High Coupling Materials For Thin Film Bulk Acoustic Wave Resonators

Author : Anli Ding
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (124 download)

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Download or read book Surface Acoustic Wave Devices Based on C-plane and A-plane AlScN written by Anli Ding and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: The global mobile data volume is projected to grow five-fold by the end of 2025 and transmission rates must keep up. 5G standard promises higher rates by increasing relative bandwidths of the frequency bands used for mobile communications. The relative bandwidth of radio frequency (RF) filters is approximately half of the effective electromechanical coupling of their components, i.e. surface acoustic wave (SAW) and bulk acoustic wave (BAW) resonators. The traditional materials, e.g. aluminum nitride (AlN), are only suitable to a limited extent for the 5G RF filters. Compared to AlN, aluminum scandium nitride (Al1-xScxN with x 0.43) exhibits up to twice as high piezoelectric coefficient e33 and up to two times lower elastic coefficient C33, leading to higher electromechanical coupling, while retaining all the advantages of AlN. Consequently, it is considered a promising material for 5G RF filters.brbrThe potential of AlScN can be evaluated by fabricating SAW resonators based on this novel material and thoroughly investigating their performance. Furthermore, as the electromechanical coupling of SAW resonators based on, e.g. AlN or GaN, can be improved by rotating the "piezoelectric axis" (i.e. polar c-axis) from out-of-plane of the thin film (i.e. c-plane) to in-the-plane (i.e. a-plane), higher coupling can also be expected with a-plane AlScN as the active material. However, so far most AlScN investigations have focused on its c-plane orientation. The aim of this thesis, therefore, is to design and fabricate AlScN-based SAW resonators, investigate their performance with regards to Sc concentration, substrate, temperature, crystal orientation of the thin film (a-plane, c-plane) and SAW propagation direction as well as find their optimal design for high effective electromechanical coupling.brbrIn this work, SAW resonators with x = {0; 0.14; 0.23; 0.32; 0.41} and wavelengths from 2 to 24 μm were fabricated on sputter-deposited c-plane Al1-xScxN/Si(001), c-plane Al1-xScxN/Al2O3(0001) as well as a-plane Al1-xScxN/Al2O3(1-102) thin films and their performance systematically characterized. The resulting resonant frequencies lie in the range of 0.18 to 2.1 GHz. The coupling was found to increase with the amount of resonator fingers and coupling maxima were found according to aperture and the bus-bar length. Compared to the resonators based on AlN, the coupling improved by up to 500 % with x = 0.41. Such an increase was caused mostly by the mentioned evolution of Al1-xScxN material parameters e33 and C33 with x. The C33, in addition to C44 and C11, also influences the resonator frequency, which slightly decreased with x. To the best of the author's knowledge, such a systematic study of SAW resonators based on Al1-xScxN with x = [0; 0.41] has not yet been done. Moreover, only one study has been reported on the performance of SAW resonators based on Al1-xScxN with x so close to the mentioned threshold of 0.43.brbrIt is also the first time, that high quality a-plane AlScN thin films were grown. Up to 1000 % higher coupling was observed for a-plane Al0.77Sc0.23N compared to c-plane Al0.77Sc0.23N-based resonators. This could be caused by the alignment of the interdigitated transducer created electric field lines and the directions along which the piezoelectric effect takes place. Moreover, the coupling of a-plane AlScN-based resonators was observed to be highly influenced by their orientation. The maxima of coupling, at 1200 % higher than minima, were observed at ±30° away from the likely orientation of c-axis. This could be explained by an additional contribution of AlScN material parameters e15 and C44.brbrFurthermore, switching from c-plane to a-plane AlScN also leads to the change of resonator wavelength at which the maximum positive e33 and C33 influence on coupling is observed. The different position of coupling maxima with regards to the wavelength then enables to cover the frequency range from 1.14 to 1.71 GHz with coupling values in the range from 3.7 to 4.1 %. Moreover, this frequency range can be expanded at both ends and higher coupling can be obtained based on the observed tendencies. Even higher frequencies of 2.05 GHz and 2.87 GHz were reached with a coupling of ~2.8 % by employing the 2nd order SAW propagation modes on a-plane and c-plane Al0.68Sc0.32N-based resonators, respectively.brbrThe temporal and thermal stability of the fabricated resonators was investigated. The temperature coefficient of frequency (TCF) of 2 μm wavelength resonators was observed to increase non-linearly but only slightly (6.8 %) with x from 0 to 0.32. The experimentally obtained phase velocity and coupling dispersion curves were used for simulations of the resonators. The obtained results improve the understanding how the thermal behaviour of each material affects the resonator frequency. The design and simulation models of SAW resonators with complex layered structures can be strengthened. For the first time, the intrinsic SAW TCF of Al1-xScxN was determined and will assist in implementing the temperature compensation for the resonators. It is important for nowaday's smartphones with internal temperatures reaching 90 °C. The agreement of the obtained high coupling with state of the art, good thermal and temporal stability of the resonators signify the high robustness of in-house developed AlScN material growth and SAW resonator fabrication technology.br