Silicon micromachined ultrasonic transducers

This paper reviews ultrasonic transducers that are made by silicon micromachining (MUTs). Transducers for both air-borne and immersion applications are made from parallel plate capacitors whose dimensions are controlled through traditional integrated circuit manufacturing methods. Typical dimensions of the capacitors are: gap <1 micron (vacuum or air), membrane thickness=1 micron (silicon nitride or poly-silicon), and diameter=50 microns. A large number of small elements are connected in parallel to make a transducer. Transducers for air borne ultrasound applications have been operated in the frequency range of 0.1-11 MHz, while immersion transducers have been operated in the frequency range of 1-20 MHz. A theoretical model for the transducers will be presented. The model is used to highlight the important parameters in the design of both airborne and immersion transducers. Theory is used to compare the receive sensitivity, the transmit power capability, and the bandwidth of the MUTs to piezoelectric transducers. We will show that MUTs are at least as good if not better performing than piezoelectric transducers. Examples of single element transducers, linear array transducers, and two dimensional arrays of transducers will be presented. The agreement between the theory and experimental measurements will be presented and will prove the validity of the model. In summary, we will present a capacitive micromachined transducer technology that is competitive to the piezoelectric technology and that carries the advantage of electronic integration and batch processing using silicon fabrication practices.