5F-5 An Assessment of the Thermal Efficiency of Capacitive Micromachined Ultrasonic Transducers

Abstract

Capacitive Micromachined Ultrasonic Transducers (CMUT's) are well known for their exceptional frequency bandwidth, short time response, and inherent flexibility in their design and fabrication process, which are all quantified and documented very well. There are other known attributes of CMUT's that make them an attractive transducer choice in various applications. One of these attributes is the thermal efficiency which is quantified in this paper as the ratio of the acoustical output power to the heat generated inside the probe. Because they don't have a significant internal loss mechanism (other than the lens which is not needed in some cases), CMUT's are expected to be thermally efficient. Thermal efficiency is critical in several applications in medical imaging. For example, in color Doppler mode, the penetration depth and resolution is ultimately limited by the heat generated in the probe. In general, in applications that use long bursts of pulsed excitations, heat generated in the probe is a limiting factor. Other examples of such applications are therapeutics and B-mode imaging using coded excitation. In these applications CMUT's have the potential to overcome the thermal barrier. Although CMUT's are commonly accepted to be thermally efficient, this property has not been investigated and quantified widely. This paper assesses the thermal efficiency of the CMUT's experimentally and compares it to that of the state-of-the-art PZT probes. In these experiments the total acoustic output power is measured by absolute pressure measurements, and the heat generated inside the probe is estimated by temperature measurements at the probe surface. Thermal efficiency of the CMUT and PZT probes are compared by measuring the total acoustic power output of the transducers, while keeping the output sound pressure levels and the temperature increases at the surface of the transducers same. The experiments revealed that for the same temperature rise in front of the transducers CMUT's were able to deliver 3.5 times more acoustic power compared to a comparable PZT probe. The experiments also revealed that thermal efficiency is not achieved readily for every CMUT probe, and requires specific electrical and packaging design.