High-performance FBAR humidity sensor based on the PI film as the multifunctional layer

For achieving the high-performance film bulk acoustic resonator (FBAR) humidity sensor with MEMS process reliability, we proposed a novel FBAR design based on the polyimide film (PI-FBAR). The PI film was employed to provide multifunction for the structural supporting and humidity sensing simultaneously, which was first used in the back-trench FBAR. Due to its flexible and low acoustic impedance characters, it can provide adequate structural robustness and high quality factor (Q = 210). According to its hygroscopic property, it would be greatly optimized the humidity sensor's sensitivity. The feasibility of this design was verified by theoretical analysis, finite element method (FEM) simulation, and combined with the indentation and various humidity response experiments. It is proved that the sensitivity reaches to + 67.3 kHz/% RH (about 64 ppm/%) between 15 % RH and 85 % RH, which is 39 times higher than the sample without PI film. The fabrication processes are more simple and controllable at the same time, so that the sensor has shown good properties of repeatability and reproducibility. The temperature coefficient of frequency (TCF) is 126 ppm/degrees C from -50 degrees C to + 50 degrees C and the hysteresis error is no more than 1.77 % RH at 50 % RH. Owing to the strong stability and higher sensitivity of the PI-FBAR sensor, it is promising to achieve high accurate humidity monitoring in practical applications.