U.S. patent application number 16/786304 was filed with the patent office on 2021-08-12 for novel lattice matched seed layer to improve pma for perpendicular magnetic pinning.
The applicant listed for this patent is RONGFU XIAO. Invention is credited to RONGFU XIAO.
Application Number | 20210249469 16/786304 |
Document ID | / |
Family ID | 1000005735981 |
Filed Date | 2021-08-12 |
United States Patent
Application |
20210249469 |
Kind Code |
A1 |
XIAO; RONGFU |
August 12, 2021 |
NOVEL LATTICE MATCHED SEED LAYER TO IMPROVE PMA FOR PERPENDICULAR
MAGNETIC PINNING
Abstract
The invention comprises a novel composite seed layer with
lattice-matched crystalline structure so that an excellent
epitaxial growth of magnetic pinning layer along its FCC (111)
orientation can be achieved, resulting in a significant enhancement
of PMA for perpendicular spin-transfer-torque
magnetic-random-access memory (pSTT-MRAM) using perpendicular
magnetoresistive elements as basic memory cells which potentially
replace the conventional semiconductor memory used in electronic
chips, especially mobile chips for power saving and
non-volatility.
Inventors: |
XIAO; RONGFU; (DUBLIN,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
XIAO; RONGFU |
DUBLIN |
CA |
US |
|
|
Family ID: |
1000005735981 |
Appl. No.: |
16/786304 |
Filed: |
February 10, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 43/02 20130101;
H01L 43/10 20130101; H01F 10/3254 20130101; H01L 27/222 20130101;
H01F 10/3272 20130101 |
International
Class: |
H01L 27/22 20060101
H01L027/22; H01L 43/02 20060101 H01L043/02; H01L 43/10 20060101
H01L043/10; H01F 10/32 20060101 H01F010/32 |
Claims
1. A perpendicular magnetic pinning element (pMPE) comprising a
lattice-matched seed layer (LmSL) having a face-center-cubic (FCC)
crystalline structure; a perpendicular magnetic pinning layer
(pMPL) provided on the top surface of the LmSL and having a
face-center-cubic (FCC) crystalline structure and having an
invariable magnetization direction, and said LMSL/pMPL forming a
strong perpendicular magnetic anisotropy (PMA); an
antiferromagnetic coupling spacer (AFCs) provided on the top
surface of the pMPL and having a single layer structure of (Ru, Rh
or Ir), bi-layer structure of (Ru, Rh or Ir)/Cr or tri-layer
structure of (Ru, Rh, or Ir)/(W, Mo, or V)/Cr; a perpendicular
magnetic reference layer (pMRL) provided on the top surface of the
AFCs and having a body-center-cubic (BCC) crystalline structure and
having an invariable magnetization direction; said pMPE forming a
strong antiferromagnetic coupling (AFC);
2. The element of claim 1, wherein said pMPL is a multilayer stack
containing [Co/(Pt, Pd or Ni)]n/Co wherein n is an integer between
2 and 6, and thickness of each said Co and (Pt, Pd, or Ni) is
between 0.25 nm-0.7 nm and 0.2 nm-0.8 nm, respectively.
3. The element of claim 1, wherein said LmSL is a bi-layer stack
containing (Rh, Cu, Al, Ag, or Au)/((Pt, Pd, or Ir) or alloy of
(Pt, Pd, or Ir)) or (alloy of Rh, Cu, Al, Ag, or Au)/((Pt, Pd, or
Ir) or alloy of (Pt, Pd, or Ir)) having a FCC crystalline structure
with its lattice constant matched with said pMPL and having a
thickness between 0.5 nm-10 nm.
4. The element of claim 1, wherein said LmSL is a bi-layer stack
containing (RhN, CuN, AlN, or AgN)/((Pt, Pd, or Ir) or alloy of
(Pt, Pd, or Ir)) or (alloy of RhN, CuN, AlN, or AgN)/((Pt, Pd, or
Ir) or alloy of (Pt, Pd, or Ir)) with nitrogen (N) content tuned so
that said RhN, CuN, AlN, AgN having its lattice constant matched
with Pt, Pd, or Ir and having a thickness between 0.5 nm-10 nm.
5. The element of claim 1, wherein said LmSL is a multi-layer stack
containing (RhN, CuN, AlN, or AgN)/X/((Pt, Pd, or Ir) or alloy of
(Pt, Pd, or Ir)) or (alloy of RhN, CuN, AlN, or AgN)/X/((Pt, Pd, or
Ir) or alloy of (Pt, Pd, or Ir)) with nitrogen (N) content tuned so
that said RhN, CuN, AlN, AgN having its lattice constant matched
with Pt, Pd, or Ir and having a thickness between 0.5 nm-10 nm,
whereas X is a thin layer consisting of one or more transition
metal elements selected from the group of Ta, Mo, Hf, W, Nb, Zr, Ru
and having a thickness no more than 2 nm.
