Abstract:
A method for making a tunnel valve head with a flux guide, a tunnel valve sensor having an isolated flux guide, and a magnetic storage system using a tunnel valve sensor having an isolated flux guide is disclosed. The tunnel valve sensors is created by forming a tunnel valve at a first shield layer, the tunnel valve comprising a free layer distal to the first shield layer, depositing a first insulation layer over the first shield layer and around the tunnel valve, depositing a flux guide over the first insulation layer and coupling to the tunnel valve at the free layer, covering the flux guide with a second insulation layer and forming a second shield layer over the second insulation, wherein the flux guide and the free layer are physically isolated by the first and second insulation layers to prevent current shunts therefrom. The structure achieves physical connection between the flux guide and the free layer and insulates the flux guide from the shields. By separating the flux guide and the free layer from the shields, the shunting of current is prevented.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to magnetoresistive (MR)heads for reading magnetically recorded data, and more particularly to a method and apparatus for providing a structure that achieves physical connection between the flux guide and the free layer and that insulates the flux guide from the shields. 
     2. Description of Related Art 
     Magnetic recording is a key and invaluable segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was applied to data storage. Since 1991, areal density has grown by the well-known 60% compound growth rate, and this is based on corresponding improvements in heads, media, drive electronics, and mechanics. 
     Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of these components to both write and subsequently read magnetically recorded data from the medium at data densities well into the Gbits/in 2  range gives hard disk drives the power to remain the dominant storage device for many years to come. 
     The heart of a computer is an assembly that is referred to as a magnetic disk drive. The disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating 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 mounted on a slider that has an air-bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating. However, when the disk rotates, air is swirled by the rotating disk adjacent the ABS causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. The write and read heads are employed for writing magnetic impressions to and reading magnetic impressions 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. 
     A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as a MR read heads for reading data in magnetic recording disk drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy (Ni 81 Fe 19 ). A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element and a corresponding change in the sensed current or voltage. 
     An MTJ device has been proposed as a magnetoresistive read head for magnetic recording in U.S. Pat. No. 5,390,061. A magnetic tunnel junction (MTJ) device is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. Such sensors are also referred to tunnel valve sensors. In such sensors, one of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers. 
     In an MTJ read head, the free ferromagnetic layer, the tunnel barrier layer and the fixed ferromagnetic layer all have their edges exposed at the sensing surface of the head, i.e., the air-bearing surface (ABS) of the air-bearing slider if the MTJ head is used in a magnetic recording disk drive. It has been discovered that when the MTJ head is lapped to form the ABS, it is possible that material from the free and fixed ferromagnetic layers will smear at the ABS and short out across the tunnel barrier layer. In addition, many antiferromagnets used to fix the magnetic moment of the fixed ferromagnetic layer contain manganese (Mn) which can corrode during the ABS lapping process. The tunnel barrier layer is typically formed of aluminum oxide, which can also corrode during the ABS lapping process. Accordingly, an tunnel valve read head for a magnetic recording system wherein the free ferromagnetic layer also acts as a flux guide to direct magnetic flux from the magnetic recording medium to the tunnel junction has been proposed to solve these problems. In a magnetic recording disk drive embodiment, the fixed ferromagnetic layer has its front edge recessed from the ABS while the sensing end of the free ferromagnetic layer is exposed at the ABS. The front edge of the tunnel barrier layer may also be recessed from the ABS. Both the fixed and free ferromagnetic layers are in contact with opposite surfaces of the tunnel barrier layer but the free ferromagnetic layer extends beyond the back edge of either the tunnel barrier layer or the fixed ferromagnetic layer, whichever back edge is closer to the sensing surface. This assures that the magnetic flux is non-zero in the tunnel junction region. The magnetization direction of the fixed ferromagnetic layer is fixed in a direction generally perpendicular to the ABS and thus to the disk surface, preferably by interfacial exchange coupling with an antiferromagnetic layer. The magnetization direction of the free ferromagnetic layer is aligned in a direction generally parallel to the surface of the ABS in the absence of an applied magnetic field and is free to rotate in the presence of applied magnetic fields from the magnetic recording disk. A layer of high coercivity hard magnetic material adjacent the sides of the free ferromagnetic layer longitudinally biases the magnetization of the free ferromagnetic layer in the preferred direction. 
     However, to prevent shunting of current, the flux guide needs to be insulated. To achieve high efficiency, it is necessary that there be no separation between the flux guide and the free layer. 
