Bleaching-induced changes in the dispersion curves of DNQ photoresists

Three sets of data are required to model a non-chemically amplified photoresist: the exposure eor ABC (Dill) parameters, the development parameters, and the refractive index. The refractive index of photoresists has often been considered constant in simulations and other work. However, the refractive index can change as the chemical composition of the photoresist is modified during the exposure reaction. This work presents a study of the refractive index as a function of exposure for a series of commercial, non-chemically amplified DNQ/novolak photoresists using spectroscopic ellipsometry. It was found that the real part of the refractive index for a photoresist changes by as much as 5% at the wavelength of exposure, in part due to the resist bleaching which removes the anomalous dispersion contributions to the refractive index. A discussion of the impact of such refractive index changes on Dill parameter extraction and the simulation of standing wave patterns will be presented.