The ever-growing mobile data transfers in the wake of 5G require the use of more and higher frequency ranges, all of which need to be accommodated within a single mobile device. Thus, the demands on radio frequency components are constantly increasing. The Fraunhofer Institute for Applied Solid State Physics IAF has developed novel, compact radio frequency filters to meet those needs.
Processed surface acoustic wave structures on silicon (left) and sapphire (right).
Fraunhofer IAF
The amount of radio frequency (RF) components built into a single smartphone has increased significantly over the past years and there is no end in sight. Predicting this trend in 2015, the project »PiTrans – Development of aluminum scandium nitride (AlScN) layers for the next generation piezoelectric RF filters« set out to develop and produce improved RF piezo-transducers with ternary AlN-based nitrides as the piezo-active layer. Within the five years of the project, the researchers succeeded in growing highly crystalline AlScN layers and realizing surface acoustic wave (SAW) resonators that meet the increasing requirements of the industry. For the growth of the material, which is also promising for other power electronic applications, a modern magnetron-sputtering infrastructure was established at Fraunhofer IAF. The project was successfully completed in January 2020.
To this day, AlScN remains the most promising new material to replace conventional aluminum nitride (AlN) in RF filter applications inside mobile phones. By introducing scandium (Sc) into AlN, the electromechanical coupling and piezoelectric coefficient of the material is increased, enabling a more efficient mechanical to electric energy conversion. This allows the production of much more efficient RF devices. However, the instability of the piezoelectric AlScN crystal phase has so far been a problem for industrial use of the material, as segregation of wurtzite-type AlN and cubic ScN usually occurs during growth.
In the course of the project, the scientists at Fraunhofer IAF managed to grow highly crystalline AlScN layers with a wide range of compositions up to a Sc content of 41 %. A good homogeneity of the layers was achieved across the entire silicon (Si) wafer up to 200 mm in diameter, which meets the requirements of industrial productions. Besides these industry-relevant results, the project team also succeeded in realizing an epitaxial growth on lattice-matched sapphire (Al2O3) substrates through a special magnetron sputter epitaxy (MSE) method of deposition, which will be useful for future material research.
“We see AlScN as a very promising candidate for future applications that capitalize on the piezoelectric effect, such as sensor technologies and high electron mobility transistors,” explains Dr. Žukauskaitė. The success of the project led to the acquisition of two further projects involving AlScN technology.