     It can be seen that there is a need for a method and apparatus for providing a structure that achieves physical connection between the flux guide and the free layer and that insulates the flux guide from the shields. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for providing a structure that achieves physical connection between the flux guide and the free layer and that insulates the flux guide from the shields. 
     The present invention solves the above-described problems by separating the flux guide and the free layer to prevent the shunting of current. 
     A method in accordance with the principles of the present invention includes forming a tunnel valve at a first shield layer, the tunnel valve comprising a free layer distal to the first shield layer, depositing a first insulation layer over the first shield layer and around the tunnel valve, depositing a flux guide over the first insulation layer and coupling to the tunnel valve at the free layer, covering the flux guide with a second insulation layer and forming a second shield layer over the second insulation, wherein the flux guide and the free layer are physically isolated by the first and second insulation layers to prevent current shunts therefrom. 
     Other embodiments of a method in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the depositing the first insulation layer over the first shield layer and around the tunnel valve is performed using a self-aligning process wherein regions of different thicknesses are formed with a single masking step. 
     Another aspect of the present invention is that the flux guide is physically connected to the free layer of the tunnel valve. 
     Another aspect of the present invention is that the covering the flux guide with a second insulation layer is performed using a self-aligning process wherein regions of different thicknesses are formed with a single masking step. 
     Another aspect of the present invention is that the flux guide increases the amount of magnetic flux in the tunnel valve. 
     Another aspect of the present invention is that the increase in the amount of magnetic flux in the tunnel valve enhances the output signal fo the tunnel valve. 
     Another aspect of the present invention is that the forming a tunnel valve at a first shield layer further includes forming an antiferromagnetic (AFM) layer of electrically insulating antiferromagnetic material, depositing a pinned layer of ferromagnetic material in contact with said AFM layer, said pinned layer making electrical contact with said first shield, forming a free layer of ferromagnetic material and forming a tunnel junction layer of electrically insulating material between said pinned and free layers. 
     In another embodiment of the present invention, a tunnel valve sensor is disclosed. The tunnel valve sensor includes a tunnel valve disposed at a first shield layer, the tunnel valve comprising a free layer distal to the first shield layer, a first insulation layer formed over the first shield layer and around the tunnel valve, a flux guide deposited over the first insulation layer, the flux guide being coupled to the tunnel valve at the free layer, a second insulation layer covering the flux guide; and a second shield layer deposited over the second insulation, wherein the flux guide and the free layer are physically isolated by the first and second insulation layers to prevent current shunts therefrom. 
     Another aspect of the present invention is that the flux guide is physically connected to the free layer of the tunnel valve. 
     Another aspect of the present invention is that the flux guide increases the amount of magnetic flux in the tunnel valve. 
     Another aspect of the present invention is that the increase in the amount of magnetic flux in the tunnel valve enhances the output signal fo the tunnel valve. 
     Another aspect of the present invention is that the tunnel valve further includes an antiferromagnetic (AFM) layer of electrically insulating antiferromagnetic material, a pinned layer of ferromagnetic material in contact with said AFM layer, said pinned layer making electrical contact with said first shield, a free layer of ferromagnetic material and a tunnel junction layer of electrically insulating material disposed between said pinned and free layers. 
     In another embodiment of the present invention, a magnetic storage device is disclosed. The magnetic storage system includes a magnetic recording medium, an actuator for moving the tunnel valve sensor across the magnetic recording disk so the tunnel valve sensor may access different regions of magnetically recorded data on the magnetic recording medium, a data channel coupled electrically to the tunnel valve sensor for detecting changes in resistance of the tunnel valve sensor caused by rotation of the magnetization axis of the free ferromagnetic layer relative to the fixed magnetization of the pinned layer in response to magnetic fields from the magnetically recorded data and a tunnel valve sensor disposed proximate the recording medium, the tunnel valve sensor, includes a tunnel valve disposed at a first shield layer, the tunnel valve comprising a free layer distal to the first shield layer, a first insulation layer formed over the first shield layer and around the tunnel valve, a flux guide deposited over the first insulation layer, the flux guide being coupled to the tunnel valve at the free layer, a second insulation layer covering the flux guide and a second shield layer deposited over the second insulation, wherein the flux guide and the free layer are physically isolated by the first and second insulation layers to prevent current shunts therefrom. 
     Another aspect of the present invention is that the flux guide is physically connected to the free layer of the tunnel valve. 
     Another aspect of the present invention is that the flux guide increases the amount of magnetic flux in the tunnel valve. 
     Another aspect of the present invention is that the increase in the amount of magnetic flux in the tunnel valve enhances the output signal fo the tunnel valve. 
