Method of providing stability of a magnetic memory cell

Details Method of providing stability of a magnetic memory cell Method of providing stability of a magnetic memory cell

2003-04-29

2004-08-31

Mai, Son (Department: 2818)

Static information storage and retrieval

Systems using particular element

Magnetoresistive

C365S171000, C365S173000

Reexamination Certificate

active

06785160

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to electronic memory. More particularly, the invention relates to a method of providing stability of a magnetic memory cell.
BACKGROUND OF THE INVENTION
Non-volatile memory is memory that retains its content (data) even when power connected to the memory is turned off. Magnetic random access memory (M RAM) is a type of non-volatile memory. MRAM includes storing a logical state, or bit, by setting magnetic field orientations of MRAM cells within the MRAM. The magnetic field orientations remain even when power to the MRAM cells is turned off.
FIG. 1
shows an MRAM cell
100
. The MRAM memory cell
100
includes a soft magnetic region
120
, a dielectric region
130
and a hard magnetic region
110
. The orientation of magnetization within the soft magnetic region
120
is non-fixed, and can assume two stable orientations as shown by the arrow M
1
. The hard magnetic region
110
(also referred to as a pinned magnetic region) has a fixed magnetic orientation as depicted by the arrow M
2
. The dielectric region
130
generally provides electrical insulation between the soft magnetic region
120
and the hard magnetic region
110
.
The MRAM memory cell generally is located proximate to a crossing point of a word line (WL) and a bit line (BL). The word line and the bit line can be used for setting the magnetic state of the memory cell, or for sensing an existing magnetic state of the memory cell.
FIG. 1
also includes a proximate word line that can also be used to set the magnetic state of the MRAM memory cell
100
. A magnetic field as depicted by the arrow
150
can be induced by a current I flowing through the proximate word line. The induced magnetic field can set the magnetic state of the MRAM memory cell
100
.
As previously stated, the orientation of magnetization of the soft magnetic region
120
can assume two stable orientations. These two orientations, which are either parallel or anti-parallel to the magnetic orientation of the hard magnetic region
110
, determine the logical state of the MRAM memory cell
100
.
The magnetic orientations of the MRAM memory cells can be set (written to) by controlling electrical currents flowing through the word lines and the bit lines, and therefore, by the corresponding magnetic fields induced by the electrical currents. Because the word line and the bit line operate in combination to switch the orientation of magnetization of the selected memory cell (that is, to write to the memory cell), the word line and the bit line can be collectively referred to as write lines. Additionally, the write lines can also be used to read the logic value stored in the memory cells. The electrical currents applied to the bit line and the word line set the orientation of the magnetization of the soft magnetic layer depending upon the directions of the currents flowing through the bit line and the word line, and therefore, the directions of the induced magnetic fields created by the currents flowing through the bit line and the word line.
The MRAM memory cells are read by sensing a resistance across the MRAM memory cells. The resistance is sensed through the word lines and the bit lines. Generally, the logical state (for example, a “0” or a “1”) of a magnetic memory cell depends on the relative orientations of magnetization in the data layer and the reference layer. For example, in a tunneling magneto-resistance memory cell (a tunnel junction memory cell), when an electrical potential bias is applied across the data layer and the reference layer, electrons migrate between the data layer and the reference layer through the intermediate layer (a thin dielectric layer typically called the tunnel barrier layer). The phenomenon that causes the migration of electrons through the barrier layer may be referred to as quantum mechanical tunneling or spin tunneling. The logic state can be determined by measuring the resistance of the memory cell. For example, the magnetic memory cell is in a state of low resistance if the overall orientation of the magnetization in its data storage layer is parallel to the pinned orientation of magnetization of the reference layer. Conversely, the tunneling junction memory cell is in a high resistance if the overall orientation of magnetization in its data storage layer is anti-parallel to the pinned orientation of magnetization of the reference layer. As mentioned, the logic state of a bit stored in a magnetic memory cell is written by applying external magnetic fields that alter the overall orientation of magnetization of the data layer. The external magnetic fields may be referred to as switching fields that switch the magnetic memory cells between high and low resistance states.
Magnetic stability of the data layer is important. That is, once the state of the data layer has been set by the externally applied magnetic fields, it is desirable that the magnetic state of the data layer remain the same until the external magnetic fields are once again applied.
Various factors can influence the stability of an MRAM memory cell. For example, certain shapes of memory cells are more stable than other shapes. Additionally, conductive line proximate to the memory cells can influence the stability of the memory cells.
While it is important to maintain MRAM memory cell stability, it is also important that the write lines and bit lines be able to change the logical state of the MRAM memory cell. That is, the stability must not be so great that externally applied write fields can not successfully write to the MRAM memory cell.
FIG. 2
shows an array
210
of MRAM memory cells. The logical states of each of the MRAM memory cells can be magnetically set by externally applied magnetic fields through bit lines (BL) and word lines (WL). Generally, the bit line and word line selections are made through a row decoder
220
and a column decoder
230
. The logical states of the memory cells are determined by a sense amplifier
240
.
It is desirable that the stability of each of the MRAM memory cells be approximately the same. That is, it is desirable that the magnetic field intensity required to write to each of the memory cells (more precisely, the magnetic field required to change the magnetic state of the memory cells) be consistent from one memory cell to another memory cell.
It is desirable to have a method and apparatus for providing desirable memory cell location with respect to conductive lines, and for providing desirable memory cell shapes, to ensure memory cell stability. It is additionally desirable that the method and apparatus provide the memory cell locations and memory cell shapes such that magnetic write fields of the memory cell can consistently change the logical state of the memory cell.
SUMMARY OF THE INVENTION
The invention includes a method and apparatus for providing desirable memory cell location with respect to conductive lines, and for providing desirable memory cell shapes, to ensure memory cell stability. The method and apparatus provides the memory cell locations and memory cell shapes such that magnetic write fields of the memory cell can consistently change the logical state of the memory cell.
An embodiment of the invention includes a method of providing magnetic stability of a memory cell. The memory cell is generally located proximate to a conductive line, and proximate to a write mechanism that can set a magnetic state of the memory cell. The method includes receiving a representation of a maximum magnetic field intensity available from the write mechanism. A desirable placement of the memory cell relative to the conductive line can be generated, for providing stability of the memory cell, while still allowing the write mechanism to change the magnetic state of the memory cell.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.


REFERENCES:
patent: 5734606 (1998-03-01), Tehrani et

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