Surface Loss Probability Research Articles

Surface loss probability of atomic oxygen on silica under atmospheric-pressure CO2

Abstract The surface loss probability γ O of atomic oxygen was measured for SiO2 in atmospheric-pressure CO2 and Ar/CO2(3.05%) mixture at 300 K. O atoms were produced via the photolysis of CO2 using a 172 nm Xe2 excimer lamp. After irradiation by Xe2 lamp, the working gas flowed through a quartz tube, causing O atoms to be lost on its inner surface. By measuring the density of O atoms at the end of the tube for different tube lengths, we obtained the value of γ O on the inner surface of the tube. The measured value was 5.5 × 10 − 4 ± 50 % , which falls within the range of referred values of γ O ( 1 × 10 − 5 to 1 × 10 − 3 ) at 4 to 1300 Pa measured using low-pressure O2 plasmas. Despite the 2 to 5 orders of magnitude pressure difference, no significant difference in γ O was obtained. The potential influence of the difference in background gas (CO2 and O2) on the value of γ O was also considered.

Oxygen atom and ozone kinetics in the afterglow of a pulse-modulated DC discharge in pure O2: an experimental and modelling study of surface mechanisms and ozone vibrational kinetics

The chemical kinetics of oxygen atoms and ozone molecules were investigated in a fully-modulated DC discharge in pure oxygen gas in a borosilicate glass tube, using cavity ringdown spectroscopy (CRDS) of the optically forbidden O(3P2)→O(1D2) absorption at 630 nm. Measurements were made over a range of tube temperatures (10 °C and 50 °C) gas pressures (0.5–4 Torr) and discharge current (10–40 mA). The discharge current was square-wave modulated (on for 0.2 s and off for 1 s), allowing the build-up to steady-state and the decay in the afterglow to be studied. This paper focusses on the afterglow period. The O atom density decays non-exponentially in the afterglow, indicating a surface loss probability dependent on incident active particle fluxes. The oxygen atom absorption peak lies on a time-varying absorption continuum due (in the afterglow) to the Chappuis bands of ozone. The ozone density passes through a maximum a few 100 ms into the afterglow, then decays slowly. An existing time-resolved self-consistent 1D radial model of O2 positive column discharges was modified to interpret the new results. The ozone behaviour in the afterglow can only be modelled by the inclusion of: (1) surface production of O3 from the reaction of O2 molecules with adsorbed O atoms, (2) reactions of vibrationally-excited ozone with O atoms and with O2(a1Δg) molecules, and (3) surface loss of ozone with a probability of around 10−5.

Molecular dynamics approach for the calculation of the surface loss probabilities of neutral species in argon–methane plasma

AbstractMolecular dynamics simulations are carried out for calculating the surface loss probabilities of neutral species from an argon–methane plasma. These probabilities are the sum of the sticking and surface recombination probabilities. This study considers both the formation of reactive and nonreactive volatile species for evaluating recombination probabilities. Results show that stable species are reflected when hydrocarbon film starts growing on the surface. CH3 is mainly lost by surface recombination leading to the formation of volatile products while very little contributes to film growth. C2H has surface loss probability in agreement with the literature. While C2H loss is usually attributed to sticking on the surface, our results show that its main loss process is due to surface recombination.

OH density, flux and loss probability measurements in a room temperature atmospheric pressure surface discharge by microscopic laser induced fluorescence

Many applications involving atmospheric pressure plasma-substrate interactions are enabled by the large fluxes of short-lived reactive species such as OH radicals to the substrate, nonetheless, the accurate measurement of radical densities and fluxes at substrates at atmospheric pressure has received little attention to date, particularly for surface ionization waves. We report the measurement of the OH density distribution in a surface discharge on a fused silica (quartz) substrate generated by an impinging atmospheric pressure plasma jet in dry and humid helium. The OH density is measured by microscopic laser induced fluorescence with a spatial resolution of 10 µm in the direction perpendicular to the quartz substrate. The measured OH diffusive flux varied for the investigated experimental conditions by almost three orders of magnitude and had a maximum value of 1.7 × 1015 cm−2 s−1. The corresponding surface loss probability of OH on the quartz surface was determined to be ∼0.01. The high spatial resolution was required to accurately resolve the near surface gradient of OH radicals.

