M M Turner

Independent control of ion current and ion impact energy onto electrodes in dual frequency plasma devices

Dual frequency capacitive discharges are designed to offer independent control of the flux and energy of ions impacting on an object immersed in a plasma. This is desirable in applications such as the processing of silicon wafers for microelectronics manufacturing. In such discharges, a low frequency component couples predominantly to the ions, while a high frequency component couples predominantly to electrons. Thus, the low frequency component controls the ion energy, while the high frequency component controls the plasma density. Clearly, this desired behaviour is not achieved for arbitrary configurations of the discharge, and in general one expects some unwanted coupling of ion flux and energy. In this paper we use computer simulations with the particle-in-cell method to show that the most important governing parameter is the ratio of the driving frequencies. If the ratio of the high and low frequencies is great enough, essentially independent control of the ion energy and flux is possible by manipulation of the high and low frequency power sources. Other operating parameters, such as pressure, discharge geometry, and absolute power, are of much less significance.

Electrostatic modelling of dual frequency rf plasma discharges

Particle-in-cell simulations have been used to study the nature of dual frequency plasma discharges. It is observed that both the ion flux on to the electrodes and the ion bombardment energy on to the electrodes can be controlled independently. There are two separate regimes in which this occurs. At large electrode separation, the ion current is controlled by varying the total discharge current, Jlf + Jhf. At small electrode separations, the ion flux can be controlled by varying the high frequency power source. In both regimes, the energy of the ions bombarding the electrodes is then determined by the low frequency voltage. A consequence of using dual frequencies to power the device is that the sheath width increases linearly as the low frequency power source is increased. This results in the dimensions of the bulk plasma decreasing, causing the electron temperature to increase for devices with electrode separations that are of comparable size to the electrode separation. In order to better understand the underlying physics involved within these devices an analytical global model has been developed which can explain many of the characteristics observed in the simulations.

Analytical model of a dual frequency capacitive sheath

In recent years, there has been an increased interest in capacitively coupled plasma discharges which are operated with two frequencies. An analytical sheath model for a capacitively coupled radio-frequency plasma discharge operated with two frequencies is proposed and studied under the assumptions of a time-independent, collisionless ion motion. Expressions are obtained for the time-average electric potential within the sheath, nonlinear motion of the electron sheath boundary and nonlinear instantaneous sheath voltage. The derived model is valid under the condition that the low frequency (lf) electric field Elf in the sheath is much higher than the high frequency (hf) electric field Ehf. This condition is fulfilled within typical dual frequency conditions. It is shown, however, that the hf electric field modifies the sheath structure significantly because of the electron response to Ehf. This model has been compared to particle-in-cell plasma simulations, finding good quantitative agreement. We present the dependence of the maximum sheath width and the dc sheath voltage drop on the hf/lf current ratio and on the hf/lf frequency ratio.

Hysteresis in the E- to H-mode transition in a planar coil, inductively coupled rf argon discharge

Inductively coupled rf discharges typically exhibit two modes of operation, namely, a low-density mode known as the E mode and a higher density mode known as the H mode. The transition between these modes exhibits hysteresis. Experimental observations of these transitions are presented. By means of a mixture of electromagnetic theory and circuit analysis and by invoking the requirement that the power absorbed and lost by the electrons should balance, the possible working points of an inductively coupled rf plasma source are identified in (P, ) space, where P is the power absorbed by the electrons, is the peak rf current in the induction coil and is the electron number density. Once the loci of the operating points have been identified in this manner, it is possible to construct a consistent explanation for all the experimental observations reported in the first part of the paper. In particular, it is possible to present an explanation for the hysteresis-like behaviour manifested by the mode transitions. Basically, the transitions occur when it is no longer possible to balance the power absorbed and lost by electrons.

Space and phase resolved plasma parameters in an industrial dual-frequency capacitively coupled radio-frequency discharge

The dynamics of high energetic electrons (11.7 eV) in a modified industrial confined dual-frequency capacitively coupled RF discharge (Exelan, Lam Research Inc.), operated at 1.937 MHz and 27.118 MHz, is investigated by means of phase resolved optical emission spectroscopy. Operating in a He–O2 plasma with small rare gas admixtures the emission is measured, with one-dimensional spatial resolution along the discharge axis. Both the low and high frequency RF cycle are resolved. The diagnostic is based on time dependent measurements of the population densities of specifically chosen excited rare gas states. A time dependent model, based on rate equations, describes the dynamics of the population densities of these levels. Based on this model and the comparison of the excitation of various rare gas states, with different excitation thresholds, time and space resolved electron temperature, propagation velocity and qualitative electron density as well as electron energy distribution functions are determined. This information leads to a better understanding of the dual-frequency sheath dynamics and shows, that separate control of ion energy and electron density is limited. (Some figures in this article are in colour only in the electronic version)

