Abstract:
Apparatus and concomitant method for performing priority based scheduling of wafer processing within a multiple chamber semiconductor wafer processing system (cluster tool) having at least one metrology chamber. The sequencer assigns priority values to the chambers and stations in a wafer processing system (i.e., a cluster tool plus a factory interface), then moves wafers from chamber to chamber in accordance with the assigned priorities. The sequencer also selects particular wafers for placement into at least one metrology chamber or station.

Description:
BACKGROUND OF THE DISCLOSURE  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to multiple chamber wafer processing systems that have integrated metrology and defect control chambers and, more particularly, the invention relates to a method and apparatus for determining wafer scheduling in a multiple chamber wafer processing system that has at least one integrated metrology and defect control chamber.  
           [0003]    2. Description of the Background Art  
           [0004]    Semiconductor wafers are processed to produce integrated circuits using a plurality of sequential process steps. These steps are performed using a plurality of process chambers. An assemblage of process chambers served by a wafer transport robot is known as a multiple chamber semiconductor wafer processing tool or cluster tool. Movement of wafers through the cluster tool is controlled by a schedule.  
           [0005]    A Factory Interface (FI), attached to the “front end” of a cluster tool, usually contains additional wafer positions (stations) for wafer orientation, metrology, and defect control, and introduces a number of challenges in wafer handling and movement. The wafer scheduling algorithms now must take into account sampling policies regarding metrology and defect control stations since these stations are integrated into the wafer processing system. The sampling policies typically require every nth wafer to be tested in a metrology or defect control station. As such, periodically cassettes of wafers are removed from the normal wafer flow for testing and the scheduling algorithm must handle such interruptions.  
           [0006]    More precisely, if FP i  represent a load position i in the factory interface (there are usually two such positions) and C i  represent a chamber i in the cluster tool, then 
           FP i →C 1 →C 2 →. . .→C N FP i , 
           [0007]    represents the wafer route through the system (FI plus cluster tool). If B(C i )ε{0,1} is a boolean variable representing whether a chamber in the tool or factory interface is visited, then wafer sampling for metrology or defect control introduces a number of sub-routes derived from the above route as 
           FP i →C 1 B(C 1 )→C 2 B(C 2 )→. . .→C N B(C N )→FP i . 
           [0008]    In other words, scheduling algorithms must take into account the change in route introduced by metrology and defect control stations that are visited by some of the wafers from a wafer cassette in the factory interface, but not by all wafers from the wafer cassette.  
           [0009]    Therefore, a need exists in the art for a method and apparatus to determine schedules for wafer movement through a wafer processing system comprising a cluster tool and a factory interface having metrology and defect control chambers.  
         SUMMARY OF THE INVENTION  
         [0010]    The invention is a method and apparatus for scheduling wafer processing in cluster tools that have integrated metrology and defect control stations or chambers. These are cluster tools with a Factory Interface (FI) (i.e., a combination of a robot and wafer cassette(s)), Integrated Particle Measurement (IPM) station or/and Integrated Metrology (IM) station as well as orient or center-find chambers. FI in the context of this invention is viewed as a cluster of stations or chambers having up to N chambers/stations, a transfer space supporting a choice of robots and scheduling algorithms that facilitate movement of the wafers, and a “mini-stocker” with capacity C wafer cassettes. The cluster tool is “connected” to the FI via one or more single-wafer load-locks.  
           [0011]    The invention computes an optimal schedule for moving wafers from the cassettes in the factory interface to the cluster tool and back to the cassette, while intermittently moving wafers into a metrology or defect control station. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 depicts a factory interface having metrology chambers coupled to a cluster tool;  
         [0014]    [0014]FIG. 2 depicts a dual robot factory interface with a pass-through chamber;  
         [0015]    [0015]FIG. 3 depicts a flow diagram of priority-based feed-first process;  
         [0016]    [0016]FIG. 4 depicts a flow diagram of a priority-based empty-first process; and  
         [0017]    [0017]FIG. 5 depicts a flow diagram of a process for selecting a destination chamber in a factory interface having dual robots and a pass-through chamber.  
