Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-06-11
2002-12-10
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S431000, C257S466000
Reexamination Certificate
active
06492703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photo diode on a semiconductor wafer, and more particularly, to a photo diode with dummy active areas.
2. Description of the Prior Art
The photo diode is a bi-electrode semiconductor device comprising a photo-conductivity cell and a junction diode and is typically used in the design and layout of optical products such as cameras and scanners. When not lit, the leakage current represents the noise but when lit the leakage current represents the signal. The photo diode manages signal information utilizing the signal-to-noise ratio. By increasing the intensity of the ratio of the signal to noise, the contrast of the signal and the quality of the photo diode are improved.
Please refer to FIG.
1
and FIG.
2
.
FIG. 1
is a schematic diagram of the structure of the prior art photo diode
10
.
FIG. 2
is a schematic diagram of the processing of the field oxide
16
on the photo diode
10
. The formation of the prior art photo diode
10
is performed on the substrate
10
of a semiconductor wafer. The photo diode
10
comprises a photo sensor area
12
positioned on a predetermined area of the substrate
14
, and a field oxide
16
positioned on the substrate
14
surrounding the photo sensor area
12
. The field oxide
16
acts as dielectric insulating material to prevent short circuiting secondary to contact between the photo sensor area and other units.
In the formation of the prior art photo diode
10
, a pad oxide layer
17
and a silicon nitride layer
18
are deposited in order on the substrate
14
and are used as a mask for the local oxidation of silicon to define an active area for the photo sensor area
12
.
When the semiconductor wafer undergoes thermal oxidation in a furnace, the field oxide layer
16
is generated only on the portion of substrate
14
not covered by the silicon nitride layer
18
due to the fact that moisture and oxygen do not penetrate the silicon nitride layer
18
and substrate
14
well.
However, moisture and oxygen can still reach the substrate
14
immediately surrounding the silicon nitride layer
18
by spreading horizontally to cause oxidation. This causes a bird's beak profile on the field oxide layer
16
, as shown in FIG.
2
and subsequent stress between the substrate
14
and the field oxide layer
16
. As the thickness of the field oxide layer
16
increases, so does the stress between the substrate
14
and the field oxide
16
. This leads to increase of leakage current in the PN junction of the photo sensor area
12
and noise of the photo diode
10
.
SUMMARY OF THE INVENTION
It is therefore a primary objective of the present invention to provide a new photo diode and its method of formation, which reduces the leakage current of the photo diode when unlit, decreases the intensity of the noise and improves the quality of the photo diode.
In a preferred embodiment, the present invention relates to a photo diode on a substrate of a semiconductor wafer comprising:
a photo sensor area positioned in a predetermined region on the substrate; and
a dielectric area positioned on the substrate surrounding the photo sensor area, the dielectric area comprising a field oxide layer covered on portion of the dielectric area and at least one dummy active area surrounding the photo sensor area which is not covered by the field oxide layer;
wherein a narrow slot of the field oxide layer exists between each dummy active area and the photo sensor area and the thickness of the field oxide layer in the slot is reduced by the dummy active area when forming the field oxide layer in the dielectric area.
It is an advantage of the present invention that stress between the substrate and the field oxide layer is reduced and the signal to noise ratio of the photo diode is improved.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.
REFERENCES:
patent: 5894059 (1999-04-01), Peidous et al.
patent: 05182960 (1993-07-01), None
Flynn Nathan
Hsu Winston
Quinto Kevin
United Microelectronics Corp.
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