Study of acid transport using IR spectroscopy and SEM

Abstract

The migration of acid catalyst molecules from exposed regions into unexposed regions in chemically amplified photoresists and the resulting image blur, has long been recognized as an important topic requiring close study. A fuller understanding of acid transport mechanisms occurring during the post exposure bake is important to help guide the development and formulation of photoresists capable of reliably resolving the increasingly small features required by the semiconductor industry. This paper reports the direct measurement of diffusion coefficients for perfluorobutane sulfonic (nonaflate) acid in poly(4-hydroxystyrene) at several elevated temperatures. These results show that the Fickian diffusion coefficient for nonaflate acid in poly(4-hydroxystyrene) is too small, at typical post exposure bake temperatures, to account for observed isofocal bias. Also reported is a new technique for investigating acid transport properties of photoresist films. This method uses selective silylation to decorate cleaved resist film stacks, so that the extent of acid catalyst migration can be measured directly by scanning electron microscope (SEM). Acid transport distances from the SEM method are compared to those obtained from infrared (IR) spectroscopic techniques.