ECR Plasma Research Articles

Studies on plasma production in a large volume system using multiple compact ECR plasma sources

This paper presents a scheme for large volume plasma production using multiple highly portable compact ECR plasma sources (CEPS) (Ganguli et al 2016 Plasma Source Sci. Technol. 25 025026). The large volume plasma system (LVPS) described in the paper is a scalable, cylindrical vessel of diameter ≈1 m, consisting of source and spacer sections with multiple CEPS mounted symmetrically on the periphery of the source sections. Scaling is achieved by altering the number of source sections/the number of sources in a source section or changing the number of spacer sections for adjusting the spacing between the source sections. A series of plasma characterization experiments using argon gas were conducted on the LVPS under different configurations of CEPS, source and spacer sections, for an operating pressure in the range 0.5–20 mTorr, and a microwave power level in the range 400–500 W per source. Using Langmuir probes (LP), it was possible to show that the plasma density (~1 − 2 × 1011 cm−3) remains fairly uniform inside the system and decreases marginally close to the chamber wall, and this uniformity increases with an increase in the number of sources. It was seen that a warm electron population (60–80 eV) is always present and is about 0.1% of the bulk plasma density. The mechanism of plasma production is discussed in light of the results obtained for a single CEPS (Ganguli et al 2016 Plasma Source Sci. Technol. 25 025026).

Asymmetry of velocity distribution function and inhomogeneity-induced flow associated with neutral depletion structure in an ECR plasma

A neutral depletion structure with strong inhomogeneity in the radial direction has been observed in an electron cyclotron resonance plasma. We have measured the velocity distribution function of neutrals with a high resolution laser-induced fluorescence system and examined the relationship between asymmetry of distribution function and flow induced by inhomogeneity. It has been revealed that the third order moment of distribution function, that is, skewness, is proportional to the inhomogeneity-induced flow, and a simple relation between the skewness and the normalized flow velocity has been obtained and confirmed in the experiment.

A new launching scheme for ECR plasma based on two‐waveguides‐array

ABSTRACTThis article describes the design, the simulations, and the tests of a microwave launcher for alternative plasma‐heating schemes in the Flexible Plasma Trap (FPT). A waveguide‐phased array has been designed to tilt the wave injection angle: this will allow modal conversion to generate Electrostatic Bernstein Waves and then to obtain overdense plasmas. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2629–2634, 2016

Improving the Performances of CdTe Gamma Ray Detectors by H2/Ar ECR Plasma Processing

H2/Ar electron cyclotron resonance plasma processing of the high-resistivity (111) CdTe crystals was studied to find its effect on the leakage current and the gamma ray detection properties of the detectors. All the crystals were chemically etched in a bromine–methanol (BM) solution prior to the plasma processing to obtain a smooth surface. A remarkable reduction in both the surface and the bulk leakage current was observed in the plasma processed detectors when compared with the unprocessed one. The gamma detection property of the detectors, both the spectral resolution and the peak shape, was also significantly improved. X-ray photoelectron spectroscopy confirmed that an excess Te-rich surface layer was present on the BM etched crystal, however, such layer was removed, and the surface atomic ratio was modified after the plasma processing. The improvement in the detector performance could be related to the change in the surface chemical states of the crystals. Hence, this could be a promising process for improving the detectors performance and their stabilities.

Diagnosing the Fine Structure of Electron Energy Within the ECRIT Ion Source**supported by National Natural Science Foundation of China (No. 11475137)

The ion source of the electron cyclotron resonance ion thruster (ECRIT) extracts ions from its ECR plasma to generate thrust, and has the property of low gas consumption (2 sccm, standard-state cubic centimeter per minute) and high durability. Due to the indispensable effects of the primary electron in gas discharge, it is important to experimentally clarify the electron energy structure within the ion source of the ECRIT through analyzing the electron energy distribution function (EEDF) of the plasma inside the thruster. In this article the Langmuir probe diagnosing method was used to diagnose the EEDF, from which the effective electron temperature, plasma density and the electron energy probability function (EEPF) were deduced. The experimental results show that the magnetic field influences the curves of EEDF and EEPF and make the effective plasma parameter nonuniform. The diagnosed electron temperature and density from sample points increased from 4 eV/2×1016 m−3 to 10 eV/4×1016 m−3 with increasing distances from both the axis and the screen grid of the ion source. Electron temperature and density peaking near the wall coincided with the discharge process. However, a double Maxwellian electron distribution was unexpectedly observed at the position near the axis of the ion source and about 30 mm from the screen grid. Besides, the double Maxwellian electron distribution was more likely to emerge at high power and a low gas flow rate. These phenomena were believed to relate to the arrangements of the gas inlets and the magnetic field where the double Maxwellian electron distribution exits. The results of this research may enhance the understanding of the plasma generation process in the ion source of this type and help to improve its performance.

