ICMCTF2014 Session TS5: Plasma Diagnostics and Modeling

In the field of hard and wear-resistant coatings, thin compound films based on the system aluminium and chromium represent the state of the art. With the addition of nitrogen and/or oxygen ceramic coatings covering a wide compositional range can be synthesised. In commercial applications, the deposition of these coatings is frequently done by means of cathodic arc deposition since high ionisation and deposition rates can be achieved that enable the optimisation of the film growth conditions and, in particular, the reduction of the process time. The employed cathodes are commonly composite cathodes containing both elements as such an arrangement facilitates easier process control and reproducibility. However, the plasma conditions in the cathodic arc plasma of composite cathodes and their erosion behaviour in the used gaseous atmospheres are scarcely investigated.

In the present study, AlCr cathodes with compositions of 100/0, 75/25, 50/50, 25/75 and 0/100 at.% were exposed to a cathodic arc plasma in Ar, N₂ and O₂ atmosphere. The cathode erosion was evaluated by analysing the cathode surface by means of light optical and electron microscopy. Due to periodic heating and cooling of the cathode’s near-surface region in the cathode spots, an intermixing of the elements Al and Cr occurred. In addition, reactions of metal atoms on the cathode with gas molecules or atoms in N₂ and O₂ atmosphere resulted in the formation of nitride and oxide phases on the cathode surface, which is commonly referred to as cathode poisoning. Such phase formations were analysed by X-ray diffraction while the spatial distribution of the elements was characterised by scanning electron microscopy. The results regarding the cathode erosion are put in context with recently reported ion charge state distributions and ion energy distribution functions obtained with the same AlCr cathodes and gas atmospheres [1].

References:

[1] R. Franz, P. Polcik, A. Anders; IEEE Transactions on Plasma Science 41(8) (2013) 1929–1937

In this contribution a complete plasma characterization including specially resolved electron density, electron temperature and gas temperature of an Al PVD process using Ar as a sputtering gas is performed by evaluating the optical emission detected with an absolute calibrated spectrometer. Also, the emission provides further information on Al density in the plasma. This data is compared to a model of the PVD process which includes TRIDYN simulations as well as drift models for sputtered particles. A comparison of the theoretical model and measured data is performed for various sputter conditions, to get a better understanding of the material transport of PVD processes. It can be shown that resputtering of deposited film at high substrate sheath voltages can significantly influence the metal density profiles in the plasma. By means of film analytic, deposition rates, predicted by the applied model, can be verified. In some cases, effective sticking of particles can be fairly low depending on the chosen conditions.

The authors would like to acknowledge the support provided by the “Deutsche Forschungsgemeinschaft” within the frame of the SFB-TR 87, as well as the “Federal Ministry of Economics and Technology” on the basis of a decision by the “German Bundestag” and the “Ruhr University Bochum Research School”.

Reactive plasmas produced in oxygen, nitrogen, hydrogen and other complex gas mixture are used for various applications including thin films, etching, ion implantation, ashing, particles growth, oxidation and other surface functionalization processes. Most of the reactive gases are also electronegative so that, the role of negative ions cannot be neglected. The continuous decrease of the features size in micro- and nanoelectronic industry requires a precise control of plasma parameters including the negative ions. Despite of a good progress in plasma diagnostics, yet more is to be done for developing techniques compatible with the strict requirements for device-making setups. Moreover the properties and possibilities to control the electronegative discharges are not completely understood. The aim of this work is to review the main electrical diagnostics techniques available to investigate reactive plasmas. Electrostatic probes have been used to diagnose electronegative plasma since 70’s. While this technique can give good results for density ratios of negative ion to electron higher than 10 its applicability for lower density ratios is questionable. In this context is was demonstrated that double hump structures observed in the electron energy probability function close to plasma potential cannot be associated with negative ion parameters because those structures are produced by a particular change in the work function over the probe surface as a result of discrete ion focusing. Another way to detect the plasma parameters in the presence of negative ions is to use the sensibility of the test function in the mid and high energy tail of the distribution function. The presence of negative ions is also associated with a lower heat flux to the probe, a fact that led to the development of a thermal probe that allows one to record at the same time not only the current bias, but also a temperature bias characteristic. The recent discovery of the discrete and modal focusing effects, associated with three-dimensional plasma-sheath-lenses has created the possibility to detect even low densities of negative ions using the sheath-lens probe. The positive ion extraction from reactive plasmas is rather easy. However, this is not the case for negative ions. The influence of biased electrodes, of small or large dimensions on plasma parameters in electronegative discharges can give more information about the possibility to control and use these plasmas for processing. Development of reactive plasma sources for both applications and basic science is rather challenging and some of these efforts will be presented in direct correlation with diagnostic approaches.

