Method of calibrating a lithographic apparatus, alignment method, computer program, data storage medium, lithographic apparatus, and device manufacturing method

A method of calibrating a lithographic apparatus, the method comprising:

• for each of a plurality of marks arranged to be observed through a single branch of front-to-backside alignment optics of the lithographic apparatus, calculating a first displacement vector between (a) the mark and (b) an image of the mark observed in an image window of said front-to-backside alignment optics;
• for each of the plurality of marks, calculating a second displacement vector between (a) the mark and (b) an image of the mark observed in an image window of said front-to-backside alignment optics subsequent to said calculating a first displacement vector;
• and
• comparing the first and second displacement vectors to detect a change in the front-to-backside alignment optics.

An alignment method comprising:

• providing an alignment beam of radiation;
• and
• providing an alignment system for projecting the alignment beam onto a substrate mark on the back side of a substrate and detecting said substrate mark,
• wherein a position of the substrate mark is calculated using a displacement vector and a plurality of correction vectors.

A data storage medium storing machine-executable instructions that describe a method comprising:

• calculating a displacement vector between (a) a position of a mark on a back side of a reference substrate as observed through a front-to-backside alignment optics of the lithographic apparatus and (b) an actual position of said mark;
• moving the reference substrate a small distance relative to said front-to-backside alignment optics and comparing a resulting displacement of an image of a point on the back side of the reference substrate to a resulting displacement of a point on the front side of the reference substrate to generate a first correction vector;
• repeating said moving and comparing for a different point on the reference substrate to generate a second correction vector;
• and
• applying calibration information based on the displacement vector and the optical correction vectors.

A lithographic apparatus comprising:

• a radiation system configured to provide a beam of radiation;
• a support structure configured to support patterning structure, the patterning structure serving to pattern the beam of radiation according to a desired pattern;
• a substrate table configured to hold a substrate;
• a projection system arranged to project the patterned beam onto a target portion of the substrate;
• an alignment system arranged to detect alignment between a reference mark and said substrate mark using an alignment beam of radiation and front-to-backside alignment optics;
• a storage device arranged to store a displacement vector and a plurality of correction vectors;
• and
• a correction device arranged to correct an alignment error of the substrate.

A lithographic projection apparatus comprising:

• a radiation system configured to provide a beam of radiation;
• a support structure configured to support patterning structure, the patterning structure serving to pattern the beam of radiation according to a desired pattern;
• a substrate table configured to hold a substrate;
• a projection system arranged to project the patterned beam onto a target portion of the substrate;
• front-to-backside alignment optics arranged to project an image of said substrate mark to the front side of said substrate;
• and
• an alignment system arranged to detect alignment between a reference mark and said substrate mark using an alignment beam of radiation and said front-to-backside alignment optics,
• wherein said front-to-backside alignment optics is configured to simultaneously project images of a plurality of marks including said substrate mark.