Patent GB-824484-A (GB)
Title: Improvements in and relating to the electrolytic treatment of semi-conductive bodies
IPC: H01L21/3063
Filed: 1955-11-09
Abstract: 824,484. Electrolytic jet-etching and -plating of semi-conductors. PHILCO CORPORATION. Nov. 9, 1955 [Dec. 3, 1954], No. 32034/55. Addition to 805,291. Classes 37 and 41. Localized electrolytic etching or plating of a semi-conductor body is produced by directing against a chosen region of the surface of the body at least one jet of an electrolyte containing ions of the type necessary for the desired type of electrolytic action and having a resistivity at least twice that of the semi-conductor body, and passing an electric current between the jet and body to produce the desired electrolytic action. In the case of materials not normally containing sufficient current carriers of the kind required for the desired electrolytic action, illumination is supplied to effect a satisfactory result by producing hole-electron pairs. Thus when N-type germanium is etched in 0.1 N KC1 in the dark the etching rate is only one quarter of that under strong illumination, while the etching rate for P-type germanium is independent of illumination, and vice versa for plating. For etching germanium non-aqueous electrolytes such as ethylene glycol solutions of zinc chloride or HF or any aqueous electrolyte may be used. For etching silicon, removal of the oxide film is necessary, as by the use as electrolyte of sodium chloride or preferably a metal fluoride, with or without free HF, or an ethylene glycol solution of HF. For plating germanium, aqueous solutions of 0.1 N zinc chloride or of 0.1 N indium sulphate with added sulphuric acid or an ethylene glycol solution of antimony trifluoride with added HF may be used, while for plating silicon, aqueous 0.1 N zinc chloride or indium sulphate may be used for producing alloy junctions by heating, or for area-contacts, the silicon is first chemically etched in an HF-NHO 3  mixture and then plated from 0.1 N ethylene glycol solutions of zinc chloride, antimony trichloride, or antimony trifluoride. In the production of a silicon area-contact transistor, a rectangular wafer blank of P-type silicon, of resistivity in the range 1 to 20 ohm/cms. and a hole lifetime greater than 10 Ásecs. and of size 160 Î 80 Î 10 mils., is chemically etched in an aqueous mixture of HF, HNO 3  and acetic acid containing a trace of bromine to reduce the wafer thickness to 3 mils. and after rinsing and soldering on a nickel tab, which is then covered with a protective resist, e.g. of polystyrene, the wafer is electrolytically etched under strong illumination between opposed jets of diameter 10 and 12 mils. supplied with 0.4 N NaF until a deep red to orange colour is visible when etching is terminated leaving 0.3 mil. separating the craters of 25 and 30 mils. diameter respectively. After a brief etch in HF-HNO 3  followed by rinsing and dried, zinc is electrodeposited in the craters by jets of an ethylene glycol solution of zinc chloride from opposed nozzles of 6 and 8 mils. diameter respectively to give deposits of 12 and 15 diameter and 0.4 mil. thickness. After plating the wafer is rinsed, immersed in HF-HNO 3 , rinsed, dried, and immersed in a clean-up etch aqueous bath containing HF, HNO 3  and acetic acid. Assembly into a holder may precede or follow the clean-up etch and contacts are applied either by quick soldering or by a spring-contact arrangement. Etching and plating may also be effected using A.C. Thus N- or P-type germanium may be etched using 0.1 N sulphuric acid and strong illumination, the negative half of each cycle releasing hydrogen, while N-type germanium may be plated using 0.1 N indium sulphate and low illumination.