Abstract:
Testing of an electronic device is carried out by combining power and signal delivery on a single pair of wires. The power delivery is decoupled from the signal delivery, using inductors, so the device power supplied does not interfere with the test signals delivered from the device and the response signals delivered to the device. Further, simultaneous bidirectional signal paths are decoupled, using capacitors, so that the tester transceiver and the device transceiver are not damaged by the power delivered to the device on the same wires. A common fixture may be used to test a number of different types of wafers, independent of the topography, size, or power requirements of the devices on the wafers, resulting in a significant cost saving, because fixture design has become very expensive, in some cases costing more than the tester whose signals it is implemented to deliver.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electronic device testing, and more particularly to integrated circuit (IC) testing using few device pins. 
   2. Description of the Related Art 
   Reduced pin count testing of electronic devices has been implemented in various ways. One way is to incorporate built-in self test (BIST) circuits into the device design. During testing, the BIST circuit translates incoming signals on a few pins into tests required to test and diagnose the device under test (DUT) and returns response signals containing test results. 
   Another way is to employ simultaneous bidirectional signaling (SBS) to combine the input to the DUT and the output from the DUT on a single line. This technique is described in a commonly owned application entitled, “A Very Small Pin Count IC Tester,” Ser. No. 10/376,025, filed Feb. 27, 2003, the entire contents of which are incorporated by reference herein. The use of SBS allows a single line to be used simultaneously for both input and output for the DUT. Hence, the time required for the test as well as the number of pins involved with the test are reduced. 
   Even with these reductions in the pin count and the resulting increase in the parallelism of the testing and decrease in the overall cost of testing multiple devices on a wafer, testing still remains very expensive. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to provide a reduced pin count or, more generally, reduced connection count, test method and apparatus that reduces the overall cost of testing electronic devices, in particular those electronic devices that employ high-speed differential serial data streams for signaling. 
   The above object is achieved by employing simultaneous bidirectional signaling for test and response signals and combining device power and signal delivery on a single pair of wires. The power delivery is decoupled from the signal delivery, using inductors, so the device power supplied does not interfere with the test signals delivered from the device and the response signals delivered to the device. Further, SBS paths are decoupled, using capacitors, so that the tester transceiver and the device transceiver are not damaged by the power delivered to the device on the same wires. 
   The invention may be applied to testing of wafers having bump arrays that are uniform. It is noted that many wafers already have uniform bump arrays, because a specific bump pattern is required for each type of device that is formed on a wafer, and a plurality of identical devices are fabricated on a single wafer. 
   As will be described in more detail below, a common fixture may be used for a number of different types of wafers, independent of the topography, size, or power requirements of the devices on the wafers. The one requirement for using a common fixture is that the bumps on the tested wafer must be applied in view of the common fixture&#39;s layout so that they are aligned with the output lines on the fixture and connect to all of the signal circuits and power grids that are used in testing the ICs on the wafer. 
   ICs with a limited number of bumps may be designed with one test circuit and one power grid connected to a single pair of bumps. During testing of a wafer containing these ICs, power and signal delivery for each of the ICs are combined on a single pair of wires. 
   Larger ICs generally have higher power requirements and are designed with more than one power grid. However, they have a greater number of bumps, so the power and signal delivery need not be combined on a single pair of wires. Therefore, in general, each of the test circuits and power grids of larger ICs has connections to a different pair of bumps, such that during testing, only power is transmitted over some pairs of bumps and only test/response signals are transmitted over some pairs of bumps. When there are more bumps aligned with the output lines on the fixture than necessary, neither power nor test/response signals are transmitted over these bumps. 
   In accordance with the invention, connection count needed for testing is reduced. Furthermore, by taking advantage of the regularity of the device bump array on a wafer, wafers having ICs of different sizes and power requirements may be tested using a common fixture. This represents a significant cost saving, because very high connection count fixtures have become very expensive, in some cases costing more than the tester whose signals it is implemented to deliver. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a block diagram showing a simplified version of the circuit design used in the invention; 
       FIG. 2  is a block diagram of a tester and a DUT in which the circuit design of  FIG. 1  is incorporated; 
       FIG. 3  shows the connections between a tester and a wafer having multiple DUTs, each of which is connected to the tester by a single pair of wires; 
       FIG. 4  shows the connections between a tester and a wafer having multiple DUTs, each of which is connected to the tester by multiple pairs of wires; 
       FIG. 5A  is a block diagram showing a connection between a tester and a DUT over which only power is transmitted; 
       FIG. 5B  is a block diagram showing a connection between a tester and a DUT over which only test/response signals are transmitted; and 
       FIG. 5C  is a block diagram showing a connection between a tester and a DUT over which neither power nor test/response signals are transmitted. 
