Abstract:
A system for measuring a displacement along a first axis includes an apparatus movable at least along a second axis perpendicular to the first axis, a measurement mirror mounted to the apparatus at an angle greater than 0° relative to the first axis, and an interferometer with a beam-splitter. The beam-splitter splits an input beam into a measurement beam and a reference beam, directs the measurement beam in at least two passes to the measurement mirror, and combining the measurement beam after said at least two passes and the reference beam into an output beam. At least exterior to the interferometer, the measurement beam travels in paths that are not parallel to the first axis.

Description:
DESCRIPTION OF RELATED ART  
       [0001]      FIG. 1  illustrates a system for acquiring the position of a stage  10  along a Z axis  20  (e.g., the exposure optical axis or the focus axis). This approach is described in detail in U.S. Pat. No. 6,208,407 to Loopstra. A wafer  12  is supported on stage  10  for exposure by projection optics or exposure tool  14 . One advantage of this system is that although the interferometer  16  is positioned at the side of the stage  10 , accurate Z axis measurements may be obtained. This is enabled by properly positioning mirrors which establish a Z measuring axis  18  that is parallel to the Z axis  20  of the exposure system. A mirror  22  is arranged at a forty-five degree angle to the movement of stage  10  along the X or Y axis. A measuring beam  24  from interferometer  16  impinges mirror  22  to establish the Z measuring axis  18 . A horizontal mirror  26  is attached to structure  28  of the exposure system, so that beam  24  is redirected to mirror  22 , which reflects the returned beam  24  to interferometer  16 . In addition to measuring beam  24 , the interferometer projects a reference beam  30  for reflection from a vertical surface  31  of the stage  10 .  
         [0002]     As can be seen in  FIG. 1 , movement of stage  10  along the Z axis  20  will result in a change in the length of the beam path segment from the forty-five degree mirror  22  to the horizontal mirror  26 . Thus, while interferometer  16  is located at the side of the stage, the measuring beam  24  has a path segment that varies in length in unity with Z axis displacements of stage  10 . In fact, the reflection from horizontal mirror  26  to the forty-five degree mirror  22  provides a second beam path segment that varies in unity with Z axis movement of stage  10 . On the other hand, the length of each beam path segment for reference beam  30  is fixed, unless the stage  10  is moved in the X direction.  
         [0003]     While the approach described with reference to  FIG. 1  operates well for its intended purposes, there are cost concerns, since horizontal mirror  26  is a relatively large reflective component that requires a high degree of planarity. Moreover, as the line widths of the features of integrated circuits decrease, the size of the projection lens of the projection optics  14  increases. In  FIG. 1 , this would result in an increase of the diameter of the projection optics  14 . As a consequence, the requirement of a horizontal mirror  26  to accommodate the entire range of motion of stage  10  imposes a potential difficulty with respect to achieving further reductions of line widths.  
       SUMMARY  
       [0004]     In one embodiment of the invention, a system for measuring a displacement along a first axis includes an apparatus movable at least along a second axis perpendicular to the first axis, a measurement mirror mounted to the apparatus at an angle (θ) greater than 0° relative to the first axis, and an interferometer with a beam-splitter. The beam-splitter splits an input beam into a measurement beam and a reference beam, directs the measurement beam in at least two passes to the measurement mirror, and combining the measurement beam after the two passes and the reference beam into, an output beam. At least exterior to the interferometer, the measurement beam travels in paths that are not parallel to the first axis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  illustrates a prior art system for acquiring the displacement of a wafer stage along the Z axis.  
         [0006]      FIGS. 2, 3A ,  3 B,  4 A, and  4 B illustrate a system for acquiring the displacement of a wafer stage along the Z axis in one embodiment of the invention.  
         [0007]      FIGS. 5, 6A ,  6 B,  7 A, and  7 B illustrate a system for acquiring the displacement of a wafer stage along the Z axis in another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0008]      FIG. 2  illustrates an interferometer system  100  using an interferometer  101  to measure the displacement of a wafer stage  102  along a Z axis (e.g., the lithographic focus axis) in one embodiment of the invention. A laser head  104  generates a coherent, collimated input beam  105  consisting of two orthogonally polarized frequency components. One frequency component f A  (e.g., a measurement beam having a P-polarization) enters the interferometer&#39;s measurement path while the other frequency component f B  (e.g., a reference beam having an S-polarization) enters the interferometer&#39;s reference path.  
