U.S. patent application number 16/339941 was filed with the patent office on 2020-02-13 for epitaxial silicon wafer and method for manufacturing epitaxial silicon wafer.
This patent application is currently assigned to SUMCO CORPORATION. The applicant listed for this patent is SUMCO CORPORATION. Invention is credited to Kazuya KODANI, Toshiaki ONO, Kazuhisa TORIGOE.
Application Number | 20200051817 16/339941 |
Document ID | / |
Family ID | 61756514 |
Filed Date | 2020-02-13 |
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United States Patent
Application |
20200051817 |
Kind Code |
A1 |
KODANI; Kazuya ; et
al. |
February 13, 2020 |
EPITAXIAL SILICON WAFER AND METHOD FOR MANUFACTURING EPITAXIAL
SILICON WAFER
Abstract
A manufacturing method of an epitaxial silicon wafer includes:
an epitaxial-film formation step for forming an epitaxial film made
of silicon on a surface of a silicon wafer in a trichlorosilane gas
atmosphere; and a nitrogen-concentration setting step for setting
the nitrogen concentration of the surface of the epitaxial film
through inward diffusion from a nitride film on the epitaxial film,
the nitride film being formed by subjecting the silicon wafer
provided with the epitaxial film through the epitaxial-film
formation step to a heat treatment in a nitrogen atmosphere.
Inventors: |
KODANI; Kazuya; (Tokyo,
JP) ; ONO; Toshiaki; (Tokyo, JP) ; TORIGOE;
Kazuhisa; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMCO CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
SUMCO CORPORATION
Tokyo
JP
|
Family ID: |
61756514 |
Appl. No.: |
16/339941 |
Filed: |
September 12, 2017 |
PCT Filed: |
September 12, 2017 |
PCT NO: |
PCT/JP2017/032846 |
371 Date: |
April 5, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/322 20130101;
H01L 29/66636 20130101; H01L 29/7848 20130101; H01L 21/324
20130101; H01L 29/34 20130101; H01L 21/02 20130101; H01L 21/205
20130101; H01L 29/165 20130101; H01L 29/161 20130101; H01L 29/16
20130101 |
International
Class: |
H01L 21/205 20060101
H01L021/205; H01L 29/16 20060101 H01L029/16 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2016 |
JP |
2016-198825 |
Claims
1. An epitaxial silicon wafer comprising: a silicon wafer; and an
epitaxial film made of silicon and formed on a surface of the
silicon wafer, wherein a nitrogen concentration of the surface of
the epitaxial film is 5.0.times.10.sup.13 atoms/cm.sup.3 or
more.
2. The epitaxial silicon wafer according to claim 1, wherein the
nitrogen concentration at a depth of 3 .mu.m from the surface of
the epitaxial film is 5.0.times.10.sup.13 atoms/cm.sup.3 or
more.
3. The epitaxial silicon wafer according to claim 1, further
comprising: a strain layer provided on the surface of the epitaxial
film, the strain layer causing a film stress ranging from 10 MPa to
1000 MPa.
4. The epitaxial silicon wafer according to claim 1, wherein an
oxygen concentration of the silicon wafer ranges from
10.times.10.sup.17 atoms/cm.sup.3 to 15.times.10.sup.17
atoms/cm.sup.3 according to ASTM 1979.
5. A manufacturing method of an epitaxial silicon wafer, the method
comprising forming an epitaxial film made of silicon on a surface
of a silicon wafer in a trichlorosilane gas atmosphere; and setting
a nitrogen concentration of the surface of the epitaxial film
through inward diffusion from a nitride film on the epitaxial film,
the nitride film being formed by subjecting the silicon wafer
provided with the epitaxial film through the forming of the
epitaxial film to a heat treatment in a nitrogen atmosphere.
6. The manufacturing method of an epitaxial silicon wafer according
to claim 5, further comprising: removing a native oxide on the
surface of the epitaxial film before the nitride film is formed,
wherein the setting of the nitrogen concentration is conducted on
the silicon wafer after removing the native oxide.
7. The manufacturing method of an epitaxial silicon wafer according
to claim 6, wherein the native oxide is removed by subjecting the
silicon wafer to a heat treatment in one of an argon atmosphere, an
ammonia atmosphere, and a hydrogen atmosphere by using the same
heat treatment equipment as used in the setting of the nitrogen
concentration.
