Low-pressure Hydrogen Plasma Research Articles

Plasma-Modification of graphene oxide for advanced ammonia sensing

The exceptional tailoriability of electrical properties in reduced graphene oxide (rGO) is a pivotal factor in unleashing its advanced gas sensing capabilities. Amidst various chemically/thermally-driven techniques, plasma reduction emerges as the fastest method with the potential for scalable treatment. This paper demonstrates a controlled plasma-enabled approach for swift and green surface reduction of graphene oxide (GO) films, specifically tailored for room-temperature ammonia (NH3) detection at ppm levels. Employing a mild hydrogen plasma treatment on a thin GO layer deposited on copper electrodes by drop casting, we were able to tailor the sensor’s sensitivity and reversibility due to the great impact of treatment time. The GO reduction experiments were conducted in a low-pressure mild hydrogen plasma discharged at 100 W, and the treatment time varied between 10 and 240 s. Structural and chemical analyses reveal an instantaneous reduction in oxygen content, dropping from approximately 30 to 20 atomic percentages within the initial 20 s. Sensitivity and recovery trade-offs are explored for different treatment durations, with the ‘rGO - 20 s’ sensor demonstrating the highest sensitivity at 23.9 % (100 ppm) and 47.1 % (1049 ppm) of NH3, albeit with a recovery time approximately four times longer than the ‘240s-rGO’ sample. This double-play behavior, attributed to chemisorption-dominated and physisorption-dominated interactions of NH3-rGO, elucidates the sensitivity and recovery time dynamics. Proposing a scalable, environmental-friendly and room-temperature H2-plasma reduction process enables one of the fastest approaches for designing advanced rGO sensors with controllable sensing behaviour towards ammonia.

The impact of hydrogen plasma on the structure and morphology of tin and lead micrometer sized particles

The stability of micrometer sized particles in hydrogen plasma is essential for extreme ultraviolet lithography, the ITER fusion program and the application of hydrogen plasma etching. We experimentally investigated the morphological evolution of tin (Sn), lead (Pb), and lead (II) oxide (PbO) micrometer sized particles on a surface that is exposed to a low pressure hydrogen plasma. Post exposure particle cross sections obtained by a scanning electron microscope accompanied by a focused ion beam demonstrated a significant influence of hydrogen plasma exposure on both the surface and the bulk material of the particles. Chemical sputtering at the surface and accumulation of pressurized hydrogen bubbles in cavities in the bulk material are the main drivers of the morphological changes. These mechanisms may influence the adhesion of particles to the surface through the introduction of asperities, increase of contact spot area, or fragmentation after the accumulation of mechanical stress.

Real-time VUV radiation monitoring in low-pressure hydrogen plasma based on fluorescence of sodium salicylate

Recently, vacuum ultra-violet (VUV) radiation emitted from plasmas has been of particular interest in semiconductor device fabrication because of the effects of its high-energy photons, such as induced damage or curing on low-k materials. Due to the difficulty of implementing conventional spectroscopic methods to monitor VUV radiation with high accuracy and time resolution in current plasma processing equipment, novel monitoring methods must be investigated. Therefore, in this work, we developed a compact VUV radiation monitoring system based on a scintillator, i.e. sodium salicylate (NaSal), for real-time VUV measurements. Compared to conventional VUV spectrometers, the system shows considerable implementation potential thanks to its compact size, higher detection accuracy and high time resolution. VUV radiation emitted by continuous and pulsed hydrogen plasmas generated at low pressure was investigated using the developed system. Using various filters, we were able to compare the VUV photon intensity in different wavelength ranges. It was found that the VUV photon intensity between 115 and 250 nm was about 2.5 times higher than in the region below 115 nm due to intense Lyman-α and molecular radiation, such as Lyman and Werner bands observed in low-pressure hydrogen plasmas.

Verified modeling of a low pressure hydrogen plasma generated by electron cyclotron resonance

A self-consistent fluid model has been successfully developed and employed to model an electron cyclotron resonance driven hydrogen plasma at low pressure. This model has enabled key insights to be made on the mutual interaction of microwave propagation, power density, plasma generation, and species transport at conditions where the critical plasma density is exceeded. The model has been verified by two experimental methods. Good agreement with the ion current density and floating potential—as measured by a retarding energy field analyzer—and excellent agreement with the atomic hydrogen density—as measured by two-photon absorption laser induced fluorescence—enables a high level of confidence in the validity of the simulation.

