U.S. patent application number 12/138294 was filed with the patent office on 2008-12-18 for back-illuminated type imaging device and fabrication method thereof.
Invention is credited to Shinji UYA.
Application Number | 20080308890 12/138294 |
Document ID | / |
Family ID | 40131500 |
Filed Date | 2008-12-18 |
United States Patent
Application |
20080308890 |
Kind Code |
A1 |
UYA; Shinji |
December 18, 2008 |
BACK-ILLUMINATED TYPE IMAGING DEVICE AND FABRICATION METHOD
THEREOF
Abstract
Light is illuminated from a back-surface side of a silicon
substrate 4. A back-illuminated type imaging device 100 reads out,
from a front-surface side of the silicon substrate 4, charges that
are generated in the silicon substrate 4 in response to the
illuminated light, so as to perform imaging. The back-illuminated
type imaging device 100 includes pad portions 17 formed on the back
surface of the semiconductor substrate 4, and a plurality of
pillars 9 that are formed in the semiconductor substrate 4, are
made of a conductive material and electrically connect wiring
portions 12 formed on the front surface of the semiconductor
substrate 4 and the pad portions 17 to each other.
Inventors: |
UYA; Shinji; (Kurokawa-gun,
JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
40131500 |
Appl. No.: |
12/138294 |
Filed: |
June 12, 2008 |
Current U.S.
Class: |
257/437 ;
257/E31.122; 438/73 |
Current CPC
Class: |
H01L 27/1464 20130101;
H01L 27/14603 20130101; H01L 27/14636 20130101 |
Class at
Publication: |
257/437 ; 438/73;
257/E31.122 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216; H01L 31/18 20060101 H01L031/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2007 |
JP |
P2007-157460 |
Claims
1. A back-illuminated type imaging device comprising: a
semiconductor substrate including a front surface and a back
surface, wherein light is illuminated from the back-surface side of
the semiconductor substrate, and charges that are generated in
response to the light are read out from the front-surface side of
the semiconductor substrate to perform imaging; a plurality of pad
portions formed on the back surface of the semiconductor substrate;
a plurality of wiring portions formed on the front surface of the
semiconductor substrate; and a plurality of contact wiring portions
that are formed in the semiconductor substrate and electrically
connect the wiring portions formed on the front surface of the
semiconductor substrate and the pad portions.
2. The back-illuminated type imaging device according to claim 1,
wherein each contact wiring portion is configured by a pillar made
of a conductive material, the pillar extending from the front
surface of the semiconductor substrate in a direction perpendicular
to the front surface of the semiconductor substrate, the pillar
reaching the back surface of the semiconductor substrate, and the
pad portions are connected to one ends of the pillars, and the
wiring portions are connected to the other ends of the pillars.
3. The back-illuminated type imaging device according to claim 2,
wherein the plurality of pillars are arranged two-dimensionally,
when viewed in plan view.
4. The back-illuminated type imaging device according to claim 2,
wherein portions of the plurality of pillars at which the pillars
have a maximum width have 2 .mu.m or less in length.
5. The back-illuminated type imaging device according to claim 1,
further comprising: an alignment mark that is formed in the
semiconductor substrate and is used to align components on the
front-surface side of the semiconductor substrate with components
on the back-surface side, wherein the alignment mark is formed by a
pattern of a plurality of pillars that are made of a conductive
material and extend in a direction perpendicular to the front
surface of the semiconductor substrate.
6. The back-illuminated type imaging device according to claim 5,
wherein the plurality of pillars, which form the alignment mark,
are arranged two-dimensionally.
7. The back-illuminated type imaging device according to claim 5,
wherein the pattern of the plurality of pillars, which form the
alignment mark, is symmetric.
8. The back-illuminated type imaging device according to claim 5,
wherein portions of the plurality of pillars at which the pillars
have a maximum width have 2 .mu.m or less in length.
9. The back-illuminated type imaging device according to claim 5,
wherein each of the pillars, which form the alignment mark, extends
from the front surface of the semiconductor substrate and reaches
the back surface of the semiconductor substrate.
10. The back-illuminated type imaging device according to claim 1,
further comprising: a supporting substrate bonded to the front
surface of the semiconductor substrate via an inorganic adhesive
layer.
11. The back-illuminated type imaging device according to claim 10,
wherein the supporting substrate is made of the same material as
the semiconductor substrate.
12. The back-illuminated type imaging device according to claim 1,
further comprising: a film that is made of the same material as the
pad portions and is formed above the back surface of the
semiconductor substrate, wherein the film includes at least one of
(i) a light shielding film for shielding a part of pixel portions
formed in the semiconductor substrate from light and (ii) a light
shielding film for shielding a boundary between the pixel portions
from light.
