Magnetic memory device

Details Magnetic memory device Magnetic memory device

2002-03-22

2003-07-08

Lam, David (Department: 2818)

Static information storage and retrieval

Systems using particular element

Magnetoresistive

C365S171000, C365S173000

Reexamination Certificate

active

06590803

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-090768, filed Mar. 27, 2001; No. 2001-095976, filed Mar. 29, 2001, the entire contents of both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic memory device using a ferromagnetic material, and more particularly to a non-volatile solid-state memory utilizing a ferromagnetic tunneling junction (MTJ).
2. Description of the Related Art
In recent years, in a sandwich film in which one layer of a dielectric body is inserted between two magnetic metallic layers, there has been discovered a magnetoresistive element which can read a change in resistance by passing a tunneling current to a film surface in the vertical direction and utilizing this tunneling current, which is a so-called MTJ element (Magnetic Tunnel Junction element).
In regard to the ferromagnetic tunneling junction, there has been reported the fact that a rate of change in magnetoresistance which is not less than 20% can be obtained (see J. Appl. Phys. 79,4724 (1996), for example). Therefore, the possibility of application to a magnetic head or a magnetic random access memory (MRAM) has been increased (see U.S. Pat. No. 5,640,343, and U.S. Pat. No. 5,734,605). This ferromagnetic tunneling junction forms a tunneling barrier layer comprising AlO
x
by forming a film of a thin Al layer having a thickness of 0.4 nm to 2.0 nm on a ferromagnetic electrode and then exposing its surface to pure oxygen or oxygen glow discharge or oxygen radical.
Further, there is proposed a ferromagnetic single tunneling junction having a structure in which an antiferromagnetic layer is given to one ferromagnetic layer of the ferromagnetic single tunneling junction and that ferromagnetic layer is determined as a magnetically pinned layer (see Jpn. Pat. Appln. KOKAI Publication No. 10-4227). This ferromagnetic tunneling junction element (ferromagnetic single tunneling junction), however, likewise has a problem that the rate of change in the magnetoresistance (MR ratio) is greatly reduced when the value of the voltage to be applied to the ferromagnetic tunneling junction element is increased in order to obtain a desired output voltage value.
Furthermore, there is proposed a ferromagnetic tunneling junction having magnetic particles dispersed in a dielectric body or a ferromagnetic double tunneling junction (see Jpn. Pat. Appln. KOKAI Publication No. 9-260743, Phys. Rev. B 56(10), R5747 (1997), Journal of the Magnetic Society of Japan 23, 4-2, (1999), Appl. Phys. Lett. 73(19), 2829 (1998)). In these junctions, since the rate of change in the magnetoresistance which is not less than 20% can be obtained, the possibility of application to a magnetic head or a magnetoresistive memory device has emerged.
In these ferromagnetic double tunneling junctions, since reduction in the MR ration involved by a bias voltage is small as compared with the ferromagnetic single tunneling junction, they have a characteristic that a large output can be obtained.
A magnetic memory element using the ferromagnetic single or double tunneling junction is non-volatile, and a writing/reading time is as fast as 10 nsec or below. It has a potential ability that a number of times of rewriting is not less than 10
15
and a cell size can be reduced as small as a DRAM (Dynamic Random Access Memory).
In particular, the magnetic memory element using the ferromagnetic double tunneling junction can suppress reduction in the rate of change in the magnetoresistance even if a value of a voltage to be applied to the ferromagnetic tunneling junction element is increased in order to obtain a desired output voltage value as described above, and hence a large output voltage can be assured, thereby demonstrating a characteristic which is preferable as the magnetic memory element.
However, since the magnetic memory element using the ferromagnetic single or double tunneling junction utilizes a ferromagnetic material, it has a problem that power consumption at the time of writing is large when a memory capacity is increased and a cell width of the ferromagnetic tunneling junction is decreased, as compared with a competing memory such as a FeRAM (Ferroelectric Random Access Memory), a flash memory or the like.
When a switching magnetic field is increased, not only power consumption during writing is increased, but also the density of an electric current caused to flow to a word line and a bit line in order to invert a spin is increased and a problem of EM (Electro-Migration) occurs when the high density of an MRAM (Magnetic Random Access Memory) is realized and the design rule is minimized.
Based on a result of the electromagnetic field simulation for an electromagnetic field distribution and the intensity in a direction within an MTJ cell plane when the design rule is 0.1 &mgr;m, it can be understood that the intensity of the electromagnetic field is the order of 10 Oersted (Oe) at the highest even in cases where the density of the electric current caused to flow to wirings is assumed to be 5×10
6
A/cm
2
.
Furthermore, when the capacity of the MRAM is approximately 1 Gbit and a distance between adjacent cells is approximately 0.1 &mgr;m, the magnetic field applied to the adjacent cells becomes approximately 80% of the magnetic field applied to cells on the wirings, and there may occur a problem of the interference between cells, i.e., so-called crosstalk.
In order to solve the problem of crosstalk, there is proposed changing a direction of a magnetization easy axis between the adjacent cells to a different direction (see U.S. Pat. No. 6,005,800). The shape of cells must be formed without irregularities in order to use this method. However, when the capacity of the MRAM is increased and the cell size is reduced, the forming accuracy is hard to be controlled, and there are irregularities in the switching magnetic field of cells, which results in a problem that the crosstalk can be hardly eliminated.
Moreover, the size of the switching magnetic field depends on a cell size of the MTJ, a cell shape, a magnetization characteristic of a material, a film thickness or the like. For example, when the cell size of the MTJ becomes small as described above, the switching magnetic field of the spin is increased due to the influence of the demagnetizing field.
As to the cell shape, a magnetic domain is produced at an end portion in case of a rectangular cell shape, the remanence is decreased, and the step-like Barkhausen jump occurs. In addition, variations are generated in the switching magnetic field depending on how the magnetic domain is produced. When the cell shape is elliptical, a single domain structure can be obtained and the MR ratio is not lowered. However, there is a problem of a large degree of the increase in the switching magnetic field as a function of the reduction in the cell width.
Additionally, in order to solve these problems, there are proposed a structure characterized in that a magnetic memory cell is provided at a part where bit and word lines cross each other substantially at right angles and shape of the cell is asymmetric with respect to the magnetization easy axis, and a structure in which the easy axis is somewhat inclined from the direction of wirings (see U.S. Pat. No. 6,104,633).
As to the shape control, however, when the density of the MRAM is increased and the cell size is reduced as described above, the forming accuracy can not be disadvantageously controlled, and irregularities are generated in the switching magnetic field of the cell.
Further, in the structure having the easy axis of the cell being somewhat inclined from the direction of wirings, the switching magnetic field is reduced, but the problem of crosstalk becomes serious when the density is increased.
In order to solve these problems (increase in the switching magnetic field involved by crosstalk and reduction in the cell width), it can be considered that

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