U.S. patent application number 10/950423 was filed with the patent office on 2005-03-03 for semiconductor device having epitaxially-filled trench and method for manufacturing semiconductor device having epitaxially-filled trench.
Invention is credited to Sakakibara, Jun, Tsuji, Nobuhiro, Yamaguchi, Hitoshi, Yamauchi, Shoichi.
Application Number | 20050045996 10/950423 |
Document ID | / |
Family ID | 29552351 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050045996 |
Kind Code |
A1 |
Yamauchi, Shoichi ; et
al. |
March 3, 2005 |
Semiconductor device having epitaxially-filled trench and method
for manufacturing semiconductor device having epitaxially-filled
trench
Abstract
A semiconductor device includes a semiconductor substrate and a
semiconductor layer. The semiconductor substrate has a main surface
that is an Si{100} surface. The substrate has a trench in the main
surface. The semiconductor layer is located on surfaces defining
the trench to have common crystallographic planes with the
semiconductor substrate. The trench is defined by a bottom surface,
two long sidewall surfaces that face each other, and two short
sidewall surfaces that face each other. The bottom surface and the
long sidewall surfaces are Si{100} surfaces.
Inventors: |
Yamauchi, Shoichi;
(Obu-city, JP) ; Yamaguchi, Hitoshi;
(Nisshin-city, JP) ; Sakakibara, Jun; (Anjo-city,
JP) ; Tsuji, Nobuhiro; (Okazaki-city, JP) |
Correspondence
Address: |
POSZ & BETHARDS, PLC
11250 ROGER BACON DRIVE
SUITE 10
RESTON
VA
20190
US
|
Family ID: |
29552351 |
Appl. No.: |
10/950423 |
Filed: |
September 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10950423 |
Sep 28, 2004 |
|
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10442196 |
May 21, 2003 |
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6836001 |
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Current U.S.
Class: |
257/627 ;
257/E21.417; 257/E29.004; 257/E29.128; 257/E29.136;
257/E29.256 |
Current CPC
Class: |
H01L 29/66674 20130101;
H01L 29/7801 20130101; H01L 29/045 20130101; H01L 29/781 20130101;
H01L 29/4232 20130101; H01L 29/0634 20130101; H01L 29/7813
20130101; H01L 29/4238 20130101 |
Class at
Publication: |
257/627 |
International
Class: |
H01L 021/332; H01L
029/04; H01L 031/036 |
Foreign Application Data
Date |
Code |
Application Number |
May 22, 2002 |
JP |
2002-148090 |
May 12, 2003 |
JP |
2003-133255 |
Claims
1. A method for manufacturing a semiconductor device, the method
comprising: preparing a semiconductor substrate, which has a main
surface that is an Si{100} surface; forming a first trench, which
is defined by a bottom surface, two long sidewall surfaces that
face each other, and two short sidewall surfaces that face each
other, in the main surface such that the long sidewall surfaces are
Si{100} surfaces; and forming a semiconductor layer on the
sidewalls and the bottom surfaces of the trench by epitaxial
growth.
2. A method for manufacturing a semiconductor device, the method
comprising: preparing a semiconductor substrate, which has a main
surface that is an Si{100} surface and which is first conductivity
type to form a source region; forming a first trench, which is
defined by a bottom surface, two long sidewall surfaces that face
each other, and two short sidewall surfaces that face each other,
in the main surface such that the bottom surface and the long
sidewall surfaces are Si{100} surfaces; forming a
base-region-forming film, which is used for forming a second
conductivity type base region, by epitaxial growth to cover the
sidewall surfaces and the bottom surface without completely filling
the first trench; forming a drift-region-forming film, which is
used for forming a first conductivity type drift region, to cover
the base-region-forming film without completely filling the first
trench; forming a drain-region-forming film, which is used for
forming a first conductivity type drain region, to cover the
drift-region-forming film; planarizing the base-region-forming
film, the drift-region-forming film, and the drain-region-forming
film to form the base region, the drift region, and the drain
region; forming a second trench to intersect the base region from
the source region in a direction parallel to the main surface and
to extend orthogonally from the main surface; forming a gate
insulating film in the second trench; and forming a gate electrode
on the gate insulating film.
3. A method for manufacturing a semiconductor device, the method
comprising: preparing a semiconductor substrate, which has a main
surface that is an Si{100} surface and which is first conductivity
type to form a drain region; forming a first trench, which is
defined by a bottom surface, two long sidewall surfaces that face
each other, and two short sidewall surfaces that face each other,
in the main surface such that the bottom surface and the long
sidewall surfaces are Si{100} surfaces; forming a
drift-region-forming film, which is used for forming a first
conductivity type drift region, by epitaxial growth to cover the
sidewall surfaces and the bottom surface without completely filling
the first trench; forming a base-region-forming film, which is used
for forming a second conductivity type base region, to cover the
drift-region-forming film without completely filling the first
trench; forming a source-region-forming film, which is used for
forming a first conductivity type source region, to cover the
base-region-forming film; planarizing the drift-region-forming
film, the base-region-forming film, and the source-region-forming
film to form the drift region, the base region, and the source
region; forming a second trench to intersect the base region from
the source region in a direction parallel to the main surface and
to extend orthogonally from the main surface; forming a gate
insulating film in the second trench; and forming a gate electrode
on the gate insulating film.
4. The method according to claim 2 further comprising: forming a
third trench, which is defined by a bottom surface, two long
sidewall surfaces that face each other, and two short sidewall
surfaces that face each other, in the drift region such that the
long sidewall surfaces extend orthogonally from the main surface to
be Si{100} surfaces; and forming a second conductivity type RESURF
layer in the third trench by epitaxial growth.
5. The method according to claim 3 further comprising: forming a
third trench, which is defined by a bottom surface, two long
sidewall surfaces that face each other, and two short sidewall
surfaces that face each other, in the drift region such that the
long sidewall surfaces extend orthogonally from the main surface to
be Si{100} surfaces; and forming a second conductivity type RESURF
layer in the third trench by epitaxial growth.
6. A method for manufacturing a semiconductor device, the method
comprising: preparing a semiconductor substrate, which has a main
surface that is an Si{100} surface; forming one layer out of a
first conductivity type layer and a second conductivity type layer
on the semiconductor substrate; forming first trenches, each of
which is defined by a bottom surface, two long sidewall surfaces
that face each other, and two short sidewall surfaces that face
each other, in a surface of the one layer such that the bottom
surfaces and the long sidewall surfaces are Si{100} surfaces;
forming the other layer out of the first conductivity type layer
and the second conductivity type layer to fill the first trenches
by epitaxial growth; forming a p/n column layer, which includes
first conductivity type drift regions and second conductivity type
first semiconductor regions in a stripe pattern layout; forming a
second conductivity type second semiconductor layer on the drift
regions and the first semiconductor regions; forming second
trenches, which extend through the second semiconductor layer; and
forming gate electrodes on surfaces defining the second trenches
with gate insulating films therebetween.
7. The method according to claim 2, wherein the second trenches are
formed such that surfaces defining the second trenches become
Si{100} surfaces.
8. The method according to claim 3, wherein the second trenches are
formed such that surfaces defining the second trenches become
Si{100} surfaces.
9. The method according to claim 6, wherein the second trenches are
formed such that surfaces defining the second trenches become
Si{100} surfaces.
10. The method according to claim 1, wherein the first trench is
formed such that the sidewall surface thereof become Si {100}
surfaces.
11. The method according to claim 1, wherein the semiconductor
substrate is a semiconductor wafer that has an indexing cut-out
that is an Si{100} surface and wherein the first trench is formed
such that the long sidewall surfaces are parallel or orthogonal to
the indexing cut-out.
12. The method according to claim 1, wherein the semiconductor
substrate is a semiconductor wafer that has an indexing cut-out
that is an Si{110} surface and wherein the first trench is formed
such that the long sidewall surfaces are at an angle of 45 degrees
with the indexing cut-out.
13. The method according to claim 1, wherein the first trench is
formed by dry etching.
14. A semiconductor device comprising: a semiconductor substrate,
which has a main surface that is an Si{100} surface, wherein the
substrate has a trench in the main surface; and a semiconductor
layer, which is located on surfaces defining the trench to have
common crystallographic planes with the semiconductor substrate,
wherein the trench is defined by a bottom surface, two long
sidewall surfaces that face each other, and two short sidewall
surfaces that face each other and wherein the bottom surface and
the long sidewall surfaces are Si{100} surfaces.
15. A semiconductor device comprising: a semiconductor substrate,
which has a main surface that is an Si{100} surface; and a
semiconductor layer, which is located in the main surface, wherein
the semiconductor layer has a bottom surface and two long sidewall
surfaces that face each other, and two short sidewall surfaces that
face each other, wherein the quotient obtained by dividing a width
of the sidewall surfaces by that of bottom surface is 1 or more,
and wherein the bottom surface and the long sidewall surfaces are
Si{100} surfaces.
16. A semiconductor device comprising: a semiconductor substrate,
which has a main surface that is an Si{100} surface and which is
first conductivity type to form a source region; a second
conductivity type base region, which extends orthogonally from the
main surface to have a bottom surface, two long sidewall surfaces
that face each other, and two short sidewall surfaces that face
each other, wherein the bottom surface and the long sidewall
surfaces are Si{100} surfaces; a first conductivity type drift
region, which extends orthogonally from the main surface in the
base region and has a lower impurity concentration than the
semiconductor substrate; a drain region, which extends orthogonally
from the main surface in the drift region to be separated from the
base region; a trench, which extends orthogonally from the main
surface and intersects the base region from the source region to
reach the drift region in a direction parallel to the main surface
and to expose an Si{100} surface in the base region; a gate
insulating film, which is located on surfaces defining the trench;
and a gate electrode, which is located on the gate insulating
film.
17. A semiconductor device comprising: a semiconductor substrate,
which has a main surface that is an Si{100} surface and which is
first conductivity type to form a drain region; a first
conductivity type drift region, which extends orthogonally from the
main surface to have a bottom surface, two long sidewall surfaces
that face each other, and two short sidewall surfaces that face
each other, wherein the bottom surface and the long sidewall
surfaces are Si{100} surfaces and wherein the drift region has a
lower impurity concentration than the semiconductor substrate; a
second conductivity type base region, which extends orthogonally
from the main surface in the drift region; a first conductivity
type source region, which extends orthogonally from the main
surface in the base region; a trench, which extends orthogonally
from the main surface and intersects the base region from the
source region in a direction parallel to the main surface to expose
an Si{100} surface in the base region; a gate insulating film,
which is located on surfaces defining the trench; and a gate
electrode, which is located on the gate insulating film.
18. The semiconductor device according to claim 16 further
comprising a second conductivity type RESURF layer, which extends
orthogonally from the main surface in the drift region to have a
bottom surface, two long sidewall surfaces that face each other,
and two short sidewall surfaces that face each other, wherein the
bottom surface and the long sidewall surfaces are Si{100}
surfaces.
19. The semiconductor device according to claim 17 further
comprising a second conductivity type RESURF layer, which extends
orthogonally from the main surface in the drift region to have a
bottom surface, two long sidewall surfaces that face each other,
and two short sidewall surfaces that face each other, wherein the
bottom surface and the long sidewall surfaces are Si{100}
surfaces.
20. A semiconductor device comprising: a semiconductor substrate,
which has a main surface that is an Si{100} surface; a p/n column
layer, which is made of first conductivity type drift regions and
second conductivity type first semiconductor regions in a stripe
pattern layout, wherein the p/n column layer is located on the
semiconductor substrate; second conductivity type second
semiconductor regions, which are located on the p/n column layer,
wherein the semiconductor device has trenches, which intersect the
second semiconductor regions, and wherein surfaces defining the
trenches are Si{100} surfaces; gate insulating films, which are
located on the surfaces defining the trenches; and gate electrodes,
which are located on the gate insulating films, wherein out of
interfacial surfaces between the drift regions and the first
semiconductor regions, the long interfacial surfaces are Si{100}
surfaces.
