Optical: systems and elements – Optical modulator – Having particular chemical composition or structure
Reexamination Certificate
1998-03-30
2001-03-27
Mai, Huy (Department: 2873)
Optical: systems and elements
Optical modulator
Having particular chemical composition or structure
C359S322000, C359S245000
Reexamination Certificate
active
06208453
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to highly textured or epitaxial niobate ferroelectric thin films and a method of depositing such films on a variety of substrates for use in electrical and optical devices.
BACKGROUND OF THE INVENTION
Conventional dielectric thin film material used in the semiconductor industry comprises silicon dioxide (SiO
2
). However, future electronic memory technology will demand thin film materials exhibiting a greater dielectric constant than that of silicon dioxide. That is, thin films having greater dielectric constant values will permit smaller individual storage device sizes and increased memory densities on integrated memory devices.
Polycrystalline ferroelectric materials are being considered for high dielectric constant thin film materials. However, ferroelectric thin film materials tend to have exceptional properties along one crystallographic direction or axis and only moderate properties along the other crystal directions or axes. Since polycrystalline ferroelectric thin films considered to-date are comprised of randomly oriented grains, the measured dielectric constant of these films will be an average of the dielectric constants in all directions. Thus, the average dielectric constant of polycrystalline ferroelectric thin films will be considerably less than the dielectric constant exhibited along the optimum crystal direction or axis of the crystal lattice.
Strontium barium niobate (Sr
x
Ba
1−x
Nb
2
O
6
) where x is greater than 0.25 and less than 0.75 (hereafter SBN) is a ferroelectric material exhibiting excellent dielectric and electro-optic properties which are highest along the c-axis of the tetragonal tungsten bronze crystal lattice as described by Prokhorov et al. in Ferroelectric
Crystals for Laser Radiation Control, (Adam Hilger, New York, 1990) p. 81. Initial studies of Sr
x
Ba
1−x
Nb
2
O
6
were carried out on Czochralski-grown single crystals as described by Neurgaonkar et al. in Ferroelectrics 15, 31 (1984). In addition, liquid-phase epitaxy, rf sputtering, and sol-gel growth of textured Sr
x
Ba
1−x
Nb
2
O
6
thin films have been reported. For example, liquid-phase epitaxy was used by Neurgaonkar et al. in Mater. Res. Bull. 22, 1095 (1987). Rf sputtering was used by Antisigin et al. in Ferroelectrics 63, 235 (1985), while sol-gel was employed by Xu et al. in J. Mater. Res. 5, 916 (1990) and by Hirano et al. in J. Am. Ceram. Soc. 75, 1697 (1992). Recently, growth of these films by conventional and single source metalorganic chemical vapor deposition (MOCVD) was reported by Greewald et al. in Mater. Res. Soc. Symp. Proc. 243, 457 (1993) and by Lu et al. in Mater. Res. Soc. Symp. Proc. 335, 59 (1994).
Potassium niobate (KNbO
3
) is promising ferroelectric material for electro-optic (EO), nonlinear optic (NLO) and photorefractive applications. The electro-optic figure of merit is 13 picometers/volt which surpasses that of LiNbO
3
. In addition, potassium niobate possesses excellent non-linear optical coefficients (d
31
=15 picometer/volt and d
33
=27 picometer/volts). The growth of KNbO
3
films also has been demonstrated by liquid phase epitaxy, sol-gel, ion beam sputtering, RF sputtering and pulsed laser deposition techniques and recently by single source metal-organic chemical vapor deposition that produced polycrystalline films with a preferred orientation. However, for practical NLO and EO applications, highly textured or epitaxial thin films are required.
An object of the present invention is to provide a highly textured or epitaxial niobate ferroelectric thin film deposited on an amorphous or crystalline substrate surface with a crystal axis exhibiting a desireable property preferentially oriented or aligned relative to the substrate surface.
