U.S. patent application number 09/864861 was filed with the patent office on 2002-02-07 for circuit-incorporating photosensitve device.
Invention is credited to Fukushima, Toshihiko, Kubo, Masaru, Tani, Zenpei.
Application Number | 20020014643 09/864861 |
Document ID | / |
Family ID | 18665311 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014643 |
Kind Code |
A1 |
Kubo, Masaru ; et
al. |
February 7, 2002 |
Circuit-incorporating photosensitve device
Abstract
A circuit-incorporating photosensitive device comprising: an SOI
wafer including a first silicon substrate, a second silicon
substrate, and an oxide film; a photodiode formed in a first region
of the SOI wafer; and a signal processing circuit formed in a
second region of the SOI wafer, wherein the photodiode includes a
photosensitive layer formed of an SiGe layer.
Inventors: |
Kubo, Masaru;
(Kitakatsuragi-gun, JP) ; Fukushima, Toshihiko;
(Nara-shi, JP) ; Tani, Zenpei; (Tondabayashi-shi,
JP) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201
US
|
Family ID: |
18665311 |
Appl. No.: |
09/864861 |
Filed: |
May 25, 2001 |
Current U.S.
Class: |
257/290 ;
257/E27.128 |
Current CPC
Class: |
H01L 27/1443
20130101 |
Class at
Publication: |
257/290 |
International
Class: |
H01L 031/062; H01L
031/113 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2000 |
JP |
2000-161260 |
Claims
What is claimed is:
1. A circuit-incorporating photosensitive device comprising: an SOI
wafer comprising a first silicon substrate, a second silicon
substrate, and an oxide film; a photodiode formed in a first region
of the SOI wafer; and a signal processing circuit formed in a
second region of the SOI wafer, wherein the photodiode comprises a
photosensitive layer comprising an SiGe layer.
2. A circuit-incorporating photosensitive device according to claim
1, wherein the photosensitive layer is formed after the signal
processing circuit is formed.
3. A circuit-incorporating photosensitive device according to claim
1, wherein the photosensitive layer is provided in a recess formed
in the SOI wafer.
4. A circuit-incorporating photosensitive device according to claim
1, wherein the signal processing circuit comprises a high-speed
transistor, and at least a portion of the high-speed transistor is
formed of an SiGe layer.
5. A circuit-incorporating photosensitive device according to claim
4, wherein the SiGe layer of the photosensitive layer and the SiGe
layer of the high-speed transistor are simultaneously formed.
6. A circuit-incorporating photosensitive device according to claim
1, wherein the photodiode has a reflection film provided on a
bottom surface thereof.
7. A circuit-incorporating photosensitive device according to claim
6, wherein the reflection film includes a high melting point metal
film.
8. A circuit-incorporating photosensitive device according to claim
1, wherein an antireflection film is provided on the photosensitive
layer.
9. A circuit-incorporating photosensitive device according to claim
8, wherein the antireflection film comprises an SiN film.
10. A circuit-incorporating photosensitive device according to
claim 9, wherein a thermal oxide film is formed between the
photosensitive layer and the SiN film.
11. A circuit-incorporating photosensitive device according to
claim 8, wherein the antireflection film is integrally formed of
the photosensitive layer.
12. A circuit-incorporating photosensitive device according to
claim 8, wherein the antireflection film includes an amorphous
carbon film.
13. A circuit-incorporating photosensitive device according to
claim 1, wherein a phase difference between light impinging upon
the photosensitive layer and light reflecting at the bottom surface
of the second silicon substrate is 1/2 of the wavelength of the
light impinging upon the photosensitive layer.
14. A circuit-incorporating photosensitive device according to
claim 1, wherein the photosensitive layer is separated into plural
photosensitive regions by a trench-type separation layer.
15. A circuit-incorporating photosensitive device according to
claim 1, wherein the photosensitive layer is separated into plural
photosensitive regions by forming the photosensitive layer with a
selective epitaxial growth method.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field Of The Invention
[0002] The present invention relates to a circuit-incorporating
photosensitive device using an SOI (Silicon on Insulator) wafer,
and especially to a circuit-incorporating photosensitive device
which has high sensibility and low power consumption.
[0003] 2. Description Of The Related Art
[0004] A circuit-incorporating photosensitive device is widely used
as an optical pickup, optical communication, or a photosensor,
e.g., a photocoupler. In recent years, there has been intense
demand for the higher sensibility, faster operation, and lower
power consumption of circuit-incorporating photosensitive devices
in all such applications.
