Influence of driving frequency on narrow-gap reactive-ion etching in SF6

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Abstract
The structure of radiofrequency discharges in a narrow-gap reactive-ion etcher with parallel-plate geometry in SF6 for frequencies 100 kHz to 13.56 MHz is investigated by the relaxation continuum model. Capacitively coupled reactive-ion etching is conventionally used for dry etching. Most etching gases are known to be strongly electronegative as well as chemically reactive. The influence of frequency on the structure and function of the discharge in electronegative gases is still unknown. The spatiotemporal discharge structure and its maintenance mechanisms are strongly changed as a function of driving frequency. The importance of a double layer to ion and radical formation on increasing the frequency is stressed, while secondary electron emission from the electrode surface as well as the double layer are essential at low frequency (100 kHz). The electron number density is three orders of magnitude less than that of positive ions, and positive and negative ions are the major charged particles in the narrow-gap reactive-ion etcher. The ion flux to the electrode surface is discussed as a function of frequency.