An optical scanning microscope system for accurate measurement of linewidth on wafers is described. The development of this system in both theory and experiment parallels the treatment of transmitted light measurements on see-through photomasks as previously described. Threshold equations for determining edge location have been developed which include corrections for contrast and phase. In reflected light, the wavelength dependence of these parameters due to thin film interference requires a much narrower spectral bandwidth. In addition, other light losses dictate the use of laser illumination. Control of the spatial coherence of the laser is discussed. A comparison of theoretical and experimental results is given along with a comparison of reflected and transmitted light measurements.
"Optical Linewidth Measurements On Wafers", Proc. SPIE 0135, Developments in Semiconductor Microlithography III, (6 September 1978); doi: 10.1117/12.956121; http://dx.doi.org/10.1117/12.956121