U.S. patent application number 11/098959 was filed with the patent office on 2005-11-03 for crystallization method and crystallization apparatus.
Invention is credited to Oyama, Yasunao, Terashiima, Kazutaka.
Application Number | 20050241569 11/098959 |
Document ID | / |
Family ID | 35185787 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050241569 |
Kind Code |
A1 |
Oyama, Yasunao ; et
al. |
November 3, 2005 |
Crystallization method and crystallization apparatus
Abstract
A crystallization method includes the steps of melting a
crystallized material in a crucible by heating, and growing a
crystal by cooling and coagulating the melted material, wherein
said melting step includes introducing a predetermined gas into the
melted material.
Inventors: |
Oyama, Yasunao; (Tokyo,
JP) ; Terashiima, Kazutaka; (Ebina-shi, JP) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
3 WORLD FINANCIAL CENTER
NEW YORK
NY
10281-2101
US
|
Family ID: |
35185787 |
Appl. No.: |
11/098959 |
Filed: |
April 4, 2005 |
Current U.S.
Class: |
117/73 |
Current CPC
Class: |
Y10T 117/102 20150115;
C30B 11/00 20130101; G02B 1/02 20130101; Y10T 117/10 20150115; C30B
29/12 20130101; G03F 7/70958 20130101; Y10T 117/1008 20150115 |
Class at
Publication: |
117/073 |
International
Class: |
C30B 021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2004 |
JP |
2004-110836 (PAT. |
Claims
What is claimed is:
1. A crystallization method comprising the steps of: melting a
crystallized material in a crucible by heating; and growing a
crystal by cooling and coagulating the melted material, wherein
said melting step includes introducing a predetermined gas into the
melted material.
2. A crystallization method according to claim 1, wherein said
crystallized material is a calcium fluoride with an added
scavenger, and the gas that introduced into the melted fluorite is
an inert gas.
3. A crystallization method according to claim 2, wherein said
inert gas is a helium gas.
4. A crystallization method according to claim 2, wherein said
crystallized material is a single crystal that does not have a
grain boundary.
5. An optical element made of a single crystal, said single crystal
being manufactured by a crystallization method that includes:
melting a crystallized material in a crucible by heating; and
growing a crystal by cooling and coagulating the melted material,
wherein said melting step includes introducing a predetermined gas
into the melted material, and wherein said crystallized material is
a calcium fluoride with an added scavenger, and the gas that
introduced into the melted fluorite is an inert gas.
6. An exposure apparatus comprising: a light source; an
illumination optical system for guiding a light from the light
source to a reticle; and a projection optical system for guiding
the light from the reticle to a wafer that is placed on a wafer
stage, wherein said illumination optical system or the projection
optical system includes an optical element, and wherein said
optical element is made of a single crystal, said single crystal
being manufactured by a crystallization method that includes:
melting a crystallized material in a crucible by heating; and
growing a crystal by cooling and coagulating the melted material,
wherein said melting step includes introducing a predetermined gas
into the melted material, and wherein said crystallized material is
a calcium fluoride with an added scavenger, and the gas that
introduced into the melted fluorite is an inert gas.
7. A crystallization method according to claim 1, wherein said
crystallized material is a lithium tantalate crystal or a lithium
niobic acid, and the gas introduced into the melted lithium
tantalate crystal or the lithium niobic acid is an oxygen or inert
gas.
8. A crystallization method according to claim 7, wherein said
inert gas is a helium gas.
9. A crystallization apparatus comprising: a crucible arranged in a
predetermined temperature distribution, and houses a melted
crystallized material; and a gas introducing part for introducing a
gas into the melted crystallized material, wherein the
crystallization apparatus coagulates the melted crystallized
material in one direction by changing the temperature distribution
relatively to the crucible.
10. A crystallization apparatus comprising: a crucible heated at a
predetermined temperature which houses a melted crystallized
material; and a gas introducing part for introducing a gas into the
melted crystallized material, wherein the crystallization apparatus
grows a crystal by lifting a member that contacts with the melted
crystallized material.
11. A crystallization apparatus according to claims 9 or 10,
wherein the gas is an inert gas.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates mainly to a method and
apparatus that grows a crystal by a solidification process that
solidifies melted crystal material.
[0002] Various existing methods to produce an excellent single
crystal by controlling a solidification process that solidifies
melted crystal material are conventional used for manufacture
various crystal materials.
