Study on the relationship between the damage of machined surface layer and the dispersion curve of laser-induced surface acoustic wave

Abstract

Laser-induced surface acoustic wave is a new surface damage detection technology, which uses the transient thermal effect of pulse laser to excite the surface acoustic wave on the sample surface. The damage of machined surface layer will cause the velocity change of laser-induced surface acoustic wave. This phenomenon is introduced into the evaluation of the surface layer damage of silicon wafers in this paper. The three-dimensional propagation model of the single-layer structure is extended to the double-layer structure, and it is proved that the gradient change of the surface layer damage will change the trend of velocity curve. In order to characterize the change of velocity curve, the surface layer damage coefficients are introduced. The cross-sectional transmission electron microscope (TEM) image of the lapped silicon wafer shows that the surface of the silicon wafer after lapping will produce a surface damage layer. The test data of the nanoindenter shows that the modulus of the surface damage layer is lower than that of the bulk material. The modulus estimated by dispersion curve is very close to the data measured by nanoindenter, indicating that the dispersion curve can truly reflect the damage degree of silicon wafer surface layer.