         [0018]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 1 depicts a schematic, block diagram of a semiconductor wafer processing system  100  comprising a cluster tool  102 , a factory interface  104  and a scheduler  118 . The cluster tool comprises a plurality of process chambers  106 A,  106 B,  106 C,  106 E),  106 E and  106 F, and a wafer transport robot  108 . The factory interface  104  comprises one or more wafer cassette stockers  110 , a plurality of stations  112 A,  112 B  112 C,  112 D,  112 E and  112 F, and a wafer transport robot  114 . Wafer cassettes  111  are arranged in a multicassette stack known as a “mini-stocker”  110 . The stations  112  comprise, for example, a metrology station  112 A, a defect location station  112 B, a wafer orienter  112 C, and a wafer center-find station  112 D. The factory interface  104  is coupled to the cluster tool  102  through one or more pass-through chambers  116  (load locks).  
         [0020]    Wafers are moved one at a time from the cassettes(s)  110  by robot  114  to the pass-through chambers  116 , the orienter  112 C or the wafer center find station  112 D. Once in chamber  116 , the robot  108  moves the wafer from chamber  116  through the various chambers  106  of the cluster tool  102 . After processing by the cluster tool  102 , the wafer is returned to the pass-through chamber  116 . The robot  114  then moves the wafer to a metrology station  112 A and/or defect location station  112 B. Lastly, the wafer is moved to a cassette  110 .  
         [0021]    The scheduling algorithm that facilitates wafer movement is implemented as an executable software routine  126 . The scheduler  118  comprises a central processing unit (CPU)  120 , memory  122  and support circuits  124 . The CPU is a general purpose computer that becomes a specific purpose computer when executing software  126  stored in the memory. The memory  122  can be any form of digital storage including read only memory, random access memory, removable memory, hard disk drive and the like. The support circuits  124  are well-known circuits such as cache, clocks, power supplies and the like.  
         [0022]    As shown in FIG. 2, multiple robots  202 ,  204  may be serving one transfer space  206  between the FI stations  112  A-F and the cluster tool  102 . The wafers are passed from one robot to another by means of the pass-through chamber  208 . As shown in FIG. 2, there may be two fixed robots  202 ,  204  in the FI transfer space  206 . These are two single blade robots  202 ,  204  (with z-motion allowed) connected by a pass-through chamber of capacity four. Robots  202 ,  204  operate independent of each other. They are fixed and centered in front of their respective load-locks  116 A,  116 B and they both can access the orient chamber  209  that is positioned mid-way between the robots  202  and  204 . Each robot services one load port  207 A or  207 B, pass-through chamber  208 , orient chamber  209 , N/ 2  metrology chambers  112  on one side, and one load-lock  116 A or  116 B. Clearly, in this case, a route for any wafer through the system should contain the robot identification (ID) visiting a chamber in the above sequence.  
         [0023]    In another embodiment of the invention, a Single Wafer Load Lock (SWLL) is used between the FI  104  and the cluster tool  102 . This load-lock is intended to hold only one wafer at a time during the pump/vent cycle of the load-lock. In addition to the existing 25-wafer load-lock logic, and single-wafer load-lock logic, this invention also contemplates a variable number of K+1 wafer slots assigned as inbound and outbound. Inbound slots are used to send up to K+1 wafers into the cluster tool and the same slots, denoted as outbound, are used for taking up to K+1 wafers out of the cluster tool. The K+1 slots are in the same volume that has to be pumped for wafers to go in and vented for wafers to go out of the tool. These K+1 slots are supposed to accommodate up to K wafers in case of single blade robots (either the tool&#39;s robot or the FI&#39;s robot) and up to K+1 wafers in case both tool and FI have dual blade robots.  
