Abstract:
A method of detecting a failure in an IC, based on spectrum which is a result of analyzing a frequency of a current running through an IC when a test signal is applied to the IC, including (a) assuming that all ICs under test define a under-test IC set, and testing each one of the ICs in the under-test IC set in a conventional manner; (b) removing ICs judged to be in failure in (a), from the under-test IC set; (c) measuring spectrum of a current supplied from a power source into each one of the ICs in the under-test IC set; (d) calculating both a mean value and standard deviation of the spectrum for the under-test IC set; (e) judging whether an IC is in failure, based on both the mean value and the standard deviation of the spectrum; (f) removing ICs judged to be in failure in (e), from the under-test IC set; and (g) judging the undertest IC set to be in no failure.

Description:
This is a divisional of application Ser. No. 09/605,978 filed Jun. 29, 2000 now U.S. Pat. No. 6,704,675, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of detecting an integrated circuit in failure among a plurality of integrated circuits, and more particularly to a method of doing so, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit. 
     The invention relates also to an apparatus for detecting an integrated circuit in failure among a plurality of integrated circuits, and more particularly to an apparatus for doing so, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit. 
     The invention relates further to a recording medium readable by a computer, storing a program therein for causing a computer to either carry out the above-mentioned method or act as the above-mentioned apparatus. 
     2. Description of the Related Art 
     A method of detecting an integrated circuit in failure among integrated circuits has usually been carried out in order to detect and remove integrated circuits which are not capable of performing desired operations due to defectiveness in fabrication steps, and ship only integrated circuits which can properly operate. 
     For instance, the inventor had suggested a method of detecting an integrated circuit in failure in Proceedings of the 1998 IEICE General Conference C-12-8 “Diagnosis of failure in an integrated circuit by analysis of a current with power spectrum”. In this method, a current running through an integrated circuit while the integrated circuit is in operation is analyzed with respect to a frequency, to thereby detect an abnormal current caused by defectiveness in fabrication steps. 
     Japanese Unexamined Patent Publication No. 9-33604 has suggested a method of identifying a failure mode, comprising the steps of detecting a logic operation test pattern in which a power source current abnormally runs through CMOS logic circuit, which is one of internal circuits of an integrated circuit, in an amount greater than a predetermined amount while the CMOS logic circuit stops its logic operation, when a logic operation test pattern is input into the CMOS logic circuit through an input terminal thereof, extracting a characteristic between a power source voltage and a power source current with the detected logic operation test pattern being input into the CMOS logic circuit, and comparing the thus extracted characteristic between a power source voltage and a power source current to similarity between a failure mode and data between a power source voltage and a power source current, stored in a database, to thereby identify a failure mode occurring in the CMOS logic circuit. 
     Japanese Unexamined Patent Publication No. 9-211088 has suggested a method of detecting a failure in CMOS integrated circuit by observing a static power source current independent of switching of a transistor, among a current running through CMOS integrated circuit when a test pattern is applied to CMOS integrated circuit. The suggested method is carried out in an apparatus including means for repeatedly applying a test pattern to CMOS integrated circuit under test, means for measuring a power source current supplied to CMOS integrated circuit under test, and means for measuring power spectrum of the detected power source current. The method includes the step of judging whether CMOS integrated circuit is in failure or not by observing a magnitude of power having a predetermined frequency band, among the power spectrum of the power source current. 
     Japanese Unexamined Patent Publication No. 10-301843 has suggested a data processor comprising a main memory including a plurality of memories, and a plurality of processors transmitting a plurality of requests of data transfer to the main memory for each unit of data, and processing data transmitted from the main memory. The plurality of memories include means for detecting bank competition which occurs by the requests transmitted from the processors, and transmit a bank competition signal to the processors, and a circuit measuring a period of time during which bank competition occurs, based on the bank competition signal. 
     Japanese Unexamined Patent Publication No. 11-2663 has suggested a method of detecting a failure in CMOS integrated circuit by observing a static power source current running CMOS integrated circuit when a series of test patterns is repeatedly applied to CMOS integrated circuit from LSI tester. The method includes the steps of repeatedly applying test patterns to an integrated circuit under test and a reference integrated circuit which is identical with the integrated circuit under test and is not in failure, measuring a difference between a current running through the integrated circuit under test and a current running through the reference integrated circuit, and judging whether the integrated circuit under test is in failure or not, based on a magnitude of spectrum component having a repetition frequency at which the test patterns are repeated. 
     Japanese Unexamined Patent Publication No. 11-94917 has suggested a method of testing a semiconductor device on which CMOS circuit is mounted, comprising the steps of inputting a clock signal into the semiconductor device, and calculating a maximum operating frequency on the basis of an inverse number of a delay time during which a power source current starts increasing at a clock operation timing and then becomes steadily equal to zero. 
     However, the above-mentioned methods are all accompanied with a problem that a reference, that is, data about a power source current of an integrated circuit having no failure has to be prepared in advance in order to test integrated circuits under test. 
     It is quite difficult to prepare such a reference. The reason is as follows. Data about a power source current is analog data, and is much influenced by processing conditions in fabrication of an integrated circuit. As a result, there is slight fluctuation among data about a power source current of integrated circuits having no failures. Hence, it is quite difficult to define a reference to be used for testing integrated circuits as to whether they are in failure or not. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problem, it is an object of the present invention to provide a method of detecting an integrated circuit in failure among integrated circuits, without preparing a reference, that is, data about a power source current of an integrated circuit having no failures, by utilizing data about a power source current of integrated circuits under test. 
     It is also an object of the present invention to provide an apparatus for doing the same. 
     Another object of the present invention is to provide a recording medium readable by a computer, storing a program therein for causing a computer to carry out the above-mentioned method or act as the above-mentioned apparatus. 
     In one aspect of the present invention, there is provided a method of detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the method including the steps of (a) assuming that all integrated circuits under test define a under-test integrated circuit set, and testing each one of the integrated circuits in the under-test integrated circuit set in a conventional manner, (b) removing integrated circuits having been judged to be in failure in the step (a), from the under-test integrated circuit set, (c) measuring spectrum of a current supplied from a power source into each one of the integrated circuits in the under-test integrated circuit set, (d) calculating both a mean value and standard deviation of the spectrum for the under-test integrated circuit set, (e) judging whether an integrated circuit is in failure or in no failure, based on both the mean value and the standard deviation of the spectrum, (f) removing integrated circuits having been judged to be in failure in the step (e), from the under-test integrated circuit set, and (g) judging the under-test integrated circuit set to be in no failure. 
     It is preferable that the method further includes the step (h) of normalizing the spectrum, the step (h) being to be carried out subsequently to the step (c). 
     It is preferable that the step (h) further includes the steps of (h 1 ) summing up spectrum for all frequencies to have a total, and (h 2 ) calculating a ratio of spectrum for each one of frequencies to the total. 
     It is preferable that the step (e) further includes the steps of (e 1 ) calculating a gap between the spectrum and the mean value, (e 2 ) dividing the gap by the standard deviation, (e 3 ) comparing a quotient obtained in the step (e 2 ) to a predetermined value, and (e 4 ) judging an integrated circuit to be in failure, if the quotient is greater than the predetermined value, and judging an integrated circuit to be in no failure, if the quotient is equal to or smaller than the predetermined value. 
     There is further provided a method of detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the method including the steps of (a) assuming that all integrated circuits under test define a under-test integrated circuit set, and testing each one of the integrated circuits in the under-test integrated circuit set in a conventional manner, (b) removing integrated circuits having been judged to be in failure in the step (a), from the under-test integrated circuit set, (c) measuring spectrum of a current supplied from a power source into each one of the integrated circuits in the under-test integrated circuit set, (d) calculating a mean value and standard deviation of the spectrum for the under-test integrated circuit set, (e) checking whether the standard deviation is equal to or smaller than a predetermined value, (f) removing an integrated circuit having specific spectrum determined based on the mean value, from the under-test integrated circuit set, if the standard deviation is greater than the predetermined value, and repeating the steps (d), (e) and (f), and (g) judging the under-test integrated circuit set to be in no failure, if the standard deviation has been judged to be equal to or smaller than the predetermined value in the step (e). 
     It is preferable that the method further includes the step (h) of normalizing the spectrum, the step (h) being to be carried out subsequently to the step (c). 
     It is preferable that the step (h) further includes the steps of (h 1 ) summing up spectrum for all frequencies to have a total, and (h 2 ) calculating a ratio of spectrum for each one of frequencies to the total. 
     It is preferable that the step (f) further includes the steps of (f 1 ) calculating a gap between the spectrum and the mean value for each one of frequencies, (f 2 ) identifying an integrated circuit having a maximum gap among gaps calculated in the step (f 1 ), and (f 3 ) removing the integrated circuit identified in the step (f 2 ), from the under-test integrated circuit set. 
     There is still further provided a method of detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the method including the steps of (a) testing integrated circuits to find an integrated circuit in no failure, (b) measuring spectrum of a current supplied from a power source into the integrated circuit, (c) judging the integrated circuit to be in failure, (d) repeating the steps (a) to (c) until spectrum is measured for N integrated circuits wherein N is a predetermined integer, (e) calculating a mean value and standard deviation for each frequencies with respect to the spectrum of the N integrated circuits, (f) judging whether the spectrum is abnormal or not, based on the mean value and the standard deviation, (g) deleting data of spectrum having been judged abnormal in the step (f), and repeating the steps (a) to (f), (h) defining the mean value and the standard deviation as a reference, (i) testing integrated circuits to find an integrated circuit in no failure, (j) measuring spectrum of a current supplied from a power source into the integrated circuit, (k) judging whether the spectrum is abnormal or not, based on the reference, and judging an integrated circuit to be either in no failure if the spectrum is abnormal or in failure if the spectrum is not abnormal, (l) carrying out the steps (i) to (j) for integrated circuits not tested yet, (m) defining integrated circuits having been judged to be in failure as integrated circuits not tested yet, and (n) repeating the steps (i) to (l). 
