INTEGRATED CIRCUIT TESTER WITH COMPENSATION FOR LEAKAGE CURRENT

Abstract: In an integrated circuit tester module (16), pin electronics circuitry (20) supplies leakage current to a circuit node which is connected to a signal pin (2) of a device under test (6). The leakage current is compensated by connecting the circuit node to a voltage source (VDD) at a first potential level, supplying current (26) to the circuit node from a second potential level, and measuring current supplied to the circuit node from the voltage source. The second potential level is selectively varied in a manner such as to reduce the current supplied from the voltage source substantially to zero. The circuit node is then disconnected from the voltage source.

Description: INTEGRATED CIRCUIT TESTER WITH COMPENSATION FOR LEAKAGE CURRENT Background of the Invention This invention relates to an integrated circuit tester with compensation for leakage current . An integrated circuit tester is used to predict how an integrated circuit device will behave in operation. A typical integrated circuit tester includes a test head having multiple tester modules, each of which has a signal terminal. Each tester module includes pin electronics circuitry which operates the module selectively in one of several operating modes, which typically include drive high, drive low, compare high and compare low. In the drive high mode, for example, the circuitry applies a logic high signal to the signal terminal, whereas in the compare low mode, the circuitry compares the voltage at the signal terminal with a low threshold value. In order to carry out a test, the test head is positioned with the signal terminals of the tester modules in contact with respective pads of a load board. The load board provides a parallel electrical interface between the signal terminals of the tester modules and signal pins of the device under test (DUT) . The load board may support connections to multiple devices in order to allow multiple devices to be tested concurrently. The tester includes a device power supply (DPS) having force and return terminals connected through the load board to the positive and negative power supply pins respectively of the DUT to supply operating current to the DUT. A test is executed in a succession of cycles, and for each cycle of the test, each tester module is placed in a selected one of its predetermined operating modes. In this manner, the DUT is exercised through a sequence of internal states, and the nature of the output of the DUT in each state is observed. In the case of a complex DUT, there may be many thousands of test cycles and therefore the testing can take a long time. Two aspects of testing of an integrated circuit device are functional testing and quiescent current testing. The purpose of functional testing is to determine whether the DUT provides the expected output in each state. The purpose of quiescent current testing is to detect anomalies in current consumption by measuring the current drawn by the DUT in steady state, when there are no changes in state occurring. In a high speed functional test, the DPS may supply several amps of current, whereas a much smaller current is drawn during quiescent current testing. FIG. 3 illustrates part of an integrated circuit tester used for testing an integrated circuit device. As shown in FIG. 3, a typical gate in a MOS integrated circuit device comprises a pull- up FET Ql and a pull-down FET Q2 having their drain/source current paths connected in series between a V DD rail and a V ss rail, which are connected to the power supply pins of the device. The output of the gate is taken from the node to which the drain of the pull-down FET Q2 and the source of the pull-up FET Ql are connected, and may be connected to the gate of another FET Q3 in order to affect the state of that transistor. In one state of the gate Ql, Q2, the pull-up transistor Ql is on and the pull- down transistor Q2 is off, and the output of the gate is accordingly at a voltage very close to V DD , whereas in another state of the gate, the pull-up transistor Ql is off and the pulldown transistor Q2 is on and the output is at a voltage very close to V ss . If the output of the gate is connected to a pin 2 of the DUT, the state of the gate Ql, Q2 can be tested by comparing the voltage at the pin 2 with a high or low threshold voltage . The tester includes a current measuring device power supply (DPS) 4 having force and return lines connected to power supply pins of the device under test (DUT) 6 to supply operating current to the DUT, and multiple tester modules 10, only one of which is shown, each having a terminal connected to a signal pin 2 of the DUT. Generally, the return line of the DPS is tied to ground. It may be desired not only to exercise the DUT through a sequence of states in order to test it, but it may also be necessary to measure quiescent current drawn by the DUT in selected states in order to detect anomalies in current consumption. The quiescent current is measured by measuring the voltage drop across a current sensing resistor when the DUT is in a target state. Ideally, the tester module 10 presents an infinite impedance to the signal pin. Therefore, if the transistor Ql is on and the transistor Q2 is off, no leakage current flows through the pin electronics circuitry. However, in practice the pin electronics circuitry does not have an infinite impedance and so there is a leakage current path from the force terminal of the DPS to the return terminal through the tester module, as indicated by the arrow in FIG . 