Positive electron-beam resist poly

Bowden and L.F. Thompson, A new family of positive electron beam resists poly(olefin sulfones), J. Electrochem. Soc. 120, 1722 (1973) Poly(styrene sulfone) A sensitive ion millable positive electron beam resist, J. Electrochem. Soc. 121 1620 (1974). 

Many papers have been published on positive electron-beam resists. These resists are mostly polymers which are degraded upon electron-beam irradiation. The resulting lower molecular weight polymer in the exposed area can be selectively removed by a solvent under certain developing conditions. The development is accomplished by the difference in the rate of dissolution between the exposed and unexposed areas, which is a function of the molecular weight of the polymer. Recently, Willson and his co-workers reported the new type of positive resist, poly(phthalaldehyde), the exposure of which in the presence of certain cationic photoinitiators resulted in the spontaneous formation of a relief image without any development step (/). 

Positive Electron-beam Resist of Poly (a-substituted Benzyl Methacrylate). The electron-beam resist behaviors of poly(a-substituted benzyl methacrylate)s are given in Table III. When the exposed resist films were developed with a mixture of MIBK and IPA, the sensitivities of these polymers were on the order of 10-4 C/cm2. When a dilute solution of sodium methoxide in methanol was used as a developer, the sensitivity was enhanced as compared with the former case, and increased with an increase in the bulkiness of the ester group of the polymer except for poly(a,a-diphenylethyl methacrylate). 

Poly(butene-l sulfone) (PBS), a sensitive, positive, electron beam resist, is highly sensitive to 185-nm radiation (Table 3.4) (9). However, PBS does not absorb above 200 nm, and the sensitization has not been successful. Incorporation of pendant aromatic rings into the polysulfone structure extends the photosensitivity to the DUV and mid-UV regions (72). Himics and Ross (73) reported that carbonyl-containing poly(olefin sulfones) such as poly(5-hexen-2-one sulfone) are sensitive to UV-induced degradation and... 

Bowden and his coworkers(j).) proposed a new type of positive electron beam resist which consists of an alkali-soluble novolac and polymeric dissolution inhibitor. The positive working mechanism of this new type positive resist( NPR ) is similar to that for the conventional positive photoresist 10). It was also found that poly(2-methylpentene-l sulfone)( PMPS ) is good as a polymeric dissolution inhibitor for NPR(lil). In addition, it was clarified that one of the difficulties with NPR is phase separation in the resist films(10)(n). 

The depolymerization mechanism from the polymer end has been recently revisited in the design of positive electron beam resists. 2-Phenylallyl-termi-nated poly(a-methylstyrene) was prepared by living anionic polymerization, which exhibited a significantly lower depolymerization temperature on TGA than the H-terminated counterpart [340]. The 2-phenylallyl-terminated polymer depolymerized completely when treated with n-BuLi in THF at room temperature. A single-component resist (without PAG) formulated with the 2-phenylallyl-terminated poly(a-methylstyrene) demonstrated a higher e-beam sensitivity (500 pC/cm2 at 20 keV) than the one based on the H-terminated polymer when developed with methanol/methyl isobutyl ketone (2/3 vol/vol) [340]. However, the sensitivity of the non-catalyzed single-component system... 

S. Matsuda, et al., Thermally reacted poly(methacrylamide) as a positive electron beam resist, Polym. Eng. Sci. 17, 410 (1977). 

Bowden and L.F. Thompson, Electron irradiation of poly(olefin sulfones) Application to electron beam resists, J. Electrochem. Soc. 120, 1722 (1973) Poly(Styrene sulfone) A sensitive ion millahle positive electron beam resist, J. Electrochem Soc. 121, 1620 (1974) D.R. McKean, U.P. Schaedeli, and S.A. MacDonald, Acid photogeneration from sulfonium salts in solid polymer matrices, J. Polym. Set Polym. Chem. Ed. 27, 3927 (1989) D.R. McKean, U.P. Schaedeli, P.H. Kasai, and S.A. MacDonald, The effect of polymer structure on the efficiency of acid generation from triarylsulfonium salts, J. Polym. Sci. Polym. Chem. Ed. 29, 309 (1991). 

Concerning the design of positive electron-beam resists [228,229], anionic polymerization was used to introduce 2-phenylallylgroups at the end of poly(a-MeSt) chains. 

Poly(glycidyl methacrylate) (PGMA), a well-known negative electron beam resist first reported by Hirai et al. (55), actually functions as a positive-tone resist upon DUV exposure (Table 3.1) (56). The epoxide functionality responsible for cross-linking under electron beam exposure does not absorb in the DUV region, and the response of PGMA to DUV radiation is determined by the absorption due to the n — tt transition of the carbonyl chromo-... 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

Photosensitized degradation of poly(olefin sulfones) similar to the Hg(3P) photosensitized reactions of olefin sulfones make them subject to photodegradation in easily accessible wavelength regions. Almost all poly(olefin sulfones) have been reported only as positive tone electron beam resists (4). As the only exception, poly(5-hexene-2-one sulfone) has been reported as a positive tone photoresist with or without a photosensitizer, benzophenone (5). Because this polymer has a carbonyl chromophore, its photosensitivity is clearly derived from the polymer structure itself. 

Poly(methyl methacrylate), PMMA, has remained the standard by which to judge positive-working electron-beam resists for over a dozen years (1>.2 .3 4) Hundreds of rival polymers have been disclosed. Most of them exceed PMMA in sensitivity. However, the combination of properties which include stability, sensitivity, contrast, adhesion, and solubility have kept PMMA in the limelight. 

Poly(methyl methacrylate) (PMMA) is the standard positive e-beam resist, usually purchased in two high molecular weight forms (495 or 950 kg moP ) in a casting solvent such as chlorobenzene or anisole. For example 950 kg mol PMMA, 4 % in anisole is a commonly used solution. Electron beam exposure breaks the polymer into fragments (as shown in Scheme 13.7) that can be dissolved in a 1 1 MIBK IPA developer (where MIBK is methyl isobutyl ketone and IPA is isopropyl alcohol). 

Interest in solution inhibition resist systems is not limited to photoresist technology. Systems that are sensitive to electron-beam irradiation have also been of active interest. While conventional positive photoresists may be used for e-beam applications (31,32), they exhibit poor sensitivity and alternatives are desirable. Bowden, et al, at AT T Bell Laboratories, developed a novel, novolac-poly(2-methyl-l-pentene sulfone) (PMPS) composite resist, NPR (Figure 9) (33,34). PMPS, which acts as a dissolution inhibitor for the novolac resin, undergoes spontaneous depolymerization upon irradiation (35). Subsequent vaporization facilitates aqueous base removal of the exposed regions. Resist systems based on this chemistry have also been reported by other workers (36,37). 

Poly(methylmethacrylate), (PMMA), is one resist which is especially favored by researchers due to its high resolution and contrast. Linewidths as small as 100 A have been produced with an extremely high dose of x-rays (10 J/cm ) using PMMA. However, even PMMA s normal sensitivity of 600-1000 mJ/cnr is too slow for commercial use. There have been many attempts to improve the sensitivity of PMMA to electron beam irradiation mainly by copolymerization (8). In the current paper, we describe an alternative approach whereby a sensitive polymer is physically blended with PMMA to increase its sensitivity and yet maintain the good film qualities associated with PMMA as a positive x-ray resist. 

Another interesting, DUV-sensitive, planarizing layer for the exposure-PCM scheme is poly(dimethyl glutarimide) (PMGI) (structure 3.7). Exposure of PMGI to DUV light or electron beam radiation results in main-chain scission therefore, this resist is positive working. 

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