UV-transparent optical coating improves CMOS image sensors

Duisburg, Germany--Image sensors as used in cell phones do not receive light of different wavelengths with equal efficiency. This is because of their protective optical coating, which prevents UV light from passing through. As a result, CMOS chips have not been previously been suitable for some uses in spectroscopy. Now, a new production process developed at Fraunhofer Institute for Microelectronic Circuits and Systems IMS makes the coating more spectrally even—and the sensors suitable for special applications.

Such sensors have been used as standard in multimedia electronics for a long time, and are now 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 photosensors have to cope with difficult operating conditions, for example high temperatures and moisture.

Improved protective coating

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 Fraunhofer IMS say they have 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," says Werner Brockherde, one of the scientists. The trick is to increase the proportion of nitrogen in the coating. "This reduces the absorption of shortwave light," notes Brockherde.

The new coating material absorbs less light of an energy higher than blue light, which means the sensor becomes more sensitive at the blue and UV range. "This makes CMOS image sensors suitable for use in wavelength ranges down to 200 nm," states Brockherde. "With standard passivation the limit was about 450 nm." To 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.

With this process development, the experts have expanded the range of applications for CMOS image technology. This could aid 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, for example in fluorescence microscopes, providing scientists with images of greater detail.