Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-06-06
2003-03-11
Jones, Deborah (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S699000, C428S700000, C428S701000
Reexamination Certificate
active
06531235
ABSTRACT:
TECHNICAL FIELD
The present application relates generally to the growth and structure of non-c-axis oriented ferroelectric materials. In particular, the present invention relates to anisotropic perovskite materials grown using a template layer formed on a buffered silicon substrate,
BACKGROUND ART
Ferroelectric perovskite materials are presently being studied as an alternative to conventional magnetic materials for use in digital memory systems. Accordingly, numerous investigations of polycrystalline, bismuth-containing layered (i.e., anisotropic) perovskite thin films, such as SrBi
2
Ta
2
O
9
(SBT) and SrBi
2
Nb
2
O
9
(SBN) thin films, have been stimulated by prospective technical applications in ferroelectric nonvolatile memories. This is due in large part to the high fatigue endurance of SBT and other layered perovskite materials. However, the application of such polycrystalline perovskite materials suffers from certain limitations. For instance, it is difficult to obtain ferroelectric properties that are homogeneous over the different cells of a large capacitor array when the lateral size of the ferroelectric cells drops below 100 nm (corresponding to cell sizes needed for Gigabit memories). Moreover, the existence of ferroelectric properties and their dependence on the cell size and material in such small structures (i.e., size effect) has been recently addressed in Alexe et al., “Patterning and switching of nanosize ferroelectric memory cells,”
Appl. Phys. Lett
. 75, 1793 (1999); and J. F. Scott: Abstracts of 12
th
International Symposium on Integrated Ferroelectrics (“Nano-Scale Ferroelectrics for Gbit Memory Application”), Aachen, Germany, Mar. 12-15, 2000, p. 102.
Successful efforts in the epitaxial growth of SBT thin films deposit pulsed laser deposition (PLD), metalorganic chemical vapor deposition (MOCVD), and RF magnetron sputtering methods, have been reported in Lettieri et al., “Epitaxial growth of (001)-oriented and (110)-oriented SrBi
2
Ta
2
O
9
thin films,”
Appl. Phys. Lett
. 73, 2923 (1998); and Pignolet et al., “Orientation dependence of ferroelectricity in pulsed-laser-deposited epitaxial bismuth-layered perovskite thin films,”
Appl. Phys
. A70, 283 (2000). In all of these works, special single crystalline substrates such as SrTiO
3
, LaAlO
3
—Sr
2
AlTaO
6
, LaSrAlO
4
, and MgO of various orientations have been used to grow epitaxial c-axis-oriented as well as non-c axis oriented SBT thin films. It was generally found that c-axis-oriented epitaxial SBT films ( i.e., films with their (001) plane parallel to the substrate surface) can be grown on SrTiO
3
(100) substrates, whereas epitaxial SBT films that have the (116) and (103) plane parallel to the substrate surface, grow on SrTiO
3
(110) and (111) substrates, respectively. In Lettieri et al., “Epitaxial growth of non-c-oriented SrBi
2
Nb
2
O
9
on (111) SrTiO
3
,” Appl. Phys. Lett
. 76, 2937 (2000), properly oriented ferroelectric films were epitaxially grown on SrTiO
3
. Specifically, it was reported that heterostructures consisting of an underlying (111) SrRuO
3
epitaxial electrode and an epitaxial (103) SrBi
2
Nb
2
O
9
overlayer were prepared, as SrRuO
3
is closely lattice matched with SrTiO
3
and chemically compatible with both SrBi
2
Nb
2
O
9
and SrTiO
3
.
The above observations are, however, not of high practical significance for memory devices, because SrTiO
3
crystals are not suitable substrates in microelectronics. For a better compatibility with silicon-based microelectronics, epitaxial SBT films should be grown on silicon substrates. The epitaxial growth of non-c-axis-oriented SBT on Si(100) has not heretofore been reported.
The growth of non-c-oriented bismuth-containing ferroelectric films having a layered perovskite structure, such as SrBi
2
Ta
2
O
9
(SBT), SrBi
2
Nb
2
O
9
(SBN) and SrBi
3
(Ta,Nb)
2
O
9
(SBTN), is of particular significance because the vector of the spontaneous electrical polarization in these layered perovskite materials is directed along the a-axis. By contrast, a c-oriented layered perovskite material does not have a polarization component along its film normal (perpendicular to the film plane). However, if the layered perovskite material is to be used in a ferroelectric thin-film capacitor with electrodes on the top and bottom film surfaces in the geometry used for dynamic random access memory, a normally oriented polarization component is essential. It would therefore be desirable to grow non-c-axis-oriented layered perovskite materials.
