Power Impulse Magnetron Sputtering Discharge Research Articles

On working gas rarefaction in high power impulse magnetron sputtering

The ionization region model (IRM) is applied to explore working gas rarefaction in high power impulse magnetron sputtering discharges operated with graphite, aluminum, copper, titanium, zirconium, and tungsten targets. For all cases the working gas rarefaction is found to be significant, the degree of working gas rarefaction reaches values of up to 83%. The various contributions to working gas rarefaction, including electron impact ionization, kick-out by the sputtered species or hot argon atoms, and diffusion, are evaluated and compared for the different target materials, and over a range of discharge current densities. The relative importance of the various processes varies between different target materials. In the case of a graphite target with argon as the working gas at 1 Pa, electron impact ionization (by both primary and secondary electrons) is the dominating contributor to working gas rarefaction, with over 90% contribution, while the contribution of sputter wind kick-out is small <10 %. In the case of copper and tungsten targets, the kick-out dominates, with up to ∼60% contribution at 1 Pa. For metallic targets the kick-out is mainly due to metal atoms sputtered from the target, while for the graphite target the small kick-out contribution is mainly due to kick-out by hot argon atoms and to a smaller extent by carbon atoms. The main factors determining the relative contribution of the kick-out by the sputtered species to working gas rarefaction appear to be the sputter yield and the working gas pressure.

Novel high-efficiency plasma nitriding process utilizing a high power impulse magnetron sputtering discharge

Lifetime and biocompatibility of orthopedic implants are crucial in meeting the new challenges brought about by the fall in the patient age and the aging population. The high-load surfaces in contact with the biological environment must display enhanced tribological properties, biocompatibility, and reduced metal ion release in long-term clinical performance. Surface modification techniques such as nitriding can significantly improve the in-service behavior of the medical-grade alloys in current use. We report on a novel approach for nitriding of CoCrMo alloys using high power impulse magnetron sputtering (HIPIMS) discharge. The new nitriding process has been successfully carried out at the National HIPIMS Technology Centre at Sheffield Hallam University, UK, in an industrial size Hauzer 1000-4 system enabled with HIPIMS technology. While the nitriding ion flux is controlled by the HIPIMS magnetron plasma source, the ion energy can be independently set via the substrate bias. Implementing the HIPIMS source allows reducing the operational pressure by one order of magnitude compared to conventional dc plasma nitriding (DCPN). Plasma analyses have identified significantly enhanced production of ions of molecular nitrogen (N2+), atomic nitrogen (N+), and N2H+ radicals in the HIPIMS discharge compared to DCPN. Because of the low pressure of operation of the HIPIMS process, the energy of ions is similar to the bias voltage, whereas the high pressures used in DCPN cause severe losses in ion energy due to scattering collisions within the sheath. The high flux and high ion energy are primarily responsible for achieving a fourfold increase in process productivity as compared to state-of-the-art plasma nitriding processes. The nitrided surface layers exhibit excellent mechanical and tribological properties, which bring about significant improvements in hardness, fracture toughness, and wear. The protective function of the nitrided layer against corrosion in the aggressive environments of simulated body fluid is remarkably augmented. The barrier properties of the nitrided layer have been demonstrated through a reduction in metal ion release by as much as a factor of 2, 4, and 10 for Co, Cr, and Mo, respectively.

Insights into the copper HiPIMS discharge: deposition rate and ionised flux fraction

The influence of pulse length, working gas pressure, and peak discharge current density on the deposition rate and ionised flux fraction in high power impulse magnetron sputtering discharges of copper is investigated experimentally using a charge-selective (electrically biasable) magnetically shielded quartz crystal microbalance (or ionmeter). The large explored parameter space covers both common process conditions and extreme cases. The measured ionised flux fraction for copper is found to be in the range from ≈10% to 80%, and to increase with increasing peak discharge current density up to a maximum at , before abruptly falling off at even higher current density values. Low working gas pressure is shown to be beneficial in terms of both ionised flux fraction and deposition rate fraction. For example, decreasing the working gas pressure from 1.0 Pa to 0.5 Pa leads on average to an increase of the ionised flux fraction by percentage points (pp) and of the deposition rate fraction by taking into account all the investigated pulse lengths.

Influence of the magnetic field on the extension of the ionization region in high power impulse magnetron sputtering discharges

The high power impulse magnetron sputtering (HiPIMS) discharge brings about increased ionization of the sputtered atoms due to an increased electron density and efficient electron energization during the active period of the pulse. The ionization is effective mainly within the electron trapping zone, an ionization region (IR), defined by the magnet configuration. Here, the average extension and the volume of the IR are determined based on measuring the optical emission from an excited level of the argon working gas atoms. For particular HiPIMS conditions, argon species ionization and excitation processes are assumed to be proportional. Hence, the light emission from certain excited atoms is assumed to reflect the IR extension. The light emission was recorded above a 100 mm diameter titanium target through a 763 nm bandpass filter using a gated camera. The recorded images directly indicate the effect of the magnet configuration on the average IR size. It is observed that the shape of the IR matches the shape of the magnetic field lines rather well. The IR is found to expand from 10 and 17 mm from the target surface when the parallel magnetic field strength 11 mm above the racetrack is lowered from 24 to 12 mT at a constant peak discharge current.

Pulse length dependence of a reactive high power impulse magnetron (HiPIMS) discharge

The pulse length dependence of a reactive high power impulse magnetron sputtering (HiPIMS) discharge with a tungsten cathode in an argon+oxygen gas mixture gas was investigated. The HiPIMS discharge is operated with a variable pulse length of 20–500 µs. Discharge current measurements, optical emission spectroscopy of neutral Ar, O, and W lines, and energy-resolved ion mass spectrometry are employed. A pronounced dependence of the discharge current on pulse length is noted while the initial discharge voltage is maintained constant. Energy-resolved mass spectrometry shows that the oxygen-to-tungsten (O+/W+) and the tungsten oxide-to-tungsten (WO+/W+) ion ratio decreases with pulse length due to target cleaning. Simulation results employing the SDTrimSP program show the formation of a non-stoichiometric sub-surface compound layer of oxygen which depends on the impinging ion composition and thus on the pulse length.

High power impulse magnetron sputtering of tungsten: a comparison of experimental and modelling results

Here, we compare the ionization region model (IRM) against experimental measurements of particle densities and electron temperature in a high power impulse magnetron sputtering discharge with a tungsten target. The semi-empirical model provides volume-averaged temporal variations of the various species densities as well as the electron energy for a particular cathode target material, when given the measured discharge current and voltage waveforms. The model results are compared to the temporal evolution of the electron density and the electron temperature determined by Thomson scattering measurements and the temporal evolution of the relative neutral and ion densities determined by optical emission spectrometry. While the model underestimates the electron density and overestimates the electron temperature, the temporal trends of the species densities and the electron temperature are well captured by the IRM.

Formations of anode double layer and ion beam in bipolar-pulse HiPIMS (BP-HiPIMS)

As an emerging ion acceleration plasma source, the bipolar-pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge provides an effective approach to improve deposited ion energy and tailor the film properties for a large range of applications. The ion acceleration mechanism in BP-HiPIMS discharge is very vital but still unclear now. In the present work, the ion acceleration mechanism is systematically investigated via the experimental measurements, particle-in-cell/Monte Carlo collision (PIC-MCC) simulation, and theoretical model together. In the experiment part, the floating potential V f and the ion velocity distribution function (IVDF) have been measured via the Langmuir probe and the retarding field energy analyser (RFEA) respectively. The measurements show that the V f at the downstream drops from +80 V to ∼+40 V after applying the positive pulse for ∼75 μs, suggesting the formation of the double layer. Correspondingly, the IVDF changes from the unimodal Maxwellian distribution to the bimodal distribution, suggesting the existence of the ion beam. The PIC-MCC simulation results clearly show the development process of the double layer and ion beam. A theoretical model is introduced to explore the complex plasma dynamics in the experiment and simulation. The theoretical results illustrate that (i) the sheath in front of the target surface prefers an ion sheath rather than an electron sheath, (ii) the stable position of the double layer boundary is in the magnetic null point, (iii) the potential drop across the boundary is influenced by the gas pressure p. These important theoretical results are well consistent with the measurements and simulation. In addition, the oscillation of the double layer boundary and the instabilities of the ions are briefly discussed by combining the previous works.

