Structural and mechanical properties of titanium–aluminium–nitride films deposited by reactive close-field unbalanced magnetron sputtering

Titanium–aluminium–nitride (Ti1−xAlxN) films were deposited onto unheated silicon (100) substrates by reactive close-field unbalanced magnetron sputtering at a pulsed-bias voltage of −50 V in an Ar–N2 gas mixture. The effects of aluminium content (x) on structural and mechanical properties of these films have been studied. The films were analyzed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), cross-sectional scanning electron spectroscopy (SEM), atomic force microscopy (AFM) and nanoindentation measurements. By XPS the stoichiometric composition of TiN and AlN was found as forming a ternary phase of Ti–Al–N, while no unbound Al and Ti atomic species were detected in films. The Raman scattering spectra of the films within the whole range of x values were investigated. Four peaks at 242, 320, 510 and 640 cm−1, arisen from the transverse acoustic, longitudinal acoustic, second-order transverse acoustic and transverse optical modes of Ti–Al–N phase, respectively, were observed in Raman spectra. The XRD θ–2θ scans exhibited the structural changes in Ti1−xAlxN films with different Al contents. The films were essentially cubic B1-NaCl TiN with (111) oriented grains in the range of x=0 to 0.48. A two-phase structure consisting of B1-NaCl (TiN) and B4-wurtzite (AlN) was observed at x=0.57, while at higher Al contents, a single-phase structure of B4-wurtzite AlN with (0002) and (101̄1) grains was formed. The films at x=0.41 had hardness of approximately 31 GPa, Youngʹs modulus of 315 GPa, and an excellent plasticity (32% in indentation deformation) at loads exceeding their elastic limit. Nanoindentation measurements combined with AFM and cross-sectional SEM revealed that the improved mechanical properties of Ti1−xAlxN films with the addition of Al into TiN compound were attributed to their densified microstructure with development of fine grains and reduced surface roughness.