Patent Publication Number: US-2005137751-A1

Title: Auto-diagnostic method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This patent application claims benefit of U.S. Provisional Patent Application Ser. No. 60/572,474, filed Dec. 5, 2003, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The embodiments of the invention generally relate to automated calibration and diagnostics of a workpiece transfer system.  
      2. Background of the Related Art  
      Semiconductor substrate processing is typically performed by subjecting a substrate to a plurality of sequential processes to create devices, conductors and insulators on the substrate. These processes are generally performed in a process chamber configured to perform a single step of the production process. In order to efficiently complete the entire sequence of processing steps, a number of process chambers are typically coupled to a central transfer chamber that houses a robot to facilitate transfer of the substrate between the surrounding process chambers. A semiconductor processing platform having this configuration is generally known as a cluster tool, examples of which are the families of PRODUCER®, CENTURA® and ENDURA® processing platforms available from Applied Materials, Inc., of Santa Clara, Calif.  
      Generally, a cluster tool consists of a central transfer chamber having a robot disposed therein. The transfer chamber is generally surrounded by one or more process chambers. The process chambers are generally utilized to process the substrate, for example, performing various processing steps such as etching, physical vapor deposition, ion implantation, lithography and the like. The transfer chamber is sometimes coupled to a factory interface that houses a plurality of removable cassettes, substrate storage, each of which houses a plurality of substrates. To facilitate transfer between a vacuum environment of the transfer chamber and a generally ambient environment of the factory interface, a load lock chamber is disposed between the transfer chamber and the factory interface.  
      As line width and feature sizes of devices formed on the substrate have decreased, the positional accuracy of the substrate in the various chambers surrounding the transfer chamber has become paramount to ensure repetitive device fabrication with low defect rates. Moreover, with the increased amount of devices formed on substrates both due to increased device density and larger substrate diameters, the value of each substrate has greatly increased. Accordingly, damage to the substrate or yield loss due to non-conformity because of substrate misalignment is highly undesirable.  
      A number of strategies have been employed in order to increase the positional accuracy of substrates throughout the processing system. For example, the interfaces are often equipped with sensors that detect substrate misalignment within the substrate storage cassette. See, U.S. patent application Ser. No. 09/562,252 filed May 2, 2000 by Chokshi, et al. Positional calibration of robots has become more sophisticated. See, U.S. Pat. No. 6,648,730 issued Nov. 18, 2003 to Chokshi, et al. Additionally, methods have been devised to compensate for substrate misplacement on the end effector of the robot. See, U.S. Pat. No. 5,980,194, issued Nov. 9, 1999 to Freerks, et al., and U.S. Pat. No. 4,944,650, issued Jul. 31, 1990 to T. Matsumoto. Methods have also been developed to compensate for thermal expansion and contraction experienced by the robot as heat is transferred to the robot from hot substrates and from hot surfaces within the process chambers. See, U.S. patent application Ser. No. 10/406,644, filed Apr. 3, 2003 by Freeman et al.  
      A fundamental principal in providing increased accuracy of substrate placement is the calibration process for teaching robot target positions (typically substrate handoff positions) of the robot&#39;s end effector. Most substrate handling robots are taught each handoff position manually. However, manual calibration relies on subjective skills of the operators and often must be performed with the systems chambers open to the FAB environment in order to allow the operator to adequately observe the target and end effector positions. If subsequent calibration is required, the processing system must again be opened, requiring cost and time consuming wipes and pump-down before production may resume.  
      Some machine vision systems supported on the end effector, such as described in U.S. Pat. No. 6,603,117 issued Aug. 5, 2003 to Corrado, et al., allow calibration to be performed under vacuum conditions. However, such systems require batteries, sensors and other electronic components that are not easily adapted for use in vacuum conditions or at elevated temperatures. These options also generally require complicated and significant programming for integration into existing robot motion code software, therefore making the cost of implementation undesirably high.  
      Therefore, there is a need for an improved method for determining a position of a robot and automatically diagnosing performance of the same.  
     SUMMARY OF THE INVENTION  
      Methods for automated calibration and diagnostics of a workpiece transfer system are provided. It is contemplated that the calibration and diagnostic methods described herein may be adapted to benefit other robotic applications. In one embodiment, a method for locating an end effector of a robot includes retrieving a workpiece located at a target location with a robot end effector, passing the workpiece disposed on the end effector through a plurality of sensors, wherein at least one of the sensors changes state in response to a position of at least one of the end effector or workpiece, recording a metric of robot position associated with the sensor change of state, determining an error for an expected metric of the end effector position from the recorded robot position metric and correcting a taught location of the robot for the target position.  
      In another embodiment, a process for monitoring a robotic transfer system is provided that includes monitoring changes in positioning errors in a robotic transfer system. In yet another embodiment, a process for monitoring a robotic transfer system includes detecting a first positional error in a robotic transfer system, and comparing the first positional error to a second positional error in the robotic transfer system.  
      In another embodiment, a method for automated teaching of a robot disposed in a processing system having a sensor based, substrate centerfinder system is provided. In one embodiment, a method for teaching of a robot includes providing a substrate in a known position, transferring the substrate to an end effector of the robot, moving the substrate through a centerfinder, resolving a difference between the substrate center and an expected position of the end effector, and correcting the robot&#39;s motion.  
      In another embodiment, the invention includes locating the position of a robot end effector with respect to a target location, where a substrate located at the target location is retrieved and transported from the target location on a robot end effector, the location of a substrate with respect to the robot end effector is determined as the end effector passes the substrate through a plurality of sensors (e.g., centerfinder) during transport, the location of the end effector with respect to the sensors has been predetermined and the error between the center of the substrate and end effector is used to correct the taught location for the target from which the substrate was received  
      In another aspect of the invention, an apparatus for determining the position of a robot is provided. In one embodiment, the apparatus includes a robot, a substrate aligner, a centerfinder and a calibration substrate, wherein a calibration substrate is utilized to remove error that may be introduced while by the interaction between an end effect or of the robot and the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.  
       FIG. 1  is a plan view of one embodiment of a semiconductor processing system in which a method for determining a position of a robot may be practiced;  
       FIG. 2  is a partial sectional view of the processing system of  FIG. 1 ;  
       FIG. 3  is a plan view of one embodiment of a semiconductor transfer robot;  
       FIG. 4  depicts one embodiment of a wrist of the robot of  FIG. 3 ;  
      FIGS.  5 A-C are flow diagrams methods for determining a position of a robot;  
       FIG. 6  is a schematic illustration of one embodiment of a method for placing a substrate in a predefined (e.g., known) position;  
       FIG. 7  is a sectional view of one embodiment of a centering lift ring;  
       FIG. 8  is a sectional view of one embodiment of a centering end effector;  
       FIG. 9  is a flow diagram of another embodiment of a method for determining a position (i.e., calibrating) of a robot;  
       FIG. 10  is a flow diagram of another embodiment of a method for determining a position (i.e., calibrating) of a robot;  
       FIG. 11  is a flow diagram of one embodiment of a method for reducing error when determining a position (i.e., calibrating) of a robot;  
       FIG. 12  is a flow diagram of another embodiment of a method for determining a position (i.e., calibrating) of a robot;  
       FIG. 13  is one embodiment of an auto-centering calibration wafer;  
      FIGS.  14 A-B are examples of kinematic substrate alignment devices suitable for aligning a substrate in a predefined position;  
      FIGS.  14 C-D are examples of passive substrate alignment devices suitable for aligning a substrate in a predefined position; and  
       FIG. 15  is another embodiment of calibration wafer; 
    
    
      It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
     DETAILED DESCRIPTION  
       FIG. 1  depicts one embodiment of a semiconductor processing system  100  wherein a method for determining a position of a robot  108  may be practiced. The exemplary processing system  100  generally-includes a transfer chamber  102  circumscribed by one or more process chambers  104 , a factory interface  110  and one or more load lock chambers  106 . The load lock chambers  106  are generally disposed between the transfer chamber  102  and the factory interface  110  to facilitate substrate transfer between a vacuum environment maintained in the transfer chamber  102  and a substantially ambient environment maintained in the factory interface  110 . One example of a processing system which may be adapted to benefit from the invention is a CENTURA® processing platform available from Applied Materials, Inc., of Santa Clara, Calif. Although the method for determining the position of a robot is described with reference to the exemplary processing system  100 , the description is one of illustration and accordingly, the method may be practiced wherever the determination or position of a robot is desired in applications where the robot or the robot&#39;s components are exposed to changes in temperature or the reference position of the substrate transferred by the robot is desired.  
      The factory interface  110  generally houses one or more substrate storage cassettes  114 . Each cassette  114  is configured to store a plurality of substrates therein. The factory interface  110  is generally maintained at or near atmospheric pressure. In one embodiment, filtered air is supplied to the factory interface  110  to minimize the concentration of particles within the factory interface and correspondingly substrate cleanliness. One example of a factory interface that may be adapted to benefit from the invention is described in U.S. patent application Ser. No. 09/161,970 filed Sep. 28, 1998 by Kroeker, which is hereby incorporated by reference in its entirety.  
      The transfer chamber  102  is generally fabricated from a single piece of material such as aluminum. The transfer chamber  102  defines an evacuable interior volume  128  through which substrates are transferred between the process chambers  104  coupled to the exterior of the transfer chamber  102 . A pumping system (not shown) is coupled to the transfer chamber  102  through a port disposed on the chamber floor to maintain vacuum within the transfer chamber  102 . In one embodiment, the pumping system includes a roughing pump coupled in tandem to a turbomolecular or a cryogenic pump.  
      The process chambers  104  are typically bolted to the exterior of the transfer chamber  102 . Examples of process chambers  104  that may be utilized include etch chambers, physical vapor deposition chambers, chemical vapor deposition chambers, ion implantation chambers, orientation chambers, lithography chambers and the like. Different process chambers  104  may be coupled to the transfer chamber  102  to provide a processing sequence necessary to form a predefined structure or feature upon the substrate surface.  
      The load lock chambers  106  are generally coupled between the factory interface  110  and the transfer chamber  102 . The load lock chambers  106  are generally used to facilitate transfer of the substrates between the vacuum environment of the transfer chamber  102  and the substantially ambient environment of the factory interface  110  without loss of vacuum within the transfer chamber  102 . Each load lock chamber  106  is selectively isolated from the transfer chamber  102  and the factory interface  110  through the use of a slit valve  226  (see  FIG. 2 ).  
      The substrate transfer robot  108  is generally disposed in the interior volume  128  of the transfer chamber  102  to facilitate transfer of the substrates  112  between the various chambers circumscribing the transfer chamber  102 . The robot  108  may include one or more end effectors, such as a blade, utilized to support the substrate during transfer. The robot  108  may have two blades, each coupled to an independently controllable motor (known as a dual blade robot) or have two blades coupled to the robot  108  through a common linkage.  
      In one embodiment, the transfer robot  108  has a single end effector  130  coupled to the robot  108  by a (frog-leg) linkage  132 . Generally, one or more sensors  116  of a centerfinding system are disposed proximate each of the processing chambers  104  to trigger data acquisition of the robot&#39;s operational parameters or metrics utilized in determining the position of the robot. The data may be used separately or in concert with the robot parameters to determine the reference position of a substrate  112  retained on the end effector  130 . The data may be also be used separately or in concert with the robot parameters to monitor the performance of substrate transfer and/or placement, along with the condition of mechanisms associated and/or affecting substrate transfer within the system.  
      Generally, a bank of sensors  116  are disposed on or in the transfer chamber  102  proximate the passages coupling the transfer chamber  102  to the load lock chamber  106  and process chambers  104 . The sensor bank  116  may comprise one or more sensors that are utilized to trigger data acquisition of robot metrics and/or substrate positional information. From the positional information of the substrate and the robot metrics acquired at the triggering events, the relative position between the substrate and the end effector may be resolved. Thus, by transferring the substrate from a predefined (e.g., known) target location to the end effector, the positional metrics of the robot may be resolved using the relative position relationship acquired using centerfind data, thereby allowing auto-calibration of the robot. Therefore, the robot may be taught to accurately move to taught locations with little or no operator interaction. As the calibration process may be performed while the system  100  is under vacuum, recalibration is much less intrusive compared to traditional calibration methods.  
      In an auto-diagnostic mode, positional error is monitored to determine trends in substrate transfer performance and/or changes in operational functionality of substrate movement devices. In one embodiment, positional error for a series of wafers (or end effector passes) may be monitored at a predefined sensor bank  116 . The change in error over time is indicative of wear or other factors that cause a drift in wafer and/or end effector position. Examples of parameters which may be monitored using this type of auto-diagnostic routine include changes in factory interface robot performance, changes in transfer chamber robot performance, changes in substrate lift mechanisms, and change in system vibration, pressure and temperature, among others. Robot performance that may be monitored include gripper changes, bearing wear, changes in robot link backlash, changes in robot friction, encoder movement, encoder reading drift, changes in motor backlash, and changes in motor performance, among others. Changes in substrate lift mechanisms performance that may be monitored include wear of lift pins, where of lift pin holes and lift pin guides, wear and/or misalignment of lift pin actuation devices, wear and/or misalignment of substrate centering mechanisms, along with other devices and/or objects effecting wafer handoff. Changes in system vibration, pressure and temperature may be monitored to determine if their change may be correlated to drift or other change in positional error over time. Identification of what is causing the change in transfer characteristics may be determined empirically, such that information derived from an analysis of the change in positional error over time may be associated with a particular type or type of system malfunction, wear, change in environmental conditions, and the like.  
