Cross Sections For Electron-impact Dissociation Research Articles

Electron impact dissociation cross sections for C2F6

Absolute total dissociation cross sections for electron impact, σt,diss, from 8 to 700 eV are reported for C2F6. A dense set of data was obtained in the technologically important 8–30 eV energy range relevant to modelling the type of plasmas used in both fundamental and applied scientific investigations. The threshold for dissociation was found to be 12.0 ± 0.2 eV and appears to be associated with a Rydberg state. Estimated values for the total neutral dissociation cross section, σneut,diss, were obtained by subtracting the ionization cross sections (all ionizing events cause dissociation) from the total dissociation cross section. It is shown that a calibration error in a paper by one of us (HFW) caused a distortion of several previous investigations. When the data from the present work are used to recalibrate data from swarm experiments, agreement becomes quite reasonable. There is now a consistent set of data obtained from several investigators which describe the dissociation of C2F6. Neutral dissociation cross sections are obtained from electron-impact excitation calculations and found to be in reasonable agreement with measurements over most of the energy range.

Electron-impact dissociation of D13CO+ molecular ions to 13CO+ ions

Absolute cross sections for electron-impact dissociation of D13CO+ ions have been measured over a collision energy range from 4 to 100 eV with a crossed electron–ion beams method. Total experimental uncertainties are about 16% near the cross section peak. The role of resonant and direct dissociative excitation for energies below 21 eV is discussed in light of the energy levels and photo-dissociation cross sections of the HCO+ formyl cation predicted by ab initio multi-reference configuration interaction calculations.

Production and loss mechanisms of SiClX etch products during silicon etching in a high density HBr∕Cl2∕O2 plasma

Si Cl X (X=0–2) radicals’ concentrations have been measured by broadband ultraviolet absorption spectroscopy during the etching of 200mm diameter silicon wafers in HBr∕Cl2∕O2 plasmas. We report the variations of the concentrations of these radicals as a function of the radio frequency (rf) source power and rf-bias power. The silicon wafer etch rate is measured simultaneously. From the measured radicals densities and using electron impact ionization cross sections found in the literature, the densities of SiClX+ ions are calculated and are found to be in good agreement with ion densities measured by mass spectrometry. The upper limit for the SiCl2 radical concentration is calculated from the wafer etch rate. By comparison with the measured SiCl2 radical concentration it is concluded that SiCl2 radicals should also be produced by the reactor walls due to the etching of silicon containing species adsorbed on the reactor walls. Finally, using electron impact dissociation cross sections, the densities of SiCl and Si are calculated from the measured densities of SiCl2 and SiCl, respectively. The comparison between the calculated and measured values of SiClX densities allowed us to conclude that SiCl (and Si) are produced both in the gas phase by electron impact dissociation of SiCl2 (SiCl) radicals and at the reactor wall surfaces by the neutralization and reflection of ≈50% of the flux of SiCl+ (Si+) ions impinging on these surfaces. At the same time SiCl and Si are estimated to be lost (adsorption and abstraction reactions) on the reactor walls with a probability ranging between 0.2 and 1.

Electron-impact dissociation ofCH+ions: Measurement ofC+fragment ions

Absolute cross sections for electron-impact dissociation of ${\mathrm{CH}}^{+}$ producing ${\mathrm{C}}^{+}$ fragment ions were measured in the 3--100 eV range using a crossed electron-ion beams technique with total uncertainties of about 10% near the cross-section peak. Although the measured energy dependence agrees well with two sets of storage ring measurements, the magnitude of the present results lies about 15--25 % below the other results at the cross-section peak near 40 eV. Below 10 eV, the present data tend to exceed the storage ring data, consistent with the presence of excited states in the ${\mathrm{CH}}^{+}$ ion beam. For energies above 29 eV, the cross sections should also include contributions from dissociative ionization, though no clear onset of this channel is evident in any of the three sets of measured data.

Calculated rates for the electron impact dissociation of molecular hydrogen: mixed isotopomers and scaling laws

Following our calculations of electron impact dissociation rates of molecular H2, D2 and T2 (Trevisan and Tennyson 2002 Plasma Phys. Control. Fusion 44 1263–76), calculations for the mixed isotopomers HD, HT and DT have been made. Total cross sections and energy differential cross sections at threshold energies as a function of vibrational states (v) for HD: v = 0–5, for HT: v = 0–5 and for DT: v = 0–7 are calculated for the electron impact dissociation through excitation to the b 3Σu+ excited electronic state, which is the dominant dissociation process at such energies. The rates of dissociation as a function of electron temperature for each state are parametrized. Coinciding with our previous results, near-threshold rates are shown to be so critically dependent on the vibrational level that dissociation from very high-lying vibrational levels must be included in calculations of the rate at local thermal equilibrium (LTE) even at low temperature. Rates for the higher vibrational levels are obtained by extrapolation following the procedure discussed in our previous publication, and are then used to calculate the LTE rate. The LTE rate is an order of magnitude greater than the v = 0 rate. A scaling law for the electron impact dissociation cross sections of vibrationally excited H2 and its isotopomers has been derived.

