Effectiveness of Thin Film Encapsulants for Reducing Evaporation during Rapid Thermal Processing of GaAs
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EFFECTIVENESS OF THIN FILM ENCAPSULANTS FOR REDUCING EVAPORATION DURING RAPID THERMAL PROCESSING OF GaAs
T. E. Haynes,* S. T. Picraux,** and W. K. Chu* * Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27514 -Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185
ABSTRACT
A technique is described for direct measurement of evaporation of Ga and As from capped GaAs during RTP. Application of this method to the study of Si, Si0 2 , and Si N4 caps with thicknesses of 20 nm to 60 nm provides a direct measure of the temperature ranges for which the caps are able to prevent evaporation during RTP. In addition, kinetic studies of the evaporation at slightly higher temperatures provides information useful for establishing the predominant evaporation mechanism. For the encapsulants studied, these measurements indicate that the observed evaporation is due to formation of cracks in the film during the initial 10 sec of RTP.
INTRODUCTION
Control of preferential evaporation of arsenic during post-implantation thermal processing is necessary to achieve high carrier mobilities and complete activation of ion-implanted
dopants in
(RTP)
CaAs.
Rapid thermal processing
provides one means for reducing the amount of such evaporation. Further can be obtained by the deposition of a thin film encapsulant onto the GaAs surface prior to RTP. It is likely that effective caps may be thinner for RTP than for conventional furnace processing, thereby relieving some of the mechanical limitations on capping materials. Although chemical and mechanical properties of the cap are important to the ultimate performance of devices, the principal requirement for a good encapsulant is that it provide an adequate evaporation barrier for the GaAs surface. Previous work has shown that the amount of semiconductor material retained in the cap during a furnace anneal can indicate the quality of the cap as a diffusion barrier for Ga and As.Il] However, we are not aware of any previous work which has determined directly the absolute amounts of material evaporated through such caps, for either furnace annealing or RTP. In this paper, we report the results of direct measurements of the amounts of Ga and As evaporated through very thin caps of Si, SiO2 , and Si3N4 during RTP. The measurements were performed using a capture technique applied previously to the measurement of the evaporation rates of native GaAs surfaces during RTP.[21 The results of such measurements allow a direct quantitative comparison of the effectiveness of various cap compositions in terms of the maximum temperature accessible to RTP before appreciable evaporation is observed and the rate of evaporation at that temperature. improvement
Mat. Res. Soc. Symp. Proc. Vol. 74. c1987 Materials Research Society
694
EXPERIMENTAL METHOD
Si0 2 and Si 3 N4 caps of nominal thicknesses 20 rnm, 40 nm, and 60 nm were deposited onto polished (111) GaAs surfaces using the technique of remote plasma enhanced chemical vapor deposition at a substrate temperatu
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