3.2-MP Back-Illuminated Polarization Image Sensor With Four-Directional Air-Gap Wire Grid and 2.5- $\mu$ m Pixels

A 3.2-MP four-directional polarization image sensor with air-gap wire-grid polarizer is described. The image sensor was fabricated using a wafer process and incorporates back-illumination and an antireflection layer to minimize optical flaring and ghosting problems. In testing, the sensor achieved a polarization transmittance of 63.3% and an extinction ratio of 85 at 550 nm, thereby outperforming conventional polarization sensors. The proposed sensor also exhibited good oblique-incidence characteristics, even with small polarization pixels of $2.5~\mu \text{m}$ . Based on these results, the proposed image sensor is suitable for various megapixel fusion-imaging applications, such as reducing surface reflections, highly accurate depth mapping, and condition-robust surveillance.