Abstract:
A method for testing integrated circuits comprises: generation of a change in an input signal of the integrated circuit, detection of a change in the output signal of the integrated circuit, the change triggered by the change in the input signal when a predetermined condition is satisfied, and a comparison of the detected output signal with at least one predetermined comparison criterion. Whereby, the predetermined condition is derived individually for each integrated circuit from a time response of the output signal.

Description:
This is a divisional application under 37 C.F.R. §1.53(b) of prior application Ser. No. 11/095,488 filed on Apr. 1, 2005, now U.S. Pat. No. 7,199,601 the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for testing integrated circuits (ICs). 
     2. Description of the Background Art 
     Integrated circuits are produced by the millions. A zero error rate is required for some applications, for example, in integrated circuits for safety-relevant functions such as the control of an airbag release. For this reason, each individual integrated circuit, which is intended for this type of application, is tested for its proper function after being manufactured. In a few safety-critical applications as well, production must be tested at least by spot checking, which in the case of the indicated quantities still produces very high numbers of integrated circuits to be tested. 
     In conventional testing methods, there is a requirement that after a change in the input signal a fixed waiting time must pass before a measured value of an output signal reaction is detected that is then used as a predetermined condition. This waiting time takes into account a production series-specific delay, with which integrated circuits respond to a stimulating input signal change. Actual delays, which occur in individual circuits, can have deviations due to deviations of the component parameters in different representatives of a series of integrated circuits from the same production series, so that some integrated circuits respond earlier and other integrated circuits respond later to an input signal change. 
     To enable a reliable differentiation between good and bad integrated circuits, the fixed waiting time must be predetermined so that, also within the scope of permitted fluctuations, integrated circuits that respond admittedly more slowly but always with still sufficient speed can still be recognized as being good. The result is that measured values for rapidly responding components within the scope of permitted fluctuations are received unnecessarily late, which in fact is not critical for the quality of the measurement, but lengthens the measuring time required for testing a large number of similar integrated circuits. This lengthening of the test time reduces the throughput of an individual testing apparatus, so that more testing apparatuses must be provided for a preset rate for testing integrated circuits. The lengthened measuring time must therefore be absorbed in higher investment for more testing apparatuses, associated manipulating systems, mounting surfaces, power supply, etc. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and a testing apparatus by which the aforementioned disadvantages are reduced. 
     This object is achieved by deriving the predetermined condition individually for each integrated circuit from a time response of the output signal. 
     Furthermore, this object is achieved in a testing apparatus in that the testing apparatus derives the predetermined condition individually for each integrated circuit from a time response of the output signal. 
     By deriving IC-individual predetermined conditions from the individual time response of the output signal of an integrated circuit, more rapidly responding integrated circuits within the scope of a test can be tested earlier than by the prior-art method. This produces a shorter testing time overall with averaging over many test cycles, so that the number of tested integrated circuits dealt with by a testing apparatus increases. As a result, at a preset quantity of integrated circuits to be tested, the number of testing apparatus, including associated manipulators, can be reduced, which reduces the space required for the entire testing unit and the average test costs for each individual integrated circuit. 
     In an example embodiment of the present invention, the output signal can be continuously monitored after a change in the input signal and the predetermined condition is regarded as having been satisfied when the output signal enters a predetermined value range. 
     This example embodiment is suitable for, for example, integrated circuits in which there is a relative strong response to the output signal. The entry of the output signal into a predetermined value range requires that the output signal first lies outside the value range. The entering into the range therefore shows that a response has generally occurred, which optionally can already be evaluated as a sign of a functional IC. 
     Alternatively or in addition, starting at this time, however, the process can also wait for a certain amount of time until the output signal form settles. The individual testing time then includes the sum of the waiting time and the preceding circuit-specific time interval between stimulation of the integrated circuit and the entry of the output signal into the predetermined value range. 
     The predetermined condition can therefore be regarded as having been satisfied when after the output signal enters a predetermined value range, a predetermined minimum waiting time has passed. 
     Furthermore, the predetermined condition can be regarded as having been satisfied when, after the output signal enters the predetermined value range, a slope of the form of the output signal over time falls below a predetermined threshold. 
