Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-03-22
2004-04-13
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S296000, C257S297000, C438S003000, C438S257000
Reexamination Certificate
active
06720599
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferroeletric memory using metal oxide layer and an electronic apparatus using the ferroelectric memory.
2. Description of Related Art
Ferroelectric memory has udergone rapid development in recent years as a form of nonvolatile memory employing a ferroelectric substance. A ferroelectric memory can be categorized into a capacitor type forming a 1T/1C structure or the like if a Ferroelectric is used as a capacitor and an MFSFET in which Ferroelectric is used type as a gate insulator for a field effect transistor instead of SiO
2
. Although an MFSFET type is advantageous in terms of high integration and non-destructive readout, this type ferroelectric memory has not be realized yet due to the structural difficulty in manufacturing; therefore, the capacitor type ferroelectric memory antecedes the MFSFET type in developing and launching into market presently.
The material for Ferroelectric used for the capacitor type ferroelectric memory is divided into PbZr
1-X
Ti
X
O
3
(PZT) and SrBi
2
Ta
2
O
9
(SBT). Although remanent polarization Pr of SBT material is smaller than the remanent polarization of PZT, SBT material has superior fatigue behavior which is a critical factor for a reliable memory element. Also, SBT material is favorable from ecological view point for pursuing lead-free ferroelectric memory.
However, when a ferroelectric capacitor is constructed using SBT as a Ferroelectric, there were following problems.
SBT has Orthorhombic structure with a property such as, a=5.522 Å, b=5.524 Å, and c=25.026 Å, the polarization moment appears only in a direction of “a” axis, in other words, the polarization moment does not appear in directions of “b” axis and “c” axis. Therefore, SBT is used for a ferroelectric capacitor composed of a substrate, a bottom electrode, a Ferroelectric, and a top electrode, in order to take advantage of polarization moment effectively, ferroelectric layer SBT should preferably be oriented in direction of “a” axis so as to make a direction of electric field and a polarization axis from the bottom electrode to the top electrode parallel each other (See Japanese Patent Application, Publication No. Hei 8-335672.) But, SBT has a layered structure; even though it is easy to be oriented in direction of “c” axis, it is very difficult to be oriented in direction of “a” axis. Also, in a non-oriented polycrystal layer, an angle between the polarization axis of each crystal grain and the charged electric field is not uniform; thus, the charged voltage when a domain inversion occur in each crystal grain is not uniform, and the angularity of hysteresis curve becomes inferior.
Here, if an epitaxial growth of SBT is possible on a Si single crystal substrate via any kind of buffer layer and a bottom electrode with a “c” axis slanting from the orthogonal direction towards the substrate, crystallinity improves and angularity of the hysteresis curve becomes improved. Also, it becomes possible to use the polarization moment which appears only in direction of “a” axis more effectively. In particular, if a metal oxide electrode having perovskite structure which is superior property in SBT and lattice-matching is used as a bottom electrode, and if epitaxial growth of such metal oxide is possible in (
110
) oriented in cubic system or pseudo-cubic system, or in (
111
)-orientation in such system, the epitaxial growth in which the “c” axis of SBT is slanting by 45 degrees or 55 degrees from the orthogonal direction towards the substrate. Furthermore, if it is assumed that “a” axis and “b” axis are oriented by poling, it is estimated that 71% (=sin 45 degrees) and 82% of the polarization moment (=Sin 55 degrees) in case “a” axis is oriented perfectly can be used effectively respectively.
An object of the present invention is to provide a capacitor type ferroelectric memory having superior remanent polarization property and superior angularity on hysteresis curve performing an epitaxial growth of SBT ferroelectric layer on Si single crystal substrate in manner of “c” axis slanting from the orthogonal direction towards the substrate by using a buffer layer and a bottom electrode layer.
BRIEF SUMMARY OF THE INVENTION
Hereafter, also in the embodiments and claims, numerals accompanied by the parenthesis like (
010
) or (
100
) indicate the condition of orientation such as disposing direction of the single crystal or the like.
A ferroelectric memory of the invention is characterized in comprising Si single crystal substrate, a buffer layer composed of metal oxide on the Si single crystal substrate, a bottom electrode layer including either a conductive oxide having perovskite structure or a metal platinum Pt on the buffer layer, a ferroelectric layer composed of SrBi
2
Ta
2
O
9
having a layered perovskite structure on the bottom electrode layer, a top electrode layer on the ferroelectric layer.
According to the above structure, epitaxaial growth of SrBi
2
Ta
2
O
9
ferroelectric layer is possible on a Si single crystal substrate, and an effect that ferroelectric memory having superior angularity in hysteresis curve is obtained.
A ferroelectric memory in a ferroelectric memory according to the above is also characterized in that Si single crystal substrate is a substrate (
100
), metal oxide having fluorite structure including either a yttrium stabilized zirconia YSZ or a cerium oxide CeO
2
is (
100
)-oriented in cubic system in a buffer layer, the bottom electrode layer is in (
110
)-orientation in cubic system or in (
110
)-orientation in pseudo-cubic system, the ferroelectric layer is in (
116
)-orientation in orthorhombic system or in (
117
)-oriented.
According to the above structure, epitaxial growth of SrBi
2
Ta
2
O
9
ferroelectric layer is possible on a Si single crystal substrate (
100
) in manner of “c” axis slanting by approximately 45 degrees from the orthogonal direction towards the substrate. Therefore, an effect that ferroelectric memory having superior angularity in hysteresis curve and obtained and 71% of polarization moment can be used.
A ferroelectric memory in a ferroelectric memory according to the above is also characterized in that Si single crystal is a substrate (
110
), a buffer layer is composed of metal oxide MO (M═Mg, Ca, Sr, Ba) having NaCl structure being in (
110
)-orientation in cubic system, a bottom electrode layer is oriented in cubic or in (
110
)-orientation in pseudo-cubic system, a ferroelectric layer is in (
116
)-orientation in orthorhombic system or in (
117
)-orientation.
According to the above structure, epitaxial growth of SrBi
2
Ta
2
O
9
ferroelectric layer is possible on a Si single crystal substrate (
110
) in manner of “c” axis slanting by approximately 45 degrees from the orthogonal direction towards the substrate. Therefore, an effect that ferroelectric memory having superior angularity in hysteresis curve and obtained and about 71% of polarization moment can be used.
A ferroelectric memory in a ferroelectric memory according to the above is also characterized in that Si single crystal is a substrate (
111
), a buffer layer is composed of metal oxide MO (M═Mg, Ca, Sr, Ba) having NaCl structure being in (
111
)-orientation in cubic system, a bottom electrode layer is oriented in cubic system or in (
111
)-orientatin in pseudo-cubic system, a ferroelectric layer is in (
103
)-orientation in orthorhombic system or in (
104
)-orientation.
According to the above structure, epitaxial growth of SrBi
2
Ta
2
O
9
ferroelectric layer is possible on a Si single crystal substrate (
111
) in manner of “c” axis slanting by approximately 55 degrees from the orthogonal direction towards the substrate. Therefore, an effect that ferroelectric memory having superior angularity in hysteresis curve and obtained and about 82% of polarization moment can be used.
A ferroelectric memory in a ferroelectric memory according to any of the above is also characterized in that a memory cell including a bottom ele
Higuchi Takamitsu
Iwashita Setsuya
Miyazawa Hiromu
Nelms David
Seiko Epson Corporation
Tran Long
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