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as offered by CMOSIS has greatly
improved. This primarily pertains to
the capacitive storage nodes need-
ed inside each pixel to hold the pixel
values for reading them out se-
quentially after the exposure stops.
Smaller storage nodes are now fea-
sible with reasonably small pixel sizes
and at lower price points. Ad-vanced
global-shutter CMOS sensor designs
feature pixels down to 5.5 x 5.5
µm² as available in the CMOSIS CMV
product family. The goal is having
3.6-µm pixels with a low noise global
shutter in the near future.
Of course, this scaled-down CMOS
layout requires fabs or foundries
with wafer-processing capabilities
that can accommodate these small
pixel dimensions. It also needs
comprehensive design know-how
to create the appropriate pixel
architecture and technology.
Eight-transistor Pixel
Architecture
Fitting a low-noise global shutter
to a CMOS image sensor requires
a complex pixel architecture. But
a specific new architecture is set
to overcome this obstacle. The
eight-transistor (8T) global-shutter
Figure 5: A rolling-shutter design (a) causes moving objects to be depicted with skewed lines, so fast-moving
objects to appear skewed. Also, a flashlight exposes only a part of a frame. This is not the case with a global-
shutter design (b)
detail.
● No image correction: All these
demands have to be implemented
without impacting the raw image
quality so that no extra off-chip cor-
rection is needed.
Global Shutter for CMOS
Sensors
Combining a much smaller pixel layout
with a global shutter is another major
progress that imagers from CMOSIS
have achieved in the last few years.
A global shutter exposes all pixels of a
sensor at the same time and over the
same duration. It is a more complex
concept - and it was ini-tially more
costly to implement in CMOS sensors
- because it requires some kind of
a local storage element (usually a
capacitor) inside each pixel, plus some
control function to start and stop the
exposure. All this enlarges pixel size.
A rolling shutter, in a marked
difference, will expose an image
sequen-tially, row by row, top to
bottom, at different moments in time,
much like the mechanical curtain
rolling down behind the lens in an
old-fashioned analog photo camera.
This rolling effect causes time-related
artifacts, which can skew the image
of fast-moving objects as the expo-
sure follows or deviates from their
horizontal or vertical position at any
given moment across the image plane
(Figure 5). Another rolling shut-ter
artifact occurs when illuminating the
scene with a short-burst flash. The
result is that only a few rows or parts
of the image are exposed, whereas
other areas remain dark.
The rolling shutter concept has been
the traditional method also in digital
imaging since it is much easier to
build a pixel architecture adapted to
the row-by-row exposure scheme.
Therefore CMOS sensors with four-
transistor pixel architecture usually
come with a rolling shutter. The lower
number of pixel transistors and state-
of-the art layout results in pixels with
excellent specs in regard of dynamic
range and dark current. CMOSIS has
developed high-end sensors for the M
camera model of Leica Camera based
on these rolling shutter pixels.
Providing a global shutter in a CMOS
sensor is more complicated since it
involves placing the storage capacity
inside each pixel. This takes up space
and leads to a larger pixel layout,
which is more expensive.
However, global-shutter technology
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