Abstract:
A method for forming a MgO barrier layer in a tunnel junction magnetoresistive sensor (TMR). The MgO barrier layer is deposited by an ion beam deposition process that results in a MgO barrier layer having exceptional, uniform properties and a well controlled oxygen content. The ion beam deposition of the barrier layer includes placing a wafer into an ion deposition chamber and placing Mg target into the chamber. An ion beam from an ion beam gun is directed at the target thereby dislodging Mg atoms from the target for deposition onto the wafer. Oxygen is introduced into the chamber by one or both of pumping molecular oxygen (O 2 ) into the chamber and/or introducing oxygen ions into the chamber from a second ion beam gun. The use of ion beam deposition avoids oxygen poisoning of the Mg target, such as would occur using a more conventional plasma vapor deposition technique.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the construction of a tunnel junction magnetoresistive sensor and more particularly to a method for constructing a barrier layer that improves the magnetic performance of the sensor. 
       BACKGROUND OF THE INVENTION 
       [0002]    The heart of a computer&#39;s long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk and when the disk rotates, air adjacent to the surface of the disk moves along with the disk. The slider flies on this moving air at a very low elevation (fly height) over the surface of the disk. This fly height can be on the order of Angstroms. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions. 
         [0003]    The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk. 
         [0004]    In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. This sensor includes a nonmagnetic conductive layer, referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. 
         [0005]    The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals. 
         [0006]    More recently, researches have focused on the development of tunnel junction sensors (TMR sensors) also referred to as tunnel valves. Tunnel valves TMR sensor offer the advantage of providing improved signal amplitude as compared with GMR sensors. TMR sensors operate based on the spin dependent tunneling of electrons through a thin, electrically insulating barrier layer. The structure of the barrier layer is critical to optimal TMR sensor performance, and certain manufacturing difficulties such as target poisoning during barrier layer deposition have limited the effectiveness of such TMR sensors. Therefore, there is a strong felt need for a tunnel junction sensor that can provide optimal TMR performance and also for a practical method for manufacturing such an optimized TMR sensor. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a method for forming a MgO barrier layer in a tunnel junction (TMR) sensor. The MgO barrier layer can be deposited by placing a wafer into an ion beam deposition chamber. An ion beam from an ion beam gun is directed at a Mg target located within the chamber, thereby dislodging Mg atoms from the target onto the wafer. While the ion beam is depositing Mg onto the wafer, oxygen is introduced into the chamber. 
         [0008]    The oxygen reacts with the deposited Mg to form a well controlled MgO layer. The oxygen can be introduced into the chamber as molecular oxygen gas (O 2 ) pumped into the chamber through a gas inlet. Alternatively or additionally, the oxygen can be introduced into the chamber as ionized oxygen emitted from an ion gun. 
         [0009]    The ion beam deposition of MgO advantageously deposits a high quality, uniform barrier layer to form TMR structure. The ion beam deposition avoids the target poisoning that occurs when using the more standard plasma vapor deposition technique to deposit MgO. Such target poisoning, which occurs with plasma vapor deposition, results when oxygen from the plasma, formed within the chamber, deposits on and reacts with the target. Since the ion beam deposition technique does not include striking a plasma within the chamber, such target poisoning does not occur when using the method of the present invention. 
         [0010]    These and other advantages and features of the present invention will be apparent upon reading the following detailed description in conjunction with the Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale. 
           [0012]      FIG. 1  is a schematic illustration of a disk drive system in which the invention might be embodied; 
           [0013]      FIG. 2  is an ABS view of a slider, taken from line  3 - 3  of  FIG. 2 , illustrating the location of a magnetic head thereon; 
           [0014]      FIG. 3  is an ABS view of a tunnel junction sensor according to an embodiment of the present invention taken from circle  3  of  FIG. 2 ; 
           [0015]      FIG. 4  is a schematic view of an ion beam deposition chamber for use in depositing a MgO barrier layer in a tunnel junction (TMR) sensor; 
           [0016]      FIG. 5  is a flow chart illustrating a method of depositing a MgO barrier layer according to an embodiment of the invention; and 
           [0017]      FIG. 6  is a flow chart illustrating a method of depositing a MgO barrier layer according to an alternate embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claims herein. 
         [0019]    Referring now to  FIG. 1 , there is shown a disk drive  100  embodying this invention. As shown in  FIG. 1 , at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic disk  112 . 
