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Ultra Low-Insertion-Loss Surface Acoustic Wave Filters Using Unidirectional Interdigital Transducers with Grating SAW Substrates

Surface acoustic wave (SAW) filters employing unidirectional interdigital transducers (UIDT) exhibit low-loss properties. We propose a new single-phase unidirectional transducer (SPUIDT) with an electrode width of approximately λ/4 of the best transducer efficiency. The new UDTs are obtained by fabricating an IDT on a very thin dielectric grating with λ/8 shifts. The electromechanical couplings (k2) of electrodes on SiO2 films decrease rapidly with film thickness. Therefore, good UIDT properties are obtained for a very thin SiO2 grating (about 0.03 µm at 2 GHz with an Al electrode thickness of 0.07 µm). In this paper we describe the experimental results of wide-band and very low-loss filters with an insertion loss of less than 1 dB in the 2 GHz-range.