A capacitive micromachined ultrasonic transducer (CMUT) is a relatively new concept in the field of
ultrasonic transducers. Most of the commercial ultrasonic transducers today are based on
piezoelectricity. CMUTs are the transducers where the energy transduction is due to change in
capacitance. CMUTs are constructed on
silicon using micromachining techniques. A cavity is formed in a silicon
substrate
Substrate may refer to:
Physical layers
*Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached
** Substrate (locomotion), the surface over which an organism lo ...
, and a thin layer suspended on the top of the cavity serves as a
membrane on which a
metallized layer acts an
electrode, together with the silicon substrate which serves as a bottom electrode.
If an
AC signal is applied across the
biased electrodes, the
vibrating membrane will produce
ultrasonic waves 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.
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 compared to other transducer technologies. To achieve a high frequency operation using CMUTs is easier due to its smaller dimensions. 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
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
, especially in an
intravascular ultrasound
Intravascular ultrasound (IVUS) or intravascular echocardiography is a medical imaging methodology using a specially designed catheter with a miniaturized ultrasound probe attached to the distal end of the catheter. The proximal end of the ca ...
(IVUS). Because of its broader bandwidth, it could be used in
second-harmonic imaging. Also some experiments have been performed to use CMUTs as
hydrophones.
Fabrication methods
Sacrificial release surface micromachining
Surface micromachining is the traditional way of manufacturing CMUTs. 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.
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.
Multi-user MUMPS (polyMUMPS) process. CMUTs manufactured in the multi-user MUMPS were reported to have reduced performance, such as relatively low resonating frequency.
Anodic bonding
In
anodic bonding Anodic bonding is a wafer bonding process to seal glass to either silicon or metal without introducing an intermediate layer; it is commonly used to seal glass to silicon wafers in electronics and microfluidics. This bonding technique, also known as ...
, wafers are sealed at high temperature and in the presence of electric field.
Top-down process
In this method the manufacturing is performed in reverse order, compared to the traditional way. 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).
[Patent US7790490]
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
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
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
devices, such as
IVUS.
References
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External links
* Simulation software for ultrasound propagation with CMUTs:
*
Field-II*
FOCUS
Ultrasound
Transducers