High performance MTJ element for STT-RAM and method for making the same

• US 20090027810A1
• Filed: 07/23/2007
• Published: 01/29/2009
• Est. Priority Date: 07/23/2007
• Status: Active Grant

• Current Assignee: Taiwan Semiconductor Manufacturing Company Limited
• Original Assignee: MagIC Technologies Inc (TDK Corporation)
• Inventors: Kula, Witold, Tong, Ru-Ying, Torng, Chyu-Jiuh, Horng, Cheng T.
• Granted Patent: US 7,750,421 B2
• Time in Patent Office: Days
• US Class Current: 360/324.200
• CPC Class Codes: B82Y 25/00 Nanomagnetism, e.g. magnetoimpedance, anisotropic magnetoresistance, giant magnetoresistance or tunneling magnetoresistance
• CPC Class Codes: B82Y 40/00 Manufacture or treatment of nanostructures
• CPC Class Codes: G11C 11/161 details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
• CPC Class Codes: H01F 10/3272 by use of anti-parallel coupled [APC] ferromagnetic layers, e.g. artificial ferrimagnets [AFI], artificial [AAF] or synthetic [SAF] anti-ferromagnets
• CPC Class Codes: H01F 10/329 Spin-exchange coupled multilayers wherein the magnetisation of the free layer is switched by a spin-polarised current, e.g. spin torque effect
• CPC Class Codes: H01F 10/3295 Spin-exchange coupled multilayers wherein the magnetic pinned or free layers are laminated without anti-parallel coupling within the pinned and free layers
• CPC Class Codes: H01F 41/307 insulating or semiconductive spacer
• CPC Class Codes: H10N 50/01 Manufacture or treatment
• CPC Class Codes: H10N 50/10 Magnetoresistive devices
• CPC Class Codes: H10N 50/85 Magnetic active materials

Abstract

We describe the structure and method of forming a STT-MTJ MRAM cell that utilizes transfer of spin angular momentum as a mechanism for changing the magnetic moment direction of a free layer. The device includes an IrMn pinning layer, a SyAP pinned layer, a naturally oxidized, crystalline MgO tunneling barrier layer that is formed on an Ar-ion plasma smoothed surface of the pinned layer and, in one embodiment, a free layer that comprises an amorphous layer of Co₆₀Fe₂₀B₂₀. of approximately 20 angstroms thickness formed between two crystalline layers of Fe of 3 and 6 angstroms thickness respectively. The free layer is characterized by a low Gilbert damping factor and by very strong polarizing action on conduction electrons. The resulting cell has a low critical current, a high dR/R and a plurality of such cells will exhibit a low variation of both resistance and pinned layer magnetization angular dispersion.

Claims

1. An STT-MTJ MRAM cell operating in a CPP configuration and utilizing the transfer, by torque, of conduction electron spin angular momentum to change a free layer magnetization direction, comprising:

a substrate;

an MTJ element formed on said substrate, said element comprising a vertically stacked lamination of horizontal parallel layers including, therein, in the following order;

an antiferromagnetic pinning layer formed of MnIr,

an SyAP pinned layer processed by a plasma process to have a smooth/flat interfacial surface,

a tunneling barrier layer formed on said smooth/flat interfacial surface, said tunneling barrier layer having a crystalline structure and comprising a naturally oxidized, sputtered layer of Mg,

a ferromagnetic free layer formed on said tunneling barrier layer, said ferromagnetic free layer having a low magnetic damping factor and producing enhanced polarization of conduction electrons, and

a capping layer formed on said free layer, and wherein

said ferromagnetic free layer is a laminate further comprising at least one interfacial layer of crystalline ferromagnetic material and an amorphous ferromagnetic layer formed thereon and whereby,

a current of conduction electrons in the vertical direction can change the direction of magnetization of said free layer relative to the magnetization direction of said pinned layer.

2. The cell of claim 1 wherein said free layer is a composite bilayer comprising a crystalline layer of Fe of approximately 3 angstroms thickness on which is formed an amorphous layer of Co₆₀Fe₂₀B₂₀. of approximately 20 angstroms thickness.

3. The cell of claim 2 wherein said crystalline layer of Fe is formed with an (001) crystal plane parallel to its plane of formation.

4. The cell of claim 1 wherein said free layer is a composite tri-layer comprising a first crystalline layer of Fe of 3 angstroms thickness on which is formed an amorphous layer of Co₆₀Fe₂₀B₂₀. of approximately 20 angstroms thickness, on which is formed a second crystalline layer of Fe of approximately 6 angstroms thickness.

5. The cell of claim 1 wherein said free layer is a composite tri-layer comprising a first crystalline layer of Fe of 3 angstroms thickness on which is formed an amorphous layer of Co₄₀Fe₄₀B₂₀. of approximately 15 angstroms thickness, on which is formed a second crystalline layer of Fe of approximately 6 angstroms thickness.

6. The cell of claim 4 wherein said crystalline layers of Fe are each formed with an (001) crystal plane parallel to its plane of formation.

7. The cell of claim 5 wherein said crystalline layers of Fe are each formed with an (001) crystal plane parallel to its plane of formation.

8. The cell of claim 1 wherein said tunneling barrier layer is a crystalline layer of naturally oxidized, sputtered Mg formed to a thickness of approximately 12 angstroms.

9. The cell of claim 1 wherein said pinning layer of IrMn is formed to a thickness of approximately 70 angstroms.

10. The cell of claim 1 wherein said SyAP pinned layer comprises a layer of Co₇₅Fe₂₅ formed to a thickness of approximately 23 angstroms, on which is formed a layer of Ru to a thickness of approximately 7.5 angstroms, on which is formed a bilayer comprising a layer of Co₆₀Fe₂₀B₂₀ of approximately 15 angstroms thickness on which is formed a layer of Co₇₅Fe₂₅ of between approximately 6 and 7 angstroms thickness.

11. The cell of claim 1 wherein said substrate is a bottom electrode and said capping layer is electrically contacted by a bit line.

12. An STT-MTJ MRAM cell operating in a CPP configuration and utilizing the transfer, by torque, of conduction electron spin angular momentum to change a free layer magnetization direction, comprising:

a substrate;

an MTJ element formed on said substrate, said element comprising a vertically stacked lamination of horizontal parallel layers including, therein, in the following order;

an antiferromagnetic pinning layer formed of MnIr,

an SyAP pinned layer processed by a plasma process to have a smooth/flat interfacial surface,

a tunneling barrier layer formed on said smooth/flat interfacial surface, said tunneling barrier layer having a crystalline structure and comprising a naturally oxidized, sputtered layer of Mg,

a ferromagnetic free layer formed on said tunneling barrier layer, said ferromagnetic free layer having a low magnetic damping factor and producing enhanced polarization of conduction electrons, and

a capping layer formed on said free layer, and wherein

said ferromagnetic free layer is a bilayer comprising a crystalline layer of a binary alloy of Fe rich FeCo on which is formed a crystalline layer of a binary alloy of Fe rich FeNi and whereby

a current of conduction electrons in the vertical direction can change the direction of magnetization of said free layer relative to the magnetization direction of said pinned layer.

13. The cell of claim 12 wherein said tunneling barrier layer is a layer of naturally oxidized, sputtered Mg formed to a thickness of approximately 12 angstroms.

14. The cell of claim 12 wherein said pinning layer of IrMn is formed to a thickness of approximately 70 angstroms.

15. The cell of claim 12 wherein said SyAP pinned layer comprises a layer of Co₇₅Fe₂₅ formed to a thickness of approximately 23 angstroms, on which is formed a layer of Ru to a thickness of approximately 7.5 angstroms, on which is formed a bilayer comprising a layer of Co₆₀Fe₂₀B₂₀ of approximately 6 angstroms thickness on which is formed a layer of Co₇₅Fe₂₅ of approximately 7 angstroms thickness.

16. A method of forming an STT-MTJ MRAM cell operating in a CPP configuration and utilizing the transfer, by torque, of conduction electron spin angular momentum to change a free layer magnetization direction, comprising:

providing a substrate;

forming an MTJ element on said substrate, said element comprising a vertically stacked lamination of horizontal parallel layers formed by the following methods and in the following order,

forming an antiferromagnetic pinning layer of MnIr, then

forming an SyAP pinned layer on said pinning layer, then

applying a plasma process to an exposed upper surface of said SyAP pinned layer, said plasma process producing a flat and smooth upper surface;

then

forming a tunneling barrier layer of MgO on said flat and smooth upper surface, said tunneling barrier layer being formed by a process comprising the sputtering of crystalline Mg and then producing its natural oxidation, whereby said tunneling barrier layer is formed directly in a crystalline state;

then

forming a free layer on said crystalline layer of MgO, said free layer comprising a first layer of ferromagnetic material having a crystalline structure matching the crystalline structure of said barrier layer and producing, thereby, an enhanced effect on the scattering of polarized electrons, on which is formed a second layer of ferromagnetic material which will be rendered amorphous so as to have a low magnetic damping factor, on which formed a third layer of ferromagnetic material having a crystalline structure and an enhanced effect on the scattering of polarized electrons;

then,

forming a capping layer on said third ferromagnetic layer and

processing the formation at a processing temperature and in a processing magnetic field and for a processing time to render said second ferromagnetic layer amorphous and to set a magnetic moment direction in said pinned and free layers, whereby

a current of conduction electrons in the vertical direction can change the direction of magnetization of said free layer relative to the magnetization direction of said pinned layer.

