Abstract:
The subject invention relates to a wafer stage, such as may be used in optical wafer metrology instruments. The stage contains a wafer-chuck that can be connected to translation stages for the purpose of clamping and translating the wafer so that a plurality of sites on the wafer surface may be measured. The chuck includes a holder for mounting a reference sample. The holder is movable between a retracted position where the reference sample is held below the chuck surface and an extended position, such as where the surface of the reference sample is co-planar with the wafer surface. Therefore the holder may be installed within the area of the chuck that is utilized for wafer clamping. By this arrangement, the size of the wafer translation system can be reduced minimizing the stage travel and enabling increased spatial resolution, increased wafer throughput and reduced capital equipment and operating costs.

Description:
CLAIM OF PRIORITY  
       [0001]    The present application is a continuation of U.S. patent application No. 10/050,653, filed Jan. 14, 2002, which claims priority to the U.S. Provisional Patent Application Ser. No. 60/287,360 filed Apr. 30, 2001, and Ser. No. 60/336,515 filed Nov. 1, 2001, both of which are incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The subject invention relates to optical metrology devices which include a movable stage for rastering a wafer with respect to a measuring probe beam. More specifically, the invention relates to a stage which includes a system for mounting a reference chip within the footprint of the wafer thereby reducing the amount of stage travel necessary to measure both the wafer and the reference chip.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical metrology instruments require periodic monitoring and calibration. The output intensity of the light sources, the nature and extent of solarization of the optical components, chemical contamination of the optical surfaces and the alignment of the system optics can all vary with system operating time. An instrument&#39;s performance must be regularly monitored to verify that the system continues to meet operational specifications and that measurements are performed with the required precision and accuracy. Frequently, this is accomplished with the aid of a reference sample. A reference sample is a well-characterized specimen with known and temporally stable optical properties. Any variation in the measurement of the reference sample optical response is indicative of a variation in performance of the instrument. It is the periodic measurement of the reference sample that indicates performance problems and the requirement for maintenance or re-calibration.  
           [0004]    Optical wafer metrology systems are characteristically configured with the wafer surface approximately coincident with the focal plane of the optical system. The focal plane is flat and perpendicular to the plane of incidence of the probe beam (typically defined as the x-y plane). The vertical or z position of the wafer should coincide with the focus position of the probing beam.  
           [0005]    High-resolution “small spot-size” optical wafer metrology tools illuminate a small portion of the wafer surface at the focal position and monitor the change in one or more properties of the reflected light caused by the interaction with the sample surface. Characteristically, measurements are made sequentially as a translating wafer stage moves the wafer surface “through” the illuminated region. Conventional wafer “mastering” or translation protocols include both bi-linear, x-y translation and single-axis translation in combination with z-axis rotation. The stage system can also include z-axis movement for raising and lowering the wafer surface to achieve focus.  
           [0006]    In the prior art it has been desirable to place the reference sample in the focal plane. If the reference surface is physically located within the same plane as the wafer surface, no substantial refocusing of the optical system is required during measurement of the standard sample. For systems employing x-y translation stages the reference sample is typically attached to the wafer chuck at the corner of the stage where it does not interfere with wafer measurements. For systems employing z-axis rotation stages, restrictions posed by rotation symmetry, the location of auxiliary metrology instrumentation and the location and design of the wafer handling equipment make locating the reference sample more difficult. Even when a suitable location can be identified this often requires a more expensive, long-travel stage to be used so that the reference sample can be moved to the measurement position. These factors increase both the complexity of the instrument and its cost and size.  
           [0007]    Accordingly it would be desirable to locate the reference sample on the wafer chuck within the wafer footprint. This offers two important advantages. First, the stage-travel requirements are determined solely by the wafer dimensions. Therefore minimum form-factor wafer-translation systems can be employed. Second, a major limitation of the prior-art approach is eliminated permitting the use of compact wafer-translation systems having rotary stages. In particular, the prior approach of placing a reference chip outside a circular chuck and connected to the chuck cannot be implemented in a rotational system where an external pin lifter mechanism is used to raise wafer. As can be appreciated, if the reference chip extended beyond the circumference of the chuck, it would prevent the chuck from rotating since it would intersect with the pins of the wafer lifter. Placing the reference sample within the footprint of the chuck allows an external pin lifter to be used with a rotational chuck.  
         BRIEF SUMMARY  
         [0008]    The subject invention relates to an apparatus for holding and translating a wafer in an optical wafer metrology tool. The apparatus incorporates a wafer-chuck that is attached to and combined with a wafer translation system. The apparatus further includes a holder for a reference specimen. The holder is installed within the body of the wafer-chuck within the area of the chuck used for wafer clamping. The holder is movable between a retracted position where the reference sample is below the chuck surface, and an extended position where the reference sample is substantially coincident with the wafer position. During wafer metrology the holder is maintained in the retracted position. Measurement of the reference sample is made with the wafer removed and the holder maintained in the extended position.  
           [0009]    Locating the reference specimen within the area of the chuck used for wafer clamping enables an economy of design. The required range of stage-travel is set by the wafer dimensions. This admits the use of extremely short-travel translation stages to access large wafer areas. For example, the entire surface of a 300 mm diameter wafer can be accessed using a single 270 degree rotary-stage in combination with two±75 mm linear-travel stages, e.g.±75 mm of travel in the x direction and±75 mm in the y direction. The economy of motion enables increased accuracy of wafer positioning and increased wafer-throughput at reduced cost. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a cross-sectional view of the vacuum chuck with the holder in the retracted position.  
         [0011]    [0011]FIG. 2 is a cross sectional view of the vacuum chuck with the holder in the extended position.  
         [0012]    [0012]FIG. 3 is a cross-sectional view of a preferred embodiment of the wafer translation system.  
         [0013]    [0013]FIG. 4 is a schematic illustration of an optical metrology tool incorporating the wafer translation system.  
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIGS. 1 and 2 are cross-sectional schematics of a preferred embodiment of the wafer-chuck  20  showing the position of holder  50  during wafer metrology and measurement of the reference sample. FIG. 1 illustrates the configuration of the chuck employed in the metrology of wafer  26 . FIG. 2 illustrates the configuration of the chuck during measurement of reference sample  52 .  
         [0015]    Wafer-chuck  20  includes a platform  22  for supporting and clamping a wafer  26 . Platform  22  includes a support surface  24  for locating and supporting the wafer. The locating surface further includes a series of intersecting radial and circular channels  28  which may be connected to a vacuum supply via orifice  30 , check valve assembly  32 , manifold  34  and supply line  36 . When supply line  36  is connected to a vacuum system, surface  22 , channels  28 , orifice  30 , check-valve assembly  32 , manifold  34  and supply line  36  comprise a vacuum wafer-chuck. In the preferred embodiment supply line  36  may be alternately connected to a vacuum system, a pressure relief-valve or a source of high-pressure gas.  
         [0016]    The chuck further includes holder  50  for supporting and clamping reference sample  52 . Holder  50  includes a reference sample  52 , mounted to a spring-loaded piston assembly  54  that is free to move within cylinder  56  between a retracted and an extended position. The cylinder  56  includes upper  58  and lower  60  locating surfaces. Piston assembly  54  further includes a seal  64  that divides cylinder  56  into upper  68  and lower  70  hydraulic chambers. The lower hydraulic chamber is connected to manifold  34 . The upper hydraulic chamber is connected to the surface  24  of platform  22  through orifice  66 . A coil spring  62  is also provided to bias the holder into the retracted position.  
         [0017]    [0017]FIG. 1 illustrates wafer-chuck  20  with holder  50  in the retracted position, the configuration used in wafer metrology, wherein spring  62  locates piston  54  at lower locating surface  60 . In this position reference sample  52  is below the surface  24  of platform  22  and wafer  26  is clamped to platform  22 .  
