Overview
Ultrasonic sensing technology is widely used in industrial and medical applications such as imaging, distance measurement, and flow sensing. Compared with traditional piezoelectric ceramic ultrasonic transducers, piezoelectric MEMS ultrasonic transducers (PMUTs) offer smaller size, lower power consumption, and better compatibility with CMOS processes. PMUT transmit/receive sensitivity and resonance frequency control are interdependent: thinner membranes increase sensitivity per unit area but make resonance frequency more sensitive to residual stress and dimensional variations. As a result, high-sensitivity PMUTs typically face significant frequency-control challenges. Air-coupled PMUTs are especially sensitive because their lower bandwidth imposes stricter requirements on frequency control.
New TCCP Approach
Researchers from the State Key Laboratory of Precision Measurement Technology and Instruments at Tianjin University recently published a letter in the IEEE Journal of Microelectromechanical Systems titled "Piezoelectric Micromachined Ultrasonic Transducers With Superior Frequency Control" (DOI: 10.1109/JMEMS.2023.3305461). The paper presents a PMUT design that addresses the trade-off between frequency control and high sensitivity, and it describes the related theory as well as the device design, fabrication, and test characterization results.
Tapered Cantilever Cluster PMUT (TCCP)
The proposed Tapered Cantilever Cluster PMUT (TCCP) design relieves residual stress in the membrane while precisely defining the membrane boundary, achieving improved frequency control. Measured results show wafer-level frequency uniformity of 0.8% and a target frequency deviation of 1.0%. Compared with the traditional Circular Clamped PMUT (CCP) design, the TCCP improves frequency-control capability by an order of magnitude. According to the authors, this is the first demonstration of frequency control at the 1% level.
Manufacturing and Application Implications
The TCCP approach does not require SOI wafers or trimming-based frequency tuning, and it reduces the demand on stress-control steps in the process flow. These characteristics make the design more amenable to high-volume, high-yield manufacturing with lower process complexity and cost. The approach could enable improved PMUT performance in applications such as ultrasonic gas flow meters and beamforming arrays.