Deposition of metal thin films using a hollow cathode hydrogen discharge

Approximately 30 to 40 years ago the group of Stan Vepřek of the University of Zurich described the fabrication of silicon thin films by formation and decomposition of silicon hydride. In their paper they described how pieces of silicon exposed to a low-pressure hydrogen plasma could promote the formation of a volatile silicon hydride. The hydride was then transported to a hot substrate which caused its thermal decomposition and the formation of the silicon film.Hollow cathodes under appropriate experimental conditions can produce a semi-resonant high-density plasma. In a previous study we used such a high-density hydrogen plasma to etch pieces of quartz but under extreme conditions, we observed that rather than etching there was deposition of a thin film of the metal used for the electrode of the hollow cathode.In this paper we describe the etching of the metal (Mo or Ta) lining of a water-cooled cylindrical hollow cathode by a high-density hydrogen plasma. The metal hydride vapour generate in the plasma by chemical sputtering was directed, by the gas flow, to the quartz substrates which were maintained at various temperatures in excess of 573 K. Here the metal hydride was thermal decomposed, and a thin film of the metal was formed. To improve the experimental reproducibility a special substrate heater was constructed such that four quartz substrates could be simultaneously exposed to the metal hydride vapour, but with each substrate at a different temperature: each approximately 30.3 K greater than the neighbouring one. In this way, depositions under identical plasma conditions could be carried out at the same time, but at four different temperatures. We report the deposition rate as a function of the substrate temperature, the hydrogen gas flow and the RF or P-DC plasma power applied to the hollow cathode.

Surface and sub-surface investigations of the plasma influence on Cs doped Mo as surface converter for negative hydrogen ion sources for fusion(a)

The influence of low-pressure and low-temperature hydrogen plasma on a pure molybdenum sample implanted with caesium is investigated by means of sputtering-XPS. A portion of the sample was exposed to the plasma, while the rest of the sample was covered by stainless steel and thus shielded from direct plasma flux. The results show that the part of the sample exposed to the plasma exhibits both on and beneath the surface an increased amount of H2O and MoO2 with respect to the covered sample part which can be explained by a chemical reduction of MoO3 oxides by the hydrogen plasma species. The XPS Cs lines show oxidation peaks. A sputtering depth of roughly 13 nm is necessary to recover an identical spectrum, indicating that the plasma changes the chemical composition of the implanted samples not only on the surface but also inside the material.

Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

In this work, negative-ion production on the surface of a sample negatively DC biased in a hydrogen plasma is studied. The negative ions created under positive ion bombardment are accelerated toward the plasma, self-extracted, and detected according to their energy and mass, by a mass spectrometer placed in front of the sample. The use of a pulsed bias allows applying a quasi-DC bias on insulating material during a short period of time and offers the possibility to extend the measurement method to nonconductive samples. The pulsed-bias tests were performed first with highly oriented pyrolytic graphite (HOPG), a conductive material, to demonstrate the feasibility of the method. By changing the pulsed-bias frequency, it was possible to obtain HOPG material with different hydrogen surface coverages and hence different surface states leading to an increase of negative-ion production by up to 30%–50% as compared to the continuous bias case. To establish a protocol for insulating materials, charge accumulation on the surface during the bias pulse and influence of the bias duration and frequency were explored using microcrystalline diamond (MCD) thin layers. By using a pulse short enough (10 μs) at 1 kHz frequency, it has been possible to measure negative-ions on the MCD sample at a quasi-constant surface bias of 130 V, with only 1 V variation during the measurement. Negative-ion surface production on MCD has been studied in pulsed mode with surface temperature from room temperature to 800 °C. It is shown that pulsing the bias and increasing the temperature allows limiting defect creation on MCD which is favorable for negative-ion production. Consequently, at 400 °C, the yield on MCD in pulsed mode is one order of magnitude higher than the yield on HOPG in continuous mode at room temperature.

Dynamics of the ion energy spectrum in EUV-induced hydrogen plasma

The dynamics of the ion energy spectrum in low-pressure (10–100 Pa) hydrogen plasma induced by extreme ultraviolet (EUV) pulses in the wavelength range of 10–20 nm was studied experimentally. The plasma was generated under cathode irradiation due to both direct gas ionization by EUV photons and impact ionization by high-energy secondary electrons. The dynamics of the spectra of ions incident on the cathode was measured using a time-resolved retarding field energy analyzer. It is shown that the ion spectrum dynamics is completely determined by the time evolution of the cathode sheath. At low gas pressures (<20 Pa), the ion spectrum at early moments after the EUV pulse has a peaked shape, typical of a collisionless plasma sheath, and is mainly determined by the cathode voltage. As the pressure increases, the peak broadens and low energy ions appear in the spectrum due to ion collisions in the cathode sheath. An increase in the role of collisions with decreasing plasma density is also observed in the time evolution of ion spectra.

