Plasma Surface Reactions Research Articles

Kinetic Monte Carlo simulations of plasma-surface reactions on heterogeneous surfaces

Reactions of atoms and molecules on chamber walls in contact with low temperature plasmas are important in various technological applications. Plasma-surface interactions are complex and relatively poorly understood. Experiments performed over the last decade by several groups prove that interactions of reactive species with relevant plasma-facing materials are characterized by distributions of adsorption energy and reactivity. In this paper, we develop a kinetic Monte Carlo (KMC) model that can effectively handle chemical kinetics on such heterogenous surfaces. Using this model, we analyse published adsorption-desorption kinetics of chlorine molecules and recombination of oxygen atoms on rotating substrates as a test case for the KMC model.

Plasma-induced physicochemical effects on a poly(amide) thin-film composite membrane

Helium and water gas plasma were used to modify the surface morphology and chemistry of commercial thin film composite membranes. Such surface treatment is a convenient tool to alter performance of the membranes and subsequently reduce interactions with contaminants in solution. Plasma reactions such as etching and re-deposition resulted in surface functionalization and texturation which were correlated with membrane flux and salt rejection changes. Investigations conducted using atomic force microscopy revealed morphology alterations which were apparent as either smoother surfaces or rougher surfaces. While the smoothing was attributed to a re-deposition of polymer material, which originated most likely from vaporization of surface polymer, the roughening was a result from balanced plasma surface reactions. The beneficial etching can occur under certain conditions when helium is used in conjunction with low excitation power, which resulted in flux enhancements up to 66% (i.e. from 30L.m−2.h−1 to 50L.m−2.h−1) with 98% rejection. The hydrophilicity was dramatically increased from 46.6° to as low as 8.9° after 5min of helium plasma duration. The impact that plasma treatments have on the nascent structure was discussed in order to highlight its application as a convenient tool to functionalize the surface of thin film composite membranes.

Dynamical Monte Carlo methods for plasma-surface reactions

Different dynamical Monte Carlo algorithms to investigate molecule formation on surfaces are developed, evaluated and compared with the deterministic approach based on reaction-rate equations. These include a null event algorithm, the n-fold way/BKL algorithm and an ‘hybrid’ variant of the latter. NO2 formation by NO oxidation on Pyrex and O recombination on silica with the formation of O2 are taken as case studies. The influence of the grid size on the CPU calculation time and the accuracy of the results is analysed. The role of Langmuir–Hinsehlwood recombination involving two physisorbed atoms and the effect of back diffusion and its inclusion in a deterministic formulation are investigated and discussed. It is shown that dynamical Monte Carlo schemes are flexible, simple to implement, describe easily elementary processes that are not straightforward to include in deterministic simulations, can run very efficiently if appropriately chosen and give highly reliable results. Moreover, the present approach provides a relatively simple procedure to describe fully coupled surface and gas phase chemistries.

Energy fluxes in a radio-frequency magnetron discharge for the deposition of superhard cubic boron nitride coatings

Energy flux measurements by a calorimetric probe in a rf-magnetron plasma used for the deposition of super-hard c-BN coatings are presented and discussed. Argon as working gas is used for sputtering a h-BN target. Adding a certain amount of N2 is essential for the formation of stoichiometric BN films, since a lack of nitrogen will lead to boron rich films. Subsequently, the contributions of different plasma species, surface reactions, and film growth to the resulting variation of the substrate temperature in dependence on nitrogen admixture are estimated and discussed. In addition, SRIM simulations are performed to estimate the energy influx by sputtered neutral atoms. The influence of magnetron target power and oxygen admixture (for comparison with nitrogen) to the process gas on the total energy flux is determined and discussed qualitatively, too. The results indicate that variation of the energy influx due to additional nitrogen flow, which causes a decrease of electron and ion densities, electron temperature and plasma potential, is negligible, while the admixture of oxygen leads to a drastic increase of the energy influx. The typical hysteresis effect which can be observed during magnetron sputtering in oxygen containing gas mixtures has also been confirmed in the energy influx measurements for the investigated system. However, the underlying mechanism is not understood yet, and will be addressed in further investigations.

Critical review: Plasma-surface reactions and the spinning wall method

This article reviews methods for studying reactions of atoms and small molecules on substrates and chamber walls that are immersed in a plasma, a relatively unexplored, yet very important area of plasma science and technology. Emphasis is placed on the “spinning wall” technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated, and the surface is periodically exposed to the plasma and then to a differentially pumped mass spectrometer, to an Auger electron spectrometer, and, optionally, to a beam of additional reactants or surface coatings. Reactants impinging on the surface can stick and react over time scales that are comparable to the substrate rotation period, which can be varied from ∼0.5 to 40 ms. Langmuir–Hinshelwood reaction probabilities can be derived from a measurement of the absolute desorption product yields as a function of the substrate rotation frequency. Auger electron spectroscopy allows the plasma-immersed surface to be monitored during plasma operation. This measurement is critical, since wall “conditioning” in the plasma changes the reaction probabilities. Mass spectrometer cracking patterns are used to identify simple desorption products such as Cl2, O2, ClO, and ClO2. Desorption products also produce a measurable pressure rise in the second differentially pumped chamber that can be used to obtain absolute desorption yields. The surface can also be coated with films that can be deposited by sputtering a target in the plasma or by evaporating material from a Knudsen cell in the differentially pumped wall chamber. Here, the authors review this new spinning wall technique in detail, describing both experimental issues and data analysis methods and interpretations. The authors have used the spinning wall method to study the recombination of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In oxygen or chlorine plasmas, these surfaces become coated with a layer containing Si, Al, and O, due to slow erosion of the reactor materials, in addition to Cl in chlorine plasmas. Similar, low recombination probabilities were found for Cl and O on anodized Al versus stainless steel surfaces, consistent with the similar chemical composition of the layer that forms on these surfaces after long exposure to the plasma. In chlorine plasmas, weakly adsorbed Cl2 was found to inhibit Cl recombination, hence the Cl recombination probability decreases with increasing Cl2-to-Cl number density ratios in the plasma. In mixed Cl2/O2 plasmas, Cl and O recombine to form Cl2 and O2 with probabilities that are similar to those in pure chlorine or oxygen plasmas, but in addition, ClO and ClO2 form on the surface and desorb from the wall. These and other results, including the catalytic enhancement of O recombination by monolayer amounts of Cu, are reviewed.

Application of quantum cascade laser absorption spectroscopy to studies of fluorocarbon molecules

The recent advent of quantum cascade lasers (QCLs) enables room-temperature mid-infrared spectrometer operation which is particularly favourable for industrial process monitoring and control, i.e. the detection of transient and stable molecular species. Conversely, fluorocarbon containing radio-frequency discharges are of special interest for plasma etching and deposition as well as for fundamental studies on gas phase and plasma surface reactions. The application of QCL absorption spectroscopy to such low pressure plasmas is typically hampered by non-linear effects connected with the pulsed mode of the lasers. Nevertheless, adequate calibration can eliminate such effects, especially in the case of complex spectra where single line parameters are not available. In order to facilitate measurements in fluorocarbon plasmas, studies on complex spectra of CF4 and C3F8 at 7.86 μm (1269 – 1275 cm-1) under low pressure conditions have been performed. The intra-pulse mode, i.e. pulses of up to 300 ns, was applied yielding highly resolved spectral scans of ∼ 1 cm-1 coverage. Effective absorption cross sections were determined and their temperature dependence was studied in the relevant range up to 400 K and found to be non-negligible.

Plasma Diagnostics for Unraveling Process Chemistry

This review focuses on the use of diagnostic tools to examine plasma processing chemistry, primarily plasma species energetics, dynamics, and molecule-surface reactions. We describe the use of optical diagnostic tools, mass spectrometry, and Langmuir probes in measuring species densities, rotational and kinetic energies, and plasma-surface reactions. Molecule-surface interactions for MX(n) species (M = C, Si, N; X = H, F, Cl) are presented and interpreted with respect to the molecule's electronic configuration and dipole moments.

The influence of substrate morphology on the growth of thin silicon films: A GISAXS study

Thin Si films, with thickness between 100 and 300 nm, were deposited by PECVD (Plasma Enhanced Chemical Vapour Deposition) in silane gas (SiH 4) highly diluted by hydrogen. The degree of dilution and the discharge power were varied in order to obtain different crystalline to amorphous fractions in the films. Two types of substrates were used. The first one was amorphous and relatively flat while the second one was polycrystalline with a roughness of a few tens of nanometers. The crystal fraction in the deposited samples, as estimated by Raman spectroscopy, varied between 0 and 40%, and the individual crystal size was between 2 and 8 nm. The larger individual crystals are usually present in those samples with the highest crystal fraction. The sample density, estimated upon the spectral distribution of the dielectric function in the infra red, was 15–25% less than the density of crystalline silicon. The GISAXS pattern of all of the examined samples indicated the presence of not-spherical-like “particles” in the “bulk” of the thin films, with an average “particle” size between 1.5 and 4 nm. These “particles” are most probably voids and their shape indicates columnar growth. By applying the GISAXS technique on samples deposited on different substrates, the borderline deposition conditions between “transport limited growth” and “growth dominantly influenced by plasma surface reactions” was estimated.

Reactive laser plasma coating formation

Reactive laser plasma surface treatments have proven to have a large potential for technical applications. Here, we present a new promising fast, flexible and clean technique for a direct laser synthesis of carbide and nitride coatings by short pulsed laser irradiation in reactive atmospheres (e.g. methane, nitrogen). The corresponding material is treated by short but intense laser pulses involving a plasma formation just above the irradiated surface. Gas–Plasma–Surface reactions lead to a fast incorporation of the gas species into the material and subsequently to the desired coating formation if the treatment parameters are chosen properly. This reactive laser synthesis can also be used for the improvement of steel surfaces, achieving drastically improved hardness, corrosion and wear resistance. We report here on the irradiation of pure iron in methane atmosphere. At low number of laser pulses, this results mostly in the formation of hexagonal ɛ-FexC, which is more and more transformed into cementite (θ-Fe3C) when increasing the number of pulses.

On the importance of ions and ion-molecule reactions to plasma-surface interface reactions

Ions are known to be key players in many plasma processes, including anisotropic etching, film deposition and surface modification. The relationship between plasma ions, film properties, and surface interactions of other plasma species is not, however, well known. Using our Imaging of Radicals Interacting with Surfaces (IRIS) technique, along with plasma ion mass spectrometry (PI-MS), and surface analysis data, we have measured the effects of ion bombardment on the surface interactions of SiF 2 in SiF 4 plasmas and of CF 2 in C 3F 8 and C 4F 8 plasmas. SiF 2 is a known product of F-atom etching of Si, and CF 2 has also been cited as a product of fluorocarbon etching of Si. With both molecules, we measure surface generation when the surface is bombarded by all the plasma species. Removal of ions from the plasma molecular beam results in a net decrease in surface generation for both molecules at all powers. Results in both systems are compared with the gas-phase ion-molecule reaction data of Armentrout and coworkers. Preliminary guided-ion beam mass spectrometry results taken in the Armentrout laboratories for the Ar + + C 3F 8 reaction system are also presented.

半導体プロセスとプラズマ表面反応

Plasma surface reactions important in the processes of semiconductor fabrication are reviewed. Plasma etching mechanisms are discussed from the relationship between the etching characteristics and the plasma diagnostics results. Oxide etching using a novel chemistry is also presented, which indicates the importance of the electron collision cross section measurement for various feedstock gases employed in plasma processing.

Gas-phase and plasma-surface reactions in radiofrequency discharges of C 2H 2–N 2–noble gas mixtures

Actinometric optical emission spectrometry was used to determine the relative concentrations of the species H, CH, N 2 and CN as a function of the ratio of the nitrogen flow rate to the acetylene flow rate in acetylene–nitrogen–helium–argon mixtures. The forms of these concentration functions are discussed in terms of gas-phase molecular fragmentation and plasma/film-surface interactions. In plasmas fed only noble gases, additional experiments revealed the occurrence of plasma/film-surface reactions that produce significant plasma concentrations of the species CN provided that a nitrogenated carbon film was previously deposited in the chamber. In contrast, there is no appreciable concentration of the species CH in plasmas of noble gases in the presence of a-C:H:N films on the chamber walls.

Mechanisms for CF2 radical generation and loss on surfaces in fluorocarbon plasmas

During fluorocarbon plasma etching, plasma-surface reactions result in the surface acting as either a source or sink for reactive species, thereby impacting the properties of the bulk plasma. For example, experiments have shown that surfaces in radio frequency (rf) capacitively coupled discharges can be either sources or sinks of CF2 depending on, among other properties, the sheath potential. The coupling of rf bulk and surface reactions, and their combined effects on the CF2 density, were investigated using an integrated plasma equipment and surface kinetics model. While CF2 sticking on surfaces led to its loss, CF2 can be generated from surfaces by energetic ion bombardment resulting in sputtering of polymeric films, or neutralization and dissociation of ions. The net effect of a surface for CF2 production depends on the relative rates of these loss and generation processes. A surface can transform from a net CF2 sink at low incident ion energies to a CF2 source at high ion energies because the CF2 yield by ion–surface interactions typically increases with increasing ion energy. The sensitivity of the model to probabilities of major surface reactions was also investigated.

Plasma-chemical vapor deposition of wide band gap a-SiC:H films: An ab initio molecular-orbital study

Plasma surface reactions for preparing a-SiC:H films system have been quantum chemically studied by using an ab initio molecular orbital method. The calculated results suggest that the sp2 carbons are preferentially formed in a-SiC:H films which have high carbon contents if silane and methane are used as source gases. It makes the band gap of a-SiC:H films with high carbon contents narrow. In order to produce a-SiC:H with wide band gap, a quantum chemical reaction design has been performed. The effect of chlorine and fluorine substitution in carbon sources on plasma surface reactions have been studied. The activation energy for sp3 network formation reactions of carbons which bonds to chlorine with dangling bond site is much lower than that of hydrocarbons with dangling bond site (20.3 and 59.2 kcal/mol, respectively). The results indicate that chloro-substituted methanes are good carbon source gases for preparing a-SiC:H films with a wide band gap.

