Choice of amines as stabilizers for chemically amplified resist systems

Chemically amplified (CA) resist systems are known to be sensitive to contamination. Environmental contaminants such as airborne amines can result in T-topping. In addition, the undesired diffusion of photogenerated acid into unexposed areas can result in linewidth slimming. To counteract these effects, amines are intentionally added to chemically amplified resist formulations. These added amines function as `buffers' or `acid traps' within the resist matrix. While the effects of strong, photogenerated acids on CA resist systems has been the focus of much research, the effects of weaker acids on these resist systems has not received as much attention. In this paper we demonstrate how the conjugate acid of some amines (amine salts) can adversely effect the lithographic performance and storage stability of CA systems. We show that salts of weak amines are sufficiently acidic at relatively low temperatures, to cause significant increases in polymer molecular weight and polydispersity. In some cases, gelation of the polymer matrix was observed. A mechanism is proposed to explain these effects. We also show how appropriate amines can provide a thermally stable salt with low acidity. Such amine additives not only improve the storage stability of the resist system but also significantly improve lithographic performance as well.