RF Plasma Cleaning Research Articles

Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit

The first mirror (FM) cleaning operations in ITER are expected to be executed in the presence of a T magnetic field. In the RF plasma cleaning configuration, this would have a significant influence on the plasma properties, ion energy, angle of incidence as well as flux spatial distribution. To this end, RF discharges were excited in an ITER-sized mock-up of a first mirror unit (FMU) consisting of a powered first mirror M1 and a grounded second mirror M2 placed in a homogeneous 3 T magnetic field. The plasma discharge was confined in a beam extending in the direction of the magnetic field, consequently wetting a limited portion of the FMU walls. In the DC-decoupled scheme (without λ/4 filter), this considerably influenced the self-bias voltage V DC that develops on M1. Changing the angle α between M1 normal and magnetic field, modified the plasma wetted wall area A g and the resulting V DC varied by over two orders of magnitude. Plasma exposure experiments were also done in the DC-coupled scheme (with λ/4 filter), wherein the angle and wetted surface determined the area of wall sputtered. Increasing α led to an increase in the sputtered wall area A g, and consequently the wall deposition on grounded M2. However, in all the cases M1 was entirely clean with the exception of edge deposits in some. In contrast, both M1 and M2 are coated with wall deposits in the absence of a magnetic field and a similar plasma exposure. The results show that plasma cleaning with λ/4 filter in a 3 T magnetic field at ITER could potentially prevent the parasitic wall deposition on FMs. The results also highlight the importance of FM orientation in the magnetic field and the wetted area in the plasma cleaning in both the DC-coupled and decoupled schemes within the ITER diagnostic systems.

Refurbishment of an Au-coated toroidal mirror by capacitively coupled RF plasma discharge.

Deposition of synchrotron-radiation-induced carbon contamination on beamline optics causes their performance to deteriorate, especially near the carbon K edge. The photon flux losses due to carbon contamination have spurred researchers to search for a suitable decontamination technique to restore the optical surface and retain its performance. Several in situ and ex situ refurbishing strategies for beamline optics are still under development to solve this serious issue. In this work, the carbon contamination is removed from a large (340 mm × 60 mm) Au-coated toroidal mirror surface using a capacitively coupled low-pressure RF plasma. Before and after RF plasma cleaning, the mirror was characterized by Raman spectroscopy, soft X-ray reflectivity (SXR) and atomic force microscopy (AFM) techniques. The Raman spectra of the contaminated mirror clearly show the G (1575-1590 cm-1) and D (1362-1380 cm-1) bands of graphitic carbon. The SXR curve of the contaminated mirror shows a clear dip near the critical momentum transfer of carbon, indicating the presence of carbon contamination on the mirror surface. This dip disappears after removal of the contamination layer by RF plasma exposure. A decrease in the intensities of the CO bands is also observed by optical emission spectrometry during plasma exposure. The AFM and SXR results suggest that the root-mean-square (r.m.s.) roughness of the mirror surface does not increase after plasma exposure.

RF plasma cleaning of water-cooled mirror equipped with notch filter based on shorted λ/4 line

Physical and engineering aspects of RF plasma cleaning developed for actively cooled first mirror serving as grounded DC-coupled electrode are under consideration. Feasibility of the water-cooling lines implemented as the coaxial notch filter for ITER in-vessel diagnostic mirrors was demonstrated for Capacitively Coupled Radio Frequency (CCRF) discharge in neon at 1–10 Pa. The impinging ions energy as a function of absorbed RF power was measured for both the DC-coupled and -decoupled schemes. Sputtering yield variation over rectangular surface of stainless steel mirror was studied experimentally. The measured sputtering yield profiles were found to be in qualitative agreement with the RF plasma numerical simulations. It was demonstrated that Al/Al2O3 used as Be/BeO proxy can be removed from the water cooled mirror surface by 100 eV neon ions with the use of the notch filter. The 14 h plasma exposure didn’t affect the mirror reflectivity.

