Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-12-30
2004-06-29
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S295000, C257S296000
Reexamination Certificate
active
06756621
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a ferroelectric capacitor device including a ferroelectric film having a bismuth layer structure as a capacitor insulating film.
In recent years, with the advance of digital technology, the tendency to process and store a massive amount of data has been accelerated. In this situation, electronic equipment has been further sophisticated, and rapid progress has been made in increasing the integration of semiconductor integrated circuit devices used for electronic equipment and attaining finer semiconductor elements.
To realize higher integration of dynamic random access memories (RAMs), there has been widely researched and developed a technology of using a dielectric having a high dielectric constant (hereinafter, simply called a high dielectric) as a capacitor film of a memory capacitor device, in place of a silicon oxide or a silicon nitride conventionally used.
Research and development have also been vigorous on ferroelectric films having the spontaneous polarization property, with the aim of commercializing a nonvolatile RAM that can operate at a lower voltage than is conventionally allowed and permit write/read at a high speed.
As a ferroelectric film used for a nonvolatile RAM, promising is a ferroelectric film having a bismuth layer structure, which is excellent in rewrite endurance and can operate at a low voltage. In general, a bismuth layer structure is represented by chemical formula (a):
(Bi
2
O
2
)(A
m−1
B
m
O
3m+1
) (a)
where m is an integer equal to or more than 1, A is a univalent, divalent or trivalent metal, and B is a tetravalent, pentavalent or hexavalent metal.
The above bismuth layer structure includes bismuth oxide layers (Bi
2
O
2
) and perovskite-like layers (A
m−1
B
m
O
3m+1
) alternately put on top of each other.
Among a group of materials having the bismuth layer structure, a material called SBT, in particular, is often used for nonvolatile memories.
The SBT is a compound represented by chemical formula (b):
(Bi
2
O
2
)(SrTa
2
O
7
) (b),
that is, m is 2, A is divalent Sr, and B is pentavalent Ta in chemical formula (a) above (hereinafter, this compound is called a normal type).
The laminated structure of the compound is as shown in
FIG. 15
, in which bismuth oxide layers
101
and perovskite-like layers
102
(m=2) are alternately put on top of each other.
The bismuth oxide layer
101
(chemical formula: Bi
2
O
2
) has a structure as shown in
FIG. 16
, in which square pyramids linked to one another extend two-dimensionally. Bismuth
111
exists at the apex of each square pyramid, and oxygen
112
exists at each corner of the bottom square of the square pyramid.
The m=2 perovskite-like layer
102
(chemical formula: SrTa
2
O
7
) has a layer structure as shown in
FIG. 17
, in which oxygen octahedra extend two-dimensionally with each two placed one upon the other vertically. Tantalum
113
exists in the B site as the center of each oxygen octahedron, and oxygen
112
exists at each apex of the oxygen octahedron. Strontium
114
exists in the A site as a space surrounded by the oxygen octahedra.
The SBT has problems to be tackled. The first problem is improving the spontaneous polarization amount, and the second problem is suppressing the leakage current and improving the breakdown voltage. As methods for improving the spontaneous polarization as the first problem, the following two crystal structures (a mixed layered superlattice type and an A-site Bi substitution type) have been proposed.
(1) Mixed layered superlattice type layer structure (first prior art)
The layer structure of this type is disclosed in U.S. Pat. No. 5,955,754 to Azuma et al. This literature describes extensively the entire of the layer structure. Herein, however, the disclosed layer structure will be described as being applied to SBT according to the purport of the present invention. As shown in
FIG. 18
, the mixed layered superlattice type layer structure (this is not a commonly-accepted name but is called herein for convenience to distinguish from other structures) includes either a perovskite-like layer
102
(m=2) or a perovskite-like layer
103
(m=1) interposed between every two adjacent bismuth oxide layers
101
. When the existence probability of the m=1 perovskite-like layer
103
is &dgr; (0<&dgr;<1), the existence probability of the m=2 perovskite-like layer
102
is 1−&dgr;.
The m=1 perovskite layer
103
, represented by TaO
4
, has a layer structure as shown in
FIG. 19
, in which a single layer of oxygen octahedra having tantalum
113
as the center extends two-dimensionally. The tantalum
113
exists in the B site as the center of each oxygen octahedron, and oxygen
112
exists at each apex of the oxygen octahedron. If valence calculation is made strictly, the chemical formula should be TaO
7/2
, indicating that the oxygen amount is short to form the structure shown in
FIG. 19. A
vacancy is therefore formed in an oxygen-lacking portion.
The feature of the mixed layered superlattice is that because the amount of bismuth having a low melting point is large compared with the normal structure, crystal grains can be easily grown large, and this can improve the spontaneous polarization property.
(2) A-site Bi substitution type layer structure (second prior art)
The layer structure of this type, disclosed in Japanese Laid-Open Patent Publication No. 9-213905 to Atsugi et al., is represented by chemical formula (c):
(Bi
2
O
2
)[(Sr
1−x
Bi
x
)Ta
2
O
7
] (c)
The A-site Bi substitution type layer structure includes the bismuth oxide layers
101
and the m=2 perovskite-like layers
102
alternately put on top of each other as shown in FIG.
15
.
The bismuth oxide layer
101
, represented by chemical formula: Bi
2
O
2
, has the structure shown in
FIG. 16
as in the normal type.
The m=2 perovskite-like layer
102
, represented by chemical formula: (Sr
1−x
Bi
x
)Ta
2
O
7
, has a structure shown in FIG.
20
. The structure shown in
FIG. 20
resembles the structure shown in
FIG. 17
, in which tantalum
113
exists in the B site as the center of each oxygen octahedron, and oxygen
112
exists at each apex of the oxygen octahedron. The difference is that the A site
115
is occupied by Sr with a probability of (1−x) or Bi with a probability of x. That is, while all the A sites
115
are occupied by Sr in the normal type, Bi substitutes for Sr in the A sites
115
with a probability of x.
In a recent research, formation of a vacancy in the A site
115
has been confirmed. The reason is that since trivalent Bi substitutes for divalent Sr, a vacancy is formed to satisfy the charge neutrality law. In this case, chemical formula (c) is changed to chemical formula (d):
(Bi
2
O
2
)[(Sr
1−x
Bi
2x/3
)Ta
2
O
7
] (d)
Thus, in the A-site Bi substitution type, the m=2 perovskite-like layer
102
is represented by chemical formula: (Sr
1−x
Bi
2x/3
)Ta
2
O
7
, where the A site shown in
FIG. 20
is occupied by Sr with a probability of (1−x), Bi with a probability of (2x/3), or a vacancy with a probability of (x/3).
The feature of the A-site Bi substitution type is that since Bi
3+
small in ion radius substitutes for Sr
2+
in the A site
115
, the lattice distortion increases, and this increases the spontaneous polarization amount. In addition, as in the mixed layered superlattice type, since the amount of Bi having a low melting point is large compared with the normal type, crystal grains can be easily grown large, and this can improve the spontaneous polarization property.
As described above, the first and second prior art structures can solve the first problem of SBT of improving the spontaneous polarization.
However, the first and second prior art structures fail to solve the second problem of SBT of reducing the leakage current and improving the breakdown voltage, for the following reason.
The first and second prior
Hayashi Shinichiro
Nasu Toru
Matsushita Electric - Industrial Co., Ltd.
Nguyen Thinh T
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