Abstract:
The present invention provides techniques for data storage. In one aspect of the invention, a semiconductor device is provided. The semiconductor device comprises at least one free layer and at least one fixed layer, with at least one barrier layer therebetween. At least one pinned magnetic layer is separated from the at least one free layer by at least one non-magnetic layer, the at least one pinned magnetic layer and non-magnetic layer being configured to cancel out at least a portion of a Neel coupling between the at least one free layer and the at least one fixed layer.

Description:
STATEMENT OF GOVERNMENT RIGHTS  
       [0001]     This invention was made with Government support under grant contract number MDA972-99-C-0009 awarded by the Defense Advanced Research Projects Agency (DARPA) of the United States Department of Defense. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to semiconductor devices and, more particularly, to semiconductor devices for data storage.  
       BACKGROUND OF THE INVENTION  
       [0003]     Certain semiconductor devices, e.g., magnetic random access memory (MRAM) devices, use magnetic memory cells to store information. Each magnetic memory cell typically comprises a submicron piece of magnetic material, e.g., having the dimensions of 300 nanometers (nm) by 600 nm in area and five nm thick.  
         [0004]     Information is stored in such semiconductor devices as an orientation of the magnetization of a free layer in the magnetic memory cell as compared to an orientation of the magnetization of a fixed (e.g., reference) layer in the memory cell. The magnetization of the free layer may be oriented parallel or anti-parallel relative to the fixed layer, representing either a logic “1” or a “0.” The orientation of the magnetization of a given layer (fixed or free) may be represented by an arrow pointing either to the left or to the right. When the magnetic memory cell is sitting in a zero applied magnetic field, the magnetization of the magnetic memory cell is stable, pointing either left or right. The application of a magnetic field can switch the magnetization of the free layer from left to right, and vice versa, to write information to the magnetic memory cell.  
         [0005]     A particular type of magnetic memory cell, a “toggle” magnetic memory cell, employs a free layer comprising two magnetic layers separated by a non-magnetic spacer layer. This multi-layer free layer is typically separated from a fixed layer of the magnetic memory cell by a tunnel barrier, such that one of the multiple layers making up the free layer is adjacent to the tunnel barrier. It is this layer adjacent to the tunnel barrier that primarily determines a resistance of the tunnel junction, which relates to the information stored.  
         [0006]     The fixed layer typically comprises one or more pinned layers. The one or more pinned layers can be formed from a single magnetic layer (e.g., a simple pinned layer), or alternatively, from two magnetic layers tightly anti-parallel coupled by a non-magnetic spacer layer (e.g., an anti-parallel (AP) pinned layer). The one or more pinned layers, either simple or AP, have a fixed direction of magnetization corresponding to an anti-ferromagnetic layer to which the one or more pinned layers are pinned.  
         [0007]     With these typical toggle magnetic memory cell configurations, however, Neel coupling often causes the one or more pinned layers to disturb the operation of the free layer. Neel coupling, also referred to as “orange peel” coupling, arises from the formation of magnetic poles on the rough magnetic interfaces of the magnetic layers, e.g., between the one or more pinned layers, the free layer and the barrier layer.  
         [0008]     The magnetic poles formed induce the directions of magnetization of the magnetic layers on either side of the barrier to line up parallel to each other. One way to reduce Neel coupling is to reduce surface interface roughness of the layers. Reducing the roughness of the layers, however, is difficult under typical manufacturing conditions, and in practice, some residual roughness always remains. Therefore, some amount of Neel coupling is always present.  
         [0009]     Neel coupling disadvantageously reduces the write margins of the semiconductor device. Therefore, it would be desirable to reduce, or eliminate, the effects of Neel coupling on the switching of toggle magnetic memory cells.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides techniques for data storage. In one aspect of the invention, a semiconductor device is provided. The semiconductor device comprises at least one free layer and at least one fixed layer, with at least one barrier layer therebetween. At least one pinned magnetic layer is separated from the at least one free layer by at least one non-magnetic layer, the at least one pinned magnetic layer and non-magnetic layer being configured to cancel out at least a portion of a Neel coupling between the at least one free layer and the at least one fixed layer.  
