Abstract:
An interferometer measuring system comprising two moveable members and a reference member that may have significantly less movement, the group having a number of attached measurement mirrors, interferometers for measuring position and two optical support blocks for the interferometers. The interferometers are used to determine the measured optical path lengths to each of the moveable members and reference member and these positions are used to calculate the misalignment, or error in the relative positions of the moveable members with respect to the reference member. This calculated error is then used to correct the misalignment by moving the appropriate members in the manner directed by the calculation.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to an interferometer system for position measurement, and more specifically, to an interferometer system and method for improving the accuracy of interferometric measurements.  
         BACKGROUND OF THE INVENTION  
         [0002]    Often a semiconductor exposure system used to manufacture semiconductor devices uses a laser interferometer to accurately measure relative displacement between two members. For example, the laser interferometer is used as an apparatus for measuring the coordinates of a wafer stage (i.e. substrate stage) or reticle stage to facilitate highly accurate positioning of a semiconductor wafer or reticle relative to stationary projection optics.  
           [0003]    [0003]FIGS. 1 and 2 show a prior art laser interferometer system. The interferometer system typically measures a change in position of measurement mirrors Mx, My attached to movable stage S relative to stationary reference mirror MR attached to projection lens PL. A laser source generates beam B of light, part of which is reflected from reference mirror MR and part of which is reflected from measurement mirror Mx (FIG. 2). The light reflected from mirrors Mx, MR is then combined and reflected into sensor SR. If measurement mirror Mx moves relative to reference mirror MR, the intensity of the combined beam periodically increases and decreases as the reflected light from the two paths alternately interferes constructively and destructively. This constructive and destructive interference is caused by the two beams moving in and out of phase.  
           [0004]    In principle, each half wavelength of movement of the measurement mirror results in a total optical path change of one wavelength and thus, one complete cycle of intensity change. The number of cycle changes indicates the number of wavelengths that the measurement mirror has moved. Therefore, by counting the number of times the intensity of the light cycles between darkest and lightest, the change in position of the measurement mirror can be estimated as an integral number of wavelengths. The simple counting of light cycles, however, does not divulge in what direction the mirrors are moving.  
           [0005]    Actual interferometers are more complicated to be able to determine the direction of movement. This information is determined in a number of ways that may include, for example, comparing the interference pattern from light of two different wavelengths. And, while data may be processed using a central processing unit, there are also interferometers that output their data optically and therefore do not communicate with a computer in the same manner.  
           [0006]    Interferometer design also may vary in the placement of certain functional elements. For example, in FIGS. 1 and 2, if the reference mirror MR were positioned a fixed distance from sensor SR (i.e. not on projection lens PL) and only measurement mirror Mx could move relative to sensor SR and reference mirror MR, the interferometer would be a type of “absolute interferometer.” An absolute interferometer measures the movement of the measurement mirror. A differential interferometer, however, determines the relative movement between measurement mirror Mx and reference mirror MR where either mirror can move relative to sensor SR. A differential interferometer does not determine which mirror moves, just that one mirror moved in relation to the other. Therefore it does not produce enough information to determine the movement of either mirror relative to a third object without some modification. The interferometer shown in FIGS. 1 and 2 is one such differential interferometer.  
           [0007]    As manufacturers of integrated circuits attempt to increase circuit density and reduce circuit feature size in the devices manufactured, interferometers are required to provide more precise measurement data. The precision with which interferometers provide position control has been improved by technical advances in the design of various optical components including lasers and photosensors. But the performance of interferometers is still limited by changes in optical path length due to environmental disturbances, such as thermal expansion, that cause movement of the optical components of the interferometer system. When optical components, such as beam splitter BS, tilt or rotate for any reason whatsoever the distance between reference mirror MR and beam splitter BS changes (FIG. 2). This movement of beam splitter BS causes an error in the position measurement of stage S which results in misalignment of circuit patterns on wafer W relative to one another.  
           [0008]    Current interferometer systems fail to account for the rotation of elements within the interferometer system. In particular, see FIG. 3., where interferometers directed to reticle stage (not shown) and wafer stage S are mounted on different blocks. For the purposes of this discussion and this patent in general these blocks are considered rigid bodies and the interferometer elements mounted on them are considered rigidly mounted. But these blocks are not rigidly joined to each other and any relative movement of these blocks will cause the interferometer to produce inaccurate data for the position of wafer W relative to reticle R. Such inaccuracy leads to poor product because the image from reticle R is not in its intended position on wafer W. Current interferometer systems do not and can not compensate for relative rotation of the blocks because they do not contain the means necessary to determine what that movement is.  
