U.S. patent application number 17/353509 was filed with the patent office on 2021-12-23 for polysilicon rod and method for manufacturing polysilicon rod.
This patent application is currently assigned to Shin-Etsu Chemical Co., Ltd.. The applicant listed for this patent is Shin-Etsu Chemical Co., Ltd.. Invention is credited to Takeshi AOYAMA, Naruhiro HOSHINO, Masahiko ISHIDA, Atsushi YOSHIDA.
Application Number | 20210395097 17/353509 |
Document ID | / |
Family ID | 1000005765958 |
Filed Date | 2021-12-23 |
United States Patent
Application |
20210395097 |
Kind Code |
A1 |
YOSHIDA; Atsushi ; et
al. |
December 23, 2021 |
POLYSILICON ROD AND METHOD FOR MANUFACTURING POLYSILICON ROD
Abstract
A polysilicon rod wherein in an area whose distance from a
center of a cross section of the polysilicon rod is within 2/3 of a
radius and that excludes a seed core, average grain boundary
characteristics have following features: a coincidence grain
boundary ratio exceeds 20%, a grain boundary length exceeds 550
mm/mm.sup.2, and a random grain boundary length does not exceed 800
mm/mm.sup.2.
Inventors: |
YOSHIDA; Atsushi; (Niigata,
JP) ; HOSHINO; Naruhiro; (Niigata, JP) ;
ISHIDA; Masahiko; (Niigata, JP) ; AOYAMA;
Takeshi; (Niigata, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shin-Etsu Chemical Co., Ltd. |
Tokyo |
|
JP |
|
|
Assignee: |
Shin-Etsu Chemical Co.,
Ltd.
Tokyo
JP
|
Family ID: |
1000005765958 |
Appl. No.: |
17/353509 |
Filed: |
June 21, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 33/035 20130101;
C01P 2002/60 20130101; C01P 2004/02 20130101 |
International
Class: |
C01B 33/035 20060101
C01B033/035 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2020 |
JP |
2020-108123 |
Claims
1. A polysilicon rod wherein in an area whose distance from a
center of a cross section of the polysilicon rod is within 2/3 of a
radius and that excludes a seed core, average grain boundary
characteristics have following features: a coincidence grain
boundary ratio exceeds 20%, a grain boundary length exceeds 550
mm/mm.sup.2, and a random grain boundary length does not exceed 800
mm/mm.sup.2.
2. The polysilicon rod according to claim 1, wherein the
coincidence grain boundary ratio exceeds 25%, the grain boundary
length exceeds 650 mm/mm.sup.2, and the random grain boundary
length does not exceed 700 mm/mm.sup.2.
3. The polysilicon rod according to claim 1, wherein the
coincidence grain boundary ratio does not exceed 90%, and the grain
boundary length does not exceed 3000 mm/mm.sup.2.
4. A polysilicon rod wherein in an area including the entire
polysilicon rod but a seed core, average grain boundary
characteristics have following features: a coincidence grain
boundary ratio exceeds 20%, a grain boundary length exceeds 550
mm/mm.sup.2, and a random grain boundary length does not exceed 800
mm/mm.sup.2.
5. The polysilicon rod according to claim 4, wherein the
coincidence grain boundary ratio exceeds 25%, the grain boundary
length exceeds 650 mm/mm.sup.2, and the random grain boundary
length does not exceed 700 mm/mm.sup.2.
6. The polysilicon rod according to claim 4, wherein the
coincidence grain boundary ratio does not exceed 90%, and the grain
boundary length does not exceed 3000 mm/mm.sup.2.
7. A method for manufacturing a polysilicon rod according to claim
1, wherein manufacturing conditions are fed back by using a ratio
of coincidence grain boundaries to all grain boundaries as an index
that expresses a feature of a grain boundary; and by using a value
obtained by dividing a length of a grain boundary that has appeared
on a surface when the polysilicon rod is cut at an arbitrary
position by a measured area, as an index of a breadth of a grain
boundary surface in polysilicon.
8. The method for manufacturing a polysilicon rod according to
claim 7, wherein a ratio of .SIGMA.3 to 9 coincidence grain
boundaries is used as an index that expresses a feature of a grain
boundary.
Description
TECHNICAL FIELD
[0001] The present invention relates to raw polysilicon for
improving the defect rate in the manufacture of single crystal and
a method for manufacturing the same.
