KEYWORDS: Analog to digital converters, Lightning, Camera shutters, Quantum efficiency, Modulation transfer functions, Silicon, Dark current, Photodiodes, Design and modelling, Engineering
As part the phase A study for the GeoXO Lightning Mapper (LMX) Teledyne e2v has been contracted by NASA to develop and characterize an Engineering Demonstrator Unit (EDU). The full-size detector specification is under consolidation; however the format should be in the region of 1500 x 1500 pixels of about 24μm pitch. The key features and requirements are high Full Well Capacity (FWC) above 500ke- with a frame rate target above 500fps, 14 bits ADC resolution in Global Shutter mode is one of the modes considered for performance demonstration. This device must be highly sensitive at 770 nm with a QE above 80% while maintaining good MTF. This is achieved via a combination of thick silicon and reverse bias HiRho technology. The demonstrator consists of a representative reduced format of 600 x 500 of the full-size detector. This demonstrator unit has been designed, manufactured and fully characterized. In this presentation, the architecture approach to the full-size detector and the EDU will be presented along with the key silicon results from the Engineering Demonstrator Unit.
A monolithic CMOS image sensor based on the pinned photodiode (PPD) and optimized for X-ray imaging in the 300 eV to 5 keV energy range is described. Featuring 40 μm square pixels and 40 μm thick, high resistivity epitaxial silicon, the sensor is fully depleted by reverse substrate bias. Backside illumination (BSI) processing has been used to achieve high X-ray QE, and a dedicated pixel design has been developed for low image lag and high conversion gain. The sensor, called CIS221-X, is manufactured in a 180 nm CMOS process and has three different 512×128-pixel arrays on 40 μm pitch, as well as a 2048×512 array of 10 μm pixels. CIS221-X also features per-column 12-bit ADCs, digital readout via four highspeed LVDS outputs, and can be read out at 45 frames per second. CIS221-X achieves readout noise of 2.6 e- RMS and full width at half maximum (FWHM) at the Mn-Kα 5.9 keV characteristic X-ray line of 153 eV at -40 °C. This paper presents the characterization results of the first backside illuminated CIS221-X, including X-ray response and readout noise. The newly developed sensor and the technology underpinning it is intended for diverse applications, including Xray astronomy, synchrotron, and X-ray free electron laser light sources.
THESEUS (Transient High Energy Sky & Early Universe Surveyor) is one of the three candidates for the M5 mission of the European Space Agency. The favoured mission will be announced in 2021 for an expected launch in 2032. THESEUS will be equipped with a Soft X-ray Imager (SXI) composed of a set of two telescopes using micro-pore optics offering an overall field of view of 0.5 sr (<2’ accuracy) for X-ray energies between 300 eV and 5 keV. The focal plane of each SXI telescope has a 16 x 16 cm2 cooled detector area. However, the limited radiator accommodation on the spacecraft prohibits the use of CCDs since cooling the focal planes to an optimal temperature for radiation hardness (<-100 ◦C) is not feasible. Therefore, the development of a suitable CMOS Image Sensor (CIS), capable of handling the expected levels of radiation at higher operating temperatures (approximately -30 ◦C) has been proposed. To demonstrate the performance required for the THESEUS SXI detector, a 2 x 2 cm2 prototype is under development using Open University pixel designs in a Teledyne-e2v digital CMOS platform. The pixel design will allow full depletion over silicon thickness of 35 µm for optimal soft X-ray quantum efficiency and instrument background suppression, and will be capable of near-Fano-limited spectral resolution that will also be of prime interest for synchrotron and Free Electron Lasers (FEL) applications. In this paper, we will present the design considerations and simulations leading to the implemented structures complying with THESEUS’ SXI requirements.
The first Earth Observation satellites used linear sensors, which have a significant limitation in the signal that can be gathered in each pixel resulting in a limited ground sampling distance (GSD) in order to achieve the required signal to noise ratio (SNR). The solution to this was to use TDI (Time delay integration) detectors. The first generations of TDI detectors were based on CCDs, which can intrinsically operate in a TDI mode by moving charge within the detector at a rate corresponding to the movement of the satellite over the ground. The CCD technology used relatively large pixels (typically 10-13μm) with low line rate of typically around 10kHz achieving a ground resolution of down to 0.5m with very high SNR. More recent CCD TDI sensors can achieve line rates of up to 30kHz with pixels as small as 7μm but the interfaces become extremely complex and power dissipation is high. Improvements to satellite technology means that a higher resolution is now achievable and this requires higher line rates, over 30kHz and pixel sizes significantly below 7μm as well as more complex sensors with higher numbers of multispectral lines to give improved spectral data. In order to achieve all of these requirements the use of a CMOS sensor with on chip digitization become essential.
The first CMOS approach was to carry the TDI functionality using digital summation. This approach quickly demonstrated limitations in terms of line rate and power consumption as the entire sensor has to be read for every line on the ground that is sampled. More recently CMOS technology has matured the charge domain CCD approach with comparable electro-optical performance to CCDs while offering higher speed, smaller pixel pitch and high level of integration.
This latest technology step has also considerably eased the integration of the sensor into the satellite, opening new opportunities to produce focal planes at significantly lower cost with much reduced power dissipation, size and weight. The challenge has been to establish a CCD on CMOS technology that can obtain a similar full well capacity and charge transfer efficiency (CTE) performance to CCDs. This CCD on CMOS technology has now reached the point where the performance is comparable to CCDs but with very much lower operating voltages.
This paper will present the evolution of earth scanning image sensors with a focus on the latest TDI CMOS technology including the recent results obtained with the latest CMOS technology using TDI in charge domain approach. These results will include FWC, CTE, radiation performance as well as results from very high speed, up to 3.6Gbps output stream, and highly integrated readout circuitry.
Finally we will provide details of new devices that will provide performance that would not have been possible with CCDs.
Earth observation (EO) is a rapidly expanding area of space science and technology, fueled by the demands for timely, comprehensive and informative data for an increasing number of applications. With the increased affordability of satellites EO is becoming accessible to a larger pool of commercial developers and users. Presently there does not exist in the market a low cost payload with the performance required to meet the growing demands of the commercial ‘New Space’ EO market (very high resolution, good quality image, low mass and low recurrent cost). The presentation will discuss the characterization results of a novel TDI-CMOS silicon prototype as well as a description of the current flight model design currently being developed under the CEOI EO technology and Instrumentation program funded by the UK Space Agency. This sensor will be a key enabling technology for the high resolution new space payload.
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