      In another embodiment of an auto-diagnostic routine, positional error determined between sensor banks  116  for wafer and/or end effector position may be monitored. The change in error is indicative activities or other events occurring between change of sensor states at each bank of sensors  116 . Functional parameters such as those described above may be monitored using a change in error detected as a substrate is moved between sensor banks. Additionally, this type of monitoring may additionally be utilized to detect changes in substrate location due to environmental factors (changes in chamber geometry due to pressure and/or temperature and/or vibration, and/or slippage of the substrate in the end effectors, among others. For example, a change in pressure and/or temperature in one processing chamber may effect the relative position of the sensor bank to the robot center. In another example, thermal changes may change the length of robot linkages. In another example, a change in the deceleration and/or acceleration of the end effector may allow the substrate to shift position during transfer. It is contemplated that other system diagnostic information may be derived from the monitored change in position, either wafer to wafer, and/or sensor to senor bank during movement of a predetermined substrate.  
      Although the auto-diagnostic and auto-calibration sequences are described with reference to improving robotic motion within a semiconductor processing system, the invention may be used to improve the operation of other robot applications, including applications outside the field of semiconductor manufacture. Moreover, the term “wafer” and “substrate” are used interchangeably herein, and are representative of any workpiece that may be moved by a robot.  
      To facilitate control of the system  100  as described above, a controller  120  is coupled to the system  100 . The controller  120  generally includes a CPU  122 , memory  124  and support circuits  126 . The CPU  122  may be one of any form of computer processor that can be used in industrial settings for controlling various chambers and subprocessors. The memory  124  is coupled to the CPU  122 . The memory  124 , or computer-readable medium, may be one or more of readily-available memory such as random access memory (RAM) read-only memory (ROM), floppy disk, hard drive, device buffer or any other form of digital storage, local or remote. The support circuits  126  are coupled to the CPU  122  for supporting the processor in a conventional manner. These circuits  126  may include cache, power supplies, clock circuits, input-output circuitry, subsystems and the like.  
       FIG. 2  depicts a sectional view of the system  100  illustrating the transfer chamber  102  having one of the load lock chambers  106  and one of the process chambers  104  coupled thereto. The illustrative process chamber  104  generally includes a bottom  242 , sidewalls  240  and lid  238  that enclose a process volume  244 . In one embodiment, the process chamber  104  may be a PVD chamber. A pedestal  246  is disposed in the process volume  244  and generally supports the substrate  112  during processing. A target  248  is coupled to the lid  238  and is biased by a power source  250 . A gas supply  252  is coupled to the process chamber  104  and supplies process and other gases to the process volume  244 . The supply  252  provides a process gas such as argon from which a plasma is formed. Ions from the plasma collide against the target  248 , removing material that is then deposited on the substrate  112 . PVD and other process chambers which may benefit from the invention are available from Applied Materials, Inc., of Santa Clara, Calif.  
      The illustrative load lock chamber  106  generally includes a chamber body  260 , a first lift ring (substrate holder)  262 , a second lift ring  264 , a temperature control pedestal  266  and an optional heater module  270 . The chamber body  260  is preferably fabricated from a singular body of material such as aluminum. The chamber body  260  includes a first side wall  268 , a second side wall  272 , a top  274  and a bottom  276  that define a chamber volume  278 . A window  280 , typically comprised of quartz, is disposed in the top  274  of the chamber body  260  and is at least partially covered by the heater module  270 .  
      The atmosphere of the chamber volume  278  is controlled so that it may be selectively evacuated or vented to substantially match the environments of the transfer chamber  102  and the factory interface  110 . Generally, the chamber body  260  includes a vent passage  282  and a pump passage  284 . Typically, the vent passage  282  and the pump passage  284  are positioned at opposite ends of the chamber body  260  to induce laminar flow within the chamber volume  278  during venting and evacuation to minimize particulate contamination. In one embodiment, the vent passage  282  is disposed through the top  274  of the chamber body  260  while the pump passage  284  is disposed through the bottom  276  of the chamber body  260 . Valves  286  are coupled to the respective passages  282 ,  284  to selectively allow flow into and out of the chamber volume  278 . Alternatively, the passages  282 ,  284  may be positioned at opposite ends of one of the chamber walls, or on opposing or adjacent walls.  
      In one embodiment, the vent passage  282  is coupled to a high efficiency air filter  288  such as available from Camfil Farr, of Riverdale, N.J. The pump passage  284  is coupled to a point-of-use pump  290  such as available from Alcatel, headquartered in Paris, France. The point-of-use pump  290  has low vibration generation to minimize the disturbance of the substrates  112  positioned within the load lock chamber  106  while promoting pump-down efficiency and time by minimizing the fluid path between the chamber  106  and pump  290  to generally less than three feet.  
      A first loading port  292  is disposed in the first wall  268  of the chamber body  260  to allow substrates  112  to be transferred between the load lock chamber  106  and the factory interface  110 . A slit valves  226  selectively seals the first loading port  292  to isolate the load lock chamber  106  from the factory interface  110 . A second loading port  294  is disposed in the second wall  272  of the chamber body  260  to allow substrates  112  to be transferred between the load lock chamber  106  and the transfer chamber  102 . Another slit valve  226  selectively seals the second loading port  294  to isolate the load lock chamber  106  from the vacuum environment of the transfer chamber  102 . One slit valve that may be used to advantage is described in U.S. Pat. No. 5,226,632, issued Jul. 13, 1993 to Tepman et al., which is hereby incorporated by reference in its entirety.  
      Generally, the first lift ring  262  is concentrically coupled to (i.e., stacked on top of) the second lift ring  264  that is disposed above the chamber bottom  276 . The lift rings  262  and  264  are generally mounted to a hoop  296  that is coupled to a shaft  298  that extends through the bottom  276  of the chamber body  260 . Typically, each lift ring  262 ,  264  is configured to retain one substrate. The shaft  298  is coupled to a lift mechanism  258  that controls the elevation of the lift rings  262  and  264  within the chamber body  260 . A bellows  256  is generally disposed around the shaft  298  to prevent leakage from or into the body  260 .  
