Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2002-06-17
2003-03-11
Elms, Richard (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S189050, C365S190000
Reexamination Certificate
active
06532168
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ferromagnetic thin film memories and, more particularly, to ferromagnetic thin film memories in which states of the memory cells based on magnetization direction are determined through magnetoresistive properties of the thin film sensed by an electronic circuit.
Digital memories of various kinds are used extensively in computers and computer system components, in 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 in magnetic materials in each memory cell, typically thin film materials. These films may be ferromagnetic thin films having information stored therein through the direction of the magnetization occurring in that film, with this information being obtained through either inductive sensing to determine the magnetization state, or by magnetoresistive sensing of such states. Such ferromagnetic thin film memories may be conveniently provided on the surface of a monolithic integrated circuit to provide easy electrical interconnection between the memory cells and the memory operating circuitry.
Ferromagnetic thin film memory cells can be made very small and packed very closely together to achieve a significant density of stored digital bits, properties which permit them to be provided on the surface of a monolithic integrated circuit as indicated above. One construction, as an example, is shown in
FIG. 1
, where a bit structure
10
for a memory cell that is presented is formed over a semiconductor material body
12
, as used in a monolithic integrated circuit, and directly on an insulating layer
13
, itself supported on a major surface of body
12
in the integrated circuit. Only a small portion of the integrated circuit is shown, and then only a small portion of the semiconductor body is shown in that integrated circuit portion.
These bit structures in an assemblage in a memory are usually provided in a series string of such bit structures often called sense lines. There are typically a plurality of such sense lines in a memory. In order to make interconnections between members of such sense lines, or between the sense lines and the external circuitry in the integrated circuit for operating the memory, terminal regions or junctures
14
are typically provided at each end of the bit structure for interconnection purposes. These interconnections might be formed of copper alloyed in aluminum.
The remainder of the bit structure disposed on the exposed major surface of insulating layer
13
includes a lower ferromagnetic thin film
15
and an upper ferromagnetic thin film
16
. Ferromagnetic thin film layers
15
and
16
typically exhibit uniaxial anisotropy, magnetoresistance, little magnetostriction, and are of an alloy composition typically comprising nickel, cobalt and iron. The magnetic device structure can be a spin valve that includes a pinned reference layer
15
spaced apart from a “free layer” that stores the digital information. The lower ferromagnetic thin film
15
is typically, but not always, thicker than the upper ferromagnetic thin film
16
. Alternatively, a pseudo-spin-valve structure can be used where the lower ferromagnetic thin film
15
is often called the hard layer, and the upper ferromagnetic thin film
16
is often called the soft layer.
Between ferromagnetic thin film layers
15
and
16
is typically a further thin layer
17
which usually would not exhibit ferromagnetism but may be either an electrical conductor or an electrical insulator. Layer
17
must, however, in this construction, minimize the exchange interaction between layers
15
and
16
so that the magnetization vectors of each layer are decoupled. A typical choice for layer
17
would be copper. An insulating layer
18
covers bit structure
10
although only a part of it is shown in FIG.
1
.
Finally, a word line
19
is shown in
FIG. 1
disposed on the major surface of insulating layer
18
. Word line
19
typically includes an aluminum layer alloyed with copper on a titanium-tungsten base layer. A protective and insulating layer over the entire structure of
FIG. 1
is often used in practice, but is not shown here.
Bit structure
10
can be operated in a longitudinal mode having its easy axis extend between internal interconnections
14
perpendicular to the direction of word line
19
. Information kept as a digital bit having one of two alternative logic values in bit structure
10
is stored therein in layer
15
by having the magnetization vector point in one direction or the other, generally along the easy axis of magnetization. If the direction of magnetization is caused to rotate from such a direction by external magnetic fields, the electrical resistance of layers
15
and
16
changes with this magnetization direction rotation because of the magnetoresistive properties of such layers. For the kinds of materials typically used in layers
15
and
16
, the maximum change in resistance is on the order of a few percent of the minimum resistance value.
Sense current refers to the current flow through bit structure
10
from one terminal
14
to the other terminal
14
thereof, and word current refers to current flowing in word line
19
adjacent to, and transverse to the orientation of, bit structure
10
. Bit structure
10
can be placed in one of the two possible magnetization states of layer
15
(pinned layer) through the selective application of sense and word currents i.e., information can be “written” in bit structure
10
. A bit structure
10
of a typical configuration can be placed in a “0” magnetization state by the application of a sense current of typically 1.0 mA , and coincidentally with the provision of a word current in one direction from 20 mA to 40 mA. The opposite magnetization state representing a “1” logic value can be provided through providing the same sense current and a word current of the same magnitude in the opposite direction. Such states typically occur fairly quickly after the proper current levels are reached, such state changes typically occurring in less than about 5 ns.
Determining which magnetization state is stored in bit structure
10
i.e., reading the information stored in bit structure
10
, is typically done by providing externally caused magnetic fields in that bit structure, through providing, for example, wordline currents and sometimes coincident sense line currents. These currents rotate the magnetization of the upper ferromagnetic thin film
16
(free layer) of the bit structure
10
, but preferably not the lower ferromagnetic thin film
15
(pinned layer). As indicated above, this causes a change in the electrical resistance encountered between terminal regions
14
in bit structure
10
for different magnetization directions in the structure, including changing from one easy axis direction magnetization state to the opposite direction state. As a result, there is detectable differences in the voltage developed across magnetic bit structure
10
by the sense current flowing therethrough, depending on the relative magnetization direction of the pinned and free layers of bit structure
10
.
As the size of the bit structure
10
decreases, the magnetic field required to rotate the upper ferromagnetic thin film
16
and the lower ferromagnetic thin film
15
also tend to increase. Accordingly, the magnitude of the word line currents and sense lines currents increase. Depending on the technology used, this may cause the electro-migration limits of the metal interconnect layers to be exceeded. To help compensate for this, a digital line is sometimes provided over the bit structure
10
parallel with the sense line. The digital line provides an additional metal layer for carrying the required current, and provides additional lateral torque at the bit structure
10
.
A limitation of many prior art magneto-resistive memories is that both sense lines and word lines are separately provided. Each of the sense lines and word lines typically requires a separa
Johnson William J.
Swanson Richard W.
Zhu Theodore
Elms Richard
Knobbe Martens Olson & Bear LLP
Micro)n Technology, Inc.
Nguyen Hien
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