A plasma chemistry model for H2/SiH4 mixtures used in PECVD processes

Plasma chemical processes in H2/SiH4 discharges are critically reviewed. A model set of reactions is proposed which includes temperature and pressure-dependent reaction rates and describes SiyHx (y ≤ 3) chemistry. Using a 2D fluid plasma simulator, the model has been tested under three different set of operating conditions. First, it has been validated against the experimental benchmark data (Horvath and Gallagher (2009) J. Appl. Phys. 105, 13304). Based on considerations of atomic hydrogen content, the branching of SiH4 dissociation channels and the H surface loss probability have been defined more accurately. Then, simulations have been also performed for the plasma source of a PECVD tool from Meyer Burger Germany. A very good agreement between the computed and experimentally determined deposition rates can be stated.

Dynamics of H atoms surface recombination in low-temperature plasma

The dynamics of H atom recombination on materials of interest for a EUV lithographer was studied under a long-term low-pressure H2 plasma exposure. The similarity of the experimental plasma with the typical EUV-induced plasma over the multilayer mirrors surface of the EUV lithographic machine is demonstrated by means of 2D PIC MC simulation. The measurement of the temporal dynamics of the H atom surface loss probability (γH) is chosen for testing the surface modification during the treatment. Time-resolved actinometry of H atoms with Kr as the actinometer gas was used to detect the dynamics of the H-atom loss probability on the surface of Al, Ru, RVS, and SiO2. It is demonstrated that significant changes of the materials surface occur only at the very beginning of the treatment and are due to surface heating and cleaning effects. After that no changes in the γH are found, indicating that the surface stays absolutely stable. A special test of sensitivity of the used method to the state of surface was carried out. Dynamics of the γH changes with small O2 addition clearly demonstrate modification of the Al surface due to oxidation with the next removal of oxygen by the H2 plasma treatment. The rate of oxide removal is shown to be determined by plasma parameters such as the ion energy and flux to the surface.

TALIF measurements of hydrogen and deuterium surface loss probabilities on quartz in low pressure high density plasmas

This article deals with surface loss on quartz of atomic hydrogen (H) and its isotope deuterium (D) in a low-pressure (10 Pa) pulsed inductively coupled plasma. The atomic temporal decay in the post discharge is measured by two-photon absorption laser-induced fluorescence (TALIF). From the loss rate, the atomic surface loss probability is determined. In pure hydrogen or pure deuterium gas, no isotopic effect on surface kinetics has been observed and the surface loss probabilities of H and D were found to be almost identical and equal to ∼1.8%. However, despite the lack of difference in surface loss probability, a net isotopic effect on surface loss rate due to the mass difference between the isotopes is measured. Hydrogen atoms diffuse faster and have higher flux to the plasma chamber walls than deuterium atoms. Hydrogen atoms are therefore lost at higher rate than deuterium atoms. Based on the observed isotopic difference and on the comparison between H and D TALIF signals, the isotopic effects on H and D atom production are discussed.

Determination of absolute O(3P) and O2(a1 Δ g) densities and kinetics in fully modulated O2 dc glow discharges from the O2(X3 Σ g −) afterglow recovery dynamics

A method is presented for the determination of the absolute densities of O(3P) atoms and O2(a1Δg) molecules in an O2 electrical discharge, which does not depend on any calibration procedure or knowledge of optical transition strengths. It is based on observing the recovery dynamics of the O2(X3Σg −) density in the afterglow of a fully-modulated discharge, and is demonstrated in a dc glow discharge in pure О2 at pressures of 0.2–4 Torr. The time-resolved O2(X3Σg −) density was measured by VUV absorption spectroscopy using the monochromator branch of the VUV DESIRS beamline at Synchrotron SOLEIL, but this methodology could be used with another density measurement technique. During the active discharge, the O2(X3Σg −) density is depleted by a combination of О2 dissociation, excitation into metastable states (principally O2(a1Δg)) and gas heating/dilation. After discharge extinction, the O2(X3Σg −) density progressively recovers to its initial (before discharge) value, with three distinct time-constants due to: (i) gas cooling (fast), (ii) O(3P) atom recombination (intermediate), and (iii) O2(a1Δg) quenching (slow). The O(3P) and O2(a1Δg) dynamics can be separated easily, allowing the O(3P) and O2(a1Δg) afterglow loss kinetics to be determined, as well as their mole fractions in the steady-state discharge. Both the O(3P) and O2(a1Δg) mole-fractions increase with current (up to the highest current studied, 40 mA) and pass through maxima with pressure at 1 Torr, reaching 16.5% and 8%, respectively. O(3P) atoms are principally lost by recombination at the borosilicate tube surface, with a loss probability in the afterglow of ∼8 × 10−4, nearly independent of gas pressure and discharge current (in contrast to previous observations in the active discharge []). The O2(a1Δg) dynamics were also measured by IR emission spectroscopy. In the late afterglow, this agrees well with the O2(X3Σg −) recovery dynamics, corresponding to an O2(a1Δg) surface loss probability of ∼2.2 × 10−4. The initial O2(a1Δg) loss is faster than in the later afterglow, indicating that it is also quenched by O atoms.

