Patent Publication Number: US-8117513-B2

Title: Test method and test program of semiconductor logic circuit device

Description:
TECHNICAL FIELD 
     The present invention relates to a test method and test program of a semiconductor logic circuit device. 
     BACKGROUND TECHNOLOGY 
     Semiconductor logic circuit devices are shipped through three steps, i.e., a design step, a production step and a test step. Here, a test is to apply a test vector upon a manufactured semiconductor logic circuit device, to observe a test response from the semiconductor logic circuit device, and to compare the test response with an expected test response, thus determining whether the semiconductor logic circuit device is good or defective. The rate of good semiconductor logic circuit devices is called a manufacturing yield which would remarkably affect the semiconductor logic circuit devices in quality, reliability and manufacturing cost. 
     Generally, a semiconductor logic circuit device (mainly, a sequential circuit) is constructed by a combinational portion formed by logic elements such as AND (AND) gates, NAND (NAND) gates, OR (OR) gates and NOR (NOR) gates, and flip-flops storing an internal state of the circuit. In this case, the combinational portion has external input lines (PI), pseudo external input lines (PPI) serving as output lines of the flip-flops, external output lines (PO), and pseudo external output lines (PPO) serving as input lines of the flip-flops. Inputs to the combinational portion are ones supplied directly from the external input lines and ones supplied by the pseudo external input lines. Also, outputs from the combinational portion are ones appearing directly at the external output lines and ones appearing at the pseudo external output lines. 
     In order to test the combinational portion of a semiconductor logic circuit device, a predetermined test vector is required to be applied thereto from the external input lines and the pseudo external input lines of the combinational portion, and a test response is required to be observed from the external output lines and the pseudo external output lines of the combinational portion. Here, one test vector is formed by bits corresponding to the external input lines and the pseudo external input lines. Also, one test response is formed by bits corresponding to the external output lines and the pseudo external output lines. 
     However, it is generally impossible to directly access the output lines of the flip-flops (the pseudo external input lines) and the input lines of the flip-flops (the pseudo external output lines) in the semiconductor logic circuit device from the exterior. Therefore, in order to test the combinational portion, there are problems in the controllability of the pseudo external input lines and the observableness of the pseudo external output lines. 
     A scan design is a technique for solving the problems of controllability and observableness in the test of the combinational portion. Such a scan design is to replace the flip-flops with scan flip-flops by which one or a plurality of scan chains are formed. The scan flip-flops are controlled by a scan enable (SE) signal. For example, when the scan enable (SE) signal is at logic value 0, the scan flip-flops carry out the same operation as conventional flip-flops, so that, if a clock pulse is given, the output values of the scan flip-flops are renewed by the combinational portion. On the other hand, when the scan enable (SE) signal is at logic value 1, the scan flip-flops with the other scan flip-flops within the same scan chain form one shift register, so that, if a clock pulse is given, new values are shifted from the exterior in the scan flip-flops, and simultaneously, values which have been in the scan flip-flops are shifted out to the exterior. Generally, scan flip-flops in the same scan chain share the same scan enable (SE) signal; however, scan enable (SE) signals in different scan chains may be the same or different from each other. 
     A test for a scan-designed semiconductor logic circuit device is carried out by repeating shift operations and capture operations. A shift operation is carried out in a shift mode where the scan enable (SE) signal is at logic value 1. In the shift operation, one or a plurality of new values given from the exterior are shifted in the scan flip-flops within the scan chain. Also, simultaneously, one or a plurality of values which have been located in the scan flip-flops within the scan chain are shifted out to the exterior. A capture operation is carried out in a capture mode where the scan enable (SE) signal is at logic value 0. In the capture mode, one clock pulse is simultaneously given to all the scan flip-flops in one scan chain, so that the values of the pseudo external output lines of the combinational portion are taken into all the scan flip-flops. 
     A shift operation is used for applying a test vector to the combinational portion through the pseudo external input lines and for observing a test vector from the combinational portion through the pseudo external output lines. Also, a capture operation is used for taking a test response of the combinational portion in the scan flip-flops. Shift operations and capture operations are repeated upon all test vectors, thus testing the combinational portion. Such a test method is called a scan test method. 
     In a scan test method, application of a test vector to the combinational portion is formed by a portion applied directly from the exterior inputs and a portion applied by shift operations. Since an arbitrary logic value is set in an arbitrary scan flip-flop, the problem of controllability of the pseudo external input lines is solved. Observation of a test response from the combinational portion is formed by a portion carried out directly from the external output lines and a portion carried out by shift operations. Since an output value of an arbitrary scan flip-flop can be observed by shift operations, the problem of observableness of the pseudo external input lines is solved. Thus, in a scan test method, it is only necessary to obtain a test vector and an expected test response for the combinational portion by using an automatic test pattern generation (ATPG) program. 
