Xiaoyang Cheng

A Capacitive Micromachined Ultrasonic Transducer Array for Minimally Invasive Medical Diagnosis

A capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive medical diagnosis has been developed. Unlike traditional ultrasonic transducers, which generally use a bulky piece of substrate, this transducer array was integrated on a 40--thick micromachined silicon substrate into a probe shape with a typical shank width of 50-80 and a shank length of 4-8 mm. For 1-D arrays, 24-96 CMUT devices were integrated on one such silicon probe and formed an accurately configured phase array. In addition to miniaturization, reduction of the substrate thickness also decreases the intertransducer crosstalk due to substrate Lamb waves. Due to its miniature size, this array can be placed or implanted close to the target tissue/organ and can perform high-resolution high-precision diagnosis and stimulation using high-frequency ultrasounds. The issue of conflict between resolution and penetration depth of ultrasonic diagnosis can therefore be resolved. A two-layer polysilicon surface micromachining process was used to fabricate this device. Suspended polysilicon membranes of diameters ranging from 20 to 90 and thicknesses from 1.0 to 2.5 were used to generate and detect ultrasounds of frequencies ranging from 1 to 10 MHz. B-mode imaging using this transducer array has been demonstrated.

A Monolithic Three-Dimensional Ultrasonic Transducer Array for Medical Imaging

This paper presents the first monolithic multidirection-looking ultrasonic imager for minimally invasive medical diagnosis. In contrast to the traditional planar ultrasonic imagers that can only view in one direction, this 3-D array is able to view in multiple directions by using seven planar imagers integrated on a hexagonal silicon prism. Each facet on the prism is integrated with a planar 1- or 2-D capacitive-micromachined ultrasonic-transducer imager array for viewing in a specific direction. Each facet is connected by a flexible dielectric membrane, which is monolithically fabricated with the transducers. The dielectric membranes also support the thin-film electrical interconnects between the arrays on different facets. The substrate is folded into a hexagonal prism after completion of the transducer microfabrication process. With this architecture, a one flip-chip bonded or monolithically integrated front-end electronic circuit will be able to manage all the imagers on the 3-D array. The number of bonding wires for a connection to external electronics can therefore be reduced. Imager prisms, which are ranging from 1 to 4 mm in diameter and 2 to 4 mm in length, and positioned to view in seven directions, have been prototyped. Preliminary testing shows that the imager transducers behaved consistently before and after the assembly process. Applications of this 3-D imager array include capsule ultrasound endoscope, intravascular ultrasound, and other internal imaging needs.

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 -m and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.

A Miniature Capacitive Micromachined Ultrasonic Transducer Array for Minimally Invasive Photoacoustic Imaging

This paper describes the development of a capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive photoacoustic imaging (PAI). Integrated on a miniature silicon bar that is approximately 100 μm thick and 2.8-5 mm × 8-18 mm in area, this CMUT array can be implanted into a tissue or placed inside an organ without causing major tissue disruption. Close proximity to the target tissue allows this CMUT array to pick up a relatively weak ultrasound signal generated in a photoacoustic process and to provide diagnostic information inaccessible from a noninvasive transducer. For invasive PAI, silicon-based CMUTs offer an additional significant advantage: The silicon and the dielectric membrane of CMUTs are relatively transparent to near infrared, and the shadowing problem associated with the piezoelectric ultrasonic transducers can be minimized or avoided. A two-layer polysilicon surface micromachining process was used to fabricate this device, followed by a double-sided deep-silicon-etching process for shaping the silicon substrate into a thin probe. Experimental characterization found that the center frequency of the CMUT devices with a 46-μm-diameter 1.0-μm-thick polysilicon membrane was 5.0 MHz, with a fractional bandwidth of 116% in reception mode. The PAI of the nerve cord of a lobster using this miniature CMUT array was demonstrated.

Design and test of a monolithic ultrasound-image-guided HIFU device using annular CMUT rings

This paper describes the design, fabrication, and characterization of a CMUT-based therapeutic ultrasound chip with built-in ultrasound imager for real-time monitoring of the object being operated on. Multiple concentric high-power (inner) CMUT rings and an annular imager CMUT array (outmost) comprising of 48 or 64 elements are integrated on a silicon substrate measuring 2 mm times 2 mm for simultaneous ultrasonic ablation/stimulation and imaging. The polysilicon membrane thickness and gap height of the high-power CMUT devices are 1.3 mum and 0.35 mum, respectively, while these of the imager CMUT integrated on the same substrate are 1.0 mum and 0.18 mum, respectively. The thicker membrane and higher gap of the high-power CMUTs make them capable of delivering high-pressure ultrasound, while the thin membrane and lower gap of imager CMUTs improves the receiver sensitivity in pulse-echo imaging. In order to maximize the overall membrane displacement of the high-power CMUT ring in transmission mode, the ring is designed as a one-chamber swimming-ring structure instead of being divided into multiple sub-chambers, like imager arrays. The high-power CMUT rings were successfully used for heating the liver tissue of a pig, creating a 2.5degC temperature increase after 6 minutes of ultrasound irradiation. Preliminary B-mode imaging using the CMUT imager element on this imager-guided therapeutic chip was also demonstrated.