Detection of OH radicals in atmospheric-pressure plasma jet by evanescent-wave laser-induced fluorescence spectroscopy

This paper reports the signal and noise in the detection of OH radicals by evanescent-wave laser-induced fluorescence (EW-LIF) spectroscopy. We adopted this method to detect OH radicals, which were produced within an atmospheric-pressure argon plasma jet, in the vicinity of a quartz surface. The dominant noise was the stray laser light which could not be eliminated by using a monochromator and an interference filter. This was because the detection optics looked at the total reflection point of the laser beam in the EW-LIF spectroscopy. It was impossible to detect the LIF signal in the duration of the laser pulse because of the intense stray light. On the other hand, thanks to the slow decay of the LIF intensity, we succeeded in detecting the LIF signal after the laser pulse. The intensity ratio between the LIF signal originated from the bulk plasma and the one induced by EW near the surface was 5000–10000, which can be explained by the difference in the observation volumes. The proportionality between the LIF intensities from the bulk plasma and the surface vicinity, when changing the discharge conditions, suggests that the surface loss probability of OH on the quartz surface was not affected by these changes.

The transport and surface reactivity of O atoms during the atmospheric plasma etching of hydrogenated amorphous carbon films

A remote microscale atmospheric pressure plasma jet with a He/O2 gas mixture is used to etch a hydrogenated amorphous carbon layer. The etched profiles are measured by means of imaging spectroscopic reflectometry, a powerful technique providing a 2D map of the film thickness (etched profile) and also film properties. Additionally, the 2D axially symmetric fluid model of the gas flow and species transport combined with the basic kinetic model of the reaction of O atoms with O2 molecules has been solved to study the transport and surface reactivity of O atoms. The model provides a spatially resolved and surface-integrated O atom loss rate at the surface. The situation with convection-dominated species transport and fast recombination reactions of O atoms in the volume leads to a strong dependence of the etched profile on the O2 admixture and O atom surface loss probability β. By comparing etched profiles with the simulation results, the O atom surface reaction probability of β = 0.2%–0.6% could be estimated. The modeled O atom loss rate at the surface was always higher and with the same trend as the etching rate, corroborating that O atoms are the main etching species. The presented data and simulation results show that the fastest surface-integrated etching rate is achieved not under conditions with the highest O density on the jet axis, but at lower O2 admixtures due to reduced recombination losses in the gas phase.

Beryllium film deposition in cavity samples in remote areas of the JET divertor during the 2011–2012 ITER-like wall campaign

Beryllium film deposition was studied with cavity samples in remote areas of the inner and outer JET divertor and below divertor tile 5 during the 2011–2012 campaign with the ITER-like wall. Predominantly beryllium films were formed inside the cavities with some additional carbon, the ratio Be/C was >2. These deposited layers had high D/(Be+C) ratios of about 0.3. The formation of these films is mainly due to sticking of beryllium-containing particles with low sticking coefficients <0.5. The observed surface loss probabilities depend on the position in the divertor. The particles responsible for film deposition originated from the location of in the divertor strike points.

