Nanoindentation and atomic force microscopy measurements on reactively sputtered TiN coatings

Titanium nitride (TiN) coatings were deposited by d.c. reactive magnetron sputtering process. The films were deposited on silicon (111) substrates at various process conditions, e.g. substrate bias voltage (V_B) and nitrogen partial pressure. Mechanical properties of the coatings were investigated by a nanoindentation technique. Force vs displacement curves generated during loading and unloading of a Berkovich diamond indenter were used to determine the hardness (H) and Young’s modulus (Y) of the films. Detailed investigations on the role of substrate bias and nitrogen partial pressure on the mechanical properties of the coatings are presented in this paper. Considerable improvement in the hardness was observed when negative bias voltage was increased from 100–250 V. Films deposited at |V _B| = 250 V exhibited hardness as high as 3300 kg/mm². This increase in hardness has been attributed to ion bombardment during the deposition. The ion bombardment considerably affects the microstructure of the coatings. Atomic force microscopy (AFM) of the coatings revealed fine-grained morphology for the films prepared at higher substrate bias voltage. The hardness of the coatings was found to increase with a decrease in nitrogen partial pressure.     

Effects of substrate temperature and substrate material on the structure of reactively sputtered TiN films

Stoichiometric TiN films were reactively magnetron sputtered in an Ar-N₂ atmosphere. The films were deposited at various substrate temperatures in the range 200–650°C onto two types of substrate material, high speed steel and stainless steel. The microstructure of the films obtained was investigated by the use of a transmission electron microscope and the morphology was studied in a scanning electron microscope. Measurements of the hardness were also performed. The analysis of the microstructure shows that the growth of the film is markedly influenced by the substrate material. In particular, the high speed steel substrates were found to have a considerable influence on the microstructure. The vanadium carbide particles in these steels, which have a good lattice match to TiN, stimulate a localized epitaxial growth to occur on these carbide particles. This results in a microstructure consisting of large grains surrounded by small grains. The shape of the large grains is influenced by the temperature. In the development of these large grains cracks and/or voids occur in and around the grains at substrate temperatures above 400°C and the hardness drops by about 20%. No large grains were found on films deposited onto stainless steel and their hardness increases slightly with temperature. High hardness for films deposited onto the high speed steel substrate at temperatures above 400°C can also be obtained if a substrate bias is used. Ion bombardment during film growth suppresses the formation of the large grains with voided or cracked boundaries because of a continuous renucleation process. The formation of the different microstructures is discussed in terms of surface energy minimization and thermally activated processes as surface and grain boundary migration.     

Photocatalytic activity of reactively sputtered and directly sputtered titania coatings

It is well known that, depending on deposition conditions, the structure of titania coatings may be amorphous, anatase or rutile, or a mixture of phases, and that the anatase phase is the most promising photocatalyst for the degradation of organic pollutants. The formation of anatase depends on the energy delivered to the growing film, which in turn depends on the operating parameters chosen. In this study, titania coatings have been deposited onto glass substrates by pulsed magnetron sputtering both from metallic targets in reactive mode and directly from oxide powder targets. The as-deposited coatings were analysed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and micro-Raman spectroscopy. Selected coatings were then annealed at temperatures in the range of 400–700 °C and re-analysed. The photocatalytic activity of the coatings has been investigated through measurements of the degradation of organic dyes, such as methyl orange, under the influence of UV and fluorescent light sources. Further sets of coatings have been produced both from metallic and powder targets in which the titania is doped with tungsten. These coatings have also been analysed and the influence of the dopant element on photocatalytic activity has been investigated. It has been found that, after annealing, both sputtering processes produced photo-active surfaces and that activity increased with increasing tungsten content over the range tested. Furthermore, the activity of these coatings under exposure to fluorescent lamps was some 50–60% of that observed under exposure to UV lamps.     

Ion beam reactively sputtered silicon nitride coatings

An ion beam source was used to deposit silicon nitride films by reactive sputtering a silicon target with an Ar+N₂ beam. The nitrogen fraction in the sputtering gas was 0.05 to 0.80 at a total pressure of 6 to 20×10^?5 torr. The ion beam current was 50 mA at 500 V. A rate of deposition of about 2 nm min^?1 (0.12 μm h^?1) was found, and the spectra indicated that Si₃N₄ was obtained for a fraction of nitrogen higher than 0.50. However, the AES spectra also indicated that the sputtered silicon nitride films were contaminated with oxygen and carbon and contained significant amounts of iron, nickel, and chromium, most probably sputtered from the holder of the substrate and target.     

