InfraRed (IR) sensor systems like night vision goggles, missile approach warning systems and telescopes have an increasing interest in decreasing their size and weight. At the same time optical aberrations are always more difficult to optimize with larger Focal Plane Arrays (FPAs) and larger field of view. Both challenges can now take advantage of a new optical parameter thanks to flexible microelectronics technologies: the FPA spherical curvature. This bio-inspired approach can correct optical aberrations and reduce the number of lenses in camera conception. Firstly, a new process to curve thin monolithic devices has been applied to uncooled microbolometers FPAs. A functional 256×320 25μm pitch (roughly 1cm2) uncooled FPA has been thinned and curved. Its electrical response showed no degradation after our process (variation of less than 2.3% on the response). Then a two lenses camera with a curved FPA is designed and characterized in comparison with a two lenses camera with a flat FPA. Their Modulation Transfer Functions (MTFs) show clearly an improvement in terms of beams dispersion. Secondly, a new process to fabricate monolithic cooled flip-chip MCT-IRCMOS FPAs was developed leading to the first spherical cooled IR FPA: with a radius of 550 mm. Other radii are achieved. A standard opto-electrical characterization at 80 K of the imager shows no additional short circuit and no mean response alteration compared to a standard IRCMOS shown in reference. Noise is also studied with a black body between 20 and 30°C.
In ground based astronomy, mainly all designs of sky survey telescopes are limited by the requirement
that the detecting surface is flat whereas the focal surface is curved. Two kinds of solution have been
investigated up to now. The first one consists in adding optical systems to flatten the image surface; however this
solution complicates the design and increases the system size. Somehow, this solution increases, in the same
time, the weight and price of the instrument. The second solution consists in curving artificially the focal surface
by using a mosaic of several detectors, which are positioned in a spherical shape. However, this attempt is
dedicated to low curvature and is limited by the technical difficulty to control the detectors alignment and tilt
between each others.
Today we would like to propose an ideal solution which is to curve the focal plane array in a spherical shape,
thanks to our monolithic process developed at CEA-LETI based on thinned silicon substrates which allows a
100% optical fill factor. Two infrared uncooled cameras have been performed, using 320 x 256 pixels and 25 μm
pitch micro-bolometer arrays curved at a bending radius of 80 mm. These two micro-cameras illustrate the
optical system simplification and miniaturization involved by curved focal plane arrays.
Moreover, the advantages of curved detectors on the optical performances (Point Spreading Function), as well as
on volume and cost savings have been highlighted by the simulation of the opto-mechanical architecture of the
spectrometer OptiMOS-EVE for the European Extremely Large Telescope (E-ELT).