Static information storage and retrieval – Read/write circuit – Including magnetic element
Reexamination Certificate
2003-08-05
2004-06-15
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Including magnetic element
C365S173000, C365S171000, C365S228000, C365S230030
Reexamination Certificate
active
06751147
ABSTRACT:
BACKGROUND OF THE INVENTION
A magnetic memory such as a MRAM typically includes an array of magnetic memory cells. A typical magnetic memory cell includes a layer of magnetic film in which magnetization is alterable and a layer of magnetic film in which magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer. The magnetic film that is pinned may be referred to as a reference layer.
The orientation of magnetization of each magnetic memory cell may assume one of two stable orientations at any given time. These two stable orientations, are referred to as “parallel” and “anti-parallel”, and representing logic states of “1” and “0,”, respectively.
FIGS. 1A and 1B
illustrate the basic structure of a conventional magnetic memory cell
100
having a reference layer
102
with a same-axis orientation of magnetization with respect to the easy-axis of the data storage layer
104
. The magnetic memory cell
100
includes a tunnel barrier
106
between the data storage layer
104
and the reference layer
102
. This structure of the magnetic memory cell
100
may be referred to as a spin tunneling device (STD) in that electrical charge migrates through the tunnel barrier
106
during read operations. This electrical charge migration through the tunnel barrier
106
is due to a phenomenon known as spin tunneling and occurs when a read voltage is applied to the magnetic memory cell
100
.
FIG. 1A
illustrates a magnetic memory cell
100
having a data storage layer
104
with parallel magnetic orientation relative to the reference layer
102
. Vector M
1
represents the overall or resulting orientation of magnetization in the data storage layer
104
. Vector M
1
includes contributions from magnetizations along the easy-axis and in the edge domains of the data storage layer
104
. The orientation of magnetization in the reference layer
102
is represented by a vector M
2
that is fixed in a direction parallel to the easy-axis of the data storage layer
104
. Thus,
FIG. 11A
is representative of a magnetic memory cell storing a logical “1” state.
Vector M
1
may be changed depending upon the logic state of the magnetic memory cell
100
. Vector M
1
is manipulated by the application of magnetic fields using conductors associated with the magnetic memory cell
100
. These magnetic fields are applied to flip or reverse the directions of the magnetizations, vector M
1
, in the data storage layer
104
including the easy-axis magnetization and the edge domains.
FIG. 1B
illustrates a magnetic memory cell
100
having a reference layer
102
with anti-parallel magnetic orientation relative to the reference layer
102
. Thus,
FIG. 1B
is representative of a magnetic memory cell storing a logical “0” state.
The logic state of a magnetic memory cell may be indicated by its resistance, which depends on the relative orientations of magnetization in its data storage and reference layers. Such a magnetic memory cell is typically in a low resistance state if the orientation of magnetization in its data storage layer is substantially 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 orientation of magnetization in its data storage layer is substantially anti-parallel to the orientation of magnetization in its reference layer.
A magnetic memory cell may be written to a desired logic state by applying magnetic fields that rotate the orientation of magnetization in its data storage layer. Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as an “easy-axis.” The magnetic fields may be applied to flip the orientation of magnetization in the data storage layer along its easy-axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer depending on the desired logic state.
Prior magnetic memories typically include an array of word lines and bit lines that are used to apply magnetic fields to the magnetic memory cells during writing. The magnetic memory cells are usually located at intersections of the word lines and bit lines. A selected magnetic memory cell may be written by applying electrical currents to the particular word and bit lines that intersect at the selected magnetic memory cell. Typically, an electrical current applied to the particular bit line generates a magnetic field substantially aligned along the easy-axis of the selected magnetic memory cell. The magnetic field aligned to the easy-axis may be referred to as a longitudinal write field. An electrical current applied to the particular word line usually generates a magnetic field substantially perpendicular to the easy-axis of the selected magnetic memory cell.
Typically, only the selected magnetic memory cell receives both the longitudinal and the perpendicular write fields. Other magnetic memory cells coupled to the particular word line usually receive only the perpendicular write field. Other magnetic memory cells coupled to the particular bit line usually receive only the longitudinal write field.
The magnitudes of the longitudinal and the perpendicular write fields are usually chosen to be high enough so that the selected magnetic memory cell switches its logic state, but low enough so that the other magnetic memory cells which are subject to either the longitudinal or the perpendicular write field do not switch. An undesirable switching of a magnetic memory cell that receives only the longitudinal or the perpendicular write field is commonly referred to as “half-select” switching.
Manufacturing variation among the magnetic memory cells may increase the likelihood of half-select switching. For example, manufacturing variation in the longitudinal or perpendicular dimensions or shapes of the magnetic memory cells may increase the likelihood of half-select switching. In addition, variation in the thicknesses or the crystalline anisotropy of data storage layers may increase the likelihood of half-select switching. Unfortunately, such manufacturing variation decreases the yield in manufacturing processes for magnetic memories and reduces the reliability of prior magnetic memories.
The reference layer of a magnetic memory cell is usually a layer of magnetic material in which magnetization is fixed or “pinned” in a particular direction. In a conventional magnetic memory cell, the reference layer may be formed with its magnetization pinned in a direction that is parallel to the easy-axis of the data storage layer. As a consequence, the orientation of magnetization in the reference layer of the conventional magnetic memory cell is typically parallel to the easy-axis of the data storage layer.
A conventional magnetic memory cell may be written by applying magnetic fields that reverse the orientation of magnetization in the data storage layer from one direction to the other along its easy-axis. This reversal causes the magnetic memory cell to switch between its high and low resistance states. The logic state of the magnetic memory cell may be determined during a read operation by measuring its resistance.
Typically, the data storage layer is fabricated as a rectangle or oval with an elongated dimension along its easy-axis. These configurations minimize the negative effects of edge domains. Such a structure usually increases easy-axis contribution to the resulting orientation of magnetization in the data storage layer in comparison to contributions from the edge domains. The rectangular or oval configuration sets the shape anisotropy for the bit cell, thereby and providing a bi-stable structure. The parallel state requires more energy to flip the orientation of magnetization in the data storage layer during write operations.
Further, the memory cells are aligned so that the easy axes are parallel with their respective word lines. One problem with this configuration is that during a write ope
Hilton Richard L.
Perner Frederick A.
Smith Kenneth K.
Hewlett--Packard Development Company, L.P.
Tran Andrew Q.
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