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

摘要

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 f at 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.