Non-destructive testing of damage layers in GaAs wafers by surface acoustic waves

Slicing of GaAs wafers from ingots causes a damage laver which must be completely removed by polishing. For this purpose a non-destructive control of the machining process is very desirable. Laser-induced surface acoustic waves are demonstrated to be a promising method to characterize the state of the surface. The dispersion of this wave mode has been used as a measure of the thickness of the damage layer. The investigations were performed on wafers which were stepwise polished to remove the damage layer. In this way the damage layer was found to have a thickness of about 10 mu m. This result is in good agreement with investigations performed with positron annihilation The damage layer was found to have a considerably lower elastic modulus (by up to about 18%) than the bulk material of GaAs. With increasing polishing depth the modulus of the damage layer approaches that of the bulk material. The elastic modulus allows us to estimate the defect volume in the damage layer by applying a theory of heterogeneous materials. A defect volume fraction of 0.48% was estimated for the as-sawn state of the wafer. It decreases to 0.05% for a polishing depth of 6.5 mu m and goes to zero at a depth of 10 mu m.