We have discovered an improved tunnel barrier process that may be applied to the formation of MgO or related oxides such as AlOx, MgTaOx, TiO, ZnO, and native CoFeB oxide. The key aspect is to insert two extra steps comprising a thin metal (Mg) layer deposition followed by a passive oxidation involving an oxygen pressure of 10.5 torr or less. Thereafter, one or more metal layers are deposited and each deposition is followed by a conventional oxidation having an oxygen pressure of at least 10⁻³torr. Passive oxidation as defined herein means that the kinetic energy of the oxygen atoms in the flow is essentially at the minimum level that is reproducible in a manufacturing environment. Typically, the pressure of the oxygen flow is less than 10⁻⁵torr, and preferably about 10⁻⁶torr during passive oxidation. In conventional oxidation methods, the pressure of oxygen flow is usually above 10⁻³torr, and at least 10 to 100 times greater than in passive oxidation. The unusually weak oxidation condition applied in the passive oxidation step is employed to minimize the extent of oxidation of the thin metal layer to prevent oxygen diffusion into an underlying (bottom) magnetic layer and to avoid cracks that extend through the thin metal layer to a top surface of the bottom magnetic layer.

According to a preferred embodiment wherein the thin metal layer is Mg, only a top surface of the Mg layer is gently oxidized by passive oxidation to form a first MgO layer whereas the bottom portion of the Mg layer and the bottom magnetic layer remain unoxidized. Moreover, the gently oxidized first MgO layer will prevent oxygen during subsequent oxidation steps from causing a substantial amount of oxidation in the bottom magnetic layer, even when standard oxidation conditions comprising ≥103 torr oxygen pressures are employed to oxidize subsequently deposited metal layers to complete the tunnel barrier formation. One or more additional metal oxide layers may be formed on the oxidized upper portion of the first metal (Mg) layer. The one or more additional metal oxide layers may be formed by (a) direct deposition of a metal oxide by a conventional method such as sputtering a metal oxide target, or by (b) depositing a metal layer and then oxidizing with a process comprising an oxygen pressure of at least 10⁻³torr. It should be understood that when two or more metal oxide layers are formed on a top surface of the oxidized portion of the first metal layer, any combination or repetition of steps (a) and (b) above may be used to form a plurality of metal oxide layers. However, passive oxidation is a key discovery that enables PMA of the bottom magnetic layer to be better preserved than in prior art tunnel barrier fabrications where only conventional methods are used to form one or more metal oxide layers.

In a first embodiment depicted in FIGS. 2-6, a series of process steps are employed to fabricate a tunnel barrier and begin with deposition of a first metal layer on a bottom magnetic layer followed by a passive oxidation. Then, a second metal layer is deposited and is oxidized by a conventional oxidation method. A third (upper) metal layer may be deposited on the oxidized second metal layer. In the exemplary embodiments, Mg is used as the metal for tunnel barrier fabrication. However, one or more other metals including Al, Ta, Zn, Ti, and Sn, may be selected instead of Mg. For example, the first (lower) metal layer and upper metal layer may be comprised of a first metal or alloy while one or more intermediate metal layers may be selected from a second metal or alloy unequal to the first metal or alloy. However, for the purpose of improving throughput, all metal layers are preferably selected from the same metal or alloy.

Referring to FIG. 2 that relates to a MTJ having a bottom spin valve configuration, a bottom magnetic layer that is reference layer 10 is provided and may be Co, CoFeB, or another alloy comprising two or more of Co, Fe, Ni, and B and deposited on a seed layer 8 (FIG. 10). Furthermore, the bottom magnetic layer may be a composite with a lower laminated stack of layers such as (Co/Ni), in which n is a lamination number. The laminated stack is preferably formed on a seed layer and there may be an upper CoFeB layer (not shown) to give a (Co/Ni) n/CoFeB configuration, for example. However, an (A1/A2)_nlaminate may be selected rather than (Co/Ni)_n. A1 may be Co, CoFe, or a CoFeR alloy where R is one of Ru, Rh, Pd, Ti, Zr, Hf, Ni, Cr, Mg, Mn, or Cu. A2 may be Ni, NiCo, NiFe, Pt or Pd. Preferably, Co, CoFeB, or the alloy that is the uppermost layer in the bottom magnetic layer is less than about 20 Angstroms thick to enable intrinsic perpendicular magnetic anisotropy (PMA) that is enhanced when forming an interface with MgO or a metal oxide tunnel barrier in a later step.

