Abstract:
The present invention provides an MRAM that includes a conductive line for generating a magnetic field. The latter is enhanced by the addition of a flux concentrator made from a single plane of soft ferromagnetic material, magnetically stabilized by means of an antiferromagnetic layer. This structure, in addition to being very easy to fabricate, facilitates close control over its magnetic properties, including uniformity and domain structure.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the general field of MRAM devices with particular reference to the write operation. 
       BACKGROUND OF THE INVENTION 
       [0002]    A significant amount of power is consumed during a write operation of an MRAM cell in an MRAM device having an array of cells. The write operation consists of passing currents through conductive lines external, but in close proximity, to the MRAM magnetic element. The magnetic fields generated by these currents are required to be sufficient to switch the magnetic state of the free layer of the magnetic element. 
         [0003]    Magnetic random access memory (MRAM) using magnetic tunneling junctions (MTJ) is a strong candidate to provide a dense (areas of 8-25 f 2  where f is the smallest feature size), fast (1-30 ns read/write speed) and non volatile storage solution for future memory applications. The MTJ utilizes at least 2 magnetic layers, that sandwich a thin dielectric insulating layer like alumina or magnesia, with one magnetic layer being pinned by an antiferromagnetic film. The magnetic vectors in the layers can be oriented to be parallel or antiparallel to each other. The top magnetic layer with switchable magnetic vector is referred to as the storage or free layer. 
         [0004]    To protect data from erasure or thermal agitation, an in-plane uniaxial magnetic anisotropy is needed for the magnetic free layer to store data. The intersection of Word and Bit line currents programs the MTJs of MRAM cells. The magnitude of the magnetic field used to switch the magnetic vector is proportional to the magnitude of the currents through these conductors. To reduce power consumption, it is desirable to increase the field to current ratio of said conductors. 
         [0005]    One conventional technique for increasing the field to current ratio is to provide a magnetic liner for the conductors. An example of this is illustrated in  FIG. 1 . Shown there is MTJ stack  11  that rests on bottom electrode  12  and is topped by capping layer/top electrode  13 . Conductive bit line  14  is seen to be in contact with cap  13 . The direction of line  14  is normal to the plane of the figure. Ferromagnetic cladding layers  15  surround the bit line on three sides and serve to increase the current induced magnetic switching field that is applied to magnetic elements sitting on top of a metal line containing such a cladding layer. Power consumption and chip size may thus be reduced as less current is needed to generate a given switching field; switching word line transistors can thus also be made smaller. This is important because, for small magnetic elements (about 0.1 micron), it is expected that the switching fields will need to increase, causing the switch transistors to consume a significant amount of chip area. 
         [0006]    Ferromagnetic cladding layers are typically made by wrapping a ferromagnetic material around the metal line. The magnetization of the ferromagnetic material is along the long axis of the metal line, thus creating poles at both ends. It has been shown that at such poles the fringing field is of the order of 50 to 300 Gauss. The magnetic cells should thus be some distance from the line ends in order not to shift the switching point of the cells. A safe distance can be calculated to be several micrometers—such a distance results in significant limitations to the chip layout since the arrays in memory chips should be packed as densely as possible. 
         [0007]    Depending on current direction, ferromagnetic cladding layers may also show a hysteresis effect. The fields for switching of the ferromagnetic cladding layer magnetization direction are normally much higher than the fields for magnetic elements. There can also be domain effects in a ferromagnetic cladding layer which allow a partially local switching of the ferromagnetic cladding layer when a current is passed through the line. This partial switching broadens the switching field distribution for the magnetic elements thereby significantly reducing the write margin. The fringing fields and hysteresis effects associated with a ferromagnetic cladding layer thus limit the use of the ferromagnetic cladding layer in a memory chip having tight element spacing and thousands of elements in a sub-array. 
         [0008]    In order to overcome these problems, liners having super-paramagnetic properties have been reported. The cladding layer in this case comprises ferromagnetic particles with a non-exchange coupled microstructure where the size of the micro domains is so small that their energy content is close to or below kT (on the order of up to 5 times kT) so such films have super paramagnetic properties and essentially behave like a paramagnet with high susceptibility. So a cladding layer of these particles acts as a super-paramagnet. 
