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Chlorine plasma system instabilities within an ICP tool driven at a frequency of 13.56 MHz

Many of today's continuous-wave plasma etch processes of semiconductor devices utilize chlorine or chlorine-based gases which require stable power delivery conditions. When the plasma process is intentionally (pulsed) or unintentionally driven into an unstable mode, the population of electronegative species become time-dependent. This paper examines the instability coupling between the external circuit and a chlorine plasma within an inductively coupled plasma tool. Standing-wave ratio measurements are used to map instabilities in a 2 mTorr pure chlorine discharge, as a function of power and the parallel capacitor (CP) of the T network (matching box and antenna). At low power (<180 W) no instabilities are observed whether the system is matched or mismatched. Instabilities are only observed at input power levels from 180 to 280 W when the system is matched. At larger powers (>280 W) instabilities are observed when the system is mismatched with respect to the system input impedance (50 Ω) and the mismatch makes the load reactance negative. The asymmetry of the match is explained using an equivalent electrical model and power balance arguments. The methodology developed allows the identification of four different types of instability and indicates operating regimes which will be free of instabilities.