, J.; Barò, A. M. Langmuir

1998, 14, 2230.

(27)

Wei, P. K.; Fann, W. S. J. Appl. Phys. 2000, 87, 2561.

(28)

CRC Handbook Of Chemistry and Physics; 84 ed.; Lide, D. R., Ed.; CRC

Press, 2003.
(29)

Czaderna, A. W.; Thomas, T. M. J. Vac. Sci. Technol. A 1987, 5, 2412.

(30)

Dragnea, B., personal communication.

59

Chapter III:
Water Vapor Uptake in Photolithographic Polymers Observed by Infrared
Near-field Scanning Optical Microscopy in a Controlled Environment

3.1 Introduction

Transmission near-field scanning optical microscopy (NSOM) uses a subwavelength aperture probe that is scanned in close proximity to the surface of asample to overcome the diffraction limit on spatial resolution. Infrared-NSOM (IRNSOM) provides the possibility of vibrational spectroscopy and band specificity withthis high spatial resolution. Although visible light is more frequently used with
NSOM, IR-NSOM has been used to study systems from semiconductors to polymers
to human cells.1-11 This work further develops the IR-NSOM by integrating it into acontrolled environment chamber so that samples can be explored in select vapor
environments. Previous work with a visible NSOM in vacuum explored the effect ofvarying temperatures on semiconductor samples.12,13 Now, additionally, samples canbe exposed to various gases, and the changes they undergo can be monitored.14
An infrared NSOM has been developed in our group to study chemically
amplified photoresists.11 The method has been successful in probing the photoresists
spatially and chemically. Chemical subgroup specificity allows for determining thelocations of regions of various chemical compounds in polymer films with highspatial resolution. Figure 3.1 shows images of a patterned photoresist consisting ofstripes of poly(t-butylmethacrylate) (PTBMA) and poly(methacrylic acid) (PMAA).
Infrared optical transmission images were taken at 2 different wavelengths with a60

Transmission (%)

100

80

60

40

unexposed

exposed20

2.85 µm2.94 µm

4000

3500

3000

2500

2000

1500

1000

500

Wavenumbers (cm-1)

Figure 3.1: Infrared near-field and topographic images (upper panels, 12.5 µm x12.5µm) of deep ultraviolet patterned and post-exposure baked poly(t-butylmethacrylate)(PTBMA), and corresponding spectral changes (lower panel) as acquired by an FTIR
on non-patterned exposed and unexposed samples. The chemical specificity of the setup is demonstrated by the difference in contrast when using (a) 2.85 µm wavelength or(b) 2.94 µm. Topographic changes visible in the shear-force image (c) occur due toshrinkage of the exposed areas of the film.

61

corresponding topographic image. The image in figure 3.1a11 was measured at 2.85
µm, a wavelength at which there is very little absorption by either the modified or
unmodified polymer. In a separate experiment, the image in figure 3.1b wasmeasured at 2.94 µm. At this wavelength there is absorption by the exposed polymer,
but not by the unexposed. Therefore, there is more contrast in figure 3.1b, indicating
the differences in the exposed and unexposed regions of the photoresist. The change
in contrast from one IR image to another (~2%) corresponds to the expected
absorption contribution from the hydroxyl groups of PMAA. Refractive index
contributions to the optical contrast are minimized with narrow field of view
collection optics, but still produce a small effect. The success of this IR-NSOM led
to the development of the microscope in an environmental chamber described in
Chapter II.
Here we study how polymer samples react when exposed to water vapor. This
topic has importance for many polymer industries, including packaging, chemicalsensors, drug delivery, artificial organs, and electronics. It is particularly important inpatterned chemically amplified photoresists, which are used in the fabrication of
micro and nanocircuits in the electronics industry.15,16 Many polymers swell andchange shape when they absorb enough water vapor. The degree of vapor uptake andswelling depends on factors such as film processing and quality, polymer compositionand density, molecular orientation of the chains, size and density of pores, and film
thickness.17-21 Because photochemically modified areas have a different affinity forwater than the unexposed areas, swelling of the polymer can cause distortions of62

patterns. This can lead to problems with reproducibility and transfer of the desiredmask for subsequent etching.
Until now, some of the better methods used to study vapor uptake in polymer
films were Fourier transform infrared spectroscopy17,22-24 and gravimetric analysis.1821

However, these methods do not provide the spatial resolution that IR-NSOM can;

300 nm spatial resolution has been achieved with transmission IR-NSOM at awavelength of 3 µm.9-11 This paper describes the results of an IR-NSOM
implemented in a controlled environment chamber to measure vapor uptake andswelling in polymers on a small spatial scale.

3.2 Experimental Materials

Samples used are patterned, acid catalyzed, photolithographic polymers
similar to those studied by Dragnea et al.9-11 A 1.0 µm thick poly(t-butylmethacrylate)(PTBMA) film containing the photoacid generator triphenylsulfonium
hexafluoroantimonate was covered by a quartz/chrome mask and exposed to UV lightin the range of 200-300 nm. The film was then baked at 130 °C for 5 minutes toactivate the acid-catalyzed thermal deprotection chemistry. In the post-exposure
bake, the ester groups are converted to a hydrogen bonded carboxylic acid,poly(methacrylic acid) (PMAA), and the volatile product isobutylene is produced,
thus shrinking the film substantially in the UV exposed regions. The resulting patternused here has alternating 2 µm wide lines of approximately 1 µm thick PTBMA and0.5 µm thick PMAA.

63

3.3 Experimental Apparatus

The infrared microscope is a variant based on the IR-NSOM apparatus
developed by Dragnea et al.9-11 It is described in detail in Chapter II. A schematic
diagram of the set-up is shown in Figure 2.1. The NSOM was built inside a bell jar
environmental chamber capable of obtaining pressures down to 0.10 Torr. A Kr+ laser
pumps a color-center laser, giving 2.6-3.2 µm infrared light, which is absorbed by the
O-H stretching vibration of water vapor. For experiments described here, the laser
was tuned to a wavelength of 2.85 µm. Figure 3.2 shows that this is a wavelength atwhich there is low absorption by either the exposed (PMAA) or unexposed (PTBMA)
polymer. There should be very little contrast due to the difference in absorbance bythese two polymers in the infrared images. However, water is strongly absorbed at
2.85 µm. Therefore, contrast in the optical transmission image can be used todetermine regions of the polymer sample with greater amounts of sorbed water. A“map” of the water sorption can be created.
The beam of infrared light enters the bell jar through a CaF2 window in thechamber’s base plate. The light is coupled into an infrared fiber composed ofzirconium aluminum fluoride, which is transparent from 0.45 – 5.0 µm. The fiber ispulled into a tip at one end using a variable-pulling method.9,10 The tip is coated withaluminum, typically leaving a 200 nm aperture. Due to the brittleness of the material,some of the tips used in this work may have fractured to sizes larger than 200 nm.
The tip is suspended above the sample, which sits on an x-y scanning stage moved bythree piezoceramic tube legs. Directly below the sample is a large area, room

64

Absorbance (arbitrary)

unexposed polymer
exposed polymer
sorbed water
2.85 µm laser

0

4000

3500

3000

2500

Wavenumbers (cm-1)

Figure 3.2: FTIR spectra of unexposed polymer (black), exposed polymer (green), water
absorbed by the exposed polymer (red); and a line denoting the wavenumber to which the FCL
laser is tuned for the IR-NSOM experiement.

65

temperature, PbS detector, which measures the transmission of the infrared light. The
fiber tip is glued to a small tuning fork, which allows the topography to be measured
via the shear-force feedback method.25 The scanning and data acquisition arecontrolled by a scanning probe microscope controller.
All the necessary alignment procedures are carried out before the chamber isevacuated. The IR beam is coupled into the fiber and the tip is positioned above thesample in the near-field. Once the alignment is established, the glass bell-jar isplaced on the base plate and the chamber is pumped down. A picomotor screw
allows for external control of the tip-sample distance in the z direction when the
NSOM is enclosed. Up to 18 Torr of water vapor can be introduced to the chamber
and is allowed to equilibrate for at least 45 minutes before a scan begins. In future
work, real time measurements of uptake may be possible. Evident problems are theneed to follow the changes in distances due to the polymer swelling and to adjust forchanges in shear force feedback signals due to the water vapor environment.

