Abstract:
A magnetoresistive (MR) transducer has at least one insulative layer made of tetrahedral amorphous carbon (ta-C). The ta-C layer is formed by filtered cathodic arc deposition, has an essentially zero concentration of hydrogen and can serve as a read gap for the transducer. The hydrogen-free t-aC read gap has high thermal conductivity, keeping an adjoining MR sensor from overheating during operation. This extends sensor lifetimes and/or improves sensor performance. The read gap also has low defects and porosity, preventing unwanted electrical conduction or shorting between a sensor and a shield. The high hardness of the read gap resists plasma and chemical etching processes such as ion milling that are used to form the sensor. The increased hardness and reduced defects and porosity allow the read gaps to be made thinner without risking electrical shorting. Other hydrogen-free t-aC layers are employed for other sensor elements where electrical insulation and reduced thickness are important.

Description:
TECHNICAL FIELD 
     The present invention relates to electromagnetic transducers or heads, and particularly to transducers that employ a magnetoresistive mechanism for sensing signals. 
     BACKGROUND OF THE INVENTION 
     The employment of magnetoresistive (MR) elements as sensors for electromagnetic transducers has led to improved performance of heads used in disk and tape drives. As is well known, the resistance of an MR element varies according to the magnetic field impinging upon the element, so that flowing an electric current through the element can be used to determine that magnetic field by measuring the change in resistance. 
     While bulk materials may exhibit some MR effect, such effects generally become more pronounced as an element becomes smaller relative to the applied electrical and magnetic flux. Thus it is known that films formed of materials such as Permalloy, which is an alloy of nickel and iron having a high permeability and low coercive force, can be useful as sensors for heads when the film thickness is less than about 500 Å. Even thinner films exhibit quantum mechanical effects which are be utilized in types of MR sensors such as spin valve (SV) or giant magnetoresistive (GMR) sensors. Higher storage density associated with smaller recorded bit size also usually requires smaller MR elements. 
     Generally speaking, the thinner the film used for MR sensing, the more important that the film have a uniform thickness and structure. As such, the material surface or template upon which the film is formed is important. Heads for hard disk drives typically position an MR sensor between a pair of magnetic shields, with the sensor separated from the shields by electrically insulative and nonmagnetic read gaps. The conventional material forming read gaps is aluminum oxide (Al 2 O 3 ), which is known to be easy to form and work with, and which provides a suitable template for forming thin MR films. Al 2 O 3 , however, has a strong affinity for moisture and tends to be porous, both of which can undermine the quality and integrity of an adjoining MR sensor. 
     U.S. Pat. No. 5,644,455 to Schultz describes forming an MR head read gap of “diamond-like carbon” or “DLC”, which is an hydrogenated carbon formed from a gas such as methane (CH 4 ), the DLC having a hydrogen content of 30 to 50 percent. DLC is known to be a hard, thermally conductive, electrically insulative material. DLC also has a high stress, however, making formation of a delicate MR sensor atop a DLC gap difficult. 
     In an article entitled Ultra-Thin Overcoats For The Head/Disk Interface Tribology, Bhatia et al. propose the use of cathodic arc deposition of carbon to form a coating for a slider or disk. This technique had formerly been used for forming a hard coating on metal tools, and involves melting and vaporizing a carbon cathode with a plasma arc, and directing the carbon ions and particles ejected from the cathode toward a target. Although filters can remove most particles, the resulting films may be rough and have much higher stress than DLC, making even adhesion to a substrate problematic. 
     SUMMARY OF THE INVENTION 
     The present invention involves forming thin layers of tetrahedral amorphous carbon (t-aC) for MR sensors. The layers of t-aC have essentially zero concentration of hydrogen and can serve as read gaps for the sensors. Such a hydrogen-free t-aC read gap has an improved thermal conductivity that helps to keep an adjoining MR sensor from overheating during operation. This improved thermal conductivity of the read gap can extend sensor lifetimes and/or improve sensor performance. Hydrogen-free t-aC has a thermal conductivity that may be more than double that of conventional DLC and more than ten times that of Al 2 O 3 . 
     Moreover, the hydrogen-free t-aC read gap of the present invention has reduced defects and porosity, which prevents unwanted electrical conduction or shorting between a sensor and a shield. Hydrogen-free t-aC also is much harder than DLC, which in turn is known to be many times harder than Al 2 O 3 . This extreme hardness renders the read gap layers of the present invention impervious to plasma and chemical etching processes such as ion milling that are used to form the sensor. The increased hardness and reduced defects and porosity allow the read gaps to be made thinner without risking electrical shorting. 
     The effects of the inherently high stress of the hydrogen-free t-aC layers can be minimized by keeping the read gap thickness preferably less than a few hundred angstroms, avoiding adhesion problems that such high stress might otherwise cause. While such thin read gaps cannot be made reliably with conventional materials due to shorting and other problems, the hydrogen-free t-aC read gaps of the present invention can form read gaps as thin as twenty angstroms. Such thin read gaps can improve the focus of the sensor and shorten the path to heat sinks provided by the shields, further improving performance. 
     Thin t-aC layers of the present invention can be beneficially employed for sensor elements beside read gaps. For example, a thin t-aC layer can be used to separate a magnetoresistive layer from an adjacent bias layer for an anisotropic magnetoresistive sensor. In addition, a relatively thin t-aC layer can be disposed between plural sensors of a dual stripe MR head. In these examples as well as others the relatively thin t-aC layers offer performance improvements that include increased resolution and reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a reading and writing head of the present invention including a magnetoresistive sensor having thin, hydrogen-free t-aC read gaps that closely focus the sensor on a relatively moving media. 
     FIG. 2 is a schematic view of a filtered cathodic arc apparatus used for producing the t-aC read gaps of the present invention. 
     FIG. 3 is a cross-sectional view of the formation of a first t-aC read gap of the present invention. 
     FIG. 4 is a cross-sectional view of the formation of plural sensor layers and a second t-aC read gap atop the first t-aC read gap. 
     FIG. 5 is a cross-sectional view of a masked etching step that forms an end to the sensor layers by rotating ion beam milling. 
     FIG. 6 is a media-facing view of a sensor connected to lead and bias layers and separated from magnetic shields by t-aC read gaps. 
     FIG. 7 is a cross-sectional view of the sensor, shields and t-aC read gaps of FIG.  6 . 
     FIG. 8 is a cross-sectional view of an anisotropic magnetoresistive sensor including t-aC sensor and read gaps layers. 
     FIG. 9 is a cross-sectional view of a dual stripe magnetoresistive sensor including t-aC sensor and read gaps layers. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a head  20  of the present invention including a magnetoresistive read transducer  22  and an inductive write transducer  25 , the head disposed in close proximity to a media  28  which is moving relative to the head as shown by arrow  30 . The media  28  may be a disk or tape, for example, which includes media layer  33  or layers atop a substrate  35 , with an overcoat layer  37  protecting the media. The write transducer  25  includes a first write pole  40  and a second write pole  42 , the poles including magnetically permeable material such as an NiFe alloy and being separated by a write gap  44  made of a nonmagnetic, electrically insulative material such as Al 2 O 3 . The write poles  40  and  42  are ends of a pair of write layers that form a magnetic circuit or loop to encourage the flow of magnetic flux across the write gap. An electrically conductive coil  46  is located between the write layers, a part of which is shown, so that an electrical current flowed through the coil induces a magnetic flux in the write layers that travels across the nonmagnetic gap  44  to write a magnetic bit in the media layer  33 . 
     The sensor  22  includes a first layer  50  of magnetic material such as NiFe, NiFe/Co or NiFe/CoFe that is biased with a magnetic direction pointing into the page and essentially perpendicular to both the head and media layers, as shown by X-marks  52 . The magnetization of first layer  50  is free to rotate within the plane of that layer in response to magnetic fields from the media. A second layer  55  of magnetic material such as NiFe, NiFe/Co, NiFe/CoFe or Co has a magnetization that is pinned in a direction shown by arrows  57 . The pinned layer  55  may have its magnetization fixed by an adjoining antiferromagnetic layer  58 . The first and second magnetic layers are separated by a conductive nonmagnetic layer  60 , which may be formed of copper or gold for instance. A magnetic shield  62  is disposed adjacent to the sensor  22 , which acts along with write pole  40  to focus the sensor  22  on media fields directly opposite the sensor, by shielding the sensor from magnetic fields emanating from other portions of the media. The shield may be formed of a magnetic material such as NiFe or other known materials. The head  20  has a media-facing surface that is coated with a layer  64  of t-aC to protect the sensor  22 . 
