Browsing by Subject "Capacitive Micromachined Ultrasonic Transducer (cMUT)"

Now showing 1 - 2 of 2

Open Access
Modeling and characterization of capacitive micromachined ultrasonic transducers
(2000) Bozkurt, Ayhan
The Capacitive Micromachined Ultrasonic Transducer (cMUT) is a device used for the generation and detection of ultrasonic sound waves. The device is constructed on a silicon substrate using a microfabrication process. Individual cells constituting the device are membranes which have dimensions in the order of tens of microns, and are made up of a mechanicalh^ strong compound of silicon. The transducer itself has dimensions measured in centimeters, thus the total number of cells that make up a transducer is in the order of thousands. The excitation/detection of acoustic waves relies on the capacitance between the substrate and membrane: The presence of acoustic waves induces a small -AlC variation on the DC bias on the device, which can be used for detection, while a small -A.C component added to the DC bias by the drive circuit changes the electro-static attraction force on the membrane causing it to vibrate, producing acoustic waves. Basic advantages of cMUT devices include easy patterning of array structures, integration of drive/detection electronics with mechanical structures, and low cost. In this study, basic theory describing the characteristics of cMUT devices were developed. The analytic formulation was used to test the validity of a Finite Element Method (FEM) model. The FEM model, then, was emplo3'ed in the analysis of structures for which no analytical models are present. Specific problems solved using the FEM model included the characterization of cMUT devices with judiciously patterned electrodes. A more specific study showed that the bandwidth of an immersion device with an active area of radius 25 /¿m can be increased by 100% by simply setting the electrode radius to 10 /rm. The FEM analysis was, then, extended to handle the effects of substrate loss, which required the incorporation of an Absorbing Boundary Condition (ABC) into the model. A Normal Mode Theory analysis was conducted to give better insight to the physical nature of the effect of substrate loss to device characteristics. The dominant wavemode for a transducer of central frequency 2.5 MHz was found to be the lowest order anti-symmetric lamb wave mode (AO), for a silicon substrate of thickness 500 //m. A microfabrication process was developed for the production of cMUT devices. Hexagonally shaped transducers of radius 40 p.m were fabrictated on a conducting silicon substrated with silicon nitride as the sacrificial la.j'er and amorphous silicon as the membrane material. Both the gap and membrane thicknesses are set to 0.5 //m. 8, 16, and 24 /im gold plates were deposited as top eletrodes. The total number of active cells were 24 thousand for a substrate size of 0.7x0.7 cm^. Some experimental results were obtained from the fabricated transducers to support the analytical cMUT model. The device is found to have a central frequency of 2 MHz.
Open Access
Radiation impedance of capacitive micromachined ultrasonic transducers
(2010) Şenlik, Muhammed N.
Capacitive micromachined ultrasonic transducers (cMUTs) are used to transmit and receive ultrasonic signals. The device is constructed from circular membranes fabricated with surface micromachining technology. They have wider bandwidth with lower transmit power and lower receive sensitivity compared to the piezoelectric transducers, which dominate the ultrasonic transducer market. In order to be commercialized, they must overcome these drawbacks or find new application areas, where piezoelectric transducers perform poorly or cannot work. In this thesis, the latter approach, finding a new application area, is followed to design wide band and highly efficient airborne transducers with high output power by maximizing the radiation resistance of the transducer. The radiation impedance describes the interaction of the transducer with the surrounding medium. The real part, radiation resistance, is a measure of the amount of the power radiated to the medium; whereas the imaginary part, radiation reactance, shows the wobbled medium near the transducer surface. The radiation impedance of cMUTs are currently not well-known. As a first step, the radiation impedance of a cMUT with a circular membrane is calculated analytically using its velocity profile up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. The radiation impedance is determined to be a strong function of the cell spacing. It is shown that excitation of nonsymmetric modes is possible in immersion applications. A higher radiation resistance improves the bandwidth as well as the efficiency and the transmit power of the cMUT. It is shown that a center-to-center cell spacing of 1.25 wavelength maximizes the radiation resistance for the most compact arrangement, if the membranes are not too thin. For the airborne applications, the bandwidth can be further increased by using smaller device dimensions, which decreases the impedance mismatch between the cMUT and the air. On the other hand, this choice leads to degradation in both efficiency and transmit power due to lowered radiation resistance. It is shown that by properly choosing the arrangement of the thin membranes within an array, it is possible to optimize the radiation resistance. To make a fair analysis, same size arrays are compared. The operating frequency and the collapse voltage of the devices are kept constant. The improvement in the bandwidth and the transmit power can be as high as three and one and a half times, respectively. This method may also improve the noise figure when cMUTs are used as receivers. A further improvement in the noise figure is possible when the cells are clustered and connected to separate receivers. The results are presented as normalized graphs to be used for arbitrary device dimensions and material properties.