8. The manufacturing method of an epitaxial silicon wafer according
to claim 5, further comprising: entirely removing the nitride
film.
9. The manufacturing method of an epitaxial silicon wafer according
to claim 8, further comprising: forming a strain layer on the
surface of the epitaxial film exposed after the removing of the
nitride film, the strain layer causing a film stress ranging from
10 MPa to 1000 MPa.
10. The manufacturing method of an epitaxial silicon wafer
according to claim 5, wherein in the setting of the nitrogen
concentration, the heat treatment is conducted so that a
temperature X (degrees C.) is in a range from 850 degrees C. to
1400 degrees C., a heat treatment time Y (seconds) second or more
and a formula (1), Y.gtoreq.1.times.10.sup.34 exp(-0.084X) (1) is
satisfied.
Description
TECHNICAL FIELD
[0001] The present invention relates to an epitaxial silicon wafer
and a manufacturing method of the epitaxial silicon wafer.
BACKGROUND ART
[0002] In order to enhance performance of silicon devices, which
have been increasingly microfabricated, strain is sometimes applied
on or near a surface (device active layer) of a wafer.
[0003] Examples of such a wafer as proposed include a strained
silicon wafer including an SiGe layer epitaxially grown on a
monocrystalline silicon substrate and a strained Si layer
epitaxially grown on the SiGe layer, a wafer whose surface is
nitrided instead of forming the SiGe layer, and an SOI wafer.
[0004] Tensile strain is caused in the above strained Si layer due
to the SiGe layer having a larger lattice constant than that of Si,
The tensile strain changes the band structure of Si to remove
degeneracy, thereby enhancing carrier mobility. Accordingly, the
use of the strained Si layer as a channel region allows for the
carrier mobility to be 1.5 times or more as fast as that in a
semiconductor substrate made of an ordinary bulk silicon. The
strained silicon wafer is thus suitable for high-speed MOSFET,
MODFET, HEMT and the like.
[0005] However, since a very large film stress is caused by the
strain applied on or near the surface of the strained silicon
wafer, dislocations sometimes occur due to the strain toward the
surface of the wafer. In view of the above, studies have been made
on a method for reducing the dislocations (see, for instance,
Patent Literature 1).
[0006] In the method disclosed in Patent Literature 1, heat
treatment for setting oxygen concentration is conducted after
epitaxial growth to enhance strength of the epitaxial film by
increasing an oxygen concentration of the surface of the epitaxial
film through outward diffusion from bulk silicon and inward
diffusion from the oxide film on the surface.
CITATION LIST
Patent Literature(s)
[0007] Patent Literature 1: JP 2013-70091 A
SUMMARY OF THE INVENTION
Problem(s) to be Solved by the Invention
[0008] However, in view of an increase in stress applied on an
epitaxial film due to recent use of a three-dimensional device
structure, sufficient strength cannot necessarily be obtained by
increasing oxygen concentration as disclosed in Patent Literature
1.
[0009] An object of the invention is to provide an epitaxial
silicon wafer capable of reducing occurrence of dislocations, and a
manufacturing method of the epitaxial silicon wafer.
Means for Solving the Problem(s)
[0010] As a result of intensive studies, the inventors of the
invention have found that the occurrence of dislocations can be
inhibited by increasing nitrogen concentration of a surface of an
epitaxial film irrespective of film stress caused by the formation
of the strain layer. The reason for the above is speculated as
follow.
[0011] It is speculated that film stress, which is caused when a
strain layer is formed on an epitaxial film, initially causes small
dislocations at an interface between the strain layer and the
epitaxial film, and then the small dislocations extends into the
epitaxial film. It is speculated that, with the increased nitrogen
concentration in the epitaxial film, nitrogen locks to the small
dislocations at the interface, thereby inhibiting the extension of
the dislocations toward into the epitaxial film.
[0012] The invention has been made based on the above findings.
[0013] An epitaxial silicon wafer according to an aspect of the
invention includes: a silicon wafer; and an epitaxial film made of
silicon and formed on a surface of the silicon wafer, in which a
nitrogen concentration of the surface of the epitaxial film is
5.0.times.10.sup.13 atoms/cm.sup.3 or more.
[0014] According to the above aspect of the invention, an epitaxial
silicon wafer capable of reducing occurrence of dislocations
irrespective of the presence of a strain layer on an epitaxial film
can be provided.
