Abstract:
A method and apparatus for transferring substrates is provided. In one embodiment, an apparatus for transferring substrates includes a first evacuable chamber and a evacuable second chamber separated by a third chamber, and a first transfer mechanism. The first transfer mechanism is movable between a first position located in the first chamber and a second position located in the third chamber. The first transfer mechanism includes a first seal plate coupled to a second seal plate and one or more substrate holders. The seal plates provides vacuum seals on opposing sides of the first chamber thereby balancing a force required to hold the first transfer mechanism and seal the first chamber from the third chamber during transfer of substrates from the substrate holder.

Description:
[0001]    This application claims benefit of U.S. Provisional Application No. 60/287,322, filed Apr. 30, 2001, which is hereby incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE DISCLOSURE  
         [0002]    1. Field of Invention  
           [0003]    The embodiments of the invention generally relate to a method and apparatus for transferring substrates in a semiconductor processing system.  
         BACKGROUND OF INVENTION  
         [0004]    Semiconductor substrate processing is typically performed by subjecting a substrate to a plurality of sequential processes to create devices, conductors and insulators on the substrate. These processes are generally performed in a process chamber configured to perform a single step of the production process. In order to efficiently complete the entire sequence of processing steps, a number of process chambers are typically coupled to a central transfer chamber that houses a robot to facilitate transfer of the substrate between the surrounding process chambers. A semiconductor processing platform having this configuration is generally known as a cluster tool, examples of which are the families of CENTURA® and ENDURA® processing platforms available from Applied Materials, Inc., of Santa Clara, Calif.  
           [0005]    Generally, a cluster tool consists of a central transfer chamber having a robot disposed therein. The transfer chamber is generally surrounded by one or more process chambers. The process chambers are generally utilized to sequentially process the substrate, for example, performing various processing steps such as etching, physical vapor deposition, chemical vapor deposition, ion implantation, lithography and the like. As the processes performed in the process chambers are generally performed at vacuum pressure, the transfer chamber is maintained at vacuum pressure as well to eliminate having to repeatedly pump down the process chamber for each substrate transfer. This is important as pumping down the transfer chamber may require as much as eight hours to reach operational vacuum levels.  
           [0006]    Load lock chambers are generally used to facilitate transfer of substrates between the vacuum environment of the transfer chamber and an environment of a factory interface wherein substrates are stored in cassettes. The factory interface is typically at or near atmospheric pressure. The load lock chambers are selectively isolated from the factory interface and transfer chamber by slit valves. Generally, at least one slit valve is maintained in a closed position to prevent loss of vacuum in the transfer chamber during substrate transfer through the load lock. For example, an interface slit valve is opened while a chamber slit valve is closed to allow an interface robot to transfer substrates between the load lock chamber and the substrate storage cassettes disposed in the factory interface. After the substrate is loaded from the interface robot, both slit valves are closed as the load lock chamber is evacuated by a pump to a vacuum level substantially equal to that of the transfer chamber. The substrate in the evacuated load lock is passed into the transfer chamber by opening the chamber slit valve while the interface slit valve remains closed. Processed substrates are returned to the factory interface in the reverse manner, wherein load lock chamber is vented to substantially equalize the pressure between the load lock chamber and the factory interface.  
           [0007]    There are generally two types of load lock chambers utilized to interface with the transfer chamber. A first type is known as a batch-type chamber. The batch-type chamber generally holds an entire substrate storage cassette within the chamber. The cassette is first loaded into the load lock chamber and the chamber is sealed and pumped down to an appropriate vacuum level. The chamber is then opened to the transfer chamber so that the robot within the transfer chamber may access any of the substrates and storage slots within the cassette until all of the substrates within the cassette have been processed. After all the substrates have been returned to the cassette, the load lock chamber is isolated from the transfer chamber to facilitate replacing the cassette with another cassette containing un-processed substrates. While the cassettes are being exchanged, the transfer robot typically draws substrates from a cassette disposed in a second load lock chamber coupled to the transfer chamber.  
           [0008]    The use of batch-type load lock chambers is generally a robust and effective system for transferring substrates into the transfer chamber. However, due to the relatively large internal volume required to accommodate the entire substrate cassette, pump-down times are long and the associated pumping hardware is large and costly. Additionally, venting of the large internal volume increases the chance of particulate contamination and condensation on the substrates.  
