Perpendicularly magnetized ferromagnetic layers having an oxide interface allowing for improved control of oxidation

Abstract

An improved magnetic tunnel junction with two oxide interfaces on each side of a ferromagnetic layer (FML) leads to higher PMA in the FML. The novel stack structure allows improved control during oxidation of the top oxide layer. This is achieved by the use of a FML with a multiplicity of ferromagnetic sub-layers deposited in alternating sequence with one or more non-magnetic layers. The use of non-magnetic layers each with a thickness of 0.5 to 10 Angstroms and with a high resputtering rate provides a smoother FML top surface, inhibits crystallization of the FML sub-layers, and reacts with oxygen to prevent detrimental oxidation of the adjoining ferromagnetic sub-layers. The FML can function as a free or reference layer in an MTJ. In an alternative embodiment, the non-magnetic material such as Mg, Al, Si, Ca, Sr, Ba, and B is embedded by co-deposition or doped in the FML layer.

Claims

1. A method, comprising: forming a magnetic tunnel junction (MTJ) stack, wherein the forming the MTJ stack includes: forming a first electrode over a substrate; depositing a reference layer over the first electrode; depositing an oxide tunnel barrier layer interfacing the reference layer; forming a free layer stack (FL) adjacent the oxide tunnel barrier layer; and forming an oxide cap layer such that a first surface of the FL interfaces the oxide tunnel barrier layer and an opposing surface of the FL interfaces the oxide cap layer, wherein the forming of the FL includes: depositing a first ferromagnetic layer; depositing a first non-magnetic layer on the first ferromagnetic layer, the first non-magnetic layer having a first resputtering rate; and depositing a second ferromagnetic layer on the first non-magnetic layer, the second ferromagnetic layer having a second resputtering rate less than the first resputtering rate, wherein the depositing of the second ferromagnetic layer causes a resputtering of a portion of the first non-magnetic layer.

2. The method of claim 1, wherein the depositing the first non-magnetic layer deposits a layer of a material selected from the group consisting of Mg, Al, Si, Ca, C, Sr, Ba, B, and combinations thereof.

3. The method of claim 1, wherein a thickness of the first non-magnetic layer is in a range from about 0.5 Angstroms to about 10 Angstroms.

4. The method of claim 1, wherein the depositing the first non-magnetic layer deposits a layer of Sr, Ba or combinations thereof.

5. The method of claim 1, wherein the depositing the reference layer includes: depositing a third ferromagnetic layer over the substrate; depositing a second non-magnetic layer directly on the third ferromagnetic layer; and depositing a fourth ferromagnetic layer directly on the second non-magnetic layer.

6. The method of claim 5, wherein the oxide tunnel barrier layer is formed directly on the fourth ferromagnetic layer.

7. A method, comprising: forming a reference layer of a magnetic tunnel junction (MTJ) stack over a substrate; forming a tunnel barrier layer of the MTJ stack on the reference layer; forming a free layer of the MTJ stack on the tunnel barrier layer, the forming of the free layer including: depositing a first ferromagnetic layer directly on the tunnel barrier layer; depositing a non-magnetic layer of a first reactive material, the first reactive material selected from the group consisting of Mg, Al, B, Ca, Ba, Sr, Si or C on the first ferromagnetic layer, the non-magnetic layer having a first resputtering rate; and depositing a second ferromagnetic layer on the non-magnetic layer, the second ferromagnetic layer having a second resputtering rate less than the first resputtering rate, wherein the depositing of the second ferromagnetic layer causes a resputtering of a portion of the non-magnetic layer; and forming a capping oxide layer of the MTJ stack directly on the second ferromagnetic layer, and diffusing oxygen from the capping oxide layer to the second ferromagnetic layer; and attracting the diffused oxygen in the second ferromagnetic layer to the first reactive material due to the first reactive material being more highly reactive to oxygen than a composition of the second ferromagnetic layer.

8. The method of claim 7, further comprising: inducing a perpendicular magnetic anisotropy in the free layer.

9. The method of claim 7, wherein the capping oxide layer includes an oxide of a material selected from the group consisting of Ru, W, Mo, NiCr, Ta, and combinations thereof.