21. A method for manufacturing a semiconductor device, the method
comprising: preparing a semiconductor substrate, which has a main
surface that is an Si{110} surface; forming one layer out of a
first conductivity type layer and a second conductivity type layer
on the semiconductor substrate; forming first trenches, each of
which is defined by a bottom surface, two long sidewall surfaces
that face each other, and two short sidewall surfaces that face
each other, in a surface of the one layer by anisotropic wet
etching such that the bottom surfaces are Si{110} surfaces and the
long sidewall surfaces are Si{111} surfaces; forming the other
layer out of the first conductivity type layer and the second
conductivity type layer to fill the first trenches by epitaxial
growth; forming a p/n column layer, which includes first
conductivity type drift regions and second conductivity type first
semiconductor regions in a stripe pattern layout; forming a second
conductivity type second semiconductor layer on the drift regions
and the first semiconductor regions; forming second trenches, each
of which is defined by a bottom surface, two long sidewall surfaces
that face each other, and two short sidewall surfaces that face
each other, in the second semiconductor layer by dry etching such
that the long sidewall surfaces are Si{100} surfaces; forming gate
oxide films on surfaces defining the second trenches by thermal
oxidization; and forming gate electrodes on the gate insulating
films.
22. The method according to claim 21, wherein the first trenches
are formed such that the sidewall surfaces of the first trenches
become Si{111} surfaces.
23. The method according to claim 21, wherein the second trenches
are formed such that the second trenches are rectangular when
viewed from above their entrance, the bottom surfaces are Si{110}
surfaces, the two long sidewall surfaces are Si{100} surfaces, and
the two short sidewall surfaces are Si{111} surfaces.
24. The method according to claim 21, wherein the semiconductor
substrate is a semiconductor wafer that has an indexing cut-out
that is an Si{111} surface, wherein the first trenches are formed
such that the long sidewall surfaces of the first trenches are
parallel to the indexing cut-out, and wherein the second trenches
are formed such that the long sidewall surfaces of the second
trenches are at an angle of 54.7 degrees or 125.2 degrees with the
indexing cut-out.
25. The method according to claim 21, wherein the semiconductor
substrate is a semiconductor wafer that has an indexing cut-out
that is an Si{112} surface, wherein the first trenches are formed
such that the long sidewall surfaces of the first trenches are
orthogonal to the indexing cut-out, and wherein the second trenches
are formed such that the long sidewall surfaces of the second
trenches are at an angle of 144.7 degrees or 35.2 degrees with the
indexing cut-out.
26. The method according to claim 21, wherein the semiconductor
substrate is a semiconductor wafer that has an indexing cut-out
that is an Si{100} surface, wherein the first trenches are formed
such that the long sidewall surfaces of the first trenches are at
an angle of 125.3 degrees or 54.8 degrees with the indexing
cut-out, and wherein the second trenches are formed such that the
long sidewall surfaces of the second trenches are parallel to the
indexing cut-out.
27. The method according to claim 21, wherein the semiconductor
substrate is a semiconductor wafer that has an indexing cut-out
that is an Si{110} surface, wherein the first trenches are formed
such that the long sidewall surfaces of the first trenches are at
an angle of 35.3 degrees or 144.8 degrees, with the indexing
cut-out, and wherein the second trenches are formed such that the
long sidewall surfaces of the second trenches are orthogonal to the
indexing cut-out.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is based on and incorporates herein by
reference Japanese Patent Application No. 2002-148090 filed on May
22, 2002 and No. 2003-133255 filed on May 12, 2003.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a semiconductor device
formed by filling a trench with an epitaxially grown film and to a
method for manufacturing the device.
[0003] Such a semiconductor device is manufactured, for example,
by: forming a trench defined by four sidewall surfaces, each of
which is an Si{110} surface, or a surface of silicon on which a
{110} plane is exposed, and a bottom surface, which is an Si(100)
surface, or a surface of silicon on which a (100) plane is exposed,
in an Si(100) surface of a silicon substrate; and depositing an
epitaxially grown film to fill the trench. Specifically, the four
sidewall surfaces defining the trench may be an Si(011) surface, an
Si(0-1-1) surface, an Si(01-1) surface, and an Si(0-11) surface.
The four surfaces of the sidewall surfaces are crystallographically
equivalent to one another, but crystallographically differ from the
bottom surface.
[0004] Alternatively, such a semiconductor device may also be
manufactured by: forming a trench having four sidewall surfaces,
each of which is an Si{111} surface, or a surface of silicon on
which a {111} plane is exposed, and a bottom surface, which is an
Si{110} surface, or a surface of silicon on which a {110} plane is
exposed, in an Si{110} surface of a silicon substrate with an
anisotropic wet etching using potassium hydroxide (KOH) aqueous
solution; and depositing an epitaxially grown film to fill the
trench, as disclosed in JP-A-2001-168327.
[0005] When a trench is formed with the anisotropic wet etching,
the etch rate difference between the planes. Therefore, the plane
orientations of the surfaces defining a trench are limited by the
plane orientation of the surface of a substrate. As a result, each
of the four sidewall surfaces and the bottom surface of the trench
automatically become an Si{111} surface and an Si{110} surface,
respectively.
[0006] The above technique for forming a trench can be applied to
what is called a three-dimensional (3-D) structure MOSFET, which is
disclosed in JP-A-2001-274398, and to a super junction MOSFET,
which is disclosed in JP-A-2001-127289. In the 3-D structure
MOSFET, a channel region has been formed such that a current mainly
flows parallel to the surface of a substrate in the channel region,
that is, the channel width direction, in which the width of the
channel region is defined, is orthogonal to the surface of the
substrate. The super junction MOSFET includes a p/n column
layer.
[0007] Some 3-D structure MOSFETs and some super junction MOSFETs
include a trench gate, which is a gate electrode formed on a
sidewall surface of a trench with a gate oxide film therebetween.
When a trench for a trench gate is formed, a trench defined by four
sidewall surfaces, each of which is an Si{110} surface, and a
bottom surface, which is an Si(100) surface, can be formed as well
as the aforementioned trench filled with an epitaxially grown film
if a silicon substrate having an Si(100) surface is used.
[0008] When a silicon substrate having an Si{110} surface is used,
a trench for a trench gate can be formed using an anisotropic wet
etching as well as the aforementioned trench filled with an
epitaxially grown film, as disclosed in JP-A-2001-168327. In that
case as well, the trench comes to have four sidewall surfaces, each
of which is an Si{111}surface, and a bottom surface, which is an
Si{110} surface
[0009] A bar (-) should have been put above the corresponding
numeral in order to show a crystallographic plane orientation.
However, a bar is put before the corresponding numeral due to the
restriction of notation in this specification. Braces { } means
every crystallographically equivalent plane as in the ordinary
notation system. For example, a {100} plane is any one of a (-100)
plane, a (010) plane, a (001) plane, a (-100) plane, a (0-10)
plane, and a (00-1) plane.
[0010] In the semiconductor devices proposed in the above
publications, there are three demands. The first demand is for
suppressing the generation of crystallographic stress, or crystal
defects, in an epitaxially grown film when the trench is filled
with the epitaxially grown film. The generation of the crystal
defects in the epitaxially grown film can be suppressed by removing
the crystal defects existing in the surfaces defining the trench
before the epitaxially grown film is formed. If crystal defects
exist in the surfaces defining the trench, crystal defects are
generated in the epitaxially grown film grown on the surfaces, too.
Therefore, the generation of the crystal defects in the epitaxially
grown film can be suppressed by removing the crystal defects
existing in the surfaces defining the trench before the epitaxially
grown film is formed.
[0011] Alternatively, the trench may be formed by wet etching, not
by dry etching. Wet etching damages the surfaces defining the
trench less than dry etching, so fewer crystal defects are
generated in the surfaces defining the trench formed by wet
etching. In addition, wet etching damages the silicon substrate
less than dry etching, and wet etching generates fewer
contamination layer, which is produced through the reaction.
Therefore, the generation of the crystal defects in the epitaxially
grown film can be more suppressed by forming the trench using wet
etching than using dry etching.
[0012] The inventors of the present invention have conducted
intensive studies on the cause of generating the crystal defects in
order to obtain a method having the effect of suppressing the
generation of crystal defects other than the aforementioned
methods. The studies have shown that the difference in
crystallographic plane between the bottom surface and the sidewall
surfaces of the trench is one of the causes generating the crystal
defects.
[0013] If the crystallographic plane exposed on the bottom surface
of the trench differs from those on the sidewall surfaces of the
trench, the growth rate of the epitaxially grown film becomes
different between on the bottom surface and on the sidewall
surfaces. Therefore, when the epitaxial grown film is formed, a
stress is generated in the epitaxial grown film near the bottom
corners of the trench. The stress causes the crystal defects.
[0014] The second demand is for increasing the breakdown voltage of
the gate oxide film, on which the trench gate is formed. If a
semiconductor device including a trench gate is manufactured by:
forming a trench defined by four sidewall surfaces, each of which
is an Si{110} surface, and a bottom surface, which is an Si(100)
surface, in an Si(100) surface of a silicon substrate; and forming
an gate oxide film on the four sidewall surfaces and the bottom
surface by thermal oxidization, the gate oxide film becomes thinner
on the bottom surface than on the sidewall surfaces. Therefore, the
breakdown voltage of the gate oxide film is substantially
determined by the thickness of the gate oxide film on the bottom
surface. Thus, it is necessary to thicken the gate oxide film on
the bottom surface in order to increase the breakdown voltage of
the gate oxide film.
[0015] However, the semiconductor device is designed on the basis
of the thickness of the gate oxide film on the sidewall surfaces
because a channel region is formed in the sidewall surfaces.
Therefore, it is difficult to thicken the gate oxide film only on
the bottom surface without thickening the gate oxide film on the
sidewall surfaces, if the trench is formed such that the above
crystallographic planes are exposed on the sidewall surfaces and
the bottom surface. From the standpoint of increasing the breakdown
voltage of the gate oxide film, it is preferred that the thickness
of the gate oxide film on the bottom surface be approximately equal
to or thicker than those on the sidewall surfaces.
[0016] The third demand is for increasing the current flowing
through the channel region without enlarging the chip size of a
power device such as a 3-D structure MOSFET and a super junction
MOSFET. The current flowing through the channel region is thought
to be increased by increasing the cross-sectional area of the
current passage. However, in that case, the chip sizes of the power
devices becomes larger, so it is not preferable to increase the
cross-sectional area of the current passage.
SUMMARY OF THE INVENTION
[0017] The present invention has been made in view of the above
aspects. A first object of the present invention is to suppress the
generation of crystal defects in an epitaxially grown film when a
trench formed in a substrate is filled with the epitaxially grown
film. A second object of the present invention is to increase the
breakdown voltage of a gate oxide film in a semiconductor device
including a trench gate. A third object of the present invention is
to increase the current flowing through the channel region without
enlarging the chip size of a power device such as a 3-D structure
MOSFET and a super junction MOSFET.
[0018] To achieve the above objects, a semiconductor device
according to the present invention includes a semiconductor
substrate and a semiconductor layer. The semiconductor substrate
has a main surface that is an Si{100} surface. The substrate has a
trench in the main surface. The semiconductor layer is located on
surfaces defining the trench to have common crystallographic planes
with the semiconductor substrate. The trench is defined by a bottom
surface, two long sidewall surfaces that face each other, and two
short sidewall surfaces that face each other. The bottom surface
and the long sidewall surfaces are Si{100} surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above and other objects, features and advantages of the
present invention will become more apparent from the following
detailed description made with reference to the accompanying
drawings. In the drawings:
[0020] FIG. 1 is a schematic partial plan view of a 3-D structure
power MOSFET as a first example of a first embodiment of the
present invention;
[0021] FIG. 2 is a schematic perspective sectional view of the
MOSFET in FIG. 1 at the area AA;
[0022] FIGS. 3A to 3E are views showing a method for manufacturing
the 3-D structure power MOSFET of FIGS. 1 and 2;
[0023] FIG. 4 is a plan view showing an arrangement of an epitaxial
trench in a semiconductor wafer;
[0024] FIG. 5 is a plan view showing another arrangement of an
epitaxial trench in a semiconductor wafer;
[0025] FIG. 6 is a schematic cross-sectional image, which is based
on the observation using a scanning electron microscope, of an
epitaxial film formed in a trench by a conventional method;
[0026] FIG. 7 is a schematic cross-sectional image, which is based
on the observation using a scanning electron microscope, of an
epitaxial film formed in a trench by the method according to the
first embodiment;
[0027] FIG. 8 is a plan view of a 3-D power MOSFET as a second
example of the first embodiment;
[0028] FIG. 9 is a perspective sectional view of the area C in FIG.
8;
[0029] FIG. 10 is a plan view of a 3-D power MOSFET as a third
example of the first embodiment;
[0030] FIG. 11 is a perspective sectional view of the area D in
FIG. 10;
[0031] FIG. 12 is a plan view of a 3-D power MOSFET as a fourth
example of the first embodiment;
[0032] FIG. 13 is a perspective sectional view of the area E in
FIG. 12;
[0033] FIG. 14 is a plan view of a power MOSFET having a super
junction structure as a first example of a second embodiment;
[0034] FIG. 15 is a cross-sectional view taken along the line XV-XV
in FIG. 14;
[0035] FIGS. 16A to 16H are views showing a method for
manufacturing a power MOSFET having a super junction structure as a
first example of a second embodiment;
[0036] FIG. 17 is a plan view of a power MOSFET having a super
junction structure as a second example of the second
embodiment;
[0037] FIG. 18A is a plan view of a power MOSFET having a super
junction structure as a third embodiment, showing the layout of a
p/n column;
[0038] FIG. 18B is a plan view of the area 62 in FIG. 18A, showing
the layout of trench gates;
[0039] FIG. 19 is a perspective sectional view of the MOSFET in
FIG. 18A, showing the cross-sectional view taken along the line
XIX-XIX in FIG. 18A;
[0040] FIGS. 20A to 20G are views showing a method for
manufacturing a power MOSFET having a super junction structure as
the third embodiment;
[0041] FIG. 21 is a table that shows the positional relations
between the orientation flats, the crystallographic planes that are
exposed on the surfaces defining epitaxial trenches, and the
crystallographic planes that are exposed on the surfaces defining
gate trenches of semiconductor wafers;
[0042] FIG. 22 is another table that shows the positional relations
between the orientation flats, the crystallographic planes that are
exposed on the surfaces defining epitaxial trenches, and the
crystallographic planes that are exposed on the surfaces defining
gate trenches of semiconductor wafers;
[0043] FIG. 23 is a table that shows the electron mobility in the
channel plane, the interface trap, and the relation in the
thickness of the gate oxide film between on the sidewall surfaces
and on the bottom surfaces of the power MOSFETs according to the
first, second, and third embodiments; and
[0044] FIG. 24 is a plan view of a power MOSFET having a super
junction structure as a fourth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] The present invention will be described in detail with
reference to various embodiments.
[0046] First Embodiment
[0047] FIG. 1 is a schematic partial plan view of a 3-D structure
power MOSFET according to a first example of a first embodiment of
the present invention. FIG. 2 is a schematic perspective sectional
view of the MOSFET in FIG. 1 at the area AA. The power MOSFET has a
structure in which the unit structure B of FIG. 1 is repeated a
plurality of times.
[0048] The power MOSFET of FIG. 1 includes a semiconductor
substrate 1, or an n.sup.+-type substrate 1. The n.sup.+-type
substrate 1 has a main surface 1a, which is an Si(100) surface, and
a rear surface 1b, which is opposite to the main surface 1a. The
n.sup.+-type substrate 1 is doped with P, As or Sb, which is an
n-type impurity, at a concentration of about 1.times.10.sup.18 to
1.times.10.sup.20 cm.sup.-3. The concentration of the impurity is
substantially uniform along the directions orthogonal to the main
surface 1a and the directions parallel to the main surface 1a. The
n.sup.+-type substrate 1 forms an n.sup.+-type source region 1.
[0049] As shown in FIG. 2, a first trench 2, or an epitaxial trench
2, has been formed to have a predetermined depth of, for example, 1
to 100 .mu.m from the main surface 1a of the n.sup.+-type substrate
1. As described later, an epitaxially grown film has filled the
epitaxial trench 2. As shown in FIG. 1, the epitaxial trench 2 is
substantially square when viewed from above the main surface 1a.
The epitaxial trench 2 is defined by long sidewall surfaces 2a and
short sidewall surfaces 2b, and the long sidewall surfaces 2a and
short sidewall surfaces 2b are Si{100} surfaces.
[0050] More specifically, one of the long sidewall surfaces 2a is
an Si(001) surface and the other surface is an Si(00-1) surface.
One of the short sidewall surfaces 2b is an Si(010) surface and the
other surface is an Si(0-10) surface. The bottom surface 2c of the
epitaxial trench 2 is parallel to the main surface 1a and is an
Si(100) surface. Therefore, the surfaces defining the trench 2 are
all Si{100} surfaces.
[0051] A p-type base region 3, or p-type well layer 3, has been
formed in the trench 2. The p-type base region 3 has been doped
with B, which is a p-type impurity, at a concentration of about
1.times.10.sup.15 to 1.times.10.sup.18 cm.sup.-3 and has a
thickness of about 0.1 to 5 .mu.m. The concentration of the
impurity in the p-type base region 3 is substantially uniform along
the directions orthogonal to the main surface 1a and the directions
parallel to the main surface 1a.
[0052] The sidewall surfaces 3a and the bottom surface 3c of the
p-type base region 3 are the same crystallographic surfaces as the
sidewall surfaces 2a, 2b and the bottom surface 2c of the trench 2,
which are in contact with the sidewall surfaces 3a and the bottom
surface 3c of the p-type base region 3, that is, Si{100} surfaces.
The p-type base region 3 in the trench 2 has an aspect ratio, or
the ratio of the dimension along the directions orthogonal to the
main surface 1a to the dimension along the directions parallel to
the main surface 1a, of 1 or more.
[0053] In general, the aspect ratio cannot be larger than 1 in the
impurity region obtained by doping an impurity using ion
implantation. However, as the p-type base region 3 is formed in the
trench 2, the aspect ratio can become 1 or more. The same can be
said of the following embodiments, which are described later.
[0054] As shown in FIG. 2, an n.sup.--type drift region 4 has been
formed in the p-type base region 3. The n.sup.--type drift region 4
has been doped with P or As, which is an n-type impurity, at a
concentration of about 1.times.10.sup.14 to 1.times.10.sup.17
cm.sup.-3. The concentration of the impurity in the n.sup.- type
drift region 4 is substantially uniform along the directions
orthogonal to the main surface 1a and the directions parallel to
the main surface 1a. The thickness of the n.sup.--type drift region
4 is determined, for example, in consideration of a required
breakdown voltage.
[0055] An n.sup.+-type drain region 5 has been formed in the
n.sup.--type drift region 4. The n.sup.+-type drain region 5 has
been doped with P or As at a concentration of about
1.times.10.sup.18 to 1.times.10.sup.20 cm.sup.-3 and has a
thickness of about 0.1 to 5 .mu.m. The concentration of the
impurity in the n.sup.+-type drain region 5 is substantially
uniform along the directions orthogonal to the main surface 1a and
the directions parallel to the main surface 1a.
[0056] As shown in FIG. 2, second trenches 6, or gate trenches 6,
extend orthogonally from the main surface 1a of the n.sup.+-type
substrate 1. The gate trenches 6 also extend in the directions
orthogonal to the long sidewall surfaces 2a of the epitaxial trench
2 formed in the n.sup.+-type substrate 1 to intersect the p-type
base region 3 from the n.sup.+-type substrate 1 to the n.sup.--type
drift region 4. As shown in FIG. 1, each of the gate trenches 6 is
defined by long sidewall surfaces 6a and short sidewall surfaces
6b. Each of the long sidewall surfaces 6a is an Si(010) surface or
an Si(0-10) surface, that is, an Si{100} surface.
[0057] A gate insulating film 7, or a gate oxide film 7, has been
formed on the surfaces that define each of the gate trenches 6. The
thickness of the gate oxide film 7 on the bottom surface of each of
the gate trenches 6 is equal to those on the sidewall surfaces of
each of the gate trenches 6. A gate electrode 8 has been formed on
the gate oxide film 7 to fill each of the trenches 6.
[0058] With the above structure, the n.sup.+-type source region 1,
the p-type base region 3, the n.sup.--type drift region 4, and
n.sup.+-type drain region 5 are arranged sequentially in a
direction parallel to the main surface 1a of the n.sup.+-type
substrate 1. The arrangement is maintained substantially down to
the depth of the gate trenches 6 in the directions orthogonal to
the main surface 1a of the n.sup.+-type substrate 1. The depths of
the p-type base region 3, the n.sup.--type drift region 4, and
n.sup.+-type drain region 5 are set corresponding to the depth of
the trenches 6. That is, they become deeper as the trenches 6
become deeper.
[0059] As shown in FIG. 1, gate wiring lines 9 have been patterned
on the main surface 1a of the n.sup.+-type substrate 1 with an
oxide film therebetween. The gate wiring lines 9 are electrically
connected to the gate electrodes 8 through contact holes formed in
the oxide film. Drain contact regions 10 have been formed in the
n.sup.+-type drain region 5. Although not shown in the figure,
drain electrodes are located above the main surface 1a of the
n.sup.+-type substrate 1 with an inter-layer insulating film
therebetween.
[0060] When a positive voltage is applied to the gate electrodes 8
of the thus constituted power MOSFET, electrons are induced in the
p-type base region 3 at the entire surfaces adjacent to the gate
oxide films 7 to form channel regions, which are inversion layers
of the p-type base region 3. The channel regions are formed such
that a current mainly flows parallel to the main surface 1a of the
n.sup.+-type substrate 1 in each of the channel regions, that is,
the channel width direction is orthogonal to the main surface 1a of
the n.sup.+-type substrate 1.
[0061] In the power MOSFET of FIGS. 1 and 2, the gate oxide films 7
are located on the sidewall surfaces, which are Si{100} surfaces,
of the gate trenches 6. In other words, the channel regions contact
the gate insulating films 7 at Si{100} surfaces. Therefore, the
interface traps at each of the interfaces between the channel
regions and the gate insulating films 7 is lower in comparison with
other structures, in which channel regions do not contact gate
insulating films at Si{100} surfaces. Therefore, it is possible to
improve the mobility of carriers in the channel regions. Thus, it
is possible to increase the current flowing through the channel
regions.
[0062] Next, the method for manufacturing the power MOSFET of FIGS.
1 and 2 will be described.
[0063] [Steps Shown in FIG. 3A]
[0064] A semiconductor wafer 1, the main surface 1a of which is an
Si(100) surface, is prepared. The semiconductor wafer 1 includes an
n.sup.+-type substrate 1, which forms an n.sup.+-source region 1. A
silicon oxide film 11 is formed on the n.sup.+-type substrate 1 by
thermal oxidation or CVD. The silicon oxide film 11 is etched at
the area where a p-type base region 3 is formed by photolithography
and etching.
[0065] [Steps Shown in FIG. 3B]
[0066] An epitaxial trench 2 having a depth of 10 to 100 .mu.m is
formed by dry etching using the silicon oxide film 11 as a mask.
The dry etching is carried out by exciting an etching gas into a
plasma state with a bias voltage applied to the substrate 1. The
etching gas includes, for example, SF.sub.6 for protecting the
sidewall surfaces of the epitaxial trench 2 and CF.sub.4 for
etching the bottom surface of the epitaxial trench 2. With the dry
etching, the n.sup.+-type substrate 1 is partially removed except
the portion that becomes the n.sup.+-type source region 1 to form
the epitaxial trench 2.
[0067] The method of selecting the plane orientations of the
epitaxial trench 2 at the steps shown in FIG. 3A and FIG. 3B will
be explained. For example, as shown in FIG. 4, when an indexing
cut-out 1c, or an orientation flat 1c, of an Si(100) surface wafer
1, which has an Si(100) surface as a main surface 1a, is an
Si(00-1) surface, the epitaxial trench 2 is formed to extend in the
directions parallel to the orientation flat 1c, as shown by the
solid lines in FIG. 4. With the above arrangement, the long
sidewall surfaces 2a of the epitaxial trench 2 become an Si(001)
surface and an Si(00-1) surface. The orientation flat 1C is a mark
for specifying the surface orientations of the wafer 1 and may be a
notch.
[0068] The short sidewall surfaces 2b of the epitaxial trench 2 are
formed to extend in the directions orthogonal to the long sidewall
surfaces 2a. With the arrangement, the short sidewall surfaces 2b
become an Si(010) surface and an Si(0-10) surface. Consequently,
all the sidewall surfaces of the epitaxial trench 2 become Si{100}
surfaces. In addition, the bottom surface of the epitaxial trench 2
is formed to become parallel to the main surface 1a of the wafer 1,
so the bottom surface is also an Si{100} surface.
[0069] The method of forming the epitaxial trench 2 is not limited
to the above method. For example, as shown by the dotted lines in
FIG. 4, the epitaxial trench 2 may be formed to become orthogonal
to the orientation flat 1c. Since the sidewall surfaces of the
epitaxial trench 2 shown by the dotted lines are all parallel to
the sidewall surfaces of the epitaxial trench 2 shown by the solid
lines, they are all Si{100} surfaces.
[0070] Alternatively, when the orientation flat 1c is formed to
become an Si(0-1-1) surface as shown in FIG. 5, the epitaxial
trench 2 may be formed to become at an angle of 45 degrees with the
orientation flat 1c. More specifically, as shown by the solid lines
in FIG. 5, the epitaxial trench 2 extends to make a 45-degree angle
with the orientation flat 1c in the counterclockwise direction. In
other words, the epitaxial trench 2 is formed such that the angles
formed by the orientation flat 1c and the long sidewall surfaces 2a
become 45 degrees.
[0071] Alternatively, as shown by the dotted lines in FIG. 5, the
epitaxial trench 2 may extend to make a 135-degree angle with the
orientation flat 1c in the counterclockwise direction. In other
words, the epitaxial trench 2 may be formed such that the angles
formed by the orientation flat 1c and the long sidewall surfaces 2a
become 135 degrees. Even in that case, the acute angles formed by
the orientation flat 1c and the long sidewall surfaces 2a become 45
degrees.
[0072] [Steps Shown in FIG. 3C]
[0073] The surfaces defining the epitaxial trench 2 are smoothed by
heating in a non-oxidizing nitrogen atmosphere. Thereafter, a
p-type film 12 is formed as a base-region-forming film 12 on the
surfaces defining the epitaxial trench 2 by the epitaxial growth
method. The p-type film 12 becomes the p-type base region 3.
Incidentally, even after the mask oxide film 11 is removed with an
etching using a hydrofluoric acid aqueous solution prior to the
heat treatment in the non-oxidizing nitrogen atmosphere, the p-type
film 12 can be formed likewise. The p-type film 12 grows
orthogonally to Si{100} surfaces. In addition, the p-type film 12
is formed so that the epitaxial trench 2 is not completely filled.
The reason is to form an n.sup.--type drift region 4 on the p-type
film 12 in the epitaxial trench 2.
[0074] Then, an n.sup.--type film 13 is formed as
adrift-region-forming film 13 by the epitaxial growth method to
cover the p-type film 12. The n.sup.--type film 13 becomes the
n.sup.--type drift region 4. Therefore, the n.sup.--type drift
region 4 is also formed on Si{100} surfaces. The n.sup.--type film
13 is formed so that the epitaxial trench 2 is not completely
filled.
[0075] [Steps Shown in FIG. 3D]
[0076] Subsequently, an n.sup.+-type film 14 is formed as a
drain-region-forming film by the epitaxial growth method to cover
the n.sup.--type film 13 and fill up the epitaxial trench 2. The
n.sup.+-type film 14 becomes the n.sup.+-type drain region 5.
Therefore, the n.sup.+-type drain region 5 is also formed on an
Si{100} surface. The growing surface of the n.sup.+-type films 14
meats itself in the epitaxial trench 2 to completely fill the
epitaxial trench 2, as shown in FIG. 3D.
[0077] [Steps Shown in FIG. 3E]
[0078] After that, the wafer 1 is polished at the main surface 1a
to planarize the wafer 1. In the polishing, the p-type film 12, the
n.sup.--type film 13, and the n.sup.+-type film 14 are planarized
using, for example, the silicon oxide film 11 as an etching
stopper. With the polishing, a p-type base region 3, an
n.sup.--type drift region 4, and an n.sup.+-drain region 5 are
formed. Then, the gate trenches 6 of FIGS. 1 and 2, each of which
extends orthogonally to the main surface 1a of the substrate 1 and
is rectangular when viewed from above their entrances, are formed
from the surface 1a of the substrate 1 by selective etching,
although the steps are not shown in the figures.
[0079] The gate trenches 6 are formed in such a manner that they
intersect the p-type base region 3 from the n.sup.+-type source
region 1, which is formed by the substrate 1, parallel to the
surface 1a of the substrate 1. That is, the gate trenches 6 are
formed to ensure that channel regions are formed in the p-type base
region 3. In addition, the gate trenches 6 are formed to extend
orthogonally to the long sidewall surfaces 2a of the epitaxial
trench. With the structure, as shown in FIG. 1, the long sidewall
surfaces 6a of each of the gate trenches 6 can respectively become
an Si(010) surface and an Si(0-10) surface, that is, the long
sidewall surfaces 6a become Si{100} surfaces. In other words, the
surfaces of the p-type base region 3 that are exposed in the gate
trenches 6 become Si{100} surfaces.
[0080] Subsequently, the gate oxide films 7 are formed in the gate
trenches 6 by, for example, thermal oxidization, and an
n.sup.+-type polysilicon film is deposited to fill the gate
trenches 6 in order to form the gate electrodes 8. After a drain
electrode, which becomes in electric contact with the n.sup.+-type
drain region 5, and a source electrode, which becomes in electric
contact with the n.sup.+-type source region 1, are respectively
formed above the main surface 1a of the substrate 1 and on the rear
surface 1b of the substrate 1, electric wiring lines are formed for
electrically connecting the gate electrodes 8 and so on to an
outside circuit. Finally, the substrate 1 is covered with a
passivation film above the main surface 1a to complete a power
MOSFET of FIGS. 1 and 2.
[0081] At the step shown in FIG. 3B, the epitaxial trench 2 is
formed in the substrate 1 such that all the surfaces defining the
epitaxial trench 2 become Si{100}surfaces. Therefore, when the
p-type base region 3 is formed on the surfaces defining the
epitaxial trench 2 by the epitaxial growth method at the step shown
in FIG. 3C, the p-type film 12, which becomes the p-type base
region 3, epitaxially grows on the bottom surface and the sidewall
surfaces of the epitaxial trench 2 at the same growth rate.
Therefore, the stress that is otherwise generated in the p-type
base region 3 near the lower corners of the epitaxial trench 2 can
be suppressed when the p-type base region is formed in the
epitaxial trench 2. This makes it possible to suppress the
generation of crystal defects in the p-type base region 3. As a
result, the p-type base region 3 becomes to have an excellent
crystallographic structure.
[0082] Conventionally, a semiconductor wafer that has an Si{100}
surface as a main surface has had an Si{110} surface as an
orientation flat. In addition, since an epitaxial trench has been
formed parallel or orthogonally to the orientation flat, the
sidewall surfaces of the epitaxial trench have been Si{110}
surfaces.
[0083] It is known that when an epitaxial film is formed on a flat
main surface of a substrate, the epitaxial film generally has a
better crystallographic structure when the main surface is an
Si{100} surface than when it is an Si{110} surface or an Si{111}
surface. The reason is that the formation of a crystal nucleus
proceeds stably on Si{100} surfaces during the epitaxial growth
because Si{100} surfaces have a higher step density than Si{110}
surfaces and Si{111} surfaces. Therefore, fewer crystal defects are
generated when an epitaxial film grows on an Si{100} surface. In
this embodiment, epitaxial films are formed on Si{100} surfaces.
Therefore, the epitaxial films formed by the method of this
embodiment have a better crystallographic structure than those
formed by the above conventional method.
[0084] In addition, in this embodiment, the surfaces defining the
epitaxial trench 2 are smoothed at the step shown in FIG. 3C before
the p-type film 12 is formed by epitaxial growth. The reason is to
remove crystal defects that exist in the surfaces defining the
epitaxial trench 2. The present invention can be used in
conjunction with another method for suppressing the generation of
crystal defects as in this embodiment, so it is possible to further
efficiently suppress the generation of crystal defects.
[0085] The p-type base region 3 is formed on an Si{100} surface,
and the n.sup.--type drift region 4 and the n.sup.+-type drain
region 5 are sequentially formed on Si{100} surfaces. Therefore,
the p-type base region 3, n.sup.--type drift region 4, and
n.sup.+-type drain region 5 become to have an excellent
crystallographic structure.
[0086] FIG. 6 is a schematic cross-sectional image of an epitaxial
film formed in a trench by the above-mentioned conventional method.
The schematic cross-sectional image is based on the observation
using a scanning electron microscope. FIG. 7 is also a schematic
cross-sectional image of an epitaxial film formed in a trench by
the method according to this embodiment. The figures have been
obtained by making crystal defects prominent by stained etching the
cross-section of each epitaxially grown sample with the mixture of
hydrofluoric acid and nitric acid and observing it using a scanning
electron microscope.
[0087] As shown in the circles of FIG. 6, the generation of crystal
defects could be confirmed in the epitaxial film formed in a trench
by the conventional method. In contrast to this, in the epitaxial
film formed in a trench by the method according to this embodiment,
no crystal defects were generated, as shown in FIG. 7. Thus, the
inventors of the present invention confirmed that the generation of
crystal defects can be suppressed by the method according to this
embodiment.
[0088] In this embodiment, after the step of FIG. 3E, the gate
trenches 6 are formed to extend orthogonally to the long sidewall
surfaces 2a of the epitaxial trench 2. Since the long sidewall
surfaces 2a of the epitaxial trench 2 are Si{100} surfaces, it is
easy to make the long sidewall surfaces 6a of the gate trenches 6
Si{100} surfaces. That is, the surfaces of the channel regions that
are in contact with the gate insulating films 7 can become Si{100}
surfaces. Hereinafter, the surfaces of the channel regions that are
in contact with the gate insulating films 7 are just referred to as
the channel surfaces.
[0089] As shown in FIG. 23, when the channel surfaces are Si{110}
surfaces, the density of the interface trap between the channel
regions and the gate insulating films 7 is 2.times.10.sup.10
cm.sup.-2 and the carrier mobility, or the electron mobility, in
the channel regions is 600 cm.sup.2/V*s. That is, the density of
the interface trap can be reduced by assigning Si{100} surfaces to
the channel surfaces compared with the case that the channel
surfaces are not Si{100} surfaces. Therefore, the loss of carriers
in the channel regions can be reduced. In other words, the mobility
of carriers in the channel regions can be improved. Therefore, it
is possible to increase the currents that flow through the channel
regions, so a larger amount of current can be applied between the
source and the drain in an ON state.
[0090] In addition, the sidewall surfaces 6a, 6b and the bottom
surfaces of the gate trenches 6 have crystallographically the same
plane orientation, so it is possible to equate the thickness of the
gate oxide films 7 on the sidewall surfaces 6a, 6b with that on the
bottom surfaces when the gate oxide films 7 are formed on the
surfaces defining the gate trenches 6 by thermal oxidization.
Therefore, if the device design is made based on the thickness of
the gate oxide films 7 on the sidewall surfaces 6a, 6b, it is
possible to thicken the gate oxide films 7 on the bottom surfaces
compared with the case that the main surface of a substrate is an
Si(100) surface and the sidewall surfaces and the bottom surfaces
are respectively Si{110} surfaces and Si{100} surfaces. Thus, it is
possible to increase the breakdown voltage of the gate oxide films
7.
[0091] In this embodiment, the main surface of the substrate is an
Si(100) surface, and the sidewall surfaces of the epitaxial trench
2 are an Si(001) surface, an Si(00-1) surface, an Si(010) surface,
and an Si(0-10) surface. However, not limiting to the above
surfaces, other surfaces may be used as long as they are Si{100}
surfaces. The epitaxial trench 2 may be formed such that only a
pair of long sidewall surfaces 2a are Si{100} surfaces. The total
area of the short sidewall surfaces 2b of the epitaxial trench 2
formed in this semiconductor device is much smaller than that of
the long sidewall surfaces 2a. Therefore, even if the epitaxial
trench 2 is formed such that only the pair of long sidewall
surfaces 2a are Si{100} surfaces, the effect of suppressing crystal
defects can be obtained. Although the gate oxide films 7 are formed
by thermal oxidization in this embodiment, the gate oxide films 7
may be formed by CVD.
[0092] FIG. 8 is a plan view of a 3-D power MOSFET as a second
example of this embodiment. FIG. 9 is a perspective sectional view
of the area C in FIG. 8. This power MOSFET has a super junction
structure with p-type RESURF layers 20 unlike the power MOSFET of
the first example. The p-type RESURF layers 20 are formed to extend
vertically from the main surface 1a in the n.sup.--type drift
region 4 with a space therebetween, that is, in a striped pattern,
and each of the p-type RESURF layers 20 extends parallel to the
current flowing direction. In other words, the n.sup.--type drift
regions 4 and the p-type RESURF layers 20 are arranged alternately
to form a p/n column layer. This power MOSFET is the same as the
power MOSFET of the first example in other constitution, and the
same elements are denoted using the same symbols as in the first
example.
[0093] This power MOSFET is formed by adding the step of forming
the p-type RESURF layers 20 to the process for manufacturing the
power MOSFET of the first example. For example, after the step
shown in FIG. 3E, a plurality of RESURF trenches 21 are formed to
extend vertically from the main surface 1a in the drift region with
a space therebetween. At that time, the RESURF trenches 21 are
formed to extend orthogonally to the long sidewall surfaces 2a of
the epitaxial trench 2. That is, the RESURF trenches 21 are formed
such that the long sidewall surfaces 21a of the RESURF trenches 21
become orthogonal to the long sidewall surfaces 2a of the epitaxial
trench 2 and the short sidewall surfaces 21b of the RESURF trenches
21 become parallel to the long sidewall surfaces 2a of the
epitaxial trench 2.
[0094] With the above arrangement, all the sidewall surfaces of the
RESURF trenches 21 become Si{100}surfaces. Since the bottom
surfaces of the RESURF trenches 21 are formed parallel to the main
surface 1a of the substrate, all the surfaces defining the RESURF
trenches 21 are Si{100} surfaces. Thereafter, the p-type RESURF
layers 20 are formed in the RESURF trenches 21 by the epitaxial
growth method. Next, trench gates are formed in the same manner as
in the first example.
[0095] In this second example, the p-type RESURF layers 20 are
formed on the surfaces defining the RESURF trenches 21, which are
all Si{100} surfaces. Therefore, the stress that is otherwise
generated near the lower corners of the RESURF trench 21 can be
reduced when the p-type RESURF layers 20 are formed in addition to
the effect obtained in the power MOSFET of FIG. 1. In addition,
since the p-type RESURF layers 20 are formed on Si{100} surfaces,
they can have an excellent crystallographic structure.
[0096] FIG. 10 is a plan view of a 3-D power MOSFET as a third
example of this embodiment. FIG. 11 is a perspective sectional view
of the area D in FIG. 10. In the first example, the source region
1, the base region 3, the drift region 4, and the drain region 5
are formed sequentially from the outer side to the inner side. In
contrast, in this power MOSFET, an n.sup.+-type substrate 1 that
forms a drain region 1, an n.sup.--type drift region 31, a p-type
base region 32, and an n.sup.+-type source region 33 are formed
sequentially from the outer side to the inner side.
[0097] Gate trenches 6 are formed to extend vertically from the
main surface 1a of the n.sup.+-type substrate 1, that is,
substantially parallel to the depth direction of the substrate. The
gate trenches 6 intersect the p-type base region 32 from the
n.sup.+-type source region 33 in a direction parallel to the main
surface 1a of the n.sup.+-type substrate and the depth direction of
the gate trenches 6. Agate electrode 8 is formed on the surfaces
defining each of the trenches 6 with a gate insulating film 7
therebetween.
[0098] In this power MOSFET, the n.sup.--type drift region 31 is
formed on the surfaces defining an epitaxial trench 2, which has
been formed in the n.sup.+-type substrate 1, the main surface 1a of
which is on an Si(100) surface. Like the first example, the bottom
surface 2c and the sidewall surfaces 2a, 2b of the epitaxial trench
2 are all Si{100} surfaces. Source contact regions 34 are formed in
the n.sup.+-type source region 33. At the trench gates, as well as
the first example, the surfaces in contact with the gate insulating
films 7 of the base region 32 are Si{100}surfaces and have the same
effect as in the first example.
[0099] Subsequently, the process for manufacturing this power
MOSFET will be explained. In this example, after the epitaxial
trench 2 is formed in the n.sup.+-type substrate 1, each layer is
formed in the epitaxial trench 2 in a different order from that of
the first example. First, an n.sup.+-type substrate 1 having an
Si(100) surface as a main surface 1a is prepared. Thereafter, the
epitaxial trench 2 having a predetermined depth is formed from the
main surface 1a. The epitaxial trench 2 is formed in such a manner
that the bottom surface 2c and the long sidewall surfaces 2a and
the short sidewall surfaces 2b of the epitaxial trench 2 all become
Si{100} surfaces.
[0100] Then, an n.sup.--type drift region 31, a p-type base region
32, and an n.sup.+-type source region 33 are sequentially formed on
the surfaces defining the epitaxial trench 2 by epitaxial growth.
Subsequently, the gate trenches 6 are formed to extend from the
n.sup.+-type source region 33 through the p-type base region 32 in
the directions orthogonal to the long sidewall surfaces 2a of the
epitaxial trench 2 and in the depth direction of the substrate.
Next, gate insulating films 7 and gate electrodes 8 are
sequentially formed. Source contact regions 34 are formed in the
n.sup.+-type source region 33. The process for manufacturing this
power MOSFET has the same effect as in the first example.
[0101] FIG. 12 is a plan view of a 3-D power MOSFET as a fourth
example of this embodiment. FIG. 13 is a perspective sectional view
of the area E in FIG. 12. In the power MOSFET of the third example,
the gate electrodes 8 are formed symmetrical to the source region
33. However, the gate electrodes 8 may be linked to one another in
the source region 33 as in the fourth example. Further, RESURF
layers 20 may be formed in the drift layer 31 to construct a
so-called super junction structure as in the second example, as
shown in FIGS. 12 and 13. The present invention can also be applied
to this power MOSFET.
[0102] Second Embodiment
[0103] FIG. 14 is a plan view of a power MOSFET having a super
junction structure as a first example of a second embodiment of the
present invention. FIG. 15 is a cross-sectional view taken along
the line XV-XV in FIG. 14. The power MOSFET has a structure in
which the unit structure G in FIG. 14 is repeated a plurality of
times. The power MOSFET includes an n.sup.+-type substrate 41,
which is a semiconductor substrate, and a p/n column layer 51. The
p/n column layer 51 includes n-type drift regions 42 and p-type
silicon regions 43, which are first semiconductor regions. The
power MOSFET further includes p-type base regions 44, which are
second semiconductor regions 44, upper n-type silicon regions 45,
n.sup.+-type source regions 46, and gate electrodes 47.
[0104] The main surface of the n.sup.+-type substrate 41 is an
Si(100) surface and the thickness of the substrate is, for example,
2 .mu.m. The n.sup.+-type substrate 41 makes up an n.sup.+-type
drain region. The n-type drift regions 42 are located on the
n.sup.+-type substrate 41 and have crystallographically the same
surface as that of the n.sup.+-type substrate 41. The n-type drift
regions 42 have, for example, an impurity concentration of
2.8.times.10.sup.16 cm.sup.-3, a width of 1 .mu.m, and a thickness
of 10 .mu.m. These values are selected to ensure that the n-type
drift regions 42 can be completely depleted when a predetermined
reverse voltage is applied. The n-type drift regions 42 and the
n.sup.+-type substrate 41 make up another semiconductor
substrate.
[0105] The n-type drift regions 42 define first trenches 50, or
epitaxial trenches 50. The epitaxial trenches 50 is rectangular
when viewed from above their entrances In this embodiment, the long
sidewall surfaces 50a and short sidewall surfaces 50b of the
epitaxial trenches 50 are Si{100} surfaces. More specifically, one
of a pair of the long sidewall surfaces 50a is an Si(001) surface
and the other surface is an Si(00-1) surface. One of a pair of the
short sidewall surfaces 50b is an Si(010) surface and the other
surface is an Si(0-10) surface. The bottom surfaces 50c of the
trenches 50 are parallel to the main surface of the n.sup.+-type
substrate 41 and Si(100) surfaces. Thus, the surfaces defining the
epitaxial trenches 50 are all Si{100} surfaces.
[0106] The p-type silicon regions 43 are located in the epitaxial
trenches 50. The p-type silicon regions 43 have an impurity
concentration of, for example, 1.times.10.sup.16 cm.sup.3. The
p-type silicon regions 43 have, for example, a width of 3 .mu.m and
a thickness of 10 .mu.m. However, the p-type silicon regions 43
have, for example, a width of 1 .mu.m at the portion between the
upper n-type silicon regions 45, which are described later. The
aforementioned values are selected to ensure that the p-type
silicon regions 43 can be completely depleted when a predetermined
reverse voltage is applied.
[0107] The bottom surfaces 43c and the sidewall surfaces 43a, 43b
of the p-type silicon regions 43 are respectively
crystallographically the same as the adjoining bottom surfaces 50c
and the adjoining sidewall surfaces 50a, 50b of the epitaxial
trenches 50, that is, Si{100}surfaces. The n-type drift regions 42
and the p-type silicon regions 43 are arranged alternately in the
directions orthogonal to the moving direction of carriers, that is,
in the horizontal directions in FIG. 15. In other words, the n-type
drift regions 42 and the p-type silicon regions 43 join at the
corresponding pn junctions 50, which are the sidewall surfaces of
the epitaxial trenches 50.
[0108] The pn junctions 50 are formed intermittently in horizontal
directions by the alternate arrangement of the n-type drift regions
42 and the p-type silicon regions 43. Out of the interfacial
surfaces 50a, 50b between the n-type drift regions 42 and the
p-type silicon regions 43, the long interfacial surfaces 50a that
are orthogonal to the horizontal direction are Si{100} surfaces. A
so-called super junction structure 51 is formed by the alternate
arrangement of the n-type drift regions 42 and the p-type silicon
regions 43.
[0109] The p-type base regions 44 are located on the corresponding
p-type silicon regions 43 in direct contact with the p-type silicon
region 43. The p-type base regions 44 have, for example, an
impurity concentration of 5.times.10.sup.16 cm.sup.-3 and a
thickness of 1.5 .mu.m. A p.sup.+-type base contact region 52
having a thickness of 0.5 .mu.m is located on the upper surface of
each of the p-type base regions 44. The upper n-type silicon
regions 45 are located between the top surfaces of the n-type drift
regions 42 and the lower surfaces of the p-type base regions 44 and
between the n-type drift regions 42 and the bottom surfaces 53c of
gate trenches 53, which are second trenches 53. The upper n-type
silicon regions 45 are arranged to include substantially all the
carrier passages that connect the n-type drift regions 42 and the
p-type base regions 44.
[0110] In this embodiment, the upper n-type silicon regions 45
have, for example, an impurity concentration of 1.times.10.sup.16
cm.sup.-3, which differs from that of the n-type drift regions 42.
However, the impurity concentration of the upper n-type silicon
regions 45 may be 2.8.times.10.sup.16 cm.sup.-3, which is the
impurity concentration of the n-type drift regions 42. The upper
n-type silicon regions 45 are also in contact with the p-type base
region 44 and further in contact with n-type channel regions 44a,
at which n-type channels are formed in the p-type base regions 44.
The upper n-type silicon regions 45 wider than the n-type drift
regions 42 and the gate trench 53. The upper n-type silicon regions
45 have, for example, a width of 3 .mu.m and a thickness of 1
.mu.m. The upper n-type silicon regions 45 wider than the n-type
drift regions 42 and the gate trench 53 substantially by the length
of the n-type drift region 42 and the gate trench 53 on both sides
in the horizontal directions of FIG. 15. The upper n-type silicon
regions 45 define the corners 53d of the bottom surfaces of the
gate trenches 53.
[0111] The n.sup.+-type source regions 46 are in contact with the
top surfaces of the p-type base regions 44. The n.sup.+-type source
regions 46 have a thickness of 0.5 .mu.m. The gate electrodes 47
are located on the surfaces defining the gate trenches 53, which
extend through the base region 44 from the main surface, with the
gate oxide films 54, or gate insulating films 54, therebetween As
shown in FIG. 14, the gate trenches 53 are rectangular when viewed
from above their entrances and parallel to the epitaxial trenches
50. That is, the long sidewall surfaces 53a of the gate trenches 53
are parallel to the long sidewall surfaces 50a of the epitaxial
trenches 50 and are Si{100} surfaces. The thickness of the gate
oxide films 54a on the sidewall surfaces of the gate trenches 53 is
equal to that on the bottom surfaces of the gate trenches 53. In
the sectional structure shown in FIG. 15, the gate electrodes 47
have, for example, a width of 1 .mu.m and a depth of 2.5 .mu.m. The
gate oxide films 54 have a width of 0.1 .mu.m.
[0112] Subsequently, the operation of the power MOSFET of this
embodiment will be explained. Positive voltage is applied to the
n.sup.+-type drain region 41 in FIG. 15, and the n.sup.+-type
source regions 46 and the p.sup.+-type base contact regions 52 are
grounded. When the power MOSFET is turned on in this state, that
is, when positive voltage is applied to the trench gate electrodes
47, electrons in the p-type base regions 44 are drawn to the n-type
channel regions 44a to form n-type channels. The electrons supplied
from the n.sup.+-type source regions 46 flow in the n-type channel
regions 44a, the upper n-type silicon regions 45, and the n-type
drift regions 42 to reach the n.sup.+-type drain region 41.
[0113] Next, the method for manufacturing the semiconductor device
according to this embodiment will be explained. FIGS. 16A to 16H
show the process for manufacturing a semiconductor device according
to this embodiment.
[0114] [Steps Shown in FIG. 16A]
[0115] An n.sup.+-type substrate 41 that has an Si(100) surface as
a main surface and makes up an n.sup.+-type drain region 41 is
prepared. An n-type drift layer 42, which becomes n-type drift
regions 42, is formed on the n.sup.+-type substrate 41 by epitaxial
growth. The surface of the n-type drift layer 42 becomes an Si(100)
surface.
[0116] [Steps Shown in FIG. 16B]
[0117] Dry etching is carried out using a resist as a mask in the
same manner as at the steps shown in FIG. 3B, FIG. 4 and FIG. 5 in
the first embodiment. With the dry etching, epitaxial trenches 50,
which reach the n.sup.+-type drain region 41 through the n-type
drift layer 42, are formed. The epitaxial trenches 50 are formed
such that the long sidewall surfaces 50a, the short sidewall
surfaces 50b, and the bottom surfaces 50c of the epitaxial trenches
50 become Si{100} surfaces.
[0118] [Steps Shown in FIG. 16C]
[0119] Subsequently, a p-type silicon layer 63 is formed to fill
the trenches 50 by epitaxial growth. At that time, since the bottom
surfaces 50c and the sidewall surfaces 50a, 50b of the epitaxial
trenches 50 are all Si{100} surfaces, the p-type silicon layer 63
grows at the same growth rate on the bottom surfaces 50c and the
sidewall surfaces 50a, 50b.
[0120] [Steps Shown in FIGS. 16D and 16E]
[0121] Then, as shown in FIG. 16D, the p-type silicon layer 63 is
planarized by Chemical Mechanical Polishing (CMP) to the height of
the top surfaces of the n-type drift regions 42 to form a p/n
column layer 51. Thereafter, as shown in FIG. 16E, an n-type layer
65 having a thickness Ta is formed by epitaxial growth. The
thickness Ta of the n-type layer 65 is equal to the total of the
thickness of upper n-type silicon regions 45 and the thickness of
p-type base regions 44, which are formed at later steps.
[0122] [Steps Shown in FIG. 16F]
[0123] A p-type impurity is added to the n-type layer 65 at the
central area of the portions located on the p-type silicon regions
43 by ion implantation. As a result, p-type connection region 55
are formed, and the upper n-type silicon regions 45 defined by the
p-type connection region 55 are formed at the same time. Then, a
p-type impurity is added to the entire regions above the upper
n-type silicon regions 45 by, for example, ion implantation. As a
result, a p-type base layer 44, or a second semiconductor layer 44,
from which p-type base regions 44 are formed, is formed. In the
above ion implantation, the implanting speed and dose of ions are
controlled to ensure that the thicknesses of the upper n-type
silicon regions 45 and the p-type connection regions 55 become Tc
and that the thickness of the p-type layer 44 becomes Tb.
[0124] [Steps Shown in FIG. 16G]
[0125] Then, gate trenches 53 that reach the upper n-type silicon
regions at their central areas 45 through the p-type base layer 44
are formed right above the n-type drift regions 42 by dry etching
using a resist as a mask. At that time, as shown in FIG. 14, the
gate trenches 53 are formed to extend parallel to the epitaxial
trenches 50. That is, the long sidewall surfaces 53a of the gate
trenches 53 and the long sidewall surfaces 50a of the epitaxial
trenches 50 become parallel to each other. With the above
arrangement, the long sidewall surfaces 53a of the gate trenches 53
become Si{100} surfaces. In other words, the surfaces of the p-type
base regions 44 that define the gate trenches 53 become Si{100}
surfaces. In this embodiment, as shown in FIG. 16G, the width of
the gate trenches 53 is wider than the width of the n-type drift
regions 42.
[0126] [Steps Shown in FIG. 16H]
[0127] Then, silicon oxide films 54 are formed on the sidewall
surfaces and bottom surfaces of the gate trenches 53 by, for
example, thermal oxidization. Thereafter, gate electrodes 47 made
of polysilicon are formed on the silicon oxide films 54 by, for
example, CVD. Finally, as shown in FIG. 15, n.sup.+-type source
regions 46 are formed in the surfaces of the base layer 44 by, for
example, implanting arsenic or phosphorus ions. Then, p.sup.+-type
base contact regions 52 are formed in the surfaces of the base
layer 44 by, for example, implanting boron ions. With the above
steps, the power MOSFET shown in FIGS. 14 and 15 is
manufactured.
[0128] In this embodiment, the epitaxial trenches 50 are formed at
the step shown in FIG. 16B such that the surfaces defining the
epitaxial trenches 50 are all Si{100}surfaces. Therefore, when the
p-type silicon layer 63 is formed to fill the trenches 50 by
epitaxial growth at the step shown in FIG. 16C, the p-type silicon
layer 63 grows on the bottom surfaces and the sidewall surfaces of
the epitaxial trenches 50 at the same growth rate. As a result, the
generation of crystal defects is suppressed as in the first
embodiment, and the p-type silicon regions 43 become to have an
excellent crystallographic structure.
[0129] In this embodiment as well, the sidewall surfaces 53a, 53b
and the bottom surfaces 53c of the gate trenches 53 have
crystallographically the same plane orientation, so it is possible
to equate the thickness of the gate oxide films 54 on the sidewall
surfaces 53a, 53b with that on the bottom surfaces 53c when the
gate oxide films 54 are formed by thermal oxidization. Therefore,
if the device design is made based on the thickness of the gate
oxide films 54 on the long sidewall surfaces 53a of the gate
trenches 53, it is possible to thicken the gate oxide films 54 on
the bottom surfaces 53c compared with the case explained in the
background of the invention, in which the main surface of a
substrate is an Si(100) surface and the sidewall surfaces and the
bottom surfaces of the gate trenches are respectively Si{110}
surfaces and Si{100} surfaces. Thus, it is possible to increase the
breakdown voltage of the gate oxide films 54.
[0130] In addition, because the surfaces defining the gate trenches
53 are all Si{100} surface, that is, the channel surfaces are
Si{100} surfaces, the density of the interface traps between the
channel regions and the gate insulating films 54 and the electron
mobility can be equal to those in the first embodiment. In this
embodiment as well as the first embodiment, the epitaxial trenches
50 may be formed such that only the pairs of long sidewall surfaces
50a of the epitaxial trenches 50 are Si{100} surfaces. In addition,
as long as the pairs of long sidewall surfaces 53a of the epitaxial
trenches 53 are Si{100} surfaces, the short sidewall surfaces 53b
of the epitaxial trenches 53 may not be Si{100} surfaces and may
have other crystallographic planes. Even in that case, it is
possible to improve the carrier mobility and increase the current
flowing through the channel regions.
[0131] In this embodiment, the trenches formed in the n-type drift
layer 42 are filled with the p-type silicon layer 63 to form the
p/n column layer. However, the p/n column layer can be formed in
the opposite order. That is, a p-type silicon layer 43 may be
formed first, and then the trenches formed in the p-type silicon
layer 43 may be filled with an n-type drift layer 42. In that case,
the following steps are carried out in place of the steps of FIGS.
16A to 16C. First, a p-type silicon layer is formed on an
n.sup.+-type substrate, the main surface of which is Si(100)
surface, by epitaxial growth. Thereafter, a plurality of epitaxial
trenches are formed in the p-type silicon layer in the same manner
as shown in FIG. 16B. The surfaces defining the epitaxial trenches
are all Si{100} surfaces.
[0132] Then, an n-type drift layer 42 is formed to fill the
epitaxial trenches by epitaxial growth and planarized by CMP to
form a p/n column layer 51, as shown in FIG. 16D. Thereafter, the
steps shown in FIG. 16E and others are carried out. Even in that
method, the n-type drift regions 42 are formed in the epitaxial
trenches defined by surfaces that are all Si{100} surfaces, so it
is possible to suppress the generation of crystal defects in the
n-type drift regions 42. As a result, the n-type drift regions 42
become to have an excellent crystallographic structure.
[0133] Although the gate oxide films 54 are formed by thermal
oxidization in this embodiment, the gate oxide films 54 may be
formed by CVD. In addition, although the epitaxial trenches 50 and
the gate trenches 53 are arranged to be parallel to each other in
this embodiment, the epitaxial trenches 50 and the gate trenches 53
may be arranged to be orthogonal to each other, as explained
below.
[0134] FIG. 17 is a plan view of a power MOSFET having a super
junction structure as a second example of the second embodiment,
and the same elements are denoted using the same symbols as in the
power MOSFET of FIG. 14. The power MOSFET shown in FIG. 17 has a
structure in which the unit structure G1 of FIG. 17 is repeated a
plurality of times at the trench gate areas and the unit structure
G2 of FIG. 17 is repeated a plurality of times at the p/n column
areas.