      Typically, the first lift ring  262  is utilized to hold an unprocessed substrate while the second lift ring  264  is utilized to hold a processed substrate returning from the transfer chamber  102 . The flow within the load lock chamber  106  during venting and evacuation is substantially laminar due to the position of the vent passage  282  and pump passage  284  and is configured to minimize particulate contamination. The processed substrate disposed in the second lift ring  264  may be lowered into close proximately to, or in contact with, the temperature control pedestal  266 . The temperature control pedestal  266  is coupled to a heat transfer system  222  that circulates a heat transfer fluid through passages formed in the pedestal  266 . In one embodiment, the temperature control pedestal  266  rapidly cools to the substrate while under vacuum, thereby reducing the chance of condensation on the substrate after the chamber volume is vented to allow transfer of the substrate to the factory interface. One load lock chamber that may be adapted to benefit from the invention is described in U.S. Pat. No. 6,558,509, filed May 6, 2003 to Kraus et al., and is hereby incorporated by reference in its entirety.  
      Generally, the transfer chamber  102  has a bottom  236 , sidewalls  234  and lid  232 . The transfer robot  108  is generally disposed on the bottom  236  of the transfer chamber  102 . A first port  202  is formed through the sidewall  234  of the transfer chamber  102  to facilitate transfer of a substrate by the transfer robot  108  between the process chamber  104  and the interior of the process chamber  104 . The first port  202  is selectively sealed by a slit valve  226  to isolate the transfer chamber  102  from the process chamber  104 . The slit valve  226  is generally moved to an open position as shown in  FIG. 2  to allow transfer of the substrate between the chambers.  
      The lid  232  of the transfer chamber  102  generally includes windows  228  disposed proximate the ports  202 ,  294 . The sensors  116  are generally disposed on or near the window  228  so that the sensors  116  may view a portion of the robot  108  and the substrate  112  as the substrate passes through a respective port  202 ,  294 . The window  228  may be fabricated of quartz or other material that does not substantially interfere with the detection mechanism of the sensor  116 , for example, a beam of light emitted and reflected back to the sensor  116  through the window  228 . In another embodiment, the sensor  116  may emit a beam through the window  228  to a second sensor positioned on the exterior side of a second window disposed in the bottom  236  of the chamber  102  (second sensor and second window not shown). It is also contemplated that sensors  116  of the centerfinding system may also be disposed in the factory interface  110 , the process chamber  104  or in the load lock chamber  106 .  
      The sensor  116  is generally disposed on the exterior of the window  228  so that the sensor  116  is isolated from the environment of the transfer chamber  102 . Alternatively, other positions of the sensor  116  may be utilized including those within the chamber  102  as long as the sensor  116  may be periodically tripped by motion of the robot  108  or substrate  112  therethrough. The sensor  116  is coupled to the controller  120  and is configured to record one or more robot or substrate metrics at each chance in sensor state. The sensor  116  may include a separate emitting and receiving unit or may be self-contained such as “thru-beam” and “reflective” sensors. The sensor  116  may be an optical sensor, a proximity sensor, mechanical limit switch, Hall-effect, reed switches or other type of detection mechanism suitable for detecting the presence of the robot  108  or the substrate.  
      In one embodiment, the sensor  116  comprises an optical emitter and receiver disposed on the exterior of the transfer chamber. One sensor suitable for use is available from Banner Engineering Corporation, located in Minneapolis, Minn. The sensor  116  is positioned such that the robot  108  or substrate  112  interrupts a signal from the sensor, such as a beam  204  of light. The interruption and return to an uninterrupted state of the beam  204  causes a change in state of the sensor  116 . For example, the sensor  116  may have a 4 to 20 ma output, where the sensor  116  outputs a 4 ma in the uninterrupted state while the sensor outputs 20 ma in the interrupted state. Sensors with other outputs may be utilized to signal the change in sensor state.  
       FIG. 3  depicts a plan view of one embodiment of the transfer robot  108 . The transfer robot  108  generally comprises a robot body  328  that is coupled by the linkage  132  to the end effector  130  that supports the substrate  112 . In one embodiment, the linkage  132  has a frog-leg configuration. Other configurations for the linkage  132 , for example, a polar configuration may be alternatively utilized. The linkage  132  generally includes two wings  310  coupled at an elbow  316  to two arms  312 . Each wing  310  is additionally coupled to an electric motor (not shown) concentrically stacked within the robot body  328 . Each arm  312  is coupled by a bushing  318  to a wrist  330 . The wrist  330  couples the linkage  132  to the end effector  130 . Typically, the linkage  132  is fabricated from aluminum, however, materials having sufficient strength and smaller coefficients of thermal expansion, for example, titanium, stainless steel or a ceramic such as titanium-doped alumina, may also be utilized.  
      At ambient temperatures, each wing  310  has a length “A”, each arm  312  has a length “B”, half the distance between the bushings  318  on the wrist  330  has a length “C” and a distance “D” is defined between the bushing  318  and a center point  320  of the end effector  130 . A reach “R” of the robot is defined as a distance between the center point  320  of the end effector  130  and a center  314  of the robot along a line “T”. Each wing  310  makes an angle θ with the line T.  
      Each wing  310  is independently controlled by one of the concentrically stacked motors. When the motors rotate in the same direction, the end effector  130  is rotated at an angle co about the center  314  of the robot body  328  at a constant radius. When both of the motors are rotated in opposite directions, the linkage  132  accordingly expands or contracts, thus moving the end effector  130  radially inward or outward along T in reference to the center  314  of the robot  108 . Of course, the robot  108  is capable of a hybrid motion resulting from combining the radially and rotational motions simultaneously. As the substrate  112  is moved by the transfer robot  108 , the sensor  116  detects the substrate or a portion of the robot upon reaching a predetermined position, for example, a position proximate the port  202 .  
      In one embodiment, the sensor  116  comprises a bank of sensors, for example four sensors, that may be tripped by different portions of the substrate and/or robot to capture a plurality of data sets during a single pass of the robot  108 . For example, an edge  332  of the wrist  330  of the robot  108  passing through the beam  204  causes the change of state of a first sensor  302  and a second sensor  304  while the substrate causes the change of state of the first sensor  302 , the second sensor  304 , a third sensor  306  and a fourth sensor  308 . Although the invention is described as having the substrate  112  activate the sensors  302 ,  304 ,  306  and  308 , the sensors may be activated by the wrist  330  or other components of the robot  108 . It is additionally contemplated that the sensor  116  may comprised a single sensor, or a bank of sensors two or more sensors, and that the sensor(s) may be positioned to changes state in response to passage of the substrate or portion of the robot. Generally, the sensors are configured to provide at least three sensor state changes per substrate pass.  
       FIG. 4  depicts one embodiment of the wrist  330  of the robot. The wrist  330  of the robot is configured to have a flat upper surface  402  and sides  404  that are generally disposed at right angles to one another. The interface between the sides  404  and upper surface  402  generally has a sharp edge or chamfer  406  to reduce the amount of light scattering by the beam  204  of the sensor  116 . The sharp edge or chamfered transition  406  between the upper surface  402  and the sides  404  provides a crisp change in sensor state which enhances the accuracy of the data acquisition if positional metrics of the end effector relative to the sensors  116  is desired.  
