1 GHz bulk acoustic wave slanted finger interdigital transducers in aluminum nitride for wideband applications

This paper presents the first experimental demonstration of bulk acoustic wave (BAW) slanted finger interdigital transducer (SFIT) delay line in aluminum nitride (AlN) thin film operating around 1 GHz. The SFIT is used to electro-acoustically transduce the Lamb wave S₀ mode. The AlN Lamb SFITs were micromachined and experimentally demonstrated to have a band-pass response with a maximum insertion loss (IL) of 26 dB, fractional bandwidth (6 dB) of 7.9%, and a shape factor (-30 dB) of 1.8. This device displays a small form factor of 150 μm × 421 μm. The transmission response of the measured devices was confirmed to match the impulse response model. However, large pass-band ripples were observed in the device due to the acoustic reflections within the interdigitated fingers. In order to verify the reflection effect, AlN lateral field excitation (LFE) SFITs were also microfabricated. Because of the absence of the bottom floating electrode and the use of a thicker AlN membrane in this configuration, the acoustic mismatch between interdigital transducers (IDTs) is reduced and so is the magnitude of the associated ripple (from 5 dB to 3.5 dB).