3.4 Experimental Procedures

Images shown here are produced by scanning a line 256 times in the x
direction, while disabling the scanning in the y direction. Each line scan also consistsof 256 points. The transmission of light through the sample and the height of the tip
above the sample are simultaneously recorded. The lines can be averaged and plotted
to obtain a profile of the topography and transmission of the sample. The averaging
produces an improvement in the signal-to-noise ratio. This work shows two sets of66

measurements that compare the topography and optical properties of the sample invacuum conditions and in a water vapor environment.

3.5 Two Micron Features

Topographic images of the sample are shown in Figure 3.3. Figure 3.3a wasrecorded in a vacuum of 0.10 Torr pressure, while Figure 3.3b was measured in 16
Torr water vapor. Their average line profiles are plotted together in Figure 3.4a toshow that when exposed to a water vapor environment the gap between the highest
point in the topography image and the lowest point is diminished. Figure 3.4a showsthat the depth of the topographical valley is 0.4 µm in vacuum; however, it is actually
deeper. Because the fiber tip is too wide, the tip cannot reach the bottom of thevalley. Therefore, it cannot produce an accurately scaled topographic image. We
know from atomic force microscopy (AFM) experiments done on the sample that thevalley is actually 0.5 µm deep. Note also that the region containing PTBMA, thethicker polymer, is not flat. This structure is discussed in detail in previous work,11
and it may be due to some combination of UV irradiation, edge enhancement andshrinkage. A schematic diagram of how the tip might follow the sample’s topography
is shown in Figure 3.4b.
Figure 3.4a shows that the depth of the valley for the sample exposed to water
vapor is only 220 nm. This indicates that the PMAA swells 280 nm more than
PTBMA when exposed to 16 Torr water vapor. It is possible that the PTBMA alsoswells slightly, but we can only measure the relative change. The measured swellingsuggests that the PMAA has absorbed more water than the PTBMA.

67

a1.00 µm

0.40 µm
PMAA

PMAA

PTBMA

b

1.00 µ m

0.72 µm
PMAA

PTBMA

PMAA

Figure 3.3: Topographic images of photolithographic polymers with 2 µm wide lines. Each image
represents a single line scanned 256 times. (a) Topographic linescan of sample in vacuum. (b)Topographic linescan of sample in 16 Torr water vapor environment.

68

a

1.0

Height (µm)

0.8

0.9

0.7
0.6
0.5
0

4
6
X Position (µm)

810Heightb

20

x position

10

Figure 3.4: (a) Average line profiles. The solid line is an average profile of Figure 3.3a and thedashed line is an average profile of Figure 3.3b. The red line portions represent PTBMA andthe black lines represent PMAA. (b) A schematic drawing of the tip following the sample
surface. The black line is a profile of the sample’s topography, and the red line shows how thetip follows that surface to produce the topography image.

69

a

100%

80%

100%

b

80%

c 96
Transmission (%)

94
92
90
88
86

0

2

4
6
8
X Position (µm)

10Figure 3.5: Transmission images of 2.85 µm light through photolithographic polymers with 2µm wide lines. Each image represents a single line scanned 256 times. (a) Transmission linescan
recorded in vacuum conditions. Darker regions represent smaller transmission intensity. (b)Transmission linescan recorded in 16 Torr water vapor environment. (c) Average line profiles ofparts a and b. The solid line represents the average of the image in part a, the dashed line
represents the average of b.

70

Figures 3.5a and 3.5b show the infrared transmission images recorded
simultaneously with the topography images in Figures 3.3a and 3.3b, respectively.
Figure 3.5a, recorded under vacuum conditions, shows an image that is almost
completely featureless, while Figure 3.5b shows large oscillations in the intensity oflight reaching the detector. The reason is that one polymer absorbs more water thanthe other, thus allowing through less light. Again, the lines were averaged andprofiles of these images are plotted in Figure 3.5c. This graph shows a 6 ± 1%
difference between the greatest and the smallest detected intensities. This number
was obtained by taking the average of the two local maxima, subtracting the averageof the two local minima, then computing error bars using their standard deviations. It
is obvious that this contrast is not due to a topographic artifact, since the transmission
contrast increases while the topographic contrast decreases. However, the difference
is not pure absorption contrast. There are several other contributing factors, which
are discussed below.
The modified and unmodified polymers have different indices of refraction.
This effect is explained by Dragnea et al.9 as a change in cone angle of detection forour particular set-up, and it is expected to produce a variation of 0.5%. It is this effectthat causes a slight oscillation of the transmission signal in the vacuum case. Anothercontrast mechanism can be the change in index of refraction with concentration ofwater. During vapor uptake, a polymer’s index will first increase as void-spaces arefilled, and then the index can decrease as the film swells.26 The changes in index ofrefraction will cause a further change in the amount of light transmitted to the PbS

71

detector. Depending on the concentration of water in the film, this process can either
enhance or diminish the transmission contrast.
Figure 3.6 shows an experiment which was carried out in order to demonstrate
this process. A broken tip with an aperture on the order of a few microns issuspended over the sample at a distance which is out of range of the near-field.
Therefore, the size of the spot of light that hits the sample is fairly large. The lighttravels through both modified and unmodified regions of the photoresist before
reaching the PbS detector. Under vacuum, the transmission of light through thesample is around 63-64%. Water is slowly introduced into the chamber. At first, thiscauses an increase in the transmission of infrared light into the sample. This is the
opposite of what would normally be expected, because water absorbs infrared light.
The result can be explained by index of refraction changes. The introduction of thewater causes the void-spaces in the polymer to be filled, and the polymer’s index ofrefraction increases. This causes light to be bent in a different manner as it travels
through the sample, and less if reflected off of the interfaces. Therefore, more lightarrives at the detector. As more water is added to the chamber, and thus to thepolymer, the film swells and causes a decrease in the index of refraction. Now thelight rays are bent in such a way that more is reflected at the interfaces. Coupled with
the increase in absorption of infrared light by the water, the effect results in adecrease of transmission of infrared light to the detector.
The OH stretching band of water can also change shape depending on theconcentration of water. Several examples are cited in the literature.22-24 Therefore, anincrease in transmission at one laser wavelength does not always correlate linearly to72

1. Water opened
2. P = 6 torr
3. P = 12.5 torr
4. P = 15 torr
5. P= 16 torr

2
70

Transmission

68
66

1

64

3
4

62

5

60
58

-5

0

5

10

15

20

25

30

35

40

Time (minutes)

Figure 3.6: Transmission of 2.85 µm laser light through a polymer photoresist sample. The laser
is emitting from a large aperture fiber tip and is far from the sample, therefore a large spot sizeof light hits the sample. A valve is opened slightly to introduce water at a time indicated by the
number 1. The partial pressure of water rises slowly and is indicated on the graph until itreaches a pressure of 16 Torr and remains constant.

73

the amount of water present. This topic will be explored further in Chapter V. In
addition, the absorption of light by the gas phase water vapor in the controlledenvironment chamber will create a baseline that must be subtracted.
Another factor that could decrease the transmission contrast is the size of the
NSOM aperture tip. When the tip’s aperture is broken or too large, the transmission
contrast is not as sharp or as great as it should be.
All of these factors combined make it difficult to determine the concentrationof water in the film. In the future, experiments with a quartz crystal microbalance
will be performed simultaneously to quantitatively correlate the water concentrationin the NSOM signals by more traditional gravimetric methods. Also, use of a tunable
laser source with a wider range of available wavelengths will make more experiments
accessible. For example, if an image was recorded using a laser tuned to 3750 cm-1, ameasurement could be taken that has no contrast contributions from either the OH
stretch of the modified polymer or the water. (see Figure 3.2) The percent contrast
found in such an image could be subtracted from the contrast in an image measured at
3500 cm-1 to minimize index of refraction effects. FTIR experiments should also be
carried out to investigate band shifts and give access to information on watermolecule interactions.
Optical and topographic images should be spatially consistent. The area thattransmits less light should correspond to the region that swells more, because these
are the regions with a greater amount of water present. However, in these particularexperiments, the optical images are shifted by 2 microns from the topographic
images. This is shown in figure 3.7. This is clearly an artifact because similar74

Transmission/Height

Topography
Transmission

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0

2

4

6

x Postion (microns)8

Figure 3.7: Corresponding topography and transmission linescans in a 16 Torr water vapor
environment. The topography line (black) is the same as the dotted line shown in Figure 3.4a.
The transmission line (red) is the same as the dotted line in Figure 3.5c.

