Patent GB-1238035-A (GB)
IPC: H03H9/56
Filed: 1968-08-12
Abstract: 1,238,035. Semi-conductor devices. GEN ERAL ELECTRIC CO. 12 Aug., 1968 [11 Aug., 1967], No. 38539/68. Heading H1K. [Also in Division H3] General description.-An electromechanical filter compatible with monolithic integrated circuitry comprises, Fig. 6, a semi-conductor crystal 10, e.g. a P-type silicon crystal, having a cavity 16 formed therein, a resonator beam 17 integral with the crystal bridging the cavity. Method of construction.-In one method of manufacture a slab 11 of silicon nitride, Fig. 1, having dimensions identical to those of the cavity 16 to be formed, is deposited on the P- type silicon crystal 10, the dimensions of the slab being determined by photo-resist techniques involving a molybdenum masking process. P- type silicon is next grown over the crystal to cover the slab 11 by an iodine epitaxy process, the thickness of the epitaxially deposited silicon being adjusted by polishing to provide the desired resonant frequency of the beam 17. The wafer 20 now has the appearance of Fig. 2 with the buried slab 11 of silicon nitride. The wafer 10 may next be passed through standard processes to produce any integrated circuit required in those regions which do not have buried silicon nitride underneath. Filter driving and filter output circuitry is added by diffusing into two regions 12 of the crystal 10 N-type impurities, e.g. phosphorus, arsenic or antimony, these deposits being made over an area to include the entire width of the resonator beam 17 to be formed. P-type impurities, e.g. boron, aluminium, gallium or indium, are diffused into the regions 12 in the form of U- shaped patterns 13 having terminal tags 18, Fig. 3. The upper surface of the crystal 10 is thereafter coated with silcon nitride 15 followed by a molybdenum coating 14 and photo-resist and etching techniques are used to remove the silicon to either side of the beam 17 down to the level of the slab 11, Fig. 4, and then to remove the slab itself. The coatings 14, 15 are also removed, the resultant structure being as shown in Fig. 6. Method of operation.-The two U-shaped regions 13 function as piezoresistive strain sensitive elements, the variation of their resistance being detected at the pairs of terminals 18 to produce in output circuits connected to the terminals signals having an amplitude and frequency proportional to the amplitude and frequency of the beam 17. In the filter driving circuit, Fig. 7, an A.C. input source 19 is connected in series with a D.C. bias supply 21 to the N-type regions 12 and to the P-type crystal 10 grounded at 7. The P-N junctions 22, reversed-biased by the supply 21, act as capacitors on the beam 17 so that electrostrictive forces due to the internal electric field within the depletion regions of junctions 22 drive the beam. The lowest flexural mode of oscillation of the beam is obtained by driving the two junctions 22 in phase whilst the first harmonic is obtained by driving junctions out of phase by inserting a phase shifter 25 by opening a switch 24. In one example of output circuit connected to a pair of the terminals 18, Fig. 8A, the circuit includes a D.C. supply 28 and a fixed resistor 27, the output signal being obtained at terminals 8, 9. In an alternative output circuit, Fig. 8B, the two regions 13 form opposite arms of a Wheatstone bridge having fixed resistors 27, the output signal being taken from terminals 29. Other embodiments.-A capacitively driven filter, Fig. 15, has metal strips 32, 33, e.g. of tungsten or molybdenum, situated on insulating strips on the base of the cavity 37 beneath the resonator beam 38. If the crystal 10 is of P- type, piezoresistive U-shaped regions 40 of N- type are diffused in the beam. The driving source 19 in series with the D.C. source 21 is connected between the strips 32, 33 and the beam, and output signals are taken from leads 41 connected to the regions 40. The varying electrostatic forces between the strips 32, 33 and the beam drive the latter in oscillation. In a modification of Fig. 15 (Fig. 20, not shown) the metal strips 32, 33, and their supporting insulating strips are replaced by N-type regions (50, 51) diffused in the P-type crystal 10. In another embodiment of capacitively driven filter, Fig. 21, piezoresistive U-shaped regions 55 are diffused in the beam 57 and the latter is overlaid with insulating material 53, e.g. silicon dioxide. The material 53 is coated with metallic film 54, the beam 57 being driven electrostrictively from signal source 19 applied across the beam and the film. In an electromagnetically driven embodiment of the filter, Figs. 23, 25, the base of the cavity 63 in a P-type silicon crystal 10 has diffused therein a U-shaped N-type region 60, the base of the U underlying the resonator beam 62. The upper surface of the beam has diffused therein an N-type conducting region 64 and a piezoresistive P-type region 65 is diffused into the region 64. A bias current is supplied from the D.C. source 21 to the N-type region 6. when switch 59 is in position F. The P-N junction between crystal 10 and region 60 is reverse-biased by D.C. supply 89 to maintain a high impedance thereof. The input signal from A.C. source 19 is applied to the ends of the N- type region 64. The energization of the regions 60, 64 drives the beam 62 at its fundamental frequency and the filter provides an output signal on leads 67, 68 of a frequency equal to that of the source 19. If the switch 59 is moved to position D to disconnect source 21 the signal from source 19 is applied to region 60 and the filter acts as frequency doubler since the force acting on the beam 62 is independent of the direction of current flow from the source 19. Thus if the resonant frequency of the beam is Fo, the output signal frequency is Fo when the input frequency of source 19 is Fo/ 2 . In another embodiment, Fig. 26, a piezoresistive region 70 is diffused in the resonator beam 72 and is of conductivity type opposite to that of the silicon crystal 10. An insulating coating 73, e.g. of silicon monoxide, is formed above the region 70 and a film of magnetostrictive metal 74 is deposited on the material 73. The input signal from source 19 is connected to the magnetostrictive material 74 so that the latter drives the beam 72 at the frequency of the source. The output signal is taken from leads 75, 76 connected to the region 70. According to another embodiment, Fig. 27, a pair of U-shaped regions 79 of magnetostrictive material 79 are formed on a layer of insulation 73 on the beam 72, the filter being driven from the input source 19 coupled through oppositely-poled D.C. bias sources 77, 84 to the regions 79. Output signals are derived from each pair of leads 75 and 76 coupled to U-shaped regions 80 of piezoelectric material in the beam. In a further embodiment, Fig. 28, a metallic conductor 88 is deposited on an insulating layer 100 on the resonator beam 81 in which a piezoresistive region 83 is diffused. A permanent magnet 87 is formed on the base of the cavity, the magnet field interacting with the current flow through the conductor 58 connected to the input signal source 19 to drive the beam 81. The output signal is taken from leads 85, 86 connected to the region 83. In another embodiment, Fig. 29, a metallic film 91 deposited on an insulating layer 92 on the beam 90 is connected to the input signal source 19. A U-shaped piezoresistive region 93 of conductivity type opposite to that of the beam is diffused in the latter, the P-N junction being reversedbiased by a D.C. supply 99. The filter is placed in the field of a permanent magnet as indicated by the arrows whereby the beam 90 is driven as described with reference to Fig. 28. The output signal from the region 93 is obtained at a terminal 96 and is derived from a D.C. source 94 connected across the region. In another embodiment, Fig. 30, a resonator 101 is supported at its modes by crosspieces 102-105, the resonator and crosspieces being integral with the crystal 10 and overlying a cavity in the latter. In a final embodiment, Fig. 31, a plurality of resonators 110-112 extend in cantilever fashion from a central stem 113 bridging a cavity 114 in the crystal 10.