Abstract:
Magnetoresistive (MR) sensors are disclosed that have leads that overlap a MR structure and distribute current to and from the MR structure so that the current is not concentrated in small portions of the leads, alleviating the problems mentioned above. For example, the leads can be formed of a body-centered cubic (bcc) form of tantalum, combined with gold or other highly conductive materials. For the situation in which a thicker bcc tantalum layer covers a highly conductive gold layer, the tantalum layer protects the gold layer during MR structure etching, so that the leads can have broad layers of electrically conductive material for connection to MR structures. The broad leads also conduct heat better than the read gap material that they replace, further reducing the temperature at the connection between the leads and the MR structure.

Description:
BACKGROUND 
     The present invention relates to magnetoresistive (MR) sensing mechanisms, which may for example be employed in information storage systems or measurement and testing systems. 
       FIG. 1  shows a media-facing view of a prior art magnetoresistive (MR) sensor  20  that may for example be used in a head of a disk drive. A MR structure  22  is formed from one or more layers so that the structure  22  has a resistance that varies in resistance in response to an applied magnetic field. Lead layers  25  have been formed that carry current through the MR structure  22  to gauge the change in resistance and thereby sense the magnetic field. Bias layers  27  abut the structure to stabilize magnetic domains at the edges of the MR structure  22  and reduce noise in the sensor  100 . A pair of magnetically soft layers  30  and  33  shield the MR structure  22  from magnetic fields that are not opposite the MR structure  22  in the media, the shields  30  and  33  isolated from the MR structure  22 , leads  25  and bias layers  27  by first and second dielectric read layers  35  and  38 . 
     The lead layers  25  may be made of gold that has been formed atop a tantalum seed layer and capped with another thin tantalum layer. The lead layers  25  overlap the MR structure  22  to contact the MR structure  22  at sharp points  40  and  42 . Because the lead layers  25  overlap the MR structure  22 , the effective sensing width of the sensor  20  is less than the width of the MR structure  22 . The effective sensing width is sometimes called the track-width of the sensor  20 . The electric current that flows through the MR structure  22  primarily flows through points  40  and  42 , which can cause excessive heating at those points, reducing the sensitivity of the sensor and leading to other problems such as electromigration and damage to the sensor. 
     SUMMARY 
     Magnetoresistive (MR) sensors are disclosed that have leads that overlap a MR structure and distribute current to and from the MR structure so that the current is not concentrated in small portions of the leads, alleviating the problems mentioned above. For example, the leads can be formed of a body-centered cubic (bcc) form of tantalum, combined with gold or other highly conductive materials. For the situation in which a thicker bcc tantalum layer covers a highly conductive gold layer, the tantalum layer protects the gold layer during MR structure etching, so that the leads can have broad layers of electrically conductive material for connection to MR structures. The broad leads also conduct heat better than the read gap material that they replace, further reducing the temperature at the connection between the leads and the MR structure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a cut-away view of a media-facing surface of a prior art MR sensor. 
         FIG. 2  is a cut-away view of a media-facing surface of a MR sensor in accordance with the present invention. 
         FIG. 3  is a cut-away opened up view of the sensor of FIG.  2 . 
         FIG. 4  is a cut-away view of a media-facing surface of another embodiment of a MR sensor. 
         FIG. 5  is a cut-away cross-sectional view of a step in the formation of the MR sensor of FIG.  4 . 
         FIG. 6  is a cut-away view of a media-facing surface of another embodiment of a MR sensor. 
         FIG. 7  is a cut-away view of a media-facing surface of another embodiment of a MR sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a view of a media-facing surface of a MR sensor  100  that has leads  102  and  104  that overlap an MR structure  106  and distribute current to the MR structure  106 . The media-facing surface may be coated with a thin layer of hard dielectric material such as diamond-like carbon (DLC) that is transparent and so not shown in FIG.  2 . The MR sensor  100  has been formed on a wafer substrate along with thousands of similar sensors and optional inductive recording transducers, not shown, before the wafer was diced into individual units, polished and coated to form the media-facing surface shown. Atop the substrate a first magnetically soft shield layer  110  has been formed, after which a first dielectric read gap layer  115  was deposited and polished. 
