Patent ID: 7894075
Filing Date: 2011-02-22
Classification: G01N,G03F

Abstract:
1. A system, comprising: a moveable stage configured to support a wafer and position the wafer relative to a projection lens; a first interferometer configured to direct a first measurement beam along a path between a first measurement object attached to the projection lens and a first interferometer assembly located remote from the projection lens, the first interferometer being configured to form a first output beam from the first measurement beam, the first output beam comprising a first interferometric phase comprising information about variations in an optical path length of the path between the first measurement object and the first interferometry assembly; a first detector configured to detect the first output beam; a second interferometer configured to direct a second measurement beam along a path between a second measurement object attached to the stage and a second interferometer assembly located remote from the stage, the second interferometer being configured to form a second output beam from the second measurement beam, the second output beam comprising a second interferometric phase comprising information about variations in an optical path length of the path between the second measurement object and the second interferometry assembly; a second detector configured to detect the second output beam; an electronic processing system in communication with the first and second detectors, the electronic processing system being programmed to monitor a degree of freedom of the stage based on the second interferometric phase and to reduce uncertainty in the monitored degree of freedom due to variations in the optical properties in a gas in the path of the second measurement beam based on the first and second interferometric phases.