RTS noise impact in CMOS image sensors readout circuit

CMOS image sensors are nowadays widely used in imaging applications even for high end applications. This is really possible thanks to a reduction of noise obtained, among others, by Correlated Double Sampling (CDS) readout. Random Telegraph Signal (RTS) noise has thus become an issue for low light level applications especially in the context of downscaling transistor dimension. This paper describes the analysis of in-pixel source follower transistor RTS noise filtering by CDS circuit. The measurement of a non Gaussian distribution with a positive skew of image sensor output noise is analysed and dimension (W and L) impact of the in-pixel source follower is analysed

Low-frequency noise impact on CMOS image sensors

CMOS image sensors are nowadays extensively used in imaging applications even for high-end applications. This is really possible thanks to a reduction of noise obtained, among others, by Correlated Double Sampling (CDS) readout. Random Telegraph Signal (RTS) noise has thus become an issue for low light level applications especially in the context of downscaling transistor size. This paper describes the analysis of in-pixel source follower transistor RTS noise filtering by CDS circuit. The measurement of a non Gaussian distribution with a positive skew of image sensor output noise is analysed. Impact of dimensions (W and L) of the in-pixel source follower is demonstrated. Circuit to circuit pixel output noise dispersion on 12 circuits coming from 3 different wafers is also analysed and weak dispersion is seen

CMOS detectors for space applications: from R&D to operational program with large volume foundry

Nowadays, CMOS image sensors are widely considered for space applications. The use of CIS (CMOS Image sensor) processes has significantly enhanced their performances such as dark current, quantum efficiency and conversion gain. However, in order to fulfil specific space mission requirements, dedicated research and development work has to be performed to address specific detector performance issues. This is especially the case for dynamic range improvement through output voltage swing optimisation, control of conversion gain and noise reduction. These issues have been addressed in a 0.35μm CIS process, based on a large volume CMOS foundry, by several joint ISAE- EADS Astrium R&D programs. These results have been applied to the development of the visible and near-infrared multi-linear imager for the SENTINEL 2 mission (LEO Earth observation mission for the Global Measurement Environment and Security program). For this high performance multi-linear device, output voltage swing improvement is achieved by process optimisation done in collaboration with foundry. Conversion gain control is also achieved for each spectral band by managing photodiode capacitance. A low noise level at sensor output is reached by the use of an architecture allowing Correlated Double Sampling readout in order to eliminate reset noise (KTC noise). KTC noise elimination reveals noisy pixels due to RTS noise. Optimisation of transistors’s dimensions, taking into account conversion gain constraints, is done to minimise these noisy pixels. Additional features have been also designed: 1) Due to different integration times between spectral bands required by mission, a specific readout mode was developed in order to avoid electrical perturbations during the integration time and readout. This readout mode leads to specific power supply architecture. 2)Post processing steps can be achieved by alignment marks design allowing a very good accuracy. These alignment marks can be used for a black coating deposition between spectral bands (pixel line) in order to minimise straight lighteffects. In conclusion a review of design improvements and performances of the final component is performed

Analysis and Optimization of Noise Response for Low-Noise CMOS Image Sensors

CMOS image sensors are nowadays widely used in imaging applications and particularly in low light flux applications. This is really possible thanks to a reduction of noise obtained, among others, by the use of pinned photodiode associated with a Correlated Double Sampling readout. It reveals new noise sources which become the major contributors. This paper presents noise measurements on low-noise CMOS image sensor. Image sensor noise is analyzed and optimization is done in order to reach an input referred noise of 1 electron rms by column gain amplifier insertion and dark current noise optimization. Pixel array noise histograms are analyzed to determine noise impact of dark current and column gain amplifier insertion. Transfer noise impact, due to the use of pinned photodiode (4T photodiode), is also measured and analyzed by a specific readout sequence

High performances monolithic CMOS detectors for space applications

During the last 10 years, research about CMOS image sensors (also called APS -Active Pixel Sensors) has been intensively carried out, in order to offer an alternative to CCDs as image sensors. This is particularly the case for space applications as CMOS image sensors feature characteristics which are obviously of interest for flight hardware: parallel or semi-parallel architecture, on chip control and processing electronics, low power dissipation, high level ofradiation tolerance... Many image sensor companies, institutes and laboratories have demonstrated the compatibility of CMOS image sensors with consumer applications: micro-cameras, video-conferencing, digital-still cameras. And recent designs have shown that APS is getting closer to the CCD in terms ofperformance level. However, the large majority ofthe existing products do not offer the specific features which are required for many space applications. ASTRI1JM and SUPAERO/CIMI have decided to work together in view of developing CMOS image sensors dedicated to space business. After a brief presentation of the team organisation for space image sensor design and production, the latest results of a high performances 512x512 pixels CMOS device characterisation are presented with emphasis on the achieved electro-optical performance. Finally, the on going and short-term coming activities of the team are discussed