     Another aspect of the present invention is that wherein the tunnel valve further includes an antiferromagnetic (AFM) layer of electrically insulating antiferromagnetic material, a pinned layer of ferromagnetic material in contact with said AFM layer, said pinned layer making electrical contact with said first shield, a free layer of ferromagnetic material and a tunnel junction layer of electrically insulating material disposed between said pinned and free layers. 
     These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  illustrates a storage system according to the present invention; 
         FIG. 2  illustrates one particular embodiment of a storage system according to the present invention; 
         FIG. 3  illustrates a slider mounted on a suspension; 
         FIG. 4  is an ABS view of the slider and the magnetic head; 
         FIG. 5  is a cross-sectional schematic view of the integrated read/write head which includes a MR read head portion and an inductive write head portion; 
         FIG. 6  is a section view of one embodiment of a tunnel valve read head as it would appear if taken through a plane whose edge is shown as line  542  in  FIG. 5  and viewed from the disk surface; 
         FIG. 7  illustrates a tunnel valve read head according to the present invention; and 
         FIG. 8  illustrates a flow chart of a method of making a tunnel valve head with a flux guide according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. 
     The present invention provides a method and apparatus for providing a structure that achieves physical connection between the flux guide and the free layer and that insulates the flux guide from the shields. By separating the flux guide and the free layer from the shields, the shunting of current is prevented. 
       FIG. 1  illustrates a storage system  100  according to the present invention. In  FIG. 1 , a transducer  110  is under control of an actuator  120 . The actuator  120  controls the position of the transducer  110 . The transducer  110  writes and reads data on magnetic media  130 . The read/write signals are passed to a data channel  140 . A system processor  150  controls the actuator  120  and processes the signals of the data channel  140 . In addition, a media translator  160  is controlled by a system processor  150  to cause the magnetic media  130  to move relative to the transducer  110 . The present invention is not meant to be limited to a particular type of storage system  100  or to the type of media  130  used in the storage system  100 . 
       FIG. 2  illustrates one particular embodiment of a storage system  200  according to the present invention. In  FIG. 2 , a hard disk drive  200  is shown. The drive  200  includes a spindle  210  that supports and rotates a magnetic disk  214 . The spindle  210  is rotated by a motor  220  that is controlled by a motor controller  230 . A combined read and write magnetic head  240  is mounted on a slider  242  that is supported by a suspension  244  and actuator arm  246 . Processing circuitry  250  exchanges signals, representing such information, with the head  240 , provides motor drive signals for rotating the magnetic disk  214 , and provides control signals for moving the slider to various tracks. A plurality of disks  214 , sliders  242  and suspensions  244  may be employed in a large capacity direct access storage device (DASD). 
     The suspension  244  and actuator arm  246  position the slider  242  so that the magnetic head  240  is in a transducing relationship with a surface of the magnetic disk  214 . When the disk  214  is rotated by the motor  220  the slider  240  is supported on a thin cushion of air (air bearing) between the surface of the disk  214  and the air-bearing surface (ABS)  248 . The magnetic head  240  may then be employed for writing information to multiple circular tracks on the surface of the disk  214 , as well as for reading information therefrom. 
       FIG. 3  illustrates a slider  310  mounted on a suspension  312 . In  FIG. 3  first and second solder connections  304  and  306  connect leads from the sensor  308  to leads  313  and  314  on the suspension  312  and third and fourth solder connections  316  and  318  connect the coil  384  to leads  324  and  326  on the suspension. 
       FIG. 4  is an ABS view of the slider  400  and the magnetic head  404 . The slider has a center rail  456  that supports the magnetic head  404 , and side rails  458  and  460 . The rails  456 ,  458  and  460  extend from a cross rail  462 . With respect to rotation of a magnetic disk, the cross rail  462  is at a leading edge  464  of the slider and the magnetic head  404  is at a trailing edge  466  of the slider. 
       FIG. 5  is a cross-sectional schematic view of the integrated read/write head  525  which includes a MR read head portion and an inductive write head portion. The head  525  is lapped to form an air-bearing surface (ABS), the ABS being spaced from the surface of the rotating disk  516  by the air bearing as discussed above. The read head includes a MR sensor  540  sandwiched between first and second gap layers G 1  and G 2  which are, in turn, sandwiched between first and second magnetic shield layers S 1  and S 2 . In a conventional disk drive, the MR sensor  540  is an AMR sensor. The write head includes a coil layer C and insulation layer  512  which are sandwiched between insulation layers  511  and  513  which are, in turn, sandwiched between first and second pole pieces P 1  and P 2 . A gap layer G 3  is sandwiched between the first and second pole pieces P 1 , P 2  at their pole tips adjacent to the ABS for providing a magnetic gap. During writing, signal current is conducted through the coil layer C and flux is induced into the first and second pole layers P 1 , P 2  causing flux to fringe across the pole tips at the ABS. This flux magnetizes circular tracks on the rotating disk during a write operation. During a read operation, magnetized regions on the rotating disk inject flux into the MR sensor  540  of the read head, causing resistance changes in the MR sensor  540 . These resistance changes are detected by detecting voltage changes across the MR sensor  540 . The combined head  525  shown in  FIG. 5  is a “merged” head in which the second shield layer S 2  of the read head is employed as a first pole piece P 1  for the write head. In a piggyback head (not shown), the second shield layer S 2  and the first pole piece P 1  are separate layers. 
     The above description of a typical magnetic recording disk drive with an AMR read head, and the accompanying  FIGS. 1–5 , are for representation purposes only. Disk drives may contain a large number of disks and actuators, and each actuator may support a number of sliders. In addition, instead of an air-bearing slider, the head carrier may be one which maintains the head in contact or near contact with the disk, such as in liquid bearing and other contact and near-contact recording disk drives. 
     According to the present invention, a MR read head uses a tunnel valve in place of the MR sensor  540  in the read/write head  525  of  FIG. 5 .  FIG. 6  is a section view of one embodiment of a tunnel valve read head  600  as it would appear if taken through a plane whose edge is shown as line  542  in  FIG. 5  and viewed from the disk surface. The tunnel valve read head  600  of  FIG. 6  is presented for the purposed of explaining the operation of the tunnel valve. 
     Referring to  FIG. 6 , the paper of  FIG. 6  is a plane parallel to the ABS and through substantially the active sensing region, i.e., the tunnel junction, of the tunnel valve read head to reveal the layers that make up the head. The tunnel valve read head includes an electrical lead  602  formed on the gap layer G 1  substrate, an electrical lead  604  below gap layer G 2 , and the tunnel valve  608  formed as a stack of layers between electrical leads  602 ,  604 . The tunnel valve  608  includes a first electrode multilayer stack  610 , an insulating tunnel barrier layer  620 , and a top electrode stack  630 . Each of the electrodes includes a ferromagnetic layer in direct contact with tunnel barrier layer  620 , i.e., ferromagnetic layers  618  and  632 . 
     The base electrode layer stack  610  formed on electrical lead  602  includes a seed or “template” layer  612  on the lead  602 , a layer of antiferromagnetic material  616  on the template layer  612 , and a “fixed” ferromagnetic layer  618  formed on and exchange coupled with the underlying antiferromagnetic layer  616 . The ferromagnetic layer  618  is called the fixed layer because its magnetic moment or magnetization direction is prevented from rotation in the presence of applied magnetic fields in the desired range of interest. The top electrode stack  630  includes a “free” or “sensing” ferromagnetic layer  632  and a protective or capping layer  634  formed on the sensing layer  632 . The sensing ferromagnetic layer  632  is not exchange coupled to an antiferromagnetic layer, and its magnetization direction is thus free to rotate in the presence of applied magnetic fields in the range of interest. The sensing ferromagnetic layer  632  is fabricated so as to have its magnetic moment or magnetization direction (shown by arrow  633 ) oriented generally parallel to the ABS (the ABS is a plane parallel to the paper in  FIG. 6 ) and generally perpendicular to the magnetization direction of the fixed ferromagnetic layer  618  in the absence of an applied magnetic field. The fixed ferromagnetic layer  618  in electrode stack  610  just beneath the tunnel barrier layer  620  has its magnetization direction fixed by interfacial exchange coupling with the immediately underlying antiferromagnetic layer  616 , which also forms part of bottom electrode stack  610 . The magnetization direction of the fixed ferromagnetic layer  618  is oriented generally perpendicular to the ABS, i.e., out of or into the paper in  FIG. 6  (as shown by arrow tail  619 ). 
