PI-Based Dual-Mode FBAR Humidity Sensor Toward Ultra-High Sensitivity and Self-Temperature-Compensation Capability

This paper presents a polyimide-based film bulk acoustic resonator (PI-FBAR) humidity sensor operating in resonant frequency and reflection coefficient S ₁₁ dual-mode. Models and experiments showed that those two modes are governed by different mechanisms, of which the former is based on the modulation of the humidity on the Young's modulus of PI, while the latter is based on the modulation of the humidity on dynamic resistance. In the full-scale relative humidity (RH) testing range (20%-80%), the resonant mode achieved a sensitivity of 29.93 kHz/%RH (28.56 ppm/%RH) with ultra-high linearity ( ${R}^{2}=0.9993$ ), and the humidity hysteresis error is only about 1.32% RH. For the S ₁₁ response, an ultra-high average humidity sensitivity of 0.044dB/%RH (2095ppm/%RH) was achieved, and the sensitivity at 20% low relative humidity reached 0.075dB/%RH (3571ppm/%RH), which is more than 2 orders of magnitude higher than that of the resonant frequency response. And the limit of detection (LOD) for the humidity response of S ₁₁ is 0.035% RH, which is half lower than that of the resonant mode. Thus, we can choose a more appropriate sensing mode according to particular application or combine these two mechanisms in one test to expand the sensor competences by taking advantages of each method. In addition, as both of the two modes have high sensitivity to humidity and temperature, we deduced a method to calibrate the temperature for the humidity sensing. Based on this dual-mode mechanism, we can further achieve either temperature-calibrated humidity sensing or dual-variable sensing of humidity and temperature with more accurate sensing results.