Polysilicon Alignment Marks for Backside Illuminated Image Sensors

The patent introduces polysilicon alignment marks as an intermediary reference structure that bridges frontside and backside processing operations. These marks are formed on the frontside substrate and serve as a common reference point for both frontside photodiode formation and backside CFA formation, enabling accurate alignment between features formed in separate processing steps on opposite sides of the substrate.

The patent extends the alignment reference from the traditional frontside-only plane to include through-substrate alignment. By forming alignment marks on the frontside that can be referenced during backside processing, the system creates a three-dimensional alignment framework that spans both sides of the substrate, enabling precise registration of features across different processing dimensions.

The patent performs preliminary formation of polysilicon alignment marks on the frontside substrate before backside processing operations. These marks are established as fixed reference points that guide subsequent backside CFA formation, ensuring that alignment is predetermined and controlled rather than relying on post-processing registration attempts.

The patent replaces conventional mechanical or optical alignment methods with a material-based alignment system using polysilicon marks. Instead of relying on mechanical positioning or optical alignment techniques during backside processing, the system uses the physical presence and position of polysilicon alignment marks as the primary alignment mechanism, which can be directly referenced during lithography.