Abstract:
A magnetic memory array. A first bit line provides a first writing magnetic field to a magnetic memory cell. A second bit line provides a second writing magnetic field to a reference magnetic memory cell. A word line provides a third writing magnetic field to the magnetic memory cell and a fourth writing magnetic field to the reference magnetic memory cell. The third writing magnetic field exceeds the fourth writing magnetic field.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The disclosure relates in general to a memory circuit. In particular, the disclosure relates to magnetic random access memory (MRAM) circuits. 
   2. Discussion of the Related Art 
   Magnetic Random Access Memory (MRAM) is a non-volatile memory used in long term data storage. The magnetic memory cell of MRAMs stores information by its magnetoresistance. When data is to be written to a magnetic memory cell, the magnetic memory cell is selected by providing magnetic fields respectively produced by a selected word line and bit line, and combined magnetic fields change the magnetic vector of the magnetic memory cell to change the resistance of the selected magnetic memory cell. When data is to be read from the selected magnetic memory cell, a sensing current passes through the selected magnetic memory cell, and the stored data is obtained according to the voltage difference across the selected magnetic memory cell. 
     FIG. 1  is a schematic diagram of a conventional MRAM array. The conventional MRAM array comprises bit lines B 1 ˜B 3  and word lines W 1 ˜W 3 . The magnetic memory cell located at the intersection of one bit line and word line comprises a soft magnetic layer (free ferromagnetic layer), a tunnel barrier layer, a hard magnetic layer (pinned ferromagnetic layer) and a non-magnetic conductor. The relative magnetic orientations of the free ferromagnetic layer and the pinned ferromagnetic layer represent logic values of “0” and “1”, for example. 
   The magnetic memory cell is written to by the magnetic field generated by the selected word line and bit line. Thus, only the magnetic dipole moment of the selected magnetic memory cell is switched. For example, the magnetic fields generated by bit line B 1  and word line W 1  switch only the magnetic dipole moment of magnetic memory cell Cs, without changing the state of non-selected magnetic memory cells C 12 , C 13 , C 21 , and C 31 . 
     FIG. 2  shows a relationship between switching state of the magnetic memory cell and magnetic fields provided by the word line and bit line, referred to as an “asteroid curve”. Horizontal magnetic field H l  is provided by the bit line, and vertical magnetic field H t  by the word line. According to the asteroid curve, the resistance of magnetic memory cell is switched when the horizontal magnetic field H l  is H 0  without vertical magnetic field H t . If vertical magnetic field H t  exists, a lower horizontal magnetic field H l , less than H 0 , is able to switch the resistance of the selected magnetic memory cell. 
   In  FIG. 2 , the resistance of the selected magnetic memory cell is not switchable in area A. The selected magnetic memory cell is switched when outside the area A, to a first state (high resistance as an example) when the applied magnetic field is in the first quadrant, and to a second state (low resistance as an example) when in the second quadrant. 
   In an MRAM array, a reference magnetic memory cell is arranged for a predetermined number of bit lines (32 or 64). To read the selected magnetic memory cell Cs, a read current through data line D 1 , magnetic memory cell Cs and bit line B 1  is provided. Another reference current through reference data line Dr, reference magnetic memory cell Cr and reference bit line Br is also provided. Data stored in magnetic memory cell Cs is obtained by comparing the voltage level at bit lines B 1  and Br. 
   The asteroid curve of the conventional reference magnetic memory cell Cr is identical to the selected magnetic memory cell Cs, that is, the switch conditions of conventional reference magnetic memory cell Cr and magnetic memory cell Cs are the same. The resistance of reference magnetic memory cell Cr can be switched by providing writing currents to word line W 1  and reference bit line Br, or providing strong writing current to reference bit line Br only. 
   However, as array size increases, it is difficult to ensure that each memory cell and reference memory cell have the same magnetic characteristics due to manufacturing process limitations, such as magnetic resistance ratio, coercivity and interlayer coupling. Most important is uniform distribution of coercivity, which affects the working range of writing currents. Sometimes, higher writing current is required as the coercivity distribution is not uniform. However, the increased writing current can switch resistance of the reference magnetic memory cell, disturbing reading results. 
