Liquid Mediated Pulsed Laser Processing of Silicon
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LIQUID MEDIATED PULSED LASER PROCESSING OF SILICON NICHOLAS BIUNNO, J. NARAYAN, R.K. SINGH, S.K. HOFMEISTER* A.F. SCHREINER, M. L. S1TO** and O.H. AUCIELLO*** * Department of Materials Science and Engineering ** Department of Chemistry ***Department of Nuclear Engineering North Carolina State University Raleigh, NC 27695 ABSTRACT We have investigated the formation of thin layers of carbides and nitrides by irradiating silicon (100) single crystal immersed in clear organic solvents or liquid ammonia. The liquids were transparent to the excimer laser (X=308 nm, --- 45 ns) used, at energy densities from 0.5 to 3.0 Jcn- 2 . Most of the laser energy was absorbed by the silicon specimens above a certain threshold to cause melting of the surface to a depth of approximately 250 nm. The pool of liquid silicon reacts with the solvents or ammonia in a saturated high pressure vapor phase for a duration of approximately 200 ns per pulse. The specimens were irradiated with a number of pulses ranging from 1 to 50. The films were then analyzed for structure and composition using TEM, AES, and IR spectroscopy. We report here calculations of laser-solid interactions, microstructure, and properties of the resulting thin films. INTRODUCTION Pulsed-laser processing of semiconductors and metals for a number of device applications and for a variety of surface modifications is now well established[l]. It has the advantage of being able to process selective surface areas and also to finely control the depth to which the process occurs. Due to the nature of laser-solid interactions it is possible to obtain metastable phases with new and novel physical properties. Pulse-laser processing was first applied to ion-implantation damage annealing and epitaxial regrowth of semiconductors[2,3]. It provides a controlled source of heat by which surface layers of semiconductors and metals can be rapidly melted and cooled with rates exceeding 108 Ks"1. However, recent emphasis is being placed on laser assisted deposition[4]. In laser assisted deposition, the mode of operation can be photolytic where the laser induces gas phase chemical reactions with subsequent deposition of the reactants onto a temperature controlled substrate. Alternatively, it can be pyrolytic where the laser is used to heat and control the temperature of a select area on a substrate. And most recently, a pulsed laser evaporation technique has been used to prepare thin films of Y-Ba-Cu-O superconductor on various substrate materials[5]. In this technique the pulse-laser energy evaporates a target pellet and the evaporated material deposits on a substrate with the same stoichiometry as the target pellet. Here we report of a new method [6], liquid mediated pulse laser processing which we have used to form carbides and nitrides on silicon substrates. EXPERIMENTAL Single crystals of boron doped silicon, 500 gim thick and having (100) orientation, were immersed in liquid and irradiated by an excimer laser. The liquids used were ammonia, cyclohexane, pentane, and petroleum ether. The laser
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