13. A method for fabricating a back-illuminated type imaging device
in which light is illuminated from a back-surface side of a
semiconductor substrate, and charges that are generated in response
to the light are read out from a front-surface side of the
semiconductor substrate to perform imaging, the method comprising:
forming a plurality of first through holes in a plane area, on
which pad portions to be formed on the back surface of the
semiconductor substrate is to be formed, from the front-surface
side of the semiconductor substrate so that the through holes
extends from the front surface of the semiconductor substrate and
reaches the back surface; filling the plurality of first through
holes with a conductive material to form a plurality of first
pillars made of the conductive material; forming wiring portions on
the front surface of the semiconductor substrate so that the wiring
portions are connected to the plurality of first pillars; and after
the wiring portions are formed, forming the pad portions on the
back surface of the semiconductor substrate so that the pad
portions are connected to the plurality of first pillars.
14. The method for fabricating the back-illuminated type imaging
device according to claim 13, wherein in the forming of the first
through holes, the plurality of first through holes are formed so
as to be arranged two-dimensionally, when viewed in plan view.
15. The method for fabricating the back-illuminated type imaging
device according to claim 13, wherein in the forming of the first
through holes, the plurality of first through holes are formed so
that portions of the first through holes at which the through holes
have a maximum width have 2 .mu.n or less in length.
16. The method for fabricating the back-illuminated type imaging
device according to claim 13, wherein in the forming of the first
through holes, a plurality of second through holes are formed in a
plane area, in which a alignment mark that is used to align
components on the front-surface side of the semiconductor substrate
with components on the back-surface side are to be formed, so that
the second through holes extends from the front surface of the
semiconductor substrate and reaches the back surface, in the
filling of the first through holes, the plurality of second through
holes are filled with the conductive material to form a plurality
of second pillars made of the conductive material, and the
alignment mark is formed by the pattern of the plurality of second
pillars.
17. The method for fabricating the back-illuminated type imaging
device according to claim 16, wherein in the forming of the first
through holes, the plurality of second through holes are formed so
as to be arranged two-dimensionally, when viewed in plan view.
18. The method for fabricating the back-illuminated type imaging
device according to claim 17, wherein in the forming of the first
through holes, a pattern of the plurality of second through hole is
formed so as to be symmetric.
19. The method for fabricating the back-illuminated type imaging
device according to claim 16, wherein in the forming of the first
through holes, the plurality of second through holes are formed so
that portions of the second through holes at which the through
holes have a maximum width have 2 .mu.m or less in length.
20. The method for fabricating the back-illuminated type imaging
device according to claim 13, further comprising forming pixel
portions in the semiconductor substrate, the pixel portions having
charge storage areas for storing therein the charges, wherein the
forming of the pad portions includes forming a film of a conductive
material having a light shielding property, on the back surface,
and removing the conductive material at portions outside at least
one of a portion above a plane area in which the pad portions are
formed, a portion above a part of the pixel portions, and a portion
above a boundary between the pixel portions.
21. The method for fabricating the back-illuminated type imaging
device according to claim 20, further comprising after the pad
portions are formed, forming color filters above the pixel
portions, so as to correspond to the pixel portions; and after the
pad portions are formed and before the color filters are formed,
heating the semiconductor substrate.
22. The method for fabricating the back-illuminated type imaging
device according to claim 13, wherein the semiconductor substrate
is a first semiconductor substrate of a SOT substrate, the SOI
substrate comprising the first semiconductor substrate, a second
semiconductor substrate, and an oxide film sandwiched between the
first and second semiconductor substrates, the method further
comprising: after the wiring portions are formed, bonding a
supporting substrate on the front-surface side of the first
semiconductor substrate via an inorganic adhesive film.
23. The method for fabricating the back-illuminated type imaging
device according to claim 22, wherein the supporting substrate is
made of the same material as the first semiconductor substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the Japanese Patent Application No. 2007-157460 filed
on Jun. 14, 2007, the entire contents of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The invention relates to a back-illuminated type imaging
device in which light is illuminated from a back-surface side of a
semiconductor substrate and charges that are generated in the
semiconductor substrate in response to the light are read from a
front-surface side of the semiconductor substrate to perform
imaging.
[0004] 2. Description of the Related Art
[0005] The following back-illuminated type imaging device has been
proposed. That is, light is illuminated from a back-surface side of
a semiconductor substrate. The back-illuminated type imaging device
stores charges that are generated in the semiconductor substrate in
response to the light, into charge storage areas formed on a
front-surface side of the semiconductor substrate. The
back-illuminated type imaging device outputs signals corresponding
to the charges stored in the charge storage areas to the outside by
a CCD or CMOS circuit formed on the front-surface side of the
semiconductor substrate so as to perform imaging.
