Abstract:
A method and system for scheduling lots for semiconductor manufacturing. The method and system comprising: determining a goal weighing factor ( 502 ); calculating the bottleneck feed factor for each lot in a tool queue ( 514 ); calculating the critical ratio for said each lot in a tool queue ( 516 ); calculating the relative rank of each lot in a tool queue; displaying the relative rank of each lot in a tool queue, for each tool queue; and selecting the lots in each tool queue for tooling according to said relative rank.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
   This application claims priority from provisional application No. 60/344,204, filed on Dec. 28, 2001 and which is hereby incorporated by reference. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to manufacturing processes such as those utilized in a semiconductor manufacturing environment and, more specifically, to apparatuses and methods for controlling the fabrication progress of semiconductor devices, including the production of lots through a manufacturing line. 
   BACKGROUND OF THE INVENTION 
   Manufacturers, such as but not limited to semiconductor manufacturers, continually strive to satisfy customer demands such as on-time delivery and lowest possible cost. Given the capital-intensive nature of the semiconductor industry, for instance, ever increasing capacity cost and longer lead-times, the manufacturing philosophy is to maximize output from its facilities while maintaining the minimum amount of Work-In-Progress (“WIP”). The scheduling of lots to fulfill the above goals is a real challenge in today&#39;s manufacturing environment of short product life cycles, complex product mix and shrinking time to market. 
   Various general rules have been used to address scheduling problems. These approaches usually select the lot to be processed based on computed parameters of lots, operations and/or manufacturing entities. Some of the commonly used computed parameters deal with processing times, due dates, setup times, and arrival times. Examples of several rules based on these parameters are FIFO (first-in, first-out), LIFO (last-in, first out), LPR (longest processing time remaining), SPT (shortest processing time first), LPT (longest processing time first), and EDD (earliest due date). 
   However, such rules do not take into account the continuously changing dynamics of the manufacturing line. Due to complex product mix, the line dynamics may change with time, thus changing the priority for the individual lots. Some known systems for scheduling lots fail to account for the current line dynamics in a manufacturing line. Another disadvantage is that many of the known methods of scheduling lots optimize the output of individual tools on the manufacturing line, thereby leading to optimized local WIP movement. This may lead to adversely affecting the overall performance of the factory due to conflicts created by local optimization. Other known methods only allow for optimizing factory efficiency at the expense of timely customer delivery, or timely customer delivery may be optimized at the expense of factory efficiency. Still other known methods of scheduling lots only allow optimization to be performed over a relatively long time span such as, for example, every 2-4 hours. 
   Based on the foregoing, it may be appreciated that a means of overcoming the disadvantages associated with prior art lot scheduling and processing systems would be advantageous. 
   SUMMARY OF THE INVENTION 
   The disclosed invention is a system for scheduling lots of items for manufacturing comprising: a manufacturing line, with a plurality of stations where a plurality of lots are processed for manufacturing; a tool at each station for processing the plurality of lots; and a lot scheduling processor communicably coupled to the stations and able to compute bottleneck feed factors and critical ratios for each lot based on time needed at bottleneck minus buffer time, planned cycle time for each lot, estimated time until end of the line for each lot, and time until due for each lot. 
   The disclosed invention is also a method for scheduling the processing of lots for manufacturing in a manufacturing line, where the manufacturing line has a plurality of stations, and tools at the stations for processing lots of items to be manufactured with a lot scheduling processor communicably coupled to the stations. The method comprises the steps of: providing, to the lot scheduling processor, the time needed at bottleneck minus buffer time; providing, to the lot scheduling processor, the planned cycle time to bottleneck for each lot; providing, to the lot scheduling processor, the estimated time until the end of the line for each lot; providing, to the lot scheduling processor, the time until due for each lot; calculating, by the lot scheduling processor, the bottleneck feed factor for each lot; calculating, by the lot scheduling processor, the critical ratio for each lot; calculating, by the lot scheduling processor, a ranking of lots based upon the bottleneck feed factors and critical ratios; and processing the lots in order according to the ranking. 
