ICMCTF2010 Session B1-1: Sputtering Coatings and Technologies

Addition of oxygen into nitride-based nanocomposite coatings represents many possibilities in microstructural design, but also challenges in processing and characterisation. In this paper, we present recent results on the formation, microstructure, and mechanical properties of Al-Cr-Si-O-N coatings deposited usingpulsed dc magnetron sputtering under Ar/(N₂+O₂) atmosphere. A wide range of oxynitrides varying from pure nitrides to pure oxides was prepared by changing the O₂/N₂ ratio of the reactive gas flow. The content of Si as well as the ratio of Al/Cr were varied within a significant range. The coatings were analysed by EPMA, SIMS, XRD, TEM, and nanoindentation to demonstrate, in particular, the role of O:N ratio and deposition temperature between 400°C-700°C on structural evolution.

Chemical analysis revealed that the incorporation of oxygen into the films increases much faster than the fraction of oxygen in the gas flow so that the pure oxides can be formed from the gas ratio of about O₂/(O₂+N₂) = 20%. Addition of oxygen alters the crystallinity of nitride phases due to the incorporation of interstitial atoms and formation of metal vacancies, but nitride lattices seem to survive up to the oxygen incorporation of about 40%. Beyond this limit, the amorphisation of films continues up to the pure oxides which show the formation of crystalline a-Al₂O₃, stimulated by the presence of a-Cr₂O₃ and SiO₂ in the films. Transition from nitride to oxide has strong consequences on films hardness, which changes from 30-33 GPa for nitrides to 12-13 GPa for amorphous layers, and increases again to 20-25 GPa for pure oxides. The relationship between structure, mechanical properties, and thermal barrier capability of coatings will be discussed.

Electrochemically deposited high contraction chromium (HC Cr) coatings have been used for decades to protect the interior surfaces of cylindrical structure for extended cycle life. However, chrome electroplating process generates hexavalent chrome, mandated for reduction or elimination. HC Cr also possesses as-deposited and operation-induced cracks to cause accelerated wear and erosion of the substrate. In this paper, we describe two approaches using plasma-enhanced cylindrical magnetron sputtering systems to deposit pollution-free tantalum on the interior surfaces of 120mm cylinders. In a stand-alone plasma-enhanced DC cylindrical magnetron sputtering system where the barrel acted as the anode, substrate biasing capability and a MPP (modulated pulse power) plasma generator power supply were incorporated. The addition of biasing and the MPP was to increase the degree of ionization and ion bombardment for improved coatings qualities. The system utilizes an in-situ plasma cleaning device for pre-deposition substrate and target cleaning to enhance adhesion. In the other plasma-enhanced DC cylindrical magnetron system which sits in a vacuum chamber, a HIPIMS (high power impulse magnetron sputter) power supply was added to generate high flux high ionization plasma. Tantalum coatings were deposited using biased DC and HIPIMS technology. The system utilizes a new double glow discharge (DGD) process for pre-deposition surface cleaning. Several cylinders of 120mm inside diameter were successfully coated directly with tantalum coatings of 100-200 µm thickness; and the coated steel cylinders are being analyzed. The Plasma-Enhanced Cylindrical Magnetron technologies using biased DC, MPP, and HIPIMS generated plasma, their effects on the hardness, phase, residual stress, topography and microstructure of the coatings will be discussed.

At the Tribology Centre at Danish Technological Institute we have performed large-scale reactive pulsed sputter deposition of TiO₂ and doped TiO₂ for photocatalytic applications. In addition cerium gadolinium oxides (CGO) and yttrium stabilized zirconium oxides (YSZ), and alternating layers hereof, have been synthesized for low temperature fuel cell applications. The various coatings were deposited on a commercial CC800/9 CemeCon Sinox unit.

For the TiO₂ system it will be shown that reproducible growth of dense, as well as highly porous TiO₂ coatings can be synthesized in the rutile and the anatase crystal phase (or mixtures hereof) as evident from SEM, RBS and XRD results. The photocatalytic activity has been quantified by photocatalytic oxidation of acetone into CO₂ and correlated to the obtained micro-/nanostructure, film thickness and choice of substrate material. The effect on the micro/-nanostructure by doping with, e.g., Cu and the impact on antimicrobial properties will be addressed.

For application of YSZ electrolytes and CGO diffusion layers it is of outmost importance to be able to control the film morphology and to avoid pinholes leading to gas penetration of the electrolyte, which is detrimental to the overall performance of the fuel cells. In order to increase the propability of depositing thin YSZ films it has been necessary to grow multi-layer YSZ/CGO coatings. The structure of single-layer YSZ and CGO films will be discussed and compared with the YSZ/CGO multilayer systems for various process parameters. The oxygen ion conductivity of the coatings and the strontium diffusion barrier ability of CGO films will be compared for different experimental process parameters.

In this study, a detailed analysis of the serial co-sputtering setup has been performed. In order to evaluate the process, it is essential to understand the dynamic processes on the rotatable target and e.g. the influence of its rotational speed. Dynamical simulations of deposition, implantation and sputter erosion on this target has been carried out by the TRIDYN code using binary collision approximation. The effect of the power ratio between the primary and auxiliary cathode and the influence of target material properties and the rotational speed has been analyzed. Transport of sputtered material and deposition at the substrate was simulated by a Direct Simulation Monte Carlo (DSMC)) model. By computational and experimental means we have evaluated optimum conditions for a sputtering yield amplification in a non-reactive process. The homogeneity of sputtered films and the functionality of the yield amplification mechanism crucially depend on the implantation and sputtering dynamics of materials A and B at the primary erosion site. This dynamical behaviour is mapped into a compositional lateral in-homogeneity in the deposited films. From our investigation of this in-homogeneity the influence of different process parameters like e.g. rotational speed or discharge voltage on the dynamics of material mixing is concluded.

Sophisticated arc management with extremely short detection time and fast recovery are of great importance in modern DC power supplies for plasma applications.

However, opportunity of improvements in hardware already almost ends. By u sing the best available elements it is possible to extinguish an arc within less then 2ms. Further improvements are still possible by software related changes.

This paper presents a method of significant (tenfold) reduction of sputtering losses caused by arc extinguishing breaks . Moreover, reduction of the detection time to a range of tens of nanoseconds is obtained by using new-developed software algorithms and advanced microprocessor techniques. Decreasing detection and reaction time minimize arc energy which results in elimination of cathode damage and prevents droplets formation on a substrate. Introduced software algorithm also focuses on new control solutions to optimize arc quenching power circuitry.

Power supplies equipped with the mentioned features are able to handle arcs with energy lower then 0.4mJ/kW, which is additional significant reduction in comparison with currently best on the market generators. Also the number of arcs per second that power supply is able to manage is substantially increased. This is crucial for some processes in which arcing rate may reach up to 10,000 arc/s and dynamically - even up to 100,000 arc/s.

All presented improvements allow using standard DC power supplies in applications reserved so far for expensive pulsed generators. Increase of deposition rate and process time savings are an additional gain.

Practical results of above mentioned improvements obtained on industrial TCO processes will be presented.