Cross-point spin-transfer torque magnetoresistive memory array and method of making the same
A memory device includes a cross-point array of spin-torque transfer MRAM cells. First rail structures laterally extend along a first horizontal direction. Each of the first rail structures includes a vertical stack including, from bottom to top, a first electrically conductive line, a reference layer having a fixed magnetization direction, and a tunnel barrier layer. Second rail structures laterally extend along a second horizontal direction. Each of the second rail structures includes a second electrically conductive line that overlies the first rail structures. A two-dimensional array of pillar structures is located between a respective one of the first rail structures and a respective one of the second rail structures. Each of the pillar structures includes a free layer having energetically stable magnetization orientations that are parallel or antiparallel to the fixed magnetization direction.
1. A method of forming a memory device including a two-dimensional array of spin- torque transfer MRAM cells, comprising:
forming a layer stack comprising a first continuous electrically conductive layer, a continuous reference layer, a continuous nonmagnetic tunnel barrier layer, and a continuous free magnetization material layer over a substrate;
forming first rail structures that laterally extend along a first horizontal direction and laterally spaced apart from each other by patterning a subset of layers within the layer stack, wherein each of the first rail structures comprises a vertical stack including, from bottom to top, a first electrically conductive line including a respective patterned portion of the first continuous electrically conductive layer, a reference layer including a respective patterned portion of the continuous reference layer, and a tunnel barrier layer including a patterned portion of the continuous nonmagnetic tunnel barrier layer;
forming a two-dimensional array of pillar structures by patterning at least the continuous free magnetization material layer, wherein each of the pillar structures comprises a free layer having energetically stable magnetization orientations that are parallel or antiparallel to the fixed magnetization direction of the reference layer; and
forming second rail structures that laterally extend along a second horizontal direction that is different from the first horizontal direction and laterally spaced apart from each other, wherein each of the second rail structures comprises a second electrically conductive line that overlies the two-dimensional array of pillar structures.
2. The method of claim 1 , further comprising:
forming a two-dimensional array of discrete masking material portions over the layer stack; and
patterning the continuous free magnetization material layer by transferring a pattern of the two-dimensional array of discrete masking material portions through the continuous free magnetization material layer employing an anisotropic etch process that stops on the continuous nonmagnetic tunnel barrier layer.
3. The method of claim 2 , further comprising:
forming a one-dimensional array of line-shaped masking material portions that laterally extend along the first horizontal direction and laterally spaced apart along the second horizontal direction; and
anisotropically etching the continuous nonmagnetic tunnel barrier layer, the continuous reference layer, and the first continuous electrically conductive layer employing the one-dimensional array of line-shaped masking material portions as an etch mask, whereby the first rail structures are formed.
4. The method of claim 1 , further comprising forming dielectric material portions between neighboring pairs of the first rail structures, wherein the second rail structures are formed on top surfaces of the dielectric material portions and on top surfaces of a respective column of pillar structures of the two-dimensional array of pillar structures.
5. The method of claim 4 , further comprising:
forming a continuous selector material layer over the continuous free magnetization material layer; and
patterning the continuous selector material layer and the continuous free magnetization material layer employing a two-dimensional array of discrete masking material portions as an etch mask, wherein each pillar structure within the two-dimensional array of pillar structures includes a respective selector that is a patterned portion of the continuous selector material layer.
6. The method of claim 1 , further comprising:
patterning the layer stack into the first rail structures and in-process rail structures that overlie the first rail structures by performing a first anisotropic etch process, wherein the in-process rail structures comprise patterned portions of the continuous free magnetization material layer;
forming dielectric rail structures between neighboring pairs of the first rail structures and between neighboring pairs of the in-process rail structures;
forming a second continuous electrically conductive layer over the in-process rail structures and the dielectric rail structures; and
patterning the second continuous electrically conductive layer and the in-process rail structures by performing a second anisotropic etch process, wherein the second continuous electrically conductive layer is patterned into the second rail structures by the second anisotropic etch process, and the in-process rail structures are patterned into the two-dimensional array of pillar structures by the second anisotropic etch process.
7. The method of claim 6 , wherein forming the two-dimensional array of pillar structures comprises patterning only the continuous free magnetization material layer, and wherein each of the pillar structures comprises only the free layer.