Current status and new insights about the capacitively coupled electronegative plasma source: injection of energetic beam-like electrons to electrode

Abstract

High-frequency capacitively coupled plasmas (HF-CCPs) have been widely investigated physically, electrically, optically, and numerically. Such research has focused on the sustaining mechanism and the interaction of the ion sheath with the material surface. Most of the reactive feed gas molecules produce a pair consisting of a negative ion and a neutral by the dissociative electron attachment. The low-temperature electronegative plasma with high electronegativity has an interesting and specific sustaining mechanism caused by the negative ions in HF-CCPs. One of the functions is a production of high energy electrons in front of the instantaneous anode under the presence of a static double layer with field reversal. Current understanding of electronegative plasmas has been advanced through the knowledge about the fundamental collision, transport processes, and related effects of negative ion and long-lived neutral on the plasma structure and function. We present new insights into the injection of energetic beam-like electrons to a biased wafer in a pulsed two-frequency mode in electronegative HF-CCP, based on a series of our previous works in CF4/Ar. Here, the criteria, consisting of plasma internal and external parameters, for the injection provide the design rule for a charging free plasma process. The magnitude of the electronegativity as a key parameter to inject negative charges is elucidated in the bulk plasma as a function of gas density times electrode distance in HF-CCP in O2 as an example. Both sides of the electronegativity correspond to the different types of the transport and the sustainability of the bulk plasma.