Abstract:
A modeling and control process for distributed factories having fabrication sequences starts with a definition of how the factory actually operates, rather than a mathematical theory which ultimately leads to a definition of the plant operation. The process begins by delineating a set of factory operating rules which define how part lots interact with machines in actual operation of the factory. A dynamic model of the factory is selected from a group of specimen models for such factories. The model defines the factory by its machines, products, fabrication sequences, collections of job sets, scheduling rules, and machine reliability parameters. The parameters that describe the specific factory are determined and defined in terms of data structures of the individual factory model. The factory specific model contains descriptions of the dynamic interactions of lots and machines. The behavior of the factory can be simulated in detail. A comparision of such a simulation against actual observation of the factory can be used to refine the model. Because the process begins with a definition of how the factory actually operates, calculations for even very complex-factory simulations, such as integrated circuit fabrication facilities, are simplified so that small computers, such as personal computers, may be employed. The models and simulations can be made accurate enough to allow automatic computer control of the factory using the models and simulations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process for the modeling and control of distributed factories which have fabrication sequences. It further relates to a class of sequenced-distributed-factory (SDF) dynamic models. It also relates to factory-specific models from the SDF class and to the use of such factory specific model to generate factory schedules and to control material movement and machine loadings in the factory. The invention further relates to a computer integrated manufacturing (CIM) system that includes a factory specific model for automatic control and to the use of SDF models for the design of CIM systems. 
     2. Description of the Prior Art 
     Most manufacturing plants or factories are distributed in that they consist of heterogeneous, unconnected workstations. The virtue of this factory design is that it provides adaptability to a varying product mix. The drawback is the resulting complexity of operations, management, and quality control. 
     A distributed manufacturing plant is capable of fabricating a variety of products through ordered-process sequences of process steps. Each process step can be performed by at least one workstation in the factory. Distributed factories are common in the manufacture of modern electronics products. Six types of distributed factories can be involved: wafer slicing, wafer fabrication, semiconductor-component assembly, circuit-board fabrication, circuit-board assembly, electronic-product assembly. 
     The archetype of a distributed factory is a wafer-fabrication plant, or &#34;wafer fab,&#34; which may manufacture products simultaneously according to over one thousand processes, averaging over one hundred steps each. An example of wafer fabrication is described in detail below. 
     We are now in position to illustrate the complexity of distributed factories. For example, wafer fabrication factories with over one thousand fabrication sequences are known. Such a large collection of processes is difficult to represent in a drawing like a fab graph. Such a factory, however, can be described to a computer system. 
     The complexity of distributed factories is further illustrated by the existence of tens of thousands of fabrication sequences in a general class of distributed factory called a &#34;job shop.&#34; The standard approach to describing the collection of sequences in a job shop is to surrender to the complexity and describe the product paths through the factory as being random. They are clearly not random, but only recently have computers provided the practical computational power to describe highly-complex factories accurately. 
     The factory is a complex, data- and information-rich entity. A data structure with tens of thousands of parameters may be required merely to describe the factory. Furthermore, in operation a dynamic factory produces orders of magnitude more data describing the production flows. The sheer volume of information has made operation and control of distributed factories a major problem. 
     MANAGING COMPLEXITY 
     Despite the large data volumes and the complexity of the problem, factory management and control is still accomplished primarily by manual methods with some assistance in scheduling from software called shop-floor controllers. The shop schedule is in practice determined by the decisions of various production supervisors or foremen, or in some cases by the workers themselves. These decisions are made more or less independently of one another on the basis of the information available to each foreman and in view of his own skills and objectives. The resulting shop schedules are not necessarily those that management would prefer. 
     Shop-floor schedulers have a very limited function. They attempt to pick the next lot to process from a queue at a work station. The choice typically depends heavily on the due date of the lot and processing time remaining. In general lots which are most behind schedule are given priority, and all lots in a queue may be ranked according to this priority function. The knowledge of the location lots in queues comes from a production tracking system. 
     There are two general types of shop-floor schedulers: infinite- and finite-loading. Infinite-loading methods are especially simplistic in that they do not consider workstation capacity. Instead a schedule is created using a standard wait time (queue time). 
     Finite-loading methods consider somewhat more information on the factory. They account for workstation capacity, and do not schedule more work than the workstation can accomplish. However, these techniques are not models and do not account for dynamic factory changes. Consequently, loading plans are typically accurate for at most two days, and then must be laboriously re-calculated. 
     In attempts to address the problem of optimal factory scheduling, a tremendous theoretical literature on production scheduling has been developed. The result of this work has been to establish that current factory practice is far from optimal and to define the degree of complexity of the factory management issues. Unfortunately, this work has not resulted in practical methods for factory control. In critical reviews, it is constantly re-iterated that all of these methods fail for factories with greater than about ten machines and ten lots. 