Development of lithium and tungsten limiters for test on T-10 tokamak at high heat load condition

Application of a complex of powerful (up to 3 MW) ECR plasma heating in T-10 tokamak is pulled down with a problem of the strong plasma pollution at power input more than 2 MW. For the solution of these problems the new W and Li limiters is developed and prepared to implementation. As it is supposed, application of W as a plasma facing material will allow excluding carbon influx into vacuum chamber. An additional Li limiter arranged in a shadow of W one will be used as a Li source for plasma periphery cooling due to a reradiation on Li that will lead to decrease in power deposition on W limiters. Parameters and design of limiters are presented. Plasma facing surface of a limiter is made of capillary-porous system (CPS) with Li. Porous matrix of CPS (W felt) provides stability of liquid Li surface under MHD force effect and an opportunity of its constant renewal due to capillary forces. The necessary Li flux from a Li limiter surface is estimated for maintenance of normal operation mode of W limiters at ECRH power of 3 MW during 400 ms. It is shown, that upgrade of limiters in tokamak T-10 will allow providing of ECR plasma heating with power up to 3 MW at reasonable Li flux.

Presentation of the results for deuterium retention and thermal release in a new type of low activation ferritic-martensitic steel EUROFER

This work presents the results of the investigation into deuterium retention and thermal release in low activation ferritic-martensitic steel EUROFER and its main components, Fe and Cr, using ion implantation, nuclear reaction depth profiling and the thermal desorption spectrometry technique. The samples were exposed to deuterium ECR plasma of 6.5*1024 Dm-2 fluence. Our NRA results show that, at 300 K, most of deuterium is trapped in the near-surface region. A part of the implanted deuterium diffuses into the bulk and is trapped by defects beyond the implantation range. The results of the TDS measurement of the Cr sample confirm the formation of an ordered Cr-hydride phase.

Investigation of bilayer HfN<sub>x</sub> gate insulator utilizing ECR plasma sputtering

Investigation of bilayer HfN<sub>x</sub> gate insulator utilizing ECR plasma sputtering

Injection of auxiliary electrons for increasing the plasma density in highly charged and high intensity ion sources

Different electron guns based on cold- or hot-cathode technologies have been developed since 2009 at INFN for operating within ECR plasma chambers as sources of auxiliary electrons, with the aim of boosting the source performances by means of a higher plasma lifetime and density. Their application to microwave discharge ion sources, where plasma is not confined, has required an improvement of the gun design, in order to "screen" the cathode from the plasma particles. Experimental tests carried out on a plasma reactor show a boost of the plasma density, ranging from 10% to 90% when the electron guns are used, as explained by plasma diffusion models.

First results of the 2.45 GHz Oshima electron cyclotron resonance ion source.

A new electron cyclotron resonance ion source has been constructed at Oshima College with a 2.45 GHz magnetron microwave source and permanent magnets employed as the main components. In addition, a solid-state power amplifier with a frequency range of 2.5-6.0 GHz was installed to study two-frequency plasma heating. Three solenoid coils were set up for adjusting the axial magnetic fields. Argon plasma generation and ion beam production have been conducted during the first year of operation. Ion current densities in the ECR plasma were measured using a biased disk. For 2.45 and 4.65 GHz two-frequency plasma heating, the ion density was approximately 1.5 times higher than that of 2.45 GHz single-frequency heating.

Microwave frequency sweep interferometer for plasma density measurements in ECR ion sources: Design and preliminary results.

The Electron Cyclotron Resonance Ion Sources (ECRISs) development is strictly related to the availability of new diagnostic tools, as the existing ones are not adequate to such compact machines and to their plasma characteristics. Microwave interferometry is a non-invasive method for plasma diagnostics and represents the best candidate for plasma density measurement in hostile environment. Interferometry in ECRISs is a challenging task mainly due to their compact size. The typical density of ECR plasmas is in the range 10(11)-10(13) cm(-3) and it needs a probing beam wavelength of the order of few centimetres, comparable to the chamber radius. The paper describes the design of a microwave interferometer developed at the LNS-INFN laboratories based on the so-called "frequency sweep" method to filter out the multipath contribution in the detected signals. The measurement technique and the preliminary results (calibration) obtained during the experimental tests will be presented.

High intensity proton injector for facility of antiproton and ion research.

