Patent GB-1355806-A (GB)
Title: Methods of manufacturing a semiconductor device
Inventor: BEALE J R A
IPC: H01L21/00
Filed: 1971-09-28
Abstract: 1355806 Semi-conductor devices MULLARD Ltd 28 Sept 1971 [9 Dec 1970] 58476/70 Heading H1K In a method of manufacturing a semi-conductor device a wafer is provided with an insulating layer having an aperture within which a metal mask is formed by a self registering method, impurities of one conductivity type are ion implanted through the insulating layer to form a region of this conductivity type, the composition and thickness of the metal mask being such that it prevents the penetration of ions to a greater extent than does the insulating layer, and a region of opposite conductivity type is provided at the aperture. As shown, Fig. 5, an NPN bi-polar transistor is produced by depositing an N-type epitaxial layer 2 on an N+ type substrate 1, oxidizing. the surface to form a thick oxide layer 4, opening a rectangular window (6) to define the base region, and depositing a thin oxide layer 5 on this window. A small aperture 7 is formed in the thin oxide layer to define the emitter region and filled with a metal mask (10) by a self registering method. Boron ions are then implanted to form a P + + type extrinsic base region below the thin oxide and a shallower P-type active base region below the metal mask which restricts the penetration of the boron ions to a greater extent than the thin oxide. The metal mask is then removed and arsenic or phosphorus is diffused through the window to form the N+ + type emitter region, the body being simultaneously annealed to activate the implanted ions. Base contact windows 11 are then opened, aluminium is deposited over the surface and etched to form emitter contact 12 and base contact 13. The self registered metal mask may be produced: (i) by depositing on top of the photoresist mask used to etch the aperture a layer of nichrome followed by a layer of gold and then boiling in acetone to remove the photoresist and break away the overlying metal; (ii) by electroplating Ni into the aperture using the body as the cathode; or (iii) by depositing Ni over the oxide and exposed Si surface, heating to ensure adhesion to the surface within the window and then ultrasonically cleaning to break away the metal over the oxide. In a modification the metal mask may be thick enough to completely stop the ions so that only the P + + type region is initially implanted. The metal mask is then removed and the active base region formed by a separate implantation step either before or after forming the emitter region. This second implantation step may be performed after opening the base contact windows so that the impurity concentration at these points is further increased. In a second embodiment, Figs. 7 to 13 (not shown), a bipolar transistor is produced by forming a thick oxide layer (25) on an N-type epitaxial layer (22) on an N + type Si substrate (21) and opening a window (24) to define the base region. Boron ions and phosphorous ions are sequentially implanted to form a buried P- type layer and an N + type surface layer parts of which are to form the active base region and the emitter region respectively. A previous inert ion bombardment may be used to reduce the tendency of the phosphorous ions to channel. A thin oxide layer (27) is deposited and covered with a layer (31) of Al. A photoresist layer (29) is applied and processed to form a window (30) exposing a part of the A1 layer on the thin oxide layer. The exposed part of the A1 layer is etched away and the aperture (28) is extended through the thin oxide layer using an etchant which does not attack the resist or A1 layers. The wafer is then subjected to further etching to side etch the A1 round the edge of the aperture, the photoresist is removed, a layer (33) of Au is deposited, and the remaining Al is etched away causing the overlying parts of the Au layer to be simultaneously broken away so that a gold masking layer (34) is left in the aperture (28) and extending on to a narrow strip of surface of the surrounding thin oxide layer. Boron ions are implanted through the thin oxide layer to form a P + + type extrinsic base region, the metal layer being thick enough to completely mask the active base region and the emitter region against these ions. The metal mask is then removed, low energy phosphorus ions are implanted through the aperture to form a shallow N+ + type emitter contact region, and the wafer is then annealed to activate all the implanted impurities. Base contact windows are opend and A1 is deposited and formed into emitter and base contacts. A plurality of devices may be produced in a single wafer which is then subdivided. The device may also comprise a diode or an IC.