Measurement and Analysis of Substrate Noise Coupling for N+/P Photodiodes on Epi-Type and Low Doped Substrate.

CMOS Image Sensors are typical mixed signal circuits because they integrate on the same die digital functions close to photodetection and amplification circuits. As a consequence, noise emitted by the digital switching nodes or power supply can be coupled to the analog circuits by the common substrate in addition to any other crosstalk phenomena. Measurement and analysis of substrate coupling in such systems are therefore necessary to prevent important performance degradations of the analog circuits including the photodetection ones. To investigate substrate noise coupling, we designed test chips with the same integrated noise source and different types of photodiodes and guard ring structures on Low Doped and Heavily Doped Substrates. The power supply noise of our source is first studied by following the ICEM methodology [1]. Then substrate noise coupling is characterized by measuring the photocurrent disturbances seen at the photodiode side. Impact of process technology and guard ring structures on substrate noise coupling is then discussed to provide a better understanding of the related physical mechanisms involved.