Abstract:
In an MRAM array based on MTJs, the size of segmented word line select transistors and their associated connections become a significant overhead, especially when the operating point is chosen deep along the hard axis of the asteroid curve. This problem has been overcome by placing the big segmented word line select transistors under the MTJ array and reducing the overall MRAM cell array down to a level comparable to a simple Cross Point MRAM array.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to magnetic random access memory (MRAM) cell array architecture and more specifically to arrays having segmented word lines where the word line programming current goes through only a segment of cells along the word line.  
       BACKGROUND OF THE INVENTION  
       [0002]     MTJs (Magnetic Tunnel Junctions) may be programmed at the intersections of Word Line and Bit Line currents in MRAM cells. The possibility of cells along the same word and bit lines being disturbed is, however, a major concern. A Segmented Word Line approach, as described in “Select Line Architecture for Magnetic Random Access Memories” (US Patent Application Publication: US 2002/0176272 A 1), eliminated disturb conditions for cells on the same word line outside the selected segment. When the operating point is chosen deep along the hard axis, the required bidirectional bit line currents to program the selected cells are significantly reduced. The possibility of a disturb along the bit line is also reduced. This is an ideal MRAM operating condition but the silicon area overhead due to the large size of the segmented word line select transistor makes widespread application impractical.  
         [0003]     In  FIG. 1 , a typical segmented word line array is shown. Since the word line programming current only goes through the selected Segmented Word Line Source, the selected word line segment and the word line segment select a transistor to Ground or a Segmented Word Line Return, any MTJs  11  outside this word line segment being unaffected by this programming current.  
         [0004]     In  FIG. 2 , the composite characteristics of MTJs within an array are shown on an asteroid chart as the shaded areas. With a segmented word line MRAM array, the word line current can be biased deep into the region where only very small bi-directional bit line currents are needed to program the MTJs. As an example, when a Word Line current is biased at point a and the bi-directional bit line currents are biased at points b and c, the margins to ensure programming of all bits within the array are shown as Im and the safety margins for not disturbing any bits along the bit lines are shown as Is.  
         [0005]     This mode of word line current biasing requires every bit within the selected word line segment to be programmed in one direction or the other. Otherwise, they will end up in random states. The Word Line bias current required in this condition is high, therefore the size of a Segmented Word Line Select Transistor will be a big overhead compared with the number of cells (or bit lines) within an selected segment.  
         [0006]     In a conventional approach, as seen in  FIG. 1 , each MTJ  11  is isolated by a transistor. This transistor has to be placed directly underneath the MTJ when laying out and constructing the MRAM array. Therefore, the big Segmented Word Line Select Transistor must be placed adjacent to the segment. This transistor and its associated connection area are a big overheads of the segmented word line approach. By using one single isolation transistor for many or all the MTJs within a word line segment, the isolation transistor (or transistors) will not take up all the area beneath the MTJs, leaving room for the Segmented Word Line Select Transistor. However, there are several undesirable effects of using a single isolation transistor for several MTJs.  
         [0007]     Since the sensing currents of all MTJs within a word line segment will flow through a single isolation transistor, the voltage on the Common Bottom Electrode node will vary depending on the resistance states (i.e. Data Pattern). The size of the Common Isolation Transistor needs to be large to reduce this effect. Another undesirable effect of using a single isolation transistor is the fairly big voltage difference, depending on bit line resistance and bit line programming current values, between bit lines in a big array during Write due to the bidirectional nature of Bit Line programming currents. This big potential difference between adjacent bit lines within a word line segment can damage MTJs and reduce programming current.  
         [0008]     A routine search of the prior art was performed with the following references of interest being found:  
         [0009]     U.S. Pat. No. 6,335,890 (Roehr et al) discloses global and local word lines where the global word lines are isolated from the memory cells, write lines and bit lines orthogonal, and a switch for each word line segment. U.S. Pat. No. 6,490,217 and U.S. Patent Application 2002/0176272 (DeBrosse et al) show 1 transistor for each MRAM element.  
         [0010]     U.S. Pat. No. 6,816,405 and U.S. Patent Applications 2004/0240265 (Lu et al) shows a local word line associated with each segment where the local word line is connected to a switch at one end and the a global word line at the other end. Local and global bit lines do not seem to be disclosed. U.S. Pat. No. 6,778,429 (Lu et al) also includes current sinks couplable to global word lines. U.S. Pat. No. 6,711,053 (Tang) discloses a switching device (transistor) for each MTJ.  
         [0011]     U.S. Patent Application 2004/0190360 (Scheuerlein) shows word line segments connected vertically without segment switching devices. U.S. Patent Application 2004/0165424 (Tsang) teaches segmented word lines and segmented bit lines.  
       SUMMARY OF THE INVENTION  
       [0012]     It has been an object of at least one embodiment of the present invention to provide a method for reducing the space overhead in a segmented MRAM array associated with the large size of the segmented word line select transistor  
         [0013]     Another object of at least one embodiment of the present invention has been to provide a circuit wherein said method has been implemented.  
         [0014]     A further object of at least one embodiment of the present invention has been to show how said circuit may be constructed.  
         [0015]     Still another object of at least one embodiment of the present invention has been to reduce the overall MRAM cell array down to a level comparable to a simple Cross Point MRAM array, while still retaining the benefits of a segmented array.  
         [0016]     These objects have been achieved by showing how to reduce silicon area overhead by using one common isolation transistor for all the bits within a segment, instead of one isolation transistor per bit. The resulting freed silicon area can be used for the segmented word line select transistor thus significantly reducing silicon area in the word line direction.  
         [0017]     The invention further teaches how to minimize the MRAM cell array in the bit line direction, such that the resulting MRAM array is comparable to the smallest Cross Point MRAM cell array while maintaining the high performance and ease of implementation characteristics of a one MTJ, one isolation transistor, MRAM cell array.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1 . A Segmented Word Line MRAM array of the Prior Art  
         [0019]      FIG. 2 . A Composite Asteroid Chart of MTJs within an MRAM array.  
         [0020]      FIG. 3 . A Highly Efficient Segmented Word Line MRAM Array Schematic.  
         [0021]      FIG. 4 . A plan view showing MTJ, Local Word Line, Global Bit Line, Local Bit Line, and Common Bottom Electrode.  
         [0022]      FIG. 5 . Cross Section  5 - 5  of  FIG. 4  (Global Bit Line on Top)  
         [0023]      FIG. 6 . Cross Section  6 - 6  of  FIG. 4  (Global Bit Line on Top)  
         [0024]      FIG. 7 . Cross Section similar to  5 - 5  but with the Global Bit Line under the MTJs  
         [0025]      FIG. 8 . Cross Section similar to  6 - 6  but with the Global Bit Line under the MTJs  
         [0026]      FIGS. 9A  and B Silicon Level layouts to minimize the cell pitch in the bit line direction.  
         [0027]      FIGS. 10A  and B Interconnect layers between MTJ related layers and Silicon Level layers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The prior art problem of the silicon area overhead due to the large size of the segmented word line select transistor has been solved by introducing separate Programming Bit Lines, which do not themselves contact the MTJs. Since the bottom sides of all MTJs within a word line segment are connected, leakage currents between Bit Lines can be significant in a large array that is in Read Mode. Connecting only a segment of the Local Bit Lines to the Global Bit Lines minimizes this leakage current. Segmented Word Line arrays often require separate Read Word Line and Write Word Line Busses to avoid damaging the MTJ during Write. By isolating the topside of the MTJ from the Write Bit Line, the Read Word Line and Write Word Line signals of a conventional segmented word architecture can be combined into a single Read Word Line/Write Word Line.  
         [0029]     These ideas and concepts are embodied  FIG. 3  to which we now refer. During the Program Cycle, only one of the Local Word Line segments carries the Word Line Programming current which is controlled by selecting one of the RWL/WWL (Read Word Line/Write Word Line)s  31  and one of the Global Word Line Sources  32 .  
         [0030]     Bidirectional Bit Line currents through one set of Global Bit lines  33  will program the bits at this intersection. All Bit Line Segment Select Transistors  34  are OFF, isolating Local Bit Lines  35  from Global Bit Lines  33 .  
         [0031]     During the Read Cycle, only one of the Read Word Line/Write Word Lines  31  is selected. All Global Word Line Sources  32  are turned off (at GND level). One set of Bit Line Segment Selects  36 , in which the selected RWL/WWL is located, is selected. The Global Bit Lines  33  within the selected word line segment are connected to (or further decoded and then connected to) sense amplifiers (not shown).  
         [0032]     This array can be implemented in two ways. A plan view (excluding the underlying semiconductor components and interconnects) is shown in  FIG. 4 . Two cross sections,  5 - 5  and  6 - 6 , illustrate exemplary bottom structures. Cross Section  5 - 5  is cut through the MTJ related layers and the drains/sources of the MOSFETs while cross section  6 - 6  avoids the MTJs to show the Global Bit Lines  33 , Local Bit Lines  35 , and Metal 1, used as GND buses that run normal to the plane of the figure. It can be shown that the structures below the MTJ-related layers may be optimized separately, with many variations possible. The two cross sections shown here serve only as examples of structures that are possible beneath the MTJ-related layers and how they might relate to one another. They are not necessarily complete in every last detail.  
         [0033]     In  FIGS. 5 and 6 , the Global Bit Line  33  is above the MTJs, Local Bit Lines  35 , and Local Word Lines  37  (seen in  FIG. 3 ).  
         [0034]     In another embodiment ( FIGS. 7 and 8 ), the Global Bit Line  33  is below the MTJs  11  as well as Common Bottom Electrode  39  to which is connected common isolation transistor  41  (see  FIG. 3 ). Thus the invention also eliminates the one contact per cell from the bottom MTJ electrode to the isolation transistor.  
         [0035]     The pitch along the bit line direction can therefore be reduced to the MTJ pitch, limited as a Cross Point MTJ MRAM cell, assuming that the underlying silicon level (MOSFETs and metal 1), and the required interconnects buses for this MRAM, can be constructed within this pitch.  
         [0036]     Based on the same generation lithography rules for MTJs and production CMOS, two embodiments for the silicon level layout are shown in  FIGS. 9A and 9B . In  FIG. 9A  the Silicon Level Layout shows how 8 MOSFETs (T1-T8) could be fitted into 4 segmented word line pitches  91 ; each word line segment will require two MOSFETs (one for the isolation transistor  41  and one for the Segmented Word Line Select Transistor  38 ). This layout normally results in minimum silicon level pitch in the direction of the bit line. It is normally smaller than 4 pitches of the MTJs.  
         [0037]      FIG. 9B  is another embodiment for fitting 8 MOSFETs in 4 pitches. In this embodiment the pitch  92  is larger than in  FIG. 9A , but the MOSFET width is doubled, therefore their ON resistance is reduced proportionally. Since both the Word Line Segment Select Transistor and the Isolation Transistor require low ON resistance, this becomes an ideal implementation.  
         [0038]     There is another pitch that could be the limiting factor of this MRAM array. That is the VIA that is needed to connect the Bottom Electrode of the MTJs and Local Word Lines to the drains of MOSFETs and the metal buses needed to shunt RWL/WWL. In  FIG. 10A . two rows of VIAs  100  are shown with 4 metal buses  101  for 4 segmented word line pitches  102 .  
         [0039]      FIG. 10B  shows two rows of VIAs  106  and 2 layers of 2 metal buses  105  for 4 segmented word line pitches  107 . In this latter embodiment, the total pitch is smaller, but 2 layers of interconnecting metal are needed.  
         [0040]     The MTJs, silicon level, and VIA/interconnect layers can be optimized individually and separately making it possible to design a high performance MRAM array, having the same order of areal efficiency as a Cross Point cell array and its associated benefits, of segmented word line MRAM cells with isolation transistors  41 .