MEAM Seminar: “Aluminum Scandium Nitride Films for Microelectromechanical Systems Resonator’s Next-Generation Cellular Technology”

Advancements in wireless and sensor technology have resulted in increasing demand for piezoelectric MEMS devices in wireless handsets and base stations, automobile technology, health monitoring sensors, and environmental sensors. Due to its high-quality factor (Q), Young’s modulus, sound velocity, and low mechanical and dielectric losses, AlN is an advantageous resonator material with applications in radio frequency filtering. AlN’s low piezoelectric coefficients, however, limit its application, especially when large electromechanical coupling is required such as wide bandwidth acoustic filters and energy harvesters. In 2009 it was discovered that alloying AlN with Scandium (Sc) increases its d33 piezoelectric coefficient by over 500% and maintains compatibility with post-CMOS integration when deposited below 〖400〗^∘{1}*. As process parameters and Sc levels are modified to enhance performance, this also impacts the built-in stress in the film. Undesired stress can degrade device performance in thin films. Undesired phases and anomalously oriented grains (AOGs) reduce piezoelectric coefficients, film orientation, and modify stress. AOGs also render etching of device structures problematic as the AOG regions etch at a much lower rate.

This presentation will discuss the origins of stress and anomalous grain formation in AlScN physical vapor deposition processes, establish methods to control the stress, and inhibit anomalous grain growth at a range of scandium alloying levels to yield high quality AlScN films with large piezoelectric coefficients. Methods for mitigation and understanding of the physical mechanism behind stress gradients in AlScN thin films to reduce out-of-plane deformations in released MEMS structures will also be discussed. I demonstrate a technique where the total process gas flow is varied during the deposition to compensate for the native through-thickness stress gradient in sputtered AlScN thin films. Finally, I will provide methods extract the AlScN stiffness, piezoelectric, and permittivity tenors from MEMS structures carefully designed to isolate various electromechanical parameters.

{1}*M. Akiyama, T. Kamohara, K. Kano, A. Teshigahara, Y. Takeuchi, and N. Kawahara. (2009). Enhancement of piezoelectric response in scandium aluminum nitride alloy thin films prepared by dual reactive cosputtering. Advanced Materials, 21(5), 593-596.