Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-11-26
2004-11-23
Thompson, Craig A. (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C438S778000, C427S126300, C427S255190
Reexamination Certificate
active
06822302
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a substrate for an electronic device, a method for manufacturing this substrate for an electronic device, and an electronic device.
2. Description of the Related Art
In the related art, ferroelectric memories which are nonvolatile memories using ferroelectrics has been used.
These ferroelectric memories are classified into a capacitor type, in which a 1T/1C structure, etc., is formed using ferroelectrics as capacitors, and an MFSFET type, in which ferroelectrics are used as gate insulating films of field-effect transistors in place of SiO
2
.
The MFSFET type ferroelectric memory has advantages over the capacitor type, for example, it increases the packing density and non-destructive read-out. However, the MFSFET type is difficult to manufacture with respect to the structure. Thus, under present circumstances, development and commercialization of the capacitor type ferroelectric memory have been more advantageous.
Ferroelectric materials adopted in the capacitor type ferroelectric memories are divided into two materials, Pb(Zr
1-x
Tix)O
3
(PZT) and SrBi
2
Ta
2
O
9
(SBT). Among them, the PZT materials having compositions in the neighborhood of the rhombohedron-tetragonal phase boundary (MPB) are superior in the residual dielectric polarization and coercive electric field property, and arc the materials which are more advantageous than the others in commercialization.
In the related art, Pt has been used as a PZT-based ferroelectric material for a lower electrode. Since Pt has a face-centered cubic lattice structure that is a close-packed structure, it has a strong self-orientation property, and therefore brings about cubic crystal orientation even on amorphous, such as SiO
2
.
However, since the orientation property is strong, there have been problems in that when a columnar crystal of Pt has grown, Pb, for example, is likely to diffuse in a substrate along grain boundaries, adhesion between Pt and SiO
2
is degraded, and the like.
Although Ti may be used to enhance this adhesion between Pt and SiO
2
, and furthermore, TiN, etc., may be used as diffusion barrier layers against Pb, etc., electrode structures become complicated, and in addition, oxidation of Ti, diffusion of Ti into Pt, and reduction in crystallinity of PZT accompanying that are brought about, and therefore degradation of the polarization electric field (P-E) hysteresis characteristic, the leakage current characteristic, and the fatigue characteristic occur.
In order to address or avoid problems of Pt electrodes as described above, RuO
x
, IrO
2
, and other conductive oxide electrode materials have been researched. Among them, in particular, SrRuO
3
having a perovskite structure has the same crystal structure as that of PZT, and therefore has superior joining property at the interface, is likely to realize epitaxial growth of PZT, and has superior characteristics as a diffusion barrier layer against Pb.
Consequently, the related art has researched ferroelectric capacitors using SrRuO
3
as an electrode.
SUMMARY OF THE INVENTION
However, when the ferroelectric capacitor is configured using a metal oxide, for example, SrRuO
3
, having a perovskite structure as a lower electrode and using PZT as a ferroelectric, there have been problems as described below.
Regarding PZT, the composition of for example, Zr:Ti=0.3:0.7, which is on the excess Ti side compared with Zr:Ti=0.48:0.52 of MPB, is important from the viewpoint of an increase in residual dielectric polarization P
r
and a decrease in coercive electric field E
c
. In this composition range, PZT exhibits a tetragonal crystal, and the polarization direction thereof is parallel to the c axis.
Consequently, regarding the ferroelectric capacitor having a structure of upper electrode/ferroelectric layer/lower electrode/substrate, in order to produce a (
001
) orientation film of the ferroelectric layer PZT, it is necessary to make a SrRuO
3
electrode as a lower electrode itself bring about pseudo-cubic crystal (
100
) orientation.
However, when the SrRuO
3
electrode, which is a perovskite type metal oxide, is deposited directly on Si (substrate), since a SiO
2
layer is formed at the interface, it is difficult to epitaxially grow SrRuO
3
.
Accordingly, it is necessary to epitaxially grow some buffer layer on Si (substrate), and to epitaxially grow a SrRuO
3
electrode thereon.
Herein, examples of buffer layers which grow epitaxially on Si (substrate) with ease include metal oxides having a fluorite structure, for example, yttria-stabilized zirconia Zr
1-x
Y
x
O
2-0.5x
(YSZ), CeO
2
, and Y
2
O
3
.
For example, it has been reported that a double buffer layers of Y
2
O
3
/YSZ (Appl. Phys. Lett., vol. 61 (1992) 1240) or CeO
2
/YSZ (Appl. Phys. Lett., vol. 64 (1994) 1573) is suitable as the buffer layer to grow epitaxially YBa
2
Cu
3
O
x
having a structure similar to the perovskite. In this case, YBa
2
Cu
3
O
x
brings about (
001
) orientation with ease.
However, (Appl. Phys. Lett., 67 (1995) 1387) discloses that SrRuO
3
(in an orthorhombic crystal, a=0.5567 nm, b=0.5530 nm, and c=0.7845 nm, and in pseudo-cubic crystal, a=0.3923 nm and 2
1/2
a=0.5548 nm) having a simple perovskite structure does not grow epitaxially with (
100
) orientation on a (
100
) plane of YSZ (a=0.514 nm) or CeO
2
(a=0.541 nm), which has a fluorite structure, but brings about (
110
) orientation (pseudo-cubic crystal).
Accordingly, the inventor of the present invention researched materials for buffer layers which had a structure other than the fluorite structure and which grew epitaxially on Si (substrate) with ease. As a result, it was discovered that metal oxides having a NaCl structure were suitable, and therefore the present invention was made.
The present invention provides a substrate for an electronic device including a conductive oxide layer which is formed by epitaxial growth with cubic crystal (
100
) orientation or pseudo-cubic crystal (
100
) orientation and which contains a metal oxide having a perovskite structure, a method for manufacturing such a substrate for an electronic device, and an electronic device provided with such a substrate for an electronic device.
The above can be addressed or achieved by the present invention as described in the following (1) to (17).
(1) A substrate for an electronic device includes:
a Si substrate;
a buffer layer which is formed by epitaxial growth on the Si substrate and which contains a metal oxide having a NaCl structure; and
a conductive oxide layer which is formed by epitaxial growth with cubic crystal (
100
) orientation or pseudo-cubic crystal (
100
) orientation on the buffer layer and which contains a metal oxide having a perovskite structure.
(2) The substrate for an electronic device according to the aforementioned (1), where the Si substrate is a (
100
) substrate or a (
110
) substrate from which a natural oxidation film is not removed.
(3) The substrate for an electronic device according to the aforementioned (1) or (2), where the metal oxide having a NaCl structure is at least one of MgO, CaO, SrO, BaO, and solid solutions containing them.
(4) The substrate for an electronic device according to the aforementioned (1) or (2), where the aforementioned buffer layer has been grown epitaxially with cubic crystal (
110
) orientation.
(5) The substrate for an electronic device according to any one of the aforementioned (1) to (4), where the buffer layer has an average thickness of 10 nm or less.
(6) The substrate for an electronic device according to any one of the aforementioned (1) to (5), where the metal oxide having a perovskite structure is at least one of CaRuO
3
, SrRuO
3
, BaRuO
3
, and solid solutions containing them.
(7) A method for manufacturing a substrate for an electronic device, including:
cleaning a Si substrate;
forming a buffer layer containing a metal oxide having a NaCl structure by epitaxial growth, in which after the Si substrate is cleaned, the Si substrate in a va
Higuchi Takamitsu
Iwashita Setsuya
Miyazawa Hiromu
Berezny Nema
Thompson Craig A.
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