Color image sensor and method for fabricating the same

Details Color image sensor and method for fabricating the same Color image sensor and method for fabricating the same

2002-03-07

2003-10-14

Thompson, Craig (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

Reexamination Certificate

active

06632702

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an image sensor; and, more particularly, to a color image sensor and a method for fabricating the same.
DESCRIPTION OF THE PRIOR ART
Generally, an image sensor, e.g., a CCD (charge coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor is employed to scan and convert an optical image into electrical signals.
Referring to
FIG. 1
, there is shown a pixel unit of a conventional image sensor including a photodiode (PD) and four NMOS transistors.
The four NMOS transistors include a transfer transistor
12
, a reset transistor
14
, a drive transistor
16
and a select transistor
18
. The transfer transistor
12
transfers photoelectric charges generated in the photodiode to a sensing node. The reset transistor
14
resets the sensing node in order to sense a next signal. The drive transistor
16
acts as a source follower. The select transistor
18
outputs data to an output terminal OUT in response to an address signal. A load transistor
20
, which is positioned between the output terminal OUT of the pixel unit and the ground voltage level, receives a biasing signal from an external device for the sake of biasing the pixel unit. A capacitance of a floating diffusion is referred to as “Cfd”.
Referring to
FIG. 2
, there is shown the photodiode of the pixel unit in FIG.
1
. The photodiode includes a P
+
silicon substrate
21
, a P-epi (epitaxial) layer
22
, field oxide layers
23
, an N
−
diffusion region
24
and a P
0
diffusion region
25
. The photodiode (PD) has a PNP junction structure in which the P-epi
22
, the N
−
diffusion region
24
and the P
0
diffusion region
25
are stacked.
Referring to
FIG. 3
, a conventional color image sensor includes pixel units and each pixel unit includes a photodiode. Photodiodes
102
a
,
102
b
and
102
c
are formed in the color image sensor. Non-photosensing regions
103
are formed between photodiodes
102
a
,
102
b
and
102
c
. An interlayer insulating layer
105
a
is formed on the photodiodes
102
a
,
102
b
and
102
c
and the non-photosensing regions
103
. Light shield layers
104
is formed on the interlayer insulating layer
105
a
to shield light incident on the non-photosensing regions
103
. An interlayer insulating layer
105
b
is formed on the light shield layers
104
. Red, green and blue filters
106
a
,
106
b
and
106
c
are formed on the interlayer insulating layer
105
b
. A buffer layer
107
is formed on the color filters
106
a
,
106
b
and
106
c
to improve the planarization of upper portions of the color filters
106
a
,
106
b
and
106
c
and the transitivity of light. Micro lenses
108
a
,
108
b
and
108
c
are formed on the buffer layer
107
.
The photodiode
102
a
receives light from an object and integrates photoelectric charges from the light via the micro lens
108
a
and the red filter
106
a
. The photodiode
102
b
receives the light of the object and integrates the photoelectric charges from the light via the micro lens
108
b
and the green filter
106
b
. The photodiode
102
c
receives the light of the object and integrates the photoelectric charges from the light through the micro lens
108
c
and the blue filter
106
c
. The pixel units consist of red, green and blue pixel units to output color signals of red, green and blue components of the light from the object. Conventionally, the red, green and blue pixel units are formed by the same fabrication process, respectively, so that the photodiodes
102
a
,
102
b
and
102
c
of the red, green and blue pixel units have substantially the same depletion region structure each other. Accordingly, there is a problem that the conventional color image sensor having photodiodes of the same depletion region structure may not improve light sensitivity since it is variable according to wavelengths of the red, green and blue components of the light.
Referring to
FIG. 4
, a graph depicts the absorption length of a light wavelength in Si, GaAs and Ge mediums when light is incident on a medium. The light wavelength of 770 nm is absorbed up to the absorption length of 7 &mgr;m in the Si medium while the light wavelength of 390 nm is absorbed up to the absorption length of 0.1 &mgr;m in the Si medium. Therefore, the light sensitivity of a photodiode depends on a kind of medium and wavelengths of red, green and blue components of light.
Referring to
FIG. 5
, a graph depicts light sensing characteristics of the conventional color image sensor in FIG.
3
. The photodiode of the color image sensor receives light from an object. The light includes red, green and blue components, which have a different wavelength from each other. The red component has the longest wavelength among the red, green and blue components. The green component has a wavelength longer than the blue component. Since the blue component has a wavelength shorter than the red or green component, a part of the blue component is lost before the blue component reaches the depletion region. Accordingly, the photodiode have low light sensitivity of the blue component. Also, when a red pixel unit reaches a saturation state, a blue pixel unit does not reach the saturation state. Furthermore, when an image process is performed on the basis of color signals from the red pixel unit, color signals from the green and blue pixel units can be lost, so that colors represented by the conventional image sensor are limited and a dynamic range for the sake of processing data is reduced. In other words, color signals from the red, green and blue pixel units may display different colors from actual colors of an object because the red, green and blue pixel units have different light sensing characteristics from each other.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a color image sensor in which a plurality of photodiodes have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.
It is another object of the present invention to provide a method for fabricating a color image sensor in which a plurality of photodiodes have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.
In accordance with an aspect of the present invention, there is provided a color image sensor for scanning and converting an optical image into electrical signals, comprising: a red photodiode including a P-type semiconductor layer and a first N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a first depletion region positioned beneath the surface of the P-type semiconductor layer; a green photodiode including the P-type semiconductor layer and a second N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a second depletion region positioned beneath the surface of the P-type semiconductor layer, wherein the second depletion region is more adjacent to the surface of the P-type semiconductor layer than the first depletion region; and a blue photodiode including the P-type semiconductor layer and a third N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a third depletion region positioned beneath the surface of the P-type semiconductor layer, wherein the third depletion region is more adjacent to the surface of the P-type semiconductor layer than the second depletion region.
In accordance with another aspect of the present invention, there is provided a method for fabricating a color image sensor for scanning and converting an optical image into electrical signals, comprising the steps of: (a) forming a P-type semiconductor layer on a substrate; (b) forming field oxide layers on the P-type semiconductor layer to define regions for red, green and blue photodiodes; (c) providing an ion implantation mask having different mask patterns for the red, the green and the bl

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