Abstract:
In one aspect, a vacuum chuck supports a substrate on an end effector, the vacuum chuck comprising a position reference structure and a suction cup. The position reference structure is mounted to the surface and comprises a reference surface. The suction cup is located proximate the reference surface and comprising a suction mount. In another aspect, a method of chucking a substrate to a vacuum chuck is provided. The vacuum chuck comprises a suction cup and a position reference structure. The method comprises attaching the suction cup to the substrate to form a seal therebetween. The suction cup is deformed such that the substrate contacts the position reference structure. The substrate is then leveled on the position reference structure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for handling substrates in a processing system and more particularly to a vacuum chuck mounted on an end effector. 
     2. Background of the Related Art 
     The advantages of using automated substrate handling devices, or robots, in the fabrication of integrated circuits to transfer substrates, including silicon substrates, throughout a cluster tool are well established. Such cluster tools typically comprise a plurality of process chambers and at least one factory interface all connected by a transfer chamber. The cluster tool processes substrates sequentially during automatic processing of substrates. Cassettes positioned in the factory interfaces will hold one, or a plurality of substrates. One or more robots in the transfer chamber sequentially remove substrates from the cassettes and transfer the substrate to one of the process chambers in the cluster tool. 
     The substrates can then be processed using a variety of processes including physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplating, metal layering, or etching. Some electroplating processes present challenges for robot transfer since substrates are typically processed in a face-down position. Electroplating in integrated circuit design was previously limited to the fabrication of lines on circuit boards. Electroplating is now used to fill vias and contact points in sub-quarter micron, multi-level metallization designs. 
     Robots typically comprise a hub about which the robot rotates, an end effector (commonly called a “blade”), and a plurality of hinged robot links that provide for relative translation of the end effector relative to the robot hub. Traditional end effectors, disposed at a distal end of a robot arm, are positioned underneath a substrate to support the substrate. The end effector may contain some type of chuck (for example vacuum or electrostatic) to hold the substrate to the end effector. During manufacture of IC&#39;s, the “face” portion of the substrate is processed such as with implantation to create devices, and with interconnect structures used to connect the devices. Therefore, the “front” side of the wafer must be maintained as clean as possible, with minimal contact between this from surface and process equipment. The back surface of the wafer, on which relatively little processing occurs, is thus used for substrate storage and transfer. 
     Cluster tools dealing with such modem processing techniques as electroplating require both face-up and face-down handling of substrates. In such cases, the robot “flips” substrates between a face-up position and a face-down position between certain successive processing steps. Flipping substrates with end effectors in which a vacuum chuck securely holds the substrate is desirable, but is difficult to perform. Also, ensuring that the substrate is held in a secured, aligned position relative to the end effector is desirable so that the robot can transfer the substrate without collisions with other known objects and equipment. Transferring substrates in a secure and aligned position increases throughput by reducing the need to align the substrate in a process chamber and decreases the possibility of dropping and damaging the expensive substrates. 
     With certain processing equipment, the robot transports a substrate in a face-down position. A vacuum chuck is typically secured as part of an end effector to allow and accelerate wafer flipping and face-down operations. The vacuum chuck uses vacuum suction applied to a vacuum line with one or more holes formed at one end of the end effector to provide vacuum chucking. After the robot inserts a substrate into an appropriate position in the electroplating process chamber in the face-down position, the substrate separates from the vacuum chuck. The robot then removes the end effector from the process chamber after which the chamber is closed and electroplating occurs. Once the substrate completes electroplating, the robot inserts the end effector into the electroplating process chamber above the substrate. The robot moves into an appropriate position and vacuum chucks the substrate to the end effector. The vacuum chucking process in the inverted orientation is called an inverted hand-off. The substrate is then removed from the chamber. The end effector then flips the wafer into a face-up position for further processing. When the robot transfers substrates in a face-up position, the end effector is located underneath the substrate. Gravity helps ensure flush contact between the end effector and a substrate when the substrate is in the face-up position. 
     One problem encountered in transferring face-down substrates is that it is difficult to align the vacuum chuck of the end effector accurately with the substrate. This difficulty in alignment makes vacuum chucking more complex. Various factors contribute to inaccurate alignment, including inexact calibration of the robot and process chamber misalignments or expansion. Accurate alignment is important in an inverted handoff to provide a good vacuum seal between the end effector and the substrate for reliable vacuum chucking. Inaccurate alignment during inverted hand-offs may result in failed hand-offs, damaged substrates and chambers, and reduced throughput. 
     Another problem with transporting substrates is that the substrate may be skewed relative to the end effector when picked up by the end effector. If the substrate is correctly aligned with respect to the end effector, then the position of the entire substrate can be determined and used by the robot. If, however, the substrate is skewed relative to the end effector, then the position of the substrate is uncertain during further substrate transporting and processing. Certainty of the substrate position is important for the controller of the robot to ensure precise robot movements and transfers of the substrate and thereby avoid colliding the wafer with the process equipment or mis-positioning the substrate in the equipment. Such collisions often result in the substrate chipping or breaking, and resultant contamination within or damage of the process chamber. Misalignments of the substrate with the end effector following inverted handoffs also increase the probability that the robot will drop the substrate during transfer or further processing. 
     Therefore, there is a need for a robot that can reliably secure a substrate to the end effector in a manner that ensures proper alignment during inverted operation or flipping of the substrate. 
     SUMMARY OF THE INVENTION 
     In one aspect, a vacuum chuck supports a substrate on an end effector, the vacuum chuck comprising a position reference structure and a suction cup. The position reference structure is mounted to the surface and comprises a reference surface. The suction cup is located proximate the reference surface and comprising a suction mount. In another aspect, a method of chucking a substrate to a vacuum chuck is provided. The vacuum chuck comprises a suction cup and a position reference structure. The method comprises attaching the suction cup to the substrate to form a seal therebetween. The suction cup is deformed such that the substrate contacts the position reference structure. The substrate is then leveled on the position reference structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of one embodiment of electroplating system; 
     FIG. 2 is a top plan view of the internal components of the electroplating system shown in FIG. 1; 
     FIG. 3 is a top view of a portion of the FIG. 2 system emphasizing one embodiment of a robot system; 
     FIG. 4 is a perspective view of one embodiment of the vacuum chuck; 
     FIG. 5 is a side cross-sectional view of the vacuum chuck in FIG. 4 in an inverted position and misaligned relative to a substrate; 
     FIG. 6 is the vacuum chuck in FIG. 5 after the vacuum chuck is coupled to a substrate; 
     FIG. 7 is a cross sectional view of the flexible suction cup of the embodiment shown in FIG. 5; 
     FIG. 8 is a side cross sectional view of an alternate embodiment of the vacuum chuck from the embodiment shown in FIG. 5; 
     FIG. 9 is a perspective view of an alternate embodiment of a flexible suction cup assembly from that shown in FIG. 7; 
     FIG. 10 is a perspective view of a final embodiment of a vacuum chuck; 
     FIG. 11 is a side cross sectional view of the FIG. 10 vacuum chuck in an inverted position positioned above a misaligned substrate; 
     FIG. 12 is a side cross sectional view of one version of the FIG. 10 embodiment in which the vacuum chuck is chucked to a substrate; and 
     FIG. 13 is a side cross sectional view of another version of the FIG. 10 embodiment in which the vacuum chuck is chucked to a substrate; 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention generally provides a robot end effector including a vacuum chuck, and systems and methods for its use. The vacuum chuck is configured for handling substrates in a processing system that provides accurate alignment between an end effector and a substrate. This alignment is highly repeatable between successive substrates. Initially, a cluster tool  10  that performs electroplating is described as one application of robot that uses an end effector having a vacuum chuck. The structure of multiple embodiments of end effectors is then detailed. Finally, the operation of the end effector is described. 
     I. Electroplating Processes and Equipment 
     FIG. 1 is a perspective view of a cluster tool  10  (shown in top plan view in FIG. 2) that performs a variety of sequential processes such as electroplating. The cluster tool  10  is a representative system. The actual cluster tool  10  may include an electroplating chamber, a physical vapor deposition (PVD) chamber, a chemical vapor deposition (CVD) chamber, or any known process chamber or combination thereof. 
     The cluster tool  10  generally comprises an electroplating segment  11 , a factory interface  12 , a thermal anneal chamber  14 , and a mainframe  18 . As shown in FIG. 2, the factory interface  12  includes one or more cassettes  30 , one or more factory interface transfer robots  32 , and at least one substrate orienter  34 . A clean environment partially enclosed by clear panels  27  encloses the electroplating system. The mainframe  18  generally comprises a spin-rinse-dry (SRD) station  16 , a mainframe transfer robot  22 , an electrolyte replenishing system  20 , a plurality of plating stations  24 , and controller  28 . Each plating station  24  includes one or more plating cells  26 . The electrolyte replenishing system  20  refreshes the electrolyte used in the plating stations  24  to ensure the chemistry is maintained. The mainframe transfer robot  22  transfers substrates between different components (for example plating cells  26  and SRD station  16 ) within the mainframe  18 , and orients the substrate  38  into a position where it can be accepted by the different components. 
     FIG. 3 is an enlarged plan view of an embodiment of a transfer robot  22  used within a cluster tool  10 . The mainframe transfer robot  22  is capable of both linear and rotational (i.e., orbit of the end effector about a pivot point) motions. The mainframe transfer robot  22  comprises a robot linkage including a plurality of robot arms  42  that comprise an inversion mechanism  40 . An end effector  44  capable of supporting a substrate is disposed at the distal end of each robot arm  42 . The mainframe transfer robot  22  includes a robot arm  42  comprising an inversion mechanism  40 . The inversion mechanism (comprising a servo motor or stepper motor to be controlled by controller  28 ) inverts or flips the end effector  44  and the substrates  38  supported thereon between a face-up position and a face-down position. Substrates  38  are then further processed in the SRD station  16 , the thermal anneal chamber  14 , the process chamber  13 , and ultimately placed back into the cassettes  30  for storage or removal. The transfer robot  22  is a representative embodiment, and it is envisioned that the term robot comprises any robot having an end effector, robot blade, or other element that supports a vacuum chuck. 
     The mainframe transfer robot  22  chucks, transport, releases, and flips the substrate  38  in both the face-up and face-down positions, at the direction of a controller. The controller  28  typically comprises programmable central processing unit (CPU)  29 , a memory  31 , support circuits  33 , and a bus (not shown). The controller  28  can be a microprocessor, a general purpose computer, or any other known type of computer. The controller  28  controls the overall operation of the cluster tool as well as the robot motions of the mainframe transfer robot  22  and the factory interface transfer robot  32 . 
     The vacuum chuck is particularly useful in cluster tools  10  comprising a plurality of process chambers, in which certain substrates are processed in a face-down position. In a face-down position, the end effector is positioned above a substrate with the vacuum chuck connected to the upwardly facing back surface of the substrate, with the face or “front” side of the substrate to be processed facing downward. In a face-up position, the end effector is positioned below the substrate with the vacuum chuck secured to the bottom surface of the substrate, and the face of the substrate to be processed facing upward. Examples of face-down process chambers include certain electroplaters in which metal (such as copper) is layered on the bottom of the substrates. Examples of face-up chambers include physical vapor deposition (PVD) and chemical vapor deposition (CVD) in which processing occurs on the top surface of the substrate. The vacuum chuck described below is useful where substrates are picked up in a face-down position (also called an inverted hand-off). In addition, the vacuum chuck is useful where substrates are “flipped” between a face-up position and a face-down position or carried in a face-down position. This vacuum chuck is applicable to metal deposition systems that typically operate under approximately atmospheric pressures. However, the pressures applied to the vacuum chuck can be slightly modified to permit operation of the vacuum chuck at different pressures. 
     II. Vacuum Chuck Embodiments 
     Multiple embodiments of vacuum chucks are now described. Any of the following embodiments of vacuum chuck may accomplish flipping of a substrate, an inverted hand-off, and carrying a substrate in both the face-up and face-down substrate positions. 
     One embodiment of vacuum chuck  400  is depicted in FIGS. 4-6. Another embodiment of vacuum chuck  800  is depicted in FIG.  8 . Another embodiment of vacuum chuck  900  is depicted in FIG.  9 . Yet another embodiment of vacuum chuck  1000  is depicted in FIGS. 10-13. While these embodiments are illustrative, they are not intended to limit the scope of vacuum chuck set forth in the claims. 
     In each vacuum chuck embodiment, the vacuum chuck comprises a suction cup member and positioning member. The positioning member has a positioning edge used by the vacuum chuck in aligning the substrate with the end effector  44 . The positioning edge acts as a reference surface that makes it possible for the robot to support a substrate in a position that is aligned in a known position with respect to the robot. Such alignment is highly repeatable. Additionally, in each embodiment of the vacuum chuck, a vacuum is generated between substrate  38  and the end effector  44  that maintains a vacuum suction on the backside of the substrate, which is sufficient to hold and support substrate  38 . 
