U.S. patent application number 10/698502 was filed with the patent office on 2004-05-13 for preparation of crystals.
Invention is credited to Meyer-Fredholm, Michele M. L..
Application Number | 20040089223 10/698502 |
Document ID | / |
Family ID | 8861679 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089223 |
Kind Code |
A1 |
Meyer-Fredholm, Michele M.
L. |
May 13, 2004 |
Preparation of crystals
Abstract
The object of the present invention is a process of preparing a
crystal, which comprises: loading a crucible with a mixture of the
appropriate starting material which contains at least one oxide as
impurity, and an effective and non-excess amount of at least one
fluorinating agent which is solid at ambient temperature, melting
said mixture within said crucible, growing the crystal, by
controlled cooling of the molten mixture, controlled cooling of
said crystal to ambient temperature, recovering said crystal; and
which is characterised in that the oxide(s) resulting from the
reaction between said fluorinating agent(s) and said oxide(s), the
impurity or impurities, can be discharged from said crucible, in
view of the dimensions of said crucible and of the intrinsic
permeability of the material constituting it. Said process is
particularly adapted for preparing (mono)crystals of CaF.sub.2 in
graphite crucibles.
Inventors: |
Meyer-Fredholm, Michele M. L.;
(Avon, FR) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
8861679 |
Appl. No.: |
10/698502 |
Filed: |
October 31, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10698502 |
Oct 31, 2003 |
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10107283 |
Mar 26, 2002 |
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6669778 |
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Current U.S.
Class: |
117/11 |
Current CPC
Class: |
C30B 29/12 20130101;
C30B 11/002 20130101; C30B 11/00 20130101 |
Class at
Publication: |
117/011 |
International
Class: |
C30B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2001 |
FR |
01 04232 |
Claims
1. A process of preparing a fluoride optical crystal comprising:
loading a crucible with a mixture of a fluoride optical crystal
starting material which contains at least one oxide as impurity,
and an effective and non-excess amount of at least one fluorinating
agent which is solid at ambient temperature, melting said mixture
within said crucible, growing the crystal, by controlled cooling of
the molten mixture, controlled cooling of said crystal to ambient
temperature, and recovering said crystal; said process being
characterised in that the oxide(s) resulting from the reaction
between said fluorinating agent(s) and said oxide(s), the impurity
or impurities, can be discharged from said crucible, in view of the
dimensions of said crucible and of the intrinsic permeability of
the material constituting it.
2. The process according to claim 1, characterised in that said
effective and non-excess amount of fluorinating agent(s) represents
5% by weight at most, advantageously between 0.1 and 2% by weight,
of the weight of said starting material.
3. The process according to one of claims 1 or 2, characterised in
that said fluorinating agent(s) is (are) selected from PbF.sub.2,
ZnF.sub.2, NH.sub.4F, NH.sub.4F.HF, PTFE, and mixtures thereof.
4. The process according to any one of claims 1 to 3, characterised
in that said crucible is a graphite crucible the permeability of
which, measured according to the DIN 51935 Standard, is greater
than 4 cm.sup.2/s.
5. The process according to any one of claims 1 to 4, characterised
in that said crucible is a graphite crucible the permeability of
which, measured according to the DIN 51935 Standard, is greater
than 10 cm.sup.2/s.
6. The process according to any one of claims 1 to 5, characterised
in that said crucible is suitable for preparing a fluoride optical
crystal with a diameter .gtoreq.200 mm and a height .gtoreq.50
mm.
7. The process according to any one of claims 1 to 6, characterised
in that it is carried out for preparing monocrystals of alkali
metal fluorides.
8. The process according to any one of claims 1 to 6, characterised
in that it is carried out for preparing monocrystals of
alkaline-earth metal fluorides.
9. The process according to any one of claims 1 to 6, characterised
in that it is carried out for preparing monocrystals of
CaF.sub.2.
