Method and apparatus for fabricating semiconductor single crystal

This invention provides a method and apparatus for fabricating semiconductor single crystals. By using the method of this invention, the temperature gradient of the single crystal being lifted can be easily controlled. The as-grown defect density can be reduced, and it is possible to manufacture high quality semiconductor single crystals with high oxidation-film breakdown strength. A shield cylinder is used for surrounding the semiconductor single crystal 7 being lifted, the shield cylinder is made to be of the telescopic type and consists of a first shield duct 4, a second shield duct 5, a third shield duct 6. A wire 8 wrapping around a wind-up reel 10 is engaged with the third shield duct 6, and the shield cylinder can be driven to extend or retract by rotating the wind-up reel 10. An ascend and descend rod 3 is connected with the first duct 4, and the shield cylinder can be driven to move upward or downward by lifting or lowering the ascend and descend rod 3. The wind-up reel 10 is driven to retract part of the shield cylinder so that the lapped portion of the shield cylinder keeps a predetermined portion of the semiconductor single crystal 7 being lifted warm, and the temperature gradient of the semiconductor single crystal 7 can be reduced when it passes through the zone whose temperature is within a range from 1000.degree. C. to 1200.degree. C.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The present invention relates to a method and an apparatus for fabricating 
semiconductor single crystals by using the Czochralski Method (the CZ 
method). 
At present, most semiconductor substrates used for fabricating 
semiconductor components are single crystals of silicon with high purity. 
FIG. 8 is a cross-sectional diagram showing a semiconductor single crystal 
fabricating apparatus provided with a shield cylinder surrounding the 
semiconductor single crystal being lifted. As shown in FIG. 8, within the 
main chamber 1, a graphite crucible 18 is disposed upon the upper end of a 
rotary crucible shaft 17 which is able to be driven to extend upward or 
downward. A cylindrical heater 16 and a keep-warm cylinder 19 are disposed 
around the crucible 18. 
Polycrystalline silicon in lumps is put into a quartz crucible 14 which is 
accommodated within the graphite crucible 18, then the polycrystalline 
silicon is heated by the heater 16 to be melted into a melt 20. A seed 
crystal in a seed holder 21 is immersed into the melt 20, and thereafter 
the seed holder 21 is slowly withdrawn and rotated in a direction the same 
as or counter to that of the rotation of the graphite crucible 18 to grow 
a single crystal silicon 7. 
A graphite shield cylinder 22 is suspended and extended to above the melt 
20 within an upper chamber 2 which is connected to the main chamber 1. The 
graphite shield cylinder 22 is engaged with an ascent and descent 
mechanism (not shown) so as to perform an upward or a downward movement 
when intended. The graphite shield cylinder 22 controls the flow of inert 
gas coming from a source above the upper chamber 2 and obstructs heat 
radiation coming from heater 16 and melt 20. By this arrangement, the 
single crystal silicon 7 being lifted can be cooled or kept warm 
throughout the whole temperature zone, thereby expediting the 
crystallization and accordingly enhancing the productivity of the single 
crystal silicon 7. 
The heat radiation coming from the parts within a hot zone (for example, 
the heater 16) toward the single crystal silicon 7 being lifted is 
obstructed by the graphite shield cylinder 22, thus the temperature 
gradients both in radial and axial directions near the solid/liquid 
boundary of the single crystal silicon 7 become large, and this leads to a 
easy crystallization of the single crystal silicon 7. In view of the 
above, it is possible to accelerate the lifting speed of the single 
crystal silicon 7, and the productivity can thus be enhanced. However, it 
is impossible to alter the thickness of the shield cylinder 22 in response 
to the surrounding circumstances within the heating furnace, nor is it 
possible to adjust the execution of cooling or heat obstruction at a 
designated portion of the single crystal silicon 7 being lifted. 
Therefore, the following disadvantages will happen: 
(a) When the single crystal silicon 7 passes through the zone whose 
temperature is within a range between 1000.degree. C. and 1200.degree. C., 
it can not be cooled slowly. As a result, the as-grown defect density can 
not be reduced sufficiently. This will reduce the oxidation-film breakdown 
strength. 
(b) In the operation of melting polycrystalline silicon in the quartz 
crucible 14, an ascent and descent mechanism is used to lift the upper 
portion of the shield cylinder 22 so as to accommodate it within the upper 
chamber 2. By this, interference between the lower end of the shield 
cylinder 22 and the polycrystalline silicon can be avoided. For this 
purpose, an accommodation space is required in the upper chamber 2, and 
the total height of the upper chamber 2 is thus increased. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a semiconductor single 
crystal fabricating apparatus in which a shield cylinder is utilized to 
surround a semiconductor single crystal being lifted by the Czochralski 
Method of fabricating semiconductor single crystals. The shield cylinder 
can be driven to move upward and downward, or to extend and retract in a 
telescopic manner at its own discretion. 
Specifically, in the above-mentioned semiconductor single crystal 
fabricating apparatus, the shield cylinder is divided into a plurality of 
telescopic ducts which can be driven to move relative to each other in a 
telescopic manner. A wire winding around a wind-up reel is engaged with 
the innermost telescopic duct and an up-down rod is connected to the 
outermost telescopic duct, the shield cylinder is thus able to be driven 
to extend or retract by way of the windup reel and able to be driven to 
move upward or downward in response to the upward or downward movement of 
the up-down rod. 
Furthermore, the method for fabricating semiconductor single crystals 
according to this invention is characterized in that : in the 
semiconductor single crystal fabricating apparatus, the wind-up reel can 
actuate the telescopic ducts of the shield cylinder to retract, and 
accordingly to lap one over another. By this, a designated portion of the 
semiconductor single crystal being lifted can be kept warm by the lapped 
telescopic ducts surrounding thereof, and the temperature gradient can 
thus be reduced when the semiconductor single crystal passes through the 
zone whose temperature is within a range from 1000.degree. C. to 
1200.degree.C. 
This invention relates a method and a apparatus for fabricating 
semiconductor single crystals by using the CZ method, in which the heat 
history of semiconductor single crystals can be easily controlled by a 
shield cylinder. In the apparatus for fabricating semiconductor single 
crystals, the shield cylinder is designed to surround the semiconductor 
single crystal being lifted, and the shield cylinder is able to be driven 
to move in a telescopic manner and to move upward or downward integrally 
so as to surround any portion of the semiconductor single crystal being 
lifted with determined thickness at its own discretion. Therefore, the 
heat history of the semiconductor single crystals can be controlled. 
Specifically, the shield cylinder is of a telescopic type, and a wire 
winding around a wind-up reel is engaged with the most inner ducts of the 
shield cylinder. By this arrangement, the shield cylinder extends if the 
wind-up reel releases the wire, and the shield cylinder retracts if the 
wind-up reel winds up the wire. As a result, the thickness in the radial 
direction of the shield cylinder can be adjusted. Furthermore, the shield 
cylinder can move up and down much more, due to the retracting movement of 
the shield cylinder. 
In the operation of fabricating semiconductor single crystals by utilizing 
the apparatus according to this invention, the shield cylinder can be 
extended and therefore the semiconductor single crystal being lifted can 
be shielded to a much greater extent along its longitudinal axis. 
Furthermore, due to the fact that the shield cylinder can be retracted to 
any predetermined length and be moved to any expected height, any portion 
of the semiconductor single crystal being lifted can be kept warm at its 
own discretion. In addition, the portion to be kept warm can also be 
altered by taking its surrounding heat circumstance into consideration. 
Especially, during the body forming process, by driving the wind-up reel 
to retract the expected portion of the shield cylinder, the ducts of the 
shield cylinder become partly lapped, and the effect of keeping warm will 
be enhanced. Therefore, by moving the lapped portion of the shield 
cylinder to shield the portion whose temperature is within a range from 
1000.degree. C. to 1200.degree.C., it is possible to reduce the 
temperature gradient of the semiconductor single crystal passing through 
the above temperature zone. Thus, the formation of as-grown defect will be 
depressed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a simplified sectional diagram showing one embodiment of the 
semiconductor single crystal fabricating apparatus according to the 
present invention. As shown in FIG. 1, an upper chamber 2 is installed 
upon the upper end portion of the main chamber 1, and two up-down rods 3 
are disposed within the upper chamber 2 by means of an ascent and descent 
mechanism (not shown) . A first shield duct 4 is engaged with the lower 
end portions of the two up-down rods 3. As shown in FIG. 2, two 
protrusions 4a are provided on the upper peripheral rim of the first 
shield duct 4 and a flange 4b is formed on the inner peripheral wall of 
the lower end of the first shield duct 4. The first shield duct 4 is 
engaged with the up-down rods 3 by means of the two protrusions 4a. 
Flanges are provided respectively on the upper and lower ends of a second 
shield duct 5 which is disposed within the first shield duct 4 in a manner 
that it can slide without restraint, and a third shield duct 6 having 
flanges on its upper and lower ends is disposed within the second shield 
duct 5 in the same manner. The up-down rods 3, the first duct 4, the 
second duct 5, and the third duct 6 are made of graphite, graphite coated 
with SiC, or metals such as Mo. The combination of the above mentioned 
materials is also acceptable. 
The third shield duct 6 is surrounding the single crystal silicon 7 with a 
predetermined gap between them. The upper end of the third shield duct 6 
is engaging with a wire 8 which is wrapping around a wind-up reel 10 by 
way of a pulley 9. A pulley 11 being coaxial with the wind-up reel 10 is 
disposed above the upper chamber 2. 
FIG. 3 is a cross sectional view along line A--A of FIG. 1. For ease of 
description, in FIG. 1, the up-down rods 3, the wire 8, the pulley 9, and 
the wind-up reel 10 are depicted as being on the same plane. In fact, as 
shown in FIG. 3, the up-down rods 3 and the wire 8 are disposed 
perpendicular to each other. Therefore, they will not interfere with each 
other during operation. A pulley 12 connected with a motor (not shown) is 
installed above the upper chamber 2, the pulley 12 drives the pulley 11 
and accordingly the wind-up reel 10 to rotate by way of a belt 13. 
Furthermore, in FIGS. 1 and 3, only one wire 8 is connecting with the 
third shield duct 6. However, this invention is not limited to such an 
arrangement, it is permissible if two or three wires 8 engage with the 
third duct 6 and each wire 8 wraps on a wind-up reel. It is also 
appropriate if the wind-up reel 10 is directly connected with the drive 
motor. 
If the wind-up reel 10 is driven to wind up the wire 8, then the third 
shield duct 6 is at first lifted, subsequently the second shield duct 5 
and the first shield duct 4 are lifted in order, and the shield ducts 4, 
5, 6 become lapped with each other. If the wind-up reel 10 is driven to 
rotate to release the wire 8, then the third shield duct 6 and the second 
shield duct 5 will go down. After the upper end of the second shield duct 
5 touches and engages with the lower end of the first shield duct 4, the 
third shield duct 6 continues to go down. The third shield duct 6 will 
stop going down when its upper end engages with the lower end of the 
second shield duct 5. Under such a circumstance, the shield cylinder 
extends to its maximum extent. 
The following is the description of the processes of fabricating the 
semiconductor single crystal. The description is following the steps of 
fabricating processes. 
(a) Material Melting Process 
As shown in FIG. 4, polycrystalline silicon 15 in lumps is put into a 
quartz crucible 14, then the polycrystalline silicon 15 is heated to be 
melted by the heater 16. At the same time, the wind-up reel 10 is driven 
to wind up the wire 8 to make the first duct 4, the second duct 5, and the 
third duct 6 lapped with each other. Then, the up-down rods 3 are driven 
to go down properly. By this, the shield ducts 4, 5, 6 can cover the 
polycrystalline silicon 15 and will not interfere with the polycrystalline 
silicon 15, and the quartz crucible 14 can be fixed and heated 
efficiently. The polycrystalline silicon 15 thus can be quickly melted 
into a melt. 
(b) Shoulder Process 
As shown in FIG. 5, the shield ducts 4, 5, 6 are kept lapped and the 
up-down rods 3 are lifted to the utmost location. To effect the lifting of 
the up-down rods 3, the wind-up reel 10 is driven to wind up the wire 8. 
Under this circumstance, radiation from the heater 16 is not obstructed 
and directly reaches the shoulder 7a of the single crystal silicon. 
(c) Body Process 
At the beginning of the body process, just like the melting process, 
measures for preventing heat dissipation through the upper portion of the 
heating furnace are taken and heat radiation coming from heater 16 toward 
the single crystal silicon 7 is not obstructed. For this purpose, the 
shield ducts 4, 5, 6 are kept lapped (see FIG. 6). Under this 
circumstance, it is better to release the wire 8 to lower down the up-down 
rods 3 and move the shield ducts 4, 5, 6 to a location slightly higher 
than the shoulder 7a of the single crystal silicon 7. 
The apparatus of the present invention comprises a shield cylinder having a 
plurality of telescopic ducts in order that the temperature gradient of 
the semiconductor single crystal can be reduced when it passes through the 
zone or region created when these ducts are lowered near the melt surface. 
The Temperature of this zone can thus be maintained within a range from 
1000.degree. C. to 1200.degree. C. 
By lowering the lapped portion of the shield cylinder near the melt 
surface, heat radiation from the melt to the semiconductor single crystal 
being pulled up is prevented and the crystal can be cooled more rapidly 
from its melting point to 1300.degree. C. than by conventional methods. 
Thus the temperature gradient of the crystal can be magnified when it 
passes through the cooling zone which has a temperature within a range 
from the crystal melting point to 1300.degree. C. 
As a result, as shown in FIGS. 9 and 10, the rate of pulling up is higher 
than that of the conventional method and higher productivity can be 
obtained. The method is especially effective at pulling up semiconductor 
single crystals having large diameters (larger than 8 inches) or a long 
length. 
Following the growing of the single crystal silicon 7, the up-down rods 3 
are lifted slowly. The first shield duct 4 rises following the lift of the 
up-down rods 3, and then the second shield duct 5 rises. However, the 
third shield duct 6 remains unmoved. Therefore, as shown in FIG. 1, the 
whole shield cylinder is extended, and the second shield duct 5 and the 
third shield duct 6 are lapped partly to form a cooling region. The lapped 
portion corresponds to a specified portion (in other words, the section 
having a temperature ranging from 1000.degree. C. to 1200.degree. C.) of 
the single crystal silicon 7. Being surrounded and insulated by the second 
shield duct 5 and the third shield duct 6, the specified portion can be 
cooled down slowly in the cooling region, and its temperature gradient is 
smaller than those of other portions. 
(d) Tail Process and Cooling Process 
To form the tail 7c, the wire 8 is wound up and the up-down rods 3 are 
lifted slowly (see FIG. 7) after the temperature of the body 7b drops 
below 1000.degree. C. Due to the fact that the body 7b is surrounded by 
the shield ducts 4, 5, 6 and heat radiation is obstructed, the body 7b is 
cooled down quickly. Following the lifting of the single crystal silicon 
7, the shield ducts 4, 5, 6 are lifted to the utmost location of the 
up-down rods 3. 
(e) Disassembling the Parts of the Furnace 
After lifting the single crystal silicon, if disassembly of the parts of 
the furnace is desired, the wind-up reel 10 is driven to wind up the wire 
8 to lift the up-down rods 3 to their highest location (same as shown in 
FIG. 5 of shoulder process). The disassembling operation will not be 
hindered by the shield ducts 4, 5, 6 and can be accomplished in a swift 
way. 
As described above, the shield cylinder used for surrounding the 
semiconductor single crystal being lifted is made to be a telescopic type 
and can be driven to ascend or descend freely, the following effects can 
be obtained. 
(a) By way of partly retracting the shield cylinder to lap over the shield 
ducts, the effect of keeping warm is enhanced, and the temperature 
gradient of the portion passing through will be thus reduced. By using the 
method of this invention to slowly cool the single crystal having a 
temperature ranging from 1200.degree. C. to 1000.degree. C., it becomes 
easy to reduce the as-grown defect density, and thus it is possible to 
manufacture high-quality semiconductor single crystals with high 
oxidation-film breakdown strength. Furthermore, by lifting or lowering the 
shield cylinder, it is possible to adjust the portion to be cooled down 
slowly in response to the surrounding heat circumstance. 
(b) If the shield cylinder is retracted, the allowance for moving along its 
longitudinal direction will be enlarged. Therefore, in the processes of 
feeding material, forming the shoulder, and disassembling the parts, the 
operation will not be hindered if the shield cylinder is retracted. 
Furthermore, if the retracted shield cylinder is lowered to a location 
near the top of the fed polycrystalline silicon lump, the time needed to 
melt the polycrystalline silicon can be reduced. In addition, electric 
power needed by the heater can be saved and the life time of the parts of 
the furnace also can be elongated.