PHOTOREACTIONS IN HALOGEN-CONTAINING NEGATIVE-TONE NOVOLAC RESISTS

The photolysis of halogenophenol novolacs is determined by the substit ution of halogens by hydrogen and the formation of quinoid groups and intermolecular crosslinks. This is concluded from elemental analyses, NMR, IR and optical absorption measurements. The rate of halogen relea se 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 f rom 4-position much more readily than trimers and tetramers. ESR measu rements at 77 K and flash photolysis studies at 296 K yielded evidence for the intermediate existence of phenoxyl and aryl radicals. Lithogr aphic tests demonstrated the high UV-sensitivity of resist formulation s based on halogen-containing novolacs. The increase in sensitivity re lative to that of formulations based on nonhalogenated novolacs is 6 t o 10fold (system: 4-chlorophenol novolac/4,4'-bisazidobiphenyl (5%)) a nd ca. 25fold (system: 4-chlorophenol/m-cresol novolac/hexamethoxymeth ylmelamine (5%)). A postulated reactions mechanism concerning the sens itivity increase takes into account that halogen elimination results i n the formation of additional radicals that accelerate the rate of cro sslinking. 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 no volac matrix.