Absolute Electron-impact Cross Sections Research Articles

Elastic Cross Sections for Electron Collisions with Molecules Relevant to Plasma Processing

Absolute electron-impact cross sections for molecular targets, including their radicals, are important in developing plasma reactors and testing various plasma processing gases. Low-energy electron collision data for these gases are sparse and only the limited cross section data are available. In this report, elastic cross sections for electron-polyatomic molecule collisions are compiled and reviewed for 17molecules relevant to plasma processing. Elastic cross sections are essential for the absolute scale conversion of inelastic cross sections, as well as for testing computational methods. Data are collected and reviewed for elastic differential, integral, and momentum transfer cross sections and, for each molecule, the recommended values of the cross section are presented. The literature has been surveyed through early 2010.

Electron Interaction Cross Sections for CF3I, C2F4, and CFx (x=1–3) Radicals

The supply of absolute electron-impact cross sections for molecular targets and radicals is extremely important for developing plasma reactors and testing different types of etching gases. Current demand for such models is high as the industry aims to replace traditional plasma processing gases with less polluting species. New theoretical electron impact cross sections at typical etching plasma energies (sub 10 eV) are presented for the CFx (x=1–3) active radical species in a form suitable for plasma modeling. The available experimental and theoretical data are summarized for two potential feed gases, CF3I and C2F4. This data cover recommended cross sections for electron scattering (total, excitation, momentum transfer, and elastic integral), electron impact dissociation, and dissociative electron attachment, wherever possible. Numerical values are given as tables in the paper and are also placed in the electronic archive.

New measurements of absolute total cross sections for electron impact on caesium using a magneto-optical trap

A new magneto-optical trap designed specifically for the measurement of absolute electron impact cross sections is described. Experimental results for electron impact total cross sections involving 62S ground state Cs are presented over the energy range 5–200 eV. Significant disagreement is obtained between our experimental data and convergent close coupling (CCC) or Breit–Pauli R-matrix calculations at the lower energies. Above 100 eV our data overlap earlier measurements from this laboratory made with a completely different apparatus and agree well with the CCC results. A comparison is made with other measurements using more conventional techniques.

Doubly differential measurements for multiple ionization of argon by electron impact: Comparison with positron impact and photoionization

Doubly differential cross sections for single and multiple ionization of Ar have been measured for 500, 750, and 1000 eV electron impact. The cross sections were measured as a function of projectile energy loss and scattering angle. The energy loss range was 0--85 % of the initial projectile energy and scattering angles were between $\ifmmode\pm\else\textpm\fi{}22\ifmmode^\circ\else\textdegree\fi{}.$ The data were put on an absolute scale by normalizing to total ionization cross sections available in the literature and found to be in good agreement with the absolute electron impact cross sections from DuBois and Rudd. For 750 eV impact, a comparison was made between the present electron impact data and positron impact data obtained using the same experimental conditions. The same energy dependence and yields for single ionization were found for both electron and positron impact. On the other hand, the double- and triple-ionization yields are smaller for positron impact as compared to electron impact. Comparisons with photoionization data showed that for outer shell ionization the fractions of double and triple ionization of argon by photon impact are in quite good agreement with the present electron impact data.

Determination of radical densities by optical emission spectroscopy during the ECR plasma deposition of Si-C-N : H films using TMS as a precursor

The ECR plasma enhanced chemical vapour deposition has been studied by optical emission spectroscopy in the substrate region. Tetramethylsilane (TMS) was used as precursor for the deposition of Si-C-N:H thin films in Ar-N2 and Ar-N2-H2 gas mixtures. Using absolute electron-impact cross sections available in the literature absolute densities of H, CH, and Si have been calculated from the emission intensities of these species according to the corona model with respect to electron temperatures obtained by Langmuir probes. The contribution of dissociative excitation out of H2 and TMS to the emission of the particles is accounted for. The investigations were carried out varying the precursor flow for different microwave powers, substrate potentials, and gas mixtures. By comparison of the radical number densities in the gas phase and the atomic deposition rate, conclusions are drawn concerning the deposition mechanisms of the Si-C-N:H films.

Total electron impact ionization cross sections of free molecular radicals: the failure of the additivity rule revisited

Using the recently developed semiclassical DM approach we have calculated the absolute electron impact cross sections for single ionization of some selected molecular radicals (CH n=1–3 and SiF n=1–3 ) from threshold up to 80 eV. The present results confirm (i) a previously noted anomaly in the ordering of the SiF n cross sections (i.e. σ(SiF) & >; σ(SiF 2) & >; σ(SiF 3)) and (ii) the failure of the simple additivity rule in predicting this anomaly.

Cross-section measurements for electron-impact ionization of atoms.

Absolute electron-impact cross sections have been measured from 0 to 200 eV for single ionization of 16 atoms (Mg, Fe, Cu, Ag, Al, Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, and Te) with an estimated accuracy of \ifmmode\pm\else\textpm\fi{}10%. Combined with our recent measurements of He, Ne, Ar, Kr, Xe, F, Cl, Br, I, Ga, and In [Wetzel et al., Phys. Rev. A 35, 559 (1987); Hayes et al., ibid. 35, 578 (1987); Shul, Wetzel, and Freund, ibid. 39, 5588 (1989)], a set of 27 atomic single-ionization cross sections has now been measured with the same apparatus. In addition, cross sections are reported for double ionization of ten atoms and triple ionization of eight atoms. The measurements are made by crossing an electron beam with a 3-keV beam of neutral atoms, prepared by charge-transfer neutralization of a mass-selected ion beam. The critical measurement of absolute neutral beam flux is made with a calibrated pyroelectric crystal. The magnitudes of the single-ionization-peak cross sections decrease monotonically across rows of the periodic table from group IIIA (Al,Ga,In) to group VIIIA (Ar,Kr,Xe), varying much more than predicted by various empirical formulas and classical and quantum-mechanical theories.

Absolute ionisation cross sections for electron impact on H2

Absolute electron impact cross sections for single ionisation leaving H2+ in its electronic ground state and double ionisation leading to dissociation into all four constituents have been determined by a direct method in the energy range 0.15-2.6 keV. The values for single ionisation are in perfect agreement with, but the double-ionisation values are much larger than, the earlier determination by Edwards et al. (1988,1989). The authors' results exhibit a close relationship between the H2 system and its atomic counterpart, helium, with respect to their response to single- and double-ionisation processes.

Electron-impact-ionization cross sections of the Ga and In atoms.

Absolute electron-impact cross sections for single, double, and triple ionization of gallium and indium have been measured from 0 to 200 eV. Beams of Ga and In atoms are formed by charge-transfer neutralization of 3-keV-ion beams with triethylamine or xenon. The cross sections are considerably larger than previous measurements and are also larger than the predictions of various empirical formulas and classical and quantum-mechanical theories, due in part to significant contributions from autoionization. The double-ionization cross sections appear to be dominated by ionization of a d electron followed by autoionization, rather than by direct double ionization.

Measurements of electron-impact ionization and dissociation cross sections in a crossed electron-supersonic molecular beam

A new approach is described for obtaining absolute electron-impact cross sections for ionization, including the partial cross sections for multiple charge excitation and ion dissociation. The method benefits from the use of a supersonic molecular beam, pulsed ion source, and multichannel mass detection. The determination of the partial cross sections for the production of Ne +, Ne 2+, Ar +, Kr +, and Kr 2+ from the neutral atoms, as well as for the production of NH 2 +, NH 2 +, and NH + from rovibrationally cold NH 3, are reported and compared with results from other recent work.