IMX Seminar Series - Microsonics for communication and sensors

Microsonics has emerged as field in microsystems technology dealing with ultrasonic applications. Considering that typical sound velocities in solids are ranging from 4’000 to 10’000 m/s we conclude that a wavelength of a micrometer corresponds to frequencies of 4 to 10 GHz. This means that mechanical resonators with micrometer dimensions resonate in the lower GHz frequency range, the same range as the transmission bands of mobile telecommunication (between 0.4 and 3 GHz). Of course, we need a link between wireless transmission based on electromagnetic waves, and the ultrasonic waves in microsonic devices. The required transformation is provided by piezoelectric materials, either as single crystal or as thin film. Modern mobile communication owes its existence indeed to piezoelectric materials, because only electromechanical resonances in selected piezoelectrics yield high enough quality factors within reasonably small dimensions.
This talk focuses on piezoelectric thin film resonators, which in the simplest configuration are just thickness mode vibrators. We note again that a typical thin film thickness fits well the required half-wavelength thickness for communication bands in the lower GHz frequency bands. Microsonics developed in parallel to mainstream MEMS technology, focusing on integrating piezoelectric films, and improving dimensional precision. There are of course very high requirements as to the quality and stability of such films. The best material found so far is AlN, having a correct size of piezoelectric coupling, a large mechanical quality factor, and also a good thermal conductivity, which is important for filters in the transmit line to support the power in the sending mode. With the discovery of the extraordinary increase of piezoelectricity when alloying non-piezoelectric ScN into AlN by a Japanese group, the field got a new thrill. The larger piezoelectric coupling allows for larger communication bands, satisfying better the ever-increasing need for higher bit rates.  Moreover, other types of resonators having inherently lower coupling factors than the simple devices exploiting longitudinal bulk waves become now also interesting.
The talk will address some growth and microstructural issues in Al_(1-x)Sc_xN thin films. The piezoelectric wurtzite phase is in fact a metastable phase, and its growth in a polarly oriented microstructure at relatively low temperatures (300 °C) is a typical achievement of sputter deposition. Nevertheless, there is a certain risk to obtain misaligned grains that are thought to be caused by secondary nucleation of ScN rich, nanometer sized rocksalt within grain boundaries at the surface of the growing wurtzite film. On the acoustic side we shall speak about a breakthrough in Lamb wave devices, for which a larger quality factor was achieved when having the wave-carrying plate not freely supported, but isolated by an acoustic Bragg mirror structure. Finally, another novel device is presented, a 3D machined micro transducer that couples bulk waves generated in periodic pillars into a surface wave that is travelling away on the substrate surface. When combined with a reflector grid at some distance, the wave comes back into the transducer and is detected as echo. Knowing the temperature coefficient of the surface wave, the travelling time reveals the temperature of the device. A wireless coupling between transducer and an antenna enables then a wireless temperature readout up to 600 °C in experiment, with prospects to reach 800 °C.

Bio: Paul Muralt is professor at Swiss Federal Institute of Technology EPFL at Lausanne, Switzerland. He leads a group working in electroceramic thin films within the Materials Science Institute, studying particularly piezoelectric and solid ionic MEMS and NEMS devices. Having a background (PhD) in solid state physics, he moved more and more into thin films, surface, and materials science for micro and nanotechnology. In his professional career he was working at the Swiss Federal Institute of Technology ETH, the IBM Research Laboratory in Zurich, the Free University of Berlin, and in a thin film coating industry (Balzers) before joining the Ceramics Laboratory at EPFL in 1993. His PhD studies dealt with incommensurate crystalline phases in an organic-inorganic layered perovskite structure. As a post-Doc, he pioneered scanning tunneling potentiometric imaging. Today, he is particularly known for his works in processing, characterization and applications of piezoelectric and pyroelectric thin films such as PbZrTiO₃ and Al_1-xSc_xN, including also works on materials integration, micro machining, and device physics. He teaches thin film deposition, micro and nano structuration, surface analysis and introduction to ceramics. He authored or co-authored over 250 scientific articles. He is IEEE Fellow, received the outstanding achievement award of the International Symposium of Integrated Ferroelectrics (ISIF) in 2005, and the C.B. Sawyer award from the IEEE International Frequency Control Symposium in 2016. He acted as co-chair of the MRS spring meeting 2008, co-organized MRS and E-MRS symposia, served in program committees of the International IEEE Symposium on Applications for Ferroelectrics (ISAF) and ISIF conferences, and of the European Meeting on Ferroelectricity (EMF). He was co-founder of the International Workshops on PiezoMEMS (IWPM) in 2010. He was an IEEE distinguished lecturer in 2017. In 2019, he was general chair of the joint meeting including ISAF, EMF, the International Conference on Electroceramics (ICE), IWPM, and the Workshop on piezo-AFM (PFM).