Abstract:
A memory cell structure. A first conductive line is cladded by at least two first ferromagnetic layers respectively having a first easy axis and a second easy axis, a nano oxide layer located between the first ferromagnetic layers, and a first pinned ferromagnetic layer. The first and second easy axes are 90 degree twisted-coupled with the first easy axis parallel to the length of the first conductive line and the second easy axis perpendicular to the length of the first conductive line. A storage device is adjacent to the first conductive line, receiving a magnetic field generated from a current flowing through the first conductive line.

Description:
BACKGROUND 
   The disclosure relates in general to Magnetoresistive Random Access Memories (MRAMs), and more particularly to magnetic tunnel junction (MTJ) MRAM arrays and a specific architecture for connecting the arrays. 
   Magnetic random access memory (MRAM) cells are often based on a magnetic tunnel junction (MTJ) cell. Basically, an MTJ configuration can be made up of three basic layers, a “free” ferromagnetic layer, an insulating tunneling barrier, and a “pinned” ferromagnetic layer. In the free layer, the magnetization moments are free to rotate under an external magnetic field, but the magnetic moments in the “pinned” layer cannot. The pinned layer can be composed of a ferromagnetic layer and/or an anti-ferromagnetic layer which “pins” the magnetic moments in the ferromagnetic layer. A very thin insulation layer forms the tunneling barrier between the pinned and free magnetic layers. In order to sense states in the MTJ configuration, a constant current can be applied through the cell. As the magneto-resistance varies according to the state stored in the cell, the voltage can be sensed over the memory cell. To write or change the state in the memory cell, an external magnetic field can be applied that is sufficient to completely switch the direction of the magnetic moments of the free magnetic layers. 
   MTJ configurations often employ the Tunneling Magneto-Resistance (TMR) effect, which allows magnetic moments to quickly switch the directions in the magnetic layer by an application of an external magnetic field. Magneto-resistance (MR) is a measure of the ease with which electrons may flow through the free layer, tunneling barrier, and the pinned layer. A minimum MR occurs in an MTJ configuration when the magnetic moments in both magnetic layers have the same direction or are “parallel”. A maximum MR occurs when the magnetic moments of both magnetic layers are in opposite directions or are “anti-parallel.” 
     FIG. 1A  is a schematic perspective view illustrating a conventional MTJ cell of an MRAM device.  FIG. 1B  is a schematic perspective view illustrative of read out operation of the MTJ cell of  FIG. 1A .  FIG. 1C  is a plane view illustrative of respective magnetization states depending on stored data of the MTJ cell of  FIG. 1A . 
   The single memory cell comprises: a first metal layer  11 ; a pinned layer  12 ; a tunnel barrier layer  13 ; a free layer  14 ; and a second metal layer  12 . The MTJ cell comprises: the pinned layer  12 ; the dielectric layer  13 ; and the free layer  14 . The tunnel barrier layer  13  is sandwiched between the pinned layer  12  and the free layer  14 . The pinned layer  12  is in contact with the first metal layer  11 . The free layer  14  is in contact with the second metal layer  12 . The pinned layer  12  and the free layer  14  are made of ferromagnetic materials. The dielectric layer  13  is made of an insulating material. The pinned layer  12  has a fixed magnetization direction. The dielectric layer  13  has a thickness of about 1.5 nanometers. The free layer  14  has a thickness of about 20 nanometers. The free layer  14  has a freely changeable magnetization direction. 
   The magnetization direction of the free layer  14  indicates stored data. The free layer  14  serves as a data storage layer. The first metal layer  11  and the second metal layer  15  extend in directions perpendicular to each other. The MTJ cell is positioned at a crossing point between the first metal layer  11  and the second metal layer  15 . In  FIG. 1B , a current  16  flows from the first metal layer  11  to the second metal layer  15  through the pinned layer  12 , the dielectric layer  13 , and the free layer  14 . The MTJ cell is capable of storing binary digit data “0” and “1”. If the magnetization directions of the pinned layer  12  and the free layer  14  are parallel to each other, then this means that the MTJ cell stores a first binary digit, for example, “0”. If the magnetization directions of the pinned layer  12  and the free layer  14  are not parallel, then this means that the MTJ cell stores a second binary digit, for example, “1”. The magnetization direction of the free layer  14  changes depending on an externally applied magnetic field. 
   An electrical resistance of the dielectric layer  13  varies by about 10-60% due to the tunneling magnetoresistance effect between in a first state where the magnetization directions of the pinned layer  12  and the free layer  14  are parallel to each other and a second state where the magnetization directions of the pinned layer  12  and the free layer  14  are not parallel. A predetermined potential difference or a predetermined voltage is applied to the first and second metal layers  11  and  15  to apply a tunneling current from the pinned layer  12  to the free layer  14  through the dielectric layer  13 . This tunneling current varies depending on the variable electrical resistance of the dielectric layer  13  due to the tunneling magnetoresistance effect. The data can be fetched from the MTJ cell by detecting the variation in the tunneling current. 
     FIG. 2A  is a fragmentary schematic perspective view illustrative of an array of MTJ cells of the MRAM of  FIG. 1A .  FIG. 2B  is a fragmentary schematic perspective view illustrative of the array of the MTJ cells during the operation shown in  FIG. 2A . 
   The first metal layers  11  extend in parallel to each other in a first direction. The second metal layers  15  extend in parallel to each other in a second direction, which is perpendicular to the first direction. The single first metal layer  11  and the single second metal layer  15  have a single crossing point, where a single MTJ cell “C” is provided. The plural first metal layers  11  and the plural second metal layers  15  have an array of crossing points where plural MTJ cells “C” are provided. The first metal layers  11  serve as word lines. The second metal layers  15  serve as bit lines. One of the plural MTJ cells “C” is selected by selecting one of the word lines and one of the bit lines, for read or write operations to the selected MTJ cell “C”. 
   The MRAM has the array of the MTJ cells, each of which comprises the tunneling magnetoresistance element utilizing the tunneling magnetoresistance effect, wherein the tunneling magnetoresistance element includes an insulating thin film sandwiched between the two or more ferromagnetic thin films. The tunneling magnetoresistance element is switched between a first state, in which the magnetization directions of the two ferromagnetic thin films are parallel to each other, and a second state, in which the magnetization directions of the two ferromagnetic thin films are anti-parallel. The resistance of the insulating film, which the tunneling current senses, is different for the first and second states. These two states correspond to binary digits, for example, the first state corresponds to the data “0”, and the second state corresponds to the data “1”. 
   The write operation is accomplished as follows. One of the word lines  11  and one of the bit lines  15  are selected. A first write current Isw is applied to the selected word line  11   s . A first magnetic field Msw is generated around the selected word line  11   s . The first write current Isw has a predetermined current value and a predetermined direction. A second write current Isb is applied to the selected bit line  15   s . The second write current Isb has a predetermined current value and a predetermined direction. A second magnetic field Msb is generated around the selected bit line  15   s . As a result, a superimposed magnetic field of both the first and second magnetic field Msw and Msb is applied to the crossing point of the selected word line  11   s  and the selected bit line  15   s . The selected MTJ cell “Cs” is positioned at the crossing point of the selected word line  11   s  and the selected bit line  15   s , for which reason the selected MTJ cell “Cs” is applied with the superimposed magnetic field. The free layer of the selected MTJ cell “Cs” is also applied with the superimposed magnetic field, whereby magnetic domains of the free layer become ordered in a first direction, for example, in a direction parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ cell “Cs” stores a binary digit data “0”. 
   Any first write current Isw or second write current Isb changes its current direction to an opposite direction, whereby the direction of the magnetic field is inverted, and the direction of the superimposed magnetic field is changed by approximately 90 degrees. As a result, the magnetic domains of the free layer become ordered in a second direction, for example, in a direction anti-parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ cell “Cs” stores another binary digit “1”. 
