Patent Publication Number: US-6989972-B1

Title: Magnetoresistive sensor with overlapping leads having distributed current

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 including one or more ferromagnetic layers so that the structure  22  has a resistance that varies 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 shield layers  30  and  33  block stray magnetic fields from the MR structure  22 , although fields that originate from the media opposite the MR structure  22  are not blocked by the shields. The shields  30  and  33  are 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 distance between the lead layers 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, an electrically resistive capping layer of tantalum or other materials can be formed to sufficient thickness on a MR structure prior to etching the structure and forming the bias and lead layers. The capping layer can have a greater thickness in portions adjoining the leads than in a central region not covered by the leads. Alternatively or in combination, the leads can be formed of a resistive material, or may have interspersed layers of resistive and conductive materials with gold or other highly conductive materials. For the situation in which a resistive lead layer also has a significantly lower milling rate, the leads can have broad layers of material for connection to MR structure, which may have a higher resistivity but lower overall resistance. 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 that has leads that overlap an MR structure and distribute current to and from the MR structure. 
         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 cross-sectional view of a step in the formation of the MR sensor subsequent to the step shown in  FIG. 5 . 
         FIG. 7  is a cut-away view of a media-facing surface of another embodiment of a MR sensor that has leads that overlap an MR structure and distribute current to and from the MR structure. 
     
    
    
     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 media-facing surface labeled  150  in  FIG. 3 . 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 (r C ) 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 50 Å and 500 Å in this example. 
     A resistive layer  138  was then formed on the conductive layer  133 , the resistive layer also having a slow ion-milling rate. The resistive layer  138  may for example include chromium, palladium, platinum or beta tantalum (β-Ta), and typically has a resistivity (r R ) that is greater than 10 −7  Ωm at 25° C. In order to encourage conduction in the resistive layer  138  as well as the conductive layer  133 , a thickness (T R ) of the resistive layer is substantially greater than a thickness (T C ) of the conductive layer. In general, a ratio of the thickness T R  of the resistive layer  138  compared to the thickness T C  of the conductive layer  133  should be greater than or about equal to a ratio of the resistivity (r R ) of the resistive layer  138  compared to the resistivity (r C ) of the conductive layer  133 . The thickness of the layers is easy to measure in an area distal to the MR structure  106  but closest to the media-facing surface  150 . Stated differently, T R /T C &gt;r R /r C  or T R /T C ≈r R /r C . Alternatively, T R /r R &gt;T C /r C  or T R /r R ≈T C /r C . The current in leads  102  and  104  is thus spread between the conductive layer  133  and the resistive layer  138 , avoiding current crowding. 
     Moreover, the resistive layer  138  (e.g., tantalum) can be much harder than the conductive layer  133  (e.g., 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 a height S H  of the structure  106  from the media-facing surface  150 . 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 gold 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  112  and  114 , so that the lead height L H  for this embodiment is substantially greater than the MR structure height S H . The gold lead layer covered by the alpha tantalum layers  138  also remains intact in this case. 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  that overlap an MR structure  106  and distribute current to an MR structure  206 . In this embodiment, leads  202  and  204  are formed of 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 or the previous embodiment the ordering of pinning, pinned and free layers may be reversed. 
     A capping layer  226  of MR structure  206 , however, has thicker portions  233  disposed beneath leads  202  and  204 , and a thinner portion  235  disposed between the thicker portions. Although for some embodiments capping layer  226  may have a greater conductivity, the capping layer  226  in this embodiment has a resistivity greater than 10 −7  Ωm at 25° C. The thicker portions of resistive capping layer  226 , which may for example be made of beta tantalum, distribute the current to MR structure  206 , providing a lead overlay sensor that avoids current crowding. The thinner portion  235  restricts current flow through capping layer  226  so that layer  226  does not shunt current flow from the MR structure. The thicker portions  235  may have a thickness in a range between about 20 Å and 500 Å, with the thinner regions having a thickness less than about half that of the thicker regions. Alternatively or in addition, the thinner region may be oxidized throughout most if not all of its thickness. It is also possible to form capping layer  226  as a pair of isolated islands at thicker regions  233 , with thinner region  235  removed. An advantage of these embodiments is that they provide closer shield-to-shield spacing and/or thicker leads without shield-to-sensor shorting. Closer shield-to-shield spacing improves the focus of the sensor  200 , and thicker leads lower the lead resistance and therefore improve the signal-to-noise ratio, both of which improve sensor resolution. 
       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 sputter-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. 
     As shown in  FIG. 6 , metal projections  260  may remain after washing with the solvent has lifted off the cap. These projections  260 , which may look like fences at the end of each lead, can create unwanted electrical connections between the leads and the second shield layer. An isotropic or anisotropic etching procedure such as ion beam etching (IBE) or reactive ion etching (RIE) can remove projections  260  while thinning the capping layer  226  in a region  264  that is uncovered by leads  238 . For example, an IBE  262  may be directed at a rotating or sweeping angle Ø to perpendicular  266  to the wafer surface. An isotropic etching process, especially an etching process that selectively removes the capping layer and projections at a higher rate than the free layer, may also be effective. 
       FIG. 7  shows a view of a media-facing surface of a MR sensor  300  that is similar to that shown in  FIG. 2 , for which a number of the elements can be substantially identical and so are not described here. Leads  302  and  304  overlap MR structure  106  and distribute current to the MR structure  106 , the leads including plural layers of conductive material and plural layers of resistive material. In this example, conductive layers  160  and  164  have a resistivity less than 6×10 −8  Ωm at 25° C., whereas resistive layers  162  and  166  have a resistivity greater than 10 −7  Ωm at 25° C. The overall thickness of the resistive layers  162  and  166  (i.e., the sum of the thickness of each layer  162  and  166 ) is substantially greater than a overall thickness (T C ) of the conductive layers  160  and  164 . In general, a ratio of the overall thickness T R  of the resistive layers  162  and  166  compared to the overall thickness T C  of the conductive layers  160  and  164  should be greater than or about equal to a ratio of the average resistivity (r R ) of the resistive layers  130 ,  162  and  166  compared to the resistivity (r C ) of the conductive layers  160  and  164 . Stated differently, T R /T C &gt;r R /r C  or T R /T C ≈r R /r C . Alternatively, T R /r R &gt;Tc/r C  or T R /r R ≈T C /r C , or a thickness-resistivity ratio of each resistive layer should be greater than or about equal to a thickness-resistivity ratio of each conductive layer. The current in leads  302  and  304  is thus spread between the conductive layers  160  and  164  and the resistive layers  162  and  166 , avoiding current crowding. Additional conductive and resistive layers can be similarly formed. 
     Instead of the lead structures described above, other lead structures that overlap a MR structure can be made to reduce current crowding in the leads. Exemplary lead structures include a single layer of Cr or laminates of Cr/Mo/Cr, β-Ta/Au/β-Ta, Cr/α-Ta/Au/Cr/α-Ta, β-Ta/Au/Cr/α-Ta, TiW/α-Ta/Au/TiW/α-Ta or β-Ta/Au/TiW/α-Ta. 
     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.