[0015] It should be noted that the nitrogen concentration of the
surface of the epitaxial film according to the aspect of the
invention refers to nitrogen concentration in a depth profile
measured using an SIMS (Secondary Ion Mass Spectrometer) at a depth
ranging from 80 nm to 200 nm, preferably at a depth of 100 nm. The
nitrogen concentration is so defined as above to exclude the
nitrogen concentration of the topmost surface of the silicon wafer,
which cannot be correctly measured by an SIMS due to influence of
sample contamination, and to correctly determine the influence of
inward diffusion.
[0016] In the epitaxial silicon wafer according to the above aspect
of the invention, it is preferable that the nitrogen concentration
at a depth of 3 .mu.m from the surface of the epitaxial film is
5.0.times.10.sup.13 atoms/cm.sup.3 or more.
[0017] According to the above arrangement, even when the nitrogen
concentration of the surface of the epitaxial film is decreased by
a heat treatment during a device process, the nitrogen
concentration at the above depth can be kept at a level capable of
reducing the occurrence of dislocations.
[0018] The epitaxial silicon wafer according to the above aspect of
the invention preferably further include: a strain layer provided
on the surface of the epitaxial film, the strain layer causing a
film stress ranging from 10 MPa to 1000 MPa.
[0019] According to the above arrangement, an epitaxial silicon
wafer capable of accelerating carrier mobility and reducing the
occurrence of dislocations can be provided.
[0020] In the epitaxial silicon wafer according to the above aspect
of the invention, it is preferable that an oxygen concentration of
the silicon wafer ranges from 10.times.10.sup.17 atoms/cm.sup.3 to
15.times.10.sup.17 atoms/cm.sup.3 according to ASTM 1979.
[0021] When the oxygen concentration in the silicon wafer is less
than 10.times.10.sup.17 atoms/cm.sup.3, the strength of the
epitaxial film may become insufficient. Meanwhile, when the oxygen
concentration in the silicon wafer exceeds 15.times.10.sup.17
atoms/cm.sup.3, defects due to oxygen may be generated in the
epitaxial film.
[0022] The above deficiencies can be restrained by the above
arrangement.
[0023] A manufacturing method of an epitaxial silicon wafer
according to another aspect of the invention includes: forming an
epitaxial film made of silicon on a surface of a silicon wafer in a
trichlorosilane gas atmosphere; and setting a nitrogen
concentration of the surface of the epitaxial film through inward
diffusion from a nitride film on the epitaxial film, the nitride
film being formed by subjecting the silicon wafer provided with the
epitaxial film through the forming of the epitaxial film to a heat
treatment in a nitrogen atmosphere.
[0024] An epitaxial film may he formed on a silicon wafer whose
nitrogen concentration is increased, and outward diffusion from the
silicon wafer may be used in order to increase the nitrogen
concentration of the surface of the epitaxial film, for instance.
However, since the diffusion rate of nitrogen is relatively fast,
the outwardly diffusing nitrogen in the above method is released to
the outside through the epitaxial film, failing to sufficiently
increasing the nitrogen concentration of the surface of the
epitaxial film.
[0025] In contrast, since the inward diffusion from the nitride
film formed on the epitaxial film is used in the above aspect of
the invention, the nitrogen concentration of the surface of the
epitaxial film can be increased to a level capable of reducing the
occurrence of the dislocations even when a strain layer is
formed.
[0026] The manufacturing method of an epitaxial silicon wafer
according to the above aspect of the invention preferably further
include: removing a native oxide on the surface of the epitaxial
film before the nitride film is formed, in which the setting of the
nitrogen concentration is conducted on the silicon wafer after
removing the native oxide.
[0027] For instance, when the epitaxial film is exposed to an
oxygen-containing atmosphere in transferring the epitaxial silicon
wafer to heat treatment equipment for forming a nitride film,
native oxide is formed on the surface of the epitaxial film. The
native oxide present on the surface of the epitaxial film may
inhibit the inward diffusion of nitrogen to the epitaxial film.
[0028] According to the above arrangement, the native oxide on the
surface of the epitaxial film is removed before the nitride film is
formed to promote the inward diffusion of nitrogen to the epitaxial
film.