     This embodiment has an additional advantage in that both time intervals, thus, the time interval between a stimulation of the circuit and the entry into the value range, as well as the time interval between the entry and the receipt of the actual measured value, e.g., the value of the output signal at the time when the predetermined condition is satisfied, are circuit-specific. This results in further shortening of the test time and thereby a further increase in the rate by which an individual testing apparatus tests integrated circuits. 
     The predetermined condition can be regarded as having been satisfied when after the output signal enters the predetermined value range, a percent change in the output signal exceeds a predetermined threshold. 
     In such an example embodiment, both aforementioned time intervals are circuit-specific, so that comparable advantages arise here. The difference between the two embodiments is that the slope depends on the difference between two output signal values, whereas the percent change results as a function of the quotient of the two output signal values. 
     In a further example embodiment, the predetermined condition can be regarded as having been satisfied when a percent change in the output signal, triggered by a change in the input signal, exceeds a predetermined threshold. 
     This example embodiment differs from the aforementioned embodiments in that it does not require that the output signal lies outside the predetermined value range at the beginning. In circuits in which the output signal is within the predetermined value range at the outset, it can be determined in this way whether a sufficient change occurs at all as a response to stimulation. This embodiment is thereby suitable particularly for integrated circuits with low output signal amplitudes. 
     The testing apparatus should continuously monitor the output signal after a change in the input signal and the predetermined condition is evaluated as having been satisfied when the output signal enters a predetermined value range. 
     It is also preferred that the testing apparatus should continuously monitor the output signal after a change in the input signal and the predetermined condition is evaluated as having been satisfied when a percent change in the output signal, triggered by a change in the input signal, exceeds a predetermined threshold. 
     It is further preferred that the testing apparatus of at least one of the aforementioned embodiments executes the method. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  illustrates a testing apparatus for testing integrated circuits; 
         FIGS. 2   a - c  show signal forms, which a testing apparatus within the scope of a first embodiment outputs and/or detects over time; 
         FIGS. 3   a - c  show signal forms, which a testing apparatus within the scope of a second embodiment outputs and/or detects over time; 
         FIG. 4  is a flowchart of an example embodiment according to the present invention; and 
         FIG. 5  is another flowchart illustrating a further example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a testing apparatus  10  for testing integrated circuits  12  with a gripping device  14 , delivery transporter  16 , a first removal transporter  18  for integrated circuits that satisfy predefined requirements, a second removal transporter  20  for integrated circuits that do not satisfy the predefined requirements, a carrying device  22 , an input signal generator  32 , an output signal detection and evaluation unit  34 , a control unit  36 , and a control connection  38 . Integrated circuits  12  that are to be tested are delivered by the delivery transporter  16 , for example, a conveyer belt, and are gripped by the gripping device  14 , which can be movable in multiple directions, such as an X and Y direction, and transported to the support plate  22 . The support plate  22  has input signal contacts  24 ,  26  and output signal contacts  28 ,  30 . The input signal contacts  24 ,  26  are connected to the input signal generator  32  and the output signal contacts  28 ,  30  are connected to the output signal detection and evaluation unit  34 . 
     The input signal generator  32  stimulates the integrated circuit  12 , which is placed on the contacts  24 ,  26 ,  28 ,  30  and responds thereto with a change in its output signal. The output signal change is detected and evaluated by the output signal detection and evaluation unit  34 . Depending on whether the tested integrated circuit  12  satisfies or does not satisfy predetermined requirements, it is transported by the gripping device  14  to the first removal transporter  18  or the second removal transporter  20 . The gripping device  14 , the delivery transporter  16 , and the first and second removal transporerst  18  and  20  can be controlled by a control  36 , which communicates via a control connection  38 , for example, a bus system, with the input signal generator  32  and/or the output signal detection and evaluation unit  34 . The testing apparatus  10  according to  FIG. 1  is distinguished by the fact that it derives the predetermined condition, at the occurrence of which a measured value of the output signal of the integrated circuit is detected to evaluate the function of the integrated circuit  12 , from a time response of the output signal. The process sequences according to the example embodiments are explained in the following with reference to  FIGS. 2 to 5 . 