The high current ion source with the low energy beam transport (LEBT) will serve as injector into the proton LINAC to provide primary proton beam for the production of antiprotons. The pulsed ion source developed and built in CEA/Saclay operates with a frequency of 2.45 GHz based on ECR plasma production with two coils with 87.5 mT magnetic field necessary for the electron cyclotron resonance. The compact LEBT consists of two solenoids with a maximum magnetic field of 500 mT including two integrated magnetic steerers to adjust the horizontal and vertical beam positions. The total length of the compact LEBT is 2.3 m and was made as short as possible to reduced emittance growth along the beam line. To measure ion beam intensity behind the pentode extraction system, between solenoids and at the end of the beam line, two current transformers and a Faraday cup are installed. To get information about the beam quality and position, the diagnostic chamber with different equipment will be installed between the two solenoids. This article reports the current status of the proton injector for the facility of antiproton and ion research.

HfO2/Al2O3/InGaAs MOSCAP Structures and InGaAs Plasma Nitridation Elaborated in a 300mm Pilot Line

The achievement of a good high-k oxide/InGaAs interface quality is a key challenge to obtain high performance MOSFET. Associating Al2O3 and HfO2 in a bilayer oxide is interesting to benefit from the good interface quality obtained with Al2O3 and the better electrostatic control achievable with HfO2. Furthermore, some recent work evidenced the reduction of the interface trap density (Dit) by using a plasma nitridation process [1]. For the first time, we evaluated the passivation properties of a Al2O3/HfO2 bilayer and a nitridation treatment on industrial equipment compatible with 300 mm Si wafers. First, we investigated the number of Al2O3 ALD cycles required to obtain a good interface. Second, we evaluated the effect of a nitridation treatment on the Al2O3/InGaAs interface properties performed on a 300 mm capacitive plasma tool.MOSCAP structures were fabricated on a 27 nm thick In0.53Ga0.47As layer grown on InP substrates. The samples were cleaned in a NH4OH solution (4%) for 1 min at room temperature and rinsed in deionized water. Then, Al2O3 and HfO2 films were deposited in a ALD chamber at 300°C with trimethylaluminium (TMA), Hafnium tetrachloride (HfCl4) and H2O as precursors. The ALD cycle numbers for Al2O3 were 0, 3, 5, 8 and 10 and 32 for HfO2. A post-deposition annealing was carried out at 370°C for 30 min in N2 ambient. For the nitridation study, one InGaAs sample was directly treated with a NH3 plasma for 120 s at 50W and two samples were treated with a NH3 plasma or a N2 plasma for 120 s at 50W after the deposition of Al2O3 (10 ALD cycles) to prevent damages on the III-V layer. After the plasma treatment, Al2O3 was again deposited to reach a 8 nm-thick layer for electrical characterization. Ni/Au gate electrode was deposited through a shadow mask by e-beam evaporation.The sample without Al2O3 (Fig1.a) presents distorted profiles, especially at 10 kHz, sign of a poor InGaAs/HfO2 interface quality. Depositing three Al2O3 cycles (fig.1.b) allows a limited improvement of the characteristics. The dispersion in accumulation of the 10 and 30 kHz curves is reduced but no clear accumulation plateau can be seen. From 5 to 10 cycles, samples (c-e), the situation is significantly improved with a clear accumulation regime. The interface trap densities (Dit) were estimated using the conductance method [2] and were found equal to ~11, 6.1 and 5.5 ×1012 cm-2eV-1 for the 5, 8 and 10 cycles samples respectively. A clear improvement is obtained from 5 to 8 cycles while only a slight improvement is obtained from 8 to 10 cycles. Compare to the Al2O3/InGaAs capacitor (fig.1.f), the best compromise between low Dit and high level capacitance appears to be 8 Al2O3 cycles. Wide frequency dispersion in accumulation can be seen after NH3 treatment without the Al2O3 protection (fig.2c) due to border traps in the dielectric. The plasma treatment prior to the dielectric deposition induces defects at the beginning of the nucleation. This dispersion is reduced when the treatment is implemented through an Al2O3 layer (fig.2b and d). N2 plasma reduces the capacitance whereas NH3 plasma does not deteriorate the C-V characteristics and keeps the same capacitance as the sample without nitridation. According to XPS analysis, N is clearly detected after plasma treatment. The N1s peak can be divided into two components, N1s A at 399.0 eV and N1s B at 397.9 eV. The N1s A component has been attributed to N-O bonds, and N1s B to N-Ga bonds [3]. Residual As oxides are reduced with NH3 (fig. 3e and f) but no As nitride is evidenced. After plasma treatment, the Ga3+ peak increases significantly (fig.3c and d), because of N incorporation at the interface. The Ga3+ component can be attributed to Ga-O and Ga-N bonds resulting in an oxynitride formation [1].In conclusion, Al2O3/HfO2 bilayer was optimized on a 300 mm equipment to achieve low Dit and high level capacitance (1.75 µF/cm²). NH3 nitridation process performed on a 300 mm capacitive plasma tool integrates nitrogen at the InGaAs surface without deterioration of the C-V characteristics. Low Dit is estimated at 2×1012 cm-2eV-1 at room temperature after NH3 plasma which is close to the value shown in [1] obtained with an ECR plasma. Measurements at lower temperature will further explain the Ditenergy distribution. [1] T. Hoshii, J. Appl. Phys., vol. 112, no. 7, p. 073702, 2012.[2] R. Engel-Herbert, Appl. Phys. Lett., vol. 97, no. 6, p. 062905, 2010.[3] T. S. Lay, J. Vac. Sci. Technol. B Microelectron. Nanom. Struct., vol. 22, no. 3, p. 1491, 2004. Figure 1

