Der-Song Lin

Wafer-bonded 2-D CMUT arrays incorporating through-wafer trench-isolated interconnects with a supporting frame

This paper reports on wafer-bonded, fully populated 2-D capacitive micromachined ultrasonic transducer (CMUT) arrays. To date, no successful through-wafer via fabrication technique has been demonstrated that is compatible with the wafer-bonding method of making CMUT arrays. As an alternative to through-wafer vias, trench isolation with a supporting frame is incorporated into the 2-D arrays to provide through-wafer electrical connections. The CMUT arrays are built on a silicon-on-insulator (SOI) wafer, and all electrical connections to the array elements are brought to the back side of the wafer through the highly conductive silicon substrate. Neighboring array elements are separated by trenches on both the device layer and the bulk silicon. A mesh frame structure, providing mechanical support, is embedded between silicon pillars, which electrically connect to individual elements. We successfully fabricated a 16 times 16-element 2-D CMUT array using wafer bonding with a yield of 100%. Across the array, the pulse-echo amplitude distribution is uniform (sigma = 6.6% of the mean amplitude). In one design, we measured a center frequency of 7.6 MHz, a peak-to-peak output pressure of 2.9 MPa at the transducer surface, and a 3-dB fractional bandwidth of 95%. Volumetric ultrasound imaging was demonstrated by chip-to-chip bonding one of the fabricated 2-D arrays to a custom-designed integrated circuit (IC). This study shows that through-wafer trench-isolation with a supporting frame is a viable solution for providing electrical interconnects to CMUT elements and that 2-D arrays fabricated using wafer-bonding deliver good performance.

Fabrication of Flexible Transducer Arrays With Through-Wafer Electrical Interconnects Based on Trench Refilling With PDMS

Flexible transducer arrays are desired to wrap around catheter tips for side-looking intravascular ultrasound imaging. We present a technique for constructing flexible capacitive micromachined ultrasonic transducer (CMUT) arrays by forming polymer-filled deep trenches in a silicon substrate. First, we etch deep trenches between the bottom electrodes of CMUT elements on a prime silicon wafer using deep reactive ion etching. Second, we fusion-bond a silicon-on-insulator (SOI) wafer to the prime silicon wafer. Once the silicon handle and buried oxide layers are removed from the back side of the SOI wafer, the remaining thin silicon device layer acts as a movable membrane and top electrode. Third, we fill the deep trenches with polydimethylsiloxane, and thin the wafer down from the back side. The 16 by 16 flexible 2-D arrays presented in this paper have a trench width that varies between 6 and 20 ; the trench depth is 150 ; the membrane thickness is 1.83 ; and the final substrate thickness is 150 . We demonstrate the flexibility of the substrate by wrapping it around a needle tip with a radius of 450 (less than catheter size of 3 French). Measurements in air validate the functionality of the arrays. The 250- by 250- transducer elements have a capacitance of 2.29 to 2.67 pF, and a resonant frequency of 5.0 to 4.3 MHz, for dc bias voltages ranging from 70 to 100 V.

Multi-row linear cMUT array using cMUTs and multiplexing electronics

A large area reconfigurable imaging array for research purposes is being developed with co-integrated cMUTs and control electronics. The goal is a 2.5cm 2D tileable module with ≫16,000 transducer sub-elements spaced at a pitch of 185um in X and Y dimensions. As a prototype demonstration of some of the goals of this effort, a multi-row linear array using cMUTs and external multiplexing electronics was designed and fabricated. In this paper the challenges of trenched cMUT attach to a laminate interposer as part of a tileable module will be discussed. The architecture of the tileable module build-up for manufacturability, reliability, acoustic planarity, and reduced spacing between tiles and cMUT chips will also be addressed. Finally, a first prototype will be shown and experimental acoustic results with the new cMUT-based probe will be presented.

Evaluation of wafer bonded CMUTs with rectangular membranes featuring high fill factor

Increasing fill factor is one design approach used to increase average output displacement, output pressure, and sensitivity of capacitive micromachined ultrasonic transducers (CMUTs). For rectangular cells, the cell-to-cell spacing and the aspect ratio determine the fill factor. In this paper, we explore the effects of these parameters on performance, in particular the nonuniformity of collapse voltage between neighboring cells and presence of higher order modes in air or immersed operation. We used a white light interferometer to measure nonuniformity in deflection between neighboring cells. We found that reducing the cell-to-cell spacing could cause bending of the center support post, which amplifies nonuniformities in collapse voltage to 18.4% between neighboring cells. Using a 2-D finite element model (FEM), we found that for our designs, increasing the support post width to 1.67 times the membrane thickness alleviated the post bending problem. Using impedance and interferometer measurements to observe the effects of aspect ratio on higher order modes, we found that the (1,3) modal frequency approached the (1,1) modal frequency as the aspect ratio of the rectangles increased. In air operation, under continuous wave (CW) excitation at the center frequency, the rectangular cells behaved in the (1,1) mode. In immersion, because of dispersive guided modes, these cells operated in a higher order mode when excited with a CW signal at the center frequency. This contributed to a loss of output pressure; for this reason our rectangular design was unsuitable for CW operation in immersion.

Forward-looking intracardiac ultrasound imaging using a 1-D CMUT array integrated with custom front-end electronics

Minimally invasive catheter-based electrophysiological (EP) interventions are becoming a standard procedure in diagnosis and treatment of cardiac arrhythmias. As a result of technological advances that enable small feature sizes and a high level of integration, nonfluoroscopic intracardiac echocardiography (ICE) imaging catheters are attracting increasing attention. ICE catheters improve EP procedural guidance while reducing the undesirable use of fluoroscopy, which is currently the common catheter guidance method. Phased-array ICE catheters have been in use for several years now, although only for side-looking imaging. We are developing a forwardlooking ICE catheter for improved visualization. In this effort, we fabricate a 24-element, fine-pitch 1-D array of capacitive micromachined ultrasonic transducers (CMUT), with a total footprint of 1.73 mm x 1.27 mm. We also design a custom integrated circuit (IC) composed of 24 identical blocks of transmit/ receive circuitry, measuring 2.1 mm x 2.1 mm. The transmit circuitry is capable of delivering 25-V unipolar pulses, and the receive circuitry includes a transimpedance preamplifier followed by an output buffer. The CMUT array and the custom IC are designed to be mounted at the tip of a 10-Fr catheter for high-frame-rate forward-looking intracardiac imaging. Through-wafer vias incorporated in the CMUT array provide access to individual array elements from the back side of the array. We successfully flip-chip bond a CMUT array to the custom IC with 100% yield. We coat the device with a layer of polydimethylsiloxane (PDMS) to electrically isolate the device for imaging in water and tissue. The pulse-echo in water from a total plane reflector has a center frequency of 9.2 MHz with a 96% fractional bandwidth. Finally, we demonstrate the imaging capability of the integrated device on commercial phantoms and on a beating ex vivo rabbit heart (Langendorff model) using a commercial ultrasound imaging system.

Encapsulation of Capacitive Micromachined Ultrasonic Transducers Using Viscoelastic Polymer

The packaging of a medical imaging or therapeutic ultrasound transducer should provide protective insulation while maintaining high performance. For a capacitive micromachined ultrasonic transducer (CMUT), an ideal encapsulation coating would therefore require a limited and predictable change on the static operation point and the dynamic performance, while insulating the high dc and dc actuation voltages from the environment. To fulfill these requirements, viscoelastic materials, such as polydimethylsiloxane (PDMS), were investigated for an encapsulation material. In addition, PDMS, with a glass-transition temperature below room temperature, provides a low Youngs modulus that preserves the static behavior; at higher frequencies for ultrasonic operation, this material becomes stiffer and acoustically matches to water. In this paper, we demonstrate the modeling and implementation of the viscoelastic polymer as the encapsulation material. We introduce a finite element model (FEM) that addresses viscoelasticity. This enables us to correctly calculate both the static operation point and the dynamic behavior of the CMUT. CMUTs designed for medical imaging and therapeutic ultrasound were fabricated and encapsulated. Static and dynamic measurements were used to verify the FEM and show excellent agreement. This paper will help in the design process for optimizing the static and the dynamic behavior of viscoelastic-polymer-coated CMUTs.

