Effect of Overcoats in Chemically Amplified Resists against Water and Organic Bases

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Abstract
We have evaluated poly-(acrylic acid) (PAA) and poly-(α-methyl-styrene) (PMST) as overcoats used to stabilize the lithographic performance of chemically amplified resist during the delay (PED) between exposure and post-exposure baking. We used water, aniline and NMP as model compounds of airborne contaminants. Both films are shown to improve the stability of resist performance during PED. PMST is more effective in preventing atmospheric water penetration than PAA, as evidenced by the finding that the sensitivity of PAA-coated resist is dependent on the relative humidity of ambient air. Conversely, the PAA-coated resist is more stable against high vapor concentration of aniline and NMP, when exposed for several hours. This is probably because the negative charge of the carboxyl groups in PAA electrically traps aniline and NMP, preventing their penetration to the resist films.