GB2340233A

GB2340233A - Current measuring method,current sensor,and IC tester using the same current sensor

Info

Publication number: GB2340233A
Application number: GB9923232A
Authority: GB
Inventors: Toshiyuki Okayasu
Original assignee: Advantest Corp
Current assignee: Advantest Corp
Priority date: 1998-02-05
Filing date: 1998-02-05
Publication date: 2000-02-16
Also published as: WO1999040446A8; CN1252130A; WO1999040446A1; TW359753B; KR20010006008A; GB9923232D0; DE19882306T1

Description

CURRENT MEASURING METHOD, CURRENT SENSOR AND IC TESTING APPARATUS USING

THE CURRENT SENSOR

TECHNICAL FIELD

The present invention relates to a current measuring method capable of providing a wide measuring range through the use of an optical modulator, a current sensor constructed by utilizing the current measuring method, and an IC (Integrated Circuit) testing appratus that utilizes the current sensor. More particularly, the present invention pertains to an IC testing apparatus that quickly measures a minute or very small current flowing through a power supply terminal of an IC under test constructed as a CMOS IC during its quiescent time and determines the quality of the IC (whether the IC is a defective article or not) depending on whether the measured current value falls within a normal range or not.

BACKGROUND ART

There has heretofore been provided an IC testing method that measures a current flowing through an IC under test and determines whether the IC is a conforming or good article (pass article) or the IC is a non-conforming or bad article (failure article) depending on whether or not the current flowing through the power supply terminal of the IC under test falls within a prescribed range.

Fig. 15 shows an example of the conventional testing method. A predetermined power supply voltageVDD from a DC power supply 12 is supplied via a current measuring means 13 to a power supply terminal TVDD of an IC under test 11. The current measuring means 13 is constituted by a shunt resistor SR for converting a current to a voltage, a differential amplifier DF for having a potential difference produced across the shunt resistor SR as a voltage value inputted thereto, and limiter diodes D connected in parallel between input terminals of the differential amplifier DF.

The voltage signal outputted from the differential amplifier DF is sampled and held at a predetermined timing in a sample-and-hold circuit 14. The sampled and held voltage is converted into digital form by an A/D converter 15, and the digital output from the sample-and-hold circuit 14 is provided as a result of the measured current to an arithmetic and logic unit or operation processor 16 which determines whether the value of the measured current falls within a predetermined range.

In general, the IC under test 11 is constructed as a CMOS type circuit. As is well known, the CMOS type circuit is formed by an n-channel FET and a p-channel FET complementarily connected with each other, which alternately turn on and off respectively, thereby to transmit signals therethrough. For this reason, when the complementarily connected FETs change their states respectively, that is, one from on state to off state and the other from off state to on state, a relatively large current flows therethrough, and when they go to stable states, the current rapidly drops to a minute value (there may be a case that this current is called leakage current). That is, the current flows just like a pulse as shown in Fig. 16 by dotted lines.

The peak value of a large-current pulse IPL flowing in the form of a pulse is as large as several amperes in the case of a large-scale IC and a leakage current Is that flows in the quiescent state is about several microamperes. A decision as to whether the IC under test I I is defectless or defective is rendered by measuring a leakage current Is flowing in the stable state of the FETs and checking whether the leakage current Is falls within a normal range or not. Accordingly, it is required to measure the leakage current Is with high accuracy.

To meet such requirement, it is conventional to connect the limiter diodes D to the input side of the differential amplifier DF. Since the limiter diodes D bypass the large-current pulse 1PL flowing through the shunt resistor SR, the current flow therethrough is limited and hence the amplitude of a voltage signal inputted to the differential amplifier DF is also limited to operate the differential amplifier DF in its unsaturated region, thereby to measure the leakage current I, Consequently, in the example shown in Fig. 16, the large current pulse IPL flowing when the IC under test I I changes its operation state is bypassed by the limiter diodes D, and during the leakage current Is is flowing, the limiter diodes D are held in the off state. As a result, a voltage induced across the shunt resistor SR when the leakage current Is is flowing through the shunt resistor SR is taken out through the differential amplifier DF, the voltage thus taken out being fed to the arithmetic and logic unit (hereinafter referred to as ALU) 16 after it is - converted into a digital signal in the sample-and-hold circuit 14 and the A/D converter 15. The ALU 16 determines whether the IC under test is defective (failure) or not defective (pass).

