Abstract:
A method for manufacturing a magnetoresistive sensor at very small dimensions with well a controlled track width and clean damage free side wall junctions. The method uses nano-imprinting rather than photolithography to pattern a resist layer. This eliminates the track width variations inherent in photolithographic patterning. The use of nano-imprinting also eliminates the need for a bottom anti-reflective coating beneath the resist layer, thereby also eliminating the need for an additional etch process to remove the bottom anti-reflective coating, which would also cause variations in track width.

Description:
RELATED INVENTIONS 
       [0001]    This invention is related to commonly assigned patent application Ser. No. ______, entitled PROCESS FOR FABRICATING AN ULTRA-NARROW DIMENSION MAGNETIC SENSORS, filed ______. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to magnetic tunneling devices and more particularly to a method for manufacturing a magnetoresistive sensor having an ultra-narrow track-width and well controlled side junction profile. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, 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. 
         [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. The sensor includes a nonmagnetic conductive layer, hereinafter 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 located parallel to the ABS, but 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]    The push for ever increased data rate and data capacity has lead a drive to increase the performance and decrease the size of magnetoresistive sensors. Such efforts have lead to an investigation into the development of tunnel junction sensors or tunnel valves. A tunnel valve operates based on the quantum mechanical tunneling of electrons through a thin electrically insulating barrier layer. A tunnel valve includes first and second magnetic layers separated by a thin, non-magnetic barrier. The probability of electrons passing through the barrier layer depends upon the relative orientations of the magnetic moment of the first and second magnetic layers. When the moments are parallel, the probability of electrons passing through the barrier is at a maximum, and when the moments are antiparallel, the probability of electrons passing through the barrier is at a minimum. 
         [0007]    In the push for ever greater data density, researchers have sought means for decreasing the dimensions of magnetoresistive sensors, especially the track-width of such sensors. However, manufacturing limitations have limited the ability to reliably reduce the track-width of such sensors, while also maintaining controllability of well defined side junction profiles of the sensors. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a method for manufacturing a magnetoresistive sensor that includes first providing a substrate and then depositing a plurality of sensor layers over the substrate. A mask structure is then deposited over the substrate and a resist layer is deposited over the mask structure. Nano-imprinting is then used to form a patterned resist layer. The image of the patterned resist layer is transferred onto the mask layer. An ion milling can then be performed to remove portions of the plurality of sensor layers that are not protected by the mask layer. 
         [0009]    The mask layer can include a first etch mask layer and a second etch mask layer formed over the first etch mask layer. The first and second etch mask layers can be constructed of materials that are removable by reactive ion etching with different chemistries. For example, the first etch mask layer can be constructed of a soluble polymer or PMGI, which is removable by reactive ion etching in an oxygen chemistry and which is resistant to removal by reactive ion etching in a fluorine chemistry and is also resistant to removal by ion milling. The second etch mask layer can be constructed of a material such as SiO 2 , SiN x , SiO x N y , SiC, or Ta, which is removable by reactive ion etching in a fluorine chemistry and may be removable by ion milling, but is resistant to removal by reactive ion etching in an oxygen chemistry. 
         [0010]    An optional protective layer, constructed of a material such as diamond like carbon (DLC) or amorphous carbon can be provided after the sensor layers and before the first etch mask layer to protect the sensor layers during subsequent processing. 
         [0011]    These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    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. 
           [0013]      FIG. 1  is a schematic illustration of a disk drive system in which the invention might be embodied; 
           [0014]      FIG. 2  is an ABS view of a slider illustrating the location of a magnetic head thereon; 
           [0015]      FIG. 3 ; is an enlarged ABS view of a magnetoresistive sensor such as can be manufactured according to an embodiment of the invention; and 
           [0016]      FIGS. 4-11  show an ABS view of a magnetoresisitve sensor in various intermediate stages of manufacture in order to illustrate a method of manufacturing a magnetoresistive sensor according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    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 claimed herein. 
         [0018]    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 . 
         [0019]    At least one slider  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 read from or written to. 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 . 
         [0020]    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. 
         [0021]    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 . 
