SPATIALLY RESOLVED OPTICAL-EMISSION AND ELECTRICAL-PROPERTIES OF SIH4 RF DISCHARGES AT 13.56 MHZ IN A SYMMETRICAL PARALLEL-PLATE CONFIGURATION

A symmetric capacitively coupled SiH4 radio-frequency discharge at 13.56 MHz is analysed by spatially resolved optical emission spectroscopy and by measurements of the RF voltage and RF current and of the ion conduction current and energy distribution through the sheath. A transition between two discharge regimes is observed, either by increasing the pressure at a constant RF voltage, or by increasing the RF voltage at a constant pressure. This transition between two discharge regimes is attributed to different mechanisms by which electrons gain energy from the sheaths and in the plasma. For a gas temperature of 200-degrees-C, at a fixed pressure of 0.185 Torr, the transition occurs as the RF voltage exceeds 140 V. When comparing both discharge regimes at the same pressure and RF voltage, we observe a drastic change of the RF current waveform. The power dissipated in the discharge, as well as the total intensity of the optical emission from the discharge, increases by a factor 3. Moreover the spatial distribution of the emission is completely modified. These results are discussed in relation to recent experimental and modelling studies of RF discharges. In the low-pressure, low-voltage regime, the power dissipation is dominated by plasma electrons pushed into the discharge by the sheath field during the cathodic phase of the RF cycle ('wave riding'). The high-pressure, high-voltage regime is probably initiated by secondary electrons emitted by ion bombardment from the electrode and accelerated through the sheaths but involves a large contribution of 'Joule heating' of electrons in the plasma by the bulk RF field. The spatial distribution of the electric field in the sheaths and in the plasma bulk region seems to be drastically affected by the presence of negative ions and/or negatively charged particles. Using the set of experimental data and simple analytical models, the bulk RF field and the electron and ion densities are determined. The negative-ion density exceeds the electron density at high pressure, in agreement with recent self-consistent modelling studies. Finally a quantitative analysis of the power dissipation by different mechanisms is performed.