Abstract:
Disclosed is a test data analyzing method and system for use in estimation of a defect cause of a product, such as, an integrated circuit, a liquid crystal display, an optical transceiver, a thin film magnetic head, etc., which is fabricated through plural processes. The estimation of a defect cause is achieved by selecting a wafer number to be analyzed, reading test data, reading fabrication line data, counting frequency of machine codes by wafers, grouping test data by machine codes or frequencies, comparing test data distributions between groups by machine codes, and comparing results between machine codes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a test data analyzing method and system for use in estimation of a defect cause (defective process) of a product which is obtained through multi-step processes, such as, fabrication processes of integrated circuits, liquid crystal displays, optical transceivers, thin film magnetic heads, etc.  
         [0003]     2. Description of the Related Art  
         [0004]     Fabrication process of integrated circuits will be explained as an example. In general, fabrication process of integrated circuits is divided into initial process steps for forming plural chips on a silicon wafer and after process (or post process) steps for cutting individual chips, molding, and product finishing. Usually, in an electrical test that is performed at the last step of the initial process, a defective chip is found and only good quality chips proceed to the post process.  
         [0005]     Particularly in the initial process, the ratio of good quality chips determined by the electrical test on every chip is called the percentage of good products, i.e., yield. Increasing this yield is very important in the initial process manufacture line especially for realizing low-cost production.  
         [0006]     Many methods have been suggested to estimate defect cause(s) (defective process) aiming to increase yield. Among them is an analysis of the relation between a fabricating machine a product (wafer) passes through and test data. This method, as described in Japanese Patent Laid-open Publication (JP-A) Nos. 2000-12640 and H11-45919, analyzes a significant difference among plural fabricating machines used in the same process.  
         [0007]     JP-A No. 2000-12640 discloses a method for analyzing, on the basis of ANOVA (Analysis of variance), the relation between fabricating machines a wafer passes through and a yield provided from an electrical test and for searching a process with a significant difference in the machines.  
         [0008]     JP-A No. H11-45919 discloses a method for discovering a significant difference among fabricating machines by comparing coordinates of particles detected through a particle inspection which is carried out by each fabricating machine a wafer passes through. However, the above-described methods for discovering a significant difference among the fabricating machines are on the basis of an assumption that plural fabricating machines exist in one process.  
         [0009]     To be short, JP-A Nos. 2000-12640 and H11-45919 provide a method for calculating a significant difference among fabricating machines. For this reason, in the case that a certain process uses one single fabricating machine for example, one cannot figure out a significant difference among fabricating machines for that process and the significant difference must be obtained outside of the estimated object of a defect cause (defective process).  
       SUMMARY OF THE INVENTION  
       [0010]     It is, therefore, an object of the invention to provide a technique for estimating a defect cause (defective process) without calculating a significant difference among machines.  
         [0011]     Particularly, the invention focuses on characteristics of a fabrication process of a product. Fabrication process of a subject with a multilayer structure, e.g., integrated circuits, liquid crystal displays, optical transceivers, thin film magnetic heads, etc., is characterized by operating the same fabricating machine plural times for similar processes. For instance, until one product is completely done, one photolithography machine may be used plural times for photolithography process. In addition, one etching machine may be used plural times for etching process. Generally, there are plural photolithography machines or plural etching machines in a plant and fabricating machines used differ by wafers or processes.  
         [0012]     To achieve the above object, in one aspect of the invention, there is provided a method for estimating a defect cause in the light of usage frequency of fabricating machines. One means for such purpose is to build a test data analyzing system including: a fabrication line data management unit, which stores fabrication line data with information of machine code numbers specifying fabrication machines of a fabrication process a product (for example, a wafer) passes through; a test data management unit, which stores test data obtained from an inspection done on the product by an inspection machine; a data analysis unit, which counts the showing frequency of the respective machine code numbers included in corresponding fabrication line data per product, analyzes correlation between the showing frequency and the test data, and estimates a defect cause of the product; and a local area network (LAN), which connects the fabrication line data management unit, the test data management unit, and the data analysis unit. More details are described in the claims.  
         [0013]     According to the aspect of the invention, it is possible to estimate a defect cause with expedition by utilizing fabrication line data which includes code numbers of machines a product passes through and test data obtained through an inspection on the product. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  is a flowchart describing how to process a test data analyzing program in accordance with an embodiment of the present invention;  
         [0016]      FIG. 2  is a block diagram of a fabrication machinery group for use in a fabrication line;  
         [0017]      FIG. 3  illustrates a definition list of machine code numbers;  
         [0018]      FIG. 4  illustrates fabrication line data by wafers;  
         [0019]      FIG. 5  illustrates test data by wafers;  
         [0020]      FIG. 6  illustrates one example of Graphical User Interface;  
         [0021]      FIG. 7  illustrates another example of Graphical User Interface; and  
         [0022]      FIG. 8  illustrates a hardware configuration for executing an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.  
         [0024]     The embodiment to be described hereinafter is to be applied to the fabrication process of an IC on a silicon wafer.  
         [0025]     To explain the embodiment,  FIG. 2  illustrates a block diagram of a fabrication machinery group for use in the fabrication line. In the fabrication line, all kinds of fabrication machines are connected to a local area network (LAN)  261  or a supplementary terminal is connected to each fabrication machine. When a certain process is carried out on a wafer by a fabrication machine, its result is transmitted and stored in a result management database  241  from each fabrication machine through the LAN  261 . In the invention, the data in the result management database  241  is searched, transmitted and analyzed by a data analysis machine  251 .  
         [0026]     Particularly the invention is characterized by recording ‘result data that indicates which fabrication machine performs which process on which wafer’ as ‘fabrication line data of a wafer’ among the process result data on a wafer transmitted from each fabrication machine. A fabrication line data management unit  242  for collecting the result data are collected from the respective fabrication machines and for storing them in the result management database  241  is shown independently. The function of the fabrication line data management unit  242 , similar to a case of recording other result data, may be given to the respective fabrication machines and the result management database  241 . In addition, the fabrication machinery group of the invention includes a test data management unit  243  for collecting test data from all types of inspection machines or testers, recording and managing the data. This function of the test data management unit  243  may be given to the respective inspection machines, testers, or the result management database  241 .  
         [0027]     As shown in this embodiment, a certain process may use plural fabrication machines performing the same process, e.g., CVD (Chemical Vapor Deposition) machine I and II, photolithography machines I to IV, etc.  
         [0028]      FIG. 3  illustrates a definition list of machine code numbers for identifying the respective fabrication machines in  FIG. 2  uniquely. The first row in the list has machine code numbers, and the second row in the list indicates has names of the machines. For example, an etching machine I is defined as a machine code number  141  and recorded as such in the list.  
         [0029]      FIG. 1  illustrates a flowchart of processing a test data analyzing program to which the invention is applied. In step  101  (Selecting wafer numbers to be analyzed), plural wafer numbers to be analyzed are selected. A wafer number is a unique identification code number of a wafer in the fabrication line. Next in step  102  (Reading test data), test data corresponding to the wafer numbers are read. In step  103  (Reading fabrication line data), fabrication line data corresponding to the plural wafer numbers selected in step  101  are read. Fabrication line data is information about machines each wafer passes through in the fabrication process or chambers in the machines. That is, fabrication line data include machine code numbers shown in  FIG. 3 , so they differ by wafers. In step  104  (Counting frequency of machine codes by wafers), frequency of machine code numbers of the respective wafers is counted from the fabrication line data read in step  103 . In step  105  (Grouping test data by machine codes and frequencies), test data are grouped by machine code numbers or by showing frequencies thereof.  
         [0030]     In step  106  (Comparing test data distributions between groups by machine codes), test data distributions between groups are compared by machine code numbers, that is, by showing frequencies thereof.  
         [0031]     In detail, for example, a yield distribution of a wafer group where the etching machine II is used once and a yield distribution of a wafer group where the same etching machine II is used twice are compared for ANOVA (Analysis of variance), and a significance probability P value is calculated. Moreover, a coefficient of correlation is calculated from a scatter diagram in which the showing frequency of the machine code numbers are set to the horizontal axis and wafer yields are set to the vertical axis. Next, in step  107  (Comparing results between machine codes), P values of machine code numbers whose coefficients of correlation calculated in step  106  are minus are compared. A machine code number with the lowest P value is taken, or machine code numbers are arranged in increasing order of P values. In this manner, one can narrow down the range of defect causes of the fabrication machines.  
         [0032]     The following will now describe a conception of narrowing down the range of defect causes of the fabrication machines by the above-described process of the test data analyzing program. According to the statistical hypothesis testing, a null hypothesis (a hypothesis to be nullified or refuted in order to support or accomplish a research or experimental object of an investigator) is set up as “a fabrication machine to be tested is operated normally, and establishment causing a defect is small within a range to be determined as normal.” Among the respective wafers processed by such fabrication machine, a yield distribution of wafer groups obtained by using the fabrication machine once (production yield of plural semiconductor chips formed on the respective wafer) and a yield distribution of wafer groups obtained by using the same fabrication machine twice are obtained from test data. If available, a yield distribution of wafer groups obtained by using the same fabrication machine three times is also studied. These yield distributions of wafer groups are regarded as ones that are obtained by gathering a sample of a certain size from a normal population. From these two sample distributions, it is tested as follows whether the distributions of two populations are the same.  
         [0033]     (1) Unbiased estimate of population variance U 1  (an estimate of population variance obtained from sample data) is obtained by 
 
 U   1 =Σ( x   1i   −x   1m ) 2 /( n   1 −1)  (1) 
 
         [0034]     where, n 1  is the number of data (number of wafers) of wafer groups that used a fabrication machine once, x 1i  is a yield of each wafer (sample data), and x 1m  is a sample mean.  
         [0035]     (2) Unbiased estimate of population variance U 2  (an estimate of population variance obtained from sample data) is obtained by 
 
 U   2 =Σ( x   2i   −x   2m ) 2 /( n   2 −1)  (2) 
 
 where, n 2  is the number of data (number of wafers) of wafer groups that used a fabrication machine twice, x 2i  is a yield of each wafer (sample data), and x 2m  is a sample mean. 
 
(3) Statistic  F   0   =U   1   /U   2   (3) 
 
         [0036]     (4) F 0  follows F distribution with the first DOF (Degree of freedom) Φ 1 =n 1 1 and the second DOF Φ 2 =n 2 −1. From the F distribution table, significance probability P=Pr{F≧≧F 0 }, which is the probability of showing the statistic F 0  (variance ratio), is obtained.  
         [0037]     (5) According to the statistical hypothesis testing, if α, the ratio of risk of testing (significance level), is 0.05 for example it is set to α/2 for two-sided testing, and α for one-side testing.  
         [0038]     When P&gt;α, a null hypothesis is selected: “One cannot say that population variances of two groups are not the same.” 
         [0039]     When P≦α, a null hypothesis is rejected: “Population variances of two groups are not the same.” 
         [0040]     The invention is not to set the ratio of risk of testing (significance level), i.e., α. Instead, it decides that a fabrication machine with the lowest significance probability P-value has a higher probability to cause a defect. This is because it is believed that fabricating wafers with a fabrication machine that causes a defect has largest influence on the yield distribution of wafers and variance of the distribution.  
         [0041]     (6) A coefficient of correlation in a scatter diagram having the showing frequency (usage frequency of fabrication machines) set to the horizontal axis and wafer yield the vertical axis is calculated.  
         [0042]     Although the coefficient of correlation is one of indices representing the strength of the relation between two variates X and Y, the coefficient of correlation R XY  may be obtained by following:  
               R   XY     =         S   XY           S   X     ⁢     S   Y           =       ∑       (       X   i     -     X   m       )     ⁢     (       Y   i     -     Y   m       )             ∑         (       X   i     -     X   m       )     2     ⁢     ∑       (       Y   i     -     Y   m       )     2                         (   4   )             
 
 where, S XY  is a covariance and S X  and S Y  are standard deviations. 
 
         [0043]     R XY  has a value between −1 and 1, i.e., −1≦R XY ≦1. The closer the absolute value of R to 1, it is decided that a strong correlation exist. However, one cannot decide only by the coefficient of correlation whether the coefficient of correlation has a meaning. The invention suggests a method for specifying a fabrication machine that causes a defect, which consequently narrows down the range of fabrication machines that have lower wafer yields despite their higher usage frequencies. That is, significance probability P values of machine code numbers with minus coefficients of correlation are compared with each other.  
         [0044]      FIG. 4  illustrates fabrication line data being read in step  103 . In this embodiment, fabrication line data corresponding to wafer numbers are shown. Fabrication line data of each wafer including grade name of the IC, wafer number and No. of processes performed on the wafer are recorded in a header unit. In addition, a data unit also records the date and hour each process is performed, process code numbers, machine code numbers, and process names. For instance, by reading the fabrication line data, one can find out that the wafer number A1540-03 underwent a LOCOS coating process on Feb. 2, 2004 and a wiring  2  coating process on Feb. 19, 2004 using the same coating machine II. Step  104  in  FIG. 1  lists machine code numbers included in the fabrication line data according to numbers.  
         [0045]     Showing frequency is calculated by machine code numbers defined in  FIG. 3 , e.g., how many times the photolithography machine I of the machine code number  131  has been used or how many times the resist stripping machine II of the machine code number  172  has been used.  
         [0046]      FIG. 5  illustrates test data being read in step  102  of  FIG. 1 . In this embodiment, test data are classified by wafers. Test data of each wafer including grade name of the IC and wafer number are recorded in the header unit, and inspection items done on the wafer are recorded on the inspection step unit. In addition, the data unit has inspection results of the respective inspection items recorded in the inspection step unit. Here, the wafer A1540-03 of the example 271 undergoes inspection J, test A and test B, whereas the wafer A1541-02 of the example 272 undergoes inspection K, test A and test B. Therefore, it is common that wafers undergo different inspections or tests.  
         [0047]      FIG. 6  illustrates a GUI (Graphical User Interface) displayed on an output unit  257  when the program in  FIG. 1  is executed in the test data analysis machine  251 . A product name is inputted to  300 , and a process being the starting point of search is set to  301 . In addition, from which month which day to which month which day a wafer underwent the process designated in  301  are inputted to  302  and  303 , respectively. Also, to specify an object wafer, two digits after the wafer number are selected. In this embodiment, ASIC- 001  is inputted to  300 , test A-Yield is selected in  301 , Feb. 1, 2004 is inputted to  302 , Feb. 3, 2004 is inputted to  303 , and  02 ,  03  and  04  after the wafer number are selected in  304 . The above is a rough choice of the wafer number in step  101  of  FIG. 1 . Next, when the SEARCH button of  306  is clicked, step  102  is executed and thus, test data shown in  FIG. 5  are read.  305  displays test data being read at the choice of the wafer number. From this display result, one can specify an object wafer by selecting the wafer number. When the ANALYSIS button of  307  is clicked, steps  103  through  107  are executed and it is transited to the image in  FIG. 7 . In addition,  310  indicates a print button and  311  indicates an end button.  
         [0048]     Similarly,  FIG. 7  illustrates a GUI (Graphical User Interface) displayed on an output unit  257  when the program in  FIG. 1  is executed in the test data analysis machine  251 . The image in the drawing shows the results of steps  103  through  107  after the ANALYSIS button of  307  is clicked. Although the input of  301  is automatically inputted to  281 , it is also possible to change to pull down. The process designated in  281  becomes an inspection item to be done on an analysis object (or target).  282  and  283  are conditions of a screen display. In this particular embodiment, in step  106 , test data are compared and sorted by P values (significance probabilities) and results by machines are displayed on the screen. That is, choosing to sort by P values in  282  corresponds to step  107 .  284  displays the result of the lowest P value, i.e. the result of a fabrication machine with the highest probability to cause a defect, and  285  displays the result of the second lowest P value, i.e. the result of a fabrication machine with the second highest probability to cause a defect. Further, one can switch the result in order of P values using the scroll bar of  286 . Inside of  284  displays an object machine code number on  287 , its machine name  288 , P value in step  106   289 , and a graph of results  290 . The example of  290  shows a box diagram, in which the vertical axis represents inspection items selected and the horizontal axis represents usage frequencies of fabrication machines the machine code numbers indicate. In the example of  290 , for instance, there are three kinds of groups for the etching machine II under the machine code number  142  among the groups prepared in step  105 , i.e., a group having only one machine code number  142  in the fabrication line data by wafers, a group having two machine code number  142   s  in the fabrication line data, and a group having three machine code number  142   s  in the fabrication line data.  
         [0049]     In the drawing,  291  indicates a return button and  292  indicates a print button.  
         [0050]      FIG. 8  is a block diagram illustrating a hardware configuration for executing the program of the invention. A test data analysis machine  251  is connected to a result management database  241  through a LAN  261 . The test data analysis machine  251  includes a communication control unit  252 , a control/operation unit  253 , a main memory  254 , a secondary memory  255 , an input unit  256  such as a keyboard or a mouse, and an output unit  257  such as a display or a printer. Fabrication history data or test data stored in the result management database  241  are inputted to the test data analysis machine  251  through the LAN  261  and the communication control unit  252 . In addition, the fabrication history data or the test data are stored in the secondary memory  255 . The program in  FIG. 1  is stored in the secondary memory  255  in advance, and is read by the main memory  254  for execution. The fabrication history data or the test data stored in the secondary memory  255  are read by the secondary memory  255  in step  102  or step  103  in  FIG. 1 . The fabrication line data management unit  242  collects fabrication line data of wafers from the respective fabrication machines, and stores them in the result management database  241 . In addition, a test data management unit  243  collects test data from all inspection machines or testers, and records and manages them in the result management database  241 .  
         [0051]     So far, one embodiment of the invention applied to the IC fabrication process has been explained. The present invention can be advantageously used not only for the fabrication process of IC on a wafer, but also for estimating defect causes in a liquid crystal display which uses a glass substrate instead of a wafer or defect causes in other types of products.  
         [0052]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.