6. The element of claim 1, wherein said LmSL is a multi-layer stack
containing (RhN, CuN, AlN, or AgN)/X/((Pt, Pd, or Ir) or alloy of
(Pt, Pd, or Ir)) or (alloy of RhN, CuN, AlN, or AgN)/X/((Pt, Pd, or
Ir) or alloy of (Pt, Pd, or Ir)) with nitrogen (N) content tuned so
that said RhN, CuN, AlN, AgN having its lattice constant matched
with Pt, Pd, or Ir and having a thickness between 0.5 nm-10 nm,
whereas X is a thin layer consisting of one or more transition
metal elements selected from the group of Ta, Mo, Hf, W, Nb, Zr, Ru
and having a thickness between 0.3 nm-1.0 nm.
7. The element of claim 1, wherein said LmSL and said pMPL both
having their closed-packed FCC (111) crystalline orientation normal
to the film surface.
8. The element of claim 1, wherein said pMRL containing a bi-layer
stack of Fe/CoFeB, Fe/FeB, FeB/CoFeB, or Fe/CoFe and said Fe having
a thickness between 0.1-0.5 nm, said CoFeB, FeB and CoFe having a
thickness between 0.7 nm 1.3 nm.
9. The element of claim 1, wherein said pMRL containing a
multilayer stack of Fe/[Co/(Pt, Pd or Ni)]m/(W or Mo)/CoFeB, with m
an integer between 2 and 4; and the thickness of said Co and (Pt,
Pd, or Ni) is between 0.25 nm-0.7 nm and 0.2 nm-0.8 nm,
respectively, said CoFeB having a thickness between 0.7 nm-1.3 nm.,
said W, Mo having a thickness between 0.1 nm to 0.5 nm.
10. The element of claim 1, wherein said strong AFC between said
pMPL and said pMRL is achieved through interfacial RKKY coupling
having a film stack configuration of Co/AFCs/Fe or Co/AFCs/FeB,
FeCo(>50%)/AFCs/Fe(>50%)Co, or Co/AFCs/Fe(>40%)CoB;
wherein said Co or FeCo(>50%) layer is an interfacial portion of
pMPL contacting with the (Ru, Rh, or Ir) of said AFCs and said Fe,
FeB, Fe(>50%)Co or Fe(>40%)CoB layer is an interfacial
portion of pMRL contacting with the Cr of said AFCs.
11. The element of claim 1, wherein said (Ru, Rh or Ir) in single
layer structure of AFCs has a thickness between 0.4 nm to 0.85 nm,
wherein said (Ru, Rh or Ir) in bi-layer or tri-layer structure has
a thickness between 0.3 nm to 0.7 nm and said Cr or (W, Mo, or
V)/Cr in said AFCs has a thickness between 0.1 nm to 0.5 nm so that
the total thickness combination of Ru/Cr or (W, Mo, or V)/Cr is at
the (effective) first peak or 2.sup.nd peak of RKKY coupling with
their interfacial magnetic layers of Co and Fe.
12. The element of claim 1, wherein said pMPE has its magnetization
direction perpendicular to the surface of said film stack, and said
pMPE further forms a perpendicular magnetic tunnel junction (pMTJ)
together with a tunnel barrier (TB) and a storage layer (SL),
whereas said TB is sandwiched between said SL and said pMPL.
13. The element of claim 12, wherein said TB is an MgO layer having
a thickness between 0.8 nm to 1.5 nm, and said SL is a single layer
CoFeB or tri-layer CoFeB/(W or Mo)/CoFeB having a total CoFeB
thickness between 1 nm-2.0 nm, wherein said W or Mo has a thickness
between 0.1 nm-0.5 nm.
14. The element of claim 12, wherein said pMTJ comprises a film
stack of LmSL/pMPL/AFCs/pMRL/TB/SL/capping layer counting from
bottom to top, forming a bottom-pinned pSTT-MRAM film element.
15. The element of claim 12, wherein said pMTJ comprises a film
stack of BCC seed layer/SL/TB/pMRL/AFCs/pMPL/LmSL cap layer
counting from bottom to top, forming a top-pinned pSTT-MRAM film
element.
16. The element of claim 12, wherein said pMTJ comprises a film
stack of LmSL/pMPL1/AFCs1/pMRL1/TB1/SL/TB2/pMRL2/AFCs2/pMPL2/LmSL
cap layer, forming a dual-pinned pSTT-MRAM film element.
17. The element of claim 14, wherein said bottom-pinned pSTT-MRAM
film element comprises a film stack of
substrate/LmSL/[Co/Pt]n/Co/(Ru, Rh, or Ir)/[Co/Pt]m/Co/(Ta, W or
Mo)/CoFeB/MgO/CoFeB/(W or Mo)/CoFeB/MgO/W/Ru/Ta, with said
superlattice repeating numbers n and m ranging from 2 to 6 and 1 to
4 respectively.
18. The element of claim 14, wherein said bottom-pinned pSTT-MRAM
film element comprises a film stack of
substrate/LmSL/[Co/Pt]n/Co/(Ru, Rh, or Ir)/Cr/Fe/CoFeB/MgO/CoFeB/W
or Mo/CoFeB/MgO/W/Ru/Ta or substrate/Pt/[Co/Pt]n/Co/(Ru, Rh, or
Ir)/(W, Mo, or V)/Cr/Fe/CoFeB/MgO/CoFeB/W,
Mo/CoFeB/MgO/W/Ru/Ta.
