Detection of CH3 radicals in an RF plasma by photoionization mass spectrometry

Abstract To investigate processes occurring during thin-film deposition by plasma chemical vapor deposition, a study was made of the radical species in an RF CH 4 H 2 plasma using photoionization mass spectrometry. The method avoids fragmentation of particles as can occur in conventional mass spectrometry by using photons for their ionization. In the present experiment, a capacitive 13.56-MHz RF plasma reactor was used and the total gas pressure was 2.7 × 10 2 Pa. Besides the CH 3 radical on whose detection emphasis was laid, CH 2 , CH, C 2 H 3 and C 2 H 4 were detected in the excited CH 4 H 2 plasma using a quadrupole mass spectrometer with a Kr or Ar resonance lamp. The lamp was microwave-operated at 2.45 GHz to produce the ultraviolet resonance radiation with two components of energies 10.03 and 10.64 eV in Kr or 11.62 and 11.83 eV in Ar. Suppression of undesirable ionization of particles by impact of accelerated photoelectrons generated by lamp radiations was achieved by voltage control of the ion-lens electrodes. The detection sensitivity of the apparatus was estimated as having a CH 3 number density of typically 2.0 × 10 11 cm −3 . Data were obtained on the dependence of CH 3 radical density upon the CH 4 mole fraction of the CH 4 H 2 source gas.