Capacitive Micromachined Ultrasonic Transducers Will Leisner and - - PDF document
12/13/2018 Capacitive Micromachined Ultrasonic Transducers Will Leisner and Mason Valdisera Background CMUTs are MEMS based structures that can be used to transmit and receive acoustic signals in the ultrasonic range advantages such as
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Will Leisner and Mason Valdisera
CMUTs are MEMS based structures that can be used to transmit and receive acoustic signals in the ultrasonic range
advantages such as large bandwidth, easy fabrication of large arrays, and integration with driver circuitry Several examples of CMUTs [6]
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1920s: Sonar Technology Developed 1950s: Ultrasound used in fetal imaging 1989: First CMUT developed 1994: First enclosed CMUT 2006: First Flexible CMUT 2017: First Transparent CMUT
Typical cross-section of a CMUT element [2]
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The principle behind how a CMUT works as an ultrasonic transducer [6]
Schematic of test set-up to determine CMUT bandwidth and pressure [1]. Example of CMUT pressure and bandwidth using the set-up described [1].
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Sacrificial Release Process [7] An illustration of surface micromachining [8] (a) Substrate doping, etch-stop layer deposition, first sacrificial layer deposition and patterning. (b) Reduced etch channel height regions. (c) Active area definition. (d) Membrane deposition. (e) Sacrificial layer etch hole definition and Si3N4 etch. (f) Membrane release in KOH. (g) Membrane sealing with more Si3N4 deposition. (h) Top electrode deposition and patterning.
(a)
Preparation of wafers.
(b)
Through thermal oxidation, oxide layers are formed on the surface of Wafer 1, then a spacer and 0.15 μm SiO2 insulating layer are formed using photolithography and wet etching.
(c)
By low-temperature bonding, Wafer 1 and Wafer 2 are bonded.
(d)
The silicon substrate of Wafer 2 is eliminated. Subsequently, the buried oxide layer of Wafer 2 is corroded away.
(e)
The rest of Wafer 2 is processed by photolithography and dry etching to form the isolation channel.
(f)
In order to effectively isolate the top electrode and vibration membrane, low pressure chemical vapor deposition (LPCVD) deposits SiO2.
(g)
The top of Wafer 2 is sputtered with metal Aluminum.
(h)
In order to form the Aluminum electrode and metal bonding pads, a lift-off process is used.
Steps of the Wafer Bonding Method [2]
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2 Step Process
First Step – Place Vibrating Membranes on PET
release layer
Second Step – Laminate Membranes onto
Sidewall Membrane acts as CMUT Structural support walls
Transparent, flexible CMUT manufactured using roll lamination [4].
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Cardiac Structures The vascular system The fetus and uterus Abdominal organs such as the liver,
the kidneys, and gall bladder
The eye Great at imaging moving systems
Examples of Medical CMUT Images [5]
Applications of Underwater Imaging [3]
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Testing of CMUT Device [1]
along with target object
display the image shown in figure b
Provide imaging in low visibility water
Better results than piezoelectric counterparts
New manufacturing methods make CMUTs more efficient, cheaper, and accurate
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