R. Sanjinés

High-temperature oxidation resistance of Cr1−xAlxN thin films deposited by reactive magnetron sputtering

The thermal stability against oxidation of Cr1−xAlxN films with 0≤x≤0.63 has been investigated by isochronal (15 min) heating in air at various temperatures up to 1173 K. Cr1−xAlxN thin films were deposited by reactive magnetron sputtering from Cr and Al targets in a mixed Ar/N2 atmosphere at a substrate temperature of 573 K. All the films crystallize in the pseudo binary, rocksalt-type cubic structure, showing a (111) preferential orientation. Oxidation proceeds by de-nitridation and the formation of a pseudo binary, mixed, Cr/Al oxide with the corundum structure. The degree of film oxidation was evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Rutherford backscattering spectroscopy (RBS). The substitution of Cr atoms by Al atoms leads to two oxidation behaviors. Cr1−xAlxN films with low Al content (x 0.2) are more resistant to high temperatures compared to pure CrN. Films with the highest Al content (x=0.63) are stable up to 1173 K due to the formation of an amorphous, aluminum-rich oxide which blocks oxygen diffusion and prevents further film oxidation.

Sputter deposited chromium nitride based ternary compounds for hard coatings

Thin films of chromium based ternary transition metal nitrides Cr12xMexNy (CrMoN, CrTiN, CrWN and CrNbN) with 0 # x # 1 were deposited on silicon, glass and high speed steel substrates by reactive magnetron sputtering. The phase, texture and lattice parameter were determined by X-ray diffraction analysis. The surface morphology was examined by scanning tunneling microscopy. The chemical composition was measured by electron probe microanalysis. The cubic phase was the only phase observed for the ternary compounds. Thin films grown at 460 K substrate temperatures exhibit grain sizes of up to 25 nm. The core levels and the valence band were analyzed using X-ray photoelectron spectroscopy. Hardness values, obtained by nanoindentation, vary strongly with the dopant and the doping level. Similarities in changes of the mechanical properties and electronic structure indicate a strong correlation between these properties. q 1998 Elsevier Science S.A. All rights reserved.

Characterization of sputter-deposited chromium nitride thin films for hard coatings

Abstract CrN and Cr 2 N thin films were deposited on silicon, quartz, HSS-steel and carbon substrates by rf reactive magnetron sputtering. The phase and texture were determined by X-ray diffraction analysis. The chemical composition was measured by electron probe microanalysis. Atomic force microscopy revealed a finely grained morphology. The nitrogen content in the sputtering gas influences the film composition and morphology. At N 2 partial pressure below 20% (0.08 Pa) of the total pressure (0.67 Pa) the hexagonal Cr 2 N phase is present, while above 40%, the cubic CrN phase only is observed. Thin films grown at high substrate temperatures (T s ≥400 K) exhibit larger grain sizes of up to 20 nm. The real and imaginary parts of the dielectric function were determined by spectroscopic ellipsometry in the photon energy range of 1.5 to 5.0 eV. The core levels and the valence band were analyzed using X-ray photoelectron spectroscopy. The degree of ionicity of the Cr-N bonding increases continuously with the N 2 partial pressure promoting the CrN phase. Hardness values of 2950 HV for Cr 2 N films and 1800 HV for CrN films were obtained by microindentation,

Correlation between processing and properties of TiOxNy thin films sputter deposited by the reactive gas pulsing technique

Titanium oxynitride thin films were deposited by d.c. reactive magnetron sputtering from titanium metallic target and from oxygen and nitrogen as reactive gases. The nitrogen mass flow rate was maintained constant whereas that of the oxygen was pulsed during the deposition. A constant pulsing period was used and the introduction time of the oxygen was systematically changed from 0 to 100% of the period time. The reactive gas pulsing technique allowed to prepare TiOxNy films with various metalloid concentrations (0≤x≤2.0 and 0≤y≤1.0) and led to changes of the crystallographic structure from f.c.c. TiN to tetragonal TiO2. The variations of the metalloid content in the films result in changes in the electrical and optical properties and the reverse evolution of the oxygen and nitrogen content correlates with the transition from metallic to semiconducting behavior. The sputtering conditions were investigated taking into account the poisoning phenomena of the surface of the target from real time measurements of the target potential and from the reactive atmosphere, followed by mass spectrometry. Such diagnostics allowed to understand and to control better the advantageous role of the reactive gas pulsing technique and conducted to close relationships between the process parameters and the film properties.

Electronic states and mechanical properties in transition metal nitrides

Abstract Thin films of hard materials are of prime importance for wear-resistant, protective and decorative coatings. Besides adhesion, hardness is the most often quoted requirement, even if doubts remain on the experimental determination of the hardness values of thin films, on their theoretical interpretation and on their significance for wear protection. Transition metal interstitial compounds are extensively used because of their broad range of functional properties in the fields of machining, microelectronics, decoration, etc. This article presents a summary of recent relevant results on the structural, mechanical, electronic and optical properties of fcc TiN, VN, CrN, NbN, W2N, hexagonal MoN, and some ternary nitrides in the form of sputtered thin films. The process parameters, e.g. the reactive gas partial pressure and the substrate bias, strongly influence the film properties. The composition and growth parameters influence the morphology, the stress state and other physical properties. The systematic investigation of the electronic density of states in valence and core states of comparable nitrides provides indications of the degree of covalency in the chemical bonding in relation to properties such as cohesive energy and hardness. For example, in molybdenum nitride the low stability of the cubic MoN phase is related to an increase in the charge transfer of Mo d electrons to nitrogen with increasing stoichiometric ratio N/Mo. Ellipsometric measurements of the dielectric function interpreted in relation to details of the band structure measure the variation of the density of conduction electrons. Vacancies and interstitials remove or add a specific number of electrons at the Fermi level. This analysis allows one to differentiate the types of defect at various compositions in, for example, TiNY films, for which the reported hardness values spread over a wide range.

