Glass manufacturing – Means shaping preform from granular material with fusion means
Reexamination Certificate
2003-01-10
2004-10-05
Griffin, Steven (Department: 1731)
Glass manufacturing
Means shaping preform from granular material with fusion means
C065S374130, C373S027000
Reexamination Certificate
active
06799441
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods and apparatus for the production of fused silica optical members. More particularly, the invention relates to methods and furnaces for the production of high purity fused silica having high internal transmission.
BACKGROUND OF THE INVENTION
As practiced commercially, fused silica optical members such as lenses, prisms, photomasks and windows, are typically manufactured from bulk pieces of fused silica made in large production furnaces. In overview, silicon-containing gas molecules are reacted in a flame to form silica soot particles. The soot particles are deposited on the hot surface of a rotating or oscillating body where they consolidate to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame deposition processes. The bulk fused silica body formed by the deposition of fused silica particles is often referred to as a “boule,” and this terminology is used herein with the understanding that the term “boule” includes any silica-containing body formed by a flame hydrolysis process.
FIG. 1
shows a prior art furnace
100
for producing fused silica glass. The furnace includes a crown
12
and a plurality of burners
14
projecting from the crown. As noted above, silica particles are generated in a flame when a silicon containing raw material together with a natural gas are passed through the plurality of burners
14
into the furnace chamber
26
. These particles are deposited on a hot collection surface of a rotating body where they consolidate to the solid, glass state. The rotating body is in the form of a refractory cup or containment vessel
15
having lateral walls
17
and a collection surface
21
which surround the boule
19
and provide insulation to the glass as it builds up. The refractory insulation ensures that the cup interior and the crown are kept at high temperatures.
The prior art standard furnace further includes a ring wall
50
which supports the crown
12
. The furnace further includes a rotatable base
18
mounted on an oscillation table
20
. The base is rotatable about an axis
3
. The crown
12
, the ring wall
3
, the base
18
and the lateral walls are all made from suitable refractory materials.
The cup or containment vessel
15
is formed on the base
18
by means of lateral cup walls or containment walls
17
mounted on the base
18
, which forms the cup or containment vessel
15
. The lateral cup or containment walls
17
and the portion of the base
18
surrounded by the walls
17
is covered with high purity bait sand
24
which provides collection surface
21
for collecting the initial silica particles produced by the burners
14
. During deposition and consolidation of the silica particles into a solid glass, the boule
19
is formed having sidewalls
23
and an upper major surface
25
. As the boule
19
is formed during the deposition process, the upper major surface
25
of the boule
19
becomes the collection surface
21
a
for the silica particles, and as the thickness of the boule
19
increases during the deposition process, the distance z between the burners
24
and the collection surface decreases. The lateral walls
17
can be made from refractory blocks such as alumina base block for forming the walls
17
and an inner liner made of a suitable refractory material such as zircon or zirconia.
Surrounding the lateral walls
17
of the cup or containment vessel
15
is a shadow wall or air inflow wall
30
. The shadow wall
30
is mounted on x-y oscillation table
20
by feet
40
, for example four feet equally spaced around the circumference of the shadow or air inflow wall
30
. Details on the construction a shadow wall and a furnace using a shadow wall may be found in U.S. Pat. No. 5,951,730, the entire contents of which are incorporated herein by reference. Other ways of mounting the air inflow wall to the oscillation table can be used if desired. The stationary ring wall
50
surrounds the ring wall and supports the crown
12
. A seal
55
is provided between the stationary ring wall
50
and the air flow wall or shadow wall
30
. The seal
55
includes an annular plate
56
, which rides in or slides in an annular channel
58
formed within the stationary ring wall
50
. The annular channel
58
can include a C-shaped annular metal plate which forms the bottom of the stationary wall, other forms of motion-accommodating seals can be used if desired, including flexible seals composed of flexible metal or refractory cloth, which, for example, can be in the form of bellows.
The products of combustion in a standard prior art furnace
100
are exhausted through ports
60
circumferentially spaced around the furnace. In a typical furnace, six ports
60
are provided, and the ports
60
are located between crown
12
and the top edge
50
a
of the stationary wall, such that the ports
60
are located above the deposition surface
21
and
21
a
during formation of the boule.
Boules typically having diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm) can be routinely produced in large production furnaces of the type shown in FIG.
1
. Multiple blanks are cut from such boules and used to make the various optical members referred to above. The blanks are generally cut in a direction parallel to the axis of rotation of the boule in the production furnace, and the optical axis of a lens element made from such a blank will also generally be parallel to the boule's axis of rotation in the furnace. For ease of reference, this direction will be referred to as the “axis
1
” or “use axis”.
As the energy and power output of lasers increase, the optical members such as lenses, prisms, photomasks and windows, which are used in conjunction with such lasers, are exposed to increased irradiation levels and energies. Fused silica members have become widely used as the manufacturing material for optics in such high energy laser systems due to their excellent optical properties and resistance to damage at higher power levels.
The next generation of fused silica glass used in the microlithography market will require ArF (193 nm) internal transmission exceeding 99.65%/cm, and preferably exceeding 99.75%/cm. The standard manufacturing processes described above is capable of consistently producing fused silica lens blanks with 99.5%/cm. Reduction of metal contaminants, which have a major impact on UV transmission, has played a major role in the production of higher transmission fused silica. The effects of metals, such as sodium, potassium and iron, are evident at the 10's of parts per billion level. The standard process has demonstrated the ability to produce fused silica having transmission of 99.65%/cm, without sacrificing glass homogeneity, but not in the quantity needed to make large production quantities of lens blanks and not with the consistency to serve as a basis for a production process. Accordingly, it would be desirable to provide methods and apparatus capable of consistently manufacturing large production quantities of fused silica having internal transmission equal to or greater than 99.65%/cm at 193 nm, and preferably greater than 99.75%/cm.
SUMMARY OF INVENTION
The invention relates to methods and apparatus for producing fused silica. According to one aspect of the invention, a method for producing fused silica is provided which includes the steps of providing a furnace including a plurality of burners disposed above a collection surface and a refractory surface surrounding at least a portion of the collection surface. According to this aspect of the invention, the method further includes collecting soot particles on the collection surface to form a fused silica boule in a generally planar shape having an upper major surface and sidewalls. Still according to this aspect, the method further includes the step of maintaining the temperature of the refractory surface at least 300° C. cooler than the temperature of the deposition surfa
Marley Floyd E.
Sempolinski Daniel R.
Sproul Merrill F.
Corning Incorporated
Griffin Steven
Lopez Carlos
Schaeberle Timothy M.
Suggs James V.
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