Abstract:
Methods and apparatus for high speed workpiece handling are provided. The method for workpiece handling includes removing a workpiece from a first cassette with a first robot, transferring the workpiece from the first robot directly to a second robot without transferring the workpiece to a transfer station, placing the workpiece on a workpiece holder at a processing station with the second robot, and transferring the workpiece from the workpiece holder to the first cassette with the first robot following processing. End effectors of the first and second robots may each have a plurality of vertical positions for efficient workpiece handling. Displacement error and rotational error of the workpiece may be sensed and corrected without use of a transfer station. The methods and apparatus may be used for handling semiconductor wafers.

Description:
FIELD OF THE INVENTION 
     This invention relates to high speed object handling and, more particularly, to methods and apparatus for moving workpieces, such as semiconductor wafers, in a vacuum chamber for high processing throughput. 
     BACKGROUND OF THE INVENTION 
     The processing of semiconductor wafers for the manufacture of microelectronic circuits involves processing tools for performing a large number of processing steps. The processing steps are usually performed in a vacuum chamber. The processing tools typically handle and process wafers one at a time in order to optimize control and reproducibility. Such processing tools utilize automated wafer handling systems. 
     The throughput of the processing tools is an important factor in achieving low cost manufacture. The overall throughput is a function of both the processing time and the efficiency of automated wafer handling. Wafer handling involves introduction of the wafers in a cassette or other wafer holder into the vacuum chamber, typically through a load lock, transfer of the wafers from the cassette to a processing station, return of the wafers to the cassette following processing and removal of the cassette from the load lock. Some processes, such as for example ion implantation, may require a specified wafer orientation during processing. In addition, a wafer may be damaged or destroyed if it is inadvertently displaced from its normal position in the wafer handling system. Accordingly, wafer handling systems may utilize wafer position sensing and correction systems. Some of the processing and wafer handling operations may be performed concurrently to achieve efficient operation and high throughput. Accordingly, careful design of wafer handling systems is required. A variety of wafer handling techniques are known in the prior art. 
     In one prior art system disclosed in U.S. Pat. No. 5,486,080, issued Jan. 23, 1996 to Sieradzki, a pair of robot arms transfers wafers from a cassette to a transfer station and then to a processing station. After wafers in a first cassette have been processed, the robots reverse their respective roles and begin processing wafers in a second cassette, while the load lock of the first cassette is vented and the first cassette is replaced with a new cassette. 
     In another prior art system disclosed in U.S. Pat. No. 6,114,705, issued Sep. 5, 2000 to Leavitt et al., robot arms transfer wafers directly from a cassette to a processing station. Wafer position errors are sensed with a camera, and displacement errors are corrected by the robot arm as it places the wafer on a wafer holder at the processing station. The wafer holder at the processing station rotates to correct rotational error. The wafer is placed in a different cassette after processing. 
     Current wafer fabrication processes typically require that wafers be returned to the same cassette following processing for increased process control. This requirement increases the difficulty of achieving high throughput. 
     All of the known prior art wafer handling systems have had one or more drawbacks, including but not limited to relatively low throughput and high cost. Accordingly, there is a need for improved methods and apparatus for high speed handling of workpieces such as semiconductor wafers. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, a method is provided for workpiece handling. The method comprises: (a) removing a workpiece from a first cassette with a first robot, (b) transferring the workpiece from the first robot directly to a second robot without transferring the workpiece to a transfer station, (c) placing the workpiece on a workpiece holder at a processing station with the second robot, and (d) transferring the workpiece from the workpiece holder to the first cassette with the first robot following processing. 
     The method may further comprise (e) repeating steps (a)-(d) for remaining workpieces in the first cassette. In addition, the method may further comprise (f) reversing roles of the first robot and the second robot and repeating steps (a)-(e) for workpieces in a second cassette. The method may be used for handling semiconductor wafers, but is not limited to wafer handling. 
     The method may further comprise sensing displacement error and rotational error of the workpiece relative to reference values and correcting the displacement error and the rotational error of the workpiece without use of a transfer station. The displacement error may be corrected with the second robot, and the rotational error may be corrected with the workpiece holder. The displacement error and the rotational error may be sensed by acquiring an image of the workpiece to provide image data and processing the image data to determine the displacement error and rotational error relative to the reference values. 
     The first and second robots may each include an end effector that is laterally and vertically movable. In some embodiments, the end effectors of the first and second robots each have a plurality of discrete vertical positions. 
     The discrete vertical positions of the first and second robots permit the second robot to position a second workpiece above the workpiece holder before a first workpiece is removed from the workpiece holder by the first robot. The workpiece can be transferred from the first robot directly to the second robot by the first robot positioning the workpiece over an end effector of the second robot and the second robot lifting the workpiece from the first robot. 
     According to another aspect of the invention, a workpiece handling system is provided. The workpiece handling system comprises a vacuum chamber, a processing station within the vacuum chamber, first and second load locks controllably coupled to the vacuum chamber through first and second isolation valves, respectively, and first and second robots within the vacuum chamber for transferring workpieces to and between the load locks and the processing station. The first and second robots each have a robot arm that is vertically movable to different levels and that is laterally movable to permit direct robot-to-robot transfer of workpieces without transferring workpieces to a transfer station. The workpiece handling system may be used for handling semiconductor wafers, but is not limited to wafer handling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
     FIG. 1 is a schematic top view of a prior art wafer handling system; 
     FIG. 2A is a schematic top view of a wafer handling system in accordance with an embodiment of the invention, showing a wafer being removed from a cassette; 
     FIGS. 2B and 2C are schematic side views of the first and second robots in the wafer handling system of FIG. 2A, showing the wafer being removed from the cassette; 
     FIG. 3 is a schematic top view of the wafer handling system of FIG. 2A, showing a wafer removed from the cassette; 
     FIG. 4A is a schematic top view of the wafer handling system of FIG. 2A, showing robot-to-robot wafer transfer; 
     FIGS. 4B and 4C are side views of the first and second robots of FIG. 4A, showing robot-to-robot wafer transfer; 
     FIG. 5A is a schematic top view of the wafer handling system of FIG. 2A, showing placement of a first wafer on a wafer holder at a processing station and removal of a second wafer from the cassette; 
     FIGS. 5B and 5C are schematic side views of the first and second robots, showing wafer placement on the wafer holder and wafer removal from the cassette; 
     FIG. 6A is a schematic block diagram of the wafer handling system of FIG. 2A, showing transfer of one wafer from the wafer holder by the first robot and positioning of another wafer above the wafer holder by the second robot; 
     FIGS. 6B-6E are side views of the first and second robots, showing exchange of wafers on the wafer holder; 
     FIG. 7 is a schematic block diagram of an embodiment of a wafer orientation system that may be utilized in the wafer handling system of FIG. 2A; 
     FIGS. 8A and 8B show a flow chart of a wafer handling process in accordance with an embodiment of the invention; and 
     FIG. 9 is a schematic block diagram of the wafer handling system of FIG.  2 A. 
    
    
     DETAILED DESCRIPTION 
     A prior art wafer handling system of the type disclosed in the aforementioned U.S. Pat. No. 5,486,080 is shown in FIG. 1. A vacuum chamber  10  contains a first robot  12 , a second robot  14 , a transfer station  16  and a processing station  18 . Load locks  20  and  22  communicate with vacuum chamber  10  through isolation valves  24  and  26 , respectively. Cassettes  30  and  32 , each holding a plurality of semiconductor wafers, are placed in the respective load locks  20  and  22 . 