           [0009]    Precise positioning of the circuit patterns is desired to prevent an imperfect final product, a potential decrease in process yield, and a corresponding increase in manufacturing costs. There is, therefore, a need for an interferometer system with the improved capability to determine and compensate for undesired rotation of the optical components of the interferometer system.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention overcomes deficiencies of the prior art by providing an interferometer measuring system that uses the interferometer system itself to compensate for movement of both the optical components and the moveable members (e.g. wafer and reticle stages) within the system. With interferometer systems that measure the stage position and attitude (rotation and tilt) of a projection exposure device, the optical components (interferometer support blocks) of the interferometer system itself tilt or rotate due to body deformation, so reticle and wafer stage position measurement errors occur. The present invention measures the position and attitude of components within the interferometer system using the interferometer system itself, and uses this to compensate for errors that occur, providing an interferometer system that can position the components of the projection exposure device with higher precision than conventional systems.  
           [0011]    The invention next uses the known relative movement between interferometer support blocks and a reference member to adjust the positions of the other moveable members that have been measured by the optical components on both blocks. Finally, after determining the relative movement of both optical components and moveable members, the invention compensates for undesired movement of the moveable members.  
           [0012]    The interferometer measuring system of a preferred embodiment of the present invention generally comprises, for example, in any single plane, two movable members with attached mirrors, one reference member with attached mirror, two optical support blocks, and the light sources, splitters and reference mirrors necessary for eight interferometer axes arrayed appropriately on the optical support blocks. On the first optical block, two interferometer beams are directed toward the first movable member, and two interferometer beams are directed toward the reference member. Similarly, on the second optical block, two interferometer beams are directed towards the second moveable member, and two interferometer beams are directed toward the reference member. In this preferred embodiment the movable members are a reticle stage and a wafer stage and the reference member is the projection lens. It should be noted that, since the positions of the moveable members are determined relative to the reference member, the reference member could also move and the invention would still compensate for undesired movement of the moveable members relative thereto.  
           [0013]    A method employing the present invention finds the positions of a plurality of movable members relative to each other and accounts and compensates for undesired movement of the movable members and interferometer system components. This method generally comprises first sensing the change in position of two points on each of a plurality of the movable members; calculating the relative change in position of a plurality of the movable members based on the sensed positions; and compensating for undesired movement.  
           [0014]    Where the interferometer system includes interferometers mounted on multiple blocks, relative movement between two blocks is determined by having additional steps where the interferometers from each block sense the position of a reference moveable member; then re-define the other moveable members&#39; positions in terms showing their position relative to the reference moveable member. The method can include a step in which a controller then compensates for undesired movement.  
           [0015]    The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a plan view of a prior art interferometer system for measuring position of a stage movable in x and y directions;  
         [0017]    [0017]FIG. 2 is a side view of the prior art interferometer system of FIG. 1 showing rotation of optical components of the system;  
         [0018]    [0018]FIG. 3 is a perspective of a portion of prior art interferometer system of FIGS. 1 and 2 applied to a wafer stage of a projection type exposure apparatus;  
         [0019]    [0019]FIG. 4 is a side view of the XZ plane component of an interferometer system of the present invention; and  
         [0020]    [0020]FIG. 5 is a side view showing the relative interferometer placement in a preferred embodiment of the present invention. 
     
    
       [0021]    Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.  
       DESCRIPTION OF THE INVENTION  
       [0022]    An interferometer system according to the present invention determines relative displacement and rotation of the optical elements and repositions the movable elements, or provides direction for their repositioning, accounting for the aforementioned movements of the optical elements. Thus, the invention provides the potential to control more precisely the positions of the wafer and reticle in an exposure device. This greater precision permits improved accuracy in circuit patterns on the wafer.  
         [0023]    Interferometer systems of the prior art are known to determine reticle and wafer stage displacement and rotation relative to the projection lens using interferometers as described in the background section. The prior art, however, does not determine whether the optical components of the interferometer system have themselves moved. The present invention, on the other hand, coordinates these determinations to create a system whereby wafer stage and reticle positions are both known with respect to projection lens while accounting for displacement and rotation of the interferometer optical components.  
