Memory structure and formation method thereof

1. A method comprising:

forming a first ferromagnetic bottom electrode over a first conductive feature of an interconnect structure, comprising forming a first ferromagnetic layer over the first conductive feature, forming a synthetic anti-ferromagnetic (SAF) spacer layer over the first ferromagnetic layer, and forming a second ferromagnetic layer over the SAF spacer layer to serve as the first ferromagnetic bottom electrode, wherein the first ferromagnetic layer has a thickness greater than a thickness of the second ferromagnetic layer;

forming a second ferromagnetic bottom electrode over a second conductive feature of the interconnect structure;

forming an spin-orbit torque (SOT) channel layer extending across the first ferromagnetic bottom electrode and the second ferromagnetic bottom electrode; and

forming a magnetic tunnel junction (MTJ) cell over the SOT channel layer.

2. The method of claim 1 , wherein the first ferromagnetic bottom electrode and the second ferromagnetic bottom electrode have anti-parallel magnetization directions.

3. The method of claim 1 , further comprising:

applying an external magnetic field to make the first ferromagnetic bottom electrode and the second ferromagnetic bottom electrode have a first magnetization direction; and

stopping the external magnetic field such that the first magnetization direction of the first ferromagnetic bottom electrode changes to a second magnetization direction opposite the first magnetization direction.

4. The method of claim 1 , wherein forming the first ferromagnetic bottom electrode further comprises:

etching the second ferromagnetic layer, the SAF spacer layer, and the first ferromagnetic layer to form a metal stack localized to the first conductive feature.

5. The method of claim 1 , wherein the second ferromagnetic bottom electrode is formed after forming the first ferromagnetic bottom electrode.

6. The method of claim 1 , further comprising:

depositing a dielectric layer over the first ferromagnetic bottom electrode;

performing a first chemical mechanical polish (CMP) process on the dielectric layer until the first ferromagnetic bottom electrode is exposed; and

etching an opening in the dielectric layer, wherein the second ferromagnetic bottom electrode is formed in the opening.

7. The method of claim 6 , wherein forming the second ferromagnetic bottom electrode comprises:

depositing a ferromagnetic material in the opening in the dielectric layer; and

performing a second CMP process on the ferromagnetic material until the dielectric layer is exposed.

8. The method of claim 7 , wherein after the second CMP process is complete, the first ferromagnetic bottom electrode is exposed.

9. A method comprising:

depositing in sequence a first ferromagnetic layer, a non-magnetic layer, and a second ferromagnetic layer over a substrate;

patterning the first ferromagnetic layer, the non-magnetic layer, and the second ferromagnetic layer into a metal stack comprising a portion of the first ferromagnetic layer, a portion of the non-magnetic layer, and a portion of the second ferromagnetic layer;

forming a dielectric layer laterally surrounding the metal stack;

etching an opening in the dielectric layer;

depositing a third ferromagnetic layer in the opening in the dielectric layer;

forming a heavy metal layer over the metal stack and the third ferromagnetic layer; and

forming an MTJ structure over the heavy metal layer.

10. The method of claim 9 , wherein the third ferromagnetic layer is further deposited on the metal stack, and the method further comprises:

performing a CMP process to remove the third ferromagnetic layer from a top surface of the metal stack.

11. The method of claim 9 , wherein the third ferromagnetic layer has a thickness greater than a thickness of the first ferromagnetic layer.

12. The method of claim 9 , wherein the third ferromagnetic layer has a thickness greater than a thickness of the second ferromagnetic layer.

13. The method of claim 9 , wherein forming the dielectric layer comprises:

depositing the dielectric layer over the metal stack; and

performing a CMP process on the dielectric layer to expose the metal stack.

14. A method comprising:

forming a dielectric layer over a substrate;

forming a first ferromagnetic bottom electrode extending through the dielectric layer;

forming a second ferromagnetic bottom electrode extending through the dielectric layer, and spaced apart from the first ferromagnetic bottom electrode, wherein the second ferromagnetic bottom electrode has a thickness different from a thickness of the first ferromagnetic bottom electrode;

forming an SOT channel layer extending from the first ferromagnetic bottom electrode to the second ferromagnetic bottom electrode; and

forming an MTJ structure over the SOT channel layer.

15. The method of claim 14 , wherein the first ferromagnetic bottom electrode and the second ferromagnetic bottom electrode are magnetically oriented in opposite directions.

16. The method of claim 14 , wherein the first ferromagnetic bottom electrode has a bottom surface lower than a bottom surface of the second ferromagnetic bottom electrode.

17. The method of claim 14 , further comprising:

forming a metal spacer layer under the second ferromagnetic bottom electrode; and

forming a ferromagnetic layer under the metal spacer layer.

18. The method of claim 14 , further comprising:

forming a passivation layer on a sidewall of the MTJ structure;

forming a top electrode on a top surface of the MTJ structure; and

forming a metal line or via over the top electrode.

19. The method of claim 16 , wherein the first ferromagnetic bottom electrode has a top surface level with a top surface of the second ferromagnetic bottom electrode.

20. The method of claim 17 , wherein the ferromagnetic layer has a thickness greater than the thickness of the second ferromagnetic bottom electrode.