c-Axis oriented ScAlN/SiO2 multilayer BAW transformer for rectifying antenna applications

Energy harvesting from surrounding electromagnetic wave is attractive for IoT passive sensor platform. A rectifying antenna (rectenna), which is composed of antenna and diode, is generally used for RF-DC conversion in energy harvesting systems. voltage-boosting devices should be integrated into rectennas for high conversion efficiency. We have previously reported the bulk acoustic wave (BAW) transformer based on c-axis zig-zag multilayers ScAlN film in order to use as a voltage-boosting device in the rectenna. However, it is tough to grow this multilayer in a large area for the present. In this study, we introduce a new type of BAW transformer based on alternate layers of piezoelectric and non-piezoelectric thin films, which can be fabricated in a large area easily by using common sputtering systems. BAW transformers, which were based on 4 layers ScAlN/ SiO2 film were fabricated. ScAlN thin films were used as piezoelectric layers and SiO2 thin films were used as non-piezoelectric layers. The experimental voltage gain was calculated using S parameters of the BAW transformer measured by a network analyzer (E5071C). The theoretical voltage gain was simulated by Mason's equivalent circuit model. As a result, voltage gain approaching +9.5dB with the 4 layers BAW transformer was obtained. This structure is suitable for BAW transformer applications.