U.S. patent application number 09/867628 was filed with the patent office on 2002-03-14 for apparatus for pulling a single crystal.
Invention is credited to Hamada, Ken, Maeda, Tokuji, Takanashi, Keiichi.
Application Number | 20020029738 09/867628 |
Document ID | / |
Family ID | 18665403 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020029738 |
Kind Code |
A1 |
Takanashi, Keiichi ; et
al. |
March 14, 2002 |
Apparatus for pulling a single crystal
Abstract
The present invention was achieved in order to provide an
apparatus for pulling a single crystal, wherein a flow of an inert
gas to a single crystal to be grown, pressure in an apparatus body,
and a temperature environment are always kept constant by keeping
the melt level at a prescribed position in spite of changes in
volume of a quartz crucible between batches and thermal deformation
of the quartz crucible, so that high quality single crystals can be
pulled, comprising a reference reflector arranged inside an
apparatus body, a level position measuring means to measure an
actual level position by detecting a mirror image position of the
reference reflector reflected in the melt surface using a
one-dimensional CCD camera arranged outside the apparatus body, a
crucible ascent speed adjustment value calculating means to
calculate an adjustment value of the crucible ascent speed based on
an output from the level position measuring means, an adjustment
value adding means to add the adjustment value to the crucible
ascent speed, and a level position controlling means to control the
level position in the crucible by controlling a motor for crucible
lifting based on an output from the adjustment value adding means
to control the crucible ascent speed.
Inventors: |
Takanashi, Keiichi;
(Nishinomiya-shi, JP) ; Maeda, Tokuji;
(Kishima-gun, JP) ; Hamada, Ken; (Saga-shi,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
18665403 |
Appl. No.: |
09/867628 |
Filed: |
May 31, 2001 |
Current U.S.
Class: |
117/217 ;
117/208; 117/218 |
Current CPC
Class: |
C30B 15/20 20130101;
C30B 15/30 20130101; Y10T 117/1072 20150115; Y10T 117/1032
20150115; Y10T 117/1068 20150115 |
Class at
Publication: |
117/217 ;
117/208; 117/218 |
International
Class: |
C30B 015/00; C30B
030/04; C30B 021/06; C30B 027/02; C30B 028/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2000 |
JP |
J.P.A.161371/2000 |
Claims
What is claimed is:
1. An apparatus for pulling a single crystal, which has a lifting
means of a crucible and a level position controlling means to
control the level position in the crucible by calculating a
crucible ascent speed based on a decrease in volume of a melt with
pulling a single crystal to control the lifting means, comprising:
a reference reflector arranged inside an apparatus body; a level
position measuring means to measure an actual level position by
detecting a mirror image position of the reference reflector
reflected in the melt surface using an optical device arranged
outside the apparatus body; a crucible ascent speed adjustment
value calculating means to calculate an adjustment value of the
crucible ascent speed based on an output from the level position
measuring means; an adjustment value adding means to add the
adjustment value to the crucible ascent speed; and the level
position controlling means to control the level position in the
crucible by controlling the lifting means based on an output from
the adjustment value adding means to control the crucible ascent
speed.
2. An apparatus for pulling a single crystal according to claim 1,
comprising an adjustment value addition propriety judging means to
judge the propriety of adding the adjustment value to the crucible
ascent speed, based on a diameter of a pulling crystal, a level
position measured by the level position measuring means, a distance
between the crystal and the reference reflector, and a crucible
rotational speed.
3. An apparatus for pulling a single crystal according to claim 1
or 2, comprising an averaging means to average level positions
measured by the level position measuring means.
4. An apparatus for pulling a single crystal according to claim 1,
wherein the optical device constituting the level position
measuring means is also used as an optical device for measuring a
crystal diameter.
5. An apparatus for pulling a single crystal according to claim 1,
comprising an automatic updating means to automatically work out a
conversion equation for converting a mirror image position of the
reference reflector on the melt surface to an actual level position
by automatically moving the crucible up and down from the initial
position to obtain the relationship between the mirror image
position of the reference reflector on the melt surface and the
level position, and making the relationship approximate to a
straight line.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an apparatus for pulling a
single crystal and, more particularly, to an apparatus for pulling
a single crystal used for pulling a single crystal of silicon or
the like from a melt of a material for forming a crystal by a
pulling method such as the Czochralski method (hereinafter,
referred to as the CZ method).
[0003] 2. Description of the Relevant Art
[0004] At present, the majority of silicon single crystals used for
manufacturing a substrate for forming a circuit component of a LSI
(large scale integrated circuit) and the like have been pulled by
the CZ method. FIG. 3 is a diagrammatic sectional view of a
conventional apparatus for pulling a single crystal used for the CZ
method, and in the figure, reference numeral 21 represents a
crucible.
[0005] The crucible 21 comprises a bottomed cylindrical quartz
crucible 21a and a bottomed cylindrical graphite crucible 21b
fitted on the outer side of the quartz crucible 21a. The crucible
21 is supported with a support shaft 28 which rotates in the
direction shown by the arrow A in the figure at a prescribed speed.
