Patent US 3,957,376

A spot of collimated monochromatic radiation is applied to a semiconductor wafer to obtain a diffraction pattern from two edges within an area, which is the size of the spot, on the wafer. The diffraction pattern is reflected to a photodiode array wherein the light intensity of the diffraction pattern at each of a plurality of positions is obtained. Each of the photodiodes is scanned separately for the same period of time to determine the light intensity at its location. The distance between the zero intensity positions on the photodiode array is determined very precisely and utilized with the wavelength of a laser, which supplies the spot of collimated monochromatic radiation, and the effective focal length of the lens system, which images the diffraction pattern to the photodiode array, to calculate the linear distance between the edges.