|
Themes > Science > Physics > Molecular Physics > Excitation Energy Transfer and Energy Migration > Ionization > Molecules |
For plasmas with complex molecules present in the feed gas, detailed understanding requires identification of the specific ions formed by electron collisions. Thus, cross sections are needed for production of parent molecular ions and dissociative ionization products. These are distinct from total ionization cross sections (the sum for all channels), which are measured in some cases but do not provide adequate information on the specific ionic species produced. Channel-specific ionization cross sections are available for a variety of plasma processing compounds, including SiH4, CF4, SF6, CCl2F2, and O2, as well as the common purge gas N2. Total ionization cross sections have been reported for Cl2 and F2. Changes in these cross sections may occur when the target molecules are vibrationally or electronically excited. Experimental techniques are not well developed to study these effects quantitatively. Experimental and theoretical results for the ionization of vibrationally excited molecules show qualitatively a shift of the ionization threshold to lower energies and a significant enhancement of the cross section in the lower energy region near threshold, two effects that can drastically affect the ionization balance in a low-temperature plasma. |
|
|