Chienliu Chang

An integrated Ring CMUT array for endoscopic ultrasound and photoacoustic imaging

This work presents our preliminary results on developing an integrated quad-ring CMUT array for endoscopic ultrasound and photoacoustic imaging. We have designed and fabricated a ring capacitive micromachined ultrasonic transducer (CMUT) array composed of 512 elements distributed among four concentric rings each having 128 elements. The operational frequency of each ring was chosen to achieve a similar pressure beam profile for all the rings. The devices inner and outer diameters measure 5.0 and 10.1 mm, respectively. The CMUT array was integrated with custom front-end ICs using a quartz fan-out board. This bench-top assembly allowed connection to a single ring (i.e., 128 elements) at a time. Thus far, we have built assemblies with connections to the two outer rings. We have successfully demonstrated real-time volumetric imaging with these assemblies using nylon wire phantom and metal spring phantom.

Dual-mode integrated circuit for imaging and HIFU with 2-D CMUT arrays

Successful high intensity focused ultrasound (HIFU) operation requires a reliable guidance and monitoring method such as magnetic resonance imaging (MRI) or ultrasound imaging. However, both widely used modalities are typically separate from the HIFU system, which makes co-registration of HIFU with cross-sectional imaging difficult. In this paper, we present a dual-mode integrated circuit (IC) that can perform both ultrasound imaging and HIFU with a single 2D capacitive micromachined ultrasonic transducer (CMUT) array, combining these two systems for ease of use. The dual-mode IC consists of pulsers, transmit beamforming circuitry, and low-noise amplifiers for imaging mode and switches for HIFU mode. By turning this switching network on and off, the system can alternately operate the imaging mode and HIFU mode on demand. The dual-mode IC was designed and fabricated in the 0.18-μm HV 4LM process provided by Maxim Inc. We fabricated a 32×32-element CMUT array that has a center frequency of 5 MHz using a sacrificial release process and flip-chip bonded this CMUT array to the IC. With the back-end system, real-time volumetric imaging on the wire phantom and HIFU ablation on ex-vivo tissue were performed respectively.

Acoustic lens for capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with traditional piezoelectric transducers in therapeutic ultrasound applications. In this paper we have designed, fabricated and developed an acoustic lens formed on the CMUT to mechanically focus ultrasound. The acoustic lens was designed based on the paraxial theory and made of silicone rubber for acoustic impedance matching and encapsulation. The CMUT was fabricated based on the local oxidation of silicon (LOCOS) and fusion-bonding. The fabricated CMUT was verified to behave like an electromechanical resonator in air and exhibited wideband response with a center frequency of 2.2 MHz in immersion. The fabrication for the acoustic lens contained two consecutive mold castings and directly formed on the surface of the CMUT. Applied with ac burst input voltages at the center frequency, the CMUT with the acoustic lens generated an output pressure of 1.89 MPa (peak-to-peak) at the focal point with an effective focal gain of 3.43 in immersion. Compared to the same CMUT without a lens, the CMUT with the acoustic lens demonstrated the ability to successfully focus ultrasound and provided a viable solution to the miniaturization of the multi-modality forward-looking endoscopes without electrical focusing.

Ex Vivo HIFU Experiments Using a

High-intensity focused ultrasound (HIFU) has been used as noninvasive treatment for various diseases. For these therapeutic applications, capacitive micromachined ultrasonic transducers (CMUTs) have advantages that make them potentially preferred transducers over traditional piezoelectric transducers. In this paper, we present the design and the fabrication process of an 8×8-mm2, 32×32-element 2-D CMUT array for HIFU applications. To reduce the system complexity for addressing the 1024 transducer elements, we propose to group the CMUT array elements into eight HIFU channels based on the phase delay from the CMUT element to the targeted focal point. Designed to focus at an 8-mm depth with a 5-MHz exciting frequency, this grouping scheme was realized using a custom application-specific integrated circuit (ASIC). With a 40-V DC bias and a 60-V peak-to-peak AC excitation, the surface pressure was measured 1.2 MPa peak-to-peak and stayed stable for a long enough time to create a lesion. With this DC and AC voltage combination, the measured peak-to-peak output pressure at the focus was 8.5 MPa, which is expected to generate a lesion in a minute according to the temperature simulation. Following ex-vivo tissue experiments successfully demonstrated its capability to make lesions in both bovine muscle and liver tissue.

32 \times 32

We are developing a capsule ultrasound (CUS) device to serve as a wireless, portable, and ultrasonic pill for investigating the multiple layers of the complete gastrointestinal (GI) tract, in particular, the small intestine. This capsule will acquire ultrasound images with 360 degrees field-of-view (FOV) and a penetration depth of 5 cm using a 128-element and cylindrically-shaped capacitive micromachined ultrasonic transducer (CMUT) array, wrapped around the center of its body. Simulation results indicate that linear array imaging with a fixed focus of F#4 and 16 active elements produces valuable images. We have designed a CMUT for this application and the fabrication process to create cylindrical CMUT arrays has been established. We report our fabrication progress and show test devices that we successfully made and bent around a glass tube. In addition, the design of the application-specific integrated circuit (ASIC) and the wireless transmitter, responsible for the acquisition and wireless transmission of ultrasonic data respectively, is described.