In an alternative embodiment relating to a top spin valve configuration in FIG. 12, the reference layer 10 becomes the top magnetic layer while the free layer 30 is the bottom magnetic layer. In this case, the top magnetic layer may have a CoFeB/(Co/Ni), or CoFeB/(A1/A2)_nconfiguration where the CoFeB layer contacts a top surface of tunnel barrier 20.

Returning to FIG. 2, a first metal layer 21 such as Mg with a thickness between 1 and 6 Angstroms is deposited by a sputter deposition method on the bottom magnetic layer. A Mg film with a thickness less than 1 Angstrom is likely to be discontinuous and comprise gaps between adjacent grains that extend vertically through the entire film. On the other hand, a Mg layer that is thicker than 6 Angstroms may not become fully oxidized during a subsequent oxidation and/or anneal step which means degraded interfacial perpendicular anisotropy along the interface 14 with the bottom magnetic layer due to the absence of a metal oxide/magnetic layer interface.

A critical feature of the tunnel barrier fabrication sequence as disclosed herein is a passive oxidation step 16 that is performed to transform an upper portion of Mg layer 21 into MgO while a lower portion of the Mg layer remains unoxidized with no cracks. The upper MgO layer 21a is advantageously used to prevent oxygen during later conventional oxidation steps with oxygen pressure ≥10⁻³torr from penetrating Mg layer 21. As indicated previously, pressure of the oxygen flow in a conventional oxidation process is generally a factor of at least 10, and preferably, about 100 greater in magnitude than employed during our passive oxidation of the first Mg layer. The extremely weak oxidation condition with a maximum oxygen pressure of 10⁻⁵torr and preferably 10⁻⁶torr for a maximum duration of 1000 seconds guarantees that only an upper portion of the first Mg layer is oxidized and no cracks are formed through the first Mg layer. Maximum oxygen pressure is determined by controlling oxygen pressure in a closed chamber, or by controlling the oxygen flow rate in a vented chamber. Note that there are oxidized indentations 15 in oxidized layer 21a but they do not touch interface 14.

Referring to FIG. 3, the following step involves the deposition of a second Mg layer 22 on oxidized layer 21a. The second Mg layer and all subsequent Mg layers have a minimum thickness of 1 Angstrom to yield a continuous film. The maximum thickness for the second Mg layer depends on the desired RA value for the tunnel barrier, the number of Mg layers deposited during tunnel barrier fabrication, and the oxidation condition employed during the second oxidation step also referred to as the first conventional oxidation process shown in FIG. 4. In general, RA tends to become larger with an increasing number of Mg layers that are oxidized with a conventional oxidation process.

In FIG. 4, a first conventional oxidation process 17 that may be a natural oxidation (NOX), for example, is performed using conditions that completely convert the second Mg layer into an oxide layer. The oxidized second Mg layer and the oxidized portion of the first Mg layer form MgO layer 21b. Preferably, the oxygen flow rate during the first conventional oxidation process is at least 1 sccm for a period of 10 seconds, an oxidation condition that is considered moderate in comparison to the weak passive oxidation 16. Optionally, a relatively strong oxidation condition may be used where a flow rate of >1 sccm is applied for a period up to 600 seconds or with a pressure in the range of 0.1 mtorr to 1 torr. However, the first conventional oxidation process should not generate cracks that extend to first Mg layer 21 or allow oxygen to diffuse through MgO layer 21a to further oxidize the first Mg layer. The second Mg layer reduces the energy of oxygen that passes through layer 22 during the first conventional oxidation process, and MgO layer 21a further prevents the NOX oxygen from reaching the first Mg layer 21. The integrity of an oxidized metal layer may be determined by transmission electron microscopy (TEM) analysis to confirm whether or not cracks are created by a particular oxidation condition that may be too extreme for a certain Mg thickness. In other words, cracking is observed by TEM analysis if oxidation conditions are too strong.

