Showing papers by "Arif Sanli Ergun published in 2003"

Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology

Abstract: Introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 /spl mu/m). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turn-around time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage.

Capacitive Micromachined Ultrasonic Transducers: Theory and Technology

Abstract: Capacitive micromachined ultrasonic transducers ~CMUTs!, introduced about a decade ago, have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. In this paper, we discuss the principles of capacitive transducer operation that underlie these aspects. Many of the key features of capacitive ultrasonic transducers are enabled with micromachining technology. Micromachining allows us to miniaturize device dimensions and produce capacitive transducers that perform comparably to their piezoelectric counterparts. The fabrication process is described briefly, and the performance of the CMUT transducers is evaluated by demonstrating characterization results. It is shown that the transduction efficiency as defined by the electromechanical coupling coefficient can be close to unity with proper device design and operating voltage. It is also shown that CMUTs provide large bandwidth ~123% fractional bandwidth! in immersion applications which translate into high temporal and axial resolution. Finally, the feasibility of using CMUTs is demonstrated by showing imaging examples in air and in immersion.

Calculation and measurement of electromechanical coupling coefficient of capacitive micromachined ultrasonic transducers

Abstract: The electromechanical coupling coefficient is an important figure of merit of ultrasonic transducers. The transducer bandwidth is determined by the electromechanical coupling efficiency. The coupling coefficient is, by definition, the ratio of delivered mechanical energy to the stored total energy in the transducer. In this paper, we present the calculation and measurement of coupling coefficient for capacitive micromachined ultrasonic transducers (CMUTs). The finite element method (FEM) is used for our calculations, and the FEM results are compared with the analytical results obtained with parallel plate approximation. The effect of series and parallel capacitances in the CMUT also is investigated. The FEM calculations of the CMUT indicate that the electromechanical coupling coefficient is independent of any series capacitance that may exist in the structure. The series capacitance, however, alters the collapse voltage of the membrane. The parallel parasitic capacitance that may exist in a CMUT or is external to the transducer reduces the coupling coefficient at a given bias voltage. At the collapse, regardless of the parasitics, the coupling coefficient reaches unity. Our experimental measurements confirm a coupling coefficient of 0.85 before collapse, and measurements are in agreement with theory.

Volumetric ultrasound imaging using 2-D CMUT arrays

Abstract: Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128/spl times/128-element 2-D CMUT array with through-wafer via interconnects and a 420-/spl mu/m element pitch. As an experimental prototype, a 32/spl times/64-element portion of the 128/spl times/128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16/spl times/16-element portion of the array to surrounding bondpads. An 8/spl times/16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4/spl times/4 group of elements in the middle of the 8/spl times/16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.

Micromachined ultrasonic transducers and method of fabrication

Abstract: There is described a micromachined ultrasonic transducers (MUTS) and a method of fabrication. The membranes of the transducers are fusion bonded to cavities to form cells. The membranes are formed on a wafer (11) of sacrificial material. This permits handling for fusions bonding. The sacrificial material is then removed to leave the membrane (14). Membranes of silicon, silicon nitride, etc. can be formed on the sacrificial material. Also described are cMUTs, pMUTs and mMUTs.

Electrical through wafer interconnects

Abstract: A wafer with through wafer interconnects. The wafer includes spaced through wafer vias which extend between the back side and front side of the wafer. A conductor within each of said vias connects to front and back side pads. Functions associated with said conductor and said pads provide a depletion region in the wafer between the pads and wafer or pads and conductor and the wafer.

Improved equivalent circuit and finite element method modeling of capacitive micromachined ultrasonic transducers

Abstract: Equivalent circuit model has been widely used to predict the bandwidth of capacitive micromachined ultrasonic transducers (CMUTs). According to this model, the lower cutoff of the bandwidth is determined by the time constant of the parallel RC where R is dictated by the radiation and C is determined by the electrical capacitance of the transducer. The higher cutoff, on the other hand, is determined by the membrane's anti-resonance. In the mechanical part of the model, the radiation impedance is simply added to the membrane impedance assuming that the membrane impedance does not change when it operates in the immersion medium. Therefore, the mass loading effect of the medium is neglected. Our finite element method calculations showed that the mass loading on the membrane impedance drastically lowers the membrane anti-resonance frequency degrading the bandwidth. In this paper, we present results of equivalent circuit modeling combined with finite element analysis. We constructed a 3D finite element model for one element of a 1D array. The element has 7 hexagonal membranes in the width dimension and it is assumed that the membranes are replicated in the length dimension infinitely by using symmetry boundary conditions. By combining membrane impedance with equivalent circuit model, we found that the center frequency of operation is 11 MHz and the bandwidth is 12.5 MHz close to the collapse voltage. We also investigated the effect of the DC bias on the center frequency. Decreasing the bias voltage increased the center frequency without affecting the bandwidth assuming the source impedance is zero.

