Detectors play a crucial role for the instrument design and the achievable instrument performance. The wavebands of interest for remote sensing are the visible and the infrared. Therefore, the European Space Agency has a strong interest in the performance enhancement of detector arrays in those spectral ranges. The Agency follows a continuous development strategy to enhance the capabilities for future Earth observation and astronomy missions. This paper presents the technical and planning status of these detector technology development activities.
Both ESA and the EC have identified the need for a supply chain of CMOS imagers for space applications which uses solely European sources. An essential requirement on this supply chain is the platformization of the process modules, in particular when it comes to very specific processing steps, such as those required for the manufacturing of backside illuminated image sensors. This is the goal of the European (EC/FP7/SPACE) funded project EUROCIS. All EUROCIS partners have excellent know-how and track record in the expertise fields required. Imec has been leading the imager chip design and the front side and backside processing. LASSE, as a major player in the laser annealing supplier sector, has been focusing on the optimization of the process related to the backside passivation of the image sensors. TNO, known worldwide as a top developer of instruments for scientific research, including space research and sensors for satellites, has contributed in the domain of optical layers for space instruments and optimized antireflective coatings. Finally, Selex ES, as a world-wide leader for manufacturing instruments with expertise in various space missions and programs, has defined the image sensor specifications and is taking care of the final device characterization. In this paper, an overview of the process flow, the results on test structures and imagers processed using this platform will be presented.
The European Space Agency has a very strong interest in the performance enhancement of detector arrays for future scientific and astronomy missions. Improvements in Visible and Infrared wavelengths are of particular interest and the Agency undertakes a programme of continuous development aimed at enhancing the capability of detectors in these wavebands. This paper presents the status of these detector technology development activities.
This paper introduces a novel imaging spectrometer subsystem concept, the Smart Slit Assembly (SSA), that improves instrument performances and enables new features for future Earth Observation. Derived from CarbonSat (ESA study) requirements, a concept of an SSA based on MEMS micro-shutters/mirrors and associated instrument design aspects are presented. The SSA replaces the classical grating spectrometer slit aperture in the focal plane of the telescope with three core elements, namely an input multimode waveguide array followed by a spatial light modulator (SLM) and an output multimode waveguide array which ends at the slit aperture viewed by the spectrometer. The SLM’s in-and-outputs being coupled to waveguide arrays leads to an enhanced SLM with light de-coherence, polarization scrambling and scene/object homogenization capabilities. The additional advantage of this subsystem’s arrangement is that waveguide level homogeneous spatial light modulation can be achieved with spatially in-homogeneous coupling from in to output multimode waveguides, allowing new, simpler and less costly designs for the SLM part of the SSA. The SSA is particularly useful for instance to reduce stray light by scene/object selection or modulation (e.g. de-clouding, intensity equalization), relax on the required dynamic range of the detectors, increase spectral stability by waveguide level intensity homogenization/scrambling, continuous in-flight monitoring of the co-registration between two or several spectrometer channels and inflight monitoring of stray light.
Remote sensing is a priority activity for the European Space Agency and detector performance is a crucial factor in determining how well this role is performed. Consequently, the Agency has a strong interest in continuous improvement of both detector capabilities and availability within Europe. To this end, ESA maintains a number of strategic detector development plans combining both technology-push and technology-pull. The visible and infrared wavebands are of particular interest for remote sensing activities and this paper sets out the requirements for current and future missions and presents details of the Agency’s current and planned detector developments.
Backside illuminated (BSI) hybrid CMOS image sensors possessing excellent spectral response
(> 80% between 400nm-800nm) have been previously reported by us. Particularly challenging with BSI imagers is to
combine such sensitivity, with low electrical inter-pixel crosstalk (or charge-dispersion). Employing thick bulk silicon
(in BSI) to maximize red response results in large crosstalk especially for blue light. In the second generation of these
imagers, we undertook the exercise of solving the crosstalk problem by a two-pronged approach: a) an optimized
epitaxial substrate that was engineered to maximize the internal electric field b) high aspect ratio trenches (30 μm deep)
with carefully tailored sidewall passivation. The results show that the proposed optimizations are effective in curtailing
crosstalk without having a major impact on other sensor parameters.
