CMOS image sensor pixel microlens array optimization using FDTD Solutions

Read the whitepaper (log in required)

The cost of CMOS image sensor pixel-based digital camera systems is being reduced through the use of smaller pixel sizes and larger fill-factors. However, CMOS pixel size reduction is only acceptable without a significant sacrifice in image quality. As CMOS pixel sizes continue to decrease, there is a reduction in image signal to noise as well as an increase in cross-talk between adjacent sensor pixels. These effects can be offset by adding microlenses above each CMOS image sensor pixel to focus the light onto the active detector regions, thereby increasing efficiency and reducing cross-talk.

How FDTD Solutions Performs

"Diffraction effects due to decreasing pixel sizes substantially affect photon collection and invalidate a ray tracing model. A more fundamental description is required, and we chose FDTD Solutions from Lumerical.
F. Hirigoyen, STMicroelectronics

Step 1. Construct the FDTD Solutions model of the CMOS image sensor pixel microlens array

Screenshot of CMOS image sensor pixel microlens array modeled in FDTD Solutions.

The layout editor shows the layout of the CMOS microlens array. Here we show a 2D model of the image sensor pixel array.

Different classes of objects (physical primitives, radiation sources, monitors) are color coded for easy identification. Objects can be moved and resized with simple mouse movements.

•	Materials with different optical properties are displayed with different colours for easy identification.

•	The orange boundary outlines the simulation region.

•	A single beam source is shown incident on the microlens of the central CMOS image sensor pixel.

•	The yellow line shows the position of a measurement monitor above the detector under the central microlens.

Step 2. Run the simulation, and watch the movie of the focusing light within the CMOS image sensor pixel

To gain insight into the sources of scattered light in the CMOS image sensor, use the built-in movie monitor in FDTD Solutions to capture the field dynamics of the simulation.

•	A properly designed image sensor microlens focuses light between the CMOS address lines, avoiding unwanted scattering and maximizing detector efficiency.

If your browser does not support mpeg movies, you can download a copy of the FDTD Solutions movie here.

Step 3. Examine the CMOS image sensor pixel response as simulated by FDTD Solutions

Examples of optical power flow simulation data produced by FDTD Solutions are shown in the figures below. The y-component of the Poynting vector is shown on the left hand side for three types of illumination. In this way, it is possible to measure quantities such as the point and line spread functions of the entire optical microlens array, including the effects of pixel cross talk.

The response of the CMOS microlens array to point, uniform, and step reponse illumination conditions.

Step 4. Optimize the radius of curvature of the CMOS image sensor microlenses

CMOS image sensor pixel throughput as a function of microlens radius of curvature.

Use the integrated scripting environment to construct and run a series of simulations to perform parameter sweeps and optimize performance

•	The results from forty simulations are plotted left and show that the CMOS image sensor detector efficiency is a strong function of the microlens radius of curvature.

We gratefully acknowledge the collaboration and assistance of Axel Crocherie, Flavien Hirigoyen, Jérôme Vaillant and Yvon Cazaux of STMicroelectronics, France.

See how easily FDTD Solutions can assist you with your design efforts! Download a free 30 day trial and request that a technical expert contact you.


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