A heater 22 of a resistance heating type and a heat insulating mold
27 arranged around the heater 22 are concentrically arranged around
the crucible 21. The crucible 21 is charged with a melt 23 of a
material for forming a crystal which is melted by the heater 22. On
the central axis of the crucible 21, a pulling shaft 24 made of a
pulling rod or wire is suspended, and at the front thereof, a seed
crystal 35 is held by a holder 24a. These members are arranged
within a water cooled type apparatus body 29 wherein pressure can
be controlled.
[0006] A window 29a is formed in the middle portion in the height
direction of the apparatus body 29. Above the window 29a in a
slanting direction, a one-dimensional CCD camera 11 is arranged in
such a position that a single crystal 36 during pulling can be seen
through the window 29a, and the one-dimensional CCD camera 11 is
connected to a diameter measuring means 8 of the single crystal 36.
The diameter measuring means 8 is connected to a crucible ascent
speed calculating means 9, which captures an input value of the
pulled single crystal 36 from the diameter measuring means 8 and
information on the pulled length from a pulling means 5 of the
pulling shaft 24 to calculate a decrease in volume of the melt
within the crucible 21, SD as to calculate an ascent speed of the
crucible 21 based on the decrease in the melt volume.
[0007] A method for pulling a single crystal 36 using the above
apparatus for pulling a single crystal is described by reference to
FIGS. 3 and 4. FIGS. 4(a)-4(d) are partial enlarged front views
diagrammatically showing a seed crystal and the vicinity thereof in
part of the steps in pulling a single crystal.
[0008] Although it is not shown in FIG. 4, the pressure within the
apparatus body 29 is reduced, and an inert gas is introduced into
the apparatus body 29 so as to make an inert gas atmosphere under
reduced pressure therein. Then, the material for forming a crystal
is melted by the heater 22 and is maintained for a period of time
so as to sufficiently release gas contained in the melt 23.
[0009] While the pulling shaft 24 is rotated on the same axis in
the reverse direction of the support shaft 28 at a prescribed
speed, the seed crystal 35 held by the holder 24a is caused to
descend and is brought into contact with the melt 23. After the
front portion of the seed crystal 35 is partially melted into the
melt 23, the single crystal 36 begins to be pulled from the melt
(the seeding step) (FIG. 4(a)).
[0010] In making a crystal grow at the front of the seed crystal
35, the pulling shaft 24 is pulled at a higher speed than the
below-described formation speed of a main body 36c, and the crystal
is narrowed to have a prescribed diameter, leading to the formation
of a neck 36a (the necking step) (FIG. 4(b)).
[0011] By slowing down the pulling speed of the pulling shaft 24
(hereinafter, simply referred to as the pulling speed), the neck
36a is made to grow to have a prescribed diameter, leading to the
formation of a shoulder 36b (the shoulder formation step) (FIG.
4(c)).
[0012] By pulling the pulling shaft 24 at a fixed rate, the main
body 36c having a uniform diameter and a prescribed length is
formed (the main body formation step) (FIG. 4(d)).
[0013] Furthermore, although it is not shown in FIG. 4, in order to
prevent induction of high density dislocation to the single crystal
36 by a steep temperature gradient at the end, the diameter of the
single crystal 36 is gradually decreased and the temperature of the
whole single crystal 36 is gradually lowered, leading to the
formation of an end-cone. Then, the single crystal 36 is separated
from the melt 23. After the above steps, cooling the single crystal
36 leads to the completion of pulling the single crystal 36.
[0014] In pulling the single crystal 36, in order to obtain a good
quality single crystal 36, it is necessary to drive the support
shaft 28 using a motor 10 for crucible elevating to move up the
crucible 21, so as to control the position of the crucible 21 so
that the melt surface is always kept in a fixed position to the
heater 22. In the crucible ascent control, a method wherein the
amount of ascent of the crucible 21 is calculated by calculating a
decrease in volume of the melt 23 from the volume of the pulled
single crystal 36 has been generally used.
[0015] Since the amount of ascent of the crucible 21 is calculated
by calculating the decrease in volume of the melt 23 from the
volume of the pulled single crystal 36 in the above crucible ascent
control, the accurate volume of the pulled single crystal 36 need
to be obtained. In addition, in order to calculate the accurate
amount of the crucible ascent from the decrease in volume of the
melt 23, the accurate internal diameter of the crucible is
required. However, in actual pulls, errors are introduced to the
crucible ascent control by errors in measuring crystal weight and
crystal diameters, changes in the internal diameter of the quartz
crucible 21a due to softening thereof during pulling, and changes
in the internal diameter of the quartz crucible 21a between batches
originating in variations in manufacturing the quartz crucible 21a.
Therefore, it is necessary to actually measure to control the level
position during pulling the single crystal 36.
[0016] In order to solve the above problems, a method for measuring
the level position has been disclosed in Japanese Kokai No.
63-281022. In the method for measuring the level position, the
level position is calculated from the position of the mirror image
of a radiation screen reflected in the melt surface. The method
makes it possible to measure the level position during crystal
pulling.