         [0009]     In the measurement path, a polarizing beam-splitter  106  transmits frequency component f A  through a quarter-wave plate  108  onto a measurement plane mirror  110 . Measurement plane mirror  110  is mounted to the side of wafer stage  102  at an angle θ from the Z axis. Measurement plane mirror  110  reflects frequency component f A  orthogonally onto a beam steering mirror  112 . In one embodiment, angle θ is less than 45 degrees so measurement plane mirror  110  reflects frequency component f A  away from wafer stage  102  and avoids the disadvantages of the prior art approach.  
         [0010]     Beam steering mirror  112  is angled at  20  to return frequency component f A  along the input path, at nominal stage orientation (without stage rotation along the Z axis), back to polarizing beam-splitter  106 . Since frequency component f A  again passes through quarter-wave plate  108 , the returning polarization is rotated 90 degrees and the newly S-polarized frequency component f A  is reflected by polarizing beam-splitter  106  into a cube corner retroreflector  114 .  
         [0011]     Retroreflector  114  returns frequency component f A  in a parallel but offset path back to polarizing beam-splitter  106 , which again reflects frequency component f A  through quarter-wave plate  108  onto measurement plane mirror  110 . As similarly described above, measurement plane mirror  110  reflects frequency component f A  orthogonally onto beam steering mirror  112 , which returns frequency component f A  along the input path back to polarizing beam-splitter  106 . Since frequency component f A  again passes through quarter-wave plate  108 , the returning polarization is rotated 90 degrees and the newly P-polarized frequency component f A  is transmitted through polarizing beam-splitter  106  onto a receiver  116 .  
         [0012]     In the reference path, polarizing beam-splitter  106  reflects frequency component f B  through a quarter-wave plate  118  and orthogonally onto to a reference plane mirror  120 . Reference plane mirror  120  returns frequency component f B  along the input path back to polarizing beam-splitter  106 . Since frequency component f B  passes again through quarter-wave plate  118 , the returning polarization is rotated 90 degrees and the newly P-polarized frequency component f B  is transmitted through polarizing beam-splitter  106  into retroreflector  114 . Retroreflector  114  returns frequency component f B  in a parallel but offset path back to polarizing beam-splitter  106 .  
         [0013]     Polarizing beam-splitter  106  again transmits frequency component f B  through quarter-wave plate  118  and orthogonally onto reference plane mirror  120 . Reference plane mirror  120  returns frequency component f B  along the input path back to polarizing beam-splitter  106 . Since frequency component f B  again passes through quarter-wave plate  118 , the returning polarization is rotated 90 degrees and the newly S-polarized frequency component f B  is reflected by polarizing beam-splitter  106  coaxially with frequency component f A  as an output beam  119  onto receiver  116 .  
         [0014]     Receiver  116  includes a mixing polarizer, a photodetector (e.g., a photodiode), an amplifier, and phase detection electronics for detecting the phase shift of output beam  119  as stage  102  translates. The phase shift is then correlated to the stage translation. Note that the measurement beam travels only in a plane defined by the Z and the X axes so that any displacement along the Y axis will not affect the measurement of Z displacements. The non-planarity of measurement mirror  110  in the Y direction can affect the measurement of Z displacement, but calibration schemes can reduce this source of error.  
         [0015]      FIGS. 3A and 3B  illustrate that if stage  102  translates along the Z axis for a distance of D Z , the distance traveled by the measurement beam is changed by a distance D M . Distance D M  is conventionally determined from the phase shift of output beam  119 . Distance D M  is then correlated to the Z displacement D Z  by trigonometry as follows:  
                   D   1     =       D   Z     ⁢     tan   ⁡     (   θ   )           ;     ⁢     
     ⁢         D   2     =       D   Z     ⁢     tan   ⁡     (   θ   )       ⁢     cos   ⁡     (     2   ⁢   θ     )           ;     ⁢     
     ⁢         D   M     =     4   ⁢     (       D   1     +     D   2       )         ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢         D   M     =     4   ⁢     D   Z     ⁢       tan   ⁡     (   θ   )       ⁡     [     1   +     cos   ⁡     (     2   ⁢   θ     )         ]           ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢         D   M     =     4   ⁢     D   Z     ⁢     sin   ⁡     (     2   ⁢   θ     )           ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢       D   Z     =         D   M       4   ⁢     sin   ⁡     (     2   ⁢   θ     )           .               (   1   )               