Capacitive micromachined ultrasonic Lamb wave transducers using rectangular membranes

Abstract: This paper details the theory, fabrication, and characterization of a new Lamb wave device. Built using capacitive micromachined ultrasonic transducers (CMUTs), the structure described uses rectangular membranes to excite and receive Lamb waves on a silicon substrate. An equivalent circuit model for the transducer is proposed that produces results, which match well with those observed by experiment. During the derivation of this model, emphasis is placed on the resistance presented to the transducer membranes by the Lamb wave modes. Finite element analysis performed in this effort shows that the dominant propagating mode in the device is the lowest order antisymmetric flexural wave (A/sub 0/). Furthermore, most of the power that couples into the Lamb wave is due to energy in the vibrating membrane that is transferred to the substrate through the supporting posts of the device. The manufacturing process of the structure, which relies solely on fundamental IC-fabrication techniques, is also discussed. The resulting device has an 18 /spl mu/m-thick substrate that is almost entirely made up of crystalline silicon and operates at a frequency of 2.1 MHz. The characterization of this device includes S-parameter and laser vibrometer measurements as well as delay-line transmission data. The insertion loss, as determined by both S-parameter and delay-line transmission measurements, is 20 dB at 2.1 MHz. When configured as a delay-line oscillator, the device functions well as a sensor with sensitivity to changes in the mass loading of its substrate.

Dynamic analysis of CMUTs in different regimes of operation

Abstract: This paper reports on dynamic analysis of an immersed single capacitive micromachined ultrasonic transducer (CMUT) cell transmitting A water loaded 24 /spl mu/m circular silicon membrane of a transducer was modeled The calculated collapse and snapback voltages were 80 V and 50 V, respectively The resonance frequency, output pressure and nonlinearity of the CMUT in three regimes of operation were determined These regimes were: a) the conventional regime in which the membrane does not make contact with the substrate, b) the collapsed regime in which the center of the membrane is in constant contact with the substrate, and c) the collapse-snapback regime in which the membrane intermittently makes contact with the substrate and releases The average membrane displacement was compared as the CMUT was operated in these regimes A displacement of 70 /spl Aring/ in the collapsed regime and 39 /spl Aring/ in conventional regime operation were predicted when a 5 V pulse was applied to the CMUT cell biased at 70 V The CMUT showed a 2/sup nd/ harmonic at -16 dB and -26 dB in conventional and collapsed regimes of operation, respectively Collapse-snapback operation provided increased output pressure at the expense of a 3/sup rd/ harmonic at -10 dB Our simulations predicted that the average output pressure at the membrane could be 90 kPa/V with collapse-snapback operation compared to 4 kPa/V with conventional operation

Collapsed regime operation of capacitive micromachined ultrasonic transducers based on wafer-bonding technique

Abstract: We report experimental results from collapsed regime operation of the capacitive micromachined ultrasonic transducers (cMUTs) fabricated by a wafer bonding technique. The results show that a cMUT operating in the collapsed regime produces a maximal output pressure higher than a cMUT operating in the precollapse regime at 90% of its collapse voltage, 1.79kPa/V vs. 9.72 kPa/V at 2.3 MHz. in collapsed regime operation the fractional bandwidth (pulse-echo) is increased compared to that obtained in precollapsed regime operation 140% vs 83% with a bias 90% of the collapse voltage. Characterization of 1-D cMUT arrays operating in oil was done by ultrasonic pulse echo and pitch catch measurements.

New fabrication process for Capacitive Micromachined Ultrasonic Transducers

Abstract: In this paper, we introduce a new method to fabricate Capacitive Micromachined Ultrasonic Transducers (CMUT) that uses a wafer-bonding technique. The transducer membrane and cavity are defined separately on a Silicon-On-Insulator (SOI) wafer and on a prime quality silicon wafer, respectively. Using silicon direct bonding in a vacuum environment, the two wafers are bonded forming the transducer. This new process offers many advantages over surface micromachining on the fabrication of the transducers with different cavity and membrane configurations. CMUTs with different dimensions have been successfully fabricated and characterized. For the first time, sub-MHz operation is achieved with CMUTs. The test results show that the new process is a promising method to fabricate CMUTs for operation in air and water at different frequency ranges.

Capacitive micromachined ultrasonic transducers for robotic sensing applications

Abstract: Ultrasonic ranging is the most common method used in robotic systems for distance or proximity sensing. The heart of the method is an ultrasonic transducer that emits and detects ultrasound. There are mainly two types of transducers existing in the market today; piezoelectric and electrostatic. In this paper, we propose the use of a new type of transducer, which is capacitive micromachined ultrasonic transducer (CMUT). CMUTs were introduced about a decade ago, and were shown to be a good alternative for conventional transducers in various aspects, such as sensitivity, efficiency and bandwidth. CMUTs are basically the micromachined versions of the electrostatic transducers. Many enhanced features of the transducer are enabled with the micromachining technology. Micromachining allows miniaturization of the device dimensions and produces capacitive transducers that outperform their piezoelectric and electrostatic counterparts. This paper describes the fabrication process briefly, and the performance of the CMUT transducers is evaluated by demonstrating characterization results. It is also shown that CMUTs can be used for pulse-echo measurements in air up to MHz frequencies.

