U.S. patent application number 09/492557 was filed with the patent office on 2002-05-09 for magnetic memory with structures that prevent disruptions to magnetization in sense layer.
Invention is credited to Anthony, Thomas C..
Application Number | 20020055190 09/492557 |
Document ID | / |
Family ID | 23956730 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020055190 |
Kind Code |
A1 |
Anthony, Thomas C. |
May 9, 2002 |
Magnetic memory with structures that prevent disruptions to
magnetization in sense layer
Abstract
A magnetic memory cell is disclosed having a structure that
prevents disruptions to the magnetization in the sense layer of the
magnetic memory cell. In one embodiment, the structure includes a
high permeability magnetic film that serves as a keeper for the
sense layer magnetization. The keeper structure provides a flux
closure path that directs demagnetization fields away from the
sense layer. In another embodiment, the structure contains a hard
ferromagnetic film that applies a local magnetic field to the sense
layer in the magnetic memory cell.
Inventors: |
Anthony, Thomas C.; (Palo
Alto, CA) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD
INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Family ID: |
23956730 |
Appl. No.: |
09/492557 |
Filed: |
January 27, 2000 |
Current U.S.
Class: |
438/3 ;
257/E21.665; 257/E27.005 |
Current CPC
Class: |
H01L 27/222 20130101;
G11C 11/161 20130101; G11C 11/15 20130101; G11C 11/16 20130101;
B82Y 10/00 20130101 |
Class at
Publication: |
438/3 |
International
Class: |
H01L 021/00 |
Claims
What is claimed is:
1. A magnetic memory cell, comprising: sense layer for storing a
magnetization state that indicates a logic state of the magnetic
memory cell; structure that prevents disruptions to the
magnetization state in the sense layer.
2. The magnetic memory cell of claim 1, wherein the structure
overlaps a pair of opposing edge regions of the sense layer and
prevents one or more demagnetization fields from forming in the
edge regions of the sense layer.
3. The magnetic memory cell of claim 1, wherein the structure is
formed from a permeable ferromagnetic material having a shape that
provides flux closure for one or more demagnetization fields in the
sense layer.
4. The magnetic memory cell of claim 1, wherein the structure is
formed from a permeable ferromagnetic material having an easy axis
that is perpendicular to an easy axis of the sense layer.
5. The magnetic memory cell of claim 1, wherein the structure
encases a conductor that provides read and write access to the
magnetic memory cell.
6. The magnetic memory cell of claim 1, further comprising a
reference layer and a tunnel barrier between the sense layer and
the reference layer.
7. The magnetic memory cell of claim 6, wherein the sense layer is
adjacent to the structure.
8. The magnetic memory cell of claim 6, wherein the reference layer
is adjacent to the structure.
9. The magnetic memory cell of claim 1, wherein the sense layer is
exchange coupled to the structure.
10. The magnetic memory cell of claim 1, wherein the structure is
formed from a hard ferromagnetic material.
11. The magnetic memory cell of claim 10, wherein the hard
ferromagnetic material is magnetized perpendicular the an easy axis
of the sense layer.
12. The magnetic memory cell of claim 10, wherein the sense layer
is exchange coupled to the structure.
13. A magnetic memory cell, comprising: sense layer for storing a
magnetization that indicates a logic state of the magnetic memory
cell; means for providing flux closure for one or more
demagnetization fields in the magnetic memory cell.
14. The magnetic memory cell of claim 13, wherein the means for
providing flux closure comprises a permeable ferromagnetic material
having a shape that provides a path for magnetic flux transport
between a pair of opposing edge regions of the sense layer.
15. The magnetic memory cell of claim 14, wherein the permeable
ferromagnetic material has an easy axis that is perpendicular to an
easy axis of the sense layer.
16. A method for forming a magnetic memory with a set of
structures, comprising the steps of: forming a set of trenches in a
substrate; depositing a layer of magnetic material for the
structures so that the magnetic material coats horizontal and
vertical surfaces of the trenches and the substrate; depositing a
layer of conductor material on the layer of magnetic material to
fill the trenches; polishing the layer of conductor material and
the layer of magnetic material to expose an upper surface of the
substrate.
17. The method of claim 16, wherein the conductor material is
copper.
18. The method of claim 16, wherein the step of polishing comprises
the step of polishing using a chem-mechanical process.
