Abstract:
The presence of a workpiece on an end effector of a vacuum robotic handler is detecting using any of a number of non-contact techniques in which some or all of the detection hardware is positioned outside a vacuum chamber that encloses the vacuum robotic handler. Various deployments include laser beam breaking, analysis of radar reflection signals, or analysis of radio frequency identification tag signatures. By providing non-physical couplings between hardware inside and outside of a vacuum environment, integrity of the vacuum is improved. These non-contact techniques are further adapted as described herein to multi-wafer and multi-end effector environments so that independent detection of multiple wafers (e.g., for each end effector) can be performed.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. App. No. 60/957,406 filed on Aug. 22, 2007, U.S. App. No. 60/957,144 filed on Aug. 21, 2007, and U.S. App. No. 60/956,834 filed on Aug. 20, 2007. Each of the foregoing applications is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This application relates to detecting the presence of a workpiece in a handling environment, and more particularly to detecting the presence of a semiconductor wafer in a vacuum wafer handling system. 
         [0003]    While numerous techniques are known for detecting the presence of objects, such as optical beam breaking, radar, sonar, and so forth, many of these techniques are not generally suitable for use in vacuum because the hardware cannot be deployed in vacuum or the hardware requires electrical, optical or other leads for which through-holes in a vacuum chamber must be provided. For high-vacuum applications such as semiconductor fabrication, there remains a need for improved techniques to detect the presence of a semiconductor wafer or the like within a vacuum chamber. 
       SUMMARY 
       [0004]    The presence of a workpiece on an end effector of a vacuum robotic handler is detected using any of a number of non-contact techniques in which some or all of the detection hardware is positioned outside a vacuum chamber that encloses the vacuum robotic handler. Various deployments include laser beam breaking, analysis of radar reflection signals, or analysis of radio frequency identification tag signatures. By providing non-physical couplings between hardware inside and outside of a vacuum environment, integrity of the vacuum is improved. These non-contact techniques are further adapted as described herein to multi-wafer and multi-end effector environments so that independent detection of multiple wafers (e.g., for each end effector) can be performed. 
         [0005]    In one aspect, a system disclosed herein includes: a first end effector of a vacuum robotic handler, the vacuum robotic handler positioned within a vacuum chamber and the first end effector having a first pair of optical couplers, one of the optical couplers positioned to receive light from a first laser and coupled to a first source fiber that directs the light across a first workpiece support plane of the first end effector, and another one of the optical couplers positioned to transmit light to a first detector and coupled to a first target fiber that has a receiving end positioned on an opposing side of the first workpiece support plane from the first source fiber and oriented to receive light therefrom; and a second end effector of the vacuum robotic handler, the second end effector having a second pair of optical couplers, one of the optical couplers positioned to receive light from a second laser and coupled to a second source fiber that directs the light across a second workpiece support plane of the second end effector, and another one of the optical couplers positioned to transmit light to a second detector and coupled to a second target fiber that has a receiving end positioned on an opposing side of the second workpiece support plane from the second source fiber and oriented to receive light therefrom. 
         [0006]    The second end effector may include a transmissive region positioned to pass light between the first pair of optical couplers and the first laser and the first detector. The first end effector and the second end effector may be independently movably, and the transmissive region may pass light between the first pair of optical couples and the first laser and the first detector only when the first end effector and the second end effector are in a specific relative orientation. The transmissive region of the first end effector may be a hole through the first end effector. The first laser may be positioned outside the vacuum chamber, with the vacuum chamber including a light transmissive wall region that passes light between an interior of the vacuum chamber and the first laser. The first laser may be mounted to the transmissive wall region. The first laser may be positioned to couple to the one of the first pair of optical couplers only when the first end effector is in a predetermined position. The first detector may be positioned outside the vacuum chamber, the vacuum chamber including a light transmissive wall region that passes light between an interior of the vacuum chamber and the first detector. The first detector may be mounted to the transmissive wall region. The second detector may be positioned outside the vacuum chamber. The first laser may be positioned outside the vacuum chamber. The second laser may be positioned outside the vacuum chamber. The system may further include a processing system that measures a light detection signal from at least one of the first detector and the second detector and determines whether a workpiece is present within the workpiece support plane based upon the light detection signal. The system may include a motion control system for the vacuum robotic handler, the motion control system configured to place the vacuum robotic handler in a home position in which wafer presence detection can be performed. The first laser and the second laser may be excited concurrently. The vacuum robotic handler may handle one or more of a reticle and a semiconductor wafer. 
