Method for fabricating a low resistance TMR read head

[0001] 1. Field of the Invention

[0002] This invention relates to the use of magnetic tunnel junction (MTJ) configurations to form tunneling magnetic read heads (TMR read heads). In particular, it relates to the formation of a low resistance MTJ by use of an under-oxidized tunneling junction layer that is naturally oxidized and formed on an electrode with an oxygen surfactant layer.

[0003] 2. Description of the Related Art

[0004] The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. One of the electrodes, the “pinned” electrode, has its magnetic moment fixed in spatial direction by an adjacent pinning layer (typically a layer of antiferromagnetic material). The other electrode, the “free” electrode, has a magnetic moment that is free to move in the presence of external magnetic fields. Although the tunnel barrier layer between the free and pinned electrodes is an insulator, it is sufficiently thin so that there is a small but finite probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. Thus, a probability dependent current can flow between the free and pinned electrodes. The tunneling probability is spin dependent, depending on the orientation of the conduction electron spin relative to the magnetization direction of each of the ferromagnetic layers. Thus, if these magnetization directions are varied relative to each other, the tunneling current will also vary as a function of the relative directions for a given applied voltage. When used as a read head, the magnetization of the free electrode varies with the external magnetic field of the recorded medium and the resulting change in current is sensed by appropriate circuitry. The MTJ is already being successfully used as a read head. Fontana, Jr. et al (U.S. Pat. No. 5,901,018) disclose a MTJ magnetoresistive read head with a free magnetic layer that also functions as a flux guide to direct magnetic flux from the recording medium to the tunnel junction. The tunnel barrier layer taught by Fontana is formed by depositing and then plasma oxidizing a 0.5-2 nm Al layer to form a layer of Al₂O₃. Wang et al. (U.S. Pat. No. 6,462,541 B1) discloses sensing arrangment formed of a plurality of magnetic field sensors, each one generally formed of two magnetically permeable films separated by an intermediate layer of non-magnetic material. Mao et al. (U.S. Pat. No. 6,411,478 B1) teaches the formation of an MTJ type read sensor whose layer structure is horizontal rather than vertical so as to achieve a thinner overall fabrication.

[0005] For a read head to operate successfully with recording densities greater than 100 Gb/in², it should have the following properties:

• • [0006] a) Very low junction resistance R for a given junction area A, (the product, RA, being less than or equal to 1 Ωμm²)
  • [0007] b) High ratio of dR/R (>10%)
  • [0008] c) High dielectric breakdown voltage (>0.5 volts)
    For a typical junction area: A=0.01 μm², R must be approximately 100 Ω for the tunneling magnetoresistive junction (MTJ). For reference, R for a typical GMR sensor (not an MTJ) of this size is approximately 50 Ω.

[0009] It is known that ultra-low junction resistance can be obtained only with an ultra-thin tunneling barrier. Al₂O₃ is known to be an insulator with a relatively wide band gap even when formed as an ultra-thin layer less than two atomic layers thick (<6 angstroms). Tunneling junctions using this material have been formed by a process of naturally oxidizing a thin layer of aluminum. In this regard, Gallagher et al. (U.S. Pat. No. 6,226,160) teach a method of thermally oxidizing a layer of Al between 5-9 angstroms thick deposited on a fixed ferromagnetic layer, which does not oxidize the surface of the fixed ferromagnetic layer on which the oxide layer is formed. HfAlOx has also been shown to have very low resistance at thin layer formations and the natural oxidation of a thin layer of HfAl has produced an RA as low as 0.6 Ωμm² and dR/R of approximately 10%. Of the three requirements, a), b) and c), mentioned above, only adequate breakdown voltage has not yet been obtained and is not disclosed in the cited prior art.

[0010] A first object of this invention is to provide a novel MTJ read-head capable of reading recorded densities exceeding 100 Gb/in².

[0011] A second object of this invention is to provide an MTJ read-head having an ultra-low junction resistance, R, where RA (resistance times junction area) is less than 1 Ωμm².

[0012] A third object of this invention is to provide such an MTJ read head having a high magnetoresistive ratio, dR/R, which exceeds 10%.

[0013] A fourth object of this invention is to provide such an MTJ read head wherein the tunneling junction has a high breakdown strength.

