Patent ID: 7176468

Claim:
A method of controllably charging a surface of an insulating substrate to a predetermined target potential, the method comprising: applying a first bias voltage to a cathode of an electron gun and a second bias voltage to the substrate so as to achieve a higher-energy electron beam having a first landing energy that is sufficiently high so that an electron yield is greater than one; applying a third bias voltage to a lower bias electrode of the electron gun to create an electrostatic field above the surface, wherein the third bias voltage is more positive than the predetermined target potential; flooding the surface with the higher-energy electron beam so that the surface of the substrate charges to a first steady state potential at approximately the third bias voltage; subsequently setting the first bias voltage to the predetermined target potential and reducing a voltage difference between the first bias voltage and the second bias voltage, the voltage difference being reduced so as to achieve a lower-energy electron beam having a second landing energy that is sufficiently low so that the electron yield is less than one; flooding the surface with the lower-energy electron beam so that the surface of the substrate charges negatively to a second steady state potential at the predetermined target potential, wherein incident electrons from the electron gun are reflected by the surface when the predetermined target potential is reached; and applying a fourth bias voltage to an extraction electrode near the surface of the substrate during flooding by the lower-energy electron beam so as to extract said reflected electrons from a vicinity above the surface.