Characterisation of nano-structured titanium and aluminium nitride coatings by indentation, transmission electron microscopy and electron energy loss spectroscopy

Titanium and aluminium nitride Ti_1 − xAl_xN films deposited by radiofrequency magnetron reactive sputtering onto steel substrate are examined by transmission electron microscopy over all the range of composition (x = 0, 0.5, 0.68, 0.86, 1). The deposition parameters are optimised in order to grow nitride films with low stress over all the composition range. Transmission electron microscopy cross-section images of Vickers indentation prints performed on that set of coatings show the evolution of their damage behaviour as increasing x Al content. Cubic Ti-rich nitrides consist of small grains clustered in rather large columns sliding along each other during indentation. Hexagonal Al-rich films grow in thinner columns which can be bent under the Vickers tip. Indentation tests carried out on TiN and AlN films are simulated using finite element modelling. Particular aspects of shear stresses and displacements in the coating/substrate are investigated. The growth mode and the nanostructure of two typical films, TiN and Ti_0.14Al_0.86N, are studied in detail by combining transmission electron microscopy cross-sections and plan views. Electron energy loss spectrum taken across Ti_0.14Al_0.86N film suggests that a part of nitrogen atoms is in cubic-like local environment though the lattice symmetry of Al-rich coatings is hexagonal. The poorly crystallised domains containing Ti and N atoms in cubic-like environment are obviously located in grain boundaries and afford protection of the coating against cracking.     

Deposition of titanium nitride films onto silicon by an ion beam assisted deposition method

Titanium nitride (TiN) films were deposited onto silicon wafers using an ion beam assisted deposition (IBAD) method with an electron cyclotron resonance (ECR) ion source for ionizing the nitrogen (N₂) gas under a condition of high nitrogen ion to titanium neutral ratio. The deposition rate of the TiN films was strongly dependent on the evaporation rate, d_Ti, of Ti metal and decreased with increasing nitrogen ion current. The deposition rate can be approximated as d=βd_Ti?_N2/{1/k+?_N2}−αI, where β, k and α are proportional constants, ?_N2 is the sum rate of neutral and ionized nitrogen impinging onto the substrate, and I is the nitrogen ion current.     

Titanium oxide thin films deposited by high-power impulse magnetron sputtering

Ionized physical vapor deposition processes are of great interest for surface modification because the flexibility of the thin film deposition process can be increased by ionizing the metallic vapor. Recently, high-power impulse magnetron discharges have been implemented to achieve high ionization rates.Thin films of titanium oxide have been deposited on glass and steel substrates using 450 × 150 mm rectangular titanium target in argon-oxygen atmosphere. The average power delivered to the plasma is ranging between 1.5 and 2 kW and peak current and voltage are respectively 200 A and 900 V.Films are characterized using Scanning Electron Microscopy, Grazing Incidence X-ray Diffraction and Optical Transmission Spectroscopy. One of the major findings is the presence of rutile deposited on steel substrate (even for 0 V bias grounded substrate) and the significant increase of the refractive index of the films deposited on glass compared to thin films deposited via conventional direct current bipolar pulsed magnetron sputtering. Films synthesized by high-power impulse magnetron sputtering are denser.     

Deposition of c-axis orientation aluminum nitride films on flexible polymer substrates by reactive direct-current magnetron sputtering

Aluminum nitride (AlN) piezoelectric thin films with c-axis crystal orientation on polymer substrates can potentially be used for development of flexible electronics and lab-on-chip systems. In this study, we investigated the effects of deposition parameters on the crystal structure of AlN thin films on polymer substrates deposited by reactive direct-current magnetron sputtering. The results show that low sputtering pressure as well as optimized N₂/Ar flow ratio and sputtering power is beneficial for AlN (002) orientation and can produce a highly (002) oriented columnar structure on polymer substrates. High sputtering power and low N₂/Ar flow ratio increase the deposition rate. In addition, the thickness of Al underlayer also has a strong influence on the film crystallography. The optimal deposition parameters in our experiments are: deposition pressure 0.38 Pa, N₂/Ar flow ratio 2:3, sputtering power 414 W, and thickness of Al underlayer less than 100 nm.     

