Abstract:
Magnetoresistive (MR) sensors are disclosed that have leads with reduced resistance, improving the signal-to-noise ratio of the sensors. The leads have broad layers of highly conductive material for connection to MR structures, as opposed to thin wires of highly conductive material or broad layers of resistive material, lowering the resistance of the leads. The low-resistance leads can be formed without increasing the shield-to-shield spacing, providing highly sensitive and focused MR sensors.

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 cutaway top view of a step in the fabrication of a prior art magnetoresistive (MR) sensor that may for example be used in a head of a disk drive. One or more MR layers  20  that vary in resistance in response to an applied magnetic field have been formed on a wafer, and then coated with a mask that has two openings separated by a small space where a MR sensor layers  20  will remain. After ion beam etching (IBE) that removes the MR layers  20  in the openings, metal bias and lead layers are deposited on the mask and openings, the mask and the metal layers atop it are removed by chemical etching, and the leads  22  remain covering the bias layers. The metal bias layer may be made of a hard magnet such as cobalt that has relatively high resistivity. The lead layer  22  may be made of a physically hard but somewhat resistive material such as tantalum or chromium, or may be a highly conductive material such as gold, which is capped with a tantalum or chromium adhesion layer. The MR layers  20  that remain between the leads  22  will define the track width of the sensor, which may be less than one micron. 
       FIG. 2  shows a step subsequent to that shown in  FIG. 1 , the subsequent step defining a height of the sensor, sometimes called the stripe height. A second mask has been created that substantially covers the lead layers  22  as well as covering part of the MR layers  20  disposed between the lead layers. An IBE is then performed that removes the MR layers  20  in areas not covered by the mask, leaving the MR layers  20  only in small region between the leads  22 , and removing part of the leads  22  that are not covered by the mask, as shown by dashed outline  25  of the original leads. 
     For the situation in which the leads are made of a physically soft material such as gold, the dashed outline  25  may represent an edge of a hard bias layer that is exposed after the gold has been milled away by IBE. For the case in which the leads are formed of physically hard materials such as tantalum or chromium, outline  25  may represent remnants of the lead layer. In either case the region outside the unmilled leads  22  and within dashed line  25  consists of material having relatively high resistivity. Later, after additional layers have been formed, the wafer will be cut and polished to line  27 , leaving MR sensor  30  connected between the leads  22  that have not been removed by IBE. 
       FIG. 3  shows an opened up view of the sensor  30  after lapping or polishing of surface  27 . Leads  22  can be seen to have long strips  33  that are connected to the sensor  30 . The scale of  FIG. 3  is magnified compared to that of  FIG. 2 , displaying the rounding of the leads  22  where the strips connect to larger lead section, the rounding due to limitations in photolithography. Similar limitations prevent the length of the strips, which may each be about one micron, from being shortened without introducing error or imperfections in defining the height of the sensor. 
     Even for the case in which the leads  22  are made of gold, the small cross sectional area and long length of the strips  33  causes measurable resistance. Since the MR sensor  30  measures a change in resistance, the lead resistance lowers the signal-to-noise ratio of the sensor. For the situation in which the leads are made of tantalum or chromium, this parasitic resistance may be worse. 
       FIG. 4  shows a view of surface  27  of the completed prior art sensing device of  FIG. 3 . The MR structure is shown generally at  30  and the leads are shown generally at  22 , each being composed of plural layers in this example. A first magnetically soft shield layer  50  has been formed of permalloy, followed by a dielectric read gap layer  52  made of alumina. An antiferromagnetic (AF) layer  55  has been formed on the read gap layer  52 , followed by a permalloy pinned layer  58 , so that the AF) layer  55  fixes the magnetic direction of the pinned layer  58 . A spacer layer  60  has been formed of copper on the pinned layer  58 , and a permalloy sense layer  62  has been formed on the spacer layer  60 . 
