ICMCTF2013 Session B1-2: PVD Coatings and Technologies

There is significant interest in the measurement of the equation of state and other parameters of bcc metals at high pressures and low temperatures. One example is Iron, where understanding its behavior at high pressures is useful for understanding planetary development. Targets are needed to perform these important measurements on large experimental platforms such as Omega (Rochester), The National Ignition Facility (Livermore) and the Z-machine (Albuquerque). Experimental design requirements have actually pushed the thickness of the “film” beyond that of the “substrate”. We have sputter deposit very thick stepped Iron and Tantalum films on diamond substrates for experiments at NIF, with steps of ~10 mm and total thicknesses up to ~ 100 mm. The diamond substrates were only 40-80 mm thick for these films. These large thickness films/targets have been fabricated while maintaining other important properties, such as a reasonably sharp rolloff of the coating at the step edges and a reasonable roughness at the film surface.

We also have sputter deposited Tantalum films up to ~2,000 microns (2 mm!) thick on aluminum substrates, enabled through control of the film stress. The substrates were subsequently removed for use as targets for experiments at Z-machine. We believe this is by far a record thickness for sputter deposited Tantalum films.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release number is: LLNL-ABS-492350

In PVD technology various different arrangements of capacitively coupled plasmas mainly magnetically enhanced are available for Al based sputtering processes. Nevertheless most techniques do not allow separate control of ion flux and ion energy distribution at the target, which limits the control range of sputtering process. However, an MFCCP process provides access to a curtain control range of plasma density and ion flux towards the target by tuning the power of a very high frequency excitation. An additional low frequency excitation effectively adjusts the self bias voltage and therefore the ion energy distribution at the target.

In this contribution a complete plasma characterization including specially resolved electron density, electron temperature and gas temperature is performed by evaluating the optical emission detected with an absolute calibrated spectrometer. From this data, the ion flux on the Al target is calculated and used in a TRIDYN simulation, which provides the sputtered Al flux from the target as a result. A Monte Carlo model delineates the transport of sputtered material through the plasma towards the substrate, which allows a theoretical prediction of the deposition rate. Furthermore, the film growth is determined experimentally by weighing a silicon wafer before and after the deposition process using a analytical balance. A comparison of the measured and predicted deposition rate can lead in some cases to rather small sticking coefficients. For verification of those results, the density of sputtered Al atoms in the plasma is determined from the photo emission of Al atoms. The spacial density distribution also implies a low sticking coefficient of sputtered material at the substrate. The low sticking coefficient can possibly explained by high ion bombarding energies at the substrate resulting from comparably high plasma sheath voltages in front of the substrate.

The authors would like to acknowledge the funding provided by the ''Deutsche Forschungsgemeinschaft'' within the frame of the SFB-TR 87 and the ''Ruhr University Bochum Research School''.

This study reports on the influence of sputter-deposition conditions on the structural, electrical properties and chemical composition of a refractory alloy (WTi) thin films. WTi thin films have been deposited using a planar DC Magnetron sputtering apparatus from WTi alloyed target (70:30 At%) in pure Ar working gas, under working pressure ranged from 0.14 to 1.4 Pa, at constant power discharge, without substrate bias and external heating. Body-centered cubic W_XTi_1-X solid solution thin films have been obtained, with x in the range 0.75<x<0.81. The films have a α-W structure with a strong {110} fiber texture. Ti depletion in WTi thin films have been observed by MEB-EDX analysis and attributed to atoms transport, re-sputtering at substrate level and disparity of W and Ti sputtering yields. For both ultra-thin (10 nm) and thin (180 nm) films a stress transition from tensile-to-compressive stress state has been observed as the working pressure increases by using Stoney and X-Ray Diffraction ex-situ methods. The stress transition has been shown to be similar for both thickness but less well defined for ultra-thin films. Preliminary in-situ stress measurements in the magnetron chamber have been performed by using Stoney method. For a low working pressure (0.5 Pa), in-situ stress state exhibits a change during deposition from tensile to compressive at a thickness of about 14 nm. For a high working pressure (1.1 Pa), a tensile stress state is observed all along the deposition whatever the thickness (up to 180 nm). Influence of thickness and working pressure on electrical properties has been revealed (range from 60 to 200 µΩ.cm). Thin films microstructure was highlighted by FIB-MET observations. WTi ultra-thin and thin films process-structure-property relations are studied and discussed in relation with the state of the art.