19. The element of claim 16, wherein said dual-pinned pSTT-MRAM
film element comprises a film stack of
substrate/LmSL/[Co/Pt]n/Co/(Ru, Rh or Ir)/Cr/Fe/CoFeB/MgO/CoFeB/W
or Mo/CoFeB/MgO/CoFeB/Fe/Cr/(Ru, Rh or Ir)/Co/[Pt/Co]n/LmSL/W/Ru or
substrate/LmSL/[Co/Pt]n/Co/(Ru, Rh, or Ir)/(W, Mo, or
V)/Cr/Fe/CoFeB/MgO/CoFeB/W or Mo/CoFeB/MgO/CoFeB/Fe/Cr/(W, Mo, or
V)(Ru, Rh or Ir)/Co/[Pt/Co]n/LmSL/W/Ru.
20. The element of claim 16, wherein said dual-pinned pSTT-MRAM
film element comprises a film stack of substrate/ or
substrate/LmSL//[Co/(Pt or Pd)].sub.m/Co/(Ru or Ir)/Co/[(Pt or
Pd]/Co]n/(W, Mo or Ta)/CoFeB/MgO/CoFeB/(W or
Mo)/CoFeB/MgO/CoFeB/(W, Mo or Ta)/Co/[Co/Pt or Pd]n/Co/(Ru or
Ir)/Co/[(Pt or Pd)/Co]m/LmSL/cap.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] This invention relates to a novel lattice-matched seed layer
(LmSL) to improve perpendicular magnetic anisotropy (PMA) for
magnetic pinning multilayer in a magnetic structure, such as a
perpendicular magnetic tunnel junction.
2. Description of the Related Art
[0002] In recent years, magnetic random access memories
(hereinafter referred to as MRAMs) using the magnetoresistive
effect of ferromagnetic tunnel junctions (also called MTJs) have
been drawing increasing attention as the next-generation
solid-state nonvolatile memories that can cope with high-speed
reading and writing, large capacities, and low-power-consumption
operations. A ferromagnetic tunnel junction has a three-layer stack
structure formed by stacking a storage layer (SL) having a
changeable magnetization direction, an insulating spacing layer,
and a fixed pinning layer (PL) that is located on the opposite side
from the SL and maintains a predetermined magnetization direction.
The insulating spacing layer sandwiched between the SL and the PL
serves as a tunneling barrier (TB) in a magnetic tunnel junction.
In a SOT MRAM, there is an additional SOT layer immediately located
on a surface of the SL, which is opposite to a surface of the SL
where the insulating spacing layer is provided. SOT can be a thin
layer made of heavy transition metal layer such as W or Ta, Pt,
etc., or a layer of topological insulating layer such as BiSB.
[0003] As a write method to be used in such magnetoresistive
elements of a STT MRAM, there has been suggested a write method
(spin torque transfer switching technique) using spin momentum
transfers. According to this method, the magnetization direction of
a storage layer (SL) is reversed by applying a spin-polarized
current to the magnetoresistive element. Furthermore, as the volume
of the magnetic layer forming the SL is smaller, the injected
spin-polarized current to write or switch can be also smaller.
Accordingly, this method is expected to be a write method that can
achieve both device miniaturization and lower currents. In a SOT
MRAM, an electric current flows in the SOT layer, which is also a
paramagnetic layer, to generate a spin-polarized current and inject
it into its adjacent recording layer, which is a ferromagnetic
layer. The spin-polarized current then exerts a torque on the
magnetic moment to reverse it.
[0004] Further, as in a so-called perpendicular pMTJ element, both
two magnetization films, i.e., the storage layer (SL) and the
pinning layer (PL), have easy axis of magnetization in a direction
perpendicular to the film plane due to their strong perpendicular
interfacial anisotropy and magnetic crystalline anisotropy (shape
anisotropies are not used), and accordingly, the device shape can
be made smaller than that of an in-plane magnetization type. Also,
variance in the easy axis of magnetization can be made smaller.
Accordingly, by using a material having a large perpendicular
magnetic crystalline anisotropy, both miniaturization and lower
currents can be expected to be achieved while a thermal disturbance
resistance is maintained.
[0005] There has been a known technique for achieving a high MR
ratio in a perpendicular MTJ element by forming an underneath MgO
tunnel barrier layer and a BCC or HCP-phase cap layer that sandwich
a thin storage layer (SL) having an amorphous CoFeB ferromagnetic
film and accelerate crystallization of the amorphous ferromagnetic
film to match interfacial grain structure to MgO layer through a
thermal annealing process. The SL crystallization starts from the
tunnel barrier layer side to the cap layer and forms a CoFe grain
structure having a perpendicular magnetic anisotropy, as Boron
elements migrate into the cap layer. Accordingly, a coherent
perpendicular magnetic tunnel junction structure is formed. By
using this technique, a high MR ratio can be achieved.