Mechanism of N2 dissociation and kinetics of N(4S) atoms in pure nitrogen plasma

This work deals with kinetics of the ground state nitrogen atoms N(4S) and N2 dissociation mechanism in pure nitrogen plasma. The experiment was carried out in positive column of DC glow discharge at a range of parameters p = 5 - 50 Torr, J = 20 - 100 mA. The use of axial homogeneous glow discharge allowed considering N(4S) balance for spatially uniform conditions controlled by only two terms: source (characterized by effective production rate kdisseff) and loss (characterized by effective loss time τNloss). Analysis of these parameters gains considerably better understanding of N2 dissociation mechanism in pure nitrogen plasma that was the main goal of the given work. So N/N2 dissociation rate as function of discharge parameters was obtained using two independent emission optical methods: actinometry on Ar atoms and N2 2+ band emission decay at discharge modulation. Measurements of N/N2 radial profiles allowed estimating N atom surface loss probability γNloss and correspondingly τNloss. It was revealed that γNloss depends on N(4S) concentration and thereby discharge conditions through the sorption balance of physisorbed N atoms. Simple phenomenological model taking into account basic surface processes provides γNloss data in good agreement with experiment. Finally, kdisseff was obtained as function of reduced electric field E/N and it was shown that even EEDF self-consistently calculated with accounting for N2 vibrational excitation is unable to provide observed values of kdisseff. Reasons of that fact are discussed in detail.

Influence of surface conditions on plasma dynamics and electron heating in a radio-frequency driven capacitively coupled oxygen plasma

The impact of changing surface condition on plasma dynamics and electron heating is investigated by means of numerical simulations, based on a semi-kinetic fluid model approach, and compared with measurements of the nanosecond electron dynamics in the plasma-surface interface region using phase resolved optical emission spectroscopy (PROES). The simulations are conducted in a one-dimensional domain and account for a geometrical asymmetry comparable to the experimental setup of a radio-frequency driven capacitively coupled plasma in a gaseous electronics conference reference cell. A simple reaction scheme is considered, including electrons, positive ions, negative ions and metastable singlet delta oxygen (SDO) as individual species. The role of surface loss and effective lifetime of SDO is discussed. To simulate different surface conditions, the SDO surface loss probability and the secondary electron emission coefficient were varied in the model. It is found that a change in surface condition significantly influences the metastable concentration, electronegativity, spatial particle distributions and densities as well as the ionization and electron heating dynamics. The excitation dynamics obtained from simulations are compared with PROES measurements. This allows to determine experimentally relevant SDO surface loss probabilities and secondary electron emission coefficient values in-situ and is demonstrated for two different surface materials, namely aluminum and Teflon.

Wall loss of atomic nitrogen determined by ionization threshold mass spectrometry

In the afterglow of an inductively coupled N2 plasma, relative N atom densities are measured by ionization threshold mass spectrometry as a function of time in order to determine the wall loss time twN from the exponential decay curves. The procedure is performed with two mass spectrometers on different positions in the plasma chamber. twN is determined for various pressures, i.e., for 3.0, 5.0, 7.5, and 10 Pa. For this conditions also the internal plasma parameters electron density ne and electron temperature Te are determined with the Langmuir probe and the rotational temperature TrotN2 of N2 is determined with the optical emission spectroscopy. For TrotN2, a procedure is presented to evaluate the spectrum of the transition υ′=0→υ″=2 of the second positive system (C3Πu→B3Πg) of N2. With this method, a gas temperature of 610 K is determined. For both mass spectrometers, an increase of the wall loss times of atomic nitrogen with increasing pressure is observed. The wall loss time measured with the first mass spectrometer in the radial center of the cylindrical plasma vessel increases linearly from 0.31 ms for 3 Pa to 0.82 ms for 10 Pa. The wall loss time measured with the second mass spectrometer (further away from the discharge) is about 4 times higher. A model is applied to describe the measured twN. The main loss mechanism of atomic nitrogen for the considered pressure is diffusion to the wall. The surface loss probability βN of atomic nitrogen on stainless steel was derived from twN and is found to be 1 for the present conditions. The difference in wall loss times measured with the mass spectrometers on different positions in the plasma chamber is attributed to the different diffusion lengths.

Hydrocarbon film deposition inside cavity samples in remote areas of the JET divertor during the 1999–2001 and 2005–2009 campaigns

Hydrocarbon film deposition was studied with cavity samples in remote areas of the inner and outer JET divertor and below the divertor septum during the 1999–2001 and 2005–2009 campaigns. Thick hydrocarbon films were formed inside the cavities. These deposited hydrocarbon layers have high D/C ratios close to 1. The formation of these films is mainly due to sticking of hydrocarbon particles with high surface loss probabilities >0.6. The observed surface loss probabilities depend on the position in the divertor and vary during different campaigns. The particles responsible for hydrocarbon layer formation originate from the divertor strike points. Except for the septum cavity the deposition of beryllium was very low and showed a very different distribution from that of deuterium and carbon.

