Abstract:
An interferometer for receiving a measurement beam from a target location on a stage of a semiconductor lithography machine and a reference beam from a reference location separated from the target location by a separation distance. The interferometer has a reference path to be traversed by the reference beam within the interferometer and a measurement path to be traversed by the measurement beam within the interferometer. Both the measurement path and the reference path are at least as long as the separation distance between the reference location and the target location.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the Apr. 24, 2002 priority date of U.S. provisional application No. 60/375,209, the contents of which are herein incorporated by reference. 
     
    
     
       FIELD OF INVENTION  
         [0002]    This invention relates to semiconductor lithography, and in particular, for interferometric measurement of position in a semiconductor lithography system.  
         BACKGROUND  
         [0003]    A semiconductor lithography machine includes a moveable stage whose position must be known with great certainty. Such measurements are conventionally provided by a system of interferometers, each of which illuminates the stage with a measurement beam and a stationary target with a reference beam. These interferometers combine the returning measurement beam and reference beam and observe the resulting interference between them. This interference is indicative of a difference in the path lengths traversed by the two beams, and hence the movement of the stage.  
           [0004]    In many semiconductor lithography machines, the stationary target is located at some distance from the stage. As a result, the measurement beam and the reference beam are separated by a considerable distance. Conventional interferometers accommodate this distance by providing a monolithic structure that is long enough so that the reference beam and the measurement beam can exit the interferometer parallel to each other. Known monolithic structures for such interferometers are prone to introducing errors resulting from thermal expansion and beam shear.  
         SUMMARY  
         [0005]    In one aspect, the invention includes an interferometer having first and second polarizing beam-splitters. The first polarizing beam-splitter directs an input beam in a direction that depends on a polarization state of the input beam. The second polarizing beam-splitter receives an output beam from the first polarizing beam-splitter and directs this received output beam in a direction that depends on its polarization state.  
           [0006]    In one embodiment, a polarization rotator, which is on an optical path between the first and second polarizing beam-splitters, rotates the polarization of the output beam received from the first polarizing beam-splitter. An exemplary polarization rotator can be a half-wave plate.  
           [0007]    In another embodiment, the interferometer also includes first and second retroreflectors. The first retroreflector is in optical communication with the first polarizing beam-splitter. The second retroreflector is in optical communication with the second polarizing beam-splitter.  
           [0008]    In another embodiment, the interferometer also includes first and second reflective polarization-rotators in optical communication with the first and second retroreflectors respectively. Exemplary reflective polarization-rotators include a mirror coated with, or otherwise in optical communication with, a quarter-wave plate.  
           [0009]    In another aspect, the invention includes an interferometer having a first polarizing beam-splitter in optical communication with a first retroreflector. A first reflective polarization-rotator lies on an optical path between the first polarizing beam-splitter and the first retroreflector. A second polarizing beam-splitter is in optical communication with a second retroreflector. A second reflective polarization-rotator lies on an optical path between the second polarizing beam-splitter and the second retroreflector. A third polarization rotator lies on an optical path between the first and second polarizing beam-splitters.  
           [0010]    In one embodiment, the first polarizing beam-splitter includes first and second beam-splitting planes in optical communication with each other. The first beam splitting plane reflects light having a first polarization toward the third polarization rotator. The second beam-splitting plane transmits light having a second polarization received from the first beam-splitting plane to the first retroreflector and also reflects light received from the first retroreflector and having the first polarization toward the third polarization rotator.  
           [0011]    In another embodiment, the second polarizing beam-splitter includes a mirror plane and a third beam splitting plane. The mirror plane redirects light received from the first polarizing beam-splitter. The third beam-splitting plane, which is in optical communication with the mirror plane, the second retroreflector, and the first polarizing beam-splitter, transmits light received from the mirror plane toward the second retroreflector, transmits light received from the second retroreflector and having the first polarization toward a detector, and transmits light received from the first polarizing beam-splitter and having the second polarization toward the detector.  
           [0012]    Another embodiment of the interferometer includes a steering wedge on the optical path between the first polarizing beam-splitter and the second polarizing beam-splitter.  
           [0013]    Another aspect of the invention includes an interferometer for receiving a measurement beam from a target location and a reference beam from a reference location separated from the target location by a separation distance. The interferometer has a reference path to be traversed by the reference beam within the interferometer and a measurement path to be traversed by the measurement beam within the interferometer. Both the measurement path and the reference path are at least as long as the separation distance between the reference location and the target location.  
           [0014]    One embodiment of this interferometer includes first and second polarizing beam-splitters. Each of these polarizing beam-splitters is disposed to intersect both the reference path and the measurement path.  
           [0015]    Another embodiment of the interferometer includes a polarization rotator disposed to intersect at least one of the reference path and the measurement path.  
