Segmented write line architecture for writing magnetic...

Details Segmented write line architecture for writing magnetic... Segmented write line architecture for writing magnetic...

2000-11-01

2002-01-01

Nguyen, Tan T. (Department: 2818)

Static information storage and retrieval

Read/write circuit

Including magnetic element

C365S055000, C365S066000

Reexamination Certificate

active

06335890

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to random access memory systems, and more particularly relates to an architecture for improving the write margin within magnetic random access memory (MRAM).
2. Description of the Prior Art
Thin film magnetic random access memory (MRAM) has been investigated since the early 1950s. However, as described in the text “Computer Storage Systems and Technology,” by Richard E. Matick, John Wiley & Sons (1977), which is incorporated herein by reference, these memories were deemed to be impractical due to narrow write margins and read margins which eroded as device dimensions were scaled. By the early 1970s, semiconductor-based memories, such as dynamic random access memory (DRAM), promised a simpler more compact memory solution than magnetic core memories, the most prevalent random access memory (RAM) available at the time. By the late 1970s, almost all development and production activity related to MRAM had been abandoned.
Recently, a renewed interest in MRAM has been sparked by its application to the non-volatile memory market. New memory devices, such as the Magnetic Tunnel Junctione (MTJ) device, which exhibits magneto resistance, overcame the earlier obstacle of inductive sensing. As summarized in Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in Each Cell,” ISSCC 2000, pp. 128-129, desirable characteristics of MTJ-based memories include high integration density, high speed, low read power, and Soft Error Rate (SER) immunity.
FIGS. 1A and 1B
depict two alternative configurations found in traditional memory architectures. Like conventional thin film RAM, MTJ RAM uses an architecture in which memory cells
100
are placed at the cross-points of intersecting word and bit lines. In the MTJ MRAM design described by Scheuerlein et. al. (see FIG.
1
A), reading of the memory cell has been simplified by the inclusion of a field effect transistor (FET)
102
within each cell for improved isolation.
FIG. 1B
depicts an alternative higher density MTJ MRAM architecture described by Frank Z. Wang in “Diode-Free Magnetic Random Access Memory Using Spin-Dependent Tunneling Effect,”
Applied Physics Letters
, Vol. 77, No. 13, pp. 2036-2038 (Sep. 25, 2000). In conventional magnetic memory architectures, however, writing individual memory cells without also writing adjacent or other non-intended cells remains a most vexing problem.
Typically, writing a memory cell involves passing electrical currents simultaneously through a bit line and word line, at the intersection of which an intended MTJ cell resides. The selected MTJ cell will experience a magnetic field which is the vector sum of the magnetic fields created by the word and bit line currents. All other MTJ devices that share the same bit line or word line as the selected MTJ device will be half-selected and will thus be subjected to either bit line or word line magnetic fields, respectively. Since the magnitude of the vector sum of the word line and bit line fields is about forty-one percent (41%) larger than the individual word line or bit line fields, the selectivity of a selected MTJ cell over half-selected MTJ cells is poor, especially when the non-uniform switching characteristics of the MTJ cells are considered.
Variations in the shape or size of an MTJ cell can give rise to variations in magnetic thresholds of the MTJ cells which are so large that it is virtually impossible to write a selected cell without also switching some of the half-selected cells, thus placing the reliability and validity of the stored data in question. There may also be environmental or other factors, such as temperature and processing variations, that adversely impact the write select margin. Additionally, creep, which generally refers to the spontaneous switching of a MTJ cell when it is subjected to repeated magnetic field excursions much smaller than its nominal switching field, narrows the acceptable write select margin even further thereby making the need for greater selectivity of individual MTJ cells even more imperative.
FIG. 2A
depicts the magnetic selectivity of an ideal thin film magnetic memory element as described by Stoner-Wohlfarth. Assuming that the word line and bit line currents generate fields along the hard magnetic axis
210
and easy magnetic axis
230
, respectively, of the magnetic element, the field (H
x
, H
y
) required to switch the magnetic state of the element must equal or exceed the solid curve or boundary
200
. This curve
200
, known by those skilled in the art as the switching asteroid, satisfies the relation H
h
⅔
+H
e
⅔
=H
k
⅔
, where H
h
is the hard axis field, H
e
is the easy axis field and H
k
is the anisotropy field. A selected cell experiences magnetic fields outside the boundary of the switching asteroid
200
(e.g., corresponding to point
220
) which are large enough to write the magnetic element to a state that aligns with the easy axis field direction. The state of a half-selected cell doesn't change since the magnetic fields acting on it (e.g., corresponding to points
210
and
230
) remain within the boundary of the switching asteroid
200
.
It is important to consider that, although depicted as a thin fixed boundary line, the switching asteroid
200
, in reality, may significantly change shape due to environmental conditions (e.g., temperature) or other factors (e.g., processing variations). Variations between individual MTJ cells substantially reduce the write select margin within the overall memory structure. Nonideal physical artifacts blur the distinction between half-selected and selected cells; the former could be written in a write operation intended only for the latter.
Hence a major hurdle to the realization of practical sub-micron MRAM architectures has been the problem of write selectivity. There is a need, therefore, in the field of magnetic memory devices and systems for an improved write selection mechanism which can be adapted readily to the present MRAM architecture as well as other alternative architectures.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a write architecture for use in a magnetic random access memory (MRAM) system that allows selection of individual MTJ cells in an array without adversely disturbing neighboring cells in the array and thus increasing the integrity of the data stored in the memory array.
It is another object of the present invention to provide an improved write selection architecture and methodology for MRM that is compatible with conventional MRAM systems.
It is yet another object of the present invention to provide a write selection architecture for MRAM that utilizes a substantially reduced bit line current, resulting in lower overall system power consumption.
It is a further object of the present invention to provide a write selection architecture for MRAM that has a substantially increased acceptable write disturb margin and is thus less sensitive to MTJ device mismatches, process variations and other environmental factors within an MRAM array.
The present invention revolutionizes the field of magnetic memory devices by providing an improved write selection architecture and methodology for use with magnetic random access memory (MRAM) that not only allows selection of individual MTJ cells in an array without adversely disturbing neighboring cells in the array, but also reduces the power consumed in the write operation and the overall sensitivity of the circuit to device mismatches, process variations and other environmental factors.
In accordance with one embodiment of the present invention, an improved architecture for selectively writing one or more magnetic memory cells in a magnetic random access memory (MRAM) device comprises one or more global write lines and a plurality of segmented write lines connected thereto. The global write lines are substantially isolated from the magnetic memory cells. A plurality of segmen

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