A new BAWR (bulk acoustic wave resonator) structure for near zero TCF (temperature coefficient of frequency)

A BAWR (bulk acoustic wave resonator) is an essential component of RF filters and duplexers in wireless communication devices. The BAWR consists of a piezoelectric layer sandwiched between bottom and top electrodes. Its resonance frequency shifts as an environment temperature changes, normally ranging −25∼−30 ppm/°C, which is referred to TCF (temperature coefficient of frequency). A large TCF value reduces the gap between adjacent bands and gives rise to interference in their operation. To overcome this problem, a low TCF value is required as keeping carrier bandwidth, high Q, and high kt². In this work, a new BAWR structure with SiO2/SiN layers enabled TCF −0.3∼−7.8 ppm/°C, Q 2400, and kt² 5.4%, respectively. This BAWR has been applied to LTE Band-7 and Band-25 filters.