         [0024]    Wafers entering a load-lock from FI are directed to either LL 1  or LL 2  depending on which load-lock is available to be loaded. If both load-locks were available, the wafer would enter the one that is closer to the wafer source station. Wafers leaving the transfer chamber are again directed to either LL 1  or LL 2 , depending on which load-lock is available. If both load-locks were available, the wafer would enter the closer loadlock. The FI will return the wafer to the source pod cassette  110  into its original position (i.e., preserving the “slot integrity”). Wafers that enter the transfer chamber through LL 2  should not be restricted to exiting the transfer chamber through LL 2 . Similarly, wafers from one cassette can enter either LL 1  or a LL 2  depending on the availability. In other words, wafers from one cassette are not restricted to entering and leaving the cluster tool  102  via a particular load-lock.  
         [0025]    When particle monitoring stations and/or metrology stations are integrated with process equipment, there are several ways of specifying scheduling of wafers for inspection. These are,  
         [0026]    1. Inspecting every N th wafer before and after processing. If N=1, every wafer is inspected, if N=2, every second wafer is inspected, if N=25, one wafer is inspected from each cassette.  
         [0027]    2. Inspecting every N th wafer only after processing.  
         [0028]    3. Assuming cluster tool with K identical chambers: inspecting before and after (or only after) every N th wafer processed in chamber J, where J ranges from 1 to K. So if N=3 every third wafer processed in chamber 1,2, . . . , K would be inspected.  
         [0029]    4. Inspect as many wafers as possible before and after (or only after) without affecting the overall throughput of the process tool. This implies “background” inspection in the sense that the robot handles wafers for inspection only when it is idle. It also implies that processing never waits for inspection to be completed.  
         [0030]    5. Inspect as many wafers as possible with up to T sec. addition to the overall processing time.  
         [0031]    The invention accommodates the requirements related to scheduling in the presence of sampling of wafers (for inspection) in cluster tools.  
         [0032]    Inspecting every N th wafer from a lot before and after processing can be implemented in two ways:  
         [0033]    1. Specify that every N th wafer from a lot can be selected for inspection. For example, if N=1, then every wafer is inspected, when N=2, every second wafer is inspected, when N=25 one wafer is inspected from each cassette. If the cassette/lot size is less than the number specified, inspect the last wafer of the lot.  
         [0034]    2. Specify by explicit enumeration of the sample wafers within a lot. For example, in a 25-wafer cassette a user can require wafers 1,2,6,9,13, and 25 to be inspected. If a particular wafer is not available, it can be ignored or can be defaulted to the last wafer in the cassette.  
         [0035]    The above methods will work when all the wafers in the lot belong to the same product. If there are multiple products in a single lot, a technique has to be developed to identify the wafers in a lot by product. Wafers may need to be inspected before processing, after processing, or both before and after processing. Therefore, wafers&#39; records within a database have to carry the necessary details about inspection before processing or after processing.  
         [0036]    In a cluster tool with K identical chambers inspecting before and after (or only after) every N th wafer which visited the chamber J, where J ranges from 1 to K . So, if N=3, every third wafer processed in chamber 1,2, . . . , K is inspected. The N th wafer visit should be counted for the following wafers:  
         [0037]    1. Of the same type of product (in case of more than one product)  
         [0038]    2. Include the number of wafer visits to a chamber (in case of chamber revisits, the N th wafer visiting a chamber may not be the N th wafer processed in the chamber).  
         [0039]    The data structure pertinent to the above-described implementation is given in Table 1. 
