Cw Plasma Research Articles

Correlation analysis for energy losses, waiting times and durations of type I edge-localized modes in the Joint European Torus

Several important ELM control techniques are in large part motivated by the empirically observed inverse relationship between average ELM energy loss and ELM frequency in a plasma. However, to ensure a reliable effect on the energy released by the ELMs, it is important that this relation is verified for individual ELM events. Therefore, in this work the relation between ELM energy loss and waiting time is investigated for individual ELMs in a set of ITER-like wall plasmas in JET. A comparison is made with the results from a set of carbon-wall and nitrogen-seeded ITER-like wall JET plasmas. It is found that the correlation between W ELM and for individual ELMs varies from strongly positive to zero. Furthermore, the effect of the extended collapse phase often accompanying ELMs from unseeded JET ILW plasmas and referred to as the slow transport event (STE) is studied on the distribution of ELM durations, and on the correlation between W ELM and . A high correlation between W ELM and , comparable to CW plasmas is only found in nitrogen-seeded ILW plasmas. Finally, a regression analysis is performed using plasma engineering parameters as predictors for determining the region of the plasma operational space with a high correlation between W ELM and .

Potential CCP Pulsed Plasma Application at Beol AIO Etch Process

As COMS is being scaled down to 28nm node and beyond, CW plasmas etching processes are reaching their limits to satisfy the multiply stringent requirements, such as better uniformity, higher selectivity, less plasma induced damage, tighter critical dimension and profile control. The RF pulsed plasma is one of promising knobs to increase the flexibility of plasma processing by enlarging the range of operating conditions. Pulsed frequency and pulsed duty cycle are two main parameters characterizing the RF pulse. As the multiple-frequency RF powers have been introduced for the better control plasma flux and energy in CCP system, the effect of different RF power pulsed varies, bias only, source only and synchronized pulsed plasmas also shows quite different performances. In this work, MHM based all-in-one (AIO) etch processes were performed in one commercial CCP etcher, and the effects of pulsed plasmas on the etch process were analyzed. The different pulsed mode, pulsed duty cycle, pulsed frequency were tested on the AIO etching process in BEOL copper interconnects system. We compared the bias only, source only and synchronized pulsed plasma performances, checked the duty cycle and pulsed frequency effects. Compared with conventional CW plasmas, the performances of AIO etching process could be significantly improved via pulsed plasma, such as depth loading, chamfer profile control and higher selectivity to metal hard mask.

Development of voids in pulsed and CW- driven reactive plasmas with large nanoparticle density

This contribution deals with the nanoparticle distribution inside of plasma with large nanoparticle density. In particular, the formation of voids i.e. dust-free regions in pulsed and CW discharge regime is analyzed. The discharge was ignited in a mixture of argon and acetylene in the conditions favorable for nanoparticles’ formation and growth. The temporal evolution of the spatial distribution of nanoparticles during their growth is studied experimentally by means of a laser light scattering technique. The experimental results show that the void expands much faster in the case of pulsed plasma than in the CW plasma operated for the same gas mixture and at the same (average) input power. In the CW plasma, we observed a rapid expansion of the void after some tens of minutes, whereas the corresponding process in the pulsed plasma occurred after some tens of seconds.

Development of a high-speed impedance measurement system for dual-frequency capacitive-coupled pulsed-plasma.

A high-speed impedance measurement system was developed, which enables the measurement of various characteristics of CW and pulsed plasmas with time resolution of less than a microsecond. For this system, a voltage and current sensor is implemented in a printed circuit board to sense the radio frequency signals. A digital board, which has a high-speed analog to digital converter and a field-programmable gate-array, is used to calculate the impedance of the signal. The final output of impedance is measured and stored with a maximum speed of 3 Msps. This sensor system was tested in a pulsed-plasma by applying it to the point between the matching box and the plasma chamber. The experimental equipment was constructed connecting the matching box, a 13.56 MHz generator, a 2 MHz generator that produced pulsed power, and a pulse-signal generator. From the temporal behavior of the measured impedance, we were able to determine the time intervals of transient states, especially of the initial active state. This information can be used to set the pulse frequency and duty for plasma processing.

