Method for reducing dark current in image sensor

Details Method for reducing dark current in image sensor Method for reducing dark current in image sensor

2000-12-21

2004-09-21

Guerrero, Maria (Department: 2822)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S048000, C438S070000, C438S066000, C438S237000

Reexamination Certificate

active

06794215

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a semiconductor device; more particularly, to a method for fabricating a CMOS image sensor having a plurality of unit pixels which are capable of reducing a dark current.
DESCRIPTION OF THE PRIOR ART
As is well known, an image sensor is an apparatus for sensing a light beam reflected from an object to generate an image data. Especially, an image sensor fabricated by using a complementary metal oxide semiconductor (CMOS) technology is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of unit pixels. Each of the unit pixels also includes a light sensing element and a plurality of transistors. The light sensing element such as a photodiode senses incident light beam to generate photoelectric charges corresponding to an amount of the incident light beam. The transistors perform switching operations to control a transfer of the photoelectric charges.
FIG. 1
is a cross-sectional view showing sequential steps of fabricating a conventional unit pixel contained in a CMOS image sensor.
Referring to
FIG. 1
, a P-type well
12
and a field oxide layer
13
are formed on a semiconductor substrate
11
, and a PN junction region
17
A and
17
B is formed in the semiconductor substrate
11
to thereby provide a photodiode
17
. Then, a floating junction region
18
A, to which photoelectric charges generated in the photodiode
17
is transferred, is formed within the semiconductor substrate
11
.
Then, a transfer transistor TX for transmitting the photodiode to the floating junction region
18
A and a reset transistor RX for resetting the floating junction region
18
A are formed on the semiconductor substrate
11
. A drive transistor DX for amplifying a voltage level corresponding to the transferred photoelectric charges and a select transistor SX for outputting amplified voltage level as the image data are formed on the P-type well
12
. At this time, the reset transistor RX and the drive transistor SX are commonly coupled to a common junction region
18
B, and an impurity junction region
19
of a lightly loped drain (DLL) structure is formed between the drive transistor DX and the select transistor SX. Also, spacers
20
are formed on sidewalls of each transistors TX, RX, DX and SX.
Then, pre-metal dielectric (PMD) layers
21
and
22
are formed on the transistors TX, RX, DX and SX, and interlayer insulating layers
23
,
24
and
25
are formed on the PMD layer
22
.
Then, the PMD layers
21
and
22
and the interlayer insulating layers
23
,
24
and
25
are selectively etched and first metal lines M
1
, M
2
and M
3
and a second metal line M
4
are formed thereon, respectively. The first metal lines M
1
, M
2
and m
3
and the second metal line M
4
used to connect the transistors TX, RX, DX and SX with an external elements are formed with staked layers of Ti/Al/TiN.
Then, a passivation layer formed with an oxide layer
29
and a nitride layer
30
is formed on the second metal line M
4
.
Then, a color filter array (CFA) operation is carried out to thereby form a color filter
31
, and a dyed photoresist
32
is formed on an entire structure. Then, a microlens
33
is formed above a portion where the color filter
31
is formed.
At this time, a number of surface energy states exist in a forbidden band due to a dangling bond of a lattice structure. The surface energy states result in a recombination of carriers. As a result, a leakage current is increased and a breakdown voltage of the image sensor may be influenced. That is, an undesired dark current is flowed so that a reliability of the image sensor is degraded.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method for fabricating a CMOS image sensor having a unit pixel which is capable of reducing a dark current.
In accordance with an aspect of the present invention, there is provided a method for fabricating a CMOS image sensor, wherein the CMOS image sensor includes a plurality of unit pixels, the method comprising the steps of: a) providing a semiconductor structure, wherein the semiconductor structure includes a photodiode and peripheral elements formed on a semiconductor substrate; b) forming an insulating layer on the semiconductor structure; c) forming a hydrogen containing dielectric layer on the insulting layer; d) diffusing hydrogen ions contained in the hydrogen containing dielectric layer into a surface of the photodiode, thereby removing a dangling bond; and e) removing the hydrogen containing dielectric layer.


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Huang et al. “Improvement in Dark Current Characteristics and Long-Term Stability of Mesa INGAAS/InP pin Photodiodes with Two-Step SiNx Surface Passivation”, Oct. 1991, IEEE, pp. 934-936.*
Korde et al. “One Gigasrad Passivatind Nitride Oxides for 100% Internal Quantum Efficiency Silicon Photodiodes”, Dec. 1993, IEEE, pp. 1655-1659.

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