     On the other hand, the world is filled with real factories that operate, however, sub-optimally. Where theory has failed to provide solutions, factories run through thousands of ad-hoc decisions made on the factory floor. Thus, there exists a substantial need for improvement in modeling and control techniques so that they can be of more practical use. 
     Both scheduling theory and shop-floor loading schemes possess two major structural faults. First they are incapable of recognizing the details of individual factories. In practical scheduling problems, details of operation often determine success or failure. Secondly, both methodologies neglect the dynamic operation of the factory. Unfortunately, it is precisely through the consideration of operational dynamics that human factory managers achieve scheduling solutions. 
     An indication of the state of the art in modeling and manufacturing control is given by the following surveys: S. S. Panwalkar and W. Iskander, &#34;A Survey of Scheduling Rules&#34;, Operations Research, Vol 25, No. 1, January-February 1977, pages 45-61; E. M. Dar-El and R. Karni, &#34;A Review of Production Scheduling and its Applications in On-Line Hierarchical Computer Control Systems,&#34; in On Line Production Scheduling and Plant-Wide Control, E. J. Compass and T. J. Williams (eds.), Tech. Publ. Co., Barrington, Ill., 1982, pages 1-25; T. E. Vollman, W. L. Berry and D. C. Whybark, Chapter 5, &#34;Shoop-Floor Control&#34;, and Chapter 13, &#34;Advanced Concepts in Scheduling&#34;, in Manufacturing Planning and Control Systems, Dow Jones-Irwin, Homewood, Ill., 1984, pages 113-145 and 373--403; E. A. Elsayed and T. O. Boucher, Chapter 7, &#34;Job Sequencing and Operations Scheduling&#34;, in Analysis and Control of Production Systems, Prentice-Hall, Englewood Cliffs, N.J., 1985, pages 227-285. 
     In summary, the prior art approach to modeling and manufacturing control is to start with a theoretical mathematical treatment of the problem and work down to characterize the operation of factories. As a result, the above literature characterizes modeling and manufacturing control as a nonpolynomial (NP) complete problem, which is a measure of problem complexity. Such a characterization means that the run time of the problem increases faster than any polynomial. As a result, classical scheduling theory holds that further research is necessary before such techniques can be used for factories with greater than about ten machines and ten lots, far too low quantities to be useful in integrated circuit fabrication. 
     In contrast to the theory of sequencing and scheduling, simulation models of factories have been used as a means of schedule generation, and this approach is reviewed in the above literature. In general, schedule generation from models is not viewed as a feasible approach in this literature. The schedule is not meaningful unless the model is accurate. An accurate model is viewed as a labor-intensive special case which is useful for only its target factory. Furthermore, in implementation the model may require a long execution time, making its use in scheduling cumbersome. 
     One of the reported successes of this approach also illustrates these drawbacks (S. K. Jain, &#34;A Simulation-Based Scheduling and Management Information System for a Machine Shop&#34;, Interface, November, 1975, pp. 81-96.) The factory was small: 35 machines, 80 jobs per week, an average of ten operations per job. The simulation developed for schedule generation was the result of intensive professional effort, and it was able to generate schedules no more frequently than eight hours. This slow scheduling response time was only adequate because of the long process times of that factory. 
     Therefore, both classical scheduling theory and standard simulation modeling have failed to meet the challenge of scheduling real factories. A fundamentally different approach is therefore required to make modeling and manufacturing control truly practical in such complex fabrication processes as are employed in integrated circuit fabrication. 
     As described above, wafer-fabrication factories are distributed factories with fabrication sequences. Models of wafer fabs have been developed by J. E. Dayhoff and R. W. Atherton, &#34;Simulation of VLSI Manufacturing Areas&#34;, VLSI Design, December 1984, pages 84-92. This work focused on the queueing dynamics associated with queues for each distinct process operation (process-step queue). To that end, the dynamics treated in these models was severely restricted, and only strict process sequences were allowed. This prior work contributed to queueing theory, and was useful for theoretical studies of the dynamics of these non-standard queues. In practice, however, such queues are secondary. The standard queue in factories is lots forming a waiting-line at a workstation (workstation queue). The earlier model does not treat workstation queues, and can not treat factory scheduling from such a queue. The factory scheduling rules discussed above all depend strongly on the attributes of individual lots. The earlier model, however, treats process-step queues, but not lots, as fundamental. Lots do not have identity, and key lot attributes such as due date and priority weight are not present. Thus, this model can not treat scheduling rules arising in theory or practice, can not be validated, and can not be used to generate schedules or control a factory. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a method for more tractable management and control of distributed factories having fabrication sequences. 
     It is another object of the invention to provide such a method which can be used with complex processes, such as integrated circuit manufacture, which require manufacturing plants having large numbers of machines and lots. 
     It is a further object of the invention to provide such a method which starts with an analysis of actual manufacturing plant operation, rather than a theoretical or mathematical approach. 
     It is still another object of the invention to provide a process for modeling a manufacturing plant which develops concrete descriptions of specific factories. 
