Source gas depletion in narrow metal tube during internal DLC coating with microwave-excited high-density near plasma

In internal DLC (Diamond-Like Carbon) coating to mm-sized narrow metal tubes by using MVP (Microwave-sheath Voltage combination Plasma) method, axially uniform distribution of film thickness can be obtained by repeating the depletion and homogenization of source gas in a coated tube during plasma-on time $T_{on}$ and plasma-off time $T_{off}$ of pulsed plasma generation, respectively. DLC was deposited to the inner surface of a stainless-steel tube 4.4 mm in inner diameter and 50 mm in length with small holes of \textit{$\Phi $}=0.4 mm fabricated at every 10 mm, where the flow rates of Ar and methane were controlled to be 14 and 2 sccm, respectively, at a total gas pressure of 80 Pa. A pulsed negative voltage of --200 V was applied to the tube at a pulse frequency of 10 Hz and duty ratio of 3.2{\%}, synchronizing a pulsed injection of 2.45-GHz microwaves at the same pulse frequency ($T_{on}$=3.2 ms and $T_{off}$=96.8 ms). The high-speed camera image showed that the emission (696, 706 nm) from Ar atom was approximately constant during plasma-on time. On the other hand, the emission (468-474 nm) from C$_{2}$ dimer was decreased until $T_{on}$ =1.5 ms, and then converged in a constant value; this is ascribed to the consumption of CH$_{4}$ gas which is considered to be a main source of C$_{2}$ dimer formation.