75

experiments on samples with 4 micron thick lines were also shifted by 2 microns. We
do not know why the optical images were shifted but hypothesize that it may be
image displacement due to the relatively large probe used. The point on the tip that isinteracting with the sample to form the shear force image may be different from theoptically transparent region for the transmission intensity. For example, a sharp grainof aluminum coating can protrude from the side of the tip and interact with thesample to form the topographic image, while light emerges from the center of the tip
to form the optical image. Future work will correlate the transmission zones with the
topographic images more rigorously.

3.6 Four Micron Features

Figure 3.8 shows an example of average linescans on four micron features.
The same procedure is carried out as for the above two micron features, but adifferent area of the sample was probed where four micron features are present. The
fiber tip used for these experiments is different from the one used in the previous
experiments. This new fiber tip made it possible to better correlate the transmission
and topography images. They match up well, with the topographic valleys (PMAA
regions) overlapping the areas with lower transmission. This image was measured in
an atmosphere of 18 Torr water vapor. The water, which is present in a higher
concentration in the PMAA, absorbs the infrared light and decreases the transmission.
The measurements were repeated several times in various environments. The
results are summarized in table 3.1. For each environment, we report the depth of thevalley (∆ H) and the transmission contrast (∆ T). In an 18 Torr water vapor

76Normalized height/transmission intensity

Transmission
Topography

1.0

0.8

0.6

0.4

0.2

0

2

4

6

8

x distance (µm)

Figure 3.8: Average linescans in transmission and topography for 4 micron features of a polymer
photoresist in 18 Torr water vapor.

77

environment, the PMAA swells 0.20 ± 0.09 microns more than the PTBMA. The
error bars were determined by multiple runs with the same tip. This matches wellwith the result of 0.28 microns swelling found on the 2 micron feature experiments inthe above section.

Table 3.1: Height and Transmission data in various experimental conditions
Environment∆ H (µm)∆ T (%)

Vacuum (0.20 Torr)

0.56 ± 0.05

0.35% ± 0.02 %10 Torr Water Vapor-

0.40% ± 0.02%

18 Torr Water Vapor

0.36 ± 0.04

0.45% ± 0.02%

The change in transmission shows that there is a 0.10% difference intransmission contrast in going from a dry to a water vapor environment. This result isquite different from the result on two micron images, which was 6 ± 1 %. The
difference could be due to differences in the tip used and its aperture. The apertureon the tip used for these experiments could be larger in size, or even slightly broken,
thus reducing the contrast. This result highlights the need for consistent tips in orderto make comparisons between experiments.
On the positive side, the result also shows the sensitivity of the IR-NSOM. It
was able to determine a range of transmission contrast for three different
environments (vacuum, 10 Torr, 18 Torr).

78

3.7 Conclusions

In order to increase the quality of the images and the information obtained
from the IR-NSOM described here, several changes to the experimental set-up are
required. First, detection should be improved by replacing the large area, room
temperature PbS detector with a liquid nitrogen cooled, small area, photovoltaic InSb
detector.10 Use of varying numerical aperture collection optics would assist in
reducing the contrast due to the cone angle effect.9 Efforts can also be made tofacilitate real-time measurements of vapor uptake. One way to achieve this would be
to operate in constant height mode, rather than constant gap mode, so that the probe
would not interact with the sample. It will also help to determine the source of thechanges in the shear-force feedback signals with change in water vapor pressure.
Perhaps the use of a non-absorptive glue to stick the fiber on the tuning fork willsolve this problem and allow for time-resolved measurements.
This work demonstrates a new method for exploring the vapor uptake ofpolymers. The shear-force topographic method produces a value for the swelling of apolymer in a vapor environment and, with calibration of a known system, can becomea precise measure of the amount of water absorbed in the sample. The transmission
data complements the topographic images, and the results can be used to quantify theamount of water in a sample if more work is completed to separate the various
contributing effects. Infrared NSOM under environmental control is an excellent
potential new tool for determining where water vapor uptake occurs in a polymer
sample on a small spatial scale.

79References for Chapter III(1)

Dragnea, B.; Leone, S. R. Int. Rev. Phys. Chem. 2001, 20, 59.

(2)

Schaller, R. D.; Saykally, R. J. Langmuir 2001, 17, 2055.

(3)

Stranick, S. J.; Chase, B.; Michaels, C. A. Abstracts of ACS 2001, 221, 173.

(4)

Cricenti, A. J. Alloys Cmpds 2001, 328, 2.

(5)Hong, M. K.; Jeung, A. G.; Dokholyan, N. V.; Smith, T. I.; Schwettman, H.

A.; Huie, P.; Erramilli, S. Nuc. Inst. Mths. Phys. Res. 1998, 144, 246.
(6)

Knoll, B.; Keilmann, F. Appl. Phys. A 1998, 66, 477.

(7)

Sahlin, J. J.; Peppas, N. A. J. Appl. Polym. Sci. 1997, 63, 103.

(8)

La Rosa, A. H.; Yakobson, B. I.; Hallen, H. D. Appl. Phys. Lett. 1997, 70,

1656.
(9)

Dragnea, B.; Preusser, J.; Schade, W.; Leone, S. R.; Hinsberg, W. D. J. Appl.

Phys. 1999, 86, 2795.
(10)

Dragnea, B.; Preusser, J.; Szarko, J. M.; Leone, S. R.; Hinsberg, W. J. Vac.

Sci. Technol. B 2001, 19, 142.
(11)

Dragnea, B.; Preusser, J.; Szarko, J. M.; McDonough, L. A.; Leone, S. R.;

Hinsberg, W. Appl. Surf. Sci. 2001, 175-176, 783.
(12)

Gray, M. H.; Hsu, J. W. P. Rev. Sci. Inst. 1999, 70, 3355.

(13)

Gray, M. H.; Hsu, J. W. P. Appl. Phys. Lett. 2000, 76, 1294.

(14)

Elbs, H.; Fukunaga, K.; Stadler, R.; Sauer, G.; Magerle, R.; Krausch, G.

Macromolecules 1999, 32, 1204.
(15)

Padmanaban, M.; Endo, H.; Inoguchi, Y.; Kinoshita, Y.; Kudo, T.; Masuda,S.; Nakajima, Y.; Pawlowski, G. Proc. SPIE 1992, 1672, 141.

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MacDonald, S. A.; Cleacak, N. J.; Wendt, H. R.; Willson, C. G.; Snyder, C.

D.; Knors, C. J.; Deyoe, N. B.; Maltabes, J. G.; Morrow, J. R.; McGuire, A. E.;
Holmes, S. J. Proc. SPIE 1991, 1466, 2.
(17)

Sutandar, P.; Ahn, D. J.; Franses, E. I. Macromolecules 1994, 27, 7316.

(18)

Buchold, R.; Nakladal, A.; Gerlach, G.; Herold, M.; Gauglitz, G.; Sahre, K.;

Eichhorn, K. J. Thin Solid Films 1999, 350, 178.
(19)

Chen, W. L.; Shull, K. R.; Papatheodorou, T.; Styrkas, D. A.; Keddi, J. L.

Macromolecules 1999, 32, 136.
(20)

Hutcheon, G. A.; Messiou, C.; Wyre, R. M.; Davies, M. C.; Downes, S.

Biomaterials 2001, 22, 667.
(21)

Neogi, P. Diffusion in Polymers; Marcel Dekker, Inc.: New York, 1996.