         [0016]      FIGS. 4A and 4B  illustrate that if stage  102  translates along both the Z and the X axes, then the measured distance D M  includes components from the Z displacement D Z  and the X displacement D X . Thus, to determine the Z displacement D Z , the X displacement D X  must first be determined. The X displacement D X  can be conventionally determined from another interferometer arranged to measure displacements along the X axis. Once the X displacement D X  is determined, the Z displacement D Z  is determined by trigonometry as follows: 
        D 1 ′=D X ;     D 2 ′=D X  cos(2θ);     D M =4[D 1 +D 1 ′+D 2 +D 2 ′], which can be rewritten as:     D M =4[D Z  tan(θ)+D X +D Z  tan(θ) cos(2θ)+D X  cos(2θ)], which can be rewritten as:  
                   D   M     =       4   ⁢     D   Z     ⁢       tan   ⁡     (   θ   )       ⁡     [     1   +     cos   ⁡     (     2   ⁢   θ     )         ]         +     4   ⁢       D   X     ⁡     [     1   +     cos   ⁡     (     2   ⁢   θ     )         ]             ,     
     ⁢     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢         D   M     =       4   ⁢     D   Z     ⁢     sin   ⁡     (     2   ⁢   θ     )         +     4   ⁢       D   X     ⁡     [     1   +     cos   ⁡     (     2   ⁢   θ     )         ]             ,     
     ⁢     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢         D   Z     =         D   M       4   ⁢     sin   ⁡     (     2   ⁢   θ     )           -       4   ⁢       D   X     ⁡     [     1   +     cos   ⁡     (     2   ⁢   θ     )         ]           4   ⁢     sin   ⁡     (     2   ⁢   θ     )               ,     
     ⁢     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢       D   Z     =         D   M       4   ⁢     sin   ⁡     (     2   ⁢   θ     )           -         D   X       tan   ⁡     (   θ   )         .                 (   2   )             
 
 The same formula can be modified if the measurement beam travels in a plane defined by the Z and the Y axes so that any displacement along the X axis will not affect the measurement of Z displacements.  
               D   Z     =         D   M       4   ⁢     sin   ⁡     (     2   ⁢   θ     )           -         D   Y       tan   ⁡     (   θ   )         .               (   3   )             
 
 where D Y  is any displacement along the Y axis. The choice of coordinate system is arbitrary but does affect the signs that appear in the previous equations. 
         
         [0021]      FIG. 5  illustrates an interferometer system  200  using an interferometer  201  to measure the displacement of a wafer stage  102  along a Z axis (e.g., a lithographic focus axis) in one embodiment of the invention. Laser head  104  generates a coherent, collimated input beam  205  consisting of two orthogonally polarized frequency components. One frequency component f A  (e.g., a measurement beam having a P-polarization) enters the interferometer&#39;s measurement path while the other frequency component f B  (e.g., a reference beam having an S-polarization) enters the interferometer&#39;s reference path.  
         [0022]     In the measurement path, a polarizing beam-splitter  206  transmits frequency component f A  through a quarter-wave plate  208  onto a measurement plane mirror  210 . Laser head  104  and interferometer  201  are arranged so that frequency component f A  impinges orthogonally onto measurement plane mirror  210 . Thus, measurement plane mirror  210  returns frequency component f A  along the input path, at nominal stage orientation, back to polarizing beam-splitter  206 . Since frequency component f A  again passes through quarter-wave plate  208 , the returning polarization is rotated 90 degrees and the newly S-polarized frequency component f A  is reflected by polarizing beam-splitter  206  into a cube corner retroreflector  214 .  
         [0023]     Retroreflector  214  returns frequency component f A  in a parallel but offset path back to polarizing beam-splitter  206 , which again reflects frequency component f A  through quarter-wave plate  208  onto measurement plane mirror  210 . Again, frequency component f A  impinges orthogonally onto measurement plane mirror  210 . Thus, measurement plane mirror  210  returns frequency component f A  along the input path back to polarizing beam-splitter  206 . Since frequency component f A  again passes through quarter-wave plate  208 , the returning polarization is rotated 90 degrees and the newly P-polarized frequency component f A  is transmitted through polarizing beam-splitter  206  onto receiver  116 .  
         [0024]     In the reference path, polarizing beam-splitter  206  reflects frequency component f B  through a quarter-wave plate  218  and orthogonally onto to a reference plane mirror  220 . Reference plane mirror  220  returns frequency component f B  along the input path back to polarizing beam-splitter  206 . Since frequency component f B  passes again through quarter-wave plate  218 , the returning polarization is rotated 90 degrees and the newly P-polarized frequency component f B  is transmitted through polarizing beam-splitter  206  into retroreflector  214 . Retroreflector  214  returns frequency component f B  in a parallel but offset path back to polarizing beam-splitter  206 .  
