Patent Publication Number: US-6335892-B1

Title: Method to electrically program antifuses

Description:
This application is a divisional of application Ser. No. 09/359,369, filed Jul. 23, 1999, and as such claims priority to the copending application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of integrated circuits. More specifically, it relates to a method for programming antifuses in dynamic random access memory (DRAM) using a signal from a global column decoder and a programming line with which the antifuse is paired. 
     2. Description of the Related Art 
     Typical DRAM circuits include arrays of memory cells arranged in rows and columns. Each of the rows and columns are driven by a respective row decoder and column decoder. Typically, these memory circuits include several redundant rows and columns that are used as substitutes for defective locations in the memory array. For example, rather than having 1024 columns (i.e., 2 10  columns), a typical array might have 1028 columns, along with 1028 column decoders. 
     As is depicted in FIG. 1, normal practice is for the column decoders CD 1 , CD 2 , CD 3 , CD 4  . . . CD N  to receive memory cell addresses (e.g., during a normal read or write operation) coming from external sites (e.g., external pads). Before the addresses reach the column decoders, they may be passed through one or all of a series of buffers, and/or redundancy decoders. The addresses then reach the column decoders where they are cascaded through the column decoders until the address data reaches the one particular column decoder that matches the incoming address. Each of the column decoders is also coupled to a corresponding Y-gate driver, or inverter I 1 , I 2 , I 3 , I 4  . . . I N . During a normal read or write operation, only one of the column decoders (i.e., only one of 1028) is set high at a time. That is, when the address is fully decoded as corresponding to a particular column decoder, that column decoder is set logic high (e.g., 1) while all others remain logic low (e.g., 0). 
     Typically, the logic high signal of the column decoder is sent across the entire memory array  10  (e.g., which may consist of as many as 16 sub-array blocks tiled side by side) where it is coupled to sense amplifiers (not shown) between every block. 
     When a defective memory array location has been identified, a repair is effected. Rather than treating the entire array as defective, one of the redundant rows or columns are substituted for a defective row or column. This substitution is performed by assigning the address of the defective row or column to the redundant row or column and decoding the address with a redundant column decoder such that, when an address signal corresponding to a defective row or column is received by one of said column decoders CD 1 , CD 2 , CD 3 , CD 4  . . . CDN, and the redundant column decoder the redundant row or column is addressed instead: This makes the substitution of the redundant row or column substantially transparent to a system employing the memory circuit. 
     An example of fuse blowing is found in the programming of sense lines. Fusebank address detection circuits employ a bank of sense lines where each sense line corresponds to a bit of an address. The sense lines are programmed by blowing fuses in the sense lines in patterns corresponding to the address of the defective row or column. 
     Traditionally, the fuses have been blown by having a laser cut the fuse conductors to remove the conductive paths through the fuses. One problem with such an approach is that the laser cutting of the fuses is time consuming, difficult and imprecise. Therefore, the cost and reliability of memory devices employing laser fuse bank circuits can be less than satisfactory. 
     More recently, memory devices have been employing antifuse banks in place of conventional fuses. Antifuses are capacitive-type structures that, in their unblown states, form open circuits. Antifuses may be blown by applying a high voltage across the antifuse. The high voltage causes the capacitive-type structure to break down, forming a conductive path through the antifuse. Therefore, blown antifuses conduct and unblown antifuses do not conduct. 
     While redundant repair is one goal associated with the blowing of antifuses, as is known in the art, antifuses may be blown e.g., to change the state of another signal associated with a bad column, or to lessen defect currents, etc. 
     Typically, the current practice is to locate antifuse banks at a periphery of a memory array block where separate antifuse electrical decode circuitry is require in order to determine which particular antifuse is to be programmed. The separate antifuse decoder circuitry comprises a second set of address decoding circuitry (i.e., in addition to the already existing column decoders) for determining which antifuse is to be programmed and eventually blown. 
     The separate antifuse decoder circuitry, located adjacent to the antifuse bank and also at a periphery of the memory array, receives address data from the address lines that are coupled to the global column decoders. That is, the separate antifuse address decoder circuitry receives the same address information as received by the global column decoder which has been selected (e.g., during normal read or write operations) only to redundantly perform the address decode operation again so as to determine which antifuse is to be programmed. 
