Abstract:
The effectiveness of an IrMn pinning layer in a CPP GMR device at high switching fields has been improved by replacing the conventional single layer seed by a layer of tantalum and either ruthenium or copper. The tantalum serves to cancel out the crystallographic influence of underlying layers while the ruthenium or copper provide a suitable base on which to grow the IrMn layer.

Description:
This application is related to U.S. patent application Ser. No. 11/043,839 filed Jan. 26, 2005, owned by a common assignee as the instant invention. 
   FIELD OF THE INVENTION 
   The invention relates to the general field of CPP GMR devices with particular reference to the layer that seeds the pinning layer. 
   BACKGROUND OF THE INVENTION 
   The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment. 
   The key elements of a spin valve are illustrated in  FIG. 1 . They are seed layer  11  (lying on lower conductive lead  10 ) on which is antiferromagnetic layer  12  whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer  14  between two antiparallel ferromagnetic layers  13  (AP 2 ) and  15  (AP 1 ). 
   Next is a non-magnetic spacer layer  16  on which is low coercivity (free) ferromagnetic layer  17 . A capping layer such as lead  18  lies atop free layer  17 . When free layer  17  is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. 
   If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%. 
   GMR devices may be designed so as to measure the resistance of the free layer for current flowing parallel or perpendicular to its two surfaces. The former is referred to as a CIP (current in plane) device while the latter is called a CPP (current perpendicular to plane) device. 
   For read head application, a CPP spin valve structure grows on a NiFe bottom shield directly, which is different from a CIP GMR reader sensor. In the latter case, the spin valve structure was grown on an insulation gap layer, typically alumina. Since NiFe is an ordered structure, it reduces the effectiveness of the seed layer/AFM combination that exists in the CIP GMR case. The present invention discloses a seed layer that is more suitable for CPP GMR spin valve structures. 
   A routine search of the prior art found the following references to be of interest: 
   In U.S. Pat. No. 6,624,985, Freitag et al. disclose a Ru/Si and NiFeCr seed layer for PtMn. In US 2004/0179311, Lin et al. (Headway) teach that a typical seed layer is Ta and NiCr. US 2003/0030945 (Heinonen et al.) shows a NiFeCr or Ta seed layer while US 2002/0051330 (Heijden et al.) shows a seed layer that may comprise Cu. 
   SUMMARY OF THE INVENTION 
   It has been an object of at least one embodiment of the present invention to provide a CPP GMR device having improved pinning by an IrMn AFM layer, particularly in the presence of high switching fields. 
   Another object of at least one embodiment of the present invention has been to provide a process for manufacturing said CPP device. 
   Still another object of at least one embodiment of the present invention has been that said process be compatible with existing processes for the manufacture of CPP devices. 
   These objects have been achieved by replacing the conventional single layer seed by a layer of tantalum and either ruthenium or copper. While the tantalum serves to cancel out the crystallographic influence of an underlying NiFe layer, the ruthenium or copper provide a suitable base on which to grow an IrMn layer without introducing any problems of a magnetic nature. In particular, the pinning strength of the IrMn in the presence of a high field environment is significantly increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a GMR stack of the prior art which has a conventional. 
       FIG. 2  shows a GMR stack according to the teachings of the present invention. 
       FIG. 3  is the same as  FIG. 2  with a different seed layer. 
       FIG. 4  is a plot of the magneto-resistance as a function of applied field. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Ta has traditionally been used to prevent propagation of the ordered structure of the bottom shield. Ta/NiFe was then used as the seed layer for IrMn, but the presence of the magnetic layer NiFe, between the Ta and the IrMn, can interfere with the magnetic performance, which is not desirable. This problem is overcome in the present invention. 
   Referring now to  FIG. 2 , we provide a description of the process of the present invention. In the course of this description, the structure of the present invention will also become apparent. 
   The process begins with what is also its key novel feature, namely the formation of the seed structure onto which is to be deposited pinning layer  12 . Instead of a single seed layer, a bilayer structure comprising tantalum layer  21  (between about 20 and 70 Angstroms thick) and ruthenium layer  22  (between about 10 and 50 Angstroms thick) is used. As noted earlier, the tantalum prevents propagation of any structural influence from layer  12  while the ruthenium layer re-establishes a structural foundation to influence the growth of antiferromagnetic layer  12  of IrMn, but without introducing complications of a magnetic nature. Layer  12  is deposited to a thickness between 50 and 100 Angstroms. 
   Layer  13  (known as AP 2  which is typically a low coercivity bilayer of FeCo 3  on FeCo to a total thickness of 20 to 50 Angstroms), the first of the two antiparallel layers that will form the synthetic AFM pinned layer, is then deposited onto layer  12 . This is followed by layer of AFM coupling material  14 , to a thickness of between 4 and 10 Angstroms, and then AP 1  layer  15  (also having low coercivity) is deposited thereon to a thickness between 20 and 50 Angstroms. The antiparallel orientations of AP 1  and AP 2  relative to one another will be effected later through use of a suitable magnetic anneal step. 
   Next, non-magnetic spacer layer  16  (Cu1/AlCu/OX/Cu2 NOL; Cu1 1-10 Å; AlCu 4-12 Å; Cu2 1-10 Å; NOL=nano-oxide layer; OX=plasma etch at 20 W in 50 sccm Ar for 35 sec., followed by plasma oxidation at 27 W in 50 sccm Ar/1 sccm O 2  for 30 sec.) is deposited on AP 1  layer  15  followed by the formation of free layer  17 . The latter is typically CoFe/NiFe deposited to a thickness of between about 30 and 70 Angstroms. The process concludes with the deposition of capping layer  18  on free layer  17 , typically Cu/Ru/Ta/Ru deposited to a thickness of between about 35 and 170 Angstroms. 
   The structure and process represented in  FIG. 3  are the same as for  FIG. 2  except that the seed now comprises copper layer  31  (between about 10 and 50 Angstroms thick) on tantalum layer  21  (between about 20 and 70 Angstroms thick). 
   Confirmatory Results 
   The most visible way in which the new seed layer structure will manifest its influence on device performance is in an increased value of Rhin. This quantity is defined in  FIG. 4  where we show a plot of (normalized) magneto-resistance MR as a function of an applied magnetic field (e.g. the switching field used during device operation). The maximum MR is seen to appear almost immediately at very low fields (curve  41 ). It remains constant till a field of about 600 Oe is reached, at which point it begins a gradual decline (curve  42 ). Rhin is then defined as the percentage (of the maximum MR) of the MR at 1,000 Oe. 
   If the process described above was correctly used to form the device, it will be found to have a GMR ratio of at least 5%, an R.A product no greater than about 0.4, and an Rh in  of at least 90%. To confirm this, four samples (one reference and three having the new seed structure) were prepared and then evaluated for CPP GMR value, R.A product, and Rhin:
     A.referenceTa5/NiCr45/IrMn70/Fe(25%)Co36/Ru7.5/[Fe(25%)Co 12/Cu2]2/Fe(25%)Co 12/Cu2.6/AICu8/OX/Cu2/CoFe12/NiFe35/Cu30/Ru10Ta60/Ru10   B.Ta50/Ru20/IrMn70/Fe(25%)Co38/Ru7.5/[Fe(25%)Co12/Cu2]2/Fe(25%)Co12/Cu3. 2/AICu8.5/OX/Cu2.0/CoFe12/NiFe35/Cu30/Ru10/Ta60/Ru10   C.Ta50/Ru20/IrMn70/Fe(25%)Co50/Ru7.5/[Fe(25%)Co16/Cu2]2/Fe(25%)Co16/Cu3. 2/AICu8.5/OX/Cu2.0/CoFe12/NiFe35/Cu30/Ru10/Ta60/Ru10   D.Ta50/Cu20/IrMn70/Fe(25%)Co38/Ru7.5/[Fe(25%)Co12/Cu2]2/Fe(25%)Co12/Cu2. 6/AICu8.0/OX/Cu2.0/CoFe12/NiFe35/Cu30/Ru10/Ta60/Ru10   

   The results are summarized in TABLE I below 
   
     
       
             
             
             
             
             
           
         
             
               TABLE I 
             
             
                 
             
             
               Sample 
               Seed 
               CPP GMR 
               RA 
               Rhin 
             
             
                 
             
           
           
             
               A 
               Ta5/NiCr45 
               5.5% 
               0.34 ohm.μm 2   
               50% 
             
             
               B 
               Ta50/Ru20 
               5.8% 
               0.38 ohm.μm 2   
               92% 
             
             
               C 
               Ta50/Ru20 
               6.2% 
               0.24 ohm.μm 2   
               78% 
             
             
               D 
               Ta50/Cu20 
               4.7% 
               0.18 ohm.μm 2   
               92% 
             
             
                 
             
           
        
       
     
   
   It is readily seen that when the conventional Ta—NiCr seed layer was replaced by Ta/Ru or Ta/Cu, there was a significant increase in Rhin with no change in either the CPP GMR ratio or the resistance area product. This confirms that the relative pinning strength of the IrMn in an SV, formed according to the present invention, is much larger than in devices based on earlier approaches.