| Due to its mechanical properties coupled with high superconducting transition temperature and chemical inertness, NbN is a candidate material for protective coating and diffusion barrier in microelectronic devices. So far, most of the work on NbN is related to its superconducting properties, little information is available on the mechanical properties of the NbN coatings. Moreover, the mechanisms on texture evolution and phase transition in NbN coatings have not yet been well understood. In addition, the investigations on multilayer coatings, in which NbN is used as one layer component, are mainly focused on CrN/NbN and TiN/NbN coatings, there have been very few reports about the influence of deposition parameter on the structure and mechanical properties for NbN/SiNx multilayers. Recent studies show that the hardness of Nb-Si-N nanocomposites can reach a high value of 53GPa. It is believed that NbN/SiNx multilayers may also exhibit the superhardness effect similar to Nb-Si-N nanocomposites and thus merits further investigations.In this thesis, the NbN films and NbN/SiNx multilayers have been prepared by reactive magnetron sputtering in discharge of a mixture of N2 and Ar gas. The influence of deposition parameters such as the N2/Ar flow ratio, pressure, substrate bias and layer thickness on the structural and mechanical properties for the NbN coatings is studied using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nanoindentation measurements. The elastic compliance tensors and surface energies of NbN coatings, based on the density functional theory (DFT) calculations, have been calculated. The influence of modulation period and modulation ratio on the phase, texture, interfacial structure and mechanical properties for fcc-NbN/SiNx multilayers and hcp-NbN/SiNx multilayers are also investigated. Furthermore, the thermal stability of NbN films and NbN/SiNx multilayer coatings is also investigated by means of annealing for 2 h at different temperatures (500, 700 and 900°C) in vacuum. The conclusions are summarized as follows:1. The effects of deposition parameters on phase, preferred orientation and mechanical properties for NbN coatings and their thermal stability:(1) Theoretical calculations for NbN coatings show that forδ-NbN phase, the calculated values of surface energies ofγ{200} andγ{111} are 1.68 and 2.92 J/m2 respectively.δ′-NbN exhibits significantly higher hardness values compared to that of the cubic phase.(2) As N2/Ar is smaller than 20:60, onlyδ-NbN phase is found, However, when N2/Ar is increased to 20:60, a mixture ofδ′-NbN with a (100) orientation andδ-NbN with a (200) orientation appears. The highest hardness value of 32GPa is obtained at N2/Ar = 60:60.(3) As the pressure increases from 0.4 to 2.0Pa, onlyδ-NbN phase is found, andδ-NbN texture is (111) at low pressure and becomes (200) with increasing pressure. The hardness and modulus of NbN coatings decrease gradually as the pressure increases from 0.4 to 2.0Pa, which may be due to the observed texture evolution and stress release.(4) The residual stress in the film is tensile as Vb is at voltage floating, while it becomes compressive as the substrate is negatively biased. The compressive stress increases with increasing the absolute value of Vb from 40 to 80 V as well as from 80 to 200 V. It has been observed that a phase transition fromδ- toδ′-NbN occurs as the absolute value of Vb increases from 80 to 120 V, whose driving force is provided by strain energy minimization. The preferred orientation inδ- orδ′-NbN phase is dependent on the competition between the strain and surface energy.(5) NbN coatings exhibit an alternating texture with increasing layer thickness. It is found that the thin layer favors the formation of ultra-fine grains in the coatings, while the thick layer promotes the formation of a typical columnar microstructure with larger diameters in the coatings.(6) Phase transition occurs for NbN coatings deposited at Vb=-120V and -160V after annealing at 500°C and 900°C, respectively, whileδ-NbN coatings deposited at Vb=-40V andδ′-NbN coatings deposited at Vb=-200V keep their single-phase up to 900°C. In addition, a sharp decrease in the hardness and modulus for NbN coatings can be observed after annealing at 900°C.2. Microstructure, mechanical properties and thermal stability for NbN/SiNx multilayers:(1) As the N2/Ar flow ratio increases from 5:60 to 30:60, only theδ-NbN phase is found. The XRR results show that with increasing the N2/Ar flow ratio the modulation periods of the multilayers decrease, which is due to the decrease in the film deposition rate. The hardness values of NbN/SiNx multilayers deposited at N2/Ar=5:60 and 10:60 are about 25 and 26GPa, respectively, which are higher than those calculated from the rule of mixture. The observed hardness enhancement for NbN/SiNx multilayers can be explained by the modulus difference mechanism proposed by Koehler.(2) For fcc-NbN/SiNx multilayers, the texture changes fromδ?NbN (200) toδ?NbN (111) and the layer structure becomes more apparent with decreasing modulation ratio (lNbN/lSi3N4). No significant influence of modulation period on the texture and interface structure can be found. The hardness enhancement is observed only when SiNx layer thickness is smaller than 0.6nm.(3) For hcp-NbN/SiNx multilayers, no hardness enhancement is observed. The interface structure becomes more apparent with decreasing modulation ratio (lNbN/lSi3N4 ).(4) The thick SiNx layer favors the improvement of thermal stability for NbN/ SiNx multilayer coatings. It is found that for NbN/SiNx multilayer coatings with the SiNx layer thickness of 1.3nm the apparent interface structure and high hardness are still maintained after annealing at 900°C.
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