ALUMINUM SCANDIUM NITRIDE (AlScN) FILMS FOR MICROELECTROMECHANICAL SYSTEMS (MEMS)
Finite Element Modeling
Physical Vapor Deposition
This thesis demonstrates methods for modeling, fabrication, and characterization of Alu- minum Scandium Nitride (AlScN) films and resonator devices for applications in Micro- electromechanical systems (MEMs) especially in Fifth Generation (5G) cellular technology. Various internal stress arise during the process for developing AlScN films. Abnormal Ori- ented Grains (AOGs) are undesired grains that grow in the off c-axis in AlScN films resulting in reduced electromechanical coupling, reduce device performance, and produce variations in lithography steps. The following study provides an understanding of the stress develop- ment, AOGs formation, and extraction of piezoelectric parameters in Aluminum Scandium Nitride (AlScN) thin films. Deposition of AlScN films grown directly on Silicon at high Sc alloying levels, with controlled stress, and free of anomalous grains (AOGs) is demonstrated using reactive co-sputtering. The stress and AOG formation consider the Sc alloying range between 20.3 and 36.6 atomic % and for film thicknesses from 400 nm to 1000 nm. The combination of a gradient seed layer and proper process gas mixture is shown to inhibit formation of AOGs in AlScN with up to 36% Sc alloying even when grown directly on Si. It is demonstrated that the total flow can be utilized to control the film stress across the 20-36% alloying range while the process gas mixture primarily impacts the density of AOG formation in the AlScN films. Studies on grain size and film thickness provide insight on the origins of AOGs and stress in these films. Bulk acoustic wave resonators fabricated from 400 nm and 600 nm thick low stress and AOG free Al0.68Sc0.32N films grown directly on Si demonstrate high frequency operation of 3.6 and 4.8 GHz, high electromechanical coupling > 23%, and quality factors in excess of 500.Thin film through-thickness stress gradients produce out-of-plane bending in released micro-electromechanical systems (MEMS) structures. This thesis reports on the effects of stress and stress gradient of Al0.68Sc0.32N thin films deposited directly on Si. Demonstrations of Al0.68Sc0.32N cantilever structures realized in films with low average film stress have sig- nificant out-of-plane bending when the Al1−xScxN material is deposited under constant sputtering conditions. This thesis establishes a method where the total process gas flow is varied during the deposition to compensate for the native through-thickness stress gradient in sputtered Al1−xScxN thin films. This method is utilized to reduce the out-of-plane bend- ing of 200 μm long, 500 nm thick Al0.68Sc0.32N MEMS cantilevers from greater than 128 μm to less than 3 μm. A suite of device structures capable of measuring the key electromechanical properties of AlScN material are demonstrated. Extraction of electromechanical properties is performed by matching solutions using COMSOL Multiphysics Finite Element Solver with the mea- surements obtained from the suite of device structures. This thesis 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 mod- els with complete material parameters to predict device performance. These are crucial innovations enabling the realization of devices in Al1−xScxN thin films.