Thermal Modulation During Solvent Annealing of PSWEB
01merARC1
C
PDMSBlok
PDMS Block
Copolymer
MASSACHUSETTS INSTITUTE
. OF TECHNOLOLGY
by
JUN 0 8 2015
Annia Pan
LIBRARIES
Submitted to the Department of Material Science and Engineering in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Material Science and Engineering
at the
Massachusetts Institute of Technology
June 2015
Annia Pan 2015. All rights reserved.
The author hereby grants to MIT permission to reproduce and to distribute publicly paper and
electronic copies of this thesis document in whole or in part in any medium now known or
hereafter created.
Signature of Author ......................................................
Signature redacted
Annia Pan
Department of Materials Science and Engineering
May 1, 2015
Certified by .....................................................................
Signature redacted
Caroline Ross
Associate Head of the Department of Materials Science and Engineering
Toyota Professor of Materials Science and Engineering
Thesis Supervisor
Signature redacted
Accepted by........................................................
. V
Geoffrey Beach
Professor of Materials Science and Engineering
Chairman, Undergraduate Thesis Committee
Thermal Modulation During Solvent Annealing of PSb-PDMS Block Copolymer
by
Annia Pan
Submitted to the Department of Material Science and Engineering on May 1, 2015
in partial fulfillment of the requirements for the degree of
Bachelor of Science in Material Science and Engineering
Abstract
The self-assembly of block copolymers (BCP) has been a promising area of research for
nanolithography applications in microelectronics because of their ability to produce nano-scale
level periodic structures with long-range order. Ideal BCPs for generating these nano-scale
patterns fall within the strong segregation limit (SSL) and have a high interaction parameter to
drive BCP phase transitions. BCP morphologies can vary from equilibrium structures such as
spheres, cylinders, and gyroid, to metastable structures such as hexagonal perforated lamellar
(HPL). A variety of processing techniques including solvent vapor annealing (SVA) have been
developed in order to facilitate the phase transitions of BCPs from disordered to ordered states.
SVA parameters which can affect the final film morphology include the swelling thickness of the
film and solvent removal rate. Thermal modulation of the substrate was used to explore the
effects of rapid solvent evaporation during the annealing process on the morphologies of the
PS16-b-PDMS 37 system. Additional cycles of solvent update and film reswelling were introduced
into the annealing procedure to induce greater long-range ordering of film morphologies.
Although a range of morphologies were explored, there was special focus on developing a
applications.
for
nanolithography
structures
HPL
for
mono-layer
procedure
3
4
Acknowledgements
First, I would like to acknowledge Professor Caroline Ross who has given me this
opportunity to work in her laboratory. She has been incredibly encouraging of this project and
provided critical insight through this project. Secondly, I would like to thank Wubin Bai who has
been instrumental in helping me understand this project and develop this research project. He has
made this thesis an enjoyable experience, even when laboratory equipment did not. Additionally,
I would like to thank Kun-Hua Tu, Tao Huang, and Patrick Boisvert for assisting me in gathering
sample images, teaching me how to use the thermal modulation set-up, and training me on the
SEM, respectively. Further thanks must be given to Professor Apostolos Avgeropoulos from the
University of Ioannina for providing the polymer used in this thesis.
Finally, I would like to acknowledge my friends and family who have supported me as I
wrote this thesis, especially to the members of the Steam Caf6 Crew.
5
6
Contents
1. Introduction ....................................................................................................................................
11
1.1.2 Overview and Contents...................................................................................................................................11
1.2. Self-Assem bly Behavior of Block Co-polym ers ......................................................................
12
1.2.1 Introduction to Block Co-polym ers......................................................................................................
1.2.2 Block copolym er Phase Separation ......................................................................................................
1.2.3 Equilibrium and M etastable M orphologies ...........................................................................................
1.3 Solvent Vapor Annealing Process ................................................................................................
12
12
16
16
1.4. Interfacial Effects .................................................................................................................................
19
2. Thermal Modulation of Thin Film PSw-b-PDMS37 di-Block Co-Polymer ........
21
2.1 Experim ental Goals...............................................................................................................................
2.1.1 Sam ple Preparation..........................................................................................................................................21
2.1.2 Therm al M odulation During Solvent Vapor Annealing ...............................................................
2.2 Effect of Tem perature on Film Sw elling Behavior .....................................................................
2.2.1 Tem perature M odulation on Sw elling Ratio .....................................................................................
2.2.2 Film Desw elling Rate ......................................................................................................................................
2.3 SR and Film Thickness on M orphology..........................................................................................
2.3.1 SR Effects on M orphology..............................................................................................................................27
2.3.2 Film Thickness on M orphology...................................................................................................................30
2.3.3 Desw elling Rate..................................................................................................................................................31
2.3.4 Resw elling Cycles ..............................................................................................................................................
21
2.4 Achieving H PL M orphology................................................................................................................
34
2.4.1 Sw elling Procedure Effects............................................................................................................................34
2.4.2 Effect of Tem plated Substrates ...................................................................................................................
3. Conclusions & Future W ork .......................................................................................................
24
25
25
26
27
33
35
38
7
8
List of Figures
Figure 1 a) Phase diagram of di-block copolymer predicted by self-consistent mean field
theory. 20 b) Diagram depicting relationship between interaction parameter, XN, and relative
volume fraction,f c) Morphologies ranging from body-centered cubic to lamellar based on
14
the relative volume fraction of each co-polymer block. ..................................................
Figure 2 Model of solvent annealing process. This diagram depicts the interactions of the solvent
molecules with the block co-polymer (BCP) during the solvent annealing process. 0)
Casting: the BCP forms disordered structures after deposition on the substrate. 1) Swelling:
the film thickness grows as solvent enters the film and causes the chains to become more
mobile. 2) Annealing: chains are able to rearrange into energetically favorable
morphologies. 3) Quenching: solvent is removed from the film to reach a final thickness,
17
and this process may also alter the final morphologies. ..................................................
