Abstract:
A method, test circuit and test system provide measurements to accurately characterize threshold voltage changes due to negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI). Both the bias temperature instability recovery profile and/or the bias temperature shifts due to rapid repetitions of stress application can be studied. In order to provide accurate measurements when stresses are applied at intervals on the order of tens of nanoseconds while avoiding unwanted recovery, and/or to achieve recovery profile sampling resolutions in the nanosecond range, multiple delay or ring oscillator frequency measurements are made using a delay line that is formed from delay elements that have delay variation substantially caused only by NBTI or PBTI effects. Devices in the delay elements are stressed, and then the delay line/ring oscillator is operated to measure a threshold voltage change for one or more measurement periods on the order of nanoseconds.

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 delay-based techniques for characterizing bias temperature instability effects. 
     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, referred to generally as bias temperature instability (BTI) are not fully understood, and measurements of their effects have been limited by their time-dependent nature, particularly due to the fast partial recovery of observed threshold voltage shifts due to BTI after stress is removed. 
     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 the removal of the stress. Similarly, PBTI effects are seen in N-channel MOS devices, particularly in those with high-k gate dielectrics. Therefore, in order to properly characterize BTI effects, in particular to simulate aging by applying a stress and measuring a change in threshold voltage before recovery, and also to gain insight into the mechanisms causing BTI, it is desirable to measure threshold voltage not only during the application of the stress and immediately after removal of the stress, but to characterize the entire transient threshold voltage recovery evolution after stress. 
     Present BTI measurement techniques provide threshold voltage recovery observation on the order of microseconds and later. Some techniques directly measure a threshold voltage change during BTI recovery by observing voltages a terminals of one or more transistors to which a stress has been previously applied, while others use techniques such as ring oscillator measurements that measure a beat frequency between a ring oscillator having stressed devices and a ring oscillator having un-stressed devices. However, existing techniques do not provide a sufficiently high resolution with respect to the recovery time to permit the BTI recovery to be characterized in the sub-microsecond range or to permit characterization of changes in recovery during repetitive stress applications at rates on the order of microseconds or faster. Such repetitive stress application is highly desirable for characterizing the long-term aging effects of BTI. Further, some of the existing techniques fail to isolate only one type of BTI effect (NBTI or PBTI without other effects such as Hot Carrier Injection), and also may fail to eliminate other factors in the measurement process caused by the application of stress. 
     Therefore, it would be desirable to provide methods, circuits and systems for BTI characterization that measures recovery characteristics from BTI effects in the sub-microsecond region, as well as the effects of continuous stress experiments while minimizing the unwanted threshold voltage recovery when stress conditions are temporarily removed to perform each measurement. It would further be desirable to provide such BTI characterization that measures the BTI effects after repetitive applications of stress, i.e. AC stress, at repetition periods of a microsecond and faster. 
     BRIEF SUMMARY OF THE INVENTION 
     Measurement of NBTI/PBTI effects, under AC (repetitive) stress conditions simulating actual aging and/or with high resolution in the sub-microsecond range, is provided in a circuit, method of measurement and a measurement system. 
     A delay line, which may form a ring oscillator, is formed from delay elements having transistors to which a stress that induces a pure NBTI or PBTI effect is applied. The stress is removed and the delay or ring oscillator frequency is measured to determine a change in threshold voltage due to the stress. If a ring oscillator is used, the ring oscillator operation is gated, and an edge detector is used to determine an absolute delay within the resolution of a delay of a single delay element at the end of a capture period. An overflow counter may be used to extend the dynamic range of the measurement without requiring a larger number of delay elements. If a delay line is used without forming a ring oscillator, then the number of delay elements is made large enough to encompass the delay range of interest and the edge is detected from its position within the delay chain at the end of the capture period. 
     The delay element may be designed to speed up the edge of the pulse that propagates more slowly through the previously stressed device (e.g., the low state propagated through a PMOS device). The result is an increase in the resolution of a ring oscillator frequency measurement by increasing rate at which the pulse propagates through the delay line. 
     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 block diagram of a test integrated circuit according to an embodiment of the present invention. 
         FIG. 2  is a block diagram of a delay line/ring oscillator within test integrated circuit  1  of  FIG. 1  according to an embodiment of the present invention. 
         FIG. 3A  is a schematic diagram of a delay element  20 A that can be used as delay element  20  in the delay line of  FIG. 2  in accordance with an embodiment of the present invention, in order to measure NBTI effects. 
         FIG. 3B  is a schematic diagram of a delay element  20 B that can be used as delay element  20  in the delay line of  FIG. 2  in accordance with an embodiment of the present invention in order to measure PBTI effects. 
         FIG. 4A  is a signal diagram showing signals within the test integrated circuit of  FIG. 1  in accordance with an embodiment of the present invention, while measuring NBTI effects. 
         FIG. 4B  is a signal diagram showing signals within the test integrated circuit of  FIG. 1  in accordance with an embodiment of the present invention, while measuring PBTI effects. 
         FIG. 5  is a block diagram of a delay line according to another embodiment of the present invention that may be used within the test integrated circuit of  FIG. 1 . 
         FIG. 6A  is a is a schematic diagram of a delay element  30 A that can be used as delay element  30  in the delay line of  FIG. 5  in accordance with an embodiment of the present invention, in order to measure NBTI effects. 
         FIG. 6B  is a is a schematic diagram of a delay element  30 B that can be used as delay element  30  in the delay line of  FIG. 5  in accordance with an embodiment of the present invention, in order to measure PBTI effects. 
         FIG. 7  is a pictorial diagram of a wafer test system in which methods in accordance with an embodiment of the present invention are performed. 
         FIG. 8  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 a measurement of NBTI and PBTI in the nanosecond range in order to avoid unwanted recovery from affecting the measurements, and so that repetitive measurements of device threshold voltage can be made as the devices recover from either DC or AC stress conditions, in which sequential measurements can be performed on the order of a microsecond. By taking the measurements at intervals in the nanosecond range, BTI effects accumulate in the stressed devices without substantial unwanted recovery, permitting simulation of longer aging periods in a short test interval. In a quasi-continuous stress mode, stress is applied to devices and is only interrupted during measurement periods, which are performed in a very short interval. Due to the power-law nature of the threshold voltage degradation, measurements can be spaced logarithmically in time, in particular to save measurement storage space, which may be located on-die, or to reduce the bandwidth requirements of the measurement interface. The present invention also provides a technique for accurately measuring fast transient threshold voltage recovery profiles, so that even the earliest portions of BTI recovery can be studied at high resolution. In a recovery characterization mode, stress is applied for some period of time, and then removed so that the recovery can be sampled at a high rate. 
     Referring now to  FIG. 1 , a test integrated circuit  1  in accordance with an embodiment of the present invention is shown. Test integrated circuit  1  includes delay line/ring oscillators  10 A,  10 B,  10 C and  10 D. Delay line/ring oscillators  10 A and  10 B are identical and include P-type transistors that exhibit a change in threshold voltage due NBTI after a stress has been applied. However, none of the devices in delay line/ring oscillator  10 B are stressed prior to operation, so that delay line/ring oscillator  10 B provides a reference measurement. Rather than applying a stress voltage to the devices in delay line/ring oscillator  10 B, the devices are provided with their nominal operating voltages, so that the stress measurements can be referenced to a delay line operating under normal operating conditions. Similarly, delay line/ring oscillators  10 C and  10 D are identical and include N-type transistors that exhibit a change in threshold voltage to PBTI after stress has been applied. Delay line/ring oscillator  10 D is not stressed in operation and serves as a reference for delay line/ring oscillator  10 C. A stress control and voltage source circuit  13  provides stress voltages to delay line/ring oscillators  10 A and  10 C under control of an external measurement processing system  18 . The stress is removed and a gate control signal is provided from an interface  17  under control of external measurement processing system  18  to initiate a pulse and provide a predetermined window during which the pulse propagates (and re-circulates if delay line/ring oscillators  10 A- 10 D are configured as ring oscillators) and at the end of which, the position of the edge of the pulse is observed for each of delay line/ring oscillators  10 A- 10 D. 
     A local storage  19  may be provided to store delay indications from each delay line/ring oscillator  10 A- 10 D due to the rate at which the measurements are made. Typical scan chain interfaces are generally not fast enough to collect the data generated by test integrated circuit  1  without the provision of local storage  19 , so unless the test results are cached (e.g., by a FIFO memory or other storage), interface  17  will need to include a high-speed interface such as a serial link operating at a rate sufficient to transfer the full resolution of the measurements for each of delay line/ring oscillators  10 A- 10 D at the rate of repetition of the measurements. For example, if the resolution of edge detection circuits/ring oscillator counters within delay line/ring oscillators  10 A- 10 D is 1000 and the measurement rate is 1 μs per stress interval, interface  17  needs to transfer data at 4 Gb/s if local storage  19  is not provided and data is provided from four delay line/ring oscillators  10 A- 10 D. Otherwise, interface  17  may be an ordinary test interface such as a scan chain interface that reads values from local storage  19  after testing has completed. Test integrated circuit  1  is provided as an example of a particular test configuration, and should not be construed as limiting the present invention to a particular configuration. For example, NBTI-only implementations can be alternatively fabricated, PBTI-only implementations can be alternatively fabricated and reference delay/ring oscillators  10 B and  10 D are not required, in particular when NBTI and PBTI variation is being observed across a die or lot. Further, while only four delay line/ring oscillators  10 A- 10 D are shown, if an entire die (or substantial portions of the die) is dedicated to the tests and circuits of the present invention, large numbers of delay line/ring oscillators may be distributed across the die. 
     Referring now to  FIG. 2 , details of a ring oscillator circuit, in accordance with an embodiment of the invention, that may be used to implement delay lines  10 A- 10 D of  FIG. 1 , is shown. The ring oscillator circuit is formed from multiple delay elements  20 , which are shown connected to stress control and voltage sources  13 . The outputs of delay elements  20  are connected to the inputs of flip-flops  14  which capture the state of the outputs of the delay element  20  when a Capture signal is asserted a predetermined time after the Gate signal is asserted. The outputs of adjacent pairs of flip-flops  14  are connected to inputs of logical exclusive-OR gates  15  that form an edge detector. The position of the edge of the pulse within the delay line formed by the chain of delay elements  20  will be indicated by a logical “1” at the output X&lt;0:N&gt; of only one of exclusive-OR gates  15  and the rest of the outputs X&lt;0:N&gt; of exclusive-OR gates  15  will be in a logical “0” state. A latch  16  captures the outputs of exclusive-OR gates  15  along with the count value of a counter  12  which counts oscillations of a ring oscillator that is formed by providing feedback from the last one of delay elements  20  to the input of a logical-NAND gate NAND 1 . A delay circuit D 1  provides the clock input to latch  16 , ensuring that the outputs of exclusive-OR gates  15  are stable at the time of edge capture. When signal Gate is de-asserted, the output of logical-NAND gate NAND 1  is in a logical “1” state, as are each of delay elements  20 . When signal Gate is asserted, the output of logical-NAND gate NAND 1  transitions to a logical “0” state, propagating a pulse through delay elements  20  and commencing oscillation of the ring oscillator circuit. Unlike frequency-only measurement circuits, the circuit of  FIG. 2  provides a “phase” indication as well, from the edge-detecting outputs of exclusive-OR gates  15 , so that the resolution of the test circuit is limited only by the delay of the individual delay elements  20 . 
     Referring now to  FIG. 3A , a schematic diagram of a delay element  20 A suitable for use in the ring oscillator of  FIG. 2  and for measuring NBTI effects is shown. A pair of inverters I 1  and I 2 , provide a non-inverting characteristic to delay element  20 A and provide a drive level for propagating the ring oscillator signal through a transistor P 1  that was previously stressed by application of a stress voltage −V STRESS  at its gate terminal. During application of the stress, logical signal STRESS is active (logical “1”) and logical signal /STRESS is also active (logical “0”), so that transistors P 2 -P 4  are “on” and the lower power supply rail of inverters I 1 -I 2  is raised to the upper power supply rail level (V CC ). Therefore, all of the terminals of transistor P 1  are at potential V CC  except for the gate terminal, which is held at potential −V STRESS . Transistor P 1  is the only stressed device in delay element  20 A and effects on its threshold voltage are substantially only due to NBTI caused by the application of potential −V STRESS  at the gate of transistor P 1 . (A typical value for −V STRESS  is −V CC  and many levels of −V STRESS  will generally be studied in different sequences of measurement.) After the stress has been applied for a predetermined time, logical signals STRESS and /STRESS are de-asserted, providing a ground level at the lower power supply rail of inverters I 1  and I 2  and turning off transistors P 2 -P 4 . Delay stage  20 A now acts as a buffer with transistor P 1  providing an active pass-gate having a rise time that varies almost linearly with variation in the threshold voltage of transistor P 1 . Since the variation in threshold voltage is substantially only caused by the NBTI effect, the delay time through the delay line of  FIG. 2  using delay elements  20 A and for a logical “1” pulse is substantially linear with NBTI effect on threshold voltage, providing a direct measurement of the threshold voltage. 
     However, in the ring oscillator of  FIG. 2 , both states of the propagating pulse contribute to the frequency (and ultimate “phase”) of the measurement. The fall time of delay element  20 A would generally be much longer than the rise time and is relatively insensitive to threshold voltage variation in transistor P 1 . In order to reduce the fall time of delay element  20 A and therefore reduce the effect of its variation on the overall frequency/phase measurement, transistor N 1  is included as a “speed-up” device. As soon as inverter I 1  begins to transition to a logical “1” state, transistor N 1  is turned on to rapidly pull down the input of inverter I 1 , reducing the fall time of delay element  20 A. 
     Referring now to  FIG. 3B , a schematic diagram of a delay element  20 B suitable for use in the ring oscillator of  FIG. 2  and for measuring PBTI effects is shown. Delay element  20 B is similar to delay element  20 A of  FIG. 3A , and therefore only differences between them will be described below. In delay element  20 B, transistor N 10  is stressed by application of a stress voltage V STRESS  at its gate terminal. During application of the stress, transistors N 11 -N 13  are “on” and the upper power supply rail of inverters I 11 -I 12  is lowered to ground. Therefore, all of the terminals of transistor N 10  are at ground except for the gate terminal, which is held at potential V STRESS . (A typical value for V STRESS  is V CC  and many levels of V STRESS  will generally be studied in different sequences of measurement.) When logical signals STRESS and /STRESS are de-asserted, V CC  is provided at the upper power supply rail of inverters I 11  and I 12  and transistors N 11 -N 13  are turned off. Delay stage  20 B acts as a buffer with transistor N 10  providing an active pass-gate having a fall time that varies almost linearly with variation in the threshold voltage of transistor PN 10 . The delay time through the delay line of  FIG. 2  using delay element  20 B and for a logical “0” pulse is substantially linear with PBTI effect on threshold voltage, providing a direct measurement of the threshold voltage. As in delay element  20 A of  FIG. 3A , both states of the propagating pulse contribute to the frequency (and ultimate “phase”) of the measurement in the circuit of  FIG. 2 . The rise time of delay element  20 B would generally be much longer than the fall time and is relatively insensitive to threshold voltage variation in transistor N 10 . In order to reduce the rise time of delay element  20 B and therefore reduce the effect of its variation on the overall frequency/phase measurement, transistor P 10  is included as a “speed-up” device. As soon as inverter I 11  begins to transition to a logical “0” state, transistor P 10  is turned on to rapidly pull up the input of inverter I 11 , reducing the fall time of delay element  20 B. 
     Referring now to  FIG. 4A , signals within the ring oscillator of  FIG. 2  using delay element  20 A of  FIG. 3A  are shown. Assertion of the logical stress control signals STRESS, /STRESS coincides with the application of stress voltage −V STRESS  to the gate of transistors P 10  in each delay element  20 . When the stress is removed, the gate control signal Gate is asserted and the ring oscillator begins to oscillate, with signals appearing on the outputs of delay elements  20  shown as d&lt;0&gt; through d&lt;N&gt;. The counter counts oscillations of signal d&lt;N&gt; and the LSB of the counter is shown as signal counter LSB. When capture signal Capture is asserted the count value and edge position are captured and stored (or transmitted to the test system). Signal Gate can be generated from signal Capture by delaying and inverting signal Capture, as can be seen from the Figure. Referring now to  FIG. 4B , signals within the ring oscillator of  FIG. 2  using delay element  20 B of  FIG. 3B  are shown.  FIG. 4B  is similar to  FIG. 4A , with the exception of the polarity of the stress voltage +V STRESS , and therefore the above description applies to  FIG. 4B , as well. After the measurement has been performed, if the measurements are being performed in quasi-continuous stress mode, then, as shown in  FIGS. 4A-4B , logical stress control signals STRESS, /STRESS are re-asserted and stress voltage −V STRESS  is reapplied after the measurement interval. If the recovery transient is being studied, then the stress is not reapplied and measurements are repeatedly taken, and may be spaced logarithmically with increasing time separation to reduce storage and bandwidth requirements. 
     Referring now to  FIG. 5 , details of a delay line circuit ring oscillator circuit, in accordance with another embodiment of the invention, that may be used to implement delay lines  10 A- 10 D of  FIG. 1 , is shown. The delay line circuit of  FIG. 5  is similar to the ring oscillator circuit of  FIG. 2 , and therefore only differences between them will be described below. In the delay line circuit of  FIG. 5 , logical-NAND gate NAND 1  and counter  12  are omitted and a pulse signal Pulse is supplied directly to the first delay element  30 . Therefore, to obtain the same measurement range, a much larger number of delay elements  30  are used. Further, since there is only a one-shot delay, delay elements  30  are slightly different in implementation than the delay elements  20  as illustrated in delay elements  20 A and  20 B of  FIG. 3A  and  FIG. 3B  above. 
     Referring now to  FIG. 6A  and to  FIG. 6B , delay elements  30 A and  30 B are illustrated, respectively. Delay element  30 A of  FIG. 6A  is used for measuring NBTI effects in transistor P 30 , but has a design similar to the PBTI measurement delay element  20 B of  FIG. 3B , in that transistors N 31 , N 32  and N 33  force the drain and source terminals of transistor P 30  to ground during the assertion of signal STRESS. A speed-up transistor is not needed, since the NTBI-insensitive state does not form part of the delay measurement in the circuit of  FIG. 5 . Inverters I 31  and I 32  buffer signal Pulse as it arrives at each delay element  30 A and have their upper power supply rail set to ground during stress application. For delay elements  30 A, pulse is a positive polarity pulse. Therefore, as mentioned above, the slow fall time of delay element  30 A does not affect the measurement. The stress voltage, shown as −2V STRESS  is increased over that supplied to delay element  20 A of  FIG. 3A  in order to obtain the same stress. (The equivalent stress voltage is actually −V STRESS −V CC , since the drain and source of transistor P 30  are held at ground instead of V CC  during stress.) Delay element  30 B of  FIG. 6B  is similarly changed with respect to the PBTI-sensitive delay element  20 B of  FIG. 3B . No speed-up device is needed, since the slow rise time of delay element  30 B does not form part of the measurement, which is initiated with a negative polarity pulse that transitions from V CC  to ground. The drain and source of transistor N 30  are held at V CC  by transistors P 31 -P 33  during stress, and the gate voltage for equivalence to delay element  20 B of  FIG. 3B  is therefore V STRESS +V CC , and is shown as 2V STRESS . Inverters I 33  and I 34  have their lower power supply rail set to V CC  during the stress application. 
     Referring now to  FIG. 7 , a test measurement system in accordance with an embodiment of the present invention is shown. A workstation computer  48  includes a processor  46  for executing program instructions forming a computer program in accordance with an embodiment of the present invention, which may be stored on a media such as compact disc CD and loaded into memory  47  by processor  48  from a CD-ROM drive  45 . A graphical display  49  is provided for displaying user interfaces for controlling measurements made by the test system of  FIG. 7  and for displaying results of the measurements in tabular and/or graphical form. Input devices such as a keyboard  44 A and a mouse  44 B are included for controlling workstation computer system  48 . Workstation computer system  48  is coupled to a wafer tester  40  having a test head  43  that is coupled by probes to a die  42 A on a wafer  42 . However, the present invention may also be practiced using packaged dies that include a test interface or other interface for controlling the test procedure and retrieving the test data. As mentioned above, wafer tester  40  may include a high-speed interface for transferring the measurement data of the present invention if local storage of the collected delay data (e.g., ring oscillator frequency and edge position for ring oscillator measurements, or delay edge position for delay-only measurements). A programmable voltage supply (PVS)  42  is included to provide the stress voltages applied to the delay elements. A scan unit  41  can be used to start and control the measurements, and to retrieve collected data when local measurement data storage is supplied on die  42 A. 
     Referring now to  FIG. 8 , a method in accordance with an embodiment of the present invention is shown in a flowchart. First, the delay line transistors are stressed (step  50 ). Next, the stress is removed (step  52 ), ring oscillator cycles are counted and the edge position of the final oscillation is captured for a predetermined capture period (step  54 ). The measurement data are stored or transmitted (step  56 ). After the last stress cycle (last capture period) is complete (decision  58 ) the collected delay indications are analyzed and displayed (step  60 ). Otherwise, if the measurement is performed in recovery characterization mode (decision  62 ), the measurement steps  54 - 56  are repeated. If the measurement is performed in a quasi-continuous stress mode (decision  62 ), the stress/measure cycles of steps  50 - 56  are repeated. 
     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.