An Antimony Planar Diffusion Source

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AN ANTIMONY PLANAR DIFFUSION SOURCE JACK WILSON*. ROBERT GUSTAFERRO*. ALAN BONNY"*. AND ED MADDOX* *BP America. Research Center Warrensville. 4440 Warrensville Center Road. Cleveland. OH 44128 "**Now with Marathon Venture Partners. San Francisco. CA 94105

ABSTRACT Antimony is a useful n-type dopant for buried layers, due to its low rate of lateral diffusion in subsequent processing steps. To date, no solid planar diffusion source has been available. Reported here is the development of such a source. The source is made by high temperature synthesis in a hot press. A mixture of antimony trioxide and silicon powders is placed in the press. and heated under pressure. The reaction 2Sb 20 3 + 3Si -

3SiO 2 + 4Sb

results in a solidified product. Wafers are sliced from this product to form the actual planar diffusion sources. The sources are used in a two-step doping process: a predeposition followed by a drive-in. Oxygen is required in both steps, due to the chemistry involved. Typical properties of the doped silicon are: sheet resistance, 10 0/square, junction depth, 4-6 microns. and antimony surface concentration. 3-8 x 101 /cm 3 . INTRODUCTION In order to diffuse antimony into silicon in a reasonable time, diffusion temperatures must be around 1200"C. Antimony melts at 630"C. and antimony oxide melts at 656°C. so these materials alone could not be suitable solid sources. However. if trapped in a solid matrix of some kind, such that the antimony evolves from it relatively slowly, antimony or antimony oxide could be useful sources of antimony. Since the matrix has to be non-contaminating, the most obvious choice for a matrix material is silicon itself, with silicon dioxide as a second choice. This suggests the possibility of using the high temperature synthesis reaction between antimony trioxide and silicon, i.e.. 2Sb 2 0 3 + 3Si

-

3SiO 2 + 4Sb

This reaction was first reported by Kahlenberg and Trautmann 11], although these authors stated that the reaction did not occur on heating with a Bunsen burner, and only reacted with difficulty on igniting with a spark. The product was a black molten mass containing a few globules of antimony. Ugai et al. [21 state that silicon and antimony trioxide react, starting at 560*C. but that only 6% of the silicon is oxidized. Most of the antimony trioxide is converted to antimony tetraoxide. Sb 204. which is inert to silicon. SOURCE PREPARATION In exploratory experiments using microwave heating, it was found that a mixture of silicon and antimony oxide powders produced a satisfactory source after heating to a temperature around 1400°C. The powders were mixed together. cold pressed to form a disc, and then heated rapidly (i.e.. in around 2 minutes) in the microwave unit to the temperature desired, when the microwave was switched off. The disc was transformed from a green compact to a solid. ceramic-like body. Mat. Res. Soc. Symp. Proc. Vol. 131. C1989 Materials Research Society

630

Repeated experimentation showed that it was only necessary to heat to 600°C for the process to occur