Claudia Lazzaroni

Comparison of a hybrid model to a global model of atmospheric pressure radio-frequency capacitive discharges

A one-dimensional hybrid analytical–numerical global model of atmospheric pressure radio-frequency (rf) driven capacitive discharges, previously developed, is compared with a basic global model. A helium feed gas with small admixtures of oxygen is studied. For the hybrid model, the electrical characteristics are calculated analytically as a current-driven homogeneous discharge. The electron power balance is solved analytically to determine a time-varying Maxwellian electron temperature, which oscillates on the rf timescale. Averaging over the rf period yields effective rate coefficients for gas phase activated processes. For the basic global model, the electron temperature is constant in time and the sheath physics is neglected. For both models, the particle balance relations for all species are integrated numerically to determine the equilibrium discharge parameters. Variations of discharge parameters with composition and rf power are determined and compared. The rate coefficients for electron-activated processes are strongly temperature dependent, leading to significantly larger neutral and charged particle densities for the hybrid model. For small devices, finite sheath widths limit the operating regimes to low O2 fractions. This is captured by the hybrid model but cannot be predicted from the basic global model.

Model of a low-pressure radio-frequency inductive discharge in Ar/O2 used for plasma spray deposition

A global (volume-averaged) model of a low-pressure radio-frequency (RF) inductive discharge used for nanostructured zinc oxide thin film deposition, the so-called spray-plasma device, is proposed. The plasma reactor is fed with an admixture of argon and oxygen and the pressure is typically several tens of mTorr. In the first step of the modelling, the injector and the substrate holder are not taken into account, and therefore zinc-containing species are not considered. The global model is based on the numerical integration of the particle balance equations and the electronic power balance equation. The model is first run until the steady state is reached to determine the equilibrium discharge parameters that are the species densities and the electron temperature. A parametric study is carried out varying the gas pressure, the RF power and the O2 fraction in the reactor. A parameter of great importance for the deposition process is the flux of the reactive species on the substrate holder and the model allows a fast exploration of this parameter. For continuous plasmas, the ratio of the reactive species flux to the total positive ion flux can be controlled varying the three basic parameters cited before (pressure, power and dilution). In the last part of the paper, we also investigate pulsed plasmas and the effect of the duty cycle variations on the neutral/ion flux ratio.

Electrical characteristics of micro-hollow cathode discharges

A cathode sheath model of micro-hollow cathode discharges is proposed to calculate the voltage–current characteristics and discuss the physics of the discharge resistance. Three different approaches are compared: (i) a self-consistent model where the electric field is determined self-consistently with the electron flux, (ii) a matrix sheath model where the electric field profile in the sheath is imposed, (iii) a constant electric field model where the electric field in the sheath is assumed to be equal to a constant fraction of the electric field at the cathode. The sheath size is found to decrease with the pressure, the voltage and the secondary emission coefficient. There is a strong effect of the secondary emission coefficient and the pressure on the voltage–current characteristics. The discharge resistance is found to be several hundreds of kΩ and decreases with the discharge current, the pressure and the secondary emission coefficient. A comparison between the matrix sheath model and experiments suggests that both the secondary emission coefficient and the surface area on which the current is collected at the cathode increase with the discharge current. This increase is related to a transition between a discharge confined in the hole at low current and a plasma abruptly expanded on the cathode backside at higher current.

A comparison between micro hollow cathode discharges and atmospheric pressure plasma jets in Ar/O2 gas mixtures

Using global models, micro hollow cathode discharges (MHCDs) are compared to radiofrequency atmospheric pressure plasma jets (APPJs) in terms of reactive oxygen species (ROS) production. Ar/O2 gas mixtures are investigated, typically with a small percentage of oxygen in argon. The same chemical reaction set, involving 17 species and 128 chemical reactions in the gas phase, is used for both devices, operated in the typical geometries previously published; the APPJ is driven by a radiofrequency voltage across a 1 mm gap, at atmospheric pressure, while the MHCD is driven by a DC voltage source, at 100 Torr and in a 400 μm hole. The MHCD may be operated either in the self-pulsing or in the normal (stationary) regime, depending on the driving voltage. The comparison shows that in both regimes, the MHCD produces larger amounts of , while the APPJ produces predominantly reactive oxygen ground state species, and . These large differences in ROS composition are mostly due to the higher plasma density produced in the MHCD. The difference in operating pressure is a second order effect.