Edgar A. H. Allanwood

Multi-level parallel clocking of CCDs for: improving charge transfer efficiency, clearing persistence, clocked anti-blooming, and generating low-noise backgrounds for pumping

A multi-level clocking scheme has been developed to improve the parallel CTE of four-phase CCDs by suppressing the effects of traps located in the transport channel under barrier phases by inverting one of these phases throughout the transfer sequence. In parallel it was apparent that persistence following optical overload in Euclid VIS detectors would lead to undesirable signal released in subsequent rows and frames and that a suitable scheme for flushing this signal would be required. With care, the negatively biased electrodes during the multi-level transfer sequence can be made to pin the entire surface, row-by-row, and annihilate the problematic charges. This process can also be extended for use during integration to significantly reduce the unusable area of the detector, as per the clocked anti-blooming techniques developed many years ago; however, with the four-phase electrodes architecture of modern CCDs, we can take precautionary measures to avoid the problem of charge pumping and clock induced charge within the science frames. Clock induced charge is not all bad! We also propose the use of on-orbit trap-pumping for Euclid VIS to provide calibration input to ground based correction algorithms and as such a uniform, low noise background is require. Clock induced charge can be manipulated to provide a very suitable, low signal and noise background to the imaging array. Here we describe and present results of multi-level parallel clocking schemes for use in four-phase CCDs that could improve performance of high precision astronomy applications such as Euclid VIS.

Point-spread function and photon transfer of a CCD for space-based astronomy

A front-illuminated development Euclid charge-coupled device (CCD) is tested to observe the CCD point-spread function (PSF) relative to signal size using a single-pixel photon transfer curve (SP-PTC) technique. In the process of generating a SP-PTC charge redistribution effects were observed. In attempting to show that charge redistribution can be caused by exposing a charge-populated well in the CCD array to further illumination, excess charge became apparent in recorded data. Excess charge is suggested to be proportionally generated in the CCD array if existing charge is subjected to further illumination before transfer and readout. The construction of an optical test bench and CCD operating variables are discussed alongside systematic error concerns and mitigation techniques.

Proton irradiation of the CIS115 for the JUICE mission

The CIS115 is one of the latest CMOS Imaging Sensors designed by e2v technologies, with 1504x2000 pixels on a 7 μm pitch. Each pixel in the array is a pinned photodiode with a 4T architecture, achieving an average dark current of 22 electrons pixel-1 s-1 at 21°C measured in a front-faced device. The sensor aims for high optical sensitivity by utilising e2v’s back-thinning and processing capabilities, providing a sensitive silicon thickness approximately 9 μm to 12 μm thick with a tuned anti-reflective coating. The sensor operates in a rolling shutter mode incorporating reset level subtraction resulting in a mean pixel readout noise of 4.25 electrons rms. The full well has been measured to be 34000 electrons in a previous study, resulting in a dynamic range of up to 8000. These performance characteristics have led to the CIS115 being chosen for JANUS, the high-resolution and wide-angle optical camera on the JUpiter ICy moon Explorer (JUICE). The three year science phase of JUICE is in the harsh radiation environment of the Jovian magnetosphere, primarily studying Jupiter and its icy moons. Analysis of the expected radiation environment and shielding levels from the spacecraft and instrument design predict the End Of Life (EOL) displacement and ionising damage for the CIS115 to be equivalent to 1010 10 MeV protons cm-2 and 100 krad(Si) respectively. Dark current and image lag characterisation results following initial proton irradiations are presented, detailing the initial phase of space qualification of the CIS115. Results are compared to the pre-irradiation performance and the instrument specifications and further qualification plans are outlined.

Proton-induced Random Telegraph Signal in the CMOS imaging sensor for JANUS, the visible imaging telescope on JUICE

JUpiter ICy moons Explorer (JUICE) is an ESA L class mission due for launch in 2022 as part of the agency’s Cosmic Vision program [1][2]. The primary science goal is to explore and characterise Jupiter and several of its potentially habitable icy moons, particularly Ganymede, Europa and Callisto. The JANUS instrument is designated to be the scientific imager on-board the spacecraft with a wavelength range between 400 nm and 1000 nm and consists of a catoptric telescope coupled to a CMOS detector [3], specifically the CIS115 monolithic active pixel sensor supplied by e2v technologies[3]. A CMOS sensor has been chosen due to a combination of the high radiation tolerance required for all systems aboard the spacecraft and its capability of operating with integration times as low as 1 ms, which is required to prevent blur when imaging the moons at fast ground velocities since the camera has no mechanical shutter. However, an important consideration of using CMOS in high radiation environments is the generation of defects or defect clusters that result in pixels exhibiting Random Telegraph Signal (RTS)[5]. A study of RTS effects in the CIS115 has been undertaken, and the method applied to identify pixels in the array that display RTS behaviour is discussed and individual RTS-exhibiting pixels are characterised. The changes observed in RTS behaviour following irradiation of the CIS115 with protons is presented and the temperature dependence of the RTS behaviour is studied. The implications on the camera design and imaging requirements of the mission are examined.

