Finite element and equivalent circuit modeling of capacitive micromachined ultrasonic transducer (CMUT) /

In this thesis, a precise finite element model (FEM) of capacitive micromachined ultrasonic transducer is developed. Trough the results of the FEM, an equivalent circuit model of a transducer is built which enables high efficient design of transceiver front-end integrated circuit with a better known transducer behavior. Consequently, more realistic simulation results of the overall system can be obtained. The FEM model is created in the ANSYS environment and all simulations are done in 3D. The model can also be used to determine the proper parameters (e.g. radius, thickness, gap height) for the target of operation without fabrication. The equivalent circuit that is constructed over the Mason model is improved for immersion applications with modeling the radiation impedance with an RLC circuit and defining an effective transformer ratio value. Modeling results match actual measurements with a very good accuracy

Design and implementation of capacitive micromachined ultrasonic transducers for high intensity focused ultrasound

High intensity focused ultrasound (HIFU) is a medical procedure for noninvasive treatment of cancers. High intensity focused ultrasound is used to heat and destroy the diseased tissue. Piezoelectricity has been the core mechanism for generation of ultrasound waves in the treatment. Focusing can be done by using spherically curved transducers or using a lens or electronically steering sound waves by using phased arrays. Current research in HIFU technology targets the development of MR-guided miniaturized ultrasonic probes for treatment of cancerous tumors. Capacitive micromachined ultrasonic transducer (CMUT) is an alternative technology to generate and detect ultrasound. CMUT consists of a suspended membrane The advances in CMUT technology, enables fabricating tiny transducer arrays with wide bandwidth makes them a strong candidate for the application. In this thesis, a new methodology is proposed to design and operate CMUTs to generate high pressures under continuous wave excitation. An accurate nonlinear circuit model of CMUT is developed and the model is carried into a SPICE (Simulation Program with Integrated Circuit Emphasis) simulator for fast simulations. The model includes the radiation impedance of the array, thus the operation in a fluid environment can be simulated. The model is verified by doing FEM simulations. The circuit model provides a novel optimization tool for CMUT operating in non-collapse mode. The optimized CMUT parameters are presented and a sample fabrication is done using anodic bonding process. With the process, a 100 m thick silicon wafer is bonded to a glass substrate. A new driving scheme is proposed without a need of DC voltage. Thus, the charge trapping problem in CMUT operation is eliminated. The fabricated device provides 1.8 MPa surface pressure with -28dB second harmonic for a maximum 125V drive voltage at 1.44 MHz which is currently a state of art performance of a CMUT under continuous wave excitation

Modeling the pulse-echo response of a 2D CMUT array element

Motivation/Background: Front-end integrated circuit design for 2D ultrasonic array elements is challenging due to reduced element sensitivity. The pulse driver and return echo amplifier circuits can be optimized by the use of a precise circuit model of the transducer array at the design stage. An equivalent circuit that models transmit and receive operations of the array enables the matching of the IC to the transducer. This helps the designer to optimize the front-end IC before fabrication. Statement of the Contribution/Methods: In this paper, we present an equivalent circuit model for a 2D CMUT array element to asses the overall pulse-echo behavior. The circuit is a modified version of the conventional model in which the radiation term in the Mason equivalent circuit has been replaced by an RLC network to model the finite size of the transducer. The new circuit includes (i) an RC network to model the frequency dependent diffraction losses and attenuation in the transmitted and reflected acoustic waves, (ii) transmission lines for propagation delays, (iii) a VCVS to model the reflection from the imaging surface (which is the oil-air interface in our experiments). Component values of this model circuit were calculated using analytic expressions for the electro mechanic transformer ratio of CMUTs, based on the work in [1] and [2]. Pulse-echo measurements were carried out using a front-end IC designed in AustriaMicroSystems H35 high-voltage CMOS technology. An array element of 14×14 CMUT elements of 20 µm radius and 1 µm membrane thickness was wire-bonded to the IC. The resulting pulse and pulse echo response responses were then compared to the simulation results based on the developed model. Results: The equivalent circuit model was verified by running simulations on Cadence Spectre. The post-layout extracted netlist of the IC is combined with the proposed equivalent circuit that models the wire bonded array elements. The elements were fired with a unipolar pulse of 20 Volts and the signal at the amplifier output was than compared to the experimental data. Figure 1 shows a comparison of the experimental data and simulation result. Discussion and Conclusions: We demonstrated that the information obtained from the pulse-echo model is consistent with experimental results and can be used for further IC designs to enhance overall system performance. [1] Yaralioglu G. G., et al, “Finite-Element Analysis of Capacitive Micromachined Ultrasonic Transducers”, Tran UFFC. Vol. 52, No: 12, pp.2185-2198, December 2005. [2] Wygant, I. O., Kupnik, M., “Analytical Calculation Membrane Displacement and the Equivalent Circuit Model of a Circular CMUT Cell”, Proc. IEEE Intl. Ultrasonics Symp. 2008

