Axel Crocherie

Uniform illumination and rigorous electromagnetic simulations applied to CMOS image sensors.

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.

FDTD-based optical simulations methodology for CMOS image sensor pixels architecture and process optimization

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&mgr;m pixel.

Three-dimensional broadband FDTD optical simulations of CMOS image sensor

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.

3D TCAD simulation of advanced CMOS image sensors

This paper presents a full 3-Dimensionnal TCAD simulation methodology for advanced CMOS image sensors. In order to consider 3D process effects, full 3D TCAD process simulations have been carried out on different advanced pixels. Based upon the obtained 3D doping distributions, 3D opto-electrical device simulation results have been compared to both 2D based approaches and experimental results. Full 3D simulation results show a qualitative agreement with measurements.

Finite-difference time domain based electro-optical methodologies to improve CMOS image sensor pixels performances

The current CMOS image sensors market trend leads to achieve good image resolution at small package size and price, thus CMOS image sensors roadmap is driven by pixel size reduction while maintaining good electro-optical performances. As both diffraction and electrical effects become of greater importance, it is mandatory to have a simulation tool able to early help process and design development of next generation pixels. We have previously introduced and developed FDTD-based optical simulations methodologies to describe diffraction phenomena. We recently achieved to couple them to an electrical simulation tool to take into account carrier diffusion and precise front-end process simulation. We propose in this paper to show the advances of this methodology. After having detailed the complete methodology, we present how we reconstruct the spectral quantum efficiency of a pixel. This methodology requires heavy-to-compute realistic 3D modeling for each wavelength: the material optical properties are described over the full spectral bandwidth by a multi-coefficient model, while the electrical properties are set by the given process and design. We optically simulate the propagation of a dozen of wavelengths at normal incidence and collect the distribution of the optical generation then we insert it in the electrical simulation tool and collect the final output quantum efficiency. Besides, we compare the off-axis performance evaluations of a pixel by simulating its relative illumination in a given wavelength. In this methodology several plane waves are propagated with different angles of incidence along a specific direction.