Electron beam charging of insulators with surface layer and leakage currents

The electron beam induced self- consistent charge transport in layered insulators ( here, bulk alumina covered by a thin silica layer ) is described by means of an electron- hole flight- drift model and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field- enhanced detrapping are described by the Poole - Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate. As a main result, the time- dependent secondary electron emission rate sigma ( t ) and the spatial distributions of currents j ( x, t ), charges rho( x, t ), field F ( x, t ), and potential V ( x, t ) are obtained. For bulk full insulating samples, the time- dependent distributions approach the final stationary state with j ( x, t )= const= 0 and sigma= 1. In the case of a measurable surface leakage current, the steady stationary state is reached for sigma < 1. First measurements are extended to the sample current measurement including nonstationary components of charge incorporation and polarization as well as dc components of leakage currents. (C) 2008 American Institute of Physics.