10. The process according to any one of claims 1 to 9,
characterised in that controlled cooling of the molten mixture, for
growing the (mono)crystals, is obtained by very slowly moving a
stack of loaded crucibles from the top to the bottom, from a hot
zone to a cold zone, of an oven having a vertical axis.
11. A method of making a calcium fluoride crystal with increased
far-ultraviolet transmission, said method comprising: providing a
calcium fluoride crystal producing graphite crucible for containing
calcium fluoride, said graphite crucible comprised of a graphite
having a permeability of which, measured according to the DIN 51935
Standard, is greater than 4 cm.sup.2/s forming a molten calcium
fluoride melt in said graphite crucible comprised of said graphite
having a permeability greater than 4 cm.sup.2/s forming a calcium
fluoride crystal from said molten calcium fluoride melt, said
formed calcium fluoride crystal having an increased far-ultraviolet
transmission with intrinsic transmission at 193 nm.gtoreq.99.9% and
intrinsic transmission at 157 nm.gtoreq.99%.
12. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 5
cm.sup.2/s.
13. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 6
cm.sup.2/s.
14. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 7
cm.sup.2/s.
15. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 8
cm.sup.2/s.
16. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 9
cm.sup.2/s.
17. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 10
cm.sup.2/s.
18. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 11
cm.sup.2/s.
19. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 12
cm.sup.2/s.
20. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 13
cm.sup.2/s.
21. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a permeability of which, measured
according to the DIN 51935 Standard, is greater than 14
cm.sup.2/s.
22. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a Hg porosity of at least
16.7%.
23. A method as claimed in claim 11 wherein said graphite crucible
is comprised of a graphite having a Hg porosity of at least
20%.
24. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission, said graphite crucible comprised of a graphite having
a permeability of which, measured according to the DIN 51935
Standard, is greater than 4 cm.sup.2/s.
25. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a Hg
porosity of at least 16.7%.
26. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a Hg
porosity of at least 20%.
27. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 5 cm.sup.2/s.
28. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 6 cm.sup.2/s.
29. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 7 cm.sup.2/s.
30. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 8 cm.sup.2/s.
31. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 9 cm.sup.2/s.
32. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 10 cm.sup.2/s.
33. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 11 cm.sup.2/s.
34. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having
permeability of which, measured according to the DIN 51935
Standard, is greater than 12 cm.sup.2/s.
35. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 13 cm.sup.2/s.
36. A calcium fluoride crystal producing graphite crucible for
making a calcium fluoride crystal with increased far-ultraviolet
transmission as claimed in claim 24, said graphite having a
permeability of which, measured according to the DIN 51935
Standard, is greater than 14 cm.sup.2/s.
Description
[0001] This application claims the benefit of French Application
No. 01 04232, filed Mar. 29, 2001 entitled "Preparation of
(mono)Crystal," by M. Meyer-Fredholm.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to the preparation
of fluoride crystals, and particularly to making of optical
fluoride crystals with improved below 200 nm wavelength optical
properties.
[0003] More specifically, said invention relates:
[0004] to a process of preparing (mono)crystals, which is improved
with reference to the purity of the (mono)crystals prepared;
and
[0005] to a process of preparing (mono)crystals which have an
increased transmission in the far-ultraviolet (.lambda.<193 nm,
even .lambda.<157 nm).
TECHNICAL BACKGROUND
[0006] Ultra-high performance optical systems are required in order
to increase the density of integration of the electronic components
on a semi-conductor plate and insofar as exposed light of low
wavelength (lower than 248 nm) is necessary in order to improve the
resolution. The most common technique up to now for obtaining such
optical systems uses molten silica. According to another technique,
which is already exploited, especially by the companies Bicron and
Schott, monocrystals of calcium fluoride and monocrystals of barium
fluoride are used. Ultra-high performance far-ultraviolet optical
systems with below 200 nm wavelengths require fluoride optical
crystals.
[0007] Said monocrystals, of calcium fluoride or of barium
fluoride, and more generally of alkali metal and/or alkaline-earth
metal fluorides, are in principle obtained according to the process
known as the Stockbarger-Bridgman process, which is familiar to the
person skilled in the art. According to said process, the crystal
is generated from an appropriate molten starting material in slowly
lowering (generally at a speed between 0.3 and 5 mm/h, more
generally between 1 and 3 mm/h) a crucible (or a stack of
crucibles) containing said molten material through a solidification
zone which is provided in an oven. The crucible(s) is (are) made
from a material which is resistant to chemical attack from the
material that it contains. In general, it is (a) crucible(s) in
graphite of high purity.
[0008] According to the teaching of US patents U.S. Pat. Nos.
5,911,824 and 6,093,245, the graphite does have the drawback of
being porous (of being a material having open porosity), and it is
recommended to coat the internal walls of such graphite crucibles
with an appropriate internal coating, in order to <<block the
porosity>> of said walls. Carbon coatings, especially
pyrolytic or vitreous carbon coatings, are described.
[0009] The (mono)crystals must imperatively be prepared in the
absence of water, of air and of any other source of oxygen. They
are thus generally prepared under vacuum in the presence of a
fluorinating agent. Said fluorinating agent ensures the elimination
of oxygen, especially of that introduced in the form of oxide as
impurity in the starting material. PbF.sub.2 is the most utilised
fluorinating agent, insofar as its manipulation does not present
any particular difficulty, insofar as it is solid at ambient
temperature and insofar as it has, itself and its corresponding
oxide (PbO), a high vapour pressure at the temperatures of use of
crystallisation ovens. Said PbF.sub.2 acts, within the context of
the preparation of CaF.sub.2 crystals, notably according to the
reaction:
CaO+PbF.sub.2.fwdarw.CaF.sub.2+PbO.
[0010] In practice, it is always delicate to optimise the
intervention of said fluorinating agent. It is especially
critical:
[0011] to adjust the rise in temperature of the mixture (for its
melting) with the view to said optimisation;
[0012] to adjust the amount of said fluorinating agent, with the
view to minimising any retention of Pb or other (according to the
nature of said fluorinating agent in question) in the crystal
prepared: such a retention has obviously disadvantageous
repercussions on the performances of transmission and resistance to
radiation of said crystal.
[0013] It is, within the context set forth above, with reference to
the optimisation of the intervention of fluorinating agents, that
the present invention has been developed.
SUMMARY OF THE INVENTION
[0014] One aspect of the invention relates to a process of
preparing a fluoride optical crystal which includes loading a
crucible with a mixture of a fluoride optical crystal starting
material which contains at least one oxide as impurity, and an
effective and non-excess amount of at least one fluorinating agent
which is solid at ambient temperature, melting said mixture within
said crucible, growing the crystal, by controlled cooling of the
molten mixture, controlled cooling of said crystal to ambient
temperature, and recovering said crystal wherein the oxide(s)
resulting from the reaction between said fluorinating agent(s) and
said oxide(s), the impurity or impurities, can be discharged from
said crucible, in view of the crucible and the intrinsic
permeability of the material constituting it.
[0015] In another embodiment, the present invention includes a
method of making an optical fluoride crystal with increased
far-ultraviolet transmission by providing a fluoride crystal
producing graphite crucible for containing the fluoride, said
graphite crucible comprised of a graphite having a permeability of
which, measured according to the DIN 51935 Standard, is greater
than 4 cm.sup.2/s, forming a molten fluoride melt in said graphite
crucible comprised of said graphite having a permeability greater
than 4 cm.sup.2/s and forming a fluoride crystal from said molten
fluoride melt, said formed fluoride crystal having an increased
far-ultraviolet transmission with intrinsic transmission at 157
nm.gtoreq.99%. In a preferred embodiment the fluoride crystal
comprises calcium fluoride. In a preferred embodiment the fluoride
crystal comprises barium fluoride.