[0006] In order to absorb almost all of visible light, the
semiconductor substrate (a photoelectric conversion area) the
back-illuminated type imaging device, the semiconductor substrate
(a photoelectric conversion area) is required to have a thickness
of about 10 .mu.m. For this reason, when the back-illuminated type
imaging device is fabricated, at first structures such as the
charge storage areas and the CCD are first formed on the front
surface of the thick semiconductor substrate. Thereafter, a wiring
layer is thereafter formed on the structures, and a supporting
substrate is bonded thereto via an adhesive layer Then, the
semiconductor substrate is etched from the back-surface side until
the thickness becomes 10 .mu.m, for example. After the etching,
structures such as color filters and microlenses are formed on the
back surface of the semiconductor substrate so as to be aligned
with the structures formed on the front-surface side of the
semiconductor substrate.
[0007] In the back-illuminated type imaging device formed in such a
manner, it is necessary to form a pad on the back-surface side or
front-surface side of the semiconductor substrate so as to be
connected to the wiring layer formed on the front-surface side of
the semiconductor substrate. A general image sensor that is
presently in wide use has such a structure that a pad opening is
provided on a light incidence side. In accordance with this
structure, test equipments for testing the functions of chips in
the wafer are installed in a fabrication line. For this reason, if
the pad opening is formed on the front-surface side of the
semiconductor substrate in the back-illuminated type imaging
device, it becomes necessary to drastically change or modify the
test equipments, which may increase the fabrication cost.
[0008] Therefore, similarly to the general image sensor, various
methods for providing the pad opening on the light incidence side
(the back-surface side) have been proposed for the back-illuminated
type imaging device. One method of them is to form a through hole
from the back-surface side of the semiconductor substrate, thereby
exposing from the back surface the pad connected to the wiring
layer formed on the front-surface side (for example, see JP
2005-285814 A and JP 2006-19653 A). However, such a method has the
following problems. That is, if the through hole is formed before
the color filter or the microlens are formed on the back-surface
side, a material for the color filter or the microlens may be left
in the through hole and cannot be removed in subsequent steps. Or,
since the substrate on which the material is to be deposited has a
large step formed thereon, the thickness of the deposited material
is not even, which may cause a fixed pattern noise that appears as
oblique lines in an image. Moreover, even if the through hole is
formed after the color filter or the microlens is formed, it is
necessary to remove a photoresist that defines the through hole
without causing any harm to the color filter or the microlens,
which is very troublesome. In particular, as described above, in
the semiconductor substrate having a thickness of 10 .mu.m, the
above problems may become conspicuous, and it becomes more
difficult to perform such a removal work.
[0009] In light of the above, as disclosed in JP 2006-339566 A, the
following method may be conceived. That is, a trench is formed in
the semiconductor substrate so as to extend from the front surface
of the semiconductor substrate to reach the back surface thereof,
the trench is filled with a conductive material, and a pad is
formed on the conductive material. It can be said that in
principle, it is possible to fill the trench corresponding to the
pad with the conductive material. However, usually, the pad has a
size of 100 .mu.m.times.100 .mu.m; therefore, in order to form a
trench having substantially the same size and fill the trench with
conductive material, if a CVD process is used, it is necessary to
form a layer of the conductive material to a thickness equal to or
larger than 50 .mu.m and then to remove the same thickness by an
anisotropic etching process. Such a process step cannot be said to
be practical as a semiconductor fabrication process.
[0010] If the size of the trench is decreased, it may not cause any
problem in view of a fabrication process. However, in such a case,
there arises a problem that the resistance of the conductive
material filled in the trench may increase, which may affect the
device characteristics.
SUMMARY OF THE INVENTION
[0011] The invention has been made in view of such circumstances,
and provides a back-illuminated type imaging device that can
prevent the fabrication cost and the resistance between pads and
wirings from increasing even if pad openings is formed on a light
incidence side.
[0012] According to an aspect of the invention, a back-illuminated
type imaging device includes a semiconductor substrate, a pad
portion, a wiring portion and a plurality of contact wiring
portions. The semiconductor substrate includes a front surface and
a back surface. Light is illuminated from the back-surface side of
the semiconductor substrate. Charges that are generated in response
to the light are read out from the front-surface side of the
semiconductor substrate to perform imaging. The pad portion is
formed on the back surface of the semiconductor substrate. The
wiring portion is formed on the front surface of the semiconductor
substrate. The plurality of contact wiring portions are formed in
the semiconductor substrate and electrically connect the wiring
portion formed on the front surface of the semiconductor substrate
and the pad portion.
[0013] In the back-illuminated type imaging device, each contact
wiring portion may be configured by a pillar made of a conductive
material. The pillar extends from the front surface of the
semiconductor substrate in a direction perpendicular to the front
surface of the semiconductor substrate. The pillar reaches the back
surface of the semiconductor substrate. The pad portion may be
connected to one ends of the pillars. The wiring portion may be
connected to the other ends of the pillars.
