High performance MTJ element for conventional MRAM and for STT-RAM and a method for making the same
View Patent ↗A STT-RAM MTJ that minimizes spin-transfer magnetization switching current (Jc) is disclosed. The MTJ has a MgO tunnel barrier layer formed with a natural oxidation process to achieve a low RA (10 ohm-um 2 ) and a Fe or Fe/CoFeB/Fe free layer which provides a lower intrinsic damping constant than a CoFeB free layer. A Fe, FeB, or Fe/CoFeB/Fe free layer when formed with a MgO tunnel barrier (radical oxidation process) and a CoFeB AP1 pinned layer in a MRAM MTJ stack annealed at 360° C. provides a high dR/R (TMR)>100% and a substantial improvement in read margin with a TMR/Rp_cov=20. High speed measurement of 100 nm×200 nm oval STT-RAM MTJs has shown a J c0 for switching a Fe free layer is one half that for switching an amorphous CO 40 Fe 40 B 20 free layer. A Fe/CoFeB/Fe free layer configuration allows the Hc value to be increased for STT-RAM applications.
1. A method of forming a MRAM MTJ on a substrate, comprising:
(a) sequentially forming a seed layer, AFM layer, and a pinned layer on said substrate, said pinned layer has an AP2/coupling layer/AP1 configuration wherein the AP2 layer is formed on an AFM layer and the AP1 layer contacts a MgO tunnel barrier layer;
(b) forming a crystalline MgO tunnel barrier having a crystal orientation on the AP1 pinned layer by a process sequence comprising:
(1) depositing a first Mg layer that contacts the AP1 layer;
(2) performing a ROX method to convert said first Mg layer to a MgO layer; and
(3) depositing a second Mg layer on the MgO layer;
(c) forming a free layer consisting of a FeB x layer, or a composite with a Fe/CoFeB/Fe composition on the MgO tunnel barrier where x is between 0 and about 5 atomic %; and
(d) forming a capping layer on the FeB x or Fe/CoFeB/Fe free layer.
2. The method of claim 1 wherein the AP2 layer is CoFe, the coupling layer is Ru, and the AP1 layer is CoFeB.
3. The method of claim 1 wherein the first Mg layer has a thickness from 8 to 12 Angstroms, the second Mg layer has a thickness from about 2 to 4 Angstroms, and the ROX process comprises a RF power of about 500 to 800 Watts, and an oxygen flow rate from about 0.4 to 0.8 slm for a period of about 80 to 150 seconds.
4. The method of claim 1 further comprised of annealing the MRAM MTJ layers along an easy axis direction by applying a magnetic field of about 5000 to 10000 Oe at a temperature between about 330° C. and 360° C. for a period of about 1 to 5 hours.
5. The method of claim 1 wherein the free layer is a Fe/CoFeB/Fe layer and wherein the Fe/CoFeB/Fe free layer has a lower Fe layer on the MgO tunnel barrier with a thickness from about 3 to 5 Angstroms, a middle CoFeB layer with a thickness of about 10 to 12 Angstroms, and an upper Fe layer with a thickness from about 5 to 6 Angstroms.
6. The method of claim 1 wherein the capping layer is comprised of a NiFeHf layer that contacts the free layer and a Ta layer on the NiFeHf layer.
7. A method of forming a STT-RAM MTJ on a substrate, comprising:
(a) sequentially forming a seed layer, AFM layer, and a pinned layer on said substrate, said pinned layer has an AP2/coupling layer/AP1 configuration wherein the AP2 layer is formed on an AFM layer and the AP1 layer contacts a MgO tunnel barrier layer;
(b) forming the MgO tunnel barrier having a crystal orientation on the AP1 pinned layer by a process sequence comprising:
(1) depositing a first Mg layer on the AP1 layer;
(2) performing a NOX method to convert said first Mg layer to a MgO layer; and
(3) depositing a second Mg layer on the MgO layer;
(c) forming a free layer consisting of a FeB x layer, or having a Fe/CoFeB/Fe composition on the MgO tunnel barrier where x is between 0 and about 5 atomic %; and
(d) forming a capping layer on the FeB x , or Fe/CoFeB/Fe free layer.
8. The method of claim 7 wherein the AP2 layer is CoFe, the coupling layer is Ru, and the AP1 layer is a composite comprised of a lower CoFeB layer on said coupling layer and an upper CoFe layer contacting the MgO tunnel barrier layer.
9. The method of claim 7 wherein the first Mg layer has a thickness from about 6 to 8 Angstroms, the second Mg layer has a thickness from about 3 to 5 Angstroms, and the NOX process comprises a 1 torr pressure and an oxygen flow rate from about 0.1 to 1.0 slm for a period of about 60 to 100 seconds.
10. The method of claim 7 further comprised of annealing the STT-RAM MTJ layers along an easy axis direction by applying a magnetic field of about 5000 to 10000 Oe at a temperature between about 300° C. and 350° C. for a period of about 1 to 5 hours.
11. The method of claim 7 wherein the free layer is a Fe/CoFeB/Fe free layer and wherein the Fe/CoFeB/Fe free layer has a lower Fe layer on the MgO tunnel barrier with a thickness from about 3 to 5 Angstroms, a middle CoFeB layer with a thickness of about 10 to 12 Angstroms, and an upper Fe layer with a thickness from about 5 to 6 Angstroms.