           [0016]    In yet another embodiment, the interferometer includes a polarization rotator disposed to intersect the reference path and the measurement path between the first and second polarizing beam-splitters. One example of such a polarization rotator is a half-wave plate.  
           [0017]    In another aspect, the invention includes a semiconductor lithography system having at least one of the foregoing interferometers. The semiconductor lithography system includes a semiconductor lithography machine having a base and a stage moveable relative to the base. The base is in optical communication with the second polarizing beam-splitter of the interferometer. The stage is in optical communication with the first polarizing beam-splitter of the interferometer.  
           [0018]    The invention also includes a method for determining the location of a moveable stage of a semiconductor lithography machine relative to a base separated from the stage by a separation distance. This method includes directing a measurement beam along a measurement path that intersects the stage and directing a reference beam along a reference path that intersects the base. The reference beam and measurement beam are both made to traverse a path length within an interferometer that is at least as long as the separation distance between stage and the base.  
           [0019]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art- to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.  
           [0020]    These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0021]    [0021]FIG. 1 shows a semiconductor lithography system incorporating an interferometer according to the invention;  
         [0022]    [0022]FIG. 2 shows one embodiment of the interferometer in FIG. 1;  
         [0023]    [0023]FIG. 3 shows the path of a reference beam through the system of FIG. 1.  
         [0024]    [0024]FIG. 4 shows the path of a measurement beam through the system of FIG. 1.  
         [0025]    [0025]FIGS. 5 and 6 show additional embodiments of the interferometer of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0026]    Referring to FIG. 1, a semiconductor lithography machine  10  includes a moveable stage  12  for holding a work piece  14 . In such a machine  10 , it is desirable to know the position of the moveable stage  12  at any time. To measure this position, an interferometer  16  having a reference beam  24  and a measurement beam  20  is mounted so that its measurement beam  20  strikes a stage retroreflector  22  mounted on the moveable stage  12  and its reference beam  24  strikes a reference retroreflector  26  mounted on a base  18 . The stage and reference retroreflectors  22 ,  26  reflect the measurement and reference beams  20 ,  24  back toward the interferometer  16 . The interferometer  16  measures the interference between the returning measurement beam  20  and the returning reference beam  24 . The extent of this interference provides a measure of the difference in path length traversed by the two beams  20 ,  24 , and hence the position of the stage retroreflector  22  relative to the reference retroreflector  26 .  
         [0027]    The stage retroreflector  22  and the reference retroreflector  26  are made as close as possible to each other to reduce measurement error within the interferometer  16 . However, because of mechanical constraints, the stage retroreflector  22  and the reference retroreflector  26  are often as much as 80 millimeters apart. An interferometer  16  according to the invention is intended to provide accurate measurements that do not depend in any significant way on the distance between the stage and reference retroreflectors  22 ,  26 .  
         [0028]    Referring now to FIG. 2, the interferometer  16  has a measurement polarizing beam-splitter  28  and a reference polarizing beam-splitter  30 . The measurement polarizing beam-splitter  28  has a planar top face  32  extending between first and second top edges  34 ,  36 , a planar bottom face  38  extending between first and second bottom edges  40 ,  42 , a planar input face  44  extending between the first top edge  34  and the first bottom edge  40 , and a planar output face  46  extending between the second top edge  36  and the second bottom edge  42 .  
         [0029]    The reference polarizing beam-splitter  30  has a planar top face  48  extending between first and second top edges  50 ,  52 , a planar bottom face  54  extending between first and second bottom edges  56 ,  58 , and a planar output face  60  extending between the second top edge  52  and the second bottom edge  58 . The bottom face  54  of the measurement polarizing beam-splitter  28  and the top face  48  of the reference polarizing beam-splitter  30  face each other across a gap  62 .  
         [0030]    The extent of the gap  62  separating the input and reference polarizing beam-splitter  30  depends on the distance between the stage and reference retroreflectors  22 ,  26 . In one embodiment, the gap is an air gap or an evacuated gap that is not subject to temperature-induced expansion or local variations in index of refraction. However, even if the gap were filled with a solid optically transmissive medium having a non-zero coefficient of thermal expansion, any errors introduced by thermal expansion would be common to both a reference beam  24  and a measurement beam  20  passing through that medium. Hence, temperature expansion and contraction will introduce no appreciable relative error between the reference beam  24  and the measurement beam  20  regardless of the extent of the gap  62 .  
         [0031]    The measurement polarizing beam-splitter  28  has a first beam-splitting plane  64  that intersects its first top edge  34  and a second beam-splitting plane  66  that intersects its second top edge  36 . The first and second beam-splitting planes  64 ,  66  intersect at a common line  68  extending along the bottom face  38  of the measurement polarizing beam-splitter  28 .  