       FIG. 2   a  shows a time form of a stimulating input signal. In  FIG. 2   b , there is shown individual and different responses of different integrated circuits  12  of a production series over time t. In  FIG. 2   c , there is shown individual test times for the integrated circuits  12 , which produce output signals according to  FIG. 2   b . The test begins with a change in the input signal  40  that is supplied by the input signal generator  32  at time t_ 0 . After a minimum waiting time has passed, at time t_ 1  the active test, i.e., a continuous monitoring of the output signal of the integrated circuit  12  that is to be tested and thereby the time response of the output signal, is started. The continuous monitoring can be performed, for example, by periodic sampling or continuous evaluation. The time t_ 1  is shown in  FIG. 2  by the falling level of the signal  42  in  FIG. 2   c , which marks the beginning of the active test time. 
       FIG. 2   b  shows starting signal forms  44 ,  46 , and  48  of three different integrated circuits  12 , which differ in their response rate. Of the three examined output signal forms  44 ,  46 , and  48 , the output signal form  48  responds most rapidly to a change of the input signal at time t_ 0 , and at time t_ 1 _ 48  enters a predetermined value range I_ 1 . In an example embodiment of the invention, the entry into the value range I_ 1  can already be evaluated as satisfying the predetermined condition. Within the scope of this embodiment, the test for these special IC can therefor end at this time. 
     Within the scope of further example embodiment, the process waits for a certain time until time t_ 2 _ 48 , and the then present value of the output signal  48  is used as the measured value for evaluating the integrated circuit  12 . Within the scope of this embodiment, the predetermined condition is regarded as having been satisfied when the time t_ 2 _ 48 =t_ 1 _ 48 +Δ_t is reached. In this case, the test measurement is ended, which is represented in  FIG. 2   c  by a rising edge  50 . Alternatively, the time t_ 2 _ 48  can also be determined by evaluating the slope of the output signal  48 . The initially steep slope declines after time t_ 1 _ 48  as it approaches time t_ 2 _ 48 , so that falling below a suitable threshold can define the time t_ 2 _ 48 . 
     The output signal forms  46  and  44 , which are obtained by measuring other integrated circuits  12 , can be evaluated very analogously to these considerations on the output signal form  48 . The output signal  46  represented by output signal form  46  at time t_ 1 _ 46  enters the value range I_ 1  and is detected, for example, at time t_ 2 _ 46  for evaluating the functionality of the integrated circuit  12 . Accordingly, the test measurement for this integrated circuit  12  can be terminated at time t_ 2 _ 46 , as represented in  FIG. 2   c  by a rising edge  52 . 
     Accordingly, an end of the test, which is represented in  FIG. 2   c  by a rising edge  54 , results from the times t_ 1 _ 44 , at which the output signal  44  enters the range value I_ 1 , and the associated time t_ 2 _ 44 , at which a measured value is received. 
     The rising edge  56  in  FIG. 2  represents an inevitable termination of the test at a time t_max. If the predetermined condition for detecting an output signal  44 ,  46 ,  48  of a special integrated circuit  12  is not yet satisfied at this time t_max, then, for example, the current value of the output signal  44 ,  46   48  can be used as the measured value for evaluating the functionality, and compared with predefined thresholds. The provision of the maximum test time t_max prevents a potentially nonfunctional IC from blocking the testing apparatus  10  for an unallowably long time. 
     The time t_max defines simultaneously an example of the time when the measured values for each individual IC  12  are received in the aforementioned prior-art method. The entire measuring time for many integrated circuits  12  according to the prior-art method therefore has a bottom limit determined by multiple time intervals between times t_ 1  and t_max. In contrast, a comparable (theoretical) bottom limit arises for a testing method according to the invention as the sum of the distances from each of the edges  50 ,  52 , and  54  at time t_ 1 , which, as is evident, results in a smaller sum and thereby overall a shortening of the test time for a multitude of integrated circuits  12 . 
