Hafnium doped cap and free layer for MRAM device
View Patent ↗A high performance MTJ, and a process for manufacturing it, are described. A capping layer of NiFeHf is used to getter oxygen out of the free layer, thereby increasing the sharpness of the free layer-tunneling layer interface. The free layer comprises two NiFe layers whose magnetostriction constants are of opposite sign, thereby largely canceling one another.
1. A process for the manufacture of an MTJ device, that includes a free layer having a magnetic moment, comprising:
providing a substrate and depositing thereon a magnetic pinning layer;
depositing a magnetically pinned layer on said pinning layer;
depositing a tunnel barrier layer on said pinned layer;
depositing on said tunnel barrier layer a composite free layer comprising first and second layers of NiFe whose magnetostriction constants are of opposite sign, thereby canceling one another;
depositing on said composite free layer a first cap layer of NiFeHf;
depositing on said first cap layer a second cap layer of Ru on Ta; and
then heating said device for a time and at a temperature that are sufficient for oxygen trapped in said composite NiFe layer to diffuse into said NiFeHf layer, thereby sharpening an interface between said tunnel barrier layer and said composite free layer.
2. The process described in claim 1 wherein said first NiFe layer comprises between about 17.5 and 21 atomic percent Fe and said second NiFe layer comprises about 12 atomic percent Fe.
3. The process described in claim 1 wherein said first NiFe layer is deposited to a thickness between about 5 and 20 Angstroms and said second NiFe layer is deposited to a thickness between about 10 and 30 Angstroms.
4. The process described in claim 1 wherein said first NiFe layer comprises about 12 atomic percent Fe and said second layer is NiFeHf(15%).
5. The process described in claim 4 wherein said first NiFe layer is deposited to a thickness between 20 and 30 Angstroms and said second NiFeHf layer is deposited to a thickness between 25 and 50 Angstroms.
6. The process described in claim 4 wherein said composite free layer has a magnetostriction value less than about 2×10 −6 .
7. The process described in claim 4 wherein said MTJ device has a dR/R value of at least 40%.
8. An MTJ device, that includes a free layer having a magnetic moment comprising:
a magnetic pinning layer on a substrate;
a magnetically pinned layer on said pinning layer;
a tunnel barrier layer on said pinned layer;
on said tunnel barrier layer, a composite free layer comprising first and second layers of NiFe whose magnetostriction constants are of opposite sign;
on said composite free layer, a first cap layer of NiFeHf; and
on said first cap layer, a second cap layer of ruthenium on tantalum.
9. The MTJ device described in claim 8 wherein said NiFeHf layer contains about 15 or less atomic percent of hafnium.
10. The MTJ device described in claim 8 wherein said tunnel barrier layer is selected from the group consisting of AlOx, MgO, AlHfOx, AlTiOx, and TiOx.
11. The MTJ device described in claim 8 wherein said first NiFe layer comprises about 17.5 atomic percent Fe and said second NiFe layer comprises about 12 or less atomic percent Fe.
12. The MTJ device described in claim 8 wherein said first NiFe layer has a thickness between about 5 and 20 Angstroms and said second NiFe layer has a thickness between about 10 and 30 Angstroms.
13. The MTJ device described in claim 12 wherein said composite free layer has a magnetostriction constant that is less than about 2×10 −6 .
14. The MTJ device described in claim 12 wherein said MTJ device has a dR/R value of at least 40%.
15. An MTJ device whose free layer is a bilayer of Ni 88 Fe 12 , between 20 and 30 Angstroms thick, and (NiFe) 85 Hf 15 , about 45 Angstroms thick, whereby said MTJ device has a dR/R ratio greater than 47%.