Abstract:
According to an embodiment of the present invention is to provide methods to evaluate the impact of scrapped wafers on the remaining wafers in a lot by using scrap codes and statistical models. An embodiment of the present invention provides a method to obtain a baseline lot population by using cluster analysis model and functional limited yields. The functional limited yields may be for example chain limited yield, dc limited yield, or ac abist limited yield. By utilizing statistical modeling it is possible to determine which failures have an impact on the lot yield and require rework for the lot. In addition by monitoring the impact of failures, it is possible to determine if corrective actions need to be taken for lots that passed through a process prior to correction of the fault.

Description:
BACKGROUND 
       [0001]    The manufacture of integrated circuits involves billions of dollars in expense and errors may result in significant losses. The inventors have observed that mis-process/events occur during chip fabrication. The impacted wafers are often scrapped when there is a mis-process or event that could potentially result in a low wafer test yield. These events may include a broken wafer which may occur in either a batch process or a single wafer process or during transport from one process to the next process. 
         [0002]    During process the impacted wafers are removed and the rest of the lot will move through the line and arrive at test. At test the inventors have noticed that significant yield variability in the yield distribution. As a result the cost of rework, scrap and decreased customer satisfaction have a significant impact on profitability. For example the inventors have determined that a wafer which fails or breaks in a batch process may impact the yield of the wafers in that lot. For example during a metallization process, a broken wafer may cause wafer fragments to be distributed onto the wafers in the batch process, thus contaminating the entire batch. Similarly, if a wafer breaks in a single wafer process, the lot as a whole may be unaffected. The inventors have determined that it would be of value to be able to determine at the time or at least prior to testing if an error or wafer break occurs what corrective actions should be taken to minimize the costs in the processing of the wafers. 
       SUMMARY 
       [0003]    According to an embodiment of the present invention is to provide methods to evaluate the impact of scrapped wafers on the remaining wafers in a lot by using scrap codes and statistical models. An embodiment of the present invention provides a method to obtain a baseline lot population by using cluster analysis model and functional limited yields. The functional limited yields may be for example chain limited yield, dc limited yield, or ac abist limited yield. 
         [0004]    By using a statistical model, such as MANOVA (Multivariate Analysis of Variance), to compare the baseline lot population and lots which have scrapped wafers, it is possible to determine which error codes, require additional actions. If there is no significant difference between the baseline lot population and lots which had scrapped wafers, no additional rework may be required. If there is a significant difference between the baseline lot population and lots which had scrapped wafers further analysis may be required to detect which process steps caused the difference. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0005]      FIG. 1  is a flow diagram of a method for determining the corrective actions to take for an error code according to an embodiment of the invention. 
           [0006]      FIG. 2  illustrates an example of Vdd shorts yield by impacted scrapped wafers. 
           [0007]      FIG. 3  is a representation of a Foup. 
           [0008]      FIG. 4  is a flow diagram of a method for determining what corrective action to take for a lot based on the error code for a wafer according to an embodiment of the invention. 
           [0009]      FIG. 5  shows a general-purpose computing device in the form of a conventional personal computer incorporating an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    With reference now to  FIG. 1  illustrates a flow diagram of a method  100  for determining the corrective actions to take for an error code according to an embodiment of the invention. Activity  110  may be to select a scrap code and query scrapped wafer and Lot_id list from a database. Activity  110  is used to identify various scrap codes and the lots that they have affected. Activity  115  is to query the functional limited yields from the database. Activity  120  is to use cluster analysis to create baseline lot populations by using the limited yields of Activity  115 . Activity  125  is to join the scrap information of  110  and the functional limited yield of  120  by using the Lot_id. Activity  130  is to use MANOVA model to compare baseline lot population and lots which had scrapped wafers. Activity  135  is to determine if there is a significant difference between baseline population and lots that had scrapped wafers. If there is a significant difference then Activity  145  uses MANOVA models to compare the baseline lot population and lots that had scrapped wafers across the process steps. Activity  150  is to identify those process steps that have significant yield differentials. Activity  155  is to change the identified processes or operations to reduce the errors. If a significant difference is not found in Activity  135 , activity  140  may be to work with the client to show why the error will not impact the quality of the remainder of the lot. 
         [0011]      FIG. 2  illustrates an example of Vdd shorts yield by impacted scrapped wafers. Using the method  100  of  FIG. 1  the impact of broken wafers can be shown. Broken wafers  210  illustrates a case where the impact from the broken wafer on the yield is minimal. Broken wafers  220  and  230  shows a case where the impact is significant. As shown in  FIG. 1 , Activity  134  would submit that the process where broken wafer  210  occurs does not have a significant difference from the baseline population and therefore Activity  140  may be to convince the clients or management to accept the lots which had the scrapped wafer. However, as shown broken wafer lots  220  and  230  indicate a significant difference from the baseline population. Therefore the processes in which the lots associated with broken wafers  220  and  230  are submitted to Activities  145 ,  150  and  155 . In addition it may be that activity  145  may indicate that the impact may affect more than one lot. 