     As an alternative to the already described embodiments, an integrated circuit  12  can also be evaluated in that after the output signal  44 ,  46 ,  48  enters the predetermined value range I_ 1 , a percent change in the output signal  44 ,  46 ,  48  can be determined and compared with a predetermined threshold. The percent change can be standardized, for example, to the value of the output signal  44 ,  46 ,  48  at the time of entry into the predetermined value range I_ 1 . 
       FIG. 3   b  shows signal forms  58 ,  60 ,  62  of individual integrated circuits  12 , which right at the beginning of the test lie within the permitted, predetermined value range I_ 1 . In this case, a reliable evaluation can be achieved in that the output signal forms  58 ,  60 ,  62  after time t_ 1 , are detected continuously and monitored for the occurrence of a percent change that exceeds a predetermined threshold. The percentage change is advantageously related to the initial level of the output signal forms  58 ,  60 ,  62 . As soon as the change in the output signal  58 ,  60 ,  62  exceeds a percent threshold, which is the case in  FIG. 3   b  at times t_ 1 _ 58 , t_ 1 _ 60 , and t_ 1 _ 62 , the circuit  12  in question can be evaluated as functional. 
     Alternatively, at these times, each of the values of the output signal  58 ,  60 ,  62  of the integrated circuit  12  that is being tested can be detected and compared with predefined thresholds, which may be identical or different from the limits of the predetermined value range I_ 1 . 
     Within the scope of another embodiment, which is explained below with reference to the output signal  58 , the process waits until time t_ 2 _ 58 . The then available value for the output signal  58  is used as the measured value for evaluating the integrated circuit  12 . Within the scope of this embodiment, the condition is therefore regarded as satisfied when the time t_ 2 _ 58 =t_ 1 _ 58 +Δ_t is reached. In this case, the test measurement is ended, which is represented in  FIG. 3   c  by a rising edge  64 . Alternatively, time t_ 2 _ 58  can also be determined by evaluating the slope of signal  58 . The steep slope after time t_ 1 _ 58  declines as the time t_ 2 _ 58  is approached, so that falling below a corresponding threshold can define the time t_ 2 _ 58 . The output forms  60  and  62  can also be evaluated analogously to these considerations on the output signal form  58 . 
     Accordingly, the testing of an integrated circuit  12  according to example embodiment shown in  FIG. 3  ends in each case at the rising edges  64 ,  66 , and  68 , all of which occur prior to the rising edge  56 , which represents an inevitable termination of the test at time t_max. 
       FIG. 4  is a flow chart in which the signal forms, shown in  FIGS. 2 and 3 , can be achieved and evaluated. The method is carried out in, for example, the testing apparatus  10  according to  FIG. 1  by the connection from the control  36  with the input signal generator  32  and the output signal detection and evaluation unit  34 . To that end, in step  70 , a test is first started when the gripping device  14  has placed IC  12  on the carrier plate  22 . After the placement of an IC  12  onto the contacts  24 ,  26 ,  28 ,  30 , an input signal change is triggered and in step  72 , the time response ZV of the resulting output signal AS;  44 ,  46 ,  48 ;  58 ,  60 ,  62  is evaluated. Next, in step  74 , a predetermined condition VB is set as a function of the time response ZV. 
     While the output signal AS;  44 ,  46 ,  48 ;  58 ,  60 ,  62 , is continuously detected further, it is checked in step  76  whether the predetermined condition is satisfied. As long as this is not the case, branching occurs in step  78 , in which it is checked whether the maximum test time t_max has been exceeded. If the answer to this query in step  78  is no, the loop of  76  and  78  is run until either the predetermined condition is satisfied in step  76  or the maximum test time in step  78  is exceeded. In both cases, a branching follows to step  80 , in which a measured value M of the output signal AS;  44 ,  46 ,  48 ;  58 ,  60 ,  62  of the integrated circuit  12  is received. 
     The received measured value M is checked in step  82  to see whether it is an element of a permitted value range I_ 2 . It is understood that I_ 2  can be identical to or different from the value range I_ 1 , which is described in regards to  FIGS. 2 and 3 . If the measured value M is within interval I_ 2 , the tested IC  12  is regarded as functional and branching to step  84  occurs, which triggers the removal of the sufficiently functional integrated circuit  12  via the first removal transporter  18 . Otherwise, if the measured value M is not within the interval I_ 2 , branching to step  86  occurs, in which, for example, the tested IC  12  is removed by the second removal transporter  20 . 
