Patent GB-1238034-A (GB)
IPC: H03H9/46
Filed: 1968-08-12
Abstract: 1,238,034. Pressure-sensitive resistors. GENERAL ELECTRIC CO. 12 Aug., 1968 [11 Aug., 1967], No.38533/68. Heading H1K. [Also in Division H3] A filter comprises a base 10 having a cavity 11, the material of the base being either a monocrystalline or polycrystalline semi-conductor wafer e.g. silicon, or ceramic e.g. aluminium oxide or a mixture of aluminium oxide and silicon dioxide termed mullite. A semi-conductor beam 12, e.g. of silicon, is bonded at its ends to the upper surface of the base 10 through metallized regions 15. A piezoresistive strain sensing region 13 is diffused into the underside of the beam 12 having widened areas 14 at each end; alternatively, regions 14 may have the same width as region 13 but be of greater diffusion depth. This ensures that only the central region 13 is significant in the functioning of the filter. A layer 16 of insulation, e.g. silicon dioxide, is formed in the upper surface of the beam 12 and a metallic conductor 17, e.g. molybdenum or aluminium, is deposited on the layer. An input signal source 19 is connected through thermo-compression bonds 23 to the ends of the metallic layer 17 and a D.C. bias supply 21 is connected across diffused regions 13, 14 through thermo-compression bonds 24 on the metallized regions 15. Preferably, gold leads are used. Output signals are taken from a terminal 26 across a resistance 22. A capacitance 25 shunting the bias source 21 provides an A.C. by-pass path for output signals from the piezoresistive diffused region 13. The filter is situated in a magnetic field directed perpendicular to the beam 12 in the plane of the filter as denoted by the arrows in Fig. 1. The interaction of the current through the layer 17 with the magnetic field causes the beam 12 to oscillate at the frequency of the input source 19 but this oscillation is insignificant except when the frequency of the source coincides with the resonant frequency of the beam. The variation in strain in the diffused region 13. produced by the flexural mode of oscillation of the beam 12 causes the resistance of the region to vary sinusoidally and hence the output signal at terminal 26 has an amplitude and frequency proportional to the amplitude and frequency of the beam 12. The beam 12 may be of P-type or N-type conductivity, the diffused regions 13, 14 being of the opposite type. When the beam is of P-type and the base 10 is of ceramic, the beam is attached to the base by coating the latter in the necessary places with molybdenum trioxide followed by a coating of an alloy of silver and tin. To make ohmic contact with the N-type piezoresistive diffused regions 13, 14, the silver-tin alloy contains an N-type dopant, e.g. phosphorus, arsenic or antimony. When the beam 12 is of N-type the silver-tin alloy contains a P-type dopant, e.g. indium, aluminium, or gallium. When the base 10 is a semi-conductor the step of coating with molybdenum trioxide is omitted. In a modification described with reference to Fig. 3 (not shown) the piezoresistive region is formed in the upper surface of the beam beneath the layer of insulation and the region has the shape of an elongated U, the two terminal ends of the region being situated at one end of the beam.