Glass manufacturing – Means shaping preform from granular material with fusion means
Reexamination Certificate
2000-10-27
2003-04-15
Vincent, Sean (Department: 1731)
Glass manufacturing
Means shaping preform from granular material with fusion means
C065S302000
Reexamination Certificate
active
06546754
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to quartz (alternatively referred to as silica) crucibles and more specifically to an apparatus for forming fused quartz crucibles used in the semiconductor industry for growing single crystal silicon articles.
In the production of silicon crystals, the Czochralski method is often employed where polycrystalline silicon is first melted down within a quartz crucible. After the polycrystalline silicon has melted, a seed crystal is dipped into the melt and subsequently extracted while the crucible is rotated to form a single crystal silicon ingot.
It is important that the crucibles used for preparing single crystal silicon, particularly for the semiconductor industry, be essentially free of impurities. In addition, the quartz crucible is preferably largely free of included bubbles and other structural imperfections.
Accordingly, it is highly desirable to have available a method for forming silica crucibles having a low bubble content and/or bubbles containing gasses having little or no negative effect on silicon crystal growth.
Traditionally, to prepare such crucibles, a raw material quartz is introduced into a rotating hollow mold which has gas pervious wall regions at the side and bottom. After introducing the raw quartz material, a heat source is introduced into the mold to melt the quartz. During heating, a vacuum is applied to the outside of the rotating mold to draw out interstitial gases. Although this process reduces bubble content, bubbles nonetheless remain. Moreover, while attempts have been made to control the atmosphere, satisfactory apparatus do not exist to adequately meet the unique requirements of quartz crucible manufacture.
Therefore, a need still exists in the semiconductor industry for a crucible with a low bubble content and/or crucible having bubbles containing gases with little destructive effect on the polycrystalline melt and the drawn crystal. This can be accomplished by reducing the number of bubbles and by controlling the composition of the gas trapped in the bubbles to reduce impact on crystal growth. Moreover, the composition of the gas in the bubbles can be controlled such that the included gas is highly soluble in the silicon melt or evolves therefrom. One important requirement to achieve this goal is the availability of an apparatus for crucible manufacture which facilitates production of crucibles with reduced bubble count and with controlled bubble gas composition.
SUMMARY OF THE INVENTION
According to an exemplary embodiment of this invention, an apparatus for manufacture of quartz crucibles is provided. The apparatus includes a hollow mold having a bottom wall portion and a side wall portion defining a hollow space therein. The walls include a plurality of passages which facilitate drawing gas through the walls of the mold under vacuum. This feature thereby draws the quartz particles against the walls and removes gas.
A rotatable support for the mold is also provided to facilitate rotation of the mold about a vertical axis. This feature also serves to enable the retention of the quartz sand on the walls of the mold via centrifugal forces.
A shroud surrounds at least a portion of the hollow mold and at least one gas inlet is provided to supply a gas to a space between the shroud and the mold. Generally, the shroud will surround the outer bottom and side walls of the mold and the outer periphery of a top of the hollow mold.
A housing, or a hood, surrounds at least a portion of the shroud and the mold, the housing having at least one gas outlet positioned to exhaust gas. Preferably, the housing will establish a crucible formation environment which is either a) sealed from the surrounding atmosphere or b) created with sufficiently close tolerances (e.g. a hood) that by maintaining a desired flow into the environment, a positive pressure relative to outside air is maintained. This ensures that there is control gas flowing out, and no surrounding atmosphere flowing into the housing.
Preferably, a compressed air source is provided in connection with the vacuum system to expose the openings in the mold to a reverse pressure to permit ejection of crucibles after melting and sintering.
Preferably, a rotary vacuum connection joint is used to couple a vacuum line to the hollow mold to permit rotation of the mold.
DESCRIPTION OF DRAWINGS
FIG. 1
is a schematic view of an apparatus for producing a crucible in accord with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Fused quartz crucibles are used by the semiconductor industry for growing single crystal silicon ingots from polycrystalline silicon according to the Czochralski Process. In conducting the process, polycrystalline silicon is placed in a quartz crucible and melted at about 1500° C. The resulting melt is touched with a seed crystal. As it is pulled out, a single crystal silicon ingot grows.
During ingot growth, the molten silicon reacts with the fused quartz crucible, dissolving part of the crucible wall. If the crucible wall which dissolves contains bubbles, the dissolution process can cause the material surrounding the bubble to fragment. In so doing, fine chips of fragments of fused quartz may be released. Fragments which break off may cause multiple growth cites and upset the equilibrium of the crystal growth process, thus limiting the crystal growing yield. In addition, any gas contained within the bubble will be dissolved in the drawn silicon, or if insoluble, may become entrained therein. Accordingly, it is desirable to control the quantity, size and gas content of crucible bubbles.
The crucible itself is formed by placing quartz sand in a rotating graphite pot. Centrifugal forces cause the sand to “cling” to the sides of the pot, taking on the shape of a bowl. An electric arc supplies heat to melt the sand. When the arc is inserted, the temperature at the arc is about 6000° C., and the temperature at the wall is about 2000° C. A series of openings at the bottom of the graphite pot supply a vacuum to remove residual gases from the sand.
Not all gases between the sand grains are removed by the vacuum. Moreover, the spaces between the sand grains will retain some residual gas even at the available vacuum levels. In addition, the sand grains typically fuse together quickly, trapping gases in the voids. The gases roughly parallel the composition of the surrounding atmosphere. Previously, poor control of the surrounding atmosphere has been maintained during the fusion process. Similarly, earlier crucible forming apparatus did not allow for effective monitoring and modification of the surrounding atmosphere
By fusing crucibles in an atmosphere with a reduced amount of undesirable gases, the number of bubbles can be reduced and the bubbles which form in the crucible voids can have a reduced content of undesirable gases. Thus, when fewer and smaller bubbles are released into the melt as the crucible dissolves during crystal growth, void defects in the crystal caused by undesirable gases are minimized or eliminated. In accordance with the present invention, it has been found that control of the crucible fusion atmosphere requires a particular apparatus to advantageously and economically control the crucible fusion process.
Referring to
FIG. 1
there is shown an apparatus
10
for producing quartz crucibles. Apparatus
10
includes a fusion atmosphere
12
. A rotatable mold
14
is provided with a rotating shaft
16
. Rotating shaft
16
is preferably hollow or otherwise equipped with a vacuum line in communication with mold
14
to facilitate drawing fusion atmosphere
12
through the walls of mold
14
. Moreover, mold
14
is provided with passages (not shown) but generally indicated by arrows
15
. In the mold
14
, there is a cavity
18
for forming a crucible
20
. The mold
14
is rotated and heat sources
22
and
24
(electric arc) produce a high temperature atmosphere in the mold
14
. A quartz powder is supplied to the cavity
18
through a feed (not shown) but disposed above the mold
14
. The quartz
Coleman George
D'Orazio Fred
Hansen Richard L.
Lou Victor
General Electric Company
Vincent Sean
LandOfFree
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