Low cost multi-state magnetic memory

• US 8,018,011 B2
• Filed: 09/24/2007
• Issued: 09/13/2011
• Est. Priority Date: 02/12/2007
• Status: Active Grant

• Current Assignee: Avalanche Technology Incorporated
• Original Assignee: Avalanche Technology Incorporated
• Inventors: Keshtbod, Parviz, Malmhall, Roger Klas, Ranjan, Rajiv Yadav
• Primary Examiner(s): Phung; Anh
• Assistant Examiner(s): BERNSTEIN, ALLISON
• Application Number:
  US11/860,467
• Publication Number:
  US 20080225585A1
• Time in Patent Office: 1,450 Days
• Field of Search: 257/421, 257/427, 257/E21.665, 365/158, 365/171
• US Class Current: 257/421
• CPC Class Codes: G11C 11/161 details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
• CPC Class Codes: G11C 11/1673 Reading or sensing circuits or methods
• CPC Class Codes: G11C 11/5607 using magnetic storage elements
• CPC Class Codes: H10N 50/10 Magnetoresistive devices

Abstract

A multi-state current-switching magnetic memory element has a magnetic tunneling junction (MTJ), for storing more than one bit of information. The MTJ includes a fixed layer, a barrier layer, and a non-uniform free layer. In one embodiment, having 2 bits per cell, when one of four different levels of current is applied to the memory element, the applied current causes the non-uniform free layer of the MTJ to switch to one of four different magnetic states. The broad switching current distribution of the MTJ is a result of the broad grain size distribution of the non-uniform free layer.

Claims

1. A multi-state current-switching non-volatile magnetic memory element comprising:

a seed layer formed on top of a bottom electrode;

a pinning layer formed on top of the seed layer;

a magnetic tunneling junction (MTJ) formed on top of the pinning layer, wherein the MTJ comprises a fixed layer formed on top of the pinning layer, a barrier layer formed on top of the fixed layer, and a non-uniform free layer formed on top of the barrier layer, the non-uniform free layer having a magnetic moment that is switchable, the non-uniform free layer responsive to a switching current of various levels, the non-uniform free layer having grains that are magnetic and of varying sizes and randomly distributed throughout the non-uniform free layer and having non-uniform grain structure, the grains being weakly exchange coupled such that each grain has a distinct magnetic moment that is switched substantially independently of the other grains, the magnetic moment of a substantial number of grains defining the magnetic moment of the non-uniform free layer, the size of the grains being determinative of the logical state preserved by the multi-state current-switching non-volatile magnetic memory element in that the size of the grains cause the non-uniform free layer to switch its magnetic moment when a predetermined switching current level flows through the non-uniform free layer, the grains having a thin outer layer, which serves as a boundary between the grains; and

a top electrode formed on top of the MTJ;

wherein different levels of switching current applied to the multi-state current-switching non-volatile magnetic memory element causes switching to different logical states,

further wherein the grains comprise of a first set of grains having a first size, a second set of grains having a second size and a third set of grains having a third size, each of the sets of grains responsive to a distinct level of the switching current flowing through the non-uniform free layer based on the size of the set of grains and each of the set of grains operative to switch its magnetic moment accordingly, wherein the multi-state current-switching non-volatile magnetic memory element is capable of preserving four logical states,

further wherein the first set of grains comprises substantially 85% of the grains and is responsive to a first level of switching current and operative to switch its magnetic moment accordingly and the second set of grains comprises substantially 10% of the grains and is responsive to a second level of switching current and operative to switch its magnetic moment accordingly and the third set of grains comprises substantially 5% of the grains and is responsive to a third level of switching current and operative to switch its magnetic moment, the first size of the first set of grains being smaller than the second size of the second set of grains and the third size of the third set of grains and the second size of the second set of grains being smaller than the third size of the third set of grains.

2. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein the size of the grains of the non-uniform free layer range in diameter from 1 nm to 20 nanometers.

3. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 2, wherein the non-uniform grain structure of the non-uniform free layer is comprised substantially of cobalt iron boron (CoFeB) and one or more of the following non-magnetic elements:

chromium, tantalum, molybdenum, titanium and tungsten;

or an oxide or nitride of chromium, tantalum, molybdenum, titanium, and tungsten.

4. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 3, wherein the barrier layer is comprised substantially of one or more of the following:

magnesium oxide (MgO), aluminum oxide, strontium oxide, ruthenium oxide, calcium oxide, and zinc oxide.

5. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 4, wherein the magnetic moments of each of the grains is switchable and the switching of the magnetic moments of the grains forms the non-uniform grain structure of the non-uniform free layer.

6. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 4, wherein each of the grains forming the non-uniform grain structure of the non-uniform free layer are capable of switching magnetic moments without affecting the magnetic moments of neighboring grains.

7. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 5, further including a cap layer formed between the MTJ and the top electrode.

8. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 7, wherein the fixed layer is comprised substantially of one or more layers of alloys of CoFe, containing less than 20 atomic percent of one or more of the following elements:

boron (B), chromium (Cr), molybdenum (Mo), silicon (Si), or zirconium (Zr); and

said layers of alloys of CoFe are separated by less than 1.5 nm of a layer comprised of alloy RuX, where X is chromium (Cr) or molybdenum (Mo), or a mixture of chromium and molybdenum.

9. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 8, wherein the top electrode is comprised substantially of tantalum.

10. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein each of the three sets of grains has substantially the same volume as the other sets of grains.

11. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein the grains are made of a cobalt iron boron (CoFeB) alloy.

12. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein the seed layer is made of material selected from a group consisting of nickel phosphorous (NiP), nickel boron (NiB), nickel niobium (NiNb), nickel zirconium (NiZr), and nickel tungsten (NiW).

13. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein the seed layer is made of material selected from a group consisting of nickel phosphorous (NiP), nickel boron (NiB), nickel niobium (NiNb), nickel zirconium (NiZr), nickel tungsten (NiW) and any combination of these compounds which contains less than 20 molar percent of oxides and nitrides of aluminum (Al), silicon (Si), titanium (Ti), tantalum (Ta), and tungsten (W).

14. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein the seed layer is made of a substantially amorphous alloy.

15. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 1, wherein the seed layer made of two elements having different melting points.

16. A multi-state current-switching non-volatile magnetic memory element comprising:

a seed layer formed on top of a bottom electrode wherein the seed layer is made of nickel (Ni)X-phosphorous (P), where X comprises one or more of Cr, Nb, Ta, Mo, W, Cu, Zn, Ti, Al, and further the seed layer is made of aluminum (Al)-Y where Y comprises one or more of Ta, W, Ti, Nb, Zr, Si;

a pinning layer formed on top of the seed layer;

a magnetic tunneling junction (MTJ) formed on top of the pinning layer, wherein the MTJ comprises a fixed layer formed on top of the pinning layer, a barrier layer formed on top of the fixed layer, and a non-uniform free layer formed on top of the barrier layer, the non-uniform free layer having a magnetic moment that is switchable, the non-uniform free layer responsive to a switching current of various levels, the non-uniform free layer having grains that are magnetic and of varying sizes and randomly distributed throughout the non-uniform free layer and having non-uniform grain structure, the grains being weakly exchange coupled such that each grain has a distinct magnetic moment that is switched substantially independently of the other grains, the magnetic moment of a substantial number of grains defining the magnetic moment of the non-uniform free layer, the size of the grains being determinative of the logical state preserved by the multi-state current-switching non-volatile magnetic memory element in that the size of the grains cause the non-uniform free layer to switch its magnetic moment when a predetermined switching current level flows through the non-uniform free layer, the grains having a thin outer layer, which serves as a boundary between the grains; and

a top electrode formed on top of the MTJ,

wherein different levels of switching current applied to the multi-state current-switching non-volatile magnetic memory element causes switching to different logical states.

17. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the size of the grains of the non-uniform free layer range in diameter from 1 nm to 20 nanometers.

18. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the non-uniform grain structure of the non-uniform free layer is comprised substantially of cobalt iron boron (CoFeB) and one or more of the following non-magnetic elements:

chromium, tantalum, molybdenum, titanium and tungsten;

or an oxide or nitride of chromium, tantalum, molybdenum, titanium, and tungsten.

19. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the barrier layer is comprised substantially of one or more of the following:

magnesium oxide (MgO), aluminum oxide, strontium oxide, ruthenium oxide, calcium oxide, and zinc oxide.

20. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein each of the grains forming the non-uniform grain structure of the non-uniform free layer are capable of switching magnetic moments without affecting the magnetic moments of neighboring grains.

21. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the magnetic moments of each of the grains is switchable and the switching of the magnetic moments of the grains forms the non-uniform grain structure of the non-uniform free layer.

22. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the non-uniform free layer is comprised of CoFeB with oxides and nitrides selected from a group consisting of:

SiO₂, TiO₂, Ta₂O₅, WO₃, Al₂O₃, TiN, TaN, WN.

23. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the non-uniform free layer further includes tantalum (Ta).

24. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the grains are made of a cobalt iron boron (CoFeB) alloy.

25. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the seed layer is made of a substantially amorphous alloy.

26. A multi-state current-switching non-volatile magnetic memory element, as recited in claim 16, wherein the seed layer made of two elements having different melting points.