Optimization of Deposition Process and Microscopic Characterization of Highly Oriented Aluminum Nitride Thin Films for Bimorph Structures of Piezoelectric Tunable Capacitors

Film deposition with RF sputtering to control residual stress in aluminum nitride (AlN) thin films has been investigated to fabricate the bimorph actuator for a piezoelectric tunable capacitor with low-voltage operation. The effects of conditions in sputtering and surface cleaning were studied both to obtain a preferable film orientation for piezoelectric actuation and to suppress structural deformation of the cantilever due to the residual stress in films. Microscopic analysis revealed that (0001)-oriented AlN and (111)-oriented Al films were epitaxially grown at each interface in the bimorph structure. The current–voltage (I–V) measurements showed a leakage current of less than 5 ×10-5 A m-2 in 500-nm thick AlN films up to 30 V. The effect of optimization of process parameters was demonstrated by the capacitance–voltage (C–V) characteristics of the folded-beam tunable capacitor, in which the suppression of residual stress in piezoelectric layers was shown to be effective to obtain low-voltage operation.