DN 21, DN 41: negative-tone photoresists for deep-UV lithography

Abstract

Advanced negative photoresists for both KrF-excimer laser lithography (DN 21) and deep-UV broadband irradiation (DN 41) have been investigated. The materials are based on the well established chemistry of an acid-sensitive melamine derivative as the crosslinking species, a phenolic polymer containing 3-methyl-4-hydroxystyrene and 4-hydroxystyrene units, and an efficient photoacid generator. This paper briefly reviews the chemistry of DN 21 and DN 41 and concentrates on some selected lithographic performance parameters. The effects of various process conditions on resist performance have been examined, especially with respect to photosensitivity, contrast, and resist profiles. DN 21/DN 41 show an excellent delay time behavior between exposure and post exposure bake and are insensitive to environmental contaminants. An optimized process gives a resolution of 0.30 micron lines and spaces using a KrF-excimer laser stepper. A good linearity down to 0.35 micrometers and depth of focus values of clearly above 1.0 micron for 0.36 micrometers lines and spaces is shown. Other important factors for the lithographic performance, namely the 3-methyl-4-hydroxystyrene/4- hydroxystyrene ratio, the polymer/crosslinker ratio, and the concentration of photoacid generator is discussed as well. In particular, metal-ion and surfactant free standard developers yield highly vertical resist sidewalls without any scumming, bridging effects or residues between the resist lines. Dry etch properties of the material are found to be equal to conventional positive systems, while the thermal stability is superior.