Abstract:
In accordance with an embodiment of the present invention, a semiconductor wafer has a plurality of dies each having a circuit and a plurality of contact pads. The plurality of contact pads include a first contact pad to receive a power supply voltage, a second contact pad to receive a ground voltage, and a third contact pad to receive a test control signal. The third contact pad is connected to a programmable self-test engine (PSTE) embedded on the corresponding die so that the test control signal activates the PSTE to initiate a self-test. A probe card has a plurality of sets of probe pins, each set of probe pins having three probe pins for contacting the first, second, and third contact pads of one of a corresponding number of the plurality of dies. During wafer test, the plurality of sets of probe pins come in contact with a corresponding number of dies so that the self-test is carried out simultaneously in the corresponding number of dies.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. application Ser. No. 10/340,558, filed Jan. 9, 2003, and issued on Feb. 6, 2007 as U.S. Pat. No. 7,173,444, entitled “Structure and Method for Parallel Testing of Dies on a Semiconductor Wafer,” which is a continuation-in-part of U.S. application Ser. No. 09/542,802, filed Apr. 4, 2000, and issued on Nov. 27, 2001 as U.S. Pat. No. 6,323,639, entitled “Powering Dies on a Semiconductor Wafer Through Wafer Scribe Line Areas,” which disclosure is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates in general to the semiconductor technology and more particularly to structures and methods for testing wafer. 
   The rapid advancement in the semiconductor industry has enabled manufacturing of highly complex integrated circuits (ICs). Such ICs are being fabricated on large wafers capable of housing a greater number of dies. Also, with the advancement in the IC packaging technology, smaller packages with significantly higher pin-count have become commercially available. A larger number of pads can thus be placed on each die. This has in turn resulted in a significantly more complex set of test requirements and a substantial increase in the time required to test all dies on a wafer. 
     FIG. 1  shows a top view of a wafer  100  having multiple dies  102  organized in rows and columns separated by horizontal scribe lines  104  and vertical scribe lines  106 . After wafer sort (i.e., testing all the dies on a wafer to identify non-functional dies), the good dies are extracted from the wafer by sawing along the vertical and horizontal scribe lines and then placed in the desired packages. Wafer sort is usually carried out using a probe card connected to a tester. The probe card sequentially contacts each die on the wafer until all dies have been tested. 
   Techniques wherein multiple dies are tested simultaneously are widely used to reduce wafer sort time. However, the number of dies that can be tested at the same time is limited by the number of pins that a tester can support divided by the number of pins on each die (the current testers are limited to few hundred pins). 
   Accordingly, there is a need for structures and methods for efficiently testing dies on a wafer. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the present invention, a semiconductor wafer has a plurality of dies each having a circuit and a plurality of contact pads. The plurality of contact pads include a first contact pad to receive a power supply voltage, a second contact pad to receive a ground voltage, and a third contact pad to receive a test control signal. The third contact pad is connected to a programmable self-test engine (PSTE) embedded on the corresponding die so that the test control signal activates the PSTE to initiate a self-test. A probe card has a plurality of sets of probe pins, each set of probe pins having three probe pins for contacting the first, second, and third contact pads of one of a corresponding number of the plurality of dies. During wafer test, the plurality of sets of probe pins come in contact with a corresponding number of dies so that the self-test is carried out simultaneously in the corresponding number of dies. 
   In accordance with another embodiment, a semiconductor wafer has a plurality of dies separated by scribe line areas, and one or more conductor lines extend through at least a portion of the scribe line areas. 
   In one embodiment, at least one of the one or more conductor lines has at least one contact pad. 
   In accordance with another embodiment, a semiconductor wafer has a plurality of dies separated by scribe line areas. A plurality of contact pads are located in the scribe line areas. At least one of the plurality of contact pads is routed into one of the plurality of dies. 
   In accordance with another embodiment, a method for testing a wafer which has a plurality of dies separated by scribe line areas, and one or more conductor lines extending through at least a portion of the scribe line areas, is as follows. A wafer is placed in a tester for testing the plurality of dies. A self-test operation is simultaneously initiated in a predetermined number of the plurality of dies by bringing a probe card coupled to the tester in electrical contact with fewer number of dies than the predetermined number. 
