Patent ID: 12249450

DETAILED DESCRIPTION

The present disclosure is a MTJ wherein at least one of a free layer, reference layer, or dipole layer has perpendicular magnetic anisotropy that is maintained during 400° C. processing of the magnetic devices such as embedded MRAM and STT-MRAM, in spintronic devices such as microwave assisted magnetic recording (MAMR) and spin torque oscillators (STO), and in various spin valve designs including those found in read head sensors.

As disclosed in related U.S. Pat. No. 8,592,927, a MTJ may be comprised of a pinned layer, a tunnel barrier layer, and a magnetic element including a composite free layer having a magnetic saturation (Ms) reducing (moment diluting) layer formed between two magnetic sub-layers (FM1and FM2). The FM, layer has a surface that forms a first interface with the tunnel barrier while the FM2layer has a surface facing away from the tunnel barrier that forms a second interface with a perpendicular Hk enhancing layer which is employed to increase the perpendicular anisotropy field within the FM2layer.

In related patent application Ser. No. 14/949,232, we disclosed an improved seed layer stack wherein a low resputtering rate layer with amorphous character such as CoFeB is deposited on a high resputtering rate layer that is Mg, for example, to provide a “smoothing effect” to reduce peak to peak roughness at a top surface of the uppermost NiCr seed layer in a Mg/CoFeB/NiCr configuration. Thus, the NiCr seed layer has a smooth top surface with a peak to peak thickness variation of about 0.5 nm over a range of 100 nm compared with a peak to peak variation of about 2 nm over a range of 100 nm in prior art seed layer films as determined by transmission electron microscope (TEM) measurements.

We have discovered that the MTJ structures disclosed in the aforementioned related applications may be further improved according to the embodiments described herein. The MTJ in the present disclosure is comprised of a stack structure with improved control of the oxidization of an oxide layer above the free layer or a reference layer. The free layer or reference layer consists of a multiplicity (n) of thin ferromagnetic layers (Fe, Co, CoFe, CoFeB or combination thereof) deposited in an alternating sequence with (n−1) NMLs having a high resputtering rate and low magnetic dilution effect. According to one embodiment, the MTJ has a FML formed between two oxide layers in a OL1/FML/OL2scheme where FML has a FML1/NML/FML2configuration. The role of the NMLs is threefold and thereby provides three advantages in performance compared with the prior art Magnetic Tunnel Junctions inFIG.1andFIG.2.

First, the resputtering of the NML having a relatively high resputtering rate during the deposition of FML2in a FML1/NML/FML2configuration leads to a smoother FML2ferromagnetic layer. In other embodiments, where a FMLnlayer is deposited on a NMLn−1layer, a similar smoothing effect is realized for the top surface of the FMLnlayer.

Secondly, the presence of an NML layer inhibits the crystallization of the FML2layer, or in more general terms, a NMLn−1layer inhibits crystallization in the overlying FMLnlayer. As a result, the FML2layer (and FMLnlayer) has smaller grains and thinner grain boundaries. This reduces the diffusion of oxygen from the top oxide layer OL2to the FML2layer below it.

Lastly, the NML is a more highly reactive material than the FML sub-layers. Therefore it attracts oxygen that has diffused from the OL2into the FML2. As a result, the FML ferromagnetic sub-layers, and especially the upper FMLnsub-layer in a stack with “n” FML sub-layers and “n−1” NML layers, are less oxidized than in the prior art which leads to a better magnetoresistive ratio and greater FML thermal stability.

According to one embodiment of the present disclosure shown inFIG.3a, the free layer20-1has a FML1/NML1/FML2configuration in which FML120amade from Fe, Co, Ni, CoFe, CoB, FeB, CoFeB, CoFeNiB, or combination thereof, is deposited on the oxide tunnel barrier layer hereafter called the tunnel barrier19. The tunnel barrier is a metal oxide or oxynitride comprised of one or more oxide or oxynitride layers made from one or more of Si, Ba, Ca, La, Mn, V, Al, Ti, Zn, Hf, Mg, Ta, B, Cu, Cr. NML120bwith a thickness from 0.5 to 10 Angstroms is then deposited over the first FML120a. The NML1is a highly reactive metal with a relatively high re-sputtering rate and is typically a metal such as Mg, Al, B, Ca, Ba, Sr, Si, or C. Next a second FML220cis deposited over the NML120band is selected from one of Fe, Co, Ni, CoFe, CoB, FeB, CoFeB, CoFeNiB, or a combination thereof.

