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

Ecografia 3-D in tempo reale

Many academic and industrial research teams are working towards the improvementof ultrasound techniques for medical diagnostic applications, in order to have volumetricechographic images in real time. The unique way to approach this problem isto use a matrix array transducer and to develop an echographic system able to handlethousands of electronic channels. A volumetric echographic system is a hard task !During the past decades huge advances have been made in the field of ultrasound imagingfor medical diagnostic applications. Conventional echographic probes are based,by now, on the mature piezoelectric technology. Recently, alternative technologiesbased on different operating principles have been proposed. In particular, the siliconmicromachined capacitive ultrasonic transducer technology (cMUT) has shown to besuitable for the replacement of the current technology. The main benefits of cMUT technologycan be found in the silicon micromachining process, commonly used in microelectronics.In order to have real time volumetric images a two dimensional matrixtransducer must be used; the afore mentioned cMUT technology is well suited for thistype of new transducers. Roma Tre has a program to develop this kind of transducersbut, because their geometrical characteristics are strongly dependent on the structureof the echographic system used, a preliminary exploration must be made in order todefine its principal features. This activity entails a huge amount of end-to-end simulationsof the whole process, from signal acquisition to the final image, to assess the impactof design choices on diagnostic capabilities

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

A portable and autonomous system for the diagnosis of the structural health of cultural heritage (PICUS)

An innovative system has been developed, named PICUS (the ‘woodpecker’ in Latin), and inspired by the auscultation method carried out by the experts in the field of conservation of cultural heritage. This device tapping the surface, controlling, acquiring and measuring some relevant parameters. In a nutshell, it performs an analysis similar to that carried out by a professional who performs a routine examination on the detachments by hand. The experimental apparatus consists of a probe made of an electromechanical percussion element that gently taps the surface producing a sound, an accelerometer to measure the impact force, and a microphone, all connected with an Arduino-like low-cost board, to record and elaborate sounds and the force sensor signal. The probe XY position on the scene is recognized using a low-cost IR camera positioned in the probe and an IR source opportunely positioned outside

PICUS: A Pocket-Sized System for Simple and Fast Non-Destructive Evaluation of the Detachments in Ancient Artifacts

An innovative, robust method has been developed, based on the use of a simple, compact, expressly designed device, named PICUS (the ‘woodpecker’ in ancient Latin), and inspired by the auscultation method carried out by the experts in the field of conservation of cultural heritage. This method entails gently knocking the surface, controlling and measuring the impact time of the stroke’s force, recording the generated sound, comparing the acquired sound with a reference sound by calculating the cross-correlation function, and its maximum, as a measure of the detachment. In a nutshell, it performs an analysis similar to that carried out by a professional who performs a routine examination on the detachments by hand. The experimental apparatus consists of a probe made of an electromechanical percussion element that gently taps the surface producing a sound, a force sensor purposely developed to measure the impact force, and a microphone, all connected with an Arduino-like low cost board, to record and elaborate the sounds and the force sensor signal. The probe XY position on the scene is recognized using an infra-red (IR) system with a low-cost IR camera and an IR light-emitting diode (IR-LED) positioned on the probe. The “tapper” and the microphone replace the hand and the ear of a conservator carrying out a detachment investigation, while the comparison with a reference is the typical mind process of a professional restorer. The result is the fusion of the microphone data and the force sensor data

An ultrasound technique for 3D palmprint extraction

In this work, an ultrasound technique for extracting 3D palmprints is experimentally evaluated. A commercial ultrasound imaging machine, provided with a high frequency (12 MHz) linear array, is employed for the experiments. The probe is moved in the elevation direction by a motorized stepper stage and at each step a B-scan is acquired and stored to form a 3D matrix representing the under skin volume. The data from the 3D matrix are elaborated to provide several renderings of the 3D ultrasonic palmprint. The results have been compared with corresponding samples obtained with conventional methods and the advantages of the ultrasound technique are underlined and discussed. © 2014 Elsevier B.V

ACUPAD: A track-pad device based on a piezoelectric bimorph

A track-pad is a pointing device, featuring a tactile sensor, able to translate the motion and position of a user's finger, or a stylus, to a relative position on a screen. In this paper a piezoelectric tactile sensor for track-pad applications is proposed; the active element of the device is a cheap piezoceramic bimorph disk, widely used in buzzers and telephone receivers, clamped all around its border. The device operating principle is the following: when a stylus is positioned on the bimorph surface, the displacement field of the contact point is modified, with a consequent variation of the device electrical input impedance; the stylus position can be therefore related to the impedance variation. The system was analyzed by FEM, obtaining a clear dependence of the device characteristic frequencies on the stylus radial position, while by moving the stylus along the angle, a clear variation of impedance values is obtained. A device prototype was realized and FEM results were experimentally confirmed, validating the proposed device performance

Depth-of-field enhancement in Filtered-Delay Multiply and Sum beamformed images using Synthetic Aperture Focusing

The Synthetic Aperture Focusing (SAF) technique makes it possible to achieve a higher and more uniform quality of ultrasound images throughout depth, as if both transmit and receive dynamic focusing were applied. In this work we combine a particular implementation of SAF, called Synthetic Transmit Aperture (STA) technique, in which a single element in turn transmits and all the array elements receive the ultrasound wave, with the Filtered-Delay Multiply and Sum (F-DMAS) non-linear beamforming algorithm that we presented in a previous paper. We show that using F-DMAS, which is based on a measure of backscattered signal spatial correlation, B-mode images have a higher contrast resolution but suffer from a loss of brightness away from the transmit focus, when a classical scan with receive-only dynamic focusing is performed. On the other hand, when synthetic transmit focusing is achieved by implementing STA, such a loss is compensated for and a higher depth of field is obtained, as signal coherence improves. A drawback of SAF/STA however is the reduced signal-to-noise ratio, due to single-element transmission; in the paper we also analyze how this influences F-DMAS images. Finally, a preliminary investigation on the use of the classical monostatic SAF technique with F-DMAS beamforming is also carried out to evaluate its potential performances