Method for testing integrated circuits

Abstract: A method for testing an integrated circuit in an automatic test environment provides for setting the automatic test environment up ( 41 ), performing a repetitive measurement of at least one electrical quantity representative of an integrated circuit response to a set of prescribed integrated circuit test conditions ( 43 , 45 , 46 , 47 , 48 ) and resetting the automatic test environment ( 411 ). The integrated circuit test conditions are changed ( 45 ) in synchrony ( 44 ) with a synchronization signal ( 38 ) having a prescribed periodicity, so that all the measurements are allotted a same time slot.

Description: The present invention relates generally to the field of integrated circuit test and measurement. In particular, the invention relates to the test and measurement of integrated circuits in an automated test environment. Test and measurement is an important phase in integrated circuit design and manufacturing. Albeit the specific tests and measurements to be conducted greatly vary depending on the type of integrated circuit and the function or functions it is intended to perform, it is in general necessary to check the absence of faults and the compliance of the circuit with the desired specifications. To do this, a relatively high number of measurements in several operating conditions is to be conducted. Normally this is done in an Automatic Test Environment ("ATE"), programmed to perform several measurements in different operating conditions on specified electrical quantities, useful to provide a characterization of the integrated circuit, by means of a sampler instrumentation. A large number of acquisitions by the sampler instrumentation is required. Generally stated, each acquisition is made up of five steps. A first step provides for setting up all the instruments involved in the measurement. A second step provides for setting the test conditions for the integrated circuit to be tested. In a third step the sampler instrumentation performs the actual acquisition, i.e. it samples the specified electrical quantities so as to measure the integrated circuit response to the set test conditions. In a fourth step mathematical calculations are performed on the acquired data. In a fifth step all the instruments are reset. Clearly, the test time rapidly increases with the number of acquisitions to be performed, that is with the number of different test conditions setups. Up to now, two techniques have been followed in order to perform this kind of measurements. A first technique, schematised by a flowchart in Figure 1, can be referred to as "multi acquisition". According to this technique, all the five steps outlined above have to be repeated for each measurement to be conducted on the integrated circuit, that is for each test condition setup. Referring to Figure 1, after all the instruments involved in the measurement have been set up (block 11, "STUP INSTR" ), an analog time acquisition routine (sampling routine) is started (block 12, "T ACQ STRT" ). The integrated circuit under test is then put in a prescribed test condition setup (block 13, "SET COND" ). Such test condition setup is chosen in a set of test condition setups corresponding to several tests to be conducted on the integrated circuit. During the sampling routine the sampler instrumentation samples the specified integrated circuit characteristic quantities so as to acquire the response of the integrated circuit to the programmed test condition setup (block 14, "MEAS" ). Once the integrated circuit response has been acquired, the analog time acquisition routine is stopped (block 15 , "T ACQ STOP" ). Mathematical calculations are then performed on the acquired data (block 16, "MTH CALC" ) and finally the measurement instruments are reset (block 17, "RST INSTR" ). The above steps are repeated until all the test condition setups have been implemented (block 18, "LST?" ). The advantage of the multi acquisition technique resides in the easiness of implementation thereof and in the possibility of predicting the overall test time given a set of test condition setups. However, this technique is inherently time consuming, due to the need of repeating the five steps for each test condition setup. Considering that each chip of the integrated circuit coming out from the manufacturing has to be tested, the time needed to perform the integrated circuit test and measurement severely impacts the overall production cost, especially in large scale production. A second technique, schematised by the flowchart of Figure 2, can instead be defined "single acquisition". According to this technique, the sampler instrumentation is started at the beginning of the test and measurement program to start the acquisition. All the test condition setups are then sequentially implemented. The sampler instrumentation is stopped when all the test condition setups have been implemented. Referring to Figure 2, after all the instruments involved in the measurement have been set up (block 21, "STUP INSTR" ), an analog time acquisition routine (sampling routine) is started (block 22, "T ACQ STRT" ). The integrated circuit is then put in a prescribed test condition setup (block 23, "SET COND"). Again, such test condition setup is chosen in a set of test condition setups corresponding to several tests to be conducted on the integrated circuit. The sampler instrumentation samples the characteristic quantities of the integrated circuit so as to acquire the response of the integrated circuit to the test condition setup (block 24, "MEAS" ). The operations corresponding to blocks 23 and 24 are repeated until all the test condition setups have been implemented (block 25, " LST? "). Once the integrated circuit response to all the test condition setups has been acquired, the analog time acquisition routine is stopped (block 26, "T ACQ STOP" ). Mathematical calculations are then performed on the acquired data (block 27, "MTH CALC" ) and finally the instruments