     In operation, a wafer is removed from cassette  30  by first robot  12  and is placed on transfer station  16 . Transfer station  16  includes a wafer support and a position sensor, which determines the displacement error and rotational error of the wafer with respect to reference values. Position sensing typically requires rotating the wafer with respect to the sensor. The rotational error is corrected by an appropriate rotation of the wafer support at transfer station  16 . The wafer is then transferred to processing station  18  by second robot  14  with an appropriate adjustment to eliminate displacement error. After processing, the wafer is returned to cassette  30  by first robot  12 . 
     Referring now to FIG. 2A, a top view of a workpiece handling system in accordance with an embodiment of the invention is shown. The workpiece handling system is advantageously used for handling semiconductor wafers, but is not limited to wafer handling. Hereinafter, the system is called a “wafer handling system.” 
     The wafer handling system includes a first robot  62 , a second robot  64  and a processing station  68  positioned within a vacuum chamber  60 . The wafer handling system further includes load locks  70  and  72  that communicate with vacuum chamber  60  through isolation valves  74  and  76 , respectively. Cassettes  80  and  82 , each holding a plurality of semiconductor wafers, may be positioned in the respective load locks  70  and  72 . It will be understood that different types of wafer holders may be utilized within the scope of the invention. Load locks  70  and  72  are provided with elevators  84  and  86 , respectively, (FIG. 9) for indexing cassettes  80  and  82  upwardly and downwardly with respect to robots  62  and  64 . 
     Each of robots  62  and  64  includes a fixed base  100 , a first arm section  102 , a second arm section  104 , and an end effector  110 . End effector  110  may be a U-shaped element that is dimensioned for supporting a semiconductor wafer of specified diameter. Arm sections  102  and  104  and end effector  110  are pivotally connected to each other and to base  100  to permit lateral movement as well as extension and retraction of end effector  110 . Arm sections  102  and  104  and end effector  110  of robot  62  constitute a robot arm  112 . Similar components of robot  64  constitute a robot arm  116 . 
     Robot arms  112  and  116  are vertically movable, typically to a plurality of discrete positions or levels. In the embodiment of FIG. 2A, each robot arm is vertically movable to one of three positions. However, the invention is not limited to three positions, and more or fewer positions may be utilized. 
     Robots  62  and  64  are positioned in vacuum chamber  60  to permit access to load locks  70  and  72 , respectively, to permit robot-to-robot transfer of wafers and to permit access to processing station  68 . The wafer handling system may include a wafer positioning system  130  (FIG.  9 ), as described below in connection with FIG.  7 . 
     Processing station  68  includes a wafer holder  120 . By way of example, wafer holder  120  may include an electrostatic wafer clamp as known in the art and may include wafer lift pins  122  for lifting a wafer above a clamping surface. The wafer lift pins  122  permit a wafer to be placed on wafer holder  120  by robots  62  and  64  and to be removed from wafer holder  120  following processing. Wafer holder  120  may further include a rotation mechanism for rotating the wafer to eliminate rotational error as described below. Processing station  68  may be part of an ion implantation system, for example. In this embodiment, wafer holder  120  may pivot the wafer to a vertical orientation for ion implantation. However, the wafer handling system described herein is not limited to ion implantation and may be utilized with different types of processing systems. 
     Load locks  70  and  72  permit cassettes  80  and  82  to access vacuum chamber  60  without requiring vacuum chamber  60  to be vented to atmospheric pressure. In particular, the respective isolation valves  74  and  76 , may be closed, thereby isolating load locks  70  and  72  from vacuum chamber  60 . Load locks  70  and  72  may be vented to atmospheric pressure, and cassettes  80  and  82  may be exchanged by a system operator or by a robot (not shown). The load lock is then sealed and vacuum pumped, and the isolation valve is opened, thereby providing access to vacuum chamber  60 . Typically, one cassette may be processed while another cassette of processed wafers is replaced with a cassette of unprocessed wafers. The wafer handling system includes a vacuum pumping system  90  (FIG. 9) for controlling the pressure level in vacuum chamber  60  and load locks  70  and  72 . 