[0029] In the manufacturing method of an epitaxial silicon wafer
according to the above aspect of the invention, it is preferable
that the native oxide is removed by subjecting the silicon wafer to
a heat treatment in one of an argon atmosphere, an ammonia
atmosphere, and a hydrogen atmosphere by using the same heat
treatment equipment as used in the setting of the nitrogen
concentration.
[0030] According to the above arrangement, the nitride film can be
formed without exposing the epitaxial film to an oxygen atmosphere
simply by changing the atmosphere in the heat treatment equipment
after removing the native oxide.
[0031] The manufacturing method of an epitaxial silicon wafer
according to the above aspect of the invention preferably further
includes: entirely removing the nitride film.
[0032] The nitride film, which is useful for the adjustment of
nitrogen concentration in the epitaxial film, needs to be removed
before fabrication of devices.
[0033] The removal process of the nitride film in a device
manufacturer can be omitted according to the above arrangement.
Further, the above arrangement allows an epitaxial silicon wafer
manufacturer to inspect the surface of the epitaxial film, so that
quality guaranteed products can be delivered to device
manufacturers.
[0034] The manufacturing method of an epitaxial silicon wafer
according to the above aspect of the invention preferably further
includes: forming a strain layer on the surface of the epitaxial
film exposed after the removing of the nitride film, the strain
layer causing a film stress ranging from 10 MPa to 1000 MPa.
[0035] In the manufacturing method of an epitaxial silicon wafer
according to the above aspect of the invention, it is preferable
that, in the setting of the nitrogen concentration, the heat
treatment is conducted so that a temperature X (degrees C.) is in a
range from 850 degrees C. to 1400 degrees C., a heat treatment time
Y (seconds) is 1 second or more and a formula (1),
Y.gtoreq.1.times.10.sup.34 exp(-0.084X) (1) is satisfied.
[0036] According to the above arrangement, the nitrogen
concentration of the surface of the epitaxial film can be increased
to a level capable of reducing the occurrence of dislocations
irrespective of the presence of the strain layer simply by
referring to the above formula (1).
BRIEF DESCRIPTION OF DRAWING(S)
[0037] FIG. 1 is a cross section showing an epitaxial silicon wafer
according to an exemplary embodiment of the invention.
[0038] FIG. 2 is a flowchart showing a manufacturing method of the
epitaxial silicon wafer.
[0039] FIG. 3 illustrates an example of a semiconductor substrate,
on which a strain layer is formed in a device process experienced
by the epitaxial silicon wafer.
[0040] FIG. 4 is a graph showing a relationship between a
temperature and time of a heat treatment in a
nitrogen-concentration setting step, and occurrence of dislocations
according to Examples of the invention.
DESCRIPTION OF EMBODIMENT(S)
[0041] An Exemplary embodiment of the invention will be described
below with reference to the attached drawings.
Structure of Epitaxial Silicon Wafer
[0042] As shown in FIG. 1, an epitaxial silicon wafer EW includes a
silicon wafer WF, and an epitaxial film EP provided on a surface of
the silicon wafer WF.
[0043] A film thickness T of the epitaxial film EP is preferably in
a range from 0.1 .mu.m to 20 .mu.m. The film thickness is
preferably 0.1 .mu.m or more because STI (Shallow Trench Isolation)
structure is usually formed at a depth of 0.1 .mu.m or more from
the surface of the epitaxial film EP. Further, the film thickness
of 20 .mu.m or less is preferable because, when the film thickness
exceeds 20 .mu.m, warpage caused by crystal lattice mismatch
between the silicon wafer WF and the epitaxial film EP exerts large
influence on the film stress at the surface, whereby it is possible
that the effect in reducing the occurrence of dislocations by
inward diffusion of nitrogen is impaired. It should be noted that
the film thickness T is further preferably more than 3 .mu.m and 10
.mu.m or less.
[0044] The nitrogen concentration of the surface of the epitaxial
film EP is preferably in a range from 5.0.times.10.sup.13
atoms/cm.sup.3 to 5.0.times.10.sup.15 atoms/cm.sup.3, more
preferably in a range from 1.times.10.sup.14 atoms/cm.sup.3 to
3.times.10.sup.15 atoms/cm.sup.3. It is preferable that nitrogen is
introduced at or below solid solubility limit of silicon crystal.
However, in view of the possibility of generating defects in the
epitaxial film EP derived from nitrogen due to long-duration heat
treatment in the device process, the above concentration range is
preferable. The nitrogen concentration is preferably measured at a
depth D of 80 nm to 200 nm from the surface, more preferably at the
depth D of 100 nm.