   Also disclosed is a method for scheduling lots for semiconductor manufacturing, comprising the steps of: determining a goal weighing factor; calculating the bottleneck feed factor for each lot in a tool queue; calculating the critical ratio for said each lot in a tool queue; calculating the relative rank of each lot in a tool queue; displaying the relative rank of each lot in a tool queue, for each tool queue; and selecting the lots in each tool queue for tooling according to said relative rank. 
   Further disclosed is a method for scheduling lots for semiconductor manufacturing comprising the steps of: determining a goal weighing factor; obtaining the time needed at bottleneck minus buffer; obtaining the planned cycle time for each lot; obtaining the estimated time until the end of the line for each lot; obtaining the time until due for each lot; calculating the bottleneck feed factor for each lot in a tool queue; calculating the critical ratio for each lot in a tool queue; calculating the relative rank of each lot in a tool queue; and selecting the lots in each tool queue for tooling according to the relative rank. 
   An advantage of the invention is that the bottleneck feed factor based scheduling algorithm has been designed to minimize the idle time of bottlenecks across the line. 
   Another advantage of the invention is that the critical ratio may be incorporated into an overall ranking of lots, thereby striking a balance between manufacturing efficiency and customer delivery. 
   Still another advantage is that the scheduling system may be continually updated (approximately every 5 minutes or less) thereby maximizing bottleneck throughput as well as promoting manufacturing line linearity. 
   Still another advantage is that the lots may be prioritized by customer due date. 
   Still another advantage is that an appropriate trade-off between increasing the efficiency of the manufacturing on the one hand and providing manufactured items by the customer due date on the other may be made. 
   Still another advantage is that the whole manufacturing facility may be optimized as opposed to only optimizing the output of individual tools. 
   Still another advantage of the invention is that upstream WIP may be prioritized as opposed to only local optimization at the bottleneck. 
   Still another advantage is that the invention allows for the optimization of manufacturing processes with moving bottlenecks. Specifically, pull points may be specified where lots may be removed from a particular tool for a certain amount of time, for example for a scheduled tool outage or for maintenance of a tool. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings. 
       FIG. 1  is an illustration of a manufacturing line with a plurality of lots, tools and bottlenecks; 
       FIG. 2  illustrates a manufacturing line with a plurality of lots, tools and bottlenecks, wherein the lots are ranked according to the invention; 
       FIG. 3   a  illustrates a manufacturing line comprising a plurality of lots, tools, bottlenecks, displays and a lot scheduling processor; 
       FIG. 3   b  illustrates the components of a lot scheduling processor according to the invention; 
       FIG. 4  is a process flow diagram illustrating the method of scheduling lots according to one embodiment of the invention. 
       FIG. 5  is a process flow diagram of a method of scheduling lots according to a further embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference now to the figures and in particular with reference to  FIG. 1 , therein is depicted system  100  for scheduling lots of items to be manufactured in accordance with one embodiment of the present invention. A manufacturing line  104  moving from left to right is shown. Tools and machines used in the manufacturing process are represented by small rectangular boxes  108 ,  110 ,  112  and  114 . Tools and machines which are known for bottlenecks and are therefore referred to as “bottlenecks” are represented by larger boxes  116  and  120 . The plurality of lots being manufactured are represented by circles  124 - 150 . 
   Each lot  124 - 150  on the manufacturing line  104  has certain properties associated with it. One property is the planned cycle time (“PCT”). The PCT is the amount of time for a particular lot  124 - 150  to move through the next tool, the PCT includes the time required to machine the lot at the next tool, the processing time, the time to transport the lot, and any waiting time. The “PCT to next bottleneck” is another important property of each lot  124 - 150  and is the amount of time for a particular lot  124 - 150  to move to the next bottleneck, this time does not include the time to machine the lot  124 - 150  at the next bottleneck. The PCT to next bottleneck may be determined by adding the PCT for all tools in between the current location of the lot to the next bottleneck. Thus the PCT to next bottleneck for lot  124  in  FIG. 1  would be the time necessary for it to be tooled at tool  108  (the PCT of lot  124  with respect to tool  108 ) plus the time necessary for it to be tooled at tool  110  (the PCT of lot  124  with respect to tool  110 ). This gives to the PCT to next bottleneck for lot  124 , one should note that the PCT to the next bottleneck does not include the tooling time at the bottleneck (the PCT of lot  124  with respect to bottleneck  116 ). 