     Despite the effectiveness of the above-mentioned scan test method, there is a problem in that the power consumption is remarkably larger in a test mode than in a usual operation mode. For example, if the semiconductor logic circuit device is constructed by CMOS circuits, the power consumption consists of static power consumption due to leakage currents and dynamic power consumption due to switching operations of the logic gates and the flip-flops. Additionally, the latter dynamic power consumption consists of shift power consumption in shift operations and capture power consumption in capture operations. 
     Generally, a large number of clock pulses are required for one test vector in shift operations. For example, in order to set new values in all the scan flip-flops of a scan chain, a number of clock pulses corresponding to the number of the scan flip-flops are required at most. As a result, the shift power consumption is increased to induce excessive heat. Therefore, semiconductor logic circuit devices would be damaged. Various techniques for decreasing the shift power consumption have been vigorously developed. 
     On the one hand, generally, a single clock pulse per one scan chain is required for one test vector in a capture operation. Therefore, heat by the capture operation mode creates no problem. However, in a capture mode, when a test response of the combinational portion appearing at the pseudo external output lines is taken in the scan flip-flops, if the values of the test response are different from the current values of the scan flip-flops, the values of the corresponding scan flip-flops change. If the number of the scan flip-flops whose output values have changed, the power supply voltage is instantaneously changed by the switching operation of the logic gates and the scan flip-flops. This is also called an IR (I: current and R: resistance) drop phenomenon. The IR drop phenomenon would erroneously operate the circuit so as to take erroneous test response values in the scan flip-flops. Thus, even semiconductor logic circuit devices normally operable in a usual state would be deemed to be defective in a test state, which can be an erroneous test. As a result, the manufacturing yield would be decreased. Particularly, when semiconductor logic circuit devices become ultra-large in scale, more fine-structured and lower in power supply voltage, the manufacturing yield caused by the erroneous test would be remarkably decreased. Therefore, it is essential to decrease the capture power consumption. 
     When a single clock signal is used in a test mode, the capture power consumption can be decreased by a clock gating technique; however, this would remarkably affect the physical design of semiconductor logic circuit devices. Also, when multiple clock signals are used in a test mode, the capture power consumption can be decreased by a one hot technique or a multiple clock technique; however, the former technique would remarkably increase test data amount, and the latter technique would require enormous memory consumption in generating test vectors which is a large burden to an ATPG program. Therefore, in view of the decrease of the capture power consumption, it is expected to decrease the impact to the physical design, to suppress the increase of test data amount and to decrease the burden of an ATPG program. 
     On the other hand, many test cubes, i.e., input vectors with unspecified bits (hereinafter, referred to as X-bits) are usually generated in a process for generating test vectors using an ATPG program. Also, when a set of test vectors without X-bits are given, some bits of some test vectors can be converted into X-bits without changing the fault detection rate of the set of test vectors. That is, test cubes can be obtained by an X-bit extracting program. The reason for the existence of test cubes is mainly to have only to set necessary logic values in a part of bits of the external input lines and the pseudo external input lines in order to detect one target fault in the combinational portion. Since assignment of 0&#39;s or 1&#39;s to the remainder bits does not affect the detection of the target fault, such remainder bits are X-bits for the target fault. 
     A test cube with X-bits is strictly an intermediate product appearing in a process for generating a test cube without X-bits. Therefore, logic values (0 or 1) finally have to be filled into the X-bits of the test cube in an appropriate method, i.e., an algorithm filling method, a merge filling method or a random filling method. 
     The algorithm filling method determines and fills optimum logic values (0 or 1) for the X-bits in a test cube in an algorithm in order to obtain an object. Such an algorithm is often mounted on an ATPG program. This algorithm filling method is used for decreasing the total number of test vectors in dynamic compaction (see: non-patent documents 1 and 2) or for decreasing the shift power consumption (see: non-patent document 3). 
     When a test cube is merged with another test cube, the merge filling method fills 0 or 1 in X-bits of the test cubes so that a bit of the test cube has the same logic value as its corresponding bit of the other test cube. For example, in order to merge a test cube  1 X 0  with a test cube  11 X, 1 is assigned to the X-bit of the test cube  1 X 0  and 0 is assigned to the X-bit of the test cube  11 X. This merge filling method is used for decreasing the total number of test vectors in static compaction (see: non-patent document 1) or for decreasing the shift power consumption (see: non-patent document 4). 
     The random filling method assigns 0 or 1 randomly to X-bits in a test cube. This random filling method is often carried out for remaining X-bits after the algorithm filling method or the merge filling method is carried out. This random filling method is used for decreasing the total number of test vectors in dynamic compaction (see: non-patent document 1) or for decreasing the shift power consumption (see: non-patent document 4).
     Non-patent document 1: M. Abramovici, M. Breuer, and A. Friedman, Digital Systems Testing and Testable Design, Computer Science Press, pp. 245-246., 1990.   Non-patent document 2: X. Lin, J. Rajski, I. Pomeranz, S. M. Reddy, “On Static Test Compaction and Test Pattern Ordering for Scan Designs”, Proc. Intl. Test Conf., pp. 1088-1097, 2001.   Non-patent document 3: S. Kajihara, K. Ishida, and K. Miyase, “Test Vector Modification for Power Reduction during Scan Testing”, Proc. VLSI Test Symp., pp. 160-165, 2002.   Non-patent document 4: R. Sankaralingam, R. Oruganti, and N. Touba, “Static Compaction Techniques to Control Scan Vector Power Dissipation”, Proc. VLSI Test Symp., pp. 35-40, 2000.   