         [0032]    The reference polarizing beam-splitter  30  has a mirror plane  70  that extends from its first top edge  50  and a beam-splitting plane  72  that extends from its second top edge  52 . The mirror plane  70  and the beam-splitting plane  72  of the reference polarizing beam-splitter  30  intersect at a common line  74  extending along the bottom face of the reference polarizing beam-splitter  30 .  
         [0033]    Between the measurement polarizing beam-splitter  28  and the reference polarizing beam-splitter  30  is a half-wave plate  76  disposed to intercept a beam traveling from the bottom face  38  of the measurement polarizing beam-splitter  28  to the top face of the reference polarizing beam-splitter  30 . Preferably, the half-wave plate  76  is on the top face  48  of the reference polarizing beam-splitter  30 . As a result of this half-wave plate  76 , any beam incident on the reference polarizing beam-splitter  30  will have its polarization rotated by ninety degrees before it enters the reference polarizing beam-splitter  30 . Optional steering wedges  78  between the input and reference polarizing beam-splitter  30  ensure that beams traveling from the measurement polarizing beam-splitter  28  to the reference polarizing beam-splitter  30  are parallel to each other.  
         [0034]    Between the output face  46  of the measurement polarizing beam-splitter  28  and the stage retroreflector  22  is a measurement mirror  80  disposed to intercept a beam reflected from the stage retroreflector  22  and to allow optical communication between the output face  46  of the measurement polarizing beam-splitter  28  and the stage retroreflector  22 . Similarly, a reference mirror  82  is disposed to intercept a beam reflected from the reference retroreflector  26  and to allow optical communication between the output face  60  of the reference polarizing beam-splitter  30  and the stage retroreflector  22 .  
         [0035]    For clarity, the measurement mirror  80  and the reference mirror  82  are shown in FIG. 2 as being some distance from the measurement and reference polarizing beam-splitters  28 ,  30 . However, the measurement mirror  80  and the reference mirror  82  can be anywhere on their respective optical paths. For example, in the embodiment shown in FIG. 5, both the reference mirror  82  and the measurement mirror  80  are secured to the reference and measurement polarizing beam-splitters  28 ,  30 .  
         [0036]    In another embodiment, shown in FIG. 6, a single bar mirror  85  has a measurement portion  80  and a reference portion  82 . To avoid covering the output faces  46 ,  60  of the polarizing beam-splitters  28 ,  30 , the bar mirror  85  is offset in a direction perpendicular to the plane of the drawing (i.e. in the y direction). The bar mirror  85  is attached to a single quarter-wave plate  87  having a reference portion  86  and a measurement portion  84  covering the respective output faces  60 ,  46  of the polarizing beam-splitters  30 ,  28 .  
         [0037]    In the configuration shown in FIG. 6, light exiting an output face  60 ,  46  experiences a 45 degree rotation in its polarization vector as it proceeds through the quarter-wave plate  87  toward a corresponding mirror portion  82 ,  80 . Light returning from the mirror portion  82 ,  80  toward a corresponding output face  46 ,  60  experiences an additional 45 degree rotation in its polarization vector as it makes a second pass through the quarter-wave plate  87 .  
         [0038]    Each beam-splitting plane  64 ,  66 ,  72  has the property of transmitting a beam having a first polarization and reflecting a beam having a second polarization. It is common to refer to these polarizations as “P” and “S” polarizations respectively. However, throughout this document, the first polarization will be referred to as the “T” (for “Transmitted”) polarization and the second polarization will be referred to as the “R” (for “Reflected”) polarization.  
         [0039]    The input face  44  of the measurement polarizing beam-splitter  28  is oriented to receive an input beam from a laser  88 . The input beam is a combination of the reference beam  24  and the measurement beam  20 . The reference beam  24  and the measurement beam  20  are coherent beams having different frequencies. In addition, the reference beam  24  and the measurement beam  20  have orthogonal linear polarizations.  
         [0040]    Referring now to FIG. 3, an R-polarized reference beam  24  enters the input face  44  of the measurement polarizing beam-splitter  28 . Being R-polarized, it cannot pass through the first beam-splitting plane  64 . The first beam-splitting plane  64  reflects the reference beam  24  toward the top face of the reference polarizing beam-splitter  30 .  
         [0041]    Before entering the reference polarizing beam-splitter  30 , the reference beam  24  passes through the half-wave plate  76 . As a result, the reference beam  24  entering the reference polarizing beam-splitter  30  is T-polarized.  
