Method of controlling oxygen concentration in single crystal and an apparatus therefor

A method for controlling an oxygen concentration of a single crystal which is pulled up in a Czochralski-method type single crystal pulling apparatus having a hermetical chamber in which the single crystal is pulled up and an inert gas supply and exhaust system by means of which an inert gas is supplied to the hermetical chamber and exhausted therefrom; the method being characterized in that the pneumatic pressure in the hermetical chamber and the supply rate of the inert gas are controlled in accordance with a prepared control pattern with respect to the proportion of the length of the as-grown crystal to the aimed final length thereof or with respect to the passage of time.

This invention relates to a method and an apparatus for controlling an 
oxygen concentration in a single crystal ingot (rod) pulled up from a 
polycrystal melt of a semiconductor substance in Czochralski (CZ) method. 
BACKGROUND OF THE INVENTION 
With reference to attached FIG. 1, in a CZ method-type single crystal 
pulling apparatus for growing and raising a single crystal silicon ingot 
(9) from a polycrystal silicon melt (10), a silicon seed crystal (12) 
fixed at the lower end of a pull wire (11) (or, alternatively, supported 
by a pull shaft) is dipped slightly in the silicon melt contained in a 
synthetic quartz crucible (4) provided in a chamber (2), and is pulled up 
at a predetermined rate while being turned about its axis by means of the 
pull wire, whereby a silicon single crystal rod is grown and raised. 
In this kind of single crystal pulling apparatus, a chemical reaction takes 
place between the quartz crucible and the Si melt to generate SiO, which 
is dissolved in the Si melt. A part of the SiO evaporates from the melt 
and mixes in the atmosphere within the chamber, and other part stays in 
the Si melt. 
When the SiO in the atmosphere builds up in the chamber, the normal growth 
of the single crystal is seriously affected. In order to prevent this 
phenomenon, the chamber is kept under reduced pressure by means of a 
vacuum pump (18) and, at the same time, it is supplied with an inert gas 
such as argon gas in order to maintain the inertness of the intrachamber 
atmosphere. Normally, therefore, the chamber is filled with an inert gas 
and the pressure in the chamber is kept at the level of ten-odd Torr. 
As the pulling of the single crystal proceeds, the amount of SiO gas 
generated in the chamber increases to thereby push up the intrachamber 
pressure, which in turn causes stagnation of the flow of the inert gas. In 
order to prevent this phenomenon, a vacuum gage is attached to the chamber 
to detect the intrachamber pressure; and based on the result of the 
pressure measurement, the opening degree of a flow control valve which is 
provided in the exhaust system is controlled in a manner such that the 
intrachamber pressure is always constant or changed arbitrarily [Japanese 
Kokai No. 61-117191 (1986)]. 
Now, turning our attention to the oxygen concentration of a single crystal, 
it is known that as the pulling of the single crystal proceeds and the 
level of the Si melt lowers, the amount of SiO in the Si melt, which 
virtually determines the oxygen amount in the melt, decreases, and this 
results in a gradual decrease in the oxygen concentration in the growing 
single crystal ingot. Consequently, the axial uniformity in the oxygen 
concentration is lost, and the tail end portion of the ingot may end up 
with an oxygen concentration less than the minimum allowable value. 
In the conventional technology, only the flow of the inert gas was 
controlled, and as such it was not possible to solve this problem of 
uneven oxygen concentration. But if the oxygen concentration of the single 
crystal is to be controlled with a sufficiently high precision, an 
extremely complicated control of the intrachamber pressure must be 
conducted with respect to the progress of the single crystal pulling 
operation. 
SUMMARY OF THE INVENTION 
This invention was made in view of this problem, and it is, therefore, an 
object of the invention to provide a method, as well as an apparatus, for 
controlling the oxygen concentration in a single crystal ingot (rod) 
pulled up from a polycrystal melt of a semiconductor substance in 
Czochralski (CZ) method with high precision. 
More particularly, an object of the invention is to provide a method and an 
apparatus which enable arbitrary controlling of axial distribution of 
oxygen concentration in the single crystal ingot and thus enable growing 
of a single crystal ingot in which the oxygen concentration is within an 
allowable range throughout the whole length of the ingot. 
According to the invention, a method is proposed for controlling an oxygen 
concentration of a single crystal which is pulled up in a 
Czochralski-method type single crystal pulling apparatus having a 
hermetical chamber in which the single crystal is pulled up and an inert 
gas supply and exhaust system by means of which an inert gas is supplied 
to the hermetical chamber and exhausted therefrom; the method being 
characterized in that the pneumatic pressure in the hermetical chamber and 
the supply rate of the inert gas are controlled in accordance with a 
prepared control pattern with respect to the proportion of the length of 
the growing crystal to the aimed final length thereof or with respect to 
the time of growing. 