Figure 3 Templated silicon substrate with trenches. Using interference lithography, trenches
were etched with a lum spacing and the width of mesa-trenches varied from 125nm to
500nm. The trench depth is roughly 40nm. The substrates were first coated with SiO 2
(40nm) and then an anti-reflective coating (ARC) (10-15 nm). The two-step shape of
gratings was achieved by partially etched the top Si0 2 and transferred to ARC layer. These
substrates are significant for exploring films with different thickness due to the nature of the
22
trenches. ................................................................................................................................
Figure 4 Experimental Set-Up for thermal modulation during solvent annealing process. a) The
control box used to control the applied voltage. b) Samples were placed within the solvent
reservoir chamber on a platform which was heated by running a current to generate resistive
23
heating ...................................................................................................................................
Figure 5 Relationship between the applied voltage and temperature of substrate. The graph
shows the temperature of the substrate at different applied voltages. Additionally, increasing
applied voltage will decrease the swelling ratio for samples annealing in pure toluene or a
mixture of toluene and heptane in a ratio of 4 to 1. Even with different solvent conditions,
the behavior of the film follows similar curves. Courtesy of Tao Huang......................... 25
Figure 6 Effect of deswelling rate on sample morphology. Samples underwent deswelling after
swelling to an equilibrium SR. Three different temperatures were applied to various
samples in order to measure the deswelling rate. Temperature values were selected from the
range of temperatures associated with film thicknesses of 36nm and I00nm.................. 26
Figure 7 Morphologies of 45nm film from SR 1.2 - 2.0. Samples were annealed for lhr at
various swelling ratios before quenching. a) Sample was annealed at SR 1.2 and shows
mostly micelle structures. b) Sample was annealed at SR 1.4 shows the beginning of
lamellar cylindrical structures. c) Sample was annealed at SR 1.6 shows increasing lamellar
cylinders. d) Sample was annealed at SR 1.8 shows mostly lamellar cylinders. e) Sample
was annealed at SR 2.0 contains nearly only lamellar cylinders. f) Ratio of micelle to
28
lamellar structure decreases as the annealing SR increases.............................................
Figure 8 a-b) Two-step and c-d) three-step sample annealing. a-b) Sample underwent annealing
at SR 1.2 & 2.0 / 2.0 & 1.2 respectively for lhr during each step. The total annealing time
for the samples was 2h. c-d) Sample underwent annealing at SR 1.2 & 2.0 & 1.2 / 2.0 & 1.2
& 2.0 respectively for lhr during each step. The total annealing time was 3h. Observed
morphologies showed that annealing for lh at the lower SR demonstrated no significant
difference in ordering. However, comparison of morphologies between sample b and c and
9
sample d show a potential increase in reordering due to film deswelling and reswelling
during the annealing process.............................................................................................
29
Figure 9 Comparison of morphologies of 45nm and 40nm film at SR of 1.8 and 2.0. Samples
with a lower initial as-cast film thickness were able to achieve more ordered structures at
lower SR. All samples underwent annealing for lh at their respective SR. Additionally, the
40nm films were able to exhibit hexagonal perforated lamellar (HPL) structures. Due to
commensurability effects, the metastable HPL structure may be easier to form without the
energy penalty associated with the polymer chains stretching to a non-characteristic
sp acin g. .................................................................................................................................
31
Figure 10 Effect of deswelling rate on sample morphology. Using a range of power, samples
underwent different deswell rates. Differences in morphologies, especially in HPL
concentration, can be attributed to the shear stress caused by the rapid shrinking of the film.
...............................................................................................................................................
32
Figure 11 Effect of deswelling rate on sample morphology. Using a range of power, samples
underwent different rates of solvent removal. Differences in morphologies, especially in
HPL concentration, can be attributed to the shear stress caused by the rapid shrinking of the
film ........................................................................................................................................
33
Figure 12 Effect of reswelling cycles and rapid deswelling on film morphology on silicon
substrates. a) The sample which annealed with 3 swelling cycles showed few HPL
structures with multilayers of lamellar cylinders instead. b) HPL structures can be seen in
one layer on top of cylinders.............................................................................................
35
Figure 13 Effect of reswelling cycles and rapid deswelling on film morphology using DSA.
Samples were initially swelled to high SR and deswelled using 0.150W and were annealed
for 30 minutes. a) After deswelling, sample continued annealing at the equilibrium SR. b)
After the initial deswelling, sample was reswelled to reach a SR of 1.8 to continue
annealing in order to examine the effects of reswelling. c) Similar to b, sample underwent
reswelling to SR of 1.8. Additionally, the sample underwent two additional deswelling
cycles. The effect of additional deswell and reswell cycles generated a large area of HPL
structures. However, the film also dewetted and caused the film to non-uniformly coat the
substrate and caused completely coverage of some trenches. .........................................
36
List of Tables
Table 1 Deswelling Rate for Various Power Applied to Sample Substrates............................
Table 2 HPL-Cylinder Ratio and Equilibrium Thicknesses for Various Thermal Modulated
Substrates ..............................................................................................................................
26
32
10
1. Introduction
As current technology strives to deliver new and advanced features while simultaneously
minimizing the device volume to create smaller electronics, there is an increasing need for
sophisticated lithographic methods on the nano-scale for microelectronics manufacturing.'
Existing technologies for producing nano-scale patterns include photolithography, electron-beam
lithography, and nano-printing. However, these methods are unable to meet the demands of
large-scale manufacturing, as the lithographic techniques are unable to achieve the desired
resolution, have low through-put, or are extremely costly. These challenges have forced the
development of other lithographic techniques for nano-scale applications.
Block copolymers (BCP) have become an increasingly important tool for generating
periodic nano-scale structures with long-range order.2 The nature of the covalently bonded
polymer blocks causes phase separation of the BCP into self-assembled structures. Because the
processing methods are fairly cost-effective and simple compared to other lithographic
techniques, there is much interest in developing lithographic patterning techniques using BCPs
for microelectronic applications such as bit patterning for high density data storage and field
effect transistors.3 4
1.1.2 Overview and Contents
Several processing techniques have been developed to facilitate the reordering of BCPs to
achieve desired morphologies.
Significant research has explored solvent, thermal, or
solvothermal annealing techniques to develop a range of morphologies for a variety of
applications.5 During solvent annealing, one of the many experimental parameters that affect the
resulting morphologies is the thickness of the film as it swells. Conventional methods of
11
controlling film thickness adjust the partial pressures of solvent vapors within the chamber in
order to drive solvent adsorption or evaporation. The work of this thesis will focus primarily on
the effects of using thermal modulation of substrates to adjust the film thickness during solvent
annealing of polystyrene-b-polydimethylsiloxane (PS-b-PDMS) BCP. Additionally, this thesis
also aimed to determine how thermal modulation could create optimal conditions for generating
hexagonal perforated lamellar (HPL) structures as this morphology is advantageous for pattern
transferring for applications of dots or bit-patterning for data.
1.2. Self-Assembly Behavior of Block Co-polymers
1.2.1 Introduction to Block Co-polymers
Block copolymers (BCP) consist of two or more chains with distinct monomers and are
covalently bonded to form a polymer with distinct monomer blocks. BCPs can have two blocks
(di-block copolymers) or more (multi-block copolymers) with varying types of polymer
geometry. Though used for a wide variety of applications, BCPs have become an attractive tool
for nanolithographic applications as they are able to self-assemble into structures on the sub1Onm
level.6 Depending on the strength of immiscibility among the polymer blocks, BCPs are
able to form nano-scale patterns with feature sizes around several nanometers.
1.2.2 Block copolymer Phase Separation
The self-assembly behavior of block co-polymers is driven by the immiscibility of the
different blocks which would lead to separation in a polymer mixture. However, using a di-block
copolymer as the most basic example, the two polymer blocks, A and B, are covalently tethered
to one another, which prevents complete separation. Instead, the polymer blocks must rearrange
to find a thermodynamically favorable morphology. The BCP behavior can be described by the
Flory-Huggins equation shown in equation (1), which includes enthalpic and entropic terms:
12
AGmix =
kbT
1-ln (fA) +
NA
1
NB
In
(JB) + fAfB
(1
where X is the Flory-Huggins interaction parameter, N is the degree of polymerization, and f is
the composition of the BCP. 7
The Flory-Huggins interaction parameter, X, describes the energy penalty of interactions
between the A and B blocks of the BCP and is also inversely affected by temperature as
described in equation (2).
T+
b
(2)
Low X BCPs will favor mixing between the polymer blocks, where high X BCPs will
preferentially segregate into regions of different polymers because of the higher energy penalty.
Thus, by taking advantage of the high X between certain polymers, BCPs can self assemble into
regions with distinct polymers on the nano-scale order. Although the high x parameter provides
the driving force for polymer block separation, it is also an obstacle when attempting to form
ordered states. One of the challenges of using high x parameter BCP include difficulties in
generating rearranging polymer blocks from the as-cast BCPs morphologies as the difflisivity of
the polymer chains decreases. 7 When the polymer is cast onto a substrate, the film morphology is
in a poorly-ordered microphase-separated structure. In order to achieve ordered structures, an
annealing process must be taken either by raising system temperature5 or adding solvent
molecules8 in order to facilitate the polymer chain self-assembly to a energetically-favorable
state. Additionally, when considering defect formation during the annealing process, high X
polymers are preferred because there is a greater driving force preventing defect nucleation.
The second BCP parameter which affects the film morphologies is N, the degree of
polymerization or the number of monomers present in the polymer chain. This term is important
13
as it relates to the configurational entropic free energy term of the system. Additionally, N
affects the domain size of structures within the final morphologies. The relationship between N
and the polymer period is as follows in equation (3):
LO ~ aN3x'
(3)
Thus, N is an important parameter for controlling the size of the final structures, which can
present segregation challenges when working with small BCPs.
Finally, the third BCP parameter which affects the phase behavior and also contributes to
the entropic component of the Flory-Huggins equation is f the composition of the BCP. The
relative volume ratio of BCPs is important in dictating the equilibrium morphologies of the
diblock BCPs. Equilibrium morphologies can range from spheres, cylinders, gyroid, or lamellar
structures, depending on f When the relative volume fractions of the polymer blocks are equal
and the blocks within the polymer are symmetric, the BCP system forms lamellar structures. The
s
s
80
-
I
C
C
L7
Z 40.
Z 40
PL
G
i
G
20
1
(b) ao
(a) Rn.
CPS
20
CPS
Disordered
Dlsordered
0
0.2
0.4
0.6
08
0
10
0,2
0.4 fA
0,6
0.8
1.0
fA
(c)
S
u
L
CS
2
Figure 1 a) Phase diagram of di-block copolymer predicted by self-consistent mean field theory. b) Diagram depicting
relationship between interaction parameter, XN, and relative volume fraction,f. c) Morphologies ranging from body-
centered cubic to lamellar based on the relative volume fraction of each co-polymer block.
14
volume fraction has a large impact on the final BCP morphology because of the interfacial
interactions between the phase-separated blocks. 7 When the volume fractions are not equal, there
is a pressure differential at the interface between the immiscible blocks as one side experiences
more stress in a lamellar structure. Thus, in order to reach a more thermodynamically favorable
state, the BCP undergoes a phase transition to morphologies such as hexagonal cylinders or
body-centered cubic spheres.7
When modeling the behavior of BCPs, the interaction parameter, XN, is used to describe
the microphase separation behavior of BCPs, and the term incorporates both the enthalpic and
entropic contributions to the free energy. Used to define segregation strength, values of XN
greater 10 define the strong segregation limits (SSL), and value XN less than 1 represent the
disordered region.7 The XN also impacts the order-disorder transition (ODT) of BCP phase
behavior, such that when xN increases, the ODT of a polymer also increases.'
In order to achieve microphase separation and achieve long-range periodicity, optimal
BCPs must remain in the SSL region. However, as mentioned above, the domain size of final
structures correlates with N. Thus, when selecting a BCP to generate nano-scale structures, the N
will be lower. In order to remain in the SSL, it is important that the x value is very high to
compensate for the lower N.
In addition to these three parameters, the thickness of the film is important when
examining BCP morphologies. For thin-film processing, the commensurability of the BCP can
impact the morphology of the film, especially at the polymer/air interface for soft confined
films. 5 Further discussion of interfacial effects will follow in section 1.4. Each BCP molecule has
a characteristic length, LO, associated with an energetically favorable molecule conformation.
