Semiconductor device test system having reduced current leakage

A test circuit tests a device under test (DUT) uses a first switching device and a second switching device. The device under test (DUT) has a terminal for receiving a test signal. The first switching device has an output terminal for use in coupling the test signal to the terminal of the DUT when the DUT is being tested. The first switching device is high impedance when the DUT is not being tested. The second switching device is high impedance when the DUT is being tested and couples a bias control signal to the output terminal of the first switching device when the DUT is not being tested. The bias control signal substantially tracks the test signal. Leakage from the first switching device when other DUTs are being tested is greatly reduced because the bias control signal results in little or no bias across the first switching device.

BACKGROUND

This disclosure relates generally to semiconductors, and more specifically, to circuitry and a process for testing semiconductor device functionality.

2. Related Art

Transistors and other semiconductor devices are often tested for functional operating characteristics by placing such devices on a semiconductor wafer between integrated circuit die. Such transistors are tested prior to singulating the integrated circuit die and are efficient by not using the area within the integrated circuit. However, such known test devices have decided disadvantages. In one known implementation, each test device has one or more wafer probing pads which consume a large amount of area per test device. To reduce the total area required for multiple test devices, test devices have been connected together in parallel. In this parallel configuration the off-state leakage current is significant, thereby preventing the measurement of a single test device's leakage current. Additionally, a parallel test structure fails when only one of the test devices is faulty and causes an electrical short-circuit. Another alternative for implementing test devices is to use CMOS transmission gates to isolate test devices that are connected in parallel. Such transmission gates have lower leakage current when implemented with low-leakage transistors, such as those with thicker gate oxide. However, such low-leakage transmission gates add significant resistance and still have some non-zero leakage current that becomes significant when large numbers of test devices are connected in parallel. Therefore, the leakage current can become greater than the small current values desired to be measured.

DETAILED DESCRIPTION

Illustrated inFIG. 1is a test system10for testing a plurality of devices under test (DUTs). Generally, a decoder12has an input connected to an output of a tester20for receiving a select signal labeled “DUT Selection”. The decoder12has a plurality of outputs for providing N select signals, where N is an integer. A first select signal, Select1, provided by the decoder12is connected to a first input of a first device under test (DUT) circuitry14. A second select signal, Select2, provided by the decoder12is connected to an input of a second device under test (DUT) circuitry16. An Nth select signal, Select N, provided by the decoder12is connected to an input of a first device under test (DUT) circuitry18. In the illustrated form any number of select signals, from one to N, may be provided by the decoder12. The DUT circuitry14has outputs that are respectively connected to each of a plurality of buses. A first bus output of the DUT circuitry14is connected to a first bus that is a Gate Force bus for providing a Gate Force test signal. A second bus output of the DUT circuitry14is connected to a second bus that is a Gate Sense bus. A third bus output of the DUT circuitry14is connected to a third bus that is a Drain Force bus for providing a Drain Force test signal. A fourth bus output of the DUT circuitry14is connected to a fourth bus that is a Drain Sense bus. A fifth bus output of the DUT circuitry14is connected to a fifth bus that is a Drain Leakage bus for providing a Drain Leakage bias control signal. In one form the Drain Leakage bias control signal is a DC bias voltage. A sixth bus output of the DUT circuitry14is connected to a sixth bus that is a Source Force bus. A seventh bus output of the DUT circuitry14is connected to a seventh bus that is a Source Sense bus. A second output of the tester20is connected to the Gate Force bus. A third output of the tester20is connected to the Drain Force bus. A fourth output of the tester20is connected to the Drain Leakage bus and a fifth output of the tester20is connected to the Source Force bus. A first input of the tester20is connected to the Gate Sense bus. A second input of the tester20is connected to the Drain Sense bus, and a third input of the tester20is connected to the Source Sense bus.

