Novel magnetic tunnel junction (MTJ) to reduce spin transfer magnetization switching current

• US 20080179699A1
• Filed: 01/30/2007
• Published: 07/31/2008
• Est. Priority Date: 01/30/2007
• Status: Active Grant

• Current Assignee: Taiwan Semiconductor Manufacturing Company Limited
• Original Assignee: MagIC Technologies Inc (TDK Corporation)
• Inventors: Tong, Ru-Ying, Horng, Cheng T.
• Granted Patent: US 7,598,579 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

Abstract

A MTJ that minimizes spin-transfer magnetization switching current (Jc) in a Spin-RAM to <1×10⁶ A/cm² is disclosed. The MTJ has a Co₆₀Fe₂₀B₂₀/MgO/Co₆₀Fe₂₀B₂₀ configuration where the CoFeB AP1 pinned and free layers are amorphous and the crystalline MgO tunnel barrier is formed by a ROX or NOX process. The capping layer preferably is a Hf/Ru composite where the lower Hf layer serves as an excellent oxygen getter material to reduce the magnetic “dead layer” at the free layer/capping layer interface and thereby increase dR/R, and lower He and Jc. The annealing temperature is lowered to about 280° C. to give a smoother CoFeB/MgO interface and a smaller offset field than with a 350° C. annealing. In a second embodiment, the AP1 layer has a CoFeB/CoFe configuration wherein the lower CoFeB layer is amorphous and the upper CoFe layer is crystalline to further improve dR/R and lower RA to ≦10 ohm/μm².

Claims

1. An MTJ element for reducing spin-transfer magnetization switching current in a magnetic device, comprising:

a pinned layer having an AP2/coupling layer/AP1 configuration wherein the AP2 layer is formed on an AFM layer and the AP1 layer is made of amorphous CoFeB;

a crystalline MgO tunnel barrier formed on the amorphous CoFeB AP1 pinned layer;

an amorphous CoFeB free layer formed on the MgO tunnel barrier;

and a composite capping layer formed on the CoFeB free layer wherein the composite capping layer is comprised of a lower layer that contacts the free layer and an upper layer, and said lower layer is made of a metal having an oxidation potential greater than that of Co, Fe, and Ta to provide a high oxygen gettering capability.

2. The MTJ element of claim 1 wherein the magnetic device is a Spin-RAM.

3. The MTJ element of claim 1 wherein the amorphous CoFeB AP1 pinned layer and amorphous CoFeB free layer have a composition represented by Co_XFe_YB_Z where x is from about 40 to 60 atomic %, y is from about 20 to 40 atomic %, and z is from about 15 to 25 atomic %.

4. The MTJ element of claim 1 wherein the lower layer in the composite capping layer is comprised of Hf or Zr.

5. The MTJ element of claim 4 wherein the composite capping layer has a Hf/Ru configuration in which the lower Hf layer has a thickness from about 5 to 50 Angstroms and the upper Ru layer has a thickness from about 20 to 100 Angstroms.

6. The MTJ element of claim 1 wherein the AP2 portion of the pinned layer is comprised of CoFe and the coupling layer is Ru.

7. An MTJ element for reducing spin-transfer magnetization switching current in a magnetic device, comprising:

a pinned layer having an AP2/coupling layer/AP1 configuration wherein the AP2 layer is formed on an AFM layer and the AP1 layer is a composite having a lower amorphous CoFeB layer and an upper crystalline CoFe layer;

a crystalline MgO tunnel barrier formed on the crystalline CoFe layer in the composite AP1 pinned layer;

an amorphous CoFeB free layer formed on the MgO tunnel barrier;

and a capping layer formed on the CoFeB free layer wherein the capping layer is comprised of a metal layer having an oxidation potential greater than that of Co and Fe to provide a high oxygen gettering capability and said metal layer contacts said amorphous CoFeB free layer.

8. The MTJ element of claim 7 wherein the magnetic device is a Spin-RAM.

9. The MTJ element of claim 7 wherein the lower amorphous CoFeB AP1 pinned layer and amorphous CoFeB free layer have a composition represented by Co_XFe_YB_Z where x is from about 40 to 60 atomic %, y is from about 20 to 40 atomic %, and z is from about 15 to 25 atomic %.

10. The MTJ element of claim 7 wherein the upper crystalline CoFe AP1 layer has a composition represented by Co_XFe_Y where y is from about 20 to 40 atomic % and said CoFe layer has a body centered cubic (bcc) crystal structure.

11. The MTJ element of claim 7 wherein the metal layer in the capping layer is comprised of Ta, Hf, or Zr.

12. The MTJ element of claim 7 wherein the AP2 portion of the pinned layer is comprised of CoFe and the coupling layer is Ru.

13. The MTJ element of claim 7 wherein the lower amorphous CoFeB layer has a thickness between about 12 and 18 Angstroms and has a composition represented by Co₆₀Fe₂₀B₂₀, and the upper crystalline CoFe layer has a thickness from about 5 to 7 Angstroms and a composition represented by Co₇₅Fe₂₅.

