Patent GB-715268-A (GB)
Title: Methods of treating semi-conductive bodies with electric discharges and devices utilizing such bodies
IPC: H01L21/00
Filed: 1951-01-10
Abstract: 715,268. Discharge apparatus. WESTERN ELECTRIC CO., Inc. Jan. 10, 1951 [Jan. 11, 1950; Jan. 31, 1950], No. 716/51. Class 39(1) [Also in Group XXXVI] The electrical characteristics of a body of germanium or silicon are altered or modified by polishing a surface of the body and bombarding it with electrically charged particles, e.g. ions of a significant impurity affecting the resistivity, photosensitivity or rectification property of the body or by glow discharge. An envelope 20 evacuated to 10-<SP>3</SP> mm. contains an ion chamber 23, a bombardment chamber 24 and a holder 25 for.the material treated. The ion chamber 23 is formed by a re-entrant tube 34, is closed by a perforated cathode 32 of graphite and contains the anode 31 which is supported by a glass or quartz tube 39 down which runs a fine lead-in wire 40. The anode may be a wire gauze basket e.g. of nickel, containing the material of a significant impurity to be included in the ion stream e.g. phosphorus, arsenic or barium. Any desired gas may be admitted through a tube 22 or evacuated via a tube 21. The main body portion is joined to the ion chamber 23 and to the end portion 44 by greased tapered joints. The work holder 25 is mounted between mica spacers 45 with a spring contact at its end resting on a contact 55 secured to the lead-in conductor 29. An induction coil heater 53 surrounds the tube and heats the conducting button 49 of the work holder. A jet 54 cools the envelope 20 at this point. Silicon prepared by reduction of silicon tetrachloride on zinc is heated in a silica crucible and progressively cooled and cut into slabs and wafers. The wafers are optically polished, cleaned in chloroform to remove grease, and in hydrofluoric acid and distilled water and dried. The wafer is mounted in the work holder and introduced into the chamber which is afterwards sealed. Gas is admitted to ion chamber 23 and the ion arc is struck, the wafer being simultaneously heated by the induction coil 53 within a critical range discussed in the Specification e.g. 60‹-400‹C. Gases admitted may be air, oxygen, hydrogen, nitrogen, helium, argon, water vapour, carbon monoxide or chloroform. The properties of the treated silicon e.g. photo resistive and photo voltaic effects, and the back-voltage characteristics, are controlled by the nature of the impurity, the temperature of the treatment and the energy of the bombarding ions and are discussed in the specification. Contact is made with the treated surface by a tungsten point or a ring of gold plated copper, or rhodium, cadmium, aluminium, silver, gold or nickel electrodes are evaporated on. Rhodium contacts may be applied to the unpolished side by evaporation of electroplating. Fig. 7 shows a photo-cell in which a cylindrical body portion of beryllium-copper contains an insulating plug 61, with a moulded plug 62 carrying a spring contact 63 of tungsten bearing on a silicon wafer 67 and forcing it against the plated contact 68. A centreing ring 69 of insulating fibre and a conductive annular disc complete the assembly. The small photo-cell elements may be made from one piece of activated material by masking the material during the evaporation of the contact material on the front face and thereafter cutting it into smaller pieces. In Fig. 8 (not shown), the silicon wafer is of larger area and the rear contact is biased by a spring plate backed by a screw threaded insulating plug so that the contact pressure on the silicon may be adjusted. The devices may be either photo-resistive or photoconductive. Point contact silicon crystal harmonic producers may have the surface against which the point bears of treated silicon containing .02 per cent of boron, Fig. 14 (not shown). The transistor in Fig. 16 is of P-conductivity-type material and the base 106 is of hard rubber with a slot 107 in which fits the silicon wafer 109. Cantilever springs 110 bearing on rhodium plated areas hold the wafer in position and spring wires 113, 114 with offset ends contact rhodium plated strips serving as emitter and collector 100, 102 respectively. In an alternative form Fig. 17 (not shown), the silicon disc is formed with spherical depressions in each face, with the thickness of the middle .002-.004 inches, and it is seated in the counterbored portions of a metal cylinder. The studs carrying the S- shaped spring contacts are slidably adjustable and are secured in position by set screws. The greatest power gain is with materials activated at 350‹C. An electrical forming treatment may improve stability. Spring contact negative resistance devices can be treated when partly assembled by means of a glow discharge, Fig. 20, a potential of 400-500 volts being applied from source 220 in a circuit containing a resistance 222 of .1-1 megohms. The gap between the electrode 214 and the body 211 breaks down giving a purple, pink or blue glow according to the atmosphere. The application of a condenser oscillation or a condenser discharge treatment improves the stability Fig. 21 (not shown); or the unit is biased beyond its peak voltage and a condenser placed across it. The glow discharge may be applied under a contact and the latter moved onto the surface afterwards, thus eliminating error in alignment.