Layer stack for magnetic tunnel junction device

1. A layer stack for a magnetic tunnel junction (MTJ) device, the layer stack comprising:

a seed layer structure;

a pinning layer structure arranged above the seed layer structure; and

a Fe-comprising reference layer structure and a free layer structure separated by a tunnel barrier layer, formed above the pinning layer structure,

wherein the Fe-comprising reference layer structure contacts the pinning layer structure, and

wherein the seed layer structure comprises a Ru-comprising layer and a Cr-comprising layer, the Cr-comprising layer forming an upper layer of the seed layer structure.

2. The layer stack according to claim 1 , wherein the Cr-comprising layer is arranged on the Ru-comprising layer.

3. The layer stack according to claim 1 , wherein the pinning layer structure is formed on the Cr-comprising layer.

4. The layer stack according to claim 1 , wherein the Ru-comprising layer has a thickness of 0.2 to 3 nm and the Cr-comprising layer has a thickness of 0.2 to 2 nm.

5. The layer stack according to claim 1 , wherein the seed layer structure further comprises a Pt-, Pd- or Ir-comprising bottom layer arranged below the Ru-comprising layer.

6. The layer stack according to claim 5 , wherein the bottom layer has a thickness of 1 to 6 nm.

7. The layer stack according to claim 5 , wherein the Ru-comprising layer forms an interfacial layer between the bottom layer and the Cr-comprising layer.

8. The layer stack according to claim 5 , wherein the bottom layer has a face-centered cubic (FCC) structure and has a (111)-textured surface.

9. The layer stack according to claim 8 , wherein the Ru-comprising layer comprises a (002) surface contacting the (111)-textured surface of the bottom layer.

10. The layer stack according to claim 1 , wherein the pinning layer structure comprises a Co/Pt laminate, a Co/Ni laminate, a CoFe/Pt laminate, a CoFe/Ni laminate, a CoPt alloy, a CoFePt alloy, a CoNi alloy or a CoFeNi alloy.

11. The layer stack according to claim 1 , wherein the pinning layer structure comprises Cr.

12. The layer stack according to claim 11 , wherein a diffusion profile characteristic of thermal diffusion extends from the Cr-comprising layer and into the pinning layer structure.

13. The layer stack according to claim 1 , wherein the reference layer structure comprises a CoFeB layer, FeB layer, CoFe layer or Fe layer.

14. The layer stack according to claim 13 , wherein the Cr-comprising layer serves to suppress diffusion of Fe from the reference layer structure to the pinning layer.

15. The layer stack according to claim 1 , wherein the pinning layer structure comprises grain boundaries occupied by Cr.

16. A method for forming a layer stack for a magnetic tunnel junction (MTJ) device, the method comprising:

forming a seed layer structure;

forming a pinning layer structure above the seed layer structure;

forming, above the pinning layer structure a Fe-comprising reference layer structure, a tunnel barrier layer, and a free layer structure,

wherein the Fe-comprising reference layer structure contacts the pinning layer structure, and

wherein forming the seed layer structure comprises forming a Ru-comprising layer and a Cr-comprising layer, wherein the Cr-comprising layer forms an upper layer of the seed layer structure; and

performing an anneal process to cause diffusion of Cr from the Cr-comprising layer into the pinning layer structure.

17. The method according to claim 16 , further comprising patterning the layer stack to form a pillar structure prior to or subsequent to performing the anneal process.

18. The method according to claim 16 , wherein the Cr-comprising layer is formed on the Ru-comprising layer.

19. The method according to claim 16 , wherein forming the seed layer structure further comprises forming a Pt-, Pd- or Ir-comprising bottom layer and subsequently forming the Ru-comprising layer above the bottom layer.

20. The method according to claim 19 , wherein the Ru-comprising layer is formed on the bottom layer.