Abstract:
A fuse circuit comprising one or more one-time programmable electrical fuses; one or more unidirectional conductive devices each coupled to one of the fuses; a programming device coupled to the unidirectional conductive devices; and a selection module coupled to the electrical fuses for selecting a predetermined electrical fuse, wherein upon a selection by the selection module, a programming current is introduced through at least one selected electrical fuse, wherein the selection module is an N-to-one multiplexer selecting one of the N number of electrical fuses to be programmed, and the unidirectional conductive devices not coupled to the selected electrical fuse to prevent the programming current from interfering with the remaining electrical fuses.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to integrated circuit designs, and more particularly to methods and systems for implementing multiple electrical fuses in a fuse cell equipped with only one programming device. 
         [0002]    Electrical fuses are often utilized for modern semiconductors. 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, the fuses can be flexibly positioned within the chip, which is much more desirable than the conventional laser fuses as it is impossible to implement many metal layers or thick dielectrics above the laser fuses. This flexibility makes electrical fuses a desirable component for higher density memory devices. 
         [0003]    However, conventional methods of programming electrical fuses in a memory device are not very efficient in utilizing precious silicon areas and thus are costly. For example, conventional methods for programming an electrical fuse require that one programming device is assigned for each fuse. In order to program an electrical fuse, a large supply current is necessary to be directed through the fuse. In order to provide this large supply current, programming devices attached to the fuses are very large. As the number of electrical fuses increase, the number of these large programming devices also increases 
         [0004]      FIG. 1A  presents a schematic diagram  102  showing how an electrical fuse is programmed using a NMOS programming device, while  FIG. 1B  presents a schematic diagram  104  showing how an electrical fuse is programmed using a PMOS programming device. In the schematic diagram  102 , an electrical fuse  106  is placed between a NMOS programming device  108  and a high voltage source VDDQ. In the schematic diagram  104 , a PMOS programming device  110  is placed between an electrical fuse  112  and a high voltage source VDDQ. A “Select” control signal enters through either a select line  114  or a select line  116 , when either the electrical fuse  106  or  112  is assigned to be programmed. 
         [0005]    The programming devices  108  and  110  are large in physical size since large currents are required to program electrical fuses, such as the electrical fuses  106  and  112 . The conventional method of programming electrical fuses requires one program device for each fuse. This method is extremely inefficient and costly since each fuse requires a separate programming device. In a large array of fuses, the large programming devices will take up massive areas within a silicon environment. 
         [0006]    It is always desirable to provide an improved programming mechanism with multiple fuses to improve silicon area utilization without causing deterioration to operational performance. 
       SUMMARY 
       [0007]    The present disclosure provides for a fuse circuit comprising one or more one-time programmable electrical fuses; one or more unidirectional conductive devices each coupled to one of the fuses; a programming device coupled to the unidirectional conductive devices; and a selection module coupled to the electrical fuses for selecting a predetermined electrical fuse, wherein upon a selection by the selection module, a programming current is introduced through at least one selected electrical fuse, wherein the selection module is an N-to-one multiplexer selecting one of the N number of electrical fuses to be programmed, and the unidirectional conductive devices not coupled to the selected electrical fuse prevent the programming current from interfering with the remaining electrical fuses. 
         [0008]    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 
         [0009]      FIGS. 1A and 1B  illustrate two schematic drawings showing conventional fuse cell programming methods. 
           [0010]      FIG. 2  illustrates a schematic for programming a fuse cell in accordance with one embodiment of the current invention. 
           [0011]      FIG. 3  illustrates a schematic of a fuse cell with circuitry to detect which fuse has been programmed. 
           [0012]      FIG. 4  illustrates a schematic for programming a fuse cell in an array of fuse cells. 
           [0013]      FIG. 5  illustrates a schematic of a multidimensional fuse cell array in accordance with one embodiment of the current invention. 
           [0014]      FIG. 6  illustrates a schematic of a multidimensional fuse cell array connected to sense amplifiers in accordance with another embodiment of the current invention. 
       
    
    
     DESCRIPTION 
       [0015]    Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
         [0016]      FIG. 2  presents a schematic diagram  200  showing how two electrical fuses  202  and  204 , and diodes  216  and  218 , share a single programming device  206  within a fuse cell  208  in accordance with the first embodiment of the present invention. Both electrical fuses  202  and  204  are connected to a high voltage source VDDQ through a selection module such as multiplexers  210  and  212 , respectively. In this example, the fuses are coupled to the programming device  206  through diodes  216  and  218 . In this example the programming device  206  is coupled between the fuses  202  and  204  and a complementary power supply such as a ground or VSS. When the electrical fuse  202  or  204  needs to be programmed, a programming selection signal will enter through a “Select” line  214 , commanding the programming device  206  to program one of the electrical fuses. Control signals coming in through the fuse select lines such as the two fuse selection signals FS 0  and FS 1  will enter the multiplexers  210  and  212  to turn on one of the multiplexers, thereby allowing one of the electrical fuses to be programmed. 
         [0017]    For example, if the electrical fuse  202  needs to be programmed, the high voltage source VDDQ will rise in order to provide enough current to break the electrical fuse  202 . The control signal will enter the programming device  206  through the “Select” line  214  and command it to program the electrical fuse  202  by turning on, in this example, the NMOS transistor within the programming device  206 . Incoming signals will also appear at the fuse select line FS 0 , which commands the multiplexer  210  to turn on. This opens up a path for the high voltage source VDDQ to provide the current necessary to program the electrical fuse  202 . The diode  218  operates to keep the fuse programming current from interfering with electrical fuse  204 , or when a plurality of cells  208  are put into an array and share the same multiplexers  210  and  212 . 
         [0018]    While programming device  206  is shown to be a single NMOS device, it is nevertheless understood by those skilled in the art that the programming device  206  may have other NMOS-type or PMOS-type configurations. Also those skilled in the art will recognize that the diodes  216  and  218  can be any unidirectional current device such as NMOS-type or PMOS-type semiconductors having their gates connected to their drains such that they only allow a current to flow in a single direction. Also combinations of zero-Vt, or low Vt MOS-type devices may be used to cause the same unidirectional current operation. 
         [0019]    In addition, the multiplexers  210  and  212  may also be NMOS-type, PMOS-type, or CMOS-type, without deviating from the spirit of this invention. In fact, the multiplexers  210  and  212  can be combined into one multiplexer in this embodiment with the fuse selection signal FS 0  and FS 1  complementary to each other so that the programming current is either passing through the fuse  202  or  204  at any round of programming. In essence, the function of the selection module is to allow VDDQ to be coupled to one and only one predetermined fuse at any particular round of programming. To meet this need, the selection module is essentially an N-to-one multiplexer where “N” is the total number of fuses in the fuse cell. In the simplest example, each fuse is controlled by one fuse selection signal and one multiplexer. 
         [0020]    References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure or characteristic, but every embodiment may not necessarily include the particular feature, structure or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one of ordinary skill in the arts to affect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. Parts of the description are presented using terminology commonly employed by those of ordinary skill in the art to convey the substance of their work to others of ordinary skill in the arts. 
         [0021]      FIG. 3  presents a schematic diagram  300  showing how two electrical fuses  354  and  356  share a single programming device  368  in accordance with one embodiment of the present invention. The fuse cell  370  is implemented with an input select device  366  and an output select device  350  to allow the construction of a one- or two-dimensional fuse array. The fuses  354  and  356  are connected to the programming device  368  through the diodes  316  and  318 , respectively. The electrical fuses  354  and  356  are also connected to the high voltage source VDDQ through, respectively, multiplexers  358  and  360 , which are controlled, respectively, by fuse signal lines FS 0  and FS 1 . This embodiment includes a sense amplifier  340  for providing an output signal “Sense Out”. The sense amplifier  340  is coupled to the fuse cell through the output select device  350  and to a reference resistor  364  through PMOS device  352  and diode  374 . The signal RWLB controls the output select device  350  and PMOS device  352  to provide signals to the sense amplifier  340 . 
         [0022]    For example, when the electrical fuse  356  needs to be programmed, a Y-select line (YSELB) will provide a low signal to command the input select device  366  to turn on, thereby allowing a high input signal from a write wordline (WWL) to enter the programming device  368  and turning on, in this example, the NMOS transistor therein. The control signal from the fuse signal line FS 1  will turn on the multiplexer  360 , thereby allowing the high voltage source VDDQ to provide the current necessary for the programming device  368  to program the electrical fuse  356 . The diode  316  acts to prevent current from interfering with the fuse  354 . After programming the electrical fuse  356 , the Y-select line YSELB is controlled to a high signal such that the input select device  366  is turned off and the NMOS transistor  372  is turned on. Turning on the NMOS transistor  372  provides a low signal to the programming device  368  ensuring that it is turned off. 
         [0023]    In this embodiment it is also possible to read data by ensuring that the NMOS transistor in the programming device  368  is off and by inputting a low signal through a read wordline (RWLB) to turn on the output select device  350  and PMOS device  352 . This can allow the signal (e.g., an electrical parameter value) at a node  334  to output to the sense amplifier  340 , which detects very small changes in the voltage of bit lines relative to a pre-determined reference voltage provided through the PMOS device  352 , the diode  374  and the reference resister  364 . The sense amplifier  340  provides an output of data as a logic state “0” or “1”. For example, if the electrical fuse  356 , which is previously programmed, needs to be checked to ensure that it is properly programmed, the fuse select line FS 1  can input a signal to turn on the multiplexer  360 . Since the electrical fuse  356  has already been blown due to previous programming, the high voltage source VDDQ will be hard to reach the node  334 . As such, the output from the sense amplifier  340  will have a low output. To check the electrical fuse  354 , the fuse select line FS 0  will need to input a signal to turn on the multiplexer  358 . This allows the high voltage supply VDDQ to reach the node  334 , thereby providing a high signal output from the fuse cell  370  through the PMOS device  350 . In this example the diode  318  prevents the voltage at the node  334  from interfering with fuse  356 . 
         [0024]    While the electrical fuses  354  and  356  are shown to connect to the programming device  368  which contains an NMOS transistor, it is nevertheless understood by those skilled in the arts that that PMOS-type devices may also be used as a programming device for this invention, whereas input and output select devices  366  and  350  can also be NMOS-type, PMOS-type, or zero threshold transistors within the scope of this invention. Also those skilled in the arts will recognize that the diodes  316  and  318  can be any unidirectional current device such as NMOS-type or PMOS-type semiconductors having their gates connected to their drains such that they only allow a current to flow in a single direction. Also combinations of zero-Vt or low Vt MOS-type devices may be used to cause the same unidirectional current operation.  FIG. 3  shows one embodiment of using one sensing device in one fuse cell. 
         [0025]      FIG. 4  presents a schematic diagram  400  showing a one-dimensional array of 8 fuse cells  402  in accordance with another embodiment of the present invention. The fuse cell array is set up in an 8-by-1 configuration. The fuse cells  402  allow two electrical fuses to share one programming device. Each electrical fuse is connected to the high voltage source VDDQ through one of the multiplexers  404  or  406 , which are respectively controlled by the fuse select lines FS 0  and FS 1 . The fuse cell that needs to be programmed will be selected by “Select” signals  0  through  7 . A select line is connected to each programming device within the array of fuse cells  402 . The programming device  410  is coupled to the fuse  408  through diode  416  and to fuse  414  through diode  418 . By selecting a certain select signal and opening the correct multiplexer, a predetermined electrical fuse can be programmed when the high voltage source VDDQ provides the necessary current for the programming device. 
         [0026]    For example, if an electrical fuse  408  is to be programmed, a control signal will enter a programming device  410  through a select line  412  and command it to program the electrical fuse  408  by turning on, in this example, the NMOS transistor within the programming device  410 . Incoming signals will also appear at the fuse select line FS 0  commanding the multiplexer  404  to turn on. This opens up a path for the high voltage source VDDQ to provide the current necessary to break the electrical fuse  408 . Diode  418  prevents the programming current from affecting fuse  414 . 
         [0027]    By implementing fuse cells where two electrical fuses share one program device, and one or more unidirectional current devices isolate the programming current, the number of available fuses is effectively doubled without wasting extra silicon area for a larger number of large programming devices. Note that fuse cells used in such array is merely an embodiment of this invention and it can vary since it is not limited to only two fuses per cell. For example a 4-to-1 selection module such as a 4-to-1 multiplexer can be used to select one of four fuses to be programmed at any time. As such, each fuse cell can have four fuses sharing one programming device. Similarly, an x-to-1 selection module combined with a programming device can control “x” number of fuses to be programmed wherein “x” is an integer or most likely an even integer. 
         [0028]      FIG. 5  presents a schematic diagram  500  illustrating a two-dimensional array of 64 fuse cells  516  in accordance with yet another embodiment of the present invention. The array is set up in an 8-by-8 configuration. All 64 fuse cells are similar to the fuse cell  370  in  FIG. 3 , which allows two electrical fuses to share one programming device in each fuse cell. As shown, the 64 fuse cells are arranged in 8 rows and 8 columns. This two-dimensional array is controlled by various selection signals. With incoming signals from various Y-select lines (YSELBs)  518 , various write wordlines (WWLs)  520 , and fuse select lines FS 0  and FS 1 , an exact electrical fuse can be located for programming. Signals from YSELBs  518  can determine the column location of the fuse cell that contains the electrical fuse that needs to be programmed, while signals from WWLs  520  can determine the row location. The fuse select lines FS 0  and FS 1  control if either the multiplexer  522  or  524  will turn on. With the exact fuse cell selected and the correct multiplexer turned on, a specific electrical fuse can be located for the programming process. 
         [0029]    For example, if an electrical fuse  526  is to be programmed, a Y-select line YSEL 7 B will provide a low signal to turn on an input select device  528 , thus allowing a high input signal from a write wordline (WWL 0 ) to enter a programming device  530 . Meanwhile, the control signal from the fuse signal line FS 1  will turn on the multiplexer  524 , thus allowing the high voltage source VDDQ to provide the current necessary for the programming device  530  to program the electrical fuse  526 . While the current for programming electrical fuse  526  is applied to the cell, the diode  512  prevents a current from interfering with fuse  514 . 
