Power spectral densities: A multiple technique study of different Si wafer surfaces

Power spectral densities (PSDs) were used to characterize a set of surfaces over a wide range of lateral as well as perpendicular dimensions. Twelve 200-mm-diameter Si wafers were prepared and the surface finishes ranged from as-ground wafers to epitaxial wafers. The wafer surfaces were then measured with different methods: atomic force microscopy, angle-resolved light scatter, interferometric microscopy, optical profiling, stylus profiling, and capacitance-based wafer thickness gaging. The data were used to compute one-dimensional PSDs and the curves were plotted as functions of spatial frequencies, comparing results for different samples or for different instruments. The useful frequency range for each method is indicated and differences in the calculated PSD values in the overlapping region of two or more methods are discussed. The method used to convert two-dimensional PSDs to one-dimensional ones is presented.