[0003] Typical manufacturing method for the single crystal is the
CZ (Czochralski) method that slowly lifts a solid crystal from
solidifying melted material in a crucible and the Bridgman method
that is solidifies the melted material in one direction by changing
a temperature distribution in the crucible that houses the melted
material. The choice of single crystal manufacturing method depends
on required characteristics and performance from the manufactured
single crystal.
[0004] The single crystal manufactured by the above crystallization
method is, generally, high-purity. Moreover, it is high demand for
the single crystal not to include defects such as air bubbles.
Then, the crystal is grown at a slow crystal growth rate by using a
melt liquid consisting of an enough high-purity crystal material so
that the impurities and air bubbles are not included in the
manufactured single crystal.
[0005] More particularly, when calcium fluoride is used as an
optical element for ultraviolet etc. with short wavelength, an
optical performance is greatly deteriorated because of extremely
small amount of impurities, so a scavenger to remove impurities is
added to the melt liquid during the crystal growth process, and
high-purity single crystal is manufactured. For instance, refer to
"Single Crystal Growth Technology (Tsuguo Fukuda, Keigo Hoshikawa.,
BAIFUKAN CO., LTD)".
[0006] However, in actual crystal growth process, impurities and
air bubbles that mix into the grown single crystal cannot always be
completely removed. This is because impurities remain such as the
solid crystal material before melt, gas molecules that adhere to
the crucible surface, and adhered particles of high melting point
that can not desorbed from the melt liquid when viscosity is
comparatively high.
[0007] Moreover, it is necessary to remove a reaction product to
outside of system by adding a high-purity scavenger as a gas for
the above calcium fluoride. However, all the gas cannot be
exhausted, and remains in the grown crystal for the above
reasons.
[0008] The present invention is invented to solve the above
problem, and provides crystallization method and crystallization
apparatus that efficiently removes gas and solid particles of high
melting point that exist in a melt liquid during the crystal growth
process to outside of system, and grows high-purity crystal that
does not include air bubbles.
BRIEF SUMMARY OF THE INVENTION
[0009] In order to achieve the above object, a crystallization
method according to one aspect of the present invention that
includes the steps of melting a crystallized material in a crucible
by heating; and growing a crystal by cooling and coagulating the
melted material, wherein said melting step includes introducing a
predetermined gas into the melted material.
[0010] An optical element according to another aspect of the
present invention made of a single crystal, said single crystal
being manufactured by a crystallization method that includes
melting a crystallized material in a crucible by heating, and
growing a crystal by cooling and coagulating the melted material,
wherein said melting step includes introducing a predetermined gas
into the melted material, and wherein said crystallized material is
a calcium fluoride with an added scavenger, and the gas introduced
into the melted fluorite is an inert gas.
[0011] An exposure apparatus according to another aspect of the
present invention includes a light source, an illumination optical
system for guiding a light from the light source to a reticle, and
a projection optical system for guiding the light from the reticle
to a wafer that is placed on a wafer stage, wherein said
illumination optical system or the projection optical system
includes an optical element, and wherein said optical element is
made of a single crystal, said single crystal being manufactured by
a crystallization method that includes, melting a crystallized
material in a crucible by heating, and growing a crystal by cooling
and coagulating the melted material, wherein said melting step
includes introducing a predetermined gas into the melted material,
and wherein said crystallized material is a calcium fluoride with
an added scavenger, and the gas introduced into the melted calcium
fluoride is an inert gas.
[0012] A crystallization apparatus according to another aspect of
the present invention includes a crucible arranged in a
predetermined temperature distribution, which houses a melted
crystallized material, and a gas introducing part for introducing a
gas into the melted crystallized material, wherein the
crystallization apparatus coagulates the melted crystallized
material in one direction by changing the temperature distribution
relatively to the crucible.
[0013] A crystallization apparatus according to another aspect of
the present invention includes a crucible heated at a predetermined
temperature, and houses a melted crystallized material, and a gas
introducing part for introducing a gas into the melted crystallized
material, wherein the crystallization apparatus grows a crystal by
lifting a member in contact with the melted crystallized
material.
[0014] The present invention can produce a crystal with low
concentration of impurities and does not include air bubbles etc.
by introducing a gas into a melt liquid used for the crystal growth
and shaking.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a typical sectional view of a crystallization
apparatus according to the present invention.
[0016] FIG. 2 is typical view of state that particles and gas
impurities in a melt liquid are removed by introducing a gas.
[0017] FIG. 3 is a view of a relationship between an introducing
time that helium gas is introduced into a melt liquid of calcium
fluoride before the crystal growth begins and a density of melt
liquid.
