MEAM Ph.D. Thesis Defense: “Aluminum Scandium Nitride (AlScN) Films for Microelectromechanical Systems”

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}*C. 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 and cause undesired deformations in released MEMS structures. 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. AlScN theoretical material models do not capture these and other fabrication related effects and often fail to accurately predict the electromechanical properties of AlScN thin films.

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 over a range of scandium alloying levels to yield high quality AlScN films with large piezoelectric coefficients. The physical mechanism behind stress gradients in AlScN thin films and methods mitigate stress gradients 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 to extract the AlScN stiffness, piezoelectric, and permittivity tenors from MEMS structures carefully designed to isolate various electromechanical parameters. A suite of device structures capable of measuring the key electromechanical properties of AlScN materials as a function of Sc alloying are demonstrated. Extraction of electromechanical properties is performed by matching solutions using COMSOL Multiphysics Finite Element Solver with the measurements obtained from the suite of device structures. This presentation provides physical vapor deposition parameters to deposit low stress, stress gradient free, and AOG free AlScN films, fabrication processes to develop AlScN devices, as well as electromechanical material models with complete material parameters to predict device performance. These are crucial innovations enabling the realization of devices in Al_1−xSc_xN thin films.

{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.