Abstract:
A sense amplifier for detecting a logic state of a selected electrical fuse cell among a number of unselected electrical fuse cells includes a bias module coupled to a power supply for generating a first current, and a tracking module coupled to the bias module for generating a second current. A current supplier is coupled to the bias module and the tracking module for generating a third current substantially equal to a sum of the first and second currents scaled by a predetermined factor, the third current being diverted into a first sub-current flowing through the selected electrical fuse cell and a second sub-current leaking through the unselected electrical fuse cells. The tracking module is so configured that the second current scaled by the predetermined factor is substantially equal to the second sub-current, thereby avoiding the first sub-current to be reduced by the second sub-current.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to integrated circuit (IC) designs, and more particularly to a sense amplifier with leakage compensation for electrical fuse cells.  
         [0002]     Electrical fuses are often utilized in modern ICs. Typically, they are designed to blow when a current through the fuses exceeds a pre-determined threshold. When the fuses are programmed or “blown,” although not necessarily physically broken, they enter into a high impedance state. Electrical fuses are commonly used for making adjustments and repairs that are performed as late as after the chip is packaged. Since wirings are allowed at the two ends of the fuses, they can be flexibly implemented within the IC. This flexibility makes the electrical fuses desirable components for IC designs.  
         [0003]     Conventionally, an IC includes a plurality of electrical fuse cells, each of which has at least one electrical fuse serially coupled to at least one switch device. A sense amplifier is typically implemented in the IC to detect a sensing current flowing through the selected electrical fuse cell in order to determine its logic state. The voltage level of the sensing current varies depending on the resistance of the fuse of the selected cell. The sense amplifier outputs a signal indicative of whether the fuse is blown or not, based on the voltage level of the sensing current.  
         [0004]     During the sensing process, only one or few electrical fuse cells are selected. Ideally, the unselected electrical fuse cells should not allow an electrical current to pass thereacross. However, in reality, part of the sensing current often leaks through the unselected electrical fuse cells. This reduces the sensitivity of the sense amplifier in responding to a change of the fuse resistance. In some cases, the leakage current would cause the sense amplifier fail to detect a fuse cell that is blown.  
         [0005]     Thus, desirable in the art of IC design is a sense amplifier with leakage current compensation features for electrical fuse cells.  
       SUMMARY  
       [0006]     The present invention discloses a sense amplifier for detecting a logic state of a selected electrical fuse cell. In one embodiment of the present invention, the sense amplifier includes a bias module coupled to a power supply for generating a first current, and a tracking module coupled to the bias module for generating a second current. A current supplier is coupled to the bias module and the tracking module for generating a third current substantially equal to a sum of the first and second currents scaled by a predetermined factor, the third current being diverted into a first sub-current flowing through the selected electrical fuse cell and a second sub-current leaking through the unselected electrical fuse cells. The tracking module is so configured that the second current scaled by the predetermined factor is substantially equal to the second sub-current.  
         [0007]     The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  schematically illustrates a conventional sense amplifier for electrical fuse cells.  
         [0009]      FIG. 2  schematically illustrates a sense amplifier for electrical fuse cells in accordance with one embodiment of the present invention.  
         [0010]      FIG. 3A  illustrates a graph showing the relationship between the voltage of output signal and the resistance of fuse cell for the conventional sense amplifier.  
         [0011]      FIG. 3B  illustrates a graph showing the relationship between the voltage of output signal and the resistance of fuse cell for the sense amplifier in accordance with the embodiment of the present invention. 
