PED-stabilized chemically amplified photoresist

The influence of the baking condition on PED stability and the resist profile of a chemically amplified (CA) photoresist was investigated. One of the most serious problems for the use of CA photoresists is the 'T-top' formation. A resist process with high temperature bake has been suggested to prevent this problem. During our development of this type of photoresists, however, a problem of standing wave was observed when the resist was processed with high temperature softbake. The role and the effect of the baking temperature for softbake and PEB were discussed. Our investigation suggests that: (1) Low temperature softbake reduces standing wave. (2) High temperature PEB prevents T-top. Another problem, pattern deformation, came out when the combination of low temperature softbake and high temperature PEB was used. Too fast deprotection rate and hence too fast gas evolution rate at high temperature PEB is thought as the reason for the pattern deformation. Stepwise PEB is effective to prevent both T-top and pattern deformation. The first PEB at low temperature decomposes the protecting groups in the body of the resist film without pattern deformation and the subsequent high temperature PEB eliminates T-top at the surface of the resist film. We also investigated the role of high temperature PEB on prevention of T-top. We found that an acid generated from photo acid generator (PAG) kept the ability to decompose the protecting group even though the acid was coupled with ammonia, a basic contaminant from the environment. The activity of the ammonium salt of the acid is thought to be the key factor on environmental stability.