  This paper presents a new FDTD-based optical simulation model dedicated to
describe the optical performances of CMOS image sensors taking into account
diffraction effects. Following market trend and industrialization constraints,
CMOS image sensors must be easily embedded into even smaller packages, which
are now equipped with auto-focus and short-term coming zoom system. Due to
miniaturization, the ray-tracing models used to evaluate pixels optical
performances are not accurate anymore to describe the light propagation inside
the sensor, because of diffraction effects. Thus we adopt a more fundamental
description to take into account these diffraction effects: we chose to use
Maxwell-Boltzmann based modeling to compute the propagation of light, and to
use a software with an FDTD-based (Finite Difference Time Domain) engine to
solve this propagation. We present in this article the complete methodology of
this modeling: on one hand incoherent plane waves are propagated to approximate
a product-use diffuse-like source, on the other hand we use periodic conditions
to limit the size of the simulated model and both memory and computation time.
After having presented the correlation of the model with measurements we will
illustrate its use in the case of the optimization of a 1.75$\mu$m pixel.
