U.S. patent application number 09/952172 was filed with the patent office on 2002-01-31 for crystal growth and annealing method and apparatus.
Invention is credited to Gianoulakis, Steven E., Sparrow, Robert.
Application Number | 20020011202 09/952172 |
Document ID | / |
Family ID | 23274886 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011202 |
Kind Code |
A1 |
Gianoulakis, Steven E. ; et
al. |
January 31, 2002 |
Crystal growth and annealing method and apparatus
Abstract
A method and apparatus for producing crystals that minimizes
birefringence even at large crystal sizes, and is suitable for
production of CaF.sub.2 crystals. The method of the present
invention comprises annealing a crystal by maintaining a minimal
temperature gradient in the crystal while slowly reducing the bulk
temperature of the crystal. An apparatus according to the present
invention includes a thermal control system added to a crystal
growth and annealing apparatus, wherein the thermal control system
allows a temperature gradient during crystal growth but minimizes
the temperature gradient during crystal annealing. An embodiment of
the present invention comprises a secondary heater incorporated
into a conventional crystal growth and annealing apparatus. The
secondary heater supplies heat to minimize the temperature
gradients in the crystal during the annealing process. The
secondary heater can mount near the bottom of the crucible to
effectively maintain appropriate temperature gradients.
Inventors: |
Gianoulakis, Steven E.;
(Albuquerque, NM) ; Sparrow, Robert; (North
Brookfield, MA) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
23274886 |
Appl. No.: |
09/952172 |
Filed: |
September 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09952172 |
Sep 15, 2001 |
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09327043 |
Jun 7, 1999 |
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6309461 |
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Current U.S.
Class: |
117/104 |
Current CPC
Class: |
C30B 29/12 20130101;
Y10T 117/1024 20150115; C30B 33/00 20130101; C30B 11/00 20130101;
C30B 11/003 20130101 |
Class at
Publication: |
117/104 |
International
Class: |
C30B 025/00; C30B
001/00; C30B 003/00; C30B 005/00; C30B 028/14; C30B 028/12; C30B
028/02; C30B 023/00 |
Goverment Interests
[0001] This invention was made with Government support under
Contract DE-AC04-94AL85000 awarded by the U.S. Department of
Energy. The Government has certain rights in the invention.
Claims
We claim:
1. A crystal annealing apparatus, comprising: a) A support
structure; b) A crucible adapted to contain a crystal within the
support structure; c) A primary heating system mounted with the
support structure proximal the top and sides of the crucible; and
d) A secondary heating system mounted with the support structure
proximal the bottom of the crucible.
2. The apparatus of claim 1, wherein the primary heating system and
secondary heating system have heating capacities that maintain the
crystal temperature uniform to within about 8.degree. C. per
inch.
3. The apparatus of claim 1, wherein the primary heating system and
secondary heating system have heating capacities that maintain the
crystal temperature uniform to within about 4.degree. C. per
inch.
4. The apparatus of claim 1, wherein the primary heating system
comprises a primary heat shield mounted with the support structure
and a primary heating element mounted between the primary heat
shield and the crucible.
5. The apparatus of claim 1, wherein the secondary heating system
comprises a secondary heat shield mounted with the support
structure and a secondary heating element mounted between the
secondary heat shield and the crucible.
6. The apparatus of claim 1, wherein the primary heating system
comprises a primary heat shield mounted with the support structure
and a primary heating element mounted between the primary heat
shield and the crucible, and wherein the secondary heating system
comprises a secondary heat shield mounted with the support
structure and a secondary heating element mounted between the
secondary heat shield and the crucible.
7. The apparatus of claim 1, wherein the crucible is at least 8
inches in diameter, the primary heating system comprises electric
resistive heaters mounted substantially surrounding the crucible,
the secondary heating system comprises an electric resistive
heater, the primary heat shield comprises graphite, and the
secondary heat shield comprises graphite.
8. The apparatus of claim 1, wherein the crystal is CaF.sub.2.
9. The apparatus of claim 1, wherein the crystal is MgF.sub.2.
10. The apparatus of claim 1, wherein the crystal is SiO.sub.2.
11. A crystal growth and annealing apparatus, comprising: a) A
support structure; b) A primary heating system mounted with the
support structure; c) A crucible support mounted with the support
structure movable between first and second positions; d) A crucible
adapted to hold liquid crystal material and solid crystal material,
mounted with the crucible support so that the crucible is proximal
the primary heating system when the crucible support is in the
first position and below the primary heating system when the
crucible support is in the second position; e) A secondary heating
system mounted with the crucible support proximal the bottom of the
crucible.
