Patent Publication Number: US-9885752-B2

Title: Test apparatus for generating reference scan chain test data and test system

Description:
CROSS REFERENCE 
     The present disclosure claims the benefit of PCT International Application No. PCT/EP2010/061772 filed on Aug. 12, 2010, which is incorporated by reference herein. 
     TECHNICAL FIELD 
     Embodiments according to the disclosure relate to the field of integrated circuit testing, and particularly to the field of scan testing of integrated circuits. 
     BACKGROUND OF THE INVENTION 
     For test of digital devices, in most cases digital random logic is tested via scan test or Logic Built-in Self Test (LBiST). Both test categories rely on scan chains that allow to shift in test patterns and shift out test results. If shifting through scan chains is blocked by a defect, scan test and LBiST is disturbed and as an outcome of that test, coverage is often considerably reduced. A fast blocked chain analysis is needed that identifies two successive scan cells with the blocking defect in between. Then further fault analysis can be used to find the root cause for the defect and to solve the problem and improve yield. 
     Additional hardware on the device under test (DUT) that is added during the design process is considered as too costly since such techniques typically require a considerable amount of die area. 
     It is inherent in the problem of blocked scan chains that without additional hardware the fault location cannot be identified correctly and unambiguously in some cases. 
     There exist software solutions that perform a blocked chain analysis, but usually require a valid simulation model of the DUT that has to be configured correctly for analysis. Furthermore, a considerable computing power is required to simulate the behavior of a modern design, so it might take minutes, hours or even days. In order to handle ambiguous results, some software solutions provide a confidence level for their results. 
     For blocked chain analysis Inovys has filed a patent (U.S. Pat. No. 7,568,139) that describes how to implement on a test apparatus a well known process to analyze scan chains that are blocked by permanent defects. However, this solution is not able to handle ambiguous results, in such a case it can report an incorrect faulty site. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the present invention to provide concept mechanism allowing to reduce the test time, the hardware efforts and/or to improve the failure localization accuracy in digital device testing. 
     Embodiments according to the invention relate to digital device testing, and particularly to a test apparatus and method for generating reference scan chain test data, a test system and a method for determining an information of a failure position and a failure type of a faulty scan chain. 
     According to one embodiment of the present disclosure, a test apparatus for generating reference scan chain test data comprises a test pattern generator and an output data modifier. The test pattern generator is configured to modify a scan chain test input bit sequence by replacing a predefined number of start bits of the scan chain test input bit sequence by a predefined start bit sequence. Further, the test pattern generator is configured to provide the modified scan chain test input bit sequence to a device under test. The output data modifier is configured to modify a scan chain test output bit sequence received from the device under test and caused by the modified scan chain test input bit sequence. The scan chain test output bit sequence is modified by replacing a predefined number of end bits of the scan chain test output bit sequence by a predefined end bit sequence to obtain the reference scan chain test data. 
     Embodiments according to the present invention are based on the central idea that a behavior of a device under test comprising a faulty scan chain is simulated by providing modified test patterns to a faultless device under test. For this, the test apparatus replaces one or more start bits of the scan chain test input bit sequence and at least one end bit of the scan chain test output bit sequence by predefined bit sequences. In this way, the output of a permanent or transient blocked scan chain can be reproduced by a faultless device. This reference scan chain test data can be compared with an output of a faulty device to obtain information about the failure position and a failure type (e.g. stuck-at-0 or stuck-at-1) of the faulty device. 
     In this way, no additional hardware may be necessary on the device under test and in comparison to software simulation solutions, amount of test logging data is reduced by just reporting the result of the comparison, the test time can be significantly reduced, it is easier to distinguish systematic from random faults and time to physical failure analysis is shortened, for example. 
     Some embodiments according to the invention relate to a test system comprising a test apparatus determining a plurality of reference scan chain test data for a plurality of different supposed failure positions, different failure types and/or different scan chain test input bit sequences. Further, the test system comprises an evaluation unit. The obtained plurality scan chain test data may be compared to an output of a faulty device to obtain an information of a failure position and a failure type of the faulty scan chain of the faulty device by the evaluation unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments according to the invention will be detailed subsequently referring to the appended drawings, in which: 
         FIG. 1  is a block diagram of an exemplary test apparatus for generating reference scan chain test data; 
         FIG. 2  is a schematic illustration of an exemplary scan chain test of a faultless scan chain; 
         FIG. 3  is a schematic illustration of an exemplary scan chain test of a faulty scan chain comprising a stuck-at-0 failure between scan flip-flop 3 and scan flip-flop 4; 
         FIG. 4  is a schematic illustration of a an exemplary scan chain test of a faultless device reproducing a stuck-at-0 failure between scan flip-flop 3 and scan flip-flop 4; 
         FIG. 5  is a schematic illustration of an exemplary scan chain test of a faultless device reproducing a stuck-at-1 failure between scan flip-flop 3 and scan flip-flop 4; 
         FIG. 6 a    shows a schematic illustration of an exemplary scan chain test of a faultless scan chain comprising an odd number of inverters reproducing a stuck-at-1 failure between scan flip-flop 3 and scan flip-flop 4; 
         FIG. 6 b    shows a schematic illustration of an exemplary scan chain test of a faultless scan chain reproducing a stuck-at-0 failure between scan flip-flop 3 and scan flip-flop 4 and a stuck-at-1 failure between scan flip-flop 3 and scan flip-flop 4; 
         FIG. 7  is a block diagram of an exemplary test system; 
         FIG. 8  is a block diagram of a further exemplary test system; 
         FIG. 9  is a flowchart of an exemplary method for generating reference scan chain test data; and 
         FIG. 10  is a flowchart of an exemplary method for determining an information of a failure position and a failure type of a faulty scan chain. 
     
    
    
     In the following, the same reference numerals are partly used for objects and functional units having the same or similar functional properties and the description thereof with regard to a figure shall apply also to other figures in order to reduce redundancy in the description of the embodiments. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of embodiments of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the embodiments of the present invention. The drawings showing embodiments of the invention are semi-diagrammatic and not to scale and, particularly, some of the dimensions are for the clarity of presentation and are shown exaggerated in the drawing Figures. Similarly, although the views in the drawings for the ease of description generally show similar orientations, this depiction in the Figures is arbitrary for the most part. Generally, the invention can be operated in any orientation. 
     Notation and Nomenclature 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “processing” or “accessing” or “executing” or “storing” or “rendering” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories and other computer readable media into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. When a component appears in several embodiments, the use of the same reference numeral signifies that the component is the same component as illustrated in the original embodiment. 
       FIG. 1  shows a block diagram of an exemplary test apparatus  100  for generating reference scan chain test data  122  according to an embodiment of the invention. The test apparatus  100  comprises a test pattern generator  110  and an output data modifier  120 . The test apparatus  100  can be connected to a device under test  130 , so that the test pattern generator  110  can provide test data to the device under test  130  and the device under test  130  can provide output test data to the output data modifier  120 . The device under test  130  is not part of the test apparatus  100 . The test pattern generator  110  modifies a scan chain test input bit sequence  102  by replacing a predefined number of start bits of the scan chain test input bit sequence  102  with a predefined start bit sequence. Further, the test pattern generator  110  provides the modified scan chain test input bit sequence  112  to the device under test  130 . The output data modifier  120  modifies a scan chain test output bit sequence  132  received from the device under test  130  caused by the modified scan chain test input bit sequence  112 . The scan chain test output bit sequence  132  is modified by replacing a predefined number of end bits of the scan chain test output bit sequence  132  with a predefined end bit sequence to obtain the reference scan chain test data  122 . 
     By modifying the scan chain test input bit sequence  102  and the scan chain test output bit sequence  132 , it may be possible to reproduce the behavior of a device comprising a faulty scan chain by a faultless device (good device). In other words, the scan chain of the device under test  130  outputting the scan chain test output bit sequence  132  caused by the modified scan chain test input bit sequence  112  may be faultless. In this way, reference scan chain test data  122  for an assessment of scan chain output data of a faulty device can be generated very fast (e.g. in comparison to failure analysis using conventional software simulations). Further, additional hardware may not be needed on the device under test  130  (DUT). 
     The scan chain test input bit sequence  102  and the scan chain test output bit sequence  132  may comprise one bit for each scan cell (e.g. flip-flop) of the scan chain of the DUT  130  the reference scan chain test data  122  is determined for. Therefore, the predefined number of start bits and the predefined number of end bits may vary between 0 and the number of bits of the scan chain test input bit sequence  102  or the scan chain test output bit sequence  132 . 
