Patent GB-983292-A (GB)
Title: Improvements in and relating to semiconductor devices
Inventor: BEALE JULIAN ROBERT ANTHONY
IPC: C23C10/22
Filed: 1961-02-20
Abstract: 983,292. Semi-conductor devices. MULLARD Ltd. Feb. 20, 1961, No. 6149/61. Heading H1K. In making a semi-conductor device amounts of the same material are applied to two sites on a P layer on an N-type body (N layer on P body) and subsequently alloying is effected to form a further PN junction at the first site and an ohmic contact at the second. The junction is obtained by ensuring that the melt at the first site contains excess of donor (acceptor) to give a recrystallized N(P) zone, and that diffusion from the liquid solid interface is predominantly of acceptor (donor) atoms derived from the layer material to reinforce the P(N) layer beneath the zone. A sufficient amount of acceptor (donor) material is included in the melt at the second site to provide a P(N) recrystallized zone constituting the ohmic contact. In a typical case boron is diffused into a 2 ohm.cm. monocrystalline phosphorus doped N-type silicon wafer by passing nitrogen successively through a water-diethylene glycol mixture, over a boron source, and then past the wafer. The source and wafer are maintained at 1200‹ C. for 2 hours to give a P-type layer of suitable thickness. After removal of the layer from one face in a specified etch the other face is etched in a series of steps to the configuration shown in Fig. 3 with three stepped sections of different surface dopings. A similar surface doping distribution on a plane surface is obtainable by performing the boron diffusion in two steps in which different areas of the surface are oxide masked. Tin arsenic pellets 31, 32, 33 are applied and the wafer mounted in the Fig. 4 apparatus as shown. After evacuation, hydrogen, purified by diffusion through heated palladium tube 18 and then passed over arsenic source 30, is flowed past the heated wafer. The boron distribution in the parts of the surface dissolved in pellets 31, 32 during the process is preselected in relation to the amount of arsenic in the pellets and the relative segregation co-efficients of arsenic and boron so that the recrystallized zones 37, 38 (Fig. 5) are respectively P and N type. Zone 38 beneath pellet 33 is also N type. Due to the steep boron concentration gradient in the surface regions the concentration of boron at the liquid-solid interface is much greater on the liquid side and preferential inward diffusion of boron therefore takes place to extend the P zone as shown. After etching the surface between pellets 31, 32 is masked, the wafer etched to line R<SP>1</SP> and nickel wires 40a, 41a, 42a soldered to the pellet residues with lead-tin eutectic. In an alternative arrangement pellet 42 is disposed on the opposite face of the wafer. In another process an N-type body with a boron diffused layer is etched to the form shown in Fig. 8 and tin pellets fused to the surface. A suspension in toluene of equal parts by weight of boron and aluminium is applied to the surface of pellet 46 and the wafer replaced in the apparatus with the arsenic source and heated. Arsenic predominates in the zone recrystallizing under pellet 45 which is therefore N type but aluminium and boron are in excess in the zone below pellet 46 to make it P type. The acceptors diffuse preferentially from both liquid-solid interfaces to deepen the layer 44 beneath the pellets. Subsequently a resist layer is applied to the surface between the pellets and the wafer etched as in Fig. 6. Finally a gold-plated nickel tab is alloyed to the opposite face of the wafer and attached to the pellets as before. Specifications 852,904, 916,881 and 983,291 are referred to.