LASER PROCESSING OF SEMICONDUCTORS
Doctor of Philosophy
The great potential for advancing the state of the art in fabrication of semiconductor devices and integrated circuits with the use of laser beam processing techniques is reviewed. The unique characteristics of this novel technology, due to its ability to heat spatially localized regions to a high temperature for a very short time duration, may open up various possibilities of microelectronic device fabrication that is truly novel, and could prove to be essential for the realization of VLSI and VHSI. Full utilization of the inherently unique advantages offered by the use of directed energy radiation is contingent upon an improved understanding of basic underlying heating mechanisms involved. We present in detail the current physical understanding regarding the various processes involved in the absorption of optical energy and its conversion into local lattice heat and subsequent transport in the semiconductor layer under the influence of a high power laser irradiation. The coupling of laser energy to the lattice is examined from the point of view of the transient depth profile of lattice temperature, T which is one of the most fundamental parameters that eventually determine the electrical characteristics of laser-processed semiconductor devices. A one-dimensional thermal model for laser annealing is developed and used to describe the thermal evolution of the sample over a wide range of laser wavelengths, pulse intensities and durations. The dynamic characteristics of T attained in Si for a given rate of energy input is described analytically, for the first time, based on a new technique developed by us for this specific purpose, namely the parametrized perturbation technique in Green's function formulation. Specifically, the kinetics of the depth profile of T in laser-annealed semiconductor and the threshold pulse energy for the onset of surface melting of Si are characterized in terms of both the material parameters such as the ambipolar diffusion length of excess charge carriers, the optical absorption coefficient and the thermal diffusivity, and the operating laser beam parameters. These analyses provide considerable insight into the transient heating phenomena and localized material modifications, and physical information concerning the initial choice of parameters required for controlling the final parameters of laser annealed devices. These are, of course, of crucial importance for facilitating optimization of laser parameters during irradiation of various types of structures encountered in real-life process implementation.
Electronics; Electrical engineering