     The test apparatus  100  may determine reference scan chain test data  122  for failures at different positions within the scan chain to be tested or different failure types. In this case, the number of start bits and the number of end bits may depend on a supposed position of a failure within the scan chain, the reference scan chain test data  122  is generated for. In other words, for different supposed failure positions, the number of start bits and the number of end bits may be different. For example, the number of start bits (#sb) plus the number of end bits (#eb) may be equal to the number of bits of the scan chain test input bit sequence  102  (#ibs=#sb+#eb) and equal to the number of bits of the scan chain test output data  132  (#obs=#sb+#eb). In other words, the sum of the number of start bits and the number of end bits may be equal to the number of scan cells of the tested scan chain. 
     The predefined start bit sequence and the predefined end bit sequence may depend on a supposed failure type the reference end chain test data  122  is generated for. A failure type may be for example a permanent or a transient stuck-at-0 or stuck-at-1 failure. The predefined start bit sequence and the predefined end bit sequence may comprise only logic 0&#39;s or only logic 1&#39;s. This may depend on the supposed failure type and/or the number of inverters (odd or even number) within the scan chain to be tested. For example, the predefined start bit sequence and the predefined end bit sequence may comprise only logic 0+s for a stuck-at-0 failure and an even number of inverters within the scan chain to be tested or the predefined start bit sequence may comprise only logic 1&#39;s and the predefined end bit sequence may comprise only logic 0&#39;s for a stuck-at-0 failure and an odd number of inverters within the scan chain to be tested. 
     In the following,  FIGS. 2 to 6  illustrate the present disclosure for a scan chain  200  comprising 10 scan flip-flops  210  (scan cells) considering different failure types and different numbers of inverters. 
     The presence of a defect causing a blocked scan chain can be reproduced on a specific test apparatus using a DUT that is fault free, meaning no faulty scan chains, where the test apparatus is configured to provide new additional functionality that allows to modify input patterns (scan chain test input bit sequences) and output patterns (scan chain test output bit sequences) accordingly. 
     For example, a good device with a scan chain length of 10 scan cells and scan input bits b 0  . . . b 9  (scan chain test input bit sequences) and scan output (response) bits r 0  . . . r 9  (scan chain test output bit sequences), as shown in  FIG. 2 , is considered. 
     If there is a blocked scan chain then this can be modeled as a stuck-at fault in the scan chain. For example, if there is a stuck-at-0 fault between SFF3 (scan flip-flop 3)  210  and SFF4 (scan flip-flop 4)  210 , then the defect modifies scan input values stored in the scan cells and scan output values observed on a test apparatus as shown in  FIG. 3 . 
     The result 0 . . . 0 r 3  . . . r 0  can be reproduced with a good device, if the test apparatus is capable of modifying scan input data  102  and output data  132  as shown in  FIG. 4  (see “mod. TM”, modified test model). 
     For an arbitrary number N (predefined number of start bits), the first N values shifted into an arbitrary scan chain are overwritten (replaced) by a pre-defined bit sequence (predefined start bit sequence), for example the first 4 values shifted into the scan chain  200  can be overwritten and set to a constant value “0”. For an arbitrary number M (predefined number of end bits), the last M values shifted out of an arbitrary scan chain can be overwritten by a pre-defined bit sequence (predefined end bit sequence), for example the last 6 values shifted out of the scan chain can be overwritten and set to a constant value “0”. 
     In a similar way the results of a scan chain blockage that can be modeled as a stuck-at-1 fault between SFF3 and SFF4, can be reproduced on the suitable test apparatus as shown in  FIG. 5 . 
     Even if there is an odd number of inverters  620  along the path from the scan input to the blocked site, this can be reproduced on the suitable test apparatus, as shown in  FIG. 6   a.    
     If there is an odd number of inverters along the path from the blocked site to the scan output or an odd number of inverters along both paths, i.e., scan input to faulty site and faulty site to scan output, then these scenarios can be reproduced similarly. 
     Further, a scan chain may comprise more than one failure position. For example,  FIG. 6 b    shows a scan chain with two stuck-at-failures. In this case, the start bit sequence may contain 0s and 1s. 
     The observations above are true for both types of blockage, stuck-at-0 and stuck-at-1. 
     The observations are not specifically related to a scan chain of 10 scan cells, and the size of the scan chain can be arbitrary. 
     The observations are not specifically related to the position of the blocked site (e.g. as shown above between SFF3 and SFF4). The position can be anywhere in the scan chain. In other words, a failure between two arbitrary neighboring scan cells of the scan chain or a failure within a scan cell itself may be reproduced. A failure within a scan cell itself may be treated similarly with a failure between the faulty scan cell and the following scan cell of the scan chain. 
