Abstract:
A magnetoresistive sensor has a read gap that is made of a slow ion milling rate material. The slow milling rate read gap allows a blunt end to be formed for the sensor without excessive overmilling into the read gap. The read gap may also be formed of plural layers with at least one of the layers having a low milling rate. This allows the other read gap layer to have complimentary attributes, such as high thermal conductivity, low stress, less pinholes and/or better dielectric properties. The electromagnetic characteristics of MR sensors having such steeply sloped ends are enhanced both in reading signals and reducing noise. The track width of such a sensor can be more accurately formed due to the blunt shape of the contiguous junction, quantizing signals and reducing errors from reading adjacent tracks. The sensor can also be made to have a sharper linear bit resolution, due to a thinner, high-integrity read gap. Barkhausen noise is reduced, as well as signal biasing improved, with blunt contiguous junctions formed between the sensor ends and lead or bias layers.

Description:
TECHNICAL FIELD 
     The present invention relates to electromagnetic transducers and magnetoresistive sensors. 
     BACKGROUND OF THE INVENTION 
     The employment of magnetoresistive (MR) sensors for reading signals from media is well known. Such sensors read signals from the media by detecting a change in resistance of the sensor due to magnetic fields from the media. Many variations of MR sensors are known, such as anisotropic magnetoresistive (AMR) sensors, dual stripe magnetoresistive (DSMR) sensors, giant magnetoresistive (GMR) sensors, spin valve (SV) sensors and dual spin valve (DSV) sensors. 
     Common to these sensors is the need to provide bias fields, both to eliminate noise and to facilitate signal readout. A known means for biasing the sensor involves abutting a permanent magnet to ends of the sensor, the magnet preferably forming a contiguous junction across plural sensor layers. Conductive leads, which may be separate from the biasing means, may also adjoin sensor layers along a contiguous junction. 
     In order to form a contiguous junction, a sensor is usually deposited in layers and then its border defined by masking and ion beam milling or etching (IBE), reactive ion etching (RIE) or the like. Ideally, milling could be performed that directs an ion beam exactly perpendicular to the surface on which the MR sensors are being formed, resulting in blunt sensor ends that terminate at a 90° angle to that surface. Redeposition of materials removed by milling and other complications interfere with this scenario, however, so that such exact etching is not practicable. Moreover, following such a perpendicular IBE the deposition of hard bias and/or conductor layers could blanket the mask, so that etching of the mask would be prevented and the necessary lift-off of these layers would become problematic. 
     A known means for forming a contiguous junction involves forming an undercut in the mask and rotating the IBE at an angle offset from 90°. FIG. 1 exemplifies this approach, in which a magnetic shield  20 , insulative layer  22  and sensor  25  has been formed atop a substrate  27 . A bi-layer resist  28  and  29  has been photolithographically patterned atop the layers that are to form the sensor  25 , leaving an undercut  30  closest to the sensor. An IBE is directed at a relatively rotating angle to perpendicular to form a curved border  33  for which part of the insulative layer as well as all of the sensor layers have been removed. Dotted lines  35  and  37  represent directions of the rotating ion beam at opposite phases, and show that etching proceeds less beneath the mask where line  37  impinges but not line  35 . The removal of part of layer  22  is termed overmilling. The undercut  30  allows etchant to remove the mask  28  even after deposition of hard bias and lead layers that cover the mask as well as adjoin the border  33  to form a contiguous junction. 
     The shallow slope of the resulting contiguous junction has a number of drawbacks. The oblique angle of the border defining the contiguous junction denigrates the bias field provided to the sensor and complicates the sensor domain structures, so that noise is not eliminated. The shallow slope also creates inaccuracy in the width of the sensor, which ideally should match the width of magnetic tracks on the media, called the “track width.” Surprisingly, the length of the contiguous junction regions on both ends of the sensor can be comparable to or even greater than the width of the sensor between the contiguous junction regions, blurring images and causing off-track errors. 
     The contiguous junction  33  could be made more blunt along the sensor with additional ion milling into the insulative layer  22 , also known as the read gap. Unfortunately this may result in electrical shorting between the shield and the hard bias and lead layers. The insulative layer  22  can be made thicker to allow for this overmilling, but the thickness of that read gap separating the sensor from the shield is a primary determinant of the resolution of the sensor. In other words, the sensor “sees” magnetic fields from the media that pass between the shields, and the closer the shields are to the sensor the more narrow the focus of the sensor. Thus an attempt to create a less oblique contiguous junction to solve the bias and resolution problems can result in other resolution problems and electrical shorting. 