           [0009]    The second type of load lock chamber is known as a single substrate-type. Generally, the single substrate-type load lock chamber shuttles one processed and one unprocessed substrate therethrough each time the load lock chamber is pumped down. To maintain high system throughput, single substrate-type load lock chambers are typically used in tandem. This allows a first load lock chamber to exchange a substrate in the vacuum environment with the transfer chamber while a second load lock chamber exchanges a substrates in the ambient environment with the factory interface.  
           [0010]    Since cluster tools often utilize more than one load lock to maintain high substrate transfer rates, the cost of ownership is more than if a single load lock chamber could be utilized. Moreover, if one of the load lock chambers could be eliminated, an additional process chamber could be utilized in the open facet of the transfer chamber, thus enhancing system throughput.  
           [0011]    Therefore, there is a need for an improved load lock chamber.  
         SUMMARY OF INVENTION  
         [0012]    One aspect of the present invention generally provides a method and apparatus for transferring substrates. In one embodiment, an apparatus for transferring substrates includes a first evacuable chamber and a evacuable second chamber separated by a third chamber, and a first transfer mechanism. The first transfer mechanism is movable between a first position located in the first chamber and a second position located in the third chamber. The first transfer mechanism includes a first seal plate coupled to a second seal plate and one or more substrate holders. The seal plates provides vacuum seals on opposing sides of the first chamber thereby balancing a force required to hold the first transfer mechanism and seal the first chamber from the third chamber during venting, pump down and transfer of substrates from the substrate holder.  
           [0013]    In another embodiment, the apparatus further comprises a second transfer mechanism. The second transfer mechanism includes a first seal plate coupled to a second seal plate and one or more substrate holders. The seal plates provides vacuum seals on opposing sides of the second chamber thereby balancing a force required to hold the second transfer mechanism and seal the second chamber from the third chamber during transfer of substrates from the substrate holder.  
           [0014]    In another aspect, a method for transferring substrates is provided. In one embodiment, the method includes providing at least one substrate disposed on a first substrate transfer mechanism disposed in an intermediate region of a load lock chamber, transferring substrates disposed on the first substrate transfer mechanism between the intermediate region of the load lock chamber and a transfer chamber, the transfer chamber having a first pressure and one or more process chambers coupled thereto, moving substrates disposed on the first substrate transfer mechanism from the intermediate region to a first region of the load lock chamber, sealing the first region from the third region by a first seal plate of the first substrate transfer mechanism, applying a secondary vacuum force to the first substrate transfer mechanism to offset a primary vacuum force applied to the first seal plate by the first pressure, and venting the first region to about the second pressure.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    The teachings of the present invention can readily be understood by considering the following detailed description in conjunction with the accompanying drawings in which:  
         [0016]    [0016]FIG. 1 depicts a plan view of a substrate processing system that includes one embodiment of a load lock chamber of the invention;  
         [0017]    [0017]FIG. 2 depicts a sectional view of one embodiment of a load lock chamber in a first position;  
         [0018]    [0018]FIG. 3 depicts a sectional view of the load lock chamber of FIG. 2 in a second position;  
         [0019]    [0019]FIG. 4 depicts a perspective view of one embodiment of a substrate holder;  
         [0020]    [0020]FIG. 5 depicts a perspective view of one embodiment of a load lock chamber; and  
         [0021]    [0021]FIG. 6 depicts another perspective view of the load lock chamber of FIG. 5.  
         [0022]    To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0023]    [0023]FIG. 1 depicts a processing system  100  that generally includes a factory interface  102 , a load lock chamber  106 , a plurality of process chambers  108  and a substrate transfer chamber  104 . The transfer chamber  104  is generally used to transfer substrates  124  between a vacuum environment maintained in the transfer chamber  104  and a substantially ambient environment maintained in the factory interface  102 . One example of a processing system that may be adapted to benefit from the invention is an ENDURA SL® processing platform, available from Applied Materials, Inc., of Santa Clara, Calif. Although the load lock chamber  106  is described disposed in the exemplary processing system  100  depicted in FIG. 1, the description is one of illustration and, accordingly, the load lock chamber  106  has utility wherever transfer of substrates between vacuum and ambient environments is desired.  
