Current status and nature of high-frequency electronegative plasmas: basis for material processing in device manufacturing

A non-equilibrium electronegative plasma serves as the reactive source for semiconductor dry processing as an advanced technology. This paper reviews the current knowledge about the fundamental processes, structures, dynamics, and functions of high-frequency electronegative plasmas investigated over the past 30 years, and discusses the hidden characteristics originating from a majority of positive and negative ions and a minority of electrons. A unique structure with a negative ion layer is emphasized in terms of the sustaining mechanism underlying capacitively coupled plasma. In a strong electronegativity, the main sustaining mechanism is caused by a cluster of ionizations placed in front of the instantaneous anode by a minority of electrons accelerated from the bulk plasma into the active double layer. A new insight is obtained for how to hold a bulk plasma. The bulk plasma is maintained by a time-averaged net ionization rate equal to the electron attachment by minority electrons under the assistance of a relatively high reduced field E ( t )/ N g in order to compensate for the large loss by ion–ion recombination. The structure is quite different from that of an electropositive plasma having a low reduced field under ambipolar diffusion. It is proposed that it will be possible to estimate the high value of E ( t )/ N g in bulk plasma in a strongly electronegative plasma on the basis of the static DC breakdown theory in electronegative gas.