     SUMMARY OF THE INVENTION 
     The present invention has a number of advantages, including creation of blunt contiguous junctions for MR heads, improving sensor performance. These advantages are achieved in part with a read gap material that has a substantially slower milling rate than conventional alumina. This slower milling rate allows a blunt contiguous junction to be formed along the sensor without excessive overmilling into the read gap. The read gap may also be formed of plural layers with at least one of the layers having a low milling rate. This can allow the other read gap layer to have complimentary attributes, such as high thermal conductivity, low stress, less pinholes and/or better dielectric properties. 
     The electromagnetic characteristics of MR sensors having such steeply sloped contiguous junctions are enhanced both in reading signals and reducing noise. The track width of such a sensor can be more accurately formed due to the blunt shape of the contiguous junction, clearing blurred signals and errors from reading adjacent tracks. The sensor can also be made to have a sharper linear bit resolution, due to a thinner, high-integrity read gap. Barkhausen noise can be reduced, as well as signal biasing improved, with blunt contiguous junctions. In sum, MR sensors of the present invention can achieve sharper resolution of both the length and width of magnetic bits, reduced noise, and enhanced signal readout. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a prior art method for forming a contiguous junction. 
     FIG. 2 is a side view of the formation of a sensor of the present invention having a blunt contiguous junction and a low milling rate read gap material. 
     FIG. 3 is a side view of the formation of a sensor of the present invention having a blunt contiguous junction and a read gap having plural layers with at least one of the layers having a low milling rate. 
     FIG. 4 is a side view of the formation of a sensor of the present invention having a blunt contiguous junction and a low milling rate read gap material along with a low milling rate cap on the sensor. 
     FIG. 5 is a view of a disk-facing portion of an AMR sensor formed according to FIG. 2 having blunt contiguous junctions with bias and conducting layers. 
     FIG. 6 is a cross-sectional view of the sensor of FIG. 5, showing a blunt junction for the sensor distal to the disk-facing surface. 
     FIG. 7 is a view of a disk-facing portion of a GMR sensor formed according to FIG. 2 having blunt contiguous junctions with bias and conducting layers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 shows the result of a directional etching step in the formation of an MR sensor of the present invention. Atop a substrate  40  which may be composed of Al 2 O 3  (alumina), AlTiC or other known materials, a first shield layer  42  of ferromagnetic material, such as NiFe, is formed. A read gap layer  44  of slow-ion-milling-rate material is then formed, and an MR sensor  46  is formed atop the read gap  44 . To facilitate understanding of the present invention, the MR sensor  46  is shown as a single structure, but typically includes plural layers. A mask is patterned atop the sensor  46 , the mask including an overcoat  50  and an undercoat  48  that forms an undercut to allow lift off after ion milling. 
     The read gap layer  44  may be formed of a variety of materials having a milling rate that is substantially less than that of conventional read gap materials such as AL 2 O 3  or AlN. Preferred slow-ion-milling-rate materials include polycrystalline diamond-like carbon (DLC), cubic-BN, ZrO 2 , MgO, Nb 2 O 5 , Y 2 O 5 , HfO 2 , BaTiO 3  and TiO 2 . A slow-ion-milling-rate material is defined in the present to be a material that is removed by conventional ion milling at less than two-thirds the rate at which alumina is removed. Polycrystalline DLC, for example, has an ion-beam-milling rate that is about one-fourth that of alumina. Cubic-BN has similar hardness, which can be significantly harder than amorphous DLC. The milling rate of the read gap is also substantially less than that of most MR sensor layers, which may be predominantly comprised of NiFe. Other similar directed impact removal or directional etching techniques such as sputter etching are generally encompassed by the term ion milling. 
     The significant difference in milling rates between the sensor  46  and the gap layer  44  creates a border  52  for the sensor that has a significantly steeper slope than the adjoining surface  55  of the gap layer. Generally the steeper slope  52  is created with a longer overmilling time. With the steep slope  52 , a lateral extent ε of the border is also greatly reduced, so that the sensor has a much more sharply defined width, which can read bits on a track much more accurately. Stated differently, the border extends substantially less in a direction of the width of the sensor than in a direction perpendicular to that width. This border  52  will subsequently form a contiguous junction with bias and/or lead layers for the sensor. 
