Integrated Ultrasonic Transducer

Abstract

The typical transducer considered consists of a piezoelectric film, and associated.electrodes, connected to one gate of a dual-gate field-effect transistor in the silicon wafer on which the piezoelectric film is situated. An individual transducer responds to various modes of excitation (flexural, surface, bulk) at frequencies which may range from far below one Hertz to hundreds of megahertz. The second gate of the field-effect transistor can be used for electrical amplitude control or for mixing purposes, Connection of a number of these small transducers together to form arrays permits realizing ultrasonic receiving devices having variable directivity, and progammable surface-wave signal processors. The transducer structures considered are shown gate transistor is achieved by application of a in cross section in Fig. 1 (the vertical scale is local oscillator signal at the second gate. In exaggerated) with schematic symbols for easy signal-processing arrays based on use of a number of representation. In the PI-DMOS transducer a singlethese transducers, separate pieoelectric films gate field-effect transistor has a piezoelectric could drive first and second gates to provide nonZnO film deposited in the gate region on top of a linear coupling of different input signals, thermally-grown Si02 layer, In the lower structure TRANSVERSAL FILTER SIGNAL PROCESSOR a commercial dual-gate double-diffused MOS transistor has one gate connected to a deposited piezoelectric ZnO film. Figure ·2 shows the four modes of excitation of this transducer. Some waveforms appear in Fig. 3. THE INDIVIDUAL TRANSDUCER Upon comparing the two transducer structures, one notes that the PI-DMOST structure offers both piezoelectric and pyroelectric response down to zero frequency owing to isolation of the ZnO film by high-quality thermally-grown Si02, Fabrication of the second transducer structure is safer as no sputtering in the gate re.gion is required, The second structure offers additional design freedom in the shaping of the piezoelectric region (hence control of directivity) and in its area for control of transducer sensitivity. Response of PI-DMOST and the dual-gate sensor to application of static strain in a cantilever fixture (Fig. 4), At top, voltage across load resistance decreased as source-drain current dropped when strain was applied and remained at constant value for duration of test, 17.8 hours, after which it returned to original value. Lower scope trace shows that no de response is observed with the second structure in which the ZnO film is not isolated by thermally-grown Si02• It appears possible to provide such isolation with the dualgate sensor if desired. In Figs. 5-7 temperature sensitivity and dimensional design criteria are given. ELECTRICAL CONTROL OF TRANSDUCER In the PI-DMOST a voltage may be applied externally to the gate, and in the dual-gate transducer (second structure) one may utilize the second gate to control the response to a signal applied via the piezoelectric film connected to the first gate. The amplitude of response can be adjusted with a control voltage VG2· Sampling of a low frequency output can be achieved by means of sampling pulses of short duration. Mixing in the dualDual-gate transducers can be connected in an array to form a transversal filter structure as shown in Fig. 11. Although dependence of transducer amplitude upon second gate bias is nonlinear over part of its range (Fig. 12), by differencing two such transistor outputs a highly linear dependence of amplitude or tap weight upon bias is obtained, simplifying setting of tap weights (Fig, 13). An array processor operating near 27 MHz is being fabricated at present. Arrays of these transducers with second-gate control of weighting also appear attractive for use as ultrasonic receiving (and perhaps also transmitting) devices for scanning in defect characterization. SUMMARY The integrated transducers based on use of a piezoelectric film and field-effect transi~tor are versatile devices offering -r.esponse to various modes of excitation -wide frequency response from de if desired to hundreds of megahertz useful electronic control within the transducer · itself possibility of interconnection into arrays for signal processing and detection. ACKNOWLEDGMENT Research sponsored by the National Science Foundation Grant ENG76-21818 and the Center for Advanced NDE operated by the Science Center, Rockwell International, for the Advanced .Research Projects Agency and the Air Force Materials Laboratory Contract F33615-74-C~5180 •. REFERENCES Detection of Acoustic Waves with a PI-DMOST Transducer, K. W. Yeh, R. S. Muller, S. H. Kwan, Proc, 8th Conference (1976 International) on Solid State Devi~es, Tokyo, 1976, Jap. J, Appl. Phys. 16, Supplement, 16-1, 527-521 (1977). Integrated Ultrasonic Transducer, S. H. Kwan, R. M, White, R. S, Muller, Proc. 1977 IEEE .. l..trasonics Symposium, 843-846. S77 ZnO Thin-Film Copacilor Sensor DEPLETION· MODE O·MOST AMPUF'IER v,,