CMOS sensor

Details CMOS sensor CMOS sensor

1998-12-23

2001-03-27

Mintel, William (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C292S294000, C292S258000, C292S258000, C438S066000

Reexamination Certificate

active

06207984

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure of a semiconductor device. More particularly, the present invention relates to a structure of complementary metal-oxide semiconductor (CMOS) sensor.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into an electrical signal that represents the intensity of the energy. The applications of the CCDs include monitors, transcription machines and cameras. Although CCDs have many strengths, CCDs also suffer from high costs and the limitation of the CCDs' volume. To overcome the weaknesses of CCDs and reduce costs and dimension, a CMOS photodiode device was developed. Because a CMOS photodiode device can be produced using conventional techniques, costs and the volume of the sensor can be reduced. The applications of CMOS photodiodes include PC cameras, digital cameras etc.
The photodiode is based on the theory that a P-N junction can convert light into an electrical signal. Before energy in the form of photons strikes the photodiode, there is an electric field in the P-N junction. The electrons in the N region do not diffuse forward to P region and the holes in the P region do not diffuse forward to N region. When enough light strikes the photodiode, the light creates a number of electron-hole pairs. The electrons and the holes diffuse forward to the P-N junction. While the electrons and the holes reach the P-N junction as a result of the effect of the inner electric field across the junction, the electrons flow to the N region and the holes flow to the P region. Thus a current is induced between the P-N junction electrodes. Ideally, a photodiode in the dark is open-circuit. In other words there is no current induced by light while the photodiode is in the dark.
FIG. 1A
is a circuit diagram of a CMOS sensor.
FIG. 1B
is a layout of the sensor cell
110
in the FIG.
1
A.
FIG. 1C
is a schematic, cross-sectional view of conventional CMOS sensor as taken along the I—I line in FIG.
1
B.
As shown in
FIG. 1A
, the sensor array used in the latest CMOS sensor is improved from a passive pixel sensor array to an active pixel sensor array. The CMOS having the active pixel sensor array cell includes at least three active transistors
104
,
106
,
108
and a photodiode
102
. The three active transistors are reset transistor
104
, sense transistor
106
and select transistor
108
. One of the source/drain regions of the transistor
104
is electrically coupled to the source voltage V
DD
. One of the source/drain regions of the transistor
106
is electrically coupled to the source voltage V
DD
. One of the source/drain regions of the transistor
108
is electrically coupled to the output. The sensor cell
110
comprises the transistor
104
and the photodiode
102
. The photodiode
102
can convert light into an electrical signal by using the P-N junction and the electrical signal is transferred to the transistor
104
.
As shown in
FIG. 1B
, the sensor cell
110
comprises the transistor
104
and the photodiode
102
. The transistor
104
comprises a gate structure
104
a,
a source/drain region
104
b
adjacent to the gate structure
104
a
in the substrate. The sensor region
102
a
of the photodiode
102
is adjacent to the source/drain region
118
in the substrate.
As shown in
FIG. 1C
, the method of manufacturing the sensor cell
110
comprises providing a substrate
100
having an isolation region
112
, an insulating layer
114
and a gate
104
a.
The insulating layer
114
can be a field oxide layer, for example. An ion implantation step is used to formed lightly doped drain (LDD) regions in portions of the substrate
100
exposed by the gate
104
a
and the isolation region
112
. A spacer
116
is formed on the sidewall of the gate
104
a.
An ion implantation step is used to form heavily doped regions in portions of the substrate
100
exposed by the gate
104
a,
the spacer
116
and the isolation region
112
. A source/drain region
104
b
is formed by a composition of the heavily doped region and the lightly doped drain region. A patterned photoresist (not shown) is formed over the substrate
100
to expose the region for the subsequently formed sensor region
102
a.
An implantation step with low energy and a high implanting dosage is performed to form a sensor region
102
a
across a portion of the source/drain region
118
and extending from the surface of the substrate
100
into the substrate
100
.
Since the bird's beak region
112
a
is present at the boundary between the sensor region
112
and the sensor region
102
a,
the stress of the interface between the isolation region
112
and sensor region
102
a
is large. Because of the large stress, many crystal defects are present at the boundary between the sensor region
112
and the sensor region
102
a.
Therefore, the crystal defects induce large junction leakage current and dark current of the sensor. Furthermore, spots of light easily occur in the display image.
In order to overcome the problems induced by the bird's beak
112
a,
another conventional method of manufacturing a CMOS sensor was developed.
FIG. 2A
is a layout of a sensor cell produced by another conventional method.
FIG. 2B
is a schematic, cross-sectional view of the conventional CMOS sensor referred to the II—II line in FIG.
2
A.
Referring to
FIG. 2A
together with
FIG. 2B
, a gate
204
a
of a reset transistor
204
is formed on a substrate
200
. A dummy shield layer
218
is formed on a isolation region
212
and covers the bird's beak region
212
a.
The gate
204
a
and the dummy shield layer
218
are formed in the same step. The region which is covered by the dummy shield layer
218
extends from the bird's beak region
212
a
extending 0.5 &mgr;m to the reset transistor
204
and to the isolation region
212
.
Because of the dummy shield layer
218
, the subsequently formed sensor region
202
a
and the bird's beak region
212
a
are staggered. Therefore, the junction leakage current is small. Since the dummy shield layer
218
extends about 0.5 &mgr;m form the bird's beak region
212
a
to the reset transistor
204
, the size of subsequently formed sensor region
202
a
is limited. Furthermore, the efficiency and the effect of the sensor are poor.
SUMMARY OF THE INVENTION
It is therefore an objective of the invention to provide a structure of a CMOS sensor. The invention can overcome the problem of junction leakage current caused by the crystal defect in the bird's beak region.
It is another an objective of the invention to provide a structure of a CMOS sensor. The invention can overcome the problems that the size of the sensor region is limited by the dummy shield layer covering the bird's beak region and the efficiency and the effect of the sensor are poor.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a structure of a CMOS sensor. The structure comprises of a substrate having a metal oxide semiconductor, wherein the metal oxide semiconductor has a source/drain region in the substrate and a gate on the substrate. A sensor is region adjacent to the source/drain region in the substrate, and a dummy shield layer is around the sensor region on the substrate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.


REFERENCES:
patent: 5712494 (1998-01-01), Akiyama et al.
patent: 5773859 (1998-06-01), Veno
patent: 5918137 (1999-06-01), Ng et al.
patent: 6051447 (2000-04-01), Lee et al.
patent: 405167412 (1993-07-01), None

Affiliated with

Also associated with

Rating

CMOS sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with CMOS sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and CMOS sensor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2504903