Chemical Mechanisms in Photoresist Systems: Part III . Crosslinking and Reciprocity Failure in Bisazide Resist

Abstract

Bis(p‐azidobenzal) cyclohexanone‐cyclized polyisoprene photoresist was exposed under different conditions and the emergence of longer wavelength absorption (410 nm) was studied. As the absorbance at 410 nm is correlated with the degree of cross‐linking, and its intensity and wavelength are similar to the spectrum of a related model bisamine compound, bis(p‐dimethylaminobenzal)‐cyclohexanone, we have identified this absorption as bisamine cross‐linking. The approximate number of bisamine cross‐links/cm2 at different stages of cross‐linking can be estimated on this basis. Reciprocity failure was also studied by monitoring bisamine cross‐linking under different exposure conditions. Reciprocity effects can be explained by the competition between nitrene‐polyisoprene and nitrene‐oxygen reactions. The reaction rate constant of the nitrene‐oxygen reaction is about times of that of the nitrene‐polyisoprene reaction and can be considered as a diffusion‐controlled reaction with respect to oxygen. The most important parameter governing the reciprocity relationship is the ratio of rates of nitrene formation and of oxygen permeation from the atmosphere into the film. Either inert gas atmosphere or rapid exposure in air can overcome oxygen permeation and yield maximum sensitivity, since the equilibrium concentration of oxygen in the film is much less than the cumulative concentration of nitrene formed by a rapid exposure. The photodecomposition of bisazide was also compared in oxygen and in nitrogen, and no appreciable difference in rate was observed. Thus reciprocity failure is not associated with bisnitrene formation, but rather with the competitive reactions which consume the nitrene, once formed.