Adjustment of high-energy ion flux in BP-HiPIMS via pulsed coil magnetic field: plasma dynamics and film deposition

As an emerging and extraordinary plasma source, bipolar pulsed high power impulse magnetron sputtering (BP-HiPIMS) discharge has been widely concerned by academia and industry due to the ability to control the deposited ion energy. In the present work, with the intension of increasing the high-energy ion fraction and flux during deposition, the BP-HiPIMS is operated together with a solenoidal coil installed in front of the target. This intension is achieved by applying a pulsed coil current so that the ions generated during the high-power negative pulse can be manipulated to diffuse towards substrate and then arrive at the substrate surface during the positive pulse. Systematic investigations of discharge characteristics and plasma parameters for Ti target discharge in Ar gas have been made, illustrating that applying a pulsed coil current prior to the positive pulse onset for ∼50 μs is an optimal selection to obtain a larger fraction of high-energy ions. The complex plasma dynamics has also been explored using the particle-in-cell/Monte Carlo collision approach. To verify the statements realized by plasma measurements, the Ti films have been deposited on a floating Si substrate, whose microstructure and surface morphology are characterized by field emission scanning electron microscope and atomic force microscope. The deposition illustrates that applying a pulsed coil current prior to the positive pulse onset for ∼50 μs can prepare a thicker, denser, and smoother Ti film. The link between the plasma parameters and film microstructure is studied using the molecular dynamics simulations which show that the high-energy ions contribute to optimizing the adatom diffusion and mobility on growing film surface, which is more beneficial to get a smaller grain size and decrease the film surface roughness.

Modeling of high power impulse magnetron sputtering discharges with tungsten target

The ionization region model (IRM) is applied to model a high power impulse magnetron sputtering discharge with a tungsten target. The IRM gives the temporal variation of the various species and the average electron energy, as well as internal discharge parameters such as the ionization probability and the back-attraction probability of the sputtered species. It is shown that an initial peak in the discharge current is due to argon ions bombarding the cathode target. After the initial peak, the W+ ions become the dominating ions and remain as such to the end of the pulse. We demonstrate how the contribution of the W+ ions to the total discharge current at the target surface increases with increased discharge voltage for peak discharge current densities J D,peak in the range 0.33–0.73 A cm−2. For the sputtered tungsten the ionization probability increases, while the back-attraction probability decreases with increasing discharge voltage. Furthermore, we discuss the findings in terms of the generalized recycling model and compare to experimentally determined deposition rates and find good agreement.

Spatially resolved study of spokes in reactive HiPIMS discharge

Understanding mechanisms behind the formation and propagation of ionisation zones, so-called spokes, have been highly important since the first observations in high power impulse magnetron sputtering discharges. To this day, however, a complete description is still lacking. This contribution gives an insight into the spokes in reactive Ar–N2 atmosphere using titanium target. The aim of this study is to describe not only global parameters of spokes such as their shape, length and propagation velocity but also provide a description of parameters evolving over the spoke, such as floating potential and spectral emission. The measurements of the latter clearly showed which species emission is most prominent and how it changes with transition from non-reactive argon discharge towards the discharge driven in a pure nitrogen atmosphere. Implications on the gas dynamics and discharge transitions are discussed.

Ionization region model of high power impulse magnetron sputtering of copper

The ionization region model (IRM) is applied to model high power impulse magnetron sputtering (HiPIMS) discharges with a Cu target. We apply the model to three discharges that were experimentally explored in the past, or applied to deposit thin copper films, with the aim to quantify internal plasma process parameters and thereby understand how these discharges differ from each other. The temporal variation of the various neutral and ionic species, the electron density and temperature, as well as internal discharge parameters, such as the ionization probability, back attraction probability, and ionized flux fraction of the sputtered species, are determined. We demonstrate that the Cu+ ions dominate the total ion current to the target surface and that all the discharges are dominated by self-sputter recycling to reach high discharge currents. Furthermore, the ion flux into the diffusion region is dominated by Cu+ ions, which represents roughly 80% of the total ion flux onto the substrate, in agreement with experimental findings. For the discharges operated with peak discharge current densities in the range 0.9 – 1.3 A cm−2, the ion back-attraction probability of the Cu+ ion ⟨βt⟩ is low compared to previously investigated HiPIMS discharges, or in the range 44 – 50%, while the ionization probability ⟨αt⟩ is in the range 61 – 69%, and the ionized flux fraction is in the range 32 – 40%. It is, furthermore, found that operating these Cu HiPIMS discharges at lower working gas pressures (in the present case around 0.5 Pa) is beneficial in terms of optimizing ionization of the sputtered species.

