Radiation resistance, poly derivatives

Greater resolution can be obtained nsing deep UV radiation, but now other resists must be used as the conventional ones are optically opaque in this region. Poly(methyl methacrylate) can be nsed as a positive resist, as can several of its derivatives. In each case, the carbonyl gronps absorb at 215 run, and this leads to chain scission and degradation. 

Most polymers that are positive resists tend to depolymerize via a monomer-unzipping action when degraded, and poly(methyl methacrylate) (PMMA) is typical of this type. Unfortunately, the sensitivity of PMMA to electron beam radiation is low, and in an attempt to improve this feature, PMMA derivatives have been prepared by replacing the a-methyl group with more polar electron-withdrawing substituents, e.g.. Cl, CN, and CF3, to assist electron capture (Figure 16.5). Modification of the... 

All attempts to copolymerize 1 with other vinyl monomers were unsuccessful until Salamone and coworkers obtained a 1,2-addition copolymer 4 of 1 (R = H] and 4-vinylbenzenesulfonate, 20, obtained by spontaneous polymerization oi the two monomers as a melt or in concentrated aqueous solution [61]. Vinyl-type copolymers of 1 and 20, and 1 and 21 were also obtained by initiation with radicals (e.g. from ACVA), light or y-radiation. These experiments are summarized in Table 6.3.1. Copolymerization of 1 (R = H) with AMPS 21 is complicated when carried out as a spontaneous (i.e. absence of an initiator) process in water, methanol, or iV,iV-dimethylformamide (DMF). Two homopolymers derived from 1 (R = H), the 1,2-addition polymer, 2 (R = H), and the polyionene, 7. are obtained along with poly AMPS. Similar attempts to form copolymer from 1 (R = H) and vinylsulfonate have been unsuccessful. In fact, the latter monomei resisted all conditions of polymerization and 1 (R = H) formed the ionene 7 from a melt or in dilute aqueous solution (< 1.0 m) and yielded 2(R = H) in concentrated aqueous solution. Table 6.3.1. Recall that this behavior duplicates that reported for 1 (R = H) by Ringsdorf and others. Ringsdorf attributed the change in structure of homopolymer to micellization of 1 (R = H) in concentrated aqueous solution. 

Sub-half-micron features could be resolved with deep-UV (35) and electron-beam (36) radiation with wide process latitude and high sensitivity using this chemistiy. An example of the resolution capability is shown in Figure 4. Very sensitive X-ray and e-beam resist formulations based on similar chemistiy using melamine and benzyl alcohol derivatives as crosslinking agents, formulated with onium salt photoacid generators in novolac or poly(hydrotystyrene) binders, have shown 0.2 fim resolution (30, 31). 

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