The CIS115: a CMOS sensor qualified for optical imaging in the Jovian environment (Conference Presentation)

The European Space Agency’s (ESA’s) Jupiter Icy Moon Explorer will spend 8 years transiting to the Jovian environment after launching from French Guiana in 2022. The spacecraft’s 10 scientific instruments, including a high resolution optical imager called JANUS, will explorer the Jovian system for a mission duration of 3 years studying the icy surfaces of Ganymede, Callisto and Europa and atmosphere of Jupiter. Using the combination of a 13 slot filter wheel and a back-illuminated CMOS image sensor, the JANUS camera will perform colour mapping and imaging at wavelengths between 350 nm and 1064 nm and resolutions of up to 10 m/pixel resolution during a Ganymede orbital phase. The CIS115 is a rolling shutter image sensor from Teledyne-e2v that has been selected for JANUS. It is back-illuminated and anti-reflection coated in order to optimise detection efficiency in its 3 MPixel imaging area. Its 4T architecture reduces the dark current in the pinned photodiode collecting area to approximately 13 pA/cm^2 at 20˚C and allows the device to be operated with correlated double sampling for a readout noise performance of 5 electrons rms. In preparation for its use in JANUS, the CIS115 has undergone a thorough qualification programme, including exposure to ionising and non-ionising radiation levels of up to 200 krad(Si) and 2x10^10 protons/cm^2 (10 MeV equivalent), and a single event effect test campaign. The CIS115 device qualification is now complete and results from the radiation test campaigns are being used to predict the expected performance at various phases of the mission as radiation damage is accumulated in the sensor. Dark current is the primary performance characteristic that has been observed to degrade with irradiation, and predicting the device’s performance at the end of life allows the maximum operating temperature of the detector to be set and justified. Additionally, behaviour observed during the qualification testing has led to optimised readout schemes that reduce the device image lag performance across the dynamic range to below the 0.1% level.

Image lag optimisation in a 4T CMOS image sensor for the JANUS camera on ESA's JUICE mission to Jupiter (Conference Presentation)

The CIS115, the imager selected for the JANUS camera on ESA’s JUICE mission to Jupiter, is a Four Transistor (4T) CMOS Image Sensor (CIS) fabricated in a 0.18 µm process. 4T CIS (like the CIS115) transfer photo generated charge collected in the pinned photodiode (PPD) to the sense node (SN) through the Transfer Gate (TG). These regions are held at different potentials and charge is transferred from the potential well under PPD to the potential well under the FD through a voltage pulse applied to the TG. Incomplete transfer of this charge can result in image lag, where signal in previous frames can manifest itself in subsequent frames, often appearing as ghosted images in successive readouts. This can seriously affect image quality in scientific instruments and must be minimised. This is important in the JANUS camera, where image quality is essential to help JUICE meet its scientific objectives. This paper presents two techniques to minimise image lag within the CIS115. An analysis of the optimal voltage for the transfer gate voltage is detailed where optimisation of this TG “ON” voltage has shown to minimise image lag in both an engineering model and gamma and proton irradiated devices. Secondly, a new readout method of the CIS115 is described, where following standard image integration, the PPD is biased to the reset voltage level (VRESET) through the transfer gate to empty charge on the PPD and has shown to reduce image lag in the CIS115.

Electro-optic and radiation damage performance of the CIS115, an imaging sensor for the JANUS optical camera onboard JUICE

The Jupiter Icy Moon Explorer (JUICE) has been officially adopted as the next Large class mission by the European Space Agency, with a launch date of 2022. The science payload includes an optical camera, JANUS, which will perform imaging and mapping observations of Jupiter, its moons and icy rings. A 13 slot filter wheel will be used to provide spectral information in order for the JANUS experiment to study the geology and physical properties of Ganymede, Europa and Io, and to investigate processes and structures in the atmosphere of Jupiter. The sensor selected for JANUS is the back-thinned CIS115, a 3 MPixel CMOS Image Sensor from e2v technologies. The CIS115 has a 4-Transistor pixel design with a pinned photodiode to improve signal to noise performance by reducing dark current and allowing for reset level subtraction. The JUICE mission will consist of an 8 year cruise phase followed by a 3 year science phase in the Jovian system. Models of the radiation environment throughout the JUICE mission predict that the End of Life (EOL) non-ionising damage will be equivalent to 1010 protons cm-2 (10 MeV) and the EOL ionising dose will be 100 krad(Si), once the shielding from the spacecraft and instrument design is taken into account. An extensive radiation campaign is therefore being carried out to qualify and characterise the CIS115 for JANUS, as well as other space and terrestrial applications. Radiation testing to take the CIS115 to twice the ionising dose and displacement damage levels was completed in 2015 and the change in sensor performance has been characterised. Good sensor performance has been observed following irradiation and a summary of the key results from the campaign using gamma irradiation (ionising dose) will be presented here, including its soft X-ray detection capabilities, flat-band voltage shift and readout noise. In 2016, further radiation campaigns on flight-representative CIS115s will be undertaken and their results will be disseminated in future publications.

Comparison of Point Spread Function in p- and n-Channel CCDs

This paper presents a brief review of charge-redistrbution effects in charge-couple devices observed in both flat-field and spot data. Experiments are described that allow such phenomenon to be measured simply in the parallel transfer direction using only equipment capable of providing flat-field illuminations. A more detailed investigation into signal-dependant PSF shape distortions are descibed for both p- and n-channel CCDs of identical design. A number of methods to minimise the effect are presented that include different image electrode configurations and bias levels during integration and novel pixel architectures.

Responsivity mapping techniques for the non-positional CCD; the swept charge device CCD236

The e2v CCD236 is a swept charge device (SCD) designed as a soft X-ray detector for spectroscopy in the range 0.8 keV to 10 keV [1]. It benefits from improvements in design over the previous generation of SCD (the e2v CCD54) [2] to allow for increased detector area, a reduction in split X-ray events and improvements to radiation hardness [3]. To enable the suppression of surface dark current the device is clocked continuously, therefore there is no positional information making responsivity variations difficult to measure. This paper describes investigated techniques to achieve a responsivity map across the device using masking and XRF, and spot illumination from an organic light-emitting diode (OLED). The results of this technique should allow a deeper understanding of the device sensitivity and allow better data interpretation in SCD applications.