MEMS raise the game of 3D ultrasonic imaging
Ultrasonic imaging enable the contactless detection of objects with millimetric precision for a wide variety of materials regardless of their state, form and colour, and in practically any environment.
Whilst today’s ultrasonic transducers are typically manufactured using piezoelectric materials, new MEMS alternatives are under development, offering greater sensitivity and broader frequency ranges than the established piezo based transducers, according to Dr. Anartz Unamuno, manager of the CMUT (Capacitive Ultrasonic Micromachined Transducers) activities at Fraunhofer IPMS.
Using wafer bonding processes, these MEMS ultrasonic transducers also have a strong potential for monolithic integration with CMOS ASICs, paving the way to very compact and more rugged systems hosting all the necessary driver circuitry underneath the micromechanical structure.
The basic MEMS structure found in these CMUTs consists of two opposing electrodes spaced apart by an insulating layer and a gap. One is static and the other is deflectable.
“As an emitter, an electric signal is applied between the electrodes which deflects the movable electrode and generates an acoustic wave. The inverse energy transformation, acoustic to electric, is used with CMUTs to sense ultrasound waves”, explains Unamuno in an introduction paper.
“Each individual plate can range between about 10um and 100um in diameter/edge. But these elements are usually grouped so that the active acoustic area is larger and therefore more acoustic power is available. This also ensures a better sensitivity”, clarified Unamuno in an email exchange with eeNews Europe.
The so-called Axel or Acoustic-pixel formed by these moving elements can regroup from one single CMUT to as many as you can fit in a wafer, and depending on the application specific requirements (frequency, sound pressure, waveform, etc.), different designs can be fabricated and arranged in 1D or 2D array configurations driven using beam steering algorithms.
With the right pulse and phase-shift combinations across a 2D MEMS array, you could pin-point a 3D volume in space where to send specific ultrasound pulses, then collect the echo from that point precisely, and this could be done in real time as long as the appropriate electronics are fitted to the CMUTs.
As for the size of the CMUTs, “when the pitch is smaller than lambda/2, maximum performance (and maximum range) can be achieved in terms of moving the focus in space”, said Unamuno. “A larger pitch allows beam forming and imaging, but with a performance loss” he added.
After proving the concept using 1D CMUT arrays, the researcher is working on the fabrication of 2D arrays. Depending on their designs, these 2D arrays could be used to perform accurate 3D mapping and spatial exploration at room level or even inside patients.
“The word “accurate” should be defined here. Low frequency signals (<1MHz) usually reach distances in the order of meters and are best suited for air borne ultrasound. Frequencies that are in the order of 10MHz, only penetrate a few centimetres.” Told us Unamuno. “On the other side, higher frequencies allow for a better resolution than lower frequencies. It is a trade-off.”
So, how fast could imaging be performed?
Probing different spatial points (serially one after the other) could be an option, but it is also possible to send a single acoustic pulse and receive all the echoes, then post-process and synthesize the images. Using so-called time-reversal processing, Dr Mathias Fink from the Langevin Institute reported acquisition speeds up to 10,000 image frames per second.
With Fraunhofer IPMS’ in-house clean room capacity to reliably fabricate application specific CMUT arrays in small to mid-volumes, the researcher hopes to take CMUTs out of the lab into real life applications. These could range from intra-vascular ultrasound (IVUS) imaging to non-destructive materials testing, to gas and chemical sensors.
Fraunhofer IPMS will present its first generation of CMUT chips and a first demonstrator showing the functionality of a CMUT component to the North American specialist community at the Sensors Expo from June 24 to 26, 2014.
Visit the Fraunhofer Institute for Photonic Microsystems at https://www.ipms.fraunhofer.de/
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