[0018] FIG. 4 is a flowchart that shows a process flow from a
material synthesis of calcium fluoride single crystal that grows in
the present invention to an exposure apparatus.
[0019] FIG. 5 is schematic sectional view of an exposure apparatus
using the calcium fluoride single crystal grown by the present
invention.
[0020] FIG. 6 is a flowchart for explaining how to fabricate
devices (such as semiconductor chips such as ICs, LCDs, CCDs, and
the like).
[0021] FIG. 7 is a detailed flowchart of a wafer process in Step 4
of FIG. 6.
[0022] FIG. 8 is a typical sectional view of a second
crystallization apparatus according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] FIG. 1 is a typical sectional view of a crystallization
apparatus according to the present invention. In FIG. 1, a side
heater 3a made of graphitized carbon with cylindrical form is
arranged in a housing 5 that forms a chamber 6. The side heater 3a
is supplied an electric power from a side heater power source 7
that is controlled by a controller 9, and forms a predetermined
temperature distribution in the chamber 6. The temperature of the
chamber 6 is measured by the temperature sensor 18, and feedback to
the controller 9. An insulator 4 made of graphitized carbon is
installed inside the housing 5, and protects the housing 5 from the
high-temperature. The chamber 6 is vacuum exhausted to the pressure
of 1E-4 or less (Torr) by the exhaust apparatus (not shown).
[0024] A crucible support rod 2 that supports a crucible 1 is
installed to penetrate through a bottom part of the housing 5. The
crucible 1 houses a material as a crystalloid. A crucible lifting
motor 2a drives the crucible support rod 2 in a vertical direction
at a predetermined speed by an electric power from a crucible
lifting motor power source 2b that is controlled by the controller
9. A bottom part of the chamber 6 is maintained at the temperature
below a melting point of the grown crystal. The crystal coagulates
from an upper side in the crucible 1 in one direction, and grows by
lowering down the crucible 1 by the crucible support rod 2.
[0025] The crystallization apparatus according to the present
invention further includes a gas introducing port 11 that
introduces a gas into the crucible 1. The gas introducing port 11
is supplied a predetermined gas from a gas tank 16 through a gas
valve 14, a valve controller 17, a gas purification apparatus 13,
and a pipe 12 etc. The gas introducing port 11 has a movable (up
and down) structure to insert into the melt liquid in the crucible
1 to introduce the gas after the material as the crystalloid is
melted, and to shelter it from the crucible 1 during the crystal
growth. The gas introducing port 11 is made of a same material as
crucible 1 so as not to pollute the melt liquid.
[0026] FIG. 2 is typical view of state that particles and gas
impurities in the melt liquid are removed by introducing the gas.
FIG. 2 shows the case where helium is introduced into the calcium
fluoride, but the present invention is not limited to this. For
instance, generally, the effect of the present invention can be
achieved by introducing the gas into the melt liquid used for the
crystal growth.
[0027] When the air bubbles and particle exist in the melt liquid,
the buoyancy F that acts on the air bubbles etc. is shown by a
reaction formula (1).
F=(.rho.-.rho.')gV (1)
[0028] Here, .rho. is a density of the melt liquid, .rho.' is a
density of the air bubbles etc., g is a gravitational acceleration,
and V is a volume of the air bubbles. When the air bubbles exists
in the melt liquid, generally, .rho.' can be disregarded for .rho.
because the density of the air bubbles is small enough compared
with the density of the melt liquid.
[0029] Originally, the air bubbles existed at the melt liquid
surface, and is exhausted to the outside of the system. Because the
buoyancy is proportional to the volume of the air bubbles as
understood from the reaction formula (1), the buoyancy is much
smaller for air bubbles that have a diameter of plural tens .mu.m
or less (micro-bubble) which causes problems during crystal growth.
Therefore, the air bubble cannot surface, and is present in the
grown crystal. Especially, when the density of the air bubbles
included in the melt liquid is below constancy by a constant
management, there is little thing to which the bubble's uniting
increases the volume, and the air bubbles of a certain amount
remains even if it dissolves for a long time.
[0030] Moreover, when the air bubbles etc. that exist in the melt
liquid reacts with the melt liquid, a secondary reaction is caused
because the air bubbles exist in the melt liquid for a long time
and becomes impurities in the melt liquid.
