Browsing by Author "Şenlik, Muhammed N."

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Open Access
Bandwidth improvement in a cMUT array with mixed sized elements
(IEEE, 2005-09) Bayram, Can; Olcum, Selim; Şenlik, Muhammed N.; Atalar, Abdullah
A capacitive micromachined ultrasonic transducer (cMUT) is typically fabricated by concatenation of several cMUT cells with identical physical dimensions. If the membrane thickness is kept fixed, the radius of the cMUT determines the center frequency of operation. A smaller radius implies a greater center frequency. Therefore, it should be possible to put cMUTs with different sizes in parallel to get a larger bandwidth at the expense of gain. In this study, we investigate the optimization of the bandwidth characteristics of a cMUT by using mixed size cells. We designed two mixed size cMUT arrays with a predicted optimized fractional bandwidth value of about 155% at 5.4 MHz, and 146% at 8.8 MHz. These values are about 55% and 58% better than what can be achieved with a uniform size array at the corresponding center frequencies. There is almost no loss in the gain bandwidth product when two different sized cMUTs are used in parallel. There is about 9% increase in gain bandwidth product when three different sized cMUTs are used in parallel. It is shown, in this study, that gain bandwidth product and bandwidth can be enhanced by use of mixed size cMUT cells. © 2005 IEEE.
Open Access
Bandwidth, power and noise considerations in airborne cMUTs
(IEEE, 2009-09) Şenlik, Muhammed N.; Olcum, Selim; Köymen, Hayrettin; Atalar, Abdullah
Capacitive micromachined ultrasonic transducers (cMUTs) offer wider bandwidth in air due to their low mechanical impedances. The impedance mismatch between the air and transducer decreases with the smaller device dimensions increasing the bandwidth at the expense of the degradation in the transmit power and the receive sensitivity. In this work, the bandwidth of cMUT is optimized by increasing its radiation resistance. This is done by properly choosing the size of cMUT membranes and their placement within an array. This selection not only brings an improvement in the transmitted power when it is used as a transmitter, but also improves the noise figure when it is used as a receiver. A further improvement in the noise figure is possible when the cells are clustered and connected to separate receivers. ©2009 IEEE.
Open Access
Experimental characterization of capacitive micromachined ultrasonic transducers
(IEEE, 2007) Ölçüm, Selim; Atalar, Abdullah; Köymen, Hayrettin; Oğuz, Kağan; Şenlik, Muhammed N.
In this paper, capacitive micromachined ultrasonic transducers are fabricated using a sacrificial surface micromachining process. A testing procedure has been established in order to measure the absolute transmit and receive sensitivity spectra of the fabricated devices. The experiments are performed in oil. Pulse-echo experiments are performed and the results are compared to the pitch-catch measurements using calibrated transducers.
Open Access
Improved performance of cMUT with nonuniform membranes
(IEEE, 2005-09) Şenlik, Muhammed N.; Olcum, Selim; Atalar, Abdullah
When capacitive micromachined ultrasonic transducers are immersed in water, the bandwidth of the device is limited by the membrane's second resonance frequency. At this frequency no mechanical power to immersion medium can be transferred. We present a membrane shape to shift the second resonance frequency to a higher value. The structure consists of a very thin membrane at the outer rim with a rigid mass at the center. The stiffness of the central region moves the second resonance to a higher frequency. This membrane configuration is shown to work better in terms of gain and bandwidth as compared to conventional uniform membranes in both transmission and reception. © 2005 IEEE.
Open Access
Interaction between a cMUT cell and a liquid medium around the parallel resonance frequency
(IEEE, 2007-10) Şenlik, Muhammed N.; Atalar, Abdullah; Olçum, Selim
In this paper, we present how a capacitive micromachined ultrasonic transducer (cMUT) couples to the immersion medium, based on an accurate parametric model. We show that the velocity of cMUT membrane can be written as a sum of an average velocity term and a residual term. We demonstrate that this residual term carries non-zero energy at the parallel resonance frequency by investigating the interaction between the cMUT cell and a liquid medium. We develop a model that is also applicable around the parallel resonance frequency. © 2007 IEEE.
Open Access
Nonuniform membranes in capacitive micromachined ultrasonic transducers
(2005) Şenlik, Muhammed N.
Capacitive micromachined ultrasonic transducers (cMUT) are used to receive and transmit ultrasonic signals. The device is constructed from many small, in the order of microns, circular membranes, which are connected in parallel. When they are immersed in water, the bandwidth of the cMUT is limited by the membrane’s second resonance frequency, which causes an increase in the mechanical impedance of the membrane. In this thesis, we propose a new membrane shape to shift the second resonance frequency to higher values, in addition to keeping the impedance of the membrane as small as possible. The structure consists of a very thin membrane with a rigid mass at the center. The stiffness of the central region moves the second resonance to a higher frequency. This membrane configuration is shown to work better compared to conventionally used uniform membranes during both reception and transmission. The improvement in the bandwidth is more than %30 with an increase in the gain.
Open Access
Radiation impedance and equivalent circuit for immersed CMUT array element
(IEEE, 2006-10) Şenlik, Muhammed N.; Atalar, Abdullah; Köymen, Hayrettin; Olcum, Selim
In this paper, we present equivalent circuit for immersed capacitive micromachined ultrasonic transducers (cMUT), based on an accurate parametric model. We also present an accurate approximation for the radiation impedance cMUT. We develop a design approach for immersed cMUTs using the equivalent circuit. We demonstrate that the equivalent circuit predicts the performance of a cMUT array element composed of many cells in parallel. We investigate the applicability of the equivalent circuit in designing cMUT array elements. © 2006 IEEE.
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.
Open Access
Stagger tuned cMUT array for wideband airborne applications
(IEEE, 2006-10) Olcum, Selim; Atalar, Abdullah; Köymen Hayrtettin; Şenlik, Muhammed N.
In this study, we explore the limits of cMUTs in air-borne applications. First we investigate the ways of increasing the bandwidth of a single cMUT cell in air. The effect of array operation is also considered in order to increase the radiation resistance seen by the transducer. We calculate the bandwidth of a stagger tuned cMUT array. It is shown in this paper that more than 60% bandwidth can be obtained by three staggered frequencies. © 2006 IEEE.
Open Access
Wafer bonded capacitive micromachined underwater transducers
(IEEE, 2009-09) Olcum, Selim; Oǧuz, Kaan; Şenlik, Muhammed N.; Yamaner F. Y.; Bozkurt, A.; Atalar, Abdullah; Köymen, Hayrettin
In this work we have designed, fabricated and tested CMUTs as underwater transducers. Single CMUT membranes with three different radii and 380 microns of thickness are fabricated for the demonstration of an underwater CMUT element. The active area of the transducer is fabricated on top of a 3″ silicon wafer. The silicon wafer is bonded to a gold electrode coated glass substrate wafer 10 cm in diameter. Thermally grown silicon oxide layer is used as the insulation layer between membrane and substrate electrodes. Electrical contacts and insulation are made by epoxy layers. Single CMUT elements are tested in air and in water. Approximately 40% bandwidth is achieved around 25 KHz with a single underwater CMUT cell. Radiated pressure field due to second harmonic generation when the CMUTs are driven with high sinusoidal voltages is measured. ©2009 IEEE.