KEYWORDS: Image sensors, High dynamic range imaging, Astronomical imaging, Semiconducting wafers, Design and modelling, CMOS sensors, Camera shutters, Assembly equipment, High dynamic range image sensors, Capacitors
ELFIS2 is the second generation of the European Low Flux Image Sensor (ELFIS), developed by Caeleste, manufactured at LFoundry and tested by Airbus on behalf of the European Space agency. As the High flux program was aborted, it was decided to continue ELFIS as a High Dynamic Range (HDR) sensor, enabling the simultaneous integration and readout of the same charge packet on a large and a small conversion capacitance. It is combined with charge domain global shutter, so that motion artefacts, typical for multi-exposure or dual photodiode architectures are completely eliminated. The ELFIS core pixel uses the Global Shutter technology pixel, developed at LFoundry, allowing charge domain Integrate-While-Read operation and on-chip CDS. In order to handle photocharge packets that exceed the full well of the core pixel, there are two low gain overflow capacitors, used alternatingly for signal integration and read-out. Whereas ELFIS1 was a fixed size device, ELFIS2 is designed for stitching. The stitch block of 512*1024 pixels can realize every n*m multiple of 1/2k by 1k pixels, as long as it fits on the wafer. The Initial prototype presented has a 2k*2k format. 4k*4k, 8k*8K, etc. can be realized with the same mask set, as well as elongated (hyperspectral) sensors up to 512*10 k pixels. As each stitch segment has its own output amplifiers, the ultimate frame rate is only determined by the number of rows to be read. Frame rate can be increased by applying row random addressing, which are especially versatile in hyperspectral applications. The initial functional tests of ELFIS2 are promising: without further optimization the noise spec of less than 5 e-rms is reached with a HDR full well of 160 ke- ; the core Global Shutter high gain full well is 10 ke- , resulting in a smooth photon shot noise limited behavior from the high gain noise floor till the low gain saturation. The full functional testing and circuit optimization is now starting and will be finished at the moment of this presentation. Also, radiation pre-qualification is planned. Due to the good SEE results obtained in another Rad-Hard by Design (RHbD) in the same technology we are also confident that the SEE will have a LET < 63 MeV/mg/cm2 for both SEL and SEU.
A new hyperspectral imager (HSI) is designed for the next-generation HY-1E ocean color monitoring satellite. This instrument requires a reduced ground sampling distance (GSD) of 100 m and increased number of spectral bands (from 8 to 10). A GSD of 100 m corresponds with a frame rate of approximately 70 Hz. The sensor has an elongated format of 2176 * 320 pixels of 30 μm to increase the sensitivity. Multiple sensors will be used in a staggered configuration to guarantee a full earth coverage in 2-3 days The overall length of the sensor is 73 mm and is subdivided in 4 segments of 16.32 mm each. Each segment is read out via two SAR ADC blocks. Each conversion takes 48 clock cycles resulting in ADC clock speed of 120 MHz. The ADC can also be read with 12 bit accuracy with increased conversion speed; and by oversampling the LSB’s it is possible to reach 16 bit accuracy. The pixel is equipped with a 4-fold High Dynamic range readout mechanism. Under low-illumination conditions all acquired signal charges are remaining in the pixel. When the illumination is increased, the additional charges will be accumulated in an additional capacitor. For even larger photon fluxes a third and a fourth capacitor are activated. In this way all intensity level are treated by the same sensitive area and in exactly the same integration time, eliminating all motion artefacts. The 4 gain stages can contain respectively 24, 120, 600 and 2500 ke- . For each gain stage the dynamic range is 5000:1 resulting in an overall dynamic range of 114dB. The Image sensor is designed in a Rad-Hard by Design (RHbD) way to cope with the LEO space environment. The dark current increase under the influence of 75 krad(Si) is a factor 3500, but the effect is largely annealed with a high temperature anneal. The SEE is higher than 37 MeV/mg/cm2 due to the use of DICE based digital blocks. The Image sensor is packaged in a custom designed ceramic package, closed with a dedicated filter window. All environmental and life time tests were executed on the assemblies.
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