However, if the thickness of a film, h, is not an integer multiple of Lo, then the BCP is
15
incommensurate and must stretch or compress in order to fit this spacing.9 Thus, this
energetically unfavorable state forces surface structures along the polymer/air interface and can
modify the equilibrium bulk morphology.' 0
1.2.3 Equilibrium and Metastable Morphologies
As mentioned above, BCPs can form equilibrium structures such as spheres, cylinders,
gyroids, and lamellar structures. However, other morphologies such as the hexagonal perforated
lamellar (HPL) structure may appear during the annealing process but are not as
thermodynamically stable. Previous experimental work has shown that the presence of these
metastable states may result from mechanical shear, as the HPL structures appear after the BCPs
underwent large shear." Additionally, in di-block systems, HPL structures may occur during low
temperature processing and transition into the more thermodynamically stable double gyroid
phase at elevated temperatures. Because of the elevated temperature, it is assumed that the
energy barrier to the double gyroid state becomes lower than that of the HPL structures, unlike in
the low temperature regime. Thus, the processing temperature can be lowered to a more
favorable range for inducing HPL structures.
1.3 Solvent Vapor Annealing Process
Solvent vapor annealing (SVA) is a technique where the polymer film absorbs solvent in
a vapor form and is a powerful method for tuning the BCP characteristics. The SVA process
provides several functions to address problems of the disordered as-cast film morphologies. First,
as the film absorbs the solvent, several characteristics, such as the X parameter and relative
polymer volume fractions, can be changed depending on the solvent mixture. These two
characteristics are important in dictating the resulting morphology, and thus, the ability to
manipulate solvent mixture to control morphologies is a powerful tool.' 2 Secondly, the swelling
16
of the film affects the characteristic thickness of the film and the commensurability of the
polymers, which also impacts the formation of target morphologies. Discussion of film thickness
and commensurability will follow in later sections that describe the polymer used in this thesis.
Thirdly, the solvent affects the interaction between the BCP and the free surface of the film.
Lastly, as the film swells, the glass transition temperature (Tg) of the polymer lowers, which
increases mobility of the chains and allows for rearrangement of the polymer blocks, because the
chain diffusivity scales with the interaction parameter exponentially. 5 This allows solvent
annealing to occur at lower temperatures than thermal annealing while still achieving block
rearrangement.
The solvent annealing process comprises of three stages: swelling, annealing, and
0) Casting
1) Swellng
Key
D
2) Annealing
Toluene
0 0
W
3) Quenching
*
W
Heptane
Figure 2 Model of solvent annealing process. This diagram depicts the interactions of the solvent molecules with the block
co-polymer (BCP) during the solvent annealing process. 0) Casting: the BCP forms disordered structures after deposition on
the substrate. 1) Swelling: the film thickness grows as solvent enters the film and causes the chains to become more mobile.
2) Annealing: chains are able to rearrange into energetically favorable morphologies. 3) Quenching: solvent is removed
from the film to reach a final thickness, and this process may also alter the final morphologies.
17
quenching. First, the sample undergoes an initial swelling process, where the film first absorbs
the solvent vapor. Secondly, the annealing stage describes the process in which the polymer
chains undergo reordering. Finally, the process of removing solvent molecules from the film is
called quenching.13 Each stage can profoundly affect the resulting morphology, as the initial
swelling rate can control the kinetics of the polymer system, and the quench rate is crucial in
preserving any morphologies created during annealing.
During the annealing process, there are several parameters that can be used to tune
sample morphologies. Typical solvents used in the annealing process include toluene and
chloroform, which are able to diffusive effectively into the PS-PDMS polymer blocks. The
volume of each solvent absorbed can vary based on each polymer, as some solvents have a
higher affinity for certain polymers than others. As mentioned above, as the film absorbs more
solvent, the x value of specific polymer blocks will continue to decrease, which lowers the ODT
barrier and encourages polymer block rearrangement. Additionally, because most solvents are
typically not equally diffusible into the different polymer blocks, the different blocks within a
BCP may swell differently depending on how preferentially the solvent is absorbed into one
polymer over another. Introducing a highly-preferential solvent into the annealing environment
can promote further phase separation between the blocks and encourage transitions between
different morphologies." Thus, by using a mixture of solvents which are somewhat selective in
the absorption process, the relative volume ratio of the BCP during the annealing process can be
significantly different than the volume ratio of the as-cast polymer. 3 Thus, the BCP can
rearrange into non-bulk morphologies by changing the ratio of solvent vapors that are exposed to
15 6
the film and can be manipulated to encourage the formation of certain morphologies. '1
18
Another important parameter of the SVA process is the overall annealing time that is
required to reach an equilibrium structure. For nanolithography applications in manufacturing of
microelectronics, it would be ideal to anneal the samples for as short of a time as possible. The
annealing time is affected by the diffusivity and x parameter of the film, as increasing both
would decrease the overall time needed to reach equilibrium. However, creating high chain
mobility and low x parameter may lead to forming disordered or other non-target morphologies
with long-range order.' 2 Thus, there is a trade-off between the processing time and the effective
ordering of the sample, as longer annealing times have shown increased order on the longdistance scale.8
Although solvent annealing presents several methods to affect the resulting film
morphologies, the process of solvent annealing is still not well understood as the specific
behavior of the polymer chains during the ordering process is incredibly complex. Rather, the
interaction of polymer chains during the solvent annealing process has been described as a black
box, and thus, monitoring of the SVA process involves measuring other parameters like film
thickness using techniques such as spectral reflectometry. However, groups such as Ross et. al
have been developing self-consistent field theory (SCFT) models to describe some of the
interactions among polymers and solvent vapor particles during the annealing process.' 3
1.4. Interfacial Effects
Additionally, there are different types of confinement which have an impact on ordering
within BCP thin films.5 Focusing on soft-confined films, there are two different interfaces, the
substrate-polymer and polymer-air interfaces, which can affect the bulk morphology of the film.
