ICMCTF 2023 Session B4-1-MoM: Properties and Characterization of Hard Coatings and Surfaces I

The number of papers in peer-reviewed journals where X-ray photoelectron spectroscopy (XPS) analysis is employed increased by a factor of 40 during last 40 years, to the level of 12000 articles published in 2021 alone. This makes XPS the most common – and for many indispensable - method for characterization of surface chemistry. From a concern within the surface science community that this rapid increase in the number of XPS studies is accompanied by a decrease of work quality,[1] we reviewed efforts towards improving correct use of the technique for reliability of data. Several sources of errors have thus been identified, including an unreliable charge referencing of the binding energy (BE) scale,[2] unrecognized sputter damage,[3] and neglected effects of sample storage.[4] By performing experiments on large sets of thin film samples such as Group IVB-VIB transition metal borides, carbides, nitrides, and oxides, we demonstrated disconcerting failure of the most popular referencing method based on the C 1s peak of adventitious carbon.[2]We appeal that it should no longer be used, with science reproducibility at stake. Furthermore, the extent of spectral changes following Ar⁺ etching, commonly applied to remove surface contaminants, was evaluated using reference spectra from in-situ capped samples.[3]We showed that changes greatly depend on the type of material system: from very subtle effects in the case of Group IVB TM carbides to a complete modification of spectral appearance for IVB-TM oxides. The effects of sample storage on XPS spectra were evaluated for commonly used storage environments such as office shelf, polypropylene wafer carrier, polystyrene box, cellulose/polyester wipers or sealed polyethylene bag.[4]Results revealed significant differences between the various storage types and provide guidance for planning all sorts of studies including those that employ Ar⁺ ion etch prior to analyses. Examples illustrating each of the above issues will be discussed during the talk and several principled practices offered.

(V,Mo)N is considered to be a promising coating material for tribological applications, owing to having high hardness and ductility deriving from its extraordinary metal-nitrogen bonding environment [1]; however, the coating is lack of relevant mechanical properties and fracture toughness data. The objective of this study was to investigate the effect of nitrogen flow rate on the microstructure and mechanical properties of (V,Mo)N nanocrystalline thin films. Five compositions of (V,Mo)N thin films were deposited by dc-unbalanced magnetron sputtering with various nitrogen flow rate. The N/Metal ratios increased from 0.47 to 0.85 with increasing nitrogen flow rate from 1.2 to 6.0 sccm. The texture of the thin films changed from (200) to random texture with increasing nitrogen flow rate. (V,Mo)N thin film deposited at the lowest nitrogen flow rate (D12) was found to contain multiple phases by transmission electron microscopy, and single phase (V,Mo)N thin films were obtained as nitrogen flow rate was above 2.5 sccm. Specimen D12 possessed the largest hardness owing to the multiphase structure, where the Mo metal phase may retard the crack propagation, thereby increasing the film hardness. Fracture toughness (G_c) of the thin films was evaluated using internal energy-induced cracking method [2]. The fracture morphology showed that the cracks initiated in the Si substrate and then propagated into the film, implying the high toughness of (V,Mo)N thin films. The resultant G_c of the (V,Mo)N thin films, ranging from 20.9 to 30.1 J/m², linearly increased with increasing nitrogen content. The results showed G_c of the (V,Mo)N thin films was more significantly affected by nitrogen content than by texture. The higher G_c for the (V,Mo)N thin films than that of VN and TiN thin films is consistent with the theoretical predictions.

Control of stress evolution during film growth is crucial in many coating applications and requires in situ analysis. One such product is a micron-thick free standing biocompatible Ti/Pt multilayer film used as diaphragms for implantable inner ear microphones [[1] [#_ftn1] ].For a given size and thickness the diaphragm compliance and hence, the deflection can only be maximized if the intrinsic stresses in the thin film structure are zero. In this work, we have measured intrinsic stresses based on dynamic wafer curvature measurement using multiple laser beam deflection optical Stress measurements. The instantaneous stress state at any stage of Ti and Pt layer’s growth and the final stress of a micron-thick film allows for direct feedback of the effects of deposition parameters. We investigated the influence of process pressure, target-to-substrate distance, target power, and influence of applied bias to the substrate- while keeping a low temperature- on the residual stress in Ti single-layered and Ti/Pt multi-layered films deposited on double-sided polished 150 mm thick Si(100) wafers using magnetron sputtering technique [[2] [#_ftn2],[3] [#_ftn3]]. The example of stress evolution during the growth of a Ti single layer compared to oscillating stress state during Ti/Pt multilayer is shown in the figure. An excellent finding here is that tuning the layer thickness ratios in Ti/Pt multilayers particularly at the onset of growth up to 100 nm can be used to engineer the final stress state between compressive and tensile. This is attributed to Ti growing as type I (low mobility) generating tensile stress while Pt grows as type II (high mobility) generating compressive stress, at room temperature. The residual stress is also compared with post-deposition XRD wafer curvature measurement technique and differences along with the structural characterization of the films will be presented at the conference.