According to one embodiment depicted in FIG. 5 that corresponds to a tunnel barrier fabrication sequence involving the fewest number of oxidation steps according to the present disclosure, an uppermost Mg layer 23 is deposited on MgO layer 21b to form a composite tunnel barrier layer 20 having a Mg/MgO/Mg configuration. As described in a later section, an anneal step (not shown) may be used following completion of the MTJ stack shown in FIG. 8 to cause diffusion of oxygen from the middle MgO layer 21b (or MgO layer 21c in an alternative embodiment where a plurality of conventional oxidation processes is performed) into Mg layers 21, 23 thereby forming a MgO layer 21d which contacts both of bottom magnetic layer 10 and top magnetic layer 30 (FIG. 9). Optionally, a passive oxidation is followed by a direct deposition of a MgO layer to give a tunnel barrier formation process with the fewest number of oxidation steps.

In a second embodiment shown in FIG. 6, the sequence illustrated in FIGS. 3-4 may be repeated one or more times to give a MgO layer 21c having a thickness t2 on the unoxidized portion of Mg layer 21 that has a thickness t1. Thus, the MgO layer in FIG. 6 may result from a second Mg layer deposition followed by a first conventional oxidation process, and then a third Mg layer deposition (not shown) followed by a second conventional oxidation process. The second conventional oxidation may also comprise NOX conditions described previously. In the exemplary embodiment, t2>t1, but the present disclosure also anticipates a structure where t1>t2 since it is well known that MgO has a smaller lattice size than that of Mg. For instance, in an embodiment where the first Mg layer has a 3 to 4 Angstrom thickness, and each of the second and third Mg layers are about 2 Angstroms thick, oxidation of the second and third Mg layers may result in a MgO layer 21c thickness less than 4 Angstroms. It should be understood that oxidation pressure and duration used to oxidize the second Mg layer may be unequal to the oxidation pressure and duration for oxidation of the third Mg layer. Furthermore, one or both of the first and second conventional oxidation processes may include more than one step. For example, a conventional oxidation sequence may have a first step with a first oxygen flow rate and pressure, and a second step with a second oxygen flow rate and pressure unequal to the conditions in the first step. It is important that all conventional processes employed during the formation of MgO layer 21c be at least 10 to 100 times stronger in terms of oxygen pressure compared with the passive oxidation of the first Mg layer in order to ensure that the second and third Mg layers are completely oxidized.

The intermediate tunnel barrier structure depicted in FIG. 6 also encompasses a third embodiment wherein a fourth Mg layer (not shown) is deposited on the oxidized third Mg layer followed by a third conventional oxidation process to oxidize the fourth Mg layer. Thereafter, an uppermost (fifth) Mg layer (not shown) may be deposited and remains unoxidized until an optional anneal process after the MTJ stack of layers is complete. As indicated earlier, the number of Mg layers that are deposited and oxidized by a conventional oxidation process, and the thickness of each Mg layer may be adjusted to influence the RA value for the tunnel barrier. An anneal process with a temperature up to 450° C. for a duration up to 90 minutes may be employed during or after the deposition of the MTJ stack in any of the aforementioned embodiments.

The present disclosure also encompasses a tunnel barrier fabrication wherein the oxidation sequence in the second embodiment is modified such that a second passive oxidation (PO) process replaces one of the conventional oxidation processes. Thus, there may be a plurality of PO steps employed during fabrication of the tunnel barrier. In one aspect, a first Mg layer is partially oxidized by a first passive oxidation, a second Mg layer is oxidized by a first NOX step, and a third Mg layer is partially oxidized by a second passive oxidation before an uppermost Mg layer is deposited. This oxidation sequence may be represented by PO/NOX/PO. However, the second Mg layer may be partially oxidized by a second PO process and the third Mg layer may be oxidized by a NOX method in a PO/PO/NOX scheme before an uppermost Mg layer is deposited and remains unoxidized until a subsequent anneal process. Preferably, at least one NOX step is retained to ensure that a sufficient amount of oxygen is contained within the oxidized Mg layers to enable diffusion into unoxidized portions of Mg layers during the anneal process and thereby forming an essentially uniform MgO tunnel barrier, or metal oxide tunnel barrier in embodiments where the metal is not Mg.

