UHF Bulk-Acoustic-Wave Filters Utilizing Thin ZnO/SiO2-Diaphragms on Silicon

This paper reports on a temperature-stable bulk-acoustic-wave (BAW) filter consisting of a thin ZnO/SiO2 composite diaphragm structure on a silicon wafer. To make a theoretical treatment of the BAW filter possible, the constants for the distributed-parameter equivalent circuit extended to the composite structure are derived from analysis of the dispersion curves for waves propagating along the ZnO/SiO2 diaphragm. Some actual filters are designed and analysed using the equivalent circuit. The measured characteristics of a 578-MHz BAW filter of the two-coupled-resonator type fabricated on silicon are reported and discussed. The results demonstrate that the measured characteristics resemble the theoretical ones but that the insertion loss is as high as 12 dB.