Abstract:
A universal leakage monitoring system (ULMS) to measure a plurality of leakage macros during the development of a manufacturing process or a normal operation period. The ULMS characterizes the leakage of both n-type and p-type CMOS devices on the gate dielectric leakage, the sub-threshold leakage, and the reverse biased junction leakage, and the like. Testing is performed sequentially from the first test macro up to the last test macro using an on-chip algorithm. When the last test macro is tested, it scans the leakage data out.

Description:
BACKGROUND OF THE INVENTION 
   This invention generally relates to the manufacture of semiconductor devices, and more particularly, to a monitoring system for detecting and characterizing various classes of leakage in CMOS devices. 
   As the scaling of integrated circuit continues to progress, CMOS devices become prone to many different types of leakage problems. Current technology having a gate dielectric thinner than 10 nm, a channel length shorter than 50 nm, a junction depth thinner than 20 nm, the threshold voltage of fast devices below 200 mV are typically subject to serious leakage related problems. 
   Handling leakage problems has been profusely described in the art, as for instance, in U.S. Pat. No. 6,844,750 to Hsu et al. Therein is described a leakage monitoring apparatus that first measures die leakage and then generates digital keeper control bits to improve the performance of the dynamic circuits. More specifically, the sub-threshold leakage of CMOS devices on multiple locations of the die is simultaneously monitored. Based on the leakage information, keepers of dynamic circuits are properly sized to compensate for the leakage while maintaining the performance. The apparatus described uses a current mirror circuit to measure the sub-threshold leakage of pull down NMOS devices whose gates are connected to 150 mV. The patent is directed toward handling one specific class of circuit from being affected by sub-threshold leakage due to process variations. However, the apparatus described does not characterize other types of leakages, such as junction leakage, gate dielectric leakage. It neither addresses the problem of monitoring and quantifying the sub-threshold leakage of PMOS devices. 
   In another related patent, U.S. Pat. No. 6,844,771, issued to Chung-Hui Chen, there is described another class of leakage monitoring apparatus which first monitors the decoupling capacitor leakage voltage and selectively switches off any decoupling capacitor that displays a predetermined amount of leakage current. The apparatus detects the presence of gate dielectric leakage at a specific threshold level, i.e., when the leakage current multiplied by the gate resistance is smaller than ½ Vdd. When detected, a switch is triggered to shut off the capacitor. The proposed approach suffers from a serious problem, in that when the gate shows either a catastrophic failure or a broken gate oxide caused by pin-holes or other defects, the voltage drop exceeds a preset threshold level (e.g., ½ Vdd). As a result, the circuit fails to shut the decoupling capacitor off. Again, the cited patent is not suited for universal leakage characterization, since it makes no mention of how to measure other classes of leakage mechanisms other than capacitor leakage. 
   Gate Dielectric Leakage Characterization Circuit 
   As previously referred to, there are two classes of gate leakage: the first is known as normal leakage, while the second relates to the broken gate oxide leakage. The first class of leakage exhibits a weak temperature dependence and an activation energy value of approximately 0.4 eV. This value approximates the barrier height of the SiO 2  to Si interface. A normal gate oxide leakage current of this class, also known as thermal emission current, is described in the article by C. R. Crowell and S. M. Sze, “Current Transport in Metal-Semiconductor Bafflers”, Solid State Electron, 1966, Vol. 9, p. 1035-1048. The leakage level ranges from 1aA to 1pA. The second class is a gate oxide leakage mechanism, commonly referred to as Fowler-Nordheim tunnel, and it, likewise, displays a weak temperature dependence. The leakage level ranges from 1 to 10 p{acute over (Å)}. This second class of leakage is caused by broken gate oxide on different substrates, and is characterized by a leakage current level ranging from 0.1 to 1 m{acute over (Å)}. 
   The behavior of an n-MOS and p-MOS capacitor abnormal leakage significantly differs from one another. Therefore, it becomes necessary that they be independently monitored and characterized. In general, the broken gate leakage of a p-MOS capacitor is known to display a two to three orders of magnitude higher leakage current level than that of an n-MOS capacitor. 
   Referring now to  FIGS. 1A and 1B , a schematic diagram and a device cross-sectional view of an n-MOS capacitor leakage monitoring device is illustrated. The gate is attached to node “B1”, while the source, drain and body are connected to ground. 
   Referring to  FIG. 2A and 2B , a schematic diagram and a device cross-sectional view of a PMOS capacitor leakage monitoring device are respectively shown. The gate is attached to node “B2”, while the source/drain and body are connected to the power supply. 
