Chapter 12 Acid-Catalyzed Thermolytic Depolymerization of Polycarbonates: A New A p p r o a c h to Dry-Developing Resist Materials J. M. J. Fréchet1, E. Eichler1, M. Stanciulescu1, T. Iizawa1, F. Bouchard1, F. M. Houlihan1, and C. G. Willson Downloaded by EAST CAROLINA UNIV on January 5, 2018 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch012
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1Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 9B4, Canada Almaden Research Center, IBM, San Jose, CA 95120-6099 2
The design of new condensation polymers which undergo acid-catalyzed thermolysis is explored with polycarbonates. The polymers are prepared by phase-transfer catalyzed polycondensation using active esters or carbonates and diols. Polycarbonates containing tertiary, allylic or benzylic diol units susceptible to elimination decompose thermally near 200° to volatile materials. Self developing resist materials can be designed by combining the active polycarbonates with photoactive triarylsulfonium salts or other similar compounds which generate strong acids upon irradiation. Exposure of the resist material creates a latent image which can be developed thermally with evolution of volatile carbon dioxide, alkenes, and alcohols. The development of new materials for application as photoresists has seen numerous advances over the past few years as researchers strive to meet the industry's needs for materials of greatly improved properties through new conceptual designs. Of special interest to the synthetic organic chemist are improvements focused on increases in resolution and sensitivity as these may be prone to chemical solutions. In practice, enhanced resolution is required to achieve dimensional control while allowing for a reduction of the overall dimensions of active devices. Enhanced sensitivity is required to compensate for the loss of flux which generally results from the use of exposure instruments capable of providing higher resolution. Other critical resist properties such as etch resistance, adhesion, processing characteristics, etc., must also be considered though initial emphasis for the development of new designs may be placed on improvements in sensitivity and resolution [1,2]. The Chemical Amplification Approach Significant advances in the field of materials which may be useful as Ε-Beam or X-Ray resists have been made based on the development of 0097-6156/87/0346-0138$06.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
12.
FRÉCHET ET A L .
new chain substituted
Acid-Catalyzed
Thermolytic
growth polymers such as the polyacrylates [4,5]·
139
Depolymerization polyalkenesulfones
[3]
and
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O v e r t h e p a s t f e w y e a r s we h a v e b e e n i n t e r e s t e d i n t h e d e s i g n o f new types of r e s i s t m a t e r i a l s which g e n e r a l l y possess h i g h sensitivities due t o s t r u c t u r a l f e a t u r e s w h i c h a l l o w f o r t h e o c c u r r e n c e of radiation i n i t i a t e d repetitive processes. The t h r e e m a i n a p p r o a c h e s we have investigated to-date a l l maximize the use of available protons through "chemical a m p l i f i c a t i o n " ; they are the f o l l o w i n g : *
Photoinduced
* *
[1,6]. Photoinduced m u l t i p l e molecular rearrangements [7]. P h o t o c a t a l y z e d d e p o l y m e r i z a t i o n or c h a i n degradation
changes
i n the
physical
properties
of
polymers
reactions
[8]. In a l l three of these designs, chemical amplification is the result of photoinitiated chain or catalytic reactions where i r r a d i a t i o n i s used o n l y t o i n i t i a t e a c h a i n r e a c t i o n or to generate a c a t a l y s t w i t h i n l o c a l i s e d areas of a r e s i s t f i l m . The most r e l e v a n t e a r l y work i n t h e c o n t e x t o f t h i s s t u d y i s t h e radiation induced d e p o l y m e r i z a t i o n of p o l y ( p h t a l a l d e h y d e ) [9]. In this case, depolymerization is due to a ceiling temperature phenomenon whereby r a d i a t i o n i n d u c e d c l e a v a g e of the polymer causes it to r e v e r t f u l l y t o monomer. Poly(phtalaldehyde) i s a material w i t h a very low c e i l i n g temperature w h i c h i s o n l y rendered s t a b l e at room temperature through the device of capping i t s chain-ends after low temperature polymerization, thereby preventing i t s spontaneous d e g r a d a t i o n when h e a t e d . This very i n t e r e s t i n g approach leads to a p o t e n t i a l l y v e r s a t i l e resist which can be imaged w i t h a l m o s t any source of energetic radiation. However, a s i g n i f i c a n t drawback of the spontaneous depolymerization process may be the fact that monomer is evolved i m m e d i a t e l y upon i r r a d i a t i o n , t h u s i n c r e a s i n g the r i s k of c o n t a m i n a ting the o p t i c s of the exposure t o o l . A better approach, w h i c h we have o r i g i n a l l y tested w i t h the p h o t o i n i t i a t e d molecular rearrangement o f c e r t a i n p o l y c a r b o n a t e s c o n t a i n i n g p r o t e c t e d g l y c i d o l m o i e t i e s [7], would involve the photochemical generation of a l a t e n t image only, followed by a thermal self-development step outside the exposure t o o l (Figure 1).