     It is a still further object of the invention to provide such a modeling process which provides a dynamic model for a distributed factory which has a fabrication sequence. 
     It is yet another object of the invention to provide such a modeling process which will simulate the behavior of the factory in detail. 
     It is another object of the invention to provide such a modeling process in which the model is validated with actual operating data for the factory. 
     It is still another object of the invention to provide such a modeling process in which the model can be used to generate factory schedules using scheduling rules that are evaluated by the model prior to being implemented in the factory. 
     It is a further object of the invention to provide such a modeling process which characterizes factory operation accurately enough so that the model can be used for automatic control of the factory. 
     It is yet another object of the invention to provide such a modeling process with automatic control of the factory which includes a computer-integrated manufacturing production control system. 
     It is another object of the invention to provide such a modeling process that will provide sizing data for determining equipment and other resource needs for the factory. 
     The attainment of these and related objects may be achieved through use of the novel process for modeling and controlling a manufacturing plant herein disclosed. In one aspect of the invention, a modeling process in accordance with this invention includes delineating a set of factory operating rules which define how part lots interact with machines in actual operation of the plant. The manufacturing plant is defined by specifying machines in the plant and at least batch size and processing time parameters of each machine. Products manufactured in the plant are defined. Fabrication sequences consisting of process steps are provided for the products manufactured in the plant. The process steps are assigned to the machines. At least time and yield characteristics of each process step are defined. Those phenomena in the manufacturing plant which are stochastic in nature are identified. Distributions and parameters of the distributions are assigned to the stochastic phenomena. 
     The model obtained by this process is used to simulate operation of the manufacturing plant. Predictions obtained with the simulation are compared with observed manufacturing trends in the plant. The comparison is used to refine choice of fundamental rules and parameters in the model. 
     The present invention provides an algorithm for factory description and specification. The algorithm develops concrete descriptions of specific factories. Described below is a class of dynamic models for distributed factories which have fabrication sequences. The first step in the algorithm is to choose one dynamic model from this class. In terms of this specimen model the factory is defined by its machines, products, fabrication sequences, collections of job sets, scheduling rules, and machine reliability parameters. The second step is to determine the parameters that describe the specific factory. These parameters are defined in terms of data structures of the individual factory model. The factory-specific model contains descriptions of the dynamic interactions of lots and machines; thus, the behavior of the factory can be simulated in detail. The third step is to simulate the dynamic behavior of the factory. 
     This invention provides a standard class of specimen models for distributed factories which have fabrication sequences. Members of this class are called sequenced-dynamic-factory (SDF) models. The class is defined by sets of fundamental rules for the definitions of fabrication sequence, queues, scheduling rules, batching, set-up times, yield, reliability, and other variables. A choice of rules from each set defines an individual model. 
     The invention provides factory-specific models for distributed factories which have fabrication sequences. A model describing actual operation of the factory is obtained. An individual model from the class above is chosen; the choice is based upon the match between the dynamic characteristics of the model and those of the factory. The parameters describing the factory&#39;s products, fabrication sequences, and machines are determined. The parameters describing such rules as equipment reliability are established from factory-specific production data provided (for example, by a computer-integrated manufacturing (CIM) production-control system that tracks lot movement, work in process, and equipment status). The model is validated by use of material movement data from the CIM system. 
     A factory-specific model obtained with this invention can overcome both of the objections to conventional scheduling theory and shop-floor loading schemes; it can treat the specific details of individual factories and it can consider complex information flows resulting from the dynamics of factory operation. Furthermore, the factory model can generate feasible schedules in the same way as successful factory managers. 
     The present invention provides the generation of factory schedules from the factory-specific models for distributed factories which have fabrication sequences. The schedule lists lot movements and machine loadings. The factory-specific model contains the scheduling rules of the factory and it simulates the detailed behavior in time of the factory. After initialization with the state of the factory, the simulation calculates and lists the lot movement and machine loadings for the indicated planning horizon. Furthermore, the scheduling rules can be evaluated by the model prior to being implemented in the factory. 
     The invention provides for use of the factory-specific model for the automatic control of the factory including feed-back and feed-forward control of lot movements, machine loadings, and processing. The CIM system provides data on the performance of the factory to the schedule generated above. The factory may deviate from the schedule due to random events like equipment failure or the loss of a lot, or unplanned events like a new product order. The factory-specific model generates control actions for the factory so that the perturbations in desired factory behavior due to unexpected events are minimized. 
     This invention provides for a computer-integrated manufacturing production control system which incorporates a factory-specific model for automatic control. The factory-specific model can also to provide sizing data for the design of the computer-integrated manufacturing (CIM) production-control system. The model provides simulated results on material movements, queue sizes, factory transactions, and other information. Such simulated results are necessary to size computers, communications networks, data bases and other aspects of the computer system for manufacturing control. 
