Method for correcting disturbances in a level sensor light path

Abstract

The invention relates to a level sensor for determining a height of a substrate. The level sensor generates one or more measurement beam and directs the measurement beam to a measurement spot on the substrate 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. The level sensor has an optical arrangement in a predetermined area close to where the substrate is to be located. The measurement beam and the reference beam propagate along substantially equal optical paths of propagation in the predetermined area. The optical arrangement deviates the reference beam from the substantially equal optical paths of propagation in the predetermined area such that the at least one reference beam does not hit the substrate.

1 . A level sensor for determining a height of a substrate, said level sensor comprising:
 an optical arrangement arranged in a predetermined area close to where the substrate is to be located;   a pair of substantially equal optical paths of propagation in the predetermined area for at least one measurement beam and at least one reference beam,   the optical arrangement being arranged to deviate the at least one reference beam from the substantially equal optical paths of propagation in the predetermined area such that the at least one reference beam does not hit said substrate.   
   
   
       2 . A level sensor according to  claim 1 , wherein the optical arrangement comprises a prism having a parallelepiped form. 
   
   
       3 . A level sensor according to  claim 2 , wherein the prism comprises:
 an input surface arranged to receive the at least one reference beam;   a reflective surface arranged to internally reflect the reference beam; and   an output surface for outputting the reflected beam, the reflective surface having two sub-surfaces intersecting along a dihedral angle.   
   
   
       4 . A level sensor according to  claim 3 , wherein the input surface, the output surface and the two sub-surfaces are arranged such that, in use, the input surface receives the input beam at a substantially normal angle and the output surface outputs the reflected beam at a substantially normal angle. 
   
   
       5 . A level sensor according to  claim 1 , wherein said optical arrangement comprises a plurality of mirrors. 
   
   
       6 . A level sensor according to  claim 4 , wherein the plurality of mirrors comprises a semi-transparent mirror arranged to split an initial beam to form the measurement beam and the reference beam. 
   
   
       7 . A level sensor according to  claim 1 , further arranged to correct for noise and drift in the measurement signal by comparing noise and drift in the measurement signal with noise and drift in the reference signal. 
   
   
       8 . A method of determining a height of a substrate, comprising:
 propagating along substantially equal paths in a predetermined area close to where the substrate is to be located, a measurement beam and a reference beam; and   deviating the reference beam with an optical arrangement located in the predetermined area, such that the reference beam does not hit the substrate.   
   
   
       9 . Imaging apparatus comprising a level sensor according to  claim 1 , and arranged to produce an image to be imaged on said substrate. 
   
   
       10 . A lithographic projection apparatus comprising an imaging apparatus according to  claim 9 . 
   
   
       11 . A lithographic projection apparatus comprising an exposure stage and a measurement stage and an imaging apparatus according to  claim 9 .