[0017] Similarly to the method disclosed in the above publication,
a method wherein the level position is controlled based on the
mirror image position of a radiation screen reflected in the melt
surface has been disclosed in Japanese Kokai No. 07-277879. In the
method, by moving a crucible up and down so that the mirror image
position of the radiation screen is kept fixed, the level position
is controlled so as to be kept fixed.
[0018] However, in those methods, the inclination of the melt
surface in which the mirror image of the radiation screen is
reflected causes a change of the mirror image position even when
the level position does not actually change, leading to the
erroneous recognition and control of the level position. Factors of
the inclination of the melt surface are changes in crystal diameter
and changes in crucible rotational speed. Therefore, errors arise
with changes in crystal diameter and changes in crucible rotational
speed when the level position is controlled based on only the
mirror image position of the radiation screen.
[0019] Particularly, the shoulder formation step wherein the
crystal diameter is increased to a desired diameter from the seed
crystal is a problem. In the shoulder formation step, the
inclination of the melt surface increases with increase in crystal
diameter. Therefore, when the level position control by which the
mirror image position of the radiation screen is kept fixed is
conducted in the shoulder formation step, the actual level
positions before and after the shoulder formation step become
different.
[0020] The same errors are also caused by changes in crystal
diameter in the main body formation step. In addition, when the
level position control is conducted based on only the mirror image
position of the radiation screen on the melt surface, the
occurrence of irregularity in measuring the level position makes it
impossible to carry out the ascent/descent control of the crucible
21.
SUMMARY OF THE INVENTION
[0021] The present invention was accomplished in order to solve the
above problems, and it is an object of the present invention to
provide an apparatus for pulling a single crystal, wherein a flow
of an inert gas to a single crystal to be grown, pressure in an
apparatus body, and a temperature environment are always kept
constant by keeping the melt level at a prescribed position in
spite of changes in volume of a quartz crucible between batches and
thermal deformation of the quartz crucible, so that high quality
single crystals can be pulled.
[0022] In order to achieve the above object, an apparatus for
pulling a single crystal (1) according to the present invention is
characterized by being an apparatus for pulling a single crystal
which has a lifting means of a crucible and a level position
controlling means to control the level position in the crucible by
calculating a crucible ascent speed based on a decrease in volume
of a melt with pulling a single crystal so as to control the
lifting means, comprising a reference reflector arranged inside an
apparatus body, a level position measuring means to measure an
actual level position by detecting a mirror image position of the
reference reflector reflected in the melt surface using an optical
device arranged outside the apparatus body, a crucible ascent speed
adjustment value calculating means to calculate an adjustment value
of the crucible ascent speed based on an output from the level
position measuring means, an adjustment value adding means to add
the adjustment value to the crucible ascent speed, and the level
position controlling means to control the level position in the
crucible by controlling the lifting means based on an output from
the adjustment value adding means so as to control the crucible
ascent speed.
[0023] Using the above apparatus for pulling a single crystal (1),
having the reference reflector, the level position measuring means,
the crucible ascent speed adjustment value calculating means and
the adjustment value adding means, the crucible ascent speed is
controlled by controlling the lifting means based on the output
from the adjustment value adding means by the level position
controlling means, while the level position in the crucible is
controlled by calculating the crucible ascent speed based on the
decrease in volume of the melt with single crystal pulling to
control the lifting means by the level position controlling means.
Therefore, even if the quartz crucible softens about the melting
point of silicon and becomes deformed during pulling, the level
position in the crucible can be controlled with high accuracy.
[0024] An apparatus for pulling a single crystal (2) according to
the present invention is characterized by comprising an adjustment
value addition propriety judging means to judge the propriety of
adding the adjustment value to the crucible ascent speed, based on
a diameter of a pulling crystal, a level position measured by the
level position measuring means, a distance between the crystal and
the reference reflector, and a crucible rotational speed in the
apparatus for pulling a single crystal (1).
[0025] Using the above apparatus for pulling a single crystal (2),
whether the adjustment value should be added to the crucible ascent
speed or not is judged by the adjustment value addition propriety
judging means, based on the diameter of a pulling crystal, the
level position measured by the level position measuring means, the
distance between the crystal and the reference reflector, and the
crucible rotational speed. These items are chiefly factors of
determining the inclination of the melt surface in the process of
single crystal pulling. The situation where the selected values
thereof for judgement are not satisfied under pulling conditions
means that the inclination of the melt surface is not suitable for
measuring the level position, and that the measurement result is
significantly affected by the inclination of the melt surface. In
that case, the adjustment value calculated from the level position
measuring means is not added to the crucible ascent speed.
Furthermore, even when there is such a problem in measuring the
actual level position by detecting the mirror image position of the
reference reflector reflected in the melt surface using the optical
device arranged outside the apparatus body, the control wherein the
level position in the crucible is controlled by calculating the
crucible ascent speed based on the decrease in volume of the melt
with single crystal pulling so as to control the lifting means is
carried out. As a result, the influence of not adding the
adjustment value to the crucible ascent speed on the level position
change can be kept to a minimum.
