Magnetically filtered low-loss scanning electron microscopy

Earlier, a method was described and demonstrated for using the magnetic field in a condenser-objective lens to energy-filter electrons scattered from a solid specimen in the high-field region of the lens. This is done by placing a detector just inside the boundary within which the fastest electrons are confined by the magnetic field. A beam stop can be inserted to remove unwanted electrons that might reach the detector after multiple turns along a spiral path. Calculations have been made for an in-lens detector to conform to the shape of the energy-filtering boundary. The accuracy with which the boundary must be located to give an energy resolution of 1% is of the order of 10 μm for a lens with a polepiece spacing of 1 cm. To obtain an energy resolution better than 1% will probably require that the position and shape of the boundary be measured with a sensing system and electrons must then be collected within a chosen distance of this measured surface. © 1992.