Stress control in AlN and Mo films for electro-acoustic devices

Piezoelectric AlN films with strong (002) crystal orientation are widely used in various resonator-based applications such as BAW and FBAR filters, oscillators and resonating sensors. Low intrinsic stress is one of the important requirements for the thin film stacks employed in electro-acoustic devices. In this paper, we describe technical and technological solutions enabling effective stress control in piezoelectric AlN films deposited with an ac (40 kHz) reactive sputtering process by a dual cathode S-Gun magnetron, and in Mo electrodes deposited by a DC powered S-Gun. In the AC powered S-Gun, a special stress adjustment unit reduces compressive stress in the AlN films by controllably suppressing the flux of charged particles to the substrate by means of discharge current redistribution between the targets and the internal shields of the magnetron. Tensile stress in the AlN and Mo films is effectively reduced by performing the thin film depositions with added rf substrate bias. To avoid texture deterioration in the Mo films deposited with RF biasing, a two-step deposition process has been developed ensuring formation of superior crystal orientation as well as near-zero or, if required, compressive stress in the Mo films. Formation of a well-textured Mo bottom electrode enables growth of highly c-axis oriented AlN films.