   SUMMARY OF THE INVENTION 
   Magnetic memory arrays are provided. An embodiment of a magnetic memory array comprises a magnetic memory cell comprising a first free ferromagnetic layer, a first pinned ferromagnetic layer, and a first tunnel barrier layer located between the first free ferromagnetic layer and the first pinned ferromagnetic layer, a reference magnetic memory cell comprising a second free ferromagnetic layer, a second pinned ferromagnetic layer, and a second tunnel barrier layer located between the second free ferromagnetic layer and the second pinned ferromagnetic layer, a first bit line providing a first writing magnetic field to the magnetic memory cell, a second bit line providing a second writing magnetic field to the reference magnetic memory cell, and a word line providing a third writing magnetic field to the magnetic memory cell and a fourth writing magnetic field to the reference magnetic memory cell, wherein the third writing magnetic field exceeds the fourth writing magnetic field. 
   Another embodiment of a magnetic memory array comprises a magnetic memory cell comprising a first free ferromagnetic layer, a first pinned ferromagnetic layer, a first tunnel barrier layer located between the first free ferromagnetic layer and the first pinned ferromagnetic layer, a first major axis and a first short axis, wherein the first major axis and the first short axis have a first aspect ratio, a reference magnetic memory cell comprising a second free ferromagnetic layer, a second pinned ferromagnetic layer, a second tunnel barrier layer located between the second free ferromagnetic layer and the second pinned ferromagnetic layer, a second major axis and a second short axis, wherein the second major axis and the second short axis have a second aspect ratio exceeding the first aspect ratio, a first bit line providing a first writing magnetic field to the magnetic memory cell, a second bit line providing a second writing magnetic field to the reference magnetic memory cell, and a word line providing a third writing magnetic field to the magnetic memory cell and a fourth writing magnetic field to the reference magnetic memory cell, wherein the third writing magnetic field exceeds the fourth writing magnetic field. 
   Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a schematic diagram of a conventional MRAM array. 
       FIG. 2  shows a relationship between switching state of the magnetic memory cell and magnetic fields provided by the word line and bit line. 
       FIG. 3  is a schematic diagram of an MRAM array according to an embodiment of the invention. 
       FIG. 4  is a cross section cut along line AA′ in  FIG. 3  according to an embodiment of the invention. 
       FIG. 5  shows asteroid curves  51  and  53  according to the embodiment shown in  FIG. 4 . 
       FIG. 6  is a cross section of an MRAM array according to another embodiment of the invention. 
       FIG. 7  shows asteroid curves  71  and  73  according to the embodiment shown in  FIG. 6 . 
       FIG. 8  is a top view of an MRAM array according to another embodiment of the invention. 
       FIG. 9  is a top view of an MRAM array according to another embodiment of the invention. 
       FIG. 10  is a top view of an MRAM array according to another embodiment of the invention. 
       FIG. 11  shows asteroid curves  93  and  95  according to the embodiment shown in  FIG. 10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3  is a schematic diagram of an MRAM array according to an embodiment of the invention. The MRAM array comprises bit lines B 1 ˜B 3  and word lines W 1 ˜W 3 . The magnetic memory cell located at the intersection of one of bit line and word line comprises a soft magnetic layer (free ferromagnetic layer), a tunnel barrier layer, a hard magnetic layer (pinned ferromagnetic layer) and a non-magnetic conductor. The relative magnetic orientations of the free ferromagnetic layer and the pinned ferromagnetic layer represent stored data of the magnetic memory cell. For example, as the magnetic moments of free ferromagnetic layer and the pinned ferromagnetic layer have the same direction or are “parallel”, the magnetic memory cell has relatively low resistance, and the stored data is “0”. Contrarily, when the magnetic moments of free ferromagnetic layer and the pinned ferromagnetic layer have opposite direction or are “anti-parallel”, the magnetic memory cell has relative high resistance, and the stored data is “1”. 