10. The method of claim 7, wherein a thickness of the non-magnetic layer is in a range from about 0.5 Angstroms to about 10 Angstroms.

11. The method of claim 7, wherein the substrate includes a material selected from the group consisting of W, Ru, Ta, Mo, NiCr, and combinations thereof.

12. The method of claim 7, wherein each of the first ferromagnetic layer and the second ferromagnetic layer includes a material selected from the group consisting of Fe, Co, CoFe, CoB, FeB, CoFeB, CoFeNiB, and combinations thereof.

13. The method of claim 7, further comprising: patterning the MTJ stack using a patterned photoresist layer as an etch mask, the patterning forming a patterned MTJ stack; depositing a dielectric layer that physically contacts sidewalls and a top surface of the patterned MTJ stack; and performing a planarizing process such that a top surface of the dielectric layer is substantially aligned with the top surface of the patterned MTJ stack.

14. A method, comprising: depositing a tunnel barrier layer on a reference layer; forming a free layer, the free layer being of a magnetic tunnel junction (MTJ) stack, the free layer comprising: depositing of a first ferromagnetic layer of the free layer directly on the tunnel barrier layer; depositing a first non-magnetic layer of the free layer directly on the first ferromagnetic layer, the first non-magnetic layer having a first resputtering rate; depositing a second ferromagnetic layer of the free layer on the first non-magnetic layer, wherein the second ferromagnetic layer has a second resputtering rate less than the first resputtering rate; depositing a second non-magnetic layer of the free layer on the second ferromagnetic layer; and depositing a third ferromagnetic layer of the free layer directly on the second non-magnetic layer, wherein depositing the first non-magnetic layer and depositing the second non-magnetic layer deposit a layer of Mg, Al, B, Ca, Ba, Sr, Si, or C; and depositing a capping layer of the MTJ stack directly on a top surface of the third ferromagnetic layer of the free layer, wherein the reference layer, tunnel barrier layer, and free layer form a magnetic structure.

15. The method of claim 14, further comprising: forming a patterned photoresist layer on a top surface of the capping layer of the MTJ stack; patterning the magnetic structure using the patterned photoresist layer as an etch mask, the patterning forming a patterned magnetic structure; depositing a dielectric layer that adjoins sidewalls and a top surface of the patterned magnetic structure; and performing a planarizing process to remove the patterned photoresist layer, wherein the planarizing process causes a top surface of the dielectric layer to be substantially aligned with the top surface of the patterned magnetic structure.

16. The method of claim 15, wherein the dielectric layer includes a material selected from the group consisting of alumina, silicon dioxide, silicon nitride, and combinations thereof.

17. The method of claim 15, wherein the patterning of the magnetic structure includes ion beam etching, reactive ion etching, or a combination thereof.

18. The method of claim 14, wherein the capping layer of the MTJ stack includes a material selected from the group consisting of Si, Ba, Ca, La, Mn, V, Al, Ti, Zn, Hf, Mg, Ta, B, Cu, Cr, and combinations thereof.

19. The method of claim 14, wherein the first non-magnetic layer of the MTJ stack includes a material selected from the group consisting of Mg, Al, Si, Ca, C, Sr, Ba, B, and combinations thereof.

20. The method of claim 14, wherein depositing the second ferromagnetic layer is depositing the second ferromagnetic layer directly on the first non-magnetic layer; and wherein the depositing the second non-magnetic layer is depositing the second non-magnetic layer directly on the second ferromagnetic layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a notional view of the prior art for a Magnetic Tunnel Junction having a bottom spin valve configuration that is utilized in an MRAM, spin transfer oscillator (STO), or read/write head.

(2) FIG. 2 is an MTJ in a prior art top spin valve configuration with the reference layer above the tunnel barrier and functionally equivalent to FIG. 1.

(3) FIG. 3a is cross-sectional view of a free layer formed between a tunnel barrier and an oxide capping layer in a MTJ with a bottom spin valve configuration wherein the free layer is a laminate comprised of a non-magnetic layer (NML) formed between two ferromagnetic layers (FMLs) according to an embodiment of the present disclosure.

(4) FIG. 3b is a cross-sectional view of a reference layer (RL) formed between a seed layer and a tunnel barrier in a MTJ with a bottom spin valve configuration wherein the RL is a laminate comprised of a non-magnetic layer (NML) formed between two ferromagnetic sub-layers (FML.sub.1 and FML.sub.2) according to an embodiment of the present disclosure.

(5) FIGS. 4a and 5a represent modifications of the FIG. 3a embodiment wherein the free layer has a plurality of n non-magnetic layers (NMLs) in the laminated stack of NMLs and n+1 FML sub-layers.

(6) FIGS. 4b and 5b represent modifications of the FIG. 3b embodiment wherein the reference layer has a plurality of n NMLs in the laminated stack of NMLs and n+1 FML sub-layers.

(7) FIG. 6 and FIG. 9 are cross-sectional views of a MTJ with a bottom spin valve and top spin valve configuration, respectively, wherein a free layer having a laminated stack of NMLs and FML sub-layers is formed between two oxide layers according to an embodiment of the present disclosure.

(8) FIGS. 7-8 are cross-sectional views of a MTJ with a bottom spin valve and top spin valve configuration, respectively, wherein the free layer has a laminated stack of NMLs and FML sub-layers formed between an oxide layer and a non-magnetic layer according to an embodiment of the present disclosure.

(9) FIG. 10 and FIG. 13 are cross-sectional views of a MTJ with a top spin valve and bottom spin valve configuration, respectively, wherein a reference layer having a laminated stack of NMLs and FML sub-layers is formed between two oxide layers according to an embodiment of the present disclosure.

(10) FIGS. 11-12 are cross-sectional views of a MTJ with a top spin valve and bottom spin valve configuration, respectively, wherein the reference layer has a laminated stack of NMLs and FML sub-layers formed between an oxide layer and a non-magnetic layer according to an embodiment of the present disclosure.

(11) FIGS. 14-15 are cross-sectional views of a MTJ with a bottom spin valve and top spin valve configuration, respectively, wherein the free layer is doped with a non-magnetic material.

(12) FIGS. 16-17 are cross-sectional views of a MTJ with a top spin valve and bottom spin valve configuration, respectively, wherein the reference layer is doped with a non-magnetic material.

(13) FIGS. 18-20 show a sequence of process steps during the fabrication of a MTJ with a free layer formed according to an embodiment of the present disclosure.

(14) FIG. 21 shows a plot of the magnetization vs. magnetic field for various free layer thicknesses t, in Angstroms, of the prior art OL/FML/OL stack structure from FIG. 1.

(15) FIG. 22 shows a plot of magnetization vs. magnetic field for various free layer thicknesses t, in Angstroms in a OL/FML.sub.1(t.sub.1)/NML/FML.sub.2(t.sub.2)/OL stack formed according to an embodiment of the present disclosure where t.sub.1=t.sub.2, and t=t.sub.1+t.sub.2.

(16) FIG. 23 shows a plot of magnetization vs. magnetic field for various free layer thicknesses t, in Angstroms, in a OL/FML.sub.1 (4 Angstroms)/NML.sub.1/FML.sub.2(t1)/NML.sub.2/FML.sub.3 (t.sub.2)/OL stack formed according to another embodiment of the present disclosure where t.sub.1/t.sub.2=3/4 and t=(4+t.sub.1+t.sub.2) Angstroms.

(17) FIG. 24 shows a plot of the degraded magnetization vs. magnetic field for a free layer stack without an NML layer that has been annealed at 400 C. for five hours and illustrates that the range of FML thickness does not exhibit the square loop characteristic of PMA.

(18) FIG. 25 shows a plot of the magnetization vs, magnetic field for a free layer stack with two NMLs that has been annealed at 400 C. for five hours.

DETAILED DESCRIPTION

(19) 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.

(20) 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 (M.sub.s) reducing (moment diluting) layer formed between two magnetic sub-layers (FM.sub.1 and FM.sub.2). The FM, layer has a surface that forms a first interface with the tunnel barrier while the FM.sub.2 layer 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 FM.sub.2 layer.

(21) 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.

(22) 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 (n1) 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 OL.sub.1/FML/OL.sub.2 scheme where FML has a FML.sub.1/NML/FML.sub.2 configuration. The role of the NMLs is threefold and thereby provides three advantages in performance compared with the prior art Magnetic Tunnel Junctions in FIG. 1 and FIG. 2.

(23) First, the resputtering of the NML having a relatively high resputtering rate during the deposition of FML.sub.2 in a FML.sub.1/NML/FML.sub.2 configuration leads to a smoother FML.sub.2 ferromagnetic layer. In other embodiments, where a FML.sub.n layer is deposited on a NML.sub.n1 layer, a similar smoothing effect is realized for the top surface of the FML.sub.n layer.

(24) Secondly, the presence of an NML layer inhibits the crystallization of the FML.sub.2 layer, or in more general terms, a NML.sub.n1 layer inhibits crystallization in the overlying FML.sub.n layer. As a result, the FML.sub.2 layer (and FML.sub.n layer) has smaller grains and thinner grain boundaries. This reduces the diffusion of oxygen from the top oxide layer OL.sub.2 to the FML.sub.2 layer below it.

(25) Lastly, the NML is a more highly reactive material than the FML sub-layers. Therefore it attracts oxygen that has diffused from the OL.sub.2 into the FML.sub.2. As a result, the FML ferromagnetic sub-layers, and especially the upper FML.sub.n sub-layer in a stack with n FML sub-layers and n1 NML layers, are less oxidized than in the prior art which leads to a better magnetoresistive ratio and greater FML thermal stability.

(26) According to one embodiment of the present disclosure shown in FIG. 3a, the free layer 20-1 has a FML.sub.1/NML.sub.1/FML.sub.2 configuration in which FML.sub.1 20a made from Fe, Co, Ni, CoFe, CoB, FeB, CoFeB, CoFeNiB, or combination thereof, is deposited on the oxide tunnel barrier layer hereafter called the tunnel barrier 19. 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. NML.sub.1 20b with a thickness from 0.5 to 10 Angstroms is then deposited over the first FML.sub.1 20a. The NML.sub.1 is 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 FML.sub.2 20c is deposited over the NML.sub.1 20b and is selected from one of Fe, Co, Ni, CoFe, CoB, FeB, CoFeB, CoFeNiB, or a combination thereof.

(27) The deposition of FML.sub.2, which has a low resputtering rate compared with NML.sub.1, resputters a portion of NML.sub.1, which leads to a smoother top surface for both of NML.sub.1 and FML.sub.2. 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.

(28) The presence of NML.sub.1 prior to the deposition of FML.sub.2 inhibits the crystallization of FML.sub.2. As a result, FML.sub.2 20c has smaller grains and thinner grain boundaries. This reduces the diffusion of oxygen from the subsequently deposited capping oxide layer 40 to the FML.sub.2 layer below it. Furthermore, NML.sub.1 20b is a more highly reactive material than the FML.sub.2 layer. As a result, NML.sub.1 20b attracts oxygen that has diffused from the top oxide layer 40 into the FML.sub.2 and thereby prevents oxidation of the FML.sub.2.

(29) Referring to FIG. 3b, an alternative embodiment of the present disclosure is depicted wherein a reference layer 10-1 having a FML.sub.1/NML.sub.1/FML.sub.2 configuration is formed between a seed layer 2 and tunnel barrier 19. 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.

(30) The composition of the FML.sub.1, NML.sub.1, and FML.sub.2 layers was described previously. In this case, the NML.sub.1 layer serves to prevent oxidation of the FML.sub.2 layer by attracting oxygen that diffuses into FML.sub.2 from the tunnel barrier. Otherwise, all of the benefits associated previously described with forming a FML.sub.1/NML.sub.1/FML.sub.2 stack apply to the reference layer 10-1.

(31) According to another embodiment shown in FIG. 4a, the free layer laminated stack 20-1 described earlier is modified to form free layer 20-2 by sequentially depositing a NML.sub.2 layer 20d and FML.sub.3 layer 20e on the FML.sub.2 layer to give a FML.sub.1/NML.sub.1/FML.sub.2/NML.sub.2/FML.sub.3 configuration. NML.sub.2 is selected from one of Mg, Al, B, Ca, Ba, Sr, Si or C, and FML.sub.3 is made of one or more of Fe, Co, Ni, CoFe, CoFeB, CoB, FeB, and CoFeNiB. Capping layer 40 contacts a top surface of FML.sub.3 20e. When the capping layer is an oxide, an oxide/FML.sub.3 interface induces or enhances PMA in the FML.sub.3 layer.

(32) In FIG. 4b, the reference layer stack 10-2 in FIG. 3b may be enhanced to form an alternative embodiment where a FML.sub.1/NML.sub.1/FML.sub.2/NML.sub.2/FML.sub.3 stack is formed between seed layer 2 and tunnel barrier 19. In other words, additional layers NML.sub.2 and FML.sub.3 are sequentially deposited on FML.sub.2 to give a reference layer having the same advantages as reference layer stack 10-1. Again, the presence of an oxide tunnel barrier 19 adjoining a top surface of the upper FML layer induces or creates PMA in the upper FML (FML.sub.3) layer.

(33) In FIG. 5a, another embodiment of the present disclosure is depicted wherein the free layer laminated stack 20-1 described earlier is modified to form free layer stack 20-3 by depositing a plurality of n1 NML layers 20b, 20n-1, and n FML sub-layers 20a, 20c, 20n in alternating fashion on the tunnel barrier 19 to give a FML.sub.1/NML.sub.1 . . . FML.sub.n1/NML.sub.n1/FML.sub.n configuration. 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 layer 40 contacts a top surface of FML.sub.n 20n and may enhance PMA therein by forming an oxide layer/FML.sub.n interface.

(34) In FIG. 5b, the reference layer stack 10-1 in FIG. 3b may be enhanced to form an alternative embodiment to form reference layer stack 10-3 wherein a plurality of n1 NML layers and n FML sub-layers are deposited on seed layer 2 in alternating fashion to give a FML.sub.1/NML.sub.1 . . . FML.sub.n1/NML.sub.n1/FML.sub.n configuration. 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 barrier 19 contacts a top surface of FML.sub.n 20n and enhances or induces PMA therein by forming an oxide layer/FML.sub.n interface. 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 FML.sub.n by reacting with oxygen that may diffuse from the tunnel barrier into the FML.sub.n.

(35) In all of the aforementioned embodiments, the present disclosure anticipates where one or more of the FML.sub.n sub-layers may be comprised of a laminated stack such as (Co/X).sub.m or (X/Co).sub.m where 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.

(36) Referring to FIG. 6, the present disclosure also encompasses an embodiment wherein a MTJ encompasses a free layer stack 20-1, 20-2, or 20-3 formed between two oxide layers. In the exemplary embodiment, the free layer contacts a top surface of the tunnel barrier 19, and adjoins a bottom surface of an oxide capping layer 40a. 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 barrier 19. In a bottom spin valve configuration, seed layer 2, reference layer 11, the tunnel barrier, the free layer, and capping layer 40a are sequentially formed on a substrate 1 that 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).sub.m or (X/Co).sub.m described earlier. A top electrode 50 is 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.

(37) Referring to FIG. 7, an alternative bottom spin valve MTJ is shown wherein all of the layers are retained from FIG. 6 except the oxide capping layer is replaced by a non-magnetic capping layer 40b. In some embodiments, capping layer 40b is one or more of Ru, W, Mo, NiCr, and Ta, including Ru/Ta and Ru/Ta/Ru configurations.

(38) In FIG. 8, a MTJ with a top spin valve configuration is shown according to an embodiment of the present disclosure. All layers are retained from FIG. 7 except the positions of the free layer 20-1 (or 20-2 or 20-3) and reference layer 11 are switched so that the seed layer 2, free layer, tunnel barrier 19, reference layer, and capping layer 40b are sequentially formed on substrate 1. The seed layer may be one or more of W, Ru, Ta, Mo, and NiCr.

