U.S. patent application number 15/399981 was filed with the patent office on 2018-07-12 for epitaxial silicon wafer.
This patent application is currently assigned to SUMCO CORPORATION. The applicant listed for this patent is SUMCO CORPORATION. Invention is credited to Tomokazu KATANO, Yasuo KOIKE, Toshiaki ONO.
Application Number | 20180197751 15/399981 |
Document ID | / |
Family ID | 62749631 |
Filed Date | 2018-07-12 |
United States Patent
Application |
20180197751 |
Kind Code |
A1 |
KOIKE; Yasuo ; et
al. |
July 12, 2018 |
EPITAXIAL SILICON WAFER
Abstract
An epitaxial silicon wafer includes a silicon wafer consisting
of a COP region in which a nitrogen concentration is
1.times.10.sup.8-3.times.10.sup.9 atoms/cm.sup.3, and an epitaxial
silicon film formed on the silicon wafer. When heat treatment for
evaluation is applied, a density of BMD formed inside the silicon
wafer is 1.times.10.sup.8-3.times.10.sup.9 atoms/cm.sup.3 over the
entire radial direction of the silicon wafer. An average density of
the BMD formed in an outer peripheral region of the silicon wafer
which is a 1-10 mm range separated inward from an outermost
periphery thereof is lower than the average density of the BMD
formed in a center region. A variation in the BMD density in the
outer peripheral region is 3 or less, and a residual oxygen
concentration in the outer peripheral region is 8.times.10.sup.17
atoms/cm.sup.3 or more.
Inventors: |
KOIKE; Yasuo; (Tokyo,
JP) ; KATANO; Tomokazu; (Tokyo, JP) ; ONO;
Toshiaki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMCO CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
SUMCO CORPORATION
Tokyo
JP
|
Family ID: |
62749631 |
Appl. No.: |
15/399981 |
Filed: |
January 6, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/32 20130101;
H01L 21/0262 20130101; H01L 21/02658 20130101; H01L 21/3225
20130101; C30B 33/02 20130101; H01L 29/16 20130101; C30B 15/203
20130101; C30B 29/06 20130101; H01L 21/02381 20130101; C30B 15/206
20130101; H01L 21/02532 20130101; H01L 29/365 20130101 |
International
Class: |
H01L 21/322 20060101
H01L021/322; H01L 29/16 20060101 H01L029/16; H01L 29/32 20060101
H01L029/32; H01L 29/36 20060101 H01L029/36; H01L 21/02 20060101
H01L021/02; C30B 29/06 20060101 C30B029/06; C30B 29/64 20060101
C30B029/64 |
Claims
1. An epitaxial silicon wafer comprising: a silicon wafer
consisting of a COP crystal oriented particle region in which a
nitrogen concentration is from 1.times.10.sup.12 atoms/cm.sup.3 to
1.times.10.sup.13 atoms/cm.sup.3; and an epitaxial silicon film
formed on a surface of the silicon wafer, wherein when treatment
for evaluating oxygen precipitates is applied to the epitaxial
silicon wafer, a density of oxygen precipitates formed inside the
silicon wafer is from 1.times.10.sup.8 atoms/cm.sup.3 to
3.times.10.sup.9 atoms/cm.sup.3 over the entire radial direction of
the silicon wafer, an average density of the oxygen precipitates
formed in an outer peripheral region of the silicon wafer within
1-10 mm separated inward from an outermost periphery thereof is
lower than an average density of the oxygen precipitates formed in
a center region of the silicon wafer which is the region of the
silicon wafer other than the outer peripheral region, a value
obtained by dividing a difference between a maximum value and a
minimum value of the density of the oxygen precipitates in the
outer peripheral region by an average value of the density of the
oxygen precipitates in the outer peripheral region is 3 or less,
and a residual oxygen concentration in the outer peripheral region
is 8.times.10 atoms/cm.sup.3 or more as measured by Old-ASTM_F121,
1979.
2. The epitaxial silicon wafer as claimed in claim 1, wherein the
maximum value of the density of the oxygen precipitates exists in 6
mm to 10 mm a region separated inward from the outermost periphery
from 6 mm to 10 mm, and the minimum value of the density of the
oxygen precipitates exists in a 0 less than 6 mm a region separated
inward from the outermost periphery from 0 to less than 6 mm.
3. The epitaxial silicon wafer as claimed in claim 2, wherein the
density of the oxygen precipitates in the outer peripheral region
is gradually reduced toward the outermost periphery of the silicon
wafer.
4. The epitaxial silicon wafer as claimed in claim 1, wherein when
heat treatment for evaluating oxidation-induced stacking fault is
applied, the density of oxidation-induced stacking fault observed
on the back surface of the epitaxial silicon wafer is 100/cm.sup.2
or less.
5. The epitaxial silicon wafer as claimed in claim 1, wherein the
silicon wafer is cut out from a silicon single crystal ingot whose
oxygen concentration is adjusted in a range of from 8.times.1017
atoms/cm.sup.3 to 14.times.10.sup.17 atoms/cm.sup.3 or less, as
measured by Old-ASTM_F121, 1979.
6. The epitaxial silicon wafer as claimed in claim 1, wherein a
diameter of the epitaxial silicon wafer is 300 mm or more.
7. The epitaxial silicon wafer as claimed in claim 1, wherein the
silicon wafer is cut out from a straight body section of a silicon
single crystal ingot having a diameter 1.02 to 1.07 times larger
than a target diameter and processed into the target diameter.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to an epitaxial silicon wafer
and, more particularly, to an epitaxial silicon wafer whose
gettering capability is enhanced by nitrogen doping.
Description of Related Art
[0002] An epitaxial silicon wafer is widely used as a substrate
material of semiconductor devices. The epitaxial silicon wafer is
obtained by forming a silicon epitaxial film on a silicon wafer and
has high crystal integrity.