     The MR structure  106  was then deposited in a series of layers atop the first read gap layer  115 , beginning with a pinning layer  118  or layers including antiferromagnetic (AF) material for pinning a magnetic moment of a first ferromagnetic layer  120 , also known as a pinned layer  120 . A nonferromagnetic spacer layer  122  was then formed, for example of copper or gold, followed by a second ferromagnetic layer  124 , also known as a free layer  124 . A capping layer  126  was then formed, for example of tantalum, after which the sensor layers were masked and etched to define MR structure  106 . 
     Bias layers  128  were then formed for example of AF or high coercivity ferromagnetic material, and the mask covering structure  106  removed, lifting off bias material that had been deposited atop the mask. Another mask was then formed that partly covered the MR structure  106 , so that leads  102  and  104  could be formed on opposite sides of the mask. An adhesion layer  130  of tantalum or chromium was formed to a thickness of between about 10 Å and 200 Å, followed by a conductive layer  133  made of materials having a resistivity of less than 6×10 −8  Ωm at 25° C., such as gold, silver, copper, aluminum, beryllium, rhodium or tungsten. The adhesion layer can also be made of a layer of chromium followed by a layer of tantalum, so that the tantalum has an alpha tantalum phase, as described below. The conductive layer  133  has a thickness in a range between about 100 Å and 800 Å in this example. 
     A bcc seed layer  135  having a lattice constant substantially matching that of tantalum, such as chromium, tungsten, tantalum-tungsten or titanium-tungsten, was then formed to a thickness between about 20 Å and 100 Å, upon which a conductive tantalum layer  138  was formed. The seed layer  135  promotes growth of the bcc phase of tantalum, also known as alpha tantalum (α-Ta). For the case in which the conductive layer  133  is made of tungsten, an additional seed layer may not be necessary to form α-Ta. Although not as easy to fabricate as beta tantalum (β-Ta), which has a tetragonal crystalline structure, α-Ta is a significantly better electrical and heat conductor than β-Ta. The current in leads  102  and  104  is thus spread between gold layer  133 , chromium layer  135  and alpha tantalum layer  138 , reducing current crowding. 
     Moreover, alpha tantalum is much harder than gold, so that less of leads  102  and  104  may be removed during a subsequent etching step that determines the height of the MR structure  106  from the media-facing surface, as explained below, further reducing current crowding and lowering lead resistance. After the MR structure  106  height was defined, a second dielectric read gap layer  140  was deposited, on top of which a second magnetically soft shield layer  144  was formed. Although not shown in this figure, an inductive transducer may be formed prior to or subsequent to the MR sensor  100 , for example to create a head that writes and reads information on a storage medium. 
       FIG. 3  is an opened up view of the sensor  100  of  FIG. 2 , which illustrates an advantage mentioned above. The media-facing surface  150  is evident in this view, as are MR structure  106  and leads  102  and  104 . Bias layers  128  are covered by the leads  102  and  104 , which partially overlap MR structure  106 . MR structure  106  has been masked and etched, for example by ion beam etching (IBE), to create a back edge  155  that defines an active height or stripe height SH of the structure  106  from the media-facing surface  150 . An inactive portion I P  of the structure  106  extends further from the media-facing surface  150 , and is covered by active portions of leads  102  and  104 . The leads  102  and  104  have been partially etched during the creation of edge  155 , as shown by dotted lines  112  and  114 , respectively. The hard alpha tantalum layers  138  protect the softer conductive layers  133  during etching so that part of the alpha tantalum layers  138  and all of the gold layers  133  remain intact. 