Characterization of cross-coupling in capacitive micromachined ultrasonic transducers

This paper analyzes element-to-element and cell- to-cell cross-coupling in capacitive micromachined ultrasonic transducers (cMUTs) using an interferometer. In a 1-D linear cMUT array immersed in oil, a single element was excited, and membrane displacements were measured at different positions along the array with an interferometer. Electrical measurements of the received voltage on each array element were also performed simultaneously to verify the optical measurements. The array was then covered with a polydimethylsiloxane (PDMS) layer, and the cross-coupling measurements were repeated. The cross-coupling levels for conventional and collapsed operation of the cMUT were compared. Since the cMUTs were immersed in oil, the optical measurements were corrected for acousto-optic interaction, and the results were reviewed in time-spatial and frequency- spatial domains. The main cross-coupling mechanism was due to the dispersive guided modes supported by the membrane periodicity. In both modes of operation, cross-coupling dispersion curves predicted a gradual reduction in phase velocity at higher frequencies. At lower frequencies, this phase velocity tended to approach 1480 m/s asymptotically. Better cross-coupling suppression was observed in the collapsed (-34 dB) than the conventional operation (-23 dB). The element-to-element cross-coupling experiments showed that a 5-µm PDMS layer reduced the measured cross- coupling levels down to -39 dB in the collapsed operation. were corrected to eliminate the acousto-optic interaction due to the refractive index of the oil and the pressure created in the oil (4). The optical time domain measurements were analyzed in the wave number-frequency (k-w) domain for the multi-mode wave propagation (5). Conventional and collapsed operations of the cMUT were compared, and the influence of a 5-µm polydimethylsiloxane (PDMS) layer covering the cMUT was investigated. The main cross-coupling mechanism was due to the dispersive guided modes. Interface waves (Stoneley-Scholte) and surface waves (Rayleigh) were relatively weak in cross-coupling (3). The dispersive guided modes were determined for conventional and collapsed operations and corresponding k-w diagrams were analyzed.

Packaging of large and low-pitch size 2D ultrasonic transducer arrays

The successful packaging and electronics integration of large 2D array devices with small pitch-sizes, such as fully populated 2D ultrasonic transducer arrays, require a flexible, simple, and reliable integration approach. One example for such electronics integration is based on through silicon vias (TSVs) with under-bump metallization (UBM) stack for solder bumping. In this paper, we demonstrate such an approach by successfully integrating a fully populated 2D ultrasonic transducer array. Our integration is based on a previously reported TSV technology (trench-frame technology), based on trench-isolated interconnects with supporting frame. We successfully combined the trench-frame technology with a simple UBM preparation technique - electro plating or chemical plating techniques with passivation layers for UBM pad definition are not required. Our results show high shear strength (26.5 g) of the UBM, which is essential for successful flip-chip bonding. The yield of the interconnections is 100% with excellent solder-ball-height uniformity (¿ = 0.9 ¿m). As demonstrated in this paper, this allows for a large-scale assembly of a tiled array by using an interposer. A design guideline for finer element-pitch design was developed suggesting that fusion bonding strength and the length of pillars are the main design parameters.

5I-6 Finite Element Analysis of Fabrication Related Thermal Effects in Capacitive Micromachined Ultrasonic Transducers

The accurate prediction of the static operation point of capacitive micromachined ultrasonic transducer (CMUT) membranes is essential for dynamic performance modeling, device design, and device fabrication. We evaluate whether fabrication-related stress effects in CMUT cells cause significant deviations observed between membrane deflection measurements and calculations. Our finite element analysis (FEA) considers the thermal structural interaction associated with the temperature stress history during device fabrication. Specific boundary conditions enable thermal expansion in all directions. Compared to conventional FEAs with one static solution step, we use seven static solution steps using the element birth and death feature of ANSYS to assemble the CMUT cell layer by layer at the appropriate deposition temperatures. We use measured static deflection profiles of metallized and non-metallized membranes with different diameters and thicknesses to extract all unknown parameters for the model. This approach improves the prediction of membrane deflections significantly, and it explains why present models often overestimate the collapse voltage

Flexible transducer arrays with through-wafer electrical interconnects based on trench refilling with PDMS

This paper reports on a method to fabricate flexible one-dimensional (1D) and two-dimensional (2D) micromachined transducer arrays that are electrically connected to flip-chip bond pads on the back side of the array. In our case, the transducers are capacitive micromachined ultrasonic transducers (CMUT) intended for medical ultrasound imaging. For ultrasound imaging, flexible arrays conform to the body part being imaged. Flexible arrays are also desired for certain catheter and fixed-focus array geometries. Electrical connection to bond pads on the back side of the array is provided for flip-chip bonding to an integrated circuit or flexible PCB. The arrays are made flexible by etching through-wafer trenches and filling the trenches with polydimethylsiloxane (PDMS). The flexibility of the substrate is demonstrated by wrapping it around a needle tip with a radius of 650 mum (French catheter size of 4).