   Another property associated with each lot on the manufacturing line is the time needed at bottleneck. A particular lot&#39;s time needed at bottleneck is the cycle time (“CT”) of all lots ahead of the particular lot to be tooled through the bottleneck minus the buffer required at the bottleneck. The buffer required at bottleneck, which may be referred to as simply “buffer”, is a certain amount of work purposefully kept at the bottleneck in order to reduce the likelihood of the bottleneck running out of lots to process. The time needed at bottleneck informs the system when the bottleneck tool will be ready for the particular lot. For instance if a particular lot  124  has a time needed at bottleneck of 4 hours, that means that if lot  124  is not ready to be tooled at the bottleneck  116  in 4 hours, the bottleneck  116  will be idle. An idle bottleneck contributes to inefficiencies in the manufacturing line. 
   The following is an example of how to determine the time needed at bottleneck of a particular lot. Assuming that when there are a plurality of lots in a queue at a tool or bottleneck, the lot at the top of the queue would be tooled first (e.g. lot  130  would be tooled first at tool  108 , lot  128  would be tooled second at tool  108 , lot  126  would be tooled third at tool  108 , and finally lot  124  would be tooled fourth at tool  108 ). Thus, the time needed at bottleneck  116  for lot  124  would estimated by the following: the CT of lot  142  through bottleneck  116 +CT of lot  140  through bottleneck  116 +CT of lot  138  through bottleneck  116 +CT of lot  136  through bottleneck  116 +CT of lot  134  through bottleneck  116 +CT of lot  132  through bottleneck  116 +CT of lot  130  through bottleneck  116 +CT of lot  142  through bottleneck  128 +CT of lot  142  through bottleneck  126 —buffer. Similarly the time needed at bottleneck  120  for lot  144  would be determined by the following: CT of lot  150  through bottleneck  120 +CT of lot  148  through bottleneck  120 +CT of lot  146  through bottleneck  120 −buffer. Those skilled in the art will appreciate that the lots may be tooled in various orders, not necessarily from top to bottom. 
   The PCT to next bottleneck and time needed at bottleneck allow for the determination of a ratio for each lot in the manufacturing line. This ratio may be called the bottleneck Feed Factor (BNFF) and is given by the expression of equation 1. 
             BNFF   =     1   -         Time   ⁢           ⁢   Needed   ⁢           ⁢   at   ⁢           ⁢   Bottleneck     -   Buffer       PCT   ⁢           ⁢   to   ⁢           ⁢   next   ⁢           ⁢   Bottleneck                 eq   .           ⁢   1             
 
Thus, if the PCT for lot  124  is 5 hours and the time needed at bottleneck  116  for lot  124  is 2 hours, and the buffer is 1 hour, then the BNFF for lot  124  is given by 1−(2−1)/5=0.8. Similarly, if the PCT for lot  144  is 4 hours and the time needed at bottleneck  120  is 2 hours and the buffer is ½ hour, then the BNFF for lot  144  is given by 1−(2−0.5)/4=0.625.
 
   Another property associated with each lot is the estimated time to the End Of the manufacturing Line (“EOL”). This is the current time estimated to finish tooling the particular lot and move to the end and out of the manufacturing line. Another property associated with each lot is the time until due, which is the original expected time to the EOL, also known as the customer due time. Thus for example, if a customer was told that lot  124  would be finished two days from particular date, then the time until due may be 2 days from the particular date. However, the estimated time to EOL for lot  124  may be 4 days from the particular date. The estimated time to EOL and time until due for each lot may be express in a ratio called a Critical Ratio (“CR”) and be given by equation 2. 
             CR   =     1   -       Time   ⁢           ⁢   Until   ⁢           ⁢   Due       Estimated   ⁢           ⁢   Time   ⁢           ⁢   to   ⁢           ⁢   EOL                 eq   .           ⁢   2             
 
Thus, for lot  144 , if the estimated time to EOL is 4 days, and the time until due is 2 days, then CR is given by 1−(2/4)=0.5. Those skilled in the art will recognize that various units of time may be used for Time Until Due and Estimated Time to EOL.
