Numerical simulation on silane plasma chemistry in pulsed plasma process to prepare a-Si :H thin films

We numerically calculated the effects of pulse modulation (plasma-on and -off times) on the concentration changes of the chemical species (SiH ₄, SiH_x, SiH_x ⁺ and polymerized negative ions) and also the growth rate of a-Si : H thin films in the pulsed SiH₄ plasmas. During the plasma-on, SiH_x is generated quickly by a fast dissociative reaction of SiH₄, but, during plasma-off, SiH_x disappears rapidly by a reaction with hydrogen and also by the deposition onto the reactor wall. During the plasma-on, the negative ions are polymerized by the reactions with SiH₄, but, during the plasma-off, they disappear by neutralization reactions with positive ions. As the plasma-on time increases or as the plasma-off time decreases, the time-averaged concentrations of SiH_x and negative ions and also the time-averaged film growth rate increase. This study shows quantitatively that polymerized negative ions, which are not considered to be preferred precursors for the high-quality thin films, can be efficiently reduced by the pulsed plasma process.