[0031] As shown in FIG. 2, the micro-bubbles that cannot surface in
the melt liquid can be effectively exhausted to the outside of the
system by introducing the predetermined gas as a bubble that has a
diameter more than certainly into the melt liquid. In other words,
when an enough big bubbles surface in the melt liquid, it is
possible to exhaust the air bubbles to the outside of the system by
catching the micro-bubbles. When a minute particle as solid exists
in the melt liquid, the particle is taken into the bubble by a
surface tension between the particle and the melt liquid, and can
be exhausted to the outside of the system. The particle carried to
the surface of the melt liquid once by the bubble does not mix
again into the melt liquid by the surface tension.
[0032] It is desirable that the gas introduced into the melt liquid
does not become a harmful impurity in the melt liquid, and is
promptly exhausted to outside of the melt liquid. As explained by
the following embodiments, an inert gas that does not generate
impurities is desirable for the melt liquid of the calcium
fluoride, and helium is more desirable because the density is low
and the diffusion speed is high in the melt liquid. On the other
hand, it is desirable for the melt liquid of the oxide crystal such
as niobic acid lithium and the tantalic acid lithium etc. to
introduce oxygen for the maintenance of stoichiometry. The
acceptable inert gas is similar to the calcium fluoride, and helium
is more desirable because the density is low and the diffusion
speed is high in the melt liquid.
[0033] The gas introduced is of a high purity to not introduce
impurities along with the introduction of the gas.
First Embodiment
[0034] The instant embodiment explains an example of manufacturing
the calcium fluoride chiefly used for an optical material by the
above crystallization method.
[0035] A material that adds ZnF.sub.2 of 0.1% by weight as the
scavenger to high-purity calcium fluoride polycrystal which is a
material of calcium fluoride single crystal is placed into the
carbon crucible 1 shown in FIG. 1. After the chamber 6 is
vacuum-exhausted, the side heater 3 is electrified to heat the
chamber 6, the crucible 1 is adjusted to about 1360.degree. C., and
the material in the crucible 1 is melted. The melt state is
maintained for about three hours for the oxygen removal reaction to
occur in the melt liquid by the scavenger, and then, the gas
introducing port 11 made of carbon is inserted in the crucible 1,
and the state is maintained for seven hours while introducing the
helium gas with 300 sccm. Meanwhile, the chamber 6 is maintained to
the pressure of 1 Torr or less to easily deaerate the introduced
helium gas.
[0036] Then, the introduction of the helium gas is ended, and the
gas introducing port 11 is taken out of the crucible 1, and the
melt state is maintained for about two hours to deaerate the
introduced helium gas. After the vacuum level of the chamber 6 is
2E-6 Torr or more, the crucible 1 is descended at the speed of 1
mm/h so as to grow the calcium fluoride single crystal.
[0037] Table 1 shows a residual concentration of Zn (analyzed with
ICP) and density of the micro-bubbles that has diameter of 30 .mu.m
or less (number for each 1 liter crystal) of the calcium fluoride
crystal grown by the instant embodiment. Table 1 shows a residual
concentration of Zn and density of micro-bubbles of a calcium
fluoride crystal that occur in the crystal after the melt state is
maintained for twelve hours without introducing the helium gas for
the comparison.
1 TABLE 1 CALCIUM FLUORIDE OF THE INSTANT COMPARISON EMBODIMENT
EXAMPLE RESIDUAL CONCENTRATION 5 57 OF Zn (ppb) DENSITY OF
MICRO-BUBBLE 3.1 32.7 (number/L)
[0038] Reference to Table 1, the residual concentration of Zn and
density of micro-bubbles have decreased when the helium gas is
introduced (the instant embodiment). It is thought that this
depends on the following actions:
[0039] The decrease in the residual concentration of Zn by the
introduction of the helium gas is thought to be a result of the
removal by the evaporation of the zinc included in the added
scavenger by the gas introduction. The added ZnF.sub.2 as the
scavenger removes oxygen in the melt liquid of calcium fluoride by
the reaction shown in the following a reaction formulas (2) and
(3).
CaO+ZnF.sub.2.fwdarw.CaF.sub.2+ZnO (2)
ZnO+C (crucible etc.).fwdarw.Zn.Arrow-up bold.+CO.Arrow-up bold.
(3)
[0040] As shown in the reaction formula 3, the metallic zinc
generated by the reaction with the scavenger evaporates and is
excluded from the melt liquid because the vapor pressure is high.
However, the concentration of zinc is actually low, and the bubble
of size that obtains the enough buoyancy to desorb from the melt
liquid can not be formed. Therefore, the desorption from the melt
liquid is limited to the desorption from the melt liquid
surface.
