ICMCTF 2025 Session PP2-1-WeM: HiPIMS, Pulsed Plasmas and Energetic Deposition I

Cathodic Arc Deposition, a commonly used PVD process of growing hard coatings, involves high fluxes of ions and electrons in a dense, expanding plasma. The composition of the arc plasma may vary significantly within the deposition chamber, and the source-to-substrate distance impacts the coating growth conditions. This study investigates the Ti-N plasma generated by a 100 mm, dc-operated arc source at 20 V and 120 A, in an 1 Pa N₂ ambient within a cylindrical, lab-scale HV chamber. A combinatorial approach of experimental probes and finite element fluid mechanical modelling is used to understand the varying plasma composition in terms of ions, neutrals, and radicals, and their corresponding fluxes. The measured and simulated ion species are Ti¹⁺, Ti²⁺, Ti³⁺, Ti⁴⁺, TiN¹⁺, TiN²⁺​, N₂¹⁺​, and N¹⁺. The dominant ion species in every probed spatial configuration is Ti^2+, while Ti¹⁺ follows closely and equalizes the energetic footprint of Ti²⁺ when the distance from the source is increased. Atomic Nitrogen ions maintain a significant presence throughout the plasma volume, largely due to a sustained emission from the nitrided Ti-source surface, N₂ dissociation and charge exchange collisions. In general, the plasma density and average charge state follow a decreasing trend for distances larger than 35 cm from the source, where the presence of Ti³⁺ and Ti⁴⁺ is suppressed. At the same time, TiN-ions retain their presence, leading to different growth conditions at the substrate, depending on the chosen distance from the source.

The highpower impulse magnetron sputtering (HiPIMS) generates highdensity plasma through higher instantaneous pulse currents, resulting in thin films with fewer defects, higher density, and densermicrostructure. In this work, a combination of HiPIMS and radio frequency power supply system was used to deposit TiZrNbTaMoN highentropy alloy (HEA) thin films with varying nitrogen contents on Si wafer,AISI304 and 420 stainless steel substrates. The cross-sectional morphology, composition, and crystal structure of thin films were analyzed using scanning electron microscopy, electron probe microanalyzer, X-ray diffraction, andtransmission electron microscope, respectively. Subsequently, potentiodynamic polarization corrosion tests were conducted on the HEA thin films in 3.5 wt.% NaCl aqueous solution using an electrochemical workstation to evaluate their corrosion resistance. We found that TiZrNbTaMoN HEA nitride coatings exhibited a hardness of up to 29 GPa, along with outstanding corrosion resistance.The effect of nitrogen content on the phase, mechanical properties, and corrosion resistance of TiZrNbTaMoN HEA thin films was discussed in this work.The potential applications for the TiZrNbTaMoN HEA thin filmsin the machining industries were proposed.

The increasing demands of big data and AI necessitate breakthroughs in energy-efficient computing and data storage. Ferroelectric nitrides, such as aluminum scandium nitride (AlScN), show great potential for non-volatile memory technologies due to their high remnant polarization, temperature stability, and compatibility with current semiconductor manufacturing processes.

The performance of ferroelectric nitrides is directly linked with their structural properties. The remnant polarization can be improved by increasing the c-axis orientation of the film, whereas the leakage current density can be improved by optimizing the microstructure. Point defects however negatively affect the breakdown behavior of the material, which represents a major challenge in the deposition of ferroelectric thin films.

Metal-ion synchronized high-power impulse magnetron sputtering (MIS-HiPIMS) can be used to accelerate film-forming metal ions onto the growing film, resulting in enhanced crystalline quality, improved c-axis texture, and a compact microstructure. Building on our recent successful demonstration of MIS-HiPIMS for piezoelectric thin films [1] we leverage the unique advantages of the technique for the deposition of ferroelectric AlScN with excellent performance.

Through a combinatorial study, we investigate the influence of HiPIMS on the ferroelectric properties of Al_1-xSc_xN films while correlating these properties with crystallinity and Sc composition. Our optimized deposition process successfully yields Al_1-xSc_xN films on Si substrates with performance that is otherwise only achieved using epitaxial growth.[2] Compared with previous reports using conventional sputtering the HiPIMS films show significantly enhanced remanent polarization with values of > 170 µC/cm² while maintaining comparable coercive fields of 5.0 MV/cm. Notably, our findings reveal that the remanent polarization remains stable even with increasing scandium concentrations. At the same time the leakage current densities are among the lowest reported to date.[3] These results can be explained by the excellent c-axis texture enabled by HiPIMS. In the future HiPIMS could enable dense ferroelectric films with low thickness to reduce the switching potential. To our knowledge this is the first report of ferroelectric switching in HiPIMS-deposited nitride thin films. Overall, the results are more than promising and highlight the potential of HiPIMS for the development of defect-sensitive electronic thin films.

[1] Patidar et al. Physical Review Materials 8 (9), 095001, 2024

[2] Deng et al. Journal of the American Ceramic Society, 107, (3), 2023

[3] Yazawa et al. arXiv preprint arXiv:2407.14037, 2024

This paper summarizes a series of experiments demonstrating the significance of energetic species during DC magnetron sputter deposition. The first example focuses on the phase composition of tungsten (W) films, which can consist of a mixture of a-W and b-W crystals. Various mechanisms have been proposed to explain phase selection, including substrate heating due to plasma exposure and residual gas pressure. However, a broad parameter scan rules out these trends and shows that the phase composition can be quantitatively correlated with the flux of reflected neutrals with energies exceeding the displacement energy threshold. To establish this correlation, the phase composition is quantitatively determined using X-ray diffraction (XRD) analysis and combined with test particle Monte Carlo simulations to evaluate the energy of the reflected neutrals. The energy of these neutrals is defined by binary collisions between argon (Ar) and tungsten (W) atoms and the initial energy of the argon ions, which is set by the discharge voltage. Increasing the target thickness results in a lower magnetic field strength and, consequently, a higher discharge voltage. This effect allows the phase composition to be tuned by just adjusting the target thickness. The role of target thickness is further illustrated in a study on the percolation film thickness during the growth of silver (Ag) thin films. In-situ four-point probe resistance measurements are used to investigate the initial nucleation of these films. A power-law correlation between the percolation thickness and the deposition flux is observed, with the correlation exponent adjustable through variations in target thickness. Both studies highlight that reporting only the discharge power during experiments omits essential information critical for other researchers.

F. Ahangarani Farahani, D. Depla. “Phase Composition of Sputter Deposited Tungsten Thin Films.” Surface and Coatings Technology, vol. 494 (2024) 131447

High vacuum is required for conventional physical vapor deposition (PVD) techniques which restricts the application space that benefits from the dimensional stability, functionality, and chemically benign processing afforded by PVD thin films. Motivated by this limitation, a high-power impulse plasma source (HiPIPS) has been developed for surface engineering at atmospheric pressure. HiPIPS technology utilizes high-voltage, low duty cycle DC pulses to generate a plasma discharge between a consumable feedstock (cathode) and a conductive plasma jet nozzle (anode). The plasma is forced out of the nozzle by high flow rates of process gas where it subsequently interacts with a substrate which may be biased to increase the kinetic energy of ionized species. The HiPIPS design enables high plasma density to be achieved while maintaining low average power. In this study, processing-microstructure-property relationships are reported for the HiPIPS deposition of metallic aluminum (Al) films. Argon (Ar) was used as the working gas, and Al thin films were deposited at atmospheric pressure by utilizing Al alloy electrodes. HiPIPS system design variables and plasma discharge characteristics were correlated with the mechanical, compositional, and microstructural properties of the Al films. Film characterization was conducted via adhesion testing, energy dispersive x-ray spectroscopy, transmission electron microscopy, and atomic force microscopy. In this presentation, HiPIPS parameters which lead to desirable film qualities are discussed.

Recently, High-Power Pulsed Magnetron Sputtering (HiPIMS) technology, due to its high ionization rate, has enabled increased ion bombardment during film growth, resulting in dense and smooth films. Previous studies using Bipulse-HiPIMS to deposit Ag-Cu and Ti-Cu films showed that, compared to unipolar mode, Ag-Cu co-sputtering significantly increased the ionization of Ag, while Ti-Cu co-sputtering greatly enhanced the ionization of Cu. These findings suggest that, in the Bipulse-HiPIMS process, the selection and configuration of targets, as well as the tuning of each target's parameters, are closely related to the ionization rate of plasma species.

CrAlN possesses excellent oxidation resistance; however, ternary metal nitride films are no longer sufficient to meet current demands. Therefore, we further investigated quaternary metal nitride films, doping with elements such as Cr, Ti, and Zr to enhance mechanical properties and oxidation resistance. This study uses Bipulse-HiPIMS co-sputtering, with CrAl as the main target at a fixed power of 1.5 kW and Cr, Ti, and Zr as auxiliary targets. Since Cr has a relatively high second ionization energy (16.49 eV), whereas Ti and Zr have second ionization energies of 13.58 eV and 13.13 eV respectively, Ti and Zr are expected to ionize more readily than Cr. The Bipulse-HiPIMS technique aims to assist the ionization of the less readily ionized material (Cr) by more easily ionized materials. Time-averaged OES was used to measure the effect of different target materials on the plasma spectrum during the process.