Capacitive Micromachined Ultrasonic Transducer Technology Research Articles

Design and Fabrication of Wideband Air-Coupled Capacitive Micromachined Ultrasonic Transducers With Varying Width Annular-Ring and Spiral Cell Structures.

Air-coupled transducers with broad bandwidth are desired for many airborne applications, such as obstacle detection, haptic feedback, and flow metering. In this article, we present a design strategy and demonstrate a fabrication process for developing improved concentric annular- and novel spiral-shaped capacitive micromachined ultrasonic transducers (CMUTs) that can generate high output pressure and provide wide bandwidth in air. We explore the ability to implement complex geometries by photolithographic definition to improve the bandwidth of air-coupled CMUTs. The ring widths in the annular design were varied so that the device can be improved in terms of bandwidth when these rings resonate in parallel. Using the same ring width parameters for the spiral-shaped design but with a smoother transition between the ring widths along the spiral, the bandwidth of the spiral-shaped device is improved. With the reduced process complexity associated with the anodic-bonding-based fabrication process, a 25- [Formula: see text] vibrating silicon plate was bonded to a borosilicate glass wafer with up to 15- [Formula: see text] deep cavities. The fabricated devices show an atmospheric deflection profile that is in agreement with the FEM results to verify the vacuum sealing of the devices. The devices show a 3-dB fractional bandwidth (FBW) of 12% and 15% for spiral- and annular-shaped CMUTs, respectively. We measured a 127-dB sound pressure level at the surface of the transducers. The angular response of the fabricated CMUTs was also characterized. The results demonstrated in this article show the possibility of improving the bandwidth of air-coupled devices by exploring the flexibility in the design process associated with CMUT technology.

Performance simulation and testing of capacitive micro-machined ultrasonic transducer

PurposeThis study aims to simulate and test the performance of a transmitting and receiving capacitive micro-machined ultrasonic transducer (CMUT). Aimed at detecting demand of the CMUT, a matched integrated adjustment circuit was designed through analyzing processing methods of transducer’s weak echo signal.Design/methodology/approachBased on the analysis of CMUT array structure and work principle, the CMUT units are designed and the dynamic performance analysis of SIMULINK is given according to the demand of underwater detecting. A transceiver isolation circuit is used to make transmission mode and receiving mode separate. A detection circuit is designed based on the transimpedance amplifier to achieve extraction of high-frequency and weak signal.FindingsThrough experimentation, the effectiveness of the CMUT performance simulation and the transceiver integrated adjustment circuit were verified. In addition, the test showed that CMUT with 400 kHz frequency has wider bandwidth and better dynamic characteristics than other similar transducers.Originality/valueThis paper provides a theoretical basis and design reference for the development and application of CMUT technology.

Advances in Capacitive Micromachined Ultrasonic Transducers.

Capacitive micromachined ultrasonic transducer (CMUT) technology has enjoyed rapid development in the last decade. Advancements both in fabrication and integration, coupled with improved modelling, has enabled CMUTs to make their way into mainstream ultrasound imaging systems and find commercial success. In this review paper, we touch upon recent advancements in CMUT technology at all levels of abstraction; modeling, fabrication, integration, and applications. Regarding applications, we discuss future trends for CMUTs and their impact within the broad field of biomedical imaging.

An Experimental Protocol for Assessing the Performance of New Ultrasound Probes Based on CMUT Technology in Application to Brain Imaging.

The possibility to perform an early and repeatable assessment of imaging performance is fundamental in the design and development process of new ultrasound (US) probes. Particularly, a more realistic analysis with application-specific imaging targets can be extremely valuable to assess the expected performance of US probes in their potential clinical field of application. The experimental protocol presented in this work was purposely designed to provide an application-specific assessment procedure for newly-developed US probe prototypes based on Capacitive Micromachined Ultrasonic Transducer (CMUT) technology in relation to brain imaging. The protocol combines the use of a bovine brain fixed in formalin as the imaging target, which ensures both realism and repeatability of the described procedures, and of neuronavigation techniques borrowed from neurosurgery. The US probe is in fact connected to a motion tracking system which acquires position data and enables the superposition of US images to reference Magnetic Resonance (MR) images of the brain. This provides a means for human experts to perform a visual qualitative assessment of the US probe imaging performance and to compare acquisitions made with different probes. Moreover, the protocol relies on the use of a complete and open research and development system for US image acquisition, i.e. the Ultrasound Advanced Open Platform (ULA-OP) scanner. The manuscript describes in detail the instruments and procedures involved in the protocol, in particular forthe calibration, image acquisition and registration of US and MR images. The obtained results prove the effectiveness of the overall protocol presented, which is entirely open (within the limits of the instrumentation involved), repeatable, and covers the entire set of acquisition and processing activities for US images.