One example of a c-axis oriented silicon/metal oxide heterostructure that, for the purposes of the present invention, is not desirable, is disclosed in U.S. Pat. No. 5,270,298. Specifically, a buffer layer of yttria-stabilized zirconia is grown on a silicon substrate. A template of a c-axis oriented anisotropic perovskite material, such as bismuth titanate (Bi
4
Ti
3
O
12
) is grown on the buffer layer. A cubic metal oxide such as a perovskite material of highly-oriented crystallinity is then able to be grown on the template layer. In the example provided in this patent, the metal oxide is Pb
1−y
La
y
Zr
1−x
Ti
x
O
3
(PLZT), where 0<x<1 and 0<y<1.
Epitaxial SrRuO
3
thin films have been found useful as electrodes for ferroelectric capacitors, due to the high thermal and chemical stability of SrRuO
3
and because of its good lattice match with SrTiO
3
and Pb(ZrTi)O
3
. SrRuO
3
is a pseudocubic perovskite with a slight orthorhombic distortion due to the tilting of the RuO
6
octahedra. High-quality epitaxial SrRuO
3
films have been successfully deposited on different substrates, such as SrTiO
3
(100) and LaAlO
3
(100) and by different methods like off-axis sputtering and PLD, as reported in Eom et al., “Single-Crystal Epitaxial Thin Films of the Isotropic Metallic Oxides Sr
1−x
Ca
x
RuO
3
(0<x<1),”
Science
258, 1766 (1992); Chen et al., “Epitaxial SrRuO
3
thin films on (001) SrTiO
3
,” Appl. Phys. Lett
. 71, 1047 (1997); and Zakharov et al., “Substrate temperature dependence of structure and resistivity of SrRuO
3
thin films grown by pulsed laser deporition on (100) SrTiO
3
,” J. Mater. Res
. 14, 4385 (1999).
Recently, epitaxial (116)- and (103)-oriented SBT thin films grown on SrRuO
3
base electrodes deposited on lattice-matched perovskite SrTiO
3
substrates have been demonstrated by Ishikawa et al., “Electrical properties of (001)- and (116)-oriented epitaxial SrBi
2
Ta
2
O
9
thin films prepared by metalorganic chemical vapor deposition,”
Appl. Phys. Lett
. 75, 1970 (1999); Zurbuchen et al.: Abstracts of 12
th
International Symposium on Integrated Ferroelectrics (“Morphology and Electrical Properties of Epitiaxial SrBi
2
Ta
2
O
9
Films”), Aachen, Germany, 12-15 Mar., 2000, p.51; Saito et al.: Abstracts of 12
th
International Symposium on Integrated Ferroelectrics (“Characterization of Residual Stress Free (001)- and (116)-oriented SrBi
2
Ta
2
O
9
Thin Films Epitaxially Grown on (001) and (110) SrTiO
3
Single Crystals”), Aachen, Germany, 12-15 Mar., 2000, p.71; Pignolet et al., Abstracts of Integrated Ferroelectrics (“Dependence of Ferroelectricity in Epitaxial Pulsed Laser Deposited Bismuth-Layered Perovskite Thin Films on the Crystallographic Orientation”), Aachen, Germany, 12-15 Mar., 2000, p.111; and Lettieri et al., “Epitaxial growth of non-c-oriented SrBi
2
Nb
2
O
9
on (111) SrTiO
3
,” Appl. Phys. Lett
. 76, 2937 (2000). However, as discussed above, these substrates are not suitable for use in the fabrication of integrated devices for microelectronic applications.
DISCLOSURE OF THE INVENTION
According to one embodiment of the present invention, a structure containing a ferroelectric material comprises a substrate comprising silicon, a buffer layer formed on the substrate, and a non-c-axis-oriented, electrically-conductive template layer formed on the buffer layer. The template layer comprises a perovskite oxide compound. A non-c-axis-oriented, anisotropic perovskite ferroelectric layer is formed on the template layer.
According to another embodiment of the pre
Gösele Ulrich
Hesse Dietrich
Lee Ho Nyung
Pignolet Alain
Senz Stephan
Jenkins & Wilson, P.A.
Jones Deborah
Max-Planck-Institute für Mikrostrukturphysik
Sperty Arden
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