The deposition of FML2, which has a low resputtering rate compared with NML1, resputters a portion of NML1, which leads to a smoother top surface for both of NML1and FML2. As described in related application Ser. No. 14/949,232, a high resputtering rate for material A vs. material B results from one or both of a higher bond energy and a higher atomic number for material B.

The presence of NML1prior to the deposition of FML2inhibits the crystallization of FML2. As a result, FML220chas smaller grains and thinner grain boundaries. This reduces the diffusion of oxygen from the subsequently deposited capping oxide layer40to the FML2layer below it. Furthermore, NML120bis a more highly reactive material than the FML2layer. As a result, NML120battracts oxygen that has diffused from the top oxide layer40into the FML2and thereby prevents oxidation of the FML2.

Referring toFIG.3b, an alternative embodiment of the present disclosure is depicted wherein a reference layer10-1having a FML1/NML1/FML2configuration is formed between a seed layer2and tunnel barrier19. The seed layer may be comprised of one or more metals or alloys such as those disclosed in related patent application Ser. No. 14/949,232, or other materials used in the art.

The composition of the FML1, NML1, and FML2layers was described previously. In this case, the NML1layer serves to prevent oxidation of the FML2layer by attracting oxygen that diffuses into FML2from the tunnel barrier. Otherwise, all of the benefits associated previously described with forming a FML1/NML1/FML2stack apply to the reference layer10-1.

According to another embodiment shown inFIG.4a, the free layer laminated stack20-1described earlier is modified to form free layer20-2by sequentially depositing a NML2layer20dand FML3layer20eon the FML2layer to give a FML1/NML1/FML2/NML2/FML3configuration. NML2is selected from one of Mg, Al, B, Ca, Ba, Sr, Si or C, and FML3is made of one or more of Fe, Co, Ni, CoFe, CoFeB, CoB, FeB, and CoFeNiB. Capping layer40contacts a top surface of FML320e. When the capping layer is an oxide, an oxide/FML3interface induces or enhances PMA in the FML3layer.

InFIG.4b, the reference layer stack10-2inFIG.3bmay be enhanced to form an alternative embodiment where a FML1/NML1/FML2/NML2/FML3stack is formed between seed layer2and tunnel barrier19. In other words, additional layers NML2and FML3are sequentially deposited on FML2to give a reference layer having the same advantages as reference layer stack10-1. Again, the presence of an oxide tunnel barrier19adjoining a top surface of the upper FML layer induces or creates PMA in the upper FML (FML3) layer.

InFIG.5a, another embodiment of the present disclosure is depicted wherein the free layer laminated stack20-1described earlier is modified to form free layer stack20-3by depositing a plurality of “n−1” NML layers20b,20n-1, and “n” FML sub-layers20a,20c,20nin alternating fashion on the tunnel barrier19to give a FML1/NML1. . . FMLn−1/NMLn−1/FMLnconfiguration. Each NML is selected from one of Mg, Al, B, Ca, Ba, Sr, Si, or C, and each FML sub-layer is made of one or more of Fe, Co, Ni, CoFe, CoFeB, CoB, FeB, and CoFeNiB. Capping layer40contacts a top surface of FMLn20nand may enhance PMA therein by forming an oxide layer/FMLninterface.

InFIG.5b, the reference layer stack10-1inFIG.3bmay be enhanced to form an alternative embodiment to form reference layer stack10-3wherein a plurality of “n−1” NML layers and “n” FML sub-layers are deposited on seed layer2in alternating fashion to give a FML1/NML1. . . FMLn−1/NMLn−1/FMLnconfiguration. Each NML is selected from one of Mg, Al, B, Ca, Ba, Sr, Si or C, and each FML sub-layer is made of one or more of Fe, Co, CoFe, CoB, FeB, CoFeB, and CoFeNiB. Tunnel barrier19contacts a top surface of FMLn20nand enhances or induces PMA therein by forming an oxide layer/FMLninterface. Thus, the process of depositing a FML sub-layer on a NML is repeated a plurality of times to reduce crystallization in each successive NML, provide a smoothing effect on a top surface of each FML sub-layer, and prevent oxidation of the FMLnby reacting with oxygen that may diffuse from the tunnel barrier into the FMLn.

In all of the aforementioned embodiments, the present disclosure anticipates where one or more of the FMLnsub-layers may be comprised of a laminated stack such as (Co/X)mor (X/Co)mwhere m is from 1 to 30, and X is Pt, Pd, Ni, NiCo, Ni/Pt, or NiFe. In another aspect, CoFe or CoFeR may replace Co in the laminated stack where R is one of Mo, Mg, Ta, W, or Cr.

Referring toFIG.6, the present disclosure also encompasses an embodiment wherein a MTJ encompasses a free layer stack20-1,20-2, or20-3formed between two oxide layers. In the exemplary embodiment, the free layer contacts a top surface of the tunnel barrier19, and adjoins a bottom surface of an oxide capping layer40a.The oxide capping layer may be comprised of one or more oxide layers that are selected from the materials previously described with respect to tunnel barrier19. In a bottom spin valve configuration, seed layer2, reference layer11, the tunnel barrier, the free layer, and capping layer40aare sequentially formed on a substrate1that may be a bottom electrode in a MRAM, a bottom shield in a read head sensor, or a main pole layer in a STO device. The reference layer may be a synthetic antiparallel (SyAP) configuration wherein an antiferromagnetic coupling layer such as Ru is formed between a lower AP2 ferromagnetic layer contacting the seed layer and an upper AP1 ferromagnetic layer (not shown) contacting the tunnel barrier. One or both of the AP2 and AP1 layers may be one or more of Co, Fe, Ni, CoB, FeB, CoFe, CoFeB, or CoFeNiB, or a laminate such as (Co/X)mor (X/Co)mdescribed earlier. A top electrode50is formed on the capping layer and there may be an optional hard mask (not shown) such as MnPt between the capping layer and top electrode. In other embodiments, the top electrode is a top shield in a read head sensor or a trailing shield in a STO device.

Referring toFIG.7, an alternative bottom spin valve MTJ is shown wherein all of the layers are retained fromFIG.6except the oxide capping layer is replaced by a non-magnetic capping layer40b.In some embodiments, capping layer40bis one or more of Ru, W, Mo, NiCr, and Ta, including Ru/Ta and Ru/Ta/Ru configurations.

InFIG.8, a MTJ with a top spin valve configuration is shown according to an embodiment of the present disclosure. All layers are retained fromFIG.7except the positions of the free layer20-1(or20-2or20-3) and reference layer11are switched so that the seed layer2, free layer, tunnel barrier19, reference layer, and capping layer40bare sequentially formed on substrate1. The seed layer may be one or more of W, Ru, Ta, Mo, and NiCr.

InFIG.9, another top spin valve configuration of the present disclosure is depicted that represents a modification ofFIG.6where the free layer20-1(or20-2or20-3), tunnel barrier19, reference layer11, and capping layer40bare sequentially formed on an oxide layer15above an optional seed layer2on substrate1. Oxide layer15may be selected from one of the oxide materials previously mentioned with regard to oxide capping layer40a. As a result, there are two oxide layer/free layer interfaces at free layer top and bottom surfaces with tunnel barrier and oxide layer, respectively, to enhance PMA within the free layer.

Referring toFIG.10, the present disclosure also anticipates the reference layer10-1(or10-2or10-3) may be formed between two oxide layers in a top spin valve MTJ. In the exemplary embodiment, seed layer2, free layer21, tunnel barrier19, the reference layer, and oxide capping layer40aare sequentially formed on substrate1. Free layer21may be selected from the same materials as previously described with regard to reference layer11. In this case, the reference layer has a first interface with the oxide tunnel barrier and a second interface with the oxide capping layer to enhance PMA in the reference layer.

InFIG.11, another top spin valve MTJ is shown that retains all of the layers inFIG.10except the oxide cap layer is replaced with a non-magnetic capping layer40bdescribed previously.

Referring toFIG.12, a bottom spin valve MTJ is shown that retains all of the layers inFIG.11. However, the positions of the free layer21and reference layer10-1(or10-2or10-3) are switched such that the reference layer, tunnel barrier19, free layer, and capping layer40bare sequentially formed on seed layer2.

InFIG.13, another bottom spin valve embodiment is illustrated that is a modification of the MTJ inFIG.12where seed layer2is replaced by an oxide layer15such that the reference layer has two oxide interfaces to enhance PMA therein.

According to another embodiment shown inFIG.14, the non-magnetic material that attracts oxygen from a ferromagnetic layer (FML) may be embedded or doped within the FML22rather than forming a laminated stack of “n” FML sub-layers and “n−1” NMLs in earlier embodiments. Depending on the doped concentration in the FML, the non-magnetic material's efficiency in reacting with oxygen that may diffuse into the FML from an adjoining oxide layer may be less than in earlier embodiments involving the lamination of “n” FML sub-layers and “n−1” NMLs.

Moreover, the advantage of inhibiting crystallization in the FML may also be reduced compared with previous embodiments. Since a low resputtering rate material is not deposited on a high resputtering rate material in this embodiment, the smoothing effect of depositing a FML on a NML described earlier does not apply here.

Free layer22is doped or embedded with one or more of Mg, Al, Si, Ca, Sr, Ba, C, or B where the non-magnetic material has a concentration from 0.1 to 30 atomic % in the free layer. The non-magnetic material may be embedded in the free layer by a co-deposition process. The non-magnetic material has a magnetic dilution effect, which means that as the concentration of the non-magnetic element is increased in the free layer, the magnetic moment of the free layer is reduced. In the exemplary embodiment, an optional seed layer2, reference layer11, tunnel barrier19, the free layer, capping layer40are sequentially formed on the substrate1. Note that capping layer may comprise one or more non-magnetic metals as in40bor an oxide material as in40a.

InFIG.15, the present disclosure also encompasses a top spin valve embodiment where oxide layer15, free layer22, tunnel barrier19, reference layer11, and capping layer40bare sequentially formed on substrate1.

FIG.16represents a modification of the top spin valve MTJ inFIG.15wherein doped free layer22is replaced by free layer21described earlier while a reference layer12is employed that is doped with one or more of Mg, Al, Si, Ca, Sr, C, Ba or B. Thus, the MTJ stack has a seed layer/free layer/tunnel barrier/doped reference layer/capping layer configuration.

Referring toFIG.17, a bottom spin valve MTJ is shown where oxide layer15, doped reference layer12, tunnel barrier19, free layer21, and cap layer40are sequentially formed on substrate1.

The present disclosure also anticipates a method of forming a MTJ wherein a ferromagnetic layer comprises a laminated stack of FML sub-layers and NML layers as shown inFIGS.3a-5b.InFIG.18, an intermediate step is shown during the fabrication of MTJ60that is formed by sequentially forming a seed layer2, reference layer11, tunnel barrier19, free layer20-1(or20-2or20-3), and oxide capping layer40aon substrate1. After all of the layers in the MTJ are formed by a conventional method, a photoresist layer55is coated and patterned on a top surface of the cap layer40ato form sidewall55swhich is transferred through MTJ60by a subsequent ion beam etch (IBE) to form sidewall60son the MTJ.

InFIG.19, a dielectric layer70such as silicon oxide, silicon nitride or alumina is deposited to a level above the capping layer, and then a chemical mechanical polish (CMP) process is performed to remove the photoresist layer and form a top surface70tthat is coplanar with a top surface40tof the capping layer40a.

Thereafter, inFIG.20, the top electrode50is formed on the dielectric layer70and capping layer40aby a method well known to those skilled in the art.

FIGS.21,22, and23show the magnetic hysteresis loop for various stacks that have been annealed at 330° C. for thirty minutes using Kerr magnetometry. Magnetization is measured for fields between +1500 and −1500 Oe. Branches measured for increasing and decreasing fields are indicated as dashed and solid lines, respectively. The Kerr magnetization signal is proportional to the perpendicular magnetization. The thickness, t, is the total thickness of one or more FML. The figures of merit on these measurements are the squareness of the loops and the value of the coercive field.

The data shows the addition of one NML (FIG.22) or two NML (FIG.23) yields improved coercivity over a wider range in thicknesses. In particular, improved PMA is achieved down to layers thinner than 12 Angstroms. This is contrary to the prior art without NML shown inFIG.21for which the FML becomes discontinuous and loses its PMA below 12 Angstroms.

Another benefit is improved thermal budget in a magnetic tunnel junction having a free layer formed according to an embodiment described herein.FIGS.24-25show magnetic hysteresis loops for a stack without NML and one with two NMLs. Both stacks were annealed at 400° C. for 5 hours. The magnetic properties of the stack without NML are strongly degraded, as indicated by the reduction of squareness and coercive field. The magnetic signal is strongly reduced and vanishes for layers thinner than 14 Angstroms. Thicker layer do not exhibit square loops characteristic of perpendicular magnetization. By contrast, the stack having 2 NMLs retains square loops and non-zero coercive fields. This indicates that the stack retains good PMA after 5 hour annealing at 400° C.