New CMOS image sensor is useful in extreme temperatures

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

A CMOS image sensor that can withstand temperatures ranging from —40 to 115 °C – making it perfectly suited for harsh industrial environments – has been developed by researchers at Fraunhofer Institute for Microelectronic Circuits and Systems IMS. The instrument also could be ideal for applications where things can get very hot or very cold; for instance, in modern cars, where automated imaging is becoming increasingly popular.

“The main issue with making cameras that can operate at high temperatures is the increase in dark current,” said Werner Brockherde, head of the optical sensor systems department and project leader at IMS.

A temperature increase of just 8 °C doubles the dark current; i.e., the residual current flowing in complete darkness. This unwanted current manifests itself in electrical noise, reducing the dynamic range of the camera and particularly affecting its ability to record images at low light levels. Furthermore, ghosting occurs in the form of artifacts or fuzziness, degrading the image. The researchers believed that if this dark current could be reduced, it would be possible to capture very high quality images, even in extreme heat.

Key features of the newly developed CMOS image sensor are a low dark current and a high dynamic range. Because of its large size, 25 x 25 mm, the device can be connected directly to an electronic image intensifier. Courtesy of Fraunhofer IMS.

They approached the problem by using a special design based on 0.5-µm CMOS technology, Brockherde said. “By working closely with our CMOS process experts, we managed to come up with a solution.”

Key features of the design are that each pixel’s photodiode is buried and that two p-n junctions are stacked on top of each other. Surface recombination, which is known to be a major contributor to dark current, is avoided by the fact that the p-n junction is not at the surface. The stacked design, on the other hand, increases the full-well capacity of the device.

The full-well capacity is defined as the amount of signal electrons that can be collected and transferred by each pixel of an image sensor; a high value helps to increase the dynamic range. Its high dynamic range of 90 dB provides increased contrast and optimized detail accuracy in both shadow and in very bright areas. Nuances of light are precisely reproduced.

Thanks to its efficient light absorption, the image sensor reacts with high sensitivity, even in weak light conditions, also making it suitable for night-vision equipment. A further special feature is its comparatively large size of 25 x 25 mm. The advantage is that the 256 x 256-pixel sensor can be connected directly to an electronic image intensifier for applications involving extremely low illumination levels or the nonvisible spectral range. The chip operates with synchronous as well as asynchronous electronic shutters.

The researchers point out that the CMOS sensor’s temperature range is quite an improvement compared with that of CCD image sensors, which degrade image quality when the temperature goes beyond ~60 °C. The new chip is resistant to heat and cold, functioning even at arctic temperatures, the researchers say. They note that the lower-temperature end is less of a problem from a dark current perspective and more of an issue with regard to packaging and assembly technologies.

The specific chip was developed for an industrial customer in IMS’ own semiconductor factory; however, the researchers are ready to use the know-how for similar designs that are customized with features such as microlenses on the CMOS chip, or that are tailored to specific applications, such as time-of-flight measurements.