Growth of AlN oriented films on insulating substrates

This work describes the structural and piezoelectric assessment of aluminum nitride (AlN) thin films deposited by pulsed-DC reactive sputtering on insulating substrates. We investigate the effect of different insulating seed layers on AlN properties (crystallinity, residual stress and piezoelectric activity). The seed layers investigated, silicon nitride (Si₃N₄), silicon dioxide (SiO₂), amorphous tantalum oxide (Ta₂O₅), and amorphous or nano-crystalline titanium oxide (TiO₂) are deposited on glass plates to a thickness lower than 100 nm. Before AlN films deposition, their surface is pre-treated with a soft ionic cleaning, either with argon or nitrogen ions. Only AlN films grown of TiO₂ seed layers exhibit a significant piezoelectric activity to be used in acoustic device applications. Pure c-axis oriented films, with FWHM of rocking curve of 6°, stress below 500 MPa, and electromechanical coupling factors measured in SAW devices of 1.25% are obtained. The best AlN films are achieved on amorphous TiO₂ seed layers deposited at high target power and low sputtering pressure. On the other hand, AlN films deposited on Si₃N₄, SiO₂ and TaO_x exhibit a mixed orientation, high stress and very low piezoelectric activity, which invalidate their use in acoustic devices.