Kinetics of the reactions involving CF2 and CF in a pure tetrafluoromethane plasma: I. Production of CF2 and CF via electron-impact dissociation

The kinetics of the production and loss of CF2 and CF radicals in a glow discharge in pure CF4 is investigated by the laser-induced fluorescence method. The effective rate constants for electron-impact dissociation of CF4 molecules along the pathways toward CF2 and CF radicals are determined within a wide range of the reduced electric field (80–250 Td). It is shown that, along with the direct electron-impact dissociation of CF4, the radicals are also produced via the dissociation of the CxFy polymer fluorocarbon particles that form in the plasma. A detailed analysis of the kinetics of the radical production and loss in a modulated discharge made it possible to evaluate the contribution of the electron-impact dissociation of CF4 to the production of radicals and, consequently, to determine the dissociation rate constants $$k_{CF_2 } $$ and k CF. A comparison of the obtained $$k_{CF_2 } $$ and k CF values with the results of calculations by the Monte Carlo method and the literature data on the cross sections for electron-impact dissociation of CF4 molecules enabled the normalization of these cross sections in the threshold region and the construction of the model cross sections for the electron-impact dissociation of CF4 into neutral products. The calculated cross sections allow a satisfactory description of the experimental results throughout the entire range of E/N under study. A significant scatter (up to 100%) in the experimental data on $$k_{CF_2 } $$ and k CF at low values of E/N is related to the considerable contribution of the CxFy polymer molecules (and, probably, CxF y + ions and fluorocarbon grains) to the production of CF2 and CF radicals both in the plasma volume and on the surface of a fluorocarbon film covering the discharge tube wall.

Electron Interactions With SF6

Sulfur hexafluoride (SF6) is commonly used as a gaseous dielectric and as a plasma etching gas. In this work, the state of knowledge on electron-interaction cross sections and electron-swarm parameters in SF6 is comprehensively reviewed and critically assessed. Cross sections are presented and discussed for the following scattering processes: total electron scattering; differential elastic; elastic integral; elastic momentum; total vibrational; total and partial ionization; total dissociative and nondissociative electron attachment; and dissociation into neutrals. Coefficients for electron-impact ionization, effective ionization, electron attachment, electron drift, and electron diffusion are also reviewed and assessed. In addition, complementary information on the electronic and molecular structure of the SF6 molecule and on electron detachment and ion transport in parent SF6 gas is provided that allows a better understanding of the nature of the cross sections and swarm parameters. The assessed data are used to deduce cross sections and coefficients for which there exist no direct measurements at the present time. The present work on electron interactions with the SF6 molecule reveals a rather simple picture which can be summarized as follows: (1) Elastic electron scattering is the most significant electron scattering process over the electron energy range from ∼0.01 to ∼1000 eV. (2) Below 15 eV the most distinct inelastic energy-loss process is vibrational excitation—direct dipole excitation involving the ν3 mode and indirect vibrational excitation via negative ion states involving the ν1 mode. (3) Below ∼0.1 eV electron attachment forming SF6− is the most dominant interaction (along with elastic scattering). Above this energy, the cross sections for dissociative electron attachment forming fragment anions [principally SFx− (x=3, 4, and 5) and F−] are appreciable, with the room temperature total electron attachment cross section dominated by the formation of SF5− between ∼0.3 and 1.5 eV and by the formation of F− beyond ∼2.0 eV. (4) Above ∼16 eV dissociative ionization becomes significant, generating principally SFx+ (x=1, 3, 4, and 5) and F+ positive-ion fragments which, together with elastic electron scattering, makes up most of the total electron scattering cross section. (5) Electron-impact dissociation into neutral fragments SFx (x=1, 2, and 3) and F occurs above ∼15 eV, with cross section values potentially exceeding those for ionization for electron energies near 20 eV. (6) The total electron scattering cross section exhibits distinct structure due to negative-ion resonances near 0.0, 2.5, 7.0, and 11.9 eV. The most significant data needs are for direct measurements of vibrational excitation cross sections, for cross sections for electron-impact dissociation into neutral fragments, and for the momentum transfer cross section at low energies.