-Element CMUT Array

Singulation of MEMS is a critical step in the transition from wafer-level to die-level devices. As is the case for capacitive micromachined ultrasound transducer (CMUT) ring arrays, an ideal singulation must protect the fragile membranes from the processing environment while maintaining a ring array geometry. The singulation process presented in this paper involves bonding a trench-patterned CMUT wafer onto a support wafer, deep reactive ion etching (DRIE) of the trenches, separating the CMUT wafer from the support wafer and de-tethering the CMUT device from the CMUT wafer. The CMUT arrays fabricated and singulated in this process were ring-shaped arrays, with inner and outer diameters of 5 mm and 10 mm, respectively. The fabricated CMUT ring arrays demonstrate the ability of this method to successfully and safely singulate the ring arrays and is applicable to any arbitrary 2D shaped MEMS device with uspended microstructures, taking advantage of the inherent planar attributes of DRIE.

Capsule ultrasound device

Real-time 3D volumetric ultrasound imaging systems require transmit and receive circuitry to generate the ultrasound beam and process the received echo signals. Since a 2D array is required for 3D imaging, the complexity of building such a system is significantly higher, e.g., front-end electronics need to be interfaced to the transducer, a large number of elements need to be interfaced to the backend system and a large dataset needs to be processed. In this work, we present a 3D imaging system using capacitive micromachined ultrasonic transducer (CMUT) technology that addresses many of the challenges in building such a system. The transducer is a 5-MHz CMUT array with an 8 mm × 8 mm aperture size. The aperture consists of 1024 elements (32×32) with an element pitch of 250 μm. An integrated circuit (IC) is integrated very close to the CMUT array. It consists of a transmit beamformer and receive circuitry to improve the noise performance of the overall system. Simultaneous multi-beam transmit is also incorporated in the IC to improve the imaging frame rate. The CMUT is flip-chip bonded to the IC and the final assembly measured 9.2 mm × 9.2 mm. The assembly was then interfaced with an FPGA and a backend system (comprising of a data acquisition system and PC). The FPGA provided the digital I/O signals for the IC and the backend system was used to process the received RF echo data (from the IC) and reconstruct the volume image using a phased array imaging approach. Imaging experiments were performed using wire phantoms. Real-time volumetric images were captured at 5 volumes per second and are presented in this paper.

Singulation for imaging ring arrays of capacitive micromachined ultrasonic transducers

Ultrasound is increasingly in demand as a medical imaging tool and can be particularly beneficial in the field of intracardiac echocardiography (ICE). However, many challenges remain in the development of a 3D ultrasound imaging system.We have designed and fabricated a quad-ring capacitive micromachined ultrasound transducer (CMUT) for real-time, volumetric medical imaging. Each CMUT array is composed of four concentric, independent ring arrays, each operating at a different frequency, with 128 elements per ring. In this project, one ring will be used for imaging. A large (5mm diameter) lumen is available for delivering other devices, including high intensity focused ultrasound transducers for therapeutic applications or optical fibers for photoacoustic imaging.We address several challenges in developing a 3D imaging system. Through wafer vias are incorporated in the fabrication process for producing 2D CMUT arrays. Device integration with electronics is achieved through solder bumping the arrays, designing a flexible PCB, and flip chip bonding CMUT and ASICs to the flexible substrate. Finally, we describe a method for integrating the flex assembly into a catheter shaft. The package, once assembled, will be used for in-vivo open chest experiments.Copyright

A 32×32 integrated CMUT array for volumetric ultrasound imaging

We are developing a capsule ultrasound (CUS) device - a pill with the capability to scan the gastrointestinal (GI) tract through ultrasound. In this paper, we discuss the design and fabrication of the main components of the CUS device including the CMUT array, front-end electronics, and the wireless transmitter. We demonstrate a successfully fabricated 128-element CMUT array with polydimethylsiloxane (PDMS)-filled trenches and show their input impedance in air. The front-end electronics, measuring 6 mm by 6 mm and the high-data rate wireless transmitter, measuring 1 mm by 1.76 mm, have been fabricated. Our preliminary power analysis indicates that our total power consumption is less than 20 mW for the CUS device. Our future work involves integrating these core components for imaging experiments.

Fabrication, Packaging, and Catheter Assembly of 2D CMUT Arrays for Endoscopic Ultrasound and Cardiac Imaging

High Intensity Focused Ultrasound (HIFU) therapy requires imaging guidance to ensure that the correct area is treated. Guidance may be provided by magnetic resonance imaging or ultrasound imaging. With piezoelectric transducers, ultrasound imaging guidance is typically done with a separate transducer array since it is difficult to obtain sufficient imaging and HIFU performance with the same transducers. However, Capacitive Micromachined Ultrasonic Transducers (CMUTs) can have sufficient bandwidth and efficiency to allow both HIFU therapy and imaging to be done with the same transducer array. Additionally, CMUTs have much lower self-heating than piezoelectric transducers and require less cooling when used for HIFU. This means that CMUT-based HIFU systems can be made smaller than other HIFU systems to date, allowing access to treatment areas that were not previously possible. In addition, using the same array for imaging and treatment eliminates challenges with co-registration. We have constructed a dual-mode HIFU and imaging probe using a 32x32 CMUT array integrated with an application-specific integrated circuit. Ultrasound imaging guided ablation of ex-vivo tissue has been demonstrated with the probe, with rapid switching between HIFU and imaging mode.