Plasma-surface reactions: an interdisciplinary complex system of technological interest

Plasma-surface reactions are currently used in various scientific and technological fields. Due to its interdisciplinary nature, it is interesting to introduce the subject in teaching. A simple model is derived, allowing the understanding of intricate roles of various fundamental and technological parameters. From the example, it is shown how to treat an interdisciplinary system. Also, this is a good case of a technological subject where parameters are interdependent. Despite this, the careful analysis of the equations governing the physico- chemistry of the system gives rise to the derivation of useful scaling laws. Numerical evaluation also gives evidence of hysteresis effects in plasma-surface reactions.

Reactive ion etching of a-SiC:H films using CCl4 and O2 gas mixture

Reactive ion etching (RIE) of a-SiC:H thin films in a CCl4/O2 plasma has been investigated. The effects of process parameters, in particular radio-frequency power, oxygen addition, gas flow rate, interelectrode spacing, and pumping speed, on a-SiC:H etch rate and a-SiC:H/oxide etch selectivity are reported. For the RIE of a-SiC:H in CCl4/O2 plasmas, the gas phase plasma chemistry and surface reactions appear to be more important than ion bombardment. Reactive species generated from neutral chemical reactions of oxygen in the discharge play a more dominant role than those generated from the electron impact dissociation processes.

Translationally hot neutrals in etching discharges

The presence and influence of translationally energetic ions in low-pressure etching discharges is well known. Neutral atoms and molecules, though known to be chemically reactive, are not generally considered to be otherwise activated in these plasmas. Neutral species may, however, become translationally hot through either charge exchange collisions or by dissociative excitation caused by electron impact. These species are important in etching discharges because they may bring an isotropic source of activation energy to the substrate which may compromise anisotropic etching mechanisms. In this paper we present a theoretical study of the sources and effects of translationally hot neutral atoms and molecules in CF4 etching plasmas. We find that ballistically hot F atoms comprise a significant fraction of the radical flux striking the substrate at pressures of <100 mTorr. In CF4 plasmas, the maximum flux of translationally hot F incident on the substrate of a parallel-plate rf etching discharge occurs between 10 and 100 mTorr. At these pressures the hot atom and ion fluxes to the substrate are comparable. The effects of translationally hot species on gas-phase plasma chemistry and surface reactions are discussed.

Reactive ion etching of plasma enhanced chemical vapor deposition amorphous silicon and silicon nitride: Feeding gas effects

Reactive ion etching of PECVD a-Si:H and silicon nitride films was investigated using CF4, C2F6, CHF3, CF3Cl, and CF2Cl2 gases. Under the same power the a-Si:H etch rate decreases in the order of: CF4∼CF3Cl>CF2Cl2>C2F6>CHF3; and the nitride etch rate decreases in the order of: CF4>CF3Cl∼C2F6>CF2Cl2>CHF3. The etch selectivity is controlled by two competitive mechanisms: plasma chemistry and surface reactions. When no surface mass-transfer-limit layer is formed, the plasma phase chemistry dominates the etch rate of the film. For example, the silicon etch rate increases with the chlorine concentration in the plasma phase. When the surface is covered with a thin film, the etch rate is controlled by the ion bombardment energy. For example, the film etch rate in the C2F6 plasma depends on the self-bias voltage only. ESCA and emission spectra results support the above theory.

Direct observation, by the matrix isolation technique, of nitric oxide effects in low-pressure silane discharges

Hydrogenated silicon radicals produced in a 0.2 mTorr silane multipole dc plasma are compared to those observed in 0.2 m Torr silane +0.3 mTorr nitric oxide plasma. The detection of plasma radicals has been performed by the solid Ar matrix isolation technique, associated with IR absorption spectroscopy. This study presents evidence for the IR absorption line assignments. When NO is added to the silane plasma, the concentration of silyl relative to silylene radicals increases while the silane and disilane decrease. The results are discussed in terms of plasma-surface chemical reactions; due to the low pressure of silane and nitric oxide, no gas phase radical reactions can occur.

Plasma phase polymerization reactions in the deposition of glow discharge deposited a-Ge:H alloy films

Films of a-Ge:H deposited from low power rf plasmas of GeH4 and H2-diluted GeH4 at substrate temperatures Ts=100 to 400 °C have been characterized by IR spectrometry to determine the levels of Ge monohydride and polyhydride bonding. Correlation of the changes in hydrogen bonding and deposition rate for these and previously studied a-Ge:H films with the plasma phase deposition parameters (power density and degree of dilution) and the Ts parameter leads to a general model of H incorporation which involves both plasma phase and surface reactions. Plasma reactions can generate either monomeric or polymeric reactive species. These species then undergo thermally activated surface reactions which further determine the degree of hydrogen elimination and subsequent incorporation as monohydride or polyhydride.