19. The method of claim 16, wherein the step of forming a set of
trenches comprises the step of forming a set of trenches using
reactive ion etching.
20. The method of claim 16, further comprising the steps of:
depositing a material for a sense layer in each of a set of
magnetic memory cells in the magnetic memory; depositing a material
for a tunnel barrier in each of the magnetic memory cells;
depositing a material for a reference layer in each of the magnetic
memory cells.
21. The method of claim 16, wherein the material for the sense
layer is deposited before the materials for the tunnel barrier and
reference layers.
22. The method of claim 21, wherein the material for the reference
layer is deposited before the materials for the tunnel barrier and
sense layers.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The present invention pertains to the field of magnetic
memories. More particularly, this invention relates to providing a
magnetic memory with structures that prevent disruptions to
magnetization in sense layers.
[0003] 2. Art Background
[0004] A magnetic memory such as a magnetic random access memory
(MRAM) typically includes an array of magnetic memory cells. Each
magnetic memory cell usually includes a sense layer and a reference
layer. The sense layer is usually a layer or film of magnetic
material that stores magnetization patterns in orientations that
may be altered by the application of external magnetic fields. The
reference layer is usually a layer of magnetic material in which
the magnetization is fixed or "pinned" in a particular
direction.
[0005] The logic state of a magnetic memory cell typically depends
on its resistance to electrical current flow. The resistance of a
magnetic memory cell usually depends on the relative orientations
of magnetization in its sense and reference layers. A magnetic
memory cell is typically in a low resistance state if the overall
orientation of magnetization in its sense layer is parallel to the
orientation of magnetization in its reference layer. In contrast, a
magnetic memory cell is typically in a high resistance state if the
overall orientation of magnetization in its sense layer is
anti-parallel to the orientation of magnetization in its reference
layer.
[0006] Typically, the overall magnetization pattern in the sense
layer of a magnetic memory cell includes magnetization in its
interior region and magnetization in its edge regions. In prior
magnetic memory cells, demagnetization fields commonly present in
the edge regions of the sense layer disrupt the overall orientation
of magnetization in the sense layer from the desired parallel and
antiparallel orientations. In addition, coupling fields and
demagnetization fields from the reference layer can disrupt the
magnetization of the sense layer from the desired parallel or
antiparallel orientations. Such disruptions may manifest as
undesirable magnetic domains.
[0007] Unfortunately, such disruptions to magnetization in the
sense layer usually obscure the high and low resistance states of a
magnetic memory cell, thereby making it difficult to determine the
logic state of the magnetic memory cell during a read operation. In
addition, the degree of disruption to sense layer magnetization may
vary among the magnetic memory cells in an MRAM array and may vary
between different MRAM arrays due to variation in the patterning
steps and/or deposition steps of device manufacture. Such variation
in the sense layer magnetization states usually leads to variations
in the threshold switching field. Such variations in the threshold
switching field typically produces uncertainty in MRAM write
operations.
SUMMARY OF THE INVENTION
[0008] A magnetic memory cell is disclosed having a structure that
prevents disruptions to the magnetization in the sense layer of the
magnetic memory cell. In one embodiment, the structure includes a
high permeability magnetic film that serves as a keeper for the
sense layer magnetization. The keeper structure provides a flux
closure path that directs demagnetization fields away from the
sense layer. In another embodiment, the structure contains a hard
ferromagnetic film that applies a local magnetic field to the sense
layer in the magnetic memory cell.
[0009] The present techniques yield greater repeatability of
magnetization characteristics among the magnetic memory cells in
MRAM arrays. The structure has an additional advantage of enlarging
the effective volume of the magnetic memory cell, thereby improving
the thermal stability of the stored magnetization state. The
structure also functions as an electromagnet to facilitate writing
of the magnetic memory cells, thereby reducing MRAM power
consumption.
[0010] Other features and advantages of the present invention will
be apparent from the detailed description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The present invention is described with respect to
particular exemplary embodiments thereof and reference is
accordingly made to the drawings in which:
[0012] FIGS. 1a-1c illustrate one embodiment of a structure for
stabilizing a magnetic memory cell;
[0013] FIG. 2 shows one embodiment of the magnetic memory cell
which is stabilized by a structure;
[0014] FIGS. 3a-3b show the "S" state and the "C" state,
respectively, of the magnetization in a sense layer of a magnetic
memory cell;
[0015] FIG. 4 is a top view of a magnetic memory, an MRAM, which
incorporates the present teachings;
[0016] FIGS. 5a-5e illustrate process steps for forming the
structures disclosed herein and the conductors of a magnetic
memory;
[0017] FIG. 6 shows another alternative embodiment of a structure
for stabilizing a magnetic memory cell;
[0018] FIG. 7 shows yet another alternative embodiment of a
structure for stabilizing a magnetic memory cell.
