ICMCTF2012 Session F2-2: High Power Impulse Magnetron Sputtering

High power impulse magnetron sputtering (HIPIMS) combines impulse glow discharges at power levels up to the MW range with conventional magnetron cathodes to achieve a highly ionised sputtered flux. If observed with a low time resolution, the optical emission from the HIPIMS discharge may appear to be homogeneous during the pulse. However, we have shown recently that the HIPIMS plasma may develop drift wave type instabilities [1]. They are characterized by well defined regions of high and low plasma emissivity along the racetrack of the magnetron and cause periodic shifts in floating potential. The structures rotate in ExB direction at velocities of ~10 kms^-1 and frequencies up to 200 kHz. It has already been shown in literature that the magnetron configuration may exhibit two-stream instabilities due to the difference in confinement of electron and ions and the associated differences in particle fluxes [2]. However the characteristic frequency of these instabilities is of the order of a few MHz, well above drift wave type instabilities we have observed.

In this paper a detailed analysis of the temporal evolution of the saturated instabilities using four consequently triggered fast ICCD cameras is presented. The influence of mass of the target material and working gas on the instability properties was investigated using titanium (Ti) and aluminium (Al) targets in either Ar or Kr gases. Furthermore working gas pressure and discharge current variation showed that the shape and the speed of the instability strongly depend on the working gas and target material combination. In order to better understand the mechanism of the instability, different optical interference band pass filters (of metal and gas atom, and ion lines) were used to observe the spatial distribution of each species within the instability. It was found that the optical emission from the instabilities comprises ion emission (both target material and gas ion lines) with strong depletion of the emission lines of the target material atom lines, concluding that instabilities are of generalised ion drift wave type.

[1] A. Ehiasarian, A. Hecimovic, T. de los Arcos, J. Winter, R. New, V. Schulz-von der Gathen, M. Böke, Appl. Phys. Lett. submitted

[2] D. Lundin, P. Larsson, E. Wallin, M. Lattemann, N. Brenning and U. Helmersson, Plasma Sources Science and Technology, 2008, (17), 035021

Acknowledgement: This work was funded within Subproject A5 of SFB-TR 87. We gratefully acknowledge valuable discussions with Prof. A. Ehiasarian and the kindness of Prof. H. Soltwisch and Dr. P. Kempkes on borrowing us the four cameras setup.

The elaboration of new nanomaterials on wires and on fibers is of great interest for industry. One possibility to manufacture these innovative products is to deposit thin films in a continuous way on wires or fibers. This solution offers possibility to synthesize new materials with a large flexibility for the process utilization.

An instrumental prototype designed as a continuous air-to-air in-line wire-coating has been set-up with the capability to run with speed lines between 5 to 30m/min. A new Inverted Cylindrical Magnetron (ICM) plasma deposition reactor operated in the HIPIMS mode has been developed. It has been demonstrated both theoretically and experimentally that this configuration allows a high ionization degree of sputtered target material while reaching the same high deposition rate as for planar magnetrons operated in the DC mode. It has been shown that this ICM configuration, which is moreover ideally adapted for a homogeneous wire coating due to its cylindrical symmetry, promotes a better self-assistance with target ions and allow for a repeated use of ionized species to maintain a high ionization degree in metal plasma. Also, the magnetron was intentionally designed with an unbalanced system of magnets to ensure the diffusion of the plasma electrons along the force lines of the magnetic field to the central part of the discharge volume. This magnetic field can permit to form a channel from the target surface to the wire, where the plasma volume can be extended down to the substrate area with minimal losses. It has also been shown that a non-stationary magnetic field allows to a better stabilization of the discharge voltage even at a high degree of the target erosion.

The outstanding performance of the ICM prototype, which is characterized by a much higher deposition rate than in conventional planar systems, was validated in direct measurements. The values of the deposition rate for the ICM were compared with a reactor equipped with 4 planar magnetrons surrounding the wire for the same power input per the cm² of the target surface. It was concluded that a 10 times higher deposition rate can be achieved with this ICM designed system. Also, in order to demonstrate the performance of the wire coated prototype instrument, elaborations of ZnO and AlTiSiN films on wires have been achieved. The structural and chemical properties of the ceramic films has been established and correlated with end-used performances.

