{{Short description|Electronics component}} {{more citations needed|date=February 2011}} {{Use dmy dates|date=February 2024}}

'''Capacitive micromachined ultrasonic transducers''' ('''CMUT''') are a relatively new concept in the field of [[ultrasonic transducer]]s. Most of the commercial ultrasonic transducers today are based on [[piezoelectricity]]. In CMUTs, the energy transduction is due to change in [[capacitance]]. CMUTs are constructed on [[silicon]] using micromachining techniques. A cavity is formed in a silicon [[coating|substrate]], and a thin layer suspended on the top of the cavity serves as a [[Acoustic membrane|membrane]] on which a [[metallizing|metallized]] layer acts an [[electrode]], together with the silicon substrate which serves as a bottom electrode.

If an [[alternating current|AC]] [[signal]] is applied across the [[current bias|biased]] electrodes, the [[Vibrations of a circular membrane|vibrating membrane]] will produce [[ultrasonic wave]]s in the medium of interest. In this way it works as a [[transmitter]]. On the other hand, if ultrasonic waves are applied on the membrane of a biased CMUT, it will generate alternating signal as the capacitance of the CMUT is varied. In this way, it works as a receiver of ultrasonic waves.<ref name=General>{{cite web|title=General Description and Advantages of CMUTs |url=http://www-kyg.stanford.edu/khuriyakub/opencms/en/research/cmuts/general/index.html |publisher=Stanford University |access-date=7 February 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110720050202/http://www-kyg.stanford.edu/khuriyakub/opencms/en/research/cmuts/general/index.html |archive-date=20 July 2011 }}</ref>

As CMUTs are micromachined devices, it is easier to construct 2D arrays of transducers using this technology. This means large numbers of CMUTs could be included in a transducer array providing larger [[bandwidth (signal processing)|bandwidth]] compared to other transducer technologies. To achieve a high frequency operation using CMUTs is easier due to its smaller dimensions.<ref>{{cite journal |doi=10.1109/TUFFC.2002.1049742|url=http://www.ieee-uffc.org/main/awards/outpapers/t02B1596.pdf |access-date=8 February 2011|archive-url=https://web.archive.org/web/20120318231148/http://www.ieee-uffc.org/main/awards/outpapers/t02B1596.pdf|archive-date=18 March 2012|url-status=dead |title=Capacitive micromachined ultrasonic transducers: Next-generation arrays for acoustic imaging? |year=2002 |last1=Oralkan |first1=O. |last2=Ergun |first2=A.S. |last3=Johnson |first3=J.A. |last4=Karaman |first4=M. |last5=Demirci |first5=U. |last6=Kaviani |first6=K. |last7=Lee |first7=T.H. |last8=Khuri-Yakub |first8=B.T. |journal=IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control |volume=49 |issue=11 |pages=1596–1610 |pmid=12484483 |s2cid=17896227 }}</ref> The frequency of operation depends on the cell size (cavity of membrane), and on the stiffness of the material used as a membrane. As it is built on silicon, the integration of electronics would be easier for the CMUTs compared to other transducer technologies. The properties to use in high frequency with large bandwidth makes it a good choice to use as a transducer in [[medical imaging]], especially in an [[intravascular ultrasound]] (IVUS). Because of its broader bandwidth, it could be used in [[second-harmonic imaging microscopy|second-harmonic imaging]]. Also some experiments have been performed to use CMUTs as [[hydrophone]]s.

==Fabrication methods==

===Sacrificial release surface micromachining=== [[Surface micromachining]] is the traditional way of manufacturing CMUTs.<ref name="ErgunAS2005"/> The major limitations of this method include complicated manufacturing process for constructing and sealing etch/drainage channels of the sacrificial material; the need for sacrificial-release channels reduces the available space for transducers, thereby reducing the achievable sound generation capability; limited control of layers' thickness during the manufacturing process; limited cavity thickness due to residues of fluid inside the cell cavity, which can cause [[stiction]] between the upper and lower parts of the cell, if the cell is not thick enough.<ref name="ErgunAS2005">{{cite journal|last1=Ergun|first1=AS|last2=Huang|first2=Y|last3=Zhuang|first3=X|title=Capacitive micromachined ultrasonic transducers: fabrication technology|journal= IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control|volume=52|issue=12|year=2005|pages=2242–58|doi=10.1109/tuffc.2005.1563267 |pmid=16463490|s2cid=3155087 }}</ref>

===Wafer bonding=== Wafer bonding is the most popular method. In this method, a CMUT is built from two separate wafers, which are later bonded to achieve cells with cavities.