     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     The above-described algorithm filling method, merge filling method and random filling method for filling 0 or 1 in X-bits, however, are intended to decrease the total number of test vectors or decrease the shift power consumption, so that an erroneous test caused by the increase of the capture power consumption cannot be avoided, and thus, the manufacturing yield is decreased, which is a problem. 
     Therefore, an object of the present invention is to provide a test method and test program of a semiconductor logic circuit device which decreases the capture power consumption by decreasing the number of output switching scan flip-flops in a capture operation, thus avoiding an erroneous test. 
     Means for Solving the Problem 
     In order to attain the above-mentioned object, in a test method of a semiconductor logic circuit device comprising a combinational portion having external input lines, pseudo external input lines, external output lines and pseudo external output lines; and a plurality of scan flip-flops connected between the pseudo external output lines and the pseudo external input lines, so that the scan flip-flops are connected in series to each other as a shift register, thus realizing at least one scan chain, a test cube including one or more unspecified bits in the external input lines and the pseudo external input lines is converted into a test vector including no unspecified bits so that the number of discrepancies between bits of the pseudo external input lines and respective bits of the pseudo external output lines is decreased. 
     Effect of the Invention 
     According to the present invention, since the number of output switching scan flip-flops in a capture operation is decreased, the capture power consumption can be decreased, thus avoiding an erroneous test caused by the decrease of the power supply voltage due to the IR drop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A block circuit diagram illustrating an embodiment of the semiconductor logic circuit device according to the present invention. 
         FIG. 2  A flowchart of a processing for converting a test cube including X-bits into a test vector including no X-bits in order to test the semiconductor logic circuit device of  FIG. 1 . 
         FIG. 3  A table showing the case types of step  202  of  FIG. 2 . 
         FIG. 4  A diagram showing the case types of step  202  of  FIG. 2 . 
         FIG. 5  A diagram explaining step  205  of  FIG. 2 . 
         FIG. 6  A diagram explaining steps  207  and  209  of  FIG. 2 . 
         FIG. 7  A table explaining the bit pair types of steps  210  of  FIG. 2 . 
         FIG. 8  A diagram explaining steps  216  and  218  of  FIG. 2 . 
     