     A mask was formed atop the sense layer  62 , as described with reference to  FIG. 1  above, and MR structure  30  defined by milling that extends slightly into the first read gap layer  52 . With the mask still present a seed layer  64  of chromium was formed to a thickness of 50 Å, followed by a 600 Å cobalt-based layer  66  that provides magnetic bias to edges of the free, layer  55 . A 100 Å tantalum adhesion layer  68  is disposed on the bias layer  66 , and a 600 Å gold lead layer  70  formed on the adhesion layer  68 . A 100 Å tantalum capping layer  72  was deposited on the gold layer  70 , after which the mask was chemically removed, lifting off the metal layers that were formed atop the mask, and a second dielectric read gap layer  75  made of alumina is deposited. For the situation in which metal layers such as bias and lead layers are thickly deposited, the metal layers may completely envelope the mask in an area over the MR sensor  30 . The metal coated mask can then be broken off, for example by ultrasonic agitation of an etchant, but this can leave metal fences protruding above the edges of the sensor  30 . 
     A second magnetically soft shield layer  77  has been formed of permalloy atop the read gap layer  75 . The shield layers  50  and  77  help to shield the MR structure  30  from magnetic flux originating from parts of a magnetized media track that are not substantially aligned with MR structure  30 , allowing the MR structure  30  to more clearly sense the flux from bits that are aligned with MR structure  30 . The spacing between shield layers  50  and  77  determines the focus of the MR structure  30  on the magnetic flux emanating from the media directly opposite the MR structure  30 , by eliminating magnetic flux that emanates from bits that are not aligned with MR structure  30 . For reading high density magnetic patterns it is therefore advantageous to reduce the thickness of the various layers between the shield layers  50  and  77 , limiting the thickness of the leads  22 . If read gap layer  52  is made too thin, however, lead or stripe etching may create a short circuit to shield layer  50 . If read gap layer  75  is made too thin, metal fences protruding above edges of sensor  30  may create a short circuit to shield layer  77 . Stated differently, the leads  22  may have a total thickness of about 800 Å, and the MR sensor  30  layers may have a combined thickness of about 500 Å, so that the lead thickness and bias layer thickness can be limiting factors in shield-to-shield spacing. 
     SUMMARY 
     Magnetoresistive (MR) sensors are disclosed that have leads with reduced resistance, improving the signal-to-noise ratio of the sensors. The leads have broad layers of highly conductive material for connection to MR structures, as opposed to thin wires of highly conductive material or broad layers of resistive material, lowering the resistance of the leads. The low-resistance leads can be formed without increasing the shield-to-shield spacing, providing highly sensitive and focused MR sensors. The broad layers of highly conductive lead material also conduct heat away from the sensors, which can further improve sensor performance and lifetime. 
     In one embodiment, a device is disclosed comprising: a solid body having a surface; a magnetoresistive structure disposed in said body adjacent to said surface, said magnetoresistive structure extending a first amount in a first direction that is substantially parallel to said surface, extending a second amount in a second direction that is substantially perpendicular to said surface, and extending a third amount in a third direction that is perpendicular to said first and second directions, with said third amount being less than each of said first and second amounts; a first lead layer disposed in said body adjacent to said surface and made of material having a resistivity of less than 6×10 −8  Ωm, said first lead layer having a first edge extending in said second direction so that a first portion of said first edge adjoins said magnetoresistive structure and a second portion of said first edge is removed from said magnetoresistive structure; and a second lead layer disposed in said body adjacent to said surface, separated from said first lead layer in said first direction and made of material having a resistivity of less than 6×10 −8  Ωm at room temperature, said second lead layer having a second edge extending in said second direction so that a first part of said second edge adjoins said magnetoresistive structure and a second part of said second edge is removed from said magnetoresistive structure; wherein said first portion is separated from said first part by a distance that is substantially identical to that separating said second portion from said second part. 
     In one embodiment, a device is disclosed comprising: a solid body having a surface; a magnetoresistive structure disposed in said body adjacent to said surface, said magnetoresistive structure extending a first amount in a first direction that is substantially parallel to said surface, extending a second amount in a second direction that is substantially perpendicular to said surface, and extending a third amount in a third direction that is perpendicular to said first and second directions, with said first amount and said second amount each being greater than said third amount; and a lead layer disposed in said body adjacent to said surface and having an edge adjoining said magnetoresistive structure, said edge extending a fourth amount in said second direction without extending as much as said second amount in said first direction, said lead layer including gold, copper, silver, aluminum, beryllium, magnesium, molybdenum or tungsten; wherein said fourth amount is significantly greater than said second amount. 
     In one embodiment, a device is disclosed comprising: a solid body having a surface; a magnetoresistive structure disposed in said body adjacent to said surface, said magnetoresistive structure extending a first amount in a first direction that is substantially parallel to said surface, extending a second amount in a second direction that is substantially perpendicular to said surface, and extending a third amount in a third direction that is perpendicular to said first and second directions, with said first amount and said second amount each being greater than said third amount; and a lead layer disposed in said body adjacent to said surface and extending a fourth amount in said third direction, said lead layer having an edge adjoining said magnetoresistive structure and extending a fifth amount in said second direction without extending as much as said second amount in said first direction, made of material having an electrical resistivity of less than 6×10 −8  Ωm at room temperature; wherein said fifth amount is at least two orders of magnitude greater than said fourth amount. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a cutaway top view of a step in the fabrication of a prior art magnetoresistive (MR) sensor that may for example be used in a head of a disk drive. 
         FIG. 2  shows a cutaway top view of a step subsequent to that shown in  FIG. 1 , the subsequent step defining a height of the MR sensor. 
         FIG. 3  shows an opened up view of the prior art sensor after additional layers have been formed and the wafer cut and polished. 
         FIG. 4  shows a view of surface  27  of the completed prior art sensor of  FIG. 3 . 
         FIG. 5  shows an opened up view of an embodiment of a sensing device having low-resistance leads. 
         FIG. 6  shows a view of the surface of the completed sensing device of  FIG. 5 . 
         FIG. 7  shows a view of the surface of an alternative completed sensing device of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 5  shows an opened up view of an embodiment of a sensing device  100  having low-resistance leads  102  and  104  connected to a MR structure  110 . A polished surface  107  defines a bottom edge or sensing side of the MR structure  110 , after a top edge  108  of the MR structure  110  has been defined by masking and removal by IBE or similar processes. Portions of the leads  102  and  104  were also trimmed by the removal process, as shown by dashed lines  112  and  114 , respectively, but the removal process was terminated after the MR structure  110  was completed and before the highly conductive leads had been removed. 
     Instead of the prior art approach of having thin leads connected to an MR sensor, the embodiment of  FIG. 5  has a lead height L H  that is significantly greater than a MR structure height S H . The greater conductive cross-section provided by the greater lead height L H  reduces the lead resistance, making any change in the MR structure  110  resistance more noticeable, increasing the sensitivity of the sensing device  100 . The lead height L H  in this embodiment is also greater than a track-width T W  of the MR structure  110 , emphasizing the series resistance of the MR structure over the parallel resistance of the leads  102  and  104 . 
     A surprising way to provide the increase in a highly conductive lead cross-section is by reducing the thickness of the highly conductive portion of the lead. This reduced thickness of the highly conductive portion of the lead affords an increased thickness of a physically hard capping layer of the lead, without decreasing the resolution of the sensor by increasing the shield-to-shield spacing. The increased thickness of the hard capping layer in turn allows an IBE that penetrates the sensor layers to define a stripe height S H  without penetrating the capping layer, leaving the physically soft but highly conductive portion of the lead intact. 
     Each of the leads  102  and  104  has an edge extending substantially perpendicular to surface  107  further than the stripe height S H . For instance, lead  104  has an edge  115  with a first portion adjoining the magnetoresistive structure and a second portion  117  removed from the magnetoresistive structure. The broad layers of highly conductive lead material also conduct heat away from the sensors, which can further improve sensor performance and lifetime. 
       FIG. 6  shows a view of the surface  107  of the completed sensing device  100  of  FIG. 5 , having leads  102  and  104  connected to a MR structure  110 . A thin coating of dielectric material such as alumina or diamond-like carbon (DLC) may coat the surface  107  to protect the sensing device  100 , with the sensing device elements visible through the coating. Leads  102  and  104  include a tantalum adhesion layer  118  having a thickness of 50 Å, a gold lead layer  120  having a thickness of only 400 Å formed on the adhesion layer  118  and a tantalum cover layer  122  having a thickness of 250 Å. Thus the leads  102  and  104  in this embodiment have the same overall thickness as those of the prior art, but the tantalum cover layer  122  is more than twice as thick. Tantalum has a milling rate that is less than half the overall milling rate of the MR layers, which currently have an overall thickness of about 300 Å, so that the stripe height of the MR structure  110  can be defined by directed etching that does not penetrate the cover layer  122 , leaving the gold layer  120  intact. 
