This paper describes a new methodology we have developed for the optical simulation of CMOS image sensors. Finite Difference Time Domain (FDTD) software is used to simulate light propagation and diffraction effects throughout the stack of dielectrics layers. With the use of an incoherent summation of plane wave sources and Bloch Periodic Boundary Conditions, this new methodology allows not only the rigorous simulation of a diffuse-like source which reproduces real conditions, but also an important gain of simulation efficiency for 2D or 3D electromagnetic simulations. This paper presents a theoretical demonstration of the methodology as well as simulation results with FDTD software from Lumerical Solutions.
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 1.75µm pixel.
In this paper, we present the results of rigorous electromagnetic broadband simulations applied to CMOS image sensors as well as experimental measurements. We firstly compare the results of 1D, 2D, and 3D broadband simulations in the visible range (380nm-720nm) of a 1.75µm CMOS image sensor, emphasizing the limitations of 1D and 2D simulations and the need of 3D modeling, particularly to rigorously simulate parameters like Quantum Efficiency. Then we illustrate broadband simulations by two proposed solutions that improve the spectral response of the sensor: an antireflective coating, and the reduction of the optical stack. We finally show that results from experimental measurements are in agreement with the simulated results.
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This paper describes a new methodology we have developed for microlens optimization for CMOS image sensors in order to achieve good optical performances. On one hand, the real pixel is simulated in an optical simulation software and on the other hand simulation results are post-processed with a numerical software.In a first part, we describe our methodology. We start from the pixel layout description from standard microelectronic CAD software and we generate a three-dimensional model on an optical ray tracing software. This optical model aims to be as realistic as possible taking into account the geometrical shape of all the components of the pixel and the optical properties of the materials. A specific ray source has also been developed to simulate the pixel illumination in real conditions (behind an objective lens). After the optical simulation itself, the results are transferred to another software for more convenient post-processing where we use as photosensitive area a weighted surface determined from the fit of angular response simulation results to the measurements. Using this surface we count the ray density inside the substrate to evaluate the simulated output signal of the sensor.Then we give some results obtained with that simulation process. At first, the optimization of the microlens parameters for different pixel pitches (from 5.6µm to 4µm). We also have studied the polarization effects inside the pixel. Finally, we compare the measured and the simulated vignetting of the sensor, demonstrating the relevance of our optical simulation process and allowing us to study solutions for a pixel pitch of 3µm and less.
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