Incidentally, such connection of the limiter diodes D to the input side of the differential amplifier DF results in a new disadvantage as will be described. That is, in case that the limiter diodes D are connected as in the above circuit configuration, the current immediately after the large-current pulse IPL does not rapidly reduce to the leakage current value due to the stored carriers and the junction capacitance inherent in the diode, which results in a phenomenon that the current comes to its stable state at a target leakage current value after a certain time duration TS has elapsed from a timing To corresponding to the trailing edge of the large-current pulse IPL as shown in Fig. 16 (this ptienomenon will hereinafter be referred to as settling). Owing to the occurrence of this settling, a measuring point PT cannot be set until at least the time duration TS (this time duration TS will be referred to as settling time, hereinafter) elapses from the timing To corresponding to the trailing edge of the large-current pulse 1PL- In other words, no correct and accurate current value can be measured during the settling time TS.

Furthermore, such conventional testing method measures the current Is flowing into the power supply terminal TVDD of the IC under test I I when it is in its quiescent state. That is, such conventional testing method measures a leakage current of a large value that cannot flow inherently through the IC under test and determines whether the IC under test is defective or not by checking the presence of the leakage current of a large value.

Therefore, whether a failure part in the IC under test is reflected on the leakage of the power supply current or not relies on the logical state of the failure part. As a result, it is necessary to change the combination of the logical states in the IC under test and measure the power supply current each time the combination of the logical states in the IC under test is changed. In order to change the combination of the logical states, it is required to cause the IC under test I I to be actuated inversely. Since the actuation of the IC under test I I inevitably results in flow of the large-current pulse IPL, to measure the leakage current Is after the combination of the logical states has been changed will be carried out after the settling time TS has passed.

Thus, the measurement period TES during which the change of the combination of the logical states and the measurement of the leakage current Is are repeated is affected by the length of the settling time TS. That is, when the settling time TS is long, the period for changing the logical states in the IC under test 11 cannot be reduced, and consequently, the measurement period TES for the leakage current Is also becomes long, resulting in a disadvantage that much time is needed to test every state of the IC under test 11. Since ICs show the tendency to increase in their scale more and more, there is a drawback that the time needed to test such ICs becomes longer.

In addition, in the prior art, the DC power supply 12 and the current measuring means 13 are placed near the IC under test 11, that is, placed in a part called a test head, and the output signal from the differential amplifier DF is transmitted via a cable or the like to the measuring instrument proper spaced apart from the test head, and in the measuring instrument proper the output signal is sampled and held, and then is converted into a digital signal to compare and determine in the ALU 16 as to whether the IC under test is defective or not. With such configuration, when the distance between the test head and the measuring instrument proper is long, the output signal transmitting therebetween is susceptible to the influences of an electrostatic capacity, a parasitic inductance and externally induced noises of the signal transmission path, which results in a shortcoming that the measuring accuracy is deteriorated.

A first object of the present invention is to provide a current measuring method which is capable of widening a current measuring range, that is, widening a dynamic range to prevent the above-mentioned settling from occurring, and hence measuring a quiescent current in an IC under test within a very short time period immediately subsequent to the large-current pulse, a current sensor using this current measuring method, and an IC testing apparatus utilizing this current sensor.

A second object of the present invention is to provide a current measuring method which is free from any adverse effects due to the signal transmission path and hence hardly suffers a deterioration in the measuring accuracy even if the distance between the test head and the measuring instrument proper is long, and an IC testing apparatus utilizin this method.

9 INT DISCLOSURE OF-THE I'VENTION

The present invention provides a current measuring method comprising the steps of: converting a current to be measured in which a pulse-like large current of large amplitude and a minute current alternately appeared in time, into a voltage W signal; supplying the voltage signal to an electric field applying electrode of an optical modulator to modulate light passing through the optical modulator; causing the modulated light to interfere with unmodulated light to obtain interference light; and converting the intensity of the interference light into an electric signal by a photodetector to be taken out as an electric signal corresponding to the value of the current to be measured.

With the current measuring method according to the present invention, the optical modulator does not saturate even if an electric field of great amplitude is applied to the optical modulator, and no settling occurs even immediately after a pulse-like voltage signal of great amplitude has been applied to the optical modulator.

Accordingly, even if the ratio between the large-current pulse IPL and the leakage current Is flowing through the IC under test in the quiescent state thereof is large and even immediately after the large-current pulse IPLhas flowed, the leakage current Is can be measured without being affected by the settling.

Moreover, the present invention provides an IC testing apparatus utilizing the above-mentioned current measuring method. The IC testing apparatus according to the present invention, which measures the leakage current flowing through a CMOS IC under test in its quiescent state of each state when the CMOS IC has inverted in its state of operation, and renders a decision that the IC under test is defective or not defective depending upon whether the measured leakage current in the quiescent state of the IC under test is larger or smaller than a prescribed value, is capable of measuring the leakage current without any influence of the settling even just after the operation state of the IC under test '.,,as been changed, and hence quickly inverting the operation state of the IC under test to measure the leakage current in each inverted operation state.

