Effects of polymer end groups on chemical amplification

Abstract

Polymer end groups could affect the sensitivity of chemical amplification resist systems based on acid catalysis in a fashion completely different from the conventional resist systems. Their acidolysis susceptibility could depend on the initiator employed in polymerization, which is illustrated by two examples in this paper. When α,α-azobis(isobutyronitrile) is used as the radical polymerization initiator, PBOCST with a lower molecular weight provides a less sensitive tBOC resist than a higher molecular weight polymer, which is due to the poisoning effect of the CN group attached to the polymer end. Low molecular weight PBOCST's were prepared also with benzoyl peroxide and via living anionic polymerization to confirm the end group effect on the tBOC acidolysis. In contrast, there are cases where certain end groups could provide reaction sites to photochemically generated acids. One such example is poly(hydroxy-α- methylstyrene) (PHOMS). The ρ-PHOMS prepared by heating the tBOC-protected polymer (made by cationic polymerization) undergoes extremely efficient acid-catalyzed depolymerization. In contrast, the ρ-PHOMS made by desilylation of anionically obtained silyl-protected polymer is very inert to such acidolysis. The high sensitivity of the cationic ρ-PHOMS is due to the presence of end groups that are introduced during accidental and/or intentional termination and that are very reactive toward acids.