Thermally Depolymerizable
Polycarbonates
Our newly d e s c r i b e d f a m i l y o f t h e r m a l l y l a b i l e p o l y c a r b o n a t e s o p e r a t e on a somewhat s i m i l a r d e s i g n [ 1 0 ] . The s y s t e m i s b a s e d on a twocomponent m i x t u r e c o n s i s t i n g of a polymer w i t h t h e r m a l l y l a b i l e bonds i n i t s m a i n - c h a i n a n d a s u b s t a n c e w h i c h c a n g e n e r a t e a c i d by e x p o s u r e to r a d i a t i o n (e.g. t r i a r y l s u l f o n i u m s a l t s [11]). T y p i c a l examples of t h e t y p e s o f r e a c t i v e p o l y c a r b o n a t e s we h a v e p r e p a r e d a r e p o l y m e r s I , I I , and I I I w h i c h a r e shown on page 1 4 1 .
Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
POLYMERS FOR HIGH T E C H N O L O G Y
140
These polycarbonates undergo thermolysis at r e l a t i v e l y low temperatures (150-250°C) while other less reactive polycarbonates are stable to much higher temperatures. For example, while polycarbonate I undergoes complete decomposition to v o l a t i l e p-benzenedimethanol, isomeric C-8-dienes, and C0 near 200°C, the homopolycarbonate of 1,4-benzenedimethanol only undergoes a p a r t i a l and complex decompo s i t i o n at higher temperatures leaving an appreciable amount of charred residue behind.
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A t y p i c a l preparation of the polycarbonates i s shown for polymer III i n Scheme I. 1,4-Benzenedimethanol i s activated by reaction with two equivalents of p-nitrophenyl chloroformate i n pyridine and the r e s u l t i n g symmetrical dicarbonate i s then used i n a polycondensation with an equimolar amount of 2-cyclohexen-l,4-diol i n a s o l i d - l i q u i d phase-transfer catalyzed reaction with 18-crown-6 as catalyst and s o l i d anhydrous potassium carbonate as base. Alternately, the same polymer can be prepared by condensation of bis(4-nitrophenyl)-2cyclohexen-1,4-ylene dicarbonate [12] with 1,4-benzenedimethanol or through a variety of similar polycondensations using d i o l b i s carbonylimidazolides [13,14]. A l l three types of polycarbonates I-III owe their thermolytic l a b i l i t y to their structural design which allows for low a c t i v a t i o n energy t e r t i a r y , benzylic, or a l l y l i c , t r a n s i t i o n states and which includes hydrogen atoms i n positions β to the carbonate oxygen to enable elimination. In a l l cases the thermolytic decompositions are very clean reactions which usually proceed quantitatively. For example, i n the case of polymer I I I , thermolysis affords only the three expected products as shown below i n Scheme I I .
Acid-Catalyzed Thermolysis of the Polycarbonates Of particular relevance to t h i s study i s the knowledge that elimina tion of carbonates proceeds through a polar t r a n s i t i o n state [15]. Thus, the thermolysis of these activated carbonates should proceed through an acid-catalyzed process and polymers I-III would be expected to undergo thermolysis at temperatures well below 100°C i n the presence of a c a t a l y t i c amount of strong acid. In terms of r e s i s t imaging, these properties would be exploited as shown i n Figure 2. These expectations were confirmed f u l l y i n a model study i n v o l ving the a c i d o l y s i s of bis(4-nitrophenyl)-2-cyclohexen-l,4-ylene dicarbonate. This b i s - a l l y l i c carbonate i s expected to undergo a clean a c i d o l y t i c cleavage to benzene, p-nitrophenol and carbon dioxide. Indeed when a sample of t h i s model compound i s treated with a c a t a l y t i c amount of a strong non-nucleophilic acid such as t r i f l u o romethanesulfonic acid, the decomposition proceeds as expected. This i s most readily monitored by performing the a c i d o l y s i s within an NMR tube on a solution of the dicarbonate as i s shown quite graphically i n Figure 3. Extension of t h i s finding to a r e s i s t material i s carried out readily as follows. A thin f i l m of an active poly carbonate such as I containing a small amount of a radiation sensi t i v e acid precursor such as a triphenylsulfonium s a l t i s cast on an
Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
12.
Acid-Catalyzed
FRÉCHET ET A L .
141
Depolymerization
volatiles
I
res i s t - £ ~ ate-£ substrate
Thermolytic
h)
LATENT IMAGE
POSITIVE IMAGE
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FIGURE 1. Thermal development o f a l a t e n t r e s i s t image
—f-O
^/Ç-O—C-O—CH,—/
CH,
CH,
I
VcH -0-C-}— 2
Ο
III
ο
ο
H0CH -^^-CH 0H-H2 2
2
0 N-^O^-0-C-C« 2
0
I °2 ^(^0-C-0-CH -^^CH ^0-C-0H(oy N
2
2
18-Crown-6 K C0 2
^ { r "
0
H
H O - H ( ^ - O H
3
C ^
0
~ r ° ~
C
H
2
H
^
C
H
2
"
0
" ^
SCHEME I. Phase transfer c a t a l y s i s i n the preparation o f polycarbonate I I I . Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
POLYMERS FOR HIGH T E C H N O L O G Y
142
^c-oHQHo-c-o-CH -^o)-cH -o-f
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2
HO-CH —^O^-CH 0H
+
2
2
2
n
2TI CO,
SCHEME I I . Thermolytic depolymerization o f Polymer I I I .
0 0 II II -(-0-hfO-C-O-R-O-C-f+
