Spatially resolved optical emission and electrical properties of SiH4RF discharges at 13.56 MHz in a symmetric 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, the authors 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.