         [0024]    To account for movements in the optical components of the interferometer system, the preferred embodiment of the present invention utilizes in each reference plane an eight beam interferometer system in conjunction with the normally existing central processing unit  6  and stage controllers  8  as illustrated in FIG. 4. The interferometers measure optical path length change for the beams directed at the movable members (projection lens assembly  22 , wafer stage  30  and reticle stage  40 ) from the optical components (interferometer blocks  50 ,  60 ) in a single plane, for example, the XZ plane. Interferometer mirrors  31 ,  32 ,  33  and  34  are mounted on reticle stage  40 , wafer stage  30  and projection lens assembly  22  as known in the art. The measured optical path length change data is processed by CPU  6  to determine both wafer stage and reticle displacement and rotation with respect to the projection lens assembly should either stage and/or the blocks upon which the interferometers are mounted change position. Controllers  8  adjust the position of the system components in response to correction information generated by CPU  6 .  
         [0025]    Another eight beam interferometer system (in addition to the existing central processing unit and stage controllers) may be employed to perform the similar function in the YZ plane and/or XY plane if desired. While the movement of the system components is in three dimensions, the description herein is limited to a single plane, i.e. as a two dimensional phenomena, in order to clarify the operation of the invention. Persons of ordinary skill in the art will appreciate that the description set forth herein may be equally applied to two or more planes for three dimensional measurement of motion in up to six degrees of freedom. Similarly, the present invention may also be practiced using a four beam interferometer system mounted on one interferometer block to align two moveable members.  
         [0026]    Referring again to FIG. 4, interferometer system  20  of the present invention is illustrated in side view in the XZ plane, with the Z axis as shown representing the vertical direction. In order to clarify the schematic representation of the invention as shown in FIG. 4, the reference beams and reference mirrors for each interferometer are not shown. Thus, in a preferred embodiment, eight absolute interferometers provide eight measurement beams,  41 - 48  as shown. Persons of ordinary skill in the art will appreciate that differential interferometers may be substituted in appropriate locations without departing from the scope of the invention.  
         [0027]    In particular, beams are preferably positioned in equidistant pairs such that beam pairs  41 - 42 ,  43 - 44 ,  45 - 46  and  47 - 48  are spaced apart by a common distance (d). Moreover, adjacent beam pair  42 - 43  and  46 - 47  are preferably spaced apart at distance (e). But distances (d) and (e) could be unique for each pair without compromising the present invention. Distances (d) and (e) are preferably the same for all pairs as this simplifies the positioning determination. However, the separate pairs may be differently spaced so long as each unique spacing is known and compensated for. Additionally, distances (d) and (e) may also represent only the component of the interferometer beam spacing that is in the plane of interest. For example, since the plane of interest in FIG. 4 is the XZ plane, and also with reference to FIG. 5, interferometer  41  may be offset from interferometer  42  in the Y direction without affecting the value of (d). As will be apparent to one of ordinary skill in the art, no two interferometers may originate or terminate at the same point without resulting in a loss of the data points necessary for the invention to function as described.  
         [0028]    The beams are also positioned with respect to a defined reference point  26  on projection lens assembly  22 . Preferably reference point  26  corresponds to the vertical center of the main lens within the projection lens assembly such that the vertical distance (Z axis) of beam  44  from reference point  26  is C  W  and the corresponding distance from beam  45  to reference point  26  is C R . Measurement beams  41 - 48  correspond positionally to optical path length changes X 1-8 , calculated according to the invention as described in greater detail below.  
         [0029]    In an initial state, specifically when the projection lens assembly  22  and the interferometer system  20  are not tilted, the Z axis matches optical axis  24  of lens assembly  22 . According to the methodology of the present invention, projection lens assembly  22  (PL), reticle stage  40  (RS), wafer stage  30  (WS), reticle stage interferometer block  50  (RIB), and wafer stage interferometer block  60  (WIB) are treated as rigid bodies. Thus, the angle of each rigid body relative to the initial optical axis (i.e. the Z axis, perpendicular to the ground) is defined as follows:  
                                                       θ W     rotation about the Y axis of wafer stage 30           θ L     rotation about the Y axis of projection lens 22           θ R     rotation about the Y axis of reticle stage 40           θ a     rotation about the Y axis of optical block 60           θ b     rotation about the Y axis of optical block 50                      
 
         [0030]    The movement distance of each rigid body from the initial position is similarly defined as follows:  
                                                       u W     x displacement of wafer stage 30           u L     x displacement of projection lens 22           u R     x displacement of reticle stage 40           u a     x displacement of optical block 60           u b     x displacement of optical block 50                      
 
         [0031]    Thus, the optical path length change X 1-8  measured by interferometer beams  41 - 48 , when any or all of the five rigid bodies  22 ,  30 ,  40 ,  50 ,  60  has translated and/or rotated, may be expressed in relation to the initial state as follows:  
           X   1   =[WS 30 position and tilt]+[ WIB 60 position and tilt]=[ u   W   −dθ   W   ]−[u   a −( d+e/ 2)θ a ]  (1)  
           X   2   =[WS 30 position and tilt]+[ WIB 60 position and tilt]=[ u   W   ]−[u   a −( e/ 2)θ a ]  (2)  
           X   3   =[PL 22 position and tilt]+[ WIB 60 position and tilt]=[ u   L −( d+c   W )θ L   −[u]   a +( e/ 2)θ a ]  (3)  
           X   4   =[PL 22 position and tilt]+[ WIB 60 position and tilt]=[ u   L   −c   W θ L   ]−[u   2 +( d+e/ 2)θ a ]  (4)  
           X   5   =[PL 22 position and tilt]+[ RIB 50 position and tilt]=[ u   L   +c   R θ L   ]−[u   b −( d+e/ 2)θ b ]  (5)  
           X   6   =[PL 22 position and tilt]+[ RIB 50 position and tilt]=[ u   L +( d+C   R )θ L   ]−[u   b −( e/ 2)θ b ]  (6)  
         i X 7   =[RS 40 position and tilt]+[ RIB 50 position and tilt]=[ u   R   ]−[u   b +( e/ 2)θ b ]  (7)  
           X   8   =[RS 40 position and tilt]+[ RIB 50 position and tilt]=[ u   R   +dθ   R   ]−[u   b +( d+e/ 2)θ b ]  (8)  
         [0032]    With these relationships defined, changes in position of the optical elements can be determined. This is accomplished by first calculating the differences between adjacent interferometer beam pairs  41 - 42 ,  43 - 44 ,  45 - 46  and  47 - 48  then dividing by the separation (d) between the beams:  
         ( X   2   −X   1 )/ d=θ   W θ a   (9)  
         ( X   4   −X   3 )/ d=ƒ   L −θ a   (10)  
         ( X   6   −X   5 )/ d=θ   L −θ   b   (11)  
         ( X   8   −X   7 )/ d=θ   R −θb   (12)  
         [0033]    Equations (10) and (11) can now be substituted into the equations describing the differences between stage mirrors  31 ,  33  and mirrors  32 ,  34  on projection lens assembly  22 . The resulting relationships show the differences as functions of moving member displacement and rotation:  
                       X   2     -     X   3       =                  [     u   w     ]     -     [       u   a     -       (     e   /   2     )          θ   a         ]     -                                [       u   L     -       (     d   +     c   w       )          θ   L         ]     +     [       u   a     +       (     e   /   2     )          θ   a         ]                   =                  u   w     +     e                   θ   a       -     u   L     +       (     d   +     c   w       )          θ   L                         (   10   )     -&gt;     =                  u   w     +     e        (       θ   L     -       (       X   4     -     X   3       )     /   d       )       -     u   L     +       (     d   +     c   w       )          θ   L                     =                  u   w     -       (     e   /   d     )          (       X   4     -     X   3       )       -     u   L     +       (     d   +     c   w     +   e     )          θ   L                       (   13   )                         X   7     -     X   6       =                  [     u   R     ]     -     [       u   b     +       (     e   /   2     )          θ   b         ]     -                                [       u   L     +       (     d   +     c   R       )          θ   L         ]     +     [       u   b     -       (     e   /   2     )          θ   b         ]                   =                  u   R     -     e                   θ   b       -     u   L     -       (     d   +     c   R       )          θ   L                         (   11   )     -&gt;     =                  u   R     -     e        (       θ   L     -       (       X   6     -     X   5       )     /   d       )       -     u   L     -       (     d   +     c   R       )          θ   L                     =                  u   R     +       (     e   /   d     )          (       X   6     -     X   5       )       -     u   L     -       (     d   +     c   R     +   e     )          θ   L                       (   14   )                               
 
         [0034]    These equations are then solved to express the element positions as functions of the measured optical path length changes.  
