Vladimir Tarnovsky

Dissociative ionization of silane by electron impact

Abstract We studied the electron impact ionization of silane (SiH4) which is widely used in the plasma deposition of different siliconcontaining thin films. Absolute partial cross-sections for the formation of all fragment ions were measured in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage for electron energies from threshold to 100 eV. No evidence for the formation of stable parent SiH4+ ions was found in agreement with previous experimental investigations. The single positive fragment ion formation is the dominant ionization process. We observed the following product ions: SiH3+, SiH2+, SiH+, Si+, H2+, and H+. The agreement between our measured absolute partial ionization crosssections and two earlier data sets obtained by different techniques is generally good for the silicon-containing fragment ions taking into account quoted uncertainties of ± 10% to ± 20%, but less satisfactory for the formation of atomic and molecular hydrogen ions which were found to be produced with significant excess kinetic energies, particularly in the case of H+. A comparison of the total SiH4 ionization cross-section derived from the measured partial ionization cross-sections and a calculated cross-section based on the Binary-Encounter-Bethe (BEB) model showed excellent agreement in the energy range above 30 eV.

Electron impact ionization of the SF5 and SF3 free radicals

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the SF5 and SF3 free radicals from threshold to 200 eV using the fast-neutral-beam technique. The total single ionization cross sections at 70 eV were found to be 5.1×10−16 cm2 (SF5) and 3.2×10−16 cm2 (SF3). The cross sections reach their maximum at about 100 eV for both radicals, with values of 5.4×10−16 cm2 (SF5) and 3.5×10−16 cm2 (SF3). Dissociative ionization is important only for SF5, where the SF5+ parent ionization cross section and the SF4+ fragment ionization cross sections have roughly the same value. By contrast, the ionization of SF3 is dominated by the formation of SF3+ parent ions. A comparison of the measured total single ionization cross sections with calculated cross sections, using a modified additivity rule, shows excellent agreement for both radicals in terms of the absolute cross section values and the cross section shapes. Total single ionization cross sections were also calculated f...

CROSS-SECTIONS FOR THE ELECTRON IMPACT IONIZATION OF NDX (X = 1-3)

Abstract Absolute cross-sections for the electron impact ionization and dissociative ionization of the ND3 molecule and the ND2 and ND free radicals were measured from threshold to 200 eV using the fast neutral beam technique. The deuterated rather than protonated targets were used in this work to facilitate a better separation of the various product ions from a given parent in our apparatus. A common feature of all three targets studied in this work is a dominant parent ionization cross-section with absolute values (at 70 eV) of 1.21 × 10−16 cm2 (ND3+), 1.44 × 10−16 cm2 (ND2+) and 1.45 × 10−16 cm2 (ND+). Dissociative ionization processes for all three targets are less significant, with a single dissociative process dominating in each case, the removal of a single D atom (NDx + e− → NDx−l+ + D + 2e−). The cross-section for this dominant dissociative ionization channel decreases from 1.03 × 10 −16 cm 2 for ND 2 + ND 3 to 0.85 × 10 −16 cm 2 for ND + ND 2 to 0.40 × 10−16 cm2 for N + ND . The measured appearance energies for the various fragment ions indicate that all fragment ions, except for D+, are formed with little excess kinetic energy. Our experimental data for ND3 are compared with existing experimental and theoretical data for NH3.

Electron-impact ionization of silicon tetrachloride (SiCl4).

We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on silicon tetrachloride (SiCl4) using two different experimental techniques, a time-of-flight mass spectrometer (TOF-MS) and a fast-neutral-beam apparatus. The energy range covered was from the threshold to 900 eV in the TOF-MS and to 200 eV in the fast-neutral-beam apparatus. The results obtained by the two different experimental techniques were found to agree very well (better than their combined margins of error). The SiCl3(+) fragment ion has the largest partial ionization cross section with a maximum value of slightly above 6x10(-20) m2 at about 100 eV. The cross sections for the formation of SiCl4(+), SiCl+, and Cl+ have maximum values around 4x10(-20) m2. Some of the cross-section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV followed by a broad second maximum at around 100 eV. This is similar to what has been observed by us earlier for another Cl-containing molecule, TiCl4 [R. Basner, M. Schmidt, V. Tamovsky, H. Deutsch, and K. Becker, Thin Solid Films 374 291 (2000)]. The maximum cross-section values for the formation of the doubly charged ions, with the exception of SiCl3(++), are 0.05x10(-20) m2 or less. The experimentally determined total single ionization cross section of SiCl4 is compared with the results of semiempirical calculations.

Electron impact ionization of the hydroxyl radical

We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10−16 cm2 at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10−16 cm2 at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D2O molecule ...