Global models of electronegative discharges: critical evaluation and practical recommendations

A first step towards understanding a complex plasma is usually to develop a zero-dimensional or global model. This is difficult when the plasma is electronegative, because the literature contains many proposed models with different and sometimes contradictory detailed assumptions, and different domains of applicability. The appropriateness of such models in a given context is often hard to assess. In this paper, we present a set of detailed kinetic simulations spanning a wide of range of parameters, especially with respect to electronegativity, collisionality and dominant negative ion destruction mechanism. We use these simulations as a benchmark to investigate the validity of the fundamental global model assumptions when used to model electronegative discharges. We reach two important conclusions: (1) that an accurate electron kinetics model is more important than detailed considerations relating to plasma dynamics in the presence of negative ions and (2) that there exists a simple and robust transport model that is in reasonable agreement with all of our benchmark simulations, when the electrons are treated properly.

Modelling of the dual frequency capacitive sheath in the intermediate pressure range

The nonlinearity of the plasma sheath in dual frequency capacitively coupled reactors is investigated for frequencies well above the ion plasma frequency. This work focuses on the behaviour of the voltage and the sheath width with respect to the driving current source and the collisionality regime. For typical plasma processing applications, the gas pressure ranges from a few milliTorrs to hundreds of milliTorrs, and the ion dynamics span different collisional regimes. To describe these different ion dynamics, we have used a collisionless model and a variable mobility model. The sheath widths and the voltages obtained from these two models have then been compared.

Concepts and characteristics of the 'COST Reference Microplasma Jet'

Biomedical applications of non-equilibrium atmospheric pressure plasmas have attracted intense interest in the past few years. Many plasma sources of diverse design have been proposed for these applications, but the relationship between source characteristics and application performance is not well-understood, and indeed many sources are poorly characterized. This circumstance is an impediment to progress in application development. A reference source with well-understood and highly reproducible characteristics may be an important tool in this context. Researchers around the world should be able to compare the characteristics of their own sources and also their results with this device. In this paper, we describe such a reference source, developed from the simple and robust micro-scaled atmospheric pressure plasma jet (μ-APPJ) concept. This development occurred under the auspices of COST Action MP1101 Biomedical Applications of Atmospheric Pressure Plasmas. Gas contamination and power measurement are shown to be major causes of irreproducible results in earlier source designs. These problems are resolved in the reference source by refinement of the mechanical and electrical design and by specifying an operating protocol. These measures are shown to be absolutely necessary for reproducible operation. They include the integration of current and voltage probes into the jet. The usual combination of matching unit and power supply is replaced by an integrated LC power coupling circuit and a 5 W single frequency generator. The design specification and operating protocol for the reference source are being made freely available.

Simulation of kinetic effects in inductive discharges

We show that, under certain conditions of practical interest, radio frequency inductive discharges can exhibit kinetic effects arising from the random thermal motion of electrons. The electrical response of the plasma is altered in this kinetic regime. More power is dissipated than is expected from cold plasma theory and the spatial distributions of the induced currents and fields change. Moment- or fluid-based simulations do not normally include these kinetic effects, but we show that the introduction of an appropriate viscosity into the moment equations can make them accurate in the kinetic regime.

Collisionless electron heating by capacitive radio-frequency plasma sheaths

Low-pressure capacitive rf plasmas can be maintained chiefly by collisionless heating in the rf modulated sheaths adjacent to the electrodes. Theoretical models dealing with this mechanism are often based on a `hard wall approximation where the electrons are considered to collide elastically with the oscillating sheath edge. The power transfer is then calculated by averaging forward and reverse power fluxes over an rf period. There are, however, several drawbacks to this approach: the models are sensitive to assumptions regarding the incident electron distribution, transit time effects in the sheath electric field are neglected, electron loss is not considered and current conservation is not satisfied. In order to examine the validity of the theoretical models, we use a Monte Carlo approach to study electron interactions with the model and self-consistent fields providing modifications that can lead to a more consistent treatment of the electron dynamics inside the sheath. Of particular importance is the presence of a small field behind the moving electron sheath edge which maintains quasi-neutrality between the electron sheath position and the bulk plasma. In addition, a semi-infinite particle-in-cell (PIC) simulation is used to investigate in detail sheath dynamics. The errors that the `hard wall approximation gives are calculated and power deposition scalings with current drive, frequency and electron temperature are provided. Our results indicate that collisionless heating cannot be attributed to the stochastic heating mechanism based on the `hard wall approximation and that in contrast electron inertia plays a dominant role as far as collisionless heating is concerned.