     It is preferable that the step (k) further includes the step of updating the reference, based on data of the spectrum. 
     It is preferable that the method further includes the step (o) of normalizing the spectrum, the step (o) being to be carried out subsequently to at least one of the steps (b) and (j). 
     It is preferable that the step (o) includes the steps of (o 1 ) summing up spectrum for all frequencies to have a total, and (o 2 ) calculating a ratio of spectrum for each one of frequencies to the total. 
     It is preferable that the method further includes the step (o) of normalizing the spectrum, the step (o) being to be carried out subsequently to at least one of the steps (b) and (j). 
     It is preferable that the step (o) includes the steps of (o 1 ) summing up spectrum for all frequencies to have a total, and (o 2 ) calculating a ratio of spectrum for each one of frequencies to the total. 
     It is preferable that the step (f) further includes the steps of (f 1 ) calculating a gap between the spectrum and the mean value, (f 2 ) dividing the gap by the standard deviation, (f 3 ) comparing a quotient obtained in the step (f 2 ) to a predetermined value, and (f 4 ) judging the spectrum to be abnormal, if the quotient is greater than the predetermined value, and judging the spectrum not to be abnormal, if the quotient is equal to or smaller than the predetermined value. 
     It is preferable that the spectrum is judged not abnormal if the spectrum is equal to or smaller than the reference, and is judged abnormal if the spectrum is greater than the reference, in the step (k). 
     There is yet further provided a method of detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the method including the steps of (a) measuring spectrum of some of integrated circuits among integrated circuits under test, to thereby establish a reference, and (b) comparing the rest of integrated circuits among integrated circuits under test, to the reference to thereby judge whether each one of the rest of integrated circuits is in failure or not. 
     It is preferable that the method further includes the step of (c) judging whether the some of integrated circuits are in failure or not, based on the reference. 
     It is preferable that the method further includes the steps of (c) assuming that the some of integrated circuits are all in failure, and (d) judging again whether integrated circuits which have been judged to be in failure are in failure or not, after all integrated circuits have been judged as to whether they are in failure or not. 
     In another aspect of the present invention, there is provided an apparatus for detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the apparatus including (a) a tester which tests an integrated circuit in a conventional manner as to whether the integrated circuit is in failure or not, (b) a spectrum measurement unit which measures spectrum of the integrated circuit, (c) a first memory storing the spectrum therein, (d) a calculator calculating both a mean value and standard deviation of spectrum of all integrated circuits under test, based on the spectrum stored in the first memory, and (e) a controller which judges whether an integrated circuit is in failure or in no failure, based on both the mean value and the standard deviation of the spectrum. 
     It is preferable that the controller judges whether an integrated circuit is in failure or in no failure, based on comparison between a value defined as G/SD and a threshold value, wherein G indicates a gap between the spectrum and the mean value, and SD indicates the standard deviation. 
     It is preferable that the controller judges an integrated circuit to be in failure, if the value is greater than the threshold value, and judges an integrated circuit to be in no failure, if the value is equal to or smaller than the threshold value. 
     It is preferable that the apparatus further includes a second memory in which the threshold value is to be stored. 
     It is preferable that the controller judges that an integrated circuit having a maximum G/SD is in failure, when the standard deviation is greater than the threshold value. 
     It is preferable that the apparatus further includes a normalizer which normalizes the spectrum and replaces the previous spectrum with the normalized spectrum. 
     There is further provided an apparatus for detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the apparatus including (a) a tester which tests an integrated circuit in a conventional manner as to whether the integrated circuit is in failure or not, (b) a spectrum measurement unit which measures spectrum of the integrated circuit, (c) a first memory storing the spectrum therein, and (d) a controller which establishes a reference, based on spectrum of the predetermined number of integrated circuits under test, and judges whether an integrated circuit is in failure or in no failure, by comparing spectrum of each one of the integrated circuits under test to the reference. 
     It is preferable that the controller updates the reference, based on spectrum of an integrated circuit having been judged to be in no failure. 
     There is still further provided an apparatus for detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the apparatus including (a) a logic tester which tests an integrated circuit in a conventional manner as to whether the integrated circuit is in failure or not, (b) a spectrum measurement unit which measures spectrum of the integrated circuit, (c) a first memory storing the spectrum therein, (d) a calculator calculating both a mean value and standard deviation of spectrum of all integrated circuits under test, for each of frequencies, based on the spectrum stored in the first memory, and (e) a controller which judges whether an integrated circuit is in failure or in no failure, based on both the mean value and the standard deviation of the spectrum. 
     There is yet further provided an apparatus for detecting an integrated circuit in failure among integrated circuits, based on spectrum which is a result of analyzing a frequency of a current running through an integrated circuit when a test signal is applied to the integrated circuit, without preparing data of an integrated circuit in no failure, as a reference, the apparatus including (a) a logic tester which tests an integrated circuit in a conventional manner as to whether the integrated circuit is in failure or not, (b) a spectrum measurement unit which measures spectrum of the integrated circuit, (c) a first memory storing the spectrum therein, (d) a calculator calculating both a mean value and standard deviation of spectrum of all integrated circuits under test, for each of frequencies, based on the spectrum stored in the first memory, and (e) a controller which establishes a reference, based on spectrum of the predetermined number of integrated circuits under test, and judges whether an integrated circuit is in failure or in no failure, by comparing spectrum of each one of the integrated circuits under test to the reference. 
     It is preferable that the apparatus further includes (f) a first container for containing therein integrated circuits not tested yet, (g) a second container for containing therein integrated circuits having been judged to be in no failure, and (h) a third container for containing therein integrated circuits having been judged to be in failure. 
     It is preferable that the controller updates the reference, based on spectrum of an integrated circuit having been judged to be in no failure. 
     In still another aspect of the present invention, there is provided a recording medium readable by a computer, storing a program therein for causing a computer to carry out the above-mentioned method. 
     There is further provided a recording medium readable by a computer, storing a program therein for causing a computer to act as the above-mentioned apparatus. 
     The advantages obtained by the aforementioned present invention will be described hereinbelow. 
     As mentioned earlier, a method of detecting an integrated circuit in failure, based on spectrum of a power source current is accompanied with a problem that it is quite difficult to prepare a reference, that is, spectrum of a power source current of an integrated circuit having no failures, because of difference in processing conditions in fabrication of integrated circuits. The inventor found out the fact that almost all integrated circuits actually have no failures among integrated circuits having been judged to have no failures in accordance with a conventional method, and that quite a small number of integrated circuits having failures could not be detected in accordance with a conventional method. Based on this discovery, the inventor herein suggests a method of detecting an integrated circuit in failure by analyzing spectrum of a power source current of all integrated circuits under test. 
     Specifically, integrated circuits are all tested in accordance with a conventional method to thereby have an under-test integrated circuit set containing integrated circuits having been judged to have no failures in accordance with a conventional method. This under-test integrated circuit set contains integrated circuits which have failures, but have not been detected in accordance with a conventional manner, at a quite small rate, though. 
     Then, power source current spectrum is observed for each one of integrated circuits belonging to the under-test integrated circuit set, to thereby calculate a mean value and standard deviation. Then, a gap between spectrum and the mean value in each one of the integrated circuits is standardized by the standard deviation to thereby quantify the gap in the under-test integrated circuit set. 
     Though spectrum of integrated circuits having no failures are expected to distribute in the vicinity of the mean value, spectrum of integrated circuits having failures distribute much far away from the mean value. Accordingly, it would be possible to select integrated circuits having no failures by removing integrated circuits having spectrum distributing far away from the mean value, even without preparation of a reference, that is, data about spectrum of integrated circuits having no failures. 
     The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the apparatus in accordance with the first embodiment. 
         FIG. 2  is a flow-chart showing an operation of the apparatus in accordance with the first embodiment. 
         FIG. 3  is a block diagram of the apparatus in accordance with an example of the first embodiment. 
         FIG. 4  is a flow-chart showing an operation of the apparatus in accordance with the example of the first embodiment. 
         FIG. 5  is a block diagram of the apparatus in accordance with the second embodiment. 
         FIG. 6  is a flow-chart showing an operation of the apparatus in accordance with the second embodiment. 
         FIG. 7  is a block diagram of the apparatus in accordance with an example of the second embodiment. 
         FIG. 8  is a flow-chart showing an operation of the apparatus in accordance with the example of the second embodiment. 
         FIG. 9  is a block diagram of the apparatus in accordance with the third embodiment. 
         FIGS. 10 and 11  are flow-charts showing an operation of the apparatus in accordance with the third embodiment. 
         FIG. 12  is a block diagram of the apparatus in accordance with an example of the third embodiment. 
         FIGS. 13 and 14  are flow-charts showing an operation of the apparatus in accordance with the example of the third embodiment. 
         FIG. 15  is a block diagram of the apparatus in accordance with the fourth embodiment. 
         FIGS. 16 and 17  are flow-charts showing an operation of the apparatus in accordance with the fourth embodiment. 
         FIG. 18  is a block diagram of the apparatus in accordance with an example of the fourth embodiment. 
         FIGS. 19 and 20  are flow-charts showing an operation of the apparatus in accordance with the example of the fourth embodiment. 
         FIG. 21  is a block diagram of the apparatus in accordance with the fifth embodiment. 
         FIG. 22  is a flow-chart showing an operation of the apparatus in accordance with the fifth embodiment. 
         FIG. 23  is a block diagram of the apparatus in accordance with an example of the fifth embodiment. 
         FIG. 24  is a flow-chart showing an operation of the apparatus in accordance with the example of the fifth embodiment. 
         FIG. 25  is a block diagram of the apparatus in accordance with the sixth embodiment. 
         FIG. 26  is a flow-chart showing an operation of the apparatus in accordance with the sixth embodiment. 
         FIG. 27  is a block diagram of the apparatus in accordance with an example of the sixth embodiment. 
         FIG. 28  is a flow-chart showing an operation of the apparatus in accordance with the example of the sixth embodiment. 
         FIG. 29  is a block diagram of the apparatus in accordance with the seventh embodiment. 