         [0020]    At least one slier  113  is positioned near the magnetic disk  112 , each slider  113  supporting one or more magnetic head assemblies  121 . As the magnetic disk rotates, slider  113  moves radially in and out over the disk surface  122  so that the magnetic head assembly  121  may access different tracks of the magnetic disk where desired data are written. Each slider  113  is attached to an actuator arm  119  by way of a suspension  115 . The suspension  115  provides a slight spring force which biases slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means  127  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  129 . 
         [0021]    During operation of the disk storage system, the rotation of the magnetic disk  112  generates an air bearing between the slider  113  and the disk surface  122  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
         [0022]    The various components of the disk storage system are controlled in operation by control signals generated by control unit  129 , such as access control signals and internal clock signals. Typically, the control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position slider  113  to the desired data track on disk  112 . Write and read signals are communicated to and from write and read heads  121  by way of recording channel  125 . 
         [0023]    With reference to  FIG. 2 , the orientation of the magnetic head  121  in a slider  113  can be seen in more detail.  FIG. 3  is an ABS view of the slider  113 , and as can be seen the magnetic head including an inductive write head and a read sensor, is located at a trailing edge of the slider. The above description of a typical magnetic disk storage system, and the accompanying illustration of  FIG. 1  are for representation purposes only. It should be apparent from disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
         [0024]    With reference now to  FIG. 3 , a tunnel junction sensor TMR  300  is described. The TMR sensor  300  includes a sensor stack  302  sandwiched between first and second electrically conductive leads  304 ,  306 . The leads  304 ,  306  can be constructed of an electrically conductive, magnetic material such as NiFe or CoFe so that they can function as magnetic shields as well as leads. The sensor stack  302  includes a magnetic pinned layer structure  308 , and a magnetic free layer structure  310 . A thin, non-magnetic, electrically insulating barrier layer  312  is sandwiched between the pinned layer structure  308  and the free layer structure  310 . The barrier layer  312  is constructed of MgO, and could have a thickness of 8 to 10 Angstroms, although other thicknesses could be used too. 
         [0025]    The pinned layer can include first and second magnetic layers AP 1   316  and AP 2   318  that are antiparallel coupled across a non-magnetic antiparallel coupling layer  320 . The AP 1  and AP 2  layers  316 ,  318  can be constructed of, for example, CoFe, CoFeB or other magnetic alloys and the antiparallel coupling layer  320  can be constructed of, for example, Ru. The free layer  310  can be constructed of a material such as CoFe, CoFeB or NiFe or may be a combination of these or other materials. 
         [0026]    The AP 1  layer  316  is in contact with and exchange coupled with a layer of antiferromagnetic material (AFM layer)  326  such as PtMn, IrMn or some other antiferromagnetic material. This exchange coupling strongly pins the magnetization of the AP 1  layer  316  in a first direction as indicated by arrow tail  328 . Antiparallel coupling between the AP 1  and AP 2  layers  316 ,  318  strongly pins the magnetization of the AP 2  layer in a second direction perpendicular to the ABS as indicated by arrowhead  330 . 
         [0027]    A capping layer  314  such as Ta, Ta/Ru or Ru/Ta/Ru may be provided at the top of the sensor stack  302  to protect the layers thereof from damage during manufacture. In addition, a seed layer  322  may be provided at the bottom of the sensor stack  302  to initiate a desired crystalline growth in the above deposited layers of the sensor stack  302 . 
         [0028]    First and second hard bias layers  324  may be provided at either side of the sensor stack  302 . The hard bias layers  324  can be constructed of a hard magnetic material such as CoPt or CoPtCr. These hard bias layers  324  are magnetostatically coupled with the free layer  310  and provide a magnetic bias field that biases the magnetization of the free layer  310  in a desired direction parallel with the ABS as indicated by arrow  326 . The hard bias layers  324  can be separated form the sensor stack  302  and from at least one of the leads  304  by a layer of electrically insulating material  328  such as alumina in order to prevent current from being shunted across the hard bias layers  324  between the leads  304 ,  306 . 
         [0029]    The MgO barrier layer  312  has excellent uniformity, and is deposited by a novel deposition method that will be described in detail herein below and which results in an improved resistance area product (RA) value and tunneling magnetoresistance (TMR) values. In fact, a TMR sensor constructed according to the present invention can have a TMR value of 81.6 to 110 for RA values of 1.5-3.1 ohms-micron 2 . 
         [0030]    With reference now to  FIG. 4 , a novel method for depositing the barrier layer  312  ( FIG. 3 ) is described. The above described layers of the sensor stack  302  ( FIG. 3 ) can be deposited in an ion beam deposition (IBD) tool  400 . The sensor layers are deposited on a wafer  402  that is held on a chuck  404  inside an ion beam deposition chamber  406 . The following description of a method for depositing a MgO barrier layer  312  ( FIG. 3 ) assumes that the AFM layer  326  and pinned layer structure  308  of the sensor stack have already been deposited, so that the barrier layer can be deposited over the pinned layer structure  308 . 
         [0031]    With reference still to  FIG. 4 , the IBD tool  400  includes an ion beam gun  408  that directs an ion beam  410  at a Mg target  412 . The ion beam gun  408  receives a noble gas, such as argon (Ar), krypton (Kr) or xenon (Xe), which is ionized within the gun and accelerated toward the target  412 . Ions from the ion beam  410  cause Mg atoms to become dislodged from the target to be deposited onto the wafer  402 . While the ion beam gun  408  is bombarding the target  412  with ions  410 , molecular oxygen O 2  is being input into the chamber  406  through an inlet  414 . An outlet  416  may also be provided for evacuating the chamber  406 . The O 2  entered into the chamber  406  mixes with the Mg from the target to deposit MgO onto the wafer  402  in an extremely controllable and uniform manner so that the relative amounts of Mg and O in the deposited MgO can be carefully controlled. 
         [0032]    The above described PVD deposition of MgO differs significantly from a more conventional plasma vapor deposition of MgO. In a plasma vapor deposition tool, a plasma would be struck in the chamber itself in the presence of oxygen. Then, MgO would be deposited from a Mg target. This method, however, does not result in a well controlled barrier layer deposition process, because of target oxidation. When the target oxidizes, the deposition rate drops significantly. This is due to the fact that oxygen from the plasma poisons the target, forming MgO so that Mg can no longer be effectively sputtered as a metal from the target. 
         [0033]    In the PVD tool  400  described above, the plasma is within the ion beam gun  408  rather than being within the chamber  406  itself. The above described ion beam deposition of MgO avoids the above described problems associated with plasma vapor deposition (PVD), resulting in a MgO barrier having excellent, well controlled properties. 
         [0034]    With continued reference to  FIG. 4 , a second ion beam gun  418  can be provided which can be directed at the wafer  402 . Whereas the first ion beam gun  408  can be used to emit ions  410  such Xe, Ar or some other element, the second ion beam gun can be used emit an ion beam  420  that includes oxygen directly onto the wafer  402 . The second ion beam gun  418  receives oxygen as oxygen gas (O 2 ) which is ionized within the gun and disseminated into the chamber which causes ionized oxygen to envelope the wafer substrate of the wafer  402  and oxidize the magnesium atoms deposited thereon to form magnesium oxide (MgO). While the ion beam gun  418  may have the capability of accelerating ionized oxygen toward the wafer substrate  402  it is preferred that the ionized oxygen be disseminated without acceleration. Without acceleration, energetic particle bombardment of the wafer substrate, which may deteriorate the barrier layer, is avoided. In another embodiment, the ionized oxygen is accelerated toward the wafer substrate  402  by the ion gun. This manner of introducing oxygen can be used in addition to or in lieu of the introduction of molecular oxygen (O 2 ) into the chamber. 
         [0035]    With reference to  FIG. 5 , a method for depositing a MgO barrier on a TMR sensor stack is described as follows. First, in a step  502  a wafer substrate is placed in a vacuum chamber of an ion beam deposition (IBD) tool. In a step  504 , a magnesium target is placed in the vacuum chamber. Then, in a step  506 , gas is provided to an ion gun. In a step  508 , an ion beam from the ion beam gun is directed at the target to sputter magnesium atoms toward the substrate. While directing the ion beam at the target, in a step  510 , oxygen is introduced into the chamber. This oxygen can react with the sputtered magnesium to deposit magnesium oxide (MgO) onto the substrate. 
         [0036]    With reference to  FIG. 6 , another method for depositing a MgO barrier on a TMR is described. In a step  602  a wafer substrate is placed in a vacuum chamber of an ion beam deposition (IBD) tool. In a step  604 , a magnesium target is placed in the vacuum chamber. Then, in a step  606 , gas is provided to an ion gun. In a step  608 , an ion beam from the ion beam gun is directed at the target to sputter magnesium atoms toward the substrate. While directing the ion beam at the target, oxygen is ionized in an ion gun and disseminated into the chamber. This ionized oxygen can be disseminated into the chamber with or without acceleration toward the substrate. The ionized oxygen reacts with the magnesium atoms to deposit magnesium oxide onto the substrate. 
         [0037]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.