The structural change and defect generation in Si by plasma exposures are investigated by spectroscopic ellipsometry (SE) and photoreflectance spectroscopy (PR). For an Ar-DC plasma exposure with 300 V bias, the SE with an optimized model identifies 1 nm-thick interfacial layer (IL) between the surface layer and the substrate. The PR indicates the mechanical strain change by approximately 0.1%. The PR was applied to an estimation of plasma-induced carrier trap site density on the basis of a model correlating surface potential to the density. Combined with plasma diagnostics, the defect generation probability was estimated for the present condition. (C) 2007 Elsevier B.V. All rights reserved.