         FIGS. 30 and 31  are flow-charts showing an operation of the apparatus in accordance with the seventh embodiment. 
         FIG. 32  is a block diagram of the apparatus in accordance with an example of the seventh embodiment. 
         FIGS. 33 and 34  are flow-charts showing an operation of the apparatus in accordance with the example of the seventh embodiment. 
         FIG. 35  is a block diagram of the apparatus in accordance with the eighth embodiment. 
         FIGS. 36 and 37  are flow-charts showing an operation of the apparatus in accordance with the eighth embodiment. 
         FIG. 38  is a block diagram of the apparatus in accordance with an example of the eighth embodiment. 
         FIGS. 39 and 40  are flow-charts showing an operation of the apparatus in accordance with the example of the eighth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings. 
     [First Embodiment] 
       FIG. 1  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the first embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  2 , a spectrum measurement unit  3 , a main controller  4 , a first memory  7 , a second memory  5 , and a calculator  6 . 
     The under-test integrated circuit set  2  contains a plurality of the same integrated circuits to be tested as to whether they are in failure or not. The tester  1  is controlled by the main controller  4 , and tests each one of integrated circuits in the under-test integrated circuit set  2  in accordance with a conventional method as to whether each one of integrated circuits is in failure or not. For instance, the tester  1  tests integrated circuits in accordance with the method mentioned in “The Electrical Handbook”, R. C. Dorf, 1993, pp. 1808-1816. 
     The test results are transmitted to the first memory  7  through the main controller  4 , and are stored in the first memory  7 . The spectrum measurement unit  3  is controlled by the main controller  4 . The spectrum measurement unit  3  applies a test signal to each one of integrated circuits in the under-test integrated circuit set  2 , observes a power source current running through the integrated circuit, and analyzes a frequency of the current. Power source current spectrum obtained by analysis of a frequency is stored in the first memory  7  through the main controller  4 . 
     The power source current spectrum is observed by the spectrum measurement unit  3  only for integrated circuits having been judged to have no failures by the tester  1 . On receipt of an instruction transmitted from the main controller  4 , the calculator  6  judges whether each one of integrated circuits in the under-test integrated circuit set is in failure or not, based on both data about power source current spectrum, stored in the first memory  7 , and data indicative of a predetermined value, stored in the second memory  5 , and outputs the results of judgement. 
       FIG. 2  is a flow-chart showing an operation of the apparatus in accordance with the first embodiment. Herein, the under-test integrated circuit set  2  containing a plurality of the same integrated circuits to be tested is indicated with “A”. At this stage, the integrated circuit set A is identical with the under-test integrated circuit set  2 . 
     First, integrated circuits in the integrated circuit set A are tested by the tester  1  as to whether they are in failure or not in accordance with conventional tests such as a performance test or a direct current test, in step S 101 . 
     The results of the test are transmitted to and stored in the first memory  7 . At the same time, integrated circuits having been judged to be in failure by the tester  1  are removed from the integrated circuit set A, in step S 102 . 
     The spectrum measurement unit  3  applies a test signal to each one of integrated circuits in the integrated circuit set A, that is, each one of the integrated circuits having been judged to be in no failure among the under-test integrated circuit set  2  in step S 101 , observes a power source current running through each one of the integrated circuits, and analyzes a frequency of the observed current. Spectrum of a power source current, obtained as a result of analysis of the frequency, is transmitted to and stored in the first memory  7 , in step S 103 . 
     Then, the calculator  6  calculates a mean value and standard deviation for each one of frequencies of the power source current spectrum of the integrated circuits in the integrated circuit set A, based on the spectrum of the integrated circuits in the integrated circuit set A, stored in the first memory  7 , in step S 104 . 
     In addition, the calculator  6  calculates G/SD for each one of the integrated circuits for each one of frequencies wherein G indicates a difference between the spectrum of each one of integrated circuits in the integrated circuit set A and the mean value, and SD indicates the standard deviation having been calculated in step S 104 . If the thus calculated G/SD is greater than the predetermined value stored in the second memory  5 , the calculator  6  judges that an integrated circuit having such G/SD is in failure, in step S 105 . 
     Then, it is judged in step S 106  whether there has been found an integrated circuit in failure in the integrated circuit set A. 
     If there has been found an integrated circuit having failures (YES in step S 106 ), such an integrated circuit is removed from the integrated circuit set A, in step S 107 , and steps S 104  to S 106  are repeated. 
     If there has not been found an integrated circuit having failures (NO in step S 106 ), all the integrated circuits in the integrated circuit set A are judged to be in no failure, in step S 108 . 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  4 . By carrying out the control program in the main controller  4 , the above-mentioned operation can be repeated. 
     Hereinbelow are explained advantages provided by the above-mentioned first embodiment. 
     When an integrated circuit is tested as to whether it is in failure or not, in accordance with a conventional method, the tested integrated circuit may have any failure, even if the integrated circuit is judged to be in no failure. In other words, it is not always possible to detect all failures in an integrated circuit in accordance with a conventional method. This means that a conventional method can detect merely about 95% of possible failures. 
     This is because ability of detecting failures is limited to practical one on the ground that if all failures in an integrated circuit are to be detected, it would take much time and cost. 
     In addition, there have been increased failure modes which could not be grouped in conventional failure models, as an integrated circuit has been fabricated in a smaller size, at a higher speed, and in higher integration. Such failure modes cannot be detected in accordance with conventional methods. 
     Thus, an integrated circuit often improperly operates due to failures which cannot be detected by conventional methods. It is quite important to detect failures as much as possible. In the apparatus in accordance with the first embodiment, data about a power source current is used in order to detect a failure in an integrated circuit. Namely, whereas the conventional methods use data about a voltage for detecting failures, the apparatus in accordance with the first embodiment uses data about a power source current to thereby make it possible to detect failures which could not be detected by the conventional methods. 
     However, since a power source current is an analog value, and is much influenced by processing conditions in fabrication of an integrated circuit, it would be necessary to prepare data about a power source current of an integrated circuit having no failures. It is quite difficult to prepare such data as a reference. 
     Such data as a reference can be obtained by carrying out simulation in which a test signal is applied to an integrated circuit. However, it would take much time to carry out such simulation for each of integrated circuits. 
     When a plurality of integrated circuits are tested in accordance with a conventional method as to whether they are in failure or not, a majority of integrated circuits having been judged to be in no failure has actually no failures, and quite a minority of integrated circuits has a failure or failures. Accordingly, it is considered that almost all spectrum of a power source current of integrated circuits having been judged to be in no failure in accordance with a conventional method exhibit almost the same value, and only a small number of integrated circuits having a failure exhibits abnormal spectrum of a power source current. 
     Thus, it is possible to identify integrated circuits having no failures among a plurality of integrated circuits without data about spectrum of a power source current of an integrated circuit having no failures, by observing spectrum of a power source current of integrated circuits having been judged to be in no failure in accordance with a conventional method, and judging that integrated circuits exhibiting power source current spectrum different from power source current spectrum exhibited by a majority of integrated circuits among tested integrated circuits are in failure. 
     In the above-mentioned first embodiment, integrated circuits exhibiting spectrum other than spectrum exhibited by a majority of integrated circuits are identified as integrated circuits having spectrum which has a great difference from a mean value. Specifically, a difference between spectrum of each one of integrated circuits and a mean value of spectrum in each of frequencies of the power source current spectrum is divided by standard deviation. If the thus calculated quotient is greater than a predetermined value, an integrated circuit associated with the spectrum is judged to be in failure. 
     By repeating the above-mentioned calculation, the mean value and standard deviation of the power source current spectrum are updated. Based on the thus updated mean value and standard deviation, an integrated circuit exhibiting power source current spectrum greater than a predetermined value is judged to be in failure and removed. At the time when no integrated circuits are judged to be in failure, it is judged that all of tested integrated circuits are in no failure. 
     As mentioned so far, the first embodiment makes it possible to select a majority of integrated circuits having no failures from a minority of integrated circuits having failures. 
     Hereinbelow is explained an example of the apparatus in accordance with the first embodiment. 
       FIG. 3  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the example of the first embodiment, and  FIG. 4  is a flow-chart showing an operation of the apparatus illustrated in FIG.  3 . 
     The illustrated apparatus is comprised of a logic tester  101 , an under-test integrated circuit set  102 , a spectrum measurement unit  103 , a main controller  104 , a first memory  107 , a second memory  105 , and a calculator  106 . 
     The under-test integrated circuit set  102  contains a plurality of the same integrated circuits to be tested as to whether they are in failure or not. The tester  101  is controlled by the main controller  104 , and tests each one of integrated circuits in the under-test integrated circuit set  102  in accordance with a conventional test such as a function test or DC test, as to whether each one of integrated circuits is in failure or not, in step S 1101 . Herein, a function test is a test by which each one of integrated circuits is tested as to whether it can satisfactorily accomplish expected performances. The function test is carried out by applying a test voltage signal to an integrated circuit through an input terminal thereof, and observing fluctuation of a voltage at an output terminal. DE test is a test by which each one of integrated circuits is tested as to whether input and output characteristic of each one of integrated circuits meets with predetermined characteristic. 
     The results of the test carried out by the logic tester  101  are transmitted to in the first memory  107 , and then, integrated circuits having been judged to be in no failure by the logic tester  101  are stored in the first memory  107  as an integrated circuit set A, in step S 1102 . 
     Then, the spectrum measurement unit  103  applies a test signal to each one of integrated circuits in the integrated circuit set A, observes a power source current running through each one of the integrated circuits, and analyzes a frequency of the observed power source current. Spectrum of a power source current, obtained as a result of analysis of the frequency, is transmitted to and stored in the first memory  107 , in step S 1103 . 
     Then, the calculator  106  calculates a mean value and standard deviation for each one of frequencies of the power source current spectrum of the integrated circuits in the integrated circuit set A, based on the spectrum of the integrated circuits in the integrated circuit set A, stored in the first memory  107 , in step S 1104 . 