Test system10further has a second DUT circuitry16. The DUT circuitry16also has outputs that are respectively connected to each of the plurality of buses. A first bus output of the DUT circuitry16is connected to the Gate Force bus. A second bus output of the DUT circuitry16is connected to the Gate Sense bus. A third bus output of the DUT circuitry16is connected to the Drain Force bus. A fourth bus output of the DUT circuitry16is connected to the Drain Sense bus. A fifth bus output of the DUT circuitry16is connected to the Drain Leakage bus. A sixth bus output of the DUT circuitry16is connected to the Source Force bus. A seventh bus output of the DUT circuitry16is connected to a seventh bus that is a Source Sense bus. Test system10further has any predetermined number of additional DUT circuitry. In the illustrated form a last or Nth DUT circuitry16has outputs that are respectively connected to each of the plurality of buses. A first bus output of the Nth DUT circuitry18is connected to the Gate Force bus. A second bus output of the Nth DUT circuitry18is connected to the Gate Sense bus. A third bus output of the Nth DUT circuitry18is connected to the Drain Force bus. A fourth bus output of the Nth DUT circuitry18is connected to the Drain Sense bus. A fifth bus output of the Nth DUT circuitry18is connected to the Drain Leakage bus. A sixth bus output of the Nth DUT circuitry18is connected to the Source Force bus. A seventh bus output of the Nth DUT circuitry18is connected to a seventh bus that is a Source Sense bus.

In the illustrated form each of the DUT circuitry14, the DUT circuitry16and the Nth DUT circuitry18includes a semiconductor device that is to be tested by bias conditions applied from the tester20. In theFIG. 1form an MOS (metal oxide semiconductor) transistor is used as the device under test. The gate force signal, the drain force signal and the source force signal apply a predetermined bias voltage to respective gate, drain and source electrodes of the MOS transistor under test in each of the DUT circuitry14, the DUT circuitry16and the Nth DUT circuitry18. The tester20functions to select one of the DUT circuitry14, the DUT circuitry16and the Nth DUT circuitry18at a time to test for functionality by use of the DUT selection signals. Each DUT selection signal may be either a multiple-bit signal or a single-bit signal. In one form each DUT selection signal is a single-bit address and decoder12decodes the single-bit address to enable one of the select signals. The tester20is an analog tester and functions to receive the gate voltage via the Gate Sense bus, the drain voltage via the Drain Sense bus and the source voltage via the Source Sense bus. In addition, the tester20may sense the current flowing to/from the Drain Force bus and the Source Force bus. These voltages and currents associated with a predetermined MOS transistor can enable the tester20to determine various operating parameters of the MOS transistor such as the drive current, threshold voltage, sub-threshold slope and leakage current. Details associated with how to minimize the parasitic drain current leakage under test of the MOS transistor are provided inFIG. 2below. In the illustrated form ofFIG. 1it will be understood below that a large number of DUTs may be efficiently implemented and that various types of devices may be tested. In one form, a portion of the test system10is implemented on a semiconductor wafer in the regions between semiconductor die (integrated circuit die). In such a form the tester20is implemented external to the semiconductor wafer and the DUTs that are being tested are positioned close on the semiconductor wafer to semiconductor die functional circuitry of the same type to get an understanding of the device characteristics within the semiconductor die. In this form the DUTs are within a wafer scribe region which is the region separating semiconductor die on a wafer and which typically get cut when the semiconductor die are singulated. However, it should be understood that various locations of the tester and the DUT circuitry may be implemented. For example, the DUT circuitry14-16may be implemented within a semiconductor die rather than within a wafer scribe region.

Illustrated inFIG. 2is further detail of one form of the DUT circuitry14and the DUT circuitry16of the test system10ofFIG. 1. In the illustrated form the DUT of the DUT circuitry14is an MOS transistor30and the DUT of the DUT circuitry16is an MOS transistor50. In the illustrated form the MOS transistors30and50are both N-channel transistors. The Gate Force, Gate Sense, Drain Force, Drain Leak, Drain Sense, Source Force and Source Sense buses ofFIG. 1are similarly labeled inFIG. 2. The Select1signal ofFIG. 1is represented as signal S1and is complemented to form signal S1B by an inverter32. Similarly, the Select2signal ofFIG. 1is represented as signal S2and is complemented to form signal S2B by an inverter52.

A switching device34has a first terminal connected to the Gate Force bus and a second terminal connected to a gate of MOS transistor30. In the illustrated form the switching devices that are illustrated herein are implemented as CMOS (complementary metal oxide semiconductor) switches or transmission gates and are conventional switching devices. An NMOS or first control terminal of switching device34is connected to signal S1, and a PMOS or second control terminal of switching device34is connected to signal S1B. Similarly, a switching device36has a first terminal connected to the Gate Sense bus and a second terminal connected to the gate of MOS transistor30. An NMOS or first control terminal of switching device36is connected to signal S1, and a PMOS or second control terminal of switching device36is connected to signal S1B. A switching device38has a first terminal connected to the Drain Force bus and a second terminal connected to a first terminal of a switching device40. An NMOS or first control terminal of switching device38is connected to signal S1, and a PMOS or second control terminal of switching device38is connected to signal S1B. A switching device42has a first terminal connected to the Drain Leak bus and a second terminal connected to the second terminal of switching device38. An NMOS or first control terminal of switching device42is connected to signal S1B, and a PMOS or second control terminal of switching device42is connected to signal S1. A second terminal of switching device40is connected to the drain of MOS transistor30at a node72. An NMOS or first control terminal of switching device40is connected to signal S1, and a PMOS or second control terminal of switching device40is connected to signal S1B. A switching device44has a first terminal connected to the Drain Sense bus and a second terminal connected to node72. An NMOS or first control terminal of switching device44is connected to signal S1, and a PMOS or second control terminal of switching device44is connected to signal S1B. A switching device46has a first terminal connected to the Source Sense bus and a second terminal connected to the source of MOS transistor30at a node74. An NMOS or first control terminal of switching device46is connected to signal S1, and a PMOS or second control terminal of switching device46is connected to signal S1B. The Source Force bus is connected to node74.

A switching device54has a first terminal connected to the Gate Force bus and a second terminal connected to a gate of MOS transistor50. An NMOS or first control terminal of switching device54is connected to signal S2, and a PMOS or second control terminal of switching device54is connected to signal S2B. Similarly, a switching device56has a first terminal connected to the Gate Sense bus and a second terminal connected to the gate of MOS transistor50. An NMOS or first control terminal of switching device56is connected to signal S2, and a PMOS or second control terminal of switching device56is connected to signal S2B. A switching device58has a first terminal connected to the Drain Force bus and a second terminal connected to a first terminal of a switching device60. An NMOS or first control terminal of switching device58is connected to signal S2, and a PMOS or second control terminal of switching device58is connected to signal S2B. A switching device62has a first terminal connected to the Drain Leak bus and a second terminal connected to the second terminal of switching device58. An NMOS or first control terminal of switching device62is connected to signal S2B, and a PMOS or second control terminal of switching device62is connected to signal S2. A second terminal of switching device60is connected to the drain of MOS transistor50at a node78. An NMOS or first control terminal of switching device60is connected to signal S2, and a PMOS or second control terminal of switching device60is connected to signal S2B. A switching device64has a first terminal connected to the Drain Sense bus and a second terminal connected to node78. An NMOS or first control terminal of switching device64is connected to signal S2, and a PMOS or second control terminal of switching device64is connected to signal S2B. A switching device66has a first terminal connected to the Source Sense bus and a second terminal connected to the source of MOS transistor50at a node80. An NMOS or first control terminal of switching device66is connected to signal S2, and a PMOS or second control terminal of switching device66is connected to signal S2B. The Source Force bus is connected to node80.

In operation, assume that the Select1signal is asserted and that the Select2signal is not asserted. When Select1signal is asserted, a selected mode of operation exists for the DUT MOS transistor30. Similarly, a non-selected mode of operation exists for the DUT MOS transistor50. As a result the signal S1is at a logic high level and signal S1B is at a logic low level. Similarly, signal S2is at a logic low level and signal S2B is at a logic high level. Under these biasing conditions, switching devices34,36,38,40,44and46are conductive within the DUT circuitry14and only switching device42is nonconductive. Within the DUT circuitry16, switching devices54,56,58,60,64and66are not conductive. Switching device62is conductive. Therefore as a result of these switching configurations, none of the Gate Force bus, the Gate Sense bus, the Drain Force bus, the Drain Sense bus or the Source Sense bus is connected to the DUT MOS transistor50. However the Drain Leak bus is connected to the second terminal of switching device58through switching device62. The purpose of the Drain Leak bus is to place a voltage potential on the second terminal of switching device58that is substantially the same as or similar to the voltage potential of the Drain Force bus which the first terminal of switching device58is connected to. The drain/source bias component of the leakage current of switching device58is removed by the action of switching device62. As a result, there is significantly less leakage current that is conducted by switching device58. Additionally, switching devices60and64are nonconductive. While switching device60will also have a nominal amount of leakage current, that leakage current detectable on the Drain Force bus is limited by the leakage of switching device58.