   

   DETAILED DESCRIPTION 
   The present invention provides a system and method for testing electronic devices such as ICs. The invention is particularly useful in testing ICs with bump arrays. However, the invention reduces the number of connections between a tester and a DUT, regardless of the type of device being tested, and is applicable to other types of ICs. 
     FIG. 1  is a block diagram showing a simplified circuit design used in the invention. The left side of  FIG. 1  shows circuit elements contained in the tester  100  and the right side of  FIG. 1  shows circuit elements contained in the DUT  200 . The tester  100  includes a test generator  110 , a DC power supply  120 , and a differential transceiver  130  that is connected to a pair of wires  140 ,  240 . The DUT  200  includes a BIST engine  210 , a power grid  220 , and a differential transceiver  230  that is connected to the pair of wires  140 ,  240 . 
   The transceiver  130  of the tester  100  and the transceiver  230  of the DUT  200 , connected to each other through the pair of wires  140 ,  240 , constitute simultaneous bidirectional signal transceivers. They are configured to transmit self-timed high-speed differential serial data streams in both directions over the wires  140 ,  240 . The use of simultaneous bidirectional signal paths embodying self-timed high-speed differential serial data streams are known in the art and are described in “A 2.4 GBPS Simultaneous Bidirectional Parallel Link with Per Pin Skew Compensation,” Proceedings of ISSCC (2000), the contents of which are incorporated by reference herein. In response to instructions from the test generator  110 , which keeps track of the test information that is required to enable the BIST engine  230 , the transceivers  130 ,  230  generate signals necessary to transmit a data packet or a series of data packets containing the required test information for enabling the BIST engine  230 . The BIST engine  230  receives the data packets, extracts the test information, and executes the test. The results of the test are then packaged by the BIST engine  230  and transmitted to the ATE  100  over the same wires  140 ,  240 . 
   The current provided by the power supply  120  to the DUT  200  flows over the same wires that are used for simultaneous bidirectional signaling. As shown in  FIG. 1 , the power supply  120  is connected to the wires  140 ,  240  and the power grid  220  is connected to the wires  140 ,  240 , so that power is supplied from the power supply  120  to the power grid  220  over the wires  140 ,  240 . Power is decoupled from the test signals transmitted over the wires  140 ,  240  by inductors. Inductors  150 ,  151  decouple the power supply  120  from the test signals transmitted over the wires  140 ,  240 , and inductors  250 ,  251  decouple the power grid  220  from the test signals transmitted over the wires  140 ,  240 . Local bypass capacitors  255 ,  256  are connected in parallel between the two wires that connect to the power grid  220 . The bypass capacitors  255 ,  256 , together with the inductors  250 ,  251 , provide a low-pass filter that keeps the DUT power at the proper level. 
   Capacitors  160 ,  161  are provided to decouple the transceiver  130  from the DC power voltages being supplied to the power grid  220 , and capacitors  260 ,  261  are provided to decouple the transceiver  230  from the DC power voltages being supplied to the power grid  220 . By blocking the DC power voltages being supplied to the power grid  220 , the capacitors  160 ,  161 ,  260 ,  261  allow the input signals to the DUT  200  and output signals from the DUT  200  to be set on average DC levels appropriate to the specific simultaneous bidirectional signal levels required by the specific DUT design, and prevent damage to the transceivers  130 ,  230  by DC voltages that are outside the tolerance of these signal circuits. 
     FIG. 2  is a block diagram of the tester  100  and the DUT  200  in which the circuit design of  FIG. 1  is incorporated. The tester includes a number of test instruments  170 , including analog test instruments and digital test instruments, that operate under the control of software, e.g., a test program  180  and a fixture  190 , which is commonly known as a loadboard. The fixture  190  is connected to the DUT  200  by a single pair of wires. As shown in  FIG. 1 , this single pair of wires is used for simultaneous bidirectional signaling as well as for supplying power to the DUT  200 . 
     FIGS. 3 and 4  show small areas of wafers containing ICs of two different types. The ICs on the wafer of  FIG. 3  are smaller than the ICs on the wafer of  FIG. 4 . The wafer area shown in  FIG. 3  contains 100 identical ICs and the wafer area shown in  FIG. 4  contains 4 identical ICs. There may be other differences in device characteristics between the ICs on the wafer of  FIG. 3  and the ICs on the wafer of  FIG. 4 . As a consequence, the power and signal needs of the two wafers will be different. 
   In a preferred embodiment of the invention, the tester  100  tests multiple DUTs. In  FIG. 3 , a small portion of the tester  100  is shown as testing a 300 mm wafer containing 60,000 identical ICs of which 100 are shown. In  FIG. 4 , the tester  100  is shown as testing a 300 mm wafer containing 2400 identical ICs of which 4 are shown. The wafer bump configurations of these two wafers are identical. Therefore, a common fixture is used to test both of these wafers. 
