Abstract:
A stage system is disclosed for supporting and positioning a semiconductor wafer for inspection in an optical metrology device. A chuck for supporting a wafer is mounted to the stage system. The stage system can move the chuck along two linear orthogonal axes. A rotational stage is also provided for rotating the chuck. A mechanism is provided for adjusting the vertical position of a chuck to allow for focusing of the probe beam of the metrology device. The vertical adjustment mechanism is designed so that it does impede the rotational positioning of the chuck.

Description:
CLAIM OF PRIORITY  
       [0001]    The present application is a continuation of U.S. patent application No. 10/178,623, filed Jun. 24, 2002, ROTATIONAL STAGE WITH VERTICAL AXIS ADJUSTMENT, which claims the benefit of U.S. Provisional Application Ser. No. 60/336,530, filed Nov. 1, 2001, ROTATIONAL STAGE WITH VERTICAL AXIS ADJUSTMENT, each of which is hereby incorporated herein by reference. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention relates to metrology tools with rotating stages having integrated Z-axis adjustment for inspecting a wafer.  
         BACKGROUND  
         [0003]    This invention relates to optical metrology tools of the type described in U.S. Pat. No. 6,278,519, incorporated herein by reference. Referring to prior art FIG. 1, these types of tools include a light source for generating a probe beam  7 , which is focused onto a semiconductor wafer  4 . Changes between the incident probe beam  7  and the reflected beam are monitored to evaluate characteristics of the sample  4 .  
           [0004]    Tools of this type typically include a motion stage for supporting the wafer  4 . Various stage motion combinations are available including full X-Y stages; R/theta stages; and ½X-½Y plus theta stages (where theta means 360 degrees of rotation). Prior art FIG. 1 exemplarily illustrates an X-stage  22 , a Y-stage  24 , and theta stage  26 . The motion of the stages is computer controlled for moving the wafer into position with respect to the focused spot of the probe beam  7 .  
           [0005]    These tools also typically include a focusing (preferably autofocusing) system, which brings the wafer into the focal plane of the focusing optics of the measurement system  2 . A number of these systems operate to translate the focusing optics in a vertical direction with respect to the sample. Alternatively, the stages themselves are provided with some form of vertical (Z-axis) movement for focusing purposes. Since the motion system needs to be designed to fit within the available height  3 , conventional Z-axis stages that utilize guide rails are difficult to integrate. The length of the profile moving along the guide rails directly affects the Z-axis&#39; stiffness against tilting movement. Where the length of the moving profile is limited by the available height, the tilting movement of the moving parts becomes hard to control. In order to reduce the tilting movement, the contact pressure between the moving profile and the guide rails needs to be increased, which results in increased friction and consequently increased actuating forces. High friction and actuating forces again reduce the movement resolution in Z-axis.  
           [0006]    Therefore, there exists a need for an apparatus and method for highly precise vertical micro adjustment of a rotating stage with minimal friction and a maximum stiffness against tilt movement and lateral movement.  
           [0007]    Conventional linear guiding systems define the movement direction by either a sliding or a rolling contact. This is feasible where an extensive movement range needs to be covered. In this application, the required Z-axis movement range is only twenty thousandths of an inch. Providing Z-axis movement over that small a range with sliding or rolling guides still requires a relatively bulky and heavy assembly, which increases the moment of inertia of the motion system. As a consequence, the motion system moves more slowly.  
           [0008]    Therefore, there exists a need for a Z-axis guiding system that is low in mass as well.  
         BRIEF SUMMARY  
         [0009]    A wafer motion system includes one or two conventional linear stages and a rotating stage, which are mounted on top of each other. The one or two linear axes are horizontal. The rotating stage is placed at the top and is configured for holding a wafer and rotating it around a vertical axis of revolution. The one or two linear stages have a travel range defined in combination with the rotating stage to position the wafer with respect to the probe beam. The wafer is placed on a chuck and held down by a vacuum provided between wafer and chuck.  
           [0010]    The chuck itself is guided along the axis of revolution within the rotating stage. Specifically configured and placed flexures or membranes elastically guide the chuck without any substantial friction. The membranes easily deflect in the vertical direction while being highly rigid in horizontal direction. Preferably, at least two ring shaped membranes are vertically positioned relative to each other. The horizontal stiffness of each membrane in combination with the vertical offset between them results in a high stiffness against tilt.  
           [0011]    The rotationally symmetric design of the membranes allows them to be easily integrated into the generally rotationally symmetric design of the rotating stage. The relatively small mounting space required for mounting the membranes results in little additional volume and mass necessary for integrating the membrane rings in the rotating portion of the rotating stage.  
           [0012]    The membranes provide a substantially friction free guidance of the chuck allowing for a smooth and precise actuation and adjustment. A horizontally oriented piezo stack is utilized to provide the vertical actuation of the chuck via a lever system, which amplifies and transforms the horizontal expansion of the piezo stack into a vertical movement of the required range. The vertical lever movement is transmitted to the chuck unit via a central linking assembly, which provides for initial adjustment and preload of the actuator to the chuck. The linking assembly also receives external vacuum and/or pressure air and transmits it into the chuck unit.  
           [0013]    The vertical movement system is actuated by a voltage applied to the piezo stack, which expands in accordance to the well-known principles of piezo elements. The amount of horizontal piezo stack movement is in the micron range. The lever system amplifies the piezo movement by a factor of approximately 15. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0014]    [0014]FIG. 1 schematically shows a metrology tool of the prior art.  
         [0015]    [0015]FIG. 2 schematically illustrates the function of the present invention  
         [0016]    [0016]FIG. 3 shows a simplified front view of a stage system having a Z-axis motion system in accordance with the present invention.  
         [0017]    [0017]FIG. 4 is a perspective view, partially in section, of the subject assembly.  
         [0018]    [0018]FIG. 5 shows a simplified section view of the rotating stage in a view direction perpendicular to the view direction of FIG. 3.  
         [0019]    [0019]FIG. 6 shows a perspective bottom view including the assembled chuck, flexures and base.  
         [0020]    [0020]FIG. 7 shows a perspective bottom view including the chuck, the flexures and the Z-axis motion system.  
         [0021]    [0021]FIG. 8 shows two exemplary flexures of the preferred embodiment in concentric arrangement as assembled for operation.  
         [0022]    [0022]FIG. 9 shows lower a perspective bottom view including the chuck, the flexures and the Z-axis motion system. 
     
    
     DETAILED DESCRIPTION  
       [0023]    [0023]FIG. 1 generally illustrates a wafer support and movement system  2  in which the subject invention can be incorporated. The system  2  is of the type that would be incorporated in a metrology tool for optically inspecting semiconductor wafers. Such metrology tools are described, for example, in U.S. Pat. Nos. 6,278,519 and 5,608,526, the disclosures of which are incorporated herein by reference. Such tools are capable of performing various measurements including spectroscopic reflectometry and spectroscopic ellipsometry.  
         [0024]    In such metrology tools, a probe beam  7  is focused onto the sample surface. The reflected probe beam is monitored to evaluate characteristics of the sample. In these systems it is important to accurately focus the probe beam on the sample. In some systems, focusing is achieved by moving the optics within system  2  (not shown). In others, focusing is achieved by moving the stage system supporting the wafer in the Z-axis perpendicular to the sample surface. The subject invention is intended to permit focusing by Z-axis movement of the stage. The mechanism for providing such Z-axis motion, while allowing complete 360 degrees of rotation of the wafer, will be discussed below.  