         [0042]    Within the reference polarizing beam-splitter  30 , the reference beam  24  strikes the mirror plane  70 , which reflects it toward the output face  60  of the reference polarizing beam-splitter  30 . On its way to the output face  60 , the reference beam  24  encounters the beam-splitting plane  72 . Because the reference beam  24  is now T-polarized, it passes through the beam-splitting plane  72  and proceeds toward the reference retroreflector  26 .  
         [0043]    The reference retroreflector  26  directs the reference beam  24  to the reference mirror  82 . The reference mirror  82  reflects the reference beam  24  back to the retroreflector. However, because the reference mirror  82  is coated with a quarter-wave plate  86 , the reference beam  24  is now R-polarized once again.  
         [0044]    The reference retroreflector  26  then directs the reference beam  24 , which is now R-polarized, back to the output face  60  of the reference polarizing beam-splitter  30 . Soon after re-entering the reference polarizing beam-splitter  30 , the reference beam  24  encounters the beam-splitting plane  72  for the second time. This time, because the reference beam  24  is R-polarized, the beam-splitting plane  72  reflects it toward the bottom face  54  of the reference polarizing beam-splitter  30 . The reference beam  24  exits the bottom face  54  and reaches a fiber optic pickup  90  by way of an optional fold mirror  92 .  
         [0045]    Meanwhile, as shown in FIG. 4, the T-polarized measurement beam  20  enters the input face  44  of the measurement polarizing beam-splitter  28  and encounters the first beam-splitting plane  64 . Because the measurement beam  20  is T-polarized, it passes through both the first and second beam-splitting planes  64 ,  66 , exits the output face  46  of the measurement polarizing beam-splitter  28 , and proceeds to the stage retroreflector  22 .  
         [0046]    The stage retroreflector  22  directs the measurement beam  20  to the measurement mirror  80 . The measurement mirror  80  reflects the measurement beam  20  back to the stage retroreflector  22 . However, because the measurement mirror  80  is coated with a quarter-wave plate  84 , the measurement beam  20  becomes R-polarized.  
         [0047]    The stage retroreflector  22  then directs the measurement beam  20 , which is now R-polarized, back to the output face  46  of the measurement polarizing beam-splitter  28 . Soon after re-entering the measurement polarizing beam-splitter  28 , the measurement beam  20  encounters the second beam-splitting plane  66  for the second time. This time, because the measurement beam  20  is R-polarized, the second beam-splitting plane  66  reflects it toward the bottom face  38  of the measurement polarizing beam-splitter  28 . The measurement beam  20  exits the bottom face  38  of the measurement polarizing beam-splitter  28  and proceeds toward the top face  48  of the reference polarizing beam-splitter  30 .  
         [0048]    An advantage of the foregoing optical configuration lies in its freedom from shear error. It is apparent that if the stage retroreflector  22  were to translate in any direction, the path traveled by the measurement beam  20  as it returns from the stage retroreflector  22  would be unchanged. A displacement in the stage retroreflector  22  would cause the measurement beam  20  to be incident on a different portion of the retroreflector  22 . However, the measurement beam  20  would continue to travel the same path relative to the polarizing beam-splitters  28 ,  30 .  
         [0049]    Before entering the reference polarizing beam-splitter  30 , the measurement beam  20  passes through the half-wave plate  76 . As a result, the measurement beam  20  entering the reference polarizing beam-splitter  30  is T-polarized.  
         [0050]    Soon after entering the reference polarizing beam-splitter  30 , the measurement beam  20  encounters the beam-splitting plane  72  of the reference polarizing beam-splitter  30 . Because the measurement beam  20  is T-polarized, it passes through the beam-splitting plane  72  and proceeds toward the bottom face  54  of the reference polarizing beam-splitter  30 . The measurement beam  20  exits the bottom face  54  and reaches the fiber optic pickup  90  by way of the fold mirror  92 .  
         [0051]    In practice, some R-polarized light inevitably leaks through the beam-splitting planes  64 ,  66 . This leakage potentially contributes to measurement errors. In an interferometer  16  according to the invention, however, a significant portion of this stray R-polarized light is reflected toward the top face  32 , harmlessly away from the second polarizing beam-splitter  30 .  
         [0052]    For example, any R-polarized light from the reference beam  24  that passes through the first beam-splitting plane  64  soon encounters the second beam-splitting plane  66 . This second beam-splitting plane  66  reflects this stray R-polarized light toward the top face  32  of the first polarizing beam-splitter  28 , and hence away from the second polarizing beam-splitter  30 . Any remaining R-polarized light returns from the measurement mirror  80  as T-polarized light. Upon re-entering the first polarizing beam-splitter  28 , this T-polarized light proceeds through the second and first beam-splitting planes  66 ,  64  and exits the first polarizing beam-splitter  28  at the input face  44 , in a direction away from the second polarizing beam-splitter  30 .  
         [0053]    It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.