Cormac Corr

Double layer formation in the expanding region of an inductively coupled electronegative plasma

Double-layers (DLs) were observed in the expanding region of an inductively coupled plasma with Ar–SF6 gas mixtures. No DL was observed in pure argon or SF6 fractions below a few percent. They exist over a wide range of power and pressure although they are only stable for a small window of electronegativity (typically between 8% and 13% of SF6 at 1 mTorr), becoming unstable at higher electronegativity. They seem to be formed at the boundary between the source tube and the diffusion chamber and act as an internal boundary [the amplitude being roughly 1.5(kTe∕e)] between a high electron density, high electron temperature, low electronegativity plasma upstream (in the source), and a low electron density, low electron temperature, high electronegativity plasma downstream.

Periodic formation and propagation of double layers in the expanding chamber of an inductive discharge operating in Ar/SF 6 mixtures

Inductive reactors are routinely used for etching of silicon and various metals in the microelectronic industry. The plasma is excited by flowing a rf current in a coil which launches a decaying wave into the plasma through a dielectric window. When operating in the high plasma density regime the inductive H mode, they provide high ion fluxes with adjustable ion energies by means of an additional rf biasing of the wafer holder. However, when the power applied to the coil is low, the H mode cannot be sustained and the discharge operates in a capacitive E mode due to the high voltage across the coil with respect to the grounded walls. The transition between the E mode and the H mode by increasing the input power is the first source of instability of inductive reactors when electronegative gases are used. This phenomenon, referred as the source instability, has been described and modeled in a series of recent papers. 1–8

Ion beam formation in a low-pressure geometrically expanding argon plasma

Supersonic ion beam formation has been observed in a geometrically expanding low-pressure inductively coupled argon plasma. It is found that the ion beam is only observed below 3 mTorr and only when the discharge is operated in inductive mode. The geometrical expansion of the plasma induces density and potential gradients leading to the ion beam formation. The ion beam energy increases with decreasing source tube radius. The results show that ion beam formation can be achieved by geometrical expansion alone and that the ion beam energy depends on the ratio of the cross-sectional area of the source and expansion region.

Discharge kinetics of inductively coupled oxygen plasmas: Experiment and model

In this paper, neutral and charged particle dynamics in both the capacitive and inductive modes of an inductively coupled oxygen discharge are presented. Langmuir probes, laser-assisted photodetachment and two-photon laser-induced fluorescence are employed to measure plasma parameters in the 13.56 MHz system for a range of plasma powers and gas pressures. It is found that the capacitive mode is more electronegative with lower molecular dissociation compared with the inductive mode. However, the negative ion density in each mode is comparable. A maximum is observed in the negative ion density and fraction with pressure for both modes. The experimental measurements are supplemented by a global model, which includes capacitive and inductive coupling effects. The model and experiments demonstrate that negative ion loss is dominated by ion–ion recombination and electron detachment at low pressures (<10 mTorr), while it is dominated by detachment processes with the metastable molecule O2(a 1Δg) and oxygen atoms at higher pressures. These findings support recent global model predictions of oxygen discharges.

Comparison between fluid simulations and experiments in inductively coupled argon/chlorine plasmas

Comparisons of 2D fluid simulations with experimental measurements of Ar/Cl2 plasmas in a low-pressure inductively coupled reactor are reported. Simulations show that the wall recombination coefficient of Cl atom (γ) is a crucial parameter of the model and that neutral densities are very sensitive to its variations. The best agreement between model and experiment is obtained for γ = 0.02, which is much lower than the value predicted for stainless steel walls (γ = 0.6). This is consistent with reactor wall contaminations classically observed in such discharges. The electron density, negative ion fraction and Cl atom density have been investigated under various conditions of chlorine and argon concentrations, gas pressure and applied rf input power. The plasma electronegativity decreases with rf power and increases with chlorine concentration. At high pressure, the power absorption and distribution of charged particles become more localized below the quartz window. Although the experimental trends are well reproduced by the simulations, the calculated charged particle densities are systematically overestimated by a factor of 3–5. The reasons for this discrepancy are discussed in the paper.

Fluorine negative ion density measurement in a dual frequency capacitive plasma etch reactor by cavity ring-down spectroscopy

The authors wish to thank Lam Research Corporation for donation of equipment and financial support.

Negative ions in single and dual frequency capacitively coupled fluorocarbon plasmas

We have studied charged particle densities and fluxes in a customized industrial etch reactor, running in Ar/O 2 /c-C 4 F 8 gas mixtures at pressures in the region of 50 mTorr and driven by 2 and 27 MHz RF power, either separately or simultaneously. Independent control of ion flux and ion energy is the aim of using dual frequency plasmas. However, little experimental data exists regarding the charged particle dynamics in complex industrial gas mixtures. Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and they may even reach the surface and participate in etching. We have measured the electron density using a microwave hairpin resonator and the positive ion flux with a RF biased ion flux probe. The ratio of these two quantities, which depends on the negative ion fractions and other factors, is seen to vary strongly with gas chemistry, giving evidence for the presence of negative ions. Our results indicate high electronegativity for high c-C 4 F 8 flow rates. We have also examined the effect of varying the 2 and 27.12 MHz RF powers on both the electron density and the positive ion flux. This allows us to estimate the effect of varying power on the negative ion density. In addition, ultra-violet cavity ring-down spectroscopy was used to measure the F density directly (Booth et al 2006 Appl. Phys. Lett. 88 151502). This optical measurement was compared with the probe technique.

Probing helium nano-bubble formation in tungsten with grazing incidence small angle x-ray scattering

Helium nano-bubble formation in plasma facing materials has emerged as a major concern for the next-step fusion experiment ITER, where helium plasmas will be used during the tokamaks start-up phase. Here, we demonstrate that grazing incidence small-angle x-ray scattering is a powerful technique for the analysis of helium nano-bubble formation in tungsten. We measured helium bubbles with sizes between 1.5–2.5 nm in tungsten exposed to helium plasma at 700 °C, where a smaller number of larger bubbles were also observed. Depth distributions can be estimated by taking successive measurements across a range of x-ray incidence angles. Compared with traditional approaches in the field, such as transmission electron microscopy, this technique provides information across a much larger volume with high statistical precision, whilst also being non-destructive.

High-beta plasma effects in a low-pressure helicon plasma

In this work, high-beta plasma effects are investigated in a low-pressure helicon plasma source attached to a large volume diffusion chamber. When operating above an input power of 900 W and a magnetic field of 30 G a narrow column of bright blue light due to Ar II radiation is observed along the axis of the diffusion chamber. With this blue mode, the plasma density is axially very uniform in the diffusion chamber; however, the radial profiles are not, suggesting that a large diamagnetic current might be induced. The diamagnetic behavior of the plasma has been investigated by measuring the temporal evolution of the magnetic field Bz and the plasma kinetic pressure when operating in a pulsed discharge mode. It is found that although the electron pressure can exceed the magnetic field pressure by a factor of 2, a complete expulsion of the magnetic field from the plasma interior is not observed. In fact, under our operating conditions with magnetized ions, the maximum diamagnetism observed is 2%. It is observed that the magnetic field displays the strongest change at the plasma centre, which corresponds to the maximum in the plasma kinetic pressure. These results suggest that the magnetic field diffuses into the plasma sufficiently quickly that on a long time scale only a slight perturbation of the magnetic field is ever observed.

Plasma parameters and electron energy distribution functions in a magnetically focused plasma

Spatially resolved measurements of ion density, electron temperature, floating potential, and the electron energy distribution function (EEDF) are presented for a magnetically focused plasma. The measurements identify a central plasma column displaying Maxwellian EEDFs at an electron temperature of about 5 eV indicating the presence of a significant fraction of electrons in the inelastic energy range (energies above 15 eV). It is observed that the EEDF remains Maxwellian along the axis of the discharge with an increase in density, at constant electron temperature, observed in the region of highest magnetic field strength. Both electron density and temperature decrease at the plasma radial edge. Electron temperature isotherms measured in the downstream region are found to coincide with the magnetic field lines.