A rolling-shutter distortion-free 3D stacked image sensor with −160dB parasitic light sensitivity in-pixel storage node

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Conventional CMOS image sensors widely used in products currently on the market are mainly equipped with a rolling exposure function. This rolling exposure causes so-called “Jell-o effect” distortion when capturing a moving target. CMOS image sensors with a global-shutter function are one of the solutions to avoid this distortion. An in-pixel storage node is required to create a global-shutter CMOS image sensor. A floating diffusion and an additional capacitor can be used as an in-pixel storage node [1,2]. The light sensitivity of the in-pixel storage node is specified by the parasitic light sensitivity (PLS), which is the ratio of the light sensitivity of an in-pixel storage node and the light sensitivity of a photodiode. The PLS should be small enough so that the in-pixel storage is not light-sensitive. Artifacts are captured in an image from bright moving objects during read-out if the PLS is not small enough. The PLS of reported global-shutter CMOS image sensors is around -100dB. That would be small enough to use those image sensors in fields where the light source can be controlled. However, for DSC usage, users can easily encounter scenes with bright objects (e.g. sunlight or car headlights). Even if the in-pixel storage node is light-shielded, it is difficult to perfectly protect the in-pixel storage node from photo-generated carriers, as long as the in-pixel storage node and a photodiode are on the same silicon substrate. Meanwhile, 3D stacking technologies have been introduced for image sensors to give them more functionality and improved performance [3,4]. The reported minimum interconnection pitches for image sensors are over 20μm. These technologies do not fit the smaller pixel pitches of the image sensors in recent DSCs. In this paper, we report a rolling-shutter distortion-free 3D stacked image sensor with an in-pixel storage node of -160dB parasitic light sensitivity. The image sensor virtually achieves a global-shutter function using a 4-times frame-shutter operation. The image sensor has 2 semiconductor substrates, where 1 substrate has a backside-illuminated photodiode array and the other a storage-node array. The image sensor achieves a PLS level of -160dB. The image sensor has 8.6μm pitched interconnections, and an interconnection yield of over 99.9% is achieved.