Diffusion Control in Silicon by Carrier Gas Composition

Abstract

A single heating process is described for producing controlled p‐n and n‐p‐n diffused structures in silicon by changing the carrier gas composition during diffusion. The process is shown to depend on large changes in the rate of evaporation from a source at a given temperature in carrier gases of different composition. The weight loss from a source at a given temperature is quite large in reducing gases, extremely small in neutral gases, and essentially zero in oxidizing gases. The weight loss from a source in reducing gases also decreases with increasing concentrations of water vapor in the carrier gas. The weight loss from a source at a given temperature is relatively large in all of these carrier gases.The diffusion apparatus consists essentially of a fused silica tube extending through separately controlled temperature zones for the location of the diffusant sources and the silicon samples. Carrier gases at controlled rates of flow carry the diffusant vapors past the silicon samples at atmospheric pressure.Data are presented for single diffusions from and sources in several carrier gas compositions. P‐type layers are formed in 5 ohm‐cm n‐type silicon in the diffusions in reducing gases but not in oxidizing gases. A controlled number of gallium atoms can be introduced into silicon by heating first in a reducing gas and then in an oxidizing gas. N‐type layers are formed in 5 ohm‐cm p‐type silicon in the diffusions in all of the carrier gases studied. Data also are presented on double diffusions from and sources to illustrate the control of layer thickness by heating first in wet N2 and then in wet . These double diffused structures are shown to be suitable for the fabrication of high alpha transistors.