Abstract:
The invention discloses how the insertion of a layer of CoFeB serves to increase the robustness of an MTF device by smoothing the interface between the tunnel barrier and the pinned layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the general field of magneto-resistive devices with special emphasis on reducing the R.A product without significant loss of device robustness. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is well known that the reliability and performance of a tunneling magneto-resistance (TMR) sensor is strongly dependent on the quality its barrier layer. As the resistance.area product (R.A) grows smaller in today&#39;s high density magnetic recording applications, the corresponding barrier thickness has also to be reduced. For example, when the R.A is in the 1-3 ohm·μm 2  range, the barrier thickness will typically need to be in a range of from about 5 to 10 Å. 
         [0003]    Referring now to  FIG. 1 , a typical tunneling magneto-resistance (TMR) sensor includes seed or buffer layer  11 , antiferromagnetic (AFM) layer  12 , outer pinned layer  13  (commonly referred to as anti-parallel 2 or AP2), AFM coupling layer  14 —typically, but not limited to, Ru, inner pinned layer  15  (AP1), usually CoFe, dielectric barrier layer  16 , free layer  17 , and capping layer  18 . CoFe is usually used for AP2 because of the strong exchange field (Hex) between CoFe and the AFM layer. (Most commonly, IrMn is used for the AFM layer in TMR sensors but it is to be understood that the invention disclosed below does not depend for its operation on any one particular AFM material). 
         [0004]    A routine search of the prior art was performed with the following references of interest being found: 
         [0005]    U.S. Patent Application 2008/0316657 (Zhang et al—Headway) teaches an AP2 pinned layer comprising CoFe/insertion layer/CoFe where the insertion layer can be CoFeB. This was found to improve the exchange field between CoFe and the AFM layer, as well as the sensor smoothness, no consideration having been given to its effect on the robustness of the device. 
         [0006]    U.S. Patent Application 2009/0269617 (Zhang et al—Headway) shows a pinned layer comprising three FeCo layers. 
         [0007]    U.S. Pat. No. 7,525,166 (Hosomi et al) discloses a pinned layer having a stacked structure where Co, Cofe, CoFeB may be used. 
         [0008]    U.S. Pat. No. 7,602,033 (Zhao et al—Headway) shows an inner pinned layer of CoFeB/Fe/Co and an outer pinned layer of CoFe. U.S. Pat. No. 7,616,475 (Yamamoto et al) teaches a pinned layer can be a stacked structure of Co, CoFe, CoFeB, or the like. Ru can be between magnetic stacks. 
       SUMMARY OF THE INVENTION 
       [0009]    It has been an object of at least one embodiment of the present invention to increase the density of MTJ devices in an MRAM. 
         [0010]    Another object of at least one embodiment of the present invention has been to be able to reliably manufacture devices whose barrier thickness is in the 5 to 10 Å range. 
         [0011]    Still another object of at least one embodiment of the present invention has been to provide a smooth interface for the underside of the barrier layer. 
         [0012]    These objects have been achieved by inserting an amorphous layer of CoFeB within the pinned layer (generally, but not necessarily, CoFe) 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of a typical TMR device of the prior art. 
           [0014]      FIG. 2  shows the device of  FIG. 1  in which the AP2 layer has been modified as disclosed below. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    As noted earlier, the quality of the barrier layer is of great importance. A key factor that is critical for controlling this quality is the smoothness of the film that underlies the barrier layer. The invention discloses how the smoothness of AP2 can be improved without diminishing the strong exchange field (Hex), between AP2 and the AFM layer, that was discussed above. 
         [0016]    The required improvement in AP2 is accomplished by inserting, within the standard CoFe layer normally used for AP2, amorphous layer  22  of (CoFe x )B y  (with x ranging from 0.1 to 0.7, with a range of from 0.1 to 0.5 being preferred, and y ranging from 0.05 to 0.4 with a range of from 0.15 to 0.3 being preferred). The thickness of this inserted layer should be in a range of from 3-15 Å, with from 4-10 Å being preferred. The resulting AP2 structure, as illustrated in  FIG. 2 , is thus: (CoFe) outer    21 /(CoFe x ) B y    22 /(CoFe) inner    23 , with (CoFe) outer  being closest to the AFM layer, as shown. The inserted amorphous CoFeB layer  23  serves to reduce the influence of (CoFe) outer  on the crystallinity of CoFe) inner  while at the same time compensating for surface roughness originating at the underlying IrMn material that is used for the AFM layer. 
         [0017]    The invention leaves AP1 unchanged. 
         [0018]    In order to confirm the effectiveness of the above arrangement, we compared the interlayer coupling, Hin, of a GMR (giant magneto-resistance) stack with and without an inserted amorphous CoFeB layer. The value of Hin was derived from its B-H loop. The Hin value of a GMR (and similarly a TMR) stack is known to increase monotonically with film roughness, making it a suitable measure of the underlayer roughness. 
         [0019]    TABLE I compares the interlayer coupling (Hin) for the pinned layer portion of a GMR stack where the pinned layer is CoFe only and where the pinned layer is CoFe/CoFeB/CoFe. The full structure on which the TABLE I data is based was: 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Ta50/Ru50/IrMn70/Pinned Layer/Cu25/CoFe10/NiFe70/Ru50 
               
             
          
           
               
                 Pinned Layer 
                 Hin (Oe) 
               
               
                   
               
             
          
           
               
                 18A CoFe-25% 
                 27.01 
                 Reference 
               
               
                 7A CoFe-25%/6A (CoFe-25%)B/7A CoFe-25% 
                 18.33 
               
               
                   
               
             
          
         
       
     
         [0020]    The data presented in TABLE I shows that Hin has been reduced by ⅓, confirming that the pinned layer did become smoother after a thin CoFeB layer was inserted within the original CoFe pinned layer. 
         [0021]    TABLE II compares Hex, Hc, and Hex/Hc [PLEASE DEFINE Hc] for the pinned layer portion of a GMR stack with CoFe only and a GMR stack having a CoFe/CoFeB/CoFe AP2 pinned layer. 
         [0022]    The full structure on which the TABLE II data is based was: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Ta50/Ru50/IrMn70/Pinned Layer/Ru50 
               
             
          
           
               
                   
                 Hex 
                   
                   
                   
               
               
                 Pinned Layer 
                 (Oe) 
                 Hc (Oe) 
                 Hex/Hc 
               
               
                   
               
             
          
           
               
                 18A CoFe(25%) 
                 2184 
                 337 
                 6.5 
                 Reference 
               
               
                 7A CoFe-25%/6A (CoFe-25%)B/ 
                 2105 
                 345 
                 6.1 
               
               
                 7A CoFe-25% 
               
               
                   
               
             
          
         
       
     
         [0023]    The data in TABLE II demonstrates that the changes made to AP2 by the invention do not significantly affect the exchange properties of the overall device. Thus, the invention provides us with a TMR device that is more robust, making it possible to build TMR sensors having a low R.A value (i.e. having a thinner barrier layer) without sacrificing reliability and/or performance. 
         [0024]    We note here that the general principles of the invention may be applied with equal effect to similar spintronic devices such as CPP (current perpendicular to plane) GMR devices, dual spin valves, etc. [ANY OTHERS?].