Abstract:
The invention comprises a magnetic random access memory (MRAM) with stackable architecture. A first word line is configured to carry electric current. A first memory column is electrically coupled to the word line and is comprised of a plurality of memory cells electrically coupled and adjacent to each other. Each memory cell is configured to store data by magnetic alignment of the memory cell. A first bit line column is electrically isolated from the first word line and is magnetically coupled to and electrically isolated from the first memory column. The first bit line column comprises a plurality of bit lines that are electrically isolated from each other and configured to carry electric current during a memory read and a memory write. The first bit line column is parallel to the first memory column.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
         [0003]     REFERENCE TO A “SEQUENTIAL LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISC (SEE 37 CFR 1.52(E)(5))  
         [0004]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0005]     1. Field of the Invention  
         [0006]     The invention relates generally to magnetic random access memory (MRAM). Specifically, the invention relates to a high-density memory architecture comprising a vertical stack of magnetic storage elements.  
       DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR 1.97 AND 1.98  
       [0007]     MRAM devices typically consist of a planar arrangement of memory cells with two magnetic layers separated by a tunnel junction. One of the magnetic layers is a fixed reference layer while the other layer is a storage layer having a magnetic polarization that is altered for storage. The storage layer can be oriented along one of two directions along a magnetic uni-axial anisotropy axis approximately parallel or anti-parallel to the magnetization of the reference layer.  
         [0008]     A memory write to a memory cell aligns the storage layer in either the parallel or the anti-parallel position with respect to the reference layer. A memory read determines the resistance of the memory cell being read and determines the alignment of the storage layer based on the resistance of the memory cell. Then the “value” of the memory cell is known.  
         [0009]     One problem with the prior art is that it is difficult to manufacture MRAM cells and they require a significant amount of space, therefore yielding a low MRAM density. Furthermore, memory write requires a narrow distribution of switching fields in order to avoid writing of half selected bits or writing adjacent bits due to crosstalk. Memory read is usually performed by comparing the resistance of the cell being read to a reference cell, again requiring a relatively tight tolerances of cell resistance values across the memory chip. MRAM are therefore difficult to manufacture and have low density.  
         [0010]     What is needed is high-density MRAM that is easy to manufacture and provides good selectivity of memory cells. The invention should reduce the area required by memory, be easily manufactured by lowering the margin requirements for memory resistance, provide improved selectivity of memory cells, and be scalable.  
       SUMMARY OF THE INVENTION  
       [0011]     The invention comprises a magnetic random access memory (MRAM) with stackable architecture. A first word line is configured to carry electrical current. A first memory column is electrically coupled to the word line and is comprised of a plurality of memory cells electrically coupled and adjacent to each other. Each memory cell is configured to store data by magnetic alignment of the memory cell. A first bit line column is electrically isolated from the first word line and is magnetically coupled to and electrically isolated from the first memory column. The first bit line column comprises a plurality of bit lines that are electrically isolated from each other and configured to carry electrical current during a memory read and a memory write. The first bit line column is parallel to the first memory column.  
         [0012]     The advantages of the invention include a reduced MRAM area achieved by reducing the number of word lines, reducing the number of switches (for example, transistors and diodes) per memory cell and improving the geometry of the bit line/memory cell relationship, improved simplicity in the manufacturing process, improved selectivity, and increased memory density. For example, the invention may apply an eight-layer design with a cell size of only 1 F2. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
       [0013]      FIG. 1  is a schematic illustrating a cross section of one embodiment of the invention.  
         [0014]      FIG. 2  is a schematic illustrating a plan view of one embodiment of the invention from  FIG. 1 .  
         [0015]      FIG. 3  is a diagram illustrating one embodiment of the invention with out-of-plane magnetization.  
         [0016]      FIG. 4  is a diagram illustrating bit lines, magnetic fields and a memory cell.  
         [0017]      FIG. 5  is a diagram illustrating the storage and readout layers of a memory cell.  
         [0018]      FIG. 6  is a diagram illustrating one embodiment of the invention with in-plane magnetization.  
         [0019]      FIG. 7  is a diagram illustrating a sensor bit line making and process flow for the invention.  
         [0020]      FIG. 8  is a flow diagram illustrating one method of executing memory write.  
         [0021]      FIG. 9  is a flow diagram illustrating one method of executing memory read. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The layering processes and materials used in the following magnetic memory cells are well known in the material processing art. Although specific embodiments have been described, one of ordinary skill in the art will recognize that other materials and other layering processes than those described may be used in accordance with the invention.  