[0003] When heavy metal impurities exist in an epitaxial silicon
wafer used for a semiconductor device, the characteristics of the
semiconductor device may be degraded, so that it is necessary to
reduce the heavy metal impurities as much as possible. As one of
the techniques for reducing the heavy metal impurities, various
gettering techniques are known. The gettering technologies include,
for example, a method called "intrinsic gettering (IG)". With this
method, an oxygen precipitate (BMD: Bulk Micro Defect) is formed in
a silicon wafer, and heavy metal impurities are trapped in the BMD.
In recent years, there is a need to provide an epitaxial silicon
wafer in which the density of the BMD formed in the wafer is
1=10.sup.8/cm.sup.3 or more.
[0004] In an epitaxial film formation process, a silicon wafer is
heated at a high temperature of 1000.degree. C. to 1200.degree. C.
At this time, minute oxygen-precipitation nuclei in the wafer are
reduced or eliminated by high-temperature heat treatment, and the
BMD cannot be sufficiently induced in the subsequent device
process. Further, the lowering of device process temperature is
promoted along with miniaturization of a semiconductor device to
make it difficult for the oxygen-precipitation nuclei to grow
during the device process, resulting in a noticeable reduction in
the BMD density. This lowers the gettering capability of the
epitaxial silicon wafer.
[0005] On the other hand, a method using a silicon wafer doped with
nitrogen is known as a method to enhance the gettering capability
of the epitaxial silicon wafer. In the silicon wafer doped with
nitrogen, a thermally stable BMD hard to be eliminated even
undergoing high-temperature heat treatment in an epitaxial process
is formed in a crystal growth stage of a single crystal ingot, so
that the gettering capability can be enhanced.
[0006] For example, Japanese Patent No. 3,760,889 describes a
production method for an epitaxial silicon wafer including
subjecting a silicon wafer doped with nitrogen to heat treatment
(pre-annealing) at 700.degree. C. to 900.degree. C. for 15 minutes
to four hours before epitaxial growth treatment. With this method,
an epitaxial silicon wafer having a BMD density of
3.times.10.sup.4/cm.sup.2 or more can be produced irrespective of a
position at which the silicon wafer is cut off from, a single
crystal ingot. Further, Japanese Patent No. 4,061,906 describes a
method that forms the outer peripheral portion of a silicon wafer
as an OSF (Oxidation induced Stacking Fault) region in order to
suppress the growth of slip dislocation in the wafer outer
peripheral portion that contacts a C ring of a vertical boat.
[0007] It is effective to increase the density of the BMD formed in
the silicon wafer in order to enhance the gettering capability; on
the other hand, however, oxygen in the silicon wafer is consumed
for BMD formation to reduce a residual oxygen concentration in the
wafer. The reduction in the residual oxygen concentration in the
wafer causes a reduction in wafer strength. Particularly, when an
oxygen concentration in the wafer outer peripheral portion is low,
the outer peripheral portion becomes easy to flaw due to contact
with a heat treatment boat during heat treatment, causing slip
dislocation in the wafer outer peripheral portion with the flaw as
a starting point.
[0008] According to the method described in Japanese Patent No.
3,760,889, the BMD density in the wafer is increased through heat
treatment applied before epitaxial growth treatment, whereby an
epitaxial silicon wafer excellent in the gettering capability can
be obtained. However, the method does not consider a BMD density
distribution in a wafer surface, that is, the BMD density in the
wafer outer peripheral portion is excessively high, so that a
residual oxygen concentration in the outer peripheral region is
reduced, resulting in easy occurrence of the slip dislocation in
the wafer outer peripheral portion.
[0009] The method described in Japanese Patent. No. 4,061,906 aims
at suppressing occurrence of the slip dislocation by forming the
wafer outer peripheral portion as a ring-shaped OSF region
(hereinafter, referred to as "R-OSF region"). However, when an
epitaxial film is grown on the R-OSF region, a defect (epitaxial
defect) may occur in the epitaxial film. Further, when a nitrogen
concentration is high, the width of the R-OSF region formed in a
crystal growth stage is increased and, accordingly, a region where
the epitaxial defect occurs may been enlarged.
SUMMARY OF THE INVENTION
[0010] The present inventors have conducted extensive studies
aiming at preventing occurrence of the slip dislocation in the
wafer outer peripheral portion, and consequently found that
application of pre-annealing to a silicon wafer having a low
nitrogen concentration resulted in formation of a region having a
low BMD density in a COP region on the side that contacts the R-OSF
region and considered that a crystal region having the low density
region is intentionally positioned in the outer peripheral portion
to complete the present invention.
[0011] The object of the present invention is to provide an
epitaxial silicon wafer capable of preventing occurrence of the
slip dislocation by enhancing strength of the wafer outer
peripheral portion while ensuring required gettering
capability.
[0012] To solve the above problems, an epitaxial silicon wafer of
the present invention is provided with a silicon wafer consisting
of a COP region in which a nitrogen concentration is adjusted in a
range of 1.times.10.+-.2 atoms/cm.sup.3 or more and
1.times.10.sup.13 atoms/cm.sup.3 or less, and an epitaxial silicon
film, formed on a surface of the silicon wafer, wherein When heat
treatment for evaluating oxygen precipitates is applied to the
epitaxial silicon wafer, a density of oxygen precipitates formed
inside the silicon wafer is 1.times.10.sup.8 atoms/cm.sup.3 or more
and 3.times.10 .sup.9 atoms/cm.sup.3 or less over the entire radial
direction of the silicon wafer, an average density of the oxygen
precipitates formed in an outer peripheral region of the silicon
wafer which is a 1-10 mm range separated inward from an outermost
periphery thereof is lower than an average density of the oxygen
precipitates formed in an center region of the silicon wafer which
is the region other than the outer peripheral region, a value
((Max-Min)/Ave) obtained by dividing a difference between a maximum
value (Max) and a minimum value (Min) of the density of the oxygen
precipitates in the outer peripheral region by an average value
(Ave) of the density of the oxygen precipitates in the outer
peripheral region is 3 or less, and a residual oxygen concentration
in the outer peripheral region is 8.times.10.sup.17 atoms/cm.sup.3
(Old-ASTM_F121, 1979) or more.
[0013] According to the present invention, it is possible to
suppress a reduction in the residual oxygen concentration in the
wafer outer peripheral region while ensuring a desired BMD density
in the wafer center region. Thus, it is possible to prevent
occurrence of slip dislocation in the outer peripheral region while
ensuring desired gettering capability, whereby an epitaxial silicon
wafer with high quality and reliability can be provided.
[0014] In the epitaxial silicon wafer according to the present
invention, it is preferable that the maximum value (Max) of the
density of the oxygen precipitates exists in 6 mm to 10 mm range of
the outer peripheral region, separated inward from, the outermost
periphery, and the minimum value (Min) of the density of the oxygen
precipitates exists in a 0-less than 6 mm range of the outer
peripheral region, separated inward from the outermost periphery.
In this case, the density of the oxygen precipitates in the outer
peripheral region is preferably gradually reduced toward the
outermost periphery of the silicon wafer. With this configuration,
it is possible to increase the BMD density in the wafer center
region and to increase the residual oxygen concentration in the
outer peripheral region, whereby both gettering capability and
wafer strength can be made compatible.
[0015] In the epitaxial silicon wafer according to the present
invention, when heat treatment for evaluating oxidation-induced
stacking fault is applied, the density of oxidation-induced
stacking fault observed on the back surface of the epitaxial
silicon wafer is preferably 100/cm.sup.2 or less. With this
configuration, the entire wafer surface consists of a COP region,
that is, the oxidation-induced stacking fault hardly exists,
thereby suppressing occurrence of an epitaxial defect. Further, the
oxidation-induced stacking fault hardly exists in the wafer outer
peripheral region, so that it is possible to prevent, an increase
in the BMD density and a reduction in the residual oxygen
concentration in the outer peripheral region. Thus, occurrence of
slip dislocation in the wafer outer peripheral region can be
prevented.
[0016] In the present invention, the silicon wafer is preferably
cut out from a silicon single crystal ingot whose oxygen
concentration is adjusted in a range of 8.times.10.sup.17
atoms/cm.sup.3 (Old-ASTM_F121, 1979) or more and 14.times.10.sup.17
atoms/cm.sup.3 (Old-ASTM_F121, 1979) or less. With this
configuration, it is possible to prevent a reduction in the
residual oxygen concentration in the outer peripheral region while
ensuring a desired BMD density in the wafer center region.
[0017] In the present invention, a diameter of the epitaxial
silicon wafer is preferably 300 mm or more. In this case, the
silicon wafer is preferably cut out from a straight body section of
a silicon single crystal ingot having a diameter 1.02 to 1.07 times
larger than a target diameter and processed into the target
diameter. Further, an outer periphery grinding region of the
straight body section of the silicon single crystal ingot outside
the target diameter preferably consists of an R-OSF region. As
described above, by setting the diameter of the straight body
section of the silicon single crystal ingot to a diameter 1.02 to
1.07 times the target diameter of the silicon wafer and positioning
the R-OSF region in the silicon single crystal within the outer
periphery grinding region for removal, it is possible to prevent
the R-OSF region from being included in the silicon -wafer and to
position a region existing in the vicinity of a boundary with the
R-OSF region, where the BMD density is reduced, in the wafer outer
peripheral region. Thus, a region where the residual oxygen
concentration is increased with a reduction in the BMD density can
be positioned in the outer peripheral region, whereby the strength
of the wafer can be enhanced.
[0018] According to the present invention, it is possible to
provide an epitaxial silicon wafer capable of preventing occurrence
of the slip dislocation by enhancing the strength of the wafer
outer peripheral portion while ensuring required gettering
capability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above features and advantages of the present invention
will be more apparent from the following description of certain
preferred embodiments taken in conjunction with the accompanying
drawings, in which:
[0020] FIG. 1 is a flowchart for explaining a manufacturing method
for an epitaxial silicon wafer according to an embodiment of the
present invention;
[0021] FIG. 2 is a plan view for explaining the cross-sectional
shape (diameter) of the silicon single crystal in comparison with
that of a silicon wafer;
[0022] FIG. 3 is a view illustrating the general relationship
between the V/G and type and distribution of the defect;
[0023] FIGS. 4A and 4B are graphs each illustrating the
relationship between the nitrogen doping amount and the R-OSF
region;
[0024] FIG. 5 is a graph illustrating a BMD density distribution in
the wafer radial direction, in which the horizontal axis represents
a distance (mm) from, the wafer center, and the vertical axis
represents a BMD density (/cm.sup.3);
[0025] FIG. 6 is a graph illustrating BMD density distributions in
the wafer radial direction of Example 4, Comparative Example 2, and
Comparative Example 4, in which the horizontal axis represents a
distance (mm) from the wafer center, and the vertical axis
represents a BMD density (/cm.sup.3);
[0026] FIG. 7 is a table showing results of the evaluation of the
characteristics of the epitaxial silicon wafers with nitrogen
concentration, oxygen concentration, and presence/absence of
pre-annealing as parameters;
[0027] FIG. 8 is a table shewing results of the BMD density
distributions in the wafer outer peripheral region of Example 4 and
Comparative Examples 2 and 4; and
[0028] FIG. 9 is a table showing results of the evaluation of the
characteristics of the epitaxial silicon -wafers produced using a
plurality of silicon single crystals having different nitrogen
concentrations and diameters.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0029] Preferred embodiments of the present invention will be
described hereinafter in detail with reference to the accompanying
drawings.
[0030] FIG. 1 is a flowchart for explaining a manufacturing method
for an epitaxial silicon wafer according to an embodiment of the
present invention.
[0031] As illustrated in FIG. 1, in the epitaxial silicon wafer
manufacturing method, a silicon single crystal growing process is
performed using the Czochralski method (CZ method) (step S11). In
the CZ method, a seed crystal is dipped into the liquid surface of
a silicon melt contained in a quartz crucible and then pulled
upward while being rotated, whereby a silicon single crystal having
the same crystal orientation as the seed crystal is grown.
Specifically, the following processes are sequentially performed: a
liquid dipping process in which the seed crystal is dipped into the
silicon melt; a necking process in which the crystal diameter is
reduced by a so-called Dash-Neck method; a shoulder section growing
process in which a single crystal diameter is gradually increased
to a desired value; a body section growing process in which growing
of the single crystal is continued with the crystal diameter kept
substantially constant; and a tail section growing process in which
the single crystal is separated from the silicon melt while the
diameter thereof is gradually reduced.
[0032] FIG. 2 is a plan view for explaining the cross-sectional
shape (diameter) of the silicon single crystal in comparison with
that of a silicon wafer.
[0033] As illustrated in FIG. 2, in the growing process of the
silicon single crystal by the CZ method, a silicon single crystal
10 having a diameter R.sub.1 which is 1.02 to 1.07 times larger
than a specified diameter R.sub.2 (e.g., 300 mm) of a silicon wafer
is pulled up. At this time, a pull-up condition is controlled such
that an oxygen concentration in the silicon single crystal 10 is
8.times.10.sup.17 atoms/cm.sup.5 or more and 14.times.10.sup.17
atoms/cm.sup.3 or less and that the R-OSF region is positioned in
an outer peripheral grinding region 11 which is positioned outside
the specified diameter R.sub.2. An outer peripheral region 21 of
the silicon wafer 20 is a region that is supported by a vertical
boat when the wafer is subjected to heat treatment in a vertical
furnace and a region between the outermost periphery and the dashed
line in FIG. 2 positioned inward there from by 10 mm.
[0034] When the silicon single crystal 10 is grown by the CZ
method, the type and distribution of the defect contained in the
single crystal depend upon a ratio V/G between a single crystal
pull-up speed V and a temperature gradient G in a single crystal
growing direction.
[0035] FIG. 3 is a view illustrating the general relationship
between the V/G and type and distribution of the defect.
[0036] As illustrated in FIG. 3, when the V/G is large, vacancies
are excessively generated, a minute void (defect generally called
"COP (Crystal Originated Particle)") which is an aggregate of
vacancies occurs. On the other hand, when the V/G is small,
interstitial silicon atoms are excessively generated, and thus a
dislocation cluster which is an aggregate of interstitial silicon
is generated. Between the region where the COP is generated and the
region where the dislocation cluster is generated, three regions:
an OSF region; a Pv region; and a Pi region exist, (which are in
the descending order of the V/G value). The OSF region is a region
where an OSF (Oxidation induced Stacking Fault) is generated when a
silicon single crystal wafer is subjected to oxidation heat
treatment at a high temperature (generally, 1000.degree. C. to
1200.degree. C.). The Pv region includes oxygen-precipitation
nuclei in an as-grown state, and when the wafer is subjected to
heat treatment including two stages at low and high temperatures
(e.g., 800.degree. C. and 1000.degree. C.), oxygen precipitates are
easily generated. The Pi region substantially does not include
oxygen-precipitation nuclei in the as-grown state, and is a region
where oxygen precipitates are not easily generated even after heat
treatment.
[0037] Normally, when the V/G is large, the OSF is generated in a
ring shape around, the center axis of a silicon single crystal, so
that it is also called "R-OSF region". The range of the V/G where
the R-OSF region appears is very narrow, so that it is necessary to
strictly manage the pull-up speed V in the manufacturing process of
the silicon single crystal containing the R-OSF region. That is,
control needs to be performed so that the V/G falls within an
adequate range in both the diameter direction and length direction
of the crystal. For the diameter direction of the single crystal,
the pull-up speed V is constant at any position in the diameter
direction, so that the structure of a hot zone in a CZ furnace
should be designed so that the temperature gradient G falls within
a predetermined range. For the length direction, the temperature
gradient G depends upon a pull-up distance, so that the V should be
changed in the crystal length direction in order to keep the V/G
within a predetermined range.
[0038] Then, a pull-up speed profile is set so that the R-OSF
region is contained in the silicon single crystal, and Cu (copper)
decoration and heat treatment for OSF evaluation are applied to a
wafer sample cut out from the pulled-up silicon single crystal to
evaluate the width or position of the OSF region. Then, based on
the width or position, subsequent pull-up speed profile is
adjusted. That is, a wider R-OSF region means an excessively large
V/G (V is excessively large), so that the V is set relatively low
as the subsequent pull-up profile; conversely, a narrower R-OSF
region means an excessively small V/G (V is excessively small), so
that the V is set relatively high as the subsequent pull-up
profile.
[0039] As described above, by adjusting the subsequent pull-up
speed profile using the width or position of the R-OSF region as an
index, the silicon single crystal 10 containing the R-OSF region in
the outer peripheral grinding region 11 can be stably pulled up.
Crystal heat hysteresis at this time is preferably as follows: a
temperature range of 1080.degree. C. to 1150.degree. C. for 40
minutes or less; a temperature range of 980.degree. C. to
1020.degree. C. for 30 minutes or less; and a temperature range of
650.degree. C. to 750.degree. C. for 150 minutes or less.
[0040] The silicon single crystal 10 is doped with nitrogen of
1.times.1012 atoms/cm.sup.3 or more and 1.times.10.sup.13
atoms/cm.sup.3 or less. The doping of nitrogen can increase the BMD
density in the wafer surface, thereby enhancing the gettering
capability of the epitaxial silicon wafer.
[0041] The nitrogen doping amount, is preferably 1.times.10.sup.13
atoms/cm.sup.3 or less. By increasing the doping amount, the BMD
density on the entire wafer surface can be increased, and thus
required gettering capability can be easily ensured; on the other
hand, however, the BMD density is further increased in the wafer
outer peripheral region 21 illustrated in FIG. 2, while a residual
oxygen concentration in the wafer is reduced to decrease wafer
strength, with the result that slip dislocation easily occurs.
Further, the more the nitrogen doping amount is, the wider the
width of the R-OSF region, which makes it difficult to control the
pull-up condition such that the R-OSF region is not contained in
the silicon wafer.
[0042] FIGS. 4A and 4B are graphs each illustrating the
relationship between the nitrogen doping amount, and the R-OSF
region. The graph of FIG. 4A illustrates the R-OSF region when the
nitrogen doping amount is large, and the graph of FIG. 4B
illustrates the R-OSF region when the nitrogen doping amount is
small. The horizontal axis of each of the graphs of FIGS. 4A and 4B
represents a distance from the center of the silicon single
crystal, and the vertical axis thereof represents the V/G.
[0043] As illustrated in FIGS. 4A and 4B, a width Wa of the R-OSF
region when the nitrogen doping amount is large is wider than a
width Wb of the R-OSF region when the nitrogen doping amount is
small. Thus, when the nitrogen doping amount is large, it is
difficult to position the R-OSF region outside the silicon wafer
20. The R-OSF region is a region containing the nuclei of the OSF
even in an as-grown state, that is, a region containing plate-like
oxygen precipitates, so that when the wafer outer peripheral region
21 contains the R-OSF region, the BMD density of the wafer outer
peripheral region 21 is increased, while the residual oxygen
concentration is reduced, with the result that the strength of the
wafer outer peripheral region 21 is reduced.
[0044] Thus, in the present embodiment, in order to position the
R-OSF region within the outer peripheral grinding region 11 outside
the silicon wafer 20, the nitrogen doping amount is reduced as much
as possible to reduce the BMD density of the wafer outer peripheral
region 21 to a low value, and the width of the R-OSF region is
narrowed as much as possible. Further, as described later, by
positioning a part of the R-OSF region near the outer peripheral
region 21 at the boundary portion thereof, the BMD density of the
outer peripheral region 21 can be reduced, so that it is possible
to enhance the strength of the outer peripheral region 21 to
thereby suppressing occurrence of the slip dislocation.
[0045] Subsequently, the silicon single crystal is processed to
obtain a silicon wafer (polished wafer) (step S12 of FIG. 1). The
silicon wafer is obtained by cutting a silicon single crystal ingot
into blocks of the same size, followed by rounding process (outer
periphery grinding) for making the diameters of all the blocks
equal to each other, then by a slice process, a beveling process, a
wrapping process, an etching process, a polishing process, a
cleaning process, and the like. In the rounding processing, the
outer peripheral grinding region 11 outside the target diameter
R.sub.2 of the wafer is removed, whereby the R-OSF region is
removed. The entire surface of the thus processed silicon wafer
consists of a COP region.
[0046] The COP region means a region where the COP is detected by
observation/evaluation to be described below. First, SC-1 cleaning
(cleaning using a liquid mixture obtained by mixing ammonia water,
hydrogen peroxide water, and ultrapure water in a ratio of 1:1:15)
is applied to the silicon wafer. Then, the surface of the
silicon-wafer after the cleaning is observed/evaluated using a
surface defect inspection apparatus (e.g., KLA-Tencor: Surfscan
SP-2) to specify a LPD (Light Point Defect) estimated as surface
pits. At this time, an observation is set to an Oblique mode
(oblique incidence mode), and estimation of the surface pits is
made based on a detection size ratio between wide and narrow
channels. Then, whether the thus specified LPD is the COP is
evaluated using an AFM (Atomic Force Microscope). Through this
observation/evaluation, a region having the COP is defined as the
COP region.
[0047] Whether the entire surface of the obtained silicon wafer is
the COP region can be confirmed by applying heat treatment for
evaluation (heat treatment for evaluation of oxidation-induced
stacking fault) to a wafer sample (step S13 of FIG. 1). The
evaluation heat treatment is treatment that makes the OSF apparent.
Specifically, as the evaluation heat treatment, oxidation heat
treatment is applied to the wafer sample in a wet oxygen atmosphere
at 1140.degree. C. for two hours. Then, the surface of the wafer is
etched by 2 .mu.m using a Wright etching liquid. After that, the
OSF density is counted by measuring etch pits observed on the wafer
surface using an optical microscope. The temperature of the
oxidation heat treatment may be 1000.degree. C. to 1200.degree. C.,
and heat treatment time may be 1 hour to 16 hours. When the
observed OSF density of the silicon wafer is 100/cm.sup.2 or less,
it can be said that the entire surface of the silicon wafer
consists of the COP region. In this case, it is possible to
suppress occurrence of an epitaxial defect.
[0048] Subsequently, annealing treatment (pre-annealing) for
growing BMD nuclei is performed (step S14 of FIG. 1). The
pre-annealing is heat treatment performed at 900.degree. C. or
lower for 10 minutes to 60 minutes. When the nitrogen doping amount
in the silicon wafer is reduced as described above, the BMD density
on the entire wafer surface is reduced, thus failing to obtain a
desired BMD density (e.g., 1.times.10.sup.8/cm.sup.3 or more) at a
center region of the wafer. In particular, when an epitaxial
process is performed without performing the pre-annealing, minute
oxygen-precipitation nuclei are eliminated during the epitaxial
process, thus failing to obtain a desired BMD density. However,
when the pre-annealing is performed before the epitaxial process,
it is possible to ensure a desired BMD density even when the
nitrogen doping amount is small.
[0049] Further, in the present embodiment, the BMD density in the
wafer outer peripheral region 21 can be reduced by positioning the
R-OSF region within the outer peripheral grinding region 11.
[0050] FIG. 5 is a graph illustrating a BMD density distribution in
the wafer radial direction, in which the horizontal axis represents
a distance (mm) from the wafer center, and the vertical axis
represents a BMD density (/cm.sup.3) . The diameter of the silicon
wafer is 300 mm, and an outermost periphery of the silicon wafer is
positioned at a distance of 150 mm from the wafer center. Thus, the
position at a distance of 1 mm from the outermost periphery is
separated from the wafer center by 149 mm, and the position at a
distance of 10 mm from the outermost periphery is separated from
the wafer center by 140 mm. The outer peripheral region 21 of the
silicon wafer 20 is defined as a 1-10 mm range separated inward
from the outermost periphery. The region outside the outermost
periphery of the silicon wafer 20 is defined as the outer
peripheral grinding region 11 of the silicon single crystal.
[0051] As illustrated in FIG. 5, the entire surface of the silicon
wafer 20 consists of the COP region, and the R-OSF region is
positioned within the outer peripheral grinding region 11. The BMD
density in the wafer radial direction is substantially constant in
a center region 22. However, it is once reduced in the wafer outer
peripheral region 21 in the vicinity of the boundary with the R-OSF
region, and is then increased in the R-OSF region.
[0052] With attention paid to such a phenomenon that the BMD
density is reduced, the present embodiment aims at reducing the BMD
density in the wafer outer peripheral region 21 by positioning the
R-OSF region within the outer peripheral grinding region 11 of the
silicon single crystal and positioning the region in the vicinity
of the boundary between the R-OSF region and the COP region where
the BMD density is reduced within the wafer outer peripheral region
21.
[0053] Subsequently, an epitaxial process for forming a silicon
epitaxial film on the surface of the silicon wafer is performed
(step S15 of FIG. 1). Although not especially limited, the
epitaxial process is preferably performed using a single wafer type
vapor phase growing apparatus for a silicon wafer having a large
diameter of 300 mm or more. The silicon wafer is set in a chamber
of the vapor phase growing apparatus, and a raw-material gas such
as trichlorosilane (SiHCl.sub.3) is introduced into the chamber
together with a carrier gas such as H.sub.2 gas and a dopant gas.
Then, silicon generated through thermal decomposition or reduction
of the raw-material gas is grown at a reaction speed of 0.5 .mu.m
to 6.0 .mu.m per minute on the silicon wafer heated to a high
temperature of 1000.degree. C. to 1200.degree. C. Thereafter, the
silicon wafer is cleaned, whereby an epitaxial silicon wafer is
completed.
[0054] The BMD density distribution of the thus produced epitaxial
silicon wafer can be confirmed by performing heat treatment for
evaluation (heat treatment for evaluation of oxygen precipitates)
simulating the device process to grow the BMD nuclei (step S16).
The BMD density is defined as follows. The heat treatment for
evaluation of oxygen precipitates is applied to the epitaxial
silicon wafer at 900.degree. C. to 1100.degree. C. for 16 hours in
an oxygen gas atmosphere. Then, the epitaxial silicon wafer is
cleaved in the thickness direction thereof, and selecting etching
is performed to etch the cleaved cross section thereof by a depth
of 2 .mu.m using a Wright etching liquid. Thereafter, the cleaved
cross section in the thickness direction center of the silicon
wafer is observed using an optical microscope, and an etch pit
density in a 100 .mu.m.times.100 .mu.m square area is defined as
the BMD density. The epitaxial silicon wafer according to the
present embodiment has the following features in terms of the BMD
density.
[0055] First, the BMD density on the entire wafer surface is
1.times.10.sup.8/cm.sup.3 or more. This makes it possible to
enhance the gettering capability of the epitaxial silicon wafer.
Preferably, the BMD density on the entire wafer surface is
3.times.10.sup.9/cm.sup.3 or less. When the BMD density of the
wafer is excessively high, the residual oxygen concentration in the
wafer is reduced to decrease wafer strength, with the result that
slip dislocation easily occurs especially in the wafer outer
peripheral region 21.
[0056] The BMD density in the wafer outer peripheral region 21
which is the 1 mm to 10 mm range separated inward from the
outermost periphery (outermost peripheral edge) of the wafer is
lower than the BMD density in the center region inside the wafer
outer peripheral region 21. With this configuration, the residual
oxygen concentration in the wafer outer peripheral region 21 can be
increased to thereby suppress occurrence of the slip
dislocation.
[0057] Further, the BMD density in the wafer outer peripheral
region 21 which is the 1-10 mm range separated inward from. the
outermost periphery is gradually reduced toward the wafer outermost
periphery, and a value ((Max-Min/Ave): variation in the BMD
density) obtained by dividing a difference between the maximum
value (Max) and the minimum value (Min) of the BMD density in the
wafer outer peripheral region 21 by an average value (Ave) of the
BMD density in the wafer outer peripheral region 21 is 3 or less.
Further, the maximum value of the BMD density exists in a 6-10 mm
range separated inward from the wafer outermost, periphery, and the
minimum value of the BMD density exists in a 0-less than 6 mm range
separated inward from the wafer outermost, periphery. Such a
reduction in the BMD density in the wafer outer peripheral region
21 can increase the residual oxygen concentration to thereby
enhance the strength of the wafer outer peripheral region 21. The
reason that the wafer outer peripheral region 21 is separated
inward from the outermost periphery by 1 mm or more is not only
that the wafer edge is chamfered, but also that it is impossible to
measure the BMD density at the outermost periphery (0 mm position
from the outermost periphery).
[0058] The residual oxygen concentration in the wafer outer
peripheral region 21 is 8.times.10.sup.17 atoms/cm.sup.3 or more.
With this configuration, it is possible to suppress occurrence of
the slip dislocation. The residual oxygen concentration in the
wafer is higher in a region where the BMD density is lower and
lower in a region where the BMD density is higher. This is
considered because oxygen in the wafer is consumed for BMD
formation, and the residual oxygen is reduced as the oxygen
precipitates increase. In the present invention, the BMD density in
the wafer outer peripheral region 21 is low, so that the residual
oxygen concentration in the wafer outer peripheral region 21 can be
increased to thereby enhance the wafer strength in the wafer outer
peripheral region 21.
[0059] As described above, in the epitaxial silicon wafer according
to the present embodiment, the BMD density in the wafer outer
peripheral region 21 is low, so that the strength of the wafer
outer peripheral region 21 can be enhanced to thereby suppress
occurrence of the slip dislocation. Thus, during heat treatment of
the wafer in a vertical furnace, it is possible to prevent
occurrence of a minute flaw at a support position for the wafer
housed in a vertical boat, which thereby prevents the slip
dislocation from occurring with the flaw as a starting point and
spreading.
[0060] While the preferred embodiment of the present invention has
been described, the present invention is not limited to the above
embodiment, but various modifications may be made without departing
from the scope of the invention, and such modifications may be
included within the scope of the present invention.
[0061] For example, although the wafer has a diameter of 300 mm in
the above embodiment, the wafer diameter only needs to be 300 mm or
more and, for example, may be 450 mm.
Examples
[0062] First, the characteristics of the epitaxial silicon wafers
were evaluated with nitrogen concentration, oxygen concentration,
and presence/absence of pre-annealing as parameters.
[0063] In this characteristic evaluation, a nitrogen-doped silicon
single crystal having a diameter of 310 mm was grown by the CZ
method, followed by outer periphery grinding to reduce the diameter
of the silicon single crystal to 300 mm to show a round shape, and
then by wafer processing. Then, before epitaxial process, heat
treatment, was applied, as pre-annealing, to some wafer samples at
890.degree. C. for 20 minutes. In the epitaxial process, an
epitaxial silicon film of 4 .mu.m thickness was formed at
1100.degree. C.
[0064] In BMD density measurement, after heat treatment for
evaluation was performed at 1000.degree. C. for 16 hours in an
oxygen gas atmosphere, the epitaxial silicon wafer was cleaved in
the thickness direction thereof, and then selecting etching was
performed to etch the cleaved cross section thereof by a depth of 2
.mu.m using a Wright etching liquid. Thereafter, the cleaved cross
section in the thickness direction center of the silicon wafer was
observed using an optical microscope to measure the BMD density.
The BMD density was measured at a 10 mm pitch radially from the
wafer center. In particular, in the wafer outer peripheral region
(1-10 mm range separated inward from the outermost periphery), the
measurement was performed at a 1 mm pitch.
[0065] The residual oxygen concentration in the epitaxial silicon
wafer was measured at a 1 mm pitch in the radial direction from the
wafer center, and the average value was calculated.
[0066] In slip-resistance test, additional heat treatment was
performed for evaluation of thermal stress loading in a boat-shaped
vertical furnace supporting the outer peripheral region of the
wafer. Conditions of the evaluation heat treatment were: feeding
temperature of 700.degree. C.; temperature rising rate of 8.degree.
C./min; holding temperature and holding time of 1100.degree. C. and
30 minutes, respectively; temperature falling rate of 3.degree.
C./min; and take-out temperature of 700.degree. C. Then, a wafer
after the heat treatment was observed by X-ray topography.
Evaluation was made in terms of the length of the slip dislocation:
".smallcircle.", 2 mm or less; ".DELTA.", 2 mm to 5 mm; and "x",
exceeding 5 mm.
[0067] In gettering capability evaluation, Ni was applied onto the
surface of the epitaxial silicon wafer after evaluation heat
treatment by 1.times.10.sup.12 atoms/cm.sup.2, followed by heat
treatment at 900.degree. C. for 30 minutes, and then, the wafer
surface was Wright-etched by 2 .mu.m. Then, the Wright-etched wafer
surface was observed through an optical microscope. Evaluation was
made in terms of the present/absence of Ni silicide pits:
".smallcircle.", absence; and "x", presence.
[0068] The results of the above evaluation test are shown in FIG.
7. Examples 1 to 6 are produced according to the technique of the
present invention, and comparative Examples 1 to 5 are produced
according to the conventional technique.
[0069] As is clear from FIG. 7, Examples 1 to 6 meeting the
conditions of the present invention can make both slip resistance
and gettering capability compatible.
[0070] On the other hand, Comparative Example 1 has a low nitrogen
concentration and has not been subjected to pre-annealing,
resulting in low BMD density and poor gettering capability.
[0071] Comparative Example 2 has not been subjected to
pre-annealing, but it shows a high nitrogen concentration of
5.times.10.sup.13 atoms/cm.sup.3, so that the BMD density
exhibiting sufficient, gettering capability is obtained, while the
minimum residual oxygen concentration in the wafer outer peripheral
region is as low as 7.8.times.10.sup.17 atoms/cm.sup.3. Further, a
variation ((Max/Min)/Ave) in the BMD density in the wafer outer
peripheral region is as large as 3.8, resulting in an increase in
slip dislocation length. This is considered, because the
excessively large variation in BMD density in the wafer outer
peripheral region changes heat transfer at the wafer support
position, causing increase in thermal stress.
[0072] Comparative Example 3 is similar to Comparative Example 2,
but the residual oxygen concentration thereof in the wafer outer
peripheral region, is 7.5.times.10.sup.17 atoms/cm.sup.3, which is
as low as lower than 8.times.10.sup.17 atoms/cm.sup.3, and a
variation ((Max/Min)/Ave) in the BMD density is as large as
5.7,resulting in a further increase in the slip dislocation
length.
[0073] Comparative Examples 4 and 5 show results obtained when
using a wafer having a nitrogen concentration higher than
1.times.10.sup.13 atoms/cm.sup.3 and applying pre-annealing to the
wafer. The BMD density exceeding 3.times.10.sup.9/cm.sup.3 is
observed in the wafer. Excessive BMD formation is undesirable since
it induces an epitaxial defect due to the BMD. In addition, the BMD
density in the wafer outer peripheral region is high, and residual
oxygen concentration in the wafer outer peripheral region is low,
resulting in poor slip resistance.
[0074] FIG. 6 is a graph illustrating BMD density distributions in
the wafer radial direction of Example 4, Comparative Example 2, and
Comparative Example 4, in which the horizontal axis represents a
distance (mm) from the wafer center, and the vertical axis
represents a BMD density (/cm.sup.3). FIG. 8 shows a BMD density
distribution in the wafer outer peripheral region.
[0075] As illustrated in FIG. 6, the BMD densities of Example 4 and
Comparative Example 4 are gradually reduced in the wafer outer
peripheral region. The Comparative Example 4 has a high BMD density
of 1.times.10.sup.9/cm.sup.3 and thus exhibits sufficient gettering
capability; however, it has a residual oxygen concentration of
8.times.10.sup.17 atoms/cm.sup.3 in the wafer outer peripheral
region, resulting in an increase in the slip dislocation length and
poor slip resistance.
[0076] In Comparative Example 2, the BMD density is increased
toward the wafer outer periphery, and the BMD density in the outer
peripheral region is as high as 1.times.10.sup.1/cm.sup.3 or more.
Thus, the residual oxygen concentration is reduced due to high BMD
density, resulting in poor slip resistance.