12. The apparatus of claim 11, wherein the primary heating system
and secondary heating system have heating capacities that maintain
the temperature of a crystal within the crucible uniform to within
about 8.degree. C. per inch.
13. The apparatus of claim 11, wherein the primary heating system
and secondary heating system have heating capacities that maintain
the temperature of a crystal within the crucible uniform to within
about 4.degree. C. per inch.
14. The apparatus of claim 11, wherein the primary heating system
comprises a primary heat shield mounted with the support structure
and a primary heating element mounted between the primary heat
shield and the crucible.
15. The apparatus of claim 11, wherein the secondary heating system
comprises a secondary heat shield mounted with the crucible support
and a secondary heating element mounted between the secondary heat
shield and the crucible.
16. The apparatus of claim 11, wherein the primary heating system
comprises a primary heat shield mounted with the support structure
and a primary heating element mounted between the primary heat
shield and the crucible, and wherein the secondary heating system
comprises a secondary heat shield mounted with the crucible support
and a secondary heating element mounted between the secondary heat
shield and the crucible.
17. The apparatus of claim 11, wherein the crucible is at least 8
inches in diameter, the primary heating system comprises electric
resistive heaters mounted substantially surrounding the crucible,
the secondary heating system comprises an electric resistive
heater, the primary heat shield comprises graphite, and the
secondary heat shield comprises graphite.
18. The apparatus of claim 11, wherein the crystal is Ca
F.sub.2.
19. The apparatus of claim 11, wherein the crystal is
MgF.sub.2.
20. The apparatus of claim 11, wherein the crystal is
SiO.sub.2.
21. A method of annealing a crystal in an apparatus having a
primary heating system proximal the top and sides of the crystal
and a secondary heating system proximal the bottom of the crystal,
comprising supplying heat using the primary heating system and the
secondary heating system to the crystal to maintain the crystal's
average temperature at a decreasing value over time without
allowing a temperature gradient of more than about 8.degree. C. per
inch.
22. The method of claim 21, wherein the temperature gradient is
kept less than about 4.degree. C. per inch.
23. The method of claim 11, wherein the crystal material is
CaF.sub.2.
24. The method of claim 11, wherein the crystal material is
MgF.sub.2.
25. The method of claim 11, wherein the crystal material is
SiO.sub.2.
26. A method of producing a crystal in an apparatus having a
primary heating system proximal the top and sides of the crystal
and a secondary heating system proximal the bottom of the crystal,
comprising: a) Forming a liquid of crystal material in a crucible
by heating the crystal material using heat from the primary heating
system; b) Lowering the crucible out of the primary heating system
so that successive portions of said liquid crystal material cool to
a temperature suitable for crystal formation; c) Reducing the
temperature of the primary heating system; d) Raising the crucible
into the primary heating system and supplying heat from the
secondary heating system; e) Reducing the heat output of the
primary and secondary heating systems so that the average
temperature of the crystal is reduced over time without allowing a
temperature gradient in the crystal more than about 8.degree. C.
per inch.
27. The method of claim 26, wherein the temperature gradient is
kept less than about 4.degree. C. per inch.
28. The method of claim 26, wherein the crystal material is
CaF.sub.2.
29. The method of claim 26, wherein the crystal material is
MgF.sub.2.
30. The method of claim 26, wherein the crystal material is
SiO.sub.2.
Description
BACKGROUND OF THE INVENTION
[0002] This invention relates to the field of crystal growth and
annealing, specifically a method and apparatus for crystal growth
and annealing with minimized residual stress and suitable for
production of calcium fluoride (CaF.sub.2) crystals. Crystals are
used in a wide variety of applications, including as lenses in
digital broadcast cameras and as optical elements in lithography
such as in semiconductor processing. Semiconductor lithography at
193 nm wavelengths commonly used fused silica optical elements.
Unfortunately, fused silica is damaged by high fluence at 193 nm.
The next generation of semiconductor lithography is expected to use
157 nm wavelength illumination. Another material will be required
since fused silica is quite opaque to 157 nm wavelength
illumination.
[0003] CaF.sub.2 is one of several candidates for optical elements
in 193 nm and 157 nm lithography. Current crystal growth and
annealing processes lead to high residual stress in large CaF.sub.2
crystals, however, limiting the applicability of CaF.sub.2
crystals. High residual stresses in a crystal can cause the crystal
to exhibit a spatially varying index of refraction. This can lead
to wavefront errors, image degradation, and birefringence, all
detrimental to the effectiveness of an optical system using
CaF.sub.2.
[0004] Contemporary crystal growth and annealing is illustrated by
FIG. 1(a,b). A powder P is placed in a crucible C. During the
growth phase, the powder P is heated to a liquid phase (roughly
1500.degree. C. for CaF.sub.2, for example). The crucible C is
slowly lowered from the heated region R1, with the crystal X
growing in the region R2 where the liquid can cool below a critical
temperature. The difference between the liquid temperature T1 and
crystal temperature T2 leads to a temperature gradient across the
crystal/liquid combination. Once the crystal growth phase is
complete, the crystal X is annealed. FIG. 1b shows the arrangement
in a conventional annealing process. The crystal X is placed back
in the heated region R1, but the temperature is less than that
required to liquefy the crystal X. The crystal loses heat through
its top, bottom, and sides. The temperature of the crystal is
slowly reduced until it reaches a certain value, typically room
temperature (annealing a CaF.sub.2 crystal conventionally takes
approximately 30 days to bring the temperature from 1000.degree. C.
to 50.degree. C., at cooling rates of less than 1.degree. C. per
hour). The temperature of the crystal is slowly reduced,
conventionally still with a vertical temperature gradient as
represented by differences between T1 and T2. After the crystal is
completely cooled, typically the top and bottom are cut off to
produce a blank. The blank can then be ground and polished to
produce an optical element such as a lens, tube, or plate. Current
CaF.sub.2 crystal production methods reliably produce CaF.sub.2
crystals of limited size, because the CaF.sub.2 crystals produced
exhibit unacceptably high birefringence at sizes over about 6 inch
diameter. The limited size crystals limit the numerical aperture
available with resulting optical elements, limiting the optical
elements' utility for high density lithography. Accordingly, there
is a need for a method and apparatus for producing crystals that
minimizes birefringence even at large crystal sizes, and is
suitable for production of CaF.sub.2 crystals.
SUMMARY OF THE INVENTION
[0005] The present invention provides a method and apparatus for
producing crystals that minimizes birefringence even at large
crystal sizes, and is suitable for production of CaF.sub.2
crystals. The method of the present invention comprises annealing a
crystal by maintaining a minimal temperature gradient across the
crystal while slowly reducing the bulk temperature of the crystal.
An apparatus according to the present invention includes a thermal
control system added to a crystal growth and annealing apparatus,
wherein the thermal control system allows a temperature gradient
during crystal growth but minimizes the temperature gradient during
crystal annealing.
[0006] An embodiment of the present invention comprises a secondary
heater incorporated into a conventional crystal growth and
annealing apparatus. The secondary heater supplies heat to minimize
the temperature gradients in the crystal during the annealing
process. The secondary heater can mount near the bottom of the
crucible to effectively maintain appropriate temperature gradients.
Advantages and novel features will become apparent to those skilled
in the art upon examination of the following description or may be
learned by practice of the invention. The objects and advantages of
the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
DESCRIPTION OF THE FIGURES
[0007] The accompanying drawings, which are incorporated into and
form part of the specification, illustrate embodiments of the
invention and, together with the description, serve to explain the
principles of the invention.
[0008] FIG. 1(a,b) are diagrams of a conventional crystal growing
and annealing apparatus.
[0009] FIG. 2 is a graph of annealing times and vertical
temperature gradient.
[0010] FIG. 3 is a graph of stress response in crystals during
annealing processes of FIG. 2.
[0011] FIG. 4 is a graph of birefringence in crystals annealed
under the conditions of FIG. 2.
[0012] FIG. 5 is a diagram of a crystal growing and annealing
apparatus according to the present invention.
[0013] FIG. 6 is a sectional view of a crystal growth and annealing
apparatus according to the present invention.
[0014] FIG. 7 is a graph of peak stress in a crystal annealed in
the apparatus of FIG. 6 compared with a crystal annealed in a
conventional apparatus.
[0015] FIG. 8 is a graph of birefringence in a crystal annealed in
the apparatus of FIG. 6 compared with a crystal annealed in a
conventional apparatus.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The present invention provides a method and apparatus for
crystal growth and annealing that minimizes residual stress and
associated birefringence, and is suitable for producing CaF.sub.2
crystals.
[0017] Method
[0018] The method of the present invention is similar to
conventional crystal production methods. With the present
invention, however, the temperature gradient across the crystal is
minimized during annealing. The temperature gradient is kept high
during growth, but minimized during annealing. Minimizing the
temperature gradient during annealing has been found by the
inventors to reduce residual stress and associated
birefringence.
[0019] A crystal can be grown at temperatures around the liquid
phase temperature of the crystal material, with a temperature
gradient during growth of 100.degree. C. or more over 6 inches.
During annealing, the crystal can be cooled from the growth
temperature to room temperature, with a temperature gradient of
less than about 8.degree. C. per inch throughout the annealing
cycle. For CaF.sub.2, the growth temperature can be around
1500.degree. C. Annealing can cool the crystal to room temperature
with temperature gradients in any direction of less than about
8.degree. C. per inch throughout the cooling process; temperature
gradients of less than about 4.degree. C. per inch can produce even
lower residual stress and consequent undesirable optical
properties. It is especially important to maintain a low
temperature gradient during the initial phases of annealing, when
the hot crystal has a relatively low yield strength.
[0020] FIG. 2 is a graph of three different vertical temperature
gradients during annealing. In a conventional crystal growth and
annealing apparatus the vertical temperature gradient exceeds the
temperature gradients in any other direction. The vertical
temperature gradient is depicted as a function of annealing time.
Case 1 is representative of conventional annealing processes,
exhibiting a high initial vertical temperature gradient slowly
decaying as the crystal temperature is reduced. Case 2 represents
an annealing process according to the present invention, exhibiting
a much lower vertical temperature gradient and also decaying as the
crystal temperature is reduced. Case 3 represents an annealing
process according to the present invention, exhibiting an even
lower initial vertical temperature gradient decaying as the crystal
temperature is reduced. For CaF.sub.2 crystal annealing, the time
scale can be roughly 20 to 40 days. The maximum vertical
temperature gradient can be 100.degree. C. or more over 6 inches
for Case 1, about 8.degree. C. per inch or less for Case 2, and
about 4.degree. C. per inch or less for Case 3. Those skilled in
the art will appreciate similar relationships for other crystal
materials such as magnesium fluoride (MgF.sub.2) and sapphire
(SiO.sub.2), with variations due to growth temperature, thermal
expansion coefficients, and yield strength.
[0021] FIG. 3 is a graph of the von Mises transient peak stress
response of a crystal during the annealing times and vertical
temperature gradients of FIG. 2. As the crystal cools, thermal
expansion and contraction produce internal stresses. If the
internal stresses are high, dislocation motion and slippage can
occur and produce residual stresses in the cooled crystal. Case 1
(conventional annealing conditions) displays increased peak stress
as compared with Case 2 and Case 3, especially in the early stages
of annealing. The early stages of annealing correspond with highest
crystal material temperature and associated minimal yield strength.
Consequently, high stress in the early stages is more likely to
exceed the crystal material's yield strength and produce
dislocation motion and slippage. Dislocation motion and slippage
produce residual stress in the cooled crystal and resulting
undesirable optical properties. For CaF.sub.2 crystals, the peak
stress for Case 1 can be about 7 MPa, for Case 2 about 5.5 MPa, for
Case 3 about 4.5 MPa. The annealing time can be about 700
hours.
[0022] FIG. 4 is a graph of the birefringence (expressed in nm/cm)
of CaF.sub.2 crystals annealed with the temperatures, times, and
vertical temperature gradients of FIG. 2, graphed as a function of
radial distance. Case 1 is the birefringence of a crystal annealed
with conventional annealing process conditions. Case 2 and Case 3
exhibit lower birefringence at shorter radial distances compared to
Case 1. Case 2 and Case 3 also exhibit birefringence below a
critical threshold level at larger radial distances than Case 1,
allowing larger diameter optical elements with desirable optical
properties to be made from crystals annealed according to the
present invention.
[0023] The method of the present invention, when practiced in
connection with a crystal annealing apparatus like that discussed
below, having a primary heating system mounted near the top and
sides of the crystal, and a secondary heating system mounted near
the bottom of the crystal, comprises supplying heat using the
primary and secondary heating systems to maintain the crystal's
temperature at a decreasing value over time while preventing any
temperature gradients of over about 8.degree. C. per inch in the
crystal (gradients of less than about 4.degree. C. per inch can
yield even better results).
[0024] The method of the present invention, when practiced in
connection with a crystal growth and annealing apparatus like that
discussed below, having a primary heating system mounted near the
top and sides of the crystal and a secondary heating system mounted
near the bottom of the crystal, comprises:
[0025] 1. Forming a liquid of crystal material in a crucible by
heating the crystal material using heat from the primary heating
system;
[0026] 2. Lowering the crucible out of the primary heating system
so that successive portions of the liquid crystal material cool to
a temperature suitable for crystal formation;
[0027] 3. Reducing the temperature of the primary heating
system;
[0028] 4. Raising the crucible into the primary heating system and
supplying heat from the secondary heating system;
[0029] 5. Reducing the heat output of the primary and secondary
heating systems so that the average temperature of the crystal is
reduced over time without allowing a temperature gradient in the
crystal more than about 8.degree. C. per inch (gradients of less
than about 4.degree. C. per inch can yield even better
results).
[0030] Apparatus
[0031] An apparatus according to the present invention is shown
schematically in FIG. 5. A heat source 21 can maintain the high
temperature needed for crystal growth. Once the growth phase is
complete, a thermal control system 22 maintains the crystal at a
substantially uniform temperature during annealing. Specifically,
thermal control system 22 maintains the vertical temperature
gradient within limits appropriate for the crystal material, for
example less than about 8.degree. C. per inch for CaF.sub.2.
Temperature gradients of less than about 4.degree. C. per inch can
provide even more desirable results for CaF.sub.2 crystals. Thermal
control system 22 gradually reduces the temperature of the crystal
during annealing without allowing a temperature gradient beyond the
bounds. Thermal control system can be implemented in various ways
that will be apparent to those skilled in the art, including by
radiant heating elements appropriately spaced proximal the crystal,
insulation, inductive heaters, monitoring and control systems, and
combinations thereof.
[0032] FIG. 6 is a sectional view of an apparatus according to the
present invention. Apparatus A comprises several conventional
elements: furnace wall 601, support structure 602, crucible support
column 603, base 604, crucible 605, primary heat shields 606, and
primary heaters 607, connected essentially as shown in the figure.
Apparatus A further comprises secondary heater 611 and secondary
heat shields 612. Secondary heater 611 can mount beneath crucible
605, with secondary heat shields 612 mounted below secondary heater
611. In operation, secondary heater 611 can be inactive during the
crystal growth phase, allowing large temperature gradients
compatible with crystal growth. Secondary heater 611 can be active,
supplying heat, during the crystal annealing phase. Secondary
heater 611 can operate in conjunction with primary heaters 607,
supplying heat from below a crystal being annealed. The multiple
heat sources provide the temperature profile required for annealing
without allowing uneven heat loss that can lead to large
temperature gradients. The power required from primary heaters 607
and secondary heater 611 is related to the furnace dimensions,
crystal material characteristics, heat shield performance, and
crucible dimensions. As an example, crucible 605 can be about 8
inches in diameter. Furnace wall 601 can define a volume about 20
to 40 inches in diameter and about 40 inches high. At the onset of
annealing primary heaters 607 can supply about 6 KW and secondary
heater 611 about 3.5 kW. The power in primary heaters 607 and
secondary heater 611 can be reduced, for example linearly, during
annealing. Primary 606 and secondary heat shields 612 can be of
graphite, 1/4 to 1/2 inch thick. Those skilled in the art will
appreciate other sizes, power ratings, and materials compatible
with the present invention.
[0033] FIG. 7 is a graph of the peak stress in a crystal annealed
in the apparatus of FIG. 6 compared with the peak stress of a
crystal annealed in a conventional apparatus. The graph was
obtained using computer models of the apparatus and process. As
shown in FIG. 7, the crystal annealed with the apparatus of FIG. 6
experiences lower stresses during annealing, because the secondary
heaters and secondary heat shields prevent large temperature
gradients that contribute to large stresses. Lower stresses during
annealing means that the crystal has a lower likelihood of
experiencing dislocation motion and slippage, and having consequent
undesirable optical properties. For CaF.sub.2 crystals, the peak
stress with a conventional apparatus can be about 7 Mpa, but only
about 4.5 MPa with the apparatus according to the present
invention. The annealing time can be about 700 hours.
[0034] FIG. 8 is a graph of the birefringence (expressed as nm/cm)
as a function of radial distance (for example, from the center to 6
inches for a CaF.sub.2 crystal) of a crystal annealed with the
apparatus of FIG. 6 compared with that of a crystal annealed with a
conventional apparatus. The graph was obtained using computer
models of the apparatus and process. As shown in FIG. 8, the
crystal annealed with the apparatus of FIG. 6 exhibits lower
birefringence, especially at large radial distances. Low
birefringence at large radial distances allows large optical
elements and improved optical performance.
[0035] The particular sizes and equipment discussed above are cited
merely to illustrate particular embodiments of the invention. It is
contemplated that the use of the invention may involve components
having different sizes and characteristics. It is intended that the
scope of the invention be defined by the claims appended
hereto.
* * * * *