ICMCTF2013 Session F2-2: High Power Impulse Magnetron Sputtering

Semiconductor oxide thin films of Fe₂O₃, TiO₂ and WO₃ were deposited by reactive sputtering in various high power pulse plasma systems and their properties were investigated for photocatalytic water splitting applications. TiO2 represents semiconductor typically used for this purpose. Fe₂O₃ and WO₃ provide a potential means to capture a relatively significant portion of the available solar light, due to their lower band-gap energies. A nanocrystalline WO₃ film on a conducting glass electrode also provides high IPCEs for photoelectrochemical water decomposition under visible light. The first system used in this study was high power pulsed magnetron sputtering system (HIPIMS) employing metallic targets and working in a gas mixture of Ar and O₂ . The influence of different magnitudes of the applied pulsed power and pulsing frequency on the formation of crystalline structure, physical properties and photocatalytical properties was investigated. The next system, hybrid pulsed magnetron working in combination of simultaneous HIPIMS discharge generation and medium frequency (MF) plasma generation with MF frequency f=350 kHz, was investigated for the depositions of these coatings. Effect of the applied MF power on physical and photocatalytical properties and on the deposition rate was evaluated. Finally, the oxide thin films were deposited by a DC pulsed hollow cathode system with additional RF field. Two metallic hollow cathodes were sputtered in argon plasma flow and reactive gas was supplied directly to the reactor. The hollow cathode discharges were supplied from the DC pulsed power supply connected in parallel with the RF power source working at frequency 13.56 MHz. The main advantage of this system was the high deposition rate which was nearly independent on the amount of used oxygen in the plasma. Deposited films were more porous from the hollow cathode system in comparison with dense and flat films deposited usually by pulsed HIPIMS magnetron. A plasma diagnostics was carried out in all the investigated systems. The most important was emission spectroscopy, Langmuir probe measurement and the investigation of ion velocity distribution function by retarding field analyzer or by energetically resolved mass spectrometry. Various forms of quartz crystal microbalance QCM with several types systems of grids were used to determine ionization fraction of sputtered and reactively sputtered particle fluxes to the substrate under different deposition conditions of these oxide thin films.

The elastic properties of Mo2BC were studied using ab initio calculations. The calculated bulk modulus of 324 GPa is 45% larger than that of Ti0.25 Al0.75 N and 14% smaller than that of c-BN, indicating a highly stiff material. The bulk modulus (B) to shear modulus (G) ratio is 1.72 at the transition from brittle to ductile behaviour. This, in combination with a positive Cauchy pressure (c12 − c44), suggests moderate ductility. When compared with a typical hard protective coating such as Ti0.25Al0.75N (B = 178GPa; B/G = 1.44; negative Cauchy pressure), Mo2BC displays considerable potential as protective coating. This prediction was critically evaluated by structural and mechanical characterization of combinatorial grown Mo2BC thin films on sapphire substrates at a substrate temperature of ∼900 °C. The calculated lattice parameters are in good agreement with values determined from x-ray diffraction. The measured Young’s modulus values of ∼460 ± 21 GPa are in excellent agreement with the 470 GPa value obtained by calculations. Structural and mechanical characterization of coatings deposited in an industrial deposition system by HPPMS also result in excellent agreement with the ab initio predictions. The reduction in synthesis temperature of 300 °C compared to the combinatorial deposition described above illustrates the applicability of this coating system on technologically relevant substrates for protective coatings and underlines the relevance of the here implemented quantum mechanically guided materials design approach for application.

Injection moulding and extrusion are effective techniques for mass production of high value plastic products. However, during production of these products adhesion and abrasion wear as well as corrosion take place in the moulding tools. Limited tool life of moulding tools represents an issue for mass production especially of plastic products with complex geometries. Concerning this, ternary nitride coatings such (Cr,Al)N deposited via physical vapor deposition (PVD) have good potential to be used as protective coatings on injection and extrusion tools. For an effective protection of coated tools a uniform layer of coating material is also required. In this regard, the HPPMS (high power pulse magnetron sputtering) technology offers possibilities to improve coating thickness uniformity as well as to adapt the chemical and mechanical properties. The present work deals with the investigation of influence of HPPMS pulse length and argon/krypton ratio on the (Cr,Al)N coating properties. For this reason, (Cr,Al)N coatings were deposited with HPPMS pulse length of 40, 80 and 200 µs at constant Ar/Kr ratio (120/80 sccm). The results of these coatings were compared with a coating deposited with DC Magnetron Sputtering (DC-MS) with the same Ar/Kr ratio. Afterwards, (Cr,Al)N coatings were deposited with constant pulse length (200 µs) in pure argon atmosphere. The chemical composition of the coatings as well as the inert gas incorporation in the samples was determined using XPS (X‑ray Photoelectron Spectroscopy). Mechanical properties, morphology, phase composition and lattice parameters were analyzed by means of Nanoindentation, SEM (Scanning Electron Microscopy) and XRD (X-ray Diffraction) measurements, respectively. It can be shown that the deposition rate of the HPPMS process reduces with decreasing pulse length. Nevertheless, short HPPMS pulse leads to an increase of the hardness from 27 GPa to 34 GPa while the DC-MS coating displays a hardness of 18 GPa. Further improvement of the hardness was also indentified in the coating deposited using argon instead of mixture of argon and krypton. In addition, EIS (electrochemical impedance spectroscopy) was employed to determine the charge carrier density, which was correlated to the defect structure of the coatings.

TiN thin films have various applications in microelectronics and coating technology. In microelectronics, it is commonly used as an adhesion layer and diffusion barrier due to high thermal stability and low bulk electrical resistance. It has also been offered as a gate metal on high-κ dielectrics in metal oxide field effect transistors (MOS) technology [1]. One of the potential high-κ candidates is MgO as it has higher dielectric constant than SiO₂ [2]. Earlier we have demonstrated that films grown by high power impulse magnetron sputtering (HiPIMS) [3] exhibited higher density, lower roughness than dc magnetron sputtered films and a growth rate that was roughly constant for all temperatures [4]. Such high-quality and low-resistance films are desirable for low-temperature device manufacturing which lead to avoid post-annealing or substrate biasing. Here, we discuss the properties of ultra-thin TiN films grown by HiPIMS on single-crystalline MgO(100) substrates at growth temperatures ranging from 30 to 600 °C. The resistance of the TiN films was measured in-situ, during growth, in order to determine the coalescence thickness and film continuity at the initial stage of growth. The film grown at room temperature coalesced at 0.92 ± 0.06 nm and became structurally continuous at 2.7 ± 0.1 nm. At 600°C, the coalescence and continuity thicknesses decreased to 0.21 ± 0.04 nm and 0.58 ± 0.05 nm, respectively. X-ray reflectivity (XRR) measurements revealed that the growth rate of the films was roughly constant for all growth temperatures. The films density increased slightly up to 5.3 g/cm^-3 at 600 °C and the surface roughness of the films decreased from 1 nm to 0.3 nm while the growth temperature increased from 30 to 600 °C. For the low-temperature grown films, grazing incident X-ray diffraction (GI-XRD) measurements showed the presence of [111], [200] and [220] crystallites in all growth temperatures. The grain size of [111] crystallites slightly reduced by increased growth temperature. The minimum [220] crystallites appeared at 400 °C while the maximum [200] grain size occurred at 400°C. It was also observed that the majority of grains corresponded to the [200] direction, which is similar to substrate orientation.

[1] R. Chau, S. Datta, M. Doczy, B. Doyle, J. Kavalieros, M. Metz, IEEE Electron Device Letter, 25 (2004) 408-410.

[2] L. Yan, C.M. Lopez, R.P. Shrestha, E.A. Irene, A.A. Suvorova, M. Saunders, Appl. Phys. Lett. 88 (2006) 142901.

[3] J. T. Gudmundsson, N. Brenning, D. Lundin and U. Helmersson, J. Vac. Sci. Technol. A, 30 (2012) 030801.

