Mode transitions in low-pressure nitrogen RF-CCP at different frequencies

Summary form only given. Low-pressure (p≈10³-1Torr) capacitively coupled RF plasmas are mostly used in material processing [1]. Different parametrical conditions in the plasma are preferred by different applications. High density plasmas in gamma mode are ideal for etching applications while stabile plasmas in gamma mode are more convenient for sputtering applications [2]. Therefore, it is better to characterize the plasma before process to get more efficient results.This study reports mode transition points in low-pressure capacitively coupled pure nitrogen RF plasma driven at 13.56 MHz and 40.68 MHz. The plasma was ignited in a home-made (500 u 400 mm2) stainless steel cylindrical reactor. Distance between the identical (200 mm in diameter) electrodes was set to 40 mm [3, 4]. Moreover, L-type automatic matching network system was connected to the 40.68 MHz RF generator to get high accuracy while matching system of 13.56 MHz was perched in its generator. In addition, the pure (99.995 %) nitrogen was selected as an activation gas because there is not a strong work about mode characterization of nitrogen gas, although it has a remarkable impact in plasma processing applications. Mode transition analyses were done according to diagnostic results that were obtained by Impedans Langmuir single and double probe systems. Pressure and power dependence of electron conduction current density, electron density and electron energy (or temperature) were investigated to determine mode transition points. These investigations revealed that a smooth alpha to gamma transition was observed at 0.1 Torr pressure in 13.56 MHz nitrogen plasma at whereas a sharp one was seen at 0.3 Torr pressure in 40.68 MHz nitrogen plasma. Also, at these pressures, another transition point (from gamma to alpha) was noticed with 150 Watt critical RF power in both plasmas. In conclusion, heating mode transition pressure of the low frequency RF-CCP is lower than that of the high frequen- y RF-CCP. On the other hand, RF power shows the same effect in both RF plasmas.