U.S. patent application number 09/864936 was filed with the patent office on 2002-03-14 for method for producing single.
Invention is credited to Maeda, Tokuji, Mizuta, Masahiko, Tabuchi, Masato.
Application Number | 20020029734 09/864936 |
Document ID | / |
Family ID | 18675884 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020029734 |
Kind Code |
A1 |
Mizuta, Masahiko ; et
al. |
March 14, 2002 |
Method for producing single
Abstract
In pulling a single crystal by CZ method, stable pulling up is
carried out in a pulling rate as fast as possible while a crystal
deformation is controlled to an aimed value and density of grown-in
defects is suppressed to a value below an upper limit value. As an
index of deformation of the single crystal from a perfect circle,
the aimed value of the crystal deformation is previously
determined. The upper limit value of a pulling rate necessary to
suppress a defect density to an allowable range is previously
calculated from distribution of grown-in defects in the crystal
section, the single crystal is pulled up according to a
predetermined pulling rate, and then deviation of the achieved
value from the aimed value of the crystal deformation in pulling is
calculated. The deviation is converted to a correction of the
pulling rate. This correction is added to a set value of the
pulling rate in the pulling and the result is used as a temporary
set value of the pulling rate in the next pulling. The temporary
set value is compared with the upper limit value of the above
described pulling rate and the smaller value is determined as the
pulling rate in the next runs.
Inventors: |
Mizuta, Masahiko; (Hyogo,
JP) ; Maeda, Tokuji; (Saga, JP) ; Tabuchi,
Masato; (Hyogo, JP) |
Correspondence
Address: |
Kevin R. Spivak
Morrison & Foerster LLP
Suite 5500
2000 Pennsylvanis Avenue, N.W.
Washington
DC
20006-1888
US
|
Family ID: |
18675884 |
Appl. No.: |
09/864936 |
Filed: |
May 25, 2001 |
Current U.S.
Class: |
117/14 ;
117/13 |
Current CPC
Class: |
C30B 29/06 20130101;
C30B 15/26 20130101 |
Class at
Publication: |
117/14 ;
117/13 |
International
Class: |
C30B 015/00; C30B
021/06; C30B 027/02; C30B 028/10; C30B 030/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 9, 2000 |
JP |
2000-173704 |
Claims
What is claimed is:
1. A method for producing a single crystal by growing a
semiconductor single crystal from a melt by CZ method, wherein as
an index of deformation of the single crystal from a perfect
circle, an aimed value d.sub.AIM of a crystal deformation defined
by (maximum diameter-minimum diameter)/minimum diameter of a
crystal section is previously calculated; an upper limit value
V.sub.MAX of a pulling rate necessary to suppress a defect density
to an allowable range is calculated from distribution of grown-in
defects in the crystal section; the single crystal is pulled up in
a predetermined pulling rate; then deviation .DELTA.d of an
achieved value d.sub.ACT of the crystal deformation from said aimed
value dam in pulling is determined, the deviation .DELTA..sub.AIM
is converted to a correction .DELTA.V of the pulling rate, this
correction .DELTA.V is added to a set value of the pulling rate in
said pulling, the pulling rate obtained by this addition is
compared with said upper limit value V.sub.MAX, and the smaller
value is selected as the set value of the pulling rate for the
following runs.
2. The method for producing a single crystal according to claim 1,
wherein the aimed value of the crystal deformation is a value
almost corresponding to an allowable upper limit of deformation
from the perfect circle of the section of the single crystal.
3. The method for producing a single crystal according to claim 1,
wherein the correction .DELTA.V (mm/min) of pulling rate is
calculated according to the following equation.
.DELTA.V=N.times.ef.times..DELTA.dN: gain being a value (-) falling
within a range of 0.1 to 1.0 ef: effect coefficient
[(mm/min)/percent].DELTA.d : deviation (%) of crystal
deformation
4. The method for producing a single crystal according to claim 1,
wherein when the correction .DELTA.V of the pulling rate is
compared with the allowable upper limit, if the correction .DELTA.V
of the pulling rate exceeds the allowable upper limit, the
allowable upper limit is used as the correction .DELTA.V of the
pulling rate.
