Abstract:
A magnetic random access memory (MRAM) is compensated for write current shunting by varying the bit size of each MRAM cell with position along the write line. The MRAM includes a plurality of magnetic tunnel junction memory cells arranged in an array of columns and rows. The width of each memory cell increases along a write line to compensate for write current shunting.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to electronic devices, especially magnetic random access memories, and more specifically to magnetic random access memory arrays compensated for write current shunting.  
         [0003]     2. Description of the Related Art  
         [0004]     Magnetic random access memory (MRAM) typically is made up of memory cells that store information based on a direction of magnetization of a ferromagnetic region. MRAM cells can hold stored information for long periods of time, and are thus nonvolatile.  
         [0005]     MRAM memory generally takes the form of an array of intersecting, electrically-conductive column and row lines formed on a substrate. An MRAM memory cell is located at each intersection of the array.  
         [0006]     Various types of MRAM memory cells are known in the prior art. U.S. Pat. No. 5,640,343, issued Jun. 17, 1997 to Gallagher et al., discloses an MRAM device that utilizes an array of Magnetic Tunnel Junction (MTJ) memory cells. Each MJT MRAM memory cell includes a magnetic tunnel junction. As shown in  FIG. 7 , a magnetic tunnel junction  70  includes three functional layers: a free ferromagnetic layer  72 , a tunnel barrier  74 , and a fixed or pinned ferromagnetic layer  76 . The magnetization direction of pinned layer  76  is fixed by exchange coupling with an antiferromagnetic material. The magnetization direction of the free ferromagnetic layer  72 , in the absence of an applied field, is oriented along either a positive or a negative axis, as indicated by the double-headed arrow.  
         [0007]     Conductance of the MTJ memory cell is determined by the alignment of the magnetization in the fixed and free ferromagnetic layers. Accordingly, binary information can be stored in each memory cell based on the relative alignment of the magnetization direction in each layer. When the alignment is parallel (both magnetizations oriented in the same direction) the MTJ has a lower magnetoresistance, representing a logic 0, for example. When the alignment is antiparallel (the magnetizations oriented in opposite directions), the resistance of the MTJ is higher, representing a logic 1. The magnetoresistance is the result of spin-polarized tunneling of conduction electrons between the ferromagnetic layers. Measuring the magnetoresistance indicates the alignment, and thus the logical state, of the cell.  
         [0008]     Changing the orientation of the magnetization of the free ferromagnetic layer changes the logic state of the cell. The magnetization of the free layer will rotate in the presence of an applied magnetic field. The magnetic field is applied by way of an electric current flowing in the conductive word and bit lines. A current in both lines generates a magnetic field of sufficient strength to change the orientation state of the memory cell. The current of at least one of the lines has to be reversible to write the two different magnetic states of the cell.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention overcomes the deficiencies of the prior art by compensating for write current shunting in an MRAM array. Write current shunting is defined as the undesired leakage current that flows through an MRAM cell during a write operation, due to the finite resistance of the cell and the difference in the potentials of write conductors on opposite sides of the cell. According to a preferred embodiment, MRAM cells are arranged along a write line such that the cells progressively increase in physical size, from the high potential side to the low potential side. Write current requirements decrease with increasing cell size. Consequently, write current shunting is compensated by making cells larger at the end of the write line farthest from the high potential voltage input.  