         [0022]    With reference to  FIG. 2 , the orientation of the magnetic head  121  in a slider  113  can be seen in more detail.  FIG. 2  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 that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
         [0023]    With reference now to  FIG. 3 , a schematic illustration is shown of a magnetoresistive sensor  300  as viewed from a plane parallel with the Air Bearing Surface (ABS). The sensor  300  includes a sensor stack  302  that is sandwiched between first and second electrically conductive shields  304 ,  306  that can be constructed of a magnetic material so that they can function as magnetic shields as well as electrical leads. 
         [0024]    The sensor stack  302  can include a magnetic pinned layer structure  308 , a magnetic free layer structure  310  and a non-magnetic spacer or barrier layer  312  sandwiched there-between. If the sensor  300  is a giant magnetoresistive sensor (GMR) the layer  312  will be an electrically conductive, non-magnetic spacer layer constructed of a material such as Cu. If the sensor  300  is a tunnel junction magnetoresistive sensor (TMR) the layer  312  will be a thin, non-magnetic electrically insulating barrier layer such as Mg—O, alumina or TiO 2 . 
         [0025]    The pinned layer structure  308  can be an antiparallel coupled structure that includes first and second magnetic layers AP 1   314  and AP 2   316 , which are antiparallel coupled across a thin, non-magnetic AP coupling layer  318  such as Ru. The AP 1  layer  314  has magnetization that is pinned in a first direction perpendicular to the ABS as indicated by arrow-head symbol  320 . Pinning of the magnetization  320  is achieved by exchange coupling with a layer of antiferromagnetic material (AFM) layer  322 , which may be a material such as PtMn, IrMn or some other suitable material. Antiparallel coupling between the AP 1  layer  314 , and AP 2  layer  316  pins the magnetization of the AP 2  layer  316  in a second direction perpendicular to the ABS as indicated by arrow tail symbol  324 . 
         [0026]    In addition to the free layer  310 , pinned layer structure  308  and spacer or barrier layer  312 , a capping layer  326  including one or more layers of Ta and/or Ru may be provided at the top of the sensor stack  302  to protect the sensor layers during manufacture. First and second hard bias layers  328 ,  330 , constructed of a material such as CoPt or CoPtCr can be provided at either side of the sensor stack  302  to provide a magnetic bias field for biasing the magnetization of the free layer  310  in a desired direction parallel with the ABS as indicated by arrow symbol  332 . The hard bias layers  328 ,  330  can each be separated from the sensor stack  302  and from at least one of the lead layers  304  by a thin insulation layer  334  in order to prevent sense current from being shunted through the hard bias layers  328 ,  330 . 
         [0027]    In operation, an electrical sense current is passed through the sensor stack  302  from one of the leads  306  to the other lead  304 . In this way, the electrical resistance across the sensor stack can be measured. This resistance across the sensor stack varies with the relative orientations of the free layer magnetization  332  and pinned or reference layer magnetization  324 . The closer these magnetizations are to being parallel to one another the lower the resistance will be, and the closer these magnetizations are to being anti-parallel the higher the resistance will be. As mentioned above, the magnetization  324  is pinned. However, the magnetization  332  is free to rotate in response to a magnetic field. Therefore, by measuring the change in electrical resistance across the sensor stack  302 , the presence and strength of an external magnetic field can be sensed. 
         [0028]    The width of the sensor stack  302  (and more specifically the width of the barrier/spacer layer  312  and free layer  310 ) determines the track width (TW) of the sensor  300 . As discussed above, the track-width of the sensor is an important parameter, because a smaller track-width is needed to increase data density. Another important design parameter is the definition of the sides of the sensor stack  302 , also referred to as the junction. Control of the side junctions  334 ,  336  includes controlling the angle of these sides and the smoothness of the side curvature, and also includes making sure that damage to the material layers at the sides is minimized and the amount of re-deposited material (re-dep) is minimized. 
         [0029]      FIGS. 4-11 , illustrate a method for manufacturing a magnetoresisitive sensor that allows the track-width of the sensor to be reduced and uniform, while also maximizing side junction definition uniformity. With particular reference to  FIG. 4 , a lower magnetic, electrically conductive lead  402  is formed, and a plurality of sensor layers  404  are deposited over the lead  402 . The lead  402  provides a substrate for the deposition of the sensor layers there-over. The sensor layers  404  can include layers that can form a sensor stack  302  such as that described with reference to  FIG. 3 . The sensor layers  404  could include layers of any of a number of other types of sensors too, with the sensor stack  302  of  FIG. 3  being merely an example. 
