Scanning Langmuir Probe Research Articles

Axial diagnosis of radio-frequency capacitively coupled Ar/O<sub>2</sub> plasma

The capacitively coupled Ar plasma containing oxygen, driven by a radio frequency of 27.12 MHz, is investigated by laser-induced photo-detachment technique assisted with a Langmuir probe. The plasmas with different amounts of oxygen are obtained by changing the flow of Ar and oxygen, each of which is controlled by a mass flow controller. The axial distribution of plasma characteristic can be measured by changing the relative axial position of the Langmuir probe between the parallel electrodes. The electron density and electron temperature are calculated from the current-voltage curve measured by the scanning Langmuir probe, and the electronegativity is obtained from the current curves of the probe with the laser-induced photo-detachment technique. The negative ion density can be calculated from the electron density and the electronegativity. It is shown that with oxygen flow rate increasing, the dissociative attachment of oxygen molecules with electrons will consume the electrons with the middle energy in the electron energy probability function (EEPF) measured by Langmuir probe. The EEPF evolves from Druyvesteyn to Maxwellian distribution due to the thermalization by the e-e interaction with applied power increasing. It is worth mentioning that a depression in the EEPF curve will appear when discharging high-pressure Ar gas containing oxygen. This depression can also be caused by the dissociative attachment of oxygen molecules with electrons where the threshold energy is around 4.5 eV. The axial profile of the electron density is calculated from the EEPF changing from a linear rise in pure Ar plasma to a flater phase of the distribution due to the negative ions such as oxygen introduced into the plasma. The electron temperature changes a little at different axial positions. The rise of negative ion density nearby the sheath of powered electrode is due to the dissociative attachment caused by the collision of oxygen molecules with energetic electrons. In addition, the axial distribution of electronegativity takes on a shape of spoon, which results from the consequence of generation and loss of negative ions in the process of the ambipolar-electric-field-driven diffusion to the plasma center.

Design of Fast Scanning Langmuir Probe Diagnostic System Suitable for Transient Arc Plasma Diagnosis in Arc Ion Plating

In order to detect the arc plasma, an in-depth analysis on the relationship between the technology and film properties is conducted in this article. In addition, a fast scanning Langmuir probe diagnostic system suitable for transient plasma diagnosis is also designed, coupled with the design scheme and schematic diagram. Regard the integrated circuit of MAX038 as the core，and utilize FET to input high-fidelity operational amplifier OPA604 as well as high power amplifier PA93 to complete the intelligent saw-tooth wave sweep frequency power with the output amplitude of ± 100V, output frequency ranging from 10Hz to 10MHz as well as the adjustable frequency and duty cycle. Then test the output waveform and the results show that the power module can precisely generate triangle wave, saw-tooth wave, rectangular wave (including square wave) and sine wave signal.

Blob/Hole Generation in the Divertor Leg of the Large Helical Device

We have analyzed ion saturation current fluctuation measured by a fast scanning Langmuir probe (FSP) in edge region of the Large Helical Device (LHD). Positive and negative spikes of the ion saturation current were observed in the private region and on the divertor leg, respectively. It was found that the boundary position between these regions corresponds to the low-field side (LFS) edge of the divertor leg where the gradient of the ion saturation current profile was the maximum. Such a positional relationship resembles that near the separatrix in the LFS in tokamaks, where blobs and holes are generated. Statistical analysis indicates similar fluctuation characteristics among different magnetic devices.

Interpretation of the Dα emission from the high field side of Alcator C-Mod

Measurements from an inner wall mounted scanning Langmuir probe and modelling of Dα emission show a sharp fall-off of plasma density in the inboard private flux region. Dα emission is modelled using a one-dimensional space, two-dimensional velocity kinetic neutral code in order to track the changes in the density profile while the magnetic topology is changed from single to double null. A sharp density decay is observed to track with the location of the secondary separatrix. The data presented are consistent with a lower level of cross-field transport on field lines in the private flux region restricted to the high field side when compared to cross-field transport on field lines in the low field side scrape-off layer.

