Method of and apparatus for casting crystalline silicon ingot by electron bean melting

This invention aims at casting a large silicon crystal grain-containing ingot (14) by melting a silicon material (20) by scanning the same with an electron beam, and gradually cooling molten silicon (5) thus obtained. A method of casting a crystalline silicon ingot by electron beam melting, involves of the steps of melting a silicon material (20) by scanning the same with an electron beam, cooling the outer lower surface of molten silicon (5) thus produced while increasing the temperature of the molten silicon (5) suitably so as to generate crystals thereof, and gradually precipitating a crystalline silicon ingot (14) by the weight of itself in accordance with the generation of the crystals. An apparatus is provided for casting crystalline silicon ingot by electron beam melting, in which silicon material supply means (2, 3) are provided at one side of a cold hearth (1), one side portion of a crucible (8, 8a) being joined to the upper portion of the other side of the cold hearth (1) via a groove (7), electron guns (16 a, 16), a cooling means (10, 10a) being attached to the wall of the crucible (8, 8a), retaining means (15, 15a) for crystalline silicon ingot (14) being provided at the lower side of the crucible (8, 8a) so that the retaining means for the crystalline silicon ingot (14) can be vertically moved or both vertically and rotationally moved. The crystalline silicon ingot is lifted up at some point at a very slow speed so as to remove some part of the molten silicon which has a high concentration of impurities.

TECHNICAL FIELD 
This invention is related to a method of and apparatus for casting of 
crystalline silicon ingot by electron beam melting for the purpose of 
casting a large silicon crystal grain-containing ingot from a molten 
silicon which is melted by electron beam. 
BACKGROUND ART 
A known, conventional method for casting crystalline silicon ingot, is 
described as follows. First, poly-silicon was charged as a raw material 
and melted in the highly-pure quartz crucible 23 in vacuum or inert gas 
atmosphere shown in FIG. 12 , and after melting down, the melt is charged 
into the mold 24 which is made of the highly-pure graphite. The silicon in 
a liquid state 25 in the mold 24 is cast in ingot form being pulled down 
in a very slow speed through the heated zone which is controlled in a 
fixed temperature, for the purpose of controlling the size of the grain of 
the cast ingot, by making the cooling rate as low as possible. This method 
is known as a Wacker Ingot Casting Method (WICP) and a sort of modified 
well-known Bridgman/Stockbarger method. Another known method for casting 
crystalline silicon ingot, is described below and is shown in FIG. 11. 
This method was known as Cold Crucible method (JP-A-64-53733). The cold 
crucible consists of several water-cooled copper segments which are 
separated from each other by narrow slides. The electro-magnetic field 
created by the induction coil penetrates through the slides and couples 
into the charged material, then the melted material is lifted up by the 
magnetic force and heated. In this FIG. 11, numerals 29, 30 and 31 
indicate a feeder, resistant heater and silicon crystal respectively. 
Electron beam bombarding melting method (EBM) shown in FIG. 10, has also 
been known as an another method for casting crystalline silicon ingot. 
Electron beam is able to heat the melt quietly without causing any 
movement in molten metal 33 because electrons are accelerated in a high 
voltage field and bombarded to the target to heat the object. The 
advantage of the EBM method is to control the energy distribution within 
any shape of the scanning area of the ingot 34. In this FIG. 10, a numeral 
35 indicates a water cooled crucible. 
In the WICP method, material cost becomes very high because the highly pure 
materials should be used for crucible and mold, and the contamination from 
the de-molding flux can be expected in a high level. The maximum size of 
the cast ingot can be less than 100 kg, because of the controlling of the 
temperature of the mold, and of the limit of the effect of the removing of 
the impurities from the ingot by the effect of the concentration of the 
impurities from the solution in the ingot. The limitation of the ingot 
size would cause some problem for the increase of the size of the 
apparatus for the future expansion of the product. Above-mentioned "cold 
crucible-continuous casting method", however, is not only very difficult 
to operate but also consumes a considerable power for maintaining the melt 
and cooling the cold crucible. The melt is also stirred very much by 
electromagnetic magnetic force which may influence slightly the crystal 
growth during the solidification. The inner diameter of the coil has some 
limits concerning the electromagnetic force, and it may be difficult to 
cast the same size of WICP's ingot. From this point, "cold 
crucible-continuous casting method" has to be developed still more to meet 
the demand of production in large scale. 
