Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1998-02-02
2003-02-04
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S312000
Reexamination Certificate
active
06515703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image pick-up apparatus provided with a solid-state image sensing device.
2. Description of the Related Art
Video cameras and electronic cameras are very popular today. Various types of apparatus are now commercially available whereas a great number of new types of apparatus are under development. CCDs are used as an image sensing device in most of these apparatus.
FIGS. 7
to
9
illustrate the structure of a conventional CCD and its reading operation, wherein
FIG. 7
illustrates the basic structure of the CCD and
FIGS. 8 and 9
illustrate waveforms of timing signals used to drive the CCD.
As shown in
FIG. 7
, the CCD includes: a photoelectric conversion part
20
for converting an optical image of an object to an electric signal (signal charge); a vertical transfer part
21
for transferring the charge of each pixel in a vertical direction; a horizontal transfer part
22
for transferring each charge obtained via the vertical transfer part
21
in a horizontal direction; and an output amplifier
23
for converting the transferred charge to a signal in the form of voltage and then outputting it. An image signal is read from the CCD in different two modes. In a field reading mode (referred to also as a field mode) signals of all pixels are acquired into the vertical transfer part, signals of two pixels adjacent in the vertical direction are added together, and the resultant signals are transferred in the vertical transfer part. On the other hand, in a frame reading mode (referred to also as a frame mode), signals of pixels on odd numbered lines and those on even numbered lines are transferred separately to the vertical transfer part.
FIG. 7
b
is a cross-sectional view of a pixel of the image sensing device.
FIG. 7
c
illustrates a potential profile, and
FIG. 7
d
is a top view of electrodes. As shown, a vertical overflow drain is employed as an anti-blooming mechanism. Image sensing devices of this type has a color filter including complementary colors arranged in a checkered pattern. During a vertical blanking period, a signal obtained at each pixel by means of photoelectric conversion is transferred to a transfer stage of a corresponding vertical transfer part. Then, signals of each pair of pixels on a vertical line are added and read out. In general, as shown in
FIG. 7
, the above reading operation is performed in a quasi interlace fashion in which odd field reading is performed by adding pairs of pixels that are shifted by one line from those in the even field reading.
FIG. 8
illustrates waveforms of four phase vertical driving pulses V
1
through V
4
used in a field mode operation. A reading pulse is superimposed on the vertical driving pulses V
1
and V
3
every vertical period during a blanking period so that signal charges of odd numbered lines and those of even numbered lines are transferred at the same time to the vertical transfer part. Then, the vertical driving pulses V
1
-V
4
are supplied at intervals of H (horizontal period) so that signals of the odd numbered lines and signals of the even numbered lines are added together in a predetermined manner and the resultant signals are transferred in the vertical direction. The manner of adding signals of the odd and even numbered lines is determined by the first pulse of the phase of the transfer pulses V
1
-V
4
during a vertical period and thus a shift by amount of one pixel occurs every vertical period thereby providing an interlace effect.
FIG. 9
illustrates waveforms of driving pulse V
1
-V
4
in a frame mode operation. In this mode, a reading pulse is added every vertical period. That is, only signals of odd numbered lines are transferred to the vertical transfer part and read out during a vertical period. Then, signals of even numbered lines are transferred to the vertical transfer part and read out during a subsequent vertical period.
In CCDs of the types used widely today, a VOD structure is employed. In this structure, unnecessary charges are swept away into deeper places of a silicon substrate thereby achieving a high sensitivity and a greater dynamic range.
In an image pick-up apparatus provided with a CCD of the above-described type, the quality of a picked-up image is sensitive to the dynamic range of the CCD. If the image sensing device (CCD) used has a narrow dynamic range, saturation occurs in the image signal when taking a picture of an object having high contrast. This results in a change in color of a portion of the image having high color saturation or results in a reduction in contrast of a portion of the image having high brightness.
In CCDs of the type widely used in video cameras, it is assumed that the reading operation is performed in the field reading mode. Therefore, the dynamic range is optimized for the field mode operation. More specifically, the maximum charge that can be stored in a stage of the vertical transfer part is set to twice the maximum charge that can be stored in a photoelectric conversion part.
If such a CCD or image sensing device is operated in a frame reading mode, one pixel of the photoelectric conversion part corresponds to one stage of the vertical transfer part and thus the dynamic range of the CCD output in the frame mode operation becomes smaller than that obtained in the field mode operation, although the vertical transfer part
21
has a high storage capacity. Since the maximum charge capacity of one pixel of the photoelectric conversion part is smaller than that of one stage of the vertical transfer part, saturation can occur in the photoelectric conversion part in the frame mode operation whereas no saturation occurs yet in the transfer part. This results in a lower saturation level in the frame mode than in the field mode.
In the CCDs of the widely-used type, as described above, since the dynamic range of one stage of the vertical transfer part is equal to the dynamic range of two combined pixels of the photoelectric conversion part, the dynamic range in the frame mode operation is as small as half that in the field mode operation.
In particular, in electronic cameras that operate in a frame mode to achieve high resolution and high picture quality, the above-described reduction in the saturation level causes serous degradation in the picture quality. To solve the above problem, image sensing devices have been developed that allow all pixels to be read at the same time.
In image sensing devices of this type, it is assumed that the reading operation is performed pixel by pixel and thus it is possible to avoid the problem arising from the above-described imbalance between the photoelectric conversion part and the transfer part. However, the image sensing device of this type is too expensive to be used in low-cost types of image pick-up apparatus.
Another known technique to solve the problem described above is to apply a lower voltage to the silicon substrate of an image sensing device during a frame mode operation than during a field mode operation thereby expanding the maximum charge storage capacity of the photoelectric conversion part.
FIG. 7
c
illustrates the potential distribution from the position just under the photoelectric conversion part of the image sensing device to a position in the silicon substrate. In
FIG. 7
c
, the solid line represents a potential distribution obtained when the substrate potential is fixed to Vsub
1
. In this case, the maximum storage capacity, and thus the dynamic range, of the photoelectric conversion part is determined by the depth of a potential well defined by the level
1
shown in the figure. The broken line represents a potential distribution obtained when the substrate potential is fixed to Vsub
2
. In this case, the maximum storage capacity is determined by the depth of a potential well defined by the level
2
. As can be-seen from the above discussion, the dynamic range of the image sensing device can be expanded by changing the substrate potential from Vsub
1
to Vsub
2
.
Thus, this property of the CCD is used to e
Kondo Kenichi
Suzuki Masao
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ho Tuan
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