Pulsed DC Mode Research Articles

Multifactorial investigations of the deposition process – Material property relationships of ZnO:Al thin films deposited by magnetron sputtering in pulsed DC, DC and RF modes using different targets for low resistance highly transparent films on unheated substrates

We present a comprehensive study of the many parameters available in magnetron sputtering (including sputtering modes and target choice) to obtain low-resistivity ZnO:Al (AZO) films. The material is widely studied by many groups, however most of the reports focus on the optimization of one parameter, making the translation to another system cumbersome. We aimed to provide a general blueprint for a quick optimization of the process parameters for low resistivity, high crystalline quality AZO films. We found that it was the total argon pressure that controlled the outcomes most and that it is very important to have it the lowest possible. We show that the deposition power and other parameters do not influence the films in such a way. We tested two different (black and white) AZO targets and also did not found many differences, which may also be relevant for the researchers starting their work and wondering if the target they ordered would work well. We obtained very low film resistivities, at 1.2 mΩcm while depositing at room temperature, which is comparable with state-of-the-art results for deposition on heated substrates. We argue, that the sputtering configuration with a short target to substrate distance is responsible for this, opening up the way for low resistivity AZO deposition on heat-sensitive substrates.

The Effect of Match between High Power Impulse and Bias Voltage: TiN Coating Deposited by High Power Impulse Magnetron Sputtering

Practical experience in the use of high power impulse magnetron sputtering (HiPIMS) technology has revealed that output bias current depends on the total energy output of the cathodes, which means that bias voltage settings do not necessarily match the actual output. In this study, we investigated the effects of bias current and voltage on the characteristics of titanium nitride thin films produced using high impulse magnetron sputtering. The bias current and voltage values were adjusted by varying the supplied cathode power and substrate bias under DC and pulsed-DC output models. Our results revealed that pulse delay (PD) and feed forward (FF) settings can be used to control bias current and voltage. Increasing the bias current from 0.56 to 0.84 was shown to alter the preferred orientation from (111) to (220), increase the deposition rate, and lead to a corresponding increase in film thickness. The surface morphology of all titanium nitride samples exhibited tapered planes attributable to the low bias current and voltage (−30 V). The maximum hardness values were as follows: DC mode (23 GPa) and pulsed-DC mode (19 GPa). The lower hardness values of pulsed-DC samples can be attributed to residual stress, preferred orientation, and surface morphology. The surface of the samples was shown to be hydrophobic, with contact angles of >100°.

Plasma electrolytic oxidation of AZ31 magnesium alloy diffusion bonded with aluminium

The corrosion resistance of AZ31 magnesium alloy is enhanced by developing a composite oxide coating through the combination of diffusion bonding with aluminium and subsequent plasma electrolytic oxidation (PEO). The diffusion bonding between AZ31 alloy and aluminium is achieved by applying a pressure of 10 MPa, at a temperature of 425 °C for 600 s. Diffusion bonded alloy is oxidised under pulsed DC mode PEO for 600 s at a current density of 60 mA cm−2. Polarisation resistance has improved by diffusion bonding to 1244 times and which is further increased to 24,585 times by PEO as compared to AZ31 alloy.

Mechanical properties and oxidation resistance of Mo-Si-B and Mo-Hf-Si-B coatings obtained by magnetron sputtering in DC and pulsed DC modes

Mo-Si-B and Mo-Hf-Si-B coatings are deposited by magnetron sputtering in two regimes (direct and pulsed current) using composite ceramic cathodes. The structure of the coatings is investigated by XRD, SEM, EDS, and GDOES. Mechanical and tribological properties are measured by nanoindentation and tribological pin-on-disc testing. The short-term oxidation resistance of the coatings in the air is evaluated at 1500 °С. Results show that pulsed current deposition decreases the coating's growth rate and the grains size of coating's main constituent h-MoSi2 phase while promoting the formation of MoB2. The introduction of hafnium increased the coating's density and suppressed the formation of unwanted columnar structure. The highest hardness of 28 GPa, Young modulus of 366 GPa, and elastic recovery of 64% were achieved for the Mo-Si-B coating deposited at direct current. Optimal tribological performance, including a stable friction coefficient and low wear rate of 6.9х10−6 mm3/(Nm), was determined for Mo-Hf-Si-B coating also obtained using direct current. All coatings were oxidation-resistant at 1500 °С, but the Mo-Hf-Si-B composition demonstrated oxidation resistance superior to that of Mo-Si-B coatings because of the formation of two-layered HfO2/SiO2 protective film. The thickness of the oxide layer was higher for the coatings deposited using pulsed current.

