ICMCTF 2023 Session B8-1-WeA: HiPIMS, Pulsed Plasmas and Energetic Deposition I

Efficient metal-ion-irradiation during film growth with the concurrent reduction of gas-ion-irradiation, are realized for high-power impulse magnetron sputtering (HiPIMS) by the use of a synchronized, but delayed, pulsed substrate bias. In this way, the growth of stress-free, single phase α-W thin films is demonstrated without additional substrate heating or post-annealing. By synchronizing the pulsed substrate bias to the metal-ion rich portion of the discharge, tungsten films with a <110> oriented crystal texture are obtained as compared to <111> orientation obtained using a continuous substrate bias. At the same time, a reduction of Ar incorporation in the films are observed, resulting in the decrease of compressive film stress from σ = 1.80 to 1.43 GPa when switching from continuous to synchronized bias. This trend is further enhanced by the increase of the synchronized bias voltage, whereby a much lower compressive stress σ = 0.71 GPa is obtained at Us = 200 V. In addition, switching the inert gas from Ar to Kr has led to fully relaxed, low tensile stress (0.03 GPa) tungsten films with no measurable concentration of trapped gas atoms. Room-temperature electrical resistivity is correlated to the microstructural properties, showing lower resistivities for higher Us and having the lowest resistivity (14.2 μΩcm) for the Kr sputtered tungsten films. Moreover, we showed a very low resistivity of 13.9 μΩcm at an ultra-thin thickness of ~10 nm grown on SiO₂ without substrate heating or post annealing. The realization of self-ion-irradiation by accelerated W⁺ ions in HiPIMS discharge has an important role at the growth phase of nucleation and island growth, and consequently to the development of larger grains. These results illustrate the clear benefit of utilizing selective metal-ion-irradiation during film growth as an effective pathway to lead grain growth and to minimize the compressive stress induced by high-energetic gas ions/neutrals during low temperature growth of high melting temperature materials.

Piezoelectric micro-electro-mechanical systems (MEMS) are one of the building blocks of modern electronics and are used in many applications such as RF filters, resonators, and sensors. AlN has been one of the standard materials for such applications due to its high-temperature stability and linear frequency response. Significant improvements in the piezoelectric response can be achieved by isovalent alloying of AlN with Sc.[1] The heterostructural nature of the alloy system, however, leads to low miscibility and a high degree of structural frustration for high Sc concentrations, and thus decreased texture of the films.

In this work, we present the utilization of synchronized-HiPIMS along with a combinatorial deposition approach to deposit highly oriented AlScN films. The combinatorial deposition approach discussed here is based on the co-sputtering of Al and Sc in a reactive environment and provides us an opportunity to probe the properties of films with different Sc contents and temperatures in fewer depositions.[2] The synchronization of substrate bias with the metal-ion-rich part of the sputter plasma is done. This effectively reduces Ar-ion incorporation and consequently helps in the growth of textured films at lower deposition temperatures.[3]

The combinatorial libraries of AlScN are deposited with different biasing conditions, essentially changing the kinetic energy of oncoming species. The solubility limit of Sc in AlN is demonstrated to change with varying bias conditions and deposition temperatures. The improvement in the texture of the films and its correlation with stress is investigated. In addition, the influence of closed-field and open-field plasma (obtained by changing the magnetic configuration of sputter guns in the chamber) is discussed in tailoring the phase, structure, and stress in the films. The combinatorial libraries are fully characterized with respect to their phase constitution, structure, composition, stress, and piezoelectric coefficients using state-of-the-art techniques. Selected libraries are mapped for d₃₃ values on doubly-polished Si wafers.

A successful deposition of textured AlScN film using synchronized HiPIMS will enable the deposition on structured as well as temperature-sensitive substrates which would be beneficial for a variety of exciting applications.

[1] Akiyama, Morito, et al. Advanced Materials2009, 21 (5), 593-596.

[2] Talley, Kevin R., et al. Physical Review Materials2018, 2 (6), 063802.

[3] Greczynski, G. et al. J. Vac. Sci. Technol. A2019, 37 (6), 060801.

In the present work, we evaluated the synthesis of AlTiN coatings through HiPIMS with variable duty cycle (3%, 6%, 12%, 18%) under synchronous pulse-DC (SP-DC) bias and with trigger delay 50 ms bias (TD50). The influence of these processing conditions on the microstructure and mechanical properties of AlTiN was investigated. FE-SEM analysis results showed a highest deposition rate of 22.1 nm/min when TD50 with 3% duty cycle. The results of EPMA and XRD show that when the Al/Ti ratio x is between 0.71 to 0.74, the h-AlN structure will be generated. The results of TEM and nanoindenter analysis show that the DC bias transforms into synchronous bias boosting the bias output time (increasing duty cycle) will refine the AlTiN grains from about 150 to 20 nm, increasing the hardness from 22.7 to 24.7 GPa.Meanwhile, the residual stress of AlTiN thin film increased from 0.2 to -1.51 GPa, and obtain a higher adhesion strength 54.8 N on synchronous bias with 6% duty cycle condition. Therefore, the both duty cycle and trigger delay under synchronous pulse-DC bias also can as an important function of HiPiMS process parameter.

Keywords: synchronous bias, duty cycle, high-power impulse magnetron sputtering, AlTiN coating

The concept of Bipolar HiPIMS was introduced some years ago by different groups and consist in applying a positive pulse with controlled pulse width and amplitude voltage after the conventional HiPIMS negative pulse [1]. This positive pulse allows the accurate acceleration of the positive metal ions towards the substrate, thus, promoting improved film properties such as reduced stress, higher film densification, improved mechanical properties - such as hardness or wear resistance- or better coverage of 3D complex parts.

