Brief History of CMUTs:

Ultrasonic transducers can be classified according to the physical mechanism upon which they are based to convert electrical energy into ultrasonic energy, and vice versa. Magnetostriction, piezoelectricity and electrostatics are some of the physical mechanisms used to generate and detect ultrasound.

 

FIGURE 1. Major milestones in the history of ultrasonic transducers.   Historically, piezoelectric crystals, ceramics, polymers and recently piezocomposite materials have been predominantly used to generate and detect ultrasound. Although the idea of electrostatic transducers is as old as the early piezoelectric transducers, piezoelectric materials have dominated ultrasonic transducer technology. F. V. Hunt writes in his book Electroacoustics: The Analysis of Transduction, and its Historical Background [1]: “After a month of careful study, during which both magnetostriction and piezoelectricity were considered and then rejected, Langevin decided that it would be better to fall back on the “singing condenser”... (March 1915) Numerical estimates indicated that, if electric field strengths on the order of a million volts per centimeter (108 V/m) could be maintained, electrostatic forces as large as a kilogram per square centimeter would come into play...” The reason why electrostatic or capacitive transducers have not been popular is the high electric fields that need to be maintained to achieve acceptable efficiencies. Recent advances in microfabrication technology have made it possible to build capacitive ultrasound transducers competing with piezoelectric transducers. There have been demonstrations of capacitive transducers in air dating back to 1950s [2], and in immersion as early as 1979 [3]. These transducers worked at kHz-range frequencies with low efficiencies. In the audible frequency range, the condenser microphones have been representative of capacitor transducers since the early 1900s. High transduction efficiencies at MHz-range frequencies are needed to use these transducers in practical imaging applications. Micromachined electrostatic transducers were first reported in the late 1980s [4, 5] and the early 1990s [6]. These devices were not well characterized, nor easily fabricated. The performance was also not at a level to compete with the piezoelectric transducers. A capacitive micromachined ultrasonic transducer with more advanced fabrication technology and improved performance was introduced in 1993 by M. Haller and B. T. Khuri-Yakub [7]. Advanced IC fabrication processes enable realization of the submicron gaps between the electrodes that makes it possible to achieve high electric fields. These processes also provide a precise control on device dimensions in the vertical and lateral directions. The wide bandwidth and the potential for integration with electronic circuits are other advantages associated with CMUTs. Since the first demonstration of CMUTs in the early 1990s, extensive research has been conducted on fabrication and modeling of this new transducer technology [7-12].
 

FIGURE 2. Major milestones in the history of CMUTs.

 

A transducer equivalent circuit based on Mason's model [13] has been developed [14] and its validity confirmed by experimental results [15]. 1-D linear CMUT arrays have been fully characterized [16]. Finite element analysis is also an important part of research to understand transducer characteristics (especially crosstalk issues), and to optimize transducer response [17-19]. The 2-D receive point spread function (PSF) of a 64-element 1-D linear CMUT array has been measured experimentally and reported in [20]. The first pulse-echo phased array images using a 16-element 1-D linear CMUT array were presented in [21]. First pulse-echo phased array B-scan sector images using a 128-element, 1-D linear CMUT array have been presented in [22], demonstrating their viability for medical ultrasound imaging. A through-wafer via interconnect technology has been developed to address densely populated 2-D transducer arrays. 2-D fully addressable CMUT arrays with as many as 128 x 128 elements have been successfully fabricated and characterized [23]. We have designed and fabricated several custom integrated circuits to interface CMUT arrays.

More recently, we have demonstrated a new interconnect structure for 2D CMUT arrays using deep trench isolation in silicon [24]. We have also proposed, and experimentally demonstrated new operating regimes for CMUTs where the membrane makes contact with the bottom electrode [25-27]. . CMUTs can function in collapsed and collapse-snapback modes in addition to the conventional mode described above. The aim of exploring these different modes is to further improve acoustic output power and receive sensitivity. To model these new operating modes, we have developed dynamic finite element analysis tools with accurate contact modeling [28].

 

 

First generation transmit-receive circuits have been designed and implemented in a 0.25-µm standard CMOS technology. This circuit can generate 5 V unipolar pulses to excite the transducer. Second generation front-end circuits have been designed and implemented in an analog high-voltage process enabling pulses with 30 V amplitude. We used these custom integrated circuits along with a variety of CMUT arrays, e.g. 2-D rectangular [29, 30], annular ring [31] and high-frequency 1D arrays [32, 33], for imaging. We have also demonstrated 3D photoacoustic images obtained by 2D CMUT arrays with integrated electronics [34]. The feasibility of using CMUT arrays for HIFU applications has been recently shown as well [35].

