Investigation on the Impact of Scandium-doping on the kt2 of ScxAl1-xN Cross-sectional Lamé Mode Resonators

Abstract

This paper reports on the impact of Scandium doping on Microelectromechanical (MEM) Aluminum Nitride (AlN) Cross-sectional Lame Mode Resonators (CLMR) electromechanical coupling coefficient (k t 2). Three different device configurations exploiting both Thickness Field Excitation (TFE) and Lateral Field Excitation (LFE) are investigated with respect to Sc-doping levels (0, 20, and 40%) and electrodes material. Tungsten (W), Platinum (Pt), Aluminum (Al), and Molybdenum (Mo) are selected as benchmark materials for the study. The impact of doping is investigated as a function of the ratio between the piezoelectric film thickness (h) normalized to the horizontal acoustic wavelength (⋋). Similarly, for each CLMR configuration and metal combination the electrode thickness is numerically optimized to provide the largest attainable k t 2. Since the sputtering of high-quality Sc-doped AlN (ScAlN) is achievable only on Pt or Mo seed layers, and metals with lower electrical resistivity are often implemented to reduce the impact of ohmic loading on the quality factor, a joint optimization of bottom Mo and top Al electrodes is also reported and compared to a non-optimized case.