System and method for controlling charge-up in an electron beam apparatus
The present invention provides means and corresponding embodiments to control charge-up in an electron beam apparatus, which can eliminate the positive charges soon after being generated on the sample surface within a frame cycle of imaging scanning. The means are to let some or all of secondary electrons emitted from the sample surface return back to neutralize positive charges built up thereon so as to reach a charge balance within a limited time period. The embodiments use control electrodes to generate retarding fields to reflect some of secondary electrons with low kinetic energies back to the sample surface.
1. An objective lens of a scanning electron microscope, comprising:
a magnetic objective lens; and
a control electrode between said magnetic objective lens and a surface of a sample to be observed,
wherein said control electrode is biased a voltage with respect to said sample,
wherein during each period of line-scanning within a frame cycle of imaging scanning, said voltage is set at a first value so as to generate an electrostatic extraction field on said surface,
wherein during each period of beam-retracing within said frame cycle of imaging scanning, said voltage is set at a second value so as to generate an electrostatic reflection field above said surface.
2. The objective lens according to claim 1 , wherein a primary electron beam of said scanning electron microscope is not blanked out during said each period of beam-retracing.
3. The objective lens according to claim 2 , wherein on said surface, a path of said primary electron beam during said each period of beam-retracing covers a path thereof during a latest period of line-scanning.
4. The objective lens according to claim 3 , wherein said second value is chosen so that said electrostatic reflection field reflects a desired amount of secondary electrons emitted from said surface back to said surface to make a charge balance thereon.
5. An electron beam apparatus, comprising:
an electron source, which emits primary electrons along a direction, wherein said direction is an optical axis of said electron beam apparatus and said primary electrons form a primary electron beam;
a condenser lens, which is located below said electron source and aligned with said optical axis;
a beam-limit aperture plate, which is located below said electron source and comprises at least one opening, wherein one opening is aligned with said optical axis and limits a current of said primary electron beam;
a magnetic objective lens, which is located below said beam-limit aperture plate and condenser lens and aligned with said optical axis;
a control electrode, which is located below said magnetic objective lens and aligned with said optical axis;
a sample stage, which is located below said control electrode and supports a sample, wherein an interested surface thereof is placed upwards and said control electrode is biased a voltage with respect to said sample;
a deflection unit, which is above said control electrode and deflects said primary electron beam so as to scan said interested surface in a raster way within a frame cycle of imaging scanning, wherein said frame cycle of imaging scanning comprises a series of alternately distributing periods of line-scanning and beam-retracing, wherein on said interested surface, a path of said primary electron beam during said each period of beam-retracing covers a path thereof during a latest period of line-scanning; and
an electron detector, which is located above said deflection unit and below said beam-limit aperture plate, comprises a hole aligned with said optical axis for said primary electron beam passing therethrough and a detection area facing to said interested surface so as to collect a first amount of secondary electrons emitted therefrom,
wherein said primary electron beam is not blanked out during said each period of beam-retracing,
wherein during said each period of line-scanning, said voltage is set at a first value so as to generate an electrostatic extraction field on said interested surface,
wherein during said each period of beam-retracing, said voltage is set at a second value so as to generate an electrostatic reflection field which reflects a second amount of said secondary electrons back to said interested surface to make a charge balance thereon.