Another object of the present invention is to provide a MOCVD method of depositing a highly textured or oriented strontium barium or other niobate ferroelectric thin film on an amorphous as well as crystalline substrate surface with a crystal axis exhibiting a desireable property preferentially oriented or aligned to the substrate surface.
Still another object of the present invention to provide a MOCVD method of depositing a highly textured or epitaxial potassium niobate ferroelectric thin film on a substrate surface using improved precursors with a crystal axis exhibiting a desireable property preferentially oriented or aligned to the substrate surface.
SUMMARY OF THE INVENTION
One embodiment of the present invention provides a method of depositing a highly textured or epitaxial niobate ferroelectric thin film on a substrate surface, which may be amorphous or crystalline, in a reactor by metalorganic chemical vapor deposition under conditions that the ferroelectric thin film is deposited with a crystal axis thereof exhibiting a desireable property, such as dielectric constant and/or electro-optic coefficient, preferentially oriented or aligned to the substrate surface. For example only, particular strontium-bearing metalorganic reactant, a barium-bearing metalorganic reactant, niobium-bearing metalorganic reactant, and oxygen reactant can be provided in proper proportions in the reactor and reacted under temperature and low pressure conditions to deposit on an amorphous substrate surface a SBN ferroelectric thin film with a high degree of preferential orientation of the c-axis of the crystal lattice perpendicular or normal to the substrate surface.
For further example only, a particular potassium-bearing metalorganic reactant, niobium-bearing metalorganic reactant, and oxygen reactant can be provided in proper proportions in the reactor and reacted under temperature and low pressure conditions to deposit on a substrate surface a highly oriented ferroelectric potassium niobate thin film.
The present invention is advantageous in that the preferentially oriented ferroelectric thin film can be deposited on amorphous as well as crystalline substrate surfaces, such as amorphous silicon dioxide in widespread use in semiconductor devices. The preferentially aligned ferroelectric thin films of the invention thus can be readily integrated into silicon base microelectronics.
The present invention provides a highly textured or epitaxial niobate ferroelectric thin film deposited by metalorganic chemical vapor depostion on an amorphous or crystalline substrate surface to provide an axis of the crystal lattice exhibiting a high dielectric constant, electro-optic coefficient, or other property with a high degree of preferential orientation or alignment relative to the substrate surface. Thus, the high dielectric constant and/or electro-optic properties along the a particular axis of a thin ferroelectric film can substantially improve performance of electrical and electro-optic devices incorporating such films.
The above and other objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.
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patent: 5204314 (1993-04-01), Kirlin et al.
patent: 5250817 (1993-10-01), Fink
patent: 5431958 (1995-07-01), Desu et al.
patent: 5536323 (1996-07-01), Krilin et al.
patent: 5576879 (1996-11-01), Nashimoto
patent: 4-301329 (1992-10-01), None
patent: 5-267196 (1993-10-01), None
Growth Studies of Ferroelectric Oxide Layers Prepared by Organometallic Chemical Vapor Deposition; J. Crystal Grow. 107(1991) pp. 712-715, L.A. Willis, et al.
Epitaxial Growth of Ferroelectric T.B. Sr.1−xBaxNb2O6Films for Optoelectronic Applications; Mat. Res. Bull., vol. 22, pp. 1095-1102, R. R. Neurgaonkar, et al.
Ferroelectric Properties of Thin Strontium Barium Niobate Films, Ferroelectrics, 1985, vol. 63, pp. 235-242, V.D. Antsigin etal.
Sol-Gel Processing of Strontium-Barium Niobate Ferroelectric Thin Film; J. Mater. Res., vol. 5, No. 5, May, 1990, pp. 916-918, Ren Xu, et al.
Preparation of Strontium Barium Niobate by Sol-Gel Method; J. Am. Cheram. Soc., 75(6) (1992) pp. 1697-1700, Shin-ichi Horano, et al.
Development and Modification of Photorefractive Properties in the
Nystrom Michael J.
Wessels Bruce W.
Mai Huy
Northwestern University
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