[0026] An apparatus for pulling a single crystal (3) according to
the present invention is characterized by comprising an averaging
means to average level positions measured by the level position
measuring means in the apparatus for pulling a single crystal (1)
or (2).
[0027] Using the above apparatus for pulling a single crystal (3),
it is possible to reject the effect of fluctuation of the melt
surface. Another factor of errors in measuring the actual level
position by detecting the mirror image position of the reference
reflector reflected in the melt surface using the optical device
arranged outside the apparatus body is the fluctuation of the melt
surface. The fluctuation of the melt surface during measurement
causes variations in measurement. The effect of the fluctuation of
the melt surface can be rejected by the action of the averaging
means to average the level positions measured by the level position
measuring means. It is desired that a period of time for averaging
should be set in the range of 1 to 30 minutes or so.
[0028] An apparatus for pulling a single crystal (4) according to
the present invention is characterized by the optical device
constituting the level position measuring means being also used as
an optical device for measuring a crystal diameter in any of the
apparatus for pulling a single crystal (1)-(3).
[0029] Using the above apparatus for pulling a single crystal (4),
the cost of manufacturing an apparatus for pulling a single crystal
can be held clown by using the optical device constituting the
level position measuring means as the optical device for measuring
a crystal diameter.
[0030] An apparatus for pulling a single crystal (5) according to
the present invention is characterized by comprising an automatic
updating means to work out a conversion equation for converting a
mirror image position of the reference reflector on the melt
surface to an actual level position by automatically moving the
crucible up and down from the initial position thereof to obtain
the relationship between the mirror image position of the reference
reflector on the melt surface and the level position and making the
relationship approximate to a straight line in any of the apparatus
for pulling a single crystal (1)-(4).
[0031] Variations in the setting position of the optical device for
detecting the mirror image position of the reference reflector and
the setting position of the reference reflector are also factors of
measurement errors in relation to the level position measurement.
If the setting situations thereof do not change, it might be
possible to accurately calculate the level position from the mirror
image position of the reference reflector based on a conversion
equation worked out from the geometric arrangement beforehand, but
it is actually difficult. Therefore, a conversion equation of the
mirror image position of the reference reflector to the level
position in the actual setting situations is required. Using the
above apparatus for pulling a single crystal (5), having the
automatic updating means to automate the activity of working out
the conversion equation, it is possible to automatically find the
conversion equation before crystal pulling using the automatic
updating means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a diagrammatic sectional view showing an apparatus
for pulling a single crystal according to an embodiment of the
present invention;
[0033] FIG. 2 is a flow chart showing the operation of a level
position controlling means in the apparatus for pulling a single
crystal according to the embodiment;
[0034] FIG. 3 is a diagrammatic sectional view showing a
conventional apparatus for pulling a single crystal; and
[0035] FIGS. 4(a)-4(d) are partial enlarged front views
diagrammatically showing a seed crystal and the vicinity thereof in
part of the steps in pulling a single crystal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The preferred embodiments of the apparatus for pulling a
single crystal according to the present invention are described
below by reference to those Figures.
[0037] FIG. 1 is a diagrammatic sectional view of an apparatus for
pulling a single crystal according to an embodiment, and in the
figure, reference numeral 21 represents a crucible which is
vertically movable. The crucible 21 comprises a bottomed
cylindrical quartz crucible 21a and a bottomed cylindrical graphite
crucible 21b fitted on the outer side of the quartz crucible 21a.
The crucible 21 is supported with a support shaft 28 which rotates
in the direction shown by the arrow A in the figure at a prescribed
speed. A heater 22 of a resistance heating type and a heat
insulating mold 27 arranged around the heater 22 are concentrically
arranged around the crucible 21. The crucible 21 is charged with a
melt 23 of a material for forming a crystal which is melted by the
heater 22. On the central axis of the crucible 21, a pulling shaft
24 made of a pulling rod or wire is suspended, and at the front
thereof, a seed crystal 35 is held by a holder 24a. These members
are arranged within a water cooled type apparatus body 29 wherein
pressure can be controlled.
[0038] A window 29a is formed in the middle portion in the height
direction of the apparatus body 29. Above the window 29a in a
slanting direction, a one-dimensional CCD camera 11 is arranged in
such a position that a single crystal 36 during pulling can be seen
through the window 29a and the one-dimensional CCD camera 11 is
connected to both a diameter measuring means 8 of the single
crystal 36 and a level position measuring means 13. The
one-dimensional CCD camera 11 has the function of measuring a
diameter of the single crystal 36 as well as the function of
measuring the level position of the melt 23. In the measurement of
the level position, a two-dimensional CCD camera can be adopted in
place of the one-dimensional CCD camera 11. Another CCD camera
specifically for measuring the level position can be placed besides
the one-dimensional CCD camera 11 for the diameter measuring means
8.
[0039] Above the crucible 21 within the apparatus body 29, a
reference reflector 12 for detecting the level position is
arranged, which also serves as a radiation screen to control the
thermal history of the pulled single crystal 36. The reference
reflector 12 is placed 22 mm or more apart from the straight body
portion of the single crystal 36. It is also possible to use the
reference reflector 12 separately from a radiation screen.
