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Synchronous and asynchronous detection of ultra-law light levels using CMOS-compatible semiconductor technologies
Auteur(s)
Lotto, Christian
Editeur(s)
Seitz, Peter
Charbon, Edoardo
Enz, Christian
Farine, Pierre-André
Date de parution
2010
Résumé
This work presents significant improvements of noise performance in synchronous CMOS image sensors and in asynchronous energy-sensitive singlephoton X-ray imaging systems. A detailed analysis of synchronous CMOS low-noise image sensors using conventional architectures reveals room for potential noise performance improvements, namely noise in switched-capacitor column-parallel amplifiers as well as imperfections in the low-pass filtering properties provided by such switched-capacitor amplifiers. A novel low-noise CMOS image sensor topology with pixel-level open-loop voltage amplification alleviates the identified issues to a large extent. An image sensor based on this concept has been designed and fabricated. Characterization shows that the image sensor achieves a readout noise equivalent charge below 0.9 electrons and an overall noise floor as low as 1.4 electrons including photodiode leakage shot noise at an exposure time of 1/60 s. Performance parameters that are potentially compromised due to the use of open-loop amplification, such as linearity and photoresponse non-uniformity, are shown to be absolutely competitive with conventional image sensors thanks to an advantageous feedback configuration for the sense node reset. Implementations of the presented topology are simple and elegant circuits requiring generally less silicon area in the periphery of the pixel field and less power consumption than conventional architectures. A second novel low-noise CMOS image sensor topology combining conventional pixel-level source-followers with column-level open-loop degenerate common-source amplifiers is proposed. This architecture provides an attractive combination of very low sense node capacitance, flawless low-pass filtering of noise from the pixel-level electronics by the use of column-level open-loop amplification, and advantageous noise performance of open-loop degenerate common-source amplifiers in comparison to OTA based switched-capacitor amplifiers. Theory, circuit simulation, and measurement of implemented test circuits indicate that image sensors based on this advantageous topology can potentially outperform the noise performance not only of conventional image sensor architectures but even of the presented topology using pixel-level voltage amplification. Analysis of charge-sensitive amplifiers (CSA), as used in conventional asynchronous single particle and high-energy photon detectors, explains the need for a sensing device with low capacitance in order to achieve good noise performance at reasonable power consumption. For instance, photogates or buried photodiodes in combination with a scintillator layer are sensing devices for X-rays with a very low capacitance. A novel charge pulse detecting circuit using a source-follower buffer plus a band-pass filter instead of a CSA with a band-pass filter is presented in this thesis. In the case of low sensing device capacitance, comparison of this circuit vs. CSAs shows that the trade-off between noise and power consumption is systematically advantageous in favor of the presented buffered charge pulse detecting circuit. A test structure implementation of a buffered charge pulse detecting circuit in conjunction with a lateral drift field photogate has achieved an equivalent noise charge of 12 electrons. This excellent noise performance will allow X-ray single-photon imaging with very low detection threshold and excellent energy resolution. This work contributes to further progress in cutting-edge noise performance of asynchronous single-photon X-ray image sensors and of synchronous lowlight CMOS image sensors. Thanks to their extremely low thermal noise, the presented novel topologies of synchronous CMOS image sensors will experience an immediate further improvement of noise performance in case of future progress in process technology either leading to reduced dark current of buried photodiodes or to a lower flicker noise power spectral density.
Notes
Thèse de doctorat : Université de Neuchâtel, 2010
Identifiants
Type de publication
doctoral thesis
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