Patent Publication Number: US-6903909-B2

Title: Magnetoresistive element including ferromagnetic sublayer having smoothed surface

Description:
BACKGROUND 
     A conventional magnetic tunnel junction includes a pinned ferromagnetic layer, a sense ferromagnetic layer and an insulating tunnel barrier sandwiched between the ferromagnetic layers. Relative orientation and magnitude of spin polarization of the ferromagnetic layers determine the resistance of the magnetic tunnel junction. Generally, the resistance of the magnetic tunnel junction is a first value (R N ) if the magnetization of the sense layer points in the same direction as the pinned layer magnetization (referred to as a “parallel” magnetization orientation). The resistance is increased to a second value (R N +ΔR N ) if the magnetization orientations of the sense and pinned layers point in opposite directions (referred to as an “anti-parallel” magnetization orientation). The magnetic tunnel junction has an intermediate resistance (R N &lt;R&lt;R N +ΔR N ) as the sense layer magnetization is rotated from one direction to the other. 
     Magnetic tunnel junctions can be used as magnetic sensors in read heads. The magnetic tunnel junction of a read head can detect data stored on a magnetic storage medium such as a hard disk drive. 
     Magnetic tunnel junctions can be used as memory elements in magnetic random access memory (MRAM) devices. The two magnetization orientations, parallel and anti-parallel, represent logic values of “0” and “1.” A logic value may be written to a magnetic tunnel junction by setting the magnetization orientation to either parallel or anti-parallel. The orientation may be changed from parallel to anti-parallel or vice-versa by applying the proper magnetic field to the magnetic tunnel junction. The logic value may be read by sensing the resistance of the magnetic tunnel junction. 
     FM coupling between the pinned and sense layers can cause problems with magnetic tunnel junctions. In read heads, the FM coupling can cause readback signal distortion. Bias techniques can be used to correct the signal distortion. However, the bias techniques tend to be complex to implement and costly to fabricate. 
     In magnetic memory elements, the FM coupling can render magnetic tunnel junctions unusable. Unusable magnetic tunnel junctions can reduce MRAM performance, increase fabrication cost, and increase the complexity of read circuits. 
     SUMMARY 
     According to one aspect of the present invention, a ferromagnetic layer of a magnetoresistive element includes a crystalline ferromagnetic sublayer and an amorphous ferromagnetic sublayer. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a magnetoresistive element according to a first embodiment of the present invention. 
         FIG. 2  is an illustration of a peak-to-valley height difference on a surface of a pinned layer of the magnetoresistive element. 
         FIG. 3  is an illustration of a magnetoresistive element according to a second embodiment of the present invention. 
         FIG. 4  is an illustration of a hard disk drive including a magnetoresistive read head according to an embodiment of the present invention. 
         FIG. 5  is an illustration of magnetized regions on a hard disk of the disk drive. 
         FIG. 6  is an illustration of a transfer curve for the magnetoresistive read head. 
         FIG. 7  is an illustration of a method of fabricating a magnetoresistive read head according to an embodiment of the present invention. 
         FIG. 8  is an illustration of a data storage device including magnetoresistive memory elements according to an embodiment of the present invention. 
         FIG. 9  is an illustration of a method of fabricating a data storage device according to an embodiment of the present invention. 
         FIG. 10  is an illustration of a response curve for the magnetoresistive memory element. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is made to  FIG. 1 , which illustrates a magnetoresistive element  110  including a multi-layer stack of materials. The stack includes seed layer(s)  112 , an antiferromagnetic (AF) pinning layer  114 , a pinned ferromagnetic (FM) layer  116 , a spacer layer  118 , and a sense FM layer  120 . The seed layer(s)  112  provides the correct crystal orientation for the AF pinning layer  114 . The AF pinning layer  114  provides a large exchange field, which holds the magnetization of the pinned layer  116  in one direction. Consequently, the pinned layer  116  has a magnetization that is oriented in a plane, but fixed so as not to rotate in the presence of an applied magnetic field in a range of interest. 
     The sense layer  120  has a magnetization orientation that is not pinned. Rather, its magnetization can be rotated between either of two directions: the same direction as the pinned layer magnetization, or the opposite direction as the pinned layer magnetization. 
     If the magnetoresistive element  110  is a magnetic tunnel junction (MTJ), the spacer layer  118  is an insulating tunnel barrier, which allows quantum mechanical tunneling to occur between the pinned and sense layers  116  and  120 . This tunneling phenomenon is electron spin dependent, making the resistance across the pinned and sense layers  116  and  120  of the MTJ a function of the orientation of the sense layer magnetization. 
     If the magnetoresistive element  110  is a giant magnetoresistive (GMR) device, the spacer layer  118  is made of a conductive metal such as copper. In-plane resistance across the sense layer  120  of a GMR device is a function of the orientations of the sense layer magnetization. 
     If the magnetoresistive element is an anisotropic magnetoresistive (AMR) device, the spacer layer  118  is made of a conductive material, and a soft adjacent layer is used instead of a pinned layer. In a dual stripe AMR device, the spacer layer is made of an insulating material. 
     Facing surfaces of the pinned layer  116  and the spacer layer  118  define an interface  122 . This interface  122  will be referred to as the “spacer interface.” During deposition of the pinned layer  116 , the pinned layer  116  exhibits columnar growth, which causes the grains to bow upward at the interface surface of the pinned layer  116  with large angle slope. This bowing produces magnetic poles on the edges of grains of the pinned layer  116 . These poles produce a magnetic field in the sense layer  120 . 
     Additional reference is made to  FIG. 2 , which illustrates the peak-to valley height variations on the interface surface of the pinned layer  116 . To prevent the bowing from causing strong FM coupling between the pinned and sense layers  116  and  120 , the spacer interface  122  is smoothed. The spacer interface  122  may be smoothed during fabrication by flattening peaks and filling valleys on the exposed surface of the pinned layer  116 . Flattening the peaks and filling the valleys reduces the peak-to-valley height variations. The pinned layer surface after smoothing is indicated in solid lines and referenced by numeral  352 , and the pinned layer surface prior to smoothing is indicated in dashed lines and referenced by numeral  354 . The height variation between a flattened peak and a valley is referenced by the letter X.  FIG. 2  is intended merely to illustrate the peak-to-valley height difference before and after the pinned layer surface has been smoothed; it is not intended to provide an accurate depiction of the surfaces  352  or  354  of the pinned layer  116 . 
     Smoothing the surface  352  to a critical flatness has been found to significantly reduce or eliminate FM coupling. It has been found that a critical flatness is achieved when the peak-to-valley height difference X is no more than about one nanometer. 
     The smoothing decreases edge grain angles on the surface of the pinned layer  116 . Shallower angles of the grains are believed to produce fewer magnetic poles at the edges. Ideally, the angle θ from the top of a grain to the intersection with an adjacent grain is between about three and six degrees. 
     Instead of eliminating the FM coupling, the level of FM coupling may be tuned to reduce or cancel AF coupling. As the magnetoresistive element  110  gets smaller, the AF coupling increases. Once the pinned layer  116  has been patterned, demagnetization fields emanate from its edges. Since this magnetic field tries to complete a circuit, it terminates on the sense layer  120  and thereby produces a field in the opposite direction of the pinned layer magnetization. This induced field, which is largest at the edges of the sense layer  120 , causes the AF coupling. Tuning the FM coupling becomes especially valuable as the magnetoresistive element becomes smaller and the AF coupling becomes more prominent. 
     Smoothing the spacer interface  122  is not limited to smoothing the surface of the pinned layer  116 . The magnetoresistive element  110  may include an FM interfacial layer between the pinned layer  116  and the spacer layer  118 . The interfacial layer would be part of the spacer interface  122 . Smoothing may be performed on the interfacial layer surface facing the spacer layer  118 . Such smoothing of the spacer interface  122  may be performed in addition to, or instead of, the smoothing of the pinned layer surface  352 . Smoothing the pinned layer  116  can result in a smoother interfacial layer, as well as a smoother spacer layer  118 . 
     The spacer interface  122  may also include sublayers of the spacer layer  118 . If the spacer layer  118  can be formed in sublayers (e.g., an insulating tunnel barrier that is formed over multiple deposition steps), at least one of the sublayers can be smoothed. Such smoothing of the spacer interface  122  may be performed in addition to, or instead of, the smoothing of the pinned layer surface  352 . Smoothing the underlying sublayers can result in a smoother spacer layer  118 . 
     The spacer interface  122  may be smoothed by using an ion etching process or other process that does not destroy the properties of the magnetoresistive element. The FM coupling can be reduced monotonically with ion etch time within certain ranges (the ranges depend upon the FM material). A typical ion etch rate is on the order of 1 nm/min. The ion etch time can be calibrated to allow the FM coupling to exactly compensate for the AF coupling, no matter what device size is used at the design center of the application. 
