Abstract
Mainly driven by automotive applications, there is an increasing interest in image sensors combining a high dynamic range (HDR) and immunity to the flicker issue. The native HDR pixel concept based on a parallel electron and hole collection for, respectively, a low signal level and a high signal level is particularly well-suited for this performance challenge. The theoretical performance of this pixel is modeled and compared to alternative HDR pixel architectures. This concept is proven with the fabrication of a 3.2 μm pixel in a back-side illuminated (BSI) process including capacitive deep trench isolation (CDTI). The electron-based image uses a standard 4T architecture with a pinned diode and provides state-of-the-art low-light performance, which is not altered by the pixel modifications introduced for the hole collection. The hole-based image reaches 750 kh+ linear storage capability thanks to a 73 fF CDTI capacitor. Both images are taken from the same integration window, so the HDR reconstruction is not only immune to the flicker issue but also to motion artifacts.
Highlights
A realistic scene for an automotive camera may include fast-moving objects, pulse-width-modulated light sources, and a very high dynamic background over 130 dB
The only draw-back difficulty of the lateral overflow (LOF) pixel with the easy integration of a high-value capacitor thanks to the capacitive deep trench isolation (CDTI) process. It offers a higher voltage swing, which can be is the need for an additional transistor (RST_PSUB) and for P-type well (PWELL) isolation, which may converted in enhanced dynamic range or in a higher signal-to-noise ratio (SNR)
The hole signal is the voltage reached by the pixel substrate (Vsub)
Summary
A realistic scene for an automotive camera may include fast-moving objects, pulse-width-modulated light sources, and a very high dynamic background over 130 dB. Such a scene requires the image sensor to be highly sensitive with high dynamic range capability and immunity to the flicker effect. Several methods of implementing high dynamic range (HDR) capabilities in a CMOS (Complementary Metal Oxide Semiconductor) image sensor have been developed far. They are listed below with their main advantages and draw-backs
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