The present invention is related to a device for measuring the quiescent current (I_DDQ) drawn by an electronic device, such as a CMOS device or an Integrated Circuit, when the device is powered by a supply voltage (V_DD).

Integrated circuits need to be thoroughly tested. The current drawn by a powered CMOS device or IC, when it is not in switching mode, is called the 'quiescent current', described by the symbol I_DDQ. It is known that the value of this current is a very sensitive criterion for identifying possible malfunctioning of the IC. The detection of the I_DDQ level, and the comparison of this level with a reference, allows a straightforward pass/fail decision to be made on the quality of the device under test. Several devices and methods for I_DDQ measurement have been described so far.

• Document EP-A-672911 describes an I_DDQ test device for a CMOS device, said test device comprising a stabilized voltage source, and a current measurement circuit, which is coupled to said source.
• Document WO-A-9815844 is related to a method for inspecting an integrated circuit, wherein the supply current is measured, by measuring the voltage over a segment of the supply line through which this supply current flows.
• Document EP-A-811850 is related to a system for the measurement of a supply current of an electronic circuit, comprising a bypass switch with a dummy transistor to avoid charge transfer.
• Document EP-A-1107013 is related to a device for testing a supply connection of an electronic device, said test device comprising a current mirror.

A problem of existing I_DDQ-monitors is related to the sensitivity of these test devices. The background leakage current (I_{Background Leakage}) of deep sub-micron devices increases drastically as process technologies are decreasing in size and the number of transistors contributing to the background leakage is increasing drastically, whereas the order of magnitude of the defect current (I_{Defect Leakage}) remains more or less constant: $${\mathrm{I}}_{\mathrm{Total\; Leakage}}\left(\mathrm{\nearrow }\right)\mathrm{=}{\mathrm{I}}_{\mathrm{Background\; Leakage}}\left(\mathrm{\nearrow }\right)\mathrm{+}{\mathrm{I}}_{\mathrm{Defect\; Leakage}}\left(\mathrm{\to }\right)$$

In order to distinguish the small defect leakage component from the background leakage current, the measurement resolution must be very high throughout the measurement range of the monitor. People skilled in the art know that increasing the measurement range is always done at the expense of measurement resolution. Yet the resolution should remain constant as the defect leakage current remains constant. Existing I_DDQ monitors do not offer a solution to this problem.

Document US6414511B1 discloses an arrangement for transient current testing of a CMOS circuit. In the article 'A fully digital controlled Off-Chip I_DDQ measurement unit', Straka et al., 1998, a measurement unit is disclosed which exhibits reduced sensitivity to charge injection, due to an auxiliary circuit which reduces the peaks approximately 5 to 10 times in comparison with an uncompensated bypass switch.

In document USS914615, a method is disclosed for improving the quality and efficiency of IDDQ testing, by calculating an upper and a lower threshold value. However, none of the above three documents suggest a solution to the problem of high background leakage current.

The present invention aims to provide a device for I_DDQ monitoring of electronic devices, which has a high resolution for higher I_DDQ levels. The device of the application is capable of being used both in on-chip and off-chip applications.

The invention is related to a device for measuring the supply current (I_DDQ) to an electronic device under test, which is powered by a supply voltage (V_DUT), said measuring device being placed in a supply line between said supply voltage and said device under test, said measuring device comprising a current measuring unit or CMU, a current bypass unit or CBU in parallel to said CMU, characterized in that said measuring device further comprises an offset current device, said offset current device comprising a current source, for providing a constant offset current to said DUT.

Said current source is preferably programmable. It may be coupled in parallel to said current measuring unit, or it may be powered by a supply voltage (V_DD) which is different from the DUT supply voltage (V_DUT).

Any device according to the invention may further comprise a processing unit, which is in connection with said current measuring unit and with an output device and which is able to acquire an I_DDQ measured value from the CMU, characterized in that the processing unit is able to perform processing actions on said measurement.

Said processing actions are preferably chosen from the group consisting of :

• subtracting a measured I_DDQ value from a reference value or vice versa,
• comparing a measured I_DDQ value with a reference value and producing a pass/fail signal on the basis of the result of said comparison,
• subtracting a measured I_DDQ value from a previously measured I_DDQ value
• comparing a calculated value, resulting from subtracting a measured I_DDQ value from a previously measured I_DDQ value or vice versa, or from subtracting a measured I_DDQ value from a reference value or vice versa, with a reference value and producing a pass/fail signal on the basis of the result of said comparison.

A device of the invention may be separate from said device under test, or it may be incorporated into said device under test.

Fig. 1 represents a schematic view of an I_DDQ monitor according to the invention.

Fig. 2a and 2b represent schematic views of two embodiments of the current offset unit according to the invention.

Fig. 3 represents a graph, illustrating the application of a virtual measurement window to the leakage current, by using a monitor according to the invention.

Figures 4 to 7 illustrate different off-chip and on-chip embodiments of a device according to the invention.

Figure 1 illustrates a schematic view of an I_DDQ monitoring device or simply named 'monitor' 1, according to the invention. In this figure, the monitor is represented as a separate device, which can for example be incorporated into the test equipment, as a load-board application. It is emphasized that the same monitor can be designed as an on-chip device.

The monitor 1 is connected by two terminals 2 and 3, between a supply voltage source 4, and the Device-Under-Test DUT 5. The supply voltage V_DUT at the terminal 2 should be present also, with a minimum error, on the terminal 3, in order to create a maximum transparency of the monitor 1.

The measurement of the I_DDQ is performed by the current measuring unit CMU 6, during a non-switching state of the DUT. Test vectors 7 are applied to the DUT at a given clock frequency, by the test equipment 8. The CMU 6 may be a unit working according to the stabilized voltage source principle or any other prior art measurement method. A current bypass unit CBU 20 is placed parallel to the CMU 6. The CBU 20 preferably comprises a power MOSFET which can be closed prior to the occurrence of the transient peak resulting from the DUT's switching action. This transient peak occurs when a test vector is applied to the DUT or when the application of a clock cycle of the DUT's operational clock causes the DUT to change state. In between transient peaks and for the desired measurement states, the MOSFET is normally opened in order to send the quiescent current I_DDQ through the current measuring unit CMU 6.

The operation of the CBU 20 and the Offset Current Unit (OCU) 21, said OCU being characteristic to the invention and described further, is controlled by the processing unit 9, via control signals 10 and 11. In particular, the PU 9 controls the opening and closing of the MOSFET incorporated in the CBU 20, on the basis of a clock signal derived from the clock with which the DUT is operated. The clock applied to the CBU is dependent on the relevant measurement sequence : there is not necessarily a measurement during every clock cycle of the DUT. When in measurement mode, the current measuring unit performs an I_DDQ measurement, during a non-switching period of the DUT and delivers a signal 12 related to the I_DDQ level, to the processing unit 9, which digitises the signal, and transmits it via the terminal 13, to the test equipment 8.

The test equipment 8 controls the processing unit 9, and processes the monitor's output 12, so that the result of the I_DDQ measurement is displayed on a screen. In the preferred set-up, the source 4 is not separate, and the supply voltage V_DUT is equally supplied by the test equipment 8. The displayed result is at least a pass/fail statement based on the comparison between the measured I_DDQ value and a predefined reference, often completed by the measured value of I_DDQ. Other measurement modes can be selected when using the preferred version of the processing unit 9. For example : the measurement of current signatures or a delta I_DDQ measurement mode wherein subsequent measurements are subtracted and the delta-values obtained are memorized and compared to a reference.

According to a preferred embodiment of the invention, the PU 9 itself performs the processing of the incoming signals, for example the subtraction of two subsequent I_DDQ measurement values, before a result is transferred to the test equipment 8. Some examples of measurement modes, performed by a PU according to this embodiment, are given further in this description.

Block 21 shown in figure 1 is the characteristic element of the invention. This is the Offset Current Unit (OCU). As the large background leakage current does not contain any defect information, it is not practical to measure this component of the leakage current. In practice, the background leakage current can be of the order of 100mA, while the defect-related leakage current I_DDQdefect is typically between 0 and 10mA. The background leakage component can therefore be regarded as an offset current (I_DDQOffset). The total quiescent current I_DDQ is the sum of both previously described components. $${\mathrm{I}}_{\mathrm{DDQ}}\mathrm{=}{\mathrm{I}}_{\mathrm{DDQBackground}}\mathrm{+}{\mathrm{I}}_{\mathrm{DDQDefect}}$$

Measuring the totality of this I_DDQ current, requires a CMU with a high dynamic range, and thus reduced sensitivity. As stated above, this is a growing problem, due to increasing background leakage current in submicron devices, while the defect leakage current remains of the same order. The total I_DDQ level becomes too high for a current measuring unit, having a high resolution, as is desired for reasons of accuracy. It would therefore be advantageous if only the defect leakage component is measured. As shown schematically in figure 2, the OCU 21 is actually a current source 40, delivering an essentially fixed offset current into the supply line to the DUT. The current source will deliver a given offset current level I_DDQoffset to the summing node 41, so that only the I_DDQ above this level is drawn from the current measuring unit 6. In that way a flexible virtual measurement window (VMW) 42 is created with a high measurement resolution (figure 3). The offset current source is preferably a programmable current source. The offset level 43 which is delivered as an offset current is programmable and variable during the measurement. It is basically controlled by the operator of the test equipment, through the processing unit 8. Two embodiments are considered, as shown in figures 2a and 2b. In figure 2a, the current source 40 is connected to the supply voltage V_DUT, placing the OCU in parallel to the CMU. However, a parallel connection is not necessary. The offset current unit may for example be connected to the device supply voltage V_DD (figure 2b). Autoranging sequences are preferably performed prior to applying a series of test vectors, in order to establish the optimal level of the applied offset current. For this purpose, a programmable current source 40 is preferably used.

The offset current unit 21, possibly in combination with a compensated switch 20 according to EP application No. 02447125.2 , can be applied in various ways. In the off-chip current measurement domain, the programmable current source 21 can be implemented either as a stand-alone module (figure 4) or it can be part of the off-chip current monitor 100 (figure 5). The block 100 in figures 4 to 7 represents an I_DDQ monitor with a CMU 6, and preferably with a CBU 20 according to EP application No. 02447125.2 . The off-chip current monitor architecture is of no importance to the windowing concept. In the on-chip domain, again the offset current unit 21 can be an add-on core to the Built-In Current Sensor (BICS) (figure 6) or be part of the BICS itself (figure 7). The BICS architecture is of no importance to the concept. In all embodiments shown in figures 4-7, the dotted line delineates what is to be understood as the device 1 of the invention.

The (programmable) current source can be controlled by the ATE 8 (figures 4a, 5a, 6a, 7a) or by the on-chip/off-chip I_DDQ monitor (figures 4b, 5b, 6b, 7b) and although it is not necessary, the offset current unit can be part of a auto-windowing procedure.

The following measurement modes comprise calculations which are performed by the processing unit itself. Results of calculations (subtraction of I_DDQ values, comparison results) are transferred to the ATE 8 which may further process them or display the results on a screen.

• Standard I_DDQ mode - I_DDQ measurements are made and compared against one predefined reference value resulting in a pass/fail result. Pass = measurement is below reference, Fail = measurement is above reference.
• Current signatures - this is a special version of the standard I_DDQ mode, for a current signature approach, I_DDQ measurements are made and compared against a predefined vector related pass/fail reference, resulting in a pass/fail result.
• Standard Delta-I_DDQ mode (vector-to-vector delta) - I_DDQ measurements are made, subsequent measurements are subtracted from each other (delta calculation). The measurement is preferably but not necessarily compared against a predefined absolute reference and the calculated delta is compared against a predefined delta reference value, resulting in a pass/fail result. Delta as well as absolute reference(s) can be set either globally or on a vector-to-vector basis.
• Vector to reference vector Delta-I_DDQ mode - A reference vector is selected of which the related I_DDQ measurement serves as reference for the following measurements. Typically the reference vector is the first I_DDQ vector (this situation is supported by the standard vector to reference vector delta I_DDQ mode firmware). I_DDQ measurements are then made, the measurement result of each subsequent measurement is subtracted from the reference value gathered during the reference vector measurement (delta calculation). The measurement is preferably but not necessarily compared against a predefined absolute reference and the calculated delta is compared against a predefined delta reference value, resulting in a pass/fail result. Delta as well as absolute reference(s) can be set either globally or on a vector-to-vector basis.
• Pre and Post stress Delta-I_DDQ mode - A first set of I_DDQ measurements are made (pre stress), then stress is applied to the device under test, followed by a second set of I_DDQ measurements (post stress). The results from the corresponding pre and post stress measurements are subtracted (delta calculation). The measurement is preferably but not necessarily compared against a predefined absolute reference and the calculated delta is compared against a predefined delta reference value, resulting in a pass/fail result. Delta as well as absolute reference(s) can be set either globally or on a vector to vector basis