      Returning to  FIG. 3 , as the substrate  112  passes through one or more of the sensors  116 , the sensors are changed from a blocked state to an unblocked state or vice versa. The change of the sensor state generally corresponds to the substrate  112  (or robot  108 ) being in a predetermined position relative to the sensor  116 . Each time the robot  108  passes through any one of these predetermined positions, the robot metrics at the time of the event are recorded in the memory  124  of the controller  120 . The robot metrics recorded at each event generally includes the sensor number, the sensor state (either blocked or unblocked), the current position of each of the two robot motors, the velocity of the two robot motors and a time stamp. Utilizing the robot metrics recorded at three events, the controller  120  can resolve an actual position the substrate  112  positioned on the end effector  130 . Generally, the center position of the substrate  112  may be resolved utilizing data corresponding to three events that define the perimeter of the substrate  112 . The controller  120  utilizes the center position data to resolve the relative position of the substrate and the end effector  130  (or other reference point) of the robot  108 . The sensors  116  may also be utilized to acquire positional data of the end effector  130  to determine the position of the robot relative to the center position of the substrate  112 . The substrate center information may be used along or in concert with the end effector  130  position information. Additionally, by comparing the actual (i.e., sensed) location of the end effector and the expected (i.e., taught or programmed) position of the end effector, the motion of the robot may be corrected in real time or over a sample period to correct for motor drift, bearing wear, linkage or motor backlash, thermal expansion or other robot error.  
      Thus, utilizing substrate center information obtained by the centerfinding sensors  116  corresponding to the position of the substrate  112  (or reference substrate as described below) retrieved by the robot from a predefined position, the substrate center information may be utilized to teach the robot how to arrive at the predefined position. It is contemplated in some alternative embodiments that the placement of the substrate in the predefined position may be realized by manually placing (aligning) of the substrate in the predefined position, mechanically aligning the substrate at the predefined position, mechanically aligning the substrate on the blade, or through an iterative process of passing the substrate through the sensor bank while moving the substrate around on the end effector, all as further described below.  
      The method for determining the position of the robot is generally stored in the memory  124 , typically as software and software routine. Software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the system or being controlled by the CPU.  
       FIG. 5A  depicts a flow diagram of one embodiment of a method  500  for determining the position of the robot. The method  500  begins at step  502  by placing a substrate in a known (e.g., predefined) position.  
      The method  500  begins at step  502  by providing a substrate in a known position. The substrate may be provided in a known position at step  502  by manually centering a substrate on a support or other object within the range of motion of the robot and able to exchange the substrate positioned thereon with the robot. Alternatively, the substrate may be placed on a substrate support and kinematically moved into a known position, for example, on an aligner or other device that mechanically centers the substrate as discussed below with reference to FIGS.  14 A-D.  
      At step  504 , the substrate  112  is transferred to the end effector  130  of the robot  108 . The substrate supported on the end effector is then moved through the centerfinder (e.g., the sensors  116 ) to acquire a set of metrics indicative of the substrate&#39;s position relative to the end effector. Generally, the robot metrics are recorded in response to a change of state (i.e., tripping of one or more of the sensors  116 ), as the robot  108  passes the sensor  116  while moving the substrate through the transfer chamber  102 . The robot metrics are recorded as the edge of the substrate trigger the sensors as the substrate passes the sensor bank. The data points from the perimeter of the substrate  112  are used to triangulate the center position of the substrate.  
      In one embodiment, the centerfind algorithm is performed by converting each latched substrate edge position to an X,Y co-ordinate system, where 0,0 is at the center of the end effector  130 , and Y extends out away from the robot center. Next, the list of points (from the latched edge position) are examined and points that are significantly not co-circular with the other points are removed from consideration. Dropped points may be due, for example, points being latched as a notch or flat present in some substrates  112  passes one of the sensors  116 . Each of the remaining points are grouped into combinations of 3 points to define both a triangle and a circle. If the area of the triangle is very small, that combination of points will be very error sensitive for circle calculation, and is excluded from further consideration. Next, the center and radius is calculated for the circle defined by each remaining combination of 3 points. The X and Y coordinates for the centers of all such circles with a radius within an acceptable range are then averaged to get the X and Y center of the substrate  112 .  
      The X and Y substrate data is compared to X and Y end effector positional obtained from the robot metrics recorded at the triggering events. If the substrate is correctly centered on the robot, the X and Y offset (dx, dy) between the substrate and end effector is zero. A non-zero dx, dy represents an offset between the substrate  112  and the center of the end effector, which is the indicative of robot positional error. The dx, dy, (e.g., substrate/robot offset) which is resolved at step  506  to correct the robot motion so that the end effector/substrate&#39;s center to center match when substrate handoff is made at the predefined position. Once the dx and dy offset is resolved at step  508 , the motion algorithm of the robot is corrected at step  510  to complete a robot calibration process.  
      Optionally, steps  502 ,  504 ,  506  and  508  may be repeated at step  512  to confirm that the calibration was successful, or to iteratively increase the accuracy of the robot motion. Alternatively, step  512  may be periodically or at each instant that a substrate travels past the sensors  116  to continually monitor and correct robot motion, such as in an auto-diagnostic mode as further described below.  
      In another embodiment of the invention, a centering device may be utilized to position the substrate in a predefined position at step  502 . For example, a substrate-centering pocket to be provided on at least one of the chamber lift, cluster tool robot end effector or dedicated substrate centering device. The methods of centerfinding in the cluster tool may also be utilized. If the robot end effector is utilized to center the substrate thereon, the steps  502  and  504  may be combined and/or reversed. The assumption is made that the robot has “sniffing” capabilities (i.e., substrate edge finding) and can mechanically center the substrate using the clamp mechanism. The basic approach is to mechanically center the substrate with respect to the end effector and target, and then determine its location using the existing centerfinder systems.  
      As previously discussed, many chamber types do already include centering lift rings or pockets to center a substrate in the event that the substrate is severely misplaced. For example, the load lock chamber  106  of  FIG. 3  may includes centering device  210  on the lift ring  264  that transfers the substrate from the end effector  130  to the temperature control pedestal  266  disposed in the load lock chamber  106 .  
      As depicted in the schematic of  FIG. 6 , the lift ring  264  includes a centering device  210  in the form of a plurality of fins that flare radially inwards towards the center of the temperature control pedestal  266 . Thus, as the substrate  112  is lifted by the lift ring, as shown in illustration (B), the substrate, when misaligned, contacts at least one of the fins of the centering device  210 , which guide the substrate into a centered position, as shown in illustration (C). In illustration (A) of  FIG. 6 , the substrate  112  is positioned at the target position by the end effector. As the substrate is then lowered onto the temperature control pedestal, the substrate is centered in a predefined position with respect to the chamber, as shown in illustration (D). When the substrate is lifted again by the lift ring, the substrate is transferred from the predefined position to the end effector. It is contemplated that the centering device  210  or similar substrate alignment mechanism, active or passive, may be incorporated in other substrate supports within the system  100 , including stand-alone alignment pedestals. It is also contemplated that the centering device  210  may be incorporated into the end effector  130 .  