are reset (block 28, "RST INSTR" ). Compared to the multi acquisition technique, the single acquisition technique has the advantage of being time saving, because the starting and stopping of the sampling routine, the setting up of the instruments involved in the measurement and the mathematical calculations have to be performed only once. However, the Applicant has observed that also this technique has some drawbacks. The drawbacks of this technique reside in the instability in time acquisition, since the time of absence of signal on the quantities under measure, i.e. the time during the switch between different test condition setups, varies when switching between different pairs of test condition setups. For example, Figure 3 is a diagram showing the response R (in mV) of an integrated circuit to six different test condition setups, for instance the response R on six different channels CH1 - CH6 of a programmable Radio-Frequency (RF) integrated circuit having several different channels all of which must be tested. The abscissa represents the number of samples S acquired by the sampler instrumentation. It appears that the time of absence of signal differs when switching between different pairs of channels. It is therefore difficult, not to say impossible, to determine whether the sampled data relate to an effective integrated circuit response to a given test condition setup, or to a time interval between two different test condition setups. This problem becomes more and more significant with the increase of the number of different test conditions to be implemented. A negative consequence of this problem is also the unpredictability of the time needed to complete the test program. Additionally, the Applicant has observed that the repeatibility of the measurement between successive runs of the test program on a same integrated circuit is scarce. This is for example shown in Figures 4A and 4B which, similarly to Figure 3, show the response R, in two successive runs of the test program, of six different channels CH1 - CH6 of a programmable Radio-Frequency (RF) integrated circuit having several different channels all of which must be tested. US-A-5,894,424 discloses a semiconductor testing apparatus allegedly able to test all functions of a semiconductor. US-A-5,398,197 describes a method of creating debug specification, and a test program creating specification. In view of the state of the art, it has been an object of the present invention to provide a method for testing integrated circuits which is not affected by the drawbacks of the known test and measurement techniques. According to the present invention, such an object has been attained by means of a method for testing an integrated circuit in an automatic test environment, providing for setting the automatic test environment up, performing a repetitive measurement of at least one electrical quantity representative of an integrated circuit response to a set of prescribed integrated circuit test conditions and resetting the automatic test environment, characterized in that the integrated circuit test conditions are changed in synchrony with a synchronization signal having a prescribed periodicity, so that all the measurements are allotted a same time slot. The features and advantages of the present invention will be made apparent by the following detailed description of one embodiment thereof, provided merely by way of nonlimiting example with reference to the annexed drawings, wherein: Figure 1 is a simplified flowchart of a test program according to the known multi acquisition technique; Figure 2 is a simplified flowchart of a test program according to the known single acquisition technique; Figure 3 is a time diagram of a characteristic quantity of an integrated circuit submitted to the test .program according to the single acquisition technique of Figure 2; Figures 4A and 48 are time diagrams of a characteristic quantity of an integrated circuit in two successive runs of a test program according to the single acquisition technique of Figure 2; Figure 5 is a simplified, schematic block diagram of an automatic test equipment adapted for implementing the method of the invention; Figure 6 is a simplified flowchart of an embodiment of the method according to the invention; Figure 7 is a diagram showing the test time saving allowed by the method of the invention with respect to the multi acquisition technique, and Figure 8 is a time diagram of a characteristic quantity of an integrated circuit submitted to a test program according to the method of the invention. With reference to Figure 5, an extremely simplified block diagram of an automatic test equipment 30 adapted to implement the testing method of the present invention is depicted. The automatic test equipment 30 comprises means 31 for putting an integrated circuit under test 32 in a desired test condition setup chosen among a plurality 33 of test condition setups TCS1...TCSn . The means 31 will not be described in detail since they are considered per-se known. The automatic test equipment 30 comprises analogue inputs 34 connected to a sampling circuitry 35 . Electrical quantities 37 useful to characterize the response of the integrated circuit 32 to the test condition setup are supplied to the analogue inputs 34 of the automatic test equipment 30 . A test and measurement program runs on a programmable control and computing unit 36 , which determines the change of test condition setup for the integrated circuit under test 32. The control and computing unit 36 also elaborates data 39 furnished by the sampling circuitry 35 . For example, the control and computing unit 36 performs mathematical calculations on said data. A synchronization generator 37 generates a synchronization signal or signals 38 for synchronizing the change of test condition setup operated by the means 31 and the sampling circuitry 35. Advantageously, in a mixed-signal automatic test equipment, such as for example the testers of the Teradyne A5 