Design Of The Vertical Dual-junction Single Photon Imaging Unit Based On CMOS Technology

In the visible and the near-infrared wavelength range,silicon single-photon avalanche diodes(SPADs)as the core devices of silicon single-photon detector have been extensively studied and implemented for detection of weak optical signals.Over the past decade,especially after the integration of SPADs in the CMOS technology,they are widely used in many fields such as fluorescence spectroscopy,biology,quantum information,and quantum optics.CMOS image sensors with the stacked junction photodiodes are well known because they allow spectral discrimination without Bayer pattern color filters.Inspired by this idea,many scholars have studied stacked junction CMOS single-photon avalanche diodes due to superior performance of spectral separation,especially in weak light condition.In this paper,refer to existing structures,a vertical stacked dual-junction structure device which can be used to detect the blue violet and near-infrared light at the same time is proposed.First of all,to the edge of the single photon avalanche photodiode effect,a photon detection efficiency and dark count rate and other performance parameters are comprehensively described,and three of SPAD quenching mode(passive quenching,active quenching and gating quenching mode)are analyzed and compared,their respective advantages and disadvantages and applicable condition are obtained.Then,a dual-junction structure based on SOI technology is proposed.The shallow junction is defined by the p+ region and n-type charge sheet,and the virtual protection ring is composed of the retrograde n-well.The deep junction is composed of deep n-well and p-type epitaxial layer,and it is dependent on the gradient of the epitaxial layer to form a virtual guard ring on the periphery of the junction,effectively avoiding the edge breakdown.The simulation is based on Silvaco Atlas 5.20.2 R semiconductor device simulation tools which released by the Silvaco company.The single photon avalanche photodiode's doping concentration,V-I characteristic curve,the field intensity distribution,the photon detection efficiency and dark count rate and other parameters is simulated and calculated,which verified the proposed structure has reached the predefined requirements,can be used for single photon detection.At the same time in order to reflect the good performance of the proposed structure,the SPAD structure proposed is compared with the existing structure of a dualjunction.The photon detection efficiency shows that the design of the single photon avalanche photodiode achieve satisfactory results in blue and near infrared light.Finally,in this paper,the SPAD quenching and resetting circuit which are located in pixel unit are designed and optimized in detail.At the same time,the equivalent circuit model of SPAD is set up,the active quenching circuit is adopted,and the delay circuit design is completed.The design circuit is simulated using Cadence simulation tool.The simulation results show that the proposed scheme effectively reduces the dead time of SPAD(approximately 2.5 ns),and improves the detection frequency of the detector.At the end of the paper,the layout of the whole pixel unit is completed by using layout software.