(22)

Ping, Z. H.; Nguyen, Q. T.; Chen, S. M.; Zhou, J. Q.; D., D. Y. Polymer 2001,

42, 8461.
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Yarwood, J.; Sammon, C.; Mura, C.; Pereira, M. J. Mol. Liq. 1999, 80, 93.

(24)

Linossier, I.; Gaillard, F.; Romand, M.; Feller, J. F.; Sci., J. A. P. J. Appl.

Polym. Sci. 1997, 66, 2465.
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Karrai, K.; Grober, R. D. Appl. Phys. Lett. 1995, 66, 1842.

(26)

Cross, G. H.; Ren, Y.; Swann, M. J. Analyst 2000, 125, 2173.

81

Chapter IV:
Atomic Force Microscopy Measurements of Humidity Effects on Polymer Films

4.1 Introduction

Atomic Force Microscopy (AFM) is used to study a variety of properties ofpolymer films, including surface morphology, structure, mechanical stability, local
elastic response, friction and adhesive forces.1-6 The ability to make thesemeasurements has led to advances in polymer nanotechnology and microfabrication
processes.1,7
The AFM measures the attractive and repulsive forces between a sample and a tip.
In “contact mode,” a probe consisting of a cantilever with a sharp tip is scanned
across the surface of a sample. The tip is in continuous contact with the surface, andthe bending of the cantilever due to surface topography is measured. High spatial
resolution images are produced. However, the forces applied to the sample surface inthese measurements can cause damage to a soft polymer film. Therefore, a reduced
force mode with intermittent contact between the tip and sample called “tappingmode” is used to image polymer surfaces. 1,2,4
In tapping mode AFM, the cantilever with tip is driven to oscillate at its resonancefrequency with a certain amplitude, A0. The cantilever is brought close to a sample
surface and “taps” the surface with a set point amplitude, Asp. As the probe scans the
sample, damping of its oscillation due to repulsive or attractive forces between thesample and the tip is monitored. A laser beam is focused on the cantilever and82

reflected onto a four-quadrant photodetector that measures the amplitude. The
vertical position is controlled by a feedback mechanism, much like the shear-force
feedback system described in Chapter II. It works to keep the amplitude of theoscillating probe constant, and it produces an image of the sample topography.
In order to understand images produced by tapping mode AFM, the tip-sample
interaction must be studied. This can be achieved by experiments carried out in the
distance-sweep mode. The amplitude of a tapping cantilever is measured as afunction of the distance between the tip and sample, while the lateral position of thecantilever remains constant. Local mechanical properties of the polymer surface,which influence the topographic image produced in tapping mode AFM, can beinvestigated.8
This work will use AFM to measure changes in polymer photoresist film
properties with humidity. Polymer films tend to absorb water and swell when
exposed to humidity. The extent of this swelling will be investigated with tapping
mode AFM imaging. The changes in polymer film properties will be explored inorder to interpret these images.

4.2 Materials

Polymer solutions of poly(t-butylcarbonylstyrene) (tBOC) with photoacid
generator are prepared by IBM using a previously described method9 and spun onto
25 mm silicon wafers. The resulting tBOC film is 193 nm thick. The resist is
exposed to a line-space pattern using a contact mask and a dose of 10 mJ/cm2

83

ultraviolet light at 254 nm. The post-exposure bake at 100°C for 90 seconds results
in a conversion of exposed areas to polyhydroxystyrene (PHOST). The resulting
lines and spaces are nominally 4 microns in width.

4.3 Instrument

The instrument used for experiments is a Digital Instruments Multimode AFM
operated in tapping mode. Probes used are standard silicon tips with a resonant
frequency around 300 Hz and a tip diameter of 5 – 10 nm. A 60 liter bell jar is placed
over the entire AFM apparatus, with a space in the base allowing a nitrogen stream topurge the experiment. The nitrogen is passed through a reservoir containing water toproduce wet nitrogen. Wet and dry nitrogen are mixed to produce the desired
humidity. Care is taken not to aim the stream towards the tip, so the flow of gas will
not affect measurements. For all experiments, equilibration of one hour at the chosen
humidity is allowed.
Distance sweep measurements were carried out on both exposed andunexposed regions of the polymer. In order to locate these regions, the sample wasscanned in tapping mode. Once the line-space pattern was identified, the tip could bepositioned either over the raised lines (tBOC) or the thinner areas (PHOST). Thenthe distance sweep could be carried out over that particular polymer.

4.4 Distance Sweep Measurements

Figure 4.1a shows a schematic drawing of a distance sweep measurement. A
cantilever with a tip oscillates at a given amplitude, A, that depends on the distance

84

a)

{

cantilever

A

}

tip

d

approachsampleAmplitude (A/A0)b)

retraction

1.0

0.5
Si (approach)Si (retract)
PHOST (approach)
PHOST (retract)
0.0
-1

0

1

2

3

tip-sample distance (d/A0)

Figure 4.1: Distance sweep measurements. a) Schematic drawing of a distance sweep
experiment. B) Curves from measurements on a silicon wafer (black lines) and the modified
photoresist polymer PHOST (red lines). The solid lines indicate the approach of the tip to thesample, the dashed lines represent the retraction of the tip from the sample.

85

between the tip and the sample, d. The amplitude is measured as the distance isdecreased (approach) and increased (retraction). The result is a plot called a distancesweep curve. Figure 4.1b shows the results of distance sweep measurements
performed on the PHOST polymer (red lines) as well as a plain silicon wafer (blacklines). These experiments were performed in a nitrogen purge environment of 11%
relative humidity (RH). The solid lines indicate measurements where the tip was
approaching the surface from far away. The dashed lines represent retractions, where
the tip started at zero distance and was moved away from the sample. The silicon
curves are used to highlight the characteristic properties of the amplitude vs. distancecurves for stiff samples as opposed to polymers.
When the tip is very far from the surface, the cantilever oscillates freely at an
amplitude of A0. The amplitude is not dependent on distance in this regime. The valueof A0 is determined by the driving force applied to the cantilever, and is constant foreach curve. In order to compare various experiments conducted with a variety ofdriving forces, it is standard practice to normalize the amplitude and the distance bydividing by A0, the freely oscillating value. All figures shown in this section have
sweeps performed at similar values of A0, unless otherwise specified. A value ofaround 30 nm would be typical.
As the tip reaches the interaction region, the amplitude is damped as it moves
closer to the sample surface. The interaction begins roughly where the distance
between tip and sample is equal to the free oscillation amplitude of the cantilever(d/A0 = 1). As the amplitude first begins to decrease, the slope of the silicon curve isgreater than that of the PHOST curve. The amplitude decreases more slowly when86

approaching softer, more rubbery samples in comparison to stiff samples when thesame value of A0 is set.2,8 When the tip – sample distance is equal to zero, the tip
ceases oscillation on both curves. Normal tapping mode images typically set the
feedback loop to an amplitude equal to 90% of A0.
A feature in need of highlighting is the difference in the retraction curves. The
PHOST retraction curve has a much larger hysteresis than the silicon curve. This isbecause the tip indents into the polymer due to its softness and becomes “stuck.”More force is needed to pull it out of the polymer film than to retract from a hardsilicon substrate.6
Another set of experiments was performed to measure differences in local tipsample interaction for the exposed and unexposed regions of the polymer. Results ofexperiments done in an 11% RH atmosphere are shown in Figure 4.2a. Distancesweeps for PHOST are red; TBOC are blue. Again, solid lines represent approaches
and dashed lines mean retractions. Results for the approach curves on each polymer
are very similar. However, there is a slight difference in the retractions. PHOST has
a larger hysteresis, meaning it is a softer, more rubbery polymer and the tip may
indent further or have more difficulty pulling out of the film. This is somewhat
expected because during the deprotection reaction the modified polymer loses volatile
products and “shrinks.” Thus its polymer chains have relaxed and the film is morerubbery as a result. In tapping mode AFM, the tips tend to indent further into rubbery
films over glassy films.10
The distance sweeps are repeated after 1 hour in a high humidity (80% RH)
environment, and the results are shown in Figure 4.2b. The results are surprising,

87

a)

11%Humidity

Amplitude (A/A0)

1.0

0.5

PHOST (approach)
PHOST (retract)
TBOC (approach)
TBOC (retract)

0.0
0

1

2

3

Tip-Sample Distance (d/A0)b)

Amplitude (A/A0)

80% Humidity

0.8

PHOST (approach)
PHOST (retract)
TBOC (approach)
TBOC (retract)

0.4

0.0
0

1

2

3

Tip-Sample Distance (d/A0)

Figure 4.2: Distance sweeps measured for PHOST (red lines) and TBOC (blue lines). Solidlines are approach curves, dashed lines are retraction curves. a) Measured with nitrogen purge
in an atmosphere of 11% RH. B) Measured at 80% humidity.

88because they look very similar to the dry sweeps. The absorption of water has notseemed to change the mechanical properties of the polymer enough to alter the tipsample interaction. It was also expected that because the PHOST polymer absorbs
more water, its mechanical properties would change, and therefore the tip-sample
interaction would change more drastically than the TBOC interaction. However, thiswas not the case.
Because the tip-sample interactions do not change noticeably with humidity,height images recorded under a variety of humidity environments can all beinterpreted the same way. Therefore, the differential swelling can be measured.
Differential swelling means the swelling amount of one polymer compared toanother. The absolute swelling cannot be determined using AFM. In other words, if
both the TBOC and the PHOST lines swell by the same amount, the differential
swelling is zero.

4.5 Differential Swelling Measurements

Tapping mode images of the patterned polymer resists were measured at 11%,23%, 43% and 56% RH. A typical 15 µm x 15 µm image is shown in Figure 4.3a. In
order to measure the swelling of the sample at each humidity, it is necessary to
determine ∆H, the change in height between the exposed and unexposed regions.
Figure 4.3b shows a profile of the topographic image which clearly shows the peaks
and valleys. The value of ∆H is quantified by first taking a histogram of the heightvalues represented in the image, shown in Figure 4.3c. Two peaks are evident. The
one at a positive height value corresponds to the height of the unexposed, TBOC

89

Figure 4.3: Differential swelling measurements of a TBOC/PHOST photoresist. a) 15 µm x
15 µm tapping mode AFM image measured at 23% RH. b) Profile of height vs. distance for one
line across the photoresist. c) Histogram of number of data points for each height represented in
the image fitted with Gaussian curves.

90

polymer, the other corresponds to the height of the exposed, thinner, PHOST
polymer. In order to systematically choose a value for the peak, the histogram wasfitted with a Gaussian curve for each polymer. The photoresists are not intended to
produce Gaussian histograms. In fact, if they were ideal, there would only be twoheight values represented on the histogram. Half of the counts would represent theheight of tBOC, the other half would be the height of PHOST. However, thephotoresists are not ideal and therefore many height values are represented. The
difference between the heights represented at the peaks of these two curves is ∆H.
This procedure was carried out on an image taken at each humidity. The
results are shown in Table 4.1. Error bars are a result of the Gaussian fit. The valueof ∆H decreases with increasing humidity, meaning the exposed, PHOST region of
the polymer swells more than the unexposed areas. This result is expected because
PHOST is the more hydrophilic polymer and would naturally absorb more water.
The percent differential swelling, ∆S, at a given humidity, x, is calculated by:∆Sx =(∆H 11 − ∆Hx )
,
∆H 11

(4.1)

where ∆H11 is the change in height between exposed and unexposed regions at 11%
RH, the lowest achievable humidity with this experimental set-up.
What is unexpected is how small the differential swelling is. In Chapter III, aphotoresist made of PTBMA/PMAA is found to have 36% differential swelling.
However, that photoresist is composed of a different polymer material. In order tohave experiments that could be compared, AFM images of the PTBMA/PMAA
sample with 4 micron lines were carried out in both a nitrogen purge environment

91

Table 4.1: Differential Swelling in patterned, TBOC photoresists.
Differential Swelling(% decrease in ∆H fromdry film)

Relative Humidity(%)∆H (nm)11

79.5 ± 0.1-

2377.5 ± 0.1

2.5 ± 0.3%

4376.2 ± 0.1

4.2 ± 0.3 %

56

75.2 ± 0.1

5.4 ± 0.3 %92

(13% RH) and a humid environment (70% RH). The histograms for these two
images are shown in Figure 4.4.
As explained in Chapter III, the region containing PTBMA, the thicker,
unexposed polymer, is not flat. This causes a double peak in the histogram. Figure
4.4c shows horizontal line profiles of the image, which clearly illustrate the reason forthe double peak in the histogram. This profile looks odd in comparison to the profileof the tBOC/PHOST photoresist shown in 4.3b. The oscillation makes it difficult todecide how to determine ∆H. Ultimately, an average of the two highest peaks wasused as the height for PTBMA. Choosing the maximum height value for the height of
PTBMA was also considered. The final result is a differential swelling of 37 ± 3% ingoing from a dry environment to a humid one. This matches well with the value of
36% that can be calculated from measurements done in Chapter III on 4 micron
samples. Distance sweep measurements could not be reliably obtained on thesesamples because of the oscillating profile. It appeared that the approach curves had
no humidity dependence, but it could not be guaranteed that the approaches took
place on the intended polymer.
Although the results match up well, the experiments described here and in
Chapter III have key differences which may make them difficult to compare. The
experiments in Chapter III were performed in a vacuum bell jar chamber, so change
in pressure could have played a role. It is also important to consider the two types of
microscopy used to measure topography. In shear-force microscopy, employed in theexperiments described in Chapter III, the tip is dithered laterally, or somewhat
parallel to the surface. If there are contributions to the height measurements from93

Figure 4.4: Height histograms of 15 µm x 15 µm tapping mode AFM images measured on
PTBMA/PMAA photoresists. a) Histogram for image measured under nitrogen purge (13%
RH). b) Histogram for image measured in 77% RH humid environment. c) Profiles of one
horizontal line in the image. Solid lines represent PTBMA regions, dashed lines represent
PMAA regions.

94differences in tip-sample interaction for the two different polymers, it may bemagnified by prolonged interaction. If the tip-sample interaction changes when apolymer becomes saturated with water, the differential swelling could be exaggerated.
The AFM tip dithers perpendicular to the surface, so effects from a hydrophilic
surface are not as strong.

4.6 Humidity Effect on Distance Sweep Curves

Models of the three possible types of distance sweep curve described in theliterature are shown in Figure 4.5. The type of curve observed depends on the sample
material, the free amplitude of the cantilever and the tip-sample interaction.11 Part ashows a Type I curve, where the amplitude decreases linearly with tip-sample
distance. In Figure 4.5b, a Type II curve depicts a step-like transition. The transition
is an observation of the formation of a “liquid neck” due to capillary forces between
the tip and a film of water on the sample surface. A Type III curve, shown in Figure
4.5c, is similar to Type II, but has a smooth transition.
There have been many distance sweep studies performed on stiff materials,
but there is still a lot to learn about polymers. A study by Zitzler, et al. looked at
amplitude vs. distance curves as a function of humidity for both hydrophilic andhydrophobic stiff surfaces.3 They found that when the free oscillation amplitude A0 is
greater than a critical amplitude Ac, the distance sweep curve is type II. Below Ac the
curve is type I. This doesn’t mean that no capillary is formed at lower amplitudes,
just that it cannot be observed. For hydrophilic surfaces, the value of Ac increases
with humidity, but there is no trend for hydrophobic surfaces. The critical amplitude

95

1.0

a) Type I

0.5

Amplitude (A/A0)

0.0
1.0

b) Type II

0.5

0.0
1.0

c) Type III

0.5

0.0
0

1

2

Tip-Sample Distance (d/A0)
Figure 4.5: Three different theoretical amplitude vs. distance curves.