[0045] The nitrogen concentration measured at the depth D of 3
.mu.m from the surface of the epitaxial film EP is preferably
5.0.times.10.sup.13 atoms/cm.sup.3 or more. The point at the depth
D of 3 .mu.m is within the epitaxial film EP when the film
thickness T of the epitaxial film EP exceeds 3 .mu.m, and within
the silicon wafer WF when the film thickness T is 3 .mu.m or
less.
[0046] With the nitrogen concentration of 5.0.times.10.sup.13
atoms/cm.sup.3 or more at the point of the depth D of 3 .mu.m
(irrespective of whether the point is within the silicon wafer WF),
even when nitrogen in the surface of the epitaxial film EP is
released through outward diffusion by the heat treatment during the
device process or the like, nitrogen is fed to the surface of the
epitaxial film EP from the depth D of 3 .mu.m, thereby reducing
dislocations.
[0047] Oxygen concentration of the silicon wafer WF is preferably
in a range from 10.times.10.sup.17 atoms/cm.sup.3 to
15.times.10.sup.17 atoms/cm.sup.3 (ASTM1979). With the oxygen
concentration being set within the above range, the sufficient
strength of the epitaxial film EP can be ensured and defects due to
oxygen can be reduced on the epitaxial film EP.
Manufacturing Method of Epitaxial Silicon Wafer
[0048] As shown in FIG. 2, a manufacturing method of the epitaxial
silicon wafer EW having the above properties includes
wafer-preparation step S1, epitaxial-film formation step S2, native
oxide removal step S3, nitrogen-concentration setting step S4,
nitride-film removal step S5, strain-layer formation step S6, and
heat-treatment step S7.
[0049] It should be noted that at least one of the native oxide
removal step S3, the nitride-film removal step S5, the strain-layer
formation step S6, and the heat-treatment step S7 may be
omitted.
[0050] In the wafer-preparation step S1, a silicon single crystal
ingot pulled up by CZ (Czochralski) method, MCZ
(Magnetic-field-applied Czochralski) method or the like is
subjected to necessary steps selected from slicing, chamfering,
grinding, lapping, etching, polishing, washing, heat treatment such
as DK (Donor Killer) treatment, and the like to be formed into the
silicon wafer WF having a mirror-polished surface.
[0051] In the epitaxial-film formation step S2, the epitaxial film
EP having a predetermined film thickness T is formed on the silicon
wafer WF to provide the epitaxial silicon wafer EW. At this time,
the epitaxial film is formed in a gas atmosphere (e.g.
trichlorosilane) at a treatment temperature ranging from 1150 to
1280 degrees C. It should be noted that necessary dopant(s) such as
boron and phosphorus may be added.
[0052] In the native oxide removal step S3, a native oxide formed
on the epitaxial film EP is removed. The removal of the native
oxide during the native oxide removal step S3 may be conducted by,
for instance, loading the epitaxial silicon wafer EW in heat
treatment equipment, which is to be used in the
nitrogen-concentration setting step S4, and subjecting the
epitaxial silicon wafer EW to a heat treatment in an atmosphere of
one of argon atmosphere, ammonia atmosphere and hydrogen
atmosphere.
[0053] The heat treatment temperature is preferably in a range from
1000 to 1350 degrees C. in any of the argon atmosphere, ammonia
atmosphere, hydrogen atmosphere, or a mixture atmosphere of
combination of at least two of the argon atmosphere, ammonia
atmosphere, and hydrogen atmosphere. When the temperature is lower
than 1000 degrees C., it is possible that the native oxide is not
completely removed. Meanwhile, the upper limit is 1350 degrees C.
in view of the performance limitations of the heat treatment
equipment. The heat treatment time is preferably in a range from 1
second to 60 seconds. In order to completely remove the native
oxide, the epitaxial silicon wafer is preferably subjected to heat
treatment of 1 or more seconds. Further, the heat treatment time is
preferably as short as possible in view of productivity. When the
heat treatment temperature is in a range from 1000 to 1350 degrees
C., it is believed that the native oxide can be sufficiently
removed by a heat treatment of 60 seconds or less.
