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Al$_{1-x}$Sc$_{x}$N is an attractive material for radio frequency microelectromechanical systems (RF-MEMS) due to higher piezoelectric coefficient d$_{33}$=27.6 pC/N (x=0.43) compared to 6 pC/N in pure AlN [1] and increased electromechanical coupling k$_{t}$$^{2}$ [2]. Mechanical properties such as elastic modulus and coefficient of thermal expansion (CTE) are important for designing RF-MEMS. However, there are very few experimental or theoretical studies of elastic modulus of Al$_{1-x}$Sc$_{x}$N in a large range of compositions (up to x=0.26) [3] and, the CTE of Al$_{1-x}$Sc$_{x}$N thin films has never been reported until now. In this work, reactive pulsed-DC magnetron sputtering process was optimized [4] to produce 1 µm thick highly c-axis oriented Al$_{1-x}$Sc$_{x}$N thin films (up to x=0.32) on 100 mm Si(001) and Al$_{2}$O$_{3}$(0001) substrates. X-ray diffraction, scanning electron microscopy, piezoresponse force microscopy, and Berlincourt method were used to analyze the film properties. To simultaneously determine the thermal expansion coefficients and the elastic modulus, a thermal cycling was performed [5] and the temperature dependent film stress was then measured. Based on the stress measurement results, CTE was calculated as a function of Sc concentration. Our measurements show average CTE αf= 5.01×10$^{-6}$/K, biaxial elastic modulus of 300 GPa, and Young’s modulus of 216 GPa for Al$_{0.7}$Sc=$_{0.3}$N. The average CTE and elastic modulus measured for AlN fits values found in literature [5]. Consequently, the experimentally determined elastic modulus will allow designing RF-MEMS based on Al$_{1-x}$Sc$_{x}$N with various Sc concentrations and the CTE will enable the device performance prediction at elevated temperatures.
[1] M. Akiyama, et al., Adv. Mater. 21(5), 593 (2009)
[2] G. Wingqvist, et al., Appl. Phys. Lett., 97(11), 112902 (2010)
[3] M. A. Caro, et al., J. Phys. Condens. Matter 27, 245901 (2015)
[4] Y. Lu, et al., Phys. Status Solidi A, 1700559 (2017)
[5] R.E. Sah, et al., J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 28, 394 (2010).
