Ion and Vacuum Ultraviolet Photon Beam Effects in 193 nm Photoresist Surface Roughening: The Role of the Adamantyl Pendant Group

Previous studies have identified a synergistic effect of ion bombardment, vacuum ultraviolet (VUV) radiation, and moderate substrate heating in 193 nm photoresist (PR) surface roughening. The degree of surface roughness is comparable to that obtained from typical plasma etch processes. Ion bombardment results in a carbon-rich surface layer with a thickness of a few nanometers, and VUV radiation causes bulk modification to a depth of about 100 nm. The probable difference in mechanical properties between the carbon-rich surface layer and the softened underlayer is proposed to be the main cause of PR surface roughening. In the same vacuum beam apparatus, we further characterize the roughening of associated homopolymers: poly-(2-methyl-2-adamantyl methacrylate) (p-MAMA) and poly-(R-functionalized adamantyl methacrylate) (p-RAMA). MAMA is the leaving group, and RAMA is the polar group in 193 nm PR, respectively. Monitored by ex situ transmission FTIR and in situ mass spectrometry, p-MAMA undergoes temperature-dependent bulk degradation during VUV exposure while p-RAMA is relatively insensitive to VUV. We identify the adamantyl leaving group as one of the main photolysis products from p-MAMA. Higher substrate temperatures increase the vapor pressure and facilitate desorption of the detached adamantyl leaving group. The adamantane loss/detachment shows strong correlation to PR surface roughening. A bi-layer wrinkling mechanism is proposed to qualitatively explain the observed surface roughening. The present results demonstrate that PR leaving group content and structure can couple to plasma etching processes and strongly alter post-etch surface morphology.