U.S. patent application number 11/339672 was filed with the patent office on 2006-06-22 for methods for manufacturing silicon wafer and silicon epitaxial wafer, and silicon epitaxial wafer.
Invention is credited to Hiroshi Takeno.
Application Number | 20060130736 11/339672 |
Document ID | / |
Family ID | 27482417 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060130736 |
Kind Code |
A1 |
Takeno; Hiroshi |
June 22, 2006 |
Methods for manufacturing silicon wafer and silicon epitaxial
wafer, and silicon epitaxial wafer
Abstract
There is provided a method for manufacturing a silicon wafer or
a silicon epitaxial wafer capable of imparting an excellent IG
capability thereto in a stable manner by simultaneously realizing
higher density of oxide precipitates and larger sizes thereof at a
stage prior to a device fabrication process. The present invention
is a method for manufacturing a silicon wafer wherein the silicon
wafer is subjected to heat treatment to impart a gettering
capability thereto comprising at least the following three steps
of: a temperature raising step A for generating oxygen
precipitation nuclei; a temperature raising step B for growing the
oxygen precipitation nuclei; and a constant temperature keeping
step C for growing the oxygen precipitation nuclei into oxide
precipitates of larger sizes.
Inventors: |
Takeno; Hiroshi;
(Annaka-shi, JP) |
Correspondence
Address: |
RADER FISHMAN & GRAUER PLLC
LION BUILDING
1233 20TH STREET N.W., SUITE 501
WASHINGTON
DC
20036
US
|
Family ID: |
27482417 |
Appl. No.: |
11/339672 |
Filed: |
January 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10482843 |
Jan 6, 2004 |
7033962 |
|
|
11339672 |
Jan 26, 2006 |
|
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Current U.S.
Class: |
117/2 ;
257/E21.321; 438/471; 438/660 |
Current CPC
Class: |
H01L 21/3225
20130101 |
Class at
Publication: |
117/002 ;
438/471; 438/660 |
International
Class: |
H01L 21/322 20060101
H01L021/322; H01L 21/44 20060101 H01L021/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 10, 2001 |
JP |
2001-209160 |
Sep 27, 2001 |
JP |
2001-296743 |
Sep 27, 2001 |
JP |
2001-296744 |
Sep 27, 2001 |
JP |
2001-296745 |
Claims
1.-8. (canceled)
9. A method for manufacturing a silicon wafer comprising the steps
of: keeping the silicon wafer having grown-in precipitation nuclei
generated in a growth step of a silicon single crystal at a
temperature T.sub.4.degree. C. in the range of from 500.degree. C.
to 700.degree. C. for a prescribed time t.sub.1; then raising the
temperature of the silicon wafer to a temperature T.sub.5.degree.
C. in the range of from 1000.degree. C. to 1230.degree. C. at a
temperature raising rate of 5.degree. C./min or less; and keeping
the temperature thereof at the temperature T.sub.5.degree. C. for a
prescribed time t.sub.2, thereby the grown-in precipitation nuclei
being grown into oxide precipitates each of a size equal to or
larger than one having a gettering capability, and oxygen in the
vicinity of a surface of the silicon wafer being out-diffused.
10. The method for manufacturing a silicon wafer according to claim
9, wherein the keeping time t.sub.1 at the T.sub.4.degree. C. is 15
min or longer.
11. The method for manufacturing a silicon wafer according to claim
9, wherein the keeping time t.sub.2 at the T.sub.5.degree. C. is 30
min or longer.
12. The method for manufacturing a silicon wafer according to claim
9, wherein oxygen concentration of the silicon wafer having the
grown-in precipitation nuclei is 16 ppma or higher.
13.-26. (canceled)
27. A method for manufacturing a silicon epitaxial wafer comprising
the steps of: keeping a silicon wafer having grown-in precipitation
nuclei generated in a growth step of a silicon single crystal at a
temperature T.sub.11.degree. C. in the range of from 500.degree. C.
to 700.degree. C. for a prescribed time t.sub.7; then raising the
temperature of the silicon wafer to a temperature T.sub.12.degree.
C. in the range of from 1000.degree. C. to 1100.degree. C. at a
temperature raising rate of 5.degree. C./min or less; keeping the
temperature thereof at the temperature T.sub.12.degree. C. for a
prescribed time t.sub.8, thereby the grown-in precipitation nuclei
being grown to sizes each equal to or larger than one not to be
annihilated in a subsequent epitaxial growth step; and thereafter
performing epitaxial growth on a surface of the silicon wafer.
28. The method for manufacturing a silicon epitaxial wafer
according to claim 27, wherein the prescribed time t.sub.7 is 15
min or longer.
29. The method for manufacturing a silicon epitaxial wafer
according to claim 27, wherein the prescribed time t.sub.8 is 30
min or longer.
30. The method for manufacturing a silicon epitaxial wafer
according to claim 27, wherein oxygen concentration of the silicon
wafer is 16 ppma or higher.
31. The method for manufacturing a silicon epitaxial wafer
according to claim 27, wherein a size of the oxide precipitate is
40 nm or larger in diameter.
32.-35. (canceled)
36. The method for manufacturing a silicon wafer according to claim
9, wherein a size of the oxide precipitate is 40 nm or larger in
diameter.
37. The method for manufacturing a silicon wafer according to claim
9, wherein a diameter of the silicon wafer is 300 mm or larger.
38. The method for manufacturing a silicon epitaxial wafer
according to claim 27, wherein a diameter of the silicon wafer is
300 mm or larger.
Description
TECHNICAL FIELD
[0001] The present invention relates to methods for manufacturing a
silicon wafer and a silicon epitaxial wafer capable of obtaining
the silicon wafer and the silicon epitaxial wafer having a
defect-free layer (a DZ layer or an epitaxial layer) in the
vicinity of a surface thereof and oxide precipitates high in
gettering capability in the bulk thereof through heat treatment
with extreme efficiency and simplicity, and a silicon epitaxial
wafer.
BACKGROUND ART
[0002] Most of silicon wafers used widely as substrates of
semiconductor devices are manufactured from a silicon single
crystal grown in a Czochralski (CZ) method. The silicon single
crystal grown in the CZ method contains interstitial oxygen as
impurities at concentration of about 10.sup.18 atoms/cm.sup.3. The
interstitial oxygen turns into a supersatulated state in a thermal
history from solidification of a melt till cooling down thereof to
room temperature during a crystal growth step (hereinafter which
may be referred to as a crystal thermal history) or in a heat
treatment step of a fabrication process of semiconductor devices to
then precipitate, precipitates of silicon oxides (hereinafter may
be referred to as oxide precipitates or simply precipitates) being
generated.
[0003] The oxide precipitates work effectively as sites capturing
heavy metal impurities contaminated in a device fabrication
process. This is called internal gettering (hereinafter may be
referred to as IG or simply gettering) and improves device
characteristics and a yield. For this reason, an IG capability is
valued highly as one of properties of a silicon wafer.
[0004] The process of oxygen precipitation includes formation of
precipitation nuclei and their growth. Usually formation of nuclei
progresses during a crystal thermal history, and then the nuclei
grow greatly in heat treatment of the following device fabrication
process and others, the grown nuclei being detected as oxide
precipitates. For this reason, the precipitation nuclei generated
during the crystal thermal history are referred to as grown-in
precipitation nuclei. As a matter of course, oxygen precipitation
nuclei may be generated in subsequent heat treatment.
[0005] A usual as-grown wafer (a wafer subjected to no heat
treatment except for general oxygen donor anihilation heat
treatment) has no IG capability because oxygen precipitation nuclei
existing at a stage prior to a device fabrication process are
extremely small. However, after the as-grown wafer has been
subjected to the device fabrication process, the nuclei grow to
large oxide precipitates and then the wafer has an IG
capability.
[0006] Since density of oxygen precipitation nuclei generated
during a crystal thermal history depends on the length of the
thermal history, there arises a problem that the density is largely
fluctuated according to crystal pulling conditions such as a
pulling rate or a position in the direction of a crystal growth
axis. For example, in a process producing one crystal, a portion
grown in a latter half thereof has lower density of oxygen
precipitation nuclei because of a shorter thermal history. This
causes a fluctuation in density of oxygen precipitation nuclei to
be generated during a device fabrication process, resulting in a
fluctuation in an IG capability.
[0007] On the other hand, when oxide precipitates exist in a device
fabrication region in the vicinity of a wafer surface, degradation
of the device characteristics occurs. For this reason, in order to
prevent progress in oxygen precipitation by out-diffusing oxygen in
the vicinity of the wafer surface, the wafer may be heat-treated at
1100.degree. C. or higher for several hours. In this case, most of
grown-in precipitation nuclei are annihilated and thereby, oxide
precipitates are not generated in the following device fabrication
process.
[0008] Therefore, in order to regenerate oxygen precipitation
nuclei, the wafer is subjected to heat treatment at about
650.degree. C. for a long time on the order of 3 hrs to 30 hrs.
Further, in order to grow the regenerated oxygen precipitation
nuclei into large oxide precipitates having an IG capability, the
wafer may be additionally subjected to heat treatment at about
1000.degree. C. There is generally called DZ-IG treatment a
combination of the three types of heat treatment, that is, the heat
treatment out-diffusing oxygen residing in the vicinity of the
wafer surface, the heat treatment generating oxygen precipitation
nuclei in the interior of the wafer, and the heat treatment growing
the oxygen precipitation nuclei.
[0009] With the DZ-IG treatment, an ideal structure is formed in
which there are no oxide precipitates in the vicinity of a wafer
surface to be a device fabrication region, while there are
generated oxide precipitates having an IG capability in the
interior thereof (hereinafter the structure may be referred to as a
DZ-IG structure). However, this treatment is long in the total
process for heat treatment, resulting in poor efficiency.
[0010] Also, a general CZ wafer contains void-like defects
generated by aggregation of atomic vacancies in addition to
grown-in precipitation nuclei. When the void defect is exposed on a
surface of a mirror-polished wafer, they are observed as surface
pits called COP (crystal originated particle). When the COPs and
voids are present in the device fabrication region, degradation of
the device characteristics occurs too. For this reason, in order to
eliminate the COPs and voids in the vicinity of the wafer surface,
the wafer may be subjected to high-temperature heat treatment at a
temperature of the order of 1200.degree. C. or higher in hydrogen
or argon atmosphere.
[0011] However, in this case, a problem arises that thermal stress
causes slip generation. In order to suppress slip generation, a
temperature of heat treatment is desirably lower, while at lower
heat treatment temperatures, the COPs and voids are harder to be
eliminated.
[0012] Further, in this case as well, an IG capability is
preferably to be imparted. Then, as a method in which annihilation
of the COPs and voids in the vicinity of a wafer surface and
generation of oxide precipitates in the interior of the wafer are
simultaneously realized, there has been known a method in which
nitrogen is added during crystal growth. In a wafer added with
nitrogen, smaller voids are easily annihilated by high-temperature
heat treatment, while larger grown-in precipitation nuclei grow
without annihilation even in high-temperature treatment into oxide
precipitates, thereby an IG capability being imparted.
[0013] However, even when a wafer added with nitrogen is used,
there is no change in that it is necessarily subjected to
high-temperature heat treatment at about 1200.degree. C. As the
wafer diameter becomes larger, the slip more easily generates;
therefore especially in a 300 mm wafer that is the mainstream
hereafter, there becomes a great problem need of high-temperature
heat treatment at about 1200.degree. C. or higher.
[0014] Further, since the area where void defects are annihilated
is in a very thin surface layer of the order of several .mu.m from
the surface, the void defects may degrade device characteristics in
a device using a surface layer deeper than the very thin surface
layer as a device fabrication region.
[0015] In order to bring a device fabrication region in the
vicinity of a wafer surface into a defect-free state, there may be
used an epitaxial wafer (hereinafter may be referred to as an epi
wafer) that is manufactured by depositing silicon single crystal on
a silicon wafer as an epitaxial growth substrate by means of vapor
phase growth. In this epi wafer, it is also important to impart an
IG capability to the substrate.
[0016] However, since a usual epitaxial step (hereinafter may be
referred to as an epi step) is operated at a high-temperature of
about 1100.degree. C. or higher, most oxygen precipitation nuclei
(grown-in precipitation nuclei) in the substrate generated in a
crystal thermal history are annihilated and thereby no oxide
precipitates are generated in the subsequent device fabrication
process. Accordingly, in an epi wafer, there arises a problem that
an IG capability deteriorates.
[0017] As a solution of this problem, there has been known a method
in which by performing heat treatment at about 800.degree. C. prior
to an epi step, grown-in precipitation nuclei generated in a
crystal thermal history are grown to sizes not to be annihilated
even in a high-temperature epi step. In this method, when a heat
treatment temperature prior to epi growth is 800.degree. C.,
grown-in precipitation nuclei with sizes equal to or larger than a
critical size (the minimum size of a precipitation nucleus that can
stably grow at the temperature) at 800.degree. C. grow and survive
during an epi step, and the survived nuclei further grow into oxide
precipitates in heat treatment such as a device fabrication process
after the epi step.
[0018] In general, density of small grown-in precipitation nuclei
is higher, while density of large grown-in precipitation nuclei is
lower. Furthermore, the lower the heat treatment temperature is,
the smaller the critical size becomes. Therefore, in order to
generate grown-in precipitation nuclei at higher density for
imparting a more excellent IG capability, it is desirable to lower
the heat treatment temperature prior to the epi step. However, if
the heat treatment temperature is lowered, a growth rate of
precipitation nuclei is slowed, so a long time of heat treatment is
required for growing the precipitation nuclei to sizes not to be
annihilated even in an epi step, which is not preferable because of
reduction in productivity.
[0019] Furthermore, if there exist oxide precipitates having sizes
not to be annihilated even in an epi step, in a recent device
fabrication process having a lower temperature and a shorter time,
no further growth of the oxide precipitates can be expected in heat
treatment of the device fabrication process. In order to exert a
gettering capability in such a recent device fabrication process,
it is necessary to generate oxide precipitates with sizes having a
gettering capability at high density prior to the device
fabrication process.
[0020] As described above, in the recent device fabrication
process, a trend of a lower temperature and a shorter time in a
device fabrication process has progressed in company with the use
of a larger-size wafer therein; for example, a series of steps in
the device fabrication process are all performed at a temperature
of 1000.degree. C. or lower, and RTP (rapid thermal processing)
requiring only a heat treatment time of the order of some tens of
seconds has frequently been employed. In such a device fabrication
process, the total heat treatment is no more than equal to heat
treatment at 1000.degree. C. for about 2 hrs so that growth of
oxide precipitates as in a conventional process cannot be expected
during the device fabrication process.
DISCLOSURE OF THE INVENTION
[0021] The present invention has been made in light of the
following prior art problems: that in general silicon wafers a
difference in a crystal thermal history creates fluctuations in
density of oxygen precipitation nuclei to thereby cause
fluctuations in an IG capability in a device fabrication process;
that in general DZ-IG treatment for bringing the vicinity of a
surface of a wafer into a defect-free state and generating oxide
precipitates having an IG capability in the interior thereof, a
heat treatment process for the purpose requires a long time and is
not efficient; and that in order to annihilate COPs and voids in
the vicinity of the wafer surface and furthermore to impart an IG
capability, high-temperature heat treatment at about 1200.degree.
C. or higher is required, so that metal contamination and slip
easily occur, the depth of the area where voids are annihilated
being shallow.
[0022] Furthermore, the present invention has been made in light of
the following prior art problems: that in general epitaxial wafers
and wafers subjected to high-temperature heat treatment at
1000.degree. C. or higher, oxygen precipitation nuclei are
annihilated in an epi step or a high-temperature heat treatment
process so that oxide precipitates are not generated in a device
fabrication process, thereby an IG capability being degraded; that
in a process where low-temperature heat treatment is applied prior
to an epi step or a high-temperature heat treatment process in
order to impart an IG capability, for imparting a more excellent IG
capability a heat treatment time becomes longer with a resulting
poor efficiency; and moreover, that if there exist oxide
precipitates of sizes not to be annihilated even in an epi step or
high-temperature heat treatment, in the recent device fabrication
process having a lower temperature and a shorter time, no further
growth of the oxide precipitates cannot be expected in heat
treatment during the device fabrication process.
[0023] It is a first object of the present invention to provide a
method for manufacturing a silicon wafer or a silicon epitaxial
wafer capable of imparting an excellent IG capability thereto in a
stable manner by simultaneously realizing higher density of oxide
precipitates and larger sizes thereof at a stage prior to a device
fabrication process.
[0024] It is a second object of the present invention to provide a
method for manufacturing a silicon wafer capable of efficiently
forming a DZ-IG structure by simultaneously realizing growth of
grown-in precipitation nuclei existing in an interior of a wafer
into oxide precipitates each of a size equal to or larger than one
having a gettering capability, and annihilation of grown-in
precipitation nuclei in the vicinity of the wafer surface by simple
heat treatment, and of efficiently forming an extremely high
quality DZ-IG structure by more perfectly annihilating oxide
precipitates in the formed DZ layer.
