Power Impulse Magnetron Sputtering Process Research Articles

Microstructure and mechanical properties of Ti-Nb alloys: comparing conventional powder metallurgy, mechanical alloying, and high power impulse magnetron sputtering processes for supporting materials screening

At the interface of thin film development and powder metallurgy technologies, this study aims to characterise the mechanical properties, lattice constants and phase formation of Ti-Nb alloys (8–30 at.%) produced by different manufacturing methods, including conventional powder metallurgy (PM), mechanical alloying (MA) and high power impulse magnetron sputtering (HiPIMS). A central aspect of this research was to investigate the different energy states achievable by each synthesis method. The findings revealed that as the Nb content increased, both the hardness and Young’s modulus of the PM samples decreased (from 4 to 1.5 and 125 to 85 GPa, respectively). For the MA alloys, the hardness and Young’s modulus varied between 3.2 and 3.9 and 100 to 116 GPa, respectively, with the lowest values recorded for 20% Nb (3.2 and 96 GPa). The Young’s modulus of the HiPIMS thin film samples did not follow a specific trend and varied between 110 and 138 GPa. However, an increase in hardness (from 3.6 to 4.8 GPa) coincided with an increase in the β2 phase contribution for films with the same chemical composition (23 at.% of Nb). This study highlights the potential of using HiPIMS gradient films for high throughput analysis for PM and MA techniques. This discovery is important as it provides a way to reduce the development time for complex alloy systems in biomaterials as well as other areas of materials engineering.Graphical abstract

Dynamics of sputtered particles in multipulse HiPIMS discharge

The behavior of the ground state neutral and singly ionized atoms is studied in multipulse high power impulse magnetron sputtering processes. The time-resolved two-dimensional laser induced fluorescence was used for imaging the discharge volume (density mapping) during the plasma-on and plasma-off time phases. The role of the number of micropulses and delay time between the micropulses in the pulse package is analyzed and discussed systematically. In addition, the propagation of the sputtered particles from the target is investigated.

Influence of a short reverse positive HPPMS pulse on the deposition of CrAlN

In conventional high power impulse magnetron sputtering processes (HPPMS) short pulses of a negative voltage are used to maintain the discharge. A new approach to accelerate ionized species towards the substrate position and subsequently improved coating properties is the use of a power supply, which generates a reverse positive pulse immediately after the negative pulse. In the present paper the influence of the frequency f and the pulse on time t on for a CrAlN process on the coating properties is investigated. All experiments were carried out on an industrial scale coating unit. In the present investigations coatings deposited by three different duty cycles are compared. For each duty cycle one coating was deposited using a conventional power supply and one coating using a power supply with a positive pulse. The positive pulse was clearly detected. It was found that the height of the positive pulse depends on the chosen pulse parameters. Furthermore, higher values of the peak power were found when using a power supply with a positive pulse, which should influence the ionization of the sputtered material. Regarding the chemical composition, a decreasing nitrogen content of the coatings was found when using a positive pulse. The investigations of the deposited coatings showed a denser morphology and increased values of the indentation hardness when using a power supply with a positive pulse compared to a conventional power supply. In addition to the higher indentation hardness, even the amount of plastic work showed increasing values for the power supply using a positive pulse compared to the conventional power supply. Furthermore, increased values of the roughness for coatings deposited on cemented carbide were found when using a positive pulse. The coatings deposited on cemented carbide showed a higher adhesion when using a positive pulse. • Higher peak power when using a short reverse positive pulse • Improved mechanical properties when using a positive pulse • Preferred (200) orientation for the use of a short reversed positive pulse and low duty cycles • Improved adhesion when using a positive pulse

Revisiting particle dynamics in HiPIMS discharges. I. General effects

A detailed experimental study of high power impulse magnetron sputtering processes is performed by time-resolved imaging of the ground state sputtered particles. New details related to the behavior of both neutral and singly ionized atoms are shown, as a result of separate treatment of the plasma-on and plasma-off time phases. In Paper I, the ion/neutral density redistribution in the ionization zone during sputtering is analyzed; the role of main discharge parameters, such as pulse repetition rate, pulse energy, etc., is discussed systematically. The time-resolved evolution of the ground state levels population for both sputtered neutrals and ions is also considered. In addition, propagation of the sputtered particles is analyzed using 2D density gradient diagrams calculated based on the measured particle density distributions. The results of this work are compared, when possible, to the data obtained previously [Britun et al., J. Appl. Phys. 117, 163302 (2015)].

