Autofocus methods and devices for lithography

Abstract

Improved autofocusing (“AF”) methods and devices for lithography are provided. Some embodiments of the invention provide an AF system that includes one or more interferometers for measuring a distance to a wafer surface or a reticle surface. The invention includes methods and devices for calibrating the interferometer(s) according to known distances to a target. According to some embodiments of the invention, a spatial filter reduces the amount of undesired signal coming from the wafer or reticle surface. In some such embodiments, higher orders of diffracted light from the wafer or reticle multilayer surfaces are eliminated with a pinhole filter oriented to reject light that is not vertically directed. Other embodiments include a spatial filtering system that passes a selected diffraction order, e.g., the first order of diffracted light, from the target.

1 . An autofocusing device for a lithography system, the autofocusing device comprising: 
 a light source;    an interferometer system configured to determine a distance to a target based on first light received from the target; and    a spatial filter system for passing selected components of the first light to an interferometer of the interferometer system.    
   
   
       2 . The autofocusing device of  claim 1 , wherein the interferometer system comprises a plurality of interferometers.  
   
   
       3 . The autofocusing device of  claim 1 , further comprising a polarizing filter for passing a selected polarization angle of the first light.  
   
   
       4 . The autofocusing device of  claim 1 , wherein the spatial filter system comprises a plurality of spatial filters.  
   
   
       5 . The autofocusing device of  claim 1 , wherein the interferometer system is further configured to determine the distance to the target based on a comparison of the first light with second light reflected from a reference mirror.  
   
   
       6 . The autofocusing device of  claim 1 , wherein the spatial filter system comprises a first spatial filter for passing a specularly reflected component of the first light.  
   
   
       7 . The autofocusing device of  claim 1 , further comprising a calibration device for determining an absolute distance to a target.  
   
   
       8 . The autofocusing device of  claim 1 , further comprising a calibration device for calibrating the autofocusing device for expected optical properties of the target.  
   
   
       9 . The autofocusing device of  claim 1 , further comprising means for causing at least a portion of the first light to illuminate the target more than once.  
   
   
       10 . The autofocusing device of  claim 6 , wherein the spatial filter system further comprises a second spatial filter for passing a selected diffraction order of the first light.  
   
   
       11 . The autofocusing device of  claim 9 , wherein the causing means comprises a beam splitter and a quarter wave plate.  
   
   
       12 . The autofocusing device of  claim 10 , wherein the spatial filter system further comprises a third spatial filter for passing both the specularly reflected component of the first light and the selected diffraction order of the first light.  
   
   
       13 . A lithography system comprising: 
 an illumination source;    an optical system;    a reticle stage arranged to retain a reticle;    a working stage arranged to retain a workpiece;    an enclosure that surrounds at least a portion of the working stage, the enclosure having a sealing surface; and    autofocusing device, comprising: 
 a light source;  
 an interferometer system configured to determine a distance to a target based on first light received from the target; and  
 a spatial filter system for passing selected components of the first light to an interferometer of the interferometer system.  
   
   
   
       14 . An object manufactured with the lithography system of  claim 13 .  
   
   
       15 . A wafer on which an image has been formed by the lithography system of  claim 13 .  
   
   
       16 . A method for making an object using a lithography process, wherein the lithography process utilizes a lithography system as recited in  claim 13 .  
   
   
       17 . A method for patterning a wafer using a lithography process, wherein the lithography process utilizes a lithography system as recited in  claim 13 .  
   
   
       18 . The lithography system of  claim 13 , wherein the optical system is a telecentric optical system.  
   
   
       19 . The lithography system of  claim 13 , wherein the optical system is a non-telecentric optical system.  
   
   
       20 . The lithography system of  claim 18 , further comprising means for disposing a fluid between the optical system and the workpiece.