Ion energy measurements during plasma immersion ion implantation of an insulator

Plasma immersion ion implantation (PIII) is a surface modification technique in which an object is placed in a plasma and negatively pulse biased. For the treatment of insulators, a metal mesh can be placed around the object and pulse biased to minimize surface charging. In this paper, the effects of surface charging of an insulator on the ion flux and ion energy distribution (IED) during PIII are investigated both with and without a mesh present. IED measurements were made through a hole in a Mylar insulator using a retarding field energy analyser and compared with the results of a two-dimensional numerical simulation. Qualitative agreement was found between experimental and simulation results. With an insulator and no mesh, at the start of the pulse, the IED is similar to the case with no insulator, with a peak close to the maximum ion energy and ions with a spread of energies between zero and the maximum ion energy. During the pulse, the number of ions measured with lower energies increases. Simulations show that this is caused by surface charging of the insulator. With a mesh and insulator, compared with the no mesh case a larger flux of ions is measured during the early stages of the pulse but during the pulse the flux decreases and more low energy ions are measured. Simulations show that this is caused by surface charging changing the potential in the mesh region which affects the trajectories of incoming ions.