     For example, assuming a typical memory array capacity of 64Mb; such an array comprises 512 global column decode lines, which correspond to 9 address bits that require decoding (i.e., 2 9 =512). Therefore, when the antifuse decoding is performed within the antifuse bank itself, by a local address decoder, a total of 18 address bit lines (i.e., 9 ADDRESS lines and 9 {overscore (ADDRESS)} lines) must be routed from an address decoding section of the DRAM to a periphery of the DRAM (where such antifuse banks are ideally located) where the addresses are locally decoded within the antifuse bank. After the address lines are locally decoded, the blowing of a particular antifuse (as determined by the address decoder) is enabled. This extraneous routing of the address lines severely complicates integrated circuit (IC) design, thereby resulting in larger die-size and increased manufacturing costs. 
     Therefore, rather than taking advantage of existing IC designs and the logic high signal (e.g., 1) that is already being produced by the column decoders, the programming of antifuses associated with, e.g., column repair, column preconditioning, etc., are typically performed as described above. Thus there exists a need for a system and method for programming antifuses, thereby selecting the antifuses to be blown, without requiring additional circuitry in the form of a local address decoder at the antifuse bank, and without requiring the extraneous routing of address lines to a periphery of a DRAM. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems associated with the prior art and provides a system and method for programming antifuses without requiring additional address decoder circuitry at the antifuse bank and without requiring the routing of address lines to a periphery of a DRAM. 
     In accordance with a preferred embodiment of the present invention, a separate antifuse is provided for each global column decoder. Therefore, setting only one column decoder to logic high (e.g., for each read or write cycle, or for an address entered by an operator through the pad during preconditioning) not only activates the corresponding column of the memory cell array, but also provides the enable signal to program a corresponding antifuse sitting on that same global select program line. That is, when a column decoder is set logic high, the particular antifuse sitting on that same line receives the same logic high signal, thereby indicating that that particular antifuse has been selected to be blown. Accordingly, a “pop voltage” (i.e., the voltage required to blow the antifuse) is routed to that particular antifuse from a source that is external to the antifuse bank. 
     The present invention provides a method and system for programming antifuses, e.g., as used in connection with DRAMs, without requiring additional address decoder circuitry at the antifuse bank. The invention utilizes already existing DRAM address decoder circuitry to effectively program a particular antifuse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention that is provided in connection with the accompanying drawings in which: 
     FIG. 1 illustrates a series of column decoders located adjacent to a typical DRAM memory cell array; 
     FIG. 2 illustrates the FIG. 1 configuration with each column decoder being coupled to a respective antifuse cell, in accordance with a preferred embodiment of the invention; and 
     FIG. 3 illustrates a detailed description of an antifuse cell, in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described with reference to FIGS. 2-3. Other embodiments may be realized and structural, or logical changes may be made to the disclosed embodiment without departing from the spirit or scope of the present invention. 
     FIG. 2 depicts a series of column decoders CD 1 , CD 2 , CD 3 ,CD 4  . . . CD N  located adjacent to a DRAM memory cell array  10  with each of the column decoders being coupled to a respective column. Located between each column decoder and its corresponding column is an inverter I 1 , I 2 , I 3 , I 4  . . . I N . In addition, each column decoder is coupled to a respective antifuse cell AF 1 , AF 2 , AF 3 , AF 4  . . . AF N . In accordance with a preferred embodiment of the invention, the signal that is present on a global y-select line GYS 1 , GYS 2 , GYS 3 , GYS 4  . . . GYSN (e.g., high or low) is forwarded to a respective antifuse cell to which it is coupled. That is, the global y-select lines GYS 1 , GYS 2 , GYS 3 , GYS 4  . . . GYS N  act to designate which antifuse is to be decoded (i.e., blown) rather than having a separate antifuse decoder circuit at the antifuse bank for decoding a particular antifuse. Once a particular antifuse has been selected to be blown, a pop voltage is routed to that particular antifuse in order to effectuate the blowing of the antifuse. 
