Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-05-04
2003-05-13
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S377000, C427S378000, C427S379000
Reexamination Certificate
active
06562408
ABSTRACT:
This application is the national phase of international application PCT/IT99/00284 filed Sep. 6, 1999 which designated the U.S.
FIELD OF THE INVENTION
The present invention relates to a sol-gel process for the preparation of thick glassy films of silicon oxide or based on silicon oxide and to the thick films thus obtained.
In the technology of solid state, with the term “film” is meant a thin layer of a material having a thickness generally comprised between a few tens of nanometers (nm) and a few tens of micrometers (&mgr;m), said layer being supported over a substrate of another material, generally a flat geometry.
The term “thick” typically refers to films of thickness larger than 1 &mgr;m.
Thick glossy films, deposited over a suitable substrate, are the object of extensive research in view of their foreseen use in the field of telecommunications, particularly telecommunications on optical and electro-optical cables.
In the past, telephone communications and data transmissions were realised transforming the signal into electronic impulses that were transmitted by means of cables of an electrically conductive material, generally copper.
Nowadays, in particular for the long distances, transmissions on electrical cables have been almost completely replaced by transmissions an optical fibers. As known, the optical fibers are glassy fibers whose structure comprises at least a central part, called nucleus, and an outer part, called mantle, made of glasses having slightly different chemical compositions; the different chemical composition gives rise to a difference in the refractive index of the two materials that allows confining the optical signal in the nucleus. Commonly the mantle is made of pure silicon oxide, whilst the nucleus is made of a mixed oxide based on silicon oxide containing from a few percent to about 10% by mole of different oxides such as germanium oxide.
The optical fibers offer several advantages over electrical cables as means for information transmission, such as a lower level of noise and lower signal attenuation as well as a higher amount of information transmitted per unit time, resulting in a higher transmission rate.
Despite these advantages, it has not been possible so far to fully exploit the potentiality of optical communications: in fact, a complete communication system requires devices for processing signals, for instance for transforming voice into signal at the two ends of the cable in telephonic transmissions, or for amplifying the signal along the fibre, that is rendered necessary due to unavoidable attenuation of the same signal. More generally, the so-called operation of signal commutation that is needed for delivering the same signal in the network requires suitable devices.
To this end, traditional electrical devices (electronic switches) are presently used, and generally any operation on the signal requires a conversion into electrical signal followed by a possible further conversion back to optical signal. In these operations time and signal quality are lost. As a consequences, a strong need is felt for optical or electro-optical devices capable of guiding an optical signal as well as of performing on it commuting operations comparable to those operated by electronic devices on electrical signals.
The main features that optical devices must have, are:
material of very high transmittance, requiring absence of inclusions and mechanical defects;
possibility of controlling through chemical composition the refractive index, that must be at least a few percent units higher than that of surrounding materials;
flat geometry, for easy fit into automated production lines;
thickness of a few &mgr;m, preferably about 2 and 20 &mgr;m.
In order to ease integration of these devices into production and communication lines, the substrate should preferably be made of silicon or silicon oxide.
BACKGROUND OF THE INVENTION
Such devices are presently produced according to physical techniques, among which thermal oxidation of silicon, and those known as Sputtering, Chemical Vapor Deposition and Flame Hydrolysis can be cited. Another method consists in the vacuum deposition on a silicon substrate of microparticles of silicon oxide obtained according to the Flame Hydrolysis technique.
However, these productions are complex, requiring costly working chambers and tools; some of these, such as silicon thermal oxidation, have a limit in the film thickness that can be obtained, while others are exceedingly slow and are often characterised by low productivity and too high costs, so as not to allow an actual industrial exploitation of optical devices.
The most economically promising technology for massive production of glassy films on substrates is sol-gel. Under the name sol-gel are gathered different procedures for the preparation of oxides of one or more elements in form of porous bodies, ceramics or glasses.
While differing from each other in the specific details, all sol-gel procedures share the following phases:
preparation of a “sol”, a solution or suspension in water, alcohol or hydroalcoholic mixtures of precursors of the elements whose oxides is to be prepared. Generally used as precursors are the alkoxides, of formula M(OR)
n
, where M represents the element whose oxide is desired, the group —OR is the alkoxide moiety, and n represents the valence of element M; soluble salts of the element M, such as chlorides; nitrates and exceptionally oxides, may be used in place of alkoxides. During this phase the precursors begin to hydrolyse, that is, alkoxide moieties or other anions bonded to element M are replaced by —OH groups;
sol gelation, requiring from a few seconds up to some days, depending on chemical composition and temperature of the solution; during this phase hydrolysis of the possibly remaining precursor is completed and condensation occurs, consisting in the reaction of —OH groups belonging to different molecules with formation of one free water molecule and an oxygen bridge between atoms M, M′ (alike or different), according to the reaction:
(HO)
n−1
M—OH+HO—M′(OH)
m−1
→(HO)
n
M—O—M′(OH)
m
+H
2
O (I)
The product obtained in this phase is called alcogel, hydrogel depending on the cases, or more generally “gel” as widely used in the English literature.
gel drying; in this phase the solvent is removed by simple evaporation or through hypercritical transformation into gas inside an autoclave; there is obtained an extremely porous dry body, that may have an apparent density ranging from about 10% to about 50% of the theoretical density of the oxide of that composition;
dry gel densification by thermal treating at a temperature generally comprised between 800° C. and 1200° C. depending on the gel chemical composition and on the parameters of the previous process phases; in this phase the porous gel densifies obtaining a glassy or ceramic compact oxide of theoretical density, with a linear shrinkage of about 50%.
If gelation phase is not too fast, it is possible to lay a liquid film of sol on a substrate, eventually resulting in a oxide supported film. Obtaining a oxide film on a substrate in this way is however easily feasible only for a thickness up to some tenths of micrometer. Up to such values of thickness, cohesive forces in the film are weak, and forces adhering the film on the substrate prevail, so that during the densification phase there is not in-plane shrinkage of the film and densification only involves its thickness decrease. At values of thickness above one micrometer, on the other hand, inner cohesive forces of the film become prevailing and during densification in-plane shrinking of the film takes place as well: the result is film fragmentation into “islands” spread over the substrate surface and poor adhesion of the film to the substrate.
This thickness of about 1 &mgr;m represents a technological limit for sol-gel technique, as indicated for instance in “SOL-GEL science: the physics and chemistry of SOL-GEL processing”, Brinker and Scherer, Academic Press, 1990, a comprehensive rev
Costa Lorenzo
Costa Pier Paolo
Grandi Stefania
Barr Michael
Novara Technology S.R.L.
Pillsbury & Winthrop LLP
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