[0025] It is a third object of the present invention to provide a
method for manufacturing a silicon epitaxial wafer capable of
efficiently growing grown-in precipitation nuclei generated in a
crystal thermal history in a short time with annihilation thereof
being suppressed to the lowest level possible, imparting an
excellent IG capability thereto even in a device fabrication
process having a lower temperature and a shorter time, and
efficiently manufacturing an epitaxial wafer exerting an enough
gettering capability even in the recent device fabrication process
having a lower temperature and a shorter time; and to provide a
silicon epitaxial wafer.
[0026] In order to solve the above problems, a first aspect of a
method for manufacturing a silicon wafer of the present invention
wherein the silicon wafer is subjected to heat treatment to impart
an excellent IG capability thereto in a stable manner, comprises at
least the following three steps of: a temperature raising step A
for generating oxygen precipitation nuclei; a temperature raising
step B for growing the oxygen precipitation nuclei; and a constant
temperature keeping step C for growing the oxygen precipitation
nuclei into oxide precipitates of larger sizes. The temperature
raising step A, the temperature raising step B and the constant
temperature keeping step C are preferably performed continuously in
the order of the above steps.
[0027] In the first aspect of the method for manufacturing a
silicon wafer of the present invention, the temperature raising
step A is a step for raising the temperature from T.sub.1.degree.
C. to T.sub.2.degree. C. at a rate of an R.sub.1.degree. C./min,
wherein it is preferable that the T.sub.1.degree. C. is about
650.degree. C. or lower, the T.sub.2.degree. C. is about
700.degree. C. or higher and the R.sub.1.degree. C./min is about
1.5.degree. C./min or less.
[0028] The lower the T.sub.1.degree. C., the higher the density of
precipitates, but a process time required for generating the
precipitates becomes longer, so the T.sub.1.degree. C. is
preferably 500.degree. C. or higher. However, when the
T.sub.1.degree. C. is higher than 650.degree. C., there are
instances where density of precipitates cannot be obtained
sufficiently. Similarly, when the T.sub.2.degree. C. is lower than
700.degree. C., there are also instances where density of
precipitates cannot be obtained sufficiently, so the
T.sub.2.degree. C. is preferably 700.degree. C. or higher. The
upper limit of the T.sub.2.degree. C. is T.sub.3.degree. C. in
light of a heat treatment process. From the viewpoints of higher
density of precipitates and a shorter process time, the
T.sub.2.degree. C. is preferably 750.degree. C. or higher and
850.degree. C. or lower.
[0029] Furthermore, the lower the rate of the R.sub.1.degree.
C./min, the higher the density of precipitates; in order to obtain
sufficient density of precipitates, the R.sub.1.degree. C./min is
preferably 1.5.degree. C./min or less. But at an excessively low
rate thereof, a required process time for generating precipitates
becomes longer; the R.sub.1.degree. C./min is preferably
0.5.degree. C./min or more.
[0030] By the temperature raising step A, oxygen precipitation
nuclei can be generated at high density. Accordingly, oxygen
precipitation nuclei can be generated at high density even in an
epi wafer containing almost no oxygen precipitation nuclei and a
silicon wafer subjected to heat treatment in advance at about
1000.degree. C. or higher. Even in a wafer in which oxygen
precipitation nuclei are not sufficiently generated during a
crystal thermal history, oxygen precipitation nuclei of high
density can be generated. Therefore, it is possible to increase
density of oxide precipitates to be generated in subsequent
steps.
[0031] Furthermore, if there is a desire for changing the density
of oxide precipitates to a level of contamination in a device
fabrication process, the density can be easily changed, for
example, by setting the T.sub.2.degree. C. to about 750.degree. C.
and an R.sub.1.degree. C./min to about 1.degree. C./min, and
changing the T.sub.1.degree. C. It is needless to say that the
higher the density of oxide precipitates, the higher the IG
capability, but when making the density higher, a longer process
time is required. Accordingly, it is an important effect to enable
the density to be easily changed in order to efficiently achieve
necessary density.
[0032] The temperature raising step B is a step for raising the
temperature from T.sub.2.degree. C. to T.sub.3.degree. C. at a rate
of an R.sub.2.degree. C./min, wherein it is preferable that the
T.sub.2.degree. C. is about 700.degree. C. or higher, the
T.sub.3.degree. C. is about 1000.degree. C. or higher and the
R.sub.2.degree. C./min is equal to the R.sub.1.degree. C./min or
more and about 7.degree. C./min or less. In the temperature raising
step, oxygen precipitation nuclei generated in the temperature
raising step A can be efficiently grown.
[0033] At the T.sub.3.degree. C. of 1000.degree. C. or higher,
precipitates can be grown to sufficiently large sizes. The higher
the T.sub.3.degree. C., the larger the precipitates, but in order
to prevent generation of slip and metal contamination, the
T.sub.3.degree. C. is preferably 1230.degree. C. or lower, more
preferably 1150.degree. C. or lower, further preferably
1100.degree. C. or lower.
[0034] While it is preferable that the R.sub.2.degree. C./min is
sufficiently low, an excessive low rate thereof leads to a longer
process time. Therefore, the R.sub.2.degree. C./min is desirably
the R.sub.1.degree. C./min or more and about 7.degree. C./min or
less. At the R.sub.2.degree. C./min of about 7.degree. C./min or
less, oxygen precipitation nuclei generated in the previous step
can be efficiently grown such that almost all oxygen precipitation
nuclei are not annihilated. Moreover, in order to increase the
density of precipitates, the R.sub.2.degree. C./min is preferably
equal to the R.sub.1.degree. C./min or more and 5.degree. C./min or
less.
[0035] The higher the temperature is, the faster the growth rate of
precipitates becomes, so that by setting the R.sub.2.degree. C./min
to a higher rate equal to or higher than the R.sub.1.degree.
C./min, it is possible to grow the precipitates in a shorter
time.
[0036] The constant temperature keeping step C is a step for
keeping the wafer at the T.sub.3.degree. C. for t time, wherein it
is preferable that the T.sub.3.degree. C. is about 1000.degree. C.
or higher and the t time is about 1 hr or longer. In the constant
temperature keeping step C, minute oxide precipitates grown in the
temperature raising step B can be further grown into large oxide
precipitates having an IG capability. Therefore, an excellent IG
capability can be exerted even in a device fabrication process
having a lower temperature and a shorter time where growth of oxide
precipitates could not be expected. Note that the longer the t time
is, the larger the size of the precipitate becomes, but a process
time becomes longer, resulting in lower productivity; therefore,
the t time is preferably 4 hrs or shorter.
[0037] Here, as for a size of an oxide precipitate having an IG
capability, there is used as a guideline the size of the oxide
precipitate detectable experimentally (about 30 nm to 40 nm in
diameter). Generally, even the oxide precipitate of the size
non-detectable experimentally is considered as having an IG
capability, so it is possible to infer that the oxide precipitate
of the size detectable experimentally has a sufficient IG
capability.
[0038] Further, by changing the keeping temperature T.sub.3.degree.
C. and the keeping time, t time, it is possible to easily change
the size of the oxide precipitate. It is considered that the larger
the size of the oxide precipitate is, the higher the IG capability
becomes; a process time lengthens. Accordingly, in order to
efficiently achieve the necessary size of the oxide precipitate,
the effect that the size is easily changeable is also
important.
[0039] While a wafer can be unloaded from a heat treatment furnace
in intervals between the temperature raising step A and the
temperature raising step B, and between the temperature raising
step B and the constant temperature keeping step C, a total process
time can be shortened by performing the above three steps
continuously.
[0040] After the constant temperature keeping step C, irrespective
of a temperature in the heat treatment furnace when the wafer is
unloaded therefrom and a temperature lowering rate down to the
temperature, the temperatures are desirably determined such that no
slip is generated owing to thermal stress. For example, after the
temperature in the heat treatment furnace is lowered from the
T.sub.3.degree. C. to 800.degree. C. at 2.degree. C./min, the wafer
can be unloaded from the heat treatment furnace.
[0041] In order to sufficiently achieve the effect of the first
aspect of the method for manufacturing a silicon wafer of the
present invention, oxygen concentration of the silicon wafer is
desirably about 17 ppma or higher. At higher oxygen concentration,
a nucleus generating rate and a nucleus growth rate become faster,
which shortens a total process time. Even at lower oxygen
concentration, for example, by adopting a lower temperature of the
T.sub.1.degree. C., or lower rates of the R.sub.1.degree. C./min
and R.sub.2.degree. C./min, the effect of the first aspect of the
method for manufacturing a silicon wafer of the present invention
can be achieved.
[0042] The first aspect of the method for manufacturing a silicon
wafer of the present invention can also be effectively applied to
an epitaxial wafer and a silicon wafer subjected to heat treatment
at 1000.degree. C. or higher (especially 1100.degree. C. or higher)
in advance.
[0043] As described above, according to the first aspect of the
method for manufacturing a silicon wafer of the present invention,
higher density of oxide precipitates and larger sizes thereof can
be simultaneously realized at a stage prior to a device fabrication
process. Accordingly, in a wafer containing almost no oxygen
precipitation nuclei, large oxide precipitates can be generated at
high density and an excellent IG capability can be exerted in a
device fabrication process having a lower temperature and a shorter
time where growth of oxide precipitates could not be expected.
[0044] As an additional effect of the first aspect of the method
for manufacturing a silicon wafer of the present invention, there
can be given an effect to suppress generation of slip dislocations
due to thermal stress. It has been known that dislocations
constituting slip can be pinned by oxide precipitates. Therefore,
by generating large oxide precipitates at higher density according
to the first aspect of the method for manufacturing a silicon wafer
of the present invention, a probability that the dislocations are
pinned becomes higher, thereby generation of slip dislocations
being suppressed.
[0045] A second aspect of the method for manufacturing a silicon
wafer of the present invention comprises the steps of: keeping the
silicon wafer having grown-in precipitation nuclei generated in a
growth step of a silicon single crystal at a temperature
T.sub.4.degree. C. in the range of from 500.degree. C. to
700.degree. C. for a prescribed time t.sub.1; then raising the
temperature of the silicon wafer to a temperature T.sub.5.degree.
C. in the range of from 1000.degree. C. to 1230.degree. C. at a
temperature raising rate R.degree. C./min of 5.degree. C./min or
less; and keeping the wafer at the temperature T.sub.5.degree. C.
for a prescribed time t.sub.2, thereby the grown-in precipitation
nuclei being grown into oxide precipitates each of a size equal to
or larger than one having a gettering capability, and oxygen in the
vicinity of a surface of the silicon wafer being out-diffused.
[0046] Thus, by raising a temperature of a silicon wafer from a low
temperature (T.sub.4.degree. C.) to a high temperature
(T.sub.5.degree. C.) at a slow temperature raising rate of
5.degree. C./min or less and keeping it at the high temperature for
a prescribed time, grown-in precipitation nuclei in the bulk of the
silicon wafer can be effectively grown into their sizes each equal
to or larger than one having a gettering capability without the
grown-in precipitation nuclei being annihilated. Furthermore,
simultaneously along with the growth, by out-diffusing oxygen in
the vicinity of a surface of the silicon wafer, oxygen
precipitation nuclei can be annihilated, so that a DZ layer free of
oxide precipitates is formed in the vicinity of the surface
thereof. That is, according to the second aspect of the method for
manufacturing a silicon wafer of the present invention, a DZ-IG
structure can be formed by simple one step heat treatment
alone.
[0047] Here, as for a size of an oxide precipitate having an IG
capability, there is used as a guideline the size of the oxide
precipitate detectable experimentally (about 30 nm to 40 nm in
diameter). Generally, even the oxide precipitate of the size
non-detectable experimentally is considered as having an IG
capability, so it is possible to infer that the oxide precipitate
of the size detectable experimentally has a sufficient IG
capability. Accordingly, the size having a gettering capability is
preferably about 40 nm or larger in diameter. While no specific
limitation is put on the upper limit of a size of an oxide
precipitate, since a heat treatment time becomes longer in order to
grow it to a larger size, it is preferably 100 nm or less in
diameter.
[0048] When the temperature of the T.sub.5.degree. C. is lower than
about 1000.degree. C., a time required for growing larger oxide
precipitates becomes longer, resulting in a longer total process
time. The higher the T.sub.5.degree. C. is, the shorter the time
for growing larger oxide precipitates becomes; therefore it is
possible to shorten a total process time, but the T.sub.5.degree.
C. is preferably 1230.degree. C. or lower since metal contamination
from a heat treatment furnace is noticeable at a high temperature
in excess of about 1230.degree. C.
[0049] The lower the temperature T.sub.4.degree. C. is, the higher
the density of oxide precipitates becomes, but a process time
lengthens, so the T.sub.4.degree. C. is preferably about
500.degree. C. or higher. At a temperature in excess of about
700.degree. C. of the T.sub.4.degree. C., there are occasions where
the density of the precipitates is insufficiently achieved.
Similarly, the lower the temperature raising rate of R.degree.
C./min is, the higher the density of precipitates becomes, but at
an excessively low rate thereof, a process time is longer;
therefore, the R.degree. C./min is preferably about 1.degree.
C./min or more. If the temperature raising rate of R.degree. C./min
is a high rate in excess of about 5.degree. C./min, the proportion
of grown-in precipitation nuclei that are annihilated without
growth becomes higher, so that there are occasions where the
density of precipitates cannot be sufficiently achieved.
[0050] By setting the temperature raising rate of the R.degree.
C./min from the T.sub.4.degree. C. to the T.sub.5.degree. C. to
5.degree. C./min or less, grown-in precipitation nuclei can be
efficiently grown, annihilation thereof being suppressed to the
lowest level possible. That is, since existing grown-in
precipitation nuclei generated in a crystal growth step are grown,
density of precipitates can be sufficiently enhanced even without a
heat treatment step for newly generating oxygen precipitation
nuclei, which enables a total process time to be short.
[0051] In the second aspect of the method for manufacturing a
silicon wafer of the present invention, prior to temperature
raising from the T.sub.4.degree. C. to the T.sub.5.degree. C., the
keeping time t.sub.1 at the T.sub.4.degree. C. may be 0 min without
hindrance, but more preferably 15 min or longer. Thereby, the
grown-in precipitation nuclei are harder to be annihilated, and
furthermore, new oxygen precipitation nuclei can be generated in
addition to the existing grown-in precipitation nuclei; higher
density of oxygen precipitation nuclei may be generated. Further,
since a process time becomes longer when the keeping time is long,
the keeping time t.sub.1 is preferably about 60 min or shorter.
[0052] Note that the lower the T.sub.4.degree. C. is or the longer
the keeping time t.sub.1 at the T.sub.4.degree. C. is or the slower
the temperature raising rate is, the more the new precipitation
nuclei are generated during the temperature raising step, and the
higher the density of precipitates becomes.
[0053] The keeping time t.sub.2 at a temperature of the
T.sub.5.degree. C. is preferably about 30 min or longer in order to
certainly grow the grown-in precipitation nuclei to sizes each
having a gettering capability or in order to form a DZ layer with a
sufficient width. The longer the keeping time t.sub.2 is, the
larger the sizes of oxide precipitates in the bulk of a silicon
wafer becomes; a DZ width in the vicinity of a surface of the
silicon wafer can be increased, but the process time becomes longer
so that the keeping time t.sub.2 is preferably about 4 hrs or
shorter and more preferably about 2 hrs or shorter. On the other
hand, when the keeping time t.sub.2 is shorter than about 30 min,
due to slight fluctuations in time, chances are that neither oxide
precipitates having desired sizes nor a desired DZ width can be
achieved.
[0054] In order to sufficiently achieve the effect of the second
aspect of the method for manufacturing a silicon wafer of the
present invention, oxygen concentration of a silicon wafer to be
heat-treated is desirably about 16 ppma or higher. The higher the
oxygen concentration is, the higher the density of precipitates
becomes; a more excellent IG capability can be imparted to the
silicon wafer. Further, the higher the oxygen concentration is, the
faster the growth rate of the precipitates becomes; a total process
time becomes shorter. However, even when the oxygen concentration
is low, for example, by lowering the temperature raising initiation
temperature of the T.sub.4.degree. C. in the temperature raising
step, or lengthening the keeping time at the T.sub.5.degree. C.,
there can be achieved the effect of the second aspect of the method
for manufacturing a silicon wafer of the present invention.
Accordingly, no specific limitation is put on the upper limit of
the oxygen concentration in the second aspect of the method for
manufacturing a silicon wafer of the present invention, but it is
preferably about 23 ppma or less in consideration for the ease in
production of silicon single crystals.
[0055] Also, according to the second aspect of the method for
manufacturing a silicon wafer of the present invention, an effect
can be achieved that suppresses generation of slip dislocations due
to the thermal stress during the device fabrication process. It has
been known that the dislocation constituting slip is pinned by an
oxide precipitate. Therefore, by generating oxide precipitates each
having a certain large size at high density according to the second
aspect of the method for manufacturing a silicon wafer of the
present invention, a probability is enhanced that dislocations are
pinned, thus generation of slip dislocations being suppressed. That
is, the second aspect of the method for manufacturing a silicon
wafer of the present invention can be especially preferably applied
to a large diameter wafer of 300 mm or larger in diameter, which
would be subject to generation of slip dislocations by heat
treatment. In order to suppress generation of slip dislocations,
the T.sub.5.degree. C. is preferably 1200.degree. C. or lower, more
preferably about 1150.degree. C. or lower.