[0002] The present application claims the priority of Japanese
Patent Application No. 2020-108123 filed on Jun. 23, 2020, the
contents of which are entirely incorporated by reference.
BACKGROUND ART
[0003] In the manufacture of semiconductor devices, the
manufacturing process of single crystal silicon is required to
control impurities, lattice defects, etc., and maintain
productivity. Examples of the currently mainstream method for
manufacturing single crystal include a floating zone (FZ) method
and a Czochralski (CZ) method. Of the two methods, the FZ method is
a method of directly heating a polysilicon rod by high-frequency
heating to obtain single crystal, which has more features favorable
for controlling impurities than the CZ method using a quartz
crucible.
[0004] A defect in the FZ method means that single crystal growth
is inhibited and dislocation occurs, and a crystal defect is caused
in a single crystal rod. One of the factors that inhibit the single
crystal growth is a phenomenon in which polysilicon is left
unmelted to cause the defect.
[0005] In this FZ method, the crystal characteristics of the raw
polysilicon rod used are greatly associated with the defect in the
FZ that occurs during the manufacture of single crystal.
[0006] In the course of the single crystal growth in the FZ method,
the occurrence of the defect in the FZ is an important problem
because it significantly lowers productivity.
[0007] Manufacture of polysilicon rods as a raw material in the FZ
method is mainly performed by a Siemens method that is a CVD method
in which silane gas as a raw material is precipitated on a heated
silicon rod in the air.
[0008] Each of JP 2008-285403 A, JP 2013-193902 A, JP 2014-28747 A,
and JP 2017-197431 A discloses a polysilicon rod characterized by
its acicular crystal, area ratio of coarse grains, and size of a
crystal grain. Each of JP 2013-217653 A, JP 2015-3844 A, and JP
2016-150885 A discloses a method for selecting a single crystal raw
material according to the peak intensities and the numbers of peaks
of Miller indices <111> and <220> by an X-ray
diffraction method. JP 2019-19010 A discloses a polysilicon rod
characterized by the size of a crystal grain and the diffraction
intensity of a Miller index <222> by an X-ray diffraction
method.
SUMMARY OF INVENTION
Problem to be Solved by Invention
[0009] (1) None of the methods of the aforementioned patent
documents can provide high quantitativeness and reproducibility.
This is because attention has been paid to coarse grains of
polysilicon (size, distribution, crystal orientation, etc.) as a
cause of the single crystallization defect in the FZ method, which
is insufficient alone.
[0010] The present invention provides a polysilicon rod in which
the single crystallization defect in the FZ method is reduced by
the ratio of the breadth of a grain boundary surface to coincidence
grain boundary, which is a feature of a grain boundary that is a
boundary surface between particles.
[0011] For example, a silicon rod having the largest crystal grain
is a single crystal silicon rod, and when a model in which this
single crystal silicon rod is single-crystallized by the FZ method
is considered, it can be said that the defect rate due to the raw
material is zero. When this single crystal is divided, a grain
boundary surface appears. A coincidence grain boundary closest to a
single crystal bond is .SIGMA.3, and a grain boundary surface
having no coincidence lattice point or having no regularity is a
random grain boundary. It can be said that a grain boundary
containing a large amount of .SIGMA.3 that is a bonding surface
closest to single crystal is close to single crystal.
[0012] (2) A reactor for performing a CVD reaction by the Siemens
method is generally a bell jar type. The inner wall of a reactor
receives radiation from a heated rod. When the inner wall is in a
mirror surface state, the reflectance is high and an effect of
returning the radiant energy from the rod to the rod can be
obtained, but when the inner wall is fogged, the reflectance is
decreased, so that the absorption of the energy into the wall
surface is increased and the energy is not returned to the rod. The
cause of the fogging is that chlorosilanes as a raw material cause
hydrolysis with the moisture in the air when the reactor is opened
between batches, so that the reflectance tends to be decreased with
each batch. As a result, it is difficult to manufacture polysilicon
rods under the same conditions at all times. Polysilicon having a
desired grain boundary can be manufactured by feeding back the
grain boundary characteristics of the previous batch to the
reaction conditions of the next batch.
Means for Solving Problem
[0013] An inhibitor for single crystallization by the FZ method is
included in the characteristics of a grain boundary surface, and by
measuring and analyzing it, and feeding back to the manufacturing
conditions, polysilicon rods suitable for single crystallization by
the FZ method can be manufactured.
