Investigation of plasma uniformity, rotational and vibrational temperature in a 162 MHz multi-electrode capacitive discharge

Abstract

The plasma density (ne) profile, rotational (Trot) and vibrational (Tvib) temperatures, and their dependence on the RF power (500–1500 W) and gas pressure (50–500 mTorr) is investigated in a high to very high frequency (VHF) (162 MHz) capacitively coupled nitrogen plasma excited by a multi-tile electrode (tiles) system. The density profile is measured in the mid-plane of the discharge using a resonance hairpin probe, and the rotational and vibrational temperatures are measured at both tile centre and tile-tile boundary using optical emission spectroscopy. It is observed that the plasma density increases monotonically with a rise in RF power and decreases with an increase in the operating gas pressure. At a low gas pressure (50 mTorr), plasma density profile shows a maximum at the tile centre and a minimum at the tile-tile boundary, whereas, at high gas pressure tile-edge effects are observed. Measured rotational temperature (∼350–450 K) is slightly above room temperature for both positions and independent of RF power and operating gas pressure. Vibrational temperature is in the range of ∼6500–9400 K, and increases with RF power, analogue to the plasma density. It is noticed that the plasma uniformity can be substantially improved, to better than 90%, by changing the power-pressure matrix. A large difference between measured vibrational and rotational gas temperature suggests that the plasma produced by VHF multi-tile electrode is under highly non-equilibrium condition and thus highly efficient to produce unique gas phase chemistry.