This paper shows experimental results for the deposition of uniform coatings solutions on 3D complex geometry components used different application sectors: cutting tools and punches for aerospace and medical industry, plastic injection moulding for packaging or Through Substrate Vias (either Silicon or Glass) in electronic engineering for high performance interconnection in 3D integrated circuits.

[1] G. Eichenhofer, I. Fernandez, and A. Wennberg, “Industrial use of HiPIMS and the hiP-V hiPlus technology,” Vak. Forsch. Prax. 29, 40 (2017).

Transition metal carbides are refractory materials possessing extremely high melting temperatures, outstanding mechanical strength, excellent electrical conductivity, and good chemical stability. Non-reactive high-power impulse magnetron sputtering (HiPIMS) of NbC films from a single compound target is an attractive deposition method because of its simplicity and scalability. The film composition (C/Nb ratio), microstructure and, consequently, mechanical properties, can be controlled by varying the power density in the pulse [1].

This presentation focuses on explaining the observed change in the composition ranging from C-rich films to stoichiometric NbC films corresponding to an increase in the pulse-averaged power density (realized by shortening the pulse length) [1]. The Nb and C atoms sputtered from the target are under the influence of multiple processes that together affect their transport towards the substrate (including the sputtering rate and angular distribution, scattering off the process gas atoms, ionization in the high-density plasma and return of ions onto the target or loss of ions to chamber walls). These processes are expected to be manifested differently for Nb and C, due to their disparate elemental properties, such as atomic mass and ionization potential.

To untangle the influence of the above-mentioned plasma processes on the C/Nb ratio in the films, Nb⁺ and C⁺ fluxes at the substrate position were measured by energy-resolved mass spectroscopy and trends in the ratios of Nb and C neutral and ion species at various positions in the discharge were monitored by optical emission spectroscopy. These experimental results are supported by a theoretical analysis of the target composition and of the transport of plasma species in the discharge, employing a pathway discharge model and particle-based simulations. The aim of this presentation is to provide a general understanding applicable to magnetron sputtering of various multi-component targets.

[1] A. Bahr, T. Glechner, T. Wojcik, A. Kirnbauer, M. Sauer, A. Foelske, O. Hunold, J. Ramm, S. Kolozsvári, E. Ntemou, E. Pitthan, D. Primetzhofer, H. Riedl, R. Hahn, Non-reactive HiPIMS deposition of NbCx thin films: Effect of the target power density on structure-mechanical properties, Surf. Coat. Technol.444 (2022) 128674.

This paper discusses discharge characteristics of a Cr cylindrical magnetron with multi-toroidal plasma geometry in Ar atmosphere. This particular shape of the magnetron enables applications of coatings on surfaces previously inaccessible to magnetron sputtering - in confined spaces, such as: the interiors of pipes or barrels. Direct current magnetron discharge sources are often characterized by current-voltage characteristics of the form I∝Vq. It has been suggested that the exponent q provides an index to the effectiveness of the magnetic electron confinement in a magnetron discharge[1]. An application of a magnetron in a confined space can significantly influence the magnetic field and thus the electron confinement. Therefore, we have examined the I-V characteristics of DC, pulsed DC and HIPIMS discharge of Cr cylindrical magnetron with multi-toroidal plasma geometry in Ar atmosphere in open and confined space realized by a A570 Gr. 36 steel pipe. The q parameter of the exponential function Id (Ud) describing current-voltage waveforms was determined. The study confirms that the type of discharge and magnetron surroundings significantly affects the value of q.

[1] G. Y. Yeom and John A. Thornton; Journal of Applied Physics 65, 3816 (1989)

Influence of different concentrations of Si into AlTiN and AlTiCrN coatings and multilayer architecture deposited by HIPIMS is discussed in two sections. The first one reports on the crystallographic phase stability investigations done by XRD, the microstructural TEM observations and the micromechanical studies to understand the role of the Si addition on these two nitride coatings. It was noticed that the Full Width at Half Maxima (FWHM) that can be correlated to the coating grain size, increases with Si addition for AlTiSiN coatings but not in the case of AlTiCrSiN coatings. The instrumented value for the wear resistance of these strain-hardened nitride coatings (Hardness/Young’s Modulus ratio) was estimated to 0.09 for both coatings independently of the Si content and compared to the multilayer coating. The second discusses the investigations on the oxidation mechanisms and the kinetics of the oxide growth at 950 °C for various durations, mainly through a detailed description of the oxide layers by combining the investigations of X-ray Diffraction, Transmission Electron Microscopy, Dynamic Secondary Ion Mass Spectrometry and the Atom Probe Tomography of the different coatings. Thank to isotopic oxidation tests with sequential time duration ¹⁶O/¹⁸O and SIMS depth profile analyses, the rule of cationic diffusion process and the growth mechanism for each oxide scales have been proposed to explain the oxidation kinetics. Concerning the thermal resistance of the AlTiN coating at 950 °C, the introduction of Si below 5 at.% has largely increase the oxidation set point, from 800 °C to 950 °C (or even highest). The oxidation study for AlTiCrSiN coating has reported a different oxide scale morphology, a pure TiO₂-rutile nearer the surface, followed by the Al-rich oxide, then mixed oxide region of (AlCr)₂O₃ with small islands of TiO₂ and an amorphous SiN_x surrounding the different oxide. A regular oxidation kinetic has been recorded for the AlTiCrSiN coating and no influence of the addition of Si has been recorded. For the moderate Si contents, thanks to an optimization of nanoscale thicknesses, a multilayer TiAlSiN/TiAlCrSiN coating architecture can inhibit the formation of TiO₂ top-layer and also promotes the formation of Al-rich protective oxide layer.