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

Abstract

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.

Ray Tracing-Guided Design of Plenoptic Cameras

“…The pixel response on a real sensor does not only depend on the wavelength of the incoming light, but also on its incident angle [24]. Accordingly, an angular threshold α can be found, so that the contribution of incoming light with a larger angle to the sensor normal can be assumed to be negligible.…”

Deep Trench Isolation and Inverted Pyramid Array Structures Used to Enhance Optical Efficiency of Photodiode in CMOS Image Sensor via Simulations

“…To calculate the power absorbed in each pixel, we integrate Poynting vector over the depletion region of the pixel. The optical efficiency (OE) is defined as the fraction of the power incident onto the pixel, that is absorbed in the depletion region of the pixel [17,18]:…”

“…An electromagnetic simulation tool based on FDTD Solutions [15,16], available from Lumerical [17], was used to describe light propagation and photon collection inside a pixel. A finite-difference time-domain (FDTD)-based optical simulation mode for describing the optical performance of CMOS image sensors considering diffraction effects has been proposed [18]. Periodic conditions are used to limit the size of the simulated model and thus reduce both memory usage and computational time.…”

Submicrometer Nanostructure-Based RGB Filters for CMOS Image Sensors

“…The biggest effect can be seen when working in the sub- It should be noted that in large-scale color filter arrays multiple ways can be employed to reduce the crosstalk. The use of a micro-lens array on top of the filters could focus the incoming light, thus limiting the surface area of interaction 57,58 . A similar effect can be achieved by adding a mask layer acting as spatial filter, though this is connected to reduction of the effective sensor area.…”

“…In the general case, the structure was excited by a normal-incidence plane wave source with a spectrum covering the visible spectral range, while the angle dependence simulations were done using a broadband fixed angle source technique (BFAST) to ensure a constant angle for all of the wavelengths within the range of interest. The simulations of the color filter array were done using the same PBC and PML boundaries, but extending the simulation domain to a pixel array of 2 x 2 pixels 57 . The simulation considered a semi-infinite substrate.…”