Abstract:
A manufacturing control system uses computer control of work flow for automatic production line control. The process is to sort (Work in Process) WIP by priority and queue time; select high priority WIP; and sort WIP by queue time and batch with other WIP by the same recipe. Calculate a dynamic dispatching ranking except for high priority batch WIP. Select the high priority stage and batch the high priority stage by recipe based upon queue time management. Test whether the line remains loaded at capacity. If the line is below capacity, interrupt the method. If the line operates at capacity, then branch to recalculate the ranking. Then return to select the high priority stage and select the batch high priority stage by recipe based upon queue time management.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to processes of manufacture of integrated circuit chips and more particularly to automatic control of production in an integrated circuit manufacturing process. 
     2. Description of Related Art 
     In a Job shop factory there are basically two types of production decisions required. The first type of decision to be made pertains to dispatching. The second type of decision to be made relates to the wafer release policy. In the past, both the dispatching decision and the wafer release policy were negotiated and handled separately in different, independent factory management centers. In such a conventional factory, dispatching decisions are carried out at each and every work center in the factory. Those dispatching decisions depend upon the status of the Work In Process (WIP), since there can be problems of unpredictable events such as machine breakdown, machine delay and scrapped work product. That is to say that the status of WIP varies based upon the quantity of product to be released and the status of equipment. 
     There are many methods of dispatching WIP which have been used in the past. Such methods of dispatching WIP include FIFO (First In First Out,) LBFS (Last Buffer First Service,) EDD (Earliest Due Date,) SLACK (Remaining Cycle Time--Remaining Process Time.), CR (Critical Ratio,) LDD (Local Due Date,) and MQT (Maximum Queue Time,) . . . and so on. All of the popular dispatching rules do not consider or include the wafer release policy being employed in the factory. Those prior art systems are based solely upon using the WIP status to dispatch work at a predicted due date based upon the pessimistic view that popular dispatching rules do not take care of WIP dispatching and wafer release to use WIP (Work In Process) management by using such popular dispatching rules. 
     U.S. Pat. No. 5,219,765 of Yoshida et al &#34;Method for Manufacturing a Semiconductor Device Including Wafer Aging, Probe Inspection, and Feeding Back the Results of the Inspection to the Device Fabrication Process&#34; describes a method for manufacturing semiconductor devices including tests from which information is fed back for fabrication process improvement. 
     U.S. Pat. No. 5,240,866 of Friedman et al &#34;Method for Characterizing Failed Circuits on Semiconductor Wafers&#34; shows a method for characterizing failed circuits on semiconductor wafers. 
     U.S. Pat. No. 5,210,041 of Kobayashi et al &#34;Process for Manufacturing Semiconductor Integrated Circuit Device&#34; shows computer control of testing/feedback in a chip manufacturing process control system. 
     SUMMARY OF THE INVENTION 
     There are three improvements obtained by using the dynamic dispatching methodology on semiconductor manufacturing plants which has the advantages as follows: 
     1) Combining of the release policy into the dispatching methodology. 
     2) Basing operation on the Kanban concept to decide stage priority, based on the Japanese Just-In-Time (JIT) production control methodology wherein a card is pulled to the front step in time and reduces the buffer WIP to &#34;0&#34;. 
     3) Real time dispatching depends upon updated WIP and 
     equipment status. 
     In accordance with this invention, a method is provided for operating a manufacturing control system with computer control of work flow in automatic control of a manufacturing production line by the steps comprising, 
     sorting WIP by priority and queue time, 
     selecting high priority WIP, 
     sorting WIP by queue time and hatching with other WIP by the same recipe, 
     performing DD --  R stage calculation except for high priority batch WIP, 
     selecting the high priority stage and hatching the high priority stage by recipe based upon queue time management, 
     testing to determine whether the production line remains loaded at capacity, and if the production line is operating below capacity, then since the dispatching system is operating below capacity the method is interrupted, but if the production line remains operating at capacity, then branching to recalculate the DD --  R and then returning to the step of selecting the high priority stage and select the batch high priority stage by recipe based upon queue time management, 
     whereby automation of production control of a manufacturing process is provided by improved automation of work scheduling. 
     It is preferred that loading is calculated; and/or that demand is calculated. 
     It is also preferred that (DD --  R) is calculated as follows: ##EQU1## where UT=updated target, 
     AO=Actual Out data, and ##EQU2##  and NEG --  WIP=Next --  Equipment Group WIP and 
     EAC=Equipment Availability Capacity. 
     Preferably, the UT ( updated target ) is calculated forming a minuend followed by subtraction of a subtrahend comprising Actual Out data therefrom yielding a remainder of NEC --  R. 
     Preferably, UT is calculated by determining the maximum of MAX (MDT, AO+HP --  WIP+S --  WIP). 
     Preferably, ##EQU3## Preferably, MDT is calculated by the equation: 
     
         MDT(k)=Min [D(k), C WIP(k)]. 
    
