Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2003-01-14
2004-11-09
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S295000, C257S315000
Reexamination Certificate
active
06815745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tunnel magnetoresistive effect element for generating so-called magnetoresistive (MR) effect in which a resistance value changes with application of a magnetic field from the outside, a method of manufacturing a tunnel magnetoresistive effect element and a magnetic memory device fabricated as a memory device capable of storing information by the use of a tunnel magnetoresistive effect element.
2. Description of the Related Art
In recent years, as information communication devices, in particular, personal small information communication devices such as portable terminal devices (e.g. personal digital assistants) are widely spreading, it is requested that devices such as memories and logic devices comprising these information communication devices or portable terminal devices should become higher in performance, such as they should become higher in integration degree, they can operate at higher speed and they can consume lesser electric power. Particularly, technologies that can make nonvolatile memories become higher in density and larger in storage capacity are becoming more and more important as complementary technologies for replacing hard disk devices and optical disk devices with nonvolatile memories because it is essentially difficult to miniaturize hard disk devices and optical disk devices because they have their movable portions (e.g. head seek mechanism and head rotation mechanism).
Flash memories using semiconductors and an FeRAM (ferro electric random access memory) using a ferro dielectric material are widely known as nonvolatile memories. However, flash memories are able to write information at speed in the order of microseconds and encounters with a defect such that they are slow in speed as compared with a DRAM (dynamic random access memory) and a SRAM (static random access memory). Further, it has been pointed out that the FeRAM cannot be rewritten so many times.
A magnetic memory device called an MRAM (magnetic random access memory), that had been described in “Wang et al IEEE Trans. Magn. 33 (1997), 4498”, receives a remarkable attention as a nonvolatile memory that can overcome these defects. The MRAM is a nonvolatile memory from which information can be read out in a nondestructive fashion and which can be accessed randomly. In addition, the MRAM has the following characteristics.
Specifically, the MRAM is simple in structure and therefore can be easily integrated at high integration degree. Further, since the MRAM is able to record information by rotation of magnetic moment, it can be rewritten a large number of times (e.g. more than 10
16
times). Furthermore, it is expected that the MRAM has very high access time and it has already been confirmed that the MRAM can be operated at speed in the order of nanoseconds (e.g. speed lower than 5 nanoseconds). From these characteristics, there is a strong possibility that MRAMs will become a main current in the field of memory devices.
Such MRAM uses a tunnel magnetoresistive effect element as a memory element for recording information. A tunnel magnetoresistive effect element has a trilayer structure composed of ferromagnetic material/insulating material/ferromagnetic material, i.e. ferromagnetic tunnel junction (MTJ (magnetic tunnel junction)) if it is of tunnel magnetoresistive effect (TMR (tunnel magnetoresistive)) type. In the MTJ structure, when the magnetization direction of one ferromagnetic material is used as a fixed layer and the magnetization direction of the other ferromagnetic material is used as a free layer, a resistance value of a tunnel current changes depending upon the magnetization direction of the free layer. To be more in detail, when an external magnetic field is applied to the ferromagnetic material layers under the condition in which a constant current flows through the ferromagnetic material layers, MR effect appears in response to a relative angle of the magnetizations of the two ferromagnetic material layers. When the magnetization directions of the two ferromagnetic material layers are anti-parallel, a resistance value becomes the maximum. When the magnetization directions of the two ferromagnetic material layers are parallel to each other, a resistance value becomes the minimum. Therefore, in response to the magnetization direction of the storage layer, the TMR type tunnel magnetoresistive effect element (hereinafter simply referred to as a “TMR element”) is able to store therein information in the form of “1” when magnetization is oriented to a certain direction and is able to store therein information in the form of “0” when magnetization is oriented to the other direction. Further, the TMR type tunnel magnetoresistive effect element becomes able to readout the states of these magnetization directions in the form of a difference current under a constant bias voltage or in the form of a difference voltage under a constant bias current through a TMR effect.
A changing ratio “of a resistance value in the TMR element is expressed as” =2·P
1
·P
2
/(1−P
1
·P
2
) where P
1
, P
2
represent spin polarizability of the respective ferromagnetic material layers. A spin polarizability represents a difference between the number of electrons (one unit of very small magnets) that are rotating (spinning) upwardly in the solid material and the number of electrons that are spinning downwardly in the solid material. A magnitude of spin polarizability is specified by compositions of magnetic materials comprising mainly a ferromagnetic material layer. Accordingly, since the changing ratio” of the resistance value increases as the spin polarizabilities P
1
, P
2
of the respective ferromagnetic material layers increase, if the ferromagnetic material layer is made of a magnetic material having a composition with high spin polarizability, then a TMR ratio (ratio between a resistance value in the high resistance state and a resistance value in the low resistance state) of the tunnel magnetoresistive effect element containing the ferromagnetic material layer can increase. As result, excellent information read characteristics can be realized in the MRAM.
To this end, in most cases, the TMR element uses any one of Fe group ferromagnetic material elements such as Fe (iron), Co (cobalt) and Ni (nickel) that are magnetic materials having compositions with high spin polarizabilities or alloy of a combination of more than two of the above-mentioned Fe group ferromagnetic material elements as a material to form the ferromagnetic material layer. As an insulating material layer sandwiched between these ferromagnetic material layers, there is generally used an Al
2
O
3
(alumina) layer that is obtained after a thin film conductive layer of Al (aluminum), which is a nonmagnetic metal material, for example, had been oxidized by native oxidation in the atmospheric pressure during a long period of time or had been oxidized by plasma oxidation or radical oxidation which are known as “strong” oxidation methods”. The reason for this is that, because the insulating material layer functions as a tunnel barrier layer to generate TMR effect, not only the spin polarizability of each ferromagnetic material layer should increase but also the insulating material layer interposed between these ferromagnetic material layers should be made uniform and thin in order to obtain a large TMR ratio.
To realize excellent read characteristics in the MRAM, it is very effective in increasing TMR ratios of respective TMR elements comprising the MRAM and is also effective in suppressing dispersions of resistance values among the TMR elements. Therefore, if dispersions of resistance values among the TMR elements are suppressed while the TMR ratios are being increased, then it becomes possible to realize an MRAM that can operate at higher speed and which can be integrated with higher integration degree.
However, the TMR ratio and the dispersion of the resistance value in the TMR element depends considerably upon the characteristics of the insulating
Bessho Kazuhiro
Higo Yutaka
Hosomi Masanori
Kano Hiroshi
Mizuguchi Tetsuya
Depke Robert J.
Holland & Knight LLP
Nguyen Thinh T
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