Abstract:
On-chip logic includes a shadow register cross-coupled with a multiple input shift/signature register (MISR). The shadow register facilitates debugging by shifting out a test signature while resetting the MISR with a fault-free signature. The on-chip logic may further include comparator circuitry to produce an output signal by comparing the test signature with the fault-free signature or by first compressing the test signature and then comparing the compressed test signature with the compressed fault-free signature.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/119,006, entitled “On-Chip Logic To Log Failures During Production Testing And Enable Debugging For Failure Diagnosis,” filed on Dec. 1, 2008, and naming Friedrich Hapke et al. as inventors, which application is incorporated entirely herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to techniques for performing fault diagnosis in integrated circuits (ICs) with built-in self-test (BIST) circuitry. 
       BACKGROUND OF THE INVENTION 
       [0003]    As integrated circuits continue to develop, they continue to have higher device densities and clocking rates. As a result, it requires ever-increasing numbers of test vectors to properly test them, which in turn requires larger and larger amounts of tester vector memory. Still further, manufacturing newer integrated circuits requires even more complex manufacturing techniques, with the corresponding increase in problems and costs related to the production of integrated circuits. To address these problems, and to allow for a self-test of integrated circuits in the field, a testing technique referred to as “built-in self-test” (BIST) is expected to be used more and more in the future. 
         [0004]    With logic built-in self-test (LBIST), test circuits for testing the functional logic of an integrated circuit are added to the circuit&#39;s design.  FIG. 1  illustrates the general configuration of an integrated circuit using LBIST. As seen in this figure, an integrated circuit  101  includes a test stimulus generator  103 , a circuit-under-test (CUT)  105 , and a test response evaluator  107 . The integrated circuit  101  also includes a test control module  109 , for controlling the operation of the test stimulus generator  103 , the circuit-under-test (CUT)  105 , and the test response evaluator  107 . With this arrangement, the test stimulus generator  103  generates test stimuli that are applied to the circuit-under-test  105  through scan chains. The scan chains may be, for example, flip-flops in the circuit-under-test  105  that can be configured into serial shift registers during a test mode. 
         [0005]    The self-test is performed by repeatedly shifting the generated test stimuli into the scan chains so that they are applied to the circuit-under-test  105 , and operating the circuit-under-test  105  for a number of clock cycles in its functional application mode. Various techniques for generating efficient stimuli are well-known in the art. These include, for example, techniques for generating test stimuli for built-in self-test applications that improve the random testability of the circuit by state-of-the-art test points insertion (TPI), by a linear feedback shift register (LFSR) reseeding, by Bit-Flipping-Logic (see, for example, U.S. Pat. No. 6,789,221, issued Sep. 7, 2004, which patent is incorporated entirely herein by reference), or by a cycle-based stimuli generation (see, for example, European Patent Application No. 06126627.6, filed on Dec. 20, 2006, which application is incorporated entirely herein by reference as well). 
         [0006]    The responses produced by the circuit-under-test  105  are captured by the scan chains, and relayed to the test response evaluator  107  where, for example, they are compacted on-chip using a compacting device, such as a multiple input shift register (MISR), to produce a compacted test signature, as illustrated in  FIG. 2 . The compacted test signature can then be compared against a corresponding fault-free signature to determine if the integrated circuit has any of the faults tested for by the test stimuli. Depending upon the implementation, the compacted test signature can be compared with the fault-free signature on-chip, or after it has been exported off of the integrated circuit for comparison by, for example, automated test equipment. When a MISR is employed, however, all the values which appear during the entire test sequence at the scan-chain-outputs are compacted into one final MISR signature. As a result, it is very difficult to calculate the position of the fault in the circuit-under-test when a faulty signature does occur. Thus, while using a standard MISR-type structure readily indicates the presence of a fault, it does not allow for an efficient debug and diagnosis of the circuit-under-test. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Aspects of the invention relate to techniques for performing fault diagnosis in integrated circuits (ICs) with built-in self-test (BIST) circuitry, without the need to test the IC with a special diagnosis pattern. According to various implementations of the invention, a cross-coupled shadow register is combined with a MISR. All fault-free MISR-signatures then are precomputed for every pattern, and all faulty MISR-signatures are precomputed for every fault identified by each pattern. Additionally, various implementations of the invention address the problem of storing the complete design data for many years, enabling a precise fault diagnosis just by using the precomputed MISR-signatures already stored in a dictionary. Furthermore, various implementations of the invention address the problem of long diagnosis runtimes. During a diagnosis, only pre-calculated MISR-signatures are looked-up, instead of performing time-consuming logic simulation or fault simulation as in previous approaches. Various implementations of the invention also address the problem of storing large amounts of diagnosis data by using MISR-signatures only, so that saving the test response data contained in the long concatenated scan chains is unnecessary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates the general configuration of an integrated circuit using LBIST. 
           [0009]      FIG. 2  illustrates an Embedded Deterministic Test (EDT) configuration that employs a MISR to compact test response data provided from scan chains. 
           [0010]      FIG. 3  illustrates an Embedded Deterministic Test (EDT) configuration that employs a MISR, a cross-coupled shadow register, and a secondary compaction circuit according to various implementations of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Built-in Self-Test System 
       [0011]      FIG. 3  illustrates an Embedded Deterministic Test (EDT) configuration  301  that employs a MISR  303 , a cross-coupled shadow register  305 , and a secondary compaction circuit  307  according to various implementations of the invention. This EDT configuration  301  may added to any desired integrated circuit. One feature of EDT configuration  301  is that it constantly produces only one output signal, identified herein as “SPY”, for the integrated circuit being test. This SPY output (labelled “spy_out” in  FIG. 3 ) always produces, in the fault-free case, a signal with a high logical value (or, with alternate implementations of the invention, a low logical value) during the whole test execution of a Logic BIST using the EDT configuration  301 . Hence, it is possible to employ the EDT configuration  301  to run a Logic BIST with only one test vector, which is repeated on the test system for as many cycles (e.g., millions of cycles) as performed during the Logic BIST execution. The constant observation of one output signal, i.e., the SPY-signal, enables the logging of the first failing pattern during production test. This is very important information for statistical failure analysis in mass production in order to improve the yield of integrated circuits. The SPY-signal results from a comparison of the actual MISR-signature and a compressed fault-free MISR-signature that may be stored, for example, on-chip in a BIST memory circuit, such as a read-only memory (ROM) circuit. 
         [0012]    Further, the illustrated example of the EDT configuration  301  according to various implementations of the invention contains the integration of a debug-register  305 , which is cross-coupled with the MISR  303 , in order to perform a detailed failure analysis. This additional debug-register  305  allows shifting-out of the actual (faulty) MISR-signature while at the same time allowing the fault-free MISR-signature for the next test pattern to be shifted in. The debug register  305  may run with a lower frequency than the on-chip BIST frequency, allowing the input data and output data to be shifted out from and shifted into the debug register  305  via a slower integrated circuit contact pad. It should be appreciated, however, that the single-step debug operation may be omitted with various embodiments of the invention. The debug test patterns need only to shift-in and shift-out the corresponding MISR-bits, and all other cycles can be compressed on the test system to one single test vector per on-chip-stored BIST pattern. 
         [0013]    Various implementations of the invention thus allow for an efficient fault diagnosis based only on data that is available during production test. Hence, time-consuming extra tests for diagnosis purposes may be omitted with various embodiments of the invention. Instead, it may be sufficient to only shift-out the small MISR-signature instead of shifting-out the complete set of flip-flop-responses of the IC (e.g., 32 bits per pattern instead of 1 million bits per pattern). Although requiring a tremendously reduced amount of test data, various implementations of the invention can provide a sufficiently high diagnosis resolution. As discussed above, various implementations of the invention employ pre-computations, which generate a fault dictionary containing the fault-free MISR-signatures for all test patterns and a set of MISR-signatures corresponding to every single fault or a set of faults which might occur in the circuit-under-test. 
         [0014]    In order to support the SPY-functionality, the fault-free signatures may be further compressed (e.g., 16 bit), and may either be stored on-chip, as shown in  FIG. 3 , or provided by the test system. In addition, various implementations of the invention enable multi-site testing with a large number of concurrently tested integrated circuits, because only one signal (labelled “spy_out” in  FIG. 3 ) need be observed per integrated circuit. In the case of a failing test pattern and hence a resulting faulty MISR-signature, it is possible to easily extract a set of fault candidates from the previously generated fault dictionary. In debug mode, the resulting MISR-signature is constantly shifted-out, and the fault-free MISR-signature is constantly shifted-in for every test pattern. 
       CONCLUSION 
       [0015]    While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. For example, while specific implementations of the invention have been discussed with regard to logic built-in self-test techniques, it should be appreciated that implementations of the invention also may be employed with other types of built-in self-test techniques, such as memory built-in self-test (MBIST) techniques.