     Preferably, UT is calculated by determining the maximum of MAX ((Min [D(k), C WIP(k)]), AO+HP --  WIP+S --  WIP). 
     Preferably, loading is calculated and/or demand is calculated. 
     A method of computer control of work flow in control of a manufacturing production line including the steps comprising, 
     sorting WIP by priority and queue time, 
     selecting high priority WIP, 
     sorting WIP by queue time and batching with other WIP by same recipe, 
     performing DD --  R stage calculation except for high priority batch WIP, 
     selecting the high priority stage and hatching the high priority stage by recipe based upon queue time management, 
     testing to determine whether the production line remains loaded at capacity, and if the production line is operating below capacity, then since the dispatching system is operating below capacity the method is interrupted, but if the production line remains operating at capacity, then branching to recalculate the DD --  R and then returning to the step of selecting the high priority stage and select the batch high priority stage by recipe based upon queue time management, 
     whereby automation of production control of a manufacturing process is provided by improved automation of work scheduling. 
     A manufacturing control system in accordance with this invention includes computer control of work flow in automatic control of a manufacturing production line, 
     means for sorting WIP by priority and queue time, 
     means for selecting high priority WIP, 
     means for sorting WIP by queue time and hatching with other WIP by same recipe, 
     means for performing DD --  R stage calculation except for high priority batch WIP, 
     means for selecting the high priority stage and batching the high priority stage by recipe based upon queue time management, 
     means for testing to determine whether the production line remains loaded at capacity, and if the production line is not operating at capacity, then since the dispatching system is operating below capacity the method is interrupted and if the production line remains operating at capacity, then 
     means for branching to recalculate the DD --  R and then return to the step of selecting the high priority stage and select the batch high priority stage by recipe based upon queue time management 
     whereby automation of production control of a manufacturing process is provided by improved automation of work scheduling. 
     Preferably, means is included for calculating loading, and/or means for calculating demand. 
     Preferably, (DD --  R) is calculated as follows: ##EQU4## 
     Preferably, the UT (updated target) is calculated forming a minuend followed by subtraction of a subtrahend comprising Actual Out data therefrom yielding a remainder of NEC --  R. 
     Preferably, UT is calculated by determining the maximum of MAX (MDT, AO+HP --  WIP+S --  WIP). 
     Preferably, ##EQU5## 
     Preferably, UT is calculated by determining the maximum of MAX (MDT, AO+HP --  WIP+S --  WIP). 
     Preferably, MDT is calculated by the equation: 
     
         MDT(k)=Min [D(k),C WIP(k)]. 
    
