Modeling and measurements of the negative ion flux from amplitude modulated rf discharges

Publisher Website
Google Scholar
Abstract
The time dependent as well as amplitude modulation frequency dependent ion flux from parallel plate radio frequency discharges have been measured by mass spectrometry and modeled using the fluid equations. The negative ion flux to the grounded electrode becomes large after the rf excitation is turned off and has complex temporal features which depend upon the ion, gas mixture, applied dc bias voltage and other externally controlled parameters. Some of these features can be explained with the assistance of the fluid equation model. In short, negative ions can be formed during the active glow as well as after the glow excitation has been turned off. The resulting negative ion flux to the grounded electrode during the afterglow is determined in part by when the negative ion is formed. For example, if the amplitude modulation frequency at which the flux of a negative ion goes to zero is larger than 100 kHz, it indicates that ion is formed in the afterglow. (Of course, it may exist during the active glow as well.) Conversely, ions with signal only at small modulation frequencies are unlikely to have been produced by attachment in the afterglow. The experimental results are from CF4 and Cl2-He discharges. One finding was that the temporal and modulation frequency dependencies of F− and SiF5− ions from CF4 discharges indicate that SiF5− exists only in a thin layer above the grounded electrode.