Multifunctional electrode
View Patent ↗A nonvolatile memory element is disclosed comprising a first electrode, a near-stoichiometric metal oxide memory layer having bistable resistance, and a second electrode in contact with the near-stoichiometric metal oxide memory layer. At least one electrode is a resistive electrode comprising a sub-stoichiometric transition metal nitride or oxynitride, and has a resistivity between 0.1 and 10 Ωcm. The resistive electrode provides the functionality of an embedded current-limiting resistor and also serves as a source and sink of oxygen vacancies for setting and resetting the resistance state of the metal oxide layer. Novel fabrication methods for the second electrode are also disclosed.
1. A method of forming a nonvolatile memory element, the method comprising:
forming a first layer operable as a first electrode; and
forming a second layer operable as a resistive switching layer;
wherein the first layer comprises a sub-stoichiometric nitride or a sub-stoichiometric oxynitride formed by reactive sputtering in an atmosphere comprising nitrogen or a combination of nitrogen and oxygen; and
wherein the second layer comprises a compound having a near-stoichiometric composition and a bi-stable resistance.
2. The method of claim 1 , wherein the reactive sputtering comprises applying a bias to a substrate; and wherein the bias is 20% higher than a minimum reactivity bias for the sub-stoichiometric nitride or the sub-stoichiometric oxynitride.
3. The method of claim 1 , wherein a sputter gun used for the reactive sputtering is positioned at an oblique angle relative to a substrate surface.
4. The method of claim 3 , wherein the sputter gun is positioned at an angle of greater than about 55° relative to a normal to the substrate surface.
5. The method of claim 4 , wherein the sputter gun is positioned at an angle of between about 55° and about 85° relative to the normal to the substrate surface.
6. The method of claim 1 , wherein the sub-stoichiometric nitride comprises a mixture of nitrides of different stoichiometries.
7. The method of claim 6 , wherein the mixture of nitrides comprises a mixture of titanium nitrides of different stoichiometries.
8. The method of claim 6 , wherein the mixture of nitrides comprises two or more of cubic β-TaN 0.05 , hexagonal γ-Ta 2 N, hexagonal δ-TaN, hexagonal ε-TaN, hexagonal Ta 2 N 6 , tetragonal Ta 4 N 5 , or tetragonal Ta 3 N 5 .
9. The method of claim 1 , wherein the first layer has a resistivity of between about 0.1 Ωcm and 10 Ωcm.
10. The method of claim 1 , wherein the atmosphere during reactive sputtering has a concentration of nitrogen of greater than about 50%.
11. The method of claim 1 , wherein the sub-stoichiometric nitride comprises hafnium nitride.
12. The method of claim 1 , wherein the sub-stoichiometric nitride comprises a mixture of hafnium nitrides of different stoichiometries.
13. The method of claim 1 , wherein the sub-stoichiometric nitride comprises a mixture of HfN and Hf 3 N 4 .
14. The method of claim 1 , wherein the first layer comprises the sub-stoichiometric oxynitride.
15. The method of claim 1 , wherein the sub-stoichiometric nitride comprises tantalum silicon nitride.
16. The method of claim 1 , further comprising a third layer operable as a second electrode, wherein the second layer is disposed between the first layer and the third layer, wherein the first layer has a resistivity of between about 0.1 Ωcm and 10 Ωcm, and wherein the third layer has a resistivity of between about 0.1 Ωcm and 10 Ωcm.
17. The method of claim 1 , wherein the second layer is in contact with the first layer.
18. The method of claim 1 , wherein the compound comprises Me 1 v Me 2 w Si x O y N z , wherein Me 1 and Me 2 are metals, v≧0, w≧0, x≧0, y>0, and z≧0, and at least one of v, w, and x is greater than zero.
19. The method of claim 1 , wherein the first layer is operable as a source of oxygen vacancies such that the oxygen vacancies migrate from the first layer and into the second layer during a set cycle, and wherein the first layer is further operable as a sink for oxygen vacancies such that the oxygen vacancies migrate out of the second layer and into the first layer during a reset cycle.
20. The method of claim 1 , wherein the sub-stoichiometric nitride or the sub-stoichiometric oxynitride comprises one of hafnium, tantalum, zirconium, lanthanum, niobium, or tungsten.