     As shown in FIG. 9, the wafer handling system includes a controller  140  for controlling the elements of the wafer handling system. Controller  140  may be coupled by a suitable control bus  142  or by separate connections to robots  62  and  64 , elevators  84  and  86 , isolation valves  74  and  76 , vacuum pumping system  90 , wafer holder,  120  and wafer positioning system  130 . Controller  140  may be a general purpose computer, such as a personal computer (PC), or a special purpose controller. Controller  140  controls the elements of the wafer handling system to perform wafer handling as described herein. 
     FIGS. 2A-6E illustrate basic operations of the wafer handling system as described below. The basic operations and other operations are combined to illustrate an example of a wafer handling process, as shown in FIGS. 8A and 8B and described below. 
     FIGS. 2A-2C and  3  illustrate removal of a wafer  150  from cassette  80  by first robot  62 . As shown in FIGS. 2A and 2B, robot arm  112  of robot  62  is moved, if necessary, to its lowest vertical position, and end effector  110  is extended into cassette  80  under wafer  150 . Robot arm  112  is then raised to an intermediate vertical position, as shown in FIG. 2C, so as to lift wafer  150  from cassette  80 , and end effector  110  carrying wafer  150  is withdrawn from cassette  80  to the position shown in FIG.  3 . To return wafer  150  to cassette  80 , the operations described above are performed in reverse. That is, robot arm  112  carrying wafer  150  is moved, if necessary, to its intermediate vertical position, and end effector  110  carrying wafer  150  is extended into cassette  80 . Robot arm  112  is then moved to its lowest vertical position, as shown in FIG. 2B, and end effector  110  is withdrawn from cassette  80 , with wafer  150  remaining in cassette  80 . The use of vertically movable robot arms  112  and  116  avoids the need for multiple indexing of cassette elevators  84  and  86  to remove and replace wafers in cassettes  80  and  82 . Nonetheless, cassette elevators having indexing capability may be utilized if desired. 
     Transfer of wafer  150  from robot  62  to robot  64  is illustrated in FIGS. 4A-4C. The transfer may take place at a position between robots  62  and  64 . As shown in FIG. 4B, end effector  110  of robot  62  extends wafer  150  over end effector  114  of robot  64 . End effectors  110  and  114  are shaped and positioned such that each end effector may be moved vertically without interference with the other end effector. In the embodiment of FIG. 4A, U-shaped end effectors  110  and  114  may have the same size and shape and are offset laterally to permit unrestricted vertical movement. In this embodiment, wafer  150  is not centered with respect to one or both of end effectors  110  and  114 . In another embodiment, end effectors  110  and  114  have different shapes and/or dimensions to permit centering of wafer  150  with respect to each end effector. For example, the spacing between the legs of the U-shaped end effectors may be different in end effectors  110  and  114  to permit both end effectors to be positioned under wafer  150  without interference. 
     In operation, robot arm  112  of robot  62  may be positioned at its intermediate vertical position, and robot arm  116  of robot  64  may be positioned at its lowest vertical position. Robot arm  112  of robot  62  is then extended to position wafer  150  over end effector  114  of robot  64 . Robot arm  116  of robot  64  is raised to its highest vertical position, as shown in FIG. 4C, such that end effector  114  lifts wafer  150  from end effector  110 . The transfer is then complete. Robot arm  112  can be retracted to perform other operations, and robot arm  116  can move wafer  150  to processing station  68 . 
     By utilizing robot-to-robot wafer transfer, the wafer handling system avoids the need for a transfer station. In cases where a wafer positioning system is utilized, wafer position sensing and correction are performed without the need for a transfer station. A suitable wafer positioning system is described below. 