Absolute total and partial cross-sections for the electron impact ionization of TiCl4

Titanium tetrachloride (TiCl4) is used for the plasma-assisted chemical vapor deposition (CVD) of titanium nitride films. We studied the electron impact ionization of TiCl4 for electron energies from threshold to 500 eV. Absolute partial cross-sections for the formation of all singly charged positive ions and for four doubly charged positive ions were measured using a time-of-flight mass spectrometer (TOF-MS), which can be operated either in the linear mode or in the reflection mode. Dissociative ionization was found to be the dominant process. At lower impact energies up to 40 eV, the ion abundance varies drastically with impact energy, whereas at higher energies, two ionization channels dominate, the formation of the TiCl3+ fragment ion with a maximum cross-section of 3.75×10−16 cm2 at 100 eV and the formation of the Cl+ fragment ion with a maximum cross-section of 4×10−16 cm2 at 70 eV. All fragment ions with the exception of TiCl3+ are formed with excess kinetic energy with the Cl+ ion showing the broadest distribution of kinetic energies. The cross-section values of the doubly charged ions are approximately one order of magnitude smaller than those of the singly charged ions. The experimentally determined total single ionization cross-section of TiCl4 is compared with results of semi-empirical calculations and good agreement is found.

Absolute cross section for the formation of ions produced by electron impact on

We employed the fast-neutral-beam technique in a measurement of the absolute partial cross section for the formation of ions following controlled electron-impact ionization of the fullerene from threshold to 200 eV. Our measurement is the first report of an absolute measurement of this cross section which does not employ any normalization procedure and which does not rely on the use of controversial vapour pressure versus temperature data for to determine the number density in the ion source. All quantities that determine the ionization cross section were measured directly in our apparatus. We found a peak value for the parent ionization cross section of in the energy range between 70 and 80 eV. This value is in very good agreement (to within 5%) with the cross section reported earlier by Matt et al (1996 J. Chem. Phys. 105 1880) who employed a novel normalization procedure. Our cross section is significantly lower than the cross sections reported by several other investigators who relied on vapour pressure data in their absolute calibration.

Electron Impact Ionization of SiClx and TiClx (x = 1–4): Contributions from Indirect Ionization Channels

We measured absolute partial cross sections for the formation of various singly charged positive ions produced by electron impact on SiClx (x = 1–4) using two different experimental techniques, a time-of-flight mass spectrometer (TOF-MS) and a fast-neutral-beam apparatus. The energy range covered was from the threshold to 900 eV in the TOF-MS and to 200 eV in the fast neutral beam apparatus. In the case of SiCl4, the absolute cross sections obtained by the two different experimental techniques were found to agree very well. The ionization of the SiClx (x = 1–3) free radicals can only be studied using the fast-beam technique. Some of the partial cross section curves were found to exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV, which is in some cases followed by a shallow minimum at about 40 eV and a broad second maximum at around 100 eV. This pronounced low-energy maximum is indicative of the presence of an indirect ionization channel and is similar to what has been observed earlier for TiCl4 and the TiClx (x = 1–3) radicals. Similarities in the contribution of the indirect ionization channel to the formation of the various SiClx+ and TiClx+ (x = 1–4) ions will be highlighted in this paper.

Electron impact ionization of the TiClx (x = 1 - 3) free radicals

Abstract Titanium tetrachloride (TiCl 4 ) is used for the plasma-assisted chemical vapor deposition of titanium nitride films. We studied the electron impact ionization of TiCl x ( x = 1 – 3) free radicals, which are abundant constitutents in TiCl 4 -containing processing plasmas, for electron energies from threshold to 100 eV. Absolute partial cross sections for the formation of all singly charged molecular ions were measured using the fast-beam technique. Dissociative ionization was found to be the dominant process for TiCl 3 whereas the formation of the respective parent ions plays a more important role for TiCl 2 and TiCl. Total single ionization cross sections for the three radicals could not be determined with reasonable accuracy because of problems in the determination of the cross sections for the formation of the atomic ions Ti + and Cl + for the three targets and, as a consequence, no comparison of the measured cross sections with calculated total single ionization cross sections is possible.

Electron-impact ionization of the C2F5 free radical

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the C2F5 free radical from threshold to 200 eV using the fast-neutral-beam technique. The electron-impact ionization of C2F5 is dominated by two channels, the formation of the CF3+ fragment ion which has the largest partial ionization cross section with a value of 3.5 × 10-16 cm2 at 70 eV and the formation of the C2F5+ parent ion with a cross section of 1.3 × 10-16 cm2 at 70 eV. Both partial cross sections have error margins of about ±15%. All other partial ionization cross sections have maximum cross section values of less than 0.1 × 10-16 cm2. The total single C2F5 ionization cross sections at 70 eV (derived from the two measured partial ionization cross sections and an estimate for the sum of the cross sections for all weaker, unobserved ionization processes) was found to be about 5 × 10-16 cm2. A comparison of the measured total single-ionization cross sections with a calculated cross section, using a modified additivity rule, shows good agreement up to an electron energy of 50 eV, but poor agreement at higher impact energies. This behaviour is somewhat similar to that observed for the CFx (x = 1-3) free radicals, but it is very different from the excellent agreement between the calculated and measured total single-ionization cross sections that was observed for the stable molecules CF4 and C2F6.