family, the synchronization generator 37 can be the digital pattern generator embedded in the equipment, and the synchronization signals 38 can thus be provided by the digital outputs of the tester. By specifying a digital pattern, that is a set of values for the vector of digital outputs of the tester, it is possible by means of the digital pattern generator to make one or more of the digital outputs of the tester to change state with a prescribed time periodicity. However, the synchronization generator could also be external to the automatic test equipment, and can for example be an external clock signal generator or digital pattern generator. This is for example the case of an analogue automatic test equipment, which does not have an embedded digital pattern generator. It is however pointed out that the nature of the synchronization generator is not limitative to the present invention. Any kind of circuit capable of generating a clock signal of sufficiently stable period can in principle be used. Neither is it strictly necessary that the synchronization generator is a physical, hardware component: the synchronization signal could in fact be generated by a software routine, for example by the control and computing unit 36. Figure 6 is a simplified flowchart illustrating an embodiment of the method according to the present invention. In this embodiment, which is not to be considered limitative, the automatic test equipment 30 is supposed to be of the mixed-signal type, and the synchronization generator is supposed to be a digital pattern generator embedded in the automatic test equipment. As a first step, the instruments involved in the measurement are set up (block 41, "STUP INSTR" ); in this step all the fixed conditions are programmed. By fixed conditions all those conditions are meant which will not change upon changing the test condition setup. After having set the instruments up, a digital pattern scan procedure is started (block 42 , " DGT PAT STRT "). A digital vector is made to periodically scan a pattern of digital values, one of which is chosen as synchronization digital vector value, with a prescribed time period. This will provide the synchronization signal for changing the test conditions. An analogue time acquisition routine (sampling routine) is then started (block 43, "T ACQ STRT" ). The synchronization signal is then waited for (block 44, "WT SYNC" ). The synchronization signal is a signal derived from the reaching on the part of the synchronization digital vector of the synchronization digital vector value. Upon receival of the synchronization signal, one test condition setup among the set of test condition setups is programmed (block 45, "SET COND" ), and the integrated circuit response to the programmed test condition setup is acquired by the measurement instruments, that is the sampling circuitry samples the characteristic electrical quantities representative of the integrated circuit response (block 46, "MEAS" ). The operations of blocks 44, 45 and 46 are repeated until all the test condition setups have been implemented (block 47, "LST?" ). After the integrated circuit response to all the desired test condition setups has been acquired by the measurement instruments, the analogue time acquisition routine (block 48, "T ACQ STOP" ) and the digital pattern scan (block 49, "DGT PAT STOP" ) are stopped. Mathematical calculations are then performed on the acquired data (block 410, "MTH CALC" ). The mathematical calculations can for example be a Fast-Fourier Transform (FFT) calculation performed by a Digital Signal Processor (DSP). The instruments are finally reset (block 411, "RST INSTR" ). It appears that in the method according to the present invention, after the response of the integrated circuit to a given test condition setup has been acquired and before putting the integrated circuit in a new test condition setup, the occurrence of a prescribed synchronization signal is waited for. By using a synchronization signal to change the test condition setup it is possible to establish a prescribed time interval for the measurement (for example the acquisition by the sampling circuitry) of the integrated circuit response to any given test condition setup. This allows to solve the problems of instability and scarce repeatability of the single acquisition technique, without the test time overhead of the multi acquisition technique. The period of the synchronization signal is chosen sufficiently long to allow the integrated circuit to develop a full response to the test condition setup. In particular, assuming to use a digital pattern generator as synchronization generator, the synchronization signal can be derived from the reaching of a prescribed synchronization digital vector value by a digital vector periodically scanning a pattern of digital vector values. The use of a digital pattern generator is particularly advantageous in mixed-signal automatic test equipments, which already embed such a functionality. Since in mixed-signal automatic test equipments it is possible to generate a digital pattern with very high stability in terms of scanning frequency, the stability of the synchronization signal is assured. The Applicant has experimented the method described above, as well as the conventional methods, in the testing of a Radio-Frequency (RF) integrated circuit transceiver for a DECT (Digital Enhanced Cordless Telecommunications) system, having a frequency band from 1881.8 MHz to 1930.2 MHz and a number of twenty-nine channels to be individually tested. The Applicant has experimentally found that in both the multi acquisition and the single acquisition techniques, the test time increases roughly linearly with the number of channels; in the case of the single acquisition technique this holds true when the number of channels to be tested is sufficiently large so that the portion of the test time necessary to set the instruments up and for setting the fixed conditions (block 21 in Figure 2) is not preponderant. However, the slope of the test time versus number of channels line in the case of the multi acquisition technique is higher (approximately 28 ms per channel) than that in the case of the single acquisition technique (approximately 6 ms per channel) . Thus, the time for testing twenty-nine channels with the multi acquisition technique amounts to approximately 800 ms, compared to a time of less than approximately. 