CIS221-X is a prototype complementary metal-oxide-semiconductor (CMOS) image sensor, optimized for soft x-ray astronomy and developed for the proposed ESA Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission. The sensor features 40 μm pitch square pixels built on a 35 μm thick, high-resistivity epitaxial silicon that is fully depleted by reverse substrate bias. Backside illumination processing has been used to achieve high x-ray quantum efficiency, and an optical light-blocking filter has been applied to mitigate the influence of stray light. A comprehensive electro-optical characterization of CIS221-X has been completed. The median readout noise is 3.3 e − RMS with 90% of pixels reporting a value <3.6 e − RMS. At −40 ° C, the dark current is 12.4 ± 0.06 e − / pixel / s. The pixel photo-response is linear to within 1% for 0.3 to 5 keV photons (82 to 1370 e − ) with <0.1 % image lag. Following per-pixel gain correction, an energy resolution of 130.2 ± 0.4 eV has been measured at 5898 eV. In the 0.3 to 1.8 keV energy range, CIS221-X achieves >80 % quantum efficiency. With the exception of dark current, these results either meet or outperform the requirements for the THESEUS mission, strongly supporting the consideration of CMOS technology for soft x-ray astronomy.
CIS221-X is the first in a new generation of monolithic CMOS image sensors optimized for soft x-ray applications. The pixels are built on 35 μm thick, high-resistivity epitaxial silicon and feature Deep Depletion Extension (DDE) implants, facilitating over depletion by reverse substrate bias. When cooled to -40 °C, CIS221-X reports a readout noise of 3.3 e- RMS and 12.4 ± 0.06 e-/pixel/s of dark current. The 40μm pixels experience near-zero image lag. Following per-pixel gain correction, an energy resolution of 130 ± 0.4 eV FWHM has been measured at 5.9 keV. In the 0.3 – 1.8 keV energy range, the sensor achieves a quantum efficiency of above 80%. Radiation tests have shown that both the readout noise and dark current increase with total ionising dose and that the OBF can help to mitigate the increase in dark current. The measured electro-optical parameters and the preliminary ionising radiation results strongly support the use of the CIS221-X in soft x-ray applications.
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.
Charge-coupled devices have been the detector of choice for soft X-ray astronomy missions for many decades due to excellent energy resolution, noise performance, and longevity in space. Newer CCD-based missions require everincreasing performance which is made challenging by radiation damage inherent to the space environment. Missions such as ESA’s upcoming EUCLID observatory is aiming to measure tiny changes in the shape of distant galaxies, created by the presence of dark matter. Such high precision (not only specific to EUCLID) necessitates significant mitigation against radiation damage effects, one of which is utilising different detector operation modes such as multilevel clocking. Multi-level clocking uses three electrode voltage levels (compared to the standard two) to encourage traps within the damaged silicon to emit their charge such that they do not contribute to charge transfer losses, improving charge transfer efficiency and overall detector performance. However, multi-level clocking requires bespoke hardware to implement, followed by significant amount of testing to show that the benefit is significant.
A recent CCD optimisation technique, called the Active Trap Model, utilises knowledge of the radiation-induced defects within a CCD to optimise charge transfer performance across a wide range of variables including temperature, clocking speeds and device operation modes. This paper presents development of the Active Trap Model to predict the performance of multi-level clocking in CCDs. The performance of the model is compared to the experimental data available, namely from ESA’s PLATO1 mission, and shows good agreement between model and experimental data. The results show the versatility of the Active Trap Model and uses of the technique in potential future CCD-based space missions such as HabEx2 and LUVIOR3.
Charge-coupled devices (CCDs) have been the detector of choice for large-scale space missions for many years. Although dominant in this field, the charge transfer performance of the technology degrades over time due to the radiation-harsh space environment. Charge transfer performance can be optimized; however, it is often time consuming and expensive due to the many operating modes of the CCD, especially considering the ever-increasing needs of detector performance. A technique that uses measurements of the trap landscape present in a CCD to predict changes in charge transfer inefficiency as a function of different experimental variables is presented and developed. Using this technique, it is possible to focus experimental lab testing on key device parameters, potentially saving many months of laboratory effort. Due to the generality of the method, it can be used to optimize the charge transfer performance of any CCD and, as such, has many uses across a wide range of fields and space missions. Future CCD variants that will be used in potential space missions (EMCCD and p-channel CCDs) can use this technique to provide feedback of the key device performance to the wider mission consortium before devices are available for experimental testing.
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