Backside-Illuminated CMOS Photodiodes With Embedded Polysilicon Grating Reflectors

In this study, we investigated the feasibility of replacing metal mirrors with polysilicon gratings to serve as compact optical back reflectors in thin backside-illuminated CMOS photodiodes (BSI CPDs). The unique reflective properties of polysilicon grating reflectors can be implemented within a very small area (12 μm2); i.e., between the contact vias of BSI CPDs. The proposed scheme achieves high optical reflectivity for TE-polarized light at wavelengths exceeding 100 nm, while improving responsivity at near-infrared wavelengths. The resulting BSI CPDs were shown to enhance photocurrent by 1.45× in a polarization-dependent manner (I
_TE
/I
_TM
= 1.148) at a wavelength of 980 nm.