Summary of Research and Development Activities for the Production of the Divertor Target Elements of Wendelstein 7-X

The realization of the high heat flux divertor of the stellarator Wendelstein 7-X requires the production of 890 target elements. Since the divertor has to operate for 30 min with an input continuous wave (cw) plasma power of 10 MW, the target elements need to be actively water cooled. A target element is made of a CuCrZr copper alloy heat sink armored with carbon fiber reinforced carbon (CFC) NB31 tiles. It is designed to remove a stationary heat flux of 10 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on its main area and 5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the end adjacent to the pumping gap. Research and development activities proved to be essential for the preparation and the support of the on-going serial fabrication. The most critical issue is the stable production of the bonding between the CFC tiles and the heat sink as the divertor surface consists of ~ 16 000 tiles. The fast unexpected failure of one tile may reduce the whole divertor performance and, consequently, restrict the stationary machine operation. Two main research and development phases allow the design improvements of the target elements compatible with industrial and economic constrains of the series production as well as the development of quality controls such as a visual inspection system able to easily detect and fast record cracks at the interface between CFC tiles and heat sink.

Hybrid simulation of a dc-enhanced radio-frequency capacitive discharge in hydrogen

A PIC-MCC/fluid hybrid model was employed to study a parallel-plate capacitively coupled radio-frequency discharge in hydrogen, under the application of a dc bias voltage. When a negative dc voltage was applied to one of the electrodes of a continuous wave (cw) plasma, a ‘beam’ of secondary electrons was formed that struck the substrate counter-electrode at nearly normal incidence. The energy distribution of the electrons striking the substrate extended all the way to VRF + |Vdc|, the sum of the peak RF voltage and the absolute value of the applied dc bias. Such directional, energetic electrons may be useful for ameliorating charging damage in etching of high aspect ratio nano-features. The vibrational distribution function of molecular hydrogen was calculated self-consistently, and was found to have a characteristic plateau for intermediate values of the vibrational quantum number, v. When a positive dc bias voltage was applied synchronously during a specified time window in the afterglow of a pulsed plasma, the ion energy distributions (IEDs) of positive ions acquired an extra peak at an energy equivalent of the applied dc voltage. The electron energy distribution function was slightly and temporarily heated during the application of the dc bias pulse. The calculated IEDs of and ions in a cw plasma without dc bias were found to be in good agreement with published experimental data.

Analysis of pulsed high-density HBr and Cl2 plasmas: Impact of the pulsing parameters on the radical densities

The dynamic of charged particles in pulsed plasma is relatively well known since the 1990s. In contrast, works reporting on the impact of the plasma modulation frequency and duty cycle on the radicals’ densities are scarce. In this work, we analyze the impact of these modulation parameters on the radicals’ composition in Cl2 and HBr plasmas. The radicals’ densities are measured by broad-band UV and vacuum-ultraviolet (VUV) absorption spectroscopy and modulated-beam mass spectrometry. We show that pulsing the rf power allows controlling the plasma chemistry and gives access to the plasma conditions that cannot be reached in continuous wave plasmas. In particular, we show that above 500 Hz, the pulsing frequency has no influence on the plasma chemistry, whereas in contrast the duty cycle is an excellent knob to control the fragmentation of the parent gas, thus the chemical reactivity of the discharge. At low duty cycle, a reduced gas fragmentation combined with a large ion flux leads to new etching conditions, compared to cw plasmas and the expected consequences on pulsed-etching processes are discussed.