96

is simply a measurement of the A0 value at which distance sweep curves change fromType I to Type II. It has no other significance other than the fact that a hydrophilic
surface has a humidity dependent Ac while a hydrophobic surface has a humidity
independent Ac value.
The work done by Zitzler and colleagues was carried out with silicon tip andmica sample, which is considered a stiff material. Both surfaces were chemicallyprocessed to obtain hydrophilic or hydrophobic properties as desired. The workshows that the distance sweeps could be used as a probe of local hydrophilicity at
different points on a sample surface.
It would be interesting to probe the local hydrophilicity of the polymers in aphotoresist using a similar experiment to Zitzler’s. Attempts were made to carry out
such measurements on TBOC and PHOST samples at a variety of humidities andamplitudes. Figure 4.6 shows an example of one set of sweeps performed on TBOC
at 13% RH. Each curve is measured at a different value of A0, but the curves arenormalized so they may all be viewed on the same scale and compared. The A0 value
ranges from as small as possible to still achieve a curve, and as large as possible
without damaging the cantilever or tip. The shape of the curve appears to be
dependent on the free oscillation amplitude. At lower values of A0, the amplitudedecreases at a slower rate at normalized tip-sample distance (d/A0) values near one.
The curves are also noisier. However, all of the curves appear to be Type I, with notransition to Type II occurring. This is consistent at each humidity explored, which
included a range between 13% and 80% RH. Because no transition is detected, there
is no Ac.

97

Amplitude (A/A0)

1.0

A0=16 nm
A0=22 nm

0.5

A0=30 nm
A0=54 nm
A0=70 nm

0.0

0

1

2

3

tip-sample distance (d/A0)

Figure 4.6: Distance sweep curves on TBOC measured at a variety of A0 values.

98Because the literature does not include many distance sweep studies on
polymers, it is not known if the type II curve has been measured on soft surfaces.
There may be a physical reason why the transition cannot be detected on softer
samples. On the other hand, perhaps more careful control of experimental parameterssuch as tip surface preparation may produce type II curves.

4.7 Conclusions

Tapping mode AFM can be used to measure polymer photoresists. Differential
swelling at can be quantified by capture of tapping mode images at a variety ofhumidities. These images can be carefully analyzed by consideration of distancesweep curves, which measure local mechanical properties of the polymer sample.
Amplitude vs. distance curves performed on exposed and unexposed regions of a
TBOC/PHOST polymer photoresist do not show a humidity dependence. Therefore,
tip-sample interaction was determined to have little effect on differential swellingmeasurements.
Attempts to measure Ac, the critical amplitude of free oscillation above which
Type II curves exist, were unsuccessful. Therefore, it was not possible to measurehumidity dependence of Ac when approaching photoresist samples, which would havebeen a way to quantify hydrophilicity.

99References for Chapter IV(1)

Ratner, B. D.; Tsukruk, V. V. Scanning Probe Microscopy of Polymers;

American Chemical Society: Washington, DC, 1996; Vol. 694.
(2)

Bar, G.; Delineau, L.; Brandsch, R.; Ganter, M.; Whangbo, M.-H. Surface

Science 2000, 457, L404.
(3)

Zitzler, L.; Herminghaus, S.; Mugele, F. Phys. Rev. B. 2002, 66, 155436.

(4)Chen, X.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M.;

Davies, J.; Dawkes, A. C.; Edwards, J. C. Ultramicroscopy 1998, 75, 171.
(5)

Opdahl, A.; Hoffer, S.; Mailhot, B.; Somorjai, G. A. The Chemical Record

2001, 1, 101.

(6)

Magonov, S., N.; Reneker, D. H. Annu. Rev. Mater. Sci. 1997, 27, 175.

(7)

Sheiko, S. S. Adv. Polym. Sci. 2000, 151, 61.

(8)

Kopp-Marsaudon, S.; Leclere, P.; Dubourg, F.; Lazzaroni, R.; Aimè, J. P.

Langmuir 2000, 16, 8432.
(9)

Burns, S. D.; Medeiros, D. R.; Johnson, H. F.; Wallraff, G. M.; Hinsberg, W.

D.; Willson, C. G. Proc. SPIE 2002, 4690, 321.
(10)

Leclere, P.; Dubourg, F.; Kopp-Marsaudon, S.; Bredas, J. L.; Lazzaroni, R.;

Aimè, J. P. Appl. Surf. Sci. 2002, 188, 524.
(11)

Garcia, R.; San Paulo, A. Ultramicroscopy 2000, 82, 79.

100

Chapter V:
Fourier Transform Infrared Spectroscopy Studies of Water-Polymer
Interactions in Chemically Amplified Photoresists

5.1 Introduction

Water sorption is an interesting process that can affect the physical, mechanicaland chemical properties of a polymer. It is important to many polymer industries,
including packaging, chemical sensors, drug delivery, artificial organs, andelectronics. The rate and extent of the water uptake are determined by a variety offactors, such as temperature, humidity, polymer chemistry, film thickness, pore sizeand density, and material processing.1-6 The structure of the sorbed water is ofparticular importance, as it can give insight into the interactions between the waterand the polymer substrate. Water is a unique penetrant because it interacts strongly
with polar groups of the polymer and can form clusters that are confined by thepolymer chains. Among the physical changes that water can induce areplasticization, hygrothermal degradation, swelling, alteration of tensile strength andhardness, and change in net dielectric constant of a film.3-5,7,8 Water can also affectchemical processes in polymers such as the deprotection reaction in photoresists usedin the fabrication of electronic circuits.3,9-11
Poly(t-butoxycarbonylstyrene) (tBOC)12 and ketal-protected
Poly(hydroxystyrene) (KRS-XE)13 are two important chemically amplified
photoresist systems. The tBOC polymer, shown in Figure 5.1a, is a thin film cast

101

(

(

)

a)

H

O

)

+

+

+

CO2

OH

O
O

)x (

(

)y

b)

(
H

)

+

O

+

H 2O2
OH

O

OR"

R

R'

OH

R"OH

+

OH

R'
R

Figure 5.1: Deprotection reactions of polymer systems studied: a) Poly(tbutoyxylcarbonylstyrene) (tBOC) b) Ketal-protected Poly(hydroxystyrene) (KRS-XE)

102

with a photoacid generator (PAG). Exposure to UV radiation forms the acid, which
reacts with the tBOC in the presence of heat and is converted to polyhydroxystyrene(PHOST). The tBOC film is initially relatively dry, but as the polymer is modified it
becomes more hydrophilic and rapidly absorbs water. The amount of water absorbedby the photoresist depends on the local humidity. Burns et al.9 have found that anincrease in the humidity will slow the deprotection reaction. This result was
unexpected. They hypothesize a “pH leveling” effect in which water reacts withphotogenerated acid, decreasing the strength of the acid and lowering the reaction
rate. This work provides an additional, complementary explanation.
The chemistry for the KRS-XE system is different.9,13 The initial exposure occurs
in vacuum, while the post exposure processing takes place in a clean room where thehumidity is carefully controlled. As shown in Figure 5.1b, a stoichiometric amount ofwater is required for the solubility switching reaction. The polymer is initiallyhydrophilic and absorbs the water needed for the deprotection. The activation energy
is low, so a post-exposure bake is unnecessary. Burns et al. found that increased
humidity in the local environment increases the reaction rate.9 Like tBOC, the KRSXE deprotection reaction results in the polymer PHOST. However, the resulting
films are not identical, because the composition and quality of the film depends on
the processing methods used. Also, the percentage of the polymer that is converted
may be different in each case.
Fourier-Transform Infrared (FTIR) spectroscopy has produced insights into thestructure of water in polymer films.4,7,14-19 The vibrational spectrum can be measured
in situ at a variety of humidities and provides details about the water-polymer

103

interactions and hydrogen bonding. In this investigation, the molecular information
obtained contributes to the understanding of transport through the polymer, which
ultimately helps explain the deprotection reaction kinetics.

5.2 Materials

Polymer solutions were prepared using a previously described method9 and spun
onto 25 mm sapphire wafers. The resulting tBOC photoresist film is 990 nm thick.
The modified tBOC, or PHOST sample, was prepared by exposing a tBOC film to a
100 mJ/cm2 dose of 257 nm ultraviolet light. The wafer was then baked at 100 °C for
90 seconds. The thickness of the resulting film is approximately 635 nm.
The KRS-XE photoresist was also spun onto 25 mm sapphire wafers, but to a
thickness of 370 nm. To prepare the modified polymer sample, the resist was
exposed to a dose of 100 mJ/cm2 of ultraviolet light. After 5 minutes of exposure toclean room conditions with controlled humidity, the thickness was 335 nm.

