Abstract:
Improved magnetic devices have been fabricated by replacing the conventional seed layer (typically Ta) with a bilayer of Ru on Ta. Although both Ru and Ta layers are ultra thin (between 5 and 20 Angstroms), good exchange bias between the seed and the AFM layer (IrMn about 70 Angstroms thick) is retained. This arrangement facilitates minimum shield-to-shield spacing and gives excellent performance in CPP, CCP-CPP, or TMR configurations.

Description:
Related application HT03-043 (file Ser. No. 10/886,288 filed on Jul. 7, 2004) discloses an ultra thin seed layer based on Ta/NiCr. It is herein incorporated, by reference, in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to the general field of magnetic read heads with particular reference to very thin seed layers for the AP2 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 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. 
     Another consideration is the R.A product. This is the resistance of the CPP device times its cross-sectional area. Normally, the transverse resistance of a layer increases as its cross-sectional area decreases so the R.A product tends to be a constant. However, by arranging for the current path to be confined to only certain parts of the free layer, a larger value for R.A can be achieved. Devices of this type are referred to as CCP (confined current path) devices. 
     If copper spacer layer  16  in  FIG. 1  is replaced by a very thin layer of insulating material, a magnetic tunneling junction (MTJ) device that depends on the TMR (tunneling magneto resistance) effect is formed. In this device, the tunneling current that passes through layer  16  (when it is an insulator) from layer  15  to layer  17 , is measurably larger when the directions of magnetization of layers  15  and  17  are parallel (as opposed to antiparallel). 
     Typically, in both TMR and CPP-GMR head structures, a bottom synthetic spin valve type film stack has been employed for biasing reasons, together with a Ta—NiCr based seed layer for the AFM and a CoFe/NiFe composite free layer. 
     In a typical TMR or CPP spin valve structure of any case shown above, the conventional seed layer thickness of Ta/NiCr is about 70 Å, and the AP1 or AP2 thickness is in the range of 20-50 Å, and the free layer thickness is in the range of 30-60 Å. For ultra-high density read head applications, a thinner total film stack thickness is preferred in order to achieve higher resolution. Since reducing the magnetic layer thickness also reduces the MR ratio of TMR or CPP, it would be preferable to reduce the thickness of the nonmagnetic layers. The present invention provides a solution to this problem. 
     A routine search of the prior art was performed with the following references of interest being found: 
     U.S. Patent Application 2004/0179311 (Lin et al) Headway states that Ta and NiCr form a typical seed layer. U.S. Pat. No. 6,936,903 (Anthony et al) and U.S. Pat. No. 6,919,594 (Sharma et al) both make reference to seed layers of Ta/Ru. but neither invention gives details concerning their thicknesses. Nor do they specify that these may be used with a pinning layer of IrMn of the thickness taught by the present invention. 
     U.S. Patent Application 2005/0164414 (Desk) teaches that the seed layer may be NiFe, Cu, or other materials and have a thickness of 5 to 50 Angstroms. U.S. Patent Application 2005/0174702 (Gill) describes a seed layer comprising 30 Angstroms of Ta and 20 Angstroms of NiFeCr. U.S. Pat. No. 6,903,904 (Li et al) Headway discloses a seed layer of Ta/NiFe of 30-70 Angstroms. 
     U.S. Pat. No. 6,806,804 (Saito et al) teaches a 30 Angstrom NiFe seed layer over a Ta underlayer. U.S. Pat. No. 6,862,159 (Hasegawa) shows a Ta/Cr or Ta/NiFe seed layer where the Cr or NiFe layer is 30 Angstroms thick. Finally, U.S. Pat. No. 6,862,158 (Hasegawa et al) teaches a Ta/Cr seed layer formed using Ar gas wherein the Cr layer is 15-50 Angstroms. It is stated that under 15 Angstroms, the Cr layer is not uniform. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a magnetic read head having improved performance together with improved spatial resolution. 
     A further object of at least one embodiment of the present invention has been to provide a process for manufacturing said read head. 
     Another object of at least one embodiment of the present invention has been to reduce total device thickness by reducing the thickness of the non-magnetic layers. 
     Yet another object of at least one embodiment of the present invention has been to reduce seed layer thickness without significantly reducing the exchange bias between the pinning and pinned layers. 
     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, CCP GMR, and TMR devices. 
     These objects have been achieved by replacing the conventional seed layer (typically Ta) with a bilayer of Ru on Ta, said seed being deposited on the NiFe layer that constitutes the lower magnetic shield. Additional improvement is obtained if the conventional non-magnetic spacer layer of copper is replaced by (1) a sandwich structure of two copper layers with an NOL (nano-oxide layer) between them, and (2) a thin insulating layer. With the ultra-thin Ta/Ru seed layer disclosed by the present invention, we are able to achieve a higher MR ratio, better exchange bias field, and thinner total film stack thickness, which are all desirable for ultra high density recording heads. 
     The thickness of the Ta layer ranges from 5 Å to 20 Å, and the thickness of the Ru layer is from 5 Å to 20 Å. Both Ta and Ru layers can be processed either using Ar gas or Kr gas. Although Ru is our preferred material for contacting the AFM layer, other materials such as Cu, Ni, NiFe, CoFe or Cr could be used in a similar thickness range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a GMR stack of the prior art which has a conventional seed layer 
         FIG. 2  shows a GMR stack according to the teachings of the present invention, including an improved seed layer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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; this makes possible a thicker AP1/AP2 which gives a better CPP GMR ratio. This requires better Hex (exchange coupling between AP2 and AFM layers). 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, which are used in CIP-GMR heads, is a concern. The present invention overcomes these difficulties by using a Ta/Ru/IrMn70 structure (as compared to Ta/NiCr/IrMn70). 
     We now disclose the present invention through a description of the 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 5 and 20 Angstroms thick, on this cleaned surface. Next, layer  31 , about 20 Angstroms thick, is deposited onto tantalum layer  21 . Our preferred material for layer  31  has been Ru but any of several other materials such as Cu, Ni, NiFe, CoFe, or Cr could also have been used. Layers  21  and  31 , together, now form the 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 a first embodiment, layer  16  is copper. In a second embodiment, layer  16  includes a nano-oxide layer that is not continuous so that the copper that connects layers  15  and  17  is confined to pass through only the pin holes in the nano-oxide layer. In a third embodiment, layer  16  is a layer of insulating material and the device is a MTJ. 
     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. 
     Confirmatory Results 
     Shown in TABLE I are the exchange bias comparisons between Ta/NiCr seed layers and Ta/Ru seed layers. The full stack structure for both was: 
     seed/IrMn70/CoFe(25%)20/Ru10/Ta50 (Ru10/Ta50 being the capping layer). 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 SEED 
                 Hex 
                 Hc 
                 Hex/Hc 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ta10/NiCr60 
                 1317 
                 444 
                 ~3 
               
