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 remains in place while the buried leads as well as longitudinal bias means are formed. Processing includes use of photoresist liftoff. The protective layer is removed as a natural byproduct of surface cleanup just prior the formation of the spin valve.

Description:
This application is related to U.S. Ser. No. 09/584,424 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 therewith, 
     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 close enough to be within the mean free path of conduction electrons in the material. If, now, 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 of 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 US Patent Office. This document discloses the structure shown in FIG. 1, along with a process for manufacturing it. Shown there is a substrate  11  (usually a dielectric material such as aluminum oxide) which has been ion milled to give its central portion the shape of a mesa (shown as  18 ). On the top of the mesa is seed layer  16  over which GMR sensor  15 , a complex of several layers, as discussed above, has been formed. On the area surrounding the mesa  18 , conductive leads  12  have been formed. These generally have the shape of a pair of stripes separated by seed layer  16 . They may comprise a single material or a laminate of several materials. 
     Seed layer  13  has been deposited onto the leads  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  16 . 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  16 . 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 a 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 include a photoresist liftoff step. 
     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 remains in place while buried leads and longitudinal bias means are formed, including use of a photoresist liftoff process. It is removed as a natural byproduct of surface cleanup just prior the formation of the spin valve. 
    
    
     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 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 formation of a mesa-like structure suitable for the introduction of buried leads. 
     FIG. 5 shows the structure just prior to liftoff. 
     FIG. 6 shows the structure at the conclusion of the process of the present invention. 
     FIG. 7 shows the structure of FIG. 6, including a spin valve structure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     We begin an account of the process of the present invention by referring to FIG.  2 . Shown there is support element  20  on which magnetic shield  21  has been deposited. This is one of the two shields that flank the GMR sensor element. It is shown in this, and subsequent figures, for reasons of completeness although it is not germane to the specific process of the invention itself. 
     The process of the present invention begins with the deposition onto shield layer  21  of dielectric layer  11 . Then, seed layer  16  is deposited over dielectric layer  11 . The material for seed layer  16  could be tantalum, nickel-chromium, nickel-iron-chromium, or zirconium and it is deposited to a thickness between about 20 and 150 Angstroms. 
     Then, in a departure from our own previous practice as well as from the known prior art, protective layer  25  of a non-magnetic material is deposited on seed layer  16 . 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 removal 
     (b) it should form an oxide that is readily soluble in a basic developer such as KOH or ammonium persulfate 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. These five metal are considered to have high sputtering yields, ranging from 2.3 atoms/ion for copper to 3.4 atoms/ion for silver (at 600V). The protective layer is deposited to a thickness between about 20 and 80 Angstroms. The deposition of both layers  16  and  25  is to be performed in a non-oxidizing atmosphere, preferably without breaking vacuum between the two depositions. 
     Referring now to FIG. 3, a liftoff mask, made up of upper and lower portions  32  and  31  respectively, is laid down on the surface of protective layer  25 . The lower mask portion  31  is a photosensitive resin such as polydimethylglutarimide (PMGI) that may be patterned in the same way as conventional photoresist but that can be easily dissolved in solvents such as NMP (N-methylpyrrolidone) or KOH. The upper mask portion  32  is a conventional photoresist pattern that uniformly overlaps lower portion  31 . The width WI of the lower mask portion is between about 0.05 and 1 microns while the amount of overhang W 2  by the upper mask portion is between about 0.1 and 1.5 microns. 
     Moving on to FIG. 4, we illustrate there the effects of subjecting the structure of FIG. 3 to uniform bombardment by ion stream  41 , a process referred to as ion beam etching or milling. The result is the removal of material from all areas not directly beneath upper mask portion  32 , leading to the formation of mesa  18 . Removed from the areas surrounding mesa  18  is all of the seed layer  16  (leaving behind seed layer  16 ′ under mask  32 ) and all of protective layer  25  (leaving behind protective layer  25 ′ under mask  32 ) as well as about half of dielectric layer  11  (now designated as layer  11 ′). 
     Referring now to FIG. 5, once mesa  18  (FIG. 4) has been given sufficient height above its surroundings (between about 30 and 450 Angstroms) the material that will form the leads to the sensor is deposited. This is shown as layer  51  in the figure. In addition to depositing in the area around the mesa, this material also deposits over the top and to some extent the sides of mask  32 . This, and the layers above it, will subsequently be patterned and etched to form of a pair of buried leads that contact protective layer  25 ′ at opposite ends. 
     Next, a second seed layer  52  (not necessarily of the same composition as seed layer  16 ) is deposited, followed by the deposition of magnetic material layer  53  which will serve to provide longitudinal bias to the sensor layer as discussed earlier. The total thickness of layers  51 ,  52 , and  53  is such that the top surface of layer  53  is too low to touch the upper mask portion  32 . If they should touch, efficient liftoff would not be possible. Thus when a suitable solvent is applied to the structure it dissolves the lower mask portion  31  so that upper mask portion  32  may be washed away along with all material adhering to  32 . It is during this step that the presence of the protective layer  18  is most important since, without it, oxidation of seed layer  16 ′ 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  25 ′ or a small amount of it (between about 5 and 25 Angstroms thick) may be left behind. If the latter option is chosen it is particularly important that protective layer  25 ′ and seed layer  16 ′ 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 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, that is contacted by the buried leads. We show a schematic representation of the finished product in FIG.  7 . Seen there are free layer  71 , separation layer  72 , pinned layer  73 , pinning layer  74 , and cap layer  75 . 
     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.