BSI sensor chip minimises reflection losses

Imec said it has developed a backside-illuminated (BSI) CMOS image sensor chip that features a new anti-reflective coating optimised for UV light.

Known for its superior enhanced light sensitivity compared with image sensors using front side illumination (FSI), BSI sensors are top candidates to further improve the performance of CMOS image sensors. Widely spread today in consumer applications such as smart phones, BSI imagers are expected to enter the higher-end application space, including industrial inspection.

BSI imagers have a clear advantage when it comes to fill factor for the pixel area, angular response, and the complete avoidance of absorption or scattering losses in the metal interconnect layers. The trade-offs for these light gathering improvements are the extra process complexity for the backside fabrication and possible electrical and optical losses at the new backside silicon interface. Therefore, tothe engineering of the backside layers and interfaces is key to develop high performance BSI devices.

Imec is tackling these challenges to exploit the benefits of BSI imagers for highly specialised customised imagers for space applications, high speed cameras, semiconductor inspection and medical applications.

To minimise reflection losses and maximise transmission of light to the sensor, specific anti-reflective coatings are developed for various applications targeting different regions of the light spectrum. The coatings are applied at wafer level as part of the BSI process flow.

Imec has already developed an ARC for visible light range (400nm to 800nm) with greater than 70 per cent quantum efficiency (QE) over the entire spectral range. Imec's new ARC, targeting the UV range, shows excellent performance at near UV wavelengths, with QE values above 50 per cent over the entire spectral range from 260nm to 400nm wavelength.

"This is an important milestone for Imec's customised 200mm CMOS process line, demonstrating our expertise and capability to design, prototype and manufacture high-end CMOS image sensors," said Rudi Cartuyvels, SVP, Smart Systems & Energy Technologies at imec. "The development widens our portfolio towards new markets, offering solutions for both visible and UV imaging in semiconductor equipment applications, such as advance lithography and wafer and mask inspection."