Atomic layer deposition of AlN using atomic layer annealing—Towards high-quality AlN on vertical sidewalls

Atomic layer deposition (ALD) of aluminum nitride (AlN) using in situ atomic layer annealing (ALA) is studied for microelectromechanical systems (MEMS). Effective piezoelectric in-plane actuation and sensing requires deposition of high crystal quality and (0002) oriented AlN on vertical sidewalls of MEMS structures. Previous studies have shown that the crystal quality of ALD AlN can be significantly improved using ALA but have not studied the conformal coverage or crystal quality on metal electrodes, which are required for piezoelectric MEMS devices. In this study, AlN thin films are deposited on Si, Al, Pt, and on vertical sidewalls etched into Si. The AlN microstructure and properties are studied using x-ray diffraction methods, transmission electron microscopy, and Fourier transform infrared spectroscopy. The conformal coverage is evaluated by measuring the film thickness on the vertical sidewalls. The effects of postdeposition annealing are studied as well. This study aims to enable effective piezoelectric actuation and sensing for MEMS sensors. The conformal coverage of the ALA ALD process is excellent and AlN has the best crystal quality and degree of orientation when deposited on Al. The as-deposited films contain oxygen impurities, which might be detrimental to the piezoelectric properties of AlN. Annealing at high temperatures reduced the number of impurities but did not improve the crystal quality.