Large-area 2-D micromachined ultrasonic transducer arrays for medical ultrasound imaging

Accurate wafer-level characterization of capacitive micromachined ultrasonic transducers (CMUTs) is essential for assessing fabrication quality, ensuring device reliability, and defining safe operating conditions for medical ultrasound applications. In particular, accurate estimation of collapse and snapback voltages is important, as these parameters are responsible for device operation and performance optimization. Although CMUT technology is mature, reliable estimation of collapse and snapback voltages remains challenging as they have high acquisition time, making them unsuitable for the characterization of large-scale (high number of elements >1000) arrays, and lack standardized measurement protocols, which leads to inaccuracy in the estimated values. In this work, an improved version of the conventional impedance-based collapse–snapback measurement technique is introduced. Second, a novel, fast, and non-invasive characterization method is proposed for extracting collapse and snapback voltages with low acquisition time, high resolution, and without compromising the device’s integrity by in-cavity charge injection and trapping phenomena. Comparison of experimental results from both techniques shows close agreement, validating the accuracy and robustness of the novel approach. This method, combined with statistical and histogram-based algorithms, was then employed to analyze uniformity across array elements and individual cells within a single element. This facilitates the detailed assessment of fabrication abnormalities and the design of optimized driving circuitry. The scope of this work is further extended to piezoelectric micromachined ultrasonic transducers (PMUTs). For this purpose, a new framework is presented to assess remanent polarization, coercive fields, and intermediate polarization states in PMUTs based on Sc-doped AlN thin films. Collectively, this research presents a practical and scalable framework for the wafer-level characterization of both electrostatic and piezoelectric acoustic devices.