Ferroelectric film and semiconductor device

Details Ferroelectric film and semiconductor device Ferroelectric film and semiconductor device

2002-11-05

2004-05-11

Tran, Minhloan (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Organic semiconductor material

C257S642000, C257S295000, C257S759000

Reexamination Certificate

active

06734456

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a ferroelectric film and a semiconductor device including the ferroelectric film.
Ferroelectric films have been widely used in a variety of semiconductor devices, such as a ferroelectric capacitor having first and second electrodes formed on respective faces of a ferroelectric film, an MFS transistor having an MFS (Metal/Ferroelectric/Semiconductor) multilayer structure, an MFIS transistor having an MFIS (Metal/Ferroelectric/Insulator/Semiconductor) multilayer structure and an MFMIS transistor having an MFMIS (Metal/Ferroelectric/Metal/Insulator/Semiconductor) multilayer structure.
Known ferroelectric materials used for forming a ferroelectric film are a PZT (PbZr
x
Ti
1−x
O
3
)-based ferroelectric material and a Bi-based ferroelectric material such as SBT (SrBi
2
Ta
2
O
9
) or BTO (Bi
4
Ti
3
O
12
), and a BTO-based ferroelectric material is regarded as a promising material because it can exhibit a good ferroelectric characteristic.
In particular, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 2000-260960, a ferroelectric material obtained by substituting a nonvolatile element A for part of Bi of Bi
4
Ti
3
O
12
and represented by a general formula, Bi
4−x
A
x
Ti
3
O
12
(wherein A is selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; and 0<x<2.8) has been recently proposed.
Japanese Laid-Open Patent Publication No. 2000-260960 describes that a ferroelectric film can be obtained by sintering a film made from the ferroelectric material represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
in an oxygen atmosphere for approximately 1 hour at a temperature of approximately 700° C.
The present inventors have found, however, that when a film made from the ferroelectric material represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
is sintered in an oxygen atmosphere at approximately 700° C., the resultant ferroelectric film has a problem in its reliability although its initial characteristic is good. Specifically, the present inventors have found that when a semiconductor device including the ferroelectric film represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
is operated for approximately 10 years or is made to repeatedly perform a rewrite operation at a temperature of 85° C. or more, the ferroelectric film cannot keep its polarization characteristic.
Furthermore, in order to realize a high degree of integration of a semiconductor device including a ferroelectric film, both compactness and a low-voltage operating characteristic are required. A ferroelectric film made from the ferroelectric material represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
has, however, a problem in the low-voltage operating characteristic. Therefore, a semiconductor device including a ferroelectric film made from the ferroelectric material represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
is disadvantageous in operating at a low voltage and realizing a high degree of integration.
SUMMARY OF THE INVENTION
In consideration of the aforementioned conventional disadvantages, an object of the invention is improving the reliability and the low-voltage operating characteristic of a ferroelectric film made from a ferroelectric material represented by a general formula, Bi
4−x
A
x
Ti
3
O
12
.
The first finding based on which the present invention was devised is as follows:
The present inventors have examined the reason why a ferroelectric film represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
is degraded in its polarization characteristic when a semiconductor device including the ferroelectric film is operated for approximately 10 years or is made to repeatedly perform a rewrite operation at a temperature of 85° C. or more. As a result, it has been found that the polarization characteristic is degraded because Bi escapes from the crystal structure with the elapse of time.
The second finding is as follows:
The present inventors have examined the reason why a ferroelectric film represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
is difficult to operate at a low voltage. As a result, it has been found that a ferroelectric film can be operated at a low voltage by reducing an interatomic distance between Ti and O in the crystal structure. Specifically, when the interatomic distance between Ti and O is large, large energy, that is, a high voltage, is necessary for moving O to cause polarization, but when the interatomic distance between Ti and O is small, energy, namely, a voltage, necessary for moving O to cause polarization is small, and therefore, the ferroelectric film can be thus operated at a low voltage.
The first aspect of the invention is on the basis of the first finding, and Bi is excessively included in a ferroelectric material represented by the general formula, Bi
4−x
A
x
Ti
3
O
12
.
Specifically, the first ferroelectric film of the invention is made from a ferroelectric material represented by a general formula (1), Bi
4−x+y
A
x
Ti
3
O
12
, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; and 0≦x≦2 and 0<y≦(4−x)×0.1.
In the first ferroelectric film of the invention, since Bi is included by an excessive amount corresponding to a composition ratio y (y>0) of the general formula (1), even though Bi escapes from the crystal structure with the elapse of time, the excessive Bi enters a portion from which Bi has escaped, and therefore, the characteristic of the ferroelectric film is never degraded. Also, the Bi excessive content y satisfies a relationship of y≦(4−x)×0.1, a leakage current flowing due to deposition of bismuth on the ferroelectric film can be prevented. Accordingly, the reliability of the ferroelectric film can be improved.
Preferably, in the first ferroelectric film of the invention, (4−x)×0.02≦y≦(4−x)×0.06 in the general formula (1).
Thus, the initial polarization value of the ferroelectric film can be 20 &mgr;C/cm
2
or more, and hence, the reliability of the ferroelectric film can be improved.
Preferably, in the first ferroelectric film of the invention, A is La and 0.65≦x≦0.85 in the general formula (1).
Thus, the initial polarization value of the ferroelectric film can be 20 &mgr;C/cm
2
or more, and hence, the reliability of the ferroelectric film can be improved.
The second aspect of the invention is on the basis of the second finding. Specifically, the second ferroelectric film of the invention is made from a ferroelectric material represented by a general formula (2), (Bi
4−x+y
A
x
Ti
3
O
12
)
z
+(DBi
2
E
2
O
9
)
1−z
, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; D is an element selected from the group consisting of Sr, Ba, Ca, Bi, Cd, Pd and La; E is an element selected from the group consisting of Ti, Ta, Hf, W, Nb, Zr and Cr; and 0≦x≦2, 0<y≦(4−x)×0.1 and 0.5<z<1.
In the second ferroelectric film of the invention, since Bi is included by an excessive amount corresponding to a composition ratio y (y>0) of the general formula (2), even though Bi escapes from the crystal structure with the elapse of time, the excessive Bi enters a portion from which Bi has escaped, and therefore, the characteristic of the ferroelectric film is never degraded. Also, the Bi excessive content y satisfies a relationship of y≦(4−x)×0.1, a leakage current flowing due to deposition of bismuth on the ferroelectric film can be prevented. Accordingly, the reliability of the ferroelectric film can be improved.
Also, since the ferroelectric material is represented by the general formula (2), (Bi
4−x+y
A
x
Ti
3
O
12
)
z
+(DBi
2
E
2
O
9
)
1−z
, the ferroelectric material has a superlattice structure in which (DBi
2
E
2
O
9
) operable at a lower voltage than

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