It should be understood that the third embodiment may be modified wherein one or more of the NOX steps are replaced by a passive oxidation. According to one fabrication sequence, a first Mg layer is deposited and partially oxidized by a first passive oxidation process, a second Mg layer is deposited and oxidized by a first NOX process, a third Mg layer is deposited and oxidized by a second NOX process, and then a fourth Mg layer is deposited and partially oxidized by a second passive oxidation before the uppermost Mg layer is deposited. This oxidation scheme is represented by PO/NOX/NOX/PO. Instead of a PO/NOX/NOX/PO sequence, a series of oxidation steps represented by PO/PO/NOX/PO, PO/PO/PO/NOX, or PO/NOX/PO/PO may be used wherein at least one oxidation involves a NOX step to ensure a sufficient quantity of oxygen within the tunnel barrier layer stack to completely oxidize all metal layers therein following free layer formation and a subsequent anneal process.

The present disclosure also anticipates that the passive oxidation process may comprise nitrogen gas so that an upper portion of the first metal layer deposited in a tunnel barrier stack becomes a metal oxynitride. The first metal layer preferably has a thickness between 1 and 6 Angstroms. In an alternative embodiment, nitrogen in the absence of oxygen is used to deposit a first metal nitride layer on the first metal layer. Typically, a metal nitride is deposited by using ionized nitrogen atoms and Ar to hit a metal target. As a result, the metal nitride is sputter deposited onto a substrate. One can control the flow rate of nitrogen gas to change the ratio between Ar and ionized nitrogen atoms and thereby change the nitrogen content in the metal nitride such as MgNx. This process may be defined as a passive nitridation process if oxygen is excluded and there is a maximum nitrogen pressure of 10⁻⁵torr. As a result, the bottom magnetic layer/first metal layer interface does not react with nitrogen and a first metal/first metal nitride stack is formed. Subsequent layers formed on the metal oxynitride or first metal nitride layer may be metal oxide layers fabricated with a conventional oxidation method of a metal layer, or by direct deposition, or one or more of the subsequent layers may have a metal oxynitride or metal nitride composition. Thereafter, an uppermost metal layer may be deposited on a top surface of an underlying metal oxide, metal oxynitride, or metal nitride layer. An anneal process at a temperature up to 450° C. and with a duration up to 90 minutes may be performed during the uppermost metal deposition or after a top magnetic layer and capping layer are sequentially formed on the uppermost metal layer.

Referring to FIG. 7, an uppermost Mg layer 23 is deposited on MgO layer 21c once the final conventional oxidation process or second passive oxidation is completed in the aforementioned embodiments related to FIG. 6. The thickness t3 of the final Mg layer is preferably at least 1 Angstrom. A maximum thickness for t3 is determined in part by the temperature and time involved in a subsequent annealing step. In particular, t3 should not be so large that oxygen from MgO layer 21c does not diffuse into all portions of Mg layer 23 and fail to oxidize a portion thereof along top surface 23s. It is important that a metal oxide/top magnetic layer interface be formed in order to maximize PMA in the top magnetic layer. As mentioned previously, an anneal process may be employed during deposition of the uppermost metal layer 23. In addition, a second anneal process may occur after the MTJ stack is completed. As temperature is increased up to 450° C. and/or process time is lengthened in any of the anneal processes, then oxygen diffuses a greater distance into layer 23. Note that a relatively thick Mg layer 23 (t3≥3 Angstroms) will provide a substantial contribution to the final RA value. Therefore, t3 is preferably kept between 1 to 3 Angstroms. It is important that unoxidized Mg layers 21, 23 are maintained on opposite surfaces of MgO layer 21c during subsequent steps related to formation of a top magnetic layer and overlying layers such as a capping layer so that oxygen does not penetrate and oxidize a portion of the bottom magnetic layer 10 and top magnetic layer 30. Thus, metal layers 21, 23 prevent oxidation of adjoining magnetic layers but serve as a pathway for the tunnel barrier to become completely oxidized at a later time after the MTJ stack is completed.

In FIG. 8 that relates to a bottom spin valve configuration, MTJ stack 40 is shown after a free layer 30 is formed as the top magnetic layer on the uppermost Mg layer 23. In this intermediate structure, a first interface 14 has a CoFeB/Mg composition, for example, while a second interface 28 may also have an Mg/CoFeB composition. In an alternative embodiment, the top magnetic layer may be comprised of Fe, or an alloy of two or more of Co, Fe, Ni, and B. The present disclosure also encompasses embodiments where the free layer has a synthetic antiferromagnetic (SAF) configuration wherein two ferromagnetic layers are separated and antiferromagnetically coupled that a layer such as Ru. Moreover, the top magnetic layer may have a moment diluting layer such as Ta or Mg formed between two magnetic layers that are ferromagnetically coupled. In yet another embodiment, the free layer may be a composite with a CoFeB, Co, or CoFe layer that has a bottom surface along interface 28, and a laminated stack such as (Co/Ni)_nor (A1/A2)n described previously formed on a top surface of the CoFeB, Co, or CoFe layer.

MTJ stack 40 may further comprise a capping layer (not shown) formed on a top surface 30s of free layer 30. For example, the capping layer may include one or more of Ru and Ta to protect the free layer during subsequent process steps such as a chemical mechanical polish process that produces a smooth top surface on the MTJ stack. In another embodiment, the capping layer may be a metal oxide to generate interfacial perpendicular anisotropy along the top surface 30s and enhance PMA within the top magnetic layer. According to one aspect of the present disclosure, a metal oxide capping layer may be formed by employing the tunnel barrier formation process disclosed herein. Thus, both of the tunnel barrier layer and capping layer may be MgO, for example, that has been fabricated by depositing a first Mg layer followed by a passive oxidation step. A bottom portion of the first metal layer in the capping layer remains unoxidized to prevent oxidation of the free layer. Thereafter, at least a second Mg layer is deposited on the partially oxidized first Mg layer followed by a conventional oxidation process. The formation of an uppermost Mg layer that is not subjected to an oxidation process may be omitted during capping layer formation since there is no subsequently deposited magnetic layer that requires protection from oxidation. During the subsequent anneal process described previously, oxygen from the oxidized second Mg layer diffuses into the bottom portion of the capping layer to form a metal oxide interface with a top surface of the top magnetic layer. In an alternative embodiment, the tunnel barrier may be an oxide made of a first metal or alloy such as MgTaO while the capping layer is made of a second metal or alloy that is MgO, for example.

Referring to FIG. 9, an anneal process is employed to cause diffusion of oxygen within MgO layer 21c (FIG. 8) into adjoining Mg layers. As a result, MgO layer 21d is formed that has a bottom surface along first interface 14 and a top surface along second interface 28 to enhance PMA within bottom magnetic layer 10 and top magnetic layer 30, respectively. The anneal process comprises applying a temperature up to 450° C. for a period of up to 90 minutes. Anneal temperatures around 400° C. are preferred when the resulting MTJ structure is incorporated in a CMOS device.

According to an embodiment depicted in FIG. 10, a MTJ nanopillar 40n is fabricated after the MTJ stack is completed and annealed by following a conventional patterning and etching sequence. In the exemplary embodiment, the MTJ nanopillar is formed on a substrate 6 such as a bottom electrode. The MTJ nanopillar comprises a seed layer 8 on the substrate and an uppermost capping layer 35 having a planar top surface 35s. A sidewall 38 extends from the top surface to substrate 6. In this drawing, the x-axis and y-axis directions are in the planes of the layers while a thickness of each MTJ layer is determined along the z-axis direction. The MTJ nanopillar top surface 35s may have a circular or elliptical shape from a top-down view along the z-axis. A plurality of MTJ nanopillars is typically arrayed in a design with columns and rows on the substrate.

Referring to FIG. 11, the present disclosure also encompasses a method of forming a tunnel barrier in a MTJ with a top spin valve structure. In this process flow, a first metal (Mg) layer 21 is deposited on a bottom magnetic layer that is a free layer 30. Thereafter, a passive oxidation as described earlier is performed to gently oxidize a top portion of the first metal layer to form a first metal oxide layer 21a while a bottom portion of the first metal layer and bottom magnetic layer remain unoxidized.

FIG. 12 depicts a MTJ structure after one or more metal layers are deposited on metal oxide layer 21a. Each of the one or more metal layers is oxidized by a conventional oxidation process as described previously to form metal oxide layer 21c that includes the first metal oxide layer. Optionally, one of the conventional oxidation processes may be replaced by a second passive oxidation. Then, an uppermost metal layer 23 is deposited. Once the uppermost metal layer in the tunnel barrier stack is laid down, a top magnetic layer that is a reference layer 10 is deposited. The top magnetic layer forms an interface 27 with metal layer 23 while the bottom magnetic layer forms an interface 16 with first metal layer 21.

FIG. 13 illustrates the MTJ structure in FIG. 12 after an anneal process is performed. As a result, metal oxide layer 21d is the tunnel barrier that has a first interface 16 with the bottom magnetic layer, and a second interface 27 with the top magnetic layer. PMA in preserved an enhanced in both magnetic layers because of the controlled oxidation processes involved in preparing the tunnel barrier, especially the passive oxidation applied to the first metal layer.

In FIG. 14, one example of a MTJ with a top spin valve structure having a tunnel barrier 21d formed according to an embodiment of the present disclosure is depicted. All layers are retained from the bottom spin valve stack in FIG. 10 except that magnetic layers 10 and 30 are switched.

Another embodiment of the present disclosure is illustrated in FIG. 15 where a MTJ nanopillar 70n has a dual spin valve structure having a first tunnel barrier 21d1 between a first reference layer 10a and free layer 30, and a second tunnel barrier 21d2 between the free layer and a second reference layer 10b. Both tunnel barrier layers may be fabricated according to a process flow that includes a passive oxidation of a first metal layer as described in one of the previously described embodiments. Alternatively, the dual spin valve may have a configuration (not shown) represented by FL1/tunnel barrier 1/reference layer/tunnel barrier 2/FL2 where FL1 is a first free layer and FL2 is a second free layer. Formation of both tunnel barriers (layers 21d1, 21d2) may proceed according to an embodiment described previously related to tunnel barrier 21d.

The dual spin valve structure may be fabricated by a sequence wherein a second tunnel barrier 21d2 is formed on stack that has a first magnetic layer 10a/first tunnel barrier 21d1/magnetic layer 30 configuration. Then a third magnetic layer 10b is deposited on the second tunnel barrier. With regard to tunnel barrier 21d2, a first metal layer (not shown) is deposited on magnetic layer 30 followed by a passive oxidation process with a maximum oxygen pressure of 10⁻⁵torr for up to 1000 seconds. The passive oxidation process oxidizes an upper portion of the first metal layer while a bottom portion of the first metal layer at an interface with a top surface of magnetic layer 30 remains unoxidized. One or more metal oxide layers may then be formed on the oxidized upper portion of the first metal layer according to methods described in previous embodiments. Thereafter, an uppermost metal layer may be deposited on a top surface of the one or more metal oxide layers before a third magnetic layer 10b is formed. There may be a capping layer 35 formed on a top surface of magnetic layer 10b to complete the MTJ stack. An anneal process with a temperature up to 450° C. for up to 90 minutes may be performed during deposition of a first metal layer in both tunnel barriers 21d1 and 21d2, or an anneal process may be performed after all layers in the dual spin valve MTJ are formed.

According to another embodiment shown in FIG. 16, a composite tunnel barrier 50 is formed according to an embodiment where a first metal layer (M₁) 21 is deposited and an upper portion thereof is partially oxidized by a passive oxidation to form a first metal oxide layer 21a represented by a M_i/M₁Ox configuration. Then, one or more metal layers made of a second metal (M₂) where M₂with a different from that of M₁may be deposited and oxidized by one or more conventional oxidation processes to form a second metal oxide layer 22a. Optionally, one of the conventional oxidation processes may be replaced by a second passive oxidation. An uppermost metal layer 23 that may have either a M₁or M₂composition is deposited on metal oxide layer 22a.

In FIG. 17, the MTJ structure from FIG. 16 is shown after an anneal is performed wherein a first metal oxide layer 21e is formed as a result of oxygen diffusion into first metal layer 21 and oxidation thereof to yield an essentially uniform M₁Ox layer from the intermediate M₁/M₁Ox stack. There is also a second metal oxide layer 22b resulting from oxygen diffusion into uppermost metal layer 23 and oxidation thereof to form an essentially uniform M₂Ox layer from the intermediate M₂Ox/uppermost metal oxide stack. Therefore, a composite tunnel barrier 50 is formed wherein at least the first metal oxide layer is fabricated by a process including a passive oxidation of a metal layer.

The present disclosure also anticipates an embodiment relating to a STO device wherein a metal oxide layer made according to a process sequence disclosed herein adjoins a spin polarization (SP) layer in order to preserve and even enhance PMA in the SP layer. Previously, we disclosed a spin torque oscillator (STO) device in U.S. Pat. No. 8,582,240 wherein non-magnetic layers formed adjacent to a spin polarization layer and oscillation layer may be metal oxides.

Referring to FIG. 18, a MAMR writer based on perpendicular magnetic recording (PMR) is depicted. There is a main pole 81 with a sufficiently large local magnetic field to write the media bit 85 in medium bit layer 84. Magnetic flux 88 in the main pole proceeds through the air bearing surface (ABS) 86-86 and into medium bit layer 84 and soft underlayer (SUL) 87. A portion of the flux (not shown) returns to the write head where it is collected by write shield 82. For a typical MAMR writer, the magnetic field generated by the main pole itself is not strong enough to flip the magnetization 89 of the medium bit in order to accomplish the write process. However, writing becomes possible when assisted by a spin torque oscillator (STO) 83 positioned between the main pole and write shield.

The STO is comprised of a high moment magnetic layer 90, and a second magnetic layer 91 that preferably has perpendicular magnetic anisotropy (PMA). Between layers 82 and 90, 90 and 91, and 91 and 81, there are nonmagnetic layers 92, 93, 94, respectively, to prevent strong magnetic coupling between adjacent magnetic layers. Non-magnetic layer 94 may be a metal oxide layer in order to form a metal oxide/magnetic layer interface with magnetic layer 91 and thereby preserving or enhancing PMA therein. Likewise, non-magnetic layer 92 may be a metal oxide layer to preserve or enhance PMA in magnetic layer 90.

An external current source 98 creates a bias current across the main pole and write shield. The applied dc results in a current flow in a direction from lead 101 into oscillation layer (OL) 90 and then through non-magnetic layer 93 and into SP layer 91 before exiting through lead 100. Direct current generated by source 98 is spin polarized by magnetic layer 91, interacts with magnetic layer 90, and produces a spin transfer torque that causes oscillation with a precession angle 95 in magnetic layer 90 hereafter called the oscillation layer (OL). The large angle oscillatory magnetization of OL 90 generates a radio frequency (1) usually with a magnitude of several to tens of GHz. This rf field (not shown) interacts with magnetization 89 of medium bit 85 and makes the magnetization oscillate into a precessional state 97 thereby reducing the coercive field of medium bit 85 to allow switching by the main pole field 88.

A key feature of the present disclosure is to provide a metal oxide composition in one or both of non-magnetic layers 92, 94 made by a passive oxidation process as disclosed in one of the previous embodiments. As a result, PMA is preserved in an adjoining magnetic layer that is SP 91 or OL 90, respectively. According to one embodiment, layer 94 is a metal oxide formed by depositing a first metal layer on the main pole layer and then performing a passive oxidation. One or more metal oxide layers are formed on the upper oxidized portion of the first metal layer before an uppermost metal layer is laid down. Then, layers 91, 93, 90, and 92 are sequentially formed before the write shield is fabricated. In one aspect, layer 92 is formed by depositing a first metal layer on OL 90 and then performing a passive oxidation. Next, one or more oxide layers are formed on an oxidized upper portion of layer 92 before an uppermost metal layer is deposited. An anneal process may be performed at this point or when each of the uppermost metal layers are deposited. As a result, oxidation processes to form metal oxide layers 92, 94 are well controlled and prevent substantial oxygen incursion into SP layer 91 and OL layer 90. In an alternative embodiment, the STO layers may be formed in reverse order on the main pole layer. Other aspects of previous embodiments are retained including the composition of metal oxide layers, methods to form one or more oxide layers on the oxidized first metal layer, and an anneal process during deposition of the uppermost metal layer or after all STO layers are laid down.

Another embodiment of the present disclosure is related to a perpendicular spin torque oscillator (PSTO) device wherein a high density STO current is isolated from a low density RF generation current that we previously disclosed in U.S. Pat. No. 8,203,389. In particular, a tunnel barrier in a RF generator portion of the three terminal device may be a metal oxide layer such as MgO that is formed by a process described in a previous embodiment.

Referring to FIG. 19, a PSTO device is shown with a STO component 103 and a RF generation component 104 hereafter referred to as “RF generator” or “MR sensor” that are separated by a non-magnetic conductive layer 114. In one aspect, STO 103 is a giant magnetoresistive junction comprised of a PMA magnetic layer 111 that serves as a magnetic reference layer (MRL), and a stack including a first junction layer also known as non-magnetic spacer 112, second PMA magnetic layer 113a, and a soft magnetic layer 113b that are sequentially formed on the MRL. PMA layer 111 and second PMA layer 113a may be comprised of a (A1/A2_nlaminate as described earlier.

Spacer 112 may be made of a conductive material such as Cu, or may have a confining current pathway (CCP) configuration in which Cu pathways are formed in an oxide matrix such as AlOx. Layers 113a, 113b are exchange coupled to each other and form a composite magnetic oscillation layer (MOL) wherein the magnetization in each layer is free to oscillate when subjected to an applied magnetic field perpendicular to the planes of the layers, and when an electric current of sufficiently high density flows in a direction perpendicular to the planes of the layers from a first electrical terminal 122 to a second electrical terminal 121. The high current density is preferably in the range of 1×10⁷to 1×10⁹Amps/cm²in order to exceed the critical current density for causing a spin torque effect on the MOL. It is believed that reflected electrons from the MRL/spacer interface excite the MOL layer and thereby induce an oscillation state in layers 113a, 113b with significant in-plane amplitude. Note that PMA layer 113a has the same oscillation frequency as soft magnetic layer 113b but a smaller in-plane magnetization component. Soft magnetic layer 113b may be made of CoFe, a CoFe alloy, or a composite thereof.

Non-magnetic conductive layer 114 is preferably a metal made of Cu or the like, or a metal alloy having a bottom surface that contacts an uppermost layer of STO 103, and with a top surface that adjoins a bottom layer in RF generator 104. Preferably, conductive layer 114 has a width in an in-plane direction along the x-axis that is greater than the width w of the layers in the STO and RF generator in order to allow an electrical connection to a first electrical terminal hereafter referred to as first terminal 122.

According to one embodiment, RF generator 104 is a magnetoresistive (MR) sensor with a TMR configuration in which a MTJ has a magnetic sensing layer 125, a second junction layer hereafter referred to as tunnel barrier 126, reference layer 127, exchange coupling layer 128, pinned layer 129, and AFM layer 130 are sequentially formed on a top surface of conductive layer