         [0007]    In another aspect, a method disclosed herein includes providing a vacuum chamber with a transmissive wall; transmitting a laser beam through the transmissive wall into an interior of the vacuum chamber; directing the laser beam across a workpiece support plane from a first side of the workpiece support plane; detecting the laser beam on an opposing side of the workpiece support plane when a wafer is present within the workpiece support plane, to provide a detected laser beam; transmitting the detected laser beam through the transmissive wall to a detector outside the vacuum chamber, thereby providing a workpiece presence signal; and analyzing the wafer presence signal to determine when the wafer is present. 
         [0008]    The method may include detecting the presence of a second wafer with a second laser beam originating outside the vacuum chamber. Directing the laser beam across a workpiece support plane may include coupling the laser beam into an optical fiber. Detecting the laser beam on an opposing side of the workpiece support plane may include coupling the laser beam into an optical fiber. 
         [0009]    In another aspect, a system disclosed herein includes: a radar transmitter disposed outside a vacuum workpiece handling enclosure, the radar transmitter producing a radar signal directed toward a plurality of workpieces within the vacuum workpiece handling enclosure; a plurality of radar detectors, two of the plurality of radar detectors disposed outside the vacuum workpiece handling enclosure to receive reflected portions of the radar signal, and one of the plurality of radar detectors disposed inside the vacuum workpiece handling enclosure to receive a non-reflected portion of the radar signal; and a processing system in communication with the radar transmitter and the plurality of radar detectors, the processing system evaluating signals from the plurality of radar detectors to determine whether one or more workpieces are present within the vacuum workpiece handling enclosure. 
         [0010]    In another aspect, a method disclosed herein includes projecting a radar signal into an interior of a vacuum workpiece enclosure; receiving portions of the radar signal from a plurality of radar detectors; and analyzing the received portions of the radar signal to determine whether one or more workpieces are present within the interior of the vacuum workpiece enclosure. 
         [0011]    Analyzing the received portions of the radar signal may include determining a portion of the radar signal reflected off a first workpiece and through a wall of the vacuum workpiece enclosure to one of the plurality of radar detectors. Analyzing the received portions of the radar signals includes determining a portion of the radar signal transmitted through a non-reflective region of a first workpiece toward a second workpiece and reflected off the second workpiece and passing through a wall of the vacuum workpiece enclosure to one of the plurality of radar detectors. The method may include determining a portion of the radar signal transmitted through the second workpiece to a second one of the plurality of radar detectors, the second one of the plurality of radar detectors positioned inside the vacuum workpiece enclosure. 
         [0012]    In another aspect, a system disclosed herein includes a radar transmitter disposed inside a vacuum workpiece handling enclosure, the radar transmitter producing a radar signal directed toward a plurality of workpieces within the vacuum workpiece handling enclosure; a plurality of radar detectors, two of the plurality of radar detectors positioned to receive reflected portions of the radar signal, at least one of the plurality of radar detectors disposed inside the vacuum workpiece handling enclosure to receive a non-reflected portion of the radar signal; and a processing system in communication with the radar transmitter and the plurality of radar detectors, the processing system evaluating signals from the plurality of radar detectors to determine whether one or more workpieces are present within the vacuum workpiece handling enclosure. 
         [0013]    At least two of the plurality of radar detectors may be disposed inside the vacuum workpiece handling enclosure. 
         [0014]    In another aspect, a system disclosed herein includes a radio frequency identification tag associated with a workpiece; a radio frequency transmitter mounted on a robotic arm of a vacuum robotic handler, the radio frequency transmitter controllable to transmit a stimulus signal for the radio frequency identification tag; a radio frequency receiver that receives an echo signal from the radio frequency identification tag in response to the stimulus signal; and a processor coupled to the radio frequency transmitter to control operation thereof, and coupled to the radio frequency receiver, the processor programmed to evaluate characteristics of the echo signal to determine a location of the radio frequency identification tag, and to infer a presence of a semiconductor wafer based upon the location of the radio frequency identification tag. 
         [0015]    In another aspect, a system disclosed herein includes a radio frequency identification tag disposed on an end effector of a robotic arm; a radio frequency transmitter disposed sufficiently near the radio frequency identification tag to activate the radio frequency identification tag with a stimulus signal; a radio frequency receiver that receives an output signal from the radio frequency identification tag in response to the stimulus signal; and a processor coupled to the radio frequency transmitter to control operation thereof, and coupled to the radio frequency receiver, the processor programmed to evaluate characteristics of the output signal to determine a location of the radio frequency identification tag, and to infer a presence of a semiconductor wafer based upon the characteristics of the output signal. 
         [0016]    The system may include a plurality of radio frequency identification tags disposed on the end effector, wherein the processor receives and evaluates a corresponding plurality of output signals to determine a presence of the semiconductor wafer. The processor may receive and evaluate the corresponding plurality of output signals to determine a position of the semiconductor wafer. The radio frequency identification tag may transmit a predetermined signal when activated. The radio frequency tag may include a sensor circuit operable to detect proximity of a semiconductor wafer, wherein the radio frequency identification tag transmits a signal indicative of an output from the sensor circuit. The sensor circuit may measure capacitance between an element of the radio frequency tag and a semiconductor wafer. 