[0016] In another embodiment, the present invention includes an
optical fluoride crystal producing graphite crucible for making an
optical fluoride crystal with increased far-ultraviolet
transmission, said graphite crucible comprised of a graphite having
a permeability of which, measured according to the DIN 51935
Standard, is greater than 4 cm.sup.2/s.
[0017] Additional features and advantages of various embodiments of
the invention will be set forth in the detailed description which
follows, and in part will be readily apparent to those skilled in
the art from that description or recognized by practicing the
invention as described herein, including the detailed description
which follows, the claims, as well as the appended drawings.
[0018] It is to be understood that both the foregoing general
description and the following detailed description present
embodiments of the invention, and are intended to provide an
overview or framework for understanding the nature and character of
the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and
are incorporated into and constitute a part of this specification.
The drawings illustrate various embodiments of the invention, and
together with the description serve to explain the principles and
operations of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The process of preparing a crystal of the invention
comprises:
[0020] loading a crucible with a mixture of the appropriate
starting material which contains at least one oxide as impurity,
and an effective and non-excess amount of at least one fluorinating
agent which is solid at ambient temperature,
[0021] melting said mixture within said crucible,
[0022] growing the crystal, by controlled cooling of the molten
mixture,
[0023] controlled cooling of said crystal to ambient temperature,
and
[0024] recovering said crystal.
[0025] In this, it can be a Stockbarger-Bridgman process, or any
other equivalent process, each of the steps of which is familiar to
the person skilled in the art, which is thus carried out in order
to obtain a mono- or polycrystalline crystal.
[0026] Thus, in order to prepare (mono)crystals of CaF.sub.2, said
crucible is in general loaded with a mixture de synthetic CaF.sub.2
powder, which contains CaO as impurity, and PbF.sub.2 (fluorinating
agent).
[0027] The crucible in question can very well not be a single one.
The process of the invention can effectively be carried out, just
as the process of the prior art, with a stack of n crucibles, which
is moved with a translatory motion along its vertical axis.
[0028] The fluorinating agent(s) which is (are) incorporated is
(are) not incorporated in an excess amount. In this way, it (they)
cannot pollute the crystals prepared.
[0029] Characteristically, within the context of the invention, the
oxide(s) (PbO, in the context specified above (in a purely
illustrative manner) of preparation of (mono)crystals of CaF.sub.2)
resulting from the reaction between said fluorinating agent(s)
(PbF.sub.2) and said oxide(s), the impurity or impurities (CaO),
can be discharged from said crucible, in view of the dimensions of
said crucible and of the intrinsic permeability of the material
constituting it.
[0030] The crucible(s) which intervene(s) for carrying out the
process of the invention is (are) optimised, as to its (their) size
and its (their) intrinsic permeability, such that the crystals
prepared no longer contain--in any case, less than 0.1 ppm--any
element corresponding to the formulation of the fluorinating agents
(element Pb, in the context specified above (in a purely
illustrative manner) of preparation of (mono)crystals of CaF.sub.2,
with intervention of PbF.sub.2 as fluorinating agent).
[0031] Within the context of the process of the invention, the
fluorinating agent (even the fluorinating agents) acts (act) and
leave no trace. By virtue of its (their) controlled amount of
intervention (effective and non-excess amount) and the original
characteristics of the crucible employed, the fluorinating agent(s)
react and the reaction products can discharge totally. There is
thus no pollution of the crystals prepared.
[0032] In a non-obvious manner, within a context of optimisation of
the intervention of the fluorinating agent(s), the inventors:
[0033] have demonstrated that the means of implementation of the
temperature rise cycle (with the view to obtaining melting of the
starting material) did not constitute the determining factor of the
purity (with reference to the fluorinating agent incorporated) of
the crystals prepared;
[0034] have demonstrated that the purity of the material
constituting the crucible was not directly responsible;
[0035] have clearly established a correlation between the intrinsic
permeability of the crucible and the purity of the crystals
prepared in said crucible. The more permeable the material
constituting the crucible is, the less pollutant (introduced by the
fluorinating agent(s) incorporated) is found in the crystals
prepared. Obviously, the permeability of said crucible remains
within a reasonable limit in order that the molten mixture be
retained, in a stable manner, in said crucible.