Photodetectors designed for the Extreme Ultraviolet (EUV) range with the Aluminum Gallium Nitride
(AlGaN) active layer are reported. AlGaN layers were grown by Molecular Beam Epitaxy (MBE) on
Si(111) wafers. Different device structures were designed and fabricated, including single pixel
detectors and 2D detector arrays. Sensitivity in different configurations was demonstrated, including
front- and backside illumination. The latter was possible after integration of the detector chips with
dedicated Si-based readouts using high-density In bump arrays and flip-chip bonding. In order to avoid
radiation absorption in silicon, the substrate was removed, leaving a submicron-thin membrane of
AlGaN active layer suspended on top of an array of In bumps. Optoelectrical characterization was
performed using different UV light sources, also in the synchrotron beamlines providing radiation
down to the EUV range. The measured cut-off wavelength of the active layer used was 280 nm, with a
rejection ratio of the visible radiation above 3 orders of magnitude. Spectral responsivity and quantum
We report on the fabrication and characterization of solar blind Metal-Semiconductor-Metal (MSM) based
photodetectors for use in the extreme ultraviolet (EUV) wavelength range. The devices were fabricated in the AlGaN-on-
Si material system, with Aluminum Gallium Nitride (AlGaN) epitaxial layers grown on Si(111) by means of Molecular
Beam Epitaxy. The detectors' IV characteristics and photoresponse were measured between 200 and 400 nm. Spectral
responsivity was calculated for comparison with the state-of-the-art ultraviolet photodetectors. It reaches the order of 0.1
A/W at the cut-off wavelength of 360 nm, for devices with Au fingers of 3 μm width and spacing of 3 μm. The rejection
ratio of visible radiation (400 nm) was more than 3 orders of magnitude. In the additional post-processing step, the Si
substrate was removed locally under the active area of the MSM photodetectors using SF6-based Reactive Ion Etching
(RIE). In such scheme, the backside illumination is allowed and there is no shadowing of the active layer by the metal
electrodes, which is advantageous for the EUV sensitivity. Completed devices were assembled and wire-bonded in
customized TO-8 packages with an opening. The sensitivity at EUV was verified at the wavelengths of 30.4 and 58.4 nm
using a He-based beamline. AlGaN photodetectors are a promising alternative for highly demanding applications such as
space science or modern EUV lithography. The backside illumination approach is suited in particular for large, 2D focal
Two types of backside illuminated CMOS Active Pixel Detectors--optimized for space-borne imaging--have been
successfully developed: monolithic and hybrid. The monolithic device is made out of CMOS imager wafers postprocessed
to enable backside illumination. The hybrid device consists of a backside thinned and illuminated diode array,
hybridized on top of an unthinned CMOS read-out. Using IMEC's innovative techniques and capabilities, 2-D arrays
with a pitch of 22.5 μm have been realized. Both the hybrid and well as the monolithic APS exhibit high pixel yield, high
quantum efficiency (QE), and low dark current. Cross-talk can be reduced to zero in the hybrid sensors utilizing special
structures: deep-isolating trenches. These trenches physically separate the pixels and curtail cross-talk. The hybrid
imagers are suitable candidates for advanced "smart" sensors envisioned to be realized as multi-layer 3D integrated
systems. The design of both these types of detectors, the key technology steps, the results of the radiometric
characterization as well as the intended future developments will be discussed in this paper.
We report on the results of fabrication and optoelectrical characterization of Gallium Nitride (GaN) based Extreme
UltraViolet (EUV) photodetectors. Our devices were Schottky photodiodes with a finger-shaped rectifying contact,
allowing better penetration of light into the active region. GaN layers were epitaxially grown on Silicon (111) by Metal-
Organic-Chemical Vapor Deposition (MOCVD). Spectral responsivity measurements in the Near UltraViolet (NUV)
wavelength range (200-400 nm) were performed to verify the solar blindness of the photodetectors. After that the
devices were exposed to the EUV focused beam of 13.5 nm wavelength using table-top EUV setup. Radiation hardness
was tested up to a dose of 3.3·1019 photons/cm2. Stability of the quantum efficiency was compared to the one measured
in the same way for a commercially available silicon based photodiode. Superior behavior of GaN devices was observed
at the wavelength of 13.5 nm.
We report first results of laboratory tests of Si:As
blocked-impurity-band (BIB) mid-infrared (4 to 28 μm) detectors developed
by IMEC. These prototypes feature 88 pixels hybridized on an integrated cryogenic readout electronics (CRE). They
were developed as part of a technology demonstration program for the future Darwin mission. In order to be able to separate
detector and readout effects, a custom build TIA circuitry was used to characterize additional single pixel detectors.
We used a newly designed test setup at the MPIA to determine the relative spectral response, the quantum efficiency, and
the dark current. All these properties were measured as a function of operating temperature and detector bias. In addition
the effects of ionizing radiation on the detector were studied. For determining the relative spectral response we used a dualgrating
monochromator and a bolometer with known response that was operated in parallel to the Si:As detectors. The
quantum efficiency was measured by using a custom-build high-precision vacuum black body together with cold (T ~ 4K)
filters of known (measured) transmission.