We have studied the impact of target material on the electron temperature of high power impulse magnetron sputtering (HiPIMS) discharges. The study is based on results from modeling 35 discharges with seven different target materials, using the ionization region (IR) model, a global plasma chemistry model for HiPIMS discharges. We find that the typical evolution of electron temperatures during a HiPIMS pulse stabilizes at the end of the pulse as a result of a balance between electron heating and electron collisional cooling. The underlying cause is a self-regulating mechanism: the monotonically increasing rate coefficients for relevant electron temperatures in HiPIMS discharges ensure that a higher electron temperature enhances electron collisional cooling, while a lower electron temperature reduces it. We furthermore find the steady state electron temperature to be inversely correlated to the sputter yield of the target material. This is a result of the atomic composition in the IR shifting from argon-dominated at low sputter yields to metal-rich at high sputter yields. As the metal ionization rate coefficients are larger at lower electron temperatures compared to that of the argon ionization rate coefficient, the self-regulating mechanism maintains a lower electron temperature in metal-rich discharges. This has implications for the metal ion escape in a HiPIMS discharge, since the ionization mean free path of sputtered atoms depends on the electron temperature. As a result, ionization in metal-rich discharges (lower electron temperature) occurs, on average, further away from the target surface, where the remaining potential hill to climb, in order for a metal ion to escape to the bulk plasma, is lower. Metal ions in those discharges can therefore escape more easily to the substrate region compared to metal ions in argon-dominated discharges.

Self organized striation structures have been observed in electronegative capacitive discharges under certain operating conditions, which include high electronegativity and an ion plasma frequency comparable to the driving frequency. In this study, striations in capacitive chlorine discharges were explored using one-dimensional particle-in-cell/Monte Carlo collisional simulations with a 2.54 cm gap driven by a sinusoidal rf voltage of 13.56 MHz. The properties of the discharges are explored focusing on the striations, as the gas pressure, driving voltage amplitude, and secondary electron emission processes are varied. The most realistic secondary electron emission model includes contribution from ions, electrons, and neutrals bombarding the electrodes. The striations start to appear at pressure around 15 Pa and increase in amplitude with increased pressure. We find that the amplitude and the number of striations increase with the addition of secondary electron emission processes to the discharge model. Furthermore, the most realistic model for secondary electron emission is used to explore the striation structures as driving voltage amplitude, driving frequency, and gas pressure is varied. As the pressure is increased, the striation amplitude increases but the number of striations remains unchanged. Higher driving voltage and higher driving frequency increase the ion critical density, resulting in the formation of striation patterns, even when the pressure is low. Increasing the driving frequency further leads to a denser arrangement of striations, with tighter striation gaps, while higher voltage results in a smaller bulk width.

High power impulse magnetron sputtering (HiPIMS) discharges with a zirconium target are studied experimentally and by applying the ionization region model (IRM). The measured ionized flux fraction lies in the range between 25% and 59% and increases with increased peak discharge current density ranging from 0.5 to 2 A/cm(2) at a working gas pressure of 1 Pa. At the same time, the sputter rate-normalized deposition rate determined by the IRM decreases in accordance with the HiPIMS compromise. For a given discharge current and voltage waveform, using the measured ionized flux fraction to lock the model, the IRM provides the temporal variation of the various species and the average electron energy within the ionization region, as well as internal discharge parameters such as the ionization probability and the back-attraction probability of the sputtered species. The ionization probability is found to be in the range 73%-91%, and the back-attraction probability is in the range 67%-77%. Significant working gas rarefaction is observed in these discharges. The degree of working gas rarefaction is in the range 45%-85%, higher for low pressure and higher peak discharge current density. We find electron impact ionization to be the main contributor to working gas rarefaction, with over 80% contribution, while kick-out by zirconium atoms and argon atoms from the target has a smaller contribution. The dominating contribution of electron impact ionization to working gas rarefaction is very similar to other low sputter yield materials.

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black-hole and neutron-star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with electron-positron pairs. Their role in the dynamics of such environments is in many cases believed to be fundamental, but their behavior differs significantly from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies, which are rather limited. We present the first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN’s Super Proton Synchrotron (SPS) accelerator. Monte Carlo simulations agree well with the experimental data and show that the characteristic scales necessary for collective plasma behavior, such as the Debye length and the collisionless skin depth, are exceeded by the measured size of the produced pair beams. Our work opens up the possibility of directly probing the microphysics of pair plasmas beyond quasi-linear evolution into regimes that are challenging to simulate or measure via astronomical observations.