Photon and high-energy–electron-impact vibronic excitation of molecular hydrogen

We have obtained generalized oscillator strengths, in the framework of the first Born approximation, for the electron-impact excitation of ${\mathrm{H}}_{2}$ through the $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}(v=0)\ensuremath{\rightarrow}B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}{(v}^{\ensuremath{'}}=0\ensuremath{-}33),$ $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}(v=0)\ensuremath{\rightarrow}{B}^{\ensuremath{'}}{}^{1}{\ensuremath{\Sigma}}_{u}^{+}{(v}^{\ensuremath{'}}=0\ensuremath{-}8)$ and $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}(v=0)\ensuremath{\rightarrow}C{}^{1}{\ensuremath{\Pi}}_{u}{(v}^{\ensuremath{'}}=0\ensuremath{-}12)$ vibronic transitions. The target electronic wave functions, previously used to determine electron-impact dissociation cross sections for the same transitions [Phys. Rev. A 57, 1025 (1998)], were calculated at the configuration-interaction level for several internuclear distances. The discrete vibrational wave functions were generated by solving the vibrational Schr\"odinger equation in the Born-Oppenheimer scheme. The Franck-Condon approximation was not invoked. Optical oscillator strengths for the same transitions were also obtained and compared favorably with previous theoretical and experimental results. The calculated generalized oscillator strengths presented good agreement with recent measurements by Zhong et al. [J. Electron Spec. and Rel. Phen. 94, 127 (1998)] for the $B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ and $C{}^{1}{\ensuremath{\Pi}}_{u}$ vibronic transitions.

Photon and electron-impact dissociation cross sections of HCl

We compute high-energy electron-impact (100-1000 eV) dissociation and photodissociation cross sections of HCl producing H and CI neutral atoms through the transition. The electron-impact dissociation cross sections are obtained in the first Born approximation, and are reported for the first time. Hartree-Fock followed by configuration interaction wavefunctions are used and vibrational degrees are considered. The dissociation cross sections as a function of the continuum energy are presented. Photodissociation cross sections are obtained and compared with previous theoretical and experimental results.

Cross Section Measurements for Electron-Impact Dissociation of C 4F 8 into Neutral and Ionic Radicals

The absolute cross sections for electron-impact dissociation of octafluoro-cyclobutane (C4F8) into the neutral radicals CF, CF2, CF3 and C3F5 are measured from the threshold to 250 eV. This measurement was accomplished by appearance mass spectrometry in a dual electron beam device. The threshold energies for the neutral dissociation into CF, CF2 and CF3 are 14.5, 10.5 and 12.8 eV, respectively. The cross sections for dissociation from C4F8 into CF2 and CF are much larger than those from CF4. Besides the neutral dissociation, the cross sections for the dissociative ionization of C4F8 are extensively measured for formation of CF+, CF2+, CF3+, C2F3+, C2F4+ and C3F5+.

Cross Section Measurements for Electron-Impact Dissociation of CHF3 into Neutral and Ionic Radicals

Absolute cross sections for electron-impact dissociation of CHF3 from the threshold to 200 eV are presented for formation of the neutral radicals CF3, CHF2, CF2, CHF and CF. This measurement was accomplished by appearance mass spectrometry in a dual electron beam device. The threshold energies for neutral dissociation into CF3, CHF2 and CF were measured to be 11.0, 13.0 and 19.5 eV, respectively. The surface loss probability of each radical and the electron-impact nitrogen dissociation were measured to calibrate the relative dissociation cross sections of CHF3. The branching ratio for dissociation at 150 eV is CF3:CF2:CF:CHF2:CHF=27:5:10:2:1. In addition to the neutral dissociation, the cross section for dissociative ionization of CHF3 was extensively measured for formation of CF3+, CHF2+, CF2+, CHF+, CF+, CH+ and F+.

Cross section measurements for electron-impact dissociation of SiF4 into neutral radicals

Absolute cross sections for electron-impact dissociation of SiF4 from threshold to 300 eV are presented for formation of the neutral radicals SiF3, SiF2, SiF and Si. This measurement was accomplished with threshold-ionization mass spectrometry in a dual electron-beam device. The threshold energy for dissociation into each neutral radical was measured for the first time: 10.8 eV for SiF3, 13.9 eV for SiF2, 20.4 eV for SiF and 24.6 eV for Si. The surface loss probability of each radical and the electron-impact nitrogen dissociation were measured to calibrate the relative dissociation cross sections of SiF4. The magnitude of the cross section at its maximum is 0.68 * 10-20 m2 at 80 eV for SiF3, 0.16 * 10-20 m2 at 100 eV for SiF2, 0.019 * 10-20 m2 at 100 eV for SiF, and 0.0086 * 10-20 m2 at 120 eV for Si.