Plasma deposition of fluorocarbon thin films using pulsed /continuous and downstream radio frequency plasmas

Fluorocarbon (FC) films deposited in continuous wave (cw) and pulsed difluoromethane radio frequency (r.f.) plasmas were characterized using Fourier transform infrared spectroscopy and atomic force microscopy. The effects of varying r.f. power, cw/pulsed discharge mode, and the distance of the substrate from the coil on the deposition rate, film structure, and surface roughness were investigated. These cw and pulsed deposition systems were characterized in-situ by means of optical emission spectroscopy. Emission intensities of H α, H β, H 2 and carbon-containing species in the coil region and downstream plasmas as a function of plasma parameters were measured. The hydrogen excitation temperature obtained from the relative emission intensities of H α and H β lines shows a clear dependence on the r.f. power and the substrate position. Correlations between film properties, gas-phase plasma diagnostic data, and film growth processes were discussed. Experimental results indicate that the film growth within the coil region in cw plasmas is controlled by the synergistic effect between energetic ions and low-energy species. The film growth in pulsed and downstream plasmas is controlled by the growth of coalesced nuclei via surface diffusion of adsorbed species, which results in the deposition of FC films with relatively rough surfaces.

Plasma parameters measurements by means of Langmuir probe

The Langmuir probe (LP) diagnostics is a powerful method for the evaluation of the plasma resistivity curve (I-V curve) and the characterization of the following plasma parameters: electron temperature, electron density, ion density, and plasma potential. In presence of a stable plasma it is possible to extrapolate the electron energy distribution function of the plasma electron population. Because of the long acquisition time (in the order of hundreds of msec or more), this method is suitable for cw plasmas in thermal equilibrium for which the physical properties vary in a time scale longer than the acquisition time. At INFN-LNS the LP diagnostics has been used in order to characterize the TRasco Intense Proton Source plasma, and the low temperature – high density plasmas of a plasma reactor designed for complex molecules dissociation. In the first case, it has been possible to evaluate the plasma properties for different magnetic field profiles and for several operating conditions. In the second case, the LP has permitted to characterize the plasma properties of the plasma reactor at different microwave powers and gas pressures, with the aim to find the optimal experimental conditions in terms of rate of molecules dissociation and of plasma stability and reliability. These series of measurements are here reported, together with measurements of the plasma reactor parameters. Finally, some considerations about the possibility to extend the LP diagnostics to the non-equilibrium plasmas in pulsed mode, as the plasmas obtained by means of laser ablation of solid targets, are given; the design of a possible experimental set-up is outlined.

BETSI, a new test bench for ion sources optimization at CEA SACLAY

In the framework of several International HPPA projects (such as IFMIF, IPHI, and Spiral2) the CEA handles the design and the developments of several electron cyclotron resonance (ECR) ion sources. For the IFMIF EVEDA demonstrator, a 140 mA cw extracted deuteron beam will be required for high yield of neutron production. For radioactive ion production in the Spiral2 project, several milliamperes of deuterons will be delivered with a permanent magnet source. The optimization of the beam quality at the entrance of the radio frequency quadropole (RFQ) accelerator system triggered the need of a new test bench for ion source optimization and beam qualification. The BETSI ion source test bench will operate up to 50 kV and ignite cw or pulsed hydrogen plasma with a 2.45 GHz magnetron. Great care has already been taken to design electrostatic optics of the extraction system to minimize the emittance growth. Plasma diagnostics will be inserted in the source chamber and several beam diagnostics (emittance and current measurements, beam species analysis) will also be implemented on the low energy beam line transport (LEBT). These diagnostics allow the simultaneous analysis of the beam quality with the plasma parameters of the source. Regional funding request will also be needed to improve the LEBT for space charge compensation measurements. The design of the present and upgraded test bench will be reported as well as the first extracted beam analysis.

Immobilization/hybridization of amino-modified DNA on plasma-polymerized allyl chloride

The present work describes the fabrication and characterization of chloride-derivatized polymer coatings prepared by continuous wave (cw) plasma polymerization as adhesion layers in DNA immobilization/hybridization. The stability of plasma-polymerized allyl chloride (ppAC) in H 2O was characterized by variation of the thickness of polymer films and its wettability was examined by water contact angle technique. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to study polymer matrix properties and oligonucleotide/DNA binding interaction. With the same carrier gas rate and process pressure, plasma polymers deposited at different input powers show various comparable immobilization properties; nevertheless, low input power plasma-polymerized films gives a lower sensitivity toward DNA binding than that from high input power plasma-deposited films. The following DNA immobilization on chloride-functionalized surfaces was found dependence on the macromolecular architecture of the plasma films. The hybridization between probe DNA and total mismatch target DNA shows no non-specific adsorption between target and ppAC.