     According to this description of a very general method, a detailed embodiment of a method is described below with reference to  FIG. 5 , with which the signal form according to  FIG. 2 , as well as the signal form according to  FIG. 3 , can be achieved and evaluated. After the start of the program in step  70 , a counter variable n is first set to the value 1. This is followed by step  72  of the evaluation of the time response ZV of the output signal AS;  44 ,  46 ,  48 ;  58 ,  60 ,  62  of an integrated circuit  12 . The evaluation of the time response ZV is shown in more detail in  FIG. 5  and begins with substep  90  of step  72 , in which it is checked whether the minimum waiting time, explained in association with  FIGS. 2 and 3 , until time t_ 1  has passed. Only when this is the case, branching to substep  92  occurs, in which an output signal AS;  44 ,  46 ,  48 ;  58 ,  60 ,  62  of the integrated circuit  12  is received. This is followed by substep  94 , in which the output signal AS;  44 ,  46 ,  48 ;  58 ,  60 ,  62  is checked to determine whether it is within the predetermined value range I_ 1 . If this is not the case, which corresponds to the output signal response shown in  FIG. 2 , step  96  follows, in which one of the predetermined conditions VB, explained in association with  FIG. 2 , is set. In addition, in step  96  the value of the counter variable n is increased by 1. 
     It is then checked in step  98  whether the maximum test time t_max has passed. As long as this is not the case, branching back to substep  92  occurs, in which a new output signal AS;  44 ,  46 ,  48  is received. This is again followed by step  94 , which means a determination whether the value AS is within the interval I_ 1 . As long as this is not the case and the maximum test time t_max is not exceeded, the loop runs through steps  92 ,  94 ,  96 , and  98 , whereby the value of the counter variable n is increased each time and thereby is always different from n=1. The loop is left only if it is determined in step  94  that the output signal AS;  44 ,  46 ,  48  enters the permitted value range I_ 1 ; because the counter variable n in this case is greater than 1, with a no answer to the corresponding query in step  102 , step  104  is reached in which it is checked whether the predetermined condition VB is satisfied. 
     As long as this is not the case, branching from step  104  to step  106  occurs, in which it is checked whether the maximum test time t_max has been reached. Branching to step  80  occurs only when the predetermined condition in step  104  is recognized as having been satisfied or if the predetermined maximum test time t_max in step  106  is recognized as having been exceeded; this has already been explained in regard to  FIG. 4  and relates to the receiving of the measured value and further branching in steps  82 ,  84 ,  86  of  FIG. 4 . Step  80  is also reached when the loop, including steps  92 ,  94 ,  96 , and  98 , is left from step  98  due to exceeding the maximum test time t_max. 
     If the output signal at time t_ 1  is within the permitted range I_ 1 , as corresponds to the situation in  FIG. 3 , the form of the process is slightly different. In this case, the query in substep  94  of step  72  is answered with yes during the first pass and step  102  is reached, in which it is checked whether the counter variable n has the value 1. Because this is the case with only a single pass through the preceding step  94 , the query  102  in this case is answered with yes and step  108  follows, in which one of the predetermined conditions VB, explained in relation to  FIG. 3 , is set. Next, in step  110  an output signal AS;  58 ,  60 ,  62  is received and evaluated in step  112  as to whether the set predetermined condition VB is satisfied. As soon as the predetermined condition VB has been satisfied, also in this embodiment of the method, branching in step  80  to receive a measured value for the output signal AS;  58 ,  60 ,  62  follows as a basis for evaluating the functionality of the integrated circuit  12 . 
     As long as the predetermined condition has not been satisfied and the maximum time t_max, checked in step  114 , has not yet been reached, the sequence includes steps  110 ,  112 , and  114  is repeatedly run through. As in the previous described embodiment, this loop is also left either because the predetermined condition in step  112  is recognized as having been satisfied or because the maximum test time t_max in step  114  has been detected as having been exceeded. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.