Patent Publication Number: US-2004056703-A1

Title: Active well-bias transistor for programming a fuse

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field of the Invention  
       [0002] The invention relates generally to the field of semiconductor devices and, more particularly, to electric fuse programming.  
       [0003] 2. Description of Related Art  
       [0004] In integrated circuits including CMOS integrated circuits, it is often desirable to be able to permanently store information, or to form permanent connections of the integrated circuit after it is manufactured. Fuses or devices forming fusible links are frequently used for this purpose. For example, fuses can be programmed to replace defective elements with identical redundant elements. Further, fuses can be used to store die identification or other such information, or to adjust the speed of a circuit by adjusting the resistance of the current path.  
       [0005] A conventional circuit for programming a fuse is shown in FIG. 1 in which the fuse is connected between the drain of a programming transistor and a supply voltage VDD. Initially, the resistance of the fuse is small (in the range of 100 ohms). The fuse is programmed by switching ON the programming transistor with a pulse applied to the gate. When the transistor is turned ON, the transistor starts to conduct and current flows through the fuse. The current flow causes the fuse to heated-up, and if sufficient current continues to flows through it, it will get programmed by melting, electromigration or other mechanisms, resulting in a much higher resistance (i.e., programmed resistance).  
       [0006] The programming transistor must have the capacity to carry the current required to program the fuse. In order to achieve that capacity, the width of the programming transistor is appropriately chosen. For poly-silicide fuses, the peak current required for programming can be of the order of 10 mA or more. This order of current requires a wide programming transistor (in the range of 40 microns for a gate oxide thickness of 6.5 nm), hence the silicon area required for implementation can be significant even if the individual fuse element itself is small.  
       [0007] Exacerbating the problem is the fact that most integrated chips include multiple fuses such as the multiple fuse circuit shown in FIG. 2. In DRAM chips, for example, a few thousand fuses are generally used. The large size of individual programming transistors combined with the large number of transistors can be quite costly because of the silicon wafer area consumed. Hence, there is a need for an innovative approach for programming electrical fuses which results in wafer area reduction and/or increased programming current capability.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention achieves technical advantages as an apparatus, system and method of programming an electrical fuse using a transistor with active well bias. With the programming transistor operated with an active well bias, more energy is enabled for programming the fuse than is available without biasing the well for the same size transistor. Thus, a smaller transistor can be used for programming the fuse. In a multiple fuse embodiment, the programming transistors can be arranged in the same “well” with a common independent Vbias applied, via a body control circuit, to the entire well during programming of a select fuse.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein:  
     [0010]FIG. 1 illustrates a conventional circuit for programming a fuse;  
     [0011]FIG. 2 illustrates a conventional circuit for programming multiple fuse;  
     [0012]FIG. 3 illustrates a circuit diagram of a fuse structure using a programming transistor with active well bias in accordance with an exemplary embodiment of the present invention;  
     [0013]FIG. 3A illustrates a fuse structure with a cross section view of an nFET transistor with active well bias for fuse programming in accordance with an exemplary embodiment of the present invention;  
     [0014]FIG. 3B illustrates a fuse structure with a cross section view of an pFET transistor with active well bias for fuse programming in accordance with an exemplary embodiment of the present invention;  
     [0015]FIG. 4 shows a graphical representation of the operating characteristics of the programming transistor illustrated in FIG. 3 in accordance with an exemplary embodiment of the present invention;  
     [0016]FIG. 5 illustrates a circuit diagram of a multiple fuse structure arrangement using active well bias in accordance with an exemplary embodiment of the present invention;  
     [0017]FIG. 6 illustrates a circuit diagram of a fuse structure using a programming transistor with active well bias in accordance with another exemplary embodiment of the present invention; and  
     [0018]FIG. 7 illustrates a circuit diagram of a multiple fuse structure arrangement using active well bias in accordance with another exemplary embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION  
     [0019] The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses and innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others. Throughout the drawings, it is noted that the same reference numerals or letters will be used to designate like or equivalent elements having the same function. Detailed descriptions of known functions and constructions unnecessarily obscuring the subject matter of the present invention have been omitted for clarity.  
     [0020] Referring now to FIG. 3 there is illustrated a circuit diagram of an embodiment of the present invention in which an electrical fuse  31  is programmed using an nFET transistor  33  with an active well bias. With active well bias, a much higher programming energy is enabled as compared to transistors of the same size with the body grounded. Thus, a smaller transistor can be used for programming the same size fuse.  
