Quantitative Analysis on the Growth of Negative Ions in Pulse-Modulated SiH4 Plasmas

The evolutions of chemical species (SiH4, SiHx, SiHx+, and polymerized negative ions) in the pulse-modulated SiH4 plasmas during the plasma-on and -off times were analyzed by solving the model equations of chemical species. During the plasma-on time, the SiHx concentration increases with time, mainly by the generation reaction from SiH4, but during the plasma-off time, it decreases because of the reaction with H2. During the plasma-on time, the concentrations of negative ions increase with time by the polymerization reactions of negative ions, and those become almost zero in the sheath regions because of the electrostatic repulsion. During the plasma-off time, the concentrations of negative ions decrease with time by the neutralization reactions with positive ions, and some negative ions can diffuse toward the sheath regions because there is no electric field inside the reactor. Our theoretical analysis shows quantitatively that the pulse-modulation plasma technique can be an efficient method to reduce the polymerized negative ion clusters, which are not good precursors for a high-quality thin film and which can also be the sources of particle contamination.