Manufacturing method and equipment of single silicon crystal

According to the present invention, the inside of a crucible in which a molten raw material is placed is partitioned off with a partition ring so that a pulled single crystal is surrounded and the molten raw material may be moved and granular silicon is supplied to the outside of the partition ring, thereby to form the whole surface of outside molten liquid as a granular silicon soluble region so as to maintain the molten liquid surface on the inside of the partition ring at almost a constant level, and also to set the temperature of the molten liquid on the outside of the partition ring higher than the temperature of the inside thereof at least by 10.degree. C. or higher by covering the partition ring and the molten liquid surface on the outside thereof with a heat keeping board.

In the drawings, reference numerals are denoted as follows: 1 . . . 
crucible; 2 . . . graphite crucible; 4 . . . molten raw material; 5 . . . 
single silicon crystal; 6 . . . heater; 8 . . . chamber; 11 . . . 
partition ring; 12 . . . small hole; 13 . . . raw material feeder; 14, 15 
. . . temperature detectors; 16 . . . granular silicon; 17 . . . heat 
resisting material; 18 . . . heat insulating block; 20 . . . heat keeping 
board; 23 . . . auxiliary heater; 24, induction coil. 
FIG. 1 is a cross-sectional view showing a typical embodiment according to 
the present invention, and FIG. 2 is a cross sectional view thereof taken 
along the line A--A. In these Figures, the reference numeral 1 denotes a 
quartz crucible and is set in a graphite crucible 2. The graphite crucible 
2 is supported movably up and down and rotatably on a pedestal 3. The 
reference numeral 4 represents molten raw materials of silicon placed in 
the crucible 1, from which a single silicon crystal 5 which is upbrought 
in a columnar form of a diameter between 12 Cm-30 Cm is pulled. 6 denotes 
a heater surrounding the graphite crucible 2, 7 denotes a hot zone heat 
insulating material surrounding this heater 6, and these are contained in 
a chamber 8. Such a structure is basically the same as a single silicon 
crystal pulling equipment by the conventional Czochralski method. 
The reference numeral 11 denotes a partition ring composed of fused quartz 
of high purity and disposed concentrically with the crucible 1. A 
plurality of small holes 12 are disposed penetrating through an area below 
the nearly central portion in the height direction (FIG. 3). 
In the case of this figure, the number of holes is 34. A larger number of 
holes may contribute to more stable crystal growth, as molten silicon from 
the raw materials melting area is mixed better with molten silicon in the 
crystal pulling area. However, as it is troublesome to make many holes; 
practically, the maximum number is 34 in this instance. 
This partition ring 11 is set in the crucible 1 together with the silicon 
raw materials when it is charged and disposed in the molten liquid 4 so as 
to surround the single silicon crystal 5 after the raw material is molten, 
and the upper edge portion thereof is exposed slightly above the molten 
liquid surface. Also, the lower edge portion presents a state of almost 
fusing and sticking to the crucible 1, and does not float off. Therefore, 
the outside and the inside of the partition ring 11 may be partitioned off 
completely. Accordingly, the molten liquid outside the partition ring can 
move quietly inward only through the small holes 12. This is one of the 
important requirements for getting temperature difference between the 
outside and the inside of the partition to greater than 10.degree. C. If 
there are comings and goings of liquid between the outside and the inside, 
which results from too large opening between both areas, accomplishment of 
the temperature difference becomes more difficult. 
The reference numeral 9 is an opening portion provided on the chamber 8 
corresponding to the molten liquid surface outside the partition ring 11. 
To this opening portion 9 is inserted and fixed a feeder 13 for small 
chunk or granule of silicon (hereinafter referred to just as granular 
silicon), and the exit of the feeder 13 is opposed to the molten liquid 
surface outside the partition ring 11. This feeder 13 is coupled with a 
raw materials feeding chamber (not shown) provided outside the chamber 8, 
and feeds granular raw materials continuously. 
Reference numerals 14 and 15 denote temperature detectors such as a 
radiation thermometer disposed in the upper part of the chamber 8. One 
temperature detector 14 measures the temperature of the molten liquid 
surface outside the partition ring 11 and the other temperature detector 
15 measures the temperature of the molten liquid surface of the inside 
thereof, respectively. 
The reference numeral 20 denotes a heat keeping board which consists of a 
flange portion 20a and a funnel-shaped shielding portion 20b. The board 20 
is made of graphite plate of which underside or whole surface is covered 
with a quartz plate having a thickness of 1 mm, or a plate material 
prevented from contamination by coating the surface thereof with SiC and 
Si.sub.3 N.sub.4 of high purity. The flange portion 20a is supported by a 
hot zone heat insulating material 7 and is set so as to cover the 
partition ring 11 and the wall surface of the crucible 1. This heat 
keeping board 20 prevents the molten liquid from solidification which 
generates from a part of the partition ring 11 exposed from the molten 
liquid surface and grows inwards, i.e. into crystal pulling area 4. For 
this purpose, the bottom portion (inner peripheral portion) thereof should 
be disposed close to the molten liquid surface (approximately 10 mm in the 
embodiment). The reference numeral 21 represents a hole provided 
correspondingly to the visual field of the temperature detector 14, and 22 
represents a hole provided on the feeding path of granular silicon 16. 
In the present invention thus constructed, the molten raw molten raw 
materials 4 is placed inside and outside the partition ring 11 immersed in 
the crucible 1, and both molten surfaces are maintained on the same level. 
When the seed crystal is pulled gradually while rotating after the contact 
with the inside molten liquid surface, the columnar single silicon crystal 
5 is obtained. In the interim, granular silicon 16 is fed onto the molten 
surface outside the partition ring 11 and this granular silicon 16 is made 
molten by the molten liquid outside the partition ring 11 and moves 
inwards through small holes 12 of the partition ring 11, thus maintaining 
the liquid surface level of the molten raw materials 4 always constant 
Besides, the reason why the lower end portion of the raw materials feeder 
13 is positioned above the molten liquid surface so that granular silicon 
drops onto the molten liquid surface is that granular silicon floats on 
the molten liquid surface in the whole area outside the partition ring 11 
so as to be molten in this whole area. If the feeder 13 is immersed in the 
molten liquid, the melting region of granular silicon is limited to the 
inside of the feeding pipe. Therefore, heat transfer from the molten 
liquid to granular silicon becomes insufficient, causing it to be 
impossible to melt the granular silicon continuously. 
In the present invention described above, when the partition ring 11 is 
exposed excessively above the molten liquid surface, solidification of the 
molten liquid will occur at this part due to big heat radiation effect. 
Accordingly, the exposure is required to meet the above-mentioned 
conditions, and the height of the exposed portion has been set within 5 mm 
in the embodiment. 
The height of exposed portion of the partition 11, however, should be made 
higher than the above value in spite of the deteriorative effect of 
solidification, when the granular silicon is fed without speed reduction 
or large diameter crystal is grown at high pulling rate. The granular 
silicon which drops with high speed hits the surface of molten silicon 
very strongly, making many splashes of molten silicon. The splashes often 
jump beyond the partition 11 and disturb crystal growth. In high speed 
crystal growth where crystal 5 is pulled at a rate higher than 0.7 mm/min, 
granular silicon is fed at a greater rate. The greater feed rate 
frequently causes the granular silicon to jump beyond the partition 11. 
From the jump resulted is such a phenomenon that granular silicon drops 
on, and is repulsed by, other granular silicon floating on the surface of 
molten silicon. Therefore, the more the feed rate of the raw materials 
increases, the more the rat of jumps increase. This jump of granular 
silicon also disturbs the crystal growth. In this case, the height of 5 mm 
of the exposed portion of partition is insufficient to prevent the 
granular silicon from splashing and jumping into the crystal pulling area. 
As shown FIG. 10, the partition 30 is required to be exposed above molten 
silicon surface by a height larger than 3 cm to prevent the jumping. The 
maximum of the exposed height is 40 cm. The crystal pulling apparatus in 
the present invention cannot be equipped with a partition having an 
exposed height larger than 40 cm, because the partition 30 is obstructed 
by the heat keeping board 31. As the partition is high as mentioned above, 
the shape of heat keeping board becomes as shown in FIG. 11, in order to 
be more effective in suppressing heat radiation from the partition 30. 
That is, the heat keeping board 31 is composed of the cylindrical portion 
32 and the flange portion 33. Heat radiation from the partition 30 is 
mainly suppressed by the cylindrical portion 32. Appropriate distance 
between the cylindrical portion 32 and the surface of molten silicon is 
from 5 mm to 60 mm. A distance larger than 5 mm is recommended for 
perfectly preventing the bottom edge of the cylindrical portion 32 from 
touching the surface of molten silicon. The upper limit 60 mm is 
determined from the view point of heat keeping effect. The heat keeping 
effect decreases with increase of the distance between the cylindrical 
portion 332 and the surface of molten silicon. Required heat keeping 
effect cannot be obtained for the distance larger than 60 mm. As materials 
for the cylindrical portion 32, metal is most suitable because of its low 
emissibility and surface cleanliness. Molybdenum and tantalum are used, as 
they have high strength at high temperature and good formability. A good 
heat keeping effect of the metal results from its low emissibility. 
Sometimes windows 34 are made on the cylindrical portion 32 in order to 
adjust the thermal circumstances for the crystal 5 (FIG. 11). 
Furthermore, the diameter of small holes 12 provided on the partition ring 
11 is set within 5 mm. This size should be small enough for the liquid in 
the crystal growing area not to flow back into the raw materials melting 
area. The minimum diameter of the holes is l mm, because it is difficult 
to make holes having a diameter smaller than this size. It is desired that 
the number of holes is more than one so that fluctuation of liquid 
temperature in the crystal pulling area is as small as possible. If a 
localized hotter flow of silicon liquid comes in from a single hole, it 
will cause a large temperature fluctuation in the silicon melt. This is 
unfavorable for growing a single crystal with a large diameter. The 
position of the holes in the depth direction is also selected in the area 
lower than the central part of the immersed portion as described 
previously so as to be kept away from the solidification front of the 
single silicon crystal 5. Besides, it is desirable that the diameter of 
the partition ring 11, 30 is twice or more of the diameter of the pulled 
single silicon crystal 5. 
Furthermore, as the result of an experiment, it has been found that the 
temperature of the molten liquid surface outside the partition ring 11, 30 
has to be higher than the temperature of the molten liquid surface inside 
it by at least 10.degree. C., preferably 20.degree. C., in order to pull a 
sound single silicon crystal 5 while melting the granular silicon 16 which 
is fed continuously. In order to get this temperature difference, the heat 
radiation from the molten liquid surface outside the partition ring 11, 30 
is depressed by the heat keeping board 20,30. The temperatures of both 
molten liquid surfaces are detected by temperature detectors 14 and 15, 
which ensures to keep the temperature difference. 
The maximum temperature difference obtainable by this invention is 
estimated to be 100.degree. according to heat transfer calculations Any 
higher temperature of silicon liquid outside the partition results in a 
larger temperature fluctuation in crystal pulling area and in acceleration 
of erosion of the quartz crucible and the partition by liquid silicon. 
Therefore, the temperature difference is desired to be less than 
100.degree. C. from a practical viewpoint as well as the above 
calculation. 
FIG. 4 is a longitudinal cross-sectional view showing typically another 
embodiment of the invention. In this embodiment, in order to maintain the 
temperature of molten liquid surfaces inside and outside the 
above-mentioned partition ring 11 more surely, heat resisting materials 17 
of high purity having low thermal conductivity, such as Si.sub.3 N.sub.4, 
are disposed at locations of the graphite crucible 2 above which the 
partition ring 11 is set in the crucible, thereby intercepting thermal 
conduction from the outer periphery of the graphite crucible 2 adjacent to 
the heater 6 to the inside thereof. 
Also, in order to reduce heat inflow from the bottom portion of the 
graphite crucible 2, there is provided, for example, a shielding block 
made of graphite underneath the graphite crucible 2 so as to reduce direct 
heat from the heater 6. Besides, if the heat insulating block 8 is made 
hollow and a carbon heater, for example, is provided inside thereof, the 
temperature at the bottom portion of the graphite crucible may be 
controlled more minutely. 
By means of this embodiment thus constructed, it is possible to control the 
temperature of the molten liquid surfaces inside and outside the partition 
ring 11 more surely, thereby maintaining the desired temperature 
difference. 
FIG. 5 is a longitudinal cross-sectional view showing typically still 
another embodiment of the invention. In the Figure, a reference numeral 23 
denotes an auxiliary heater having a ring form in general, which is 
disposed not in the area just under the raw materials feeder 13 and the 
visual field areas of temperature detectors 14 and 15 as shown in FIG. 7. 
This auxiliary heater 23 consists of a carbon heating element for 
instance, and is surrounded by quartz of high purity so as to prevent the 
single silicon crystal 5 from contamination with impurity. 
According to the present invention, the auxiliary heater 23 is disposed on 
the molten liquid surface outside the partition ring 11 and the inner 
periphery thereof is surrounded with the heat keeping board 20. Therefore, 
the temperature of the molten liquid surfaces inside and outside the 
partition ring 11 may not only be maintained at a desired value, but also 
may be controlled by the measurement of temperature detectors 14 and 15. 
FIG. 8 shows another embodiment of the above-mentioned invention, wherein a 
high frequency induction heating system is adopted as the auxiliary 
heater. In this embodiment, a plurality of induction coils 24 formed in a 
spiral are disposed on the molten liquid surface outside the partition 
ring 11 and high frequency current (100 KHz in the embodiment) is applied 
to these induction coils 24, thereby heating directly the molten liquid 
surface. In this case, granular silicon 16 is fed from the raw materials 
feeder 3 through the gap at the central portion of induction coils 24. 
Besides, a case in which a plurality of small-sized induction coils 24 are 
disposed on the molten liquid surface has been described in the 
above-mentioned embodiment, but one piece of induction coils in a spiral 
form corresponding to the size of the raw materials melting area outside 
the partition ring 11 may be disposed instead. 
In each of the embodiments described above, a case wherein one set of 
feeders 13 for feeding granular silicon 16 onto the molten liquid surface 
outside the partition ring 11 has described, but 2 sets or more of feeders 
may be provided. 
Also, each embodiment may be executed independently or in appropriate 
combination of embodiments. 
The fed silicon raw materials 16 contains a doping material in the quantity 
which is equal to that in the pulled single crystal 5, of which 
explanation has been omitted in the method of pulling a single silicon 
crystal because the process is executed necessarily. Practically this 
doping is carried out by mixing pure silicon granule which is raw 
materials and highly doped piece of silicon. Also, it has been confirmed 
that the present invention may be executed perfectly even when a magnetic 
field is applied to the molten liquid from the outside of the chamber 8. 
As is apparent from the foregoing, the present invention is constructed in 
such a manner that granular silicon is fed to the outside molten liquid 
surface while preventing the wave pattern from propagating to the inside 
of the partition ring by dividing the crucible inside and outside with a 
partition ring and covering the partition ring and the wall surface of the 
crucible with a heat keeping board, the fed raw material is molten and 
moved inside so as to maintain the liquid surface of the molten raw 
materials at a constant level, and the temperature of the outside molten 
surface is higher than the temperature of the inside molten surface, thus 
enabling to pull a sound single silicon crystal. As the result, 
improvement of the yield and improvement of productivity may be realized 
by uniformalizing the quality in the pulling direction, which is very 
effectual in execution.