Temperature compensated bulk acoustic wave resonator (BAWR) for near zero temperature coefficient of frequency (TCF)

A bulk acoustic wave resonator (BAWR) 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, of which phenomenon is referred to temperature coefficient of frequency (TCF). When a resonator has a large TCF value the gap between adjacent bands is reduced and interference occurs. To overcome this problem, a low TCF value is required while maintaining the characteristics such as carrier bandwidth, high quality factor (Q), and high effective coupling coefficient (kt²). In this work, a new BAWR structure with SiO₂/SiN layers both on the top electrode and under the bottom electrode enabled TCF of -0.3~-7.8 ppm/°C, Q of 2400, and kt² of 5.4%, respectively. This BAWR has been applied to transmission filters for LTE Band-7 and Band-25.