Researchers crack CMOS sensor ultraviolet problem

Researchers at Germany's Fraunhofer Institute have come up with a way to make CMOS image sensors pick up ultraviolet and blue light, a breakthrough that they say could lead to new uses for the technology in microscopy and spectroscopy.

On Wednesday, the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg — the Fraunhofer Institute is actually a network of research centres around the country — said it had overcome key limitations in the silicon nitride coating that is used to protect CMOS (complementary metal oxide semiconductor) image sensors from high temperatures and moisture. This coating has until now been impermeable to UV and blue light, but the researchers fixed this problem by increasing the proportion of nitrogen in the coating.

"We've developed a new process step that allows us to produce a protective coating with the same properties but which is permeable to blue and UV light," Werner Brockherde, department head at Fraunhofer IMS, said in a statement, explaining that boosting the nitrogen proportion "reduces the absorption of shortwave light" — while the standard coating process put a 450nm limit on the wavelength ranges that could be picked up by the sensor, the new method allows wavelength ranges down to 200nm.

According to the Fraunhofer, the new process "could revolutionize UV spectroscopic methods, which are used in laboratories around the world, significantly improving their accuracy", while allowing CMOS image sensors to "take up a new role in professional microscopy, e.g. in fluorescence microscopes, providing scientists with images of even greater detail".

CMOS image sensors are already commonplace in cameras and mobile phones, but are also increasingly being used in applications such as automotive driver assistance, where the environmental issues that necessitate the silicon nitride coating come into play.