Abstract:
A method and test circuit provide measurements to aid in the understanding of time-varying threshold voltage changes such as negative bias temperature instability and positive bias temperature instability. In order to provide accurate measurements during an early stage in the threshold variation, a current generating circuit is integrated on a substrate with the device under test, which may be a device selected from among an array of devices. The current generating circuit may be a current mirror that responds to an externally-supplied current provided by a test system. A voltage source circuit may be included to hold the drain-source voltage of the transistor constant, although not required. A stress is applied prior to the measurement phase, which may include a controllable relaxation period after the stress is removed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to device characterization methods and circuits, and more particularly to array-based techniques for measuring early threshold voltage recovery. 
     2. Description of Related Art 
     As geometry and power supply voltages in very large-scale integrated circuits (VLSI) such as semiconductor memories and microprocessors are decreased, the effect of threshold voltage variation has become increasingly significant. Not only do process variation changes in threshold voltage cause variation from device-to-device, but effects such as negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) cause changes in performance that are time and stress dependent. The mechanisms behind NBTI and PBTI are not fully understood, and measurements of their effects have been limited by their time-dependent nature. 
     NBTI effects are seen when a negative gate voltage stress is applied to a P-channel metal-oxide semiconductor (MOS) transistor, and the effects diminish rapidly during the recovery time immediately following removal of the stress. Similarly, PBTI effects are seen in N-channel MOS devices. Therefore, in order to properly characterize NBTI/PBTI effects and gain insight thereby into the mechanisms causing NBTI/PBTI, it is desirable to measure threshold voltage not only during the application of the stress and after removal of the stress, but to characterize the threshold voltage variation during the time period between removal of the stress and recovery of the initial (non-stressed) threshold voltage. 
     Present measurement techniques provide threshold voltage observation in the range of 100 microseconds and later by measuring the drain current of a transistor having fixed drain and source voltages and responding to a step voltage at the gate of the transistor that transitions from the stressed condition (negative gate voltage) to an unstressed condition. The NBTI/PBTI effects are masked during the early portion of such measurements by the delays in both the operation of the transistor, i.e., delays due to the transition time of the transistor, and the test instrumentation, i.e., the delays inherent in making a current measurement. It would be desirable to eliminate as many of the measurement delays as possible. Further, current-based measurement of threshold voltage relies on a model of the drain current versus gate voltage in order to determine the actual change in threshold voltage due corresponding to the drain current changes. As a result, drain current-based NBTI/PBTI measurements are typically not reflective of the true dynamic operation of logic circuits and the transient nature of the effect of NBTI/PBTI on logic circuits. Finally, drain current-based measurements typically operate the drain-source terminals near their full on-state current level, which makes it difficult to simultaneously test a large number of devices in an array due to the high current requirement when multiple devices are turned on. 
     Therefore, it would be desirable to provide threshold voltage characterization that measures early effects of threshold voltage change due to NBTI and PBTI. It would further be desirable to provide such threshold voltage characterization that measures the NBTI/PBTI effect under transistor terminal conditions reflecting actual operating conditions in a logic circuit. It would further be desirable to perform such measurements in an array environment, so that multiple measurements can be performed across a die. 
     BRIEF SUMMARY OF THE INVENTION 
     Early effects of threshold voltage change due to NBTI/PBTI, under conditions reflecting actual operating conditions of devices in a logic circuit, are measured via a characterization circuit and a test methodology. The characterization circuit may be included within a characterization array. 
     A current-generating circuit integrated on a substrate with a device under test is is enabled to provide a constant drain-source current to the device under test. The gate voltage is set to a predetermined measurement value, and a voltage source may also be included to maintain the drain-source voltage constant. A time varying source voltage waveform is measured and a time varying threshold voltage characteristic is determined from the source voltage waveform. 
     To measure NBTI/PBTI effects, a stress gate voltage is applied to the gate of the device under test. Drain and source voltage may also be pre-set to alternate values during the stress period. After the stress voltage(s) is/are removed, a measurement gate voltage is applied and the constant drain-source current applied while the source voltage waveform is measured. 
     The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of the invention when read in conjunction with the accompanying Figures, wherein like reference numerals indicate like components, and: 
         FIG. 1  is a schematic diagram of a test circuit according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a characterization array in accordance with an embodiment of the present invention. 
         FIG. 3  is a pictorial diagram of a wafer test system in which methods in accordance with an embodiment of the present invention are performed. 
