Extended Photodiode Scheme for Enhancement of Demodulation Contrast in Indirect Time-of-Flight Sensors

Abstract

Recent indirect time-of-flight (iToF) sensors utilize backside structure technology (BST) to improve quantum efficiency by increasing the absorption of infrared light. However, this technology causes electrons to be generated far from the pixel center, leading to degraded demodulation contrast (DC) due to inefficient charge transfer. This study presents the first in-depth analysis of how the spatial distribution of electron generation affects DC in iToF sensors using TCAD simulations, analyzing both electron transfer ratios and optical generation profiles. We introduce the concept of transfer contrast (TrC), defined as the electron transfer ratio to the memory nodes (MNs), and examine it in conjunction with the probability of optical generation to quantify localized charge transfer inefficiencies. To address the performance degradation, we propose an extended photodiode scheme with vertical and lateral expansion. This design accelerates electrons generated even at the edges of the pixel by introducing additional electric fields across the pixel region, ensuring efficient charge transport to the MN within the pulse time. The proposed scheme enhances DC by 12% and reduces parasitic light sensitivity (PLS) by 18%, with minimal fabrication complexity. This approach is compatible with various pixel sizes and offers improved depth accuracy for infrared imaging applications.