     The observations are shown for a single scan chain, but is also valid, if more (good) scan chains are in the device. Reference scan chain data may be generated for one, some or all scan chains of a faultless device under test. 
     The modification of input patterns as shown above might bring the DUT into an unknown state, which can be tolerated by the DUT in accordance with the present disclosure. 
     Thus, reference scan chain test data  122  can be obtained from a faultless device under test. This reference scan chain test data  122  may be compared with scan chain test data obtained from a faulty device. For this, the test apparatus  100  may provide the scan chain test input bit sequence  102  (without modification) to a faulty device under test comprising a faulty scan chain and may obtain a faulty scan chain test output bit sequence from the faulty device under test. This scan chain test output bit sequence or information determined from the scan chain test output bit sequence (e.g. frequency of logic 0&#39;s or logic 1&#39;s) may be compared with the reference scan chain test data  122  obtained from the faultless device under test. 
       FIG. 7  shows a block diagram of an exemplary test system  700  according to an embodiment of the invention. The test system  700  comprises a test apparatus  100  according to the mechanism described before and an evaluation unit  740 . The evaluation unit  740  can compare information of a faulty scan chain test output bit sequence obtained from a faulty device under test comprising a faulty scan chain with the reference scan chain test data  122  to determine an information of a failure position  742  and a failure type  744  of the faulty scan chain. 
     The evaluation unit  740  may be connected to the test apparatus  100  as shown in  FIG. 7  or may be independent from the test apparatus  100 . An independent evaluation unit  740  may receive the reference scan chain test data  122  and/or the information of the faulty scan chain test output bit sequence from a storage device. In this way, a real time failure analysis, monitoring of manufacturing or volume testing may be possible. 
     The reference scan chain test data  122  may be, for example, the modified scan chain test output bit sequence itself, a frequency of occurring logic 1&#39;s or logic 0&#39;s of each bit position of the scan chain test output bit sequence  132  or a last toggled bit position indicating a position of the last bit within the scan chain test output bit sequence, a logic 1 as well as a logic 0 is obtained for different scan chain test output bit sequences. In other words, one could compare each single test response bit of the faulty DUT (for multiple patterns) with each single test response bit of the faultfree DUT for multiple fault types and multiple fault positions. This would end up in a huge amount of data, therefore using the bit frequencies or toggle bit positions may be of advantage. 
     Correspondingly, the information of the faulty scan chain test output bit sequence obtained from the faulty device under test may be the faulty scan chain test output bit sequence itself, a frequency of occurring logic 1&#39;s or logic 0&#39;s for each bit position of the scan chain test output bit sequence  132  or a last toggle bit position of the faulty scan chain test output bit sequence. 
     The information of the failure position  742  and the failure type  744  of the faulty scan chain may be the exact failure position (e.g. scan cell or between two scan cells) and the failure type (stuck-at-0 or stuck-at-1), if the comparison provides an unambiguous result. For transient blocked scan chains, the result of the comparison may be ambiguous. In this case, the evaluation unit  740  may determine a failure candidate list containing possible failure positions and failure types together with a corresponding confidence level information (e.g. probability of indicating the actual failure position and/or failure type of the faulty scan chain) representing the determined information of the failure position  742  and the failure type  744  of the faulty scan chain. 
     Often test patterns comprise a plurality of different scan chain test input bit sequences (e.g. pseudo random bit sequences or a predefined sequence of different scan chain test input bit sequences). In this case, the test pattern generator  110  may modify the plurality of different scan chain test input bit sequences  102  by replacing the same predefined number of start bits of each scan chain test input bit sequence  102  of the plurality of scan chain test input bit sequences with the same predefined start bit sequence. Further, the output data modifier  120  may modify a plurality of scan chain test output bit sequences  132  caused by the plurality of modified scan chain test input bit sequences  112  by replacing the same predefined number of end bits of each scan chain test output bit sequence  132  of the plurality of scan chain test output bit sequences  132  with the same predefined end bit sequence. Based on this plurality of modified scan chain test output bit sequences the output data modifier  120  may obtain the reference scan chain test data. In other words, the test apparatus  100  may determine for each scan chain test input bit sequence  102  a modified scan chain test output bit sequence. 
     The number of bits of each scan chain test input bit sequence of the plurality of input bit sequences and the number of bits of each scan chain test output bit sequence of the plurality of scan chain test output bit sequences may be the same and may be equal to the number of scan cells of the scan chain to be tested. 
