Measurement of ion current on target biased with a high negative voltage

Abstract

Summary form only given. Analysis of the conduction current on the negatively biased target is important to develop the pulse switch system and to improve the material surface treatment using the plasma source ion implantation. In case of highly negative biased target immersed in the plasma, the current on the target is composed of the incident ion current and the electron emission current. The virtual area of collecting current on the target is proportional to the sheath area formed over the target electrode which is expanded considerably compared with the size of electrode with increasing the high voltage. The sheath developed near the edge of the target should be considered in the analysis of the target ion current. The spatial distribution of plasma in front of the target also affects the sheath formation because the density distribution is not uniform in real situation which may shrink the sheath expansion. In this study, the effects of sheath geometry and plasma distribution with the secondary electron emission are investigated. Experiments were carried out with the planar stainless steel and aluminum targets having a diameter 100 mm, negatively bias potential ranging in 2 kV-11 kV, and various plasma gases of Ar and He, respectively. Also the target is enshrouded by the quartz tube of 100 mm long which may reduce the effect of the sheath formation at the edge of the target. Spatial plasma density was measured by Langmuir probe and compared with the values obtained from the target current which were analyzed with the current model considered with the Bohm current, the edge effect of sheath formation on the target, and spatial plasma distribution with secondary electron emission coefficient. Results will be discussed