Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Havin growth from molten state
Reexamination Certificate
2001-03-02
2002-04-02
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
C117S083000
Reexamination Certificate
active
06364946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for growing highly homogeneous large-volume single crystals from calcium fluoride by tempering at elevated temperature, especially in an oven, and their uses.
2. Prior Art
Single crystals, especially calcium fluoride single crystals, are required as starting materials for optical components in DUV-photolithography, such as steppers or excimer lasers. They are conventionally used as lenses or prisms. They are especially used to optically copy fine structures into integrated circuits, computer chips and/or photo-lacquer-coated wafers. For these purposes it is necessary that these optical components have a high degree of uniformity, i.e. that their index of refraction n must be exactly constant throughout the entire crystal. This means that the index of refraction difference &Dgr;n should vary by at most 5×10
−6
, preferably not more than 2×10
−6
. Since, on the one hand, the requirements for circuits of this type and computer chips are continuously increasing, their production rate also must continuously increase. Also ever large wafers are frequently required for applications. For example, wafer diameters of 200 mm have been required in the meantime. This however presupposes that the diameters of the optical components required for illumination of the wafer are similarly larger. In order to be able to make minute circuits and/or computer chips with high throughput, the differences in the index of refraction at different positions throughout the crystal volume should not be larger than 1×10
−6
.
Growth of single crystals from the melt is known per se. Crystals, in principle, can be grown from the gas phase, the melt, from solution and even from a solid phase by recrystallization or solid body diffusion. Different processes for crystal growth are described in text books for crystal growth, such as the 1088 page work of K.-Th. Wilke and J. Bohm, entitled “Crystal Growth”, Harri Deutsch Press, Thun, Frankfurt/Main, 1988 (ISBN 3-87144-971-7).
However single crystals for industrial applications are usually grown by solidification from a melt. The so-called Stockbarger-Bridgeman and the vertical gradient freeze processes are used for industrial manufacture of single crystals. The crystals are grown in a drawing oven and in a vacuum of 10
−4
to 10
−5
mbar in the Stockbarger-Bridgeman method. A crystalline raw material is melted, so that a homogeneous single crystal is obtained with exacting control of temperature.
In order to make the single crystals up to now the crystalline raw material is slowly heated in a vessel to the volatilization or evaporation temperature of water of about 400° C. and is maintained at this temperature in order to keep it free of water for some time. Additive scavengers, such as PbF
2
, SnF
2
or ZnF
2
, are used to remove oxygen from the raw material. The added scavengers react with the oxygen present in the raw material and arising partially by oxidation and/or hydrolysis to form easily volatile oxides, which escape at these temperatures. After that a so-called refinement during a single week at 1450° C. usually is performed followed by a multi-week cooling to about 1200° C., in which the desired crystal is solidified from the melt. The single crystal so obtained is then cooled in a first slow cooling phase and then cooled to room temperature in a second accelerated cooling phase after the first cooling phase.
Since the heat flow is not completely under control in a crystal growth unit of this type, so-called stress birefringence usually arises during crystallization. Furthermore various crystal orientations, so-called block formations, also form. Although the CaF
2
single crystals are held for a considerable time at a high temperature after crystallization, the stress birefringence usually still amounts to 5 to 20 nm/cm, which is too large for later use. The high stress birefringence is still further increased by sawing or cutting the single crystal and by mechanical working of the optical elements, for example by milling and polishing.
For this reason attempts have already been made to improve the uniformity of the single crystals produced by heating them to a temperature under their melting point for an extended duration. The desired homogeneity can thus be attained by keeping the single crystal at an elevated temperature for an extended time after its formation so that the mechanical stresses and optical faults are reduced or eliminated by rearrangement of the atoms of the crystal lattice. This process is usually referred to as tempering.
For example, a tempering process is described in DD-PS 213514, in which a calcium fluoride crystal is heated at a temperature of 1200° C. in a PbF
2
-containing atmosphere, at a PbF
2
partial pressure of 0.01 to Torr (1.33 Pa to 133.3 Pa). The purpose of the PbF
2
is to getter the residual oxygen present in the calcium fluoride crystal. In this process the stress birefringence present of 10 to 25 nm/cm is reduced to only 1 nm/cm by a two to three hour heating at 1200° C. for a crystal of a diameter of 20 nm and a thickness of 10 mm. With this process however it was not possible to improve the stress birefringence of substantially larger calcium fluoride single crystals, i.e. single crystals of a diameter greater than 200 mm, especially greater than 300 nm, and a thickness of 50 to 400 mm, or also of already polished and worked optical elements during economical processing times. During experiments, which shorten the tempering time and/or improve the tempering, the tempering temperature is still further increased, which leads to a great loss in volume for the single crystals. Also finished optics rapidly loose their worked shape in these latter tempering processes.
A method for making calcium fluoride single crystals, especially for photolithography, is described in EP A 939 147. Large single crystals are introduced into a closed container and heated to a first temperature between 1020° C. and 1150° C. and after that are cooled in a first stage with a cooling rate of at most 1.2° C./h to 2° C./h at temperatures of from 600 to 900° C. After that they are cooled to room temperature with a cooling rate of at most 5° C./h. In preferred embodiments the tempering is performed in a fluorine gas-containing atmosphere and under protective gas.
It has been shown however that no satisfactory uniformity can be attained with sufficient yield with the known methods.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the above-described problems and to provide a method of preparation for highly homogeneous single crystals, which have a constant index of refraction and small stress birefringence, and optical elements made from them in a satisfactory yield.
It is another object of the present invention to overcome the above-described problems and to provide a method of preparation for highly homogeneous single crystals, which have an index of refraction that does not vary more than &Dgr;n=5×10
−6
.
It is an additional object of the present invention to avoid or eliminate the formation of small-angle grain boundaries or other low-dimensional crystal defects.
It is a further object of the present invention to avoid or reduce fogging or opacity formation during tempering crystals.
According to the invention a single precursor crystal of calcium fluoride is placed in a tempering vessel provided with a cover in the presence of, preferably in contact with, a calcium fluoride powder and subsequently heated for at least two hours at a temperature above 1150° C. to temper the precursor crystal and thus form a uniform, large-scale single crystal of calcium fluoride.
It has been surprisingly found that mechanical stress, small angle grain boundaries and stress birefringence can be reduced and/or eliminated, when a finished single crystal is heated to a temperature over 1150° C. in the presence of a finely divided calcium fluoride powder.
It has been shown t
Parthier Lutz
Speit Burkhard
Staeblein Joerg
Wehrhan Gunther
Weisleder Andreas
Hiteshew Felisa
Schott Glas
Striker Michael J.
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