Transient Ion Sheath in Pulsed Plasmas

In plasma-based ion implantation and deposition (PBII&D), the use of pulsed-plasma sources such as a shunting arc might have some advantages over a conventional plasma source used to produce high-density plasmas. It is desirable for a conformal implantation into the materials (targets) with complicated surfaces. In some cases, the pulsed plasmas may be surrounded by a vacuum region in the vessel, when the lifetime of each pulsed plasma is short compared with the plasma-expansion time. When a target is immersed in such isolated plasmas and a negative voltage pulse is applied for ion implantation, the ion-sheath potential may be different from what is expected in a plasma contacting the vacuum vessel of the ground potential. The ion-sheath potential, the difference between the plasma space potential and the target potential, determines the maximum energy of the implanted ions. A simplified one-dimensional model, in which the target is immersed in a plasma surrounded by a vacuum (or low density) region and an outside vessel of the ground potential, has been developed. It is shown that, when a negative high-voltage pulse is applied to the target for ion implantation, the resultant sheath potential can be substantially lower than the applied potential depending upon the ion current to the target and the distance of the plasma edge from the vessel wall. This effect on the ion current and implanted-ion energy distribution is discussed in this paper