    
    
     
       
         
           
               
             
               
                   
               
               
                 DESCRIPTION OF THE SYMBOLS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1L: 
                 combinational portion 
               
               
                   
                 12-1, 12-2, . . . , 12-n: 
                 scan flip-flops 
               
               
                   
                 PI: 
                 external input lines 
               
               
                   
                 PPI: 
                 pseudo external input lines 
               
               
                   
                 PO: 
                 external output lines 
               
               
                   
                 PPO: 
                 pseudo external output lines 
               
               
                   
                 SC:  
                 scan chain 
               
               
                   
                   
               
            
           
         
       
     
     BEST MODE CARRYING OUT THE INVENTION 
       FIG. 1  is a block circuit diagram illustrating an embodiment of the semiconductor logic circuit device according to the present invention. 
     The semiconductor logic circuit device of  FIG. 1  is constructed by a combinational portion  11  formed by logic elements such as AND gates, OR gates, NAND gates, and NOR gates, and scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n.    
     The combinational portion  11  has external input lines PI, pseudo external input lines PPI serving as output lines of the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n , external output lines PO, and pseudo external output lines PPO serving as input lines of the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n . Note that the number of the external input lines PI is not always the same as that of the external output lines PO; however, the number of the pseudo external input lines PPI is always the same as that of the pseudo external output lines PPO. 
     In each of the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n , a first input (lower input) terminal is used for a shift mode, while a second input (upper input) terminal is used for a capture mode. Therefore, when a scan enable (SE) signal is at logic value 1, the first inputs of the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n  are selected, while, when the scan enable (SE) signal is at logic value 0, the second inputs of the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n  are selected. Note that the first input terminal of the scan flip-flop  12 - 1  is connected to a scan-in terminal SI, and the output terminal of the scan flip-flop  12 - n  is connected to a scan-out terminal SO; however, the present invention is applied to not only a case where only one scan chain is provided, but also a case where a plurality of scan chains are provided. 
     That is, in a shift mode, the SE signal is set at logic value 1, so that the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n  read data from their lower inputs. Therefore, the scan flip-flop  12 - 1 , the scan flip-flop  12 - 2 , . . . , the scan flip-flop  12 - n  and the scan-out terminal SO are connected so that a scan chain SC is realized between the scan-in terminal SI and the scan-out terminal SO. At this time, the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n  serve as a shifter register operated by clock pulses of a clock CLK, so that a new test vector is shifted in, and simultaneously, a test response to a previous test vector is shifted out. 
     On the other hand, in a capture mode, the SE signal is set at logic value 0, so that, when clock pulses are given to the clock CLK, the logic values at the pseudo external output lines PPO of the combinational portion  11  are simultaneously taken in the scan flip-flops  12 - 1 ,  12 - 2 , . . . ,  12 - n.    
       FIG. 2  shows a process for converting a test cube including X-bits into a test vector including no X-bits by filling 0 or 1 in the X-bits of the test cube in the semiconductor logic circuit device of  FIG. 1 . Note that the test cube is assumed to be generated by an APTG program or an X-bit extracting program. Here, a test cube is defined as an input vector including X-bits corresponding to the external input lines PI and the pseudo external input lines PPI. In this case, since the external input lines PI and the pseudo external input lines PPI include X-bits, the external output lines PO and the pseudo external output lines PPO include X-bits. In other words, when the external input lines PI and the pseudo external input lines PPI include no X-bits, the external output lines PO and the pseudo external output lines PPO automatically include no X-bits. Thus, the routine of  FIG. 2  converts a test cube including X-bits into a test vector including no X-bits. 
