Giosuè Caliano

An accurate model for capacitive micromachined ultrasonic transducers

Modeling of capacitive micromachined ultrasonic transducers (cMUTs) is based on a two-port network with an electrical and a mechanical side. To obtain a distributed model, a solution of the differential equation of motion of the diaphragm for each element of the transducer has to be found. Previous works omit the mechanical load of the cavity behind the diaphragm, i.e., the effect of the gas inside. In this paper, we propose a distributed model for cMUTs that takes this effect into account. A closed-form solution of the mechanical impedance of the membranes has been obtained, including the effect of the restoring forces because of the stiffness of the membrane and because of the compression of the air in the cavity. Simulation results based on the presented model are compared with the experimental data for two types of cMUTs reported in the recent literature. It is demonstrated that the compression of the air has a significant effect on the fundamental frequency of the air transducer, with a deviation of about 22% from the prediction of a model that does not consider the interaction between the vibrating diaphragm and the air cushion.

Capacitive micromachined ultrasonic transducer (CMUT) arrays for medical imaging

Abstract 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.

A CMUT probe for medical ultrasonography: from microfabrication to system integration

Medical ultrasonography is a powerful and costeffective diagnostic technique. To date, high-end medical imaging systems are able to efficiently implement real-time image formation techniques that can dramatically improve the diagnostic capabilities of ultrasound. Highly performing and thermally efficient ultrasound probes are then required to successfully enable the most advanced techniques. In this context, ultrasound transducer technology is the current limiting factor. Capacitive micromachined ultrasonic transducers (CMUTs) are micro-electro-mechanical systems (MEMS)-based devices that have been widely recognized as a valuable alternative to piezoelectric transducer technology in a variety of medical imaging applications. Wideband operation, good thermal efficiency, and low fabrication cost, especially for those applications requiring high-volume production of small-area dice, are strength factors that may justify the adoption of this MEMS technology in the medical ultrasound imaging field. This paper presents the design, development, fabrication, and characterization of a 12-MHz ultrasound probe for medical imaging, based on a CMUT array. The CMUT array is microfabricated and packed using a novel fabrication concept specifically conceived for imaging transducer arrays. The performance of the developed probe is optimized by including analog front-end reception electronics. Characterization and imaging results are used to assess the performance of CMUTs with respect to conventional piezoelectric transducers.

The Delay Multiply and Sum Beamforming Algorithm in Ultrasound B-Mode Medical Imaging

Most of ultrasound medical imaging systems currently on the market implement standard Delay and Sum (DAS) beamforming to form B-mode images. However, image resolution and contrast achievable with DAS are limited by the aperture size and by the operating frequency. For this reason, different beamformers have been presented in the literature that are mainly based on adaptive algorithms, which allow achieving higher performance at the cost of an increased computational complexity. In this paper, we propose the use of an alternative nonlinear beamforming algorithm for medical ultrasound imaging, which is called Delay Multiply and Sum (DMAS) and that was originally conceived for a RADAR microwave system for breast cancer detection. We modify the DMAS beamformer and test its performance on both simulated and experimentally collected linear-scan data, by comparing the Point Spread Functions, beampatterns, synthetic phantom and in vivo carotid artery images obtained with standard DAS and with the proposed algorithm. Results show that the DMAS beamformer outperforms DAS in both simulated and experimental trials and that the main improvement brought about by this new method is a significantly higher contrast resolution (i.e., narrower main lobe and lower side lobes), which turns out into an increased dynamic range and better quality of B-mode images.

Design, fabrication and characterization of a capacitive micromachined ultrasonic probe for medical imaging

In this paper we report the design, fabrication process, and characterization of a 64-elements capacitive micromachined ultrasonic transducer (cMUT), 3 MHz center frequency, 100% fractional bandwidth. Using this transducer, we developed a linear probe for application in medical echographic imaging. The probe was fully characterized and tested with a commercial echographic scanner to obtain first images from phantoms and in vivo human body. The results, which quickly follow similar results obtained by other researchers, clearly show the great potentiality of this new emerging technology. The cMUT probe works better than the standard piezoelectric probe as far as the axial resolution is concerned, but it suffers from low sensitivity. At present this can be a limit, especially for in depth operation. But we are strongly confident that significant improvements can be obtained in the very near future to overcome this limitation, with a better transducer design, the use of an acoustic lens, and using well matched, front-end electronics between the transducer and the echographic system.