                                       TABLE 1                               3       5   6           1   2   Inspection Type (By   4   Go to an   Go to a   7       Process   Family   Frequency, By Wafer #,   Inspection   Inspection   Non Inspec-   Inspect       Step   Name   By Wafer visit in Chamber)   Wafers   Step   tion Step   When?                   1   FOUPs*   By frequency   N = 2**   2   3   Before       2   FI inspect           5       After       3   LL   By Wafer Number   X = 1,2,5***   4   5   Both       4   Inspect2           5       5   CVD (A,B,C)   By Wafer Visit in   y = 4****   6   7   7               Chamber       6   Inspect2           7       7   LL       N = 1,2,5           8       8   FI Inspect           8                                                  
 
         [0040]    The above sampling requires the “inspection type” (in column three of the above table) to be a function with the following four arguments:  
         [0041]    “inspection wafers”,  
         [0042]    “go to inspection step”,  
         [0043]    “go to non-inspection step”,  
         [0044]    “inspect when”  
         [0045]    which are columns 4-7 in the table. There are three “inspection type” methods:  
         [0046]    a) “By number of wafer visits to chambers” (i.e., every K th wafer)  
         [0047]    b) “By Frequency” (i.e., inspect every second wafer coming out from a cassette)  
         [0048]    c) “By Wafer Number” (i.e., a particular wafer from a cassette)  
         [0049]    Alternatively, as a wafer enters the system (FI plus cluster tool), the invention can associate a binary 2-tuple to the wafer record with the following meaning:  
         [0050]    00 not inspected before and not after processing  
         [0051]    01 not inspected before but inspected after processing  
         [0052]    10 inspected before but not after processing  
         [0053]    11 inspected before as well as after processing  
         [0054]    When a process chamber is in the cluster tool and a metrology chamber is integrated into the FI, the requirement of measuring every K th wafer from a chamber requires setting a “metrology” bit, marked visited, in a wafer record to 1. This means that upon leaving the load-lock, that particular wafer (the K th wafer from chamber A) must visit the metrology chamber. This is an example of altering the wafer route based on the outcome in processing. The process chamber has a counter whose variable (content) count is reset after every K wafers, i.e.,  
                                                   if count == K {           visited = 1; count = 0;           }           else {           visited = 0; count = count + 1;           }                      
 
         [0055]    To each wafer the invention thus associates a record in which various fields correspond to chambers being visited and are modified by the control system prior to or during the wafer processing. This data structure is instrumental in scheduling of wafers in case of integrated metrology or/and particle monitoring.  
       Scheduling Algorithms  
       [0056]    Wafers, that are marked “metrology” or “IPM” (i.e., have the corresponding bits set to one) visit their respective chambers according to a given scheduling logic. A priority based scheduling logic, which may be different than the logic used for “special” wafers, is then applied to “ordinary” wafers (i.e., wafers having no metrology field in their data structure). The following embodiments of the invention illustrate the modification on general versions of priority-based scheduling for both “feed-first” and “empty-first” types of scheduling algorithms.  
         [0057]    Denote by T the length of a cassette stay in the system (cluster tool plus FI). It is assumed that pump and vent time for a cassette in a load-lock are overlapped with processing time of other cassettes, then, by Little&#39;s formula, it follows that in steady-state T=N/S, where N is the number of wafers in the cassette and S is the steady-state throughput. Thus, the length of a cassette stay in the system is minimized when the throughput is maximized and hence a scheduling logic that minimizes the length of a cassette&#39;s stay in the tool should be the best attainable.  
         [0058]    A priority based scheduling routine should then assign the highest priority to a robot move that takes a wafer out of the cassette and puts the wafer into the first stage of a wafer&#39;s flow. A “stage” is a set of chambers that are executing the same process. Reasoning inductively, such an algorithm should give priorities n,n−1,n−2, . . . , 2,1 to stages 1,2, . . . , n−1,n, respectively. The load-lock should have the highest priority, n+1, when the wafer is to be taken out from the load-lock and the lowest priority, 0, when the (processed) wafer is to be returned to the load-lock. The above described algorithm is known as “wafer packing”, which is a variant of feed-first class of algorithms, and is optimum for serial configurations with process limited throughput. For a description of “wafer packing” and other priority based scheduling algorithms, see U.S. Pat. No. 5,928,389, issued Jul. 27, 1999.  
