Fraunhofer Institute for Photonic Microsystems
Fraunhofer Institute for Photonic Microsystems

CMUT Chip

128 channel chip (5 MHz) of a capacitive micromachined ultrasonic transducers (CMUT).

© Fraunhofer IPMS

CMUT Array

Illustration of a two-dimensional CMUT array in the deflected state.

© Fraunhofer IPMS

CMUT Probe

128 channel CMUT ultrasonic probe for applications in liquids.

© Fraunhofer IPMS

Capacitive Pressure Sensors

Capacitive pressure sensors in surface micro mechanics integrable as post CMOS module.

© Fraunhofer IPMS

CMUT Chip

High frequency 128 channel chip (20 MHz) of a capacitive micromachined ultrasonic transducers (CMUT).

© Fraunhofer IPMS

Micromachined Ultrasonic Transducers

Ultrasonography is a well-established technology. Common areas of application include material characterization and testing as well as medical diagnostics. Currently, ultrasonic transducers are manufactured based mainly on a piezoelectric material or piezo composite. These transducers possess properties adequate for applications across spaces of open air (distance measurement) or in the field of high-frequency material testing. However, they fall short particularly with high-resolution imaging, invasive testing and material testing of liquids.

Ultrasonic transducers manufactured with micromachining technology (MUT) can overcome these drawbacks. MUTs are basically MEMS structures which consist of two electrodes. One of the electrodes is fixed, the other is movable. An insulating layer and a vacuum-sealed gap separate the electrodes. The movable electrode in piezoelectric ultrasonic transducers (PMUT) are stimulated by a piezoelectrical film, e.g. AIN. In capacitive micromachined ultrasonic transducers (CMUT), stimulation occurs through electrostatic deflection of the movable electrode. 

Fraunhofer IPMS offers research as well as development and pilot production of micromachined components for innovative areas of application.

Micromachined Ultrasonic Transponders in Post-CMOS Technology

CMUT in receive operation mode.
© Fraunhofer IPMS
CMUT in receive operation mode.
CMUT in transmit operation mode.
© Fraunhofer IPMS
CMUT in transmit operation mode.

Fraunhofer IPMS develops micromachined ultrasonic components for various areas of application. These elements are uniquely capable of being integrated into CMOS processes as post-CMOS modules, paving the way for highly-miniaturized components with integrated evaluation electronics. Therefore, Fraunhofer IPMS is focused on the development of capacitive micromachined ultrasonic transducers (CMUTs). 

CMUTs are basically MEMS structures which consist of two electrodes. One of the electrodes is fixed, the other is movable. The electrodes are separated by an insulating layer and a vacuum-sealed gap. CMUTs can both send and receive by converting electric energy to acoustic energy or vice versa through the displacement of the movable electrode. When a CMUT is sending sound, an electronic potential is established between the electrodes in order to deflect the electrostatic force from the movable electrode to its fixed counterpart. This movement generates a sound wave. Using the opposite procedure, the CMUT is also able to act as a receiver. 

At Fraunhofer IPMS, CMUT production implements a unique method that allows for CMUTs to be manufactured as so-called Back-end-of-Line (BeoL) process modules. Here, amorphous metals in particular are used for the CMUT plates to provide elements high long-term stability and reproducibility. Furthermore, the CMUT module can be integrated into standard CMOS processes, a unique feature of this technology. In the institute's clean room, highly-integrated CMUTs are developed on 200mm wafers and manufactured for pilot production.

Acoustic Spectroscopy

At the testing station developed by Fraunhofer IPMS, liquids are radiated in cuvettes via a CMUT and frequency-dependent attenuation is determined by means of a Fourier analysis.
© Fraunhofer IPMS
At the testing station developed by Fraunhofer IPMS, liquids are radiated in cuvettes via a CMUT and frequency-dependent attenuation is determined by means of a Fourier analysis.

Spectroscopic examination by means of ultrasound provides information about the physical characteristics of materials as well as the chemical analysis of dispersions. By analyzing the frequency-dependent attenuation and speed of sound, quality conclusions and the composition of oils, alcohol-water mixtures or other liquids can be determined, providing an ideal complement to optical spectroscopy. 

In this area of application, the use of capacitive micromachined ultrasonic transducers can lead to new highly-compact environment measurement systems. In contrast to conventional ultrasonic piezoelectric elements, CMUTs are realized through a micromachining manufacturing process and allow for an extremely compact structure. Through a monolithic integration with CMOS circuits, sensors allow for the realization of complete analysis systems on one chip. CMUTs are ideal for acoustic spectroscopy because they can radiate sound in liquid media with extreme efficiency, detection is highly sensitive and a wide frequency bandwidth can be used.

Medical Imaging

Sonography is a well-established field of analysis, especially in medical technology. In the form of ultrasonic arrays, the use of of ultrasonic transducers is crucial for imaging techniques. The majority of ultrasonic arrays produced in medical technology today use the piezoelectric ceramic-lead-zirconate-titanate (PZT) according to the utilization of the reverse piezoelectric effect to generate sound. 

High-frequency, high-resolution arrays based on PZT are, however, difficult to produce and therefore expensive. Both PMUT and CMUT micromachined ultrasonic transducers are providing new opportunities. The micromachining manufacturing process now allows for the economical production of high-frequency, high-resolution ultrasonic arrays. In addition, the capability for high miniaturization makes it possible to use MUTs in invasive applications such as intravascular ultrasound (IVUS).

Results from current development demonstrate the beneficial features of MUTs for the production of high-frequency arrays. MUTs offer:

  • a very high bandwidth
  • an extremely low mechanical coupling between elements
  • integration together with electronic components (CMOS)
  • no toxic materials

A high bandwidth and low coupling are fundamental for the compatibility of MUT-based imaging with conventional medical imaging standards. For the first time, highly-integrated MEMS technology enables the signal of an array to locally connect with read-out electronics to achieve simple and compact contact between the elements. The implementation of this connection technique allows for highly planar surfaces to be used as a contact to the medium.

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