     A thickness of the read gap  44  under the sensor  46  is labeled G, an overmill depth is labeled O, and a thickness of the read gap  44  away from the sensor is labeled H, where G=H+O. The thickness G directly contributes to sensor resolution, such that a thinner G leads to better resolution. H is determined by the requirement of electrical insulation between the shield  42  and bias or lead layers that will be formed atop border  55  to abut border  52  and provide electrical connections to the sensor  46 . O is equal to the time spent overmilling multiplied by the ion-milling rate of the read gap layer  44 . A longer overmilling time generally results in a steeper slope for border  52 , so that the slower ion milling rate materials of the present invention allow O and H to be standard dimensions and still obtain a steep slope for that border  52 . A steeper slope for border  52  provides increased accuracy to the sensor&#39;s track-width dimension, measured essentially perpendicular to the path of recorded tracks, improving resolution in that direction. 
     On the other hand, overmilling for a standard length of time will leave the overmill depth O for the low-ion-milling rate gap materials of the present invention less than that for a conventional alumina gap, so that the thickness of the read gap G can be reduced without reducing the thickness H. The thickness H can also be reduced, however, since the materials chosen for the gap layer  44  can be more impervious to electrical shorting by having reduced defects or porosity, for instance. Since both O and H can be reduced, the overall gap G can be made much smaller. Such a thinner read gap layer  44  provides higher resolution for the sensor along the direction of recorded tracks, improving linear resolution. While conventional read gaps may average 50 nm-100 nm, the impervious read gap materials employed in the present invention are preferably formed to a thickness of 200 Å-800 Å currently, while gap layers of less than 100 Å are clearly possible, and gaps as thin as 60 Å may be achievable. 
     It is also possible to have a longer overmilling time and a reduced overmilling depth to form a sensor with a steeper border  52  but smaller read gap G. Such a sensor has improved resolution in both linear and track-width directions, greatly increasing overall resolution. 
     Since the read gap  44  is formed prior to formation of the sensor layers, the gap  44  may be formed under conditions that would not be possible once the delicate sensor has been formed. For example, a DLC read gap may be formed at temperatures ranging between 400° C. and 900° C. in order to create crystalline or polycrystalline carbon having tetrahedral bonds, as opposed to amorphous DLC that is formed at lower temperatures (typically room temperature). Cubic-BN can be deposited at 400° C. and has a hardness that may be several times that of amorphous DLC. MR sensor layers are likely to be damaged at temperatures of 250° C. or higher, while polycrystalline carbon may be formed at temperatures as low as 300° C., although higher temperatures are preferable and generally result in better quality films. Crystalline carbon, polycrystalline carbon and cubic-BN have lower ion milling rates than amorphous carbon, and thus are preferred for the read gaps of the present invention. Other slow milling rate read gap materials may also benefit from high temperature formation. Another means for forming tetrahedral carbon (ta-C) for the read gap  44  is by filtered cathodic arc deposition, as described in commonly assigned application entitled Improved Insulator Layers For Magnetoresistive Transducers, invented by Knapp et al., which is incorporated herein by reference. 
     A variation of the present invention is shown in FIG. 3, in which plural read gap layers are provided, at least one of the layers having a significantly lower milling rate than is conventional. In this embodiment, which has similar elements as disclosed above, a first layer  60  and a second layer  62  together form a read gap  65  that separates the sensor  46  from the shield  42 . A thin adhesion layer, not shown, may be deposited before or between the gap layers. The two layers  60  and  62  are designed to compliment each other and improve the characteristics of the read gap  65 . For instance, the second layer  62  may be relatively impervious to ion milling but electrically more conductive and/or thermally less conductive, while the first layer  60  may have superior dielectric properties, high thermal conductivity, and/or low stress but high ion milling rate. 
     Examples of slow-ion-milling-rate materials that may be employed for the second layer  62  include TiC, TaC, SiC, B 4 C, WC, TiB 2 , TaB 2 , AlB 12 , C, DLC, cubic-BN, B 4 N, BCN, β-C 3 N 4 , ZrC, VC, NbB 2 , W 2 B 5 , LaB 6 , or SiB 6 . The first layer  60  can be formed of complimentary materials that are, for example, electrically insulative, such as AL 2 O 3 , AIN or SiN x . The second layer can be formed to a thickness corresponding to a desired overmilling time, so that after overmilling the exposed portion of the second layer is removed but the first layer is left substantially intact. Such a thickness may be as low as about 20 Å to 100 Å, although a greater thickness is possible. 
