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Traditional images sensors, both CCD and CMOS, are made by building
circuitry on one side of a silicon wafer.
This puts the sensing areas and the circuitry in the same plane
and requires that interconnecting conductors overlay part of the silicon
area. The result is that
only a fraction of the silicon surface can be used to detect light, and
effect called low fill factor.
Worse, as the pixels are made smaller, the fill factor drops even
more.
Techniques have been developed to improve the fill factor such as
microlenses and backside thinning but these have their own disadvantages
and, especially with CMOS, don’t provide much improvement.
The Lumiense
SiLM
technology avoids the fill factor limitation by positioning all of the
circuitry below the photo sensing areas, thus providing nearly 100% fill
factor at all pixel sizes without microlenses.
The SiLM technology confers many other benefits that can be seen
from the illustration.
The SiLM structure consists of two bonded wafers:
The Sense Wafer, consisting of:
-
A
photodiode (PD) layer in which the photodiodes are formed.
-
A
dichroic reflector to give the incoming light a second chance to be
absorbed.
-
A transistor (TR) layer, which contains the scan and readout
circuit.
The Mount Wafer, which:
As an added benefit,
the
SiLm technology provides
lower production costs than traditional backside thinned CMOS sensors
because only 180 nm fabrication processes are required.
For a full description
of the
SiLM technology, please
download the
pdf presentation
here.
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