[0056] Further, by applying the second aspect of the method for
manufacturing a silicon wafer of the present invention to a wafer
having a few defects with few COPs and voids which is manufactured
from a silicon single crystal pulled in a condition that suppresses
generation of void defects in growth thereof, an extremely high
quality DZ-IG structure having hardly not only oxide precipitates
but also COPs and voids can be formed without applying
high-temperature treatment at a high temperature of about
1200.degree. C. This is especially effective for a 300 mm diameter
wafer that is a future main stream from the viewpoint of
suppression of generation of slip.
[0057] The above described wafer having a few defects can be
manufactured by using a known technique that a single crystal is
pulled while controlling a ratio V/G between a crystal pulling rate
V and a temperature gradient G in the vicinity of a solid liquid
interface in a growing crystal as is described in, for example, JP
A 99-147786 and JP A 99-157996.
[0058] A third aspect of a method for manufacturing a silicon wafer
of the present invention wherein the silicon wafer having grown-in
precipitation nuclei generated in a growth step of a silicon single
crystal is subjected to heat treatment to make a surface layer
thereof defect-free and impart a gettering capability to an
interior thereof, comprises at least the following three steps of:
a temperature raising step A.sub.1 for growing the grown-in
precipitation nuclei; a temperature raising step B.sub.1 for
further raising a temperature of the silicon wafer to a keeping
temperature higher than that in the temperature raising step
A.sub.1; and a constant temperature keeping step C.sub.1 for
growing the grown-in precipitation nuclei into oxide precipitates
each of a size equal to or larger than one having a gettering
capability and for out-diffusing oxygen in the vicinity of a
surface of the silicon wafer. It is desirable that the temperature
raising step A.sub.1, the temperature raising step B.sub.1 and the
constant temperature keeping step C.sub.1 are performed
continuously in this order.
[0059] In the third aspect of a method for manufacturing a silicon
wafer of the present invention, the temperature raising step
A.sub.1 is a step of raising a temperature of the silicon wafer
from T.sub.6.degree. C. to T.sub.7.degree. C. at a rate of
R.sub.3.degree. C./min, wherein it is preferable that the
T.sub.6.degree. C. is 700.degree. C. or lower, the T.sub.7.degree.
C. is 800.degree. C. to 1000.degree. C. and the R.sub.3.degree.
C./min is 3.degree. C./min or less.
[0060] The lower the T.sub.6.degree. C. is, the higher the density
of grown-in precipitation nuclei becomes and then the higher the
density of precipitates becomes, but a process time required for
growing the grown-in precipitation nuclei lengthens; the
T.sub.6.degree. C. is preferably T.sub.6.degree. C. or higher. When
the T.sub.6.degree. C. is higher than 700.degree. C., there are
occasions where the density of precipitates cannot be sufficiently
achieved.
[0061] When the T.sub.7.degree. C. is lower than 800.degree. C.,
the grown-in precipitation nuclei cannot be sufficiently grown in
the temperature raising step A.sub.1 and the proportion of grown-in
precipitation nuclei to be annihilated in the following temperature
raising step B.sub.1 becomes higher; there are occasions where the
density of precipitates cannot be sufficiently achieved. When the
T.sub.7.degree. C. is higher than 1000.degree. C., grown-in
precipitation nuclei in the vicinity of a surface of the silicon
wafer grow larger and survive in the vicinity of the surface of the
silicon wafer even after being subjected to the subsequent
temperature raising step B.sub.1 and the constant temperature
keeping step C.sub.1; there are occasions where the density of
precipitates in a DZ layer increases.
[0062] Moreover, as the rate of the R.sub.3.degree. C./min is
lower, there becomes higher the proportion that the grown-in
precipitation nuclei grow without annihilation and then the density
of precipitates is raised, so that the R.sub.3.degree. C./min is
preferably 3.degree. C./min or less in order to achieve sufficient
density of precipitates, but at a excessively low rate a process
time becomes longer with resulting poor efficiency; the
R.sub.3.degree. C./min is preferably 0.5.degree. C./min or
more.
[0063] By the temperature raising step A.sub.1, grown-in
precipitation nuclei can be efficiently grown, annihilation thereof
being suppressed to the lowest level possible. That is, since
existing grown-in precipitation nuclei generated in the crystal
growth step are grown, the density of precipitates can be
sufficiently enhanced even without a heat treatment step for
generating new oxygen precipitation nuclei, yet a total process
time can be shortened.
[0064] Further, prior to raising temperature from the
T.sub.6.degree. C. to the T.sub.7.degree. C. in the temperature
raising step A.sub.1, the keeping time t.sub.3 at the
T.sub.6.degree. C. may be 0 min without hindrance, but more
preferably 30 min or longer. Thereby, the grown-in precipitation
nuclei are harder to be annihilated, and furthermore, new oxygen
precipitation nuclei can be generated in addition to the existing
grown-in precipitation nuclei; higher density of oxygen
precipitation nuclei may be generated. Since a process time becomes
longer when the keeping time t.sub.3 is long, the keeping time
t.sub.3 is preferably about 4 hrs or shorter.
[0065] The temperature raising step B.sub.1 is a step for raising a
temperature of the silicon wafer from the T.sub.7.degree. C. to the
T.sub.8.degree. C. at a rate of R.sub.4.degree. C./min, wherein it
is preferable that the T.sub.7.degree. C. is 800.degree. C. to
1000.degree. C., the T.sub.8.degree. C. is 1050.degree. C. to
1230.degree. C. and the R.sub.4.degree. C./min is 5.degree. C./min
or more. In the temperature raising step B.sub.1, by raising a
temperature of the silicon wafer up to a high temperature in a
short time, growth of oxide precipitates in the vicinity of a
surface of the silicon wafer can be suppressed; the oxide
precipitates in the vicinity of the surface thereof can be easily
annihilated in the subsequent constant temperature keeping step
C.sub.1.
[0066] When the T.sub.8.degree. C. is 1050.degree. C. or higher, by
growing efficiently oxide precipitates in the bulk of the silicon
wafer to sufficiently large sizes and out-diffusing oxygen in the
vicinity of a surface of the wafer, oxide precipitates in the
vicinity of the surface of the silicon wafer can be annihilated.
Furthermore, the higher the T.sub.8.degree. C. is, the larger the
precipitates in the bulk of the silicon wafer becomes and the wider
the DZ width becomes, but at a temperature in excess of about
1230.degree. C. of the T.sub.8.degree. C., metal contamination from
a heat treatment furnace becomes remarkable; the T.sub.8.degree. C.
is preferably 1230.degree. C. or lower.
[0067] When the R.sub.4.degree. C./min is less than 5.degree.
C./min, oxide precipitates in the vicinity of the surface of the
silicon wafer grow large; they are hard to be annihilated in the
subsequent constant temperature keeping step C.sub.1. However, if
the R.sub.4.degree. C./min is excessively high, the proportion of
oxide precipitates in the bulk of the silicon wafer that are
annihilated becomes higher, so that the density of precipitates is
lowered; the R.sub.4.degree. C./min is desirably 10.degree. C./min
or less.
[0068] The constant temperature keeping step C.sub.1 is a step for
keeping the wafer at a temperature T.sub.8.degree. C. for t.sub.4
time, wherein it is preferable that the T.sub.8.degree. C. is
1050.degree. C. to 1230.degree. C. and the t.sub.4 time is 30 min
or longer. By the constant temperature keeping step C.sub.1, minute
oxide precipitates grown in the previous temperature raising steps
A.sub.1 and B.sub.1 can be grown into large oxide precipitates each
having an IG capability in the bulk of the silicon wafer and can be
annihilated in the vicinity of a surface thereof. Accordingly,
there can be formed a high quality DZ-IG structure combining a
defect-free DZ layer and an IG layer having an excellent IG
capability.
[0069] Here, as for a size of an oxide precipitate having an IG
capability, there is used as a guideline the size of the oxide
precipitate detectable experimentally (about 30 nm to 40 nm in
diameter). Generally, even the oxide precipitate of the size
non-detectable experimentally is considered as having an IG
capability, so it is possible to infer that the oxide precipitate
of the size detectable experimentally has a sufficient IG
capability. Accordingly, the size having a gettering capability is
preferably about 40 nm or larger in diameter. While no specific
limitation is put on the upper limit of a size of an oxide
precipitate, since a heat treatment time becomes longer in order to
grow it to a larger size, it is preferably 100 nm or less in
diameter.
[0070] As the keeping time t.sub.4 at the T.sub.8.degree. C. is
longer, there becomes larger the sizes of oxide precipitates in the
bulk of the silicon wafer and the DZ width becomes wider, but a
process time lengthens; the keeping time t.sub.4 is preferably
about 4 hrs or shorter. On the other hand, when the keeping time
t.sub.4 is shorter than about 30 min, due to slight fluctuations in
time, chances are that neither oxide precipitates having desired
sizes nor a desired DZ width can be achieved.
[0071] Moreover, by changing the keeping temperature
T.sub.8.degree. C. and the keeping time t.sub.4, the size of the
oxide precipitate and the DZ width can be easily changed. It is
considered that the larger the size of the oxide precipitate is,
the higher the IG capability becomes, but a process time lengthens.
Accordingly, in order to efficiently achieve the necessary size of
the oxide precipitate and the DZ width, the effect that the size
and the DZ width are easily changeable is also important.
[0072] While a wafer can be unloaded from a heat treatment furnace
in intervals between the temperature raising step A.sub.1 and the
temperature raising step B.sub.1, and between the temperature
raising step B.sub.1 and the constant temperature keeping step
C.sub.1, a total process time can be shortened by performing the
above three steps continuously.
[0073] After the constant temperature keeping step C.sub.1,
irrespective of a temperature in a heat treatment furnace when a
wafer is unloaded therefrom and a temperature lowering rate down to
the temperature, the temperatures are desirably determined such
that no slip is generated owing to thermal stress. For example,
after the temperature in the heat treatment furnace is lowered from
the T.sub.8.degree. C. to 700.degree. C. at 3.degree. C./min, the
wafer can be unloaded from the heat treatment furnace.
[0074] In order to sufficiently achieve the effect of the third
aspect of the method for manufacturing a silicon wafer of the
present invention, oxygen concentration of the silicon wafer to be
heat-treated is desirably about 14 ppma to 17 ppma. While at higher
oxygen concentration, the density of precipitates becomes higher
and a more excellent IG capability can be imparted to the silicon
wafer, at excessively high oxygen concentration, precipitates in
the vicinity of a surface of the silicon wafer is difficult to be
annihilated. Further, at lower oxygen concentration, the density of
grown-in precipitation nuclei generated in a growth step of a
silicon single crystal becomes lower, and then the density of
precipitates becomes lower. However, even at lower oxygen
concentration, for example, by lowering the temperature raising
initiation temperature T.sub.6.degree. C. in the temperature
raising step A.sub.1 or reducing the rate of the R.sub.3.degree.
C./min, the effect of the third aspect of the method for
manufacturing a silicon wafer of the present invention can be
achieved.
[0075] Furthermore, according to the third aspect of a method for
manufacturing a silicon wafer of the present invention, an effect
can be achieved that suppresses generation of slip dislocations due
to the thermal stress during the device fabrication process. It has
been known that the dislocation constituting slip is pinned by an
oxide precipitate. Therefore, by generating oxide precipitates each
having a large size at high density according to the third aspect
of the method for manufacturing a silicon wafer of the present
invention, a probability is enhanced that dislocations are pinned,
thus generation of slip dislocations being suppressed. That is, the
third aspect of the method for manufacturing silicon wafer of the
present invention can be especially preferably applied to a large
diameter wafer of 300 mm or larger in diameter, which would be
subject to generation of slip dislocations by heat treatment.
[0076] In order to suppress generation of slip dislocations, the
T.sub.8.degree. C. is preferably 1200.degree. C. or lower, more
preferably about 1150.degree. C. or lower.
[0077] Further, by applying the third aspect of the method for
manufacturing a silicon wafer of the present invention to a wafer
having a few defects with few COPs and voids which is manufactured
from a silicon single crystal pulled in a condition that suppresses
generation of void defects in growth thereof, there can be formed
an extremely high quality DZ-IG structure having hardly not only
oxide precipitates but also COPs and voids. This is especially
effective for a 300 mm diameter wafer that is a future main stream
from the viewpoint of suppression of generation of slip.
[0078] Here, the above described wafer having a few defects can be
manufactured by using a known technique that a single crystal is
pulled while controlling a ratio V/G between a crystal pulling rate
V and a temperature gradient G in the vicinity of a solid liquid
interface in a growing crystal as is described in, for example, JP
A 99-147786 and JP A 99-157996.
[0079] A first aspect of a method for manufacturing a silicon
epitaxial wafer of the present invention comprises the steps of:
keeping a silicon wafer having grown-in precipitation nuclei
generated in a growth step of a silicon single crystal at a
temperature T.sub.8.degree. C. in the range of from 500.degree. C.
to 700.degree. C. for a prescribed time t.sub.5; then raising the
temperature of the silicon wafer to T.sub.10.degree. C. in the
range of from 800.degree. C. to 900.degree. C. at a temperature
raising rate R.sub.5.degree. C./min of 5.degree. C./min or less;
keeping the wafer at the T.sub.10.degree. C. for a prescribed time
t.sub.6, thereby the grown-in precipitation nuclei being grown to
sizes each equal to or larger than one not to be annihilated in a
subsequent epitaxial growth step; and thereafter performing
epitaxial growth on a surface of the silicon wafer.
[0080] Thus, by performing epitaxial growth on a silicon wafer
after grown-in precipitation nuclei have been grown to sizes not to
be annihilated in an epitaxial growth step, oxide precipitates each
which is not so large as to have a sufficient IG capability
immediately after the epi step but grown to a size having an IG
capability during a device fabrication process after the epi step,
so that an excellent IG capability is imparted to the silicon
epitaxial wafer.
[0081] While the temperature T.sub.10.degree. C. may be higher than
about 900.degree. C., there become longer a time required for
raising temperature and a time for a temperature lowering step
after the keeping step at the T.sub.10.degree. C. Furthermore, when
the T.sub.10.degree. C. is lower than about 800.degree. C., a
growth rate of an oxide precipitate is slowed down; therefore,
there is a longer time required for growing oxide precipitates to
sizes not to be annihilated in the epitaxial growth step, thereby a
total process time being longer.
[0082] The lower the temperature T.sub.9.degree. C. is, the higher
the density of precipitates becomes, but a process time lengthens;
the T.sub.9.degree. C. is preferably about 500.degree. C. or
higher. If the T.sub.9.degree. C. is more than about 700.degree.
C., there are occasions where the density of precipitates cannot be
sufficiently attained. Likewise, the lower the temperature raising
rate R.sub.5.degree. C./min is, the higher the density of
precipitates becomes, but at an excessively low rate of the
R.sub.5.degree. C./min, a process time becomes longer; the
R.sub.5.degree. C./min is preferably about 1.degree. C./min or
more. When the temperature raising rate R.sub.5.degree. C./min is a
high rate in excess of about 5.degree. C./min, the proportion of
grown-in precipitation nuclei that are annihilated without growth
becomes higher, so that there are occasions where the density of
precipitates cannot be sufficiently achieved.
[0083] By setting the temperature raising rate from the
T.sub.9.degree. C. to the T.sub.10.degree. C. to 5.degree. C./min
or less, grown-in precipitation nuclei can be efficiently grown,
annihilation thereof being suppressed to the lowest level possible.
That is, since existing grown-in precipitation nuclei generated in
the crystal growth step are grown, the density of precipitates can
be sufficiently enhanced even without a heat treatment step for
generating new oxygen precipitation nuclei, yet a total process
time can be shortened.
[0084] In the first aspect of the method for manufacturing a
silicon epitaxial wafer of the present invention, prior to
temperature raising from the T.sub.9.degree. C. to the
T.sub.10.degree. C., the keeping time t.sub.5 at the
T.sub.9.degree. C. may be 0 min without hindrance, but more
preferably 15 min or longer. Thereby, grown-in precipitation nuclei
are harder to be annihilated in a temperature raising step, and
furthermore, new oxygen precipitation nuclei can be generated in
addition to the existing grown-in precipitation nuclei; oxygen
precipitation nuclei of higher density may be generated. Since a
process time becomes longer when the keeping time t.sub.5 is long,
the keeping time t.sub.5 is preferably about 60 min or shorter.
[0085] Further, the lower the temperature T.sub.9.degree. C. is or
the longer the keeping time t.sub.5 at the T.sub.9.degree. C. is or
the slower the R.sub.5.degree. C./min is, the more the new
precipitation nuclei generated during the temperature raising step
become; the density of precipitates becomes higher.