     In some cases it may be desirable to form a relatively impervious gap layer  60  first and then coat it with another more workable layer  62 , for instance to serve as a template for sensor formation. Certain highly stressed materials, such as cubic-BN or DLC, may be used as a thin first gap layer  60  and then coated with an even thinner second layer  62  of AL 2 O 3 , AlN or SiN x  for example. Depending on a number of factors, the first and second layers  60  and  62  may each have a thickness in a range between about 40 Å and 400 Å. Further, the high-stress layer may be formed on a thin adhesion layer of AL 2 O 3 , AlN or SiN x , so that the high-stress, low ion milling rate layer is sandwiched by the lower stress, faster ion milling rate layers. For this situation, the high-stress, low ion milling rate layer may have a thickness in a range between about 40 Å and 400 Å, while the lower stress, faster ion milling rate layers have a thickness in a range between about 10 Å and 100 Å. By monitoring for the detection of materials from the first layer  60 , second layer  62  and/or any other layers, for example by optical emission spectroscopy, termination of milling can be accurately controlled and the gap  65  made thinner. 
     As shown in FIG. 4 very thin, slow-ion-milling-rate cap layer  70  may be formed atop the sensor layers prior to ion milling, in order to provide protection for the top of the sensor  46 , causing a further increase in the steepness of the sensor end  72  that is to form a contiguous junction. For sensor having a soft adjacent layer adjoining this layer  70 , the cap layer  70  can be composed of conductive materials, such as about 20 Å-80 Å TiC or TaC. Such a layer  70  may instead be formed of other nonconductive materials mentioned above, for instance about 30 Å-100 Å DLC. 
     FIG. 5 shows an AMR sensor that has been constructed according to the present invention. A shield  80  of Permalloy or similar material has been formed atop a substrate, not shown in this figure. For the situation in which the shield  80  may be heated to elevated temperatures during formation of a read gap, the shield may be formed of CoZn, CoTa or FeAlN based materials. An a magnetic layer  82  of slow-ion-milling-rate material such as those mentioned above has been formed on the shield, the amagnetic layer  82  providing a mill stop as well as a read gap of the present invention. Although shown as a single layer  82  for clarity, the read gap may instead be formed of plural layers, as described above. The layer(s)  82  preferably have a thickness ranging between 80 Å and 800 Å, although greater and lesser thicknesses are possible. Layers  84 ,  86  and  88  form an AMR sensor element with a soft adjacent layer (SAL) bias element. The SAL layer  84 , made of a low-coercivity, high-permeability magnetic material such as NiFeRh, is disposed adjacent to the AMR sensing layer  88 . To prevent electrical shorting, a thin insulating or highly resistive layer  86  is interposed. The layer  86  may be composed of insulators such as SiO2 2 , Ta 2 O 5  or AL 2 O 3 , or for example a high-resistivity phase of Ta. The AMR sensing layer  88  may be conventionally formed of a material having an AMR effect, such as Permalloy, and may have a thickness preferably between 80 Å and 400 Å. A sensing current flowing in layer  88  in a direction that is generally parallel to the air bearing surface (ABS) of the head, for example from left to right in FIG. 5, produces a vertical (with respect to the ABS) magnetic field in the SAL layer  84 . This magnetic field magnetizes the SAL layer  84  in the vertical direction (into the page in the example of FIG.  5 ). The magnetization of the SAL layer  84  in turn generates a magnetic bias field for the MR element  88  (out of the page in the example of FIG.  5 ). This magnetic field complements the magnetic field from the medium that is sensed in the MR element to produce a linear signal. 
     Sensor ends  74  and  76  are formed by ion milling and have a relatively steep slope due to overmilling of the read gap layer  82 , but the slow-ion-milling-rate of the read gap prevents the milling from piercing that layer  82 . A bias layer  90  of hard magnetic materials such as CoCrPt is formed on the read gap  82 , abutting layers  84 ,  86  and  88  of the sensor, and covering a mask not shown in this figure but similar to layers  48  and  50  of FIGS. 2-4. A thin underlayer such as Cr may be formed prior to the hard bias layer of CoCrPt. A conductive layer  92 , which may be composed of Au, Cu, Ta or other known materials is then formed, and mask layers such as  48  and  50  are etched away, leaving layers  90  and  92  as shown in FIG.  5 . Thin adhesion or cap layers may be formed before or after the hard bias and lead layers. A second amagnetic read gap  94  is formed, followed by a second magnetic shield  96 . The second shield may serve as part of a write transducer, not shown, as is known. The sensor shown in FIG. 5 provides a view from a storage media surface, although an overcoat of material such as diamond-like carbon (DLC) may protect the sensor from corrosion and from contact with the disk, tape or other media. 