               
                   
                 Ta10/NiCr40 
                 ~100 
                 ~100 
                 ~1 
               
               
                   
                 Ta50/Ru20 
                 2110 
                 259 
                 ~8 
               
               
                   
                 Ta20/Ru20 
                 2257 
                 227 
                 ~10 
               
               
                   
                 Ta10/Ru10 
                 2000 
                 270 
                 ~7.5 
               
               
                   
                   
               
               
                   
                 where Hc is the coercive field 
               
             
          
         
       
     
     It can be clearly seen that exchange bias properties such as Hex, Hex/Hc are significantly improved with Ta/Ru seed layers. Also the Ta/Ru seed layers can be thinned down to an exceptionally low value without sacrificing the exchange properties. By contrast, the Ta/NiCr seed layers cannot establish significant Hex once the NiCr thickness is less than about 45 Å. 
     Furthermore, when this ultra-thin Ta/Ru seed layer was used as part of a TMR stack, a truly outstanding TMR ratio, even at a very small R.A, was obtained; when compared with a Ta/NiCr seeded TMR film with a similar structure, the gain in MR ratio was about 40%, as listed in TABLE II: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 CONFIGURATION 
                 R.A 
                 dR/R 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ta10/NiCr60/IrMn70/CoFe25%(19)/Ru7.5/CoFeB13/ 
                 1.2 
                 12% 
               