[0041] On the other hand, when the helium gas is introduced (the
instant embodiment), the micro-bubble of the zinc (or vacuole) that
exists in the melt liquid contacts the helium gas, is taken into
the helium gas as vapor, and is exhausted to the outside of the
melt liquid.
[0042] The decrease in the micro-bubbles by the introduction of the
helium gas is understood as follows: The generation cause of the
micro-bubble is not clear. However, it is thought that the
micro-bubble is caused by the gas's that exist in the melt liquid,
and the micro-bubble is caused by the crystal growth's being
locally obstructed.
[0043] For instance, CO etc. generated by the reaction with the
scavenger do not have the enough buoyancy to desorb from the melt
liquid because they are generated as the micro-bubble in the melt
liquid, and is taken into the grown crystal easily. CO that exists
in the melt liquid for a long time as the micro-bubbles generates a
minute amount of solid carbon by heat-resolving, and causes the
micro-bubble of the crystal being present in the crystal.
[0044] On the other hand, when the helium gas is introduced (the
instant embodiment), the micro-bubble that exists in the melt
liquid contacts the helium gas, is taken into the helium gas as
vapor, and is exhausted to the outside of the melt liquid. When
remaining as the air bubbles in the melt liquid for certain time
after mixing with the helium gas, a reactive speed with the melt
liquid etc. decreases due to the decrease in the partial pressure
due to the mixing, and a generation of a secondary particles and a
mixing of impurities can be prevented.
[0045] Moreover, a particle that mixes from a refractory material
such as crucible etc. is exhausted to the melt liquid surface by
introducing the helium gas. It is thought that the amount of
micro-bubbles in the grown crystal decreases by these effects.
[0046] FIG. 3 is a view of a relationship between an introducing
time that helium gas is introduced into the melt liquid of calcium
fluoride before the crystal growth and the density of the melt
liquid. The density is measured by Archimedes method that uses a
gauge head made of the carbon at the melting point of 1360.degree.
C. The density of the melt liquid increases as the introducing time
of the gas becomes long. The cause of the density changing of the
melt liquid shown in FIG. 3 is not clear. However, it is thought
that it is the result of the promotion of the exhaust of the air
bubbles that exist in the melt liquid.
[0047] FIG. 4 is a flowchart that shows a process flow from a
synthesis of material of calcium fluoride single crystal that grows
in the present invention to a device assembly.
[0048] A high-purity calcium fluoride polycrystal above, used as
material of calcium fluoride single crystal, is synthesized by the
following processes. First, a calcium carbonate and a hydrogen
fluoride are made to react as in a reaction formula (4), and a
powdery calcium fluoride is synthesized.
CaCO.sub.3+2HF.fwdarw.CaF.sub.2+H.sub.2O+CO.sub.2 (4)
[0049] In the purification process that bakes the calcium fluoride
generated by the reaction formula (4), the baked calcium fluoride
mixes with the scavenger, and melts in the carbon crucible, for
example, the oxygen contained in the calcium fluoride as a burnt
lime is removed as shown in reaction formula (5).
CaO+ZnF.sub.2.fwdarw.CaF.sub.2+ZnO.Arrow-up bold. (5)
[0050] The scavenger preferably includes zinc fluoride, bismuth
fluoride, sodium fluoride, lithium fluoride, and others which are
more easily combinable with oxygen mixed in the calcium fluoride
than the calcium fluoride, and easily decomposes and evaporates.
The zinc fluoride is preferable. In the purification process, the
scavenger is added by 0.05 mol % to 5.0 mol %, desirably 0.1 mol %
to 1.0 mol %. The calcium fluoride polycrystal obtained thus is
used to manufacture the calcium fluoride single crystal.
[0051] The instant embodiment explained the gas introduction in the
growth process of the calcium fluoride crystal, but even if the gas
is introduced in the above purification process, a similar effect
can be achieved. Therefore, the purity of the material used for the
crystal growth is improved. Moreover, it is applicable when
scavengers other than the zinc fluoride are used.
[0052] In addition, the present invention is not limited to
manufacturing the calcium fluoride crystal. When the material in
the crucible is coagulated in one direction and the crystal is
manufactured, the present invention can be applied.
[0053] Heat treatment processing is performed on the grown calcium
fluoride single crystal in an anneal chamber. This process heats
the calcium fluoride single crystal to 900.degree. C. to
1300.degree. C. in the crucible. The heating time is 20 hours or
more, desirably 20 hours to 30 hours, and then the calcium fluoride
single crystal is cooled at cooling speed of about 1.degree.
