ICMCTF2012 Session F2-1: High Power Impulse Magnetron Sputtering

The microstructure and the resulting properties of coatings and thin films strongly depend on the growth mechanism which in turn is closely related to the energetic aspects of surface reactions during the material’s synthesis. It has been accepted that the microstructural evolution of films grown in ionized and plasma environments can be well described in terms of the ion energy and ion flux, or specifically, in terms of the “universal parameter: the energy per deposited atom”.

Film growth, while under ion bombardment, leads to growth-related effects such as interfacial atom mixing, high surface mobility (diffusion) of deposited species, resputtering of loosely bound species, and deep penetration of ions below the surface, leading to the displacement of atoms (forward sputtering or knock-in effects). The energy and flux of ions can generally be controlled, to different levels of selectivity, by the use of ion beams, by surface biasing, and by the control of plasma density. Compared to more traditional PECVD and PVD techniques (including DC, pulsed DC, and medium-, radio- and microwave frequency discharges), there has been a lot of progress in generating very dense plasmas in pulsed discharges, more recently in High Power Impulse Magnetron Sputtering (HiPIMS). The latter technique offers a unique possibility to obtain films from a high flux of highly ionized materials.

In this presentation, we will critically evaluate the energetic aspects of the film growth in HiPIMS plasmas, and compare the resulting film characteristics with those obtained by other techniques. Examples will include hard protective coatings (conductive or partially conductive), as well as optical (generally non-conductive) coatings. We will review different strategies for the control of the deposition process, and discuss various effects related to the pulse management, to the suppressions of hysteresis, and to the magnetic field configuration at the target surface.

We introduce a brand new technology based on High Power Pulse Sputtering at industrial scale. The New Technology is a magnetron discharge process like conventional sputtering, however, momentarily input power is approximately ten times higher in magnitude. Conventional magnetron sputtering is a low current - high voltage discharge and the ionization rate of the target material is quite low, usually in the order of a few percent and the plasma is mainly consisting of gas ions. On the other hand the New Technology realized stable operation with high current - high voltage discharge with high ionization rate of the target material and high deposition rate compared to conventional sputtering. This unique property is characterized by controlling pulse shape of input power, this means the film property generated by the New Technology can be modified with great flexibility.

In this study, we analyzed film property by changing coating condition and compared with another coating process. For example, different types of nitride coatings including standard TiAlN were deposited by industrial arc ion plating (AIP), unbalanced magnetron sputtering(UBMS) and New Technology . TiAlN coatings deposited by the New Technology process show strong preferred (111) orientation and a relatively high hardness up to 35 GPa. Whereas AIP TiAlN coatings are characterized by a moderate hardness up to 30 GPa and (200) or nearly random orientation at an equivalent substrate bias condition. High magnification image of cross sectional TEM observations of both coatings revealed that many lattice defects can be observed for the New Technology coating and is hardened by many atomic defects, possibly highly stressed as a consequence.

In the presentation, a comparison between AIP, UBMS and the New Technology coating by different power supply, arc source and deposition conditions will be shown, not only from property of the coating but also from industrial perspective such as productivity.

The physical vapor deposition methods, characterized by highly ionized deposition fluxes of the film forming species, provide added means for the synthesis of tailor-made materials. Cathodic arc and pulsed laser deposition are examples of such discharges where electron densities in the order of 10²¹ m^-3 can be obtained. These techniques, while providing a very high degree of ionization of the deposition flux, often exhibit several drawbacks, such as macroparticle ejection from the target, lack of lateral film uniformity, and difficulty to scale up. Magnetron sputtering based techniques are technologically interesting, owing to their inherent advantages of conceptual simplicity, scalability, and film uniformity. However, electron densities in magnetron discharges are significantly smaller, in the range of 10¹⁴-10¹⁶ m^-3 and therefore generation of a highly ionized deposition flux is often difficult. This difficulty is overcome by high power impulse magnetron sputtering (HiPIMS), where plasma densities on the order of 10¹⁹ m^-3 are achieved. HiPIMS has been successful in enhancing the ionization for most common metals (Cu, Al, Ta, Ti), but it is challenged by C. Previous investigations have shown that C⁺/C ratio in HiPIMS does not exceed 5% [1]. In the present study we address the low degree of C ionization by increasing the electron temperature of the plasma. This is achieved in the HiPIMS discharge by using Ne as sputtering gas instead of Ar. It resulted in an energetic C⁺ ion population with a three-fold increase in the total number of C⁺ ions as compared to a conventional HiPIMS process. The enhanced ionized fraction of carbon facilitates the growth of carbon films with mass densities as high as approx. 2.8 g/cm³ as determined by high resolution x-ray reflectively measurements.

[1] B.M. DeKoven, P.R. Ward, and R.E. Weiss, D.J. Christie, R.A. Scholl, W.D. Sproul, F. Tomasel, and A. Anders, Proceedings of the 46th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, May 3-8, 2003, San Francisco, CA, USA, vol., p.158

High Power Impulse Magnetron Sputtering (HIPIMS) has been of interest over the past decade owing to its ability to ionize sputtering materials at higher ionization energy. It is possible to modify coating properties in ways which are not easily possible with DC sputtering due to the higher ionization of HIPIMS. Therefore, HIPIMS technology is expectedly applicable in the field of hard coatings for cutting tools. The aim of this work is to study the effect of deposition parameters of coatings applied by HIPIMS and to study its applicability in the field of cutting tools.

In this study, bias voltage during coating was investigated in details. In the same regard, chemical composition, morphology and crystal structure of coatings were analyzed using Electron Probe Micro Analyzer (EPMA), Scanning electron microscope (SEM) and X-Ray Diffraction (XRD) under different deposition parameters. Furthermore, cutting tests were made with different deposition parameters. The coatings made by HIPIMS system showed better cutting performance than the coatings made by DC sputtering showing good possibility of application in the field of cutting tools.

Metastable cubic structure Ti_1-xAl_xN alloy thin films are grown in an industrial scale coating unit by high-power pulsed magnetron sputtering (HIPIMS or HPPMS) using segmented Ti-Al targets. The properties of resulting films are analyzed and compared to layers grown from elemental Al and Ti targets using a hybrid approach in which HIPIMS was combined with dc magnetron sputtering (DCMS). All films are analyzed by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, elastic recoil detection analysis, and nanoindentation. Ion fluxes at the substrate position are determined using in-situ mass spectroscopy. Hardness of Ti_1-xAl_xN films grown from segmented targets peaks at 31 GPa for AlN concentrations well below x = 0.6, above which it stays low around 20 GPa. Residual stresses determined by sin²ψ analyses are compressive and high, reaching -4 GPa at x = 0.50 for a Si substrate. Relaxed cubic lattice parameters a_o(x), obtained from θ-2θ scans acquired at the strain-free tilt angle ψ^*, decreases monotonically with increasing AlN concentration up to x = 0.58, corresponding to a kinetic solubility limit of AlN in TiN. Comparison to Ti_1‑xAl_xN alloys grown in a hybrid configuration with either Al or Ti target operated in HIPIMS mode (Al-HIPIMS/Ti-DCMS or Ti-HIPIMS/Al-DCMS, respectively) reveals that properties of purely HIPIMS films obtained from segmented targets are better than in the case of Ti-HIPIMS/Al-DCMS films and worse than for Al-HIPIMS/Ti-DCMS films. These results fully support our recent assessment [1] concerning the detrimental role of Ti²⁺ ions, created in large concentrations during HIPIMS operation of the Ti target, which upon application of a mild substrate bias lead to creation of residual defects, high compressive stresses, and precipitation of hexagonal AlN phase

[1] Selection of Metal Ion Irradiation for Controlling Ti_1-xAl_xN Alloy Growth via Hybrid HIPIMS/Magnetron Co-sputtering, G. Greczynski, J. Lu, M. P. Johansson, J. Jensen, I. Petrov, J.E. Greene, and L. Hultman, Vacuum 2012, in press.

Modulated pulsed power (MPP) magnetron sputtering and middle frequency pulsed dc magnetron sputtering (PMS) techniques have been used to synthesize thick CrN/AlN multilayer coatings (up to 10 µm). The Cr target was powered by the MPP technique while the Al target was powered by the PMS technique simultaneously in an Ar/N₂ mixture. The bilayer period of the coatings was varied in a range of 10 nm to 2.5 nm by varying the ratio of the N₂ flow rate to the total gas flow rate. The microstructure and properties of the CrN/AlN coatings were characterized using electron probe microanalysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, scratch test, nanoindentation, and ball-on-disk wear test. The thick CrN/AlN coatings has been annealed in the ambient air at 900 ^oC and 1000 ^oC for 2 hours and at 850 ^oC for 200 hours. The MPP+PMS CrN/AlN coatings showed good adhesion. Superhardness values of 40-45 GPa and low wear rates in the low 10^-8 mm³N^-1m^-1 range have been achieved in the coatings with the bilayer period in a small range of 2.5 to 3.2 nm. No oxides were identified in the coatings and the coating maintained cubic structure after annealing at 1000 ^oC for 2 hours and at 850 ^oC for 200 hours.