[0040] The one-dimensional CCD camera 11 is connected to the
diameter measuring means 8, which is connected to a crucible ascent
speed calculating means 9. The crucible ascent speed calculating
means 9 captures an input value from the diameter measuring means 8
of the pulled single crystal 36 and information on the pulled
length from a pulling means 5 of the pulling shaft 24 to calculate
a decrease in volume of the melt within the crucible 21, so as to
calculate an ascent speed of the crucible 21 based on the decrease
in volume of the melt.
[0041] Besides the diameter measuring means 8, the level position
measuring means 13 is also connected to the one-dimensional CCD
camera 11. The level position measuring means 13 receives an output
from the one-dimensional CCD camera 11 to calculate the level
position from the mirror image position of the reference reflector
12 reflected in the melt surface 23a of the melt 23. To the level
position measuring means 13, both an averaging means 14 and an
automatic updating means 18 are connected. The averaging means 14
receives an output from the level position measuring means 13 to
average the measured values in a prescribed period of time, which
is desirable to be set in the range of 1 to 30 minutes or so.
[0042] The automatic updating means 18 is used for automatically
working out a conversion equation for converting the mirror image
position of the reference reflector 12 on the melt surface 23a to
the actual level position by automatically moving the crucible 21
up and down from the initial position thereof to obtain the
relationship between the mirror image position of the reference
reflector 12 on the melt surface 23a and the level position and
making the relationship approximate to a straight line. Variations
in the setting position of the one-dimensional CCD camera 11 for
detecting the mirror image position of the reference reflector 12
and the setting position of the reference reflector 12 are also
factors of measurement errors in the level position measurement. If
the setting situations thereof do not change, it might be possible
to calculate the level position from the mirror image position of
the reference reflector 12 based on a conversion equation worked
out from the geometric arrangement beforehand, but it is actually
difficult. Therefore, a conversion equation of the mirror image
position of the reference reflector 12 to the level position in the
actual setting situations is obtained by the automatic updating
means 18.
[0043] In the present embodiment, the level position is actually
changed by moving the crucible 21 up and down after the melting of
a material for forming a crystal is completed and the initial level
position becomes stable, and a conversion equation is obtained by
making the relationship between the mirror image position of the
reference reflector 12 and the level position approximate to a
straight line using the automatic updating means 18. It is desired
that the ascent/descent range of the crucible 21 at that time
should be set to be .+-.5 mm to the initial position.
[0044] A crucible ascent speed adjustment value calculating means
15 is connected to the averaging means 14. By the crucible ascent
speed adjustment value calculating means 15, an adjustment value
for adjusting the control error of the melt position which arises
in the case of the control based on a crucible ascent speed
calculated in the crucible ascent speed calculating means 9 is
calculated. How to calculate the adjustment value is that after the
measured value of the level position at the first point when it is
judged to be able to be added by a below-described adjustment value
addition propriety judging means 16 is set to be the initial
position, an adjustment value of the crucible ascent speed is
calculated by calculating the level position shifted from the
initial position.
[0045] The adjustment value addition propriety judging means 16 to
judge irregularity in measuring the level position is connected to
the crucible ascent speed adjustment value calculating means 15.
The criteria for judgement in the adjustment value addition
propriety judging means 16 are: the upper and lower limits of a
crystal diameter are within the range of .+-.2 mm to a desired
diameter of a main body 36c of the single crystal 36; the upper and
lower limits of the level position are within the range of .+-.5 mm
to the initial value of the level position; the lower limit of the
distance between the single crystal 36 and the reference reflector
12 is 22 mm; and the upper and lower limits of the rotational speed
of the crucible 21 are within the range of .+-.10 rpm to the
rotational speed of the crucible 21 at the starting point of
pulling the main body 36c of the single crystal 36. When these
conditions are not satisfied, the adjustment value calculated in
the crucible ascent speed adjustment value calculating means 15 is
not allowed to be added to the ascent speed of the crucible 21
calculated in the crucible ascent speed calculating means 9 by an
adjustment value adding means 17.
[0046] In the settings of the present embodiment, the above upper
and lower limits of a crystal diameter do not satisfy the condition
of within the range of .+-.2 mm to a desired diameter of the main
body 36c of the single crystal 36 during pulling a shoulder 36b of
the single crystal 36. Therefore, the level position control is
preformed based on the crucible ascent speed calculated in the
crucible ascent speed calculating means 9. However, the shoulder
36b of the single crystal 36 is very small in volume, so that the
control error of the level position thereby can be kept in a very
small range.
[0047] According to the above construction, in the apparatus for
pulling a single crystal according to the embodiment, two systems
of controlling the level position during pulling a single crystal
36, a crucible ascent speed based on a decrease in volume of the
melt 23 with pulling the single crystal 36 and an output from the
crucible ascent speed adjustment value calculating means 15 based
on the measurement result of the level position measured from the
mirror image position of the reference reflector 12 on the melt
surface 23a, are included.