         [0024]    The factory interface  102  generally includes an interface robot  120  and a plurality of bays  128 . Each bay  128  is adapted to receive a substrate storage cassette  130 . Generally, the factory interface  102  is coupled to the load lock chamber  106  through a port  136  that is positioned opposite the bays  128 . The interface robot  120  includes a first gripper and a second gripper coupled thereto. Generally, each gripper may be an edge gripper, vacuum gripper or other substrate securing device used to hold the substrate  124  during transfer. The interface robot  120  is generally positioned between the port  136  and bays  128  to facilitate transfer of substrates between the cassettes  130  and the load lock  106 . An example of one factory interface that may be adapted to benefit from the invention is described in U.S. patent application Ser. No. 09/161,970, filed Sep. 28, 1998 by Kroeker, which is hereby incorporated by reference in its entirety.  
         [0025]    The transfer chamber  104  is generally fabricated from a single piece of material such as aluminum. The transfer chamber  104  generally includes side walls  150  and chamber bottom  156 . A lid  138  is supported by the side walls  150  and, with the side wall  150  and chamber bottom  156 , define an evacuable interior volume  122  therebetween. Substrates  124  are transferred between the process chambers  108  and load lock chambers  106  coupled to the exterior of the chamber  104  through the vacuum maintained within the volume  122 .  
         [0026]    At least one transfer robot is disposed in the transfer chamber  104  to facilitate transfer of substrates between the process chambers  108 . In the illustrative embodiment depicted in FIG. 1, a first transfer robot  112 A and a second transfer robot  112 B are disposed in the interior volume  122  of the transfer chamber  104 . The robots  112 A,  112 B may be of the dual or single blade variety. The robots  112 A,  112 B typically have a “frog-leg” linkage that is commonly used to transfer substrates in vacuum environments. The first robot  112 A is generally disposed in an end of the transfer chamber  104  adjacent the load locks  106 . The second robot  112 B is disposed in an opposite end of the transfer chamber  104  such that each robot  112 A,  112 B services the adjacent process chambers  108 .  
         [0027]    One or more transfer platforms  118  are generally provided in the interior  122  of the chamber  104  to facilitate substrate transfer between robots  112 A,  112 B. For example, a substrate retrieved from one of the load locks  106  by the first robot  112 A is set down on one of the platforms  118 . After the first robot  112 A is cleared from the platform  118  supporting the substrate  124 , the second robot  112 B retrieves the substrate from the platform  118 . The second robot  112 B may then transfer the substrate to one of the process chambers  108  serviced by the second robot  112 B at that end of the transfer chamber  104 .  
         [0028]    The process chambers  108  are typically bolted to an exterior side  152  of the side walls  150  of the transfer chamber  104 . Examples of process chambers  108  that may be utilized are etching chambers, physical vapor deposition chambers, chemical vapor deposition chambers, ion implantation chambers, lithography chambers and the like. Different process chambers  108  may be coupled to the transfer chamber  104  to provide a processing sequence necessary to form a predefined structure or feature upon the substrate&#39;s surface. An aperture (not shown) is disposed in the side wall between each process chamber  108  (or other chamber) to allow the substrate to be passed between the process chamber  108  and interior volume  122  of the transfer chamber  104 . A slit valve  132  selectively seals each aperture to maintain isolation between the environments of the chambers  108 ,  104  between substrate transfers and during processing within the process chambers  108 . One slit valve that may be used to advantage is described in U.S. Pat. No. 5,226,632, issued Jul. 13, 1993 to Tepman, et al., and is hereby incorporated by reference in its entirety.  
         [0029]    Generally, a pumping system  142  is coupled to the transfer chamber  104  to evacuate and maintain the chamber at a predetermined vacuum level. Typically, a pumping port  140  is disposed in the chamber bottom  156  to fluidly couple the interior volume  122  to the pumping system  142 . The pumping system  142  may include one or more pumps such as a roughing pump, a turbomolecular pump or a cryogenic pump.  
         [0030]    The load lock chamber  106  is generally coupled between the factory interface  102  and the transfer chamber  104 . The load lock chamber  106  is generally used to facilitate transfer of the substrates  124  between the transfer chamber  104  and the factory interface  102  rapidly without loss of vacuum within the transfer chamber.  
         [0031]    [0031]FIG. 2 depicts one embodiment of the load lock chamber  106 . The load lock chamber  106  generally comprises a chamber body  202 , a first substrate transfer mechanism  204  and a second substrate transfer mechanism  206 . The chamber body  202  is preferably fabricated from a single body of material such as aluminum. The chamber body  202  includes a first side wall  208 , a second side wall  210 , lateral walls  212 A and  212 B, a top  214  and a bottom  216  that define a chamber volume  218 . A first interior wall  220  is disposed between the first side wall  208  and the second side wall  210 . A first region  224  is defined between the first interior wall  220  and the top  214  of the chamber body  202 . A second interior wall  222  is disposed between the first side wall  208  and the second side wall  210 . The second interior wall  222  is positioned between the first interior side wall  220  and the bottom  216  of the chamber body  202 . A second region  226  is defined between the second interior wall  222  and the bottom  216  of the chamber body  202 . A third region  228  is defined between the first interior wall  220  and the second interior wall  222 .  
         [0032]    [0032]FIGS. 5 and 6 depict one embodiment of pressure control for both the first and second regions  224 ,  226 . Typically, both the first and second regions  224 ,  226  have separate pressure control systems used to evacuate and vent the regions. Generally, the first region  224  is coupled by a first vent passage  502  to the atmosphere outside the load lock chamber  204  (see FIG. 5). A first pump passage  602  couples the first region  224  to a pumping system  604  (see FIG. 6). Typically, the vent passage  502  and the pump passage  602  are positioned at opposing ends of the first region  224  to induce laminar flow within the region during venting and evacuation to minimize particulate contamination while minimized substrate motion.  
         [0033]    In one embodiment, the vent passage  502  is disposed in lateral wall  212 A and coupled to a high efficiency air filter  508  such as available from Camfil-Farr, of Riverdale, N.J. A shut-off valve  510  is disposed between the filter  508  and the vent passage  502  to provide vacuum isolation within the region  224 . Optionally, a window  520  may also be disposed in the lateral wall  212 A to allow viewing of the interior of the load lock  204 .  
         [0034]    In one embodiment, the pump passage  602  is disposed on lateral wall  212 B and coupled to a point-of-use pump  606  such as available from Alcatel, headquartered in Paris, France. The point-of-use pump  606  typically is positioned within three feet of the load lock  106  and has low vibration generation as not to disturb the substrates  124  positioned within the load lock chamber  106 . Additionally, the pump&#39;s proximity to the load lock  106  promoting pump-down efficiency and time by minimizing the fluid path between the chamber  106  and pump  606 .  
         [0035]    The second region  226  is coupled by a second vent passage  514  to the atmosphere outside the load lock chamber  106 . A second pump passage  608  is disposed on lateral wall  212 B and couples the second region  226  to the pumping system  604 . Generally, a diverter valve  610  allows for both passages  602 ,  608  to utilize a single pumping system  604 . Optionally, each region  224 ,  226  may utilize a dedicated pump. Typically, the second vent passage  608  and the pump passage  516  are positioned at opposing ends of the second region  226 .  
         [0036]    In one embodiment, the second vent passage  514  is disposed in lateral wall  212 A and is coupled to a high efficiency air filter  516 . A shut-off valve  518  is disposed between the filter  516  and the vent passage  514  to provide vacuum isolation within the second region  226 .  
         [0037]    Optionally, a cryogenic pump  612  or other vacuum pump may be coupled to the third region  228  via a port  614  disposed in the lateral wall  212 B. The pump  612  allows for additional pumping and pressure control within the third region  228 .  
         [0038]    Returning to FIG. 2, a first loading port  238  is disposed in one of the walls of the chamber body  202  to allow substrates  124  to be transferred between the first region  224  of the load lock  106  and the factory interface  102 . A slit valve  244  selectively seals the first loading port  238  to isolate the load lock  106  from the factory interface  102 . One slit valve that may be used to advantage is described in U.S. Pat. No. 5,226,632, issued Jul. 13, 1993 to Tepman et al., which is hereby incorporated by reference in its entirety.  
         [0039]    A second loading port  240  is disposed in one of the walls of the chamber body  202  to allow substrates  124  to be transferred between the second region  226  of the load lock  106  and the factory interface  102 . A slit valve  246  selectively seals the second loading port  240  to isolate the load lock  106  from the factory interface  102 . Although the first loading port  238  and the second loading port  240  are typically disposed on the first side wall  208  of the load lock chamber  106 , the ports  238 ,  240  may be both disposed on another wall or each on different walls of the chamber body  202 .  
         [0040]    A third loading port  242  is disposed in one of the walls of the chamber body  202  to allow substrates to be transferred between the load lock  106  and the transfer chamber  104 . The third loading port  242  is generally disposed between the first interior wall  220  and the second interior wall  222  to allow fluid communication between the third region  228  of the load lock  106  and the transfer chamber  104 . Although the third loading port  242  is typically disposed on the second wall  210  of the load lock chamber  106  opposite the first and the second loading ports  238 ,  240 , the third loading port  242  may be disposed on any of the walls comprising the chamber body  202 . As the third loading port  242  remains open to the transfer chamber  104  (i.e., there is no slit valve therebetween) the vacuum environment maintained in the transfer chamber  104  extends into the third region  228 . Optionally, a slit valve (not shown) may be disposed between the third loading port  242  and the transfer chamber  104  to selectively seal the third region  228  and allow additional pump down of the third region  228  of the load lock  106 .  
         [0041]    A first aperture  248  is disposed in the first interior wall  220  to allow fluid communication between the first and the third regions  224 ,  228  of the chamber body  202 . Similarly, a second aperture  250  is disposed in the second interior wall  222  to allow fluid communication between the second and the third regions  226 ,  228  of the chamber body  202 .  
         [0042]    In one embodiment, the first substrate transfer mechanism  204  generally includes a first actuator  252 , a first seal plate  254 , a second seal plate  256  and a substrate holder  258 . The first substrate transfer mechanism  204  is generally moved between a first position (as depicted in FIG. 2) and a second position by the first actuator  252 . In the first position, the substrate holder  258  is positioned in the first region  224  aligned with the first loading port  238 . In the first position, the interface  120  robot may exchange substrates  124  through the first loading port  238  when the slit valve  244  is opened.  
         [0043]    [0043]FIG. 3 depicts the first substrate transfer mechanism  204  in a second position. In the second position, the substrate holder  258  is positioned in the third region  228  aligned with the third loading port  242 . In the second position, the transfer robot  112  disposed in the transfer chamber  104  may exchange substrates  124  through the third loading port  242 .  
         [0044]    The first actuator  252  is generally positioned exterior to the load lock chamber  106  and coupled to the second seal plate  256  by a shaft  302 . The first actuator  252  may be a pneumatic cylinder, a hydraulic cylinder, lead screw, cam or other motion device configured to provide linear motion to the second seal plate  256 .  
         [0045]    The first seal plate  254  is generally cylindrical in form and includes sealing surface  308 . The first seal plate  254  is generally greater in diameter than the first aperture  248  such that when the first substrate transfer mechanism  204  is in the first position, the sealing surface  308  seats against a lower side  304  of the first interior wall  220  to provide a vacuum seal between the first region  224  and the third region  228  as depicted in FIG. 1. The first seal plate  254  typically includes a seal  306  disposed on or in the sealing surface  308 . The seal  306  may comprise a gasket, o-ring, lip seal or the like that provides a reliable vacuum seal between the first seal plate  254  and the first interior wall  220 . Alternative geometric configurations for the first aperture  248  and first seal plate  254  may be utilized as long as a vacuum seal is established between the first seal plate  254  and the first interior wall  220  when the first substrate transfer mechanism is in the first position.  
         [0046]    [0046]FIG. 4 depicts one embodiment of the substrate holder  258 . Generally, the substrate holder  258  includes at least a first substrate holder  410  coupled to the first seal plate  254 . A second substrate holder  412  preferably is concentrically coupled to (i.e., stacked on top of the first substrate holder  410 . Each substrate holder  410 ,  412  is configured to retain one substrate  124 . Optionally, additional substrate holders may be stacked on the second substrate holder  412  to retain a plurality of substrates. Alternatively, the first substrate holder  410  may be configured to accept a substrate storage cassette  130 .  
         [0047]    In one embodiment, each substrate holder  410 ,  412  comprises a pair of substrate support hoops  414 . Each hoop includes a first member  416  and a second member  418  that are coupled to the first seal plate  254 . Each member  416 ,  418  may have a “L-shaped” configuration, or be spaced from the first seal plate by a standoff. Each member  416 ,  418  includes a curved inner portion  420  that has a lip  422  extending radially inwards therefrom. The curved inner portion  420  is generally configured to allow the substrate  124  to pass therebetween and rest on the lip  422 . The curved inner portion  420  captures the substrate  124  therebetween, thus preventing the substrate  124  from falling off the lip  422 .  
         [0048]    The pair of tabs  416 ,  418  comprising each substrate support hoop  414  is disposed in a spaced apart relation to allow the transfer or factory interface robot  112 A,  120  to pass therebetween when retrieving and depositing substrates on the substrate holders  258 .  
         [0049]    A pair of stanchions  424  generally supports the second seal plate  256  above the first seal plate  254 . Typically, the stanchions  424  are positioned outward of the substrate holders  258  to allow the substrates  124  to be disposed on the substrate holders  258  disposed between the seal plates  254 ,  256 .  
         [0050]    The second seal plate  256  is coupled to the first seal plate  254  by the stanchions  410 . Generally, the second seal plate  256  is generally smaller in diameter (or area when the seal plates are configured in non-circular geometry) than the first seal plate  254 . The second seal plate  256  is additionally smaller in diameter than the first aperture  248 , thus allowing the second seal plate  256  to pass through the first aperture  248  as the first substrate transfer mechanism moves between the first and the second position.  
         [0051]    Referring back to FIG. 3, in one embodiment, the second seal plate  256  includes a sealing surface  326  on a side  328  of the second seal plate  256  facing away from the first seal plate  254 . The second seal plate  256  typically includes a seal  330  disposed on or in the sealing surface  326 . The seal  330  may comprise a gasket, o-ring, lip seal or the like to provide a reliable vacuum seal between the second seal plate  256  and the top  214  of the chamber body  202  when the first substrate transfer mechanism  204  is in the first position.  
         [0052]    Referring back to FIG. 2, a passage  260  is disposed in the chamber body  202  that has a first end  262  in communication with the third region  228  and a second end  264  disposed in the top  214  of the chamber body  202 . Typically, the first end  262  is disposed in the first interior wall  220  radially outward of the interface between the seal  306  of the first seal plate  254  and the first interior wall  220 . Alternatively, the first end  262  may be disposed in other areas of the chamber body  202  that maintain fluid communication with the third region  228  of the load lock chamber  106  regardless of the position of the first transfer mechanism  204 .  
         [0053]    The second end  264  is disposed in the top  214  of the chamber body  202  inwards of the interface between the seal  330  of the second seal plate  256  the top  214  of the chamber body  202  when the first substrate transfer mechanism  204  is in the second position. The second end  264  of the passage  260  is in communication with a plenum  268  disposed proximate the top  214  of the chamber body  202 . Optionally, a counter bore  266  may be disposed in the top  214  of the chamber body  202  in communication with the passage  260  to define the plenum  268  between the second seal plate  256  and the top  214  of the chamber body  202 .  
         [0054]    When the first transfer device  204  is in the first position, the first seal plate  254  creates a vacuum seal between the first region  224  and the third region  228 . Additionally, the second seal plate  256  creates a vacuum seal between the second seal plate  256  and the top  214  of the chamber body  202 . As the passage  260  allows a vacuum to be established between the second seal plate  256  and the top  214  of the chamber body  202 . Thus, as the first region  224  is vented to allow the atmosphere within the first region  224  to substantially equal the atmosphere of the factory interface  102 , a first vacuum force  270  is generated on the second seal plate  256  while a second vacuum force  272  is generated on the first seal plate  254 . The difference in the vacuum forces  270 ,  272  is generally proportional to the ratio of areas within the contact areas the respected seal area of the seal plates  254 ,  256  (as defined by the seals  306 ,  330 ). As the second vacuum force  272  opposes the first vacuum force  270 , the net force required to maintain the first transfer mechanism  204  in the first position without compromising the vacuum integrity of the seals  306 ,  330  is substantially reduced as compared to conventional designs. Thus, a force required by the first actuator  252  to move and seal the first transfer mechanism  204  is accordingly minimized. Optionally, the first actuator  254  may be coupled to the second seal plate  256  by an over-center linkage  274  that allows the seal plates  254 ,  256  (i.e., the first transfer mechanism  204 ) to be locked in a predetermined position, preferably in the first position so that vacuum integrity of the transfer chamber  104  is maintained even if the first actuator  254  loses power while in the first position.  
         [0055]    When the first transfer mechanism  204  is disposed in the second position, both seal plates  254 ,  256  are positioned in the third region  228 . The spacing between the first interior wall  220  and the second interior wall  222  allows the substrate holders  258  to be positioned adjacent the third loading port  242  to allow access to the substrates  124  by the transfer robot  112 A. Although second seal plate  256  is not required to pass through the first aperture  248  into the third region  228 , having the second seal plate  256  move at least partially through the first aperture  248  allows the overall distance between the top  214  and the bottom  216  of the chamber body  252  to be minimized. Thus, minimizing the distance between the first seal plate  254  and the second seal plate  256  reduces pump down time.  
         [0056]    The second transfer mechanism  206  is generally configured similar to the first transfer mechanism  204 . In one embodiment, the second substrate transfer mechanism  206  generally includes a second actuator  276 , a first seal plate  278 , a second seal plate  280  and a substrate holder  282 . The second substrate transfer mechanism  206  is generally moved between a first position and a second position by the second actuator  276 . In the second position shown in FIG. 2, the substrate holder  282  is positioned in the third region  228  aligned with the third loading port  242 . In the second position, the transfer robot  112 A disposed in the transfer chamber  104  may exchange substrates  124  held in the second substrate transfer mechanism  206  through the third loading port  242 .  
         [0057]    In the first position shown in FIG. 3, the substrate holder  282  is positioned in the second region  226  aligned with the second loading port  240 . In the first position, the factory interface robot  120  may exchange substrates  124  through the second loading port  240  when the slit valve  246  is opened.  
         [0058]    The second actuator  276  is generally positioned exterior to the load lock chamber  106 . Typically, the second actuator  276  controlling the movement of the second substrate transfer mechanism  206  is positioned on the opposite side of the chamber body  202  than the first actuator  252  that controls the first substrate transfer mechanism  204 . The second substrate transfer mechanism  206  is coupled to the second seal plate  280  by a shaft  284  that passages through the bottom  216  of the chamber body  202 . A dynamic seal or bellows (not shown) prevents fluid (i.e., gas) passage through the bottom  216  of the chamber body  202  around the shaft  284 . A similar seal (also not shown) is disposed in the top  214  of the chamber body  202 . The actuator  276  and its connection to the transfer mechanism  206  is similarly configured to the first actuator  252 .  
         [0059]    Referring back to FIG. 2, the first seal plate  278  is generally cylindrical in form and includes sealing surface  286 . The first seal plate  278  is generally larger than the second aperture  250  such that when the second substrate transfer mechanism  206  is in the second position, the sealing surface seats against an upper side  288  of the second interior wall  222  to provide a vacuum seal between the second region  226  and the third region  228 . The first seal plate  278  typically includes a seal  290  disposed on or in the sealing surface  286 . The seal  290  may comprise a gasket, o-ring, lip seal or the like that provides a reliable vacuum seal between the first seal plate  278  and the second interior wall  222 . Alternative configurations for the second aperture  250  and first seal plate  278  may be utilized as long as the vacuum seal is established between the first seal plate  278  and the second interior wall  222  when the second substrate transfer mechanism  206  is in the second position.  
         [0060]    Generally, the substrate holder  282  is coupled to the second seal plate  280 . The substrate holder  282  preferably is configured similarly to the substrate holder  258  described above.  
         [0061]    A pair of stanchions  292  generally supports the first seal plate  278  above the second seal plate  280 . Typically, the stanchions  292  are positioned outward of the substrate holder  282  to allow the substrates  124  to be disposed on the substrate holder  282  between the seal plates  278 ,  280  by the robots  112 A,  130 .  
         [0062]    The second seal plate  280  is coupled to the first seal plate  278  by the stanchions  292 . Generally, the second seal plate  280  is generally smaller in diameter (or area when the seal plates are configured in non-circular geometry) than the first seal plate  278 . The second seal plate  280  is additionally smaller in diameter than the second aperture  250 , thus allowing the second seal plate  280  to pass through the second aperture  250  as the second substrate transfer mechanism  206  moves between the first and the second position.  
         [0063]    In one embodiment, the second seal plate  280  includes a sealing surface  294  on a side of the second seal plate  280  facing away from the first seal plate  278 . The second seal plate  280  typically includes a seal  296  disposed on or in the sealing surface  294 . The seal  296  may comprise a gasket, o-ring, lip seal or the like to provide a reliable vacuum seal between the second seal plate  280  and the bottom of the chamber body  202 .  
         [0064]    Referring to FIG. 3, a passage is disposed in the chamber body  202  that has a first end  334  in communication with the third region  228  and a second end  336  disposed in the bottom  216  of the chamber body  202 . Typically, the first end  334  is disposed in the second interior wall  222  radially outward of the interface between the seal  290  of the first seal plate  278  and the second interior wall  222 . Alternatively, the first end  334  may be disposed in other areas of the chamber body  202  that maintain fluid communication with the third region  228  of the chamber body  202 .  
         [0065]    The second end  336  is disposed in the bottom  216  of the chamber body  202  inwards of the interface between the seal  296  of the second seal plate  280  the bottom  216  of the chamber body  202 . Optionally, a counter bore  338  may be disposed in the bottom  216  of the chamber body  202  in communication with the passage  332  to define a plenum  240  between the second seal plate  280  and the bottom of the chamber body  202 .  
         [0066]    When the second transfer device  206  is in the first position, the first seal plate  278  creates a vacuum seal between the second region  226  and the third region  228 . Additionally, the second seal plate  280  creates a vacuum seal between the second seal plate  280  and the bottom of the chamber body  202 . As the passage  332  allows a vacuum to be established between the second seal plate  280  and the bottom  216  of the chamber body  202 . Thus, as the second region  226  is vented to allow the atmosphere within the second region  226  to substantially equal the atmosphere of the factory interface  102 , a first vacuum force  342  generated on the first seal plate  278  while a second vacuum force  344  is generated on the second seal plate  280 . The difference in the vacuum forces is generally proportional to the areas defined within the area of the seal plates  278 ,  280  bounded by the respected seals  290 ,  296 . As the second vacuum force  344  opposes the first vacuum forces  342 , the net force required to maintain the second substrate transfer mechanism  206  in the second position without compromising the vacuum integrity of the seals  290 ,  296  is substantially reduced. Thus, a force required by the second actuator  276  to move and seal the second substrate transfer mechanism  206  is reduced over conventional designs.  
         [0067]    Referring back to FIG. 2, when the second substrate transfer mechanism  206  is disposed in the first position, both seal plates  278 ,  280  are positioned in the third region  228 . Although second seal plate  280  is not required to pass through the second aperture  280  into the third region  228 , having the second seal plate  280  move at least partially through the second aperture  250  allows the overall distance between the top  214  and the bottom  216  of the chamber body  206  to be minimized. Thus, minimizing the distance between the first seal plate  278  and the second seal plate  280  reduces pump down time.  
         [0068]    Alternatively, the load lock chamber  106  may be described as having a first chamber, a second chamber and a third chamber. The first chamber is defined by the first region  224 . The second chamber is defined by the second region  226  and the third chamber is defined by the third region  228 . The first substrate transfer mechanism  204  moves the substrates between the first and third chambers while the second substrate transfer mechanism  206  moves the substrates between the second and third chambers.  
         [0069]    Referring to FIGS. 2 and 3, in one mode of operation, the load lock  106  shuttles substrates utilizing both the first substrate transfer mechanism  204  and the second substrate transfer mechanism  206  to maximize substrate throughput into and out of the transfer chamber  104 . Moreover, the load lock chamber  106  only requires one position on the exterior of the transfer chamber  104 , thus allowing an additional process chambers  108  to be employed as compared to systems having two conventional load lock chambers.  
         [0070]    Generally, in operation, the slit valve  244  is opened with the first substrate transfer mechanism  204  in the first position and the first region  224  vented to substantially atmospheric pressure. The factory interface robot  120  transfers an unprocessed substrate from one of the cassettes  130  to the second substrate holder  412 . A processed substrate is removed from the first substrate holder  410  by the factory interface robot  120  and returned to one of the cassettes  130 . After completion of the substrate transfer, the slit valve  144  is closed and the first region is pumped down to the pressure substantially equal to the pressure of the transfer chamber  104 .  
         [0071]    While the first substrate transfer mechanism  204  is in the first position, the second substrate transfer mechanism  206  is in the second position. A processed substrate is disposed in the first substrate holder by one blade of the transfer robot  112 A while a second blade of the transfer robot  112 A retrieves an unprocessed substrate for processing in one or more of the process chambers  108  circumscribing the transfer chamber  104 . After substrate transfer is completed, the second substrate transfer mechanism  206  is moved to the second region  226  wherein the first seal plate  278  seals the second region  226  from the third region  228 . The vent is opened in the second region to allow the pressure in the second region to rise to substantially match the pressure in the factory interface. Once, the pressures are matched, the slit valve  246  is opened to allow the second substrate transfer mechanism  206  to interface with the factory interface robot  120  as described above with reference to the first substrate transfer mechanism  204 .  
         [0072]    After the second substrate transfer mechanism  206  has cleared the third region  228 , the first substrate transfer mechanism  204  is moved from the first region  224  to the third region  228 . The transfer robot  112 A then interfaces with the first substrate transfer mechanism  204  as described above with reference to the second substrate transfer mechanism  206 .  
         [0073]    Although the teachings of the present invention that have been shown and described in detail herein, those skilled in the art can readily devise other varied embodiments that still incorporate the teachings and do not depart from the scope and spirit of the invention.