A Wide Spectral Response Single Photon Avalanche Diode for Backside-Illumination in 55-nm CMOS Process

This article presents a wide-spectral response single photon avalanche diode (SPAD) designed and fabricated in advanced 55-nm CMOS image sensor technology. SPADs with different active areas and doping profiles are simulated by Sentaurus-TCAD to optimize their electrical and optical performances. A global well-sharing technique is employed to deliver a pixel pitch of $16.4 ~\mu \text{m}$ and a fill factor of 50.96% for a device with a $6 ~\mu \text{m}$ radius. The proposed structure is based on a p ⁺ /deep n-well (DNW) multiplication junction, extending its spectral response as much as possible. Compared to the existing BSI SPADs, a triple protection method is innovatively used to suppress premature edge breakdown and to reduce the dark count rate (DCR) through a combination of a virtual retrograde DNW, p-well guard ring, and a poly gate ring located above the shallow trench isolation. Furthermore, deep trench isolation is employed to suppress crosstalk. Samples of different radii from 2 to $6 ~\mu \text{m}$ are manufactured. The SPADs exhibit a low DCR below 20 cps/ $\mu \text{m}$ ² at room temperature and with a 2-V excess bias. The peak photon detection probability is 20.3% at 660 nm and is maintained at a high value, more than 10%, in the spectral range of 550–820 nm.