Analysis of Low-Dark-Current, High-Sensitivity, Low-Optical-Crosstalk, and Small-Pixel-Size Pinned Photodiode With No-Gap Microlens

A low-dark-current, high-sensitivity, low-optical-crosstalk, and small-pixel-size pinned photodiode with no-gap microlens is designed, fabricated, and measured. Compared with nonpinned photodiodes of the same size, the dark current of a 2μm × 2μm pinned photodiode is reduced about 41.2%, while the photocurrent is reduced less than only 1% at the low reverse biased voltage Vpd=2 V under illumination of the same light source (470 nm, 450 μw/cm²). Due to the use of the pinning layer, the equivalent capacitance of the 2-m pinned photodiode is increased about 1.1 fF. In addition, the no-gap microlens technology has been developed here to improve the photodiode performance. Prototype complementary metal-oxide-semiconductor (CMOS) imagers consisting of a 640 (row) by 480 (column) array of pinned photodiodes with no-gap or conventional microlens have been designed and fabricated based on 0.18-μm CMOS process. The test results show that, compared with the photodiode with a conventional microlens, the sensitivity is increased about 74% and the optical crosstalk is reduced up to 7.2% for the photodiode with a no-gap microlens. Thus, the small-pixel-size pinned photodiode with no-gap microlens shows advantages in applications of high-resolution optical detectors and high-quality CMOS image sensors.