Surface Loss Probability Research Articles

Surface loss probability of H radicals on silicon thin films in SiH4/H2 plasma

The surface loss probability of H radicals was investigated in SiH4/H2 plasma using vacuum ultraviolet resonance absorption spectroscopy. The surface loss probability was calculated from the decay curve of the H radical density in the plasma afterglow and increased with the SiH4 flow rate. Silicon thin films deposited on the chamber wall were analyzed to investigate the relation between the surface loss probability and the surface condition. The surface reaction of H radicals is influenced by deposition precursors, such as SiH3 radicals. The density of H radicals significantly decreased with heating of the chamber wall up to 473 K. The surface loss probability of H radicals was estimated to be ca. 1 at 473 K. Quantitative measurements of the surface loss probability of H radicals in SiH4/H2 plasma are expected to be particularly important for understanding the surface reactions that occur during the deposition of silicon thin films.

Loss of hydrogen atoms in H2 plasma on the surfaces of materials used in EUV lithography

Lowpressure hydrogen is an important component of the working medium in extreme ultraviolet (EUV) projection lithography. Under the action of EUV photons and fast secondary electrons on the gas medium, plasma and atomic hydrogen, actively interacting with the surface, are produced. This interaction is very important, because it largely determines the lifetime of the multilayered EUV optics. In this study, the loss of atomic hydrogen under the conditions of a low pressure (<10 Torr) RF plasma discharge on the sur� faces of materials used in EUV lithography is investigated. The surface loss probabilities of H atoms on these materials are measured. It is shown that surface recombination of atomic hydrogen goes according to the Eley-Rideal mechanism via direct recombination of H atoms from the gas phase with chemically and phys� ically adsorbed atoms. In this case, the surface recombination probability is mainly determined by the density of chemical adsorption sites. The density of adsorption sites and the desorption energy of H atoms are esti� mated. The desorption energy of physically adsorbed H atoms on pure metal surfaces (or surfaces exposed to plasma) is about 0.5 eV, and the density of sorption sites is close to the surface density of atoms. This results in a high loss probability of H atoms on metals (~0.1). Therefore, to provide efficient transportation of hydro� gen atoms, it is necessary to use materials with the lowest loss probability of H atoms, i.e., dielectrics.

Surface loss probability of atomic oxygen

Measurements of surface loss probabilities of oxygen atoms on Simax glass, PET, Teflon, and Mica at room temperature are presented. The surface loss probability was measured by using a stationary afterglow double pulse discharge technique. The main discharge pulse created oxygen atoms by dissociation of O2 molecules. The second (probe) discharge pulse excited existing atoms in the afterglow. The decay of the density of the oxygen atoms in volume was monitored by discharge induced emission spectroscopy of O lines and Ar actinometric lines. The rate of decay of oxygen atoms was used to determine their surface loss probability. The determined surface loss probabilities are: Simax (3.9 ± 0.65) × 10−3, PET (9.3 ± 1.3) × 10−4, Teflon (6.6 ± 1.3) × 10−4, and Mica (1.2 ± 0.3) × 10−3.

Modeling Cl2/O2/Ar inductively coupled plasmas used for silicon etching: effects of SiO2 chamber wall coating

In this paper, simulations are performed to gain a better insight into the properties of a Cl2/Ar plasma, with and without O2, during plasma etching of Si. Both plasma and surface properties are calculated in a self-consistent manner. Special attention is paid to the behavior of etch products coming from the wafer or the walls, and how the chamber walls can affect the plasma and the resulting etch process. Two modeling cases are considered. In the first case, the reactor walls are defined as clean (Al2O3), whereas in the second case a SiO2 coating is introduced on the reactor walls before the etching process, so that oxygen will be sputtered from the walls and introduced into the plasma. For this reason, a detailed reaction set is presented for a Cl2/O2/Ar plasma containing etched species, as well as an extensive reaction set for surface processes, including physical and chemical sputtering, chemical etching and deposition processes. Density and flux profiles of various species are presented for a better understanding of the bulk plasma during the etching process. Detailed information is also given on the composition of the surfaces at various locations of the reactor, on the etch products in the plasma and on the surface loss probabilities of the plasma species at the walls, with different compositions. It is found that in the clean chamber, walls are mostly chlorinated (Al2Cl3), with a thin layer of etch products residing on the wall. In the coated chamber, an oxy-chloride layer is grown on the walls for a few nanometers during the etching process. The Cl atom wall loss probability is found to decrease significantly in the coated chamber, hence increasing the etch rate. SiCl2, SiCl4 and SiCl3 are found to be the main etch products in the plasma, with the fraction of SiCl2 being always slightly higher. The simulation results compare well with experimental data available from the literature.

Spatial Distributions of Electron, CF, and CF2 Radical Densities and Gas Temperature in DC-Superposed Dual-Frequency Capacitively Coupled Plasma Etch Reactor Employing Cyclic-C4F8/N2/Ar Gas

On a plasma etch reactor for a wafer of 300 mm in diameter, the spatial distributions of the absolute densities of CF and CF2 radicals, electron density (ne), and the gas temperature (Tg) of N2 were measured employing the dual frequency of negative dc voltage superposed to a very high frequency (VHF) of 60 MHz capacitively coupled plasma (DS-2f-CCP) with the cyclic- (c-)C4F8/Ar/N2 gas mixture. The dc bias was superposed on the upper electrode with a frequency of 60 MHz. The distributions of electron and radical densities were uniform within a diameter of about 260 mm, and took a monotonic decay in regions outside a diameter of 260 mm on the reactor for 300 mm wafers in the reactor. It was found that only CF2 density at the radial position between 150 and 180 mm, corresponding to the position of the Si focus ring, dropped, while CF density took a uniform distribution over a diameter of 260 mm. Additionally, at this position, the rotational temperature of N2 gas increased to be 100 K larger than that at the center position. CF2 radical density was markedly affected by the modified surface loss probability of the material owing to coupling with surface temperature.

Surface Loss Probability of Nitrogen Atom on Stainless-Steel in N2 Plasma Afterglow

We investigated the loss kinetics of nitrogen (N) atoms in a N2 plasma afterglow using a vacuum ultraviolet absorption spectroscopy technique with an atmospheric-pressure microdischarge hollow cathode lamp. The decay curves of N atom density were fitted with single exponential functions at pressures from 1.33 to 13.3 Pa. The dependence of the decay time constant on the pressure showed that the N atoms were predominantly lost through diffusion to the wall surface. The surface loss probability of N atoms on stainless-steel based on the decay time constant as a function of pressure was estimated to be 0.03.

A comparative study of the surface recombination of nitrogen atoms on various materials at atmospheric pressure

The surface recombination of nitrogen atoms on various materials was investigated at atmospheric pressure. The afterglows of pure nitrogen discharge are injected into a surface loss test unit, whose path wall is composed of the materials to be examined. Measurement of the nitrogen atom densities at several points in the flowing afterglows reveals the temporal decay characteristics. The results are compared with two-dimensional numerical simulations to obtain the surface loss probability γ. Among the seven materials we studied (stainless steel, aluminium, titanium, alumina, Pyrex glass, aluminium nitride and boron nitride), stainless steel was found to have the highest γ value: 1.8 × 10−3; the lowest was for boron nitride: 4.8 × 10−5. The dependence of γ on surface roughness and temperature was also examined.

Surface loss probabilities of H and N radicals on different materials in afterglow plasmas employing H2 and N2 mixture gases

Surface loss probabilities of hydrogen (H) and nitrogen (N) radicals on different wall materials in H2/N2 mixture plasmas have been investigated by employing vacuum ultraviolet (VUV) absorption spectroscopy with a high pressure microdischarge hollow cathode lamp as a light source. The surface loss probability of a radical was calculated by using the lifetime obtained from the decay curve of the radical density in afterglow plasmas. The surface loss probabilities on different walls in the H2/N2 mixture plasmas were higher than those in a pure H2 or N2 plasma. The behaviors of species such as ions and VUV photons as a function of the gas mixture ratio were measured to investigate those influences on plasma-surface interactions. In addition, changes on the surface exposed to the plasma were analyzed by x-ray photoelectron spectroscopy. Quantitative measurements of surface loss probabilities of radicals on various wall materials are expected to be crucially important in achieving good understanding of the interaction between the surface and the plasma.

Fuel removal from tile gaps with oxygen discharges: reactivity of neutrals

Carbon removal in the so-called remote areas, where no energetic species can reach the surface, is investigated with low-temperature glow discharges in pure oxygen. Plasma-deposited amorphous hydrogenated carbon thin films are used as a model system for redeposited films. Erosion measurements are performed on flat substrates as well as on two different three-dimensional (3D) test structures. One design consists of 19 mm deep gaps with widths ranging from 0.5 to 4 mm to simulate ITER tile gaps. A second design consists of a flat box-like structure where particles can enter only through a narrow slit. Measurements are performed for substrate temperatures ranging from 290 to 580 K. Erosion rates follow an Arrhenius-type dependence on substrate temperature with an effective activation energy of 0.25 eV. While at room temperature the surface loss probability of the dominant eroding species is 50% it becomes smaller at elevated temperatures. At elevated temperatures, the films are not only removed faster but simultaneously erosion penetrates deeper into the gaps.

Influence of the reactor wall composition on radicals’ densities and total pressure in Cl2 inductively coupled plasmas: II. During silicon etching

In an industrial inductively coupled plasma reactor dedicated to silicon etching in chlorine-based chemistry, the density of Cl2 molecules and the gas temperature are measured by means of laser absorption techniques, the density of SiClx (x⩽2) radicals by broadband absorption spectroscopy, the density of SiCl4 and ions by mass spectrometry, and the total gas pressure with a capacitance gauge. These measurements permit us to estimate the mole fractions of Cl, SiCl4, and etch product radicals when etching a 200mm diameter silicon wafer. The pure Cl2 plasma is operated in well prepared chamber wall coating with a thin film of SiOCl, AlF, CCl, or TiOCl. The impact of the chemical nature of the reactor wall’s coatings on these mole fractions is studied systematically. We show that the reactor wall coatings have a huge influence on the radicals densities, but this is not only from the difference on Cl–Cl recombination coefficient on different surfaces. During silicon etching, SiClx radicals sticking on the reactor walls are etched by Cl atoms and recycled into the plasma by forming volatile SiCl4. Hence, the loss of Cl atoms in etching the wall deposited silicon is at least as important as their wall recombination in controlling the Cl atoms density. Furthermore, because SiCl4 is produced at high rate by both the wafer and reactor walls, it is the predominant etching product in the gas phase. However, the percentage of redeposited silicon that can be recycled into the plasma depends on the amount of oxygen present in the plasma: O atoms produced by etching the quartz roof window fix Si on the reactor walls by forming a SiOCl deposit. Hence, the higher the O density is, the lower the SiCl4 density will be, because silicon is pumped by the reactor walls and the SiOCl layer formed is not isotropically etched by chlorine. As a result, in the same pure Cl2 plasma at 20mTorr, the SiClx mole fraction can vary from 18% in a SiOCl-coated reactor, where the O density is the highest, to 62% in a carbon-coated reactor, where there is no O. In the latter case, most of the Cl mass injected in the reactor is stored in SiCl4 molecules, which results in a low silicon etch rate. In this condition, the Cl mass balance is verified within 10%, and from the silicon mass balance we concluded that SiClx radicals have a high surface loss probability. The impact of the reactor wall coating on the etching process is thus important, but the mechanisms by which the walls control the plasma chemistry is much more complicated than a simple control through recombination reaction of halogen atoms on these surfaces.

Role of the wall reactor material on the species density distributions in an N2–O2 post-discharge for plasma sterilization

The species density distributions in a large post-discharge reactor placed downstream from a flowing microwave discharge in N2–O2 are calculated using a three-dimensional hydrodynamic model. The effects of surface losses of N(4S) and O(3P) atoms on the density distributions of different species in the reactor are investigated for three different wall materials: (i) Pyrex, (ii) aluminium and (iii) stainless steel. The effects produced by considering different surface loss probabilities corresponding to each one of these materials, as well as by assuming the production of NO from the wall, are evaluated and discussed. The simulation is conducted for the case of a 65 × 25 × 25 cm3 post-discharge reactor fed from a 2450 MHz discharge, at 2 Torr and 2 × 103 sccm flow rate, in an N2–xO2 mixture composition, with x = 0.5–7%.

Insights into sticking of radicals on surfaces for smart plasma nano-processing

In reactive plasma processing, species produced in the plasma reach the surface of a substrate and cause etching, deposition and surface modification through surface reactions. These reactions are characterized by the densities and energies of species incident on the surfaces. In order to realize nano-scale plasma processing, important species for plasma processing have been identified and characterized, and their behavior, not only in the gas phase, but also on the surface, have been clarified and controlled. One of the most critical parameters for insights into surface reaction kinetics of radicals is sticking and surface loss probability. On the basis of radical densities measured by various methods, the sticking and surface reaction loss probabilities have been compiled, and they enable the quantitative understanding of the kinetics of radicals on the surface in the plasma. In this article, the sticking and surface reaction loss probabilities measured thus far are reviewed focusing on fluorocarbon gas, silane gas and methane gas based plasma processes. The establishment of a smart plasma process and the development of an autonomous production device with control of radicals on the basis of insights into the surface reactions for nano-scale plasma processing are presented.

Further insight into the mechanism of hydrocarbon layer formation below the divertor of ASDEX Upgrade

The surface loss probability of hydrocarbon radicals was measured below the roof baffle of the ASDEX Upgrade divertor using the cavity technique. Hydrocarbon layers are mainly formed by sticking of hydrocarbon radicals with high surface loss probabilities of about 0.2 and close to unity. In addition to sticking, re-erosion by atomic hydrogen plays an important role in layer formation. The temperature dependence of layer formation was measured with heated and cooled long term samples from 77 to 475 K. The layer growth rate is larger by a factor of about 40 at 77 K compared with room temperature, while it is lower by a factor of about 70 at 475 K than at room temperature due to enhanced re-erosion. Implications of the results for predictions of tritium retention in future fusion devices and hydrocarbon layer formation on mirror surfaces are discussed.

Discharged generator of singlet oxygen for oxygen-iodine laser. Transport kinetics of O2(a 1δg) and O2(b 1σ g + ) molecules and O(3 P) atoms in Ar:O2 and He:O2 flows excited by a 13.56-MHz discharge

To understand and reveal the basic physical factors providing the possibility of scaling of a discharged singlet oxygen generator (DSOG) in an oxygen-iodine laser, the production, and transport kinetics of metastable O2(a 1δg) and O2(b 1σ g + ) molecules, as well as O(3 P) atoms, were investigated in Ar:O2 and He:O2 gas flows excited by a 13.56-MHz discharge in a wide range of pressures (4–40 Torr) and oxygen percentages. It is shown that the densities and transport kinetics of O2(a 1δg), O2(b 1σ g + ), and O(3 P) appear similar for oxygen mixtures with argon and helium in the same conditions independent of discharge mode. Compared to pure O2, the dilution of oxygen with an inert gas allows higher energy inputs per an oxygen molecule to achieved, especially under conditions of the homogeneous discharge mode (α-mode), which gives a higher efficiency of O2(a 1δg) excitation in Ar:O2 and He:O2 mixtures. But the maximum attainable yield of singlet oxygen in Ar:O2 and He:O2 at fixed partial O2 pressure is found to be comparable with the O2(a 1δg) yield in pure oxygen at the same pressure. The reason for this is the increased three-body deactivation of O2(a 1δg) by atomic oxygen in the mixtures because of the greater total pressure. The estimation of the rate constant of O2(a 1δg) three-body quenching by O(3 P) in Ar:O2 and He:O2 mixtures as (1.5 ± 0.5) × 10−32 cm6/s was carried out from the analysis of transport kinetics of singlet and atomic oxygen in the discharge afterglow at high pressures exceeding ∼10 Torr. A similar analysis for the lower pressures has revealed that losses both of metastable O2(a 1δg) and O2(b 1σ g + ) molecules, and of O(3 P) atoms on the surface of the discharge tube, are determined by the density of each of the components. The obtained loss probabilities of O2(a 1δg), O2(b 1σ g + ), and O(3 P) on the silica surface show that the surface loss probabilities of all the species can increase noticeably under the discharge exposure. Thus, the key parameters determining the maximal O2(a 1δg) yield in the DSOG are a homogeneous volumetric mode of the discharge, energy input per oxygen molecule in this mode, and a low rate of O2(a 1δg) quenching. Just three-body quenching of O2(a 1δg) by O(3 P) limits the singlet oxygen yield with increasing pressure. The fast removal of atomic oxygen both in discharge and in the earlier afterglow could provide DSOG scaling with pressure.

Atomic oxygen surface loss probability on silica in microwave plasmas studied by a pulsed induced fluorescence technique

The aim of this paper is to determine the atomic oxygen surface loss probability on silica under microwave plasma conditions around 133 Pa (1 Torr). A pulsed induced fluorescence technique where a main long pulse creates the plasma and a shorter one re-excites atoms in the time post-discharge was used. The method and its validity under the present experimental conditions are discussed at large. The oxygen surface loss probability on silica is found to be around 3% under plasma conditions, while it is estimated to be two orders of magnitude lower for a surface not submitted to the plasma.

Measurement of the differential electron surface and volume excitation probability in Cu, CuO and Cu2O

AbstractReflection electron energy loss spectra (REELS) were measured for Cu and in situ‐grown Cu2O and CuO. The differential surface and volume inelastic excitation probability is extracted from the REELS using a rigorous procedure developed recently. The resulting surface and volume loss probabilities for Cu agree quantitatively with theoretical calculations based on optical data. The resulting loss distributions can be used for a rigorous line shape analysis of XPS and AES spectra of these materials. Copyright © 2006 John Wiley &amp; Sons, Ltd.

06/00104 Sticking coefficient and surface loss probability of eroded species during bombardment of carbon with deuterium: Roth, J. and Hopf, C. Journal of Nuclear Materials, 2004, 334, (2–3),97–103

06/00104 Sticking coefficient and surface loss probability of eroded species during bombardment of carbon with deuterium: Roth, J. and Hopf, C. Journal of Nuclear Materials, 2004, 334, (2–3),97–103

Atomic hydrogen densities in capacitively coupled very high-frequency plasmas in H2: Effect of excitation frequency

Absolute hydrogen atom densities in pure hydrogen capacitive discharges were measured as a function of excitation frequency (13.56, 27.12, and 40.68 MHz), nominal electrical power, and gas pressure (between 0.1 and 1 Torr). Quantitative measurements were made using two-photon absorption laser-induced fluorescence (TALIF), put on an absolute scale by comparison with the TALIF signal from a known density of krypton gas, as proposed by Niemi, Schultz von Gathen, and Döbele [J. Phys. D 34, 2330 (2001)]. The H atom density increases with gas pressure and electrical power, and at a given power and pressure it increases significantly with excitation frequency. The latter can be attributed in part to increased electron density. However, time-resolved TALIF measurements in the afterglow showed that the H atom surface loss probabilities are not constant, becoming somewhat smaller when the sheath voltage is lowered, as is the case when the excitation frequency is increased, contributing to the increase in H density.

Correlation and Interaction between Sidewall Passivation and Chamber Walls Deposition During Silicon Gate Etching

AbstractThe aim of the present paper is to review our work on plasma‐surface interactions during the etching of silicon gates in low pressure high density HBr/Cl2/O2 based plasmas. In integrated circuit manufacturing, the transfer of patterns onto the polysilicon layer relies on the formation of a passivation layer on sidewalls of the etched features, which protects the silicon gate sidewall fromisotropic etching. At the same time, a layer with an almost identical chemical composition is also deposited on the reactor walls and can modify the process parameters by changing the radicals' surface loss probability. In this work we use a new “plasma etching‐sputtering” diagnostic technique, involving a subsequent “mild etching” of the layer deposited on the chamber walls by a few % SF6‐containing Ar plasma. Thus we investigate the chemical composition of this layer. This chemical composition is deduced from the time variation of the gas phase concentration of different atoms and radicals, the etch products of the wall deposited layer, monitored during the “mild etching” of the layer by time‐resolved optical emission spectroscopy and mass spectrometry.Optical absorption spectroscopy and mass spectrometry have been employed to characterize the HBr/Cl2/O2 plasma, currently used for the gate etching, and to estimate the absolute flux of neutral and ionic deposition precursors reaching the reactor walls. It comes out that the silicon wafer etch byproducts, SiClx radicals (x &lt; 2) and SiOyCl+x ions, are the main depositing agents.Finally, the chemical topography analysis method, based on the X‐ray photoelectron spectroscopy, has been used to determine the chemical composition of the layer formed on the sidewalls of the silicon etched features. We show that the layers deposited on the sidewalls and on the chamber walls are both silicon oxyhalogenides (SiOyClx) compounds, formed by the subsequent oxidation of the deposited silicon chloride etching by‐products. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Sticking coefficient and surface loss probability of eroded species during bombardment of carbon with deuterium

Investigations on the deposition behaviour of eroded carbon species under controlled ion beam conditions were performed to evaluate absolute sticking coefficients. Pyrolytic carbon was eroded by D ions at energies between 45 eV and 1 keV and temperatures between 20 and 475 °C. The sticking coefficient of the eroded species was obtained from a quantitative comparison of the total deposition inside a cavity with the flux of carbon entering the cavity as well as from the distribution of deposited atoms inside the cavity. High sticking coefficients of the order of 0.6 were obtained at room temperature and high ion energy, as expected from physically eroded atoms. In contrast, hydrocarbon molecules chemically emitted at either elevated temperature or low ion energy have typical sticking coefficients around 0.02. At elevated temperatures, the quantitative comparison with weight-loss measurements indicate the dominance of saturated hydrocarbons with zero sticking to the cavity walls.