ScAlN for microacoustic devices

Within a short time, our communication systems have evolved from one-way radio systems to complete mobile solutions. We can speak and hear at once and use GPS, Internet, mobile radio and Bluetooth at the same time. Each communication requires its own frequency band, which has to be filtered out of the user's frequency spectrum by a power-efficient and compact bandpass filter. Today there are already up to 12 bandpass filters based on surface or bulk wave components using piezoelectric excitation in every smartphone.

Mechanical motion into electrical signals is used in electroacoustic filters. Due to its good compatibility with silicon technologies, high thermal stability and high sound velocity, piezoelectric aluminum nitride (AlN) dominates the active layers in surface or volume wave based frequency filters. However, the relatively low electromechanical coupling and the low piezoelectric coefficients are limiting factors for future applications at the high operating frequencies required in the future.

The decisive motivation for this project is a recently published Japanese publication which shows that ScAlN with scandium concentrations of up to 40 % has significantly higher piezoelectric coefficients and a higher electromechanical coupling compared to AlN. For this reason, manufacturers of bulk wave and surface wave devices have a great interest in replacing AlN with the ScAlN developed in "COMET" in the next generation of high frequency filter applications.

Aluminium finger contacts (cutout) of a surface acoustic wave device