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

Fabrication and characterization of a multimodal 3D printed mouse phantom for ionoacoustic quality assurance in image-guided pre-clinical proton radiation research

Objective. Image guidance and precise irradiation are fundamental to ensure the reliability of small animal oncology studies. Accurate positioning of the animal and the in-beam monitoring of the delivered radio-therapeutic treatment necessitate several imaging modalities. In the particular context of proton therapy with a pulsed beam, information on the delivered dose can be retrieved by monitoring the thermoacoustic waves resulting from the brief and local energy deposition induced by a proton beam (ionoacoustics). The objective of this work was to fabricate a multimodal phantom (x-ray, proton, ultrasound, and ionoacoustics) allowing for sufficient imaging contrast for all the modalities. Approach. The phantom anatomical parts were extracted from mouse computed tomography scans and printed using polylactic acid (organs) and a granite/polylactic acid composite (skeleton). The anatomical pieces were encapsulated in silicone rubber to ensure long term stability. The phantom was imaged using x-ray cone-beam computed tomography, proton radiography, ultrasound imaging, and monitoring of a 20 MeV pulsed proton beam using ionoacoustics. Main results. The anatomical parts could be visualized in all the imaging modalities validating the phantom capability to be used for multimodal imaging. Ultrasound images were simulated from the x-ray cone-beam computed tomography and co-registered with ultrasound images obtained before the phantom irradiation and low-resolution ultrasound images of the mouse phantom in the irradiation position, co-registered with ionoacoustic measurements. The latter confirmed the irradiation of a tumor surrogate for which the reconstructed range was found to be in reasonable agreement with the expectation. Significance. This study reports on a realistic small animal phantom which can be used to investigate ionoacoustic range (or dose) verification together with ultrasound, x-ray, and proton imaging. The co-registration between ionoacoustic reconstructions of the impinging proton beam and x-ray imaging is assessed for the first time in a pre-clinical scenario

Advancements on Silicon Ultrasound Probes (CMUT) for Medical Imaging Applications

Capacitive micromachined ultrasonic transducers (CMUTs) are micro-electromechanical devices (MEMS) fabricated using silicon micromachining techniques. The interest of this technology relies in its full compatibility with the microelectronic technology that makes possible to integrate on the same chip the transducer and the controlling/conditioning electronics, so as to achieve low-cost and high-performance devices. The design and fabrication of a 192-element linear array CMUT probe operating in the range 6–18 MHz is here presented. The CMUT array is micro-fabricated and packed using a novel fabrication concept specifically conceived for imaging transducer arrays. The performance optimization of the probe is performed by connecting the CMUT array with multichannel analog front-end electronic circuits housed into the probe body. Characterization and imaging results are used to assess the performance of CMUTs with respect to conventional piezoelectric transducers. This paper is a review on the activities of our group in this field

MEMS-based transducers (CMUT) for medical ultrasound imaging

Capacitive micromachined ultrasonic transducers (CMUTs) are micro-electromechanical devices (MEMS) fabricated using silicon micromachining techniques. In the past decade, their use has proved to be attractive mainly in the field of medical ultrasound imaging as active elements in ultrasound probes. The interest of this novel technology relies on its full compatibility with standard integrated circuit technology that makes it possible to integrate, on the same chip, the transducers and the electronics, thus enabling the realization of extremely low-cost and high-performance devices. From an operational point of view, CMUTs have been widely recognized as a valuable alternative to piezoelectric transducer technology in a variety of medical imaging applications, thanks to a higher sensitivity, a wider bandwidth, and an improved thermal efficiency. In this chapter, the design and fabrication of a 192-element linear array CMUT probe operating in the range 6-18 MHz, designed for vascular, small parts, rheumatology and anesthesiology imaging applications, is reported. The CMUT array is microfabricated and packed using a novel fabrication concept specifically conceived for imaging transducer arrays. The performance optimization of the probe is performed by connecting the CMUT array with multichannel analog front-end electronic circuits housed into the probe body. Characterization and imaging results are used to assess the performance of CMUTs with respect to conventional piezoelectric transducers

