A back-illuminated 6 μm SPAD depth sensor with PDE 36.5% at 940 nm via combination of dual diffraction structure and 2×2 on-chip lens

We present a back-illuminated 3D-stacked 6 $\mu \mathrm{m}$ single-photon avalanche diode (SPAD) sensor with very high photon detection efficiency (PDE) performance. To enhance PDE, a dual diffraction structure was combined with $2\times 2$ on-chip lens (OCL) for the first time. A dual diffraction structure comprises a pyramid surface for diffraction (PSD) and periodic uneven structures by shallow trench for diffraction formed on the Si surface of light-facing and opposite sides, respectively. Additionally, PSD pitch and SiO2 film thickness buried in full trench isolation were optimized. Consequently, a PDE of 36.5% was achieved at $\lambda=940$ nm, the world’s highest value. Owing to shield ring contact, crosstalk was reduced by about half compared to a conventionally plugged one.