The role of radiation in melt stability in zone-melt recrystallization of SOI
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L. R. Thompson and G. J. Collins Colorado State University, Ft. Collins, Colorado 80523 (Received 2 June 1989; accepted 17 January 1990)
Under circumstances in Zone-Melt-Recrystallization (ZMR) of Si-on-Insulator (SOI) structures where radiative heat loss is significant, the —50% decrease in emissivity when Si melts destabilizes the Si molten zone. We have demonstrated this both experimentally using a slowly scanned e-beam line source and numerically with a finite-element computational simulation. The resulting instability narrows the process window and tightens requirements on beam control and background heating uniformity, both for e-beam ZMR systems and optically-coupled systems such as a graphite strip heater.
I. INTRODUCTION
The use of an energetic beam or hot radiant source to melt and recrystallize an isolated layer of Si deposited on an insulator is known as Zone Melt Recrystallization (ZMR) and is a promising technique for forming Si-on-Insulator (SOI) wafers to be used in the next generation of microelectronics. Several approaches have been explored, from rapidly scanned high-power spot or line beams1'2 to slow scanning of long lineshaped sources, covering a wafer in one pass.3 It has long been known that a useful phenomenon in ZMR is the increase in reflectivity of Si upon melting,3"7 which self-regulates the input power for heating sources optically coupled to the Si layer, such as a laser or graphite strip system. However, accompanying the increase in reflectivity is the change in emissivity, whose effects on melt stability have been heretofore neglected. For nonoptical heating sources such as electron beams which are not self-regulated (by the change in reflectivity), it is particularly important to understand the effect of emissivity changes in order to control the melt/recrystallization process. Moreover, even for optically coupled heating the effects of radiation loss need to be included for an accurate understanding of the recrystallization process, particularly when the background temperature is very high and thermal gradients are kept to a minimum. This latter set of conditions describes the graphite strip heater and analogous approaches to ZMR, where a long lineshaped beam is used to recrystallize an entire wafer in one pass at relatively low sweep speed and high background temperature.3 In particular, we have been exploring the use of line-shaped electron beams for ZMR in this low thermal gradient, low sweep speed regime, and have found that under circumstances where radiation loss is a sig998
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nificant fraction of the total heat leaving the sample, the shape of the molten zone tends to be unstable. We attribute this to the effects of the -50% drop in emissivity that Si undergoes when it melts. Finite-element computational simulations of the experiment confirm the effect and predict a similar instability for optically coupled ZMR systems. II. EXPERIMENTAL
The experimental arrangement we have been using to explore the process of ZMR is a line-source, coldcatho
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