A nonlinear lumped model for ultrasound systems using CMUT arrays

An Adaptive Element-Level Impedance-Matched ASIC With Improved Acoustic Reflectivity for Medical Ultrasound Imaging

This paper presents an active impedance matching scheme that tries to optimize electrical power transfer and acoustic reflectivity in ultrasound transducers. Leveraging negative capacitance-based impedance matching would potentially improve the bandwidth and electrical power transfer while minimizing acoustic reflection of transducer elements and improve uniformity while reducing acoustic crosstalk of transducer arrays. A 16-element transceiver front-end is designed which employs an element-level active capacitive impedance cancellation scheme using an element-level negative impedance converter. The ASIC fabricated in 180-nm HVBCD technology provides high-voltage pulses up to 60 V consuming 3.6 mW and occupying 2.5 mm2. The front-end ASIC is used with a 1-D capacitive micromachined ultrasonic transducer (CMUT) array and its acoustical reflectivity reduction and imaging capabilities have successfully been demonstrated through pulse-echo measurements and acoustic imaging experiments.

Analysis of Negative Capacitance-Based Broadband Impedance Matching for CMUTs

Tight integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with integrated circuits can make active impedance matching feasible for practical imaging devices. In this article, negative capacitance-based impedance matching for CMUTs is investigated. Simple equivalent circuit model-based calculations show the potential of negative capacitance matching for improving the bandwidth along with electrical power transfer and acoustic reflectivity, but the model has limitations especially for acoustic reflectivity evaluation. For more realistic results, an experimentally validated CMUT array model is applied to a small 1-D CMUT array operating in the 5-15 MHz range. The results highlight the difference between electrical power transfer and acoustic reflectivity as well as the tradeoffs in signal-to-noise ratio (SNR). According to the results, ideal negative capacitance termination matched to the CMUT capacitance provides the broadest bandwidth and highest SNR if acoustic or electrical reflections are of no concern. On the other hand, negative capacitance and resistance matching to minimize acoustic reflectivity provides both lower reflection and closer to ideal SNR as compared with electrical power matching. It is observed that acoustic matching also reduces acoustic crosstalk and improves array uniformity. While several challenges for integrated circuit implementation are present, negative capacitance-based impedance matching can be a viable broadband active impedance matching method for CMUTs operating in conventional and collapsed mode as well as other ultrasound transducers with mainly capacitive impedance.

Highly Integrated Guidewire Ultrasound Imaging System-on-a-Chip

In this article, we present a highly integrated guidewire ultrasound (US) imaging system-on-a-chip (GUISoC) for vascular imaging. The SoC consists of a 16-channel US transmitter (Tx) and receiver (Rx) electronics, on-chip power management IC (PMIC), and quadrature sampler. Using a synthetic aperture imaging algorithm, a Tx/Rx pair, connected to capacitive micromachined ultrasound transducers (CMUTs), can be activated at any time. The Tx generates acoustic waves by driving the CMUT, while the Rx picks up the echo signal and amplify it to be delivered through an interconnect that is driven by a buffer. On-chip logic controls the pulsers that generate the high-voltage (HV)-pulse for Tx. An on-chip PMIC provides 1.8-, 5-, 39-, and 44-V supplies and a clock signal from the two interconnects besides GND. A quadrature sampler down-converts the Rx echo signal to baseband, reducing its bandwidth requirement for the output interconnect. The system design, including transimpedance amplifier (TIA) optimization, based on the equivalent circuit of a specific CMUT is presented. The SoC was fabricated by a 0.18-μm HV CMOS process, occupying 1.5-mm2 active area and consuming 25.2 and 44 mW from 1.8 to 44 V supplies, respectively. The US Tx and Rx show bandwidths of 32-42 and 32.7-37.5 MHz, respectively. The input-referred noise of the system was measured as 9.66 nA in band with 2-m-long 52 American Wire Gauge (AWG) wire interconnects. The functionality of the GUISoC was verified in vitro by imaging wire targets.

Supply-Inverted Bipolar Pulser and Tx/Rx Switch for CMUTs Above the Process Limit for High Pressure Pulse Generation

A combined supply-inverted bipolar pulser and a Tx/Rx switch is proposed to drive capacitive micromachined ultrasonic transducers (CMUTs). The supply-inverted bipolar pulser adopts a bootstrap circuit combined with stacked transistors, which guarantees high voltage (HV) operation above the process limit without lowering device reliability. This circuit generates an output signal with a peak-to-peak voltage that is almost twice the supply level. It generates a bipolar pulse with only positive supply voltages. The Tx/Rx switch adopts a diode-bridge structure with the protection scheme dedicated to this proposed pulser. A proof- of-concept ASIC prototype has been implemented in 0.18-μm HV CMOS/DMOS technology with 60 V devices. Measurement results show that the proposed pulser can safely generate a bipolar pulse of -34.6 to 45 V, from a single 45 V supply voltage. The Tx/Rx switch blocks the HV bipolar pulse, resulting in less than 1.6 V at the input of the receiver. Acoustic measurements are performed connecting the pulser to CMUTs with 2 pF capacitance and 8 MHz center frequency. The variation of acoustic output pressures for different pulse shapes were simulated with the large signal CMUT model and compared with the experimental results for transmit pressure optimization. A potential implementation of the methods using MEMS fabrication methods is also described.

An Analysis Method for Capacitive Micromachined Ultrasound Transducer (CMUT) Energy Conversion during Large Signal Operation

With Capacitive Micromachined Ultrasound Transducers (CMUTs) increasingly being used for high intensity, large signal ultrasound applications and several drive methods being proposed, the efficiency of these devices in this operation regime have not been quantitatively evaluated. Since well-known frequency and capacitance-based coupling coefficients definitions are not valid for large signal, nonlinear operation, an energy-based definition should be used. In this paper, an expression for mechanical energy in a CMUT is obtained based on the assumption that CMUT is a linear time varying capacitor in all regimes of operation. This expression is evaluated by the help of an experimentally verified nonlinear CMUT model to define an energy conversion ratio (ECR) which can be considered as a coupling coefficient valid for all regimes of operation. This parameter is validated in the small signal regime and then used to evaluate CMUT performance with various large drive signals. The quantitative modeling results show that CMUTs do not need DC bias to achieve high efficiency large signal transduction: AC only signals at half the operation frequency with amplitudes beyond the collapse voltage can provide efficiencies (ECR) above 0.9 with harmonic content below -25 dB. Based on these results, ECR variation with membrane geometry and parasitic capacitance are given as examples for device optimization. The overall modeling approach is also qualitatively validated by experiments.

Analysis and Design of High-Frequency 1-D CMUT Imaging Arrays in Noncollapsed Mode

High-frequency ultrasound imaging arrays are important for a broad range of applications, from small animal imaging to photoacoustics. Capacitive micromachined ultrasonic transducer (CMUT) arrays are particularly attractive for these applications as low noise receiver electronics can be integrated for an overall improved performance. In this paper, we present a comprehensive analysis of high-frequency CMUT arrays based on an experimentally verified CMUT array simulation tool. The results obtained on an example, a 40-MHz 1-D CMUT array for intravascular imaging, are used to obtain key design insights and tradeoffs for receive only and pulse-echo imaging. For the receiver side, thermal mechanical current noise, plane wave pressure sensitivity, and pressure noise spectrum are extracted from simulations. Using these parameters, we find that the receiver performance of CMUT arrays can be close to an ideal piston, independent of gap thickness, and applied dc bias, when coupled to low noise electronics with arrays utilizing smaller membranes performing better. For pulse-echo imaging, thermal mechanical current noise limited signal-to-noise ratio is observed to be dependent on the maximum available voltage and gap thickness. In terms of bandwidth, we find that the Bragg resonance of the array, related to the fill factor, is a significant determinant of the high frequency limit and the fluid loaded single membrane resonance determines the lower limit. Based on these results, we present design guidelines requiring only fluid loaded single membrane simulations and membrane pitch to achieve a desired pulse-echo response. We also provide a design example and discuss limitations of the approach.

Low Temperature CMUT Fabrication Process with Dielectric Lift-off Membrane Support for Improved Reliability

This paper reports an improved CMOS compatible low temperature sacrificial layer fabrication process for Capacitive Micromachined Ultrasonic Transducers (CMUTs). The process adds the fabrication step of silicon oxide evaporation which is followed by a lift-off step to define the membrane support area without a need for an extra mask. This simple addition improves reliability by reducing the electric field between the top and bottom electrodes everywhere except the moving membrane without affecting the vacuum gap thickness. Furthermore, the parasitic capacitance which degrades the CMUT receive performance is reduced. A 1-D CMUT array suitable for Intracardiac Echocardiography (ICE) imaging with 9MHz center frequency is fabricated using this method. Detailed electrical and acoustic testing indicates adequate performance of the devices for ICE in agreement with simulations. Long term output pressure testing with more than 2×10¹¹ pulsing cycles and environmental testing demonstrate the efficacy of the approach for improved reliability as compared to devices without the additional membrane support layer.

