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Composition Insensitive Solids as a Diffusion Source for Capsule Diffusion

IP.com Disclosure Number: IPCOM000077141D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 4 page(s) / 39K

Publishing Venue

IBM

Related People

Reisman, A: AUTHOR

Abstract

In conducting capsule diffusions involving doped sources, two distinct phenomena make it difficult to obtain reproducible diffusions. The first of these is associated with the time to build up the vapor phase concentration of dopant to the desired quasi-equilibrium value. The second is associated with depletion of dopant in the source, as the partial pressure of dopant in the vapor builds up. One or both of these deleterious phenomena is present to a significant degree whether mechanical mixtures of, for example, As and Si, or quenched As in Si, or doped single-crystal sources or the currently employed homogeneous polycrystalline sources are employed.

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Composition Insensitive Solids as a Diffusion Source for Capsule Diffusion

In conducting capsule diffusions involving doped sources, two distinct phenomena make it difficult to obtain reproducible diffusions. The first of these is associated with the time to build up the vapor phase concentration of dopant to the desired quasi-equilibrium value. The second is associated with depletion of dopant in the source, as the partial pressure of dopant in the vapor builds up. One or both of these deleterious phenomena is present to a significant degree whether mechanical mixtures of, for example, As and Si, or quenched As in Si, or doped single-crystal sources or the currently employed homogeneous polycrystalline sources are employed. In the case of the last named, the first phenomenon (vapor pressure buildup) is obviated to a considerable extent, but the source depletion behavior can be significant and is always undesirable. This results in the necessity for computing an average vapor phase concentration, (in terms of pressure) based on source size, capsule size, and temperature. In effect. one has to zero-in for each new set of time-temperature-geometry conditions in order to compensate for the depletion phenomenon.

A method for preparing a source with the advantages of a homogeneous source but without the problem of depletion is disclosed herein. It is based on the use of an invariant three-phase source, the characteristics of which are best realized with reference the figure.

The figure shows schematically a portion of the Si-As phase diagram according to V. W. Klemm and P. Pirscher-Z. anorg. Allg. Chem. 247, 211 (1941). According to this work, in the composition interval 0-50 atomic 8 As, the system is completely solid below ca. 1070 degrees Cr this solid being in equilibrium with vapor. As the vapor pressure of Si below this temperature (the eutectic temperature) is small and unimportant from a doping viewpoint, one can concentrate on the variation of As pressure in the composition interval in question.

According to the Phase Rule, a generalized thermodynamic relationship which defines the number of independent variables possessed by a system consisting of C components distributed amongst P phases, a two component-two phase system is isothermally univariant, and a two component-three phase system is isothermally invariant. In simpler terms, this means that if a two component-two phase equilibrium exists, as for example between a solid solution of As in Si and its vapor, the vapor pressure varies monotonically with concentration. If a three-phase situation prevails, as for example in a system consisting of a solid solution of As in Si, a solid solution of Si in the compound SiAs and their vapor, the vapor pressure does not vary with composition at constant temperature, independent of the quantity of each solid phase present. With the above in mind, one is now in a position to examine the consequences of either of these equilibria, as t...