         [0025]    As seen in FIG. 1, the stage system can include linear stages  22 ,  24  and a rotating stage  26 . The first linear stage  22  provides movement along a linear axis  21  perpendicular to the view plane of FIG. 1. On top of the first stage  22  is mounted the second stage  24  moving along a linear axis  23 . On top of the second stage is mounted the rotating stage  26  rotating around an axis of revolution  25 . The present invention introduces a Z-axis motion system that is preferably integrated in the rotating stage  26 , while permitting a full 360-degrees of rotation of the rotating stage  26 . It is noted that the scope of the invention is not limited by a specific configuration and/or number of stages. The detailed configuration and operation of the inventive Z-axis motion system is described in the following.  
         [0026]    FIGS.  2  to  9  illustrate features of the invention. FIG. 2 is the most simplified schematic which is useful for illustrating the mechanical Z-axis operation of the system. FIGS. 3 and 5 are also somewhat simplified to illustrate the overall structure. FIGS. 4 and 6 to  9  illustrate specific details of the construction of the preferred embodiment.  
         [0027]    As seen in these Figures, an actuator, illustrated generally as  30 , is provided for controlling the vertical movement of the chuck  20  which supports the wafer  4 . The vertical movement is indicated by the arrow A. The actuator acts via cup  40  directly on push rod  60  which is preferably substantially coincident with the axis of revolution  25  of the chuck (which is co-axial with the axis of the rotary stage  26 ). The vertical movement is transmitted from the push rod  60  via adjustor  62 , coupling body  64 , and a bearing  80  onto the central element  70 , which is attached to a chuck  20 . The chuck  20  is attached to and coupled with the base  50  via a flexure  54 . The central element  70  is attached to and coupled with the base  50  via a flexure  52 . The central element includes channel  19  which transmits vacuum received through bore  76  in coupling body  64  to grooves  81  in the chuck  20  for holding the wafer in place during measurement.  
         [0028]    The flexures  52 ,  54  hold the chuck  20  together with the central element  70  and any other attached elements substantially rigid against lateral movement and/or tilt movement with respect to base  50 . Only movement in vertical direction along the axis of revolution  25  is provided. The scope of the invention includes embodiments, where the central element  70  and the chuck  20  are one piece or separate pieces mechanically connected to one another.  
         [0029]    The base  50  is attached onto a rotating portion  27  of the stage  26 . Rotational movement induced by the rotating stage  26  is transmitted via the base  50  and the flexures  52 ,  54  onto the central element  70  and the chuck  20 . The non-rotating portions  28  and  53  of the stage  26  are affixed on the linear stage  24  as is shown in FIGS. 1 and 3. Rotational movement is decoupled from the elements  60 ,  62 , and  64  by the bearing  80 .  
         [0030]    In the preferred embodiment, the flexures  52 ,  54  are concentrically arranged with respect to the axis of revolution  25 . Each of the flexures  52 ,  54  provide rigidity against lateral movement. The concentric and vertically offset arrangement of the flexures  52 ,  54  combines the lateral stiffness of each of the flexure  52 ,  54  advantageously to make the chuck  20  and the central element  70  additionally highly stiff against tilt movement and provides for a highly precise linear movement performed by the chuck  20  in direction along the axis of revolution  25 .  
         [0031]    The actuator  30  includes a PZT stack translator  38  to which members  43  and  44  are attached. Members  43  and  44  have spherical ends (and are shown as balls in FIG. 2). The lever  34  of the actuator is a single piece incorporating a stationary base section  34 A and a rotating lever section  34 B connected by a thin hinge element  36 . One end of the PZT stack translator is held rigid against the base section of the lever  34 B via member  43 , adjusting screw  35  and clamping plate  39 . The opposite end of the PZT stack translator contacts the lever section of the lever  34 B via member  44 , and retainer plate  45 .  
         [0032]    The PZT stack  38  is a well known horizontally stacked number of piezo elements. Increasing the voltage applied to the PZT stack  38  causes it to expand; decreasing the applied voltage causes the PZT to contract. The expansion movement of the PZT stack  38  is transmitted via the member  44  onto the lever  34 B, which transforms the horizontal movement into an angular movement around the fixed hinge  36 , while at the same time amplifying it by a factor of approximately  15 . Consequently, the point of contact  41  moves upward. The lever  34 B thereby redirects the horizontal expansion movement of the PZT stack  38  into a vertical movement suitable for Z-axis adjustment of the chuck  20  and a wafer  4 . The upward positioning of the chuck and the bending motion of the flexures is shown in phantom line in FIG. 2.  
