Method for correcting disturbances in a level sensor light path

Abstract

A level sensor for determining a height of a substrate. In one configuration, the level sensor forms part of a lithographic apparatus that includes a projection lens system. The level sensor generates one or more measurement beams and directs the measurement beam to a measurement spot on a substrate having a first reflecting surface, and produces a reflected measurement beam. The level sensor also generates one or more reference beams. A detector detects both the reflected measurement beam and the reference beam, respectively, and produces a measurement signal and a reference signal, respectively, the measurement signal being indicative for the height at the measurement spot. A processor that receives these signals and corrects the measurement signal based on the reference signal. In one configuration, the level sensor has an optical element arranged to direct the reference beam towards a detection branch via an optical reference path arranged to be insensitive to the position of the projection lens system and the first reflecting surface. In one configuration, the level sensor is configured to direct the measurement beam and reference beam along optical paths that are at least partially substantially equal in at least one of the reference branch and the projection branch.