[0006] A core structure of the pMTJ stack comprises (see FIG. 1) a
fixed perpendicular magnetic pinning element (pMPE-70), a tunnel
harrier, and a variable storage layer (SL-90). The pMPE is
typically formed by a relatively thick perpendicular synthetic
antiferromagnetic (pSAF) stack of composition:
seed-layer[Co/X].sub.m/Co/Ru/Co/[X/Co].sub.n/crystal-structure-breaking-l-
ayer (15)/FeCoB reference layer (16)/tunnel barrier (17), where X
represents Pt, Pd or Ni metals, m and n are integers (normally
m>n), and Ru is a spacer to provide perpendicular RKKY coupling
between [Co/X].sub.m/Co and Co/[X/Co].sub.n. Here and thereafter
throughout this application, each element written in the left side
of "/" is stacked below an element written in the right side
thereof. A typical film stack of bottom-pinned pMTJ (100) is shown
in FIG. 1 which starts on a substrate (10), a seed layer (111) such
as Pt, a perpendicular synthetic antiferromagnetic (pSAF)
multilayer stack containing a perpendicular magnetic pinning layer
(pMPL-12) [Co/Pt].sub.m/Co, a Ru spacer (113), an upper magnetic
multilayer (14) such as Co/[Pt/Co].sub.n, a crystal structure
transition layer (115) such as W, Mo or Ta, a magnetic reference
layer (16) such as amorphous CoFeB, a TB MgO (17), a tri-layer SL
formed with a first storage layer (18) such as CoFeB, a
non-magnetic B absorption layer (19) such as W, Mo or Ta and a
second storage layer (20) such as CoFeB, a capping layer (21), such
as MgO, W or W/Ru. This pMTJ comprises a thick pMPE film stack
although it has a strong pSAF.
[0007] Recently a French research group proposed (see Scientific
Reports 8, Article number: 11724, 2018) a thin synthetic
antiferromagnetic (tSAF) structure (see FIG. 2) comprising a Pt
seed-layer (11) on which pinning layer [Co/M].sub.m/Co (12),
hi-layer Ru/W spacer (23) and magnetic reference layer CoFeB (16).
Although the authors claimed multi-functionalities of their
bi-layer Ru/W spacer (23): (i) absorbing boron out of the magnetic
layer (FeCoB) in contact with W layer upon annealing, (ii) allowing
the crystalline transition between the FCC part of the stack
[Co/Pt]m/Co of 3-fold symmetry and the BCC part of the stack FeCoB
next to the MgO barrier (of 4-fold symmetry) and (iii) preventing
interdiffusion between the two parts of the SAF during high
temperature annealing. RKKY coupling at W/CoFeB interface is not as
strong as at the Co/Ru interface, such tSAF exhibits a serious
magnetic instability during information writing.
[0008] No matter whether it is a thick pSAF or thin tSAF film
stack, a key factor to achieve stable magnetic pinning is
perpendicular magnetic anisotropy (PMA) of the perpendicular
magnetic pinning layer (pMPL) [Co/Pt].sub.m/Co (12), which provides
a magnetic anchoring force to prevent the entire pSAF (or tSAF)
film stack from a concurrent rotation under the influence of spin
transfer torque or an external magnetic field. It was reported (see
Article: Appl. Phys. Lett. 96, 152505 (2010)) that the PMA of Co/Pt
(or Co/Pd) magnetic multilayer is closely dependent on the lattice
constant of the multilayer itself, and a positive (perpendicular)
PMA occurs only when Co/Pt (or Co/Pd) multilayer has FCC
crystalline structure with a lattice constant larger than
.about.0.372 nm, and the larger the lattice constant, the higher is
the PMA of Co/Pt (or Co/Pd) multilayer. Without an external factor,
increase of the PMA of Co/Pt (or Co/Pd) can only be achieved by
increasing the thickness of Pt (or Pd) spacer. However, a research
group found (see their report: Sensors, 17(12): 2743, December
2017) that the effective energy per bilayer starts to decrease
linearly after a lattice constant value of .about.0.383 nm. They
attributed this to the enhanced increase in the Pd fraction
compared to the Co, which weakens the ferromagnetic coupling
between the adjacent ultrathin Co layers.
SUMMARY OF THE PRESENT INVENTION
[0009] The present invention discloses a lattice-matched seed layer
(LmSL) having FCC crystalline structure to promote a perfect FCC
(111) growth for above perpendicular magnetic pinning layer (pMPL)
to enhance its PMA needed for magnetic stabilization. Said LmSL
comprises:
a bi-layer stack containing (Rh, Cu, Al, Ag, or Au)/(Pt, Pd, or Ir)
or (alloy of Rh, Cu, Al, Ag, or Au)/(Pt, Pd, or Ir) having a FCC
crystalline structure with its lattice constant matched with said
pMPL; or bi-layer stack containing (RhN, CuN, AlN, or AgN)/(Pt, Pd,
or Ir) or (alloy of RhN, CuN, AlN, or AgN)/(Pt, Pd, or Ir) with
nitrogen (N) content tuned so that said RhN, CuN, AlN, AgN having
its lattice constant matched with said pMPL; or a multi-layer stack
containing (RhN, CuN, AlN, or AgN)/X/(Pt, Pd, or Ir) or (alloy of
RhN, CuN, AlN, or AgN)/X/(Pt, Pd, or Ir) with nitrogen (N) content
tuned so that said RhN, CuN, AlN, AgN having its lattice constant
matched with Pt, Pd, or Ir and having a thickness between 0.5 nm-10
nm, whereas X is a thin layer consisting of one or more transition
metal elements selected from the group of Ta, Mo, Hf, W, Nb, Zr, Ru
and having a thickness between 0.1 nm-2 nm.
