Capacitive Micromachined Ultrasonic Transducers
B. T. Khuri-Yakub, Fellow, IEEE
E. L. Ginzton Laboratory, Stanford University, Stanford, CA
IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society
Tuesday, March 7, 2000
4 pm 3269 Beckman Institute
Capacitive micromachined ultrasonic transducers (cMUT) can match and even outperform piezoelectric transducers in terms of efficiency and bandwidth. With the advent of silicon micromachining, it is now possible to make capacitors with very thin gaps that sustain electric fields of the order of 109 V/m or more. At these levels of electric field, the transformer coupling between the electrical and mechanical parts of the capacitor transducer and thus its performance, are comparable to that of piezoelectric transducers. Because cMUTs are made using integrated circuit manufacturing processes, it is possible to control the dimensions of transducers to submicron tolerance, thus enabling operation in any frequency range, ease and reproducibility of manufacture, making 1-D or 2-D arrays, and integration with electronic circuits.
We will review the design and performance of cMUTs for both immersion (medical imaging, nondestructive evaluation...) and airborne (nondestructive evaluation, range sensing, gas flow and composition...) ultrasound applications. We will show examples of airborne ultrasound transducers that deliver 100s of angstroms of displacement per volt in the MHz frequency range and immersion transducers that deliver 10s of angstroms of displacement per volt while operating from dc to tens of MHz. In both airborne (single element) and immersion (single element, 1-D and 2-D arrays of elements) applications, we will present transducer systems with dynamic ranges of over 150 dB/Volt/√Hz, all of which are in excellent agreement with the theoretical prediction for the performance of both types of transducers.
Finally, we will show that vibrating membranes generate both surface waves and Lamb waves in silicon wafers. The edges of the vibrating membranes, where they attach to the substrate, act as point sources of ultrasonic energy that couple into the modes of propagation in a plate. By proper design, this energy can be harnessed to couple selectively into surface or Lamb wave modes. We will present theoretical studies showing the viability of this approach for making surface or Lamb wave filters and other devices.
Butrus T. Khuri-Yakub received the B.S. degree in 1970 from the American University of Beirut, the M.S. degree in 1972 from Dartmouth College, and the Ph.D. degree in 1975 from Stanford University, all in electrical engineering. He joined the research staff at the E. L. Ginzton Laboratory of Stanford University in 1976 and has been Professor of Electrical Engineering (Research) since 1982. He is the deputy director of the E. L. Ginzton Laboratory. Prof. Khuri-Yakub's current research interests include in situ acoustic sensors (temperature, film thickness, resist cure,...) for monitoring and control of integrated circuits manufacturing processes, micromachining silicon to make acoustic materials and devices such as air borne and water immersion ultrasonic transducers and arrays, and fluid ejectors, and in the field of ultrasonic nondestructive evaluation and acoustic imaging and microscopy. Professor Khuri-Yakub has authored over 340 publications and has been principal inventor or co-inventor of 46 issued patents.
Sponsors: Bioacoustics Research Laboratory, Department of Electrical and Computer Engineering; Department of Theoretical and Applied Mechanics; and Bioengineering Program.