     Preferably, loading is calculated and/or demand is calculated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages of this invention are explained and described below with reference to the accompanying drawings, in which: 
     FIG. 1 shows a block diagram of a portion of a computer controlled integrated circuit manufacturing factory. 
     FIG. 2A shows the method (algorithm) for dispatching by lot in accordance with this invention. 
     FIG. 2B shows the program for calculating LOADING (K). 
     FIG. 2C shows the program for calculating DEMAND (K). 
     FIG. 2D shows the equations calculated in the calculation of DD --  R. 
     FIG. 3 shows a bar graph of LOADING and DEMAND or WIP (Work in Process) vs. Cycle Time as described above with reference to FIG. 2C. 
     FIG. 4 shows a block diagram similar to FIG. 1 of a portion of a computer controlled integrated circuit manufacturing factory. 
     FIG. 5 shows a chart of Stages, WIP and CYCLE Time for stage S1 . . . Sn, WIP for W1 . . . Wn, and Cycle time CT1 . . . CTn in connection with Demand. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. INTRODUCTION 
     The dynamic dispatching method of this invention combines the PULL concept for next service equipment and the PUSH concept for updated WIP and equipment status. The concepts of PULL and PUSH are described by N. E. Larsen and L. Alting in &#34;Criteria for Selecting a Production Control Philosophy&#34;, Production Planning and Control, (1993) Vol. 4, No. 1, pp 54-68. 
     FIG. 1 shows a block diagram of a portion of a computer controlled integrated circuit manufacturing factory. The supervisor CPU 20 sends output signals on line 26 to control several work processing centers including the centers defined as follows: 
     CVD . . . Chemical Vapor Deposition 
     DIF . . . Diffusion 
     DRY . . . Plasma,etc. 
     IMP . . . Ion Implanting 
     PHO . . . Photolithography 
     SPU . . . Sputtering 
     WET . . . Wet Chemical Processing 
     The above centers provide return signals on line 28 to CPU 20. There are a number of &#34;PHO&#34; units for photolithographic processing of a work piece which includes depositing of photoresist, exposing the photoresist with a master mask and developing the photoresist to form a mask on the surface of the workpiece. 
     The first group of units which are connected by bus line 22 with CPU 20 are as follows: 
     CONT-PHO 
     CODE1-PHO 
     WELL-PHO 
     CONT-P-PHO 
     P-FLD --  PHO 
     1POLY-1-PHO 
     POLY-2-PHO 
     VT2-PHO 
     The second group of units which are connected by bus line 32 with CPU 20 which are as follows: 
     PLDD-PHO 
     BRC-2-PHO 
     POLY-3-PHO 
     VIA-PHO 
     MET-1-PHO 
     ONO-PHO 
     ESD-PHO 
     PASS-2-PHO 
     CELL-PHO 
     SIN-2-PHO 
     LDD-PHO 
     MET-2PHO 
     N+ --  S/D --  PHO 
     2. LOADING AND DEMAND CONCEPT 
     FIG. 2B shows the program for calculating LOADING (K). The program begins at step 70 which leads by line 71 to steps 72 and 73. The numerator X1 of the ratio of the summation of work in progress WIP from i to K is calculated in step 72 (where WIP i  is written as &#34;W i  &#34; for convenience of illustration. Then X2, the sum of the stage process cycle time is calculated as a denominator in step 73. The outputs of steps 72 and 73 pass on lines 74 and 75 to step 76 where the ratio of X1 to X2 is calculated which is the LOADING(K). The program proceeds on line 77 to stop in step 78. ##EQU6## K: Loading. CT: stage process cycle time. 
     WIP: stage work-in-process quantity. 
     FIG. 2C shows the program for calculating DEMAND (K). The program begins at step 80 which leads by line 81 to steps 82 and 83. The numerator Y1 of the ratio of the summation of work in progress WIP from i to K is calculated in step 82 (where WIP i  is written as &#34;W i  &#34; for convenience of illustration). Then Y2, the sum of the stage process cycle time is calculated as a denominator in step 83. The outputs of steps 82 and 83 pass on lines 84 and 85 to step 86 where the ratio of Y1 to Y2 is calculated which is the DEMAND (K). ##EQU7## n: total process stages. CTi: stage process cycle time. 
     WIP: stage work-in-process quantity. 
     The loading equation from above is restated here. ##EQU8## 
     LOADING Intensity=LOADING/Equipment Capacity 
     DEMAND Intensity=DEMAND/Equipment Capacity 
     3. MINIMA DEMAND TARGET 
     
         MDT(k)=Min{D(k),C.sub.-- WIP(k)} 
    
     MDT - - - Minimum DEMAND Target 
     D - - - DEMAND 
     C --  WIP - - Coming WIP 
     Coming WIP is defined as all of the WIP that will arrive at a given stage in a one day cycle time. DEMAND is defined above. 
     Such a &#34;stage&#34; at which the WIP will arrive can be one of many different stages. 
     4. DYNAMIC DISPATCHING METHOD 
     Definitions: 
     1) Hi-WIP - - - High priority WIP 
     2) Slow-WIP - - WIP queue time over maximum queue time 
     3) Updated target=Max {Min. demand target, Actual out+Hi-WIP+Slow-WIP} 
     4) Next machine Capacity Ratio (NEC --  R)=Next Equipment Group WIP/Equipment Available Capacity 
     5) Dynamic Dispatching Ranking (DD --  R) =(Updated Target Actual-Actual Out)/NEC --  R 
     CALCULATION OF DYNAMIC DISPATCH RANKING 
     Updated Target: (UT) 
     
         UT=MAX (MDT,AO+HP.sub.-- WIP+S.sub.-- WIP) 
    
