Radio frequency hollow cathode jet: optical emission study

Hydrogen radicals are crucial for: remote plasma enhanced chemical vapor deposition; metal–organic chemical vapor deposition; or plasma cleaning. A new type of 13.56 MHz hollow cathode jet shows high efficiency of radical generation. For better understanding of plasma heating mechanisms, the source is characterized by optical emission spectroscopy from hydrogen plasma in the wavelength range from 200 to 800 nm. The examined parameter range was: pressure from 0.1 to 4 torr; gas flow up to 100 sccm; and rf power from 20 to 180 W. The emission intensity in the jet zone is 2 orders of magnitude higher than in the anode or cathode zone. The axial emission intensity distributions in the remote zone indicate the propagation and absorption of rf power along the remote plasma column. The emission intensities of lines related to C1Πu–XA1Σg+ transition, are compared with the intensities of Hα, Hβ and Hγ atomic lines. The molecular-to-atomic emission intensity ratio increases with increasing pressure and with increasing distance from the plasma source, due to the increase of vibrational excitation.