[0036] The correlation established was, a priori, in no way
obvious, and is entirely against the teaching of US patents U.S.
Pat. Nos. 5,911,824 and 6,093,245 set forth further up in the
present text.
[0037] The permeability of a porous material (in this case of the
crucibles used, which are in general graphite crucibles) is a
parameter which is perfectly defined by the DIN 51935 Standard:
1993-08 (entitled "Determination of the coefficient of permeability
by means of the vacuum--decay method with air as experimental
gas"), which is familiar to the person skilled in the art. Said
permeability, which is generally expressed in cm.sup.2/s, is in
fact the resultant of several factors which relate to the porosity,
such as:
[0038] the size of the pores,
[0039] their distribution within the mass,
[0040] the fact that they unblock or not in a given proportion.
[0041] Characteristically, the process of the invention thus
enables very pure crystals to be prepared (less than 0.1 ppm of
contaminant in general, and especially less than 0.1 ppm of Pb when
PbF.sub.2 is used as fluorinating agent), insofar as the
discharging of the impurities, which are generated following the
intervention of the fluorinating agents, is mastered perfectly. The
mastering of this discharging is based jointly on the dimensions of
the crucible in question (said dimensions are inevitably limited
such that the vapours have the possibility of diffusing (and of
being discharged) within the molten material before its
crystallisation (its solidification) and on the permeability of the
material constituting said crucible. The vapours in question (PbO,
in the context specified above (in a purely illustrative manner) of
preparation of (mono)crystals of CaF.sub.2, with intervention of
PbF.sub.2 as fluorinating agent) diffuse within the molten material
and discharge through the permeable walls of the crucible and
through the surface of said molten material.
[0042] Mention has been made of the intervention of an effective
and non-excess amount of at least one fluorinating agent which is
solid (at ambient temperature). In general, one sole such agent
intervenes. It is however in no way excluded from the context of
the invention that several of them intervene.
[0043] With reference to said effective and non-excess amount, it
is indicated in a totally non-limiting way that said amount is
rarely greater than 5% by weight of the starting material which
intervenes, that it is advantageously between 0.1 and 2% by weight
of said starting material.
[0044] With reference to the nature of said fluorinating agent(s),
it is specified in the same way, i.e. in a totally non-limiting
manner, that said fluorinating agent(s) is (are) selected from:
PbF.sub.2, ZnF.sub.2, NH.sub.4F, NH.sub.4F.HF, PTFE
(polytetrafluoroethylene: Teflon.RTM.), and mixtures thereof. It
has already been seen, in the introduction of the present text,
that PbF.sub.2 is the most used fluorinating agent up to now. Its
intervention is particularly recommended within the context of the
present invention.
[0045] In a preferred alternative embodiment of the invention, the
high permeability graphite crucible that are comprised of graphite
having a DIN Standard (DIN 51935) greater than 4 cm.sup.2/s are
utilized in conjunction with a gaseous fluorinating agent such as
CF.sub.4. Within the context of a preferred embodiment of the
process of the invention, the crucible(s) which intervene(s) is
(are) graphite crucible(s) the permeability of which, measured in
accordance with the DIN Standard identified above (DIN 51935), is
greater than 4 cm.sup.2/s. Within the context of a particularly
preferred variant, said crucible(s) is (are) in a graphite, the
permeability of which, in accordance with the same Standard, is
greater than 10 cm.sup.2/s.
[0046] Generally, the intervention is recommended, in the process
of the invention, of crucibles which are suitable for preparing
crystals which have the following dimensions:
[0047] 200 mm diameter, for 50 mm height,
[0048] 300 mm diameter, for 80 mm height.
[0049] The intervention is particularly recommended of such
graphite crucibles, the permeability of which is as indicated
above.
