Sheath reversal during transient radio-frequency bias

Optical imaging is performed with temporal and spatial resolution in a capacitively coupled plasma. The region imaged is in front of an RF biased planar probe embedded in the center of the ground electrode of a standard Gaseous Electronics Conference (GEC) reference cell. Two main periods of interest stand out. The local sheath induced by the biasing and the main plasma bulk are affected. The first interesting period is at the onset of the RF burst on the planar probe. The voltage applied to the surface can locally reverse the sheath in front of this surface. A second interesting period is after the build up of self bias and before the extinction of the RF burst. During the steady self-bias phase, the local perturbation of optical emission amounts to less than 10%, whereas in the sheath reversal phase it reaches 70

Plasma processing of materials at the atomic scale

Plasma etching is finding new applications beyond the microelectronics industry. There are new challenges in the devising and controlling of plasma-surface interactions. Atomic monolayers offer opportunities for passivation and etching. Maintaining appropriate plasma conditions to exploit such opportunities requires intelligent strategies for the control of processing plasmas

Transient RF self-bias in electropositive and electronegative plasmas

The transient self-biasing of surfaces has been modelled to extend the utility of an isolated probe technique. The biasing is effected by the arrival of electrons drawn from the adjacent plasma but proceeds at a rate determined by the positive ion flux. Electron temperature and ion flux can be extracted from the initial stages of transient biasing. The model has been used to interpret data from a helicon plasma in argon.Shorter transients occur within the period of applied radio frequency (RF). Sheath reversal occurs during the initial stages of RF bias when the RF amplitude exceeds the normal DC floating potential. Very large RF bias signals, even after the primary transient phase, can reverse the sign of potential across the space charge sheath briefly during the cycle. The onset of this stage is mass dependent and may arise in hydrogen when the RF amplitude is only 47 times the electron temperature.The development of self-bias is also modelled for an electronegative plasma. Here, sheath reversal sets in at lower RF amplitude and the self-bias takes longer to establish than in equivalent electropositive plasmas. The model has been applied to data from a helicon plasma in sulphur hexafluoride, leading to a quantification of its electronegativity

The electron distribution function at an RF planar probe due to an incident electron beam

Experiments with a planar probe embedded in the grounded electrode of the GEC cell run in the capacitive mode suggest the presence of extra-energetic electrons. In particular, under a wide range of filling pressure and power the I–V characteristic rises linearly towards the ion saturation current. To model this analytically, an electron beam is taken to be incident at the plasma–sheath boundary of the planar probe. The beam electrons are retarded, as they approach the probe surface, by a combination of a steady component of potential and that fraction of applied RF which is distributed between the plasma and ground; the electron transit time is assumed short compared to the RF period. The electron distribution function received at the probe is expressed in terms of a convolution integral. The distribution received at the probe is used to calculate the resulting component of beam current at the probe surface for the special case that the incident beam distribution has a top hat form. Combining this current with the ion sound and electron thermal currents leads to the I–V characteristic. Given a specified range of energies for the incident beam it is shown that the characteristic rises linearly towards the ion saturation current. It is concluded that this feature of the characteristic can be plausibly interpreted as due to extra-energetic electrons

Analysis of the excited argon atoms in the GEC RF reference cell by means of one-dimensional PIC simulations

We examine the question of whether the excited states in argon contribute significantly to ionization in a capacitively coupled plasma through metastable pooling and step-ionization processes. We look at this issue by means of a one-dimensional particle-in-cell (PIC) code, with collisions treated by a Monte Carlo collision package. In the range explored, 50–1000 mTorr, the main source of ionization, in the absence of secondary emission, is direct ionization from the ground state with a contribution from excited states that is negligible at lower pressures, but increases in importance at higher pressures. When secondary electrons are included, their interaction with ground state neutrals dominates the ionization. At higher pressures the metastable profiles can reveal useful information on the non-uniform mean electron energy across the discharge, even though these states do not necessarily play a significant role in ionization

2D fluid simulations of acoustic waves in pulsed ICP discharge: Comparison with experiments

International audienc

2D fluid simulations of acoustic waves in pulsed ICP discharge: Comparison with experiments

International audienc

2D fluid simulations of acoustic waves in pulsed ICP discharges: Comparison with experiments

International audienc

An electrostatic probe technique for r.f. plasma

Available from British Library Document Supply Centre- DSC:7623.47(OUEL--1637/86) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo