Wafer-level experimental study of residual stress in AlN-based bimorph piezoelectric micromachined ultrasonic transducer

During the last decade, piezoelectric micromachined ultrasonic transducer (pMUT) have raised a great interest for various applications. In particular, AlN material has been used to build bimorph structures made of two AlN layers sandwiched between metallic electrodes. This kind of pMUT offer good performances for both sensing and actuating because of the multi-electrodes scheme of the design. Yet, this pMUT architecture is expected to be particularly sensitive to residual stress within the AlN layers. At the wafer level, stress dispersion is likely to cause important performance variations. In this paper, we characterize bimorph pMUT working in the 50-250 kHz range to investigate how much stress variations at the wafer level (8 '') impact critical pMUT features such as resonant frequency, sensitivity and quality factor. Drive sensitivity up to 216 nm/V are obtained with a single electrode scheme. Two approaches are combined, respectively based on pMUT simulation and curvature measurement, to assess the contribution of AlN residual stress to the overall tension of the diaphragm, ranging from -300 to 900 N m(-1). The resulting dispersion of resonant frequency is higher than 100%. We highlight how to obtain satisfactory yield.