Ante Hecimovic

Quadrupole mass spectrometry of reactive plasmas

Reactive plasmas are highly valued for their ability to produce large amounts of reactive radicals and of energetic ions bombarding surrounding surfaces. The non-equilibrium electron driven plasma chemistry is utilized in many applications such as anisotropic etching or deposition of thin films of high-quality materials with unique properties. However, the non-equilibrium character and the high power densities make plasmas very complex and hard to understand. Mass spectrometry (MS) is a very versatile diagnostic method, which has, therefore, a prominent role in the characterization of reactive plasmas. It can access almost all plasma generated species: stable gas-phase products, reactive radicals, positive and negative ions or even internally excited species such as metastables. It can provide absolute densities of neutral particles or energy distribution functions of energetic ions. In particular, plasmas with a rich chemistry, such as hydrocarbon plasmas, could not be understood without MS. This review focuses on quadrupole MS with an electron impact ionization ion source as the most common MS technique applied in plasma analysis. Necessary information for the understanding of this diagnostic and its application and for the proper design and calibration procedure of an MS diagnostic system for quantitative plasma analysis is provided. Important differences between measurements of neutral particles and energetic ions and between the analysis of low pressure and atmospheric pressure plasmas are described and discussed in detail. Moreover, MS-measured ion energy distribution functions in different discharges are discussed and the ability of MS to analyse these distribution functions with time resolution of several microseconds is presented.

Current–voltage characteristics and fast imaging of HPPMS plasmas: transition from self-organized to homogeneous plasma regimes

Current–voltage characteristics within the temporal pulse were recorded in high-power pulsed magnetron sputtering discharges for different target materials. These curves allowed identifying at a first glance the existence of separated plasma regimes clearly differentiated by the plasma conductivity and by the spatial arrangement of the plasma emission. We could establish that regimes of high plasma conductivity are univocally associated to the self-organization of the plasma in well-defined ionization zones. As the applied power is gradually increased, the high conductivity regime is abruptly replaced by a regime of high current and low plasma conductivity, associated to homogeneous plasma emission.

Temporal evolution of the radial plasma emissivity profile in HIPIMS plasma discharges

High power impulse magnetron sputtering (HIPIMS) is a plasma vapour deposition technique used for deposition of dense coatings. In order to contribute to a better understanding of the dynamics of a HIPIMS plasma discharge, time- and wavelength-resolved measurements of the light emitted from the plasma were performed. Lateral images of the HIPIMS plasma were recorded using an ICCD camera with a gate width of 1??s. For each picture, Abel inversion was performed to compute the radial emissivity profile of the plasma. Band-pass interference filters were used to isolate the desired wavelength in order to observe lines of ions and neutrals of metal and argon (Ar) in HIPIMS plasma discharges with an aluminium (Al), titanium (Ti) or chromium (Cr) target in Ar atmosphere. The result is the temporal evolution of the radial emissivity profile of metal and gas neutrals and singly charged ions.

Influence of Ar/Kr ratio and pulse parameters in a Cr-N high power pulse magnetron sputtering process on plasma and coating properties

Krypton is sometimes used in physical vapor deposition processes due to its greater atomic mass and size compared to argon, which leads to a lower gas incorporation and may have beneficial effects on kinetics of the coating growth. In this paper, the authors investigate the plasma composition and properties of deposited high power pulse magnetron sputtering Cr-N coatings for discharges with various Ar/Kr ratios and for various pulse lengths of 40 μs, 80 μs, and 200 μs, keeping the average discharge power constant. The results show that an addition of Kr influences the discharge process by altering the ignition and peak values of the discharge current. This influences the metal ion generation and growth conditions on the substrate by reducing the nucleation site densities, leading to a predominantly columnar grow. However, the deposition rate is highest for an Ar/Kr ratio of 120/80. The integral of the metal ion and atom emission exhibits the same trend, having a maximum for Ar/Kr ratio of 120/80. By decreasing the pulse length, the deposition rate of coatings decreases, while the hardness increases.

Fundamental study of an industrial reactive HPPMS (Cr,Al)N process

In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the pl...

Two dimensional spatial Argon metastable dynamics in HiPIMS discharges

The spatial and temporal dynamics of the Ar metastable () density and temperature is studied by an extended Tunable Diode Laser Absorption Spectroscopy set-up (TDLAS) within a High Power Impulse Magnetron Sputtering (HiPIMS) pulse. A beam expander in combination with a photo diode array is used to simultaneously measure the spatial and time resolved absorption profile of an transition. HiPIMS discharges are known for their complex physics, e.g. drifting ionisation zones called spokes, that occur in a discharge with crossed magnetic and electric fields. By analysing the Doppler shift of the absorption line, we found evidence for a small azimuthal velocity of the in direction. However, this observed azimuthal velocity is three orders of magnitude slower than the reported ion drift velocities and four orders of magnitudes slower than the drift velocity. This finding may be caused by the weak coupling between the plasma rotation driven by drift and the background gas, providing an estimate of the coupling between the plasma drifting in direction and the neutral background gas.