     During operation, an electric current flows through the conductive layer  60 , and the resistance to that current is decreased or increased depending upon whether the free layer  50  magnetization is more or less parallel with the pinned layer, the free layer magnetization rotating based upon signals from the media. Although the mechanism for varying magnetoresistance may not be fully understood, it is believed that when the magnetization of the free and pinned layers are parallel, electrons in the current are more free to cross the interfaces between the conductive layer  60  and the free and pinned layers, essentially broadening the conductive cross-section and lowering resistance. When the magnetization of the free and pinned layers are antiparallel, on the other hand, electrons in the current are restricted from crossing the interfaces between the conductive layer  60  and the free and pinned layers, essentially narrowing the conductive cross-section and increasing resistance. A voltage difference thus may sensed based upon whether magnetic spins of electrons in the free and pinned layers are more or less parallel, earning this type of magnetoresistive sensor the name spin-valve sensor. 
     Separating the sensor  22  from NiFe layers  40  and  62  are layers  66  and  68  of hydrogen-free t-aC. The t-aC layers  66  and  68  are preferably formed by filtered cathodic arc deposition, as will be explained in more detail below. The layers  66  and  68  can be much thinner than conventional read gap materials while maintaining electrical insulation between the sensor  22  and shields  40  and  62 . Current design specifications for the thickness of t-aC layers such as layers  66  and  68  can range between about 100 Å and 500 Å, although the t-aC read gaps of the present invention may be formed to thicknesses between about 10 Å and 100 Å, and such thinner gaps are likely to be employed in the future. Such thin read gaps can help the sensor focus more accurately on fields from individual media bits, enhancing resolution and allowing greater areal density of stored information. Forming thinner read gaps also helps to keep the high-stress t-aC from peeling away from the shield or sensor, as one might have otherwise expected for a read gap of highly stressed t-aC formed to a conventional thickness of about 1000 Å. 
     The improved thermal conductivity of hydrogen-free t-aC read gaps helps to keep the sensor from overheating during operation. The relatively thin read gaps also provide a short path for heat from the sensor to travel before reaching the heat sinks provided by the shields. This improved thermal conductivity and heat transfer of the read gaps can extend sensor lifetimes, since diffusion of sensor materials which can degrade and eventually destroy a sensor generally proceeds at a higher rate at higher temperatures. Lower temperature sensor operation afforded by the read gaps of the present invention may extend sensor lifetimes several fold. Alternatively, a sensor may be operated at conventional temperatures but with increased current, since the increased thermal conductivity of the read gaps allows the greater heat that is produced by that higher current to be better dissipated. This higher current provides greater voltage output for a given change in resistance felt by the sensor, magnifying signal output. Overall sensor performance can be improved by operating at lower temperatures and/or higher current. The hydrogen-free t-aC layers of the present invention can have a thermal conductivity that is more than double that of conventional DLC and more than ten times that of Al 2 O 3 . 
     The read gaps such as layers  66  and  68  preferably are composed of a type of diamond-like carbon having zero hydrogen content according to secondary ion mass-spectroscopy measurements, although the inventors recognize that trace amounts of hydrogen (up to about three percent) may be tolerable. Carbon has a natural affinity for hydrogen in forming sp 3  bonds rather than sp 2  bonds such as graphite, and so the formation of defect-free sp 3 -bonded carbon that is substantially devoid of hydrogen is not trivial. Although t-aC is labeled amorphous, a large majority (over 80%) of the carbon in the t-aC layers of the present invention may be polycrystalline. Thus a more accurate characterization of the t-aC material formed in the present invention may be that of carbon having primarily tetrahedral molecular bonds that is substantially devoid of hydrogen. 
     The t-aC deposition of layer  68  is preferably highly filtered to provide a smooth template for the sensor. Otherwise, any roughness of the read gap  68  is likely replicated in sensor layers  50 ,  55  and  60 . Such roughness would be expected to cause electron scattering at the interfaces between those layers  50 ,  55  and  60  regardless of any field from the media, increasing noise and lowering signal resolution and thereby denigrating sensor performance. The finding that filtered t-aC could be used for such a sensitive initial layer of a sensor was surprising. On the other hand, formation of layer  68  has an advantage over that of layer  66  since layer  68  is buried further by sensor  22  when the wafer upon which the layers are being formed is removed from a cathodic arc formation chamber for electroplating, and so layer  68  may be less prone than layer  66  to delamination caused by the high stress of t-aC. Although preferable to form both read gap layers  66  and  68  from hydrogen-free t-aC, it is possible to form only one of those layers  66  or  68  from this material and still obtain important benefits, while avoiding some of the challenges of using this material for both layers. 
