This paper describes a surface characterization method of an ultra-precision machined surface of Si wafer by means of photo-reflectance spectra in the region from visible to ultraviolet range. To detect small change of reflectance spectrum, differential reflectance (D.R.) spectrum is applied which means the relative differences of reflectance between a testing specimen and reference. A reflectometer system is designed to detect D.R. of the order of 0.01%. The reproducibility of this system is tested by measuring D.R. spectrum between two surfaces which are simultaneously cleaned by aqueous solution of HF, and the magnitude of the obtained D.R. spectrum is less than 0.05%. To investigate the influence of storage of Si wafer, D.R. spectrum is measured between the specimen stored in nitrogen gas ambients for one week after the HF cleaning and just HF cleaned. The maximum magnitude of the obtained D.R. spectrum is less than 0.3%. The influence of storage can be detected using this system. It is concluded that the measurement of D.R. spectrum is useful for the characterization of ultra-precision machined surface of Si wafer.
