Patent GB-785467-A (GB)
Title: Improvements in or relating to the manufacture of semi-conductor devices
Inventor: COLSON IAN DOUGLAS
IPC: H01L21/00
Filed: 1954-12-23
Abstract: 785,467. Semi-conductor devices. GENERAL ELECTRIC CO., Ltd. Dec. 14, 1955 [Dec. 23, 1954], No. 37200/54. Class 37. An alloy junction electrode on a semiconductor body is formed by first causing the impurity material to adhere to the body by pressing together freshly formed surfaces of the body and the impurity, and then heating to melt the impurity material. The impurity may be such as to produce either a PN junction, or P or N regions. In one example, a rectifier of the type described in Specifications 775,121 and 775,191 is produced with the additional step of causing the indium to adhere to the germanium before fusion takes place. As shown in Fig. 3, a nickel wire 6 carrying a blot of indium 18 slides in a hole in punch 19, and is placed in contact with a germanium plate 1. The plate 1 is held in a central position on a block 14 by means of cylinder 17 guided by an iris diaphragm 16. Before being placed in position, the indium is sliced to provide a fresh, clean surface, and the surface of the germanium is etched. Lever 21, limited by stop 23, is used to press down punch 19 so that the hemispherical indium blob 18 adheres to germanium plate 1. Punch 19 is then raised and a shield 24 which fits loosely on punch 19, falls until lifted by bolt 26, to provide protection for the germanium assembly. Ejector rod 27 is then used to eject wire 6 and the germanium body 1. The body is then placed on a disc of soft solder on a base-plate, and the assembly heated to 500 C. in dry hydrogen, to provide the alloy junction electrode and to solder the germanium to the base-plate. A copper cap is then cold welded to the base-plate. Fig. 4 shows a transistor comprising emitter 36, collector 37 and a nickel base electrode 34 on a germanium wafer 33. The emitter and collector electrodes are provided by means of a jig shown in Fig. 5 consisting of a steel'block 38 containing punches 42 and 39. A mounting member 47 which slides over the end of punch 39 is recessed to carry the germanium wafer. A short length of indium wire is placed in hole 43 in the end of punch 42 and member 45 pressed while the punch is held against a ground glass plate; the excess indium is cut off to leave a short indium cylinder with a fresh surface, in hole 43. The punch is then inserted into the block 38, and 45 again pressed so that the indium adheres to the germanium. Details are given as to the quantities and measurements involved, to ensure accurate control of the size and shape of the electrode. Punch 39 which is similar to punch 42, is then withdrawn, leaving the germanium wafer suspended in position by the indium cylinder, and the process repeated to provide a second indium cylinder attached to the germanium. The germanium assembly is then withdrawn and placed adjacent the base electrode 34 and heated, as in the case of the rectifier, to provide the alloy junction electrodes and to solder the base electrode. After providing leads 52, 53, 54 sealed in a glass bead 55, itself sealed to a copper skirt 56, a copper cap 57 is cold welded to skirt 56. The etching solution for the germanium may consist of three volumes of glacial acetic acid, 5 of nitric acid and 3 of hydrofluoric acid and 0.3 per cent of bromine. The surfaces of the indium and germanium must have freshly cut or etched surfaces, or be in the equivalent condition which may be achieved by storing such elements in sealed air-filled containers with anhydrous magnesium perchlorate. To facilitate adherence of the indium to the germanium with low pressure, the indium may be prepared by heating in a vacuum to 600 C. for 30 minutes in an alumina crucible, cast under vacuum in a stainless-steel mould and then extruded into a rod. Indium and antimony may be used instead of pure indium.