         [0017]    In another aspect, a method disclosed herein includes transmitting a radio frequency stimulus signal into an interior of a vacuum robotic handler; reflecting the predefined radio frequency stimulus signal off a radio frequency identification tag disposed on an end effector to produce an echo identification signal; and receiving the echo identification signal at a predetermined location, wherein the echo identification signal comprises characteristics based at least in part on a distance from the predetermined location to the radio frequency identification tag. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]    The invention and the following detailed description of certain embodiments thereof may be understood by reference to the following figures: 
           [0019]      FIG. 1  shows a laser-based dual wafer detection system. 
           [0020]      FIG. 2  shows a radar-based dual wafer detection system. 
           [0021]      FIG. 3  shows a single radio-frequency-based wafer detection system. 
           [0022]      FIG. 4  shows the wafer detection system of  FIG. 3  without a workpiece. 
           [0023]      FIG. 5  shows a dual workpiece detection system. 
           [0024]      FIGS. 6A and 6B  depict a dual detector configuration of the workpiece detection system of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  shows a laser-based dual wafer detection system for use with a vacuum robotic handler. In general, the system  100  can detect the presence of a wafer  150  on an upper robotic arm end effector  144  and of a wafer (not shown) on a lower robotic arm end effector  148  while the robotic arms are stationary, such as in a home position. By adapting a robot arm to include optical couplers, light fibers, and suitably placed transparent portions, wafers on an upper and a lower robotic arm end effector may be detected without relying on the coordinated movement of the arms. It will be understood that, while the following description generally refers to “workpieces”, the principles of the invention may be suitably adapted to any substantially planar media that is being handled and/or processed including by way of example semiconductor wafers, reticles, disc media (CD&#39;s, DVD&#39;s, HD-DVD&#39;s, Blu-ray, etc.) and any other similar substrate or the like. Similarly, it will be understood that the principles of the invention may be suitably adapted to a wide range of vacuum handling equipment and end effectors, and all such variations, modifications, and adaptations that would be apparent to one of ordinary skill in the art are intended to fall within the scope of this disclosure. 
         [0026]    The dual wafer handling and detection system  100  may include a pair of lasers  102  and  104 , which may be positioned outside a vacuum chamber that has a light transmissive top  105 , such that light from the lasers  102 ,  104  is directed through the top  105  into the chamber. The system may also include a pair of laser detectors  112  and  114  positioned outside the vacuum chamber to receive any portion of the laser light that is returned from the dual wafer detection elements inside the vacuum chamber. 
         [0027]    The detection elements and method may be substantially similar for detecting a wafer on the upper arm  144  and the lower arm  148 . In the following description, the first reference number in each parenthetical set references an element associated with the upper arm  144  and the second reference number references elements associated with the lower arm  148 . Differences between the configuration for the upper arm  144  and the lower arm  148  will be noted, although it will be understood that more generally, the techniques described herein may be readily adapted to single wafer detection systems, triple wafer detection systems, other double wafer detection systems (e.g., side-by-side, offset stacked, etc.), and so forth. It will also be understood that with suitable modifications, the lasers  102 ,  104  and detectors  112 ,  114  may be positioned within the vacuum environment, which generally entails a trade off between the disadvantages of constructing a wall of a vacuum chamber from a transmissive material and the disadvantages of vacuum-sealing a conduit into the vacuum chamber for optical/electrical connections, or conversely, the advantages of using a conventional aluminum construction for the walls of a vacuum chamber and the advantages of avoiding through-holes and related sealing requirements for the vacuum chamber. 
         [0028]    Within the vacuum chamber, a first pair of optical couplers  122 ,  120  and a second pair of optical couplers  132 ,  130  may be positioned on a pair of robotic arms  144 ,  148  so that they couple source light from the lasers  102 ,  104  to source fibers  124 ,  128  positioned to direct a beam of light along a plane—referred to herein as a workpiece support plane—where wafer would rest on the robotic arm end effectors if present. The resulting workpiece detection beams  142 ,  140  are received by target fibers  134 ,  139  when a wafer is not present. The workpiece detection beams  142 ,  140  are carried by the target fibers  134 ,  138  to the second pair of optical couplers which transmit the received beams  142 ,  140  through the top  105  to laser collectors  112 ,  114 . Suitable lasers, couplers, fibers, and the like will be readily apparent to one of skill in the art, and are not described in detail here. 
