Structural, optical and mechanical properties of aluminium nitride films prepared by reactive DC magnetron sputtering

Aluminium nitride (AlN) is a wide band gap III–V semiconductor material which is often used for optical applications. We have deposited aluminium nitride films by reactive DC magnetron sputtering in an Ar–N2 atmosphere on Si (100) and glass substrates. The total pressure was kept constant at 0.8 Pa. For thin film preparation the N2 flow was varied from 0 to 8 sccm, while the Ar flow has been adjusted to maintain a constant total pressure. The corresponding films have been characterized by a variety of techniques including Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD), X-ray reflectometry (XRR), optical spectroscopy, spectroscopic ellipsometry and wafer curvature measurements to determine the deposition stress in the films. The stoichiometry of the films has been determined by RBS, which shows that stoichiometric AlN can be prepared for N2 flows above 4.75 sccm. The structure of the films has been determined by XRD, which shows that crystalline aluminium nitride can be formed above 4.75 sccm N2. XRR was used to determine the thickness, density and surface roughness of the films. The sputter rate decreases upon increasing N2 flow, while at the same time the density of the films increases from 2.7 g/cm3 for metallic films to 3.1 g/cm3 for the stoichiometric and transparent films. Optical spectroscopy and spectroscopic ellipsometry measurements determine that the stoichiometric AlN films prepared above 4.75 sccm possess a refractive index of n ≈ 1.9 at 533 nm (2.3 eV). Wafer curvature measurements reveal that stoichiometric AlN films are characterized by high tensile stresses.