The Effect Of Secondary Electron On RF Discharge And Its Application

Inductively coupled plasma (ICP) generated at low gas pressure has beenextensively employed in plasma surface treatment, in which, high plasma density issuggested to improve the functionality of surface modification. So much attention hasbeen paid on achieving the high plasma density.Here the negative DC pulsed voltages applied on an Al target are proposed toimprove the plasma density in ICP discharges by secondary electrons emitted from Altarget. The secondary electrons are induced by the bombardment of ions, which aregenerated in ICP discharge and accelerated in the sheath region established bynegative pulse voltage. In this study, the effects on plasma parameter in terms ofplasma density are investigated under different voltage bias and the plasma surfacemodification on materials is also explored. The contents are as follows:Firstly, the spatio-temporal evolution of discharge characteristics and thefunctionality of the secondary electrons were studied by the time-resolved Langmuirprobe and the emission spectrum diagnosis. The generation of secondary electronswith DC pulsed voltage bias was proved by the measurements of electron current inLangmuir probe. These secondary electrons are considered to improve the plasmadensity by enhancing the collision ionization in discharge region, which suggests thatthe plasma density is manipulated by the parameters of applied DC pulsed voltagebias. Furtherly, the generation of secondary electrons was also confirmed by thein-situ diagnosis of fluorescence from GaN film, in which the fluorescence intensitywas found to be dependent on the DC pulsed voltage bias. Secondary, the surface modification of PET film by the secondary electroncollaborated ICP discharge was carried out. The wettability of the film was measuredby the contact angle test, and the morphological structure and chemical compositionof the modified PET membrane were characterized by the atomic force microscope(AFM) and X-ray photoelectron spectroscopy (XPS), respectively. It shows that thesurface hydrophilicity of PET film can be improved quickly and efficiently by theplasma treatment within15s with the water contact angle dropping from72.4°to21.8°. The better hydrophilicity can be achieved with enhanced secondary electronemission. The AFM test showed that the surface roughness increased to be13.356nm,which is about1.7times as much as the original one. It suggests that the etchingphenomenon is one of the mechanism of improving the hydrophilicity of PET film bythe secondary electron collaborated RF plasma. In addition, the oxygen wasintroduced with the ratio between oxygen and argon of1:14in plasma surfacemodification to add the functional group of O-C=O on PET surface. It wascharacterized that the C=O content reached69.7%and the contact angle dropped to19.3°. It suggests that the introduction of polar functional groups can also improvethe hydrophilicity of PET film in plasma surface modification.