Abstract:
A centrally-controlled correlation system for testing a correlation wafer and comparing the testing results with the wafer&#39;s reference data that has been determined previously. The testing instructions and the correlation criteria are stored and transmitted from a central database. Such centrally-controlled correlation system improves the reliability of the correlation results and reduces the time to correlate a correlation wafer.

Description:
TECHNICAL FIELD 
   This disclosure relates to testing of semiconductor circuitry and more in particular to testing of semiconductor circuitry using correlation techniques. 
   BACKGROUND 
   As semiconductor devices become increasingly complex, so does the time required to properly test these devices. Considering the high volume production of some semiconductor devices, such an increase in testing time and the corresponding testing cost can significantly increase the cost of the final product. Additionally, semiconductor devices are often manufactured in more than one location, often by different subcontractors or fabrication companies. This often results in different testing specifications between the sites because different tests and testing procedures may be used. Consequently, minimizing the testing time and standardizing the testing specifications are two challenges for any wafer-testing process. 
   Wafer-testing is physically conducted by a device known as prober. The prober holds a probe card, on which needle-like probe pins are aligned in a configuration that corresponds to a specific semiconductor device. Through the probe pins, the prober makes electrical contact with the dice on a wafer and performs various test patterns, which are also called test programs. Different semiconductor devices require different testing programs. A die may pass all or just some of the test programs, and based on such testing results, the die is then categorized as good, bad, or even some other classifications. The process for wafer testing has already been addressed by prior art such as the Taiwanese patent issued to Jeng et al. (TW 516149). 
   During the wafer-testing process, high failure rate at specific die locations or low overall yield rate are often indications that there might be a defect in the wafer fabrication process. The defect may locate either in the production stage or in the testing stage of the wafer manufacturing process. There are several possible defects that can occur in the testing stage of the process. One possible defect is the faulty design of the test programs. Another possible defect is the mechanical failure of testing equipments. The probe pins on the probe card may be misaligned or the prober&#39;s hold of the probe card may become loose. The prober itself may also suffer mechanical failures such as leaking hydraulic fluid or coolant. Often times, these problems arise simply due to the wear and tear of the testing process. 
   Moreover, an existing defect may come with a new piece of testing equipment. Improper maintenance of the testing equipments may also cause an equipment defect. Thus, it is important to ensure that there is no defect before starting up a new testing equipment or restarting an existing testing equipment that has been serviced. 
   A technique known as wafer correlation has been used to diagnose the existence and location of a defect. Correlation involves first choosing a test wafer and determining the locations of the good and bad dice on the correlation wafer. The test wafer is often called a correlation wafer. One way to make such determination is to test the correlation wafer using testing equipments that are working properly. Using the testing results of the correlation wafer, a reference map recording the location of the good and bad dice is generated and stored as reference data. Before startup or restart, or after a defect is suspected, an operator would test the same correlation wafer to again determine the locations of the good and bad dice. The operator then applies a set of correlation criteria to compare the test results with the previously-determined reference data and determine the number of matching dice. The testing results and reference data correlate only if the number of matching dice exceeds a threshold number. If the testing results correlate with the previously-determined reference data, then there is either no defect, or a defect is located in the production stage. If the test results do not correlate with the previously-determined reference data, then a defect is likely located in the testing stage. 
   Although wafer correlation is an important diagnostic technique, it has several disadvantages. First, it increases the cost of production because the correlation process takes up labor and machine time that can otherwise be used for production. Running correlation is a labor-intensive task because the comparison of testing results and reference data has been done manually by an operator or engineer. Running correlation also requires machine time, which is proportionate to the number of dice that are sampled for testing. Thus, an inefficient sample size would increase the machine time required. An inefficient sample size can also reduce the life span of a correlation wafer as each wafer can only be tested for a limited number of times before becoming damaged; in turn, a shorten life span would require more labor and machine time to be spent to prepare another correlation wafer. 
   Second, the correlation results are sometimes inconsistent and unreliable. Often times, wafers are manufactured and tested by independent subcontractors at different sites and the subcontractor at each site may have developed different sets of correlation criteria. For example, subcontractors may have different correlation-passing requirement. A subcontractor may require 97% of the dice to have matching testing results while a different subcontractor may lower the requirement to 95%. As another example, some subcontractors do not treat a good die becoming bad die as non-matching because this change may be attributed to the normal wear and tear involved in wafer-testing; but some other subcontractors treat it as non-matching. Applying different sets of correlation criteria, a subcontractor may conclude that a defect exists in the testing equipments while another subcontractor at another site may conclude otherwise. The inconsistent correlation results may lead to more inaccurate wafer-testing results. 
   Because wafer correlation is an important diagnostic technique, there exists a need for an efficient and reliable correlation system and a method of using such correlation system. In particular, there is a need to minimize labor and machine time and turn out consistent correlation results. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of an exemplary embodiment of a centrally-controlled correlation system in accordance with the present disclosure; 
       FIG. 2  is a schematic block diagram illustrating an exemplary embodiment of a testing-control module in accordance with the present disclosure; 
       FIG. 3  is a schematic block diagram illustrating an exemplary embodiment of an integrated machine operable to obtain testing instructions for a correlation wafer and output the testing instructions as electrical signals; and 
       FIG. 4  is a schematic block diagram illustrating an exemplary correlation process in accordance with the present disclosure. 
   

   SUMMARY 
   The centrally-controlled correlation system substantially meets these needs and others. Disclosed herein is a wafer correlation diagnostic technique in which a correlation wafer is tested and its testing results are compared to its previously-determined reference data. Using a centrally-controlled correlation system, the process time for the wafer correlation can be reduced and the consistency of the wafer correlation results can be improved. 
   In an embodiment, the centrally-controlled correlation system includes a database, an instruction-server module, a testing-control module, and a prober. The database stores the testing instructions for testing the correlation wafer and correlation criteria for comparing the testing results with the previously-determined reference data. The instruction-server module identifies, at the database, the testing instructions and transmits them to the testing-control module. According to the testing instructions, the instruction-server module outputs electrical signals to the prober, which tests the correlation wafer accordingly. Either the instruction-server module or the testing-control module may apply the correlation criteria identified by the instruction-server module and determines whether the testing results correlates with the previously-determined reference data. 
   According to another aspect of the disclosure, a method for wafer correlation includes identifying at a database, testing instructions for testing the correlation wafer and correlation criteria for comparing the testing results with the previously-determined reference data. The method further includes transmitting the testing instructions from an instruction-server module to a testing-control module. 
   In yet another aspect of the disclosure, an instruction system for wafer correlation is provided that includes a database operable to store the testing instructions for testing the correlation wafer and correlation criteria for comparing the testing results with the previously-determined reference data. The system further includes an instruction-server module operable to identify, at the database, the testing instructions and correlation criteria, and to transmit the testing instructions. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates an exemplary embodiment of a centrally controlled correlation system  5 . The centrally controlled correlation system  5  may include a central instruction subsystem  90  in communication with remote testing subsystems  80 ,  82 ,  84  over a communications network  55 . 
   The central instruction subsystem  90  is operable to provide testing instructions and correlation criteria to one or more remote testing subsystems  80 ,  82 , and  84 . Central instruction subsystem  90  may include an instruction-server module  30  and a database  40 , which are communicatively coupled as shown. As used herein, “communicatively coupled” refers to the coupling of functional modules and/or subcomponents such that signals may be passed from one functional module to another. Such signals may be passed directly or indirectly, via direct connection, wireless connection, local area network(s), and/or wide area network(s) using conventional communication techniques. The database  40  holds the wafer-testing instructions and correlation criteria for a correlation wafer. The instruction-server module  30  retrieves the testing instructions and correlation criteria from the database  40 . 