   The magnetic memory cell is written to by the magnetic fields generated by the selected word line and bit line. Thus, only the magnetic dipole moment of the selected magnetic memory cell is switched. For example, the magnetic fields generated by bit line B 1  and word line W 1  only switch the magnetic dipole moment of magnetic memory cell Cs, without changing the state of non-selected magnetic memory cells C 12 , C 13 , C 21 , and C 31 . In  FIG. 3 , a single memory cell comprises one magnetic memory cell, and the reference magnetic memory cell can comprise a plurality of magnetic memory cells connected in parallel or serial, to obtain a middle state resistance. 
     FIG. 4  is a cross section cut along line AA′ in  FIG. 3  according to an embodiment of the invention. In this embodiment, the distance between reference magnetic memory cell C ref  and word line WL exceeds that between magnetic memory cell C data  and word line WL, and the distance between reference magnetic memory cell C ref  and reference bit line Br is less than that between magnetic memory cell C data  and bit line BL. Here, a combination magnetic field of the magnetic fields respectfully generated by bit line BL and word line WL can be different from that generated by bit line Br and word line WL. 
   A manufacturing process forms a dielectric layer  41  on word line WL, patterning a salient  43  on dielectric layer  41  by etching using a mask. Next, reference magnetic memory cell C ref  and magnetic memory cell C data  are formed by etching a plurality of magnetic metal layers using two masks. As shown in  FIG. 4 , the position of the magnetic memory cell C data  is lower than that of reference magnetic memory cell C ref . Isolation layer  45  is formed, and a contact via  47  connected to the magnetic memory cell C data  is formed by etching, plating conductive materials and chemical mechanical polishing (CMP). Isolation layer  49  is formed on the isolation layer  45 , and bit lines BL and Br are patterned. Bit lines BL and Br are formed by etching, plating conductive materials and performing chemical mechanical polishing (CMP). 
     FIG. 5  shows asteroid curve  51  of reference magnetic memory cell C ref  for bit line Br and word line WL, and asteroid curve  53  of magnetic memory cell C data  for bit line BL and word line WL according to the embodiment shown in  FIG. 4 . Reference magnetic memory cell C ref  is close to bit line Br. In addition, the distance between word line WL and reference magnetic memory cell C ref  exceeds that between word line WL and magnetic memory cell C data . Thus, current through bit line Br to define middle state resistance of reference magnetic memory cell C ref  is reduced. In addition, magnetic field generated from the current through word line WL influences reference magnetic memory cell C ref  less than magnetic memory cell C data  when writing data to magnetic memory cell C data . Thus, middle state resistance of reference magnetic memory cell C ref  is stabilized, decreasing data reading disturbance. 
     FIG. 6  is a cross section of an MRAM array according to another embodiment of the invention. In this embodiment, the current path of word line WL under reference magnetic memory cell C ref  is changed.  FIG. 6  shows another current path branch WLa. As the magnetic field generated by the current is in direct proportion to the current value, and inverse proportion to the distance therebetween, the total magnetic field generated by currents through word line WL and current path brunch WLa is less than that generated by a single word line WL at reference magnetic memory cell C ref . Here, a combination magnetic field of the magnetic fields respectively generated by bit line BL and word line WL can be different from that generated by bit line Br and word line WL. 
   A manufacturing process forms current path branch WLa in dielectric layer  61  under the reference magnetic memory cell C ref  by etching dielectric layer  61 , plating metal materials and chemical mechanical polishing. Contact vias connected to word line WL and current path branch WLa are formed by etching isolation layer  63 , plating metal materials and chemical mechanical polishing. Isolation layer  65  is formed after forming word line WL, and reference magnetic memory cell C ref , magnetic memory cell C data  and bit lines are formed using conventional process. 
     FIG. 7  shows asteroid curve  71  of reference magnetic memory cell C ref  for bit line Br and word line WL, and asteroid curve  73  of magnetic memory cell C data  for bit line BL and word line WL according to the embodiment shown in  FIG. 6 . The total magnetic field generated by currents through word line WL and current path branch WLa at reference magnetic memory cell C ref  is less than that generated by a single word line WL at magnetic memory cell C data . Thus, when writing data to magnetic memory cell C data , reference magnetic memory cell C ref  influenced by the magnetic field generated from the current through word line WL is mitigated. Therefore, middle state resistance of reference magnetic memory cell C ref  is stabilized, decreasing data reading disturbance. 
