Stress control in reactively sputtered AlN and TiN films

Al and Ti planar magnetron targets have been sputtered in Ar/N2 mixtures to deposit AlN and TiN films on unheated substrates; rf and dc discharges were used for the AlN and TiN deposition, respectively. The N2 flow was always sufficient to produce a nitrided target layer and x-ray diffraction showed single-phase films of AlN and TiN1.2. In N2 discharges, the AlN film stress varied from −19 GPa (compressive) to +2.5 GPa (tensile) as the pressure increased from 0.2 to 5 Pa. Intrinsic tensile stress in reactively sputtered compound films has not previously been reported. A similar change was observed in the stress of TiN films. Zero-stress AlN and TiN films were obtained at ∼1 Pa for the sputtering conditions used. The rapid decrease in compressive stress with increasing pressure is associated with a rapid decrease in both target voltage (for constant applied power) and deposition rate. Calorimetry measurements of the power flux at the substrates show an increase with pressure. With increasing Ar content, at a constant pressure, the compressive stress decreases although the target voltage and deposition rate increase; the substrate power flux decreases. The surface coverage of N on the targets increases with increasing N2 pressure because the N+2 flux increases. The compressive stress is probably due to bombardment of the film by neutral N atoms reflected with high energy from the targets. The decrease in deposition rates and increase in substrate power density may result from increased sputtering rates of N atoms from the target with a consequent decrease in metal atom sputtering.