An implementation of a microfluidic mixer and switch using micromachined acoustic transducers

Abstract: This paper presents the results of fluidic switching and mixing, performed in microfluidic channels, integrated with micromachined acoustic transducers. The transducers operate at 400 MHz while the microfluidic channels are made by casting polydimethylsiloxane (PDMS) on silicon moulds. Rapid switching is incorporated by applying pressure waves in one of the two outlet channels to get preferential flow. Radiation pressure is also utilized to obtain active mixing of fluids within the channel. This method of switching and mixing has the advantages of simple design and ease of integration into larger systems.

Wideband micromachined acoustic sensors with radio frequency detection

Abstract: Silicon micromachining techniques permit batch fabrication of microphones that are small, reproducible, and inexpensive. However, many such sensors have limited bandwidth or are too fragile to be used in a humid, wet, or dusty outdoor environment. Microphones using capacitive micromachined ultrasonic transducer (CMUT) membranes and radio frequency (RF) detection overcome some of the problems associated with conventional micromachined microphones. CMUTmembranes can be vacuum-sealed and still withstand atmospheric pressure and submersion in water. In addition, the membrane mechanical response is very flat from dc up to hundreds of kilohertz. A very sensitive RF detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing smaller membranes. In this paper, we present the theory and recent experimental results of RF detection with CMUTmembranes. Measurements of a sensor with 1-mm 2 area demonstrate a flat output response of the acoustic sensor from a fraction of 1 Hz to over 100 kHz, with a sensitivity at 1 kHz of 65 dB/Pa in a 1-Hz noise bandwidth.

Acoustic heating and thermometry in microfluidic channels

Abstract: This paper presents the results of the work performed in establishing closed loop temperature control of fluids in microfluidic channels. Acoustic energy is used to introduce heat into localized regions of the channel. The temperature of the fluid in these regions is determined by using the relationship between the attenuation and speed of sound propagating in the fluid with temperature. The above two operations of heating and temperature measurement when used in unison serve as a very powerful tool in microfluidics. The system requires milli-watts of power for heating and has a temperature measurement accuracy of 0.1 degrees.

Improved modeling and fabrication techniques for capacitive micromachined ultrasonic Lamb wave transducers

Abstract: This paper discusses improvements in the theoretical and experimental framework of capacitive micromachined ultrasonic Lamb wave transducers. Theoretically, a new method for the analysis of these Lamb wave devices is proposed using the electro-mechanical capabilities of ANSYS, a commercial finite element package. The model used in these simulations has been verified by comparing its predictions when configured as a clamped transducer to those predicted by the standard equivalent circuit model; the input impedances obtained using the two methods agree to within 1%. This method performs harmonic and transient analyses to predict device performance metrics in the presence of electrostatic forces. Experimentally, this paper introduces a new manufacturing process for the fabrication of Lamb wave devices using CMUT wafer bonding technology. Devices have successfully been built using this technique and preliminary results show significantly improved uniformity in membrane to membrane performance and successful Lamb wave propagation between transducers. The membranes fabricated are 1 cm long and 60 /spl mu/m wide have an insertion loss of 16.2 dB near 2.6 MHz.

Phased subarray imaging for low-cost, wideband coherent array imaging

Abstract: The front-end hardware complexity of conventional full phased array (FPA) imaging is proportional to the number of array elements. Phased subarray (PSA) imaging has been proposed as a method of reducing the hardware complexity $and therefore system cost and size - while achieving near-FPA image quality. A new method is presented for designing the subarray-dependent interpolation filters suitable for wideband PSA imaging. The method was tested experimentally using pulse-echo data of a wire target phantom acquired using a 3.2-cm, 128-element capacitive micromachines ultrasonic transducer (CMUT) array with 85% fractional bandwidth at 3 MHz. A specific PSA configuration using seven 32-element subarrays was compared to FPA imaging, representing a 4-fold reduction in front-end hardware complexity and a 43% decrease in frame rate. For targets near the fixed transmit focal distance, the mean 6-dB lateral resolution was identical to that of FPA, the axial resolution improved by 4%, and the SNR decreased by 5 dB. Measurements were repeated for 10 different PSA configurations with subarray sizes ranging from 4 to 60. The lateral and axial resolutions did not vary significantly with subarray size; both the SNR and contrast-to-noise ration (CNR) improved with increased subarray size.