         [0033]    The expansion range of the PZT stack  38  is approximately 33 microns and the angular movement of the lever  34 B is 0.30 degrees, such that the point of contact  41  moves off-axis of the rotary stage and chuck. This misalignment is allowed and absorbed by the cup  40  and links  60  and  62 , which has a ball and socket joint at each end.  
         [0034]    The lever  34 B serves to amplify the expansion of the PZT stack  38  by a factor that is essentially defined by the proportion between distance  31  and  32 . Distance  31  is between hinges  36  and  44 . Distance  32  is between  36  and  41 . In the preferred embodiment, the amplification is approximately 15 times the expansion of the PZT stack  38 . The movement amplification and redirection introduced by the lever  34 B provides for sufficient design space for the PZT stack  38  without compromising the overall height of the motion system. In the preferred embodiment, the PZT stack has a diameter of ⅜ of an inch and a length of about three inches.  
         [0035]    The PZT stack  38  expands proportionally in response to the applied voltage. In practice, it is difficult to control height directly based on the applied voltage due to friction and and/or deformation along the movement path between the PZT stack  38  and the chuck  20 . Therefore, in the preferred embodiment, the vertical positioning is controlled by a feed back system wherein the vertical height of the wafer is monitored with a focusing mechanism  110  above the wafer. Errors in focusing are monitored and used is a feedback loop to the voltage controlling the PZT stack. In the preferred embodiment, the focus system  110  provides a signal that is processed by a processor  112  into a second signal, which is transformed preferably by a linear variable differential transformer  114  (LVDT) into a voltage applied to the PZT stack  38 . It is clear to one of ordinary skills in the art, that any device for precise distance or position measurement may be utilized instead or in addition to the auto focus system  11 .  
         [0036]    To initially setup the vertical position of the chuck  20 , adjustor  62  may be threaded in or out of the push rod  60 , which lowers or lifts the chuck  20 . Adjustor  62  is seated and held in coupling body  64 , the detailed function of which will be explained with respect to FIGS. 4 and 5.  
         [0037]    As seen in FIG. 4, the lever  34 B is preferably configured as a partly separated portion of an enclosure  34 A- 34 B enclosing the PZT stack  38 . The enclosure  34 A- 34 B has a central cavity configured to encapsulate the PZT stack  38  and the elements  43 ,  44 . The hinge  36  is preferably configured as a membrane that bridges the fixed portion  34 A of the enclosure with the moveable portion  34 B. The membrane hinge  36  is sufficiently flexible to absorb the angular movement of the lever  34 B. A frame  37  is mounted on the enclosure and hold anti-rotation pins  72  (see also FIGS. 4 and 7). Pins  72  keep the elements  60 ,  62 , and  64  aligned and non-rotating.  
         [0038]    In the preferred embodiment, the PZT stack  38  is biased at a central voltage value, which is defined as the mid-range or central Z-axis position of the chuck at which the flexures  52 ,  54  may be in a neutral non deflected state. By increasing or decreasing the voltage to the PZT stack  38 , the chuck  20  can be raised or lowered. This is compensated by the flexures  52 ,  54  by bending with their inner portion upwards or downwards. In the preferred embodiment, the chuck can be moved ± ten thousandths of an inch. The flexures  52 ,  54  are configured to absorb this vertical movement well within their elastic deformation range. The chuck  20  is pre-loaded via springs (not shown) vertically downward relative to the base  50 . This ensures that, as the PZT stack expands and contracts as a result of the varying the applied voltage, intimate mechanical contact is maintained through parts  34 B,  41 ,  60 ,  62 ,  64 ,  80 ,  78 , ( 52 ) and  20 . The movement can be used to focus the probe beam of light onto the wafer  4 .  
