Reactive deposition of Al-N coatings in Ar/N2 atmospheres using pulsed-DC or high power impulse magnetron sputtering discharges

In this paper, the metal to ceramic transition of the Al-N₂ system was investigated using classical reactive pulsed-DC magnetron sputtering and HIgh Power Impulse Magnetron Sputtering (HIPIMS) at a constant average current of 3 A. Optical emission spectroscopy measurements revealed more ionised aluminium species in the HIPIMS discharge compared to pulsed-DC sputtering. It also showed excited N⁰ and ionised N⁺ species in reactive Ar/N₂ HIPIMS discharges. The corresponding evolution of the consumed nitrogen flow as a function of the N₂ partial pressure revealed that a higher amount of reactive gas is needed to achieve stoichiometric AlN with HIPIMS. Electron probe micro-analysis and X-ray diffraction measurements confirmed that a partially poisoned aluminium target is enough to allow the deposition of stoichiometric hcp-AlN thin films via HIPIMS. To go further in the comparison of both processes, two stoichiometric hexagonal aluminium nitride thin films have been deposited. High power impulse magnetron sputtered hcp-AlN exhibits a higher nano-hardness (18 GPa) than that of the coating realised with conventional pulsed-DC sputtering (8 GPa).     

Time and energy resolved ion mass spectroscopy studies of the ion flux during high power pulsed magnetron sputtering of Cr in Ar and Ar/N2 atmospheres

Mass spectroscopy was used to analyze the energy and composition of the ion flux during high power pulsed magnetron sputtering (HIPIMS/HPPMS) of a Cr target in an industrial deposition system. The ion energy distribution functions were recorded in the time-averaged and time-resolved mode for Ar⁺, Ar²⁺, Cr⁺, Cr²⁺, N₂⁺ and N⁺ ions. In the metallic mode the dependence on pulse energy (equivalent of peak target current) was studied. In the case of reactive sputtering in an Ar/N₂ atmosphere, variations in ion flux composition were investigated for varying N₂-to-Ar flow ratio at constant pressure and HIPIMS power settings. The number of doubly charged Cr ions is found to increase linearly with increasing pulse energy. An intense flux of energetic N⁺ ions was observed during deposition in the reactive mode. The time evolution of ion flux composition is analyzed in detail and related to the film growth process. The ionization of working gas mixture is hampered during the most energetic phase of discharge by a high flux of sputter-ejected species entering the plasma, causing both gas rarefaction and quenching of the electron energy distribution function. It is suggested that the properties (composition and energy) of the ion flux incident on the substrate can be intentionally adjusted not only by varying the pulse energy (discharge peak current), but also by taking advantage of the observed time variations in the composition of ion flux.     

Reactive sputtering of titanium in Ar/CH4 gas mixture: Target poisoning and film characteristics

Reactive sputtering of titanium target in the presence of Ar/CH₄ gas mixture has been investigated. With the addition of methane gas to above 1.5% of the process gas a transition from the metallic sputtering mode to the poison mode was observed as indicated by the change in cathode current. As the methane gas flow concentration increased up to 10%, the target was gradually poisoned. The hysteresis in the cathode current could be plotted by first increasing and then subsequently decreasing the methane concentration. X-ray diffraction and X-ray photoelectron spectroscopy analyses of the deposited films confirmed the formation of carbide phases and the transition of the process from the metallic to compound sputtering mode as the methane concentration in the sputtering gas is increased. The paper discusses a sputtering model that gives a rational explanation of the target poisoning phenomenon and shows an agreement between the experimental observations and calculated results.     