[0086] The keeping time t.sub.6 at the T.sub.10.degree. C. is
preferably about 30 min or longer in order to certainly grow the
grown-in precipitation nuclei to each of a size (about 3 nm to 4
nm) equal to or larger than one not to be annihilated in a
subsequent epitaxial growth step performed at a high temperature on
the order of 1050.degree. C. to 1200.degree. C. The sizes ranging
from 3 nm to 4 nm are critical sizes in diameter at an epi step
temperature of 1150.degree. C., which have been calculated
according to a classical theory. The longer the keeping time
t.sub.6 is, the larger the sizes of oxide precipitates become, but
the process time also lengthens; the keeping time t.sub.6 is
preferably about 4 hrs or shorter, more preferably about 2 hrs or
shorter. On the other hand, when the keeping time t.sub.6 is
shorter than about 30 min, due to slight fluctuations in time,
chances are that an oxide precipitate having a desired size cannot
be generated. If the keeping time t.sub.6 is 0 min, growth of
grown-in precipitation nuclei is insufficient; therefore, the
proportion of the grown-in precipitation nuclei that are
annihilated in the epi step becomes higher, so that there is
decreased the density of oxide precipitates detectable after the
device fabrication process.
[0087] In order to sufficiently achieve the effect of the first
aspect of the method for manufacturing a silicon epitaxial wafer of
the present invention, oxygen concentration of the silicon wafer to
be heat-treated is desirably about 16 ppma or higher. At the higher
oxygen concentration, the density of precipitates becomes higher; a
more excellent IG capability can be imparted to the silicon wafer.
The higher the oxygen concentration is, the faster the growth rate
of precipitates becomes; a total process time becomes shorter.
However, even at lower oxygen concentration, for example, by
lowering the temperature raising initiation temperature
T.sub.9.degree. C. in the temperature raising step, or lengthening
the keeping time at the T.sub.10.degree. C., there can be achieved
the effect of the first aspect of the method for manufacturing a
silicon epitaxial wafer of the present invention. Accordingly,
while no specific limitation is put on the upper limit of oxygen
concentration in the first aspect of the method for manufacturing a
silicon epitaxial wafer of the present invention, it is preferably
about 23 ppma or less in consideration for the ease in production
of silicon single crystals.
[0088] From the above description, according to the first aspect of
the method for manufacturing a silicon epitaxial wafer of the
present invention, a silicon epitaxial wafer having an excellent
gettering capability can be efficiently obtained.
[0089] A second aspect of a method for manufacturing a silicon
epitaxial wafer of the present invention comprises the steps of:
keeping a silicon wafer having grown-in precipitation nuclei
generated in a growth step of a silicon single crystal at a
temperature T.sub.11.degree. C. in the range of from 500.degree. C.
to 700.degree. C. for a prescribed time t.sub.7; then raising the
temperature of the silicon wafer to T.sub.12.degree. C. in the
range of from 1000.degree. C. to 1100.degree. C. at a temperature
raising rate R.sub.6.degree. C./min of 5.degree. C./min or less;
keeping the wafer at the T.sub.12.degree. C. for a prescribed time
t.sub.8, thereby the grown-in precipitation nuclei being grown into
oxide precipitates of sizes each equal to or larger than one having
a gettering capability; and thereafter, performing epitaxial growth
on a surface of the silicon wafer.
[0090] Thus, by performing an epitaxial growth after the grown-in
precipitation nuclei have been grown into oxide precipitates of
sizes each equal to or larger than one having a gettering
capability, no re-dissolution of oxide precipitates occurs during
an epitaxial growth step; there is obtained a silicon epitaxial
wafer having oxide precipitates of sizes each having a gettering
capability generated prior to loading it into a device fabrication
process.
[0091] Here, as for a size of an oxide precipitate having an IG
capability, there is used as a guideline the size of the oxide
precipitate detectable experimentally (about 30 nm to 40 nm in
diameter). Generally, even the oxide precipitate of the size
non-detectable experimentally is considered as having an IG
capability, so it is possible to infer that the oxide precipitate
of the size detectable experimentally has a sufficient IG
capability. Accordingly, the size having a gettering capability is
preferably about 40 nm or larger in diameter. If the size of the
oxide precipitate is equal to or larger than this value, it is not
annihilated during the epitaxial step, the IG capability being kept
sufficiently. While no specific limitation is put on the upper
limit of a size of an oxide precipitate, since a heat treatment
time becomes longer in order to grow it to a larger size, it is
preferably 100 nm or less in diameter.
[0092] When the T.sub.12.degree. C. is lower than about
1000.degree. C., a time for obtaining large oxide precipitates
becomes longer and hence a total process time lengthens. The higher
the T.sub.12.degree. C. is, the shorter the time for growing large
oxide precipitates becomes; a total process time may be shorter,
but in order to sufficiently suppress generation of slip
dislocations in the heat treatment or in a subsequent epi step, the
T.sub.12.degree. C. is preferably 1100.degree. C. or lower.
[0093] The lower the temperature T.sub.11.degree. C. is, the higher
the density of precipitates becomes, but a process time lengthens;
the T.sub.11.degree. C. is preferably about 500.degree. C. or
higher. Also, when the T.sub.11.degree. C. is in excess of about
700.degree. C., there are occasions where the density of
precipitates is not sufficiently high. Likewise, while the slower
the temperature raising rate R.sub.6.degree. C./min is, the higher
the density of precipitates becomes, but an excessively slow rate
causes a process time to be too long; the R.sub.6.degree. C./min is
preferably about 1.degree. C./min or more. If the temperature
raising rate R.sub.6.degree. C./min is a high rate in excess of
about 5.degree. C./min, the proportion of grown-in precipitation
nuclei that are annihilated without growth becomes higher, so that
there are occasions where the density of precipitates cannot be
sufficiently achieved.
[0094] By setting the temperature raising rate R.sub.6.degree.
C./min from the T.sub.11.degree. C. to the T.sub.12.degree. C. to
5.degree. C./min or less, grown-in precipitation nuclei can be
efficiently grown, annihilation thereof being suppressed to the
lowest level possible. That is, since existing grown-in
precipitation nuclei generated in a crystal growth step are grown,
the density of precipitates can be sufficiently enhanced even
without a heat treatment step for generating new oxygen
precipitation nuclei, yet a total process time can be
shortened.
[0095] In the second aspect of the method for manufacturing a
silicon epitaxial wafer of the present invention, prior to raising
temperature from the T.sub.11.degree. C. to the T.sub.12.degree.
C., the keeping time t.sub.7 at the T.sub.11.degree. C. may be 0
min without hindrance, but more preferably 15 min or longer.
Thereby, the grown-in precipitation nuclei are harder to be
annihilated in the temperature raising step, and furthermore, new
oxygen precipitation nuclei can be generated in addition to the
existing grown-in precipitation nuclei; oxygen precipitation nuclei
of higher density may be generated. Further, since a process time
becomes longer when the keeping time t, is long, the keeping time
t, is preferably about 60 min or shorter.
[0096] Note that the lower the temperature T.sub.11.degree. C. is
or the longer the keeping time t.sub.7 at the T.sub.11.degree. C.
is or the slower the temperature raising rate is, the more the new
precipitation nuclei are generated during the temperature raising
step; the density of precipitates becomes higher.
[0097] The keeping time t.sub.8 at the temperature T.sub.12.degree.
C. is preferably about 30 min or longer in order to certainly grow
the grown-in precipitation nuclei to sizes each having a gettering
capability. The longer the keeping time t.sub.8 is, the larger the
sizes of oxide precipitates becomes, but the process time
lengthens; the keeping time t.sub.8 is preferably about 4 hrs or
shorter, more preferably about 2 hrs or shorter. On the other hand,
when the keeping time t.sub.8 is shorter than about 30 min, due to
slight fluctuations in time, chances are that no oxide precipitates
having desired sizes can be achieved.
[0098] In order to sufficiently obtain the effect of the second
aspect of the method for manufacturing a silicon epitaxial wafer of
the present invention, oxygen concentration of a silicon wafer to
be heat-treated is preferably about 16 ppma or higher. At the
higher oxygen concentration, the density of precipitates becomes
higher; a more excellent IG capability can be imparted to the
silicon wafer. Further, the higher the oxygen concentration is, the
faster the growth rate of the precipitates becomes; a total process
time can be shorter. However, it is easily inferred that even when
the oxygen concentration is low, for example, by lowering the
temperature raising initiation temperature T.sub.11.degree. C. in
the temperature raising step, or lengthening the keeping time at
the T.sub.12.degree. C., it would be conceived that there can be
achieved the effect of the second aspect of the method for
manufacturing a silicon epitaxial wafer of the present invention.
Accordingly, no specific limitation is put on the upper limit of
the oxygen concentration in the second aspect of the method for
manufacturing a silicon epitaxial wafer of the present invention,
but it is preferably about 23 ppma or less in consideration for the
ease in production of silicon single crystals.
[0099] From the above description, according to the second aspect
of the method for manufacturing a silicon epitaxial wafer of the
present invention, a silicon epitaxial wafer having an excellent
gettering capability can be efficiently obtained.
[0100] As an additional effect of the second aspect of the method
for manufacturing a silicon epitaxial wafer of the present
invention, there can be given an effect to suppress generation of
slip dislocations due to the thermal stress during a device
fabrication process. It has been known that a dislocation
constituting slip is pinned by an oxide precipitate. Therefore, by
generating oxide precipitates each having a certain large size at
high density prior to loading a silicon epitaxial wafer into a
device fabrication process according to the second aspect of the
method for manufacturing a silicon epitaxial wafer of the present
invention, a probability is enhanced that dislocations are pinned,
thus suppressing generation of slip dislocations. Accordingly, this
is especially effective for a 300 mm wafer (epitaxial wafer) that
is the future mainstream.
[0101] In a first aspect of a silicon epitaxial wafer of the
present invention, density of oxide precipitates detected in the
bulk thereof prior to the heat treatment at 1000.degree. C. for 2
hrs is less than 1.times.10.sup.9/cm.sup.3 and density of oxide
precipitates detected in the bulk thereof after the heat treatment
at 1000.degree. C. for 2 hrs is equal to or higher than
1.times.10.sup.9/cm.sup.3.
[0102] Thus, according to the first aspect of the silicon epitaxial
wafer of the present invention, in the silicon epitaxial wafer
subjected to the heat treatment at 1000.degree. C. for 2 hrs
simulating a device fabrication process with a lower temperature
and a shorter time, the density of oxide precipitates of sizes each
surely having a gettering capability (about 40 nm or more in
diameter detectable by means of an optical measuring instrument) is
1.times.10.sup.9/cm.sup.3 or higher, so that it has a sufficient
gettering capability in a device fabrication process with a low
temperature and a short time. Note that the higher the density of
oxide precipitates is, the more excellent the IG capability
becomes, but the wafer strength may be reduced due to a decrease in
dissolved oxygen concentration and hence the density of oxide
precipitates after the heat treatment at 1000.degree. C. for 2 hrs
is preferably 1.times.10.sup.12/cm.sup.3 or lower.
[0103] In a second aspect of a silicon epitaxial wafer of the
present invention, density of oxide precipitates detected in the
bulk thereof in a non-heat treatment state after formation of an
epitaxial layer is 1.times.10.sup.9/cm.sup.3 or more.
[0104] Thus, according to the second aspect of the silicon
epitaxial wafer of the present invention, in the silicon epitaxial
wafer being in a non-heat treatment state (the non-heat treatment
means that any heat treatment is not applied after the epitaxial
growth) prior to loading the silicon epitaxial wafer into a device
fabrication process with a low temperature and a short time, the
density of oxide precipitates of sizes each surely having a
gettering capability (about 40 nm or more in diameter and
preferably 100 nm or less in diameter detectable by means of an
optical measuring instrument) is 1.times.10.sup.9 /cm.sup.3 or
higher, so that it has a sufficient gettering capability in a
device fabrication process with a low temperature and a short time
from the initial stage thereof. Note that the higher the density of
oxide precipitates, the more excellent the IG capability, but the
wafer strength may be reduced due to a decrease in solution oxygen
concentration and hence the above density of oxide precipitates is
preferably 1.times.10.sup.12/cm.sup.3 or lower.
BRIEF DESCRIPTION OF THE DRAWINGS
[0105] FIG. 1 is a flow chart showing an example of step orders of
a first aspect of a method for manufacturing a silicon wafer of the
present invention;
[0106] FIG. 2 is a descriptive schematic view showing essential
parts of the step order of FIG. 1;
[0107] FIG. 3 is a flow chart showing an example of step orders of
a second aspect of a method for manufacturing a silicon wafer of
the present invention;
[0108] FIG. 4 is a descriptive schematic view showing essential
parts of the step order of FIG. 3;
[0109] FIG. 5 is a flow chart showing an example of step orders of
a third aspect of a method for manufacturing a silicon wafer of the
present invention;
[0110] FIG. 6 is a descriptive schematic view showing essential
parts of the step order of FIG. 5;
[0111] FIG. 7 is a flow chart showing an example of step orders of
a first aspect of a method for manufacturing a silicon epitaxial
wafer of the present invention;
[0112] FIG. 8 is a descriptive schematic view showing essential
parts of the step order of FIG. 7;
[0113] FIG. 9 is a flow chart showing an example of step orders of
a second aspect of a method for manufacturing a silicon epitaxial
wafer of the present invention;
[0114] FIG. 10 is a descriptive schematic view showing essential
parts of the step order of FIG. 9;
[0115] FIG. 11 is a graph showing a relationship between T.sub.1
and density of oxide precipitates in Experimental Example 1;
[0116] FIG. 12 is a graph showing a relationship between T.sub.2
and density of oxide precipitates in Experimental Example 2;
[0117] FIG. 13 is a graph showing a relationship between R.sub.1
and density of oxide precipitates in Experimental Example 3;
[0118] FIG. 14 is a graph showing a relationship between R.sub.2
and density of oxide precipitates in Experimental Example 4;
[0119] FIG. 15 is a graph showing a relationship between T.sub.3
and density of oxide precipitates in Experimental Example 5;
[0120] FIG. 16 is a graph showing a relationship between T.sub.4
and density of oxide precipitates in Experimental Example 6;
[0121] FIG. 17 is a graph showing a relationship between oxygen
concentration and density of oxide precipitates in Experimental
Example 7;
[0122] FIG. 18 is a graph showing a relationship between a keeping
time t.sub.1 and density of oxide precipitates in Experimental
Example 8;
[0123] FIG. 19 is a graph showing a relationship between a
temperature raising rate R and density of oxide precipitates in
Experimental Example 9;
[0124] FIG. 20 is a graph showing a relationship between a keeping
time t.sub.2 and density of oxide precipitates in Experimental
Example 10;
[0125] FIG. 21 is a graph showing a relationship between T.sub.5
and density of oxide precipitates in Experimental Example 11;
[0126] FIG. 22 is a graph showing a relationship between T.sub.5
and a DZ width in Experimental Example 12;
[0127] FIG. 23 is a descriptive view showing a measuring
arrangement in an infrared scattering tomography;
[0128] FIG. 24 is a graph showing a distribution of density of
oxide precipitates in a wafer depth direction in Experimental
Example 13;
[0129] FIG. 25 is a graph showing a distribution of density of
oxide precipitates in a wafer depth direction in Experimental
Example 14;
[0130] FIG. 26 is a graph showing a relationship between T.sub.9
and density of oxide precipitates in Experimental Example 15;
[0131] FIG. 27 is a graph showing a relationship between oxygen
concentration and density of oxide precipitates in Experimental
Example 16;
[0132] FIG. 28 is a graph showing a relationship between a keeping
time t.sub.5 and density of oxide precipitates in Experimental
Example 17;
[0133] FIG. 29 is a graph showing a relationship between a
temperature raising rate R.sub.5 and density of oxide precipitates
in Experimental Example 18;
[0134] FIG. 30 is a graph showing a relationship between T.sub.10
and density of oxide precipitates in Experimental Example 19;
[0135] FIG. 31 is a graph showing a relationship between a keeping
time t.sub.6 and density of oxide precipitates in Experimental
Example 20;
[0136] FIG. 32 is a graph showing a relationship between T.sub.11
and density of oxide precipitates in Experimental Example 21;
[0137] FIG. 33 is a graph showing a relationship between oxygen
concentration and density of oxide precipitates in Experimental
Example 22;
[0138] FIG. 34 is a graph showing a relationship between a keeping
time t.sub.7 and density of oxide precipitates in Experimental
Example 23;
[0139] FIG. 35 is a graph showing a relationship between a
temperature raising rate R.sub.6 and density of oxide precipitates
in Experimental Example 24;
[0140] FIG. 36 is a graph showing a relationship between a keeping
time t.sub.8 and density of oxide precipitates in Experimental
Example 25; and
[0141] FIG. 37 is a graph showing a relationship between T.sub.12
and density of oxide precipitates in Experimental Example 26.
BEST MODE FOR CARRYING OUT THE INVENTION
[0142] Detailed description will be given of embodiments of the
present invention with reference to the accompanying drawings and
it is needless to say that in addition to examples in the figures
various modification or variations may be made without departing
from the technical concept of the present invention.
