Thermal Conductivity of Al1-xScxN for 5G RF MEMS Filters

Abstract

Abstract Body: Global fifth-generation (5G) coverage has the potential to underpin the internet-of-things (IoT) ecosystem. Radio frequency (RF) microelectromechanical system (MEMS) filters based on Al1-xScxN are replacing AlN-based devices because of the higher achievable bandwidths. [1] However, overheating of Al1-xScxN film bulk acoustic wave resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone’s ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. Despite this, the factors limiting thermal transport within Al1-xScxN have not been examined. This work investigated the physics that govern the thermal transport within Al1-xScxN, a fundamental building block for 5G RF MEMS. The thermal conductivities of c-axis textured Al1-xScxN films were found to be one order of magnitude lower than similarly textured polycrystalline AlN films and two orders of magnitude lower than single crystal and/or bulk AlN. This abrupt reduction of thermal conductivity with incorporation of Sc atoms into the AlN crystal can be understood in terms of phonon-alloy/disorder scattering, in the context of the phonon gas theory. Increasing the Sc composition results in a further decrease in the thermal conductivity due to structural frustration and lattice softening, which is an effect absent in isomorphs such as Al1-xGaxN. [2] A relatively strong film thickness dependence of the thermal conductivity was observed for the Al1-xScxN films. However, thermal conductivity measurement at varying ambient temperatures exhibited a weak temperature dependence. The impact of abnormally oriented grains on the cross-plane thermal conductivity was found to be negligible for the piezoelectrically functional Al1-xScxN films tested in this work. Outcomes of this work support the electro-thermo-mechanical co-design of 5G Al1-xScxN-based RF acoustic filters. From a thermal standpoint, for Al1-xScxN-based bulk acoustic wave (BAW) filters, the solidly mounted resonator (SAW) configuration would be preferred over the FBAR (free-standing membrane) configuration due to the poor thermal conductivity of Al1-xScxN. The thermal property data set generated in this work reveals design trade-offs for (i) increasing the Sc composition of Al1-xScxN to maximize the electromechanical coupling factor, (ii) decreasing the film thickness to achieve higher GHz-range resonance frequencies, (iii) higher operating temperatures resulting from higher integration density and RF input powers. The thermal conductivity data will allow construction of multi-physics device models that will enable the design and development of Al1-xScxN RF filter technologies with enhanced device performance and improved lifetime. Acknowledgment: This material is based upon work supported by the National Science Foundation, as part of the Center for Dielectrics and Piezoelectrics under Grant Nos. IIP-1361571, IIP-1361503, IIP-1841453, and IIP-1841466. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. G.E. would like to gratefully thank Sara Dickens and Joseph Michael at Sandia for conducting the EBSD analysis. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.