5. The method for producing a single crystal according to claim 1,
wherein the pulling rate is set at a plurality of points with the
transition of a pulling length and the pulling rate at each point
is set so that it decreases monotonously with the progress of
pulling.
6. The method for producing a single crystal according to claim 1,
wherein the crystal deformation is smoothed on the basis of
achievement of a plurality of pulling runs to use it in setting the
pulling rate for the following runs.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for producing a
single crystal by growing a semiconductor single crystal such as a
silicon single crystal from a melt by the Czochralski method (CZ
method), and particularly relates to a method for producing a
single crystal, in which setting of a pulling rate of the single
crystal from the melt is improved.
[0003] 2. Description of the Prior Art
[0004] In growing the silicon single crystal by CZ method, as
publicly known, a seed crystal is dipped in the melt of silicon and
the seed crystal is pulled up in this state while controlling a
pulling rate to obtain a cylindrical silicon single crystal on a
lower end of the seed crystal. Then, a silicon wafer, which is to
be used as a material for a semiconductor device, is taken from the
silicon single crystal grown.
[0005] It has been known that the pulling rate of the silicon
single crystal, in other words, the pulling rate of a body portion
has a large effect on the defect distribution in a radial direction
of the silicon single crystal. This means that in the silicon
wafer, when subjected to thermal oxidization treatment, oxidation
induced stacking faults called OSF may occur in a form of a ring. A
region (hereafter, this region is referred to as an OSF
ring-occurring region) where this OSF may occur is known to move
toward the periphery with increase in pulling rate.
[0006] While the OSF ring is a kind of thermal treatment induced
defect, infrared scattering defect and dislocation cluster formed
during crystal growth are called grown-in defects. Among grown-in
defects, infrared scattering defects occur inside the OSF ring and
degrade the gate oxide integrity. On the other hand, dislocation
clusters occur outside the OSF ring appearing on both sides of a
non defect region.
[0007] With a change of a diameter of the region where the OSF ring
occurs due to a change of the pulling rate, the area of the region,
where these grown-in defects occur, changes. Specifically, a high
pulling rate will increase the diameter of the region where the OSF
ring occurs and expand the area inside this region, where these
grown-in defects occur. A lower pulling rate will decrease the
diameter of the region where the OSF ring occurs and reduce the
area inside this region, where these grown-in defects occur.
[0008] In recent years, with a lower room temperature and a lower
content of oxygen in the single crystal in semiconductor producing
processes, an adverse effect of the OSF on a device is being
suppressed. Therefore, it has become important to reduce the
density of grown-in defects, namely, infrared scattering defects
occurred inside the OSF ring-occurring region, which have an
adverse effect on gate oxide integrity Therefore, it is proposed to
set a pulling rate such that the density of grown-in defects
occurred inside the OSF ring-occurring region may be reduced.
[0009] It has been well known that the pulling rate of the silicon
single crystal has also a large effect on a shape of the single
crystal section. Specifically, the crystal deformation
(ellipticity) expressed by (maximum diameter-minimum
diameter)/minimum diameter of the same section of the single
crystal increases with increase in pulling rate. When the crystal
deformation increases, a product portion taken from the grown
crystal decreases to lower the yield. Therefore, it is also
practiced to set the pulling rate to make the crystal deformation
fall within a predetermined range.
[0010] With respect to such setting of the pulling rate of the
silicon single crystal, the following prior art has been disclosed
in Japanese Patent Laid-Open No. 11-189489. According to Japanese
Patent Laid-Open No. 11-189489, a technique for reducing the
density of the grown-in defects is disclosed, wherein a profile of
the pulling rate is determined in advance such that the crystal
deformation is maintained within the range of from 1.5 to 2.0
percent over the full length from a top to a bottom of the body
portion in the direction of pulling axis, and the profile of the
pulling rate multiplied by .alpha. (.ltoreq.0.8) is used as an
aimed profile of the pulling rate in actual crystal.
