System for pulling-up monocrystal and method of exhausting silicon oxide

A monocrystal pulling apparatus according to the Czochralski technique, provided with a flow controller which guides a carrier gas supplied from the top of a pulling chamber to the surface of a melt of a material forming the monocrystal and exhausts the silicon oxide vaporizing from the surface of the melt to the outside of the pulling chamber and which surrounds the pulled monocrystal near the surface of the melt and is provided partially inside a crucible, wherein the flow controller has a tubular portion which has an outer diameter smaller than the inner diameter of the crucible and extends substantially perpendicularly along the direction of downward flow of the carrier gas, a constricted diameter portion which constricts in diameter from the bottom end of the tubular portion and forms a bottom gap with the pulled monocrystal, and an engagement portion which projects out from the top of the tubular portion and forms a top gap at the outer circumference of the tubular portion of the flow controller by supporting the flow controller partially in the pulling chamber. As a result, a first flow path through which the carrier gas flows toward said bottom gap is defined between the inside of the tubular portion and the pulled monocrystal, a second flow path is defined comprised of a flow path of the carrier gas passing through the top gap and a flow path of the carrier gas passing from the first flow path through the bottom gap and then passing between the surface of the silicon melt and flow controller. The silicon oxide is exhausted together with the carrier gas through the second flow path to the outside of the pulling chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a monocrystal pulling-up system which 
pulls up and grows a monocrystal of silicon etc. by the Czochralski 
technique (hereinafter referred to as the "CZ technique"), in particular 
relates to a monocrystal pulling-up system which can pull a large diameter 
and high quality monocrystal while controlling the oxygen concentration 
(density) by providing a carrier gas flow controller. 
The present invention also relates to a method of exhausting silicon 
oxide(SiO) produced from the melt silicon in a quartz crucible to the 
outside of a pulling chamber by controlling the flow of the carrier gas. 
2. Description of the Related Art 
To produce a silicon monocrystal by the CZ technique, polycrystalline 
silicon and the necessary dopant(s), for example, P, B, Sb, As, are 
inserted into a quartz crucible provided rotatably at a bottom of a 
pulling chamber, the chamber is evacuated to a vacuum, then a heater 
arranged around the quartz crucible is used to melt the polycrystalline 
silicon and the dopant(s). A carrier gas is then passed from an upper 
portion of the chamber to the quartz crucible. At the same time a seed 
crystal (starting crystal) attached to and supported by a chuck on a 
pulling shaft is immersed in the melt silicon in the quartz crucible under 
conditions of a vacuum of 10 to 20 Torr. Then the pulling shaft is pulled 
up at a predetermined speed while relatively rotating it with respect to 
the quartz crucible. 
U.S. Pat. No. 4,330,362 discloses a pulling system provided with a member 
(hereinafter called a "heat cap") comprised of a material able to reflect 
ultraviolet rays above the crucible so as to partially cover the crucible 
and the melt silicon in the crucible, to thereby block the radiant heat 
from the melt surface, promote the formation of the monocrystal, and raise 
the pulling speed and to keep down the concentration of carbon in the 
monocrystal. 
When pulling a silicon monocrystal using the above-mentioned pulling 
system, however, there are the following disadvantages. 
First, the above heat cap can be expected to a certain extent to have the 
effect of controlling the flow of the carrier gas, for example, argon 
gas(Ar), being passed to remove the silicon oxide(SiO) produced from the 
melt and efficiently eliminating the silicon oxide depositing on the inner 
wall of the top end of the crucible, but silicon oxide ends up depositing 
and condensing on the top of the heat cap itself. This is a drawback in 
that it would fall onto the melt silicon surface and thus obstruct the 
formation of the silicon monocrystal. This is believed to be because the 
heat cap has as its main object the literal blocking of ultraviolet rays 
and is not designed with the intention of control of the flow of the argon 
gas. 
Second, the melt surface near the inner peripheral wall of the quartz 
crucible ends up being covered by the heat cap, so an operator cannot 
visually inspect the melt surface from a peephole etc. provided in the 
pulling chamber. As a result, there is the disadvantage that it is not 
possible to quickly deal with any heat deformation of the top end of the 
crucible, recrystallization or deposition of silicon near the inner 
peripheral wall of the crucible, or any other disadvantages when they 
occurred. 
In addition, there are the following disadvantages when trying to pull a 
silicon monocrystal for use for the manufacture of a large diameter VLSI 
device, for example, a diameter of 6 inches, 8 inches, or more. 
With a large diameter crystal, the most important thing is the control of 
the oxygen concentration (density). This is generally classified in the 
manufacturing process of an LSI device into high oxygen of, for example, 
1.55.times.10.sup.18 atoms/cm.sup.3, medium oxygen of, for example, 
1.35.times.10.sup.18 atoms/cm.sup.3, and low oxygen of, for example, 
1.15.times.10.sup..about. atoms/cm.sup.3. Further, in some cases, 
extremely high oxygen and extremely low oxygen are demanded and such 
classifications are selectively used. For example, when using the 
intrinsic gettering technique utilizing oxygen precipitation, much use is 
made of monocrystals from a high oxygen to a medium oxygen concentration. 
On the other hand, when strength and reduction of lattice faults are 
required, much use is made of monocrystals from a medium oxygen to a low 
oxygen concentration. Thus, it is necessary to control the variation in 
the oxygen concentrations in the axial direction of the crystal and in the 
silicon wafer surface so that the oxygen concentration of the pulled 
monocrystal becomes in the designated narrow range. 
The "oxygen in the crystal" described here means the oxygen dissolving out 
from the quartz crucible. Almost of the oxygen, for example, 95% of the 
oxygen, becomes silicon oxide and is exhausted by the carrier gas to the 
outside of the pulling chamber. Therefore, the following techniques are 
known for the control of the oxygen concentration in the crystal: 
Approach (1): Changing the rotational speed of the crucible so as to 
control the supply of oxygen from the wall of the quartz crucible. By this 
technique, if the rotational speed of the crucible is increased, the 
amount of oxygen of the pulled monocrystal becomes higher. However, if the 
rotational speed of the crucible is made lower, the temperature 
fluctuations of the melt become great and crystal faults become easier to 
occur at a low oxygen concentration. If the rotational speed of the 
crucible is raised to obtain a high oxygen pulled monocrystal, it is 
necessary to raise the rotational speed of the pulled crystal along with 
the same. There is a problem, however, of the resonance point in the case 
of pulling a pulled crystal by a wire. Further, if the rotational speed of 
the pulled crystal is made too high, deformation occurs in the monocrystal 
and there are problems in the control of the diameter of the monocrystal 
as well. 
Approach (2): Control of the pressure of the carrier gas. If the pressure 
of the carrier gas is increased, the vaporization of the silicon oxide is 
suppressed, so the amount of oxygen of the pulled monocrystal becomes 
higher. However, this approach is governed largely by the structure inside 
the pulling furnace, so not much can be expected in terms of the response 
of the control of the oxygen concentration. 
Approach (3): Spraying carrier gas on the melt silicon surface in the 
crucible so as to control the temperature of the melt silicon surface and 
control the amount of vaporization of the silicon oxide. When the heat cap 
is used, for example, the carrier gap between the heat cap and the melt 
silicon surface and the gap between the heat cap and the pulled 
monocrystal (hereinafter referred to all together as the "bottom gap") are 
controlled. By this technique, if the bottom gap is made smaller, the 
temperature of the melt surface falls, so the amount of vaporization of 
the silicon oxide is held down and as a result the amount of oxygen of the 
pulled monocrystal becomes higher. This approach is relatively effective 
to obtain a high oxygen crystal, but if the flow of the carrier gas is 
increased and the bottom gap is made too small, the carrier gas will 
strike the melt hard and therefore cause bubbles in the melt. As a result, 
there are the problems that the crystal growth will no longer be uniform 
and further the variations in the oxygen concentration in the surface will 
become greater. 
Approach (4): Control of the discharge of the vaporized silicon oxide by 
the flow of the carrier gas. If the vaporized silicon oxide is efficiently 
discharged by the carrier gas from the melt silicon surface to outside of 
the pulling chamber, the vaporization of the silicon oxide is promoted and 
as a result the oxygen concentration in the melt is lowered and the amount 
of oxygen in the pulled crystal becomes lower. There is a gas diffusion 
layer of the vaporized silicon oxide directly above the melt surface. By 
using the heat cap, the flow rate of the same is increased by the flow of 
the carrier gas introduced from above the pulling chamber in the narrowed 
gap between the heat cap and the melt surface and therefore the thickness 
of the gas diffusion layer is reduced. As a result, the partial pressure 
of the silicon oxide on the melt surface becomes lower and vaporization of 
the silicon oxide is promoted, but if a heat cap is used, an opposing 
phenomenon simultaneously occurs. Further, if the crucible deforms and the 
melt surface drops, the subsequent oxygen concentration in the crystal 
will change. 
In addition to the disadvantages in the approach (4), in the approach (3), 
if the size of the bottom gap is increased, the effect of the approach (4) 
becomes stronger and the oxygen concentration rapidly decreases. 
Therefore, to control the pulled monocrystal to within the target range of 
oxygen concentration, it is necessary to continuously control the size of 
the bottom gap precisely. 
In this way, when using a heat sink, there is a problem that it is always 
difficult to set and manage the conditions. 
Further, with a silicon monocrystal used for the production of a large 
diameter VLSI device, it is desirable that the crystal as a whole have the 
same heat history as much as possible so that the concentration of the 
oxygen taken into the crystal becomes uniform, even during the subsequent 
cooling process. Therefore, a heat cap blocking the radiant heat and/or a 
water cooling tube are provided. 
Further, there are phenomena believed to be related to the behavior of 
clusters of point faults directly on the growth surface having an effect 
on the pressure resistance of the oxide film of the device. Therefore, 
Japanese Unexamined Patent Publication (Kokai) No. 3-275586 discloses the 
production of a crystal with a high oxide film pressure resistance by 
lowering the pulling speed to 0.5 mm/min or less in a furnace structure 
with a usual pulling speed of 1.5 mm/min. This is because it is guessed 
that by lengthening the residence time in the temperature region of over 
1300.degree. C. from the crystal growth interface, the faults relating to 
the pressure resistance of the oxide film diffuse and disappear. 
In view of these problems, the present inventor started studies from a 
completely new viewpoint smashing fixed conceptions about the heat cap 
disclosed in U.S. Pat. No. 4,330,362 and designed to block (shield) 
ultraviolet rays, that is, from the viewpoint of a "carrier gas flow 
controller", and analyzed the state of flow of the carrier gas using 
computer simulation to find a numerical solution to the Navier-Stokes 
equation, i.e., a non-linear fluid diffusion equation based on fluid 
dynamics and thermodynamics. 
First, if the state of flow of carrier gas in the case of pulling a 
monocrystal by a pulling apparatus equipped with the heat cap disclosed in 
U.S. Pat. No. 4,330,362 (hereinafter referred to as a "closed type heat 
cap") is considered, the result becomes as shown in FIG. 1 to FIG. 3. 
FIG. 1 is a view showing the state of flow of carrier gas in a pulling 
system equipped with a closed type heat cap, FIG. 2 is a view of the state 
of flow of carrier gas analyzed by computer simulation of the 
Navier-Stokes equation, and FIG. 3 is a view of the temperature 
distribution obtained by analysis by the same computer simulation. 
The heat cap 30 shown in FIG. 1 completely partitions the flow path of the 
carrier gas G into the top (shown by region X) and bottom (shown by region 
Y) of a pulling chamber, so the carrier gas G introduced from the top of 
the pulling chamber passes through the narrow bottom gap 33 between the 
heat cap 30 and the pulled monocrystal 31 and surface of the melt silicon 
32 to be increased in speed. By this colliding with the melt surface 
positioned directly under the bottom gap 33, the temperature of the melt 
directly under the gap 33 falls, the vaporization of silicon oxide is 
suppressed, and the melt 32 of the hatched portion 34 shown in FIG. 1 
becomes high in oxygen, but on the other hand, the carrier gas forcibly 
removes the diffusion layer including the silicon oxide from the melt 
surface, so vaporization of silicon oxide is promoted. 
In the other area of the melt 32, however, the degree of contact with the 
carrier gas G is smaller than with the portion 34 directly under the 
bottom gap 33, so the melt becomes relatively low in oxygen concentration. 
Therefore, the distribution of the oxygen concentration of the melt in the 
crucible becomes nonuniform and there is an adverse effect on the oxygen 
distribution (ORG) in the silicon wafer surface of the pulled monocrystal 
31. 
Note that this state is verified by the results of computer simulation 
shown in FIG. 2 and FIG. 3. 
This problem, it may be concluded, derives from the way the carrier gas 
flows. Since the heat cap partitions the pulling chamber into a top and 
bottom section, the carrier gas passing through the bottom gap flows in a 
so-called "squished" manner. 
Based on these studies and the results of analyses, the present inventor 
took note of the "flow-control of the carrier gas" and discovered that if 
the carrier gas is suitably guided in the pulling chamber, the temperature 
region directly above the crystal growth interface can be expanded, the 
control of the oxygen concentration and ORG can be improved, and the 
condensation and falling of silicon oxide can be prevented and thereby 
completed the invention disclosed in Japanese Unexamined Patent 
Publication (Kokai) No. 1-100,086. 
A heat-cap disclosed in JPP 1-100,086 comprises a reflector body and 
projected stops. The reflector consists of a tube and an inclined cylinder 
provided at a lower portion of the tube with a tip (end) which is reduced 
in diameter inward. The tube and the inclined cylinder may be formed 
integrally or together. Projections are provided at the top of the tube 
and are affixed to the top of a heat retaining member provided around the 
crucible. The carrier gas is branched by the tube to flow, on one hand, 
through a gap between the pulling monocrystal and the tube, and, other 
hand, through a gap between the tube and the heat retaining member. That 
is, the heat cap can form a flow path between the tube and the heat 
holding member. Thus, the heat cap can be called an open-type heat cap. 
The heat cap disclosed in JPP 1-100,086 can overcome the disadvantage of 
U.S. Pat. No. 4,330,362 as a basic idea, but JPP 1-100,086 does not 
disclose specific conditions. In addition, the heat cap of JPP 1-100,086 
requires some improvements. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a pulling system which 
enables control of the oxygen concentration of the pulled monocrystal, can 
make the oxygen concentration (ORG) in a wafer surface uniform, can pull a 
monocrystal with a uniform oxygen concentration in the axial direction, 
and in particular can pull a large diameter monocrystal with a high 
quality and a uniform heat history. 
Another object of the present invention is to provide a method of 
exhausting silicon oxide by a carrier gas effectively and a system 
thereof. 
Still another object of the present invention is to provide an improved 
heat cap. 
According to the present invention, there is provided a monocrystal pulling 
system according to the Czochralski technique, comprising: a pulling 
chamber; a crucible provided inside the pulling chamber; a heating means 
provided at the circumference of the crucible and for melting a material 
accommodated in the crucible for forming the monocrystal; a heat retention 
means provided a predetermined distance away from the heating means and 
for retaining the heat of the crucible; a means for supplying a carrier 
gas from the top of the pulling chamber toward the surface of the melt 
solution in the crucible; and a flow controller which guides the carrier 
gas to the surface of the melt of the material forming the monocrystal and 
exhausts the silicon oxide vaporizing from the surface of the melt 
solution to the outside of the pulling chamber and which surrounds the 
pulled monocrystal near the surface of the melt and is provided partially 
inside the crucible. The flow controller comprises a tubular portion which 
has an outer diameter smaller than the inner diameter of the crucible and 
extends substantially perpendicularly along the direction of downward flow 
of the carrier gas, a constricted diameter portion which constricts in 
diameter from the bottom end of the tubular portion and forms a bottom gap 
with the pulled monocrystal, and an engagement portion which projects out 
from the top of the tubular portion and forms a top gap at the outer 
circumference of the tubular portion of said flow controller by supporting 
the flow controller partially in the pulling chamber. As a result, a first 
flow path through which the carrier gas flows toward the bottom gap is 
defined between the inside of the tubular portion and the pulled up 
monocrystal, and a second flow path is defined comprised of a flow path of 
the carrier gas passing through the top gap and a flow path of the carrier 
gas passing from the first flow path through the bottom gap and then 
passing between the surface of the silicon melt solution and the flow 
controller. The silicon oxide is exhausted together with the carrier gas 
through the second flow path to the outside of the pulling chamber. 
Preferably, the bottom gap and the top gap are formed so that the amount of 
the carrier gas flowing through the bottom gap becomes greater than the 
amount of the carrier gas flowing through the top gap. 
Specifically, the engagement portion is affixed to the top of a heat 
retaining tube provided at the outer circumference of the crucible. 
Preferably, a heating means for heating the crucible is arranged between 
the crucible and the heat retaining tube, a gap is formed between the 
heating means and the heat retaining tube, the gap is connected to the 
second flow path, and the silicon oxide is exhausted together with the 
carrier gas to the outside of the pulling chamber through the gap. 
Preferably, the area of the opening of the constricted diameter portion is 
1.5 to 2.0 times the sectional area of the pulled monocrystal. 
Also, preferably, the sectional area (Ru) of the top gap is 0.5 to 1.4 
times the sectional area (Rd) of the bottom gap. 
Preferably, the top gap is positioned at least partially to the inside of 
the top end of the crucible. 
Preferably, the flow controller is made of carbon. 
Further preferably, the surface of said carbon flow controller is covered 
with silicon carbide. 
The length of the tubular portion of said flow controller is greater than 
the range of vertical movement of said crucible. 
Preferably, the tubular portion, the constricted diameter portion, and the 
engagement portion are integrally formed, or, 
the tubular portion and the constricted diameter portion are integrally 
formed and the engagement portion is attached detachably to the tubular 
portion. 
Preferably, provision is made, in the pulling chamber at the top of the 
flow controller, of a cooling means for cooling the pulled monocrystal and 
introducing the carrier gas into the pulling chamber and 
a peephole is provided at the outer wall of the pulling chamber on the line 
connecting the gap between the front end of the cooling means and the 
front end of the top of the tubular portion of said flow controller and 
the bottom gap. 
Also, according to the present invention, there is provided a silicon oxide 
exhaust method for guiding a carrier gas, supplied from the top of a 
pulling chamber for pulling up a monocrystal according to the Czochralski 
technique, to a surface of a melt of a material for forming a monocrystal 
and exhausting to the outside of the pulling chamber a silicon oxide 
vaporized from the surface of the melt accommodated in a crucible. The 
silicon oxide exhaust method characterized by 
defining by a carrier gas branching means a bottom gap of a predetermined 
size between a circumference of the pulled monocrystal and the surface of 
the melt and defining a top gap between the crucible and a heat retaining 
tube provided at the outside of the same, 
defining a first flow path through which the carrier gas flows toward the 
bottom gap between the carrier gas branching means and pulled monocrystal, 
defining a second flow path comprised of a flow path of the carrier gas 
passing through the top gap and a flow path of said carrier gas passing 
from the first flow path through the bottom gap and then passing between 
the surface of the silicon melt and a flow controller, 
forming the bottom gap and the top gap so that the amount of the carrier 
gas flowing through the bottom gap becomes greater than the amount of the 
carrier gas flowing through the top gap, and 
exhausting said silicon oxide together with said carrier gas through said 
second flow path to the outside of the pulling chamber. 
Preferably, the crucible is heated by a heating means arranged between the 
crucible and the heat retaining tube and a gap is formed between the 
heating means and the heat retaining tube, the gap is connected to the 
second flow path, and the silicon oxide is exhausted together with the 
carrier gas to the outside of said pulling chamber through the gap. 
Preferably, the diameter of the carrier gas branching means near the 
surface of the melt solution is constricted and the area of the opening of 
the constricted diameter portion is 1.5 to 2.0 times the sectional area of 
the pulled monocrystal. 
Also, preferably, the sectional area (Ru) of said top gap is 0.5 to 1.4 
times the sectional area (Rd) of the bottom gap. 
Preferably, the top gap is positioned at least partially to the inside of 
the top end of the crucible. 
According to the present invention, there is provided a monocrystal pulling 
system, further provision is made of a rotational control means for 
controlling the relative rotational speed of the crucible or the pulled 
monocrystal so as to control the concentration of oxygen included in the 
monocrystal. 
Further, according to the present invention, there is provided a 
monocrystal pulling method, wherein the relative rotational speed of the 
crucible or the pulled monocrystal is controlled so as to control the 
concentration of oxygen included in the monocrystal. 
To pull a large diameter and high quality monocrystal, it is necessary to 
(1) adjust the rotational speed of the crucible to regulate the amount of 
oxygen entering the melt from the wall surface of the crucible, (2) change 
the rotational speed of the crucible in accordance with changes in the 
area of the crucible wall surface so as to control the distribution of the 
oxygen concentration of the resultant monocrystal to be uniform, and (3) 
raise the temperature of the carrier gas removing the silicon oxide from 
the melt surface to reduce the drop in temperature at the crystal growth 
interface and control the radiant heat received by the crystal so as to 
establish a uniform heat history environment. 
When pulling the monocrystal, however, the silicon oxide which condenses at 
the top of the pulling apparatus and at the crystal surface and falls into 
the melt becomes a cause of ruin of the monocrystal, so it is critical to 
exhaust the vaporized silicon oxide by the carrier gas out of the system 
quickly and smoothly. In the present invention, by providing a suitably 
constructed flow controller, the optimal flow of carrier gas is realized, 
the amount of vaporization of the silicon oxide is made constant, and the 
particles of the silicon oxide pass between a graphite susceptor and heat 
shield and between the heat shield and heater and are exhausted by a 
vacuum pump without condensing and solidifying. 
That is, the carrier gas does not flow in a squished manner as in the 
conventional pulling apparatus. Rather, the aspiration effect of the 
carrier gas passing through the second flow path formed at the outer 
portion of the flow controller is utilized and the carrier gas that passes 
through the inside of the flow controller and sweeps up and exhausts the 
atmosphere including the silicon oxide particles from the melt surface is 
used to draw this to the outer portion of the crucible. 
Therefore, the carrier gas flowing down along the inner portion of the flow 
controller (first flow path) is heated by the carbon flow controller, 
passes through the gap, then is led to the melt surface without 
excessively cooling the melt, and promotes the vaporization of the silicon 
oxide. The atmosphere including the silicon oxide is led uniformly outside 
by the aspiration effect of the large energy carrier gas passing through 
the outer portion of the flow controller (second flow path). 
Consequently, by uniformly exhausting the silicon oxide vaporizing from the 
melt surface by a smaller amount of gas, the distribution of the oxygen 
concentration at the melt surface is maintained uniform, the cooling of 
the crystal growth surface is reduced, and as a result the pulled 
monocrystal is given a uniform distribution of the oxygen concentration 
and becomes high in quality.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The pulling system of the present embodiment, as shown in FIG. 4, provides 
a quartz crucible 3 in a pulling chamber 1. The quartz crucible 3 is 
attached to a rotatable bottom shaft 9 through a graphite susceptor 8. 
Further, around the quartz crucible 3 is provided a heater for heating and 
controlling the temperature of the silicon melt M in the quartz crucible 
3. Between the heater 10 and the pulling chamber 1 is provided a heat 
retaining tube 11. 
At the top surface of the heat retaining tube 11 is attached an annular 
supporting member 12. By placing an engagement portion 7 of a flow 
controller 2 on the supporting member 12, the flow controller 2 is affixed 
inside the pulling chamber. 
Note that reference numeral 13 denotes a cooling tube for cooling the 
pulled monocrystal, and reference numeral denotes a peephole for observing 
the surface of the melt M and the top end edge of the quartz crucible 3. 
As shown in FIG. 5, FIG. 6, and FIG. 9, the flow controller 2 according to 
the present embodiment has an outer diameter d smaller than the inner 
diameter D of the quartz crucible 3 and is comprised of a tubular portion 
which extends substantially perpendicularly along the direction of 
downward flow of the carrier gas G and a constricted diameter portion 6 
which constricts in diameter gradually from the bottom end of the tubular 
portion 4 and forms the small bottom gap 5 with the pulled monocrystal S. 
The tubular portion 4 is particularly effective in the case of pulling the 
monocrystal by the batch-type CZ technique. Namely, the height of the 
tubular portion 4 in the present embodiment is set so that the area of the 
opening of the opening 17 becomes constant even if the graphite susceptor 
8 is moved up and down between the limit of descent and the limit of rise. 
Therefore, even if the graphite susceptor 8 rises along with the pulling 
of the monocrystal, the area of the opening 17 can be held constant, so 
there is no fluctuation in the flow of the carrier gas G mentioned later. 
At the bottom end of the constricted diameter portion 6, an opening 16 is 
made. This has an opening diameter B in accordance with the diameter A of 
the pulled monocrystal S and in consideration of the area of the bottom 
gap 5. With an opening area (=.pi.B.sup.2 /4) less than 1.5 times the 
crystal area, if the crystal suspended by the wire deviates off center, it 
may contact the flow controller 2. On the other hand, if greater than 2.0 
times, the flow rate of the gas on the melt will fall. Therefore, the area 
of the opening at the front end of the constricted diameter portion 
(lateral sectional area) is preferably 1.5 to 2.0 times the area of the 
pulled monocrystal S (lateral sectional area). 
By way of note, the above numerical figures limit the ratio between the 
area of the opening at the front end of the constricted diameter portion 
and the lateral sectional area of the monocrystal, but in the flow 
controller according to the present invention, the same effect is obtained 
even if the ratio between the area of the bottom gap and the lateral 
sectional area of the monocrystal is limited. The ratio in this case is 
0.5 to 1.0. 
The flow controller 2 must withstand high temperatures and not discharge 
any heavy metal elements since it is directly above the melt. Use is made 
of a refractory metal like molybdenum(Mo) or tungsten(W) or carbon(C). A 
refractory metal reflects radiant heat and has a large shielding effect, 
while carbon absorbs radiant heat and conversely discharges radiant heat 
to the crystal. 
FIG. 16 is a graph showing the results of measurement of the temperature 
for comparing the effects of radiant heat of molybdenum and carbon at a 
position 100 mm off from the center of the pulling system. 
As shown by this temperature distribution, if use is made of a carbon flow 
controller, the temperature at the crystal growth interface becomes high, 
so in the past it was considered that this was detrimental to increasing 
the pulling speed, but since the crystal quality is considered important 
now, use of carbon is effective. In addition, a carbon flow controller has 
the effect of heating the carrier gas flowing through the first flow path 
P.sub.1. 
Further, it is preferable that the surface of the carbon flow controller be 
coated with silicon carbide(SiC). Giving this silicon carbide coating is 
advantageous to the lifetime of the flow controller 2 and, further, is 
effective in inhibiting the discharge of the heavy metal elements included 
in small amounts in the carbon. 
The engagement portion 7 for placing the flow controller 2 on the 
supporting member 12 may be formed integrally with the tubular portion 4 
as shown in FIG. 5 and FIG. 6. Further, as shown in FIG. 7 and FIG. 8, the 
engagement portion 7 may be formed separately and be affixed to the 
tubular portion 4 by carbon bolts 15. 
In the pulling system of this embodiment, by affixing the above-mentioned 
flow controller 2 inside the pulling chamber 1, the flow paths of the 
carrier gas G supplied from the top of the pulling chamber 1 become as 
follows: 
As shown in FIG. 10, first, as the first flow path, there is the flow path 
P.sub.1 in which the carrier gas G branches at the top end of the tubular 
portion 4 of the flow controller 2 and reaches the inside of the tubular 
portion 4. Connecting to the first flow path P.sub.1, there is the second 
flow path P.sub.2, in which the carrier gas G passes from the first flow 
path P.sub.1 through the gap between the bottom end of the constricted 
diameter portion 6 and the pulled monocrystal S (hereinafter referred to 
as the "bottom gap 5") and through the gap between the bottom end of the 
constricted diameter portion 6 and the surface of the melt M, then passes 
between the surface of the melt M and the flow controller 2, and further 
forms the atmosphere including the silicon oxide accompanying the rising 
air flow between the flow controller 2 and the inner surface 3a of the 
quartz crucible. 
The ratio of the carrier gas G led through the first flow path P.sub.1 and 
the second flow path P.sub.2 is related to the areas of the top and bottom 
gaps, that is, the area Ru of the gap at the outer portion of the tubular 
portion of the flow controller 2 in the second flow path P.sub.2 
(hereinafter this gap is referred to as the "top gap 18") and the area Rd 
of the bottom gap 5 between the constricted diameter portion 6 of the flow 
controller 2 and the pulled monocrystal S in the first flow path P.sub.1, 
but the carrier gas flowing through the bottom gap 5 encounters the 
resistance of the rising air flow from the melt, so judging from the 
results of simulation of the embodiments shown in FIG. 11 and FIG. 13, 
when the area ratio Ru/Rd is about 0.7, the carrier gas G flows in equal 
amounts (branching ratio 50% ) in the first flow path P.sub.1 and the 
second flow path P.sub.2. 
By way of note, if the branching ratio of the gas flowing through the first 
flow path P.sub.1 is less than 20% of the overall amount, it is not 
possible to suppress the rising air flow from the melt surface, the 
silicon oxide deposits on the cooled monocrystal, and, if it condenses and 
falls, it will ruin the crystal. Conversely, if the branching ratio of the 
gas flowing through the first flow path P.sub.1 is over 80% of the overall 
amount, the atmosphere including the silicon oxide above the melt surface 
will not be able to be effectively exhausted. Therefore, in terms of the 
branching ratio, the carrier gas flowing through the first flow path 
P.sub.1 preferably should be 20% to 80% of the total amount. Converting 
this to the area ratio Ru/Rd between the top gap 18 and the bottom gap 5, 
the preferable region of the ratio may be said to be 0.4 to 1.4 (see FIG. 
17). 
The carrier gas G led into the second flow path P.sub.2 is heated by the 
carbon flow controller 2 and strikes the melt surface M without 
overcooling the crystal growth interface, replaces the atmosphere 
including the silicon oxide removed by the carrier gas from the first flow 
path P.sub.1, and diffuses over the melt surface as a whole. This 
determines the amount of vaporization of the silicon oxide, defines the 
oxygen concentration of the melt surface from which the crystal is raised, 
and stabilizes it at the low oxygen side. 
Therefore, to obtain a medium oxygen and high oxygen monocrystal, rather 
than make the bottom gap 5 narrower, the rotational speed of the crucible 
is rather increased. 
If the rotational speed of the crucible is increased, as clear from the 
temperature change of the melt surface shown in FIGS. 15a-15d the 
stability of the temperature change increases and the stability of the 
solid-liquid interface increases. In this case, in the state with no flow 
controller 2, if the rotation of the crystal is not increased so that the 
ratio of the crystal rotation/crucible rotation becomes constant, numerous 
crystal faults occur, experience shows, but with this construction, it was 
confirmed that it was sufficient to make the crystal rotation a maximum of 
20 rpm. 
Further, as the second flow path P.sub.2, there is the flow path in which 
the carrier gas G supplied from the top of the pulling chamber 1 branches 
at the top end of the tubular portion 4 of the flow controller 2 and 
passes through the top gap 18 to flow to the outer portion of the tubular 
portion 4. The carrier gas G led to this second flow path P.sub.2 ends up 
working with the carrier gas G passing through the first flow path P.sub.1 
and the other second flow path P.sub.2 to function to exhaust the silicon 
oxide outside the system and, further, has the following functions: 
When the carrier gas G branching at the top end of the tubular portion 4 of 
the flow controller 2 and passing through the top gap 18 to be led into 
the second flow path P.sub.2 flows down along the outer surface of the 
flow controller 2 and then flows further down along the outer surface of 
the graphite susceptor 8, that is, when passing through the opening 18 
between the graphite susceptor 8 and the flow controller 2, it sweeps up 
the atmosphere including the silicon oxide at the outside of the flow 
controller 2 by the aspiration effect and reduces the air pressure. 
Therefore, the silicon oxide vaporizing from the surface of the melt M is 
exhausted outside of the quartz crucible 3 along with the large energy gas 
from the flow path P.sub.1, so will not condense and solidify at the top 
of the flow controller facing the melt and fall into the melt. As a 
result, the yield of the monocrystal is improved. 
Since the atmosphere including the silicon oxide is exhausted in this way 
from the second flow path P.sub.2 by the aspiration effect, the carrier 
gas G led to the first flow path P.sub.1 uniformly contacts the entire 
surface of the melt M as rectified. 
Therefore, the amount of vaporization of the silicon oxide vaporizing from 
the melt surface becomes uniform and it becomes possible to suppress the 
adverse effects on the oxygen concentration caused by turbulence of the 
carrier gas G, that is, to improve the oxygen distribution (ORG) in the 
wafer surface of the pulled monocrystal. 
Further, the carrier gas G supplied from the top of the pulling chamber 1 
branches into the first flow path P.sub.1 to and the second flow path 
P.sub.2 at the top end of the tubular portion 4 of the flow controller 2, 
so for example if the resistance through the first flow path P.sub.1 
fluctuates, there is a function of absorbing that fluctuation. That is, 
when the monocrystal S is started to be pulled, the top of the pulled 
monocrystal S is still not positioned at the constricted diameter portion 
6 of the flow controller 2, so the gap dimension of the bottom gap 5 is 
large. 
In this way, the flow area of the first flow path P.sub.1 is large, so a 
large amount of carrier gas G is led to the first flow path P.sub.1. When 
the monocrystal S is pulled, the pulled monocrystal rises to the inside of 
the flow controller 2 and the flow area rapidly becomes smaller. 
By this, the flow resistance of the first flow path P.sub.1 rapidly 
increases, the flow rate of the carrier gas G passing through the bottom 
gap 5 becomes faster, and the top of the pulled monocrystal S becomes 
relatively high in oxygen concentration, but in the present invention, an 
amount of carrier gas G equal to the increase in the flow resistance of 
the bottom gap 5 is led into the second flow path P.sub.2, so as a result 
the flow rate of the carrier gas G passing through the bottom gap 5 does 
not fluctuate that much. 
When there is no branching, with a closed heat cap disclosed in U.S. Pat. 
No. 4,330,362, if the clearance of the bottom gap is made larger, the 
cooling of the melt surface decreases, vaporization of the silicon oxide 
is promoted, and a low oxygen concentration results. In a construction as 
with the flow controller of the present invention, however, where the gas 
branches to the first flow path P.sub.1 and the second flow path P.sub.2, 
the flow rate does not fluctuate that much, so it becomes unnecessary to 
finely adjust the bottom gap and otherwise be strict in the settings. 
Therefore, the distribution of the oxygen concentration in the axial 
direction of the pulled monocrystal S becomes constant. 
After this, the rotation of the crucible is adjusted to control the oxygen 
concentration. 
However, to pull a large diameter, high quality monocrystal without overly 
reducing the pulling speed, it is necessary to lengthen the region of over 
1300 .degree. C. near the crystal growth interface. To achieve this, it is 
advantageous to reduce the flow of the carrier gas or to raise the 
temperature. Therefore, in the present invention, an effect can be 
expected by having the radiant heat from the melt received by the carbon 
flow controller 2 and re-radiated to the pulled monocrystal. Further, 
after passing this region, it is possible to efficiently produce the high 
quality monocrystal by raising the cooling ability of the pulled 
monocrystal by the top water cooling tube. 
In addition to this, the melt surface near the inner peripheral wall of the 
quartz crucible 3 is not covered by the flow controller 2, so by observing 
the melt surface from a peephole 14 etc. provided in the pulling chamber 
1, it is possible to quickly deal with any heat deformation in the top end 
of the quartz crucible 3, recrystallization or silicon deposition near the 
inner peripheral wall of the quartz crucible 3, or other problems when 
they occur. 
Further, when pulling a monocrystal by the batch type CZ technique, the 
graphite susceptor 8 on which the quartz crucible 3 is carried is raised 
in accordance with the pulling of the monocrystal S so as to maintain the 
dimensions of the bottom gap. In the flow controller 2 of the present 
embodiment, a carbon flow controller 2 absorbs the heat irradiated from 
the melt surface, then radiates that heat to the pulled monocrystal. 
Considering this, the tubular portion 4 is formed in the flow controller 2 
so that the temperature will not fall to the extent where silicon oxide 
condenses. Therefore, even if the quartz crucible 3 and the graphite 
susceptor 8 are raised, the gap formed between the tubular portion 4 and 
the inner surface of the quartz crucible 3, that is, the area of the 
opening of the opening 18, can be maintained constant. Consequently, even 
if the quartz crucible 3 rises, there is no fluctuation caused in the flow 
of the carrier gas G passing through. 
The present invention will be explained in further detail to help the 
effect of the flow controller to be understood. 
FIG. 17 is a graph showing the branching of the carrier gas with respect to 
a ratio Ru/Rd of the bottom gap area Rd and the top gap ratio Ru. In the 
figure, when the area ratio Ru/Rd is 0.4, the flow of the first flow path 
becomes 20% while when it is 1.4, the flow of the first flow path becomes 
80%. 
At this time, if the area ratio Ru/Rd is smaller than 0.4, the crystal is 
overcooled and a high quality crystal cannot be obtained. Further, if the 
area ratio Ru/Rd is over 1.4, the carrier gas flowing through the first 
flow path becomes too little, so the gas including silicon oxide from the 
bottom gap rises and adheres to the top of the crystal. This condenses and 
falls into the melt, thereby creating the problem of the ruining of the 
monocrystal. 
This relationship will be explained with reference to a pulling apparatus 
for a 6 inch crystal. 
First, the crystal diameter A is 156 mm and the constricted diameter 
portion opening B of the flow controller is 210 mm, so the ratio of the 
area of the opening to the area of the crystal becomes 1.8. 
Further, the area Rd of the bottom gap after the pulled monocrystal passes 
through the opening of the constricted diameter portion of the flow 
controller becomes 15,523 mm.sup.2. The diameter d of the tubular portion 
of the flow controller is 342 mm, the opening width C is 12 mm, and the 
width of the engagement portion 7 is 60 mm, so the area of the top gap 18 
is 10,465 mm.sup.2. Therefore, the area ratio Ru/Rd becomes 0.67. Using 
such a pulling system, monocrystals were produced under conditions of 40 
nl/min of argon gas and 10 to 20 Torr of vacuum. The standard deviations 
in the target of the oxygen concentrations were as shown in Table 1. It 
was possible to obtain the monocrystals aimed for in the present 
invention. 
TABLE 1 
______________________________________ 
Standard Deviation in Oxygen Concentrations 
Class Target value 
Power Standard deviation 
______________________________________ 
Low oxygen 
1.15 .times. 10.sup.18 
121 0.062 
Medim oxygen 
1.39 .times. 10.sup.18 
221 0.044 
High oxygen 
1.55 .times. 10.sup.18 
110 0.039 
______________________________________ 
The embodiments explained above were described to facilitate the 
understanding of the present invention and were not meant to limit the 
present invention in any way. Therefore, the elements disclosed in the 
above embodiment include all design modifications and equivalents falling 
under the technical scope of the present invention. 
As explained above, according to the present invention, control of the 
concentration of oxygen in the pulled monocrystal is possible, achievement 
of a uniform oxygen distribution (ORG) in the wafer surface is possible, 
it is possible to pull a monocrystal with a uniform oxygen concentration 
with respect to the axial direction, and it is possible to pull a high 
quality monocrystal having a particularly large diameter.