Cross-point magnetoresistive random memory array and method of making thereof using self-aligned patterning
A memory device includes a cross-point array of magnetoresistive memory cells. Each magnetoresistive memory cell includes a vertical stack of a selector-containing pillar structure and a magnetic tunnel junction pillar structure. The lateral spacing between neighboring pairs of magnetoresistive memory cells may be smaller along a first horizontal direction than along a second horizontal direction, and a dielectric spacer or a tapered etch process may be used to provide a pattern of an etch mask for patterning first electrically conductive lines underneath the magnetoresistive memory cells. Alternatively, a resist layer may be employed to pattern first electrically conductive lines underneath the cross-point array. Alternatively, a protective dielectric liner may be provided to protect selector-containing pillar structures during formation of the magnetic tunnel junction pillar structures.
1. A memory array, comprising:
first electrically conductive lines laterally extending along a first horizontal direction and laterally spaced apart along a second horizontal direction;
rows of selector-magnetic tunnel junction (selector-MTJ) assemblies located on a respective one of the first electrically conductive lines, wherein each of the selector-MTJ assemblies comprises a respective row of magnetic tunnel junction (MTJ) pillar structures and a respective row of selector-containing pillar structures that are arranged along the first horizontal direction, and a lateral spacing between neighboring pairs of selector-containing pillar structures that are laterally spaced apart along the first horizontal direction is less than a lateral spacing between neighboring pairs of selector-containing pillar structures that are laterally spaced apart along the second horizontal direction; and
second electrically conductive lines laterally extending along the second horizontal direction and overlying a respective column of the selector-MTJ assemblies.
2. The memory array of claim 1 , wherein each of the first electrically conductive lines has a respective variable width along the second horizontal direction that varies along the first horizontal direction.
3. The memory device of claim 1 , wherein:
each of the selector-MTJ assemblies includes a respective selector-containing pillar structure overlying a respective one of the MTJ pillar structures; and
the selector-containing pillar structures are arranged as a two-dimensional periodic array of the selector-containing pillar structures having a first pitch along the first horizontal direction and having a second pitch along the second horizontal direction.
4. The memory device of claim 3 , wherein:
each of the selector-containing pillar structures has a first lateral dimension along the first horizontal direction and has a second lateral dimension along the second horizontal direction that is less than the first lateral dimension; and
the second pitch is the same as the first pitch.
5. The memory device of claim 3 , wherein:
each of the selector-containing pillar structures has a same lateral extent along the second horizontal direction as along the first horizontal direction; and
the second pitch is greater than the first pitch.
6. The memory device of claim 5 , wherein each of the selector-containing pillar structures has a circular horizontal cross-sectional shape.
7. The memory device of claim 1 , wherein:
tapered sidewalls of each neighboring pair of MTJ pillar structures within the respective row of MTJ pillar structures are adjoined to each other at a respective edge located above a horizontal plane including top surfaces of the first electrically conductive lines and laterally extending along the second horizontal direction;
each of the MTJ pillar structures comprises a vertical stack including a reference layer, a tunnel barrier layer, and a free layer; and
free layers within the respective row of MTJ pillar structures are laterally spaced apart from each other along the first horizontal direction and do not contact one another.
8. The memory device of claim 7 , wherein:
reference layers within the respective row of MTJ pillar structures are interconnected as a single continuous reference structure underlying the respective row of selector-containing pillar structures; and
the edges at which tapered sidewalls of a respective neighboring pair of MTJ pillar structures are adjoined are located at V-shaped indentations in a top surface of the single continuous reference structure.
9. The memory device of claim 7 , wherein:
nonmagnetic tunnel barrier layers within the respective row of MTJ pillar structures are interconnected as a single continuous nonmagnetic tunnel barrier structure underlying the respective row of selector-containing pillar structures; and
the edges at which tapered sidewalls of a respective neighboring pair of MTJ pillar structures are adjoined are located at V-shaped indentations in a top surface of the single continuous nonmagnetic tunnel barrier structure.
10. The memory device of claim 7 , wherein:
a superlattice layer is located underneath the reference layer within each vertical stack;
superlattice layers within the respective row of MTJ pillar structures are interconnected as a single continuous superlattice structure that extends underneath the respective row of selector-containing pillar structures; and
the edges at which tapered sidewalls of a respective neighboring pair of MTJ pillar structures are adjoined are located at V-shaped indentations in a top surface of the single continuous superlattice structure.