[0014] In the back-illuminated type imaging device, the plurality
of pillars may be arranged two-dimensionally, when viewed in plan
view.
[0015] In the back-illuminated type imaging device, portions of the
plurality of pillars at which the pillars have a maximum width may
have 2 .mu.m or less in length.
[0016] The back-illuminated type imaging device may further include
an alignment mark that is formed in the semiconductor substrate and
is used to align components on the front-surface side of the
semiconductor substrate with components on the back-surface side.
The alignment mark may be formed by a pattern of a plurality of
pillars that are made of a conductive material and extend in a
direction perpendicular to the front surface of the semiconductor
substrate.
[0017] In the back-illuminated type imaging device, the plurality
of pillars, which form the alignment mark, may be arranged
two-dimensionally.
[0018] In the back-illuminated type imaging device, the pattern of
the plurality of pillars, which form the alignment mark, may be
symmetric.
[0019] In the back-illuminated type imaging device, portions of the
plurality of pillars at which the pillars have a maximum width may
have 2 .mu.m or less in length.
[0020] In the back-illuminated type imaging device, each of the
pillars, which form the alignment mark, may extend from the front
surface of the semiconductor substrate and may reache the back
surface of the semiconductor substrate.
[0021] The back-illuminated type imaging device may further include
a supporting substrate bonded to the front surface of the
semiconductor substrate via an inorganic adhesive layer.
[0022] In the back-illuminated type imaging device, the supporting
substrate may be made of the same material as the semiconductor
substrate.
[0023] The back-illuminated type imaging device may further include
a film that is made of the same material as the pad portion and is
formed above the back surface of the semiconductor substrate. The
film includes at least one of (i) a light shielding film for
shielding a part of pixel portions formed in the semiconductor
substrate from light and (ii) a light shielding film for shielding
a boundary between the pixel portions from light.
[0024] According to another aspect of the invention, a method for
fabricating a back-illuminated type imaging device in which light
is illuminated from a back-surface side of a semiconductor
substrate, and charges that are generated in response to the light
are read out from a front-surface side of the semiconductor
substrate to perform imaging, includes: forming a plurality of
first through holes in a plane area, on which a pad portion to be
formed on the back surface of the semiconductor substrate is to be
formed, from the front-surface side of the semiconductor substrate
so that the through holes extends from the front surface of the
semiconductor substrate and reaches the back surface; filling the
plurality of first through holes with a conductive material to form
a plurality of first pillars made of the conductive material;
forming a wiring portion on the front surface of the semiconductor
substrate so that the wiring portion is connected to the plurality
of first pillars; and after the wiring portion is formed, forming
the pad portion on the back surface of the semiconductor substrate
so that the pad portion is connected to the plurality of first
pillars.
[0025] In the method for fabricating the back-illuminated type
imaging device, in the forming of the first through holes, the
plurality of first through holes may be formed so as to be arranged
two-dimensionally, when viewed in plan view.
[0026] In the method for fabricating the back-illuminated type
imaging device, in the forming of the first through holes, the
plurality of first through holes may be formed so that portions of
the first through holes at which the through holes have a maximum
width have 2 .mu.m or less in length.
[0027] In the method for fabricating the back-illuminated type
imaging device, in the forming of the first through holes, a
plurality of second through holes may be formed in a plane area, in
which a alignment mark that is used to align components on the
front-surface side of the semiconductor substrate with components
on the back-surface side are to be formed, so that the second
through holes extends from the front surface of the semiconductor
substrate and reaches the back surface. In the filling of the first
through holes, the plurality of second through holes may be filled
with the conductive material to form a plurality of second pillars
made of the conductive material. The alignment mark may be formed
by the pattern of the plurality of second pillars.
[0028] In the method for fabricating the back-illuminated type
imaging device in the forming of the first through holes, the
plurality of second through holes may be formed so as to be
arranged two-dimensionally, when viewed in plan view.
[0029] In the method for fabricating the back-illuminated type
imaging device, in the forming of the first through holes, a
pattern of the plurality of second through hole may be formed so as
to be symmetric.
[0030] In the method for fabricating the back-illuminated type
imaging device, in the forming of the first through holes, the
plurality of second through holes may be formed so that portions of
the second through holes at which the through holes have a maximum
width have 2 .mu.m or less in length.
[0031] The method for fabricating the back-illuminated type imaging
device may further include forming pixel portions in the
semiconductor substrate, the pixel portions having charge storage
areas for storing therein the charges. The forming of the pad
portion may include forming a film of a conductive material having
a light shielding property, on the back surface, and removing the
conductive material at portions outside at least one of a portion
above a plane area in which the pad portion is formed, a portion
above a part of the pixel portions, and a portion above a boundary
between the pixel portions.
[0032] The method for fabricating the back-illuminated type imaging
device may further include: after the pad portion is formed,
forming color filters above the pixel portions, so as to correspond
to the pixel portions; and after the pad portion is formed and
before the color filters are formed, heating the semiconductor
substrate.
[0033] In the method for fabricating the back-illuminated type
imaging device, the semiconductor substrate may be a first
semiconductor substrate of a SOI substrate. The SOI substrate
includes the first semiconductor substrate, a second semiconductor
substrate, and an oxide film sandwiched between the first and
second semiconductor substrates. The method may further include:
after the wiring portion is formed, bonding a supporting substrate
on the front-surface side of the first semiconductor substrate via
an inorganic adhesive film.
[0034] In the method for fabricating the back-illuminated type
imaging device, the supporting substrate may be made of the same
material as the first semiconductor substrate.
[0035] According to the above configurations and methods, it is
possible to provide a back-illuminated type imaging device capable
of preventing the fabrication cost and the resistance between pads
and wirings from increasing even if a pad opening is provided on a
light incidence side.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a schematic plan view of a back-illuminated type
imaging device according to an embodiment of the invention, when
viewed from a back-surface side.
[0037] FIG. 2 is a schematic sectional view taken along the line
A-A of FIG. 1.
[0038] FIG. 3 is a schematic sectional view showing the
back-illuminated type imaging device 100 during fabrication process
steps.
[0039] FIG. 4 is a schematic sectional view showing the
back-illuminated type imaging device 100 during the fabrication
process steps.
[0040] FIG. 5 is a schematic sectional view showing the
back-illuminated type imaging device 100 during the fabrication
process steps.
[0041] FIG. 6 is a schematic sectional view showing the
back-illuminated type imaging device 100 during the fabrication
process steps.
[0042] FIG. 7 is a schematic sectional view showing the
back-illuminated type imaging device 100 during the fabrication
process steps.
[0043] FIG. 8 is a schematic sectional view showing the
back-illuminated type imaging device 100 during the fabrication
process steps.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0044] Hereinafter, embodiments of the invention will be described
with reference to the accompanying drawings.
[0045] FIG. 1 is a schematic plan view of a back-illuminated type
imaging device according to an embodiment of the invention, when
viewed from a light incidence side (a back-surface side). FIG. 2 is
a schematic sectional view taken along the line A-A of FIG. 1.
[0046] As shown in FIG. 1, the back-illuminated type imaging device
100 includes an imaging area 30 for receiving light to perform
imaging; a mark forming area in which formed are a large number of
alignment marks M that are used to align components (charge storage
areas for storing charges, a signal output portion for outputting
signals corresponding to the charges stored in the charge storage
areas, and the like) on a front-surface side opposite to the back
surface, in which the imaging area 30 is formed, with components on
the back-surface side; and a pad forming area in which a large
number of pad portions 17 that are connected to wiring portions
formed on the front-surface side are formed for inputting and
outputting signals from and to the outside.
[0047] As shown by the enlarged view in FIG. 1, each alignment mark
M is formed of a pattern of nine pillars 8 which are made of a
conductive material (e.g., doped polysilicon) and which are
two-dimensionally arranged in the vertical and horizontal
directions. As shown in the figure, the nine pillars 8 are
symmetric in the vertical and horizontal directions. Each pillar is
a circular column. Since the pattern of the nine pillars 8 has the
symmetrical arrangement, it is possible to detect one alignment
mark M by using the nine pillars 8. As shown in FIG. 2, the pillars
8 are formed in an n-type silicon substrate 4, which is a
semiconductor substrate, so as to extend from the back surface and
reach the front surface.
[0048] As shown in FIG. 1 (enlarged view) and FIG. 2, the pad
portions 17 are connected to the plurality of pillars 9, which are
made of the conductive material (for example, the doped
polysilicon) and are formed in the silicon substrate 4 below the
pad portions 17 so as to extend from the back surface and reach the
front surface. The plurality of pillars 9 are connected to wiring
portions 12 formed on the front-surface side of the silicon
substrate 4. As shown in the enlarged view of FIG. 1, the pillars 9
are arranged two-dimensionally so as to be symmetrical in the
vertical and horizontal directions. Each pillar 9 is a circular
column. Since the pillars 9 only need to electrically connect the
pad portions 17 and the wiring portions 12 to each other, it is not
necessary that the pillars 9 are arranged symmetrically. Moreover,
it is not necessary that the pillars 9 are arranged
two-dimensionally.
[0049] As shown in FIG. 2, when the back-illuminated type imaging
device 100 is used, light is incident thereon from the back-surface
side of the silicon substrate 4. Specifically, light is focused by
microlenses 21 and filtered by color filters 20, and then the light
is incident to the silicon substrate 4. Charges that are generated
in the silicon substrate 4 are stored in charge storage areas 10
and signals corresponding to the stored charges are output to the
outside by a MOS circuit including MOS transistors. In this
specification, a positional relationship among components will be
defined using the front and back surfaces of the silicon substrate
4 as a reference. For example, when the front surface of the
silicon substrate 4 is used as the reference, the light incidence
direction is defined as an upper direction of the front surface,
while when the back surface of the silicon substrate 4 is used as
the references, the direction opposite to the light incidence
direction will be defined as an upper direction of the back
surface.
[0050] The back-illuminated type imaging device 100 includes the
silicon substrate 4, an insulating film 5 that is made, for
example, Of SiO.sub.2 and is formed on the front surface of the
silicon substrate 4, an insulating film 14 formed on the insulating
film 5, and a supporting substrate 16 formed on the insulating film
14 via an adhesive film 15.
[0051] The adhesive film 15 is formed of an organic material film
made such as SiO.sub.2 and is formed by a CVD process. The
supporting substrate 16 is preferably made of a material having a
thermal expansion coefficient close to that of the silicon
substrate 4. This is because Such selection of material can
decrease damages to the silicon substrate 4, which may be caused by
a sintering process described later. However, if the supporting
substrate 16 is made of a material such as Pyrex which is included
in such materials, white defects may occur due to the radioactive
rays (especially, an a ray) emitted from radioactive isotopes
contained in the material. The white defects may occur even after
the back-illuminated type imaging device 100 is assembled into a
camera, and the defects are referred to as post-generated white
defects. The post-generated white defects may cause a serious
problem since they are not subjected to an image correction
process. For this reason, it is more preferable that the supporting
substrate 16 is made of a material that has a thermal expansion
coefficient close to that of the silicon substrate 4 and contains
few radioactive isotopes.
[0052] Furthermore, the supporting substrate 16 is preferably made
of a highly transparent material. If the supporting substrate 16 is
transparent, it becomes possible to detect the alignment marks M
from the front-surface side of the back-illuminated type imaging
device 100 by using an exposure apparatus having alignment
mechanisms above and below the exposure apparatus, thereby forming
the components on the back-surface side with reference to the
alignment marks M. In this case, it is not necessary to form the
alignment marks M in the silicon substrate 4 but the alignment
marks M may be formed on the front surface of the silicon substrate
4.
[0053] A glass substrate can be exemplified as the highly
transparent material. However, it is hard to be used since its
thermal expansion coefficient differs from that of silicon and it
contains many radioactive isotopes. Therefore, silicon is
preferably used for the supporting substrate 16 since the silicon's
thermal expansion coefficient is close to that of the silicon
substrate 4 and the silicon contains few radioactive isotopes.
However, since the silicon is not transparent, it cannot be used
with the exposure apparatus having the alignment mechanisms above
and below the exposure apparatus. Therefore, in this embodiment,
the alignment marks M that are visible from the back-surface side
are formed.
[0054] In a portion of the silicon substrate 4 where the imaging
area 30 is located, a large number of pixel portions are formed
two-dimensionally. Each pixel portion includes a charge storage
area 10 for storing the charges, which are generated in the silicon
substrate 4 in response to the incidence light, and components (not
shown) of a MOS circuit having MOS transistors for reading out
signals corresponding to the charges stored in the charge storage
area 10. Moreover, pixel separation areas 11 for separating the
pixel portions 10 from each other are formed on an inner side of
the front surface of the silicon substrate 4.
[0055] A highly doped p-type impurity region 3 for blocking dark
current is formed on an inner side of the back surface of the
silicon substrate 4. In the pad forming areas of the silicon
substrate 4, a plurality of the pillars 9 (only nine pillars are
shown in the drawing for the sake of simplicity) extending from the
back surface of the silicon substrate 4 and reaching the front
surface are formed below the large number of pad portions 17,
thereby electrically connecting the pad portions 17 to the
plurality of pillars 9. In the mark forming area of the silicon
substrate 4, the plurality of pillars 8 (only five pillars are
shown in the drawing for the sake of simplicity) extending from the
back surface of the silicon substrate 4 and reaching the front
surface are formed in positions where the plurality of alignment
marks M are to be formed. The pillars 8 and 9 have the same size,
when viewed in a plan view. Preferably, in view of easiness of
fabrication, portions of the pillars where they have the maximum
width have 2 .mu.m or less in length (in the case of a circular
column, they have a diameter equal to or less than 2 .mu.m). The
side walls of the pillars 8 and 9 are covered with insulating
films.
[0056] Also, the pillars 8 and 9 may have mutually different sizes.
Furthermore, the respective pillars 9 connected to the pad portions
17 may not have the same size, that is, may have different sizes.
However; in any case, it is preferable that the size of the pillars
8 and 9 is set equal to or smaller than 2 .mu.m. Also, the pillars
8 do not necessarily extend from the back surface of the silicon
substrate 4 and reach the front surface. The pillars 8 may be
buried in the silicon substrate 4 so that the pillars 8 are
detectable from both the front and back-surface sides of the
silicon substrate 4. From the same reason, the pillars 8 are not
necessarily made of the conductive material similar to the pillars
9. However, it is to be noted that if the pillars 8 and 9 are made
formed of a columnar conductive material that extends from the back
surface of the silicon substrate 4 and reach the front surface,
they can be formed simultaneously with each other, which is
advantageous for the fabrication process.
[0057] The back-illuminated type imaging device 100 further
includes: an oxide film 2 formed on the back surface of the silicon
substrate 4 except the pad portions 17; a light shielding film 18
formed on the oxide film 2 above pixel portions for black level
detection; a light shielding film 19 for preventing color mixing
between pixels, formed on the oxide film 2 above the pixel
separation areas 11, which separate the pixel portions, excluding
the pixel portions for black level detection; the color filters 20
formed on the oxide film 2 above the pixel portions; the
microlenses 21 formed on the color filters 20; and a material film
22 made of the materials, which are used to form the color filters
20 and the microlenses 21. In the material film 22, openings are
formed on the pad portions 17, and the pad portions 17 are exposed
through the openings.
[0058] It is preferable that the pad portions 17, the light
shielding film 18, and the light shielding film 19 are made of the
same material. If they are made of the same material, they can be
formed at the same process step, which advantageous for the
fabrication process. As a material that has light shielding
property and allows the pad portions 17 to function properly,
aluminum can be exemplified.
[0059] In the insulating film 14, formed are the wiring portions 12
such as various wrings connected to the components of the MOS
circuit, which outputs signals corresponding to the charges stored
in the charge storage areas 10 of the silicon substrate 4, and
connected to a peripheral circuit. In the example shown in the
figure, the wiring portions 12 have a three-layer wiring structure.
The wiring portions 12 include contact wirings 13 that (i) connect
various wirings connected to the components of the MOS circuit and
the peripheral circuit and (ii) the pillars 9 to each other. With
this configuration, the wiring portions 12 and the pad portions 17
are electrically connected to each other via the pillars 9.
[0060] Next, a method for fabricating the back-illuminated type
imaging device 100 having such configuration will be described.
[0061] FIGS. 3 to 8 are schematic sectional views showing the
back-illuminated type imaging device 100 at respective fabrication
process steps.
[0062] First, as shown in FIG. 3, an SOI substrate is prepared to
include the n-type silicon substrate 4 in which a p-type impurity
layer 3 is doped in an inner side from the back surface, an n-type
silicon substrate 1, and an oxide film 2 formed between the back
surface of the silicon substrate 4 and the silicon substrate 1.
[0063] Next, a SiO.sub.2 film, for example, is formed on the front
surface of the silicon substrate 4 to form the insulating film 5,
and a resist mask pattern is formed on the insulating film 5 by a
photolithographic process. The resist mask pattern has openings
formed thereon in positions where the pillars 8 and 9 are to be
formed. Subsequently, portions of the insulating film 5, the
silicon substrate 4 and the oxide film 2 which are below the mask
openings are selectively etched and removed via the resist mask
pattern to form through holes T having a circular column shape in a
mark forming area 6 and a pad forming area 7 (see FIG. 4). The
resist mask pattern is designed so that the through holes T have a
diameter of 0.8 .mu.m, for example.
[0064] Next, the side walls of the through holes T are oxidized to
form an insulating film on the side walls. A doped polysilicon film
is formed as a conductive material layer on the insulating film 5.
Subsequently, the doped polysilicon film is etched back to bury the
dope polysilicon in the through holes T, and exposed surfaces of
the doped polysilicon are oxidized to form the pillars 8 and 9 (see
FIG. 5).
[0065] Next, the alignment marks M formed of the pillars 8 are
detected from the front-surface side of the silicon substrate 4,
and with reference to the detected alignment marks M, the pixel
portions including the charge storage areas 10 and the pixel
separation areas 11 are formed in the inner side of the silicon
substrate 4 from the front surface of the silicon substrate 4 by a
conventional process. Subsequently, the components of the MOS
circuit, the peripheral circuit, and the wiring portions 12 are
formed on the insulating film 5, and the insulating film 14 is
formed thereon to planarize the surface (see FIG. 6). In this case,
the contact wirings 13 of the wiring portions 12 are formed by
forming openings in portions of the insulating film 5 above the
pillars 9 and filling the openings with polysilicon.
[0066] Next, the supporting substrate 16 (made of silicon) is
prepared to have on a front surface thereof an adhesive film 15
that is made of SiO.sub.2 and formed by the CVD process. The
supporting substrate 16 is bonded to the insulating film 14 via the
adhesive film 15 by a direct bonding method (see FIG. 7).
Subsequently, the silicon substrate 1 is etched and removed to
expose the oxide film 2 with the supporting substrate 16 being used
as a base layer (see FIG. 8).
[0067] Next, portions of the oxide film 2 on the pad forming areas
7 are etched and removed to expose the pillars 9. Thereafter, an
aluminum film, for example, is formed as a conductive material
layer on the oxide film 2. Subsequently, a resist mask pattern is
formed on the aluminum film so that the resist mask pattern has
openings in positions outside (i) the pad forming areas 7, (ii) a
portion above the pixel portions for black level detection and
(iii) a portion above the pixel separation areas 11 between the
pixel portions. Then, the aluminum film is etched via the resist
mask pattern to form pad portions 17 and the light shielding films
18 and 19.
[0068] Next, in order to secure the electrical connection between
the doped polysilicon constituting the pillars 9 and the aluminum
film constituting the pad portions 17, a heating treatment
(sintering) is performed in a hydrogen-containing atmosphere at a
temperature around 400.degree. C. Subsequently, the color filters
20 and the microlenses 21 are formed. Finally, openings are formed
at portions of the material film 22, which is formed when the color
filters 20 and the microlenses 21 are formed, above the pad
portions 17 to expose the pad portions 17. Thereby, the
back-illuminated type imaging device 100 shown in FIG. 1 is
obtained.
[0069] As described above, according to the back-illuminated type
imaging device 100, the device has the structure that the pad
portions 17 formed on the back-surface side of the semiconductor
substrate 4 and the wiring portions 12 on the front-surface side
are electrically connected to each other via the plurality of
pillars 9. Therefore, it is possible to perform the electrical
connection between the pad portions 17 and the wiring portions 12
while suppressing the resistance between the pad portions 17 and
the wiring portions 12 to a low level even if the size of the
pillars 9 is not increased. Since the size of the pillars 9 can be
decreased (for example, to 2 .mu.m or less), if the above-described
fabrication method is employed, it is possible to decrease the
process load of filling the through hole T with the conductive
material, which reduces the fabrication cost.
[0070] Moreover, according to the back-illuminated type imaging
device 100, the alignment marks M are formed of the pattern of the
plurality of smaller pillars 8. Therefore, it is possible to
decrease the process load of forming the alignment marks M, which
decreases the fabrication cost. In addition, since the pillars 8
are made of the same material and have the same length as the
pillars 9, it is possible to form the pillars 8 simultaneously with
the pillar 9 as described in the fabrication method, which reduces
the fabrication cost.
[0071] In addition, according to the back-illuminated type imaging
device 100, the semiconductor substrate 4 and the supporting
substrate 16 are bonded to each other via an inorganic material.
Therefore, a sintering process for securing the electrical
connection between the pad portions 17 and the pillars 9 does not
weaken the adhesion force between the semiconductor substrate 4 and
the supporting substrate 16 or bend the supporting substrate 16.
That is, it is possible to form the color filter 20 or the
microlens 21 in a state where the supporting substrate 16 is
completely parallel to the semiconductor substrate 4. Therefore, it
is possible to form the color filter 20 or the microlens 21 as
originally designed, which improves a yield ratio and reduces the
fabrication cost.
[0072] In addition, according to the back-illuminated type imaging
device 100, the plurality of pillars 9 connected to the pad
portions 17 are arranged two-dimensionally. Therefore, it is
possible to provide advantages that the electrical resistance is
decreased and that bonding properties to the pad electrode are
improved.
[0073] In addition, according to the back-illuminated type imaging
device 100, the supporting substrate 16 and the semiconductor
substrate 4 are made of the same material. Therefore, it is
possible to provide advantages that post-generated white defects
can be prevented and that damages to the semiconductor substrate 4,
which may be caused during a sintering process, can be reduced.
[0074] In addition, according to the back-illuminated type imaging
device 100, the pad portions 17 and the light shielding films 18
and 19 are made of the same material. Therefore, it is possible to
form the pad portions simultaneously with the light shielding films
as described in the above fabrication method, which reduces the
fabrication cost.
[0075] In addition, although the back-illuminated type imaging
device 100 has been described as being a MOS type imaging device,
the imaging device may be a CCD type imaging device. Also, marks
for inspecting misalignment of the components on the back-surface
side may be formed in the semiconductor substrate 4. These marks
may be formed of a plurality of pillars.
* * * * *