     FIG. 8  is a top view of an MRAM array according to another embodiment of the invention. In  FIG. 8 , line width of word line WL near magnetic memory cell C data  is W 1 , near reference magnetic memory cell C ref  is W 2 , wherein line width W 2  exceeds W 1 . The expanded word line results in the generated magnetic field decreasing as the current density is decreased. Thus, the generated magnetic field on reference magnetic memory cell C ref  is decreased with the same current. Here, a combination magnetic field of the magnetic fields respectively generated by bit line BL and word line WL can be different from that generated by bit line Br and word line WL. 
     FIG. 7  shows asteroid curve  71  of reference magnetic memory cell C ref  for bit line Br and word line WL, and asteroid curve  73  of magnetic memory cell C data  for bit line BL and word line WL according to the embodiment shown in  FIG. 8 . In this embodiment, the magnetic field generated from the current through word line WL influences reference magnetic memory cell C ref  less than magnetic memory cell C data  when writing data to magnetic memory cell. Thus, middle state resistance of reference magnetic memory cell C ref  is stabilized, decreasing data reading disturbance. 
     FIG. 9  is a top view of an MRAM may according to another embodiment of the invention. In  FIG. 9 , a cladding layer  91  is formed around word line WL near magnetic memory cell C data  with an opening between the magnetic memory cell C data  and word line WL. According to the technology disclosed in U.S. Pat. No. 6,351,409, a stronger magnetic field is obtained at an opening of a cladding layer by cladding the cladding layer around a metal line. Thus, in this embodiment, the generated magnetic field on reference magnetic memory cell C ref  is less than magnetic memory cell C data  with the same current. Here, a combination magnetic field of the magnetic fields respectively generated by bit line BL and word line WL can be different from that generated by bit line Br and word line WL. 
     FIG. 7  shows asteroid curve  71  of reference magnetic memory cell C ref  for bit line Br and word line WL, and asteroid curve  73  of magnetic memory cell C data  for bit line BL and word line WL according to the embodiment shown in  FIG. 9 . In this embodiment, the magnetic field generated from the current through word line WL influences reference magnetic memory cell C ref  less than magnetic memory cell C data  when writing data to magnetic memory cell. Thus, middle state resistance of reference magnetic memory cell C ref  is stabilized, decreasing data reading disturbance. 
     FIG. 10  is a top view of an MRAM array according to another embodiment of the invention. In this embodiment, the aspect ratio of reference magnetic memory cell C ref  exceeds that of magnetic memory cell C data . As a magnetic memory cell with larger aspect ratio has a larger coercivity, a larger writing current is required to switch the resistance of reference magnetic memory cell C ref  with larger aspect ratio. Thus, in this embodiment, it is harder to switch the resistance of reference magnetic memory cell C ref  with the same magnetic field. Here, a combination magnetic field of the magnetic fields respectively generated by bit line BL and word line WL can be equal to that generated by bit line Br and word line WL. 
   A manufacturing process increases the major axis and decreases the short axis of magnetic memory cell C ref  when defining the magnetic memory cell C ref . Thus, the target of the reference magnetic memory cell C ref  having a larger aspect ratio than magnetic memory cell C data  is achieved. In addition, in another embodiment, the size of magnetic memory cell C ref  and magnetic memory cell C data  can be defined as a identical value. Thus, the resistances of the magnetic memory cells at high and low states are not changed, and the peripheral circuit can retain its original design when the aspect ratio of the reference magnetic memory cell C ref  is changed. 
     FIG. 11  shows asteroid curve  93  of reference magnetic memory cell C ref  for bit line Br and word line WL, and asteroid curve  95  of magnetic memory cell C data  for bit line BL and word line WL according to the embodiment shown in  FIG. 10 . In this embodiment, as the coercivity of reference magnetic memory cell C ref  is increased, the magnetic field generated from the current through word line WL influences reference magnetic memory cell C ref  less than magnetic memory cell C data  when writing data to magnetic memory cell. Thus, middle state resistance of reference magnetic memory cell Cref is stabilized, decreasing data reading disturbance. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.