D Marić

Plasma–liquid interactions: a review and roadmap

Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.

Breakdown, scaling and volt-ampere characteristics of low current micro-discharges

We give preliminary results on the breakdown and low current limit of volt‐ampere characteristics of simple parallel plate non-equilibrium dc discharges at standard (centimetre size) and micro-discharge conditions. Experiments with micro-discharges are reported attempting to establish the maintenance of E/N, pd and j/p 2 scalings at small dimensions down to 20 µm. It was found that it may not be possible to obtain properly the left-hand side of the Paschen curve. The possible causes are numerous but we believe that it is possible that long path prevention techniques do not work at high pressures. Nevertheless, the standard scaling laws seem to be maintained down to these dimensions which are consistent with simulations that predict violation of scaling below 10 µm. Volt‐ampere characteristics are also presented and compared with those of the standard size discharges.

Measurements and modelling of axial emission profiles in abnormal glow discharges in argon: heavy-particle processes

We report studies on argon glow discharges established between flat disc electrodes, at pressure × electrode separation (pd) values between 45 and 150 Pa cm, with special attention to heavy-particle processes including heavy-particle excitation induced light emission. The discharges are investigated experimentally and also through self-consistent hybrid modelling. The comparison of the experimental and computed light intensity distributions verifies the correctness of the model, which gives a detailed insight into the discharge operation. The efficiency of heavy-particle excitation shows a universal dependence on the reduced electric field. At the higher pd values the scaling of electrical characteristics and light emission intensity with electrode separation is verified, however, additional processes (radial losses of charged particles and reduction of the active cathode area) result in the violation of scaling at the lowest pd value when the discharge tube diameter is kept constant.

Measurements and analysis of excitation coefficients and secondary electron yields in Townsend dark discharges

In this paper, we review our study of excitation coefficients in rare gases and in methane, some of the excitation cross sections that were obtained, the spatial profiles of emission (with absolute calibration) and secondary electron yields. The data for excitation coefficients have been analysed to produce the cross section data in some cases. The spatial profiles of emission at the low currents were used to establish the importance of the non-hydrodynamic relaxation and the contributions of heavy particles and reflected electrons. These data were also used to get more reliable secondary electron yields for rare gases. The spatial emission profiles at higher currents have been applied to obtain field profiles and make comparisons with hybrid models. In particular, we present in this paper, the emission coefficients in krypton and we discuss the wide range of interconnected applications of excitation coefficients and spatial emission profiles.

On the possibility of long path breakdown affecting the Paschen curves for microdischarges

We discuss the effect of electrode shape on Paschen curves and our ability to seal off microdischarges to prevent long path breakdown. It was found that for structured electrodes at high pressures and small gaps, the left-hand side of the Paschen curve is relatively flat, extending the minimum to lower pd values. At high pd values the curves are almost identical to those at standard pressures/gaps and the discharge runs between the top plane of the cathode and the anode. For intermediate pd values the higher electric field at the edge attracts most of the current and the discharge extends along the side wall maintaining the same low breakdown voltage. When the length of the discharge reaches the longest path the voltage starts a rapid increase. We have selected the dimension of the segmented electrode so as to have the same losses to the walls that block or allow the long path breakdown, thus being able to represent situations when the Paschen curve may be properly determined. In general, however, this shows that recording of the left-hand side for open structures (without enclosure by a dielectric) is impossible and conclusions about secondary emission should be focused on the well-defined conditions.

Oscillation modes of direct current microdischarges with parallel-plate geometry

Two different oscillation modes in microdischarge with parallel-plate geometry have been observed: relaxation oscillations with frequency range between 1.23 and 2.1 kHz and free-running oscillations with 7 kHz frequency. The oscillation modes are induced by increasing power supply voltage or discharge current. For a given power supply voltage, there is a spontaneous transition from one to other oscillation mode and vice versa. Before the transition from relaxation to free-running oscillations, the spontaneous increase of oscillation frequency of relaxation oscillations form 1.3 kHz to 2.1 kHz is measured. Fourier transform spectra of relaxation oscillations reveal chaotic behavior of microdischarges. Volt-ampere (V-A) characteristics associated with relaxation oscillations describes periodical transition between low current, diffuse discharge, and normal glow. However, free-running oscillations appear in subnormal glow only.