     Turning now to FIG. 3, a detailed description of an antifuse cell  30  is depicted, in accordance with a preferred embodiment of the invention. Each antifuse cell  30  contains an antifuse  32 , a first side of which is coupled to a source of a metal-oxide-semiconductor field-effect transistor (MOSFET)  38 , and is also coupled to a source of MOSFET  42 . A drain of MOSFET  38  is coupled to ground and a gate of MOSFET  38  is coupled to a respective global y-select line GYS 1 , GYS 2 , GYS 3 , GYS 4  . . . GYS N  of a column decoder CD 1 , CD 2 , CD 3 , CD 4  . . . CDN that corresponds to this particular antifuse cell  30 . 
     A gate of MOSFET  42  is coupled to an inverter  40  which inverts the signal that is present on the PROGRAM line. A drain of MOSFET  42  is coupled to a source of MOSFET  44 . A gate of MOSFET  44  is coupled to ground and a drain of MOSFET  44  is floating. A drain of MOSFET  42  is also coupled to a source of MOSFET  46  and an input of inverter  48 . A drain of MOSFET  46  is floating. An output of inverter  48  is coupled to a gate of MOSFET  46 . An output of the inverter  48  is also the output signal of the antifuse cell  30 . 
     Coupled to the antifuse cell  30  is MOSFET  34  and MOSFET  36 . A source of MOSFET  34  is coupled to a second side of the antifuse  32 . A drain of MOSFET  34  is coupled to ground. A gate of MOSFET  34  is coupled to an inverted program line (i.e., {overscore (PROGRAM)}) A gate of MOSFET  36  is coupled to the PROGRAM line and a source of MOSFET  36  is coupled to the pad where e.g., an external pop voltage source is located. Furthermore, the PROGRAM lines may be coupled together via connector  50  and similarly, the {overscore (PROGRAM)} lines may be coupled together via connector  52 . 
     The PROGRAM line comes from a PROGRAM circuit that outputs a PROGRAM signal (e.g., a high signal) when the DRAM is in antifuse programming mode. That is, when the DRAM is in an antifuse programming mode (e.g., when designating a redundant column, or during preconditioning), a corresponding PROGRAM signal is sent to the gate of MOSFET  36 . Additionally, {overscore (PROGRAM)} is sent to the gates of MOSFETs  42  and  34 . 
     Under normal operation, in accordance with a preferred embodiment of the invention, the pop voltage sits on the pad terminal of the source of MOSFET  36 . When the DRAM is in the program mode, and a PROGRAM signal is set e.g., high, the pop voltage also sits on the second side of antifuse  32 . When the global y-select line GYS 1 , GYS 2 , GYS 3 , GYS 4  . . . GYSN (of FIG. 2) that corresponds to this particular antifuse cell  30  is also set e.g., high, the second side of antifuse  32  is effectively coupled to ground, via MOSFET  38 , and the difference of potential between the plates of the antifuse  32  is sufficient for that antifuse  30  to blow. The output OUT of the antifuse cell  30  may then be utilized for its usual applications as are well known in the art. 
     Moreover, since MOSFETs  34  and  36 , and the pad voltage are not part of the antifuse cell  30 , the MOSFET pair of  34  and  36  and the pad voltage may be joined and utilized as described above for more than one antifuse cell  30 . That is, while each antifuse cell  30  has one antifuse  32 , one inverted program signal and one respective global y-select line GYS 1 , GYS 2 , GYS 3 , GYS 4  . . . GYSN at its input, several antifuse cells  30  (or all 1028 antifuse cells) may share the MOSFET pair of  34  and  36  and the voltage pad. 
     The present invention provides both a system and method for programming antifuses without requiring a local address decode operation at the antifuse bank and without requiring address lines be routed to a periphery of a DRAM. Therefore, as a result of the invention, DRAM designs are greatly simplified since there is no need for a local address decoder and corresponding address lines running from the primary address decoders, at the column decoders, to a secondary address decoder at the antifuse bank. 
     While a preferred embodiment of the invention has been described and illustrated, it should be apparent that many modifications can be made to the invention without departing from its spirit or scope. For example, although the invention is depicted as being used for decoding antifuses, the invention may be used in order to decode any other programmable elements or components. In addition, while a particular circuit diagram employing specific electronic components is depicted for practicing die invention, it should be readily apparent that many other combinations of the same or similar components may be substituted for those described above, while still providing a method and system for programming antifuses by using existing circuitry, and without deviating from tie spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of die appended claims.