     FIG. 2 shows an apparatus  100  for forming essentially pure carbon having a primarily tetrahedral bond structure on a substrate  103  on which thousands of sensors are being formed. A carbon cathode  105  is surrounded by an anode  108 , the anode and cathode separated by a vacuum in a chamber  110  that includes the substrate. The cathode  105  and anode  108  are provided greatly different voltages such that an arc of current is generated that flows from one to the other. A focusing solenoid  112  wraps around the anode to direct carbon ions generated by the arc outward from the anode and cathode. The chamber includes a serpentine guiding solenoid  115  that filters out most macroparticles that are generated by the arc, while guiding the carbon ions toward the substrate. The filtering occurs since the charge to mass ratio of the carbon ions is much higher than that of the macroparticles, which causes the ions to be guided through the path defined by the magnetic field within serpentine solenoid  115 , while the macroparticles fly out of that path. 
     A more detailed description of such an apparatus can be found in an article entitled: “S-Shaped Magnetic Macroparticle Filter For Cathodic Arc Deposition,” by Anders et al. in  IEEE Transactions on Plasma Science , Vol. 25, No. 4, August 1997, pp. 670-674, which is incorporated herein by reference. Other cathodic arc apparatuses may instead be employed, such as a 45° solenoid filter, which is described, in U.S. Pat. No. 5,279,723 to Falabella et al., which is also incorporated herein by reference. An inner wall of the chamber  110  may be fitted with a lining  116  that traps the macroparticles so that they do not bounce back into the path of the guiding solenoid  115 . An adjoining chamber  118  may be opened for a separate step of providing sputtered NiFe or other materials used for forming magnetoresistive sensors, with the carbon source walled off and the substrate rotated to face that chamber  118 . Although a single chamber  118  is shown for brevity and clarity, similar chambers for forming other sensor materials, for example Cu or FeMn, may also adjoin the chamber  110 . 
     FIG. 3 shows a small portion of a wafer substrate  150  on which many thousand sensors of the present invention may be formed. A shield  152  of Permalloy or similar material has been formed on the substrate, for instance by sputtering a seed layer followed by electroplating the remainder of layer  152 . After polishing and cleaning of the surface of shield  152 , a layer  155  of essentially hydrogen-free carbon having primarily tetrahedral bonds is formed on the shield using an apparatus such as shown in FIG.  2 . Layer  155 , which is designed to become a read gap layer for the sensors, is formed by initially providing a large bias between the cathode and the substrate, which may help to clean the shield  152  as well as implanting carbon ions in the shield and adhering the carbon layer to the shield layer. This initial bias voltage may range between about −100 V and −2000 V, and is preferably about −1000 V. During this optional initial stage, which may last between about 20 seconds to about 2 minutes depending upon factors such as the arc current, bombardment by high-energy carbon ions may etch the shield layer  152  slightly, or may result in zero or slight growth as shield atoms or ions are replaced with or infused with carbon ions or atoms. The bias between the target and substrate is then reduced to a range between about −50 V and −500 V, and preferably approximately −100 V, which is believed to favor formation of sp 3  bonds in layer  155 . In order to reduce the possibility of macroparticle impingement into layer  155 , further filtering may be employed. For instance, the substrate  103  may be turned about 90°, more or less, from the orientation shown in FIG. 2 so that the relatively massive macroparticles fly past the substrate while carbon ions are diverted to the shield layer  152  by the bias voltage. Depending upon the desired application, the thickness of layer  155  may range between about 20 Å and 500 Å. 
     In FIG. 4, a number of sensor layers  160  have been formed atop the t-aC layer  155 , followed by an optional t-aC layer  162  that protects the sensor layers, provides a hard top which after etching results in a blunter end to the sensor layers, and which forms a part of another read gap. In a chamber such as  118  of FIG. 2, a layer  165  of NiFe or other magnetically permeable material is sputtered onto layer  155 , after evacuating carbon plasma from the chamber containing the wafer substrate and facing the substrate toward the NiFe source. A nonmagnetic, electrically conductive layer  168  made of a material such as Au or Cu is then similarly formed on top of layer  165 , which is immediately followed by another layer  170  of NiFe. An antiferromagnetic layer  172  such as FeMn is then formed atop layer  170  for pinning layer  170 . This figure provides an example of one arrangement of sensor layers  160  for a spin valve sensor, while other known types of sensors may instead be created. Optional layer  162 , which may be formed of t-aC by cathodic arc deposition, may have a thickness less than that of layer  155  and in a range between about 10 Å and 100 Å. 
     FIG. 5 depicts an ion beam etching (IBE) step in forming sensors of the present invention. Prior to this step, a resist having two layers  175  and  177  has been photolithographically patterned atop the t-aC read gap layer  162 , leaving an undercut  180  closest to the sensor. The undercut has been formed by selectively etching layer  175  compared to layer  177 . An IBE is directed at a relatively rotating angle to the substrate, as shown by lines  182  and  185 , forming a curved border  188  of the sensor layers  160 . The IBE may also remove a small part of the read gap layer  155 , but due to the extreme hardness of the semi-amorphous diamond forming that layer  155 , relatively little of the read gap is removed. The undercut  180  and angled IBE allow etchant to remove the mask layers  175  and  177  after deposition of a hard bias and lead layers that cover the mask layers as well as adjoin the border  188  to form a contiguous junction. The border  188  has a much steeper slope than is conventional, which is advantageous for magnetic domains that may be formed in the sensor layers  160  as well as for providing bias fields via the border  188 . Further, the steep slope of border  188  removes ambiguity in the width of the sensor, allowing the sensor to more accurately match (or undercut) the width of media tracks, reducing noise and increasing resolution as noted in a commonly assigned application entitled MR Sensor with Blunt Contiguous Junction and Slow-Milling-Rate Read Gap, invented by Hong et al., filed on even date herewith and incorporated herein by reference. 
     FIG. 6 shows another steeply sloped border  190  for the sensor layers  160 , a hard bias layer  192  and a conductive layer  194  that have been formed, while mask layers  175  and  177  have been etched away, lifting off in the process any part of layers  192  and  194  that were atop the mask. Another layer  196  of hydrogen-free tetrahedral carbon is then formed by cathodic arc deposition, creating a read gap either alone or in combination with layer  162 . Read gap layers  162  and  196  may be formed with a lower bias than that used to form read gap  155 , to avoid damage to delicate sensor layers  160 . A second shield  198  is then formed, which most commonly includes NiFe formed by sputtering and electroplating. The view of the sensor layers  160  and other layers depicted in FIG. 6 is essentially that which would be seen from the media, although a coating of t-aC may be formed on the media-facing side to protect the sensor. The sensor layers  160 , shield layers  152  and  198 , bias layer  192  and conductive layer  194  are all substantially parallel with each other. FIG. 7 is a cross-sectional view of the sensor of FIG. 6, taken along the dashed lines in the direction shown by the double-headed arrow labeled  7 . A giant magnetoresistive (GMR) sensor may be formed by adding additional free, pinned and conductive layers to sensor layers  160 . 
     In addition to the employing the electrically insulative t-aC layers as read gaps for the spin-valve sensor described above, MR sensors may include t-aC sensor layers for other functions. For example, FIG. 8 shows an anisotropic magnetoresistive (AMR) sensor  200  that uses a soft adjacent layer (SAL)  202  to bias an MR layer  205 . Disposed between the SAL layer  202  and the MR layer  205  is a layer of t-aC  210 . Also shown are magnetic shields  212  and  215 , and read gap layers  218  preferably formed of t-aC. Employment of a thin, approximately 20 Å to 100 Å, defect-free electrically-insulative carbon layer  210  separating the SAL  202  and MR  205  layers reduces current shunting through the SAL layer, which increases the current conducted through the MR layer. This increased current leads to an increased output voltage for a given change in resistance of the MR layer, extending the use of this relatively simple sensor into high-density applications. In an alternative embodiment, layer  202  can carry electrical current that is used to magnetically bias MR layer  205 . 
     FIG. 9 shows a dual stripe magnetoresistive (DSMR) sensor  300  that includes a thin t-aC layer  301  separating a first and second MR layers  303  and  305 . A second t-aC layer  308  separates MR layer  303  from a bias layer  312 , and a third t-aC layer  318  separates MR layer  305  from its bias layer  320 . A fourth t-aC layer  322  separates bias layer  320  from a first magnetic shield  325 , while a fifth t-aC layer  330  separates bias layer  312  from a second magnetic shield  333 . Reducing the insulating layer  301  thickness separating the two MR stripes  303  and  305 , which is provided by the defect and hydrogen-free t-aC employed in the present invention, allows the stripes to more closely read the same bits from the media, which helps with the differential amplification of signal over noise that is a primary advantage of DSMR heads. T-aC layer  301  may have a thickness of from about 100 Å to 500 Å in this embodiment, while other types of DSMR sensors may have a thinner t-aC layer separating the sensors. 
     While several embodiments of the transducers of the present invention have been illustrated, other implementations of the present invention will become apparent to those of ordinary skill in the art, those implementations falling within the scope of the invention as defined in the following claims.