         [0029]    With a workpiece properly positioned on the robotic arm  144 ,  148 , and the robotic arm  144 ,  148  properly positioned in a stationary position, such as a home position, the workpiece detection beam  142 , 140  may be interrupted or attenuated by the workpiece so that a reduced amount of the beam impacts a receiving area on an end of the receiving light fibers  134 ,  138 . The workpiece may entirely or partially block the workpiece detection beam  142 ,  140 . In other embodiments, the workpiece detection beam  142 ,  140  may reflect off of an edge of the workpiece, or may reflect off of surface features of the workpiece. Some portion of the projected light and any reflected light may reach the receiving light fibers  134 ,  138 . When the projected light in the workpiece detection beam  142 ,  140  reaches the receiving light fibers  134 ,  138  substantially undisturbed (e.g. the workpiece is not present, the workpiece is not properly positioned, or the robotic arm  144 ,  148  is not properly positioned), a processing system  170  that analyzes signals from the detectors  112 ,  114  may infer that a workpiece is absent. The processing system  170  may in general measure the detected laser light, with any differences or variations noted and corresponding inferences drawn. For example, a substantial difference, such as a reduction beyond a predetermined threshold, may indicate a presence of a wafer on the robotic arm  144 ,  148 . To facilitate distinguishing an error condition that causes complete interruption of the laser light, such as an object blocking the light or the light fiber  124 ,  128  being misaligned, the dual wafer handling and detection system  100  may be configured to ensure that at least a portion of the projected light reaches the receiving light fibers  134 ,  138  when a properly positioned workpiece is on an end effector. 
         [0030]    A presence of a workpiece on the upper robotic arm  144  and the lower robotic arm  148  may be determined at the same time and/or independently by properly positioning both of the robotic arms in a stationary position such as a home position. The upper robotic arm  144  may include optically transparent portions, such as through-holes or any translucent/transparent material suitable for in-vacuum use, positioned directly in the light path from the laser  104  to the coupler  120  and the light path from the coupler  130  to the detector  114 . In an example, the optically transparent portion  108  of the upper arm  144  may facilitate a laser  104  transferring light directly to the detector  120  even when the upper arm  144  is positioned between the laser  104  and the coupler  120 . 
         [0031]    With the upper robotic arm  144  and the lower robotic arm  148  positioned substantially as shown in the dual wafer handling and detection system  100  of  FIG. 1 , a presence of two wafers (a workpiece supported by the upper robotic arm  144  and a workpiece supported by the lower robotic arm  148 ) may be detected without requiring either the upper robotic arm  144  or the lower robotic arm  148  to be moved out of the way of the laser light paths. The dual wafer handling and detection system  100  may also facilitate detecting both an upper workpiece and a lower workpiece independently of each other. Each laser-coupler-fiber-beam-fiber-coupler-detector path can be excited independently or at the same time for faster, substantially simultaneous wafer detection. 
         [0032]    The dual wafer handling and detection system  100  may communicate with the robotic arms  144 ,  148  motion control system to determine when the robotic arms  144 ,  148  are in position prior to exciting the lasers  102 ,  104 , or to position the robotic arms  144 ,  148  in a location for wafer detection. In general the lasers  102 ,  104  are stationary relative to the robotic arm vacuum enclosure. Typically, the elements of the dual wafer handling and detection system  100  are also held in a stationary position on the robotic arms  144 ,  148  during wafer detection. However, in embodiments the lasers  102 ,  104  and related hardware may move in coordination with the robotic arm to facilitate wafer detection in a range of locations. 
         [0033]    Although the optically transparent portions  108 ,  118  of the upper robotic arm  144  facilitate projecting light through the upper robotic arm  144 , the dual wafer handling and detection system  100  may detect a workpiece on the lower robotic arm  148  even when the upper robotic arm  144  is in a position other than the home position. As long as a non-transparent portion of the upper arm  144  does not interfere with the light path associated with detecting a workpiece on the lower robotic arm  148 , the upper robotic arm  144  may be in any position or may be moving relative to the lower arm  148  while the presence of a workpiece on the lower robotic arm  148  is detected. Similarly, detecting a workpiece on the upper arm  144  can be done independently of the position or existence of the lower robotic arm  148 . Likewise, the lasers  102 ,  104  may be positioned such that the robotic arm  144 ,  148  may be stopped at any other position in its range of motion under a transparent portion of the robotic vacuum enclosure cover. Through an appropriate selection of properties for the lasers  102 ,  104 , the optical couplers  120 ,  122 ,  130 ,  132 , the optically transparent portions  108 ,  118  of the upper arm  144 , and the laser detectors  112 ,  114 , the wafer detection may be performed while the robotic arm  144 ,  148  is in motion. Therefore, although a home position of the robotic arms  144 ,  148  may be a convenient position for detecting the presence of workpieces, the invention is not limited to this position, nor to any particular robotic arm  144 ,  148  position except as herein described. 
         [0034]    In an alternative configuration of the dual wafer handling and detection system  100 , a single laser  102  and a single detector  112  may be used to detect the presence of a wafer supported by the upper arm  144  and a workpiece supported by the lower arm  148 . In this configuration, the upper robotic arm  144  is positioned so that the optical couplers  122  and  132  are positioned to form a light path between the laser  102  and the detector  112 . The laser  102  is excited and the detector  112  is measured to determine an amount of light passing from the laser  102  to the detector  112  through the dual wafer handling and detection system  100  elements associated with the upper robotic arm  144 . The upper robotic arm  144  may then be moved so that the light path between the optical couplers  120  and  130  associated with the lower robotic arm  148  forms a light path with the laser  102  and detector  112 . The laser  102  is again excited and the detector  112  is measured to determine an amount of light passing from the laser  102  through the optical elements associated with the lower robotic arm  148 . The upper robotic arm  144  may be moved so that no portion of the upper robotic arm  148  affects the path of laser  102  by positioning the transparent portions  108  and  118  in the laser  102  light path. In this configuration, a single laser may be used to detect both an upper workpiece and a lower workpiece while requiring that only the upper robotic arm  144  be moved in a coordinated manner. 