[0011] Here, .alpha. is a coefficient for setting the aimed value
to the diameter (defect region diameter D) of the region, where
grown-in defects occur, located inside the OSF ring-occurring
region, and is calculated from the change rate of the defect region
diameter D.
[0012] In growing the silicon single crystal, growing conditions
change, for example, an amount of melt remained decreases and a
heater power changes with the progress of growth and thus, if the
pulling rate is maintained constant throughout the entire period of
pulling, quality is likely to be deteriorated in later stages of
pulling. In addition, for either the defect distribution in the
crystal radial direction or the crystal deformation, the pulling
rate must be gradually lowered with the progress of growth in order
to yield the same profile in the full length of pulling axis
direction of the body portion.
BRIEF SUMMARY OF THE INVENTION
Object of the Invention
[0013] According to the prior art disclosed in the Japanese Patent
Laid-Open No. 11-189489, for the following reasons, it has been
described that reduction of density of grown-in defects is
possible.
[0014] The crystal deformation increases with increase in
crystal-pulling rate. Maintaining the crystal deformation to such
relatively high range of from 1.5 to 2.0 percent fixes the OSF
ring-occurring region to the outer circumferential portion of the
crystal. This is used as a standard profile of the pulling rate and
this value multiplied by .alpha. (.ltoreq.0.8) is used as the aimed
profile of the pulling rate in actual crystal growth. Thus, the
defect region diameter D is determined in accordance with .alpha..
When the .alpha. is selected from the range equal to or smaller
than 0.8, the defect region diameter D is minimized; and the non
defect region occurring in adjacent region outside the OSF
ring-occurring region is effectively used. As a result, density of
grown-in defects is reduced.
[0015] Though the crystal deformation and the change rate of the
defect region diameter D can be actually used as an index in
setting the pulling rate, in practice of crystal growth, these are
insufficient. For example, when the change rate of the defect
region diameter D is small, even if the crystal deformation is
maintained in the range of from 1.5 to 2.0 percent, a set value of
the pulling rate possibly increases unlimitedly. This is because
when the crystal deformation is smaller than the aimed value of 1.5
to 2.0 percent, some pulling rate is always added to the aimed
pulling rate profile. When the pulling rate becomes too high, a
stable operation cannot be performed and a situation is expected in
which density of grown-in defects exceeds an assumed value,.
[0016] The object of the present invention is to provide a method
for producing a single crystal, allowing for minimizing density of
grown-in defects as well as stable practical operation.
SUMMARY OF THE INVENTION
[0017] To achieve the above described purpose, according to this
invention, a method for producing a single crystal by growing a
semiconductor single crystal from a melt by CZ method is provided,
wherein as an index of deformation of the single crystal from a
perfect circle, an aimed value d.sub.AIM of a crystal deformation
defined by (maximum diameter-minimum diameter)/minimum diameter of
a crystal section is previously calculated; an upper limit value
V.sub.MAX of a pulling rate necessary for suppressing a defect
density to an allowable range is calculated from distribution of
grown-in defect in the crystal section, the single crystal is
pulled up according to a predetermined pulling rate, then deviation
.DELTA.d of the achieved value d.sub.ACT of the crystal deformation
from said aimed value d.sub.AIM in pulling is calculated, the
deviation .DELTA.d is converted to a correction .DELTA.V of the
pulling rate, this correction .DELTA.V is added to a set value of
the pulling rate in said pulling; the pulling rate obtained by this
addition is compared with said upper limit value V.sub.MAX, and a
smaller pulling rate is determined as the set value of the pulling
rate in the next and following runs.
[0018] According to the method for producing the single crystal of
the present invention, the crystal deformation is controlled to
maintain the aimed value d.sub.AIM thereof Density of grown-in
defects in the crystal section is suppressed to the allowable
range. Satisfying these conditions, pulling is carried out in a
pulling rate as fast as possible. In addition, in CZ process,
product specifications, such as the crystal deformation are
sensitively influenced by a difference in thermal conditions in a
furnace and sensitivity is very high and thus, change in pulling
rate causes a big effect. However, because the value of the pulling
rate added by the correction .DELTA.V is compared with the upper
limit value V.sub.MAX thereof, and the smaller value is used as the
set value of the pulling rate in the following runs, an abrupt
change of pulling rate can be avoided resulting in a more stable
operation.
