DETAILED DESCRIPTION OF THE INVENTION

[0015] Prior Art

[0016] FIG. 1 illustrates in cross-sectional view a conventional prior art MTJ device. The device includes a substrate 9 , a base multilayer stack 10 , a spacer layer of an insulating tunnel barrier layer 20 , a top stack 30 , an insulating layer 40 surrounding top stack 30 and bottom stack 10 , and a top wiring layer or electrical lead 50 . The tunnel barrier layer 20 is sandwiched between the two stacks 10 and 30 .

[0017] The base stack 10 formed on substrate 9 includes a first seed layer 12 deposited on substrate 9 , an optional “template” ferromagnetic layer 14 on the seed layer 12 , a layer of antiferromagnetic material 16 on the template layer 14 , and a “pinned” ferromagnetic layer 18 formed on and exchange coupled with the underlying antiferromagnetic layer 16 . The ferromagnetic layer 18 is called the pinned layer because its magnetization direction (shown by arrow 19 ) is prevented from rotation in the presence of applied magnetic fields in the desired range of interest for the MTJ device, i.e., the field from the write current if the device is a MTJ memory cell, or the field from the data recorded on the disk if the device is a MTJ read head. The top electrode stack 30 includes a “free” ferromagnetic layer 32 and a protective or capping layer 34 formed on the free layer 32 . The magnetization direction of the ferromagnetic layer 32 is not pinned by exchange coupling, and is thus free to rotate in the presence of applied magnetic fields in the range of interest. When the device is an MTJ memory cell, the magnetization direction of ferromagnetic layer 32 will be either parallel or antiparallel to the magnetization direction 19 of pinned ferromagnetic layer 18 . When the device is an MTJ sensor, such as a disk drive read head, the magnetization direction of pinned ferromagnetic layer 18 will be oriented into the paper in FIG. 1 and the magnetization direction of free ferromagnetic layer 32 will be oriented in the plane of the paper in FIG. 1 (perpendicular to the magnetization direction of pinned ferromagnetic layer 18 ) in the absence of an applied magnetic field, but will rotate slightly when exposed to a magnetic field from the recorded data on the disk, as described in the previously cited '410 patent.

[0018] The materials for MTJ devices with the structure illustrated in FIG. 1 are well known, and representative ones will be described. The MTJ base stack 10 comprises a stack of 200 Å Pt/40 Å Ni ₈₁ Fe ₁₉ /100 Å Mn ₅₀ Fe ₅₀ /80 Å Ni ₈₁ Fe ₁₉ (layers 12 , 14 , 16 , 18 , respectively) grown on substrate 9 . In addition to Pt, other conducting underlayers include Ta, Cu and Au. Other CoFe and NiFe alloys may be used for the ferromagnetic layers and other antiferromagnetic materials include NiMn, PtMn and IrMn. Substrate 9 would be a silicon wafer if the device is a memory cell. Substrate 9 would typically be the bottom electrically conductive lead located on either the alumina gap material or the magnetic shield material on the trailing surface of the head carrier if the device is a read head, as shown in the previously cited '410 patent. The stack 10 is grown in the presence of a magnetic field applied parallel to the surface of the substrate wafer. The magnetic field serves to orient the Mn ₅₀ Fe ₅₀ antiferromagnetic layer 16 . Layer 16 pins the magnetization direction of the NiFe free ferromagnetic layer 18 by exchange coupling. Next, the tunnel barrier layer 20 is formed by depositing and then oxidizing a 5-15 Å Al layer. This creates the Al ₂ O ₃ insulating tunnel barrier layer 20 . While Al ₂ O ₃ is the most common tunnel barrier material, a wide range of other materials may be used, including MgO, AlN, aluminum oxynitride, oxides and nitrides of gallium and indium, and bilayers and trilayers of such materials. The MTJ top stack 30 is an 80 Å Co/200 Å Pt stack (layers 32 , 34 , respectively) having a cross-sectional area of a few microns or less. The free ferromagnetic layer 32 is preferably either a single layer of an alloy of Fe and one or more of Co and Ni, or a bilayer of a CoFe alloy and a NiFe alloy. The top stack 30 is surrounded by an insulation layer 40 , which is typically SiO ₂ if the device is a memory cell and alumina if the device is a read head. The junction is contacted by a 200 Å Ag/3000 Å Au contact layer 50 that serves as the top wiring lead. Other capping or lead materials include Ta, Ti, Ru and Rh.