     First, at step  201 , a test cube including X-bits obtained by an APTG program or an X-bit extracting program is given. 
     Next, at step  202 , the case type is determined. Here, there are four case types as the case type as shown in  FIG. 3 . That is, 
     Case type 1: A state where the pseudo external input lines PPI and the pseudo external output lines PPO have no X-bits as shown in (A) of  FIG. 4 ; 
     Case type 2: A state where the pseudo external input lines PPI have at least one X-bit and the pseudo external output lines PPO have no X-bits as shown in (B) of  FIG. 4 ; 
     Case type 3: A state where the pseudo external input lines PPI have no X-bits and the pseudo external output lines PPO have at least one X-bit as shown in (C) of  FIG. 4 ; and 
     Case type 4: A state where the pseudo external input lines PPI have at least one X-bit and the pseudo external output lines PPO have at least one X-bit as shown in (D) of  FIG. 4 . 
     In the case of Case type 1, the control proceeds to step  203 . At step  203 , the number of the output switching scan flip-flops cannot be decreased; however, since Case type 1 shows a test cube, there are always one or more X-bits in the external input lines PI as shown in (A) of  FIG. 4 . As a result, the external output lines P 0  may have one or more X-bits. At step  203 , 0 or 1 is filled in all the X-bits of the external input lines PI in order to decrease the number of test vectors and decrease the shift power consumption. As a result, at step  204 , a test vector with no X-bits can be obtained. 
     In the case of Case type 2, the control proceeds to step  205  which assigns to all the X-bits of the pseudo external input lines PPI logic values at corresponding bits of the pseudo external output lines PPO. For example, as shown in  FIG. 5 , logic value 0 at the corresponding bit “e” of the pseudo external output lines PPO is assigned to the X-bit “a” of the pseudo external input lines PPI, and also, logic value 1 at the corresponding bit “g” of the pseudo external output lines PPO is assigned to the X-bit “c” of the pseudo external input lines PPI. Thus, the output of the scan flip-flop corresponding to the bit “a” and the bit “e” and the output of the scan flip-flop corresponding to the bit “c” and the bit “g” are not switched in a capture operation. no X-bit. Then, the control proceeds to step  219 . 
     In the case of Case type 3, the control proceeds to step  206  which selects a target X-bit. First, it is determined whether the number of X-bits of the pseudo external output lines PPO is 1 or more than 1. When there is only one X-bit, this X-bit is selected as the target X-bit. When there are more than 1 X-bit, one of the X-bits to be processed is selected and this selected X-bit is a target X-bit. Generally, such an X-bit is selected in accordance with the sequence of success rates of X-bits in the justifying operation at next step  207 . 
     Next, at step  207 , a first justifying operation is carried out. That is, as shown in (A) of  FIG. 6 , a required logic value is assigned to one X-bit of the test cube so that the logic value V (for example, 1) of the bit of the pseudo external input lines PPI corresponding to the target X-bit may appear at the target X-bit of the test cube. As a result, if this assignment is successful, the control proceeds via step  208  to step  209 . 
     On the other hand, in the above-described first justifying operation, a necessary logic value may not be assigned and therefore, the assignment may fail. When this first justifying operation has failed, the control proceeds from step  208  to step  209  which carries out a second justifying operation. That is, as shown in (B) of  FIG. 6 , a necessary logic value is assigned to one X-bit of the test cube so that the logic value P (for example, 0) of the bit of the pseudo external input lines PPI corresponding to the target X-bit may appear at the target X-bit of the test cube. Then, the control proceeds to step  209 . 
       FIG. 6  is explained briefly below. In  FIG. 6 , assume that the X-bit “g” of the pseudo external output lines PPO is a target X-bit. In the first justifying operation ((A) of  FIG. 6 ), logic value 1 is determined for the X-bit “a” within the test cube, so that the logic value 1 at bit “c” of the pseudo external input lines PPI corresponding to the target X-bit “g” may appear at the target X-bit “g”. In this case, logic value 1 is assigned to the X-bit “a” within the test cube. In the second justifying operation ((B) of  FIG. 6 ), logic value 0 is determined for the X-bit “a” within the test cube, so that the value 0 opposite to the logic value 1 at bit “c” of the pseudo external input lines PPI corresponding to the target X-bit “g” may appear at the target X-bit “g”. In this case, logic value 0 is assigned to the X-bit “a” within the test cube. 
     In the case of Case type 4, first, at step  210 , attention is paid to each bit pair (ppi, ppo) of the pseudo external input lines PPI and the pseudo external output lines PPO, and the following types A, B, C and D as defined in  FIG. 7 : 
     Type A: A state where any of “ppi” and “ppo” is not an X-bit; 
     Type B: A state where “ppi” is an X-bit and “ppo” is not an X-bit; 
     Type C: A state where “ppi” is not an X-bit and “ppo” is an X-bit; and 
     Type D: A state where both of “ppi” and “ppo” are X-bits. 
     No processing is performed upon bit pairs of Type A. 
     Step  211  determines whether or not the number of bit pairs of Type B is zero, step  212  determines whether or not the number of bit pairs of Type C is zero, and step  213  determines whether or not the number of bit pairs of Type C is zero. As a result, when there are one or more bit pairs of Type B, the control proceeds to step  205  which performs an assigning operation upon all the bit pairs of Type B, and then, the control proceeds to step  219 . On the other hand, when there is no bit pair of Type B and there are one or more bit pairs of Type C, the control proceeds to step  213  which one bit pair of Type C as a target bit pair. Then, a first justifying operation at step  207  is performed upon the target bit pair, and also, when the first justifying operation has failed, the control proceeds from step  208  to step  209  which performs a second justifying operation upon the target bit pair. After that, the control proceeds to step  219 . Additionally, when there is no bit pair of Type B and no bit pair of Type C, the control proceeds to steps  214  and  218 . 
     At step  214 , it is determined whether or not the number of bit pairs of Type D is zero. As a result, when there is no bit pair of Type B and Type C and there are one or more bit pairs of Type D, the control proceeds to step  215 . 
     At step  215 , first, one of the bit pairs of Type D is selected as one target bit pair. Generally, a bit pair having a high success rate of a first assigning/justifying operation at next step  216  is selected. 
     At step  215 , a first assigning/justifying operation formed by a sub 1 operation and a sub 2 operation is carried out. For example, as shown in (A) of  FIG. 8 , the sub 1 operation uses logic value V (for example, 0), and the sub 2 operation uses its opposite logic value P (for example, 1). First, in the sub 1 operation, the logic value V is assigned to the X-bit “ppi” of the pseudo external input lines of the target bit pair, and a necessary logic value is determined for the X-bit of the test cube so that the logic value V may appear in the X-bit “ppo” of the pseudo external output lines of the target bit pair. In this sub 1 operation, the setting of the necessary logic value is impossible and therefore, the setting may fail. When the sub 1 operation has failed, the sub 2 operation is carried out using the above-mentioned operation by replacing the logic value V with the logic value P. 
     When the above-mentioned first assigning/justifying operation formed by the sub 1 operation and the sub 2 operation has failed, the control proceeds from step  217  to step  218  which carries out a second assigning/justifying operation formed by a sub 3 operation and a sub 4 operation. For example, as shown in (B) of  FIG. 8 , at the sub 3 operation, the logic value V is assigned to the X-bit “ppi” of the pseudo external input lines of the target bit pair, and a necessary logic value is determined for the X-bit of the test cube so that the logic value P may appear in the X-bit ppo of the pseudo external output lines of the target bit pair. When the sub 3 operation has failed, in the sub 4 operation, the logic value P is assigned to the X-bit “ppi” of the pseudo external input lines of the target bit pair, and a necessary logic value is determined for the X-bit of the test cube so that the logic value V may appear in the X-bit ppo of the pseudo external output lines of the target bit pair. 
     At step  219 , a logic simulation is carried out so that some or all of the X-bits at the external output lines PO and the pseudo external output lines PPO are determined. Then, the control returns to step  202 . 
     The above-described program of  FIG. 2  is stored in a memory medium. For example, if the memory medium is a non-volatile memory such as a ROM, the program is incorporated thereinto in advance, while, if the memory medium is a volatile memory such as a RAM, the program is written thereinto as occasion demands.