C./hour. Then, the calcium fluoride single crystal is formed into a
shape of a prescribed optical element, and used for an optical
system.
[0054] FIG. 5 is schematic sectional view of an exposure apparatus
using the calcium fluoride single crystal grown by the present
invention. Referring now to FIG. 5, a description will be given of
the exposure apparatus 500. The exposure apparatus 500 includes, as
shown in FIG. 5, an illumination apparatus 510 for illuminating a
reticle 520 which forms a circuit pattern, a projection optical
system 530 that projects diffracted light created from the
illuminated reticle pattern onto a plate 540, and a stage 545 for
supporting the plate 540.
[0055] The exposure apparatus 500 is a projection exposure
apparatus that exposes onto the plate 540 a circuit pattern created
on the reticle 520, e.g., in a step-and-repeat or a step-and-scan
manner. Such an exposure apparatus is suitable for a sub-micron or
quarter-micron lithography process. This embodiment exemplarily
describes as a step-and-scan exposure apparatus (which is also
called "a scanner"). The "step-and-scan manner", as used herein, is
an exposure method that exposes a reticle pattern onto a wafer by
continuously scanning the wafer relative to the reticle, and by
moving, after an exposure shot, the wafer stepwise to the next
exposure area to be shot. The "step-and-repeat manner" is another
mode of exposure method that moves a wafer stepwise to an exposure
area for the next shot, for every cell projection shot.
[0056] The illumination apparatus 510 which illuminates the reticle
520 that forms a circuit pattern to be transferred, includes a
light source unit 512 and an illumination optical system 514.
[0057] As an example, the light source unit 512 uses a light source
such as ArF excimer laser with a wavelength of approximately 193
[nm] and KrF excimer laser with a wavelength of approximately 248
[nm]. However, the laser type is not limited to excimer lasers
because for example, F.sub.2 laser with a wavelength of
approximately 157 [nm] and a YAG laser may be used. Similarly, the
number of laser units is not limited. For example, two
independently acting solid lasers would cause no coherence between
these solid lasers and significantly reduces speckles resulting
from the coherence. An optical system for reducing speckles may
swing linearly or rotationally. When the light source unit 512 uses
laser, it is desirable to employ a beam shaping optical system that
shapes a parallel beam from a laser source to a desired beam shape,
and an incoherently turning optical system that turns a coherent
laser beam into an incoherent one. A light source applicable for
the light source unit 512 is not limited to a laser, and may use
one or more lamps such as a mercury lamp and a xenon lamp.
[0058] The illumination optical system 514 is an optical system
that illuminates the reticle 520, and includes a lens, a mirror, a
light integrator, a stop, and the like, for example, a condenser
lens, a fly-eye lens, an aperture stop, a condenser lens, a slit,
and an image-forming optical system in this order. The illumination
optical system 514 can use any light regardless of whether it is
axial or non-axial light. The light integrator may include a
fly-eye lens or an integrator formed by stacking two sets of
cylindrical lens array plates (or lenticular lenses), and can be
replaced with an optical rod or a diffractive element. The
inventive calcium fluoride crystal is applicable to optical
elements, such as, a lens in the illumination optical system
514.
[0059] The reticle 520 is made, for example, of quartz, forms a
circuit pattern (or an image) to be transferred, and is supported
and driven by a mask stage (not shown). Diffracted light emitted
from the reticle 520 passes through the projection optical system
530 and is then projected onto the plate 540. The reticle 520 and
the plate 540 are located in an optically conjugate relationship.
Since the exposure apparatus 500 of this embodiment is a scanner,
the reticle 520 and the plate 540 are scanned at the speed ratio of
the reduction ratio of the projection optical system 530, thus
transferring the pattern from the reticle 520 to the plate 540. If
it is a step-and-repeat exposure apparatus (referred to as a
"stepper"), the reticle 520 and the plate 540 remains still when
exposing the mask pattern.
[0060] The projection optical system 530 is an optical system that
projects light that reflects a pattern on the reticle 520 located
on an object surface onto the plate 540 located on an image
surface. The projection optical system 530 may use an optical
system comprising solely of a plurality of lens elements, an
optical system including a plurality of lens elements and at least
one concave mirror (a catadioptric optical system), an optical
system including a plurality of lens elements and at least one
diffractive optical element such as a kinoform, a full mirror type
optical system, and so on. Any necessary correction of the
chromatic aberration may be accomplished by using a plurality of
lens units made from glass materials having different dispersion
values (Abbe values) or arranging a diffractive optical element
such that it disperses light in a direction opposite to that of the
lens unit. An optical element made of the inventive calcium
fluoride crystal is applicable to any optical element, such as a
lens in the projection optical system 530.