DETAILED DESCRIPTION
[0019] FIGS. 1a-1c illustrate one embodiment of a structure 56 for
stabilizing a magnetic memory cell 40. The structure 56 encases a
conductor 20 which provides a path for electrical current flow
during read and write operations on the magnetic memory cell
40.
[0020] FIG. 1a shows a cross-sectional side view of the structure
56 and the magnetic memory cell 40 in a direction parallel to the
length of the conductor 20. FIG. 1b shows a cut-away top view of
the structure 56 and the conductor 20 through the magnetic memory
cell 40. Portions of the structure 56 overlap a pair of edge
regions 157-158 of the magnetic memory cell 40. FIG. 1c l shows a
perspective view of the structure 56 and the magnetic memory cell
40.
[0021] FIG. 2 shows one embodiment of the magnetic memory cell 40.
The magnetic memory cell 40 includes a sense layer 50 that has an
alterable magnetization state and a reference layer 54 having a
pinned orientation of magnetization. In this embodiment, the
magnetic memory cell 40 includes a tunnel barrier 52 between the
sense layer 50 and the reference layer 54.
[0022] This embodiment of the magnetic memory cell 40 is a spin
tunneling device in which an electrical charge migrates through the
tunnel barrier 52 during read operations. This electrical charge
migration through the tunnel barrier 52 occurs when a read voltage
is applied to the magnetic memory cell 40. In an alternative
embodiment, a giant magneto-resistive (GMR) structure may be used
in the magnetic memory cell 40 in which the tunnel barrier 52 is
replaced with a conductor such as Cu.
[0023] In one embodiment, the structure 56 serves as a keeper for
the sense layer 50 magnetization and may be referred to as the
keeper structure 56. The keeper structure 56 is a soft magnetic
material that provides a mechanism for flux closure, thereby
preventing the formation of demagnetization fields in the edge
regions 157-158. The keeper structure 56 is a high permeability
ferromagnetic film that is magnetized with an easy axis
substantially perpendicular to the easy axis of the sense layer 50
of the magnetic memory cell 40. The proximity of the keeper
structure 56 to the magnetic memory cell 40 causes any
demagnetization fields that would have been produced in the absence
of the keeper structure 56 to be directed through the keeper
structure 56. This provides a path for flux that substantially
eliminates demagnetizing fields from acting on the sense layer 50
in the magnetic memory cell 40. This prevents the overall
magnetization in the sense layer 50 of the magnetic memory cell 40
from straying from the desired parallel or antiparallel directions
with respect to the pinned reference layer 54 in the magnetic
memory cell 40. The keeper structure 56 stabilizes the magnetic
memory cell 40 in that it provides a pair of stable and discernable
high and low resistance states for storing a data bit.
[0024] The keeper structure 56 reduces the electrical current level
needed to write the magnetic memory cell 40 to a desired logic
state. The keeper structure 56 is analogous to a single-turn
electromagnet. Electrical current flowing through the conductor 20
rotates the magnetization of the keeper structure 56 from its
quiescent state along its length to a direction perpendicular to
the direction of electrical current flow according to the right
hand rule. This creates a magnetic field along the easy axis of the
sense layer 50 in the magnetic memory cell 40 which is useful for
rotating the magnetization in the sense layer 50 to either the
parallel or antiparallel state with respect to the pinned reference
layer 54 of the magnetic memory cell 40.
[0025] A reduction in the electrical current level needed to write
the magnetic memory cell 40 is desirable because it reduces power
consumption in a magnetic memory such as an MRAM. A reduction in
power consumption is particularly advantageous for portable
applications. In addition, a reduction in the electrical current
level needed to write the magnetic memory cell 40 reduces the
integrated circuit chip area consumed by the power transistors that
supply write currents. The chip area savings lowers the cost of a
magnetic memory.