[0014] When the single crystallization process of the FZ method is
looked at, an area near the center of a polysilicon rod is easily
affected by a grain boundary because it reaches a single crystal
growth surface immediately after being melted, while an area near
the outer periphery of the polysilicon rod is less affected than
the area near the center because it passes through a heating zone
by an induced current.
[0015] Specifically, for the area to be the center at the time of
single crystallization by the FZ method, an area having a small
random grain boundary length and a large grain boundary length is
favorable, and as the distance from the center becomes larger, even
an area having a smaller grain boundary length becomes
acceptable.
[0016] Therefore, the rod containing polysilicon is beneficial, in
which in an area whose distance from the center of the cross
section of the polysilicon rod is within 2/3 of the radius and that
excludes the seed core, the average coincidence grain boundary
ratio exceeds 20%, the average grain boundary length exceeds 550
mm/mm.sup.2, and the average random grain boundary length does not
exceed 800 mm/mm.sup.2. Further, the polysilicon rod is favorable,
in which the coincidence grain boundary ratio exceeds 25%, the
grain boundary length exceeds 650 mm/mm.sup.2, and the random grain
boundary length does not exceed 700 mm/mm.sup.2.
[0017] When it is applied to the entire polysilicon rod, the rod
containing polysilicon is beneficial, in which in an area including
the entire polysilicon rod but the seed core, the average
coincidence grain boundary ratio exceeds 20%, the average grain
boundary length exceeds 550 mm/mm.sup.2, and the average random
grain boundary length does not exceed 800 mm/mm.sup.2. Further, the
polysilicon rod is favorable, in which the coincidence grain
boundary ratio exceeds 25%, the grain boundary length exceeds 650
mm/mm.sup.2, and the random grain boundary length does not exceed
700 mm/mm.sup.2.
[0018] The closer the coincidence grain boundary ratio is to 100%,
the better. However, the manufacturing conditions for realizing
this is close to those for epitaxial film growth, so that there is
little cost advantage with current technology. In addition, when
the grain boundary length is intended to be increased, it is also
necessary to increase the coincidence grain boundary ratio in order
to reduce the random grain boundary length to 700 mm/mm.sup.2 or
less, so that it is realistic from the above reason that the grain
boundary length is 3000 mm/mm.sup.2 or less.
[0019] In the method for manufacturing a polysilicon rod by the
Siemens method, the environment inside a reactor gradually changes
as described above. Therefore, it is considered that polysilicon is
analyzed at constant intervals and the results are fed back to the
CVD conditions. The coincidence grain boundary ratio that is a
feature of a grain boundary, the grain boundary length that is an
index of the breadth of a grain boundary, and the random grain
boundary length obtained from them are quantitative values and
characterized by being able to be associated with manufacturing
conditions. Also, in product design, the grain boundary
characteristics from the inner periphery to the outer periphery of
a polysilicon rod can be controlled, so that polysilicon rods
according to the requirements of customers can be provided.
[0020] According to one aspect of the present invention,
[0021] 1. the defect rate in single crystallization by the FZ
method can be reduced, and the yield and productivity can be
improved, and
[0022] 2. polysilicon rods can be stably manufactured by feedback
from the grain boundary characteristics to the manufacturing
conditions.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a graph showing the relationship between a grain
boundary length and a coincidence grain boundary ratio;
[0024] FIG. 2 is a graph showing the relationship between a grain
boundary length and a .SIGMA.3 coincidence grain boundary
ratio;
[0025] FIG. 3A is a view showing images of .SIGMA.3 coincidence
grain boundaries;
[0026] FIG. 3B is a view showing images of .SIGMA.9 coincidence
grain boundaries,
[0027] FIG. 3C is a view showing images of random grain boundaries,
and .SIGMA.3 to 49 coincidence grain boundaries;
[0028] FIG. 4 is a schematic view for explaining the outline of a
measurement method 1 in an embodiment of the present invention;
and
[0029] FIG. 5 is a schematic view for explaining the outline of a
measurement method 2 in the embodiment of the present
invention.