Within the DUT circuitry14each of the Gate Force bus, the Gate Sense bus, the Drain Force bus, the Drain Sense bus and the Source Sense bus are electrically connected to the DUT MOS transistor30via switching devices34,36,38,44and46, respectively. The Source Force bus is hard-wire connected to the source of the DUT MOS transistor30. Switching device42is nonconductive and thus the Drain Leak bus is not connected to the drain of the DUT MOS transistor30. In the selected mode of operation a predetermined bias is applied to the DUT MOS transistor30by the tester20which also measures the voltages and currents of each bus. If the predetermined gate bias is zero volts and a higher voltage is applied to the drain, any drain current that is measured is considered to be a leakage current for that DUT MOS transistor. The cumulative parasitic leakage current on the Drain Force bus determines the minimum leakage current for the selected DUT MOS transistor30that can measured. As stated above, the addition of switching device62significantly lowers the parasitic current leakage of switching device58.

Thus the leakage current that can be measured for MOS transistor30is significantly lower than what would otherwise have been possible to measure if switching device62were not present. Another advantage that the use of switching device62provides is that a larger number of additional DUT circuitry can be directly connected to transistor test buses. Since the leakage current per DUT drops, more DUTs can be added to the test system while not exceeding the original amount of leakage current that would have been present without switching device62.

Illustrated inFIG. 3is another form of DUT circuitry for use in test system10. For convenience of illustration, elements inFIG. 3that are common with the elements ofFIG. 1are assigned the same reference number. Inverter32receives signal S1and provides signal S1B. A DUT circuitry91has a diode90to be tested and a DUT circuitry93has a diode92to be tested. In the illustrated form switching devices38,40and42are connected as inFIG. 1with several exceptions. A second terminal of switching device40is connected to node72and to a cathode of diode90. The Drain Force bus, Drain Leak bus, Drain Sense bus, Source Force bus and Source Sense bus are replaced with a Cathode Force bus, a Cathode Leak bus, a Cathode Sense bus, an Anode Force bus and an Anode Sense bus. A first terminal switching device38is connected to the Cathode Force bus. A first terminal of switching device42is connected to the Cathode Leak bus. A first terminal of switching device44is connected to the Cathode Sense bus. A first terminal of switching device46is connected to the Anode Sense bus and the second terminal thereof is connected to node74and to the Anode Force bus. An anode of diode90is connected to node74. Further, with the use of diode90rather than MOS transistor30, switching devices34and36and the Gate Force and Gate Sense buses are removed.

Within the DUT circuitry93, diode92has a cathode that is connected to node78and has an anode connected to node80. Inverter52receives signal S2and provides signal S2B. A first terminal of switching device58is connected to the Cathode Force bus. A first terminal of switching device64is connected to the Cathode Sense bus. A first terminal of switching device62is connected to the Cathode Leak bus. A first terminal of switching device66is connected to the Anode Sense bus, and a second terminal of switching device66is connected to node80and to the Anode Force bus. A second terminal of switching device58is connected to a first terminal of switching device60. A second terminal of switching device62is also connected to the first terminal of switching device60. A second terminal of switching device60is connected to node78and to the cathode of diode92. A second terminal of switching device64is connected to node78. Each of switching devices38,40,42,44,46,58,60,62,64and66are CMOS transmission gates having complementary control inputs (i.e. first and second control inputs or N-conductivity and P-conductivity control inputs) for respectively receiving the same complementary select control signals as each did inFIG. 2. The N-conductivity control input of all of the switching devices except switching device42and switching device62receives control signal S1. The N-conductivity control input of switching device42and switching device62receives control signal S1B.

In operation assume that the Select1signal is asserted and that the Select2signal is not asserted. When Select1signal is asserted, a selected mode of operation exists for the diode90. Similarly, a non-selected mode of operation exists for the diode92. As a result the signal S1is at a logic high level and signal S1B is at a logic low level. Similarly, signal S2is at a logic low level and signal S2B is at a logic high level. Under these biasing conditions, switching devices38,40,44and46are conductive within the DUT circuitry91and only switching device42is nonconductive. Within the DUT circuitry93, switching devices58,60,64and66are not conductive. Switching device62is conductive. Therefore as a result of these switching configurations none of the Cathode Force bus, the Cathode Sense bus, the Anode Sense bus or the Cathode Leak bus is connected to the diode92. However the Cathode Leak bus is connected to the second terminal of switching device58. The purpose of the Cathode Leak bus is to place a voltage potential on the second terminal of switching device58that is substantially the same as or similar to the voltage potential of the Cathode Force bus which the first terminal of switching device58is connected to. The drain/source bias component of the leakage current of switching device58is removed by the action of switching device62. As a result, there is significantly less leakage current that is conducted by switching device58. Additionally, switching devices60and64are nonconductive. While switching device60will also have a nominal amount of leakage current, that leakage current detectable on the Cathode Force bus is limited by the leakage of switching device58.

Within the DUT circuitry91each of the Cathode Force bus, the Cathode Sense bus and the Anode Sense bus are electrically connected to the diode90via switching devices38,44and46, respectively. Switching device42is nonconductive and thus the Cathode Leak bus is not connected to the cathode of the diode90. In the selected mode of operation a predetermined bias is applied to the diode90by the tester20which also measures the voltages and currents of each bus. As stated above, the addition of switching device62significantly lowers the parasitic current leakage of switching device58. Thus the leakage current is significantly lower than what would otherwise have been possible to measure if switching device62were not present. Another advantage that the use of switching device62provides is that a larger number of additional DUT circuitry can be directly connected to diode test buses. Since the leakage current per DUT drops, more DUTs can be added to the test system while not exceeding the original amount of leakage current that would have been present without switching device62.

By now it should be appreciated that there has been provided a biasing method to reduce leakage currents associated with switching devices in a test circuit. By minimizing leakage current using switching devices, such as pass gates or transmission gates, any of a large number of DUTs can be selected for testing while electrically isolating the other DUTs. The leakage current compensation technique that is used on those DUTs that are not selected prevents a current drain from the test sensing buses, such as those illustrated in each ofFIGS. 1-3. Therefore, the currents which are measured by a tester are more accurate. The minimization of leakage current in non-selected DUTs enables more accurate measurement of all DC (direct current) parameters of a large number of DUTs connected to the same buses. Numerous embodiments described herein may be used and the selection of which embodiment may depend upon processing requirements and desired semiconductor device specifications.

In one form there is herein provided a test circuit having a device under test (DUT) having a terminal for receiving a test signal. A first switching device has an output terminal for use in coupling the test signal to the terminal of the DUT when the DUT is being tested. The first switching device has high impedance when the DUT is not being tested. A second switching device has high impedance when the DUT is being tested and couples a bias control signal to the output terminal of the first switching device when the DUT is not being tested. The bias control signal substantially tracks the test signal. In another form the test circuit further includes a third switching device for coupling the test signal from the first switching device to the terminal of the DUT when the DUT is being tested and for being high impedance when the DUT is not being tested. In one form the first switching device is a first transmission gate having an input terminal for receiving the test signal and control terminal for receiving a select signal that indicates if the DUT has been selected for being tested. In another form the DUT is a transistor having a drain as the terminal, a source coupled to ground, and a gate for receiving a gate force signal when the DUT is being tested. In another form the test circuit includes a second transmission gate having a signal input terminal for receiving the gate force signal, a signal output coupled to the gate, and a control terminal coupled to the control terminal of the first transmission gate. In yet another form the test circuit has a third transmission gate having a signal input terminal coupled to the drain, an output for providing a drain sense signal, and a control terminal coupled to the control terminal of the first transmission gate. In one form the second switching device is a fourth transmission gate having a signal input terminal for receiving the bias control signal, an output coupled to the output of the first switching device, and a control terminal for receiving a signal complementary to the select signal. In another form the fourth transmission gate has a complementary control terminal for receiving the select signal. In another form the DUT is a diode. In yet another form the test signal is a first DC bias voltage of a first magnitude and the bias control signal is a second DC bias voltage of substantially the first magnitude.

