Observation of source gas depletion in narrow metal tube during internal diamond-like carbon coating with microwaves

For stable plasma generation inside mm-sized metal tubes and holes, we developed a new method of plasma generation using microwave propagation along plasma–sheath interface; “Microwave–sheath Voltage combination Plasma (MVP)” method. In this work, we tried to homogenize the axial distribution of film thickness and property in internal diamond-like carbon (DLC) coating using MVP by further employing the method proposed by Segner; the characteristic of the method is to repeat the depletion and refill of source gas in a coated tube during plasma-on time and plasma-off time of pulsed plasma generation inside the tube, respectively. The variability of film thickness was improved from 35% in our previous result to 14% in this work. It was further demonstrated that the source gas depletion can be visualized by observing optical emission from Ar atom and C2 dimer.

► Microwave-excited high-density plasma was generated inside a 1/4 in. steel tube. ► Diamond-like carbon (DLC) was coated inside the tube. ► The axial variability of film thickness was improved from 35% to 14%. ► The source gas depletion in the tube during internal DLC coating was observed. ► The time of source gas depletion was identified to be 1.6 ms.