[0143] FIG. 1 is a flow chart showing an example of step orders of
a first aspect of a method for manufacturing a silicon wafer of the
present invention and FIG. 2 is a descriptive schematic view
showing essential parts of the step order of FIG. 1. The first
aspect of the method for manufacturing a silicon wafer of the
present invention, as shown in FIGS. 1 and 2, comprises the
following indispensable three steps: a temperature raising step A
for generating oxygen precipitation nuclei (step 104); a
temperature raising step B for growing the oxygen precipitation
nuclei (step 106); and a constant temperature keeping step C for
growing the oxygen precipitation nuclei into oxide precipitates of
larger sizes (step 108). Note that in FIGS. 1 and 2, there is shown
a preferable case where the temperature raising step A (104), the
temperature raising step B (106) and the constant temperature
keeping step C (108) are continuously performed.
[0144] As shown in FIG. 1, at first a wafer to be heat-treated is
prepared (step 100). The wafer is loaded into a heat treatment
furnace kept at T.sub.1.degree. C. (step 102). Here, the
T.sub.1.degree. C. is preferably about 650.degree. C. or lower.
Then, as well shown in FIG. 2, a temperature in the furnace is
raised from the T.sub.1.degree. C. to T.sub.2.degree. C. at a rate
of R.sub.1.degree. C./min (a temperature raising step A: step 104).
Here, it is preferably that the T.sub.2.degree. C. is about
700.degree. C. or higher and the R.sub.1.degree. C./min is about
1.5.degree. C./min or lower. In the temperature raising step A
(step 104), oxygen precipitation nuclei can be formed at high
density. Thereby, even in a wafer containing almost no oxygen
precipitation nuclei at a stage prior to heat treatment, oxygen
precipitation nuclei can be generated at high density. In a wafer
containing oxygen precipitation nuclei in advance, oxygen
precipitation nuclei can be generated at higher density.
[0145] If there is a desire for changing the density of oxide
precipitates to a level of contamination in a device fabrication
process, the density can be easily changed, for example, by setting
the T.sub.2.degree. C. to about 750.degree. C. and the
R.sub.1.degree. C./min to about 1.degree. C./min, and changing the
T.sub.1.degree. C.
[0146] Then, a temperature in the heat treatment furnace is raised
from the T.sub.2.degree. C. to T.sub.3.degree. C. at a rate of
R.sub.2.degree. C./min (a temperature raising step B: step 106).
Here, it is preferable that the T.sub.3.degree. C. is about
1000.degree. C. or higher and the R.sub.2.degree. C./min is the
R.sub.1.degree. C./min or higher and about 7.degree. C./min or
lower. In the temperature raising step B (step 106), oxygen
precipitation nuclei generated in the previous step can be
efficiently grown.
[0147] Note that, if the temperature raising rates R.sub.1.degree.
C./min and R.sub.2.degree. C./min in the temperature raising steps
A and B are constant, respectively, the temperature raising steps
become preferably uncomplicated, but it is possible to switch the
temperature raising rates in the course of the respective steps.
For example, in the temperature raising step B, temperature raising
is initiated at the temperature raising rate R.sub.2.degree. C./min
and at a temperature prior to reaching the T.sub.3.degree. C. the
temperature raising rate may be switched to a temperature raising
rate higher than the R.sub.2.degree. C./min, and thereby it is
possible to shorten a process time to some extent.
[0148] Then, the wafer is kept at the T.sub.3.degree. C. for t time
(a constant temperature keeping step C: step 108). Here, it is
preferable that the T.sub.3.degree. C. is about 1000.degree. C. or
higher and the t time is about 1 hr or longer. In the constant
temperature keeping step C (step 108), minute oxide precipitates
generated in the previous temperature raising steps A and B can be
grown into large oxide precipitates each having an IG capability.
Thus, it is an object of the constant temperature keeping step C to
further grow precipitates generated (grown) in the previous steps.
Therefore, as far as the object can be achieved, this step is not
limited literally to keeping a constant temperature but can be
modified to a step accompanied with some temperature changes
(raising or lowering temperatures or the like).
[0149] Furthermore, by changing the T.sub.3.degree. C. and the t
time in the constant temperature keeping step C (step 108), sizes
of oxide precipitates can be easily changed. It is considered that
the larger the sizes of oxide precipitates are, the higher the IG
capability becomes, but a process time lengthens. Therefore, in
order to attain efficiently oxide precipitates of necessary sizes,
there is important the effect that the sizes are easily
changeable.
[0150] Here, when manufacturing a silicon wafer loaded into a
device fabrication process in which no growth of oxide precipitates
is expected, the T.sub.3.degree. C. in the constant temperature
keeping step C is preferably 1000.degree. C. or higher, but when
the present invention is applied in the case where using a device
fabrication process in which growth of oxide precipitates can be
expected, oxide precipitates are not required to be experimentally
detected after heat treatment, the T.sub.3.degree. C. in the
constant temperature keeping step may be 1000.degree. C. or lower,
for example, 800.degree. C.
[0151] After the above heat treatment, for example, a temperature
in the heat treatment furnace is lowered from the T.sub.3.degree.
C. to 800.degree. C. at a temperature lowering rate of 2.degree.
C./min (a temperature lowering step D: step 110), and after that
the wafer is unloaded from the heat treatment furnace (step
112).
[0152] In order to sufficiently achieve the effect of the
heat-treating method of the present invention, oxygen concentration
of a silicon wafer is desirably about 17 ppma or higher. When the
oxygen concentration is higher, the nucleus generating rate and the
nucleus growth rate are faster, so a total process time is
shorter.
[0153] FIG. 3 is a flow chart showing an example of step orders of
a second aspect of a method for manufacturing a silicon wafer of
the present invention and FIG. 4 is a descriptive schematic view
showing essential parts of the step order of FIG. 3.
[0154] As shown in FIG. 3, at first there is prepared a wafer
having grown-in precipitation nuclei to be heat-treated (step 200).
The wafer can be obtained by processing a silicon single crystal
grown by means of an ordinary CZ method. The wafer is loaded into a
heat treatment furnace (step 202). The heat treatment furnace is
maintained at T.sub.4.degree. C. (500.degree. C. to 700.degree. C.)
and prior to the next temperature raising step, the loaded wafer is
kept at the T.sub.4.degree. C. for a prescribed time (t.sub.1
time), preferably for 15 min or longer (a keeping step before
temperature raising: step 204). Then, as well shown in FIG. 4, a
furnace temperature is raised from the T.sub.4.degree. C. to
T.sub.5.degree. C. set in the range of from 1000.degree. C. to
1230.degree. C. at a temperature raising rate R.degree. C./min of
5.degree. C./min or less (a temperature raising step: step 206). In
the temperature raising step (step 206), grown-in precipitation
nuclei of high density can be effectively grown without
annihilation.
[0155] If there is a desire for changing the density of oxide
precipitates to a level of contamination in a device fabrication
process, the density can be easily changed, for example, by setting
the T.sub.5.degree. C. to about 1100.degree. C. and the temperature
raising rate R.degree. C./min to about 3.degree. C./min, and
changing the T.sub.4.degree. C.
[0156] Then, the wafer is kept at the T.sub.5.degree. C. for a
prescribed time (t.sub.2 time) (a keeping step after temperature
raising: step 208). Here, the keeping time is preferably about 30
min or longer. In the keeping step after temperature raising (step
208), minute oxide precipitates in the bulk of a wafer grown in the
previous temperature raising step (step 206) can be grown into
oxide precipitates of desired sizes each on the order of from 30 nm
to 40 nm in diameter, preferably about 50 nm or larger, and at the
same time, by out-diffusing oxygen in the vicinity of a surface of
the wafer, oxygen precipitation nuclei can be annihilated, so that
a DZ layer free of oxide precipitates can be formed.
[0157] Thus, it is an object of the keeping step after temperature
raising (step 208) at the T.sub.5.degree. C. to further grow oxide
precipitates in the bulk of the wafer grown in the temperature
raising step (step 206) and to out-diffuse oxygen in the vicinity
of a surface of the wafer. Therefore, as far as the object can be
achieved, this step is not limited to keeping a constant
temperature but can be modified to a step accompanied with some
temperature changes (raising or lowering temperatures or the like).
Furthermore, by changing the T.sub.5.degree. C. and the keeping
time t.sub.2 in the keeping step after temperature raising (step
208), sizes of oxide precipitates can be easily changed.
[0158] Note that the wafer to be heat-treated is irrespective of
the presence or absence of addition of impurities and concentration
of impurities. For example, even a nitrogen-doped wafer can achieve
an almost similar effect to a nitrogen un-doped wafer.
[0159] After the above heat treatment, for example, a temperature
in the heat treatment furnace is lowered from T.sub.5.degree. C. to
700.degree. C. at a rate of 2.degree. C./min (a temperature
lowering step: step 210), and after that the wafer is unloaded from
the heat treatment furnace (step 212). Note that no specific
limitation is put on the temperature lowering rate and the lowest
temperature after the temperature has been lowered.
[0160] No specific limitation is put on atmosphere in the above
heat treatment. For example, there can be applied oxygen
atmosphere, mixed atmosphere of oxygen and nitrogen, argon
atmosphere, hydrogen atmosphere and others. In case of
non-oxidative atmosphere such as argon and hydrogen, no oxide film
is formed on a surface of a wafer; therefore out-diffusion of
oxygen may be more preferably accelerated as compared with in case
of oxidative atmosphere.
[0161] In order to sufficiently achieve the effect of the
heat-treating method of the present invention, oxygen concentration
in a silicon wafer to be heat-treated is desirably about 16 ppma or
higher. The higher the oxygen concentration, the higher the density
of precipitates; it is possible to impart a more excellent IG
capability to a wafer. Furthermore, the higher the oxygen
concentration, the faster the growth rate of precipitates; a total
process time becomes shorter.
[0162] FIG. 5 is a flow chart showing an example of step orders of
a third aspect of a method for manufacturing a silicon wafer of the
present invention, and FIG. 6 is a descriptive schematic view
showing essential parts of the step order of FIG. 5. The third
aspect of the method for manufacturing a silicon wafer of the
present invention, as shown in FIGS. 5 and 6, comprises the
following three indispensable steps: a temperature raising step
A.sub.1 for growing grown-in precipitation nuclei generated in a
silicon single crystal growth step (step 304); a temperature
raising step B.sub.1 for efficiently raising a temperature to a
higher keeping temperature (step 306); and a constant temperature
keeping step C.sub.1 for growing the grown-in precipitation nuclei
to oxide precipitates of sizes each equal to or larger than one
having a gettering capacity and for out-diffusing oxygen in the
vicinity of a surface of the silicon wafer (step 308). Note that in
FIGS. 5 and 6, there is shown a preferable case where the
temperature raising step A.sub.1(step 304), the temperature raising
step B.sub.1(step 306) and the constant temperature keeping step
C.sub.1(step 308) are continuously performed.
[0163] As shown in FIG. 5, at first there is prepared a wafer
having grown-in precipitation nuclei to be heat-treated (step 300).
This wafer can be obtained by processing a silicon single crystal
grown by means of an ordinary CZ method. The wafer is loaded into a
heat treatment furnace maintained at T.sub.6.degree. C. (step 302).
Here, the T.sub.6.degree. C. is preferably 700.degree. C. or lower.
Then, as well shown in FIG. 6, a temperature in the heat treatment
furnace is raised from the T.sub.6.degree. C. to T.sub.7.degree. C.
at a rate of R.sub.3.degree. C./min (a temperature raising step
A.sub.1: step 304). Here, it is preferable that the T.sub.7.degree.
C. is 800.degree. C. to 1000.degree. C. and the R.sub.3.degree.
C./min is 3.degree. C./min or less. Furthermore, prior to
temperature raising from the T.sub.6.degree. C. to the
T.sub.7.degree. C. in the temperature raising step A.sub.1(step
304), a keeping time t.sub.3 at the T.sub.6.degree. C. may be 0 min
without hindrance, but more preferably 30 min or longer. In the
temperature raising step A.sub.1(step 304), grown-in precipitation
nuclei of high density can be efficiently grown without
annihilation.
[0164] Then, a temperature in the heat treatment furnace is raised
from the T.sub.7.degree. C. to T.sub.8.degree. C. at a rate of
R.sub.4.degree. C./min (a temperature raising step B.sub.1: step
306). Here, it is preferable that the T.sub.8.degree. C. is
1050.degree. C. to 1230.degree. C. and the R.sub.4.degree. C./min
is 5.degree. C./min or more. In the temperature raising step
B.sub.1(step 306), by raising a temperature to a higher keeping
temperature for a short time, there is avoided growth of more
precipitates than necessary so that precipitates in the vicinity of
a surface of the wafer can be easily annihilated.
[0165] Then, the wafer is kept at the T.sub.8.degree. C. for
t.sub.4 time (a constant temperature keeping step C.sub.1: step
308). Here, it is preferable that the T.sub.8.degree. C. is
1050.degree. C. to 1230.degree. C. and the t.sub.4 time is about 30
min or longer. In the constant temperature keeping step C.sub.1
(step 308), minute oxide precipitates in the bulk of the wafer
grown in the previous temperature raising steps (steps 304 and 306)
are grown into oxide precipitates of desired sizes each of about 40
nm or larger in diameter and preferably about 50 nm or larger, and
at the same time, by out-diffusing oxygen in the vicinity of a
surface of the wafer, oxide precipitates are annihilated, so that a
DZ layer free of oxide precipitates can be formed.
[0166] In this case, as compared with a DZ layer formed in the
second aspect of the present invention, oxide precipitates in the
vicinity of the surface region of a wafer can be annihilated more
perfectly; therefore, an extremely high quality DZ layer can be
efficiently obtained.
[0167] Thus, it is an object of the constant temperature keeping
step C.sub.1 at the T.sub.8.degree. C. (step 308) to further grow
oxide precipitates in the bulk of the wafer grown in the
temperature raising steps (steps 304 and 306) and to out-diffuse
oxygen in the vicinity of a surface of the wafer. Therefore, as far
as the purposes can be achieved, the constant temperature keeping
step C.sub.1 is not limited to keeping a wafer at a constant
temperature but the step can be modified to a step accompanied with
some temperature changes (raising or lowering temperatures or the
like). Furthermore, by changing the T.sub.8.degree. C. and the
keeping time t.sub.4 in the constant keeping step C.sub.1(step
308), sizes of oxide precipitates and a DZ width can be easily
changed.
[0168] Note that the wafer to be heat-treated is irrespective of
the presence or absence of addition of impurities and concentration
of impurities. For example, even a nitrogen-doped wafer can achieve
an almost similar effect to a nitrogen un-doped wafer.
[0169] After the above heat treatment, for example, a temperature
in the heat treatment furnace is lowered from the T.sub.8.degree.
C. to 700.degree. C. at 3.degree. C./min (a temperature lowering
step D.sub.1: step 310), and after that the wafer is unloaded from
the heat treatment furnace (step 312). Note that no specific
limitation is put on the temperature lowering rate and the lowest
temperature after the temperature has been lowered.
[0170] No specific limitation is put on atmosphere in the above
heat treatment. For example, there can be applied oxygen
atmosphere, mixed atmosphere of oxygen and nitrogen, argon
atmosphere, hydrogen atmosphere and others. In case of
non-oxidative atmosphere such as argon and hydrogen, no oxide film
is formed on a surface of a wafer; therefore out-diffusion of
oxygen may be more preferably accelerated as compared with in case
of oxidative atmosphere.
[0171] In order to sufficiently achieve the effect of the third
aspect of the method for manufacturing a silicon wafer of the
present invention, oxygen concentration in a silicon wafer to be
heat-treated is desirably about 14 ppma to 17 ppma. When the oxygen
concentration is higher, the density of precipitates is raised; it
is possible to impart a more excellent IG capability to a wafer,
but if the oxygen concentration is excessively high, precipitates
in the vicinity of a surface of the wafer are hard to be
annihilated. On the other hand, when the oxygen concentration is
lower, the density of grown-in precipitates generated in a silicon
single crystal growth step becomes lower; the density of
precipitates is reduced. However, even when oxygen concentration is
low, for example, by lowering the temperature raising initiation
temperature T.sub.6.degree. C. or slowing the R.sub.3.degree.
C./min in the temperature raising step A.sub.1, the effect of the
third aspect of the method for manufacturing a silicon wafer of the
present invention can be achieved.
[0172] FIG. 7 is a flow chart showing an example of step orders of
a first aspect of a method for manufacturing a silicon epitaxial
wafer of the present invention and FIG. 8 is a descriptive
schematic view showing essential parts of the step order of FIG.
7.
[0173] As shown in FIG. 7, at first there is prepared a wafer
having grown-in precipitation nuclei to be heat-treated (step 400).
This wafer can be obtained by processing a silicon single crystal
grown by means of an ordinary CZ method. The wafer is loaded into a
heat treatment furnace (step 402). The heat treatment furnace is
maintained at T.sub.9.degree. C. (500.degree. C. to 700.degree. C.)
and the loaded wafer is kept at the T.sub.9.degree. C. for a
prescribed time (t.sub.5 time), preferably 15 min or longer, prior
to performing the next temperature raising step (a keeping step
before temperature raising: step 404). Then, as well shown in FIG.
8, a temperature in the heat treatment furnace is raised from the
T.sub.9.degree. C. to T.sub.10.degree. C. set in the range of from
800.degree. C. to 900.degree. C. at a temperature raising rate
R.sub.5.degree. C./min of 5.degree. C./min or less (a temperature
raising step: step 406). In the temperature raising step (step
406), grown-in precipitation nuclei of high density can be
efficiently grown without annihilation.
[0174] If there is a desire for changing the density of oxide
precipitates to a level of contamination in a device fabrication
process, the density can be easily changed, for example, by setting
the T.sub.10.degree. C. to about 800.degree. C. and the temperature
raising rate R.sub.5.degree. C./min to about 3.degree. C./min, and
changing the T.sub.9.degree. C.
[0175] Then, the wafer is kept at the T.sub.10.degree. C. for a
prescribed time (t.sub.6 time) (a keeping step after temperature
raising: step 408). Here, the keeping time is preferably about 30
min or longer. In the keeping step after temperature raising (step
408), minute oxide precipitates grown in the previous temperature
raising step (step 406) can be grown into oxide precipitates of
desired sizes each of 30 nm to 40 nm, preferably 50 nm or larger.
Thus, it is an object of the keeping step after temperature raising
(step 408) at the T.sub.10.degree. C. to further grow oxide
precipitates grown in the temperature raising step (step 406).
Therefore, as far as the object can be achieved, this step is not
limited to keeping a constant temperature but can be modified to a
step accompanied with some temperature changes (raising or lowering
temperatures or the like).
[0176] Furthermore, by changing the T.sub.10.degree. C. and the
keeping time t.sub.6 in the keeping step after temperature raising
(step 408), sizes of oxide precipitates can be easily changed.
[0177] Note that the wafer to be heat-treated is irrespective of
the presence or absence of addition of impurities and concentration
of impurities. For example, even a nitrogen-doped wafer can achieve
an almost similar effect to a nitrogen un-doped wafer.
[0178] After the above heat treatment, for example, a temperature
in the heat treatment furnace is lowered from the T.sub.10.degree.
C. to 700.degree. C. at a rate of 2.degree. C./min (a temperature
lowering step: step 410), and after that the wafer is unloaded from
the heat treatment furnace (step 412). Note that no specific
limitation is put on the temperature lowering rate and the lowest
temperature after the temperature has been lowered.
[0179] Then, if required, there are performed such processes as
wafer cleaning and oxide film removing, and after that epitaxial
growth is carried out (step 414). In this case, while sizes of
oxide precipitates immediately after the epi step are not so large
as to have an enough IG capability, the oxide precipitates grow to
sizes each having an IG capability by heat treatment in a device
fabrication process into which the wafer is loaded after the epi
step; an excellent IG capability is imparted to the wafer.
[0180] The above heat treatment according to the present invention
may be applied to a wafer to be heat-treated before or after mirror
polishing (may be referred to as mechanical chemical polishing). In
the case where the heat treatment is performed before the mirror
polishing, the mirror polishing is applied after the heat treatment
and then epitaxial growth follows.
[0181] In order to sufficiently achieve the effect of the
heat-treating method of the present invention, oxygen concentration
of a silicon wafer to be heat-treated is desirably about 16 ppma or
higher. When the oxygen concentration is higher, the density of
precipitates is raised; it is possible to impart a more excellent
IG capability to a wafer. Furthermore, the higher the oxygen
concentration is, the faster the growth rate of the precipitates
becomes, so a total process time is shortened.
[0182] FIG. 9 is a flow chart showing an example of step orders of
a second aspect of a method for manufacturing a silicon epitaxial
wafer of the present invention and FIG. 10 is a descriptive
schematic view showing essential parts of the step order of FIG.
9.
[0183] As shown in FIG. 9, at first there is prepared a wafer
having grown-in precipitation nuclei to be heat-treated (step 500).
This wafer can be obtained by processing a silicon single crystal
grown by means of an ordinary CZ method. The wafer is loaded into a
heat treatment furnace (step 502). The heat treatment furnace is
maintained at T.sub.1.degree. C. (500.degree. C. to 700.degree. C.)
and the loaded wafer is kept at the T.sub.11.degree. C. for a
prescribed time (t.sub.7 time), preferably 15 min or longer, prior
to performing the next temperature raising step (a keeping step
before temperature raising: step 504). Then, as well shown in FIG.
10, a temperature in the heat treatment furnace is raised from the
T.sub.11.degree. C. to T.sub.12.degree. C. set in the range of from
1000.degree. C. to 1100.degree. C. at a temperature raising rate
R.sub.6.degree. C./min of 5.degree. C./min or less (a temperature
raising step: step 506). In the temperature raising step (step
506), grown-in precipitation nuclei of high density can be
efficiently grown without annihilation.
[0184] If there is a desire for changing the density of oxide
precipitates to a level of contamination in a device fabrication
process, the density can be easily changed, for example, by setting
the T.sub.12.degree. C. to about 1000.degree. C. and the
temperature raising rate R.sub.6.degree. C./min to about 3.degree.
C./min, and changing the T.sub.11.degree. C.
[0185] Then, the wafer is kept at the T.sub.12.degree. C. for a
prescribed time (t.sub.8 time) (a keeping step after temperature
raising: step 508). Here, the keeping time is preferably about 30
min or longer. In the keeping step after temperature raising (step
508), minute oxide precipitates grown in the previous temperature
raising step (step 506) can be grown into oxide precipitates of
desired sizes each of the order of from 30 nm to 40 nm in diameter,
preferably of 50 nm or larger. Thus, it is an object of the keeping
step after temperature raising (step 508) at the T.sub.12.degree.
C. to further grow oxide precipitates grown in the temperature
raising step (step 506). Therefore, as far as the object can be
achieved, this step is not limited to keeping a constant
temperature but can be modified to a step accompanied with some
temperature changes (raising or lowering temperatures or the
like).
[0186] Furthermore, by changing the T.sub.12.degree. C. and the
keeping time t.sub.8 in the keeping step after temperature raising
(step 508), sizes of oxide precipitates can be easily changed.
[0187] Note that the wafer to be heat-treated is irrespective of
the presence or absence of addition of impurities and concentration
of impurities. For example, even a nitrogen-doped wafer can achieve
an almost similar effect to a nitrogen un-doped wafer.
[0188] After the above heat treatment, for example, a temperature
in the heat treatment furnace is lowered from the T.sub.12.degree.
C. to 700.degree. C. at a rate of 2.degree. C./min (a temperature
lowering step: step 510), and after that the wafer is unloaded from
the heat treatment furnace (step 512). Note that no specific
limitation is put on the temperature lowering rate and the lowest
temperature after the temperature has been lowered.
[0189] Then, if required, there are performed such processes as
wafer cleaning and oxide film removing, and after that epitaxial
growth is carried out (step 514). In this case, no re-dissolution
of oxide precipitates occurs during the epitaxial growth step;
there can be obtained a silicon epitaxial wafer in which oxide
precipitates of sizes each having an IG capability are generated
before being loaded into a device fabrication process.
[0190] The heat treatment may be applied to a wafer to be
heat-treated before or after mirror polishing (may be referred to
as mechanical chemical polishing). In the case where the heat
treatment is performed before the mirror polishing, the mirror
polishing is applied after the heat treatment and then epitaxial
growth follows.
[0191] In order to sufficiently achieve the effect of the
heat-treating method of the present invention, oxygen concentration
of a silicon wafer to be heat-treated is desirably about 16 ppma or
higher. When the oxygen concentration is higher, the density of
precipitates is raised; it is possible to impart a more excellent
IG capability to a wafer. Furthermore, the higher the oxygen
concentration is, the faster the growth rate of the precipitates
becomes, so a total process time is shortened.
EXAMPLES
[0192] While description will be given of the present invention
showing Experimental Examples below, the present invention is not
limited to the Experimental Examples.
Experimental Examples 1 to 5
[0193] There were prepared boron-doped silicon wafers each having a
diameter of 8 inches, a plane orientation of <100> and
resistivity of about 10 .OMEGA.cm grown by means of a CZ method.
Oxygen concentrations of the wafers were 16.0, 17.0, 18.5 and 19.5
ppma, respectively (JEIDA scale). The wafers were all subjected to
heat treatment at 1050.degree. C. for 1 hr. With the aid of the
heat treatment, almost all oxygen precipitation nuclei formed in a
crystal thermal history are annihilated. By this treatment, for
example, there can be simulated states of an epi wafer containing
almost no oxygen precipitation nuclei and a wafer heat-treated at a
temperature of 1000.degree. C. or higher in advance. Note that
JEIDA is an abbreviation for Japan Electronic Industry Development
Association (now renamed JEITA: Japan Electronics and Information
Technology Industries Association).
[0194] Next, heat treatment shown in FIGS. 1 and 2 was applied to
the wafers. That is, a temperature was raised from T.sub.1.degree.
C. to T.sub.2.degree. C. at a rate of R.sub.1.degree. C./min, from
the T.sub.2.degree. C. to T.sub.3.degree. C. at a rate of
R.sub.2.degree. C./min and kept at the T.sub.3.degree. C. for t
time. After the keeping, a temperature in a heat treatment furnace
was lowered to 800.degree. C. at a rate of 2.degree. C./min and
then the wafer was unloaded from the heat treatment furnace.
[0195] The density of oxide precipitates in the bulk of the
heat-treated wafer was measured with an infrared scattering
tomography method (hereinafter may be referred to LST), which is
one of infrared scattering methods. Note that, while Experimental
Examples 1 to 5 include ones carried out in the same heat treatment
condition, they are performed as independent experiments using
separate wafers; there are some differences among measured values
of the density of oxide precipitates, the differences being
inessential.
[0196] FIG. 11 shows the density of oxide precipitates at
550.degree. C., 600.degree. C. and 650.degree. C. of the
T.sub.1.degree. C. (Experimental Example 1). In this case, the
other conditions were as follows: T.sub.2=750.degree. C.,
R.sub.1=1.degree. C./min, T.sub.3=1050.degree. C.,
R.sub.2=3.degree. C./min and t=2 hrs. The lower the T.sub.1.degree.
C. is, the higher the density of oxide precipitates becomes. This
is because the lower the temperature is, the faster the
precipitation nucleus forming rate becomes and the longer the
temperature raising time becomes, thereby a time for generating
precipitation nuclei being longer. Furthermore, the higher the
oxygen concentration is, the higher the density of oxide
precipitates becomes. This is because the higher the oxygen
concentration is, the faster the precipitation nucleus forming rate
becomes. In order to obtain an IG capability, the density of oxide
precipitates is desirably on the order of 10.sup.8/cm.sup.3 or
higher. This indicates that the T.sub.1.degree. C. is preferably
about 650.degree. C. or lower. Further, it is found that the oxygen
concentration is preferably about 17 ppma or higher in order to
attain oxide precipitates of high density. Furthermore, as shown in
FIG. 11, by changing the T.sub.1.degree. C., the density of oxide
precipitates can be easily changed. This is effective where there
is a desire for obtaining requested density of oxide precipitates
to a level of contamination in a device fabrication process.
[0197] FIG. 12 shows the density of oxide precipitates at
700.degree. C., 750.degree. C., 800.degree. C. and 850.degree. C.
of the T.sub.2.degree. C. (Experimental Example 2). In this case,
the other conditions were as follows: T.sub.1=600.degree. C.,
R.sub.1=1.degree. C./min, T.sub.3=1050.degree. C.,
R.sub.2=3.degree. C./min and t=2 hrs. When the oxygen concentration
is about 17 ppma or higher, the density of oxide precipitates shows
almost no change at the T.sub.2.degree. C. of 750.degree. C. or
higher. This is because since formation of precipitation nuclei is
hard to progress in a temperature raising step at a temperature of
750.degree. C. or higher, the density of oxygen precipitation
nucleus is almost unchanged. The fact that the density of oxide
precipitates is low at 700.degree. C. is because the density of
oxygen precipitation nuclei formed is low due to a short time of
the temperature raising step A. This indicates that the density of
oxide precipitates can be higher by setting the T.sub.2.degree. C.
to about 700.degree. C. or higher, preferably about 750.degree. C.
or higher.
[0198] FIG. 13 shows the density of oxide precipitates 1.degree.
C./min, 1.5.degree. C./min, 2.degree. C./min, 2.5.degree. C./min
and 4.degree. C./min of the R.sub.1.degree. C./min (Experimental
Example 3). In this case, the other conditions were as follows:
T.sub.1=600.degree. C., T.sub.2=750.degree. C.,
T.sub.3=1050.degree. C., R.sub.2=3.degree. C./min and t=2 hrs. The
lower the R.sub.1.degree. C./min is, the higher the density of
oxide precipitates becomes. This is because the lower the rate is,
the longer the time for generating precipitation nuclei becomes.
From FIG. 13, it is found that the R.sub.1.degree. C./min is
preferably about 1.5.degree. C./min or less in order to attain
oxide precipitates of high density.
[0199] FIG. 14 shows the density of oxide precipitates at 1.degree.
C./min, 3.degree. C./min, 5.degree. C./min, and 7.degree. C./min of
the R.sub.2.degree. C./min (Experimental Example 4). In this case,
the other conditions were as follows: T.sub.1=600.degree. C.,
T.sub.2=750.degree. C., R.sub.1=1.degree. C./min,
T.sub.3=1050.degree. C., and t=2 hrs. The slower the
R.sub.2.degree. C./min is, the higher the density of oxide
precipitates becomes. This is because at a slower rate, more of
oxygen precipitation nuclei formed in the previous stage can grow
into oxide precipitates without annihilation. To the contrary, when
the rate is faster, the proportion of oxygen precipitation nuclei
to be annihilated becomes higher; the density of oxide precipitates
is reduced. From FIG. 14, it is found that the R.sub.2.degree.
C./min is about 7.degree. C./min or less, preferably about
5.degree. C./min or less in order to attain oxygen precipitates of
high density.
[0200] FIG. 15 shows the density of oxide precipitates at
950.degree. C., 1000.degree. C., 1050.degree. C. and 1100.degree.
C. of the T.sub.3.degree. C. (Experimental Example 5). In this
case, the other conditions were as follows: T.sub.1=600.degree. C.,
T.sub.2=750.degree. C., R.sub.1=1.degree. C./min, R.sub.2=3.degree.
C./min and t=1 hr. At the T.sub.3 of 1000.degree. C. or higher, the
density of oxide precipitates is almost unchanged. This is because
at 1000.degree. C. or higher, oxide precipitates have been
sufficiently grown to detectable sizes. The fact that the density
of oxide precipitates is low at 950.degree. C. is because at the
keeping time of 1 hr, the proportion of oxide precipitates not to
grow up to detectable sizes is high. In this case, if the keeping
time is longer, the density of detectable oxide precipitates
becomes to the same level in case of 1000.degree. C. or higher but
a longer process time is required. This indicates that the
T.sub.3.degree. C. is preferably about 1000.degree. C. or
higher.
[0201] As can be seen from the above description, by setting the
T.sub.1 to about 650.degree. C. or lower, the T.sub.2.degree. C. to
about 700.degree. C. or higher, the R.sub.1.degree. C./min to about
1.5.degree. C./min or less, the R.sub.2.degree. C./min to about
7.degree. C./min or less, the T.sub.3.degree. C. to about
1000.degree. C. or higher and the t time to about 1 hr or longer,
respectively, oxide precipitates of large sizes can be efficiently
generated even in a wafer containing almost no oxygen precipitation
nuclei. That is, there can be stably provided a wafer having an
excellent IG capability at a stage prior to a device fabrication
process. Further, even in wafers having fluctuations in density of
oxygen precipitation nuclei due to different crystal thermal
histories, by performing the heat treatment in the method for
manufacturing a silicon wafer of the present invention after the
heat treatment has been carried out for annihilating the oxygen
precipitation nuclei thereof as in these Experimental Examples,
fluctuations in the IG capabilities thereof can be reduced.
Experimental Examples 6 to 10
[0202] There were prepared mirror-polished silicon wafers each
having a diameter of 8 inches, a plane orientation of <100>
and resistivity of about 10 .OMEGA.cm, which were manufactured from
different two portions (these portions belong to positions grown in
the former half and the latter half of the crystal growth step and
hereinafter may be referred to as a crystal position A and a
crystal position B, respectively) of a boron-doped silicon single
crystal grown by means of a CZ method. Oxygen concentrations of the
wafers are 16 ppma to 20 ppma (JEIDA scale).
[0203] Next, the wafers were subjected to heat treatment in
atmosphere of a mixture of oxygen and nitrogen according to a
procedure shown in FIGS. 3 and 4. That is, the wafer was kept at
T.sub.4.degree. C. for t.sub.1 time, thereafter the temperature was
raised from the T.sub.4.degree. C. to T.sub.5.degree. C. at a rate
of R.degree. C./min and kept at the T.sub.5.degree. C. for t.sub.2
time. After the keeping, the temperature in a heat treatment
furnace was lowered to 700.degree. C. at a rate of 2.degree. C./min
and then the wafers were unloaded from the heat treatment furnace.