         [0018]    [0018]FIG. 2 illustrates wafer-chuck  20  with holder  50  in the extended position, the configuration used for measurement of reference sample  52 , wherein spring  62  is compressed and the piston  54  is driven upwards so that the upper locating surface  58  abuts the surface of shelf  72 . In this position reference sample  52  is located at the measurement position, e.g. the upper surface is substantially co-planar with the upper surface of the wafer  26  as illustrated in FIG. 1.  
         [0019]    Supply  36 , manifold  34 , check valve  32 , piston  54 , seal  64  and spring  62  comprise a hydro-mechanical actuation mechanism for moving holder  50  between the extended and retracted positions. Connecting supply  36  to a source of high-pressure gas causes holder  50  to move to the extended position. Initial pressurization of manifold  34  produces a differential pressure across ball  40  raising the ball and pressing it against seal  38  sealing check valve  32 . With check-valve  32  sealed, manifold  34  and lower hydraulic chamber  70  fill with high-pressure gas. The pressurization of lower hydraulic chamber  70 , compresses spring  62  raising piston  54  to the point where the piston locates at upper locating surface  58 . In this position reference sample  52  is substantially at the measurement position, e.g. substantially the same position as the wafer illustrated in FIG. 1. This is the configuration illustrated in FIG. 2.  
         [0020]    The holder is moved to the retracted position by connection of supply line  36  to a pressure relief valve which vents lower hydraulic chamber  70  and manifold  34 . In this configuration, spring  62  forces the piston  54  against lower locating surface  60 , and the holder is maintained in the retracted position with reference sample  52  below the surface  24  of platform  22 . In the absence of pressurization of manifold  34 , ball  40  moves downward, away from seal  38  opening check-valve  32  and connecting manifold  34  to channels  28  through orifice  30 . With holder  50  in the retracted position wafer  26  can be located on surface  24  of platform  22  and clamped by connecting supply  36  to a vacuum system. In this configuration channels  28  are evacuated and the differential pressure established across the wafer  26  clamps wafer  26  to surface  24  of platform  22 . This is the configuration illustrated in FIG. 1.  
         [0021]    In the preferred embodiment illustrated in FIGS. 1 and 2 manifold  34  is connected to supply line  36  through a rotary bearing assembly  74 . Assembly  74  consists of a fixed hollow shaft  76  mounted in a housing  78  that connects to supply line  36 , and a rotary bearing  80  mounted within the body of manifold  34 , which is fixed to platform  22 . Bearing assembly  74  is arranged such that a hermetic rotary seal  100  is formed between the exterior surface of the hollow shaft and the inner surface of the rotating manifold. In the preferred embodiment, bearing system  74  also serves as a thrust bearing and supports the weight of platform  22 . In this fashion platform  22  may be rotated about hollow shaft  76  while supply line  36  remains fixed and connects, through bearing assembly  74 , manifold  34  to a vacuum system, a pressure-relief valve or a source of high-pressure gas.  
         [0022]    It should be noted that in the preferred embodiment, a single fluid line  36  is used to supply vacuum to the chuck surface to “clamp” down the wafer and to provide the pressure to raise of the reference chip. This dual function is important since access to the rotating stage is limited to the rotation axis of the system.  
         [0023]    [0023]FIG. 3 illustrates a preferred embodiment of a three-axis wafer-translation system  88  incorporating the wafer-chuck  20  shown in FIGS. 1 and 2 and described in the preceding discussion. Translation system  88  is comprised of wafer-chuck  20 , rotary stage  86  and linear translation stages  82  and  84 . Stages  82  and  84  are configured to provide translation in orthogonal directions within the x-y plane. Rotary stage  86  is arranged to provide rotation about the z-axis (perpendicular to the x-y plane). In the preferred embodiment, the rotary stage has 360 degrees of rotation. In addition, a mechanism (not shown) for raising and lowering the stage system in the vertical, z-axis is provided to permit the wafer to be brought into the focal plane.  
         [0024]    As illustrated in the FIG. 3, the use of holder  50  permits the reference sample  52  to be located within the footprint of the wafer. This allows a minimum form-factor platform to be employed with dimensions determined by the wafer size.  
         [0025]    [0025]FIG. 4 illustrates a preferred embodiment of the wafer translation system  88  incorporated in an optical metrology system  90 . Optical metrology system  90  is configured to derive the characteristics of sample  26  by measurement and analysis of the changes in the incident illumination produced by reflection from and interaction with the sample  26 . Optical metrology system  90  includes an illuminator  92 , wafer translation system  88 , sample  26 , reference sample  52 , detector  94  and processor  96 .  
         [0026]    Optical metrology system  90  may employ a plurality of measurement techniques either alone or in combination and including detection of the change in amplitude and the change in polarization state of the incident illumination upon reflection from and interaction with sample  26 . Further these measurements may be made using both bright-field (e.g. reflectometry) and dark-field (e.g. scatterometry) detection strategies at a single wavelength, or at a plurality of wavelengths. Consequently, illuminator  92  and detector  94  may include one or more instruments selected from the group consisting of reflectometers, ellipsometers, spectroscopic reflectometers, spectroscopic ellipsometers, polarized beam reflectometers, polarized beam spectroscopic reflectometers, scatterometers, spectroscopic scatterometers and optical CD measurement tools. Consequently, it is advantageous to provide processor  96  to analyze the output signals generated by the various detectors. These outputs correspond to changes in magnitude, changes in polarization state, changes in magnitude of polarized radiation and scatter measured at a plurality of wavelengths. The analysis protocols can treat the signals individually or in combination to evaluate the characteristics of a sample.  
         [0027]    Examples of metrology tools having one or more of these measurement systems are described in U.S. Pat. Nos. 5,608,526 and 6,278,519, incorporated herein by reference. Systems of this type include at least one broadband light source generating a polychromatic probe beam which is directed to the surface of the sample. The reflected probe beam is measured to provide both reflectometry and ellipsometric information as a function of wavelength. U.S. Pat. No. 6,278,519 also illustrates the use of single wavelength lasers for measuring a sample.  
         [0028]    It should be noted that reference sample  52  can be used to facilitate calibration of the wafer stage coordinates. In particular, the location of the edges of the reference sample can be accurately measured and compared to stage coordinates to calibrate measurement points with respect to a known coordinate system. In addition, measurement of the reference sample can also be used for focus adjustment in the Z-axis. In particular, the probe beam spot can be scanned over an edge of the reference sample while monitoring the reflected intensity. The distance over which the intensity moves from a minimum to a maximum gives a measure of spot size. This measurement is performed at a number of different z-positions, with the smallest measured spot size defining the focal plane.  
         [0029]    While the preceding discussion of the preferred embodiments has focused on the use of a vacuum-chuck for clamping the wafer, the invention can also employ mechanical or electrostatic means to accomplish both the functions of wafer-clamping and holder actuation. Furthermore, mechanical and electrostatic means can be used in place of or in combination with the preferred vacuum-clamping embodiment. In systems employing vacuum-clamping of the wafer the addition of hydro-mechanical actuation may be accomplished cost-effectively. Particularly in those cases where the required hardware, e.g. vacuum systems, manifolds, pressure relief valves, sources of high-pressure gas, etc. are already incorporated in the existing wafer-clamping system. The ability to locate the reference sample within the wafer footprint also allows considerable reduction in the cost of the wafer translation systems, and the implementation of new, high-precision translation systems at a cost comparable to existing low-precision systems. These economic benefits accrue from the ability to utilize lower-cost, reduced-travel stages. For example, in the preferred embodiment of FIG. 3 the entire surface of a 300 mm diameter wafer can be measured using at least a 270° z-axis rotation stage in combination with two±75 mm linear x and y translation stages.