A Miniature Capacitive Ultrasonic Imager Array

This paper describes the development of a miniature capacitive micromachined ultrasonic transducer (CMUT) array suitable for minimally invasive medical imaging and diagnosis. In contrast to conventional laboratory-scale CMUT platforms, which are generally integrated on a silicon substrate thicker than 550 mum, this imager array is integrated on a probe shaped silicon substrate with a typical shank dimension of 60 mum(width) times 40 mum(thickness) times 4-10 mm(length) for 1-D arrays, and 0.4-2.3 mm(width) times 100 mum(thickness) times 6-12 mm(length) for 2-D arrays. Such miniature CMUT arrays are suitable for implantation into tissue through a fine incision or by being placed inside an organ for close-range imaging. In a close-range diagnosis made possible by using such miniature CMUT arrays, ultrasound of a higher frequency can be used and the conflict associated with the penetration depth and image resolution can be resolved. This imager array was fabricated using a two-layer polysilicon surface micromachining process followed by a double-sided deep silicon etching for substrate shaping. The total mask count was eight. The central frequency of ultrasound transmitted by a circular 46 mum-diameter transducer was 3.8 MHz, while its fractional bandwidth was 116% in water. A simple transducer-fluid model was used to predict the acoustic characteristics of this device in water. Preliminary B-mode imaging using a 21-element 1-D array was demonstrated.

Fabrication and characterization of surface micromachined CMUT with a bossed membrane

This paper describes the fabrication and characterization of surface-micromachined capacitive ultrasonic transducers with a bossed membrane. The boss was formed using 3mum-thick deposited tetraethoxysilane (TEOS) oxide on top of a suspended polysilicon membrane. This same oxide layer was also used to seal the release holes along the peripheral of the polysilicon membrane. No extra mask or processing step in addition to that used for fabricating planar-membrane capacitive micromachined ultrasonic transducer (CMUT) is needed for the addition of a boss on the polysilicon membrane. It was found from device characterization that a bossed device shifted the center frequency to a higher value, improved the fractional bandwidth in transmission mode, increased the receiver sensitivity, and augmented the electromechanical coupling efficiency compared to their planar-membrane counterpart with the same membrane diameter.

P6B-7 The Initial Doppler Blood Flow Measurement Using an Implantable CMUT Array

This paper describes the initial Doppler blood flow velocity measurement using an implantable capacitive micromachined ultrasonic transducer (CMUT) array. This CMUT array was integrated on a probe-shaped silicon substrate of a typical shank cross-section of 40 mum (thickness) times 60 mum (width) and a length of 4-10 millimeters. Smaller in diameter than a humans hair, this CMUT probe can be implanted inside the tissue with a minimal tissue disruption and is potentially useful for monitoring blood flow deep inside the tissue. Pulsed-wave approach was used in this blood flow velocity measurement experiment using ultrasound with a central frequency of 2.2 MHz. The time domain method was used for data analysis in order to achieve a higher blood flow rate resolution using a broadband ultrasound signal. The pulse Doppler changes on successive echoes scattered from the moving red blood cells were counted as a progressive time shift. Using correlation procedure, the time difference or distance the red blood cells travel between the two transmitted pulses can be determined. From the correlation curve as a function of period numbers and the time lag between two successive echoes, the blood flow velocity was derived with a resolution of 1.5 mm/s.

6F-3 Fabrication and Assembly of a Monolithic 3D CMUT Array for Imaging Applications

This paper describes a novel architecture for integration of polysilicon capacitive micromachined ultrasonic imager array on a 3D hexagonal silicon platform for simultaneous multi-directional viewing. Integrated with seven CMUT arrays oriented at seven different directions, this three-dimensional hexagonal imager prism is capable of viewing the front direction and 360deg side directions concurrently. The seven CMUT-array plates were monolithically fabricated on one silicon substrate using a surface micromachining process. The silicon substrate under the connection areas between the imager plates was then completely etched away using a selective backside deep silicon etching, leaving behind flexible dielectric films for inter-plate connection. In addition to providing mechanical linkage, these dielectric connecting films also support the thin-film electrical interconnects between the imagers on different plates. The seven CMUT arrays were then folded into a miniature prism and glued together using epoxy. Hexagonal imager prisms with width across corner of the hexagon, ranging from 500 mum to 4 millimeters in width, and 2 to 4 millimeters in length have been prototyped and tested. The CMUT devices behaved consistently before and after the assembly process. This architecture allows one monolithically integrated or flip-chip bonded beam-forming circuit to control all the imagers on this miniature prism and the number of bonding wires can be minimized. The multi- direction viewing capability and miniature size make this device capable of delivering more comprehensive information for internal medical imaging applications.

5H-4 An Implantable Ultrasonic Doppler Blood Flowmeter

A miniature ultrasonic transducer array for minimally invasive blood flow measurement has been developed. Compare with laser Doppler flowmeter, this tool is much smaller in size and is capable of reaching deep inside the tissue to monitor blood flow not accessible by the laser beam. Compared with traditional ultrasound Doppler flowmeter, this miniature tool is able to achieve more accurate measurement by using high frequency ultrasounds. The silicon substrate of the transducers array was micromachined into a probe shape using double-sided dry etching. The typical cross-sectional dimension of the silicon probe is 40 mum times 80 mum. Arrays comprising 24-32 transducers were prototyped and tested. The 46 mum-diameter drum shaped ultrasound transducers operate at a voltage lower than 40 volts, generate and receive ultrasound at a central frequency of 5.8 MHz, and has a bandwidth of about 80%. A new poly silicon surface micromachining process was developed for fabrication of the transducer. The new process separates all the conductors from the silicon nitride film and reduces the charging problem associated with nitride-conductor interface