A Study on the Etching Residue of Nano-Scale Patterns in C4F8/CH2F2/O2/Ar Plasma

The demand for miniaturization and high integration of semiconductor devices has ever been increasing. In order to achieve the requirements of the semiconductor industry, plasma etching processes using fluorocarbon-based gases are being designed to have higher selectivity and ion energy. Generally, to have a higher selectivity to the mask material, a more polymerizing gas (i.e. low F/C ratio gas) is used [1, 2], and therefore the removal of etching residues formed inside the pattern after the etching process has a crucial effect on the performance of the semiconductor devices. As the pattern size becomes smaller, there is an area that cannot be cleaned by the conventional wet cleaning method. Recently, various dry-cleaning techniques using plasma radicals and ions have been proposed [3, 4]. However, for precise control and understanding of residue removal, a detailed compositional analysis of the etching residue must be performed. These studies are also very important in evaluating the effectiveness of dry-cleaning. However, little research has been done to investigate the characteristics of etching residues formed inside the nano-scale patterns (i.e. sidewall and bottom) in much detail. In this work, we conducted a study on the characteristics of the etching residues formed inside the nano-scale patterns. A C4F8/CH2F2/O2/Ar mixture gas plasma was used to form a high-aspect-ratio silicon dioxide contacts (HARCs) on a silicon wafer. The chemical compositions and binding states of etching residues inside the patterns were examined by an x-ray photoelectron spectroscopy (XPS) with angle changes in detail. To investigate the formation of etching residues, the plasma characteristics (i.e. radicals, ions and electrons) were examined in parallel using an optical emission spectroscopy (OES) and a Langmuir probe (LP) system. As a result of the analysis, it was observed that the etching residues formed on the sidewalls of the line and hole patterns changed to the same tendency. At the same time, it was confirmed that depending the etched depth, the composition of the residue on the sidewall of holes and lines changed. [1] Takahashi, Kunimasa, Masaru Hori, and Toshio Goto. "Fluorocarbon radicals and surface reactions in fluorocarbon high density etching plasma. I. O2 addition to electron cyclotron resonance plasma employing CHF3." Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 14.4 (1996): 2004-2010. [2] Kwon, B. S., et al. "Ultrahigh selective etching of SiO2 using an amorphous carbon mask in dual-frequency capacitively coupled C4F8/CH2F2/O2/Ar plasmas." Journal of The Electrochemical Society 157.3 (2010): D135-D141. [3] Koh, Kyongbeom, et al. "Quasi atomic layer etching of SiO2 using plasma fluorination for surface cleaning." Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 36.1 (2018): 01B106. [4] Cagomoc, Charisse Marie D., et al. "RF plasma cleaning of silicon substrates with high-density polyethylene contamination." Japanese Journal of Applied Physics 57.1S (2017): 01AB04.

Contribution of the backstreaming ions to the self-magnetic pinch (SMP) diode current

The results presented here were obtained with a self-magnetic pinch (SMP) diode mounted at the front high voltage end of the RITS accelerator. RITS is a Self-Magnetically Insulated Transmission Line (MITL) voltage adder that adds the voltage pulse of six 1.3 MV inductively insulated cavities. The RITS driver together with the SMP diode has produced x-ray spots of the order of 1 mm in diameter and doses adequate for the radiographic imaging of high area density objects. Although, through the years, a number of different types of radiographic electron diodes have been utilized with SABER, HERMES III and RITS accelerators, the SMP diode appears to be the most successful and simplest diode for the radiographic investigation of various objects. Our experiments had two objectives: first to measure the contribution of the back-streaming ion currents emitted from the anode target and second to try to evaluate the energy of those ions and hence the Anode-Cathode (A-K) gap actual voltage. In any very high voltage inductive voltage adder utilizing MITLs to transmit the power to the diode load, the precise knowledge of the accelerating voltage applied on the A-K gap is problematic. This is even more difficult in an SMP diode where the A-K gap is very small (∼1 cm) and the diode region very hostile. The accelerating voltage quoted in the literature is from estimates based on the measurements of the anode and cathode currents of the MITL far upstream from the diode and utilizing the para-potential flow theories and inductive corrections. Thus, it would be interesting to have another independent measurement to evaluate the A-K voltage. The diode's anode is made of a number of high-Z metals in order to produce copious and energetic flash x-rays. It was established experimentally that the back-streaming ion currents are a strong function of the anode materials and their stage of cleanness. We have measured the back-streaming ion currents emitted from the anode and propagating through a hollow cathode tip for various diode configurations and different techniques of target cleaning treatment: namely, heating at very high temperatures with DC and pulsed current, with RF plasma cleaning, and with both plasma cleaning and heating. We have also evaluated the A-K gap voltage by energy filtering technique. Experimental results in comparison with LSP simulations are presented.