Accordingly, the IC testing apparatus according to the present invention has an advantage that the time duration needed for testing ICs can be reduced even if they are large scale integrated circuits.

Further, according to the present invention, it is possible to use an optical waveguide for connecting between the test head with the IC under test mounted thereon and the measuring instrument proper which performs processings of comparison operation of current values and so forth. There is no possibility that the optical waveguide is adversely affected by an electrostatic capacity, a parasitic inductance, externally induced noises or the like. As a result, there is obtained an advantage that the accuracy of measurement can be maintained high even if the distance between the test head and the measuring instrument proper becomes long.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. I is a plan view for explaining a current measuring method according to the present invention and a current sensor using this current measuring method; Fig. 2 is a plan view for explaining the operation of an optical modulator for use in the current measuring method and the current sensor according to the present invention; Fig. 3 is a waveform diagram for explaining the operation of the optical modulator depicted in Fig. 2; Fig. 4 is a waveform diagram for explaining the operation of the embodiment shown in Fig. 1; Fig. 5 is a plan view illustrating a modified embodiment of the current sensor shown in Fig. 1; Fig. 6 is a plan view for explaining another example of the current measuring method according to the present invention; Fig. 7 is a perspective view illustrating an example of the specific structure of the optical modulator for use in the current sensor according to the present invention; Fig. 8 is a plan view for explaining an example of the IC testing apparatus using the current sensor according to the present invention; Fig. 9 is a waveform diagram for explaining the operation of the embodiment shown in Fig. 8; Fig. 10 is a plan view for explaining a modified form of the embodiment shown in Fig. 8; Fig. I I is a waveform diagram for explaining the operation of the modified embodiment shown in Fig. 10; Fig. 12 is a plan view for explaining a modified form of the embodiment shown in Fig. 8; Fig. 13 is a perspective view for explaining a still another modification of the embodiment shown in Fig. 8; Fig. 14 is a plan view for explaining a modified -:ng apparatus according to the present embodiment of the IC test invention; Fig. 15 is a schematic diagram for explaining an example of the prior art; and

Fig. 16 is a waveform diagram for explaining the operation of the prior art shown in Fig. 15.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be given, with reference to Fig. 1, of a current measuring method according to the present invention, the configuration and operation of a current sensor utilizing the current measuring method. In Fig. 1, reference numeral 10 denotes a circuit under measurement which outputs a current to be measured Im, and 20 denotes a current sensor for measuring the value of the current to be measured Im by use of the current measuring method proposed by the present invention.

The present invention provides the current measuring method which measures the current to be measu--ed Im by applying it to a current-voltage converter 30, inputting a voltage signal VS generated therefrom into an optical modulator 40 to convert the voltage signal VS into interference light, and converting the intensity of the interference light into an electric signal.

Specifically, the current sensor 20 according to the present invention is constructed by a substrate 21, the current voltage converter 30 mounted on the substrate 21, and the optical modulator 40 also mounted on the substrate 21 so that the current-voltage converter 30 generates the voltage VS corresponding to the value of the current to be measured Im, the voltage VS being applied to the optical modulator 40.

As the optical modulator 40, a branched interferometric modulator shown in Fig. 2 may be used, for example. The branched interferometric modulator 40 comprises an optical branching part 42 for branching an optical waveguide, an optical coupling part 43, two optical waveguides 44A and 44B formed between the optical branching part 42 and the optical coupling part 43, and electric field applying electrodes 45, 46 and 47 formed along both sides of each of the two optical waveguides 44A and 44B.

The optical branching part 42, the optical coupling part 43 and the optical waveguides 44A and 44B can be formed by diffusing, for example, titanium or the like into a dielectric substrate 41 made of lithium niobate (LiNbQ3) or the like, for instance. Optical waveguides such as optical fibers are optically coupled as an input optical waveguide 22 and an output optical waveguide 23 to a light receiving end 49A and a light emitting end 49B formed and exposed on the end surfaces of the dielectric substrate 41, respectively, and a light source 51 such as a laser diode is coupled to the other end of the input optical waveguide 22 coupled to the light receiving end 49A, and a photodetector 53 such as a photodiode is coupled to the other end of the output optical waveguide 23 coupled to the light emitting end 49B.

The light source 51 is driven to its lighting state by a light source driving circuit 52. This example shows that the light source 51 is driven by a DC power supply. Accordingly, the light source 5 1 emits a fixed quantity of laser light into the input optical waveguide 22. To the photodetector 533 is connected a detecting circuit 54 which converts the intensity of light emitted from the output optical waveguide 23 into an electric signal to be taken out.

The voltage VS generated from the current-voltage converter 30 is applied to one of pairs of the electric field applying electrodes 45, 46, and 47. The example shown in Fig.

1 shows the case that the voltage VS generated from the current voltage converter 30 is applied between the electric field applying electrodes 45 and 46, and the electric field applying electrodes 45 and 47 of the other pair are connected in common and no electric field is formed therebetween.

By applying an electric field to the one optical waveguide

44A and no electric field to the other optical waveguide 44B as mentioned above, light passing through the optical waveguide 44A applied with the electric field is phase modulated therein and light passing through the optical waveguide 44B applied with no electric field is not modulated. Due to the phase modulation of light given in the optical waveguide 44B, interference of light occurs in the optical branching part 43, and the intensity of light emitted to the output optical waveguide 23 varies accordingly.

This manner will be described with reference to Figs. 2 and 3. Let the intensity of light incident to the input optical waveguide 22, the intensity of light outputted to the output optical waveguide 23 and the voltage applied to the electric field applying electrodes 45 and 46 be represented by Pin, P,, and VS, respectively. With a change of the applied voltage VS, the intensity P,,,, of light which is emitted to the output optical waveguide 23 varies along a cosine curve as depicted in Fig. 3A.

That is, when the applied voltage VS is 0 (VS=O), P(,,, equals to 0 -13 Pin (Po,t = Pin), and when the voltage VS is gradually varied in the positive (+) direction or negative (-) direction, the quantity of output light gradually decreases along the cosine curve and at a certain voltage the quantity of output light goes down to zero.

When the applied voltage VS is further increased therefrom, the output light intensity Pout gradually increases along the cosine curve and when it is increase to a certain voltage, the output light intensity Pout goes to "I", namely, Pout = Pin. With the subsequent change in the voltage VS, the output light intensity Pout shows an optical modulation characteristic that it goes up and down between I and 0. Further, the optical modulation characteristic shown in Fig. 3A is one obtained from the case that the optical waveguides 44A and 44B have the same optical path length. When one of the optical waveguides 44A and 44B has its optical path length 'I onger than that of the other by a quarter of the wavelength of light propagating through the one optical waveguide, or when a bias electric field is applied to either one of the optical waveguides 44A and 44B, an optical modulation characteristic is obtained, as shown in Fig. 3B, that the output light intensity varies along the sine curve with a change in the applied voltage VS. That is, this optical modulation characteristic shows that the output light intensity sharply varies about the applied voltage VS = 0. The following description will be given on the assumption that the optical modulator 40 has the optical waveguides 44A and 44B the optical path length of one of which is longer than that of the other by a quarter of the wavelength of light passing through the one optical waveguide and hence it operates with the modulation characteristic shown in Fig. 3B.

As is evident from the modulation characteristics of the optical modulator 40 described above, the intensity P,,u, of light outputted from the optical -inodulator 40 is exprcssed by a value between the state P,,ut = Pi, and 0, that is, between P,,,t/Pj,=1 and Pout/Pj,,=O, no matter what value an inputted electric field to the optical modulator may have, and therefore, the detecting circuit 54 shown in Fig. 1 needs only to measure a voltage between P,,ut/Pj,,=l and P,,ut/Pi,,=O to specify the value of the current to be measured Im.

In other words, when the value of the current to be measured Im is large, the value thereof can be obtained by counting the number of times that the output light intensity P"ut reaches, for example, 0 and by measuring the value between Pout/Pin=l and P,,u,/Pin=O after it has finally reached 0. The value of the minute current Is can also be obtained from a value between Pout/Pin=1 and Pout/Pin=O. Accordingly, the detecting circuit 54 is inputted with only a voltage between Pout/Pin=l and Pou,/Pin=O, no matter what value the input current to the current voltage converter 30 may have. As a result, the detecting circuit 54 cannot saturate even without any limiter circuit, and can measure the voltage corresponding to the current value.

Consequently, no settling phenomenon occurs even immediately after the large-current pulse IPL has appeared as shown in Fig. 4, and hence the minute current Is can be measured when a minute time interval has passed from the timing To of the trailing edge of the large-current pulse IPL. By applying this current sensor to the IC testing apparatus for measuring a leakage current which flows through a CMOS IC under test in its quiescent state, thereby to determine whether the CMOS IC is defective or not defective, there is obtained an advantage that a high-speed testing can be carried out, Further, since the detecting sensitivity of the current sensor 20 is proportional to the electrode lengths L of the electric field applying electrodes 45, 46 and 47 and inversely proportional to the spacing between adjacent electrodes, a desired sensitivity can be obtained by reducing the electrode spacing and increasing the electrode lengths L. In addition, the detecting sensitivity of the current sensor 20 can also be enhanced by increasing the intensity of light emitted from the light source 5 1.

Moreover, it is possible to double the detecting sensitivity of the current sensor by differentially applying the voltage VS to the two optical waveguides 44A and 44B as shown in Fig. 5.

Fig. 6 illustrates another embodiment of the current measuring method according to the present invention. In this embodiment an optical switch 55 is disposed at the light source 51 side, and switching pulses SWP are applied to the optical switch 55, thereby to input light incident to the optical modulator 40 thereinto as an optical pulse 56 in accord with a timing of the measurement, and the amount of the optical pulse 56 transmitted through the optical modulator 40 is detected by the photodetector 53. Thus, the current value to be measured at the intended timing point can be measured with high accuracy.

That is, the timing point for measuring the current value is determined by the timing of the application of the optical pulse 56. Accordingly, the timing point for measuring the current value can be set with fine resolution by reducing the pulse duration or width of the optical pulse 56. Since the photodetector 53 needs only to measure the total quantity of light transmitted through the optical modulator, the value of the current Im to be measured can be measured by measuring an integration voltage integrated in an integration circuit 57 provided at the output side of the detecting circuit 54. Therefore, no fast response is required at the photodetector 53 side. Thus, a measurement with fine resolution in measuring timing can be carried out by requiring only the optical switch 55 to operate at high-speed and without requiring fast operations for other elements and circuits.

Fig. 7 illustrates an example of the specific structure of the current sensor 20. The substrate 21 can be formed of an insulating material such as ceramic. The current-voltage converter 3 0 is constructed by depositing a resistance film 3 1 on one surface of the substrate 21 a resistance film 3 1 which constitutes the current-voltage converter 30, depositing electrodes 32 and 33 on opposite ends of the resistance film 3 1 respectively, and electrically connecting one ends of the electrodes 32 and 33 to current measuring terminals 34 and 35 respectively.

The substrate 21 can be made of a member having its size of approximately 10 mm by 10 mm. The current sensor 20 can be constructed by mounting a dielectric substrate 41 constituting the optical modulator 40 on the blank space on the surface of the substrate 21 on which the current-voltage converter 30 is formed.

0 -17 Between the electrodes 32, 33 and the electric field applying electrodes 46, 45 constituting the optical modulator 40 can be electrically connected by, for example, bonding wires BF or the like. The current sensor 20 having a card-like shape as shown in Fig. 7 is effective in its application to an IC testing apparatus described hereinafter.

Fig. 8 illustrates an example of the IC testing apparatus to which the current measuring method and the current sensor 20 according to the present invention are applied. This embodiment shows the IC testing apparatus to which the current sensor 20 described above with reference to Figs. I through 6 is applied.

A power supply voltageVDD is given from a DC power supply 12 to a power supply terminal TVDD of an IC under test 11 and a common terminal Tvss thereof is connected to a common potential point. The current-voltage converter 30 constituting the current sensor 20 according to the present invention is inserted in this power supply path in series therewith so that a current IDD flowing to the power supply terminal TVDD of the IC under test 11 is fed to the current-voltage converter 30, thereby to generate therein the voltage VS corresponding to the value of the current IDD. The voltage VS is applied between the electric field applying electrodes 45 and 46 to operate the optical modulator 40 to give to light an optical modulation corresponding to the value of the current flow to the power supply terminal TVDD of the IC under test 11.

The photodetector 53) converts the intensity of light outputted from the output optical waveguide 23 into an electric signal which is, in turn, outputted as a voltage signal from the C detecting circuit 54. The voltage signal outputted from the detecting circuit 54 is supplied to a sample-and-hold circuit 14 where it is sampled by a sampling pulse TGP and held therein, the sampling pulse being outputted from, for example, a pattern generator 58 at the timing at which the IC under test 11 is in its quiescent state. The value of the sampled and held voltage is converted in an A/D converter means 15 to digital form, and the digital signal is inputted into an ALU 16 which in turn determines whether the value of the input digital signal falls within a desired voltage range or not, therebyrendering a decision that the IC under test I I is defective (failure) or not defective (pass).

The pattern generator 58 applies a driving signal to the IC under test I I to change or invert its state step by step, and a test is made to determine whether the value of the power supply current IDD in the quiescent state of the IC under test at each step falls within a predetermined range or not.

The sample-and-hold circuit 14 is supplied with the sampling pulse TGP at a timing TST after a slight time interval has passed from the trailing edge of the large-current pulse IPL which flows through the IC under test 11, as shown in Fig. 9. As a result, the sample-and-hold circuit 14 measures the minute current Is in each quiescent state of the IC under test 11 after the IC under test I I has done its inverse operation step by step, and the measured current value is compared with a preset value in the ALU 16 to determine whether the IC under test is defective or not defective.

As shown in Fig. 8, by using the current sensor 20 according to the present invention in the IC testing apparatus, it is possible to operate the current sensor 20 and the detecting circuit 54 without saturating them even if the IC under test 11 intermittently consumes the large-current pulse IPL. Accordingly, the embodiment shown in Fig. 8 makes it possible that the sample-and-hold circuit 14 is activated at the timing TST immediately following the large-current pulse IPL to sample and hold, the sampled signal is converted into a digital signal in the A/D converter 14, and the ALU 16 determines whether the IC under test I I is defective or not defective on the basis of the digital signal. In this instance, the sample-and-hold circuit 14 can sample and hold the current signal at a timing point just after a slight time interval TST has passed from the falling edge of the large-current pulse IPL, and it is possible to carry out the measurement in a short time period. Thus, the period TES that the IC under test I I is inverted in its operation state can be reduced, and hence a practical benefit is obtained that the time duration needed to test an IC under test can be shortened even if the IC under test is tested in its every operation state. In addition, since the embodiment shown in Fig. 8 is insusceptible to external electromagnetic waves even if the input optical waveguide 22 and the output optical waveguide 23 are made long, the measuring accuracy does not deteriorate even in the case that the photodetector 53 is placed at a position greatly spaced apart from the IC under test 11. Accordingly, the accuracy of test does not deteriorate even if the IC testing apparatus proper has the photodetector 53 housed therein.

Fig. 10 illustrates another IC testing apparatus in which an optical switch 55 is inserted in the input optical waveguide 22 coupling between the light source 51 and the optical modulator 40, and the optical switch 55 is turned on and off in synchronism with a timing for the measurement of current, thereby to apply the optical pulse 56 (see Fig. 11B) to the optical modulator 40 in synchronism with the timing for the current measurement.

In this case, at the output side of the detecting circuit 54 is provided the integration circuit 57 which integrates the total quantity of light received by the photodetector 53, the resulting integration voltage INTV (see Fig. I I C) is sampled in synchronism with the sampling pulse TGP (see Fig. 11D) and held in the sample-and-hold circuit 14, and the sampled voltage is converted to a digital signal in the A/D converter 15 to be inputted into the ALU 16. The integration voltage INTV in the integration circuit 57 is reset by the reset pulse RSP shown in Fig. 11E upon each completion of sampling.

As described above, in the case of employing the configuration in which the optical pulse 56 is inputted into the optical modulator 40, as already discussed with reference to Fig.

6, the measurement timing is determined by the timing of application of the optical pulse 56 to the optical modulator.

Therefore, there is obtained an advantage that by reducing the pulse duration or width of the optical pulse 56, the resolution of the measurement timing in the direction of time base can be made fine. Moreover, according to this embodiment, the photodetector 53, the detecting circuit 54 and the sample-and hold circuit 14 need only to obtain the value corresponding to the total quantity of light received by the photodetector 53, and hence they need not have fast-response characteristic. This also provides an advantage that the IC testing apparatus can be constructed. by using elements which need not operate at high speed and hence are inexpensive.

Fig. 12 illustrates a still another embodiment of the IC testing apparatus used in t1csting an IC of the type having a plurality of power supply terminals TVDD- In Fig. 12 there are not shown the pattern generator 58, the driving circuit for the light source 51, and the internal configuration of the current sensor 20.

In this embodiment, there are provided a plurality of current sensors 20 the number of which is equal to that of the power supply terminals TVDD of the IC under test 11.

Specifically, in this embodiment, four current sensors 20 are provided, and one end of the current-voltage converter 30 in each of the four current sensors 20 is connected to the positive voltage terminal of the DC power supply 12, and the other end of -voltage converter 30 is connected to an associated each current power supply terminal TVDD- Furthermore, the input and output optical waveguides 22 and 23 to be connected to each of the optical modulators 40 are connected in series, and the light source 5 1 is optically coupled to one end of the series-connected -optical waveguides and the photodetector 53) is optically coupled to the other end thereof.

With such arrangement, the quantity of light received by the photodetector 53) corresponds to the sum total of amounts of optical modulations that are given to light by the four current 0 -22- sensors 20 respectively when the light has passed therethrough, and also that quantity of light corresponds to the sum total of the values of currents each flowing to the associated power supply terminal TVDD. Accordingly, an adding means is constituted by the optical system, and the sum of the values of currents flowing through the plurality of power supply terminals TVDD can be measured by one light source 5 1, one photodetector 53 and one detecting circuit 54. Consequently, it is possible to determine whether the sum of the values of currents flowing through the respective power supply terminals TVDD falls within the range of normal values or not.

Fig. 13 illustrates a modified form of the embodiment shown in Fig. 12. This modified embodiment shows the case that probes 63 are brought into contact with electrode portions of IC chips 62 formed on a wafer 61 thereby directly testing the ICs under their chip state.

Conventionally, the probes 63) are supported by a ringshaped probe card 64 so as to extend toward the center thereof, and the tip of each probe 63 is brought into contact with the electrode portion of each chip formed on the wafer 61. A power supply current and a driving signal are supplied through each probe 63, and the measurement of the power supply current is performed at the side of the power supply 12.

In contrast to the above, according to the present invention, the current sensor 20 is inserted in each probe 63 at the intermediate way thereof, and an optical signal optically modulated by the current sensor 20 is transmitted through the optical waveguide 23 and is received by the photodetector 53.

0 -23 In this case, too, no disturbance is caused by external electromagnetic waves and the like even if the length of the optical waveguide 23 is extended. Therefore, it is possible to test ICs in chip form without being affected by external electromagnetic waves and the like even if the probe card 64, the light source 5 1, the photodetector 53, the detecting circuit 54, the sample-and-hold circuit 14 and the like are housed in the measuring instrument proper disposed at a position spaced apart from the position (test head) where the IC under test I I is disposed.

Fig. 14 shows a modification of the current measuring method. This modified embodiment is intended, for example, to exclude the influence due to a drift of the quantity of light emitted from the light source 5 1 and to eliminate an offset voltage of the photodetector 5").

In this embodiment, an optical coupler 65 is connected to the input optical waveguide 22 at the side of the light source 5 L The optical coupler 65 branches an input light into two parts one of which is inputted into the optical modulator 40, and the other of which is inputted into a correcting optical waveguide 66 formed adjacent to the optical modulator 40. The output light from the correcting optical waveguide 66 is received by a second photodetector 53B, and a voltage signal corresponding to the quantity of light received by the second photodetector 53B is fed back to the driving circuit 52 for the light source 5 1, thereby to t:o C> control the intensity of light emitted from the light source 5 1 to be stabilized. The voltage signals corresponding to the quantities of light received by the photodetectors 53A and 53B 0 -24- respectively are supplied to a subtracting circuit 67 where one is subtracted from the other. With such arrangement, offset voltages produced in the photodetector 53A and the detecting circuit 54A can be removed.

Accordingly, the voltage signal from which the offset voltages have been removed in the subtracting circuit 67 is inputted via the sample-and-hold circuit 14 and the A/D converter 15 into the ALU 16, and hence it is possible to obtain the measured value from which the offset voltages have been removed. Furthermore, since this embodiment is configured such that light branched by the optical coupler 65 passes through the correcting optical waveguide 66 formed adjacent to the optical modulator 40, the light passing through the correcting optical waveguide 66 is subject to the same influence in temperature as that of the light passing through the optical IP modulator 40. Therefore, an additional advantage is obtained that there is eliminated variations in the optical modulation characteristic of the optical modulator 40 due to temperature variations of the dielectric substrate 41.

INDUSTRIAL APPLICABILITY

As described above, according to the current measuring method according to the present invention, since an electric signal which is detected in relation to a current flow of excess 25 amplitude results in that the ratio between the intensity Pi, of li ht emitted from the light source 5 1 and the intensity P 9 out Of licrht received by the photodetector 53' goes up and down only C) between the state of Pout/Pi,,=1 and the state Of Pou,/Pin=0, no limiter circuit is needed at the detecting circuit side.

Accordingly, even if a current value returns to 0 immediately after an excess amount of current has flowed, there occurs no settling phenomenon, and hence the current immediately reaches a target value and is stabilized at that value. Consequently, even if the phenomenon that the large-current pulse IPL and the minute current 1, alternately flow is repeated at high speed, the minute current Is can be measured with high accuracy and in stable condition even just after the large-current pulse IPL has flowed.

As a result, the current measuring method according to the present invention is suitable for use in measuring a leakage current that flows through a power supply terminal of an IC having CMOS structure, and the current measuring method is suitably applied to an IC testing apparatus of the type that renders a decision that an IC under test is defective or not defective depending upon whether the leakage current is within a prescribed range or not.

Moreover, according to the present invention, the measured value of current detected by the current sensor 20 is transmitted through the output optical waveguide 23, and hence it is free from the possibility of suffering a variety of electrical interferences in the output optical waveguide 23. Accordingly, there is obtained an advantage that the accuracy of measuring the minute current can be maintained high even if the test head and the measuring instrument proper are greatly distanced from each other.