         [0009]    Without an external magnetic field induced by the current, local magnetic moments cancel each other out (super paramagnetic effect); with an external magnetic field the local magnetic moments are aligned by the current induced field to give the desired field enhancement effect. Thus, when a super-paramagnetic material is used, no hysteresis effects and no fringing fields occur. 
         [0010]    The super-paramagnetic films discussed above are made either from small ferromagnetic particles in a polymeric matrix, as a particle film, or evaporated multi-layer films made out of repeated layers of Terbium and ferromagnetic particles, or deposition of ferromagnetic nano-particles. 
         [0011]    A routine search of the prior art was performed with the following references of interest being found: 
         [0012]    U.S. Pat. No. 6,943,038, U.S. Patent Applications 2005/0208681, 2005/0164413 (Meixner et al) disclose a cladding layer formed within a trench over a bit line. The cladding layer is flux-concentrating. No insulator is formed between the bit line and the cladding layer. U.S. Pat. No. 6,885,074, U.S. Patent Applications 2004/0099908, 2005/0158992 (Durlam et al) and U.S. Pat. No. 6,559,511 (Rizzo) describe cladding on the top and sides of bit lines. 
         [0013]    U.S. Patent Application 2005/0026308 (Ingvarsson et al) shows a super-paramagnetic lining on the top and sides of a conductor. U.S. Patent Application 2005/0274997 (Gaidis et al) teaches a magnetic flux layer on the top and sides of the write line. U.S. Patent Application 2006/0083053 (Hosotani) shows a soft magnetic flux concentrating layer formed around the bit line. 
       SUMMARY OF THE INVENTION 
       [0014]    It has been an object of at least one embodiment of the present invention to provide an effective flux concentrator for use with an MRAM Bit line 
         [0015]    Another object of at least one embodiment of the present invention has been to provide a low cost process for the manufacture of said flux concentrator. 
         [0016]    Still another object of at least one embodiment of the present invention has been that said flux concentrator be free of hysteresis effects. 
         [0017]    These objects have been achieved by constructing the flux concentrator from a single plane of soft ferromagnetic material, magnetically stabilized by means of an antiferromagnetic layer. This structure, in addition to being very easy to fabricate, facilitates close control over its magnetic properties, including uniformity and domain structure. Magnetic liners of the type described in the prior art may be optionally included but this makes for a more expensive structure. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  illustrates a prior art structure for concentrating the write field 
           [0019]      FIGS. 2-4  illustrate steps in the process of the present invention 
           [0020]      FIG. 5  explains the basic principle of the present invention. 
           [0021]      FIGS. 6 and 7  are computed curves showing the effects of layout density on field interference between adjacent devices in an array. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    In this invention, a planar soft ferromagnetic layer that serves as a flux concentrator for the conductive bit line, as part of a magnetic random access memory, is disclosed. Its fabrication process is very simple and its magnetic properties, including uniformity, and domain structure are easily controlled. The magnetic liners described in the prior art for use as flux concentrators are difficult to fabricate. Magnetic material/domains may not be sufficiently uniform and stable to serve as good flux concentrators. 
         [0023]    We now describe the process of the present invention which will also make clear the structure of the present invention. Referring now to  FIG. 2 , the process of the present invention begins with the provision of MTJ stack  11 , including capping layer  13  and bottom electrode  12 , all these elements being encapsulated in dielectric layer  21 . 
         [0024]    As shown in  FIG. 2 , trench  22  is then etched (using RIE or other suitable etching process) in dielectric layer  21  its dimensions being such that it fully overlaps capping layer  13 . It extends downwards as far as the top surface of capping layer  13 . 
         [0025]    At this point, as an optional step, a layer may be deposited on the vertical sidewalls of trench  22 . This layer may be either a super-paramagnetic material (having a thickness between about 200 and 1,000 Angstroms) or a ferromagnetic material (between about 50 and 1,000 Angstroms thick). 