DETAILED DESCRIPTION
[0030] A horizontal plane orthogonal to the growth direction of a
polysilicon rod is cut out, and the crystal orientations of the
crystal grains exposed on a measurement surface are entirely
measured at an electron backscatter diffraction (EBSD) step of 1
.mu.m, whereby the state of a grain boundary is calculated from
differences between the orientations/angles of adjacent crystals of
the obtained data matrix. A .SIGMA.3 coincidence grain boundary
means a grain boundary surface where one coincidence lattice point
appears with respect to three atoms, which can be said to be a
grain boundary surface closest to single crystal among coincidence
grain boundaries. It can be said that when a grain boundary has
more coincidence lattice points, the thermal and physical
properties of the grain boundary are closer to those of single
crystal.
[0031] Coincidence Grain Boundary Ratio
[0032] The .SIGMA.3 to 49 detected using an EBSD analysis software
(TSL Solutions KK) are defined as coincidence grain boundaries.
About 80% of all the coincidence grain boundaries of .SIGMA.3 to 49
are occupied by .SIGMA.3 and .SIGMA.9, in which .SIGMA.3 is
slightly more than .SIGMA.9. As the .SIGMA. value becomes larger,
the interval between coincidence lattice points becomes larger,
which becomes closer to a random grain boundary. Therefore, in the
present embodiment, a coincidence grain boundary ratio is
calculated by using the sum of .SIGMA.3 to .SIGMA.9 coincidence
grain boundaries, which is adopted as an index. Here, .SIGMA.1
means single crystal.
[0033] Since the grain boundary is a boundary between grains, it is
obtained as a surface when surface observation is performed, so
that the grain boundary is indicated as an area. However, the
information obtained by a measurement using an actual apparatus
becomes a line (becomes the length of a boundary line around when
surface observation is performed).
[0034] Therefore, in the present embodiment, the coincidence grain
boundary ratio is defined as follows (see FIGS. 3A to 3C):
coincidence grain boundary ratio (%)=boundary lines of observed
coincidence grain boundaries/boundary lines of observed grain
boundaries.
[0035] The boundary lines include boundary lines exceeding
.SIGMA.49. The "boundary lines of observed grain boundaries" in the
above expression are all the grain boundaries observed by the above
EBSD analysis software. In the present embodiment, the ".SIGMA.3 to
49" are referred to as coincidence grain boundaries as described
above. The boundary lines in the coincidence grain boundaries are
about 50 to 60% of the "boundary lines of observed grain
boundaries."
[0036] In the EBSD analysis software, the orientations (angles) of
crystals on an observation surface are measured at intervals of,
for example, 1 .mu.m in the case of .times.150. When there is a
difference of a certain angle or more in the changes in the
obtained continuous data, it is regarded as a grain boundary. The
coincidence grain boundaries of ".SIGMA.3 to 49" can be obtained
from the orientations and directions of the crystals with the grain
boundary interposed therebetween.
[0037] It becomes boundary lines of observed grain
boundaries>boundary lines of ".SIGMA.3 to 49 coincidence grain
boundaries">boundary lines of ".SIGMA.3 to .SIGMA.9 coincidence
grain boundaries". The boundary lines of observed grain boundaries
include the coincidence grain boundaries and grain boundaries that
are not the coincidence grain boundaries. Therefore, the
coincidence grain boundary ratio is obtained by dividing the sum of
the boundary lines of ".SIGMA.3 to .SIGMA.9 coincidence grain
boundaries" by the sum of the boundary lines of ".SIGMA.3 to 49
grain boundaries" and the boundary lines exceeding .SIGMA.49.
[0038] A grain boundary having a low coincidence lattice point
density (a grain boundary close to a random grain boundary) has
high energy and is unstable. Therefore, when there are many grain
boundaries each having a low coincidence lattice point density, it
triggers falling off of unmelted particles on an FZ melt surface,
causing an FZ defect. On the other hand, when a polysilicon rod
having physical properties close to those of single crystal is used
as a raw material in the FZ method, stable melting can be
obtained.
[0039] Grain Boundary Length
[0040] It is difficult to accurately measure the grain size of
single crystal in polysilicon because a grain boundary surface
cannot currently be determined by images of SEM or the like. By
measuring the crystal orientation for each particle using the above
EBSD or the like, the length of a grain boundary on the measurement
surface can be obtained, so that the average size of particles can
be indirectly expressed. When the sum of the length of grain
boundaries on the measurement surface is divided by the measured
area, a grain boundary length per unit area can be obtained. In the
present embodiment, this is defined as a grain boundary length
(unit: length/area) that is an index of the breadth of a grain
boundary surface.
