Reduction of Barrier Resistance X Area (RA) Product and Protection of Perpendicular Magnetic Anisotropy (PMA) for Magnetic Device Applications

Abstract

A method of forming a MTJ with a tunnel barrier having a high tunneling magnetoresistance ratio, and low resistance x area value is disclosed. The method preserves perpendicular magnetic anisotropy in bottom and top magnetic layers that adjoin bottom and top surfaces of the tunnel barrier. A key feature is a passive oxidation step of a first Mg layer that is deposited on the bottom magnetic layer wherein a maximum oxygen pressure is 10 −5 torr. A bottom portion of the first Mg layer remains unoxidized thereby protecting the bottom magnetic layer from substantial oxidation during subsequent oxidation and anneal processes that are employed to complete the fabrication of the tunnel barrier and MTJ. An uppermost Mg layer may be formed as the top layer in the tunnel barrier stack before a top magnetic layer is deposited.

We claim: 
     
         1 . A method of forming a magnetic tunnel junction (MTJ) stack of layers including a tunnel barrier layer between two magnetic layers, comprising:
 (a) providing a bottom magnetic layer with perpendicular magnetic anisotropy (PMA);   (b) depositing a first metal layer that forms a bottom magnetic layer/first metal layer interface;   (c) performing a first passive oxidation process with a maximum oxygen pressure of about 10 −5  torr, the first passive oxidation process oxidizes an upper portion of the first metal layer while a bottom portion of the first metal layer along the bottom magnetic layer/first metal layer interface remains unoxidized;   (d) forming one or more metal or metal oxide layers on the oxidized portion of the first metal layer wherein steps (b)-(d) form a tunnel barrier layer; and   (e) depositing a top magnetic layer on a top surface of the tunnel barrier layer.   
     
     
         2 . The method of  claim 1  wherein the first passive oxidation process has a maximum duration of about 1000 seconds. 
     
     
         3 . The method of  claim 1  wherein the first metal layer has a thickness from about 1 to 6 Angstroms. 
     
     
         4 . The method of  claim 1  where the one or more metal oxide layers formed on the oxidized portion of the first metal layer are formed by one or more conventional methods comprising:
 (a) direct deposition of a metal oxide layer; 
 (b) depositing a metal layer and then oxidizing all or part of the metal layer with an oxygen pressure that is at least 10 −3  torr; and 
 (c) any combination or repetition of steps (a) and (b) above. 
 
     
     
         5 . The method of  claim 1  wherein the first metal layer, and the one or more metal oxide layers are comprised of a metal or alloy selected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa. 
     
     
         6 . The method of  claim 1  wherein the bottom magnetic layer is part of a synthetic antiferromagnetic (SAF) layer and is antiferromagnetically coupled to a second magnetic layer through a coupling layer in a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. 
     
     
         7 . The method of  claim 1  wherein the top magnetic layer is part of a SAF layer and is antiferromagnetically coupled to a second magnetic layer through a coupling layer in a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. 
     
     
         8 . The method of  claim 1  wherein the passive oxidation process is further comprised of nitrogen. 
     
     
         9 . The method of  claim 1  wherein at least one of the one or more metal oxide layers formed on the top surface of the oxidized portion of the first metal layer is further comprised of nitrogen and has a metal oxynitride composition. 
     
     
         10 . The method of  claim 1  further comprising an anneal process during or following the deposition of the MTJ stack, the anneal process comprises a temperature up to about 450° C. for a duration up to about 90 minutes. 
     
     
         11 . The method of  claim 1  wherein the first metal layer is comprised of a different metal than a metal in the one or more metal or metal oxide layers. 
     
     
         12 . The method of  claim 1  further comprised of forming a capping layer on the top magnetic layer. 
     
     
         13 . The method of  claim 12  wherein the capping layer is a metal oxide layer that is formed by a process sequence comprising one or more of a direct deposition method, a passive oxidation process, or an oxidation process comprising an oxygen pressure of at least 10 −3  torr. 
     
     
         14 . The method of  claim 1  further comprising the formation of a dual spin valve MTJ stack by a process comprising:
 (a) forming a second stack of layers on a top surface of the top magnetic layer, the second stack is formed by a process comprising;
 (1) depositing a second metal layer that contacts a top surface of the top magnetic layer; 
 (2) performing a second passive oxidation process with a maximum oxygen pressure of about 10 −5  torr, the second passive oxidation process oxidizes an upper portion of the second metal layer while a bottom portion of the second metal layer at an interface with the top magnetic layer remains unoxidized; and 
 (3) forming one or more metal or metal oxide layers on the oxidized portion of the second metal layer, steps (1)-(3) form a second tunnel barrier; and 
 
 (b) depositing a third magnetic layer on a top surface of the second tunnel barrier. 
 
