Transient laser-induced surface deformation of silicon in relation to damage

摘要

Transient thermal expansion, heat generation and conduction, and nonlinear laser heating of single crystal Si wafers below and above the damage onset has been investigated via a laser pump-probe configuration. The transient photothermal deflection (TPD) technique that we employed consisted of a 10 ns Nd:YAG pump source at 1064 nm with 10 Hz repetition rate. A cw HeNe probe beam was used to probe the localized time dependent slope change of illuminated surface. The deflection of the reflected probe beam, centered at the maximum slope of the irradiated spot, was detected by a fast bicell photodiode. The deformation signals were recorded by a digital camera system in conjunction with a high-speed oscilloscope. The waveforms were later analyzed for surface angular deflection and vertical displacement based on the geometry of the Gaussian irradiation profile. Vertical displacement down to a few nm could be detected and converted into instantaneous peak surface temperature by a first- order, approximate thermal model. Measured displacement and surface temperature were then compared to computer simulations at different fluence levels. They were found to be in excellent agreement to each other. In addition, single and multiple shot experiments were performed to obtain their respective damage onsets and thresholds. Measurement of peak surface deformation at subthreshold fluences gives insight into the thermomechanical processes which may play an important role in multi-pulse damage.