[0041] Random Grain Boundary Length
[0042] Various coincidence grain boundaries are included in the
grain boundaries other than the .SIGMA.3 to .SIGMA.9 coincidence
grain boundaries. As the .SIGMA. value becomes larger, the interval
between the coincidence lattice points becomes larger, so that the
features of a grain boundary having a low .SIGMA. value (having low
grain boundary energy and being stable) are lost. Therefore, the
sum of .SIGMA.s larger than .SIGMA.9 is defined as a random grain
boundary, for convenience, and a random grain boundary length is
determined from the grain boundary length per unit area. That is,
when the sum of the length of grain boundaries which have .SIGMA.s
larger than .SIGMA.9 on the measurement surface is divided by the
measured area, a random grain boundary length per unit area can be
obtained.
[0043] In order to reduce crystal defects and increase the yield in
the FZ method, it is favorable to use, as a raw material, one
having a grain boundary length as long as possible, a low .SIGMA.
value, a large coincidence grain boundary ratio, and a small random
grain boundary length. However, the coincidence grain boundary
ratio and the grain boundary length are in a contradictory
relationship for the most part. For example, in the polysilicon
manufactured under the condition of increasing the coincidence
grain boundary ratio, its grain boundary length is small.
Therefore, it is important to find the best point for both the
grain boundary characteristics.
[0044] The cause of the falling off of crystal particles, which
inhibits single crystal growth, is that the bonding at a grain
boundary surface is weak and unstable. As the number of random
grain boundaries with less bonding of coincidence lattice points
becomes larger, peeling off and falling off from the melt surface
are more likely to occur. It can be said that when of the grain
boundary characteristics, the .SIGMA. value is small and the
coincidence grain boundary ratio is large, the bonding at a grain
boundary surface is strong and stable, so that the falling off of
crystal particles is less likely to occur. The falling off of
crystal particles due to a random grain boundary occurs at a
temperature lower than the melting temperature of the single
crystal because the energy at the grain boundary surface is high.
Therefore, the single crystal particles that have fallen off are
not sufficiently heated and melted, and reach the single crystal
growth surface while the unmelted and semi-melted particles are in
a cluster form, causing a crystal defect. The unmelted and
semi-melted particles depend on the sizes of the crystal particles
that have fallen off. The larger the size is, the longer the
existence time is, so that they are more likely to reach the single
crystal growth surface.
[0045] Factors of the manufacturing conditions to obtain desired
grain boundary characteristics include the temperature of the
surface of a rod, reaction pressure, the concentration of silane as
a raw material, etc. When a regression analysis is performed on
these, a correlation of R.sup.2=0.8 or more (R: coefficient of
determination) is obtained. The same applies to a case where the
number of parameters is further increased and machine learning is
used. The obtained correlation is used as feedback to an apparatus,
so that optimum reaction conditions that follows the changes in the
state inside the reactor can be set.
[0046] While the diameters of the apparatuses are becoming larger
In the FZ method, conventional small-diameter apparatuses are also
often used. The grain boundary characteristics required for each
apparatus are different. In even an apparatus of the same type,
there are so-called equipment peculiarities, but polysilicon rods
that meet needs can be manufactured by performing the present
analysis.
[0047] As the measurement method, for example, an aspect
(hereinafter, also referred to as a "measurement method 1") as
shown in FIG. 4 may be used. The prepared silicon rod is cut at
arbitrary positions (three positions in the aspect shown in FIG. 4)
and sliced, whereby samples are cut out. The samples thus obtained
are measured. Since the characteristics of both the feet of a U-rod
are basically the same, the measurement may be performed only on
one foot.
[0048] The measurement results show that in all the cut-out
samples, the yield in the FZ becomes good according to the
following conditions in which: in an area whose distance from the
center of the cross section of the polysilicon rod is within 2/3 of
the radius and that excludes the seed core, the average coincidence
grain boundary ratio exceeds 20%, the average grain boundary length
exceeds 550 mm/mm.sup.2, and the average random grain boundary
length does not exceed 800 mm/mm.sup.2; or in an area including the
entire polysilicon rod but the seed core, the average coincidence
grain boundary ratio exceeds 20%, the average grain boundary length
exceeds 550 mm/mm.sup.2, and the average random grain boundary
length does not exceed 800 mm/mm.sup.2.
