Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-12-31
2004-06-29
Munson, Gene M. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S440000, C257S463000, C257S464000
Reexamination Certificate
active
06756618
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor; and, more particularly, to a CMOS image sensor with reduced interference and cross-talk phenomena occurring between closely located pixels by forming a differently structured photodiode in a pixel of a low power dissipation and high density CMOS image sensor and a method for fabricating the same.
DESCRIPTION OF RELATED ARTS
Image sensor is a semiconductor device converting an optical image into an electric signal. Particularly, a charge coupled device (CCD) is a device, wherein each metal-oxide-silicon (hereinafter referred as to MOS) capacitor is closely located and carriers are stored into the MOS capacitor and transferred. A complementary metal oxide semiconductor (hereinafter referred as to CMOS) image sensor employs CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits and adopts a switching mode sensing outputs sequentially. The MOS transistors are formed as the same number of existing pixels in the peripheral circuit.
There are several problems in using the CCD due to its complex driving mode, high power dissipation, a complex process having lots of steps for a mask process and a difficulty in one chip realization since the signal processing circuit cannot be constructed on a CCD chip. Therefore, there has been actively researched on the CMOS image sensor that uses sub-micron CMOS technology to overcome the above problems. The CMOS image sensor obtains an image by forming a photodiode and a MOS transistor in a unit pixel and then detecting signals sequentially through a switching mode. The use of the CMOS technology results in less power dissipation and an enabled one chip process for the signal processing circuit. Also, compared to the CCD process that requires approximately 30 to 40 masks, the CMOS image sensor implemented with the CMOS technology needs approximately 20 masks because of a simplified process. Hence, the CMOS image sensor is currently highlighted as a next generation image sensor.
FIG. 1
is a cross-sectional view showing a photodiode formed in each unit pixel of a CMOS image sensor and a doping profile of ion implantation regions of the photodiode in accordance with a prior art.
Typically, a color image sensor has a plurality of arrayed pixels for red, green and blue colors. Hereinafter, a pixel for red is expressed as a red pixel and the same is applied for the other two colors. A photodiode of each pixel in accordance with the prior art has the identical structure for all red, green and blue pixels. Any one of three color filters (not shown) is formed on a top portion of this photodiode, and thus, each pixel is able to sense any one light among red, green and blue lights.
Referring to
FIG. 1
, among the red, green and blue lights, the blue light has the shortest penetration depth while the red light has the longest penetration depth. The red light is able to penetrate into neighboring pixels, and this ability further induces noises. The more detailed explanation on this effect will be provided in the following.
With reference to
FIG. 1
, a structure of a photodiode in accordance with a prior art will be described in detail. A field oxide layer
11
defining an active area and a field area is formed on a p-type substrate
10
. Next, a p-type ion implantation region
12
is formed with a consistent depth from a surface of the p-type substrate
10
.
Beneath the p-type ion implantation region
12
, a first n-type ion implantation region
13
contacting to the p-type ion implantation region
12
is formed. Herein, the first n-type ion implantation region
13
has a high concentration and a consistent depth. A second n-type ion implantation region
14
contacting to the first n-type ion implantation region
13
with a consistent depth is formed beneath the first n-type ion implantation region
13
. Herein, the second n-type ion implantation region
14
has a low concentration.
Generally, the field oxide layer
11
has a thickness ranging from about 0.3 &mgr;m to about 0.8 &mgr;m. Also, the second n-type ion implantation region
14
has a thickness in a range between about 0.3 &mgr;m and about 0.8 &mgr;m.
The p-type ion implantation region
12
formed on a near surface of the p-type substrate
10
, the first n-type ion implantation region
13
formed below the p-type ion implantation region
12
, the second n-type ion implantation region
14
and the p-type substrate
10
constitute a pn junction, constructing a p
/p photodiode.
FIG. 1
provides another diagram showing a doping profile of the ion implantation regions measured in a logarithmic scale in accordance with the cross-sectional view of the photodiode illustrated in the left side of FIG.
1
. This doping profile includes doping concentrations of the p-type ion implantation region P
0
12
, the first n-type ion implantation region N+
13
, the second n-type ion implantation region N
14
and the p-type substrate P-sub
10
.
Also, this diagram shows a scale of a depletion region formed when a predetermined voltage is supplied to the pn junction having the above doping profile. It is indicated that the depletion region has a depth in several &mgr; ms by being formed deeply into a deep region of the p-type substrate
10
.
As well known, photodiode is a device that stores optical charges of light into the depletion region and uses the stored optical charges for generating an image as the photodiode supplies a predetermined voltage to the pn junction so that the depletion region is formed.
The photodiode constructed with a conventional structure has all identical depths for the red pixel, blue pixel and green pixel even though each color light has a different penetration depth. Therefore, since the depletion regions are formed even in deeper regions of the p-type substrate
10
of the blue pixel and the green pixel, red light penetrated into the neighboring red pixel induces light interference.
Furthermore, it is a current trend of increasing demands for developing a color image sensor that can be installed in a highly integrated and low power consuming mobile telecommunication terminal. However, this image sensor has a unit pixel of which size is decreased in about half of the conventional unit pixel. In case of applying the 0.18 &mgr;m technology, the unit pixel size is below about 4.0 &mgr;m×4.0 &mgr;m.
As the unit pixel size decreases, it is focused to solve such problems of a signal distortion with respect to the blue pixel and the green pixel caused by red light having a deep penetration depth and electric interference between neighboring pixels.
In case of using the 0.18 &mgr;m technology instead of using the generally used 0.5 &mgr;m or 0.35 &mgr;m technology, it is much difficult to isolate devices. Also, there is another difficulty when using the 0.18 &mgr;m technology as an allowable noise level decreases to about half of a conventional noise level.
Furthermore, in case of employing the 0.18 &mgr;m technology, the photodiode area decreases below about 70%, and a driving voltage also decreases below about 75% compared to the area acquired when using the 0.35 &mgr;m technology. Therefore, efficiency on optical charge generation is expected to be below 50% compared to the 0.35 &mgr;n technology.
In order to compensate the efficiency on optical charge generation, it is essential to increase ion implantation energy and ion implantation concentration so to increase generations of an electron-hole pair. However, this increase in the ion implantation energy conversely decreases an insulating distance between pn junctions of nearly located photodiodes. Hence, there occur electric noises between nearly located pixels due to a weakened insulating characteristic. For this reason, it is much emphasized to compensate the insulating characteristic.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a complementary metal-oxide semiconductor (CMOS) image sensor with decreased electric and optical no
Birch, Kolasch Stewart & Birch, LLP
Hynix / Semiconductor Inc.
Munson Gene M.
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