11. The memory device of claim 7 , wherein:
an antiferromagnetic coupling layer is located underneath the reference layer within each vertical stack;
antiferromagnetic coupling layers within the respective row of MTJ pillar structures are interconnected as a single continuous antiferromagnetic coupling structure that extends underneath the respective row of selector-containing pillar structures; and
the edges at which tapered sidewalls of a respective neighboring pair of MTJ pillar structures are adjoined are located at V-shaped indentations in a top surface of the single continuous antiferromagnetic coupling structure.
12. The memory device of claim 1 , wherein each of the first electrically conductive lines comprises a laterally alternating sequence of uniform thickness segments that underlie a respective one of the MTJ pillar structures and indented segments that includes a V-shaped indentation in a respective top surface segment.
13. The memory device of claim 1 , wherein:
each of the first electrically conductive lines comprises a pair of contoured lengthwise sidewalls that generally extend along the first horizontal direction with a lateral undulation along the second horizontal direction; and
each of the contoured lengthwise sidewalls comprises straight segments laterally extending along the first horizontal direction and pairs of adjoined convex sidewalls adjoined at a respective vertically-extending edge.
14. The memory device of claim 1 , wherein:
each of the first electrically conductive lines comprises a periodic repetition of wider bulging regions and narrower neck regions; and
each bulging region has a bottom surface having a uniform radius of curvature.
15. A method of forming a memory device, comprising:
forming a first electrically conductive layer, magnetic-tunnel-junction-level (MTJ-level) material layers that include magnetic tunnel junction material layers, and selector-level material layers over a substrate;
forming a two-dimensional array of discrete patterned resist material portions, wherein a first nearest-neighbor spacing along a first horizontal direction of the two-dimensional array of discrete patterned resist material portions is less than a second nearest-neighbor spacing along a second horizontal direction of the two-dimensional array of discrete patterned resist material portions; and
transferring a pattern in the two-dimensional array of discrete patterned resist material portions through the selector material layers, the magnetic tunnel junction material layers, and the first electrically conductive layer such that physically exposed surfaces of remaining portions of the MTJ-level material layers are formed with taper angles, wherein patterned portions of the selector-level material layers comprise a two-dimensional array of selector-containing pillar structures including a respective selector element, patterned portions of the MTJ-level material layers comprise a two-dimensional array of magnetic tunnel junction (MTJ) pillar structures, and patterned portions of the first electrically conductive layer comprise first electrically conductive lines that laterally extend along the first horizontal direction and laterally spaced apart from each other along the second horizontal direction.
16. The method of claim 15 , further comprising forming second electrically conductive lines laterally extending along the second horizontal direction and laterally spaced apart from each other along the first horizontal direction over the two-dimensional array of selector-containing pillar structures.
17. The method of claim 15 , wherein:
the two-dimensional array of MTJ pillar structures comprises rows of MTJ pillar structures that are arranged along the first horizontal direction; and
tapered sidewalls of each neighboring pair of MTJ pillar structures within the respective row of MTJs are adjoined to each other at a respective edge located above a horizontal plane including top surfaces of the first electrically conductive lines and laterally extending along the second horizontal direction during patterning of the MTJ-level material layers.
18. The method of claim 17 , wherein:
the magnetic tunnel junction material layers comprise a continuous reference layer, a continuous nonmagnetic tunnel barrier layer, and a continuous free layer; and
the edges at which a respective pair of tapered sidewalls of the neighboring pairs of the MTJ pillar structures are adjoined to each other are located within the continuous nonmagnetic tunnel barrier layer, within the continuous reference layer, or below the continuous reference layer.
19. The method of claim 15 , wherein:
each of the first electrically conductive lines is patterned with a respective pair of contoured and tapered lengthwise sidewalls; and
each of the first electrically conductive lines is formed with a respective bottom surface having a respective variable width along the second horizontal direction that varies along the first horizontal direction.
20. The method of claim 15 , wherein:
a lateral spacing between neighboring pairs of selector-containing pillar structures that are laterally spaced apart along the first horizontal direction is less than a lateral spacing between neighboring pairs of selector-containing pillar structures that are laterally spaced apart along the second horizontal direction;
transfer of the pattern in the two-dimensional array of discrete patterned resist material portions through the magnetic tunnel junction material layers is performed using an ion beam etch process that etches materials of the MTJ-level material layers at a variable etch rate that decreases with a local aspect ratio; and
the discrete patterned resist material portions comprise photoresist or electron beam resist portions.