Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Havin growth from molten state
Reexamination Certificate
2002-03-26
2003-12-30
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Havin growth from molten state
C117S070000, C117S083000
Reexamination Certificate
active
06669778
ABSTRACT:
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
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.
More specifically, said invention relates:
to a process of preparing (mono)crystals, which is improved with reference to the purity of the (mono)crystals prepared; and
to a process of preparing (mono)crystals which have an increased transmission in the far-ultraviolet (&lgr;<193 nm, even &lgr;<157 nm).
TECHNICAL BACKGROUND
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.
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.
According to the teaching of 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.
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
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
2
acts, within the context of the preparation of CaF
2
crystals, notably according to the reaction:
CaO+PbF
2
→CaF
2
+PbO.
In practice, it is always delicate to optimise the intervention of said fluorinating agent. It is especially critical:
to adjust the rise in temperature of the mixture (for its melting) with the view to said optimisation;
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.
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
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.
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
2
/s, forming a molten fluoride melt in said graphite crucible comprised of said graphite having a permeability greater than 4 cm
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>99%. In a preferred embodiment the fluoride crystal comprises calcium fluoride. In a preferred embodiment the fluoride crystal comprises barium fluoride.
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
2
/s.
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.
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
The process of preparing a crystal of the invention 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, and
recovering said crystal.
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.
Thus, in order to prepare (mono)crystals of CaF
2
, said crucible is in general loaded with a mixture de synthetic CaF
2
powder, which contains CaO as impurity, and PbF
2
(fluorinating agent).
The crucible in question
Corning Incorporated
Douglas Walter M.
Hiteshew Felisa
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