On the population density of the argon excited levels in a high power impulse magnetron sputtering discharge

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p*)−6, where p* is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron–ion recombination.

Ion energy distribution and non-linear ion dynamics in BP-HiPIMS and ACBP-HiPIMS discharge

Investigating the ion dynamics in the emerging bipolar pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge is necessary and important for broadening its industrial applications. Recently, an optimized plasma source operating the BP-HiPIMS with an auxiliary anode and a solenoidal coil is proposed to enhance the plasma flux and energy, named as ACBP-HiPIMS (‘A’-anode, ‘C’-coil). In the present work, the temporal evolutions of the ion velocity distribution functions (IVDF) in BP-HiPIMS and ACBP-HiPIMS discharges are measured using a retarding field energy analyser (RFEA). For the BP-HiPIMS discharge, operated at various positive pulse voltages U +, the temporal evolutions of IVDFs illustrate that there are two high-energy peaks, E 1 and E 2, which are both lower than the applied U +. The ratio of the mean ion energy E i,mean to the applied U + is around 0.55–0.6 at various U +. In ACBP-HiPIMS discharge, the IVDF evolution shows three distinguishable stages which has the similar evolution trend with the floating potential V f on the RFEA frontplate: (i) the stable stage with two high-energy peaks (E 2 and E 3 with energy respectively lower and higher than the applied U + amplitude) when the floating potential V f is close to the applied positive pulse voltage; (ii) the transition stage with low-energy populations when the V f drops by ∼20 V within ∼10 μs; and (iii) the oscillation stage with alternating E 2 and E 3 populations and ever-present E 1 population when the V f slightly decreases until to the end of positive pulse. The comparison of IVDFs in BP-HiPIMS and ACBP-HiPIMS suggests that both the mean ion energy and high-energy ion flux have been effectively improved in ACBP-HiPIMS discharge. The formation of floating potential drop is explored using the Langmuir probe which may be attributed to the establishment of anode double layer structure. The acceleration of ion at the double layer boundary is analysed using a theoretical model, in this way to clarify the oscillation in IVDF evolutions in ACBP-HiPIMS discharge.

Electron transport in high power impulse magnetron sputtering at low and high working gas pressure

The magnetic field of a magnetron serves to increase the residence time of electrons in the ionization region and thereby enables the discharge to be sustained at low working gas pressures. This hinders the electrons to reach the anode which is necessary to close the electrical circuit. At high atom densities in the ionization region, and in the presence of an electric field, collisions of electrons with heavy species consecutively push electrons across the magnetic field lines, which is known as the classical cross-field transport mechanism. At low atom densities in the ionization region, collisions are rare and the classical cross-field transport mechanism is insufficient to carry the discharge current. This gives rise to plasma instabilities, called spokes, that locally provide pathways for electrons to escape from the near-target region and across the magnetic field lines. Here, we show experimentally, for the case of a high power impulse magnetron sputtering discharge with an aluminum target, how spokes gradually disappear with the increase in local gas density. We present an analytical model that shows that under these high gas density conditions, the classical electron transport mechanism is indeed strong enough to solely carry the discharge current. This highlights the importance of the local gas density in the ionization region for the intensity of spokes in a magnetron sputtering discharge and suggests ways for process optimization.

Simulation results of the gas rarefaction and target ion evolution in a chopped high power impulse magnetron sputtering discharge

The high power impulse magnetron sputtering (HiPIMS) technique has recently been improved experimentally to deposit titanium films with several short micro-pulses that are decomposed from one single pulse. The additional control parameters cause a totally different current characteristic and result in a high deposition rate in these so-called chopped-HiPIMS (c-HiPIMS) sequences. Owing to the difficulty in digging deeply into the details of parameter variations through the experimental method, simulation works are adopted to exploit the complex mechanism. Here, the ionization region model is used to simulate the short micro-pulse discharge in HiPIMS. It is found that short pulse on-time tμon, long pause off-time tμoff, and the large number of micro-pulses within a single voltage pulse are factors that result in a higher deposition rate and are relevant to the gas rarefaction and the metal ion running away behavior.