Axial light emission and Ar metastable densities in a parallel plate dc microdischarge in the steady state and transient regimes

Axial emission profiles in a parallel plate dc microdischarge (feedgas: argon; discharge gap d = 1 mm; pressure p = 10 Torr) were studied by means of time-resolved imaging with a fast ICCD camera. Additionally, volt–ampere (V–A) characteristics were recorded and Ar* metastable densities were measured by tunable diode laser absorption spectroscopy (TDLAS). Axial emission profiles in the steady-state regime are similar to corresponding profiles in standard size discharges (d ≈ 1 cm, p ≈ 1 Torr). For some discharge conditions relaxation oscillations are present when the microdischarge switches periodically between the low current Townsend-like mode and the normal glow. At the same time the axial emission profile shows transient behavior, starting with peak distribution at the anode, which gradually moves toward the cathode during the normal glow. The development of argon metastable densities highly correlates with the oscillating discharge current. Gas temperatures in the low current Townsend-like mode (Tg = 320–400 K) and the high current glow mode (Tg = 469–526 K) were determined by the broadening of the recorded spectral profiles as a function of the discharge current.

On Explanation of the Double-Valued Paschen-Like Curve for RF Breakdown in Argon

This paper represents an investigation of the dependence of the breakdown voltage on the gas pressure in radio-frequency argon discharges under conditions when ion-induced secondary electron production is negligible. Calculations were performed by using a Monte Carlo collision code including electrons only. Our simulation results clearly show a region, occurring at low pressure, where multiple values of the breakdown voltage exist at a given pressure, in agreement with previous experimental observations. The two different regimes of operation, each satisfying the breakdown condition, may be best analyzed in contour plots of electron density, ionization rate, and mean energy.

On Application of Plasmas in Nanotechnologies

In this chapter we give a review of the application of non-equilibrium plasmas in the field of nanotechnologies, and nanotechnology-related science. The field of applications of plasma in general in nanotechnologies is extensive, and the field of non-equilibrium plasmas in the same context is almost as large with many possibilities covering both top-down and bottom-up fabrication approaches. Thus a single review cannot give it justice. We opted here, as may be expected, to give a review of possibilities in general, ranging from the growth and functionalization of aligned nanotubes, carbon walls and sheaths of graphene, through to the growth of gas-phase nanocrystals and dust particles; surface deposition of different structures, functionalization of the surface and conditions for developing means to prepare hyperhydrophobic surfaces; treatment of textiles, organic materials, living cells, to applications in nanoelectronics for the manufacture of future generations of integrated circuits to meet the Moore’s law driven semiconductor roadmap. In a text like this it is not possible to cover all issues and even less to cover all sources. We thus focus on several topics and mostly on the plasma physics problems related to improving the plasma technologies. They include charging as a source of errors in integrated circuit fabrication to the generation of non-equilibrium plasmas at atmospheric pressure for even more convenient application and for applications in biomedicine. The message we hope to convey to all readers is an understanding as to the explicit advantages that non-equilibrium plasmas have in a large number of plasma-assisted nanotechnologies and other modern technologies—an advantage that other emerging technologies will struggle to match at the levels of integration, cost and quality required of future fabrication.

Breakdown and dc discharge in low-pressure water vapour

In this paper we report studies of basic properties of breakdown, low-current Townsend discharge and high-current discharge regimes in water vapour. Paschen curves and the corresponding distributions of emission intensities at low current were recorded in the range of pd (pressure x electrode gap) from 0.1 to 10 Torrcm covering the region of Paschen minimum. From the experimental profiles we obtained effective ionization coefficient of water vapour for the E/N range 650 Td–7 kTd and fitted the results by using the extended Townsend analytical formula. Using the obtained ionization coefficient, we calculated the effective yield of secondary electrons from the copper cathode. Results of the measurements of Volt-Ampere characteristics in water vapour were presented together with the images of the axial structure of the discharge in a wide range of discharge currents for two pd values. Recorded profiles showed development of the spatial structure of the discharge in different operating regimes. We were able to identify conditions where processes induced by heavy particles, probably fast hydrogen atoms, are dominant in inducing emission from the discharge. Finally, standard scaling laws were tested for low current and glow discharges in water vapour.