Two-dimensional optical simulation on a visible ray passing through inter-metal dielectric layers of CMOS image sensor device

Two-dimensional optical simulation has been performed for investigating light propagation through a micro lens and inter-metal dielectric (IMD) layers in an Al and Cu back-end of line (BEOL) onto a Si photodiode, and its effects on the wave power, as well as optical carriers generated by a visible ray in the silicon substrate area, i.e. photodiode of a CMOS image sensor pixel. The number of optically generated carriers in an Al-BEOL has been compared to a Cu-BEOL. It is shown that more optical carriers are generated in the Cu-BEOL for the red color because a higher permittivity dielectric material like SiC is used in the Cu-BEOL to prevent Cu from diffusing into the dielectric material, resulting in higher optical loss in the higher- permittivity dielectric layers. Thus, the optical power density arriving in the silicon substrate is higher in the Al-BEOL than in the Cu-BEOL when the wavelength is blue (470 nm) or green (550 nm) in the visible ray spectrum. In conclusion, the structure of a Cu-BEOL in a CMOS image sensor has to be optimized for generating more optical carriers through lower-permittivity IMD materials or by reducing the permittivity difference between SiC (or SiN) and IMD materials, without deteriorating the capability as a barrier to Cu diffusion.