Polymer cross-linking negative resists

Acid-C t lyzed Chemistry. Acid-catalyzed reactions form the basis for essentially all chemically amplified resist systems for microlithography appHcations (61). These reactions can be generally classified as either cross-linking (photopolymerization) or deprotection reactions. The latter are used to unmask acidic functionality such as phenohc or pendent carboxyhc acid groups, and thus lend themselves to positive tone resist apphcations. Acid-catalyzed polymer cross-linking and photopolymerization reactions, on the other hand, find appHcation in negative tone resist systems. Representative examples of each type of chemistry are Hsted below. 

After the absorption step, it appears that the cross-linking or degradation chemistries of X-ray and e-beam resists are similar. This is demonstrated in Figure 9 where the sensitivities of several polymer resists to both 20 keV electron beam exposures and Mo (5.41 A) X-rays are plotted. Both degrading (positive) and cross-linking (negative) polymers are included in this plot. 

Almost all polymers are more sensitive (require fewer exposure units) when exposed to ion beams than when they are exposed to e-beams. Polymers such as styrene and novolac, which are relatively insensitive to e-beams, have a much more sensitive response to ion beams. However, resist speed is important for focused ion beam lithography and several high speed resists have been found, Poly(2,2,2-trifluoroethyl a-chloroacrylate), a scissioning positive tone polymer, and two cross-linking negative tone polymers, poly(2,5-dichlorostyrene) and poly(4-bromostyrene), have been found to function at high speed as ion beam resists. ... 

Fig. 24. Representative cross-linking systems employed in negative tone CA resists, (a) Epoxy polymers requiring organic solvent development, (b) PHOST-based cross-linking systems requiring aqueous development, (c) Monomeric cross-linking agents used in PHOST matrix polymers.

Two types of resists are used negative and positive resists. Negative resists have a chemically inert polymer component, such as mbber, and a photoreactive agent that reacts with light to form cross-links in the mbber. When placed in an oiganic developer solvent, the unexposed, unpolymerized resist dissolves, leaving a polymeric pattern in the exposed regions. Because the polymer swells in the solvent, the resolution is limited to two to three times... 

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