Effect of argon seeding on the negative ion yield of the Kamaboko III ion source

It has been previously reported that the addition of argon to a hydrogen plasma in an RF driven ion source can substantially increase (up to a factor 4) the extracted and accelerated negative ion (H −) current (W. Kraus et al., Development of large radio frequency negative-ion sources for nuclear fusion, Rev. Sci. Instrum. 73(2) (2002)). Realizing such an increase in the filamented arc discharge negative ion sources used for neutral beam injection systems would have significant benefits. Unfortunately the reported studies of argon addition to filamented sources have not shown a similar gain, but so far these have been carried out with arc powers and plasma densities far from those typical of the plasma in the negative ion sources used on neutral beam injectors (N. Nishiura et al., Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002); N. Curran et al ., The effect of the addition of noble gases on H- production in a dc filament discharge in hydrogen, Plasma Scources, Sci. Technol. 9 (2000)). The Kamaboko III ion source operates at the pressure and plasma density close to those anticipated in the ion source proposed for the ITER neutral beam injectors. Measurements have been made of the plasma density, electron temperature and the negative ion yield as a function of the argon seeding rate. The plasma parameters are determined with a fast spatially scanning Langmuir probe system. The effect on the H − yield is determined from the effect on the current extracted and accelerated from the source. Data will be presented for source filling pressures between 0.1 and 0.5 Pa of hydrogen, additions of argon from 0 to 30%, and a discharge power of 38 kW. All these data are collected in pure volume non-caesiated discharges. Some increase in the extracted H − yield is measured for small percentage additions of argon, (0–20%), but only at the highest H 2 pressure used, 0.5 Pa.

Spatial profiles of neutral, ion, and etch uniformity in a large-area high-density plasma reactor

Spatial profiles of neutral density, ion density, and etch rate have been measured in a large-area high-density plasma reactor. Blanket photoresist films on 200 mm wafers are etched by oxygen plasmas in a magnetized inductively coupled plasma reactor. Ion density and relative neutral density are measured by a scanning Langmuir probe and optical probe, respectively. Spatially resolved atomic oxygen density is then measured by optical emission spectroscopy and spatially resolved actinometry. The etch rate is calculated from film thickness measurements taken before and after the wafer is exposed to the oxygen plasma. Ion energy and wafer surface temperature were also measured. The mechanism linking neutral density, ion density, and etching rate is discussed.

Langmuir probe studies of a transformer-coupled plasma, aluminum etcher

Spatially resolved positive ion densities (ni+), electron densities (ne), electron temperatures (Te), plasma potentials (Vp), and floating potentials (Vf) were measured with a scanning Langmuir probe (PMT FastProbe) in Cl2 and BCl3/Cl2, inductively coupled plasmas (Lam Research Alliance, transformer-coupled plasma (TCP) metal etcher with a high-flow chamber). Time-resolved ion saturation current was measured during etching of Al/TiN metal stacks. Device damage during the metal stack etching was also studied. Positive ion densities increase nearly linearly with power for all of the gases. The maximum plasma density in the reactor is independent of pressure. The density profiles in the plane of the wafer are peaked above the center of the wafer at low pressure and off center at high pressure. Peaking off center is enhanced for smaller height-to-radius ratio chamber configurations, varied by changing the TCP window–wafer chuck gap. The ni+ uniformity across the wafer depends weakly on power, more strongly on feed gases and radio frequency bias, and most strongly on pressure and the TCP window–wafer gap. Within experimental error, Te is uniform across the reactor at most pressures with a slight fall off beyond the wafer edge. At the lowest pressure, Te dips slightly in the center of the reactor. Addition of 28% BCl3 to a Cl2 plasma causes a 20% decrease in Te due to a decrease in the effective ionization potential of the gas. A small, grounded aluminum electrode was inserted into the plasma to eliminate perturbations from the Langmuir probe on the plasma, caused by charging and discharging of the insulating walls of the reactor. Such perturbations make apparent Te, Vf, and Vp, values too high, and at least partly explain why Te’s measured with the Langmuir probe were higher than those obtained from optical emission spectroscopy.

A fast scanning Langmuir probe system for ASDEX-Upgrade divertor

A fast scanning Langmuir probe system (LPS) for ASDEX-Upgrade's divertor plasma investigations was designed, manufactured and operated. Profiles of ion saturation current density, J sat, electron temperature, T ed, electron density, N ed, floating potential, V fl, and Mach number have been recorded for ohmic (OH) and low/medium power neutral beam heated (NI) discharges. In these cases the probe can access both divertor legs, allowing comparison of plasma parameters in the two divertors and investigations of the private flux region. Strong divertor asymmetries, complex plasma flow patterns and changes in the power flow to the divertor targets, depending on ion grad B drift direction, density and divertor geometry, have been observed.

