Extraction of Secondary Electron Emission Coefficients from the Collapse Ion Sheath Current Curve in Plasma Immersion Ion Implantation

The traditional method to measure the secondary electron emission coefficient (SEEC) is to shoot a continuous beam of charged particles such as electrons or ions toward a metal surface. A hemi-spherical stainless steel chamber is used to collect the secondary electrons released from the surface. The target current (Ip - Is) and the collector current ls can be measured by pico-ammeters with Ip being the primary beam current. The SEEC can be calculated as Is/Ip at different impact energies. When the ions hit a dielectric surface, the deposited positive charges cannot be drained away readily. Surface charging is thus exacerbated by secondary electron emission and consequently, the impact energy of the incoming ions is reduced due to the reduced negative surface potential. In order to overcome surface charging, the system must be modified, for example, by using two electron guns. The modified method only reduces but not totally eliminates surface charging. It has been shown that the pulse duration of the primary electron/ion beam is limited giving an uncertainty of 10% in the primary electron/ion energy. An ion sheath will collapse in front of a dielectric substrate during argon plasma immersion ion implantation. The probe signals during the buildup and collapse of the ion sheath are recorded from a lime glass substrate with a magnesium metal plate placed on top. The collapsing speed of the ion sheath is shown to strongly depend on the secondary electrons emission coefficient of the substrate. Our results show that it is possible to extract secondary electron emission coefficients from insulating materials based on the probe signals.