     FIG. 6 is a cross-sectional view of the sensor taken along arrows  6 — 6  of FIG.  5 . Another blunt junction  98  can be seen from this view, which may be ion milled during different steps from that which created ends  74  and  76 . The junction is coated with the second read gap layer  94 , and blunt junction  98  enables better stripe height definition and process control, without residue. As with the contiguous junction formed by ends  74  and  76 , junction  98  can be seen to form an obtuse, nearly 90° angle to the ion milled surface of read gap layer  84 . 
     FIG. 7 shows a spin-valve head formed with blunt contiguous junctions  100  and  102 . Like the AMR head described above, the SV head begins with formation of a magnetic shield layer  105  atop a substrate, not shown. Atop the shield layer a read gap  108  of amagnetic material is formed. Similar to the above discussion regarding AMR sensors, this gap  108  may be formed as an homogenous layer of slow-milling-rate material or may include plural layers of amagnetic material, at least one of which is a slow-milling-rate material, and may include thin adhesion layers. Atop the read gap layer a first layer  110  of ferromagnetic material is formed, which may comprise CoFe for example. A thin amagnetic conductor layer  112 , which may comprise Cu for example is then formed, followed by a second layer  115  of ferromagnetic material such as CoFe. A pinning layer  118 , which may be formed of antiferromagnetic material such as MnFe, is then formed on the second ferromagnetic layer  115 , to fix the magnetization direction of pinned layer  115 . 
     An alternative formation of the SV sensor which may be preferred essentially reverses the sequence of forming the sensor layers, beginning with forming a Ta or other seed layer, followed by forming an antiferromagnetic layer, a pinned layer, a spacer layer and then a free layer in that order. Also, the free layer may include plural layers, and/or the pinned layer may include plural layers to form a synthetic sensor. 
     As described above with reference to FIG. 2, a mask is formed over the sensor layers  110 ,  112 ,  115  and  118 , including undercuts near the desired ends of the sensor, the ends disposed at contiguous junctions  100  and  102 . The ends are formed by ion milling and having a relatively steep slope due to milling the read gap layer  108  as well as the sensor layers, but the slow-ion-milling-rate of the read gap prevents the milling from piercing that layer  108  even while the sensor ends are made blunt. A bias layer  120  of hard magnetic materials such as CoCrPt is formed on the read gap  108 , abutting layers  110 ,  112 ,  115  and  118  of the sensor, and covering the mask. As mentioned above, an underlayer of Cr may be deposited prior to the hard bias layer  120 . A conductive layer  122 , which may be composed of Au, Cu or other known materials is then formed, and the mask layers etched away, leaving bias layer  120  abutting sensor layers  110 ,  112 ,  115  and  118  at steep contiguous junctions  100  and  102 . An underlayer and/or cap layer of Ta for example may deposited before and/or after the lead layer. A second amagnetic read gap  125  is formed, followed by a second magnetic shield  127 . The second shield may serve as part of a write transducer, not shown, as is known. The sensor shown in FIG. 7 provides a view from a storage media surface, although an overcoat of material such as diamond-like carbon (DLC) may protect the sensor from corrosion and from contact with the disk, tape or other media. 
     An active region A of the sensor can be seen to extend much further in a direction parallel to the sensor layers  110 ,  112 ,  115  and  118  than the lateral extent ε 1  and ε 2  of contiguous junctions  100  and  102 . Moreover, contiguous junctions  100  and  102  extend much further in a direction perpendicular to those layers  110 ,  112 ,  115  and  118  than in a direction parallel to those layers. 
     Construction of a GMR head follows similar sequences of steps as described above for a SV head, although with additional interleaved magnetoresistive and amagnetic conductor layers. Likewise, DSMR and DSV heads can be formed with the above process for forming steeply sloped contiguous junctions. While the above description is meant to illustrate a few types of MR sensors, many variations in forming sequences of layers similar to those described above are possible, in order to produce other varieties of MR heads with blunt contiguous junctions. In sum, the improved tailoring of various MR sensors achieved by the present invention provides advantages in magnetic characteristics and resolution that greatly enhance transducer performance.