Simultaneous characterization of static and induced magnetic fields in high power impulse magnetron sputtering discharges

Hall currents generated in circular planar magnetrons can induce magnetic fields of several mT when operating in high power impulse magnetron sputtering (HiPIMS) mode. Near the target surface, the induced magnetic field is negligible compared to the static one. However, as we move away from the target surface the induced field becomes comparable or even stronger than the static field. In this paper, we investigate the induced magnetic field using Hall sensors, which can directly measure the effective magnetic field (the sum of the static and the induced magnetic fields) and with sufficient time resolution to observe the temporal evolution of the induced field during the HiPIMS pulse. We present the 2D temporal evolution of an induced magnetic field, showing its influence on the effective field. Based on the 2D induced magnetic field map, we calculate numerically the spatial distribution of the Hall currents generating the field, resulting in a current density up to 7 A cm−2. We present these results for both an unbalanced and balanced magnetron configuration.

Target implantation and redeposition processes during high-power impulse magnetron sputtering of aluminum

The processes of argon retention by the target and redeposition of target material were investigated by x-ray photoelectron spectroscopy as a function of radial position for different plasma conditions in high-power impulse magnetron sputtering of aluminum targets. Significant differences in Ar radial concentration profiles were observed for different discharge conditions. Inside the racetrack area, Ar ion flux-dominated implantation is compensated by radiation-enhanced diffusion loss terms. Outside the racetrack, the role of ion implantation is diminished, and Ar retention by the target may stem from a balance between gettering by redeposited Al and ion-induced Ar desorption.

Investigation of plasma spokes in reactive high power impulse magnetron sputtering discharge

Spokes, localised ionisation zones, are commonly observed in magnetron sputtering plasmas, appearing either with a triangular shape or with a diffuse shape, exhibiting self-organisation patterns. In this paper, we investigate the spoke properties (shape and emission) in a high power impulse magnetron sputtering (HiPIMS) discharge when reactive gas (N2 or O2) is added to the Ar gas, for three target materials; Al, Cr, and Ti. Peak discharge current and total pressure were kept constant, and the discharge voltage and mass flow ratios of Ar and the reactive gas were adjusted. The variation of the discharge voltage is used as an indication of a change of the secondary electron yield. The optical emission spectroscopy data demonstrate that by addition of reactive gas, the HiPIMS plasma exhibits a transition from a metal dominated plasma to the plasma dominated by Ar ions and, at high reactive gas partial pressures, to the plasma dominated by reactive gas ions. For all investigated materials, the spoke shape changed to the diffuse spoke shape in the poisoned mode. The change from the metal to the reactive gas dominated plasma and increase in the secondary electron production observed as the decrease of the discharge voltage corroborate our model of the spoke, where the diffuse spoke appears when the plasma is dominated by species capable of generating secondary electrons from the target. Behaviour of the discharge voltage and maximum plasma emission is strongly dependant on the target/reactive gas combination and does not fully match the behaviour observed in DC magnetron sputtering.Spokes, localised ionisation zones, are commonly observed in magnetron sputtering plasmas, appearing either with a triangular shape or with a diffuse shape, exhibiting self-organisation patterns. In this paper, we investigate the spoke properties (shape and emission) in a high power impulse magnetron sputtering (HiPIMS) discharge when reactive gas (N2 or O2) is added to the Ar gas, for three target materials; Al, Cr, and Ti. Peak discharge current and total pressure were kept constant, and the discharge voltage and mass flow ratios of Ar and the reactive gas were adjusted. The variation of the discharge voltage is used as an indication of a change of the secondary electron yield. The optical emission spectroscopy data demonstrate that by addition of reactive gas, the HiPIMS plasma exhibits a transition from a metal dominated plasma to the plasma dominated by Ar ions and, at high reac...

Correlative plasma-surface model for metastable Cr-Al-N: Frenkel pair formation and influence of the stress state on the elastic properties

Correlatively employing density functional theory and experiments congregated around high power pulsed magnetron sputtering, a plasma-surface model for metastable Cr0.8Al0.2N (space group Fm 3 ¯m) is developed. This plasma-surface model relates plasma energetics with film composition, crystal structure, mass density, stress state, and elastic properties. It is predicted that N Frenkel pairs form during Cr0.8Al0.2N growth due to high-energy ion irradiation, yielding a mass density of 5.69 g cm−3 at room temperature and Youngs modulus of 358–130 GPa in the temperature range of 50–700 K for the stress-free state and about 150 GPa larger values for the compressive stress of 4 GPa. Our measurements are consistent with the quantum mechanical predictions within 5% for the mass density and 3% for Youngs modulus. The hypothesis of a stress-induced Youngs modulus change may at least in part explain the spread in the reported elasticity data ranging from 250 to 420 GPa.