5.3 FTIR Instrument

Figure 5.2 shows the apparatus used for the experiments. It consists of amodified Bruker Optics IFS 66v/s Fourier Transform Infrared (FTIR) spectrometerwith a tungsten IR source and a liquid nitrogen cooled MCT (HgCdTe) detector. Boththe sample and instrument chambers can be pumped down to 2.0 kPa. The sample
chamber is isolated by vacuum flanges with sapphire windows to allow the passing of
the infrared radiation. Gas inlets and outlets were added to the faceplate of thesample chamber to allow the sample environment to be humidity controlled. Dry

104

Figure 5.2: Schematic diagram of FTIR spectrometer apparatus.

105

nitrogen is used to purge the sample. The nitrogen can be passed through a reservoir
containing water to produce wet nitrogen. Mixing different ratios of wet nitrogen anddry nitrogen produces the variety of humidities used in this work, which are recorded
by a thermohygrometer at the gas outlet.
Samples are loaded onto a holder with mounting positions for two samples.
An external manipulator can move samples into the IR beam, as well as move themout of the way to allow for a background scan. For the experiments performed here,
the two samples mounted at the same time are the modified and unmodified polymers
for either the tBOC or the KRS-XE system. The background scan is taken
immediately before or after the sample scan in order to best match the humidity
conditions and allow for elimination of the gas phase water features in the spectrum.
The final spectra, which show only the absorbance of the water sorbed into thepolymer, are obtained through the following subtraction:
 I0 
 I 0' 
A = log  − log '  ,
 I 
I 

(5.1)

where I 0 is the background spectrum under humid conditions, I is the spectrum of thesample in humid conditions, I 0' is the background spectrum in dry conditions, and I 'is the spectrum of the sample in dry conditions.

5.4 Results and Discussion - tBOC system

Spectra of water sorbed in the modified tBOC polymer were obtained at 10%,21%, 37%, 67% and 79%, all ± 2%, relative humidity (RH). Figure 5.3a shows the
OH stretching region of these spectra, which is the most sensitive region to hydrogen

106

a)

Absorbance

0.015

0.010

0.005

0.000

4000

3500

3000

2500
-1

Wavenumber (cm )
b)

3257

Absorbance

0.015

0.010

3623
3513

2953

0.005

0.000
3500

3000

-1

2500

Wavenumber (cm )Figure 5.3: Spectra of water absorbed by the modified tBOC system. a) Black lines show spectra
taken in modified polymer at 10, 21, 37, 67 and 79% RH. The arrow points in the direction ofincreasing humidity. b) Deconvoluted spectrum of OH stretching band at 67% RH. Four subbands, which represent four different water environments, are shown by dotted lines. The subband with a peak at 2953 cm-1 represents strongly-bound water molecules, 3257 cm-1 is mediumbound, 3513 cm-1 is weakly-bound, and 3623 cm-1 shows free water molecules.

107bonding (H-bonding), with the arrow pointing in the direction of increasing humidity.
The band can be deconvoluted into distinct sub-bands within the OH stretching band,
which reflect different types of water molecular environments.4,14,17,18,20 This type ofanalysis was performed on our sorbed water spectra. We chose four sub-bands anddesignated them strongly-bound, medium-bound, weakly-bound and free water. The
results for modified tBOC are shown in Figure 5.3b.
The lowest frequency band is commonly attributed to the most strongly boundwater molecules.4,7,14,17-21 This peak at 2953 cm-1 is due to water molecules H-bonded
onto specific polar sites along the polymer network. In the tBOC photoresist system,
the OH group formed during the deprotection would be a good candidate for a strong
H-bonding site. The strongly-bound peak is usually the broadest, due toheterogeneity of the bonding sites. The largest sub-band, found at 3257 cm,-1
corresponds to medium-strength hydrogen bonds. These water molecules are still
associated with the polymer chains, but they are not as tightly hydrogen-bonded as
the strongly bound ones. In the case of modified tBOC, we believe the medium andstrongly-bound bands are larger because of the hydrophilicity of the polymer. Theband at 3513 cm-1 shows weakly-bonded water molecules, and the highest frequency
peak at 3623 cm-1 is characteristic of free or unassociated water. These less-boundwater molecules can be found in the pores formed by spaces between the polymer
chains that make up the hole free volume. The bands are narrower, and because thewater molecules are relatively isolated, there is less heterogeneous broadening.
Water molecules may also form clusters in these spaces.

108

The fraction of each type of water is calculated by integrating the indicated subband and dividing by the area of the total OH stretching band from 2500 – 3750 cm -1.
However, it is not very straightforward to convert these fractions into actual amountsof water molecules. The molar absorptivity significantly increases with the degree ofhydrogen bonding, so the cross section would be different for each sub-band.7,14,17
Therefore, we look only at the change in each fraction with humidity to see how theinteraction inside the polymer network is affected when more water is present. The
fraction of each type of water present depends on various chemical and physicalproperties of the polymer, such as structure, cross-linking, molecular weight, poresize and density.4,14,16,19,22
The intensity of all four sub-bands, and therefore the amount of each type ofwater, increases with local humidity. However, the fraction of each of the four subbands changes with humidity in a different way, as clearly illustrated in Figure 5.4.
Error bars represent one standard deviation from multiple measurements. The
deviations are due to the reproducibility of the experiment, particularly because ofdrift in the humidity conditions. The accuracy of the spectral fit also plays a role.
The amounts of free and weakly bound water molecules increase more rapidly
than the medium and strongly-bound water. This is manifested by an increase in thefraction of these subbands with increasing humidity, as seen in Figure 5.4. The
fraction of medium-bound water remains fairly constant with humidity, and thefraction of strongly-bound water decreases.
Similar behavior has been observed in other polymers and can be explained by thefollowing considerations.3,7,8,14,17,18,23 Enthalpy mandates that at the lower

109

0.6

Free
Weakly-boundMedium-boundStrongly-bound

0.5

Fraction

0.4
0.3
0.2
0.1
0.0
0

10

20

30

40

50

60

70

80

90

100

% Relative Humidity

Figure 5.4: Fraction of total OH stretching band vs. relative humidity for the four sub-bands
representing strongly-bound (), medium-bound (), weakly-bound (▲) and free (●) watermolecules in the modified tBOC polymer sample of Figure 5.3.

110

humidities, when there is less water available, the absorbed water molecules would be
more strongly bound. Water hydrogen bonds to polar sites along the polymer chains.
As the sites for water association begin to saturate, the water molecules increasingly
tend towards locations in the pores formed by spaces between polymer chains andbegin to come together to form dimers and larger clusters.
When water forms clusters in polymer chains, the rates of transport of water andother species in the film can be decreased.8,17,23,24 Berger et al.3 found that in the caseof PHOST, diffusion is impeded when there is one water molecule present for every
two OH sites in the polymer. This is when the hydrogen bonding to hydrophilic sites
is saturated enough that water molecules begin to cluster together. The clustering ofwater molecules may also serve to slow the diffusion of the photoacid species.
Interaction with water could decrease the mobility of the large acid counter ion andlimit transport to active sites.25 Therefore, in addition to the dilution of the acid by
the water9, this limiting of transport within the polymer may be an additional cause
for the slowed reaction rate with increasing humidity reported by Burns, et al.
Similar experiments were carried out on unmodified tBOC films. The tBOC ishydrophobic and has been reported to take up less than one fourth as much water as
PHOST, the modified polymer.9 In our experiments, it was difficult to measure thespectra because of the small amounts of water present and the low signal-to-noise
ratio caused by error in the subtraction of the gas phase water signal.26 Therefore, wewere not able to carry out an analysis of the different water environments on thispolymer. We believe that the water molecules would be less strongly-bound than the111

waters in the modified polymer because of the lack of hydrophilic sites on thepolymer chain of unmodified tBOC.

