Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2003-10-15
2004-10-19
Nguyen, Van Thu (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S171000, C365S145000
Reexamination Certificate
active
06807093
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to magnetic structures. More particularly, it pertains to enhancing memory devices using magnetic material so that a desired memory cell is selected while other memory cells are unselected.
BACKGROUND OF THE INVENTION
A memory device is a device where information typically in the form of binary digits can be stored and retrieved. Such a device includes dynamic random access memory (DRAM), static random access memory (SRAM), and flash memory. Despite being slower, DRAMs are more commonly used than other memory types because they can be fabricated in high density to store a large amount of information. SRAMs are usually reserved for use in caches because they can operate at high speed. Unlike both DRAMs and SRAMs, which retain information as long as there is applied power, flash memory is a type of nonvolatile memory, which will keep information even if power is no longer applied. Flash memory is typically not used as main memory, however, because its block-oriented architecture prevents memory access in single-byte increments.
Another memory type has emerged that can be fabricated in high density, operated at high speed, and retain information even after power is no longer applied. This memory type is magnetic random access memory (MRAM).
FIG. 1A
is a block diagram showing a portion of an MRAM array
100
according to the prior art. The MRAM array
100
includes a number of memory cells, such as memory cells
106
1,1
to
106
3,4
, which are arranged in a number of rows (word lines),
104
1
to
104
3
, and a number of columns (bit lines),
102
1
to
102
4
. Each of these memory cells
106
1,1
to
106
3,4
stores information magnetically instead of electronically as in DRAMs, SRAMs, and flash memory. As an example, to select the memory cell
106
2,3
for reading and writing, a row current I
row
is issued over the row
104
2
and a column current (I
col
) is issued over the column
102
3
.
FIG. 1B
is a partial cross-sectional isometric view of the portion of the MRAM array
100
according to the prior art. Each memory cell is sandwiched between a portion of a row and a portion of a column. Rows and columns are formed from strips of conductive material. Following the example above, when the row current I
row
is present in the row
104
2
, the magnetic field H
y
that is generated by this current partially selects memory cells
106
2,1
to
106
2,4
. When the column current I
col
, is present in the row
102
3
, the magnetic field H
x
that is generated by this current partially selects memory cells
106
1,3
to
106
3,3
. Because memory cell
106
2,3
is exposed to both magnetic fields (H
x
and H
y
), it is fully selected for reading or writing information.
FIG. 1C
is an exploded isometric view of the memory cell
106
2,3
and a portion of the row
104
2
and the column
102
3
according to the prior art. The row current I
row
creates the magnetic field H
y
that comprises a magnetic flux line
108
and the column current I
col
, creates the magnetic field H
x
that comprises a magnetic flux line
110
. These magnetic flux lines,
108
and
110
, change the dipolar orientation of the memory cell (north or south)
106
2,3
. In this way, by taking advantage of the dipolar nature of a magnetic material that comprises the memory cell
106
2,3
, a bit of information can be represented as a 0 or a 1.
FIG. 1D
is a graph showing the ferromagnetic nature of the memory cell
106
2,3
according to the prior art. The graph shows a hysteresis loop
112
, which shows the relationship of induction B as a function of magnetic field strength, H. With a sufficient coercive field H
c
applied to the memory cell
106
2,3
, the magnitude of the induction B rises until it levels off at a saturation induction, B
s0
. The coercive field H
c
is a combination of the magnetic fields H
x
and H
y
. As the coercive field H
c
is removed by withdrawing power to the memory cell
106
2,3
, much of the induction B is retained by dropping its magnitude to a remanent induction B
r0
. This ability to retain the induction B even after power is no longer applied allows each memory cell of the MRAM array
100
to be nonvolatile. The induction B can be moved to another saturation induction, B
s1
, by the application of the coercive field H
c
. When power is again withdrawn, the magnitude of the induction B drops slightly to settle at a remanent reduction B
r1
. A bit of information can be magnetically represented as a 0 or a 1 by forcing the induction B to settle at the remanent induction B
r0
or B
r1
.
FIG. 1E
is a graph showing the ferromagnetic nature of the memory cell
106
2,3
as a relationship between resistance R and coercive field H
c
according to the prior art. This relationship is shown as a hysteresis loop
114
, which illustrates that the memory cell
106
2,3
exhibits a high resistance R
H
at one magnetized orientation (remanent induction B
r0
) and a low resistance R
L
at another magnetized orientation (remanent induction B
r1
). As a practical matter, it is less complicated to measure resistance to determine whether a 0 or a 1 is being stored by the memory cell
106
2,3
than to measure the induction B as shown in FIG.
1
D.
FIG. 1F
is a graph showing the coercive field H
c
that defines the relationship between the magnetic field H
y
, which is formed from the row current I
row
, and the magnetic field H
x
, which is formed from the column current l
col
according to the prior art. The shaded area
116
0
, which is underneath the curve of the coercive field H
c
, defines a region where the memory cell
106
2,3
is partially selected but is not sufficiently selected for reading and writing information despite the application of one or both the magnetic fields H
x
and H
y
. The area
118
0
, which is above the curve of the coercive field H
c
, defines a region where the memory cell
106
2,3
is fully selected because both the magnetic fields H
x
and H
y
are of a sufficient magnitude. The dashed line
120
illustrates an application of both the magnetic fields H
x
and H
y
at the same magnitude to select the memory cell
106
2,3
and to unselect (or partially select) memory cell
106
2,3
when only one of the magnetic fields H
x
and H
y
is applied.
FIG. 1G
is a graph showing a full-select probability distribution
118
1
, which represents a range of H
x
where the memory cell is fully selected, and a partial-select probability distribution
116
1
, which represents another range of H
x
where the memory cell is partially selected, according to one embodiment of the present invention. The probability distribution
118
1
reflects the application of both the magnetic fields H
x
and H
y
at the same magnitude to fully select the memory cell
106
2,3
. The probability distribution
116
1
reflects the application of only the magnetic field H
x
but not H
y
to unselect (or partially select) the memory cell
106
2,3
. As shown, a portion of the area under the probability distribution
118
1
overlaps with a portion of the area under the probability distribution
116
1
. This overlapped area indicates that an ambiguity exists in the process of selecting the memory cell
106
2,3
. For example, in certain circumstances, the memory cell
106
2,3
may be fully selected even though only the magnetic field H
x
is applied. This accidental selection of a memory cell may compromise the integrity of the data stored by the memory cells.
Without a solution to unambiguously select a magnetic memory cell for reading and writing information, consumers may question the reliability of this type of memory device, which may lead to its eventual lack of acceptance in the marketplace. Thus, there is a need for structures and methods to increase the reliability of magnetic memory devices.
SUMMARY OF THE INVENTION
An illustrative aspect of the present invention includes various methods for increasing a magnetic field to unambiguously select a magnetic memory cell structure. One method includes folding a current line into two portions around a magnetic memory
Dorsey & Whitney LLP
Micro)n Technology, Inc.
Nguyen Tuan T.
Nguyen Van Thu
LandOfFree
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