Abstract:
An improved seed/AFM structure is formed by first depositing a layer of tantalum on the lower shield. A NiCr layer is then deposited on the Ta followed by a layer of IrMn. The latter functions effectively as an AFM for thicknesses in the 40-80 Angstrom range, enabling a reduced shield-to-shield spacing.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of CPP GMR read heads with particular reference to the pinning layer and its associated seed 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 lower magnetic shield  10  on which is seed layer  11 . Antiferromagnetic (AFM) layer  12  is on seed layer  11 . Its purpose is to act as a pinning agent for a magnetically pinned layer. The latter is typically a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer  14  between two antiparallel ferromagnetic layers  13  (AP2) and  15  (AP1). 
     Next is a copper spacer layer  16  on which is low coercivity (free) ferromagnetic layer  17 . Capping layer  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 10-20%. 
     Earlier GMR devices were designed to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also emerged. CPP GMR heads are considered to be promising candidates for the over 100 Gb/in 2  recording density domain (see references 1-3 below). 
     A routine search of the prior art was performed with the following references of interest being found: 
     U.S. Pat. No. 6,683,762 (Li et al) disclose Ta/NiFe as an example of a seed layer and IrMn as the possible composition of the AFM layer. Note that, although a Ta/NiCr seed layer is mentioned in the specification, this appears to have been a typographical error since only Ta/NiFe seeds are specified in the claims. 
     In U.S. Pat. No. 6,574,079, Sun et al. discuss Ta and NiCr alloys for the seed layer. However, they disclose Ta only, NiCr only or a Ta—NiCr alloy; which behaves differently from a Ta/NiCr two layer structure. 
     U.S. Pat. Publication 2002/0191356, Hasegawa et al. teach an underlayer of Ta, seed layer with NiCr together with an AFM layer of IrMn. However, this is for a CIP GMR structure where the Ta/NiCr seed layer is grown on alumina. In the present invention, the seed layer is grown on NiFe bottom shield and is for CPP application. Additionally the present invention teaches a Ta thickness range of around 3-10 A, a thickness range not claimed in 2002/0191356. 
     In U.S. Pat. No. 6,636,389, Gill shows a deposited NOL layer between two free layers which is a different use of a NOL from that disclosed by the present invention. In U.S. Pat. Publication 2004/0004261, Takahashi et al. mention an NOL-GMR but provide no details. What is discussed is a half-metal oxide layer as the AP1 or free layer in a CPP GMR. It has no bearing on the present invention. 
     In U.S. Pat. Publication 2002/0048127, Fukuzawa et al. discloses Ta as the seed layer and IrMn as the AFM layer. Cu is not mentioned as the base metal, but plasma etch and oxidation to form the NOL layer are discussed in great detail. This invention is directed to specular NOL layer formation in AP1 for CIP GMR applications. The NOL structure and plasma etch/oxidation conditions are totally different from those of the present invention. 
     REFERENCES 
     
         
         [1] M. Lederman et al. U.S. Pat. No. 5,627,704. 
         [2] J. W. Dykes et al. U.S. Pat. No. 5,668,688 
       