Characterisation of plasma polymers by thermoluminescence

Thin plasma polymer films were deposited using the pulsed plasma mode. These plasma polymers should possess a more regular structure than those produced by the conventional continuous-wave (cw) mode, because of lower monomer fragmentation caused by the plasma pulses and the chemical chain propagation during the plasmaless (free!) periods. The thermoluminescence method was applied to functional groups carrying plasma polymer layers which are used in medical technology. Examples are formation of biocompatible, biosensoric and bioactive coatings or in metal polymer composites such as adhesion-promoting interlayers. In addition to the use of the conventional X-ray Photoelectron Spectroscopy for thin film characterization, the new method of thermoluminescence was applied to characterize undesired defects and structural specifics produced in the polymer films by pp or cw plasma mode. The main areas of focus were oxygen-containing groups produced by post-plasma oxygen introduction via auto-oxidation, oxidation of implemented unsaturations and trapped radical sites known as typical irregular structures in plasma polymers.

Comparison of pulsed and downstream deposition of fluorocarbon materials from C3F8 and c-C4F8 plasmas

Materials deposited in continuous wave (cw) and pulsed low-pressure octafluoropropane (C3F8) and octafluorocyclobutane (c-C4F8) plasmas were characterized using Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy, static contact angle measurements, spectroscopic ellipsometry, and scanning electron microscopy (SEM). Fluorocarbon (FC) materials deposited in pulsed plasmas were less crosslinked than those deposited in cw plasmas with equivalent input powers. Within each system, higher F/C ratio materials were deposited by lowering the plasma input power/duty cycle. Using downstream depositions had a similar effect on film composition, but also resulted in decreased deposition rates. SEM analysis showed that decreases in the flexibility of the fluorocarbon films were correlated with increases in the percent of crosslinking. Additionally, the smoothness of the film surfaces suggests that polymerization processes occur on the substrate surface. Overall, films deposited in C4F8 plasmas were more crosslinked and less flexible than those deposited in C3F8 plasmas under the same conditions. The composition of the deposited FC films is correlated to the nature and relative concentrations of nascent ions previously measured in these plasma systems using plasma ion mass spectrometry.

The effect of topographical local charging on the etching of deep-submicron structures in SiO2 as a function of aspect ratio

Physical and electrical influences on plasma etching on the inside of a microtrench in SiO2 were numerically investigated using Monte Carlo simulation of ions and electrons with the aid of surface charge continuity and Poisson’s equation. When the aspect ratio is greater than seven, the bottom is charged up to a potential sufficient to prevent the influence of all the incident ions, with a realistic initial energy of 300 eV for SiO2 etching within the period required for monolayer stripping, resulting in etch stop. The cause of etch stop is purely the result of the electrical local charging due to the topography of the trench, and of the initial conditions for incident charged particles. The etch stop caused by a cw plasma will be disorganized or prevented within a short time by the aid of ion–ion plasma in an afterglow phase.

Dominance of Cl2+ or Cl+ Ions in Time-Modulated Inductively Coupled Cl2 Plasma Investigated with Laser-Induced Fluorescence Technique and Probe Measurements

Relative densities of chlorine molecular ions Cl2+ and metastable chlorine atomic ions Cl+* were measured by a laser-induced fluorescence technique in cw and time-modulated inductively coupled chlorine plasma. In the discharge-off period of the time-modulated plasma, a considerable amount of Cl2+ persisted. Under the modulation frequency of 100 kHz, the decrease of the Cl2+ density in the discharge-off period was less than 30% and the decay time constant of Cl2+ density was longer than that of Cl+*. In order to investigate the dominance between Cl2+ and chlorine atomic ions Cl+, the relative densities of Cl2+ and Cl+* were calibrated as absolute densities along with the Langmuir probe measurements. The dominant ions in the cw plasma were Cl+, however, the Cl2+ density became comparable to the Cl+ density in the time-modulated plasma.

