Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Dopant introduction into semiconductor region
Reexamination Certificate
1998-12-08
2001-02-06
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Dopant introduction into semiconductor region
C438S022000, C438S443000, C438S450000, C438S923000, C438S981000
Reexamination Certificate
active
06184050
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to the fabrication of semiconductor integrated circuits (ICs), and more particularly to a method for forming a photodiode.
2. Description of the Related Art
A photodiode is a light-sensitive semiconductor device having a P-N junction that coverts light into an electrical signal (also known as a photo-detecting device). Due to the presence of an electric field at the P-N junction, electrons in the N-doped layer and holes in the P-doped layer cannot normally diffuse across the junction in the absence of light. However, when sufficient light falls on the P-N junction, electron-hole pairs are generated by energy from the light. These electrons and holes are able to diffuse towards the junction. Due to the presence of an electric field at the junction, electrons will separate out towards the N-side and holes will separate out towards the P-side of the junction and accumulate there. Therefore, a current is able to flow across the P-N junction. Ideally, a photodiode should remain in open-circuit condition in the dark until light is shone on the junction.
In general, photodiode devices are used as imaging sensors in different types of equipment, for example, PC cameras and digital cameras. One major defect of a conventional photodiode is its relatively large junction leakage current. Junction leakage current often leads to the build-up of a large dark current in products that employ a large number of imaging sensors. Furthermore, the large dark current is capable of producing abnormal bright spots on an imaging screen.
FIGS. 1A-1C
are schematic, cross-sectional views showing the process of forming a conventional photodiode.
In
FIG. 1A
, a substrate
100
having a pad oxide layer
102
thereon is provided. The pad oxide layer
102
is used as sacrificial layer to protect the substrate
100
form being destroyed during following steps. A patterned silicon nitride layer (Si
3
N
4
)
104
is formed on the pad oxide layer
102
. An ion implantation I
1
is performed to form a P-well
106
in the substrate
100
.
In
FIG. 1B
, the silicon nitride layer
104
is used as a mask in a local oxidation of silicon (LOCOS) operation. The LOCOS operation is carried out in an atmosphere comprising oxygen (O), such as water vapor or O
2
, to form a field oxide (FOX) layer
108
in the substrate
100
. In other words, an insulating layer is formed surrounding a device region.
Since water vapor and oxygen cannot penetrate the silicon nitride layer
104
easily, the field oxide layer
108
does not form in regions covered by the silicon nitride layer
104
. However, water vapor and oxygen still can diffuse horizontally into the substrate
100
at the edges of the silicon nitride layer
104
. Bird's beaks
110
are formed at the edge of the field oxide layer
108
.
In
FIG. 1C
, a wet etching is carried out to remove the silicon nitride layer
104
and the pad oxide layer
102
between the field oxide layer
108
. An N
+
-doped region
110
is formed in the P-well
106
by an ion implantation I
2
. The N
+
means that the doped region has a heavy N-type dopant. The N
+
-doped region
110
is annealed to drive the implanted N-type ions into the P-well
106
. A P-N junction forms between the N
+
-doped region
110
and the P-well
106
, so that a photodiode device is thus completed.
However, in the conventional photodiode device described above, bird's beaks form on each side of the field oxide layer of the device structure. Since stress at the bird's beaks is higher and more crystal defects occur from the stress at the bird's beaks than at other portion of the device structure, a large junction leakage current is generated there. A unit of imaging equipment that employs a large number of photodiodes thus has a large leakage current and produces a large dark current that results in abnormal bright spots on an imaging screen.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved method for forming a photodiode to prevent the leakage current, which forms at the junction of a conventional photodiode, from occurring. The defects of the conventional photodiode, such as a large dark current and abnormal bright spots are thus avoided.
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 method for forming a photodiode. A substrate having a well with a first electric type therein is provided. An insulating layer is formed on the substrate. The insulating layer is patterned to form an opening. The insulating layer still remains with a thin thickness below the bottom of the opening. A heavily doped region with a second electric type is formed in the well in the position below the opening. A junction is thus formed between the heavily doped region and the well.
REFERENCES:
patent: 4984047 (1991-01-01), Stevens
patent: 5679602 (1997-10-01), Lin et al.
patent: 5853626 (1998-12-01), Kato
patent: 353031978 (1978-03-01), None
patent: 356048168 (1981-05-01), None
patent: 362120018 (1987-06-01), None
patent: 401283944 (1989-11-01), None
Pham Long
United Microelectronics Corp.
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