Properties of chromium thin films deposited in a hollow cathode magnetron powered by pulsed DC or HiPIMS

Pure chromium coatings have been deposited within a hollow cathode powered by means of a pulsed DC or a High Power Impulse Magnetron Sputtering (HiPIMS) supply. The discharge characterization by Optical Emission Spectroscopy (OES) has permitted to highlight that more Cr+ are obtained with the HiPIMS mode. A deposition rate of 28μm/h has been reached in pulsed DC mode and, in this case, the films present a columnar morphology whereas, in HiPIMS mode, a maximum deposition rate of 3μm/h is obtained and the films have a compact morphology. X-ray diffraction (XRD) has shown a transition between {211} to random texture with pulsed DC mode when discharge current increases and a {110} texture for coatings deposited in the HiPIMS mode. The hardness is higher for the films deposited in the HiPIMS mode than for those obtained in the pulsed DC mode (1200HV0.025 and 400HV0.025 respectively). Oxidation resistance investigations show a barrier effect for all coatings.

Plasma analysis of inductively coupled impulse sputtering of Cu, Ti and Ni

Inductively coupled impulse sputtering (ICIS) is a new development in the field of highly ionised pulsed PVD processes. For ICIS the plasma is generated by an internal inductive coil, replacing the need for a magnetron. To understand the plasma properties, measurements of the current and voltage waveforms at the cathode were conducted. The ion energy distribution functions (IEDFs) were measured by energy resolved MS and plasma chemistry was analysed by OES and then compared to a model. The target was operated in pulsed DC mode and the coil was energised by pulsed RF power, with a duty cycle of 7.5%. At a constant pressure (14 Pa) the set peak RF power was varied from 1000–4000 W. The DC voltage to the target was kept constant at 1900 V. OES measurements have shown a monotonic increase in intensity with increasing power. Excitation and ionisation processes were single step for ICIS of Ti and Ni and multi-step for Cu. The latter exhibited an unexpectedly steep rise in ionisation efficiency with power. The IEDFs measured by MS show the material- and time-dependant plasma potential in the range of 10–30 eV, ideal for increased surface mobility without inducing lattice defects. A lower intensity peak, of high energetic ions, is visible at 170 eV during the pulse.

Influence of composition on the wear properties of boron carbonitride (BCN) coatings deposited by high power impulse magnetron sputtering

Abstract We investigate boron-carbon-nitride (BCN) coatings deposited with high power impulse magnetron sputtering (HiPIMS) technology and conventional pulsed DC sputtering for their application as wear resistant coatings. Especially for BCN coatings, HiPIMS qualifies as a promising deposition technology as the short pulses with very high power density result in a high ionization degree of the plasma species, allowing a manipulation of the film structure and properties. Both, carbon and boron nitride can hybridize in sp2 or sp3 configuration or a mixture of both, so that coating features such as hardness and coefficient of friction can be tailored to meet the requirements of special applications. We studied the influence of different carbon sources and deposition modes on the composition and tribological properties of BCN films, including microhardness, friction coefficient, and thermal stability. Additionally, we investigated thermal degradation mechanisms by interpreting X-ray photoelectron spectra (XPS). We observed that the application of either pulsed DC or HiPIMS pulse mode has a significant influence on the microhardness and thermal stability of the coatings, in which HiPIMS mode generally provides higher hardness and better thermal stability. Furthermore, we found that samples co-sputtered from B4C and graphite target show superior hardness and thermal stability compared to those sputtered reactively with acetylene.

Spectroscopic diagnostics of active screen plasma nitriding processes: on the interplay of active screen and model probe plasmas

In a reactor used for active screen plasma nitriding (ASPN) the interplay of two plasma types, (i) the plasma of the cylindrical active screen driven in a pulsed dc mode (f = 1 kHz, 60% duty cycle) and (ii) the plasma at an internal model probe driven in a cw dc mode, ignited in a low pressure H2–N2 gas mixture (p = 3 mbar) containing small amounts of CH4 and CO2 have been studied by tunable diode laser infrared absorption (TDLAS) and optical emission spectroscopy (OES) techniques. Applying in situ TDLAS the evolution of the carbon containing precursors, CH4 and CO2, and of the reaction products, NH3, HCN, CO and H2O, has been monitored. The degree of dissociation of the carbon containing precursor molecules varied between 70% and 92%. The concentrations of the reaction products were found to be in the range 1012…1015 molecules cm−3. By analyzing the development of the molecular concentrations at changes of gas mixtures and plasma power values, it was found that (i) HCN and NH3 are the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (RF(CH4) ≈ 1…2 × 1015 molecules J−1, RF(CO2) ≈ 0.5…1.0 × 1016 molecules J−1) and the respective conversion efficiencies to the product molecules (RC(product) ≈ 1013–1015 molecules J−1) have been determined for different gas mixtures and plasma power values, while the influence of probe and screen plasmas, i.e. the phenomena caused by the interplay of both plasma sources, was analyzed. The additional usage of the plasma at the model probe has a sensitive influence on the generation of the reaction products, in particular that of NH3 and HCN. With the help of OES the rotational temperature of the screen plasma could be determined, which increases with power from 770 K to 950 K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the -(0-0) band of the first negative system, increases strongly in relation to the intensity of the neutral component, represented by the N2-(0-0) band of the second positive system. In addition, the behavior of the emission of the plasma at the model probe has been studied during the off phase of the plasma of the active screen leading to a direct comparison of the emission characteristics of both plasma sources.

Correlation between plasma electrolytic oxidation treatment stages and coating microstructure on aluminum under unipolar pulsed DC mode

Alumina coatings were deposited on 6061 aluminum alloy substrates using plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte. Different electrical parameters, including frequency, duty cycle, and current density were applied to obtain PEO coatings. Scanning electron microscopy (SEM) and microhardness tests were used to investigate the coating microstructure and properties. For each sample, the voltage–time response of the PEO process was obtained and compared with surface morphology, and coating cross sections and thicknesses. Special consideration was given to the connection between the electrical parameters, different stages of the PEO process, and the morphology and microstructure of the coatings. Applying different electrical parameters changed the duration and ratio of the PEO stages in the voltage–time response curve and affected the growth rate, surface morphology, microstructure and microhardness of the coatings. We report the new finding of a correlation between the stage of the PEO process and the microstructure and morphology of the coating.

Control of particle flux and energy on substrate in an inverted cylindrical magnetron for plasma PVD

Inverted cylindrical magnetrons (ICMs) are often used in dc, pulsed dc or mid-frequency ac mode for coating complex objects with thin films deposited by plasma PVD. Since in such a configuration the substrate is inherently surrounded by the target and hence by the plasma, the energy flux of the impinging particles represents the main contribution to the substrate heating. This can readily constitute a limiting factor in the deposition process, especially when it is not possible to cool and bias the substrate. This work concerns a dc-driven ICM configuration subjected to several constraints: not only is the substrate surface area small by comparison to the cathode surface area, but its imposed potential is the ground one, thus itself constituting the anode surface of the considered setup. Several important substrate heating factors are highlighted and, in order to reduce the most prominent of them, a means to raise the plasma potential is proposed. This is achieved by positively polarizing two additional electrodes with respect to the ground. This additional surface generates a redistribution of the current and consequently regulates the electron flux on the substrate. The results are shown as a function of bias applied on the auxiliary electrodes and discussed in terms of the impact on the substrate heating.

Factors affecting the formation of the radiation pre-peak at the operation of a Grimm-type source in pulsed DC mode.

The plasma emission pre-peaks of many atomic and ionic spectral lines of Cu and Ar were systematically investigated in a Grimm-type pulsed glow discharge (PGD). To register the pre-peaks with sufficient time resolution, a monochromator with photomultiplier detection was used. When the applied power exceeded a specific threshold, pre-peaks were found in all spectral lines investigated, and it was revealed that the electrical pre-peak was the cause of the atomic emission pre-peak. The form and intensity of the pre-peak radiation were, however, found to be different for different atomic emission lines. The excitation energy of the upper energy level of the atomic line transition, and factors related to recombination and self-absorption, were found to affect the emission pre-peak. Pre-peaks observed when using pulsed DC and pulsed radio-frequency power were compared. This investigation provides insight into best practice when selecting spectral lines most suitable for analytical spectrometry using PGD.

Gas Permeation, Mechanical Behavior and Cytocompatibility of Ultrathin Pure and Doped Diamond-Like Carbon and Silicon Oxide Films

Protective ultra-thin barrier films gather increasing economic interest for controlling permeation and diffusion from the biological surrounding in implanted sensor and electronic devices in future medicine. Thus, the aim of this work was a benchmarking of the mechanical oxygen permeation barrier, cytocompatibility, and microbiological properties of inorganic ~25 nm thin films, deposited by vacuum deposition techniques on 50 µm thin polyetheretherketone (PEEK) foils. Plasma-activated chemical vapor deposition (direct deposition from an ion source) was applied to deposit pure and nitrogen doped diamond-like carbon films, while physical vapor deposition (magnetron sputtering in pulsed DC mode) was used for the formation of silicon as well as titanium doped diamond-like carbon films. Silicon oxide films were deposited by radio frequency magnetron sputtering. The results indicate a strong influence of nanoporosity on the oxygen transmission rate for all coating types, while the low content of microporosity (particulates, etc.) is shown to be of lesser importance. Due to the low thickness of the foil substrates, being easily bent, the toughness as a measure of tendency to film fracture together with the elasticity index of the thin films influence the oxygen barrier. All investigated coatings are non-pyrogenic, cause no cytotoxic effects and do not influence bacterial growth.

