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.
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.