         [0023]      FIG. 1  is a schematic illustrating a cross section of one embodiment of MRAM  100 . MRAM  100  includes electrically conducting word line  102  connected to memory columns  104 . Each memory column  104  consists of one or more memory cells  106  stacked on top of each other and electrically connected.  
         [0024]     Memory cell  106  is a magnetic tunnel junction (MTJ) composed of two layers: a storage layer and a readout layer. Both layers are magnetic with either in-plane magnetization or out-of-plane magnetization. The storage layer has a higher coercivity than the readout layer. Layers with in-plane magnetization may be made with, for example, approximately Ni 80 Fe 20 , Co, or CoFe alloys. Layers made with out-of-plane magnetization may be made with, for example, Co/Pt multilayers or Rare-Earth Transition Metal alloys. Although four memory cells  106  are illustrated in each memory column  104 , fewer or more memory cells may be included.  
         [0025]     Switch  108  is opposite word line  102  on memory column  104 . Switch  108  turns on one or more of memory columns  104  during an operation. Word line  102  carries electrical current that flows through, for example, memory column  104 - 1  when switch  108 - 1  is activated. One aspect of the invention is that only one switch  108  is needed for multiple memory cells  106 . In one embodiment, switches  108  are transistors. In the prior art, each memory cell typically has one transistor per memory cell.  
         [0026]     Bit line columns  110  are positioned alongside memory columns  104 . Although  FIG. 1  illustrates a bit line column between each memory column, one skilled in the art will recognize that fewer bit line columns may be used in the invention. Bit line columns include bit lines  112 . Bit lines  112  are conductors that carry electric current in order to generate a magnetic field that switches the polarity of the storage and readout layers in memory cells  106 . Positioning bit lines  112  on the sides of memory cells  110  reduces masking layers and process steps during memory fabrication. The invention is applicable to both 1T1MTJ and 1D1MTJ architecture to achieve 1TnMTJ and 1DnMTJ respectively.  
         [0027]     In one embodiment, MRAM  100  is fabricated on a substrate which conta.  
         [0028]     ins transistors  108  for addressing memory columns  104  and peripheral circuitry providing power, sensing, address registers, etc. (not shown). A silicon wafer may be used as a substrate, however, other materials with appropriate electrical and thermal properties may also be used as a substrate. The substrate must be such that the address transistor  108  and all the peripheral electronics can be built in it. It should have sufficient thermal conductivity to dissipate the heat produced by the MRAM cells. Other materials include SiO, SiC, polySi.  
         [0029]      FIG. 2  is a schematic illustrating a plan view of one embodiment of the invention from  FIG. 1 . MRAM  200  has word lines  210  connected to memory columns  220 . Bit lines  230  are perpendicular to word lines  210  and stacked as illustrated in  FIG. 1 . Each word line  210  activates several memory columns  220 , according to the design and architecture of MRAM  200 .  
         [0030]      FIG. 3  is a cross-sectional diagram illustrating one embodiment of the invention with out-of-plane magnetization. MRAM  300  includes word line  305  connected to memory columns  310  with memory cells  315 . Connected opposite word line  305  are switches  320 . Bit line columns  325  include bit lines  330 .  
         [0031]     In one embodiment, memory cells  315  and bit lines  330  in MRAM  300  are composed of several layers. Element  340  illustrates the layers. Layers  345  may include Ni 80 Fe 20  that is approximately 30 nm thick. Top and bottom NiFe layers  345  may be included as cladding layers to avoid leakage of magnetic field above and below the bit lines when activated. One purpose of layers  345  is to concentrate the flux on either side of the bit lines  330 , rather than above and below.  
         [0032]     Layer  350  is a multilayer of (CuTa), shown as 4 repeating layers in  340 , which may be included to adjust the conductivity of bit lines  330 . The Cu and Ta of each layer may be 10 nm and 5 nm thick respectively.  
         [0033]     Layer  355  is a multilayer (4 repeats) of (CoPt), which is the storage layer with higher coercivity (harder) than the readout layer. The Co may be approximately 0.5 nm thick and the Pt may be approximately 2 nm thick.  
         [0034]     Layer  360  is a multilayer (2 repeats) of (PtCo), which is the readout layer with lower coercivity (softer) than the storage layer. The Co may be approximately 0.5 nm thick and the Pt may be approximately 2 nm thick. In CoPt multilayers, the coercive field can be adjusted by varying the thickness of the layers and number of repeats. Generally, the coercive field increases with the number of repeats. One example of values of the coercive field for the readout and storage layers is 20 Oe for the readout layer and 60 Oe for the storage layer. The magnetic polarization of both the readout and storage layers are aligned during a memory write, while the magnetic polarization of only the readout layer is switched during a memory read.  
         [0035]     Layer  365  is Al 2 O 3 , which is an insulating layer that forms a tunnel barrier between layers  355  and  360 . The Al 2 O 3  may be approximately 1 nm thick. Layer  365  may be formed, for example, by depositing about 0.8 nm of metallic aluminum and oxidizing it with plasma or natural oxidation. The other layers in element  340  may be deposited by sputtering. Elements  340  making up memory cells  315  may be connected by copper, aluminum, or other conductors. Elements  340  making up bit lines  330  may be connected by insulators, for example SiO 2 , Al 2 O 3 , or other oxides. One skilled in the art will recognize that memory cells  315  and bit lines  330  need not be fabricated in the same way or with the same material as each other, however manufacturing is simplified if they are the same.  
         [0036]     For out-of-plane magnetization, the magnetic storage and readout layers may be made from Co/Pt multilayers, CoFeNi/Pt multilayers, Co/Pt alloys, Co/Pd multilayers, Co/Pd alloys, CoFeNi/Pd multilayers, Co/Au multilayers, CoFeNi/Au multilayers, Co/Ni multilayers, Ni/Cu multilayers or rare earth-transition metal alloys. If heating is used as a select method (see below), the storage layer can be exchange biased with an antiferromagnetic layer with low blocking temperature, for example Ir 20 Mn 80  with a thickness of 6 nm.  
         [0037]     Layer  370  between the memory cells may be formed from vapor-deposited or sputtered Cu, which facilitates current transmission through the memory columns. Layer  370  is approximately 100-300 nm thick. The thickness of layer  370  may vary according to selectivity desired between the memory cells. Greater space between memory cells will help selectivity using a single bit line, while less space between memory cells is needed when using two bit lines for selection. Although the various layers have a specific purpose for memory cells, they are not as relevant in the bit lines, where storage does not occur. Rather, bit lines should contain material that will carry current in order to select the memory cells. Layering the bit lines with the same material as the memory cells simplifies manufacturing of the MRAM.  
         [0038]      FIG. 4  is a diagram illustrating bit lines, magnetic fields and a memory cell. One method for performing a memory write, or storing data within MRAM  300  for out-of-plane magnetization, follows for memory cell  400 . A memory write operation for in-plane magnetization differs and will be described below. Electric current flowing in opposite directions is feed to bit lines  410  and  420 . Current in bit line  410  flows perpendicular and into the plane of  FIG. 4  while current in bit line  420  flows perpendicular and out of the plane of  FIG. 2 . Because the current is in opposite directions, the magnetic field around each of bit lines  410  and  420  are in opposite directions. The magnetic field around bit line  410  is clockwise while the magnetic field around bit line  420  is counter clockwise.  
         [0039]     Memory cell  400  is selected by powering the associated word line and selecting the memory column to which memory cell  400  belongs by turning on the appropriate switch.  
         [0040]     Current flowing through a memory cell reduces the switching field by one of several effects. First, thermal heating of the storage layer due to current flowing through the tunnel barrier causes the switching field to decrease. Heating of memory cell  400  makes it possible to align the magnetic polarity in the storage and readout layers with the cumulative effect of magnetic fields around bit lines  410  and  420 . Pulses of current through bit lines  410  and  420  can be adjusted so that only the heated junction switches, rather than other memory cells which are at a standby temperature.  
         [0041]     Furthermore, the Oersted field due to the vertical current flow through the memory column of memory cell  400  reduces the switching field because it favors the formation of an in-plane vortex state that, due to cylindrical symmetry, helps the reversal of magnetization.  
         [0042]     Finally, injection of spin-polarized electrons from the readout layer or from a polarizing additional layer into the storage layer can be used to decrease the switching field of the storage layer magnetization.  
         [0043]     Memory cell  400  must be far enough from bit lines  410  and  420  so that current through bit lines  410  and  420  does not leak into memory cell  400 , and close enough for a magnetic field around bit lines  410  and  420  to affect the magnetic polarity of memory cell  400 . In one embodiment, memory cell  400  is approximately 100 nm from bit line  410 . Typically, current through bit lines  410  and  420  (and word lines, see  FIG. 1 ) ranges from 1-5 mA. One skilled in the art will recognize that distances between memory cells and bit lines changes relative to the state of the art and is not a limiting factor of the invention. Decreasing the distance between memory cells increases the density, which is usually a goal in memory design. Also, current levels may differ depending on the particular application.  
         [0044]      FIG. 5  is a diagram illustrating the storage and readout layers of a memory cell with out-of-plane magnetization. After a memory write, memory cell  500  has the magnetic polarization for both readout layer  510  and storage layer  520  aligned in the same direction. Readout layer  510  is separated from storage layer  520  by dielectric layer  530 , which creates a tunnel junction between readout layer  510  and storage layer  520 . Although  FIG. 5  illustrates the readout layer on top, either the readout layer or the storage layer may be on top or bottom. Also, storage values of “1” and “0” may be arbitrarily assigned to storage layer  520  being in the up (parallel) and down (anti-parallel) position.  
         [0045]     One method a performing a memory read, or retrieving data from a memory cell, follows for memory cell  106 - 2 , assuming out-of-plane magnetization. One problem with connecting multiple memory cells between a single word line and a transistor is that it is more difficult to detect the resistance of individual memory cells and therefore the polarization of the storage layer within the memory cell. The invention overcomes this difficulty with differential readout.  
         [0046]     In a manner similar to memory write, counter-directed current flows through each of bit lines  112 - 1  and  112 - 2  (also see  FIG. 4 ). The strength of the magnetic field through memory cell  106 - 2  (also see  400 ) is directly related to the strength of current through bit line  112 - 1  and  112 - 2  (also see  410  and  420 ). A magnetic field through memory cell  106 - 2  (also see  400 ) is made strong enough to switch readout layer  510  (see  FIG. 5 ), which has lower coercivity than storage layer  520  and is therefore easier to switch its magnetic polarity, but weak enough not to switch storage layer  520 . Whether magnetic polarization in readout layer  510  actually switches is irrelevant; rather, it enters a predetermined state based on current flow through bit lines  112 - 1  and  112 - 2  (also see  410  and  420 ).  
         [0047]     The resistance of memory column  104 - 1  is then determined by well know methods. Then, the direction of current through bit lines  112 - 1  and  112 - 2  is switched and the respective magnetic fields switch alignment of the magnetic polarization for readout layer  510 , again without switching storage layer  520 . Resistance of memory column  104 - 1  is again determined. Based on the difference in resistance between the first reading and the second, and the known magnetic polarization of readout layer  510  during the first and second reading, the magnetic polarization of storage layer  320  becomes known. Resistance through a memory cell is lower when both the storage layer and readout layer are aligned in the same direction, higher when they are aligned in opposite directions.  
         [0048]     For example, if readout layer  510  is first aligned in the up direction and then in the down direction, and the resistance during the second reading increases, then storage layer  520  is aligned in the up direction (parallel). Conversely, if readout layer  510  is first aligned in the up direction and then in the down direction, and the resistance during the second reading decreases, then storage layer  520  is aligned in the down direction (anti-parallel). Although a two-part memory read may require more time than a one-part memory read, when used in a NAND mode the stackable arrangement simplifies the read process and improves performance.  
         [0049]     There are two commonly used architectures in non-volatile memory cells: NOR and NAND. In the NOR architecture, each bit cell is individually addressed by a separate word line and a separate bit line. In the NAND architecture several memory cells are connected in series to one common word line, for example. The common word line remains in the “on” state while the individual bit lines address each of the connected cells. NOR architectures are often used for programming while NAND memories are typically used for storage applications. The stackable architecture described here lends itself to be used in an NAND configuration.  
         [0050]     In another embodiment, the readout layer is biased (with an exchange layer for example) so that the standby direction of its magnetization is always fixed, for example in the upward direction. The readout scheme then involves applying a pulse of opposite currents in the adjacent bit lines to temporarily switch the magnetization of the readout layer in the downward direction (without switching the storage layer) and measure the resulting voltage across the memory stack. If the pulse corresponds to a temporary increase in resistance of the stack, the storage layer is magnetized in the up direction. Conversely, if the pulse corresponds to temporary decrease in the resistance of the stack, the storage layer is magnetized in the down direction. In this embodiment, the read process consists of only one step to determine the state of magnetization of the storage layer.  
         [0051]     In another embodiment, selectivity of the memory cell is achieved with current running through a single bit line, rather than both bit lines. One skilled in the art will recognize that the invention encompasses the position of the bit lines with respect to the memory cell and word line. Two bit lines carrying current improves selectivity, but is not necessary to practice the invention. This is applicable to both memory read and memory write.  
         [0052]      FIG. 6  is a cross-sectional diagram illustrating one embodiment of the invention with in-plane magnetization. MRAM  600  includes word line  605  connected to memory columns  610  with memory cells  615 . Connected opposite word line  605  are switches  620 . Bit line columns  625  include bit lines  630 .  
         [0053]     In one embodiment, memory cells  615  and bit lines  630  in MRAM  600  are composed of several layers, described below. Element  640  illustrates the layers. Layer  645  may include a multilayer of (Cu/Ta) (4 repeats) with the Cu approximately 10 nm thick and the Ta approximately 3 nm thick. Layer  645  may be used to adjust the conductivity of the bit lines to the appropriate value, which depends on the length and width of the line. The resistance of layer  645  will not greatly affect the MTJ because the tunnel barrier has a much greater resistance.  
         [0054]     Layer  650  is a crystalline lattice of Ir 20 Mn 80 , with the IrMn approximately 5 nm thick. Layer  655  is Co 90 Fe 10 , which together with IrMn (layer  650 ) constitutes the storage layer. Layer  655  is approximately 10-50 nm thick.  
         [0055]     Layer  660  is Al 2 O 3  with a thickness of approximately 1.2 nm. Layer  660  forms the tunnel barrier between the storage and readout layers. Layer  660  may be formed, for example, by depositing about 0.8 nm of metallic aluminum and oxidizing it with plasma or natural oxidation. The other layers in element  640  may be deposited by sputtering.  
         [0056]     Layer  670  is Ni 80 Fe 20  that is approximately 25 nm thick. Layer  670  forms a free layer that is magnetostatically coupled parallel to the magnetization of the top NiFe layers of the adjacent bit lines.  
         [0057]     Elements  640  making up bit lines  630  may be connected by insulators, for example SiO 2 , Al 2 O 3 , or other oxides. One skilled in the art will recognize that memory cells  615  and bit lines  630  need not be fabricated in the same way or with the same material as each other, however manufacturing is simplified if they are the same.  
         [0058]     Layer  680  between the memory cells is Cu, which facilitates current transmission through the memory columns. Although the various layers have a specific purpose for memory cells, they are not as relevant in the bit lines, where storage does not occur. Rather, bit lines should contain material that will carry current in order to select the memory cells. Layering the bit lines with the same material as the memory cells simplifies manufacturing of the MRAM.  
         [0059]     One method of performing a memory write, or storing data within MRAM  600  for out-of-plane magnetization, follows for memory cell  615 . Electric current flowing (or pulsed) in the same direction is feed to bit lines  630 - 1  and  630 - 2 . Current in bit lines  630 - 1  and  630 - 2  is shown flowing perpendicular and into the plane of  FIG. 6 . Layer  670  on each of bit lines  630 - 1  and  630 - 2  act as cladding layers. Layers  670  are polarized in an Oersted magnetic field generated by the current through bit lines  630 - 1  and  630 - 2 . Due to the parallel magnetostatic coupling between these layers  670  on bit lines  630 - 1  and  630 - 2 , and layer  670  on memory cell  615 , the magnetization of layer  670  in memory cell  615  aligns parallel to the magnetic field of bit lines  630 - 1  and  630 - 2 .  
         [0060]     In order to select memory cell  615 , current flows through word line  605 , memory column  610 - 1 , and switch  620 - 1 . Then, because of anti-parallel magnetostatic coupling with the storage layer, the storage layer switches in the anti-parallel direction.  
         [0061]     Current flowing through a memory cell reduces the switching field by one of several effects. First, thermal heating of the storage layer due to current flowing through the tunnel barrier causes the switching field to decrease. If the current is large enough the storage layer is heated above its blocking temperature. In Ir 20 Mn 80 , it is known that the blocking temperature can be adjusted from 150C to 300C by varying the thickness of this layer. The Co magnetization becomes anti-parallel to the NiFe layer and freezes in this direction when the temperature decreases back to the standby temperature. Therefore, the direction of magnetic field created by the pulses of current in the bit lines determines the alignment of the magnetization of the storage layer.  
         [0062]     Furthermore, the Oersted field due to the vertical current flow through the memory column of the memory cell reduces the switching field because it favors the formation of an in-plane vortex state that, due to cylindrical symmetry, helps the reversal of magnetization. In one embodiment, the storage layer is made with CO 50 Fe 50 , or of a CoFe/IrMn bilayer with reduced IrMn thickness, for example 4 nm, so that the storage layer will have greater coercivity but no loop shift.  
         [0063]     Finally, injection of spin-polarized electrons from the readout layer or from a polarizing additional layer into the storage layer can be used to decrease the switching field of the storage layer magnetization.  
         [0064]     A memory read for in-plane magnetization operates in the same manner as for out-of-plane magnetization, with the exception that current carried through the bit lines travels in the same direction, rather than in opposite directions, during the initial setting of the readout layer and the switching of the readout layer.  
         [0065]     In another embodiment, selectivity of the memory cell is achieved with current running through a single bit line, rather than both bit lines. One skilled in the art will recognize that the invention encompasses the position of the bit lines with respect to the memory cell and word line. Two bit lines carrying current improves selectivity, but is not necessary to practice the invention. This is applicable to both memory read and memory write.  
         [0066]      FIG. 7  is a diagram illustrating the making and process flow for the invention. In block  700 , start with planarized dielectric surface  705  with an embedded conductor pad to connect to the memory cell. In block  710 , deposit buffer/sensor/conductor stack  715  and spin coat photoresist  720 . In block  725 , expose and develop memory cell and bit line pattern  730 . In block  735 , etch with an ion beam through sensor stack  740 , fill with dielectric  740 , and lift off photo-resist. In block  750 , planarize buffer  755 , blank deposit a dielectric, and spin resist, expose and develop conductor pad for the next conductor stack by photoresist. In block  760 , etch to the buffer, remove resist, blank deposit conductor  765 , planarize to dielectric  770 , and remove resist. This method requires only two photomasking plus one planarization per sensor layer.  
         [0067]      FIG. 8  is a flow diagram illustrating one method of executing memory write to a MRAM with a word line, a memory cell electrically coupled to the word line, and a bit line coupled to, adjacent to and electrically isolated from the memory cell. In block  800 , generate an electric current in the word line. In block  810 , receive an electric current in the memory cell. In block  820 , generate a magnetic field around the bit line. In block  830 , align a magnetic polarization within a readout layer in the memory cell according to the direction of the magnetic field. In block  840 , align a magnetic polarization within a storage layer according to the direction of the magnetic field, the storage layer coupled to the readout layer and having a higher coercivity than the readout layer.  
         [0068]      FIG. 9  is a flow diagram illustrating one method of executing memory read in a MRAM with a word line, a memory cell electrically coupled to the word line, and a bit line coupled to, adjacent to and electrically isolated from the memory cell. In block  900 , generate a magnetic field around the bit line. In block  910 , generate an electric current in the word line. In block  920 , receive an electric current in the memory cell. In block  930 , align a magnetic polarization within a readout layer in the memory cell according to the direction of the magnetic field. In block  940 , measure a resistance of the memory cell. In block  950 , reverse the magnetic field around the bit line. In block  960 , reverse the magnetic polarization within the readout layer. In block  970 , measure the resistance of the memory cell.  
         [0069]     The advantages of the invention include a reduced MRAM cell area achieved by the stackable architecture which reduces the number of word lines, improves the geometry of the bit line/memory cell relationship and uses only one transistor per memory stack rather one transistor per cell. In addition, the readout process is simplified requiring only the determination of the polarity when changing the magnetic states of one cell. In contrast, the prior art compares the resistance of a cell to a distinct reference cell requiring a narrow distribution resistance values. The manufacturing process is simplified by the co-planar metallization process of bit lines and storage cells as well as the repeated application of only 2 masks per layer. Other advantages include improved write selectivity by the use of two adjacent bit lines and increased memory density. For example, the invention may apply an eight-layer design with a cell size of only 1 F2.  
         [0070]     One of ordinary skill in the art will recognize that configurations of different materials may be used without straying from the invention. The illustrated embodiments of the invention include, for example transistors, but one skilled in the art recognizes that these may be interchanged and/or replaced by components with similar functionality, for example diodes, applying appropriate circuit rerouting. Additionally, certain combinations of elements have been disclosed in certain thicknesses or certain ratios. However one of ordinary skill in the art will recognize that other ratios will work and other thicknesses may be used, as well as other materials. The embodiments described herein are meant to provide an enabling disclosure only and not meant as limiting features of the invention. As any person skilled in the art will recognize from the previous description and from the figures and claims that modifications and changes can be made to the invention without departing from the scope of the invention defined in the following claims.