CMOS processing advances imaging
December 2014 – At the 60th IEDM conference in San Francisco, IMEC presented a talk on using traditional CMOS processing for enhancing imaging systems. The presentation focused on the the creation of an integrated hyperspectral and multispectral imager. Typical consumer imagers have a three color spectral pickup – Red, Green, and Blue. Many industrial, medical and scientific application need additional spectral separation and color identification.
IMEC has developed a hyperspectral sensor concept in which the spectral unit is monolithically integrated on top of a standard CMOS sensor at wafer level. This solution enables an extremely compact form factor and a path to high volume manufacturing in a traditional semiconductor wafer fab. The concept design incorporates different filter arrangements that enable a variety of system level usage modes. The integrated filters can reach high transmission efficiencies and at the same time eliminate the need for camera level alignment of high-end lenses or gratings. It additionally can eliminate stray light generated by the unwanted reflections from external discrete components or substrate layers.
This approach leverages on CMOS manufacturing technology, like deposition, high accuracy lithography and etching. This ensures the quality of the filter layers and enables pixel level filter definition, in terms of layout (covering different groups of pixels or depositing filters per pixel) and performance (i.e. number of bands, filter width), to match the requirements of specific applications. To prove the versatility of this technology, IMEC has successfully demonstrated three sensor types with different filter arrangements: a line-scan imager and two non-scanning, frame-based spectral sensors.
The presentation included results from cameras built with these 3 imagers which used common processing techniques, but different design layouts. The layout options shown were (1) Line scan/wedge later where the filters are arranged in a staircase-like structure over the pixel array, (2) a Snapshot/Tiled layout where the filters are laid out in large squares on top of groups of pixels and (3) a pixel level mosaic layout that has the filers arranged over individual pixel and in a 4×4 repeated filter configuration. This last configuration allows for extending the standard Bayer color imaging concept to multi-spectral images at video rates without linear scanning or dedicated fore-optics. The concept design had 16 bands and did not require scanning or an optical duplicator for spectral image capture. The test designs had the first configuration as a fully fabricated 100-band imager and the the second configuration (tiled design) as a 32-band sensor.
Their presentation did not discuss the details of the manufacturing processes other than it was standard CMOS processing steps and standard lithography steps. The goal is to help bring the ability to create higher reliability and repeatably at a lower cost for these industrial, medical and specialty sensors.