The density of oxide precipitates in the bulk of the wafer was
measured with the LST without any heat treatment. According to the
LST, there are detectable oxide precipitates having sizes of about
40 nm or larger in diameter.
[0204] FIG. 16 shows the density of precipitates at 600.degree. C.,
700.degree. C., and 800.degree. C. of the T.sub.4.degree. C.
(Experimental Example 6). In this case, the other conditions were
as follows: t.sub.1=30 min, R=3.degree. C./min, T.sub.5.degree.
C.=1050.degree. C. and t.sub.2=2 hrs. Oxygen concentrations are 16
ppma to 18 ppma. The lower the T.sub.4.degree. C. is, the higher
the density of precipitates becomes. This is because there is
increased the density of grown-in precipitation nuclei which grow
at a lower temperature without annihilation. In order to obtain an
excellent IG capability, the density of precipitate is desirably on
the order of 10.sup.9/cm.sup.3 or higher. This indicates that the
T.sub.4.degree. C. is about 700.degree. C. or lower. Moreover, as
shown in FIG. 16, by changing the T.sub.4.degree. C., the density
of precipitates can be easily changed. This is effective where
there is a desire for obtaining requested density of precipitates
to a level of contamination in a device fabrication process.
[0205] FIG. 17 shows the oxygen concentration dependence of density
of precipitates at T.sub.4=700.degree. C., t.sub.1=30 min,
R=3.degree. C./min, T.sub.5=1050.degree. C. and t.sub.2=2 hrs
(Experimental Example 7). This result indicates that in either
crystal position there is a trend that as the oxygen concentration
increases, the density of oxide precipitates increases. It is also
found that when an oxygen concentration is about 16 ppma or higher,
the high density of precipitates of the order of 10.sup.9/cm.sup.3
or higher can be surely achieved. This is because the higher the
oxygen concentration is, the higher the density of grown-in
precipitation nuclei becomes and the larger the sizes of oxide
precipitates becomes, thereby the detectable proportion thereof
being increased.
[0206] FIG. 18 shows the density of precipitates at 0 min, 5 min,
15 min, 30 min and 45 min of the t.sub.1 time (Experimental Example
8). In this case, the other conditions were as follows:
T.sub.4=700.degree. C., R=3.degree. C./min, T.sub.5=1050.degree. C.
and t.sub.2=2 hrs. Oxygen concentrations are 18 ppma to 20 ppma.
When the t.sub.1 time is about 15 min or longer, the density of
precipitates increases with time. This is because as the t.sub.1
time is longer, the proportion of grown-in precipitation nuclei to
be grown without annihilation becomes higher and new precipitation
nuclei are generated for the keeping of the t.sub.1 time. This
result indicates that even in the case where the t.sub.1 time is
shorter than 15 min, the high density of precipitates can be
achieved, but in the case where the t.sub.1 time is 15 min or
longer, the density of precipitates is further increased; it is
found that the latter case is more preferable.
[0207] FIG. 19 shows the density of precipitates at 1.degree.
C./min, 3.degree. C./min and 5.degree. C./min of the R.degree.
C./min (Experimental Example 9). In this case, the other conditions
were as follows: T.sub.4=700.degree. C., t.sub.1=30 min,
T.sub.5=1050.degree. C. and t.sub.2=2 hrs. Oxygen concentrations
are 18 ppma to 20 ppma. The slower the R.degree. C./min is, the
higher the density of precipitates becomes. The reason why when the
R.degree. C./min is faster, the density of precipitates becomes
lower is that there becomes higher the proportion of precipitation
nuclei which are annihilated without growth in the temperature
raising step. This result indicates that in order to achieve the
high density of precipitates of the order of 10.sup.9/cm.sup.3 or
higher, the R.degree. C./min is preferably about 5.degree. C./min
or less, more preferably about 3.degree. C./min or less.
[0208] FIG. 20 shows the density of oxide precipitates at 30 min,
60 min and 120 min of the t.sub.2 time (Experimental Example 10).
In this case, the other conditions were as follows:
T.sub.4=700.degree. C., t.sub.5=30 min, R=3.degree. C./min,
T.sub.5=1100.degree. C. Oxygen concentrations are 18 ppma to 20
ppma. The density of precipitates is high almost independently of
the t.sub.2 time. This result indicates that when the t.sub.2 time
is about 30 min or longer, the high density of precipitates is
surely achieved.
Experimental Example 11
[0209] Mirror-polished wafers similar to the wafers used in
Experimental Examples 6 to 10 were prepared. Then, according to a
heat treatment procedure shown in FIGS. 3 and 4, the following
conditions were applied: T.sub.4=700.degree. C., t.sub.1=30 min,
R=3.degree. C./min and t.sub.2=2 hrs, and there were performed 4
levels of heat treatment where T.sub.5.degree. C. was changed to
950.degree. C., 1000.degree. C., 1050.degree. C. and 1100.degree.
C., respectively. After the keeping at the T.sub.5.degree. C., the
temperature in a heat treatment furnace was lowered to 700.degree.
C. at 2.degree. C./min and then the wafers were unloaded from the
heat treatment furnace. The density of oxide precipitates in the
bulk of the wafer was measured with the LST without any heat
treatment.
[0210] FIG. 21 shows a relationship between density of precipitates
and T.sub.5.degree. C. in the Experimental Example. Oxygen
concentrations are 18 ppma to 20 ppma. When the T.sub.5.degree. C.
is 1000.degree. C. or higher, the density of precipitates becomes
high. The reason why the density of oxide precipitates decreases at
950.degree. C. is that a growth rate of the precipitates is slower
than at 1000.degree. C. or higher and hence sizes of precipitates
are too small to be detectable by the LST. This result indicates
that in order to generate precipitates of large sizes at high
density at a stage prior to a device fabrication process, the
T.sub.5.degree. C. is preferably about 1000.degree. C. or
higher.
Experimental Example 12
[0211] Mirror-polished wafers similar to the wafers used in
Experimental Examples 6 to 11 were prepared. Then, according to a
heat treatment procedure shown in FIGS. 3 and 4, the following
conditions were applied: T.sub.4 700.degree. C., t.sub.1=30 min,
R=3.degree. C./min and t.sub.2=2 hrs, and there were performed 3
levels of heat treatment where the T.sub.5.degree. C. was changed
to 1000.degree. C., 1050.degree. C. and 1100.degree. C.,
respectively. After the keeping at the T.sub.5.degree. C., the
temperature in a heat treatment furnace was lowered to 700.degree.
C. at 2.degree. C./min and then the wafers were unloaded from the
heat treatment furnace.
[0212] The heat-treated wafers were cleaved and then subjected to
chemical preferential etching. Thereafter, by observing a cleaved
surface thereof through an optical microscope, there was measured a
depth from a wafer surface of the region where no etch pits due to
oxide precipitates were observed (hereinafter may be referred to as
a DZ width).
[0213] A relationship between a DZ width obtained in the above
measurement and the T.sub.5.degree. C. is shown in FIG. 22. Oxygen
concentrations are 18 ppma to 20 ppma. The higher the
T.sub.5.degree. C. is, the wider the DZ width becomes. This is
because the higher the temperature is, the faster the diffusion
rate of oxygen becomes; oxygen in the vicinity of a wafer surface
is out-diffused, so that oxygen precipitation nuclei in the
vicinity of the wafer surface are annihilated.
[0214] As described above, by setting the T.sub.4.degree. C. to
about 700.degree. C. or lower, the R.degree. C./min to about
5.degree. C./min or less, the T.sub.5.degree. C. to about
1000.degree. C. or higher and the t.sub.2 time to about 30 min or
longer, respectively, a silicon wafer having a defect-free layer
(DZ layer) in the vicinity of a surface thereof and oxide
precipitates each having a high gettering capability in the bulk
thereof can be attained with the aid of an extremely efficient and
simple heat treatment.
Experimental Example 13
[0215] There were prepared boron-doped silicon wafers each having a
diameter of 8 inches, a plane orientation <100> and
resistivity of about 10 .OMEGA.cm grown by means of a CZ method.
Each of these wafers is one having a few defects with few COPs and
voids which is manufactured from a silicon single crystal pulled in
a condition that suppresses generation of void defects in growth
thereof. Oxygen concentrations of the wafers were 14.5, 15.4 and
17.0 ppma, respectively (JEIDA scale).
[0216] Next, the wafers were subjected to heat treatment in
atmosphere of a mixture of oxygen and nitrogen according to a
procedure shown in FIGS. 3 and 4. That is, the wafer was kept at
700.degree. C. for 1 hr, then the temperature was raised up to
1150.degree. C. at a rate of 3.degree. C./min and kept at
1150.degree. C. for 4 hrs. After the keeping, the temperature in a
heat treatment furnace was lowered to 700.degree. C. at a rate of
3.degree. C./min and then the wafers were unloaded from the heat
treatment furnace. There was measured a depthwise distribution of
density of oxide precipitates from a position in the vicinity of a
surface of each wafer to another position in the bulk thereof with
the LST without any heat treatment.
[0217] There is shown a measuring arrangement with the LST in
measurement of a depthwise distribution of density of oxide
precipitates in FIG. 23. An infrared beam 12 is incident upon a
cleaved surface 10 of a cleaved wafer W and there is detected
scattering light 16 from precipitates coming out of a wafer surface
14. In this measurement, by changing a distance from the surface of
the incident point 18 (a depth from the surface) at the intervals
of 5 .mu.m in the range of 5 .mu.m to 35 .mu.m, the depthwise
distribution of density of precipitates was measured. The depth
resolution in this case is on the order of .+-.5 .mu.m.
Accordingly, for example, the density of precipitates at a point of
10 .mu.m in depth actually shows an average value thereof in a
region of the order of 5 .mu.m to 15 .mu.m in depth.
[0218] FIG. 24 shows results of measurement of the depthwise
distribution of density of precipitates. In this case, the density
of precipitates at the positions of 5 .mu.m in depth in the wafers
having any oxygen concentrations was on the order of
10.sup.6/cm.sup.3 to 10.sup.7/cm.sup.3. These results indicate that
a region on the order of 0 .mu.m to 10 .mu.m deep is effective as a
DZ layer, but precipitates of low density still remain therein.
[0219] Further, there was separately prepared a wafer similar to
the above wafers except for the oxygen concentration of 18 ppma.
The wafer was subjected to heat treatment where the wafer was kept
at 700.degree. C. for 1 hr, then the temperature was raised to
1200.degree. C. at a rate of 3.degree. C./min and kept at
1200.degree. C. for 1 hr.
[0220] Furthermore, the density of oxide precipitates of the
heat-treated wafer was observed, and the results indicate that in a
region of about 10 .mu.m deep from a wafer surface, there was
formed a DZ layer where precipitates remain at low density of the
order of 10.sup.7/cm.sup.3 or lower, and in the bulk portion deeper
than the layer, the density of oxide precipitates of about
5.times.10.sup.9/cm.sup.3 were obtained.
Experimental Example 14
[0221] There were prepared wafers similar to the wafers used in
Experimental Example 13. Then, the wafers were subjected to heat
treatment in atmosphere of a mixture of oxygen and nitrogen
according to a procedure shown in FIGS. 5 and 6. That is, the wafer
was kept at 700.degree. C. for 1 hr, then the temperature was
raised to 900.degree. C. at a rate of 3.degree. C./min, further
raised from 900.degree. C. to 1150.degree. C. at a rate of
5.degree. C./min and kept at 1150.degree. C. for 4 hrs. After the
keeping, the temperature in a heat treatment furnace was lowered to
700.degree. C. at a rate of 3.degree. C./min and the wafers were
unloaded from the heat treatment furnace. That is, unlike in
Experimental Example 13 (the second embodiment of the present
invention), there was carried out such heat treatment as the
temperature raising rate was faster from the middle of the
temperature raising step (which corresponds to the third embodiment
of the present invention). There was measured a depthwise
distribution of density of precipitates of each wafer without any
heat treatment according to the method similar to Experimental
Example 13 with the LST.
[0222] FIG. 25 shows results of measurement of a depthwise
distribution of a density of precipitates. In a wafer having any
oxygen concentrations, the density of precipitates at the position
5 .mu.m deep was the detection lower limit (about
1.times.10.sup.6/cm.sup.3) or less. This results indicate that
there exists no detectable precipitates in the region of the order
of 0 .mu.m to 10 .mu.m deep. That is, it is found that there is
formed a DZ layer of an extremely high quality. Further, it is
found that when the oxygen concentration is low, the DZ layer is
wider. This is because the lower the oxygen concentration prior to
the heat treatment, the lower the oxygen concentration in a surface
region in the vicinity of a surface of the heat-treated wafer,
thereby precipitates in the vicinity of the surface thereof being
easily annihilated. On the other hand, it is found that the density
of precipitates in the interior of the wafer is
1.times.10.sup.9/cm.sup.3 or higher; an excellent IG capability can
be expected.
[0223] Furthermore, there was separately prepared a wafer similar
to the wafers used in Experimental Example 13 except for the oxygen
concentration of 16 ppma. The wafer was subjected to heat treatment
where the wafer was kept at 700.degree. C. for 1 hr, then the
temperature was raised to 900.degree. C. at a rate of 3.degree.
C./min, further raised from 900.degree. C. to 1200.degree. C. at a
rate of 5.degree. C./min and kept at 1200.degree. C. for 1 hr.
[0224] Then, the density of oxide precipitates of the heat-treated
wafer was observed, and the results indicate that in a region of
about 10 .mu.m deep from a wafer surface, there was formed a DZ
layer having density of oxide precipitates of
1.times.10.sup.6/cm.sup.3 or lower, and in the bulk portion deeper
than the layer, the density of oxide precipitates of about
3.times.10.sup.9/cm.sup.3 were obtained.
[0225] As described above, according to the third aspect of the
method for manufacturing a silicon wafer of the present invention,
there can be formed a high quality DZ-IG structure which combines a
DZ layer containing few precipitates and an IG layer having an
excellent IG capability.
Experimental Examples 15 to 20
[0226] There were prepared silicon wafers each having a diameter of
8 inches, a plane orientation of <100> and resistivity of
about 10 .OMEGA.cm, which were sliced from different two portions
(these portions belong to positions grown in the former half and
the latter half of the crystal growth step and hereinafter may be
referred to as a crystal position A and crystal position B,
respectively) of a boron-doped silicon single crystal grown by
means of a CZ method. The wafers prepared here were chemical etched
wafers that are not subjected to mirror polishing. Oxygen
concentrations of the wafers are 16 ppma to 20 ppma (JEIDA
scale).
[0227] Then, the wafers were subjected to heat treatment according
to the procedure shown in FIGS. 7 and 8. That is, the wafer was
kept at T.sub.9.degree. C. for t.sub.5 time, thereafter the
temperature was raised from the T.sub.9.degree. C. to
T.sub.10.degree. C. at a rate of R.sub.5.degree. C./min and kept at
the T.sub.10.degree. C. for t.sub.6 time. After the keeping, the
temperature in a heat treatment furnace was lowered to 700.degree.
C. at a rate of 2.degree. C./min and then the wafers were unloaded
from the heat treatment furnace.
[0228] After the heat-treated wafers were mirror-polished, a
silicon single crystal layer about 3 .mu.m thick was deposited on
each of the mirror-polished wafers by epitaxial growth at about
1100.degree. C. to thereby obtain epi wafers.
[0229] After the epi wafers were subjected to heat treatment at
1000.degree. C. for 2 hrs that simulates a device fabrication
process with a low temperature and a short time, the density of
oxide precipitates in the bulk of the wafer below the epitaxial
layer of each epi wafer was measured with the LST. With the LST,
there are detectable oxide precipitates having sizes of about 40 nm
or larger in diameter.
[0230] FIG. 26 shows the density of precipitate at values
600.degree. C., 700.degree. C., and 800.degree. C. of the
T.sub.9.degree. C. (Experimental Example 15). In this case, the
other conditions were as follows: t.sub.5=30 min, R.sub.5=3.degree.
C./min, T.sub.10.degree. C.=850.degree. C. and t.sub.6=1 hr. Oxygen
concentrations are in the range of from 16 ppma to 18 ppma. The
lower the T.sub.9.degree. C. is, the higher the density of
precipitates becomes. This is because there is increased the
density of grown-in precipitation nuclei which grow at a lower
temperature without annihilation. In order to obtain an excellent
IG capability, the density of precipitates is desirably on the
order of 10.sup.9/cm.sup.3 or higher. This indicates that the
T.sub.9.degree. C. is preferably about 700.degree. C. or lower.
Moreover, as shown in FIG. 26, by changing the T.sub.9.degree. C.,
the density of precipitates can be easily changed. This is
effective where there is a desire for obtaining requested density
of precipitates to a level of contamination in a device fabrication
process.
