High Precision Optical Profilometer Using the Differential Method

A scanning type optical precision profilometer is proposed. By Calculating the difference of the measured height at a point and its surrounding areas, it gives the roughness profile nearly independent of vertical vibrations. The height variation of a point and areas are measured from the interfered light intensity variation using a photomultiplier tube and three Si photo diodes arranged on the image plane of the interference microscope. The performance of the profilometer is checked from the roughness measurements of the aluminum substrate for hard disc, the aluminized optical flat, a Si-wafer, and the step standard calibrated by a stylus instrument. As a result, the reproducibility of 0.5nm was verified and the step height at the grain boundary of diamond-turned aluminum disc was measured as about 2.0nm. The errors, caused by the distribution of the slight reflectivity, were reduced by using the dual polarized light interference method. The filtering effect on the profile determined by the sensor arrangement is calculated and corrected by the digital signal processing. By this correction, not only the precise roughness profile but also, the figure profile can be measured independently of vertical vibrations.