عنوان مقاله
ترانسدیوسر فراصوت میکروماشین شده خازنی
فهرست مطالب
مقدمه
ولتاژ فروپاشی
پهنای باند فرکانس
تکنولوژی ساخت
اندازه گیری امپدانس ورودی الکتریکی
کاربردها و نتیجه گیری
بخشی از مقاله
اندازه گیریهای امپدانس ورودی الکتریکی اولین پروکسی برای فعالیت صوتی ترانسدیوسر فراهم می کنند. اندازه گیریها با آنالیزور شبکه برداری انجام شدند ( HP 8751A) که امپدانس ورودی پیچیده ترانسدیوسر به عنوان تابع فرکانس را اندازه گیری کرد. راه اندازی اندازه گیری در شکل زیر نشان داده شده است.
کلمات کلیدی:
Capacitive Micromachined Ultrasonic Transducers:Theory and TechnologyArif S. Ergun1; Goksen G. Yaralioglu2; and Butrus T. Khuri-Yakub3Abstract: Capacitive micromachined ultrasonic transducers ~CMUTs!, introduced about a decade ago, have been shown to be a goodalternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. In thispaper, we discuss the principles of capacitive transducer operation that underlie these aspects. Many of the key features of capacitiveultrasonic transducers are enabled with micromachining technology. Micromachining allows us to miniaturize device dimensions andproduce capacitive transducers that perform comparably to their piezoelectric counterparts. The fabrication process is described briefly,and the performance of the CMUT transducers is evaluated by demonstrating characterization results. It is shown that the transductionefficiency as defined by the electromechanical coupling coefficient can be close to unity with proper device design and operating voltage.It is also shown that CMUTs provide large bandwidth ~123% fractional bandwidth! in immersion applications which translate into hightemporal and axial resolution. Finally, the feasibility of using CMUTs is demonstrated by showing imaging examples in air and inimmersion.DOI: 10.1061/~ASCE!0893-1321~2003!16:2~76!CE Database subject headings: Transducers; Fabrication; Imaging techniques; Electronic equipment.IntroductionElectrostatic transducers have long been in use for sound waveexcitation and detection ~Kuhl 1954; Hunt 1982!. The fundamentalmechanism of the transduction is the vibration of a thin plateunder electrostatic forces. Many macroscale devices use thismechanism for generating and sensing sonic waves. A condensermicrophone is the most well-known example. In the simplestform of this device, a thin metal membrane is stretched above aback electrode forming a small gap. This structure constitutes acapacitor, which is charged by a dc voltage applied through alarge resistor. When the device is exposed to sound waves, thegap height is modulated at the same frequency of the incomingpressure field. This induces a change in the device capacitance,generating an output voltage proportional to the amplitude of thefield. The capacitor structure can also be used to generate soundwaves. If the biased membrane is driven by an ac voltage, aharmonic field is generated in the sound-bearing medium.The striking advantage of the electrostatic devices comparedto the other types of transducers such as piezoelectric and magnetostrictis the inherent impedance match between the transducerand the surrounding medium. The low-mechanical impedance ofthe membrane is usually negligible. This results in very efficientcoupling of the sound waves into and from the sound-bearingmedium.Recent advances in the silicon micromachining techniques enabledthe fabrication of microelectro-mechanical systems~MEMS! based electrostatic transducers ~Haller and Khuri-Yakub1996; Soh et al. 1996; Ladabaum et al. 1998!. Miniaturizationcapability of the silicon micromachining process made the fabricationof devices working at ultrasonic frequencies possible.These devices are called capacitive micromachined ultrasonictransducers ~CMUTs!. CMUTs are made of small and thin membranesthat are suspended over a conductive silicon substrate byinsulating posts. The diameter of the membrane ranges from 10mm to hundreds of micrometers. The gap between the membraneand the substrate is vacuum sealed or left unsealed at will and itcan be as small as 500 Å. The membranes are either conductive orcoated with a conductive electrode and essentially create smallcapacitors together with the substrate. This structure results invery efficient transducers that can compete with their piezoelectriccounterparts in terms of efficiency and bandwidth.The small and thin membranes that constitute the CMUTtransducer are micromachined onto a silicon substrate. Micromachininghas evolved from the integrated circuit manufacturingtechnology as a means of fabricating microelectromechanical systems,and therefore has all the abilities that the integrated circuittechnology has. These abilities include, but are not limited to,batch fabrication, and a high level of integration and scalability.Thus, with this technology, batch fabrication of high-densitytransducer arrays, as well as single elements are enabled. In addition,the scalability provides the ability to fabricate transducerswith a wide range of size and shapes. Because the transducerresponse is primarily determined by the size and the shape of themembranes, scalability translates into the ability to fabricate awide range of devices for operation at different frequency spansand regimes.This paper summarizes the work that has been done onCMUTs. In the next section, the principle of operation and under-1E. L. Ginzton Laboratory, Stanford Univ., Stanford, CA 94305-4088.E-mail: sanli@stanford.edu2E. L. Ginzton Laboratory, Stanford Univ., Stanford, CA 94305-4088.E-mail: goksenin@stanford.edu3E. L. Ginzton Laboratory, Stanford Univ., Stanford, CA 94305-4088.E-mail: khuri-yakub@stanford.eduNote. Discussion open until September 1, 2003. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and possiblepublication on November 6, 2002; approved on November 6, 2002.This paper is part of the Journal of Aerospace Engineering, Vol. 16, No.2, April 1, 2003. ©ASCE, ISSN 0893-1321/2003/2-76–84/$18.00.76 / JOURNAL OF AEROSPACE ENGINEERING © ASCE / APRIL 2003
