ICMCTF2017 Session F2-1: HiPIMS, Pulsed Plasmas and Energetic Deposition

[1] J. T. Gudmundsson, Plasma Phys. Contr. Fusion, 58(1) (2016) 014002

[2] F. Magnus, T. K. Tryggvason, S. Olafsson and J. T. Gudmundsson, J. Vac. Sci. Technol. A, 30(5) (2012) 050601

[3] J. T. Gudmundsson, D. Lundin, N. Brenning, M. A. Raadu, Chunqing Huo and T. M. Minea, Plasma Sources Sci. Technol. accepted for publication 2016

Energetic deposition provides increased control over the micro-structural, electrical and optical properties of thin films. In our work, high power impulse magnetron sputtering (HiPIMS) and filtered cathodic vacuum arc deposition systems have been employed to form high quality metal oxide and carbonaceous materials. Electronic and optical applications for these materials including transparent thin film transistors, photo-detection and resistive memory storage devices have been explored. Process and film characteristics will be discussed for materials including ZnO, HfO₂, HfO_xN_y, BN and C. In particular, the plasma and target conditions during HiPIMS growth of metal oxides and carbonaceous materials will be related to microstructural, electronic and optical measurements performed on the films produced. In addition, work will be presented in which ab initio calculations and X-ray absorption spectroscopy have been employed to characterise defects in energetically grown hBN.

Vanadium dioxide (VO₂) is the most interesting thermochromic material due to its reversible phase transition from semiconducting IR transparent state (monoclinic structure) to metallic IR reflective state (tetragonal structure) at around 68 °C. A high IR transmittance modulation makes the VO₂–based films a suitable candidate for optical switching applications, such as self-tunable infrared filters, temperature sensing devices and “smart” windows regulating the solar transmission. Current drawbacks limiting the application potential of the VO₂ films include high deposition temperatures (> 400 °C) of the films and the necessity to use a substrate bias potential in the case of their magnetron sputter deposition.

Reactive HiPIMS with a feed-back pulsed reactive gas (oxygen) flow control and an optimized location of the oxygen gas inlets in front of the target and their orientation toward the substrate made it possible to form crystalline thermochromic VO₂ films at very high values of the maximum target power density of up to 5 kWcm^-2 in a pulse. The thermochromic VO₂ films (80 nm thick) were deposited onto floating glass substrates without any nucleation–promoting “seed” layer at the temperature of 300 °C. The depositions were performed using a strongly unbalanced magnetron with an indirectly water-cooled planar vanadium target (50.8 mm in diameter) in argon-oxygen gas mixtures at the argon pressure of 1 Pa. The duty cycle was set to a constant value of 1%, the voltage pulse durations were 50 µs and 80 µs, and the corresponding repetition frequencies were 200 Hz and 125 Hz, respectively. The deposition-averaged target power density was approximately 13 Wcm^–2. The target-to-substrate distance was 150 mm.

The phase composition of the VO₂ films was determined by X-ray diffraction. The thermochromic behavior of the films was investigated using a spectrophotometer and spectroscopic ellipsometer equipped with custom designed heat cells to control the measurement temperature of the samples from 25 °C to 100 °C. The temperature-dependent electrical resistivity of the films was measured by a four-point probe. The time-averaged energy distributions of positive ions were measured with an energy-resolved mass spectrometer placed at the substrate position.

Since the first presentation of the High Power Impulse Magnetron Sputtering (HiPIMS) idea by Kouznetsov and co-workers in 1999 the basic architecture of a DC-charged capacitor bank dissipating periodically its energy into the plasma in pulses evolved to a sophisticated electronic device commercially available for industry. In order to meet rigorous requirements of industrial application, engineers have proposed different modifications of HiPIMS power delivery units to make the pulse shape and duration independent on the size of the capacitor bank and time-dependent plasma impedance. Since typically pulses longer than 100 μs are required to reach metal self-sputtering regime, a precise control of voltage and current pulse shape is required during the whole pulse length. Furthermore, for tuning ionization of the plasma species and deposition rate flexibility in power delivery regulation is of a key importance.

This contribution provides a series of case-study examples where HIPIMS was tested and qualified for industrial applications. First, the examples from the hard and decorative coating applications will be discussed. Using the basic facts about the HIPIMS plasma parameters the influence of the modified rectangular-like current peak on the plasma composition and dynamics will analyzed. As next, a novel approach of a flexible and application selectable HiPIMS power delivery control will be presented: (i) peak current regulation, (ii) pulse frequency, and (iii) pulse length. The in-field experiments data will be used to discuss the influence of peak current regulation on the ionization degree of the sputtered species as well as the influence of frequency regulation on the deposition rate.

CrN is used in many industrial applications. It provides good electrical conductivity, high hardness, corrosion resistance and high temperature stability. As known from many other investigations high power impulse magnetron sputtering (HIPIMS) lead to significant improved properties.

This presentation will discuss the influence of different HIPIMS techniques on the adhesion, hardness and composition of the thin film. Therefore, the gas composition (ratio between Ar and N2), the working pressure were changed. Additionally, the influence of the sputtering plant and target geometry was investigated. Two sputtering plants were used. The first one, a lab scale one, was equipped with a planar target (132 mm x 224 mm) and the second one with a planar (130 mm x 470 mm) and a rotating (Length of 500 mm) target.

While conventional processes require temperatures in the range of 250 °C and additional biasing the presented films all showed hardness values up to 2900 HV without substrate bias or additional heating.

The heteroepitaxial growth of metallic thin films on semiconductor substrates is usually required to provide a special growth direction for subsequent deposition of magnetic ultrathin layers or to reduce the dislocation density of lattice mismatched heterostructures. However, epitaxial relationship may be only obtained if the substrate surface is free of native oxides and contaminants, i.e., an atomic cleaning process is required. Conventional pre-treatment methods include the heating of the substrate at relatively high temperatures (800 °C) and the creation of a hydrogen-termination layer on the substrate surface through chemical etching with hydrofluoric acid. In this contribution we report a new route to grow epitaxial copper thin films at room temperature on silicon(001) wafers covered with native oxide without any prior substrate cleaning process. This method consists in a single-step deposition using high power impulse magnetron sputtering (HiPIMS) and substrate biasing. The studied Cu thin films were deposited onto Si(001) wafers by HiPIMS and conventional direct current magnetron sputtering (DCMS) at different substrate bias voltages. The stress evolution during deposition was monitored in situ using the real-time wafer curvature method. The as-deposited Cu films were characterized ex situ by X-ray diffraction, focused ion beam scanning electron microscopy, electron backscattering diffraction and atomic force microscopy. It was found that for higher bias voltages, a Cu/Si epitaxial growth is achieved following the Cu(001) [100] // Si(001) [110] orientation relationship in HiPIMS films, while polycrystalline Cu films with [111] preferred orientation are obtained using a DCMS discharge under the same deposition conditions. Detailed investigation of the film structure correlated with the intrinsic stress measurements shows that the substrate bias voltage affects the early growth stages of HiPIMS Cu films on Si, and thus their final microstructures.