The high ionization of sputtered metal particles is a main advantage of High Power Impulse Magnetron Sputtering (HiPIMS) discharges. Large quantity of ionized sputtered material leads to the growth of smooth and dense films, allows control of the crystallography phase, mechanical and optical properties etc. Because of these positive effects there is reason to develop sputtering sources with high level of metal ionization. It is already known that namely energy of ions and incoming particles to growing film is a key parameter which influences film property. In our contribution we report two novel HiPIMS-based techniques which allow energy control in wide range.

The first system is a unipolar hybrid-dual HiPIMS based on a combination of dual-HiPIMS (the system where two magnetically and electrically confined magnetrons are alternately operated in HiPIMS mode, f = 100 Hz, duty cycle 1 %) with mid-frequency (MF) discharge operated at f = 94 kHz and duty cycle 30 %. The second system is based on sputtering of HiPIMS driven electrode inserted in RF discharge with an additionally superimposed magnetic field (ECWR). The most important feature of these hybrid methods is the pre-ionization effect which causes/allows: (i) significant reduction of working pressure by nearly two orders of magnitude, (ii) intensive ionization of metal atoms with substantial amount of double ionized species, (iii) increase of HiPIMS power density and other discharge parameters, (iv) faster ignition and development of HiPIMS pulses.

Enhanced time-resolved diagnostic of developed plasma sources has been done. From time-resolved Langmuir probe measurements was estimated mean electron energy, electron density and electron energy probability function (EEPF). The plasma density reached values about 5.10¹⁸ m^-3 during HiPIMS pulses at low pressure (0.04 Pa). The time–resolved measurements of Ion Velocity Distribution Functions (IVDFs) and Ion Energy Distribution Functions (IEDFs) were performed. It was found that ion energies during HiPIMS pulses are strongly enhanced (about 20-30 eV) while in background (MF or RF) discharge were measured much lower values. Parameters from Langmuir probe diagnostic serve also as input for calculation of influx contributions of particular species, e.g. neutral particles. The study of plasma transport effects was done by fast optical emission imaging and spectroscopy.

This work was supported by Deutsche Forschungsgemeinschaft through SFB/TR 24 and by the German Federal Ministry of Education and Research (BMBF) through Campus PlasmaMed. Further projects KAN301370701 of ASCR, 1M06002 of MSMT and project 202/09/P159 of GACR are acknowledged.

High Power Pulse Magnetron Sputtering, HPPMS (also known as HiPIMS), is a promising pulsed plasma technology for deposition of high quality thin films. HPPMS has been shown to exhibit very high electron density and ionization of the sputtered material. The plasma charged particles are easily affected by magnetic and electric fields and present therefore conditions for the thin film developer to control the energy bombardment and subsequently the properties of the deposited thin films. Even if HPPMS has a great potential for bringing new and interesting solutions to the PVD market, it is still inconsistent and not standardized as is always the case for new technologies or techniques. It has been established that in order for HPPMS to function in an optimal fashion, it is of importance to understand not only the plasma condition of the process but also how the different parts of a coating system interact with each other in order to give the right process conditions needed for the deposition.

A number of works have characterized and attempted to correlate the effect of pulsing on the HPPMS process, especially on the deposition rate. These have used different power supplies as well as different power supply combinations in order to achieve the different modes of operation. They found that the pulsing configuration and frequency affect to a large extent the coating process as well as the resulting thin film properties.

Recent approaches supply power to the magnetron source by super-imposing a medium frequency (MF) and a HPPMS power unit. The main advantage of such an approach is to provide means to even better architect the plasma conditions that best enable a suitable compromise between deposition rate and ionization. Moreover, the combination of MF and HPPMS allows better tuning of the process parameters and therewith the resulting thin film’s properties. This new approach for using the HPPMS technique is appropriate for single and dual magnetron sputtering and synchronized pulsed bias. It is therefore possible to perform stable reactive depositions in continuous operating systems such as inline glass coaters.

The present work investigates the influence of the pulsed power controllers and the mechanisms for producing the very high peak currents needed for the HPPMS process. The influence of MF and HPPMS superimposed pulse packages on the deposition rate during reactive sputtering of titanium in an Ar/N₂ atmosphere will be presented. Additionally the resulting TiN film microstructure and phase/texture composition will be discussed with respect to the corresponding plasma conditions.