Negative electron beam resist

C The Epoxy Resists. The first negative tone electron beam resist materials with useful sensitivity were based on utilizing the radiation chemistry of the oxirane or epoxy moiety. The most widely used of these materials, COP (Figure 32) is a copolymer of glycidyl methacrylate and ethyl acrylate and was developed at Bell Laboratories (43,44). COP has found wide applicability in the manufacturing of photomasks. The active element... 

Figure 3. Size of clear and opaque 2.0 fim features as a function of exposure dose for a negative electron beam resist. The dose Dp that results in the correct feature size is denoted as the "sensitivity .

Methods 1 and 3 have been utilized in dry developed resist systems. To our knowledge, there are no resist systems commercially available that depend on post-exposure treatment other than the post-curing effect in negative electron beam resists mentioned earlier. Since such systems are still largely in the research phase we will not discuss them here but rather refer the reader to the literature for more detailed descriptions (44-50). 

As a general rule, the sensitivity of conventional electron beam resists is not sufficient for economic throughput in an x-ray lithographic system. This is particularly true of positive electron resists such as PMMA, the most widely used x-ray resist for experimental purposes, whose sensitivity of >500 mJ/cm2 is some 100 times too slow for practical application. Even PBS only shows a sensitivity of 94 mJ/cm2 to PdLa x-rays. Consequently, the major research effort has concentrated on negative resists because of their higher inherent sensitivity. 

A Novel Technique for Determining Radiation Chemical Yields of Negative Electron-Beam Resists... 

From this result on MRS, we expected that a combination of phenolic-resin-based resist and aqueous alkaline developer would lead to etching-type dissolution and non-swelling resist patterns. In this paper, we report on a new non-swelling negative electron beam resist consisting of an epoxy novolac, azide compound and phenolic resin matrix (EAP) and discuss the radiation chemistry of this resist. 

To achieve a manufacturable system for sub-0.5-pm patterning, extremely precise control of the molecular properties, structure, composition, and purity of the polymer is required (Table 4). Meeting these requirements provides intellectual challenges in ultrapurification reaction engineering and chemical synthesis. An illustration of the control required in this synthesis process can be found in a negative electron beam resist, GMC. 

The major reaction pathways for SNR operating as a negative electron beam resist are general known, but several problems still remain. 

Many reports have been published on negative electron-beam resists. Most of these resists utilize radiation-induced gel-formation as the insolubilzation reaction. However, a major problem with these resists, is that their resolution is limited by swelling which is induced by the developer during development. 

Novolac- or phenolic resin-based resists usually show no pattern deformation induced by swelling during development in aqueous alkaline solution. Examples of such resists are naphtho-quinonediazide/novolac positive photoresists, novolac-based positive electron-beam resist (NPR) (1), and azide/phenolic negative deep-UV resist (MRS) (2). Iwayanagi et al.(2) reported that the development of MRS proceeds in the same manner as the etching process. This resist, consisting of a deep-UV sensitive azide and phenolic resis matrix, is also sensitive to electron-beams. This paper deals with the development mechanism of non-swelling MRS and its electron-beam exposure characteristics. 

We have successfully employed the trimethylsilylmethyl appendage to effect oxygen RIE resistance in both positive and negative acting electron-beam resist systems (10,11). The relatively compact nature of this substituent allows the preparation of glassy polymers useful for lithographic applications. The preparation and characterization of select trimethylsilylmethyl substituted resists will be presented in addition to a study of their radiation chemistry and lithographic properties. 

Materials Synthesis and Characterization. In addition to the requirements of etching resistance, sensitivity, solubility and high glass transition temperature (Tg), one of the criteria used in designing both a negative and positive electron-beam resist system was synthetic simplicity. The trimethylsilylmethyl appendage allows the incorporation of silicon into polymeric resists without adverse synthetic complications. Standard free radical or condensation polymerization techniques can be employed with appropriately substituted monomers that are readily available. 

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-... 

The negative electron beam resists as shown in Table VII are based on vinyl polymers —... 

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). 

The surface photo absorption for contrast enhancement (SPACE) process has been recently reported for use with a negative-working electron-beam resist (42). By addition of a controlled UV-flood exposure step, enhancements in both contrast and sensitivity of MRS RD2000N resist, a negative working resist composed of poly(p-hydroxystyrene) and 3,3 diazido-diphenylsulfone, have been obtained. However, its application is iimited to negative working resists. 

Taniguchi, Y. Hatano, S. Shiraishi, S. Horigone, S. Nonogaki, and K. Naraoka, PGMA as a high resolution, high sensitivity negative electron beam resist, Jpn. J. Appl. Phys. 18, 1143 (1979). 

Z.C.H. Tan, R.C. Daly, and S.S. Georgia, Novel, negative working electron beam resist, Proc. SPIE 469, 135 (1984) R.C. Daly, M. Hanrahan, and R.W. Blevins, Negative working e beam copolymers, Proc. SPIE 539, 138 (1985). 

When some or all of the phenyl rings are substituted with halogen (Scheme 6.11), the radiation sensitivity and the cross-linking efficiency of polystyrene can be enhanced significantly. Negative electron-beam resist formulated from iodi-nated and chlorinated polystyrene and based on this approach have been reported. Sensitivity of about 2 p.C/cm and resolution of about 1-p.m features have been demonstrated with these materials. ... 

Ueno, H. Shiriashi, and S. Nonogaki, Insolubilization mechanism and lithographic character istics of a negative electron beam resist iodinated polystyrene, J. Appl. Polym. Sci. 29, 223 (1984). "ibid. 

Ito, C.G. Willson, and J.M.J. Frechet, Positive/negative mid UV resists with high thermal stability, Proc. SPIE 771, 24 (1987) H. Ito, M. L.A. Pederson, K.N. Chiong, S. Sonchik, and C. Tsai, Sensitive electron beam resist systems based on acid catalyzed deprotection, Proc. SPIEW86, 11 (1989). 

The major advantages of electron-beam lithography over conventional photolithography are a higher potential resolution and the possibility of direct beam writing on the resist surface. In addition to the usual requirements discussed earlier, positive or negative electron-beam resists have to possess the following properties ... 

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