MRAM structure with raised edge of tunnel barrier layer

1 . A MRAM structure comprising:

a magnetic tunnel junction (MTJ) stack, the MTJ stack comprising, vertically from a bottom to a top thereof, a bottom electrode layer, a first ferromagnetic layer, a tunnel barrier layer, a sidewall spacer, a second ferromagnetic layer, and a top electrode layer,

wherein the second ferromagnetic layer is directly on top of the tunnel barrier layer and has a sidewall, a central portion of the tunnel barrier layer is directly on top of the first ferromagnetic layer, and a peripheral portion of the tunnel barrier layer surrounds at least a lower portion of the sidewall of the second ferromagnetic layer and is surrounded by the sidewall spacer, and

wherein the first ferromagnetic layer has a sidewall and a conformal liner covers the sidewall of the first ferromagnetic layer.

2 . The MRAM structure of claim 1 , wherein the sidewall of the first ferromagnetic layer is not coplanar with the sidewall of the second ferromagnetic layer.

3 . The MRAM structure of claim 1 , wherein the sidewall of the second ferromagnetic layer is angled relative to a bottom surface of the second ferromagnetic layer, forming an obtuse angle between about 120 degrees and about 150 degrees.

4 . The MRAM structure of claim 1 , wherein the first ferromagnetic layer is a reference layer and the second ferromagnetic layer is a free layer.

5 . The MRAM structure of claim 1 , wherein the first ferromagnetic layer is a free layer and the second ferromagnetic layer is a reference layer.

6 . The MRAM structure of claim 1 , wherein the sidewall of the second ferromagnetic layer has an upper portion that is not surrounded by the tunnel barrier layer and is not coplanar with the lower portion of the sidewall of the second ferromagnetic layer.

7 . The MRAM structure of claim 6 , further comprising a metal hard mask layer between the second ferromagnetic layer and the top electrode layer.

8 . A MRAM structure comprising:

a magnetic tunnel junction (MTJ) stack, the MTJ stack comprising a bottom electrode layer, a first ferromagnetic layer, a tunnel barrier layer, a sidewall spacer, a second ferromagnetic layer, and a top electrode layer,

wherein the second ferromagnetic layer has a sidewall, and the tunnel barrier layer has a central portion and a peripheral portion, the central portion being directly on top of the first ferromagnetic layer and the peripheral portion surrounding at least a lower portion of the sidewall of the second ferromagnetic layer and being surrounded by the sidewall spacer, and

wherein the first ferromagnetic layer has a sidewall and a conformal liner covers the sidewall of the first ferromagnetic layer.

9 . The MRAM structure of claim 8 , wherein the sidewall of the second ferromagnetic layer has an upper portion, the upper portion being not coplanar with the lower portion of the sidewall of the second ferromagnetic layer.

10 . The MRAM structure of claim 8 , wherein the sidewall of the first ferromagnetic layer is not coplanar with the lower portion of the sidewall of the second ferromagnetic layer.

11 . The MRAM structure of claim 8 , wherein the second ferromagnetic layer has a non-planar top surface.

12 . The MRAM structure of claim 8 , wherein the first ferromagnetic layer is either a reference layer or a free layer.

13 . A method of forming a MRAM structure, the method comprising:

patterning a bottom electrode layer and a first ferromagnetic layer on top of the bottom electrode layer;

depositing a dielectric layer, the dielectric layer covering the bottom electrode layer and the first ferromagnetic layer;

creating an opening in the dielectric layer, the opening exposing a portion of the first ferromagnetic layer;

forming a sidewall spacer at a sidewall of the opening;

forming a tunnel barrier layer inside the opening;

forming a second ferromagnetic layer on top of the tunnel barrier layer;

patterning the tunnel barrier layer and the second ferromagnetic layer; and

forming a top electrode layer on top of the second ferromagnetic layer,

wherein the second ferromagnetic layer has a sidewall and the tunnel barrier layer has a central portion and a peripheral portion, the central portion being directly on top of the first ferromagnetic layer and the peripheral portion surrounding at least a lower portion of the sidewall of the second ferromagnetic layer and being surrounded by the sidewall spacer, and the first ferromagnetic layer has a sidewall and a conformal liner covers the sidewall of the first ferromagnetic layer.

14 . The method of claim 13 , wherein forming the tunnel barrier layer comprises forming the central portion of the tunnel barrier layer on top of the exposed portion of the first ferromagnetic layer and forming the peripheral portion of the tunnel barrier layer against the sidewall of the opening in the dielectric layer, wherein the sidewall of the opening is angled relative to the exposed portion of the first ferromagnetic layer to have an obtuse angle between about 120 degrees to about 150 degrees.

15 . The method of claim 13 , wherein patterning the tunnel barrier layer and the second ferromagnetic layer comprises applying a chemical-mechanic-polishing process to remove portions of the second ferromagnetic layer and the tunnel barrier layer that are above a top surface of the dielectric layer.

16 . The method of claim 13 , wherein patterning the tunnel barrier layer and the second ferromagnetic layer comprises forming a metal hard mask layer covering a portion of the second ferromagnetic layer and the tunnel barrier layer, and selectively removing the second ferromagnetic layer and the tunnel barrier layer not covered by the metal hard mask layer through an anisotropic etching process.