CMUT

A capacitive micromachined ultrasonic transducer (CMUT) is a type of ultrasonic transducer that uses a micromachined capacitor with a suspended membrane as one electrode. CMUTs are fabricated with MEMS processes and are used as transmitters, receivers, or combined transducers in ultrasonic systems.

Each CMUT element consists of a dense array of parallel-plate capacitors formed by a thin, compliant membrane separated from a fixed bottom electrode by a small gas or vacuum gap. A DC bias preloads the membrane. When an AC voltage is applied, the membrane vibrates and emits ultrasound; when acoustic waves arrive, membrane deflection alters the capacitance, producing an electrical signal that can be read by a preamplifier.

CMUTs are manufactured using standard MEMS techniques, such as surface micromachining and wafer bonding. The membranes are often silicon or silicon nitride, with gaps measured in hundreds of nanometers to a few micrometers. This microfabrication enables integration with complementary metal–oxide–semiconductor (CMOS) electronics, allowing co-fabricated transducers and readout circuits.

Advantages of CMUTs include wide electrical and acoustic bandwidth, large array scalability, and the potential for seamless integration with silicon-based electronics, which can reduce interconnect complexity and packaging costs. They also offer closer acoustic impedance matching to water or tissue in some configurations. Limitations include the need for relatively high DC bias, pull-in instabilities, device aging, and packaging challenges that affect reliability and yield.

CMUTs are used in medical ultrasound imaging, high-frequency diagnostic probes, endoscopy, and non-destructive testing, and are actively studied for integration with other modalities such as photoacoustic imaging and therapeutic ultrasound.