Chapter 7
New Design for Self-Developing Imaging Systems Based on Thermally Labile Polyformals 1
2
3,4
3
Jean M . J . Fréchet , C. Grant Willson , T. Iizawa , T. Nishikubo , Κ. Igarashi , and J . Fahey 3
1
1
2
Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301 IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 Department of Applied Chemistry, Kanagawa University, Yokohama 221, Japan 3
The polycondensation of bis-allylic or bis-benzylic diols with dibro momethane under phase transfer catalysis may be used to generate some polyformal polymers and copolymers which depolymerize readily when heated or when subjected to acidolysis. The polymers are best prepared by condensation of diols such as 1,4-dihydroxy-2-cyclohex ene or 1,4-dihydroxy-1,2,3,4-tetrahydronaphthalene and CH Br2 under phase transfer conditions. The polymers which are obtained by this process generally have fairly broad polydispersities and are stable to temperatures higher than 150°C. Under acid catalysis, they decom pose below 80°C to afford mainly the aromatic elimination product (benzene or naphthalene), and formaldehyde hydrate. As the poly mers absorb very weakly even in the deep-UV (E < 300), imaging is possible through formulations incorporating both the polyformals and a small amount of a triarylsulfonium salt or similar photoacid generator. The polyformals can be used as self-developing and chemi cally amplified imaging systems which operate in the deep-UV and possess good sensitivities due to the catalytic nature of their photoini tiated thermal decomposition. 2
max
The initial work on a new generation of deep-UV and Ε-beam resist materials which incorporate chemical amplification [1] was carried out in 1979 when Frechet and Willson first prepared poly(4-t-butyloxycarbonyloxystyrene), poly(4-allyloxystyrene, end-capped poly(o-phthalaldehyde), and copolymers of a-phthalaldehyde and its 3nitro derivative. The first of these materials was designed for its ability to lose its t-BOC phenolic protecting groups under thermolytic conditions, the second for its potential to undergo a catalyzed Claisen rearrangement, while the two cyclic polyacetals would undergo chain depolymerization under a variety of conditions. In particular the copolymers containing 3-nitro-phthalaldehyde units have o-nitrobenzyl acetal groups which are readily cleaved by U V irradiation [2] thereby rendering the copolymers photodepolymerizable (Figure 1 ) . This basic approach to chemical amplification was subsequently extended by Ito et al. [3] through resist formulations incorporating appropriately chosen triaryl sulfonium or diaryliodonium salt. For example, end-capped poly(phthalaldehyde) used in combination with an onium salt photoacid generator is an excellent self-
4Current address: Department of Chemical Engineering, Hiroshima University, Japan 0097-6156/89/0412-0100$06.00/0 o 1989 American Chemical Society
7. FRECHET ET AL.
New Design for Self-Developing Imaging Systems 101
developing imaging material. While this material is interesting, its usefulness is restricted as depolymerization of the polymer after exposure to radiation does not require any thermal activation; therefore, the phthalaldehyde monomer may be evolved within the confines of the exposure tool where it might well interfere with optical and mechanical components. This study will show an approach to materials which have many of the desirable features of the poly(phthaladehyde)-onium salt imaging system but do not suffer from the problem of spontaneous gaseous material evolution upon irradiation. The new polyformal-based imaging systems all have a "built-in" thermal activation requirement which allows for image self-development outside of the exposure tool. Imagine via Thermolvtic Main-Chain Cleavage. A significant part of our recent work with imaging systems which incorporate chemical amplification has involved the design of polymers which can undergo thermally activated multiple main-chain cleavages as the result of a phototriggered process. Imagine via Main-chain Cleavage of Polycarbonates. Polyesters and Polvethers. We have designed, prepared, and tested dozens of new imaging materials based on polycarbonates [4-10], polyethers [11] and polyesters [12] which are all susceptible to acid-catalyzed thermolytic cleavage. With all of these systems, imaging is accom plished through irradiation of a film of the polymer containing a small amount of a photoactive compound which can generate acid upon irradiation. Imagewise expo sure to a source of the appropriate wavelength causes formation of a latent image consisting of acid dispersed only in those areas of the polymer film which were exposed to radiation. The latent image can subsequently be developed by a baking step in which the latent