Full well capacity is one of the major characteristics of Complementary Metal-Oxide-Semiconductor (CMOS) image sensors. The full well capacity is a measurement of how much charges an image sensor may store before the charges overflow. The full well capacity determines the dynamic range of the image sensors. A high full well capacity means that the respective image sensor may have a great difference between the brightest level and darkest level of sensed signals. Hence, the full well capacity needs to be known to determine the quality of the image sensor chip. If necessary, the manufacturing process of the image sensor chips may be adjusted when the respective full well capacity does not meet design requirement.