     
    
     DESCRIPTION  
       [0012]      FIG. 1  schematically illustrates a conventional sense amplifier  100  for a plurality of electrical fuse cells. The sense amplifier  100  includes an output module  101 , a bias module  102 , and a current supplier  104 , which is coupled to a selected electrical fuse cell  106  and a number of unselected electrical fuse cells  108 . The bias module  102  and the current supplier  104  work together as a current mirror, such that the current supplier  106  generates a sensing current Is, which is substantially equal to a first current I 1  generated by the bias module  102  multiplied by a predetermined constant. The switch device  110  in the selected cell  106  is turned on for allowing the sensing current Is to flow therethrough to ground, and the switch devices  112  in the unselected electrical fuse cells  108  are all turned off to prevent the sensing current Is to pass thereacross. Note that while there is only one set of fuse and switch device is depicted in the box showing the unselected electrical fuse cells  108 , it conceptually represents n−1 cells where n is the total number of the electrical fuse cells.  
         [0013]     Ideally, the sensing current Is should flow through the selected electrical fuse cell  106 , without leaking through the unselected cells  108 . However, in reality, the switch devices  112  cannot be turned off perfectly, and part of the sensing current Is would leak therethrough. The current passing through the selected electrical fuse cell  106  is denoted as I L , and the current leaking through the unselected electrical fuse cells  108  is denoted as I R , where Is is substantially equal to the sum of I L  and I R . As a result, the actual current flowing through the switch device  110  is less than the sensing current Is. This reduces the sensitivity of the sense amplifier  100 , which detects whether the fuse  114  within the selected cell  106  is blown or not based on the voltage level of the sensing current Is. As such, the output module  101  may fail to generate an output signal properly indicating the state of the electrical fuse  114  in the selected cell  106 .  
         [0014]      FIG. 2  schematically illustrates a sense amplifier  200  for a plurality of electrical fuse cells in accordance with one embodiment of the present invention. The sense amplifier  200  includes an output module  201 , a bias module  202 , and a current supplier  204 , which is coupled to a selected electrical fuse cell  206  and a number of unselected electrical fuse cells  208 . The sense amplifier  200  further includes a tracking module  209  coupled to the bias module  202  and the current supplier  204 .  
         [0015]     The bias module  202  includes a PMOS transistor  210  having a gate and a drain coupled together and a source coupled to a power supply denoted as VDD. A load  121  is coupled to the drain and gate of the PMOS transistor  210 . A switch device  214 , such as an NMOS transistor, is coupled between the load  212  and a complementary power supply such as ground. The tracking module  209  includes a number of parallelly connected switch devices, which are depicted and represented by one NMOS transistor  216 . The drain of the NMOS transistor  216  is coupled to the gate and drain of the PMOS transistor  210 . The source of the NMOS transistor  216  is coupled to the drain of the switch device  214 . The gate of the NMOS transistor  216  is coupled to a complementary power supply such as ground and remains constantly off. It noted that the NMOS transistor  216  can be substituted by a PMOS transistor, bipolar transistor or other devices, as long as it functions as a switch that can be constantly turned off.  
         [0016]     The current supplier  204  includes a PMOS transistor  218  having a source coupled to the power supply VDD, a gate coupled to the gate of the PMOS transistor  210  and the drain of the NMOS transistor  216 . The drain of the PMOS transistor  218  is coupled to the output module  201  and further coupled to the selected electrical fuse cell  206  and the unselected fuse cells  208 . The selected electrical fuse cell  206  includes an electrical fuse  220  coupled to the NMOS transistor  218 . A switch device, such as an NMOS transistor  220 , is coupled between the electrical fuse  220  and a complementary power supply such as ground. The unselected electrical fuse cells  208  include a set of electrical fuses, represented by one fuse  224 , and switch devices, represented by one NMOS transistor  226 . Note that while there is only one set of fuse and switch device is depicted in the box showing the unselected electrical fuse cells  208 , it conceptually represents n−1 unselected electrical fuse cells where n is the total number of the electrical fuse cells.  
         [0017]     In operation, the switch device  214  is turned on to allow an electrical current flowing from the power supply VDD to the complementary power supply, such as ground, through the PMOS transistor  210  and the load  212 . Ideally, the electrical current would not flow through the tracking module  209 , since the switch devices, which are represented by the NMOS transistor  216 , are turned off. However, in reality, the switch devices would not remain off perfectly, and the electrical current would leak therethrough to the complementary power supply. The current flowing through the load  212  is denoted as I 1 , and the current leaking through the tracking module  209  is denoted as I 2 .  
