Differential amplitude scanning optical microscope: theory and applications.

A differential amplitude scanning optical microscope is described. The operation of the system depends on the sinusoidal interrogation of the sample, at any scan coordinate, by moving the beam through a fraction of the focal spot on the object surface. The technique is capable of detecting variations in sample reflectivity down to 3 x10(-7) in a 10-Hz bandwidth. The image formation mechanism of the microscope is presented and applied to a number of important structures. Experimental results on grain structure of metals, surface integrity of polished diamond, and doped silicon wafers are presented. The microscope is well suited to the accurate measurement of linewidths. Theoretical and experimental results are presented to demonstrate this capability. It is further shown that the main limitations in providing accurate linewidth measurements are due to the accuracy of the scan coordinates rather than the optical aspects of the system.