Characterization of diffused ECR plasma—Application to pulsed plasma ion implantation of nitrogen in titanium

A compact surface electron cyclotron resonance (ECR) plasma source, developed at Institut National de la Recherche Scientifique (INRS)-Énergie et Matériaux, has been used as a versatile surface treatment tool. The source has been operated with nitrogen for application to plasma assisted nitriding near room temperature. A negatively biased titanium target is immersed in the processing chamber (PC) plasma. The pulsed plasma diffuses from the ECR source across a transparent grid and fills the PC. The ions are accelerated and implanted into the Ti target. The electric field distribution in the PC, and therefore the uniformity of the implantation, was controlled using a highly transparent grounded grid. For a typical operating pressure of about 0.1–0.2 Pa, and with ~300W of microwave power coupled to the plasma source, a flat density profile with n c = 2.5 × 10 15 m −3 (over a diameter > 0.07 m) was measured by a scanning Langmuir probe in the PC. The electron temperature was 6.5 eV, and it also had a flat profile. In these preliminary experiments, ion nitriding of titanium samples was achieved with 30 keV nitrogen ion implantation. The hardness of the Ti target surface changed from 3.4 GPa to 5.8 GPa following the implantation. The depth profile of the implanted ions, measured by the X-ray photoelectron spectroscopy technique, combined with the simulation results from the TRIM code suggests that ion-neutral collisions in the PC were not frequent enough to alter the ion energy distribution significantly.

Generation and behavior of particulates in a radio frequency excited CH4 plasma

The behavior of particulates in a capacitively coupled, 13.56 MHz radio frequency (rf) excited CH4 plasma was investigated by time-resolved laser light scattering (LLS). The ballistic motion of the large particulates causes the LLS signal to fluctuate with peaks at the plasma sheath boundary. In an Ar plasma with a substrate which had been coated by diamondlike carbon film in the CH4 plasma process, a quasistatic particulate cloud formed and the LLS signal, the self-bias voltage, and the optical emission oscillated slowly, all with the same period. Also, the shape of the particulate cloud changed periodically. The time variation of the particulate’s mean size in the cloud was obtained by the angular dissymmetry measurement of scattered light intensities. The behavior of the particulates seemed to depend on the particulate size. The time-resolved plasma potential obtained from the heated, fast scanning Langmuir probe indicates that the formation of particulates influences the plasma state. The optical microscope and scanning electron microscopy were used to observe the morphology and the size of the particulates. Transmission electron microscopy and electron probe x-ray microanalysis studies indicate that they have an armorphous structure. Together with the observation of the collected particulates, the results of LLS studies showed that two different groups of particulates are generated in CH4 plasma. The optical emission spectra of the particulate-contaminated Ar plasma and the normal clean Ar plasma were obtained and compared. The difference between the two spectra indicates that the mean energy of the secondary electrons which come out of the powered electrode changes as a result of the interaction with the particulate cloud formed around the plasma-sheath boundary. The distribution of the particulate cloud was highly dependant on the rf power. The ion drag force due to the ambipolar diffusion flux from the central high plasma density region to the wall is thought to play an important role in particulate cloud distribution. A modified turn-off process is suggested to prevent particulates from falling on the wafer surface.

Investigation of the wake due to a large probe using a spatially scanning Langmuir probe

This paper describes direct experimental investigation of the region of plasma perturbed by a probe. Two adjacent probes have been mounted on the DITE tokamak. The large “disturber” probe is a rectangular graphite plate that creates the main particle flow pattern and can be biased with respect to the torus. The smaller “search” Langmuir probe is located 0.5 m away on the same magnetic field line. It can be swept back and forth mechanically through the shadow of the main probe many times during each discharge. Through measurements of ion saturation current at many distinct radii, a complete map of the perturbed region has been built up and transformed into radial and poloidal coordinates. A numerical solution of the two-dimensional diffusion equations is compared with experimental data in the limiter shadow. Two zones of substantially different interconnection length are included in the model. The assumption of equal radial and poloidal cross-field diffusion coefficients ( D ⊥ = 0.18 m 2 s −1) is found to give a good fit with the data.