Quantitative comparison of PZT and CMUT probes for photoacoustic imaging: Experimental validation

Photoacoustic (PA) signals are short ultrasound (US) pulses typically characterized by a single-cycle shape, often referred to as N-shape. The spectral content of such wideband signals ranges from a few hundred kilohertz to several tens of megahertz. Typical reception frequency responses of classical piezoelectric US imaging transducers, based on PZT technology, are not sufficiently broadband to fully preserve the entire information contained in PA signals, which are then filtered, thus limiting PA imaging performance. Capacitive micromachined ultrasonic transducers (CMUT) are rapidly emerging as a valid alternative to conventional PZT transducers in several medical ultrasound imaging applications. As compared to PZT transducers, CMUTs exhibit both higher sensitivity and significantly broader frequency response in reception, making their use attractive in PA imaging applications. This paper explores the advantages of the CMUT larger bandwidth in PA imaging by carrying out an experimental comparative study using various CMUT and PZT probes from different research laboratories and manufacturers. PA acquisitions are performed on a suture wire and on several home-made bimodal phantoms with both PZT and CMUT probes. Three criteria, based on the evaluation of pure receive impulse response, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) respectively, have been used for a quantitative comparison of imaging results. The measured fractional bandwidths of the CMUT arrays are larger compared to PZT probes. Moreover, both SNR and CNR are enhanced by at least 6dB with CMUT technology. This work highlights the potential of CMUT technology for PA imaging through qualitative and quantitative parameters.

Glass-SOI-Based Hybrid-Bonded Capacitive Micromachined Ultrasonic Transducer With Hermetic Cavities for Immersion Applications

Capacitive micromachined ultrasonic transducer (CMUT) can be widely used in ultrasonic devices. After a discussion of the advantages and challenges for the existing CMUT technology especially for immersion applications, this paper presents a glass-silicon-on-insulator (SOI)-based and hybrid-bonded CMUT with hermetic cavities for the first time. It is understood that while anodic bonding can provide good mechanical strength, it is poor in providing sealing. Hence, Au–Au thermal compression bonding is needed. With the hybrid-bonded vacuum-sealing-line, the bond pad can be opened without damaging the vacuum level of the hermetic cavities, which effectively addresses the difficulty in cavity vacuum sealing. Due to structure design and the insulation characteristic of the glass wafer, the CMUT also has highlight in the aspect of no additional stray parasitic capacitances. Furthermore, the low-temperature compatibility of this hybrid-bonded CMUT improves the performance uniformity for both cells and dies. Capacitance–voltage test of device certificates the feasibility of electrical connections and confirms the low parasitic capacitance of device. Topography of the prototype is measured by using white light interferometer to verify the hermetic performance. Frequency response characterization, including dynamic vibration mode, resonance frequency, vibrating deflection and velocity, are carried out with a laser Doppler vibrometer. The immersion test is carried out and demonstrates the transmitting and receiving of ultrasound from fabricated devices. This glass-SOI-based and hybrid-bonded hermetic CMUT shows a great potential for immersion applications. [2015-0280]

PECVD low stress silicon nitride analysis and optimization for the fabrication of CMUT devices

Two technological options to achieve a high deposition rate, low stress plasma-enhanced chemical vapor deposition (PECVD) silicon nitride to be used in capacitive micromachined ultrasonic transducers (CMUT) fabrication are investigated and presented. Both options are developed and implemented on standard production line PECVD equipment in the framework of a CMUT technology transfer from R & D to production. A tradeoff between deposition rate, residual stress and electrical properties is showed.The first option consists in a double layer of silicon nitride with a relatively high deposition rate of ~100 nm min−1 and low compressive residual stress, which is suitable for the fabrication of the thick nitride layer used as a mechanical support of the CMUTs. The second option involves the use of a mixed frequency low-stress silicon nitride with outstanding electrical insulation capability, providing improved mechanical and electrical integrity of the CMUT active layers. The behavior of the nitride is analyzed as a function of deposition parameters and subsequent annealing. The nitride layer characterization is reported in terms of interfaces density influence on residual stress, refractive index, deposition rate, and thickness variation both as deposited and after thermal treatment. A sweet spot for stress stability is identified at an interfaces density of 0.1 nm−1, yielding 87 MPa residual stress after annealing. A complete CMUT device fabrication is reported using the optimized nitrides. The CMUT performance is tested, demonstrating full functionality in ultrasound imaging applications and an overall performance improvement with respect to previous devices fabricated with non-optimized silicon nitride.

Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 mum and 150 mum diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.

Capacitive micromachined ultrasonic transducer (CMUT) arrays for medical imaging

Capacitive micromachined ultrasonic transducers (CMUTs) bring the fabrication technology of standard integrated circuits into the field of ultrasound medical imaging. This unique property, combined with the inherent advantages of CMUTs in terms of increased bandwidth and suitability for new imaging modalities and high frequency applications, have indicated these devices as new generation arrays for acoustic imaging. The advances in microfabrication have made possible to fabricate, in few years, silicon-based electrostatic transducers competing in performance with the piezoelectric transducers. This paper summarizes the fabrication, design, modeling, and characterization of 1D CMUT linear arrays for medical imaging, established in our laboratories during the past 3 years. Although the viability of our CMUT technology for applications in diagnostic echographic imaging is demonstrated, the whole process from silicon die to final probe is not fully mature yet for successful practical applications.