Curious Morphology of Silicon-Containing Polymer Films on Exposure to Oxygen Plasma

Thin films of silicon-containing polymers were studied to investigate changes in surface composition and morphology on exposure to an oxygen plasma. For low molecular weight poly(pentamethyldisilylstyrene) (P(PMDSS)), a reticulated structure was observed by atomic force microscopy (AFM) that could limit future lithographic applications of these materials. The reticulations were of approximately 1 μm in width and 5 μm in length, though a higher molecular weight polymer resulted in smaller feature sizes. In polysilane polymers containing silicon in the backbone and molecular weights significantly larger than the entanglement molecular weight, the feature dimensions were even smaller. Films etched at lower temperature (0 °C) displayed none of the reticulated morphology, retaining instead the smooth appearance of pre-etched films. It was found by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) that a thin (<100 Å) layer of SiO_x formed on the surface of all of the studied silicon-containing polymer films. Appearance of the reticulated morphology required the combined presence of heating, oxygen plasma, and silicon in the polymer. The reticulated structures are believed to result from the destabilization of the thin films as they undergo the transition from a nonpolar organosilane to a polar oxide.