Fabrication technology and design for CMUTS on CMOS for IVUS catheters

The objective of this research is to develop novel capacitive micromachined ultrasonic transducer (CMUT) arrays for intravascular ultrasonic (IVUS) imaging along with the fabrication processes to allow for monolithic integration of CMUTs with custom CMOS electronics for improved performance. The IVUS imaging arrays include dual-ring arrays for forward-looking volumetric imaging in coronary arteries and annular-ring arrays with dynamic focusing capabilities for side-looking cross sectional imaging applications. Both are capable of integration into an IVUS catheter 1-2 mm in diameter. The research aim of monolithic integration of CMUTs with custom CMOS electronics has been realized mainly through the use of sloped sidewall vias less than 5 µm in diameter, with only one additional masking layer as compared to regular CMUT fabrication. Fabrication of CMUTs has been accomplished with a copper sacrificial layer reducing isolation layers by 50%. Modeling techniques for computational efficient analysis of CMUT arrays were developed for arbitrary geometries and further expanded for use with larger signal analysis. Dual-ring CMUT arrays for forward-looking volumetric imaging have been fabricated with diameters of less than 2 mm with center frequencies at 10 MHz and 20 MHz, respectively, for an imaging range from 1 mm to 1 cm. These arrays, successfully integrated with custom CMOS electronics, have generated 3D volumetric images with only 13 cables necessary. Performance from optimized fabrication has reduced the bias required for a dual-ring array element from 80 V to 42 V and in conjunction with a full electrode transmit array, it was shown that the SNR can be improved by 14 dB. Simulations were shown to be in agreement with experimental characterization indicated transmit surface pressure in excess of 8 MPa. For side-looking IVUS, three versions of annular CMUT arrays with dynamic focusing capabilities have been fabricated for imaging 1 mm to 6 mm in tissue. These arrays are 840 µm in diameter membranes linked to form 8 ring elements with areas that deviate by less than 25 %. Through modeling and simulation undesirable acoustic cross between ring elements was reduced from -13 dB to -22 dB.Ph.D

Dual electrode capacitive micromachined ultrasonic transducer array for 1-D intracardiac echocardiography (ICE)

We designed and fabricated a 64 element 1-D linear dual electrode Capacitive Micromachined Ultrasonic Transducer (CMUT) array operating at 9.5 MHz for Intracardiac Echocardiography (ICE). The dual electrode CMUT structure increases the overall sensitivity by 12.6dB (6.2dB in receive sensitivity; 6.4dB in output pressure) when compared to optimized single electrode CMUT. We report peak output pressure of 2.3MPa on the CMUT surface when 170V AC and 180V DC is applied. This significant performance increase makes the CMUT more competitive with their piezoelectric counterparts.Publisher's Versio

Multiple annular ring capacitive micromachined ultrasonic transducer arrays for forward-looking intravascular ultrasound imaging catheters

We investigate multiple-annular-ring CMUT array configuration for forward-looking intravascular ultrasound (FL-IVUS) imaging. This configuration has the potential for independent optimization of each ring and uses the silicon area more effectively without any particular drawback. We designed and fabricated a sample 1mm diameter dual annular ring CMUT test array which consists of 24 transmit and 32 receive elements. For imaging experiments, we designed IC chips that contain 8 transimpedance amplifiers, a multiplexer and a buffer. The real time-pulse echo experiments obtained with designed IC electronics show 26dB Signal to Noise Ratio (SNR) from a 3.5 mm away aluminum reflector in oil. This paper presents our first efforts in obtaining real time imaging with designed IC chips which is one step before CMUT on CMOS implementation.Publisher's Versio

