Plasma dynamics in an inductively coupled magnetic neutral loop discharge

Magnetic neutral loop discharges (NLDs) can be operated at significantly lower pressures than conventional radio-frequency (rf) inductively coupled plasmas (ICPs). These low pressure conditions are favourable for technological applications, in particular anisotropic etching. An ICP–NLD has been designed providing excellent diagnostics access for detailed investigations of fundamental mechanisms. Spatially resolved Langmuir probe measurements have been performed in the plasma production region (NL region) as well as in the remote application region downstream from the NL region. Depending on the NL gradient two different operation modes have been observed exhibiting different opportunities for control of plasma uniformity. The efficient operation at comparatively low pressures results in ionization degrees exceeding 1%. In this regime neutral dynamics has to be considered and can influence neutral gas and process uniformity. Neutral gas depletion through elevated gas temperatures and high ionization rates have been quantified. At pressures above 0.1 Pa, gas heating is the dominant depletion mechanism. At lower pressures neutral gas is predominantly depleted through high ionization rates and rapid transport of ions by ambipolar diffusion along the magnetic field lines. Non-uniform profiles of the ionization rate can, therefore, result in localized neutral gas depletion and non-uniform processing. We have also investigated the electron dynamics within the radio-frequency cycle using phase resolved optical emission spectroscopy and Thomson scattering. In these measurements electron drift phenomena along the NL torus have been identified.

Space and phase resolved plasma parameters in an industrial dual-frequency capacitively coupled radio-frequency discharge

The dynamics of high energetic electrons (>= 11.7 eV) in a modified industrial confined dual-frequency capacitively coupled RF discharge (Exelan, Lam Research Inc.), operated at 1.937 MHz and 27.118 MHz, is investigated by means of phase resolved optical emission spectroscopy. Operating in a He-O-2. plasma with small rare gas admixtures the emission is measured, with one-dimensional spatial resolution along the discharge axis. Both the low and high frequency RF cycle are resolved. The diagnostic is based on time dependent measurements of the population densities of specifically chosen excited rare gas states. A time dependent model, based on rate equations, describes the dynamics of the population densities of these levels. Based on this model and the comparison of the excitation of various rare gas states, with different excitation thresholds, time and space resolved electron temperature, propagation velocity and qualitative electron density as well as electron energy distribution functions are determined. This information leads to a better understanding of the dual-frequency sheath dynamics and shows, that separate control of ion energy and electron density is limited.

Spatial structures of plasma parameters in a magnetic neutral loop discharge

Spatial structures of plasma parameters in a radio-frequency inductively coupled magnetic neutral loop discharge are investigated under various parameter variations using spatially resolved Langmuir probe measurements. A strong coupling between the plasma production region, in the neutral loop (NL) plane, and the axially remote substrate region is observed. The two regions are connected through the separatrices and therefore, spatial profiles in the substrate region are strongly influenced by the plasma production region and the structure of the separatrices. The electron temperature in the plasma production region peaks in the centre of the NL while the maximum in electron density is shifted radially inwards due to diffusion. Details of the structures in both regions, the production region and the substrate region, are determined through the position of the NL and the gradient of the inhomogeneous magnetic field around the NL confinement region. Parameter combinations are found providing higher plasma densities and better uniformity than in common inductively coupled plasmas without applying an additional magnetic field. The uniformity can be further improved using temporal variations of the magnetic field structure.

A planar inductively coupled radio-frequency magnetic neutral loop discharge

A planar inductively coupled radio-frequency (rf) magnetic neutral loop discharge has been designed. It provides diagnostic access to both the main plasma production region as well as a remote plane for applications. Three coaxial coils are arranged to generate a specially designed inhomogeneous magnetic field structure with vanishing field along a ring in the discharge-the so-called neutral loop (NL). The plasma is generated by applying an oscillating rf electric field along the NL, induced through a four-turn, planar antenna operated at 13.56 MHz. Electron density and temperature measurements are performed under various parameter variations. Collisionless electron heating in the NL region allows plasma operation at comparatively low pressures, down to 10(-2) Pa, with a degree of ionization in the order of several per cent. Conventional plasma operation in inductive mode without applying the magnetic field is less efficient, in particular in the low pressure regime where the plasma cannot be sustained without magnetic fields.

Plasma ionization through wave-particle interaction in a capacitively coupled radio-frequency discharge

Phase resolved optical emission spectroscopy, with high temporal resolution, shows that wave-particle interactions play a fundamental role in sustaining capacitively coupled rf plasmas. The measurements are in excellent agreement with a simple particle-in-cell simulation. Excitation and ionization mechanisms are dominated by beam-like electrons, energized through the advancing and retreating electric fields of the rf sheath. The associated large-amplitude electron waves, driven by a form of two-stream instability, result in power dissipation through electron trapping and phase mixing. (c) 2007 American Institute of Physics.

Transition phenomena in a radio-frequency inductively coupled plasma

Changes of the electron dynamics during the mode transition (E- to H-mode) in a hydrogen radio-frequency (rf) inductively coupled plasma are investigated using space and phase resolved optical emission spectroscopy. The E- mode is characterized through relatively weak optical emission which is strongly modulated on a nanosecond time scale during the rf-cycle, with one pronounced maximum per cycle. The modulation in H-mode, with twice the rf-frequency, is significantly weaker while the emission intensities are about two orders of magnitude higher. In particular the transition between these two modes is studied under variations of rf-power input and gas pressure. Characteristic spatio-temporal structures are observed and can be understood in the frame of a simple model combining both coupling mechanisms in the transition regime.

Ultrasmall radio frequency driven microhollow cathode discharge

We have operated 25-100 mu m diameter radio frequency microhollow cathode discharges stably, for many hours, in neon and in argon. Electrical and spectroscopic measurements were used to explore three possible electron heating modes and obtain detail regarding the electron energy distribution. Analysis points to the possibility of pendular electron heating at low voltages.