Electrical, optical and mechanical properties of sputtered CrNy and Cr1−xSixN1.02 thin films

We have investigated the electrical, optical and mechanical properties of CrN y and Cr 1-x Si x N 1.02 films as a function of N and Si contents near the fcc-CrN stoichiometric composition. Polycrystalline CrN, with 0.93<y<1.15 and Cr 1-x Si x N 1.02 with 0<x< 0.16 were deposited by reactive magnetron sputtering. Optical reflectivity and electrical resistivity measurements indicate that the electronic properties of CrN y and Cr 1-x Si x N 1.02 thin films are strongly dependent on their chemical composition. The main changes in the optical properties of CrN y and Cr 1-x Si x N 1.02 films as a function of N or Si content mainly occur below 1.5 eV. Substoichiometric CrN y films with 0.93<y<0.98 exhibit room temperature resistivity ρ RT values of (1.2-7)×10 -3 Ω cm, metallic behavior and an antiferromagnetic orthorhombic phase transition at approximately 260K. In contrast, overstoichiometric CrN y (1.05<y<1.15) and Cr 1-x Si x N 1.02 films exhibit ρ RT values of (1.2-4)×10 -2 Ω cm and negative temperature coefficients of resistivity. Finally, the hardness values of CrN y films depend little on the chemical composition but are influenced by the film morphology: the nanohardness values of (111) CrN y are typically 12-14 GPa while (002) CrN y exhibit nanohardness values of 18 GPa. The addition of small amounts of Si increases the hardness values up to 22 GPa for Cr 0.94 Si 0.06 N 1.02 .

Preparation and characterization of highly oriented, photoconducting WS2 thin films

A novel combination of methods is shown to produce semiconducting WS2 thin films with properties close to those of a single crystal. The first step requires the deposition of a very thin Ni layer on a quartz substrate. On top of it an amorphous, sulphur rich, (WS3 +x) thin film is deposited by reactive rf sputtering. The final annealing step in an argon atmosphere yields 200 nm thick WS2 films. X-ray diffraction shows that the films crystallize in the 2H-WS2 phase and are perfectly oriented with the (002) basal planes parallel to the substrate. Residual W18O49 needles andβ-NiS grains are detected by transmission electron microscopy. The dc conductivity and its activation energy have values typical of bulk crystals. The optical absorption spectrum measured at Room Temperature (RT) shows excitonic peaks at the same energies as in a single crystal. RT photoconductivity measured as a function of wavelength is shown to result from interband transitions.

Hexagonal nitride coatings: electronic and mechanical properties of V2N, Cr2N and δ-MoN

The electronic structure of hexagonal V 2 N, Cr 2 N and δ-MoN thin film nitrides, deposited by rf magnetron sputtering, have been investigated by X-ray photoemission spectroscopy. The binding energy values and the shape of the core level peaks are representative of the chemical bonding between the elements, and thus related to the mechanical properties. Comparing hexagonal and fcc structures, both V 2 N and Cr 2 N are more covalent than the cubic phases VN and CrN. In the case of molybdenum nitrides, the fcc and the hexagonal close-packed (hcp) structures exhibit comparable covalency level. The prominent covalent bonding in hexagonal Cr 2 N and V 2 N nitrides can be related with their higher hardness values compared to that of the fcc phases.

Enhanced sputtering of titanium oxide, nitride and oxynitride thin films by the reactive gas pulsing technique

The reactive gas pulsing technique was used to deposit titanium oxide, nitride and oxynitride thin films by d.c. reactive magnetron sputtering. A pure titanium target was sputtered in a mixture of Ar + O 2 for TiO x , Ar + N 2 for TiN y and Ar + O 2 + N 2 for TiO x N y . The reactive gas was injected with a well-controlled pulsing method for titanium oxide and nitride whereas titanium oxynitride were prepared with a constant flow rate of nitrogen and a pulsing flow rate of oxygen. A constant pulsing period was used for every deposition and the injection time of the reactive gas was systematically changed. Instability phenomena typical to the reactive process were prevented by this technique. An improvement of the deposition rate of TiO 2 and TiN thin films was achieved. The modulation of the reactive gas injection time allowed to change the crystallographic structure of the material as well as the chemical composition (1.4 <x < 2.0 for TiO x and 0.45 < y < 1.06 for TiN y ). With two reactive gases, the pulsing technique appeared as an original way to prepare titanium oxynitride with every x, y composition. Real time measurements of the Ti target potential were used as process parameters in relation to the changes in TiO x , TiN y and TiO x N y thin film properties.

Electrical and optical properties of TiOx thin films deposited by reactive magnetron sputtering

The present study of the electronic properties of titanium monoxide thin films is centered on the electrical and optical properties of nano-grain material. TiOx thin films with x ranging from 0.75 to 1.45 have been deposited by r.f. reactive magnetron sputtering in a mixed Ar/O2 or Ar/H2O atmosphere. All films show a negative temperature coefficient of the resistivity. Spectroscopic ellipsometry measurements were performed in the Vis-UV spectral range. The free carrier and interband contributions to the dielectric function have been sorted out. The most striking feature of the free carrier optical response is the very short scattering time of the order of 10−15 s. Such an intense impurity scattering is beyond the validity range of the semi-classical Boltzmann equation and remains an open problem.