[0026] The keeper structure 56 obviates the need to reduce the
thickness of the sense layer 50 in the magnetic memory cell 40 or
to increase or elongate the d.sub.x and d.sub.y dimensions of the
magnetic memory cell 40 in an attempt to reduce the effects of
demagnetization fields in the sense layer 50. This enables magnetic
memories to be formed with thicker sense layers which increases the
thermal stability of the magnetic memory by increasing the magnetic
volume of the magnetic memory cell 40 and enhances uniformity in
the switching behavior among the magnetic memory cells of a
magnetic memory. This also enables the formation of magnetic memory
cells with smaller d.sub.x, and d.sub.y dimensions which increases
the data storage density of a magnetic memory. In addition, the
keeper structure 56 itself adds effective magnetic volume to the
magnetic cell 40 which increases the thermal stability of the
stored magnetization state.
[0027] In one embodiment, the dimensions d.sub.xand d.sub.y of the
magnetic memory cell 40 are selected to be substantially equal and
form a square shape for its sense layer 50. The square shape of the
sense layer 50 enhances the density that may be obtained in an MRAM
in comparison to that which may be obtained when using rectangular
memory cells. This is so because for a given minimum feature size
more square magnetic memory cells may be formed on a given
substrate area than rectangular magnetic memory cells. In other
embodiments, rectangular or other shapes may be used.
[0028] The sense layer 50 or the reference layer 54 may be directly
exchange coupled to the keeper structure 56 or magnetically
decoupled from the keeper structure 56 by spacer layers.
[0029] In one embodiment, the magnetic memory cell 40 is positioned
so that the sense layer 50 is adjacent to the keeper structure 56.
The sense layer 50 is directly exchange coupled to the keeper
structure 56 at the edge regions 157 and 158. The sense layer 50 is
influenced by the magnitude and direction of the magnetic
anisotropy of the keeper structure 56.
[0030] FIGS. 3a-3b show the "S" state and the "C" state,
respectively, of the magnetization in the sense layer 50. Since the
easy axis of the keeper structure 56 lies along the length of the
conductor 20, the sense layer 50 has a local exchange field applied
to the edge regions 157 and 158 that is perpendicular to the easy
axis of the sense layer 50. Application of this orthogonal field in
the edge regions 157 and 158 forces the sense layer 50
magnetization to be in a "S" state as opposed to an "C" state. The
"S" state may have more reproducible switching characteristics.
[0031] Alternatively, the magnetic memory cell 40 is flipped over
so that the reference layer 54 is adjacent to the keeper structure
56. The sense layer 50 is not exchange coupled to the keeper
structure 56 but is influenced by the proximity of the permeable
keeper structure 56 and no orthogonal field is generated in the
edge regions 157-158.
[0032] FIG. 4 is a top view of a magnetic memory 10, an MRAM, which
incorporates the present teachings. The magnetic memory 10 includes
an array of magnetic memory cells including the magnetic memory
cell 40 along with additional magnetic memory cells 41-43. The
magnetic memory 10 includes an arrangement of conductors 20-21 and
30-31 that enable read and write access to the magnetic memory
cells 40-43.
[0033] The conductors 30-31 are top conductors and the conductors
20-21 are orthogonal bottom conductors encased in corresponding
structures 56-57. The conductor 20 provides a bottom conductor for
both magnetic memory cells 40 and 42 and the structure 56 provides
a structure for both magnetic memory cells 40 and 42. Similarly,
the conductor 21 provides a bottom conductor for both magnetic
memory cells 41 and 43 and the structure 57 provides a structure
for both magnetic memory cells 41 and 43.
[0034] The structures 56 and 57 are each magnetized with an easy
axis that is substantially parallel to the y axis. The easy axes of
the sense layers in the magnetic memory cells 40-43 are
substantially parallel to the x axis. Electrical current flowing
through the conductor 20 creates magnetic writing fields which are
parallel to the x axis and parallel to the easy axes of the sense
layers in the corresponding magnetic memory cells 40 and 42.
Similarly, electrical current flowing through the conductor 21
creates magnetic writing fields parallel to the easy axes of the
sense layers in the corresponding magnetic memory cells 41 and 43.
Electrical current flow through the conductor 30 or 31 generates a
magnetic field in the y direction. Only the magnetic memory cells
that experience a combination of x and y magnetic fields are
written.