         [0030]    With continued reference to  FIG. 4 , an optional protective layer  406  can be deposited over the sensor layers. The optional protective layer  406  can be constructed of a material such as Diamond Like Carbon (DLC) or amorphous carbon. A first etch mask  410  is deposited over the optional protective layer, and a second etch mask  412  is deposited over the first etch mask  410 . The first etch mask  410  is deposited to a thickness T that, together with the second etch mask layer  412  will define a desired mask height for a future ion milling operation that will be described herein below. The second etch mask  412  can be made significantly thinner than the first etch mask  410 . A layer of photoresist  414  is deposited over the second etch mask  412 . No bottom antireflective coating (BARC) is needed under the photoresist mask  414 , for reasons that will become apparent below. The first etch mask layer  410 , and second etch mask layer  412  are constructed of materials that are removable by different reactive ion etching processes. In other words, the first etch mask  410  is constructed of a material that can be selectively removed by a reactive ion etching that will leave the second etch mask  412  substantially intact. Similarly, the second etch mask  412  is selectively removable by a reactive ion etching process that will leave the first etch mask substantially intact. In addition, the first etch mask  410  is constructed of a material that is resistant to ion milling. To this end, the first etch mask layer  410  can be constructed of a soluble polymer material (preferably a polymer that is soluble in NMP solution) such as DURIMIDE® or polymethylglutarimide (PMGI). The second etch mask  412  can be constructed of a material such as SiO 2 , SiN x , SiO x N y , SiC, or Ta. NMP is the more commonly used acronym for the chemical C5H9NO, also known as N-Methylpyrrolidone. This chemical is also known by other names, such as N-Methyl-2-pyrrolidone, and for simplicity&#39;s sake will be referred to herein simply as “NMP”. 
         [0031]    With reference now to  FIG. 5 , the photoresist layer  414  is patterned by nano-imprinting. This is performed using a nano-imprinting mold  416  having a patterned imprint or groove  418 . The mold  418  is pressed onto the resist layer  414 , resulting in a desired pattern as shown in  FIG. 5 . Heat may be applied during the nano-imprinting process to cure or harden the resist layer  414  somewhat. The nano-imprinting process results in a certain amount of resist residue  420  extending from the patterned portion, as shown in  FIG. 5 . 
         [0032]    Prior art methods for manufacturing magnetoresistive sensors have used photolithographic techniques to pattern and develop the resist layer  414 . This also required the use of a bottom anti-reflective coating (not shown) directly beneath the resist layer  414 . This BARC layer would then be etched away after the resist layer had been patterned. This extra etching step resulted in unwanted variation in the width of the resist mask, resulting in sensor track width variation. Another major source of track width variation using such a prior art method resulted from variations in the photolithographic process itself, both flash field to flash field, within wafer and wafer to wafer. This variation increased substantially when the print resist critical dimension (i.e. width) went below a certain limit, such as 60-75 nm. The above described nano-imprinting method eliminates these sources of track-width variation, because the same mold is used for all flash fields within wafer and for many wafers, allowing a sensor to be constructed at very narrow track widths with an extremely consistent, well controlled track width. 
         [0033]    A first reactive ion etching (RIE) is performed to remove the residual portion  420  of the patterned resist  414 . This first RIE is preferably performed in an oxygen containing atmosphere. This leaves a structure as shown in  FIG. 6 , with the resist mask  414  formed above the second hard mask layer  412 . 
         [0034]    Then, with reference to  FIG. 7 , a second reactive ion etching is performed to remove portions of the second etch mask layer  412  that are not protected by the resist mask  414 , thereby transferring the image of the resist mask  414  onto the underlying second etch mask  412 . This second RIE is performed using a gas that preferentially removes the second etch mask layer  412  while leaving the first etch mask layer  410  substantially intact. For example if the second etch mask  412  is constructed of SiO 2 , SiN x , SiO x N y  or SiC, then the second RIE is performed in an atmosphere that contains fluorine. 
         [0035]    Then, with reference to  FIG. 8 , a third reactive ion etching (RIE) is performed to remove portions of the first etch mask layer  410  that are not protected by the second etch mask layer  412 , thereby transferring the image of the second etch mask layer  412  onto the underlying first etch mask layer. This third RIE is performed using a chemistry that selectively removes portions of the first etch mask layer  410  that are not protected by the second etch mask layer  412 , while removing little, if any, of the second etch mask layer  412 . To this end, the third RIE is preferably performed in an atmosphere that contains oxygen. This third RIE can also be used to remove portions of the optional protective layer  406  (if the protective layer  406  is used), in order to transfer the image of the over-lying mask layers  410 ,  412  onto the protective layer  406 . 
         [0036]    With the first etch mask  410 , and optional protective layer  406  patterned, an ion milling process can be performed to remove portions of the sensor material  404  that are not protected by the mask layers  406 ,  410 , thereby forming a sensor  404  with clean, well defined sides as shown in  FIG. 9 . The ion milling process actually involves a series of ion milling operations performed at various angles relative to normal so as to form a sensor  404  with well defined sides, repeatable, uniform side walls that have little or no damage or re-deposited material (re-dep). This ion milling also removes any of the second etch mask layer  412  ( FIG. 8 ) that remained after the third RIE. 
         [0037]    The formation of a read sensor has unique requirements that are not shared by the formation of other devices such as magnetic write heads or semiconductor devices, such as the necessity to form the sensor  404  with clean, well defined side junctions  902 ,  904 . In order to accurately define the side junctions  902 ,  904 , a certain well defined amount of shadowing from the mask layers  406 ,  410  must be present during the ion milling, and this amount of shadowing must be consistent and well controlled. According to the present invention, the thickness of the protective layer  406 , thickness T of the first etch mask layer  410 , and thickness of the second mask layer  412  (shown in  FIG. 9 ) can be easily and accurately controlled to desired design thicknesses through the above processes. The optional protective layer  406  is much thinner than the mask layer  410 , so that its thickness is a very small portion of the total mask thickness. Therefore, any variation in the thickness of the protective layer  406  has little impact in the overall mask thickness. The thickness of the second mask layer  412  is much thinner than that of the thickness T of the first etch mask layer  410  and it is substantially unchanged during the third RIE process used to etch the first etch mask layer  410 , and its variation has a very small impact on the overall mask thickness. Previously disclosed processes resulted in a reduction in the height of the overall mask thickness during formation of the mask itself. This made it impossible to control the overall thickness of the mask layers, especially at extremely narrow track widths at the start of the ion milling process. In the method of the present invention, the ion milling process reduces the mask height, but the process is repeatable in that the starting mask height is consistent and easily controlled. 
         [0038]    The above described process makes it possible to control mask thickness precisely and controllably from wafer to wafer for the ion milling process that defines the sensor junction. The ion milling mask consists of the first etch mask  410 , second etch mask  412  and protective layer  406 . The thickness T of the first mask  410  remains the exact thickness at which it was deposited. In other words, the thickness T is controlled by deposition of the layer  410 , which can be accurately and consistently controlled. This is also true of the protective layer  406 . The thickness of the second mask  412  is little changed by the third RIE process in  FIG. 8  and its thickness at the start of ion milling process is substantially controlled by the deposition process that deposits it. 
         [0039]    With reference now to  FIG. 10 , a thin layer of non-magnetic, electrically insulating material  1002  can be deposited, followed by a hard magnetic material  1004 . The deposition of the hard magnetic layer  1004  can be preceded by the deposition of one or more seed layers (not shown) that initiate a desired grain structure in the above deposited hard magnetic bias layer  1004 . The insulation layer  1002  can be alumina and can be deposited by atomic layer deposition. The hard bias material layer  1004  can be a material such as CoPt or CoPtCr and can be deposited by sputter deposition. One or more capping layers (not shown) may be deposited after the hard magnetic material  1004 . Then, a liftoff process can be performed to remove the mask layers  410 . A chemical mechanical polishing process may be used as well to assist lift-off of the mask and to planarize the surface of the structure. The optional protective layer  406  may be removed. This leaves a structure as shown in  FIG. 11 . Thereafter, a second shield can be deposited such as the shield  306  shown in  FIG. 3 . 
         [0040]    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.