     A. Embodiment depicted in FIGS. 4-6 
     FIGS. 4,  5 , and  6  are respectively perspective, side cross sectional, and side cross sectional views of one embodiment of vacuum chuck  400 . The vacuum chuck  400  comprises a position reference structure  404 , a flexible suction cup  406 , a vacuum port  408 , and a controllable vacuum device  409 . The position reference structure  404  has a reference surface  412  formed thereon. The flexible suction cup  406  has a suction cup surface or seal  410  formed thereon. The vacuum chuck  400  is affixed to a surface  414  of the end effector  44 . In FIG. 5, the vacuum chuck  400  is inverted, similar to as it would appear when approaching substrate  38  to enter a process tool. FIG. 6 shows the vacuum chuck as it appears when engaged with a substrate (such as when inserting substrate  38  into an electroplating chamber where inverted substrate processing occurs). 
     The term “rigid” and “flexible” are used in this disclosure as a relative term, and are not meant to imply that the respective element is completely rigid or completely flexible. For example, certain hard rubber or plastic elements may provide sufficient structural rigidity to be considered rigid. Other relatively softer elements may provide sufficient structural resilience to be considered flexible. 
     The compliancy of the seal enhances its sealing capability, and permits the vacuum chuck to pick up a substrate even if the substrate is not aligned parallel with the chucking surface of the end effector, or where the substrate has contamination or slight discontinuities thereon. With respect to copper plating, it is known that crystals of copper and other contaminants conform on the substrate (sometimes as thick as {fraction (1/16)} th  inch thick) making vacuum chucking difficult. Materials of the suction cup and coatings applied to the suction cup preferably allow operation under corrosive and other types of hazardous chemical environments without undue erosion or degradation of the suction cup. The seal extends about the periphery of the suction cup surface  410  and is configured to be deformable. Thus if the backside of a wafer being picked up by the suction cup has a slight irregularity, the seal may deform slightly from its circular-planar outline to conform to the outline of the irregularity. When vacuum is applied within the seal, the suction cup surface further conforms to the shape of the backside of the wafer. 
     The flexible suction cup  406 , when contacted with the backside of a substrate, forms and maintains a vacuum in the volume between the seal and the substrate  38 . A vacuum port  408  extends through both the bottom of the flexible suction cup  406  and the position reference structure  404 . The vacuum port  408  is in communication via line  421  (built into the end effector) to a vacuum pump  422 . In one example, the flexible suction cup  406  is approximately an inch in diameter. The necessary pressure to be applied to the suction cup  406  to support the substrate depends upon the size of the substrate and the configuration of the suction cup  406  and position reference structure  404  i.e., a greater diameter cup will require less vacuum where a larger substrate will require more vacuum. The flexible suction cup  406  comprises a base portion  424  and a cup portion  426  shown in detail in FIG.  7 . The cup portion  426  includes a suction mount surface  410 . The suction cup is formed of such an elastomeric material as Buna-N, VITON™, EPDM, silicone rubber, CHEMRAZ® (CHEMRAZ is a trademark owned by Green, Tweed, &amp; Co.), KALRAZ® (KALRAZ is a trademark owned by Green, Tweed, &amp; Co.), or TEFLON® (TEFLON is a trademark owned by E. I. duPont Nemoirs and Company) impregnated rubber. Alternately, the suction cup is formed from a thin compliant metallic material, which may also be coated, in particular where it contacts the substrate, with a more compliant material. 
     The flexible suction cup  406  may include a coating  428  such as TEFLON® formed on the interior surface, around the suction mount surface  410 , and/or possibly even around the outside of the flexible suction cup  406 . The TEFLON™ may be applied in different thicknesses depending upon the desired suction cup characteristics. TEFLON® is highly lubrous and enhances the sliding action of the flexible suction cup  406 . Portions of the flexible suction cup  406  coated with TEFLON® such as the suction mount surface  410  may slide along a substrate surface  420  when a seal is being formed between the suction cup and the substrate surface, i.e., as the substrate  38  and reference surface are brought together, the surface  410  may expand in circumference as it is pushed toward the recess in the reference surface within which the cup resides. 
     The coating on the flexible suction cup  406  limits mechanical resistance between the suction cup  406  and the substrate surface  420 . Debris or other contaminants and irregularities on the backside of the substrate often make it difficult to vacuum chuck against substrate surface  420 , where the contaminants limits the substrate from adhering with a firm contact to the substrate surrounding the vacuum source or port. The above sliding action assists in forming an adequate seal between the substrate surface  420  and the suction cup  406 . 
