 Wafer Thinning Technology |
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In the current CMOS image sensor structure,
light receiving layers (photodiodes and
transistors) that collect light are formed on
the surface of a silicon substrate. While the
silicon substrate itself is about 600 to 800 µm
thick, in the Sony "Exmor R" CMOS image
sensors, it was necessary to make of the silicon
substrate, including the metal wiring layer,
have a thickness of about 8 µm to allow light
to be received through the back of the substrate.
However, this thinning can cause wafer surface
distortion and warping that cannot be seen by the
naked eye as well as micrometer level positional discrepancies and height differences. If we
were to see a 1 µm pixel as a person, then these
height differences would be like a bump the
height of a person on a skating rink that was
expected to be smooth. This meant that it would
not be possible to form the on-chip lenses and
color filters over 1 µm pixels. To resolve this
issue, Sony developed a new wafer thinning
technology to assure that such distortion and
warping does not occur.
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| Unique Sony Photodiode Structure |
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In image sensors, it is easy for electrons that are
unrelated to the incident light to be generated
from the silicon surface, and even a single
electron can become noise of a level that is
visible to the naked eye. To avoid this problem,
Sony developed a technology that prevents the
generation of electrons that occurs at surface
defects for this back-illuminated structure in
which light is received on the back surface of
the wafer.
Furthermore, since light is received on the
back surface, it is necessary to transport all
the photoelectrons that are generated at the back
surface to the circuits on the front surface of the
device. Sony also succeeded in developing a
unique photodiode with a structure that makes
this possible.
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| Future Developments |
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Back-illuminated CMOS image sensors not only
make high-sensitivity and low-noise imaging
possible, but they also make it possible to use
multi-layer metal wirings and arbitrary transistor
structures. This means that we can expect even
further improvements in speed and performance.
Sony is committed to taking advantage of the
superlative characteristics only available in
back-illuminated CMOS image sensors and
continuing our development efforts to create
even easier to use and even higher picture quality
image sensors.
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Figure 2 : Image Comparison (100 lx, F4.0, 1/30 s accumulation)
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See
all articles with figures and tables.  |
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Vol.59 |
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