Jaroslav Hnilica

Characterization of a periodic instability in filamentary surface wave discharge at atmospheric pressure in argon

This study reports on a periodic instability in a microwave atmospheric pressure surface wave plasma. Time-resolved observation by means of a high-speed camera reveals that the discharge can operate in two different regimes. The discharge operates either in a static mode of a controlled number of filaments propagating alongside the wall of the fused silica tube or in a dynamic mode of one or more straight filaments together with one revolving filament. Microwave power and argon flow rate are interpreted as the scaling parameters that govern the self-organization in the discharge. The operating diagram is formed by alternating stripes of static and dynamic mode with well-defined borders.

PECVD of nanostructured SiO2 in a modulated microwave plasma jet at atmospheric pressure

Atmospheric pressure plasma enhanced chemical vapour deposition (AP-PECVD) of thin films by means of a microwave plasma jet operating with mixtures of argon and tetrakis(trimethylsilyloxy)silane (TTMS) is reported for the first time. In contrast to other siloxy-alkanes that are commonly used for PECVD, the molecule of TTMS (C12H36O4Si5) exhibits a complex and symmetric molecular structure which is presumably essential for a large scale nanostructuring of the films. Deposited films have been characterized by means of electron microscopy (SEM), x-ray spectroscopy (EDX), and infra-red spectroscopy (FTIR). The applied methods demonstrate the prevalent inorganic SiO2-like character of the films and their highly fractalized nanostructure over a wide range of dimension 100?104?nm. Contact angle measurements show the superhydrophobicity of the films, while the dispersive component of the surface energy can be varied in a controlled way by low-frequency amplitude modulation of the excitation power of the MW discharge. The modulation regimes of the jet have been investigated by means of time-resolved optical emission spectroscopy in order to describe the oscillations of plasma parameters e.g. rotational temperature from OH and relative emission of silicon atoms to substantiate the reproducibility of the deposition conditions and to correlate the plasma properties with the resulting film properties.

Surface Treatment by Atmospheric-Pressure Surfatron Jet

Using an atmospheric-pressure surface-wave-driven jet, the surfatron, we carried out the surface treatment of the silicon, polyamide, and wood. The changes in surface energy of the samples were determined by the contact angle measurements. Some of the results show rather quick treatment time, probably due to a high power density in the surfatron device.

Time-resolved study of amplitude modulation effects in surface-wave atmospheric pressure argon plasma jet

The effects of amplitude modulation (AM) on an atmospheric pressure microwave argon jet is investigated using time-resolved optical emission spectroscopy, passive acoustic diagnostic and digital camera imaging. These techniques show significant changes of the effluent plasma properties with varying AM frequency. Operation in AM mode can enhance the plasma jet length or width over continuous-wave mode with the same mean power, which could be advantageous in many practical applications of plasma jets.

Determination of titanium atom and ion densities in sputter deposition plasmas by optical emission spectroscopy

The thorough characterizations of deposition plasma lead to important achievements in the fundamental understanding of the deposition process, with a clear impact on the development of technology. Measurement of the spatial and, in the case of pulse excited plasma, also temporal evolution, of the concentrations of sputtered atoms and ions is a primary task in the diagnostics of any sputter deposition plasma. However, it is difficult to estimate absolute number densities of the sputtered species (atoms and ions) in ground states directly from optical emission spectroscopy, because the species in the ground levels do not produce any optical signal. A method using effective branching fractions enables us to determine the density of non-radiating species from the intensities of self-absorbed spectral lines. The branching fractions method described in the first part of this paper was applied to determine the ground state densities of the sputtered titanium atoms and ions. The method is based on fitting the theoretically calculated branching fractions to experimentally measured ratios of the relative intensities of carefully selected resonant titanium atomic and ionic lines. The sputtered species density is determined in our experimental setup with a relative uncertainty of less than 5% for the dc driven magnetron and typically 15% for time-resolved measurements of high- power impulse magnetron sputtering (HiPIMS) discharge. In the second part of the paper, the method was applied to determine the evolution of titanium atom and ion densities in three typical cases ranging from the dc driven sputter process to HiPIMS.