      One embodiment of a lift ring  264  with a substrate-centering device  710  is shown in  FIG. 7 . The device  710  includes a centering pocket  712  having flared walls. The centering pocket diameter, D CP  is sufficiently larger than the substrate diameter, D W , such that it does not affect the position of the substrate  112  in normal system operation. The outer-most diameter of the lift pocket, D LP , is sized large enough to center a substrate placed at the default chamber location.  
      Similarly, each cluster tool robot end effector  130  also includes a substrate-centering pocket  812  as shown in  FIG. 8 . Again, the centering pocket diameter, D CP , is sufficiently larger than the substrate diameter, D W , such that it does not affect the position of the substrate in normal system operation. The outer-most diameter of the end effector pocket, D EP , is sized large enough to handle the error between the end effector and a substrate placed at the default chamber location.  
       FIG. 5B  depicts a flow diagram another embodiment of a method  550  for determining a position of a robot. Assuming that the centerfinding system has been calibrated, the sensors  116  can be used to determine the error between a wafer on the end effector and the center of the end effector pocket. To teach the robot end effector to a target location, the wafer must first be physically located at the desired location at step  552 . The robot is extended to the desired location at step  554  and then picks the wafer up at step  556 . At step  558 , the robot transports the substrate through a centerfinder sensor bank at step  504 . The wafer correction system is then used to establish the error of the wafer position with respect to the end effector, which is also the same as the error between the actual target location and current taught target location at step  560 . Using this information, the robot calibration values for the target location are updated at step  562 , such that the taught location is then coincident with the actual target location. The proposed semi-automatic teaching method removes all subjectivity from the calibration process.  
      The described process also automates the process, with the exception of the first step of initially placing the calibration wafer at the desired target location. There are a number of ways to automate this step as well, resulting in a fully automatic calibration process. A fully automated calibration method is beneficial, as it can be performed without removing the chamber lids or venting the system to atmospheric pressure. The basic steps for automating a calibration process  570  are illustrated in  FIG. 5C . The process  570  includes first placing a wafer or calibration wafer at a taught target location at step  572 , and kinematically aligning the wafer with the actual target location at step  574 . It is also contemplated that the wafer may be passively aligned at the target location.  
      These two additions to the current system hardware, a substrate-centering end effector and lift ring, can perform the required functions when used in conjunction with the existing centerfinder systems. The process by which this is accomplished is described in greater detail below.  
      Robot-To-Load Lock Calibration  
      It is contemplated that the entire calibration may be automated by the present invention. In one embodiment, the robot, load lock substrate lift fins, and/or a centering feature located on the temperature control pedestal perform the function of positioning the substrate automatically in a predefined position as illustrated in the flow diagram illustrated in  FIG. 9 .  
       FIG. 9  is a functional flow diagram for placement of a wafer in the loadlock for calibration using a method  900 . The method  900  begins at step  902  by removing a wafer from a FOUP on the end effector of the robot. At step  904 , the robot moves the substrate to a predefined default location (e.g., target location). The default location is a location having a kinematic or passive alignment mechanism for positioning the wafer in a known position with respect to the end effector. At step  906 , the wafer is lifted from the end effector. The end effector is retracted clear of the wafer at step  908 . At step  910 , the wafer is lowered onto the centering device. At step  912 , the wafer is raised back to the exchange position from the centering device. In the raised position, the wafer is positioned in a predefined position, from which the actual position of the substrate may be determined using the substrate as a reference.  
      With the process of initially locating a substrate in the load lock now automated, the remainder of the procedure is much the same as described in  FIG. 5A . However, the entire sequence can now be automated as illustrated in  FIG. 10 .  
       FIG. 10  depicts a functional flow diagram for one embodiment of a loadlock calibration process  1000 . The process  1000  begins at step  1002  wherein the wafer is positioned in a known location relative to the end effector. In the embodiment depicted in  FIG. 10 , step  1002  may be performed using the method  900  described above. At step  1004 , the end effector is extended back to the target location of the wafer in the loadlock chamber over the centering device and receives the wafer. At step  1006 , the end effector, having the substrate positioned thereon, is raised slightly to a position that changes the state of a sensor. At step  1008 , the robot motor position is latched (i.e., stored in the memory of the controller) for each sensor transition (i.e., change of sensor state). If less than two sensor transitions are observed, the method  1000  proceeds to step  1010  where the end effector is extended by a small distance. At step  1012 , the end effector is lowered slightly to change the state of at least one sensor. At step  1013 , the position of the robot motor is latched for each sensor transition. If less than two transitions are observed, the method  1000  proceeds to step  1014 , wherein the end effector is extended by a small distance. Steps  1006  and  1008  are then repeated.  
      If two sensor transitions are observed after steps  1008  or  1013 , the method  1000  proceeds to step  1016  wherein the location and thickness of the wafer is calculated from the latched motor data. At step  1018 , the calculated location and thickness of the wafer are compared to thickness and position thresholds for the wafer. If the calculated location and thickness are not acceptable, the method  1000  proceeds to step  1020 , wherein the wafer is picked up from the default location on the loadlock on the end effector and moved to the default location for repositioning at step  1002 . If the calculated location and thickness data is acceptable, the method  1000  proceeds to step  1022  where the controller stores the height of the bottom surface of the wafer. At step  1024 , the end effector is retracted.  
      At step  1026 , the end effector is extended with the wafer thereon to the wafer location. At step  1028 , the end effector is moved such that at least one sensor is blocked by the wafer. At step  1030 , the end effector is retracted in order to unblock the sensor. At step  1032 , the end effector is moved so that a sensor once again blocked by the wafer. At step  1034 , the robot motor position is latched. At step  1036 , the radial distance or error of the expected robot extension and the actual robot extension needed to change the sensor state is determined. In one embodiment, the radial distance is the distance from the wrist moves from the expected position to a position where the edge of the wafer trips the sensor. Assuming that the robot extension required to trip the sensors has been increasing and the minimum radial distance has not been found, the method  1000  proceeds to step  1038 , wherein the controller calculates an angle based on the previous wrist angle, such that the other points are not duplicated. At step  1040 , the robot linkage is rotated about the wrist small angle. Following step  1040 , steps  1030 ,  1032 ,  1034  and  1036  are repeated until either a predetermined number of data points are obtained, the minimum radial distance is found, or one of the wafers centerline or edge has been found. If the minimum radial extension is found at step  1036 , the method proceeds to step  1042  where the controller estimates a wafer center from the minimum reach and angle. At step  1044 , the robot target location is stored based on derived wafer center location. It is contemplated that this procedure may be performed using the other wafer to trip the sensors.  