         [0059]    Recall that the wafer record contains metrology and defect control fields according to the above description of the data structure needed in scheduling of the wafers. If these fields have variables set to 1, hereafter, these wafers are referred to as M-wafers. Clearly, M-wafers should not receive any special treatment in scheduling in the sense of initiating the movement of these wafers out of order dictated by the scheduling algorithm. However, once they become a source wafer (that is, a wafer to be moved according to the algorithm), their target chamber is different than the ones for “ordinary” wafers. For example, while an ordinary wafer is moved from a load-lock back to its position in the cassette, an M-wafer first visits metrology chamber and then the wafer returns to the cassette. So, all scheduling algorithms are augmented by first reading a metrology or defect control field in the data structure associated with scheduling needs of a wafer that is to be moved.  
         [0060]    In a priority-based feed-first algorithm, a wafer transfer starts by identifying a chamber pair (C S ,C D ) , C S  and C D  being a source and a destination (also called target) chamber, respectively. In feed-first algorithms, in particular, chamber C D  is chosen first. In empty-first algorithms, chamber C S  is chosen first. An example of such a data structure (without metrology and defect detection fields) for implementation of priority-based heuristics is given in [1].  
         [0061]    If C D  is chosen first and C D  happens to be a metrology chamber, upstream stages are scanned for a wafer whose “metrology bit” is set to 1 (and thus whose target chamber is C D  ). If such a wafer is identified, the transfer is made; else, the priority number is decreased by one and the search is repeated. FIG. 3 depicts a flow diagram of a priority-based, feed first algorithm  300 . The following algorithm is repeated for each independent robot space:  
         [0062]    Step  302  and  302 B. If all stages are full, preposition the robot at the chamber in the last stage whose wafer is first ready to leave the chamber. Wait if necessary, and then move that wafer into its position in the cassette (cassette is sitting on the load-port). Go to Step  304 .  
         [0063]    Step  304 . Set the stage priority P to one (P←1) and go to Step  308 . (This is a usual assignment to a variable “stage priority”.)  
         [0064]    Step  306 A and  306 B. If P&lt;L ( 306 A), then P←P+1 ( 306 B) (decrease priority) and go to Step  308 . Else (P≧L), go to Step  318 A.  
         [0065]    Step  308 A and  308 B. If all chambers in the current priority stage are busy either processing or cleaning, go to Step  306 A. Else if the current priority stage (i.e., stage with priority P) has an empty metrology chamber (empty means ready to receive a wafer), go to Step  310 . Else (there is an empty non-metrology chamber), go to Step  312 .  
         [0066]    Step  310 . Scan all upstream chambers for a wafer whose metrology bit is set to one and whose (next) target chamber is a metrology chamber identified in Step  308 B. If there is no such a wafer, go to Step  306 A.  
         [0067]    Else, go to Step  318 A.  
         [0068]    Step  312 . If the stage or load-lock that is right before the current priority stage has at least one chamber with (product) wafer in it, go to Step  314 A. Else (the stage is empty), go to Step  306 A.  
         [0069]    Step  314 A and  314 B. Preposition ( 314 A) the robot at a chamber in the stage right before the current priority stage (found in Step  6 ) whose wafer is first ready to go. Wait if necessary, and move ( 314 B) that wafer into an empty chamber in the current priority stage. Go to Step  302 A.  
         [0070]    Step  316 A and  316 B. Preposition ( 316 A) the robot at a chamber found in Step  313 . Wait if necessary, and move ( 316 B) the wafer within into an empty metrology chamber in the current priority stage. Go to Step  302 A.  
         [0071]    Step  318 A and  318 B. If there are any wafers left in the system ( 318 A), move ( 318 B) them into their target chambers or FA in the order of completion. Else, STOP at step  320 .  
         [0072]    As already mentioned, the above algorithm searches for a pair of stages S p  and S q  such that the following two conditions hold:  
         [0073]    S p  is the current highest priority stage and has at least one empty chamber,  
         [0074]    For “ordinary wafers”, S q  is the stage right before S p  (i.e. chambers in S p  are target for the wafers from S q ) which has at least one non-empty chamber (with a wafer ready to go into stage S p  at some point in time). For M-wafers, S q  is a stage prior to s p  (not necessarily right before S p ) which contains a wafer whose target chamber is a metrology chamber.  
         [0075]    The above algorithm can be extended into a gamma-tolerant version in a way similar to that described in U.S. Pat. No. 5,928,389, issued Jul. 27, 1999.  
         [0076]    In a priority-based empty-first algorithm, the highest priority non-empty source chamber is first identified. In case of single blade transporters, the move is made only if the target chamber is available. In case of dual (multiple) blade transporters, the move is made regardless (because one of the blades can serve as a temporary wafer-holding position). FIG. 4 depicts a flow diagram of a priority based, empty first algorithm  400  that pertains to dual-blade robots.  
         [0077]    Step  502 . Scan each stage of the system to find a chamber that has the highest priority and a wafer in it. Position the robot (any blade) in front of the highest priority chamber. Go to Step  504 .  
         [0078]    Step  504 . Wait if necessary and pick up a wafer from the chamber found in Step  502 . Go to Step  506 .  
         [0079]    Step  506 . If the target chamber for the wafer on the blade is empty, go to Step  508 . Else, go to Step  510 .  
         [0080]    Step  508 . Position the full blade in front of the target chamber and put the wafer into the chamber. Go to Step  502 .  
         [0081]    Step  510 . Position the empty blade in front of the target chamber. If necessary, wait until wafer in the target chamber is ready to move. Swap the wafer on the blade with the wafer in the target chamber (according to the type of a robot). Go to Step  506 .  
         [0082]    Note that neither M-chambers nor wafers requesting such chambers have a separate treatment in the above algorithm. It is only that target chamber in the wafer exchange is determined by first looking at the “metrology field” of a data structure associated with the wafer.  
       Managing the Pass-Through Chamber in FI  
       [0083]    A transfer space in FI may contain one or two robots. In case of two fixed robots, as described previously, robots service their respective regions and exchange the material (wafers) through either an orient chamber or through a multiple slot pass--through chamber. Below is described a data structure and algorithms needed for an effective management of a pass-through chamber.  
         [0084]    As already mentioned the two fixed robots in the FI transfer space are single blade robots (with z-motion allowed) connected by a pass-through chamber of capacity four. Robots are independent of each other and centered in front of their respective load-lock positions and they both can access the orient chamber that is positioned mid-way between them. Each robot services one load port, pass-through chamber, orient chamber, metrology chambers on one side, and one load-lock. So, if RS, represents robot space i, where i=1,2, then 
           RS   i   ={FP   i   O   i   ,M   1   (i)   , M   2   (i)   , M   3   (i)   ,LL   i   }, i= 1,2. 
         [0085]    In the above set, FP i  stands for the FOUP position i (also, load position i), O i  is the orient position accessible by robot i i  M k   (i)  is the kth metrology chamber, and LL i  is the load-lock i. Clearly, the Pass Through (PT) chamber is visited whenever wafer goes from RS i  to RS j  and i≠j. With SC, TC, and PTC, denoting a Source Chamber, Target Chamber and Pass-Through Chamber, respectively, as such,  
                                                                       if SC ε RS 1  {           if TC ε RS 1                  SC ← PTC;                }                      
 
         [0086]    which sets pass-through chamber (PTC) as a new source chamber (i.e., after the robot moves wafer to pass-through). Target chamber remains unchanged. The robot deployment is then described by the following  
                                                                                                     if SC == PTC ∥ SC == O {                if TC ε RS 1                  return ROBOT 1                  else if SC ε RS 1 &amp;&amp;TC ε RS 1                  return ROBOT 1                  else return ROBOT 2             }                      
 
         [0087]    Once wafer is in pass-through chamber, the wafer&#39;s target chamber (i.e., the wafer&#39;s next move) is determined from a previous source chamber (this is the chamber that had PTC as a target). Thus the algorithm becomes,  
                                                                                           If SC == PTC {           If TC == FA_Load           SC ← FA_Load           else if PSC ε RS 1                  TC ε RS 2                  else if PSC ε RS 2                  TC ε RS 1                  }                      
 
         [0088]    [0088]FIG. 5 depicts a flow diagram representing an algorithm  500  that handles a pass-through chamber. A wafer to be moved is sitting either in a pass-through chamber or orient chamber (also called an aligner) or elsewhere (e.g., FOUP load position, load-lock, an IPM or metrology chamber). At step  502 , the process  500  queries whether the source chamber is a pass through chamber. If the query is negatively answered, the process proceeds to step  506  where the process queries whether the target chamber or load-lock in the same robot space as the source chamber is busy. If that target chamber or load-lock is not busy, the routine proceeds to step  504 . If the query at step  502  is affirmatively answered, the process proceeds to step  504 .  
         [0089]    At step  504 , the process takes the wafer W from the source chamber or load-lock and places the wafer W into the target chamber or load-lock, where both the target and source chambers or load-locks are in the same robot space. The process then returns to step  502 .  
         [0090]    If, at step  506 , the query was affirmatively answered, the process  500  proceeds to step  508 . At step  508 , the process queries whether the target chamber or load-lock in an adjacent robot space is busy. If the query is affirmatively answered, the wafer W cannot be moved at this time, so the process returns to step  502 . If the query is positively negatively answered, then the process  500  proceeds to step  510 . In step  510 , the process  500  queries whether the passthrough chamber is busy. If the query is affirmatively answered, the wafer W cannot be moved from one robot space to the other through the pass-through chamber. As such, the process returns to step  502 .  
         [0091]    If the query at step  510  is negatively answered, then the process  500  proceeds to step  512 , where the wafer W is moved into the pass-through chamber. Additionally, the target chamber in the adjacent robot space is reserved for the wafer W that is now positioned in the pass-through chamber. The process  500  then returns to step  502 .  
         [0092]    An alternative way to express the above scheduling logic is via the sub-routes of a complete wafer route. The full wafer route involves the following stages:  
                                                       FA Load Ports   {FP1, FP2}           Orient Chamber   {0}           Pass Through Chamber   {PTC1, . . . , PTC4}           Metrology Chamber   {MC1, . . . , MC6}           Pass Through Chamber   {PTC1, . . . , PTC4}           Load-Locks   {LL1, LL2}           Cluster Tool   {List Wafer Flow}           Load-Locks   {LL, LL2}           Pass Through Chamber   {PTC1, . . . , PTC4}           Metrology Chamber   {MC1, . . . , MC6}           Pass Through Chamber   {PTC1, . . . , PTC4}           FA Load Ports   {FP1, FP2}                      
 
         [0093]    A wafer starts and ends with the same FA Load Port. The sub-route is obtained by deleting a stage (e.g. pass-through chamber or metrology chamber) from the above route and specifying location in the stage (e.g. LL 1  or LL 2 , FP 1  or FP 2 , etc.) See U.S. patent application Ser. No. 09/523,409 filed Mar. 10, 2000, which is hereby incorporated herein by reference, for detailed description of a calculation of timing in wafer arrivals (departures) to (from) pass-through chamber.  
       Deadlock Handling  
       [0094]    When there is only one metrology or defect control chamber in the FI and the sampling policy is to inspect wafers before entering and after leaving the tool, a “simple” processing loop is formed. A load-lock, pass-through chamber, orient, metrology or IPM chamber, and the FA_Load position form the loop. Here, metrology or inspection chamber is the knot chamber. Algorithms which handle processing loops in cluster tools are described in U.S. patent application Ser. No. 09/074,122, filed May 7, 1998, (attorney docket number 2331), which is herein incorporated by reference. Depending on the type of deadlock handling (i.e., avoidance or resolution), these algorithms may require slight adaptation to new conditions induced by sampling.  
         [0095]    Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.