     Next Equipment Capacity Satisfaction Ratio: (NEC --  R) ##EQU9## Dynamic dispatching ranking: (DD --  R) 
     Referring to FIG. 2D, the equations as follows are calculated in the calculation of DD --  R as follows: ##EQU10## where the values are as follows: AO........ Actual Out 
     DD --  R...... Dynamic Dispatching Ranking 
     EAC....... Equipment Available Capacity 
     HP --  WIP.... High Priority WIP 
     MAX....... Maximum 
     MDT....... Minimum DEMAND Target 
     NEC --  R...... Next Equipment Capacity Satisfaction Ratio 
     NEG --  WIP.... Next --  Equipment Group WIP 
     S --  WIP..... Slow WIP 
     UT........ Updated Target 
     Referring to FIG. 2D, the equations as follows are calculated in the calculation of DD --  R by the steps which follow: 
     Step 60 is the start of calculation of DD --  R. Step 60 leads by line 61 to step 64 which provides calculation of UT (Updated Target) by determining the maximum of MDT, AO+HP --  WIP+S --  WIP. The result of step 64 then goes by line 65 to step 66. 
     In step 66 the value AO (Actual Out) is subtracted from the UT (Updated Target.) The result of step 66 then goes by line 67 to step 68. 
     In step 68 the result of step 66 is multiplied by the EAC (Equipment Available Capacity.) The result of step 68 then goes by line 69 to step 70&#39;. 
     In step 70&#39; the result of step 68 is divided by NEG --  WIP Next --  Equipment Group WIP. The result of step 70&#39; then goes by line 71&#39; to step 72&#39;. 
     FIG. 2A shows the method (algorithm) for dispatching by lot in accordance with this invention. The beginning of the process is the start 40 which leads by line 41 into the step 42 of sorting the Work In Process (WIP.) 
     From step 42, line 43 leads to step 44 where the system selects the WIP units which have a high priority in terms of schedule requirements. Then the WIP is sorted by queue time and the WIP is sorted into different batches of WIP having the same recipe of processing steps. Having the same recipe mean that the process steps and process conditions are the same for all WIP in a given batch. 
     From step 44, line 45 leads to step 46 where the system does a Dynamic Dispatching --  Ranking (DD --  R) stage calculation except for the WIP which is of high priority. Note that 
     
         (DD.sub.-- R)=(Updated demand-Actual Out/NEC.sub.-- R) 
    
     From step 46, line 47 leads to step 48 where the system selects the high priority stage and high priority WIP is placed into batches by recipe based upon queue time management. Step 48 first selects the high priority stage by computer sorting. 
     Then step 48 batches the high priority stage by recipe based on queue time management by computer sorting. 
     From step 48, line 49 leads to decision block 50 where the system determines whether the WIP remains at capacity level. 
     Based upon the equipment number one can determine the total capacity. 
     If the test results in NO, then the program stops until the next time the computer 20 calls it into operation again. 
     If YES, then the system goes on line 51 to step 52. 
     In step 52, the system recalculates the dynamic dispatching ranking DD --  R of the WIP for feedback to step 48. 
     From step 52, line 53 leads back to step 48 where the same operation is repeated from step 48 to step 50. 
     FIG. 3 shows a bar graph of LOADING and DEMAND or WIP vs. Cycle Time as described above with reference to FIG. 2C. 
     The LOADING is as follows: 
     LOADING 1  =W 1  /CT 1   
     LOADING 2  =(W 1  +W 2 )/(CT 1  +CT 2 ) 
     In this case there are factors to be considered as possible: 
     LOADING Intensity=LOADING/Capacity 
     DEMAND Intensity=DEMAND/Capacity 
     Min DEMAND Target=Min(Coming WIP, DEMAND) 
     W 1 . . . W 12  designate WIP. 
     C 1 . . . C 12  designate cycle time. ##EQU11## For example if today is 27 Mar. 1995, then the Remaining Cycle Time=31/Mar -27/Mar=4 days. 
     FIG. 5 shows chart of Stages, WIP and CYCLE Time for stage S1 . . . Sn, WIP for W1 . . . Wn, and Cycle time CT1 . . . CTn as described in connection with Demand. 
     1. LOADING intensity is defined to be greater than 1 when the wafer arrival speed exceeds the capacity of the manufacturing system. The solution to the problem of excessive loading intensity is to control release, reduce release quantity or to improve the rate of throughput. 
     2. DEMAND intensity is defined to be greater than 1 when the demand exceeds the capacity. 
     3. The loading intensity is greater than the demand intensity when the wafer start quantity exceeds the demand. 
     4. LOADING intensity is less than demand intensity if wafer starts are insufficient to meet demand. 
     
                                           TABLE A__________________________________________________________________________PHOTO AREA DYNAMIC DISPATCHING REPORT BY STAGE:PHO.sub.-- STG    PHO.sub.-- WIP          H.sub.-- PUSH                SLOW                    INSP                        RUN.sub.-- IN                              PHO.sub.-- OUT                                     NNTAR                                          N.sub.-- CR                                              DD.sub.-- R__________________________________________________________________________CONT-PHO 293   50    0   7   0     334    504  0.20                                              809.6CODE1-PHO    96    96    0   0   0     0      96   0.23                                              420.4WELL-PHO 84    84    0   0   0     48     132  0.23                                              367.9CONT-P-PHO    48    48    0   0   0     94     142  0.16                                              295.1P-FLD.sub.-- PHO    24    24    24  0   0     216    264  0.23                                              210.2POLY-1-PHO    192   72    0   0   24    96     216  0.66                                              175.6POLY-2-PHO    642   187   77  108 97    288    552  0.55                                              107.9VT2-PHO  144   72    0   48  48    0      120  0.23                                              105.1PLDD-PHO 93    21    24  24  0     600    645  0.23                                              92.0BRC-2-PHO    168   0     24  24  0     24     72   0.55                                              43.9POLY-3-PHO    48    0     0   0   0     48     72   0.55                                              43.9VIA-PHO  166   66    0   56  6     322    389  0.20                                              39.7MET-1-PHO    589   96    24  23  96    372    492  0.35                                              2.9ONO-PHO  23    0     0   0   0     24     24   0.41                                              0.0ESD-PHO  24    0     0   0   0     324    324  0.16                                              0.0PASS-2-PHO    383   105   10  36  117   629    744  1.49                                              -25.5CELL-PHO 138   0     0   24  48    120    168  0.23                                              -105.1SIN-2-PHO    216   0     0   24  72    384    432  0.41                                              -116.4LDD-PHO  395   11    0   35  48    144    155  0.23                                              -315.3MET-2-PHO    312   48    0   72  120   312    384  0.35                                              -343.1N+.sub.-- S/D.sub.-- PHO    407   57    0   48  72    300    347  0.16                                              -448.8__________________________________________________________________________ NOTES H.sub.-- PUSH = (HIGH PRIORITY PUSH??)  INCLUDES ALL WIP WHICH IS OF PRIORITY 1 AND PRIORITY 2 SLOW = WAITING WIP AND QUEUE TIME OVER 24 HOURS DD.sub.-- R = (NEW.sub.-- TARGET - PHO.sub.-- OUT - INSP - RUN.sub.-- IN) * ((CAP + MC.sub.-- NO)/NWIP) NEW.sub.-- TARGET (NTAR) = MAX (TARGET, (PHO.sub.-- OUT + SLOW WIP + HIGH PRIO WIP)) NEC.sub.-- R = NEXT MACHINE GROUP WIP/CAPACITY RUN.sub.-- IN = RUNNING QUANTITY ON INLINE SYSTEM NNTAR = UPDATED DEMAND PHOTOLITHOGRAPHY AREA PROCESS PROCEDURE: INLINE SYSTEM → INSPECTON → OUT TO OTHER AREA DETAIL DISPATCHING METHOD BY LOT 
    
     FIG. 4 shows a block diagram similar to FIG. 1 of a portion of a computer controlled integrated circuit manufacturing factory. The CPU 20 sends output signals on line 26 to control processing centers including the centers as follows: 
     CVD, DIF, DRY,IMP, PHO, SPU, and WET. 
     Those centers provide return signals on line 28 to CPU 20. There are a number of PHO units for photolithography processing of a work piece. 
     The first group of units are connected by bus line 122 with CPU 20 which are as follows: 
     PHO --  STG . . . Photo Stage 
     PHO --  WIP . . . Photo Work-In --  Process 
     H --  PUSH . . . High Priority Pushing WIP 
     SLOW . . . Slow Moving 
     INSP . . . Inspection Quantity (Include ADI(After Developing Inspection) and CD Measure) 
     RUN --  IN . . . Running WIP 
     PHO --  OUT . . . PHOTO Stage MOVE OUT Quantity 
     NNTAR . . . New Target (Updated Target) 
     N --  CR (Defined Above under Dynamic Dispatching Method) 
     DD --  R (Defined Above under Dynamic Dispatching Method) 
     P+ --  S/D --  PHO 
     N+ --  S/D --  PHO 
     CONT-PHO 
     MET-2-PHO 
     P-FLD --  PHO 
     WELL-PHO 
     PLDD-PHO 
     LDD-PHO 
     BRC-2-PHO 
     The second group of units are connected by bus line 132 with CPU 20 which are as follows: 
     POLY-3-PHO 
     CONT-P-PHO 
     MET-1-PHO 
     PASS-2-PHO 
     SIN-1-PHO 
     SIN-2-PHO 
     POLY-1-PHO 
     PI-PHO 
     VIA-PHO 
     POLY-2-PHO 
     SELFAL-PHO 
     CONT-N-PHO 
     PWELL-PHO 
     ESD-PHO 
     BRC-3-PHO 
     PHOTO (Coater+Stepper+Developer→Inspection→CD 
     
                                           TABLE B__________________________________________________________________________PHO.sub.-- STG    PHO.sub.-- WIP          H.sub.-- PUSH                SLOW                    INSP                        RUN.sub.-- IN                              PHO.sub.-- OUT                                     UPD.sub.-- DEM                                            N.sub.-- CR                                                DD.sub.-- R__________________________________________________________________________P+.sub.-- S/D.sub.-- PHO    636   18    30  0   72    288    674    0.13                                                2468.8N+.sub.-- S/D.sub.-- PHO    454   47    72  0   0     0      96     0.13                                                1695.3CONT-P-PHO    196   4     4   0   0     0      144    0.13                                                1125.0MET-2-PHO    873   288   113 97  85    578    979    0.23                                                934.6P-.sub.-- FLD.sub.-- PHO    312   0     0   48  48    228    500    0.19                                                916.7WELL-PHO 591   63    84  0   0     0      147    0.19                                                765.6PLDD-PHO 443   36    22  54  72    240    500    0.19                                                697.9LDD-PHO  343   0     0   0   0     24     144    0.19                                                625.0BRC2-PHO 396   48    48  0   0     12     240    0.40                                                569.3POLY-3-PHO    46    14    2   32  0     69     288    0.40                                                466.9CONT-PHO 729   96    36  0   0     168    500    0.71                                                466.0MET-1-PHO    450   55    0   26  120   48     300    0.23                                                452.3PASS-2-PHO    275   108   72  71  48    113    432    0.44                                                450.7SIN-1-PHO    564   84    0   24  72    120    500    0.66                                                430.8SIN-2-PHO    635   12    0   108 72    72     500    0.66                                                376.2POLY-1-PHO    120   24    120 0   0     0      144    0.40                                                359.6PI-PHO   192   24    24  0   24    0      48     0.13                                                187.5VIA-PHO  285   58    0   10  0     239    297    0.71                                                67.4POLY-2-PHO    627   78    24  54  72    144    288    0.40                                                44.9SELFAL-PHO    310   0     72  24  72    48     144    0.40                                                0.0CONT-N-PHO    168   0     24  24  0     0      24     0.13                                                0.0PWELL-PHO    72    24    0   0   24    96     24     0.19                                                0.0ESD-PHO  288   0     24  24  0     24     48     0.13                                                0.0BRC-3-PHO    288   0     0   48  0     168    168    0.71                                                -67.4__________________________________________________________________________ NOTES UPDATED.sub.-- DEMAND = (UPD.sub.-- DEMAND) UPD.sub.-- DEMAND = MAX (MIN.sub.-- DEMAND.sub.--  TARGET, (PHO.sub.-- OU + SLOW WIP + HIGH PRIO WIP)) ##STR1## ##STR2## ##STR3##-  Advantages: 
    
     1. High priority WIP can be push first. 
     2. Based on next equipment WIP status and demand, decide stage priority. 
     3. Queue time management under stage selection. 
     Disadvantages: 
     1. High CPU time requirement. 
     2. High computerization integration needed. 
     While this invention has been described in terms of the above specific embodiment(s), those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims, i.e. that changes can be made in form and detail, without departing from the spirit and scope of the invention. Accordingly all such changes come within the purview of the present invention and the invention encompasses the subject matter of the claims which follow.