17. The method of claim 16 wherein said tunneling barrier layer is formed to a thickness of approximately 12 angstroms as a crystalline layer of naturally oxidized, sputtered Mg by a process comprising the sputtering of an Mg target to form a first crystalline sputtered layer of Mg, naturally oxidizing said first crystalline sputtered layer, then sputtering a second crystalline sputtered layer onto said oxidized layer.

18. The method of claim 16 wherein said pinning layer of IrMn is formed to a thickness of approximately 70 angstroms.

19. The method of claim 16 wherein said SyAP pinned layer comprises a layer of Co₇₅Fe₂₅ formed to a thickness of approximately 23 angstroms, on which is formed a layer of Ru to a thickness of approximately 7.5 angstroms, on which is formed a bilayer comprising a layer of Co₆₀Fe₂₀B₂₀ of approximately 15 angstroms thickness on which is formed a layer of Co₇₅Fe₂₅ of between approximately 6 and 7 angstroms thickness.

20. The method of claim 16 wherein said plasma process is a low power, 20 W, Ar ion-milling process that smoothes and flattens the said upper surface of said SyAP layer while said ion energy is sufficiently low so that it does not damage said upper surface.

21. The method of claim 16 wherein said free layer is formed as a composite tri-layer comprising the formation of a first crystalline layer of Fe of 3 angstroms thickness on which is formed an amorphous layer of Co₆₀Fe₂₀B₂₀. of approximately 20 angstroms thickness, on which is formed a second crystalline layer of Fe of approximately 6 angstroms thickness.

22. The method of claim 16 wherein said free layer is formed as a composite tri-layer comprising the formation of a first crystalline layer of Fe of 3 angstroms thickness on which is formed an amorphous layer of Co₄₀Fe₄₀B₂₀. of approximately 15 angstroms thickness, on which is formed a second crystalline layer of Fe of approximately 6 angstroms thickness.

23. The method of claim 21 wherein each of said crystalline layers of Fe is formed with an (001) crystal plane parallel to its plane of formation.

24. The method of claim 22 wherein each of said crystalline layers of Fe are formed with an (001) crystal plane parallel to its plane of formation.

25. The method of claim 16 wherein said processing is carried out at a temperature between approximately 250 and 265°

C. in a magnetic field of approximately 10 KOe for a processing time between approximately 1 and 2 hours.

26. A method for forming an STT-MTJ MRAM cell operating in a CPP configuration and utilizing the transfer, by torque, of conduction electron spin angular momentum to change a free layer magnetization direction, comprising:

providing a substrate;

forming an MTJ element on said substrate, said element comprising a vertically stacked lamination of horizontal parallel layers formed by the following methods and in the following order;

forming an antiferromagnetic pinning layer of MnIr, then

forming an SyAP pinned layer on said pinning layer, then

applying a plasma process to an exposed upper surface of said SyAP pinned layer, said plasma process producing a flat and smooth upper surface;

then

forming a tunneling barrier layer of MgO on said flat and smooth upper surface, said tunneling barrier layer being formed by a process of sputtering of crystalline Mg and then producing its natural oxidation, whereby said tunneling barrier layer is formed directly in a crystalline state;

then

forming a ferromagnetic free layer on said tunneling barrier layer, said ferromagnetic free layer having a low magnetic damping factor and producing enhanced polarization of conduction electrons, and

forming a capping layer on said free layer, and wherein

said ferromagnetic free layer is formed as a bilayer comprising a crystalline layer of a binary alloy of Fe rich FeCo on which is formed a crystalline layer of a binary alloy of Fe rich FeNi;

processing said formation at a processing temperature, in a processing magnetic field and for a processing time, said processing being at a temperature of greater than approximately 330°

C., whereby

a current of conduction electrons in the vertical direction can change the direction of magnetization of said free layer relative to the magnetization direction of said pinned layer.

27. The method of claim 26 wherein said tunneling barrier layer is formed to a thickness of approximately 12 angstroms as a crystalline layer of naturally oxidized, sputtered Mg by a process comprising the sputtering of an Mg target to form a first crystalline sputtered layer of Mg, naturally oxidizing said first crystalline sputtered layer, then sputtering a second crystalline sputtered layer onto said oxidized layer.

28. The process of claim 26 wherein said pinning layer of IrMn is formed to a thickness of approximately 70 angstroms.

29. The process of claim 26 wherein said SyAP pinned layer comprises a layer of Co₇₅Fe₂₅ formed to a thickness of approximately 23 angstroms, on which is formed a layer of Ru to a thickness of approximately 7.5 angstroms, on which is formed a bilayer comprising a layer of Co₆₀Fe₂₀B₂₀ of approximately 6 angstroms thickness on which is formed a layer of Co₇₅Fe₂₅ of approximately 7 angstroms thickness.