Photoreactions in halogen‐containing negative‐tone novolac resists

Abstract

AbstractThe photolysis of halogenophenol novolacs is determined by the substitution of halogens by hydrogen and the formation of quinoid groups and intermolecular crosslinks. This is concluded from elemental analysis, NMR, IR and optical absorption measurements. The rate of halogen release depends on the chemical nature of the halogen. It increases in the order F < Cl < Br < I. Chlorine elimination from 4‐position is favored over that from 2‐ and 3‐position. Moreover, dimers release chlorine from 4‐position much more readily than trimers and tetramers. ESR measurements at 77 K and flash photolysis studies at 296 K yielded evidence for the intermediate existence of phenoxyl and aryl radicals. Lithographic tests demonstrated the high UV‐sensitivity of resist formulations based on halogen‐containing novolacs. The increase in sensitivity relative to that of formulations based on nonhalogenated novolacs is 6 to 10fold system: 4‐chlorophenol novolac/4,4′‐bisazidobiphenyl (5%) and ca. 25fold system: 4‐chlorophenol/m‐cresol novolac/hexamethoxymethylmelamine (5%). A postulated reactions mechanism concerning the sensitivity increase takes into account that halogen elimination results in the formation of additional radicals that accelerate the rate of crosslinking. Moreover, hydrogen halide generated by hydrogen abstraction of halogen radicals (Hal˙ + RH → H‐Hal + R˙) provides for the acid required to catalyze the reaction of the melamine compound with the novolac matrix.