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Environmental stability has been a major concern in chemically amplified resist systems. These are the major chemistry platforms adopted in DUV resists. To resolve this issue, two extreme approaches were proposed recently. One is using the annealing concept through high temperature bake to densify the film and reduce free volume in the resist, thus minimizing the uptake of airborne contaminants. In this approach, high post exposure bake temperature is used which then bakes away major portion of airborne base from top of the resist, thus minimizing (or eliminating) T-top formation. To be able to execute this concept, a highly thermally stable copolymer of 4-hydroxystyrene and t-butyl acrylate was adopted. The nature of the thermal stability is due to the high activation energy of t-butyl ester group during the deprotection process. The other approach in achieving the environmental stability is using an extremely low activation energy protecting group which will deprotect during the exposure step. Since T-top formation usually occurs between exposure and PEB, the spontaneous room temperature deblocking eliminates any delay, thus PED effect free resist can be obtained. In this approach, the ketal resist system (KRS) using methoxy propene protected polyvinylphenol was used. The advantages of low activation energy are not only environmental stability but also large bake latitudes with PEB variation as low as 0 nm/degrees Celsius. Besides ketal, silyl ethers and acetals are the other two distinct resist families which have lower activation energy than t-butoxycarbonyl and t-butyl ester systems. According to many recent publications, the acetal resist systems have demonstrated some environmental robustness and insensitivity to bake temperature variation. In order to shed some light on the lithographic performance of these low activation energy resist systems, we present here some of our work on acetal resist family in comparison to ketal family. In the acetal resist family, we have evaluated a few protecting groups which include ethoxy ethyl, propyloxy ethyl and isobutyloxy ethyl. Since ethoxy ethyl had better polymer properties than other acetal groups, most lithographic work was done on this protecting group in comparison to ketal groups. The experimental findings suggest that the environmental stability in the acetal system is inferior to the ketal system. Since the reaction is completed during exposure, ketal system is completely insensitive to the bake temperature. Acetal system can have large bake latitudes (PAB & PEB) only if the resist formulation contains large amount of basic additive, e.g. ethoxy ethyl protected polyvinylphenol resist with significant amount of basic additive gives only 3 - 4% change in Eo when PEB temperature changes from 90 to 130 degrees Celsius (PAB 120 degrees Celsius), and 4 - 5% change when PAB temperature changes from 100 to 120 degrees Celsius (PEB 110 degrees Celsius).
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