Pulsed and continuous wave plasma deposition of amorphous, hydrogenated silicon carbide from SiH4/CH4 plasmas

Continuous wave (cw) and equivalently powered, pulsed radio frequency plasmas are used to deposit a-Si1−xCx:H films. Films produced from SiH4/CH4 and SiH4/CH4/H2 gas mixtures were analyzed with Fourier-transform infrared, x-ray photoelectron spectroscopy, scanning electron microscopy, and profilometery. Gas-phase plasma species were identified using optical emission spectroscopy. The effects of biasing (±1000 V) and grounding the substrates, pulse peak power, pulse on time and off time, and duty cycle on film composition were examined. Films deposited with cw plasmas show an increase in hydrogen incorporation compared to films deposited in the pulsed systems. In the pulsed plasmas, deposition rates depend on both the on time and off time of the plasma pulse cycle, while grounding the substrate causes a significant reduction in oxidation rates for films deposited under all conditions.

Effect of the Nanoparticles on the Structure and Crystallization of Amorphous Silicon Thin Films Produced by rf Glow Discharge

Thin films of nanostructured silicon (ns-Si:H) were deposited by plasma-enhanced chemical vapor deposition in the presence of silicon nanoparticles at 100 °C substrate temperature using a silane and hydrogen gas mixture under continuous wave (cw) plasma conditions. The nanostructure of the films has been demonstrated by diverse ways: transmission electron microscopy, Raman spectroscopy, and x-ray diffraction, which have shown the presence of ordered silicon clusters (1–2 nm) embedded in an amorphous silicon matrix. Because of the presence of these ordered domains, the films crystallize faster than standard hydrogenated amorphous silicon samples, as evidenced by electrical measurements during the thermal annealing.

Spatially averaged (global) model of time modulated high density argon plasmas

The behavior of argon plasmas driven by time modulated power in ‘‘high density’’ plasma reactors is investigated using a global model. The time evolution of the electron temperature and the plasma density is calculated by solving the particle and energy balance equations. In the first stage of power application during the ‘‘on’’ time, the electron temperature rapidly increases above the steady state value. In this region, charged particles accumulate in the plasma due to the relatively higher power applied than for the continuous wave (cw) case. In the first stage of the ‘‘off’’ time, the electron temperature drops quickly, yielding a smaller particle loss (Bohm) velocity. These effects give rise to higher time-average plasma densities than for the cw plasma driven by the same average power. The highest average plasma density obtained was more than twice the density of the cw plasma for a duty ratio of 25%. Even higher plasma densities were obtained for shorter duty ratios. The possibility of controlling chemical reactions in the plasma by changing the modulation period is also shown.

Comparison of oxides prepared in CW and pulse microwave plasma

The electrical properties of oxides prepared in continuous and pulse microwave magnetoactive plasma on silicon are analysed. It is shown that better oxides can be prepared in the continuous plasma. The properties of these oxides, created at temperatures lower than 500°C, are comparable with those of oxides prepared by thermal oxidation at temperatures of about 1100°C. [Russian text ignored].

Ignition and maintenance of a cw plasma in atmospheric-pressure air with CO2 laser radiation

The focused output from a high-power cw CO2 laser is used to study the maintenance of a plasma in atmospheric-pressure air. The plasma is initiated externally by an electric spark and maintained by the absorbed laser beam energy. Ignition has also been observed when the laser was focused on a solid target. The maintenance threshold is examined as a function of beam diameter; for beams smaller than 1 mm the threshold power is ∼2 kW. When focused with lenses of 14-cm focal length and shorter, the plasma ball is stationary, situated slightly ahead of the focal point. For this focusing system, the transmission through the plasma was a function of incident power and increased with decreasing laser power. The value of the measured plasma absorption coefficient corresponds to an electron temperature of 14 000°K. When the laser beam was focused with 25-cm—focal-length mirrors or larger, the plasma was not stationary but propagated up the beam at a velocity of ∼10 m/sec toward the laser before extinguishing. The maintenance threshold of the plasma is discussed in terms of the loss channels from the plasma.