Measurement of electron energy probability function in weakly magnetized plasma

Knowledge of the real EEDF is of great importance in understanding the underlying physics of processes occurring at the magnetized plasma, such as the formation of transport barriers, cross-field diffusion coefficients and plasma–substrate interactions. In the present experiment, the application of Langmuir probe to evaluate EEPF in presence of magnetic fields within the range 594–32 G is investigated. The data recorded for EEPFs in magnetic fields and in dust is acquired using current–voltage characteristics measured in low pressure hydrogen plasma. The values of plasma density, electron temperature and EEPF are evaluated with a single cylindrical Langmuir probe at different axial positions (1 cm to 6 cm) from the magnet. From the recent EEPF observations in presence of magnetic field, it shows a bi-Maxwellian EEPF structure at different magnetic fields. But at different magnetic field, it is observed that the low energy electron population changes whereas the high-energy electron population remains almost constant. EEPF measurement shows almost identical behaviour with the unmagnetized plasma when the larmour radius of electron is greater than or equal to 10 times of the probe radius.

Reduction of a thin chromium oxide film on Inconel surface upon treatment with hydrogen plasma

Inconel samples with a surface oxide film composed of solely chromium oxide with a thickness of approximately 700nm were exposed to low-pressure hydrogen plasma at elevated temperatures to determine the suitable parameters for reduction of the oxide film. The hydrogen pressure during treatment was set to 60Pa. Plasma was created by a surfaguide microwave discharge in a quartz glass tube to allow for a high dissociation fraction of hydrogen molecules. Auger electron depth profiling (AES) was used to determine the decay of the oxygen in the surface film and X-ray diffraction (XRD) to measure structural modifications. During hydrogen plasma treatment, the oxidized Inconel samples were heated to elevated temperatures. The reduction of the oxide film started at temperatures of approximately 1300K (considering the emissivity of 0.85) and the oxide was reduced in about 10s of treatment as revealed by AES. The XRD showed sharper substrate peaks after the reduction. Samples treated in hydrogen atmosphere under the same conditions have not been reduced up to approximately 1500K indicating usefulness of plasma treatment.

Progress of the ELISE test facility: towards one hour pulses in hydrogen

In order to fulfil the ITER requirements, the negative hydrogen ion source used for NBI has to deliver a high source performance, i.e. a high extracted negative ion current and simultaneously a low co-extracted electron current over a pulse length up to 1 h. Negative ions will be generated by the surface process in a low-temperature low-pressure hydrogen or deuterium plasma. Therefore, a certain amount of caesium has to be deposited on the plasma grid in order to obtain a low surface work function and consequently a high negative ion production yield. This caesium is re-distributed by the influence of the plasma, resulting in temporal instabilities of the extracted negative ion current and the co-extracted electrons over long pulses. This paper describes experiments performed in hydrogen operation at the half-ITER-size NNBI test facility ELISE in order to develop a caesium conditioning technique for more stable long pulses at an ITER relevant filling pressure of 0.3 Pa. A significant improvement of the long pulse stability is achieved. Together with different plasma diagnostics it is demonstrated that this improvement is correlated to the interplay of very small variations of parameters like the electrostatic potential and the particle densities close to the extraction system.

Stability of AlGaN/GaN heterostructures after hydrogen plasma treatment

We report on the investigation of low temperature (300°C) hydrogen plasma treatment influence on the AlGaN/GaN heterostructures. This issue was raised in the frame of study on processes related to hybrid integration of diamond with GaN-based devices. At the same time, the capabilities of thin SiNx covering were investigated. The samples were exposed to low pressure hydrogen plasma ignited in the linear plasma system at low temperature. We analyze the surface morphology of samples by scanning electron microscopy while microstructural changes down to AlGaN/GaN interface were studied using secondary ion mass spectrometry. The sheet resistance, monitored using circular transmission line measurements, increases more than 3.5 times after 60min treatment. The basic transport properties of the fabricated circular high electron mobility transistors after H2 plasma treatment were analyzed. The sheet resistance increasing was attributed to the decrease of effective mobility. Whilst, the observed Schottky barrier lowering indicates necessity of gate contact protection.

Understanding of the importance of the spore coat structure and pigmentation in the Bacillus subtilis spore resistance to low-pressure plasma sterilization

Low-pressure plasmas have been evaluated for their potential in biomedical and defense purposes. The sterilizing effect of plasma can be attributed to several active agents, including (V)UV radiation, charged particles, radical species, neutral and excited atoms and molecules, and the electric field. Spores of Bacillus subtilis were used as a bioindicator and a genetic model system to study the sporicidal effects of low-pressure plasma decontamination. Wild-type spores, spores lacking the major protective coat layers (inner, outer, and crust), pigmentation-deficient spores or spore impaired in encasement (a late step in coat assembly) were systematically tested for their resistance to low-pressure argon, hydrogen, and oxygen plasmas with and without admixtures. We demonstrate that low-pressure plasma discharges of argon and oxygen discharges cause significant physical damage to spore surface structures as visualized by atomic force microscopy. Spore resistance to low-pressure plasma was primarily dependent on the presence of the inner, and outer spore coat layers as well as spore encasement, with minor or less importance of the crust and spore pigmentation, whereas spore inactivation itself was strongly influenced by the gas composition and operational settings.

Quantification of the VUV radiation in low pressure hydrogen and nitrogen plasmas

Hydrogen and nitrogen containing discharges emit intense radiation in a broad wavelength region in the VUV. The measured radiant power of individual molecular transitions and atomic lines between 117 nm and 280 nm are compared to those obtained in the visible spectral range and moreover to the RF power supplied to the ICP discharge. In hydrogen plasmas driven at 540 W of RF power up to 110 W are radiated in the VUV, whereas less than 2 W is emitted in the VIS. In nitrogen plasmas the power level of about 25 W is emitted both in the VUV and in the VIS. In hydrogen–nitrogen mixtures, the NH radiation increases the VUV amount. The analysis of molecular and atomic hydrogen emission supported by a collisional radiative model allowed determining plasma parameters and particle densities and thus particle fluxes. A comparison of the fluxes showed that the photon fluxes determined from the measured emission are similar to the ion fluxes, whereas the atomic hydrogen fluxes are by far dominant. Photon fluxes up to 5 × 1020 m−2 s−1 are obtained, demonstrating that the VUV radiation should not be neglected in surface modifications processes, whereas the radiant power converted to VUV photons is to be considered in power balances. Varying the admixture of nitrogen to hydrogen offers a possibility to tune photon fluxes in the respective wavelength intervals.

Plasmachemical Conservation of Corroded Metallic Objects

Plasmachemical process for conservation of metallic objects is a new way of effective and fast treatment of corroded objects. This process consists of two main steps: corrosion removal and deposition of a protecting film. Removal of corrosion products is based on plasmachemical reduction of corrosion layers by radio-frequency (RF) low pressure hydrogen plasma. Chosen barrier films are parylene (poly-para-xylylene) coatings and SiO2- like high density films. Parylene coatings are prepared by a standard chemical vapor deposition (CVD) method. Plasma enhanced chemical vapour deposition (PECVD) enables preparation of SiOx based thin films with higher flexibility due to variable incorporated organics groups. The coatings were characterized by various methods in order to obtain information about their chemical structure (FTIR) and barrier properties (OTR). The results from standard corrosion test were compared with those on samples treated by conventional conservation procedures.

Effect of Hydrogen Plasma on Model Corrosion Layers of Bronze

Our work is about plasmachemical reduction of model corrosion layers. The model corrosion layers were produced on bronze samples with size of 10 × 10 × 5 mm3, containing Cu and Sn. Concentrated hydrochloric acid was used as a corrosive environment. The application of reduction process in low-pressure low-temperature hydrogen plasma followed. A quartz cylindrical reactor with two outer copper electrodes was used. Plasma discharge was generated in pure hydrogen by a RF generator. Each corroded sample was treated in different conditions (supplied power and a continual or pulsed regime with a variable duty cycle mode). Process monitoring was ensured by optical emission spectroscopy. After treatment, samples were analyzed by SEM and EDX.

Low-pressure hydrogen plasmas explored using a global model

Low-pressure hydrogen plasmas have found applications in a variety of technology areas including fusion, neutral beam injection and material processing applications. To better understand these discharges, a global model is developed to predict the behaviour of electrons, ground-state atomic and molecular hydrogen, three positive ion species (H+, , and ), a single negative ion species (H−), and fourteen vibrationally excited states of molecular hydrogen (–14)). The model is validated by comparison with experimental results from a planar inductively coupled GEC reference cell and subsequently applied to the MAGPIE linear helicon reactor.The MAGPIE reactor is investigated for a range of pressures from 1 to 100 mTorr and powers up to 5 kW. With increasing power between 50 W and 5 kW at 10 mTorr the density of all charged species increases as well as the dissociative fraction while the electron temperature remains almost constant at around 3 eV. For gas pressures from 1–100 mTorr at an input power of 1 kW, the electron density remains almost constant, the electron temperature and dissociative fraction decreases, while density increases in density and also dominates amongst ion species. Across these power and pressure scans, electronegativity remains approximately constant at around 2.5%. The power and pressure determines the dominant ion species in the plasma with observed to dominate at high pressures and low powers whereas H+ tends to be dominant at low pressures and high powers.A sensitivity analysis is used to demonstrate how experimental parameters (power, pressure, reactor wall material, geometry etc) influence individual species’ density as well as the electron temperature. Physical reactor changes including the length, radius and wall recombination coefficient are found to have the largest influence on outputs obtained from the model.

Effect of magnetic field on dust charging and corresponding probe measurement

The effect of external magnetic field on the Langmuir probe measurement and dust charging are studied in low-pressure hydrogen plasma. The experiment is performed in a dusty plasma device where plasma is created by the hot cathode filament discharge technique. A strong Strontium ferrite magnet is used inside the plasma, near the dust zone. The plasma parameters are measured at different distances from the magnet with the help of Langmuir probe system. It is observed that even at “low magnetic field case,” where rLe≥rP, rLi〉〉rP; the electron collection by the probe deviates strongly from the actual value, until rLe≥10rP. The observations of electron energy probability function show that at higher magnetic field, the Langmuir probe collects only the higher energy electrons compared to the low energy electrons. Both Quasi-neutrality condition and capacitance model are used separately to calculate the charge accumulated on the dust grain. Introducing the reduction factor on quasi-neutrality condition, it is observed that the influence of magnetic field on dust charge is almost negligible for “low magnetic field” case. The dust charge calculated from quasi-neutrality condition matches well with the experimentally observed dust current results, within the experimental error range. However, capacitance model deviates from the experimental results at higher magnetic field.

A collisional radiative model of hydrogen plasmas developed for diagnostic purposes of negative ion sources.

A collisional radiative model of low-pressure hydrogen plasmas is elaborated and applied in optical emission spectroscopy diagnostics of a single element of a matrix source of negative hydrogen ions. The model accounts for the main processes determining both the population densities of the first ten states of the hydrogen atom and the densities of the positive hydrogen ions H(+), H2 (+), and H3 (+). In the calculations, the electron density and electron temperature are varied whereas the atomic and molecular temperatures are included as experimentally obtained external parameters. The ratio of the Hα to Hβ line intensities is calculated from the numerical results for the excited state population densities, obtained as a solution of the set of the steady-state rate balance equations. The comparison of measured and theoretically obtained ratios of line intensities yields the values of the electron density and temperature as well as of the degree of dissociation, i.e., of the parameters which have a crucial role for the volume production of the negative ions.

Traceability of mass II: a study of procedures and materials

The surface contamination of various metallic samples upon cyclic transfer from vacuum to ambient pressure air or dry nitrogen was studied using x-ray photoelectron spectroscopy (XPS) for surface chemical analysis. The venting methods were vacuum–air–vacuum, vacuum–N2–air–vacuum and vacuum–N2–vacuum. In addition, the dependence on the initial surface chemical state was investigated by using different cleaning methods such as the traditional washing and cleaning (nettoyage-lavage), low-pressure hydrogen plasma and UV/ozone as starting conditions. We found that cyclic exposure to air after UV/ozone and nettoyage-lavage has very little effect on the surface contamination. In contrast, hydrogen plasma removes the initial contamination almost completely. Until a first layer of contaminants is built up, the growth is very rapid but slows down exponentially with the number of cycles. Exposure directly to ambient pressure air causes the fastest and largest recontamination, followed by the cyclic transfer vacuum–N2–air–vacuum. Venting with dry nitrogen exclusively exhibits the slowest growth of contaminants but continues with further cycling until the first layer is completed. A list of precedence for existing and potential new materials and for the processes applied is presented in order to test and evaluate the best materials for future primary mass standards and to find the best working practice.