[0054] In the nitrogen-concentration setting step S4, the epitaxial
silicon wafer EW, from which the native oxide is removed, is
subjected to a heat treatment in a nitrogen atmosphere to form a
nitride film on the epitaxial film EP and the nitrogen
concentration of the surface of the epitaxial film EP is set
through inward diffusion from the nitride film. At this time, the
nitrogen concentration in the silicon wafer WF is set as well as
the nitrogen concentration on and inside of the surface of the
epitaxial film EP.
[0055] In the nitrogen-concentration setting step S4, it is
preferable that the heat treatment is conducted such that the
temperature X (degrees C.) is in a range from 850 degrees C. to
1400 degrees C., and the heat treatment time Y (seconds) is 1
second or more and satisfies the following formula (1).
Y.gtoreq.1.times.10.sup.34 exp(-0.084X) (1)
[0056] The heat treatment may be conducted in any manner (e.g.
batch process using a vertical furnace, rapid heating/cooling heat
treatment using a single wafer processing furnace, and the like) as
long as the nitrogen concentration of the epitaxial film EP can be
set in the above-described range. The atmosphere gas for the heat
treatment may consist of ammonia or may be a mixture gas of argon
and ammonia. The heat treatment equipment used for the
nitrogen-concentration setting step S4 is preferably the same heat
treatment equipment as used in the native oxide removal step S3,
since the nitride film can be formed only by changing the
atmosphere in the heat treatment equipment after removing the
native oxide.
[0057] In the nitride-film removal step S5, the nitride film used
for the setting of the nitrogen concentration of the epitaxial film
EP is entirely removed. In the nitride-film removal step S5, a part
of the epitaxial film EP near the surface thereof may be removed
together with the nitride film. At this time, the nitrogen
concentration of the surface of the epitaxial film EP after the
removal of the nitride film (and the part of the surface of the
epitaxial film EP) needs to be kept at a level capable of reducing
the occurrence of dislocations when a strain layer is formed. The
nitride film is exemplarily removed by polishing and/or
etching.
[0058] In the strain-layer formation step S6, the strain layer
causing a film stress ranging from 10 MPa to 1000 MPa is formed on
the surface of the epitaxial film EP exposed after the nitride-film
removal step S5.
[0059] The strain layer is partially formed on the surface of the
epitaxial film EP to form a part of devices. Specifically, as shown
in FIG. 3, the strain layer is SiGe film, SiC film or the like,
which causes film stress in a plane direction (shown by arrows TE)
of the surface of the epitaxial film EP and forms a source region
SC and a drain region DR at a part except for a part immediately
below a gate region GT. The strain layer as shown in FIG. 3 is not
exhaustive, but may be in any form and may be formed in any manner
as long as the film stress is caused.
[0060] It should be noted that the epitaxial silicon wafer EW of
the exemplary embodiment is subjected to the device process, a part
of which may be the strain-layer formation step S6.
[0061] The heat-treatment step S7 is performed, for instance, under
the same conditions as the heat treatment in the device process.
Since the nitrogen concentration of the surface of the epitaxial
film EP is set within the above range, the occurrence of
dislocations can be reduced even when a film stress ranging from 10
MPa to 1000 MPa is caused by the strain layer in the heat-treatment
step S7.
Advantages of Exemplary Embodiment(s)
[0062] As described above, since the nitrogen concentration of the
surface of the epitaxial film EP of the epitaxial silicon wafer EW
is set at 5.0.times.10.sup.13 atoms/cm.sup.3 or more, the
occurrence of the dislocations can be reduced even when the strain
layer is formed on the epitaxial film EP.
[0063] Further, since the inward diffusion from the nitride film
formed on the epitaxial film EP is used in order to set the
nitrogen concentration of the epitaxial silicon wafer EW, the
nitrogen concentration of the surface of the epitaxial film EP can
be increased to a level capable of reducing the occurrence of the
dislocations in the manufacturing method of the invention.
EXAMPLES
[0064] Next, the invention will be described in more detail below
with reference to Examples. However, it should be noted that the
scope of the invention is by no means limited by these
Examples.
Manufacturing Method of Samples
Experimental Example 1
[0065] A 2-.mu.m thick epitaxial film was grown on a silicon wafer,
which was mirror-polished after being cut from a 300-mm silicon
single crystal grown by Czochraiski (CZ) method, at a temperature
of approximately 1100 degrees C.
[0066] Subsequently, as shown in Table 1, the silicon wafer was
subjected to the native oxide removal step, as necessary, and the
heat treatment (nitrogen-concentration setting step) in a nitrogen
atmosphere under conditions shown in Table 1. The silicon wafer was
subjected to the treatment in an RTA (Rapid Thermal Annealing)
furnace when the treatment time was 180 seconds or less, or to the
treatment in a vertical furnace when the treatment time was more
than 180 seconds. After the heat treatment, the formed nitride film
was entirely removed to provide the epitaxial silicon wafers
according to Experimental Example 1 under 25 conditions shown in
Table 1.
[0067] It should be noted that heat treatment equipment used in the
nitrogen-concentration setting step was also used in the native
oxide removal step to apply a heat treatment for 60 seconds in a
hydrogen atmosphere of 1000 degrees C.
Experimental Examples 2, 3
[0068] Epitaxial silicon wafers according to Experimental Example 2
were obtained under 25 conditions shown in Table 1 in the same
process as in Experimental Example 1 except that the epitaxial film
was grown to be 4-.mu.m thick.
[0069] Epitaxial silicon wafers according to Experimental Example 3
were obtained under 25 conditions shown in Table 1 in the same
process as in Experimental
[0070] Example 1 except that the epitaxial film was grown to be
6-.mu.m thick.
TABLE-US-00001 TABLE 1 Nitrogen-concentration Nitrogen Result of
setting step Native concentration three- Temperature Time oxide at
3 .mu.m depth point X (degrees Y removal (atoms/ bending Conditions
C.) (sec.) step cm.sup.-3) test 1 800 3600 Yes <LOD (limit of NG
detection) 2 800 10800 Yes <LOD NG 3 850 12 Yes <LOD NG 4 850
120 Yes <LOD NG 5 850 300 Yes <LOD NG 6 850 600 Yes <LOD
NG 7 850 1200 No <LOD NG 8 850 1200 Yes 5.1 .times. 10.sup.13 OK
9 850 3600 Yes 5.5 .times. 10.sup.13 OK 10 850 10800 Yes 5.8
.times. 10.sup.13 OK 11 900 6 Yes <LOD NG 12 900 18 No <LOD
NG 13 900 18 Yes 5.0 .times. 10.sup.13 OK 14 900 180 No <LOD NG
15 900 180 Yes 1.1 .times. 10.sup.14 OK 16 900 1200 Yes 1.3 .times.
10.sup.14 OK 17 950 6 No <LOD NG 18 950 6 Yes 1.2 .times.
10.sup.14 OK 19 950 60 Yes 2.5 .times. 10.sup.14 OK 20 1000 6 No
<LOD NG 21 1000 6 Yes 3.9 .times. 10.sup.14 OK 22 1000 18 Yes
5.0 .times. 10.sup.14 OK 23 1100 6 No <LOD NG 24 1100 6 Yes 1.5
.times. 10.sup.15 OK 25 1200 6 Yes 3.0 .times. 10.sup.15 OK
Evaluation
Nitrogen Concentration of Epitaxial Silicon Wafer
[0071] The nitrogen concentration of the top layer of the epitaxial
film of the epitaxial silicon wafer prepared through the process of
Experimental Examples 1, 2, 3 (specifically, the concentration at
the depth of 3 .mu.m from the surface of the epitaxial film) was
measured using an SIMS.
[0072] The measurements were the same as in Experimental Examples
1, 2, 3. The results are shown in Table 1.
[0073] It should be noted that, since the thickness of the
epitaxial film in Experimental Example 1 was 2 .mu.m, the results
in Table 1 show the nitrogen concentration in the silicon wafer.
Since the thicknesses of the epitaxial films in Experimental
Examples 2, 3 were 4 .mu.m and 6 .mu.m, respectively, the results
in Table 1 show the nitrogen concentration in the epitaxial film.
The lower limit of detection was 5.0.times.10.sup.13
atoms/cm.sup.3.
[0074] As shown in Table 1, the nitrogen concentrations under the
conditions 7,12,14,17,20 and 23, in which the native oxide removal
step was not conducted, were less than the lower limit of
detection. In contrast, all of the nitrogen concentrations under
conditions 8,13,15,18,21 and 24, in which the
nitrogen-concentration setting step under the same conditions as in
the conditions 7,12,14,17,20 and 23 and the native oxide removal
step were conducted, were the lower limit of detection or more.
[0075] Accordingly, it has been demonstrated that the native oxide
removal step before the nitrogen-concentration setting step
promotes the inward diffusion of nitrogen into the epitaxial film
to increase the nitrogen concentration to a desired level.
[0076] It can be assumed that the samples with the nitrogen
concentration of the lower limit of detection or more in Table 1
have the nitrogen concentration of the lower limit of detection or
more in the surface of the epitaxial film. The reason for the above
is as follows.
[0077] With the use of outward diffusion from the silicon wafer,
the nitrogen concentration becomes lower toward the surface of the
epitaxial silicon wafer. However, in Experimental Examples, with
the use of inward diffusion from the nitride film formed on the
surface of the epitaxial silicon wafer, the nitrogen concentration
becomes higher toward the top layer.
Effect in Reducing Occurrence of Dislocations in Epitaxial Silicon
Wafer
[0078] Stress-applying test was conducted on the epitaxial silicon
wafers according to Experimental Example 2.
[0079] Initially, a measurement sample of 3-cm length and 1.5-cm
width was cut out from each of epitaxial silicon wafers. Next, a
linear impression with 100-nm depth, 50-.mu.m width and 1-mm length
was formed on the surface of each measurement sample (i.e. on the
surface of the epitaxial film). Then, a three-point bending test
was conducted on each measurement sample with a distance between
supports being 2 cm at a test temperature of 800 degrees C. At this
time, 2N load was applied so that tensile stress acts on the
surface of the measurement sample.
[0080] Subsequently, each measurement sample having been cooled to
the ambient temperature was light-etched by approximately 1 .mu.m
to check the presence of dislocation pits generated from the linear
impression using an optical microscope.
[0081] The results are shown in Table 1. It should be noted that
"NG" indicates that the dislocation pits were detected and "OK"
indicates that the dislocation pits were riot detected.
[0082] As shown in Table 1, the dislocation pits were detected in
all the samples with the nitrogen concentration of less than the
lower limit of detection at the depth of 3 .mu.m from the surface
of the epitaxial film. In contrast, the dislocation pits were not
detected in all the samples with the nitrogen concentration of the
epitaxial film having the lower limit of detection or more at the
depth of 3 .mu.m from the surface.
[0083] This is speculated to be because a high nitrogen
concentration increases critical stress for the occurrence of
dislocations and, consequently, reduces the occurrence of
dislocations, which is caused at the linear impression formed on
the surface of the silicon wafer as a result of concentration of
the stress applied in the three-point bending test. It is also
supposed that the same results are obtained in Examples 1,3 because
of the same nitrogen concentration.
[0084] From the above, it has been demonstrated that the nitrogen
concentration of 5.0.times.10.sup.13 atoms/cm.sup.3 or more
measured at the depth of 3 .mu.m from the surface of the epitaxial
film can reduce the occurrence of dislocations.
[0085] Further, it is speculated that the above range of the
nitrogen concentration at the depth of 3 .mu.m allows nitrogen to
be supplied from the 3-.mu.m depth to the surface of the epitaxial
film even when nitrogen in the surface of the epitaxial film is
released through outward diffusion due to the heat treatment during
the device process or the like, resulting in the reduction of the
occurrence of dislocations.
Mathematical Formulation of Heat Treatment Conditions in Nitrogen
Concentration Setting Step
[0086] The relationship between temperature X and time Y of the
heat treatment during the nitrogen-concentration setting step is
plotted in a graph shown in FIG. 4 for the conditions other than 7,
12, 14, 17, 20, and 23 in Table 1, in which the native oxide
removal step is not conducted. In the graph, the result "NG" of the
three-point bending test is plotted in "x", whereas the result "OK"
is plotted in ".smallcircle.."
[0087] The border between "OK" and "NG" is approximated by a linear
line represented by a formula (2) below, where the left and right
sides with respect to the linear line are "NG" region and "OK"
region, respectively.
Y=1.times.10.sup.34 exp(-0.084X) (2)
[0088] As a result, it has been found that the heat treatment
during the nitrogen-concentration setting step, which satisfies the
above formula (1), allows the nitrogen concentration at the depth
of 3 .mu.m from the surface of the epitaxial film to be
5.0.times.10.sup.13 atoms/cm.sup.3 or more.
EXPLANATION OF CODES
[0089] EP . . . epitaxial film, EW . . . epitaxial silicon wafer,
WF . . . silicon wafer
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