The effect of a closed back cavity on air-coupled piezoelectric micromachined ultrasonic transducer performance

In conventional piezoelectric micromachined ultrasonic transducers (PMUTs), the backside acoustic energy is often used inefficiently, resulting in up to half of the energy being wasted. Vacuum encapsulation can improve the energy utilization efficiency, but this technique is not compatible with state-of-the-art devices such as cantilever-based PMUTs. A closed back cavity provides an alternative method for effectively utilizing the backside acoustic energy. This paper investigates the effects of a closed back cavity on PMUT performance through theoretical analysis, simulations, and experimental verification. Increasing the cavity depth produces a periodic modulation of several key PMUT metrics, such as the relative frequency deviation and quality factor. The optimal cavity depth for PMUTs that ensures a robust resonant frequency and high quality factor is defined as a function of the acoustic wavelength. A closed back cavity also provides an effective method for continuously tuning the quality factor, and thus the bandwidth, of PMUTs. This work paves the way for air-coupled PMUTs with adjustable performance for various applications.