         [0011]     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a diagram illustrating a toggle magnetic memory cell;  
         [0013]      FIG. 2  is a diagram illustrating an exemplary toggle magnetic memory cell in a “1” state configuration having an anti-parallel (AP) pinned layer to counteract Neel coupling effects according to an embodiment of the present invention;  
         [0014]      FIG. 3  is a diagram illustrating an exemplary toggle magnetic memory cell in a “0” state configuration having an AP pinned layer to counteract Neel coupling effects according to an embodiment of the present invention; and  
         [0015]      FIG. 4  is a graph illustrating an exemplary toggle magnetic memory cell having a single pinned layer to counteract Neel coupling effects according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIG. 1  is a diagram illustrating a toggle magnetic memory cell. As used herein, the term “toggle magnetic memory cell” refers to magnetic memory cells having a free layer comprising multiple magnetic layers. In  FIG. 1  toggle magnetic memory cell  100  comprises free magnetic layers  102  and  106 , labeled “free 2” and “free 1,” respectively, separated by non-magnetic spacer layer  104 , labeled “free non-magnetic spacer.” Toggle magnetic memory cell  100  also comprises barrier layer  108 , labeled “barrier.” As will be described in detail below, barrier layer  108  separates a free layer from a fixed layer of toggle magnetic memory cell  100 . Below barrier layer  108 , are pinned magnetic layers  110  and  114 , labeled “pinned 2” and “pinned 1,” respectively, anti-parallel coupled by non-magnetic spacer layer  112 , labeled “pinned non-magnetic spacer.” Each of pinned magnetic layers  110  and  114 , being anti-parallel coupled to each other, are thus each also pinned to anti-ferromagnetic layer  116 , labeled “anti-ferromagnet.” 
         [0017]     Free magnetic layers  102  and  106  with non-magnetic spacer layer  104  therebetween, form a free layer of toggle magnetic memory cell  100 . Pinned magnetic layers  110  and  114 , and non-magnetic spacer layer  112  therebetween, as well as, associated anti-ferromagnetic layer  116  form a fixed layer of toggle magnetic memory cell  100 . The free layer and the fixed layer of toggle magnetic memory cell  100  are thus separated by barrier layer  108 .  
         [0018]     Although the layers in  FIG. 1 , for simplicity, are shown to have smooth surfaces, typical manufacturing and fabrication processes result in the layers having a rough surface similar to that found on the surface of an orange peel. This interface roughness that exists, e.g., between free magnetic layer  106  and barrier layer  108 , and between barrier layer  108  and pinned magnetic layer  110 , results in Neel coupling of the free and fixed layers, such that the fixed layer disturbs the operation of the free layer. Namely, due to the striations on the surfaces of these layers, small magnetic poles form, as shown in the magnified view of the interface between free magnetic layer  106 , barrier layer  108  and pinned magnetic layer  110 . These poles attract each other causing coupling of, e.g., free magnetic layer  106  and pinned magnetic layer  110 . As will be described in detail below, this coupling affects the switching properties of the free layer.  
         [0019]     According to the techniques of the invention presented herein, the effects of Neel coupling in toggle magnetic memory cells may be reduced or eliminated by employing an anti-parallel (AP) pinned layer associated with a free layer of a toggle magnetic memory cell.  FIG. 2  is a diagram illustrating an exemplary toggle magnetic memory cell in a ‘1’ state configuration having an AP pinned layer to counteract Neel coupling effects.  
         [0020]     In  FIG. 2 , toggle magnetic memory cell  200  comprises anti-ferromagnetic layer  202  (labeled “anti-ferromagnet 2”) for pinning the direction of magnetization of pinned layers  204  and  208 , labeled “pinned 4” and “pinned 3,” respectively, tightly anti-parallel coupled by non-magnetic spacer layer  206  (labeled “pinned non-magnetic spacer 2”) therebetween, e.g., having a coupling field strength greater than, for example, the applied field and the saturation field.  
         [0021]     Adjacent to pinned layer  208  is non-magnetic spacer layer  210  (labeled “Neel non-magnetic spacer”). Adjacent to non-magnetic spacer layer  210 , opposite pinned layer  208 , is free layer  212  (labeled “free 2”) separated from free layer  216  (labeled “free 1”) by non-magnetic spacer layer  214  (labeled “free non-magnetic spacer”).  
         [0022]     Adjacent to free layer  216  is barrier layer  218  (labeled “barrier”). Adjacent to barrier layer  218 , opposite free layer  216 , is pinned layer  220  (labeled “pinned 2”) tightly anti-parallel coupled to pinned layer  224  (labeled “pinned 1”) by non-magnetic spacer layer  222  (labeled “pinned non-magnetic spacer 1”). Adjacent to pinned layer  224 , opposite non-magnetic spacer layer  222 , is anti-ferromagnetic layer  226  (labeled “anti-ferromagnet 1”).  
         [0023]     According to an exemplary embodiment of the present invention, pinned layers  220  and  224  and non-magnetic spacer layer  222  therebetween, as well as anti-ferromagnetic layer  226  associated therewith form a fixed layer of toggle magnetic memory cell  200 . Free layers  212  and  216  and non-magnetic spacer layer  214  therebetween form a free layer of toggle magnetic memory cell  200 . Pinned layers  204  and  208  and non-magnetic spacer layer  206  therebetween, as well as anti-ferromagnetic layer  202  associated therewith, form a Neel coupling counteractive layer of toggle magnetic memory cell  200 . Each of the fixed layer, free layer and Neel coupling counteractive layer, are not limited to a particular number or configuration of respective layers.  
         [0024]     The Neel coupling counteractive layer and the free layer are separated by non-magnetic spacer layer  210 . The free layer and the fixed layer are separated by barrier layer  218 .  
         [0025]     Anti-ferromagnetic layer  202  and anti-ferromagnetic layer  226  each may comprise a metal alloy, including, but not limited to, platinum/manganese alloys (PtMn), iridium/manganese alloys (IrMn) and combinations comprising at least one of the foregoing alloys. Anti-ferromagnetic layer  202  and anti-ferromagnetic layer  226  may have the same, or different, compositions. In an exemplary embodiment, anti-ferromagnetic layer  202  and anti-ferromagnetic layer  226  have the same composition.  
         [0026]     Pinned layers  204  and  208  each may comprise a ferromagnetic material. Suitable ferromagnetic materials include, but are not limited to, cobalt/iron alloys (CoFe). Pinned layers  204  and  208  may have the same, or different, compositions. In an exemplary embodiment, pinned layers  204  and  208  have the same composition.  
         [0027]     Non-magnetic spacer layer  206  may comprise a non-magnetic material. Suitable non-magnetic materials include, but are not limited to, ruthenium (Ru).  
         [0028]     Non-magnetic spacer layer  210  may comprise any non-magnetic material that adds little, if any, resistance to the toggle magnetic memory cell structure, or a tunnel barrier material where the tunnel magnetoresistance between pinned layer  208  and free layer  212  is inverse magnetoresistance. Suitable non-magnetic materials include, for example, non-magnetic metallic materials, including, but not limited to, tantalum (Ta), tantalum nitride (TaN) and combinations comprising at least one of the foregoing non-magnetic metallic materials. Further, as will be described in detail below, the thickness of non-magnetic spacer layer  210  may be chosen so as to counteract at least a portion of the Neel coupling experienced between the free layer and the fixed layer of the device.  
         [0029]     Free layers  212  and  216  may comprise any suitable material, including, but not limited to, nickel/iron alloys (NiFe), cobalt/iron/boron alloys (CoFeB) and combinations comprising at least one of the foregoing alloys. Free layers  212  and  216  may have the same, or different, compositions. In an exemplary embodiment, free layers  212  and  216  have the same composition.  
         [0030]     Non-magnetic spacer layer  214  may comprise a non-magnetic material including, but not limited to, Ru. Barrier layer  218  may comprise, for example, aluminum (Al) oxides of the general formula, AlO x , including, but not limited to, Al 2 O 3 . Alternative materials may also be employed (e.g., tantalum (Ta) oxides of the general formula, TaO x  and magnesium (Mg) oxides of the general formula, MgO x ).  
         [0031]     As with pinned layers  204  and  208 , each of pinned layers  220  and  224  may comprise a ferromagnetic material including, but not limited to, CoFe, and may have the same, or different, compositions. In an exemplary embodiment, pinned layers  220  and  224  have the same composition. Further, as with non-magnetic spacer layer  206 , non-magnetic spacer layer  222  may comprise a non-magnetic material, including, but not limited to Ru.  
         [0032]     As shown in  FIG. 2 , when toggle magnetic memory cell  200  is in a particular state, e.g., a “1” state, free layer  216  has a direction of magnetization opposite to the direction of magnetization of pinned layer  220 . As described above, for example, in conjunction with the description of  FIG. 1 , Neel coupling between free layer  216  and pinned layer  220  can result in the direction of magnetization of free layer  216  being influenced by, and lining up parallel to, the direction of magnetization of pinned layer  220 .  
         [0033]     The Neel coupling counteractive layer, however, counteracts at least a portion of the Neel coupling between free layer  216  and pinned layer  220 . Specifically, anti-ferromagnetic layer  202  pins (i.e., fixes) the direction of magnetization of pinned layer  204 . Since pinned layer  204  is anti-parallel coupled to pinned layer  208 , e.g., by non-magnetic spacer layer  206 , its direction of magnetization will remain opposite to that of pinned layer  208 .  
         [0034]     Similar to free layer  216  and pinned layer  220 , surface interface roughness between free layer  212  and non-magnetic spacer layer  210 , and between non-magnetic spacer layer  210  and pinned layer  208  results in Neel coupling of free layer  212  and pinned layer  208 . Since, as was mentioned above, free layers  212  and  216  are anti-parallel coupled to one another, e.g., by non-magnetic spacer layer  214 , and thus will have directions of magnetization that will remain opposite to each other, the Neel coupling between pinned layer  208  and free layer  212  can counteract at least a portion of the effects of the Neel coupling between free layer  216  and pinned layer  220 .  
         [0035]     According to this exemplary embodiment, pinned layer  208  should have a direction of magnetization the same as the direction of magnetization of pinned layer  220 . Obtaining this particular direction of magnetization for pinned layer  208  is easy since, as mentioned above, pinned layer  208  is anti-parallel coupled to pinned layer  204 , pinned layer  204  having a direction of magnetization set by anti-ferromagnetic layer  202 .  
         [0036]     Further, as was mentioned above, the thickness of non-magnetic spacer layer  210  may be chosen to increase or decrease an intensity of the Neel coupling between pinned layer  208  and free layer  212  as desired, so as to achieve a proper counterbalancing of the Neel coupling between free layer  216  and pinned layer  220 . Namely, a thicker non-magnetic spacer layer  210  will result in less Neel coupling between pinned layer  208  and free layer  212 , while a thinner non-magnetic spacer layer  210  will result in greater Neel coupling between pinned layer  208  and free layer  212 . In an exemplary embodiment of the present invention, non-magnetic spacer layer  210  has a thickness that is substantially similar to the thickness of barrier layer  218 , e.g., ±5 angstroms (Å), such that the amount of Neel coupling experienced between pinned layer  208  and free layer  212  is substantially similar to, e.g., the same as, that experienced between free layer  216  and pinned layer  220 , thus reducing or eliminating the net effective Neel coupling (e.g., zero net Neel coupling). In this exemplary embodiment, barrier layer  218  has a thickness of, e.g., from about ten Å to about 15 Å.  
         [0037]      FIG. 3  is a diagram illustrating the exemplary toggle magnetic memory cell  200  shown in  FIG. 2  in a “0” state configuration having an AP pinned layer to counteract Neel coupling effects. In  FIG. 3 , toggle magnetic memory cell  200  has the same composition and configuration of layers as that shown illustrated in  FIG. 2 . The directions of magnetization of free layers  212  and  216  are, however, both opposite to those shown in  FIG. 2  (as well as being opposite to each other).  
         [0038]     The effects of the Neel coupling counteractive layer are the same as described above in conjunction with the description of  FIG. 2 . Namely, while the direction of magnetization of free layer  216  is now parallel to the direction of magnetization of pinned layer  220 , the direction of magnetization of free layer  212  (anti-parallel coupled to free layer  216 ) is now anti-parallel to pinned layer  208 . Therefore, the effects of Neel coupling between free layer  216  and pinned layer  220  are sufficiently counteracted in both the exemplary “1” state, as in  FIG. 2 , and the exemplary “0” state, as in  FIG. 3 .  
         [0039]     In another exemplary embodiment of the present invention, the Neel coupling counteractive layer and the fixed layer comprise a single pinned layer, as shown, for example, in  FIG. 4 . Employing single pinned layers allows for the production of an easy axis offset in the free layer. In  FIG. 4 , toggle magnetic memory cell  400  comprises pinned layer  402  (labeled “pinned 2a”) pinned to anti-ferromagnetic layer  202 . Adjacent to pinned layer  402  opposite anti-ferromagnet layer  202  is non-magnetic spacer layer  210 . Adjacent to non-magnetic spacer layer  210  opposite pinned layer  402  is free layer  212  anti-parallel coupled to free layer  216  by non-magnetic spacer layer  214 . Adjacent to free layer  216  opposite non-magnetic spacer layer  214  is barrier layer  218 . Adjacent to barrier layer  218  opposite free layer  216  is pinned layer  404  (labeled “pinned 1a”) pinned to anti-ferromagnetic layer  226 .  
         [0040]     Like pinned layers  204 ,  208 ,  220  and  224  of toggle magnetic memory cell  200  shown in  FIG. 2  and  FIG. 3 , pinned layers  402  and  404  may comprise a ferromagnetic material including, but not limited to, CoFe, and may have the same, or different, compositions. In an exemplary embodiment, pinned layers  402  and  404  have the same composition.  
         [0041]     In the exemplary configuration shown illustrated in  FIG. 4 , anti-ferromagnetic layer  202  and pinned layer  402  form a Neel coupling counteractive layer of toggle magnetic memory cell  400 . Free layers  212  and  216  and non-magnetic spacer layer  214  therebetween form a free layer of toggle magnetic memory cell  400 . Pinned layer  404  and anti-ferromagnetic layer  226  form a fixed layer of toggle magnetic memory cell  400 .  
         [0042]     In the exemplary toggle magnetic memory cell configuration shown in  FIG. 4 , Neel coupling would be present between free layer  216  and pinned layer  404  due to surface interface roughness between free layer  216  and barrier layer  218 , and between barrier layer  218  and pinned layer  404 . However, at least a portion of this Neel coupling effect can be counteracted by the Neel coupling that is present between pinned layer  402  and free layer  212 . Namely, as was described above in conjunction with the description of  FIG. 2 , surface interface roughness between pinned layer  402  and non-magnetic spacer layer  210 , and between non-magnetic spacer layer  210  and free layer  212  results in Neel coupling between pinned layer  402  and free layer  212 .  
         [0043]     Further, as was described above, non-magnetic spacer layer  210  can be chosen to have a thickness such that the resulting Neel coupling between pinned layer  402  and free layer  212  will counteract at least a portion of the Neel coupling present between free layer  216  and pinned layer  404 . It is important to note that pinned layer  402  and pinned layer  404  have the same direction of magnetization. This orientation thus can effectively cancel out at least a portion of the effects of Neel coupling for either state, e.g., “1” or “0,” that the free layer might be in.  
         [0044]     In an exemplary embodiment, to have the directions of magnetization of pinned layer  402  and pinned layer  404  be the same, anti-ferromagnetic layers  202  and  226 , used to pin pinned layers  402  and  404 , respectively, are the same, e.g., have the same composition. Also, having the directions of magnetization of pinned layer  402  and pinned layer  404  be the same makes setting the magnetization of anti-ferromagnetic layers  202  and  226  easy using conventional annealing processes.  
         [0045]     Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope or spirit of the invention.