(Invited) Plasma Etching Technology Challenges for Future CMOS Fabrication Based on Microwave ECR Plasma

As dimensions of semiconductor devices shrink further into the nanometer range, transistor performance is improved not only by scaling and adopting boosting technology, but also by adopting new channel material and structure. It is also required to reduce power consumption for mobile devices. In MOS logic devices, 3D-trasistor (FinFET) was adopted to improve switching property. When fabricating, more accurate and higher aspect ratio patterning is required, e.g. fin width reaches 3.7 nm at the 5nm node, where nanowire FET and Ge/III-V channel materials are other candidates [1]. Thus, required accuracy will reach atomic level in the etching process. To realize nm scale etching, new etching process functions are necessary to suppress wiggling, roughening, and degradation of etched layer. At the same time, reasonable productivity is required for mass production. To improve etched profiles, we have been investigated and improved microwave ECR plasma etch tool, e.g. installation of time modulation (pulsing of RF bias, discharge, and gas) technologies, accurate power control of wafer RF bias, end-point monitor, and so on. TM bias (wafer RF bias pulsing) technology is already used in semiconductor fabrication [2]. Lower RF bias power and zero bias condition were effective for extremely high-selective etching. Microwave ECR plasma tool seems to have good affinity to pulsing discharge because it was designed to avoid the drift of plasma density. By using the pulsing discharge, wiggling was suppressed dramatically in DSA mask etching [3]. Gas pulsing technology was also investigated for fin etching. Etching progress and sidewall protection steps were changed cyclically without interruption of the discharge. It was found that etched profile (vertical, etch front flatness, and mask selectivity) was improved [4]. Atomic level precision etching is required in future MOS device fabrication where etched material, etch stop layer, and structure become more complicated. It will be necessary to control lower ion incident energy with vertical direction, lower the gas dissociation to reduce aggressive radical generation and reduce sidewall deposition. We believe multi-pulsing technology (discharge, bias, and gas) and low bias power control at low pressure operation are key technologies to realize atomic level precision etching. [1] ITRS 2013. [2] T.Ono el al, JJAP. 38, 5292 (1999). [3] M. Morimoto et al., SPIE 2014 [9054-18]. [4] M. Tanaka et al., JSAP Autumn meeting, 19p-S10-7, 2014.

The effect of ion flux on plasma-induced modification and deuterium retention in tungsten and tungsten–tantalum alloys

The paper presents the results of an experimental study of deuterium retention in W and W–Ta alloy that were exposed to first-wall relevant low flux (∼1020m−2s−1) deuterium plasma in the ECR plasma generator PlaQ. Subsequent analysis included surface imaging by optical microscopy, deuterium depth profiling by nuclear reaction analysis (NRA) and measurements of deuterium content by thermal desorption spectroscopy (TDS). It was found that under investigated exposure conditions the deuterium content was higher in W–Ta alloy than in W. Combined with the previously reported results showing that under high-flux (∼1024m−2s−1) retention is higher in W instead, this gives rise to a peculiar flux effect – dependence of relative retention between different materials on exposure flux. We interpret this effect as evidence that at different flux ranges different populations of trapping sites determine the retention, namely pre-existing microstructural traps at low-flux exposure and plasma-induced ones at high-flux exposure.