Production of transition metal nitrides with dense structures in industrial relevant conditions well below the homologous temperature requires assistance by ion bombarding flux and ion energy additional to that of a sputter process. In conventional direct current magnetron sputtering (DCMS) the metal species remain in a non-ionised state and have low energies. Thus, even at high ion-to-neutral ratios of 30, microstructures can be porous. Recently, fully dense CrN films have been deposited at low temperature without substrate bias by high power impulse magnetron sputtering (HIPIMS) technology which ionizes the metal flux and dissociates reactive nitrogen molecules within its plasma.

It is not clear if the additional ionization provided by HIPIMS can outweigh a simple addition of energy by substrate biasing in DCMS. Therefore we compare floating and biased growth of CrN films by DCMS and HIPIMS technologies.

Mass spectroscopic analyses showed that HIPIMS deposition produces a factor of 10 higher flux of dissociated N¹⁺ compared to DCMS. A ratio of N¹⁺ : N₂¹⁺ = 0.4 in the HIPIMS plasma is factor 5 greater than in DCMS.

The crystallinity of the DCMS layers improved with ion energy to a certain extent. However, a step change was observed when ionization degree increased in HIPIMS-process. The texture evolved from random towards (200) as energy and ionisation increased. (220) growth was eliminated altogether.

HIPIMS layers had a laterally-homogeneous texture as observed by transmission electron microscopy. Nucleation and competitive growth were resolved very quickly and the film structure was very dense. HIPIMS deposition on top of DCMS layers resulted in fast closure of voids and establishing of a fully dense structure within 50 nm.

Growth of DCMS layers on HIPIMS layers was unable to sustain textured growth because of the low ionization of the DCMS process. Island coalescence was poor resulting low boundary density.

Results from DCMS-deposited films indicate that ion energy alone may be insufficient to promote a dense structure and a dominant (200) texture if N¹⁺ : N₂¹⁺ ratio is too low. The HIPIMS results show that elevating the N¹⁺ : N₂¹⁺ ratio to a moderate amount promotes the growth of (100) surfaces. Complementing this with moderate ion energy produces highly textured films with a fully dense structure.

Ti_1-xSi_xN alloy thin films are grown by high-power pulsed magnetron (HIPIMS) and dc magnetron (DCMS) co-sputtering. Elemental Ti and Si targets are operated in HIPIMS and DCMS, co-sputtering mode. The properties of resulting films are analyzed by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, elastic recoil detection analysis, and nanoindentation. Ion fluxes at the substrate position are determined using time-resolved in-situ mass spectrometry. The distinctly different flux distributions obtained from targets driven in HIPIMS vs. DCMS modes allow the effects of Si⁺/Si²⁺ and Ti⁺/Ti²⁺ ion irradiation on resulting film properties to be investigated separately. [1] Interesting results are presented showing the dependency of coating microstructure, phase composition, hardness, elastic modulus, residual stresses and others from process parameters and silicon content.

[1] G. Greczynski, J. Lu, M. Johansson, J. Jensen, I. Petrov, J.E. Greene, and L. Hultman, “Role of Tiⁿ⁺ and Alⁿ⁺ ion irradiation (n = 1, 2) during Ti_1-xAl_xN alloy film growth in a hybrid HIPIMS/magnetron mode”, Surf. Coat. Technol. 206 (2012) 4202

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 that are not easily possible with DC sputtering due to the higher ionization of HIPIMS. Therefore, HIPIMS technology is expected to be of interest applicable in the field of hard coatings for cutting tools. However, it was reported that the deposition rate in HIPIMS is lower than the DC sputtering rate. The low deposition rate in HIPIMS has been explained in terms of ions attraction back toward the target and ion capture by the negative potential on the cathode. In recent years, an alternative HIPIMS technique known as Modulated Pulse Power (MPP) has been developed to overcome the rate loss problem with a high degree of ionization of the sputtered material. The aim of this work is to study the effect of deposition parameters of coatings applied by constant voltage HIPIMS and MPP sputtering systems 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. Both of them coatings made by constant voltage HIPIMS and MPP sputtering systems showed better cutting performance than the coatings made by DC sputtering. Coatings deposited by HIPIMS and MPP sputtering systems showed good possibility of application in the field of cutting tools.