A theoretical model for electron‐impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary‐encounter theory and the Bethe theory for electron‐impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public‐domain molecular structure codes such as GAMESS. The theory is called the binary‐encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals.

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New model for electron-impact ionization cross sections of molecules

A model for the plasma enhanced chemical vapor deposition of amorphous hydrogenated silicon (a‐Si:H) in rf and dc discharges is presented. The model deals primarily with the plasma chemistry of discharges sustained in gas mixtures containing silane (SiH4). The plasma chemistry model uses as input the electron impact rate coefficients generated in a separate simulation for the electron kinetics and therefore makes no a priori assumptions as to the manner of power deposition. Radical densities and contributions to film growth are discussed as a function of gas mixture, electrode separation, and locale of power deposition, and comparisons are made to experiment. A compendium of reactions and rate constants for silane neutral and ion chemistry is also presented.

A model for the discharge kinetics and plasma chemistry during plasma enhanced chemical vapor deposition of amorphous silicon

This paper presents a critical review of the basic data concerning the physics and chemistry of low pressure SiH 4 glow discharges used to deposit hydrogenated amorphous silicon films (a-Si:H). Starting with an updated table of thermochemical data, we analyze the gas-phase elementary processes consisting of i) electron-molecule collisions, ii) ion-molecule collisions, iii) neutral-neutral collisions, iv) other electron and ion collisions involving electron-ion and ion-ion recombination, electron attachment on radicals and detachment of anions, and v) cluster growth kinetics in dusty plasmas. Experimental data or theoretical estimates are given and discussed in terms of cross-sections. collision and reaction rate constants, and transport coefficients. We also analyze the surface processes and reaction probabilities of ions, radicals and molecules.

Cross-Sections, Rate Constants and Transport Coefficients in Silane Plasma Chemistry

The results of numerical simulations, optical emission spectroscopy (OES), and quadrupole mass spectrometry (QMS) of inductively coupled Ar/CH4/H2 plasmas in the plasma enhanced chemical vapor deposition (PECVD) of self-assembled vertically aligned carbon nanostructures (CNs) are presented. A spatially averaged (global) discharge model is developed to study the densities and fluxes of the radical neutrals and charged species, the effective electron temperature, methane conversion factor under various growth conditions. The numerical results show a remarkable agreement with the OES and QMS data. It is found that the deposited cation fluxes in the PECVD of CNs generally exceed those of the radical neutrals.

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Inductively coupled Ar/CH4/H2 plasmas for low-temperature deposition of ordered carbon nanostructures

Theoretical determination of absolute electron-impact ionization cross sections of molecules

The total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6 have been measured for electron energies from threshold to 300 eV. Comparisons are made to earlier measurements.

Total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6

SiHn radical densities at the surface of discharges in Ar‐SiH4 mixtures have been measured by low‐energy, electron‐collisional ionization and mass spectrometer detection of SiH+n. The principal radical seen at the substrate surface of a dc proximity discharge is SiH3.

Radical species in argon‐silane discharges

The rate coefficients and product‐ion distributions for the reactions of He+ and Ar+ with silane and disilane have been measured in a drift tube, typically for collision energies of 0.01–1 eV. The total charge‐exchange rate coefficients are found to be roughly independent of E/N, or collision energy, and are about equal to the Langevin values for the reactions of He+ with SiH4 and C2H6 and Ar+ with CH4 and C2H6. The He+ rate coefficients on CH4 and Si2H6, and the Ne+ rate coefficients on SiH4 and Si2H6 are 50% to 80% of the Langevin values, while the Ar+ rate coefficients on SiH4 and Si2H6 are much smaller. Product ions tend to be hydrogen poor with very infrequent breaking of the C–C or Si–Si bonds. Furthermore, hydrogen stripping is more severe for the silanes than the alkanes. These product‐ion distributions bear no resemblance to the product‐ion distributions of either photoionization or electron collisional ionization.

D. Hils

Papers

Total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6

Radical species in argon‐silane discharges

Reactions of He+, Ne+, and Ar+ with CH4, C2H6, SiH4, and Si2H6