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LOW-TEMPERATURE DIFFUSION OF DOPANTS IN SILICON

P. FAHEY AND M. WITTMER IBM T. J. Watson Research Center, Yorktown Heights, New York 10598

ABSTRACT It has been reported that diffusion of substitutional dopant atoms in silicon occurs during

the formation of transition-metal silicides at temperatures below 3000 C. By observing the diffusion enhancements of buried marker layers of Sb-, Ga-, Ge-, and B-doped silicon layers, we provide solid experimental evidence that the diffusion enhancement

induced by Pd 2Si formation at low temperatures is due to point defects generated by the silicide reactions. Diffusion enhancement is observed at temperatures as low as 200 0 C. We have found the surprising result that diffusion is asymmetric: diffusion occurs preferentially towards the siliciding interface.

1. INTRODUCTION Silicides are being used at an increasing rate in modern integrated circuit technologies, primarily as contacts, gate materials and interconnect lines. A number of reports in the literature have indicated that silicide reactions generate point defects which are injected into the silicon, inducing dopant diffusion and sometimes shrinking of dislocation loops. Wittmer and Tu [1] showed that dopant diffusion is induced at the surprisingly low temperature of 300'C. In their experiment, dopants implanted at the Si surface are pushed ahead into the substrate by the advancing Pd 2Si interface; this has been termed the "snowplow" effect. Ohdomari et al. [2] have also demonstrated the snowplow effect with Pd 2Si - see [2] for a summary of systems exhibiting the snowplow effect - and in addition found that this silicide reaction resulted in the shrinkage of extrinsic dislocation loops. The shrinkage of extrinsic loops implies the injection of vacancies from the silicide reaction. More recently, Wen et al. [3] demonstrated that formation of TiSi2 also eliminates extrinsic dislocation loops. Hu [4] has shown that formation of TaSi 2 can enhance the diffusion of buried marker layers of B- and Sb-doped Si and proposed vacancy injection by the silicide reactions as the explanation. In this study, we have prepared special structures of thin (=200A) lightly doped buried marker layers grown at temperatures of about 500 0 C. These structures are much more sensitive to point-defect changes than conventional structures formed by ion implantation and high-temperature epitaxy. Since the buried marker layers are physically separated from the silicide interface, they are ideally suited to studies of surface point-defect injection phenomena. We have chosen to study the effect of Pd 2Si formation on dopant diffusion. Our specialized structures have allowed us to observe new diffusion phenomena not seen in previous studies.

Mat. Res. Soc. Symp. Proc. Vol. 163. ©1990 Materials Research Society

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2. SAMPLE PREPARATION The special sample structures used in this work consisted of narrow buried marker layers of different dopants formed in silicon by either molecular beam epitaxy or an ultra-high vacuum low temperature epitaxy (LTE)