Electrometers are the most common tool used to measure the accumulation of charge flowing through a circuit. In standard raster scan ion implantation, the Faraday cup is used to measure the flow of charge to the substrates. It is important to measure the flow of charges in order to determine the dose, i.e. ions per square centimeter, implanted.
Modern electronic devices are based on semiconductor materials. Semiconductor materials in their native state have very low intrinsic conductivity and it is necessary to add small amounts of certain types of impurities into the crystal lattice to provide current carriers.
High temperature gas diffusion and raster scan ion implantations have been the known techniques for doping selective impurity into semiconductors. Very recently, it has become recognized that raster scan ion implantation has limitations at low energy beam conditions, under 10 KV, especially where dose and production wafer throughput are of concern. Shallow junction devices have become more important and a new implantation approach known as Plasma Immersion Ion Implantation (PI.sup.3) is being studied. PI.sup.3 uses lower energy and provides high dose rate. In PI.sup.3 the use of an electrometer to measure dose is not practical because of the difficulty of inserting an electrometer into an electrode especially if a biasing voltage is used. Also, the electrometer typically has a slow response time and the electrometer cannot distinguish between unwanted and wanted portions of current flow into the electrode.