Inter-tier Coupling Analysis in Back-illuminated Monolithic 3DSI Image Sensor Pixels

This study investigates the inter-tier coupling, for a Back-Side Illuminated (BSI) 4-Transistor (4T) pixel with its diode and Transfer Gate on the bottom tier and the rest of its circuitry on the top tier of a 3D Sequential Integration (3DSI) process. Variations due to coupling are compared with variations due to temperature, showing that both effects may result in a readout error of the same order of magnitude for the top-tier readout circuit. Nevertheless, we demonstrate that in a typical rolling readout, the sequence of the pixel control signals makes the coupling effect nearly negligible. As a result, we suggest that the fabrication of an inter-tier ground plane for electrical isolation is not strictly necessary for Monolithic 3D pixels when the readout top tier is directly stacked on the photodiode bottom tier.