Substrate heating by the plasma during magnetron sputtering is known to occur, however, there have been very few detailed studies of this process which involves a combination of bombardment by ions, excited and neutral species and UV radiation incident on the substrate. We have studied the heating of the substrate during DC magnetron sputtering of a 4” titanium target as a function of a variety of experimental conditions; plasma power, Ar gas pressure, floating, grounded and biased substrates. We have also studied the plasma heating during two reactive sputtering processes of the titanium target using a gas mixture of argon with either nitrogen or oxygen. It is known that the crystalline orientation of titanium nitride depends on the sputtering conditions, whilst titanium oxide prepared at low temperatures is normally amorphous. In this work we report the effect of the plasma substrate heating on the morphology and the crystalline structure of titanium oxide and nitride. The properties of the films were analyzed using EDX, SEM and X-ray Diffraction and the film thickness was measured using a stylus perfilometer. The measurements of the non-reactive sputtering showed that the substrate temperature could reach temperatures higher than 200ºC with a plasma power of 200W and showed a non uniform temperature distribution over the substrate, with the highest temperature in front at the race track and the lowest temperatures in front of the target edge.

In spite of its long term history and broad technical applicability, the dynamics of magnetron sputter discharges are not yet fully understood in all details. Recent high speed image processing experiments on High Power Impulse (HiPIMS) magnetron discharges reveal that the ring plasma of a PVD sputter source is not homogeneous but consists of several propagating plasma waves, which are also referred to as »spokes«.

In order to improve the knowledge of the inherent mechanisms, particle based simulation methods, namely »Direct Simulation Monte Carlo« (DSMC) for rarefied gas dynamics and »Particle-in-Cell Monte Carlo« (PIC-MC) for non-equilibrium plasma discharges are developed. In recent years, due to the increasing availability of high performance computing hardware and their exploitation by massive parallel algorithms, the feasibility of both methods even for large and complicated reactor geometries is significantly improved.

We present three-dimensional PIC-MC simulations of a circular magnetron source operating at DC power in Ar gas under different process conditions regarding power and total pressure. Even for moderate DC power levels we find similar propagating features in the simulated plasma which can qualitatively explain the experimentally observed spokes. By analyzing the electron and ion flux distributions, a more detailed picture about the role of the plasma fluctuations in the overall discharge dynamics is obtained.

Besides of the 3D PIC-MC discharge simulations, further approaches towards modeling different aspects of magnetron sputtering e. g. the gas heating effect in front of the target, transport of ionized sputter particles and the use of global models are discussed.

It is now commonly accepted that the ionization rate of metal atoms is dramatically increased during High-Power Impulse Magnetron Sputtering discharges as compared to conventional DC magnetron plasmas. It is therefore assumed that the dissociation rate of molecular gases might also be significantly increased during reactive HiPIMS processes, hence leading to formation of a very reactive environment. In this study, titanium was sputtered in an Ar+O₂ gas mixture in a HiPIMS discharge. Several discharge parameters such as the pulse duration, discharge frequency, Ar pressure, etc. were examined.

In this contribution, we first show how it is possible to determine the absolute densities of metastable atomic oxygen atoms during the pulse ON and OFF times by utilizing resonant optical absorption spectroscopy (ROAS) technique. The photons corresponding to 2s²2p³(⁴S)3s – (⁴S)3p transition (λ~777 nm, the lower energy state is metastable) necessary for performing the ROAS measurements were produced in a microwave Ar/O₂ discharge and guided to the HiPIMS chamber by a fiber optics. ROAS measurements were carried out 5 cm above the sputter target.

It is demonstrated that the absolute densities vary in time and present a maximum after the end of the plasma pulse. Our results are consistent with relative density data reported by Vitelaru et al. (Appl. Phys. Lett. 103, 104105 (2013)) who used a laser spectroscopy method. The maximum of the density value measured in this work is ~5x10⁹ cm^-3 for the 20 µs long pulse. Increasing the pulse energy allows for and increased dissociation rate of the oxygen molecules. The obtained results are compared to the time-resolved mass-spectrometry data obtained in the same system, as well as to the ROAS results corresponding to non-reactive HIPIMS.

Metal oxide thin films like ZnO, Al₂O₃ and TiO₂ are widely used in industry in microelectronics, catalysts and display devices. Plasma-based deposition techniques for the production of these films such as pulsed laser deposition (PLD), suffer from a lack of fundamental understanding of the underlying physical plasma processes and hence a lack of control of the deposited film properties.

This research focuses on establishing a detailed understanding of the plasma that is used for thin film deposition in PLD. This includes the interaction of the laser with the solid target, the creation of the ablation plasma, the expansion in the vacuum chamber or surrounding atmosphere and the plasma-surface interaction for film growth at the substrate. By employing a combination of plasma modelling and experimental diagnostic techniques such as laser-induced fluorescence and optical emission spectroscopy, we are able to accurately measure and subsequently predict the plasma properties such as species densities and temperatures, as well as the formation of the thin film in the plasma-surface interaction. This understanding will be used to tailor and control the Zn and ZnO plasma to obtain the desired film properties.

Additionally, a new form of PLD is introduced, plasma-enhanced PLD (PE-PLD). In PE-PLD we combine the laser-produced metal ablation plasma with an electrically produced oxygen plasma. With this method a Zn target is ablated which subsequently expands in an oxygen plasma rather than neutral oxygen gas. This oxygen plasma contains well-characterised and controllable, reactive oxygen species such as atomic oxygen, ozone and singlet delta oxygen. This method offers additional control of the Zn and O interactions and concentrations in the depositing ZnO plasma.