Dragos Crintea

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional‐radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05Pa‐5Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56MHz and powers varied between P = 1kW and 2kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p 0.1Pa and rf powers P> 1kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p> 1Pa the degree of ionization is relatively low (<10 −3 ) and neutral gas depletion is dominated by gas heating. (Some figures in this article are in colour only in the electronic version)

Neutral gas depletion mechanisms in dense low-temperature argon plasmas

Neutral gas depletion mechanisms are investigated in a dense low-temperature argon plasma—an inductively coupled magnetic neutral loop (NL) discharge. Gas temperatures are deduced from the Doppler profile of the 772.38 nm line absorbed by argon metastable atoms. Electron density and temperature measurements reveal that at pressures below 0.1 Pa, relatively high degrees of ionization (exceeding 1%) result in electron pressures, pe = kTene, exceeding the neutral gas pressure. In this regime, neutral dynamics has to be taken into account and depletion through comparatively high ionization rates becomes important. This additional depletion mechanism can be spatially separated due to non-uniform electron temperature and density profiles (non-uniform ionization rate), while the gas temperature is rather uniform within the discharge region. Spatial profiles of the depletion of metastable argon atoms in the NL region are observed by laser induced fluorescence spectroscopy. In this region, the depletion of ground state argon atoms is expected to be even more pronounced since in the investigated high electron density regime the ratio of metastable and ground state argon atom densities is governed by the electron temperature, which peaks in the NL region. This neutral gas depletion is attributed to a high ionization rate in the NL zone and fast ion loss through ambipolar diffusion along the magnetic field lines. This is totally different from what is observed at pressures above 10 Pa where the degree of ionization is relatively low (<10 −3 ) and neutral gas depletion is dominated by gas heating.

Phase resolved measurement of anisotropic electron velocity distribution functions in a radio-frequency discharge

Oscillating radio-frequency (RF) electric fields penetrating into a plasma lead to a corresponding oscillation of the electron velocity distribution function. Here we report on the first time measurement of this oscillation by Thomson scattering in a low-pressure inductively coupled plasma. The local current density is inferred by combining the oscillation amplitude with the plasma density, also resulting from Thomson scattering. The results are compared with a novel emission spectroscopic technique, RF modulation spectroscopy, which also provides direct determination of the electron oscillation velocity.

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.

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.

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.

Wave propagation and noncollisional heating in neutral loop and helicon discharges

Heating mechanisms in two types of magnetized low pressure rf (13.56 MHz) discharges are investigated: a helicon discharge and a neutral loop discharge. Radial B-dot probe measurements demonstrate that the neutral loop discharge is sustained by helicon waves as well. Axial B-dot probe measurements reveal standing wave and beat patterns depending on the dc magnetic field strength and plasma density. In modes showing a strong wave damping, the plasma refractive index attains values around 100, leading to electron-wave interactions. In strongly damped modes, the radial plasma density profiles are mainly determined by power absorption of the propagating helicon wave, whereas in weakly damped modes, inductive coupling dominates. Furthermore, an azimuthal diamagnetic drift is identified. Measurements of the helicon wave phase demonstrate that initial plane wave fronts are bent during their axial propagation due to the inhomogeneous density profile. A developed analytical standing wave model including Landau damping reproduces very well the damping of the axial helicon wave field. This comparison underlines the theory whereupon Landau damping of electrons traveling along the field lines at speeds close to the helicon phase velocity is the main damping mechanism in both discharges.

Collisionless wave damping in neutral loop discharges

Heating mechanisms in a low pressure magnetized argon radio-frequency neutral loop discharge (NLD) are investigated. Results obtained in the NLD are compared with results obtained in an azimuthally isotropic m = 0 helicon discharge operated in the same setup. B-dot probe measurements are carried out showing that helicon waves are excited in the NLD. Phase resolved optical emission spectroscopic measurements demonstrate that electrons oscillate effectively not in the neutral loop (NL) but in a toroidal cusp located above the NL. In a simple transformer model we consider a diamagnetic current induced from the antenna windings into this cusp. The resulting calculated fields reproduce qualitatively well the measured helicon-NLD field. Furthermore, standing wave and beat patterns emerge in the axial direction. Depending on the dc magnetic field strength and plasma density different damping of the axial wave takes place. At strong damping, measured and calculated wave phase velocities approach the electron thermal velocity. Observed optical emission patterns corroborate the hypothesis of electron Landau damping being the main heating mechanism in both discharges. (Some figures in this article are in colour only in the electronic version)

Helicon-Type Discharge With a Flat Spiral Antenna

A helicon-type discharge is realized in a curved magnetic field with end point launching of whistler waves by a flat RF antenna. Standing modes are observed by monitoring conditions of zero reflected power. There are indications of Landau damping, with the phase velocity matching the thermal velocity.