[0077] The above results show that the BMD density is desirably
1.times.10.sup.8/cm.sup.3 or more (and 1.times.10.sup.9/cm.sup.5 or
less) in the wafer surface and has a surface distribution in which
it is gradually reduced in the wafer outer peripheral region.
[0078] FIG. 8 shows results obtained when investigating in detail
the BMD density distributions of the wafers of Example 4 and
Comparative Examples 2 and 4 at a 1 mm pitch in the radial
direction. As is clear from FIG. 8, when the maximum value of the
BMD density exists in a 140-144 mm (6-10 mm range separated inward
from the outermost periphery) range separated outward from the
wafer center, and the minimum value of the BMD density exists in a
range exceeding 144 mm (0-less than 6 mm range separated inward
from the outermost periphery) separated outward from the wafer
center, the slip resistance becomes satisfactory.
[0079] Secondly, the characteristics of the epitaxial silicon
wafers produced using a plurality of silicon single crystals having
different nitrogen concentrations and diameters were evaluated.
[0080] In this characteristic evaluation, a plurality of silicon
single crystals having different nitrogen concentrations were grown
by the CZ method, followed by outer periphery grinding to reduce
the diameter of each silicon single crystal to 300 mm, and then by
wafer processing. In the growing process of the silicon single
crystal, a portion having a wide outer periphery grinding region
and a portion having a narrow outer periphery grinding region were
made to be included in one silicon single crystal ingot by changing
the crystal diameter.
[0081] Then, the maximum value of the OSF density of the processed
wafer (polished wafer) was measured. In the OSF density
measurement, heat treatment for evaluation was performed at
1140.degree. C. for two hours, followed by 2 .mu.m Wright etching.
After that, the OSF density on the wafer surface was measured using
an optical microscope.
[0082] Then, before epitaxial process, heat treatment was applied,
as pre-annealing, to the silicon wafer at 890.degree. C. for 20
minutes. In the epitaxial process, an epitaxial silicon film of 4
.mu.m thickness was formed at 1100.degree. C.
[0083] In BMD density measurement, after heat treatment for
evaluation was performed at 1000.degree. C. for 16 hours in an
oxygen gas atmosphere, the epitaxial silicon wafer was cleaved in
the thickness direction thereof, and then selecting etching was
performed to etch the cleaved cross section thereof by a depth of 2
.mu.m using a Wright etching liquid. Thereafter, the cleaved cross
section in the thickness direction center of the silicon wafer was
observed using an optical microscope to measure the BMD density.
The BMD density was measured at a 10 mm pitch radially from the
wafer center. In particular, in the wafer outer peripheral region
(1-10 mm range separated inward from the outermost periphery), the
measurement was performed at a 1 mm pitch.
[0084] In slip-resistance test, additional neat treatment was
performed for evaluation of thermal stress loading in a boat-shaped
vertical furnace supporting the outer peripheral region of the
wafer. Conditions of the evaluation heat treatment were: feeding
temperature of 700.degree. C.; temperature rising rate of 8.degree.
C./min; holding temperature and holding time of 1100.degree. C. and
30 minutes, respectively; temperature falling rate of 3.degree.
C./min; and take-out temperature of 700.degree. C. Then, a wafer
after the heat treatment was observed by X-ray topography.
Evaluation was made in terms of the length of the slip dislocation:
".smallcircle.", 2 mm or less; ".DELTA.", 2 mm to 5 mm; and "x":
exceeding 5 mm.
[0085] The results of the above evaluation test are shown in FIG.
9. Examples 1 to 9 are produced according to the technique of the
present invention, and Comparative Examples 1 to 9 are produced
according to the conventional technique. The wafers (e.g., Examples
1, 4, and 7 and Comparative Example 1) having the same nitrogen
concentration are those cut out from the same silicon single
crystal ingot.
[0086] As is clear from FIG. 9, in Examples 1 to 9 meeting the
conditions of the present invention, the nitrogen concentration is
as low as 1.0.times.10.sup.12 to 1.0.times.10.sup.13
atoms/cm.sup.3, and the ratio of the silicon single crystal
diameter to the wafer diameter is 1.02 to 1.07. The OSF density of
these wafers is 100/cm.sup.2 or less, and the wafer entire surface
consists of the COP region crystal. Further, a variation
((Max/Min)/Ave) in the BMD density in the outer peripheral region
of the epitaxial silicon wafer is 3 or less, resulting in
satisfactory slip resistance.
[0087] On the other hand, in the Comparative Examples 1 to 3, the
diameter ratio of the silicon single crystal is as low as 1.01, and
the maximum value of the OSF density is as large as 109 to
163/cm.sup.2. Thus, it can be understood that the R-OSF region
exists not only in the outer periphery grinding region of the
silicon single crystal, but also inside the silicon wafer, with the
result that an epitaxial defect is induced to degrade epitaxial
quality.
[0088] In Comparative Examples 4 to 6, the nitrogen concentration
is as high as 1.9.times.10.sup.13 to 9.5.times.10.sup.13
atoms/cm.sup.3, so that the width of the OSF ring is increased.
Thus, even when a silicon single crystal having the diameter ratio
of 1.05 is subjected to rounding processing, the R-OSF region
cannot be removed, resulting in very high OSF density, which in
turn induces an epitaxial defect. Further, the BMD density in the
wafer outer peripheral region is high, and its variation is 3 or
more, resulting in poor slip resistance.
[0089] In Comparative Examples 7 to 9 as well, the nitrogen
concentration is as high as 1.9.times.10.sup.13 to
9.5.times.10.sup.13 atoms/cm.sup.3, while the diameter ratio of the
silicon single crystal is as large as 1.08, so that the R-OSF
region can be removed by rounding processing. However, the BMD
density in the wafer outer peripheral region is high, and the
residual oxygen concentration in the wafer outer peripheral region
is low, resulting in poor slip resistance. Further, the diameter of
the silicon single crystal needs to be 324 mm. Accordingly, more
silicon raw material is required for wafer manufacturing, resulting
in poor productivity and thus in low suitability for mass
production.
* * * * *