The polymer with a lower surface energy preferentially forms at these two interfaces. Thus, some
groups have explored how substrate parameters such as polymer brush coating or substrate
19
lithography can guide the polymer reordering during annealing. 9 Using interference lithography,
trenches can be engraved on the surface of the substrate in order to explore the effects of
polymer confinement on the resulting film morphologies. With the more sophisticated mesatrench, the deposited film will vary in thickness and could affect the difference in structures as
well.
Polymer brush layers have also been explored as a method of altering the interfacial
effects of the substrate surface. By using a brush layer, there will be preferential formation of one
polymer block at the substrate surface. Thus, the brush layer can affect the commensurate
thickness of the film. Although this thesis will not explore the effects of using a brush layer, they
can be another effective parameter for controlling the resultant film morphologies.
20
2. Thermal Modulation of Thin Film PS16-b-PDMS 37 diBlock Co-Polymer
2.1 Experimental Goals
Using the solvent vapor annealing process to facilitate the formation of ordered
morphologies, there are several parameters that can be controlled to achieve the target
morphology. Previous work on SVA using both reservoir and flow systems17 have explored the
effects of swelling ratio on the film morphology. However, in order to manipulate the swelling
ratio of the film, the relative partial pressures of the solvent vapor are altered usually with a
carrier gas (such as nitrogen) to control the mass flow of the system and maintain equilibrium
conditions of the solvent vapor environment. There are several advantages to using thermal
modulation of the substrate rather than a carrier gas to control the film swelling. First, thermal
modulation provides a simpler method to controlling the swelling ratio. When using the carrier
gas flow system, it can be more difficult to achieve a target swelling ratio as the entire chamber
must reach steady state along with the film. Secondly, the rate of change in the film thickness
can be quicker when the substrate is heated as opposed to changes in the solvent vapor pressures.
Additionally, though this thesis does not specifically explore this effect, the temperature increase
(though no more than 5 to 10 degrees Celsius) could have some effect on the diffusivity and
behavior of the polymer chains.
2.1.1 Sample Preparation
The
di-block
BCP
explored
in
this
thesis
consisted
of
a
polystyrene6-
polydimethylsiloxane37 (PS 16-b-PDMS 37) with a molecular weight of 53 kg/mol and a relative
volume fraction, f = 0.4. PS and PDMS are highly immiscible with a X parameter of 0.2718 and,
thus, are an attractive combination for self-assembly applications. Additionally, the combination
of PS and PDMS provides good etch resistance and etch selectivity which makes it ideal for
preferentially etching one polymer such that the film can be used for future nanolithography
applications.12 The PS-b-PDMS polymer was dissolved at 1 wt % cyclohexane and stored at
room temperature.
To prepare the thin film samples, PS-b-PDMS was spin-cast onto uncoated silicon
substrates and uncoated double trenched substrates to examine the morphologies of selfassembled and directed self-assembled structures respectively. Substrate sizes were roughly lcm
by lcm such that they could rest on the resistive heating bar completely. Figure 3 shows an
image of the template substrate used for directed self-assembly. For the trenched samples, the
trench width was lum, and the mesa-trenches width ranged from 125-500nm. The initial
Mxg(4w5P*oIO@
ddig DWs
I ime
Dit 4M
Figure 3 Templated silicon substrate with trenches. Using
interference lithography, trenches were etched with a lum spacing
and the width of mesa-trenches varied from 125nm to 500nm. The
trench depth is roughly 40nm. The substrates were first coated with
Si0 2 (40nm) and then an anti-reflective coating (ARC) (10-15 nm).
The two-step shape of gratings was achieved by partially etched
the top SiO2 and transferred to ARC layer. These substrates are
significant for exploring films with different thickness due to the
nature of the trenches.
22
thickness of the film is determined by this spin-cast step, and a range of film thicknesses was
explored to determine the effects of commensurability on morphologies. Spin-cast speeds were
3500, 4000 and 4500 rpm respectively for 45, 40 and 36nm thick films.
Once the samples were prepared to the target thickness, the substrates were placed into
the solvent annealing chamber. There are several types of systems to deliver solvent vapor to the
sample, such as a flow or reservoir system, each with distinct advantages and disadvantages.
Although a flow solvent vapor system allows for finer tuning of the partial pressures of the gases,
a reservoir system can provide greater vapor pressure, which is advantageous for swelling films
to SR greater than 2. Thus, for the purposes of examining the effects of the film deswelling rate
from high SR, a reservoir system was used for all solvent annealing experiments. Figure 4 shows
the reservoir chamber used, which was roughly 8cm in diameter and 12cm tall.
For the solvent vapor solution, a mixture of toluene and heptane was used because of
Figure 4 Experimental Set-Up for thermal modulation during solvent annealing process. a) The control box used to
control the applied voltage. b) Samples were placed within the solvent reservoir chamber on a platform which was heated by
running a current to generate resistive heating.
23
their relative selectivity for PS and PDMS respectively. Additionally, heptane is highly
preferential for PDMS and can further promote the phase separation of the two polymers during
the annealing process. A ratio of three parts of toluene to two parts of heptane was used and 3ml
of the toluene-heptane mixture was used to swell the sample to SR of around 2. In order to
achieve higher SR, a total solvent volume of 5ml was used to create a more saturated solvent
environment.
After annealing, the samples were etched using a reactive ion etching (RIE) technique to
expose the film morphology. As mentioned previously, due to the effects of substrate surface
interactions, the top layer of the film consists of PS during the annealing process. Because 02 ion
gas selectively etches PS, the RIE method is able to expose the film morphology without
damaging the PDMS layer. Sample morphologies were imaged using either a scanning electron
microscope (SEM) or helium ion microscope (HIM).
2.1.2 Thermal Modulation During Solvent Vapor Annealing
During the annealing process, there are several techniques that can be used to control the
thickness of the sample film, such as using nitrogen to adjust the partial pressures of the various
solvent vapors within the chamber. However, this thesis focuses on controlling the swelling ratio
using thermal modulation of the film substrate. Samples were placed on a metal platform which
was connected to a power source. The substrate temperature was modulated using resistive
heating by running a current through the platform. Figure 4 shows the relationship between the
applied power and temperature of the platform. Sample thickness was measured using
ultraviolet-visible spectrophotometry (UV-Vis).