Unbiased Charged Circular CMUT Microphone: Lumped-Element Modeling and Performance

An energy-consistent lumped-element equivalent circuit model for charged circular capacitive micromachined ultrasonic transducer (CMUT) cell is derived and presented. It is analytically shown and experimentally verified that a series dc voltage source at the electrical terminals is sufficient to model the charging in CMUT. A model-based method for determining this potential from impedance measurements at low bias voltages is presented. The model is validated experimentally using an airborne CMUT, which resonates at 103 kHz. Impedance measurements, reception measurements at resonance and off-resonance, and the transient response of the CMUT are compared with the model predictions.

A capacitive ultrasonic transducer based on parametric resonance

A capacitive ultrasonic transducer based on a parametric resonator structure is described and experimentally demonstrated. The transducer structure, which we call capacitive parametric ultrasonic transducer (CPUT), uses a parallel plate capacitor with a movable membrane as part of a degenerate parametric series RLC resonator circuit with a resonance frequency of f_o. When the capacitor plate is driven with an incident harmonic ultrasonic wave at the pump frequency of 2f_o with sufficient amplitude, the RLC circuit becomes unstable and ultrasonic energy can be efficiently converted to an electrical signal at f_o frequency in the RLC circuit. An important characteristic of the CPUT is that unlike other electrostatic transducers, it does not require DC bias or permanent charging to be used as a receiver. We describe the operation of the CPUT using an analytical model and numerical simulations, which shows drive amplitude dependent operation regimes including parametric resonance when a certain threshold is exceeded. We verify these predictions by experiments with a micromachined membrane based capacitor structure in immersion where ultrasonic waves incident at 4.28 MHz parametrically drive a signal with significant amplitude in the 2.14 MHz RLC circuit. With its unique features, the CPUT can be particularly advantageous for applications such as wireless power transfer for biomedical implants and acoustic sensing.

A capacitive ultrasonic transducer based on parametric resonance

A capacitive ultrasonic transducer based on a parametric resonator structure is described and experimentally demonstrated. The transducer structure, which we call capacitive parametric ultrasonic transducer (CPUT), uses a parallel plate capacitor with a movable membrane as part of a degenerate parametric series RLC resonator circuit with a resonance frequency of f_o. When the capacitor plate is driven with an incident harmonic ultrasonic wave at the pump frequency of 2f_o with sufficient amplitude, the RLC circuit becomes unstable and ultrasonic energy can be efficiently converted to an electrical signal at f_o frequency in the RLC circuit. An important characteristic of the CPUT is that unlike other electrostatic transducers, it does not require DC bias or permanent charging to be used as a receiver. We describe the operation of the CPUT using an analytical model and numerical simulations, which shows drive amplitude dependent operation regimes including parametric resonance when a certain threshold is exceeded. We verify these predictions by experiments with a micromachined membrane based capacitor structure in immersion where ultrasonic waves incident at 4.28 MHz parametrically drive a signal with significant amplitude in the 2.14 MHz RLC circuit. With its unique features, the CPUT can be particularly advantageous for applications such as wireless power transfer for biomedical implants and acoustic sensing.

Efficient Broadband Simulation of Fluid-Structure Coupling for Membrane-Type Acoustic Transducer Arrays Using the Multilevel Fast Multipole Algorithm

A boundary element model provides great flexibility for the simulation of membrane-type micromachined ultrasonic transducers (MUTs) in terms of membrane shape, actuating mechanism, and array layout. Acoustic crosstalk is accounted for through a mutual impedance matrix that captures the primary crosstalk mechanism of dispersive-guided modes generated at the fluid-solid interface. However, finding the solution to the fully populated boundary element matrix equation using standard techniques requires computation time and memory usage that scales by the cube and by the square of the number of nodes, respectively, limiting simulation to a small number of membranes. We implement a solver with improved speed and efficiency through the application of a multilevel fast multipole algorithm (FMA). By approximating the fields of collections of nodes using multipole expansions of the free-space Green's function, an FMA solver can enable the simulation of hundreds of thousands of nodes while incurring an approximation error that is controllable. Convergence is drastically improved using a problem-specific block-diagonal preconditioner. We demonstrate the solver's capabilities by simulating a 32-element 7-MHz 1-D capacitive MUT (CMUT) phased array with 2880 membranes. The array is simulated using 233280 nodes for a very wide frequency band up to 50 MHz. For a simulation with 15210 nodes, the FMA solver performed ten times faster and used 32 times less memory than a standard solver based on LU decomposition. We investigate the effects of mesh density and phasing on the predicted array response and find that it is necessary to use about seven nodes over the width of the membrane to observe convergence of the solution-even below the first membrane resonance frequency-due to the influence of higher order membrane modes.