Understanding Substrate Loss in Microwave Acoustic Resonators

This work studies the influence of substrate loss on the performance of acoustic resonators and characterizes the effective substrate resistivity of LiNbO
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thin-film on a high-resistivity Si (HR-Si) wafer. Coplanar waveguides (CPWs) are utilized to extract the effective substrate resistivity of LiNbO
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film on Si over a frequency range from 1 to 40 GHz with two different film thicknesses: 400 nm and 1.6 µm. The effective resistivities of released and unreleased CPW structures are experimentally extracted and analyzed to show that a suspended structure on a thinner LiNbO
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film has less substrate loss due to a smaller electric field filling factor in the film. As a result, the removal of Si wafer by a release process brings more substrate loss reduction for a thin film than that on a thick film.