LASER PROCESSING OF ELECTRONIC MATERIALS

Abstract

The laser is increasingly used as a tool for microprocessing of materials. The temperature rise induced on the material substrate is an important parameter for laser processing. The heat originates almost instantaneously as the laser irradiation interacts with the substrate material and has a direct influence on many of the processes that are subsequently induced. A better understanding of these processes can be gained through a heat transfer analysis. This will in turn help in optimising the techniques used in laser processing. This thesis deals with the use of both semi-analytical and numerical methods in the analysis of heat transfer during laser processing and the verification of some of these methods through experimental investigation. A total of four specific cases are examined. The first of these looks at the problem of inducing a square-shaped temperature profile using an argon­ ion (Ar+) laser which has a Gaussian-shaped beam intensity. It illustrates the use of a semi­ analytical method, based on the principle of superposition, in the solution of the non-linear problem. The semi-analytical method was subsequently used in the next problem of deducing the temperature distribution induced in a Gaussian-shaped etched hole. By comparing with the results obtained using a separate finite-element formulation, inaccuracies due to the method of image adopted in the formulation of the problem were investigated. Effects of polarisation and multiple reflections occurring along the slope of the etched hole were also examined. The finite-element model developed for the above study was further modified to predict the final etched profile obtained experimentally in the pyrolytic etching of silicon and gallium arsenide substrates using an Ar+ laser. Further insights to the etching process were obtained as a result. The fourth case deals with the experimental and theoretical investigations of the melt reliefs formed on a moving silicon substrate by pulses of the Ar+ laser. A finite-element analysis has been carried out for the melting problem to explain the formation of the peculiar topographical features obtained in the experiments.