 

References

[1] F. V. Hunt, Electroacoustics: The Analysis of Transduction, and Its Historical Background. New York: Acoustical Society of America, 1982.

[2] W. Kuhl, G. R. Schodder, and F. K. Schodder, "Condenser transmitters and microphones with solid dielectric for airborne ultrasonics," Acustica, vol. 4, pp. 520-532, 1954.

[3] J. H. Cantrell and J. S. Heyman, "Broadband electrostatic acoustic transducer for ultrasonic measurements in liquid," Rev. Sci. Instrum., vol. 50, pp. 31-33, Jan. 1979.

[4] D. Hohm and G. Hess, "A subminiature condenser microphone with silicon nitride membrane and silicon backplate," J. Acoust. Soc. Amer., vol. 85, pp. 476-480, Jan. 1989.

[5] K. Suzuki, K. Higuchi, and H. Tanigawa, "A silicon electrostatic ultrasonic transducer," IEEE Trans. Ultrason., Ferroelect., Freq. Cont., vol. UFFC-36, pp. 620-627, Nov. 1989.

[6] M. Rafiq and C.Wykes, "The performance of capacitive ultrasonic transducers using v-grooved backplates," Meas. Sci. Technol., vol. 2, pp. 168-174, Feb. 1991.

[7] M. I. Haller and B. T. Khuri-Yakub, "A surface micromachined electrostatic ultrasonic air transducer," in Proc. IEEE Ultrason. Symp., 1994, pp. 1241-1244.

[8] Schindel DW and Hutchins DA, "The design and characterization of Micromachined air-coupled capacitance transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 42, pp. 42-50, Jan. 1995.

[9] Haller MI and Khuri-Yakub BT, "A Surface Micromachined Electrostatic Ultrasonic Air Transducer," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 43, pp.1-6, Jan. 1996.

[10] Eccardt PC, Niederer K, Scheiter T, and Hierhold C, "Surface micromachined ultrasound transducers in CMOS technology," Proceedings of the 1996 IEEE International Ultrasonics Symposium, pp. 959-962, 1996.

[11] Eccardt PC, Niederer K, and Fischer B, "Micromachined transducers for ultrasound applications," Proceedings of the 1997 IEEE International Ultrasonics Symposium, pp. 1609-1618, 1997.

[12] Khuri-Yakub BT, Cheng CH, Degertekin FL, Ergun S, Hansen S, Jin XC, and Oralkan Ö, "Silicon Micromachined Ultrasonic Transducers," Japanese Journal of Applied Physics, vol. 39, pp. 2883-2887, 2000.

[13] Mason WP, "Electromechanical Transducers and Wave Filters". New York, NY: Van Nostrand, 1948.

[14] Ladabaum I, Jin X, Soh HT, Atalar A, and Khuri-Yakub BT, "Surface Micromachined Capacitive Ultrasonic Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 45, pp. 678-690, May 1998.

[15] Oralkan O, Jin X, Degertekin FL, and Khuri-Yakub BT, "Simulation and Experimental Characterization of a 2-D Capacitive Micromachined Ultrasonic Transducer Array Element," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 46, pp. 1337-1340, Nov. 1999.

[16] Jin X, Oralkan Ö, Degertekin FL, and Khuri-Yakub BT, "Characterization of One-Dimensional Capacitive Micromachined Ultrasonic Immersion Transducer Arrays," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 48, 750-760, May 2001.

[17] Bozkurt A, Ladabaum I, Atalar A, and Khuri-Yakub BT, "Theory and Analysis of Electrode Size Optimization for Capacitive Microfabricated Ultrasonic Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 46, pp. 1364-1374, Nov. 1999.

[18] Wojcik G, Mould J, Reynolds P, Fitzgerald A, Wagner P, and Ladabaum I, "Time-domain models of MUT array cross-talk in silicon substrates," Proceedings of the 2000 IEEE Ultrasonics Symposium, pp. 909-914.

[19] Roh Y and Khuri-Yakub BT, "Finite Element Analysis of Underwater Micromachined Ultrasonic Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 49, pp. 293-298, March 2002.

[20] Jin XC, Degertekin FL, Calmes S, Zhang XJ, Ladabaum I, and Khuri-Yakub BT, "Micromachined Capacitive Transducer Arrays for Medical Ultrasound Imaging," Proceedings of the 1998 IEEE International Ultrasonics Symposium, pp. 1877-1880, 1998.

[21] Oralkan Ö, Jin XC, Kaviani K, Ergun AS, Degertekin FL, Karaman M, and Khuri-Yakub BT, "Initial Pulse-Echo Imaging Results with One-Dimensional Capacitive Micromachined Ultrasonic Transducer Arrays," Proceedings of the 2000 IEEE International Ultrasonics Symposium, pp. 959-962, 2000.