     The calculator  106  calculates G/SD for each one of the integrated circuits for each one of frequencies wherein G indicates a difference between the spectrum of each one of integrated circuits in the integrated circuit set A and the mean value, and SD indicates the standard deviation having been calculated in step S 1104 . If the thus calculated G/SD is greater than a predetermined value stored in the second memory  105 , the calculator  106  judges that an integrated circuit having such G/SD is in failure, in step S 1105 . 
     Then, it is judged in step S 1106  whether there has been found an integrated circuit in failure in the integrated circuit set A. 
     If there has been found an integrated circuit having failures (YES in step S 1106 ), such an integrated circuit is removed from the integrated circuit set A in step S 1107 , and steps S 1104  to S 1106  are repeated. 
     Steps S 1104  to S 1106  are repeated until there is no integrated circuit judged to be in failure. Thus, if there has not been found an integrated circuit having failures (NO in step S 1106 ), all the integrated circuits in the integrated circuit set A are judged to be in no failure, in step S 1108 . 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104 . By carrying out the control program in the main controller  104 , the above-mentioned operation can be repeated. 
     [Second Embodiment] 
       FIG. 5  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the second embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  2 , a spectrum measurement unit  3 , a main controller  4   a , a first memory  7 , a second memory  5 , and a calculator  6 . 
     In comparison with the apparatus in accordance with the first embodiment, the apparatus in accordance with the second embodiment is designed to include the main controller  4   a  in place of the main controller  4 . The main controller  4   a  has different functions from those of the main controller  4 . 
       FIG. 6  is a flow-chart showing an operation of the apparatus in accordance with the second embodiment. Operation to be carried out in steps S 201 , S 202 , S 203  and S 204  in  FIG. 6  are the same as operation to be carried out in steps S 101 , S 102 , S 103  and S 104  in FIG.  2 . Hence, explanation about steps S 201 , S 202 , S 203  and S 204  is omitted. 
     The calculator  6  calculates a mean value and standard deviation for each one of frequencies of the power source current spectrum of integrated circuits in the integrated circuit set A, based on the spectrum of the integrated circuits in the integrated circuit set A, stored in the first memory  7 , in step S 204 . 
     Then, it is judged whether the standard deviation is greater than a predetermined value stored in the second memory  5 , in step S 205 . This judgement is carried out for each one of frequencies of the power source current spectrum. 
     If the standard deviation is greater than the predetermined value in a certain frequency (YES in step S 205 ), an integrated circuit having a maximum gap among gaps between spectrum of each one of the integrated circuits and the mean value of the spectrum of all the integrated circuits in the frequency is removed from the integrated circuit set A, in step S 206 . 
     Then, steps S 204  and S 205  are repeated until the standard deviation is equal to or smaller than the predetermined value in each one of the frequencies of the power source current. Thus, when the standard deviation becomes equal to or smaller than the predetermined value in each one of the frequencies of the power source current (NO in step S 205 ), all the integrated circuits in the integrated circuit set A are judged to be in no failure, in step S 207 . 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  4   a . By carrying out the control program in the main controller  4   a , the above-mentioned operation can be repeated. 
     Hereinbelow is explained advantages obtained by the above-mentioned second embodiment. 
     Almost all integrated circuits judged to be in no failure in accordance with a conventional test actually have no failures. If a test signal is applied to integrated circuits judged to be in no failure, a current behaves in the almost same manner in those integrated circuits, and hence, those integrated circuits have almost the same power source current spectrum. 
     Dispersion in power source current spectrum is caused by fluctuation in processing conditions in fabrication integrated circuits. Since such fluctuation can be predicted in advance, dispersion in power source current spectrum of integrated circuits judged to be in no failure can also be predicted. Dispersion in power source current spectrum can be indicated with standard deviation. Hence, in the second embodiment, degree of dispersion in power source current spectrum of integrated circuits judged to be in no failure is indicated with standard deviation, and is compared to a predetermined value. 
     Hence, if the integrated circuit set A comprised of integrated circuits having been judged to be in no failure in accordance with a conventional test contains integrated circuits having failures, the standard deviation of the power source current spectrum would be greater than a predetermined value. Thus, if the standard deviation is over a predetermined value, it is considered that an integrated circuit having a maximum gap among gaps between power source current spectrum of each one of integrated circuits and a mean value of power source current spectrum of all integrated circuits has any failures. By removing such an integrated circuit, there is obtained the integrated circuit set A comprised only of integrated circuits 
     In addition, the apparatus in accordance with the second embodiment also has the same advantages as those provided by the apparatus in accordance with the first embodiment. 
     Hereinbelow is explained an operation of the apparatus in accordance with the second embodiment with reference to a specific example. 
       FIG. 7  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the example of the second embodiment. 
     The illustrated apparatus is comprised of a logic tester  101 , an under-test integrated circuit set  102 , a spectrum measurement unit  103 , a main controller  104   a , a first memory  107 , a second memory  105  and a calculator  106 . 
     In comparison with the apparatus in accordance with the example of the first embodiment, illustrated in  FIG. 3 , the apparatus in accordance with the example of the second embodiment is designed to include the main controller  104   a  in place of the main controller  104 . The main controller  104   a  has different functions from those of the main controller  104 . 
       FIG. 8  is a flow-chart showing an operation of the apparatus in accordance with the example. Operation to be carried out in steps S 1201 , S 1202 , S 1203  and S 1204  in  FIG. 8  are the same as operation to be carried out in steps S 1101 , S 1102 , S 1103  and S 1104  in FIG.  4 . Hence, explanation about steps S 1201 , S 1202 , S 1203  and S 1204  is omitted. 
     The calculator  6  calculates standard deviation for each one of frequencies of the power source current spectrum of integrated circuits in the integrated circuit set A, based on the spectrum of the integrated circuits in the integrated circuit set A, stored in the first memory  107 , in step S 1204 . 
     Then, it is judged in the main controller  104   a  whether the standard deviation is greater than a predetermined value stored in the second memory  105 , in step S 1205 . This judgement is carried out for each one of frequencies of the power source current spectrum. 
     If the standard deviation is greater than the predetermined value in a certain frequency (YES in step S 1205 ), an integrated circuit having a maximum gap among gaps between spectrum of each one of integrated circuits and the mean value of the spectrum of all the integrated circuits in the frequency is removed from the integrated circuit set A, in step S 1206 . 
     Then, steps S 1204  and S 1205  are repeated until the standard deviation is equal to or smaller than the predetermined value in each one of the frequencies of the power source current. Thus, when the standard deviation becomes equal to or smaller than the predetermined value in each one of the frequencies of the power source current (NO in step S 1205 ), all the integrated circuits in the integrated circuit set A are judged to be in no failure, in step S 1207 . 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104   a . By carrying out the control program in the main controller  104   a , the above-mentioned operation can be repeated. 
     [Third Embodiment] 
       FIG. 9  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the third embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  12 , a spectrum measurement unit  3 , a main controller  4   b , a first memory  7 , a second memory  5 , a calculator  6 , a first set  9  comprised of integrated circuits having been judged to have no failures, and a second set  10  comprised of integrated circuits judged to have failures. 
     The under-test integrated circuit set  12  is comprised of a plurality of the same integrated circuits not tested yet. An integrated circuit  8  is selected one by one from the under-test integrated circuit set  12 , and is tested by the tester  1 . If the integrated circuit  8  is judged to have no failures, the integrated circuit  8  is introduced into the first set  9 . In contrast, if the integrated circuit  8  is judged to have failures, the integrated circuit  8  is introduced into the second set  10 . 
     The tester  1  is controlled by the main controller  4   b , and tests the integrated circuit  8  in accordance with conventional tests as to whether the integrated circuit  8  is in failure or not. 
     The test results are transmitted to the first memory  7  through the main controller  4   b , and are stored in the first memory  7 . The spectrum measurement unit  3  is controlled by the main controller  4   b . The spectrum measurement unit  3  applies a test signal to the integrated circuit  8 , observes a power source current running through the integrated circuit  8 , and analyzes a frequency of the power source current. Power source current spectrum obtained by analysis of a frequency is stored in the first memory  7  through the main controller  4   b.    
     On receipt of an instruction transmitted from the main controller  4   b , the calculator  6  judges whether the integrated circuit  8  in failure or not, based on both data about power source current spectrum of the integrated circuit  8 , stored in the first memory  7 , and data indicative of a predetermined value, stored in the second memory  5 . 
       FIGS. 10 and 11  are flow-charts showing an operation of the apparatus in accordance with the third embodiment. 
     As mentioned earlier, the under-test integrated circuit set  12  is comprised of integrated circuits not yet judged as to whether they are in failure or not. An integrated circuit is selected one by one from the under-test integrated circuit set  12 , and the thus selected integrated circuit  8  is tested by the tester  1  in accordance with conventional tests, in step S 301 . 
     Then, it is judged in step S 302  that the tested integrated circuit  8  is whether in failure or not. 
     If the tested integrated circuit  8  is judged to be in failure (YES in step S 302 ), the tested integrated circuit  8  is introduced into the second set  10  in step S 304 . Then, steps S 301  and S 302  are repeated. 
     In contrast, if the tested integrated circuit  8  is judged to be in no failure (NO in step S 302 ), the spectrum measurement unit  3  applies a test signal to the integrated circuit  8 , observes a power source current running through the integrated circuit, and analyzes a frequency of the observed power source current, in step S 303 . Spectrum of the power source current obtained as a result of analysis of the frequency of the observed current is transmitted to and stored in the first memory  7 . 
     Then, the integrated circuit  8  is introduced into the second set  10 , in step S 305 . 
     Then, it is judged in step S 306  whether the predetermined number N of the integrated circuits  8  have already been judged to be in no failure, in step S 306 . 
     If the number of the integrated circuits  8  having been judged to be in no failure is smaller than N (NO in step S 306 ), steps S 301 , S 302 , S 303  and S 305  are repeated. 
     In contrast, if the number of the integrated circuits  8  having been judged to be in no failure is equal to N (YES in step S 306 ), a mean value and standard deviation are calculated by the calculator  6  for each one of the frequencies of the power source current, based on the power source current spectrum of all the integrated circuits, stored in the first memory  7 , in step S 307 . 
     Then, the calculator  6  calculates a value defined as G/SD wherein G indicates a difference between the mean value of spectrum of all the integrated circuits and spectrum of each one of the integrated circuits, and SD indicates the standard deviation, in step S 308 . This calculation is carried out for each one of the frequencies of the power source current. 
     Then, it is judged in step S 309  whether the value G/SD is greater than a predetermined value stored in the second memory  5 . 
     If the value G/SD is greater than the predetermined value (YES in step S 309 ), the integrated circuit having the value G/SD is judged to be in failure, and data about power source current spectrum of the integrated circuit is removed from the first memory  7 , in step S 311 . Then, steps S 301 , S 302 , S 305 , S 306 , S 307 , S 309  are repeated. 
     If the value G/SD is equal to or smaller than the predetermined value (NO in step S 309 ), the mean value and the standard deviation are determined as a reference, and this reference is stored in the first memory  7 , in step S 310 . 
     Then, a next integrated circuit  8  is selected from the under-test integrated circuit set  12 , and is tested by the tester  1  as to whether it is in failure or not in accordance with conventional tests, in step S 312 . 
     Then, it is judged in step S 313  that the tested integrated circuit  8  is whether in failure or not. 
     If the tested integrated circuit  8  is judged to be in failure (YES in step S 313 ), the tested integrated circuit  8  is introduced into the second set  10  in step S 314 . Then, steps S 312  and S 313  are repeated. 
     In contrast, if the tested integrated circuit  8  is judged to be in no failure (NO in step S 313 ), the spectrum measurement unit  3  applies a test signal to the integrated circuit  8 , observes a power source current running through the integrated circuit, and analyzes a frequency of the observed power source current, to thereby obtain spectrum of the power source current as a result of analysis of the frequency of the observed current, in step S 315 . 
     Then, the calculator  6  evaluates the thus obtained spectrum in comparison with the reference stored in the first memory  7 , as follows. 
     First, there is calculated a value defined as G/SD wherein G indicates a difference between a mean value of the reference and the power source current spectrum, and SD indicates standard deviation of the reference, in step S 316 . Then, it is judged in step S 317  whether the value G/SD is equal to or smaller than a predetermined value stored in the second memory  5  in each one of the frequencies of the power source current. 
     If the value G/SD is equal to or smaller than the predetermined value (YES in step S 317 ), an integrated circuit having the value G/SD is judged to be in no failure, in step S 318 . Then, the integrated circuit is introduced into the first set  9 . 
     In contrast, if the value G/SD is greater than the predetermined value (NO in step S 317 ), an integrated circuit having the value G/SD is judged to be in failure, in step S 319 . Then, the integrated circuit is introduced into the second set  10 . 
     Then, it is checked in step S 320  whether the under-test integrated circuit set  12  is not empty, that is, the under-test integrated circuit set  12  still contains integrated circuits not yet tested. 
     If the under-test integrated circuit set  12  still contains integrated circuits not yet tested (YES in step S 320 ), steps S 312 , S 313 , S 315 , S 316  and S 317  are repeatedly carried out. 
     If the under-test integrated circuit set  12  is empty, that is, the under-test integrated circuit set  12  does no longer contain integrated circuits not tested (NO in step S 320 ), the integrated circuits in the second set  10  are tested again in step S 321 . Specifically, the integrated circuits contained in the second set  10  are all transferred into the under-test integrated circuit set  12 . As a result, the second set  10  becomes empty. 
     Then, a next integrated circuit  8  is selected from the under-test integrated circuit set  12 , and is tested by the tester  1  as to whether it is in failure or not in accordance with conventional tests, in step S 322 . 
     Then, it is judged in step S 323  that the tested integrated circuit  8  is whether in failure or not. 
     If the tested integrated circuit  8  is judged to be in failure (YES in step S 323 ), the tested integrated circuit  8  is introduced into the second set  10  in step S 324 . Then, steps S 322  and S 323  are repeated. 
     In contrast, if the tested integrated circuit  8  is judged to be in no failure (NO in step S 323 ), the spectrum measurement unit  3  applies a test signal to the integrated circuit  8 , observes a power source current running through the integrated circuit, and analyzes a frequency of the observed power source current, to thereby obtain spectrum of the power source current as a result of analysis of the frequency of the observed current, in step S 325 . 
     Then, the calculator  6  evaluates the thus obtained spectrum in comparison with the reference stored in the first memory  7 , as follows. 
     First, there is calculated a value defined as G/SD wherein G indicates a difference between a mean value of the reference and the power source current spectrum, and SD indicates standard deviation of the reference, in step S 326 . Then, it is judged in step S 327  whether the value G/SD is equal to or smaller than a predetermined value stored in the second memory  5  in each one of the frequencies of the power source current. 
     If the value G/SD is equal to or smaller than the predetermined value (YES in step S 327 ), an integrated circuit having the value G/SD is judged to be in no failure, in step S 328 . Then, the integrated circuit is introduced into the first set  9 . 
     In contrast, if the value G/SD is greater than the predetermined value (NO in step S 327 ), an integrated circuit having the value G/SD is judged to be in failure, in step S 329 . Then, the integrated circuit is introduced into the second set  10 . 
     Then, it is checked in step S 330  whether the under-test integrated circuit set  12  is not empty, that is, the under-test integrated circuit set  12  still contains integrated circuits not yet tested. 
     If the under-test integrated circuit set  12  still contains integrated circuits not yet tested (YES in step S 330 ), steps S 322 , S 323 , S 325 , S 326  and S 327  are repeatedly carried out. 
     If the under-test integrated circuit set  12  is empty, that is, the under-test integrated circuit set  12  does no longer contain integrated circuits not tested (NO in step S 330 ), the test is ended. 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  4   b . By carrying out the control program in the main controller  4   b , the above-mentioned operation can be repeated. 
     Hereinbelow are explained advantages obtained by the above-mentioned third embodiment. 
     In a conventional method of identifying an integrated circuit in failure among a plurality of integrated circuits, a reference is in advance prepared. Hence, each one of integrated circuits is judged whether it is in failure or not by comparing it to the reference. 
     However, it is generally difficult for the above-mentioned reasons to prepare an accurate reference. Hence, in accordance with the third embodiment, integrated circuits are all tested once. Then, there is established a reference, based on the result of the test. Then, all integrated circuits are judged whether they are in failure or not by comparing them to the thus established reference. 
     In the above-mentioned first and second embodiments, each one of integrated circuits to be tested is numbered, and thus, data about power source current spectrum is stored in the first memory  7  in association with each one of the integrated circuits. Hence, all the integrated circuits are judged whether they are in failure or not after data of power source current spectrum of all the integrated circuits has been collected, and integrated circuits in failure can be identified by detecting abnormal spectrum. 
     However, in accordance with the first and second embodiments, each one of integrated circuits to be tested has to be numbered and correlated with power source current spectrum, which might take extra time to test all the integrated circuits. 
     Hence, in accordance with the third embodiment, power source current spectrum is observed in a certain number of integrated circuits only for the purpose of establishing a reference. After power source current spectrum has been observed in a number of integrated circuits sufficient to establish a reference, the rest of integrated circuits are judged as to whether they are in failure or not, based on the thus established reference. 
     However, the integrated circuits used only for establishing a reference are not yet judged as to whether they are in failure or not. Hence, they are preferably judged as to whether they are in failure or not, based on the established reference. To this end, they are provisionally determined to be in failure when a reference is established. Then, after integrated circuits have been all judged as to whether they are in failure or not, integrated circuits having been judged to be in failure are judged again as to whether they are in failure or not. Thus, it is possible to identify all integrated circuits having no failures, and to establish a reference with a small number of steps. 
     In place of judging integrated circuits used for establishing a reference, to be in failure, they might be provisionally determined to be in semi-failure to distinguish from integrated circuits judged to be in failure. When all integrated circuits are judged again as to whether they are in failure or not after a reference has been established, only those integrated circuits in semi-failure are judged as to whether they are in failure or not. As a result, it is no longer necessary to judge again whether integrated circuits having been once judged to be in failure, are in failure or not. 
     Hereinbelow is explained an operation of the third embodiment with reference to an example. 
       FIG. 12  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with an example of the third embodiment. The illustrated apparatus is comprised of a logic tester  101 , a spectrum measurement unit  103 , a main controller  104   b , a first memory  107 , a second memory  105 , a calculator  106 , a first container  109  containing integrated circuits having been judged to have no failures, a second container  110  containing integrated circuits judged to have failures, and a third container  112  containing integrated circuits to be tested. 
       FIGS. 13 and 14  are flow-charts showing an operation of the apparatus in accordance with the example of the third embodiment. 
     As mentioned earlier, the third container  112  contains all of integrated circuits which are to be judged as to whether they are in failure or not. An integrated circuit is selected one by one from the third container  112 , and the thus selected integrated circuit  108  is tested by the tester  101  in accordance with conventional tests such as a function test and DC test, in step S 1301 . 
     Then, it is judged in step S 1302  that the tested integrated circuit  8  is whether in failure or not. 
     If the tested integrated circuit  108  is judged to be in failure (YES in step S 1302 ), the tested integrated circuit  108  is introduced into the second container  110  in step S 1304 . Then, steps S 1301  and S 1302  are repeated. 
     In contrast, if the tested integrated circuit  108  is judged to be in no failure (NO in step S 1302 ), the spectrum measurement unit  103  applies a test signal to the integrated circuit  108 , observes a power source current running through the integrated circuit, and analyzes a frequency of the observed power source current, in step S 1303 . Spectrum of the power source current obtained as a result of analysis of the frequency of the observed current is transmitted to and stored in the first memory  107 . 
     Then, the integrated circuit  108  having been judged to be in no failure is introduced into the second container  110 , in step S 1305 . 
     Then, it is judged in step S 1306  whether the predetermined number N of the integrated circuits  108  have already been judged to be in no failure, in step S 1306 . 
     If the number of the integrated circuits  108  having been judged to be in no failure is smaller than N (NO in step S 1306 ), steps S 1301 , S 1302 , S 1303  and S 1305  are repeated. 
     In contrast, if the number of the integrated circuits  8  having been judged to be in no failure is equal to N (YES in step S 1306 ), a mean value and standard deviation are calculated by the calculator  106  for each one of the frequencies of the power source current, based on the power source current spectrum of all the integrated circuits, stored in the first memory  107 , in step S 1307 . 
     Then, the calculator  106  calculates a value defined as G/SD wherein G indicates a difference between the mean value of spectrum of all the integrated circuits and spectrum of each one of the integrated circuits, and SD indicates the standard deviation, in step S 1308 . This calculation is carried out for each one of the frequencies of the power source current. 
     Then, it is judged in step S 1309  whether the value G/SD is greater than a predetermined value stored in the second memory  105 . 
     If the value G/SD is greater than the predetermined value (YES in step S 1309 ), the integrated circuit  108  having the value G/SD is judged to be in failure, and data about power source current spectrum of the integrated circuit is removed from the first memory  107 , in step S 1311 . Then, steps S 1301 , S 1302 , S 1305 , S 1306 , S 1307 , S 1309  are repeated. 
     If the value G/SD is equal to or smaller than the predetermined value (NO in step S 1309 ), the mean value and the standard deviation are determined as a reference, and this reference is stored in the first memory  107 , in step S 1310 . 
     Then, a next integrated circuit  108  is selected from the third container  112 , and is tested by the logic tester  101  as to whether it is in failure or not in accordance with conventional tests such as a function test and DC test, in step S 1312 . 
     Then, it is judged in step S 1313  that the tested integrated circuit  108  is whether in failure or not. 
     If the tested integrated circuit  108  is judged to be in failure (YES in step S 1313 ), the tested integrated circuit  108  is introduced into the second container  110  in step S 1314 . Then, steps S 1312  and S 1313  are repeated. 
     In contrast, if the tested integrated circuit  108  is judged to be in no failure (NO in step S 1313 ), the spectrum measurement unit  103  applies a test signal to the integrated circuit  108 , observes a power source current running through the integrated circuit, and analyzes a frequency of the observed power source current, to thereby obtain spectrum of the power source current as a result of analysis of the frequency of the observed current, in step S 1315 . 
     Then, the calculator  106  evaluates the thus obtained spectrum in comparison with the reference stored in the first memory  107 , as follows. 
     First, there is calculated a value defined as G/SD wherein G indicates a difference between a mean value of the reference and the power source current spectrum, and SD indicates standard deviation of the reference, in step S 1316 . Then, it is judged in step S 1317  whether the value G/SD is equal to or smaller than a predetermined value stored in the second memory  105  in each one of the frequencies of the power source current. 
     If the value G/SD is equal to or smaller than the predetermined value (YES in step S 1317 ), an integrated circuit having the value G/SD is judged to be in no failure, in step S 1318 . Then, the integrated circuit is introduced into the first container  109 . 
     In contrast, if the value G/SD is greater than the predetermined value (NO in step S 1317 ), an integrated circuit having the value G/SD is judged to be in failure, in step S 1319 . Then, the integrated circuit is introduced into the second container  110 . 
     Then, it is checked in step S 1320  whether the third container  112  still contains integrated circuits not yet tested. 
     If the third container  112  still contains integrated circuits not yet tested (YES in step S 1320 ), steps S 1312 , S 1313 , S 1315 , S 1316  and S 1317  are repeatedly carried out. 
     If the third container  112  does no longer contain integrated circuits not tested (NO in step S 1320 ), the integrated circuits in the second container  110  are tested again in step S 1321 . Specifically, the integrated circuits contained in the second container  110  are all transferred into the third container  112 . 
     Then, a next integrated circuit  108  is selected from the third container  112 , and is tested by the logic tester  101  as to whether it is in failure or not in accordance with conventional tests such as a function test and DC test, in step S 1322 . 
     Then, it is judged in step S 1323  that the tested integrated circuit  108  is whether in failure or not. 
     If the tested integrated circuit  108  is judged to be in failure (YES in step S 1323 ), the tested integrated circuit  108  is introduced into the second container  110  in step S 1324 . Then, steps S 1322  and S 1323  are repeated. 
     In contrast, if the tested integrated circuit  108  is judged to be in no failure (NO in step S 1323 ), the spectrum measurement unit  103  applies a test signal to the integrated circuit  108 , observes a power source current running through the integrated circuit, and analyzes a frequency of the observed power source current, to thereby obtain spectrum of the power source current as a result of analysis of the frequency of the observed current, in step S 1325 . 
     Then, the calculator  106  evaluates the thus obtained spectrum in comparison with the reference stored in the first memory  107 , as follows. 
     First, there is calculated a value defined as G/SD wherein G indicates a difference between a mean value of the reference and the power source current spectrum, and SD indicates standard deviation of the reference, in step S 1326 . Then, it is judged in step S 1327  whether the value G/SD is equal to or smaller than a predetermined value stored in the second memory  105  in each one of the frequencies of the power source current. 
     If the value G/SD is equal to or smaller than the predetermined value (YES in step S 1327 ), the integrated circuit having the value G/SD is judged to be in no failure, in step S 1328 . Then, the integrated circuit is introduced into the first container  109 . 
     In contrast, if the value G/SD is greater than the predetermined value (NO in step S 1327 ), an integrated circuit having the value G/SD is judged to be in failure, in step S 1329 . Then, the integrated circuit is introduced into the second container  110 . 
     Then, it is checked in step S 1330  whether the third container  112  still contains integrated circuits not yet tested. 
     If the third container  112  still contains integrated circuits not yet tested (YES in step S 1330 ), steps S 1322 , S 1323 , S 1325 , S 1326  and S 1327  are repeatedly carried out. 
     If the third container  112  does no longer contain integrated circuits not tested (NO in step S 1330 ), the test is ended. 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104   b . By carrying out the control program in the main controller  104   b , the above-mentioned operation can be repeated. 
     [Fourth Embodiment] 
       FIG. 15  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the fourth embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  12 , a spectrum measurement unit  3 , a main controller  4   c , a first memory  7 , a second memory  5 , a calculator  6 , a first set  9  comprised of integrated circuits having been judged to have no failures, and a second set  10  comprised of integrated circuits judged to have failures. 
     In comparison with the apparatus in accordance with the third embodiment, illustrated in  FIG. 9 , the apparatus in accordance with the fourth embodiment is designed to include the main controller  4   c  in place of the main controller  4   b . The main controller  4   c  has different functions from those of the main controller  4   b.    
       FIGS. 16 and 17  are flow-charts showing an operation of the apparatus in accordance with the fourth embodiment. 
     An operation of the apparatus in accordance with the fourth embodiment is identical with the operation of the apparatus in accordance with the third embodiment except the following step. 
     As shown in step S 420 , when an integrated circuit  8  has been judged to be in no failure in step S 418 , a reference is updated. Specifically, the power source current spectrum of the integrated circuit  8  having been judged to be in no failure in step S 418  is transmitted into and stored in the first memory  7 . The first memory  7  already stores data about power source current spectrum of integrated circuits used for establishing a reference. The calculator  6  calculates again a mean value and standard deviation of power source current spectrum of all integrated circuits, based on both data about the spectrum already stored in the first memory  7  and data about the spectrum additionally stored in the first memory  7 . The thus calculated mean value and standard deviation are stored in the first memory  7  as an updated reference, in step S 420 . 
     The steps other than step S 420  are identical with the associated steps in the third embodiment. Specifically, steps S 401  to S 419  and steps S 421  to S 431  in the fourth embodiment, illustrated in  FIGS. 16 and 17  are identical with steps S 301  to S 319  and steps S 320  to S 330  in the third embodiment, illustrated in  FIGS. 10 and 11 , respectively. 
     Hereinbelow is explained advantages obtained by the above-mentioned fourth embodiment. 
     In the third embodiment, the power source current spectrum of N integrated circuits having been judged to be in no failure was used to establish a reference. Though it is preferable that a reference is established based on the power source current spectrum of all of integrated circuits to be tested, it is quite difficult to establish such a reference, because observation of power source current spectrum is successively carried out, and at that time an integrated circuit has to be judged as to whether it is in failure or not. 
     Hence, in accordance with the fourth embodiment, a reference is successively updated, based on power source current spectrum of the integrated circuits having been judged to be in no failure. Thus, the fourth embodiment can update a reference and finally establish an ideal reference. 
     Hereinbelow is explained an example of the fourth embodiment. 
       FIG. 18  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with an example of the fourth embodiment. The illustrated apparatus is comprised of a logic tester  101 , a spectrum measurement unit  103 , a main controller  104   c , a first memory  107 , a second memory  105 , a calculator  106 , a first container  109  containing integrated circuits having been judged to have no failures, a second container  110  containing integrated circuits judged to have failures, and a third container  112  containing integrated circuits to be tested. 
     In comparison with the apparatus in accordance with the third embodiment, illustrated in  FIG. 12 , the apparatus in accordance with the example of the fourth embodiment is designed to include the main controller  104   c  in place of the main controller  104   b . The main controller  104   c  has different functions from those of the main controller  104   b.    
       FIGS. 19 and 20  are flow-charts showing an operation of the apparatus in accordance with the example of the fourth embodiment. 
     As shown in step S 1420 , when an integrated circuit  108  has been judged to be in no failure in step S 1418 , a reference is updated. Specifically, the power source current spectrum of the integrated circuit  108  having been judged to be in no failure in step S 1418  is transmitted into and stored in the first memory  107 . The first memory  107  already stores data about power source current spectrum of integrated circuits used for establishing a reference. The calculator  106  calculates again a mean value and standard deviation of power source current spectrum of all integrated circuits, based on both data about the spectrum already stored in the first memory  107  and data about the spectrum additionally stored in the first memory  107 . The thus calculated mean value and standard deviation are stored in the first memory  107  as an updated reference, in step S 1420 . 
