On/off Pulse Modulation of Atmospheric-Pressure Helium/Argon Inductively Coupled Plasma Microjet for Plasma Processing of Polymers

To explore the low-temperature surface treatment of polymers using high-density atmospheric-pressure inductively coupled plasma (AP-ICP) microjet, the critical parameters determining the breakdown and sustainment of the pulse-modulated (PM) helium/argon (He/Ar) AP-ICP microjet were studied by varying the Ar flow rate, pulse-modulation frequency, f, VHF power, P, and duty ratio, DR. The AP-ICP microjet source used in this study consists of a planar serpentine-shaped antenna and an alumina discharge tube of 0.8 mm inner diameter. Although the minimum VHF power, P, required for the breakdown was 95 W for the pure helium plasma, it could be reduced to 51 W owing to the Penning effect by adding Ar (4-7 sccm) to He (3 slm). The lower limits of DR for the He/Ar PM plasma breakdown and sustainment were investigated by changing fat the constant P= 90 W. It was found that the minimum DR for the generation of the PM plasma without using the auxiliary ignitor was proportional to f The linear relation indicates the existence of minimal on-time period that is presumably a requisite rise time of PM VHF voltage in the plasma circuit. The minimum DR for the breakdown became higher than that for the sustainment at f> 26 kHz, where the latter DR was 5-10 W. Consequently, it was found that the plasma operation with low net power is possible particularly under the low-f pulse modulation condition. Furthermore, a preliminary experiment on the surface treatment of polycarbonate by scanning the PM AP-ICP microjet under low-f conditions demonstrated the applicability of this plasma system to the surface processing of polymers without the thermal damage.