Aluminum chromium nitride (Al_xCr_1-xN) coatings (within the AlN-CrN quasibinary system) are known for their excellent oxidation and wear resistance, thermal stability and high hardness. In general, these properties are correlated to their Al content, x. Recent studies indicated that by increasing x up to ~ 0.75 (hence, 75 at% Al of the metal sublattice), which corresponds to the ab initio suggested solubility limit within the cubic (NaCl type) structure, the film properties can be enhanced significantly. Exceeding this Al content, a cubic/hexagonal or a single phase hexagonal (wurtzite ZnS type) structure will be formed, resulting in decreased properties. However, stabilizing Al-rich Al_xCr_1-xN coatings in a single phase cubic structure should result in a further increase in mechanical properties and thermal behavior. Therefore, monolithic as well as multilayered Al_xCr_1-xN coatings were deposited by cathodic arc evaporation (in N₂ atmosphere) using powder metallurgically prepared Al_xCr_1-x targets with compositions of x = 0.7, 0.75, 0.85, and 0.9. X-ray diffraction studies clearly exhibit a single phase cubic structure for the monolithically grown nitride coatings using Al_0.7Cr_0.3 and Al₀₇₅Cr_0.25 targets, a mixed cubic/hexagonal structure for the coatings prepared from Al_0.85Cr_0.15 targets, and a single phase hexagonal structure when using Al_0.9Cr_0.1 targets, independent from the dc bias voltage applied to the substrates. Multilayer variations of the single phase cubic layers with the mixed cubic/hexagonal or hexagonal layers (by combining the individual targets during the preparation by an industrial plant) contain also a hexagonal phase fraction in addition to the cubic phases when applying low bias voltages of -40 V. However, when increasing the bias voltage up to -120 V, the hexagonal phase formation can be suppressed. This is even valid for the multilayer arrangements combining Al_0.75Cr_0.25N layers with the highest Al containing layers prepared from Al_0.9Cr_0.1 targets. These multilayers (Al_0.75Cr_0.25N/Al_0.9Cr_0.1N) exhibit a pronounced hexagonal phase content when applying -40 V bias but a cubic structure for -120 V bias. The structural modification from pronounced hexagonal to cubic results in increased hardness values, from ~17 to 32 GPa, as well as increased thermal stability and oxidation resistance.

Our results highlight the importance of an architectural design in addition to the alloy development for optimized material properties and performance.

The production of stoichiometric oxide ceramics by reactive magnetron sputtering is usually only possible with low deposition rates. This is due to oxygen poisoning of the sputter source at high oxygen flow rates [1, 2]. This study investigates the influence of argon flow on the transition zone between metallic mode sputtering with high sputter rates and oxidic mode sputtering with significantly reduced sputter rates. The aim is to produce stoichiometric and, thus, transparent thin films of various oxides in metallic mode sputtering. Rutherford backscattering, ellipsometry and profilometry measurements confirm that high argon flow rates help to prevent unwanted reaction of the sputter source with the reactive gas and allow for fast deposition of optically transparent materials such as TiO₂, ZrO₂ and SiO₂.

The findings from the previous investigations are used for the production of a novel ceramic coating for dental implants. Due to its inherent dark grey color, titanium might cause undesired darkening of the peri-implant mucosa when used for transgingival dental implant screws [3]. Spectrophotometric measurements show that the presented coating significantly increases the lightness of the implant and prevents mucosal discolorations completely.

References:

[1] S Venkataraj, O Kappertz, H Weis, R Drese, R Jayavel, M Wuttig (2002) Journal of Applied Physics 92: 3599.

[2] T Kubart, D Depla, DM Martin, T Nyberg, S Berg (2008) Applied Physics Letters 92.

[3] SE Park, JD Da Silva, H-P Weber, S Ishikawa-Nagai (2007) Clinical Oral Implants Research 18: 569.