Visualization of Revolving Modes in RF and MW Nonthermal Atmospheric Pressure Plasma Jets

The high-speed sequence imaging of two different atmospheric pressure plasma jets illuminates the low-frequency dynamics of discharge filaments. Usually, atmospheric plasma jets consist of several contracted filaments occurring in the active zone of the plasma randomly in space and stochastically in time. However, particular plasma excitation schemes support collective regimes where the filaments are self-organized at large spatial and temporal scales, even at atmospheric pressure. Moreover, self-organized dynamics can act in a similar manner at different types of plasma. In this paper, evident similarities in filament patterning are pointed out for radio-frequency and microwave atmospheric pressure jets by publishing images of short exposure time (6 ms) from which the filament movement can be derived.

Simultaneous electrical and optical study of spoke rotation,merging and splitting in HiPIMS plasma

To gain more information on the temporal and spatial behaviour of self-organized spoke structures in HiPIMS plasmas, a correlation between the broadband optical image of an individual spoke (taken over 200 ns) and the current it delivers to the target has been made for a range of magnetron operating conditions. As a spoke passes over a set of embedded probes in the niobium cathode target, a distinct modulation in the local current density is observed, (typically up to twice the average value), matching very well the radially integrated optical emission intensities (obtained remotely with an ICCD camera). The dual diagnostic system allows the merging and splitting of a set of spokes to be studied as they rotate. It is observed that in the merger of two spokes, the trailing spoke maintains its velocity while the leading spoke either decreases its velocity or increases its azimuthal length. In the spoke splitting process, the total charge collected by an embedded probe is conserved. A simple phenomenological model is developed that relates the spoke mode number m to the spoke dimensions, spoke velocity and gas atom velocity. The results are discussed in the context of the observations of spoke dynamics made by Hecimovic et al (2015 Plasma Sources Sci. Technol. 24 045005).

Gas mixing enhanced by power modulations in atmospheric pressure microwave plasma jet

Microwave plasma jet operating in atmospheric pressure argon was power modulated by audio frequency sine envelope in the 10^2 W power range. Its effluent was imaged using interference filters and ICCD camera for several different phases of the modulating signal. The combination of this fast imaging with spatially resolved optical emission spectroscopy provides useful insights into the plasmachemical processes involved. Phase-resolved schlieren photography was performed to visualize the gas dynamics. The results show that for higher modulation frequencies the plasma chemistry is strongly influenced by formation of transient flow perturbation resembling a vortex during each period. The perturbation formation and speed are strongly influenced by the frequency and power variations while they depend only weakly on the working gas flow rate. From application point of view, the perturbation presence significantly broadened lateral distribution of active species, effectively increasing cross-sectional area suitable for applications.

Spatially resolved spectroscopy of an atmospheric pressure microwave plasma jet used for surface treatment

Abstract In this study, the variations of properties of a microwave plasma jet (surfatron) along the discharge axis have been investigated using optical emission spectroscopy. As the argon jet is not enclosed, the spatial distribution of individual species in effluent plasma is the result of rather complicated interplay between energy loss and gradual mixing with the air. Spatial 2D relative intensity profiles of atomic lines and molecular bands at 310 nm, 336 nm, 391 nm and 656 nm are presented in the form of colour maps revealing different positions of maximum emission intensity for 310 nm and 336 nm (in the effluent plasma) and for 391 nm and 656 nm (inside the discharge tube). The plasma jet was used for surface treatment of heat resistant samples (stainless steel, aluminium, silicon wafer) and the effectiveness of the plasma treatment was evaluated by measuring the sessile drop contact angle, with water and glycerol as testing liquids. The optimal position for plasma treatment (close to the tube nozzle) combined with longer treatment time (10 s) lead to hydrophilic properties of samples with contact angles as low as 10°. Graphical Abstract

Electron density in amplitude modulated microwave atmospheric plasma jet as determined from microwave interferometry and emission spectroscopy

Time resolved electron density in an atmospheric pressure amplitude modulated microwave plasma jet is determined using the microwave interferometry method, refined by numerical modelling of the propagation of non-planar electromagnetic waves in the vicinity of a small diameter, dense collisional plasma filament. The results are compared to those from the Stark broadening of the H-beta emission line. Both techniques show, both qualitatively and quantitatively, a similar temporal evolution of electron density during one modulation period.