     The steps other than step S 1420  are identical with the associated steps in the third embodiment. Specifically, steps S 1401  to S 1419  and steps S 1421  to S 1431  in the example of the fourth embodiment, illustrated in  FIGS. 19 and 20  are identical with steps S 1301  to S 1319  and steps S 1320  to S 1330  in the third embodiment, illustrated in  FIGS. 13 and 14 , respectively. 
     [Fifth Embodiment] 
       FIG. 21  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the fifth embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  2 , a spectrum measurement unit  3 , a main controller  4   d , a first memory  7 , a second memory  5 , a calculator  6 , and a normalizer  11 . 
     In comparison with the apparatus in accordance with the first embodiment, illustrated in  FIG. 1 , the apparatus in accordance with the fifth embodiment is designed to include the main controller  4   d  in place of the main controller  4  and additionally include the normalizer  11 . The main controller  4   d  has different functions from those of the main controller  4 . The normalizer  11  is electrically connected to the spectrum measurement unit  3 , normalizes power source current spectrum measured by the spectrum measurement unit  3 , and transmits the result of normalization to the first memory  7 . 
       FIG. 22  is a flow-chart showing an operation of the apparatus in accordance with the fifth embodiment. Herein, the under-test integrated circuit set  2  containing a plurality of the same integrated circuits to be tested is indicated with “A”. At this stage, the integrated circuit set A is identical with the under-test integrated circuit set  2 . 
     First, integrated circuits in the integrated circuit set A are tested by the tester  1  as to whether they are in failure or not in accordance with conventional tests, in step S 501 . 
     The results of the test are transmitted to and stored in the first memory  7 . At the same time, integrated circuits having been judged to be in failure by the tester  1  are removed from the integrated circuit set A, in step S 502 . 
     The spectrum measurement unit  3  applies a test signal to each one of integrated circuits in the integrated circuit set A, that is, each one of the integrated circuits having been judged to be in no failure among the under-test integrated circuit set  2  in step S 501 , observes a power source current running through each one of the integrated circuits, and analyzes a frequency of the observed current, in step S 503 . 
     Spectrum of a power source current, obtained as a result of analysis of the frequency, is transmitted to the normalizer  11 , and then, is normalized by the normalizer  11 , in step S 504 . For instance, the normalizer  11  calculates a sum of the power source current spectrum for each one of the frequencies of the observed spectrum, and divides the sum by the power source current spectrum for each one of the frequencies. The thus calculated quotient is defined as normalized spectrum. 
     Then, the calculator  6  calculates a mean value and standard deviation for each one of frequencies of the power source current spectrum of the integrated circuits in the integrated circuit set A, based on the spectrum of the integrated circuits in the integrated circuit set A, stored in the first memory  7 , in step S 505 . 
     In addition, the calculator  6  calculates G/SD for each one of the integrated circuits for each one of frequencies wherein G indicates a difference between the spectrum of each one of integrated circuits in the integrated circuit set A and the mean value, and SD indicates the standard deviation having been calculated in step S 505 . If the thus calculated G/SD is greater than the predetermined value stored in the second memory  5 , the calculator  6  judges that an integrated circuit having such G/SD is in failure, in step S 506 . 
     Then, it is judged in step S 507  whether there has been found an integrated circuit in failure in the integrated circuit set A. 
     If there has been found an integrated circuit having failures (YES in step S 507 ), such an integrated circuit is removed from the integrated circuit set A, in step S 508 , and steps S 505  to S 507  are repeated. 
     If there has not been found an integrated circuit having failures (NO in step S 507 ), all the integrated circuits in the integrated circuit set A are judged to be in no failure, in step S 509 . 
     Hereinbelow is explained advantaged obtained by the above-mentioned fifth embodiment. 
     As mentioned earlier, power source current spectrum is obtained by analyzing a frequency of a power source current running through an integrated circuit when a test signal is applied to the integrated circuit. A power source current is an analog value, and hence, is likely to be much influenced by fluctuation in processing conditions in fabrication of integrated circuits. Since fluctuation in processing conditions is unavoidable, it is also unavoidable for a power source current or power source current spectrum to be influenced by fluctuation in processing conditions. 
     In the fifth embodiment, since an integrated circuit is judged as to whether it is in failure or not by detecting an abnormal power source current caused by a failure in an integrated circuit, it is necessary to distinguish fluctuation in power source current spectrum caused by a failure in an integrated circuit from fluctuation in power source current spectrum caused by fluctuation in processing conditions. 
     It is considered that fluctuation in processing conditions merely causes uniform fluctuation in power source current spectrum in each of the frequencies of the spectrum. Thus, by normalizing the power source current spectrum in each one of the frequencies with a sum of the power source current spectrum in all of the frequencies, it would be possible to avoid the power source current spectrum from being influenced by fluctuation in processing conditions, ensuring higher accuracy with which a failure is detected. 
     Hereinbelow is explained one example of the fifth embodiment. 
       FIG. 23  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with an example of the fifth embodiment. The illustrated apparatus is comprised of a logic tester  101 , an under-test integrated circuit set  102 , a spectrum measurement unit  103 , a main controller  104   d , a first memory  107 , a second memory  105 , a calculator  106 , and a normalizer  111 . 
     In comparison with the apparatus in accordance with the first embodiment, illustrated in  FIG. 1 , the apparatus in accordance with the example of the fifth embodiment is designed to include the main controller  104   d  in place of the main controller  4  and additionally include the normalizer  111 . The main controller  4   d  has different functions from those of the main controller  4 . 
     The normalizer  111  is electrically connected to the spectrum measurement unit  103 , normalizes power source current spectrum measured by the spectrum measurement unit  103 , and transmits the result of normalization to the first memory  107 . 
       FIG. 24  is a flow-chart showing an operation of the apparatus in accordance with the example of the fifth embodiment. 
     As shown in step S 1504 , power source current spectrum observed by the spectrum measurement unit  103  is normalized by the normalizer  111 . The thus normalized spectrum is transmitted to and stored in the first memory  107 . 
     The normalizer  111  normalizes power source current spectrum as follows, for instance. 
     Herein, it is assumed that power source current spectrum in each of frequencies is indicated as p( 1 ), p( 2 ), p( 3 ), - - - , p(n), wherein p(i) indicates power source current spectrum at a frequency i. It is also assumed that a sum of power source current spectrum in all of the frequencies is indicated as S. That is, S is defined as follows.
 
 S=p ( 1 )+ p ( 2 )+ p ( 3 )+ - - - + p ( n )
 
     By dividing p(i) by S, there are obtained p( 1 )/S, p( 2 )/S, p( 3 )/S, - - - , p(n)/S, with which the previous spectrum are replaced. Namely, p( 1 )/S, p( 2 )/S, p( 3 )/S, - - - , p(n)/S are used as power source current spectrum hereinafter. 
     The steps other than step S 1504  are identical with the associated steps in the first embodiment. Specifically, steps S 1501  to S 1503  and steps S 1505  to S 1509  in the example of the fifth embodiment, illustrated in  FIG. 24  are identical with steps S 1101  to S 1103  and steps S 1104  to S 1108  in the first embodiment, illustrated in  FIG. 4 , respectively. 
     The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104   d . By carrying out the control program in the main controller  104   d , the above-mentioned operation can be repeated. 
     [Sixth Embodiment] 
       FIG. 25  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the sixth embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  2 , a spectrum measurement unit  3 , a main controller  4   e , a first memory  7 , a second memory  5 , a calculator  6 , and a normalizer  11 . 
     In comparison with the apparatus in accordance with the second embodiment, illustrated in  FIG. 5 , the apparatus in accordance with the sixth embodiment is designed to include the main controller  4   e  in place of the main controller  4   a  and additionally include the normalizer  11 . The main controller  4   e  has different functions from those of the main controller  4   a . The normalizer  11  is electrically connected to the spectrum measurement unit  3 , normalizes power source current spectrum measured by the spectrum measurement unit  3 , and transmits the result of normalization to the first memory  7 . 
       FIG. 26  is a flow-chart showing an operation of the apparatus in accordance with the sixth embodiment. 
     An operation to be carried out in step S 604  in  FIG. 26  is identical with an operation to be carried out in step S 504  in FIG.  22 . An operation of the apparatus in accordance with the sixth embodiment, to be carried out in other steps is identical with an operation of the apparatus in accordance with the second embodiment. Specifically, an operation to be carried out in steps S 601 , S 602 , S 603 , S 605 , S 606 , S 607  and S 608  in  FIG. 26  is identical with an operation to be carried out in steps S 201 , S 202 , S 203 , S 204 , S 205 , S 206  and S 207  in FIG.  6 . 
     The above-mentioned operation of the apparatus in accordance with the sixth embodiment is controlled by the main controller  4   e . The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  4   e . By carrying out the control program in the main controller  4   e , the above-mentioned operation can be repeated. 
     The sixth embodiment provides the same advantages as those obtained by the fifth embodiment. 
     Hereinbelow is explained an example of the sixth embodiment. 
       FIG. 27  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the example of the sixth embodiment. The illustrated apparatus is comprised of a logic tester  101 , an under-test integrated circuit set  102 , a spectrum measurement unit  103 , a main controller  104   e , a first memory  107 , a second memory  105 , a calculator  106 , and a normalizer  111 . 
     In comparison with the apparatus in accordance with the second embodiment, illustrated in  FIG. 5 , the apparatus illustrated in  FIG. 27  is designed to include the main controller  104   e  in place of the main controller  4   a  and additionally include the normalizer  111 . The main controller  104   e  has different functions from those of the main controller  4   a . The normalizer  111  is electrically connected to the spectrum measurement unit  103 , normalizes power source current spectrum measured by the spectrum measurement unit  103 , and transmits the result of normalization to the first memory  107 . 
       FIG. 28  is a flow-chart showing an operation of the apparatus in accordance with the example of the sixth embodiment. 