[0019] This MTJ structure is fabricated by sputtering all the layers in the junction stack (layers 12 , 14 , 16 , 18 , 20 , 32 , 34 ) onto the substrate 9 , followed by ion milling down through the free ferromagnetic layer 32 to the barrier layer 20 . This process of direct subtractive removal of the free ferromagnetic layer by ion milling or reactive ion etching (RIE) suffers from the disadvantages of redeposition of conductive material, inability to precisely control the removal process, and ion damage that can extend 20-40 Å below the etched surface. The ion milling or RIE of the free layer and pinned layer can cause redeposition of the material in these layers onto the edges of the tunnel barrier layer 20 and electrically “short” the insulating tunnel barrier at its edges. In addition, uncertainties in the ion milling rate and film thicknesses make it difficult to avoid damaging the underlying layers. Typical ion milling rates are 1 Å/sec for capping material (Ta) and free layer material (NiFe or CoFe). Typical capping layer thickness is 100 Å to 200 Å and typical free layer thickness is 30 Å to 40 Å. The film thickness uniformity and ion mill removal rate uniformity are each approximately 5%. Thus the use of ion milling or RIE to precisely remove the capping layer 34 and free layer 32 and stop at the tunnel barrier layer 20 has an inherent uncertainty of 13 Å to 24 Å in the removal process. This uncertainty is greater than or equivalent to the thickness of the tunnel barrier layer 20 .

[0020] The Invention

[0021] The MTJ device of the present invention is shown in FIG. 2 in the form of an MTJ read head for a magnetic recording disk drive. The cross-sectional view of FIG. 2 is essentially the read head as would be viewed from the disk with “TW” representing the trackwidth of the data tracks on the disk. The layers formed on the substrate 109 , which is typically the permalloy (NiFe) bottom shield or the alumina gap material in the head structure, are the bottom electrical lead layer 102 , seed layer 112 , antiferromagnetic layer 116 , fixed or pinned ferromagnetic layer 118 with its magnetization direction 119 being shown as into the paper, nonmagnetic insulating tunnel barrier layer 120 , free ferromagnetic layer 132 with its magnetization direction 135 being in the plane of the paper and perpendicular to direction 119 in the absence of an applied field from the recorded data on the disk, capping layer 134 and top electrical lead 150 . The top magnetic shield (not shown) or alumina gap material (not shown) would then be formed on top lead 150 , as depicted in the previously cited '410 and '548 patents. The bottom lead 102 and top lead 150 are formed of nonmagnetic materials and thus serve as first and second spacer layers to separate the ferromagnetic layers of the device from the bottom magnetic shield 109 and top magnetic shield, respectively.

[0022] Typical material compositions and thicknesses for layers 102 through 134 are as follows:

[0023] 20-50 Å Ru or Ta lead layer/20-50 Å NiFe or NiFeCr seed layer/200 Å PtMn or IrMn antiferromagnetic layer/30 Å NiFe or CoFe or NiFe—CoFe bilayer pinned layer/10-20 Å Al ₂ O ₃ tunnel barrier layer/30 Å NiFe or CoFe or CoFe—NiFe bilayer free layer/50-100 Å Ta, Ru or Ti capping layer.

[0024] The device is similar to the prior art of FIG. 1 with the primary difference being that there are nonmagnetic side regions 142 adjacent free layer 132 that are formed of oxides of the ferromagnetic material making up free layer 132 . The side regions 142 are formed by selectively oxidizing the free layer to render it locally nonmagnetic (substantially incapable of conducting magnetic flux) and electrically insulating. Insulating alumina covers 140 are formed on top of the oxidized side regions 142 . Additional alumina is located in outer regions 147 surrounding the outer edges of the tunnel barrier layer 120 and the layers beneath it. Covers 140 and outer regions 147 may be formed of other insulating material, such as SiO ₂ .

[0025] FIG. 3 is a perspective view of the MTJ read head with the top lead 150 and alumina covers 140 and outer regions 147 removed. Because the pinned layer 118 has a width W wider than the width (trackwidth TW) of the free layer 132 in the trackwidth direction, better longitudinal biasing of the free layer in this direction is achieved since the edge domain effects of the pinned layer are physically separated from the edges of the free layer by the nonmagnetic side regions 142 . FIG. 3 also illustrates that the pinned layer 118 can have a height H greater than the height (stripe height SH) of the free layer 132 in the stripe height dimension perpendicular to the trackwidth dimension. Because the pinned layer 118 can be formed with an aspect ratio (H/W) greater than unity, better stabilization of the pinned layer magnetization direction 119 along its height H can be achieved.

[0026] The MTJ device of the present invention is fabricated using controlled oxidation of selected regions 142 of the free layer 132 to render the free layer nonmagnetic and non-conducting in these selected regions above the pinned layer 118 . The oxidation process does not penetrate the previously oxidized tunnel barrier layer 120 and therefore can not damage the underlying pinned layer.

[0027] FIGS. 4 A- 4 I illustrate the process sequence for forming the MTJ device with the selective oxidation process. The process begins ( FIG. 4A ) by sputter depositing on the substrate (not shown) the MTJ layers 102 through 118 followed by a layer of Al, typically 5-15 Å thick. The Al is then oxidized by evacuation of the Ar sputtering gas and then either introduction of oxygen or exposure to an oxygen plasma. This forms the alumina (Al ₂ O ₃ ) tunnel barrier layer 120 , typically 10-20 Å thick. The remaining layers 132 and 134 are then sputter deposited over the tunnel barrier layer, resulting in the stack shown in FIG. 4A . A mask of resist 800 is then patterned on a central region of the capping layer 134 to define the lateral edges (TW and SH) of the free layer 132 , as shown in FIG. 4B . The stack is then moved to the RIE or ion milling tool where it is etched through the capping layer 134 and ending at or into the free layer 132 ( FIG. 4C ).

[0028] Next, the exposed portions of free layer 132 are oxidized in the RIE or ion milling tool to render these regions 142 of the free layer nonmagnetic and non-conducting ( FIG. 4D ). Suitable oxidation processes include ozone treatment, air oxidation, thermal oxidation, plasma oxidation, electrolytic oxidation, implantation of oxygen or molecular oxygen (O ₂ , O ₃ ) ions or neutrals. Reactive oxygen plasma induced oxidation can be performed in a RF coupled plasma, electron cyclotron resonance coupled plasma, or an inductively coupled plasma (ICP) system. A preferred process for oxidation of the free layer is with an ICP tool, which generates a dense plasma of oxygen radicals and allows the substrate bias to be controlled separately from the plasma source. When etching a test wafer with photoresist in the ICP system in an oxygen plasma under our typical plasma oxidation conditions, the etch rate is uniform across an entire 5 inch wafer to within 3%. The ICP oxidation process that induced demagnetization of the ferromagnetic layer had parameters of 30 scc O ₂ /min, 20° C. substrate temperature, 10 mT chamber pressure, 50W @ 13.56 MHz applied to the source coils, and 18W @ 13.56 MHz applied to the substrate.

[0029] The regions 142 become oxides of the material in the free layer 132 , e.g., one or more oxides of Fe and Co and/or Ni. Because the underlying layer 120 is already oxidized, the oxidation process is self-limiting, so control of the oxidation time is not critical. Next, insulation material, typically alumina or SiO ₂ , is deposited to form covers 140 above the nonmagnetic side regions 142 , followed by lift-of of the resist 800 , resulting in the structure shown in FIG. 4E . While the deposition of the covers 140 is preferred, this additional step may not be necessary if the oxide regions 142 are sufficiently free of pin holes. New resist 820 is then patterned to define the outside lateral extent of the pinned layer 118 as well as the other layers in the stack ( FIG. 4F ). The stack is then ion milled or RIE to remove all the layers down to the substrate ( FIG. 4G ), and an insulating layer of alumina or SiO ₂ is then deposited to form the outer regions 147 ( FIG. 4H ). Finally the top electrical lead 150 , typically 200 Å of Ta, Au or Cu, is deposited and patterned over the capping layer 134 ( FIG. 4I ). FIG. 5 is a graph showing the effect of oxidation on the effective ferromagnetic layer thickness (as calculated from measurements of magnetic moment) vs. time. Shown in the plot are data points for two compositions: a 31 Å CoFe single layer and a 10 Å CoFe/32 Å NiFe bilayer. While these two compositions have different physical thicknesses, they each have substantially the same effective magnetic thicknesses as a 36 Å film of NiFe. The oxidation time was approximately 6 minutes to demagnetize the CoFe single layer, and approximately 14 minutes to demagnetize the CoFe/NiFe bilayer.

[0030] While the device and method for its fabrication have been described above with respect to an MTJ device, particularly an MTJ sensor in the form an MTJ read head, the invention is also applicable to current-perpendicular-to-the-plane or CPP spin-valve (SV) sensors. A CPP SV read head has a structure substantially the same as the above-described MTJ read head, with the exception that the spacer layer is electrically conductive instead of insulating. For example, a copper spacer layer can replace the alumina tunneling barrier layer.

[0031] While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.