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

The plasma density (n_e) profile, rotational (T_rot) and vibrational (T_vib) 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. (paper)