High textured AlN thin films grown by RF magnetron sputtering; composition, structure, morphology and hardness

High quality thin aluminum nitride (AlN) films have been deposited onto a silicon (1 0 0) substrate by radio frequency magnetron sputtering of a pure Al target using different gas (Ar, N₂) mixtures. The depositions were carried out at substrate temperatures varying from room temperature (plasma heating) up to 400°C. The crystalline structures were investigated by X-ray diffractometry (XRD) revealing a pronounced texture of the deposited films. Some of the compounds investigated were deposited onto a thin buffer layer of pure Al. The film surface morphology was investigated by Atom Force Microscopy (AFM) (SPM-9500J3 from Shimadzu Co), and was found to depend distinctively upon the different deposition conditions. Generally, two kinds of structures were found—a columnar one, which was densely packed or organized in planar parallel sheets, and a flat structure, (typical for mono-crystals), with rms roughness below 0.2 nm. In this paper, the influence of argon added to the sputtering gas environment on the film properties is investigated and discussed. The depth elemental distributions were calculated using 2.4 MeV ⁴He⁺ Rutherford Backscattering Spectrometry (RBS). Finally, the mechanical characteristics were described using hardness tests.     

Compositional and structural evolution of sputtered Ti-Al-N

The compositional and structural evolution of Ti-Al-N thin films as a function of the total working gas pressure (p_T), the N₂-to-total pressure ratio (p_N2/p_T), the substrate-to-target distance (ST), the substrate position, the magnetron power current (I_m), the externally applied magnetic field, and the energy and the ion-to-metal flux ratio of the ion bombardment during reactive sputtering of a Ti_0.5Al_0.5 target is investigated in detail. Based on this variation we propose that the different poisoning states of the Ti and Al particles of the powder-metallurgically prepared Ti_0.5Al_0.5 target in addition to scattering and angular losses of the sputter flux cause a significant modification in the Al/Ti ratio of the deposited thin films ranging from ~ 1.05 to 2.15.The compositional variation induces a corresponding structural modification between single-phase cubic, mixed cubic-hexagonal and single-phase hexagonal. However, the maximum Al content for single-phase cubic Ti_1−xAl_xN strongly depends on the deposition conditions and was obtained with x = 0.66, for the coating deposited at 500 °C, p_T = 0.4 Pa, ST = 85 mm, and p_N2/p_T = 17%. Our results show, that in particular, the N₂-to-total pressure ratio in combination with the sputtering power density of the Ti_0.5Al_0.5 compound target has a pronounced effect on the Al/Ti ratio and the structure development of the coatings prepared.     

Deposition of diamond-like carbon films on the inner surface of narrow stainless steel tubes

Diamond-like carbon (DLC) films were deposited on the inner surface of 304-type stainless steel tube with an inner diameter of 10 mm by DC glow discharge plasma. The influence of the deposition time, pressure and the ratios of CH₄ in CH₄/Ar gas mixture on the DLC film deposition were investigated. The images of Scanning Electron Microscopy (SEM) show that the DLC films are featureless and free of porosity. Fibre-like structure was recognized on the film surface by Atomic Force Microscopy (AFM). The film deposition rate decreases with increasing the deposition time. Relative higher deposition rate (40 nm/min) can be obtained at 20-30 Pa, higher and lower pressure will significantly decrease the deposition rate. Raman spectrum analysis shows that the films deposited in 30 min at 20-30 Pa have more sp³ content. The corrosion resistance of the films was measured by potentiodynamic polarization test. The DLC films deposited on the inner surface of the 304-type stainless steel tube significantly improve its corrosion resistance.     

Synthesis of c-axis oriented AlN thin films on different substrates: A review

Highly c-axis oriented AlN thin films have been deposited by reactive sputtering on different substrates. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on a variety of substrates, have been examined. Aluminum nitride thin films have been deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N₂ gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the rocking curve Full-Width at Half Maximum (FWHM) of the AlN-(0 0 0 2) peak. The relationship between the substrate, the synthesis parameters and the crystallographic orientation of the AlN thin films is discussed. A guide is provided showing how to optimize these conditions to obtain highly c-axis oriented AlN thin films on substrates of different nature.     

