GB1178130A - Improvements relating to Semiconductor Strain Transducers - Google Patents

Abstract

1,178,130. Pressure-sensitive resistors. FERRANTI Ltd. 3 Dec., 1968 [8 Dec., 1967], No. 55819/67. Heading H1K. A pressure-sensitive transducer comprising a plurality of piezo-resistors formed in one face of a semi-conductor wafer and defined by PN junctions is provided on its opposite face with a conductive coating overlying an insulating layer. The insulating layer prevents D.C. earth loop currents and the conductive layer is connected to a potential source (e.g. earth) to provide a screen for A.C. currents. A wafe 11 of N-type Si doped with Sb is is provided with an epitaxially deposited layer of P-type Si doped with B. A silicon dioxide mask is formed on the epitaxial layer and P is diffused-in, the remaining parts of the P-type layer constituting a plurality of piezo-resistors. Four straight resistors are provided at the centre of the wafer and four U-shaped resistors are provided near the periphery of the wafer in line with the central resistors. The ends (14) of the resistors are connected by conductive tracks (15) to contact points (16) on a neutral circle of the wafer and bridging members (17) forming parts of the U-shaped resistors are produced by depositing Al over the surface by masking and etching. The wafer is mounted in a housing. Fig. 2, by alloying a Ni-Fe alloy ring 22 to the edge of the wafer 11 and securing this ring to a Ti body 20 by means of epoxy resin so that the wafer is insulated from the body. Two of the inner resistors and two of the outer resistors are selected for their matched characteristics and are connected in a bridge circuit by Au leads 18 thermo-compression bonded to the contact points 16 and gold-ball bonded to Cu leads 19. A layer 28 of SiO 2 is deposited on the face of wafer 11 opposite to the resistors before it is mounted in the housing and after mounting a layer 29 of Ti is deposited over the insulating layer 28 and into contact with the housing body 20. Temperaturesensitive elements and a thin film trimming resistor (not shown) are provided in the housing and the temperature-sensitive elements may be formed in the wafer 11. Ancillary apparatus such as an amplifier or voltage regulator may also be mounted in the housing. The metal screening layer may be of Ti, Au, A1 or Pt and may be produced by evaporation or by chemical deposition.