Calibrating a lithographic apparatus
View Patent ↗The X, Y and Rz positions of a mask stage are measured using two optical encoder-reading heads measuring displacements of respective grid gratings mounted on the mask stage. The grid gratings are preferably provided on cut-away portions of the mask table so as to be coplanar with the pattern on the mask itself. Measurements of the table position in the other degrees of freedom can be measured with capacitive or optical height sensors.
1. A method of calibrating a lithographic apparatus comprising:
providing a reference pattern to a patterning device held in a moveable support structure, said reference pattern having a plurality of reference marks at pre-calibrated positions;
positioning said movable support structure so as to project an image of each of said reference marks in turn onto an image sensor arranged on a substrate table at a constant position relative to a projection system;
measuring position of said movable support structure in at least a first degree of freedom when each of the reference marks is projected onto said image sensor; and
comparing the measured position of said movable support structure with pre-calibrated position of respective one of the reference marks to generate an error map.
2. The method according to claim 1 , wherein said image sensor is positioned under a center line of the projection system.
3. The method according to claim 1 , wherein said image sensor is positioned at an extreme position of an exposure field of the projection system.
4. The method according to claim 1 , wherein the measuring comprises:
measuring the position of said moveable support structure using at least one grid grating mounted thereon and at least one sensor head.