[0050] The material constituting the crucibles used is not forced
to be graphite, but, obviously, said material is adapted to the
constraints of the process carried out within it (presence of
corrosive products, high temperatures . . . ).
[0051] In any case, the pollutant oxide(s) generated during the
crystallisation within the crystallisation crucible is (are),
according to the invention, capable of being discharged from said
crucible, by virtue of the dimensions of said crucible and the
permeability of the material constituting it (them).
[0052] The process of the invention is particularly suitable for
preparing (mono)crystals of alkali metal and/or alkaline-earth
metal fluorides. It enables the preparation of (mono)crystals,
which are very pure, of alkali metal or alkaline-earth metal
fluorides, and even the preparation of mixed (mono)crystals of
fluorides of alkali metals and/or alkaline-earth metals, which are
very pure, (mixtures of at least two alkali metals, of at least two
alkaline-earth metals or of at least one alkali metal and at least
one alkaline-earth metal).
[0053] In accordance with the invention, (mono)crystals of
fluorides have been prepared of high optical quality; especially
(mono)crystals of calcium and barium fluorides which have, at the
wavelengths (.lambda.) indicated below, the intrinsic transmissions
(T.sub.i) specified below:
.ltoreq.193 nm, T.sub.i.gtoreq.99.9% and
.ltoreq.157 nm,T.sub.i.gtoreq.99.0%.
[0054] Such monocrystals have obvious potential in laser and
lithography industries.
[0055] The process of the invention is more particularly suitable
for preparing (mono)crystals of calcium fluoride (CaF.sub.2).
[0056] The process of the invention is advantageously carried out
with a stack of crucibles, according to the Stockbarger-Bridgman
method, i.e. that in its context, the controlled cooling of the
molten mixture (for growing the (mono)crystals) is obtained by very
slowly moving a stack of loaded crucibles from the top to the
bottom, from a hot zone to a cold zone, of an oven having a
vertical axis.
[0057] The process of the invention is very advantageously carried
out according to the improved Stockbarger-Bridgman method, as
described in the French patent application FR 00 03 771 (Mar. 24,
2000) not published as yet, Le. with a translatory motion of the
stack of loaded crucibles, continuously, the operations of loading
of said crucibles being carried out without stopping the
translatory motion (along the vertical axis) of the stack of
crucibles.
[0058] Said process of the invention is classically carried out
with starting material in the form of a powder, especially a
synthetic powder (e.g. CaF.sub.2). It may also advantageously be
carried out with starting material which intervenes in the form of
beads. Such alkali metal or alkaline-earth metal fluoride beads,
their preparation and their use for preparing monocrystals are
described in French patent application FR-A-2,799,194.
[0059] The person skilled in the art has understood perfectly that
the presently claimed invention provides an advantage as regards
the purity of the crystal prepared, that said crystal be obtained
in a mono- or polycrystalline form.
[0060] The process of the invention is advantageously carried out
for preparing (mono)crystals of calcium fluoride (CaF.sub.2), in
the presence of PbF.sub.2 (fluorinating agent); said calcium
fluoride (starting material) containing calcium oxide (CaO) as
impurity.
[0061] Alternatively the process of the invention is advantageously
carried out for preparing (mono)crystals of calcium fluoride
(CaF.sub.2), in the presence of CF.sub.4 (gaseous fluorinating
agent); said calcium fluoride (starting material) containing
calcium oxide (CaO) as impurity.
[0062] This advantageous variant of implementation of the process
of the invention is illustrated by the following Examples.
EXAMPLE I
[0063] The Stockbarger-Bridgman process was carried out, starting
with synthetic CaF.sub.2 powder, under the same conditions, in
using graphite crucibles (stacks of such crucibles); the graphites
(type A to D) not having the same characteristics. The
characteristics in question--density, porosity, average pore
diameter, permeability--are indicated in Table I below.
[0064] The crucibles used had the same geometry (cylindrical) and
the same dimensions: 200 mm diameter for 50 mm height.