[0135] In the power MOSFET shown in FIG. 17, gate trenches 53 are
arranged such that the long sidewall surfaces 53a of the gate
trenches 53 are orthogonal to the long sidewall surfaces 50a of
epitaxial trenches 50. In addition, the long sidewall surfaces 53a
of the gate trenches 53 are orthogonal to the short surfaces 53b of
the gate trenches 53, although not shown in the figure. With this
arrangement as well, it is possible to make all the surfaces
defining the gate trenches 53 Si{100} surfaces.
[0136] In the first and second examples of this embodiment, upper
n-type silicon regions 45 are located on n-type drift regions 42.
However, the upper n-type silicon regions 45 may not be located on
the n-type drift regions 42. In that case, the gate trenches 53
reach the n-type drift regions 42. However, the structure that the
upper n-type silicon regions 45 are located on the n-type drift
regions 42 is better than the structure that the upper n-type
silicon regions 45 are not employed due to the following two
reasons.
[0137] In the structure otherwise shown in FIG. 15, if the upper
n-type silicon regions 45 are not employed, the corners 53d of the
bottom surfaces of the gate trenches 53 are located in the p-type
silicon regions 43. In that case, the breakdown voltage decreases
because the electric field concentrates at the proximities of the
corners 53d of the bottom surfaces of the gate trenches 53 located
in the p-type silicon regions 43. On the other hand, if the upper
n-type silicon regions 45 are located on the n-type drift regions
42 as shown in FIG. 15, the electric field does not concentrate in
the p-type silicon regions 43, but in the upper n-type silicon
regions 45. As a result, it is possible to increase the breakdown
voltage in comparison with the case that the upper n-type silicon
regions 45 are not employed.
[0138] In addition to that, even if an error happens in the mask
alignment when the gate trenches 53 are formed, there are no
problems as long as the gate trenches are located within the upper
n-type silicon regions 45. For this reason, it is possible to
decrease the influence of the alignment error on device
performances.
[0139] Third Embodiment
[0140] In the second embodiment, a power MOSFET having a super
junction structure is formed using a substrate that has an Si(100)
surface as its main surface. On the other hand, in this embodiment,
a power MOSFET having a super junction structure is formed using a
substrate that has an Si{110} surface as its main surface.
[0141] FIG. 18A is a plan view of a power MOSFET having a super
junction structure as a third embodiment, showing the layout of p/n
columns. FIG. 18B is a plan view of the area 62 in FIG. 18A,
showing the layout of trench gates. FIG. 19 is a perspective
sectional view of the MOSFET in FIG. 18A, showing the
cross-sectional view taken along the line XIX-XIX in FIG. 18A. In
FIGS. 18A, 18B, and 19, the same elements are denoted using the
same symbols as in the power MOSFET of FIGS. 14 and 15.
[0142] The power MOSFET in this embodiment differs from those in
the second embodiment in the crystallographic plane orientation of
an n.sup.+-type substrate 41 and the crystallographic plane
orientations of the surfaces defining epitaxial trenches 50 and
gate trenches 53. Specifically, the power MOSFET shown in FIGS. 21
and 22 includes an n.sup.+-type substrate 41 that has an Si(100)
surface as its main surface, a p/n column layer 51, P.sup.--type
base regions 44, p-type channel regions 61, n.sup.+-type source
regions 46, and gate electrodes 47. This power MOSFET differs from
those in the second embodiment in: that the base regions 44 are of
P.sup.--type instead of P-type; that no upper n-type silicon
regions 45 are employed; and that the p-type channel regions 61 are
employed. The base regions 44 and the p-type channel regions 61
make up second semiconductor regions of second conductivity type in
the present invention.
[0143] In this embodiment as well, the p/n column layer 51 is made
up of n-type drift regions 42 and p-type silicon regions 43. The
n-type drift regions 42 are located on the n.sup.+-type substrate
41, and the upper and lower surfaces of the n-type drift regions 42
have crystallographically the same plane orientation as the upper
surface of the n.sup.+-type substrate 41 in FIG. 19. The p-type
silicon regions 43 are formed by filling the epitaxial trenches 50
defined by the n-type drift regions 42 with an epitaxial growth
film.
[0144] As shown in FIG. 18A, each of the epitaxial trenches 50 is a
parallelogram, adjoining sides of which have a length different
from each other and an internal angle of which is 70.5 degrees,
when viewed from above their entrances. The epitaxial trenches 50
are arranged in a stripe pattern layout, in which the epitaxial
trenches 50 extend parallel to one another. Each side of the
parallelogram, that is, the long sidewall surfaces 50a and the
short sidewall surfaces 50b are both Si{111}surfaces. Specifically,
one of the long sidewall surfaces 50a is Si(-111) surface, and one
of the short sidewall surfaces 50b is Si(-11-1) surface. The bottom
surfaces 50c of the epitaxial trenches 50 are parallel to the upper
surface of the n.sup.+-type substrate 41 and Si(110) surface.
[0145] Therefore, the p-type silicon regions 43 located in the
epitaxial trenches 50 are also arranged in a stripe pattern layout,
in which the p-type silicon regions 43 in the shape of a thin
parallelogram extend parallel to one another. The sidewall surfaces
43a, 43b and the bottom surface 43c of the p-type silicon regions
43 respectively have crystallographically the same plane
orientations as the adjoining surfaces 50a, 50b and the bottom
surface 50c of the epitaxial trenches 50.
[0146] Therefore, in this embodiment, out of the interfaces 50a,
50b between the n-type drift regions 42 and the p-type silicon
regions 43, which makeup the p/n column layer 51, the long sidewall
surfaces 50a are Si{111} surfaces, and the short sidewall surfaces
50b are also Si{111} surfaces.
[0147] As shown in FIG. 18B, the gate electrodes 47 are arranged in
a stripe pattern layout in an active area 62. The gate electrodes
47 are located on the surfaces defining the gate trenches 53, which
extend from the surface to the p/n column layer 51 through the
p-type channel regions 61 and the P.sup.--type base regions 44,
with the gate oxide films 54 therebetween. As shown in FIG. 18B,
the gate trenches 53 are rectangular when viewed from above their
entrance and in a stripe pattern layout. The gate trenches 53 are
arranged such that the long sidewall surfaces 53a of the gate
trenches 53 become Si{100} surfaces.
[0148] Specifically, the long sidewall surfaces 53a of the gate
trenches 53 are Si(001) surfaces, and the short sidewall surfaces
53b of the gate trenches 53 are Si(110) surfaces. The long sidewall
surfaces 53a of the gate trenches 53 are at an angle of 54.7
degrees with the long sidewall surfaces 50a of the epitaxial
trenches 50. The gate oxide films 54 are thicker at the portions
54c located on the bottom surfaces 53c of the gate trenches 53 than
at the portions 54a, 54b respectively located on the long and short
sidewall surfaces 53a, 53b of the gate trenches 53.
[0149] As shown in FIG. 19, the n.sup.+-type source regions 46 and
the p.sup.+-type base contact regions 52 are also arranged in a
stripe pattern layout as well as the gate electrodes 47. As
described above, the power MOSFET in this embodiment, the p-type
silicon regions 43 and the gate electrodes 47 are arranged in a
stripe pattern layout. However, the stripe pattern layouts are
different in directions from each other.
[0150] Next, the method for manufacturing a semiconductor device in
this embodiment will be explained. FIGS. 20A to 20G show a method
for manufacturing the semiconductor device in this embodiment.
[0151] [Steps Shown in FIG. 20A]
[0152] An n.sup.+-type substrate 41 that has an Si(100) surface as
a main surface and makes up an n.sup.+-type drain region 41 is
prepared. An n-type drift layer 42, which becomes n-type drift
regions 42, is formed on the n.sup.+-type substrate 41 by epitaxial
growth.
[0153] [Steps Shown in FIG. 20B]
[0154] Although not illustrated in the figure, a silicon oxide film
is formed on a surface of the n-type drift layer 42 and patterned
by photolithography. Then, the n-type drift layer 42 is etched
using this patterned silicon oxide film as a mask to form epitaxial
trenches 50, which are in the shape of, for example, a
parallelogram when viewed from above their entrances, and to form
n-type drift regions 42. The n-type drift layer 42 is
anisotropically etched using an anisotropic wet etchant containing
TMAH or KOH.
[0155] Here, an explanation will be made on the crystallographic
planes that are exposed on the surfaces defining the epitaxial
trenches 50. FIGS. 21 and 22 are tables that show the positional
relations between the orientation flats 41c, the epitaxial trenches
50, and the gate trenches 53, on which a description will be made
later, of semiconductor wafers. In FIG. 21, the orientation flats
41c are parallel or orthogonal to the long sidewall surfaces 50a of
the epitaxial trenches 50 in consideration of the readiness in mask
designing. In FIG. 22, the orientation flats 41c are parallel or
orthogonal to the long sidewall surfaces 53a of the gate trenches
53 in consideration of the readiness in mask designing. In FIGS. 21
and 22, "OF" stands for orientation flat.
[0156] The following is an example for the formation of the
epitaxial trenches 50, in which epitaxial trenches 50 are formed in
the positional relations shown in (1) of FIG. 21.
[0157] In (1) of FIG. 21, a semiconductor wafer 41 has an Si(110)
surface and an Si(-111) surface as a main surface and an
orientation flat 41c, respectively. Epitaxial trenches 50 are
formed such that the long sidewall surfaces 50a of the epitaxial
trenches 50 are parallel to the orientation flat 41c and the short
sidewall surfaces 50b of the epitaxial trenches 50 are at an angle
of 109.5 degrees with the orientation flat 41c when measured
counterclockwise from the orientation flat 41c. With this
arrangement, the long sidewall surfaces 50a and the short sidewall
surfaces 50b become Si(-111) surfaces and Si(-11-1) surfaces,
respectively. That is, the sidewall surfaces 50a, 50b become
Si{111} surfaces.
[0158] The bottom surfaces 50c of the epitaxial trenches 50 are
formed to become Si(110) surfaces by forming the epitaxial trenches
50 such that the bottom surfaces 50c become parallel to the main
surface. The width of the epitaxial trenches 50 is set to be 0.1 to
50 .mu.m. The depth of the epitaxial trenches 50 is set to be 1 to
50 .mu.m and smaller than the thickness of the n-type drift regions
42. After the epitaxial trenches 50 are formed, the silicon oxide
film that has been used as an etching mask is removed by etching
using a hydrofluoric acid aqueous solution.
[0159] [Steps Shown in FIG. 20C]
[0160] Then, a p-type silicon layer 63 is deposited to fill the
epitaxial trenches 50 by epitaxial growth using LP-CVD.
[0161] [Steps Shown in FIG. 20D]
[0162] The p-type silicon layer 63 is planarized by polishing it to
the same height as the n-type drift regions 42 using CMP. With the
planarization, a p/n column layer 51, which are made up of the
n-type drift regions 42 and p-type silicon regions 43, is formed.
Instead of CMP, the p-type silicon layer 63 may be planarized by
etchback using dry etching.
[0163] [Steps Shown in FIG. 20E]
[0164] Next, a p-type silicon layer 64, which becomes p-type base
regions 44, is deposited on the p/n column layer 51 by epitaxial
growth using LP-CVD.
[0165] [Steps Shown in FIG. 20F]
[0166] Then, a p-type channel layer 61, which becomes p-type
channel regions 61, and an n.sup.+-type source layer 46, which
becomes n.sup.+-type source regions 46, are formed by ion
implantation and thermal diffusion. The steps shown in FIGS. 20C to
20F can be modified. For example, it is possible to omit the steps
shown in FIG. 20E by leaving the p-type silicon layer 63 above the
n-type drift regions 42, not completely removing the p-type silicon
layer 63 above the n-type drift regions 42. In that case, the
p-type silicon layer 63 can be used as a p-type channel layer
61.
[0167] It is also possible to directly form a p-type channel layer
61 and an n.sup.+-type source layer 46 in a surface of a p/n column
layer 51 by implanting ions into the p/n column layer 51 after the
steps shown in FIG. 20D without the steps shown in FIG. 20E.
[0168] [Steps Shown in FIG. 20G]
[0169] Then, gate trenches 53 for gate electrodes are formed by
etching. The etching is a dry etching using RIE. The gate trenches
53 are formed to become rectangular when viewed from above their
entrances. The long sidewall surfaces 53a and the bottom surfaces
53c of the gate trenches 53 are Si{100} surfaces and Si(110)
surfaces, respectively. Specifically, when the orientation flat 41C
of the n.sup.+-type substrate 41 is Si(-111) surface, the gate
trenches 53 are formed such that the long sidewall surfaces 53a are
at an angle of 54.7 degrees with the orientation flat 41C, as shown
in (1) of FIG. 21. With this arrangement, the long sidewall
surfaces 53a become Si(001) surfaces, that is, Si{100} surfaces.
The short sidewall surfaces 53b of the gate trenches 53 are
arranged to be orthogonal to the long sidewall surfaces 53a. With
this arrangement, the short sidewall surfaces 53b become Si(110)
surfaces as well as the bottom surfaces 53c of the gate trenches
53.
[0170] Subsequently, gate oxide films 54 are formed on the surfaces
defining the gate trenches 53 by thermal oxidization. Then, gate
electrodes 47 are formed on the surfaces defining the gate trenches
53 with the gate oxide films 54 therebetween. P.sup.+-type base
contact regions 52 are formed in the n.sup.+-type source layer 46.
Then, although not illustrated in the figure, source electrodes,
drain electrodes, and so on are formed to complete a power MOSFET
show in FIGS. 18A, 18B, and 19.
[0171] In this embodiment, the epitaxial trenches 50 are formed by
wet etching at the step shown in FIG. 20B. Therefore, it is
possible to lessen the damage to the surfaces defining the
epitaxial trenches 50 in comparison with the case that the
epitaxial trenches 50 are formed by dry etching. As a result, it is
possible to suppress the generation of crystallographic defects.
Thus, when the p-type silicon layer 63 is formed to fill the
epitaxial trenches 50 by epitaxial growth, it is possible to
suppress the generation of crystallographic defects in the p-type
silicon layer 63 in comparison with the case that the epitaxial
trenches 50 formed by dry etching.
[0172] In addition, an etching bath capable of a batch wafer
processing can be used for the above wet etching. Therefore, it is
possible to reduce production costs in comparison with the case
that the epitaxial trenches 50 are formed by dry etching such as
RIE, which is usually a single wafer processing. Moreover, in this
embodiment, the long sidewall surfaces 53a and the bottom surfaces
53c of the gate trenches 53 for the gate electrodes are Si{100}
surfaces and Si{110} surfaces, respectively. That is, in the power
MOSFET of this embodiment as well as the first and second
embodiments, the channel surfaces are Si{100} surfaces.
[0173] FIG. 23 is a table that shows the electron mobility in the
channel regions, the density of interface trap, and the relation in
the thickness of the gate oxide film between on the sidewall
surfaces and on the bottom surfaces of the power MOSFETs according
to the first, second, and third embodiments. FIG. 23 also shows the
same items with respect to the proposed method, which is described
in the background section and in which the channel surfaces are
Si{111} surfaces.
[0174] In this embodiment, the long sidewall surfaces 53a and the
bottom surfaces 53c of the gate trenches 53 for the gate electrodes
are Si{100} surfaces and Si{110} surfaces, respectively. Therefore,
when the gate oxide films 54 are formed by thermal oxidization, the
thickness of the gate oxide films 54c on the bottom surfaces 53c of
the gate trenches 53 can be thicker than that of the gate oxide
films 54a on the long sidewall surfaces 53a of the gate trenches
53.
[0175] In addition, since the short sidewall surfaces 53b of the
gate trenches 53 are Si(110), the thickness of the gate oxide films
54c on the bottom surfaces 53c of the gate trenches 53 can be equal
to that of the gate oxide films 54b on the short sidewall surfaces
53b of the gate trenches 53. Therefore, even if the devise
designing is made on the basis of the thickness of the gate oxide
films 54a on the long sidewall surfaces 53a of the gate trenches
53, it is possible to thicken the gate oxide films 54c on the
bottom surfaces 53c of the gate trenches 53 in comparison with the
case that the main surface is an Si(110) surface and the sidewall
surfaces of the gate trenches are Si{111} surfaces, which is the
case described in the background section. Therefore, it is possible
to improve the breakdown voltage.
[0176] As shown in FIG. 23, when the channel surfaces are Si{111}
surfaces as in the proposed method, the density of the interface
traps between the channel regions and the gate insulating films is
1.7.times.10.sup.11 cm.sup.-2 and the electron mobility in the
channel regions is 430 cm.sup.2/V*s. In contrast, in this
embodiment, they are 2.times.10.sup.10 cm.sup.-2 and 600
cm.sup.2/V*s, respectively. In this embodiment, the channel
surfaces are Si{100} surfaces, so it is possible to reduce the
density of the interface traps and increase the electron mobility
in comparison with the case that the channel surfaces are Si{111}
surfaces.
[0177] Moreover, in this embodiment, the epitaxial trenches 50 and
the gate trenches 53 are formed using a substrate, the main surface
of which is Si(110) surface, in the arrangement shown in (1) of
FIG. 21. However, the epitaxial trenches 50 and the gate trenches
53 can be formed in the arrangements shown in (2) to (4) of FIG. 21
and (5) to (8) of FIG. 22. In the arrangements shown in (2) to (8)
of FIGS. 21 and 22, each of the epitaxial trenches 50 is a
parallelogram, an internal angle of which is 70.5 degrees, when
viewed from above their entrances, and the gate trenches 53 are
rectangular when viewed from above their entrances. In (1) and (2)
of FIG. 21, the orientation flat 41c is an Si{111}surface. In (3)
and (4) of FIG. 21, the orientation flat 41c is an Si{112} surface.
In (5) and (6) of FIG. 22, the orientation flat 41c is an Si{100}
surface. In (7) and (8) of FIG. 22, the orientation flat 41c is an
Si{110} surface.
[0178] As shown in (2) of FIG. 21, when the orientation flat 41c is
an Si(-11-1) surface, the long sidewall surfaces 50a of the
epitaxial trenches 50 are set to be parallel to the orientation
flat 41c and the long sidewall surfaces 53a of the gate trenches 53
are set to be at an angle of 125.2 degrees with the orientation
flat 41c. Here, the angle is defined as the one between the
extended line of the orientation flat 41c and the extended line of
each sidewall surface when measured counterclockwise from the
orientation flat 41c. The same definition will be used
hereinafter.
[0179] As shown in (3) of FIG. 21, when the orientation flat 41c is
an Si(2-1-1) surface, the long sidewall surfaces 50a of the
epitaxial trenches 50 are set to be orthogonal to the orientation
flat 41c and the long sidewall surfaces 53a of the gate trenches 53
are set to be at an angle of 144.7 degrees with the orientation
flat 41c.
[0180] As shown in (4) of FIG. 21, when the orientation flat 41c is
an Si(121) surface, the long sidewall surfaces 50a of the epitaxial
trenches 50 are set to be orthogonal to the orientation flat 41c
and the long sidewall surfaces 53a of the gate trenches 53 are set
to be at an angle of 35.2 degrees with the orientation flat
41c.
[0181] As shown in (5), (6) of FIG. 22, when the orientation flat
41c is an Si(001) surface, the long sidewall surfaces 50a of the
epitaxial trenches 50 are set to be at an angle of 125.3 degrees or
54.8 degrees with the orientation flat 41c and the long sidewall
surfaces 53a of the gate trenches 53 are set to be parallel to the
orientation flat 41c.
[0182] As shown in (7), (8) of FIG. 22, when the orientation flat
41c is an Si(-1-10) surface, the long sidewall surfaces 50a of the
epitaxial trenches 50 are set to be at an angle of 35.3 degrees or
144.8 degrees with the orientation flat 41c and the long sidewall
surfaces 53a of the gate trenches 53 are set to be orthogonal to
the orientation flat 41c.
[0183] In the arrangements shown in (1) and (3) of FIG. 21 and (5)
and (7) of FIG. 22, the long sidewall surfaces 50a of the epitaxial
trenches 50 become Si(-111) surfaces and the angles between the
long sidewall surfaces 50a of the epitaxial trenches 50 and the
long sidewall surfaces 53a of the gate trenches 53 become 54. 7
degrees. In the arrangements shown in (2) and (4) of FIG. 21 and
(6) and (8) of FIG. 22, the long sidewall surfaces 50a of the
epitaxial trenches 50 become Si(-11-1) surfaces and the angles
between the long sidewall surfaces 50a of the epitaxial trenches 50
and the long sidewall surfaces 53a of the gate trenches 53 become
125.2 degrees.
[0184] In this embodiment as well, the epitaxial trenches 50 may be
formed such that only the long sidewall surfaces 50a of the
epitaxial trenches 50 are Si{100} surfaces. In addition, as long as
the long sidewall surfaces 53a of the epitaxial trenches 53 are
Si{100} surfaces, the short sidewall surfaces 53b of the epitaxial
trenches 53 may not be Si{100} surfaces and may have other
crystallographic planes. Even in that case, it is possible to
improve the carrier mobility and increase the current flowing
through the channel regions in comparison with the proposed
method.
[0185] In this embodiment, the trenches formed in the n-type drift
layer 42 are filled with the p-type silicon layer 63 through the
steps shown in FIGS. 20A to 20D to form the p/n column layer.
However, the p/n column layer can be formed in the opposite order.
That is, a p-type silicon layer 43 may be formed first on an
n.sup.+-type substrate 41 by epitaxial growth, and then the
trenches formed in the p-type silicon layer 43 may be filled with
an n-type drift layer 42. In that case, the crystallographic plane
orientations of the surfaces are set to be the same as in this
embodiment.
[0186] Fourth Embodiment
[0187] FIG. 24 is a plan view of a power MOSFET having a super
junction structure as a fourth embodiment. The cross-sectional view
taken along the line I-I' in FIG. 24 is similar to FIG. 14. In FIG.
24, the same elements are denoted using the same symbols as in
FIGS. 17, 18A, 18B, and 19.
[0188] The power MOSFET according to this embodiment has the
structure in which upper n-type silicon regions 45 are added to the
power MOSFET shown in FIGS. 18a, 18b, and 19 such that the n-type
silicon regions 45 are wider than the n-type drift regions 42 of a
p/n column layer in the lateral directions and located on the p/n
column layer 51. That is, the upper n-type silicon regions 45 can
be added to the power MOSFET in the third embodiment such that the
n-type silicon regions 45 are located between the p-type base
regions 44, in which channels are formed, and the p/n column layer
51, as in the same manner shown in FIG. 14.
[0189] The method for manufacturing the power MOSFET in this
embodiment basically differs from that in the second embodiment in
the arrangement of the trench gates, and there are no differences
in other aspects. Therefore, the explanation on the method is
omitted.
[0190] Other Embodiment
[0191] In the second, third, and fourth embodiments, the n-type
drift layer 42 are formed on the n.sup.+-type substrate 41 by
epitaxial growth, and the epitaxial trenches 50 are formed in the
n-type drift layer 42. However, an n-type substrate may be used
instead of the n.sup.+-type substrate 41 and the n-type drift layer
42. In that case, the portion that is located within the same depth
of the epitaxial trenches 50 from the surface functions as an
n-type drift layer 42, and the rest of then-type substrate becomes
a drain region.
[0192] In each power MOSFET of the above embodiments is an n
channel power MOSFET, in which the first conductivity type is
n-type and the second conductivity type is p-type. However, the
present invention can be applied to a p channel power MOSFET, in
which the conductivity types of the elements are opposite to those
in the n channel power MOSFET.
[0193] In the above embodiments, the present invention has been
applied to power MOSFETs. However, the present invention can be
applied to IGBT, in which a collector is substituted for the drain
and an emitter is substituted for the source, and thyristor.
[0194] The foregoing invention has been described in terms of
preferred embodiments. However, those skilled, in the art will
recognize that many variations of such embodiments exist. Such
variations are intended to be within the scope of the present
invention and the appended claims.
* * * * *