U.S. patent application number 12/007900 was filed with the patent office on 2008-08-28 for cmos image sensor and method for fabricating the same.
Invention is credited to Chang Hun Han.
Application Number | 20080203451 12/007900 |
Document ID | / |
Family ID | 36639441 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080203451 |
Kind Code |
A1 |
Han; Chang Hun |
August 28, 2008 |
CMOS image sensor and method for fabricating the same
Abstract
A CMOS image sensor and a method for fabricating the same are
provided, in which an N type region of a photodiode is prevented
from adjoining a device isolation film and a dark current is
reduced. The CMOS image sensor includes an interlayer dielectric
film formed between a gate poly and a power line, a contact formed
in the interlayer dielectric film, and an epitaxial layer connected
with the contact and formed only in a blue photodiode region.
Inventors: |
Han; Chang Hun;
(Icheon-city, KR) |
Correspondence
Address: |
MCKENNA LONG & ALDRIDGE LLP
1900 K STREET, NW
WASHINGTON
DC
20006
US
|
Family ID: |
36639441 |
Appl. No.: |
12/007900 |
Filed: |
January 16, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11319585 |
Dec 29, 2005 |
7344911 |
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12007900 |
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Current U.S.
Class: |
257/292 ;
257/E27.131; 257/E27.132; 257/E27.133 |
Current CPC
Class: |
H01L 27/14621 20130101;
H01L 27/14627 20130101; H01L 27/14687 20130101; H01L 27/14603
20130101; H01L 27/14609 20130101 |
Class at
Publication: |
257/292 ;
257/E27.133 |
International
Class: |
H01L 27/146 20060101
H01L027/146 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 30, 2004 |
KR |
10-2004-0116519 |
Claims
1. A CMOS image sensor comprising: a first conductive type
semiconductor substrate defined by a photodiode area and a
transistor area; a second conductive type impurity ion area formed
in the semiconductor substrate of the photodiode area; a gate
electrode formed on the semiconductor substrate of the transistor
area; an insulating film formed on an entire surface of the
semiconductor substrate including the gate electrode and excluding
the second conductive type impurity ion area; and a silicon
epitaxial layer formed on the second conductive type impurity ion
area and doped with first conductive type impurity ions.
2. The CMOS image sensor of claim 1, wherein the silicon epitaxial
layer is formed on the second conductive type impurity ion area
using a selective epitaxial growth process.
3. The CMOS image sensor of claim 1, wherein the gate electrode is
isolated from the silicon epitaxial layer by the insulating film
formed at sidewalls of the gate electrode.
4. The CMOS image sensor of claim 1, further comprising spacers
formed at sidewalls of the gate electrode.
5. The CMOS image sensor of claim 4, wherein the spacers formed at
the sidewalls of the gate electrode are adjacent to the silicon
epitaxial layer and are formed on the silicon epitaxial layer at
the sidewalls of the gate electrode.
6-9. (canceled)
Description
[0001] This application claims the benefit of Korean Patent
Application No. 10-2004-0116519, filed on Dec. 30, 2004, which is
hereby incorporated by reference for all purposes as if fully set
forth herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to complementary
metal-oxide-semiconductor (CMOS) image sensors, and more
particularly, to a CMOS image sensor and a method for fabricating
the same, which improves color reproduction by preventing an N type
region of a photodiode from adjoining a device isolation film. The
CMOS image sensor and the method for fabricating the same also
reduces dark current.
[0004] 2. Discussion of the Related Art
[0005] An image sensor is a semiconductor device that converts
optical images to electrical signals. The image sensor is
classified into a charge-coupled device (CCD) and a CMOS image
sensor. The CCD stores charge carriers in MOS capacitors and
transfers the charge carriers to the MOS capacitors. The MOS
capacitors are approximate to one another. The CMOS image sensor
employs a switching mode that sequentially detects outputs of unit
pixels using MOS transistors by forming the MOS transistors to
correspond to the number of the unit pixels using CMOS technology
that uses a control circuit and a signal processing circuit as
peripheral circuits.
[0006] The CCD has drawbacks in that its driving mode is
complicated, power consumption is high, and process steps are
complicated due to many mask process steps. Also, it is difficult
to integrate signal processing circuits in one chip of the CCD. To
solve such drawbacks, a CMOS image sensor based on a method for
manufacturing a sub-micron CMOS has been developed.
[0007] To display images, the CMOS image sensor sequentially
detects signals in a switching mode by forming a photodiode and a
MOS transistor in a unit pixel. Also, since the CMOS image sensor
uses the CMOS technology, low power consumption is required.
Furthermore, the number of masks required is less than the number
of masks required for the CCD. For example, the number of masks for
a CMOS image sensor is fewer by twenty than the thirty to forty
masks required for the CCD. In this way, in the CMOS image sensor,
process steps are simplified and various signal processing circuits
can be integrated in one chip. Therefore, the CMOS image sensor has
received much attention as an image sensor for next generation. The
CMOS image sensor is widely used in digital still cameras, PC
cameras, mobile cameras, etc.
[0008] FIG. 1 is a circuit diagram illustrating a unit pixel of a
related art CMOS image sensor. As shown in FIG. 1, the unit pixel
of the CMOS image sensor includes a photodiode (PD) 10 and four
NMOS transistors. A ground GND is shown. The four NMOS transistors
are comprised of a transfer gate (Tx) 30 transferring optical
charges collected in the photodiode 10 to a floating diffusion (FD)
region 20, a reset gate (Rx) 40 resetting the floating diffusion
region 20 by setting the potential of the floating diffusion region
20 at a desired value and discharging the charges, a drive gate
(Dx) 50 serving as a source follower buffer amplifier, and a
selection gate (Sx) 60 serving as an addresser. A load transistor
V.sub.b is formed to allow an output signal to be read outside the
unit pixel. Power (V.sub.DD) is applied.
[0009] FIG. 2 is a sectional view illustrating a layout of the
related art CMOS image sensor. As shown in FIG. 2, a pnp type
photodiode is provided with a p type epitaxial layer grown on a p+
type substrate. A device isolation film Fox is also provided. An n-
type region 70 is formed in the p type epitaxial layer, and a
P.sup.0 type region 80 is formed on the n- type region 70 and a
surface of the p type epitaxial layer.
[0010] A reverse bias is applied between the n- type region 70 and
the p type region (P.sup.0 type region 80 and p type epitaxial
layer) so that the n- type region 70 is fully depleted when an
impurity ion of the n- type region 70 is properly mixed with an
impurity ion of the p type region. Thus, the depletion region is
enlarged to the p type epitaxial layer below the n- type region 70
and the P.sup.0 type region 80 on the n- type region 70. The
depletion region is more abundantly enlarged to the p type
epitaxial layer having a relatively low dopant concentration. The
depletion region is used for image reproduction because optical
charges generated by incident light can be accumulated and stored
in the depletion region.
[0011] FIG. 3 illustrates absorption coefficient and penetration
depth depending on the wavelength of incident light for the related
art photodiode. As shown in FIG. 3, in accordance with the increase
of wavelength of the incident light to the photodiode, the
absorption coefficient decreases constantly while the penetration
depth increases constantly. In a related art pixel structure, a
blue wavelength has a penetration depth of 0.3 .mu.m and causes a
difficulty in the color reproduction of images. By contrast, a red
light has a penetration depth of 110 .mu.m. Thus, it is difficult
to reproduce respective colors at a ratio of 1:1 in color
reproduction using red, green, and blue colors. The failure in
obtaining an ideal ratio of 1:1:1 of the respective colors in color
reproduction reduces color reproduction.
[0012] FIG. 4 illustrates a penetration depth in a related art
silicon substrate depending on the wavelength of incident light to
the silicon substrate. As shown in FIG. 4, the penetration depth is
marked by percentage. A main penetration region of a red light
having a wavelength of 700 nm has a penetration depth of 4000 .ANG.
to 15000 .ANG.. By contrast, main penetration regions of a blue
light and a green light have a penetration depth less than 4000
.ANG..
SUMMARY OF THE INVENTION
[0013] Accordingly, the present invention is directed to a CMOS
image sensor and method for fabricating the same that substantially
obviates one or more problems due to limitations and disadvantages
of the related art.
[0014] An advantage of the present invention is to provide a CMOS
image sensor and method for fabricating the same, in which an N
type region of a photodiode is prevented from adjoining a device
isolation film.
[0015] Another advantage of the present invention is to provide a
CMOS image sensor and method for fabricating the same, in which a
dark current is reduced.
[0016] Additional features and advantages of the invention will be
set forth in the description which follows, and in part will be
apparent from the description, or may be learned by practice of the
invention. The objectives and other advantages of the invention
will be realized and attained by the structure and method
particularly pointed out in the written description and claims
hereof as well as the appended drawings.
[0017] To achieve these and other advantages in accordance with the
purpose of the present invention, as embodied and broadly
described, there is provided a CMOS image sensor comprising an
interlayer dielectric film formed between a gate poly and a power
line; a contact formed in the interlayer dielectric film; and an
epitaxial layer connected with the contact and formed only in a
blue photodiode region.
[0018] In another aspect of the present invention, there is
provided a method for manufacturing a CMOS image sensor. The method
comprising forming an interlayer dielectric film between a gate
poly and a power line; forming a contact in the interlayer
dielectric film; forming an epitaxial layer connected with the
contact; and allowing the epitaxial layer to remain in a blue
photodiode region having a low wavelength and removing the
epitaxial layer in a region other than the blue photodiode
region.
[0019] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiment(s)
of the invention and together with the description serve to explain
the principles of the invention. In the drawings:
[0021] FIG. 1 is a circuit diagram illustrating a unit pixel of a
related art CMOS image sensor;
[0022] FIG. 2 is a sectional view illustrating a layout of a
related art CMOS image sensor;
[0023] FIG. 3 illustrates absorption coefficient and penetration
depth depending on the wavelength of incident light for a related
art photodiode;
[0024] FIG. 4 illustrates a penetration depth in a related art
silicon substrate depending on the wavelength of incident light to
the silicon substrate; and
[0025] FIG. 5 is a cross-sectional view of a CMOS image sensor
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Reference will now be made in detail to embodiments of the
present invention, examples of which are illustrated in the
accompanying drawings. Wherever possible, like reference
designations will be used throughout the drawings to refer to the
same or similar parts.
[0027] As shown in FIG. 5, illustrating a CMOS image sensor
according to the present invention, a photodiode 100 is formed, and
a power line layer 110 is formed on the photodiode 100. A gate poly
120 is connected between the power line layer 110 and a first metal
layer 140. An interlayer dielectric film 130 is formed between the
photodiode 100 and the first metal layer 140. The first metal layer
140 and a second metal layer 160 are sequentially deposited on the
interlayer dielectric film 130. A first inter-metal dielectric
layer 150 insulates the first metal layer 140 from the second metal
layer 160. A second inter-metal dielectric layer 170 is formed on
the second metal layer 160. The second inter-metal dielectric layer
170 insulates the second metal layer 160 from a nitride layer 180
deposited on the second inter-metal dielectric layer 170. A color
filter array 190 is deposited on the nitride layer 180. A
planarization layer 200 is deposited on the color filter array 190.
As an example, a blue color filter array is used as the color
filter array 190 because the photodiode is described based on a
blue photodiode region. Further, a microlens 210 is deposited on
the planarization layer 200.
[0028] A method for fabricating the CMOS image sensor of the
present invention will now be described.
[0029] First, the interlayer dielectric film 130 is formed between
the gate poly 120 and the power line layer 110.
[0030] A contact 230 connected with an N type epitaxial layer 220
is formed, and the N type epitaxial layer 220 is formed to have a
thickness of 300 .ANG. to 500 .ANG.. As an example, the N type
epitaxial layer 220 has a thickness of 400 .ANG..
[0031] The epitaxial layer 220 remains in the blue photodiode
region corresponding to a low wavelength region. The epitaxial
layer 220 is removed in a region other than the blue photodiode
region. That is, the blue photodiode region is formed in a range
corresponding to a width of the color filter array 190. Therefore,
the epitaxial layer 220 is made to have the same width as the color
filter array 190 and is removed in a region other than the width of
the color filter array 190, that is where the color filter array
190 does not exist. The area outside of the width of the color
filter array 190 does not correspond to the blue photodiode region.
Thus, the epitaxial layer 220 remains in the blue photodiode
region. Unlike the related art photodiode that uses a silicon
substrate as a sensitive layer of a blue light having a low
wavelength, in the present invention, photosensitivity is improved
by using the epitaxial layer 220 as the sensitive layer of the blue
light.
[0032] The CMOS image sensor manufactured as above is constructed
as follows. The interlayer dielectric film 130 is formed between
the gate poly 120 and the power line 110. The contact 230 is formed
in the interlayer dielectric film 130 and connected with the
epitaxial layer 220. Also, the epitaxial layer 220 remains only in
the blue photodiode region.
[0033] As described above, in the present invention, when an N type
region of the photodiode is formed, a mask oxide is used to prevent
the N type region of the photodiode from adjoining a device
isolation film, thereby reducing a dark current. Also, to heavily
dope the device, ions are implanted into a device isolation film
and the N type region of the photodiode at a specific angle to
further reduce the dark current, thereby improving characteristics
of the CMOS image sensor.
[0034] Unlike the related art photodiode, in the present invention,
the N type epitaxial layer, not a silicon substrate, is used as the
sensitive layer of the blue light having the low wavelength to
improve photosensitivity. The photodiode region is enlarged because
a portion above the device isolation film is used as the sensitive
area of the blue light. Also, since the N type epitaxial layer is
formed to be higher than the related art silicon substrate, the
depth of a focus becomes low to effectively collect the light.
[0035] As described above, the CMOS image sensor and the method of
fabricating the same improves color reproduction. The CMOS image
sensor and the method of fabricating the same has an improved
resolution by improving reproduction of the blue light having the
low wavelength.
[0036] It will be apparent to those skilled in the art that various
modifications and variation can be made in the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *