Process for vapor-solid diffusion of a conductivity-type determining impurity in semiconductors

摘要

823,317. Semi-conductor devices. WESTERN ELECTRIC CO. Inc. May 17, 1957 [May 18, 1956], No. 15756/57. Class 37. In a method of diffusing a significant impurity into a solid semiconductor body, the body is heated in an atmosphere of the vaporized impurity to a temperature at which the impurity diffuses into the body while the surface of the body is evaporated to provide continuous etching thereof. The evaporation and diffusion rates are acceptably comparable if a layer at least 0.05 mil. thick is evaporated in the time taken to form a diffused layer in the range 100 Angstroms to 5 mils. thick. As shown, twenty etched wafers 23 (five only shown) of P-type silicon are mounted on a support 22 in a previously baked tantalum bucket 16 comprising a small hole 17 in its wall, and an extension 16B which makes a tight fit within the extension 12 of a quartz housing 11, a quantity of red phosphorous 25 being provided in the extension 12. The bucket, which also comprises radiation shields 21, is suspended from a bell-jar 14 containing liquid nitrogen, and the housing 11 is continuously evacuated by vacuum equipment (not shown) through an opening 13. The bucket is heated by R.F. coils 19 surrounding the housing 11 to 1250‹ C., and the phosphorous is heated by an auxiliary heater 20 to 330‹ C. Modification of the housing 11 will enable the heater 20 to be dispensed with, use then being made of the temperature gradient along the bucket. Diffusion and evaporation attain an equilibrium in about two hours, producing an N-type surface layer on the silicon bodies which may be provided with ohmic electrodes to make PN diodes, and adapted for use in transistors as described in Specification 809,642. In a similar arrangement for the simultaneous diffusion of two impurities into N-type silicon wafers, the bucket comprises two extensions, quantities of gallium and arsenic being placed in inserts in the extensions at points attaining temperature of 950‹ C. and 250‹ C. respectively, the mouth of the extension containing arsenic being constricted. Alternatively, separate auxiliary heaters may be used around each extension. After 2¢ hours a steady state is reached in which each wafer comprises an N-type surface layer and a P-type intermediate layer. The method is applicable to other semi-conductor bodies, including germanium-silicon alloys and selected Group III-V intermetallic compounds, and to the formation of a diffused layer of the same conductivity type as the wafer but of lower specific resistivity; also, it may be modified to obtain diffusion of an impurity out of a surface layer of a uniformly doped wafer. Mechanical evacuation of the furnace may be replaced or supplemented by placing in the furnace a material which will continuously absorb the semiconductor vapour. The surface concentration of the diffusant is influenced by controlling the temperature of the vapour source, the size of the openings in the main part of the bucket, and, to a lesser extent, the temperature of the wafer, while the time taken to reach a stable diffusion-evaporation condition depends on the temperature of the wafer, and very little on the temperature of the vapour source; the extent of the total exposed silicon surface affects both concentration and time. Specification 809,643 also is referred to.