Abstract:
An improved process for manufacturing a spin valve structure that has buried leads is disclosed. A key feature is the inclusion in the process of a temporary protective layer over the seed layer on which the spin valve structure will be grown. This protective layer is in place at the time that photoresist (used to define the location of the spin valve relative to the buried leads and longitudinal bias layers) is removed. The protective layer is later removed as a natural byproduct of surface cleanup just prior to the formation of the spin valve itself.

Description:
This application is related to attorney docket number HT99-025, Ser. No. 09/584,426 filed on Jun. 5, 2000, assigned to a common assignee. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the general field of GMR read heads for magnetic disk systems with particular reference to use of buried leads and photoresist processes therwith. 
     BACKGROUND OF THE INVENTION 
     Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. The magnetic field that ‘writes’ a bit at the surface of a recording medium is generated by a flat coil whose magnetic flux is concentrated within two pole pieces that are separated by a small gap (the write gap). Thus, most of the magnetic flux generated by the flat coil passes across this gap with peripheral fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of the recoding medium. 
     The present invention is concerned with the manufacture of the read element. This is a thin slice of material located between two magnetic shields, one of which is also one of the two pole pieces mentioned above. The principle governing operation of the read sensor is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance. 
     It is now known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting 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 the solid as a whole. 
     The key elements of a spin valve structure are, in addition to a seed layer and a cap layer, two magnetic layers separated by a non-magnetic layer. The thickness of the non magnetic layer is chosen so that the magnetic layers are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other&#39;s magnetic behavior at the atomic level) but are dose enough to be within the mean free path of conduction electrons in the material. If, now, layers of these two magnetic layers are magnetized in opposite directions and a current is passed through them along the direction of magnetization, half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing the non magnetic layer. However, once these electron ‘switch sides’, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure. 
     In order to make use of the GMR effect, the direction of magnetization of one the layers must be permanently fixed, or pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material. The other layer, by contrast, is a “free layer” whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface of a magnetic disk). 
     On Feb. 5, 1999, application Ser. No. 09,244,882, entitled “Magnetoresistive (MR) sensor element with sunken lead structure” was filed with the U.S. Patent Office. This document discloses a structure similar to the one shown in FIG.  1 . Shown there is a substrate  11  (usually a dielectric material such as aluminum oxide) on whose upper surface is a seed layer  12 . GMR sensor  15  has been grown over seed layer  12  and contact to this GMR layer is made through buried lead structure  13 ′. This generally has the shape of a pair of stripes separated by seed layer  12 . It may comprise a single material or a laminate of several materials. 
     Leads  13 ′ have been deposited onto seed layer  12  and then overcoated with a pair of longitudinal bias stripes  14 ′. The latter are made of a suitable magnetic material and, in the finished device, are permanently magnetized in a direction parallel to the surface of seed layer  12 . Their purpose is to prevent the formation of multiple magnetic domains in the free layer portion of the GMR sensor, particularly near its ends. 
     While the structure shown in FIG. 1 has proven to be an effective package for a GMR sensor and its leads, early versions of said structure were found to exhibit lower than expected GMR ratios. The cause of this problem was found to be the presence of an oxide layer at the interface between layers  15  and  12 . The present invention is directed to finding a solution to this problem 
     A routine search of the prior art was performed but no references that describe the solution disclosed in the present invention were encountered. Several references of interest were found, however. For example in U.S. Pat. No. 5,985,162, Han et al. show conductive lead process using a PMGI/PR bilayer structure. Chen et al. (U.S. Pat. No. 5,491,600) and Pinarbasi (U.S. Pat. No. 5,883,764) show other conductive lead processes/ etches using a PMGI/PR bilayer structure while Lee et al. (U.S. Pat. No. 5,731,936) show a seed layer for a MR. 
     SUMMARY OF THE INVENTION 
     It has been an object of the present invention to provide an improved process for the manufacture of a sensing element for a magnetic disk system. 
     Another object of the invention has been that said sensing element be based on the GMR effect and have buried leads. 
     A further object has been that said process utilize reverse photoresist masking. 
     These objects have been achieved by including in the process deposition of a protective layer over the seed layer on which the spin valve structure will be grown. This protective layer is in place at the time that photoresist (used to define the location of the spin valve relative to the buried leads and longitudinal bias layers) is removed. The protective layer is removed as a natural byproduct of surface cleanup just prior to the formation of the spin valve itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-section of the structure whose performance is to be improved through the process of the present invention. 
     FIG. 2 shows the starting point of the process of a first embodiment of the present invention. 
     FIG. 3 illustrates the placement of a liftoff mask on the surface of the structure seen in FIG.  2 . 
     FIG. 4 illustrates the structure of FIG. 3 after etching to expose the seed layer. 
     FIG. 5 shows the structure of FIG. 4 after a protective layer has been deposited over the seed layer. 
     FIG. 6 shows the structure of FIG. 5 after sputter cleaning has been used to remove all or most of the protective layer. 
     FIG. 7 shows the starting point of the process for a second embodiment of the present invention. 
     FIG. 8 illustrates the placement of a conventional photoresist mask on the surface the structure seen in FIG.  7 . 
     FIG. 9 illustrates the structure of FIG. 8 after etching to expose the protective layer. 
     FIG. 10 shows the structure of FIG. 9 after sputter cleaning has been used to remove all or most of the exposed portions of the protective layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     We disclose two embodiments of the process of the present invention. 
     First Embodiment 
     Referring now to FIG. 2, the process of the first embodiment of the present invention begins with the deposition onto substrate  11  of seed layer  12 . The material for seed layer could be tantalum, nickel-chromium, nickel-iron-chromium, or zirconium and it is deposited to a thickness between about 10 and 200 Angstroms. With the seed layer in place, blanket layers  13  (buried leads) and  14  (longitudinal bias providers) are deposited to thicknesses between about 100 and 1,500 Angstroms and between about 50 and 650 Angstroms, respectively. 
     Next, as seen in FIG. 3, a liftoff mask is formed on the longitudinal bias layer  14 . Said mask consists of a lower portion  31 , made of a soluble resin such as poly dimethylglutarimide (PMGI) that may be patterned in the same way as conventional photoresist but that can be easily dissolved in any basic solvent having a pH in excess of 0.5 such as KOH. and an upper portion  32 , made of a photoresist material. The upper portion  32  uniformly overlaps lower portion  31  by a certain amount. 
     With the mask in place, all areas of the longitudinal bias and buried lead layers that are not directly beneath upper portion  32 , are removed, as shown in FIG. 4, leaving behind layers  13 ′ and  14 ′ and exposing seed layer  12 . Then, as illustrated in FIG. 5, in a departure from our own previous practice as well as from the known prior art, protective layer  51  of a non-magnetic material is deposited onto seed layer  12  as well as the sidewalls of layers  13 ′ and  14 ′. This protective layer is required to have certain properties. These include: 
     (a) a high sputtering yield so that it may be removed more rapidly than surrounding material, thereby eliminating any need to mask surrounding material during its later removal 
     (b) it should form an oxide that is readily soluble in ammonium persulfate or ammonium hydroxide so that any oxide that should happen to form during processing can be readily removed. 
     (c) it should have a crystal structure similar to that of the seed layer so that if some of it should be left behind, inadvertently or intentionally (see below), proper seeding of the next layer will still occur. 
     Our preferred choice for the protective layer has been copper but other materials such as gold, platinum, silver, or palladium could have been used. The protective layer is deposited to a thickness between about 20 and 150 Angstroms. 
     After deposition of protective layer  51 , liftoff of the mask is effected. This is accomplished by applying a suitable solvent such as NMP to the structure in order to dissolve lower mask portion  31  allowing upper mask portion  32  to be easily washed away along with all material adhering to  32 . It is during this step that the presence of the protective layer is essential since, without it, oxidation of seed layer  12  is liable to occur. 
     Once all masking material has been removed, the structure is subjected to sputter cleaning, symbolized by ion stream  61  in FIG.  6 . Sputter cleaning may be carried on for long enough to completely remove protective layer  51  or a small amount of it (less than about 30 Angstroms) may be left behind. If the latter option is chosen it is particularly important that protective layer  51  and seed layer  12  have similar crystal structures (lattice constants within ±0.5 Angstroms of each other). FIG. 6 illustrates the structure after the protective layer has been fully removed. 
     The process of the first embodiment of the present invention essentially ends when the structure of FIG. 6 has been achieved. In practice, of course, processing would continue with the formation of a spin valve structure on the exposed seed layer in order to form the structure that was shown in FIG.  1 . 
     Second Embodiment 
     Referring now to FIG. 7, the process of the second embodiment of the present invention begins with the deposition onto substrate  11  of seed layer  12 . The material for seed layer  12  could be tantalum, nickel-chromium, nickel-iron-chromium, or zirconium and it is deposited to a thickness between about 10 and 200 Angstroms. This is followed, as quickly as possible, with the deposition of protective layer  151 , to a thickness between about 10 and 150 Angstroms. As in the first embodiment, the protective layer should have a high sputtering yield, it should form an oxide that is readily soluble, and it should have a crystal structure similar to that of the seed layer. Our preferred choice for the protective layer has been copper but other materials such as gold, platinum, silver, or palladium could have been used. The protective layer is deposited to a thickness between about 10 and 150 Angstroms. 
     With protective layer  151  in place, blanket layers  13  (buried leads) and  14  (longitudinal bias providers) are deposited to thicknesses between about 100 and 1,500 Angstroms and between about 50 and 650 Angstroms, respectively. 
     Next, as seen in FIG. 8, a conventional photoresist mask  132  is formed on the longitudinal bias layer  14 . With the mask in place, all areas of the longitudinal bias and buried lead layers that are not protected by mask  132  are removed, as shown in FIG. 9, leaving behind layers  13 ′ and  14 ′ and exposing protective layer  151 . 
     The photoresist layer  132  is then removed using KOH developer. It is during this step that the presence of the protective layer is essential since, without it, oxidation of seed layer  12  is liable to occur. 
     Following the removal of mask  132 , the structure is subjected to sputter cleaning, symbolized by ion stream  61  in FIG.  10 . Sputter cleaning may be carried on for long enough to completely remove the exposed portion of protective layer  151  or a small amount of it (less than about 30 Angstroms) may be left behind. If the latter option is chosen it is particularly important that protective layer  151  and seed layer  12  have similar crystal structures (lattice constants within ±0.5 Angstroms of each other). 
     The process of the second embodiment of the present invention essentially ends when the structure of FIG. 10 has been achieved. In practice, of course, processing would continue with the formation of a spin valve structure on the exposed seed layer in order to form the structure that was shown in FIG.  1 . 
     A comparison was made, for both tantalum seeded and NiCr seeded structures, of the GMR ratios (ΔR/R) of devices that were manufactured both with and without the protective layer step. The results are summarized in TABLE I below: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Tantalum seeded 
                 NiCr-seeded 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 without protective layer 
                  4% 
                 2-3% 
               
               
                   
                 with protective layer 
                 5-6% 
                 8-9% 
               
               
                   
                   
               
             
          
         
       
     
     This confirms that there is a significant improvement in performance of the structure when a protective layer step is added, particularly for NiCr seeded devices. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.