Synthesis of novolac resin by acid removal after condensation (ARAC) process

Novolak resins are synthesized by a condensation reaction using a mixture of cresylic acids with a formaldehyde source [paraformaldehyde, trioxane or aqueous (37%) formaldehyde] in the presence of an acid catalyst. After the condensation process, the unreacted cresols, water and solvents are removed using a high temperature (approximately 210 degrees Celsius) vacuum (approximately 20 mm Hg) distillation. The resin is isolated as a molten solid or in solvents such as propylene glycol methyl ether acetate, ethyl lactate, or ethoxy ethyl propionate. During this process, molecular weight changes were noticed. The molecular weight changes were attributed to the presence of residual acid catalyst and the high temperature distillation process. In order to minimize the molecular weight changes, an effort has been made in our laboratories to study the effect of acid removal after the condensation process. Novolak resins with consistent molecular weights could be synthesized using the ARAC process. This paper deals with the novel ARAC process, which has made a major impact on consistency of novolak resins and lithographic characteristics of the resist formulations made with them.