Acoustic coupling in capacitive microfabricated ultrasonic transducers: modeling and experiments

In the design of low-frequency transducer arrays for active sonar systems, the acoustic interactions that occur between the transducer elements have received much attention. Because of these interactions, the acoustic loading on each transducer depends on its position in the array, and the radiated acoustic power may vary considerably from one element to another. Capacitive microfabricated ultrasonic transducers (CMUT) are made of a two-dimensional array of metallized micromembranes, all electrically connected in parallel, arid driven into flexural motion by the electrostatic force produced by an applied voltage. The mechanical impedance of these membranes is typically much lower than the acoustic impedance of water. In our investigations of acoustic coupling in CMUTs, interaction effects between the membranes in immersion were observed, similar to those reported in sonar arrays. Because CMUTs have many promising applications in the field of medical ultrasound imaging, understanding of cross-coupling mechanisms and acoustic interaction effects is especially important for reducing cross-talk between array elements, which can produce artifacts and degrade image quality. In this paper, we report a finite-element study of acoustic interactions in CMUTs and experimental results obtained by laser interferometry measurements. The good agreement found between finite element modeling (FEM) results and optical displacement measurements demonstrates that acoustic interactions through the liquid represent a major source of cross coupling in CMUTs.

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 of capacitive micromechanical ultrasonic transducers by low-temperature process

This work is focused on the development of an innovative process for the realization of capacitive micromachined ultrasonic transducers (cMUTs) using surface micromachining on silicon with the possibility of integrating front-end electronic. We describe the processing steps and the materials used to obtain an entirely low-temperature (<510°C) fabrication process. The structural SiO x layer and the SiN, membrane layer are deposited, respectively, by thermal evaporation and by plasma-enhanced chemical vapor deposition (PECVD). These techniques are compatible with the introduction of polyimide as the sacrificial layer. The 100% etch selectivity of the polyimide with respect to SiO x and SiN x allows the fabrication of membranes with a wide variety of shapes and air-gap dimensions. We have optimized thermal annealing treatment to control stress of the PECVD silicon nitride film to obtain optimal mechanical properties of membranes. Transducers have been characterized by electrical impedance analysis, showing resonance frequencies ranging from 4 to 6 MHz.

Capacitive micromachined ultrasonic transducer (cMUT) made by a novel "reverse fabrication process"

We report a novel fabrication process of a cMUT array based on the electrostatic effect and realized by silicon micromachining technique. Several fabrication technologies for 1D and 2D cMUT have been presented in the last ten years, differing from each other in the materials used and the process steps involved. They all have in common the presence of micro-holes on the surface of the transducer necessary to evacuate the cavities under the membranes or, in the 2D array, to electrically connect upper to lower pads and to allow the electrical connection to external circuits. The authors of the present work designed and realized a cMUT transducer using a new concept. In the standard process, successive layers are deposited on the silicon wafer up to the silicon nitride structural layer of the micro membranes; our different approach consists in inverting the function of each layer and to build the cMUT capacitive cell starting from the membrane, made of LPCVD silicon nitride coating the silicon wafer, up to the bottom electrode and the backplate. By working on the back of the device, there is no need to use holes in the structural silicon nitride layer to evacuate the cavities and the pads for the electrical connections are on the bottom surface of the device.

Performance optimization of a high frequency CMUT probe for medical imaging

In this work we report on the performance optimization of a recently developed high frequency CMUT probe. By leveraging the advantages offered by our proprietary “Reverse Fabrication Process (RFP)” CMUT technology, we have developed an efficient and reliable packaging process, useful for the fabrication of small sized ultrasound probe-heads. We have maximized the performance by connecting the CMUT probe-head to multichannel analog front-end electronic circuits housed into the probe. An experimental comparison with an equivalent piezoelectric probe, the LA435 commercialized by Esaote S.p.A. (Italy), carried out by means of pulse-echo measurements, showed superior performance for the CMUT probe. Real time in-vivo ultrasound imaging capability demonstrates the actual usability of CMUT technology for medical imaging.