[0010] Said LmSL and pMPL both having an FCC crystalline structure
together with a composite non-magnetic spacer (CnmS) and a
perpendicular magnetic reference layer (pMRL) having a
body-center-cubic (BCC) crystalline structure constitute a strong
perpendicular magnetic pinning element (pMPE): LmSL/pMPL/CnmS)/pMRL
with enhanced synthetic antiferromagnetic (eSAF) coupling.
[0011] Said pMRL comprises a multilayer stack containing [Co/(Pt,
Pd or Ni)]m/Co, and said CnmS comprises either a single layer of
Ru, or Ir or a bi-layer of (Ru, Rh or Ir)/Cr or tri-layer of (Ru,
Rh, or Ir)/(W, Mo, or V)/Cr, and said pMRL comprises a multilayer
stack either of Co/[(Pt, Pd or Ni)]n/Co/(W, Mo, or Ta)/CoFeB for
single layer Ru spacer, or Fe/CoFeB, Fe/FeB, FeB/CoFe for bilayer
or tri-layer CnmS.
[0012] Said pMPE with large PMA are employed to form a
perpendicular magnetoresistive element (pMRE) comprising
LmSL/pMPL/CnmS/pMRL/TB/SL/CL or a reverse structure of BCC seed
layer/SL/TB/PMRL/CnmS/pMPL/LmSL, and wherein said TB is a tunnel
barrier, SL is a storage layer (SL) having magnetic anisotropy in a
direction perpendicular to a film surface and having a variable
magnetization direction on the tunnel barrier layer and CL is a
capping layer.
[0013] Application of said pMRE including bottom-pinned pSTT-MRAM,
top-pinned pSTT-MRAM and dual-pinned pSTT-MRAM. Said pMRE is
sandwiched between an upper electrode and a lower electrode of each
MRAM memory cell, which also comprises a write circuit which
bi-directionally supplies a spin polarized current to the
magnetoresistive element and a select transistor electrically
connected between the magnetoresistive element and the write
circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 A conventional pSTT-MRAM film stack with a
perpendicular synthetic antiferromagnetic (pSAF) pinning layer.
[0015] FIG. 2 A pSTT-MRAM film stack with a thin synthetic
antiferromagnetic (tSAF) pinning layer.
[0016] FIG. 3 Various configurations of said LmSL.
[0017] FIG. 4 A bottom-pinned pSTT-MRAM with a thick pSAF film
stack.
[0018] FIG. 5 A bottom-pinned pSTT-MRAM with a thin tSAF film
stack.
[0019] FIG. 6 Experimental results of magnetization for two thin
pSTT-MRAM film stacks without and with LmSL.
[0020] FIG. 7 A top-pinned pSTT-MRAM containing a thick pSAF capped
by LmSL.
[0021] FIG. 8 A top-pinned pSTT-MRAM containing a thin tSAF capped
by LmSL.
[0022] FIG. 9 A dual-pinned pSTT-MRAM containing two thick pSAF
film stacks.
[0023] FIG. 10 A dual-pinned pSTT-MRAM containing two thick tSAF
film stacks.
[0024] Table 1 List of crystalline structure and corresponding
lattice constant for some selected metallic elements.