Influence of the nitrogen partial pressure on the properties of d.c.-sputtered titanium and titanium nitride films

The properties of titanium and titanium nitride films deposited onto biased substrates in a d.c. sputtering system were studied as a function of the partial nitrogen pressure. The deposition rate was deduced from film thickness measurements. The film composition was determined by Rutherford backscattering analysis and the structure was studied using X-ray diffraction. The resistivity was measured by the four-probe method and the temperature coefficient of resistivity (TCR) was determined in the temperature range from ?196 to 25 °C.Around a critical nitrogen pressure p_c of 4 × 10^-2 Pa the deposition rate decreases rapidly, the film structure changes from h.c.p. titanium to nearly stoichiometric f.c.c. TiN. At the same pressure, the film resistivity and the TCR present minimum values.A general sputtering model which takes into account the gettering action of the deposited material is proposed. This model allows the calculation of the surface coverage of the target by the reactive gas or the metallic compound and the determination of the deposition rate as a function of the reactive partial pressure. A good agreement is found with the deposition rates measured experimentally.     

Influence of the growth conditions on the stoichiometry and on the optical properties of titanium oxide thin films prepared by reactive sputtering

Titanium oxide thin films are deposited at room temperature by reactive DC sputtering onto glass and Si (100) substrates. Different conditions of deposition were varied such as sputtering power, deposition time and oxygen partial pressure to study their influence on the titanium oxide thin films growth. The absolute amount of oxygen and the relative O/Ti composition of films have been determined by Nuclear Reaction Analysis and Rutherford Backscattering Spectroscopy, respectively. Additionally, the band-gap was determined by measuring the optical absorption and its behavior correlated with the oxygen film content. From the present study, it is possible to establish that the optical band-gap energy depends mainly on the sputtering oxygen partial pressure used at the preparation and that films prepared with a partial oxygen pressure of 4 × 10^− 2 Pa allows titanium oxide with near stoichiometric composition. Additionally, from the optical point of view, band-gap energies of 3.4 eV are obtained for near stoichiometric films and a decrease is observed for samples prepared with higher oxygen concentrations.     

Microstructural, mechanical and electrochemical corrosion properties of sputtered titanium-aluminum-nitride films for bio-implants

The effect of aluminum (Al) addition to titanium nitride (TiN) matrix on the structural, mechanical and corrosion resistance properties of titanium-aluminum-nitride was studied. Ti_1−xAl_xN where x = 0, 0.5 and 1 films were coated onto substrates like Si wafer, AISI 316L stainless steel and low carbon steel by a direct current magnetron sputtering process. The layers were sputtered in pure Argon with a substrate temperature maintained at 400 °C, power of 250 W and a sputtering time of 120 min. XRD, TEM-SAED pattern and XPS analyses were made to study the structural properties of these films. Laser Raman spectrum showed the characteristic peaks at 249 and 659 cm⁻¹ for the Ti_0.5Al_0.5N film. AFM analysis showed a relatively smooth surface for the ternary film. Corrosion performance analysis indicated that the Ti_0.5Al_0.5N coated specimen had superior corrosion resistance when compared to TiN and AlN coated substrates. Higher values of nanohardness and lower coefficient of friction were observed for the Ti _0.5Al_0.5N specimen. Blood platelet adhesion experiments were made to examine the interaction between human blood and the materials in vitro.     

Ti2Al(O,N) formation by solid-state reaction between substoichiometric TiN thin films and Al2O3 (0001) substrates

Titanium nitride TiN_x (0.1 ≤ x ≤ 1) thin films were deposited onto Al₂O₃(0001) substrates using reactive magnetron sputtering at substrate temperatures (T_s) ranging from 800 to 1000 °C and N₂ partial pressures (pN₂) between 13.3 and 133 mPa. It is found that Al and O from the substrates diffuse into the substoichiometric TiN_x films during deposition. Solid-state reactions between the film and substrate result in the formation of Ti₂O and Ti₃Al domains at low N₂ partial pressures, while for increasing pN₂, the Ti₂AlN MAX phase nucleates and grows together with TiN_x. Depositions at increasingly stoichiometric conditions result in a decreasing incorporation of substrate species into the growing film. Eventually, a stoichiometric deposition gives a stable TiN(111) || Al₂O₃(0001) structure without the incorporation of substrate species. Growth at T_s 1000 °C yields Ti₂AlN(0001), leading to a reduced incorporation of substrate species compared to films grown at 900 °C, which contain also Ti₂AlN(101?3) grains. Finally, the Ti₂AlN domains incorporate O, likely on the N site, such that a MAX phase oxynitride Ti₂Al(O,N) is formed. The results were obtained by a combination of structural methods, including X-ray diffraction and (scanning) transmission electron microscopy, together with spectroscopy methods, which comprise elastic recoil detection analysis, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy.     

Dielectric properties of sol-gel derived high-k titanium silicate thin films

High-k dielectric titanium silicate (Ti_xSi_1 − xO₂) thin films have been deposited by means of an optimized sol-gel process. At the optimal firing temperature of 600 °C, the Ti_0.5Si_0.5O₂ films are shown to exhibit not only a dielectric constant (k) as high as ∼ 23 but more importantly the lowest leakage current and dielectric losses. Fourier transform infrared spectroscopy shows an absorbance peak at 930 cm^− 1, which is a clear signature of the formation of Ti-O-Si bondings in all the silicate films. The developed sol-gel process offers the required latitude to grow Ti_xSi_1 − xO₂ with any composition (x) in the whole 0 ≤ x ≤ 1 range. Thus, the k value of the Ti_xSi_1 − xO₂ films can be tuned at any value between that of SiO₂ (3.8) to that of TiO₂ (k ∼ 60) by simply controlling the TiO₂ content of the films. The composition dependence of the dielectric constant of the Ti_xSi_1 − xO₂ films is analyzed in the light of existing models for dielectric composites.     

Growth of highly textured aluminum films on LiTaO3 with optimized titanium intermediate layers

Ultra thin titanium films in the range of a few nanometers have been deposited on monocrystalline lithiumtantalate (LiTaO₃) followed by deposition of 400 nm pure aluminum (Al). Texture measurements by means of electron backscatter diffraction show that the thickness of the intermediate titanium (Ti) layer significantly influences texture and grain structure of the overlying Al film. Increasing the thickness of the Ti layer from 0 nm to 20 nm leads to a change of aluminums texture from unoriented polycrystalline over highly oriented in single direction to highly oriented in twin structure.     

Deposition and characterization of c-axis oriented aluminum nitride films by radio frequency magnetron sputtering without external substrate heating

Aluminum nitride (AlN) films were deposited on a variety of substrates (glass, Si, oxidized Si, Al-SiO₂-Si, Cr-SiO₂-Si, and Au-Cr-SiO₂-Si) by radio frequency (RF) magnetron sputtering using an AlN target. The films were deposited without external substrate heating. The effect of RF power, ambient gas (Ar and Ar-N₂) and sputtering pressure on deposition rate and crystallinity were investigated. The structure and morphology of the films were studied by X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. These investigations revealed that the AlN films prepared in mixed gas ambient (Ar-N₂) were highly c-axis oriented with moderate surface roughness on all the substrate. A strong IR absorption band was observed around 670 cm^− 1 which confirms the presence of Al-N bond in the film. The dc resistivity of the films was measured to be in the range of 10¹¹ to 10¹² Ω-cm at moderate electric fields. The application of these films in piezoelectric based micro-electro-mechanical systems is discussed.     

Influence of aluminum nitride interlayers on crystal orientation and piezoelectric property of aluminum nitride thin films prepared on titanium electrodes

Highly c-axis-oriented aluminum nitride (AlN) thin films have been prepared on titanium (Ti) bottom electrodes by using AlN interlayers. The AlN interlayers were deposited between Ti electrodes and silicon (Si) substrates, such as AlN/Ti/AlN/Si. The crystallinity and crystal orientation of the AlN films and Ti electrodes strongly depended on the thickness of the AlN interlayers. Although the sputtering conditions were the same, the X-ray diffraction intensity of AlN (0002) and Ti (0002) planes drastically increased, and the full-width at half-maximum (FWHM) of the X-ray rocking curves decreased from 5.1° to 2.6° and from 3.3° to 2.0°, respectively. Furthermore, the piezoelectric constant d₃₃ of the AlN films was significantly improved from − 0.2 to − 4.5 pC/N.     

