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
Y. Fujisaki, H. Tsugawa, K. Sakai, Hiroaki Kumagai, R. Nakamura, Tomoharu Ogita, Shunsuke Endo, Toshiki Iwase, Hiroshi Takase, Keiichi Yokochi, S. Yoshida, Shoichi Shimada, Y. Otake, T. Wakano, H. Hiyama, Kenta Hagiwara, M. Arakawal, S. Matsumotol, H. Maeda, K. Sugihara, K. Takabayashi, Masayoshi Ono, Koji Ishibashi, K. Yamamoto
Abstract
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 SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> 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.