         (13)→ u   W   −[u   L −( d+c   W   +e θ L   ]=X   2   −X   3 +( e/d )( X   4   −X   3 )  (15)  
         (14)→ u   R   −[u   L +( d+c   R   +e )θ L   ]=X   7   −X   6 −( e/d )( X   6   −X   5 )  (16)  
         (9)−(10)−θ W −θ L =( X   2   −X   1 )/ d −( X   4   −X   3 )/ d   (17)  
         (12)−(11)→θ R −θ L =( X   8   −X   7 )/ d −(i X6 −X   5 )/ d   (18)  
         [0035]    The left sides of equations (15) through (18) are intermediate steps that begin to show the transition from the position and angle values (u, θ) of the coordinate system created by the initial optical axis Z to the coordinate system created by the optical axis  24  of projection lens assembly  22  after movement or rotation of system elements. For example, (d+c W +e) of equation (15) is the distance from projection lens  26  to the wafer surface, so u W −[u L −(d+c W +e)θ L ] shows the transition from the coordinate system created by the initial optical axis to the coordinate system created by the projection lens optical axis  24  after elements of the system have been moved and rotated. Similarly, θ W −θ L  of equation (17) shows the transition from angle θ W  measured from the initial optical axis Z to the relative angle between wafer stage  30  and projection lens assembly  22  after movement or rotation of system elements.  
         [0036]    After solution these positions are still described with respect to the Z axis. But with respect to a coordinate system defined around reference point  26  (i.e. optical axis  24  of lens  26 ) some terms drop out (u L =θ L =0). Therefore, using reference point  26  as a reference moveable member allows a projection system employing the present invention to position both wafer stage and reticle stage relative to the projection lens assembly (specifically reference point  26 ) according to the following:  
                                                       U W  = X 2  − X 3  + (e/d)(X 4  − X 3 ) = wafer stage position   (19)           U R  = X 7  − X 6  − (e/d)(X 6  − X 5 ) = reticle stage position   (20)           Θ W  = (X 2  − X 1 )/d − (X 4  − X 3 )/d = wafer stage angle   (21)           Θ R  = (X 8  − X 7 )/d − (X 6  − X 5 )/d = reticle stage angle   (22)                      
 
         [0037]    Thus, if the optical axis of projection lens assembly  22  as described above is made the reference standard, then with the eight beam interferometer system of a preferred embodiment, even if projection lens assembly  22 , wafer stage  30 , reticle stage  40 , reticle stage interferometer block  50 , and wafer stage interferometer block  60  are moved and tilted, it is possible to measure and correct for the relative movement and tilt when positioning wafer stage  30  and reticle stage  40 .  
         [0038]    As depicted, equations (19)-(22) may be viewed as having two components. The first component, a “nominal” component, determines the basic translation of the stage relative to the projection lens assembly  22 . The second component, a “correction,” accounts for the rotation and movement that cause the various components of the system to deviate in position from the “nominal” position. For example, in equation (19) the nominal component is “X 2 −X 3 ” and the correction is “+(e/d)(X 4 −X 3 ).” Similarly, in equation (21) the nominal component is “(X 2 −X 1 )/d” and the correction is “−(X 4 −X 3 )/d.” These nominal and correction components are representations designed to simplify the discussion of the invention. Depending on the specific hardware and software implementation, the present invention does not necessarily differentiate between “nominal” and “correction” during either data acquisition or computation.  
         [0039]    It will be observed from the above description that the interferometer system of the present invention provides a number of advantages over prior art systems. Importantly, the interferometer system accounts for displacement of the optical components of the interferometer system due to environmental effects such as thermal fluctuations or vibrations (or translations, since vibration can be considered translation with a time component), thus providing more accurate stage position measurement than possible with prior art interferometer systems.  
         [0040]    A further alternative embodiment of the present invention addresses rotation about the Z-axis with the addition of four interferometer reference beams  72 ,  74 ,  76 ,  78  as shown in FIG. 5. (FIG. 5 schematically depicts the relative position of mirrors  31 - 34  in the YZ plane. The interferometer beams which lie in the XZ plane therefore appear in cross-section in FIG. 5.) Each of beams  72 - 78  is directed to one of mirrors  31 ,  32 ,  33 ,  34 , at a point displaced in the Y direction from the other beams already incident on each mirror. Beams  72 ,  74 ,  76 ,  78  are each paired with one other interferometer beam directed to each mirror. For example the difference between the displacement from axes  72 ,  74 ,  76 ,  78  and its partner used as previously described to account for rotation about the Z axis of both moveable members and optical components. For example, interferometer  70  can be paired with either interferometer  52  or  54  as long as it is offset in the Y direction.  
         [0041]    The interferometer system of the present invention may be used as a position measurement system of a stage apparatus such as a wafer stage or a mask stage in a one-shot or scan type exposure apparatus for which highly precise driving control is required. But the invention is not limited to use with an exposure apparatus. For example, it may be used to accurately measure the relative displacement between two members in various high precision tools.  
         [0042]    As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. In particular, although the invention is described using interferometers to supply the necessary data, any sufficiently precise method of acquiring such data would suffice.