Characteristics of Recrystallised Polysilicon on SiO 2 Produced by Dual Electron Beam Processing
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CHARACTERISTICS OF RECRYSTALLISED POLYSILICON ON SiO2 PRODUCED BY DUAL ELECTRON BEAM PROCESSING J.R. DAVIS, R.A. McMAHON AND H. AHMED Cambridge University Engineering Department, Cambridge, England, CB2 IPZ
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ABSTRACT This paper describes the production of large areas of precisely oriented, defect-free, single crystal silicon films on Si02 by dual electron beam heating of deposited polysilicon Defects which occur in the film far using lateral epitaxy. from the seeding window are characterised, and the dependence of the area of the defect-free region on the processing conditions is discussed. INTRODUCTION The conversion of polysilicon layers on insulators to single crystal Si would be an important technological advance (C). In this paper we report the production of extended areas of precisely oriented, defect-free single crystal films by the lateral seeding technique (2), and using a dual electron beam processing system (3,4). This system, with its precisely controllable high power beams, which have good energy coupling to the silicon, allows us to investigate a variety of heating techniques. The two most commonly reported recrystallisation systems employ focussed laser beams or large graphite strip heaters. In the former system (2), which has been shown to be capable of producing small islands of defect free silicon, the specimen is heat-sinked at a relatively low temperature, creating large thermal gradients from the laser spot to the backside of the wafer. Because the small spot of a low power beam has to be rastered over the surface of the cool specimen, the laser system has a poor wafer throughput. The graphite strip heater (5), on the other hand, has a high throughput and does not produce large temperature gradients, but it does hold the substrate at high temperatures for long enough to cause changes to any previously processed substrate structures. The characteristic low angle grain boundaries in the recrystallised silicon do not prevent the fabrication of MOS transistors, but the material would be unacceptable in VLSI circuits where leakage and lifetime are critical. TECHNIQUES The specimen is supported in thermal isolation so that both its surfaces are accessible to electron beams of 20-3OkeV energy. One beam provides isothermal heating of the specimen by multiply scanning its back surface. A total power of 500 W is available in the bottom beam, which can bring the specimen temperature to a radiation-limited thermal equilibrium of, typically, iiOO C in about 4 seconds. The upper beam provides the small additional amount of heat, required to give localised melting of the polysilicon film, by means of a line beam (3,4). In one method of forming the line-beam, a line source of electrons is imaged directly onto the specimen. In other methods the emission from a point source gun may be either focussed into a small circular spot which is rapidly deflected in one direction by a triangular waveform to time-synthesise a line, or it may be shaped by electromagnetic optics into a stationary line sp
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