200 ms when the single acquisition technique is used. The test time that can be saved using the single acquisition technique over the multi acquisition one increases with the number of channels to be tested, at a rate of roughly 22 ms per channel. This is schematically shown in Figure 7, which is a diagram of test time saving as a function of the number of channels to be tested. Figure 8 is a diagram showing the response R of the integrated circuit to six different test condition setups, that is, in the example, the response of six different channels of the integrated circuit. The abscissa represents the number of samples S . TCC indicates the instants at which the test conditions are changed, in synchrony with the synchronization signal, while the measurement time for each channel is indicated as M . Using the method of the invention, the time required for testing all the twenty-nine channels is approximately equal to that obtainable by using the single acquisition technique, thus significantly lower than that obtainable by using the multi acquisition technique. Therefore, the method according to the present invention combines the advantages in terms of test time saving of the single acquisition technique, with the good performance in terms of stability in time acquisition and repeatability of the multi acquisition technique. Another advantage of the method according to the present invention resides in the possibility of forecasting the test time, or the test time saving with respect to a multi acquisition approach, for any number of measurements to be conducted. The Applicant has found that the time required for performing a test using the method of the invention can in fact be expressed by the following relation: Ttest = T1 + Nch * (T2 + T3) + T4 where T1 is the time required to perform the instrumentation set up, Nch is the number of different test condition setups to be implemented, T2 is the time necessary to perform the acquisition of the integrated circuit response to a given test condition setup, T3 is the time required to perform the mathematical calculation of the data acquired for a given test condition setup and T4 is the time required to reset the instruments. Considering again the practical example given above, the Applicant has experimentally found the following values: T1: 31.6 ms; T2: 3.2 ms; T3: 2.5 ms; T4: 2.7 ms. The invention is applicable in general to the testing of any integrated circuit, in particular where a same measurement is to be conducted in different test conditions. Additionally, the test method of the invention is not limited to the nature, analogue or digital, of the electrical quantity or quantities to be measured: the sampling operated by the sampler instrument can be performed on either an analogue quantity or a digital quantity.

Claims: Method for testing an integrated circuit in an automatic test environment, comprising the steps: setting the automatic test environment up (41), performing a repetitive measurement of at least one electrical quantity representative of an integrated circuit response to a set of prescribed integrated circuit test conditions (43,45,46,47,48) and resetting the automatic test environment (411) , characterized in that the integrated circuit test conditions are changed (45) in synchrony (44) with a synchronization signal (38) having a prescribed periodicity, so that all the measurements are allotted a same time slot. Method according to claim 1, in which said synchronization signal is derived from a clock signal generated by a clock signal generator (37) . Method according to claim 2, in which said clock generating signal generator is a digital pattern generator a digital vector that is made to scan a pattern of digital values, said pattern of digital values acting as a synchronization pattern. Method according to claim 3, in which the automatic test environment comprises a mixed-signal automatic test equipment embedding the digital pattern generator. Method according to claim 3 or 4, in which the setting of a new prescribed test condition for the integrated circuit is made in synchrony with the occurrence value synchronization of a preselected digital vector in said digital pattern Method according to claim 5, in which the steps of setting the automatic test environment up, performing a repetitive measurement and resetting the automatic test environment involve: a) setting up instruments of the automatic test environment; b) starting in the automatic test environment a synchronization digital pattern scan; c) starting an analogue time acquisition procedure; d) setting a prescribed integrated circuit test condition in synchronism with the synchronization digital pattern; e) measuring a response of the integrated circuit to the set test condition; f) repeating steps d) and e) for all prescribed test conditions; g) terminating the analogue time acquisition procedure; h) terminating the generation of the synchronization digital pattern; i) performing mathematical calculations on the measured analogue time responses of the integrated circuit to the set of test conditions; 1) resetting the instruments. Method according to any one of the preceding claims, including forecasting an integrated circuit test time by multiplying the time slot allotted to each measurement for a number of prescribed integrated circuit test conditions.

Inventors: DI GREGORIO GIUSEPPE, RUSSO BIAGIO, TUTTOBENE GIUSEPPE, DI GREGORIO, GIUSEPPE, RUSSO, BIAGIO, TUTTOBENE, GIUSEPPE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2001
Main classification: G01R31
IPC codes: G01R31