         FIG. 4  is a flow chart of a method in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to test circuits and methodologies for measuring time-variant effects on threshold voltage due to stress. In particular, the present invention provides information on the early stages of recovery from NBTI and PBTI in the sub-microsecond range, so that the causes and effects of NBTI and PBTI can be studied in further detail. The circuits and methods of the present invention can also be used to study other time-variant effects on threshold voltage. The method may be a computer-performed method embodied in a computer program having program instructions for controlling a test system and characterization array to carry out the method. The present invention overcomes difficulties in measuring the early portions of threshold voltage recovery by including a current-generating circuit on the same substrate as the device under test. Generally, the current source and device under test will be integrated on the same die. By co-locating the current source used to set the drain-source current of the device under test during threshold voltage measurement with the device under test itself, parasitic impedances that would otherwise affect the response of the current source to changes in the device under test and initial application of current are overcome. Therefore, the present invention provides for threshold voltage waveform information that is accurate much earlier than data available from previous test circuits and measurement methodologies. 
     The present invention measures the source voltage of a device under test while maintaining the gate of the device under test at a constant voltage and providing a constant current from the drain terminal to the source terminal of the device under test. U.S. Published Patent Application US20080030220-A1, filed on Aug. 3, 2006 having at least one common inventor and assigned to the same Assignee, discloses the use of source voltage measurements to study threshold voltage variation and is incorporated herein by reference. The above-incorporated U.S. patent application provides exact threshold voltage values for studying device-to-device variation and holds the drain-source voltage constant in order to fully characterize the device under test. In the present invention, it is not necessary to maintain the drain-source voltage constant, and since the measurement is of a dynamic waveform, it is also not necessary to compute the actual threshold voltage to study the dynamic variation of threshold voltage, as the source voltage and threshold voltage differ only by a constant, as explained in detail in the above-incorporated U.S. patent application. However, the drain-source voltage can be maintained at a constant level and the actual threshold voltage computed from an offset determined by fully characterizing at least one device under test, in order to provide further information on static threshold voltage values. The techniques of the present invention are particularly useful when applied in combination with the techniques disclosed in the above-incorporated U.S. patent application, so that both the setting of input conditions to and output measurements from the device under test are isolated from off-chip test equipment, thus isolating the device under test from the influences of probe and line impedances that introduce delays and/or error in the measurements. 
     Referring now to  FIG. 1 , a test integrated circuit  10  in accordance with an embodiment of the present invention is shown. Test integrated circuit  10  includes a device under test P DUT  for which a NBTI recovery measurement waveform is generated. An N-channel device can be alternatively used to study PBTI. In general, the technique employed is the same: after initial stress conditions have been imposed and then ended, the dynamic change in threshold voltage of device under test P DUT  is observed while device under test P DUT  is operated according to a fixed gate voltage set via a test pad V GATE  and a fixed (constant) drain-source current generated locally within test integrated circuit  10 . To generate the constant current, which is supplied to the source terminal of device under test P DUT  in the exemplary embodiment, but may be alternative supplied from the drain terminal in other circuit configurations, a current mirror M 1  is included within test integrated circuit  10 . Current mirror M 1  is controlled by an externally-supplied current at test pad I REF , which is provided for connection to an external current source. While current control is illustrated and current mirror M 1  used for that purpose, it is understood that alternative techniques for providing a local current source may alternatively be employed within test integrated circuit, including voltage controlled current sources or fixed current sources. Including of current mirror M 1 , or another local current-generating device, dramatically improves the response time of the test circuit, as the internal impedances of the signal paths and shunt capacitances are much smaller within test integrated circuit  10  than along the external tester connection paths, which include probes, test pads and the like. Therefore, when measurement is initiated after the stress is removed, current mirror M 1  will be stably providing a constant current through device under test P DUT  much earlier than would otherwise be possible. 
     The source terminal of device under test P DUT  is provided to a source follower circuit formed by transistor N 2  and current source I 12 . Transistor N 2  is generally a thick oxide device having a long channel and operated in the saturation region. Current source I 12  fixes the channel current I DS  through transistor N 2 . During measurement, since the voltage at the gate of device under test P DUT  is fixed at a constant voltage supplied through test pad V GATE  and the drain-source current of device under test P DUT  is held constant, dynamic changes in the threshold voltage of device under test P DUT  appear directly as an opposite change in source voltage of device under test P DUT  and therefore as an opposite change in the voltage provided at test pad V SOURCE . A threshold voltage waveform can then be computed by inverting the waveform captured from test pad V SOURCE  by a test system and may be adjusted for offset as described in the above-incorporated U.S. patent application to obtain an absolute threshold voltage waveform, if needed. 
