Dynamic focus adjustment with optical height detection apparatus in electron beam system
The present invention generally relates to dynamic focus adjustment for an image system. With the assistance of a height detection sub-system, present invention provides an apparatus and methods for micro adjusting an image focusing according the specimen surface height variation by altering the field strength of an electrostatic lens between objective lens and sample stage/or a bias voltage applied to the sample surface. Merely by way of example, the invention has been applied to a scanning electron inspection system. But it would be recognized that the invention could apply to other system using charged particle beam as observation tool with a height detection apparatus.
1. A method for dynamic adjusting a focus of an electron beam image while inspecting a specimen, the method comprising:
providing an electron beam inspection system;
providing an optical illumination flux through a pattern plate and a lens to a surface of the specimen to project a pattern onto the surface of the specimen, the pattern being associated with the pattern plate;
detecting the illumination flux reflected from the surface of the specimen with a detector;
processing information associated with the detected illumination flux;
generating optical images based on a portion of the information associated with the detected illumination flux, a first image including a first image part for the pattern and a second image part for the specimen;
determining a local height variation of the surface of the specimen within an electron beam field of view according to a position variation of the pattern between these optical images;
comparing the local height variation with a depth of focus of an electron beam image;
calculating correction factors to perform primary beam shift compensation in order to compensate the local height variation with respect to the depth of focus of the electron beam image within the field of view;
generating a control signal based on the portion of the information associated with these optical images;
providing the control signal to a controlled objective;
adjusting the focus of the electron beam image of the specimen in response to the control signal that apply to the controlled objective; and
performing an electron beam inspection for the specimen.
2. The method of claim 1 , wherein the controlled objective is the specimen on a sample stage.
3. The method of claim 2 , wherein the control signal applies to the specimen is a voltage bias on the specimen surface.
4. The method of claim 3 , wherein the calculating scanning correction factors further includes image rotation compensation and image magnification compensation.
5. The method of claim 1 , wherein the controlled objective for the image focuses adjustment is a control electrode.
6. The method of claim 5 , wherein the control signal applies to the control electrode is a voltage to generate electrostatic field strength.
7. The method of claim 6 , wherein the calculating scanning correction factors further includes image rotation compensation and image magnification compensation.