Image sensors as used in cell phones are partially color-blind, due to their coating, which prevents UV light from passing through. Because of this, CMOS chips have not been suitable for spectroscopy. However, a new production process makes the coating transparent, and the sensors suitable for special applications, and may allow for use in spectroscopy after all. 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, such as high temperatures and moisture. For 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 partially color-blind. Scientists at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg have found a solution to this problem. “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. This reduces the absorption of shortwave light,” said Werner Brockherde, head of department at Fraunhofer IMS. In 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. With 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.

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