5.5 Results and Discussion - KRS-XE

Spectra of the water in the KRS-XE system, measured at 12%, 21%, 41%, and
51% (all ± 2%) relative humidity, are shown in Figure 5.5a. In this system, theoriginal polymer needs to absorb water so that it is available as reactant in the
deprotection reaction. After the reaction is carried out, the polymer is slightly morehydrophilic and holds more water. This is illustrated in Figure 5.5a. The adjacentsolid and dotted lines are spectra of the unmodified and modified polymers at thesame humidity, and the arrow points in the direction of increasing humidity. The
absorbance is higher for the modified polymer, denoting a greater amount of water
present. The shape of the band is similar to the shape of the water band in themodified tBOC, as expected because of the similarity of the chemical structures ofthese polymers.
Figure 5.5b was constructed in a similar fashion to Figure 5.4 and illustrates thechanging shape of the water band with humidity. The KRS-XE system follows apattern similar to the tBOC system, in that the fractions of free and weakly boundwater seem to increase as the medium and strongly-bound water molecule fractions
decrease. For the unmodified polymer, there is a big change from 12%RH to
21%RH, and then the fractions stay fairly constant with humidity. This is aninteresting point because Burns et al. found that the deprotection reaction rate

112

a)

0.010

Absorbance

0.008

Unmodified KRS-XE
Modified KRS-XE

0.006

0.004

0.002

0.000

-0.002
3600

3400

3200

3000

-1

2800

2600

40

60

Wavenumber (cm )

b)

Free
Weakly-boundMedium-boundStrongly-bound

Fraction

0.7

c)
0.7

0.6

0.6

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0
0

20

40

60

80

20

80

% Relative Humidity

Figure 5.5: Results of KRS-XE experiments. a) Spectra of absorbed water taken at
12%, 21%, 41% and 51% relative humidity. Solid lines show spectra of the unmodified
polymer; dotted lines show the modified polymer. The arrow points in the direction ofincreasing humidity. b) Fraction of total OH stretching band vs. relative humidity forthe four sub-bands representing strongly-bound (), medium-bound (), weaklybound (▲)and free (●) water molecules for the unmodified polymer. c) Fraction of total
OH stretching band vs. relative humidity for the modified polymer.

113

increases with humidity until the humidity reaches 25%, and then it stays constant.9
Because water is a reactant in the deprotection, the leveling off of the reaction rate
can occur when there is an excess of water, meaning the polymer becomes thelimiting reagent. Transport could also play a role. As with the modified tBOC, athigher humidities the free and weakly-bound water molecule fractions are increasing
because the available hydrophilic sites are becoming saturated. Therefore, it ispossible that water molecules are clustering, which may slow diffusion of water
and/or the acid to available reactive sites.
In the modified KRS-XE polymer, the increase in free and weakly-bound water
species and the decrease in strongly-bound species continues with increasing
humidity beyond 21%RH. Another interesting aspect of Figure 5.5b is that in theunmodified polymer, the fraction of free water is greater than the fraction of weaklybound water. The opposite is true of the modified polymer. This is evidence that a
subtle change in the polymer structure can change the water-polymer interactions.

5.6 Comparison of Photoresists

Figure 5.6 shows spectra of water sorbed in three different polymer photoresists,
poly (methacrylic acid) (PMAA), KRS-XE and PHOST, all taken at 68% humidity.
The basic features of the spectra are very similar. All have the same four sub-bands
as presented in the previous sections. Therefore, the polymer-water interactions must
be very similar.
Figure 5.7 presents the structures of each of these photoresists. The common
aspect of their structures is an OH group off the polymer chain. This is additional

114

Normalized AbsorbancePMAA
KRS-XE
PHOST

0.00
4000

3500

3000

2500

Wavenumber (cm-1)

Figure 5.6: FTIR spectra of water sorbed in three different polymers taken at 68% humidity.

115

(

a) PMAA

CH3

)

OH

O

)x (

(

)y

b) KRS-XE
O

OR" OH

R

R'

(
c)

)

PHOST
OH

Figure 5.7: Structures of three polymer photoresists.

116evidence that it is the polar OH group on the polymer that attracts and binds the watermolecules. The KRS-XE water spectrum shows medium and strongly-bound peaks
that look slightly shifted from the peaks of the other two polymers. The structure ofthis polymer has C – O – C bonds that are not present in the other two photoresists.
Perhaps they have some influence on the binding of the water molecules.

5.7 Conclusions

This work has provided information regarding the water – polymer interactions inhydrated chemically amplified photoresists. The sorption of water in the polymer
films is found to be highly dependent on the humidity of the sample environment.
The OH stretching band, found at 2500 – 3750 cm,-1 deconvolutes into four subbands, which represent four different types of water present in the wet photoresists.
These water molecules undergo varying degrees of hydrogen bonding to the polymer
chains. The amount of each type of water is humidity dependent, with the fraction offree and weakly-bound water increasing, and the fraction of medium and stronglybound water decreasing, with increasing humidity. In the case of the tBOC
photoresist system, this increase in free and weakly bound water could indicate
slowed transport of acid to reaction sites on the polymer, resulting in a decreased
deprotection reaction rate. In the case of the unmodified KRS-XE photoresist,similar changes occur at low humidites, but the fractions stay relatively constant withincreasing humidity after 21%RH. This mirrors the trend of the deprotection reaction
rate with humidity, and it provides additional evidence that the polymer-water117interactions may affect transport and ultimately reaction dynamics within a polymer
network.

118

References for Chapter V(1)Buchold, R.; Nakladal, A.; Gerlach, G.; Herold, M.; Gauglitz, G.; Sahre, K.;

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(26)

Much of the noise comes from the subtraction of the gas phase water. If thelocal humidity conditions present during the measurement of the I and I0 spectra are
not exactly the same, subtraction noise due to gaseous species is introduced.
Performing more averages only gives a greater chance that the local humidity that the
sample is exposed to will drift and cause more noise.

120

Conclusions

Three methods have been used to study water uptake in polymer photoresist
films. Infrared Near Field Scanning Optical Microscopy (IR-NSOM) helped
determine the regions of the photoresist that absorbed water. Atomic ForceMicroscopy (AFM) was employed to further understand the water-induced
differential swelling of the photoresist pattern and changes in mechanical propertiesof the polymers. Fourier Transform Infrared (FTIR) spectroscopy measurements
were performed to characterize the interactions between the water molecules and thepolymer chains in the photoresist. This helped provide an explanation of deprotection
reaction kinetics.
An interesting direction for future work would be to combine the microscopy
and spectroscopy aspects. Micro-FTIR has been developed by other researchers andcurrently has a spatial resolution limit of about 10 µm.1,2 Unfortunately this is not
small enough to resolve the features of the photoresists studied in this work, but if themethod is improved enough to do so, measurements could provide spatially resolved
information on water-polymer interactions in different regions of the patterned
polymer.
Work is currently being done to improve the IR-NSOM. Because a large
obstacle is the fabrication of a reproducible fiber tip, an improved probe is being
investigated. Apertureless NSOM uses an AFM as a base instrument with ametallized commercial Si tip. Infrared radiation illuminates the tip and scatters on to
the sample with a small spot size. An optical image of the sample with spatial
121

resolution better than 30 nm can be obtained. The probe is more reliable and robust,so multiple experiments under a variety of conditions can be obtained, possibly
leading to more quantitative information on water sorption in polymers. There is also
improved resolution over the fiber-based IR-NSOM. However, it is still a challenge
to interpret the optical images, and many researchers are investigating the contrast
mechanisms at work.3-6
Another method under investigation which shows promise is Scanning
Transmission X-ray Microscopy (STXM). This technique boasts chemical
selectivity, high spatial resolution, and fast measurements. Investigations on latentimages of polymer photoresists have already shown spatial resolution below 50 nm.7
A goal is to use the technique to monitor acid diffusion during the post-exposurebake. With a few modifications, water diffusion through the film could also be
investigated. This method could contribute more quantitative information which
could aid in the understanding of photoresist processing and dynamics.

122

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