N St J Braithwaite

Introduction to gas discharges

This is a tutorial article. An introductory discussion of direct current gas discharges is presented. Beginning with basic ideas from kinetic theory, gas discharge plasmas are described in terms of phenomena observed in the laboratory. Various models are introduced to account for electrical breakdown, plasma boundaries and the longitudinal and transverse structure of discharges.

A novel electrostatic probe method for ion flux measurements

A novel electrostatic probe method is described in which the ion flux is determined from the discharging of an RF-biased capacitance in series with the probe. By using a large-area planar probe, with a guard ring and located in or on other surfaces, edge effects and perturbations to the plasma volume can be kept small. The ion flux to the probe can be determined even when its surface is coated with insulating material from the plasma itself. Results are reported for ion fluxes in RF-excited plasmas in Ar and in in a RIE reactor. In Ar, ion fluxes to the earthed surfaces increase with pressure and power over the ranges 50 - 200 mTorr and 30 - 200 W. In , over the same ranges the ion fluxes to the surfaces decrease with increasing pressure.

Low temperature plasma-assisted chemical vapour deposition of amorphous carbon films for biomedical-polymeric substrates

Preliminary results have been obtained on the biocompatibility of amorphous carbon hydrogen (αC:H) coatings deposited on polystyrene. Deposition was carried out at low substrate temperatures using pulsed r.f. plasma-assisted chemical vapour deposition from a methane-hydrogen gas mixture. Cytotoxicity tests using a standard cell line indicate a high degree of biocompatibility. Specifically, αC:H is not toxic to cells, appears to increase cell attachment and affords normal cell growth rates. Wear and other tests have revealed no significant differences between these αC:H coatings and those deposited on a stainless steel at a higher substrate temperature, except for a more pronounced surface texture. However, position in the r.f. plasma was found to be critical for the deposition of good, adherent low temperature coatings.

Floating potential in a bi-Maxwellian RF plasma

The floating potential is investigated for a planar probe/electrode in a DC/RF plasma with a bi-Maxwellian electron distribution. A form of Bohm velocity is adopted appropriate to a two-temperature (T1, T2) plasma with corresponding electron densities n1, n2. Taking hydrogen as an example, a numerical study shows that for given values of T1/T2, n1/n2 and RF amplitude (normalized to T1), the floating potential (normalized to T1) is unique. On the other hand, using the same normalizations for prescribed values of floating potential and RF amplitude, and given n1/n2, there are two values of T1/T2. Conversely for given values of floating potential, RF amplitude and T1/T2, n1/n2 may be single or double valued. Under conditions of sheath inversion RF plasmas again show uniqueness or two valuedness, according to the prescription.

Hairpin resonator probe measurements in RF plasmas

This work investigates the use of hairpin probes in plasma where RF plasma potential is present. The microwave resonance of the hairpin is used to determine electron density. Two types of hairpin probe were used. One type was dc coupled: its dc potential could be varied while monitoring its resonance frequency and collected current. The other probe was designed to be fully floating, being (dc) isolated from ground and able to float with RF variations in the plasma potential. Additional measurements of the RF plasma potential and its effect on the dc floating potential of the former probe were made using a wire loop probe. The resonant frequency of the dc coupled probe at zero current (nominal floating potential) was less than that determined from the fully floating probe. This is attributed to the wider sheath around the former caused by RF plasma potential across it. The presence of the electron-free sheath around the wires of the hairpin is included in the analysis that links the resonant frequency to the electron density in the bulk plasma. When the dc coupled probe was biased at the true floating potential (determined from independent loop probe measurements) its resonant frequency was closer to, though still consistently higher than, that of the floating probe. This work shows that RF potential across the probe sheath affects the resonance of a hairpin probe and should be accounted for when using hairpin probes in discharges where RF plasma potential variations are even as low as a few times the electron temperature (in volts).

Plasma engineering of graphene

Recently, there have been enormous efforts to tailor the properties of graphene. These improved properties extend the prospect of graphene for a broad range of applications. Plasmas find applications in various fields including materials science and have been emerging in the field of nanotechnology. This review focuses on different plasma functionalization processes of graphene and its oxide counterpart. The review aims at the advantages of plasma functionalization over the conventional doping techniques. Selectivity and controllability of the plasma techniques opens up future pathways for large scale, rapid functionalization of graphene for advanced applications. We also emphasize on atmospheric pressure plasma jet as the future prospect of plasma based functionalization processes.

Measurements of characteristic transients of planar electrostatic probes in cold plasmas

This article describes how to extract accurate information about a plasma from a capacitively coupled planar probe that is biased using pulsed radio-frequency excitation. The conditions necessary to observe correct saturation of the probe current are investigated, particularly the use of correct geometry and biasing for the guard ring. With these precautions the probe is an effective diagnostic for electron tail temperature at energies beyond those probed by conventional cylindrical probes. The dynamic response of the probe is investigated using conventional sweep voltages and shows the onset of displacement current and inertial effects associated with ions and electrons. In addition the effect of insulating films on the probe surface is examined, showing how the probe continues to operate even when it is coated. Characteristic changes caused by the presence of an insulating film give information about its electrical properties and its thickness.

Reflections on electrical probes

Langmuir probes are relatively simple to construct and to use for establishing local and time-resolved measurements of charged particles in ionized gases. The analysis requires the adoption of assumptions and implies constraints on operation that can often be met in laboratory plasmas that have been primarily devised for the testing of probe theories. Probe modelling can also be tested against simulations. More generally, the analysis of data from probes operated in less ideal plasmas, such as those used in materials processing, is more challenging. In these circumstances alternative strategies are required that are tolerant of the processing environment. Techniques for immediate, local measurements include the use of structures that involve standing waves and travelling waves, typically at microwave frequencies. Non-invasive electrical probing can also conveniently be done from isolated planar sections of the bounding surfaces.

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.

Fuzzy logic in a blackboard system for controlling plasma deposition processes

Abstract A blackboard system, ARBS, has been used to control a plasma processing unit, which is used for depositing coatings on the surface of electronic or mechanical components. Previous applications of ARBS have been based on crisp logic, but fuzzy logic was added in this study for plasma deposition control. Fuzzy rules have been introduced into ARBS without changing either the rule syntax or the existing inference engines, thereby demonstrating the flexibility of the software. Consequently crisp and fuzzy rules can coexist within a single knowledge source (i.e. module). An efficient technique for defuzzification has been employed in which the membership functions are replaced by Dirac delta functions. The technique is equivalent to standard methods of defuzzification, without loss of precision or accuracy, but with a reduced number of calculations. Multi-variable control of DC-bias (an electrical parameter) by automatic adjustment of pressure and RF (radio frequency) electrical power is demonstrated.