We have investigated adsorption process of hydrocarbon species on a silicon surface during methane plasma investigated by in-situ and real time infrared absorption spectroscopy (IRAS) in multiple internal reflection (MIR) geometry. We found that what kinds of hydrocarbon species are deposited strongly depends on the working pressure under which the methane plasma is generated. IRAS data also showed that hydrocarbon species randomly stick on an Si surface at a low hydrocarbon coverage, but does not stick at the high coverage when the methane plasma is generated at low pressure. (c) 2005 Elsevier B.V. All rights reserved.