Z. Lj. Petrović

Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons

We review the data and models describing the production of the electrons, termed secondary electrons, that initiate the secondary and subsequent feedback avalanches required for the growth of current during breakdown and for the maintenance of low-current, cold-cathode discharges in argon. First we correlate measurements of the production of secondary electrons at metallic cathodes, i.e. the yields of electrons induced by Ar+ ions, fast Ar atoms, metastable atoms and vuv photons. The yields of electrons per ion, fast atom and photon vary greatly with particle energy and surface condition. Then models of electron, ion, fast atom, excited atom and photon transport and kinetics are fitted to electrical-breakdown and low-current, discharge-maintenance data to determine the contributions of various cathode-directed species to the secondary electron production. Our model explains measured breakdown and low-current discharge voltages for Ar over a very wide range of electric field to gas density ratios E/n, i.e. 15 Td to 100 kTd. We review corrections for nonequilibrium electron motion near the cathode that apply to our local-field model of these discharges. Analytic expressions for the cross sections and reaction coefficients used by this and related models are summarized.

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.

Functional separation of biasing and sustaining voltages in two-frequency capacitively coupled plasma

Separation of the effects of rf sources used for biasing the wafer and for sustaining the plasma is studied by measuring the space profiles of net excitation rate of Ar(3p5) for a two-frequency capacitively coupled plasma as a representation of a typical oxide etcher. Measurements were performed in Ar and in CF4/Ar mixtures. For biasing supply operating at low frequency, 700 kHz, it was shown that the effect of the voltage becomes significantly smaller as the sustaining voltage is changed from high frequency, 13.56 MHz, to very high frequency (VHF), 100 MHz, and it even disappears for pulsed operation in mixtures. This is the result of the low dc self-bias at the VHF electrode that allows the high energy secondary electrons to leave the plasma without excessive contribution to ionization and dissociation.

Surface recombination of atoms in a nitrogen afterglow

The surface recombination of nitrogen atoms in afterglow is studied by the time delay method, accompanied by the macrokinetic diffusive model. The method consists of the measurement of the dependence of the mean value of the breakdown time delay on afterglow period td=f(τ) and fitting of the data by the model that was developed. Excited N2(A 3∑+u) nitrogen molecules formed in the surface‐catalyzed recombination on cathode produce secondary electrons. The electrons entering the interelectrode space determine the time delay in electrical breakdown. The time delay method is very efficient in nitrogen atom detection down to a natural radioactivity level. By fitting the calculated curve to the experimental data, we have: (1) shown that the nitrogen atom recombination on the glass container walls is second‐order in N while the recombination on the copper electrode is the first order; (2) determined the value of the surface recombination coefficient for molybdenum glass; (3) determined the combined probability o...

Kinetic phenomena in charged particle transport in gases, swarm parameters and cross section data*

In this review we discuss the current status of the physics of charged particle swarms, mainly electrons. The whole field is analysed mainly through its relationship to plasma modelling and illustrated by some recent examples developed mainly by our group. The measurements of the swarm coefficients and the availability of the data are briefly discussed. More time is devoted to the development of complete electron?molecule cross section sets along with recent examples such as NO, CF4 and HBr. We extend the discussion to the availability of ion and fast neutral data and how swarm experiments may serve to provide new data. As a point where new insight into the kinetics of charge particle transport is provided, the role of kinetic phenomena is discussed and recent examples are listed. We focus here on giving two examples on how non-conservative processes make dramatic effects in transport, the negative absolute mobility and the negative differential conductivity for positrons in argon. Finally we discuss the applicability of swarm data in plasma modelling and the relationship to other fields where swarm experiments and analysis make significant contributions.

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.

Monte Carlo studies of non-conservative electron transport in the steady-state Townsend experiment

An investigation of the spatial relaxation of the electrons and benchmark calculations of spatially resolved non-conservative electron transport in model gases has been carried out using a Monte Carlo simulation technique. The Monte Carlo code has been specifically developed to study the spatial relaxation of electrons in an idealized steady-state Townsend (SST) experiment in the presence of non-conservative collisions. Calculations have been performed for electron transport properties with the aim of providing the benchmark required to verify the codes used in plasma modelling. Both the spatially uniform values and the relaxation profiles of the electron transport properties may serve as an accurate test for such codes. The explicit effects of ionization and attachment on the spatial relaxation profiles are considered using physical arguments. We identify the relations for the conversion of hydrodynamic transport properties to those found in the SST experiment. Our Monte Carlo simulation code and sampling techniques appropriate to these experiments have provided us with a way to test these conversion formulae and their convergence.

Development of optical computerized tomography in capacitively coupled plasmas and inductively coupled plasmas for plasma etching

In this paper we review the results of optical emission spectroscopy (OES) in diagnostics of radio frequency (rf) discharges. The status of OES diagnostics of rf plasmas used primarily for plasma etching has been reviewed for the past 15 years. The results obtained at Keio University have been reviewed in greater detail. Time resolved and time-averaged computerized tomography (CT) of OES has been applied to obtain 2D and 3D profiles of emission for a large number of gases (Ar, H 2 , SiH 4 , Cl 2 , CF 4 , O 2 , SF 6 ) and gas mixtures. Application of OES-CT has become essential in understanding the sustaining mechanisms in capacitively coupled plasmas (CCPs) and in inductively coupled plasmas (ICPs) operating in rf. Those plasmas are produced in a wide range of frequencies, or combinations of frequencies and pulsed conditions. In addition, a wide range of geometries has been covered, providing an insight into spatial profiles of excitation (i.e. production of radicals) and therefore the uniformity of plasma etching. In our laboratory the technique was first applied to magnetrons. In CCP spatially resolved emission profiles were obtained for complex geometries and furthermore the effects of high frequency, two frequencies and pulsed operation were studied. Control of dust particles could be implemented by the same technique. Absolute values as well as the time resolved data showed the importance of the double layers in front of the instantaneous anode in maintaining the rf discharges in electro-negative gases. In ICP, CT was used to study the azimuthal anisotropy and the axial dependence of emission. Transition from capacitively to inductively coupled mode of operation could be proved directly by OES-CT profiles. Time resolved data for emission close to the coil showed two peaks with 45° phase difference, one of which was unexpected. In addition the effect of electro-negative gas and pulsing of the rf field were studied. It has been concluded that OES supplemented by fast, detailed models may be used for efficient real time control of plasma etching devices of the next generation.

Time resolved optical emission images of an atmospheric pressure plasma jet with transparent electrodes

We study development of plasma packages in atmospheric pressure plasma jet from their formation as a discharge close to the instantaneous cathode, following their motion between and inside the electrodes up to their emergence at the edge of the glass tube and formation of a plasma bullet. Inside both electrodes, plasma is concentrated close to the walls and is bright, while outside it is located at the axis. This paper opens issues of the geometry of electrodes, fields, and atomic processes, allowing some predictions to be made about pertinent mechanisms.

Dissociative electron attachment to some chlorine‐containing molecules

The electron‐attachment rate constants of CH3Cl, C2H5Cl, and C2H3Cl in N2 and Ar were measured as a function of reduced electric field (E/N). These data and the previous data of SOCl2 and CCl2F2 were converted to the electron‐attachment cross sections as a function of electron energy. The present results are compared with existing fragmentary data. The dissociative electron‐attachment processes of the studied molecules are discussed.