[0049] Therefore, it can be expected that favorable results will be
obtained even with the polysilicon rods of subsequent batches that
will be manufactured under the same conditions. Since the wall
surface of the reactor loses its luster and the efficiency of the
radiant heat changes every time a batch is processed, the
environment inside the reactor gradually changes even under the
same conditions, but this change is not dramatic. Therefore, for a
certain period of time (e.g., for about one month), it can be
expected that favorable results will be obtained even with
polysilicon rods of batches that will be manufactured under the
same conditions.
[0050] When measurement results are favorable by meeting the above
conditions, products with a good yield can be manufactured when the
FZ is performed by using the foot that has not been cut out into
slices in FIG. 4. The foot, from which the sliced samples are
obtained in FIG. 4, may be used as a chunk for the CZ.
[0051] For example, the procedure as described below can be
taken.
[0052] Measurement is performed using the measurement method 1, and
the foot, which is opposite to the foot that has passed by meeting
the above conditions, is subjected to single crystal growth by the
FZ (if the manufacturing apparatus is of the same type, one is
regarded as a representative.).
[0053] At this time:
[0054] the measurement method 1 may be performed on every silicon
rod in the same chamber that has grown into a silicon core wire,
and if they pass by meeting the above conditions, the foot, which
is opposite to the foot that has passed, may be subjected to single
crystal growth by the FZ;
[0055] the measurement method 1 may be performed on a
representative of those outside the chamber, like the
representative of those inside the chamber, and if it passes by
meeting the above conditions, the rest of those may be subjected to
single crystal growth by the FZ; or
[0056] one representative may be measured by the measurement method
1, and if it passes by meeting the above conditions, the rest may
be subjected to single crystal growth by the FZ.
[0057] In addition, areas near the electrode and the bridge are
excluded from the viewpoint of quality, and the central portion
having no cracks is used as an ingot for the FZ, so that, for
example, an aspect as shown in FIG. 5 may be adopted as another
measurement method.
[0058] According to the aspect (hereinafter, also referred to as a
"measurement method 2") in which an upper portion near the bridge
and a lower portion near the electrode are only cut out to make
samples, as shown in FIG. 5, a rod for the FZ can also be acquired
in the foot from which the samples have been made. A sample for
quality evaluation is taken from a portion outside the effective
length of the ingot for the FZ, mainly from the vicinity of the
electrode. The sample is analyzed to determine a resistance value,
metal components, etc.
[0059] In the measurement method 2, for example, the following
procedure can be taken.
[0060] A foot is measured by the measurement method 2, and the
foot, which has passed by meeting the above conditions, and its
opposite foot are both subjected to single crystal growth by the FZ
(if the manufacturing apparatus is of the same type, one is
regarded as a representative.).
[0061] At this time:
[0062] all samples may be inspected by the measurement method 2,
and rods, which have passed by meeting the above conditions, may be
subjected to single crystal growth by the FZ;
[0063] the measurement method 2 may be performed on a
representative of those outside the chamber, like the
representative of those inside the chamber, and if it passes by
meeting the above conditions, all of the those may be subjected to
single crystal growth by the FZ method; or
[0064] one presentative may be measured by the measurement method
2, and if it passes by meeting the above conditions, all may be
subjected to single crystal growth by FZ.
[0065] If inspection results are different even when a
manufacturing apparatus of the same type is used, or if the same
silicon rods cannot be manufactured even when manufactured under
the same manufacturing conditions, either of the measurement
methods 1 and 2 may be performed in each apparatus.
[0066] The cause of the case where characteristics are gradually
lost as the lot is increased with the same apparatus is thought to
be that deposits are deposited inside the bell jar, which leads to
a decrease in the radiant heat.
[0067] Even in this case, the manufacturing conditions may be
continuously reviewed, or the inside of the bell jar may be cleaned
to return to the initial state. However, if electropolishing is
performed to clean the inside of the bell jar, the cost is highly
expensive. Therefore, it is a realistic choice to continue to
review the manufacturing conditions.
EXAMPLES
[0068] <Relationship Between FZ Results and Grain Boundary
Characteristics>
[0069] Preparation of Polysilicon Rod
[0070] A crystal sample was prepared by the Siemens method using
trichlorosilane and hydrogen as raw materials, the grain boundary
characteristics were measured by EBSD, and the results of actual
pull-out experiments by the FZ method are shown below. A sample, in
which dislocation occurred in its crystal as a result of a single
crystallization experiment by the FZ method, was determined as x
(failure). The measurement results are also shown in FIG. 1.
TABLE-US-00001 TABLE 1 Coincidence Grain grain boundary Random
grain boundary length boundary length FZ ratio % mm/mm.sup.2
mm/mm.sup.2 Determination 1 65 530 470 x 2 54 1030 190
.smallcircle. 3 39 1400 850 x 4 50 1100 550 .smallcircle. 5 41 1050
620 .smallcircle. 6 45 1500 830 x 7 50 830 420 .smallcircle.
[0071] Sampling for Grain Boundary Characteristics Measurement
[0072] Since it was not realistic to measure the grain boundary
characteristics of the entire rod, average grain boundary
characteristics were determined by sampling.
[0073] 1) Wafers each having a thickness of 10 mm were cut out from
both ends of an effective length (the electrode side and the bridge
side were removed) of a U rod taken out from a Siemens method CVD
apparatus (see FIG. 5).
[0074] 2) A line segment a was drawn from the outer periphery to
the seed core of the wafer, the line segment a bisecting, on the
acute angle side, the angle formed between lines that are drawn to
extend from the outer periphery to the seed core of the wafer in a
portion where the line is the largest and a portion where the line
is shortest.
[0075] 3) Samples were cut out at intervals of 20 mm from the core
wire along the line segment a. For each sample, a measurement range
of 0.5 mm.times.0.5 mm or more was measured with an EBSD apparatus
(manufactured by TIM Inc.) Step of 1.0 microns, and average grain
boundary characteristics were determined. Note that calculation was
performed in consideration of performing cylindrical grinding in a
later step.
[0076] 4) For sections in which reaction conditions (factors that
affect a grain boundary, such as the temperature of a rod, reaction
pressure, raw material concentration, raw material supply speed,
CVD apparatus, and radiant heat that a rod receives from outside)
were the same in the radial growing direction throughout a reaction
batch, the characteristics of the entire sections were determined
by measuring a representative point.
[0077] <Example of Analysis of Reaction Conditions and
Feedback>
[0078] An example is taken, in which: in an apparatus for
manufacturing polysilicon by the Siemens method, the apparatus
having a function of keeping a constant temperature by generating
heat by making a current flow through the seed core of the
polysilicon that is connected to the electrode, and the gas phase
portion of the apparatus being filled with hydrogen and
chlorosilane, a deposited layer of polysilicon is formed on the
surface of the seed core of heated silicon, thereby forming a
polysilicon rod.
[0079] As the reaction conditions, a chlorosilane concentration and
the temperature of the surface of the polysilicon rod during the
CVD reaction were taken, and the relationships with grain boundary
characteristics were analyzed. Results of the analysis are
schematically shown in FIG. 2. Assuming that the point A is the
current condition, the grain boundary characteristics change in the
direction of the point B when the chlorosilane concentration is
only changed to be higher from the condition of the point A. When
the temperature of the rod is only lowered from the condition of
the point A, the grain boundary characteristics change in the
direction of the point C. When the chlorosilane concentration is
set to the condition of the point B and the temperature of the rod
is set to the condition of the point C, polysilicon having the
grain boundary characteristics of the point D is obtained.
[0080] By applying the actual measurement results to FIG. 2 and
maintaining the latest state, optimal reaction conditions for
obtaining a polysilicon rod with desired grain boundary
characteristics can always be adjusted.
[0081] Method 1 of Feedback to Manufacturing Conditions:
[0082] As a method for designing the grain boundary characteristics
from the center to the outer periphery of a polysilicon rod, the
temperature of the surface of the rod is relatively raised in order
to increase a region where the coincidence grain boundary ratio is
increased when the diameter is small, while the temperature of the
surface of the silicon rod is made lower and the chlorosilane
concentration is made higher (to prevent the heat inside the
silicon rod from rising) in order to increase the grain boundary
length as the diameter becomes larger. Thereby, a polysilicon rod
having a "proper region" can be manufactured.
[0083] Method 2 of Feedback to Manufacturing Conditions:
[0084] As the diameter becomes larger, a higher frequency is
applied to raise the temperature of the surface (by preventing the
heat inside the silicon rod from rising, the temperature of the
surface can be raised), and the chlorosilane concentration in the
chamber is made higher. Thereby, a polysilicon rod having a "proper
region" can be manufactured.
* * * * *