Abstract:
The invention provides an apparatus and method for positioning a moveable mass, such as the moveable components found in a typical XY table. The apparatus comprises driving means producing a driving force to move the mass along an axis, which mass has a variable center of gravity position perpendicular to said axis. The driving force from the driving means is operative to act through the center of gravity of the mass as the position of the center of gravity changes.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a motor system for driving and positioning a load, such as a stage of an XY table.  
         BACKGROUND AND PRIOR ART  
         [0002]    In a conventional XY table, a first motor is used to drive a first stage of the XY table and a second motor is used to drive a second stage of the XY table. The second stage is usually mounted onto the first stage. A load, such as that comprising a bond head and bonding member, is attached to the second stage and moves with it. Commonly, the first stage is called an X stage which moves the XY table in an X-axis and the second stage is called a Y stage which moves the XY table in a Y-axis. A combination of movement of the X and Y stages result in the positioning of the load in an X-Y plane.  
           [0003]    The load, together with the X and Y stages of the XY table, comprise a moving mass of the XY table which mass has a combined weight that is concentrated in a center of gravity approximately located centrally in the XY table structure when it is in an equilibrium position. At the equilibrium position, when the X stage is driven, the orientation of the X stage is such that a motor driving the X stage (“X motor”) generates a force that is aligned with the center of gravity of the mass when the Y stage is in a home or central position.  
           [0004]    However, a problem with mounting the Y stage onto the X stage is that the said center of gravity of the moving mass shifts when the Y stage moves. This will happen no matter how well the driving force is aligned with the center of gravity at the equilibrium position. Once the center of gravity moves, the driving force will not be aligned with the center of gravity. When the driving force is not aligned with the center of gravity, a turning moment is generated that is equal to the driving force multiplied by the distance that the center of gravity is shifted from the equilibrium position. This undesirable turning moment causes the moving mass to vibrate as the mass is moved, thereby increasing the performance differential and increasing the amount of time required for accurate positioning at a given location, since such vibration will cause some positional offset. The further the Y stage moves away from the equilibrium position, the greater the turning moment that induces vibration.  
           [0005]    Problems associated with displacement of the center of gravity as an object or load is moved are identified in U.S. Pat. No. 5,844,664 for “Positioning Device with a Force Actuator, System for Compensating Center-of Gravity Displacements, and Lithographic Drive provided with such a Positioning Device”. The patent discloses a positioning device and a method to compensate for shifts in the center of gravity. A plurality of vertically-arranged force actuator systems generate compensation forces to balance an object table when the center of gravity moves as the object table is displaced. Although a balancing force is generated as a passive response to the movement of the center of gravity, there is no disclosure of how to align the driving force to the center of gravity as the object table is moved, that would improve the driving efficiency and performance.  
         SUMMARY OF THE INVENTION  
         [0006]    It is thus an object of the invention to seek to align a driving force with a center of gravity of a load being driven when positioning such a load.  
           [0007]    According to a first aspect of the invention there is provided apparatus for positioning a mass, comprising driving means to move the mass along an axis, which mass has a variable center of gravity position perpendicular to said axis, wherein a driving force from the driving means is operative to act through the center of gravity of the mass as the position of the center of gravity changes.  
           [0008]    According to a second aspect of the invention there is provided a method for positioning a mass, comprising moving the mass along an axis, which mass has a variable center of gravity position perpendicular to said axis and whereby a force is maintained operative to act through the center of gravity of the mass as the position of the center of gravity changes.  
           [0009]    Using a single driving force, it is difficult to move the direction of the driving force so that it is aligned with the mass each time. Using the invention it is possible to use a combination of forces to drive the said mass, wherein the combination of forces results in a force acting through the center of gravity of a moving mass as the mass is being driven along an axis.  
           [0010]    Preferably, each driving force may be provided by a separate motor.  
           [0011]    It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is an isometric view of an XY table assembly with a bond head attached;  
         [0013]    [0013]FIG. 2 is a plan view of the XY table assembly of FIG. 1;  
         [0014]    [0014]FIG. 3 is an isometric view of only an X stage of the XY table assembly and attached coil brackets;  
         [0015]    [0015]FIG. 4 is a plan view of the X stage of FIG. 3; and  
         [0016]    [0016]FIG. 5 is a schematic illustration of the combination of forces applied to the X stage of the XY table assembly.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Referring to the drawings, FIG. 1 is an isometric view of an apparatus having an XY table assembly  10  with a bond head  18  attached. The XY table assembly  10  comprises a first stage, usually called an X stage  12 , which is driven by driving means such as an X motor assembly  22  mounted on a platform  11 . A second stage, usually called a Y stage  14  is in turn mounted on the X stage  12  and is driven by a Y motor assembly  24  mounted on the platform  11 . An X-axis linear encoder  15  detects a position of the X stage  12 , whereas a Y-axis encoder  16  detects a position of the Y stage  14  at any given time.  
         [0018]    The described embodiment relates to the use of the XY table assembly  10  for semiconductor wire bonding. Thus, a mass or load comprising a bond head  18  with a bonding member  20  which has a bond tip  21  is attached to a top surface of the Y stage  14 . The contemporaneous movement of the X stage  12  and Y stage  14  function to position the bond tip  21  of the bonding member  20  in the X and Y axes to a position on a semiconductor device whereat a bonding wire is to be bonded. Generally, the X stage  12 , Y stage  14  and load  18 ,  20 ,  21  and associated components may be referred to as a movable mass.  
         [0019]    The X motor assembly  22  comprises a plurality of motors, which are illustrated in the form of two linear motors in this embodiment. A front (in use) linear motor includes a front set of magnets  26  and front coil bracket  30  holding a coil (or set of coils)  34  (see FIG. 3). A rear (in use) linear motor includes a rear set of magnets  28  and rear coil bracket  32  holding a rear coil (or set of coils)  36 . Each linear motor works on a moving-coil principle, each moving coil  34 ,  36  being disposed between a set of permanent magnets  26 ,  28 . As each coil  34 ,  36  is energized by current flowing through it, a force actuated by electromagnetic induction causes the coil  34 ,  36 , coil bracket  26 ,  28  and therefore the connected X stage  12  to move in the X axis.  
         [0020]    Correspondingly, the Y motor assembly also comprises a linear motor (not shown) to drive the Y stage  14 . The Y stage  14  is mounted on the X stage  12 , such that the weight of the Y stage  14  is carried by the X stage  12 . For the Y stage  14 , the Y motor driving force is designed to always act through the center of gravity of the Y stage  14 , and there is generally no variation in its center of gravity. Thus, the Y stage  14  will not be further discussed.  
         [0021]    [0021]FIG. 2 is a top view of the XY table assembly  10  of FIG. 1. In this view, the relative orientations of the X stage  12 , Y stage  14  and bonding member  20  in the XY axes are shown.  
         [0022]    [0022]FIG. 3 is an isometric view of only the X stage  12  and attached coil brackets  26 ,  28 . The front and rear coil brackets  26 ,  28  have at their ends, a front coil  34  and a rear coil  36 . FIG. 4 is a plan or view from on top of the X stage  12  of FIG. 3.  
         [0023]    It will be understood that since the Y stage  14  is mounted on the X stage  12 , the center of gravity of the movable mass of the XY table comprising the combined weight of the X stage  12 , Y stage  14 , load  18 ,  20 ,  21  and  20  associated components supported by them will shift as the Y stage  14  is moved. If there were only a single motor applying a single force, the non-alignment between the driving force and the center of gravity would result in a turning moment applied to the XY table that would cause the movable mass to vibrate when in motion, making the application of force to position the bonding  25  member  20  less efficient. This is very undesirable especially in the case of small semiconductor devices, wherein the position of the bonding member  20  must be precise in order to bond wires accurately.  
         [0024]    To illustrate the principle of the invention, FIG. 5 is a schematic illustration of  30  the combination of forces F 1 , F 2  applied to the X stage of the XY table in order to ensure that the effective driving force from the combination of forces always acts through the center of gravity at two positions O and O′ of the XY table. A driving force generated by the front (in use) linear motor is indicated by F 1  whereas a force generated by the rear (in use) linear motor is indicated by F 2 .  
         [0025]    The original center of gravity of the XY table is indicated by  0  in an equilibrium position, and by O′ in a position wherein the Y stage  14  has moved in direction A in the Y axis.  
         [0026]    Generally, when the Y stage  14  moves forwards (in direction A), more current is provided to the front coil  34  and less current to the rear coil  36 . Conversely, when the Y stage  14  moves rearwards (in direction B) more current is provided to the rear coil  36  and less current to the front coil  34 .  
         [0027]    The respective forces F 1 , F 2  that should drive the X stage is represented by the representative formulae:  
               F   1     =     M                   a        (       1   2     +       m   M          y   L         )                       F   2     =     M                   a        (       1   2     -       m   M          y   L         )                                     
 
         [0028]    Where  
         [0029]    F 1 —driving force from the coil  34  of the front X motor;  
         [0030]    F 2 —driving force from the coil  36  of the rear X motor,  
         [0031]    M—the total moving mass in the X axis. Since the Y stage is mounted on the X stage, it also includes the moving mass (m) of the Y stage;  
         [0032]    m—total moving mass of the Y stage.  
         [0033]    a—acceleration required to move the table in the X axis;  
         [0034]    y—the distance moved by the Y stage from equilibrium or center position. It can be obtained from the reading of the Y linear encoder  16 ;  
         [0035]    L—the distance between driving forces F 1 , F 2  of the X motor assembly;  
         [0036]    O—center of gravity position of the XY table in the X axis when the Y stage is at equilibrium location. At this position, it is arranged such that  
         F   o1     =       F   o2     =       1   2        M                 a                             
 
         [0037]    F o1 —F 1  when Y stage is at equilibrium position;  
         [0038]    F 02 —F 2  when Y stage is at equilibrium position;  
         [0039]    O′—center of gravity position of the XY table in the X axis when Y stage moves distance y;  
         [0040]    By maintaining this relationship, it could be ensured that the combined X motor assembly driving force (F 1 +F 2 ) would be through the center of gravity of the moving mass at all of the Y stage  14  positions. The relative forces F 1 , F 2  may be controlled by varying the electric current ratio flowing through the front and rear coils  34 ,  36 . Thus, the determined electric current ratio between the front and rear coils  34 ,  36  is accordingly based on the position of the Y stage  14 . This helps to reduce any undesired moment generated by misalignment of the driving force and center of gravity, whatever the position of the bonding member  20 .  
         [0041]    In apparatus embodying the present invention, it will be understood that with less vibration of the movable mass when in motion, easier control and better performance of the XY table may be attained at different top-table (Y stage  14 ) positions. The difference in bottom-table (X stage  12 ) performance in the X-axis for the whole bonding area may be reduced.  
         [0042]    The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.