         [0030]    Since the fuse cells  516  are implemented with both input and output select devices, it is possible to read the state of a certain fuse if the specific address of the fuse is provided. The RWLs  532  can provide a low signal to turn on the output select devices for a row of fuse cells  516 . The YSELBs  518  will provide the column location of the certain electrical fuse. With the row and the column location along with a proper signal from the fuse select lines FS 0  or FS 1 , a specific electrical fuse can be precisely located for in a read operation. The results from this reading process may exit through various read bit lines (RBLs)  534 . 
         [0031]    If the previously programmed electrical fuse  526  is to be read for its state, the read wordline RWL 0 B will input a low signal, thereby turning on the output select devices of an entire row of the fuse cells  516 , including the output select device  536 . The Y-select line YSEL 7 B will assist in locating the specific fuse cell by providing the column address of the fuse cell. The fuse select line FS 1  will also command the multiplexer  524  to turn on. Since the electrical fuse  526  has already been programmed due to previous programming, the high voltage supply VDDQ will be hard to reach a node  538 . As such, a read bit line RBL 7  will have a low output. The diode  512  prevents the high voltage supply VDDQ from interfering with fuse  514 . 
         [0032]      FIG. 6  illustrates yet another embodiment of a two-dimensional fuse cell array in accordance with another embodiment of the current invention. The array is set up in an 8-by-8 configuration. All 64 fuse cells are similar to the fuse cell  370  in  FIG. 3 , which allows two electrical fuses to share one programming device in each fuse cell. As shown, the 64 fuse cells are arranged in 8 rows and 8 columns. This two-dimensional array is controlled by various selection signals. With incoming signals from various Y-select lines (YSEL 0 B-YSEL 7 B), various write wordlines (WWL 0 -WWL 7 ) and fuse select lines (FS 0   a  and FS 0   b -FS 7   a  and FS 7   b ), an exact electrical fuse can be located for programming. The Signals YSEL 0 B-YSEL 7 B can determine the column location of the fuse cell that contains the electrical fuse that needs to be programmed, while signals from write wordlines WWL 0 -WWL 7  can determine the row location. The fuse select lines FS 0   a -FS 7   b  control if either the multiplexers MX 0   a -MX 7   b  will turn on. With the exact fuse cell selected and the correct multiplexer turned on, a specific electrical fuse can be located for the programming process. 
         [0033]    For example, if an electrical fuse  626  is to be programmed, a Y-select line YSEL 7 B will provide a low signal to turn on an input select device  628 , thus allowing a high input signal from a write wordline (WWL 0 ) to enter a programming device  630 . Meanwhile, the control signal from the fuse signal line FS 1  will turn on the multiplexer MX 7   b , thus allowing the high voltage source VDDQ to provide the current necessary for the programming device  630  to program the electrical fuse  626 . While the current for programming electrical fuse  626  is applied to the fuse cell, the diode  612  prevents a current from interfering with fuse  614 . Once programming is complete, the YSEL 7 B signal returns to a high signal turning on NMOS device  640  such that a low signal is applied to the programming device  630  ensuring the programming device  630  is shut off. 
         [0034]    Since the fuse cell  616  are implemented with both input and output select devices, it is possible to read the state of a certain fuse if the specific address of the fuse is provided. The read wordline WWL 0 -WWL 7  and YSEL 0 B-YSEL 7 B can provide a low signal to turn on the select devices for a row of fuse cells. The read select lines Rsel 0   a -Rsel 7   b  will provide the column location of the certain electrical fuse. With the row and the column location along with a proper signal from the fuse select lines FS 0   a  to FS 7   b  to turn off multiplexers MX 0   a -Mx 1   a , MX 0   b -MX 1   b , a specific electrical fuse can be precisely located for a read operation. The results from this reading process may be applied to various sense amplifiers, which amplify the signal present on the read line. The various sense amplifiers generate the signals Q 1 -Q 3  which exit the circuit, thus each sense amplifier s is responsible to amplify 4 out of a total of 16 outputs. 
         [0035]    If the previously programmed electrical fuse  626  is to be read for its state, the read wordline Rsel 7   b  will input a high signal, thereby turning on the output select device  642 . The Y-select line YSEL 7 B will assist in locating the specific fuse cell by providing the column address of the fuse cell. The fuse select line FS 7   b  will also command the multiplexer MX 7   b  to turn off. The signal on read bit line RBL 7   b  is coupled to the sense amplifier  646  through the output select device  642 . The sense amplifier  646  generates the signal Q 3  which exits the circuit. 
         [0036]    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. 
         [0037]    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.