Advanced technologies of FBAR for tuning effective electromechanical coupling coefficient

Abstract

The film bulk acoustic wave resonator (FBAR) is one of the most popular devices in the radio frequency field. Numerous researchers are simultaneously working to develop an effective electromechanical coupling coefficient (keff2) tuning technology, aiming to meet the diverse bandwidth requirements of the 5 G era. Based on a traditional FBAR process, this work prepares several different devices and then analyzes the four factors that influence keff2 from the perspective of process, material, and design. The pillar structure achieves the largest keff2 tuning range of 2.84% (33 MHz). The composite piezoelectric film can tune the overall resonant frequency in a wide range, and its keff2 tuning range is 1.75% (12.5 MHz). The area effect tunes the fs in a small range, ultimately achieving a keff2 tuning range of 1.16% (11 MHz). Film stress regulation achieves keff2 tuning range of 2.73% (30 MHz), but it has the greatest difficulty. The integration of various keff2 tunable methods has important guiding significance for the design of FBAR filters in the future.