Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1999-10-22
2001-07-24
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
Reexamination Certificate
active
06265241
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of forming a photo diode on a semiconductor wafer, and more particularly, to a method of forming 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 method of forming a new photo diode, 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 method of forming a photo diode on a semiconductor wafer comprising:
forming a pad silicon oxide layer on the substrate and a silicon nitride layer on the pad silicon oxide layer;
removing portion of the silicon nitride layer over which the remaining silicon nitride layer defines a photo sensor area and at least one dummy active area on the pad silicon oxide layer, each dummy active area being positioned surrounding the photo sensor area with a narrow slot in between which is not covered by the silicon nitride layer;
performing a thermal oxidation process over the slot to form a field oxide layer, the dummy active area being used to reduce the thickness of the field oxide layer in the slot;
removing the remaining silicon nitride layer by using a wet etching process; and
forming a P/N junction in the photo sensor area to form a photo diode.
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: 5585653 (1996-12-01), Nakashiba
patent: 5600173 (1997-02-01), Suzanga
Christianson Keith
Hsu Winston
United Microelectronics Corp.
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