Abstract:
Throughput in a semiconductor wafer processing system, such as in-line photolithography equipment, is improved by identifying and correcting bottlenecks in the flow of wafers through multiple, associative segments of the equipment. Segmental rather than total processing times are monitored in order to identify the segments having the longest processing times. The theory of constraints is applied to identify the process segment representing a bottleneck in the process, and effect improvements in the bottleneck segment that provide greater overall throughput.

Description:
TECHNICAL FIELD 
   The present invention broadly relates to techniques for processing semiconductor wafers, and deals more particularly with a method for identifying bottlenecks in wafer processing flow, and improving wafer throughput. 
   BACKGROUND OF THE INVENTION 
   The current, highly competitive semiconductor market is forcing semiconductor companies to constantly seek improvements in productivity by reducing manufacturing time while maintaining or increasing production output. The small feature sizes and the large number of steps required to fabricate state-of-the-art integrated circuits on semiconductor wafers makes it essential that each of the process steps meet a tight set of specifications. Since process variations are inevitable, performance monitoring techniques such as statistical process control (SPC) are commonly used to control fabrication processes. In addition to statistical techniques for controlling process quality, a number of other techniques have been developed to measure the performance of equipment in terms of reliability, availability, maintainability, and utilization of process tools. For example, it is well known to measure tool performance based on status tracking using common indices such as WPH (Wafers Per Hour), MTTR (Mean Time To Repair), MTBF (Mean Time Between Failure), etc. More recently, an industry accepted performance measurement known as OEE (Overall Equipment Effectiveness) has been used as a performance metric which takes into consideration the availability, operational efficiency, rate efficiency and rate of quality when computing the effectiveness of process tools. 
   Although highly effective in some applications such as a single process tool, these techniques and indices do not lend themselves for effective use in the case of multiple tools that are combined into a single piece of equipment, where the tools are arranged in-line to perform sequential processing steps that are associative. Sequential tools are sometimes referred to as serial tools because they process wafers in a series of sub-steps formed in separate modules or “tool units” of the equipment. One example of combined, associative tools is a so-called scanner and track for carrying out photolithographic processing of the wafers. This process is carried out in three basic steps. First, a photoresist is applied to each wafer in a coater. The wafers are then exposed to a radiation source in a stepper, and finally each exposed wafer is developed in a photoresist developer. Since the IC&#39;s are typically multilayered, this process is repeated a number of times. The “track” referred to above includes both a coater and a developer. The scanner, which is combined with the track in a single cluster tool, is used to scan defects in the coated wafers, prior to developing. 
   In the past, because the scanner and track are formed in a single, combined cluster tool, the traditional indices have been inadequate for analyzing and tracking in-line performance of the equipment. Although computer integrated manufacturing (CIM) techniques are capable of measuring total processing time for wafers flowing through the scanner and track, this collected data provides little information regarding the processing efficiency of individual equipment components (tool units), and the bottlenecks in flow that may exist within the cluster tool. 
   Accordingly, there is a need in the art for a method of identifying bottlenecks and improving throughput of wafer processing equipment having in-line, associated tools. The present invention is directed towards providing a solution to this problem 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, a method is provided for determining a bottleneck in the flow of products through a cluster tool having a plurality of differing process segments wherein each of the segments includes multiple process machines or tool units. The method includes determining the number of tool units in each segment, determining the segmental process time for each of the segments based on a preselected lot size, calculating the throughput of the wafers, and identifying the bottleneck based on the segment having the longest segmental process time. The throughput is calculated by dividing the total segment process times by the number of process machines in the corresponding segment. 
   According to another aspect of the invention, a method is provided for improving the throughput performance of a photolithography cluster tool having multiple process segments for processing semiconductor wafers, comprising the steps of: determining the segmental process time for each of the segments based on a wafer lot of predetermined size; identifying a segment causing a bottleneck in the flow of wafers through the segments based on the determined times, and making process changes in the segment identified as being the bottleneck. 
   Accordingly, it is a primary object of the present invention to provide a method for analyzing the performance of in-line, associative process tools in order to eliminate bottlenecks and improve throughput. 
   Another aspect of the invention is to provide a method as described above which employs the application of the theory of constraints to analyze individual equipment performance and eliminate bottlenecks in order to improve process flow. 
   A still further object of the invention is to provide a method as described above which provides a means for analyzing the performance differences among associative in-line tools. 
   Another object of the invention is to provide a method as generally described above that yields process performance and flow information that can be employed for improved dispatching in order to increase the equipment utilization and improve product throughput. 
   These, and further objects and advantage of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which form an integral part of the specification, and are to be read in conjunction therewith, and in which like components are used to designate identical components in the various views: 
       FIG. 1  is a diagrammatic view showing the flow of wafers through a track and scanner cluster tool; 
       FIG. 2  is a chart showing the wafer processing times for each segment of the scanner based on a continuous run; 
       FIG. 3  is a view similar to  FIG. 2 , but depicting processing times for a discontinuous run; 
       FIG. 4  is a chart showing wafer processing times for each of the segments and depicting a bottleneck; 
       FIG. 5  is a table showing the minimum processing times for units among a lot of wafers for each of the segments; and, 
       FIG. 6  is an improvement trend chart for multiple cluster tools. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is concerned with equipment that includes a plurality of in-line, associative processing tools. In the illustrated embodiment, the equipment comprises a conventional photolithography station in the form of a well known cluster tool  13  that provides the functions of coating, scanning, and developing. This equipment is diagrammatically represented in  FIG. 1 , and includes a conventional scanner  15  and a track  17 . From the standpoint of the process flow of wafers, the equipment can be viewed as having a plurality of in-line segments through which wafers flow in a serial manner. Specifically, it includes a coating segment  12  (COT), scanning segment  20  (SCN) and a developing segment  26  (DEV). Segments  12 ,  20 ,  26  include one or more tool units for carrying out identical process functions within the segments. For example, COT segment  12  includes a plurality of coating tool units  14 . Similarly, SCM segment  20  includes a plurality of scanning tool unit  22  and the DEV segment  26  includes a plurality of developing tool units  28 . Wafers are delivered from a starting point  10  to each of the coating units  14 . Coated wafers are passed through a cooling processing station (CPL)  16  and thence to a WEE operation  18 . WEE (Wafer Edge Exposure) is a process in which the edge of the wafer is fabricated such that polishing contaminants and slurry residues are prevented from entering the scribed channels of the wafer. 
   Following WEE, the wafers are delivered to a buffer (not shown) before passing on to the SCN segment  20  where the wafers are scanned at any of the plurality of the scanning tool units  22 . Following scanning, the wafers are passed through another buffer (not shown) before being delivered to a second cooling processing station  24 . Finally, the wafers are delivered to the developing tool units  28  forming the DEV segment  26 . After being developed, the wafers are passed through a transfer and chill plate  30  before reaching the end  32  of the process. 
   Heretofore, computer integrated manufacturing (CIM) techniques assessed the effectiveness of the scanner  15  and track  17  as a single unit. In other words, the time for a wafer to pass from start  10  to the end  32  was measured, and this was the sole metric determining the throughput performance of the scanner and track. In fact, however, the processes carried out in the various segments are interrelated and/or can influence each other. Because of this interrelationship, the throughput through the scanner  15  and track  17  sometimes decreased unexplainably, and it was not possible to correct the problem. The use of the above mentioned buffers, while improving throughput, did not entirely solve the problem. In contrast, the present invention advantageously utilizes the so-called “theory of constraints” to monitor the processing time of each of the segments  12 ,  20 ,  26  rather than the total processing time through the cluster tool  13 , and provides an indication of the slowest segment which indicates a bottleneck. According to the theory of constraints, attention is focused on the weakest link in the process chain, since the overall process can be no better than its weakest link. By focusing attention on improving the weakest link or “constraint”, the overall process can be improved. In essence, the present inventive method monitors the segmental processing times, determines the location of the bottleneck and then adjusts the performance of all the tools within a given segment to be as good as the tool having the best performance. 
   In accordance with the present method, the track and scanner are viewed as three separate segments  12 ,  20 ,  26 , each including a plurality of identical tool units. Using conventional CIM techniques, CEID (Collected Event ID) events related to the tool units are selected and the times of each wafer entering or leaving each tool unit is recorded. Each of the tool units (e.g.  14 ,  22 ,  28 ) within a given segment  12 ,  20 ,  26  has two CEID events, including an “in” time and “out” time. In  FIG. 1 , segment COT includes units COT 1  . . . COTi. Segment SCN  22  includes tool units SCN 1  . . . SCNj, and finally segment DEV  26  includes tool units DEV 1  . . . DEVk. Thus, the total tool unit quantity is i+j+k, and the total CEID quantity is therefore (i+j+k+)×2. 
   The ideal processing times for the segments  12 , 20 ,  26  are determined as follows: 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               TCm = OutC (m) - InC (m) 
               Processing time of wafer m in unit COT1 
             
             
                 
               COT2, . . . COTi 
             
             
               TSm = OutS (m) - InS (m) 
               Processing Time of wafer m in unit SCN1, 
             
             
                 
               SCN2, . . . SCNj 
             
             
               TDm = OutD (m) - InD (m) 
               Processing Time of wafer m in unit DEV1, 
             
             
                 
               DEV2, . . . DEVk 
             
             
               where: 
               Out (m) is the time wafer m leaves the unit, 
             
             
                 
               and 
             
             
                 
               In (m) is the time wafer m enters the unit. 
             
             
                 
             
           
        
       
     
   
   The ideal processing times (i.e. the fastest segment) are given by: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               TCOT = Min (TCm) 
               m = 1,2,3, . . . n 
             
             
                 
               TSCN = Min (TSm) 
               m = 1,2,3, . . . n 
             
             
                 
               TDEV = Min (TDm) 
               m = 1,2,3, . . . n 
             
             
                 
                 
             
           
        
       
     
   
   In order to confirm that the ideal processing times are the minimum processing times, Applicants the following experiment was conducted and in this connection reference is made now to  FIGS. 2 and 3 . In this experiment, 25, wafers were selected as the lot sample used to determine the ideal (minimum) processing time. It is important to observe that the pull type effect created by wafers ahead in the processing sequence may block the wafers behind, although in an ideal situation the time wafers stay in a tool unit should be regular. The “block effect” may force a wafer to remain in a tool unit for a prolonged period. Thus, in order to determine bottlenecks for purposes of the present method, it is necessary to consider situations that exclude the block effect. The bottleneck is obviously the slowest segment by contrast. 
   The experiment consisted of running two batches of wafers in continuous and discontinuous operations, respectively. These two batches are referred to as lot A (continuous run) and Lot B (discontinuous run). Wafer lots A and B ran the same recipe on the same tool. Lot A was processed closely following a previous lot, and therefore is referred to as a “continuous run”. Lot B, however, was run after a previous lot had been completed and left the cluster tool, and therefore is referred to as a “discontinuous run”.  FIG. 2  is a graph showing the processing times for each of the 25 wafers in Lot A, and depicts the times required to pass through each of the segments. Specifically, plot  36  shows the processing time in segment DEV  26 , plot  38  shows the processing time in segment COT  10  and plot  40  shows the processing time in segment SCM  20 . In the present example, segment COT  10  comprised two tool units. Segment SCM  20  comprised a single tool unit and segment DEV  26  comprised four tool units. Similar plots  36   a ,  38   a ,  48   a  are shown in FIG.  3 . 
   From  FIG. 2  it is apparent that there is a variation of the segment COT processing time due to the block effect. In contrast, as shown in  FIG. 3 , the segment COT processing time of the first fourteen wafers is nearly constant with no block effect. In the case of the discontinuous run of lot B shown in  FIG. 3  the ideal processing time may be directly inferred to be the minimum time. For the continuous run of lot A shown in  FIG. 2  the minimum processing time is equal to or close to the ideal processing time. Based on this information, it was concluded that choosing the minimum processing time as the ideal time is appropriate regardless of whether the lot is run in a continuous or discontinuous manner. Based on the empirical data shown in  FIGS. 2 and 3 , it was concluded that the ideal processing times of segments COT, SCM and DEV are  76 ,  38  and  218  respectively, regardless of whether the data of lot A or lot B runs is used. 
   The next step in the present invention is to determine the location of the bottleneck and the process flow through the scanner  15  and track  17  shown in FIG.  1 . Specifically the objective is to determine which of the segments  12 ,  20 ,  26  has the longest processing time and is thus a bottleneck in the process flow. The bottleneck is given by the formula: 
   
     
       
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Max {TCOT/i, TSCN/j, TDEV/k} = Max {Min(TCm)/i, Min(TSm)/j, 
             
           
        
         
             
                 
               Min (TDm) /k} 
             
           
        
         
             
                 
               If Max {TCOT/i, TSCN/j, TDEV/k} = TCOT/i, then the 
             
           
        
         
             
                 
               bottleneck is segment COT. 
             
           
        
         
             
                 
               Similarly, if Max {TCOT/i, TSCN/j, TDEV/k} = TSCN/j, then 
             
           
        
         
             
                 
               the bottleneck is segment SCN, and 
             
           
        
         
             
                 
               if Max {TCOT/i, TSCN/j, TDEV/k} = TDEV/k, then the 
             
           
        
         
             
                 
               bottleneck is segment DEV 
             
             
                 
                 
             
           
        
       
     
   
   Reference is also now made to  FIGS. 4 and 5  which provide the basis for determining the bottleneck.  FIG. 4  is a plot of the individual segmental processing times, which is essentially the reciprocal of the throughput.  FIG. 5  depicts statistical information showing the processing time through each of the segments for a number of machines. In order to determine the actual processing times for each of the segments, the times shown in  FIG. 5  must be divided by the number of machines in the corresponding segment. Thus, for example the processing times “TC” must be divided by 2 since in the present example, segment COT  12  includes two coater tool units. Similarly, the processing times, “PD” for segment  26  must be divided by 4 since there are four developing tool units in that segment. In effect, this procedure provides the average processing time for the tool units in the segment, thus enabling the processing times of the three segments to be compared for the purpose of identifying the bottleneck. It should also be noted here that in determining the bottleneck in accordance with the method of the present invention, it is important to employ a sufficiently large lot of wafers, and in this connection it has been found that the lot should comprise no less than 10 wafers, although 25 wafers has been found to provide the most satisfactory results. 
   The bottleneck can be determined by dividing the processing times by the number of tool units in the corresponding segment and determining which segmental processing time is largest. The results showing which segment is the bottleneck is designated in the last column as “B/N”. The lot sizes used to generate the information for the table shown on  FIG. 5  were all 25 wafers.  FIG. 4  shows the processing times for each of the segments COT, SCN and DEV, wherein plots  42 ,  44  and  46  respectively designate segments DEV, SCN and COT. From the plot shown in  FIG. 4 , it may be seen that the segment DEV is the bottleneck at 55 seconds, compared to segment COT and SCN which are both 38 seconds. 
   Consistent with the theory of constraints, the actual bottleneck is determined for each historical lot of wafers processed. With this information in hand, it is then possible to determine how the processes or tool units may be adjusted to improve the processing times. In the illustrated example, the data shown in the table of  FIG. 5  indicates that tool A consistently was the slowest in the segment SCN compared with tool B and C. Based on this information, steps were taken to improve the tool units in segment SCN of tool A so that the performance of tool A and segment SCN was equal to or better than that for tool B or C. 
     FIG. 6  is a plot of the throughput in terms of wafers per hour for tool A, B, and C shown in FIG.  5 .  FIG. 6  shows four plots for each of the tools A, B, C respectively representing throughput performance for each of four consecutive weeks. In the case of tool A, it may be observed that notable improvement was achieved during the third week as a result of having identified the bottleneck of segment SCN in tool A and making appropriate corrections to reduce the processing times through segment SCN. 
   From the foregoing, it is apparent that the method described above not only provides for the reliable accomplishment of the objects of the invention, but does so in a particularly simple and economical manner. It is recognized, of course, that those skilled in the art may make various modifications or additions chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.