Compositions – Piezoelectric
Reexamination Certificate
2000-02-16
2002-01-08
Koslow, C. Melissa (Department: 1755)
Compositions
Piezoelectric
C252S06290R, C501S134000, C501S135000, C501S136000, C501S138000, C427S100000, C427S226000
Reexamination Certificate
active
06337032
ABSTRACT:
FIELD OF INVENTION
This invention relates to a sol-gel precursor solution for forming a perovskite ferroelectric material and a method for forming ferroelectric materials for integrated circuits, with particular application for fabrication of ferroelectric dielectric and piezoelectric materials. Particular aspects of the invention relate to lead containing ferroelectric dielectric materials, including lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), lead magnesium niobate (PMN), lead iron niobate (PFN).
BACKGROUND OF THE INVENTION
In recent years, the use of the ferroelectric materials for random access memory (RAM) elements has reached commercial applications in the semiconductor industry. Ferroelectric dielectric materials have very high dielectric constants (typically &egr;>500) providing for memory elements that have high charge storage capacity. Ferroelectric memory elements are non-volatile, consume low power and are programmable with low voltage, e.g. less than 5V. Other advantages include fast access times, (<40 ns), radiation hardness, and robustness with respect to virtually unlimited read and write cycles.
Ferroelectric dielectrics are of also of interest for applications as coupling and decoupling capacitors, and for filter elements operating at low frequency (<1 Hz) up to microwave (GHz) frequencies. The relatively low value of the dielectric constant of conventional dielectrics, typically silicon dioxide and silicon nitride (&egr;<10) limits the capacitance attainable to about 2 to 3 fF/&mgr;m
2
. The high dielectric constant of ferroelectric dielectric materials allows for capacitances greater than 30 fF/&mgr;m
2
. A number of integrated circuit applications would benefit from large on-chip capacitances in the nF range. Consequently, there is much interest in ferroelectric dielectric materials for larger, high value capacitors, as well as for smaller memory elements.
Since ferroelectric materials may also exhibit useful piezoelectric and non-linear optical properties, there is also much interest in providing improved methods for making thin film ferroelectric materials for optoelectronics and other applications, e.g. waveguides, electro-acoustic transducers, surface acoustic wave devices, non-linear optical devices, optical modulators and piezoelectric devices. The high piezoelectric coupling coefficient of ferroelectric materials make them suitable as actuators for micromachine structures incorporated in silicon micro-circuits.
Ferroelectric dielectric materials with large dielectric constants include ferroelectric perovskites, which are complex metal oxides of the general structure ABO
3
in which the A and B sites of the perovskite structure are occupied by one or more different metals. Particular perovskite ferroelectric materials which have made the breakthrough in integrated circuit applications include, for example, lead zirconate titanate PbZr
x
Ti
1−x
O
3
(PZT), lead lanthanum zirconium titanate (PLZT), barium titanate (BT), and barium strontium titanate (BST).
PZT has a higher dielectric constant and may be formed at lower temperature than BST. On the other hand, PZT formed by conventional methods shows dispersion, at frequencies above ~100 MHz, above which the dielectric constant drops to a low value. BST has a flat dielectric response up to ~5 GHz and is favoured for high frequency GaAs integrated circuit applications.
The interest in using ferroelectric materials for applications in non-volatile DRAMs has led to rapid development of improved processes for deposition of thin layers of ferroelectric dielectric materials. Known deposition methods which have been investigated include, for example, metallo-organic sol-gel processes, and other spin-on liquid processes based on metallo-organic decomposition, chemical vapour deposition (CVD) and sputtering, laser ablation, electron beam deposition and ion beam deposition.
The integration of ferroelectric materials for capacitor dielectrics for integrated circuits, or for other device structures, requires a process which is compatible with known semiconductor process technologies. Furthermore, the properties of ferroelectric materials provided as thin films are found to differ from bulk ferroelectric materials. In comparison with preparation of bulk ferroelectric materials, factors including film stress, interactions with substrate materials, and restrictions on process temperatures may significantly influence the characteristics of thin films of ferroelectric materials. Thus, much work has been devoted to developing low temperature processes for formation of thin films of ferroelectric dielectrics compatible with semiconductor processing for CMOS, bipolar and bipolar CMOS technologies.
For integrated circuit applications, a preferred known process for forming thin films of ferroelectric materials is based on a technique, generally known as a sol-gel process, in which a complex oxide is prepared from a sol-gel precursor solution comprising a mixture of metallo-organic
1
compounds, e.g. alkoxides dissolved in an organic solvent, and/or organic metal salts dissolved in an appropriate solvent e.g. an acid or alcohol. The sol-gel process for preparing metal oxides proceeds by the hydrolysis of a metal organic compound to form a sol comprising metal oxide precursors. This process is well known for forming single component oxide glasses and multi-component oxide glasses from a precursor mixture of metal alkoxides. Formation of metal oxide bonds and growth of metal oxide chains and networks in the solution eventually lead to gelation. The hydrolysis and polymerization by condensation (polycondensation) reactions are controlled by factors such as the amount of water, pH, presence of acid or base catalysts, and reaction sequence, for example, as described in U.S. Statutory invention registration no. H 626, published Apr. 4, 1989, entitled “Sol-Gel Ceramic Oxides” to Covino which relates to formation of silicate glasses. In the latter disclosure, it is described how it is known that lowering of pH tends to form oxide networks and chains, forming a polymer network, and leading to gelation without formation of colloidal oxides.
1
In the context of sol-gel processing of complex oxide ceramics, the term organo-metallic or metallo-organic has often used to denote metal containing organic compounds used as precursors, including metal alkoxides, metal carboxylates and metal beta diketonates. In organic chemistry, the term “organo-metallic” or metallo-organic is more generally used to denote a compound having a metal-carbon bond.
In a conventional method of sol-gel processing of piezo-electric and ferroelectric dielectric thin films of the general formula ABO
3
using a sol-gel precursor solution, a precursor solution is provided comprising metal A as an organo-metallic salt, e.g. a metal acetate, and a mixture of metals B as alkoxides, provided in the required stoichiometric proportions. For example, to make PZT, a precursor mixture of a soluble organic lead salt, e.g. lead acetate tri-hydrate, and a mixture of zirconium propoxide Zr(OC
3
H
7
) and a titanium iso-propoxide Ti(OC
3
H
7
)
4
is dissolved in a suitable solvent e.g. an alcohol, or mixture of solvents. The lead salt is dissolved in a suitable anhydrous solvent such as methoxy-ethanol, and the solution is dehydrated, and then the zirconium and titanium propoxides, also dissolved methoxy-ethanol, are added in stoichiometric ratio to provide the B metal cations.
The metal oxide precursor solution forms a mixture of metallo-organic intermediate compounds which react to form a metal oxide precursor. The viscosity and surface tension of the precursor solution is adjusted to allow a layer with a controlled thickness to be spin-coated or dip-coated onto a substrate, as required, depending on the particular application.
Organic metal oxide precursors other than metal alkoxides which have been reported include metal beta-diketonate (e.g. acetyl acetonate) or metal carboxylates, e.g. acetates. For example U.S. Pat. No. 4,946,710
Chivukula Vasanta
Emesh Ismail T.
McDonald David R.
Sayer Michael
Koslow C. Melissa
Nortel Networks Limited
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