Abstract:
A electrically blowable fuse is programmed using an electro-migration effect and is reprogrammed using a reverse electro-migration effect. The state (i.e., “opened” or “closed”) of the electrically blowable fuse is determined by a sensing system which compares a resistance of the electrically blowable fuse to a reference resistance.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to fuses included within semiconductor structures. More particularly, the present invention provides an electrical fuse that can be reprogrammed using a reverse electro-migration effect. 
   2. Related Art 
   As is known in the art, many modern semiconductor integrated circuits include fuses to protect sensitive parts during the manufacturing process, and for the activation of redundant circuits, such as redundant memory cells in the case of Dynamic Random Access Memories (DRAMs). There are typically two types of fuses, a laser-blowable fuse, and an electrically (e.g., current) blowable-fuse. Electrically blowable fuses provide an advantage over laser-blowable fuses in terms of size. 
   With laser blowable fuses, the fuses are typically formed at or near the surface of the integrated circuit. A laser beam striking the fuse material renders the fuse non-conductive, thereby inhibiting current from flowing through the fuse. Although laser blowable fuses are relatively simple to fabricate, there are disadvantages associated with them. For example, laser blowable fuses tend to be surface oriented, which places a limitation on the design of the integrated circuit. Further, laser blowable fuses tend to occupy a large amount of space on the surface of an integrated circuit, since adjacent fuses or devices must not be placed too close to the fuse or risk being inadvertently damaged by the laser beam during the fuse blowing operation. 
   Electrically blowable fuses, on the other hand, do not have to be placed at or near the surface of the integrated circuit. Accordingly, they give designers greater latitude in fuse placement. In general, electrically blowable fuses tend to be smaller than laser blowable fuses, which render them highly suitable for use in modern high density integrated circuits. Further, electrically blowable fuses have a high programming speed compared to conventional laser blowable fuses. 
   Various means have been used in the past to blow electrically blowable fuses. One recently used technique for opening the connection at the fuse employs the electro-migration effect, which has long been identified as a major metal failure mechanism. Electro-migration is the process whereby the ions of a metal conductor move in response to the passage of a high density current flow though the conductor. Such motion can lead to the formation of “voids” in the conductor, which can grow to a size where the conductor is unable to pass current. One can take advantage of the electro-migration effect to selectively open up metal connections (e.g., fuses) at desired locations within an integrated circuit. 
   One limitation of such electrically blowable fuses is they can be programmed only once (e.g., from a state “1” (conducting) to a state “0” (non-conducting)). In other words, once an electrically blowable fuse has been opened using the electro-migration effect it can not be closed again. Therefore, to reprogram or reconfigure an integrated circuit, redundant electrically blowable fuses and complicated supporting circuitry would be necessary. 
   Studies have been made regarding the healing of electro-migration related damage using a current reversal method. Evidence of such healing has been reported by E. Castano, et al, in a paper entitled “In Situ Observation of DC and AC Electro-migration in Passivated Al Lines,” Applied Physics Letters, Volume 59, Issue 1, Jul. 1, 1991, pp. 129-131. In this paper, it was shown that void size could be decreased by applying current stress in a reverse direction. As depicted in  FIG. 1 , for example, it was found that the average void size was reduced from 5.0 μm 2  (point A) to 1.5 μm 2  (point B) in less than one hour. A similar study was presented by J. Tao, et al. in a paper entitled, “An Electro-migration Failure Model for Interconnects under Pulsed and Bi-directional Current Stressing,” IEEE Trans on Electron Devices, Vol. 41, No. 4, April 1994, pp. 539-545. In this paper, it was shown that the resistance of a conductor made of Al/Si could be altered back and forth during forward and reverse current stressing as shown in  FIG. 2 . These and other such studies, however, have not provided a solution to the “programming only” nature of electrically blowable fuses that are programmed using the phenomenon of electro-migration. 
   SUMMARY OF THE INVENTION 
   The present invention provides an electrical fuse that can be reprogrammed using a reverse electro-migration effect. Electro-migration is used to open a connection in the electrical fuse, while reverse electro-migration is used to subsequently close the opened connection. A programming/reprogramming circuit is provided to enable the use of such a reprogrammable electrical fuse. 
   A first aspect of the present invention is directed to a method, comprising: programming an electrically blowable fuse using an electro-migration effect; and reprogramming the electrically blowable fuse using a reverse electro-migration effect. 
