Growth, Integration, and Functional Characterization of AlScN Thin Films for RF MEMS Devices

Mobile communication has grown explosively in recent years, leading to a strong demand for cheap, but high-performing bandpass filters with low insertion loss and smaller size. Surface acoustic wave (SAW) and thin film bulk acoustic wave (BAW) resonators realized with piezoelectric materials are currently the only ones used in radio frequency (RF) passband filters for mobile communication in the LTE bands situated in the range of 0.3 to 3 GHz. SAW devices are more suitable for the lower frequency range, while BAW devices are more suited for the higher frequency range. It is of high interest to dispose of a microresonator type that would combine the advantages of these two technologies, and that would cover the complete frequency range. The lowest order symmetric Lamb wave mode (S0) propagating in a thin film structure appears to be a good solution for this challenge. The frequency is mostly defined by the periodicity of an interdigitated electrode system, and to a lesser extent by the thickness of the piezoelectric thin film. Realizations with AlN thin films were studied since 2002. Advantageous properties of such devices were demonstrated. However, only moderate electromechanical coupling factors (k^2) of around 1%, combined with too low quality factors (Q) were achieved. The so-called figure of merit (FoM) for filter applications, Qk^2, turned out to be much too low and never achieved more than 8. Recently, it was shown that the piezoelectric coefficients of the wurtzite structure AlN can be strongly increased by partial substitution of Al by Sc. In this work, we investigated Lamb wave resonators (LWR) by theory and experiment using thin films of AlxSc1-xN with Sc concentrations of x = 0.15, and 0.3 for application frequencies around 1.4 GHz. The device design contained a floating bottom electrode and an interdigitated top electrode. Free standing (free LWR) and solidly mounted (SM-LWR) LWRs were studied. The latter were grown on an acoustic Bragg mirror containing 5 alternating layers of SiO2 and W. The free LWRs based on 15% AlScN thin films showed k^2 values of typically 2.5%, which clearly shows the enhanced piezoelectric response of Sc doped AlN. However, the moderate quality factor in the range of 200 to 400 resulted in a FoM that again did not exceed 8. However, the SM-LWR has lead to a significantly enhanced Q factor, while k^2 was almost unchanged. With a Q of 1300, and a k^2 of 2.3% , a FoM of 30 was reached. We could further improve the results by increasing the Sc content to 30%. An impressive FoM of 70 (Q=1350, k^2=5.2%) was achieved with the SMR-LWR. Using AlN instead of W in the Bragg reflector has resulted in a FoM of 59 (Q=1300, k^2=4.6%). These FoM values are by far the best ones ever achieved in MEMS-type LWRs. The design of LWRs requires a full set of elastic, dielectric, and piezoelectric material constants. Shape resonators vibrating at well-defined modes constitute the most precise tools for their derivation. We designed and fabricated dual mode BAW resonators for the extraction of longitudinal constants (c33E , e33) and shear mode constants (c44E , e15). The AlScN thin film grew in a slightly tilted c-axis texture during the first 200 nm, as planned. But then, massive secondary nucleation lead to a film dominated by abnormally oriented grains. Combining TEM nano diffraction mapping and finite element modelling, we nevertheless managed to derive the targeted material constants.