The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films

Aluminium nitride (AlN) reactively sputter-deposited from an aluminium target is an interesting piezoelectric thin film material with high CMOS compatibility. A good c-axis orientation is essential for obtaining high piezoelectric coefficients. Therefore, the influence of different sputtering conditions on the microstructure of AlN thin films with a typical thickness of about 500 nm was investigated. In this study it is demonstrated that highly c-axis oriented AlN thin films can be deposited on nominally unheated (1 0 0) silicon substrates, most preferentially when using a pure nitrogen atmosphere. The degree of c-axis orientation increases with higher nitrogen concentration and with decreasing the sputtering pressure, whereas the influence of plasma power on the microstructure was found to be negligible. A low sputtering pressure is also useful for minimizing the amount of oxygen contaminations in the deposition chamber and hence for reducing the incorporation of impurities into the AlN films. Intrinsic stress values of AlN thin films were determined by wafer bow measurements and were found to be between −3.5 and 750 MPa depending on choice of deposition parameters. Finally, the piezoelectric coefficients d33 and d31 were determined experimentally by laser scanning vibrometry in conjunction with a theoretical model. Effective values in c-axis oriented 500 nm films with FWHM of 0.33° are 3.0 and −1.0 pm/V. For a film of 2.4 μm thickness, values of 5.0 and −1.8 pm/V were measured, which are near the bulk values.