ICMCTF2011 Session B1-1: PVD Coatings and Technologies

Magnetron sputtering is a mature and well establish PVD deposition technique. Since the introduction of commercial planar magnetrons in the 1970s there are few vacuum coating sectors that haven’t been touched by successful implementations of this deposition technique. In the 1970s the semiconductor industry was revolutinonized by the introduction of planar magnetron sputtering as an alternative to evaporation and diode sputtering. Even today more than a quarter of a century later, magnetron sputtering is at the heart of many of the advances in the new wave of the semiconductor revolution. In commercial terms we have seen very strong performances of magnetron sputtering technology in sectors like glass, web, thermal solar, photovoltaic thin film, display and decorative coatings., This presentation will give a historic overview of magnetron sputtering with its main breakthroughs, the current status of the technology in important PVD coating sectors and will look at the current and future challenges ahead.

Physical vapour deposition techniques (PVD) such as magnetron sputtering and electron-beam vapour deposition operate in high vacuum. The mean free path of the sputtered or evaporated atoms is much larger than the source to substrate distance. The atoms arrive nearly without collision at the substrate. As a result, only areas are coated which are in line of sight to the deposition source. In contrast to these PVD techniques gas flow sputtering (GFS) is know for its capability to coat non line of sight areas (NLOS) without substrate manipulation. Atoms are sputtered from a hollow cathode by glow discharge. A high inert gas flow streams through the cathode and provides transportation of sputtered material to the substrate. The mixture of inert gas and sputtered atoms performs a circulation around the contour of the geometry and reaches NLOS areas. Coating thickness distribution depends on fluid dynamics.

In this investigation a basic understanding for the observed thickness distributions and a model for the mass transport was generated. Stripes, cylinders and turbine blades were coated with pure titanium. Thickness distributions and microstructure were examined by scanning electron microscopy. Some coating procedures were simulated by computational fluid dynamics to understand the correlation between gas flow and coating thickness distribution.

Argon is the process gas of choice for most magnetron sputtering applications due to its large atomic mass, inert chemistry, and relatively low cost. Other inert gases are available for use in sputtering deposition that have varying mass and hence different momentum behaviour during ion bombardment of solid surfaces - affecting sputter yield, particle implantation and incorporation of process gas into deposited films. The plasma discharges generated from these gases vary in terms of the nature and energy of species incident at both target and substrate. In particular, the contribution from energetic neutrals varies as a consequence of the atomic mass of the process gas in comparison to the target material to be sputtered.

Magnetron plasma discharges were generated from neon, argon, krypton and xenon gases in DC and mid-frequency pulsed-DC modes with different transition metal cathode materials. The electrical characteristics, such as potential and current at the cathode, substrate and in the plasma bulk were measured and compared. Thin metallic films were then deposited and analysed in terms of structure, properties and process gas incorporation. The data generated is used to establish the relationship between process gas species, plasma discharge characteristics of those gases, and the subsequent growth and properties of deposited coatings.

Electron beam physical vapor deposition (EB-PVD) is an industrially well established large area PVD technology. Highly productive air-to-air coaters for metal strip, roll-to-roll coaters for plastic web or thin metal foils as well as inline carrier coaters for substrates or lots of substrates have been introduced into industrial production. Axial type high power EB guns having a nominal power of several 100 kW at 60 kV are available as technological key components in order to generate high vapor densities.

Great efforts have been made to combine high rate EB-PVD with plasma generation processes in order to activate the generated vapor. The industrial break through can be expected in the near future especially as result of current developments in the field of spotless arc activated deposition.

Electron beams are excellent tools for vacuum coating as well as for materials melting and refining purposes.

EB technology provides high power densities, fast and precise control of the beam spot combined with energy efficiency, and low contamination affinity.

High-voltage glow-discharge (HVGD)-based EB sources represent a compact and cost-efficient yet versatile variant of beam generation technologies. In the past, they have been used in fields as different as CRT or pumping of gas lasers for a long time. The high discharge voltage, compared to ordinary glow discharges, results in new effects, most importantly in runaway-electrons carrying the major part of the discharge power. Recently, significant progress has been achieved in understanding the underlying mechanisms of the discharge and of the beam generation as well as in enhancing the stability of operation by advanced technical means. This made it possible to design high-power cold-cathode EB guns of axial type capable of producing beam currents up to 15 A at accelerating voltages up to 45 kV and of focusing these beams into spots of 20 mm or less. These features brought applications such as PVD and vacuum metallurgy into the reach of cold-cathode EB technology.

Computer simulation appeared to be an important tool for gaining insight into the physics of high voltage glow discharges and for optimizing practical beam sources. Conventional ray-tracing codes can neither consider space charge effects of multiple species nor particle generation or loss by stochastic collisions despite these effects are essential in HVGD-based beam sources. Particle-in-cell (PIC) codes previously developed for simulating kinetic effects in plasma physics, however, proved to be able of correctly modeling HVGD. Congruence of the predictions of the simulation and of the experimental findings supports the plausibility of the numeric results. Additionally, new regimes of the HVGD might be able to overcome disadvantages of established designs. First experiences and explanations will be presented.

TiN was deposited on tungsten carbide (WC) cutting inserts and high speed steel (HSS) test drills using the Innova deposition system from Oerlikon Balzers. The process was modified to interrupt film growth during deposition. The new coating was contrasted with an existing standard TiN coating architecture.

Microhardness of both TiN coatings was measured using a UMIS2000 microhardness tester. A significant increase in hardness was observed. The modified coating displayed a 25% increase in median values of hardness and Young’s modulus when compared with the current TiN coating.

Coating performance was then compared by drill testing on a Haas vertical machining centre (VMC) using coated HSS test drills. Coatings were further analysed with scanning electron microscopy (SEM). Fracture cross-sections of the new coating revealed no distinct multilayer structure but continuous grain growth was observed. Transmission electron microscopy (TEM) of focussed ion beam (FIB) cross-sections revealed detailed grain features of the coating.

Glancing angle X-ray diffraction (XRD) was used to identify the coating phase structure across its thickness and showed fcc TiN with a predominant (111) orientation. Residual stress analysis was undertaken on a PANalytical X’Pert Pro materials research diffractometer (MRD), utilising the psi-tilt method and compared with standard TiN coatings. Additionally the X’Pert Pro MRD was used to examine the phase evolution of both TiN coatings at high temperatures.

Most of these problems could be solved by using a AlCr(Si)N/TiN nanolayered film on cemented carbide substrates. The single layer thickness is in the range of 20 nm with fluent interfaces which results in a nanocrystalline structure. The deposition process is very stable and deposition rates from 15 to 20 micrometer per hour with rotating substrates could be obtained. Very thick films (up to 50 microns) with a low level of intrinsic stress were deposited by using adapted process parameters. It could be shown, that a film thickness distribution on complexly shaped substrates and the suppression of film defects can be well controlled by process guiding.

The hardness measurements by nano-indentation show only a small decrease with increasing film thickness. Hardness values of 30 – 33 GPa at 5 µm thick films were obtained in comparison to 29 – 31 GPa at 50 µm thick films.