Analysis of Ion Beam Generated from Sheath Field of Grid Electrode and Its Application

Summary form only given. In plasma immersion ion implantation and ion assisted materials processing, it is difficult for the ions to have enough energy and flux when they inject into the surface of thick insulating materials due to the large voltage drop and charging in the materials. For this application, we used a conducting grid above the insulating materials and applied voltage to the grid to accelerate the ions. There are some critical factors associated with the mesh design such as the distance between a mesh and a material, and the mesh size. In addition, we also need to solve the ionic space charge and accumulated charge on the surface. They make it impossible that a large number of energetic ions are able to reach the material surface from the sheath. With these backgrounds, we analyzed ion dynamics in biased grid-mesh. Characteristics of ion beam generated from sheath field are solved using fluid equations and the behaviors of ions between mesh electrode and materials are investigated by particle simulation. Maximum travel distance of ions, the distance between mesh electrode and virtual electrode, was found to be a strong function of current and energy of ion beam generated from sheath field. As the plasma density increase and bias decreases the materials should be located closer to the mesh electrode for effective treatment. Ion current limitation problem by surface charging was solved by applying asymmetric alternative voltage pulses to the optimally designed grid. The electron flux allowed during higher potential period of pulse bias neutralizes the accumulated charge effectively. Biased grid mesh designed by these analyses has been applied to surface treatment of thick polyurethane materials. Correlations between calculated ion dynamics and experimental results of ion-assisted surface treatment will be discussed