      Since the substrate-centering pocket is slightly oversized, some amount of error will be introduced; however, iterating the handoff process as shown in  FIG. 11  will reduce this error. In this approach, the robot end effector presents the substrate in a slightly different position each time it is placed. By sniffing each time after the placed substrate has been centered by the chamber lift, the variation in the correction value can be obtained. A number of techniques can then be used to convert this set of points into one location to which the robot is taught.  
       FIG. 11  depicts a functional diagram for averaging a position to reduce error using a method  1100 . The method  1100  may be employed selectively when the substrate, kinematically and/or passively positioned in a known location, is transferred to the end effector.  
      The system  1100  begins at step  1102  by transferring the wafer onto the end effector. At step  1104 , the end effector is moved a small distance. The distance moved by the end effector may be either an extension, rotation or both. At step  1106 , the wafer is lifted from the end effector, and the end effector is retracted clear of the wafer at step  1108 . At step  1110 , the wafer is lowered to wafer centering device, such as a kinematic centering or passive centering device, which positions the substrate in a known location. At step  1112 , the substrate is lifted and the end effector is extended back to the taught location to receive the wafer. At a sniffing step  1114 , the wafer on the end effector is moved through one or more sensors to determine a relative position between the end effector and the wafer. The end effector is moved to an expected position which is proximate the sensors. The difference between the latching of the robot motors in response to the actual end effector position and the expected robot motor position is indicative of a movement or positional error. Steps  102  through  1114  are iteratively repeated a predetermined number of times to collect a plurality of data points indicative of the relative positions of the end effector and wafer. At step  1116 , after the data points have been collected, an error between the taught location and the known wafer position is determined based on an average position errors derived from collected data.  
      Cluster Tool Robot-To-Load Lock Calibration  
      Another method of automating the cluster tool calibration is similar to that of the robot to load lock described above, with the robot having the ability to center a substrate with the clamping mechanism. However, the cluster tool robot does not initially know where a substrate is located on the end effector. The centerfinder system (e.g., the sensors  116 ) could be used to determine the substrate position. However, the centerfinder system must be calibrated prior to use. In order to calibrate the centerfinding system, a substrate must be centered on the end effector; but, a substrate cannot be centered on the end effector without the use of the centerfinder system.  
      Two methods are presented for calibrating the cluster tool. The first requires the centerfinder system to be calibrated first. Once the centerfinder is calibrated, it can then be used to calibrate the robot in a process similar to that proposed for calibrating in the previous section. In the second approach, the end effector is taught to the load lock first. Once taught to this position, a centered substrate can be removed from the load lock and used to calibrate the centerfinder system.  
      Centerfinder-First Approach  
      A special tool resembling an oversized substrate is loaded into the load lock by the robot, which is retrieved by the cluster tool robot and used to calibrate the centerfinder system. The diameter of the tool is matched to the pocket diameter of the end effector, such that the tool fits tightly in the pocket. Alternatively, a specially designed end effector can be used with some other kinematic mounting feature provided to interface with a centerfinder calibration tool. The oversized substrate approach is most likely the easiest to implement with the existing hardware. Once the centerfinding system is calibrated, the transfer chamber robot is then taught to the target locations in a manner similar to that presented for the load lock calibration.  
      Robot-First Approach  
      This approach is also similar to the load lock calibration process; however, a different method must be first used to locate the end effector ( FIG. 12 ). The procedure begins with the assumption that a substrate has been placed in the center of the load lock by the robot. The cluster tool robot moves to the default location of the load lock, where a centered substrate is lowered onto the end effector. The substrate then slides into place in the substrate-centering pocket on the end effector. The robot retracts and uses the centerfinder sensors to determine the position of the substrate with respect to the sensors.  
      Since it is not yet calibrated, the centerfinder system cannot be used to determine if the substrate is in the center of the end effector; but it can be used to determine how much a substrate moves from one operation to the next. Using this basic principle, the transfer chamber robot iterates the pick and drop of the substrate in the load lock; retracting each time to determine how much the substrate has moved. During this initial process, the fins are used to lift the substrate from the end effector, but the substrate is not lowered onto the centering ring within the load lock. This first step is only required to locate the end effector with respect to the substrate.  
       FIG. 12  is a functional flow diagram for a method  1200  for locating the robotic end effector. The method  1200  begins at step  1202  by rotating the end effector to face a default loadlock location. At step  1204 , the end effector is extended slowly, and the state of a bank of centerfind sensors are monitored at  1206 . If no sensor transitions are detected, steps  1204 ,  1206  are repeated after a small end effector rotational displacement. At step  1208 , the extension of the end effector is stopped in response to a detected sensor transmitter.  
      At step  1210 , the end effector is rotated slowly while the state of the sensors is monitored at step  1212 . If no sensor transition is detected, steps  1210  and  1212  are repeated. At step  1214 , the rotation of the end effector is stopped.  
      At step  1216 , the end effector is rotated one-half the distance to center the end effector in the loadlock chamber opening. At step  1218 , the end effector is extended to reach the full default reach position.  
      At step  1220 , the end effector is moved by a small distance. The distance may be either an extension, rotation or a combination of both extension and rotation. At step  1222 , a wafer is lowered onto the end effector. At step  1224 , the end effector is retracted from the target chamber. At step  1226 , the wafer position is recorded with respect to the end effector as the wafer passes through the sensors. At step  1228 , the wafer is extended back into the loadlock chamber, and the wafer is lifted from the end effector at step  1230 . This process is iteratively repeated a predefined number of times, as described with reference to the method  1100 , to further reduce the error in the robot position. In one embodiment, the end effector is rotated 45 degrees iteratively such that 8 data points are obtained from handoff position 360 degrees around the target position.  
      At step  1232 , the robot is retracted from the loadlock chamber. At step  1234 , the position of the centered wafer is calculated using the corrected wafer center points collected at step  1226 . At step  1236 , the calculated error from the default loadlock position is subtracted from the taught position of the end effector and stored as a new taught position for the loadlock. At step  1238 , the end effector is extended back into the loadlock chamber. At step  1240 , the wafer is lowered onto the end effector.  
      In one embodiment, step  1234  may be resolved using a method  1260 . The method  1260  is performed during the method  1200  to ensure that the offset of the substrate relative to the end effector is within a predefined range or a threshold. The method  1260  begins at step  1262  by subtracting the magnitude of the wafer movement from the magnitude of end effector movement, which was determined at step  1226 . At step  1264 , the difference in magnitude is compared to a predefined or established threshold. If the movement difference is within an established threshold, the error is set to zero at step  1266 . If not all of the differences are within the established threshold, the robot movement with the largest error is determined at step  1268 . At step  1270 , the target position is corrected by the error plus one half the clearance distance, wherein the clearance distance is the difference between the pocket size in a centering device and the diameter of the wafer.  