   Sub-Threshold Leakage Characterization Circuit 
   The sub-threshold characteristics of a MOSFET device have strong temperature dependence, i.e., the lower the temperature, the lower the leakage current. The sub-threshold leakage current is conventionally measured at Vg=0V or at any lower bias level, typically, of the order of about 150 mV. 
   Referring to  FIGS. 3A and 3B , there are shown a schematic diagram accompanied by a device cross-section of an NMOS sub-threshold leakage monitoring device. The gate is connected to ground or biased at 150 mV, while the drain is attached to node “A1” and the source and body to ground. 
   Correspondingly,  FIGS. 4A and 4B  show a schematic diagram and a cross-sectional view of a p-MOS sub-threshold leakage monitoring device. The gate is connected to VDD or biased at (VDD-150 mV). The drain is shorted to node “A2” while the source and body are connected to VDD. 
   Junction Leakage Characterization Circuit 
   The reserve leakage current for a normal p-n junction displays a strong temperature dependence which is governed by the relationship exp(−Eg/kT), wherein Eg is the band-gap energy of the order of 1.1 eV. However, for an abnormal p-n junction, the leakage level is two to five orders of magnitudes higher than that of a normal device. Its activation energy remains below 0.1 eV. 
   Referring to  FIGS. 5A and 5B , a schematic diagram and a device cross-sectional view of an NMOS reverse p-n junction leakage monitoring device are shown. The gate is connected to node “C1”, while the source, drain and body are attached to ground. 
   With reference to  FIGS. 6A and 6B  there are shown a schematic diagram and a device cross-sectional view of a p-MOS reverse p-n junction leakage monitoring device. The body is connected to node “C1”, while the gate, source and drain are connected to VDD. 
   It is conceivable that a single p-type or n-type CMOS device or a plurality of CMOS devices can be advantageously used for monitoring. By way of example, if the leakage is proportional to the size (also referred to as the width) of the device, one may use as many devices as the area allocated thereto permits it. 
   In summary, while the prior art has addressed the problem of measuring the sub-threshold leakage and how to generate feedback controls to maintain keeper strength of dynamic circuits, there still remains a problem of how to monitor the effect of different classes of leakages, such as gate leakage, junction leakage and device leakage across a chip. 
   Furthermore, while the prior art has provided means for measuring leakage in n-type devices, it has not been able to expand this teaching to other classes of CMOS devices, regardless whether p-type or n-type. 
   Finally, and referring to monitor circuits capable of achieving the aforementioned goals, they typically require complex and bulky circuitry, each monitor device requiring its own comparator, which are difficult to integrate in a CMOS fabrication facility. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to provide an on-chip circuit capable of measuring and quantifying a plurality of classes of leakage in CMOS semiconductor devices. 
   It is another object to provide a universal circuit for characterizing leakage for both p-type and n-type CMOS devices. 
   It is a further object to characterize the gate dielectric leakage, junction leakage and sub-threshold leakage in CMOS devices. 
   It is still another object to provide a simple and small monitoring system that comprises a plurality of leakage sensors distributed across an integrated circuit chip. 
   It is yet a further object to provide a monitoring system that occupies less than 1% of the chip area, and which is preferably achieved by using only one comparator. 
   The invention provides a universal leakage monitoring system, hereinafter referred to as ULMS, to measure each leakage class during process development and/or normal operation period. The ULMS of the present invention is able to characterize CMOS devices on: (1) the gate dielectric leakage through the thin gate dielectric, (2) the sub-threshold leakage through a very short channel region even when the gate is completely turned off, (3) the reverse biased junction leakage, and the like. The ULMS further is advantageously used to qualify the fabrication process, devices and circuits by investigating, for instance, the gate leakage while using a new gate dielectric; junction leakage while using a raised source-drain structure; and sub-threshold leakage while providing ultra-low threshold voltage CMOS devices. 
   The present invention further provides a system for monitoring a plurality of classes of leakages in an integrated circuit, the circuit including: a) a plurality of monitoring macros, each of the monitoring cells detecting one class of the leakages; b) a set of measuring macros respectively measuring one class of the leakages to generate respective leakage data; c) a set of data collecting devices, collecting the leakage data: and a scan chain to scan leakage data out when the measuring is completed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and which constitute part of the specification, illustrate the presently preferred embodiments of the invention which, together with the general description given above and the detailed description of the preferred embodiments given below serve to explain the principles of the invention. 
       FIGS. 1A-1B  show respectively a schematic diagram and device cross-section of a conventional n-MOS capacitor monitor device. 
       FIGS. 2A-2B  show respectively a schematic diagram and a cross-sectional view of a prior art p-MOS capacitor leakage monitoring device. 
       FIGS. 3A-3B  show respectively a schematic diagram and a cross-sectional view of a prior art n-MOS sub-threshold leakage monitoring device. 
       FIGS. 4A-4B  show respectively a schematic diagram and a cross-sectional view of a prior art p-MOS sub-threshold leakage monitoring device. 
       FIGS. 5A-5B  show respectively a schematic diagram and a cross-sectional view of a prior art n-MOS reverse p-n junction leakage monitoring device. 
       FIGS. 6A-6B  show respectively a schematic diagram and a cross-sectional view of a prior art p-MOS reverse p-n junction leakage monitoring device. 
       FIG. 7A  shows a schematic diagram of an n-type MOS device leakage monitoring circuit according to the invention, 
       FIG. 7B  illustrates a schematic diagram of an n-type MOS current controlled current digital to analog converter (IDAC), according to the present invention. 
       FIGS. 8A-8B  respectively show a schematic diagram of a p-type MOS device leakage monitoring circuit and corresponding IDAC incorporated therein, in accordance with the invention. 
       FIG. 9  illustrates a built-in algorithm to perform a binary search to determine the presence of leakage of any source, and more particularly, to perform a binary search for sets of digital bits while probing the leakage current (Ileak). 
       FIG. 10  shows a combination test site that includes an n-type test macro and a p-type test macro. 
       FIG. 11  is a top level representation of a test system which includes a plurality of test units, as shown in  FIG. 10 . 
       FIG. 12  is a flow chart diagram of a preferred measurement technique wherein testing is performed sequentially from the first test site to the last test site using the on-chip algorithm shown in  FIG. 9 . 
   

   DETAILED DESCRIPTION 
   Hereinafter will be described the n-MOS and p-MOS leakage monitoring apparatus of the present invention. Since the circuit conFIGurations for testing n-MOS and p-MOS differ significantly from each other, they will be described independently. 
   N-MOS Leakage Measuring Apparatus 
   As previously stated, leakage monitoring devices of the class shown with reference to the circuits illustrated in  FIGS. 1 ,  3  and  5 , only one class of leakage, whether testing the gate, sub-threshold and junction leakage, respectively, can be tested at one time. The circuit is designed to test both normal and abnormal types of leakage for any classes of CMOS devices, which differ from each other by several orders of magnitude. 
   Regarding the present invention, and with reference to  FIG. 7A , the digital bits DIG&lt; 0 &gt;, DIG&lt; 1 &gt; . . . DIG&lt;n&gt; are used to size the reference current for sensing the leakage current. A binary decoding technique is, preferably, used to achieve high resolution when taking measurements. 
   The total monitoring current “Imon” is generated as follows: the monitor current level is to coincide with the highest reachable leakage level, e.g., 1 mA. This level can be downsized by way of a mirror device positioned between P 0  and P 1 . If the size of P 0  is N multiplied by the size of P 1 , (i.e, N=size P 0 /size P 1 ), the mirrored current to P 1  becomes Imon/N. In order to achieve a reduction of the order of several orders of magnitude, more than one mirror stage (not shown) becomes necessary. If the leakage current Ileak drawn by the leakage monitor device is greater than Imon/N, the node voltage at the input of the inverter formed by P 2  and N 5  will decrease to ground, and the output of the sensor will remain at “low”. One may then increase the digital setting to raise the value of Imonl until Imon/N &gt;Ileak, while the output remains at high. The leakage level is measured as the ratio Imon/N. For monitoring different leakage mechanisms, a different size ratio N is selected to achieve an effective sensing. 
   When measuring p-MOS leakage, a different class of leakage monitoring device becomes necessary, namely, one that is better adapted to handle p-type leakage, as previously described with reference to  FIGS. 2 ,  4 , and  6  in which case, a p-type binary decoder is used to tune Imon. The ratio of the size of the n-MOS devices N 0  to N 1  determines the ratio N of the leakage measurement. When Imon/N&gt;Ileak, then Vout switches to high, and the leakage current is measured. 
