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.
Microlens arrays are used on CMOS image sensors to focus incident light onto the appropriate photodiode and thus
improve the device quantum efficiency. As the pixel size shrinks, the fill factor of the sensor (i.e. ratio of the
photosensitive area to the total pixel area) decreases and one way to compensate this loss of sensibility is to improve the
microlens photon collection efficiency. This can be achieved by developing zero-gap microlens processes. One elegant
solution to pattern zero-gap microlenses is to use a grayscale reticle with varying optical densities which locally
modulate the UV light intensity, allowing the creation of continuous relief structure in the resist layer after development.
Contrary to conventional lithography for which high resist contrast is appreciated to achieve straight resist pattern
profiles, grayscale lithography requires smooth resist contrast curve. In this study we demonstrate the efficiency of
grayscale lithography to generate sub-2μm diameter microlens with a positive-tone photoresist. We also show that this
technique is resist and process (film thickness, development normality and exposure conditions) dependent. Under the
best conditions, spherical zero-gap microlenses as well as aspherical and off-axis microlenses, which are impossible to
obtain with the conventional reflow method, were obtained with satisfying process latitude.
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.
In this paper, we present a versatile characterization method we developed at STMicroelectronics for off-axis pixels (i.e. over the image plane) on CMOS image sensor. The solution does not require optics, making it suitable for early design phases as for optimizations and investigations. It is based on a specific design of color filters and microlens masks, which consists in several blocks. Inside each block, the filters and the microlens are shifted by a given amount, relatively to the pixel. Each block is related to a given chief ray and then defines a point in the chief ray angle space. Then, the performances of these angular points can be measured by rotating the sensor, using conventional uniform illumination setup with controlled f-number. Then it is possible to map these data on the image plane, knowing the chief ray angle versus focal plane coordinate function. Finally, we present some characterizations and optimizations based on the fact that the shift is arbitrary defined during circuit layout step, so it is possible to test the sensor with higher chief ray angles than those present in the product, or to optimize the shift of the microlens versus the chief ray angle for a given pixel architecture.
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.
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 micro-electronic 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 5.6um to 4um). 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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