         [0010]     According to one aspect of the invention, the array is designed so that the cell width increases monotonically from high to low potential side of the write line. According to another aspect of the invention, compensating for current shunt by distributing cell size in the array allows more bits to be provided on the same write line, thereby reducing the number of transistors in the array and increasing bit density. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a plan view of a magnetic random access memory device according to the present invention, illustrating current shunting;  
         [0012]      FIG. 2  is graph illustrating the effect of shunting on the write margin;  
         [0013]      FIG. 3  is a graph illustrating the relationship between switching field and MRAM bit width;  
         [0014]      FIG. 4  is a diagram illustrating an MRAM array according to the present invention;  
         [0015]      FIG. 5  is a graph illustrating the effect of bit size compensation on shunting current losses;  
         [0016]      FIG. 6  illustrates an alternative version of the present invention;  
         [0017]      FIG. 7  illustrates a microprocessor based system including an MRAM according to the present invention; and  
         [0018]      FIG. 8  illustrates a prior art magnetic tunnel junction memory cell.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     MRAM arrays, such as MTJ array  2  shown in  FIG. 1 , exhibit measurable amounts of write current shunting through the memory cells to adjacent lines, as illustrated by the arrows in  FIG. 1 . Consequently, the write current applied to the write conductors  4  drops with distance along a write conductor, from the high potential side to the low potential side (left to right as shown in  FIG. 1 ), due to the current shunting that takes place through the MTJ cell to adjacent read or write lines. The effective write field applied to an MRAM cell for a fixed value of I, applied in a write conductor thus decreases along the length of the conductor. As a result, bit yield is degraded in MTJ MRAM arrays.  
         [0020]     The effect is described with reference to  FIGS. 1-3 . Referring initially to  FIG. 1 , an MRAM array  2  is shown formed on a substrate  3 . The MRAM array is made up of a plurality of column and row lines, with a memory cell formed at each line junction. A voltage potential V is applied to a column line  4  having n memory cells representing n logic bits b i  (b 1  through b n ). When a write current I 1  is applied at the high potential side of column line  4 , an effective write current I n  is available at the low potential side of column line  4 . Write current I n  is less than I 1  (I n &lt;I 1 ) due to currents I s1 , I s2 , I s3  . . . I sn  which shunt through bits b 1  into the adjacent row line conductors Row i  (Row 1  through Row n ) used to select bits for writing.  
         [0021]     Referring to  FIG. 2 , the effect of shunting on the write current margin is illustrated graphically.  FIG. 2  represents the distribution of write currents along a column line that exhibits significant write current shunting. Line  6  represents write current distributions for selected bits, and line  8  represents write current distributions for unselected bits. An overlap of lines  6  and  8  is apparent between dotted lines  10  and  12 , representing the bit yield degradation due to unselected bits having the same write currents as selected bits.  
         [0022]     Referring to  FIG. 3 , the dependence of MRAM cell switching field (Hc) on bit width (W) is illustrated graphically. Line  14  shows a substantially linear inverse relationship between decreasing Hc/Hco 0  and increasing bit width W/W 0 .  
         [0023]     An MRAM array  20  according to a preferred embodiment of the present invention is illustrated in  FIG. 4 . Array  20  includes column lines  22 ,  24 ,  26 ,  28  and  30 , and row lines  32 ,  34 ,  36 ,  38  and  40 . Columns and rows are selected by line circuits  42  and  44 , respectively. At the intersection of each column and row line is an MTJ cell, MTJ cells in column  22  being designated  52 ,  54 ,  56 ,  58 , and  60 . The width of cells  52 - 60  increases along the length of column line  22  to compensate for shunt currents.  
         [0024]     Referring to  FIG. 5 , the effect of increasing bit width along the length of a write line on the distribution of write currents is shown graphically. Line  62  shows the distribution of currents for selected cells, and line  64  shows current distributions for unselected cells. Significantly, no overlap exists between lines  62  and  64 .  
         [0025]     Referring to  FIG. 6 , a variation of the present invention as applied in a memory array architecture  62  that includes floating read conductors, for example. A current I injected on write conductor  64  results in a lowest write current at the center of the write conductor (I 2 &lt;I 1 &lt;I). Accordingly, compensation for shunting is greatest at the center of the write conductor, as shown.  
         [0026]      FIG. 7  illustrates a microprocessor based system  70  in which an MRAM  80  according to the present invention is utilized. System  70  includes a CPU  72  and a user input/output (I/O) device  74  connected to a system bus  76 . System  70  also includes DRAM  78 . MRAM  80  communicates with the other components by way of system bus  76 . Other peripheral devices include a disk drive  82  and a CD ROM drive  84 .  
         [0027]     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. For example, although an exemplary embodiment of the present invention has been described in connection with an MTJ MRAM, the invention is not so limited, and the arrangement of different sized bits can be adjusted to compensate current shunting in any architecture. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.