[0019] The aimed value d.sub.AIM of the crystal deformation can be
the value almost corresponding to the upper limit of deformation
from the perfect circle of the section of the single crystal.
Consequently, deformation can be suppressed to the allowable range
to enable faster pulling.
[0020] The correction .DELTA.V (mm/min) of pulling rate can be
calculated by the following equation For this calculation a gain N
is introduced and thus, the abrupt change of pulling rate can be
avoided resulting in a more stable operation.
.DELTA.V=N.times.ef.times..DELTA.d
[0021] N: gain being the value (-) falling within the range of 0.1
to 1.0
[0022] ef: effect coefficient [(mm/min)/percent]
[0023] .DELTA.d: deviation (%) of crystal deformation
[0024] For the correction .DELTA.V of the pulling rate, a
calculated value of the correction .DELTA.V of the pulling rate is
compared with the allowable upper limit thereof and if the
calculated value exceeds the allowable upper limit, the allowable
upper limit can be used as the correction .DELTA.V of the pulling
rate. Consequently, an abrupt change of pulling rate can be avoided
resulting in a more stable operation.
[0025] The pulling rate can be set at a plurality of points with
the transition of the pulling length and the pulling rate in each
point can be set to decrease monotonously according to the progress
of pulling. Consequently, fluctuations of the crystal pulled in the
later stage of pulling can be suppressed to avoid quality
deterioration. Even if the pulling rate is monotonously decreased
with the progress of pulling, the diameter of the crystal can be
kept constant by regulation of the heater power and the like.
[0026] The crystal deformation can be smoothed on the basis of
achievement of a plurality of pulling runs to use it in setting the
pulling rate for the following runs. Consequently, an abrupt change
of pulling rate can be avoided resulting in a more stable
operation. As a method for smoothing, calculating a simple average,
employing an weighted average, and employing a median can be
employed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a block diagram of a crystal growing apparatus
used for the method for producing a single crystal according to an
embodiment of this invention;
[0028] FIG. 2 is an illustration showing the pulling rate pattern
of the single crystal;
[0029] FIG. 3 is a flow chart showing the steps of setting the
pulling rate; and
[0030] FIG. 4 is a chart showing a comparison of a result of the
process according to this invention with that of the conventional
process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] Embodiments of the present invention are given below by way
of illustration FIG. 1 is a block diagram of a crystal growing
apparatus used for the method for producing a single crystal
according to embodiments of the present invention.
[0032] The crystal growing apparatus has a cylindrical main body 11
of a furnace. In a central part of the furnace main body 11, a
crucible 12 is provided. The crucible 12 has a double structure
comprising a quartz crucible 12a and a graphite crucible 12b
outside thereof and mounted on a rotating shaft 14 called pedestal.
By driving the rotating shaft 14, the crucible 12 is rotated and
lifted. In the quartz crucible 12a, a melt 13 prepared by melting a
silicon material for the single crystal is contained.
[0033] For preparation of the melt 13 and control of a temperature,
a heater 15a is disposed outside the crucible 12. In further
outside thereof, an insulating material 15c is provided along an
inner periphery face of the furnace main body 11. The heater 15a
together with a controlling unit 15b constitutes a heating means
15.
[0034] On the other hand, to the upper side of the furnace main
body 11, a cylindrical casing 11a more slender than the furnace
main body 11 is connected. In the casing 11a, a wire 17 is
suspended concentrically with the rotating shaft 14. To a lower end
portion of the wire 17, a seed holder 17a is mounted and a seed
crystal 17b is attached thereto. Subsequently, the seed crystal 17b
is dipped into the melt 13 contained in the quartz crucible 12a,
the crucible 12 and the seed crystal 17b are rotated and the seed
crystal 17b is raised and the silicon single crystal 20 is grown on
the lower end thereof.