3. The leakage current I l ge introduces an error term into the measurement of I DDQ . It has been proposed that an electromechanical relay switch 14 should be connected between the signal terminal of the tester module and the pin electronics circuitry and that the switch should be opened (rendered non-conductive) when carrying out a quiescent current measurement in order to prevent flow of leakage current through the pin electronics circuitry. However, this is not an optimum solution because of the finite time taken to open the relay switch prior to the measurement and to close the relay switch after the measurement, to resume functional testing of the DUT. Although this time is fairly short, it might be desirable to make several thousand measurements of quiescent current on any particular device, and the total time taken to open and close the relay switch then adds significantly to the time taken for testing the device. Summary of the Invention According to a first aspect of the present invention there is provided an improved tester module for an integrated circuit tester, said tester module having a signal terminal for connecting to a signal pin of a device under test, said tester module comprising pin electronics circuitry connected to a circuit node of the tester module and a current measurement circuit connected to said circuit node and providing an output signal representative of leakage current supplied by the pin electronics circuitry, and wherein the improvement resides in a controllable current source connected to said circuit node and responsive to a control signal to supply current to the pin electronics circuitry. According to a second aspect of the present invention there is provided an integrated circuit tester comprising a tester module having a signal terminal for connecting to a signal pin of a device under test, said tester module including pin electronics circuitry connected to a circuit node, a current measurement circuit connectable to said circuit node and providing an output signal representative of leakage current supplied by the pin electronics circuitry when it is so connected, and a controllable current source connected to said circuit node and responsive to a control signal to supply current to the pin electronics circuitry, and a control means connected to receive the output signal provided by the current measurement circuit and generate the control signal supplied to the current source. According to a third aspect of the present invention there is provided a method of compensating for leakage current in pin electronics circuitry of a tester module of an integrated circuit tester, the tester module having a circuit node for connection to a signal pin of a device under test and the pin electronics circuitry being connected to said circuit node, said method comprising connecting the circuit node to a voltage source at a first potential level, supplying current to said circuit node from a second potential level, measuring current supplied to said circuit node from said voltage source, (d) selectively varying the second potential level in a manner such as to reduce the current supplied from said voltage source substantially to zero, and disconnecting the circuit node from said voltage source. Brief Description of the Drawings For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a partial schematic block diagram of an integrated circuit tester in accordance with the present invention in a first configuration, FIG. 2 is a similar view of the tester in a second configuration, and FIG. 3 is a partial schematic block diagram of a tester in accordance with the prior art. In the several figures, like reference numerals designate corresponding elements . Detailed Description Each tester module 16 of the tester shown in FIG. 1 includes pin electronics circuitry 20 having a terminal connected to a circuit node 22. During a test of the DUT 6, the node 22 is connected through a normally closed (conductive) switch SI to the signal pin 2 of the DUT for selectively forcing the pin 2 to a high or low voltage or comparing the voltage at the pin with a high or low threshold voltage. The node 22 is also connected to a controllable current source 26. The node can be connected through the switch SI, a normally open (non-conductive) switch S2 and a current sensing resistor 32 to a differential amplifier 34. The tester shown in FIG. 1 operates under control of a server 28. A second differential amplifier 36 provides an output signal dependent on the current flowing through the resistor 32 and an analog-to-digital converter (ADC) 40 converts the analog output signal of the amplifier 36 to digital form. The digital output signal of the ADC 40 is supplied to the server 28. The controllable current source 26 comprises a digital-to- analog converter (DAC) 44, which receives a digital control signal from the server 28, and a resistor 46. The DAC 44 generates an output voltage which depends on the digital control signal received from the server. Accordingly, the current source 26 supplies current to the node 22 at a level which depends on the digital control signal supplied to the DAC 44 by the server 28. During a calibration phase, the tester module is not connected to the pin 2 of the DUT, so that the pin electronics circuitry 20 is isolated from the DUT 6, and the switches SI and S2 are closed. The voltage V DD for the specific DUT 6 is applied to one input of the differential amplifier 34 and the differential amplifier 34 supplies sufficient current to the node 22 through the resistor 32 and the switches SI and S2 to force the node 22 to the voltage V DD , i.e. the voltage that would be present at the node 22 during measurement of I DDQ with the FET Ql on and the FET Q2 off. Consequently, the total current supplied to the node 22 is equal to I l ge - The current supplied by the differential amplifier 34 is measured by the differential amplifier 36 and ADC 40, which provide the server 28 with a digital signal representative of the current supplied by the amplifier 34. The server adjusts the output voltage of the DAC 44 until no current flows through the current sensing resistor 32. At this point, the current supplied by the DAC 44 is equal to I lkge . The current supplied to the node 22 holds the node at V DD . During the measurement phase, the tester module is connected to the pin 2. The switch SI remains closed and the switch S2 is open. Since the node 22 is at V DD/ no current flows from the pin 2 into the pin electronics circuitry and consequently the accuracy of a measurement of I DDQ is not impaired by the leakage current. Therefore, it is possible to measure I DDQ when the transistor Ql is on and the transistor Q2 is off without its being necessary to isolate the pin electronics circuitry from the pin of the DUT and accurate measurements of I DDQ can be made more rapidly than when using the technique described with reference to FIG. 3. It will be understood by those skilled in the art that the value of V DD depends on the particular device that is to be tested, and accordingly the calibration phase may involve determining the proper output voltages of the DAC 44 for several different values of V DD , such that the proper voltage can be invoked when carrying out a test. 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Claims: Claims 1. An improved tester module for an integrated circuit tester, said tester module having a signal terminal for connecting to a signal pin of a device under test, said tester module comprising pin electronics circuitry connected to a circuit node of the tester module and a current measurement circuit connected to said circuit node and providing an output signal representative of leakage current supplied by the pin electronics circuitry, and wherein the improvement resides in a controllable current source connected to said circuit node and responsive to a control signal to supply current to the pin electronics circuitry. 2. A tester module according to claim 1, including a switch connected between said circuit node and the signal terminal of the tester module. 3. An integrated circuit tester comprising: a tester module having a signal terminal for connecting to a signal pin of a device under test, said tester module including pin electronics circuitry connected to a circuit node, a current measurement circuit connectable to said circuit node and providing an output signal representative of leakage current supplied by the pin electronics circuitry when it is so connected, and a controllable current source connected to said circuit node and responsive to a control signal to supply current to the pin electronics circuitry, and a control means connected to receive the output signal provided by the current measurement circuit and generate the control signal supplied to the current source. 4. A method of compensating for leakage current in pin electronics circuitry of a tester module of an integrated circuit tester, the tester module having a circuit node for connection to a signal pin of a device under test and the pin electronics circuitry being connected to said circuit node, said method comprising: (a) connecting the circuit node to a voltage source at a first potential level, (b) supplying current to said circuit node from a second potential level, (c) measuring current supplied to said circuit node from said voltage source, (d) selectively varying the second potential level in a manner such as to reduce the current supplied from said voltage source substantially to zero, and (e) disconnecting the circuit node from said voltage source . 5. A method according to claim 4, wherein the first potential level is substantially equal to a positive potential level applied to a power supply pin of the device under test. 6. A method according to claim 4, wherein the first potential level is substantially equal to a negative potential level applied to a power supply pin of the device under test. 7. A method according to claim 4, wherein the circuit node is connected to a signal pin of a device under test, and the method comprises, before step (a) , disconnecting the circuit node from the signal pin of the device under test. 8. A method according to claim 7, further comprising: (f) reconnecting the circuit node to the signal pin of the device under test.

Inventors: CURRIN JEFFREY D, PUN HENRY Y, CURRIN, JEFFREY, D., PUN, HENRY, Y.
Applicants: CREDENCE SYSTEMS CORP, CREDENCE SYSTEMS CORPORATION
Application year: 1998
Main classification: G01R1
IPC codes: G01R31