Composite spacer layer for magnetoresistive memory

1. A method of forming a device comprising:

providing a substrate;

performing back-end-of-line (BEOL) processing to form an inter-level dielectric (ILD) layer on the substrate, wherein the ILD layer comprises a plurality of ILD levels; and

forming a magnetic tunneling junction (MTJ) stack in between adjacent ILD levels, wherein the MTJ stack comprises

a magnetic fixed layer, the magnetic fixed layer comprises

a synthetic antiferromagnetic (SAF) layer,

a composite spacer layer disposed on the SAF layer, the composite spacer layer comprises

a first non-magnetic (NM) spacer layer,

a magnetic (M) spacer layer disposed on the first NM spacer layer, and

a second NM spacer layer disposed on the M spacer layer, and

a reference layer disposed on the composite spacer layer,

a tunneling barrier layer disposed on the magnetic fixed layer, and

a magnetic free layer disposed on the tunneling barrier layer.

2. The method of claim 1 wherein the MTJ stack is disposed between the adjacent ILD levels of an upper ILD layer.

3. The method of claim 1 wherein:

the M spacer layer comprises a cobalt-based (Co-based) magnetic layer; and

the first and second NM spacer layers comprise tantalum (Ta), molybdenum (Mo), tungsten (W), niobium (Nb), ruthenium (Ru), titanium (Ti) or a combination thereof.

4. The method of claim 3 wherein the Co-based M spacer layer comprises cobalt-iron/nickel-boron alloy (Co(Fe, Ni)B).

5. The method of claim 3 wherein the Co-based M spacer layer comprises a Co-based magnetically continuous amorphous layer.

6. The method of claim 4 wherein M spacer layer comprises:

a concentration of Boron (B) comprising about 0-40%; and

a concentration of Cobalt (Co) comprising about 20-60%.

7. The method of claim 3 wherein the first and second NM spacer layers comprise Ta.

8. The method of claim 1 wherein the M spacer layer comprises a monolayer.

9. The method of claim 8 wherein the M spacer layer comprises a discontinuous layer.

10. The method of claim 1 wherein forming the composite spacer layer comprises co-sputtering using a sputter target comprising materials of the M and NM spacer layers.

11. The method of claim 1 wherein:

the NM spacer layers are formed by sputtering using krpton (Kr) or xenon (Xe) gas at 75 W; and

the M spacer layer is formed by sputtering using argon (Ar) gas at 600 W.

12. The method of claim 1 wherein:

the first NM spacer layer serves as a base layer (BL);

the M spacer layer and second NM spacer layer form a bilayer (M/NM); and

the composite spacer layer comprises (BL)/(M/NM)n, wherein n is the number of bilayers on the BL in the composite stack and n≥1.

13. The method of claim 12 wherein n is equal to 1-5.

14. The method of claim 1 wherein the MTJ stack comprises:

a cap layer disposed on the magnetic free layer;

a seed layer disposed below the magnetic fixed layer; and

the MTJ stack is disposed between top and bottom electrodes.

15. The method of claim 14 further comprises a second tunneling barrier layer disposed between the magnetic free layer and cap layer.

16. The method of claim 1 wherein the magnetic free layer comprises a magnetic coupling stack, the magnetic coupling stack comprises:

a first magnetic free layer;

a free spacer layer disposed on the first magnetic free layer; and

a second magnetic free layer.

17. The method of claim 16 wherein the free spacer layer comprises a composite free spacer layer, the composite free spacer layer comprises:

a first NM free spacer layer;

a M free spacer layer disposed on the first NM free spacer layer; and

a second NM free spacer layer disposed on the M free layer.

18. A method of forming a device comprising:

providing a substrate comprising circuit component formed on a substrate surface;

performing BEOL processing to form an inter-level dielectric (ILD) layer on the substrate, wherein the ILD layer comprises a plurality of ILD levels; and

forming a magnetic tunneling junction (MTJ) stack in between adjacent ILD levels of an upper ILD layer, wherein the MTJ stack comprises

a bottom electrode layer,

a seed layer disposed on the bottom electrode,

a magnetic fixed layer, the magnetic fixed layer comprises

a synthetic antiferromagnetic (SAF) layer,

a composite spacer layer disposed on the SAF layer, the composite spacer layer comprises

a first non-magnetic (NM) spacer layer,

a magnetic (M) spacer layer disposed on the first NM spacer layer, and

a second NM spacer layer disposed on the magnetic spacer layer, and

a reference layer disposed on the composite spacer layer,

a tunneling barrier layer disposed on the magnetic fixed layer,

a magnetic free layer disposed on the tunneling barrier layer,

a cap layer disposed on the magnetic free layer, and

a top electrode disposed on the cap layer.

19. A device comprising:

a substrate;

an inter level dielectric (ILD) layer disposed on the substrate, wherein the ILD layer comprises a plurality of ILD levels; and

a magnetic tunneling junction (MTJ) stack disposed between adjacent ILD levels, wherein the MTJ stack comprises

a magnetic fixed layer, the magnetic fixed layer comprises

a synthetic antiferromagnetic (SAF) layer,

a composite spacer layer disposed on the SAF layer, the composite spacer layer comprises

a first non-magnetic (NM) spacer layer,

a magnetic (M) spacer layer disposed on the first NM spacer layer, and

a second NM spacer layer disposed on the M spacer layer, and

a reference layer disposed on the composite spacer layer,

a tunneling barrier layer disposed on the magnetic fixed layer, and

a magnetic free layer disposed on the tunneling barrier layer.

20. The device of claim 19 wherein:

the first NM spacer layer serves as a base layer (BL);

the M spacer layer and second NM spacer layer form a bilayer (M/NM); and

the composite spacer layer comprises (BL)/(M/NM)n, wherein n is the number of bilayers on the BL in the composite stack and n≥1.