Methods of increasing write selectivity in an MRAM

Details Methods of increasing write selectivity in an MRAM Methods of increasing write selectivity in an MRAM

2002-12-20

2004-09-14

Auduong, Gene (Department: 2818)

Static information storage and retrieval

Interconnection arrangements

C365S158000, C365S171000, C365S173000

Reexamination Certificate

active

06791856

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to non-volatile memories, and more particularly to Giant Magneto Resistive (GMR) memories that use one or more word lines and one or more digital lines to select and write individual memory bits.
Digital memories of various kinds are used extensively in computer and computer system components, digital processing systems and the like. Such memories can be formed, to considerable advantage, based on the storage of digital bits as alternative states of magnetization of magnetic materials in each memory cell, typically thin-film materials. These films may be thin magneto-resistive films having information stored therein based on the direction of the magnetization occurring in those films. The information is typically obtained either by inductive sensing to determine the magnetization state, or by magneto-resistive sensing of each state.
Such thin-film magneto-resistive memories may be conveniently provided on the surface of a monolithic integrated circuit to thereby provide easy electrical interconnection between the memory cells and the memory operating circuitry on the monolithic integrated circuit. When so provided, it is desirable to reduce the size and increase the packing density of the thin-film magneto-resistive memory cells to achieve a significant density of stored digital bits.
Many thin-film magneto-resistive memories include a number of parallel word lines intersected by a number of parallel digital lines. A thin magneto-resistive film is provided at the intersection of each word line and digital line. As such, the thin film magneto-resistive memory cells typically are configured an array configuration having a number of rows and a number of columns.
FIG. 1
is a schematic diagram illustrating a conventional thin film Magnetic Random Access Memory (MRAM) architecture. Parallel word lines
12
,
14
,
16
,
18
and
20
are provided in a vertical direction and parallel digital lines
22
and
24
are provided in a horizontal direction. In the diagram shown, only a portion of the MRAM array is shown. A thin film magneto-resistive memory cell is provided at the intersection of each word line and digital line. Referring specifically to
FIG. 1
, thin film magneto-resistive memory cells
28
a
,
28
b
,
28
c
,
28
d
and
28
e
are provided at the intersection of digital line
22
and word lines
12
,
14
,
16
,
18
and
20
, respectively. Likewise, thin film magneto-resistive memory cells
30
a
,
30
b
,
30
c
,
30
d
and
30
e
are provided at the intersection of digital line
24
and word lines
12
,
14
,
16
,
18
and
20
, respectively.
The thin film magneto-resistive memory cells in each row are typically connected in a string configuration to form a corresponding sense line. For example, thin film magneto-resistive memory cells
28
a
,
28
b
,
28
c
,
28
d
and
28
e
, which correspond to row
32
, are connected in a string configuration to form sense line
34
. Sense line
34
typically includes a number of non-magnetic connectors
34
a
,
34
b
,
34
c
,
34
d
,
34
e
, and
34
f
to connect each end of the thin film magneto-resistive memory cells to the end of the adjacent thin film magneto-resistive memory cells. The non-magnetic connectors
34
a
,
34
b
,
34
c
,
34
d
,
34
e
, and
34
f
are typically formed using a conventional metal interconnect layer. The sense lines are used to provide current to a particular row of thin film magneto-resistive memory cells, and ultimately, to sense the resistance of a selected one of the cells.
To write a value (i.e. zero or one) to a selected memory cell, a word line current is provided to the word line that extends adjacent the selected memory cell. Likewise, a digital line current is provided to the digital line that extends adjacent the selected memory cell. In some instances, a sense line current is also provided to the sense line that that includes the selected memory cell.
The polarity of the word line current typically determines the value to be written into the selected memory cell. To illustrate this further, the magnetic fields produced by word line current
40
, digital line current
42
and sense current
44
at memory cell
30
a
are shown in
FIG. 1
, assuming digital line
46
and word line
12
extend above memory cell
40
. The polarity of the various currents would change if the corresponding word or digital line extend below the memory cell.
The magnetic field H
w1
48
produced by word line current
40
extends to the right and along the major axis of the memory cell
40
as shown. The magnetic field H
d1
50
produced by digital line current
42
extends upward and along the minor axis of the memory cell
40
. Finally, the magnetic field H
s1
52
produced by sense line current
44
extends upward and along the minor axis of the memory cell
40
.
The magnetic field H
w1
48
produced by word line current
40
provides the longitudinal force to switch the magnetization vector of the selected memory cell to the right, which in the example shown, corresponds to the desired value to be written. The magnetic fields H
d1
50
and H
s1
52
produced by digital line current
42
and sense line current
44
, respectively, provide the lateral torque necessary to initiate the switching of the magnetic vector of the selected memory cell.
FIG. 2
is a graph showing a typical write margin curve for an MRAM memory cell. The x-axis of the graph represents the magnetic field component H
w1
48
that extends down the major axis of the memory cell
30
a
, typically provided by the word line current. The y-axis represents the magnetic field component H
d1
50
that extends across the minor axis of the memory cell
30
a
, typically provided by the digital line current (and sense line current when so provided). The various combinations of H
w1
48
and H
d1
50
that are required to write the memory cell
30
a
are represented by curve
56
.
To provide some write margin, the sum of H
w1
48
and H
d1
50
(which produce a vector
58
) must extend to the right of curve
56
. The closer that the sum of H
w1
48
and H
d1
50
is to curve
56
, the less write margin is present. As the write margin decreases, it becomes more difficult to reliably write a selected memory cell. It also becomes more difficult to prevent other non-selected memory cells from being inadvertently written. To overcome these limitations, there are often very stringent process requirements for control of bit dimensions, edge roughness, and bit end contamination levels in the memory cells. These process requirements can become particularly burdensome as the memory cell size decreases to increase packing density.
The magneto-resistive memory cells are often GMR type memory cells. GMR type cells typically include a number of magnetically layers separated by a number of non-magnetic coercive layers. To considerable advantage, the magnetic vectors in one or all of the layers of a GMR type cell can often be switched very quickly from one direction to an opposite direction when a magnetic field is applied over a certain threshold. The states stored in a GMR type cell can typically be read by passing a sense current through the memory cell via the sense line and sensing the difference between the resistances (GMR ratio) when one or both of the magnetic vectors switch.
A limitation of many GMR type cells is that the magnetic field required to switch the magnetic vectors can be relatively high, which means that relatively high switching currents are required. This increase in current, or magnetic field, can result in a substantial operating power, especially in large memory arrays. As the size of the GMR cells shrink to accommodate higher density applications, the switching fields that are required also increase. It is expected under such circumstances that the current density in the word and/or digital lines may become too high even for Cu metallization.
One way to increase the write margin and reduce the current density requirements of such a device is shown in
FIG. 5
of the article “Experimenta

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