Application of optical emission spectroscopy for detection of alpha-gamma transition of RF capacitive discharge in nitrogen

In this paper, we use optical emission spectroscopy to determine the conditions for the transition from alpha to gamma mode of an RF capacitive discharge. The experiments were carried out at a distance of 2 cm between flat stainless-steel electrodes in the nitrogen pressure range from 0.01 to 10 Torr. The emission spectra of the discharge were measured from the central region and the region near the boundary of the near-electrode sheath. In the low-current alpha mode, radiation from the first (predominantly) and second positive systems of excited nitrogen molecules and low-intensity lines of nitrogen molecular ions emerge from the central region of the discharge. Here, electrons are heated in RF electric field in the plasma. Stochastic heating of electrons dominates near the sheath boundary, which enhances the lines of nitrogen molecules and ions listed above. The transition to the high-current gamma mode leads to the appearance of fast electron flows from the near-electrode sheaths. The RF electric field in the plasma decreases, due to which the intensities of the first positive system of nitrogen molecules are reduced significantly. The intensities of other lines both in the plasma and in the vicinity of the sheath are enhanced. From the breaks in the dependences of the line intensities on the RF voltage, the alpha-gamma transition curve was plotted in a wide range of nitrogen pressures.