[0048] In the apparatus for pulling a single crystal according to
the embodiment, as one system, the level position is controlled by
calculating the ascent speed of the crucible 21 from both the
measured value of the crystal diameter of the single crystal 36 and
the pulling speed thereof. The crucible ascent speed is calculated
based on the below equation (1).
V.sub.f=(D.sub.si.sup.2P.sub.s/D.sub.cur.sup.2P.sub.L).times.(V.sub.si31
V.sub.cur)+V.sub.cur (1)
[0049] where
[0050] V.sub.f: crucible ascent speed
[0051] P.sub.s: specific gravity of solid silicon
[0052] P.sub.L: specific gravity of liquid silicon
[0053] D.sub.si: crystal diameter
[0054] D.sub.cur: quartz crucible internal diameter
[0055] V.sub.si: crystal pulling speed
[0056] V.sub.cur: preceding crucible ascent speed
[0057] Thus, an accurate value of the internal diameter of the
quartz crucible 21a is required for calculating the crucible ascent
speed. However, in some cases, the quartz crucible 21a softens
about the melting point of silicon and becomes deformed during
pulling. Therefore, in the apparatus for pulling a single crystal
according to the embodiment, the level position is measured from
the mirror image position on the melt surface 23a of the reference
reflector 12 arranged inside the apparatus body 29, and the control
error in the level position control based on the crucible ascent
speed calculated from the decrease of the melt 23 is calculated.
Then, an adjustment value of the ascent speed of the crucible 21 is
calculated from the calculated control error and is added to the
crucible ascent speed, so that the level position is controlled
thereby with higher accuracy. The crucible ascent speed adjustment
value is calculated based on the below equation (2).
V(mm/min)=V.sub.f(mm/min)+V.sub.adj(mm/min) (2)
V.sub.adj=.SIGMA..DELTA.V.sub.adj(i)
V.sub.adj(i)=P.times.{H.sub.i(mm)-H.sub.i-1(mm)}+I.times.{H.sub.i(mm)-H.su-
b.0(mm)}+D.times.{H.sub.i(mm)-2H.sub.i-1(mm)+H.sub.i-2(mm)}
[0058] where
[0059] V: crucible ascent speed after adjusting level position
change
[0060] V.sub.adj: crucible ascent speed adjustment value
[0061] V.sub.f: crucible ascent speed
[0062] H.sub.0: reference level position
[0063] H.sub.i: ith measured value of level position
[0064] P, I, D: control constants
[0065] P.sub.S: specific gravity of solid silicon
[0066] P.sub.L: specific gravity of liquid silicon
[0067] However, in some cases, errors are introduced to the level
position measurement from the mirror image position of the
reference reflector 12 on the melt surface 23a. The prime cause of
the measurement errors is inclination of the melt surface 23a. When
the inclination of the melt surface 23a changes during measurement,
the mirror image position of the reference reflector 12 reflected
in the melt surface 23a changes, resulting in an error in measuring
the level position. The inclination of the melt surface 23a is
varied by two factors.
[0068] The first factor is the surface tension caused by the
pulling single crystal 36. During crystal pulling, the melt surface
23a is lifted by the surface tension and becomes inclined. The melt
surface 23a closer to the single crystal 36 is more inclined, while
the melt surface 23a farther from the single crystal 36 is less
inclined. Therefore, the larger the crystal diameter becomes, the
more greatly the mirror image position of the reference reflector
12 during crystal pulling is influenced by the inclination of the
melt surface 23a. Here, the inclination of the melt surface 23a on
the same melt surface 23a does not change when the distance between
the single crystal 36 and the reference reflector 12 is fixed.
Therefore, the measurement error of the level position caused by
the change of the inclination of the melt surface 23a during
crystal pulling is influenced by the change of the crystal diameter
and the change of the crystal position during pulling.
[0069] The second factor of changing the inclination of the melt
surface 23a is the influence of centrifugal force which arises with
the rotation of the crucible 21. Since the melt 23 within the
crucible 21 rotates with the rotation of the crucible 21, the melt
23 is pushed to the outer regions by the centrifugal force, leading
to the inclination of the melt surface 23a. Since the inclination
is chiefly determined by the rotational speed of the melt 23, the
inclination of the melt 23a changes with the change of the
rotational speed of the crucible 21 during pulling, so that the
mirror image position of the reference reflector 12 also
changes.
[0070] Therefore, in the adjustment value addition propriety
judging means 16 of the ascent speed of the crucible 21 of the
apparatus according to the embodiment, whether adding the
adjustment value is right or not is judged based on the selected
upper and lower limits of the crystal diameter, the upper and lower
limits of the level position, the selected lower limit of the
distance between the single crystal 36 and the reference reflector
12, and the selected upper and lower limits of the rotational speed
of the crucible 21, which are factors of changing the inclination
of the melt 23a, so that the measurement error caused by the change
of the inclination of the melt 23a becomes below allowance.
