Evaluation of Silicon Wafer Surface Quality by Coherent Radiation Scattering

The inspection of surface quality of opaque materials by optical methods is now a very attractive topic with many people involved and many papers covering this subject. The theories describing phenomena produced by interaction of a laser beam with a conductive surface are studying two main aspects: specular reflection and scattering of the incident beam. The spatial distribution of scattered and specularly reflected flux is strongly dependent of the surface quality. Reciprocally, the analysis of this distribution will give an information about surface quality. The analysis of the interaction between a light beam and a rough surface started long time ago. The theoretical analysis and measurement of the scattered beam and of the specularly reflected beam are reported in many papers. The specular reflective component R s produced by a light beam incident normally to a perfect conductive surface can be expressed by: $${R_s} = {R_0}\,\exp \left[ { - {{(4\pi \sigma )}^2}/{\lambda ^2}} \right]$$ (1) where R 0 is the reflectance of a perfect surface of the same material, σ is the RMS value of the roughness, defined as the RAIS deviation of the surface from the mean surface level, λ is the wavelength of the incident light beam. In formula (1) is considered the total specularly reflected light flux.