Dissociative excitation of Tetramethylsilane (TMS) and Hexamethyldisiloxane (HMDSO) by controlled electron impact

The optical emission spectra in the wavelength region 200-800 nm produced by electron impact on the silicon-organic molecules TMS (tetramethylsilane) and HMDSO (hexamethyldisiloxane) under controlled single-collision conditions have been analyzed. Absolute emission cross sections from threshold to 200 eV impact energy were determined for a variety of emission features. For both targets, the CH (A²Δ → X²Π) emission, the so-called CH "4300 Å" band, was found to have the largest emission cross section with values (at 100 eV) of 5.5×10^-19cm² and 6.1×10^-19 cm² for TMS and HMDSO, respectively. Relatively high onset energies of 28.0 ± 1.5 eV (TMS) and 33.1 ± 1.5 eV (HMDSO) were measured for these emissions. Weaker emission features in both spectra were identified as CH bands corresponding to the B²Σ^- → X²Π transition (the CH "3900 Å" system) and the C²Σ → X²Π transition, and as the atomic Si line emissions at 253 nm and 288 nm. Near-threshold studies indicate an onset for the Si emissions of 29.0 ± 2.0 eV (TMS) and 44.6 ± 2.0 eV (HMDSO). Absolute cross sections and appearance energies were also determined for the strongest lines of the hydrogen Balmer series for both targets. The comparatively high onset energies and small emission cross sections for all observed emissions indicate that single-step dissociative excitation processes are unlikely to play a dominant role in low-temperature processing plasmas containing TMS and HMDSO.