[0014] These objects will be achieved by a fabrication method that forms an ultra-thin tunneling barrier layer of an Al oxide or an Hf/Al alloy oxide on a bottom electrode (pinned electrode) that has first been treated by an oxygen ambient to form an oxygen surfactant layer. The importance of such surfactant layers in the improvement of specular reflection scattering at interfaces of GMR spin valve structures has been demonstrated by W. F. Egelhoff et al., “Oxygen as a surfactant in the growth of giant magnetoresistance spin valves,” J. Appl. Phys. 82(12), 15 Dec. 1997 p. 6142. We have found that the prior surfactant formation is also essential to creating the thin barrier layer that has high breakdown voltage in the TMR structure. If the layer is just naturally oxidized, but not formed on a surfactant layer, it will have a tendency to form an aluminum oxide without the proper Al₂O₃ stoichiometry and stability to insure high breakdown voltage. The low breakdown strength of such layers which are not formed in accord with the method of the present invention have already been reported by Jianguo Wang et al. “Continuous thin barriers for low-resistance spin-dependent tunnel junctions,” J. Appl. Phys. (93)10, 15 May 2003, p. 8367. In the present invention it is found that by combining natural oxidation of an ultra-thin layer which has been formed on an oxygen surfactant layer, the requisite Al₂O₃ stoichiometry and barrier layer properties are obtained because both the natural oxidation and the oxygen surfactant layer provides a source of oxygen for the aluminum layer. It is determined experimentally that upon depositing an Al layer on an electrode having an oxygen surfactant layer containing less than an oxygen monolayer, a high quality Al₂O₃ is immediately obtained at the interface. During the subsequent natural oxidation to complete the oxidation of the Al layer, the Al₂O₃ already formed serves as an oxygen diffusion layer to prevent oxygen from diffusing into the bottom electrode.

[0015] More particularly, two structures and methods of fabricating them are provided. Structure I) uses an oxidized aluminum layer of approximately 5.75 angstroms thickness, while structure II) uses an oxidized Hf—Al layer wherein the Hf is between approximately 1.5 and 2.5 angstroms thick and the Al is between approximately 3 and 4.5 angstroms thick.

• [0016] I) NiCr/MnPt/CoFe(10%)/Ru/CoFe(10%)-10Å NiFe(60%)/OSL/5.75Å Al/NOX/10Å NiFe(60%)-NiFe(20%)/Ta or: NiCr/MnPt/CoFe(10%)/Ru/CoFe(25%)-10Å NiFe(60%)/OSL/5.75Å Al/NOX/10Å NiFe(60%)-NiFe(20%)/Ta
• [0017] II) Ta/NiCr/MnPt/CoFe(10%)/Ru/CoFe(10%)-10Å NiFe(60%)/OSL/Hf—Al/NOX/10 ÅNiFe(60%)-NiFe(20%)/Ta

[0018] The formation of the structures I) and II) will be further discussed and explained in the context of the description of the preferred embodiments presented below.

[0021] The two preferred embodiments of the present invention (I and II above) are each a method of forming a magnetoresistive read head in a magnetic tunneling junction (MTJ) configuration in which an ultra-thin barrier layer is formed with a more stable stoichiometry that gives it a high breakdown voltage together with an ultra-low junction resistance. In the first preferred embodiment the barrier layer is formed by the natural oxidation (NOX) of an ultra-thin aluminum layer deposited on a sub-atomic monolayer of oxygen (an oxygen surfactant layer (OSL)) that was formed on a bottom electrode surface. The combination of the OSL beneath the Al layer and the NOX of the Al layer produces an oxidized Al layer with Al₂O₃ stoichiometry. In the second preferred embodiment, the read head is formed using a naturally oxidized HfAl layer.

[0022] Referring first to FIG. 1a, there is shown a cross-sectional view an initial step in the formation of an MTJ device designed to efficiently achieve the objects of the present invention while using a naturally oxidized Al layer as a tunneling barrier layer. There is seen a substrate (10), which in a preferred embodiment of this invention could contain structures to allow electrical connection to a bottom electrode of the MTJ device. On the substrate is then formed a bottom electrode (20), which includes a magnetically pinned layer (22) and which comprises a Ta/NiCr(40%) seed layer (21), wherein the Ta layer provides an advantageous crystalline growth direction for the NiCr, and an MnPt/CoFe(10%)/Ru/CoFe(10%)-NiFe(60%) synthetic antiferromagnetic (SyAF) pinned layer formed on the seed layer, wherein the antiferromagnetic material MnPt (22) provides pinning of two antiferromagnetically coupled ferromagnetic layers CoFe(10%) (24) and the composite layer CoFe(10% or 25%)-NiFe(60%)(26 and 27) coupled by a Ru layer (25). The indicated percentages represent atom percentages of the Fe and Cr in the material composition of the layers. The NiCr(40%) seed layer (21) is formed to a thickness between approximately 30 and 40 angstroms. The CoFe(10%) layer (24) contacting the MnPt layer (22) is formed to a thickness of between approximately 20 and 25 angstroms. The Ru coupling layer (25) is formed to a thickness between approximately 7 and 8 angstroms, the CoFe layer (26) formed on the Ru layer is formed to a thickness between approximately 20 and 25 angstroms. A layer of NiFe(60%) (27) of thickness between approximately 5 and 15 angstroms, with 10 angstroms being preferred, is formed on the CoFe layer (26). It is this NiFe layer (27) which is treated in an oxygen ambient to form an oxygen surfactant layer (OSL) approximately less than 1 atomic monolayer in thickness (30) on its upper surface. The OSL on the upper surface of (27) is indicated as the shaded layer (30). Our experiments suggest that forming the additional NiFe layer is particularly advantageous for achieving the objectives of the present invention because a surfactant monolayer (or sub-monolayer) of oxygen adsorbed on a NiFe layer is more effective for the purposes of the present invention than a surfactant layer of oxygen adsorbed in a CoFe layer. This is because the lower binding energy of oxygen with Fe (as compared to its binding energy with Co) allows the oxygen to be more easily incorporated into the subsequently deposited Al layer and to achieve the stable stoichiometry discussed below. A method for forming an OSL on an exposed surface that achieves the objectives of the invention is as follows:

• • [0023] 1. After NiFe(60%) (27) deposition on CoFe layer (26), the entire fabrication is moved into a separate vacuum chamber.
  • [0024] 2. The chamber is fed with a small amount of oxygen (approx. 0.02 sccm) to obtain an oxygen pressure of about 2×10⁻⁷ torr.
  • [0025] 3. The NiFe(60%) surface is exposed to the oxygen for about 100 s., forming less than a monolayer of oxygen on the surface.

[0026] Referring next to FIG. 1b, there is shown the fabrication of FIG. 1a wherein an ultra-thin (2 atomic layers) layer of aluminum (40), between approximately 5 and 6 angstroms thick, with approximately 5.75 angstroms being preferable, is formed on the oxygen surfactant layer (30). The oxygen in the surfactant layer is immediately incorporated within the atomic structure of the Al, creating an Al₂O₃ stoichiometry at the interface of the layers (35). The region of oxygen adsorption and Al₂O₃ stoichiometry is shown as shaded (35).

[0027] Referring next to FIG. 1c, there is shown the Al layer ((40) in FIG. 1b) after a natural oxidation process (NOX) to complete the formation of an aluminum oxide tunneling barrier layer (50) with approximately Al₂O₃ stoichiometry throughout its thickness. During the natural oxidation, the interface portion of the barrier layer ((35) in FIG. 1b) acts as a barrier to oxygen diffusion into the pinned electrode (20). The NOX process occurs in the same chamber as the OSL formation, except that the oxygen is present at higher pressure (between 0.1 torr and 1 torr) and the surface is exposed for corresponding times (between approximately 500 and 200 seconds), with the shorter time being required for the higher oxygen pressure. The OSL layer ((30) in FIG. 1b) is no longer indicated by shading, because ideally the oxygen in that layer would have been adsorbed into the aluminum oxide layer (50) to complete the Al₂O₃ stoichiometry. Similarly, the interfacial region ((35) in FIG. 1b) has now merged with the NOX region and layer (50) is, therefore, shown uniformly shaded.

[0028] Referring next to FIG. 1d, there is shown schematically the fabrication of FIG. 1c wherein there has been the formation of an upper electrode, which is a ferromagnetic free layer (60), on the tunneling barrier layer (50). Note that layer (50) is now shown with uniform shading to indicate the uniform Al₂O₃ stoichiometry that has been achieved by the natural oxidation and the absorption of the oxygen in the OSL. The free layer (60) is a layer of NiFe(60%)-NiFe(20%) having a total thickness between approximately 30 and 50 angstroms, with between approximately 5-10 angstroms of the NiFe(60%) being preferred. A capping layer of Ta (70) is formed on the free layer. It is understood that the fabrication described in the foregoing description will be formed between upper and lower NiFe shield layers which also serve as conducting leads so that a sense current can pass through. An illustration of this lead structure is not provided herein as it can be formed using different methods well known to practitioners of the art.

[0029] Referring next to FIG. 2, there is shown the structure of FIG. 1a wherein an alloy layer of Hf—Al (45) has been formed on the oxygen surfactant layer (30). In this, the second embodiment of the present invention, the Hf—Al layer will be naturally oxidized in place of the Al layer of the first embodiment. The Hf—Al layer is formed by first depositing a layer of Hf (41) to a thickness between approximately 1 and 3 angstroms, with 2 angstroms being preferred and then depositing a layer of Al (43) to a thickness between approximately 4 and 5 angstroms on the Hf layer. As a result of this deposition process, the Hf removes oxygen from the surfactant layer more reactively than the Al, since the heat of formation of HfO is higher than that of AlO, to form a stable HfO stoichiometry at the interface (46). In a manner similar to that described with reference to FIG. 1c, the Hf—Al layer is then subjected to a natural oxidation to complete the fully oxidized barrier layer. The subsequent formation of an upper electrode, which is a ferromagnetic free layer, and capping layer on the barrier layer proceed in the same manner as that described in FIG. 1d.

[0030] As is understood by a person skilled in the art, the preferred embodiment of the present invention is illustrative of the present invention rather than being limiting of the present invention. Revisions and modifications may be made to processes, materials, structures, and dimensions through which is formed an MTJ read head with ultra-low junction resistance and high junction breakdown voltage, while still providing a method of forming such an MTJ read head in a manner which is in accord with the present invention as defined by the appended claims.