Diagnostics of low-frequency ch₄ and h₂ discharge by optical emission spectroscopy

Low–frequency glow discharge for the deposition of hydrogenated amorphous carbon has been investigated by using time– and space–resolved emission spectroscopy as the parameters of pressure (0.3and<pand<3.0 Torr) and gas mixture ratio between CH₄ and H₂ at 100 kHz and 10 SCCM. The authors ascertain the temporal structure of the discharge. The CH(B² Sigma ⁻; v’=0 to X² Pi; v”=0) band head in CH₄/H₂ is selected as a probe for monitoring the dynamics of the electron transport. The Ar I (3p⁵5p:3p₁ to 3p⁵4s:1s₂) line in pure Ar is also observed for comparison with the discharge structure in CH₄/H₂. The glow discharge property changes significantly with total gas pressure in the CH₄/H₂ mixture under investigation. Considerable variation is observed between the sustaining voltage and the emission profile of the discharge with the increase of mixture ratio of CH₄ to H₂ at constant power. Remarkable differences in the emission profile exist between CH₄/H₂ and Ar. The authors show that the drift velocity of the dominant ions plays an important role in deciding the structure of the discharge especially at low frequency, and that the plasma bulk potential in Ar is higher than that in H₂ at the same applied voltage.