   A second aspect of the present invention is directed to a fuse system, comprising: an electrically blowable fuse; means for programming the electrically blowable fuse using an electro-migration effect; and means for reprogramming the electrically blowable fuse using a reverse electro-migration effect. 
   A third aspect of the present invention is directed to an integrated circuit comprising: an electrically blowable fuse; and means for programming and reprogramming the electrically blowable fuse using an electro-migration effect and a reverse electro-migration effect, respectively. 
   A fourth aspect of the present invention is directed to a method, comprising: reprogramming an electrically blowable fuse using a reverse electro-migration effect. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
       FIGS. 1-2  depict the healing of electro-migration related damage using a current reversal method. 
       FIGS. 3A-3C ,  4 A- 4 C,  5 A- 5 C, and  6 A- 6 C illustrate the operation of a reprogrammable electrical fuse in accordance with an embodiment of the present invention. 
       FIGS. 7A-7C  illustrate the operation of a reprogrammable electrical fuse in accordance with another embodiment of the present invention. 
       FIG. 8  depicts programming and reprogramming circuitry for a reprogrammable electrical fuse in accordance with the present invention. 
       FIG. 9  depicts a sensing circuit for a reprogrammable electrical fuse in accordance with an embodiment of the present invention. 
       FIG. 10  depicts a sensing circuit for a reprogrammable electrical fuse in accordance with another embodiment of the present invention. 
       FIG. 11  depicts an illustrative physical layout of a reprogrammable electrical fuse system in accordance with an embodiment of the present invention. 
       FIGS. 12-16  depict an analytical model for illustrating the predicted electro-migration behavior in a tapered structure in accordance with the concepts of the present invention. 
   

   The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements. 
   DETAILED DESCRIPTION 
   The general operation of a reprogrammable electrical fuse  10 , hereafter referred to as an “e-fuse  10 ” in accordance with an embodiment of the present invention is depicted in  FIGS. 3A-3C . In this example, the e-fuse  10  has a “dog-bone” shape to facilitate programming and reprogramming operations. As depicted in  FIG. 3A , the e-fuse  10  includes a first conductive body region  12 , a second conductive body region  14 , and a conductive neck region  16  extending between the first and second body regions  12 ,  14 . During programming, as shown in  FIG. 3B , a void  18  forms in the neck region  16  of the e-fuse  10  in response to the application of a current  1 . This occurs because of the so-called “crowding effect,” which is caused by the flow of electrons from a larger cross-sectional area (e.g., body region  12 ) into a smaller cross-sectional area (e.g., neck region  16 ). This leads to a large temperature gradient, which expedites the electromagnetic effect. The formation of the void  18  causes the resistance of the e-fuse  10  to increase significantly. It should be noted that the neck region  16  must remain conductive to allow a reverse current flow required for reprogramming of the e-fuse  10 . 
   During reprogramming, as shown in  FIG. 3C , the void  18  is refilled and pushed toward the larger cross-sectional area of the body region  12  in response to the application of an oppositely directed current I, thereby restoring the resistance of the e-fuse  10 . This occurs because of a so-called current “de-crowding effect.” If too much current stress is applied in the reverse direction, however, the void  18  may start to increase in size again at a different location. To this extent, to make the e-fuse  10  of the present invention reprogrammable, one must: 
   (1) Provide a structure that allows metal to migrate in a controllable manner in both forward and reverse current flow directions; 
   (2) Create a large resistive ratio change; and 
   (3) Provide a reference resistance R ref  to determine the state of the e-fuse  10 . Prior to programming (and after reprogramming) the reference resistance R ref  is higher than the resistance of the e-fuse  10 . After programming, the reference resistance R ref  is much lower than the resistance of the e-fuse  10 . 
   Different voltages and currents may be applied to the e-fuse  10  to perform programming and reprogramming. During programming, the resistance of the e-fuse  10  will rise higher than the reference resistance R ref , while during reprogramming the resistance of the e-fuse  10  will drop lower than the reference resistance R ref . 