[0035] For rotation and lifting of the seed crystal 17b, a wire
lifting apparatus 19 is provided on the uppermost portion of the
casing 11a though a wire rotating apparatus 18. The wire rotating
apparatus 18 has a motor 18a and the wire 17 is rotated by driving
the motor 18a. A wire lifting apparatus 19 has the motor 19a and
the wire 17 is lifted by driving the motor 19a. Lifting means 16 is
constituted by the wire 17, wire rotating apparatus 18, and wire
lifting apparatus 19.
[0036] On a top portion of the furnace main body 11, an observation
window 11b is provided. On the side opposite to the single crystal
20 with the observation window 11b inserted inbetween, a two
dimensional CCD camera 31 is mounted. This CCD camera 31 is
connected to an image processing unit 32 and a measuring and
detecting unit 30 is constituted by these parts. The measuring and
detecting unit 30 detects a brightness distribution in the vicinity
of a fusion ring 21 formed around the single crystal 20 with the
CCD camera 31 and processes this brightness distribution in the
image processing unit 32 to calculate the diameter and the crystal
deformation of the single crystal 20.
[0037] The diameter and the crystal deformation of the single
crystal 20, calculated, are fed to temperature
calculation/regulating means 33 and pulling rate
computation/controlling means 34. Temperature
calculation/regulating means 33 uses the measured diameter and
crystal deformation of the single crystal to calculate an aimed
temperature of the melt 13 and feeds this to a controlling unit 15b
of the heating means 15. The controlling unit 15b controls a power
of the heater 15a to achieve the aimed temperature. The pulling
rate computing means 34 uses the measured crystal diameter and
deformation to calculate the set value of the pulling rate for the
next pulling run and supplies it to the wire lifting apparatus 19.
The wire lifting apparatus 19 lifts the wire 17 in the next pulling
run according to the given set value of the pulling rate.
[0038] Now, a method for setting the pulling rate in the next
pulling run, important in the present invention, will be described
below with reference to FIG. 2 and FIG. 3. This setting is carried
out by the pulling rate computing means 34 as described above.
[0039] First, the single crystal 20 is pulled up with a standard
pulling rate pattern. This pulling is called current pulling to
distinguish it from the next pulling.
[0040] The standard pulling rate pattern is illustrated in the FIG.
2. The pulling rate pattern is expressed by the pulling rate Va,
Vb, Vc, . . . set at a plurality of points A, B, C, . . . with the
transition of the pulling length. The pulling rate Va, Vb, Vc, . .
. at the plurality of points A, B, C, . . . decreases monotonously.
The number of these set points A, B, C, . . . are selected to make
the characteristics of the pulling rate pattern clear. In other
words, in case of large change in pulling rate, set points are
densely provided and in case of small change in pulling rate, set
points are coarsely provided. The pulling rate between adjacent set
points is determined by interpolation of the pulling rate at set
points of both sides.
[0041] In the current pulling, an operation of the FIG. 3 is
conducted for the plurality of points A, B, C, . . . .
[0042] First, the achieved value d.sub.ACT of the crystal
deformation is calculated from the measured crystal diameter. The
achieved value d.sub.ACT calculated of the crystal deformation is
compared with the aimed value d.sub.AIM thereof and calculate
deviation .DELTA.d (d.sub.AIM-d.sub.ACT) is determined. The aimed
value d.sub.AIM of the crystal deformation is the value almost
corresponding to the upper limit of deformation from the perfect
circle of the section of the single crystal and has been previously
calculated from a relation between deformation state and the
deformation of an actual crystal pulled up.
[0043] Once the deviation .DELTA.d of the crystal deformation is
determined, this value is converted to the correction .DELTA.V of
the pulling rate. This conversion is carried out according to the
following equation.
.DELTA.V=N.times.ef.times..DELTA.d
[0044] .DELTA.V: deviation of pulling rate (mm/min)
[0045] N: gain being the value (-) falling within the range of 0.1
to 1.0
[0046] ef: effect coefficient [(mm/min)/percent]
[0047] .DELTA.d: deviation (%) of crystal deformation
[0048] When the correction .DELTA.V of the pulling rate is
calculated, this value is compared with the upper limit value
V.sub.MAX of the correction .DELTA.V. Here, the upper limit value
V.sub.MAX is the upper limit value of the pulling rate necessary to
suppress the defect density of the crystal to the allowable range.
If the correction .DELTA.V calculated is below the upper limit
value V.sub.MAX, the correction .DELTA.V is added to the set value
of current pulling rate and the resulting value is used temporarily
as the pulling rate of the next run. If the correction .DELTA.V
calculated is above the upper limit value V.sub.MAX thereof, the
upper limit value is added to the predetermined value of current
pulling rate and resulting value is used temporarily as the pulling
rate of the next run.
[0049] After the set value of the pulling rate of the next run is
temporarily determined in this way, this set value is compared with
the upper limit value V.sub.MAX of the pulling rate. The upper
limit value V.sub.MAX of the pulling rate is the upper limit value
of the pulling rate necessary to suppress the density of the
grown-in defects in the crystal section to the allowable range.
This upper limit value V.sub.MAX has been previously calculated
from the relation between distribution of the grown-in defects in
the section of the actual crystal and pulling rate.
[0050] As a result of comparison of the temporary set value of the
pulling rate of the next run with the upper limit value V.sub.MAX,
if the temporary set value is smaller than the upper limit value
V.sub.MAX, the set value is determined as the pulling rate of the
next run. On the contrary, if the temporary set value is larger
than the upper limit value V.sub.MAX, the upper limit value
V.sub.MAX is determined as the pulling rate of the next run. In
other words, the temporary set value of the pulling rate of the
next run is compared with the upper limit value V.sub.MAX and the
smaller value is used as the set value of the pulling rate in the
next run.
[0051] In this way, the set values of the pulling rate in the next
pulling run are determined for the plurality of set points A, B, C,
. . . .
[0052] In the next pulling run, pulling is carried out according to
these set values of the pulling rate. And using the set values of
the pulling rate in this pulling as a reference, the set values of
the pulling rate for the pulling run after next are determined by a
similar procedure and this process is repeated for following
runs.
[0053] In this way, an eight-inch silicon single crystal was
produced. As the aimed value d.sub.AIM of the crystal deformation,
the value of 1.4 percent obtained on the basis of a deformed shape
of the single crystal pulled in the past without any problem was
used as the upper limit of the crystal deformation The gain N was
set to 0.5. The gain N is ideally 1. However, because change in
pulling rate has a sensitive effect on the crystal deformation and
the like, an effect on the crystal deformation and the like is
reduced by this gain N. Excessively low gain N causes insufficient
reflection of current pulling achievement to setting process of the
next pulling rate. A particularly preferable gain N ranges from 0.4
to 0.8.
[0054] The result of operation is shown in FIG. 4 in comparison
with that of conventional process. In comparison with the
conventional process, the pulling rate increased 5 percent. The
density of the grown-in defects did not exceed the upper limit. The
crystal deformation fluctuated around the aimed value and fell
within the allowable range. The pulling rate increased 5 percent
and the OSF ring-occurring region moved to the outer periphery
resulted in 50-percent reduction of OSF density. For the sake of
reference, the conventional process means specifically the pulling
method using the pulling profile without the present method and in
which the pulling rate is 5-percent smaller than the present
method.
[0055] In the conventional process, if the prior art disclosed in
the Japanese Patent Laid-Open No. 11-189489 is carried out, which
is a method using only the pulling rate of the crystal as an index
of the pulling rate setting, the crystal deformation is excessively
relied on and thus, the pulling rate becomes too high so that the
density of the grown-in defects exceeds the allowable upper
limit.
[0056] As described above, the method for producing a single
crystal according to the present invention can control the crystal
deformation to the aimed value d.sub.AIM thereof resulting in a
high yield. In addition, the density of the grown-in defects in the
crystal section can be suppressed to the allowable range resulting
in high quality. While satisfying these conditions, stable pulling
can be carried out in a pulling rate as fast as possible resulting
in a high productivity and reduction of the OSF density.
Consequently, a semiconductor single crystal of high quality can be
produced at low cost.
* * * * *