Thin-film microwave acoustic transducers of piezoelectric aluminum nitride (AlN) have been vacuum deposited by two methods. Aluminum nitride was deposited on metallic film substrates at 300°–1200°C by evaporating aluminum in the presence of either nitrogen gas dissociated in an ac discharge or in the presence of ammonia gas. Longitudinal ultrasonic waves were generated in a sapphire rod with a one-way-tuned conversion loss of 10 dB at 1700 MHz.

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Spurious triple-transit signals can be rejected through phase cancellation by rotation of one transducer with respect to the other or by use of a triangular-shaped metal film in the path between parallel transducers. Rejection is facilitated by slightly limiting the aperture via mechanical surface loading. The technique is demonstrated to give high triple-transit suppression in delay lines with 20-electrode-pair transducers that give specular acoustic reflections. The technique is not found to be useful for conjugate-matched 5-electrode-pair transducers, which are shown to give acoustic reflections normal to the electrodes.

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