         [0035]    The laser  102  and  104  may be any type of laser that generates light that is compatible with detection by optical couplers and transmission through light fiber. Laser types may include gas, chemical, dye, metal-vapor, solid-state, semiconductor, and the like. Visible light that is emitted from laser  102  and  104  may be any color, such as red, yellow, and the like. The light fiber  124 ,  128 ,  134 , and  138  may be flexible glass, plastic, or the like designed to guide light along it length by confining as much light as possible in a propagating form. The light fiber may comprise a large core that relies on total internal reflection, or a smaller core that confines light by establishing a waveguide. The fiber may include one or more of graded-index optical fibers, step-index optical fibers, birefringent polarization-maintaining fibers, photonic crystal fibers, and the like. 
         [0036]    In another embodiment of a wafer detection system, radar may provide a medium for detecting a presence of one or more workpieces in a vacuum workpiece handling environment such as a vacuum semiconductor handling and fabrication system. Radar transmission in atmosphere and vacuum may facilitate detecting one or more workpieces that are vertically displaced from one another. Because radar may both reflect and pass through materials, the reflecting and transmitting properties of radar may be employed to identify the one or more workpieces. 
         [0037]    The radar-based dual workpiece sensing system  200  of  FIG. 2  incorporates radar signal reflection and transmission properties to facilitate detecting at least a first and a second wafer being handled in a vacuum environment using a single radar transmitter. Because radar does not require a line of sight presentation of the item being detected by the radar, the radar-based dual workpiece sensing system  200  may be employed with vacuum enclosures that do not have a glass cover. 
         [0038]      FIG. 2  shows a radar-based dual wafer detection system. In the embodiment of  FIG. 2  the radar-based dual workpiece sensing system  200  comprises a radar transmitter  202 , atmosphere radar detectors  208  and  210 , vacuum radar detector  220 , and a processing system (not shown) for controlling the system  200  and processing the radar signals detected by the detectors  208 ,  210 , and  220 . A radar signal  204  generated by the radar transmitter  202  may pass through the enclosure top cover  212  substantially intact and impact a first wafer  214 . A portion of the radar signal  204  may be reflected from the top cover  212 . A portion of the radar signal  204  will be reflected off of the first workpiece  214  and a portion will be transmitted through the first workpiece  214 . The reflected portion may pass through the top cover  212  substantially unchanged to become a first reflected radar signal  228 , while a portion of the reflected portion may further be reflected from the top cover  212 . The first radar detector  208  may receive the first reflected radar signal  228  and communicate information representing the detected first radar signal  228  to the processing system. The transmitted portion  232  may be attenuated by the first workpiece  214  and may impact a second workpiece  218 . The transmitted portion  232  may be further attenuated as it impacts the second workpiece  218  and may be split into at least a second reflected radar signal  224  and a residual radar signal  230 . The second reflected radar signal  224  may be further attenuated as it transmits through the first workpiece  214  and then through the top cover  212  before impacting the second atmosphere radar detector  210 . The residual radar signal  230  may be detected by internal radar detector  220 . 
         [0039]    As shown in  FIG. 2 , each radar signal impacting an object such as a workpiece or a cover may be attenuated and reflected by the object. The amount of attenuation and any reflection are dependent on at least the presence of the object. In an example, the first reflected radar signal  228  would not be present if the first workpiece  214  were not present. Also attenuated radar signal  232  would not be attenuated and therefore would be the same as radar signal  204 . In this example, the first detector radar detector  208  would receive no radar signal while the second radar detector  210  would receive a signal reflected off of the second workpiece  218 . The signal reflected off of the second workpiece  218  may result in detector  210  indicating a larger detected value than if the first workpiece  214  were present. Likewise, in this example, the residual radar detector  220  would indicate a larger detected value. 
         [0040]    A generalization of the principals described and exemplified herein may be represented by the following grid. Each column Wx indicates the presence (P) or absence (A) of the first workpiece  214  (W 1 ) and the second workpiece  218  (W 2 ). Ax represents attenuation of workpiece ‘x’ and Rx represents reflection from workpiece ‘x’. The formulas in the grid represent how the presence of each workpiece may impact detection of a radar signal by the first radar detector  208  (FD), the second radar detector  210  (SD), and the residual radar detector (RD). S represents the source radar signal  204 , and f represents a formula adapted to compute an expected radar signal value at each detector. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 W1 
                 W2 
                 RD 
                 FD 
                 SD 
               
