Low switching current MTJ element for ultra-high STT-RAM and a method for making the same

• US 20090256220A1
• Filed: 04/09/2008
• Published: 10/15/2009
• Est. Priority Date: 04/09/2008
• Status: Active Grant

• Current Assignee: Taiwan Semiconductor Manufacturing Company Limited
• Original Assignee: MagiQ Technologies, Incorporated
• Inventors: Tong, Ru-Ying, Horng, Cheng T., Guo, Yimin
• Granted Patent: US 7,948,044 B2
• Time in Patent Office: Days
• US Class Current: 257/421
• CPC Class Codes: H10N 50/01 Manufacture or treatment
• CPC Class Codes: H10N 50/10 Magnetoresistive devices
• CPC Class Codes: Y10S 977/883 Fluidic self-assembly, FSA

Abstract

A STT-RAM MTJ that minimizes spin-transfer magnetization switching current (Jc) while achieving a high dR/R is disclosed. The MTJ has a MgO tunnel barrier formed by natural oxidation to achieve a low RA, and a CoFeB/FeSiO/CoFeB composite free layer with a middle nanocurrent channel layer to minimize Jc₀. There is a thin Ru capping layer for a spin scattering effect. The reference layer has a shape anisotropy and Hc substantially greater than that of the free layer to establish a “self-pinned” state. The free layer, capping layer and hard mask are formed in an upper section of a nanopillar that has an area substantially less than a lower pedestal section which includes a bottom electrode, reference layer, seed layer, and tunnel barrier layer. The reference layer is comprised of an enhanced damping constant material that may be an insertion layer, and the free layer has a low damping constant.

Claims

1. A MTJ nanopillar structure in a STT-RAM device, comprising:

(a) a first stack of layers with sidewalls formed on a bottom electrode and having a thickness in a direction perpendicular to said bottom electrode and a first shape with a first area in a plane orthogonal to said thickness direction, comprising;

(1) a seed layer on the bottom electrode;

(2) a composite reference magnetic layer formed on the seed layer and comprised of at least one magnetic layer and an insertion layer that induces a high damping constant in said at least one magnetic layer, and wherein the at least one magnetic layer with a high damping constant is a reference layer in a “

self-pinned”

state with a first thickness and a magnetization direction along an easy axis direction in a plane orthogonal to said thickness direction; and

(3) a tunnel barrier layer on the “

self-pinned”

reference magnetic layer

(b) a second stack of layers with sidewalls formed on the first stack of layers and having a thickness in a direction perpendicular to said bottom electrode and a second shape with a second area substantially less than the first area in a plane orthogonal to said thickness direction;

comprising

(1) a composite free layer having a FM1/NCC/FM2 configuration wherein NCC is a nanocurrent channel layer comprised of R(Si) grains formed in an oxide or nitride insulator matrix and R is Fe, Ni, Co, or B, and the FM1 and FM2 layers are magnetic layers having a low damping constant and a combined thickness substantially less than the first thickness of the reference layer;

(2) a Ru capping layer on the composite free layer; and

(3) a hard mask formed on the capping layer.

2. The MTJ nanopillar structure of claim 1 wherein the bottom electrode is comprised of Ta/Ru/Ta or Ta and has the same first shape as the first stack of layers.

3. The MTJ nanopillar structure of claim 1 wherein the composite reference layer is a comprised of a lower insertion layer made of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru formed on the seed layer, and an upper “

self-pinned”

reference layer comprised of Co, Fe, Ni, B or an alloy thereof that contacts said tunnel barrier layer.

4. The MTJ nanopillar structure of claim 1 wherein the composite reference layer has a M₁/X/M₂ configuration where M₁ and M₂ are magnetic layers made of Co, Fe, Ni, B, or an alloy thereof, X is an insertion layer comprised of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru that induces an enhanced damping constant in the M₁ and M₂ layers, and the M₂ magnetic layer is a “

self-pinned”

reference layer.

5. The MTJ nanopillar structure of claim 1 wherein the composite reference layer is a synthetic anti-ferromagnetic (SyAF) layer represented by a M₁/coupling/X/M₂ configuration where M₁ and M₂ are magnetic layers made of Co, Fe, Ni, B, or an alloy thereof, X is an insertion layer comprised of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru that induces an enhanced damping constant in the M₂ layer, the coupling layer is a non-magnetic layer made of Ru, Rh, or Ir, and the M₂ layer is a “

self-pinned”

reference layer.

