R. L. Champion

Collisional electron detachment and decomposition rates of SF−6, SF−5, and F− in SF6: Implications for ion transport and electrical discharges

Measured cross sections for prompt collisional detachment and decomposition of SF−6, SF−5, and F− in SF6 reported in the preceding companion paper are used to calculate detachment coefficients and ion‐conversion reaction coefficients as functions of electric field‐to‐gas density ratio (E/N) for ion drift in SF6. Analysis from a model presented here using these coefficients suggests that prompt electron detachment from SF−6 and SF−5 in SF6 are insignificant processes in such ion‐drift experiments. Calculated rates for ion‐conversion processes indicate the necessity to: (1) reexamine the previously measured rates in SF6 from drift‐tube experiments, and (2) use ion kinetic‐energy distributions with larger high‐energy tails than the standard distributions assumed in earlier calculations. The calculated detachment and reaction coefficients are used in a model which invokes detachment from long‐lived energetically unstable states of collisionally excited SF−6 to explain the pressure dependence of previously mea...

Collisional electron detachment and decomposition cross sections for SF−6, SF−5, and F− on SF6 and rare gas targets

Absolute total cross sections for collisional electron detachment and collision‐induced dissociation (CID) have been measured for binary collisions of SF−6 and SF−5 with rare gas and SF6 targets for laboratory collision energies ranging from about 10 up to 500 eV. The cross sections for electron detachment of SF−6 are found to be surprisingly small, especially for the SF6 target, for relative collision energies below several tens of electron volts. Specifically, detachment onsets are found to occur at around 30 and 90 eV for the rare gas and SF6 targets, respectively. The CID channel which leads to F− as a product is observed to dominate detachment for relative collision energies below 100 eV. The results for the SF−5 projectile are remarkably similar to those exhibited for SF−6. The role of long‐lived excited states in the reactant SF6 ion beam is discussed.

Destruction cross sections for low energy collisions of H+3 and D+3 with rare gas atoms

Absolute total cross sections for collisional dissociation and charge transfer have been measured for collisions of H+3 and D+3 with He, Ar, and Xe for projectile energies ranging from 15 to 400 eV. The cross sections for collision‐induced dissociation of H+3 into H++H2 or H+H+2 are a few A2 and exhibit a weak target dependence. It is suggested that the charge transfer cross sections, which vary markedly with target species, are due primarily to proton transfer to the rare gas target rather than electron transfer from the target. At the lower collision energies, proton abstraction is favored over deuteron abstraction for the He and Ar targets.

Total cross sections for low energy collisions of H3+ with molecular hydrogen and rare gases

Measurements of total cross sections for collision induced dissociation (CID) and proton abstraction have been made for the reactants H3++H2, Ar, and He. The laboratory collision energies range from a few up to 400 eV and D3+ has been substituted to investigate possible isotope effects. The CID cross sections of H3+→H++H2 or H2++H maximize at a value of a few A2. Proton abstraction is the dominant process for relative collision energies below 5 eV and is observed to lead to a highly excited H3+ product which often autodissociates. Dissociative charge transfer at higher collision energies is responsible for producing H2+ product ions. The role of internal energy contained within the H3+ primaries in previous experiments and the effects of internal energy on the cross section measurements presented here are also discussed.

Measured cross sections and ion energies for a CHF3 discharge.

Trifluoromethane (CHF3) is used in semiconductor plasma processing chambers to achieve high-etch selectivity of an oxide layer over a silicon substrate. Such surface etching is governed by the ion and molecule fluxes near the surface, the concentrations of which are dependent upon species interactions in and their transport through the plasma. In order to assist in the interpretation of ion flux measurements and to provide fundamental data required for plasma modeling, we report the first total cross sections for significant ion-molecule reactions occurring in CHF3 discharges. The reactions studied include collision-induced dissociation for CF3+ on CHF3, dissociative charge transfer for CF3+ and F+ on CHF3, and electron detachment from F− on CHF3. Collision energies range from a few to a few hundred electron volts. In addition, ion-flux energy distributions and relative ion intensities have been measured and are presented for dc townsend discharges with E/N values ranging from 5×10−18 to 25×10−18 V m2 [5 ...

Reactive scattering and electron detachment in O− collisions with H2. Nascent product energy distributions

Results of the measurements of the energy distributions of secondary electrons and H− ions produced in low relative energy (1 to 10 eV) collisions of O− with H2 are presented. The electron distribution for the associative detachment reaction is found to be narrow and peaks at almost zero energies, indicating the formation of a highly vibrorotationally excited molecule. The H− energy distributions are structured and indicate the formation of vibrationally excited OH. For low collision energies this excitation is small. The H− energy distributions are found to resemble closely the ones obtained in dissociative attachment in electron–H2O scattering.

Collisional decomposition of the sulfur hexaflouride anion (SF6

Insulating gas mixtures containing SF6 have been promoted to serve as replacements for pure SF6 in order to reduce SF6 atmospheric emission. It has been argued that some synergism may be achieved by choosing proper buffer gases in mixtures with SF6 such that the buffer gases efficiently slow down electrons into an energy range where the electron attachment cross section for SF6 is large. A complete understanding of the dielectric properties of SF6 mixtures obviously requires information about electron detachment from SF6− as collisional electron detachment may be the principal source of discharge initiation in SF6 mixtures. In this paper, we report total cross-section measurements for electron detachment and collision induced dissociation for collisions of SF6− with N2 for collision energies ranging up to a few hundred eV. The experimental results are analyzed using a two-step collision model where the unimolecular decomposition of collisionally excited SF6− ions is described in a statistical framework.

Detachment and charge transfer for collisions of negative ions with ozone

Cross sections for electron detachment and charge transfer have been measured for collisions of O−, S−, and halogen anions with neutral ozone for laboratory collision energies ranging from 3 to 500 eV. Both electron detachment and charge transfer are found to be large over the entire range of collision energies for O− and S−+O3 whereas the cross sections for the halogen anions colliding with O3 exhibit threshold behaviors characteristic of endothermic scattering processes.

Collision induced dissociation, proton abstraction, and charge transfer for low energy collisions involving CH4+

Measurements of total cross sections for collision induced dissociation, proton abstraction, and charge transfer have been made for collisions of CH4+ with CD4, H2, and Ar. The laboratory collision energies range from a few up to 400 eV, and isotopic substitutions have been made where possible to investigate any possible isotope effects, and for some reactants, to more closely identify the product ions. Cross sections for all reaction channels are observed to be small; ≲5 A2 for the methane target, ≲0.5 A2 for the hydrogen target, and ≲2 A2 for the argon target. For the methane and hydrogen targets, proton abstraction is observed at low energies while charge transfer occurs for energies ≳10 eV. The implications of internal energy present in the CH4+ projectile for the cross section measurements is discussed. In addition, decomposition of CD4H+ is addressed in terms of recent calculations related to the structure of the CH5+ ion.

Cross Section Measurements for Various Reactions Occurring in CF 4 and CHF 3 Discharges

In an attempt to expand the somewhat limited database for collisional processes relevant to plasma chemistry and its modeling, absolute cross sections have been measured for positive and negative ion-molecule reactions for species typically found in discharges which contain CF4and CHF3. the reactions investigated include electron detachment, collision induced dissociation (CID), and dissociative electron transfer resulting from collisions of CF3+, F+, and F with the targets CF4 and CHF3. Impact energies range from near threshold to ≳ 200 eV. in general, the cross sections for all inelastic processes studied here reach maximum values and are constant for E ≳ 40 eV. the importance and implications of these measurements to discharge modeling will be discussed.