     The obtained reference scan chain test data  122  may be the plurality of modified scan chain test output bit sequences or an information obtained from them. 
     For example, the test apparatus  100  or the evaluation unit  740  may comprise a frequency determiner  850 . This frequency determiner  850  may determine a frequency  852  of occurring logic 1&#39;s or logic 0&#39;s for each bit position of the scan chain test output bit sequence based on the plurality of modified scan chain test output bit sequences. In other words, the frequency determiner  850  may determine a ratio  852  of occurring logic 1&#39;s and occurring logic 0&#39;s for each scan cell of the scan chain to be tested. This may be done for the faultless device and the faulty device, so that the evaluation unit  740  can compare the frequency  852  of occurring logic 1&#39;s or logic 0&#39;s (or the ratio of logic 1&#39;s to logic 0&#39;s) of the good device and the faulty device to obtain the information of the failure position  742  and the failure type  744  of the faulty scan chain. Alternatively, instead of comparing frequencies of all bits of a scan chain only the bit number and frequencies of the biggest outliers (e.g. the biggest 16, 32, 64 or another number) or most characteristic bits (e.g. the 16, 32, 64 or another number of most characteristic bits) may be stored as failure characteristic and may be compared to the corresponding frequencies of the faulty device. 
     Determining the frequency of occurring logic 1&#39;s or logic 0&#39;s may be especially useful for determining failure positions and failure types of transient stuck at failures (transient blocked scan chains). 
     Alternatively, the test apparatus  100  or the evaluation unit  740  may comprise a toggle bit determiner  860 . This toggle bit determiner  860  may determine a last toggle bit position  862  within a scan chain test output bit sequence. The last toggle bit position  862  may indicate a position of the last bit within the scan chain test output bit sequence outputted by the scan chain to be tested comprising for at least one scan chain test output bit sequence of the plurality of scan chain test output bit sequences a logic 0 and for at least one scan chain test output bit sequence of the plurality of scan chain test output bit sequences a logic 1. In this way, the last faultless scan cell before the failure position may be determined. In this connection, the last toggle bit position  862  may be counted starting from the output of the scan chain. In the examples shown in  FIGS. 2 to 6  the last toggle bit position  862  would be a scan cell with the highest number (SFF 0 to SFF 9) before the failure position (e.g. SFF3 in  FIG. 3 ). This kind of reference scan chain test data  122  may be especially useful for permanent stuck at failures (permanent blocked scan chains). Alternatively, instead of the last toggle bit position, a first stuck-at bit position  864  may be determined. The first stuck-at bit position  864  may indicate a position of the first bit within the scan chain test output bit sequence outputted by the scan chain to be tested comprising for all scan chain test output bit sequences of the plurality of scan chain test output bit sequences a logic 0 or for all scan chain test output bit sequences of the plurality of scan chain test output bit sequences a logic 1. 
     Fittingly,  FIG. 8  shows a block diagram of an exemplary test system according to an embodiment of the invention. The test system  800  is similar to the test system shown in  FIG. 7 , but comprises additionally a frequency determiner  850  and/or a toggle bit determiner  860  as mentioned above, located between the output data modifier  120  of the test apparatus  100  and the evaluation unit  740 . 
     The frequency determiner  850  and/or the toggle bit determiner  860  may be part of the test apparatus  100  or part of the evaluation unit  740 . In the example shown in  FIG. 8 , the frequency determiner  850  and/or the toggle bit determiner  860  are part of the test apparatus  100  and provide a frequency  862  of occurring logic 1&#39;s or logic 0&#39;s, a last toggle bit position  862  or a first stuck-at bit position  864  as mentioned above. 
     The test system  100  may comprise both the frequency determiner  850  and the toggle bit determiner  860  or only one of them. 
     Since the failure position and failure type of a faulty scan chain may be unknown, it may be desired to generate reference scan chain test data  122  for a plurality of different possible failure positions and/or failure types or even for all possible failure positions and failure types. For this, the test pattern generator  110  may modify the same scan chain test input bit sequence  102  by replacing a plurality of different predefined numbers of start bits (e.g. depending on the failure position) with a plurality of different start bit sequences (e.g. depending on the failure type and/or the number of inverters within the scan chain) to obtain a plurality of modified scan chain test input bit sequences  112 . Then, the output data modifier  120  may modify a plurality of scan chain test output bit sequences  132  caused by the plurality of modified scan chain test input bit sequences  112  by replacing a plurality of different predefined numbers of end bits (e.g. depending on the failure position) corresponding to the plurality of different numbers of start bits with a plurality of different end bit sequences (e.g. depending on the failure type and/or the number of inverters within the scan chain) to obtain a plurality of reference scan chain test data  122  representing different supposed failure positions or failure types of the scan chain. 