Characterisation Studies of the Structure and Properties of As-Deposited and Annealed Pulsed Magnetron Sputtered Titania Coatings

Titanium dioxide thin films are durable, chemically stable, have a high refractive index and good electro/photochemical proprieties. Consequently, they are widely used as anti-reflective layers in optical devices and large area glazing products, dielectric layers in microelectronic devices and photo catalytic layers in self-cleaning surfaces. Titania coatings may have amorphous or crystalline structures, where three crystalline phases of TiO2 can be obtained: anatase, rutile and brookite, although the latter is rarely found. It is known, however, that the structure of TiO2 coatings is sensitive to deposition conditions and can also be modified by post-deposition heat treatments. In this study, titania coatings have been deposited onto soda-lime glass substrates by reactive sputtering from a metallic target. The magnetron was driven in mid-frequency pulsed DC mode. The as-deposited coatings were analysed by micro Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Selected coatings were annealed at temperatures in the range 200–700 °C and re-analysed. Whilst there was weak evidence of a nanocrystallinity in the as-deposited films, it was observed that these largely amorphous low temperature structures converted into strongly crystalline structures at annealing temperatures above 400 °C.

Sputter crater formation in the case of microsecond pulsed glow discharge in a Grimm-type source. Comparison of direct current and radio frequency modes

Depth profiling with pulsed glow discharge is a promising technique. The application of pulsed voltage for sputtering reduces the sputtering rate and thermal stress and hereby improves the analysis of thin layered and thermally fragile samples. However pulsed glow discharge is not well studied and this limits its practical use. The current work deals with the questions which usually arise when the pulsed mode is applied: Which duty cycle, frequency and pulse length must be chosen to get the optimal sputtering rate and crater shape? Are the well-known sputtering effects of the continuous mode valid also for the pulsed regime? Is there any difference between dc and rf pulsing in terms of sputtering? It is found that the pulse length is a crucial parameter for the crater shape and thermal effects. Sputtering with pulsed dc and rf modes is found to be similar. The observed sputtering effects at various pulsing parameters helped to interpret and optimize the depth resolution of GD OES depth profiles.

Deposition of photocatalytic titania coatings on polymeric substrates by HiPIMS

Titania coatings have been deposited onto PET substrates by reactive magnetron sputtering in the HiPIMS (high power impulse magnetron sputtering) mode and for comparison, pulsed DC mode. In the latter case, the substrate showed evidence of melting, but the HiPIMS results were dependent on the characteristics of the power supply when operating under nominally identical conditions. A coating deposited by one of the HiPIMS supplies was found to have a mixed phase structure and to demonstrate a level of photocatalytic activity comparable to conventional coatings which had been post-deposition annealed.

Low Frictional TiO<sub>2</sub> and MoS<sub>2</sub>/TiO<sub>2</sub> Coatings on Ti-6Al-4V Alloy

TiO2 and MoS2/TiO2 coatings were prepared using a Plasma Electrolytic Oxidation (PEO) process at a unipolar pulsed-DC mode. Both coatings, mainly comprised of anatase TiO2, exhibited a low coefficient of friction, C.O.F < 0.21 for the TiO2 coating and C.O.F < 0.16 for the MoS2/TiO2 coating, while the MoS2/TiO2 coating had a longer wear life than TiO2-coated and uncoated Ti alloy (Ti-6Al-4V). This result showed that the solid lubricant MoS2 may play a significant role in the tribological behavior and it can be applied to many high load-bearing systems in aerospace and automotive industries.

Corrosion mechanism and model of pulsed DC microarc oxidation treated AZ31 alloy in simulated body fluid

This paper addresses the effect of pulse frequency on the corrosion behavior of microarc oxidation (MAO) coatings on AZ31 Mg alloys in simulated body fluid (SBF). The MAO coatings were deposited by a pulsed DC mode at four different pulse frequencies of 300Hz, 500Hz, 1000Hz and 3000Hz with a constant pulse ratio. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were used for corrosion rate and electrochemical impedance evaluation. The corroded surfaces were examined by X-ray diffraction (XRD), X-ray fluorescence (XRF) and optical microscopy. All the results exhibited that the corrosion resistance of MAO coating produced at 3000Hz is superior among the four frequencies used. The XRD spectra showed that the corrosion products contain hydroxyapatite, brucite and quintinite. A model for corrosion mechanism and corrosion process of the MAO coating on AZ31 Mg alloy in the SBF is proposed.

Investigations of diffusion behaviour in Al-doped zinc oxide and zinc stannate coatings

Multi-layer dielectric/silver/dielectric coating systems have excellent proprieties as heat insulators and for solar energy reflection and electrical conductivity. The largest market is dominated by low-emissivity (low-E) coatings, which are applied to large area architectural glazing to reduce heat losses from buildings. They combine high visible transparency with high reflectance in the far-infrared region, where the thin (~ 10 nm) silver layer reflects long wavelength IR back into the building and the dielectric layers both protect the silver and act as anti reflectance layers. In this study, a range of dielectric coatings has been deposited onto soda-lime glass substrates by reactive sputtering from metallic targets. The magnetrons were driven in DC mode and also in mid-frequency pulsed DC and AC modes. Process variables investigated include operating pressure, oxygen flow rate and magnetron configuration. Selected coatings were annealed at 650 °C and analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscope (AFM). The oxide samples were then over-coated with silver and annealed for a second time. These coatings were analysed by secondary ion mass spectroscopy (SIMS) to determine the diffusion rates of silver and sodium (from the substrate) through the oxide coatings. The results to date, presented here, show the diffusion of silver and sodium atoms through zinc oxide and zinc stannate thin films deposited under a vast range of conditions. Preliminary attempts have been made to estimate diffusion coefficients for these coating systems and to relate these values to processing conditions and the structural variations observed.

Influence of concentration of Ag on electron and crystallographic structure of reactively co-sputtered CFUBMS TiN/Ag nanocomposite coatings

<title/>TiN/Ag coatings of 2·5-3 μm thickness were deposited on 304 austenitic stainless steel substrates by reactive closed field unbalanced magnetron sputtering (CFUBMS) in a Teer Coatings UDP350 rig. The titanium and silver targets were co-sputtered in pulsed DC mode in Ar-N2 atmospheres. In order to obtain different Ag contents in the coatings, the power applied to the Ag target was varied between 0 and 120 W, while the Ti target was sputtered at an average power of 1500 W. The concentration and behaviour of the silver atoms in the TiN matrix and their influence on the electron and crystallographic structure of the applied nanocomposite TiN/Ag coatings was studied by X-ray diffraction, density functional theory, scanning electron microscopy, energy dispersive X-ray spectroscopy and nanoindentation. The results show that the applied pure TiN and the composite coatings have polycrystalline structures built up by nanocrystals. A well pronounced {111} in-plane crystallographic texture was observed in all the coatings. Depending on their concentration the silver atoms are either dissolved into the TiN lattice forming substitutional solid solutions or in addition to that are also distributed as second phase nanoparticles along the boundaries of the nanocrystallites. In the pure TiN coating and in the coating with lower silver concentration compressive residual macrostresses formed, while in the coatings with higher silver content the stresses are tensile. The addition of silver decreases the valence electron density of TiN and thus weakens the metal bonding. This, together with the tensile residual stresses and softer Ag particles along the crystalline boundaries observed in the coatings with higher Ag concentration, is expected to cause the previously observed decrease in hardness with increasing silver content.

Coating growth behavior during the plasma electrolytic oxidation process

In this study, aluminum oxide was deposited on an Al alloy substrate to produce hard ceramic coatings using a plasma electrolytic oxidation (PEO) process working at atmospheric pressure. The process utilizes dc and unipolar pulsed dc in the frequency range 0.2–20 kHz. Optical emission spectroscopy was employed to study the species and electron temperature of the plasma. The morphology and microstructure of the coatings were investigated using scanning electron microscopy. It was found that in the first 12 min of the PEO process, the plasma electron temperature increased with the applied voltage during the experiments, the plasma electron temperature was found to be in the range 4000–9000 K, and the applied voltage to the electrodes ranged up to 550–600 V for the different current modes. The plasma temperature profile exhibits a wider peak temperature spike for the dc power mode than for the pulsed dc mode, indicating that the dc plasma discharges would provide longer sintering time. The pulsed dc mode increases the spike temperature up to 8700 K but does not necessarily enhance the coating growth. The high spike temperature generated by strong discharges likely melts the oxide and then traps gas into the melt pool, resulting in some porosity at the interface. By eliminating the high temperature spike, a denser interface layer and homogenous coating morphology are produced.