CSEM Scientific and Technical Report 2008

CMOS Color Sensing
M. Schnieper, A. Luu-Dinh, P. Schueepp
The proposal is to replace dye based pixel filters in color image sensors by replicated Zero Order Devices arranged in a Bayer pattern. These novel
color filters are manufactured by wafer scale UV-replication. The manufacturing process is greatly simplified as only one single replication step and
one thin film evaporation are required to manufacture all color filters and lift-off processes are completely avoided. In addition, the zero-order filters
proposed here are completely stable even under harsh conditions. They completely avoid the bleaching problems associated with chemical dyes.
Beside their obvious use as picture or recording cameras,
color image sensors are used in a very wide range of
applications. Today almost all cellular phones and many
portable computers and similar electronic equipment have
built in color sensors. The most common image sensors are
CMOS and CCD devices which can be manufactured in large
quantities at low cost. To convert these solid-state image
sensors into color sensors, chemical color filters, i.e. dyes, are
deposited directly onto the surface of the CMOS or CCD
image sensor in a mosaic array of pixel-size filter elements.
Photoresists are used to produce these filters by successively
depositing and patterning each color layer. The choice of
colors can vary from additive (R, G, B) to subtractive (C, M, Y)
or combinations of both. Examples of such patterns which
match the vision of the human eye are shown in Figure 1.
Figure 1: Classical Bayer pattern layout a) CMY, b) RGB (Bayer
pattern)
Chemical dyes are susceptible to bleaching by UV-radiation
which severely limits the useful lifetime of dye-based color
filters. As the markets for color sensors are usually very price
sensitive, a simpler and reliable color post processing is a
further requirement. A new approach for the fabrication of
subtractive (C, M, Y) color patterns on top of vision sensors is
therefore proposed. Instead of dyes or resists, zero-order
devices to fabricate the color patterns are used. Zero-order
color filters are made by combining specific subwavelength
gratings with an inorganic wave-guiding layer. The grating can
be replicated on top of the image sensor by UV casting and
the wave-guiding layer is deposited through thermal
evaporation. The transmitted and reflected color depends on
the correct choice of the grating parameters. By changing the
grating period, the reflected color can be tuned over the entire
visible spectrum. No dyes or pigments are needed in this
approach, which therefore produces long term stable color
filters, which are not prone to UV-bleaching.
Combining several different gratings in a single replication tool
allows fabrication of the zero-order filters with just one
replication and one thermal evaporation step. The complexity
of the patterning is integrated into one tool. Therefore, the
fabrication of the replication tool is more difficult. Several
imprint replication steps followed by etching and thermal
evaporation steps are required to combine three different
subwavelength gratings in the same tool. To avoid scattering
losses, it is very important to minimize the step height
between adjacent gratings. A SEM picture of a ORMOCER ®
replication of this tool is presented in Figure 2.
Figure 2: SEM pictures of a sol-gel replicated mould master made of
a 3 gratings structure
ZOD color filters replicated in ORMOCER ® are shown in
Figure 3. These filters are very reliable and insensitive to UV
exposure. Encapsulated samples are also not sensitive to
mechanical use.
It is possible to combine the process described above with
microlens arrays replication using the same sol-gel materials
but different tools. In such a way complex, highly integrated
image sensors with customized spectral characteristics and
optimized optical fill factors are obtained.
Figure 3: Microscope images made in transmission mode of two final
replicated ZOD color filter (CMY) patterns. a) a 60 µm pixel size;
b) 30 µm pixel size
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