Detection of CH3 radicals in an RF CH4/H2 plasma by photoionization mass spectrometry

To investigate processes occurring during thin-film deposition by plasma chemical vapor deposition, a study was made of the radical species in an RF CH4/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 x 10(2) Pa. Besides the CH3 radical on whose detection emphasis was laid, CH2 CH, C2H3 and C2H4 were detected in the excited CH4/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 CH3 number density of typically 2.0 x 10(11) cm(-3). Data were obtained on the dependence of CH3 radical density upon the CH4 mole fraction of the CH4/H-2 source gas. (C) 1998 Elsevier Science Ltd.