====Fusion-bonding==== Fusion-bonding of wafers.<ref>{{cite journal | doi=10.1109/JMEMS.2003.809968 | title=Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology | year=2003 | last1=Yongli Huang | last2=Ergun | first2=A.S. | last3=Haggstrom | first3=E. | last4=Badi | first4=M.H. | last5=Khuri-Yakub | first5=B.T. | journal=Journal of Microelectromechanical Systems | volume=12 | issue=2 | pages=128–137 | s2cid=73596830 }}</ref><ref>{{cite journal | doi=10.1109/TUFFC.2009.1141 | title=Fabricating capacitive micromachined ultrasonic transducers with a novel silicon-nitride-Based wafer bonding process | year=2009 | last1=Logan | first1=A. | last2=Yeow | first2=J.T.W. | journal=IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control | volume=56 | issue=5 | pages=1074–1084 | pmid=19473926 | s2cid=12058311 }}</ref><ref>{{cite book | doi=10.1109/ULTSYM.2006.249 | chapter=Fabrication and Characterization of CMUTs realized by Wafer Bonding | title=2006 IEEE Ultrasonics Symposium | year=2006 | last1=Midtbo | first1=K. | last2=Ronnekleiv | first2=A. | last3=Wang | first3=D.T. | pages=938–941 | isbn=1-4244-0201-8 | s2cid=20519164 }}</ref><ref>{{cite book | doi=10.1109/MEMSYS.2008.4443662 | chapter=Fabricating capacitive micromachined ultrasonic transducers with direct wafer-bonding and LOCOS technology | title=2008 IEEE 21st International Conference on Micro Electro Mechanical Systems | year=2008 | last1=Park | first1=K.K. | last2=Lee | first2=H.J. | last3=Kupnik | first3=M. | last4=Oralkan | first4=O. | last5=Khuri-Yakub | first5=B.T. | pages=339–342 | isbn=978-1-4244-1792-6 | s2cid=9508355 }}</ref>

Multi-user MUMPS (polyMUMPS) process. CMUTs manufactured in the multi-user MUMPS were reported to have reduced performance, such as relatively low resonating frequency.<ref name="LiuOakley2009">{{cite journal|last1=Liu|first1=Jessica|last2=Oakley|first2=Clyde|last3=Shandas|first3=Robin|title=Capacitive micromachined ultrasonic transducers using commercial multi-user MUMPs process: Capability and limitations|journal=Ultrasonics|volume=49|issue=8|year=2009|pages=765–773|issn=0041-624X|doi=10.1016/j.ultras.2009.06.003|pmid=19640557|pmc=2783530}}</ref>

====Anodic bonding==== In [[anodic bonding]], wafers are sealed at high temperature and in the presence of electric field.<ref>{{cite book | doi=10.1109/ULTSYM.2009.5441699 | chapter=Wafer bonded capacitive micromachined underwater transducers | title=2009 IEEE International Ultrasonics Symposium | year=2009 | last1=Olcum | first1=Selim | last2=Oguz | first2=Kagan | last3=Senlik | first3=Muhammed N. | last4=Yamaner | first4=F. Yalcin | last5=Bozkurt | first5=Ayhan | last6=Atalar | first6=Abdullah | last7=Koymen | first7=Hayrettin | pages=976–979 | hdl=11693/28638 | isbn=978-1-4244-4389-5 | s2cid=7143784 }}</ref>

===Top-down process=== In this method the manufacturing is performed in reverse order, compared to the traditional way.<ref>{{cite journal | doi=10.1016/j.mee.2007.01.211 | title=Building CMUTs for imaging applications from top to bottom | year=2007 | last1=Coppa | first1=A. | last2=Cianci | first2=E. | last3=Foglietti | first3=V. | last4=Caliano | first4=G. | last5=Pappalardo | first5=M. | journal=Microelectronic Engineering | volume=84 | issue=5–8 | pages=1312–1315 }}</ref><ref>{{cite book | doi=10.1109/ULTSYM.2008.0517 | chapter=Curvilinear capacitive micromachined ultrasonic transducer (CMUT) array fabricated using a reverse process | title=2008 IEEE Ultrasonics Symposium | year=2008 | last1=Caronti | first1=Alessandro | last2=Coppa | first2=Andrea | last3=Savoia | first3=Alessandro | last4=Longo | first4=Cristina | last5=Gatta | first5=Philipp | last6=Mauti | first6=Barbara | last7=Corbo | first7=Antonio | last8=Calabrese | first8=Beatrice | last9=Bollino | first9=Giulio | last10=Paz | first10=Alejandro | last11=Caliano | first11=Giosue | last12=Pappalardo | first12=Massimo | pages=2092–2095 | isbn=978-1-4244-2428-3 | s2cid=6900919 }}</ref> The structural membrane is in silicon-nitride LPCVD, but the entire process is low-temperature, so it is CMOS-compatible. There are no etch-hole on the radiating surface of the device. The connection pads are on the back of the device, without using of through VIAs in the silicon, and the silicon substrate is completely removed. A custom acoustic backing is used to improve acoustic performances of the device. The process uses few masks (7–8).<ref>Patent US7790490</ref>

===Integration with electrical circuits=== As mentioned earlier, one of the significant advantages of CMUTs over piezoelectric transducers is the ability to integrate CMUTs with electrical circuits, using existing manufacturing methods.

==Benchmarking== CMUT performance is [[Benchmarking|benchmarked]] using pitch-catch and pulse-echo experiments, and operation uniformity is tested in air and in immersion. In a pitch-catch experiment, the transducer is benchmarked using a [[hydrophone]], and in a pulse-echo experiment, the transducer is used both for transmitting and receiving, while comparing the measured signal to the hydrophone response.

==Applications== The CMUT-on-CMOS technology and the flip-chip process allows tight integration of CMUTs with front-end electronics, which is necessary for miniature [[medical imaging]] devices, such as [[IVUS]].

== References == {{Reflist}}

== See also == * [[PMUT]], a similar technology based on piezoelectricity

==External links== * [https://spectrum.ieee.org/mems-ultrasound-history How Ultrasound Became Small], in IEEE Spectrum * Simulation software for ultrasound propagation with CMUTs: ** [http://field-ii.dk/ Field-II] ** [http://www.egr.msu.edu/~fultras-web/ FOCUS]

[[Category:Ultrasound]] [[Category:Transducers]]