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.

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.

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.