Ion-to-CH ₃ flux ratio in diamond chemical-vapor deposition

Methyl radicals (CH ₃) and positive ionic species in a low-pressure inductively coupled plasma under diamond-depositing conditions have been detected by using a quadrupole mass spectrometer. Absolute calibration of the fluxes of CH ₃ and ionic species was made by the threshold ionization technique and Langmuir probe measurement, respectively. The CH ₃ density increased by two to three times with a small addition of carbon monoxide to a methane-hydrogen plasma and was on the order of 10 ¹¹-10 ¹² cm ^-3. As the pressure decreased from 60 to 10 mTorr, the ion-to-CH ₃ flux ratio increased from 0.2 to 4.3, accompanied by an increase in the fraction of light ions such as H _x ⁺ (x=1-3). The average ion energy in the ion energy distribution at a grounded electrode was compared with the sheath potential and the discrepancy was found to be 0.5-2 eV depending on pressure and ion mass. The results were used to describe the specific surface process dominated by energetic (∼ several eV) ions rather than thermal neutrals.