Coating processes – Direct application of electrical – magnetic – wave – or... – Electromagnetic or particulate radiation utilized
Reexamination Certificate
2000-09-14
2002-04-16
Jones, Deborah (Department: 1775)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electromagnetic or particulate radiation utilized
C427S255110, C427S595000
Reexamination Certificate
active
06372306
ABSTRACT:
TECHNICAL FIELD
The present invention relates to ferroelectric materials and methods for fabrication thereof, and more specifically to ferroelectric materials with a chemical formula:
A
(1−x)
B
x
C
(1−y)
D
y
F
3
;
such as NaCF
3
, or Na
1−x
K
x
CaF
3
, or which are fabricated by deposition of source materials onto a substrate with a (111) orientation and a cubic lattice constant of between about 3.8 to 4.3 Angstroms, or a substrate with a (001) orientation and a hexagonal lattice with an a-axis parameter of between about 5.4 to 6.2 Angstroms.
BACKGROUND
It is first to be appreciated that ferroelectric materials are noted for having large permittivities and for demonstrating hysteresis-type retention of residual polarization after an electric field which has been applied, is removed. These properties make ferroelectric materials attractive for application in, for instance, thin film capacitors and random access memories (RAM's), infrared sensors based upon pyroelectric properties, and possibly thermal infra-red switches based on electro-optic properties, as well as microactuators based upon piezoelectric properties.
All ferroelectric materials are related by having noncentrosymmetric crystalline structure and attendant polarization due to displacement of metal cations from the center of their respective coordination spheres. Many ferroelectric materials with formulae ABX
3
occur in distorted perovskite structures composed of octahedra connected through shared vertices. Typically small, highly charged, metal cations, (such as Ti
4+
) are located within the octahedra and large cations, (such as Ba
2+
), are located in the spheres between octahedra. Polarization occurs when the small cation is displaced toward one corner or face of the octahedral environment. BaTiO
3
is the archetypical ferroelectric material of this variety.
Because ferroelectric properties are related to noncentrosymmetric crystalline structure, phase transitions to higher symmetry centrosymmetric structures typically destroy their ferroelectric properties. Consequently, most ferroelectric materials have a phase transition Curie temperature (Tc), above which the materials no longer act as a ferroelectric, but might retain good dielectric properties.
It is also noted that most ferroelectric materials are oxides and are opaque in ultraviolet spectral region. A ferroelectric material which is transparent in the ultraviolet spectral region could therefore provide additional utility, and, as taught by the present invention, candidates for such a ferroelectric material include fluorides.
Continuing, as early as 1984 Dr. J. W. Flocken, Dr. R. A. Guenther at the University of Nebraska at Omaha, and Dr. L. L. Boyer of the Naval Research Lab determined that of approximately sixty (60) halide-based perovskites of the form ABX
3
, where A is an alkali metal, and B is an alkaline earth metal, and X is a halogen, three (3) were candidates for study as ferroelectrics. One of these compounds is NaCaF
3
. This is because NaCaF
3
was determined to be of a distorted perovskite structure, the distortion being from a cubic symmetry to a rhombohedral symmetry by elongation along the cubic body-centered diagonal. The body-diagonal of the cubic cell being the trigonal axis of the ferroelectric structure, along which polarization is expected to occur by displacement of sodium cations and fluoride anions in opposite directions from the cubic geometry.
In view of the potential benefits associated with NaCaF
3
, several attempts to fabricate NaCaF
3
over a period of years were made, however, none produced results. Unsuccessful approaches included solid-state co-precipitation, freeze-drying, and decomposition reactions.
With the present invention in mind, a Search of Patents was performed, with the result being that very little was found. Perhaps the best reference is U.S. Pat. No. 3,238,015 to Pessahovitz et al., which describes the preparation of KFMgF
2
by dissolving thixotropic MgF
2
in a hot solution of KF. The resultant material, however, is not identified as being ferroelectric. Another U.S. Pat. No. 3,682,727 to Phillips describes fabrication of photochromic materials by melting CaF
2
, LaF
2
and NaF in a crucible at between 630 and 725 degrees Centigrade. U.S. Pat. No. 5,888,296 to Ooms et al. describes formation of a layer of ferroelectric bismuth on a lattice matched semiconductor. U.S. Pat. Nos. 5,667,725 and 5,552,083 to Wanatabe et al. describe the preparation of colloidal NaFMgF
2
. A U.S. Pat. No. 5,356,831, to Calveillo et al., describes lattice matching a semiconductor to a substrate such as MgO with a lattice constant in the range of three (3) to six (6) Angstroms. Recent Patents which describe application of ferroelectric materials in thin film capacitors and RAM devices are U.S. Pat. Nos. 5,889,299 and 5,889,696.
