Capacitive Micromachined Ultrasonic Transducers (CMUTs)

























































Capacitive Micromachined Ultrasonic Transducers (CMUTs) are tiny transducers that is expected to replace traditional piezoelectric transducers for high frequency ultrasonic imaging due to the flexibility offered by the microfabrication, wide bandwidth, CMOS compatibility, very high level of integration and batch fabrication (low cost)

 I: Dual electrode CMUTs:

CMUTs have so far been fabricated with a single top electrode whose size and location can be optimized for coupling efficiency. However, the flexibility of microfabrication processes used for making CMUTs, and the control of transducer parameters with bias voltage offer a much larger design space that can be explored to realize transducers with improved performance and functionality. Along this line, we propose CMUTs with a dual-electrode structure, where multiple electrically isolated electrodes are embedded in the dielectric CMUT membrane as shown in Figure 1. By locating the transmit electrodes near the edges of a rectangular CMUT membrane, the stable displacement range, hence the maximum pressure amplitude during transmit mode is increased without collapsing the membrane when operated within static collapse voltage range. In the receive mode, the center receive electrode is brought closer to the substrate by biasing the side electrodes and a higher electromechanical transformer ratio is obtained at low DC bias. Therefore, dual-electrode CMUT has an effectively larger gap as compared to conventional CMUT during transmit, and it has an effectively smaller gap during receive. In addition this study was mainly motivated by recent findings suggesting that although CMUTs offer superior performance in terms of bandwidth, their overall sensitivity is inferior as compared to piezoelectric transducers by about 10dB, especially due to lower output pressure. Figure 2 illustrates a picture of a 1-D dual electrode CMUT array designed for intracardiac ultrasonic imaging.


Figure 1: Comparison of dual and single electrode CMUT and membrane shape during transmit and receive cycles

Figure 2: 64 element dual electrode CMUT array operating at 7.5MHz for intracardiac imaging.


II: Dual Annular Ring array for Intravascular imaging:

Forward-looking intravascular ultrasound (FL-IVUS) has become attractive in recent years since it enables 3-D volumetric imaging for guiding interventions and extracting Doppler information. In particular, CMUT seems to be a promising technology for FL-IVUS imaging applications since it offers high bandwidth and good sensitivity while enabling electronics integration. In order to avoid grating lobes in radiation pattern and to have wide viewing angle, the FL-IVUS transducer array elements need to be small in both radial and lateral dimensions. Consequently, the small radial element size leaves significant area unused outside of the guidewire opening. By taking advantage of the flexibility offered by CMUT technology, it is possible to utilize this area efficiently. This approach opens up a room for designing multiple ring arrays without any particular drawbacks. These array configurations has potential for optimization each ring transducer array independently for transmit (Tx) or receive (Rx) operation, such as changing the gap or adjusting the DC bias required for collapse. Also the area underneath the CMUT array element may be utilized more effectively for CMUT on CMOS implementation as the demand for T/R switch is either relaxed or eliminated. In addition, by adjusting the lateral size of the elements of different rings, it is possible to adjust the operation frequencies of the rings independently for harmonic imaging or broadband operation. A particular example of multiple ring array architecture where separate Tx and Rx rings are employed to form a dual-annular-ring array configuration as illustrated in Fig. 1 and 2.


Figure 1: Schematic of a FL- IVUS probe with a dual-annular-ring array


Figure 2.  Sample picture of experimentally fabricated arrays


People: J Zahorian, G Gurun, S Satir, M Hochman

Collaborators: Dr. Paul Hasler (GT)

Dr. Mustafa Karaman (Isik University, Istanbul)

Funding: National Institute of Health, Boston Scientific