Magnetic field-applied fabrication method for a semiconductor single crystal and an apparatus therefor

A method for fabricating a semiconductor single crystal by the MCZ method by which it is possible to pull large diameter and heavy semiconductor single crystals without breaking the contraction portion, is provided. In the contracting step, change the shape of the crystal growth interface by making the range of the temperature fluctuation caused by convection in the vicinity of the melt surface more than 5.degree. C. so as to eliminate the dislocation in the contracted portion. When a transverse magnetic field is applied by magnets 6,6, the magnetic field intensity is set below 2000 Gauss to properly change the shape of the crystal growth interface to form the contracted portion 10. Thus,even though the diameter of the contracted portion 10 is larger than normal, free dislocation is achieved. After the dislocation is eliminated, the magnetic field intensity is recovered and shoulder 11 is formed. When a cusp magnetic field is applied, the contracting is performed when the magnetic field intensity of one of the upper-and-lower magnets being increased while the magnetic field intensity of another magnet is decreased or the upper-and-lower magnets are moved in a vertical direction to make the vicinity of the melt surface similar to a longitudinal magnetic field. After dislocation is eliminated, the magnetic field intensity of the upper-and-lower magnets or the position of the magnets is recovered.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic field-applied fabrication 
method for a semiconductor single crystal and an apparatus therefor, and 
in particular to a fabrication method which is possible to pull heavy 
semiconductor single crystal without dislocation even though the diameter 
of the contracted portion of the semiconductor single crystal is large. 
2. Description of the Prior Art 
The magnetic field-applied Czochralski crystal growth method (hereinafter 
called the MCZ method) is a method by which silicon single crystals are 
grown by the Czochralski method (hereinafter called the CZ method) while a 
magnetic field is applied to the melt by magnets surrounding a 
semiconductor single crystal fabrication apparatus to increase the dynamic 
viscosity coefficient of the melt. As the convection of the melt is 
inhibited by the action of the magnetic field, the temperature fluctuation 
near the melt surface is reduced, and thus it is possible to grow silicon 
single crystals stably. Moreover, as the reaction between melt and quartz 
crucible (SiO.sub.2) is inhibited or promoted, it is an effective method 
for controlling the oxygen concentration of the silicon single crystals. 
The pulling of silicon single crystals by the MCZ method like that of the 
CZ method, includes immersing seed crystals mounted on seed chucks in the 
melt of a polycrystal silicon, followed by pulling the seed chucks and 
then growing the silicon single crystals. As dislocations occur in the 
seed crystal due to the heat impact imparted thereto during the dipping of 
it in the melt, the silicon single crystal is first grown thinner and 
longer in the contracting step to prevent the dislocation, then the 
diameter of the silicon single crystal is enlarged to a predetermined size 
in the shoulder-forming step, and finally the silicon single crystal is 
delivered to the body-forming step. The magnetic field intensity during 
each steps is maintained at a predetermined value. 
However, in the fabrication of silicon single crystal by the MCZ method, 
the convection of the melt is inhibited by the application of the magnetic 
field, resulting in a reduction of the fluctuation of the temperature near 
the melt surface. This stabilize the solid-liquid interface, and thus the 
dislocation in the seed crystals can not be eliminated in the right and 
left directions and thus remains in the interior of the silicon single 
crystal. Accordingly, in order to completely eliminate any dislocation, 
when the MCZ method is used for pulling silicon single crystals, the 
diameter of the contracted portion must be further contracted until the 
dislocation is eliminated. In the fabrication of a silicon single crystal 
by the CZ method, the weight of the pulled silicon single crystal is 
limited by the diameter of the contracted portion. If the weight exceeds 
the limit, the contracted portion breaks, and there is the danger that the 
single crystal may drop. The possibility of the single crystal dropping is 
higher when using the MCZ method than when using the CZ method. Moreover, 
with the enlargement of silicon single crystals in recent years, the 
weight thereof increases, and thus it is more difficult to pull 
large-diameter silicon crystals by the MCZ method. 
SUMMARY OF THE INVENTION 
In view of the above mentioned problems in the prior art, an object of the 
invention is to provide a method for fabricating a semiconductor single 
crystal by the MCZ method by which it is possible to safely pull 
large-diameter and heavy semiconductor single crystals without breaking 
the contracted portion. 
In order to attain the above object, the method for fabricating a 
semiconductor single crystal of the present invention is characterized by, 
in the contracting step prior to the shoulder-forming step, changing the 
shape of the crystal growth interface by keeping the range of the 
temperature fluctuation caused by convection in the vicinity of the melt 
surface, more than 5.degree. C. so as to eliminate the dislocation. 
In the fabrication of the semiconductor single crystal, when a transversal 
magnetic field is applied, the contracting is performed with a magnetic 
field intensity reduced to less than 2000 Gauss, and after dislocation is 
eliminated, the magnetic field intensity is returned to the original 
intensity and the semiconductor single crystal is pulled. Moreover, when a 
cusp magnetic field is applied, the contracting is performed by increasing 
the magnetic field intensity of one of the upper-and-lower magnets which 
form the cusp magnetic field while reducing the magnetic field intensity 
of other magnet or the upper-and-lower magnets are moved in a vertical 
direction to make the vicinity of the melt surface similar to a 
longitudinal magnetic field, and after dislocation is eliminated, the 
magnetic field intensity of the upperand-lower magnets or the position of 
the magnets is returned to the original state and the semiconductor single 
crystal is pulled. 
A first method of the present invention shows, magnetic field applied 
method of fabricating a semiconductor single crystal, comprising steps of: 
immersing a seed crystal in a melt; forming a contracted portion by 
pulling up the seed crystal at a predetermined velocity so as to have a 
small diameter while applying a magnetic field to the melt; forming a 
shoulder portion by pulling up the seed crystal so as to have a diameter 
grown larger gradually while applying the magnetic field to the melt; and 
forming a straight cylindrical portion by pulling up the seed crystal so 
as to have a same diameter longitudinally while applying the magnetic 
field to the melt; wherein the step of forming a contracted portion is 
performed in the state of changing the shape of the crystal growth 
interface by keeping the range of the temperature fluctuation caused by 
convection in the vicinity of the melt surface more than 5.degree. C. so 
as to eliminate dislocation in the contracted portion. 
The second method of the present invention shows, that the step of forming 
the contracted portion is performed while applying a transversal magnetic 
field having intensity below 2000 Gauss, and the step of forming the 
shoulder portion is performed while applying the transversal magnetic 
field having a predetermined intensity more than that in the step of 
forming the contracted portion. 
The third method of the present invention shows, that the magnetic field is 
a cusp magnetic field formed by upperand-lower magnets and the step of 
forming the contracted portion is performed while increasing a magnetic 
field intensity of one of the upper-and-lower magnets and decreasing a 
magnetic field intensity of the other of the upper-and-lower magnets so as 
to make the vicinity of the melt surface similar to a longitudinal 
magnetic field, and the step of forming the shoulder portion is performed 
while applying the cusp magnetic field having a predetermined intensity. 
The forth method of the present invention shows, that the magnetic field 
is a cusp magnetic field formed by upper-and-lower magnets and the step of 
forming the contracted portion is performed while moving the 
upper-and-lower magnets in a vertical direction so as to make the vicinity 
of the melt surface similar to a longitudinal magnetic field, and the step 
of forming the shoulder portion is performed after recovering the position 
of the upper-and-lower magnets. 
The fifth of the present invention shows an apparatus for fabricating a 
semiconductor single crystal, comprising: a crucible filled with a raw 
material; a heater for melting the raw material to a melt; a magnetic 
means for applying a magnetic field to the melt in the crucible; a lifting 
means for lifting a seed crystal immersed in the melt; and for controlling 
the magnetic means and changing the magnetic filed applied to the melt 
with syncronizing the lifting means; wherein in the step of forming a 
contracted portion, the magnet controlling means is controllable to keep 
the range of the temperature fluctuation caused by convection in the 
vicinity of the melt surface, more than 5.degree. C., thereby eliminate 
dislocation in the contracted portion. 
The sixth apparatus of the present invention shows that the controlling 
means controls the step of forming the contracted portion to be performed 
while applying a transversal magnetic field having intensity below 2000 
Gauss, and the step of forming the shoulder portion is performed while 
applying the transversal magnetic field having a predetermined intensity 
more than that in the step of forming the contracted portion. 
The seventh apparatus of the present invention shows that the magnetic 
field is a cusp magnetic field formed by upper-and-lower magnets and the 
step of forming the contracted portion is performed while increasing a 
magnetic field intensity of one of the upper-and-lower magnets and 
decreasing a magnetic field intensity of the other of the upper-and-lower 
magnets so as to make the vicinity of the melt surface similar to a 
longitudinal magnetic field, and the step of forming the shoulder portion 
is performed while applying the cusp magnetic field having a predetermined 
intensity. 
The eighth apparatus of the present invention shows that the magnetic field 
is a cusp magnetic field formed by upperand-lower magnets and the step of 
forming the contracted portion is performed while moving the 
upper-and-lower magnets in a vertical direction so as to make the vicinity 
of the melt surface similar to a longitudinal magnetic field, and the step 
of forming the shoulder portion is performed after recovering the position 
of the upper-and-lower magnets.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention relates to a fabrication method for semiconductor single 
crystal by which when a large- diameter, heavy semiconductor single 
crystal is pulled by the MCZ method, the contracted portion does not break 
and propagation of dislocation in seed crystals can be prevented. 
According to the invention, in the contracting step of the fabrication of 
semiconductor single crystal by the MCZ method, as the temperature 
fluctuation in the vicinity of the melt surface is properly generated to 
change the shape of the crystal growth interface, dislocation is thus 
easily eliminated at the peripheral surface of the contracted portion. 
Accordingly, even though the contracted portion has a diameter that is 
suitable for pulling heavy semiconductor single crystal, dislocation is 
eliminated. 
When a transversal magnetic field is applied in the fabrication of a 
semiconductor single crystal, in the contracting step, by setting the 
magnetic field intensity below 2000 Gauss, the shape of the crystal growth 
interface is properly changed. Accordingly, even though the diameter of 
the contracted portion is larger than normal, dislocation is eliminated. 
Moreover, when a cusp magnetic field is applied in the fabrication of 
semiconductor single crystal, in contracting step, by changing the 
magnetic field intensity of the up-and-low magnets or changing the 
positions of the up-and-low magnets, in the vicinity of the melt surface a 
magnetic field similar to a longitudinal magnetic field is formed, and 
thus the shape of the crystal growth interface is properly changed. 
Accordingly, even though the diameter of the contraction portion is larger 
than normal, dislocation is eliminated. 
Reference is made to FIG. 1 which is a sectional schematic view of the main 
portions of a transversal magnetic field applied fabrication apparatus for 
a semiconductor single crystal. This fabrication apparatus for a 
semiconductor single crystal is the same as the normal apparatus for 
fabricating a semiconductor single crystal by the CZ method. A graphite 
crucible 2 is rotatably and elevatedly mounted in the main chamber 1, and 
a quartz crucible 3 is received in the graphite crucible 2. The periphery 
of the graphite crucible 3 is provided with a cylindrical heater 4 and an 
adiabatic cylinder 5. Moreover, vertically-mounted ring-shaped magnets 6,6 
are provided at the right and left sides of the main chamber 1 so that the 
ring-shaped magnets 6,6 are facing each other and a face perpendicular to 
the both surface have a diameter of the crucible. 
The quartz crucible 3 is filled with raw polysilicon material. The raw 
material is heated by the heater 4, and melted into the melt 7. 
Thereafter, the seed crystal 9 mounted on the seed chuck 8 is immersed in 
the melt 7, and then the seed chuck 8 and the graphite crucible 2 is 
rotated in the same direction or opposite direction while the seed chuck 8 
is pulled to grow the single crystal. When electrical current flow through 
the the magnets 6,6, a transverse magnetic field having a predetermined 
intensity, is horizontally applied to the melt 7 and thus the convection 
of the melt in the upper-and-lower direction orthogonal with the magnetic 
lines is inhibited, and accordingly the temperature fluctuationis reduced. 
At the beginning of the contracting step, as the temperature fluctuation 
range in the vicinity of the surface of the melt 7 is allowed to be 
greater than 5.degree. C., the magnetic field intensity is reduced to less 
than 2000 gauss. Accordingly, convection in the upper-and-lower direction 
of the melt 7 is activated, and the shape of the crystal growth interface, 
have been kept stable by then, changes properly. The seed crystal 9 
gradually contracts in this state, and finally the diameter of the 
contracted portion 10 becomes 4 mm .PHI.. In this period, any dislocation 
in the vertical direction of the seed crystal 9 disappears at the 
periphery thereof even though the diameter of the contracted portion 10 is 
larger than 4 mm, and thus does not propagate to the shoulder 11 formed 
below. 
After it is confirmed that dislocation-free state is achieved in the 
contracted portion 10, the single crystal is moved to the shoulder-forming 
step. At this time, the magnetic field intensity is gradually increased to 
its original predetermined value. In the shoulder-forming step, the 
diameter of the grown single crystal silicon is gradually increased to a 
predetermined dimension, and finally becomes the body portion 12. The 
magnetic field intensity after the shoulder-forming step is maintained at 
a predetermined value. In the case that a fluctuation range of a 
temperature near a surface of a melt 7, is lower than 5.degree. C. at the 
beginning of contracting step, because of being the shape of the crystal 
growth interface stable it is difficult to eliminate the dislocation. 
Reference is then made to FIG. 2 which is a sectional schematic view of the 
main portions of a cusp magnetic field applied fabrication apparatus for a 
semiconductor single crystal. The periphery of the main chamber 1 is 
provided a ring-shaped upper magnet 13 and a ring-shaped lower magnet 14. 
In a normal state, the magnetic lines of the upper magnet 13 are curved in 
an almost horizontal direction above the surface of the melt 9 as shown in 
the dot lines in FIG. 2. The magnetic lines of the lower magnet 14 are 
curved in an almost horizontal direction below the surface of the melt 9 
and penetrate the side wall of the quartz crucible 3 almost vertically. 
Accordingly, the melt convection along the side wall of the quartz 
crucible 3 is inhibited. On the other hand, the magnetic field in the 
vicinity of the crystal growth interface is almost zero, and thus the 
convection of the melt is not inhibited. 
At the beginning of contracting step, according to the first method in 
which the temperature fluctuation of the melt 7 is allowed to be greater 
than 5.degree. C., one of the magnetic field of the upper magnet 13 and 
the lower magnet 14 is strong and the other is weak. FIG. 3 is a schematic 
view showing that the magnetic lines wherein the magnetic field intensity 
of the upper magnet 13 is strong and the magnetic field intensity of the 
lower magnet 14 is weak. In FIG. 3, only the quartz crucible, melt and 
magnets are shown, the other parts of the apparatus are omitted. When the 
magnetic field intensity of the upper magnet 13 is B.sub.U and the 
magnetic field intensity of the lower magnet 14 is B.sub.L, the amount of 
respective increase and decrease of the magnetic field intensities should 
satisfy the following formula: 
EQU 1.0&lt;B.sub.U .div.B.sub.L .ltoreq.3.0 
In the case that the value of B.sub.U /.div.B.sub.L is not more than 1.0, a 
center of the magnetic field is located above the melt surface and 
magnetic field at the melt surface is vertical magnetic field which is 
vertical to the melt, it is difficult to control a temperature near the 
melt surface. 
On the other hand, in the case that the value of B.sub.U /.div.B.sub.L 
excess 3.0, a center of the magnetic field is located under the melt 
surface and magnetic field at the melt surface is vertical magnetic field 
which is vertical to the melt, it is difficult to control a temperature 
near the melt surface. The magnetic lines of the upper magnet 13 enter the 
melt 7 almost vertically from the top, and curve in an almost horizontal 
direction from the beneath of the surface of the melt 7 and penetrate the 
side wall of the quartz crucible 3 almost orthogonally. Accordingly, the 
vicinity of the surface of the melt 7 is almost similar to a longitudinal 
magnetic field and thus the up and down convection of the melt 7 is 
activated. And thus the shape of the crystal growth interface changes 
properly and even the diameter of the contracted portion is larger than 4 
mm, dislocation free state is achieved. 
FIG. 4 is a schematic view showing the magnetic lines wherein the magnetic 
field intensity of the upper magnet 13 is weak and the magnetic field 
intensity of the lower magnet 14 is strong. When the magnetic field 
intensity of the upper magnet 13 is B.sub.U and the magnetic field 
intensity of the lower magnet 14 is B.sub.L, the amount of respective 
increase and decrease of the magnetic field intensity should satisfy the 
following formula: 
EQU 1. 0&lt;B.sub.U .div.B.sub.L .ltoreq.3. 0 
The magnetic lines of the lower magnet 14 enter the melt 7 almost 
vertically from the lower part of the quartz crucible 3, penetrate the 
melt 7 and curve in an almost horizontal direction from the top of the 
surface of the melt 7. Accordingly, the vicinity of the surface of the 
melt 7 is almost similar to a longitudinal magnetic field and thus the up 
and down convection of the melt 7 is activated. Accordingly, the shape of 
the crystal growth interface changes properly and even though the diameter 
of the contracted portion is larger than 4 mm, dislocation free state is 
achieved. 
In the cusp magnetic field-applied semiconductor single crystal fabrication 
apparatus, in case the second method in which the temperature fluctuation 
range of the melt is set larger than 5.degree. C., the upper magnets and 
the lower magnets can be moved simultaneously in the vertical direction. 
FIG. 5 is a schematic view showing the position relationship between the 
melt surface and the magnets. If the space between the upper magnet 13 and 
the lower magnet 14 is "2d", the distance from the central line of the 
upper magnet and the lower magnet to the melt surface is "a", then the 
moving of the upper and lower magnets should satisfy the following formula 
: 
EQU 0&lt;a.ltoreq.2d 
If the level of the melt surface excess between the upper magnet 13 and the 
lower magnet 14, a magnetic field in the vicinity of the melt surface is 
similar to a longitudinal magnetic field and it is difficult to eliminate 
a convection in a vertical direction and fabricate a contracted portion. 
Referring to FIG. 6 which shows the upper magnets 13 and the lower magnets 
14 moved to the below, the magnetic lines of the upper magnets 13 enter 
the melt 7 almost vertically from the top, as in the FIG. 3, and curve at 
the beneath of the surface of the melt 7 almost horizontally, penetrating 
the side wall of the quartz crucible 3 almost orthogonally. Referring to 
FIG. 7 which shows the upper magnets 13 and the lower magnets 14 moved to 
the top, the magnetic lines of the lower magnet 14 enter the melt 7 almost 
vertically from the lower part of the crucible, as in FIG. 4, and 
penetrate the melt 7 and curve in at the top of the surface of the melt 
almost horizontally. In all cases, the vicinity of the melt surface is 
similar to a longitudinal magnetic field, the up and down convection of 
the melt 7 is activated, and thus the shape of the crystal growth 
interface changes properly and even though the diameter of the contracted 
portion is larger than 4 mm, dislocation free state is achieved. 
As indicated above, in the contracting step, when a transverse magnetic 
field is used, by lowering the magnetic field intensity below 2000 gauss, 
and when a cusp magnetic field is used, by increasing and decreasing the 
magnetic field intensity of the upper and lower magnets or moving the 
upper and lower magnets in the vertical direction to make the vicinity of 
the melt surface similar to a longitudinal magnetic field, the change of 
the shape of the crystal growth interface becomes large. In all cases, the 
rate of free dislocation is increased to 90% as compared with the 50% when 
pulling by applying magnetic field in the prior art. 
As stated above, compared to pulling a semiconductor single crystal by the 
CZ method, the MCZ method necessitates thinning the diameter of the 
contracted portion to attain the free dislocation. According to the 
invention, by weakening the magnetic field intensity or changing the 
positions of the magnets without changing the magnetic field intensity to 
make proper change of the crystal growth interface, the free dislocation 
is easily attained even though the diameter of the contracted portion is 
larger than that of the prior art. Accordingly, a large diameter, heavy 
semiconductor single crystal can be safely pulled without breaking the 
contracted portion, and the semiconductor single crystal yield can be 
increased.