Tech Briefs Magazine - September 2021 - PIT-8

Imaging Sensors
the circuitry out of the way, this new
BSI sensor has a fill factor of close to
100%.
Figure 1. BSI sensors improve fill factor by providing a direct route for light to reach the light-receiving
surface.
* Increased processing speed: The basic
speed of the pixel array is limited by
resistor-capacitor (RC) time constants,
and adding metal reduces the
resistance and increases the speed. In
FSI sensors, the amount of metal on
the sensor front is limited to allow
light to reach the photodiodes. This
constraint leads to overhead in the
processing speed. As frame rates
increase and resolutions decrease, the
camera cannot provide maximum
Gpx/sec throughput because of losses
to overhead. BSI sensors do not have
this constraint and can have significantly
increased metal circuitry, substantially
reducing or even eliminating
overhead. This capability allows a
BSI sensor to maintain its maximum
Gpx/sec throughput even at very
high frame rate/low resolution combinations.
Specialized
10
M FPS
1 M FPS
Custom High-Speed
300 FPS
Frame rate gap
BSI sensors have been available for
more than 10 years in a variety of cellphone
and standard digital cameras.
They've
offered
proven
advantages
Mainstream Camera Technology (COTS)
Production Volumes
Figure 2. Back side illuminated (BSI) technology applied to high-speed imaging starts to close the frame
rate performance gap between custom high-speed capability and specialized imaging capability.
which in turn affects the fill factor and
reduces the sensor's sensitivity.
BSI sensors are designed with a thick
carrier wafer attached to the top of the
metal stack. This arrangement allows
the bulk silicon to be thinned and
flipped to expose the diodes facing the
light source and the metal surface
behind them. There are two significant
advantages
to BSI
sensors
in highspeed:
improved fill factor, by providing
a direct route for light to reach the
light-receiving surface (see Figure 1),
and improved processing speed, by
8
Cov
ToC
adding more metal to the sensor's
metal surface.
* Improved fill factor: This effectiveness
at capturing incident light is
expressed in terms of the sensor's fill
factor - or the percentage of the
pixel surface area that is able to capture
photons. With its metal circuitry
blocking or reflecting some of the
light, a typical FSI sensor used in
high-speed imaging will have a fill factor
between 50 and 60%, partially
compensated for by a microlens in
typical current FSI sensors. By moving
when it comes to improving low light
performance and dynamic range of
these consumer-focused cameras. Why
did it take so long to bring these sensors
to high-speed imaging? In a word, size.
The sensors and pixels used in highspeed
cameras are much larger than
standard cameras to minimize speedresolution-sensitivity
trade-offs. For
instance, while a cellphone camera
may have a pixel that measures less
than 2 µm per side, pixels on this new
image sensor are typically more than 6
µm and as much as 28 µm per side.
The manufacturing process for BSI
inherently more difficult
sensors
is
than comparable
FSI
sensors and
requires additional manufacturing
steps. Among them is a wafer backthinning
step to remove the bulk silicon,
bringing the photodiodes closer to the
light source. There are also additional
processing steps on the back side of
the wafer to anneal the surface and
provide electrical contacts to the front
side. The size of high-speed image sensors
only exacerbates manufacturing
difficulties. The realities of semiconductor
economics also made it difficult
to transfer the technology from the
high production volumes of standard
cameras to the comparatively low volumes
of high-speed imaging sensors. It
Photonics & Imaging Technology, September 2021

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Tech Briefs Magazine - September 2021

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