6. The MTJ nanopillar structure of claim 1 wherein the NCC layer is comprised of FeSiO or FeSiN and the FM1 and FM2 magnetic layers are comprised of CoFeB, CoFe, FeB, or Fe.

7. The MTJ nanopillar structure of claim 1 wherein the first area is about 400% to 900% greater than the second area and the first thickness is at least two times larger than the combined thickness of the FM1 and FM2 layers.

8. The MTJ nanopillar structure of claim 1 wherein the Ru capping layer has a thickness from about 5 to 15 Angstroms and the hard mask is comprised of Ta, TaN, Ti, or TiN.

9. The MTJ nanopillar structure of claim 1 further comprised of a passivation layer formed along the sidewalls of the second stack of layers.

10. A MTJ nanopillar structure in a STT-RAM device, comprising:

(a) a first stack of layers formed on a bottom electrode and having a thickness in a direction perpendicular to said bottom electrode and a first shape with a first area in a plane orthogonal to said thickness direction, comprising;

(1) a seed layer formed on said bottom electrode;

(2) a composite SyAF reference magnetic layer formed on the seed layer and comprised of a AP1 and AP2 magnetic layers, an anti-ferromagnetic coupling layer, and an insertion layer that induces a high damping constant in at least one of the AP1 and AP2 magnetic layers, and wherein the AP1 magnetic layer contacts an overlying tunnel barrier layer and is in a “

self-pinned”

state with a first thickness and a magnetization direction along an easy axis direction in a plane orthogonal to said thickness direction; and

(3) a tunnel barrier layer on the “

self-pinned”

reference magnetic layer

(b) a second stack of layers with sidewalls formed on the first stack of layers and having a thickness in a direction perpendicular to said bottom electrode and a second shape with a second area in a plane orthogonal to said thickness direction, said first area is from 0% to about 900% greater than the second area;

comprising

(1) a composite free layer having a FM1/NCC/FM2 configuration wherein NCC is a nanocurrent channel layer comprised of R(Si) grains formed in an oxide or nitride insulator matrix and R is Fe, Ni, Co, or B, and the FM1 and FM2 layers are magnetic layers having a low damping constant and a combined thickness substantially less than the first thickness of the reference layer;

(2) a Ru capping layer on the composite free layer; and

(3) a hard mask formed on the capping layer.

11. The MTJ nanopillar structure of claim 10 wherein the bottom electrode is comprised of Ta or Ta/Ru/Ta and has the same first shape as the first stack of layers.

12. The MTJ nanopillar structure of claim 10 wherein the composite SyAF reference layer has a X/M₁/coupling/M₂ configuration where M₁is an AP2 layer, M₂ is a “

self-pinned”

AP1 reference layer, M₁ and M₂ are comprised of Co, Fe, Ni, B, or an alloy thereof, X is an insertion layer comprised of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru that induces an enhanced damping constant in the M₁ layer, and the coupling layer is a non-magnetic layer comprised of Ru, Rh, or Ir.

13. The MTJ nanopillar structure of claim 10 wherein the composite SyAF reference layer has a M₁/coupling/X/M₂ configuration where M₁is an AP2 layer, M₂ is a “

self-pinned”

AP1 reference layer, M₁ and M₂ are comprised of Co, Fe, Ni, B, or an alloy thereof, X is an insertion layer comprised of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru that induces an enhanced damping constant in the M₂ layer, and the coupling layer is a non-magnetic layer comprised of Ru, Rh, or Ir.

14. The MTJ nanopillar structure of claim 10 wherein the NCC layer is made of FeSiO or FeSiN and the FM1 and FM2 magnetic layers are comprised of CoFeB, CoFe, FeB, or Fe.

15. The MTJ nanopillar structure of claim 10 wherein the Ru capping layer has a thickness from about 5 to 15 Angstroms and the hard mask is comprised of Ta or TaN, Ti, or TiN.

16. The MTJ nanopillar structure of claim 10 further comprised of a passivation layer formed along the sidewalls of the second stack of layers.

