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Proceedings Paper

Microlens performance limits in sub-2um pixel CMOS image sensors
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Paper Abstract

CMOS image sensors are now widely used in digital imaging systems as pixel size has steadily decreased to allow higher-resolution imaging. When the pixel size scales below 2 &mgr;m, however, microlens performance is significantly affected by diffraction from the edges of the image sensor pixel. This results not only in quantitative performance degradation, but also in a qualitative shift in functionality. We perform a systematic analysis of microlens design during lateral scaling of CMOS image sensor pixels. The optical efficiency and optical crosstalk are calculated with a first-principles finite-difference time-domain (FDTD) method. We find that there are two regimes of operation for three-metal-layer pixels depending on pixel size and wavelength: a refraction-dominated regime for pixel sizes larger than 1.45 &mgr;m and a diffraction-dominated regime for pixel sizes smaller than 1.45 &mgr;m. In the refraction-dominated regime, the microlens can be designed and optimized to perform its concentrating function. In the diffraction-dominated regime, the optimal radii of curvature for microlenses are very large and a flat microlens layer, in fact, becomes the best choice and performance is severely degraded. Under these circumstances, the microlens no longer fulfills its optical function as a focusing element. To extend the functionality of the microlens beyond the 1.45 &mgr;m node, we predict that a one-metal-layer dielectric stack or shorter will be required.

Paper Details

Date Published: 19 January 2009
PDF: 10 pages
Proc. SPIE 7250, Digital Photography V, 725005 (19 January 2009); doi: 10.1117/12.807007
Show Author Affiliations
Yijie Huo, Stanford Univ. (United States)
Christian C. Fesenmaier, Stanford Univ. (United States)
Peter B. Catrysse, Stanford Univ. (United States)

Published in SPIE Proceedings Vol. 7250:
Digital Photography V
Brian G. Rodricks; Sabine E. Süsstrunk, Editor(s)

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