Electron-impact dissociation of carbon monoxide

The electron-impact dissociation of CO to form C and O atoms is observed in a crossed beam experiment at electron energies between the dissociation threshold (14 eV) and 198.5 eV. The center-of-mass energy released in the dissociation of individual molecules is explicitly measured using a position and time sensitive detector for the correlated neutral fragments. The observed energy release distribution is found to be highly structured, reflecting electron-impact excitation to Rydberg states converging to CO+(X 2Σ+) which predissociate to ground state atoms. Little or no dissociation is observed from states above the first ionization limit. Total electron impact dissociation cross sections, exclusive of dissociative ionization contributions, and partial cross sections for the dissociative excitation of specific CO electronic states are presented.

Inelastic processes in e–HCl collisions in the energy range including the dissociation threshold

Inelastic processes in low-energy collisions of electrons with HCl molecules are considered. We present results of calculations of vibrational excitation cross sections from the ground state and vibrationally excited states, dissociative attachment cross sections, and electron-impact dissociation cross sections near threshold. Our approach is based on the resonance R-matrix theory with the quasiclassical treatment of the nuclear motion. We use a rigorous procedure for the inclusion of the vibrational continuum. This allows for calculations of the cross sections in the energy range including dissociation threshold. The results for the dissociation cross sections are analyzed from the point of view of the threshold law for the three-particle break-up process.

Infrared detection by the matrix-isolation technique of radicals produced in deposition plasmas: germane

The matrix-isolation technique is used to study the neutral species (molecules and radicals) issued from a low pressure (0.2 mTorr) germane multipole dc discharge. Hydrogenated germanium GeH x and Ge 2H n radicals are observed by infrared absorption spectroscopy. Assignments are proposed based on relative intensity behaviour. Double doping of the germane plasma with hydrogen shows no effect upon the GeH 3 to GeH 2 relative concentration. The total electron-impact dissociation cross section of germane is measured for quasi-monoenergetic primary electrons from 45 to 150 eV. The shape of the partial yield of GeH 2 and GeH 3 formation in e − + GeH 4 reactions and the branching ratio GeH 3/GeH 2 are discussed in the same electron-energy range.

Electron impact dissociation cross sections in hydrogen and deuterium, leading to Balmer alpha and beta emission

Dissociation of H2and D2by electron impact, leading to the emission of Balmer alpha and beta radiation, has been studied for electron energies up to 100 eV. Normalised excitation functions, their isotope ratio, and polarisation are reported. Care was taken to eliminate magnetic and electric fields, and to allow for molecular radiation near the Balmer lines. Compared with previous reported work the shape of the cross sections found is substantially the same, while showing an additional threshold near 43 eV, but the isotope ratios are markedly different at low energies while agreeing at higher energies. Complete polarisation measurements are reported for the first time.

Dissociation of the hydrogen molecule by electron impact

The electron-impact dissociation cross sections of the ${\mathrm{H}}_{2}$ molecule have been calculated for the processes leading to $\mathrm{H}(1s)+\mathrm{H}(1s)$ and to $\mathrm{H}(1s)+\mathrm{H}(2s)$ by means of the Born-Rudge and Born-Ochkur methods. In addition to direct excitation to the repulsive $b^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state for the first process, cross sections are also computed for excitation of the discrete levels of the $a{^{3}\ensuremath{\Sigma}_{g}}^{+}$, $c^{3}\ensuremath{\Pi}_{u}$, $d^{3}\ensuremath{\Pi}_{u}$, and $e{^{3}\ensuremath{\Sigma}_{u}}^{+}$ states in order to account for cascades. These two mechanisms are found to be of equal importance. The second process is found to proceed mainly through excitation of the ${B}^{\ensuremath{'}}{^{1}\ensuremath{\Sigma}_{g}}^{+}$ state except near the threshold where $e{^{3}\ensuremath{\Sigma}_{u}}^{+}$ is an important contributor. The theoretical cross sections are in reasonably good agreement with experimental data for the first process. The cross sections of the second process account for nearly two-thirds of the measured cross sections of $\mathrm{H}(2s)$-atom production by electron impact. The difference is attributed mainly to predissociation and dissociative excitation through the doubly excited states.