   In accordance with another embodiment, a wafer has a plurality of dies separated by scribe line areas, and a plurality of contact pads in the scribe line areas. At least one of the plurality of contact pads is routed into one of the plurality of dies. A method of testing such wafer is as follows. The wafer is placed in a tester for testing the plurality of dies. A probe card coupled to the tester is brought in electrical contact with the plurality of pads in the scribe line areas to initiate a self-test operation in a predetermined number of the plurality of dies. 
   In accordance with another embodiment of the invention, a wafer has a plurality of dies separated by scribe line areas, and a plurality of conductor lines extending through at least a portion of the scribe line areas. The plurality of conductor lines are selectively routed into predesignated groups of dies. One of more of the plurality of conductor lines has at least one contact pad. A method of testing such a wafer is as follows. The wafer is placed in a tester for testing the plurality of dies. A probe card coupled to the tester is brought in electrical contact with the at least one contact pad of the one or more of the plurality of conductor lines extending in the scribe line areas to initiate a self-test operation in at least one of the predesignated groups of dies. 
   Other features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a conventional wafer containing dies separated by scribe lines areas; 
       FIGS. 2   a  and  2   b  are schematic diagrams of a portion of a wafer showing dies with internal test pads in accordance with two embodiments of the invention; 
       FIG. 3  is a schematic diagram of a portion of a wafer showing dies with test pads located in the scribe line areas in accordance with another embodiment of the invention; 
       FIGS. 4 and 5  are schematic diagrams showing conductor lines extending through scribe line areas in accordance with two embodiments of the invention; and 
       FIG. 6  is a schematic diagram of a portion of a wafer showing conductor lines with contact pads extending through scribe line areas in accordance with yet another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance with the invention, methods and structures for wafer level test of semiconductor dies organized on the wafer in a matrix of rows and columns are provided. A programmable self-test engine (PSTE) housed in each die enables self-test to be performed in each die by only supplying power, ground, and one or more test signals. This in turn enables simultaneous testing of a larger number of dies than was previously possible since the tester needs to contact only few (e.g., 3 or 4) contact pads on each die to initiate the self-test. In one embodiment, the few contact pads are located in the scribe line areas rather than inside each die. In another embodiment, the few contact pads on each die are coupled to corresponding metal lines extending through the scribe line areas. In yet another embodiment, contact pads are provided on the metal lines extending through the scribe line areas so that the tester can directly contact these metal lines. 
     FIG. 2A  shows a portion of a wafer having dies  202  separated from each other by horizontal scribe lines  204  and vertical scribe lines  206 . Each die has, in addition to the pads normally used for the purpose of electrically connecting the die to the pins of a package, three pads  208 ,  210 ,  212  that are used for the purpose of wafer sort. Alternatively, one, two, or all of pads  208 ,  210 , and  212  are used both for the purpose of wafer sort as well as for normal die functions (i.e., are connected to pins of the package). 
   Pad  208  is configured to receive the power supply otherwise referred to as Vcc, Vdd, or other common notations of the industry, and hereinafter referred to as Vcc. Pad  210  is configured to receive the ground potential, otherwise referred to as GND, Vss, or other common notations of the industry, and hereinafter referred to as Vss. Pad  212  is configured to receive a signal for activating a programmable self-test engine (PSTE) incorporated in die  202 . More details of the PSTE are provided further below. 
   During wafer test, only pads  208 ,  210  and  212  are coupled to the tester through a probe card. Multiple dies, in fact as many as can be handled satisfactorily by the probe card and the tester, can be tested simultaneously. The tester supplies both Vcc and Vss on respective pads  208  and  210 , and then supplies a self-test activate signal on pad  212  to activate PSTE. The Probe card remains in contact with the dies until the test is complete. In another embodiment, pad  212  is also an input/out (IO) pad capable of indicating whether the self-test is successfully completed. In one embodiment, pads  208 ,  210  and  212  are routed to the PSTE only, and thus are not used once the die is packaged. The invention, however, is not limited as such, and the Vcc pad  208  and Vss pad  210  may be routed to supply power to both the PSTE and other circuit blocks on a die. 
   Unlike built-in self-test (BIST) mechanisms, the PSTE allows changing certain test parameters through the tester (i.e., the PSTE has programming capability). This feature is essential in testing of most semiconductor devices and is commonly used during the development of test programs or for the purpose of debugging a faulty die. The programmability of the PSTE allows for, but is not limited to, changing the order in which certain portions of the die are tested, changing the sequences of test conditions, and changing the test parameters. Accordingly, pin  212 , other than being used for activating the PSTE and providing test result information, may also be used for the purpose of loading data to be programmed into the PSTE memory. 
   Designing an interface that allows the extraction of a clock from the data provided on pin  212  would be obvious to one skilled in this art. Schemes such as Manchester coding could be used for this purpose. In another embodiment, PSTE memory may be pre-loaded with initial self-test sequences and parameters. In yet another embodiment, the PSTE can be designed to provide information about the test results based on information stored in PSTE internal registers. This information may include, but is not limited to, success or failure of the test, identification of failure, position of failed bits in memory tests (where the dies contain memory), action taken to override bit failures in memory, reference to operational voltages, and so on. Information may be transmitted in a manner requiring clock extraction, if necessary. 
     FIG. 2B  shows another embodiment of the present invention wherein in addition to the three pads  208 ,  210 ,  212 , pad  214  is provided for wafer sort purposes. Pad  214  is used to supply a clock signal to the PSTE. The clock signal helps make both loading of the test data as well as extracting test results simpler, thus simplifying the on-die circuitry. 
     FIG. 3  shows a portion of a wafer having dies  302  separated from each other by horizontal scribe lines  304  and vertical scribe lines  306 . The wafer layout is similar to the embodiment in  FIG. 2A , except that the three pads (used solely for wafer sort) on each die are placed in scribe line areas rather than on the die. Each of three pads  308 ,  310 ,  312  is electrically connected to circuitry inside the corresponding die via short interconnect lines. Pad  308  is configured to receive Vcc, pad  310  is configured to receive Vss, and pad  312  is configured to receive a signal for activating PSTE in the respective die. 
   The wafer sort operation is similar to that described in connection with  FIG. 2A  except that the probe card must be modified to contact pads in the scribe line areas. Also, in the  FIG. 3  embodiment, when extracting good dies from the wafer by sawing along the scribe lines, the pads in the scribe line areas are eliminated or destroyed. The variations of the  FIG. 2A  embodiment described above may also be implemented in the  FIG. 3  embodiment. Further, similar to the  FIG. 2B  embodiment, a fourth pad (not shown) may be added for each die in the scribe line area for the purpose of supplying a dedicated clock signal to the PSTE. In yet another embodiment, pad  312  is used for the purpose of providing a reference voltage. In yet another embodiment, in the case where a reference voltage input is required, another pad is added. Different voltages may be provided to the dies via this additional pad for testing purposes. This eliminates the need for incorporating on-chip reference voltage generators. In a further embodiment, the voltage reference pad is also used for the purpose of providing a clock while the PSTE is being programmed, simplifying the design of PSTE so that no self-extraction of a clock is required. 
     FIG. 4  shows a portion of a wafer with dies  402  separated by vertical and horizontal scribe line areas, in accordance with another embodiment of the present invention. Interconnect metal lines  420  and  422  are routed through the scribe line areas and extend perpendicularly to one another. Metal lines  420  extend vertically through the scribe line areas and are connected to pads  408  (Vcc) of different dies, and metal lines  422  extend horizontally through the scribe line areas and are connected to pads  410  (Vss) of different dies. Vertically-extending lines  420  may be from a different layer metal than horizontally-extending lines  422 . This achieves physical separation between Vcc and Vss power signals. Further, by running Vcc lines and the Vss lines perpendicular to each other (rather than in parallel through same scribe line areas), the possibility of electrical shorts between the Vcc and Vss lines during the sawing process is significantly reduced. As can be seen, this embodiment reduces the number of probe-card pins required to simultaneously test multiple dies as compared to previous embodiments. 
   In one embodiment, metal lines  420  and  422  extend across the entire length of the wafer. In another embodiment, metal lines  420  and  422  extend only through a specified field of the wafer. For example, a field containing 12 dies organized in three rows by four columns may be repeated throughout the wafer. 
   As in previous embodiments, pad  412  may be used as an input/out (IO) pad, capable of indicating if a die  402  under test has completed successfully the self test. Alternatively, pad  430  may be used for the purpose of providing a reference voltage. 