[0065] The process of the invention was carried out with crucibles
in graphite of type C and D.
[0066] Upon completion of the implementation of the process, the
crystals obtained were analysed chemically with the view to
determining their lead (Pb) content.
[0067] Said lead content is indicated in said Table I below (last
line).
[0068] The presence of lead, within the crystals prepared in the
crucibles in graphite of type A and B, was further confirmed by
examination of the absorption band at 205 nm. In the same way, the
<<absence>> of lead (the absence of said absorption
band) within the crystals prepared in the crucibles in graphite of
type C and D was confirmed.
[0069] It emerges without ambiguity from the consideration of the
values indicated in said Table I that the more the graphite is
permeable, the lower the residual lead content is. The crystals
obtained in the crucibles in graphite of type A and B are not
acceptable (due to their residual lead content, which is too
high).
[0070] These results were not foreseeable in the light of the prior
art teaching.
[0071] Thus, the inventors have themselves carried out considerable
experimental work before identifying the critical parameter--the
intrinsic permeability of the material constituting the crucible,
the dimensions of said crucible being fixed--. The inventors have
especially demonstrated that the means of implementation of the
heating cycle (with the view to obtaining the melting of the
starting material) was not itself critical. This is the subject of
the Comparative Example below.
1TABLE I Graphite A B C D Density 1.745 1.723 1.704 1.590
(g/cm.sup.3) Porosity (Hg) 15.8 16.1 16.7 22.6 (%) Average pore
diameter 2.2 19.1 6 21 (.mu.m) Perm ability 0.13 2.6 4.6 14.7
(cm.sup.2/s) Pb content 1,000 to 1,500 5 to 20 <0.2* <0.2*
(ppm) *below the limit of detection.
EXAMPLE II
[0072] Graphite crucibles, having the dimensions indicated in
Example I, of type A and C were used (in stacks) to prepare
crystals according to the Stockbarger-Bridgman method. Said method
was carried out with different temperature rise cycles which are
specified in Table II below.
[0073] It is seen that the results, in terms of pollution (lead
content of the crystals prepared), are not linked to the means of
implementation of the heating, but only to the nature of the
graphite constituting the crystallisation crucibles.
[0074] Preferably the increased far-ultraviolet transmission
fluoride optical crystal producing graphite crucible is comprised
of a graphite having a permeability of which, measured according to
the DIN 51935 Standard, is greater than 4 cm.sup.2/s. Preferably
the graphite permeability is greater than 5 cm.sup.2/s, more
preferably greater than 6 cm.sup.2/s, more preferably greater than
7 cm.sup.2/s, , more preferably greater than 8 cm.sup.2/s, more
preferably greater than 9 cm.sup.2/s more preferably greater than
10 cm.sup.2/s, more preferably greater than 1 cm.sup.2/s, more
preferably greater than 12 cm.sup.2/s., more preferably greater
than 13 cm.sup.2/s, more preferably greater than 14 cm.sup.2/s.
Preferably the increased far-ultraviolet transmission fluoride
optical crystal producing graphite crucible is comprised of a
graphite having a Hg porosity of at least 16.7%, more preferably at
least 17%, more preferably at least 18%, more preferably at least
19%, and more preferably a Hg porosity of at least 20%.
2 TABLE II {circle over (1)} (low temperature) {circle over (2)}
(high temperature) Temperature 0 to 600.degree. C., at 50.degree.
C./h 0 to 850.degree. C., at 50.degree. C./h rise cycle 600 to
800.degree. C., at 850 to 1200.degree. C., at 10.degree. C./h
30.degree. C./h 800.degree. C., for 12 h 1,200.degree. C., for 12 h
800 to 1,100.degree. C., at 1,200.degree. C. to 1,520.degree. C.,
at 20.degree. C./h 50.degree. C./h 1,100 to 1,520.degree. C., at
50.degree. C./h Graphite A Presence of lead Presence of lead
Graphite C No lead No lead
[0075] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit and scope of the invention. Thus
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *