An infrared differential interference contrast method using a YLF laser (1.3 mu m) was used to evaluate the surface roughness of polished silicon wafers and wafers oxidized at 700, 900, and 1000 degrees C. For polished wafers, the root-mean-square optical phase shifts (Delta phi(rms)) from this method exhibit a nonlinear relation with the average roughness, R(a), obtained by a conventional laser interferometry technique. The surface roughness of the thermally oxidized wafers is found to have a nonmonotonic trend that evolves with time. This trend is also dependent on the oxidation temperature.