               
                   
                   
               
             
             
               
                   
                 P 
                 P 
                 f(S, A1, A2) 
                 f(S, R1) 
                 f(S, A1, R2) 
               
               
                   
                 P 
                 A 
                 f(S, A1) 
                 f(S, R1) 
                 f(0) 
               
               
                   
                 A 
                 P 
                 f(S, A2) 
                 f(0) 
                 f(S, R2) 
               
               
                   
                 A 
                 A 
                 f(S) 
                 f(0) 
                 f(0) 
               
               
                   
                   
               
             
          
         
       
     
         [0041]    The above grid provides a visualization of potential differences in detected values at each of the detectors based on the presence of each of the workpieces. As is shown in the rows of the grid, each combination of workpieces  214  and  218  results in a different set of formulas representing the detected radar signals. Consequently, a processing system may determine the presence of each workpiece based on a comparison of the resulting detected radar signal at each detector with the expected radar signal value based on the formulas in the grid. 
         [0042]    The radar transmitter  202  may include a conical antenna to direct the radar signal  204  in a narrow path, thereby reducing the potential of random reflections affecting the detection of the first reflected radar signal  228  or the second reflected radar signal  224 . The radar transmitter  202  may be pulsed and may be timed to transmit the radar signal  204  when a workpiece handling robotic arm has positioned the workpieces for detection. It may be possible to perform radar-based dual workpiece detection when the workpieces are stationary or moving. 
         [0043]    The residual radar signal detector  220  may be positioned inside the enclosure as shown in  FIG. 2 . Alternatively the residual radar signal detector  220  may be positioned outside the enclosure as represented by element  234 . When the residual radar signal detector is positioned as element  234 , the residual radar signal  230  may pass through the bottom cover  222  before being detected. 
         [0044]    Alternatively, the residual radar signal detector  220  may be left out of the configuration so that the presence of the workpieces is detected by analyzing the detection of the first reflected radar signal  228  and the detection of the second reflected radar signal  224 . In this configuration, the residual radar signal  230  is not detected. 
         [0045]    In another embodiment, detecting workpiece presence in a vacuum processing system may be facilitated by a contactless, wireless, passive proximity detection system positioned on a robotic arm so that a workpiece properly supported by the robotic arm is in close proximity to the detection system. The detection system may include a circuit that, when stimulated by a radio signal, produces a radio frequency echo. The radio frequency echo signal may vary based on a distance between the workpiece and the circuit. The circuit may include elements that interact with the workpiece and therefore produce an echo signal characterized by the interaction. 
         [0046]      FIGS. 3 through 6  show various configurations of a radio frequency proximity-based workpiece detection system  300 . 
         [0047]      FIG. 3  shows a single radio-frequency-based wafer detection system. In  FIG. 3 , a workpiece is detected using a proximity-based technique wherein an adaptable identification tag  302 , positioned on a robotic arm  304  so that a workpiece  318  properly supported by the robotic arm  304  is in close proximity to the identification tag  302 . A radio transmitter  314  may transmit a radio frequency stimulus signal  312  so that the stimulus signal  312  reaches the identification tag  302 . The identification tag  302  may echo or reflect the stimulus signal  312  using a circuit. The radio stimulus signal  312  may enter the circuit and interact with elements of the circuit to produce an echo signal  308  that may be detected by a radio receiver  310 . Based on the proximity of the workpiece  318  to the circuit, the echo signal  308  may include characteristics that are detectable by the receiver  310 . When the workpiece  318  is close to the identification tag  302 , the detectable characteristics of the echo signal  308  may be different than when the workpiece  318  is not in close proximity to the identification tag  302 . 
         [0048]    A circuit may be affected by a proximity to a workpiece in a variety of ways. Capacitance-based proximity detection is an example of how the workpiece  318  may impact the circuit. In this example, a capacitance is established between elements of the circuit and when the workpiece  318  comprises a conductive material, (e.g. a semiconductor wafer) the capacitance established in the circuit is changed when the workpiece  318  is close to the circuit. The identification tag  302  uses the circuit, and therefore the established capacitance in the circuit to produce an echo signal. A frequency characteristic of the echo signal may be based on the established capacitance and therefore will vary based on the proximity of the workpiece. 
         [0049]    The identification tag  302  may be passive or active. A passive identification tag  302  may derive the energy needed to power it from the stimulus signal  312 . The energy obtained by the tag  302  may facilitate the tag  302  transmitting the response or echo signal  308 . The response or echo signal  308  may be generated by modulating impedance the tag  302  presents to the stimulus signal  312 . In the example capacitance-based example above, the modulation of the impedance may be impacted by the established frequency that is based on the proximity of the workpiece  318 . 
         [0050]      FIG. 4  shows the wafer detection system of  FIG. 3  without a workpiece. The response or echo signal  308  of the configuration depicted in  FIG. 3  is distinguished from the echo signal  408  of the configuration depicted in  FIG. 4  by the characters shown. Echo signal  308  of  FIG. 3  is impacted by the workpiece  318  and is consequently depicted as comprising “B−”. Echo signal  408  of  FIG. 4  is not impacted by the workpiece  318  (which has been removed) and is consequently depicted as comprising “b−”. An actual signal may include a variety of characters, attributes, characteristics, frequencies, energies, and the like. These signal representations are to indicate a difference between the echo signal  308  when the workpiece  318  is in close proximity to the tag  302  and the echo signal  408  when the workpiece  318  is not in close proximity to the tag  302 .  FIG. 4  also depicts a different orientation of the stimulus signal  312  and transmitter  314  than that shown in  FIG. 3  because the orientation of the transmitter  314  and tag  302  is not critical for the invention. A directional transmitter, as depicted in  FIGS. 3 ,  4 , and  5 , may be directed toward the tag  302 . An omni-direction transmitter may not need to be thusly directed. Additionally, the echo signal  308  may not be a directional signal and therefore the position of the radio receiver  310  may not be limited. Factors, such as the echo signal  308  strength and any absorption or reflection of the signal by the aspects of the configuration may determine a preferred position of the receiver  310 . 
         [0051]    Referring to  FIG. 5 , the proximity-based detection system  300  may be configured with more than one identification tag to facilitate determining the presence of a plurality of workpieces. A robotic handling system, such as a vacuum semiconductor handling system, may include handling a plurality of workpieces on independent or dependent robotic arms. The embodiment of  FIG. 5  depicts a configuration wherein two workpieces  318 ,  518  are supported on robotic arms, one above the other. A first robotic arm  304  supports a first workpiece  318  that is sensed by a first identification tag  302 . A second robotic arm  504 , positioned below the first robotic arm  304 , supports a second workpiece  518  that is sensed by a second identification tag  502 . A single radio transmitter  314  may transmit a stimulus signal  312  that interacts with each tag. In response to the stimulus signal  314 , each tag may transmit an echo signal that may include characteristics that distinguish an echo signal  308  associated with tag  302  from an echo signal  508  associated with tag  502 . Radio receiver  310  may receive one or more of the transmitted echo signals  308 ,  508  for detection of a presence of workpiece  318  and/or workpiece  518 . 
         [0052]    Each tag may be disposed on a robotic arm so that a workpiece properly supported by the robotic arm may influence how each tag responds to the stimulus signal  312 . Although the configuration of  FIG. 5  depicts the upper robotic arm  304  and lower robotic arm  504  vertically aligned, no such alignment is required for detection of a workpiece on either arm. Therefore, the proximity-based workpiece detection system  300  may facilitate the detection of workpieces on a plurality of robotic arms without requiring predetermined positioning or coordination of movement of the robotic arms. Even if the workpieces are not visible to an operator of the robotic handling system, their presence while being properly supported by a robotic arm may be detected. Although  FIG. 5  depicts a two workpiece configuration, the proximity-based workpiece detection system  300  is not limited to only one or two workpiece detection. Because each tag may be preconfigured with a unique identifier that may impact an echo signal transmitted by the tag in response to a stimulus signal, any number of tags may be configured in the system  300 . 
         [0053]    Radio signals detected by radio receiver  310  may be processed by a processing system, such as a computing system, to determine the presence of the workpieces as they are being handled. The proximity-based workpiece detection system  300  may also be beneficially applied during a workpiece pickup or drop-off operation to facilitate detecting a workpiece prior to pickup, after pickup, prior to drop-off, and ensure the workpiece is no longer supported by the robotic arm after drop-off. 
