Coupled photo-thermal and time resolved reflectivity methods to original investigation of laser/material nanosecond interaction

Abstract

A high number of papers were published on the simulation of laser/surface interaction at the level of nanosecond scale. Several assumptions on thermal properties data, laser spot homogeneity, were assumed for describing as well as possible the boundary conditions, the mathematical writing and finally the numerical or the analytical results. A few tentative of surface temperature monitoring during laser processing were proposed for the numerical validation. Also, simulation of the melting kinetics is rarely directly compared to in situ experiments. It is very hard to determine the time duration of a melting pool by in situ experiments. It should be the same for the surface temperature. A new method to plot the thermal history of the surface by using a combination of the Time Resolved Reflectivity (TRR) and the Pulsed Photo-Thermal (PPT) or Infrared Radiometry (IR) methods is proposed in this paper. Surface temperature, melting kinetics, threshold of melting and threshold of plasma formation are determined in the case of KrF laser spot in interaction with several materials. In the first step, the experimental setup including fast detectors (IR, UV, Vis.) and related optical devices is described. In the second step, typical results (TRR and IR spectra) for monocrystaline silicon are presented and discussed. Namely, phase change transitions (melting and resolidification) are detected versus fluence change and number of laser shots change. TRR and IR spectra of metallic surfaces (Cu, Mo, Ni, Stainless steel 15330 and 17246, Sn, Ti), are measured. For each sample the surface temperature during heating, the threshold of melting, melting duration and the threshold of plasma formation are directly deduced.