System and method to determine focus parameters during an electron beam inspection
This invention relates to apparatus and method to fast determine focus parameters in one pre-scan during an e-beam inspection practice. More specifically, embodiments of the present invention provide an apparatus and method that provide accurate focus tuning after primary focusing has been done.
1. An electron beam apparatus, comprising:
an electron source for generating a primary electron beam;
at least one condenser lens for pre-focusing the primary electron beam;
an objective lens for forming an magnetic field and an electrostatic field to focus the primary electron beam onto a specimen in the electron beam path;
a detection system for detecting signal electrons emanated from the specimen;
a deflection system for deflecting the primary electron beam over surface of the specimen to form a scanning pattern;
wherein a scan is performed over a predetermined field of view with the best focus current condition;
a stage system for supporting and moving the specimen; and
a fast respond auxiliary lens for varying the e-beam focusing position along the z direction.
2. The auxiliary lens of claim 1 , wherein core material of the auxiliary lens is a low hysteresis material.
3. The auxiliary lens of claim 2 , wherein the low hysteresis material is ferrite.
4. The method of claim 3 wherein the scan is performed one time.
5. The auxiliary lens of claim 1 wherein a scan can be performed over a field of view (FOV) on one line.
6. A method of determining the best focus condition in a predetermined field of view (FOV), comprising:
select a pre-scan area within or close to the predetermined FOV;
utilize auxiliary lens and deflection system perform x (position), y (position) and z (focusing current) 3-dimensional scan image over the selected area;
examining the xyz scan image to locate the best focus point (x,y);
determine the z focus current according to the best focus point (x,y); and
perform the scan over the predetermined FOV with the best focus current condition.
7. A computer readable medium containing program instructions for determining the best focus condition in a predetermined field of view (FOV), the program instructions for:
select a pre-scan area within or close to the predetermined FOV;
utilize auxiliary lens and deflection system perform x (position), y (position) and z (focusing current) 3-dimensional scan image over the selected area;
examining the xyz scan image to locate the best focus point (x,y);
determine the z focus current according to the best focus point (x,y); and
perform the scan over the predetermined FOV with the best focus current condition.
8. The computer readable medium of claim 7 , wherein core material of the auxiliary lens is a low hysteresis material.
9. The computer readable medium of claim 8 , wherein the low hysteresis material is ferrite.
10. The computer readable medium of claim 7 , wherein a scan can be performed over a field of view (FOV) on one line.