One embodiment disclosed relates to a method of setting a surface charge of
an area on a substrate to a desired level. The substrate is held on a
stage, and a stage bias voltage applied to the stage is controlled. A
flood of electrons is directed to the area. The stage bias voltage is
controlled such that the surface charge of the area reaches an equilibrium
at the desired level. Another embodiment disclosed relates to a method of
auto-focusing a main electron beam incident upon an imaging area of a
substrate. A monitor electron beam is generated and directed towards a
monitoring area of the substrate at a non-perpendicular incidence angle.
An in-focus band in data collected from the monitor beam is detected. The
focal length of an objective lens focusing the main beam is adjusted based
upon a position of the in-focus band.