Thermal modulation was selected for several objectives. First, in order to examine the
effects of varying the SR on sample morphology, thermal modulation was used to change the
24
film thickness during the annealing process. Second, thermal modulation was used to explore the
relationship between the deswelling rate during annealing and the resulting sample morphology.
Third, to examine how the polymer blocks behave under stress during the annealing process,
samples underwent repeated deswelling and reswelling cycles via thermal modulation.
For all the samples in this thesis, the platforms were not preheated such that all samples
were able to initially swell at similar rates in order to isolate the effects of the annealing
procedure on the film morphologies, as the initial swelling rate during the overall annealing
process could also have an impact on the final morphology.
2.2 Effect of Temperature on Film Swelling Behavior
2.2.1 Temperature Modulation on Swelling Ratio
After the initial swelling period, the equilibrium film SR can be adjusted by heating the
substrate to drive solvent from the film. Figure 5 shows the effects of temperature on the SR
50
2.80
-- Temperature
-*Swelling Ratio in T:H=4:1
2.60
4'-Swelling Ratio in Toluene
2.40
2.20
-40
0)0
2.00
I-
1.80
E
~30
1.60
1.40
25
1.20
. .............
1.00
20
-
0
1
2
3
4
5
6
Voltage (V)
Figure 5 Relationship between the applied voltage and temperature of substrate. The graph shows the
temperature of the substrate at different applied voltages. Additionally, increasing applied voltage will decrease the
swelling ratio for samples annealing in pure toluene or a mixture of toluene and heptane in a ratio of 4 to 1. Even
with different solvent conditions, the behavior of the film follows similar curves. Courtesy of Tao Huang.
25
during annealing for samples with an initial thickness of 45nm and 40nm. Thus, regardless of
initial film thickness, there is an associated film thickness at each temperature value. This can be
significantly advantageous for developing annealing procedures as it provides an easy method to
control the film thickness consistently. Consequent experiments will discuss the effects of
multiple SR annealing steps on the resultant film morphology.
2.2.2 Film Deswelling Rate
3
,
-
-,-
7-
--
-
2.s
0.
1*
0
1
2
3
4
Time (min)
of deswelling rate on sample morphology. Samples underwent deswelling after swelling to an equilibrium
SR. Three different temperatures were applied to various samples in order to measure the deswelling rate. Temperature
values were selected from the range of temperatures associated with film thicknesses of 36nm and lO0nm.
Figure 6 Effect
Voltage (V)
_ ____
___ ___ ___
___ _______
___ ___
_
Sample A
Sample B
Sample C
Current (A)
__ ___ ___ ___
Power (W)
Deswell Rate
26.5 +/- 1.2
102.5 +/- 2.3
174.5 +/- 14.6
(nm/s)
0.9
0.02
0.0 18
1.7
0.04
0.06
0.068
0.15
2.5
Table 1 Deswelling Rate for Various Power Applied to Sample Substrates
The application of heat to the substrate can control the equilibrium SR during the
annealing process, but because the kinetics of the solvent vapor in the system can have a major
impact on the resultant film morphologies, it was important to examine the effects of temperature
on the deswelling rate of the films. Figure 6 shows the swelling behavior of samples which were
26
heated after reaching an equilibrium swelling ratio of around 2.8. The deswelling rate for each
sample was derived from the linear region of the curve shown in Figure 6 in order to describe the
initial film response to the applied heat before reaching an equilibrium thickness. Samples which
were heated to higher temperatures experienced faster initial deswelling, and the film thicknesses
were very responsive to this thermal application. Previous work has shown that flow systems
may take up to 15 minutes to reach steady state which would have an effect on the equilibrium
swelling ratio." Thus, this thermal application demonstrates a method to control the deswelling
rate without the steady state concerns associated with using a carrier gas to evaporate solvent
from the sample. Because the rate at which solvent is removed from the film has been shown to
affect the ordering within samples after annealing," this type of thermal application can be a
powerful method for rapid solvent removable. The observed morphologies for these samples will
be discussed in section 2.3.3.
2.3 SR and Film Thickness on Morphology
2.3.1 SR Effects on Morphology
Figure 7 shows the resulting morphologies of the PS-PDMS sample after annealing for
lh at various swelling ratios with an initial thickness of 45 nm. There was little variation in the
critical pitch length, which was roughly 12nm, and the diameter of the micelle and lamellar
structures were roughly 28nm. Although there was little change in these characteristic lengths for
the different SRs, there is a clear difference in the ordering of the samples. Micelle morphologies
are similar to the typical disordered state of initial as-cast films where polymer blocks may
exhibit some mixing and form micelle-like structures with rough edges. The ratio of micelles to
cylinder structures decreases as the SR increases because at high SR, xN decreases and promotes
chain mobility. As a result, the micelles are able to rearrange into the more-ordered cylinder
27
Effect of SR on Micelle-LamellarPhase Transition
0.8
O.6
-
% 0.4
-
- --
1.4
1.6
0.2
0:
1
1.2
1.8
2
Swelling Ratio
Figure 7 Morphologies of 45nm film from SR 1.2 - 2.0. Samples were annealed for lhr at various swelling ratios
before quenching. a) Sample was annealed at SR 1.2 and shows mostly micelle structures. b) Sample was annealed at SR
1.4 shows the beginning of lamellar cylindrical structures. c) Sample was annealed at SR 1.6 shows increasing lamellar
cylinders. d) Sample was annealed at SR 1.8 shows mostly lamellar cylinders. e) Sample was annealed at SR 2.0 contains
nearly only lamellar cylinders. f) Ratio of micelle to lamellar structure decreases as the annealing SR increases.
structures faster.
Thus, even though the samples all had the same characteristic starting
thickness, annealing at high SR can increase the effectiveness of the SVA process.
In order to explore the potential effects of various SR on the film morphology, a stepwise annealing process was used such that samples were annealed for lh at various SR during a
single annealing run. The motivation behind these experiments was to explore how the BCPs
undergo the ordering process during SVA and whether altering the SR during the annealing
process could induce a change in morphology after the BCP has already undergone reordering.
