Abstract:
An integrated circuit includes switching means for selectively connecting the bond pads to functional core logic and isolating the bond pads from second conductors, and the switch means for selectively connecting the bond pads to the second conductors to provide bi-directional connections between the bond pads on opposite sides of the substrate and isolating the bond pads from the functional core logic.

Description:
[0001]     This application is a divisional of prior application Ser. No. 11/214,088, filed Aug. 29, 2005, now pending; 
    which was a divisional of prior application Ser. No. 10/832,919, filed Apr. 26, 2004; now U.S. Pat. No. 6,954,080, issued Oct. 11, 2005;     which was a divisional of prior application Ser. No. 09/835,802, filed Apr. 16, 2001, now U.S. Pat. No. 6,727,722, issued Apr. 27, 2004;     which was a divisional of prior application Ser. No. 09/325,487, filed Jun. 3, 1999, now U.S. Pat. No. 6,262,587, issued Jul. 17, 2001;     which was a divisional of prior application Ser. No. 08/742,189, filed Oct. 31, 1996, now U.S. Pat. No. 5,969,538, issued Oct. 19, 1999.   
 
     
    
     FIELD OF THE INVENTION  
       [0006]     The invention relates generally to integrated circuit manufacturing and, more particularly, to testing die on wafer.  
       BACKGROUND OF THE INVENTION  
       [0007]     Integrated circuit (ICs) manufacturers produce die on typically circular substrates referred to as wafers. A wafer may contain hundreds of individual rectangular or square die. Die on wafer, or unsingulated die, must be tested to determine good from bad before the dies are singulated. Unsingulated die testing traditionally occurs by physically probing each die at the die pads, which allows a tester connected to the probe to determine good or bad die. This type of probing is relatively slow and requires expensive mechanical mechanisms to accurately step and position the probe at each die location on the wafer. The probing step can damage the die pads which may interfere with the bonding process during IC packaging or assembly of bare die on MCM substrates. Also, as die sizes shrink, pads are positioned closer and closer together and it becomes more difficult and costly to design precision probing instruments to access them.  
         [0008]     Alternate conventional methods for testing unsingulated die on wafers include: (1) designing each die to test itself using built-in-self-test (BIST) circuitry on each die and providing a way to enable each die BIST circuitry to test the die, (2) widening the scribe lanes between the die to allow for: (a) test probe points, (b) test access conductors, and/or (c) test circuitry, and (3) processing an overlying layer of semiconductor material with test circuitry over the die on wafers and providing via connections, from the overlying layer, to the pads of each die on the wafer. Method 1 disadvantageously requires BIST circuitry on the die which takes up area, and the BIST circuitry may not be able to adequately test the I/O of the die. Method 2 disadvantageously reduces the number of die that can be produced on a wafer since the widening of the scribe lanes takes up wafer area which could be used for additional die. Method 3 disadvantageously requires additional wafer processing steps to form the overlying test connectivity layer on top of the die on wafers, and also the overlying layer needs to be removed from the wafer after testing is complete. This overlying layer removal step is additive in the process and the underlying die could be damaged during the removal step.  
         [0009]     Ideally, only good die are singulated and packaged into ICs. The cost of packaging die is expensive and therefore the packaging of bad die into ICs increases the manufacturing cost of the IC vendor and results in a higher cost to the consumer.  
         [0010]      FIG. 1  illustrates a schematic of a die containing functional core logic (FCL) and input and output buffering to pad locations. The variety of pad buffering shown includes: inputs (I), 2-state outputs (2SO), 3-state outputs (3SO), open drain outputs (ODO), input and 3SO bidirectionals (I/O1), and input and ODO bidirectionals (I/O2). The FCL could be a custom or semicustom (ASIC) implementation comprising: microprocessors, combinational logic, sequential logic, analog, mixed signal, programmable logic, RAMs, ROMs, Caches, Arrays, DSPs, or combinations of these and/or other functions. The die is shown having a top side A, right side B, bottom side C, and left side D for convenience of description in regard to its position on the wafer. The die also has at least one voltage supply (V) pad and at least one ground (G) pad for supplying power to the die. Side A has pad locations  1 - 7 , B has pad locations  1 - 8 , C has pad locations  1 - 8 , and D has pad locations  1 - 9 . The arrangement of the buffer/pad combinations on each side (A, B, C, D) corresponds to the desired pinout of the package that the die will be assembled into, or to signal terminals on a multi-chip module (MCM) substrate onto which the die will be connected.  FIG. 2  is a cutaway side view of the die showing an input pad at D 2  and an output pad at B 2  both connected to the FCL.  
         [0011]      FIG. 3A  shows an example wafer containing  64  of the die of  FIG. 1 .  FIG. 3B  shows the position of each die on the wafer with respect to sides A, D, C, and D. The phantom die in dotted line shows how the wafer would be packed to yield more die per wafer. Notice that even when the die is tightly packed on the wafer (i.e. the phantom die locations utilized), there is still area at the periphery of the wafer where die cannot be placed. This is due to the circular shape of the wafer versus the square/rectangular shape of the die. This unusable peripheral area of the wafer can be used to place test points (pads), test circuitry, and conductors for routing test signals and power and ground to die.  
         [0012]      FIG. 4  shows how conventional die testing is performed using a tester and pad probe assembly. The probe assembly is positioned over a selected die and lowered to make contact with the die pads. Once contact is made the tester applies power and checks for high current. If current is high a short exists on the die and test is aborted and the die is marked (usually by an ink color) as bad. If current is normal, then testing proceeds by applying test patterns to the die and receiving test response from the die. If the test fails the die is marked as bad. If the test passes the die is good and not marked, or if marked, marked with a different ink color. During testing the die current can be monitored to see if it stays within a specified range during the test. An out of range current may be marked as a high current functional failure.  
         [0013]     Such conventional wafer testing has several disadvantages. The act of probing the die scars the metal die pads. Thus, using physical probing, it is essential that dies be tested only once, since re-probing a die to repeat a test may further damage the pads. Even a single probing of a die may cause enough pad damage to adversely affect subsequent assembly of the die in IC packages or on MCM substrates. With the extremely small target provided by a die pad, the equipment used for positioning the probe on a die pad must be designed with great care and is therefore very expensive to purchase/build and maintain and calibrate. Also the stepping of the probe to each die location on the wafer takes time due to the three dimensional motions the probe must be moved through to access and test each die on the wafer.  
