Patent GB-823559-A (GB)
Title: Improvements in or relating to silicon semiconductor devices
IPC: C25D7/12
Filed: 1956-07-27
Abstract: 823,559. Semi-conductor devices. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 27, 1956 [Aug. 1, 1955], No. 23230/56. Class 37. A semi-conductor device consists of a wafer of silicon attached to an end contact member of molybdenum, tungsten, or alloys containing one of these materials as a base, by means of a solder consisting of at least 72% by weight of silver and 0.5% to 8% of antimony. The balance may comprise one of the elements germanium, silicon, lead and tin. A list of suitable compositions for the solder is given in the Specification. As shown in Fig. 1 a silicon PN junction rectifier comprises an end contact 12 of molybdenum, tungsten or other base alloy from 20 to 100 mils. in thickness and  to 2 inches in diameter and a silicon wafer 18. Both the molybdenum and tungsten may be alloyed with minor amounts of other metals, thus molybdenum may be alloyed with from 5% to 25% of osmium, platinum, chromium, nickel, cobalt, silicon copper and silver. A thin coating 13, 14 of silver or silver alloy is applied to the face surfaces of contact 12. This may be done by coating the faces with a thin sheet or a fine powder and heating in a hydrogen atmosphere at 1200 C. Alternatively, the molybdenum may be coated with a nickel phosphide coating deposited from an aqueous solution containing nickel sulphate NiCl 2  and sodium hypophosphite. Immersion of the molybdenum in the solution will produce a coating of nickel phosphide containing 95% nickel to which by silver cyanide electro-plating a coating of 1 mil. thickness of silver may be applied. The molybdenum may also be coated with " Kovar " (Registered Trade Mark). Upon the coated molybdenum end contact is placed a foil of the silver-antimony base solder referred to above. On the upper surface of the silicon wafer 18 is a foil 20 of a thickness 1 to 2 mils. of aluminium or an aluminium based alloy with an element from Group III or IV which function both to enable soldering to the upper contact 22 and the formation of the PN junction by diffusion of P-type material into the upper portion of the N-type wafer. Layer 20 may consist of pure aluminium, with slight amounts of impurities such as magnesium, sodium, zinc or an alloy containing aluminium, the balance being silicon, gallium, indium and germanium. Various examples of suitable alloys are given in the Specification. Upper contact 22 is of the same material and consists of a flat disc portion of smaller diameter than the silicon. A well 28 of the contact 22 has a thin coating 29 of a suitable solder such as a silver-gold alloy, silver-germanium, gold alone or silver-germanium or nickel phosphide. During manufacture the layers described with reference to Fig. 1 are placed in a furnace 50 shown in Fig. 3. The furnace comprises a base through which pass conduits 54, one of which is connected with a pump to produce a vacuum and the other may introduce a protective gas such as helium or argon. The components are mounted in a recess in a graphite block 64 on a refractory support 62. A weight 68 is applied to contact 22. A heater 72 may be lowered around the bell to heat the block 64 by radiation or a high-frequency heating coil may be used instead. During manufacture a number of assemblies is placed in the graphite block, bell 58 placed in position and the graphite block heated to 850 to 1000 C. The resulting rectifiers are then placed in a hermetically sealed casing. The base may be soldered to the silicon in a separate stage from the top contact.