Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Havin growth from molten state
Reexamination Certificate
2001-03-27
2002-09-17
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, C252S301500, C423S328200
Reexamination Certificate
active
06451111
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to processes for producing CaF
2
crystals. More specifically, the invention relates to a nucleant seed for epitaxial growth of single-crystal CaF
2
.
2. Background Art
Single-crystal CaF
2
is commonly grown using the Bridgman-Stockbarger crystal growth process. For epitaxial growth of CaF
2
, the process starts, as illustrated in
FIG. 1
, with a seed crystal
2
made of CaF
2
and having the desired crystallographic orientation. For deep-ultraviolet microlithography applications, for example, the desired crystallographic orientation is <111>, i.e., cubic (octahedral or cubic forms) crystal structure. The seed crystal
2
is placed at the base of a crucible
4
. A starting material
6
comprising CaF
2
powder (or beads) is placed in the crucible
4
, on top of the seed crystal
2
. The crucible
4
is then placed in a vertical furnace
8
and heated to a temperature sufficient to melt the starting material
6
. To prevent oxidation of the starting material
6
and the components of the furnace
8
, the furnace
8
is typically maintained under vacuum and/or the process is carried out in an inert atmosphere.
After melting the starting material
6
, the crucible
4
is moved downwardly at a predetermined rate (typically 0.3 to 5 mm/h), from a hot zone
10
into a cold zone
12
. An insulating barrier 14 separates the hot zone
10
from the cold zone
12
.
FIG. 2
shows a typical temperature distribution along the vertical axis of the furnace (
8
in FIG.
1
). A single crystal of CaF
2
forms on the seed crystal (
2
in
FIG. 1
) when the molten material reaches the zone
12
in which the furnace temperature is below the melting point of CaF
2
. The CaF
2
crystal front propagates inside the crucible
4
, within the material
6
, as long as the crucible
4
is caused to move downwardly. The CaF
2
crystal conforms to the crystallographic orientation of the seed crystal
2
as it propagates inside the crucible
4
.
To enhance the optical properties of the CaF
2
crystal, a scavenger is typically added to the starting material
6
to remove oxygen and hydroxyl ions. These impurities have been known to reduce transmission in the deep-ultraviolet region. The most common scavenger used is PbF
2
. PbF
2
is solid and can be added directly to the starting material
6
. Typically, a specific amount of PbF
2
, typically 1 to 2% by weight, is mixed into the starting material
6
. The mixture is then gradually heated to approximately 800° C. to 900° C., at which point PbF
2
reacts with the starting material
6
to form PbO. After the reaction is complete, the more volatile PbO is evaporated from the mixture by heating the mixture to the melting point of CaF
2
or higher. In an attempt to remove as much of the PbO as possible through volatization, the CaF
2
melt may become overheated and cause the seed crystal
2
, which is also made of CaF
2
, to completely melt and lose its crystallographic orientation.
SUMMARY OF THE INVENTION
In one aspect, the invention relates to a nucleant seed for epitaxial growth of single-crystal CaF
2
which comprises SrF
2
. In some embodiments, a second fluoride is substituted in the SrF
2
structure, the second fluoride being selected from the group consisting of YF
3
, LaF
3
, rare-earth fluoride, and combinations thereof. In some embodiments, the rare-earth fluoride comprises one selected from the group consisting of YF
3
, LaF
3
, CeF
3
, NdF
3
, PrF
3
, DyF
3
, SmF
3
, EuF
3
, TbF
3
, and GdF
3
.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
REFERENCES:
patent: 2149076 (1939-02-01), Stockbarger
patent: 2214976 (1940-09-01), Stockbarger
patent: 4038201 (1977-07-01), Hargreaves
patent: 4053572 (1977-10-01), Moss et al.
patent: 5215631 (1993-06-01), Westfall
patent: 6201634 (2001-03-01), Sakuma et al.
patent: WO 00/75405 (2000-12-01), None
patent: WO 00/75697 (2000-12-01), None
Chernevskaya et al., Optical Characteristics of Large Single Crystals of Fluorides, Optical Technology, vol. 40, No. 6, Jun. 1973.
Beall George H.
Deneka Charles W.
Kar Gitmoy
Corning Incorporated
Hiteshew Felisa
Murphy Edward F.
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