U.S. patent application number 11/294411 was filed with the patent office on 2006-07-06 for semiconductor device having vertical metal insulator semiconductor transistors having plural spatially overlapping regions of different conductivity type.
Invention is credited to Hitoshi Kobayashi, Hideki Okumura, Ichiro Omura, Akihiko Osawa, Wataru Saito, Masanobu Tsuchitani, Masakazu Yamaguchi.
Application Number | 20060145290 11/294411 |
Document ID | / |
Family ID | 19189010 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060145290 |
Kind Code |
A1 |
Okumura; Hideki ; et
al. |
July 6, 2006 |
Semiconductor device having vertical metal insulator semiconductor
transistors having plural spatially overlapping regions of
different conductivity type
Abstract
A semiconductor device includes a diffusion area formed in a
semiconductor layer of a first conductive type. The diffusion area
comprises first and second impurity diffusion areas of the first
and second conductive types, respectively. The diffusion area has a
first and second areas which are defined by an impurity
concentration of the first and second impurity diffusion areas. A
junction between the first and second area is formed in a portion
in which the first and second impurity diffusion areas overlap each
other. A period of the impurity concentration, in a planar
direction of the semiconductor layer, of the first or second area
is smaller than the maximum width, in the planar direction of the
semiconductor layer, of the first and second impurity diffusion
areas constituting the first or second area.
Inventors: |
Okumura; Hideki;
(Yokohama-shi, JP) ; Kobayashi; Hitoshi;
(Yokohama-shi, JP) ; Tsuchitani; Masanobu;
(Kawasaki-shi, JP) ; Osawa; Akihiko; (Himeji-shi,
JP) ; Saito; Wataru; (Kawasaki-shi, JP) ;
Yamaguchi; Masakazu; (Kawasaki-shi, JP) ; Omura;
Ichiro; (Yokohama-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
19189010 |
Appl. No.: |
11/294411 |
Filed: |
December 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10327937 |
Dec 26, 2002 |
6995426 |
|
|
11294411 |
Dec 6, 2005 |
|
|
|
Current U.S.
Class: |
257/529 ;
257/E29.027; 257/E29.04; 257/E29.257; 438/268; 438/527 |
Current CPC
Class: |
H01L 29/7802 20130101;
H01L 29/7811 20130101; H01L 29/0634 20130101; H01L 29/0696
20130101; H01L 29/0878 20130101 |
Class at
Publication: |
257/529 ;
438/527; 438/268 |
International
Class: |
H01L 29/00 20060101
H01L029/00; H01L 21/425 20060101 H01L021/425; H01L 21/336 20060101
H01L021/336 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2001 |
JP |
2001-395558 |
Claims
1. (canceled)
2. A method of manufacturing a semiconductor device comprising:
injecting first impurities of a first conductive type and second
impurities of a second conductive type into a surface of a first
semiconductor layer of a first conductive type at a plurality of
points; and diffusing the first and second impurities to form a
first diffusion area of the first conductive type and a second
diffusion area of the second conductive type, the first diffusion
area and the second diffusion area being defined by a first
concentration profile of the first impurities and a second
concentration profile of the second impurities, a junction between
the first diffusion area and the second diffusion area being formed
where a concentration of the first impurities and a concentration
of the second impurities are same, a period of the first
concentration profile in a planar direction of the first
semiconductor layer being smaller than a maximum diffusion width of
the first impurities in the planar direction of the first
semiconductor layer.
3. The method according to claim 2, further comprising: forming a
second semiconductor layer of the first conductive type on the
first semiconductor layer after injecting the first and second
impurities into the first semiconductor layer; injecting third
impurities of the first conductive type into the second
semiconductor layer over points where the first impurities are
injected in the first semiconductor layer; and injecting fourth
impurities of the second conductive type into the second
semiconductor layer over points where the second impurities are
injected in the first semiconductor layer, wherein diffusing the
first and second impurities further includes diffusing the first to
fourth impurities until the first diffusion area and the second
diffusion area extend over the first semiconductor layer and the
second semiconductor layer.
4. The method according to claim 2, wherein the first impurities
include phosphorous and the second impurities include boron.
5. The method according to claim 3, wherein the first impurities
and the third impurities include phosphorous and the second
impurities and the fourth impurities include boron.
6. A method of manufacturing a semiconductor device comprising:
injecting first impurities of a first conductive type into a first
semiconductor layer of the first conductive type at least at two
first points; injecting second impurities of a second conductive
type into the first semiconductor layer between the two first
points; and diffusing the first impurities and the second
impurities until a first concentration profile of the first
impurities having a first period and a second concentration profile
overlap to form a first diffusion area and a second diffusion area
which are defined by the first concentration profile and the second
concentration profile, the first period being smaller in a planar
direction of the first semiconductor layer than a maximum diffusion
width of the first impurities.
7. The method according to claim 6, wherein the first impurities
include phosphorous and the second impurities include boron.
8. The method according to claim 6, further comprising forming the
first semiconductor layer by epitaxial growth above a semiconductor
substrate before injecting the first impurities and the second
impurities, the first semiconductor layer having an impurity
concentration equal to or less than a fifth of an impurity
concentration of the first diffusion area.
9. The method according to claim 6, further comprising: forming a
second semiconductor layer of the first conductive type on the
first semiconductor layer after injecting the first impurities and
the second impurities into the first semiconductor layer; injecting
third impurities of the first conductive type into the second
semiconductor layer over points where the first impurities are
injected in the first semiconductor layer; and injecting fourth
impurities of the second conductive type into the second
semiconductor layer over points where the second impurities are
injected in the first semiconductor layer, wherein diffusing the
first impurities and the second impurities further includes
diffusing the first to fourth impurities until the first diffusion
area and the second diffusion area extend over the first
semiconductor layer and the second semiconductor layer.
10. The method according to claim 9, wherein the first impurities
and the third impurities include phosphorous and the second
impurities and the fourth impurities include boron.
11. The method according to claim 9, further comprising forming the
first semiconductor layer by epitaxial growth above a semiconductor
substrate before injecting the first impurities and the second
impurities, the first semiconductor layer having an impurity
concentration equal to or less than a fifth of an impurity
concentration of the first diffusion area, wherein forming a second
semiconductor layer includes forming by epitaxial growth the second
semiconductor layer having an impurity concentration equal to or
less than a fifth of an impurity concentration of the first
diffusion area.
12. The method according to claim 6, wherein injecting the first
impurities includes injecting the first impurities at a plurality
of points, injecting the second impurities includes injecting the
second impurities at a plurality of points between the plurality of
points where the first impurities are injected, the second
concentration profile has a second period which is smaller in the
planar direction of the first semiconductor layer than a maximum
diffusion width of the second impurities, and plural of said first
diffusion area and said second diffusion area are formed in the
first semiconductor layer.
13. The method according to claim 6, wherein the plurality of
points where the first impurities are injected are apart from each
other at 6 to 18 .mu.m and the plurality of points where the second
impurities are injected are apart from each other at 6 to 18
.mu.m.
14. The method according to claim 6, further comprising forming a
third diffusion area in an end region which is formed between an
end of an first semiconductor layer and a MISFET region where the
plural of the first diffusion area and the plural of the second
diffusion area are formed.
15. The method according to claim 6, further comprising: forming a
base area of the second conductive type in a surface of the first
semiconductor layer and connected to the second diffusion area;
forming a source area of the first conductive type in the base
area; forming a source electrode on the surface of the first
semiconductor layer so as to cover a part of the source area;
forming a gate electrode on the surface of the first semiconductor
layer with a gate insulating film interposed therebetween so as to
cover a part of the base area, source area, and first diffusion
area; forming an insulating film in the end region and on the
surface of the first semiconductor layer, the insulating film
having a height increasing toward the end of the first
semiconductor layer; and forming a first electrode on the
insulating film, the first electrode being connected to the source
electrode or the gate electrode.
16. The method according to claim 6, further comprising forming a
third diffusion area and a fourth diffusion area having
substantially a same structure as the first diffusion area and the
second diffusion area, respectively, in an end region which is
formed between an end of an first semiconductor layer and a MISFET
region where a plural of the first diffusion area and a plural of
the second diffusion area are formed
17. The method according to claim 6, wherein the third diffusion
area and the fourth diffusion area meet:
0.5<(S1.times.Qd1)/(S2.times.Qd2)<1.5 where Qd1: dose of
impurities used when ions are injected to form the third diffusion
area, Qd2: dose of impurities used when ions are injected to form
the fourth diffusion area, S1: area in which ions are injected to
form the third diffusion area, and S2: area in which ions are
injected to form the fourth diffusion area.
18. The method according to claim 16, wherein the third diffusion
area and the fourth diffusion area are formed to be substantially
linear in a plane of the first semiconductor layer.
19. The method according to claim 16, wherein the third diffusion
area and the fourth diffusion area are formed to be substantially
radial in a plane of the first semiconductor layer.
20. The method according to claim 16, wherein the third diffusion
area and the fourth diffusion area form a substantial lattice in a
plane of the first semiconductor layer.
21. A method of manufacturing a semiconductor device comprising:
forming a first semiconductor layer by epitaxial growth on a
semiconductor substrate; injecting first impurities of the first
conductive type and second impurities of a second conductive type
into the first semiconductor layer, points where the first
impurities are injected and points where the second impurities are
injected being apart from each other; conducting formation of an
i-th semiconductor layer on the (i-1)-th semiconductor layer by
epitaxial growth and i-th injection of the first impurities and the
second impurities into the i-th semiconductor layer over points
where the first impurities and the second impurities are injected
in the (i-1)-th semiconductor layer respectively, i being a natural
number at least two, a set of the formation of an i-th
semiconductor layer and the i-th injection being repeated with
i=i+1 for every set until n-th semiconductor layer is formed and
n-th injection is conducted, n being a natural number at least two;
diffusing the first impurities and the second impurities to form a
first diffusion area of the first conductive type and a second
diffusion area of the second conductive type, the first diffusion
area and the second diffusion area extending over the first
semiconductor layer to the n-th semiconductor layer, the first
diffusion area and the second diffusion area being defined by a
first concentration profile of the first impurities and a second
concentration profile of the second impurities, a junction between
the first diffusion area and the second diffusion area being formed
where a concentration of the first impurities and a concentration
of the second impurities are same.
22. The method according to claim 21, wherein the first impurities
include phosphorous and the second impurities include boron.
23. The method according to claim 21, wherein the first
semiconductor layer to the n-th semiconductor layer have an
impurity concentration equal to or less than a fifth of an impurity
concentration of the first diffusion area.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2001-395558, filed Dec. 27, 2001, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a substrate structure of a
semiconductor device having vertical power MISFETs (Metal Insulator
Field Effect Transistors) each having a gate electrode formed on a
semiconductor substrate, as well as a method of manufacturing this
substrate structure.
[0004] 2. Description of the Related Art
[0005] In a vertical power MIS (including a MOS (Metal Oxide
Semiconductor) FET formed on a semiconductor substrate, a drain
current flows between a source and drain electrodes formed on a top
and bottom surfaces, respectively, of a semiconductor substrate.
Such an element allows the resistance of a current passage to be
reduced and is thus often used as a power device.
[0006] FIG. 34 shows the sectional structure of a super junction
type MISFET currently put to practical use. A semiconductor
substrate 100 is composed of a first semiconductor substrate and a
second semiconductor substrate consisting of an epitaxial growth
layer. The first semiconductor substrate, which functions as an
N.sup.+ drain area 101, contacts with a drain electrode 105. The
second semiconductor substrate, which functions as N.sup.- drain
areas 102, is provided with first P base areas 103.
[0007] Second P base areas 106, which contact with the first P base
areas 103, are formed under a surface of the second semiconductor
substrate. Reference numerals 107, 108, 109, and 110 denote an N
source area, a gate insulating film, a gate area, and a source
area.
[0008] The width of the P base area 103 and the N.sup.- drain area
102 located between the P base areas 103 (a P and N type pillar
layers, respectively) and the amounts of P and N type impurities
contained in these areas are optimally designed. Thus, if a reverse
bias voltage is applied to the MISFET, the P and N type pillar
layers are depleted. This structure enables on resistance to be
reduced compared to other vertical MISFETs.
[0009] Other known examples of a MISFET improved so as to reduce
the on resistance is described in U.S. Pat. No. 5,216,275 and Jpn.
Pat. Appln. KOKAI Publication No. 2000-40822. In this U.S. Patent,
pillar-like P-type areas 7 (corresponding to 103 in FIG. 34 of the
specification) connected to base areas are formed of trenches as
shown in FIG. 2 or the like. However, this patent does not clearly
state that it can completely deplete the pillar layers and reduce
the on resistance. Further, the latter publication describes the
formation of both P and N layers in a drift layer by diffusion.
However, a non-diffusion area remains between the P and N layers.
That is, an area with a low concentration remains in a substrate.
Accordingly, in this structure, the maximum width of a first or
second diffusion area is larger than the thickness of a single
epitaxial growth layer. Thus, this patent fails to form a fine
structure in a substrate planar direction and thus does not serve
to reduce the on resistance.
[0010] The structure shown in FIG. 34 is formed as follows: First,
a P type impurity diffusion area is formed in a first epitaxial
growth layer formed on the first semiconductor substrate. Then, a P
type impurity diffusion area is formed in a second epitaxial growth
layer formed on the first epitaxial growth layer. This step is
repeated for about five to seven layers. Then, the P type
impurities in the epitaxial growth layers are thermally diffused
and thus connected together in a depth direction to form the first
P base area 103. At this time, adjacent P impurity diffusion areas
must be formed at a specified distance so as not to be joined
together.
[0011] A MISFET having the structure shown in FIG. 34 allows the
concentration of impurities to be increased by reducing the widths
of the P and N type pillar layers. This enables the on resistance
to be further reduced. However, to reduce the widths of the pillar
layers, it is necessary to join the impurity diffusion areas 102
together lengthwise with a small amount of diffusion. As a result,
the number of epitaxial growth layers (102a to 102k) increases as
shown in FIG. 35, thus increasing manufacturing costs.
[0012] Further, the manufacturing costs can be cut down by reducing
the number of epitaxial growth layers. However, in this case, the
diffusion areas 120 must be enlarged as shown in FIG. 36. Thus, the
width of the pillar layers increases, and the concentration of
impurities decreases. This may degrade the on resistance.
[0013] The present invention is provided in view of these
circumstances. It is an object of the present invention to provide
a semiconductor device having a drift area structure with a reduced
pitch between each area (P type area) exhibiting the same polarity
as that of a P type and a corresponding area (N type area)
exhibiting the same polarity as that of an N type and terminal area
structure, in order to form MISFET elements having a fine structure
and achieve complete depletion.
BRIEF SUMMARY OF THE INVENTION
[0014] According to a first aspect of the present invention, there
is provided a semiconductor device comprising a semiconductor layer
of a first conductive type and a diffusion area formed the
semiconductor layer, the diffusion area comprising first impurity
diffusion areas of the first conductive type and second impurity
diffusion areas of a second conductive type which are alternately
formed, the diffusion area having first areas of the first
conductive type and second areas of the second conductive type
which are defined by the impurity concentrations of the first and
second impurity diffusion areas, respectively, wherein a junction
between each of the first areas and the corresponding second area
is formed in a portion in which the corresponding first and second
impurity diffusion areas overlap each other, and the period of the
impurity concentration, in a planar direction of the semiconductor
layer, of the areas selected from a group consisting of the first
and second areas is smaller than the maximum width, in the planar
direction of the semiconductor layer, of the first and second
impurity diffusion areas constituting the selected areas.
[0015] According to a second aspect of the present invention, there
is provided a method of manufacturing a semiconductor device, the
method comprising injecting first impurities of a first conductive
type and second impurities of a second conductive type into a
surface of a semiconductor layer of a first conductive type; and
diffusing the first and second impurities to form a diffusion area,
the diffusion area having a first area and a second area, the first
and second areas defined by an impurity concentration of a first
impurity diffusion area of the first conductive type and a second
impurity diffusion area of the second conductive type, the first
and second impurity diffusion area overlapping each other, and a
period of the impurity concentration, in a planar direction of the
semiconductor layer, of an area selected from a group consisting of
the first and second areas being smaller than the maximum width, in
the planar direction of the semiconductor layer, of the first and
second impurity diffusion areas constituting the selected
areas.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0016] FIG. 1 is a diagram showing the sectional structure of a
semiconductor device according to a first embodiment of the present
invention;
[0017] FIGS. 2 and 3 are plan views of a surface of the
semiconductor substrate shown in FIG. 1;
[0018] FIGS. 4 and 5 are plan views of a second semiconductor
substrate on which a vertical MISFET, shown in FIG. 2 is
formed;
[0019] FIGS. 6 to 9 are diagrams illustrating first and second
diffusion layers 13 and 14, shown in FIG. 1, and a method of
manufacturing the same;
[0020] FIG. 10 is a plan view illustrating diffusion areas in the
semiconductor substrate in FIGS. 8 and 9;
[0021] FIG. 11 is a graph showing an impurity concentration profile
of the semiconductor device in FIGS. 8 and 9;
[0022] FIG. 12 is a graph showing an NET concentration profile of
the semiconductor device in FIGS. 8 and 9;
[0023] FIG. 13 is a diagram showing the sectional structure of a
semiconductor device according to a second embodiment of the
present invention;
[0024] FIGS. 14 to 16 are diagrams illustrating first and second
diffusion layers 13 and 14, shown in FIG. 13, and a method of
manufacturing the same;
[0025] FIG. 17 is a graph showing an impurity concentration profile
of the semiconductor device in FIG. 16;
[0026] FIG. 18 is a graph showing an NET concentration profile of
the semiconductor device in FIG. 16;
[0027] FIGS. 19 and 20 are graphs showing the concentration of
impurities in a depth direction of a second semiconductor substrate
2;
[0028] FIG. 21 is a diagram showing the relationship between the
epi-number and on resistance of the semiconductor substrate;
[0029] FIG. 22 is a diagram showing the sectional structure of a
semiconductor device according to a third embodiment of the present
invention;
[0030] FIG. 23 is a graph showing an impurity concentration profile
of the semiconductor device in FIG. 22;
[0031] FIG. 24 is a graph showing an NET concentration profile of
the semiconductor device in FIG. 22;
[0032] FIG. 25 is a diagram showing the planar structure of a
semiconductor device according to a fourth embodiment of the
present invention;
[0033] FIG. 26 is a diagram showing the sectional structure of the
semiconductor device in FIG. 25;
[0034] FIG. 27 is a diagram showing the planar structure of a
semiconductor device according to a fifth embodiment of the present
invention;
[0035] FIG. 28 is a diagram showing the sectional structure of the
semiconductor device in FIG. 27;
[0036] FIG. 29 is a diagram showing the planar structure of a
semiconductor device according to a sixth embodiment of the present
invention;
[0037] FIG. 30 is a diagram showing the sectional structure of the
semiconductor device in FIG. 29;
[0038] FIG. 31 is a diagram showing the planar structure of a
semiconductor device according to a seventh embodiment of the
present invention;
[0039] FIG. 32 is a diagram showing the sectional structure of the
semiconductor device in FIG. 31;
[0040] FIG. 33 is a diagram showing the planar structure of a
semiconductor device according to a variation of the seventh
embodiment of the present invention;
[0041] FIGS. 34 to 36 are sectional views of a conventional
vertical MISFET; and
[0042] FIG. 37 is a graph showing a NET dose amount of the
semiconductor device in FIG. 10.
DETAILED DESCRIPTION OF THE INVENTION
[0043] Embodiments of the present invention will be described below
with reference to the drawings. In the description below,
components having substantially the same functions and
configurations are denoted by the same reference numerals.
Duplicate description will be given only when required.
First Embodiment
[0044] A first embodiment will be described with reference to FIGS.
1 to 12.
[0045] FIG. 1 is a diagram showing the sectional structure of a
semiconductor device according to the first embodiment of the
present invention. This semiconductor device is a vertical MISFET
in which PN junctions are formed to extend in a depth direction. In
each of the embodiments described below, for example, a first
conductive type is N and, whereas a second conductive type is
P.
[0046] As shown in FIG. 1, a semiconductor substrate (layer) 10
consisting of, for example, silicon is composed of a first
semiconductor substrate 1 and a second semiconductor substrate 2.
The first semiconductor substrate 1 has impurities of a high
concentration and an N type conductivity. The second semiconductor
substrate 2 is formed on the first semiconductor substrate 1 and
has an N type conductivity with an impurity concentration lower
than that of the first semiconductor substrate 1. The second
semiconductor substrate 2 may be, for example, a single epitaxial
layer.
[0047] An N.sup.+ drain area 11 is formed in the first
semiconductor substrate 1. The N.sup.+ drain area 11 is connected
to a drain area 20 formed on a back surface of the first
semiconductor substrate 1.
[0048] An N.sup.- drain area 12 which contacts with the N.sup.+
drain area 11 is formed in the second semiconductor substrate 2. An
impurity diffusion area is formed in the N.sup.- drain area 12 by
diffusing impurities and has an impurity concentration higher than
that of the second semiconductor substrate 2. This impurity
diffusion area is composed of first diffusion areas 13 and second
diffusion areas 14 formed inside the first diffusion area 13. The
first diffusion areas 13 have the same polarity as that of the N
type. The second diffusion areas 14 have the same polarity as that
of the P type. The end of each of the second diffusion areas 14 is
adjacent to the corresponding first diffusion area 13. The junction
between each of the first diffusion areas 13 and the corresponding
second diffusion area 14 in a substrate planar direction is
perpendicular to the substrates.
[0049] N and P type impurities are mixed in the first and second
diffusion areas 13 and 14. In each portion of the impurity
diffusion area, the concentrations of these impurities define the
first or second diffusion areas 13 or 14 as described below in
detail. There are differences in impurity concentration among the
portions of the first and second diffusion areas 13 and 14.
However, in terms of an average value, the impurity concentration
of the second semiconductor substrate 2 is set to be greatly lower
than those of the first and second diffusion areas 13 and 14. More
specifically, the impurity concentrations are set so that the
concentration in the second semiconductor substrate is equal to or
smaller than one-fifths of those in the first and second diffusion
areas 13 and 14. Preferably, the concentration in the second
semiconductor area 2 is one-two-hundredth to one-fifth and more
preferably one-one-hundredth to one-fifth of those in the first and
second diffusion areas.
[0050] The first diffusion areas 13 each function as an N drain
area. The second diffusion areas 14 each function as a first P base
area.
[0051] Second P base areas 15 are formed on a surface of the
semiconductor substrate 10 which is located on the respective first
P base area (second diffusion area) 14. The second P base areas 15
are connected to the respective first P base areas 14 and formed by
diffusing impurities. N source areas 16 are formed inside each of
the P base areas 15. The first diffusion areas 13, the second P
base areas 15, and the N source areas 16 are exposed from a main
surface of the semiconductor substrate 10 (the N.sup.- drain areas
12 is normally passivated by an oxide film).
[0052] Gate electrodes 18 are each formed on the main surface of
the semiconductor substrate 10 via a gate insulating film 19 such
as a silicon oxide film. The gate insulating film 19 and the gate
electrode 18 cover a part of the second P base area 15 and areas
extending from the second P base area 15 to the N drain area 13 and
the N source area 16. Source-base leader electrodes (hereinafter
referred to as "source electrodes") 17 are formed on the main
surface of the semiconductor substrate 10. The source electrodes 17
each have a central portion formed on the P base area 15 and
opposite ends each covering a part of the N source area 16.
[0053] FIGS. 2 and 3 are plan views of the structures of MISFET
elements formed on a surface area of the semiconductor substrate
10. In these figures, the gate electrodes and the source electrodes
are omitted. FIG. 1 is a sectional view of a portion of the
semiconductor device taken along the line I-I in FIG. 2. In the
example shown in FIG. 2, lengthwise long MISFET elements (in FIG.
2, two) are formed in the semiconductor substrate. Further, in the
example shown in FIG. 3, MISFET elements have a substantially
square planar shape and are arranged on the semiconductor substrate
10 in a matrix. The sectional structure is the same as that shown
in FIG. 1.
[0054] FIGS. 4 and 5 are plan views of a substrate surface
illustrating the diffusion areas formed in the second semiconductor
substrate 2. FIG. 4 corresponds to FIG. 2, and FIG. 5 corresponds
to FIG. 3. As shown in FIG. 4, the first diffusion area 13 and 14
are arranged adjacent to each other in the N.sup.- drain area 12,
constituting the second semiconductor substrate 2. The adjacent
first and second diffusion areas 13 and 14 form a junction
lengthwise in the plane of the semiconductor substrate 10. Further,
as shown in FIG. 5, the first and second diffusion areas 13 and 14
have a substantially square planar shape. The first and second
diffusion areas 13 and 14 are alternately arranged lengthwise and
breadthwise within the N.sup.- drain area 12. The second diffusion
areas 14 are each surrounded by the first diffusion areas 13. The
junction between the second diffusion area 14 and the adjacent
first diffusion area 13 is formed along the periphery of the second
diffusion area 14.
[0055] Now, the first and second diffusion areas 13 and 14 will be
described below in detail with reference to FIGS. 6 to 9. FIGS. 6
to 9 illustrate the first and second diffusion layers 13 and 14 in
FIG. 1 and a method of manufacturing the same. A method of forming
these portions will also described. First, as shown in FIG. 6, the
second semiconductor substrate 2 is formed on the first
semiconductor substrate 1. Then, a photo resist 36 is formed on a
surface of the second semiconductor substrate 2. Then, a
photolithography step and an etching technique are used, forming
openings in the photo resist 36 at positions corresponding to those
at which boron injection areas 31 are to be formed. The diameter of
these openings is determined by the widths of the first and second
diffusion areas 13 and 14 and the like. The appropriate diameter
is, for example, between about 0.3 and 2.0 .mu.m. Further, the
appropriate pitch of the openings is, for example, between about 6
and 18 .mu.m. Then, boron (P type impurities) ions are injected
through these openings at a dose Qd of 2 to 10.times.10.sup.13
cm.sup.-2. As a result, the boron injection areas 31 are formed at
the predetermined positions of a surface area of the second
semiconductor substrate 2.
[0056] Then, as shown in FIG. 7, the photo resist 36 is removed. A
photo resist 37 is then formed on the surface of the semiconductor
substrate 2. Then, a photolithography step and an etching technique
are used, forming openings each of which is located between the
areas in which the boron injection areas 31 are formed. The
diameter of these openings is determined by the widths of the first
and second diffusion areas 13 and 14 and the like. The appropriate
diameter is, for example, between about 0.3 and 2.0 .mu.m. Further,
the appropriate pitch of the openings is, for example, between
about 6 and 18 .mu.m. Then, phosphorus (N type impurities) ions are
injected through these openings at a dose Qd of 2 to
10.times.10.sup.13 cm.sup.-2. As a result, the phosphorus injection
areas 32 are formed at the predetermined positions of the surface
area of the second semiconductor substrate 2. This processing
allows the boron injection areas 31 and the phosphorous injection
areas 32 to be formed in the surface area of the second
semiconductor substrate 2 so as to be alternately arranged. When
the photo resist is formed, a thin oxide film may be formed between
the photo resist and the silicon.
[0057] Then, as shown in FIG. 8, the semiconductor substrate 10 is
thermally treated, diffusing the boron and phosphorous in the boron
injection areas 31 and the phosphorous injection areas 32,
respectively. As a result, boron diffusion areas 33 and phosphorous
diffusion areas 34 are formed. At this time, junctions 35 are each
formed in the center of an overlapping portion of the corresponding
boron diffusion area 33 and phosphorous diffusion area 34 in a
direction perpendicular to the substrate. As a result, as shown in
FIG. 9, the first and second diffusion areas 13 and 14 are formed.
The junction 35 is formed at a middle position between the each
center or the phosphorous diffusion area 34 and the adjacent boron
diffusion area 33 and the smaller a cell pitch becomes, the closer
the junction 35 becomes to a center of the first and second
diffusion areas.
[0058] FIG. 10 is a plan view illustrating the diffusion areas in
the semiconductor substrate shown in FIGS. 8 and 9. In the impurity
diffusion areas formed as shown in FIGS. 8 and 9, the P and N type
impurities cancel each other in areas 39. As a result, N type areas
21 and P type areas 22 are alternately arranged. The PN junctions
35 are formed perpendicularly in a depth direction of the
substrate. Those portions of the P type areas 22 which are located
in an area "A" lying at the top of the semiconductor substrate 2
shown in FIG. 10 are shown offset from the substrate surface for
the convenience of description but actually rest on it. The first
diffusion areas with a high phosphorous concentration exhibit the
same polarity as that of the N type. The second diffusion areas 14
with a high phosphorous concentration exhibit the same polarity as
that of the P type.
[0059] FIGS. 11 and 12 are characteristic diagrams showing an
impurity and NET concentration profiles of the impurities injected
into the semiconductor substrate shown in FIGS. 8 and 9, in a
portion of the semiconductor substrate taken along line X-X in
these figures. The boron and phosphorous (hereinafter collectively
referred to as "impurities") injected into the semiconductor
substrate 10 are diffused and exhibit an impurity and NET
concentration profiles such as those shown in FIGS. 11 and 12. As
shown in FIGS. 11 and 12, P type areas (having the same polarity as
that of the P type) and N type areas (having the same polarity as
that of the N type) are alternately formed. The adjacent individual
boron diffusion areas 33 are joined together and the concentration
distribution (B concentration profile) of the boron diffusion areas
33 in the planar direction of the semiconductor substrate 10
(hereinafter referred to as the "substrate planar direction") has a
period "a".
[0060] The period "a" substantially corresponds to the period of
concentration of the impurities in the first or second diffusion
area 13 or 14, or the pitch of the diffusion areas 13 or 14, or the
spacing between the phosphorous or boron injection areas 32 or 31.
These descriptions also apply to the P concentration profile. The
junctions 35 are each formed at the position where the phosphorous
(P) concentration profile equals the boron (B) concentration
profile.
[0061] The boron injection areas 31 and the phosphorous injection
areas 32 are formed, for example, under the above described
conditions. As a result, the period "a" of the boron diffusion
areas 33 and phosphorous diffusion areas 34 is smaller than the
maximum diffusion length (diffusion width) of the individual
diffusion areas 33 and 34 in the substrate planar direction. Thus,
a high impurity concentration area extends widely in the first and
second diffusion areas 13 and 14.
[0062] The high impurity concentration area extending widely will
now be explained with examples. FIG. 37 shows a NET dose amount
along the line XI-XI in FIG. 10 and a comparison between the
embodiment and a prior art. Only either of P profile and N profile
is shown in the figure. Also, the solid line exhibits the
embodiment and the broken line shows the prior art. A concrete
condition for the figure is that the period "a" is 8 .mu.m in the
embodiment and 161m in the prior art. Other conditions remain the
same in both cases.
[0063] As shown in FIG. 37, an area (70% area) in which a
concentration is 70% of the peak concentration extends over 50% of
the first and second diffusion areas 13 and 14 in the embodiment,
while 25% in the prior art. In a case of an area (50% area) in
which the concentration is 50% of the peak concentration extends
over 65% of the first and second diffusion areas 13 and 14 in the
embodiment, while 40% in the prior art. That is, an area in which a
concentration is over 50% of the peak concentration extends over
50% to 65% of the first and second diffusion areas 13 and 14 in the
embodiment.
[0064] According to the first embodiment, the first and second
diffusion areas 13 and 14 are formed in the second semiconductor
substrate 2 with a low impurity concentration, using impurities
formed by ion injection and diffusion. The first and second
diffusion areas 13 and 14 are defined by the concentrations and
overlapping portions in the second substrate 2. Thus, the first and
second diffusion areas 13 and 14 can be formed to be narrower while
avoiding joining the adjacent second diffusion areas 14 together.
This serves to provide a semiconductor device with reduced on
resistance.
[0065] According to the first embodiment, the period a of impurity
concentration of each of the first and second diffusion areas 13
and 14 is smaller than the maximum diffusion length of the boron
diffusion areas 33 and phosphorous diffusion areas 34 in the
substrate planar direction. Thus, junction 35 is formed at the
vicinity of the center of the boron diffusion areas 33 and
phosphorous diffusion areas 34. As a result, most part of the first
and second diffusion layers 13 and 14 are formed at the vicinity of
the center of the phosphorous diffusion areas 34 and the boron
diffusion areas 33, and this part has a high impurity
concentration. Thus, the impurity concentration of the first
diffusion areas 13, which constitute a current passage, is high
while the MISFET is on. This serves to provide a semiconductor
device with reduced on resistance. Further, narrow width (small
period "a") of the first and second diffusion layers 13 and 14 help
these diffusion layers 13 and 14 deplete completely. This serves to
provide a semiconductor device with a high withstand voltage, while
reducing a cell pitch.
[0066] Further, the balance of total sum of impurity concentrations
in the first and second diffusion areas 13 and 14 is important to
obtain a high withstand voltage. According to the present
application, adding an N type dopant during epitaxial growth
conventionally forms N type impurities corresponding to the first
diffusion areas 13. On the other hand, an ion injection forms the
first and second diffusion areas 13 and 14 in the first embodiment.
The ion injection improves concentration controllability, thus
allowing the balance to be maintained easily even with finer
design.
Second Embodiment
[0067] A second embodiment will be described with reference to
FIGS. 13 to 20. In the first embodiment, the second semiconductor
substrate 2 is composed of, for example, a single epitaxial growth
layer or the like. In contrast, a semiconductor device according to
the second embodiment has a structure in which the second
semiconductor substrate 2 has a plurality of layers and in which PN
junctions are formed to be deeper by repeating the manufacturing
method of the first embodiment.
[0068] FIG. 13 shows the sectional structure of the semiconductor
device according to the second embodiment of the present invention.
This semiconductor device is a vertical MISFET in which PN
junctions are formed to extend in the depth direction. In the
second embodiment, the second semiconductor substrate 2 is composed
of a plurality of epitaxial growth layers consisting of, for
example, silicon. The first and second diffusion areas 13 and 14
are formed by forming a plurality of different impurity diffusion
areas in the respective layers and joining the impurity diffusion
areas with the same polarity together lengthwise. As a result, the
PN junctions are formed to be deeper than those in the first
embodiment as shown in FIG. 13.
[0069] Now, with reference to FIGS. 14 and 15, detailed description
will be given of the second semiconductor substrate 2 and the first
and second diffusion areas 13 and 14. FIGS. 14 and 15 are sectional
views illustrating the first and second diffusion areas 13 and 14.
Description will also be given of a method of forming these
portions. In FIGS. 14 and 15, the second semiconductor substrate 2
is formed by repeating a single epitaxial layer configured as
described in Embodiment 1, for example, six times.
[0070] As shown in FIGS. 14 and 15, the second semiconductor
substrate 2 is composed of a plurality of epitaxial layers (2a to
2f). These epitaxial layers 2a to 2f are formed as described below.
First, the boron injection areas 31 and the phosphorous injection
areas 32 are formed in the surface area of the first epitaxial
layer 2a as described in the first embodiment. Then, the second
epitaxial layer 2b is formed on the first epitaxial layer 2a. Then,
the boron injection areas 31 and the phosphorous injection areas 32
are formed in the surface area of the second epitaxial layer 2b so
as to join with the injection areas 31 and 32, respectively, in the
first layer 2a lengthwise of the substrate. Subsequently, the above
steps are repeated until the sixth layer 2f is formed. Then, the
phosphorous diffusion areas 34 and the boron diffusion areas 33 are
formed from the phosphorous and boron injection areas,
respectively, in each layer by thermal treatment.
[0071] The thermal treatment makes the first and second diffusion
layers 13 and 14 from the phosphorous diffusion areas 34 and the
boron diffusion areas 33. PN junctions are formed in the
semiconductor substrate 10 in the vertical direction.
[0072] Further, in FIGS. 14 and 15, the thickness of a single
epitaxial growth layer constituting the second semiconductor
substrate 2 (the period of the concentration of impurities in the
substrate depth direction) is defined as "b". Further, the
diffusion length of P type impurities (boron) or N type impurities
(phosphorous) in the depth direction is defined as "r", and then
the diffusion length (spread width) of P type impurities or N type
impurities in the substrate planar direction is defined as "L". In
this case, relationships shown below are established between the
periods "a" and "b" of the boron diffusion area 33 or phosphorous
diffusion area 34, between "a" and "L", and between "b" and "r",
respectively. L>a (1) r>b/2 (2)
[0073] Now, the diffusion structure of the second semiconductor
substrate 2 will be described with reference to FIGS. 17 to 20.
FIG. 17 is a characteristic diagram showing an impurity
concentration profile of that portion of the second semiconductor
substrate 2 shown in FIG. 16 takes along the line XVII-XVII. The
first and second diffusion areas 13 and 14 are formed at a pitch
(period) "a". FIG. 18 is a characteristic diagram in which the
impurity concentration profile shown in FIG. 17 is replaced with a
NET concentration profile.
[0074] FIG. 19 is a characteristic diagram showing the impurity
concentration of a portion of the second semiconductor substrate 2
taken along the line XIX-XIX. The P type impurity concentration is
higher than the N type impurity concentration in this area, and the
area exhibits the second diffusion area 14 which has the same
polarity as that of the P type.
[0075] FIG. 20 is a characteristic diagram showing the impurity
concentration of a portion of the second semiconductor substrate 2
taken along the line XX-XX.
[0076] The N type impurity concentration is higher than the P type
impurity concentration in this area, and the area exhibits the
first diffusion area 13 which has the same polarity as that of the
N type. As shown in FIGS. 19 and 20, the concentrations of the N
and P type impurities vary with the period "b".
[0077] Now, description will be given below of a comparison of the
second embodiment with a conventional example. FIG. 21 is a
characteristic diagram showing the relationship between the number
of epitaxial growths carried out to form the epitaxial growth
layers (hereinafter referred to as the "epitaxial number") and on
resistance of the semiconductor substrate. The epitaxial number
affects on resistance of the element, as shown in FIG. 21. The axis
of abscissas in FIG. 21 indicates the epitaxial number, while the
axis of ordinates indicates the on resistance Ron
(m.OMEGA.cm.sup.2) Ron denotes the on resistance normalized by the
area of the FET. The characteristic curve in FIG. 21 shows the
dependence of the on resistance on the epitaxial number in a method
described in the second embodiment (a fine multi-epitaxial method)
and in a method (normal multi-epitaxial method) according to the
conventional example shown in FIGS. 34 to 36.
[0078] As described in the first embodiment, narrowing the first
and second diffusion layers 13 and 14 can increase the impurity
concentrations of these diffusion layers 13 and 14, and thus the on
resistance can be reduced. This is shown in FIG. 21. As shown in
this figure, with the same epitaxial number, the method according
to the present embodiment allows the first and second diffusion
layers 13 and 14 to be narrowed. Accordingly, the on resistance
thus obtained is half of that obtained in the conventional example.
The figure also indicates that the same on resistance can be
accomplished using half the epitaxial number compared to the
conventional example.
[0079] According to the second embodiment, the second semiconductor
substrate 2 is configured similarly to the first embodiment. Thus,
the second embodiment produces effects similar to those of the
first embodiment.
[0080] Further, the second embodiment 2 has a structure in which a
plurality of epitaxial layers are stacked together a number of
times. Furthermore, the concentration period "a" of each first
diffusion area 13 or second diffusion area 14 in the substrate
planar direction is greater than the concentration period "b" (the
thickness of a single epitaxial layer) in the substrate depth
direction (a>b). This also serves to increase the impurity
concentrations of the first and second diffusion areas 13 and 14
and provide a semiconductor device with a high withstand voltage
and reduced on resistance, as in the first embodiment. It is noted
that the advantages brought about by the second embodiment can be
obtained while the relationship between "a" and "b" is a<b.
However, design and implementation can be performed easily when the
relationship is a>b than a<b.
[0081] Further, the second semiconductor substrate 2 having such
characteristics has a structure in which a plurality of epitaxial
layers are stacked together a number of times. Thus, if a
semiconductor device is formed with the same epitaxial number as
that in the conventional example, about half the on resistance is
obtained compared to the conventional example. On the other hand,
the same on resistance can be accomplished using half the epitaxial
number compared to the conventional example.
Third Embodiment
[0082] A third embodiment will be described with reference to FIGS.
22 to 24. In addition the structure of the second embodiment, the
third embodiment has a structure in which diffusion areas are
further repeatedly formed breadthwise.
[0083] FIG. 22 shows the sectional structure of a semiconductor
device according to the third embodiment of the present invention,
i.e. the sectional structure of a semiconductor substrate provided
with vertical MISFET elements. As shown in FIG. 22, for example,
three second diffusion areas (P type areas) 14 are formed inside
the semiconductor substrate 2 so as to be each sandwiched between
the first diffusion areas (N type areas) 13. It is possible to
further increase the number of second diffusion areas 14.
[0084] FIG. 23 shows an impurity concentration profile of a portion
of the semiconductor device taken along the line XXIII-XXIII in
FIG. 22. FIG. 24 shows a NET concentration profile indicating the
total concentration distribution of the same portion.
[0085] According to the third embodiment, the second embodiment 2
and the first and second diffusion areas 13 and 14 are structured
similarly to the second embodiment. Thus, the third embodiment
produces effects similar to those of the first and second
embodiments.
[0086] Furthermore, according to the third embodiment, three or
more second diffusion areas 14 are formed. Thus, the MISFET
elements can be formed with a high density. This provides a
semiconductor device that can be highly integrated.
Fourth Embodiment
[0087] A fourth embodiment relates to a structure used in addition
to those of the first to third embodiments, and is directed to a
terminal structure of a semiconductor device. As described above,
according to the first to third embodiments of the present
invention, the concentration of the second semiconductor substrate
2 can be maintained at a low level. This is because, injecting ions
into an N type semiconductor substrate with a low concentration
makes N and P type pillar-like diffusion layers as opposed to
injecting P type impurities into an N type semiconductor substrate
with a high concentration in the conventional example.
[0088] FIG. 25 shows the planar structure of a semiconductor device
according to the fourth embodiment of the present invention. FIG.
26 shows the sectional structure of a portion of the semiconductor
device taken along the line XXVI-XXVI in FIG. 25. In FIGS. 25 and
26, a portion of the semiconductor device provided with a MISFET
has a structure similar to that in the second or third embodiment.
In addition, in FIG. 26, the first impurity diffusion layers 13,
the N source areas 16, the gate electrodes 18, and insulating films
44 and N.sup.+ stopper electrodes 43, described later, are
omitted.
[0089] As shown in FIGS. 25 and 26, the first and second diffusion
layers 13 and 14 are not formed near a terminal of the
semiconductor device. That is, the first and second diffusion
layers 13 and 14 are spaced from the terminal of the semiconductor
device. For example, three guard rings 41 of an appropriate width
are formed around a MISFET element at predetermined intervals. The
guard rings 41 are each formed on the surface of the second
semiconductor substrate 2 in the area (hereinafter referred to as
the "terminal area of the semiconductor device") between the
terminal of the MISFET element, i.e. the corresponding end of the
first diffusion layer 13 and the corresponding end of the
semiconductor device. Further, the guard rings 41 are formed of an
impurity diffusion area of the second conductive type.
[0090] An N.sup.+ stopper layer 42 with a high concentration is
formed at the end of the semiconductor device and on the surface of
the second semiconductor substrate 2. An N.sup.+ stopper electrode
43 is formed on the N.sup.+ stopper layer 42. An insulating film
(interlayer film) 44 is formed in the terminal area of the
semiconductor device and on the surface of the second semiconductor
substrate 2.
[0091] The effects of the fourth embodiment will be described
below. In the terminal area of the semiconductor device, a
depletion layer must be formed to an appropriate extent in order to
obtain a withstand voltage in this area. However, if a
semiconductor layer (corresponding to the second semiconductor
substrate 2 in the present embodiments) provided with N and P type
diffusion layers has a high concentration as in the prior art, a
depletion layer extending to the terminal is not sufficiently
formed. Accordingly, separate measures are required in order to
sufficiently extend the depletion layer. However, according to the
first to third embodiments of the present application, the impurity
concentration of the second semiconductor substrate 2 can be
reduced. Consequently, it is possible to form a depletion layer
extending to the terminal of the semiconductor without any special
measures. Thus, when, in addition to such a structure, the guard
rings 41 are formed as in the fourth embodiment, a depletion layer
can be formed to a larger extent.
[0092] According to the fourth embodiment, the second semiconductor
substrate 2 and the first and second diffusion layers 13 and 14 are
structured similarly to the first to third embodiments. The fourth
embodiment thus produces effects similar to those of the first to
third embodiments.
[0093] Furthermore, according to the fourth embodiment, the
terminal area not provided with the first or second diffusion
layers 13 or 14 is formed, and the second semiconductor substrate
2, provided with the first and second diffusion layers 13 and 14,
have a low impurity concentration. Thus, a depletion layer
extending to the terminal of the semiconductor device is formed in
this area. This serves to provide a semiconductor device with a
high withstand voltage. Furthermore, the formation of the guard
rings 41 allows a depletion layer to be formed to a larger
extent.
Fifth Embodiment
[0094] A fifth embodiment relates to a variation of the fourth
embodiment.
[0095] FIG. 27 shows the planar structure of a semiconductor device
according to the fifth embodiment of the present invention. FIG. 28
shows the sectional structure of a portion of the semiconductor
device taken along the line XXVIII-XXVIII in FIG. 27. As shown in
FIGS. 27 and 28, an insulating film 51 with an appropriate number
of (in FIG. 28, three, for example) steps is formed in the terminal
area of the semiconductor device and on the surface of the second
semiconductor substrate 2. The height of each step of the
insulating film increases toward the terminal of the semiconductor
device. A field plate electrode 52 extends on the insulating film
51. The field plate electrode 52 is connected to the source
electrode 17 or the gate electrode 18 (in FIG. 28, it is connected
to the source electrode 17). An end of the field plate electrode 52
is arranged on, for example, a portion of the insulating film 51
which is highest. The number of steps of the insulating film 51 is
not limited to three. Furthermore, the insulating film 51 may be
inclined instead of having the steps.
[0096] According to the fifth embodiment, the second semiconductor
substrate 2 and the first and second diffusion layers 13 and 14 are
structured similarly to the first to fourth embodiments. The fifth
embodiment thus produces effects similar to those of the first to
fourth embodiments.
[0097] Moreover, according to the fifth embodiment, the insulating
film 51, which is thicker toward the end of the semiconductor
device, is formed on the surface of the second semiconductor
substrate 2. Further, the field plate electrode 52, connected to
the source electrode 17 or the gate electrode 18, is formed on the
insulating film 51. Thus, electric fields concentrate in a thicker
part of the insulating film 51 which is located closer to the end
of the field plate electrode 52. The insulating film has a higher
withstand voltage than the semiconductor substrate such as silicon,
thus serving to provide a semiconductor device having a high
withstand voltage as a whole.
Sixth Embodiment
[0098] As described above, for a semiconductor device in which a
semiconductor layer (corresponding to the second semiconductor
substrate 2 in the present embodiments) provided with N and P type
diffusion layers has a high concentration, measures are required in
order to sufficiently form a depletion layer extending to the
terminal of the semiconductor device. One possible method for this
purpose is to form an impurity diffusion layer in the semiconductor
layer which does not function as a MISFET.
[0099] FIG. 29 shows the planar structure of a semiconductor device
according to a sixth embodiment of the present invention. FIG. 30
shows the sectional structure of a portion of the semiconductor
storage device taken along the line XXX-XXX. As shown in FIGS. 29
and 30, substantially linear third diffusion layers 61 and fourth
diffusion layers 62 are formed in the second semiconductor
substrate 2. The third diffusion layers 61 are of the N type, while
the fourth diffusion layers 62 are of the P type. The third
diffusion layers 61 and the fourth diffusion layers 62 reach, for
example, the N.sup.- drain area 12 located at an end of the
semiconductor substrate 10 and are alternately formed. The third
and fourth diffusion layers 61 and 62 may be formed in a steps in
which the first and second diffusion layers 13 and 14 are formed at
the same time. Accordingly, the third and fourth diffusion layers
61 and 62 are configured substantially similarly to the first and
second diffusion layers 12 and 13.
[0100] In the terminal area of a semiconductor device configured as
described above, depletion layers are formed along the junctions
between the third diffusion layers 61 and the fourth diffusion
layers 62. Accordingly, in the planar breadthwise direction and a
depth direction of the semiconductor substrate, depletion layers
are formed so as to correspond to the positions at which the third
and fourth diffusion layers 61 and 62 are formed. In this regard,
the planar shapes of the third and fourth diffusion layers 61 and
62 (the shapes in FIG. 29) are determined according to the
positions at which depletion layers are to be formed. These planar
shapes are not limited to those shown in FIG. 29.
[0101] Now, the effects of the sixth embodiment will be described.
In the present embodiments, which allow the maintenance of impurity
concentration of the second semiconductor substrate 2 at a low
level, a common structure such as the one shown in the fourth and
fifth embodiments is used to obtain the desired withstand voltage.
However, if such a method still fails to form depletion layers to a
sufficient extent, the sixth embodiment can be effectively
applied.
[0102] Furthermore, the impurity concentration of the second
semiconductor substrate 2 can be reduced, so that the concentration
of impurities can be controlled more easily than in the case in
which impurity diffusion layers are formed in a semiconductor
substrate with a high impurity concentration.
[0103] According to the sixth embodiment, the second semiconductor
substrate 2 and the first and second diffusion layers 13 and 14 are
structured similarly to the first to fourth embodiments. The sixth
embodiment thus produces effects similar to those of the first to
fourth embodiments.
[0104] According to the sixth embodiment, furthermore, the third
and fourth diffusion layers 61 and 62, which are used to form
depletion layers, are formed inside the second semiconductor
substrate 2 with a low impurity concentration. Thus, the third and
fourth diffusion layers 61 and 62 can be formed easily and
depletion layers can be formed to a larger extent, which serves to
provide a semiconductor device with a high withstand voltage.
[0105] Further, the third and fourth diffusion layers 61 and 62 can
be formed when the first and second diffusion layers 13 and 14 are
formed at the same time. Therefore, a semiconductor device with a
high withstand voltage can be obtained by fewer manufacturing steps
than ones in the fourth and fifth embodiments.
Seventh Embodiment
[0106] A seventh embodiment relates to a variation of the sixth
embodiment.
[0107] FIG. 31 shows the planar structure of a semiconductor device
according to the seventh embodiment of the present invention. FIG.
32 shows the sectional structure of a portion of the semiconductor
device taken along the line XXXII-XXXII in FIG. 31. As shown in
FIGS. 31 and 32, the fourth diffusion layers 62 are formed in the
respective third diffusion layers 61 in the terminal area, for
example, so as to radiate from the center of the semiconductor
device.
[0108] The third and fourth diffusion layers 61 and 62 are formed
to meet the following equation:
0.5<(S1.times.Qd1)/(S2.times.Qd2)<1.5 (3)
[0109] where Qd1: dose of impurities used when ions are injected to
form the third diffusion layers 61,
[0110] Qd2: dose of impurities used when ions are injected to form
the fourth diffusion layers 62,
[0111] S1: area in which ions are injected to form the third
diffusion layers 61, and
[0112] S2: area in which ions are injected to form the fourth
diffusion layers 62.
[0113] By forming the third and fourth diffusion layers 61 and 62
so as to meet Equation (3), depletion layers are extended far from
the junctions between the diffusion layers 61 and the diffusion
layers 62. Thus, the third and fourth diffusion layers 61 and 62
may be formed, for example, like a lattice as shown in FIG. 33 as
long as the Equation (3) is met. This lattice shape need not lie
along the edges of the semiconductor device but may extend at an
appropriate angle from them.
[0114] According to the seventh embodiment, the second
semiconductor substrate 2 and the first and second diffusion layers
13 and 14 are structured similarly to the first to fourth
embodiments. The seventh embodiment thus produces effects similar
to those of the first to fourth embodiments.
[0115] Furthermore, according to the seventh embodiment, the third
and fourth embodiments 61 and 62, used to form depletion layers,
are formed radially or like a lattice under the predetermined
conditions. Depletion layers can be formed to a large extent in the
terminal area. This serves to provide a semiconductor device with a
high withstand voltage.
[0116] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *