Abstract:
A method and apparatus for substrate processing at lower cost than existing processing systems are disclosed, which by implementing an arrangement using load locks of smaller dimensions or of a non-indexing type, as compared to existing large dimension or indexing load locks, along with a substrate loading and unloading technique can achieve the fast throughput of existing systems while reducing the size and cost of the load lock apparatus required. A processed substrate is returned by an internal robot from one of its processing modules to the shelf or slot in the small load lock from which the last substrate was removed for processing by the robot, rather than being returned to the original source shelf or slot from which it was removed for processing, as in the prior art. Also venting for a first one of the load locks is started as soon as the second load lock becomes the substrate source for the internal robot rather than waiting until the first load lock has been refilled with processed substrates. By virtue of these improved operations, small dimension load locks, whether of the indexing or non-indexing type, can be used in place of more expensive large dimension indexing type load locks while maintaining comparable substrate throughputs.

Description:
CLAIM OF PRIORITY BASED ON CO-PENDING PROVISIONAL APPLICATION 
     The present application is related to co-pending Provisional patent application Ser. No. 60/067,036 of Ruth A. Hendrickson and Peter F. Van Der Meulen, filed Dec. 1, 1997, entitled “APPARATUS AND METHOD ° FOR TRANSPORTING SUBSTRATES”, and based on which priority is herewith claimed under 35 U.S.C. 119(e) and the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to transporting substrates and, more particularly, to moving substrates into and out of areas with limited individual holding areas. 
     2. Prior Art 
     Substrate processing apparatus, commonly referred to as cluster tools, are known that include a module for supplying substrates from the exterior into a main processing section or chamber from which they are transferred into substrate processing modules communicating with the main section. The main chamber may be maintained in a vacuum and has a substrate transport for moving substrates among the processing modules, which transport may be of the type of transfer device described in PCT patent publication No. WO 94/23911. The processing modules may be of various forms familiar to the art. The substrate supply module is connected to a front end of the main section and typically has a frame, a substrate transport, and means for holding two substrate cassettes. The front end of the main section has two load locks that function as compartments for transporting the substrates between the vacuum chamber and the supply module  16 , i.e., between a vacuum environment and an atmospheric pressure environment. An external or atmospheric robot transports the substrates from the cassettes to the load locks, and an internal or vacuum chamber robot transports the substrates from the load locks to the processing modules. When the substrate processing is finished, the vacuum chamber robot transports the substrates from the processing modules back to the load locks and the atmospheric robot transports the substrates from the load locks back to the cassettes. Typically, the load locks are indexing load locks which have numerous substrate support shelves and an elevator mechanism to move the shelves up and down. The shelves in the load locks may be as many as 30, depending upon the corresponding number of substrates held in a single cassette. The external robot loads a full cassette of substrates into each load lock. The internal robot loads and unloads the substrates between the load locks and the processing modules and then returns the processed substrates back to their cassettes. In the prior art apparatus, a computer controller is programmed to move the vacuum robots such that a substrate moved from a first location, such as a shelf in one of the the load locks, will be returned to the same location after being transported to a second location. Recently, substrate processing apparatus are being manufactured to process newer larger substrates, such as 300 mm diameter semiconductor wafers or flat panel display substrates which could be as large as a square meter. Indexing load locks for such large substrates can hold a large quantity of substrates and have the advantage of providing very good substrate throughput. Large size substrates must also be relatively slowly exposed to environmental change in the load locks in order to prevent undesired effects such as vapor condensation on the substrates. Indexing load locks are again of advantage as they can effectively compensate or the longer load lock environment change time to retain good substrate throughput. However, indexing load locks are very expensive, and thus a problem is presented regarding how to maintain good substrate throughput, but nonetheless reduce the costs associated with large substrate indexing load locks. 
     3. Object 
     It is therefore an object of the present invention to provide a substrate processing apparatus which can achieve a comparable substrate throughput to that of an apparatus with large substrate indexing load locks while being of considerably lower cost. 
     SUMMARY OF THE INVENTION 
     The present invention embodies a method and apparatus for substrate processing at lower cost than existing systems that use large dimension or indexing load locks, by implementing an arrangement using load locks of smaller dimensions or of a non-indexing type, along with a substrate loading and unloading technique that maintains the fast throughput while reducing the size and cost of the load lock apparatus required. In accordance with the invention, a processed substrate is returned by the internal robot from one of the processing modules to the shelf or slot in the load lock from which the last substrate was removed for processing by the robot, rather than being returned to the original source shelf or slot from which it was removed for processing, as in the prior art. Also venting for a first one of the load locks can start as soon as the second load lock becomes the substrate source for the internal robot rather than waiting until the first load lock has been refilled with processed substrates. By virtue of these improved operations, small dimension load locks, whether of the indexing or non-indexing type, can be used in place of more expensive large dimension indexing type load locks while maintaining comparable substrate throughputs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a schematic top plan view of a prior art substrate processing apparatus or cluster tool; 
     FIG. 2 is a schematic top plan view of a substrate processing apparatus comprising features of the present invention; 
     FIG. 3 is a partial schematic side view of one of the load lock areas of the apparatus shown in FIG. 2; 
     FIG. 3A is a partial schematic side view of the load lock area and substrate supply section of an alternate embodiment; 
     FIG. 3B is a partial schematic side view of the load lock area and substrate supply section of another alternate embodiment; 
     FIG. 4 is a schematic view of general components which the present invention can be used with; 
     FIG. 5 is a schematic top plan view of another alternate embodiment of the invention; and 
     FIG. 6 is a plot illustrating a comparison of the throughputs (wafers/hr.) achievable with the system of the invention versus that achievable with the prior art systems wherein the substrates are returned to the same slots from which they are picked up. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a schematic top plan view of a prior art substrate processing apparatus  10  or, as commonly referred to, a cluster tool. The apparatus  10  includes a main section  12 , substrate processing modules  14  and a substrate supply module  16 . The main section  12  has a substrate transport  18  for moving substrates among the modules  14 ,  16 . The substrate transport  18  is substantially the same as the transfer device described in PCT patent publication No. WO 94/23911 which is hereby incorporated by reference in its entirety. However, any suitable type of transport could be used. The chamber  30  formed by the main section  12  is preferably maintained in a vacuum. The substrate supply module  16  is connected to a front end of the main section  12 . The supply module  16  has a frame  20 , a substrate transport  22 , and means for holding two substrate cassettes  24 ,  25 . However, in alternate embodiments, any suitable type of substrate supply module could be provided. The substrate processing modules  14  are well known in the art and, therefore, will not be described further. 
     The front end of the main section  12  has two load locks  26 ,  28 . The load locks function as compartments for transporting the substrates between the vacuum chamber  30  and the supply module  16 ; namely, between a vacuum environment and an atmospheric pressure environment. The atmospheric robot  22  transports the substrates from the cassettes  24 ,  25  to the load locks  26 ,  28 . The vacuum chamber robot  18  transports the substrates from the load locks  26 ,  28  to the processing modules  14 . Similarly, when the substrates are finished being processed, the vacuum chamber robot  18  transports the substrates from the modules  14  to the load locks  26 ,  28  and, the atmospheric robot  22  transports the substrates from the load locks  26 ,  28  back to the cassettes  24 ,  25 . 
     Typically, the load locks  26 ,  28  are indexing load locks. Indexing load locks have numerous substrate support shelves and an elevator mechanism to move the shelves up and down. The shelves in the load locks could be as many as  13 ,  25 , or  30 , preferably corresponding to the number of substrates held in a single cassette  24 ,  25 . The atmospheric robot  22  loads a full cassette of substrates into each load lock. The vacuum chamber robot  18  loads and unloads the substrate between the load locks  26 ,  28  and the modules  14 . The atmospheric robot  22  then returns the processed substrates back to their cassettes  24 ,  25 . In the prior art, the computer controller  11  was programmed to move the robots  18 ,  22  such that a substrate moved from a first location, such as a shelf in one of the cassette  24 ,  25  or a shelf in one of the load locks  26 ,  28 , would be returned to the same location after being transported to a second location. 
     Recently, substrate processing apparatus are being manufactured for newer larger substrates, such as 300 mm diameter semiconductor wafers and flat panel display substrates which could be as large as 2 feet square. Indexing load locks for such large substrates are very expensive. However, indexing load locks have the advantage of providing very good substrate throughput. Large size substrates must also be relatively slowly exposed to environmental change in the load locks in order to prevent undesired effects on the large size substrates, such as vapor condensation on the substrates. Indexing load locks, which can hold a large quantity of substrates can effectively compensate for the longer load lock environment change time to retain a good substrate throughput. Thus, a problem existed regarding how to maintain good substrate throughput, but nonetheless reduce the costs associated with large substrate indexing load locks. 
     Referring now to FIG. 2, a schematic top view of a substrate processing apparatus  50  incorporating features of the present invention is shown. Although the present invention will be described with reference to the single embodiment shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. The apparatus  50  includes a main section  52 , substrate processing modules  54 , a substrate supply section  56  and load locks  58  connecting the main section  52  to the supply section  56 . The apparatus  50  also includes a computer controller  62 . In the embodiment shown, the apparatus  50  has three of the substrate processing modules  54 ; separately numbered P 1 , P 2 , P 3 . The apparatus  50  also has two of the load locks  58 ; separately labeled LA and LB. The main section  52  is preferably maintained in a vacuum or inert gas environment. The main section robot  60  has two end effectors  64  for individually supporting two separate substrates thereon. The main section robot  60  can move substrates between or among the various processing modules P 1 , P 2 , P 3  and load locks LA, LB. Preferably, the main section robot  60  is also adapted to vertically move the end effectors  64  up and down by moving the movable arm assembly  61  up and down. Doors  66  are provided between the main section  52  and the substrate processing modules  54  and load locks  58 . 
     The substrate supply section  56  includes a frame  68 , an atmospheric substrate transport mechanism  70 , and substrate cassette holders  72 . The transport mechanism  70  includes a car  74  movably mounted on rails of the frame  68  for linear movement on the rails as indicated by arrow X. The transport mechanism  70  also includes a robot  76  mounted to the car  74 . Referring also to FIG. 3, the atmospheric robot  76  has a drive system  78 , a movable arm assembly  80  connected to the drive system  78 , and an end effector  82  attached to the end of the movable arm assembly  80 . In the embodiment shown, the movable arm assembly is a scara arm assembly. In the embodiment shown, the end effector is adapted to individually move only one substrate at a time. However, in alternate embodiments, the end effector could be sized and shaped to move multiple substrates at the same time. In other alternate embodiments, any suitable atmospheric substrate transport mechanism could be provided. In the embodiment shown the drive system  78  is adapted to vertically move the movable arm assembly  80  and the end effector  82  as indicated by arrow Z 1 . The substrate cassette holders  72  are adapted to vertically move the cassettes  73  relative to the frame  68  as indicated by arrow Z 2 . 
     The load locks  58  are non-indexing load locks. In other words, the load locks do not have an elevator mechanism to vertically move substrates in the load locks up and down. The load locks each have four stationary substrate support shelves  84 . In alternate embodiments any suitable number of support shelves in the load locks could be provided, including only one shelf. However, with the non-indexing load locks, the number of shelves is generally limited to their relative spacing and the amount of vertical movement Z 1 , and Z 3  available to the two robots  76 ,  60 . Doors  86  are provided at the atmospheric section side of the load locks. By providing the load locks  58  as a non-indexing type, a significant cost saving is obtained. For applications where the locks separate a vacuum environment from an atmospheric environment, the equalization cycle whereby the load lock is vented to atmospheric pressure or pumped to the required vacuum pressure must be done slowly to avoid transporting particles onto the substrate surfaces from turbulent gas flow or moisture condensation. For controlled atmospheric environments, the purge rate flow must be slow enough to avoid turbulence. For thermal equilibration, larger size substrates generally need to be exposed to environmental changes at a slower rate than smaller size substrates. Because the number of substrates which a non-indexing type load lock holds is significantly less than the number of substrates which an indexing type load lock can hold, substrate throughput would be expected to be significantly less when using the non-indexing type load locks. However, to overcome this problem, the present invention uses a new method to load substrates into the load locks  58  and unload substrates from the load locks. 
     As noted above, the apparatus  50  has a controller  62 . The controller  62  preferably comprises a computer. The controller  62  is operably connected to the two robots  60 ,  76 , the doors  66 ,  86 , the transport mechanism  70 , the movable substrate cassette holders  72 , and the processing modules  54  to control their functions. A unique feature of the present invention is that a processed substrate is returned by the robot  60  from one of the processing modules  54  to the shelf or slot  84  of the last substrate removed from the load lock by the robot  60  rather than to the processed substrate&#39;s original source shelf or slot  84 . 
     Another unique feature is that venting for a first one of the load locks can start as soon as the second load lock becomes the substrate source for the robot  60  rather than waiting until the first load lock has been refilled with processed substrates. 
     Referring also to FIGS. 3A and 3B, two alternate embodiments of the substrate supply section are shown connected to the load lock  58 . In FIG. 3A, the supply section  156  has the transport mechanism  170  with robot  176 . The cassette  124  is stationarily, but removably mounted to the frame  168 . The cassette  124  could be an open  13  or  25  wafer cassette. The robot  176  can vertically move its end effector  182  as indicated by arrow Z 2  to load and unload substrates between the cassette  124  and the load lock  58 . In FIG. 3B the supply section  256  has the same transport mechanism  170  with robot  176 . The cassette  224  is a Front Opening Universal Pod (FOUP), such as an Infab  13  or  25  wafer capsil which is stationarily, but removably mounted to the frame  268 . The frame  268  includes a movable door  267  that can be moved up and down when the wafer capsil  224  is changed. 
     FIG. 4 shows one of the most basic adaptations for the present invention. The first environment  300  and the second environment  302  could be two transport chambers, such as the main section  12  shown in FIG. 1, or one of the main sections  12  and an atmospheric section. The pass-through chambers  304 ,  306  have one or more positions for holding material. Each pass-through chamber has two doors  308 ,  310  that serve to isolate the pass-through chamber from the two environments. For example, one door might open to a standard semiconductor clean room; the other might open to a semiconductor vacuum transport chamber. Because the pass-through chamber connects the two different environments, the environment in the pass-through chamber must change to match the environment in the neighboring chamber before the isolation door opens. Consequently, the pass-through chamber is often called a pass-through lock or load lock. 
     A certain period of time must pass to equalize the environment in the pass-through chamber to the neighboring environment before the isolation door can be opened. The amount of material that can move through the pass-through chamber in a given time period is called “throughput”. To maximize throughput, the pass-through chambers are used in an alternating mode; one pass-through chamber will be receiving or sending material while the other pass-through chamber is equalizing. If the equalization time is short enough, one pass-through chamber may finish equalizing before the other finishes accepting or sending material. In that case, the equalization activity is “in the background”, and therefore does not inhibit throughput. 
     Typically, material handling robots move the material from one environment, through the pass-through chamber, and into the second environment. One material handling robot moves wafers from the first environment into the pass-through chamber; a second robot moves wafers from the pass-through chamber into a second environment. In practice, both the second and the first environments may have more than one handling robot. 
     FIG. 5 shows another alternate of the present invention. The main section  400  has more than three processing modules  402  attached to it. The main section  400  also has a substrate aligner  404  and a substrate cooler  406  located in paths to the load locks  58 . The substrate supply section  408  also includes a substrate buffer  410  between two cassettes. The present invention could be used with any suitable type of substrate processing apparatus. 
     The material moved includes, but is not limited to, wafers, substrates, and glass panels. The controlled environment includes, but is not limited to, vacuum (significantly less than atmospheric pressure), near atmospheric pressure but with controlled gas constituents, or any pressure with controlled temperature. The movement of the robot and the material is under the control of “scheduling” software. The presently described method pertains to the scheduling algorithm for this material movement. 
     In order to optimize the total tool throughput, using a substrate processing apparatus or cluster tool in accordance with the present invention, e.g., as shown in FIG. 2, the following exemplary scheduling algorithm may be followed for scheduling through two alternating pass through locks LA and LB. The preferred steps are set forth for a single-pan i.e., single holder or end effector, robot and a dual-pan robot. 
     EXAMPLES 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Assume 
                 Key 
                   
               
               
                   
                 Two 4 slot pass through 
                 LAn 
                 Lock A, shelf n 
               
               
                   
                 locks 
               
               
                   
                 3 parallel PMs 
                 LBn 
                 Lock B, shelf n 
               
               
                   
                 10 substrates in cassette 
                 Pn 
                 Process Module n 
               
               
                   
                 Batch of 1 cassette to be 
                 w1 
                 substrate 1 
               
               
                   
                 processed 
               
               
                   
                   
                 pk 
                 pick up 
               
               
                   
                   
                 pl 
                 place 
               
               
                   
                   
                 phm 
                 pump, home, map 
               
               
                   
                   
                 F1 cycle 
                 vent, empty 
               
               
                   
                   
                   
                 and refill, pump, 
               
               
                   
                   
                   
                 home, map 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 1 
     
       
         
               
               
             
               
             
               
               
             
           
               
                   
               
             
             
               
                 Single-pan Robot 
                   
               
               
                 pl w1 to LA1 
                 (Atmospheric robot 76 fills the locks) 
               
               
                 pl w2 to LA2 
               
               
                 pl w3 to LA3 
               
               
                 pl w4 to LA4 ----------&gt; 
                 start LA phm 
               
               
                 pl w5 to LB1 
               
               
                 pl w6 to LB2 
               
               
                 pl w7 to LB3 
               
               
                 pl w8 to LB4 ----------&gt; 
                 start LB phm 
               
               
                 pk w1 LA1 
                 (fill cluster pipeline) 
               
               
                 pl w1 P1 
               
               
                 pk w2 LA2 
               
               
                 pl w2 P2 
               
               
                 pk w3 LA3 
               
               
                 pl w3 P3 
                 (pipeline full) 
               
               
                 wait for P1 to finish 
               
               
                 pk w1 P1 
               
               
                 pl w1 LA3 
                 (same slot as the most recent input 
               
               
                   
                 substrate came from) Note that in an indexing 
               
               
                   
                 lock, this means no index is required between 
               
               
                   
                 pick and place at the load lock. 
               
               
                 pk w4 LA4 
               
               
                 pl w4 P1 
               
               
                 wait for P2 to finish 
               
               
                 pk w2 P2 
               
               
                 pl w2 LA4 
               
               
                 pk w5 LB1 ----------&gt; 
                 start F1 cycle for LA EVEN THOUGH 
               
               
                   
                 IT 
               
               
                   
                 HAS ONLY TWO SUBSTRATES because a 
               
               
                   
                 substrate was picked from the other 
               
               
                   
                 lock, refill LA with w9 and w10 
               
               
                 pl w5 P2 
               
               
                 wait for P3 to finish 
               
               
                 pk w3 P3 
               
               
                 pl w3 LB1 
               
               
                 pk w6 LB2 
               
               
                 pl w6 P3 
               
               
                 wait for P1 to finish 
               
               
                 pk w4 P1 
               
               
                 pl w4 LB2 
               
               
                 pk w7 LB3 
               
               
                 pl w7 P1 
               
               
                 wait for P2 to finish 
               
               
                 pk w5 P2 
               
               
                 pl w5 LB3 
               
               
                 pk w8 LB4 
               
               
                 pl w8 P2 
               
               
                 wait for P3 to finish 
               
               
                 pk w6 P3 
               
               
                 pl w6 LB4 ----------&gt; 
                 start F1 cycle for LB because LB is 
               
               
                   
                 full, wait until LA F1 finishes (LA 
               
               
                   
                 has w9 in LA1 and w10 in LA2) 
               
               
                 pk w9 LA1 
               
               
                 pl w9 P3 
               
               
                 wait for P1 to finish 
               
               
                 pk w7 P1 
               
               
                 pl w7 LA1 
               
               
                 pk w10 LA2 
               
               
                 pl w10 P1 
               
             
          
           
               
                 No more substrates to pick - start pipeline drain 
               
               
                 wait for P2 to finish 
               
               
                 pk w8 P2 
               
               
                 pl w8 LA2 
               
               
                 wait for P3 to finish 
               
               
                 pk w9 P3 
               
               
                 pl w9 LA3 
               
               
                 wait for P1 to finish 
               
               
                 pk w10 P1 
               
             
          
           
               
                 pl w10 LA4 ----------&gt; 
                 start F1 cycle for LA because LA is 
               
               
                   
                 full 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 2 
     
       
         
               
               
               
             
               
               
             
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Dual-pan robot 
                   
               
               
                   
                 pl w1 to LA1 
                 (Atmospheric robot 76 fills the locks) 
               
               
                   
                 pl w2 to LA2 
               
               
                   
                 pl w3 to LA3 
               
               
                   
                 pl w4 to LA4 ----------&gt; 
                 start LA phm 
               
               
                   
                 pl w5 to LB1 
               
               
                   
                 pl w6 to LB2 
               
               
                   
                 pl w7 to LB3 
               
               
                   
                 pl w8 to LB4 ----------&gt; 
                 start LB phm (locks full) 
               
               
                   
                 pk w1 LA1 
                 (fill cluster pipeline) 
               
               
                   
                 pl w1 P1 
               
               
                   
                 pk w2 LA2 
               
               
                   
                 pl w2 P2 
               
               
                   
                 pk w3 LA3 
               
               
                   
                 pl w3 P3 
               
               
                   
                 pk w4 LA4 
                 (pipeline full) 
               
               
                   
                 wait for P1 to finish 
               
               
                   
                 pk w1 PI 
               
               
                   
                 pl w4 P1 
               
               
                   
                 pk w5 LB1 ----------&gt; 
                 start LA F1 cycle because substrate 
               
               
                   
                   
                 was picked from the other lock, 
               
               
                   
                   
                 refill LA1 with w9 and LA2 with w10 
               
               
                   
                 pl w1 LB1 
                 (same slot as the most recent input 
               
               
                   
                   
                 substrate came from) 
               
               
                   
                 wait for P2 to finish 
               
               
                   
                 pk w2 P2 
               
               
                   
                 pl w5 P2 
               
               
                   
                 pk w6 LB2 
               
               
                   
                 pl w2 LB2 
               
               
                   
                 wait for P3 to finish 
               
               
                   
                 pk w3 P3 
               
               
                   
                 pl w6 P3 
               
               
                   
                 pk w7 LB3 
               
               
                   
                 pl w3 LB3 
               
               
                   
                 wait far P1 to finish 
               
               
                   
                 pk w4 P1 
               
               
                   
                 pl w7 P1 
               
               
                   
                 pk w8 LB4 
               
               
                   
                 pl w4 LB4 ----------&gt; 
                 start LB F1 cycle because LB is full 
               
               
                   
                 wait for P2 to finish 
               
               
                   
                 pk w5 P2 
               
               
                   
                 pl w8 P2 
               
               
                   
                 pk w9 LA1 
               
               
                   
                 pl w5 LA1 
               
               
                   
                 wait for P3 to finish 
               
               
                   
                 pk w6 P3 
               
               
                   
                 pl w9 P3 
               
               
                   
                 pk w10 LA2 
               
               
                   
                 pl w6 LA2 
               
               
                   
                 wait for P1 to finish 
               
               
                   
                 pk w7 P1 
               
               
                   
                 pl w10 P1 
               
             
          
           
               
                   
                 No more substrates to pick - start pipeline drain 
               
             
          
           
               
                   
                 pl w7 LA3 
                 (fill slots in active lock) 
               
               
                   
                 wait for P2 to finish 
               
               
                   
                 pk w8 P2 
               
               
                   
                 pl w8 LA4 ---------&gt; 
                 start LA F1 cycle because LA is full 
               
               
                   
                 wait for P3 to finish 
               
               
                   
                 pk w9 P3 
               
               
                   
                 wait for LB F1 to finish 
               
               
                   
                 pl w9 LB1 
               
               
                   
                 wait for P1 to finish 
               
               
                   
                 pk w10 P1 
               
               
                   
                 pl w10 LB2 ----------&gt; 
                 start LB F1 cycle because no more 
               
               
                   
                   
                 unprocessed substrates in batch 
               
               
                   
                   
               
             
          
         
       
     
     Based on the preceding steps it will be appreciated that the rules for scheduling through two alternating pass through locks are: 
     1) It is preferred only for a dual-pan vacuum robot with 2 positions on the arms that carry the substrates, to swap at the lock (pick first, then place to the same slot); 
     2) Always put an output substrate to the same slot from which the most recent input substrate came, 
     ELSE, put it in the next empty slot in the lock that the last output substrate went to, 
     ELSE, put it in the next empty slot in the other lock; 
     3) As soon as the lock is filled with processed substrates, 
     OR as soon as the vacuum robot picks a substrate from the other lock (whichever comes first), 
     OR if no more unprocessed substrates are in the batch, 
     start the F 1  cycle (vent, empty using a separate robot in an atmospheric buffer station and refill, pump, home, map). 
     The same algorithm can be used for transferring substrates from one cluster to another and with clusters that are non-vacuum or that have a controlled atmosphere such as an inert gas. It can also be used with more than two load locks and applied to other pass through modules, such as heaters or coolers connecting two chambers of a 2-TM (transport module) chamber cluster. 
     It will be seen that unlike the operation of most prior art cluster tools, which use two load locks that each accept a full cassette (or SMIF pod) of wafers and wherein the substrates must return to the same cassette slots that they came from so that each ends up in its slot of origin, the present invention instead returns a substrate to the load lock slot of the substrate most recently sent into the tool for processing (although the external robot may still return the substrate to the cassette slot of origin). Also, venting is started on one lock as soon as the other lock becomes the substrate source rather than waiting until it has been refilled with processed substrates. Thus, the alternate load lock may be vented, refilled, and pumped down, much sooner than before, thereby maximizing throughput and eliminating indexing between the pick and place operations. A comparison of the throughputs (wafers/hr.) achievable with the system of the invention versus that achievable with the prior art systems wherein the substrates are returned to the same load lock slots is shown in FIG.  6 . 
     As described above, the primary advantage of the present invention is the reduction in manufacturing costs by using lower cost load locks, but maintaining substantially the same throughput as prior art devices. Preferably, the lower cost load locks are non-indexing load locks. However, the method of the present invention could be used with any suitable type of load lock including load locks capable of indexing. The cost saving comes by providing a smaller volume load lock than would otherwise be necessary for a specific throughput with a large volume load lock not using the method of the invention. The invention can be used with a large capacity lock that is smaller than a full batch load lock and still have the same or faster throughput as the full batch load lock. If an indexing load lock is provided, one or both of the substrate transport robots can lack Z-motion capability. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.