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Fraunhofer Group for Microelectronics

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Acoustics with MEMS

Flow meters, quality control, medical imaging – the uses of ultrasound are many and varied. Fraunhofer IPMS has now developed a new MEMS technology for making CMUTs that has the potential of increasing the range of uses of ultrasound even further.


Capacitive micromechanical ultrasound transducers, or CMUTs for short, are MEMS-based structures that can be used to generate and capture acoustic signals in the ultrasound range. Their basic structure comprises two opposed electrodes. One of the electrodes is rigid; the other can move. The electrodes are separated from another electrically by means of insulator layers and an air gap. CMUTs can both transmit and receive by moving the movable electrode to convert electrical energy to acoustic energy, or back again. Thanks to the quality of the acoustic signals that can be generated and the option of integrating them with CMOS, interest in CMUTs is growing. Since the end of 2012, the Fraunhofer Institute for Photonic Microsystems IPMS has been working on the development of CMUT devices in order to bring these from laboratory stage to market readiness.


The challenge of homogeneity


Despite all attempts to make CMUTs commercially viable, there is as yet no product on the European or US market that makes use of this technology. One possible explanation could be the high demands with regard to aspects such as the homogeneity of converter elements on large arrays and particle density when manufacturing MEMS. R&D-only clean rooms have difficulties fulfilling these requirements. Fraunhofer IPMS, on the other hand, operates a clean room under industrial conditions and is therefore in a position to advance the manufacture of CMUTs. The Institute also has extensive experience in integrating MEMS on CMOS wafers – a decisive factor in further CMUT development.


So far, the researchers at Fraunhofer IPMS have been able to clarify many salient points. The technologists selected a CMUT manufacturing process that involves the etching of sacrificial layers. At the same time, they worked out a complete CMUT process, including the choice of materials and possible layer thicknesses. The first FEM models of the CMUT elements have been created to determine the space required. The scientists also selected different CMUT designs for resonance frequencies in the range between 1 and 50 MHz. The dimensions of the CMUT elements are between 10 and 100 μm. Groups, each of several hundred adjoining CMUT elements on a single wafer, act together as a single structure.


New applications in the ultrasound range


The scientists at Fraunhofer IPMS are currently subjecting the first generation of CMUT arrays to electrical tests and are analyzing them. The results so far are very promising; the first acoustic measurements will be carried out soon. The researchers are also investigating the requirements for various potential uses of CMUTs. Of particular interest are applications where the use of CMUTs would have particular benefit compared with today‘s ultrasound transducers. These include, for example, imaging ultrasound systems in cardiac catheters, gas flow sensors in severe environments, nondestructive testing procedures, and gesture detection that could one day replace mice and keyboards.




Dr. Michael Scholles
Phone +49 351 8823-201
Fraunhofer Institute for Photonic Microsystems IPMS
Maria-Reiche-Strasse 2
01109 Dresden