   The read operation is accomplished as follows. One of the word lines  11  and one of the bit lines  15  are selected. A potential difference is applied between the selected word line  11   s  and the selected bit line  15   s  for measuring a current value to detect a resistance value of the selected memory cell “Cs” to the tunneling current. Namely, a predetermined potential difference or a predetermined voltage is applied to the selected word line  11   s  and the selected bit line  15   s  to provide a tunneling current from the pinned layer through the insulating layer to the free layer of the selected memory cell “Cs”. This tunneling current varies depending on the variable electrical resistance of the insulating layer due to the tunneling magnetoresistance effect. The binary digit data can be detected from the selected memory cell “Cs” by detecting the variation in the tunneling current. 
     FIG. 3  is a diagram illustrative of a conventional array structure of the MTJ cells in the MRAM. An array  21  includes 2m word lines (W 1 , W 2 , W 3 , - - - Wm, Wm+1, - - - , W 2   m ) and 2n bit lines (B 1 , B 2 , B 3 , - - - Bn, Bn+1, - - - , B 2   n ) as well as 2 m×2n MTJ cells (C 11 , C 12 , - - - , C 2   m   2   n ) that are positioned at crossing points of the word and bit lines. A word line Wig and a bit line Bj are selected to select a MTJ cell Cij positioned at the crossing point between the selected word line Wi and the selected bit line Bj for read or write operation as described above. 
   As mentioned above, the first and second write current are provided to generate the first and second magnetic fields around the selected conductive lines, respectively. For reducing the current density, a ferromagnetic layer adjacent to the conductive line would dramatically enhance the magnetic field generated by the conductive line. For linear response in the magnetic field by the conductive lines, the hard axis of the ferromagnetic layer is parallel to the magnetic field. Due to the enhancement in magnetic field by the ferromagnetic layer, less power will be consumed during a writing process. In  FIG. 4 , for linear response and zero remnant magnetization of the ferromagnetic layer, the easy axis EA of the ferromagnetic layer  32  would be set parallel to the current direction Id along the conductive line  31 . Here, the easy axis EA of the ferromagnetic layer  32  is pinned by the anti-ferromagnetic layer  33  and substantially perpendicular to the circular magnetic field Hd generated by the current Id. Therefore, the ferromagnetic layer  32  inserted adjacent to the conductive line  31  will force the magnetization of the ferromagnetic layer  32  parallel to the current direction. Here, the ferromagnetic layer  32  and the anti-ferromagnetic layer  33  comprise a ferromagnetic cladding layer. 
   U.S. Patent Application 20020182557 discloses a method to heat-treat (or annealing process) a substrate provided with thin magnetic layers in a magnetic field to magnetize the pinned layer in one direction. Typically, an oriented magnetic field of 0.5 T (tesla) or more must typically be applied, and an oriented magnetic field of more than 1.0 T is often necessary depending on the materials of the pinned layer. To apply an oriented magnetic field to wafer substrates, a vacuum heat-treating furnace has conventionally been used. This vacuum heat-treating furnace comprises a magnetic field-generating coil equipped with a cooling pipe, a high-frequency coil disposed inside the coil, and a vacuum container for holding a plurality of wafer substrates disposed inside the high-frequency coil. 
   However, due to the substantially orthogonal write line structure, the ferromagnetic cladding layers, which respectively clad the word line and the bit line, are substantially perpendicular to each other. Here, the easy axes of the substantially perpendicular word line and bit line are also substantially perpendicular. Thus, the ferromagnetic cladding layers have to be exchange-biased with anti-ferromagnets exhibiting different anneal temperatures to form the ferromagnetic cladding layers of the bit line and the word line with perpendicular easy axes. 
   In addition, a complicated annealing process with at least two annealing steps in mutually perpendicular annealing processing-fields is typically necessary. The obvious disadvantage is that one ferromagnetic cladding layer in which anisotropy is set in the first annealing process is affected by the following second annealing process. 
   SUMMARY 
   One object of the present invention is thus to provide a MRAM cell having ferromagnetic cladding layers respectively cladding the word line and the bit line perpendicular to each other. In addition, the easy axes of the perpendicular word line and bit line are set perpendicularly by single annealing process step. 
   To achieve the above-mentioned object, embodiments of the present invention provide a memory cell structure. A first conductive line is cladded by at least two first ferromagnetic layers respectively having a first easy axis and a second easy axis, a nano oxide layer located between the first ferromagnetic layers, and a first pinned ferromagnetic layer. The first and second easy axes are 90 degree twisted-coupled with the first easy axis parallel to the length of the first conductive line and the second easy axis perpendicular to the length of the first conductive line. A storage device is adjacent to the first conductive line, receiving a magnetic field generated from a current flowing through the first conductive line. 
   In addition, embodiments of the present invention provide a memory cell structure. A first conductive line is cladded by a plurality of films, including a first ferromagnetic layer having a first easy axis and cladding the first conductive line. The first easy axis is parallel to the length of the first conductive line. The first ferromagnetic layer is cladded by a nano oxide layer. A synthetic anti-ferromagnetic layer cladding the nano oxide layer, includes a second ferromagnetic layer having a second easy axis perpendicular to the first easy axis, a third ferromagnetic layer having a third easy axis anti-parallel to the second easy axis, and a dielectric layer located between the second ferromagnetic layer and the third ferromagnetic layer. The synthetic anti-ferromagnetic layer is cladded by a first pinned ferromagnetic layer. A storage device is adjacent to the first conductive line, receiving a magnetic field generated from a current flowing through the first conductive line. 

   
     DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention. 
       FIG. 1A  is a schematic perspective view illustrative of a conventional MTJ cell of the MRAM device. 
       FIG. 1B  is a schematic perspective view illustrative of a read out operation of the MTJ cell of  FIG. 1A . 
       FIG. 1C  is a plan view illustrative of respective magnetization states depending on stored data of the MTJ cell of  FIG. 1A . 
       FIG. 2A  is a fragmentary schematic perspective view illustrative of an array of MTJ cells of the MRAM of  FIG. 1A . 
       FIG. 2B  is a fragmentary schematic perspective view illustrative of the array of the MTJ cells during a write operation shown in  FIG. 2A . 
       FIG. 3  is a diagram illustrative of a conventional array structure of the MTJ cells in the MRAM. 
       FIG. 4  is a schematic perspective view illustrative of the program line of the MTJ cell. 
       FIG. 5  is a schematic perspective view illustrative of one memory cell of the MRAM device. 
       FIG. 6A  and  FIG. 6B  are schematic perspective views illustrative of the word line and the bit line, respectively. 
       FIG. 7  is a schematic perspective view illustrative of the annealing process. 
       FIG. 8A  and  FIG. 8B  are plan views illustrative of magnetization states of two program lines, respectively. 
       FIG. 9  is a plan view illustrative of magnetization states of the alternative structure of the program line using synthetic anti-ferromagnetic layer. 
   

   DETAILED DESCRIPTION 
     FIG. 5  is a schematic perspective view illustrative of one memory cell of the MRAM device. The memory cell comprises a first metal layer  51 , an MTJ cell  50 , and a second metal layer  55 . The MTJ cell  50  comprises the pinned layer  52 , the dielectric layer  53 , and the free layer  54 . The tunnel barrier layer  53  is sandwiched between the pinned layer  52  and the free layer  54 . The pinned layer  52  is in contact with the first metal layer  51 . Here, the first metal layer  51  can be coupled or isolated with the MTJ cell  50  in different implementations. 
   The free layer  54  is in coupled to the second metal layer  55  through a switch  58 . The pinned layer  52  and the free layer  54  are made of ferromagnetic materials. The tunnel barrier layer  53  is made of an insulating material. The pinned layer  52  has a fixed magnetization direction. The dielectric layer  53  has a thickness of about 1.5 nanometers. The free layer  54  has a thickness of about 20 nanometers. The free layer  54  has a freely changeable magnetization direction. 
   The magnetization direction of the free layer  54  indicates stored data. The free layer  54  serves as a data storage layer. The first metal layer  51  and the second metal layer  55  extend in directions substantially perpendicular to each other. The MTJ cell  50  is positioned at a crossing point between the first metal layer  51  and the second metal layer  55 . The MTJ cell  50  is capable of storing binary digit data “0” and “1”. If the magnetization directions of the pinned layer  52  and the free layer  54  are parallel to each other, then this means that the MTJ cell  50  stores a first binary digit, for example, data “0”. If the magnetization directions of the pinned layer  52  and the free layer  54  are not parallel, then this means that the MTJ cell  50  stores a second binary digit, for example, “1”. The magnetization direction of the free layer  54  is changed depending on an externally applied magnetic field. 
   An electrical resistance of the dielectric layer  53  varies by about 10-60% due to the tunneling magnetoresistance effect between a first state, where the magnetization directions of the pinned layer  52  and the free layer  54  are parallel to each other, and a second state, where the magnetization directions of the pinned layer  52  and the free layer  54  are anti-parallel. A predetermined potential difference or a predetermined voltage is applied to the first and second metal layers  51  and  55  to provide a tunneling current from the pinned layer  52  to the free layer  54  through the dielectric layer  53 . This tunneling current varies depending on the variable electrical resistance of the dielectric layer  53  due to the tunneling magnetoresistance effect. The data can be fetched from the MTJ cell  50  by detecting the variation in the tunneling current. 
   During a write operation, the first metal line (word line)  51  and the second metal line (bit line)  55  are selected. A first write current Isw is applied to the selected word line  51 . A first magnetic field Msw is generated around the selected word line  51 . The first write current Isw has a predetermined current value and a predetermined direction. A second write current Isb is applied to the selected bit line  55 . The second write current Isb has a predetermined current value and a predetermined direction. A second magnetic field Msb is generated around the selected bit line  55 . As a result, a superimposed magnetic field of both the first and second magnetic fields Msw and Msb is applied to the crossing point of the selected word line  51  and the selected bit line  55 . The selected MTJ cell  50  is positioned at the crossing point of the selected word line  51  and the selected bit line  55 , for which reason the superimposed magnetic field is applied to the selected MTJ cell  50 . The free layer of the selected MTJ cell  50  is also applied with the superimposed magnetic field, whereby magnetic domains of the free layer become ordered in a first direction, for example, in a direction parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ cell  50  stores a binary digit data “0”. 
   To enhance the magnetic fields generated by the selected word line and bit line, metal lines  56 A and  56 B of the word line and the bit line are respectively clad by ferromagnetic cladding layers  57 A and  57 B. Thus, the metal line  56 A and the cladding layer  57 A comprise a first program line. In addition, the metal line  56 B and the cladding layer  57 B comprise a second program line. 
   As mentioned above, the ferromagnetic layer (cladding layer) adjacent to the metal line dramatically enhances the magnetic field generated by the metal line. For linear response in the magnetic field by the conductive lines, the easy axis of the ferromagnetic layer must typically be substantially perpendicular to the magnetic field. Thus, less power will be consumed during a writing process by setting the easy axis of the ferromagnetic cladding layers substantially parallel to the current direction of the selected word line and bit line, respectively. 
     FIGS. 6A and 6B  are schematic perspective views illustrative of the word line and the bit line, respectively. The structures of the word line and the bit line may also be respectively illustrated in the  FIGS. 6A and 6B . 
   In  FIG. 6A , the word line comprises, sequentially, a metal line  56 A, a ferromagnetic layer  571 , and an anti-ferromagnetic layer  572 , which is a pinned ferromagnetic layer for sustaining the magnetization of the ferromagnetic layer  571 . In  FIG. 6B , the bit line comprises, sequentially, a metal line  56 B, a ferromagnetic layer  573 , a nano oxide layer (known as NOL)  574 , a ferromagnetic layer  575 , and an anti-ferromagnetic layer  576 , which is a pinned ferromagnetic layer for sustaining the magnetization of the ferromagnetic layer  575 . Here, the oxide spacer  574  is comprised of magnetic oxide material such as FeOx. Some embodiments of the metal lines  56 A and  56 B can be made of alloy, copper, tantalum, titanium, cobalt, or ferruginous. 
   To develop the magnetization of the magnetic cladding layers of the word line and the bit line, only one annealing process step is performed in some embodiments of the present invention.  FIG. 7  is a schematic perspective view illustrative of one embodiment of the annealing process. Here, the word line  51  is set along the X axis and the bit line  55  is set along the Y axis. An annealing field M pointed along the positive X direction is applied during the annealing process. Due to the exchange anisotropy pinning, the magnetization direction of the ferromagnetic layers  571 ,  573  and  575  will point to the direction of the annealing process field (X axis) M. In other words, the Y direction is the hard axis of the ferromagnetic layers  571 ,  573  and  575 . Thus, the magnetization direction of the ferromagnetic layer  571  is pinned by the anti-ferromagnetic layer  572 , and the magnetization direction of the ferromagnetic layer  575  is pinned by the anti-ferromagnetic layer  576 , as shown in  FIGS. 8A and 8B . In  FIG. 8A , the arrows represent the magnetization direction of the ferromagnetic layer  571  and the anti-ferromagnetic layer  572 . In  FIG. 8B , the arrows represent the magnetization direction of the ferromagnetic layer  575  and the anti-ferromagnetic layer  576 . After the annealing process field is removed, the magnetization directions of the ferromagnetic layers  571  and  575  are still pinned by the anti-ferromagnetic layer  572  and  576  with exchange anisotropy and are aligned along the annealing process field M. However, the magnetization direction of the ferromagnetic layer  573  is perpendicular to the ferromagnetic layer  575  by spin flop through the oxide spacer, as shown in  FIG. 8B . In addition, the magnetization direction of the ferromagnetic layer  573  is along the Y axis. Furthermore, the alternative layer structure of the ferromagnetic layer  575  uses a synthetic anti-ferromagnetic layer (SAF) shown in  FIG. 9 . In the SAF structure, the ferromagnetic layer  575  is divided into a ferromagnetic layer  575 A, an oxide layer  575 C, and a ferromagnetic layer  575 B. Thus, the ferromagnetic layers  575 A and  575 B are anti-ferromagnetically coupled through an ultra thin oxide layer (Ru) owning to Ruderman-Kittel-Kasuya-Yoshida(RKKY) coupling. Due to the magnetization of the ferromagnetic layers  575 A and  575 B being equal but in opposite directions, the self-closed flux loops between the terminals of the ferromagnetic layers  575 A and  575 B at both side are respectively created, eliminating the effect of the magnetization on the free layer of the memory cell due to the magnetic flux between both terminals of the ferromagnetic layer  575  in  FIG. 8B . 
   Thus, the magnetic cladding layers of the word line and the bit line according to the present invention set the ferromagnetic layers perpendicular to each other with only one annealing processing step by adding an oxide spacer (oxide layer). The magnetization direction of each layer in the multilayer ferromagnetic cladding layer is along the word line  51  and the bit line. As mentioned above, in  FIG. 7 , the magnetization direction of the ferromagnetic layers  571  is pinned by the anti-ferromagnetic layers  572  along the annealing process field direction M. With a spin flip mechanism, the magnetic direction of the ferromagnetic layers  573  is orthogonal to the magnetization direction of the ferromagnetic layer  571  after a single annealing process. Therefore, the easy axis of the ferromagnetic cladding layer of bit line and the word line are substantially perpendicular to each other after only one annealing process. Since there is only one annealing processing step to be carried out, the mutual affection in ferromagnetic cladding layers of the bit line and the word line from two annealing process steps by the conventional method is avoided. 
   The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.