     A further basic operation, including transfer of wafer  150  by robot  64  to wafer holder  120  and removal of another wafer from cassette  80 , is shown in FIGS. 5A-5C. As shown in FIG. 5B, robot arm  116  of robot  64  is positioned at its highest vertical position with wafer  150  located over wafer holder  120 . The lift pins in wafer holder  120  are raised above the platen surface for receiving wafer  150 . The lift pins are located on wafer holder  120  to avoid interference with end effector  114  as robot arm  116  is raised and lowered. Robot arm  116  is then moved to its lowest vertical position, as shown in FIG. 5C, thereby transferring wafer  150  to the lift pins of wafer holder  120 . Robot arm  116  may then be retracted from wafer holder  120 , and lift the lift pins may be lowered to thereby position wafer  150  on the wafer clamping surface of wafer holder  120 . 
     Concurrently with positioning wafer  150  on wafer holder  120 , robot  62  may remove a second wafer  152  from cassette  80 . As shown in FIG. 5B, robot arm  112  is moved at its lowest vertical position and end effector  110  is positioned under wafer  152 . Robot arm  112  is then raised to its intermediate vertical position, as shown in FIG. 5C, and wafer  152  is removed from cassette  80 . 
     An exchange of wafers on wafer holder  120  is illustrated in FIGS. 6A-6E. Robot  62  removes wafer  150  from wafer holder  120  following processing, and robot  64  then places wafer  152  on wafer holder  120  for processing. As shown in FIG. 6B, robot arm  112  of robot  62  is positioned at its lowest vertical position, and robot arm  116  carrying wafer  152  is positioned at its highest vertical position. The lift pins of wafer holder  120  are raised, thereby lifting wafer  150  from the clamping surface of wafer holder  120 . Robot arm  112  of robot  62  is extended to position end effector  110  between wafer  150  and the clamping surface of wafer holder  120 , and robot arm  116  of robot  64  is extended to position wafer  152  above wafer holder  120 , as shown in FIG.  6 C. Robot arm  112  is then retracted, so as to remove wafer  150  from wafer holder  120 . When wafer  150  is clear of wafer holder  120 , robot arm  116  of robot  64  its lowered to its lowest vertical position, and wafer  152  is positioned on the lift pins of wafer holder  120 , as shown in FIG.  6 D. Robot arm  116  of robot  64  is then retracted, and the lift pins of wafer holder  120  are lowered to thereby position wafer  152  on the clamping surface of wafer holder  120 . The exchange of wafers is thereby completed. 
     Ion implantation typically requires orientation of the wafer with respect to the ion beam in order to control channeling effects. Because the positions of wafers in cassettes are not tightly controlled and because wafer handling systems may produce undesired wafer movements as the wafer is transferred from the cassette to the processing station, wafer positioning systems may be used to sense and correct wafer position errors. Such position errors may include displacement errors (eccentricity) and rotational errors relative to desired positions. 
     An embodiment of wafer positioning  130  system suitable for use in the wafer handling system of FIG. 2A is shown in FIG. 7. A camera  180  acquires an image of wafer  150  on end effector  114  of robot  64 . An optional light source  184  may illuminate wafer  150  from below as shown. In another embodiment, a light source (not shown) may illuminate wafer  150  from above. Image data from camera  180  is supplied to the controller  140  of the wafer handling system. Image analysis analysis software in controller  140  analyzes the image data to determine displacement error of wafer  150  with respect a desired position on end effector  114  and rotational error of wafer  150  with respect to a desired rotational position. Controller  140  supplies a control signal to motor  186  of robot  64  to correct sensed displacement error as wafer  150  is placed on wafer holder  120 . In particular, the placement of wafer  150  on wafer holder  120  is adjusted to compensate for the sensed displacement error. In addition, controller  140  supplies a control signal to a motor  188  in wafer holder  120 . After wafer  150  is positioned on wafer holder  120 , wafer holder  120  is rotated by motor  188  to compensate for sensed rotational error. Additional details regarding the wafer positioning system are disclosed in the aforementioned U.S. Pat. No. 6,144,705, which is hereby incorporated by reference. As an alternative, an image of wafer  150  can be obtained while wafer  150  is positioned on robot  62 . However, any slippage of wafer  150  during and after robot-to-robot transfer would not be taken into account in the position correction process. 
