Multilayer Structure for Reducing Film Roughness in Magnetic Devices

Abstract

A seed layer stack with a uniform top surface having a peak to peak roughness of 0.5 nm is formed by sputter depositing an amorphous layer on a smoothing layer such as Mg where the latter has a resputtering rate 2 to 30× that of the amorphous layer. The seed layer stack may be repeated to give a laminate of two amorphous layers and two smoothing layers, and is advantageous for enhancing performance in magnetic tunnel junctions in embedded MRAMs, spintronic devices, or in read head sensors. A template layer such as NiCr may be formed on the uppermost smoothing layer to promote and maintain perpendicular magnetic anisotropy in an overlying magnetic layer during high temperature processing up to 400° C. The amorphous seed layer is SiN, TaN, or CoFeM where M is B or another element with a content that makes CoFeM amorphous as deposited.

We claim: 
     
         1 . A multilayer structure for reducing film roughness in a magnetic device, comprising:
 (a) a buffer layer that is one or more of Zr, ZrN, Nb, NbN, Mo, MoN, TiN, W, WN, and Ru, or one of more of the aforementioned materials with Ta or TaN that is formed on a substrate;   (b) a first smoothing (S 1 ) layer made of a material with a first bond energy, and having a first surface with an “as deposited” first peak to peak roughness, the S 1  layer is formed on the buffer layer;   (c) a second smoothing (S 2 ) layer that is non-crystalline or nano-crystalline and is made of a material with a second bond energy that is greater than the first bond energy such that deposition of the S 2  layer results in resputtering of the S 1  layer to give a S 1  layer with a second surface having a second peak to peak roughness substantially less than the “as deposited” first peak to peak roughness, and the S 2  layer formed on the second surface, the S 2  layer has a third surface with the second peak to peak roughness;   (d) a third smoothing (S 3 ) layer with the first bond energy that is formed on the S 2  layer; and   (e) a fourth smoothing (S 4 ) layer that is non-crystalline or nano-crystalline with the second bond energy, and formed on the S 3  layer.   
     
     
         2 . The multilayer structure of  claim 1  further comprised of a template layer that is an uppermost layer in the multilayer structure, and with a top surface having the second peak to peak roughness, the template layer has a (111) crystal orientation to promote PMA in an overlying magnetic layer. 
     
     
         3 . The multilayer structure of  claim 2  wherein the template layer is one of NiW, NiMo, NiCr, NiFeCr, Hf, Hf/NiCr, Hf/NiFeCr, NiCr/Hf, or NiFeCr/Hf. 
     
     
         4 . The multilayer structure of  claim 1  wherein each of the S 1  and S 3  layers is one or more of Mg, Al, Si, C, B, Mn, Rb, Zn, and Ti. 
     
     
         5 . The multilayer structure of  claim 1  wherein the each of the S 2  and S 4  layers is one of TaN, SiN, and a CoFeM alloy wherein M is one of B, P, Ta, Zr, Si, Cu, Hf, Mo, W, and Nb with a content which makes the CoFeM alloy amorphous as deposited. 
     
     
         6 . The multilayer structure of  claim 1  wherein each of the S 1  and S 3  layers has a thickness from about 3 to 20 Angstroms. 
     
     
         7 . The multilayer structure of  claim 1  wherein each of the S 2  and S 4  layers has a thickness from about 2 to 15 Angstroms. 
     
     
         8 . The multilayer structure of  claim 2  wherein the overlying magnetic layer contacts the top surface of the template layer, and is a reference layer in a magnetic tunnel junction (MTJ) having a bottom spin valve configuration, or is a free layer in a MTJ with a top spin valve configuration. 
     
     
         10 . The multilayer structure of  claim 2  wherein the overlying magnetic layer is a reference layer, free layer, or dipole layer in a magnetic random access memory (MRAM) device, spin torque oscillator (STO), spintronic device, or a read head sensor. 
     
     
         11 . A method of forming a magnetic tunnel junction (MTJ), comprising:
 (a) forming a buffer layer on a substrate wherein the buffer layer is one or more of Zr, ZrN, Nb, NbN, Mo, MoN, TiN, W, WN, and Ru, or one of more of the aforementioned materials with Ta or TaN; and   (b) forming a seed layer stack (SL 1 ) on the buffer layer, comprising:
 (1) sputter depositing a first seed layer with a first bond energy on the buffer layer, the first seed layer has a first surface with a first peak to peak roughness; 
 (2) sputter depositing a second seed layer that is non-crystalline or nano-crystalline and with a second bond energy on the first seed layer, the second bond energy is greater than the first bond energy, and results in a second surface on the first seed layer with a second peak to peak roughness substantially less than the first peak to peak roughness, and the second seed layer formed on the second surface, the second seed layer has a third surface with the second peak to roughness; and 
 (3) sputter depositing a template seed layer to form an uppermost layer in the SL 1  stack wherein the template seed layer has a (111) crystal orientation with a top surface having the second peak to peak roughness, and promotes perpendicular magnetic anisotropy (PMA) in an overlying magnetic layer; and 
   (c) depositing the overlying magnetic layer that contacts the top surface of the template layer.   
     
     
         12 . The method of  claim 11  wherein the second seed layer is sputter deposited with a process comprising an inert gas that is Ar, a base pressure between 5×10 −8  and 5×10 −9  torr, a sputter deposition chamber temperature up to 400° C., and a forward power of 20 W to 5000 W applied to one or more targets. 
     
     
         13 . The method of  claim 11  wherein the template seed layer is one of NiW, NiMo, NiCr, NiFeCr, Hf, Hf/NiCr, Hf/NiFeCr, NiCr/Hf, or NiFeCr/Hf. 
     
     
         14 . The method of  claim 11  wherein the overlying magnetic layer is a reference layer in a magnetic tunnel junction (MTJ) with a bottom spin valve configuration and the MTJ is further comprised of a tunnel barrier on the reference layer, a free layer on the tunnel barrier layer, and an uppermost capping layer. 
     
     
         15 . The method of  claim 11  wherein the overlying magnetic layer is a free layer in a magnetic tunnel junction (MTJ) with a top spin valve configuration and the MTJ is further comprised of a tunnel barrier on the free layer, a reference layer on the tunnel barrier layer, and an uppermost capping layer. 
     
     
         16 . The method of  claim 11  wherein forming the SL 1  stack further comprises sputter depositing a third seed layer with the first bond energy that contacts a top surface of the second seed layer, and then sputter depositing a fourth layer with the second bond energy on the third seed layer before depositing the template layer. 
     
     
         17 . The method of  claim 11  wherein the first seed layer is one of Mg, Al, Si, C, B, Mn, Rb, Zn, and Ti. 
     
     
         18 . The method of  claim 11  wherein the second seed layer is one of TaN, SiN, and a CoFeM alloy wherein M is one of B, P, Ta, Zr, Si, Cu, Hf, Mo, W, and Nb with a content which makes the CoFeM alloy amorphous as deposited.