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CCDs

Working in partnership with Teledyne-e2v and major space agencies, the CEI performs fundamental research on CCD technology with the aim of improving the performance of devices for specialised scientific applications.

Cryogenic Irradiations

Historically, the end of life (EOL) performance estimate of CCD based instruments has been provided through irradiation to the expected proton fluence at room temperature. Recent studies have shown that the temperature at which devices are irradiated has a significant impact on the relative abundances of silicon defects responsible for signal loss. As a result, performance estimates from a room temperature irradiation are not necessarily representative of what a device may exhibit in-flight, were device are typically cooled to cryogenic temperatures.

The CEI has now successfully completed multiple performance investigations for space missions, such as the WFIRST CGI and EUCLID, where sensors were irradiated at the nominal mission temperature. The differences in performance between the equivalent room temperature and cryogenic irradiations were significant, indicating that irradiation at the nominal mission operating temperature is necessary to accurately predict performance.

As a result of this research, cryogenic irradiations are now being widely adopted by major space agencies in order to provide accurate estimations of device performance at the end of the nominal mission lifetime.

Studies of fundamental silicon defects

Radiation induced trapping sites within CCD based technologies have the potential to capture and defer charge to later pixels; increasing detector noise and limiting the lifetime of an instrument in-flight. The CEI has adapted a technique known as “pocket pumping”, originally developed by James Janesick, to study the fundamental properties of silicon defects within radiation damaged CCDs. This technique, now referred to as “trap-pumping”, has provided a new level of detail on the nature of silicon defects responsible for charge loss. The technique has been instrumental in improving the charge transfer performance of irradiated devices and explaining the differences in performance between room temperature and cryogenic irradiations. The new level of knowledge provided by this technique has not only been useful for CCD-based missions, but also to the solid-state physics community where detailed information on individual silicon defects can otherwise be difficult to obtain.

Simulation of CCD structure and operation

The CEI has a rich heritage in the accurate simulation of CCD and CMOS structures using state-of-the simulation tools both developed “in-house” and available commercially. The capability to accurately simulate manufacturing processes has inspired better device designs and allowed a more comprehensive understanding of fundamental CCD performance phenomena. Examples include charge transfer inefficiency within EMCCDs operated with high signal, the “brighter fatter” effect where device linearity deviates from the expected relation due to charge re-distribution effects and charge trailing effects due to radiation induced trapping sites. The increased level of understanding provided by these tools has allowed for more intelligent optimisation of devices for specific applications.

“Rad-Hard” CCD designs

Design modifications have been developed by the scientific community that can improve a detectors tolerance to radiation induced damage and extend mission lifetime. One such modification is referred to as a “narrow channel”, where the effective width of pixels is reduced through modifications to the manufacturing process.   A “narrow channel” works by constricting the size of a charge packet such that it encounters fewer trapping sites on its path to the output node. Less signal is therefore lost due to radiation induced damage and mission lifetime is increased by an appreciable factor. Based on recommendations provided by the CEI, narrow channel technology is being utilised within the serial register of ESA’s EUCLID devices and within the detectors baseline for the SMILE mission.

P-Channel development programme

Working in partnership with Teledyne-e2v and the European Space Agency, the CEI has recently completed a near-decade long study investigating the potential of P-Channel CCD technology for use in space missions. Traditional CCDs are manufactured on an “N-Channel” process that has rich space heritage but suffers severe performance degradation in the presence of radiation induced damage due to known impurities that limit efficient charge transfer. “P-Channel” CCDs behave in a near identical manner to N-Channel CCDs, but do not suffer from degradation due to the same impurities that limit N-Channel performance. P-Channel devices therefore have the potential to significantly improve the lifetime of CCD based space missions as well as providing other benefits such as increased frame cadence and relaxed cooling requirements.

Through the results of the study, the optimum manufacturing techniques for the production of flight quality CCDs have been identified. Customised devices are now being developed that are expected to provide a significant performance improvement compared to the current generation of N-Channel CCD technology.