In addition, a number of Scientific Papers have been identified as follows:
“The Physics Of Ferroelectric Memories”, by Auciello, Scott and Ramamoorthy, Physics Today, (July 1998).
“Theory For Forces Between Closed-Shell Atoms And Molecules”, Gordon and Kim, J. Chem. Phys., Vol. 56, No. 6, (March 1972).
“Phase Transitions In Mixed Alkali Calcium Trifluoride Solid Solutions”, Flocken, Smith, Hardy, Stevenson and Swearingen, Mat. Research Bull., Vol 31, No. 9, (1996).
“Molecular Dynamics Simulation Of Superionicity In Neighboring NaMgF
3
”, Zhou, Hardy and Cao, Geophysical Research Lett., Vol. 24, No. 7, (April 1997).
“Synthesis Of Novel Thin-Film Materials By Pulsed Laser Deposition”, Lowndes, Geohegan, Puretzky, Norton and Rouleau, Science, Vol 273 (August 1996).
“Thin Film Synthesis Of Metastable And Artificially Structured Oxides”, Gupta, Elect. Mat., Curr. Opin. Solid State Mater Sci., Vol 2 (1997).
“Stabilization of YMnO
3
In A Perovskite Structure As A Thin Film”, Salvador, Doan, Mercey and Raveau, Chem. Mater., Vol. 10, No. 10, (1998).
“Pulsed Laser Ablation Synthesis Of NbNx (0 (x (1.3) Thin Films”, Chem. Mater., Vol. 6, No. 12, (1994).
“New Phase Superconducting NbN Stabilized By Heteroeptaxial Film Growth”, Phys. Rev. B, Vol. 51, No. 14, (April 1995).
“Pulsed Laser Deposition Of Oriented PbZr
54
Ti
46
O
3
”, Grabowski, Horwitz, and Chrisey, Ferroelectrics, Vol. 116, (1991).
“Microwave Properties Of Sr
0.5
Ba
0.5
TiO
3
Thin-Film Interdigitated Capacitors”, Kirchoefer, Pond, Carter, Chang, Agrwal, Horowitz and Chrisey, Microwave and Opt. Tech. Lett., Vol. 18, No. 3 (June 1998).
“Epitaxial Growth Of Metal Fluoride Thin Films By Pulsed-Laser Deposition”, Norton, Budai, Chakoumakos, Geohegan and Puretzky, Mat. Res. Soc. Symp. Proc. Vol. 387, (1996).
“Predicting New Materials”, Boyer, Computers in Phys. Vol 8, No. 1, (January/February 1994).
“First-Principals Study Of Structural Instabilities In Halide-Based Perovskites: Competition Between Ferroelectricity and Ferroelasticity”, Guenther, Hardy, Boyer, Phys. Rev. B, Vol. 31, No. 11, (June 1985).
“Ferroelectricity In Perovskites like NaCaF
3
Predicted Ab Initio”, Phys. Rev. B, Vol 39, No. 13, (May 1989).
“Ferroelectric Phase Transitions In NaCa-Halide Perovskites”, Flocken, Guenther, Hardy, Edwardson and Boyer, Phase Transactions, Vol. 20, (1990).
“Ferroelectric Phase Transactions In NA-CA-Halide Perovskites”, Flocken, Mei, Guenther, Hardy, Edwardson and Boyer, Ferroelectrics, Vol. 104, (1990).
“Perovskite To Antiperovskite In ABF
3
Compounds”, Boyer and Edwardson, Ferroelectrics, Vol. 104, (1990).
“The Effect Of K Defect Clusters On The Ferroelectric Phase Transition In NaCaF
3
”, Flocken, Mei, Guenther, Hardy and Boyer, Ferroelectrics, Vol. 120, (1991).
“Revised Effective Ionic Tadii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides”, Acta Cryst A 32 (1976).
“New Phase of Superconducting NbN Stabilized by Hetroepitaxial Film Growth”, Treece et al., Phys. Rev. B, Vol 51, No. 14 (April 1995).
“Perovskite to Antiperovskite in ABF
3
Compounds”, Boyer et al., Ferroelectrics, Vol. 104, (1990).
“The Crossover of Phase Transitions From NaCaF
3
to KCaF
3
”, Flocken et al., Ferroelectrics, Vol.
Horwitz James S.
Smith Robert W.
Bahta Abraham
Board of Regents of the University of Nebraska
Jones Deborah
Welch James D.
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