Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-03-30
2003-02-18
Mai, Son L. (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C438S048000
Reexamination Certificate
active
06521925
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-091726, filed Mar. 31, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a solid-state image sensor, and more particularly to a solid-state image sensor with an improved read transistor portion for reading signal charges from a photoelectric conversion section, such as a photodiode.
In the field of solid-state image sensors, various techniques for amplifying MOS image sensors with an amplifying function in their pixels have been proposed. MOS image sensors of this type have been expected to be suitable for the reduction of the pixel size following an increase in the number of pixels or a reduction in the chip size. In addition, the MOS image sensors have the advantages that they consume less electric power and make it easier to integrate the sensor section with its peripheral circuit by the same CMOS process than CCD image sensors.
A MOS image sensor is composed of unit cells, each unit cell constituting one pixel, arranged two-dimensionally. A unit cell is composed of a photoelectric conversion element and a transistor. The signal charges generated by photoelectric conversion at the photoelectric conversion element modulate the potential at a signal storage section (a photodiode acting as a photoelectric conversion element generally also serves as a signal storage section). The amplifying transistor in a pixel is modulated according to the potential, thereby providing the inside of the pixel with an amplifying function.
One of the most important evaluation items concerning this type of device is a brightly shining (or a whitely shining monochrome) point (white defect) caused by an extremely high output among the pixels activated during a dark period. One of the causes of a white defect is leakage current from the photoelectric conversion section. To decrease the leakage current, it is necessary to keep a photodiode (PD) acting as a photoelectric conversion element away from the surface of the semiconductor substrate where many causes of leakage current exist. That is, it is necessary to form a PD in a place deep down from the substrate surface.
With a PD formed in a place deep down in the substrate, however, even when a maximum voltage of 3.3V applied to a device using CMOS transistors is applied to a read gate electrode, the potential below the gate does not rise sufficiently because there is a limit to the extent of the depletion layer. As a result, the electric charges have been read partially or not been read at all. The left-over charges or the processing thereof permit another noise to occur in the activated pixels.
To overcome the problem, a method of providing a second signal storage section under the gate electrode in the semiconductor substrate has been proposed (refer to Jpn. Pat. Appln. KOKAI Publication No. 11-274457). Use of the second signal storage section makes the effective gate length shorter and therefore can involve a short-channel effect.
Conventional CCD image sensors employ n-type substrates, whereas MOS image sensors use p/p
+
substrates obtained by growing an epitaxial layer at a low B concentration of, for example, 1×10
14
cm
−3
to a thickness of about 5 to 10 &mgr;m on the surface of the substrate with a very high B concentration of, for example, 1 to 3×10
18
cm
−3
. The reason why conventional CCD image sensors employ n-type substrates is to prevent blooming and color cross talks from taking place by allowing those of the carriers generated by photoelectric conversion not gathered in the PD, particularly those generated deep in the substrate or those leaking from the PD due to strong incident light to be easily discarded to the substrate side, although they tend to leak into adjacent pixels by diffusion. Discarding carriers generated by photoelectric conversion, however, leads to a decrease in the sensitivity.
To solve the sensitivity decrease problem, CCD image sensors use a method of applying a higher read voltage (e.g., 5V) to widen the depletion layer and gathering carriers from a wider region. MOS image sensors are characterized by operating on a lower voltage than CCD image sensors. Because of the lower voltage driving, the depletion layer under the gate electrode does not get wider than that in CCD image sensors. Therefore, an improvement in the sensitivity by this method cannot be expected in MOS image sensors.
With this backdrop, MOS image sensors employ the aforementioned p/p
+
substrate and gather generated carriers in the PD to increase the sensitivity without discarding them to the substrate side.
FIG. 1
shows an impurity concentration distribution in the direction of depth in the PD of a MOS image sensor.
FIG. 2
shows a potential distribution in the direction of depth. As shown in
FIG. 1
, such a profile as has the lowest B concentration at a specific depth (about 3 &mgr;m) of the substrate is used. Use of the profile enables carries generated at a place deeper than the PD portion to bounce back toward the surface of the substrate by a low potential at a position deeper than the place with the lowest B concentration, even when the carriers attempt to diffuse more deeply. As a result, because part of the electrons rebounded gather in the PD by diffusion or the like, an improvement in the sensitivity can be expected as compared with an equivalent formed on an ordinary p-type Si substrate. Moreover, by increasing the concentration on the substrate side and shortening the lifetime of carriers, carriers generated at still deeper places can be prevented from leaking into adjacent pixels by diffusion.
In the impurity profile of the PD in an amplifying solid-state image sensor using a p/p
+
substrate, the B concentration is high even at the surface of the substrate to provide a surface shield layer or the like and the B concentration is the lowest at a depth deeper than the depth at which the P (phosphorus) concentration of the PD peaks. Specifically, in this profile, even if electrons generated in the vicinity of the photodiode tend to flow toward a deeper place in the substrate, they are caused to bounce back to the surface side of the substrate at the aforesaid high B concentration place and diffuse sidewise in the substrate at the minimum point of B concentration. The flow of electrons is the cause of color cross talks. In any case, the diffusion of electrons rebounded at the high B concentration place might improve the sensitivity or cause color cross talks. Thus, the technical problem of MOS image sensors is to realize a PD structure capable of gathering carriers in the PD more efficiently.
In addition, MOS image sensors also have a noise feedback problem. At the impurity concentration in a conventional PD, even when the PD was operated on 3.3V, the signal charge stored in the PD could not be read out completely. Because of this, the capacitance C at the PD portion caused kTC noise. If noise charge is Q, the square mean of the noise charge is expressed as:
Q
2
=kTC.
As described above, in a MOS image sensor, the photoelectric conversion section must be kept away from the surface of the semiconductor substrate where many causes of leak current exist, that is, the photoelectric conversion section must be formed at a deep place from the substrate surface. In this case, low voltage driving at about 3.3V puts a limit on the extent of the depletion layer, which permits some of the signal charges to be left over or completely prevents the signal charges from being read.
In addition, although providing a second signal storage section under the gate has been proposed, this method might cause a short-channel effect.
In a MOS image sensor formed on a p/p
+
substrate, use of low-voltage driving prevents the depletion layer from getting wider in the PD. As a result, an improvement in the sensitivity cannot be expected using a similar means to that in a CCD
Ihara Hisanori
Ishiwata Hiroaki
Mori Akiko
Nozaki Hidetoshi
Yamaguchi Tetsuya
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Mai Son L.
Nguyen Dao H.
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