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EP-4740716-A1 - FREE LAYER IN MAGNETORESISTIVE RANDOM-ACCESS MEMORY

EP4740716A1EP 4740716 A1EP4740716 A1EP 4740716A1EP-4740716-A1

Abstract

Embodiments of present invention provide a magnetoresistive random-access-memory (MRAM). The MRAM includes a reference layer; a tunnel barrier layer of magnesium-oxide (MgO); and a free layer, where the free layer includes a first cobalt-iron-boron (CoFeB) layer on top of the tunnel barrier layer; a spacer layer on top of the first CoFeB layer; a second CoFeB layer on top of the spacer layer; and a capping layer of MgO on top of the second CoFeB layer. Additionally, the first and the second CoFeB layer are substantially depleted of boron (B) to include respectively a first region adjacent to the tunnel barrier layer and the capping layer respectively and a second region adjacent to the spacer layer, where the first regions of the first and the second CoFeB layer include crystallized cobalt-iron (CoFe) and the second regions of the first and the second CoFeB layer include amorphous CoFe alloy.

Inventors

  • GOTTWALD, Matthias Georg
  • HU, GUOHAN
  • MEHTA, Virat, Vasav
  • BRULEY, JOHN
  • REZNICEK, ALEXANDER

Assignees

  • International Business Machines Corporation

Dates

Publication Date
20260513
Application Date
20240704

Claims (20)

  1. 1 . A magnetoresistive random-access-memory (MRAM) comprising: a reference layer; a tunnel barrier layer of magnesium-oxide (MgO); and a free layer, wherein the free layer comprises: a first cobalt-iron-boron (CoFeB) layer on top of the tunnel barrier layer; a spacer layer on top of the first CoFeB layer; a second CoFeB layer on top of the spacer layer; and a capping layer of MgO on top of the second CoFeB layer, wherein the first and the second CoFeB layer are substantially depleted of boron (B) to include respectively a first region adjacent to the tunnel barrier layer and the capping layer respectively and a second region adjacent to the spacer layer, wherein the first regions of the first and the second CoFeB layer include crystallized cobalt-iron (CoFe) and the second regions of the first and the second CoFeB layer include amorphous CoFe alloy.
  2. 2. The MRAM of claim 1, wherein the spacer layer includes an alloy of a refractory metal and iron, the refractory metal being zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), rhenium (Re), or tungsten (W).
  3. 3. The MRAM of claim 1, wherein the spacer layer includes a concentration level of iron ranging from about 20 at. % to about 80 at. %.
  4. 4. The MRAM of claim 1 , wherein the first and the second CoFeB layer include less than 5 at. % of boron.
  5. 5. The MRAM of claim 1 , wherein the first region of the first CoFeB layer has a thickness that is between about 20% to about 40% of a thickness of the first CoFeB layer.
  6. 6. The MRAM of claim 1 , wherein a thickness of the first region of the first CoFeB layer is between about 0.2nm and about 0.5nm, and a thickness of the second region of the first CoFeB layer is between about 0.2nm and about 0.7nm.
  7. 7. A magnetoresistive random-access-memory (MRAM) comprising: a reference layer; a tunnel barrier layer above the reference layer; and a free layer above the tunnel barrier layer, wherein the free layer includes: a first cobalt-iron-boron (CoFeB) layer on top of the tunnel barrier layer; a spacer layer on top of the first CoFeB layer; a second CoFeB layer on top of the spacer layer; and a capping layer on top of the second CoFeB layer; wherein the spacer layer is an alloy of a refractory metal and iron, with the iron having a concentration level ranging from about 20 at. % to about 80 at. %.
  8. 8. The MRAM of claim 7, wherein the refractory metal is selected from a group consisting of zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), rhenium (Re), and tungsten (W).
  9. 9. The MRAM of claim 7, wherein the tunnel barrier layer is a layer of magnesium-oxide (MgO) and the first CoFeB layer is directly on top of the tunnel barrier layer, the first CoFeB layer is substantially depleted of boron (B) to include a first region adjacent to the tunnel barrier layer and a second region adjacent to the spacer layer; wherein the first region includes crystallized cobalt-iron (CoFe) and the second region includes amorphous CoFe alloy.
  10. 10. The MRAM of claim 7, wherein the capping layer is a layer of magnesium-oxide (MgO) and is directly on top of the second CoFeB layer, the second CoFeB layer is substantially depleted of boron (B) to include a first region adjacent to the capping layer and a second region adjacent to the spacer layer; wherein the first region includes crystallized cobalt-iron (CoFe) and the second region includes amorphous CoFe alloy.
  11. 11 . The MRAM of claim 7, wherein the first region of the first CoFeB layer has a thickness that is between about 20% to about 40% of a thickness of the first CoFeB layer.
  12. 12. The MRAM of claim 7, wherein a thickness of the first region of the first CoFeB layer is between about 0.2nm and about 0.5nm, and a thickness of the second region of the first CoFeB layer is between about 0.2nm and about 0.7nm.
  13. 13. The MRAM of claim 7, wherein the spacer layer is formed from a stack of alternating layers of the refractory metal and iron, with each of the alternating layers having a thickness between about 0.3nm and about 1.0nm.
  14. 14. The MRAM of claim 7, wherein the spacer layer is formed through a sputtering deposition process using an alloy target, the alloy target comprising the refractory metal and iron, and the iron having a concentration level ranging from about 20 at. % to about 80 at. %.
  15. 15. A magnetic tunnel junction (MTJ) stack comprising: a reference layer; a tunnel barrier layer above the reference layer; and a free layer above the tunnel barrier layer, wherein the free layer includes a first cobalt-iron-boron (CoFeB) layer; a spacer layer of an alloy comprising a refractory metal and iron; a second CoFeB layer; and a capping layer, wherein the refractory metal is zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), rhenium (Re), or tungsten (W), and the spacer layer being sandwiched between the first and the second CoFeB layer.
  16. 16. The MTJ stack of claim 15, wherein the tunnel barrier layer is a layer of magnesium-oxide (MgO), and the first CoFeB layer is directly on top of the tunnel barrier layer to include a first region adjacent to the tunnel barrier layer and a second region adjacent to the spacer layer; wherein the first region is substantially depleted of boron (B) to include crystallized cobalt-iron (CoFe) and the second region is substantially depleted of boron to include amorphous CoFe alloy.
  17. 17. The MTJ stack of claim 16, wherein both the first region and the second region of the first CoFeB layer include less than 5 at. % of boron.
  18. 18. The MTJ stack of claim 16, wherein a thickness of the first region of the first CoFeB layer is between about 0.2nm and about 0.5nm, and a thickness of the second region of the first CoFeB layer is between about 0.2nm and about 0.7nm.
  19. 19. The MTJ stack of claim 16, wherein the spacer layer is formed from a stack of alternating layers of the refractory metal and iron, with each of the alternating layers having a thickness between about 0.3nm and about 1.0nm.
  20. 20. The MTJ stack of claim 15, wherein the spacer layer includes a concentration level of iron ranging from about 20 at. % to about 80 at. %.

Description

FREE LAYER IN MAGNETORESISTIVE RANDOM-ACCESS MEMORY BACKGROUND [0001] The present application relates to manufacturing of semiconductor integrated circuits. More particularly, it relates to method of forming a free layer in a magnetoresistive random-access memory and the structure formed thereby. [0002] Semiconductor memory devices are well recognized as playing an extreme important role in managing and organizing digital information which, in recent years, has experienced explosive growth and is constantly transforming our society. Magnetoresistive random-access memory (MRAM) is a type of non-volatile memory (NVM) and particularly a spin-transfer torque MRAM (STT-MRAM) is known as an embedded NVM (eNVM) that is capable of holding saved digital information without losing them even in the event that supply of power to the STT-MRAM device is down or accidentally cut off. The use of STT-MRAM enables higher densities, lower power consumption, and reduced manufacturing cost of memory devices when it is compared with other types of MRAM devices. [0003] MRAM technology, including STT-MRAM, is based on a magnetic tunnel junction (MTJ) stack that usually includes a tunnel barrier layer that is placed or sandwiched between a reference layer and a free layer. Conventionally, the free layer is made of a cobalt-iron-boron alloy material. The property of the cobalt-iron-boron alloy material has a significant influence on the performance of the MRAM and is under study for further improvement. [0004] STT-MRAM is a type of advanced eNVM device that relies on a MTJ stack for device functionality. A cobalt-iron-boron (CoFeB) alloy is usually used as material for forming a free layer in the MTJ stack. It is known to the inventors that properties of the CoFeB alloy, together with the formation process of the free layer using the CoFeB alloy, have a significant influence on the function of the MTJ stack and the overall operation of the STT- MRAM. For example, when forming the STT-MRAM, in particular the MTJ stack, the free layer needs to be deposited in an amorphous state onto a crystalline tunnel barrier layer which is usually a magnesium-oxide (MgO). To keep the CoFeB in its amorphous state during and after the deposition, the CoFeB alloy is usually deposited from a target containing about 20 to 40 at. % of boron (B). [0005] High content of boron helps in wetting the MgO surface resulting in better uniformity during the formation of the CoFeB free layer. On the other hand, the high content of boron causes the CoFe to stay in an amorphous state, thus reduces the magnetoresistance (TMR) of the free layer formed thereby, which is not desirable for the performance of MTJ. A free layer structure with a refractory metal layer placed between a first and a second CoFeB layers have been introduced to help absorb the excess amount of boron. The refractory metal layer is usually made sufficiently thin such that it does not result in magnetic decoupling of the first and the second CoFeB layer to cause the MTJ device become dysfunctional. However, the thin refractory metal layer is not sufficient to absorb the excessive boron in the free layer. SUMMARY [0006] Embodiments of present invention provide a magnetoresistive random-access-memory (MRAM). The MRAM includes a reference layer; a tunnel barrier layer of magnesium-oxide (MgO); and a free layer, where the free layer includes a first cobalt-iron-boron (CoFeB) layer on top of the tunnel barrier layer; a spacer layer on top of the first CoFeB layer; a second CoFeB layer on top of the spacer layer; and a capping layer of MgO on top of the second CoFeB layer. Additionally, the first and the second CoFeB layer are substantially depleted of boron (B) to include respectively a first region adjacent to the tunnel barrier layer and the capping layer respectively and a second region adjacent to the spacer layer, wherein the first regions of the first and the second CoFeB layer include crystallized cobalt-iron (CoFe) and the second regions of the first and the second CoFeB layer include amorphous CoFe alloy. The local crystallinity of the first regions helps lead to high tunnel magnetoresistance (TMR), high perpendicular magnetic anisotropy (PMA), and high retention of states as compared with current art. The local amorphous state of the second regions result in less magnetic moment and thus enable faster write time and improved device property distribution. [0007] In one embodiment, the spacer layer includes an alloy of a refractory metal and iron, the refractory metal being zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), rhenium (Re), or tungsten (W). [0008] In another embodiment, the spacer layer includes a concentration level of iron ranging from about 20 atomic percent (at. %) to about 80 at. %. [0009] In yet another embodiment, the first and the second CoFeB layer include less than 5 at. % of boron. [0010] In one embodiment, the first region of the first CoFeB la