     While not required to provide an early indication of threshold voltage change, the drain-source voltage of device under test P DUT  can be further controlled to maintain the drain-source voltage of device under test P DUT  at a constant level. Amplifier A 1  provides a voltage source that maintains the drain-source voltage of device under test P DUT  constant by offsetting the source voltage at the gate of a transistor N 1  by a voltage determined by the magnitude of current source I 10  and the channel resistance of transistor N 1 . Since the drain terminal of device under test P DUT  is connected to the inverting input and the output of amplifier A 1 , amplifier A 1  forms a buffer that applies a voltage equal to the source voltage of device under test P DUT  plus the offset provided by transistor N 1  and current source I 10 , maintaining the drain-source voltage of device under test P DUT  equal to the offset. 
     Referring now to  FIG. 2 , a characterization array  20  in accordance with an embodiment of the present invention is shown. Characterization array  20  is a test integrated circuit integrated on a die, a wafer kerf or other integrated circuit location that may be experimental only, or occupy one or more die or kerf locations in a production wafer. An array of transistors including device under test DUT is operated in a controlled manner via signals provided by scan latches  22 . Although the exemplary embodiment uses scan latches  22  to apply the control signals, it is understood that registers controlled via a control interface or other suitable circuit may be provided to control the operation of characterization array  20 . Further, it is understood that although the exemplary embodiment supplies signals to external equipment via pads VGP, I REF  and V SOURCE , one or more of the external devices used to operate and evaluate device under test DUT may be integrated within characterization array  20 . For example, any or all of voltage source V G , current source I 21  and a voltage measurement circuit for measuring the voltage at pad VSP can be integrated on a wafer including characterization array  20 . Stress may be manipulated by adjusting the voltage applied at test pad VGP and optionally by other circuitry added to manipulate the drain and/or source voltages of transistors within the array. 
     Signals provided from scan latches  22  select a unique row and column associated with one of the transistors, e.g., device under test DUT, illustrated as an N-channel FET. For N-channel FETs, PBTI is the effect that is studied, but an NBTI measurement circuit can be similarly constructed for NBTI measurements in the manner illustrated in  FIG. 1 . Device under test DUT may be of either P-type or N-type. The selection of a row is made by a logical “1” applied to the gate of one of current steering transistors NI 1 -NI 4  and simultaneously to a gate of a corresponding one of source voltage sense transistors NS 1 -NS 4 . Scan latches  22  are programmed such that only one row is selected at a time, i.e., all gates of transistors NI 1 -NI 4  and NS 1 -NS 4  are set to logical “0” other than the gates corresponding to the selected row. The selection of a column is made by enabling a buffer, e.g., buffer  24  that applies a reference gate voltage provided at pad VGP to the gates of all of the transistors in a column of the transistor array. A corresponding buffer  23  is also enabled and applies the output of amplifier A 1  to the drain of each transistor in the selected column. The gate of a corresponding drain voltage sense transistor ND 1 -ND 4  for the selected column is also set to a logic “1”, and provides a sense path for sensing the drain voltage of a column at the inverting input of amplifier A 1 . Scan latches  22  are programmed such that only one column is selected at a time, i.e., all buffer enable inputs and drain voltage sense transistor ND 1 -ND 4  gates are set to logical “0” other the enable inputs of the buffers corresponding to the selected column and the gate of the corresponding drain voltage sense transistor ND 1 -ND 4 . 
     Characterization array includes a current mirror M 10  having a function similar to that described above with respect current source M 1  to  FIG. 1 , which forces the drain-source current of the selected device under test DUT to a constant level after stress is removed. The source follower circuit comprising transistor N 2  and current source I 22  having a function similar to that described above with reference to current source I 12  of  FIG. 1  is also included within characterization array  20 , and optionally the voltage source provided by amplifier A 1  may be included to force the drain-source voltage (V DS ) to be a constant value for each selected transistor in the array. For example, when device under test DUT is selected by enabling buffers  23  and  24  and transistors ND 4 , NI 2  and NS 2 , transistor ND 4  applies the drain voltage of device under test DUT to the inverting input of amplifier A 1 . Simultaneously, transistor NS 2  applies the source voltage of transistor DUT to the gate of source-follower transistor P 10 , which controls the voltage at the non-inverting input of amplifier A 1 . The feedback loop acts to hold the drain-source voltage of transistor DUT constant by tracking any changes in the source voltage sensed from the selected row and adjusting the drain voltage supplied to the transistors in the column by an equal amount. Only one of the transistors in the array is conducting current at any time. Current provided from the output of A 1  is directed through buffer  23  through the channel of transistor DUT and through transistor NI 2  to current mirror M 10 , which is controlled by an external stable current source I 21 . Since the current output of amplifier A 1  is supplied to the drains of each transistor in a selected column, but only one selected row has a return path enabled via one of transistors NI 1 -NI 4 , only one device is selected for characterization for each valid combination of row and column selection signals provided from scan latches  22 . Alternatively, if the drain-source voltage control voltage source formed by amplifier A 1  is omitted, then the inputs to buffers  23  can be connected to a constant voltage source for supplying the drain voltage to selected device under test DUT. 
