Search

Among the numerous advantages of the high-power impulse magnetron sputtering (HiPIMS) technique, the most important is the enhanced ionization degree of sputtered species contributing to the film growth. Consequently, the quality of deposited thin films is highly improved. Still, the optimization process is challenging due to the complexity associated with the intricate transport of the sputtered species, ionized or neutrals. The scarce knowledge available on the spatial distribution of these species when operating a HiPIMS discharge makes the quantitative prediction of any deposition feature particularly difficult. In this paper, we discuss the influence of experimentally controllable quantities, such as gas pressure and target current density, on the transport of sputtered titanium in non-reactive (argon) HiPIMS, namely, on the behavior of metal atoms and metal ion fluxes intercepting the substrate. Systematic quantitative measurements were performed in a diameter normal plane on a circular planar target. Hence, the 2D spatial distribution of the ionized flux fraction (IFF) and the total flux of titanium sputtered particles (deposition rate) are evaluated by biasing a quartz crystal microbalance equipped with an electron magnetic filter. The wide range of parameters we examined allows us to predict and optimize the flux of sputtered species based on complete mapping of the IFF of sputtered particles. [ABSTRACT FROM AUTHOR]

The glow-to-arc transition is a critical phenomenon in plasma discharges, commonly leading to detrimental effects. The physical mechanisms triggering this transition remain poorly understood. The advent of a discharge called Hyper-Power Impulse Magnetron has opened possibilities. Hyper-Power Impulse Magnetron allows the glow mode to be maintained over long periods (1 ms) and at high-current densities (>5 A . cm − 2 ), which has unveiled certain features in the glow-to-arc transition. This work focuses on a graphite target that transits easily in the arc regime. The high-speed video-camera analysis revealed specific properties of graphite in ExB discharges, and the statistical study of the arc transition revealed differences from other refractory target materials. The early stage of cathodic spot formation, observed as bright dots, will be presented and analyzed within the known "ecton" and "vaporization" models for spot formation. This experimental study highlights the role of luminous spot formation prior to arc transition, with possible optimization on the stability of magnetron discharges. [ABSTRACT FROM AUTHOR]

Glow discharges are known to be limited for running at high current densities (typically below 0.1 A ⋅ c m − 2 ). Magnetically enhanced plasmas overcome this limitation, reaching up to two orders of magnitude higher current densities. However, the highest reported discharge current density hardly reaches 10 A ⋅ c m − 2 even in high power impulse magnetron sputtering (HiPIMS). Here, we report on the long (∼1 ms) pulsed operation of the magnetron in helium with an over-dense current (∼30 A ⋅ c m − 2 , usual for arcs) while preserving the glow mode. This regime defying the arcs was called hyper power impulse magnetron (HyPIM) because the power injected per pulse is several times higher than in HiPIMS for the same average power and pulse duration. HyPIM operation mode is distinguished from HiPIMS by the limited pulse voltage and is unsuitable for thin film deposition but may interest plasma switches. These huge currents were obtained for two cathode materials, molybdenum (Mo) and tungsten (W). Different experimental diagnostics bring insights into the physical phenomena governing the HyPIM plasma. A discussion on the role played by the metal atoms on the current unveils the origin of such exceptional glow current all along the pulse and the key role of the helium metastable atoms. A schematic of the kinetic steps of the HyPIM discharge is proposed. [ABSTRACT FROM AUTHOR]

Conventional magnetron discharge is a widely used technology for many applications. In the last decade, the high current density sputtering regime has been an interesting alternative for tailoring thin film properties. In this paper, we focused on the electrical characterization of the helium magnetron plasma operated at average gas pressure (5 Pa) with a molybdenum target. Optical emission spectroscopy diagnostics also supports this study by providing information on electron density evolution. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (6 A cm−2), corresponding to different discharge regimes. The self-sputtering yield plays a key role in high-power impulse magnetron sputtering discharge operated with helium. The electron density is highly dependent on the presence of a metal. [ABSTRACT FROM AUTHOR]

Electron powder bed fusion (E-PBF) is an additive manufacturing technology used to produce parts layer-wise for advanced aerospace, biomedical, and other applications. Precise control over the energy transferred to the powder by the electron beam is key to further process improvements. Here, we used tunable diode laser absorption spectroscopy to characterize the evaporated titanium atoms above the molten area of a TA6V powder alloy, and, thus, the effects of the energy transferred to it by the electron beam. This unconventional diagnostic tool achieves analyses at very high temporal (<1 μs) and spatial (<100 μm) resolutions, thus, offering valuable information on the microsecond-scale dynamics of the micro-melting zone and the effectiveness of the electron beam spot at diameters as small as ∼200 μm. Our measurements highlighted sharp fluctuations during the evaporation process that were independent of the power and scan speed of the electron beam; instead, the molten pool surface itself seems to drive these fluctuations. Our analysis also documented the shape and density of the vapor plume, which was oriented perpendicular to the surface under common E-PBF conditions. [ABSTRACT FROM AUTHOR]

In the last decades, numerical simulation has become a precious tool to assist the design and study of electron sources based on regular arrays of field emitters. Simulations of field emitter arrays (FEAs) require 3D treatment to account for the interactions between neighbor emitters. Therefore, modeling the thermal evolution of FEAs involves high computational resources due to the multi-physics approach and time dependency. The present paper proposes an algorithm which gives a fast prediction of the self-heating of a large array of N axisymmetric field emitters. It consists in finding for each emitter the equivalent 2D axisymmetric situation yielding the same electron current at 300 K as in the 3D array. The 3D modeling is thus efficiently split into N simulations in 2D, with a significant computation time reduction by at least one order of magnitude. The proof of concept uses 3 × 3 arrays of ideal emitters. Our results show a correct prediction, within a few percent, of the array thermo-field current and maximum temperature—two quantities of high interest for thermal failure and breakdown voltage considerations. The algorithm paves the way for including thermal effects in future optimization studies of realistic FEAs. [ABSTRACT FROM AUTHOR]

Electron beam powder bed fusion (E-PBF) is an attractive technology for the additive manufacturing of metal parts. However, process improvements require precise control of the energy transferred to the powder by the electron beam. Here, we used tunable diode laser absorption spectroscopy (TD-LAS) to measure the velocity distribution functions of titanium atoms evaporated during E-PBF. The narrow spectral ranges emitted by laser diodes allow for high-resolution absorption profiles of the evaporated atoms and thus accurate determinations of their Doppler broadening, density, and temperature during melting. The obtained vapor temperature reveals overheating at the surface of the melt pool relative to the low-pressure (0.1 Pa) boiling point of titanium, indicating that evaporation occurs under nonequilibrium conditions. We characterized the influence of the linear energy density on titanium evaporation and found it to be consistent with the saturation vapor pressure. Our characterization of the vapor properties provides reliable inputs for melt pool simulations. Furthermore, TD-LAS may be further exploited to prevent the evaporation of low-concentration alloy elements, which can induce defects in the printed part. [ABSTRACT FROM AUTHOR]

A coaxial configuration of discharge is proposed for plasma surface treatment and possibly other applications. The reactor is based on a cylindrical structure, with the inner cylinder radio frequency powered (RF, 13.56 MHz) and the outer cylinder grounded, playing the role of a guard ring. The charged species can escape from the inner cavity through two longitudinal slits made in both cylinders, aligned to each other, and producing a linear slab of plasma. Hence, it is possible to project the plasma directly onto a surface placed under the slits, called external plate. The operation of this device is uniform and stable in argon for a large pressure range (0.8–50 mbar). Furthermore, simulations using the Plasimo™ software package were performed to evaluate the plasma parameters and to explain the experimental results. The ion flux on the surface exposed to this plasma increases when RF power increases, and the pressure or gap distance to the plate decreases. This cylindrical capacitive coupled plasma configuration can be very effective for surface treatment of different materials (conductors or insulators) on large area (when the plate or the system is moving perpendicular to the slits) due to energetic ions and active species released from the plasma. [ABSTRACT FROM AUTHOR]

High-power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition technique that provides a high flux of ionized target species for thin film growth. Optimization of HiPIMS processes is, however, often difficult, since the influence of external process parameters, such as working gas pressure, magnetic field strength, and pulse configuration, on the deposition process characteristics is not well understood. The reason is that these external parameters are only indirectly connected to the two key flux parameters, the deposition rate and ionized flux fraction, via two internal discharge parameters: the target atom ionization probability αt and the target ion back-attraction probability βt. Until now, it has been difficult to assess αt and βt without resorting to computational modeling, which has hampered knowledge-based optimization. Here, we present a simple method to deduce αt and βt based on measured deposition rates of neutrals and ions. The core of the method is a refined analytical model, which is described in detail. This approach is furthermore validated by independent calculations of αt and βt using the considerably more complex ionization region model, which is a plasma-chemical global discharge model. [ABSTRACT FROM AUTHOR]