Depth of Saw and Lap Damage in Germanium

A study utilizing an application of X-ray diffraction analysis has been made to determine the depth of surface damage in s ingle-crystal wafers of germanium. Damage was introduced by mechanical lapping and sawing. A scintillation-counter double-crystal diffractometer system was used with a 3-m evacuated collitnator. This experimental arrangement employed the (220) reflection, from a silicon monochromator, using Cu Kα1 radiation. All wafers were taken from dislocation-free crystals which were subsequently centerless ground, sawed, lapped, and etched. Half-widths were determined for the sawed and lapped conditions. Successive etchings were used to remove the damaged surface layer with the resulting halfwidths being plotted as a function of thickness removed and etching time.The depths of damage—8.0 ± 0.7 μ for sawing and 3.0 ± 0.7 μ for lapping—were indicated by the depth at which the line breadth measurements reached a limiting value. For crystals used in this study, the median value for this limiting half-width was 28″. The experimental data expressing the rate of etching indicates the presence of two separate rates—one for the damaged material and one for the undamaged substrate. An unexpected finding is a faster etch rate for the undamaged material than that observed for the damaged layer. Quite good correlation is obtained between the depth of damage as determined by X-ray and etching techniques.