     Also shown in  FIG. 6  is a biasing ferromagnetic layer  650  for longitudinally biasing the magnetization of the sensing ferromagnetic layer  632 , and an insulating layer  660  separating and isolating the biasing layer  650  from the sensing ferromagnetic layer  632  and the other layers of the tunnel valve  608 . The biasing ferromagnetic layer  650  is a hard magnetic material, such as a CoPtCr alloy, that has its magnetic moment (shown by arrow  651 ) aligned in the same direction as the magnetic moment  633  of the sensing ferromagnetic layer  632  in the absence of an applied magnetic field. The insulating layer  660 , which is preferably alumina (Al 2 O 3 ) or silica (SiO 2 ), has a thickness sufficient to electrically isolate the biasing ferromagnetic layer  650  from the tunnel valve  608  and the electrical leads  602 ,  604 , but is still thin enough to permit magnetostatic coupling (shown by dashed arrow  653 ) with the sensing ferromagnetic layer  632 . The product M*t (where M is the magnetic moment per unit area of the material in the ferromagnetic layer and t is the thickness of the ferromagnetic layer) of the biasing ferromagnetic layer  650  must be greater than or equal to the M*t of the sensing ferromagnetic layer  632  to assure stable longitudinal biasing. Since the magnetic moment of Ni (100 −x) —Fe (x)  that is typically used in the sensing ferromagnetic layer  632  is about twice that of the magnetic moment of a typical hard magnetic material suitable for the biasing ferromagnetic layer  650 , such as Co 75 Pt 13 Cr 12 , the thickness of the biasing ferromagnetic layer  650  is at least approximately twice that of the sensing ferromagnetic layer  632 . 
     A sense current I is directed from first electrical lead  602  perpendicularly through the antiferromagnetic layer  616 , the fixed ferromagnetic layer  618 , the tunnel barrier layer  620 , and the sensing ferromagnetic layer  632  and then out through the second electrical lead  604 . As described previously, the amount of tunneling current through the tunnel barrier layer  620  is a function of the relative orientations of the magnetizations of the fixed and sensing ferromagnetic layers  618 ,  632  that are adjacent to and in contact with the tunnel barrier layer  620 . The magnetic field from the recorded data causes the magnetization direction of sensing ferromagnetic layer  632  to rotate away from the direction  633 , i.e., either into or out of the paper of  FIG. 6 . This changes the relative orientation of the magnetic moments of the ferromagnetic layers  618 ,  632  and thus the amount of tunneling current, which is reflected as a change in electrical resistance of the tunnel valve  608 . This change in resistance is detected by the disk drive electronics and processed into data read back from the disk. The sense current is prevented from reaching the biasing ferromagnetic layer  650  by the electrical insulating layer  660 , which also insulates the biasing ferromagnetic layer  650  from the electrical leads  602 ,  604 . 
     In a magnetic recording device the read head senses flux from small magnetized regions or magnetic bits written into a thin film magnetic medium above which the head is suspended. Increased capacity disk drives are achieved in part by higher magnetic bit areal densities. Thus the area of each magnetic region or bit must be decreased but this thereby gives rise to reduced magnetic flux. Magnetic recording heads which can sense reduced magnetic flux with greater output signal are thereby required for higher performance and higher capacity magnetic recording disk drives. 
       FIG. 7  illustrates a tunnel valve read head  700  according to the present invention. In  FIG. 7 , a tunnel valve  710  is patterned at a first shield layer  712 . Insulation  720  is deposited using the self-aligned process. A flux guide  730  is deposited so that it overlaps with and is coupled to the tunnel valve  710 , but does not completely extend over the tunnel valve  710 . The flux guide  730  is then covered with insulation  740  using a self-aligned process. Self-aligned processes allow a feature to be defined without precise contact alignment. As with the other self-aligned processes, a first feature is patterned by a backside process step. Then a second feature is defined by a second backside process step. This second backside process step uses an aspect of the first process step to define the second feature. After depositing the insulation over the flux guide, the second shield  750  is formed. 
     The flux guide  730  thus makes physical connection with the free layer  732  of the tunnel valve  710  while the flux guide  730  is insulated from the shields  712 ,  750 . By separating the flux guide  730  and the free layer  732  from the shields  712 ,  750 , the shunting of current is prevented. Thus, the flux guide  730  increases the amount of magnetic flux in the active region of the sensor  700  . Consequently, the output signal of the tunnel valve sensor  700  with the flux guide  730  is enhanced by the amount of extra flux in the active region of the sensor  700 . 
       FIG. 8  illustrates a flow chart  800  of a method of making a tunnel valve head with a flux guide according to the present invention. In  FIG. 8 , a tunnel valve is patterned at a first shield layer  810 . Insulation is deposited using a self-aligned process  820 . A flux guide is deposited  830  so that it overlaps with tunnel valve, but does not completely extend over the tunnel valve. The flux guide is then covered with insulation using a self-aligned process  840 . After depositing the insulation over the flux guide, the second shield is formed  850 . 
     The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.