On the Amorphization of Silicon by Light Ion Implantation: Ion Flux and Substrate Temperature Effects
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ON THE AMORPHIZATION OF SILICON BY LIGHT ION IMPLANTATION: ION FLUX AND SUBSTRATE TEMPERATURE EFFECTS A. Claverie, A. Roumili, N. Gessinn and J. Beauvillain CEMES/CNRS, 29 rue J. Marvig, BP 4347, 31055 Toulouse France.
Abstract: In this work, we have observed by cross-sectionalelectron microscopy (XTEM) and high resolutionelectron microscopy (HREM) the kinetics of silicon amorphization during nitrogen and helium bombardmentsfor various dose rates and substrate temperatures. It is shown that the progressionof the cla interfaces can be accurately describedby the "CriticalDamage Energy Density" (CDED) model for both ions at 100'K. At this temperature however, dose rate effects are unimportant. When increasing the substrate temperature up to 300'K, the amorphization efficiency is lowered. At 300'K dose rate effects are importantand we show thatfor higher dose rates, the efficiency of the amorphizationprocess is improved. In this case, the amorphous layers createdby helium implantation are situated in the near surface region in apparentcontradiction with damage calculations.The present experimental study demonstrates the competition which exists between the generation rate of point defects and the anihilationrate of these defects. Therefore, it is shown that the position of the a-layers createdby light ion implantationat room temperature can be predictedby using a three-dimensionaldamage energy calculation.
INTRODUCTION AND BACKGROUND The presence of a continuous amorphous layer created before or during dopant implantation is sometimes desirable because chanelling may be strongly reduced which allows the formation of sharp dopant profiles. Afterwards it is possible to activate the dopant through a low temperature (500-600*C) SPE regrowth of the a-layer during which negligible redistribution occurs. A basic prerequisite for a more precise, reproducible use of ion implantation in VLSI technology is that it be rendered independent of machine parameters. Among many facts, the amorphization of silicon targets by bombardment is still a somewhat mysterious phenomenon. Amorphization is known to occur at "low" temperatures depending on the ion, energy and dose used for the implantation. Little attention is generally paid to ion flux and to substrate temperature effects. Some earlier studies (1, 2) have already reported that increasing ion flux may result in an enhancement of damage generation while others claimed that the efficiency of the damage buildup increases as a function of ion flux (for low ion fluxes) then reaches a maximum and decreases for higher fluxes (3). This paper will focus on the efficiency of the amorphization process as a function of both ion flux and substrate temperature. Up to now, different approaches have been used to describe the amorphization mechanism due to ion bombardment. Disregarding spike effects (spikes occur for heavy ions), the most popular is certainly the critical damage energy density (CDED) model (4). This model assumes that a crystal containing a high density of defects will spontaneous
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