     [0021] The body  36  of the transistor  33  is biased with Vbias independent of the supply voltage  38  and the gate control circuit  35  which operates to activate the gate  39 . For programming the fuse  31 , a programming voltage (VDD) is applied across the series coupled fuse  31  and transistor  33  of sufficient magnitude to operate the transistor  33  in its saturation region. The transistor body  36 , which is grounded in typical approaches, is then turned high to Vbias by operation of a body control circuit  37 . Cooperable with the body control circuit  37 , the gate  39  is then turned on with a voltage pulse (VGS) for the duration of fuse programming. After the programming operation is complete, the body bias can be brought back to ground. In one embodiment, values for the body bias are greater than zero and less than approximately 0.7 volts. In one embodiment, the fuse  31 , transistor  33 , body control circuit  37  and gate control circuit are integrated on a silicon chip.  
     [0022] Referring now to FIG. 3A there is shown a schematic representation of cross-section of the nFET device in an integrated circuit connected to a fuse. In this embodiment, a first end of the fuse  31  is coupled to the transistor drain  32  and the second end is coupled to VDD supply, the transistor source  34  is coupled to ground. For programming, Vbias is provided to the body  36  for actively biasing the p-well of the NFET where Vbias is greater than zero volts.  
     [0023] Referring now to FIG. 3B there is shown a graphical representation of an exemplary embodiment of the present invention in which the programming transistor  33  is an pFET. In this embodiment, a first end of the fuse  31  is coupled to the transistor drain  32  and the second end is coupled to ground, the transistor source  34  is coupled to VDD supply. For programming, Vbias is provided to the body  36  for actively biasing the n-well of the pFET in which Vbias is less than the programming voltage VDD.  
     [0024] Referring now to FIG. 4 there is shown a graphical representation of the operating characteristics of the programming transistor illustrated in FIG. 3 in accordance with an exemplary embodiment of the present invention. Here, the load-line of the fuse at two times, 0 and t, are shown as straight lines, R(0) and R(t). This change in the load line from time period 0 to t is due to the increase in the fuse resistance as the fuse gets heated-up during programming. This in combination with increasing Vbias results in increased energy coupled into the fuse. For example, a bias of approximately 0.6 volts enables an increase of as much as 30% over conventional approaches.  
     [0025] The effectiveness of fuse programming is dependent on the amount of energy that is coupled to the fuse during programming. The amount of power coupled into the fuse at time t for Vbias=0 is given by the area marked as item  43 . The amount of power coupled into the fuse at time t for Vbias&gt;0 is shown by the area marked as item  41  which includes the item  43  area. Thus, it can be seen that more energy is coupled to the fuse by operating the transistor with the body forward biased at any time t.  
     [0026] The present invention can also be used in multiple fuse arrangements. An exemplary multiple fuse arrangement is illustrated in FIG. 5. Here, the fuses  31  are coupled to the VDD supply  38  via a common node and the source  34  of each nFET is coupled to ground. Each gate  39  is coupled to a gate control circuit  35  and the bodies  36  are coupled to the body control circuit  37 . For programming a select fuse, the programming voltage (VDD) is applied across the series coupled fuse  31  and nFET. The body  36 , which is grounded in typical approaches, is then turned high to Vbias by operation of a body control circuit  37 . Using suitable logic, the gate control circuit  35  turns on the gate  39  of the nFET associated with the select fuse with a voltage pulse (VGS) for the duration of the fuse programming. After the programming operation is complete, the body bias can be brought back to ground.  
     [0027] The NFET transistors can be arranged in the same “well” with a common independent Vbias applied to the entire well during the programming of the fuse. The maximum number of transistors that may be placed in single well for this embodiment depends on the maximum current leakage due the forward bias of the well that can be tolerate for a specific application. In addition, the bank of transistors can be placed in a triple well, which is available with current technology, offering for improved isolation from other circuits when programming using active well bias.  
     [0028] Although the preceding description focused on programming using nFETs, the presently described approach can be implemented using pFETs. FIG. 6 shows a circuit diagram of an embodiment of the present invention in which an electrical fuse  31  is programmed using an pFET using an active well bias. Here, the fuse  31  is coupled between the drain  32  and ground. In one embodiment, VDD−0.7 volts&lt;Vbias&lt;VDD. In a pFET for example, the source is held at the highest potential (VDD). Vbias is selected to prevent the p+ source-n-well junction diode from turning ON when forward well bias is provided. Otherwise the leakage is disadvantageously high.  
     [0029]FIG. 7 shows a multiple fuse programming embodiment using the pFET arrangement shown in FIG. 6. As with the NFET transistors, the pFETs can be arranged in the same “well” with a common independent Vbias applied to the entire well during the programming of the fuse.  
     [0030] Although a preferred embodiment of the method and system of the present invention has been illustrated in the accompanied drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.