A Lumped circuit model for the mutual radiation impedance of acoustic array elements

Closely spaced transducer elements used in ultrasonic imaging suffer from cross-coupling via the fluid medium. Circuit models for transducer elements in ultrasonic arrays have to account for the coupling effect for an accurate representation of their radiation impedance. In this paper, we present a circuit model for the cross-coupling effects among capacitive micromachined ultrasonic transducer (cMUT) elements. Using the finite element method (FEM) we first show that the mutual radiation impedance of cMUT cells with a physical distance much smaller that the acoustic wavelength obey the analytic results derived for plane piston transducers. Then, we show that this mutual impedance can be modeled by an electrical RLC tank circuit suitable to be used in general circuit simulators. The circuit shows 80% percent accuracy in between 5 to 15 MHz for a cMUT resonant frequency of 11.8 MHz. Found element values for R, L and, C components are 0.268m , 4.156pH and, 49.946´F respectively. We then combine this circuit with a previously proposed model for the self radiation impedance of a cMUT cell to yield an equivalent circuit representation that accounts for the cross-coupling effect. The proposed equivalent circuit proves itself to be useful in the analysis of transceiver front-end integrated circuits where an accurate transducer model is compulsory for optimizing circuit performance

Front-end IC Design for 2D cMUT Arrays: Modeling and Experimental Verification

In this paper, we present the modeling, design and test of a front-end IC for 2D cMUT arrays. In the modeling phase, present simulation results for a front-end circuit integrated with an array transducer element, and compare these with the experimental result for a front-end IC for 2D cMUT array. The pulse-echo model for the transducer is a modified version of the Mason Equivalent Circuit where the radiation impedance term has been replaced by an RLC network to include the effects of finite transducer size and diffraction loss. The model has been verified by running transient simulations using ANSYS. The circuit was composed of a high voltage (50 Volt) pulse driver, a NMOS protection switch and a trans impedance amplifier. The IC was manufactured by AustriaMicroSystems AG, Graz, Austria, in 0.35 μm high-voltage CMOS technology. We wire bonded the IC to a cMUT element to test the overall circuit performance. The cMUT elements that we used in the experiments had an operating frequency of 10 MHz and consisted of 49 CMUT cells with an overall transducer area of 200×200 µm2. The applied DC bias was 70 Volts. The cMUTs were driven by a 40 Volts unipolar pulse. We first performed hydrophone measurements to verify the functionality of the driver circuit. A droplet of vegetable oil was used as the propagation medium for pulse-echo measurements. The echo was observed from the air-oil interface. The results show that the performance of the circuit was consistent with the simulations. We were able to receive an echo from the surface of an oil layer of thickness less than 0.5 mm. (approximately 1 μs round-trip flight time.) The overall layout size of the manufactured circuit is 170×170 µm2 and it is suitable for integration to 3-5 MHz cMUT elements

Ultrasonic Phased Array Device for Real-Time Acoustic Imaging in Air

A real-time acoustic imaging system is developed as a prototype electronic travel aid (ETA) device. The design is implemented on a field programmable gate array (FPGA). A 6 channel transmit and 4 channel receive digital beamforming algorithm with dynamic focusing is accommodated in a FPGA. The developed system consists of a FPGA, pulser and receiver circuitry and separate transmitter and receiver arrays, which are constructed by using commercially available transducers. The transducer elements have a physical dimension of 1.9 wavelengths and a half-power beamwidth of 43◦ at 40.8 kHz center frequency. The transmitter array is formed by aligning the transducers with minimum spacing between the elements, which is 2 wavelengths. Obviously, more than one wavelength inter-element spacing leads to the occurrence of grating lobes in the array response and decreases the Field of View (FOV) below the half-power beamwidth of transducers. To extend the FOV and eliminate the grating lobe, the receiver array is formed with 3 wavelength inter-element spacing. The non identical element spacing makes the grating lobes of transmitter and receiver array to appear at different places. The described placement strategy and the functionality of the system is tested with several experiments. The results of these experiments prove the grating lobe suppression capability of the applied placement strategy

Fabrication of vacuum-sealed capacitive micromachined ultrasonic transducers with through-glass-via interconnects using anodic bonding

WOS: 000397049500023This paper presents a novel fabrication method for vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays that are amenable to 3D integration. This paper demonstrates that MEMS structures can be directly built on a glass substrate with preformed through-glass-via (TGV) interconnects. The key feature of this new approach is the combination of copper through-glass interconnects with a vibrating silicon-plate structure suspended over a vacuum-sealed cavity by using anodic bonding. This method simplifies the overall fabrication process for CMUTs with through-wafer interconnects by eliminating the need for an insulating lining for vias or isolation trenches that are often employed for implementing through-wafer interconnects in silicon. Anodic bonding is a low-temperature bonding technique that tolerates high surface roughness. Fabrication of CMUTs on a glass substrate and use of copper-filled vias as interconnects reduce the parasitic interconnect capacitance and resistance, and improve device performance and reliability. A 16x16-element 2D CMUT array has been successfully fabricated. The fabricated device performs as the finite-element and equivalent circuit models predict. A TGV interconnect shows a 2-Omega parasitic resistance and a 20-fF shunt parasitic capacitance for 250-mu m via pitch. A critical achievement presented in this paper is the sealing of the CMUT cavities in vacuum using a PECVD silicon nitride layer. By mechanically isolating the via structure from the active cells, vacuum sealing can be ensured even when hermetic sealing of the via is compromised. Vacuum sealing is confirmed by measuring the deflection of the edge-clamped thin plate of a CMUT cell under atmospheric pressure. The resonance frequency of an 8-cell 2D array element with 78-mu m diameter circular cells and a 1.5-mu m plate thickness is measured as 3.32 MHz at 15-V dc voltage (80% Vpull-in).National Science Foundation, National Nanotechnology Coordinated Infrastructure (NNCI) [ECCS-1542015]; State of North Carolina; National Science Foundation [ECCS-1542015]The authors would like to thank Tim Mobley and John Maki from Triton Microtechnologies for helping with the fabrication of TGV substrates. The device fabrication was performed in part at the NCSU Nanofabrication Facility (NNF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). The device characterization was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI)

Front-end IC design for intravascular ultrasound imaging

Capacitive micromachined ultrasonic transducers(cMUT) technology is a new trend for intravascular ultrasound (IVUS) imaging. Large bandwidth, high sensitivity and compatibility to CMOS processes makes the cMUT a better choice compared to the conventional piezoelectric transducer. To exploit the merits of cMUT technology, an accurately designed front end circuit is required. The circuit functions as an output pulse driver for the generation of the acoustic signal and buffers the return echo. For an accurate evaluation before tape-out, the circuit has to be simulated using the post-layout extracted netlist of the IC with the electrical equivalent circuit that models the transducer pulse-echo behavior. In this paper, we present two different designs of front-end IC for 2D cMUT arrays that can be used for intravascular ultrasound imaging system. To simulate the response of the front-end circuit, we first developed a pulse-echo model for an array element using Mason Equivalent Circuit. The model is then combined with the front-end circuit using Cadence Spectre. The simulation results are verified by comparing them to experimental data obtained from the manufactured front-end IC. The results show that the front-end circuit tested with the equivalent circuit model of the cMUT elements is promising for the optimization of the overall system performance before manufacturing