Surface loss probability of atomic hydrogen for different electrode cover materials investigated in H2-Ar low-pressure plasmas

In an inductively coupled H2-Ar plasma at a total pressure of 1.5 Pa, the influence of the electrode cover material on selected line intensities of H, H2, and Ar are determined by optical emission spectroscopy and actinometry for the electrode cover materials stainless steel, copper, tungsten, Macor®, and aluminum. Hydrogen dissociation degrees for the considered conditions are determined experimentally from the measured emission intensity ratios. The surface loss probability βH of atomic hydrogen is correlated with the measured line intensities, and βH values are determined for the considered materials. Without the knowledge of the atomic hydrogen temperature, βH cannot be determined exactly. However, ratios of βH values for different surface materials are in first order approximation independent of the atomic hydrogen temperature. Our results show that βH of copper is equal to the value of stainless steel, βH of Macor® and tungsten is about 2 times smaller and βH of aluminum about 5 times smaller compared with stainless steel. The latter ratio is in reasonable agreement with literature. The influence of the atomic hydrogen temperature TH on the absolute value is thoroughly discussed. For our assumption of TH = 600 K, we determine a βH for stainless steel of 0.39 ± 0.13.

The influence of surface properties on the plasma dynamics in radio-frequency driven oxygen plasmas: Measurements and simulations

Plasma parameters and dynamics in capacitively coupled oxygen plasmas are investigated for different surface conditions. Metastable species concentration,electronegativity, spatial distribution of particle densities as well as the ionization dynamics are significantly influenced by the surface loss probability of metastable singlet delta oxygen (SDO). Simulated surface conditions are compared to experiments in the plasma-surface interface region using phase resolved optical emission spectroscopy. It is demonstrated how in-situ measurements of excitation features can be used to determine SDO surface loss probabilities for different surface materials.

Influence of Oxygen Addition and Wafer Bias Voltage on Bromine Atom Surface Reaction in a HBr/Ar Inductively Coupled Plasma

The influence of the wafer surface material and wafer bias voltage on the Br radical density in HBr/Ar and HBr/Ar/O2 inductively coupled plasmas was investigated by appearance mass spectrometry. By increasing the bias voltage, a monotonic decrease in the Br radical density was observed irrespective of the surface material (Si, Al2O3) of the wafer. A drastic increase in Br radical density was observed after O2 addition to HBr/Ar plasma in the case of a bare Si wafer, whereas almost the same density was observed in the case of an Al2O3-sputtered Si wafer. X-ray photoelectron spectroscopy (XPS) analysis indicated that O2 addition promotes oxide formation on the Si surface. Measurement of the decay time constant for a Br radical after turning off the plasma indicated that O2 addition results in a longer decay time constant, suggesting the decrease of the surface loss probability of Br radicals for the surface-oxidized Si surface.

H atom surface loss kinetics in pulsed inductively coupled plasmas

Pulsed plasmas are very powerful tools to investigate mechanisms. This paper is focused on H atom kinetics in low-pressure high-density inductively coupled pulsed plasmas. We explore pure H2, H2/N2, CH4/H2 and CH4/N2 mixtures. These gas mixtures offer two very different kinds of wall conditions, which are stainless-steel and hydrocarbon-coated walls. It shows that H loss probability (β) is sensitive to wall conditions. Efforts are made to understand β evolutions with the different parameters. The effect of pressure in non-depositing plasmas is also investigated. Evolution of H atom surface loss probability is linked to ion flux measurements. Ion bombardment promotes H surface loss.

Deposition/erosion and H/D retention characteristics in gaps of PFCs in KSTAR studied by cavity technique

Stainless steel coupons with cavity structure were installed at different poloidal locations in KSTAR during 2009 campaign to study deposition and H/D retention inside the gap of PFCs. The deposition profiles at different poloidal locations indicate that the incoming species have high surface loss probability (β) and they are probably charge exchange neutral particles. The (H + D)/C ratios of layers inside the coupons were in a range from 0.04 to 0.37 at different poloidal locations which might be caused by different contribution of different hydrocarbon species. Campaign integrated, time averaged net C, H/D atom flux density towards outer walls are in the similar range with other carbon dominated machines.