Preferably, the prepared control pattern is programmed in a storage of a 
CPU, and the control operation is conducted in accordance with the command 
signals from the CPU. 
In a preferred embodiment of the method, the prepared control pattern is 
programmed such that the pneumatic pressure in the hermetical chamber is 
varied while the supply rate of the inert gas is kept unchanged. 
Or alternatively, the prepared control pattern is programmed such that the 
supply rate of the inert gas is varied while the pneumatic pressure in the 
hermetical chamber is kept unchanged. 
According to the invention, an apparatus is also provided for controlling 
the oxygen concentration of a single crystal which apparatus comprises a 
control valve means provided across an exhaust line of the inert gas 
supply and exhaust system for controlling the pressure in the hermetical 
chamber, a pressure sensor for detecting the pressure in the hermetical 
chamber, and a control means for controlling the supply rate of the inert 
gas to the hermetical chamber. 
OPERATION 
According to the invention, it is now possible to regulate the growth of a 
single crystal rod in a manner such that the oxygen concentration of the 
single crystal rod throughout its entire length becomes within a desired 
range. For example, in the case of growing a Si single crystal rod; as the 
proportion of the grown length of the crystal to the aimed final length 
thereof increases, the pressure in the chamber is increased under control 
and/or the flow rate of the inert gas supplied to the chamber is decreased 
under control, such that the evaporation rate of SiO in the chamber is 
decreased whereby the concentration of SiO remaining in the Si melt in the 
crucible is increased, with a result that the lengthwise decrease in the 
oxygen concentration of the single crystal is checked.

EMBODIMENT 
An embodiment of the invention will now be described with reference to the 
attached drawings. 
Reference numeral 1 designates a single crystal pulling apparatus based on 
CZ method, in which a pull chamber 3 is provided above and in coaxial 
alignment with the chamber 2, both made of a stainless steel. A quartz 
crucible 4 for containing a semiconductor substance 10 is supported by a 
support shaft 5 in the chamber 2. The support shaft 5 is adapted to rotate 
about its axis and shift in the axial direction. If the quartz crucible 4 
were always held at the original level, the melt level in the quartz 
crucible 4 would gradually shift downwards relative to a heater 6 with the 
growth of a single crystal 9, thus resulting in an instability of the 
thermal field around the crystal and the melt 10. Therefore, the support 
shaft 5 is adapted to axially displace the crucible such that the 
displacement of the melt level (solid-liquid interface) downwards is 
compensated for by continuous rise of the crucible 4 so that the interface 
level is stationary relative to the heater 6 during the growing. 
Around the heater 6 is provided a cylindrical thermal insulator 7, both 
made of carbon. 
In the upper part of the chamber 2 a purge tube 8 for supplying Ar gas is 
provided vertically and in coaxial alignment with the growing single 
crystal 9, which is raised into the purge tube 8. During the single 
crystal pulling operation, the altitudinal position of the crucible is 
controlled such that the lower end of the purge tube 8 is always in the 
upward vicinity of the surface of the melt of the polycrystal Si melt 10. 
A seed crystal 12 is fixed at the lower end of a pull wire 11, which hangs 
within the purge tube 8. The wire 11 is rotated and vertically moved at a 
rate by means of a drive mechanism, not shown for the sake of simplicity. 
Argon gas is supplied to the pull chamber 3 and the chamber 2 from an argon 
gas supply source 13, e.g. a gas cylinder, by way of a supply line 14, 
across which are provided a mass flow controller (MFC) 15 and a valve 16. 
The MFC 15 is capable of controlling the Ar gas flow rate to a set value. 
The thus supplied Ar gas and the SiO gas evaporating from the melt are 
drawn from the chamber 2 by means of a vacuum pump 18. In an exhaust line 
19 connecting the chamber 2 with the vacuum pump 18 is provided a 
conductance valve 20, which consists of an electrically operated butterfly 
valve 21 and an electrically operated needle valve 22, the valves 21 and 
22 being arranged in parallel. The needle valve 22 is driven by a pulse 
motor 23 such that its opening is precisely controlled. 
Incidentally, it is possible to replace the conductance valve 20 with a 
single electrically controlled butterfly valve or an electrically 
controlled ball valve. 
A pressure sensor 24 is affixed to the chamber 2 for detecting the internal 
pressure (negative pressure) of the chamber 2. 
In FIG. 1, reference numeral 25 designates a central processing unit (CPU) 
25 which constitutes control means adapted to control the internal 
pressure of the chamber 2 and/or the Ar gas flow rate by controlling the 
opening of the needle valve 22 chiefly in response to the pressure value 
detected by the pressure sensor 24 in accordance with a control pattern 
corresponding to the length of the growing single crystal ingot. 
