A CMOS-compatible, monolithically integrated snapshot-mosaic multispectral imager

Introduction S pectral imaging can reveal a lot of hidden details about the world around us, but is currently confined to laboratory environments due to the need for complicated, costly and bulky cameras. Imec has developed a unique hyperspectral sensor concept in which the spectral unit is monolithically integrated on top of a standard complementary metal–oxide semiconductor (CMOS) sensor at wafer level (Figure 1), hence enabling an extremely compact form factor and the potential for mass manufacturing. Different filter arrangements 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, as well as stray light generated by the unwanted reflections on discrete components or substrate layers. The use of semiconductor process technology heavily reduces the cost and improves the compactness of the hyperspectral camera. Moreover, the use of tools and processes borrowed from the integrated circuit industry, like deposition, high accuracy lithography and etching, ensures the quality of the filter layers and allows pixel level accuracies in filter alignment. Thanks to this, the filter layout (covering different groups of pixels or depositing filters per pixel) and performance (i.e. bandwidth, full-width at half-maximum etc.) can be customised to match the requirements of specific applications. To prove the versatility of this technology, Imec has successfully demonstrated in the past two sensor types with different filter arrangements: a wedge-based line-scan imager and a non-scanning tiled-layout snapshot sensor. Combined with an optical sub-system to replicate the scene onto each filter, this second sensor enables real-time, low latency operation at video-rates, particularly suited for dynamic scenes. This article presents the latest addition to Imec’s family of monolithically-integrated hyperspectral sensors: a snapshot-mosaic per-pixel sensor (Figure 2). In this equipment, the filters are arranged onto individual pixels, on a 5.5 μm pitch, extending the traditional Bayer colour imaging concept to multior hyperspectral imaging at video rates without the need for dedicated fore-optics or linear scanning. Two mosaic sensors have been successfully fabricated: a visible (Vis) (470–620 nm range) with a 4 × 4 filter repeated configuration and a near infrared (NIR) (600–1000 nm) with a 5 × 5 configuration.