In another form there is provided a method of testing a first device under test (DUT) having a terminal and a second DUT having a terminal. A test signal is applied to a first terminal of a first switching device and a first terminal of a second switching device. The test signal is applied to the terminal of the first DUT from a second terminal of the first switching device when the first DUT is being tested. The test signal is applied to the terminal of the second DUT from a second terminal of the second switching device when the second DUT is being tested. A leakage control signal is applied that substantially tracks the test signal to the second terminal of the first switching device when the second DUT is being tested. The leakage control signal is applied to the second terminal of the second switching device when the first DUT is being tested. In another form the bias control signal is blocked from the terminal of the first DUT when the second DUT is being tested. The bias control signal is blocked from the terminal of the second DUT when the first DUT is being tested. In another form the first DUT is a first transistor and the second DUT is a second transistor. A disabling bias is applied to a gate of the first transistor when the first transistor is being tested. The disabling bias is applied to a gate of the second transistor when the second transistor is being tested. A voltage present on the terminal of the first DUT is measured when the first DUT is being tested. A voltage on the terminal of the second DUT is measured when the second DUT is being tested. In one form applying a test signal to a first terminal of a first switching device is implemented by the first switching device having a transmission gate. In another form the terminal of the first DUT that the test signal is applied to is a cathode of a diode. In another form applying a leakage control signal that substantially tracks the test signal to the second terminal of the first switch when the second DUT is being tested is performed by the leakage control signal having a voltage level higher than that of the test signal.

In yet another form there is provided a test circuit having a first device under test (DUT) having a first terminal coupled to a reference terminal and having a second terminal. A first switching device has a control input for receiving a first select signal, a signal input for receiving a test signal, and a signal output coupled to the second terminal of the first DUT. A second switching device has a control input for receiving a complement of the first select signal, a signal input for receiving a leakage control signal, and a signal output coupled to the output terminal of the first switching device. A second DUT has a first terminal coupled to the reference terminal and a second terminal. A third switching device has a control input for receiving a second select signal, a signal input for receiving the test signal, and a signal output coupled to the second terminal of the second DUT. A fourth switching device has a control input for receiving a complement of the second select signal, a signal input for receiving the leakage control signal, and a signal output coupled to the output terminal of the third switching device. In another form there is provided a fifth switching device coupled between the first switching device and the terminal of the first DUT. The fifth switching device has a signal input coupled to the output terminal of the first switching device, a signal output coupled to the terminal of the first DUT, and a control input for receiving the first select signal. A sixth switching device is coupled between the third switching device and the terminal of the second DUT. The sixth switching device has a signal input coupled to the output terminal of the third switching device, a signal output coupled to the terminal of the second DUT, and a control input for receiving the second select signal. In one form the first, second, third, fourth, fifth, and sixth switching devices each have a complementary control input. The complementary control inputs of the first and fifth switching devices are for receiving the complement of the first select signal. The complementary control inputs of the third and sixth switching devices are for receiving the complement of the second select signal. The complementary control input of the second switching device is for receiving the first select signal, and the complementary control input of the fourth switching device is for receiving the second select signal.

Although the invention has been described with respect to specific conductivity types or polarity of potentials, skilled artisans appreciated that conductivity types and polarities of potentials may be reversed. Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under”, “above”, “below” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, embodiments that are discussed apply to both N-channel transistors and P-channel transistors. While N-channel transistor embodiments are illustrated, it should be understood that modifications regarding the logic value of control signals may be readily made to account for a change in conductivity when P-channel transistor embodiments are used. Various types of devices under test may be used in addition to transistors and diodes. For example, resistors, capacitors, inductors, thyristors and other semiconductor devices may be used as a DUT. When transistors are used, each transistor may represent a transistor in an addressable array in one form.

Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.