[0061] The plate 540, such as a wafer and a LCD, is an exemplary
object to be exposed. Photoresist is applied to the plate 540. A
photoresist application step includes a pretreatment, an adhesion
accelerator application treatment, a photo-resist application
treatment, and a pre-bake treatment. The pretreatment includes
cleaning, drying, etc. The adhesion accelerator application
treatment is a surface reforming process to enhance the adhesion
between the photoresist and a base (i.e., a process to increase the
hydrophobicity by applying a surface active agent), through a coat
or vaporous process using an organic coating such as HMDS
(Hexamethyl-disilazane). The pre-bake treatment is a baking (or
burning) step, which makes the photoresist softer than after
development and removes the solvent.
[0062] The stage 545 supports the plate 540. The stage 545 may use
any structure known in the art, thus, a detailed description of its
structure and operation is omitted. The stage 545 may use, for
example, a linear motor to move the plate 540 in the XY directions.
The reticle 520 and plate 540 are, for example, scanned
synchronously, and the positions of the stage 545 and a mask stage
(not shown) are monitored, for example, by a laser interferometer
and the like, so that both are driven at a constant speed
ratio.
[0063] The stage 545 is installed on a stage stool supported on the
floor and the like, for example, via a dampener. The mask stage and
the projection optical system 530 are installed on a lens barrel
stool (not shown) support, for example, via a dampener, to the base
frame placed on the floor.
[0064] In exposure, light is emitted from the light source 512,
e.g., Koehler-illuminates the reticle 520 via the illumination
optical system 514. Light that passes through the reticle 520 and
reflects the mask pattern is imaged onto the plate 540 by the
projection optical system 530. The illumination and projection
optical systems 514 and 530 in the exposure apparatus 500 include
an optical element made of inventive calcium fluoride crystal that
transmits the UV light, FUV light, and VUV light with high
transmittance, and provide high-quality devices (such as
semiconductor devices, LCD devices, photographing devices (such as
CCDs, etc.), thin film magnetic heads, and the like) with high
throughput and economic efficiency.
[0065] Referring now to FIGS. 6 and 7, a description will be given
of an embodiment of a device fabrication method using the above
mentioned exposure apparatus 500. FIG. 6 is a flowchart for
explaining how to fabricate devices (i.e., semiconductor chips such
as IC and LSI, LCDs, CCDs, and the like). Here, a description will
be given of the fabrication of a semiconductor chip as an example.
Step 1 (circuit design) designs a semiconductor device circuit.
Step 2 (mask fabrication) forms a mask having a designed circuit
pattern. Step 3 (wafer making) manufactures a wafer using materials
such as silicon. Step 4 (wafer process), which is also referred to
as a pretreatment, forms the actual circuitry on the wafer through
lithography using the mask and wafer. Step 5 (assembly), which is
also referred to as a post-treatment, forms into a semiconductor
chip the wafer formed in Step 4 and includes an assembly step
(e.g., dicing, bonding), a packaging step (chip sealing), and the
like. Step 6 (inspection) performs various tests on the
semiconductor device made in Step 5, such as a validity test and a
durability test. Through these steps, a semiconductor device is
finished and shipped (Step 7).
[0066] FIG. 7 is a detailed flowchart of the wafer process in Step
4. Step 11 (oxidation) oxidizes the wafer's surface. Step 12 (CVD)
forms an insulating layer on the wafer's surface. Step 13
(electrode formation) forms electrodes on the wafer by vapor
disposition and the like. Step 14 (ion implantation) implants ion
into the wafer. Step 15 (resist process) applies a photosensitive
material onto the wafer. Step 16 (exposure) uses the exposure
apparatus 500 to expose a circuit pattern from the mask onto the
wafer. Step 17 (development) develops the exposed wafer. Step 18
(etching) etches parts other than a developed resist image. Step 19
(resist stripping) removes unused resist after etching. These steps
are repeated to form multi-layer circuit patterns on the wafer. Use
of the fabrication method in this embodiment helps fabricate
higher-quality devices than conventional methods. Thus, the device
fabrication method using the exposure apparatus 500, and resultant
devices constitute one aspect of the present invention.
Second Embodiment
[0067] FIG. 8 is a typical sectional view of a second
crystallization apparatus according to the present invention. FIG.