     
     
         15 . A method of forming a magnetic tunnel junction (MTJ) stack of layers including a tunnel barrier layer between two magnetic layers, comprising:
 (a) providing a bottom magnetic layer with perpendicular magnetic anisotropy (PMA);   (b) depositing a first metal layer that forms a bottom magnetic layer/first metal layer interface;   (c) performing a first passive metal nitride deposition process with a maximum nitrogen pressure of about 10 −5  torr, the first passive metal nitride deposition process deposits a first metal nitride layer on the first metal layer while keeping the bottom magnetic layer/first metal layer interface from reacting with nitrogen;   (d) forming one or more metal, metal oxide, metal oxynitride, or metal nitride layers on the first metal nitride layer, steps (b)-(d) form a tunnel barrier layer; and   (e) depositing a top magnetic layer on a top surface of the tunnel barrier layer.   
     
     
         16 . The method of  claim 15  wherein the first metal layer has a thickness from about 1 to 6 Angstroms. 
     
     
         17 . The method of  claim 15  wherein the first metal layer, the first metal nitride layer, and the one or more metal, metal oxide, metal oxynitride, or metal nitride layers comprise a metal selected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa. 
     
     
         18 . The method of  claim 13  wherein the first metal layer is comprised of a different metal than in the first metal nitride layer, and in the one or more metal, metal oxide, metal oxynitride, or metal nitride layers. 
     
     
         19 . A method of forming a spin torque oscillator (STO) device, comprising:
 (a) forming a first metal oxide layer on a substrate that includes the steps of:
 (1) forming a first metal layer on the substrate and oxidizing an upper portion thereof with a first passive oxidation having a maximum oxygen pressure of about 10 −5  torr; and 
 (2) forming one or more metal or metal oxide layers on the oxidized upper portion of the first metal layer; 
   (b) forming a spin polarization (SP) layer on a top surface of the first metal oxide layer;   (c) forming a non-magnetic layer on the SP layer;   (d) forming an oscillation layer (OL) on the non-magnetic layer; and   (e) forming a second metal oxide layer on the OL with a process comprising:
 (1) forming a second metal layer on the OL and oxidizing an upper portion thereof with a second passive oxidation process having a maximum oxygen pressure of 10 −5  torr; and 
 (2) forming one or more metal or metal oxide layers on the oxidized upper portion of the second metal layer. 
   
     
     
         20 . The method of  claim 19  wherein the first and second passive oxidation processes have a maximum duration of about 1000 seconds, the maximum oxygen pressure is determined by directly controlling the oxygen pressure in a closed chamber, or by controlling an oxygen flow rate in a vented chamber. 
     
     
         21 . The method of  claim 19  wherein the first metal layer and the second metal layer each have a thickness from about 1 to 6 Angstroms. 
     
     
         22 . The method of  claim 19  where the one or more metal oxide layers formed on the oxidized upper portion of the first metal layer and the second metal layer are formed by one or more conventional methods comprising:
 (a) direct deposition of a metal oxide layer; 
 (b) depositing a metal layer and then oxidizing all or part of the metal layer with an oxygen pressure that is at least 10 −3  torr; and 
 (c) any combination or repetition of steps (a) and (b) above. 
 
     
     
         23 . The method of  claim 19  wherein the first and second metal layers, and the one or more metal or metal oxide layers formed on the oxidized upper portion of the first metal layer and the second metal layer are comprised of a metal selected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa. 
     
     
         24 . The method of  claim 19  wherein the first and second passive oxidation processes are further comprised of nitrogen. 
     
     
         25 . The method of  claim 19  wherein at least one of the one or more metal oxide layers formed on the top surface of the oxidized portion of the first metal layer is further comprised of nitrogen and has a metal oxynitride composition. 
     
     
         26 . The method of  claim 19  further comprising an anneal process following the formation of the second metal oxide layer, the anneal process comprises a temperature up to about 450° C. for a duration up to about 90 minutes. 
     
     
         27 . The method of  claim 19  wherein the first metal layer is comprised of a different metal than in the second metal layer or in the one or more metal or metal oxide layers formed on an oxidized upper portion of the first metal layer. 
     
     
         28 . The method of  claim 19  wherein the second metal layer is comprised of a different metal than in the first metal layer or in the one or more metal or metal oxide layers formed on an oxidized upper portion of the second metal layer. 
     
     
         29 . A method of forming an RF signal generation device, comprising:
 (a) forming a spin torque oscillator (STO) with a top surface and having at least one magnetic reference layer (MRL) that contacts a first terminal, a magnetic oscillation layer (MOL), and a first junction layer formed between the MRL and MOL;   (b) forming a non-magnetic spacer layer on the MOL, the non-magnetic spacer layer is connected to a second terminal;   (c) forming a magnetoresistive (MR) sensor on the non-magnetic spacer, the MR sensor has at least one magnetic sensing layer that is magnetostatically coupled with said MOL, a second magnetic reference layer, and a second junction layer that is a metal oxide formed between the magnetic sensing layer and the second magnetic reference layer, the metal oxide is formed by a process comprising:
 (1) depositing a first metal layer on the magnetic sensing layer; 
 (2) oxidizing an upper portion of the first metal layer with a passive oxidation process having a maximum oxygen pressure of 10 −5  torr; and 
 (3) forming one or more metal or metal oxide layers on an oxidized upper portion of the first metal layer; and 
   (d) forming a third terminal on the MR sensor, the magnetic sensing layer has an oscillation state with an oscillation frequency that is induced in said MOL when a magnetic field is applied to said STO and MR sensor in a direction perpendicular to the STO top surface concurrently with a first electric current flowing between the first and second terminals, and the magnetostatic coupling generates magnetic oscillation with an RF frequency in the magnetic sensing layer that produces a varying voltage across the MR sensor when a second electric current flows between the second and third terminals.   
     
