Gokce Gurun

Front-end receiver electronics for high-frequency monolithic CMUT-on-CMOS imaging arrays

This paper describes the design of CMOS receiver electronics for monolithic integration with capacitive micromachined ultrasonic transducer (CMUT) arrays for high-frequency intravascular ultrasound imaging. A custom 8-inch (20-cm) wafer is fabricated in a 0.35-μm two-poly, four-metal CMOS process and then CMUT arrays are built on top of the application specific integrated circuits (ASICs) on the wafer. We discuss advantages of the single-chip CMUT-on-CMOS approach in terms of receive sensitivity and SNR. Low-noise and high-gain design of a transimpedance amplifier (TIA) optimized for a forward-looking volumetric-imaging CMUT array element is discussed as a challenging design example. Amplifier gain, bandwidth, dynamic range, and power consumption trade-offs are discussed in detail. With minimized parasitics provided by the CMUT-on-CMOS approach, the optimized TIA design achieves a 90 fA/√Hz input-referred current noise, which is less than the thermal-mechanical noise of the CMUT element. We show successful system operation with a pulseecho measurement. Transducer-noise-dominated detection in immersion is also demonstrated through output noise spectrum measurement of the integrated system at different CMUT bias voltages. A noise figure of 1.8 dB is obtained in the designed CMUT bandwidth of 10 to 20 MHz.

Single-chip CMUT-on-CMOS front-end system for real-time volumetric IVUS and ICE imaging

Intravascular ultrasound (IVUS) and intracardiac echography (ICE) catheters with real-time volumetric ultrasound imaging capability can provide unique benefits to many interventional procedures used in the diagnosis and treatment of coronary and structural heart diseases. Integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with front-end electronics in single-chip configuration allows for implementation of such catheter probes with reduced interconnect complexity, miniaturization, and high mechanical flexibility. We implemented a single-chip forward-looking (FL) ultrasound imaging system by fabricating a 1.4-mm-diameter dual-ring CMUT array using CMUT-on-CMOS technology on a front-end IC implemented in 0.35-μm CMOS process. The dual-ring array has 56 transmit elements and 48 receive elements on two separate concentric annular rings. The IC incorporates a 25-V pulser for each transmitter and a low-noise capacitive transimpedance amplifier (TIA) for each receiver, along with digital control and smart power management. The final shape of the silicon chip is a 1.5-mm-diameter donut with a 430-μm center hole for a guide wire. The overall front-end system requires only 13 external connections and provides 4 parallel RF outputs while consuming an average power of 20 mW. We measured RF A-scans from the integrated single- chip array which show full functionality at 20.1 MHz with 43% fractional bandwidth. We also tested and demonstrated the image quality of the system on a wire phantom and an ex vivo chicken heart sample. The measured axial and lateral point resolutions are 92 μm and 251 μm, respectively. We successfully acquired volumetric imaging data from the ex vivo chicken heart at 60 frames per second without any signal averaging. These demonstrative results indicate that single-chip CMUT-on-CMOS systems have the potential to produce realtime volumetric images with image quality and speed suitable for catheter-based clinical applications.

Monolithic CMUT-on-CMOS integration for intravascular ultrasound applications

One of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach is required to minimize the parasitic capacitances in the receive mode, especially in catheter-based volumetric imaging arrays, for which the elements must be small. Furthermore, optimization of the available silicon area and minimized number of connections occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication and performance evaluation of CMUT arrays for intravascular imaging on custom-designed CMOS receiver electronics from a commercial IC foundry. The CMUT-on-CMOS process starts with surface isolation and mechanical planarization of the CMOS electronics to reduce topography. The rest of the CMUT fabrication is achieved by modifying a low-temperature micromachining process through the addition of a single mask and developing a dry etching step to produce sloped sidewalls for simple and reliable CMUT-to-CMOS interconnection. This CMUT-to-CMOS interconnect method reduced the parasitic capacitance by a factor of 200 when compared with a standard wire-bonding method. Characterization experiments indicate that the CMUT-on-CMOS elements are uniform in frequency response and are similar to CMUTs simultaneously fabricated on standard silicon wafers without electronics integration. Ex- periments on a 1.6-mm-diameter dual-ring CMUT array with a center frequency of 15 MHz show that both the CMUTs and the integrated CMOS electronics are fully functional. The SNR measurements indicate that the performance is adequate for imaging chronic total occlusions located 1 cm from the CMUT array.

Single chip CMUT arrays with integrated CMOS electronics: Fabrication process development and experimental results

One of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach maximizes transducer sensitivity by minimizing parasitic capacitances and ultimately improves the signal to noise ratio. Additionally, due to physical size limitations required for catheter based imaging devices, optimization of area occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication of CMUTs on custom designed CMOS electronics from a commercial IC foundry. This is achieved by modifying a low temperature micromachining process by adding one additional mask and developing a sloped wall dry etching step on isolation oxide layer for metal interconnect. Experiments show that both the CMUT and the integrated CMOS electronics are functional, resulting combination generating pulse-echo response suitable for ultrasound imaging.

An Analog Integrated Circuit Beamformer for High-Frequency Medical Ultrasound Imaging

We designed and fabricated a dynamic receive beamformer integrated circuit (IC) in 0.35-μm CMOS technology. This beamformer IC is suitable for integration with an annular array transducer for high-frequency (30-50 MHz) intravascular ultrasound (IVUS) imaging. The beamformer IC consists of receive preamplifiers, an analog dynamic delay-and-sum beamformer, and buffers for 8 receive channels. To form an analog dynamic delay line we designed an analog delay cell based on the current-mode first-order all-pass filter topology, as the basic building block. To increase the bandwidth of the delay cell, we explored an enhancement technique on the current mirrors. This technique improved the overall bandwidth of the delay line by a factor of 6. Each delay cell consumes 2.1-mW of power and is capable of generating a tunable time delay between 1.75 ns to 2.5 ns. We successfully integrated the fabricated beamformer IC with an 8-element annular array. Experimental test results demonstrated the desired buffering, preamplification and delaying capabilities of the beamformer.