     The spacer interface  122  can be further smoothed by adding a layer of amorphous FM material to the magnetoresistive element. A magnetoresistive element having an amorphous FM layer is illustrated in FIG.  3 . 
     Referring now to  FIG. 3 , the magnetoresistive element  310  includes seed layers  312 , an AF pinning layer  314 , a pinned layer  316 , a spacer layer  318 , and a sense layer  320 . The element  310  further includes first and second interfacial FM layers  324   a  and  324   b  on opposite sides of the spacer layer  318 . The pinned layer  316  includes a sublayer  316   a  of amorphous ferromagnetic material between sublayers  316   b  and  316   c  of crystalline ferromagnetic material (the interfacial layer  324   a  and the sublayer  316   b  may be formed as a single layer instead of separate layers). For the purpose of this application, crystalline FM material refers to both monocrystalline and polycrystalline FM material. The amorphous sublayer  316   a  may be an FM material (e.g., NiFe, NiFeCo alloys, CoFe) to which an amorphizing agent (e.g., B, Nb, Hf, Zr) is added. The crystalline sublayers  316   b  and  316   c  may be made of a material such as NiFe. Final thickness of these sublayers  316   a ,  316   b  and  316   c  may be between one and three nm. 
     The amorphous sublayer  316   a  can break the grain boundary structure at the AF pinning layer  316 . This deliberate interruption of the grain structure reduces the severity of the peaks and valleys formed at the spacer interface  122 . If the AF pinning layer includes a material (e.g., manganese) that can diffuse into crystalline ferromagnetic material, the amorphous sublayer  316   a  provides the additional advantage of blocking the diffusion. 
     The crystalline sublayers  316   b  and  316   c  of the pinned layer preferably have a high moment/polarization. Increasing the moment/polarization of these crystalline sublayers  316   b  and  316   c  can increase the TMR of the magnetoresistive element  310 . 
     The pinned layer  316  may instead include only a single layer  316   a  of amorphous ferromagnetic material, or an amorphous sublayer  316   c  and a single crystalline sublayer  316   b . However, eliminating one or both of the crystalline sublayers  316   b ,  316   c  would weaken the pinning field (reducing the crystallinity would reduce the pinning field) in addition to reducing the TMR of the magnetoresistive element  310 . 
     The upper surface of the amorphous sublayer  316   a  and the lower surface of the insulating tunnel barrier  318  define a spacer interface  322 . The interface may be smoothed by smoothing the upper surfaces of at least one of the following layers: the amorphous sublayer  316   a , the crystalline sublayer  316   b , and the interfacial layer  324   a . The spacer layer interface  322  may also be smoothed by smoothing the lower surface of the insulating tunnel  318 . 
     As an alternative, none of these surfaces are smoothed. However, smoothing the surface of the amorphous sublayer  316   a  improves the gain structure of the overlaying crystalline sublayer  316   b . Moreover, smoothing any of these surfaces further reduces FM coupling or allows the FM coupling to be tuned. 
     The magnetoresistive elements just described are not limited to any particular application. Two exemplary applications will now be described: a hard disk drive, and an MRAM device. 
     Reference is now made to  FIG. 4 , which shows a hard disk drive  410  including magnetic media disks  412 . User data is stored in concentric circular tracks on the surface of each disk  412 . The disk drive  410  also includes transducers  414  for performing read and write operations on the disks  412 . Each transducer  414  includes a magnetoresistive read head for the read operations (each transducer  414  may also include a thin film inductive head for the write operations). 
     Additional reference is made to FIG.  5 . During read operations, magnetized regions  510  of the disk are passed under the read head of the transducer  414 . The read head detects transitions in the magnetized regions  510 . Fringe fields  512  emanate from the grains at the grain boundaries. Three net magnetization states occur: where the fringe fields add out of the plane of the disk  412  (the net field is indicated by arrow  514 ); where the fringe fields add into the plane of the disk  412  (the net field is indicated by an arrow  516 ); and where the fringe fields cancel out (no net field and, therefore, no arrow). The net fields  514  adding out of the disk  412  cause the sense layer magnetization to rotate toward one direction, and the net fields  516  adding into the disk  412  cause the sense layer magnetization to rotate toward the other direction. 
     The read head generates a readback signal during detection of the net fields. Amplitude of the readback signal depends upon the magnetoresistance of the read head. The smoothing can be controlled to adjust the position of the transfer curve so that the amplitude is linearly related to the read head resistance. This, in turn, reduces readback signal distortion during read operations. 
     Additional reference is made to  FIG. 6 , which illustrates a transfer curve  610  for the read head. The transfer curve  610  has a region that is roughly linear between points A and B. It may be desirable to center this linear region about a zero magnetic field (H=0). The magnetic tunnel junction has a nominal resistance (R 1 =R N ) when the magnetization of the pinned and sense layers  116  and  120  are in the same direction. The magnetic tunnel junction has a higher resistance (R 2 =R N +ΔR N ) when the magnetization of the pinned and sense layers  116  and  120  are in opposite directions. The magnetic tunnel junction has an intermediate resistance (R 1 &lt;R&lt;R 2 ) as the sense layer magnetization is rotated from one direction to the other. 
     The FM coupling can be tuned to improve the centering of the transfer curve  610  about the zero magnetic field. The AF coupling tends to move the transfer curve  610  toward the left in  FIG. 6 , such that the resistance of the magnetoresistive element  110  at zero field is a high resistance state. 
     Smoothing can provide advantages other than adjusting magnetic coupling. In the case of a magnetic tunnel junction, the insulating tunnel barrier is distributed more evenly over the pinned layer. Distributing the barrier material more evenly allows the thickness of the insulating tunnel barrier to be reduced without creating pinholes (the pinholes greatly increase magnetic coupling and short the junctions). Reducing the barrier thickness, in turn, reduces the resistance of the read head, which reduces the RC constant of the read head. Consequently, response time of the read head is faster. Reducing the resistance of the read head can also reduce power consumption. 
     Smoothing the spacer interface can also improve the uniformity of read head resistance. Read head uniformity simplifies read head design, and reduces the need to tune the disk drives for read head properties. Smoothing the interface can also reduce the number of shorted read heads and thereby improve manufacturing yield. 
     Amorphous FM material only may be used in the pinned layer. However, adding the crystalline FM material to the pinned layer will increase signal strength. 
     The magnetoresistive elements are not limited to longitudinal recording, as illustrated in FIG.  5 . The magnetoresistive elements may be used for perpendicular recording, in which magnetic fields from grains are measured. 
     Reference is now made to  FIG. 7 , which illustrates an exemplary method of fabricating a plurality of read heads. A stack of the following materials is deposited on a wafer: shield material, seed layer material, AF pinning layer material, and pinned FM layer material ( 710 ). The thickness of the deposited pinned layer material is increased to compensate for the ion etching that follows. 
     The upper, exposed surface of the pinned layer is ion etched to a critical flatness (block  712 ). The ion etching may be performed by bombarding the pinned layer with argon ions or any other non-reactive ions. The ion etch may be performed in-situ in a deposition chamber. The FM coupling can be reduced monotonically with ion etch time within a particular range (the range depends upon the material). This permits adjustment of the magnetic interactions such that the read head operates linearly. 
     Interfacial layer material is then deposited on the etched surface, and an upper surface of the interfacial layer material is ion etched ( 714 ). Insulating tunnel barrier material is then deposited ( 716 ). A barrier material such as Al 2 O 3 , for example, may be formed by r-f sputtering, or by depositing aluminum and then oxidizing the aluminum by a process such as plasma oxidation. 
     Interfacial layer material, sense layer material, and shield material are then deposited ( 718 ). An isolation dielectric layer is formed between the sense layer material and the shield for GMR and AMR devices. The resulting stack is then patterned into a plurality of read heads (block  720 ). 
     The shield material may be an electrically conductive and magnetically permeable material such as NiFe. The seed layer material may be any material that establishes the desired grain orientation of the AF pinning layer. Candidate materials for the seed layer(s) include titanium (Ti), tantalum (Ta), and platinum (Pt). Candidate materials for the AF pinning layer  36  include platinum-manganese (PtMn), manganese-iron (MnFe), nickel manganese (NiMn), and iridium-manganese (IrMn). Candidate materials for the insulating tunnel barrier include aluminum oxide (Al 2 O 3 ), silicon dioxide (SiO 2 ), tantalum oxide (Ta 2 O 5 ), silicon nitride (SiN 4 ), other dielectrics, and certain semiconducting materials. Candidate materials for the pinned and sense layers and include NiFe, iron oxide (Fe 3 O 4 ), chromium oxide (CrO 2 ), alloys of any of Ni, Fe and Co (e.g., CoFe, NiCoFe), and other ferromagnetic and ferrimagnetic materials. 
     Reference is now made to  FIG. 8 , which shows an exemplary MRAM device  810  including an array  812  of memory cells  814 . Each memory cell  814  may include one or more magnetoresistive elements. Only a relatively small number of magnetoresistive elements  814  are shown to simplify the description of the MRAM device  810 . In practice, arrays  812  of any size may be used. 
     Word lines  816  extend along rows of the memory cells  814 , and bit lines  818  extend along columns of the memory cells  814 . There may be one word line  816  for each row of the array  812  and one bit line  818  for each column of the array  812 . Each memory cell  814  is located at a cross point of a word line  816  and bit line  818 . 
     The MRAM device  810  also includes a read/write circuit  820  for performing read and write operations on selected memory cells  814 . The read/write circuit  820  senses resistance of selected memory cells  814  during read operations, and it orients the magnetization of the selected memory cells  814  during write operations. 
       FIG. 9  shows a method of manufacturing the MRAM device. The read/write circuit and other circuits are fabricated on a substrate (block  910 ). Conductor material is then deposited onto the substrate and patterned into bit lines (block  912 ). A dielectric may then be deposited between the bit lines. Materials for the seed layer(s), and the AF pinning layer are deposited (block  914 ). 
     Pinned layer material is then deposited. Material for an amorphous FM sublayer is deposited, and an upper, exposed surface is ion etched to a critical flatness (block  916 ). Material for a crystalline FM sublayer is deposited, and an upper, exposed surface is ion etched to a critical flatness (block  918 ). Ion etching the upper surface of the amorphous layer can improve grain structure of the crystalline sublayer. The thickness of the deposited material is increased to compensate for the ion etching. 
     Insulating tunnel barrier material is formed on the etched material (block  920 ). A barrier material such as Al 2 O 3 , for example, may be deposited by r-f sputtering, or by depositing aluminum and then oxidizing the aluminum by a process such as plasma oxidation. If the insulating tunnel barrier material is formed in multiple stages, at least one stage may be ion etched. 
     Material for the sense FM layer is then deposited (block  922 ). The resulting stack is then patterned into bits, and spaces between the bits are filled with dielectric (block  924 ). Conductor material is then deposited on the dielectric and patterned into word lines (block  924 ). 
     The resulting array may then be planarized. A new array may be formed atop the planarized array. 
       FIG. 9  describes the manufacture of a single MRAM device. In practice, however, many MRAM devices will be fabricated simultaneously on a single wafer. 
     Reference is made to  FIG. 10 , which illustrates a response curve  1010  for a magnetoresistive element of the MRAM device. The smoothing can be performed to adjust the position of the response curve and improve the uniformity of switching across the array. The switching points A and B may be centered about a zero magnetic field. Improving the switching uniformity can reduce the number of magnetic tunnel junctions that are not switched in the presence of a sufficient magnetic field, and that are inadvertently switched even though the magnetic field is not sufficient. Thus, the smoothing reduces the number of unusable magnetic tunnel junctions. 
     The smoothing further reduces the number of unusable tunnel junctions by reducing the number of magnetic tunnel junctions that are shorted due to pinholes. 
     Reducing the number of unusable magnetic tunnel junctions can increase the storage capacity of the MRAM device. It can also reduce the burden on error correction. Consequently, reducing the number of unusable magnetic tunnel junctions can improve MRAM performance and reduce fabrication cost. 
     The smoothing can increase uniformity of the tunnel junction resistance across the array. Increasing the uniformity can reduce the complexity of reading selected memory cells among multiple columns of memory cells. 
     The amorphous ferromagnetic sublayer can further reduce interface roughness. If the antiferromagnetic pinning layer includes manganese, the amorphous sublayer can also block diffusion of the manganese into the crystalline ferromagnetic layer. 
     Although the memory cells were described as each having only a single magnetoresistive device, the MRAM device is not so limited. Each memory cell may include more than one magnetoresistive device. 
     The memory cells are not limited to magnetoresistive devices such as magnetic tunnel junctions. Other magnetoresistive devices may be used. 
     The present invention is not limited to the magnetoresistive elements described above. Any ferromagnetic layer (pinned or unpinned, crystalline or amorphous) of a magnetoresistive element may be replaced with the combination of crystalline and amorphous ferromagnetic sublayers. 
     The AF pinning layer may be placed near the top of the stack instead of the bottom of the stack, whereby the pinned layer is deposited after the sense layer. If the sense layer material is deposited before the pinned layer material, the sense layer would have an interface surface having peaks and valleys. Thus the interface surface of the sense layer would be smoothed. 
     The present invention is not limited to the specific embodiments described and illustrated above. Instead, the present invention is construed according to the claims that follow.