ICMCTF2014 Session F2-1: High Power Impulse Magnetron Sputtering (HIPIMS)

It was recently discovered that HiPIMS plasma close to the magnetron surface is not uniformly distributed; instead it forms azimuthally organized structures that rotate in the direction of E×B electron drift [1-3]. These structures were named ionization zones or spokes and play an important role in the ionization process and transport of charged particles, as demonstrated by recent measurements [4,5].

In the present work, we show existence of organized plasma structures also in the DC magnetron sputtering discharge at current densities several magnitudes lower than in the HiPIMS regime. A high speed camera was used to study morphology and evolution of plasma structures from the top and side views. A more or less periodic plasma structures containing one or two ionization zones were observed at extremely low discharge currents (i.e. 50 mA for 4 inch magnetron). Structures were stable and reproducible for the same discharge conditions. The periodicity and morphology of patterns was found to depend on the discharge current and the working gas pressure. Using a copper spacer between the magnetron and the niobium target we also show that the intensity and morphology of light patterns is more pronounced at higher magnetic fields indicating that self-organized plasma structures are related to the E×B electron drift.

[1] A. Kozyrev, N. Sochugov, K. Oskomov, A. Zakharov, A. Odivanova, Plasma Physics Reports, 37 (2011) 621-627.

[2] A. Anders, P. Ni, A. Rauch, Journal of Applied Physics, 111 (2012) 053304-053313.

[3] A.P. Ehiasarian, A. Hecimovic, T.d.l. Arcos, R. New, V.S.-v. der Gathen, M. Boke, J. Winter, Applied Physics Letters, 100 (2012) 114101-114104.

[4] A. Anders, M. Panjan, R. Franz, J. Andersson, P. Ni, accapted for publication in Applied Physics Letters

[5] M. Panjan, R. Franz, A. Anders, in preparation for Plasma Sources Science & Technology

High Power Impulse Magnetron Sputtering (HiPIMS) during the last decade has developed from an exotic idea towards a technology well understood and controlled to a large extent in laboratory scale, and now being on the cusp of industrial application. The high degree of ionization of the plasma species allows for efficient pre-cleaning processes and the deposition of layers with outstanding properties. However, at levels of 2-3 A/cm² and primarily caused by effects like gas rarefaction and electron losses across the magnetic field lines, a limit in peak current density becomes apparent that cannot be exceeded for the majority of target materials.

To overcome this limit, the hollow cathode magnetron might represent a promising alternative to the commonly used planar magnetron design. It combines the cup-shaped target and the annular plasma zone along the inner cylinder wall of an inverted magnetron with a special design of the magnetic field to control the fluxes of charged particles.

In the work to be presented, a hollow cathode magnetron has been operated with a HiPIMS power supply at duty cycles around 1 %, and the non-reactive sputtering of several target materials like copper, aluminum and carbon has been investigated. Systematically varying pulse length, frequency, discharge voltage, pressure and geometry of the magnetic field, waveforms of discharge voltage and current have been recorded and the state of the plasma has been characterized by different methods. Beyond a material-dependent voltage level the pulse current always shows a runaway behavior stopped only at current densities between 10 and 20 A/cm² due to the limits of the power supply.

Optical emission spectroscopy in time-averaged as well as in time-resolved mode has been utilized and showed the contribution of self-sputtering and the portion of metal ions rising with elapsing time during the pulse. Due to the special geometry of the sputtering cathode gas rarefaction seems to play only a minor role.

Measurement of ion currents and energy distribution functions of different species have been carried out which allow to draw conclusions on the elementary processes in the plasma zone.

In spite of several successful applications of the high-power impulse magnetron sputtering (HiPIMS) systems to reactive sputter depositions of dielectric films, there are still substantial problems with arcing on target surfaces during the reactive deposition processes at high target power densities, particularly for voltage pulses longer than 40 µs, and with low deposition rates achieved.

To avoid these problems, we have proposed a pulsed reactive gas flow control (RGFC) of the reactive HiPIMS processes. Using this process control, we are able to maintain sputter deposition of the dielectric stoichiometric films in the region between a more and less metallic mode, and to utilize exclusive benefits of the HiPIMS discharges, such as intense sputtering of atoms from the target, very high degrees of dissociation of RG molecules in the flux onto the substrate, strong “sputtering wind” of the sputtered atoms, highly ionized fluxes of particles to substrate and enhanced energies of the ions bombarding the growing films, in preparation of the films.

In the presentation, we report on details of deposition processes, including an energy-resolved mass spectrometry at the substrate position (energy distribution functions of positive and negative ions, and compositions of the integral fluxes of positive ions), and on film structure and properties.

HiPIMS with the pulsed RGFC was used for high-rate reactive deposition of densified, optically transparent zirconium dioxide films. The depositions were performed using a strongly unbalanced magnetron with a planar zirconium target of 100 mm diameter in argon-oxygen gas mixtures at the argon pressure of 2 Pa. The repetition frequency of a pulsed dc power supply was 500 Hz at the average target power density from 5 Wcm^-2 to 50 Wcm^-2 during a deposition with duty cycles from 2.5% to 10%. Typical substrate temperatures were less than 130°C during the depositions of films on a floating substrate at the distance of 100 mm from the target. Usual deposition rates, being around 10 nm/min, were achieved for the target power density of 5 Wcm^-2. An optimized location of the oxygen gas inlets above the target and their orientation to the substrate surface made it possible to improve quality of the films due to minimized arcing at the sputtered target and to enhance their deposition rates up to 120 nm/min for the target power density of 50 Wcm^-2 at the duty cycle of 10%. The zirconium dioxide films were crystalline with a predominant monoclinic structure. They exhibited a hardness of 16 GPa, refractive index of 2.19 and extinction coefficient of 0.002 (both at the wavelength of 550 nm).

CuInSe₂-based thin film photovoltaics are gaining popularity in industry due to high conversion efficiency, high productivity of the deposition process and competitive pricing against Si-based processes.

In this work CuInSe deposition experiments were carried out in a UHV system enabled with HIPIMS technology.

Energy-resolved mass spectroscopy measurements showed that the deposition plasma contained significant quantities of Cu¹⁺ and In¹⁺ ions which were on par with those of Ar¹⁺, whereas the Se ion flux constituted 1% of the total. The temperature of Ar¹⁺ ions was 7 eV in the pulse on-time and 2 eV in the pulse off-time with maximum energy of 20 eV.

CuInSe films were deposited by HIPIMS with different composition. Films with high Cu content Cu₂In_0.6Se, had faceted columnar tops and low intensity broad CuInSe₂ (112) peak. A secondary phase was detected by Raman spectroscopy.

Films with near-stoichiometric composition had smooth column tops. CuInSe₂ phase was confirmed at 350°C by Raman spectroscopy and XRD and a high ratio of (112):(220) diffraction peaks was measured. No Cu₂Se was found by Raman with excitation wavelength of 532 nm. Regardless of Cu content, columns were well defined, ranging in size from 200 to 700 nm at film thickness of 1.5 µm.

A time resolved analysis of the emission of HiPIMS plasmas reveals inhomogenities in the form of rotating spokes. The frequency spectrum analysis showed transition of the spokes from stochastic to periodic behaviour within each HiPIMS pulse. The shape of these spokes is very characteristic depending on the target material. The localised enhanced light emission has been correlated with the ion production. Optical emission spectroscopy using the bandpass interference filters isolating the emission of a single species showing enhanced ionisation within the spoke and depletion of the neutral species. Additionally, using the double flat probe, the transport of the ions and electrons outward from the closed magnetic field region has been correlated with the ionisation zone of the spoke. Based on these data, the peculiar shape of the emission profiles can be explained by the localised generation of secondary electrons (SE), resulting in an energetic electron pressure exceeding the magnetic pressure. This general picture is able to explain the observed emission profile for different target materials including gas rarefaction and second ionization potential of the sputtered elements.

This work is funded by the DFG within the framework of the SFB-TR 87.

CN_x thin films were deposited on grade AISI52100 steel substrates by high power impulse magnetron sputtering (HiPIMS) in an industrial vacuum chamber. Mixed metal ion/neutral fluxes from elemental Cr and Ta targets operated in HiPIMS mode in an Ar-based plasma were used for substrate etching, metal implantation, and/or interlayer deposition in order to improve the film adhesion. The metal ion-to-neutral ratio in the material flux to the substrate was controlled by varying the pulse energy (5 J, 10 J, and 15 J). Additionally, the effects of pretreatment at different negative bias voltages ranging between 300 V and 900 V were investigated. Other process settings, including the pulse width of 200 μs and the frequency of 100 Hz as well as the deposition pressure of 200 mPa and substrate temperature of 150 °C were kept constant during interlayer formation. CN_x films with a thickness of ~1000 nm were subsequently deposited by reactive HiPIMS from a pure graphite target at an Ar/N₂ flow ratio of 0.16. Corresponding deposition processes were carried out at a pressure of 400 mPa, a pulse energy of 4.5 J and a pulsed negative bias voltage of 150 V. Transmission electron microscopy in combination with energy dispersive X-ray spectroscopy and selective area electron diffraction provided insights to which extent interlayer and substrate intermixing occurred as well as into the structural evolution of the interlayers and CN_x films. The chemical bonding structure at the interface between Cr or Ta interlayer and CN_x film was investigated by X-ray photoelectron spectroscopy. Increasing the pulse energy and the negative bias voltage during interlayer growth resulted in an effective cleaning of substrate surface oxides, pronounced interlayer-substrate intermixing, and metal carbide as well as metal nitride formation. Rockwell C tests revealed an improved adhesion (HF = 1 to 2) of CN_x films on steel with a Cr interlayer. The resultant mechanical properties of the film were assessed by nanoindentation, revealing a moderate hardness and a considerable resiliency.

Multilayer coatings based on transition metal nitrides such as CrN, AlN and TiN deposited via physical vapor deposition (PVD) have shown great advantage as protective coatings on tools and components subject to high loads in tribological applications. By varying the individual layer material and their thicknesses it is possible to optimize the coating properties, e.g. hardness, Young’s modulus and thermal stability. Other alternative for further improvement of the coating properties is the use of different PVD technology. High power pulse magnetron sputtering (HPPMS) is an advancement of pulsed magnetron sputtering (MS). The use of HPPMS allows a better control of the energetic bombardment of the substrate. It offers the possibility to influence chemical and mechanical properties by variation of the process parameters. The present work deals with the development of CrN/AlN multilayer coatings in an industrial scale unit by using two different PVD technologies. In this work middle frequency pulsed (MF) MS and high power pulse magnetron sputtering (HPPMS) technology were used. The bilayer period Λ, thickness of a CrN/AlN, was varied between 37 and 6 nm by varying the rotational speed of the substrate holders. In a second step one rotational speed was chosen and further HPPMS CrN/AlN coatings were deposited applying different HPPMS pulse lengths (200, 80 and 40 µs) at the same cathode power and frequency. The chemical composition of the coatings was determined using Glow Discharge Optical Emission Spectroscopy (GDOES). Morphology, roughness and phase composition were analyzed by means of Scanning Electron Microscopy (SEM), confocal laser microscopy, and X-ray Diffraction (XRD), respectively. Detailed characterization of the multilayers was conducted by Transmission Electron Microscopy (TEM). Hardness and Young’s modulus were analyzed by nanoindentation measurements. The results of the phase analysis show the formation of h-Cr₂N and c-AlN for the MF CrN/AlN coatings while in the HPPMS coatings only cubic phases (c-CrN, c-AlN) were detected. A high hardness of 31 GPa was measured for the HPPMS with a bilayer period of 6 nm. Decreasing of HPPMS pulse length at constant mean power leads to a considerable increase of cathode current. It could be observed that the deposition rate of the CrN and AlN reduces with decreasing pulse length, so that a CrN/AlN coating with a bilayer period of 3 nm and a high hardness of 40 GPa was achieved using a short pulse length of 40 µs.

High power impulse magnetron sputtering (HiPIMS) is a promising technique for the large scale deposition of nanocrystalline ZnO thin films on temperature sensitive substrates. The most common techniques for depositing semiconducting ZnO films currently include DC and pulse DC magnetron sputtering, rf sputtering, MBE and PLD. The highest reported mobilities (110 cm²V^-1s^-1) and on-of ratios ( up to 10¹²)[1] are for nanocrystalline ZnO films deposited with PLD. However, these techniques are limited in that they either require high deposition temperature and epitaxy (MBE), produce films with lower electron mobilities (DC and pulsed DC sputtering) or are difficult to adapt for large-scale depositions (rf sputtering and PLD).

In this study, we investigate the interrelationship between HiPIMS plasma characteristics and resulting film microstructures for nanocrystalline ZnO films grown from ceramic ZnO and metallic Zn targets onto amorphous SiO₂. For depositions from both the ZnO and Zn targets, XRD and SEM measurements indicate highly crystalline, (002) oriented films with a layer at the SiO₂ interface where different ZnO orientations are in competition. Alignment of the (002) orientation in the films improves with decreased growth pressure until 5 mTorr. Films grown below 5 mTorr had a bimodal microstructure with larger (~25 nm) grains among the small crystals. The effect of plasma conditions on the competitive growth layer and (002) alignment was investigated by correlating time-resolved current measurements of the target and ion energy distributions to film microstructure. The results were used to optimize HiPIMS growth conditions of ZnO channels for back-gated thin film transistors. XRD, SEM and AFM studies of ZnO films deposited at optimized growth conditions show microstructures comparable to those deposited with PLD. The I-V characteristics of the produced devices are measured to determine the electrical transport properties of the ZnO films.

[1] B.Bayraktaroglu, K. Leedy, R. Neidhard. Microwave ZnO Thin Film Transistors. IEEE Electronic Device Letters V 29 Iss. 9 2008