Low-loss BAW filters on high resistivity silicon for mobile radio

Integration of bulk acoustic wave (BAW) filters into W-CDMA mobile communication applications is of high interest, as the BAW technology allows for low-loss and high Q signal forming in these 2 GHz devices. During the design phase of low- loss BAW filters it is very important that we not only control the most major loss mechanisms, like metal conductivity and viscosity of piezo material. We also have to focus the less well known losses. In this paper, we describe an interface loss effect which appears between the Si substrate and the first SiO₂ layer of the BAW structure. Its influence on the filter characteristics of our 2 GHz filter is shown. This loss effect is caused by channel currents, which are induced by static charges (similar in FET transistors). For investigating the loss and its impact on the filter performance, we designed test structures and filters, performed and analyzed measurements, and adapted the loss model in our filter simulations accordingly.