Comparison of Optical Properties in Al- and Cu-BEOL of CMOS Image Sensor Devices

Abstract

Two-dimensional optical simulation and experiments have been performed to investigate light propagation through a microlens and intermetal 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 Si substrate area, i.e., photodiode of complementary metal oxide semiconductor (CMOS) image sensor (CIS) pixel. It is shown that more optical carriers are generated in the Cu-BEOL for the red color due to the use of higher permittivity SiC 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 Si 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. Therefore, the structure of a Cu-BEOL in a CIS must 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.