Characterization of Carbon-Contaminated B4C-Coated Optics after Chemically Selective Cleaning with Low-Pressure RF Plasma

Boron carbide (B4C) is one of the few materials that is expected to be most resilient with respect to the extremely high brilliance of the photon beam generated by free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. Carbon contaminations represent a serious issue for the operation of FEL beamlines due to severe reduction of photon flux, beam coherence, creation of destructive interference, and scattering losses. A variety of B4C cleaning technologies were developed at different laboratories with varying success. We present a study regarding the low-pressure RF plasma cleaning of carbon contaminated B4C test samples via inductively coupled O2/Ar, H2/Ar, and pure O2 RF plasma produced following previous studies using the same ibss GV10x downstream plasma source. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coating before and after the plasma cleaning are reported. We conclude that among the above plasma processes only plasma based on pure O2 feedstock gas exhibits the required chemical selectivity for maintaining the integrity of the B4C optical coatings.

Eliminating carbon contamination on oxidized Si surfaces using a VUV excimer lamp

In the VUV spectrum we see a significant decrease in reflection due to organic contamination on the surface of mirrors. To study VUV mirrors it is requisite to have calibration standards. Such standards are useless as calibration tools if the surface has organic contamination. For our standard, we use a thermally oxidized silicon wafer with a 27 nm oxide overlayer. We found that silicon wafer samples capped with native oxide acquire 0.1 to 0.2 nm of organic contamination within two hours of being cleaned with stored in closed, but nonvacuum, conditions. After a week there is an additional 0.2 to 0.5 nm deposition after which no further significant deposition is measured up to 90 days. We place the samples in air within one cm of a xenon excimer lamp that radiates 7.2 eV photons which remove half of the remaining contamination every minute. Five minutes exposure is sufficient to clean both fresh and stored samples. Data are determined using spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). Additionally this paper addresses the need to ensure that these characterization tools are not a source of organic contamination. We determined that the antechamber of our XPS was contaminating samples at a rate of 0.6 nm/30 min as they waited for transfer to the analysis chamber. This contamination was virtually eliminated by attaching an oxygen radical source (ORS) device (Evactron® C De-Contaminator RF Plasma Cleaning System) directly to the antechamber.

Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron

Results are reported demonstrating that radio-frequency (rf) plasma cleaning is an effective technique for mitigating microwave-pulse shortening (i.e., lengthening the pulse) in a multimegawatt, large-orbit, coaxial gyrotron. Cleaning plasmas were generated by 50 W of rf power at 13.56 MHz in nitrogen fill gas in the pressure range 15–25 mTorr. Improvements in the averaged microwave energy output of this high-power-microwave device ranged from 15% to 245% for different initial conditions and cleaning protocols. The mechanism for this improvement is believed to be rf plasma sputtering of excess water vapor from the cavity/waveguide and subsequent removal of the contaminant by cryogenic vacuum pumps.

High-Tc ramp-edge junctions and dc SQUIDs with a Ga-doped YBCO barrier

We report on high-Tc ramp-edge junctions and dc superconducting quantum interference devices (SQUIDs) with a Ga-doped YBCO barrier. The interface resistances of the junctions were drastically reduced by in situ RF plasma cleaning treatment. The plasma gas and pressure were Ar, O2 and 50-100 mTorr, respectively. The lattice images of the interface of the junctions were analysed by high-resolution transmission electron microscopy. The effects of RF plasma treatment and barrier layer material on the junction properties were systematically investigated. These junctions were fabricated uniformly and reproducibly, and they displayed clear RSJ-like I-V characteristics with high values of IcRn products at 65 K. Dc SQUIDs fabricated with the Ga-doped YBCO junctions exhibited excellent voltage modulations in response to applied fields at 65 K.

RF plasma cleaning of the oxide surface as a possibility for contamination control in mos structures

The creation of defects and release of mobile contaminants in thermal SiO 2 after a metal layer deposition in different vacuum systems are considered. The electrical characteristics of the (metal-oxide-semiconductor) MOS structures in oxides with evaporated and sputtered gates are compared. Sputter metallization process induces radiation instabilities that cannot be neglected. The RF plasma cleaning of the oxide surface in O 2 and CF 4⧹O 2 mixture before vacuum evaporation of the metal gate is studied. Precleaning of the oxide surface in oxygen RF plasma is found to provide a stable oxide surface, resistant against permeation and movement of contaminants in MOS device technology. It is suggested that this is due to a plasma induced modification of the free outer oxide surface. The effect is enhanced by application of additional CF 4⧹O 2 plasma treatment.