Ionisation fractions of sputtered titanium species at target and substrate region in HiPIMS

In this paper, the ground state number densities of titanium atom and ion, ionised density fraction and ionised flux fraction were determined in a high power impulse magnetron sputtering process. Three regions of interest within the deposition chamber were studied; the magnetised plasma region, middle region, and substrate region. Pulse length and applied power were kept constant and working pressure and duty cycle were varied. The effective branching fractions method was utilised to evaluate the number densities of sputtered species from a measured optical emission signal. A biasable quartz crystal microbalance system was used to evaluate the ionised flux fraction and deposition rate. The highest ionised density fraction of 90% was reached in the target region for 1.2% duty cycle and it was mostly independent on working pressure. Moving from the target, the ionised density fraction was observed to decrease. In the substrate region for a duty cycle below 2% and the whole studied working pressure range, the ionised density fraction always reached at least 50%.

Process stabilization during reactive high power impulse magnetron sputtering of Ce/Gd target

In this study, a simple approach to stabilize reactive high power impulse magnetron sputtering process in the transition zone between the metallic and oxide modes was investigated. The method is based on the use of peak current value as feedback and control signal. It was shown that the change in the state of the target surface causes almost instantaneous change in the amplitude and shape of the current pulses. To stabilize the deposition process, the pulse frequency was regulated to maintain a constant maximum discharge current. The hysteresis behavior and the variations of the pulse current waveforms over a wide range of O2 flow rates and pulse frequencies during a reactive HiPIMS deposition of gadolinia-doped ceria (GDC) thin films in an Ar-O2 atmosphere were examined. Stable process conditions were maintained at the O2 flow from 0.9 to 3.9 sccm by adjustment of the pulse frequency from 1.45 to 2.9 kHz. GDC films deposited using peak current regulation exhibited a stable stoichiometry and high deposition rate in comparison with films obtained without process stabilization.

A comparative study of direct current magnetron sputtering and high power impulse magnetron sputtering processes for CNx thin film growth with different inert gases

Reactive direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) discharges of carbon in different inert gas mixtures (N2/Ne, N2/Ar, and N2/Kr) were investigated for the growth of carbon-nitride (CNx) thin films. Ion mass spectrometry showed that energies of abundant plasma cations are governed by the inert gas and the N2-to-inert gas flow ratios. The population of ion species depends on the sputter mode; HiPIMS yields approximately ten times higher flux ratios of ions originating from the target to process gas ions than DCMS. Exceptional are discharges in Ne with N2-to-Ne flow ratios <20%. Here, cation energies and the amount of target ions are highest without influence on the sputter mode. CNx thin films were deposited in 14% N2/inert gas mixtures at substrate temperatures of 110°C and 430°C. The film properties show a correlation to the substrate temperature, the applied inert gas and sputter mode. The mechanical performance of the films is mainly governed by their morphology and composition, but not by their microstructure. Amorphous and fullerene-like CN0.14 films exhibiting a hardness of ~15GPa and an elastic recovery of ~90% were deposited at 110°C in reactive Kr atmosphere by DCMS and HiPIMS.

Modified high power impulse magnetron sputtering process for increased deposition rate of titanium

A modified version of high power impulse magnetron sputtering (HiPIMS) has been used to deposit titanium films at higher deposition rates than for conventional HiPIMS while maintaining similar pulse voltages and peak currents. In the present study, additional control parameters are explored through the chopping of the HiPIMS pulse into a pulse sequence. Experiments show that the use of sequences allows for an increase of the deposition rate of more than 45% compared to conventional HiPIMS. The increase in deposition rate is ascribed to a combination of reduced gas rarefaction effects, prevention of sustained self-sputtering, and a relaxation of ion trapping.

Reactive high power impulse magnetron sputtering of CFx thin films in mixed Ar/CF4 and Ar/C4F8 discharges

The reactive high power impulse magnetron sputtering processes of carbon in argon/tetrafluoromethane (CF4) and argon/octafluorocyclobutane (c-C4F8) have been characterized. Amorphous carbon fluoride (CFx) films were synthesized at deposition pressure and substrate temperature of 400mPa and 110°C, respectively. The CFx film composition was controlled in the range of 0.15<x<0.35 by varying the partial pressure of the F-containing gases from 0mPa to 110mPa. The reactive plasma was studied employing time averaged positive ion mass spectrometry and the resulting thin films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by elastic recoil detection analysis, X-ray photoelectron spectroscopy, transmission electron microscopy, nanoindentation, and water droplet contact angle measurements, respectively. The experimental results were compared to results obtained by first-principles calculations based on density functional theory. The modeling of the most abundant precursor fragment from the dissociation of CF4 and C4F8 provided their relative stability, abundance, and reactivity, thus permitting to evaluate the role of each precursor during film growth. Positive ion mass spectrometry of both fluorine plasmas shows an abundance of CF+, C+, CF2+, and CF3+ (in this order) as corroborated by first-principles calculations. Only CF3+ exceeded the Ar+ signal in a CF4 plasma. Two deposition regimes are found depending on the partial pressure of the fluorine-containing reactive gas, where films with fluorine contents below 24at.% exhibit a graphitic nature, whereas a polymeric structure applies to films with fluorine contents exceeding 27at.%. Moreover, abundant precursors in the plasma are correlated to the mechanical response of the different CFx thin films. The decreasing hardness with increasing fluorine content can be attributed to the abundance of CF3+ precursor species, weakening the carbon matrix.

The effect of Ti sputter target oxidation level on reactive High Power Impulse Magnetron Sputtering process behaviour

This paper investigates the effect of sputter target oxidation level on reactive process behaviour during High Power Impulse Magnetron Sputtering of transition metal target (Ti) in Ar/O2 atmosphere. It was found by this study that the sputter target state depends on the history of its use in reactive HIPIMS mode. The effect appears to be pronounced strongly due to the significance of ion irradiation induced effects leading to enhanced sputter target oxidation. The target sputter cleaning times (e.g. after a reactive deposition run) are long (compared to DC sputtering) due to the same reason. The abovementioned effect has serious implications for the hysteresis behaviour and reactive deposition process window. It is shown in this paper that the hysteresis loop can be artificially suppressed or even eliminated and the reactive deposition process window can be reduced a number of times if the starting target surface is not a clean metal surface but a partially oxidised metal surface. Such target surface condition is readily obtainable for the targets that have a history of prolonged use in reactive HIPIMS mode. Processing parameters, such as pulse frequency and peak voltage pulse values are shown to have influence on the degree to what the abovementioned effects occur.

Investigation of reactive high power impulse magnetron sputtering processes using various target material–reactive gas combinations

The two most important issues limiting reactive high power impulse magnetron sputtering (HIPIMS) process applicability until recently were the absence of suitable reactive HIPIMS control means and the limited capability of HIPIMS power supplies in terms of arc handling. The significant advancement has been made recently by the development of the optical plasma monitoring (PM)-based process control technology for reactive HIPIMS [Surface & Coatings Technology 204 (2010) 2159–2164]. The initial studies of reactive HIPIMS processes however have only covered Ti–O2 target material–reactive gas system.In this paper the recently developed PM-based active feedback control technology was applied to explore further reactive HIPIMS processes now using a variety of different target material and reactive gas combinations. Data for hysteresis behaviour and process control using either PM or constant gas flow methods for Ti–O2, Ti–CO2, Cr–O2, Cr–C2H4, Al–O2, and Zn:Al–O2 material–gas systems is presented and compared. In all cases the processes were found to exhibit hysteresis behaviour. The magnitude and features of the hysteresis loop were found to depend strongly on a particular metal–reactive gas pair. Similar to AC and DC reactive sputtering processes the hysteresis behaviour in reactive HIPIMS was found to be more pronounced for the gases that have high chemical affinity for a metal sputter target. The PM-based process control technology monitoring either metal or gas plasma emissions was shown to provide accurate control and stable operation of reactive HIPIMS discharges.

Control of reactive high power impulse magnetron sputtering processes

High power impulse magnetron sputtering (HIPIMS) is a technologically important physical vapour deposition (PVD) process that is able to provide a highly ionised flux of sputtered species. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor applications). HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to better coating structure–property relationship control through self-species (sputtered metal) plasma/ion assistance. Many of the technologically important thin films (e.g. transparent conductive oxides, permeation barrier coatings, etc.) are sputtered from metal targets in a reactive gas atmosphere, usually Ar + O 2 or N 2, to ensure industrially relevant coating deposition rates. Enhanced structure–property relationship control of these thin film materials is highly desirable; hence, it also is desirable to use HIPIMS in a reactive deposition mode. Preliminary trials of reactive HIPIMS however have indicated that the control of this process using conventional means, such as conventional plasma emission monitoring (PEM) is difficult. Thus, the application of reactive HIPIMS is rather limited and the potential benefits are not realised, especially in the areas where precise process control and long term stability in a reactive environment are required. In this paper reactive HIPIMS process (Ti in Ar/O 2 atmosphere) is investigated and various control options are evaluated. The application of a recently developed PEM based reactive HIPIMS control method is reported. Performance of the developed technique is compared to that of the conventional PEM, Penning-PEM and λ-sensor based methods. It is shown that conventional PEM is impractical to control reactive HIPIMS, while the constant reactive gas flow method does not lead to a stable deposition process. The new PEM based process control technology was shown to provide precise control and stable operation of reactive HIPIMS discharges anywhere within the hysteresis loop. It was also found to be superior when compared to oxygen partial pressure control based techniques.