     The flexible suction cup may include a chemically resistant coating such as PFA or TEFLON thereon that allows the end-effector to perform in a chemically aggressive environment without undue corrosion or erosion thereof. The coating  428  is also believed to limit the degradation of the suction cup  406  when exposed to harsh chemicals that may exist on substrate  38 . Consequently, the coating extends the expected lifetime of the flexible suction cup  406 . 
     The coating is also believed to regulate the characteristics of the flexible suction cup  406  depending upon the coating&#39;s thickness. For example, a flexible suction cup  406  having a thicker coating may be used with substrates expected to have a thick crystalline layer on the backside, such that the coating would be able to displace more of the crystals and minimize suction cup leaks. Coating of the flexible suction cup  406  may limit its sticking. By comparison, a flexible suction cup  406  having a thinner coating may be used with substrates that are free of a thick crystalline formation or harsh chemicals, such that a more flexible suction cup  406  may provide an improved seal. 
     The position reference structure  404  has a reference surface  412  and a recess  416  formed therein. The position reference structure  404  is formed from material such as hard elastomerics and metals. The base portion  424  of the flexible suction cup  406  mounts to a wall of recess  416  formed in the position reference structure  404 . The position reference structure  404  is used as a repeatable attitude reference for locating a substrate relative to a vacuum chuck. Robots can precisely position a vacuum chuck relative to a substrate that is maintained at a known three-dimensional location. Known orienter devices are used to locate substrates at a desired three-dimensional position such that a robot can easily attach a vacuum chuck to the backside of a substrate as desired. When the flexible suction cup attaches to a desired and known location on the backside of the substrate, the attitude and the position of the substrate is known relative to the vacuum chuck are known. Thus, any substrate abutting the position reference structure  404  is substantially aligned to the surface  414  of the end effector  44 . The controller  28  shown in FIG. 2 can readily determine the position of the outline of a substrate based upon its relation to the end effector because each substrate is held in a repeatable position by the vacuum chuck  400 . 
     FIG. 5 shows a vacuum chuck approaching a substrate  38 . The suction mount surface  410  of the flexible suction cup  406  is spaced from the surface  414  of the end effector  44  by a first distance shown by the arrow  502 . The reference surface  412  of the position reference structure  404  is spaced from the surface  414  by a second distance shown by he arrow  504 . The first distance is greater than the second distance. Thus, the suction mount surface  410  contacts a substrate  38  before the reference surface  412  contacts the substrate. Due to the flexibility of the flexible suction cup  406 , as the suction cup  406  engages with substrate, a larger percentage of the suction mount surface  410  will contact the substrate until the entire circumference of surface  410  contacts the substrate. 
     As the vacuum chuck  400  progresses downward against the substrate backside surface, the entire periphery of the suction mount surface  410  will contact substrate  38  due to the compliancy of the flexible suction cup  406 . In this position, the vacuum pump  422  creates a vacuum between the interior surface  418  of the suction cup  406  and the substrate  38 . The seal is improved by the pressure difference created by the vacuum pump  422  that pumps out the internal space  419 . The flexible suction cup adapts its form to the substrate surface creating the seal necessary to maintain a vacuum in the volume between the wafer and the seal even if the end effector  44  and the substrate surface are not parallel. 
     Once the vacuum seal is formed, the vacuum created between a substrate  38  and the flexible suction cup  406  deforms the suction cup. This deformation “pulls” the substrate upward (i.e., against reference surface  412 ) until the substrate abuts with the reference surface  412 . Since reference surface  412  is aligned with the end effector  44 , having the substrate  38  abut the reference surface  412  results in the substrate  38  positioning (or aligning) with the end effector  44 . Generation of a vacuum in the interior space  419  acts to bias the substrate  38  against the reference surface  412 . 
     Substrate  38  is considered parallel or aligned to the surface  414  of the end effector  44  when its surface abuts with the reference surface  412 . Though the face of the end effector  44  may be selected such that the substrate surface is not exactly parallel to the face, the term “parallel” within this specification indicates that the substrate is aligned with the end effector in a known, repeatable manner. Thus, the controller  28  that controls operation of the robot can determine or compensate for some calibrated location of a substrate relative to the end effector  44 . 
     The vacuum chuck  400  provides for a highly repeatable positioning arrangement, in which each successive substrate  38  that is vacuum chucked has a back surface which is aligned with the end effector  44 . This feature of repeatability applies to the latter embodiments described below as well. Such repeatability is important where further processing follows the vacuum chucking. 
     B. Embodiment depicted in FIG. 8 
     FIG. 8 shows a side cross-sectional view of another embodiment of vacuum chuck  800 . The vacuum chuck  800  is attached to the end effector  44 . The vacuum chuck  800  comprises a position reference structure  802 , a flexible suction cup  806 , a vacuum port  814 , and an air shield  820 . The position reference structure  802  includes a reference surface  808 . The flexible suction cup  806  includes a suction mount surface  810 . The vacuum chuck  800  comprises a flexible suction cup  806  including a suction mount surface  810  and a position reference structure  802  having a reference surface  808 . The flexible suction cup  806  extends around and outwardly of the plane of the reference surface  808 . Additionally, the reference surface  808  preferably extends outwardly of and above the plane defined by the surrounding face  816 , but may be on the same plane. This configuration permits a substrate  38  to abut the reference surface  808  before is abuts the surrounding face  816 . 
     Thus, in the embodiment shown in FIG. 8, as the end effector is positioned to engage a substrate, the contact is initially made between the suction mount surface  810  and the substrate, before the reference surface  808  contacts substrate. The suction cup thus initially deforms on contact with a substrate  38  as a seal is created between the substrate and the flexible suction cup  806  and a vacuum is created in the resulting volume formed. However, when a substrate is coupled to the vacuum chuck  800 , substrate  38  is positioned in a repeatable position vis-a-vis the robot components against the reference surface  808 . The reference surface  808  contacting the substrate provides a high degree of positioning repeatability, in which a substrate is aligned relative to the end effector  44  each time that the vacuum chuck couples to a substrate. The reference surface  808  is within an enclosure  812  defined between a substrate and the suction cup  806  (when the substrate is in position on the end effector). To provide a pumping aperture, the vacuum port  814  opens to the side of the position reference structure  802  and into the volume formed by the flexible suction cup and the substrate. Thus, fluid communication with the enclosure  812  is maintained regardless of whether a substrate abuts the reference surface  808 . 
     The vacuum chuck  800  also may comprise an air shield  820  that directs air into the recess  818  circumferentially formed between the surrounding face  816  and the flexible suction cup  806 . The air shield  820  comprises air nozzles  822 , tubing  824 , controllable valve  826 , and air source  828 . An air source  828  directs air selectively (under the control of controllable valve  826 ) via the tubing  824  to the air nozzles  822 . The air nozzle  822  projects air at a sufficient velocity to drive off or dry liquids disposed on substrate  38  adjacent the vacuum chuck  800 . In various processes, when a substrate is vertically orientated (as they are being flipped between the face-up position and a face-down position) liquid about the periphery of the substrate sometimes runs into the center of the substrate to a position where it is desired to attach or detach the vacuum chuck. A wet substrate chucked by the vacuum chuck  800  can increase surface tension between the flexible suction cup  806  and substrate  38 , and may increase the force required to dechuck substrate  38 . The air chuck acts as a shield to limit passage of liquid (either by diversion or evaporation) along the surface of the substrate to where it is desired to vacuum chuck the substrate. As the vacuum chuck  800  is moved to chuck the water, the air shield  820  can be used to blow off liquids and other impurities from the surface of the wafer. Thus, there is a reduced possibility of damaged or broken chucks (which can also damage the processing equipment) when using the air shield  820 . The air shield  820  may be easily adopted with the other embodiments of the invention described herein. 
     C. Embodiment depicted in FIG. 9 
     FIG. 9 illustrates a perspective view of yet another embodiment of vacuum chuck  900  attached to the end effector  44 . The vacuum chuck  900  comprises sliding sealing suction cup  902 , a positioning chuck  904 , and a flexible biasing element  906 . The sliding sealing suction cup  902  includes a suction mount surface  910  that is capable of forming a seal with a substrate. The positioning chuck  904  comprises a generally planar reference surface  912 . The sliding sealing suction cup  902  has a generally cylindrical outer surface  920 , and the positioning chuck  904  has a generally cylindrical inner surface  922 . The cylindrical outer surface  920  can slide relative to the cylindrical inner surface  922  to provide motion between the sliding sealing suction cup  902  and the positioning chuck  904  in a direction indicated by arrow  908 . The flexible biasing element  906  preferably comprises a canted spring that biases the sliding sealing suction cup  902  upwardly. The suction mount surface  910  extends above the reference surface  912  (the directions are as indicated in FIG.  9 ). An upper vacuum chamber  914  is defined within the sliding sealing suction cup  902 . A lower vacuum chamber  916  is defined between the edge effector  44  and a lower surface of the sliding sealing suction cup  902 , and within the reference surface  912  of the positioning chuck  904 . Upper vacuum chamber  914  and lower vacuum chamber  916  combine to connect a vacuum pump to an enclosure  918  formed between the sliding sealing suction cup  902  and substrate. 
     The clearance between the sliding sealing suction cup  902  and the positioning chuck  904  (between relative cylindrical surfaces  920  and  922 ), in combination with the flat annular surface of cup  902 , enables establishing of a seal when the plane of the substrate  38  is not parallel to the vacuum chuck  900 . This making of this seal is enhanced by the radial clearance between the inner peripheral surface  922  (of the rigid reference chuck  904 ) and the outer peripheral surface  920  (of the sliding sealing suction cup  902 ) permits “tipping” of the sliding sealing suction cup  902  relative to the rigid reference chuck i.e. the axis of the cylindrical section of the positioning member can be set as set forth in the positioning member  904 . This tipping permits the suction mount surface  910  to align with a substrate  38  that is slightly skewed or misaligned during the establishing of a seal between the substrate and the reference surface. The flexible biasing element  906  biases the sliding sealing suction cup  902  in a vertical upward direction. A compliant sealed volume is formed between the sliding seal suction cup  902  and the internal face  926 . The compliant sealed volume allows for a creation of a vacuum in the vacuum chamber  910  by vacuum pump  422 . The creation of the vacuum biases the suction mount surface  918  against the substrate. The flexible biasing element  906  is preferably formed from a hard elastomeric material that can be deformed slightly to form a seal against a substrate when the biasing element  906  biases the sliding sealing suction cup  902  against the substrate. This seal can be formed even when the sliding seal suction cup  902  is skewed relative to the substrate. 
     Thus, when the vacuum chuck  900  shown in FIG. 9 initially chucks a substrate, a first point of the suction mount surface  910  of the sliding sealing suction cup  902  contacts the substrate except in the case where the substrate plane and the plane of the mount surface  910  are parallel. As the substrate and the end effector move closer together, the suction mount surface  910  cants into alignment with the plane of the substrate  38 , thus permitting the entire suction mount surface  910  to contact substrate  38 . The vacuum device  422  then creates a vacuum in the enclosure  918  defined between the substrate aligned with the sliding sealing suction cup  902  and an internal face  926  of the end effector  44  by pumping air there between. This vacuum is sufficient to displace the substrate  38  and the sliding sealing suction cup  902  toward the end effector  44 , against the upward bias of the flexible biasing element  906 . This sliding continues until the substrate  38  abuts with the reference surface  912 , thereby positioning the substrate relative to the vacuum chuck  900 . Based upon this positioning, the controller  28  that controls the operation of the transfer robot  22  including the vacuum chuck  900  can be assured of the horizontal position of a substrate that is vacuum chucked during transfer and processing. Furthermore, inverted hand-offs and flipping of substrates are secure. 
     D. Embodiment depicted in FIGS. 10-13 
     FIG. 10 shows a perspective view of another embodiment of vacuum chuck  1000 . Vacuum chuck  1000  comprises a flexible suction cup  1002 , a plurality of inner positioning mounts  1004 , and a plurality of outer positioning mounts  1006 . The inner positioning mounts  1004  comprise an inner reference surface  1008  and spaces  1012  between each one of the inner reference surface  1008 . The outer positioning mounts  1006  comprise outer reference surface  1010 . FIGS. 12 and 13 show a cross sectional view of two modifications of the FIG. 10 embodiment in which the vacuum chuck  1000  is chucked to a substrate in a face-down position. In FIG. 12, the inner reference surface  1008  is closer to the end effector  44  than the outer reference surface  1010 . In FIG. 13, the inner reference surface  1008  is further from the end effector  44  than the outer reference surface  1010 . 
     FIG. 11 shows the FIG. 10 embodiment in a cross sectional view. Both the outer reference surface  1010  and the inner reference surface  1008  are spaced from a face  414  of the end effector  44  by respective distances shown as  1020  and  1022 . Both distances  1020  and  1022  are smaller than a distance shown as  1021  between suction mount surface  1016  of the flexible suction cup  1002  and face  414 . Therefore, the flexible suction cup  1002  typically contacts a misaligned substrate  38  prior to contact of the substrate with the outer reference surface  1010  or the inner reference surface  1008 . The vacuum pump  422  will generate a vacuum within the flexible suction cup  1002  and the substrate by evacuating air therefrom. The vacuum will deform the flexible suction cup  1002 , thereby bringing the substrate  38  closer to the end effector  44 . The substrate  38  will level against either the outer reference surface as depicted in FIG. 11, or the inner reference surface as depicted in FIG. 12 depending upon which is located closer to the substrate, as will be discussed further herein. Alternatively, the distance  1020  can be selected to equal the distance shown as  1022  such the substrate  38  will level against both the inner reference surface or the outer reference surface simultaneously. 
     If the inner reference surface  1008  is closer to the end effector  44  than the outer reference surface  1010  as shown in FIG. 12, then substrate  38  will level against the outer reference surface. As the flexible suction cup  1002  deforms during the positioning process, the distance  1021  shown in FIG. 11 will decrease until it equals  1020 . When the substrate is leveled in the vacuum chuck, the substrate  38  is biased against the outer reference surface  1010 . This positioning can be repeatably performed between successive substrates. In FIG. 12, where the substrate is chucked and the inner reference surfaces  1018  are still spaced from the substrate, they will only contact the substrate if the substrate is bowed, which may damage a substrate. Thus, the vertical dimension between the inner reference surface  1008  and the outer reference surface  1010  can be selected to limit bowing. Alternatively, in the FIG. 12 embodiment, the inner positioning mounts  1004  can be eliminated. 
     FIG. 13 shows a vacuum chucked substrate where the inner reference surface  1008  extends further from the end effector  44  than the outer reference surface  1010 . A substrate being chucked will be supported by the inner reference surface  1008  before it is supported by the outer reference surface  1010 . When the substrate is positioned on the end effector, the spaces  1012  between the inner reference surfaces  1008  maintain a fluid communication between the vacuum pump  422  and volume  1018  defined between the flexible suction cup  1002  and a substrate  38 . The outer reference surface  1010  limits the warping and the amount of tilt that can occur between the vacuum chuck and an engaged substrate. If there is a vacuum failure, the wafer will tilt and rest on the outer reference surface  1010  without the substrate falling from the end effector when the wafer is in a face-up position. In the FIG. 13 embodiment, the outer positioning mound can be eliminated while still allowing vacuum chucking of the substrate. 
     III. Operation 
     One representative example of operation of a transfer robot  22  comprising end effectors having a vacuum chamber as described above is now described. 
     In cluster tool  10 , the factory interface transport robot  32  transports substrates  38  contained in the cassettes  30  (stored in a face-up position) into the electroplating segment  11 . The substrate orienter  34  positions each substrate  38  in an orientation to ensure that the substrate  38  properly aligns with the end effector during processing. The factory interface transfer robot  32  transports substrates  38  between the factory interface  12 , the spin-rinse-dry (SRD) station  16 , and the thermal anneal chamber  14  as desired. 
     After the factory interface transfer robot  32  inserts the substrate  38  face-up into the SRD chamber  16 , the transfer robot  22  picks up substrate  38  using end effector  44  (by vacuum chucking the substrate to the end effector) in a face-up position. The transfer robot  22  transfers the substrate  38  to a position in the mainframe  18 . The inversion mechanism  40  of the end effector  44  then flips or inverts the substrate  38  into the face-down position. The robot arm  42  then inserts the substrate  38  face-down into the plating cell  26 . The vacuum chuck of the end effector  44  is then dechucked from substrate  38 , and the robot removes the end effector from the plating cell  26  while the substrate remains in the plating cell. Substrate  38  is then processed within the plating cell  26 . 
     After processing, the transfer robot  22  re-enters the plating cell  26  and chucks the substrate  38 , and removes the substrate from the plating cell. The transfer robot  22  then transfers substrate  38  from the plating chamber  24  to the mainframe  18 . The end effector  44  then flips substrate into a face-up position and transfers the substrate  38  for further face-up processing (if necessary). The transfer robot  22  then transfers substrate  38  into the SRD chamber  16 . The factory interface transfer robot  32  then transfers substrate  38  into thermal anneal chamber  14  into the factory interface  12  and onto further processing if necessary. The order and type of substrate processing is dependent upon the processing to be performed. 
     While 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.