         [0039]    One important aspect of this invention is that the structure allows the chuck  20  to be rotated by 360 degrees. Bearing  80  serves thereby to transfer the upward thrust forces while at the same time minimizing friction torque and eliminating surface-to-surface rubbing during rotation. The pins  72  prevent push rod  60  and coupling  64  from being rotated by the torque resulting from the remaining friction in the bearing  80 . In this manner, hose  74  (see FIG. 5) does not become wrapped around the mechanism when the stage  26  is rotating. Adjustment element  62  is threaded and can be rotated with a tool inserted into the hex recess  63  at the top of element  62  to vary its vertical position. The hex recess  63  is accessible through the top of chuck  20  and the central bore  19 . Adjustor  62  is used in the initial set up to adjust the height of the system and compensate for tolerances. Coupling body  64  is connected to a source of vacuum delivered by hose  74  (see FIGS. 5 and 7). Through opening  76  the fluid flow communicates to the bore  19 .  
         [0040]    In FIGS. 6 and 7, the flexures  52 ,  54  are visible in their assembled position. The flexures  52 ,  54  are preferably configured as thin membranes made, for example, from full hard stainless steel with a thickness 0.004 inch. Such membranes may feature circular perforations  79  (see FIGS. 6, 7 and  8 ) to reduce resistive axial forces resulting from their elastic deformation. In the preferred embodiment, the flexures  52 ,  54  have an annular width of approximately 1.25 inch. The width is defined as the difference between the outside diameter and inside diameter of the flexures  52 ,  54 . FIG. 8 shows in that context two exemplary flexures  52 ,  54 . The hole pattern along their inside and outside edges correspond to the screw holes of the clamping rings  73 ,  75  for the flexure  54  and clamping rings  71 ,  78  for the flexure  52  (FIG. 5). The clamping rings  71 ,  73 ,  75 ,  78  assist in mounting flexures  52 ,  54  since they are too thin to be screwed down directly without risk of damage. Clamping ring  78  also holds the bearing  80  and the seal  77  at the rotating portion of the assembly.  
         [0041]    In the preferred embodiment, the stage assembly is provided with three leveling feet (one of which is shown in FIG. 4 at 90). The leveling feet allow the chuck to be set to the required height relative to the optics. In addition, the leveling feet allow the chuck to be leveled, or made parallel to the optics and at the same time make the chuck perpendicular to the axis of rotation of the rotational stage. A screw (not shown) passes through the center of each of the leveling feet to attach the Z-stage to the rotary stage.  
         [0042]    In the preferred embodiment, the stage assembly is further provided with three damper assemblies (one of which is shown at 92). The damper assemblies provide critical damping to the mechanical “mass-spring” system, such that optimum closed-loop servo control is achieved.  
         [0043]    The Z-axis motion system may operate as follows. To receive the wafer  4 , the chuck  20  may be brought into a predetermined receiving position by having the processor apply an initial voltage to the PZT stack  38 . After the wafer  4  has been placed on the chuck  20 , a vacuum is communicated to the gap between wafer  4  and chuck  20  along the hose  74 , the opening  76 , central bore  19  and the grooves  81 . Once the wafer  4  is secured to the chuck, the processor  112  controls one or more of the stages  22 ,  24 ,  26  to bring a predetermined measurement area of the wafer  4  within the range of the probe beam. The auto focus system  110  recognizes and communicates the offset between focal plane and the measurement area to the processor  112 , which in turn adjusts the voltage correspondingly until the offset is substantially eliminated. In the preferred embodiment, the movement resolution and consequently the adjustment precision in vertical direction is within a range of 0.02-0.003 microns, depending upon particular elements incorporated into the feedback loop.  
         [0044]    While the subject invention has been described with reference to a preferred embodiment, various changes and modifications could be made therein, by one skilled in the art, without varying from the scope and spirit of the subject invention as defined by the appended claims