Niobium nitride based thin films deposited by DC reactive magnetron sputtering: NbN, NbSiN and NbAIN
Due to their high hardness, high melting point and high chemical stability, transition metal nitrides present a great interest for various applications. This work constitutes a contribution to the understanding of the properties of Nb based binary and ternary nitride materials. It deals with the study of the deposition and characterization of the niobium nitride system. Single and mixed phase thin films of niobium nitride in addition to niobium silicon nitride and niobium aluminum nitride were deposited by DC reactive magnetron sputtering. The properties of these thin films are investigated using several experimental techniques: X-ray diffraction, scanning and transmission electronic microscopy, optical reflectivity and spectroscopic ellipsometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electrical measurements and nanoindentation. The influence of the nitrogen partial pressure and substrate temperature on the phase composition is studied. Single and mixed phase of niobium nitride films: β-Nb2N, δ-NbN and δ'-NbN were successfully deposited. The single phase niobium nitride films are characterized. Properties of mixed phase films are interpreted in the light of that of single phase. All NbN thin films have a columnar morphology. The columnar structure in the hexagonal phases is more pronounced than in the cubic. The hexagonal δ' and β phases are more covalent than the cubic one. The physical parameters (carrier charge density N* and free electron relaxation time τ) for each single phase were calculated by fitting the optical properties using a Drude model with a set of Lorentz oscillators. High hardness values of 35 and 40 GPa are measured for the β and δ' phases, respectively. They are larger than that of the cubic δ phase, 25 GPa. This hardness values is related to the high covalent character of the hexagonal phases compared to that of the cubic. Hardness of mixed phase is determined by the hardness of the majority phase. Influence of the addition of a third element, Si and Al, into NbN is studied. Nb-Si-N and Nb-Al-N thin films were deposited and characterized. A model for the film formation of Nb-Si-N thin films deposited by DC magnetron sputtering is proposed. Three distinct concentration domains were pointed out. In Domain 1 (1 ≤ CSi ≤ 4 at.%) the Si atoms substitute Nb in the NbN lattice and polycrystalline films of NbN:Si are deposited. In Domain 2 (4 ≤ CSi ≤ 7 at.%) a fraction of Si atoms segregates to the grain boundaries. A SiNx layer forms on the NbN:Si crystallite surfaces. The covering ratios increase with Si content up to 100% (formation of a monolayer). For further increase of Si content (Domain 3), the NbN:Si crystallites, surrounded by a monolayer of SiNx, reduce their size from 18 to 2 nm. The increasing amount of the SiNx phase in the films is realized by increasing the surface to volume ratio of the NbN:Si nanocrystallites. The formation of the SiNx layer explains the change observed in the electrical and optical properties of Nb-Si-N films with increasing the Si content. The electrical resistivity measured as a function of temperature is proposed to provide an experimental mean for determining the limit of Si solubility in the Nb-Si-N system and for following the thickness evolution of the SiNx coverage layer in the composite films. For NbzAlyNx films, the solubility limit of the Al in the NbN lattice is in the range: y/(y+z) = 0.5±0.1. Passing this value an insulating hexagonal AlNx phase is formed. The electronic properties of the NbzAlyNx are significantly altered by the changes in the value of y and x. The resistivity increases with increasing y. The hardness of NbzAlyNx is maximum in films with y = 0.19 (solubility limit). At higher y the formation of the AlNx phase reduces the hardness. The increase of hardness observed in the NbzAlyNx films is attributed to the solid solution hardening mechanism. If the system NbN presents many phases, it appears that the addition of a third element, Si and Al, allows the stabilization of the cubic δ phase. In both Nb-Si-N and Nb-Al-N systems Si and Al are soluble up to a certain limit. Passing this limit the third element segregates at the grain boundaries forming SiNx and hexagonal AlNx. If in the case of Nb-Si-N, the formed SiNx cannot be detected by X-ray diffraction due to the fact that it segregates as a thin layer at the grain boundaries of the NbN grains. In the case of the Nb-Al-N the hexagonal AlNx phase is clearly detected. The high Al solubility in the Nb-Al-N gives rise to a large change of the electronic properties with increasing the Al content.