General Description and Advantages of CMUTs:

Piezoelectric transducers have long dominated ultrasonic transducer technology, but capacitive micromachined ultrasound transducers (CMUTs) have recently emerged as an alternative offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with supporting electronic circuits. CMUT technology can potentially produce the type of integrated catheter arrays and external probes enabling advanced 3-D imaging. A CMUT is simply a device with two plate-like electrodes biased with a DC voltage and driven with an additional AC signal to harmonically move one of the plates. The main components are the cavity, the membrane, and the electrode. Using common integrated circuit (IC) fabrication processes, a capacitor cell appears as a metallized membrane (top electrode) suspended above a heavily doped silicon substrate (bottom electrode), as shown in Fig. 1. An insulating layer is included to prevent the two electrodes from shorting in case of contact. A single transducer element uses many small capacitor cells connected in parallel. By organizing transducer elements in different geometries, any array shape is possible (Fig. 2).


FIGURE 1. Basic CMUT structure.


CMUTs provide several advantages over piezoelectric transducers: they can be batch-produced with micromachining processes to tight parameter specifications, which is difficult for piezoelectrics; they are easier to fabricate than piezoelectric transducers; batch processing also enables the fabrication of transducer arrays with different geometries and operating frequencies on a single wafer. Fig. 3 shows transducer arrays including 1-D linear, 2-D rectangular and ring-annular arrays all fabricated on a single 4” silicon wafer; the use of standard IC processes also makes integration of CMUT arrays with supporting electronics convenient; and CMUTs can operate over a wider temperature range than piezoelectric devices. Results over the last decade demonstrate that traditionally fabricated CMUTs optimized with respect to such design parameters as device size, membrane radius, thickness, shape, gap height, and operating mode compare favorably to piezoelectric transducers in terms of bandwidth (170%), frequency range (100 kHz-70 MHz), dynamic range (130 dB/V), maximum output pressure (35 kPa/V) and receive sensitivity (50 dB/Pa/Hz).


FIGURE 2. CMUT arrays. (a) A 64-element 30-MHz 1D array with 36-μm element pitch.
(b) A 128x128-element 3-MHz 2D CMUT array with 420-μm element pitch.
(c) A 64-element 10-MHz annular ring CMUT array with 100-μm element size.



FIGURE 3. 1D and 2D arrays, ring-annular arrays, and single element transducers
with different specifications, designed for different applications fabricated on a
single silicon wafer and diced for individual use.