Optical Reflectance Method for Determining the Surface uality of Sapphire ( Al2 O 3 )

Abstract

Polished sapphire wafers are used as substrates for the growth of crystalline silicon. The quality of the substrate surface is an important factor in determining the crystalline quality of the silicon film and, in turn, relates to silicon device performance and reliability. To improve silicon‐on‐sapphire device manufacture, a sensitive and nondestructive optical reflectance technique has been developed to determine the quality of polished sapphire surfaces. The correlation between surface damage and single or multiple specular reflectance of sapphire in the lattice mode region, 900–300 cm−1, is reported. As a result of surface damage, the vibrational modes of sapphire are distorted and the optical constants associated with these modes change. To interpret the single and multiple reflectance spectra the optical constants of undamaged and damaged (11̅02) sapphire surfaces have been calculated from reflectance data using the Kramers‐Kronig method. From this analysis, the reflectance of sapphire at about 600 cm−1 is found to be very sensitive to surface quality. In this spectral region multiple reflectance can be used to measure the surface quality of sapphire with a high degree of sensitivity. The technique can, in principle, be used to measure the quality of any semiconductor or dielectric surface in spectral regions of high reflectance, such as regions of lattice band or bandgap absorption.