Apparatus and methods of controlling surface charge and focus

A method of setting a surface charge of an area on a substrate to a desired voltage level, the method comprising:

• holding the substrate in a stage;
• controlling a stage bias voltage applied to the stage;
• and
• directing a flood of electrons to the area such that the surface charge of the area reaches an equilibrium at the desired voltage level, and
• wherein the stage bias voltage is controlled so as to be substantially proportional to the desired voltage level.

An apparatus configured with a capability to maintain focus of a main electron beam incident upon a substrate, the apparatus comprising:

• a column configured to generate and direct the main beam towards an imaging area of the substrate;
• an objective lens with a variable focal length that is configured to focus the main beam onto the imaging area;
• a monitor beam gun configured to generate and direct a monitor electron beam towards a monitoring area of the substrate at a non-perpendicular incidence angle;
• and
• an in-focus detector configured to detect an in-focus band in data collected from the monitor beam,
• wherein the monitoring area of the substrate has sufficient edge content so as to locate the in-focus band between out-of-focus areas.

A method of auto-focusing a main electron beam incident upon an imaging area of a substrate, the method comprising:

• generating a monitor electron beam;
• directing the monitor beam towards a monitoring area of the substrate at a non-perpendicular incidence angle;
• detecting an in-focus band in data collected from the monitor beam;
• and
• adjusting a focal length of an objective lens focusing the main beam based upon a position of the in-focus band,
• wherein the monitoring area of the substrate has sufficient edge content so as to locate the in-focus band between out-of-focus areas.

A method of setting a surface charge of an area on a substrate to a desired voltage level and maintaining focus of a main electron beam incident upon the area, the method comprising:

• holding the substrate in a stage;
• controlling a stage bias voltage applied to the stage wherein the stage bias voltage is controlled so as to be substantially proportional to the desired voltage level of the surface charge;
• directing a flood of electrons to the area such that the surface charge of the area reaches an equilibrium at the desired voltage level;
• imaging the area with the main beam;
• generating a monitor electron beam;
• directing the monitor beam towards a monitoring area of the substrate at a non-perpendicular incidence angle wherein the monitoring area of the substrate has sufficient edge content so as to locate an in-focus band between out-of-focus areas;
• detecting the in-focus band in data collected from the monitor beam;
• and
• adjusting the stage bias voltage based upon a position of the in-focus band to effectively adjust the focus of the main beam.