MTJ cell for an MRAM device and a manufacturing method thereof

An MTJ cell comprising:

• a first metal layer disposed on the surface of a substrate aligned with an XY plane and elongated in the X-direction (one end in the elongated direction having a terminal R);
• a second metal layer separated from the first metal layer in a Z-direction (hereinafter “above”) and elongated in the Y-direction orthogonal to the X-direction (ends in the elongated direction having terminals Wp and Wn, respectively);
• a magnetic tunnel junction (hereinafter “MTJ”) interposed between overlapping parts of the first and second metal layers, the MTJ comprising a ferromagnetic pinned layer, a nonmagnetic barrier layer, and a ferromagnetic storage layer (hereinafter simply called the “pinned layer,” “barrier layer,” and “storage layer,” respectively) stacked in the order given and including extended parts not covered by the second metal layer, and producing a small resistance or a large resistance in the Z-direction, respectively assigned a “0” state and “1” state, depending on whether the magnetization states of the pinned layer and the storage layer are parallel or antiparallel;
• a yoke comprising a ferromagnetic material or a high-permeability magnetic material and having a YZ cross-sectional area greater than the YZ cross-sectional area of the storage layer, the yoke spanning across and covering a portion of the second metal layer, with both ends of the yoke in the X-direction contacting extended parts of the storage layer not covered by the second metal layer, either directly or through a nonmagnetic insulating layer;
• and
• a third metal layer elongated in the X-direction;
• wherein the storage layer of the MTJ has a shape in an XY plane possessing a quantum easy axis in the X-direction, and the yoke has a shape in an XY plane possessing a quantum easy axis in the X-direction;
• the storage layer exhibits a relaxed coupling with the yoke at the extended parts not covered by the second metal layer that essentially satisfies the continuity of flux return directly or indirectly through a nonmagnetic insulating layer and maintains a quantum easy axis orthogonal to that of the yoke;
• and
• Y-direction quantum anisotropy is produced in the yoke by a magnetic field induced by a write bias current applied to the third metal layer.

A method of fabricating an MTJ cell, including steps of:

• forming, elongated in an X-direction, a first metal layer on the surface of a substrate;
• forming a magnetic tunnel junction (“MTJ”) on the first metal layer, the MTJ comprising a ferromagnetic pinned layer, a nonmagnetic barrier layer, and a ferromagnetic storage layer stacked in the order given;
• forming, elongated in the Y-direction (orthogonal to the X-direction), a second metal layer on the MTJ such that the MTJ includes extended parts not covered by the second metal layer;
• forming a nonmagnetic insulating layer comprising a first insulating layer and a second insulating layer;
• wherein:
• the first insulating layer is formed, shaped with a bird's beak, the tip of the bird's beak reaching the sloped surface of the storage layer, through deposition at a first angle of incidence, while
• the second insulating layer is formed on top of the first insulating layer and shaped with a bird's beak, the tip of the bird's beak not reaching above the sloped surface of the storage layer, through deposition at a second angle of incidence different from the first angle of incidence, and
• forming a yoke comprising a ferromagnetic material or a high-permeability magnetic material, spanning across and covering a portion of the second metal layer, with both ends of the yoke in the X-direction contacting the extended parts of the storage layer not covered by the second metal layer, indirectly through the nonmagnetic insulating layer;
• wherein the storage layer of the MTJ has a shape in an XY plane possessing a quantum easy axis in the X-direction, while the yoke has a shape in an XY plane possessing a quantum easy axis in the Y-direction.

A method of fabricating the MTJ cell, including steps of:

• forming, elongated in an X-direction, a first metal layer on the surface of a substrate;
• forming a magnetic tunnel junction (“MTJ”) on the first metal layers, the MTJ comprising a ferromagnetic pinned layer, a nonmagnetic barrier layer, and a ferromagnetic storage layer stacked in the order given;
• forming, elongated in the Y-direction (orthogonal to the X-direction), a second metal layer on the MTJ such that the MTJ includes extended parts not covered by the second metal layer;
• forming a nonmagnetic insulating layer comprising a first insulating layer and a second insulating layer;
• wherein:
• the first insulating layer is formed, shaped with a bird's beak, the tip of the bird's beak reaching the sloped surface of the storage layer, through deposition at a first angle of incidence, while
• the second insulating layer is formed on top of the first insulating layer and shaped with a bird's beak, the tip of the bird's beak not reaching above the sloped surface of the storage layer, through deposition at a second angle of incidence different from the first angle of incidence,
• forming a yoke comprising a ferromagnetic material or a high-permeability magnetic material, spanning across and covering a portion of the second metal layer, with both ends of the yoke in the X-direction contacting the extended parts of the storage layer not covered by the second metal layer, indirectly through the nonmagnetic insulating layer;
• and
• forming a third metal layer elongated in the X-direction, between any of the neighboring steps mentioned above;
• wherein the storage layer of the MTJ has a shape in an XY plane possessing a quantum easy axis in the X-direction, and the yoke has a shape in an XY plane possessing a quantum easy axis in the X-direction.