Scandium aluminum nitride for high frequency filters

Within a short time, our communication systems have evolved from one-way radio systems to complete mobile solutions. We can talk and listen at the same time, use GPS, Internet, mobile communications and Bluetooth at the same time. This also has its price: each communication requires its own frequency band, which must be filtered out of the frequency spectrum of the radio network at the user's end by a power-efficient and compact bandpass filter. Already today, up to 12 bandpass filters based on surface- or bulk-wave devices with piezoelectric excitation can be found in every smart phone.

The ability of piezoelectric materials to convert electrical charge into mechanical vibration and mechanical motion into electrical signals is used in electroacoustic filters. Due to its good compatibility with silicon technologies, high thermal stability and high switching speed, piezoelectric aluminum nitride (AlN) dominates the active layers in surface or bulk wave based frequency filters. However, the relatively low electromechanical coupling and low piezoelectric coefficients are limiting factors for future applications, at the high operating frequencies required in the future.

In order to develop the next generation of piezoelectric materials, the "COMET" project has been launched. It aims to determine the fundamental physical properties of ScAlN such as the tensors of elastic, piezoelectric and dielectric coefficients for the design of power efficient bandpass filters for high operating frequencies from 2 to 6 GHz.

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