     Because the modeling process of this invention starts with a definition of how a manufacturing plant actually operates, rather than a mathematical theory which ultimately leads to a definition of the plant operation, this modeling process is able to handle very complex manufacturing plants and processes, such as those used in the manufacture of integrated circuits. However, the results obtained with his process should make it of use in a wide variety of other manufacturing plants and processes as well. 
     The attainment of the foregoing and related objects, advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention, taken together with the drawings, in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram in the form of a directed multigraph, showing fabrication sequences representative of a first embodiment of the invention. 
     FIG. 2 is another flow diagram in the form of a directed multigraph, showing fabrication sequences representative of a second embodiment of the invention. 
     FIG. 3 is a further flow diagram in the form of a directed multigraph, showing fabrication sequences representative of a third embodiment of the invention. 
     FIG. 4 is another flow diagram in the form of a directed multigraph, showing fabrication sequences representative of a fourth embodiment of the invention. 
     FIG. 5 is still another flow diagram in the form of a directed multigraph, showing fabrication sequences representative of a fifth embodiment of the invention. 
     FIG. 6 is yet another flow diagram in the form of a directed multigraph, showing fabrication sequences representative of a sixth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following nonlimiting examples represent preferred embodiments of the invention and describe the invention further. 
     EXAMPLE 1: Machine Shop 
     Turning now to the drawings, more particularly to FIG. 1, there is shown a machine shop 10, with ten workstations 12-30, labeled as #1-#10, and four fabrication sequences A1-A10, B1-B6, C1-C7 and D1-D6. The convention used in this directed multigraph is that directed arcs indicate a transition from one process step to another and that the process step is performed at the workstation at the end of the arc. Thus, start 32 is present only as a convenience prior to the beginning of the sequence. The process sequences consist of all the arcs from start to completion. Table 1 below shows important characteristics of each workstation 12-30. The column headed Set-up Parameter gives a time in the same units as process time to account for changes in the machines that are necessary for different products and/or process conditions in operation of the machine shop. 
     
                       TABLE 1______________________________________Workstations in a Machine ShopLabel Type      # Machines                     Load Size                             Set-up Parameter______________________________________1.    Cleaning  5         1       .012.    Centering 2         2       .253.    Turning   3         1       .54.    Milling   4         1       .255.    Drilling  4         2       .156.    Shaping   5         1       .257.    Treatment 2         4       .158.    Grinding  4         1       .159.    Finishing 6         1       .1510.   Cleaning  5         1       .01______________________________________ 
    
     A number of products may be associated with each fabrication sequence. Each product is differentiated by a change in the process-step parameters for one or more steps. While this diagram is becoming complex with only four fabrication sequences, real-world machine shops may have over one thousand. This real world complexity has prevented accurate representation of the fabrication sequences. This problem is surmounted by this invention. While we can not usefully draw such a fab graph, the computer can input, store, and compute with a mathematical representation of the 1000 fabication sequences. 
     EXAMPLE 2: Wafer Fabrication 
     FIG. 2 provides a representation of two simplified semiconductor wafer fabrication processes. Fabrication sequences A1-A26 and B1-B18 have respectively 26 and 18 steps. Twenty workstations 34-72, labeled #1-#20 are shown. In this drawing the number of machines at each work station is indicated. Again we have reached the limit of practical pictorial representation with a simple example. Each fabrication sequence may have dozens of products. Real semiconductor fabrication sequences may have 250 process steps using 50 workstations and 200 pieces of equipment. The two fabrication sequences A1-A26 and B1-B18 are for building integrated circuits on the surface of a silicon wafer. Fabrication sequence A1-A26 is a simplified bipolar process, and fabrication sequence B1-B16 is a simplified metal-oxide-silicon (MOS) process. 
     An integrated circuit is implemented as a three-dimensional micro-structure on the surface of a single-crystal silicon. Each wafer serves as the site for the fabrication of hundreds or even thousands of integrated circuits, such as memories. Each fabrication sequence consists of four general categories of process operations: deposition, patterning, etching, and doping. Circuit elements, such as transistors, capacitors, and resistors, are formed in layers of five or more different materials including dielectics, conductors, and semiconductors. These layers, low-defect thin films, are deposited to thicknesses less than three microns by a variety of techniques, such as chemical-vapor deposition, sputtering, or plasma-assisted deposition. 
     At each layer, the two-dimensional structure of the device is fabricated by sequential steps of patterning and etching. The patterning process involves applying an organic resist, irradiating the resist through a mask, and removing the positive or negative image. The etching process involves either liquid-phase dissolution (wet-etching) or gas-phase reactions (dry etching). The remaining class of process steps is doping; in current manufacturing this step is usually performed using ion-implantation. 
     In the process sequence variations of these four steps are repeated for each material layer. Each process operation is performed by specialized equipment. Equipment is grouped into workstations of like machines. Workstations may perform more than one step in a single process sequence or process steps in more than one sequence. 
