Non c-axis oriented bismuth-layered perovskite ferroelectric...

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Reexamination Certificate

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C428S701000, C365S145000

Reexamination Certificate

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06663989

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. In particular, non-volatile memory devices are important computer components due to their ability to retain information after power has been removed or interrupted. Non-volatile memories that use ferroelectric thin films have been termed ferroelectric random access memories, or FRAMs or FERAMs. The structure of a FRAM cell can be similar to that of a conventional dynamic random access memory (DRAM) cell, but with the ferroelectric film replacing the dielectric material in the capacitor. Binary digital information is stored in the polarization states of the ferroelectric film.
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, and La-substituted Bi
4
Ti
3
O
12
such as Bi
3.25
La
0.75
Ti
3
O
12
(BLT), have been stimulated by prospective technical applications in ferroelectric nonvolatile memories. See, e.g., Arauio et al., “Fatigue-free ferroelectric capacitors with platinum electrodes,”
Nature
374, 627 (1995); Park et al., “Lanthanum-substituted bismuth titanate for use in non-volatile memories,”
Nature
401, 682 (1999). 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). See Gruverman, “Scaling effect on statistical behavior of switching parameters of ferroelectric capacitors,”
Appl. Phys. Lett.
75,1452 (1999). It is believed that the use of epitaxial films is a way to overcome this non-uniformity problem of ferroelectric properties. See Kingon, “Memories are made of . . . ,”
Nature
401,658(1999). 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 deposited by 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. As a general matter, it is widely acknowledged that the integration of complex feroelectric materials with silicon-based devices has been elusive to date. See, e.g., Kingon, “Memories are made of . . . ,”
Nature
401, 658 (1999).
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
2
(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 as 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 Integrate

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