 
   There are two competing goals in a manufacturing environment, 1) one is to operate the manufacturing facility in as efficient manner as possible, e.g. keep the bottlenecks from starving, 2) another is to provide to the customer his or her products as close to the promised date (or earlier) as possible. These two goals are often conflicting goals, and a plant manager may need to determine which of the two goals he or she must weigh more favorably and by how much to weigh one goal more favorably than other, if at all. One disclosed method of assigning weight to these two goals is to rank each lot by both the BNFF and CR and include a goal weighing factor (“G”). Such a method would be governed by the expression of equation 3.
 
RANK= G ×Bottleneck Feed Factor+(1 −G )×Critical Ratio  eq. 3
 
In equation 3, G is number between 1 and 0 assigned by someone such as plant manager or production coordinator. (1−G) is of course the complement of G, thus if G is set to 0.2 (a relatively low weight factor applied to plant efficiency), then (1−G) would be 0.8 (a relatively high weight factor applied to customer due date. Thus, by using eq. 3 for each lot  124 - 130 ,  134 - 142 ,  144 - 146  in a queue on a manufacturing line  104 , one would obtain a ranking of each lot  124 - 130 ,  134 - 142 ,  144 - 146  and one would use said ranking to determine the order of manufacturing the lots  124 - 130 ,  134 - 142 ,  144 - 146  from a queue, wherein the lot with the largest value for its RANK would be the first to be manufactured out of its queue and the lot with the smallest value for its RANK would be the last lot to manufactured out of its queue.
 
   With reference now to  FIG. 2 , therein is depicted a manufacturing system in accordance with one embodiment of the present invention. A manufacturing line moving from left to right is shown. Tools and machines  108 ,  110 ,  112  and  114  used in the manufacturing process are represented by small rectangular boxes. Bottlenecks  116  and  120  are represented by larger boxes. The plurality of lots  200 ,  204  and  208  being manufactured are now shown with hypothetical rankings in their respective queues after being hypothetically ranked by applying eq. 3 to each lot. For instance, in the first queue of lots  200 , the lots are ranked with respect to each other as follows: top lot is ranked 2 nd , the next lot is ranked 1 st , the next lot is ranked third and the next lot is ranked 4th. Similarly lots  204  and  208  are shown with each lot in the queue ranked relative to other lots in the same queue. The lots in queue  204  are ranked from top to bottom as follows: 3 rd , 4 th , 5 th , 2 nd , and 1 st . The lots in queue  208  are ranked from top to bottom as follows: 1 st  and 2 nd . 
   Referring now to  FIG. 3   a , an alternative embodiment of the invention is shown. A manufacturing line  300  is shown. Tools  302 ,  304 ,  306  and  308  are shown as rectangular cubes on the manufacturing line. The bottlenecks  310  and  312  are shown as larger cubes on the manufacturing line. Those skilled in the art will recognize that manufacturing lines may have more or less tools and bottlenecks, and that this invention is equally applicable to manufacturing processes with multiple manufacturing lines. 
   Still referring to  FIG. 3   a , each tool  302 ,  304 ,  306 ,  308  and bottleneck  310 ,  312  location may be referred to as a “station”  320 ,  322 ,  326 ,  328 ,  330 ,  332 . Associated with each station  320 ,  322 ,  326 ,  328 ,  330  and  332  are lots  314 . Each tool  302 ,  304 ,  306 ,  308  and bottleneck  310 ,  312  may have a display  316  located at their respective stations  320 ,  322 ,  326 ,  328 ,  330 ,  332 . Each display  316  may be communicably coupled via a coupling  317  to a lot scheduling processor (“LSP”)  318 . The LSP  318  may be any sort of CPU, computer system, or machine able to perform the calculations described in this specification. The displays  316  may be coupled directly to the LSP  318 , or may be coupled through an intranet, internet, or any other communications network. Thus, data may be inputted into the LSP  318 , and the LSP  318  may then perform calculations in order to determine the rankings for each lot in a queue at a station. The ranking may be displayed on the displays  316 , so that an operator is informed as to which lot to select from the queue for further processing in the manufacturing process. In another embodiment, the operator may be a robotic operator, and the LSP  318  may be communicably coupled to the robotic operator, and then the robotic operator selects the proper lot from the queue based on the ranking communicated to it from the LSP  318 . 
