Effect of the degree of high power impulse magnetron sputtering utilisation on the structure and properties of TiN films
Introduction
High power impulse magnetron sputtering (HIPIMS) is a fast developing technology, which utilises extremely high power impulses (short pulses) to ionise the sputtered metal atom flux. It is characterised with power densities of about 3 kW cm− 2 and current densities of about 2 A cm− 2 applied at low duty cycle of < 0.25% [1]. Each power impulse undergoes a cycle of breakdown-ignition, gas sputtering and self-sputtering and produces highly dense plasma of the order of 1013 cm− 3. This has the effect of ionising and activating reactive and inert gases in the plasma while sputtered metal atoms traversing the plasma are ionised with a high probability and charge states of 2+ or higher are observed for many target materials. The resulting deposition flux is rich in metal ions and highly activated gas ions which results in Me+/Me0 (metal ion to metal neutral) and G+/Me+ (gas ion to metal ion) ratios of 1 near the substrate. The mean energy of metal and gas ions of approximately 6 eV is factor 3 greater than in conventional sputtering in the same conditions [2]. The high ion-to-neutral ratios, high degree of metal ionisation and gas activation in the deposition flux are prerequisites for the build-up of a dense microstructure and specific preferred orientation of the coatings. Thus HIPIMS coatings have improved wear, corrosion and oxidation resistance. Oxide coatings have improved optical and electrical properties [3]. The HIPIMS has also been used for pre-treatment of the substrate prior to coating deposition to improve adhesion by intensive sputter-cleaning of impurities and metal ion implantation [1], [4]. Extensive reviews on the sputter-cleaning process and technology can be found in the literatures [3], [5].
However, one of the drawbacks of HIPIMS is the lower deposition rate when compared to the conventional magnetron sputtering. Major contribution to the reduction is the back attraction of the positively charged metal ions to the cathode, an effect observed and described already with the arc discharges [6]. One way to improve the deposition rate is to use lower strength magnetic fields, for example lower than 40 mT as suggested in [7], which reduces the magnetic confinement of the plasma and allows more positive ions to reach the substrate. Following this approach, deposition rates reaching 90% of the deposition rate of the conventional magnetron sputtering were achieved for Nb and other metals [7], [8]. With deposition systems equipped with multiple and different plasma sources however, there is a possibility to run combined processes where only parts of the sources are operated in HIPIMS mode. A schematic cross section of a Hauzer system equipped with one HIPIMS source and three conventional unbalanced magnetrons was published already in 2004, soon after the upscaling of the HIPIMS technology [9]. In such configuration HIPIMS was made available to be used for surface pre-treatment to enhance the coating-substrate adhesion and also to deposit the coatings, either as a single source or in combination with the standard unbalanced magnetrons with improved microstructure due to the higher ionisation of the sputtered material. Indeed, highly dense TiN coatings with excellent mechanical properties deposited by HIPIMS can be seen elsewhere in the literature [10], [11], [12]. Nanoscale multilayer coatings of CrN/NbN with enhanced wear and corrosion resistance [13], CrN/TiN with improved wear resistance [14] and CrAlYN/CrN with improved high temperature oxidation resistance [15] were produced by the combined HIPIMS and conventional direct current (dc)-unbalanced magnetron (UBM) sputtering technology. The performance enhancement of the above said coatings was attributed to the fully dense microstructure with reduced multilayer waviness and sharp interfaces resulting from the higher ad atom mobility (energy) of the condensing species provided by HIPIMS.
Furthermore, the penalty due to the lower deposition rate of HIPIMS is expected to be significantly reduced with combined HIPIMS/UBM sputtering. However, the contribution, the extent of the HIPIMS utilisation in such combined processes, needs to be well understood and carefully considered. The answer to the question: “how much HIPIMS is needed in the process for the production of high quality coating” is not straightforward. The answer to this question has implications also on the physical vapour deposition (PVD) system manufacturers as well as HIPIMS power supply manufacturers who shall consider the design and the specifications of the next generation of systems enabled to deliver the combined technology.
The aim of this research is to shed more light on the effect of combining HIPIMS with UBM sputtering in one process by studying the ionisation degree in the plasma, coating microstructure and the deposition rate. TiN has been chosen as a model coating, which presents a simpler case to study the above mentioned effects in the HIPIMS/UBM combination. The authors however, believe that the conclusions of this study will be transferable to other more sophisticated coatings too.
Section snippets
Deposition of TiN coatings
TiN coatings were deposited in an industrial size PVD coating machine (chamber volume — 1 m3, HAUZER 1000 — 4 HTC, Hauzer Techno Coating, The Netherlands) enabled with HIPIMS technology at Sheffield Hallam University. The original system was equipped with four dc-unbalanced magnetrons with target area of 1200 cm2 each. In the modified version of the HTC 1000 — 4 system, two of the magnetrons were connected to HIPIMS power supplies allowing operation selectively either in UBM or in HIPIMS mode (
Effect of different HIPIMS/UBM source combination
The effect of various HIPIMS/UBM source combinations on {Ti1 +} and {N20} is illustrated in Fig. 2. All the measurements were carried out with coil current (Icoil): 3 A, bias voltage (Ub): 0 V and working pressure: 0.3 Pa.
The {Ti1 +} starts at a low value of 0.13 for the pure UBM process and rises continuously as more HIPIMS cathodes are energised; reaching a maximum of 0.75 for the pure HIPIMS process. This increase is related to the increased production of Ti1 + ions in the high peak power HIPIMS
Conclusions
Combining high power impulse magnetron sputtering with dc-unbalanced magnetron sputtering in one deposition process in a multiple source deposition system such as Hauzer 1000/4 is an effective approach which allows manipulation of the ionisation degree in the plasma therefore widens the process window for coating structure, texture, residual stress and properties control. The OES measurements revealed that the {Ti1 +} in the deposition flux was found to be increased with increasing number of
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