     The above-described characterization array  20  thus provides a mechanism for uniquely selecting each device in the array and sensing changes in the source voltage V S  at pad V SOURCE  due to application of the current supplied by current mirror M 10  after stress is applied to test pad VGP and optionally to other terminals of device under test DUT prior to measurement. By setting different valid selection combinations in scan latches  22 , each transistor in the array is selected and waveform of values of V S  is measured and collected, for example by an external computer-controlled digital voltmeter (DVM). The threshold voltage waveform is the invert of the source voltage waveform, since changes in the threshold voltage of device under test DUT cause an opposing change in the source voltage of device under test DUT. If absolute threshold voltage values are needed, the difference between V S  and V T  need only be measured for one device, by fully characterizing the I DS  versus V GS  behavior of one of the transistors in the array, e.g. transistor DUT and then subtracting the measured offset from inverted versions of the source voltage waveforms. 
     Referring now to  FIG. 3 , a wafer test system in which a method according to an embodiment of the invention is performed, is shown. A wafer tester  30  includes a boundary scan unit  31  for providing stimulus to a die or kerf circuit  32 A on a wafer under test  32 , via a probe head  33  having electrical test connections  33 A to die  32 A. Wafer tester  30  also includes a digital voltmeter DVM, which may be part of a parametric measurement unit that also includes a programmable voltage source PVS, a programmable current source PCS, and a digital current meter DCM, that are all coupled to die  32 A via probe head  33  electrical test connections  33 A. The output of programmable voltage source is connected to pad VGP, the output of programmable current source PCS is connected to pad I REF  and the input of digital voltmeter DVM is connected to pad V SOURCE . 
     A workstation computer  38 , having a processor  36  coupled to a memory  37 , for executing program instructions from memory  37 , wherein the program instructions include program instructions for executing one or more methods in accordance with an embodiment of the present invention, is coupled to wafer tester  30 , whereby the measurements described above are performed and measurements collected and stored in memory  37  and/or other media storage such as a hard disk. A CD-ROM drive  35  provides for import of program instructions in accordance with embodiments of the present invention that are stored on media such as compact disc CD. Workstation computer  38  is also coupled to a graphical display  39  for displaying program output such as the threshold voltage waveform for devices in the characterization array provided by embodiments of the present invention. Workstation computer  38  is further coupled to input devices such as a mouse  34 B and a keyboard  34 A for receiving user input. Workstation computer may be coupled to a public network such as the Internet, or may be a private network such as the various “intra-nets” and software containing program instructions embodying methods in accordance with embodiments of the present invention may be located on remote computers or locally within workstation computer  38 . Further, workstation computer  38  may be coupled to wafer tester  30  by such a network connection. 
     While the system of  FIG. 3  depicts a configuration suitable for sequential test of a plurality of dies on a wafer, the depicted system is illustrative and not a limitation of the present invention. Probe head  33  may be a multi-die full wafer probe system, or may comprise multiple probe heads for simultaneously testing multiple wafers on a single or multiple die basis. Additionally, while boundary scan control of the characterization array is illustrated, the techniques of the present invention may also be applied to execution of test code from a processor incorporated on wafer  32  with appropriate current and voltage sources and voltage measurement circuitry provided on wafer  32 , as well. The resultant generated display or data exported from workstation computer  38  may take the form of graphical depictions of the threshold voltage waveform variation across the characterization array, or may graphical or numerical statistical distribution information that describes changes in threshold voltage over time. 
     Referring now to  FIG. 4 , a method in accordance with an embodiment of the invention is depicted in a flowchart. First, stress is applied to the device under test (step  40 ). Next, the device under test can be selected for relaxation prior to test (step  42 ). The gate voltage is forced and the internally-generated drain-source current is applied to the device under test (step  44 ). The source voltage waveform of the selected device is measured (step  46 ). Until the source voltage has been measured for all devices (decision  48 ), steps  42 - 46  are repeated, selecting a different device each repetition of step  42 . After the source voltage waveforms for all of the devices have been captured, the threshold voltage waveform may be computed, optionally correcting for the offset between the source voltage waveform measured in step  46  using an offset measured for a fully-characterized device (step  50 ). Finally, the resulting threshold voltage waveforms are displayed (step  52 ). 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.