And, EBM has also a problem in casting operation. In general, the casting 
process of EBM is discontinuous in the ingot pulling down method. It is 
very difficult to maintain the casting operation because the first part of 
the ingot which solidifies is always contacting the water-cooled copper 
crucible, which is different from "cold crucible-continuous casting 
method", and the casting face is cooled down at a rapid speed so that 
stress is always applied to the ingot forming cracks, and the cast ingot 
is broken off. 
DISCLOSURE OF INVENTION 
This invention which is modified from the above-mentioned conventional EBM 
method can successfully produce an ingot with large grain size 
continuously, and at relatively low cost, to control and release the 
stress which would act on the ingot by heating the first part of the 
solidifies, and avoiding the cracks by precipitating the ingot with the 
weight of itself. 
The present invention is a method of casting a crystalline silicon ingot by 
electron beam melting, comprising of the steps of melting a silicon 
material by scanning the same with an electron beam, processing the outer 
rim of the bottom side of the molten silicon thus produced so as to 
generate a crystalline silicon ingot, and gradually precipitating the 
crystalline silicon ingot in a very slow speed by the weight of itself in 
accordance with the generation of the crystalline silicons. The invention 
also a method of casting a crystalline silicon ingot by electron beam 
melting, comprising the steps of melting a silicon material which is 
charged into a cold hearth by scanning the same with an electron beam, 
pouring the molten silicon thus produced into a crucible so as to generate 
a crystalline silicon ingot from the bottom side of the crucible 
successively, and gradually precipitating the crystalline silicon ingot in 
a very slow speed by the weight of itself. Another embodiment is a method 
of casting a crystalline silicon ingot by electron beam melting, 
comprising the steps of melting a silicon material which is charged 
through a means for protecting the material from the contamination into 
the cold hearth by scanning the same with an electron beam, pouring the 
molten silicon thus produced into the crucible so as to generate a 
crystalline silicon ingot successively by chilling the surface of the 
molten silicon exposed from the bottom side of the crucible, and gradually 
precipitating the crystalline silicon ingot in a very slow speed by the 
weight of itself. A further embodiment is a method of casting a 
crystalline silicon ingot by electron beam melting, comprising the steps 
of melting a silicon material which is charged through the means for 
protection from the contamination into the cold hearth by scanning the 
same with an electron beam, pouring the molten silicon thus produced into 
the crucible so as to generate a crystalline silicon ingot successively by 
chilling the surface of the molten silicon exposed from the bottom side of 
the crucible, precipitating the crystalline silicon ingot in a very slow 
speed, and removing some part of the molten silicon which has a high 
concentration of impurities from the upper part of the crucible by lifting 
up the crystalline silicon ingot in a very slow speed at regular intervals 
in order to avoid the concentration of the impurities in the crystalline 
silicon ingot. In the before described methods, the molten silicon is 
flowing down from the cold hearth to the crucible by the head between the 
surface of molten silicon in the cold hearth and the surface of molten 
silicon in the crucible, and helium gas is introduced by blowing to the 
surface of the molten silicon at the bottom of the crucible in order to 
promote the crystal growth. Also, the crystalline silicon ingot is 
precipitated in a very slow speed and rotated during said precipitation. 
The before described protecting means is a lining or coating which is 
applied on the surface of the supplying means for the silicon material in 
order to prevent the molten silicon from the intermixing of iron, etc. 
The present invention also provides an apparatus for casting crystalline 
silicon ingot by electron beam melting, in which silicon material supply 
means is provided at one side of a cold hearth, one side portion of a 
crucible being joined to the upper portion of the other side of the cold 
hearth via a groove, an electron gun which is capable of scanning the 
whole area of the cold hearth and crucible being provided above the cold 
hearth and crucible, a cooling means being attached to the wall of the 
crucible, a retaining means for crystalline silicon ingot which can move 
vertically being provided just under the crucible. Another apparatus for 
casting crystalline silicon ingot by electron beam melting, in which 
silicon material supply means is provided at one side of a cold hearth 
which has a rectangular shape toward the crucible and being attached with 
a preventing means for the impurities located at the upper part of the 
cold hearth and near the surface of the molten silicon, one side portion 
of a crucible being joined to the upper portion of the other side of the 
cold hearth via a groove, an electron gun which is capable of scanning the 
whole area of the cold hearth and crucible being provided above the cold 
hearth and crucible, a cooling means being attached to the wall of the 
crucible, a retaining means for crystalline silicon ingot which can move 
vertically being provided just under the crucible. In the before described 
apparatuses, the silicon material supply means consists of a drum screw 
conveyer and chute which are operational under vacuum condition and being 
lined or coated in order to prevent contamination such as the intermixing 
of iron, etc. into the silicon material. The before described cooling 
means of the crucible is water-cooled, and the thermal conductivity is 
done by introducing helium gas. Further, the retaining means for 
crystalline silicon ingot is made of a water-cooled plate which can move 
vertically or a water cooled-plate which can move vertically and is 
coupled with a rotating means. And the preventing means for the impurities 
is a water-cooled copper tube which is installed on the upper part of the 
cold hearth at right angles to the moving direction of the molten silicon, 
and a part of said tube is dipped in the surface of molten silicon. 
This invention, as a casting method by electron beam melting, can be 
thought to have advantages mentioned below: 
(1) high energy density, 
(2) heat up the silicon material quietly, 
(3) scanning a wide area with any shape and speed by electromagnetic 
deflection, 
(4) control the energy density which is distributed into the silicon 
material freely, 
(5) good refining effect with melting in a high vacuum, and 
(6) no contamination from the parts which contact with the melt such as 
crucible and hearth made of water cooled copper. 
In this invention, because the material silicon can be melted by electron 
beam, it is possible to heat the determined area in determined temperature 
range comparably quietly, and it is also possible to control the 
penetration depth of heating. 
In this invention, because the crystalline silicon ingot is precipitated 
with the weight of itself, it is possible to precipitate the ingot in a 
precisely controlled speed which corresponds to the generation of the 
crystalline silicon without any partial strain. 
In this invention, because the melting of the silicon material and the 
growth of the crystalline silicon proceed under the precise control of 
removing the impurities and avoiding the contamination, it is possible to 
produce a highly pure ingot with large grain size continuously and in a 
large scale. 
In this invention, because the silicon material can be melted by electron 
beam, it is possible to melt the silicon material in a programmed 
sequence, and as a result, it is possible to cast effectively the 
crystalline silicon ingot with a comparably large grain size with a good 
character by the precise control of the precipitation speed of the ingot. 
In this invention, because the casting capacity can be improved for the 
production of large ingot, it is possible to improve the productivity and 
contribute to the drastic cost reduction for the production of the ingot. 
And by the apparatus, it is possible to produce in a large scale the 
crystalline silicon ingot with a good character successively which is free 
from inner induced crack and strain.

BEST MODE FOR CARRYING OUT THE INVENTION 
(Embodiment 1) 
The embodiment of this invention will now be described in detail. 
The granule material of silicon (grain size 3-20 mm) is charged into the 
cold hearth in a constant speed of 6 kg/min. in a vacuum range of 
10.sup.-4 mbar. 
The cold hearth has a rectangular shape whose length is long to the moving 
direction of the melt, made of water cooled copper and whose area and 
depth is 2,700 cm.sup.2 and 5 cm respectively. The charged material is 
melt down by the scanned electron beam which is generated from an electron 
beam gun of, for example 250 kw, which is located above the cold hearth. 
The melting rate is about 330 kg/Hr. The resulted melt flows to the 
crucible after the impurities are removed in the cold hearth. The melt in 
the crucible is also heated up and kept in a same condition by the scanned 
electron beam which is generated from electron beam gun which is located 
above the crucible. The surface of the melt which is contacting the wall 
of the crucible is frozen because the wall of the crucible is made of 
water cooled copper. There is a volume expansion of approx. 9% in 
solidification of silicon, and the shape of the lower part of the casting 
crucible should be good enough not to disturb the crystalline silicon 
ingot to go down. For the purpose of improving the thermal conductivity 
under the crucible, forming a solid chilled layer in a short time and 
promoting the crystal growth, helium gas is introduced between the inside 
of the lower part of the crucible and crystalline silicon ingot. At the 
same time, electron beam is programmed to sweep near the edge of crucible 
to avoid growth of the chilled layer. And electron beam is programmed to 
concentrate in the center of the crucible to keep the deep pool. In this 
way, the crystalline silicon ingot is generated by precipitating the ingot 
in a very slow speed with the weight of itself. It is favorable for the 
smooth precipitating of the crystalline silicon ingot, that the weight of 
the starting ingot should be more than a certain amount, for example, more 
than 200 kg. For this purpose, the total surface area of the crucible is 
designed as more than 4,700 cm.sup.2. In this way, poly-crystalline 
silicon ingot of 680 mm long .times. 680 mm wide .times. 900 mm height, 
total weight of which is 970 kg without starting block was successfully 
cast with the input EB power of 535 kw. The preferable range of input EB 
power is from 300 kw to 600 kw. The melting of silicon and the holding of 
the melt can be done at the same time within this range of input EB power. 
The range of the crystal grain size of above mentioned ingot was 3-30 mm 
and the average was 6-10 mm. Specific impurity level was Fe 15 ppbw, A1&lt;8 
ppbw. The conversion rate of solar energy of a solar cell module made from 
this ingot was 14.5% and this figure is almost the same level of the 
current top data of the well known poly silicon type solar cell. 
In this embodiment, it is favorable to remove some amount of the melt from 
the pool in the crucible in a certain interval which has been 
investigated, for example at intervals of a little under two hours, in 
order to avoid the penetration of the impurities to the crystalline 
silicon ingot from the pools where impurities are concentrated during the 
long casting operation. For example, some ditches are carved in the upper 
edge of the crucible and stoppers are put on it in a normal casting 
operation. These stoppers are removed in case of need, and some part of 
the melt in the pool in which the impurities is concentrated is removed. 
In this operation, the melt is effectively over-flowed by moving up the 
cast ingot slowly. It is preferable to conduct this moving up operation at 
intervals of a little under two hours. 
According to this embodiment, production cost of the crystalline silicon 
ingot can be drastically reduced, for example, about one fourth of the 
cost of the conventional method, because a high quality ingot can be cast 
effectively with this method. 
In this embodiment, the crystalline silicon ingot is heated from the side 
and just under the crucible, to avoid the crystalline silicon ingot from 
crack generation. The range of preferable heating temperature is from 1000 
deg. C. to 1350 g. C. It is well known that the critical cooling speed to 
avoid cracks during the casting of silicon is less than 15 deg. C./min. In 
this embodiment, the ingot should be heated in the range above mentioned, 
because the speed of precipitating ingot is in the range of from 1 mm/min. 
to 8 mm/min. Usually, an the ingot retraction system is controlled by 
lifting the ingot up and down, and this method may cause the generation of 
stress which may originate cracks and break down the ingot. In order to 
avoid such problem, an ingot including the starting block should be heavy 
enough to be precipitated by the weight of itself and the whole ingot 
should not be fixed in the retraction unit on which the ingot is placed. 
This retraction unit can move up and down. The speed of precipitating the 
ingot is in the range of from 1 mm/min. to 3 mm/min. This data is based on 
the embodiment of WICP method and cold crucible method. The preferable 
weight of the starting ingot should be more than 200 kg. It is also 
possible to precipitate the ingot with rotating. In this case, the shape 
of the cross section of the crucible should be round. The rotating speed 
is about 5 rpm, while the diameter of the ingot is 76 cm and the weight is 
about 1000 kg. If the rotation speed may exceed more than 5 rpm, the 
mechanical stress may be increased and cause cracks and break the ingot. 
The starting block should also be made from the finished product prior to 
the operation, in order to start the ingot precipitating smoothly. Also in 
order to avoid the ingot from sticking or hanging with the crucible, the 
upper edge of the crucible should be free from the frozen splash by 
programmed electron beam bombarding. 
Once a silicon melt is contaminated with iron (Fe), Fe can not be removed 
during the EB melting. The charging means such as drum screw conveyer or 
chute which silicon material passes through before charged into the cold 
hearth, is usually made of steel bearing materials like stainless steel. 
It is a preferable countermeasure to protect the silicon material from the 
contamination of Fe, to coat the surface of the charging means which is 
directly contacted with the silicon material, with quartz or silicon 
nitride. 
(Embodiment 2) 
The apparatus of this invention is described with FIG. 1-FIG. 3. 
In FIG. 1, the lower part of the chute 3 of the vacuum tighten screw 
conveyer 2 is located on an upper side of cold hearth 1. The surface of 
the screw conveyer 2 and chute 3 is coated with quartz or silicon nitride 
in order to prevent the silicon material which passes through them from 
the contamination of Fe. Above-mentioned cold hearth 1 is a vessel made of 
water cooled copper with rectangular shape, and the water cooled tube 4 is 
installed at right angles to the moving direction of the silicon melt 5 
(as shown by an arrow 6) on the center of the cold hearth 1. This water 
cooled tube 4 is the preventing means for the impurities on the surface of 
the silicon melt. Each end of the tube 4 is fixed on the wall of the cold 
hearth 1 (see FIG. 2), and bottom part of the tube 4 is dipped in the 
silicon melt 5 (see FIG. 1). The groove 7 is set up on the other upper 
side of above-mentioned cold hearth 1 and connected to a side of the 
crucible 8. The pool levels of the cold hearth 1 and the crucible 8 are 
the same. If the pool level of the cold hearth 1 is going up, the silicon 
melt is flowing through the groove 7 toward the crucible 8 as shown by an 
arrow 9 and the silicon melt 5 is moving at a very slow speed according to 
the difference in the pool levels. The above mentioned crucible 8 is a 
rectangular shape shown from the top view, and made of water cooled copper 
wall 10 as shown in FIG. 2. There are ditches 11, 11a, on side walls 10a, 
10b, respectively, and detachable stoppers 12, 12a are installed on the 
ditches. The cross section area of the crucible 8 is designed in more than 
4700 cm.sup.2. A resistant heater assembly made of three sets of resistant 
heating elements, 13a, 13b, 13c, from the top, is installed just under the 
water cooled copper wall 10. The pipe set 17 of helium gas is installed 
between the above mentioned three sets of resistant heating elements, 13a, 
13b, 13c and the water cooled copper wall 10. Helium gas is introduced 
through this pipe 17 and blown against the whole surface around a solid 
chilled layer 36 which is generated at the surface of crystalline silicon 
ingot just under the crucible for the purpose of promoting the crystal 
growth and improving the thermal conductivity under the crucible. The 
bottom plate 15 with ingot retraction system is installed under the 
crucible 8 and the starting block 14a for withdrawing the silicon ingot 14 
is put on the bottom plate 15. Above mentioned bottom plate 15 is water 
cooled and mirror-polished on the surface for a good thermal conductivity 
which can absorb a heat from the starting block 14a. The bottom of the 
starting block 14a is also mirror-polished for the same reason. The 
electron beam guns 16a, 16 are installed above the cold hearth 1 and the 
crucible 8, respectively. The type of the above mentioned electron beam 
guns 16a, 16 should be a pierce type because of the good reliability for 
high power and high frequency oscillation. The upper space of the above 
mentioned cold hearth 1 and the crucible 8 is controlled to maintain 
vacuum and at least, the silicon melt can not be exposed in the air. The 
method and the apparatus for the above mentioned controlling in vacuum are 
publicly well known but the mechanical pump set which may generate some 
vibration should be installed in a distant place and several 
countermeasure means should be introduced to protect the whole melting 
vessel from the vibration for the purpose of maintaining the stability of 
the production of crystalline silicon ingot. 
The embodiment of the use of this apparatus will now be described in 
detail. The cylindrical screw conveyer 2 is started and the silicon 
material 20 is charged in a constant amount from the raw material hopper 
(not illustrated in the FIG. 1) to the direction indicated as arrow 18, 
and charged in a side of cold hearth 1 through the chute 3 to the 
direction indicated as arrow 19. In the meantime, the electron beam gun 
16a is installed above the cold hearth 1 and the beam emitted from the 
electron beam gun 16a is scanning over the silicon material on the cold 
hearth 1. Then, the silicon material 20 is melted down successively by 
bombarding of the electron beam as shown by an arrow 21 and the melt is 
collected in the cold hearth 1. The pool level 5a of the silicon melt 5 
collected in the cold hearth 1 goes up higher than the bottom of the 
groove 7, then the melt flows to the crucible 8 as the direction shown in 
arrow 9. The melt going into the crucible 8 has no chance of pouring out 
from the crucible because the starting block 14a is already set in the 
lower part of the crucible 8 prior to the melting operation. The silicon 
melt 5 which entered into the crucible 8 is cooled down by the water 
cooled copper wall 10 of the crucible 8 and crystallized successively, and 
then the bottom plate 15 is withdrawn down to the direction indicated as 
arrow 36 with the growth of the crystalline silicon ingot. The bottom 
plate 15 is withdrawn down in a very slow speed such as 1 mm/min to 3 
mm/min with the weight of the formed ingot itself so that there is no 
possibility that the crystalline silicon ingot is pulled by the bottom 
plate 15 and cracks or inner stress are generated in the crystalline 
silicon ingot. Three sets of resistant heating elements, 13a, 13b, 13c are 
installed just under and around the crucible 8 with sufficient space, and 
the crystalline silicon ingot is heated by these heating element, for 
example, in the range of from 1000 deg. C. to 1350 deg. C., to avoid a 
rapid solidification of silicon. Helium gas is introduced into the space 
22 between the above mentioned three sets of resistant heating elements, 
13a, 13b, 13c and the water cooled copper wall 10 shown in FIG. 3, for the 
purpose of improving the thermal conductivity under the crucible and 
giving a good influence to the growth of the crystalline silicon. 
As mentioned above, while the silicon melt 5 is flowing from the cold 
hearth 1, the impurities floating on the surface of the silicon melt 5 are 
removed by the water cooled tube 4. And during the long operation of the 
crystal growth, a lot of the impurities come out and float on the surface 
of the silicon melt 5 of the crucible 8. In such a case, these impurities 
are removed by discharging some part of the melt in the pool over the 
crucible by taking detachable stoppers 12, 12a from the ditches 11, 11a. 
The bottom plate 15 is lifted at intervals of a little under two hours in 
a very slow speed, and consequently, the melting part becomes shallow and 
it becomes easier to discharge some part of the melt. It is also effective 
for the control of the ingot formation with the volume expansion during 
solidification that the silicon melt 5 from the cold hearth is maintained 
in the shape of upside-down cone 23 shown in FIG. 1, by increasing the 
energy density in the center part of the melt by properly programming the 
scanning speed of the electron beam gun 16 which is installed above the 
crucible 8. 
(Embodiment 3) 
In this invention, the impurities are concentrated in the melt of the 
crucible 8 , and then some part of the melt in the pool is discharged over 
the crucible in a certain interval which has been investigated, for 
example at intervals of a little under two hours. In this case, the 
ditches 11, 11a, are provided on the upper rim of the crucible 8 and the 
detachable stoppers made of water cooled copper 12, 12a are installed on 
the ditches or solidified silicon lump is provided on the ditches 
beforehand, and detachable stoppers 12, 12a are put off mechanically, or 
the solidified silicon lump is removed by the electron beam bombarding, 
and then some part of the melt in the pool, which has a high concentration 
of impurities, is discharged from the ditches 11, 11a as shown in FIG. 2 
and FIG. 4. In this time, the bottom plate 15 is lifted in a very slow 
speed, and consequently, the pool level of the melt in which impurities 
are concentrated goes up (as shown by an arrow 43 in FIG. 1) and the melt 
is discharged effectively. 
Being shown in the graph a of FIG. 5, Fe content is constantly less than 20 
ppbw even in the point which is 90 cm distant from the bottom of the 
crystalline silicon, because the melt in which impurities are concentrated 
is discharged at the point (1) and (2) , respectively. On the contrary, in 
the case that there is no discharging operation, being shown in the graph 
b of FIG. 5, Fe content is increasing suddenly and Fe content is more than 
80 ppbw in the point which is 60 cm distant from the bottom of the 
crystalline silicon. Fe content in the crystalline silicon ingot, which 
may be applied for the silicon device of a solar cell, should be lower 
constantly. So, it becomes clear that if the ingot has been processed 
without the discharging operation, only the part of the crystalline 
silicon ingot whose distance from the bottom of the crystalline silicon 
ingot is less than 40 cm should be available, and this fact may have a 
strong influence on the yield of the crystalline silicon ingot. 
(Embodiment 4) 
As shown in FIG. 4, the electron beam is programmed to repeat to repeatly 
move slowly to the direction indicated by an arrow 38 and let the 
impurities floating on the melt stay in the back of the cold hearth 1 for 
the purpose of preventing the impurities from going to the crucible 8. 
Some part of the impurities which is still going through the barrier of 
the cold hearth 1 is concentrated in the center of the melt in the 
crucible 8 for the purpose of preventing the impurities from going into 
the crystalline silicon ingot during casting, by the method that the 
electron beam is programmed to repeat to repeatly move slowly to the 
direction indicated in arrow 39, 39. 
(Embodiment 5) 
Another apparatus of this invention is described with reference to FIG. 6 
and FIG. 7. 
In this embodiment, the crucible 8a whose cross section is round is 
introduced instead of the crucible whose cross section is rectangular in 
embodiment 2 which is mentioned above. The bottom plate 15a with ingot 
retraction system is also equipped with the rotating system(not shown in 
the FIG. 6). This apparatus enables the crystalline silicon ingot to 
precipitate and rotate at the same time. The crucible 8a shown in FIG. 6 
and FIG. 7 is not equipped with the detachable stoppers and the ditches 
such as in the crucible 8 in the embodiment 2. But, it is possible to 
equip with these devices. 
The other parts of the apparatus of this embodiment are the same as those 
of the apparatus of the embodiment 2. So, the explanation about this part 
can be skipped. It is also favorable in this embodiment, for the smooth 
precipitating of the crystalline silicon ingot, that the weight of the 
starting ingot should be more than a certain amount, for example, more 
than 200 kg, and the total surface area of the crucible 8a is designed as 
more than 4,700 cm.sup.2. 
The pool level 5a of the silicon melt 5 collected in the cold hearth 1 goes 
up higher than the bottom of the groove 7, then the melt flows to the 
crucible 8a by the direction shown in arrow 9 according to the difference 
in the pool levels. The silicon melt 5 which entered into the crucible 8a 
is cooled down by the water cooled copper wall 10a of the crucible 8a and 
crystallized successively, and then the bottom plate 15a is withdrawn down 
in a very slow speed with the weight of the formed ingot itself. In this 
time, the bottom plate 15a is rotated by the above-mentioned rotating 
system (not shown in the FIG. 6), and then, the crystallized silicon ingot 
is rotated and withdrawn down in a very slow speed. 
The resulted ingot has a better quality compared with the ingot which is 
cast only by withdrawing down in a very slow speed with the weight of the 
formed ingot itself. 
The rotating speed of the bottom plate 15a is at about 5 rpm, while the 
diameter of the crystallized silicon ingot is 76 cm and the weight is 
about 1000 kg. According to this rotating speed, there is little 
possibility that any stress which may originate cracks is generated in the 
ingot. The rotating speed at about 5 rpm is the upper limit. If the 
rotation speed may exceed more than 5 rpm, the mechanical stress may be 
increased. If the rotating speed is less than this range, the quality of 
the resulted ingot has almost no difference compared with the quality of 
the ingot which is cast only by withdrawing down in a very slow speed with 
the weight of the formed ingot itself. 
As shown in FIG. 7, the electron beam is programmed in a rectangular shape 
37 to repeat to move slowly to the direction indicated in arrow 38 and let 
the impurities floating on the melt stay in the back of the cold hearth 1 
for the purpose of preventing the impurities from going to the crucible 8a 
. Some part of the impurities which still go through the barrier of the 
cold hearth 1 is concentrated in the center of the melt of the crucible 8a 
for the purpose of preventing the impurities from going into the 
crystalline silicon ingot during casting, by the method that the electron 
beam is programmed to repeatedly to move slowly to the direction indicated 
in arrow 39, 39a. 
(Embodiment 6) 
The grain size of the cast ingot is deeply influenced by the grain size of 
the starting block so that the starting block with large grain size should 
be arranged before the casting operation. As shown in FIG. 8, single 
crystal 41 of semiconductor grade made by Czochralski method is put in the 
mold made of highly pure graphite 40 for casting an ingot of large grain 
size, and as shown in FIG. 9, a seed crystal 42 of silicon which is cut 
from a single crystal of semiconductor grade made by Czochralski method, 
for casting an ingot of single crystal is put in the mold made of highly 
pure graphite 40. The whole mold is set in the heater and heated up in a 
vacuum for degassing and maintained in the temperature near about the 
melting point of silicon. Then, silicon melt is poured from cold hearth 1 
into the mold and after the mold is filled up, the mold is lowered 
gradually for the crystal growth. The material of the melt should be a 
silicon of semiconductor grade in consideration of the contamination from 
the mold. As shown in FIG. 8, the bottom of the mold should be fabricated 
in a cone shape in order to assist the orientation of the crystal to go up 
during the crystal growth. As shown in FIG. 9, a channel from the seed 
crystal to the bottom of the mold should be fabricated in small and 
winding, to assist the crystal to go up securely to the orientation during 
the crystal growth, in order to get the single crystal of the same 
orientation as that of the mold, even if the orientation of the seed 
crystal is not the same as the vertical direction of the mold. 
As has been described above about the favorable embodiments of this 
invention, various alternations are of course possible within the spirit 
of this invention.