     In other words, the predefined numbers of start bits and predefined number of end bits as well as the predefined start bit sequence and the predefined end bit sequence may be varied to reproduce a failure at different failure positions and of the different failure types. The plurality of reference scan chain test data may be compared with the information of the faulty scan chain test output bit sequence obtained from a faulty device under test to determine the information of the failure position  742  and the failure type  744  of the faulty scan chain. In other words, the reference scan chain test data  122  matching (or matching best) the information of the faulty scan chain test output bit sequence may reveal the failure position and failure type of the faulty scan chain, since each reference scan chain test data  122  is generated for a specific failure position (e.g. considered by the predefined number of start bits and the predefined number of end bits) and a specific failure type (e.g. considered by the predefined start bit sequence and the predefined end bit sequence). 
     In some embodiments of the invention different failure positions and failure types as well as different scan chain test input bit sequences can be taken into consideration. In this case, the test apparatus may generate a plurality of reference scan chain test data for a plurality of different supposed failure positions, different failure types and different scan chain test input bit sequences. Based on this data, the evaluation unit may compare the faulty scan chain test output bit sequence (or a sequence of a plurality of faulty scan chain test output bit sequences corresponding to the plurality of different scan chain test input bit sequences) with each reference scan chain test data of the plurality of reference scan chain test data or may compare a frequency of occurring logic 1&#39;s or logic 0&#39;s of the plurality of faulty scan chain test output bit sequences determined for different scan chain test input bit sequences with a frequency of logic 1&#39;s or logic 0&#39;s of each reference scan chain test data of the plurality of reference scan chain test data, for example. In other words, the test apparatus may reproduce reference scan chain test data for different supposed failure positions, different failure types and different scan chain test input bit sequences, which may be compared with the corresponding output of the faulty device to determine an information of a failure position and a failure type of the faulty scan chain of the faulty device under test. 
     Sometimes failures within scan chains exist not permanently. Such failures may appear randomly or may be data depending, for example. For these transient failures, the failure position and failure type is difficult to determine. Using the described mechanism, such transient failures can also be identified and localized. 
     For this, the test pattern generator  110  may provide a plurality of unmodified, different scan chain test input bit sequences  102  subsequently to a faultless device under test and a faulty device under test. The term unmodified means in this case, that the predefined number of start bits is not replaced by the predefined start bit sequence. Then, the frequency determiner determines a frequency of occurring logic 1&#39;s or logic 0&#39;s for each bit position of the scan chain test output bit sequence for the faultless device under test and for the faulty device under test based on a plurality of unmodified scan chain test output bit sequences received from the faultless device under test and a plurality of unmodified scan chain test output bit sequences from the faulty device under test (caused by the plurality of unmodified, different scan chain test input bit sequences). Further, the evaluation unit may determine a difference between the frequency of logic 0&#39;s or logic 1&#39;s determined for the faulty device under test and the frequency of logic 0&#39;s or logic 1&#39;s determined for the faultless device under test for each position of the scan chain test output bit sequence to obtain a failure characteristic. This failure characteristic may indicate a failure position and or a failure type of the faulty device under test. 
     Optionally, the determined differences may be normalized (e.g. by dividing the differences by the number of unmodified, different scan chain test input bit sequences). 
     Additionally, the evaluation unit  740  may determine a difference between each frequency of logic 0&#39;s or logic 1&#39;s determined from a plurality of modified scan chain test output bit sequences obtained by considering a plurality of different supposed failure positions and different failure types and the frequency of logic 0&#39;s or logic 1&#39;s determined from the plurality of unmodified scan chain test output bit sequences of the faultless device under test to obtain a reference characteristic for each considered failure position and failure type. In other words, the evaluation unit may compare the faultless behavior of the faultless device under test by using unmodified scan chain test input bit sequences with the simulated faulty behavior of the faultless device under test by using modified scan chain test input bit sequences for different supposed failure positions and different failure types. 
     Optionally, the determined differences may be normalized (e.g. by dividing the differences by the number of unmodified or modified, different scan chain test input bit sequences). 
     Further, the evaluation unit  740  may compare the determined failure characteristic with each determined reference characteristic to obtain the information of the failure position and the failure type. 
       FIG. 9  shows a flowchart of a method  900  for generating reference scan chain test data according to an embodiment of the invention. The method  900  comprises modifying  910  a scan chain test input bit sequence by replacing a predefined number of start bits of the scan chain test input bit sequence with a predefined start bit sequence. Then, the modified scan chain test input bit sequence is provided  920  to a device under test and a scan chain test output bit sequence caused by the modified scan chain test input bit sequence is received  930  from the device under test. Further, the method  900  comprises modifying  940  the scan chain test output bit sequence by replacing a predefined number of end bits of the scan chain test output bit sequence with a predefined end bit sequence to obtained the reference scan chain test data. 
     Optionally, a plurality of different scan chain test input bit sequences may be modified by replacing the same predefined number of start bits with the same predefined start bit sequence to consider a test pattern comprising several different scan chain test input bit sequences as for example pseudo random bit sequences. Accordingly, the plurality of scan chain test output bit sequences caused by the plurality of modified scan chain test input bit sequences may be modified by replacing the same predefined number of end bits with the same predefined end bit sequence. 
     Further optionally, different failure positions and/or failure types may be considered by replacing a plurality of different predefined numbers of start bits with a plurality of different predefined start bit sequences to obtain a plurality of modified scan chain test input bit sequences. Accordingly, the plurality of scan chain test output bit sequences caused by the plurality of modified scan chain test input bit sequences is modified by replacing a plurality of different predefined numbers of end bits corresponding to the plurality of different numbers of start bits with a plurality of different predefined end bit sequences. Optionally, a frequency of occurring logic 1&#39;s or logic 0&#39;s may be determined for each bit position of the scan chain test output bit sequence based on the plurality of modified scan chain test output bit sequences. 
       FIG. 10  shows a flowchart of a method  1000  for determining an information of a failure position and a failure type of a faulty scan chain according to an embodiment of the invention. The method  1000  is similar to the method shown in  FIG. 9 , but comprises additionally comparing  1050  information of a faulty scan chain test output bit sequence obtained from a faulty device under test comprising the faulty scan chain with the reference scan chain test data to determine information of a failure position and a failure type of the faulty scan chain. 
     For example, the observation mentioned above may be utilized for block chain analysis in six steps. 
     The first three steps may prepare blocked chain analysis using a good device in order to generate an intended data (reference scan chain test data):
         1. Define an appropriate set of test patterns SP (this step is optionally, since scan patterns SP may be provided by a storage device).   2. Run the set SP (scan pattern, test pattern, plurality of scan chain test input bit sequences) on the specific test apparatus for a good device and store for each scan chain and for each bit of the scan chain (each bit of the scan chain test output bit sequence) the frequency of observed “1”s.   3. For each scan chain, for each possible fault location and for each inversion polarity (odd or even number of inverters) along the scan chain—assuming either stuck-at-0 or stuck-at-1—(i.e. for each possible fault candidate of a blocked chain, e.g. considered by the respective predefined start bit sequence and the predefined end bit sequence) re-produce the faulty scan results for SP on the specific test apparatus. Store for each bit of the scan chain, that contains the assumed blocking fault candidate, the frequency of observed “1”s.       

     The next steps may be executed once a DUT shows the behavior of a blocked scan chain and blocked chain analysis is required:
         4. Run the set SP on the specific test apparatus for the faulty device and store for each bit of the faulty scan chain the frequency of observed “1”s.   5. Firstly a permanent blocked scan chain may be assumed. Compare for each bit of the faulty scan chain the frequencies obtained in step 4 with that obtained reproducing fault candidates&#39; behavior in step 3. The lower the differences of the frequencies, the higher the confidence rating (confidence level). Finally, the output of this analysis step may be a list of fault candidates sorted according to the confidence rating.   6. If the results of step 5 give no clear indication then a transient defect may be assumed. Then compute the difference between the frequencies of a good device (step 2) and the faulty device (step 4) for all scan bits of the faulty scan chain and normalize over these differences. Analog compute for each fault candidate the normalized differences between the frequencies of a good device and the values obtained for reproducing faulty behavior in step 3 for the faulty scan chain. The lower the differences of the values of the faulty device and of a reproduced faulty behavior, the higher the confidence rating. Again, the output of this analysis step may be a list of fault candidates sorted according to the confidence rating.       