     An operation to be carried out in step S 1604  in  FIG. 28  is identical with an operation to be carried out in step S 1504  in FIG.  24 . An operation of the apparatus in accordance with the example of the sixth embodiment, to be carried out in other steps is identical with an operation of the apparatus in accordance with the second embodiment. Specifically, an operation to be carried out in steps S 1601 , S 1602 , S 1603 , S 1605 , S 1606 , S 1607  and S 1608  in  FIG. 28  is identical with an operation to be carried out in steps S 1201 , S 1202 , S 1203 , S 1204 , S 1205 , S 1206  and S 1207  in FIG.  8 . 
     The above-mentioned operation of the apparatus in accordance with the example of the sixth embodiment is controlled by the main controller  104   e . The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104   e . By carrying out the control program in the main controller  104   e , the above-mentioned operation can be repeated. 
     [Seventh Embodiment] 
       FIG. 29  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the seventh embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  12 , a spectrum measurement unit  3 , a main controller  4   f , a first memory  7 , a second memory  5 , a calculator  6 , a normalizer  11 , a first set  9  comprised of integrated circuits having been judged to have no failures, and a second set  10  comprised of integrated circuits judged to have failures. 
     In comparison with the apparatus in accordance with the third embodiment, illustrated in  FIG. 9 , the apparatus in accordance with the seventh embodiment is designed to include the main controller  4   f  in place of the main controller  4   b  and additionally include the normalizer  11 . The main controller  4   f  has different functions from those of the main controller  4   b . The normalizer  11  is electrically connected to the spectrum measurement unit  3 , normalizes power source current spectrum measured by the spectrum measurement unit  3 , and transmits the result of normalization to the first memory  7  through the main controller  4   f.    
       FIGS. 30 and 31  are flow-charts showing an operation of the apparatus in accordance with the seventh embodiment. 
     An operation to be carried out in steps S 705 , S 717  and S 728  in  FIGS. 30 and 31  is identical with an operation to be carried out in step S 504  in the fifth embodiment illustrated in FIG.  22 . An operation of the apparatus in accordance with the seventh embodiment, to be carried out in other steps is identical with an operation of the apparatus in accordance with the third embodiment. Specifically, an operation to be carried out in steps S 701  to S 704 , S 706  to S 716 , S 718  to S 727  and S 729  to S 733  in  FIGS. 30 and 31  is identical with an operation to be carried out in steps S 301  to S 304 , S 305  to S 315 , S 316  to S 325  and S 326  to S 330  in  FIGS. 10 and 11 , respectively. 
     The above-mentioned operation of the apparatus in accordance with the seventh embodiment is controlled by the main controller  4   f . The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  4   f . By carrying out the control program in the main controller  4   f , the above-mentioned operation can be repeated. 
     The seventh embodiment provides the same advantages as a sum of the advantages obtained by the third embodiment and the advantages obtained by the fifth embodiment. 
     Hereinbelow is explained an example of the seventh embodiment. 
       FIG. 32  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the example of the seventh embodiment. The illustrated apparatus is comprised of a logic tester  101 , a spectrum measurement unit  103 , a main controller  104   f , a first memory  107 , a second memory  105 , a calculator  106 , a first container  109  containing integrated circuits having been judged to have no failures, a second container  110  containing integrated circuits judged to have failures, a third container  112  containing integrated circuits to be tested and a normalizer  111 . 
     In comparison with the apparatus in accordance with the example of the third embodiment, illustrated in  FIG. 9 , the apparatus in accordance with the example of the seventh embodiment is designed to include the main controller  104   f  in place of the main controller  104   b  and additionally include the normalizer  111 . The main controller  104   f  has different functions from those of the main controller  104   b . The normalizer  111  is electrically connected to the spectrum measurement unit  103 , normalizes power source current spectrum measured by the spectrum measurement unit  103 , and transmits the result of normalization to the first memory  107  through the main controller  104   f.    
       FIGS. 33 and 34  are flow-charts showing an operation of the apparatus in accordance with the example of the seventh embodiment. 
     An operation to be carried out in steps S 1705 , S 1717  and S 1728  in  FIGS. 33 and 34  is identical with an operation to be carried out in step S 1504  in the example of the fifth embodiment, illustrated in FIG.  24 . An operation of the apparatus in accordance with the example of the seventh embodiment, to be carried out in other steps is identical with an operation of the apparatus in accordance with the example of the fifth embodiment. Specifically, an operation to be carried out in steps S 1701  to S 1704 , S 1706  to S 1716 , S 1718  to S 1727  and S 1729  to S 1733  in  FIGS. 33 and 34  is identical with an operation to be carried out in steps S 1301  to S 1304 , S 1305  to S 1315 , S 1316  to S 1325  and S 1326  to S 1330  in  FIGS. 13 and 14 , respectively. 
     The above-mentioned operation of the apparatus in accordance with the example of the seventh embodiment is controlled by the main controller  104   f . The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104   f . By carrying out the control program in the main controller  104   f , the above-mentioned operation can be repeated. 
     [Eighth Embodiment] 
       FIG. 35  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the eighth embodiment. The illustrated apparatus is comprised of a tester  1 , an under-test integrated circuit set  12 , a spectrum measurement unit  3 , a main controller  4   g , a first memory  7 , a second memory  5 , a calculator  6 , a first set  9  comprised of integrated circuits having been judged to have no failures, a second set  10  comprised of integrated circuits judged to have failures, and a normalizer  11 . 
     In comparison with the apparatus in accordance with the fourth embodiment, illustrated in  FIG. 15 , the apparatus in accordance with the eighth embodiment is designed to include the main controller  4   g  in place of the main controller  4   c  and additionally include the normalizer  11 . The main controller  4   g  has different functions from those of the main controller  4   c . The normalizer  11  is electrically connected to the spectrum measurement unit  3 , normalizes power source current spectrum measured by the spectrum measurement unit  3 , and transmits the result of normalization to the first memory  7  through the main controller  4   g.    
       FIGS. 36 and 37  are flow-charts showing an operation of the apparatus in accordance with the example of the eighth embodiment. 
     An operation to be carried out in steps S 805 , S 817  and S 828  in  FIGS. 36 and 37  is identical with an operation to be carried out in step S 504  in the fifth embodiment, illustrated in FIG.  22 . An operation of the apparatus in accordance with the eighth embodiment, to be carried out in other steps is identical with an operation of the apparatus in accordance with the example of the fourth embodiment. Specifically, an operation to be carried out in steps S 801  to S 804 , S 806  to S 816 , S 818  to S 828  and S 830  to S 834  in  FIGS. 36 and 37  is identical with an operation to be carried out in steps S 401  to S 404 , S 405  to S 415 , S 416  to S 426  and S 427  to S 431  in  FIGS. 16 and 17 , respectively. 
     The above-mentioned operation of the apparatus in accordance with the eighth embodiment is controlled by the main controller  4   g . The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  4   g . By carrying out the control program in the main controller  4   g , the above-mentioned operation can be repeated. 
     The eighth embodiment provides the same advantages as a sum of the advantages obtained by the fourth embodiment and the advantages obtained by the fifth embodiment. 
     Hereinbelow is explained an example of the eighth embodiment. 
       FIG. 38  is a block diagram of an apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits, in accordance with the example of the eighth embodiment. The illustrated apparatus is comprised of a logic tester  101 , a spectrum measurement unit  103 , a main controller  104   g , a first memory  107 , a second memory  105 , a calculator  106 , a first container  109  containing integrated circuits having been judged to have no failures, a second container  110  containing integrated circuits judged to have failures, a third container  112  containing integrated circuits to be tested and a normalizer  111 . 
     In comparison with the apparatus in accordance with the example of the fourth embodiment, illustrated in  FIG. 18 , the apparatus in accordance with the example of the eighth embodiment is designed to include the main controller  104   g  in place of the main controller  104   c  and additionally include the normalizer  111 . The main controller  104   g  has different functions from those of the main controller  104   c . The normalizer  111  is electrically connected to the spectrum measurement unit  103 , normalizes power source current spectrum measured by the spectrum measurement unit  103 , and transmits the result of normalization to the first memory  107  through the main controller  104   g.    
       FIGS. 39 and 40  are flow-charts showing an operation of the apparatus in accordance with the example of the eighth embodiment. 
     An operation to be carried out in steps S 1805 , S 1817  and S 1829  in  FIGS. 39 and 40  is identical with an operation to be carried out in step S 1504  in the example of the fifth embodiment, illustrated in FIG.  24 . An operation of the apparatus in accordance with the example of the eighth embodiment, to be carried out in other steps is identical with an operation of the apparatus in accordance with the example of the fourth embodiment. Specifically, an operation to be carried out in steps S 1801  to S 1804 , S 1806  to S 1816 , S 1818  to S 1828  and S 1830  to S 1834  in  FIGS. 39 and 40  is identical with an operation to be carried out in steps S 1401  to S 1404 , S 1405  to S 1415 , S 1416  to S 1426  and S 1427  to S 1431  in  FIGS. 19 and 20 , respectively. 
     The above-mentioned operation of the apparatus in accordance with the example of the eighth embodiment is controlled by the main controller  104   g . The above-mentioned operation may be described as a control program and stored in a recording medium such as a floppy disc or ROM equipped with the main controller  104   g . By carrying out the control program in the main controller  104   g , the above-mentioned operation can be repeated. 
     Hereinbelow is explained an embodiment of a recording medium storing a program therein for accomplishing the above-mentioned apparatus and method. 
     A recording medium storing a program for accomplishing the above-mentioned apparatus for detecting an integrated circuit having failures among a plurality of integrated circuits may be accomplished by programming functions of the above-mentioned apparatuses and systems with a programming language readable by a computer, and recording the program in a recording medium such as CD-ROM, a floppy disc, a magnetic tape, and any other suitable means for storing a program therein. 
     A hard disc equipped in a server may be employed as a recording medium. It is also possible to accomplish the recording medium in accordance with the present invention by storing the above-mentioned computer program in such a recording medium as mentioned above, and reading the computer program by other computers through a network. 
     While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims. 
     The entire disclosure of Japanese Patent Application No. 11-182726 filed on Jun. 29, 1999 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.