Optical characterization of vanadium-titanium oxide films

Composite films of titanium and vanadium oxides (TVO) with various compositional ratios were prepared by sputtering deposition. The optical properties, crystalline structure and film morphology were investigated as a function of the composition. The results of thermochromism and X-ray diffraction suggest that the TVO films at any compositional ratios form substitutional solid solution of Ti and V, that is, Ti_xV_1−xO₂, where 0 ≤ x ≤ 1. Dielectric constants of the TVO films at any compositions were consistently determined at photon energies between 0.75 to 3 eV by employing the Lorentz-oscillator formula. With wide variation in x, the dielectric constants at visible and near-infrared wavelengths monotonically decrease down to the values of TiO₂, which suggests that dielectric constants of the TVO film can be precisely controlled by adjusting rf power in co-sputtering deposition.     

Reactive deposition of Al-N coatings in Ar/N2 atmospheres using pulsed-DC or high power impulse magnetron sputtering discharges

In this paper, the metal to ceramic transition of the Al-N₂ system was investigated using classical reactive pulsed-DC magnetron sputtering and HIgh Power Impulse Magnetron Sputtering (HIPIMS) at a constant average current of 3 A. Optical emission spectroscopy measurements revealed more ionised aluminium species in the HIPIMS discharge compared to pulsed-DC sputtering. It also showed excited N⁰ and ionised N⁺ species in reactive Ar/N₂ HIPIMS discharges. The corresponding evolution of the consumed nitrogen flow as a function of the N₂ partial pressure revealed that a higher amount of reactive gas is needed to achieve stoichiometric AlN with HIPIMS. Electron probe micro-analysis and X-ray diffraction measurements confirmed that a partially poisoned aluminium target is enough to allow the deposition of stoichiometric hcp-AlN thin films via HIPIMS. To go further in the comparison of both processes, two stoichiometric hexagonal aluminium nitride thin films have been deposited. High power impulse magnetron sputtered hcp-AlN exhibits a higher nano-hardness (18 GPa) than that of the coating realised with conventional pulsed-DC sputtering (8 GPa).     

Spectroscopic ellipsometry and positron annihilation investigation of sputtered HfO2 films

Hafnium oxide (HfO₂) films were prepared using a pulsed sputtering method and different O₂/(O₂ + Ar) ratios, deposition pressures, and sputtering powers. Spectroscopic ellipsometry (SE) and positron annihilation spectroscopy (PAS) were used to investigate the influence of the deposition parameters on the number of open volume defects (OVDs) in the HfO₂ films. The results reveal that a low O₂/(O₂ + Ar) ratio is critical for obtaining films with a dense structure and low OVDs. The film density increased and OVDs decreased when the deposition pressure was increased. The film deposited at high sputtering power showed a denser structure and lower OVDs. Our results suggest that SE and PAS are effective techniques for studying the optical properties of and defects in HfO₂ and provide an insight into the fabrication of high-quality HfO₂ thin films for optical applications.     

Structural and mechanical properties of multi-element (AlCrMoTaTiZr)Nx coatings by reactive magnetron sputtering

(AlCrMoTaTiZr)N_x high-entropy films were deposited on silicon wafer and cemented carbide substrates from a single alloy target by reactive RF magnetron sputtering under a mixed atmosphere of Ar and N₂. The effect of nitrogen flow ratio R_N on chemical composition, morphology, microstructure, and mechanical properties of the (AlCrMoTaTiZr)N_x films was investigated. Nitrogen-free alloy film had an amorphous structure, while nitride films with at least 37 at.% N exhibited a simple NaCl-type FCC (face-centered cubic) structure. Mixed structures occurred in films with lower nitrogen contents. Films with the FCC structure were thermally stable without phase decomposition at 1000 °C after 10 h. The (AlCrMoTaTiZr)N film deposited at R_N = 40% exhibited the highest hardness of 40.2 GPa which attains the superhard grade. The main strengthening mechanisms for this film were grain-size and solid-solution strengthening. A residual compressive stress of 1.04 GPa was small to account for the observed hardness. The nitride film was wear resistant, with a wear rate of 2.8 × 10^− 6 mm³/N m against a loaded 100Cr6 steel ball in the sliding wear test. These high-entropy films have potential in hard coating applications.