Lithographic apparatus and method for determining Z position errors/variations and substrate table flatness

A method for measuring mirror flatness and/or chuck flatness in a lithographic system that has a substrate table configured to hold a substrate, the substrate table defining a Z-direction substantially perpendicular to a surface of the substrate table that is configured to hold the substrate;

• a plurality of level sensors for sensing a level of a substrate carried on the substrate table at a plurality of different positions, to determine coordinates of the substrate along the Z-direction, and a system for determining the position of the substrate table in at least the Z-direction, the method comprising:
• generating relative movement between the substrate table and the plurality of level sensors, so that each of the plurality of the level sensors is able to assume a first position relative to the substrate to make a measurement at a first measurement point on the substrate, to provide a first set of overlapping measurements, and so that each of the plurality of level sensors is able to assume a second position relative to the substrate to make a measurement at a second_measurement point on the substrate, to provide a second set of overlapping measurements;
• and
• determining at least one of a measure of position errors, substrate table unflatness or a measure of the level sensor position/offset using the first and second sets of overlapping measurements and position determinations from the system for determining the position of the substrate table.

A computer program on at least one physical, computer-readable medium, the computer program having code or instructions for determining mirror and/or chuck flatness, the code or instructions for performing the method of:

• generating relative movement between a substrate table and a plurality of level sensors, so that each of the plurality of the level sensors is able to assume a first position relative to the substrate to make a measurement at a first measurement point on the substrate, to provide a first set of overlapping measurements;
• generating relative movement between the substrate table and the plurality of level sensors, so that each of the plurality of level sensors is able to assume a second position relative to the substrate to make a measurement at a second measurement point on the substrate, to provide a second set of overlapping measurements;
• and
• determining at least one of a measure of position errors, substrate table unflatness or a measure of the level sensor position/offset using the first and second sets of overlapping measurements.