[0071] Another factor of errors in relation to the measurement of
the level position is fluctuation of the melt surface 23a. When the
melt surface 23a fluctuates during measurement, variations in
measurement are caused. The influence of the fluctuation of the
melt surface 23a can be rejected by the operation of the averaging
means 14 to average the measured values of the level positions. It
is appropriate that the period of time for averaging should be set
in the range of 1 to 30 minutes or so.
[0072] Variations in the setting position of the one-dimensional
CCD camera 11 for detecting the mirror image position of the
reference reflector 12 and the setting position of the reference
reflector 12 are also factors of measurement errors in relation to
the level position measurement. If the setting situations thereof
do not change, it might be possible to uniformly calculate the
level position from the mirror image position of the reference
reflector 12 based on a conversion equation worked out from the
geometric arrangement beforehand, but it is actually difficult.
Therefore, a conversion equation of the mirror image position of
the reference reflector 12 to the level position in the actual
setting situations of the one-dimensional CCD camera 11 and the
reference reflector 12 is required. The conversion equation can be
obtained by moving the crucible 21 up and down to change the level
position. Using the apparatus for pulling a single crystal
according to the embodiment, having the automatic updating means 18
to automate the activity of working out the conversion equation, it
is possible to automatically find the conversion equation before
pulling the single crystal 36 using the automatic updating means
18.
[0073] Since the reference reflector 12 is a structure above and in
the proximity of the melt surface 23a, it needs to be made of a
material which does not contaminate the melt 23 and can endure high
temperatures. As a component of the reference reflector 12, carbon
can be exemplified. In addition, if the distance between the single
crystal 36 and a radiation screen can be ensured to be above the
selected lower limit of the distance between the single crystal 36
and the reference reflector 12, the radiation screen for
controlling the thermal history of the single crystal 36 can be
also used as the reference reflector 12.
[0074] As an optical device for measuring the mirror image position
of the reference reflector 12, a normal two-dimensional CCD camera,
one-dimensional CCD camera, or the like can be used. When the
crystal diameter is measured during crystal pulling using the
one-dimensional CCD camera 11 and the reference reflector 12 is
placed within the field of view of the measurement, the
one-dimensional CCD camera 11 for measuring a crystal diameter can
be also used as that for measuring the level position, if the
distance between the single crystal 36 and the reference reflector
12 is ensured to be above the selected lower limit thereof.
[0075] The operation of the level position controlling means 20 of
the apparatus for pulling a single crystal according to the
embodiment is described below by reference to the flow chart shown
in FIG. 2.
[0076] In Step 1, whether the melting of the material for forming a
crystal is completed or not is judged. When the melting of the
material for forming a crystal is judged to be not completed in S1,
the means waits for the completion of the melting thereof. On the
other hand, when the melting of the material for forming a crystal
is judged to be completed, the operation proceeds to Step 2,
wherein the crucible 21 is caused to move up and down within the
range of .+-.5 mm to the initial position. Then, it goes to Step 3,
wherein a conversion equation of the mirror image position to the
level position is obtained from the change of the mirror image
position of the reference reflector 12 with the ascent/descent of
the crucible 21.
[0077] After the conversion equation is obtained, pulling a single
crystal 36 is started (Step 4). With the start of pulling, measured
values of the pulling speed of the pulling shaft 24 and the crystal
diameter are captured (Step 5), and the ascent speed of the
crucible 21 is calculated based on these values using the equation
(1) (Step 6). Then, the level position is calculated from the
mirror image position on the melt surface 23a of the reference
reflector 12 placed inside the apparatus body 29 (Step 7),
averaging is conducted on the calculated values of the level
positions (Step 8), and an adjustment value of the ascent speed of
the crucible 21 is calculated from the averaged measured value
based on the equation (2) (Step 9).
[0078] The operation proceeds to Step 10, wherein the propriety of
adding the adjustment value calculated in Step 9 to the ascent
speed of the crucible 21 is judged. The judgement is carried out
based on whether or not the upper and lower limits of the crystal
diameter are within the range of .+-.2 mm to a desired diameter of
the main body 36c, the upper and lower limits of the level position
are within the range of .+-.5 mm to the initial level position, the
lower limit of the distance between the single crystal 36 and the
reference reflector 12 is 22 mm, and the upper and lower limits of
the crucible rotational speed are within the range of .+-.10 rpm to
the crucible rotational speed at the early stage of forming the
main body 36c. When adding the adjustment value to the ascent speed
of the crucible 21 is judged to be right in Step 10, the operation
goes to Step 11, wherein adding the adjustment value is conducted,
and then, the ascent speed of the crucible 21 is controlled based
on the adjusted value (Step 12). On the other hand, when adding the
adjustment value to the ascent speed of the crucible 21 is judged
to be wrong in Step 10, it bypasses Step 11 and goes to Step 12,
wherein the ascent speed of the crucible 21 is controlled based on
the value calculated in Step 6.
[0079] Using the apparatus for pulling a single crystal according
to the embodiment, it is possible to control the level position in
the crucible 21 by the level position controlling means 20, which
calculates the crucible ascent speed based on a decrease in the
melt volume with pulling a single crystal 36 so as to control a
motor 10 for crucible lifting. Furthermore, using the apparatus for
pulling a single crystal according to the embodiment, having the
reference reflector 12, level position measuring means 13, crucible
ascent speed adjustment value calculating means 15, and adjustment
value adding means 17, the level position controlling means 20
controls the crucible ascent speed by controlling the motor 10 for
crucible lifting based on an output from the adjustment value
adding means 17. As a result, even if the quartz crucible 21a
softens about the melting point of silicon and becomes deformed
during pulling, the level position in the quartz crucible 21a can
be controlled with higher accuracy.
[0080] Using the apparatus for pulling a single crystal according
to the embodiment, the adjustment value addition propriety judging
means 16 judges whether adding the adjustment value to the crucible
ascent speed is appropriate or not based on the crystal diameter of
the pulling single crystal 36, the level position measured by the
level position measuring means 13, the distance between the single
crystal 36 and the reference reflector 12, and the crucible
rotational speed. These items are chiefly factors of determining
the inclination of the melt surface 23a in the apparatus for
pulling a single crystal. Therefore, the situation where the
selected values thereof for judgement are not met under pulling
conditions means that the inclination of the melt surface 23a is
not adequate to the level position measurement, and that the
measurement result is significantly influenced by the inclination
of the melt surface 23a. In that case, the adjustment value
calculated from the level position measuring means 13 is not added
to the crucible ascent speed. In addition, even if there is such a
problem in measuring the actual level position by detecting the
mirror image position of the reference reflector 12 reflected in
the melt surface 23a using the one-dimensional CCD camera 11
arranged outside the apparatus body 29, the control wherein the
level position in the crucible 21 is controlled by calculating the
crucible ascent speed based on a decrease in the melt volume with
pulling a single crystal 36 so as to control the motor 10 for
crucible lifting is conducted, so that the influence on the level
position change can be kept to a minimum.
[0081] Using the apparatus for pulling a single crystal according
to the embodiment, it is possible to reject the influence of
fluctuation of the melt surface 23a. Since the mirror image
position of the reference reflector 12 reflected in the melt
surface 23a is detected using the one-dimensional CCD camera 11
arranged outside the apparatus body 29, another factor of errors in
the level position measurement wherein the actual level position is
measured is the fluctuation of the melt surface 23a. The
fluctuation of the melt surface 23a during measurement causes
variations in measured values. The effect of the fluctuation of the
melt surface 23a is rejected by the action of the averaging means
14 to average the level positions measured by the level position
measuring means 13.
[0082] Using the apparatus for pulling a single crystal according
to the embodiment, wherein the one-dimensional CCD camera 11
constituting the level position measuring means 13 is also used as
a one-dimensional CCD camera 11 for measuring a crystal diameter,
the cost of manufacturing the apparatus for pulling a single
crystal can be held down. In addition, using the apparatus for
pulling a single crystal according to the embodiment, having the
automatic updating means 18 to automate the activity of working out
a conversion equation of the mirror image position of the reference
reflector 12 in the actually placed situation to the level
position, it is possible to automatically find the conversion
equation before crystal pulling using the automatic updating means
18.
EXAMPLES AND COMPARATIVE EXAMPLES
[0083] An example of the level position control actually conducted
using an apparatus for pulling a single crystal according to an
example of the present invention is described below.
[0084] Apparatus for pulling a single crystal according to Example:
apparatus shown in FIG. 1
[0085] Internal diameter of crucible 21: 22 inches
[0086] Weight of material for forming a single crystal: 100 kg
[0087] Mean diameter of single crystal 36: 8 inches
[0088] Distance between reference reflector 12 and main body 36c of
single crystal 36: 22 mm or more
[0089] Period of time for averaging in averaging means 14: 10
minutes
[0090] Criteria for judgement in adjustment value addition
propriety judging means 16
[0091] the upper and lower limits of the crystal diameter are
within the range of .+-.2 mm to a desired diameter of the main body
36c,
[0092] the upper and lower limits of the level position are within
the range of .+-.5 mm to the initial level position,
[0093] the lower limit of the distance between the single crystal
36 and the reference reflector 12 is 22 mm, and
[0094] the upper and lower limits of the crucible rotational speed
are within the range of .+-.10 rpm to the crucible rotational speed
at an early stage of forming the main body 36c.
[0095] The conversion equation was obtained by the automatic
updating means 18 whereby the level position was actually changed
by moving the crucible 21 up and down after the melting of the
material was finished and the initial level position was set, and
the relationship between the mirror image position of the reference
reflector 12 and the level position was made approximate to a
straight line. The ascent/descent range of the crucible 21 at that
time was set within the range of .+-.5 mm to the initial
position.
[0096] In the settings of the Example, since the condition of the
upper and lower limits of the crystal diameter was not satisfied
during pulling the shoulder 36b, the level position control was
conducted based on the output from the crucible ascent speed
calculating means 9. However, the shoulder 36b was very small in
volume, so that the error in controlling the level position was
very small.
[0097] In the Example, it could be confirmed that it was possible
to control the level position during pulling the single crystal 36
within the range of 11 mm to the initial level position.
* * * * *