Hysteresis behaviour of reactive high power impulse magnetron sputtering

Hysteresis behaviour during reactive High Power Impulse Magnetron Sputtering (HIPIMS) has been investigated in detail. Such analysis has been made possible by the recently developed plasma emission monitoring based reactive HIPIMS monitoring and control technology. Hysteresis curves were recorded at frequencies of 300, 450 and 600 Hz at an average power of 3.0 kW during reactive HIPIMS of Ti in Ar/O₂ atmosphere. It is shown that the target pulsing parameters, such as frequency, pulse voltage, and duty cycle do affect the overall shape of the hysteresis loop. Analysis of the hysteresis behaviour at different target pulsing parameters reveals how different regions of the hysteresis loop are affected by different pulsing parameters. The outcomes of this work demonstrate trends and explain relationships between the pulsing parameters and the hysteresis behaviour. Although the overall picture is rather complicated, it is quite clear that the hysteresis effect is induced by the same processes as in direct current magnetron sputtering, while the influence of the reactive ion implantation oxidation mechanism appears to be far more significant in reactive HIPIMS.     

Deposition rate characteristics for steady state high power impulse magnetron sputtering (HIPIMS) discharges generated with a modulated pulsed power (MPP) generator

High power impulse magnetron sputtering (HIPIMS) pulses have been of great interest over the last decade. With such sputtering techniques a substantial amount of target material can be ionized and used for the engineering of surfaces and coatings. Depending on voltage, system configuration and target material, such discharges can be either transient or reach steady state currents during the pulse. The used HIPIMS power supply was a constant voltage supplies. Similarly, HIPIMS pulses with multiple steady state current phases can be generated using a modulated pulsed power (MPP) generator. A typical pulse consists of an ignition, low current and high current phase. The contribution of these phases to the deposition rate is presented. The ionization rate of single charge chromium ions has been found to increase linearly with increasing peak current density. An increase in deposition rate with lower magnetic field strength at the target surface can be attributed to a higher sputter yield due to a higher cathode voltage due to increasing system impedance in HIPIMS case, weaker trapping of deposition flux and to enhanced ion flux towards the substrate.     

Rotatable magnetron sputtering of aluminium in continuous and high power pulse modes using different strength magnetic arrays

This paper reports preliminary results of industrial size (152 mm target O.D.) rotatable magnetron sputtering of Al target in direct current (DC) and High Power Impulse Magnetron Sputtering (HIPIMS) modes using two standard commercially available magnetic arrays: standard strength array (as used for DC and AC processing) and a lower strength ‘RF’ array [i.e. as used for radio frequency (RF) magnetron sputtering]. A comparison of processes resulted in by combining the different magnetic arrays and power modes is made in terms of magnetic field distribution on the cathode surface, magnetron characteristics, process characteristics and deposition rates.Optical emission spectroscopy (OES) revealed enhanced sputtered Al flux ionisation in the HIPIMS discharge monitored 64 mm away from the target surface when using the ‘RF’ array. Importantly, the results of this work (at the processing conditions investigated) demonstrate that at the same average power the deposition rate of Al using HIPIMS in conjunction with the ‘RF’ array is substantially the same as that obtained for the ‘standard’ strength balanced array and DC power. This indicates that the magnetic field design of the ‘RF’ magnetic array affects favourably the sputtered flux transport perpendicular to the target surface by altering mass transport direction and minimising effects that reduce deposition rate (e.g. ion return effect). Arc rate is also reduced significantly (approximately ten times) if the low strength ‘RF’ array is used.     

The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge

The energy distribution of sputtered and ionized metal atoms as well as ions from the sputtering gas is reported for a high power impulse magnetron sputtering (HIPIMS) discharge. High power pulses were applied to a conventional planar circular magnetron Ti target. The peak power on the target surface was 1-2 kW/cm² with a duty factor of about 0.5%. Time resolved, and time averaged ion energy distributions were recorded with an energy resolving quadrupole mass spectrometer. The ion energy distributions recorded for the HIPIMS discharge are broader with maximum detected energy of 100 eV and contain a larger fraction of highly energetic ions (about 50% with E_i > 20 eV) as compared to a conventional direct current magnetron sputtering discharge. The composition of the ion flux was also determined, and reveals a high metal fraction. During the most intense moment of the discharge, the ionic flux consisted of approximately 50% Ti¹⁺, 24% Ti²⁺, 23% Ar¹⁺, and 3% Ar²⁺ ions.     

Enhancement of the crystalline quality of reactively sputtered yttria-stabilized zirconia by oxidation of the metallic target surface

Yttria-stabilized zirconia (YSZ) thin films were fabricated on glass substrates by direct current magnetron reactive sputtering. We found out that the crystalline quality of the YSZ film was improved by an oxidation process of the metallic target surface prior to the sputtering deposition process. It is speculated that, at the initial stage of the sputtering, the sputtered particles from the oxidized target surface form a layer with higher degree of crystallization on the substrate, compared with those particles from the metallic target surface. This crystallized layer can enhance the crystallization of the film deposited subsequently. Other sputtering conditions such as sputtering pressure, oxygen flow rate, substrate temperature, and Y₂O₃ content in the film were investigated, for optimization of the crystalline quality of the deposited YSZ film.     

High-power impulse magnetron sputtering of Ti-Si-C thin films from a Ti3SiC2 compound target

We have deposited Ti-Si-C thin films using high-power impulse magnetron sputtering (HIPIMS) from a Ti₃SiC₂ compound target. The as-deposited films were composite materials with TiC as the main crystalline constituent. X-ray diffraction and photoelectron spectroscopy indicated that they also contained amorphous SiC, and for films deposited on inclined substrates, crystalline Ti₅Si₃C_x. The film morphology was dense and flat, while films deposited with direct-current magnetron sputtering under comparable conditions were rough and porous. We show that, due to the high degree of ionization of the sputtered species obtained in HIPIMS, the film composition, in particular the C content, depends on substrate inclination angle and Ar process pressure.     

Target voltage measurements during DC sputtering of silver in a nitrogen/argon plasma

During reactive sputtering, addition of the reactive gas results in a target voltage change. This effect finds its origin in the modification of the gas composition but also in the change of the target condition. In this paper, we focus on the target voltage changes during magnetron sputtering of silver in an argon/nitrogen plasma. In the first second during the nitrogen addition, we notice a decrease of the target voltage followed by an increase in target voltage. The target voltage decrease can be easily explained from the increased volume ionisation. The change of the target condition seems to be responsible for the target voltage increase. However, this effect cannot be explained from the formation of a silver nitride phase on the target surface as generally accepted during reactive sputtering of metal nitrides. Indeed, silver is a poor nitride former. To explain the target voltage increase, we have studied in this paper the influence of ion implantation of N₂⁺ ions on the target voltage during magnetron sputtering. The ions were implanted in situ in a silver target and the target voltage of this modified target was registered under the same conditions as during the sputtering experiments. The implantation of the N₂⁺ ions results in a target voltage increase. Hence, during sputtering of a silver target in an argon/nitrogen plasma, the target voltage increases by the presence of non-reacted N atoms in the target top surface layers.     

HIPIMS Power for Improved Thin Film Coatings

High Power Impulse Sputtering (HIPIMS) has received increasing attention as a new sputtering technique. The main feature of the HIPIMS process is the high ionization percentages in the sputter flux. This can be used for film densification, surface modification, trench filling and other applications. Layers produced with HIPIMS show superior properties in many applications. The most investigated and promising HIPIMS application is for hard coatings in wear and corrosion protection (e.g., CrN or TiN). This application and the related coating equippment has been discussed in detail by W.‐D. Münz in a previous issue [1]. The significantly lower heat transfer to the substrate compared to standard magnetron sputtering is another advantage of the HIPIMS process. This enables high rate coating even on temperature sensitive substrates. HIPIMS also significantly changes the hysteresis curve in reactive sputtering, offering a much easier control of the reactive process. HIPIMS power supplies can be added to existing sputter systems with little or no system modification. By this way the process capability can be extended easily. Production power supplies with pulse energies up to 16MW and pulse frequencies up to 1 kHz are available. Design and features of these power supplies are discussed in this article.     

Structure and properties of high power impulse magnetron sputtering and DC magnetron sputtering CrN and TiN films deposited in an industrial scale unit

Deposition of complex shaped or round-symmetric samples requires multi-fold substrate rotations during deposition or multiple cathode arrangements. The present paper investigates the influence of the high power impulse magnetron sputtering (HIPIMS) and DC magnetron sputtering (DCMS) process on the mechanical and tribological properties as well as the resulting structure of CrN and TiN coatings using static (0-fold) and dynamic (1-, 2- and 3-fold) depositions in an industrial scale unit. Furthermore, to increase the deposition rate without losing the high ion density in the plasma a hybrid HIPIMS/DCMS deposition technique is investigated. The results demonstrate the advantage of the HIPIMS technique when using multi-fold substrate rotation during deposition as it enables depositions of CrN_HIPIMS and TiN_HIPIMS coatings with hardness values around 23 and 35 GPa, respectively, compared with around 15 GPa for CrN_DCMS and TiN_DCMS coatings. Hardness values of 35 GPa for TiN_DCMS coatings prepared with substrate rotations could only be obtained when introducing an additional anode or using a multilayered CrN_HIPIMS/TiN_DCMS base layer as a template.Based on our results we can conclude that especially for up-scaling and multi-fold substrate rotations the HIPIMS process offers an improved performance as compared to DCMS.     

Formation of ZnO nanostructures in energy-controlled hollow-type magnetron RF plasma

In this study, we investigated how zinc, sputtered from a zinc target, reacts with oxygen on the substrate to form ZnO nanostructures when the discharge parameters, such as gas flow ratio and target bias voltage, are controlled in O₂/Ar plasma. The deposits were estimated by SEM and Raman spectroscopy. Under conditions of a Zn to Ar optical emission intensity ratio of 2/1, a target voltage of − 550 V, a total pressure of 40 Pa, a substrate temperature of 150 °C, an RF power of 50 W, and a deposition time of 30 min, many vertically aligned ZnO nanorods were observed to be deposited on the substrate. The diameter of the rods was typically 50 nm. It was found that the film morphology can be controlled by the sputtering rate of zinc varied by the target bias voltage and gas flow rate.     

Effect of oxygen concentration and system geometry on the current-voltage relations during reactive sputter deposition of titanium dioxide thin films

Current-voltage relations at different magnetron sputtering systems and gas mixtures were studied during reactive sputter deposition of titanium dioxide thin films. The main goal of this work was to investigate the influence of reactive gas mixture (Ar + O₂) and system geometry on the electrical characteristics of the discharge. The geometries utilized were the conventional magnetron sputtering, hollow cathode magnetron sputtering and triode magnetron sputtering. A change in the system geometry leads to a change in the electric field distribution, which alters the working range of the discharge voltage and magnetron efficiency. It is noticed that the discharge voltage at constant current can be reduced when the geometry is altered from conventional magnetron to hollow cathode magnetron or triode magnetron, at the same time the magnetron efficiency is increased when hollow cathode magnetron or triode magnetron are used instead of conventional magnetron sputtering.     

Control of microstructures and properties of dc magnetron sputtering deposited chromium nitride films

Chromium nitride coatings have been prepared by a conventional DC magnetron reactive sputtering process in nitrogen-argon mixed atmospheres. The sputtering pressure and target voltage versus nitrogen flow rate curves were established in order to control the structures and properties of chromium nitride coatings. A good correspondence among the sputtering pressure, target voltage evolutions and the phase developments with respect to nitrogen flow rate has been found. The stoichiometric Cr₂N and CrN coatings were confirmed by EPMA and XRD analysis. Cryogenic fracture cross-section SEM images show columnar growth morphologies. Stoichiometric chromium nitrides present high hardness and elastic modulus as well as high H³/E² ratio in a nano-indenter test. Adhesion and tribological properties were evaluated by scratch and pin-on-disk tests, respectively. Chromium nitrides present normal adhesion failure critical load (Lc₂) between 10 and 20 N and friction coefficients ranging from 0.5 to 0.75.     

Novel TiAlCN/VCN nanoscale multilayer PVD coatings deposited by the combined high-power impulse magnetron sputtering/unbalanced magnetron sputtering (HIPIMS/UBM) technology

A new TiAlCN/VCN coating combining high hardness, low friction coefficient and chemical inertness has been developed for dry machining of “Sticky” (Al-, Ti- and Ni-based) alloys as well as advanced Metal-Matrix-Composite (MMC) materials used in aerospace and automotive industries. Excellent performance was achieved due to the synergy between V and C as main coating elements and the nanoscale multilayer structure of the coating. TiAlCN/VCN was deposited by the combined High-Power Impulse Magnetron Sputtering/Unbalanced Magnetron sputtering (HIPIMS/UBM) technology. Macroparticle free V⁺ ion flux generated by HIPIMS discharge was used to sputter clean the substrates prior to the coating deposition. A 0.4 μm thick TiAlN base layer followed by 3 μm thick TiAlCN/VCN nanoscale multilayer coating was deposited by unbalanced magnetron sputtering. The sputtering was carried out in a mixed CH₄, N₂ and Ar atmosphere. In dry milling of Al7010-T7651 alloy, TiAlCN/VCN nanoscale multilayer PVD coating outperformed state of the art Diamond Like Carbon (DLC, Cr/WC/a-CH) coating by factor of 4. In drilling Al-alloy enforced MMC materials, cemented carbide drills coated with TiAlCN/VCN produced 130 holes compared to 1-2 holes with uncoated drills.     

Magnetron sputtering process control by medium-frequency power supply parameter

In this paper, the influence of the sputtering technology (reactive gas partial pressure, magnetic field intensity) on the power supply parameters has been investigated. The reactive sputtering of aluminium target in Ar + O₂ atmosphere has been used as an example. The experiments have shown that the formation of the Al₂O₃ layers can be observed by the parameter of the power supply. It allowed estimating the point at which the sputtered material surface was poisoned by the reactive compound i.e. distinguishing magnetron sputtering modes.     

Spatial and temporal investigation of high power pulsed magnetron discharges by optical 2D-imaging

Optical 2D-imaging in combination with Abel inversion was used to study the spatial and temporal evolution of the plasma-induced emission of HIPIMS discharges. A titanium target, as well as an aluminium-doped zinc target was sputtered in an argon atmosphere of pressures 0.53 Pa and 1.33 Pa and an average power of 650 W and 400 W, respectively. The discharge was observed optically employing various wavelength filters to investigate the development of selected species, namely argon and titanium neutrals, as well as argon and zinc neutrals and ions.The argon neutral emission did not only differ substantially from the DC case but also underwent a significant development during the pulse ‘on’-time, showing a structure similar to an ion acoustic wave travelling away from the target, as well as rarefaction of the working gas while sputtering with high discharge peak currents. In addition, the intensity profile of argon and zinc ions revealed an increased width in front of the target which is then reflected by a wider sputter distribution of the target material. Indeed, it was found that the width of the metal neutral emission increased with increasing discharge current. Their emission revealed two distinct maxima and the loss of intensity in between these maxima could be explained by an increased ionisation of the sputtered metal flux.     

Resputtering-induced chemical inhomogeneity during the growth of highT c superconductor thin film

HighT _c films belonging to Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems have been fabricated by reactive sputtering of single targets in a planar magnetron and Ar + O₂ sputtering atmosphere. Although it was possible to deposit films of correct composition, resputtering related composition variation was a problem. The key to obtaining correct chemistry was a proper control of the deposition parameters. The pressure and oxygen content of the sputtering gas were found to be the most critical parameters. Results of the variation of these parameters on the cation chemistry are presented in this paper. Results from Monte-Carlo simulation of the sputtering process are also presented showing that low pressure and oxygen content of the sputtering gas result in a higher yield of energetic reflected neutrals which can cause compositional variation in the film mainly due to preferential sputtering of the growing film. The effect was particularly noticeable directly underneath the target. The energetic particle bombardment can be controlled by using moderately high pressures and low oxygen concentration in the gas.