[0005] FIG. 8 is a cross-sectional view illustrating the structure
of a conventional circuit-incorporating photosensitive device 400.
The conventional circuit-incorporating photosensitive device 400
shown in FIG. 8 has a laminated structure of a P-type silicon
substrate 1 and an N-type silicon substrate 4 epitaxially grown on
the P-type silicon substrate 1. In this laminated structure, a
photodiode 270, and a bipolar transistor 280 which is a circuit for
processing signals output from the photodiode 270 are integrally
provided. The N-type silicon substrate 4 is separated into plural
regions by P-type embedded diffusion layers 13. The photodiode 270
and the bipolar transistor 280 are respectively provided in the
regions separated by the P-type embedded diffusion layers 13.
[0006] The photodiode 270 is of a PN junction type, formed with the
laminated structure of the P-type silicon substrate 1 and the
N-type silicon substrate 4.
[0007] The bipolar transistor 280 has a P-type diffusion layer 7
formed in the N-type silicon substrate 4 near the surface thereof.
An N-type diffusion layer 8 is formed in the P-type diffusion layer
7. Furthermore, the N-type silicon substrate 4 includes an N-type
diffusion layer 6 which extends from the surface of the N-type
silicon substrate 4 to an N-type diffusion layer 12.
[0008] An oxide film layer 9 is provided on the entire surface of
the N-type silicon substrate 4. In the bipolar transistor 280
region, the oxide film layer 9 is provided with wiring 10a
connected to the N-type diffusion layer 6, wiring 10b connected to
the P-type diffusion layer 7, and wiring 10c connected to the
N-type diffusion layer 8 (which is embedded near the surface of the
P-type diffusion layer 7).
[0009] In the circuit-incorporating photosensitive device 400
having such a structure, the photosensitivity of the photosensitive
portion of the photodiode 270 depends on the photosensitivity at
the PN junction, as well as the amount of the light absorption
corresponding to the size and thickness of the photodiode 270.
[0010] In a circuit-incorporating photosensitive device used as an
optical pickup, light having a wavelength of about 635 nm for DVD
applications, about 780 nm for CD applications, about 850 nm for
space optical transmission, or about 950 nm for a photosensor
(e.g., a photocoupler) is normally used. The light absorption
coefficients of silicon (Si) and light penetration depths into
silicon for these wavelengths are shown in Table 1.
1TABLE 1 Light Light absorptance absorption Penetration in an
active Wavelength coefficient depth layer of 1 .mu.m 650 nm 2500
cm.sup.-1 4 .mu.m 22% 780 nm 1200 cm.sup.-1 8.5 .mu.m 11% 850 nm
800 cm.sup.-1 12.5 .mu.m 8% 950 nm 400 cm.sup.-1 25 .mu.m 4%
[0011] As shown in Table 1, the depths to which these light
wavelengths penetrate into silicon are no less than 4 .mu.m.
Normally, the depths are greater than the thickness of the N-type
silicon substrate 4 which forms the circuit-incorporating
photosensitive device 400. Thus, the PN junction between the N-type
silicon substrate 4 and the P-type silicon substrate 1 is used to
improve the photosensitivity of the photodiode 270 and to improve
the absorptance at these light wavelengths.
[0012] On the other hand, for faster operation and lower power
consumption, it is effective to use a SiGe layer (which has a
higher light absorptance) as a base layer, as well as an SOI
(Silicon on Insulator) wafer, as shown in, for example, Japanese
Laid-Open Publication No. 6-61434.
[0013] FIG. 9 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device 410 in which an SOI
wafer 290 is used. The SOI wafer 290 includes a silicon substrate 1
and an N-type silicon substrate 4, with an N-type diffusion layer 3
formed on a lower surface thereof and an oxide film 2 interposed
therebetween.
[0014] The N-type silicon substrate 4 of the SOI wafer 290 is
separated into plural regions by trench-type separation layers 5. A
photodiode 270 and a bipolar transistor 280 are respectively
provided in the regions separated by the trench-type separation
layers 5. The trench-type separation layers 5 extend from the
surface of the N-type silicon substrate 4, through the N-type
diffusion layer 3, so as to reach the oxide film 2.
[0015] In the photodiode 270, a P-type diffusion layer 7a, which
serves as an active layer, is formed near the surface of the N-type
silicon substrate 4. An N-type diffusion layer 6 is provided so as
to extend from the surface of the N-type silicon substrate 4 to the
N-type diffusion layer 3.
[0016] In an NPN-type bipolar transistor 280 which is a signal
processing circuit of the photodiode 270, a base layer 7b formed of
SiGe is embedded as a P-type diffusion layer near the surface of
the N-type silicon substrate 4. An N-type diffusion layer 8 is
provided near the surface of the base layer 7b. Furthermore, in the
N-type silicon substrate 4, an N-type diffusion layer 6 is provided
so as to extend from the N-type silicon substrate 4 to the N-type
diffusion layer 3.
[0017] An oxide film 9 is provided on the entire surface of the
N-type silicon substrate 4. In the NPN-type bipolar transistor 280
region, the oxide film 9 is provided with an electrode boa
connected to the N-type diffusion layer 6, a base electrode 10b
connected to the base layer 7b, and an electrode 10c connected to
the N-type diffusion layer 8 (which is embedded near the surface of
the base layer 7b).
[0018] In the circuit-incorporating photosensitive device 410
having such a structure, the thickness of the silicon layer 7a,
which serves as an active layer forming the photosensitive portion
of the photodiode 270, is normally about 1 .mu.m, so that there is
a problem in that the amount of light absorption is small. Table 1
also shows the light absorptance at different wavelengths in the
case where the thickness of the silicon active layer 7a is 1 .mu.m.
The light absorptance is 22% for a light wavelength of 650 nm, 11%
for a light wavelength of 780 nm, 8% for a light wavelength of 850
nm, and 4% for a light wavelength of 950 nm.
[0019] The photodiode 270 has a low photosensitivity because the
amount of the light absorption of each of the silicon
photosensitive layers 3a, 4a, and 7a is small.
[0020] The output of the photodiode 270 having such a low
photosensitivity could be subjected to gain compensation by the
signal processing circuit. However, when the gain of the output is
compensated, the response speed of the signal processing circuit
and the signal-to-noise ratio (S/N ratio) may decrease.
SUMMARY OF THE INVENTION
[0021] According to one aspect of the invention, there is provided
a circuit-incorporating photosensitive device comprising: an SOI
wafer comprising a first silicon substrate, a second silicon
substrate, and an oxide film; a photodiode formed in a first region
of the SOI wafer; and a signal processing circuit formed in a
second region of the SOI wafer, wherein the photodiode comprises a
photosensitive layer comprising an SiGe layer.
[0022] In one embodiment of the invention, the photosensitive layer
is formed after the signal processing circuit is formed.
[0023] In one embodiment of the invention, the photosensitive layer
is provided in a recess formed in the SOI wafer.
[0024] In one embodiment of the invention, the signal processing
circuit comprises a high-speed transistor, and at least a portion
of the high-speed transistor is formed of an SiGe layer.
[0025] In one embodiment of the invention, the SiGe layer of the
photosensitive layer and the SiGe layer of the high-speed
transistor are simultaneously formed.
[0026] In one embodiment of the invention, the photodiode has a
reflection film provided on a bottom surface thereof.
[0027] In one embodiment of the invention, the reflection film
includes a high melting point metal film.
[0028] In one embodiment of the invention, an antireflection film
is provided on the photosensitive layer.
[0029] In one embodiment of the invention, the antireflection film
comprises an SiN film.
[0030] In one embodiment of the invention, a thermal oxide film is
formed between the photosensitive layer and the SiN layer.
[0031] In one embodiment of the invention, the antireflection film
is integrally formed of the photosensitive layer.
[0032] In one embodiment of the invention, the antireflection film
includes an amorphous carbon film.
[0033] In one embodiment of the invention, a phase difference
between light impinging upon the photosensitive layer and light
reflecting at the bottom surface of the second silicon substrate is
1/2 of the wavelength of the light impinging upon the
photosensitive layer.
[0034] In one embodiment of the invention, the photosensitive layer
is separated into plural photosensitive regions by a trench-type
separation layer.
[0035] In one embodiment of the invention, the photosensitive layer
is separated into plural photosensitive regions by forming the
photosensitive layer with a selective epitaxial growth method.
[0036] Thus, the invention described herein makes possible the
advantage of providing a circuit-incorporating photosensitive
device with high-sensibility, fast operation, and low power
consumption, which prevents a decrease in the SIN ratio.
[0037] This and other advantages of the present invention will
become apparent to those skilled in the art upon reading and
understanding the following detailed description with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device according to an example
of the present invention.
[0039] FIG. 2 is a diagram illustrating a relationship between a
composition ratio of an SiGe layer and a bandgap thereof.
[0040] FIG. 3 is a graph illustrating a relationship between
wavelengths of irradiated light and absorption coefficients of an
Si layer and a Ge layer.
[0041] FIG. 4 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device according to an example
of the present invention.
[0042] FIGS. 5A to 5F are cross-sectional views illustrating steps
of a fabrication method of a circuit-incorporating photosensitive
device according to an example of the present invention.
[0043] FIG. 5G is a cross-sectional view illustrating a
circuit-incorporating photosensitive device according to an example
of the present invention.
[0044] FIG. 6 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device according to an example
of the present invention.
[0045] FIG. 7 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device according to an example
of the present invention.
[0046] FIG. 8 is a cross-sectional view illustrating an exemplary
conventional circuit-incorporating itiphotosensitive deice.
[0047] FIG. 9 is a cross-sectional view illustrating an exemplary
conventional circuit-incorporating photosensitive device having an
SOI structure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] Hereinafter, embodiments of the present invention will be
described in detail with reference to the figures.
(Example 1)
[0049] FIG. 1 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device 300 according to
Example 1 of the present invention. The circuit-incorporating
photosensitive device 300 shown in FIG. 1 comprises an SOI wafer
29. In the circuit-incorporating photosensitive device 300, an
N-type silicon substrate 4 having an N-type diffusion layer 3
formed on a lower surface thereof is provided on a silicon
substrate 1, with an oxide film 2 (e.g., SiO.sub.2) interposed
between the N-type diffusion layer 3 and the silicon substrate 1.
Since the SOI wafer 29 has a small parasitic capacitance, the
circuit-incorporating photosensitive device 300 using the SOI wafer
29 can operate fast while requiring low power consumption.
[0050] The SOI wafer 29 includes a photodiode 27 and a bipolar
transistor 28, which are formed in regions separated by trench-type
separation layers 5 provided in the N-type silicon substrate 4. The
photodiode 27 comprises a photosensitive layer (formed of SiGe) 17a
for receiving light. The bipolar transistor 28 constitutes a signal
processing circuit of the photodiode 27. The trench-type separation
layers 5 extend from the surface of the N-type silicon substrate 4,
through the N-type diffusion layer 3, so as to reach the oxide film
2.
[0051] The SiGe photosensitive layer 17a is formed near the surface
of the N-type silicon substrate 4 in the photodiode 27.
Specifically, the region near the surface of the N-type silicon
substrate 4 is etched away to a depth corresponding to the
thickness of the photosensitive layer 17a, thereby forming a recess
35. The photosensitive layer 17a is formed by growing a crystal of
SiGe in the recess 35. The photosensitive layer 17a serves as the
photosensitive region of the photodiode 27. Furthermore, in the
N-type silicon substrate 4 of the photodiode 27, an N-type
diffusion layer 6 is provided which extends from the surface of the
N-type silicon substrate 4 to the N-type diffusion layer 3.
[0052] The photosensitive layer 17a of the photodiode 27 is
embedded in the recess 35 provided in the N-type silicon substrate
4, and the surface of the photosensitive layer 17a is planed so as
to become flush with the surface of the N-type silicon substrate 4.
Any wiring provided on the surface is similarly planed.
[0053] The bipolar transistor 28, which serves as a signal
processing circuit of the photodiode 27, includes a base layer
(formed of SiGe) 17b near the surface of the N-type silicon
substrate 4. An N-type diffusion 15 layer 8 is formed in the base
layer 17b. Furthermore, the N-type silicon substrate 4 includes an
N-type diffusion layer 6 which extends from the surface of the
N-type silicon substrate 4 to the N-type diffusion layer 3.
[0054] An oxide film layer 9 is provided on the entire surface of
the N-type silicon substrate 4. In the bipolar transistor 28
region, the oxide film 9 is provided with metal wiring 10a
connected to the N-type diffusion layer 6, metal wiring 10b
connected to the base layer 17b, and metal wiring 10c connected to
the N-type diffusion layer 8, which is formed in the base layer
17b.
[0055] The circuit-incorporating photosensitive device 300
includes; a photodiode 27 which has significantly improved
photosensitivity because the photosensitive layer 17a which is
formed of SiGe having a high light absorptance is provided in the
photosensitive region of the photodiode 27. Furthermore, since the
base layer 17b of the bipolar transistor 28 is also formed of SiGe,
the carrier injection efficiency of the bipolar transistor 28 is
increased. Thus, the current amplification hfe is increased,
thereby enabling faster operation than in conventional
circuit-incorporating devices.
[0056] FIG. 2 is a graph illustrating a relationship between the
composition ratio between Si and Ge in the SiGe layer and the
bandgap thereof. FIG. 3 is a graph illustrating a relationship
between the absorption coefficient and the wavelength of light with
which the Si layer and the Ge layer are irradiated. As shown in
FIG. 2, the bandgap of the SiGe layer varies in accordance with the
composition ratio between Si and Ge. As the concentration of Ge
relative to Si increases, the bandgap of the SiGe layer becomes
narrower. As the bandgap becomes narrower, the absorption
coefficient increases. Thus, the photosensitivity of the photodiode
27 featuring the SiGe photosensitive layer 17a increases, and
faster operation can be obtained.
[0057] Furthermore, the Si layer and Ge layer have different
absorption coefficients as shown in FIG. 3. Thus, by using an SiGe
layer which is intercrystallized with Si and Ge, the overall
absorption coefficient significantly increases at certain
wavelengths as compared to the case of using Si alone.
[0058] In the circuit-incorporating photosensitive device 300 shown
in FIG. 1, the photosensitive layer 17a of the photodiode 27 is
designed with a thickness of not more than about 1 .mu.m from the
perspective of crystallinity. In order to ensure that a light
amount of 1-1/e is absorbed in such a photosensitive layer 17a
having a thickness of 1 .mu.m, an absorption coefficient of about
10000 cm.sup.-1 will be required. Therefore, in order to improve
the photosensitivity of the photodiode 27 as shown in FIG. 1, the
composition ratio between Si and Ge in the photosensitive layer 17a
is set so that the absorption coefficient in the wavelength band of
received light is about 10000 cm.sup.-1 or more.
[0059] The SiGe layer forming the photosensitive layer 17a (i.e.,
the active layer of the photodiode 27) and the SiGe layer forming
the base layer 17b of the bipolar transistor 28 are provided near
the surface of the N-type silicon substrate 4. Thus, the SiGe
layers can be formed simultaneously and the number of steps
required for fabricating the circuit-incorporating photosensitive
device 300 is not increased.
[0060] Furthermore, each of the SiGe layers can be a multilayer
film or a superlattice layer. By employing a multilayer film or a
superlattice layer, the carrier injection efficiency can be
increased without increasing the thickness of the layers.
[0061] It is preferable to avoid a heat treatment after the SiGe
layer is formed because the composition and the properties of the
SiGe layer will be changed if it is subjected to a high temperature
after it is formed. Thus, in the circuit-incorporating
photosensitive device 300 shown in FIG. 1, it is preferable to form
the photosensitive layer 17a and the base layer 17b (both of which
are SiGe layers) after completing a heat diffusion process which
may be used to form the bipolar transistor 28.
(Example 2)
[0062] FIG. 4 is a cross-sectional view illustrating a
circuit-incorporating photosensitive device 310 according to
Example 2 of the present invention. Similarly to the
circuit-incorporating photosensitive device 300, the
circuit-incorporating photosensitive device 310 shown in FIG. 4
comprises an SOI wafer 29. In the SOI wafer 29, a silicon substrate
4 having an N-type diffusion layer 3 formed on a lower surface
thereof is provided on a silicon substrate 1, with an oxide film 2
interposed between the N-type diffusion layer 3 and the silicon
substrate 1.
[0063] The SOI wafer 29 includes a photodiode 27 and a bipolar
transistor 28, which are formed in regions separated by trench-type
separation layers 5 provided in the N-type silicon substrate 4. The
photodiode 27 comprises a photosensitive layer (formed of SiGe) 17a
for receiving light. The bipolar transistor 28 constitutes a signal
processing circuit of the photodiode 27. The trench-type separation
layers 5 extend from the N-type silicon substrate 4 through an
N-type diffusion layer 3 so as to reach the oxide film 2. In each
of the regions where the photodiode 27 and the bipolar transistor
28 are formed, an N-type diffusion layer 6 is provided along each
trench-type separation layer 5 so as to extend from the surface of
the N-type silicon substrate 4 to the N-type diffusion layer 3.
[0064] In the photodiode 27, a photosensitive layer 17a formed of
SiGe is laminated on the surface of the N-type silicon substrate 4.
On the photosensitive layer 17a, an antireflection film 21 is
laminated. To a side of the photosensitive layer 17a and the
antireflection film 21 which are laminated together, a polysilicon
layer 16 doped with P-type impurities is provided on the surface of
the N-type silicon substrate 4 to bring the anode of the photodiode
27 into conduction. The regions other than the laminated portion of
the photosensitive layer 17a and the antireflection film 21 are
covered with an oxide insulator film 15. On the edge of each side
of the photosensitive layer 17a and the antireflection film 21, a
sidewall spacer 18 is provided.
[0065] In the oxide insulator film 15, metal wiring 22d and 22e are
provided as electrodes extend through the oxide insulator film 15
and contacting the polysilicon layer 16 and the N-type diffusion
layer 6, respectively.
[0066] In the bipolar transistor 28, a base layer 17b formed of
SiGe layer is laminated on the surface of the N-type silicon
substrate 4. To each side of the base layer 17b, a polysilicon
layer 16 doped with P-type impurities is provided on the surface of
the N-type silicon substrate 4 to bring the base electrode of the
bipolar transistor 28 into conduction. The regions other than the
base layer 17b and the polysilicon layer 16 are covered with an
oxide insulator film 15.
[0067] On the base layer 17b, a polysilicon layer 19 doped with
N-type impurities is laminated so as to form an emitter. Sidewall
spacers 18 are respectively interposed between either end of the
base layer 17b and the polysilicon layer 19. Each side of the
polysilicon layer 19 is embedded in the oxide insulator layer 15.
The surface of the polysilicon layer 19 is also covered with the
oxide insulator layer 15.
[0068] In the oxide insulator film 15, metal wiring 22a, 22b and
22c are respectively provided as electrodes extending through the
oxide insulator film 15 and contacting the N-type diffusion layer
6, the base layer 17b and the polysilicon layer 16.
[0069] In order to simplify description, multilayer wiring,
overcoat, etc., provided in the circuit-incorporating
photosensitive device 310 are omitted in FIG. 4.
[0070] The photosensitive layer 17a and the base layer 17b can be
formed of a multilayer film or a superlattice layer of SiGe.
[0071] FIGS. 5A to 5F are cross-sectional views illustrating a
fabrication process of the circuit-incorporating photosensitive
device 310 shown in FIG. 4. The method for fabricating the
circuit-incorporating photosensitive device 310 is described with
reference to FIGS. 5A to 5F.
[0072] First, as shown in FIG. 5A, the N-type silicon substrate 4
having the N-type diffusion layer 3 formed on a lower surface
thereof is provided on the silicon substrate 1, with the oxide film
2 interposed between the N-type diffusion layer 3 and the silicon
substrate 1, thus forming the SOI wafer 29.
[0073] In the case where the bipolar transistor 28 formed in the
SOI wafer 29 is a CMOS transistor, the N-type diffusion layer 3 is
not required. The substrate 4 is not necessarily N-type, but may be
of P-type. Furthermore, the SOI wafer 29 may be formed by adhering
the silicon substrate 1 and the silicon substrate 4 together, or by
a method such as SIMOX.
[0074] Next, as shown in FIG. 5B, at the border of the regions of
the silicon substrate 4 where the photodiode 27 and transistor 28
are to be formed, the trench-type separation layers 5 are formed.
Each separation layer 5 is formed along the direction of thickness
of the silicon substrate 4 so as to extend from the surface of the
silicon substrate 4, through the N-type diffusion layer 3, to the
oxide film 2. After each separation layer 5 is formed, in the
regions where the photodiode 27 and the bipolar transistor 28 are
respectively formed, the N-type diffusion layers 6 are formed along
each trench-type separation layer 5. Then, the oxide film 15 is
formed on the entire surface of the N-type silicon substrate 4.
[0075] After formation, the oxide film 15 of a central portion of
the regions where the photodiode 27 and the bipolar transistor 28
are respectively formed is removed by etching to expose the surface
of the N-type silicon substrate 4. On the exposed surface of the
N-type silicon substrate 4 in the region defining the photodiode
27, the polysilicon layer 16 doped with N-type impurities (see FIG.
5C) is formed to bring the anode of the photodiode 27 into
conduction. On the exposed surface of the N-type silicon substrate
4 in the region defining the bipolar transistor 28, the polysilicon
layer 16 doped with P-type impurities (see FIG. 5C) is formed to
bring the base electrode of the bipolar transistor 28 into
conduction.
[0076] Next, as shown in FIG. 5C, portions of the oxide film 15 and
the polysilicon layers 16 are removed by etching to expose the
surface of the N-type silicon substrate 4 except at the farther end
(from the N-type diffusion layer 6) of the polysilicon layer 16 in
the region defining the photodiode 27. Also, the oxide film 15 and
the polysilicon layers 16 are removed by etching to expose the
surface of the N-type silicon substrate 4 except at both ends of
the polysilicon layer 16 in the region defining the bipolar
transistor 28.
[0077] Next, as shown in FIG. 5D, SiGe layers are simultaneously
formed on the exposed surface of the N-type silicon substrate 4 in
the regions defining the photodiode 27 and the bipolar transistor
28, through selective growth by a method such as MBE. Thus, the
photosensitive layer 17a as a photosensitive region of the
photodiode 27 and the base layer 17b of the bipolar transistor 28
are formed at the same time. By simultaneously forming the
photosensitive layer 17a and base layer 17b with SiGe, the number
of fabrication steps can be decreased. Alternatively, the
photosensitive layer 17a may be formed by a selective epitaxial
method. In this case, as shown in FIG. 7, the photosensitive layer
17a can be formed into separate plural photosensitive regions.
[0078] Next, sidewall spacers 18 are provided on the respective
edge portions of at both ends of the photosensitive layer 17a and
the base layer 17b. Then, as shown in FIG. 5E, the polysilicon
layer 19 doped with N-type impurities is formed on the base layer
17b and on a portion of the oxide film 15 covering the polysilicon
layer 16 next to the base layer 17b.
[0079] Next, as shown in FIG. 5F, the antireflection film 21 is
formed on the photosensitive layer 17a in the region where the
photodiode 27 is located. Then, an oxide film 20 is formed on
portions other than the antireflection film 21 by CVD, or the
like.
[0080] By adjusting the Ge concentration of the surface side of the
photosensitive layer 17a formed of SiGe so as to be smaller than
that of the internal side, a potential barrier is formed. Thus,
surface recombination is restrained to prevent the photosensitivity
from being lowered.
[0081] Thus, by adjusting the Ge concentration of the SiGe layer
forming the photosensitive layer 17a to be small, the surface
recombination can be restrained. Thus, an SiN film can be
integrally formed on the surface of the photosensitive layer 17a as
the antireflection film 21. As the circuit-incorporating
photosensitive device 310' shown in FIG. 5G, a thermal oxide film
21', such as SiO.sub.2, may be formed on the surface of the
photosensitive layer 17a first, and then an SiN film may be formed
as the antireflection film 21. An amorphous carbon film which can
be formed at a low temperature may be formed as the antireflection
film 21.
[0082] Contact holes are provided in the oxide film 15 in the
region defining the photodiode 27, such that the contact holes
reach the surfaces of the polysilicon layer 16 and the N-type
diffusion layer 6, respectively. In each of the formed contact
holes, metal wiring 22d or 22e is provided to form an electrode. In
the region defining the bipolar transistor 28, contact holes which
reach one of the polysilicon layer 16, the polysilicon layer 19 on
the base layer 17b and the N-type diffusion layer 6 are formed. In
each of the formed contact holes, metal wiring 22a, 22b or 22c is
provided to form an electrode. Thus, the circuit-incorporating
photosensitive device 310 shown in FIG. 4 is completed.
[0083] The circuit-incorporating photosensitive device 310 includes
the photosensitive layer 17a, which is formed of an SiGe layer
having a high light absorptance serving as the photosensitive
region of the photodiode 27. Thus, the photosensitivity of the
photodiode 27 is significantly improved. Also, since the base layer
17b of the bipolar transistor 28 is formed of SiGe, the injection
efficiency of the carriers in the bipolar transistor 28 is
increased. Therefore, the current amplification hfe can be higher
and a faster operation is achieved.
[0084] Moreover, since the antireflection layer 21 is provided on
the photosensitive layer 17a of the photodiode 27, light impinging
upon the photosensitive layer 17a can be absorbed effectively,
which also improves the photosensitivity of the photodiode 27.
[0085] For the antireflection film 21 provided on the
photosensitive layer 17a, an amorphous carbon film which can be
grown at a low temperature of 100.degree. C. or less is
particularly preferable. The composition and the properties of SiGe
which forms the photosensitive layer 17a will be affected by a
high-temperature heat treatment. Therefore, if the amorphous carbon
film is grown at the temperature of 100.degree. C. or less, the
composition of SiGe which forms the photosensitive layer 17a will
not change.
[0086] Furthermore, to reduce the reflection of the light at the
surface of the photosensitive layer 17a, the thickness of the
photosensitive layer 17a and the silicon substrate 4 may be
adjusted to be an integer multiple of .lambda./4n (.lambda.: the
wavelength of the light, n: the index of the refraction), so that
the phase difference between the light impinging upon the
photosensitive layer 17a formed of SiGe and the light through the
photosensitive layer 17a reflecting from the bottom surface of the
silicon substrate 4 will be about .lambda./2. Thus, the
photosensitivity of the photodiode 27 can be further improved.
(Example 3)
[0087] FIG. 6 is a cross-sectional view of a circuit-incorporating
photosensitive device 320 according to Example 3 of the present
invention. In the circuit-incorporating photosensitive device 320,
a reflection film 23 which is formed of a high melting point metal
film is provided on the silicon substrate 1. The structure is
similar to that of the circuit-incorporating photosensitive device
310 except for the reflection film 23.
[0088] The light transmitted through the photosensitive layer 17a
is reflected from the reflection film 23 so as to return to the
photosensitive layer 17a. Thus, the photosensitive layer 17a can
obtain a level of photosensitivity which is equal to that of a
twice-as-thick photosensitive layer. Furthermore, a high melting
point metal film which forms the reflection film 23 can be
sputtered to the surface of either one of the silicon substrate 1
or the N-type silicon substrate 4 which are adhered together when
forming the SOI wafer 29.
[0089] When the proportion of Ge is increased so as to increase the
light absorption coefficient of the photosensitive layer 17a formed
of SiGe, the distortion of the Si layer will be greater and the
crystallinity will be lowered. Therefore, the photosensitive layer
17a may not be thick enough to sufficiently absorb light. However,
with the circuit-incorporating photosensitive device 320, the light
can be absorbed sufficiently with a thin photosensitive layer 17a
by providing the reflection film 23.
[0090] In the above-described Examples 1-3, the description is made
with respect to the photodiode 27 having a single photosensitive
layer 17a which is not segmented. However, as in the
circuit-incorporating photosensitive device 330 shown in FIG. 7,
the N-type silicon substrate 4 and the photosensitive layer 17a may
be separated into plural photosensitive regions by the trench-type
separation layers 5. In this case, a segmented photodiode,
preferably used as an optical pickup or a camera device, is
provided. By separating the photosensitive regions of a photodiode
27a into a plurality of regions by the trench-type separation
layers 5, the photodiode 27a becomes free from crosstalk and thus,
high resolution can be achieved. Since the photosensitive layer 17a
formed of SiGe may be formed by a selective epitaxial method, the
photosensitive layer 17a may be separated into plural regions when
the photosensitive layer 17a is formed thorough such selective
epitaxial growth.
[0091] As described above, in the circuit-incorporating
photosensitive device according to the present invention, a
photodiode having a photosensitive layer formed of SiGe and a
signal processing circuit are provided on an SOI wafer which has
lower power consumption. Thus, the photodiode can achieve higher
photosensitivity, and the signal processing circuit can have lower
gain. Accordingly, the response speed in the signal processing
circuit, the S/N ratio, etc., can be prevented from decreasing.
Furthermore, by providing a high-speed transistor having a portion
formed of SiGe on the same SOI wafer as a signal processing
circuit, the speed of the signal processing is increased, and a
photosensitive device with fast operation, high sensitivity and low
power consumption can be provided.
[0092] Various other modifications will be apparent to and can be
readily made by those skilled in the art without departing from the
scope and spirit of this invention.
[0093] Accordingly, it is not intended that the scope of the claims
appended hereto be limited to the description as set forth herein,
but rather that the claims be broadly construed.
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