CMOS image sensors can now be covered with a transparent protective coating that is permeable to light in the UV and blue spectral range. Image: Fraunhofer IMS
sensors as used in cell phones are partially color-blind. This is
because of their coating, which prevents UV light from passing through.
CMOS chips have as a result not been suitable for spectroscopy up to
now. A new production process makes the coating transparent – and the
sensors suitable for special applications.
have been used as standard in multimedia electronics for a long time,
and now they are making rapid inroads in high performance applications:
CMOS image sensors are no longer only used in cell phones and digital
cameras. The automotive industry, for instance, has discovered the
potential of optical semiconductor chips and is increasingly using them
in driver assistance systems – from parking aids and road lane detection
to blind-spot warning devices. In special applications, however, the
sensors that convert light into electrical signals have to cope with
difficult operating conditions, for example high temperatures and
this reason, CMOS devices are covered with a silicon nitride coating.
This chemical compound forms hard layers which protect the sensor from
mechanical influences and the penetration of moisture and other
impurities. The protective coating is applied to the sensor in the final
stage of CMOS semiconductor production. The process is called
passivation, and is an industry requirement. Unfortunately, up to now
this passivation has entailed a problem: the silicon nitride coating
limits the range of optical applications because it is impermeable to
light in the UV and blue spectral range. CMOS sensors for
high-performance applications, used in special cameras are therefore
at the Fraunhofer Institute for Microelectronic Circuits and Systems
IMS in Duisburg have found a solution to this problem.
developed a new process step,” says Werner Brockherde, head of
department at Fraunhofer IMS, “that allows us to produce a protective
coating with the same properties but which is permeable to blue and UV
The trick is to increase the proportion of nitrogen in the coating.
“This reduces the absorption of shortwave light,” explains Brockherde.
simplified terms, the new coating material will absorbless light of an
energy higher than blue light, which means the sensor becomes more
sensitive at the blue and UV range.
makes CMOS image sensors suitable for use in wavelength ranges down to
200 nanometers,” states Brockherde. “With standard passivation the limit
was about 450 nanometers.”
change the structure of the silicon nitride for the coating, the
Fraunhofer research scientists had to fine-tune the deposition
parameters such as pressure and temperature.
this process development the experts have expanded the range of
applications for CMOS image technology. This could revolutionize UV
spectroscopic methods, which are used in laboratories around the world,
significantly improving their accuracy. Likewise, CMOS image sensors
stand to take up a new role in professional microscopy, e.g. in
fluorescence microscopes, providing scientists with images of even