   In general, a common fixture may be used to test wafers containing ICs of different types, so long as the wafers employ the same wafer bump configuration. Wafers can be configured to have the same bump configuration, because bump technology has no dependence on underlying device characteristics. The bumps are applied to the wafer in a series of manufacturing steps. This series of steps does not depend on the circuits being “bumped.” In order to employ a common fixture for different types of wafers, the bumps on the wafer are applied in view of the common fixture&#39;s layout so that they are aligned with the output lines on the fixture and connect to all of the test circuits and power grids that are used in testing the devices on the wafer. 
   In  FIG. 3 , 100 ICs, each with 16 bumps, are shown. During test, each IC is connected to the tester  100  by a single pair of wires, but for simplicity only ten pairs of these connections are shown. Because each IC is connected to the tester by only a single pair of wires, the wires are used for both simultaneous bidirectional signaling and power transmission. Therefore, in the example of  FIG. 3 , each IC has the internal circuit design of the DUT  200  shown in  FIG. 1 , and the bumps are applied to the wafer so that during test the power grid and the transceiver of each IC are connected to the tester  100  through that IC&#39;s corresponding pair of wires. 
   On the tester side, each pair of wires is connected to a power supply  120  and a transceiver  130  as shown in  FIG. 1 . The power supplies  120  are housed in one or more test instruments  170  and the transceivers are housed in one or more test instruments  170 . The fixture  190  is configured to provide the decoupling between the power and test signals (e.g., provision of inductors  150 ,  151  and capacitors  160 ,  161 ) that is shown in  FIG. 1 . 
   In  FIG. 4 , 4 ICs, each with 400 bumps, are shown. The fixture  190  that is designed for the wafer of  FIG. 3  is also used to connect the wafer of  FIG. 4  to the tester  100 . As in  FIG. 3 , there are 100 pairs of wires connecting the tester  100  and the wafer being tested. Each IC in  FIG. 4  has 25 pairs of wires connecting it to the tester  100 , but for simplicity only 5 pairs of wires are shown for IC  401  and IC  402 . Because each IC is connected to the tester  100  by multiple pairs of wires, depending on the IC design, one or more pairs of wires may be designated to only transmit power (see  FIG. 5A ), and one or more pairs of wires may be designated to only transmit test and response signals (see  FIG. 5B ). Also, one or more pairs of wires may be designated to transmit both power and test/response signals (see  FIG. 1 ), or neither power nor test/response signals (see  FIG. 5C ). 
   The designation is carried out under the control of the test program and is dependent on what components of the IC that the wires are connected to. If the wires are connected to a power grid  220  of the IC as shown in  FIG. 5A , the wires are designated to only transmit power. If the wires are connected to a BIST engine  210  of the IC through a transceiver  230  as shown in  FIG. 5B , the wires are designated to only transmit test and response signals. If the wires are connected to both the power grid  220  and the BIST engine  210  through a transceiver  230 , as shown in  FIG. 1 , the wires are designated to transmit both power and test/response signals. The remaining wires are designated to transmit neither power nor test/response signals as shown in  FIG. 5C . 
   For clarity, the following specific example is provided in connection with the wafer of  FIG. 4 . In this example, it is assumed that each IC that is being tested requires power supplied to nine power grids, and test signals supplied to four BIST engines through corresponding transceivers. Because there are 25 available pairs of connections for each IC and only 13 pairs of connections are necessary to test one IC, it is determined that nine pairs of wires will be used for supplying power and four pairs of wires will be used for transmitting test/response signals. Twelve pairs of wires will be unused. The bumps are applied to the IC with the desired connections in mind so that, after the wafer is attached to the tester  100  for testing, nine pairs of wires are connected to the power grid of the IC and four pairs of wires are connected to the BIST engine through corresponding transceivers, while twelve pairs of wires are left open. 
   In another example, the tester  100  has all of the test transceivers contained in twenty-five instruments, each having two thousand transceivers. The tester  100  has all of the power supplies contained in ten instruments, each having two hundred power supplies. All of the power supplies are ganged and then distributed to the DUTs. In this example, the individual signal pairs are connected to fifty thousand (25×2000=50,000) individual sites directly, while each power supply is distributed to twenty-five signal pairs in parallel. The power distribution and a technique for disconnecting power connection to one or more of the DUTs are described in “Simultaneous Bidirectional Test Data Flow for a Low-cost Wafer Test Strategy,” ITC 2003 General Proceedings (2003), the contents of which are incorporated by reference herein. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.