Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2000-10-02
2003-04-15
Dinh, Son T. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000, C365S173000
Reexamination Certificate
active
06549454
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a magnetic material, and more particularly to a tunneling magneto resistance (TMR) material.
BACKGROUND OF THE INVENTION
Magnetoelectronic, spin electronic or spintronic devices have drawn a great deal of attention in the field of magnetics. These devices, which include magnetic random access memory (MRAM), magnetic field sensors, read/write heads for disk drives, and other magnetic applications, use giant magneto resistance (GMR) and tunneling magneto resistance (TMR) effects predominantly caused by electron spin rather than electron charge.
One class of spintronic device is formed of a GMR material or a TMR material. The basic structure of these two materials includes two magnetic layers separated by a spacer layer. In a GMR material, the spacer layer is conductive, while in a TMR material, the spacer layer is insulating.
FIG. 1
illustrates an enlarged cross-sectional view of a TMR material
20
according to the prior art.
The TMR material
20
, which is also referred to as a Magnetic Tunnel Junction (MTJ), has a first magnetic layer
22
and second magnetic layer
24
separated by an insulating spacer layer
26
, which is also referred to as a tunnel barrier layer. The first and second magnetic layers
22
,
24
can be single layers of magnetic materials such as nickel, iron, copper, cobalt or alloys thereof. The tunnel barrier layer
26
is typically aluminum oxide (Al
2
O
3
), but may include any number of insulators, such as aluminum nitride or oxides of nickel, iron, cobalt or alloys thereof.
Without intending to be bound by theory and with particular reference to the enlarged view
28
of the interface
31
between the tunnel barrier layer
26
and second magnetic layer
24
, as adsorbate atoms
30
are deposited on the surface
32
of the tunnel barrier layer
26
, several types of layer growth are possible, and even in the absence of mixing between the atoms
30
and the surface
32
at the interface
31
, the layer growth of atoms
30
that forms on the surface
32
may not be a preferable thin film.
The growth mode of the atoms
30
on the surface
32
is determined by several factors including: the mobility of the atoms
30
on the surface
32
, the surface energy of the surface
32
, the surface energy of atoms
30
, and the binding energy of the atoms
30
to the surface
32
at the interface
31
. In a majority of physical vapor deposition processes, the atoms
30
have sufficient energy for significant mobility on the surface
32
, moving numerous atomic spacings before coming to rest. In this medium to high mobility environment, the atoms
30
will naturally form a film morphology, which minimizes the total energy of the atoms
30
on the surface
32
. Thus, when the surface energy of the atoms
30
is high compared to the energy of the surface
32
, a configuration will be favored, which minimizes the surface area of the atoms
30
at the expense of exposing some area of the surface
32
, resulting in the formation of three-dimensional islands
34
of atoms
30
on the surface
32
during the initial stages of film growth. Conversely, if the energy of the surface
32
is higher than the atoms
30
, the growth of the atoms
30
in an atomic layer-by-layer fashion over the surface
32
is preferred since this quickly covers the surface
32
with atoms
33
that form a surface that has a lower energy.
Strong bonding of the atoms
30
to the surface
32
favors the growth of an atomic layer of atoms
30
by limiting the mobility of the atoms
30
and by decreasing the total system energy through maximization of the contact between the atoms
30
and the surface
32
. During layer-by-layer growth, the atoms
30
nearly complete a first atomic layer of atoms
30
on the surface
32
before forming the second atomic layer of atoms
30
on atoms
30
. Three-dimensional growth (i.e., island growth) occurs when the atoms
30
tend to grow additional atom
30
on atom
30
layers rather than completing the first atomic layer of atoms
30
on the surface
32
.
A film is generally considered to be continuous when it has covered over about 80% of a surface. When the growth mode is a layer-by-layer growth, the film is more likely to become continuous much faster than for an island growth mode. For island growth, it may take the equivalent of ten or more atomic layers of deposition before the film becomes continuous or substantially continuous. Such films are generally considered to be discontinuous and are composed of disconnected islands before enough material is deposited to make islands large enough to connect and form a substantially continuous layer. Furthermore, once a continuous film is formed it will be rougher than a film that is grown in a layer-by-layer manner.
It is often desirable to form a smooth and substantially continuous film on a substrate that is less than about ten atomic layers (i.e., less than about 20 Å thick). Prior to the present invention, it was impossible to form a smooth and substantially continuous layer that was less than about 20 Å if the film of material forms by island growth or any growth mode that is similarly three-dimensional. Even though a film with substantial island growth may become continuous with ten atomic layers, it will be much rougher than a film grown in a layer-by-layer manner as some areas will be only one or two atomic layers thick while other areas will be well over 10 atomic layers thick. While this layer-by-layer formation provides proper operation of a TMR material, it is desirable to form a TMR material having a substantially smooth and continuous magnetic layer that is less than about 20 Å as significant benefits would be realized with such a TMR material structure.
For example, double MTJs would significantly benefit from a substantially smooth and continuous ultra-thin magnetic layer as resonant effects in a double MTJ would be tunable if a magnetic layer is available having a 1-3 atomic layer thickness. (See, Xiangdong Zhang, Bo-Zang Li, Gang Sun, & Fu-Cho Pu, Phys. Rev. B, vol. 56, p 5484 (1997), and S. Takahashi & S. Maekawa, Phys. Rev. Lett. vol. 80, p 1758 (1998) for theoretical predictions of resonant effects that give a higher MR, etc. which are hereby incorporated by reference). In addition, magnetic bi-layers (i.e., two magnetic materials forming the first or second magnetic layer) in a single or multiple tunnel junction would increase thermal endurance if composed of a substantially smooth and continuous ultra-thin diffusion tunnel barrier layer grown on another tunnel barrier layer and a soft magnetic layer combination such that switching characteristics would not be adversely affect during device operation. Furthermore, a specific crystallographic phase could be obtained with the selection of the two magnetic materials forming the magnetic bi-layer in order to obtain desired magnetic properties, including, but not limited to coercivity, anisotropy, and magneto resistive ratio considerations. As may be appreciated, there are many desirable applications and attributes for a TMR material having a substantially smooth and continuous uniform ultra-thin magnetic layer.
Accordingly, it is -desirable to have a TMR material that includes a substantially smooth and continuous uniform magnetic layer with a thickness that does not exceed about 20 Å, preferably does not exceed 15 Å, and more preferably does not exceed about 10 Å.
REFERENCES:
patent: 6083764 (2000-07-01), Chen
patent: 6232777 (2001-05-01), Sato
patent: 6344954 (2002-02-01), Redon et al.
Dinh Son T.
Koch William H.
Motorola Inc.
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