Abstract:
A magnetoresistive sensor having bias stabilization tabs includes a protective cap layer. The protective cap layer prevents oxidation, avoids potential damage from using ion milling for oxidation removal, and lowers parasitic resistance. In one embodiment, a bias layer, having a central portion with quenched magnetic moment, is formed over the free layer with an intervening coupling layer. A disk drive is provided with the magnetoresistive sensor including a protective cap layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a magnetic spin valve sensor typically used in a magnetic disk drive; and, more specifically, to an antiparallel tab magnetic spin valve sensor having a very low resistance conducting path. 
   2. Description of the Background Art 
   Disk drives using magnetic recording of digital information store most of the data in contemporary computer systems. A disk drive has at least one rotating disk with discrete concentric tracks of data. Each disk drive also has at least one recording head typically having a separate write element and read element for writing and reading the data on the tracks. The recording head is constructed on a slider and the slider is attached to a suspension. The combination of the recording head, slider, and suspension is called a head gimbal assembly. In addition, there is an actuator which positions the recording head over a specific track of interest. The actuator first rotates to seek the track of interest. After positioning the recording head over the track, the actuator maintains the recording head in close registration to the track. The disk in a disk drive has a substrate and a magnetic layer formed over the substrate for magnetic recording. The slider carrying the recording head has a disk facing surface upon which an air bearing is constructed. The air bearing allows the slider to float on a cushion of air and to be positioned close to the disk surface. Alternatively, the slider surface facing the disk can be adapted for partial or continuous contact with the disk. The read element in most contemporary disk drives include a magnetic spin valve sensor. A magnetic spin valve sensor is a sandwich of layers including a ferromagnetic pinned layer, a nonmagnetic electrically conducting layer, and a ferromagnetic free layer. The resistance of the spin valve sensor changes with respect to the direction and magnitude of an applied magnetic field such as the field from a written magnetic transition on a disk. To detect the change in resistance, sense current is passed through the sensor. 
   The free layer in a magnetic spin valve sensor is usually operated in the presence of a constant weak magnetic field to insure magnetic stability and prevent spurious signals. The application of a weak magnetic field to the sensor is sometimes referred to as magnetically biasing or magnetically stabilizing the sensor. One structure which may effectively be used for magnetic biasing is a pair of magnetic stabilization tabs which are antiparallel coupled to portions of the free layer. This biasing structure is effective. However, there are two practical problems. One, during annealing an oxide layer typically forms on the cap layer. This oxide layer must be thoroughly removed by ion milling before formation of the lead structures. If the oxide layer is not completely removed the resistance of the sensor increases. Since this additional resistance, sometimes called parasitic resistance, is not related to the sensor response to an external magnetic field, the effective sensitivity of the sensor is degraded. Two, the above mentioned ion milling operation easily causes damage to the magnetic biasing layer in the magnetic tabs. The most common damage from ion milling is a loss of magnetic moment of the magnetic biasing layer. It is important that the magnetic biasing layer have a slightly higher magnetic moment than the adjacent portion of the free layer. The magnetic biasing layers are very sensitive to damage from ion milling and therefore this sensor architecture is difficult to manufacture. 
   What is needed is a magnetoresistive sensor having antiparallel coupled bias tabs which has low resistance and is easy to manufacture. 
   SUMMARY OF THE INVENTION 
   A preferred embodiment of the invention provides a magnetoresistive spin valve sensor which has novel antiparallel coupled bias tabs. Each antiparallel biasing tab includes a ferromagnetic biasing layer which is antiparallel coupled with a portion of the free layer. Each antiparallel biasing tab also includes both a cap layer and a protective cap layer. The presence of the protective cap layer prevents oxidation of the cap layer during annealing. The presence of the protective cap layer also avoids an ion milling operation to remove oxidized material thus preventing possible damage to the bias layer. 
   An embodiment of a magnetoresistive sensor thus provided by the invention has a lower resistance and has very low risk for damage to occur to the biasing layers during manufacture. Another embodiment of the invention provides a disk drive having a read element including a magnetoresistive sensor with antiparallel coupled bias tabs with a protective cap layer. Other aspects and advantages of the invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a view of a disk drive having a magnetoresistive sensor according to the present invention; 
       FIG. 2  illustrates a view (not necessarily to scale) of a slider and recording head having a magnetoresistive sensor according to the present invention; 
       FIG. 3   a  illustrates a disk facing view (not necessarily to scale) of a magnetoresistive sensor before annealing according to the prior art; 
       FIG. 3   b  illustrates the magnetoresistive sensor after annealing according to the prior art; 
       FIG. 4   a  illustrates a disk facing view (not necessarily to scale) of a magnetoresistive sensor having a protective cap layer before annealing; 
       FIG. 4   b  illustrates a view of the magnetoresistive sensor having a protective cap layer after annealing and formation of the photoresist liftoff structure; 
       FIG. 4   c  illustrates a view of the magnetoresistive sensor after deposition of lead material; 
       FIG. 4   d  illustrates a view of the magnetoresistive sensor after liftoff of the photoresist; 
       FIG. 4   e  illustrates a view of the magnetoresistive sensor undergoing ion milling; 
       FIG. 4   f  illustrates a view of the magnetoresistive sensor undergoing fluorine reactive ion etching; 
       FIG. 4   g  illustrates a view of the magnetoresistive sensor undergoing oxygen reactive ion etching; and, 
       FIG. 4   h  illustrates a view of the completed magnetoresistive sensor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A magnetoresistive sensor having antiparallel coupled biasing tabs according to a preferred embodiment of the invention includes a protective cap layer for each biasing tab. A sensor according to the invention has low parasitic resistance. During manufacture, a sensor having a bias tab structure according to the invention is effectively protected from damage due to ion milling. 
   Referring to  FIG. 1 , a magnetic disk drive  100  has at least one rotatable magnetic disk  102  supported by a spindle  104  and rotated by a motor (not shown). There is at least one slider  106  with an attached recording head  108  positioned over the disk  102  surface while reading and writing. The recording head  108  includes a write element for writing data onto the disk  102 . The recording head also includes a magnetic spin valve sensor according to the present invention (shown in detail below) used as a read element for reading data from the disk. The slider  106  is attached to a suspension  110  and the suspension  110  is attached to an actuator  112 . The actuator  112  is pivotally attached  114  to the housing  116  of the disk drive  100  and is pivoted by a voice coil motor  118 . As the disk is rotating, the actuator  112  positions the slider  106  and suspension  110  along a radial arcuate path  120  over the disk  102  surface to access the data track of interest. 
   Again referring to  FIG. 1 , during operation of the disk drive  100 , the motion of the rotating disk  102  relative to the slider  106  generates an air bearing between the slider  106  and the disk  102  surface which exerts an upward force on the slider  106 . This upward force is balanced by a spring force from the suspension  110  urging the slider  106  toward the surface of the disk  102 . Alternatively, the slider  106  may be in either partial or continuous contact with the disk  102  surface during operation. 
     FIG. 2  illustrates a more detailed view of a slider  200 . The recording head  218  is preferably constructed on the trailing surface  206  of the slider  200 .  FIG. 2  illustrates the upper pole  208  and the turns  210  of the coil  214  of the write element of the recording head  218 . The read element includes a read sensor  204  disposed between two magnetic shields  220  is formed between the slider body  202  and the write element. The electrical connection pads  212  which allow connection with the write element and read element are illustrated. 
     FIG. 3   a  illustrates a partially completed magnetoresistive sensor  300  according to the prior art. The pinned layer  302  may be a simple layer of ferromagnetic material. Alternatively, the pinned layer  302  may be an assembly of antiparallel coupled ferromagnetic layers. The pinned layer  302  may be formed over an antiferromagnetic layer (not shown). Alternatively, a antiferromagnetic layer may not be necessary if the pinned layer  302  is self-pinned. A nonmagnetic conducting layer  304  is formed over the pinned layer  302 . A ferromagnetic free layer  306  is formed over the nonmagnetic conducting layer  304 . A thin nonmagnetic layer  308 , usually ruthenium, is formed over the free layer  306  and promotes antiparallel coupling with the ferromagnetic bias layer  310  which is formed over the thin nonmagnetic layer  308 . A cap layer  312 , usually of tantalum, is formed over the bias layer  310 . The structure  300  as illustrated in  FIG. 3   a  is now removed from vacuum and annealed at elevated temperature. 
     FIG. 3   b  illustrates a formation of oxidized material  314  in the cap layer  312  which resulted from the annealing operation. The oxide layer  314  must be removed with ion milling  316  to insure good electrical connection with subsequently formed lead structures (not shown). This ion milling operation  316  must be relatively aggressive to adequately remove the oxide layer  314  thus risking damage to the bias layer  310  and necessitating a thicker cap layer  312 . 
     FIG. 4   a  illustrates a view of a partially completed magnetoresistive sensor  400  according to the present invention. The sensor  400  includes a pinned layer  402  which may be a simple layer of ferromagnetic material. Alternatively, the pinned layer  402  may be an assembly of antiparallel coupled ferromagnetic layers. The pinned layer  402  may be formed over an antiferromagnetic layer (not shown). Alternatively, an antiferromagnetic layer may not be necessary if the pinned layer  402  is self-pinned. A nonmagnetic conducting layer  404 , typically of copper, is formed over the pinned layer  402 . A ferromagnetic free layer  406  is formed over the nonmagnetic conducting layer  404 . The free layer  406  may be a single layer of ferromagnetic alloy or alternatively multiple layers of ferromagnetic alloys. Appropriate ferromagnetic alloys are typically formed from binary or tertiary combinations of iron, nickel, and cobalt. A thin nonmagnetic coupling layer  408 , usually ruthenium, is formed over the free layer  406 . This thin nonmagnetic coupling layer  408  promotes antiparallel coupling between a ferromagnetic bias layer  410  formed over the thin nonmagnetic layer  408  and the underlying free layer  404 . The magnetic moment of the bias layer  410  should ordinarily be somewhat greater than the magnetic moment of the free layer  406 . A cap layer  412 , usually of tantalum, is formed over the bias layer  410 . Importantly, a protective cap layer  414  is formed over the cap layer  412 . The protective cap layer  414  may be formed from rhodium (Rh), gold (Au), ruthenium (Ru), or other material which protects the cap layer against oxidation and is not readily oxidized itself. The protective cap layer  414  is effective in a thickness range of about 10 to 30 Angstroms. A protective cap layer thicker than 30 Angstroms is also likely to be effective but could add undesirable thickness to the sensor stack. The magnetoresistive sensor  400  as illustrated in  FIG. 4   a  is now removed from vacuum and annealed at elevated temperature. 
     FIG. 4   b  illustrates a view of the magnetoresistive sensor after annealing and after the formation of a photoresist liftoff structure  450 . The cap layer  412  was protected from oxidation during annealing by the protective cap layer  414 . The material chosen for the protective cap layer  414  is not readily oxidized during annealing. An advantage of the present invention is that the cap layer can be relatively thin with a thickness range of about 30 to 50 Angstroms. In contrast, the thickness of the cap layer of the prior art ( 312  in  FIGS. 3   a  and  3   b ) was typically greater than about 80 Angstroms. 
     FIG. 4   c  illustrates a view of the deposited lead layers  416   a ,  416   b . The leads  416   a ,  416   b  are formed on the protective cap layer  414 . Lead material  416   c  is also formed on the photoresist  450  and will be removed along with the photoresist  450  during liftoff (not shown). Because of the absence of oxidation, there is very low resistance between the lead layers  416   a    416   b  and the underlying protective cap layer  414 . 
     FIG. 4   d  illustrates a view the sensor after liftoff of the photoresist ( 450  in  FIG. 4   c ). 
     FIG. 4   e  illustrates the use of an ion milling operation  452  to remove an exposed portion, indicated by reference number  415 , of the protective cap layer  414 . The ion milling operation  452  renders the original protective cap layer  414  into two remaining portions. In subsequent Figures the two remaining portions of the original protective cap layer  414  shall be referred to as two separate layers and are labeled with reference numbers  414   a  and  414   b . Typically, the ion milling operation  452  is only used to remove material  415  which is not covered by the leads  416   a ,  416   b . Thus the possibility of damaging the bias layer  410  other than the exposed area  415  is greatly minimized. 
     FIG. 4   f  illustrates the use of a fluorine reactive ion etch  454  to remove an exposed portion  413  of the cap layer  412 . The fluorine reactive ion etch  454  renders the original cap layer  412  into two remaining portions. In subsequent Figures the two remaining portions of the cap layer are labeled with reference numbers  412   a  and  412   b . The bias layer  410  serves as an effective etch stop for fluorine reactive ion etching  454 . 
     FIG. 4   g  illustrates the use of an oxygen reactive ion etch  456  to quench the magnetic moment of an exposed portion  411  of the bias layer  410 . Some of the material in the exposed portion  411  may be removed and some may remain after the oxygen reactive ion etch  456 . The magnetic moment of the exposed portion of the bias layer  410  is destroyed regardless of whether material is removed. The thin nonmagnetic coupling layer  408  serves as an etch stop during oxygen reactive ion etching  456  thus protecting the free layer  406  from damage. Once the magnetic moment of the exposed portion  411  of the bias layer  410  is quenched, the portion  407  of the free layer  406  directly opposite the quenched portion  411  becomes responsive to an external magnetic field. The width  460  of the quenched portion  411  of the bias layer  410  determines the width  462  of the active portion  407  of the free layer  406 . 
     FIG. 4   h  illustrates a view of the completed sensor  400 . The pinned layer  402 , the nonmetallic conducting layer  404 , the free layer  406 , and the nonmagnetic coupling layer  408  have remained during the construction of the sensor  400 . Some of the exposed portion  411  of the biasing layer ( 410  in  FIG. 4   a ) may remain after exposure to the oxygen reactive ion etch ( 456  in  FIG. 4   g ). Overcoat layers  434   a ,  434   b , typically formed from tantalum have been formed over the leads  416   a ,  416   b . The magnetoresistive sensor  400  as illustrated in  FIG. 4   h  includes two bias stabilization tabs  438   a ,  438   b . Although the bias layer, the cap layer and the protective cap layer were originally deposited as continuous layers, the construction of the sensor has rendered each of these layers into two portions. Therefore, it is convenient to describe the remaining portions of these layers separately. Accordingly, the first bias stabilization tab  438   a  includes a first bias layer  410   a  formed over a portion  408   a  of the nonmagnetic coupling layer  408 , a first cap layer  412   a , and a first protective cap layer  414   a . The second bias stabilization tab  438   b  includes a second bias layer  410   b  formed over a portion  408   b  of the nonmagnetic coupling layer  408 , a second cap layer  412   b , and a second protective cap layer  414   b.    
   A read element according to the present invention includes a protective cap layer. This protective cap layer effectively protects the cap layer from oxidation. Since the cap layer is protected from oxidation, ion milling is not necessary to remove oxide and the bias layer in the bias stabilization tabs are protected from ion milling damage. The parasitic resistance is significantly reduced. Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements thus described. Those skilled in the art will readily recognize other embodiments which fall within the scope of the invention.