Thermoreflectance and photodeflection combined for microscopic characterization of metallic surfaces

Abstract

A non-contact technique is presented that provides information on thermal diffusivity at the cubic micron scale in metal surfaces. It relies on the simultaneous fit of the frequency dependence of two mechanisms that appear when heating the surface of the sample with a modulated laser beam, the thermoreflectance (change in reflectivity with temperature) and photodeflection (the deflection of the beam due to the surface deformation). A complete analytical model is presented that takes both mechanisms into account, showing that the relative importance of both mechanisms depends strongly on the material under test and varies in several orders of magnitude between different metallic alloys. By filtering the reflected signal with an adjustable knife edge, the photodeflection signal can be enhanced. It is also shown how the signal arising from each mechanism depends strongly on the relative position of the pump and probe beams. A characteristic modulating frequency appears at which a drop in the signal with frequency is observed that is coincident for both mechanisms. From the determination of the characteristic frequency, the local heat diffusivity can be determined as it is shown for the case of an AISI304 stainless steel sample.