[0035] FIGS. 5a-5e illustrate process steps for forming the
structures 56-57 and the conductors 20-21 of the magnetic memory
10. The magnetic memory 10 is formed on a substrate 100 (FIG. 5a)
which in one embodiment is a dielectric such as silicon-dioxide
(SiO.sub.2).
[0036] A set of trenches 102-104 (FIG. 5b) are formed in the
substrate 100. The trenches 102-104 may be formed using, for
example, reactive ion etching.
[0037] Next, a stabilization layer 106 (FIG. 5c) is deposited on
the substrate 100 and its trenches 102-104. The stabilization layer
106 is a layer of ferromagnetic material which may be a soft
magnetic material such as nickel-iron(NiFe) in a keeper structure
embodiment or hard material such as CoPt, CoPtCr, or CoPtTa in the
alternative embodiment. The stabilization layer 106 is preferably
deposited using a technique such as sputtering which coats both
horizontal and vertical surfaces of the substrate 100 and its
trenches 102-104.
[0038] A layer of conductor material 108 (FIG. 5d) such as copper
is then deposited on the stabilization layer 106. The conductor
material 108 may be deposited using sputtering, evaporation, or
plating steps.
[0039] A chem-mechanical polishing (CMP) step is then applied to
planarize the surface and expose the substrate 100 (FIG. 5e).
[0040] The layers of the magnetic memory cells 40-43 are then
deposited on the polished surface of the substrate 100 and
patterned over the structures 56-57. The layers for the magnetic
memory cells 40-43 in one embodiment include the following. First,
a set of seed layers of tantalum, nickel-iron, and iron-manganese
are deposited. Next, a layer of nickel-iron is deposited which
serves as the reference layers of the magnetic memory cells 40-43.
A dielectric layer such as aluminum-oxide (Al.sub.2O.sub.3) is then
deposited which serves as the tunnel barriers within the magnetic
memory cells 40-43. Next, a layer of nickel-iron is deposited which
is to be patterned into the sense layers of the magnetic memory
cells 40-43. Finally, tantalum is deposited as an encapsulating
layer.
[0041] In an alternative embodiment of a structure for stabilizing
a magnetic memory cell 40, the structure 56 is a hard ferromagnetic
material that is magnetized along the length of the conductor 20, a
direction that is substantially perpendicular to the easy axis of
the sense layer 50. In this alternative embodiment, the structure
56 does not function as a keeper but is instead a source of
magnetic field for stabilizing the edge regions 157 and 158. The
structure 56 is directly exchange coupled to the under side of the
sense layer 50. As a result, the longitudinally magnetized hard
magnetic material of the structure 56 interacts with the sense
layer 50. Such an exchange coupled configuration generates the
desired "S" state in the magnetization of the sense layer 50 by
forcing the magnetization in the edge regions 157 and 158 to be
aligned parallel to the direction of magnetization of the structure
56. Exchange coupling the sense layer 50 to the structure 56 forces
the magnetization into the "S" state.
[0042] FIG. 6 shows another alternative embodiment of a structure
for stabilizing a magnetic memory cell 40. In this alternative
embodiment, the structure 56 is a soft magnetic film of uniform
thickness which is patterned to substantially the same width as the
conductor 20. The magnetization of the structure 56 lies parallel
to the length of the conductor 20 and substantially perpendicular
to the easy axis of the sense layer 50. The soft magnetic film that
forms the structure 56 may be located anywhere through the
thickness of the conductor 20. The total thickness of the conductor
20 is t which is equal to t.sub.1+t.sub.2 and the position of the
structure 56 can range from t.sub.1=0 to t.sub.2=0.
[0043] FIG. 7 shows another alternative embodiment of a structure
for stabilizing the magnetic memory cell 40. In this alternative,
the keeper structure 56 is inverted in comparison to the embodiment
shown in FIGS. 1a-1c. A thin layer 200 of, for example, tantalum
lies between the keeper structure 56 and the magnetic memory cell
40. The magnetization of the keeper structure 56 lies parallel to
the length of the conductor 20 and substantially perpendicular to
the easy axis of the sense layer 50.
[0044] The foregoing detailed description of the present invention
is provided for the purposes of illustration and is not intended to
be exhaustive or to limit the invention to the precise embodiment
disclosed. Accordingly, the scope of the present invention is
defined by the appended claims.
* * * * *