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Environmentally stable lithography with acidity-optimized TARC material

We have found the Post Coat Delay (PCD) problem in an environment, where ammonia contamination is not controlled, with Top Anti Reflective Coating (TARC) process. The Chemically Amplified (CA) resist coated wafer using the conventional TARC with pH equals 3.2 was kept in the ammonia contaminated environment between the TARC coating and the exposure, the 'T-top' shape resist pattern was formed. To solve the PCD problem, we are considering the advantage to optimize the acidity in TARC material. In the case of using a higher pH TARC (e.g. pH >= 2.5), the 'T-top' shape resist pattern was formed. The TARC was neutralized, after contact with ammonia contamination during the PCD duration, the generated acid by the exposure in the CA resist diffused into the TARC without attacking the protective groups of resist polymer during the Post Exposure Bake (PEB) step. The lack of acid in the resist surface resulted in the formation of the 'T-top' shape resist pattern. On the other hand, in the case of using a lower pH TARC (e.g. pH >= 1.9), the remarkable film thickness loss occurred. Once a lower pH TARC was coated, the acid in the TARC diffused to the CA resist surface. Excess acid existence near the resist surface enhanced CA reaction and resulted in the film thickness loss. Thus, optimizing the acidity of TARC material is very important. A higher pH TARC results in the 'T-top' shape resist pattern, and a lower pH TARC results in the film thickness loss. We have found that a TARC with pH equals 2.2 shows the optimum result in achieving the environmentally stable lithography.