Modeling of high power impulse magnetron sputtering discharges with graphite target

The ionization region model (IRM) is applied to model a high power impulse magnetron sputtering discharge in argon with a graphite target. Using the IRM, the temporal variation of the various species and the average electron energy, as well as internal parameters such as the ionization probability, back-attraction probability, and the ionized flux fraction of the sputtered species, is determined. It is found that thedischarge develops into working gas recycling and most of the discharge current at the cathode target surface is composed of Ar+ ions, which constitute over 90% of the discharge current, while the contribution of the C+ ions is always small (%), even for peak current densities close to 3 A cm−2. For the target species, the time-averaged ionization probability ⟨α t,pulse⟩ is low, or 13–27%, the ion back-attraction probability during the pulse β t,pulse is high (%), and the ionized flux fraction is about 2%. It is concluded that in the operation range studied here it is a challenge to ionize carbon atoms, that are sputtered off of a graphite target in a magnetron sputtering discharge, when depositing amorphous carbon films.

Single-shot spatial-resolved optical emission spectroscopy of argon and titanium species within the spoke

The rotating plasma patterns, also known as ionisation zones or spokes, observed, among other discharges, in high power impulse magnetron sputtering discharge (HiPIMS) require non-invasive diagnostics favourable for a precise characterisation of their properties. In this contribution, the single-shot spatial-resolved optical emission spectroscopy of the spoke was conducted in non-reactive HiPIMS discharge using a titanium target. Investigated working pressures cover the conditions with the presence of localised, well-defined spokes. A fast photodiode and a cylindrical Langmuir probe were utilised to capture and determine the passing spoke position. These signals were synchronised with the acquisition of the optical emission spectrum by the intensified charge-coupled device detector. A large amount of single-shot data enabled the statistical analysis of the spoke. The optical emissions of argon atoms and ions and titanium atoms and ions were investigated in the passing spoke. It was found that the intensities of the spectral lines of the Ar and Ti species have the characteristic evolution for all studied spectral lines of this specific species within the spoke. The intensity evolutions are independent of the applied pressure. The evolution of the excitation temperatures determined by the Boltzmann plot method using the Ar II and Ti I and Ti II spectral lines remains constant within the spoke in the margin of standard error for all investigated pressures.

Influence of the magnetic field on the discharge physics of a high power impulse magnetron sputtering discharge

The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization regionleads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability.

Direct current and high power impulse magnetron sputtering discharges with a positively biased anode

A magnetron sputtering discharge with a positively biased anode in argon gas is investigated by Langmuir probe diagnostics and by energy-resolved mass spectrometry. The discharge is operated in continuous (direct current) and in pulsed (high power impulse magnetron sputtering, Hi) mode with a Ti target and in Ar gas. Singly-charged Ar+, Ti+, and Ar2+ and doubly-charged Ar2+ and Ti2+ ions are observed. A novel approach is to bias the magnetron anode. Application of a positive anode voltage shifts the kinetic energies of plasma ions by qe0Va, where Va is the anode voltage and qe0 is the ion charge. It allows for an effective control of plasma ion energies.

A high-power impulse magnetron sputtering global model for argon plasma–chromium target interactions

A high power impulse magnetron sputtering (HiPIMS) discharge process is analyzed numerically and experimentally so that one may better understand and explain the effect of the pressure and pulse width on HiPIMS discharges and the deposited thin films. For this reason, a time-dependent global plasma model is developed for the ionization region in a HiPIMS discharge of a Cr target in Ar gas. It is based on the solving of a nonlinear equation system composed of the continuity equations of neutral and charged species in the ionization region considered in the reaction scheme. The pulse widths are about tens of microseconds for a frequency cycle of 1 KHz. The simulations are performed for a 150 W average power and 5–30 mTorr pressure range. In these average power and pressure ranges, a double peak of the electron temperature is observed. The high first peak is due to a high rapid increase of negative voltage during the rising time of pulses, while the second one is due to a diminution of the injected gas Ar in the ionization region. On the other hand, gas rarefaction characterized by the diminution of the Ar density during time-on is due to the high ionization degree and the sputtering wind effect. During plasma off, the density of Ar returns to the stationary state corresponding to the initial pressure. In addition, the simulations reveal that the Cr+ population is more important than that of Ar+ caused by the high ionization degree of Cr compared with Ar. This confirms the high ionization degree during time-on, leading to a high efficiency of sputtering of Cr material. However, Cr2+ is still weak. In addition, a good agreement is shown between the calculated time current evolution and the measured one.