      Once the location of the substrate is known relative to the end effector, the process for calibrating the chamber positions is the same as the previously presented. The centerfinder system can either be calibrated using the same standard substrate used in the initial end effector location process, or it can be calibrated using a calibration tool once the robot teaching process is complete. In the later case, a specially designed calibration substrate can be automatically installed once the robot has been taught to load lock position.  
      Once the end effector is taught to a load lock chamber using this technique, the centerfinder system itself must then be calibrated. The conventional approaches require the chamber to be vented to atmospheric pressure so that the chamber lid can be removed. However, once the end effector has been precisely taught to a load lock, it should be possible to pass a special centerfinder calibration substrate into the cluster tool without venting the system. The simplest approach identified uses a pinned substrate  1300  designed to interface with a hole  1302  in the center of the end effector ( FIG. 13 ), which is currently used in the manual calibration process. If this simple approach proves to be insufficient, a more robust kinematic mounting alternative can be used; however, a specially designed end effector would most likely be required.  
      FIGS.  14 A-D depict examples of devices suitable for aligning the substrate in a predefined position, thereby enhancing the calibration processes described above. In FIGS.  14 A-B, kinematic devices are shown that mechanical move the substrate to the predefined position. For example,  FIG. 14A  depicts an end effector  1402  having a lip  1404  at a distal end and a pusher  1406  proximate the wrist of the end effector. The pusher  1406  may be actuated, for example by a pneumatic cylinder or solenoid, to force the substrate  112  (shown in phantom) against the lip  1404 , thereby centering the substrate with respect to the end effector.  
       FIG. 14B  depicts a substrate support  1412  having a plurality of pushers  1414  disposed around the circumference of the support  1412 . The pushers  1414  may be actuated, for example by a pneumatic cylinder or solenoid, to center the substrate (not shown) on the support  1412 . Lift pins have been omitted, here and in other embodiments, for the sake of brevity.  
      The substrate may alternatively be aligned by passive devices. For example, in  FIG. 14C , a substrate support  1422  is configured to engage a calibration wafer  1424 . The support  1422  and wafer  1424  include mating features that passively position the wafer  1424  relative to the support  1422 . In the embodiment depicted in  FIG. 14C , the substrate support  1422  includes a plurality of grooves  1428  that engage a respective pin  1426  extending from the calibration wafer  1424 . It is contemplated that other mating features or geometry may be utilized to position the wafer  1424  in a predefined position relative to the support  1422 .  
       FIG. 14D  depicts another embodiment of a substrate support  1432  having a passive alignment mechanism. The support  1432  includes a substrate receiving pocket  1434  having flaring sidewalls  1436 . The flaring sidewalls  1436  are configured to urge a misaligned substrate into a predefined position relative to the support  1432 .  
       FIG. 15  is one embodiment of a calibration wafer  1500  configured to prevent error (i.e., movement of the substrate) introduced by features of the end effector during transfer between the substrate supporting component and the end effector. The calibration wafer  1500  itself must interface with the centerfinding sensors (sensing paths of which are shown by dashed lines), but must not be affected by the end effector pocket or lips  1506  in any way. Therefore, the calibration wafer  1500  has one or more perimeter sections  1502  for tripping the sensors  116  and one or more cutout sections  1504  designed such that there is adequate clearance  1508  between the sections  1504  and lips  1506  when it is placed onto the end effector. The calibration wafer may also possesses friction pads on the bottom surface, which make contact with the end effector to prevent sliding during transport.  
      Both the function of the passive and active centering devices can be verified using an interactive approach similar to the method described in  FIG. 11 . Once a calibration or process wafer is passively (or actively) centered by such a centering device, the operator may not be able to visual verify the alignment is correct. In order to detect misalignment errors in centering, such as total misalignment of the kinematic features, some form or verification that the centering process worked correctly is desirable. Therefore, once a wafer has been aligned to a target location by the centering device, the alignment can be verified by iteratively repeating pick and drop operations with small known offsets in various directions. Each time the wafer is placed at a slightly offset position, the alignment mechanism should re-align the wafer to the same location. If during the iterative process, the centerfinding system observes the wafer to be off by an amount larger than expected for a properly functioning centering device, then gross errors in the passive centering can be detected.  
      Another method for verifying the detect misalignment errors in centering may be practiced by handing off the substrate to the end effector where the end effector is offset by a small predefined offset in a known direction prior to accepting the substrate. The centerfinder should confirm that the substrate and end effector is misaligned by the predefined offset if the centering mechanism is functioning properly. If the centerfinding system observes the wafer to be off by an amount larger or in a different direction than expected for a properly functioning centering device, then gross errors in the centering can be detected.  
      Thus, a method for automated teaching of a robot disposed in a processing system having a sensor based, substrate centerfinder system is provided. In some embodiment, the invention includes locating the position of a robot end effector with respect to a target location, where a substrate located at the target location is retrieved and transported from the target location on a robot end effector, the location of a substrate with respect to the robot end effector is determined as the end effector passes the substrate through a plurality of sensors (e.g., centerfinder) during transport, the location of the end effector with respect to the sensors has been predetermined and the error between the center of the substrate and end effector is used to correct the taught location for the target from which the substrate was received. The location of the end effector may be predetermined through a calibration step wherein the calibration is performed by precisely aligning a device resembling a substrate to the end effector and the device is passed though the sensors to determine the location of the end effector itself. The substrate in the target location may be mechanically aligned such that the center of the substrate and center of the target location is coincident before the substrate transferred to the end effector.  
      In other embodiments, a method for teaching a robot may include locating the position of a robot end effector with respect to a substrate at a target location, where a substrate located near the target location is retrieved and transported from the target location on a robot end effector, the location of a substrate with respect to the robot end effector is determined as the end effector passes the substrate through a plurality of sensors during transport, the location of the end effector with respect to the sensors has been predetermined, and the error between the center of the substrate and end effector is used to continually monitor parameters indicating functional performance of the system. The functional parameter may include substrate movement prior to the hand-off, substrate movement during to the hand-off, substrate misalignment as a result of the previous hand-off, friction within the robotic arm, and backlash within the robotic arm among other functional parameters effecting repeatable robot motion.  
      Although the process of the present invention is discussed as being implemented as the software routine, some of the method steps disclosed herein may be performed in hardware as well as by itself or controller. As such, the invention may be implemented in software as executed upon a computer system in hardware as in applications, specific integrated circuit or other type of hardware implementation or a combination of software and hardware.  
      While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.