Claims

WHAT IS CLAIMED IS:

1. A current measuring method comprising the steps of: converting a current to be measured into a voltage signal by a current-voltage converter; phase modulating light being transmitted through an optical modulator by the voltage signal; causing the phase modulated light to interfere with light that is not phase modulated to obtain interference light; and converting the intensity of the interference light into an electric signal to specify the value of the current to be measured.

2. The current measuring method as set forth in claim 1, wherein light inputted into said optical modulator is an optical pulse which is generated in synchronism with a measuring timing, and the transmission quantity of the optical pulse is integrated to obtain the value of the current to be measured.

3. A current sensor comprising: a branched interfero-metric optical modulator comprising an optical branching part, an optical coupling part, two optical waveguides formed between said optical branching part and said optical coupling part, and two pairs of electric field applying electrodes one pair being formed at both sides of one of said two optical waveguides to put it between these two electrodes, the other pair being formed at both sides of the other of said two optical waveguides to put it between these two electrodes, said optical branching part, said optical coupling part, said two optical waveguides, and said two pairs of electric field applying electrodes being formed on a dielectric substrate; a current-voltage converter for converting a current to be measured to a voltage signal; a substrate for supporting said optical modulator and said current-voltage converter; an input optical waveguide for applying light into said optical modulator; and an output optical waveguide for taking out interference light emitted from said optical modulator.

4. The current sensor as set forth in claim 3, wherein a correcting optical waveguide is provided on the dielectric substrate on which said optical modulator is formed adjacent to said optical modulator, and an electric signal derived from light transmitted through said correcting optical waveguide is used to compensate an electric signal derived from the light transmitted through said optical modulator.

5. An IC testing apparatus which measures a current flowing to a power supply terminal of an IC under test and renders a decision that the IC under test is defective or not defective depending upon whether the current flowing to the power supply terminal is larger than or smaller than a prescribed value, said IC testing apparatus being characterized in: that the current flowing to the power supply terminal is supplied to the current-voltage converter of the current sensor defined by claim 3; that the intensity of interference light emitted from the output optical waveguide of the current sensor is converted into an electric signal by a photodetector; and that the electric signal is compared with a preset value to determine whether the electric signal is larger than or smaller than the preset value, thereby to render a decision that the IC under test is defective or not defective.

6. An IC testing apparatus which measures a current flowing to a power supply terminal of an IC under test and renders a decision that the IC under test is defective or not defective depending upon whether the current flowing to the power supply terminal is larger than or smaller than a prescribed value, said IC testing apparatus being characterized in: that the current flowing to the power supply terminal is supplied to the current-voltage converter of the current sensor defined by claim 4; that the intensity of interference light emitted from the output optical waveguide of the current sensor is converted into a first electric signal by a first photodetector, and the intensity of light transmitted througliI the correcting optical waveguide of the current sensor is converted into a second electric signal by a second photodetector; that said first and second electric signals are processed by operation thereof to compensate the first electric signal; and that the first electric signal is compared with a preset value to determine whether the first electric signal is larger than or smaller than the preset value, thereby to render a decision that the IC under test is defective or not defective.

7. The IC testing apparatus as set forth in claim 5 or 6, wherein the IC under test repeats at high speed the state of consuming a large current and the state of consuming a minute current, and in the measurement of current, the minute current is mainly measured.

8. The IC testing apparatus as set forth in claim 5 or 6, wherein when the IC under test has a plurality c 'L power supply terminals, the current sensor is provided for each of the power supply terminals of the IC under test, and a current is measured for each power supply terminal.

9. The IC testing apparatus as set forth in claim 5 or 6, wherein when the IC under test has a plurality of power supply terminals, the current sensor is provided for each of the power supply terminals of the IC under test and the optical waveguides of the current sensors are optically connected in series, and the sum total of currents flowing to the plurality of power supply terminals respectively is found from the quantity of optical modulation for light.

10. The IC testing apparatus as set forth in claim 5 or 6, wherein an optical pulse is applied to the optical modulator at each measuring timing, and the quantity of optical modulation for each optical pulse is detected by a photodetector, thereby to measure the current value at each measuring timing.

11. The IC testing apparatus as set forth in claim 5 or 6, wherein the current sensor is mounted on a measuring probe supported by and protruding from a probe card, and the tip of said measuring probe is brought into contact with a terminal portion of an IC chip formed on a semiconductor wafer, thereby to test the IC chip.

12. The IC testing apparatus as set forth in claim 6, wherein the electric signal, derived from the light transmitted through the correcting optical waveguide provided adjacent to the optical modulator, is applied to a light source driving circuit to stabilize the intensity of light emitted from a light source, thereby to measure a current flowing through the IC under test.