               
                 CoFe12.5/SL/AI4.5/NOX/Fe70%Co(10)/Ni90%Fe(40)/ 
               
               
                 Ru10/Ta60/Ru30 
               
               
                 Ta20/Ru20/IrMn70/CoFe25%(19)/Ru7.5/CoFeB13/ 
                 1.2 
                 17% 
               
               
                 CoFe12.5/SL/AI4.5/NOX/Fe70%Co(I0)/Ni90%Fe(40)/ 
               
               
                 Ru10/Ta60/Ru30 
               
               
                   
               
             
          
         
       
     
     This represents a significant improvement over the state of the art. Prior to the deposition and oxidation of the 4.5 Å Al layer, there a surfactant layer (SL) which is used to provide a smoother and well-oxidized bottom interface for the AIOx barrier layer. 
     We also observed that with ultra-thin Ta/Ru seed layers, the AIOx barrier becomes much smoother than in the Ta/NiCr case. Additionally, the ultra-thin Ta/Ru seed layers lead to higher breakdown voltage, better high-state ratio and improved wafer MR ratio uniformity. All these improvements are derived from the improved barrier smoothness caused by ultra-thin Ta/Ru seed layers. 
     In the CCP-CPP case, this ultra-thin Ta/Ru seed layer also brings obvious advantages, not only in the much reduced film thickness but also in the much enhanced MR ratio. As shown TABLE III, a Ta10/Ru10 seeded CCP-CPP structure has been shown to have a large dR/R ratio of 11.4% for an R.A of 0.15 ohms-sq. microns. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                 CONFIGURATION 
                 R.A 
                 dR/R 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ta10/Ru10/IrMn70/CoFe8/Fe70%Co10/CoFe14/Ru7.5/ 
                 0.15 
                 11.43% 
               
               
                 Fe70%Co12/Cu2/Fe70/Cu12/AICu8.5/ 
               
               
                 RFPIT(20W50sccm34s)/RFIAO(27WAr/O=35/ 
               
               
                 0.5640s)/Cu3/Fe70%Co12/CoFeB(10)/Ni95%Fe(50)/ 
               
               
                 Ru10/Ta60/Ru30 
               
               
                   
               
             
          
         
       
     
     In TABLE III above, Ta10/Ru10 serves as the seed layer, and IrMn is used as the antiferromagnetic pinning layer. In the synthetic AP structure, FCC-like trilayer CoFe8/Fe70% Co/CoFe14 is used as AP2 for better EM performance while the Fe70% Co laminated with Cu is used as the AP1 layer. AlCu is employed as the CCP-layer, and its oxide formation and segregated metal path is defined by the following RF-PIT, as 20 W 50 sccm for 34 sec, and RF-IAO processes, i.e. 27 W, Ar/O 2 =35/0.56. Fe70% Co(12)/CoFeB(10)/Ni95% Fe(50) is the free layer and the Ru/Ta/Ru is applied as the capping layer. 
     In summary, the present invention offers the following advantages:
     (a) Significant improvement in TMR ratio, especially at lower R.A.   (b) Improved AlOx barrier smoothness for better breakdown voltage, high-state ratio and wafer uniformity.   (c) Better exchange bias strength   (d) Thinner full film thickness, enabling a reduced shield-to-shield spacing for resolution improvement. The film thicknesses that are claimed for the Ta/Ru seed bilayer are critical for the success of the present invention.   

     While the thickness of the Ta and Ru layers may range from 5 to 30 Å, our preferred thicknesses have been 20 Å or less as this provides the best overall design. Such very thin seed layers had not been previously used because of the poor Hex values obtained when using materials such as NiCr. We note here that both the Ta and the Ru layers can be processed through sputtering using either Ar gas or Kr gas.