Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state
Reexamination Certificate
2000-10-04
2002-09-17
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
C252S005000, C252S006500, C252S008050
Reexamination Certificate
active
06451106
ABSTRACT:
The present invention has the following objectives:
polycrystalline alkali-metal or alkaline-earth metal fluorides, produced in an original form, namely in the form of beads;
a process for the preparation (the conditioning) of said fluorides;
a process for the preparation of single crystals of the corresponding alkali-metal or alkaline-earth metal fluorides that uses polycrystalline fluorides in the aforementioned original form
providing an economic process for producing optical fluoride crystal raw material feedstock beads and manufacture of optical fluoride single crystals with optical fluoride crystal raw material feedstock beads.
The present invention was specifically developed for the preparation of fluoride single crystals, particularly optical fluoride single crystals and optical lithography element fluoride single crystal blanks and notably for single crystals of CaF
2
. Whereas its description more particularly references single crystals of CaF
2
, the invention is not limited to said crystals.
Single crystals of CaF
2
are grown using the Stockbarger technique, which is familiar to those skilled in the art. They are the material of choice as optical element blanks for preparing UV (&lgr;<250 nm) optical lithography element lenses used in microlithography at 157 and 193 nm, as these lenses must have a below 200 nm high transmission, good laser resistance, low birefringence, and exhibit minimal fluorescence. These crystals must be prepared in the absence of water, air, and all sources of oxygen. This is well known to those skilled in the art. These single crystals are usually prepared in the presence of a fluoridating agent. PbF
2
is the most commonly used fluorinating agent, as it is relatively easy to handle, is solid at room temperature, and has a high vapor pressure at the melting point of CaF
2
. Its reaction with CaO generates PbO, which is subsequently eliminated.
The process for growing large crystals of CaF
2
(or any other alkali-metal or alkaline-earth metal fluoride) that are relatively free of defects typically lasts several weeks, particularly for high optical quality optical fluoride single crystals and below 200 nm optical lithography element fluoride single crystal blanks. The cost of the equipment as well as the staging of the crystal-growth process is significant and there is no guarantee of having a successful result at the end of the growth process. There has therefore long been a concerted effort to augment the yield of this crystal-growth process.
The problem of optimizing the mass of raw material feedstock introduced into the growth furnace with respect to the volume of said furnace has been faced. In practice, highpurity synthetic CaF
2
is used as the raw material. This powder typically has an apparent density of only ca. 1.1 g/cm
3
, whereas the crystals produced have an apparent density close to the theoretical density, i.e. 3.18 g/cm
3
. Thus if synthetic powder is used directly as the raw material, ⅔ of the volume of the crystal-growth furnace is not used, or at least, not used efficiently. This same problem exists for all of the fluorides, as the crystals grown are denser than the raw material used.
An attempt has therefore been made to increase the density of said raw material before proceeding with the crystal-growth process. Increasing the density of the powder of a mineral salt typically entails its fusion and/or compression.
In the present case:
Compression is not a desirable solution: it requires special equipment, it is liable to introduce impurities, and the compressed masses produced can not be placed in intimate contact with a fluorinating agent before the start of and during the crystal-growth process. In any case, compression alone does not result in material having near theoretical density.
Nor is fusion a satisfactory technique: the products obtained from this process must be ground and grinding invariably affects the purity of the product.
Confronted with this technical problem, namely the optimization of the volume occupied by the raw material in the crystal-growth furnace and more generally, the optimization of the crystal growth process of alkali-metal and alkaline-earth metal fluorides, the inventors developed the original treatment process disclosed herein of said fluorides. Fluorides prepared by this method can be used for crystal growth in an optimized crystal-growth process.
The inventors propose an original procedure for increasing the density of the said fluorides. An advantageous variation of this original process assures both the densification and purification of these fluoride salts.
Said process of densification yields, starting from low-density and notably synthetic raw-material powder, beads with an apparent density that approaches the theoretical density of the corresponding fluoride. These beads have a small diameter. Therefore they do not need to be ground to be brought into efficient contact with a fluorinating agent when they are ultimately loaded into the crystal-growth furnace. Moreover, owing to their shape (quasi-perfect spheres), they can fill the crystal-growth crucible with minimal volume loss. The invention includes optical fluoride crystal raw material feedstock beads, the making of optical fluoride crystal feedstock beads and manufacture of optical fluoride single crystals and UV optical lithography element fluoride crystal blanks with optical fluoride crystal feedstock beads.
Those skilled in the art have certainly already understood the interest of the present invention. Its three aspects—beads, their preparation, and their use in preparing single crystals—are hereafter presented in detail.
As its first object, the present invention concerns a polycrystalline alkali-metal or alkaline-earth metal fluoride, prepared in the shape of beads. These beads have:
a diameter or equivalent diameter (their preparation procedure generates more or less perfect spheres) greater than or equal to 100 &mgr;m, advantageously between 100 &mgr;m and 2 cm; and
an apparent density greater than or equal to 60% of the theoretical density of the appropriate fluoride, advantageously at least 90% of the aforementioned theoretical density.
In the context of the invention, there is little interest in optical fluoride crystal feedstock beads of diameter (the word “diameter” is used from now on to cover both the notion of diameter and equivalent diameter) less than 100 &mgr;m or greater than 2 cm. More precisely:
beads with diameter less than 100 &mgr;m are almost equivalent, in terms of density, to powder;
beads with diameter greater than 2 cm are not useful as such for the crystal-growth process. They must be ground to optimize packing and to increase their surface area to volume ratio (so they can be in intimate contact with the fluorinating agent). Such beads are equivalent to the “pieces” obtained from the compression and/or fusion of a powder.
The beads of this invention have an apparent density greater than their corresponding powders. Their apparent density tends toward the theoretical density of the constituent material. The beads of this invention have an apparent density at least equal to 60%, advantageously at least equal to 90%, of the theoretical density of the fluoride in question. Therefore, beads of CaF
2
produced according to the present invention have a density greater than or equal to 1.9 g/cm
3
, advantageously greater than or equal to 3 g/cm
3
.
The term “apparent density” is familiar to those skilled in the art. The density of a material is constant under given conditions of temperature and pressure. The term “apparent density” is referred to for a solid material dispersed in the form of particles in such a way that its “density” is sensitive to the size and surface state of said particles. In the present context, the apparent density of powders or beads can be defined as the mass of said powders or beads (g) that can be placed into a given volume (cm
3
), at ambient temperature and without application of pressure.
The optical crystal feedstock densified beads of the invention constitute
Mayolet Alexandre M.
Pell Michael A.
Corning Incorporated
Hiteshew Felisa
Murphy Edward F.
LandOfFree
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