    
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a CPP GMR magnetic read head having an improved GMR ratio. 
     Another object of at least one embodiment of the present invention has been to provide a magnetic read head having an improved Resistance Area Product. 
     A further object of at least one embodiment of the present invention has been to provide a process for manufacturing said read head. 
     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 GMR devices. 
     These objects have been achieved by replacing the conventional seed layer (typically Ta) with a bilayer of NiCr on Ta, said seed being deposited on the NiFe layer that constitutes a magnetic shield. Additional improvement is also obtained if the conventional non-magnetic spacer layer of copper is replaced by a sandwich structure of two copper layers with an NOL (nano-oxide layer) between them. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a GMR stack of the prior art in which has conventional seed layer and spacer layers. 
         FIG. 2  shows a GMR stack according to the teachings of the present invention. having an improved seed layer. 
         FIG. 3  shows the GMR stack of  FIG. 2  with an improved spacer layer. 
         FIGS. 4 and 5  compare, respectively, GMR ratio curves for CPP devices having a conventional seed layers and devices with seed layers of the type taught by the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the CPP GMR head structure, a synthetic spin valve type film stack is employed for bias and stability reasons. The generic CPP spin valve structure has already been discussed above in relation to  FIG. 1 . One of the important parameters that defines a CPP device is R.A, the resistance area product. Currently, CPP devices have been achieving R.A values of 0.05 to 0.1 ohm.μm 2 . Higher values than this are to be desired for improved performance. 
     One solution has been to use a NOL (nano-oxide layer) sandwiched between two layers of copper as the non-magnetic spacer layer. Since conduction between the two copper layer occurs only where there are pin-holes in the NOL, a higher transverse resistance is obtained without any apparent reduction in area. This can increase the R.A to about 0.6-1 ohm.μm 2  with CPP GMR=3-5%. 
     However, passing current via pinholes may result in head reliability problems. In the present invention, a new seed layer/AFM structure is disclosed together with the use of ion assisted oxidation to oxidize a layer of AlCu. Additionally, prior to AlCu oxidation, a pre-ion beam treatment is used to precipitate Cu out of the AlCu so as to form reproducible metallic conduction paths. 
     It is desirable for a CPP spin valve structure to have a thin seed layer and a thin AFM layer so as to reduce shield to shield spacing; a thicker AP1/AP2 gives a better CPP GMR ratio, which requires better exchange coupling between AP2 and AFM. Previously, Ta50/Ru20/PtMn was used. The resulting thickness of the seed layer and AFM is undesirably large for high density applications. Also, the reliability of Ta/Ru/PtMn type spin valves, as in CIP-GMR heads, is a concern in reader sensors. The present invention overcomes these difficulties by using a Ta/NiCr/IrMn70 structure (as compared to Ta/NiCr/MnPt125). 
     We will further disclose the present invention through a description of a process for its manufacture. Referring now to  FIG. 2 , the process begins with the provision of lower shield layer  10  whose upper surface is then cleaned through plasma etching using argon flowing at a rate of about 30 SCCM at a power level of about 50 W for about 10 minutes, followed by the deposition of tantalum layer  21 , between about 3 and 10 Angstroms thick, on this cleaned surface. Next, NiCr layer  31 , between about 30 and 60 Angstroms thick, is deposited onto tantalum layer  21 . Layers  21  and  31 , together, now form a seed layer. 
     Once this seed layer has been formed, as described above, layer  22  of IrMn is deposited thereon to a thickness between 40 and 80 Angstroms. This is followed by the formation of the pinned layer—AP2 layer  13  is deposited on the IrMn layer, followed by AFM coupling layer  14  and then by AP1 layer  15 . 
     Next is the deposition of the non-magnetic spacer layer. In one embodiment, illustrated in  FIG. 2 , the is copper layer  16 . In a second embodiment, the spacer layer can be further improved, as follows: 
     Referring now to  FIG. 3 , lower copper layer  32  is deposited onto AP1 layer  15 . to a thickness between about 1.5 and 6 Angstroms followed by the deposition thereon of AlCu layer  33 , between about 6 and 10 Angstroms thick. This AlCu layer is then subjected to a low power plasma etch so that about 1-3 Angstroms of it get removed. Details of this step include using argon flowing at a rate of about 50 SCCM at a power level of about 17-20 W for about 20-60 seconds. 
     Then, AlCu layer  33  is subjected to plasma oxidation which converts the AlCu layer into a porous aluminum oxide layer whose pores are filled with copper. Details of the plasma oxidation process include using argon flowing at a rate of about 30 SCCM, together with oxygen flowing at about 1 SCCM, at a power level of about 20-30 W for about 15-45 seconds. Formation of the non-magnetic spacer layer is completed with the deposition of copper layer  34  which is connected to layer  32  through said copper filled pores. 
     The process concludes with the deposition of free layer  17  (of a material such as CoFe, CoFe/NiFe, or (CoFeCu) 2 /CoFe to a thickness between about 15 and 60 Angstroms) onto layer  34  followed by the formation, on free layer  17 , of capping layer  18  (of a material such as Cu/Ta, CuRu, or Cu/Ru/Ta/Ru. 
     CPP GMR devices made according to the process we have just described have a GMR ratio greater than 5% and a R.A product that is greater than 0.4 ohm.μm 2 . 
     Confirmatory Results 
     Referring now to  FIG. 4 , we show the GMR curve for a CIP device having a conventional seed (45 Å of NiCr). The CIP device whose GMR curve is shown in  FIG. 5  had a seed layer made up of 45 Å of NiCr on 5 Å of Ta. The remaining structure for both devices was as follows:
 
NiFe200/seed/IrMn70/Fe 25 Co30/Ru7.5/CoFe35/Cu30/CoFe30/Cu10/NiCr50
 
     The CIP GMR of Ta/NiCr seed layer is higher than that of NiCr only, which may indicate better crystalline structure so it is expected that the CPP GMR will also be better. Structure with Ta/NiCr seed layer showed better pinning field than NiCr only seed layer, which potentially can help to make AP1/AP2 thicker for better CPP GMR; 
     A typical CPP structure with NOL in spacer is:
     Seed/AFM/AP2/Ru/AP1/Cu/Cu/NOL/free layer/cap.   

     In TABLE I below we compare CPP GMR properties between devices having a Ta/NiCr/IrMn/. . . and Ta/Ru/PtMn . . . structure. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Seed &amp; pinning layer 
                 AP2 
                 AP1 
                 Cu/NOL/Cu 
                 RA(ohmμm 2 ) 
                 DR/R(%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ta50/Ru20/MnPt150 
                 CoFe16/FeTa10/CoFe10 
                 CoFe36 
                 2.6/8.0/5 
                 0.54 
                 2.93 
               
               
                 ″ 
                 ″ 
                 CoFeCu36 
                 2.6/8.0/5 
                 0.45 
                 3.8 
               
               
                 ″ 
                 ″ 
                 ″ 
                 2.6/8.0/2 
                 0.45 
                 4.3 
               
               
                 ″ 
                 ″ 
                 Fe 25 Co36 
                 2.6/8.0/2 
                 0.2 
                 4.5 
               
               
                 Ta5NiCr45/IrMn70 
                 Fe 25 Co36 
                 CoFe36 
                 2.6/8.0/5 
                 0.54 
                 3.2 
               
               
                 ″ 
                 ″ 
                 CoFeCu36 
                 2.6/8.0/5 
                 0.86 
                 3.57 
               
               
                 ″ 
                 ″ 
                 ″ 
                 2.6/8.0/2 
                 0.9 
                 4.18 
               
               
                 ″ 
                 ″ 
                 Fe 25 CoCu36 
                 2.6/8.0/2 
                 0.5 
                 5.58 
               
               
                   
               
             
          
         
       
     
     Free layer in all cases was CoFe12/NiFe35 
     From Table I, it is clearly seen that Ta/NiCr/IrMn gave a better CPP GMR ratio than Ta/Ru/PtMn, which is very desirable for higher signal in CPP GMR head. In comparison, Ta/NiCr/IrMn seed/AFM combination offers the following advantages over the reference case: 
     1) thinner seed layer/AFM thickness; with narrower shield-shield spacer better extendability for higher density; 
     2) higher GMR ratio; 
     3) less hysteresis in the pinned layer (not shown)