Investigation of reactively sputtered TiN films for diffusion barriers

Two groups of reactively sputtered TiN films, gold-yellow films (G films) with low resistivity and high compressive internal stress and brown-black films (B films) with high resistivity, which are formed with and without a negative substrate bias, were examined as potential diffusion barriers.The microstructures and compositions were investigated by Auger electron spectroscopy, electron probe microanalysis and X-ray diffraction. The G films and the B films have a fine-grained and a columnar-arranged morphological structure respectively. Both films exhibit the cubic NaCl-type crystallographic structure strongly oriented towards the (111) plane which is parallel to the substrate surface. They also exhibit a superstoichiometric composition having excess nitrogen atoms. In addition, the B films contain a large number of oxygen atoms.The compressive internal stress in the G film originates from lattice expansion which is probably due to the incorporation of the excess nitrogen atoms. Using the experimental value of the Young's modulus, the expanded lattice parameter in this film was calculated from the corrected interplanar spacing after the silicon substrate had been removed.The superior diffusion barrier capability of the B film is demonstrated through the application of the TiN films to an Au/Pt/TiN/Ti metal system on thick polysilicon. It is assumed that the higher diffusion barrier capability of the B film is due to the reduction of grain boundary diffusion by the oxygen atoms located in the intercolumnar layers. This is supported by film analysis, the peculiar etching behaviour and hardness studies.     

R.F. reactively sputtered TiN: Characterization and adhesion to materials of technical interest

Films of TiN were prepared by reactive r.f. sputtering of titanium in an ArN₂ atmosphere. Materials of great technological importance, such as high speed steel, stainless steel and a nickel alloy were coated. Characteristic properties of the deposits such as the deposition rate, the microhardness, the electrical resistivity and the adhesion to the substrate were investigated in relation to the partial pressures of N₂ and argon during deposition. The composition of the deposit which was found to give the maximum hardness was determined by microprobe analysis. It was shown to be nearly stoichiometric. The suitable N₂ partial pressure range which gives golden yellow films of stoichiometric composition was shown to depend mainly on the applied r.f. power and argon partial pressure. The adhesion of the coatings to different substrates was investigated by the scratch test. On high speed steel it was found that the critical load at which the coating is stripped becomes constant for coating thicknesses above about 4 μm.     

Scratch adhesion testing of hard coatings

The scratch test for adhesion is reviewed as the only method currently available for testing thin, hard and well-adhering coatings such as TiC on steel or cemented carbide substrates. The critical load, mode of coating removal and acoustic signals are discussed. It is found that the combination of acoustic signal with microscopic observations can indicate whether failure occurs following a cohesive or an adhesive mode. The critical loads increase with increasing coating thickness in a manner which is a characteristic of the coating-substrate combination being studied. Critical loads are higher for harder tougher substrate materials; they also appear to depend on the elastic modulus and the coefficient of friction of the coating itself.     

Reactively sputtered TiN coatings for tribological applications

The results of tribological studies carried out on TiN sputter-coated ferrous and non-ferrous materials are presented. A disc magnetron system was used to sputter TiN reactively onto samples suitable for standard friction and wear tests. The tests carried out show that thin films of reactively sputtered TiN, typically 5 microm thick, provide good wear protection for ferrous and non-ferrous surfaces subjected to sliding and rolling contacts. Cutting tests with coated cermet tool inserts show that reactive sputtering is also useful for improving the performance of cutting tool inserts. The results obtained suggest that magnetron reactive sputtering can be useful to enhance the wear resistance of finished precision components.     

Crystallography of magnetron sputtered TiN coatings on steel substrates

Structure formation processes in TiN coatings deposited by reactive CFUBMS on steel substrates have been investigated by X-ray diffraction experiments in symmetric Bragg-Brentano (B-B) and grazing incidence asymmetric Bragg diffraction (GIABD) modes and by SEM. The results show that the deposits with thicknesses of 500 and 4000 nm are built-up of polycrystalline stoichiometric TiN, in addition to which, some negligible amount of Ti-O and Ti-N-O phases have also been observed predominantly at their surfaces. In the thinner 500 nm films only columnar crystallites with {1 1 1}, {2 0 0} and {2 2 0} crystallographic planes parallel to the surface were formed. The share of the micro-volumes belonging to the 〈1 1 1〉 out-of-plane texture component varied between 70% and 80% depending on the target current (I_d) used (4 or 8 A in the present experiments). During the more advanced stages of growth the 〈1 1 1〉 texture weakens and new texture components appear; the process being more pronounced when the application has been performed at higher I_d values. The obtained crystallographic texture results for the thinner films and their changes during the more advanced stages of the coatings formation are discussed with particular consideration of the crystallography of the TiN lattice and the anisotropy of its elastic parameters. Based on a precise estimation of the interplanar distances, d_{〈u v w〉}, corresponding to the main texture components of the investigated films in the direction along the surface macro-normal, it has been revealed that the elastic strain, ε_{〈u v w〉}, caused by the compressive residual macro-stresses acting parallel to the film surfaces and the corresponding elastic stored energy, U_{〈u v w〉}, values obey the following relationships: ε_{〈1 1 1〉}>ε_{〈2 2 0〉}> ε_{〈2 0 0〉} and U_{〈1 1 1〉}>U_{〈2 2 0〉}>U_{〈2 0 0〉}, respectively. The observed ε_{〈u v w〉} and U_{〈u v w〉} anisotropy is found to be more pronounced in the thinner coatings and is such that, at more advanced stages of growth, it would be expected to favour the transition from 〈1 1 1〉 to 〈2 0 0〉 out-of-plane preferred orientation. However, the experimental results do not confirm this expectation, which points out that the texture-formation at these stages is not governed solely by the minimization of the stored elastic energy, but is a rather complicated process depending on a larger number of factors, some of which are discussed in the paper.     

Polymer pre-treatment by linear anode layer source plasma for adhesion improvement of sputtered TiN coatings

The pre-treatment of work-piece surfaces is decisive for improved adhesion of tribological, decorative, sensor, biocompatible, etc. coatings subsequently deposited by vacuum coating techniques. Most current industrial techniques (mainly glow discharges) miss the requirements for activating temperature-sensitive and electrically insulating materials. Gridless plasma sources like the linear anode layer ion source are an excellent alternative due to their low investment and operating costs and scalability to many industrial applications, and also due to the measured plasma characteristics (low surface charging, broad energy distribution). The appreciable increase of adhesion by anode layer source plasma pre-treatment and the effects of ion energy and gas composition are presented for room temperature sputtered titanium nitride coatings polyamide, polycarbonate, and poly(ethyleneterephtalate). For these polymers, the oxygen-based functionalization (chain-scissoring and cross-linking) strongly influence the wetting behaviour and improve the adhesion by suppressing adhesive cracking at low scratching loads. Low O₂ contents in Ar-O₂ discharge of ∼400 eV average ion energy lead to best coating adhesion.     

Mechanical, microstructural and oxidation properties of reactively sputtered thin CrN coatings on steel

Thin (40 nm and 160 nm) CrN coatings were deposited on steel by reactive magnetron sputtering deposition, varying the N₂ flow. The coatings were characterized in the as-deposited condition and after annealing in air at 500 °C for 1 h, by X-Ray Diffraction, Transmission Electron Microscopy, Raman and Fourier Transform Infrared spectroscopies. Hardness was measured by nanoindentation. Coatings have a nanocrystalline microstructure with the phase shifting from Cr₂N to CrN, increasing grain size, thermal stability and resistance to oxidation with increasing N₂. Also intrinsic coating hardness is influenced by both N₂ flow during deposition and film thickness, as a result of changes in phase composition and microstructural properties.     

Influence of substrate temperature on the growth and properties of reactively sputtered In-rich InAlN films

Indium-rich InAlN films were prepared on Si (111) substrates by using reactive co-sputtering in a mixed Ar-N₂ atmosphere. The substrate temperature was varied from room temperature to 300 °C to investigate the film’s growth and properties at different temperatures. Structural and optical properties of the films were evaluated through high resolution XRD and Raman spectroscopy respectively, surface morphology and roughness analysis was performed by using FE-SEM and AFM respectively, whereas the electrical characterizations were made through resistivity and current–voltage (I–V) measurements respectively. Highly c-axis oriented nanocrystalline InAlN films with wurtzite structure were obtained at a substrate temperature of 100 °C and above. Structural quality of the films was improved with increase of the substrate temperature. The Raman spectroscopy revealed A₁ (LO) modes which became more intense by the increasing the substrate temperature. The electrical studies indicated n-type nature of InAlN film having electron concentration in the range 3 × 10¹⁹–20 × 10¹⁹ cm^?3. The electrical resistivity exhibited a decreasing trend with increase of the deposition temperature. The I–V measurements showed a noticeable increase in the value of current by increasing the substrate temperature to 300 °C.     

Improved adhesion between a sputtered alumina coating and a copper substrate

The influence of various metallic intermediate layers on the adhesion between a copper substrate and an alumina coating was studied. The alumina coatings and the intermediate layers were prepared by r.f. sputtering. Titanium and combined Ni/Ti intermediate layers were used. The adhesion properties of alumina with different combinations of coatings were compared by an interrupted tensile testing method and by thermal cycling from room temperature to 600°C.The adhesion of a sputtered alumina coating to a copper substrate without any intermediate layer appeared to be rather poor. The use of titanium as an intermediate layer enhanced the adhesion significantly. The adhesion was further increased when the deposition temperature of the titanium was increased from 200 to 350°C. The combined Ni/Ti intermediate layer led to better adhesion than the titanium layer alone did.     

Characterization and adhesion study of thin alumina coatings sputtered on PET

Thin amorphous alumina coatings have been deposited on polyethylene terephthalate (PET) by rf magnetron sputtering in a pure argon plasma. Their microstructure, composition, stresses and adhesion to the PET films have been studied. SEM microstructure study showed a good agreement with Thornton's structure zone model, i.e. a progression from a dense structure to a columnar structure was observed as the pressure was increased. The composition of deposits, determined by RBS, showed that oxygen-rich alumina was obtained when high plasma pressures (more than 1 Pa) were used. FTIR spectra of alumina indicate that the excess oxygen was essentially due to the presence of hydroxyl groups in the coatings. The stress evolution in alumina deposits, determined by the bending beam method, was correlated with the microstructural change. Adhesion of alumina on PET has been studied by a peel test. Best results were obtained when sputtering parameters combined moderate power (1 W cm⁻²) and pressure of about 1 Pa. XPS analysis of the alumina/PET interface showed that bonding between the ceramic and the polymer occurred primarily via Al-O-C bonds.     

The microstructure of reactively sputtered Ti-N films

Ti-N films with nitrogen concentrations ranging from 0 to 52 at.% prepared by both d.c. magnetron and r.f. reactive sputtering are examined using transmission electron microscopy. The observed microstructures are correlated with physical properties of the films. In particular, the maximum in the microhardness and density and the minimum in the electrical resistivity are found to correspond to a fully dense structure. The sample prepared by magnetron sputtering has a very inhomogeneous and voided microstructure and a lower hardness than the r.f.-sputtered sample with a similar nitrogen content. A theory is proposed for the development of a lamellar microstructure in the films containing two phases.     

Microstructural characteristics and mechanical properties of magnetron sputtered nanocrystalline TiN films on glass substrate

Nanocrystalline TiN thin films were deposited on glass substrate by d.c. magnetron sputtering. The microstructural characteristics of the thin films were characterized by XRD, FE-SEM and AFM. XRD analysis of the thin films, with increasing thickness, showed the (200) preferred orientation up to 1·26 μm thickness and then it transformed into (220) and (200) peaks with further increase in thickness up to 2·83 μm. The variation in preferred orientation was due to the competition between surface energy and strain energy during film growth. The deposited films were found to be very dense nanocrystalline film with less porosity as evident from their FE-SEM and AFM images. The surface roughness of the TiN films has increased slightly with the film thickness as observed from its AFM images. The mechanical properties of TiN films such as hardness and modulus of elasticity (E) were investigated by nanoindentation technique. The hardness of TiN thin film was found to be thickness dependent. The highest hardness value (24 GPa) was observed for the TiN thin films with less positive micro strain.     

Dielectric breakdown of reactively sputtered silicon nitride

Silicon nitride layers on silicon substrates were prepared by reactive sputtering of silicon in nitrogen under conditions which led to layers of maximum dielectric strenght. Dielectric breakdown and its dependence on temperature, pulse width and layer thickness were investigated. It is shown that conversion from thermal to electronic breakdown can be achieved at low temperatures and short pulse widths. This enables the thickness dependence of breakdown to be observed. From this dependence the mean free time of carriers was found to be 1.1×10^-15 sec.     

Investigation of Tin films reactively sputtered using a sputter gun

TiN films were prepared by reactive sputtering of titanium in a gas mixture of argon and nitrogen using a sputter gun. The properties of the sputtered films were investigated with Rutherford backscattering spectrometry, electrical resistivity and optical reflectivity measurements, X-ray diffractometry and transmission electron microscopy. For a given input power to the sputter gun the film properties depend on the gas composition, the bias voltage applied to the substrates and oxygen contamination during sputtering. In addition, it was found that powering the sputter gun with r.f. causes an iron contamination in the films by material sputtered off the plasma confinement shield. This contamination is very much reduced when d.c. is employed to power the sputter gun.