A large-scale Reconfigurable Smart Sensory Chip

The Reconfigurable Smart Sensory Chip (RSSC) is a powerful tool for fast prototyping sensory microsystems. Innovative design ideas can be quickly realized and tested in hardware without doing time-consuming and expensive silicon fabrication. The RSSC is a large-scale floating-gate based IC containing 8 universal sensor interface blocks, each of which can be configured for voltage sensing, capacitive sensing, or current sensing, and 28 configurable analog blocks. The outputs of the interface circuits can be multiplexed out in a time-division sequence or can be routed to the configurable analog blocks for further analog signal processing or data conversion. With more than 50,000 programmable elements and on-chip programming circuitry, RSSC is an extremely powerful tool to develop and test a great variety of smart sensory microsystems in minutes.

A 1.5-mm diameter single-chip CMOS front-end system with transmit-receive capability for CMUT-on-CMOS forward-looking IVUS

Integration of receive and transmit electronics and transducer array elements on a single chip is critical for successful implementation of a highly-flexible forward-looking (FL) IVUS imaging catheter. Single-chip integration reduces the interconnect complexity significantly, and enables the ultimate miniaturization of IVUS arrays. Here we describe a single-chip FL-IVUS system based on a front-end IC implemented in 0.35-μm CMOS that is integrated with a 1.4-mm diameter CMUT array using CMUT-on-CMOS technology. The IC incorporates a pulser capable of generating 25-V pulses and a low-noise receiver transimpedance amplifier dedicated to each of the 56 Tx and 48 Rx CMUT array elements, respectively. The chip also includes a digital control circuitry that is designed to synchronize transmitting and receiving sequence during the data acquisition. All of the active circuitry fits into a size of 1.5-mm diameter silicon donut shape with a 430-μm gap left inside for a guide wire. The single-chip FL-IVUS system requires only 13 external connections and provides 4 parallel outputs. The average power consumption of the chip is reduced to 20 mW by biasing off the unused receive amplifiers using the digital logic. Characterization results of the low-noise preamplifiers in the IC after monolithic integration with the FL-IVUS CMUT array are presented. Successful implementation of the single-chip FL-IVUS system is also demonstrated through a volumetric imaging experiment.

P0-18 Forward-Looking IVUS Imaging Using a Dual-Annular Ring CMUT Array: Experimental Results

This paper presents the experimental results on forward-looking intravascular ultrasound (FL-IVUS) using dual- annular-ring CMUT arrays. The array has a diameter of 1mm including bondpads which consists of separate, concentric 24 transmit and 32 receive ring arrays built on the same silicon substrate. This configuration has the potential for independent optimization of each array and uses the silicon area more effectively without any drawback. For imaging experiments, we designed and constructed a custom integrated circuit using a standard 0.5mum CMOS process for data acquisition. A sample pulse-echo signal received from the oil-air interface (plane reflector) at 6 mm had a center frequency of 11 MHz with 95*% fractional 6-dB bandwidth. The measured SNR of the echo was 24 dB with no averaging. B-scan image of a wire-phantom was generated to test the resolution.

High frequency ultrasonic imaging using thermal mechanical noise recorded on capacitive micromachined transducer arrays

The cross-correlation of diffuse thermal-mechanical noise recorded by two sensors yields an estimate of the ultrasonic waves propagating between them. We used this approach at high frequencies (1-30 MHz) on a capacitive micromachined ultrasonic transducer (CMUT) ring array (d = 725 μm), monolithically integrated with low noise complementary metal oxide semiconductor electronics. The thermal-mechanical noise cross-correlations between the CMUT array elements in immersion reveal both evanescent surface waves (below 10 MHz) and waves propagating primarily in the fluid (above 10 MHz). These propagating waves may allow passive imaging of scatterers closer to the array as compared to conventional pulse-echo systems, providing potentially higher resolution.

Thermal-mechanical-noise-based CMUT characterization and sensing

When capacitive micromachined ultrasonic transducers (CMUTs) are monolithically integrated with custom-designed low-noise electronics, the output noise of the system can be dominated by the CMUT thermal-mechanical noise both in air and in immersion even for devices with low capacitance. Because the thermal-mechanical noise can be related to the electrical admittance of the CMUTs, this provides an effective means of device characterization. This approach yields a novel method to test the functionality and uniformity of CMUT arrays and the integrated electronics when a direct connection to CMUT array element terminals is not available. Because these measurements can be performed in air at the wafer level, the approach is suitable for batch manufacturing and testing. We demonstrate this method on the elements of an 800-μm-diameter CMUT-on-CMOS array designed for intravascular imaging in the 10 to 20 MHz range. Noise measurements in air show the expected resonance behavior and spring softening effects. Noise measurements in immersion for the same array provide useful information on both the acoustic cross talk and radiation properties of the CMUT array elements. The good agreement between a CMUT model based on finite difference and boundary element methods and the noise measurements validates the model and indicates that the output noise is indeed dominated by thermal-mechanical noise. The measurement method can be exploited to implement CMUT-based passive sensors to measure immersion medium properties, or other parameters affecting the electro-mechanics of the CMUT structure.