Image sensors are semiconductor devices capable of converting electric signals into optical images. Image sensors are classified according to their various types, which include the charge coupled device (CCD) type and the complementary metal oxide semiconductor (CMOS) type. A CMOS image sensor (CIS) includes pixels arranged in two dimensions. Each of the pixels includes a photodiode (PD) component, which converts incident light into an electric signal.
As semiconductor devices become more highly integrated, image sensors become highly integrated. Accordingly, the corresponding pixels are scaled down, and the resolution becomes better.
However, existing image sensors have not been satisfactory in every respect. Therefore, it is a challenge to form a reliable image sensor.