Quartz glass crucibles

A flat-bottomed quartz glass crucible for the manufacture of monocrystalline silicon is made by disposing granular quartz particles on a flat base in a cylindrical mold. A vacuum is drawn on the particles through vacuum holes in the base to retain the particles thereon while the base and mold are rotated, and while the particles are heated to fuse them into a quartz glass crucible shape.

This invention concerns quartz glass crucibles for use in making 
monocrystalline silicon for semiconductors. A method of making such 
crucibles involves disposing granular quartz particles on the interior 
surface of a rotating cylindrical mold, drawing a vacuum on the granular 
quartz through the mold in order to remove gas bubbles from the quartz, 
and then heating and melting an inner layer of the quartz to fuse the 
quartz into a crucible shape. Such a method is disclosed in U.S. Pat. No. 
4,416,680. The crucibles made by such a method have a substantially curved 
bottom, as shown in the patent drawing. 
This invention is concerned with making a crucible having a substantially 
flat bottom. Such a crucible can be used advantageously in the manufacture 
of monocrystalline silicon by providing silicon having improved 
properties. The improvement is probably due to a decrease in thermal 
gradient currents occurring in the molten silicon during the drawing of 
the monocrystalline silicon therefrom. 
In order to provide the substantially flat bottom, provision must be made 
to retain the granular quartz on the flat base of the rotating mold during 
the heating and melting of the quartz, so that centrifugal force does not 
throw the quartz against the walls of the mold. In the mold used in U.S. 
Pat. No. 4,416,680, the retention of quartz on the bottom is not a problem 
because the bottom is curved. 
We have found that in order to retain the quartz on a flat bottom, there 
must be sufficient vacuum to hold the granular quartz there and overcome 
centrifugal force tending to throw the quartz outward. To accomplish this, 
the vacuum holes in the base must be larger than those disclosed in U.S. 
Pat. No. 4,416,680 where the hole size is small enough so that the 
granular quartz will bridge the opening in order to prevent clogging of 
the opening. In said patent, 0.5 mm holes are used, which are successfully 
bridged by the 160 to 250 micron size granular quartz disclosed in the 
patent. 
In our invention, a larger hole is used than will be bridged by the 
granular quartz. We dispose a porous material in the hole such that vacuum 
is readily transmitted through the porous material, but the granular 
quartz will not pass through. Furthermore, the porous material is not 
readily clogged by the granular quartz. 
The use of a non-porous material for the mold, with a porous material only 
in the vacuum holes as per this invention, is superior to the use of a 
porous material for the entire mold, the reason being that the useful life 
of a mold as per this invention is much longer than when substantially the 
entire mold is made of a porous material. Furthermore, our arrangement 
permits better distribution of the vacuum where it is needed more, that is 
to say, where centrifugal force is greater.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In one example of this invention, as shown in FIG. 1, the apparatus 
comprises a base 1 having flat upper surface 15 and having vacuum holes 2 
through the base. The sides of the mold comprise an open-ended cylinder 3 
resting in an undercut in base 1. Base 1 rests on metal plate 4 supported 
on spindle 14 through which vacuum can be transmitted to vacuum holes 2. 
Plate 4 and spindle 14 are rotated by motor 5. Vacuum is supplied to 
spindle 14 by vacuum pump 6 through valve 7 through pipe 8. Means 9 are 
provided for circulating cooling water within spindle 14. 
In one embodiment, base 1 was made of graphite and was 23" in diameter by 
6" thick. The undercut region in which cylinder 3 rested, which also 
comprised flat surface 15, was 20.1" in diameter by 1/4" deep. There were 
1,104 holes 2 through base 1 in twenty circular rows. The first circular 
row comprised sixteen equally spaced holes on a 4" body circle. It was not 
necessary to locate any vacuum holes within the 4" body circle because 
centrifugal force thereat was slight. Thus, granular quartz 10 could be 
satisfactorily retained therein by means of inverted frustums of granular 
quartz drawn and held by vacuum which extend above holes 2 in the first 
circular row. The second circular row also had sixteen equally spaced 
holes staggered or located between the holes in the first row, on a 5" 
body circle. The third circular row also had sixteen equally spaced 
staggered holes on a 6" body circle. Thus these circular rows were 1/2" 
apart, edge-to-edge, which spacing was maintained for the first ten rows. 
Row four had thirty-two equally spaced holes and rows five through twenty 
each had sixty-four equally spaced holes. In rows five through ten, the 
holes were not staggered but were radially aligned. Row eleven was on a 
131/2" body circle, and the edge-to-edge spacing between circular rows for 
rows eleven through twenty was reduced to 1/4", the holes in adjacent rows 
being staggered. Row twenty was on an 18" body circle. 
As shown in FIG. 2, each hole 2 had a diameter of 3/16" for a distance of 
1/4" below upper surface 11 of base 2 and then a reduced diameter of 1/8" 
for a distance of 11/2". The reason for the reduced diameter was to 
provide a shoulder 12 on which porous graphite plug 13 would rest when 
inserted into the hole. Each hole then expanded to a diameter of 1/4" for 
the remaining thickness of base 2 in order to not unnecessarily constrict 
the vacuum passages. 
In this example cylinder 3 had an outside diameter of 20", a thickness of 
1" and a height of 12". There were no vacuum holes in cylinder 3 because 
the crucible was going to be cut down in height to about 3" and the vacuum 
being drawn through holes 2 in base 1 could adequately remove gas bubbles 
from the 3" height. 
The arrangement of metal plate 4 and spindle 14 was such that there was 
considerable surface contact therebetween, so that water cooled spindle 14 
would also cool plate 4, while vacuum through plate 4 could still be 
drawn. The arrangement of base 1 and plate 4 was similar, i.e., to provide 
cooling of base 1 while still providing for the vacuum. Thus, the bottom 
surface of base 1 had grooves or passages therein (not shown) leading from 
the vacuum opening in plate 4 to all the vacuum holes 2 in base 1. Such 
grooves permitted surface contact of plate 4 to base 1 between the grooves 
in order to provide cooling surfaces therebetween. Similarly, there was 
surface contact between plate 4 and base 1 within the 4" circle at the 
center of base 1 where there were no vacuum holes. 
This invention permits use of a dense graphite material machined into shape 
for the mold base and cylinders, such as high purity graphite 890 RL 
having a density of 1.68 gm/cc made by Airco Carbon, St. Marys, Pa. Such a 
material is more oxidation resistant and lasts much longer than the porous 
graphite referred to in U.S. Pat. No. 4,416,680. A fibrous, very porous 
graphite, however, was used for plugs 13. In this example, plugs 13 were 
made of Fiberform, which is made by Fiber Materials, Inc., Biddeford, Me. 
Fiberform is a low density (11.9 lbs./cu.ft), rigid, carbon bonded carbon 
fiber insulation material designed to operate at temperatures up to 
5000.degree. F. in a vacuum or inert environment and up to 660.degree. F. 
in an oxidizing atmosphere. Plugs 13 were cemented in place with a 
carbonaceous cement such as Ucar C-34 made by Union Carbide Corporation, 
Danbury, Conn. 
The vacuum tends to hold the granular quartz in the shape of an inverted 
frustum at the top of each hole 2, the angle of the frustum being about 
45.degree.. In order to ensure that the quartz is held in place on the 
flat surface of base 1, without being spun off by centrifugal force during 
rotation, holes 2 cannot be spaced too far apart, relative to their 
diameter. In the embodiment above, the maximum edge-to-edge spacing 
between two adjacent holes 2 in a circular row was 1". Thus bridging of 
the granular quartz frustums at these two holes occurred at a height above 
surface of about 3/8" which is about 11/2 times the diameter of a hole 2. 
Said maximum edge-to-edge spacing should not substantially exceed said 
amount of 11/2. 
In one example, the apparatus was prepared as follows. Base 1 was mounted 
on metal plate 4 and spindle 14, as shown in FIG. 1, and cylinder 3 was 
placed on base 1. Rotation was commenced and granular quartz particles 
were poured into cylinder 3. A suitable jig shaped the granular quartz 
particles into a thick layer, as shown in FIG. 1, about 3/4" thick. With 
vacuum and rotation being maintained, about half of the thickness of the 
layer of granular quartz particles 10 was melted and fused into quartz 
glass by heat from electrodes 10. Most of the remaining half of the layer 
thickness, except for a thin coating of sintered quartz particles adhering 
to the fused quartz, remained unfused and unsintered as loose particles, 
thus facilitating removal of the rough quartz glass crucible shape from 
the mold. The rough crucible was machined into a finished crucible by 
removing the loose particles, cutting off the upper portion of the 
crucible shape to produce the desired height, and grinding, beveling or 
polishing the upper edge of the crucible. 
An 18" diameter substantially flat-bottomed crucible made from the above 
mold had a flat bottom of about 15" diameter and was slightly radiused for 
the remaining 11/2" perimeter. Because the flat bottom is advantageous in 
a semi-continuous process of drawing monocrystalline silicon, the crucible 
height could be considerably less than that of prior art crucibles having 
a curved bottom. In this example, the height of the finished, cut-down 
crucible was only three inches. About 5/6ths of the diameter of the 
crucible bottom was substantially flat.