ICMCTF2014 Session F2-2: High Power Impulse Magnetron Sputtering (HIPIMS)

Titanium dioxide is a material that has been used in a range of thin-film applications, particularly as on optical coating, but more recently in the exploitation of its inherent photocatalytic and hydrophilic properties – self cleaning, anti-fogging, or antimicrobial surfaces, for example. In order to exhibit photocatalytic activity, it requires the necessary anatase, or mixed anatase/rutile crystalline phase to be achieved during film growth, or via post-deposition treatments. However, these growth or post-deposition treatment conditions generally require the substrate to withstand elevated temperatures (>400^oC), which precludes the use of thermally sensitive substrates such as polymeric web. The ability to produce crystalline photocatalytic titania onto such substrates would provide significant opportunities for commercial exploitation in a range of industries.

HiPIMS deposition has been shown previously to be able to deliver high energy to a depositing film, whilst maintaining a relatively low substrate temperature, enabling the deposition of high-quality functional films on thermally sensitive substrates such a polymeric web. This paper provides a study of the growth of as-deposited crystalline titania coatings via the HiPIMS process, the effect on photocatalytic and hydrophilic activity, and their application onto polymer substrates.

Activation of titania photocatalysts normally requires incident UV radiation, due to the high band gap of titania (~3.2 eV). In order to achieve activity at wavelengths approaching or including visible light, a variety of dopant materials can be used to modify the bandgap of the titania coating. In addition to the study of HiPIMS deposition of pure titania, an investigation of the growth and properties of coatings reactively sputtered from a tungsten-doped titanium target, also via HiPIMS, is presented.

In High Power Impulse Magnetron Sputtering (HiPIMS), high degree of ionization of the sputtered material is achieved thanks to the high instantaneous peak powers and thus high plasma densities. HiPIMS is therefore beneficial for deposition of dense films, uniform coatings on complex-shaped surfaces as well as interface engineering for improved adhesion. In reactive HiPIMS, compound thin films are deposited from metal targets in an atmosphere of argon and a suitable reactive gas, such as oxygen. As a result of the interaction between sputtered metal and the reactive gas, the relation between the deposition rate and reactive gas flow is nonlinear and typically exhibits hysteresis behaviour.

This contribution deals with reactive HiPIMS, mainly with the effect of HiPIMS on the hysteresis. First, results from various hysteresis studies are summarized. We show that the frequency dependence indicates an influence of gas rarefaction. The optimum frequency is related with the gas refill time as demonstrated by experiments with targets of different dimensions. Formation of compound at the target surface is also accompanied by a pronounced change in the shape of discharge current waveforms. In HiPIMS, the discharge behaviour is dominated by ionized oxygen sputtered from the target. This is shown from the ion energy distribution functions of different species characterized by energy and time resolved mass spectroscopy. Finally, we simulate the target surface sputtering by TRIDYN code. The predicted time to completely remove the compound from a target is in reasonable agreement with measurements.

Chopped-HiPIMS (c-HiPIMS), a modified version of High Powered Impulse Magnetron Sputtering, HiPIMS, has been employed for depositing Ti containing thin films on various substrates. The technique consists of decomposing single HiPIMS pulses into a sequence of pulses. In this manner, the pulse is a ‘chopped’ variant of HiPIMS, thus termed chopped-HiPIMS. This technique has been used to deposit films of metallic titanium, titanium nitride, TiN, and titanium silicon nitride, Ti-Si-N. These coatings were characterized by XPS, XRD, SEM and nanoindentation. Higher deposition rates than under equivalent HiPIMS parameters were attained, with increases in deposition rates of up to 150 % by c-HiPIMS for Ti deposition. At the same time, stoichiometric TiN and Ti-Si-N films could be grown without the need for a substrate bias or additional heating of the substrate during deposition, as is required when using traditional direct current magnetron sputtering, DCMS. The c-HiPIMS TiN films have comparable hardnesses to DCMS films grown with heat or bias, at around 25 GPa (compared to ~ 20 GPa for HiPIMS with similar conditions).

Data from optical emission spectroscopy, OES, and atomic absorption spectroscopy, AAS, showed little differences between HiPIMS and c-HiPIMS. This suggests that the ion density and ion-to-atom ratios reaching the substrate, produced by both methods, are similar. Thus the increase of sputtered particle flux in c-HiPIMS appears not to be a result of relaxed ion retention (ion trapping) in the near-cathode region. Further, temperatures recorded at the substrate are similar for both methods, indicating similar ion energies. Thus, in c-HiPIMS the reduced gas rarefaction and increased gas refill at the substrate leads to the prevention of self-sputtering and thereby favours increased argon bombardment of the target.

Magnetron sputtering technologies are widely used for the deposition of thin films in many commercial applications. In recent years, high power impulse magnetron sputtering (HiPIMS) and modulated pulse power magnetron sputtering (MPP), a variation of HIPIMS, have shown great advantages as compared to the conventional (DCMS) and pulsed dc magnetron sputtering (PMS) techniques. Unlike the simple one pulse shape in HIPMS, MPP generates a high density metal ion plasma by first producing a weakly ionized plasma followed by a transition to a strongly ionized plasma within one overall pulse. However, commercially available HiPIMS plasma generators have not been able to create stable and arc-free discharges in many reactive processes.

In this work a new method of generating an arc free discharge for reactive HiPIMS has been used. A Cyprium plasma generator from ZPulser has been used to deposit both Cr and CrN thin films on silicon and steel substrates using the new deep oscillation magnetron sputtering (DOMS) technique. The effect of the peak current and deposition pressure on the structural (X-Ray diffraction), chemical (Electron Probe Micro-Analysis) and morphological (Scanning Electron Microscopy) properties of the films was studied. All depositions were done at constant average power (1.2 KW) and a constant thickness of 1 micrometer was deposited for all the films. The peak current increases with the deposition pressure while the peak voltage behaves in a symmetrical way. Increasing the peak current at constant pressure (0.8 Pa) allows the deposition of more compact Cr films. The columnar structure similar to the one obtains by DCMS slowly transforms in a dense morphology without any porosity. The hardness of the Cr films also increases from 9 to 16 GPa. The mechanical properties of the CrN thin films (hardness and friction coefficient) were studied by nano-indentation and pin-on-disk tests on the films deposited on steel substrates. The wear rate of the films was evaluated from the inspection of the pin-on-disk tracks by white light interferometry.

SIPP – technology (SIPPSuper Imposed Pulse Power) offers significant advantages in HiPIMS sputtering combined with a synchronized pulsed BIAS. The synchronized pulsed BIAS is able to operate fully synchronized or phase shifted. The phase shift can be set at any time of the pulse period. Variances of the pulse duration, pulse period or phase shift settings directly result in different thin film properties like microstructure, morphology or mechanical properties especially with respect to residual stresses. Using the example of reactive sputtered Al-Cr-N thin films we demonstrate the influence of Highly Charged Metal Ions Particle Extractions using SIPP on the texture, film morphology and stress development. The characterization of the plasma with respect to the ionization moment of the sputtered metal particles was carried out by spectrometer analysis. In order to examine and compare the metal ion density the spectrometer analysis was performed both at the target and the substrate.

Among the design of experiment (DOE) methods, Taguchi experimental design is becoming a popular one for industrial application due to its efficiency and convenience.

The aluminum nitride thin film is an important coating material due to its unique properties, such as wide band gap, good thermal and chemical stability and dielectric property. In this work, the aluminum nitride thin films were fabricated using the high power impulse magnetron sputtering (HIPIMS) process through the Taguchi experiments to determine the optimum deposition condition. The duty cycle, pulse frequency and substrate bias were varied to systematically study the structure and property evaluation of AlN coatings. The crystalline phase and microstructure of AlN coatings were evaluated by X-ray diffractometer (XRD) and scanning electron microscopy (SEM), respectively. The mechanical properties of AlN coatings were explored by nanoindenter, scratch tester and wear tester. Influences of duty cycle, pulse frequency and substrate bias of HIPIMS on the peak power density, microstructure and mechanical properties of coatings were discussed in this study through the Taguchi analysis.

Keywords: high power impulse magnetron sputtering, aluminum nitride thin film, duty cycle, pulse frequency, mechanical properties, Taguchi analysis

During reactive magnetron sputter deposition of oxides, bombardment by high-energy negative oxygen ions can have detrimental effects on the structural, optical and electrical properties of developing thin films. The amount of high-energy atomic oxygen negative ions (O^-) detected at the substrate has been found to be strongly correlated to the secondary electron emission coefficient of the target surface.

By means of energy-resolved mass spectrometry, time-averaged O^- ion energy distributions (IED) have been obtained during reactive high power impulse magnetron sputtering (HiPIMS) of titanium in the presence of oxygen and different inert gases: X/O₂ where X = Ne, Ar, Kr or Xe. The high-energy population of O^- ions was found to peak close to the value of the absolute applied target potential averaged over the pulse on-time. The high-energy O^- ion yield was estimated by integrating over the high-energy population of the IEDs and was found to decrease markedly for heavier inert gases. The decrease in negative ion emission is believed to be a consequence of lower potential secondary electron emission coefficients for heavier gases due to lower ionization potentials.

Surface analysis techniques were applied to titania samples deposited onto both glass and silicon substrates via reactive HiPIMS of titanium in Ar/O₂ and Kr/O₂ gas mixtures for a number of different discharge conditions.