Overview of ECR plasma heating experiment in the GDT magnetic mirror

This paper summarizes the results of experiments on electron cyclotron resonance heating (ECRH) of plasma obtained at the axially symmetric magnetic mirror device gas dynamic trap (GDT) (Budker Institute, Novosibirsk). The main achievement is the demonstration of plasma discharges with extremely high temperatures of bulk electrons. According to the Thomson scattering measurements, the on-axis electron temperature averaged over several sequential shots is 660 ± 50 eV with peak values exceeding 900 eV in a few shots. This corresponds to an at least threefold increase as compared to previous experiments both at the GDT and at other comparable machines, thus demonstrating the maximum quasi-stationary (∼0.6 ms) electron temperature achieved in open traps. The breakthrough is made possible with the successful implementation of a sophisticated ECRH scheme in addition to standard heating by neutral beams (NBs). Another important result is the demonstration of the significantly increased lifetime of NB-driven fast particles with the application of ECRH, leading to a 30% higher plasma energy content at the end of the discharge. All available data including the previously demonstrated possibility of plasma confinement with β as high as 60%, allows us to consider fusion applications of axially symmetric magnetic mirror machines on a realistic basis.

Influence of Working Pressure on Ion Sensitive Probe Measurement in Microwave ECR Plasmas**supported by National Natural Science Foundation of China (No. 10875093)

In order to precisely measure the ion parameters in a microwave electron cyclotron resonance plasma using an ion sensitive probe, the dependences of the current-voltage (I-V) characteristics on the shielding height (h) and the potential difference between inner and outer electrodes (VB) have been investigated at different working pressures of 0.03 Pa and 0.8 Pa. Results show that the I-V curves at higher pressure are more sensitive to the variation of h than those at lower pressure. The influence of VB on ion temperature (Ti) measurement becomes more prominent when the pressure is increased from 0.03 Pa to 0.8 Pa. Under both pressures, the optimized h is obtained at the condition where the current reaches zero in the positive voltage region with a suitable VB of −1.5 V because of effective shielding of the electron E×B drift.

Experimental studies of the interactions between a hydrogen plasma and a carbon or tungsten wall

We present work done at LSPM (Laboratory of Sciences of Processes and Material Sciences), using the CASIMIR ECR plasma reactor device, aimed at answering questions about hydrogen isotope fuel retention and dust production in the context of the plasma-facing components (PFCs) of the International Thermonuclear Experimental Reactor (ITER). The plasma is characterized by means of optical spectroscopy, mass spectrometry and electrostatic probe; furthermore the dust density and size distribution will be measured by a laser diagnostic system. We present some early results obtained from hydrogen plasma exposure of pure tungsten samples, as well as samples of ITER-relevant tungsten-rich powders, produced inhouse by the ball-milling technique, which are likely to be a by-product of material erosion and migration during tokamak operation. In particular, we have performed measurements of the specific surface area of these powders as a proxy to their capacity to absorb hydrogen.

Highly photoluminescent low-temperature SiNx films in situ-deposited via SLAN ECR PECVD

Highly photoluminescent SiNx films were in situ-deposited at low temperatures using a slot-antenna electron-cyclotron-resonance (SLAN ECR) plasma. Spectroscopic ellipsometry, photoluminescence (PL) spectroscopy, high-resolution transmission electron microscopy (HRTEM), and UV–vis–NIR spectroscopy measurements were used to characterize the film. The SLAN ECR plasma, the ionization rate of which exceeds that of an RF plasma and a plasma density of approximately 1010–1011cm−3, is useful for high-quality film deposition at low temperatures. The deposited SiNx film displayed a high PL intensity, and as the SiH4/N2 ratio increased, the PL peak energy shifted from 2.8 to 1.7eV. These characteristics are useful for fabricating optoelectronic devices, such as LEDs and solar cells. As the silicon content in the SiNx film increased, the PL FWHM decreased. This effect deviated from the predictions based on the quantum confinement effect. The quantum dots did not properly form, as confirmed based on the HRTEM image. The linear relationship between the PL peak energy and the optical band gap energy suggested that bandtail recombination offered a more appropriate description of the PL mechanism than quantum confinement effects. Annealing and hyper-thermal neutral beam (HNB) treatment did not produce a shift in the PL peak, although the PL intensity decreased. These results also support the bandtail recombination model.

Status of Plasma Spectroscopy Method for CNS Hyper-ECR Ion Source at RIKEN

The optical line spectra of multi-charged gaseous and metal ion beams from ECR plasma have been observed using a grating monochromator with photomultiplier. This new method simplifies the observation of the targeted ion species in the plasma during beam tuning. In this paper we describe present condition of the Hyper-ECR ion source tuning with this plasma spectroscopy method. This data is important because separation of ion species of the same charge to mass ratio with an electromagnetic mass analyzer is almost impossible.