Image Processing Reference
In-Depth Information
In each type of imager
Many noise sources
noise
noise
noise
noise
Limit the SNR
noise
noise
noise
noise
Originating from
Physical essentials
Technology levels available
Unavoidable
At the time
FIGURE 5.83
What should be taken into account when choosing which technologies to adopt.
hoped further progress will be made using stacked BSIs, advanced FSIs, and organic
sensors.
In CCDs, noise reduction pixel techniques were pursued to make good use of low-
noise performance due to complete charge transfer. It was a technological necessity
that CMOS sensors had low-noise pixels in the same way as CCDs, because they aim
for high SNR through high multiplication gain of signal charge quantity by pixel
amplifier.
While CCDs were a major player in the imaging field for about two decades, CMOS
sensors have made advances in low-noise performance. The present situation of CCDs is
that they can capture images with quality suitable for practical use at reasonable cost and
realize global shutter mode at no additional cost.
CCDs are of good birth and beauty without makeup: they use only basic cosmetology
such as device structures and process technologies. On the other hand, CMOS sensors
have progressed by using basic cosmetology, that is, the pixel technology of CCDs. This
means that high-performance CMOS sensors cannot be manufactured by a low-cost
vanilla CMOS logic process. It could be said that the beauty of CMOS sensors is further
evolving with use of cosmetic surgery, namely circuit technology.
Figure 5.83 summarizes what should be taken into account when technologies are cho-
sen and is not restricted to image sensors. Each type of image sensor has many noise ori-
gins that restrict the SNR. Some of them originate from unavoidable physical essentials,
such as kTC noise, and others come from the level of technology available at the time. The
most important thing to evaluate is which ones are essential, because noise is the main
factor that determines what level of performance can be reached, which dictates the value
of the image sensor.
In MOS and CMOS sensors, MOS and CMOS technology are available. MOS sensors have
become a leading competitor because of this. Both sensors can share in the bounty of the
progress of MOS/CMOS technologies, such as transistor size shrinkage, low power, and
low voltage. As the column-parallel ADC in CMOS sensors shows, a lot is expected from
on-chip functional circuits. An example is the one-chip camera
69
named ASIC VISION,
70
realized with a pioneering spirit in 1990.