   The present invention provides a sensor circuit (described in greater detail below) to sense the change of resistance of the e-fuse  10  and latch the results into a corresponding register. One reference resistance can be shared by a bank of e-fuses  10  to save power and area overhead. In this case, sensing can be done in sequential manner, for example, during a power-on sequence to read the bank of e-fuses  10 , and the results stored in one or more registers. The stored results can be used to provide information regarding programming state. The registers can comprise a small cache memory like that of DRAMs, or local registers. 
   As shown in  FIG. 4A , an illustrative e-fuse  10  has a “dog-bone” shape with a taper angle θ of about 45 to 75 degrees and a neck region  16  with a width/body ratio from about 1/10 to ⅓, depending on the width of the body regions  12 ,  14 . As will be presented in greater detail below, the taper facilitates the programming/reprogramming of the e-fuse  10 . 
   Cross-sectional views of the e-fuse  10  taken along line  4 B- 4 B and  4 C- 4 C are illustrated in  FIGS. 4B and 4C , respectively. A uniform thickness of a barrier film formed of a material such as titanium (Ti), tantalum (Ta), tungsten (W), titanium nitride (TiN), or a combination thereof is deposited in a conventional manner at the bottom of the e-fuse  10  to form a barrier layer  20 . Due to the reduced surface area within the neck region  16 , the deposited barrier film will be much thicker in the neck region  16  as compared to the body regions  12 ,  14 . The thicker barrier layer  20  in the neck region  18  shrinks the cross-section of the neck region  16  in the third (i.e., Z) dimension. This increases the current density inside the neck region  16 , thereby enhancing the electro-migration effect in the e-fuse  10 . 
   A metal material  22 , such as aluminum (Al), copper (Cu), aluminum-copper (Al/Cu) alloy, or other suitable metal material susceptible to electro-migration, is then deposited and planarized (e.g., using chemical-mechanical-polishing (CMP)). A depletion region d 1  is formed at the surface of the body regions  12 ,  14 . A dielectric material (not shown) is deposited to cap the top surface of the e-fuse  10 . 
   The sidewall  24  of the e-fuse  10  comprises a barrier liner formed of a material such as Ti, Ta, W, TiN, or a combination thereof. Other conductive materials such as doped poly-silicon or a silicided diffusion region can also be used. The barrier liner can be used to provide the reference resistance R ref  described above with regard to the programming/reprogramming of the e-fuse  10 . The material of the barrier liner is not sensitive to the electro-migration effect and has a resistance value higher than that of the e-fuse  10  prior to programming (and after reprogramming) and a resistance value much lower than that of the e-fuse  10  after programming. The material of the barrier liner is preferably compatible with back end of line (BEOL) metallization processes to limit processing costs. 
   After programming, as shown in  FIGS. 5A-5C , a void  18  is created in the neck region  16  of the e-fuse  10  due to the electro-migration effect. As a result, the resistance of the e-fuse  10  is drastically increased, even though the void  18  may only be located partially within the neck region  16 . Programming conditions (e.g., voltage, current, temperature, etc.) are controlled so that that a desired void  18  size is formed. If the void  18  size is too small, the e-fuse  10  will be under-programmed. If the void  18  size is too large, it may not be possible to reprogram the e-fuse  10 . Neither of these conditions is desirable. 
   As shown in  FIG. 5B , during the programming of the e-fuse  10 , metal migrates from the neck region  16  of the e-fuse  10  toward the body region  14  and accumulates to a depth d 2 . As a result, this area of the e-fuse  10  has a higher atomic density and is more stressed than before programming. The migration of the metal results in the creation of a void  18  located at least partially within the neck region  16  of the e-fuse  10 . Different degrees of programming can be used to create different sized voids  18 ′,  18 ″, etc., with different depths, resulting in different resistance values for the programmed e-fuse  10 . The resistance of the e-fuse  10  after programming is much greater than the reference resistance R ref . 
   During the programming of the e-fuse  10 , high-voltage and high-current are applied to the e-fuse  10  at room or high temperature (e.g., 100 to 250° C.) to “open” the e-fuse  10  in a relatively short period of time. Care must be taken, however, to ensure that at least some of the metal material  22  remains within the neck region  16  to allow a reverse current to be applied during a subsequent reprogramming of the e-fuse  10 . 