MEMS-based transducers (CMUT) for medical ultrasound imaging

Capacitive micromachined ultrasonic transducers (CMUTs) are micro-electromechanical devices (MEMS) fabricated using silicon micromachining techniques. In the past decade, their use has proved to be attractive mainly in the field of medical ultrasound imaging as active elements in ultrasound probes. The interest of this novel technology relies on its full compatibility with standard integrated circuit technology that makes it possible to integrate, on the same chip, the transducers and the electronics, thus enabling the realization of extremely low-cost and high-performance devices. From an operational point of view, CMUTs have been widely recognized as a valuable alternative to piezoelectric transducer technology in a variety of medical imaging applications, thanks to a higher sensitivity, a wider bandwidth, and an improved thermal efficiency. In this chapter, the design and fabrication of a 192-element linear array CMUT probe operating in the range 6-18 MHz, designed for vascular, small parts, rheumatology and anesthesiology imaging applications, is reported. The CMUT array is microfabricated and packed using a novel fabrication concept specifically conceived for imaging transducer arrays. The performance optimization of the probe is performed by connecting the CMUT array with multichannel analog front-end electronic circuits housed into the probe body. Characterization and imaging results are used to assess the performance of CMUTs with respect to conventional piezoelectric transducers

Density-tapered spiral arrays for ultrasound 3-D imaging

The current high interest in 3-D ultrasound imaging is pushing the development of 2-D probes with a challenging number of active elements. The most popular approach to limit this number is the sparse array technique, which designs the array layout by means of complex optimization algorithms. These algorithms are typically constrained by a few steering conditions, and, as such, cannot guarantee uniform side-lobe performance at all angles. The performance may be improved by the ungridded extensions of the sparse array technique, but this result is achieved at the expense of a further complication of the optimization process. In this paper, a method to design the layout of large circular arrays with a limited number of elements according to Fermat's spiral seeds and spatial density modulation is proposed and shown to be suitable for application to 3-D ultrasound imaging. This deterministic, aperiodic, and balanced positioning procedure attempts to guarantee uniform performance over a wide range of steering angles. The capabilities of the method are demonstrated by simulating and comparing the performance of spiral and dense arrays. A good trade-off for small vessel imaging is found, e.g., in the 60Λ spiral array with 1.0Λ elements and Blackman density tapering window. Here, the grating lobe level is -16 dB, the lateral resolution is lower than 6λ the depth of field is 120Λ and, the average contrast is 10.3 dB, while the sensitivity remains in a 5 dB range for a wide selection of steering angles. The simulation results may represent a reference guide to the design of spiral sparse array probes for different application fields

An Automatic Compact Schlieren Imaging System for Ultrasound Transducers Testing

The current standard used for the characterization of ultrasonic transducers is the hydrophonic technique able to measure the acoustic pressure profile. This technique allows a quantitative analysis, though marred by several problems. The scan of the region of interest appears to be a very costly operation in terms of time, especially when we want to measure a long acoustic beam. Furthermore, a hydrophone placed near the radiating surface is certainly a nuisance to the free propagation of the field. Off-axis measurements can be inaccurate because of the angular response of the hydrophone. These problems together with the costs have encouraged the search for a complementary, quick, and inexpensive test system. The well known Schlieren technique allows a real time visualization of the whole pressure range of the transducers, but to display the entire beam emitted by the transducers it is necessary to use very large-diameter lenses, with focal lengths of several meters. Such systems are very cumbersome, and make their usage very difficult. The system developed in this paper allows the image of an acoustic beam up to 200 mm in length, but the system is compact, being only about 1 meter long and 0.30 meter wide. A similar system based on a classic Schlieren effect would size several meters, with lenses of 200 mm in diameter. Finally, the system can reconstruct the section of the beam at any height, using an acoustic tomography technique, and can also implement a quantitative analysis. Since it uses only commercial components, the developed ultrasonic beam analyzer fabricated is a very low-cost imaging system. This work is aimed at creating a compact, low cost system based on this technique to test a wide range of ultrasonic transducers up to 40 MHz, and above