     In contrast, during the creation of a back edge for the prior art MR structure  22  shown in  FIG. 1 , the soft gold leads  25  would have been fully removed at areas such as those bounded by dotted lines  112  and  114 , exposing bias layers  27  and leaving only thin leads connected to the MR structure  22 . The thicker alpha tantalum layers  138  shown in  FIG. 3  have not been completely removed above lines  102  and  104 , so that the lead height L H  for this embodiment is substantially greater than the MR stripe height S H . The conductive layer  133  covered by the alpha tantalum layers  138  also remains intact in this case, even for the case in which that layer  133  is made of soft materials such as gold or copper. This greater lead height L H  decreases the electrical resistance of the leads  102  and  104  and increases the thermal conductivity of the material directly adjoining the contact between the leads  102  and  104  and the MR structure  106 . A track width T W  of the sensor  100  is slightly less than the spacing between leads  102  and  104 , due to the broadened contacts of those leads with the MR structure  106 . 
       FIG. 4  shows another embodiment of a MR sensor  200  that has leads  202  and  204  containing alpha tantalum that overlap and distribute current to an MR structure  206 . In this embodiment, leads  202  and  204  include a layer  238  of alpha tantalum formed on a bcc seed layer  230  such as Cr, W, TaW or TiW that promotes the formation of alpha tantalum, although leads  202  and  204  could instead be formed of a multilayer structure described above or below. Similar to the embodiment described above, MR sensor  200  has first and second magnetically soft shield layers  210  and  244 , first and second dielectric read gap layers  215  and  240 , a pinning layer  218  or layers, a pinned ferromagnetic layer  220 , a nonferromagnetic spacer layer  222 , a free ferromagnetic layer  224  and bias layers  228 . Note that in this embodiment as well as other embodiments the ordering of pinning, pinned and free layers may be reversed. Also note that various other magnetoresistive structures may be used, such as multilayer giant magnetoresistive (GMR) structures. A capping layer  226  is disposed atop MR structure  206 , the capping layer  226  formed for example of tantalum. 
     The alpha tantalum lead layers  238  have resistivity that is four to five times greater than that of gold but almost an order of magnitude less than that of beta tantalum. Although the higher resistivity should result in greater ohmic heating and therefore exacerbated electromigration, the alpha tantalum leads  138  have reduced electromigration compared to the prior art sensor of FIG.  1 . The reduced electromigration may result from a greater distribution of current in the layers  238 , and as discussed with reference to  FIG. 3 , the track width of the sensor may be greater than the separation between lead layers  238 . 
       FIG. 5  is a cross-sectional view of a step in the formation of the transducer  200  of FIG.  4 . In  FIG. 5 , a bi-layer mask  236  has been formed of PMGI  250  and photoresist  252 , the mask partly covering beta tantalum capping layer  226 . Bcc seed layers  230  and alpha tantalum lead layers  238  have been deposited on bias layers  228  and also on and around the mask  236 . The overhanging photoresist  252  allows undercut PMGI layer  250  to remain exposed, provided that the lead layers  238  are not deposited too thickly, allowing the mask to be chemically dissolved and the metal atop the mask to be lifted off. For the situation depicted in  FIG. 5 , however, metal leads  238  and seed layer  230  have completely enveloped mask  236 . In this case a metal cap  255  covering mask  236  can be removed by breaking the cap off during washing with the resist solvent, for example by agitating the solvent and/or the wafer. 
     The seed layers  230  and lead layers  238  have been formed by ion beam deposition (IBD), as depicted with arrows  262  indicating a direction at which atoms are sputtered toward substrate  210 . For example, sputtering may be directed at an angle θ to perpendicular  266  to the wafer surface, angle θ also being known as the deposition angle. The angle θ can be changed by adjusting the fixture holding the substrate. It has been discovered that depositing the seed layer  230  at a large angle θ and depositing the lead layer  238  at a small angle θ is preferable for formation of alpha tantalum with minimal seed thickness. For example, the angle θ for seed layer  230  deposition may be about 20° or more, and the angle θ for lead layer  238  deposition may be about 10° or less. 
       FIG. 6  is a cross-sectional view of another embodiment in accordance with the present invention. In this embodiment MR sensor  300  has leads  302  and  304  each containing plural layers of alpha tantalum that overlap and distribute current to an MR structure  306 . Similar to the embodiments described above, MR sensor  300  has first and second magnetically soft shield layers  310  and  344 , first and second dielectric read gap layers  315  and  340 , a pinning layer  318  or layers, a pinned ferromagnetic layer  320 , a nonferromagnetic spacer layer  322 , a free ferromagnetic layer  324  and bias layers  328 . A capping layer  326  is disposed atop MR structure  306 , the capping layer  326  formed for example of tantalum. 
     A layer  333  of alpha tantalum is formed on a bcc seed layer  330  such as Cr, W, TaW or TiW that promotes the formation of alpha tantalum. The seed layer  330  should have sufficient thickness, for example at least about 20 Å, so that the lattice-matched bcc crystalline structure of the seed layer is adopted by the tantalum layer  333 . The alpha tantalum layer  333  can have various thicknesses, ranging for example from less than 40 Å to greater than 200 Å. A highly conductive layer  335  is then formed, for example of gold, silver, copper, aluminum, beryllium, rhodium or tungsten, that has a resistivity of less than 6×10 −8  Ωm at 25° C. A bcc seed layer  337  such as Cr, W, TaW or TiW is then formed to a thickness of at least about 20 Å, upon which an alpha tantalum layer  339  is formed. This layer may have a thickness calculated to withstand trimming of the MR structure  306 , protecting the highly conductive layer  335 . This also protects the MR structure  306  in the area overlapped by leads  302  and  304 , so that the MR structure  306  is shaped like a letter U, similar to the depiction in  FIG. 3 , but despite this unusual shape the sensor  300  functions well. Having the highly conductive layers  335  extend beyond the stripe height of the structure reduces lead resistivity and increases heat dissipation, both improving electromigration. 
       FIG. 7  shows another embodiment of a MR sensor  400  that has leads  402  and  404  containing alpha tantalum that distribute current to an MR structure  406 . In this embodiment, however, leads  402  and  404  abut the MR sensor  406  in a design that may be known as a contiguous junction. Leads  402  and  404  include a layer  438  of alpha tantalum formed on a bcc seed layer  435  such as Cr, W, TaW or TiW that promotes the formation of alpha tantalum. Gold, copper or other highly conductive materials may form layer  433 , deposited atop a seed layer  430  such as Cr or Ta. Instead of a multistep process used to form overlapping leads, lead layers and hard bias layers  428  are deposited using a single mask. The stripe height, however, may defined by the same process as described above with reference to FIG.  3 . Due to the slow etch rate of Ta on top of Au or Cu layer  433 , the more electrically conductive Au or Cu is preserved during stripe height milling. The extended electrically conductive layer  433  offers the advantage of reduced device-level parasitic resistance and provides a more effective heat dissipation path to improve device reliability. Similar to embodiments described above, MR sensor  400  has first and second magnetically soft shield layers  410  and  444 , first and second dielectric read gap layers  415  and  440 , a pinning layer  418  or layers, a pinned ferromagnetic layer  420 , a nonferromagnetic spacer layer  422 , and a free ferromagnetic layer  424 . A capping layer  426  is disposed atop MR structure  406 , the capping layer  426  formed for example of tantalum. 
     Although the present disclosure has focused on teaching the preferred embodiments, other embodiments and modifications of this invention will be apparent to persons of ordinary skill in the art in view of these teachings. For example, the sensing device may be part of a magnetic head that includes a write element that may be previously or subsequently formed. Alternatively, the sensing device may be used for measuring or testing for magnetic fields. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.