Patent Publication Number: US-2023133957-A1

Title: Z-rotary wafer positioning stage comprising a lift-pin mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Application No. 21205342.5, filed in the European Patent Office on Oct. 28, 2021, which is expressly incorporated herein in its entirety by reference thereto. 
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor processing equipment and more particularly to a Z-rotary wafer positioning stage that includes a lift-pin mechanism, e.g., adapted to raise a wafer off the stage surface for wafer removal. The present invention also relates to a method of operating the Z-rotary wafer positioning stage, e.g., to toggle the lift-pin mechanism between a first position, in which the wafer chuck holds a wafer, and a second position, in which the wafer is lifted off the stage. 
     BACKGROUND INFORMATION 
     Wafer positioning stages are often used to hold wafers in place while they are being tested. Most wafers are held in place by applying a small amount of vacuum to the backside of the wafer. Once the processing is completed, the wafer needs to be unloaded from the wafer chuck. To that end, the stage includes a lift-pin mechanism configured to be actuated in a Z-direction to raise the wafer off the wafer chuck surface so that a wafer removing tool can be inserted under the wafer to remove it from the stage. 
     European Patent Document No. 4 047 250 describes a Z-rotary wafer positioning stage including a stationary unit, a Z-unit, a linear motor for fine upward and downward movements of the Z-unit relative to the stationary unit, a rotatable unit with a wafer chuck rotatably mounted inside the Z-unit, and a rotary motor arranged to rotate the rotary unit relative to the Z-unit. The rotary unit includes a lift-pin mechanism actuatable by a dedicated magnetic Z-actuator to move lift-pins from a retracted position to an extended position and vice versa. 
     Another example of lift-pin mechanism which relies on a dedicated actuator is described, for example, in U.S. Patent Application Publication No. 2017/0133260. 
     High-end wafer positioning stages with lift-pin mechanisms require at least a first actuator and a rotary encoder to control fine angular movement of the stage, a second actuator and a linear encoder to control fine vertical movements of the stage, and a third actuator to actuate the lift-pin mechanism to move lift-pins from a retracted position to an extended position and vice versa. 
     At least three independent actuators are therefore needed which increase the complexity, the footprint, and the cost of such high-end wafer positioning stages. 
     SUMMARY 
     Example embodiments of the present invention provide a cost-effective Z-rotary positioning stage, provide a Z-rotary positioning stage having a reduced footprint, and/or provide a Z-rotary positioning stage with a simplified architecture to achieve high-reliability. 
     According to an example embodiment of the present invention, a Z-rotary wafer positioning stage includes a stationary unit, a Z-unit actuatable upwardly and downwardly relative to the stationary unit, a rotary unit with a wafer chuck rotatably mounted in the Z-unit, a rotary motor arranged to rotate the rotary unit relative to the Z-unit, and a lift-pin mechanism including lift-pins aligned with through-holes extending through the wafer chuck. The lift-pin mechanism is configured to be toggled between a first stable position, in which distal ends of the lift-pins are below or at the level of the wafer chuck surface, and a second stable position, in which the distal ends of the lift-pins extend above the wafer chuck surface. The Z-rotary wafer positioning stage further includes at least one upper magnet to hold the lift-pin mechanism in the first stable position, at least one lower magnet to hold the lift-pin mechanism in the second stable position, and at least one electromagnet configured to be energized to toggle the lift-pin mechanism from the second stable position to the first stable position. The electromagnet may also be configured to be energized to toggle the lift-pin mechanism from the first stable position to the second stable position. 
     According to example embodiments, the upper and lower magnets are mounted on the rotary unit. 
     According to example embodiments, the upper magnet is arranged to magnetically connect to an upper part of the lift-pin mechanism to maintain the lift-pins in the first stable position. The lower magnet is arranged to magnetically connect to a lower part of the lift-pin mechanism to maintain the lift-pins in the second stable position. 
     According to example embodiments, the electromagnet is mounted on the stationary unit and is arranged to come into contact with the lower part of the lift-pin mechanism and to be energized to maintain the lift-pin mechanism in contact with the electromagnet to disconnect the lower magnet from the lift-pin mechanism to toggle the lift-pin mechanism from the second stable position to the first stable position. 
     According to example embodiments, the rotary unit includes three upper magnets and three lower magnets. The lift pin-mechanism includes three lift-pins and three lift-pin holders to hold respective lift-pins. The lift-pin holders are made of a magnetic material and are shaped so as to be in contact with respective: (a) upper magnets to maintain the lift-pin mechanism in the first stable position, or (b) lower magnets to maintain the lift-pin mechanism in the second stable position. 
     According to example embodiments, the stationary unit includes three electromagnets arranged to magnetically connect to the lower part of the lift-pin mechanism. 
     According to example embodiments, the lift-pin mechanism includes a ring having a top side arranged to magnetically connect to the lower magnets and a bottom side arranged to magnetically connect to the electromagnets. 
     According to example embodiments, the stationary unit includes a cylindrical guide around which is mounted the rotary unity. One or more hard stops are connected to the inner wall of the cylindrical guide and are shaped to limit the amplitude of the upward moment of the lift-pin mechanism. 
     According to example embodiments, the upper magnets and lower magnets are mounted on at least one upper magnet support and at least one lower magnet support, respectively. The axial position of the upper and/or lower magnets are adjustable relative to their supports so as to modify the amplitude of the axial displacement of the lift-pin mechanism between the first stable position and second stable position. 
     According to an example embodiment of the present invention, a method of operating a Z-rotary wafer positioning stage includes: actuating the Z-unit to move the rotary unit downwardly to bring the lift-pin mechanism in contact with the electromagnet; and energizing the electromagnet while actuating the Z-unit to move the rotary unit upwardly to produce a holding force on the lift-pin mechanism exceeding the magnetic force of the lower magnet in order to disconnect the lower magnet from the lift-pin mechanism. 
     According to example embodiments, the electromagnet is also energized during the actuating to firmly hold the lift-pin mechanism to counteract the attracting force of the lower magnet on the lift-pin mechanism as the lower magnet is about to contact a lower part of the lift-pin mechanism. 
     According to example embodiments, the electromagnet is energized during the whole transition phase from the second stable position to the first stable position to firmly hold the lift-pin mechanism to counteract the attracting force of the upper magnet on the lift-pin mechanism as the upper magnet approaches and is about to contact an upper part of the lift-pin mechanism. 
     Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended schematic Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1   a  to  1   d    illustrate a sequence of movements of the Z-rotary wafer positioning stage according to an example embodiment of the present invention, when the lift-pin mechanism is toggled from the first stable position to the second stable position and vice versa. 
         FIG.  2    illustrates the main steps to toggle the lift-pin mechanism of the Z-rotary wafer positioning stage between the first stable position and second stable position. 
         FIG.  3    is a perspective view of the Z-rotary wafer positioning stage. 
         FIG.  4    is another perspective view of the Z-rotary water positioning stage without the wafer chuck. 
         FIG.  5    is a top view of the Z-rotary wafer positioning stage. 
         FIG.  6    is a perspective cross-sectional view of the Z-rotary wafer positioning stage. 
         FIG.  7    is another perspective cross-sectional view of the Z-rotary wafer positioning stage. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   a   , the Z-rotary wafer positioning stage  10  includes a stationary unit  20 , a Z-unit  30  moveable upwardly and downwardly relative to the stationary unit  20 , a rotary unit  40  rotatably mounted in the Z-unit  30 , and a rotary motor  70  arranged to rotate the rotary unit  40  relative the Z-unit  30  about a rotation axis. The rotary unit  40  includes a chuck support  48  (see, e.g.,  FIG.  4   ) holding a wafer chuck  42 . The wafer chuck  42  includes through-holes  43  extending through the chuck from a top surface to a bottom surface. The through-holes  43  accommodate lift-pins  52  of a lift-pin mechanism  50 , which is described in more detail below. 
     As illustrated in  FIGS.  3  to  7   , the Z-unit  30  is actuated along the rotation axis by a linear motor  60 . To that end, the Z-unit  30  includes a casing  32  having on two opposite sides a magnetic track extending along the vertical direction and a first and second set of coils disposed on the stationary unit  20  to face respective magnetic track. 
     In a variant, the Z-rotary wafer positioning stage  10  may include two concentric rotary motors as described in European Patent Document No. 1 748 537, which is expressly incorporated herein in its entirety by reference thereto. One of the two rotary motors is adapted to rotate the stage about the rotation axis, and the other of the two rotary motors is adapted to move the stage along the rotation axis. 
     As illustrated, for example, in  FIG.  5   , the casing  32  of the Z-unit  30  includes two rails  34  extending along the vertical direction on both sides of a respective magnetic track. Each rail  34  is slidably engaged with a corresponding carriage  22  mounted on the stationary unit  20  to guide the upward and downward movements of the Z-unit  30 . 
     Referring to  FIGS.  5  to  7   , the lift-pin mechanism  50  of the Z-rotary wafer positioning stage  10  includes three lift-pin holders  54  angularly spaced apart from each other by 120°. Each lift-pin holder  54  has a first portion  54   a  extending radially within a plane perpendicular to the rotation axis of the rotary unit  40  and a second portion  54   b  extending perpendicularly from an end portion of the first portion  54   a  to form a L-shaped lift-pin holder. A lift-pin  52  is mounted at a distal end of the first portion  54   a  partly inside the corresponding through-hole  43  of the wafer chuck  42 . 
     The lift-pin mechanism  50  further includes a ring  56  on which an end of the second portion  54   b  of each pin holder  52  is connected to form an integral lift-pin mechanism. A vertical rail  55  is mounted on the second portion  54   b  of each pin holder  54  and is slidably engaged with a carriage  47 . Pin holder supports  46 , connected to the rotary unit  40 , support the carriage  47  of respective lift-pin holders  54 . 
     Referring to  FIG.  6   , the rotary unit  40  is arranged inside the casing  32  of the Z-unit  30  to be rotated by a rotary motor  70 . An upper and a lower bearing  80 ,  84  are mounted between the casing  32  and the rotary unit  40  with the outer rings  82   a ,  86   a  and inner rings  82   b ,  86   b  of respective bearings  80 ,  84  forming an integral part with the casing  32  and with the rotary unit  40  respectively. 
     The stationary unit  20  further includes a cylindrical guide  24  around which is rotatably mounted the rotary unit  40 . Several hard stops  28 , for example, three hard stops spaced apart from each other by 120°, are connected to the inner wall of the cylindrical guide  24 . These hard stops  28  extend upwardly and include a bent-over distal portion  28   a  arranged to come into contact with a top side of the ring  56  to limit the amplitude of the upward moment of the lift-pin mechanism  50 . 
     One or more upper magnets  44   a  are mounted on the rotary unit  40  to hold the lift-pin mechanism  50  in a first stable position, in which the distal ends of the lift-pins  52  are below or at the level of the wafer chuck surface. In this first stable position, wafers may be held on the wafer chuck  42  for wafer positioning. One or more lower magnets  44   b  are mounted on the rotary unit  40  to hold the lift-pin mechanism  50  in a second stable position, in which the distal ends of the lift-pins  52  extend above the wafer chuck surface. In this second position, the wafer is lifted off the wafer chuck  42  for wafer removal. One or more electromagnet  58  are mounted on the stationary unit  20  to toggle the lift-pin mechanism  50 , as described in more detail below, from the first stable position to the second stable position in vice versa. 
     In the example embodiment illustrated in  FIGS.  3  to  7   , three upper magnets  44   a  are mounted on upper magnet supports  45   a  on an upper part of the rotary unit  40  (see, e.g.,  FIG.  4   ). These upper magnets  44   a  are spaced apart from each other by 120° and are in contact with the first portion  54   a  of respective lift-pin holder  54  when the lift-pin mechanism  50  is in the first stable position. Three lower magnets  44   b  are mounted on lower magnet supports  45   b  (see, e.g.,  FIG.  5   ). These lower magnets  44   b  are in contact with the top side of the ring  56  when the lift-pin mechanism  50  is in the second stable position. 
     The lift-pin holders  54  and the ring  56  are made of a ferromagnetic metal, such as iron, to be magnetically connected to the upper and lower magnets  44   a ,  44   b  in respective first and second stable positions. 
     The upper and lower magnets  44   a ,  44   b  are, for example, engaged with a thread on the upper and lower support magnets  45   a ,  45   b  so that their axial position may be adjusted in order to modify the amplitude of the axial movement of the lift-pin mechanism  50  between the first and second positions. 
     As illustrated in  FIGS.  6  and  7   , three electromagnets  58  are mounted on an upper circular edge of the cylindrical guide  24  of the stationary unit  20 . The electromagnets  58  are spaced apart from each other by 120° and are arranged to come into contact with a bottom side of the ring  56 . These electromagnets  58  are configured to be energized to toggle the lift pin mechanism  50  from the first to the second stable positions while operating the Z-rotary wafer positioning stage  10  as described below with reference to  FIGS.  1   a  to  1   d    and  2 . 
     Referring to  FIG.  1   a   , the lift pin-mechanism  50  is in its first stable position, in which the top surface of the lift pins  52  is set substantially at the level of the surface of the wafer chuck  42 . In this position, the upper magnets  44   a  of the rotary unit  40  are magnetically connected to an upper part of respective lift-pin holders  54 , whereas the bottom side of the ring  56  of the lift-pin mechanism  50  is distant from the electromagnets  58  connected to the stationary unit  20 . 
     To toggle the lift-pin mechanism  50  in its second stable position, the following actions are preformed. 
     Step  100  includes driving the linear motors  60  to actuate the rotary unit  40  downwardly to bring the bottom side of the ring  56  in contact with the electromagnets  58  as illustrated in  FIG.  1   b   , whereupon the rotary unit  40  is further actuated downwardly to disconnect the upper magnets  44   a  from respective pin holders  54  as illustrated in  FIG.  1   c   . This produces a relative motion between the rotary unit  40  and the lift pin mechanism. 
     Step  102  includes turning on the electromagnets  58  before the lower magnets  44   b  of the rotary unit  40  connect to the ring top side of the lift-pin mechanism  50  to firmly hold the ring  56  in order to counteract the attracting force of the lower magnets  44   b  on the lift-pin mechanism  50  as they approach and are about to contact the ring top side of the lift-pin mechanism  50 . 
     Step  104  includes turning off the electromagnets  58  as soon as the lower magnets  44   b  contact the ring top side of the lift-pin mechanism. The lift-pin mechanism is thus in the second stable position, in which the lift-pins  52  protrude from their corresponding through-holes  43  above the surface of the wafer chuck  42 . In this position, the rotary unit  40  can be moved upwardly, downwardly, and/or in rotation. 
     Step  106  includes turning on again the electromagnets  58  while actuating the Z-unit  30  to move the rotary unit  40  upwardly to produce a holding force on the lift-pin mechanism  50  exceeding the magnetic force of the lower magnet  44   b  in order to disconnect the lower magnets  44   b  from the ring top side of the lift-pin mechanism  50  as illustrated in  FIG.  1   d   . The electromagnets  58  are energized during the whole transition phase from the second stable position to the first stable position to firmly hold the ring  56  of the lift-pin mechanism  50  in order to counteract the attracting force of the upper magnets  44   a  on the lift pin mechanism  50  as they approach and are about to contact the pin holder  54  of the lift pin mechanism  50 . 
     Step  108  includes turning off again the electromagnets  58  as soon as the upper magnets  44   a  contact the lift-pin mechanism  50 . The lift-pin mechanism  50  is thus in the first stable position (see, e.g.,  FIG.  1   a   ), in which the rotary unit  40  can be moved upwardly, downwardly, and/or in rotation. 
     Various modifications and variations to the described exemplary embodiments may be made without departing from the spirit and scope hereof. For example, the position and the number of the upper magnets, lower magnets, and electromagnets may vary according to the configuration of the lift-pin mechanism. 
     In addition, the upper and lower magnets may be mounted on respective upper and lower parts of the stationary unit instead being mounted on the rotary unit. In that case, the upper and lower magnets are arranged to contact, respectively, upper and lower parts of the rotary unit made of ferromagnetic metal to ensure a magnetic coupling in the first and second stable positions of the pin-lift mechanism. 
     LIST OF REFERENCE CHARACTERS 
     
         
           10  Z-Rotary wafer positioning stage 
           20  Stationary unit 
           22  Carriages 
           24  Cylindrical guide 
           28  Hard stop 
           28   a  Bent-over distal portion 
           30  Z-unit 
           32  Casing 
           34  Vertical rails 
           40  Rotary unit 
           42  Wafer chuck 
           43  Through-hole 
           44   a  Upper magnets 
           44   b  Lower magnets 
           45   a  Upper magnet support 
           45   b  Lower magnet support 
           46  Pin holder support 
           47  Carriage 
           48  Chuck support 
           50  Lift-pin mechanism 
           52  Lift-pin 
           54  Lift-pin holder (e.g., L-shaped) 
           54   a  First portion 
           54   b  Second portion 
           55  Vertical rail 
           56  Ring 
           58  Electromagnets 
           60  Linear motor 
           70  Rotary motor 
           80  Upper bearing 
           82   a ,  82   b  Outer and inner ring 
           84  Lower bearing 
           86   a ,  86   b  Outer and inner ring