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The optimization of zero-spaced microlenses for 2.2um pixel CMOS image sensor

In CMOS image sensor, microlens arrays are generally used as light propagation carrier onto photo diode to increase collection efficiency and reduce optical cross-talk. Today, the scaling trend of CMOS technology drives reduction of the pixel size for higher integration density and resolution improvement. Microlenses are typically formed by photo resist patterning and thermal reflowing, and the space between photo resist is necessary to avoid merging of microlenses during thermal reflow process. With the shrinking sizes, microlenses become more and more difficult to manufacture without their merging. Hence, the key of light loss free microlens fabrication is still zero-space between microlenses. In this paper, we report the selection of the optimum shape of microlens by the dead space and the curvature of radius. The improvements of critical dimension and thickness uniformities of microlens are also reported.