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SiLM quantum efficiency
Image sensor quantum efficiency depends on several design and
construction factors. The
SiLM
technology was designed to optimize all of these.
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Effective anti-reflectance coating
Silicon has a high index of refraction so it looks like a mirror to the
eye. In practice, uncoated
silicon reflects around 40% of the incoming light.
This can be reduced to less than 1% by the application of an
anti-reflectance coating but almost all CCD and CMOS imagers have a
thick
SiO
layer on top that makes AR coatings ineffective.
The SiLM photodiodes, however, are covered by only 50nm of
passivation/protection materials and nothing else, permitting deposition
of highly effective AR coatings.
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Very high fill factor
The construction of most sensors requires that much of the input surface
be used for scan circuitry and electrical conductors.
In small-pixel sensors, the insensitive area can exceed 60%.
Microlenses can help but these introduce directional effects that
complicate selection of optics.
In SiLM sensors, the fill factor is always essentially 100%.
Nothing is located on the illuminated surface except photodiodes.
This means that every incoming photon enters a sensitive area.
The SiLM reflector
Some of the photons entering a sensor pass right through the photodiodes
without being converted to charge.
The longer the wavelength, the less absorption occurs.
In both CCDs and CMOS sensors, the typical absorption thickness
is only a few microns.
As a result, much of the red and most of the near infrared
photons are not captured.
Since the SiLM sense wafer is constructed of two fused layers, a mirror
can be fabricated between the layers before they are fused.
This mirror reflects the photons that would normally pass through
the silicon back through for a second chance to be absorbed.
Better NIR response is the result.
The graph below shows the effect of adding the reflector to the
already high QE of the
SiLM
sensor.
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SiLM-R is a process in which a
dichroic
mirror is fabricated below the photodiodes to reflect the light
back through the silicon.
The reflectance band can be tuned for each application.
The curve shown uses a filter designed for flat response
in the visible range providing the equivalent if 12 µm of
silicon thickness.
Tuning for longer wavelengths can double the NIR response from
900 nm out.
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