     With data of steps 1-3 prepared, step 4 can be performed on the specific test apparatus. Then the resulting data can be uploaded to a computer (evaluation unit) that afterwards executes steps 5 and 6. Thus, the expected run-time (test time) for steps 4-6, that have to be executed for each occurrence of a blocked chain, is only seconds and this analysis is applicable for high volume manufacturing. 
     Step 2 and step 3 might generate large amounts of data. The amount of data used for the blocked chain analysis can be reduced, if only that part of data is generated that is necessary for analyzing a certain blocked scan chain. Although the analysis may not be ideal during high volume manufacturing, it is still very valuable in less time critical steps, for example during bring-up of test and validation or characterization of the DUT. 
     For the modification the six steps given above may be reordered such that step 2 and step 3 are performed no earlier than a DUT showing the behavior of a blocked scan chain. Then only the faulty scan chain is considered, and in step 2 “1” frequencies for a good device can be determined only regarding this chain. In step 3 the behavior of fault candidates can be reproduced only regarding this chain as well. Step 2 and step 3 might be performed after step 4. 
     The method can be simplified, if only permanently blocked scan chains are considered. Then step 2 and 6 may be skipped. In step 3 and step 4 no longer bit frequencies of observed “1”s are stored, but a single number per chain may be sufficient. This number (last toggle bit position) may be the highest (counting from scan outputs as shown in the pictures above) bit, for that the specific test apparatus could observe “0” and “1” while applying patterns SP. Alternatively, also a first stuck-at bit position may be determined. During analysis in step 5, rating is computed by comparing the numbers. The expected run time (test time) for this simplification can be considerably smaller. 
     Some embodiments according to the invention relate to a blocked chain analysis providing confidence levels on results. 
     Blocked chain analysis often delivers ambiguous results. One goal is, beyond identifying possibly faulty sites, to provide a confidence level to the analysis results—for both, transient and permanent defects blocking scan chains. The analysis usually takes only seconds. 
     For the new blocked chain analysis it is assumed, that at least one device exists, for that a scan test can be set up such that shifting through scan chains is successful. Furthermore it is assumed, that filling scan chains with arbitrary values and running in normal operation mode does not produce new defects in the device. 
     The described mechanism utilizes a test apparatus capable of addressing both, transient and permanent defects blocking scan chains. For example, beside three steps being performed only once in order to prepare intended data (generate reference scan chain test data), the blocked chain analysis for a fault runs in seconds and may provide additionally a confidence level for the analysis results to address ambiguousness. 
     The specific test apparatus may provide a way to identify the structure of the scan pattern in order to distinguish shift and launch/capture cycles of a scan pattern. For this, the test pattern generator may be triggered by a trigger pin (similar to a clock signal) or a list of vector numbers stored by a storage device to identify the start of a scan chain test input bit sequence, wherein a vector represents for each pin of the test apparatus the current logic value (e.g. logic 1 or logic 0). 
     The specific test apparatus must be able to modify scan input data (scan chain test input bit sequence) such that for an arbitrary number N, the first N values shifted into an arbitrary scan chain are overwritten by a pre-defined bit sequence, for example constant values—“0” or “1”. 
     The specific test apparatus must be able to modify scan output data (scan chain test output bit sequence) such that for an arbitrary number M, the last M values shifted out of an arbitrary scan chain are overwritten by a pre-defined bit sequence, for example constant values—“0” or “1”. 
     Additionally, the specific test apparatus may be able to compute the “1” frequencies in step 2, 3 and 4. 
     For the simplified analysis (only permanently blocked scan chains are considered), the specific test apparatus may be able to determine the highest number of scan bits producing toggling output (last toggle bit position). 
     In some embodiments according to the invention, the test apparatus is an automatic test equipment. 
     An aspect of the present invention is a setup to reproduce faulty behavior with a good device. This includes a specific test apparatus that supports the full process or that only supports the simplified process according to requirements mentioned above, for example. 
     Another aspect of the invention is a method (process) as described above to compute blocked scan chain analysis results sorted regarding a confidence level, applicable to transient defects as well. Alternatively, a modified method (process) with reordered steps reducing amount of data to be stored may be used. Optionally, also the simplified process mentioned above may be used for reducing steps, amount of data to be stored and/or runtime (test time). 
     Although some aspects of the described concept have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. 
     Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable. 
     Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed. 
     Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier. 
     Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. 
     In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer. 
     A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. 
     A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet. 
     A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. 
     A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein. 
     In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus. 
     Although certain preferred embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. It is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.