Ching-Hsiang Cheng

Volumetric ultrasound imaging using 2-D CMUT arrays

Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128/spl times/128-element 2-D CMUT array with through-wafer via interconnects and a 420-/spl mu/m element pitch. As an experimental prototype, a 32/spl times/64-element portion of the 128/spl times/128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16/spl times/16-element portion of the array to surrounding bondpads. An 8/spl times/16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4/spl times/4 group of elements in the middle of the 8/spl times/16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.

Silicon Micromachined Ultrasonic Transducers

This paper reviews capacitor micromachined ultrasonic transducers (cMUTs). Transducers for air-borne and immersion applications are made from parallel-plate capacitors whose dimensions are controlled through traditional integrated circuit manufacturing methods. Transducers for airborne ultrasound applications have been operated in the frequency range of 0.1–11 MHz, while immersion transducers have been operated in the frequency range of 1–20 MHz. The Mason model is used to represent the cMUT and highlight the important parameters in the design of both airborne and immersion transducers. Theory is used to compare the dynamic range and the bandwidth of the cMUTs to piezoelectric transducers. It is seen that cMUTs perform at least as well if not better than piezoelectric transducers. Examples of single-element transducers, linear-array transducers, and two-dimensional arrays of transducers will be presented.

Electrical through-wafer interconnects with sub-picofarad parasitic capacitance [MEMS packaging]

This paper presents a technology for high density and low parasitic capacitance electrical through-wafer interconnects to an array of capacitive micromachined ultrasonic transducers (CMUTs) on a silicon wafer. Vertical wafer feedthroughs (interconnects) connect an array of sensors or actuators from the front side (transducer side) to the backside (packaging side) of the wafer. A 20 to 1 high aspect ratio 400 /spl mu/m long and 20 /spl mu/m diameter interconnect is achieved by using deep reactive ion etching (DRIE). Reduction of the parasitic capacitance of the polysilicon pads to the substrate can be achieved by using reversed-biased pn-junction diodes operating in the depletion region. A parasitic capacitance of 0.3 pF has been achieved by this means. This three-dimensional architecture allows for elegant packaging through simple flip-chip bonding of the chips back side to a printed circuit board (PCB) or a signal processing wafer.

Highly integrated 2-D capacitive micromachined ultrasonic transducers

Two dimensional (2-D) silicon based capacitive micromachined ultrasonic transducer (cMUT) arrays are fabricated efficiently using standard integrated circuit (IC) processing techniques. Furthermore, high density interconnects are implemented using through-chip vias which bring the signal from the front surface of the transducer chip to the back side. The transducer chip then can be flip-chip bonded to a signal processing chip. This results in a very compact two chip hybrid package that can be easily sealed for immersion applications. The electrical interconnects are realized by high aspect ratio vias (50 /spl mu/m in diameter, 550 /spl mu/m in depth) that are etched into the silicon substrate by a deep reactive ion etcher (DRIE). The vias are then coated by polysilicon which is subsequently doped to achieve good conductivity. The transducer arrays are then built on the front side using the conventional process developed for cMUTs. Measurements indicate that the through wafer vias have a typical resistance of 7 /spl Omega/. The parasitic via capacitances are reduced to 2 pF by reverse biasing the p-n junction formed by the n++ doped polysilicon via coating and the near-intrinsic p-type silicon substrate. Ultrasonic measurements on the 2-D array cMUT elements with these interconnects shows more than 100% fractional bandwidth and high sensitivity.

Comparison of conventional and collapsed region operation of capacitive micromachined ultrasonic transducers

We report experimental results from a comparative study on collapsed region and conventional region operation of capacitive micromachined ultrasonic transducers (CMUTs) fabricated with a wafer bonding technique. Using ultrasonic pulse-echo and pitch-catch measurements, we characterized single elements of 1-D CMUT arrays operating in oil. The experimental results from this study agreed with the simulation results: a CMUT operating in the collapsed region produced a higher maximum output pressure than a CMUT operated in the conventional region at 90% of its collapse voltage (3 kPa/V vs. 16.1 kPa/V at 2.3 MHz). While the pulse-echo fractional bandwidth (126%) was higher in the collapsed region operation than in the conventional operation (117%), the pulse-echo amplitude in collapsed region operation was 11 dB higher than in conventional region operation. Furthermore, within the range of tested bias voltages, the output pressure monotonously increased with increased bias during collapsed region operation. It was also found that in the conventional mode, short AC pulses (larger than the collapse voltage) could be applied without collapsing the membranes. Finally, while no significant difference was observed in reflectivity of the CMUT face between the two regions of operation, hysteretic behavior of the devices was identified in the collapsed region operation

Capacitive micromachined ultrasonic transducers with piston-shaped membranes: fabrication and experimental characterization

Capacitive micromachined ultrasonic transducers (CMUTs) featuring piston-shaped membranes (piston CMUTs) were developed to improve device performance in terms of transmission efficiency, reception sensitivity, and fractional bandwidth (FBW). A piston CMUT has a relatively flat active moving surface whose membrane motion is closer to ideal piston-type motion compared with a CMUT with uniformly thick membranes (classical CMUT). Piston CMUTs with a more uniform surface displacement profile can achieve high output pressure with a relatively small electrode separation. The improved device capacitance and gap uniformity also enhance detection sensitivity. By adding a center mass to the membrane, a large ratio of second-order resonant frequency to first-order resonant frequency was achieved. This improved the FBW. Piston CMUTs featuring membranes of different geometric shapes were designed and fabricated using wafer bonding. Fabricating piston CMUTs is a more complex process than fabricating CMUTs with uniformly thick membranes. However, no yield loss was observed. These devices achieved ~100% improvement in transduction performance (transmission and reception) over classical CMUTs. For CMUTs with square and rectangular membranes, the FBW increased from ~110% to ~150% and from ~140% to ~175%, respectively, compared with classical CMUTs. The new devices produced a maximum output pressure exceeding 1 MPa at the transducer surface. Performance optimization using geometric membrane shape configurations was the same in both piston CMUTs and classical CMUTs.

Lamb wave devices using capacitive micromachined ultrasonic transducers

Lamb wave devices based on capacitive micromachined ultrasonic transducers (CMUTs) have been built on 500-μm-thick silicon wafers for frequencies in the vicinity of 1 MHz. CMUTs have been used to both excite and detect Lamb waves in the substrate. This configuration eliminates the need for piezoelectric materials, which are not compatible with the existing integrated circuit (IC) fabrication techniques, and allows easy integration of Lamb wave devices and electronics on the same wafer. Finite element analysis of the devices shows that the lowest order antisymmetric Lamb wave (A0) is the dominant mode in the substrate in this frequency range. This result is also confirmed by demonstration experiments.

Broadband capacitive micromachined ultrasonic transducers ranging from 10 kHz to 60 MHz for imaging arrays and more

Capacitive micromachined ultrasonic transducers (CMUTs) have long been studied. Past research has shown that CMUTs indeed have remarkable features such as wide bandwidth and high efficiency. This paper introduces an inclusion to the CMUT technology that uses the wafer-bonding technique to fabricate membranes on silicon. This new technology enables the fabrication of large membranes with large gaps, and expands the frequency span of CMUTs to 10 kHz in the low end. CMUT devices with different frequency spans are fabricated using both technologies, and tested. Electromechanical coupling efficiency, k/sub T//sup 2/, value as high as 0.85 and fractional immersion bandwidth as wide as 175 % are measured.

Volumetric imaging using 2D capacitive micromachined ultrasonic transducer arrays (CMUTs): initial results

This paper presents the first volumetric images obtained using a 2D CMUT array with through-wafer via interconnects. An 8/spl times/6-element portion of a 32/spl times/64-element array flip-chip bonded onto a glass fanout chip was used in the experiments. This study experimentally demonstrates that 2D CMUT arrays can be fabricated with high yield using silicon micromachining processes, individual electrical connections can be provided using through-wafer interconnects, and the flip-chip bonding technique can be used to integrate the dense 2D arrays with electronic circuits for practical imaging applications.

Two-dimensional capacitive micromachined ultrasonic transducer (CMUT) arrays for a miniature integrated volumetric ultrasonic imaging system

We have designed, fabricated, and characterized two-dimensional 16x16-element capacitive micromachined ultrasonic transducer (CMUT) arrays. The CMUT array elements have a 250-μm pitch, and when tested in immersion, have a 5 MHz center frequency and 99% fractional bandwidth. The fabrication process is based on standard silicon micromachining techniques and therefore has the advantages of high yield, low cost, and ease of integration. The transducers have a Si3N4 membrane and are fabricated on a 400-μm thick silicon substrate. A low parasitic capacitance through-wafer via connects each CMUT element to a flip-chip bond pad on the back side of the wafer. Each through wafer via is 20 μm in diameter and 400 μm deep. The interconnects form metal-insulator-semiconductor (MIS) junctions with the surrounding high-resistivity silicon substrate to establish isolation and to reduce parasitic capacitance. Each through-wafer via has less than 0.06 pF of parasitic capacitance. We have investigated a Au-In flip-chip bonding process to connect the 2D CMUT array to a custom integrated circuit (IC) with transmit and receive electronics. To develop this process, we fabricated fanout structures on silicon, and flip-chip bonded these test dies to a flat surface coated with gold. The average series resistance per bump is about 3 Ohms, and 100% yield is obtained for a total of 30 bumps.