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 $\text{Ar}/\text{SF}\_6$ gas mixtures. No DL was observed in pure argon or $\text{SF}\_6$ fractions below 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 $\text{SF}\_6$ at 1mTorr), 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$\frac{kT\_e}{e}$)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

Experimental investigation of double layers in expanding plasmas

Double layers (DLs) have been observed in a plasma reactor composed of a source chamber attached to a larger expanding chamber. Positive ion beams generated across the DL were characterized in the low plasma potential region using retarding field energy analyzers. In electropositive gases, DLs were formed at very low pressures between 0.1 and 1 mTorr with the plasma expansion forced by a strongly diverging magnetic field. The DL remains static, robust to changes in boundary conditions, and its position is related to the magnetic field lines. The voltage drop across the DL increases with decreasing pressure, i.e., with increasing electron temperature around 20 V at 0.17 mTorr. DLs were also observed in electronegative gases without a magnetic field over a greater range of pressure 0.5 to 10 mTorr. The actual profile of the electronegative DL is very sensitive to external parameters and intrusive elements, and they propagate at high negative ion fraction. Electrostatic probes measurements and laser-induced photodetachment show discontinuities in all plasma parameters electron density, electron temperature, negative ion fraction at the DL position. The voltage drop across the electronegative DL is about 8 V, is independent of the gas pressure and therefore of the electron temperature

Real space investigation of structural changes at the metal-insulator transition in VO2

Synchrotron X-ray total scattering studies of structural changes in rutile VO2 at the metal-insulator transition temperature of 340 K reveal that monoclinic and tetragonal phases of VO2 coexist in equilibrium, as expected for a first-order phase transition. No evidence for any distinct intermediate phase is seen. Unbiased local structure studies of the changes in V--V distances through the phase transition, using reverse Monte Carlo methods, support the idea of phase coexistence and point to the high degree of correlation in the dimerized low-temperature structure. No evidence for short range V--V correlations that would be suggestive of local dimers is found in the metallic phase.Comment: 4 pages, 5 figure

A flowing plasma model to describe drift waves in a cylindrical helicon discharge

A two-fluid model developed originally to describe wave oscillations in the vacuum arc centrifuge, a cylindrical, rapidly rotating, low temperature and confined plasma column, is applied to interpret plasma oscillations in a RF generated linear magnetised plasma (WOMBAT), with similar density and field strength. Compared to typical centrifuge plasmas, WOMBAT plasmas have slower normalised rotation frequency, lower temperature and lower axial velocity. Despite these differences, the two-fluid model provides a consistent description of the WOMBAT plasma configuration and yields qualitative agreement between measured and predicted wave oscillation frequencies with axial field strength. In addition, the radial profile of the density perturbation predicted by this model is consistent with the data. Parameter scans show that the dispersion curve is sensitive to the axial field strength and the electron temperature, and the dependence of oscillation frequency with electron temperature matches the experiment. These results consolidate earlier claims that the density and floating potential oscillations are a resistive drift mode, driven by the density gradient. To our knowledge, this is the first detailed physics model of flowing plasmas in the diffusion region away from the RF source. Possible extensions to the model, including temperature non-uniformity and magnetic field oscillations, are also discussed

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

Nonlinear instability dynamics in a high-density, high-beta plasma

Entrainment and periodic pulling of an ion acoustic instability have been observed in the power spectra of a low-pressure high-beta plasma. The observed nonlinear phenomena can be modeled by using the van der Pol equation with a forcing term. Experimental results of the nonlinear processes are presented. Ion density fluctuations are detected on a negatively biased Langmuir probe for magnetic fields and input powers above 30 G and 900 W at 7.2 MHz respectively, and gas pressure below 1.5 mTorr. This low-frequency instability is observed in the central plasma blue core (argon II emission) and can be controlled by amplitude modulation of the radio frequency input power at frequencies close to the instability frequency

Transition from unstable electrostatic confinement to stable magnetic confinement in a helicon reactor operating with Ar∕SF₆ gas mixtures

Two types of instabilities were previously identified in inductive discharges having an expanding chamber when negative ions are present: (i) the sourceinstability, occurring in the neighborhood of the capacitive-to-inductive (E to H) transition, and (ii) the downstream instability, which was shown to be the periodic formation and propagation of double layers. These unstable double layers were found over the entire parameter space (pressure/power) of interest, and they were born at the interface of the source and diffusion chambers. They acted as an internal electrostatic barrier separating a low-electronegativity, high-electron-density plasma upstream (in the source) and a high-electronegativity, low-electron-density plasma downstream. In this paper we have investigated the effect of adding a static axial magnetic field, classically used to increase the confinement and the plasma heating via helicon wave propagation. This had the following consequences: (i) the unstable double layers, and therefore the axial electrostatic confinement, were suppressed in a large part of the parameter space, and (ii) the magnetic confinement leads to a radially stratified plasma, the center being a low-electronegativity, high-density plasma and the edges being essentially an ion-ion plasma

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 3mTorr 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

Spatial evolution of an ion beam created by a geometrically expanding low-pressure argon plasma

The spatial distribution of an ion beam—created at the interface of a small diameter plasma source and much larger diameter diffusion chamber—is studied in a low-pressure inductively coupled plasma using a retarding field energy analyzer. It is found that the ion beam density decays axially and radially in the diffusion chamber following the expansion of the plasma from the source region. The radial distribution of the ion beam indicates that the acceleration region has a convex shape and is located just outside the source exit, giving rise to a hemispherical plasma expansion into the diffusion chamber

Optimization of a Cl₂–H₂ inductively coupled plasma etching process adapted to nonthermalized InP wafers for the realization of deep ridge heterostructures

Inductively coupled plasmaetching using Cl₂–H₂ chemistry with no additive gas (CH₄, Ar, or N₂) is studied to realize deep (>5μm) ridges with smooth and vertical sidewalls. The process is optimized for nonthermalized InP wafers to avoid the use of thermal grease. Cleaning of the rear side of the wafer after etching is avoided, which is suitable for an industrial process or for critical subsequent steps such as epitaxial regrowth. The influence of the Cl₂∕H₂ ratio on the etching mechanism is investigated for both InP bulk layers and InGaAs∕InP or InGaAlAs∕InPheterostructures. The authors show that this ratio is the main parameter controlling the ridge profile, in a similar way for both bulk InP and InGa(Al)As∕InP samples. Smooth and vertical sidewalls with neither undercuts nor notches can be obtained in the 0.5–1mTpressure range for a hydrogen percentage of 35%–45% in the gas mixture. Etching rates from 900to1300nm∕min together with a selectivity over SiNx dielectric mask as high as 24:1–29:1 are measured for the InP bulk layers under these conditions. Etching does not affect the optical quality of the heterostructures as evidenced from micro-photoluminescence measurements performed on 1.6‐to0.85‐μm-wide deep etched ridge waveguides. The process is well adapted to the realization of low loss deep ridge waveguides or buried heterostructures