     An asociation of the two process sequences with workstations is shown in FIG. 2. Each circle 34-72 represents a workstation; each arrow represents a transition from one process step to another. The process sequence is the complete path (or flow) over the set of workstations. This diagram is a type of directed multi-graph, called a fab graph. Further details on the workstations 34-72 are presented below in Table 2. 
     
                       TABLE 2______________________________________Workstations for Wafer FabricationLabel Machine Type # Machines                        Load Set-Up Parameter______________________________________1.    Masking      12        25   .22.    Oxide Furnace              8         50   .33.    Film Spinner 2         25   .154.    Diffusion Furnace              4         50   .35.    Wet-Etch     4         25   .156.    Epitaxy      6         20   .57.    Oxide Furnace              4         50   .38.    Deposition   2         50   .259.    Diffusion Furnace              4         50   .310.   Deposition   2         50   .311.   Diffusion Furnace              4         50   .312.   Deposition   2         50   .313.   Diffusion Furnace              4         50   .314.   Metal Deposit 1              2         30   .215.   Diffusion Furnace              2         50   .316.   CV Deposition              2         50   .317.   Bake Furnace 2         50   .1518.   Prober       2         25   .219.   Ion-Implanter              2         25   .520.   Metal Deposit 2              2         50   .3______________________________________ 
    
     EXAMPLE 3: Wafer Sort 
     Wafer sort, shown in FIG. 3, illustrates the case where a pair of process steps 74 and 76 may be interchanged to give an equivalent fabrication sequence. Since parametric test 74 and functional test 76 may be done in either order, the two process sequences A1-A3 and B1-B3 shown are equivalent. In wafer fabrication such process sequences allow no changes in the order of steps. Loosely constrained order of steps is common in machine shops. The three process steps 74, 76 and 78, labeled 1-3 in FIG. 3, are identified in Table 3 below. 
     TABLE 3 
     Workstations for Semiconductor Component Test (Wafer Sort) 
     1. Parametric Test 
     2. Functional Test 
     3. Wafer Sort 
     EXAMPLE 4: Semiconductor Component Assembly 
     In this example the path followed by fabrication among the workstations is simple. Five identical process sequences A1-A7 through E1-E7 are shown; however, each process step has different parameters. This example might be viewed as an extreme example of one process flow and four attached product flows. The seven workstations 80-92, labeled 1-7, are identified in Table 4 below. 
     TABLE 4 
     Workstations for Semiconductor Component Assembly 
     1. Wafer Saw 
     2. Die Attach 
     3. Die Bond 
     4. Mold &amp; Cure 
     5. Inspect/Test 
     6. Functional Test 
     7. Mark &amp; Pack 
     EXAMPLE 5: Circuit Board Fabrication 
     This fabrication sequence 94 is marked by extensive opportunities for re-work. Each workstation 96-110 labeled &#34;R&#34; indicates the beginning and end of a possible re-work subsequence of process steps. Not shown are feedback flow paths from the rework workstations 96-110 to earlier workstations 112-134. The workstations 112-134, labeled 0-11, are identified below in Table 5. 
     TABLE 5 
     Workstations for Circuit Board Fabrication 
     0. Layer Fabrication 
     1. Pattern 
     2. Etch 
     3. Inspect 
     4. Laminate 
     5. Drill 
     6. Smear Removal 
     7. Pattern 
     8. Electroplate 
     9. Edge-Connector 
     10. Board Contouring 
     11. Test &amp; Inspect 
     Example 6: Electronic System Assembly 
     FIG. 6 shows electronic system assembly with three fabrication sequences A1-A10 through C1-C10. Not shown are the different material requirements for each sequence. This is a simple basic flow, but it contains the opportunity for re-work loops at workstations 136 and 138. The workstations 140-158, labeled 1-10, are identified below in Table 6. 
     TABLE 6 
     Workstations for Electronic Systems Assembly 
     1. Circuit Board Inspection 
     2. Circuit Board Tester 
     3. Auto Insertion (Components) 
     4. Manual Insertion (Components) 
     5. Solder 
     6. Visual Inspection 
     7. Electrical Test 
     8. Sub-System Assembly 
     9. System Assembly 
     10. System Test 
     1. Algorithm for Sequential-Distributed Factory Description and Specification 
     Described below is a class of dynamic models for distributed factories which have fabrication sequences. The first step in the algorithm is to choose one dynamic model from this class. In terms of this specimen model the factory is defined by its machines, products, fabrication sequences, collections of job sets, scheduling rules, and machine reliability parameters. The second step is to determine the parameters that describe the specific factory. These parameters are defined in terms of data structures of the individual factory model. The factory-specific model contains descriptions of the dynamic interactions of lots and machines; thus, the behavior of the factory can be simulated in detail. The third step is to simulate the dynamic behavior of the factory. 