(39) In FIG. 9, another top spin valve configuration of the present disclosure is depicted that represents a modification of FIG. 6 where the free layer 20-1 (or 20-2 or 20-3), tunnel barrier 19, reference layer 11, and capping layer 40b are sequentially formed on an oxide layer 15 above an optional seed layer 2 on substrate 1. Oxide layer 15 may be selected from one of the oxide materials previously mentioned with regard to oxide capping layer 40a. 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.

(40) Referring to FIG. 10, the present disclosure also anticipates the reference layer 10-1 (or 10-2 or 10-3) may be formed between two oxide layers in a top spin valve MTJ. In the exemplary embodiment, seed layer 2, free layer 21, tunnel barrier 19, the reference layer, and oxide capping layer 40a are sequentially formed on substrate 1. Free layer 21 may be selected from the same materials as previously described with regard to reference layer 11. 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.

(41) In FIG. 11, another top spin valve MTJ is shown that retains all of the layers in FIG. 10 except the oxide cap layer is replaced with a non-magnetic capping layer 40b described previously.

(42) Referring to FIG. 12, a bottom spin valve MTJ is shown that retains all of the layers in FIG. 11. However, the positions of the free layer 21 and reference layer 10-1 (or 10-2 or 10-3) are switched such that the reference layer, tunnel barrier 19, free layer, and capping layer 40b are sequentially formed on seed layer 2.

(43) In FIG. 13, another bottom spin valve embodiment is illustrated that is a modification of the MTJ in FIG. 12 where seed layer 2 is replaced by an oxide layer 15 such that the reference layer has two oxide interfaces to enhance PMA therein.

(44) According to another embodiment shown in FIG. 14, the non-magnetic material that attracts oxygen from a ferromagnetic layer (FML) may be embedded or doped within the FML 22 rather than forming a laminated stack of n FML sub-layers and n1 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 n1 NMLs.

(45) 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.

(46) Free layer 22 is 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 layer 2, reference layer 11, tunnel barrier 19, the free layer, capping layer 40 are sequentially formed on the substrate 1. Note that capping layer may comprise one or more non-magnetic metals as in 40b or an oxide material as in 40a.

(47) In FIG. 15, the present disclosure also encompasses a top spin valve embodiment where oxide layer 15, free layer 22, tunnel barrier 19, reference layer 11, and capping layer 40b are sequentially formed on substrate 1.

(48) FIG. 16 represents a modification of the top spin valve MTJ in FIG. 15 wherein doped free layer 22 is replaced by free layer 21 described earlier while a reference layer 12 is 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.

(49) Referring to FIG. 17, a bottom spin valve MTJ is shown where oxide layer 15, doped reference layer 12, tunnel barrier 19, free layer 21, and cap layer 40 are sequentially formed on substrate 1.

(50) 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 in FIGS. 3a-5b. In FIG. 18, an intermediate step is shown during the fabrication of MTJ 60 that is formed by sequentially forming a seed layer 2, reference layer 11, tunnel barrier 19, free layer 20-1 (or 20-2 or 20-3), and oxide capping layer 40a on substrate 1. After all of the layers in the MTJ are formed by a conventional method, a photoresist layer 55 is coated and patterned on a top surface of the cap layer 40a to form sidewall 55s which is transferred through MTJ 60 by a subsequent ion beam etch (IBE) to form sidewall 60s on the MTJ.

(51) In FIG. 19, a dielectric layer 70 such 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 surface 70t that is coplanar with a top surface 40t of the capping layer 40a.

(52) Thereafter, in FIG. 20, the top electrode 50 is formed on the dielectric layer 70 and capping layer 40a by a method well known to those skilled in the art.

(53) FIGS. 21, 22, and 23 show 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.

(54) 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 in FIG. 21 for which the FML becomes discontinuous and loses its PMA below 12 Angstroms.

(55) Another benefit is improved thermal budget in a magnetic tunnel junction having a free layer formed according to an embodiment described herein. FIGS. 24-25 show 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.