Klaas Tack

A CMOS-compatible, integrated approach to hyper- and multispectral imaging

Imec has developed a unique hyperspectral sensor concept in which the spectral unit is monolithically integrated on top of a standard CMOS sensor at wafer level, hence enabling the design of compact, low cost and high speed spectral cameras with a high design flexibility. This paper presents the various demonstrated prototype sensors, with different filter arrangements and performance, linked to different usage modes and application domains. It also reviews the key aspects and challenges of imecs hyperspectral technology.

Platform independent optimisation of multi-resolution 3D content to enable universal media access

Flurries of terminals with large differences in terminal capabilities currently consume information and multi-media content. Their different processing capabilities make it challenging to guarantee satisfactory quality in all possible situations. This paper proposes a systematic methodology for interactive 3D graphics applications to adapt the complexity of the content automatically to the terminal’s available resources. Our contribution is an off-line/online partitioned optimisation that increases the visual quality with respect to previous work at the same rendering cost, while the overhead of the optimisation is minimal.

Derivative-Based Scale Invariant Image Feature Detector With Error Resilience

We present a novel scale-invariant image feature detection algorithm (D-SIFER) using a newly proposed scale-space optimal 10th-order Gaussian derivative (GDO-10) filter, which reaches the jointly optimal Heisenbergs uncertainty of its impulse response in scale and space simultaneously (i.e., we minimize the maximum of the two moments). The D-SIFER algorithm using this filter leads to an outstanding quality of image feature detection, with a factor of three quality improvement over state-of-the-art scale-invariant feature transform (SIFT) and speeded up robust features (SURF) methods that use the second-order Gaussian derivative filters. To reach low computational complexity, we also present a technique approximating the GDO-10 filters with a fixed-length implementation, which is independent of the scale. The final approximation error remains far below the noise margin, providing constant time, low cost, but nevertheless high-quality feature detection and registration capabilities. D-SIFER is validated on a real-life hyperspectral image registration application, precisely aligning up to hundreds of successive narrowband color images, despite their strong artifacts (blurring, low-light noise) typically occurring in such delicate optical system setups.

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

Imec is a research centre located in Belgium. Specialising in nanoelectronics, it is mostly known for advanced lithography and CMOS scaling research. However, building on that equipment and material knowledge, Imec works in a number of different application-oriented domains. Hyperspectral imaging, to which this article is devoted, is one of them.

Spatially variable filters — Expanding the spectral dimension of compact cameras for remotely piloted aircraft systems

Recent advances in hyperspectral imaging techniques using spectral filters deposited directly onto an image sensor chip, and suitable for RPAS platforms, are reported in this paper. New filter configurations for a compact spectral camera are described and a prototype of the compact hyperspectral payload developed for small RPAS systems, as well as the first data acquired with the new camera are presented. The spectral range of the payload was optimized for earth observations such as vegetation monitoring or water quality studies. Although the spatially variable filters have to date only been used in small satellite sensors, this technology has a clear potential for RPAS platforms.

Linear variable filters — A camera system requirement analysis for hyperspectral imaging sensors onboard small Remotely Piloted Aircraft Systems

Improving the spectral detail of earth observation imaging from Remotely Piloted Aircraft Systems (RPAS) can greatly expand its potential for use in vegetation monitoring and specifically in precision agriculture. Spatially variable interference filters which can be placed very close to the image sensor offer an excellent opportunity for reducing the size, mass and complexity of hyperspectral imagers, allowing them to be mounted onboard small RPAS. Recent advances in filter deposition techniques allow to directly deposit interference filters on an image sensor. The monolithic integration of optical hyperspectral filters on top of a standard CMOS image sensor has been demonstrated by IMEC. Compared to the more conventional deposition of filters onto an external glass substrate, this new approach offers advantages in terms of cost, alignment accuracy, straylight, etc. A hyperspectral camera prototype compatible with small RPAS has been developed by VITO to demonstrate the potential of LVF-based compact spectral cameras. Whereas application of the filter technology offers major advantages for RPAS systems, it still faces some important challenges. The prototype system specifications need to fit a fixed wing RPAS platform that is able to cover several km2 in a single flight with hyperspectral geo-information. It remains challenging to make a sufficiently compact camera system, achieve precise spectral band registration, handle the amount of data to be processed and cope with limited integration times possible during acquisition.

Quantum efficiency and dark current evaluation of a backside illuminated CMOS image sensor

We report on the development and characterization of monolithic backside illuminated (BSI) imagers at imec. Different surface passivation, anti-reflective coatings (ARCs), and anneal conditions were implemented and their effect on dark current (DC) and quantum efficiency (QE) are analyzed. Two different single layer ARC materials were developed for visible light and near UV applications, respectively. QE above 75% over the entire visible spectrum range from 400 to 700 nm is measured. In the spectral range from 260 to 400 nm wavelength, QE values above 50% over the entire range are achieved. A new technique, high pressure hydrogen anneal at 20 atm, was applied on photodiodes and improvement in DC of 30% for the BSI imager with HfO2 as ARC as well as for the front side imager was observed. The entire BSI process was developed 200 mm wafers and evaluated on test diode structures. The knowhow is then transferred to real imager sensors arrays.