     To complete the description of the embodiment shown in  FIG. 6 , a first magnetically soft shield layer  126  has been formed of permalloy, followed by a dielectric read gap layer  132  made of alumina. An antiferromagnetic (AF) layer  135  has been formed of permalloy on the read gap layer  132 , and a pinned layer  138  has been formed of permalloy or other magnetic materials on the AF layer, the AF layer pinning the magnetic direction of the pinned layer. A spacer layer  140  of copper or other highly conductive materials has been formed on the pinned layer  138 . A free layer or sense layer  142  has been formed of permalloy or other magnetic materials on the spacer layer  140 . 
     A mask was formed atop the sense layer  142 , as described with reference to  FIG. 1  above, and MR structure  110  defined by milling that extends slightly into the first read gap layer  132 . With the mask still present a seed layer  144  of chromium was formed to a thickness of 50 Å, followed by a 600 Å cobalt-based layer  146  that provides magnetic bias to edges of the free layer  55 . The tantalum adhesion layer  118 , gold lead layer  120  and the cover layer  122  were deposited as mentioned above. The mask was then chemically removed, lifting off the metal layers that were formed atop the mask, which may in addition be removed by ultrasonic agitation of the etchant, and a second dielectric read gap layer  145  made of alumina was deposited. A second magnetically soft shield layer  147  has been formed of permalloy atop the read gap layer  145 . 
     Although the above-described embodiment has been shown to work well, variations in many of the parameters are possible. For example, the leads may overlap the top of the MR structure  110 , effectively narrowing the track width of the MR structure  110  compared to the amount that the sensor layers extend in the track width direction. The pinning and pinned layers of the MR structure may be formed after the free layer, and/or the MR structure may have additional pinned or free layers, or the MR structure may consist of a single sense layer. The pinning structure may include antiferromagnetic material and may include exchange-coupled layers. Use of an antiferromagnetic material for the bias layer may allow the bias layer to be made thinner, allowing the capping layer to be thicker without having the highly conductive lead layer thinner. 
       FIG. 7  shows an alternative embodiment in which the bias layer  146  is thinner, allowing the capping layer  122  to be thicker without the highly conductive lead  120  to be thinner. The resistance of the leads  102  and  104  is further reduced in this case, further improving the signal-to-noise ratio of the sensing device. The bias layer  146  may be formed with an angled sputtering or other deposition techniques that provide coverage of the MR structure  110  edges despite the reduced bias layer thickness. 
     The cover layer  122  may be formed of materials selected for physical hardness rather than conductivity. For example, tungsten or chromium can serve as a capping layer and are at least as hard as tantalum. Moreover, the cover layer  122  may be formed of α-tantalum, which has a body-centered cubic crystalline structure, instead of conventional β-tantalum, increasing conductivity as well as hardness, by first forming a chromium seed for the capping layer. The cover layer  122  may be formed of an adhesive layer and a hard layer, such as a tantalum layer covered by a DLC, SiC, Aln or Al 2 o 3  layer, in which the exposed hard layer is removed during stripe-defining etching, leaving the adhesive layer covering the highly conductive layer. The thickness of the cover layer may be chosen to be almost completely removed during stripe-defining etching, so that the thickness of the highly conductive layer can be increased. 
     The highly conductive leads may be made of gold, copper, silver, aluminum, beryllium, magnesium, molybdenum, tungsten or other materials known for electrical conductivity. The lead height L H  may be in a range between slightly greater than the MR structure height S H  to an order of magnitude greater than the MR structure height S H . The highly conductive leads may have a lead height L H  that is more than an order of magnitude greater than their lead thickness L T . A highly conductive lead cross-section L X  may be determined by multiplying the lead thickness L T  by the lead height L H . This may be compared with an active element cross section MR X  of the MR structure  110 , which is determined by multiplying the combined thickness of the pinned, free and spacer layers by the MR structure height S H . A ratio of the highly conductive lead cross-section L X  to the active element cross section MR X  may be greater than two orders of magnitude. 
     The MR structure height S H  may range from less than 100 nm to more than one-quarter micron, and the track-width T W  of the MR structure may range from less than 100 nm to more than one-half micron, although other ranges are possible in the future. The pinned layer  140  and the free layer  135  may each be formed of a nickel-iron alloy or other materials known in the art of MR sensors, and may each have a thickness in a range between less than one nanometer and hundreds of nanometers. The conductive layer  106  may be formed of copper or other materials known in the art, and may have a thickness in a range between less than one nanometer and tens of nanometers. 
     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.