     The wafer positioning system is not limited to image sensing of wafer position errors. Sensing techniques including but not limited to RF electric field sensing, magnetic resonance sensing, laser scanning, and sensing with photodetector arrays, for example, may be utilized for position sensing. Furthermore, some processing systems may be tolerant of wafer position errors and may not require the use of a wafer positioning system. In addition, systems for handling workpieces other than semiconductor wafers may or may not require accurate workpiece positioning and thus may or may not require workpiece positioning systems. 
     A flow chart of an example of a process for handling wafers in cassettes  80  and  82  is shown in FIGS. 8A and 8B. The process may be controlled by software in controller  140 . The process involves basic operations shown in FIGS. 2A-2C,  3 ,  4 A- 4 C,  5 A- 5 C,  6 A- 6 E and  7  and described above. In step  200 , a first wafer is transferred from cassette  80  to processing station  68 . The transfer of the first wafer in step  200  involves: (1) removal of the wafer from cassette  80 , as shown in FIGS. 2A-2C and  3  and described above, (2) robot-to-robot transfer from robot  62  to robot  64 , as shown in FIGS. 4A-4C and described above, (3) wafer position sensing and correction, as described above in connection with FIG. 7, and (4) and transfer of the wafer to wafer holder  120  by robot  64 , as described above in connection with FIGS. 5A-5C. 
     When the first wafer has been loaded into the processing station  68 , the process proceeds to step  202 . In step  202 , robot  62  removes wafer n (where n is a wafer number or index) from cassette  80 , as shown in FIGS. 2A-2C and  3  and described above. In step  204 , robot  62  transfers wafer n to robot  64 , as shown in FIGS. 4A-4C and described above. Also in step  204 , wafer positioning system  130  senses the displacement error and rotational error of wafer n, as described above in connection with FIG.  7 . In step  206 , robot  62  removes wafer n−1 from wafer holder  120 , as described above in connection with FIGS. 6A-6C. In step  208 , robot  64  places wafer n on wafer holder  120 , as described above in connection with FIGS. 6A,  6 D and  6 E, and the placement is adjusted to correct the sensed displacement error of wafer n, as described above in connection with FIG.  7 . In step  210 , wafer holder  120  rotates to correct the sensed rotational error of wafer n, as described above in connection with FIG.  7 . In step  212 , robot  62  returns wafer n−1 to cassette  80 , as described above in connection with FIGS. 2A-2C. In step  216 , a determination is made as to whether wafer n is the last wafer in the cassette (n=n max ). If wafer n is not the last wafer, wafer index n is incremented in step  218  and elevator  84  indexes cassette  80  for access to the next wafer. The process then returns to step  202  to remove the next wafer from cassette  80 . The next wafer is processed in the same manner. If a determination is made in step  216  that wafer n (currently on the wafer holder) is the last wafer, wafer n is unloaded from the processing station  68  and returned to cassette  80  in step  220 , as described above in connection with steps  206  and  212 . 
     With the completion of step  220 , all wafers in cassette  80  have been processed and returned to cassette  80 . In step  230 , the roles of robots  62  and  64  are reversed in the wafer handling process to permit processing of wafers in cassette  82 . In step  232 , the process of steps  200 - 220  is repeated for cassette  82 , and cassette  80  can be exchanged for a new cassette containing unprocessed wafers. The process thus continues with processing of alternate cassettes and with wafers being returned to the cassettes from which they were removed. 
     The workpiece handling system described herein is advantageously used for handling semiconductor wafers and other disk-shaped workpieces. However, the invention is not limited in this regard and may be utilized for handling other types of workpieces, with appropriate modifications to the system elements that support and move the workpieces. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.