In an RF-discharge methane plasma, the dissociation rate of CH4 in collisions with electrons has been measured using a laser absorption technique. The measured rate increases as the RF power increases and as the gas pressure decreases. The electron energy distribution function f( epsilon ) and the electron density ne have also been measured by a heated electrical probe. The measured f( epsilon ) is quite different from the Maxwellian distribution and rather close to the Druyvesteynian distribution. The dissociation rate constant deduced from f( epsilon ) and the reported cross-section when multiplied by the measured ne is consistent with the results of the laser absorption method. Based on these measurements a modelling of the plasma has been performed using a set of rate equations for CH4 and the various neutral and ionic species produced in the plasma. The calculated degree of dissociation for CH4 in the plasma agrees well with the measured results. The most abundant neutral radical in the plasma predicted by the model is CH3, while CH5+ is the most abundant ionic species.

https://iopscience.iop.org/article/10.1088/0022-3727/17/8/026/pdf

Diagnostics and modelling of a methane plasma used in the chemical vapour deposition of amorphous carbon films

Extensions of the electrostatic probe technique to situations with negative ions, both theoretical and experimental, are described. Charged particle distributions and the plasma-sheath boundary are discussed in relation to probe measurements. Plasmas with negative ions, their occurrence and interest, particularly situations where the negative ion density becomes comparable to or exceeds the electron density are described. Some data in negative ion containing plasmas are shown and discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/23/8/001/pdf

Plasmas with negative ions-probe measurements and charge equilibrium

The Glow Discharge at Low Pressure

A partial analysis has been made of the effects of fluctuations in a plasma on the data obtained by conventional use of Langmuir probes. Consideration has been given to effects of periodic fluctuations of electron temperature, random current and space-potential, supposed independent of probe potential. For a wide range of conditions, the values of electron temperature obtained will be correct, but values of electron concentration underestimated. Experiments are described to check part of the theory, and the bearing of the experimental and theoretical results obtained on the probe analysis of discharges in which there are plasma electron oscillations is discussed briefly.

https://iopscience.iop.org/article/10.1088/0370-1328/79/3/311/pdf

Notes on the effect of noise on langmuir probe characteristics

A model has been constructed to describe the electrical characteristics of the central bulk plasma region in a 13.56-MHz parallel-plate discharge in chlorine at pressures of about 1 torr. This region is modeled as a volume-controlled plasma with the electron balance dominated by single-step electron-impact ionization and attachment and with the electron energy distribution function in equilibrium with the local instantaneous electric field. Relationships between the ionization frequency, the attachment frequency, the electron drift velocity, and the electric field are provided by solutions of the Boltzmann equation for mixtures of Cl2 and Cl which result from Cl2 dissociation. From a measured current waveform and Cl2/Cl density ratio, the model generates the local electric-field waveform, the time-varying electron density, and the power density in the central portion of the bulk plasma. The calculated time-averaged power input per unit discharge length compares well with experimentally determined values.

A Model for the Bulk Plasma in an RF Chlorne Discharge

The theory of the positive column is commonly based on the assumption of a Maxwellian energy distribution for the electrons, though in many cases it is far from clear how such a condition may be brought about. Moreover, past work has shown that the distribution is by no means always Maxwellian. Indeed, in any detailed consideration of the rates of inelastic collision processes occurring in the plasma it is desirable to be able to determine the distribution experimentally. This paper reports a method of carrying out a Druyvesteyn analysis electronically, and gives a critical account of its performance. A high-frequency voltage of small amplitude chopped with a certain lower frequency is applied to a spherical probe, and the second derivative of the current-voltage characteristic is found from the amplitude of the sine wave with the chopping frequency present in the probe current. An adequate signal-to-noise ratio is obtained by using narrow band amplification with phase-sensitive detection. A large amount of data has now been obtained with this method in a variety of striated discharges. It is found that in every case studied so far the energy distribution takes the form of two well-separated groups of electrons with sometimes a very small third group. The high-energy group is generated by the potential difference across the striation head and becomes progressively attenuated towards its tail. An effect of the varying distribution is a sudden increase in the net rate of loss of electrons from the low-energy group resulting from the fall in number of electrons capable of ionizing and the change in potential difference between wall and discharge at the end of the striation. This loss of electrons causes a fall in the local discharge conductivity and so gives rise to another potential step and striation head.

Electron Energy Distributions in Plasmas. I

This paper is the second in a series of studies of low-pressure discharges in which an electronic method was used for the Druyvesteyn analysis of electron energy distribution. The method reported in the first paper (Boyd & Twiddy 1959) has been applied to the study of the mechanism of striation structure in hydrogen discharges, in the pressure range 0.01 to 0.1 mmHg and the current range 0.016 to 0.800 A. The basic relations governing the current flow, ionization rate and energy flux have been established and verified. These relations and this mechanism is not specific to hydrogen, as later papers will show. The measurements reveal insufficient electrons with energy capable of producing H$^+$ and H$\_2^+$ directly, though it is possible that electrons having passed through two striation heads may make an important contribution. In view of earlier reports of the predominance of H$\_3^+$ in such discharges it is tentatively suggested that the process H' + H$\_2 \rightarrow$ H$\_3^+$ + e may be important.

Electron energy distributions in plasmas. ii. hydrogen

The Druyvesteyn method of measuring electron energy distributions in low-pressure plasmas has been applied to the cathode region of hot cathode discharges in helium, neon and argon at a pressure of 0 05 mm Hg. In all gases the column was striated. In helium and neon, measurements in the striation nearest the cathode show two well separated groups of electrons, the more energetic or primary electrons arising from the Langmuir double space-charge sheath at the filament; the lower-energy group or secondary electrons being produced by the primary electrons in inelastic collisions. The concentrations of both groups fall off with increasing distance from the filament, but their separation and energy spread is constant throughout the striation indicating the absence of any energy exchanging process capable of appreciably modifying the energy distribution. It would appear therefore that the energy-exchanging processes observed by Langmuir (I925), by Merrill & Webb (I939), and by Gabor, Ash & Dracott (I955) were inoperative in the discharges employed in the present experiments. In the absence of an energy-exchanging process a uniform column cannot arise at the pressures employed, since in order to give rise to a truly uniform column the axial field must extend over many mean free paths, a condition which is not fulfilled in these discharges, since the entire discharge tube is less than 100 mean free paths long and thus on the average an electron makes fewer than 1000 collisions between cathode and anode. The application of an axial field simply augments the energy of the electrons without modifying the broad features of the energy distribution. If therefore the energy distribution consists of two separate groups of electrons a uniform field can maintain the discharge while primary electrons remain, but when these are exhausted, because of the distinct separation between the primary and secondary groups, new primary electrons can only be created by a step of potential. Under these conditions the striated column will exist and a transition to a stable uniform column is only to be expected when a mechanism exists for modifying the energy distribution in a few mean free paths. Measurements at higher currents in helium and neon show a definite tendency for the trough between the two groups to be eliminated; this is presumably due to secondary processes. The presence of a high metastable concentration enables both primary and secondary groups to be modified by secondary collisions thus eliminating the sharp demarcation between the two groups which was evident at lower currents. It seems possible also that the disposition of the first critical potential in a gas may influence its ability to develop a uniform column, since if the first excitation potential is appreciably less than half the ionization potential the primary and secondary groups will overlap. This might be effective in the metal vapours. The probable influence of impurities on the appearance of the discharge and the energy distribution are considered on the basis of the data available from measurements in hydrogen, helium and neon. Measurements in the negative regions in argon, while showing features characteristic of the striated column, were quite different in character from the other rare gases in that a fairly strong but decreasing field existed in the tube. Measurements here show very clearly how a uniform field augments the energy of the electrons without modifying the broad features of the distribution.

Electron Energy Distributions in Plasmas. III. The Cathode Regions in Helium, Neon and Argon

THE distribution of energy among the plasma electrons is a factor of primary importance in determining the processes occurring in a discharge. Two matters of special interest in our work are (a) the processes determining the equilibrium of negative ions in electronegative plasmas, and (b) the mechanism of production of atomic hydrogen in discharges in dry and wet hydrogen. In the first, the case of oxygen is well known to be somewhat anomalous in that the negative ion concentration is very much less than it is in the halogen plasmas, although the electron affinity is not vastly different. In case b, the much greater efficiency of radio-frequency discharges for dissociating H2 and the fact that increased dissociation resulting from an introduction of water vapour does not seem to be entirely attributable to wall poisoning, suggests that the difference between the energy distribution for electrons in a striated positive column and that in the radio-frequency discharge may be concerned in the changed efficiency. The purpose of this communication is to direct attention to some preliminary results for energy distributions in the positive column of a d.c. discharge in hydrogen.

Electron Energy Distribution in the Striated Hydrogen Discharge

Since earlier measurements of electron energy distributions obtained with the Druyvesteyn probe method showed no evidence of the scattering observed in mercury by Langmuir (1925) and Merrill & Webb (1939), the method has now been used to examine discharges in hydrogen, neon, krypton and mercury in a search for evidence of such a high rate of energy exchange between the electrons. This was, in fact, observed in neon when, as the discharge current was increased, the well-defined group of primary electrons from the filament disappeared, giving rise to a uniform distribution. In krypton and mercury, the distribution near the filament was usually uniform, although frequently there was evidence of a strongly broadened primary group and electrons were often observed with energies greater than the potential difference between the filament and the measuring probe. This behaviour is almost certainly to be explained in terms of Langmuir scattering near the filament, but the scattering was always restricted to regions very close to the filament and was never observed in the positive column. Indeed, after the initial scattering the distribution is influenced by the applied field in a similar manner to that observed in earlier measurements and thus, in these gases, although the distribution is quite uniform near the filament there are very marked departures from uniformity further from the cathode as the distribution is accelerated in either a uniform field or through a step of potential, giving rise to well defined primary and secondary groups of electrons. These newly created primaries are not scattered. Measurements in high-field regions in krypton show a high ratio of primary to secondary electrons. This is due to the pronounced Ramsauer effect in this gas and was observed earlier in argon. The change of energy distribution resulting from the contamination of a discharge in neon by argon and mercury has been investigated.

Electron Energy Distributions in Plasmas. V. A Search for Evidence of a High Anomalous Rate of Energy Exchange between the Electrons of a Low-Pressure Discharge

N. D. Twiddy

Papers

Electron Energy Distributions in Plasmas. I

Electron energy distributions in plasmas. ii. hydrogen

Electron Energy Distributions in Plasmas. III. The Cathode Regions in Helium, Neon and Argon

Electron Energy Distribution in the Striated Hydrogen Discharge

Electron Energy Distributions in Plasmas. V. A Search for Evidence of a High Anomalous Rate of Energy Exchange between the Electrons of a Low-Pressure Discharge