Hybridized oxide capping layer for perpendicular magnetic anisotropy
View Patent ↗A method of forming a hybrid oxide capping layer (HOCL) is disclosed and used in a magnetic tunnel junction to enhance thermal stability and perpendicular magnetic anisotropy in an adjoining free layer. The HOCL has a lower interface oxide layer and one or more transition metal oxide layers wherein each of the metal layers selected to form a transition metal oxide has an absolute value of free energy of oxide formation less than that of the metal used to make the interface oxide layer. One or more of the HOCL layers is under oxidized. Oxygen from one or more transition metal oxide layers preferably migrates into the interface oxide layer during annealing to further oxidize the interface oxide. As a result, a less strenuous oxidation step is required to initially oxidize the lower HOCL layer and minimizes oxidative damage to the free layer.
1. A method of forming a hybrid oxide capping layer in a magnetic device, comprising:
(a) providing a substrate;
(b) depositing a first metal or first alloy layer on the substrate and performing a first oxidation step that at least partially oxidizes the first metal or first alloy layer to form an interface oxide layer;
(c) depositing a second metal or second alloy layer on the interface oxide layer and performing a second oxidation step that at least partially oxidizes the second metal or second alloy layer to form a first transition metal oxide layer, the second metal or second alloy has an absolute value of free energy of oxide formation that is less than that of the first metal or first alloy;
(d) depositing a third metal or third alloy layer on the first transition metal oxide layer and performing a third oxidation step that at least partially oxidizes the third metal or third alloy layer to form a second transition metal oxide layer, the third metal or third alloy has an absolute value of free energy of oxide formation that is less than that of the first metal or first alloy; and
(e) performing an anneal process that increases an oxidation state of the interface oxide layer to form a hybrid oxide capping layer (HOCL) comprised of a lower interface oxide layer, a first transition metal oxide layer, and a second transition metal oxide layer.
2. The method of claim 1 wherein the substrate is a free layer in a magnetic tunnel junction (MTJ), the free layer is comprised of one or more of CoFeB, CoFe, Co, Fe, CoB, FeB, and CoFeNiB, and exhibits perpendicular magnetic anisotropy.
3. The method of claim 1 wherein the HOCL has a resistance x area (RA) value that is reduced by decreasing a thickness of the HOCL or by reducing an oxidation state in one or more of the interface oxide layer, first transition metal oxide layer, and second transition metal oxide layer.
4. The method of claim 1 wherein the interface oxide layer is comprised of one or more of MgTaOx, SrTiOx, BaTiOx, CaTiOx, LaAlOx, MgO, TaOx, MnOx, VOx, and BOx.
5. The method of claim 1 wherein each of the first transition metal oxide layer and second transition metal oxide layer is comprised of RuOx, PtOx, RhOx, MoOx, WOx, SnOx, or InSnOx, and the second metal or second alloy is different from the third metal or third alloy.
6. The method of claim 1 further comprised of doping one or both of the first and second transition metal oxide layers with one or more of Fe, Co, Ni, Ru, Cr, Au, Ag, and Cu to increase conductivity therein.
7. The method of claim 1 wherein the anneal process is performed for a period of about 10 minutes to 2 hours at a temperature between about 250° C. and 500° C.
8. The method of claim 1 wherein the interface oxide layer is MgO/TaOx and is formed by oxidation of a Mg/Ta stack wherein Ta has an α-Ta phase, or is MgTaOx and is formed by oxidation of a Mg:Ta alloy wherein Ta has an α-phase.
9. The method of claim 1 wherein the interface oxide layer has a resistance x area (RA) value less than that of an equivalent thickness of MgO.