Using the resistive heating to modulate the substrate temperature and film thickness, samples
were annealed with a two-step and three-step SR procedure, where the two-step procedure
transitioned from a high to low SR and vice versa, and the three-step procedure included a high
to low to high and vice versa. The as-cast thickness of the sample was 45nm.
28
2.2
2
1.8
1.6
1.4
1.2
0.8
0
20
40
60
80
100
120
140
2.2
160
180
----------------
2
1.6
1.4
~
1.20
0.8-------------.
20
0
C
2.2
--
40
60
80
100
120
140
160
180
___
2
-q
1.8
--
1.6
-- -----------------
------
1.4
-------- ----------------
------- -
--
-
1.2
1
0.8
o
20
40
60
80
100
120
140
160
180
2.2
2
1.8
---
1.4
-
-- --
-
1.6
1.2
0.
0
20
40
60
80
100
120
140
160
180
Time
Figure 8 a-b) Two-step and c-d) three-step sample annealing. a-b) Sample underwent annealing at SR 1.2 & 2.0 / 2.0
& 1.2 respectively for lhr during each step. The total annealing time for the samples was 2h. c-d) Sample underwent
annealing at SR 1.2 & 2.0 & 1.2 / 2.0 & 1.2 & 2.0 respectively for Ihr during each step. The total annealing time was 3h.
Observed morphologies showed that annealing for lh at the lower SR demonstrated no significant difference in ordering.
However, comparison of morphologies between sample b and c and sample d show a potential increase in reordering due
to film deswelling and reswelling during the annealing process.
29
Rather than exhibiting hysteric effects where samples contained the morphology
associated with the SR of the last annealing step, the samples retained morphologies formed
during the annealing step where the SR had the lowest xN. Figure 8a shows the image of a
sample that underwent annealing at SR 1.2 for lhr and then SR 2.0 for lhr, and the morphologies
are very similar to that of a sample annealed for lhr at SR 2.0. The morphologies of Figure 8b
and 8c are also very similar, where the procedural difference was an initial lhr anneal at SR 1.2
for 8c. Thus, it can be concluded that annealing at SR 1.2 during the stepwise function was not
effective in allowing the film to form ordered structures, presumably due to a relatively high XN
value at that SR. However, the morphologies of the three-step sample with an initial SR of 2.0
showed significantly more ordered cylinder structures. Thus, it was hypothesized that the stepwise annealing procedure introduced stresses on the polymer chains which may have aided the
ordering process. Additionally, the sample annealed for a longer time at SR 2.0, which may have
also contributed to the increased order.
2.3.
TFilm ThPLkness nn
1drph1lmg
Figure 9 shows the effects of the as-cast thickness on the resulting morphology of the
sample after annealing. All films were annealed for lhr at a specific SR. A comparison of the
films with initial thicknesses of 45nm and 40nm shows a difference in morphology after
annealing at the same SR. At a SR of 1.8, the 40nm films were able to achieve a higher ratio of
cylinder structures than the 45nm films, and at a SR of 2.0, the 40nm films contained some
regions of HPL structures as well. The difference in morphology is likely due to
commensurability effects, as the characteristic thickness of the PS16-b-PDMS37 BCP is 36nm.
Because the BCP experiences increased stretching within the 45nm system, this may impact the
30
ability of polymers to rearrange as they pay an energy penalty, and ordering requires higher SR
to further lower the XN of the system.
c) 40 nm, SR =1.8
d) 40 nm, SR =2.0
Figure 9 Comparison of morphologies of 45nm and 40nm film at SR of 1.8 and 2.0. Samples with a lower
initial as-cast film thickness were able to achieve more ordered structures at lower SR. All samples underwent
annealing for lh at their respective SR. Additionally, the 40nm films were able to exhibit hexagonal perforated
lamellar (HPL) structures. Due to commensurability effects, the metastable HPL structure may be easier to form
without the energy penalty associated with the polymer chains stretching to a non-characteristic spacing.
2.3.3 Deswelling Rate
Even though the equilibrium SR varied for the films, the samples did not undergo
annealing and were immediately quenched after reaching their equilibrium thickness. All
morphologies appeared to be a mixture of cylinders and HPL structures, and samples that
deswelled faster have a greater area of HPL structures and the larger domain sizes as well. There
are several possibilities for this phenomenon. Previous work done by the Fredrickson group
discusses the effects of rapid solvent evaporation on the formation and growth of cylinder
31
structures. Using dynamic field theory simulations to describe the behavior of the film as the
solvent evaporates, fast solvent removal for systems with a large XN lead to the formation of
cylinder structures, whereas systems with a lower values XN are more prone to forming spheres
(which can be interpreted as inverted version of HPL structures). Thus, the formation of HPL
structures can be explained by the initial swelling step which allowed the system to reach lower
values of XN as the SR exceeded 2.8.
Table 2 HPL-Cylinder Ratio and Equilibrium Thicknesses for Various Thermal Modulated Substrates
Power (W)
HPL-Cylinder
Equilibrium
Ratio
Thickness
(nm)
85 +/- 0.8
61+/- 0.9
49+/- 0.7
Sample A
Sample B
0.018
0.14 +/- 0.1
0.068
0.21 +/- 0.09
Sample C
0.150
0.47 +/- 0.1
Figure 10 Effect of deswelling rate on sample morphology. Using a range of power, samples
underwent different deswell rates. Differences in morphologies, especially in HPL concentration,
can be attributed to the shear stress caused by the rapid shrinking of the film.
-
25
0
en
b
CC
0.5
0
1
2
3
4
Time (min)
32
However, this effect does not explain the larger HPL domain sizes present in the sample
that underwent the fastest solvent removal according to the work done by the Fredrickson group.
Instead, a potential reason for the prevalence of HPL structures within the sample is the fact that
the samples reached different equilibrium thicknesses after the initial deswelling rate, such that
the fastest deswelling rate was also associated with the lowest equilibrium thickness. Even
though the samples did not anneal for very long before quenching, the difference in the
equilibrium swelling thickness could account for the difference in morphology as the induced
HPL structures could have transitioned into the more stable cylinder structure at greater SR
values.
2.3.4 Reswelling Cycles
To observe the effects of film deswelling and reswelling during the annealing process,
Figure 11 shows samples that underwent multiple swelling cycles. Additionally, the as-cast film
22
..
..............
...
2
=
1.24,
1
Figure 11 Effect of deswelling rate on sample morphology. Using a range of power, samples underwent
different rates of solvent removal. Differences in morphologies, especially in HPL concentration, can be
attributed to the shear stress caused by the rapid shrinking of the film.
33
thickness was 36nm in order to be commensurate and encourage the formation of the metastable
HPL structures. Figure 1la-c show the morphologies of films that underwent 0, 1, or 3
reswelling cycles. Although there were no HPL structures exhibited and only cylinder structures
formed, the number of reswelling cycles does have an impact the ordering of the system, which
can be described by the correlation length. Defined as the number of cylinder structures that
remain parallel in relation to each other, the correlation length was largely improved with greater
reswelling cycles. For the sample which underwent the most number of reswell cycles, the
correlation length for number of cylinder structures ranges up to 9 and is only about 4 for the
sample without any reswell cycles. Additionally, an increase in the number of reswell cycles also
lengthens the in-plane cylinder structures, which is another measure of increased ordering.
2.4 Achieving HPL Morphology
2.4.1 Swelling Procedure Effects
Previous research has shown that the metastable HPL state can be induced by the
presence of mechanical shear." As mentioned in section 2.3.3, rapid deswelling from a high SR
was able to create many regions of HPL structures. It was hypothesized that by introducing
multiple deswelling and reswelling cycles into the annealing procedure after a rapid deswelling,
the BCP would be more likely to form HPL structures with long-range order. In Figure 12, there
are clear effects of introducing reswelling cycles into the procedure, as the morphology of
sample 12a shows more ordering than the 12b and contains cylinder structures. However, more
HPL structures were exhibited in sample 12b, which did not undergo any reswelling. This
behavior can be explained by the fact that HPL structures could have formed during the initial
period of deswelling, but subsequent reswelling resulted in the formation of more energetically
stable structures as the chains continued to rearrange during the annealing process. Because the
34
samples were annealed at higher SR values after the initial deswelling, the HPL structures may
have transitioned into cylinders and consequently, additional reswelling cycles would not have
been able to induce long-range ordering of HPL structures.
b)
a) 3 reswell
0
reswell
2.8
2.6
2.4
--------------
2.2
-
o
2+
''I
.~4
b
1.2
0.8.
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34
Time Elapsed After Deswelling (min)
Figure 12 Effect of reswelling cycles and rapid deswelling on film
morphology on silicon substrates. a) The sample which annealed with 3
swelling cycles showed few HPL structures with multilayers of lamellar
cylinders instead. b) HPL structures can be seen in one layer on top of cylinders
2.4.2 Effect of Templated Substrates
The use of templates during the annealing process can be very effective for forming
target film morphologies. Figure 13 contains SEM images from samples that were able to
achieve HPL structures regardless of the number of reswelling cycles, unlike the samples in
Figure 12 with identical annealing procedures. For the sample which did not undergo reswelling
cycles, the film showed some areas with HPL structures and was monolayer. Samples that did
undergo reswelling had greater areas of HPL structures, but because they reswelled and
35
underwent annealing at elevated SRs, the resulting film contains multilayers with mixture a
mixture of HPL and in-plane cylinders. The differences in morphology between the selfassembled samples and the directed self-assembled (DSA) structures could result from the extra
constraints provided by the trenched samples. Thus, during reswelling, the constrained behavior
of the chains may not allow for HPL structures to transition into in-plane cylinders compared to
the polymer chains on pure silicon substrates. However, the presence of multilayers within the
DSA samples is not ideal for pattern transferring techniques. The sample which underwent 3
reswelling cycles may have dewetted during the annealing process and caused the film to
completely cover the trenches as the polymer aggregated to form thicker regions.
Additionally, the reswelling cycles may not have been effective in inducing long-range
HPL ordering because difference in SR during each reswell might not have been able to alter the
a)
0 reswell
b)
1
c) 3
reswell
reswell
2.6
2.4
0
2 4
6 8
10 12
14 16 18
20 22 24 26 2
30 32 34
Thue Elapsed After Deswelling (min}
Figure 13 Effect of reswelling cycles and rapid deswelling on film morphology using DSA.
Samples were initially swelled to high SR and deswelled using 0.150W and were annealed for 30
minutes. a) After deswelling, sample continued annealing at the equilibrium SR. b) After the initial
deswelling, sample was reswelled to reach a SR of 1.8 to continue annealing in order to examine the
effects of reswelling. c) Similar to b, sample underwent reswelling to SR of 1.8. Additionally, the
sample underwent two additional deswelling cycles. The effect of additional deswell and reswell
cycles generated a large area of HPL structures. However, the film also dewetted and caused the film
to non-uniformly coat the substrate and caused completely coverage of some trenches.
36
xN of the system significantly to induce a phase transition from cylinders to HPL. The change in
SR for these samples was 0.4, and at a SR of 1.8, the polymers are more likely to form cylinders
rather than HPL. Thus, a larger difference in film thickness may be required in order to see a
difference in HPL formation.
37
3. Conclusions & Future Work
Solvent vapor annealing of block co-polymers has proven to be an effective method of
generating a variety of morphologies on the nano-scale level. Although this thesis did not
achieve the target film structure after annealing, the use of thermal modulation to heat the
substrate has provided some promising avenues for developing a procedure to form a uniform
monolayer of HPL. Using the thermal modulation to introduce rapidly deswell the polymer
during the annealing procedure can be used to aid the formation of the metastable HPL structure
for directed self-assembled structures.
Future work can continue to explore the effects of reswell cycles during the annealing
process. Although not ideal for forming HPL structures, this method of reswelling and
deswelling the film during annealing can be used to induce greater in-plane cylinder order and
alignment while reducing the overall annealing time. There are several ways to modify the
reswelling technique used in this thesis. It may be interesting to see whether an increase in the
SR difference during reswelling can induce a phase transition. Additionally, the samples
annealed at elevated swelling ratios after reswelling, and it may be useful to annealing at a lower
SR after introducing a reswell cycle such that any resultant morphologies can be preserved
through the annealing process.
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