Photochemical reaction of organosilane self-assembled monolayer as studied by scanning probe microscopy

Abstract

A photopatterning process of an alkylsilane self-assembled monolayer (SAM) using vacuum ultraviolet (VUV) light at 172 nm has been studied by characterizing the VUV-irradiated SAM surfaces through lateral force microscopy (LFM) and Kelvin-probe force microscopy (KFM). The SAM was formed from n-octadecyltrimethoxysilane on a Si substrate covered with a 2-nm thick oxide layer. Due to VUV irradiation in the presence of atmospheric oxygen, polar functional groups, e.g., COOH, were generated through photochemical oxidation of the SAM which was gradually etched. Finally, the SAM was completely removed from the VUV-irradiated region. Relative frictional forces and surface potentials of the VUV-irradiated regions at various irradiation dose rates were measured by LFM and KFM using the unirradiated regions on each sample as a reference. Changes in the surface chemical composition of the SAM could be measured by these scanning probe microscopic methods. The LFM contrast increased monotonically with the increase in the dose rate, while the surface potential contrast measured by KFM showed a characteristic feature. At a certain dose rate of 10–15 J/cm2, VUV-irradiated SAMs showed particularly lower surface potentials than the other SAM surfaces with different dose rates. This is most likely because COOH groups, which were considered negatively charged in part, had been accumulated on the surfaces. LFM and KFM measurements are promising in order to probe chemistries on organic thin films proceeding in small domains with a scale.