DETAILED DESCRIPTION OF THE INVENTION
[0025] From prior art discussion, the PMA of the perpendicular
magnetic pinning layer (pMPL) (Co/Pt (or Co/Pd) multilayer) is
closely related to its lattice constant, and the larger is the
lattice constant, the higher is its PMA. In this invention, we
employ a lattice-matched seed layer (LmSL) with FCC (111)
crystalline structure at the bottom of Co/Pt or Co/Pd multilayer to
provide a specially engineered lattice mold (bedding) for the
growth of Co/Pt or Co/Pd multilayer to maximize its PMA. Among the
various materials in periodical table, there are some metallic
elements (see Table 1) which naturally form an FCC crystalline
structure in their solid phase with lattice constant close to or
slightly larger than that of Co/Pt (or Co/Pd). Said LmSL is in
direct contact with said pMPL either from below or on the top as a
cap with a thickness between 0.5 nm-10 nm; and said LmSL comprise
several film configurations as below: [0026] (1) said LmSL is a
bi-layer stack containing (Rh, Cu, Al, Ag, or Au)/(Pt, Pd, or Ir)
(stack 113 in FIG. 3) or (alloy of Rh, Cu, Al, Ag, or Au)/(Pt, Pd,
or Ir) (stack 114 in FIG. 3) having a FCC crystalline structure
with its lattice constant matched with said pMPL; wherein for the
alloyed LmSL, an element with a larger lattice constant (such as
Al, Ag, Au) can be mixed with element having a smaller lattice
constant (such as Ni or Cu) to form a just-right lattice constant
to maximize the PMA for Co/Pt (or Co/Pd) multilayer; [0027] (2)
said LmSL is a bi-layer stack containing (RhN, CuN, AlN, or
AgN)/(Pt, Pd, or Ir) or (alloy of RhN, CuN, AlN, or AgN)/(Pt, Pd,
or Ir) (stack 115 in FIG. 3) with nitrogen (N) content tuned so
that said RhN, CuN, AlN, AgN having its lattice constant matched
with or larger than Pt (or Pd); wherein the metal nitridation is
done by ion assisted physical vapor deposition (PVD) or ion bean
deposition (IBD); wherein said lattice constant of nitrides (RhN,
CuN, AlN, or AgN) can be increased by adding more nitrogen (N2) gas
to Ar gas during deposition, for example to a nitrogen-rich CuN can
have a lattice constant of 0.388 nm. [0028] (3) said LmSL is a
tri-layer stack (stack 116 in FIG. 3) containing (RhN, CuN, AlN, or
AgN)/X/(Pt, Pd, or Ir) or (alloy of RhN, CuN, AlN, or AgN)/X/(Pt,
Pd, or Ir) with nitrogen (N) content tuned so that said RhN, CuN,
AlN, AgN having its lattice constant matched with Pt (or Pd),
whereas X is a thin layer consisting of one or more transition
metal elements selected from the group of Ta, Mo, Hf, W, Nb, Zr, Ru
and has a thickness no more than 2 nm. Adding of such transition
metal in between, crystalline structure (lattice constant and
crystalline orientation) said LmSL can be optimized to further
enhance PMA for said pMPL.
[0029] The following lists are some typical embodiments to
illustrate the use of said LmSL to increase PMA for perpendicular
magnetic stabilization for bottom-pined, top-pined and dual-pinned
pSTT-MRAM having either a thick pSAF or thin tSAF film stack:
First Embodiment
[0030] FIG. 4 is bottom-pinned pSTT-MRAM (300) with a thick pASF
film stack. A lattice-matched seed layer (LmSL) (11) was first
grown on substrate with a thickness between 0.5-10 nm on a
substrate, followed by a thick pSAF stack (12/13/14) of [Co/Pt or
Pd].sub.mCo/(Ru or Ir)/Co/[(Pt or Pd)/Co].sub.n with thickness
values of (0.3-0.7) for Co and 0.2-0.8 for Pt (or Pd) and
repetition number (m>n) between 2-6 for m and 1 to 4 for n,
followed by a crystalline structure transition layer (15) of Ta, W,
or Mo with thickness between 0.1-0.5 nm, and magnetic reference
layer (16) of CoFeB with thickness between 0.9-1.3 nm, a tunnel
barrier (17) of MgO between 0.8-1.2 nm, a composite storage layer
of first magnetic CoFeB (18) with thickness between 1.0-1.5 nm, B
absorption layer of Ta, W, Mo) (19) with thickness between 0.15-0.5
nm, second magnetic CoFeB (20) with thickness between 0.5-1.0 nm
and a capping stack of MgO/W/Ru (21) with thickness of (1.0-1.5
nm)/1-3 nm)/(2-5 nm) respectively. The annealing temperature of
above film stack is between 350 C-450 C for 30 min to 150 min. With
the help of said LmSL, after annealing the bottom portion of the
stack between 11-14 will be converted into FCC crystalline
structure with (111) orientation normal to substrate surface and
upper portion of the stack (16-21) into a BCC (100) crystalline
structure to achieve a large PMA while maintaining high tunnel
magnetoresistive (TMR) value. During annealing the layer (15) of
Ta, W, or Mo helped the crystalline transition between bottom FCC
to top BCC structure.
Second Embodiment
[0031] FIG. 5 is a bottom-pinned pSTT-MRAM with a thin film stack
(400) with an enhanced synthetic antiferromagnetic (eSAF) coupling.