   It should be noted that same test procedure would apply even if only specific fields containing a number of dies as mentioned above are tested. In this case, the test procedure will repeat several times until all tests are completed. A person skilled in the art could easily modify the scheme of pads internal to die  402  to pads placed in the scribe line. The pads would be connected directly to lines  420  and  422  respectively and then routed into each die. In yet another embodiment, test pads  412  are also placed in the scribe line and routed to corresponding dies. In yet another embodiment, specifically in the case where a reference voltage input is required, another pad is added. 
   In the embodiment where the test program and/or parameters, collectively the “test data”, are to be loaded into the PSTE, this can be done by loading such test data in parallel in all dies. However, after all dies are tested, each die may contain different test results requiring each die to be individually accessed for the test result information. In one approach, the probe card can provide power and ground to one column and one row of dies at a time, resulting in the ability to read the test result from a single die. The process can be repeated until all dies are read. In the case where the power and ground lines cover a predefined field, the probe card would be able to examine in parallel test information from multiple dies. 
   Reference is now made to  FIG. 5  where an additional horizontally-extending metal line  524  is added in the scribe line areas, and which is connected to the test pad  512  of the dies along a corresponding row. In this case, the self-test may be activated simultaneously in a whole row of dies. This advantageously further reduces the number of probe-card pins required. A person skilled in the art could easily modify the scheme of pads internal to dies  502  to pads placed in the scribe lines in view of this disclosure. The pads would be connected directly to lines  520 ,  522  and  524  respectively. In yet another embodiment, where a reference voltage input is required, another pad is added for this purpose. 
   Reference is now made to  FIG. 6  where an additional vertically-extending metal line  626  is added in the scribe line areas, and which is connected to clock pads  628  in the dies along a corresponding column of dies. In this case, a whole column of devices may receive a clock signal simultaneously and operate in the manner described above where a separate clock pad is provided on each die. Pads  630  connected to lines  620 ,  622 ,  624 , and  626  enable a probe card to touch these lines and activate multiple dies in parallel or one at a time as desired. 
   The above embodiments wherein metal lines extend through scribe line areas are not limited to the particular orientation of the metal lines. For example, the Vcc lines extending vertically may be changed to extend horizontally. 
   Following are a set of examples intended to illustrate the efficiencies achieved in testing wafers using the above-described embodiments. In all the following examples, it is assumed that the wafer has 1200 dies organized in 30 rows and 40 columns, and testing of each die takes one minute. Thus, if each die is tested separately, a total time of 1200 minutes is necessary to complete testing of all 1200 dies. 
   Using, for example, the  FIG. 4  embodiment, a column (30 dies) can be tested simultaneously with a probe card having a total of 61 pins (one supply pin, 30 ground pins, and 30 test enable pins). The wafer test will take 1 minute per column and a total of 40 minutes for the wafer, a significant reduction using a simpler probe card and tester. The dies can be tested in rows in a similar manner. 
   If the probe card pins and the tester are capable of high current drive, it is possible to test the entire wafer (e.g., using the embodiment shown in  FIG. 5 ) with a total of 100 pins (40 supply pins, one per column of dies; 30 ground pins, one per row of dies; and 30 test enable pins, one per row of dies). Test time would thus be one minute for the whole wafer. It is possible to test the entire wafer simultaneously using a probe card with even fewer than 100 pins. In a three-layer metal process, metal layers  1 ,  2 , and  3  may be used to form lines  520 ,  522 , and  524 , respectively. This enables connecting together all vertically-extending lines  520  with metal layer  1 , connecting all horizontally-extending lines  522  with metal layer  2 , and connecting all horizontally-extending lines  524  with metal layer  3 . Thus, with only three probe pins the entire wafer can be tested at once. This embodiment is not limited to the above assignment of the three metal layers to the three metal lines. 
   It is further possible to test the wafer in fields. A field could be an array of dies having, for example, six rows and eight columns. By testing all the dies in a field simultaneously, it would take one minute to test the entire field. The probe card would then move to continue the test on an untested field and repeat this process 25 times, hence, the test time would be 25 minutes. If each die in a field is to have its own three probe pins connected, a total of 144 pins would be required. However, it is possible to use either the row or column scheme described above so that a probe card with fewer number of pins may be used, e.g., a probe card with 20 pins (8 supply pins, 6 ground pins, and 6 test enable pins). In another embodiment, five fields in a row could be tested in parallel resulting, for example, in the use of a total of 100 pins. In this case the total test time would be eight minutes. Many other variations can be envisioned by one skilled in this art. 
   The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and it should be understood that many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.