14. A method of forming an MTJ element on a substrate in a magnetic device, comprising:

(a) forming a pinned layer in a MTJ stack of layers wherein the pinned layer has an AP2/coupling layer/AP1 configuration in which the AP2 layer is formed on an AFM layer and the AP1 layer is made of amorphous CoFeB;

(b) forming a crystalline MgO tunnel barrier on the amorphous CoFeB AP1 pinned layer by a method that comprises a radical oxidation (ROX) or natural oxidation (NOX) of a Mg layer;

(c) forming an amorphous CoFeB free layer on the MgO barrier layer; and

(d) forming a composite capping layer on said amorphous CoFeB free layer wherein said composite capping layer is comprised of a lower layer made of a metal having an oxidation potential greater than that of Co, Fe, and Ta to provide a high oxygen gettering capability, and an upper layer.

15. The method of claim 14 wherein the substrate is a bottom conductor in a Spin-RAM structure and the MTJ stack is further comprised of a seed layer on the substrate and an AFM layer on the seed layer.

16. The method of claim 14 wherein all layers in the MTJ stack are laid down in a low pressure sputter deposition system having multiple chambers including an oxidation chamber for the ROX or NOX step.

17. The method of claim 14 wherein the amorphous CoFeB AP1 pinned layer and amorphous CoFeB free layer have a composition represented by Co_XFe_YB_Z where x is from about 40 to 60 atomic %, y is from about 20 to 40 atomic %, and z is from about 15 to 25 atomic %.

18. The method of claim 14 further comprised of annealing the MTJ stack of layers at a temperature of about 250°

C. to 300°

C. with an applied magnetic field of about 10000 Oersted for about 1 to 5 hours.

19. The method of claim 14 wherein forming the MgO layer comprises depositing a first Mg layer about 8 Angstroms thick on the amorphous CoFeB AP1 pinned layer followed by a ROX process comprising about 500 Watts RF power and with a flow rate of about 0.6 slm O₂ for about 15 to 50 seconds, and then deposition of a second Mg layer about 2 to 6 Angstroms thick on the oxidized first Mg layer.

20. The method of claim 14 wherein forming the MgO layer comprises depositing a first Mg layer about 8 Angstroms thick on the amorphous CoFeB pinned layer followed by a NOX oxidation comprising exposure of the first Mg layer to about 1 torr pressure and with a flow rate of about 1 slm O₂ for about 50 to 100 seconds, and then deposition of a second Mg layer about 2 to 6 Angstroms thick on the oxidized first Mg layer.

21. The method of claim 14 wherein the lower layer of the composite capping layer is comprised of Hf or Zr with a thickness of about 5 to 50 Angstroms and the upper layer is made of Ru having a thickness of about 20 to 100 Angstroms.

22. A method of forming an MTJ element on a substrate in a magnetic device, comprising:

(a) forming a pinned layer in a MTJ stack of layers wherein the pinned layer has an AP2/coupling layer/AP1 configuration in which the AP2 layer is formed on an AFM layer and the AP1 layer is a composite having a lower amorphous CoFeB layer and an upper crystalline CoFe layer;

(b) forming a crystalline MgO tunnel barrier on the crystalline CoFe layer in the composite AP1 pinned layer by a method that comprises a radical oxidation (ROX) or natural oxidation (NOX) of a Mg layer;

(c) forming an amorphous CoFeB free layer on the MgO barrier layer; and

(d) forming a capping layer on the CoFeB free layer wherein the capping layer is comprised of a metal layer having an oxidation potential greater than that of Co and Fe to provide a high oxygen gettering capability and said metal layer contacts said amorphous CoFeB layer.

23. The method of claim 22 wherein the substrate is a bottom conductor in a Spin-RAM structure and the MTJ stack is further comprised of a seed layer on the substrate and an AFM layer on the seed layer.

24. The method of claim 22 wherein all layers in the MTJ stack are laid down in a low pressure sputter deposition system having multiple chambers including an oxidation chamber for the ROX or NOX step.

25. The method of claim 22 wherein the lower amorphous CoFeB AP1 pinned layer and amorphous CoFeB free layer have a composition represented by Co_XFe_YB_Z where x is from about 40 to 60 atomic %, y is from about 20 to 40 atomic %, and z is from about 15 to 25 atomic %.

26. The method of claim 22 wherein the upper crystalline CoFe AP1 layer has a composition represented by Co_XFe_Y where y is from about 20 to 40 atomic % and said CoFe AP1 layer has a body centered cubic (bcc) crystal structure.

27. The method of claim 22 wherein the metal layer in the capping layer is comprised of Ta, Hf, or Zr.

28. The method of claim 22 further comprised of annealing the MTJ stack of layers at a temperature from about 250°

C. to 300°

C. with an applied magnetic field of about 10000 Oersted for about 1 to 5 hours.

29. The method of claim 22 wherein forming the MgO layer comprises depositing a first Mg layer about 8 Angstroms thick on the upper crystalline CoFe layer in the composite AP1 pinned layer followed by a ROX process comprising about 500 Watts RF power and a flow rate of about 0.6 slm O₂ for about 15 to 50 seconds, and then deposition of a second Mg layer about 2 to 6 Angstroms thick on the oxidized first Mg layer.

30. The method of claim 22 wherein forming the MgO layer comprises depositing a first Mg layer about 8 Angstroms thick on the upper crystalline CoFe layer in the composite AP1 pinned layer followed by a NOX oxidation comprising exposure of the first Mg layer to about 1 torr pressure and a O₂ flow rate of about 1 slm for about 80 to 300 seconds, and then deposition of a second Mg layer about 2 to 6 Angstroms thick on the oxidized first Mg layer.

31. The method of claim 27 wherein the capping layer is further comprised of a layer of Ru on the Ta, Hf, or Zr metal layer.