8 shows a structure when this invention is executed with the
crystallization apparatus by Czochralski method. In the instant
embodiment, a description will be given of manufacturing of lithium
tantalate (LiTaO.sub.3) (hereafter, LT) crystal as one example of
the grown crystal.
[0068] In FIG. 8, a crucible 1a that houses a melting material as a
crystalloid is installed in the housing 5 that form the chamber 6.
An insulator 4 is installed inside the housing 5, and protects the
housing 5 from the high-temperature. The chamber 6 is vacuum
exhausted to the pressure of 1E-4 or less (Torr) by the exhaust
apparatus (not shown).
[0069] A RF (radiofrequency generation) coil 21 that heats the
material in the crucible 1a is arranged outside of the chamber 6.
The RF coil 21 is supplied an electric power from a radiofrequency
generation power source 20 that is controlled by a power source
controller 19. The crucible 1a is made of platinum, and an
induction heating is possible by an induced current caused by the
RF coil 21.
[0070] A material support rod 2c that fixes a seed crystal to an
edge is installed in an upper part of the crucible 1a. A support
rod lifting motor 2a is supplied the electric power from a support
rod power source 2b that is controlled by the controller 9, and
drives the material support rod 2c.
[0071] The crystallization apparatus according to the present
invention further includes the gas introducing port 11 that
introduces the gas in the crucible 1a. The gas introducing port 11
is supplied a predetermined gas from the gas tank 16 through the
gas valve 14, the valve controller 17, the gas purification
apparatus 13, and the pipe 12 etc. The gas introducing port 11 has
a movable (up and down) structure to insert into the melt liquid in
the crucible 1a to introduce the gas after the crystalloid material
is melted, and to shelter from the crucible 1a during the crystal
growth. The gas introducing port 11 is made of a same material as
the crucible 1a so as not to pollute the melt liquid.
[0072] Powdery lithium tantalate (LT) is put in the crucible 1a,
the chamber 6 is vacuum-exhausted, and the crucible 1a is heated up
to 1670.degree. C. of the melting point, and the material in the
crucible 1a is melted. Then, the vacuum exhaust is stopped, and the
chamber 6 is returned to the atmospheric pressure by inserting the
gas introducing port 11 in the crucible 1a, and introducing oxygen
at the rate of 200 sccm. In addition, oxygen is introduced for
three hours while the chamber 6 maintained to the atmospheric
pressure.
[0073] Then, the atmospheric pressure is maintained for one hour
after the introduction of oxygen is stopped, the material support
rod 2c that fixes the seed crystal to the edge is moved so that the
seed crystal contacts the melt liquid, and the crystal growth is
begun. The crystal growth is done while lifting the material
support rod 2c at speed of 1 mm/h and rotating the material support
rod 2c at 40 rpm.
[0074] Table 2 shows a transmittance (wavelength: 300 nm) and a
density of micro-bubbles of the lithium tantalate crystal obtained
by the instant embodiment, that compared with a lithium tantalate
crystal grown under other conditions. The comparison examples are
an example of introducing the helium gas instead of oxygen and an
example of growing the crystal without introducing gas under the
oxygen atmosphere.
2 TABLE 2 DENSITY OF MICRO- BUBBLE TRANSMITTANCE (%) (piece/L) LT
CRYSTAL OF THE INSTANT 65 5.2 EMBODIMENT LT CRYSTAL INTRODUCED He
52 6.7 LT CRYSTAL WITHOUT 45 27.8 INTRODUCING GAS
[0075] Reference to Table 2, the transmittance has improved and the
density of micro-bubbles has decreased when the oxygen is
introduced (the instant embodiment). On the other hand, when the
helium gas is introduced, the transmittance is not improved enough
even though the decrease in the density of the micro-bubbles is
achieved. When the helium gas is introduced, a part of the oxygen
in the crystal material melts into the melt liquid, and the oxygen
defect is taken into the grown crystal even though the gas is
removed.
[0076] The instant embodiment explains the crystal growth of the
lithium tantalate crystal, but the same effect can be achieved with
the lithium niobic acid crystal that has a same structure.
[0077] The lithium tantalate crystal and the lithium niobic acid
crystal manufactured by the above method has a composition near a
stoichiometric composition has excellent transmittance, and
therefore, can be used as a wavelength sensing element, an optical
modulator, an optical switch element, and a digital hologram memory
element besides for just a laser medium.
[0078] This application claims foreign priority benefits based on
Japanese Patent Application No. 2004-110836, filed on Apr. 5, 2004,
which is hereby incorporated by reference herein in its entirety as
if fully set forth herein.
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