   The reprogramming of the e-fuse  10  is illustrated in  FIGS. 6A-6C . As with programming, the reprogramming of the e-fuse  10  is carried out under high-current, voltage and at room or high temperature, but in the opposite direction. As shown, excessive metal that accumulated on the body region  14  of the e-fuse  10  migrates toward the neck region  16  and at least partially fills the void  18 . It may be desirable to perform in-situ monitoring during reprogramming to minimize the depth of the void  18  inside the neck region  16 . The resistance of the e-fuse  10  after reprogramming is once again much lower than the reference resistance R ref . 
   Another embodiment of the present invention is depicted in  FIGS. 7A-7C . As shown, a reprogrammable e-fuse  30  can be formed using a conductive double-layer metal structure  32 . In particular, a top metal layer  34  of the double-layer metal structure  32  can be formed using a metal material that is susceptible to electro-migration, while the bottom metal layer  36  of the double-layer metal structure  32  can be formed using a metal material that is much less susceptible (or not susceptible) to electro-migration. For example, since pure copper (Cu) is at least 2 to 4 times more susceptible to electro-migration than pure aluminum (Al) or certain alloys of Al, the top metal layer  34  can be formed of copper, while the bottom metal layer  36  can be formed of Al or an alloy thereof. In another embodiment of the present invention, the metal layers  34  and  36  can be reversed such that the metal material that is susceptible to electro-migration is located below the metal material that is much less susceptible (or not susceptible) to electro-migration. The metal material that is susceptible to electro-migration can also be sandwiched between layers of, or surrounded by, the metal material that is much less susceptible (or not susceptible) to electro-migration. 
   During programming of the e-fuse  30 , as shown in  FIG. 7B , a void  38  is formed in the top metal layer  34 , which increases the resistance of the reprogrammable e-fuse  30  such that it is much greater than a reference resistance R ref  of the e-fuse  30 . During reprogramming, as shown in  FIG. 7C , the void  38  is at least partially refilled and the resistance of the e-fuse  30  is reduced. Any suitable bi-layer or multi-layer metal structure can be used to form the reprogrammable e-fuse  30 . The bottom metal layer  36  can also be used to provide the reference resistance R ref  instead of using a barrier liner as detailed above. 
   Illustrative programming and reprogramming circuitry  40  for a reprogrammable e-fuse  10  in accordance with the present invention is depicted in  FIG. 8 . During programming, the control pin “F” is set to high, so that the two nMOS devices N 11  and N 12  are on, but the other two nMOS devices N 10  and N 13  are off. A programming current with a preset current pulse height and width is applied to the e-fuse  10  from net A to net B. Similarly, during reprogramming, the control pin “F” is set to low, so that the two nMOS devices N 11  and N 12  are off, but the other two nMOS devices N 10  and N 13  are on. A reprogramming current with another preset pulse height and width is applied to the e-fuse  10  element in the opposite direction, from net B to net A. 
   An illustrative sensing circuit  50  for a reprogrammable e-fuse  10  in accordance with an embodiment of the present invention is depicted in  FIG. 9 . As shown, the e-fuse  10  and a reference element R ref  having a resistance R ref  serve as the load for cross-coupled nMOS devices N 1  and N 2 . A control pin “Sample” is used to activate the sensing operation. The “Sample” signal is tied to the gate of a pMOS device P 1  and a nMOS device N 3 . After programming, most of the current will flow through N 1  and cause node B to go “high,” since the resistance of e-fuse  10  after programming should be substantially higher than that of the reference element R ref . The final “high” state is latched in a latch register  52 . On the other hand, after reprogramming, the resistance of the e-fuse  10  should be substantially lower than that of the reference element R ref , and more current will flow through N 2  and cause node B to go “low.” The final “low” state is latched in the latch register  52 . During programming and reprogramming, the “Sample” signal is off, so that both node A and B will be at float. Then, as shown in  FIG. 8 , either N 12  (program) or N 13  (reprogram) is on and a path to ground is provided to allow current to flow only through the e-fuse  10 . 
   The sensing circuit  50  does not allow the reference element R ref  to be shared among a plurality of e-fuses  10 . If the size of the reference element R ref  is relative small this approach is acceptable—a separate reference element R ref  can be provided for each e-fuse  10 . Otherwise, a sensing circuit  60  such as that shown in  FIG. 10  can be used, where a single reference element  62  in a reference unit  64  is shared by a plurality of e-fuse units  66  (only one is shown). In sensing circuit  60 , the reference unit  64  is used to generate a reference voltage equal to (Vdd-I*R ref ), where I is the current flow through the reference path formed by pMOS device P 51  and nMOS devices N 51  and N 61 , and R ref  is the resistance of the reference element R ref . The current I is mirrored by a shared current source  68 . 
   Each e-fuse unit  66  includes an e-fuse  70 . An identical amount of current I is mirrored via nMOS device N 62 . The output voltage at node B is Vdd-I*R f , where R f  is the resistance of the e-fuse  70 . A comparator  72  is formed by two pMOS devices P 53  and P 54 , two nMOS devices N 52  and N 54 , and a tail device N 63 . 
   The output from the reference unit  64  (node C) is tied to the gate of the nMOS device N 53  and the output of the e-fuse path (node B) is tied to the gate of the nMOS device N 54 . After programming, R f &gt;R ref , and the voltage at node B is lower than at node C, so that output of the comparator  72  will go high and the high state will be latched by latch  74 . Otherwise, after reprogramming, a low state will be latched by latch  74 . 
   An illustrative physical layout of a reprogrammable e-fuse system  80  in accordance with the present invention is depicted in  FIG. 11 . As shown, the physical layout of the e-fuse system  80  includes a programming power and timing generator  82 , a reprogramming power and timing generator  84 , and a plurality of e-fuse units  86 . Each e-fuse unit  86  further includes an e-fuse module  88 , a sensing element  90  and a latch  92 . The programming power and timing generator  82  and reprogramming power and timing generator  84  can be merged into a single unit. 
   As mentioned above, the use of a tapered neck region  16  facilitates the programming/reprogramming of the e-fuse  10 . An analytical model illustrating the predicted electro-migration behavior in a tapered structure is presented below. 
   The tapered structure  100  used in this analysis is illustrated in  FIG. 12 . As shown, the tapered structure  100  includes first and second body regions  102 ,  104 , and a neck region  106  that extends between the first and second body regions  102 ,  104 . The first and second body regions  102 ,  104  each have a thickness of 0.5 μm, while the thickness of the neck region  106  varies from a minimum of 0.14 μm to a maximum of 0.5 μm. The taper of the neck region  106  is specified by a taper angle β. Based on the tapered structure  100 , the following items were examined: 
   (A) Growth of void during forward current stressing conditions and void shrinkage during reverse current stressing conditions; 
   (B) Effects of taper geometry (β)——vary length of tapered neck region  106 , keeping same minimum and maximum widths; and 
   (C) Time required to form a void and remove the void for selected void sizes (e.g., 0.125, 0.25, and 0.5 μm). 
   The following analytic modeling assumptions were used: 
   (A) Void growth emanates from the beginning (i.e., narrowest width region) of tapered neck region  106  (no incubation time); 
   (B) Metal (e.g., Cu) removed from void is deposited at the end of tapered neck region  106 ; 
   (c) Metal is redeposited in the void during reverse current; 
   (D) Void growth kinetics from data on uncapped structures, T=225° C.; 
   (E) Growth velocity has exponential dependence on temperature; and 
   (F) Growth velocity has linear dependence on current density. 
   Based on these modeling assumptions, for a tapered neck region  106  with a length of 1.0 μm, the predicted void growth during forward current stressing (J 0 =70 mA/μm 2  through 0.5 μm wide line) is illustrated in  FIG. 13 . The predicted void growth and shrinkage for a tapered neck region 1.0 μm long and with a taper angle β=10° is illustrated in  FIG. 14 . The predicted void growth and shrinkage for a tapered neck region 106 0.5 μm long and with a taper angle β=20° is illustrated in  FIG. 15 . The predicted void growth and shrinkage for a tapered neck region 2.0 μm long and with a taper angle β=5° is illustrated in  FIG. 16 . 
   From the above graphs, it can be seen that: 
   (A) Predicted electro-migration behavior in the tapered structure  100  follows asymmetric void growth and shrinkage during forward and reverse current. 
   (B) Taper Angle (β): 
   (1) Larger taper angle β increases time required to reach equivalent void size and increases nonlinearity of void growth rate. 
   (2) Total time for void growth and shrinkage is roughly equivalent. 
   (C) Time to reach equal void size is roughly proportional to current density (J). 
   (D) Strong dependence of temperature on void growth (activated process). 
   (Joule heating not accounted for in analytical model). 
   The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.