         [0018]     The bias module  202 , the tracking module  209  and the current supplier  204  together work as a current mirror, such that the sum of I 1  and I 2  multiplied by a predetermined constant c is substantially equal to the sensing current Is generated by the current supplier  204 , where the constant can be set by adjusting the value of the load  212 . The following equation can be obtained: Is=c(I 1 +I 2 ).  
         [0019]     Ideally, the electrical current would not flow through the unselected electrical fuse cells  208 , since the switch devices therein are turned off. However, in reality, the switch devices would not remain off perfectly, and the electrical current would leak therethrough to the complementary power supply. As a result, the sensing current Is is diverted into a first sub-current I L  flowing to the selected electrical fuse cell  206  in which the switch device  222  is turned on, and a second sub-current I R  flowing to the unselected fuse cells  208  in which the switch devices represented by the NMOS transistor  226  are turned off. Thus, the sensing current Is is substantially equal to the sum of the first sub-current I L  and the second sub-current I R . Algebraically, Is=(I L +I R ). The following equation can therefore be obtained: cI 1 +cI 2 =I L +I R .  
         [0020]     Since c is a known constant, cI 2  can be set equal to I R  by adjusting the number of the switch devices within the tacking module  209 . For example, the switch devices represented by the NMOS transistor  216  can be made identical to the switch devices represented by the NMOS transistor  226  in the unselected electrical fuse cells  208 , with its number equal to the number of switch devices represented by the NMOS transistor  226  divided by the predetermined constant c. In this embodiment, the total number of the electrical fuse cells is n, and the number of the unselected electrical fuse cells  208  is m, which equals to n−1. Thus, the number of the switch device in the tacking module  209  can be set as m/c. If the effect of the selected electrical fuse cell  206  is negligible, the number of the switch devices in the tacking module  209  can be set as n/c. For example, for an electrical fuse array with  128  cells and the predetermined constant c equal to 8, the number of the switch devices in the tacking module  209  can be set as 16, thereby rendering cI 2  substantially equal to I R . As such, the influence of the leakage current I R  can be compensated by properly setting the number of the switch devices in the tracking module  209 .  
         [0021]     Referring to  FIG. 3A  and  FIG. 1 , a graph  300  shows the conventional relationship between the voltage of output signal generated by the output module  101  and the resistance of electrical fuse  114  in the selected electrical fuse cell  106 . The output signal voltage is at a low level when the electrical fuse  114  is programmed below about 750 ohms, while it is at a high level when the electrical fuse is programmed above about 1,500 ohms. Due to the leakage current I R , the transition from the low level to the high level takes a rather large range of resistance from about 750 to 1,500 ohms. This means that it would be difficult to ascertain if the fuse  114  is “blown” or not, when its resistance falls in the transition range.  
         [0022]     Referring to  FIG. 3B  and  FIG. 2 , a graph  400  shows the relationship between the voltage of output signal generated by the output module  201  and the resistance of electrical fuse  220  in the selected electrical fuse cell  222  in accordance with one embodiment of the present invention. The output signal voltage is at a low level when the electrical fuse  220  is programmed below about 750 ohms, while it is at a high level when the electrical fuse is programmed above about 1,000 ohms. Since the leakage current I R  has been compensated, the transition from the low level to the high level takes a rather small range of resistance from about 750 to 1,000 ohms. This means that it would be easier to ascertain if the fuse  220  is “blown” or not, when its resistance falls in this rather small transition range.  
         [0023]     It is understood by those skilled in the art of circuit design that while the transistors  210  and  218  are P-type in this embodiment, they can be substituted by N-type NMOS transistors with proper adjustments as an alternative embodiment of the present invention.  
         [0024]     The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.  
         [0025]     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.