17. A method of forming a STT-RAM MTJ nanopillar on a substrate wherein said substrate has a via stud formed in a first dielectric layer and an overlying bottom electrode layer that contacts said via stud and has a planar top surface, comprising:

(a) sequentially forming a MTJ stack of layers comprised of a seed layer, composite reference layer having an upper “

self-pinned”

magnetic layer with a thickness and an insertion layer that induces a high damping constant in the “

self-pinned”

magnetic layer, a tunnel barrier layer, a composite free layer having a FM1/RSiO/FM2 configuration, a capping layer, and a hard mask on the bottom electrode layer wherein said RSiO layer is comprised of R(Si) grains in a silicon oxide matrix and FM1 and FM2 are magnetic layers;

(b) patterning the composite free layer, capping layer, and hard mask to form an upper section of said nanopillar having sidewalls, a certain thickness in a direction perpendicular to said planar top surface and a first shape with a first area in a plane parallel to said planar top surface, said patterning process exposes a portion of the tunnel barrier layer;

(c) depositing a passivation layer on said upper section of the nanopillar, along said sidewalls, and on the exposed portion of the tunnel barrier layer; and

(d) patterning the tunnel barrier layer, composite reference layer, and seed layer to form a lower pedestal section having sidewalls and a second shape with a second area in a plane parallel to said planar top surface, said lower pedestal section and the upper section are formed above said via stud, and said passivation layer remains only along the sidewalls in the second stack of layers.

18. The method of claim 17 further comprising:

(a) forming a second dielectric layer along the sidewalls of the lower pedestal section and adjacent to the passivation layer, said second dielectric layer is planarized to become coplanar with the hard mask; and

(b) forming a bit line on the second dielectric layer and on the upper section of the nanopillar that contacts the hard mask layer.

19. The method of claim 17 wherein the second area is from about 400% to 900% greater than the first area and the thickness of the “

self-pinned”

magnetic layer is at least 2 times greater than a combined thickness of the FM1 and FM2 layers which provides a Hc and shape anisotropy in the self-pinned magnetic layer that is substantially greater than that in the free layer.

20. The method of claim 17 wherein the second area is from 0% to about 20% greater than the first area and the thickness of the “

self-pinned”

magnetic layer is from 2 to 3 times greater than a combined thickness of the FM1 and FM2 layers.

21. The method of claim 17 wherein the tunnel barrier layer is made of MgO which is formed by depositing a first Mg layer on the fixed layer, performing a natural oxidation of the first Mg layer to form a MgO layer, and then depositing a second Mg layer on the MgO layer.

22. The method of claim 17 wherein the passivation layer is made of silicon nitride and has an essentially conformal shape following the deposition step.

23. The method of claim 17 wherein the RSiO layer is one of FeSiO or FeSiN and has a thickness from about 8 to 15 Angstroms, and said FM1 and FM2 magnetic layers are made of CoFeB, CoFe, FeB, or Fe.

24. The method of claim 17 wherein the cap layer is Ru with a thickness from about 5 to 15 Angstroms and the hard mask is made of Ta, TaN, Ti, or TiN.

25. The method of claim 17 wherein the composite reference layer is comprised of a lower insertion layer made of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru formed on the seed layer, and an upper “

self-pinned”

reference layer comprised of Co, Fe, Ni, B or an alloy thereof that contacts said tunnel barrier layer.

26. The method of claim 17 wherein the composite reference layer has a M₁/X/M₂ configuration where M₁ and M₂ are ferromagnetically coupled layers made of Co, Fe, Ni, B, or an alloy thereof, X is an insertion layer comprised of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru that induces an enhanced damping constant in the M₁ and M₂ layers, and the M₂ magnetic layer is a “

self-pinned”

reference layer.

27. The method of claim 17 wherein the composite reference layer is a synthetic anti-ferromagnetic (SyAF) layer represented by a M₁/coupling/X/M₂ configuration where M₁ and M₂ are magnetic layers made of Co, Fe, Ni, B, or an alloy thereof, X is an insertion layer comprised of Tb, Gd, Pt, Pd, Ta, Hf, Os, Nb, Rh, or Ru that induces an enhanced damping constant in a “

self-pinned M₂ reference layer, and the coupling layer is a non-magnetic layer made of Ru, Rh, or Ir.