         [0054]      FIGS. 6A and 6B  depicts an alternate configuration of the proximity-based workpiece detection system  300  wherein two tags are configured to facilitate detecting a workpiece size in addition to the workpiece proximity. Based on characteristics of an echo signal  608 , the workpiece  318  presence may be detected. Based on characteristics of the echo signal  308 , the workpiece  318  size may be determined. The following combinations of echo signals indicate the presence and size of the workpiece as shown in  FIGS. 6A and 6B . In one encoding scheme, an echo signal  608 =“a−” may indicate no workpiece present (not shown). An echo signal  608 =“A−” may indicate a workpiece is present ( FIGS. 6A and 6B ). An echo signal  608 =“A−” and echo signal  308 =“b−” may indicate a small workpiece  628  is present ( FIG. 6B ). An echo signal  608 =“A−” and echo signal  308 =“B−” may indicate a large workpiece  618  is present ( FIG. 6A ). It will be understood that the echo signal or output signal may be adapted in various ways to assist in wafer detection and centering. For example, in one aspect, each tag may simply transmit a predetermined response when activated, using conventional RFID tag technology. The output may be detected (either from within a vacuum chamber, or using RF detectors positioned outside the vacuum chamber) and a processor may infer the presence/position of a wafer based upon the characteristics of the detected output signal. Thus, where a wafer affects propagation of the output signal, these effects can be measured and used to draw inferences concerning wafer presence. In another aspect, each tag may include wafer detection circuitry, such as based upon measurements of capacitance between the tag (or an element of the tag such as a metal pad or the like) and a wafer. This wafer sensor may in turn be used to selectively control output from the tag, such as by selecting one of a discrete set of output signals, or by encoding the measured capacitance in digital or analog form and transmitting the measured capacitance to an RF detector. Where a number of such tags are disposed on an end effector, accurate position data may also be obtained for a wafer. It will be understood that a single tag system may also be employed for detecting position; however, a simple point measurement of capacitance may not be readily converted into wafer position, and may be more generally employed to draw general conclusions about whether a wafer is on center than specific conclusions about two or three dimensional wafer position. A number of other similar techniques may be suitably adapted for use with a radio frequency identification tag system for determining wafer presence and/or position. 
         [0055]    The elements depicted in flow charts and block diagrams throughout the figures imply logical boundaries between the elements. However, according to software or hardware engineering practices, the depicted elements and the functions thereof may be implemented as parts of a monolithic software structure, as standalone software modules, or as modules that employ external routines, code, services, and so forth, or any combination of these, and all such implementations are within the scope of the present disclosure. Thus, while the foregoing drawings and description set forth functional aspects of the disclosed systems, no particular arrangement of software for implementing these functional aspects should be inferred from these descriptions unless explicitly stated or otherwise clear from the context. 
         [0056]    Similarly, it will be appreciated that the various steps identified and described above may be varied, and that the order of steps may be adapted to particular applications of the techniques disclosed herein. All such variations and modifications are intended to fall within the scope of this disclosure. As such, the depiction and/or description of an order for various steps should not be understood to require a particular order of execution for those steps, unless required by a particular application, or explicitly stated or otherwise clear from the context. 
         [0057]    The methods or processes described above, and steps thereof, may be realized in hardware, software, or any combination of these suitable for a particular application. The hardware may include a general-purpose computer and/or dedicated computing device. The processes may be realized in one or more microprocessors, microcontrollers, embedded microcontrollers, programmable digital signal processors or other programmable device, along with internal and/or external memory. The processes may also, or instead, be embodied in an application specific integrated circuit, a programmable gate array, programmable array logic, or any other device or combination of devices that may be configured to process electronic signals. It will further be appreciated that one or more of the processes may be realized as computer executable code created using a structured programming language such as C, an object oriented programming language such as C++, or any other high-level or low-level programming language (including assembly languages, hardware description languages, and database programming languages and technologies) that may be stored, compiled or interpreted to run on one of the above devices, as well as heterogeneous combinations of processors, processor architectures, or combinations of different hardware and software. 
         [0058]    Thus, in one aspect, each method described above and combinations thereof may be embodied in computer executable code that, when executing on one or more computing devices, performs the steps thereof. In another aspect, the methods may be embodied in systems that perform the steps thereof, and may be distributed across devices in a number of ways, or all of the functionality may be integrated into a dedicated, standalone device or other hardware. In another aspect, means for performing the steps associated with the processes described above may include any of the hardware and/or software described above. All such permutations and combinations are intended to fall within the scope of the present disclosure. 
         [0059]    While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is not to be limited by the foregoing examples, but is to be understood in the broadest sense allowable by law. 
         [0060]    All documents referenced herein are hereby incorporated by reference.