Chemical amplification based on acid-catalyzed depolymerization.

Abstract

The chemical amplification concept proposed in 1982 to support high resolution short wavelength lithographic technologies that demand highly sensitive advanced resist systems is based on acid-catalyzed crosslinking, deprotection, and depolymerization reactions. Each approach has shown a tremendous progress. In this paper is reviewed the depolymerization concept for the design of sensitive positive resist systems formulated with triarylsulfonium salt acid generators. Three modes of the initiation of acid-catalyzed depolymerization are discussed; (1) attack of acids onto backbone oxygens as exemplified by depolymerization of polyphthalaldehyde (PPA), (2) scission of pendant ester groups followed by depolymerization with poly(α-acetoxystyrene) (PACOST) as an example, and (3) unzipping from the polymer ends as demonstrated by depolymerization of cationically obtained poly(p-hydroxy-α-methylstyrene) (pPHOMS). The positive resist systems based on depolymerization of PPA derivatives include a thermally- evelopable O2 reactive ion etch (RIE) barrier resist for use in the bilayer scheme and the use of PPA as a polymeric dissolution inhibitor for novolac resins. The deesterification of PACOST results in formation of poly(phenylacetylene) (PPA) and simultaneous depolymerization to provide positive images upon development with xylenes, which is briefly discussed. Cationically prepared pPHOMS exhibits 80% thickness loss at ca. 1mJ/cm2 upon postbake at 130°C whereas anionic pPHOMS and cationic meta-PHOST are very stable toward acidolysis, indicating that the depolymerization propagates from the termination side of the chain.