Determination of surface roughness of silicon wafers using soft-x-ray scattering*

Abstract

Characterization of the morphology of polished surfaces, particularly on the nanoscale, has become increasingly important for a wide range of technologies. One method that is sensitive to roughness on this scale is hard x-ray scattering. However, the technique has limitations because measurements must be performed at grazing incidence to obtain sufficient signal. These limitations can be partly overcome by using soft x rays. In this study, we use soft-x-ray scattering to examine the morphology of Si wafers before and after the final chemical polishing step used in wafer fabrication. We use transverse (rocking) scans to map the diffuse intensity distribution normal to the surface. Both sample preparations yield a two-component profile, an instrument limited peak in the specular direction and broad diffuse component, suggesting a roughness with a long-wavelength cutoff. The samples that did not receive the final polishing step showed much greater diffuse intensity, indicating that they are much rougher. The soft-x-ray measurements are in qualitative agreement with optical scatter measurements. Results will be compared with atomic force microscopy measurements and discussed in terms of the band pass of the respective measurements. Efforts will be made to compare the rms roughness and the power spectral density extracted from these techniques.