image is provided with the activation energy which is required for the catalyzed thermolysis of the polymer to occur. The unexposed areas of the film do not undergo thermolysis as the uncatalyzed process requires much higher baking temperatures in order to take place. If the fragments resulting from multiple main-chain cleavages are somewhat volatile, self-development may be achieved by carrying out the thermolytic development step in an evacuated envi ronment. Mechanism of Cleavage of Polyesters and Polycarbonates. It is useful at this point to review the mechanism of cleavage of esters and carbonates [13]. For the sake of simplicity, this will be considered only in the context of a polycarbonate, and only a single cleavage step will be considered. It must be remembered that a similar mechanism would also apply to appropriately designed polyesters, and that, in the case of our polymers, the cleavage step would eventually involve all the carbonate functionalities which constitute the polymer chain; thus a thermolytic cleavage would result in complete depolymerization of a chain. The uncatalyzed thermolytic cleavage of a carbonate proceeds through a cis-elimination which requires that the carbonate possess a p-hydrogen. Early studies of this reaction have shown that the transition state has significant polarity, (suggesting much Ε j-like character for the elimination) especially in the case of carbonates. While it is well-known that the ease of elimination increases from primary to tertiary carbonates [13], we have based our novel designs of polymers containing bis-allylic or bis-benzylic carbonates on the premise that strong allylic or benzylic stabilization of the polar transition state would occur, thereby greatly facilitating the reaction. This assumption proved to be correct as the elimination reaction of allylic (Scheme I) and benzylic polycarbonates is qualitatively as facile as that of polycarbonates of tertiary alcohols [7, 9] . If a catalytic amount of acid is present, the thermolysis reaction is notably facilitated and therefore occurs at a much reduced temperature. This is key to our resist
POLYMERS IN MICROLITHOGRAPHY
102
Cleavage of benzylic C - 0 b o n d ortho to N 0 . 2
Figure 1.
/vw(-R-0-C
Preparation and photocleavage of a poly(phthalaldehyde)
N
2b
2a
Scheme I Η OH
w
u
Η Ο
2a
3
2c
Η
η HO — R - O H
4- H + 2n C O , + η +
V
vs^R-OH
\
•
Scheme II
2d
C0 + H 2
+
^—( / / \
Ο / - O ^ v w ο
2b
New Design for Self-Developing Imaging Systems 103
7. FRECHET ET XL
design as acid is generated within the coating by irradiation rendering those areas which have been irradiated more susceptible to low temperature thermolytic decomposition than areas which have remained unexposed. Scheme II shows the El-like elimination process which may prevail in the case of the acid-catalyzed thermolytic cleavage of polycarbonate 1. The reaction starts with protonation of a carbonate carbonyl group to afford 3, this is followed by breaking of the adjacent allylic carbon-oxygen bond to produce two fragments: a fragment containing a monoester of carbonic acid 2a, and another containing an allylic carbocationic moiety 2ç_. Elimination of a proton from this carbocationic moiety results in regeneration of the acid catalyst (the "eliminated" proton) and formation of a terminal diene-containing fragment 2i- The unstable monoester of carbonic acid 2a decarboxylates releasing a terminal alcohol fragment 2ά· The proc ess continues at other carbonate sites with complete breakdown of the polymer chain resulting in the eventual release of benzene (two successive eliminations on the bisallylic moiety), additional carbon dioxide, and a diol H O - R - O H . The protons ini tially generated by irradiation are not consumed in the process (except by possible side-reactions or impurities) which explains the high quantum efficiency or chemi cal amplification of this resist design. In contrast, the clean thermolysis of appropriately chosen ethers into alkene and alcohol components was little known until our work in this area demonstrated its applicability to transformations involving both side-chain ether groups [14] or the main chain of poly ethers [11]. Thermolytic Cleavage of Allvlic and Benzvlic Ethers and Polvethers. The thermal lability of the types of ethers which are of interest in the context of this study was discovered during a thorough study of the application of certain benzylic carbonates as labile protecting groups for alcohols and phenols [15]. It was observed that, under acid catalysis, the bis-methylcarbonate derivative of l-(4hydroxyphenyl)ethanol (4, in Equation 1) was transformed into the benzylic ether (5) which then underwent clean acid-catalyzed thermolysis to the corresponding styrene (6) in very high yield [14]. CH,
