Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-09-22
2001-07-03
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S436000
Reexamination Certificate
active
06255681
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a complementary metal-oxide-semiconductor (CMOS) sensor structure and method of manufacture. More particularly, the present invention relates to a method of manufacturing a CMOS sensor having a silicon-on-insulator (SOI) substrate formed by a nitrogen implant.
2. Description of Related Art
A charge-coupled device (CCD) is a commonly used digital sensing device for extracting images. Applications of CCD are many and have been used in security systems, image recorders and video cameras. However, CCDs are generally expensive and difficult to miniaturize. Due to the current trend for miniaturization, low power consumption and low cost of production, a type of CMOS photo diode device has been recently developed. Employing the latest semiconductor manufacturing techniques to fabricate the CMOS photo diode device. cost of production is low. Moreover, the diode consumes very little energy and is very compact. At present, these CMOS photo diodes are used in PC cameras, digital cameras and some other photographic instruments.
A photo diode is a photosensitive semiconductor device (or light sensing device) that makes use of a P-N junction to convert photonic energy into electrical signals. Due to the presence of an electric field at the P-N junction, electrons in the N-type layer and holes in the P-type layer cannot diffuse across the junction in the absence of light. However, when light of sufficient energy is shone on the junction, molecules near the junction are be activated to generate electron-hole pairs. These electrons and holes then diffuse towards the junction.
In the presence of an internal electric field, the electrons move across the junction into the N-type region while the holes also move across the junction into the P-type region. Consequently, a current is able to flow through the P-N junction. Ideally, the photo diode is in an open-circuit state when surrounded by total darkness.
FIG. 1
is a cross-sectional view showing the structure of a conventional CMOS sensor. As shown in
FIG. 1
, the CMOS sensor has a P-type semiconductor substrate
100
, a field oxide layer
104
, a P-well
110
, a gate structure
120
, an N-type source/drain region
122
, an N-type sensor region
124
, a depletion region
126
and a borophosphosilicate glass
itride-silicide glass dielectric protective layer
134
.
Since the depletion region
126
is at the P-N junction, electron-hole pairs are generated when a beam of light
140
falls on this region. Hence, light energy is transformed to a current signal. However, the light beam
140
can activate the molecules in the depletion region
126
in a single pass so that its effective interaction length is rather short.
Consequently, only a very small current signal is generated leading to a low contrast ratio. In other words, sensitivity of the photo diode is small. Moreover, a portion of the light ray may penetrate through the depletion region
126
to reach the substrate region
100
. When the light ray is absorbed by the substrate
100
, undesirable current may be produced.
In light of the foregoing, there is a need to improve the CMOS sensor structure.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a CMOS sensor structure having a reflecting surface embedded in the substrate so that the effective interaction length of a light beam is increased.
Another aspect of this invention is to improve the contrast ratio for the CMOS sensor so that the sensor is more sensitive to light.
A third aspect of this invention is to reduce substrate leakage current produced by the CMOS sensor.
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 of manufacturing CMOS sensor. The method comprises the steps of forming a silicon nitride layer within a first conductive type substrate, and then forming a gate structure over the substrate.
Thereafter, a source/drain region of the second conductive type is formed in the substrate on one side of the gate structure, whereas a sensing region of the second conductive type is formed in the substrate on the other side of the gate structure. The sensing region is located above the silicon nitride layer. Finally, a dielectric protection layer is formed over the sensing region and the gate structure.
The sensor is fabricated using conventional semiconductor manufacturing techniques. A depletion region is formed at the junction between the sensing region and the substrate. Because the refractive index in the sensing region and the depletion region is larger than the silicon nitride layer, incident light coming from above is able to reflect portion of the light back at the silicon nitride/silicon substrate interface. Consequently, the effective interaction length for an incoming light ray is increased, and the probability of absorption by the substrate is reduced.
This invention also provides a CMOS sensor structure. The structure comprises a substrate of the first conductive type, a gate structure above the substrate, a source/drain region of the second conductive type and a sensing region of the second conductive type in the substrate, one on each side of the gate structure, a silicon nitride layer within the substrate below the sensing region, and a dielectric passivation layer over the gate structure and the sensing region.
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: 6100551 (2000-08-01), Lee et al.
patent: 6147373 (2000-11-01), Moon
Ngo Ngan V.
United Microelectronics Corp.
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