Deposition and characterization of aluminum nitride thin films for piezoelectric MEMS

Piezoelectric actuation and sensing would improve many sensors based on microelectromechanical systems (MEMS), such as gyroscopes used in inertial measurement units. Inertial MEMS sensors, especially gyroscopes, require in-plane actuation and sensing, in addition to out-of-plane actuation and sensing in order to measure movement in all three directions. Effective piezoelectric implementation requires the deposition of high crystal quality material on the vertical sidewalls of MEMS structures, which can then move in the in-plane direction. Currently used methods for thin film deposition and processing do not have adequate conformal coverage on vertical sidewalls. Previous research on the deposition of piezoelectric aluminum nitride (AlN) has focused on using AlN in applications, such as resonators, where conformal coverage is not needed, using line-of-sight physical vapor deposition (PVD), which generally has poor conformal coverage and results in tilted crystallites when deposited on vertical surfaces. Chemical vapor deposition (CVD) on the other hand, should result in better conformal coverage on the vertical sidewalls of the three-dimensional structures needed for optimum in-plane actuation and sensing, and metalorganic CVD (MOCVD) and atomic layer deposition (ALD) were used to deposit AlN thin films on vertical sidewalls. The conformal coverage of the MOCVD and ALD processes were studied by growing AlN thin films on patterned silicon substrates, and the crystal quality and microstructure of the films were studied using a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). Furthermore, the mechanical properties, reliability, and microstructural stability of AlN and Sc-alloyed AlN (AlScN) thin films were studied. The purpose of this dissertation was to the find a suitable method and process parameters for the deposition of high crystal-quality piezoelectric AlN on vertical sidewalls that can then be used in the fabrication of piezoelectric inertial MEMS sensors and enable the use of AlN and AlScN in new applications by studying their mechanical reliability and microstructural stability.