Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-08-10
2003-12-02
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S421000, C257S660000, C257S666000, C257S678000, C257S704000, C257S710000
Reexamination Certificate
active
06657246
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a memory device and, in particular, to a magnetic random access memory (hereinafter abbreviated to MRAM) using a nonvolatile magnetic memory cell.
At present, development of an MRAM is making rapid progress as a large-capacity memory device capable of carrying out high-speed reading and writing operations. As the MRAM, use is predominantly made of a structure comprising a number of memory cells and utilizing so-called tunnel junction. The tunnel junction makes use of the fact that an electric resistance between two magnetic layers with a nonmagnetic film interposed therebetween is different depending upon whether spins known in the art are parallel or anti-parallel to each other between the magnetic layers. The MRAM of the type includes a transistor for selecting a specific memory cell to be accessed.
Referring to
FIG. 1
, description will be made of an existing MRAM. The MRAM illustrated in
FIG. 1
comprises a substrate
1
and an integrated device portion
2
mounted on the substrate
1
. The integrated device portion
2
is connected through lead wires
3
to lead terminals
4
. The substrate
1
, the integrated device portion
2
, the lead wires
3
, and the lead terminals
4
are molded by a resin mold
5
.
The integrated device portion
2
includes a number of transistor portions and a number of memory device portions. Each of the memory device portions serves as a memory cell. Each of the transistor portions serves to select a particular memory cell.
Referring to
FIG. 2
, description will be made of the structure of the integrated device portion
2
. The integrated device portion
2
comprises a first conductor
11
on the substrate
1
, a first insulator layer
12
on the first conductor
11
, a first ferromagnetic member or layer
13
on the first insulator layer
12
, a second insulator layer
14
covering the first ferromagnetic member
13
, a second ferromagnetic member
15
on the second insulator layer
14
, a third insulator layer
16
covering the second ferromagnetic member
15
, and a fourth insulator layer
17
and a second conductor
18
both of which are formed on the third insulator layer
16
. A combination of the first ferromagnetic member
13
, the second insulator layer
14
, and the second ferromagnetic member
15
forms a magnetic tunnel function device as one of the memory device portions.
Each of the first and the second conductors
11
and
18
is arranged so that the second ferromagnetic member
15
is applied with a magnetic field when an electric current is supplied thereto. In case where both of the first and the second conductors
11
and
18
are supplied with electric currents, magnetic fields are produced by the electric currents and combined into a composite magnetic field. Under the composite magnetic field, magnetization of the second ferromagnetic member
15
is rotated and reversed. On the other hand, the first ferromagnetic member
13
is fixed in magnetization, for example, by the use a ferromagnetic material having high saturation magnetization.
The first ferromagnetic member
13
is made of a CoPt alloy while the second ferromagnetic member
15
is made of a NiFe alloy. The second insulator layer
14
is made of Al
2
O
3
or the like.
In the meanwhile, highly-integrated semiconductor devices capable of carrying out high-speed operations include not only the MRAM but also a dynamic random access memory (DRAM), a read-only memory (ROM), a microprocessor unit (MPU), and an image processor arithmetic logic unit (IPALU), and so on. Recently, these devices are remarkably increased in calculation speed and signal processing speed. Such increase in calculation speed and signal processing speed results in drastic or quick change in electric current flowing in these devices. The quick change in electric current is a major factor causing high-frequency inductive noise.
On the other hand, reduction in weight, thickness, and size of electronic components and electronic apparatuses is making rapid progress also. Therefore, the degree of integration of the semiconductor device as an electronic component and the density of mounting the same on a printed wiring board are increased. As a consequence, if the electronic component is highly integrated or the electronic components are mounted at a high density, signal lines are very close to each other. In combination with the above-mentioned increase in signal processing speed, high-frequency radiation noise is readily induced.
In the above-mentioned electronic circuit, an attempt to suppress the noise has been made by optimizing the design in arrangement of components on the printed wiring board and wiring therebetween or by inserting a lumped-constant component such as a decoupling capacitor into a power supply line.
However, in the semiconductor device or the printed wiring board increased in operation speed, the noise generated therefrom contains harmonics components so that the behavior of a signal path is similar to that of a distributed-constant circuit. In this event, the existing noise countermeasure assuming a lumped-constant circuit is no longer effective. In addition, limitation is imposed upon reduction of the noise by optimizing the arrangement of the electronic parts and the wiring.
In the operation of the above-mentioned MRAM, harmonic distortion produced upon high-speed change in electric current is a major factor causing the high-frequency radiation noise, like other types of semiconductor random access memories (RAM). On the other hand, the noise superposed on a writing current or a magnetic layer causes the fluctuation in magnitude of magnetization of the magnetic layer. As a result, an additional operation will be required upon writing. If the noise is mixed in a signal upon reading, an additional process such as repetition of a reading operation is required. In other words, if the noise countermeasure is not effective, substantial writing and reading speeds are decreased upon writing and reading data. Therefore, in the operation of the MRAM, it is important not only to prevent the noise from being propagated to other components or portions but also to prevent reduction in substantial writing and reading speeds due to the noise.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a large-capacity non-volatile memory which is suppressed in generation of noise and is excellent in noise resistance so as to carry out writing and reading operations at a substantially high speed.
Other objects of the present invention will become clear as the description proceeds.
The present inventors have already invented a composite magnetic material having a high magnetic loss at a high frequency and found out a method of effectively suppressing extraneous emission or unnecessary radiation produced from a semiconductor device or an electronic circuit by arranging the composite magnetic material in the vicinity of an extraneous emission source. From the recent research, it has been found out that the effect of attenuating the extraneous emission utilizing the magnetic loss is based on a mechanism in which an equivalent resistance component is added to the electronic circuit as the extraneous emission source. Herein, the magnitude of the equivalent resistance component depends upon the magnitude of a magnetic loss term &mgr;″ of a magnetic material. Specifically, the magnitude of the resistance component equivalently inserted into the electronic circuit is substantially proportional to &mgr;″ and the thickness of the magnetic material as far as the area of the magnetic material is fixed. It is noted here that the magnetic loss term &mgr;″ is an imaginary part of the relative permeability of the magnetic material. Therefore, in order to achieve a desired level of attenuation of the extraneous emission by the use of a smaller or a thinner magnetic material, the value of &mgr;″ must be greater. For example, in order to prevent the extraneous emission utilizing a magnetic loss material in a very
Masumoto Toshiaki
Ono Hiroshi
Yoshida Shigeyoshi
Frishauf Holtz Goodman & Chick P.C.
Mandala Jr. Victor A.
NEC Tokin Corporation
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