[0231] FIG. 27 shows the oxygen concentration dependence of density
of precipitates at T.sub.9=700.degree. C., t.sub.5=30 min,
R.sub.5=3.degree. C./min, T.sub.10=850.degree. C. and t.sub.6=1 hr
(Experimental Example 16). This result indicates that in either
crystal positions there is a trend that as the oxygen concentration
increases, the density of oxide precipitates increases. It is also
found that when the oxygen concentration is about 16 ppma or
higher, the high density of precipitates of the order of
10.sup.9/cm.sup.3 or higher can be surely achieved. This is because
the higher the oxygen concentration is, the higher the density of
grown-in precipitation nuclei becomes and the larger the sizes of
oxide precipitates becomes, thereby a proportion of the detectable
precipitates being increased after the heat treatment at
1000.degree. C. for 2 hrs.
[0232] FIG. 28 shows the density of precipitates at 0 min, 5 min,
15 min, 30 min and 45 min of the t.sub.5 time (Experimental Example
17). In this case, the other conditions were as follows:
T.sub.9=700.degree. C., R.sub.5=3.degree. C./min, T.sub.10.degree.
C.=850.degree. C. and t.sub.6=1 hr. Oxygen concentrations are 16
ppma to 18 ppma. When the t.sub.5 time is 15 min or longer, there
is a trend that the density of precipitates increases with time.
This is because as the t.sub.5 time is longer, the proportion of
grown-in precipitation nuclei to be grown without annihilation
becomes higher and new precipitation nuclei are generated during
the keeping of the t.sub.5 time. This result indicates that even in
the case where the t.sub.5 time is shorter than 15 min, the high
density of precipitates can be achieved, but in the case where the
t.sub.5 time is 15 min or longer, the density of precipitates is
further increased; it is found that the latter case is more
preferable.
[0233] FIG. 29 shows the density of oxide precipitates at 1.degree.
C./min, 3.degree. C./min and 5.degree. C./min of the
R.sub.5.degree. C./min (Experimental Example 18). In this case, the
other conditions were as follows: T.sub.9=700.degree. C.,
t.sub.5=30 min, T.sub.10=850.degree. C., t.sub.6=1 hr. Oxygen
concentrations are 16 ppma to 18 ppma. The lower the
R.sub.5.degree. C./min is, the higher the density of precipitates
becomes. The reason why when the R.sub.5.degree. C./min is faster,
the density of precipitates becomes lower is that there becomes
higher the proportion of precipitation nuclei which are annihilated
without growth in the temperature raising step. This result
indicates that the high density of precipitates of the order of
10.sup.9/cm.sup.3 or higher can be surely attained at the
R.sub.5.degree. C./min of about 5.degree. C./min or less.
[0234] FIG. 30 shows the density of oxide precipitates at
750.degree. C., 800.degree. C., 850.degree. C. and 900.degree. C.
of the T.sub.10 (Experimental Example 19). In this case, the other
conditions were as follows: T.sub.9=700.degree. C., t.sub.5=30 min,
R.sub.5=3.degree. C./min and t.sub.6=2 hrs. Oxygen concentrations
are 18 ppma to 20 ppma. When the T.sub.10.degree. C. is 800.degree.
C. or higher, the density of precipitates becomes high
independently of the temperature. The reason why in case of
750.degree. C., the density of precipitates is low is that due to a
slow growth rate of precipitates, the precipitates cannot grow to
sizes at which they can survive in an epi step; they are
annihilated in the epi step. This result shows that in order to
obtain high density of precipitates, the T.sub.10.degree. C. is
preferably about 800.degree. C. or higher.
[0235] FIG. 31 shows the density of oxide precipitates at 30 min,
60 min and 120 min of the t.sub.6 time (Experimental Example 20).
In this case, the other conditions were as follows:
T.sub.9=700.degree. C., t.sub.5=30 min, R.sub.5=3.degree. C./min,
T.sub.10=850.degree. C. Oxygen concentrations are 18 ppma to 20
ppma. The density of oxide precipitates becomes high almost
independently of the t.sub.6 time. This result indicates that the
high density of oxide precipitates can be surely attained in the
t.sub.6 time of about 30 min or longer. Furthermore, in
Experimental Example 20, when the t.sub.6 time is 0 min, it turned
out that the density of oxide precipitates detected with the LST
was lower than 1.times.10.sup.9/cm.sup.3.
[0236] As described above, by setting the T.sub.9.degree. C. to
about 700.degree. C. or lower, the R.sub.5.degree. C./min to about
5.degree. C./min or less, the T.sub.10.degree. C. to about
800.degree. C. or higher and the t.sub.6 time to about 30 min or
longer, respectively, the higher density of oxide precipitates in
an epi wafer generated in a device fabrication process can be
achieved. That is, there can be provided an epi wafer with an
excellent IG capability. Note that the density of oxide
precipitates each of the epi wafers manufactured in Experimental
Examples 15 to 20 was measured with the LST prior to heat treatment
at 1000.degree. C. for 2 hrs, and it is confirmed that any wafer
had the density of 1.times.10.sup.7/cm.sup.3 or lower
Comparative Example 1
[0237] There were prepared wafers similar to the wafers prepared in
the Experimental Examples 15 to 20, which were mirror-polished
without heat treatment and then epi wafers were manufactured
therefrom. The epi wafers were subjected to heat treatment at
1000.degree. C. for 2 hrs, which simulates a device fabrication
process and thereafter the density of precipitates thereof was
measured with the LST.
[0238] As a result of the measurement, even at the oxygen
concentration of about 19 ppma, the density of precipitates was
about 1.times.10.sup.7/cm.sup.3 or lower. This indicates that if a
silicon epitaxial wafer is not subjected to the heat treatment
according to the method for manufacturing a silicon epitaxial wafer
of the present invention, the silicon epitaxial wafer does not have
an IG capability.
Experimental Examples 21 to 25
[0239] There were prepared silicon wafers each having a diameter of
8 inches, a plane orientation of <100> and resistivity of
about 10 .OMEGA.cm, which were sliced from different two portions
(these portions belong to positions grown in the former half and
latter half of the crystal growth step and hereinafter may be
referred to as a crystal position A and crystal position B,
respectively) of a boron-doped silicon single crystal grown by
means of a CZ method. The wafers prepared here were chemical etched
wafers that are not subjected to mirror polishing. Oxygen
concentrations of the wafers are 16 ppma to 20 ppma (JEIDA
scale).
[0240] Then, the wafers were subjected to heat treatment according
to the procedure shown in FIGS. 9 and 10. That is, the wafer was
kept at T.sub.11.degree. C. for t.sub.7 time, thereafter the
temperature was raised from the T.sub.11.degree. C. to
T.sub.12.degree. C. at a rate of R.sub.6.degree. C./min and kept at
the T.sub.12.degree. C. for t.sub.8 time. After the keeping, the
temperature in a heat treatment furnace was lowered to 700.degree.
C. at a rate of 2.degree. C./min and then the wafers were unloaded
from the heat treatment furnace.
[0241] After the heat-treated wafers were mirror-polished, a
silicon single crystal layer about 3 .mu.m thick was deposited on
each of the wafers by epitaxial growth at about 1100.degree. C. to
thereby obtain epi wafers.
[0242] Without the epi wafers being subjected to any heat
treatment, the density of oxide precipitates in the wafer bulk
below the epitaxial layer of each epi wafer was measured with the
LST. With the LST, there are detectable oxide precipitates having
sizes of about 40 nm or larger in diameter.
[0243] FIG. 32 shows the density of precipitate at 600.degree. C.,
700.degree. C. and 800.degree. C. of the T.sub.11.degree. C.
(Experimental Example 21). In this case, the other conditions were
as follows: t.sub.7=30 min, R.sub.6=3.degree. C./min,
T.sub.12=1000.degree. C. and t.sub.8=2 hrs. Oxygen concentrations
are 16 ppma to 18 ppma. The lower the T.sub.11.degree. C. is, the
higher the density of precipitates becomes. This is because there
is increased the density of grown-in precipitation nuclei which
grow at a lower temperature without annihilation. In order to
obtain an excellent IG capability, the density of precipitates is
desirably on the order of 10.sup.9/cm.sup.3 or higher. This
indicates that the T.sub.11.degree. C. is preferably about
700.degree. C. or lower. Moreover, as shown in FIG. 32, by changing
the T.sub.11.degree. C., the density of precipitates can be easily
changed. This is effective where there is a desire for obtaining
requested density of precipitates to a level of contamination in a
device fabrication process.
[0244] FIG. 33 shows the oxygen concentration dependence of density
of precipitates at T.sub.11=700.degree. C., t.sub.7=30 min,
R.sub.6=3.degree. C./min, T.sub.12=1000.degree. C. and t.sub.8=2
hrs (Experimental Example 22). This result indicates that in either
crystal positions there is a trend that as the oxygen concentration
increases, the density of oxide precipitates increases. It is also
found that when the oxygen concentration is about 16 ppma or
higher, the high density of precipitates of the order of
10.sup.9/cm.sup.3 or higher can be surely achieved. This is because
the higher the oxygen concentration is, the higher the density of
grown-in precipitation nuclei becomes and the larger the sizes of
oxide precipitates becomes, thereby a proportion of the detectable
precipitates being increased.
[0245] FIG. 34 shows the density of precipitates at 0 min, 5 min,
15 min, 30 min and 45 min of the t.sub.7 time (Experimental Example
23). In this case, the other conditions were as follows:
T.sub.11=700.degree. C., R.sub.6=3.degree. C./min,
T.sub.12=1050.degree. C. and t.sub.8=1 hr. Oxygen concentrations
are 18 ppma to 20 ppma. When the t.sub.7 time is about 15 min or
longer, the density of precipitates increases with time. This is
because as the t.sub.7 time is longer, the proportion of grown-in
precipitation nuclei to be grown without annihilation becomes
higher and new precipitation nuclei are generated during the
keeping of the t.sub.7 time. This result indicates that even in the
case where the t.sub.7 time is shorter than 15 min, the high
density of precipitates can be achieved, but in the case where the
t.sub.7 time is 15 min or longer, the density of precipitates is
further increased; it is found that the later case is more
preferable.
[0246] FIG. 35 shows the density of oxide precipitates at 1.degree.
C./min, 3.degree. C./min and 5.degree. C./min of the
R.sub.6.degree. C./min (Experimental Example 24). In this case, the
other conditions were as follows: T.sub.11=700.degree. C.,
t.sub.7=30 min, T.sub.12=1050.degree. C., t.sub.8=1 hr. Oxygen
concentrations are 18 ppma to 20 ppma. The lower the
R.sub.6.degree. C./min is, the higher the density of precipitates
becomes. The reason why when the R.sub.6.degree. C./min is faster,
the density of precipitates becomes lower is that there becomes
higher the proportion of precipitation nuclei which are annihilated
without growth in the temperature raising step. This result
indicates that in order to achieve the high density of precipitates
of the order of 10.sup.9/cm.sup.3 or higher, the R.sub.6.degree.
C./min is preferably about 5.degree. C./min or lower, more
preferably about 3.degree. C./min or lower.
[0247] FIG. 36 shows the density of oxide precipitates at 30 min,
60 min and 120 min of the t.sub.8 time (Experimental Example 25).
In this case, the other conditions were as follows:
T.sub.11=700.degree. C., t.sub.7=30 min, R.sub.6.degree.
C./min=3.degree. C./min and T.sub.12=1050.degree. C. Oxygen
concentrations are 18 ppma to 20 ppma. The density of oxide
precipitates is high almost independently of the t.sub.8 time. This
result indicates that the high density of oxide precipitates can be
surely attained in the t.sub.8 time of about 30 min or longer.
Experimental Example 26
[0248] There were prepared wafers similar to the wafers used in
Experimental Examples 21 to 25. The wafers were
mirror-polished.
[0249] Then, according to a heat treatment procedure shown in FIGS.
9 and 10, the following conditions were applied:
T.sub.11=700.degree. C., t.sub.7=30 min, R.sub.6=3.degree. C./min
and t.sub.8=2 hrs, and there were performed 4 levels of heat
treatment where the T.sub.12.degree. C. was changed to 950.degree.
C., 1000.degree. C., 1050.degree. C. and 1100.degree. C.,
respectively. After the keeping at the T.sub.12.degree. C., the
temperature in a heat treatment furnace was lowered to 700.degree.
C. at 2.degree. C./min and then the wafers were unloaded from the
heat treatment furnace.
[0250] A silicon single crystal layer about 3 .mu.m thick was
deposited on each of the heat-treated wafers by epitaxial growth at
about 1100.degree. C. to thereby obtain epi wafers. Without the epi
wafers being subjected to any heat treatment, the density of oxide
precipitates was measured with the LST.
[0251] FIG. 37 shows a relationship between the density of
precipitates and the T.sub.12.degree. C. in this Experimental
Example. Oxygen concentrations are 18 ppma to 20 ppma. When the
T.sub.12.degree. C. is 1000.degree. C. or higher, the density of
precipitates becomes higher. The reason why the density of oxide
precipitates lowers at 950.degree. C. is that a growth rate of
precipitates is slower than at 1000.degree. C. or higher and hence
sizes of precipitates are too small to be detectable with the LST.
This result indicates that in order to generate precipitates of
large sizes at high density at a stage prior to a device
fabrication process, the T.sub.12.degree. C. is preferably about
1000.degree. C. or higher.
[0252] As described above, by setting the T.sub.11.degree. C. to
about 700.degree. C. or lower, the R.sub.6.degree. C./min to about
5.degree. C./min or less, the T.sub.12.degree. C. to about
1000.degree. C. or higher and the t.sub.8 time to about 30 min or
longer, respectively, there can be achieved the higher density of
oxide precipitates of sizes each having a gettering capability in
an epi wafer prior to loading it into a device fabrication process.
That is, an epi wafer having an excellent IG capability can be
provided.
Comparative Example 2
[0253] There were prepared wafers similar to the wafers prepared in
Experimental Examples 21 to 25, which were mirror-polished without
heat treatment and then epi wafers were manufactured therefrom. The
epi wafers were subjected to heat treatment at 1000.degree. C. for
2 hrs, which simulates a device fabrication process and thereafter
the density of precipitates thereof was measured with the LST.
[0254] As a result of the measurement, the density of precipitates
in the epi wafer having the relatively higher oxygen concentration
of about 19 ppma was about 1.times.10.sup.7/cm.sup.3 or lower even
after the wafer was subjected to the heat treatment which simulates
a device fabrication process. This indicates that if a silicon
epitaxial wafer is not subjected to the heat treatment according to
the heat-treating method of the present invention, the silicon
epitaxial wafer does not have an IG capability.
Capability of Exploitation in Industry:
[0255] As described above, the first aspect of the method for
manufacturing a silicon wafer of the present invention makes it
possible to generate large oxide precipitates of high density in
even a wafer containing almost no oxygen precipitation nuclei, so
it is possible to impart an excellent IG capability even to a wafer
to be used in a device fabrication process having a low temperature
and a short time in a stable manner at a stage prior to the device
fabrication process.
[0256] The second aspect of the method for manufacturing a silicon
wafer of the present invention makes it possible by simple heat
treatment to simultaneously realize growth of oxide precipitates in
an interior of a wafer and more perfect annihilation of oxygen
precipitation nuclei in the vicinity of a surface of the wafer due
to out-diffusion of oxygen, thereby a DZ-IG structure being
effectively formed, and in addition, by applying a wafer containing
almost no COPs nor voids, to form a high quality DZ-IG structure
containing almost not only no oxide precipitates but also no COPs
nor voids.
[0257] The third aspect of the method for manufacturing a silicon
wafer of the present invention makes it possible by simple heat
treatment to simultaneously realize growth of oxide precipitates in
an interior of a wafer and annihilation of oxygen precipitation
nuclei in the vicinity of a surface of the wafer due to
out-diffusion of oxygen, thereby a DZ-IG structure being
effectively formed. Furthermore, by applying a wafer containing
almost no COPs nor voids to this method, it is possible to form a
DZ-IG structure containing almost not only no oxide precipitates
but also no COPs nor voids.
[0258] The first aspect of the method for manufacturing a silicon
epitaxial wafer of the present invention makes it possible to
effectively use grown-in precipitation nuclei with annihilation
thereof being suppressed to the lowest level possible, furthermore
to efficiently grow sizes thereof for a short time, and to provide
an epi wafer to which an excellent IG capability is imparted even
in a device fabrication process having a low temperature and a
short time.
[0259] The second aspect of the method for manufacturing a silicon
epitaxial wafer of the present invention makes it possible by
applying simple and short heat treatment prior to epitaxial growth
to obtain a silicon epitaxial wafer in which there were generated
oxide precipitates of sizes each having a gettering capability
prior to loading the wafer into a device fabrication process, and
therefore, to efficiently provide an epitaxial wafer capable of
exerting a sufficient gettering capability even in a recent device
fabrication process having a low temperature and a short time.
[0260] The first aspect of the silicon epitaxial wafer of the
present invention has a sufficient gettering capability even in a
device fabrication process having a low temperature and a short
time.
[0261] The second aspect of the silicon epitaxial wafer of the
present invention has a sufficient gettering capability even in a
device fabrication process having a low temperature and a short
time from the initial stage of the process.
* * * * *