One-dimensional particle-in-cell/Monte Carlo collisional simulations are performed on capacitive chlorine discharges with 2.54 cm gap rf driven by a sinusoidal with voltage amplitude of 222 V at driving frequency of 13.56 MHz. The properties of the discharge, the reaction rates for creation and loss of a few key species, the electron energy probability function, and the primary electron power absorption processes are explored as the gas pressure and the inclusion of secondary electron emission processes in the discharge model is varied. Five cases are investigated, including and neglecting electron, ion, and fast neutrals induced secondary electron emission. The negative ion Cl− is almost entirely created by dissociative attachment and lost through ion-ion recombination, and therefore the capacitive chlorine discharge is recombination dominated.

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.

The detailed mechanism of bonding in the cold spray process has remained elusive for both experimental and theoretical parties. Adiabatic shear instability and hydrodynamic plasticity models have been so far the most popular explanations. Here, using molecular dynamics simulation, we investigate their validity at the nanoscale. The present study has potential applications in the fabrication of ultrathin layers in the electronics industry. For this aim, we considered Ti nanoparticles of different diameters and Si substrates of different orientations. It is shown that very high spray velocities are required for a jet to be observed at the nanoscale. We propose a method for thermostating the substrate that enables utilizing high spray velocities. For the first time, we demonstrate an oscillatory behavior in both the normal and radial stress components within the substrate that can propagate into the particle. We have shown that neither the adiabatic shear instability model nor the hydrodynamic plasticity model can be ignored at the nanoscale. In addition, the formation of a low-resistance titanium silicide proper for electronic application is illustrated.

We present a comparative molecular dynamics simulation study of copper film growth between various physical vapor deposition (PVD) techniques: a constant energy neutral beam, thermal evaporation, dc magnetron sputtering, high-power impulse magnetron sputtering (HiPIMS), and bipolar HiPIMS. Experimentally determined energy distribution functions were utilized to model the deposition processes. Our results indicate significant differences in the film quality, growth rate, and substrate erosion. Bipolar HiPIMS shows the potential for an improved film structure under certain conditions, albeit with increased substrate erosion. Bipolar HiPIMS (+180 V and 10% Cu+ ions) exhibited the best film properties in terms of crystallinity and atomic stress among the PVD processes investigated.

The angular dependence of the deposition rates due to ions and neutrals in high-power impulse magnetron sputtering (HiPIMS) discharges with a titanium target were determined experimentally using a magnetically shielded and charge-selective quartz crystal microbalance (or ionmeter). These rates have been established as a function of the argon working gas pressure, the peak discharge current density, and the pulse length. For all explored cases, the total deposition rate exhibits a heart-shaped profile and the ionized flux fraction peaks on the discharge axis normal to the cathode target surface. This heart-shaped pattern is found to be amplified at increasing current densities and reduced at increased working gas pressures. Furthermore, it is confirmed that a low working gas pressure is beneficial for achieving high deposition rates and high ionized flux fractions in HiPIMS operation.

We present optimization of [(15 angstrom) Ni80Fe20/(5 angstrom) M]20 single crystal multilayers on (001) MgO substrates, with M being Cu, Cu50Pt50 and Pt. These superlattices were characterized by high resolution X-ray reflectivity (XRR) and diffraction (XRD) as well as polar mapping of important crystal planes. It is shown that cube on cube epitaxial relationship can be obtained when depositing at substrate temperature of 100 degrees C regardless of the lattice mismatch (5% and 14% for Cu and Pt, respectively). At lower substrate temperatures poly-crystalline multilayers were obtained while at higher substrate temperatures {111} planes appear at similar to 10 degrees off normal to the film plane. It is also shown that as the epitaxial strain increases, the easy magnetization axis rotates towards the direction that previously was assumed to be harder, i.e. from [110] to [100], and eventually further increase in the strain makes the magnetic hysteresis loops isotropic in the film plane. Higher epitaxial strain is also accompanied with increased coercivity values. Thus, the effect of epitaxial strain on the magnetocrystalline anisotropy is much larger than what was observed previously in similar, but polycrystalline samples with uniaxial anisotropy (Kateb et al. 2021).