[22] Oralkan Ö, Ergun S, Johnson JA, Karaman M, Demirci U, Kaviani K, Lee TH, and Khuri-Yakub BT, "Capacitive Micromachined Ultrasonic Transducers: Next Generation Arrays for Acoustic Imaging?", IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 49, pp. 1596-1610, Nov. 2002.

[23] Cheng CH., Ergun AS, and Khuri-Yakub BT, "Electrical Through Wafer Interconnects with 0.05 Pico Farads Parasitic Capacitance on 400 um Thick Silicon Substrate," presented at the Solid- State Sensor, Actuator, and Microsystems Workshop, Hilton Head Island, South Carolina, June 2-6, 2002.

[24] Zhuang X, Ergun AS, Oralkan Ö, Wygant IO, and Khuri-Yakub BT, "Interconnection and Packaging for 2D Capacitive Micromachined Ultrasonic Transducer Arrays Based on Through-Wafer Trench Isolation," Digest of Microelectromechanical Systems Conference, pp. 270-273, Istanbul, Turkey, Jan. 22-26, 2006.

[25] Bayram B, Hæggström E, Yaralioglu GG, and Khuri-Yakub BT, "A New Regime for Operating Capacitive Micromachined Ultrasonic Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 50, no. 9, pp. 1184-1190, Sep. 2003.

[26] Bayram B, Oralkan Ö, Ergun AS, Haeggström E, Yaralioglu GG, and Khuri-Yakub BT, "Capacitive micromachined ultrasonic transducer design for high power transmission," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 52, pp. 326-339, Feb. 2005.

[27] Oralkan Ö, Bayram B, Yaralioglu GG, Ergun AS, Kupnik M, Yeh D, Wygant IO, and Khuri-Yakub BT, "Collapse-Mode Operation of Capacitive Micromachined Ultrasonic Transducers," accepted for publication in IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2006.

[28] Bayram B, Yaralioglu GG, Kupnik M, Ergun AS, Oralkan Ö, Nikoozadeh A, and Khuri-Yakub BT, "Dynamic Analysis of Capacitive Micromachined Ultrasonic Transducers, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 52, no.12, pp. 2242-2258, Dec. 2005.

[29] Wygant IO, Yeh DT, Zhuang X, Nikoozadeh A, Oralkan Ö, Ergun AS, Karaman M, and Khuri-Yakub BT, "A miniature real-time volumetric ultrasonic imaging system," Proceedings of SPIE Medical Imaging Conference, pp. 26-36, 2005.

[30] Wygant IO, Zhuang X, Yeh DT, Vaithilingam S, Nikoozadeh A, Oralkan Ö, Ergun AS, Karaman M, and Khuri-Yakub BT, "An Endoscopic Imaging System Based on a Two-Dimensional CMUT Array: Real-Time Imaging Results," presented at the 2005 IEEE International Ultrasonics Symposium, Rotterdam, The Netherlands, Sept. 18 - 21, 2005.

[31] Yeh DT, Oralkan Ö, Wygant IO, O'Donnell M, and Khuri-Yakub BT, "3D ultrasound imaging using a forward-looking CMUT ring array for intravascular/intracardiac applications," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, accepted for publication, 2006.

[32] Yeh D, Oralkan Ö, Ergun AS, Zhuang X, Wygant IO, Cheng CH, Huang Y, Yaralioglu GG, and Khuri-Yakub BT, "High-frequency CMUT arrays for high-resolution medical imaging," Proc. SPIE Medical Imaging Conference, pp. 87-98, 2005.

[33] Yeh D, Oralkan Ö, Wygant IO, Ergun AS, Wong JH, and Khuri-Yakub BT, “ High-Resolution Imaging with High-Frequency 1-D Linear CMUT Arrays,” presented at the 2005 IEEE International Ultrasonics Symposium, Rotterdam, The Netherlands, Sept. 18 - 21, 2005.

[34] Vaithilingam S, Wygant IO, Kuo PS, Zhuang X, Oralkan Ö, Olcott PD, and Khuri-Yakub BT, “Capacitive Micromachined Ultrasonic Transducers CMUTs for Photoacoustic Imaging,” presented at the SPIE Photonics West Symposium, Jan. 24-26 2006, San Jose, CA.

[35] Wong SH, Wygant IO, Yeh DT, Zhuang X, Bayram B, Kupnik M, Oralkan Ö, Ergun AS, Yaralioglu GG, and Khuri-Yakub BT, “Capacitive Micromachined Ultrasonic Transducer Arrays for Integrated Diagnostic/Therapeutic Catheters,” presented in International Symposium on Therapeutic Ultrasound, Boston, MA, Oct. 27-29,2005.