Re-growth of c-axis oriented AlN thin films

Re-growth of AlN films deposited on a first set of differently treated AlN films has been studied. All films were deposited by DC pulsed sputtering at 300 /spl deg/C. The first layer consisted of smooth, pure c-axis oriented AlN mono-layers, with a narrow rocking curve width and excellent piezoelectric coefficient, grown on a platinized substrate. Prior to the re-growth of a 1000 nm AlN layer, a number of conditioning steps have been performed on the surface of the first layer in order to simulate exposure to fabrication conditions. The use of a strongly alkaline developer during intermediate lithography steps and the time elapsed between the two steps of AlN re-growth were found to degrade the overall quality of the films. It has been shown that the alkaline developer etches down, increases the roughness and contaminates the surface of the AlN monolayer with mainly oxides and hydroxides, while the exposure of the films to air develops a native oxide layer. Low-level oxygen contamination was found to invert the polar direction, thus reducing very much the piezoelectric coefficient of the total layer. RF plasma cleaning prior to the regrowth process helps to recover a good piezoelectric coefficient.