         [0012]      FIG. 3  is a representation of a foup  300 . Foups such as foup  300  are one of the common means by which wafers are transported from one process to another. One means by which wafer breaks are detected is visual inspection during transportation. Foup  300  is holds a number of wafers  330  within a retainer  340  and seals the wafers during transportation. 
         [0013]      FIG. 4  is a flow diagram of a method  400  for determining what corrective action to take for a lot based on the error code for a wafer according to an embodiment of the invention. Activity  410  may be to utilize statistical analysis to identify key process steps and reason codes. Activity  420  may be to tag lots with reason codes in key process steps. Activity  430  may be to have the lot tagged for 100% process limited yield (PLY) inspection at the next step. Activity  440  is to determine if the defect density is high. If the defect density is high Activity  450  may be to subject the lot to an extra process such as WET process. If the defect density is not high, then Activity  460  may be to continue the normal route process. In addition to sending the specific lot to an extra process in Activity  440 , it may be that additional lots may need to be process through a corrective process step such as WET. 
         [0014]      FIG. 5  shows a general-purpose computing device such as a personal computer  1220 , which includes a processing unit  1221 , a system memory  1222 , and a system bus  1223  that couples the system memory  1222  and other system components to processing unit  1221 . System bus  1223  may be any of several types, including a memory bus or memory controller, a peripheral bus or a local bus, and may use any of a variety of bus structures. System memory  1222  includes read-only memory (ROM)  1224  and random-access memory (RAM)  1225 . A basic input/output system (BIOS)  1226 , stored in ROM  1224 , contains the basic routines that transfer information between components of personal computer  1220 . BIOS  1226  may also contain start-up routines for the system. Personal computer  1220  further includes hard disk drive  1239 , magnetic disk drive  1228  for reading from and writing to a removable magnetic disk  1229 , and optical disk drive  1230  for reading from and writing to a removable optical disk  1231  such as a CD-ROM (compact disc-read only memory), DVD (digital versatile disc or digital video disc) or other optical medium. Hard disk drive  1239 , magnetic disk drive  1228 , and optical disk drive  1230  are connected to system bus  1223  by a hard-disk drive interface  1232 , a magnetic-disk drive interface  1233 , and an optical-drive interface  1234 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer-readable instructions, data structures, program modules and other data for personal computer  1220 . Although the environment described herein employs a hard disk drive  1239 , a removable magnetic disk  1229  and a removable optical disk  1231 , those skilled in the art will appreciate that other types of computer-readable media that may store data accessible by a computer may also be used in the operating environment. Such media may include magnetic cassettes, flash-memory cards, DVD, Bernoulli cartridges, RAMs (random access memory), ROMs, and the like. 
         [0015]    Program modules may be stored on the hard disk drive  1239 , magnetic disk  1229 , optical disk  1231 , ROM  1224  and RAM  1225 . Program modules may include operating system  1235 , one or more application programs  1236 , other program modules  1237 , and program data  1238 . The application programs  1236  may include such programs as MANOVA. A user may enter commands and information into personal computer  1220  through input devices such as a keyboard  1240  and a pointing device  1242 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  1221  through a serial-port interface  1246  coupled to system bus  1223 , but they may be connected through other interfaces not shown in  FIG. 5 , such as a parallel port, a game port, a universal serial bus (USB), Institution of electrical and electronics engineers IEEE 1394 port, etc. A monitor  1247  or other display device also connects to system bus  1223  via an interface such as a video adapter  1248 . In addition to the monitor, personal computer  1221  may include other peripheral output devices such as printer  1275 . A print interface  1270  may be connected to system bus  1223  and to printer  1275 . 
         [0016]    Personal computer  1220  may operate in a networked environment using logical connections to one or more remote computers such as remote computer  1249 . Remote computer  1249  may be another personal computer, a server, a router, a network PC, a peer device, or other common network node. It typically includes many or all of the components described above in connection with personal computer  1220 ; however, only a remote storage device  1250  is illustrated in  FIG. 5 . The logical connections depicted in  FIG. 5  include local-area network (LAN)  1251  and wide-area network (WAN)  1252 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
         [0017]    When placed in a LAN networking environment, personal computer  1220  connects to LAN  1251  through a network interface or adapter  1253 . When used in a WAN networking environment such as the Internet, personal computer  1220  typically includes modem  1254  or other means for establishing communications over WAN  1252 . Modem  1254  may be internal or external to personal computer  1220 , and it may connect to system bus  1223  via serial-port interface  1246 . In a networked environment, program modules, such as those comprising Microsoft® Word, which are depicted as residing within personal computer  1220  or portions thereof, may be stored in remote storage device  1250 . Of course, the network connections shown are illustrative, and other means of establishing a communications link between the computers may be substituted. 
         [0018]    An imager  1201  may be connected to system bus  1223 . Embodiments of the invention may be operated by personal computer  1220 . The imager  1201  may monitor foups as they are transported between processes identifying bad lots. 
         [0019]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0020]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.