Image Processing Reference
In-Depth Information
Exposure zone
Exposure zone runs
from top to bottom
with time.
(a)
(b)
(c)
FIGURE 5.81
Example of still picture of a rotary fan using electronic shutter mode: (a) still object image; (b) still image of
rotating fan captured by CMOS sensor; (c) shift of exposure zone with time.
Why can't MOS and CMOS sensors take still images using global shutter mode? In CCDs,
signal charges integrated in all pixels are transferred to the VCCD at the same time. This
means the VCCD can accept and store signal charges of the same number with pixels; that
is, it plays the role of frame memory. On the other hand, in the case of MOS sensors, when
signal charges are read out to vertical signal lines, each signal line can accept only one
signal charge packet independently at a time, because it is a metallic wiring. Thus, signal
lines can accept signal charge packets corresponding to only one row; it has a line memory
but not a frame memory as a whole sensor.
How about 4-Tr pixel configuration CMOS sensors? As they have a FD at each pixel, they
can play the role of frame memory by transferring a signal charge packet from each PD to
each FD in each pixel. But the FD is connected to metallic wiring by ohmic contact with a
high enough impurity concentration, and this fact results in higher dark current. Image
quality deterioration due to dark current generated during storage in FD is so severe that
images obtained this way cannot be practically used. Therefore, in spite of functioning as a
frame memory, the performance level is useless. On the other hand, approaches have been
proposed that use a similar structure
66
with a buried PD, MOS capacitor,
67
and stacked PIP
capacitor for high capacitance in a narrower area with low impurity concentration diffu-
sion
68
for low leakage current, in addition to a MOS capacitor forming out of the image area.
There is another problem for sensors in which the memory part is formed in each pixel.
This is the issue of noise due to mixture of charge or light to memory caused by high illu-
mination to neighboring pixels during the signal charge storage period. The mechanism
is similar to smear in CCD and MOS sensors. Since image quality debasement due to this
phenomenon is severe, efforts to decrease it to a practical use level are being made.
As an aside, the reason smear is not seen in CMOS sensors, unlike CCD and MOS sensors,
is not that charges or light do not mix into pixel drains, but because pixel output voltage
is decided by the SFA independent of mixture level, that is, unaffected by charge mixture.
5.5 Comparison of Situation and Prospects of Each Sensor Type
Figure 5.82 compares the three types of image sensors from the viewpoint of noise and
SNR.