A lattice-matched seed layer (LmSL) (11) was first grown with a
thickness between 1-8 nm on a substrate (10), followed by a
magnetic superlattice [Co/X].sub.m (m is an integer between 2-6)
with thickness of Co (0.25-0.6 nm)/X (0.2-0.4 nm) (12) on top of
the seed layer where X is selected among Pt, Pd or Ni, a composite
eSAF Co/(Ru or Ir)/Cr/Fe (33) or Co/(Ru, Rh, or Ir)/(W, or
Mo)/Cr/Fe, an amorphous FeB or CoFeB (16) reference layer in
contact with Fe from below, a tunnel barrier MgO (17), a tri-layer
recording layer formed with a first magnetic layer (18), a
non-magnetic bridging layer (19) and a second magnetic layer (20),
a capping layer, such as MgO, W or W/Ru (21). In the above stack,
the thickness of Ru, Ru or Ir is between 0.3 to 0.7 nm and Cr or (W
or Mo)/Cr thickness is between 0.1 to 0.5 nm, with a combined (Ru,
Rh, or Ir)/Cr or (Ru, Rh, or Ir)/(W, or Mo)/Cr thickness chosen to
reach the first or second peak for an effective RKKY coupling, the
amorphous FeB or CoFeB reference layer (16) has a B composition
between 15-35% with a thickness between 0.8 to 1.4 nm, the
thickness of MgO TB is between 0.8-1.2 nm, the thickness of the
first magnetic memory layer (20) can be selected among CoFeB, FeB,
Fe/CoFeB with a B composition between 15-30% and preferably at 20%
and a thickness between 14.6 nm, the non-magnetic bridging layer is
selected among W, Mo, Ta with a thickness between 0.1-0.6 nm, the
second magnetic memory layer (20) is selected from CoFeB, FeB with
a B composition between 15-30% with a thickness between 0.4-0.8 nm,
the capping layer is can be either (1-1.5 nm)MgO/(2-5 nm)W, (2-5
nm)W/(2-4 nm)Ru or MgO/W/Ru. The use of Fe at the Cr interface, not
only increases the RKKY coupling hence improving magnetic stability
for the device, but also creates a good BCC structure right
starting from the CoFeB reference layer, throughout the barrier MgO
layer to the entire memory tri-layer layer owing to the intrinsic
BCC structure of Fe. Such a bottom-pinned pSTT-MRAM film stack will
have strong magnetic pinning with sharp layer interfaces and higher
and stable TMR characteristics, which is good for pSTT-MRAM device
application.
[0032] The annealing temperature of above bottom-pinned film
pSTT-MRAM stacks are between 350 C-450 C for 30 min to 150 min.
With the help of said LmSL, after annealing the low portion (11-13)
of the stack including Ru will be converted into FCC crystalline
structure with (111) orientation normal to film surface and upper
portion of the stack (16-21) above Cr into a BCC (100) crystalline
structure to achieve a large PMA while having a high tunnel
magnetoresistive (TMR) value. For comparison, FIG. 6 shows actual
two VSM test results obtained by growing two pSTT-MRAM both having
an exactly same film stack of seed layer/[Co(0.5 nm)/Pt(0.25
nm)]3/Ru(0.4 nm)/W(0.25 nm)/CoFeB(1.1 nm)/MgO(1 nm)/CoFeB(1.3
nm)/W(0.25 nm)/CoFeB(0.6 nm)/MgO(1.0 nm)/W(2 nm)/Ru(14 nm) except
sample-1 using a conventional seed layer and sample-2 using a
tri-layer LmSL (curve-2) of CuN(10 nm)/Ta(1 nm)/Pt(2 nm) as a seed
layer. While curve-1 exhibits a concurrent tSAF rotation under an
external magnetic field during VSM measurement while curve-2 not
only showing a larger RKKY coupling field but also exhibiting no
concurrent rotation throughout the entire measurement range.
Third Embodiment
[0033] FIG. 7 is a top-pinned pSTT-MRAM film stack (500) with a
thick pASF. Instead of using LmSL as seed layer, a material with
BCC structure such as W or Ta is used as seed layer and LmSL is
used as a top capping layer to promote formation of FCC crystalline
structure for the reversed pMPL. The full film stack is in a
reverse order of the above bottom-pinned pSTT-MRAM: i.e., starting
a seed layer(30) such as (W or Ta) with a thickness between 2-10
nm, and MgO (1-1.5 nm)/tri-layer memory layer of first CoFeB(31)
thickness between 0.5-0.8 nm/B absorption layer (Ta, W, or Mo)(32)
thickness between 0.1-0.5 nm/second CoFeB(33) thickness between
0.9-1.5 nm/MgO tunnel barrier(34) thickness between 1 nm-1.5
nm/CoFeB reference layer(35) thickness between 0.9-1.3 nm/(Ta, W,
Mo) crystalline structure transition layer (36) thickness between
0.1-0.5 nm/and a thick pSAF stack of [Co/Pt or Pd]n/Co(37)/(Ru or
Ir)(38)/Co/[Pt/Co]m(39)/capped with Pt/LmSL(40)/cap(41). The
annealing temperature of above film stack is between 350 C-450 C
for 30 min to 150 min. With the help of crystalline FCC structure
of LmSL, the multilayer [Pt/Co]m (39) will start to crystalline
from the surface in contact with said LmSL (40) and convert the
entire pSAF [Co/Pt or Pd]n/Co(37)/(Ru or Ir)(38)/Co/[Pt/Co]m stack
(37-41) into a FCC crystalline structure with (111) orientation
normal to the film surface and the lower portion of the stack
(30-35) into a BCC (100) crystalline structure obtaining a large
PMA while maintaining high tunnel magnetoresistive (TMR) value.
During annealing the layer (36) of Ta, W, or Mo helped the
crystalline transition between top FCC to bottom BCC structure.