An Analog beamformer for integrated high-frequency medical ultrasound imaging

We designed and fabricated a dynamic receive beamforming integrated circuit (IC) in 0.35-mu m CMOS technology. This beamformer is suitable for integration with an ultrasound annular array for high-frequency (30-50 MHz) intravascular ultrasound (IVUS) imaging. The beamformer IC is capable of buffering, delaying and preamplification for 8 receive channels. We explored an analog delay cell based on a currentmode first-order all-pass filter, which is used as the basic building block to form an analog dynamic delay line. We also explored a bandwidth enhancement method on the delay cell that 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 delay between 1.75 ns to 2.5 ns, enabling dynamic receive beamforming over a focal range from 1.4 mm to 2 mm. We successfully integrated the fabricated beamformer IC with an 8-element annular array. Our experimental test results demonstrated the desired buffering, preamplification and delaying capabilities of the beamformer.Publisher's Versio

An annular CMUT array beamforming system for high-frequency side looking IVUS imaging

A CMUT annular array system for Side-Looking Intravascular Ultrasound (SL-IVUS) with fixed transmit and dynamic receive focusing capabilities has been developed. The system was experimentally characterized and validated through analytical models that simulate the beamformed transducer behavior. An 840 m diameter, 35MHz array was fabricated, characterized, and used in experiments. The array consists of curved 18m by 60m CMUT membranes that form 8 ring transducer elements with approximately equal areas. The beamforming system uses an IC chip consisting of 8 transimpedance amplifiers and delay elements for receive beamforming with adjustable delays between 2ns and 4ns, that are constant up to 50 MHz with close to unity gain. Transmit focusing is implemented with an FPGA controlled, high voltage pulser board that can generate adjustable electrical pulses with delays as small as 2ns. The system is characterized by measuring the radiation patterns of individual CMUT annular array elements as well as the unfocused and fixed transmit focused arrays. The results show predicted behavior including acoustic crosstalk effects at certain frequencies. For transmit-receive beamforming characterization, a 25m gold wire was imaged using 4 beamformed transmit elements and 4 beamformed receive elements with different delay values. The results show improved lateral resolution and lower side lobes with proper beamforming.Publisher's Versio

Monolithic CMUT-on-CMOS integration for intravascular ultrasound applications

PubMed ID: 23443701work was supported by the National Institutes of Health under the grants HL082811 and EB010070One 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. Experiments 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.Publisher's Versio

Dual-annular-ring CMUT array for forward-looking IVUS imaging

We investigate a dual-annular-ring CMUT array configuration for forward-looking intravascular ultrasound (FL-IVUS) imaging. The array consists of separate, concentric transmit and 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 particular drawback. We designed and fabricated a 1mm diameter test array which consists of 24 transmit and 32 receive elements. We investigated synthetic phased array beamforming with a non-redundant subset (if transmit-receive element pairs of the dual-annular-ring array. For imaging experiments, we designed and constructed a programmable FPGA-based data acquisition and phased array beamforming system. Pulse-echo measurements along with imaging simulations suggest that dual-ring-annular array should provide performance suitable for real-time FLAVUS applications.This research is funded by NIH-NHLBI (Grant No: 1 R01 HL082811-01) and Boston Scientific Corp. Support of Isik University through the internal grant BAP-05B301 is also acknowledged.Publisher's Versio

Evaluation of CMUT annular arrays for side-looking IVUS

Side-looking (SL) IVUS probes are extensively used for management of cardiovascular diseases. Currently SL-IVUS imaging probes use either a single rotating transducer element or solid-state arrays. Probes with single rotating piezoelectric transducer have simple front-end, but have fixed focused operation, and suffers from motion artifacts. Solid-state SL-IVUS imaging probes use piezoelectric transducer arrays and electronic beam-forming. Synthetic phased array processing of signals detected with small-sized elements in these arrays limits the SNR achievable with these probes. In this study, we explore a new SL-IVUS probe architecture employing rotating phased annular CMUT arrays. We tested and compared imaging performance of the existing and proposed probe configurations through simulated point spread functions. We also two fabricated sample annular array designs operating at 20-MHz and 50-MHz. Our experimental measurements on the 20-MHz array in oil shows 105% fractional bandwidth. The 50-MHz array with parylene coating shows approximately 40% fractional bandwidth measured in water. We also present imaging results acquired from wire-targets to test the experimental point-spread functions.IEEE Ultrasonics, Ferroelectrics, and Frequency Control SocietyPublisher's Versio