   Referring to  FIG. 7A  depicting a schematic diagram of an n-type leakage measuring device  100 , there is also shown a circuit formed by an n-type IDAC  103  (current controlled current digital to analog converter), a current mirror circuit  104 , two output inverters, INV 101  and INV 102 , and an n-type leakage monitoring device  108 . The n-type leakage monitor circuit further includes three classes of monitoring structures to measure various types of leakage, preferably including gate, junction and sub-threshold leakages. The current mirror circuit  104  mirrors the monitor current Imon from diode P 0  to P 1  having a width ratio N. More particularly, Imon/N is mirrored to probe the leakage current Ileak. If Imon/N&gt;Ileak, then the input node voltage at the output inverter INV 102  will rise to power supply level and subsequently force a low at the output of inverter  102 . Moreover, the output Vout of the second inverter INV 101  remains at high. However, when adjusting IDAC  103  until Imon/N&gt;Ileak, the output voltage Vout remains at low. Thus, the n-type leakage current is measured digitally. The detailed measuring technique will be explained hereinafter. Leakage levels are sampled using digital bits DIG&lt;i&gt;. Generally, DIG&lt;l&gt;. . . . DIG&lt;n&gt; are preferably generated locally within the n-type leakage device  100  (as will be further described with reference to  FIG. 10 ) via a preset algorithm or a state machine to sample the leakage current of each monitor. 
   Referring to  FIG. 7B , there is shown a schematic diagram of an n-type IDAC  103 . The DAC receives a reference current Iref, (e.g., 50 μ{acute over (Å)}), which is mirrored from Nr to Ni (where I=1, 2 . . . , n). If the size of Nr=10 μm and Ni is configured in binary form, e.g., if the width of N 1  is 1 μm, then the width of N 2  is multiplied by  2  (i.e., 4 μm), and N 3  multiplied by 4 (i.e., 8 μm), etc. When N 1  is activated by asserting a first digital vector, or when DIG&lt; 0 &gt;=1, the output of inverter I 1  will be at low and the output of inverter I 0  will stand at high, which in turn switches n-MOS device N 21  on and n-MOS N 22  off. At this instant, the gate of N 1  becomes connected to the gate of the mirror device Nr. The current mirrored from Nr to N 1  will be 50 μ{acute over (Å)} ( 1/10)=5 μ{circumflex over (Å)}. Similarly, if DIG&lt; 1 &gt;=1 while is at 0, the current that mirrors Nr to N 2  will be 10 μ{acute over (Å)}, and the like. The binary setting allows Imon to vary from 5 μ{circumflex over (Å)} to 80 μ{circumflex over (Å)} when 4-bit vectors are used. That is, when DIG&lt; 0 : 3 &gt;=&lt;1,1,1,1&gt;, then Imon=80 μ{circumflex over (Å)}. This technique as described enables the leakage current of the n-type leakage devices to be digitized. 
   Referring to  FIG. 8A , there is shown a schematic diagram  110  for measuring a p-type leakage device. It is formed by p-type IDAC  113  (current controlled current digital to analog converter), a current mirror circuit  114 , an output inverter INV  111 , and a p-type leakage monitor device  118 . The p-type leakage monitor device further includes three different classes of monitor devices to detect among others, gate, junction and sub-threshold leakages. The current mirror circuit  114  mirrors the monitor current Imon from a diode device comprised of N 0  to N 1  and having a width ratio of N. In other words, Imon/N is mirrored in order to probe the leakage current Ileak. If Imon/N&gt;Ileak, then the input node voltage of output inverter INV 102  decreases to 0, subsequently setting Vout at high at the output of the inverter. However, when adjusting IDAC  113  until Imon/N is smaller than Ileak, the output voltage Vout will stand at low and the P-type leakage current may then be measured digitally. The measuring technique will be explained in more detail hereinafter. 
   Referring to  FIG. 8B , there is shown a schematic diagram of an n-type IDAC  113 . It receives a reference current Iref, (in the present example, 50 μ{circumflex over (Å)}), which is mirrored from Pr to Pi (where i=1,2, . . . n). If the width of Pr=10 μm, and the width of Pi is in binary format, e.g., if the width of P 1 =μm,the width of P 2  will then be twice the width of P 1 , and P 3  will be four times, and so on. When P 1  is activated by asserting the first digital vector, or when DIG&lt; 0 &gt;=1, the output of the inverter I 1  will be at low, and the output of the inverter I 0  switch will be at high, which in turn switches the p-MOS device P 21  on and the p-MOS device P 22  off, connecting the gate of P 1  to the gate of mirror device Pr. The current mirrored from Pr to P 1  is of the order of 50 μ{circumflex over (Å)} ( 1/10)=5 μ{circumflex over (Å)}. Similarly, if DIG&lt; 1 &gt;=1 while the remaining are  0 , the current mirrors from Pr to P 2  will be 10 u{circumflex over (Å)}, and the like. The binary setting makes it possible for Imon to vary from 5 μ{circumflex over (Å)}to 80 μ{circumflex over (Å)} when 4-bit vectors are used, i.e., when DIG&lt; 0 : 3 &gt;=&lt;1,1,1,1&gt; and Imon=80 μ{circumflex over (Å)}. By way of this technique, the leakage current level of the p-type leakage devices can be digitized. 
   Referring to  FIG. 9 , there is shown a technique for executing a binary search to set the digital bits while probing the leakage device. A built-in algorithm is used to probe the leakage current Ileak. At the start, if all the 4-bit vectors (i.e., starting with &lt;0,0,0,0&gt;; then &lt;0,0,0,1&gt;; and the like up to &lt;1,1,1,1&gt;) are set to high, &lt;1,1,1,1&gt; and Imon&gt;Ileak, the output voltage Vout remains at high. In step  2 . Imon is lowered by 50%, which sets the vectors to &lt;0,1,1,1&gt;, with the most significant bit set to 0. Vout is at low, and since Imon &lt;Ileak, then Imon must increase by 25%, which is equivalent to setting the vectors to &lt;1,0,1,1&gt;, while Vout remains at low, which indicates that Imon&gt;Ileak. The third step is to reduce current by 12.5%, or setting the vectors to &lt;1,0,0,1&gt; which results in Vout remaining at low. The final step is to further reduce Imon by 6.25%, or setting the vectors to &lt;1,0,0,0&gt;, since all the bits have been used, and the measured Ileak approximates the value of Imon. However, if 4-bit vectors are not sufficient, preferably, one can provide additional bits to obtain better results. 
   Still referring to  FIG. 9 , by sizing the width of Ni, the adjacent path will flow twice the current of the existing path. For example, if the target is 7.3, the search range is 10. Then, the first step starts with 5 (mid-point of 10), and since 5 is less than 7.3, in the next step, it goes up half-way between 5 and 10, i.e., 7.5. However, since 7.5 is greater than 7.3, it is lowered down half-way between 7.5 and 5, i.e., 7.25 and the like, until the final result is sufficiently close to the target. Having described the operating steps in  FIG. 9 , the algorithm can be expressed in hardware, by way of a circuit design (not shown) or, alternatively, it can take the form of a state-machine (not shown). Practitioners of the art will readily realize that binary search is a known algorithm, and therefore, no further description will be provided. 
   The circuits respectively shown in  FIGS. 7B and 8B  are designed to perform a binary search. 
   Referring now to  FIG. 10 , there is shown a test site formed by a combination of an n-type and a p-type test macro. Each test macro preferably includes a test circuit  100 , a multiplexer, three leakage classes test of monitors N 1  to N 3  and P 1  to P 3  and corresponding three register groups SN 1  to SN 3  and SP 1  to SP 3 . During testing, the test controller (not shown) selects the first class of test monitor to be connected to the test circuit via a multiplexer for measuring the first class of leakage. When completed, the final digital vectors representing the first class leakage level are stored in corresponding register groups. The test controller selects the second class of test monitor, and the like. When all the leakage information is collected, the data is scanned out sequentially. 
   Referring to  FIG. 11 , there is shown a high level view of the test system formed by a plurality of the test units shown in  FIG. 10 . Each unit is placed strategically throughout the chip and linked by way of a scan chain to facilitate the collection of leakage information. The size of each leakage monitor is designed to properly recognize any statistical significances. Herein, the recommended size ranges from 50 μm 2  to 1000 μm 2 . 
   Referring now to  FIG. 12 , there is shown a flowchart of the preferred measurement technique according to the invention. In this case, testing is performed sequentially from the first test site up to the last test site using an on-chip algorithm. It starts with test site  1  (step  1200 ), simultaneously testing N 1  and P 1  and feeding the test result to register group SN 1  and SP 1 , respectively (step  1201 ). It then tests N 2  and P 2  ( 1202 ) and so on, until the first site is completed ( 1203 ), and then moves to the second test site ( 1204 ). When the last test site is tested ( 1205 ), it scans the data out (step  1206 ). 
   While the present invention has been particularly described in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the present description. For instance, in addition to the classes of leakages that were described and handled in the present invention, other classes may be considered and handled with equal success. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.