Measuring the power of electromagnetic waves and their phases is a classical method for the study of microwave circuits. With new apparatus we may use this technique in the 13.56-MHz range. By computation we may derive currents, voltages, and impedances with very good accuracy. In an industrial etching system (GIR 100 made by Alcatel), we measured impedances of glow discharges in argon and SF6 as a function of pressure and power input. We compute voltages on the powered electrode in addition to the current and power actually dissipated in the discharge. We find that the efficiency Weff/W of the generator may be between 10% and 98% depending on the plasma parameters. Real and imaginary parts of impedances relate to the dissipated power and the thickness of the sheaths, respectively. Therefore, one can compare measurements to models of sheaths (Child–Langmuir law, mobility of ions limited by scattering) and to models of glow discharges (power dissipated in the sheath alone or dissipated in the heart of the glow). From our results we discern two types of discharges: discharges in electropositive gases like argon and discharges in very electronegative gases like SF6.
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Andries et al. (1989) studied this question.