Growth study of AlN on amorphous films with defined roughness

(001)-textured AlN thin films as needed for longitudinal bulk acoustic wave (BAW) devices exhibit large mechanical stress variations and large orientation variation as a function of growth substrate properties. We prepared thin silicon and silicon dioxide layers by sputter deposition to increase the roughness of thermal oxide films. The amorphous silicon films exhibited an rms roughness of 0.1 to 1.1 nm and the amorphous SiO₂ an rms roughness ranging from 0.2 to 3.8 nm. In the following we studied stress, orientation, and rocking curve width of AlN films grown on these substrates. Mechanical stress varied from -700 to + 350 MPa and X-ray rocking curve width of AlN increased from 1.3 to 4.5 deg with increasing roughness. The effect of substrate RF bias during AlN deposition was studied as well. The piezoelectric d_33,f coefficient varied together with the rocking curve width and the mechanical stress. Optimal conditions so far identified yielded a d_33,f of 4.2 pm/V in case of a 1.2 mum piezoelectric AlN film grown on a amorphous sputtered SiO₂ layer.