Such a top-pinned pSTT-MRAM film stack will have strong magnetic
pinning with high TMR value, which is good for spin-orbit torque
(SOT) type MRAM device application.
Forth Embodiment
[0034] FIG. 8 is a top-pinned pSTT-MRAM film stack (600) with a
thin eSAF coupling with a reversed layer structure (except the seed
and capping layers) as compared with the above bottom-pinned
pSTT-MRAM shown in FIG. 5 of stack 400, seed layer(50)/memory
tri-layer(51, 52,53)/MgO(54)/CoFeB(55)/Fe/Cr/(Ru, Rh, or
Ir)(56)/Co/[Co/X]m(57)/LmSL(58)/cap layer(59). A seed layer (50)
with BCC structure selected among (W, Ta or PO/MgO or BiSB/MgO was
first grown on a substrate, followed a hi-layer memory layer (ML)
stack (51,52,53) similar to the memory layers (31,32, 33) shown in
FIG. 7, a tunnel barrier MgO(54), and a thin stack of
CoFeB/Fe/Cr/(Ru, Rh, or Ir)/Co with a thickness of Ru, Rh, or Ir is
between 0.3 to 0.7 nm and Cr thickness between 0.1 to 0.5 nm, in
contact with a magnetic superlattice pinning layer (PL) of [Co/Pt,
Pd, or Ni].sub.m/Co (12) (m is an integer between 2-6), which is
immediately covered a said LmSL and a final cap of ((W/Ru/Ta). The
annealing temperature of above film stack is between 350 C-450 C
for 30 min to 150 min which will convert the lower portion of the
stack below Cr into BCC crystalline structure under influence of
the bottom BCC seed layer (50) of W or T while changing the upper
portion of the pMPL stack into an FCC crystalline structure under
influence of said LmSL crystalline FCC capping structure. Such a
top-pinned pSTT-MRAM film stack will have strong magnetic pinning
with sharp layer interfaces and higher and stable TMR
characteristics, which is good for spin-orbit torque (SOT) type
MRAM device application.
Fifth Embodiment
[0035] FIG. 9 is a dual-pinned pSTT-MRAM (700) with two thick pSAF
and two LmSL stacks at bottom and top respectively, and FIG. 10 is
a dual-pinned pSTT-MRAM (800) with two thin eSAF and two LmSL
stacks at bottom and top respectively. A typical structure of stack
of 700 including substrate (10)/LmSL(11)//[Co/(Pt or
Pd)].sub.m/Co(12)/(Ru or Ir)(13)/Co/[(Pt or Pd]/Co]n(14)/(W, Mo or
Ta)(15)/CoFeB(16)/MgO(17)/CoFeB(18)/(W or
Mo)(19)/CoFeB(20)/MgO(34)/CoFeB(35)/(W, Mo or Ta)(36)/Co/[Co/Pt or
Pd]n(37)/Co/(Ru or Ir)(38)/Co/[(Pt or
Pd)/Co]m(39)/LmSL(40)/cap(41), and a typical structure of stack 800
including substrate (10)LmSL(11)//[Co/(Pt or
Pd)].sub.m/Co(12)/Ru/Cr/Fe/CoFeB(16)/MgO(17)/CoFeB(18)/(W or
Mo)(19)/CoFeB (20)/MgO(54)/CoFeB/(55)/Fe/Cr/(Ru(56)/Co/[(Pt or
Pd)/Co]m(57)/LmSL(58)/cap(59). In these dual pSTT-MRAM structures,
the two LmSL sandwich the two SAF pinning stacks. During high
temperature annealing, said LmSL will force the two multilayers
Co/(Pt or Pd)].sub.m in contact with from the bottom (layers
11,12,13, 14 in sack 700 and layers 11,12,33 in stack 800) and top
(layers 37, 38,39, 40 in stack 700 and layers 56,57,58 in stack
800) to transform into FCC crystalline structure with (111)
orientation aligned normal to the film surface and all the middle
layers (16,17,18,19,20,34,35 in stack 700 and layers
16,17,18,19,20,54,55 in stack 800 will transform into BCC
crystalline structure with (100) orientation, which not only allows
to further increase the perpendicular anisotropy (PMA) to the
middle composite storage layer (CSL) and thus increase the thermal
stability and prolong the retention time of MRAM device but also
allows to increase the thickness of the CSL (from 1.8 nm to 4 nm)
which further increases TMR value.
[0036] While certain embodiments have been described above, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
TABLE-US-00001 TABLE 1 Atomic No Element X-Structure Lattice const
[A] 13 Al FCC 4.05 28 Ni FCC 3.52 29 Cu FCC 3.61 45 Rh FCC 3.8 46
Pd FCC 3.89 47 Ag FCC 4.09 77 Ir FCC 3.84 78 Pt FCC 3.92 79 Au FCC
4.08 26 Fe BCC 2.87 42 Mo BCC 3.15 73 Ta BCC 3.31 74 W BCC 3.16 22
Ti HCP 2.95/4.68 27 Co HCP 2.51/4.07 44 Ru HCP 2.70/4.28
* * * * *