In particular, the pressure value (analog value) detected by the pressure 
sensor 24 is digitized through an A/D converter 26, and inputted to the 
CPU 25, which 25 outputs a control signal responsive to the detected 
pressure value. This control signal is amplified through a pulse amplifier 
27, and inputted to the pulse motor 23, which, thereupon, drives the 
needle valve 22 to control the opening thereof based on the control signal 
received. As a result, the internal pressure of the chamber 2 and/or the 
Ar gas flow rate is controlled in accordance with the control pattern. 
When the opening of the needle valve 22 is decreased, the intrachamber 
pressure increases, and the flow rate of the Ar gas is lowered. 
When it is not possible to control the pressure of the chamber 2 by only 
adjusting the opening degree of the needle valve 22, the opening of the 
MFC 15 is adjusted to change the Ar flow rate to thereby control the 
pressure in the chamber 2. 
The flow rate of Ar gas (analog value) detected by the mass flow controller 
15 is digitized through the A/D converter 29, and supplied to the CPU 25 
as a feedback. The CPU 25 compares this detected Ar gas flow rate with a 
reference value for Ar gas flow rate, and generates a control signal 
(digital signal) based on the result of the comparison. Then, this digital 
control signal is converted into an analog signal through a D/A converter 
30 and inputted to the MFC 15 to renew the set value to which the Ar gas 
flow rate is controlled. 
The Ar gas flow rate detected by the MFC 15 and the internal pressure of 
the chamber 2 detected by the pressure sensor 24 are alternatively 
displayed on the display device 32 by alternating the switch 31. 
To pull up the Si single crystal 9 based on the CZ method in the single 
crystal pulling apparatus 1, lumps of polycrystal silicon having 
appropriate sizes are charged into the crucible 4, and melted by the 
heater 6; then the seed crystal 12 connected to the lower end of the wire 
11 is lowered until it comes in contact with the Si melt 10 contained in 
the crucible 4. After the lower end of the seed crystal 12 is partially 
melted, the melt temperature is lowered to such an extent that the seed 
crystal would not be melted any more. Then, the wire carrying the seed 
crystal is pulled up at a rate, and a single crystal grows on the seed 
crystal, while the crystal and the melt are counter-rotated with respect 
to each other. During this operation, Ar gas is supplied to the chamber 2 
through the purge tube 8, and this Ar gas and the SiO gas evaporating from 
the melt are drawn out of the chamber 2 by the vacuum pump 18. 
As the single crystal pulling operation proceeds, the melt level shifts 
downwards relative to the crucible, and, as a result, the ratio of the 
surface area to volume of Si melt increases gradually whereby the 
concentration of SiO, or oxygen concentration, in the Si melt 10 
decreases, which results in progressively decreasing oxygen concentration 
in the single crystal 9. 
With the view of preventing this phenomenon of gradual decrease in the 
oxygen concentration of the single crystal ingot with time, experiments 
were conducted and the result in one of them is as follows: the opening of 
the needle valve 22 was controlled in a manner such that the pressure in 
the chamber 2 was maintained constant (100 millibar), and the flow rate of 
the Ar gas was gradually decreased along a curve shown in FIG. 2 by means 
of the MFC 15 as the single crystal ingot grows longer; the analysis of 
the oxygen concentration of the resulting silicon single crystal ingot 
(FIG. 3) revealed that the oxygen concentration did not decrease with 
time, but increased at a low rate as the crystallization proceeded. 
Incidentally, the axis of abscissa represents the percentage of the grown 
length of the ingot based on the final length thereof. It is postulated 
that the oxygen concentration picked up, rather than kept decreasing, 
because as the Ar gas flow rate decreased, the evaporation amount of the 
SiO gas also decreased in the chamber 2 whereby the amount of SiO staying 
in the Si melt 10 increased and more oxygen could enter the single crystal 
9. 
As described above, it is possible to control the Ar gas flow rate in 
accordance with a control pattern, like that of FIG. 2, stored in the CPU 
25, so that it is possible to arbitrarily control the oxygen concentration 
of the single crystal ingot after conducting trial operations. 
Consequently, the oxygen concentration throughout the whole length of the 
single crystal 9 can be controlled to be within a desired range. 
Since by increasing the pressure in the chamber 2 the evaporation rate of 
the SiO decreases, it is also possible to attain a similar result as above 
by controlling the opening of the needle valve 22, instead of controlling 
the Ar gas flow rate at the MFC 15, in accordance with a pattern stored in 
CPU. 
Incidentally, the control pattern, according to which the changes in the 
pressure in the chamber 2 and/or the Ar gas flow rate are ruled, is 
programmed in advance in a memory of the CPU, and the manner of 
determining such a pattern is carried out in the following procedure. 
Let P represent the pressure in the chamber 2, F the Ar gas flow rate, S 
the grown length of the single crystal ingot divide by the final length 
thereof O.sub.i the oxygen concentration, then it is empirically confirmed 
that O.sub.i at S is given by the following linear Equation (1): 
EQU O.sub.i =aS+bP+cF+d (1) 
where a, b, c, and d are empirically obtained coefficients. An example of 
Equation (1) can be as follows: 
EQU O.sub.i =-0.1S+0.03P-0.01F+20 (2) 
Since the coefficients a, b, c, and d are modified depending on different 
conditions, such as different kinds of pulling apparatus and pulling 
condition, these coefficients must be determined responsive to various 
operational and other conditions. 
For example, in a trial operation, while the pressure P and the Ar gas flow 
rate F are kept constant, the oxygen concentration O.sub.i is measured 
with respect to different percentages S. Then, the measured oxygen 
concentration O.sub.i is plotted and the coefficient value a is obtained 
as the general gradient of the curve (i.e., .differential.O.sub.i 
/.differential.S=a). Similarly, while the Ar gas flow rate F and the 
percentage S are kept constant, the oxygen concentration O.sub.i is 
measured with respect to different pressure P. And the value of 
coefficient b is obtained as the gradient of the curve of O.sub.i vs. P 
(i.e., .differential.O.sub.i /.differential.P=b). Next, while the pressure 
P and the percentage S are kept constant, the oxygen concentration O.sub.i 
is measured with respect to different flow rate F. The value of 
coefficient c is determined as the gradient of the curve of O.sub.i vs. F 
(i.e., .differential.O.sub.i /.differential.F=c). 
Different tentative values for d are obtained from the equation (1) by 
substituting in the equation the values of a, b, c, and the actually 
measured values of S, P, and F. The desired value for the constant d is 
determined as the arithmetic mean of these tentative d values. 
Next, a simplified method will be described for obtaining a control pattern 
which, when observed, will produce a single crystal ingot in which the 
oxygen concentration is uniform with respect to the axis of the ingot. 
First, a single crystal ingot is grown under a condition where the pressure 
P of the chamber 2 and the Ar gas flow rate F are kept constant, and the 
O.sub.i is measured and plotted with respect to S to thereby an O.sub.i 
profile. Thereafter, S is divided up into small segments having a width of 
S, and the oxygen concentration O.sub.i corresponding to the S value at 
each increment end of the segment S is obtained. If it is intended that 
the oxygen concentration O.sub.i O is controlled to and maintained at a 
desired constant value O.sub.i O through variation of the pressure P alone 
while the Ar flow rate is kept unchanged at Fc, the following procedure is 
taken. Let the initial condition be such that P=PO, O.sub.i =O.sub.i O, 
and S=SO. Then, the increment P from PO that would cause the oxygen 
concentration to increase by O.sub.i at Fc which is the difference between 
O.sub.i O and the measured value O.sub.i 1 at the increment end of the 
first segment S, is obtained from Equation (1) or (2) and the O.sub.i 
profile. Namely, the value P is obtained in the following procedure: 
EQU O.sub.i 0=aSO+bPO+cFc+d (3) 
EQU O.sub.i 1=aS+bP+cFc+d (4) 
Subtracting Equation (3) from Equation (4) gives: 
EQU O.sub.i 1-O.sub.i 0=a(S-SO)+b(P-PO) 
or 
EQU O.sub.i 1-O.sub.i 0=aS+bP 
and hence: 
EQU P=(O.sub.i 1-O.sub.i 0-aS)/b 
The same operation is repeated with respect to the second segment S, and so 
on. Each resultant increment P is added to the preceding PO+.SIGMA.P 
value, and thus the desired control pattern of P is obtained. Such a 
control pattern is stored in a memory of the CPU 25, and in accordance 
with the control pattern, the pressure P of the chamber 2 is controlled to 
the aimed values which give constant O.sub.i 0. 
Similarly, the like controlling of the Ar gas flow rate F while the 
pressure P of the chamber 2 is kept constant can give a uniform O.sub.i 
throughout the ingot. However, it should be noted that in this case, 
unlike the case of controlling by the chamber pressure P, the Ar flow rate 
F ought to be decreased, rather than increased, to obtain constant oxygen 
concentration. Although in the above embodiment of the invention, the 
controlling of the oxygen concentration throughout the length of the 
single crystal ingot is conducted with respect to the ratio of the grown 
length per the aimed final length, it is possible to conduct the same with 
respect to the time that passes from the start of the pulling operation. 
EFFECTS OF THE INVENTION 
As is clear from the above description, the invention provides a method and 
an apparatus which enable arbitrary controlling of oxygen concentration 
with respect to the axis of the single crystal ingot raised in a CZ type 
pulling apparatus.