     
         30 . The method of  claim 29  wherein the passive oxidation process has a maximum duration of about 1000 seconds, and the maximum oxygen pressure is controlled by directly controlling the oxygen pressure in a closed chamber, or by controlling an oxygen flow rate in a vented chamber. 
     
     
         31 . The method of  claim 29  wherein the first metal layer has a thickness from about 1 to 6 Angstroms. 
     
     
         32 . The method of  claim 29  where the one or more metal oxide layers formed on the oxidized upper portion of the first metal layer is formed by one or more conventional methods comprising:
 (a) direct deposition of a metal oxide layer; 
 (b) depositing a metal layer and then oxidizing all or part of the metal layer with an oxygen pressure that is at least 10 −3  torr; and 
 (c) any combination or repetition of steps (a) and (b) above. 
 
     
     
         33 . The method of  claim 29  wherein the first metal layer, and the one or more metal or metal oxide layers formed on the oxidized upper portion of the first metal layer are comprised of a metal selected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa. 
     
     
         34 . The method of  claim 29  wherein the passive oxidation process is further comprised of nitrogen. 
     
     
         35 . The method of  claim 29  wherein at least one of the one or more metal oxide layers formed on the top surface of the oxidized portion of the first metal layer is further comprised of nitrogen and has a metal oxynitride composition. 
     
     
         36 . The method of  claim 29  further comprising an anneal process following formation of the MR sensor, the anneal process comprises a temperature up to about 450° C. for a duration up to about 90 minutes. 
     
     
         37 . The method of  claim 29  wherein the first metal layer is comprised of a different metal than in the one or more metal or metal oxide layers formed on an oxidized upper portion of the first metal layer. 
     
     
         38 . A method of forming a three terminal device, comprising:
 (a) forming a bottom magnetic layer as a polarizing layer that contacts a first terminal;   (b) forming a non-magnetic metal layer or a low RA tunnel barrier on the polarizing layer, where the low RA tunnel barrier is formed by a process comprising:
 (1) depositing a first metal layer on the bottom magnetic layer; 
 (2) oxidizing an upper portion of the first metal layer with a passive oxidation process having a maximum oxygen pressure of 10 −5  torr; and 
 (3) forming one or more metal or metal oxide layers on the oxidized upper portion of the first metal layer; 
   (c) depositing a middle magnetic layer as a free layer on the non-magnetic metal layer or the low RA tunnel barrier, the free layer contacts a second terminal, a current flowing between the first terminal and second terminal is used during a write operation;   (d) forming a tunnel barrier on the free layer where the tunnel barrier formation process comprises:
 (1) depositing a second metal layer on the free layer; 
 (2) oxidizing an upper portion of the second metal layer with a passive oxidation process having a maximum oxygen pressure of 10 −5  torr; and 
 (3) forming one or more metal or metal oxide layers on an oxidized upper portion of the second metal layer; and 
   (e) depositing a top magnetic layer as a reference layer on the tunnel barrier, the reference layer contacts a third terminal, a current flowing between the second terminal and third terminal is employed during a read operation.   
     
     
         39 . A method of forming a three terminal device, comprising:
 (a) forming a bottom magnetic layer as a reference layer that contacts a first terminal;   (b) forming a tunnel barrier on the reference layer where the tunnel barrier formation process comprises:
 (1) depositing a first metal layer on the reference layer; 
 (2) oxidizing an upper portion of the first metal layer with a passive oxidation process having a maximum oxygen pressure of 10 −5  torr; and 
 (3) forming one or more metal or metal oxide layers on an oxidized upper portion of the first metal layer; 
   (c) depositing a middle magnetic layer as a free layer on the tunnel barrier, the free layer contacts a second terminal, a current flowing between the first terminal and second terminal is used during a read operation;   (d) forming a non-magnetic metal layer or a low RA tunnel barrier on the free layer, where the low RA tunnel barrier is formed by a process comprising:
 (1) depositing a second metal layer on the free magnetic layer; 
 (2) oxidizing an upper portion of the second metal layer with a passive oxidation process having a maximum oxygen pressure of 10 −5  torr; and 
 (3) forming one or more metal or metal oxide layers on the oxidized upper portion of the second metal layer; and 
   (e) depositing a top magnetic layer as a polarizing layer on the non-magnetic metal layer or low RA barrier, the polarizing layer contacts a third terminal, a current flowing between the second terminal and third terminal is employed during a write operation.