     2. Standard Class of Sequential-Dynamic-Factory Dynamic Models. 
     For this invention, the factory is described in terms of discrete entities such as machines, attributes such as load size, and activities such as machine unload or reload. Associated with each entity are data-structures; the collection of all such data structures provides the factory system image. The state of the factory at any point in time is given by the values of the parameters of all such entity data-structures. 
     In this invention, series of simulated events change the factory state, and thus, the system image. These changes, associated with the performance of the factory, are tracked. Details or summaries of the performance variables describe the behavior of the factory as a result of the simulated-event scenario. 
     In this embodiment, the fundamental physical entities in the factory are machines (pieces of equipment), and lots (jobs). The fundamental informational entities are fabrication sequences, groups of machines called workstations, and queues. The fundamental dynamics are governed by the interactions of lots with machines, grouped in workstations. Lot movements through the workstations are determined by the fabrication sequences. There is a hierarchy of fabrication sequences, and the assignment of a sequence to lot may be dynamic. 
     In this embodiment the factory dynamics are determined by the lot-sets (which may be dynamic or static), machine process times, setup times, yields, batching operations, and equipment failure and repair. A detailed description of these entities follows: 
     Lots 
     Lots are composed of like parts and follow a fabrication sequence. The assignment of a lot to a fabrication sequence may be static or dynamic. In this embodiment the attributes of lots include fabrication-sequence parameters (which define allowable process flows), start-time, part-count, current process step, current workstation and machine, due-time, priority weight, and action time. In the general form of the invention, other attributes may be of interest. Lots are sometimes called &#34;jobs&#34;. Lot-size or part-count is a key variable in the interaction of lots with machines; see &#34;batching&#34; below. Beginning with an initial lot-size, the part-count may decrease due to yield losses (defined below) at process steps. The lot-set is the total collection of lots assigned to the factory for a time interval. The population of this set may be static or dynamic. 
     Machines and Workstations 
     Workstations are groupings of like machines. Workstations contain at least one machine. In this embodiment the attributes of machines include batch-size, standard operating time to maintenance, standard time of maintenance, and parameters of distributions describing sporadic failure and repair. In the general form of the invention, machine-specific set-up parameters and other attributes may be defined. In this embodiment each workstation is qualified to perform only a specified set of process steps or operations; the population of this set may be dynamically defined. 
     Process Steps 
     The process step is the fundamental operation performed on a machine. In this embodiment, the attributes of each process step include a process time, a vector of parameters describing conditional setup times, a standard yield, and parameters for distributions describing random yield and process-time fluctuations. Each process step is associated with a set of qualified workstations. 
     Fabrication Sequences 
     A fabrication sequence is a sequence of process operations that are required to complete a product. In this embodiment there is hierarchy of fabrication sequences, and fabrication sequences may be altered dynamically. A process flow is a restricted class of fabrication sequence. In a process flow, each of the ordered sequence of process steps is assigned to a workstation. A process-flow group is a set of closely related process flows. A set of product flows may be associated with each process flow. Each product flow is a minor variation of the basic process flow. In this embodiment, allowable variations include changes in the attributes of small set of process steps and the addition or deletion of one or more process steps. Use of product sequences allows process tailoring in that minor changes of process steps and process sequence may define a specific product flow. Major deviations require defining a new process sequence. In a more general form, a more extensive hierarchy of fabrication sequences may be defined. In this embodiment, some fabrication sequences may be weakly constrained: the interchange of certain steps results in an equivalent fabrication sequence. Each interchange and/or assignment of process steps to workstations results in a distinct process sequence. However, these process sequences may be equivalent in that they fabicate the same product. As a given lot that is assigned to fabrication sequence moves through the factory, it may be dynamically reassigned to any equivalent process sequence. In this embodiment, rework also results in dynamic changes in process sequences. In rework, certain lots may repeat a single process step or sub-sequence of steps. 
     Yield 
     Yield is a measure of the imprecision of manufacturing operation. In this embodiment, the following models for yield are defined. In the simplest yield model, each operation successfully processes a standard fraction, yield, of parts in each lot. In a refinement, yield becomes a random variable, the parameters of whose distribution can be defined by each process step. In a second refinement a distinction is made between single-part machines and batch machines. In single-part machines each part has a probability of failed processing. In batch processing each batch may have a probability of failure. Each workstation may use a different yield model. In a more general embodiment other yield models may be defined. 
     Machine Availability 
     A machine is available if it is capable of processing. It becomes unavailable as a result of planned downtime of a fixed duration due, for example, to preventive maintenance. A machine may also become unavailable as a result of unplanned, random failures, which require a repair of random duration. In the general invention, sporadic failure and repair can be modeled by probability distributions. 
     Batching 
     A batch is the number of parts loaded into a machine for simultaneous processing. The maximum batch is the load-size of the machine. The lot-size may be less than, equal to, or greater than the load-size. These three cases require the definition of batching rules, which can significantly affect the dynamics of lot movement. Some examples of batching rules are given below. 
     Mixed-lot batches are batches composed of parts from different lots. Mixed-lot batches may or may not be allowed at each process step. Batching-up means loading two or more lots into one batch for large load sizes. A modification of this rule requires defining the specific wait-time which is required for sufficient lots to complete the batch. The wait may be zero, in which case many batches may be significantly less than the load-size. A second modification is the decision on alowing mixed-lot batches. Batching-down involves breaking a lot into batches, when the lot-size excedes the load-size. The last batch may be significantly smaller than the load-size, thus requiring a mixed-lot rule. In dynamic lot management lots with the same product attributes are merged or split for processing convenience and minimization of set-up time. In a more general form of the invention, specific batching rules eliminate the need for dynamic lot-management. 
     Set-up Time 
     Set-up time is the time required to prepare the machine for processing. Set-up time for a machine is conditional on the previous lot processed by that machine. Different set-up times are associated with changes in process step, product, and process sequence. 
     Queues 
     Lots which are not in machines being processed are in waiting-lines or queues. Queues may be defined in a variety of ways: by machine, by workstation, by process step, by process and by product, and so forth. The definition of queues and the assignment of lots into queues does not change the inventory, but only its classification. Different queue assignments may be appropriate depending on the scheduling rule being applied. 
     Scheduling Rules 
     The existence of inventory and queues requires dispatch, priority, or scheduling rules to determine which lot to process next. Over one hundred scheduling rules have been catalogued in the above-referenced S. S. Panwalkar and W. Iskander paper. Additional rules are limited only by the imagination of factory managers. Complex scheduling rules may require a large set of parameters describing the attributes of lots and machines and the status of queues. 
     Implementation in terms of a an algorithm for computation: The model structure has been defined above in terms of entities, attributes, and fundamental rules. The model can be implemented as a concrete computer algorithm by using the technique of discrete-event simulation (G. S. Fishman, Concepts and Methods in Discrete Event Digital Simulation, Wiley, New York, 1973; J. A. Payne, Introduction to Simulation, McGraw-Hill, New York, 1982.) The algorithm can be defined as follows: Data structures are defined for each entity and its attributes. Events are defined which follow the fundamental rules of the model. As a consequence of the fundamental rules, events occur which change the system state. The changes are tracked. As a consequence of the occurence of events, new events are scheduled. An event-scheduler sorts and orders all scheduled events. The simulation is initialized by the occurrence of initial events and moves forward in time by performing the next scheduled event. The system time clock moves forward in discrete increments as required by the next event. The simulation ends when the predefined final time is reached. 
     Factory System Image 
     The factory system image provides the state of the factory at any point in time. The system image is given by the values of all attributes of all entities defining the factory. The entities and attributes are arranged into tables. Each table is composed of data structures built from linked lists. The following tables are present in the system image. Other tables may be included in the general invention. Lot Table: Set of linked-lists detailing lot properties. Machine Table: Set of linked-lists detailing machine properties and workstation assignments. Fabrication Sequence Table: Set of linked lists detailing fabrication paths including information on process steps. Queue Table: Set of linked-lists giving queue structure. Event Table: Set of linked-lists detailing all scheduled events. System Performance Table: Set of linked-lists containing summaries of factory performance. 
     Events 
     Each event causes changes in the system-image tables affected by that event. 
     Start a lot: A lot is initialized and becomes active in the factory. It is assigned to a process sequence from its fabrication sequence and moves to a queue at the workstation of its first process operation. 
     Move a lot: After completion of processing at a workstation, a lot moves to the workstation assigned to the next process step in the process sequence. 
     Load a machine: Following batching and scheduling rules, a machine is loaded from one of the queue or queues assigned to its workstation. 
     Unload a machine: A machine is unloaded. If its lot is complete, it is returned to the list of available machines. 
     Machines fails: Machine becomes unavailable. A repair time is scheduled. 
     Reassign process sequence: A lot is reassigned from its current process sequence to a second process sequence from its fabrication sequence. 
     In another form of the invention, other events may be defined. 
     Implementation in Computer Software 
     In this embodiment the algorithm, an SDF dynamic model, is written in a high-level computer language such as Fortran, Pascal, PL/I, or the like. The general embodiment includes any implementation in assembly language, a higher-level language, or a simulation language. 
     3. Factory-specific Models for Distributed Factories which Have Fabrication. 
     An individual model from the class above is chosen; the choice is based upon the match between the dynamic characteristics of the model and those of the factory. The parameters describing the factory&#39;s products, fabrication sequences, and machines are determined. The parameters describing such dynamic rules as equipment reliability are established from factory-specific production data provided (for example, by a computer-integrated manufacturing (CIM) production-control system that tracks lot movement, work in process, and equipment status). The model is validated by use of material movement data from the CIM system. Defined above is a class of models for distributed factories which contain fabrication sequences. In this section a specified model from this class is given for a wafer-fabrication factory with ten processes and up to ten products associated with each process. 
     The system modeled is the entire wafer-fabrication plant, or &#34;fab&#34;. The fab is described in terms of wafer-processing equipment, wafer lots, process sequences, and product sequences. Data-structures are associated with each entity, and the system image or system state at any point in time is given by the values of all such entity data-structures. 
     Lots 
     Lots are composed of wafers and follow a specified process and productflow. The attributes of lots include start-time, process, product, part-count, current process step, current workstation and machine, due-time, priority weight, and action time. The lot-set is fixed or static. 
     Machines and Workstations 
     Workstations are composed of like machines. The attributes of machines include batch-size, standard operating time to maintenance, standard time of maintenance, and parameters of distributions describing sporadic failure and repair. Each workstation is qualified to perform only specified set of process steps or operations. 
     Process Steps 
     The process step is the fundamental operation performed on a machine. The attributes of each process step include a process time, a vector of parameters describing conditional setup times, a standard yield, and parameters for random yield distributions. Each process step is assigned to a workstation. 
     Process 
     The process flow is the ordered sequence of process steps required to complete fabrication. A process sequence may have as many as 250 steps. 
     Product 
     A set of one or more products is associated with each process. Process tailoring is allowed in that minor changes of process steps and process sequence may define a specific product flow. Major deviations require defining a new process. The specific changes allowed are as follows: The parameters of up to five process steps may be changed. A single sequence of up to five steps may be added or deleted as long as the limit of 250 steps is not exceded. 
     Dynamic fabrication Sequences 
     With the exception of re-work defined below, dynamic fabrication sequences are not allowed. Specifying the process and product completely defines the fabrication sequence. 
     Equipment Availability 
     Parameters are provided for scheduled equipment maintenance. Sporadic failure is modeled by exponential distributions for time to failure and time to repair with different parameters for each work station. 
     Scheduling Rules 
     Queues are primarily associated with work stations. Lots are processed on first-in, first-out basis. Over-riding this priority are &#34;hot-lots&#34; associated with up to six products. The class of hot-lots are processed in preference to regular lots. Within the hot-lot class lots closest to completion are given priority. 
     Batching 
     Mixed-lot batches are not allowed. In batching-up, a maximum wait equal to one-half the process time is allowed to complete the batch. 
     Set-up Time 
     Set-up time is the time required to prepare the machine for processing. Set-up time is conditional on the previous lot processed by the machine. Different set-up times are associated with changes in process step, product, and process sequence. 
     4. Generation of factory schedules from the factory-specific models for distributed factories which have fabrication sequences. 
     The schedule lists lot movements and machine loadings. The factory-specific model contains the scheduling rules of the factory and it simulates the detailed behavior in time of the factory. After initialization with the state of the factory, the simulation calculates and lists the lot movement and machine loadings for the indicated planning horizon. 
     5. Use of the factory-specific model for the automatic control of the factory including feed-back and feed-forward control of lot movements, machine loadings, and processing. 
     The CIM system provides data on the performance of the factory to the schedule generated above. The factory may deviate from from the schedule due to random events like equipment failure or the loss of a lot, or unplanned event like a new product order. The factory-specific model generates control actions for the factory so that the perturbations in desired factory behavior due to unexpected events is minimized. 
     6. Computer-integrated manufacturing production control system which incorporates a factory-specific model for automatic control. 
     Given a model that produces an accurate simulation of the manufacturing plant, computer control of the operation of that plant becomes possible with the model. Without an accurate stimulation, human intervention between the outputs of the model and equipment operation is required. 
     7. Use of a factory-specific model to provide sizing data for the design of the computer-integrated manufacturing (CIM) production-control system. 
     The model provides simulated results on material movements, queue sizes, factory transactions, and other information. Such simulated results are necessary to size computers, communications networks, data bases and other aspects of the computer system for manufacturing control. 
     Attached hereto and forming a part of this specification is an Appendix, comprising a source code listing written in Fortran 77 with comments, showing an implementation of a model in accordance with the invention. 
     It should now be readily apparent to those skilled in the art that a modeling and control process capable of achieving the stated objects of the invention has been provided. The invention gives a realistic, validatable, model for wafer fabrication factories. The invention also gives a realistic validatable model for the other types of factories in the electronics industry. Each of these factory models is a subset of the wafer fab model. The invention gives a realistic, validatable model for distributed factories in general, sometimes referenced as &#34;job shops&#34;. Such models are a subset of the fab model. The model can be used to control material movement in the factory, including scheduling. The invention permits a CIM system that includes model-controlled lot-movement and machine loading. The model can be used for aggregate planning. The model can further be used for the design and sizing of a CIM system. While the prior art discussed above indicates that modeling and control of manufacturing plants is, in general, too complex for practical implementation because of the large amounts of computation required with the prior art approaches, the present invention simplifies the computations involved sufficiently so that software implementing the invention can be run on a personal computer, such as an IBM PC. 
     It should further be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto. ##SPC1##