   Remote testing subsystem  80  provides an exemplary representation of a remote testing subsystem, and as illustrated, there may be one or more remote testing subsystems  80 ,  82 ,  84  in communication with central instruction subsystem  90 . Each remote testing subsystem  80 ,  82 ,  84  is operable to test a correlation wafer and may include a prober  10  communicatively coupled to a testing-control module  20 , as described in further detail later in this specification. In operation, within remote testing subsystem  80 , the testing-control module  20  receives the testing instructions from the central instruction subsystem  90 , and provides commands to the prober  10 , to test the correlation wafer according to the testing instructions. 
   After having tested the correlation wafer, either the remote testing subsystem  80  or the central instruction subsystem  90  may apply the correlation criteria to determine the correlation data, such as the location and number of matching dice and whether the number of matching dice exceeds a threshold number. Using these correlation data, the correlation result may be determined. If the number of matching dice exceeds a threshold number, then the testing results and the previously-determined data correlate. In one embodiment, the central instruction subsystem  90  provides through communication network  55  the correlation criteria to the remote testing subsystem  80 . The remote testing subsystem  80  then applies the correlation criteria to determine the correlation result. The remote testing subsystem  80  may also transmit the correlation result to the central instruction subsystem  90 , which may then modify the testing instructions for future testing based on the correlation result. In another embodiment, the testing-subsystem  80  conducts the wafer-testing and transmits the testing results to the instruction subsystem  90  over communication network  55 . The instruction subsystem  90  then compares the testing results with the previously-determined reference data and may modify the testing instructions for future testing based on the correlation result. By centralizing the distribution of the correlation criteria, every subcontractor may be able to apply substantially uniform correlation criteria and to reach more consistent correlation results. Moreover, the modification of the testing instructions at the central instruction subsystem  90  may allow subcontractors to test correlation wafers more efficiently. 
   As mentioned briefly above, communications network  55  may provide a conduit for communication of testing instructions and correlation criteria to the remote testing subsystems  80 ,  82 ,  84 . The communication network  55  through which the central instruction subsystem  90  communicates with the remote subsystems may be a wide-area-network (WAN), a local-area-network (LAN), or a combination thereof. For example, in an embodiment, the remote testing subsystems such as subsystems  80 ,  82 , and  84  are physically located at one or more geographically remote testing facilities and the central instruction subsystem  90  is located in a different central facility. The communication network  55  in this embodiment may include a wide-area-network. In an alternative embodiment, the central instruction subsystem  90  is geographically collocated in the same testing facility where one or more remote testing subsystems are located. In this embodiment, the communication network  55  through which the central instruction subsystem  90  communicates with the remote testing subsystems may comprise LAN(s) for the remote testing subsystem(s) that are geographically collocated with the central instruction subsystem  90 , and a WAN to communicate between the geographically remotely-located remote testing subsystem(s) and the central instruction subsystem  90 . In another alternative embodiment, the central instruction subsystem  90  and the remote testing subsystems  80 ,  82 ,  84  are located at one facility. The communication network in this embodiment may comprise at least one local-area-network. 
   System Components 
   Prober  10  includes a mechanical device that holds a probe card and probes the dice on a semiconductor wafer. It receives testing instructions from the testing-control module  20  and tests the correlation wafer according to the testing instructions. In an embodiment, the prober  10  may be a stand-alone device that communicates with the testing-control module  20  over a direct link, or a communication network, which can be either a LAN or WAN. Alternatively, in another embodiment, the prober  10  may be a part of a larger device that integrates the prober  10  and the testing-control module  20 . The integrated device may comprise the entire testing-control module  20  or just some of its components.  FIG. 2  shows an exemplary embodiment of the testing-control module  20 . The testing-control module  20  may include one or more microcontrollers which collectively communicate with the central instruction subsystem  90  and control the prober  10 . The one or more microcontrollers provide various functions, such as provision of the manufacturing executive system (MES)  22 , equipment server  24 , tester server  26 , and the tester  28 . 
   MES  22  is operable to identify a correlation wafer and initiate the testing of a correlation wafer. The MES  22  may initiate the correlation process by instructing other components of the testing-control module  20  to gather testing instructions. Equipment server  24  is operable to retrieve the previously-determined reference data and testing instructions from the central instruction subsystem  90 . The reference data includes the previously-determined position information of the good and bad dice on the correlation wafer, and the testing instructions includes the position information of the specific dice that are going to be sampled. Tester server  26  is operable to store the test programs for testing a correlation wafer and the testing results. Tester  28  is operable to retrieve test programs from the tester server  26  and controls the prober  10  to test the dice according to the testing instructions. The tester  28  controls the prober  10  by executing the testing programs and outputting testing instructions. In an embodiment in which the remote testing subsystems conduct the correlation of the testing results to the reference data, the tester  28  may further be operable to retrieve, from the central instruction subsystem  90 , the correlation criteria and apply them to determine the correlation results. The correlation criteria may be in the form of a computer script or in the form of a pointer pointing to a script stored in the tester server  26 . 
   In embodiments of the testing-control module  20 , any two or more components  22 - 28  may be integrated into one microprocessor that performs the same tasks. For example, equipment server  24  and tester  28  may be integrated into one machine that is operable to obtain the testing instructions for a correlation wafer and output the testing instructions as electrical signals. A schematic block illustration of this exemplary embodiment is shown in  FIG. 3 . Equipment server  24 , tester  28 , and prober  10  are incorporated into an integrated machine  50 , which retrieves the reference data and testing instructions from the central instruction subsystem  90 , retrieves the testing programs from the tester server  26 , and probes the correlation wafer (not shown). In an embodiment in which the remote testing subsystems conduct the correlation of testing results to reference data, the integrated machine  50  may further be operable to retrieve the correlation criteria from the central instruction system  90  and apply them to determine the correlation results. 
   Referring to  FIG. 4 , the instruction-server module  30  of  FIG. 1  may include a correlation server  32  and an instruction modifier  34 . Correlation server  32  is operable to communicate with the database  40  and at least one remote testing subsystem, which is represented by subsystem  80 . The correlation server  32  retrieves from the database  40  the testing instructions for a correlation wafer and provides them to remote testing subsystem  80 . The testing instructions may be in the form of a location map indicating the position of the dice that require testing. In an embodiment in which the central instruction subsystem  90  conducts the correlation of testing results to reference data, the correlation server  32  is further operable to retrieve the correlation criteria from the database  40  and apply them to determine the correlation result. The correlation criteria in this embodiment may be in the form of a computer script. In another embodiment in which the remote testing subsystem  80  conducts the correlation of the testing results to the reference data, the correlation server  32  is operable to retrieve the correlation criteria from the database  40  and provide the instructions to the remote testing subsystem  80 . The correlation criteria in this embodiment may be in the form of a computer script or in the form of a pointer pointing to a script stored in the tester server  26 . In any embodiment, the instruction-server module  30  may be operable to obtain and use the correlation results to modify the testing instructions for testing the same correlation wafer in the future. The modifications can by done by the instruction modifier  34 , which is dedicated to making modifications to the testing instructions. The modified testing instructions are transmitted to and saved in the database  40 . 
   The database  40  comprises any storage media that holds information or a database. Database  40  is communicatively coupled to the instruction-server module  30 , and may include any memory device known in the art, such as random access memory, a hard drive, removable computer media such as floppy disk, CD, DVD, solid state flash memory, or a combination thereof. 
   Correlation Process Steps 
   In an embodiment, the correlation system  5  involves the following general steps, which may be provided in this or another order: 1) transmitting, from the central instruction subsystem  90  to the remote testing subsystem  80 , the reference data and testing instructions; 2) the remote testing subsystem  80  testing the correlation wafer according to the testing instructions; 3) transmitting, from the central instruction subsystem  90  to the remote testing subsystem  80 , the correlation criteria; 4) the remote testing subsystem  80  applying the correlation criteria to determine the correlation result; and 5) transmitting, from the remote testing subsystem  80  to the central instruction subsystem  90 , the correlation result and using them to modify the testing instruction for testing the correlation wafer in the future. 
   An exemplary correlation process is illustrated in  FIG. 4 , in which the process steps are labeled. When the MES  22  identifies a correlation wafer in the remote testing subsystem  80 , it proceeds to initiate the correlation process. To initiate the process in step A, the MES  22  instructs the equipment server  24  to retrieve, from the central instruction subsystem  90 , the previously-determined reference data and the testing instructions. The MES  22  also instructs the tester server  26  to provide the appropriate test programs to the tester  28 . In step B, the equipment server  24  transmits, to the central instruction subsystem  90 , a request for the previously determined reference data and the testing instructions for the correlation wafer that was identified by the MES  22 . In step C, the correlation server  32  in the central instruction subsystem  90  receives the request that was sent from the equipment server  24  and identifies the requested reference data and testing instructions in the database  40 . In step D, the correlation server  32  transmits the reference data and testing instructions to the equipment server  24  in the remote testing subsystem  80 . In step E, the equipment server  24  transmits the reference data and testing instructions to the tester  28 . In step F, the tester server  26  identifies the appropriate test programs for testing the correlation wafer that was identified by the MES  22 ; the tester server  26  then transmits the test programs to the tester  28 . Step F does not have to be performed in sequence with any of steps B, C, D, and E. Thus, step F can be performed before, concurrently with, or after any of these steps. In step G, the tester  28  collects the reference data, testing instructions, and test programs for the correlation wafer and executes the testing instructions, which are the position information of the dice that are to be tested. The tester  28  then transmits signals to the prober  10  to test the correlation wafer. In step H, the prober physically makes electrical contact with the dice (one die at a time, or several dice at once) on the correlation wafer to test the functionality of the dice. As the prober  10  tests the dice on the correlation wafer, it sends the results back to the tester server  26  for storage. 
   After the testing is completed, the tester  28  in step I may send a request to the central instruction subsystem  90  for the correlation criteria. In step J, the correlation server  32  may identify within the database  40  the requested correlation criteria and in step K, transmit the correlation criteria to the tester  28 . The correlation criteria may be in the form of a computer script or in the form of a pointer pointing to a script stored in the tester server  26 . In step L, the tester  28  may retrieve the testing results from the tester server  26  and apply the correlation criteria to determine the correlation result. In an alternative embodiment, steps I, J, K, and L may be eliminated. Instead, the correlation server  32  may retrieve from the tester server  26 , the testing results, and from the database  40 , the correlation criteria and the previously-determined reference data. The correlation server  32  then apply the correlation criteria to determine the correlation result. 
   After the correlation result is determined, the correlation result may be transmitted to an instruction modifier  34 , which may use the correlation result to modify the testing instructions for testing the same correlation wafer in the future. Based on the correlation result, the instruction modifier  34  may apply a statistical model to determine a more efficient sampling of the correlation wafer. In one embodiment, the statistical model used for calculating sample size is based on the equation:
 
 n=[p*q ( N /( N− 1))]/[(ε/ Z ) 2 +( p*q )/( N− 1)]
 
where “n” is the sample size, “N” is the number of gross dice, “p” is the percentages of non-matching dice, “q” is the value of “p” subtracted from 1, “ε” is the acceptable measurement resolution, and “Z” is the confidence level. The acceptable measurement resolution and confidence level are arbitrary values that can be varied to achieve different testing qualities. Since the number of gross dice is fixed and the acceptable measurement resolution and the confidence level values are arbitrary, a new sample size may be determined after determining the percentage of non-matching dice. The application of this model may generate a sample size that is smaller while maintaining the same testing qualities.
 
   While various embodiments of central correlation systems and methods of centrally correlating and testing semiconductor wafers according to the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Accordingly, the following claims should be construed broadly to cover any embodiment tailored to achieve the principles disclosed herein. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
   Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.