         [0026]    Then, as seen in  FIG. 3 , trench  22  is just filled with a conductive material, typically Cu, thereby forming bit line  31  that passes over MTJ stack  11  and that has a top surface coplanar with the top surface of dielectric  21 . This is accomplished by first over-filling the trench, following which a chemical mechanical polishing (CMP) process is used to remove all conductive material (and liner material if used) above the top surface of dielectric  21 , thus electrically isolating individual conductors from each other. 
         [0027]    We next refer to  FIG. 4  where we illustrate the key features of the invention. Insulating layer  41  is deposited onto the monoplanar surface shared by dielectric layer  21  and bit line  31 . Insulating layer  41  is deposited to a thickness between about 100 and 500 Angstroms. Optionally, layer  41  may also be highly permeable which would serve to further enhance the magnetic field. 
         [0028]    This is followed by the deposition, in succession, of ferromagnetic layer  42  (to a thickness of between about 50 and 500 Angstroms} and antiferromagnetic layer  43 . Examples of layer  43  include IrMn while examples of ferromagnetic layer  42  include NiFe, NiFeCo, and NiFeX, where X is a non-magnetic metal known to be a magnetic moment dilution agent. 
         [0029]    Note that it is important that ferromagnetic layer  42  be given a permeability normal to the bit line direction of between about 10 and 300, with around 100 being preferred. This can be achieved by adjusting the thickness of layer  42  and/or through the addition to ferromagnetic layer  42  of one or more dopants such as Ta, Cr, Pt, Ti, B, W etc. 
         [0000]    Note that, as an option, a seed layer of NiCr or NiFeCr [ANY OTHERS?] may be deposited on insulating layer  41  immediately prior to the deposition of ferromagnetic layer  42 . This seed layer would typically have a thickness of between about 30 and 50 Angstroms. 
         [0030]    This concludes the process for forming the flux concentrator except for a required annealing step. This comprises heating at a temperature between about 150 and 250° C. for between about 5 and 200 minutes in the presence of a magnetic field of between about 50 and 2,000 Oe, whose direction is parallel to that of the bit line. This results in the formation of a domain stabilizing field for ferromagnetic layer  42 . 
         [0031]    The working principle of the present invention is illustrated in  FIG. 5 . Before a current is applied along conductive bit lines, the magnetization in the planar flux concentrating layer  52  is uniformly aligned in the direction of the bit line by the pinning field of its AFM component. As a current is applied in the bit line, the magnetization in region  53  of the planar layer (which is located close to the top of the bit line) rotates while the magnetization in other regions  54  stays essentially unchanged, yielding magnetic charges near the two edges of the bit line. These magnetic charges generate an additional magnetic field  55  on the MTJ free layer, enhancing the total write field by a significant amount. 
         [0032]    A computed example is given in  FIG. 6 . The flux concentrating layer is 25 nm thick and separated by an insulator 25 nm thick from the bit line top surface. The bit line is 0.7 micron wide and 0.225 micron thick. The MTJ free element with 0.3×0.6 sq. microns of area is 50 nm below the bit line bottom surface. The field enhancement by the flux concentrating layer as a percentage is represented by curve  61 , while the field interferences by its two adjacent bit line currents are represented by curve  62  (bit line pitch of 1.0 micron) and curve  63  (bit line pitch of 1.2 micron). 
         [0033]    Another calculated example is given in  FIG. 7 . The flux concentrating layer is 25 nm thick and separated by an insulator 25 nm thick from the bit line top surface. On two side surfaces of the bit line, 50 nm super-paramagnetic material (or ferromagnetic material) is formed as side liners with relative permeability of 10. The bit line width is 0.7 micron wide and 0.45 micron thick. The MTJ free element with 0.3×0.6 micron 2  dimension is 50 nm below the bit line bottom surface. The field enhancement by the flux concentrating layer is represented as a percentage by curve  71 , while the field interferences by its two adjacent bit line currents are represented by curve  72  (bit line pitch of 1.0 micron). 
         [0034]    We note here that the structure of the present invention is easily made, having well controllable, uniform and stable magnetic properties, and having much higher write field and smaller adjacent bit line current interference which is well-suited for magnetic cladding function.