         [0014]     It is therefore desirable to test die on wafers without the disadvantages described above.  
         [0015]     The present invention provides: a die framework comprising die resident circuitry and connections to selectively provide either a bypass mode wherein the die has direct pad-to-pad connectivity or a functional mode wherein the die has die pad to functional core logic connectivity; a fault tolerant circuit and method to select a die on a wafer to be placed in functional mode while other die remain in bypass mode; a method and apparatus for (1) electronically selecting one die on a wafer to be placed in functional mode for testing while other die on the wafer are in bypass mode, (2) testing that selected die, and (3) repeating the electronic selection and testing steps on other die; and a method and apparatus for (1) electronically selecting a plurality of diagonally positioned die on the wafer to be placed in functional mode for testing while other die on the wafer are in bypass mode, (2) testing the selected group of diagonally positioned die in parallel, and (3) repeating the electronic selection and testing steps on other groups of diagonally positioned die.  
         [0016]     The present invention provides improved testing of unsingulated die on wafer. The invention provides the following exemplary improvements: (1) electronic selection and testing of unsingulated die on wafer, (2) faster testing of dies on wafer, (3) elimination of expensive, finely designed mechanical wafer probes, (4) the ability to at-speed test unsingulated die on wafer. (5) the ability to test a plurality of unsingulated die in parallel, and (6) the ability to simplify the burn-in testing of unsingulated die.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  diagramatically illustrates the functional core logic, input and output pads and pad buffering structures of a conventional integrated circuit die.  
         [0018]      FIG. 2  is a cutaway side view of a portion of the conventional die of  FIG. 1 .  
         [0019]      FIGS. 3A-3B  illustrate the placement and orientation of a plurality of die on a conventional wafer.  
         [0020]      FIG. 4  illustrates a conventional arrangement for testing die on a wafer.  
         [0021]      FIG. 5  diagramatically illustrates the functional core logic, input and output pads and pad buffering of an exemplary integrated circuit die according to the present invention.  
         [0022]      FIG. 6  schematically illustrates pad-to-pad connections that exist in the die of  FIG. 5  when in a bypass mode.  
         [0023]      FIGS. 7A-11B  are cutaway side views of various portions of the  FIG. 5  die in both functional and bypass modes.  
         [0024]      FIG. 12A  illustrates an exemplary embodiment of the input state holder of  FIG. 7A .  
         [0025]      FIG. 12B  illustrates an alternative to the input state holder of  FIG. 12A .  
         [0026]      FIGS. 13A-14B  are further cutaway side views similar to  FIGS. 7A-11B .  
         [0027]      FIGS. 15A-15B  illustrate an exemplary arrangement for bussing power and ground to each die on a wafer.  
         [0028]      FIGS. 16A-16B  illustrate an exemplary die selection scheme according to the present invention.  
         [0029]      FIGS. 17A-17B  illustrate the operation of the die selectors of  FIG. 16A .  
         [0030]      FIGS. 18A-18C  illustrate the structure and operation of the die selection scheme of  FIG. 16A .  
         [0031]      FIGS. 19A-19C  illustrate a fault tolerant feature of the die selection scheme of  FIG. 16A .  
         [0032]      FIG. 19D  illustrates the die selection scheme of  FIG. 16A  applied to a plurality of die on a plurality of wafers.  
         [0033]      FIGS. 20A-20B  illustrate another exemplary die selection scheme according to the present invention.  
         [0034]      FIGS. 21A-21B  illustrate the operation of the die selectors of  FIG. 20A .  
         [0035]      FIGS. 22A-23B  illustrate the structure and operation of the die selection scheme of  FIG. 20A .  
         [0036]      FIGS. 24A-24C  illustrate a fault tolerant feature of the die selection scheme of  FIG. 20A .  
         [0037]      FIGS. 24D and 24E  diagramatically illustrate an exemplary implementation of the die selector defined in  FIGS. 21A-23B .  
         [0038]      FIGS. 24F and 24G  diagrammatically illustrate an exemplary implementation of the die selector defined in  FIGS. 17A-18C .  
         [0039]      FIGS. 25A-25E  illustrate an exemplary arrangement according to the present invention for testing die on wafer.  
         [0040]      FIG. 26  illustrates a portion of  FIG. 25A  in greater detail.  
         [0041]      FIG. 27  illustrates an exemplary arrangement according to the present invention for testing a plurality of die on wafer in parallel.  
         [0042]      FIG. 28  illustrates an exemplary power and ground bussing arrangement for use with the testing arrangement of  FIG. 27 .  
         [0043]      FIG. 29  illustrates an exemplary die selection scheme for use with the testing arrangement of  FIG. 27 .  
         [0044]      FIG. 30  illustrates a portion of  FIG. 27  in greater detail.  
         [0045]      FIGS. 31A-310  illustrate a sequence of testing steps supported by the arrangement of  FIG. 27 , wherein groups of diagonally positioned die are tested in parallel fashion.  
     
    
     DETAILED DESCRIPTION  
       [0046]     In  FIG. 5 a  die schematic similar to that of  FIG. 1  is shown. Like  FIG. 1 , the die has sides A, B, C, and D and corresponding pad sites for input, output, input/output, V and G.  FIG. 5  includes additional pad sites A 8  and B 9  referred to as bypass, and an additional pad site C 9  referred to as mode. The mode pad is buffered like a data input. When mode is at a predetermined logic level, say high, the die schematic appears as shown in  FIG. 5 , and the die is in its functional mode which is exactly the equivalent of the die in  FIG. 1 . In functional mode, the FCL, input, output, and input/output pads are enabled and the die performs its intended function. In functional mode, the bypass pads are not used.  
         [0047]     In exemplary  FIG. 6 , the die of  FIG. 5  is schematically shown as it would operate in the bypass mode of the present invention. The die is placed in bypass mode by taking the mode pad to a logic state opposite that of the functional mode logic state, in this case a logic low. In bypass mode, the die&#39;s FCL, input, output, and input/output buffers are disabled and pad sites of corresponding position between sides A and C and between sides D and B are electrically connected. In bypass mode the die is transformed into a simple interconnect structure between sides A and C and between sides D and B. The interconnect structure includes a plurality of conductors extending parallel to one another between sides A and C, and a further plurality of conductors extending parallel to one another between sides D and B. While in bypass mode, signals from a tester apparatus can flow through the interconnects between A and C and between D and B to access and test a selected die on a wafer.  
         [0048]     While most bypass connections can be made between existing functionally required pad sites, the number of functional pad sites on one side may not equal the number of functional pad sites on the opposite side. Thus the bypass pads of  FIG. 5  provide pad-to-pad connectivity when the number of pads on opposite sides are not equal. For example, in  FIGS. 5 and 6 , bypass pads A 8  and B 9  provide connecting pads for functional pads C 8  and D 9  respectively. The bypass connections between opposite side die pads form a low impedance, bidirectional signaling path through the die from pad to pad. The bypass connections between two sides are preferably designed to have an equal propagation delay between opposite side pads to avoid skewing of test signals passed through bypassed die.  
         [0049]     Assuming for example the die positioning shown on the wafer of  FIG. 3A , the sides of a die selected for testing need to be driven by signals from the adjacent sides of top, right, bottom, and left neighboring die which are in bypass mode ( FIG. 6 ). In order for the neighboring die to be tested, it is placed in functional mode, and: (1) all signals required at its A side are provided at the C side of the top neighboring bypassed die, (2) all signals required at its B side are provided at the D side of the right neighboring bypassed die, (3) all signals required at its C side are provided at the A side of the bottom neighboring bypassed die, and (4) all signals required at its D side are provided at the B side of the left neighboring bypassed die.  
         [0050]      FIGS. 7 through 14  depict cross section views of example circuitry and connections which can achieve the framework for selective die functional and bypass modes.  
         [0051]     Exemplary  FIGS. 7A and 7B  illustrate side views of the D 1  input pad and the B 1  3-state output pad of the die in  FIGS. 5 and 6 . A switch  71  is provided between the input pad and input buffer to allow isolating the input pad from the input buffer during bypass mode, and an input state holder (ISH) circuit is provided between the switch and input buffer to allow holding a predetermined input state to the input buffer (which drives the FCL) while the switch is open during bypass mode. Gating circuitry, such as an AND gate (A), is provided in the control path between the FCL and 3-state output buffer to allow the 3-state output buffer to be disabled during bypass mode. A selectable connection path  73  between the input and output pad includes a conductor  75  connected between a switch  77  associated with the input pad and a switch  79  associated with the output pad, which switches are operable to connect conductor  75  to G or to the input and output pads. The mode pad is connected to the switches. ISH and gate A as shown such that when the mode pad is in one logic state the die is in functional mode and when in the opposite logic state the die is in bypass mode. The mode pad can be connected to FCL as shown to permit disabling of clocks or other operations in FCL during bypass mode.  
         [0052]     As shown in exemplary  FIG. 12A , ISH can be realized with a 3-state data buffer having a data input connected to a desired logic level (logic “1” in this example) and a data output connected to the input of the input buffer and a 3-state control input connected to the mode pad. The desired logic level for a given FCL input could be, for example, a logic level which minimizes current flow in the FCL during bypass mode. The 3-state buffer is enabled during bypass mode and 3-stated during functional mode. If the desired logic level is a don&#39;t care condition, then the bus holder BH of exemplary  FIG. 12B  can be used to hold the last input logic level during bypass mode.  
         [0053]     When in functional mode ( FIG. 7A ), the switches  77  and  79  connect the conductor  75  to G which provides a ground plane on the conductor and prevents AC coupling between the input and output pads. When in bypass mode ( FIG. 7B ), the switches  77  and  79  and the conductor  75  provide a low impedance, bidirectional signaling path connection between the input and output pads. In bypass mode, switch  71  is open to isolate FCL from the input pad, and the 3-state output buffer is disabled (3-stated) via AND gate A to isolate FCL from the output pad.  
         [0054]     The examples of  FIGS. 8-11  show the use of the bypass circuitry with other types of pad buffers.  FIGS. 13 and 14  show the use of the bypass circuitry between functional input (D 9 ) and bypass (B 9 ) pads, and functional output (C 8 ) and bypass (A 8 ) pads.  
         [0055]     In  FIGS. 8A and 8B , a further switch  81  is used to isolate the 2-state output buffer from output pad B 2  during bypass mode.  FIGS. 8C and 8D  are similar to  FIGS. 8A and 8B  except a 3-state output buffer is used instead of a 2-state output buffer and switch  81 , in order to eliminate the impedance of switch  81  during functional mode.  
         [0056]     The input pads in  FIGS. 9A and 9B  and the 3-state output pads in  FIGS. 10A and 10B  are arranged in the manner described above with respect to  FIGS. 7A and 7B .  
         [0057]      FIGS. 11A and 11B  illustrate I/O pads with 3-state (I/O1) and open drain (I/O2) outputs. The input buffers and the 3-state output buffer of  FIGS. 11A and 11B  are arranged as described above with respect to  FIGS. 7A and 7B . The open drain output buffer of  FIGS. 11A and 11B  has its input connected to an output of an OR gate (O) which has one input driven by FCL and another input driven by the logical inverse of the mode signal, whereby the open drain output will float high during bypass mode assuming that the mode signal selects bypass mode when low.  
         [0058]     The input pad in  FIGS. 13A and 13B , and the 3-state output pad in  FIGS. 14A and 14B  are arranged in the manner described above with respect to  FIGS. 7A and 7B .  
         [0059]      FIG. 15A  illustrates an example of how wafer voltage (WV) and wafer ground (WG) bussing can be distributed to the V and G pads of each die on the wafer. The WV bussing is shown originating from areas of the wafer designated as probe area PA 1  and probe area PA 2 . The WG bussing is shown originating from probe area PA 3  and probe area PA 4 . Probe areas PA 1 - 4  are positioned at the periphery of the wafer and in areas where die cannot be placed, as mentioned in regard to  FIG. 3A .  FIG. 15B  illustrates how WV and WG are coupled to the V and G die pads (see  FIGS. 1 and 5 ) through diodes. By placing diodes between WG and G and WV and V, conventional localized probing and power up of an individual die can occur without powering up neighboring dies.  
         [0060]      FIG. 16A  illustrates an exemplary scheme for performing fault tolerant selection of unsingulated die on wafer. The scheme involves the placement of a small circuit, referred to as a die selector  161 , in the scribe lane adjacent each die on the wafer. The die selector  161  shown in  FIG. 16B  includes an I/O terminal S 1 , an I/O terminal S 2 , a mode output terminal, and connections to WV and WG for power. The die selector&#39;s mode output is connected to the mode pad of an associated die. The die selectors are connected in series via their S 1  and S 2  terminals. In the example of  FIG. 16A , S 1  of the first die selector in the series (at die  1 ) is connected to PA 4 , and S 2  of the last die selector in the series (at die  64 ) is connected to PA 3 . Because the die selector is placed in the scribe lane instead of on the die, the mode pad of the die can be physically probed if required, to override the die selector mode output. This feature permits any die to be tested using the conventional probe testing technique. Because the mode output of the die selector drives only the mode pad of a single die, it can be designed with a relatively weak output drive so that the conventional probe tester can easily override the mode output without any damage to the mode output.  
         [0061]     Power is applied to WV and WG by probing PA 1 -PA 4 . When power is first applied, all the die selectors get reset to a state that forces their mode outputs low, which causes all die to be placed in bypass mode. If excess current is detected at power up (indicating perhaps a short between WV and WG), the wafer can be powered down and tested using the traditional mechanical probing technique (note that the diodes of  FIG. 15B  allow for this). If normal current is detected (meaning that all die have successfully powered up in bypass mode) further testing according to the present invention may be performed.  
         [0062]     Before testing die, the integrity of the serially connected die selectors  161  can be tested. Testing of the die selectors can occur by injecting clock pulses from PA 4  to S 1  of the upper left die selector (adjacent die  1 ) and monitoring S 2  of the lower left die selector (adjacent die  64 ) at PA 3 . If the serial path between the die selectors is intact, a clock pulse output will occur on lower left S 2  after  65  clock pulses have been applied to upper left SI. On the falling edge of the first injected clock pulse, die  1  is switched from bypass mode to functional mode by the mode output of the associated die selector going high. All other die are forced to remain in bypass mode by their die selectors&#39; mode outputs being low. Also on the falling edge of the first injected clock pulse, the upper left die selector connects its S 1  and S 2  terminals so that subsequent S 1  clocks are output on S 2 . On the rising edge of the second injected clock pulse, die  1  is placed back into bypass mode by its die selector&#39;s mode output going low. This second clock pulse is transferred through the upper left die selector to the next die selector via the S 1  to S 2  connection. On the falling edge of the second clock pulse, the die  2  selector connects its S 1  and S 2  terminals and switches die  2  from bypass to functional mode by driving the mode output high. This process continues on to die  64  and its die selector. On the rising edge of the 65th injected clock pulse, die  64  is placed back into the bypass mode by its die selector&#39;s mode output going low, and the 65th clock pulse is output from S 2  to PA 3 .  
         [0063]     Also, during the die selector test the current flow to and/or from the wafer via WV and WG can be monitored during each rising and falling clock edge to see if the expected current increase and decrease occurs as each die transfers in sequence between bypass and functional modes. By sensing the wafer current fluctuations, it is possible to detect when a die that should be selected (i.e. in functional mode) is not selected, which could indicate a defect in the die selector arrangement as discussed further below.  
         [0064]     The above description illustrates how to test and operate the die selector path from PA 4  to PA 3 . The same test and operation mode is possible by clocking S 2  of the lower left die selector from PA 3  and monitoring S 1  of the upper left die selector at PA 4 . The die selector model of exemplary  FIG. 17A  and state diagram of exemplary  FIG. 17B  illustrate die selector operation modes in detail. From  FIG. 17B  it is seen that the die selector responds to a first received S 1  or S 2  clock pulse to output mode control (on the falling edge) to place the connected die in functional mode so that it can be tested. After the die is tested, a rising edge on the same signal (say S 1 ) causes the tested die to be placed back into bypass mode and also drives the S 1  input of the next die selector. On the next successive falling edge the die associated with the next die selector is switched into functional mode for testing. And so on.  
         [0065]     Exemplary  FIGS. 18A-18C  illustrate in detail the die selector operation described above. PS 1  and PS 2  in  FIG. 18  are externally accessible terminals (like PA 3  and PA 4 ) for injecting and receiving clock pulses. Note that the die selectors operate bidirectionally as mentioned above. The reason for the bidirectional operation is for fault tolerance, i.e. a broken connection between two die selectors can be tolerated. An example of the fault tolerant operation of the die selector is shown in  FIGS. 19A-19C . In  FIG. 19A  an open circuit fault exists between the 2nd and 3rd die selectors. PS 1  clock activations can only select die  1  and  2  ( FIG. 19B ). However, PS 2  clock activations can select die  5 ,  4 , and  3  ( FIG. 19C ). Thus even with an open circuit the die selector arrangement is able to select and place a given die in functional mode for testing.  
         [0066]     Wafers such as shown in  FIG. 16A  may also be connected in series via the S 1 /S 2  signals to allow selection of die on many wafers as shown in  FIG. 19D . S 2  of the lower left die of wafer  191  is connected, via PA 3  of wafer  191  and external conductor  193  and PA 4  of wafer  195 , to S 1  of the upper left die of wafer  195 . An analogous connection also exists between wafers  195  and  197 . External probe connections at PA 4  of wafer  191  and PA 3  of wafer  197  permit the die selection scheme described above with respect to  FIGS. 16A-18C  to be applied to die on plural wafers.  
         [0067]     Exemplary  FIGS. 20 and 21  illustrate how to further improve die selector fault tolerance by the addition of a second pair of I/O terminals S 3  and S 4  in die selector  201 . In  FIG. 20A , the S 3  and S 4  serial connection path is shown routed between PA 1  and PA 2  in the vertical scribe lanes. Separating the S 1 /S 2  (horizontal scribe lanes) and S 3 /S 4  (vertical scribe lanes) routing is not required, and both routings could be in the same horizontal or vertical lanes if desired. It is clear in the example of  FIG. 20A  that routing S 1  and S 2  in the horizontal lanes and routing S 3  and S 4  in the vertical lanes will result in different die selection orders, i.e. S 1  and S 2  select die order  1 ,  2 ,  3  . . .  64  or die order  64 ,  63 ,  62  . . .  1 , whereas S 3  and S 4  select die order  1 ,  16 ,  17 , . . .  64  . . .  8  or die order  8 ,  9 ,  24  . . .  1 .  
         [0068]     Exemplary  FIGS. 21A and 21B  illustrate the model and state diagram of the improved fault tolerant die selector  201  of  FIGS. 20A and 20B . The operation of the die selector  201  of  FIG. 21A  is similar to that of the die selector  161  of  FIG. 17A  except that the die selector  201  has redundant bidirectional selection paths. Redundant selection paths allow the die selector  201  to maintain operation even when one of its selection paths is rendered inoperable by gross defects that defeat the fault tolerance features provided in the single path die selector  161  of  FIG. 17A .  
         [0069]     In  FIGS. 22A-24C  operational examples using dual selection path die selectors  201  are shown. For clarity, the examples show both paths (S 1  and S 2 , and S 3  and S 4 ) routed together (in same scribe lanes) to the same sequence of die  1  through  5 . This differs from the example routing of  FIG. 20A  where S 1  and S 2  are routed in horizontal lanes and S 3  and S 4  are routed in vertical lanes, and thus each path has a different sequence of die selection.  FIG. 22B  shows PS 1  selecting die in the order  1 ,  2 ,  3 ,  4  &amp;  5 .  FIG. 22C  shows PS 2  selecting die in the order  5 ,  4 ,  3 ,  2  &amp;  1 .  FIG. 23A  shows PS 3  of  FIG. 22A  redundantly selecting die in the same order as PS 1  ( FIG. 22B ).  FIG. 23B  shows PS 4  of  FIG. 22A  redundantly selecting die in the same order as PS 2  ( FIG. 22C ). Both paths can tolerate a single defect (open circuit) as shown in  FIGS. 19A-19C .  
         [0070]     However,  FIG. 24A  shows a multiple defect example (two open circuits) on the S 1  and S 2  path that would disable access to intermediate die  2 ,  3  &amp;  4  if only the S 1  and S 2  path were provided.  FIGS. 24B-24C  illustrate that PS 1  can only select die  1 , and PS 2  can only select die  5  with the defects shown in  FIG. 24A . However, since redundant selection paths are provided in the die selectors  201  of  FIG. 24A , the S 3  and S 4  path can be used to select die  2 ,  3  &amp;  4  as illustrated in  FIGS. 23A-23B . Thus an advantage of die selector  201  is that it can maintain access to die even if one of the paths is critically disabled by multiple defects.  
         [0071]      FIGS. 24F and 24G  illustrate an exemplary implementation of the die selector  161  defined in  FIGS. 17A-18C . In  FIG. 24F , input terminals S 1  and S 2  are respectively connected to inputs S 1 IN and S 2 IN of a die selector state machine  241  via respective input data buffers  243  and  245 . The die selector state machine  241  outputs the mode signal and enable signals S 1 ENA and S 2 ENA. Enable signals S 1 ENA and S 2 ENA respectively control output data buffers  247  and  249 . The output of input data buffer  243  is connected to the input of output data buffer  249  to permit signals received at terminal S 1  to be output on terminal S 2  when enable signal S 2 ENA enables output data buffer  249 . Similarly, the output of input data buffer  245  is connected to the input of output data buffer  247  to permit signals received at terminal S 2  to be output on terminal S 1  when enable signal S 1 ENA enables output data buffer  247 .  
         [0072]     Exemplary  FIG. 24G  illustrates the die selector state machine  241  of  FIG. 24F  in greater detail. A conventional power-up reset circuit initially clears D flip-flops  251 ,  253  and  255  when the die selector is initially powered up. The pass signal output from flip-flop  255  is inverted at one input of AND gate  259 . The other input of AND gate  259 , which is driven by the output of OR gate  257 , is thus qualified at gate  259  by the pass signal upon initial power up. Because flip-flop outputs QS 1  and QS 2  are low after initial power-up, the mode signal is therefore low after power-up. Noting that QS 1  is connected to S 2 ENA and QS 2  is connected to S 1 ENA, it is seen from  FIG. 24F  that output data buffers  247  and  249  are initially disabled after power-up. Because signal QS 1  is initially low, signal S 2 IN is initially qualified at AND gate  261 , and because signal QS 2  is initially low, signal S 1 IN is also initially qualified at AND gate  263 . The low levels of QS 1  and QS 2  also drive the D input of flip-flop  255  low via OR gate  265 . The outputs of AND gates  261  and  263  are connected to respective inputs of OR gate  271  whose output drives the clock inputs of flip-flops  251 ,  253  and  255 . The output of AND gate  261  is connected to the D input of flip-flop  253  via delay element  267 , and the output of AND gate  263  is connected to the D input of flip-flop  251  via delay element  269 . Delay elements  267  and  269  are designed to have a propagation delay which is greater than the propagation delay of OR gate  271 .  
         [0073]     A rising edge of a first clock pulse on S 1  IN causes a logic zero to be clocked through flip-flop  255 , thereby maintaining the pass signal at its initial low state. When the falling edge of the clock pulse occurs and propagates through OR gate  271  to clock flip-flop  251 , the D input of flip-flop  251  will still be high due to the delay element  269 , thus causing flip-flop output QS 1  to go high. With QS 1  high, the mode signal is driven high via OR gate  257  and AND gate  259 . Also with QS 1  high, the output data buffer  249  of  FIG. 24F  is enabled via signal S 2 ENA, the input S 2 IN is disqualified at AND gate  261 , and the D input of flip-flop  255  is driven high via OR gate  265 . Thus, the rising edge of the second clock pulse on terminal S 1  of  FIG. 24F  passes directly to terminal S 2  via output data buffer  249 , and also passes through AND gate  263  and OR gate  271  of  FIG. 24G  to clock flip-flop  255  and take the pass output thereof high, thereby driving the mode signal low. The next falling edge on terminal S 1  will pass through data output buffer  249  to terminal S 2 , and will maintain the QS 1  output of flip-flop  251  in the high logic state. The positive edge of the third clock pulse received on terminal S 1  will pass through data output buffer  249  to terminal S 2 , and will also clock a logic one through flip-flop  255  so that the pass signal will maintain the mode output low via AND gate  259 . The negative edge of the third clock pulse will maintain the logic one at the QS 1  output of flip-flop  255 . Each successive clock pulse after the third clock pulse on terminal S 1  will achieve the same results as described with respect to the third clock pulse.  
         [0074]     The bidirectional feature of die selector  161  should be apparent from  FIGS. 24F and 24G . That is, if a succession of clock pulses had occurred on terminal S 2  rather than on terminal S 1 , then output QS 2  of flip-flop  253  would have been driven high to enable data output buffer  247  and disable the S 1 IN signal via AND gate  263 . The mode signal behaves exactly the same in response to a succession of clock pulses on terminal S 2  as described above with respect to the succession of clock pulses on terminal S 1 , and the terminal S 1  will receive the second and all successive clock pulses input on terminal S 2 .  
         [0075]     Exemplary  FIGS. 24D and 24E  show an implementation of die selector  201  which is similar to the implementation of die selector  161  illustrated in  FIGS. 24F and 24G . Referencing  FIG. 24D , the output of data input buffer  243  is connected to the input of data output buffer  249  as in  FIG. 24F , and the output of data input buffer  245  is connected to the input of data output buffer  247  as in.  FIG. 24F . Similarly, the output of data input buffer  275  is connected to the input of data output buffer  277 , and the output of data input buffer  281  is connected to the input of data output buffer  279 .  
         [0076]     The die selector state machine  273  of  FIG. 24D  is shown in greater detail in  FIG. 24E . As seen from  FIG. 24E , the die selector state machine  273  of  FIG. 24D  represents an extension of the die selector state machine of  241  of  FIG. 24G . An additional AND gate  287 , delay element  293 , and flip-flop  283  have been added for terminal S 3 , and an additional AND gate  289 , delay element  291  and flip-flop  285  have been added for terminal S 4 . The operation of these additional elements is identical to the operation described above with respect to the corresponding elements in  FIG. 24G . Similarly to the operation described above with reference to  FIG. 24G , a first falling clock pulse edge on terminal S 3  will result in the QS 3  output of flip-flop  283  going high to drive the mode signal high and to enable the data output buffer  277  to connect terminal S 3  to terminal S 4 . The rising edge of the second clock pulse on terminal S 3  will clock a logic one through flip-flop  255  so that the pass signal will drive the mode signal low again via AND gate  259 . Similarly, the falling edge of a first clock pulse on terminal S 4  will drive high the QS 4  output of flip-flop  285 , which drives the mode signal high and enables data output buffer  279  to connect terminal S 4  to terminal S 3 . The decoder circuit  291  receives QS 1 -QS 4  as inputs and provides DS 1 -DS 4  as outputs. When QS 1  is active high, the decoder circuit  291  drives DS 2 -DS 4  active high, which disables signals S 2 IN, S 3 IN and S 4 IN at AND gates  261 ,  287  and  289 . Similarly, when signal QS 2  is active high, the decoder circuit drives signals DS 1 , DS 3  and DS 4  active high, when signal QS 3 , is active high, the decoder circuit drives signals DS 1 , DS 2  and DS 4  active high, and when QS 4  is active high, the decoder circuit drives signals DS 1 -DS 3  active high.  
         [0077]     Referencing exemplary  FIGS. 25A and 25D , probe test pads in PA 1  are bussed (via A-Bus) to one side of eight top column switch groups (TC 1 - 8 ), representative switch group TC 8  being shown in  FIG. 25D . Each top column switch group also receives a select top column signal (such as STC 8 ) from PA 1  that opens or closes the switches. The other side of each top column switch group is bussed to the A side (recall  FIG. 5 ) pads of die  1 , 2 ,  3 , 4 ,  5 ,  6 ,  7 , and  8 .  
         [0078]     Also referencing  FIG. 25C , probe test pads in PA 2  are bussed (via B-Bus) to one side of eight right row switch groups (RR 1 - 8 ), representative switch group RR 8  being shown in  FIG. 25C . Each right row switch group also receives a select right row signal (such as SRR 8 ) from PA 2  that opens or closes the switches. The other side of each right row switch group is bussed to the B side pads of die  8 ,  9 ,  24 ,  25 ,  40 ,  41 ,  56 , and  57 .  
         [0079]     Referencing also  FIG. 25E , probe test pads in PA 3  are bussed (via C-Bus) to one side of eight bottom column switch groups (BC 1 - 8 ), representative switch group BC 1  being shown in  FIG. 25E . Each bottom column switch group also receives a select bottom column signal (such as SBC 1 ) from PA 3  that opens or closes the switches. The other side of each bottom column switch group is bussed to the C side pads of die  57 ,  58 ,  59 ,  60 ,  61 ,  62 ,  63 , and  64 .  
         [0080]     Referencing also  FIG. 25B , probe test pads in PA 4  are bussed (via D-Bus) to one side of eight left row switch groups (LR 1 - 8 ), representative switch group LR 1  being shown in  FIG. 25B . Each left row switch group also receives a select left row signal (such as SLR 1 ) from PA 4  that opens or closes the switches. The other side of each left row switch group is bussed to the D side pads of die  1 ,  16 ,  17 ,  32 ,  33 ,  48 ,  49 , and  64 .  
         [0081]     PA 1 - 4 , the switch groups, and bussing to correct them can all be located in the unusable peripheral area (recall  FIG. 3A ) of the wafer.  
         [0082]     As shown in the detailed example of  FIG. 26 , each die on the wafer, excluding the boundary die, such as die  1 ,  2 ,  3 ,  16 ,  17  etc. is connected at its top (A), right (B), bottom (C) and left (D) side pad sites to neighboring die pad sites by way of short busses that bridge across the scribe lanes between the die. Due to the regularity of the die and their positioning on the wafer, vertical pad bussing is provided between each neighboring die on sides A and C, and horizontal pad bussing is provided between each neighboring die on sides B and D. The pads of boundary die are similarly bussed to neighboring die pads, but only on at most three sides, since at least one of the boundary die sides will always be connected to a switch group.  
         [0083]     Although not shown in  FIG. 25A , the wafer also comprises: (1) die having selectable functional and bypass modes as described in  FIGS. 5-14 , (2) WV and WG bussing as shown in  FIGS. 15A-15B , and (3) fault tolerant die selectors as described in  FIGS. 16-24 .  
         [0084]     Each switch group, when selected (switches closed), provides a low impedance, bidirectional signaling path. Also the bussing connections between PA 1 - 4  and the switch groups (TC 1 - 8 , LR 1 - 8 , BC 1 - 8 , RR 1 - 8 ), between the switch groups and the die sides (A, B, C, D), and between the die sides, provide a low impedance, bidirectional signaling path. As previously mentioned, the die&#39;s internal bypass pad-to-pad connections also provide low impedance, bidirectional signaling paths.  
         [0085]     When testing is to be performed, a probe is positioned onto the wafer at the pad areas PA 1 - 4 . PA 1 - 4  are large compared to the pad area of each die, and therefore the mechanical requirements of the probe design are simpler ad less costly than conventional probes which are elegantly designed for contacting tiny die pads. Also, since the present invention allows for a die to be electronically selected for testing, the probe needs to be positioned onto the wafer only once, which reduces test time when compared to conventional multiple probing of a wafer. This test time reduction can significantly decrease the cost of wafer testing, which in turn decreases the cost of the die and packaged IC. Also, since the probe does not contact any die pads, no damage to die pads occurs during the wafer probe and die test procedure. Furthermore, the relatively large probe target area provided by PA 1 - 4  lends itself well to computer controlled and automated test probing processes.  
         [0086]     After the probe contacts the wafer at PA 1 - 4 , power is applied to the wafer to power up the die and die selectors. The tester can quickly detect a high current situation and remove power from the wafer as necessary. Wafer processing faults could cause shorts between WG and WV bussing or a die or die selector could have a V and G short. If the wafer fails the high current test at power up, die testing can still be done by conventional die probing techniques.  
         [0087]     If the wafer exhibits normal current flow at power up, the die selectors can be tested as previously described with regard to  FIGS. 16-24 . If the die selectors fail in all fault tolerant modes, the wafer can still be tested conventionally. If the die selectors pass, the row and column bussing paths can be tested. To test row  1  and column  1  (FIGS.  25  and  26 ), the LR 1 , RR 1 , TC 1  and BC 1  switch groups are closed and, with all die in bypass mode, an external tester (such as in  FIG. 4 ) passes signals between PA 4  and PA 2  to test row  1  bussing and between PA 1  and PA 3  to test column  1  bussing. This step tests, (1) the PA 1 - 4  to switch group bussing, (2) the switch group closures, (3) the switch group to boundary die bussing, (4) the die bypass mode, and (5) the die-to-die pad bussing. This step is repeated on all rows and columns. If a row or column fails, die associated with that row and column can be tested conventionally. After testing row and column connectivity, the die can be tested.  
         [0088]     The die test starts by outputting a first pulse to S 1  (could be S 2 , or S 3  or S 4  if die selector  201  is used) from PA 4  to cause the upper left die selector to switch die  1  from bypass to functional mode, and then closing switch groups LR 1 , TC 1 , RR 1  and BC 1 , and then testing die  1  using the external tester which is connected to die  1  via PA 1 - 4 , the closed switch groups and the row  1  and column  1  bussing paths. This test sequence is repeated on all die on the wafer.  FIG. 26  illustrates in detail the testing of die  15  via the row  2  and column  2  bussing paths. Different types of testing can be performed on a selected die. A first test is a DC test where the objective is to verify the I/O parametrics and the logical correctness of the die. A second test is a functional test wherein the die is functionally tested at its intended operating speed. Some high reliability applications require an environmental (or burn in) test where the die is tested in chambers where temperature, humidity, and vibration can be cycled during testing. Die that pass DC testing may fail functional and environmental testing, so at wafer level it is important to test die in DC, functional, and perhaps environmental test mode to prevent bad die from being packaged into IC form or assembled on MCMs.  
         [0089]     To perform die testing, it is important to provide relatively high performance bussing paths, i.e. all the wafer routed bussing, the die bypass mode pad-to-pad connectivity bussing, and the switch group switches are preferably designed for low impedance and bidirectional signaling. In the die  15  test example of  FIG. 26 , the D and A sides of die  15  receive test signaling from PA 4  and PA 1  through only bypassed die  16  and  2  respectively, whereas test signaling at sides B and C of die  15  must traverse more than one bypassed die (see  FIG. 25A ) before arriving from PA 2  and PA 3 , respectively. The die bypass signaling delay and die-to-die bussing delays can easily be modeled in tester software so that the tester can compensate for the delays through row and column bussing paths that traverse different numbers of die in bypass mode. In this way, test signaling between the tester and target die under test will occur correctly, independent of the number of bypassed die that exists in the row and column bussing paths connected to the A, B, C, and D sides of the die under test.  
         [0090]     In exemplary  FIG. 27 , a wafer bussing structure is shown where each row and column has its own pair of probe areas. For example probe area left row  1  (PALR 1 ) and probe area right row  1  (PARR 1 ) serve as the row  1  probe areas, and probe area top column  1  (PATC 1 ) and probe area bottom column  1  (PABC 1 ) serve as the column  1  probe areas. The die-to-die bussing is the same as described previously relative to  FIGS. 25-26 . Also the probe areas can exist in the unused peripheral area of the wafer. Optionally, the probe areas could be eliminated altogether and the pad sites at the A, B, C and D sides of the top, right, bottom, and left boundary die could be probed if desired.  FIG. 28  illustrates an example of how each row can be supplied, via its left and right probe areas PALRn and PARRn, with a unique V and G connection.  FIG. 29  illustrates how each row can be supplied, via its left and right probe areas PALRn and PARRn, with a unique die selector signaling connection. The power and die selector connections could also be arranged column-wise so that PATCn and PABCn would provide each column with unique power supply and die selection.  
         [0091]     Exemplary  FIG. 30  illustrates in detail how diagonally positioned die  17 ,  15 , and  3  are tested in parallel. If a group of diagonally positioned die are placed in functional mode (via each row&#39;s independently operated die selectors of  FIG. 29 ) while all other die are in bypass mode, then further test time reduction can be achieved by parallel (i.e. simultaneous) testing of the group of diagonally positioned die via the dedicated row and column bussing paths and probe areas shown in  FIG. 30 .  FIGS. 31A through 31O  illustrate the parallel die testing approach as it proceeds across all groups of diagonally positioned die on the wafer. These steps of parallel die testing are listed below, using the die numbering of  FIG. 27 .  
         [0092]     Step  1 —Select and Test die  1  ( FIG. 31A ).  
         [0093]     Step  2 —Select and Test die  16  and  2  ( FIG. 31B ).  
         [0094]     Step  3 —Select and Test die  17 ,  15 , and  3  ( FIG. 31C ).  
         [0095]     Step  4 —Select and Test die  32 ,  18 ,  14 , and  4  ( FIG. 3D ).  
         [0096]     Step  5 —Select and Test die  33 ,  31 ,  19 ,  13 , and  5  ( FIG. 3 E ).  
         [0097]     Step  6 —Select and Test die  48 ,  34 ,  30 ,  20 ,  12 , and  6  ( FIG. 31F ).  
         [0098]     Step  7 —Select and Test die  49 ,  47 ,  35 ,  29 ,  21 ,  11 , and  7  ( FIG. 31G ).  
         [0099]     Step  8 —Select and Test die  64 ,  50 ,  46 ,  36 ,  28 ,  22 ,  10 , and  8  ( FIG. 31H ).  
         [0100]     Step  9 —Select and Test die  63 ,  51 ,  45 ,  37 ,  27 ,  23 , and  9  ( FIG. 311 ).  
         [0101]     Step  10 —Select and Test die  62 ,  52 ,  44 ,  38 ,  26 , and  24  ( FIG. 31J ).  
         [0102]     Step  11 —Select and Test die  61 ,  53 ,  43 ,  39 , and  25  ( FIG. 31K ).  
         [0103]     Step  12 —Select and Test die  60 ,  54 ,  42 , and  40  ( FIG. 31L ).  
         [0104]     Step  13 —Select and Test die  59 ,  55 , and  41  ( FIG. 31M ).  
         [0105]     Step  14 —Select and Test die  58  and  56  ( FIG. 31N ).  
         [0106]     Step  15 —Select and Test die  57  ( FIG. 310 ).  
         [0107]     The foregoing die test sequence notwithstanding, the die can be grouped as desired for parallel testing, so long as each die of the group is row and column accessible independently of all other die of the group. As another example, and using the die numbering of  FIG. 27 , each of the following eight die groups can be tested in parallel to achieve an eight-step test sequence.  
         [0108]     Step  1 —Select and Test die  1 ,  9 ,  23 ,  27 ,  37 ,  45 ,  51  and  63 .  
         [0109]     Step  2 —Select and Test die  2 ,  16 ,  24 ,  26 ,  38 ,  44 ,  52  and  62 .  
         [0110]     Step  3 —Select and Test die  3 ,  15 ,  17 ,  25 ,  39 ,  43 ,  53  and  61 .  
         [0111]     Step  4 —Select and Test die  4 ,  14 ,  18 ,  32 ,  40 ,  42 ,  54  and  60 .  
         [0112]     Step  5 —Select and Test die  5 ,  13 ,  19 ,  31 ,  33 ,  41 ,  55  and  59 .  
         [0113]     Step  6 —Select and Test die  6 ,  12 ,  20 ,  30 ,  34 ,  48 ,  56  and  58 .  
         [0114]     Step  7 —Select and Test die  7 ,  11 ,  21 ,  29 ,  35 ,  47 ,  49  and  57 .  
         [0115]     Step  8 —Select and Test die  8 ,  10 ,  22 ,  28 ,  36 ,  46 ,  50  and  64 .  
         [0116]     The above-described parallel testing of die on wafer can reduce wafer test time as compared to individual, sequential testing of die on wafer.  
         [0117]     The present invention is also applicable to IDDQ testing of each die on the wafer. IDDQ testing is the monitoring of current to an IC/die during the application of test patterns. A higher than expected current at a particular test pattern may indicate a defect. The WV and WG bussing arrangement of  FIG. 15A  is adequate when performing IDDQ testing in the one-die-at-a-time arrangement of  FIGS. 25-26 , because any unexpected current on WV and/or WG can be attributed to the one die that is in functional mode. As to the parallel die testing arrangement of  FIGS. 30-31 , row-specific V and G bussing of the type shown in  FIG. 28  permits unexpected V and G current to be attributed to the correct die of the diagonal grouping being tested. If this capability is not desired in the test arrangement of  FIGS. 30-31 , then the WV and WG bussing of the type shown in  FIG. 15A  can be used in  FIGS. 30-31 . For example, an additional probe access area could be provided for power supply bussing, in which case PALRn and PARRn need not provide power.  
         [0118]     As mentioned above, the present invention permits the tester probe design to be greatly simplified relative to prior art designs, resulting in less expensive testers. Thus, even the IC vendor&#39;s customers can afford to maintain their own wafer tester. This permits the vendor to sell complete wafers (rather than singulated die) to customers, who can then repeat the vendor&#39;s wafer test and verify the results, and then advantageously singulate the die for themselves. The vendor is thus relieved of the risk of damaging die during singulation, while the customers can advantageously obtain unpackaged die (on wafer), verify that the die have not been damaged in transit from the vendor, and then singulate the die themselves.  
         [0119]     Although exemplary embodiments of the present invention are described above, this description does not limit the scope of the invention, which can be practiced in a variety of embodiments.