   Referring now to  FIG. 3   b , one detailed embodiment of an LSP  318  is shown. The communications coupling is shown as  317 . Five inputs are shown for the LSP  318 : (1) the goal weighing factor (“G”); (2) an input for the time needed at bottleneck-buffer; (3) an input for the PCT to next bottleneck for each lot; (4) the time until due for each lot and (5) the estimated time to EOL for each lot. Once the LSP  318  has received the inputs, the LSP  318  can calculate the BNFF, CR and the rank according to the equations shown inside the LSP  318  on  FIG. 3   b . Once the rank for each lot is calculated, the LSP may transmit the rank information through the coupling  317 . 
   This invention need be applied at every tool, but may be applied only at complex tools and bottlenecks. That is, only complex tools and bottlenecks would be associated with a station, thus reducing the complexity of the invention, by reducing the number of stations. Complex tools may be those tools selected as such because those tools have a certain minimum number of lots that must be processed through them, those tools that require an automated manager, or those tools that have different types of lots (product mix) going through them. Complex tools may be selected for other non-listed reasons. 
   Referring now to  FIG. 4 , a flow chart illustrating a method for scheduling the manufacturing of lots according to the present invention is shown, but with stations only at the bottlenecks and at complex tools. At step  400  certain tools are selected as complex tools. At step  401 , the G is provided to the LSP. At step  402 , the time needed at bottleneck-buffer time is provided to the LSP. At step  404  the PCT to next Bottleneck for each lot is provided to the LSP  318  (from  FIGS. 3   a  and  3   b ). At step  408  the estimated time until EOL for each lot is provided to the LSP  318 . At step  412  the time until due for each lot is provided to the LSP  318 . Those skilled in the art will recognize that steps  400 ,  402 ,  404 ,  408  and  412  may be accomplished in numerous ways including using a menu driven program, manually inserting values for certain variables directly into a program code, and remotely sending the data to the LSP. At step  416  the LSP calculates the BNFF for each lot. At step  420  the LSP calculates the CR for each lot. At step  424  the LSP calculates the ranking for each lot. In step  426  the lot rankings are provided to an operator. The operator may be human or robotic. The means of providing the lot ranking to the operator may be via a display located at the tool station, or may be directly communicated to a robotic operator, or may be communicated to an operator via an ear piece or portable data devise such as a PDA. Finally at step  428 , the lots are processed according the ranking calculated by the LSP. Those skilled in the art will recognize that this embodiment may be applicable to more tools up to all the tools in a manufacturing line, depending on how one selects the complex tools. 
   Referring now to  FIG. 5 , a flow chart depicting another method for scheduling the manufacturing of lots according to the present invention is shown. In this embodiment, each tool has a station associated with it, not just complex tools and bottlenecks. However, other embodiments may have fewer tools associated with stations. At step  502 , a goal weighing factor is determined. At step  504  the method obtains the time needed at bottleneck-buffer time. At step  506  the method obtains the PCT to next bottleneck for each lot. At step  508  the method obtains the estimated time until EOL for each lot. At step  510  the method obtains the time until due for each lot. Those skilled in the art will recognize that steps  504 ,  506 ,  508  and  510  may be accomplished in numerous ways including inputting data using a menu driven program, manually inserting values for certain variables directly into a program code, and remotely sending the data to the LSP  318  (from  FIGS. 3   a  and  3   b ). At step  512  a counter N is set to 1. At step  514  the BNFF is calculated for lot at the Nth tool. At step  516 , the CR is calculated for each lot at the Nth tool. At step  518  the rank for each lot at the Nth tool is calculated. At step  520  the ranking for each lot at the Nth tool is displayed. At step  522  the lots are selected for processing at the Nth tool according to the ranking. At step  524  the counter N is advanced by 1. At query  526 , it is determined whether there is an Nth tool. If there is an Nth tool, the method goes back to step  514 . If there is not an Nth tool, the method ends. 
   The embodiments and examples set forth herein are presented in order to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. 
   Those skilled in the art will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims.