Patent GB-919211-A (GB)
Title: Improvements in and relating to semi-conductor devices
Inventor: BEALE JULIAN ROBERT ANTHONY
IPC: H01L21/22
Filed: 1959-04-10
Abstract: 919,211. Semi-conductor devices; etching and plating. MULLABD Ltd. April 10, 1959, No. 12229/59. Classes 37 and 41. A thin semi-conductor body comprising a PI, NI, or PN junction adjacent one of its major surfaces, a zone of high resistivity material extending from the junction towards the opposite face of the body, and a contact on each side of the junction is connected via these contacts in a circuit for monitoring an electrical property of the body and the high resistivity zone eroded and the surface damaged by the action of abrasive particles until a desired change in the monitored property is obtained. In one embodiment (Fig. 2) comprising a wafer of P-type germanium of 2 ohm cm. resistivity, P- and N- type layers 2b<SP>1</SP>, 2a, respectively, are formed by alloying on top of a diffused N-type base zone 12. An ohmic collector contact 14, 15, for which an annular electrode may be substituted, is made to the edge of the other face of the wafer. After filling the slot between layers 2a and 2b<SP>1</SP> with a protective lacquer the body is electrolytically etched in 5% caustic soda to the dotted line 23. The upper face is then covered with lacquer 24 and the lower face bombarded with silicon carbide particles carried by a jet of air. The zone is eroded down to line 26 and the minority carrier lifetime is greatly reduced in the underlying zone between lines 26 and 28 due to the bombardment. During bombardment the switching characteristic of the collectorbase diode is continuously monitored by means of the circuit shown in Fig. 3. The output waveform across resistor 32 which is displayed on an oscilloscope is shown in Fig. 4. Initially on reversal of the applied voltage an appreciable reverse current 35 is obtained due to charge storage in the collector zone, but as the volume of the zone is reduced the reverse current is reduced both in intensity and duration (36). When the characteristic 34, 36 is of desired form the eroding jet is cut off manually. Alternatively the negative-going pulses of the output across resistor 33 are fed via a rectifier and amplifier to an integrating circuit the output of which controls a relay to automatically cut off the jet when the output falls to a certain level. The bombardment damaged surface is subsequently electroplated with nickel up to contact 15, and the device encapsulated. Electroplating is effected from an aqueous solution of nickel sulphate, nickel chloride and boric acid in the proportions 150 grams, 30 grams and 20 grams, respectively, per litre. In another embodiment (Fig. 6) material is removed from the base crystal 40 of an alloy PN junction diode opposite the alloy junction. In this case the reverse resistance of the diode at maximum reverse voltage is continuously monitored and the erosion stopped either manually or automatically when the resistance falls to a certain value, preferably before the bombardment damaged layer extends across the boundary 52 of the depletion layer associated with the junction. As before, the treated surface is plated and the device encapsulated. Specification 911,292 is referred to.