Corundum type (Al_xCr_1-x)₂O₃ oxides are of major interest especially when synthesized using physical vapour deposition techniques at low temperatures around 500°C. First studies on cathodic arc-evaporated solid solution Al-Cr-O oxides showed promising results if the Al content is not exceeding 50 at%. However, detailed investigations demonstrated that not only the increased tendency of droplet formation due to oxide island formation on the target surface, but also the development of an fcc-structure is limiting the quality of these films.

Recent studies showed that using Al_xCr_1-x-ySi_y targets reduces the oxide island formation on the target surface and influences the structure and morphology of the oxide formed. Furthermore, oxidation tests of Al-Cr-Fe-N indicated that the FeO₂ oxide is promoting the Cr₂O₃ formation, which is needed to stabilize a corundum-type α-alumina in the solid solution oxide. Therefore powder metallurgical Al_xCr_1-x-ySi_y targets, with Si contents y of 0.01, 0.02, 0.05 and 0.1 (1, 2, 5 and 10 at.%) as well as (Al_xCr_1-x)_zFe_1-z targets with Fe contents z of 0.01, 0.02 and 0.05 (1, 2 and 5 at%) were studied in detail on their arc-evaporation behaviour and their suitability to prepare corundum-type (Al_xCr_1-x)₂O₃ oxide coatings.

Recently, huge interest has arisen in the synthesis of transition metal oxynitride thin films, due to their excellent mechanical and optical properties as well as chemical stability. Within this work, we explored the evolution of structure and properties of Al-Ti-O-N films over a wide composition range from the nitride to the oxide side. Films were grown on silicon wafers in a laboratory-scale pulsed d.c. unbalanced magnetron sputtering system from powder metallurgical TiAl targets with an Al/Ti atomic ratio of 60/40, using either a constant level of nitrogen with rising oxygen partial pressure or vice versa. Coating composition and structure were investigated by energy- and wavelength-dispersive X-ray spectroscopy, scanning electron microscopy, atomic force microscopy, X-ray photo-electron microscopy, Raman spectroscopy and X-ray diffraction. Furthermore, hardness and elastic modulus were evaluated by nanoindendation. At constant nitrogen partial pressure, oxygen concentrations of up to 55 at.-% within the films could be established. In contrast, nitrogen incorporation at constant oxygen partial pressure is significantly hindered, resulting in nitrogen concentrations not exceeding a few percent. While oxygen-free coatings show a single-phase face-centered cubic phase, oxide coatings without nitrogen addition are based on the rutile TiO₂ structure. Low nitrogen or oxygen contents, respectively, result in the additional formation of a face-centered cubic TiAlON phase, whereas for high oxygen and nitrogen concentrations amorphous structures are formed.

Coatings of (Cr_xAl_1-x)_δ(O_yN_1-y)_ξ with 0.33≤x≤0.96, 0≤y≤1, and 0.63≤δ/ξ≤1.30 were deposited using cathodic arc evaporation in N₂/O₂ reactive gas mixtures on 50 V negatively-biased WC-10 wt.% Co substrates from different Cr and Al alloys with three different Cr/Al compositional ratios. For N₂ less than 63 % of the total gas, ternary (Cr,Al)₂O₃ films containing <1 at. % of N forms; as determined by elastic recoil detection analysis. Increasing the N₂ fraction to 75 % and above result in formation of quaternary oxynitride films. Phase analyses of the films by x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy show the predominance of cubic Cr-Al-N and cubic-(Cr,Al)₂O₃ solid solutions and secondary hexagonal α-(Cr,Al)₂O₃ solid solution. High Cr and Al contents result in films with higher roughness while high N and O contents result in smoother surfaces. Nanoindentation hardness measurements showed that Al-rich oxide or nitride films have hardness values of 24-28 GPa whereas the oxynitride films have a hardness of ~ 30 GPa regardless of the Cr and Al contents. Metal cutting performance tests showed that the good wear properties are mainly correlated to the oxygen-rich coatings, regardless of the cubic or corundum fractions.This abstract is based on an artile published with the same title in Acta Materialia*.

Keywords

Physical vapor deposition (PVD); Solid solution; Face-centered cubic crystals; α-Al₂O₃; Oxynitride