         [0025]     Polarizing beam-splitter  206  again transmits frequency component f B  through quarter-wave plate  218  and orthogonally onto reference plane mirror  220 . Reference plane mirror  220  returns frequency component f B  along the input path back to polarizing beam-splitter  206 . Since frequency component f B  again passes through quarter-wave plate  218 , the returning polarization is rotated 90 degrees and the newly S-polarized frequency component f B  is reflected by polarizing beam-splitter  206  coaxially with frequency component f A  as output beam  219  onto receiver  116 .  
         [0026]     Similarly described above, receiver  116  includes a photodetector (e.g., a photodiode), an amplifier, and phase detection electronics for detecting the phase shift of the recombined frequency components f A  and f B  as stage  102  translates. The phase shift is then correlated to the stage translation. Note that the measurement beam travels only in a plane defined by the Z and the X axis so that any displacement along the Y axis will not affect the measurement of Z displacements. The non-planarity of the measurement mirror  210  in the Y direction can affect the measurement of Z displacement, but calibration schemes can reduce this source of error.  
         [0027]      FIGS. 6A and 6B  illustrate that if stage  102  translates along the Z axis for a distance of D Z , the distance traveled by the measurement beam is changed a distance D M . Distance D M  is determined from the phase shift of output beam  219 . Distance D M  is then correlated to the Z displacement D Z  by trigonometry as follows:  
                     D   3     =       -     D   Z       ⁢     sin   ⁡     (   θ   )           ;     ⁢     
     ⁢         D   M     =     4   ⁢     D   3         ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢       D   M     =       -   4     ⁢     D   Z     ⁢     sin   ⁡     (   θ   )           ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   written   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢       D   Z     =     -         D   M       4   ⁢     sin   ⁡     (   θ   )           .                 (   4   )               
         [0028]      FIGS. 7A and 7B  illustrate that if stage  102  translates along both the Z and the X axes, then the measured distance D M  includes components from the Z displacement D Z  and the X displacement D X . Thus, to determine the Z displacement D Z , the X displacement D X  must first be determined. The X displacement D X  can be conventionally determined from another interferometer arranged to measure displacement along the X axis. Once the X displacement D X  is determined, the Z displacement D Z  is determined from by trigonometry as follows.  
                   D   4     =       -     D   X       ⁢     cos   ⁡     (   θ   )           ;     ⁢     
     ⁢         D   M     =     4   ⁢     (       D   3     +     D   4       )         ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢         D   M     =         -   4     ⁢     D   Z     ⁢     sin   ⁡     (   θ   )         -     4   ⁢     D   X     ⁢     cos   ⁡     (   θ   )             ,     which   ⁢           ⁢   can   ⁢           ⁢   be   ⁢           ⁢   rewritten   ⁢           ⁢   as   ⁢     :         ⁢     
     ⁢       D   Z     =       -       D   M       4   ⁢     sin   ⁡     (   θ   )             -         D   X       tan   ⁡     (   θ   )         .                 (   5   )               
         [0029]     Similarly, equation (5) can be used to determine the Z displacement D Z  if the measurement beam travels in a plane defined by the Z and the Y axes so that any displacement along the X axis will not affect the measurement of Z displacements.  
               D   Z     =       -       D   M       4   ⁢     sin   ⁡     (   θ   )             -         D   Y       tan   ⁡     (   θ   )         .               (   6   )             
 
 The choice of coordinate system is arbitrary but does affect the signs that appear in the previous equations. 
 
         [0030]     One advantage of the invention results directly from the placement of the measurement plane mirror (e.g., mirror  110  or  210 ) on the side of the movable apparatus (e.g., wafer stage  102 ), which allows for a smaller and, more importantly, less costly plane mirror than the prior art. This is because the measurement plane mirror in Loopstra has to accommodate the full range of motion along the X, Y, and Z axes, whereas the measurement plane mirror in the current invention only has to accommodate the full range of motion along two of the axes (e.g., the X and Z axes).  
         [0031]     In lithography and other possible optical applications, air showers are provided for purposes such as cooling and reducing contamination. Uniformity of the air shower can be important, since disruption in the air shower can cause fluctuation in the index refraction of air, which in turns cause fluctuations in the phase shift measured by a laser interferometer. By eliminating a horizontal measurement plane mirror as described in Loopstra, the current invention improves the uniformity of the air shower and reduces errors in the interferometry measurements.  
         [0032]     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. While the illustrated embodiments utilize plane mirrors and cube corner retroreflectors, other reflective, refractive, diffractive, and holographic components may be substituted. Furthermore, although nominal stage alignment is described above, the stage may rotate about the Z axis within an angle range that provides enough of a signal to the detector. Numerous embodiments are encompassed by the following claims.