Plasma-Surface Interaction Limits for Remote H-Plasma Cleaning of Si(100)

ABSTRACTAn investigation of the parameters in H-plasma cleaning influencing H-diffusion and surface etching is described. The Si surface and subsurface regions were characterized with Raman spectroscopy and high resolution transmission electron microscopy (HRTEM), and the plasma parameters were monitored with a double Langmuir probe and optical emission spectroscopy. The parameters varied in the rf plasma cleaning were the substrate temperature, the rf power, and the plasma exposure time. It was found that low pressure and low power H-plasma exposure was effective in terms of cleaning the surface. In the 300°C, 20 Watts, 2 min. H-plasma exposure case, the Raman spectra indicated that there was no detectable H incorporation into the Si bulk and HRTEM showed no obvious defect microstructure in the near surface region and that the surface was smooth. In contrast, in the 150°C, 50 Watts, 60 min. H-plasma exposure case, HRTEM indicated that H-induced platelet defects formed in the Si(100) subsurface region and that the surface morphology was rough. Raman scattering revealed Si-H features at ∼2100 cm−1.

Fabrication and characteristics of NbN-based Josephson junctions for logic LSI circuits

The characteristics of a 1.5 μm square NbN-based Josephson junction were investigated and improved to make large scale application of these junctions to logic LSI circuits possible. An amorphous-like Si thin film was newly examined in order to define a junction area consistent with the pattern size of the resist mask. A Nb-oxide barrier was formed on a NbN base electrode by rf plasma cleaning and oxidation. The low leakage current (the characteristic voltage Vm, that corresponds to converting the leak into the maximum junction currents, being larger than 40 mV) and the small critical current variation were achieved by low rf-voltage plasma cleaning. By using Si-insulating layers and optimizing surface-cleaning conditions, the maximum critical current variation was reduced within ±12% for 850 junctions. The change in the normal tunneling resistance R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nn</inf> due to heat-treating below 200 °C was kept within ±5%. To confirm the feasibility of applying the junctions to logic LSIs, a 3K-logic-gate array was fabricated using about 23,000 junctions and its operation was successfully demonstrated.

Nb-Oxide-Pb Josephson Tunnel Junctions Fabricated Using CF4 Cleaning Process (II)

The rf plasma cleaning conditions using a CF4 cleaning process (CFCP) giving high-quality junctions are discussed. The C-F radical chemical reaction provides a clean, damage-free Nb surface which is protected from oxygen dissolution by a carbon layer. The carbon layer is possibly effective in forming a high-quality tunnel oxide. The dependence of the maximum dc Josephson current on an external magnetic field shows a uniform current distribution. The total critical current spread was measured in arrays of 50 series-connected junctions with an area of 10×10 µm2 and was found to be within ±2.5% for the best sample at a mean current density of 10.6 kA/cm2.

Characterization of Nb/Nb oxide structures in Josephson tunnel junctions

The subgap conductance of Nb, Nb oxide/Pb-alloy Josephson tunnel junctions was found to strongly depend on the rf plasma processing used to form the Nb oxide tunnel barrier. We have therefore studied the Nb/Nb oxide structures after rf plasma processing using x-ray photoelectron spectroscopy (XPS) and transmission electron microscropy (TEM) data. A crystalline Nb-C-O layer was formed as a transition region between the polycrystalline Nb base electrode and the Nb 2 O 5 tunnel barrier. The thickness of this transition region is sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. An unexpected relation between junction electrical properties and the Nb/Nb oxide structure exists. The lowest subgap conductances are those obtained for tunnel junctions with transition regions about 30A thick. An abrupt interface between Nb and Nb 2 O 5 leads to junctions with high subgap conductances.

Structure of a Nb oxide tunnel barrier in a Josephson junction

Oxides grown on the surface of a polycrystalline Nb thin film using rf plasma cleaning and plasma oxidation processes were examined by transmission electron microscopy. The oxides were prepared on the Nb surface using the same rf plasma processes developed for fabricating the tunnel barriers of Nb/Nb oxide/Pb alloy Josephson tunnel junctions. Transmission electron diffraction patterns obtained from the tunnel barrier formed on the Nb surface indicated the presence of a crystalline NbC0.25O0.75 layer between the amorphous Nb2O5 tunnel barrier layer and the Nb base electrode. The thickness of this transition layer was found to be sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. The formation of this NbC0.25O0.75 layer of proper thickness is correlated to good junction quality. An epitaxial relationship between the NbC0.25O0.75 layer and the underlying Nb crystal was also observed.