Design Of SPAD Laser Ranging Pixel Based On Direct-TOF Method

Single Photon Avalanche Diode(Single Photon Avalanche Diode,SPAD)has several advantages of high detection efficiency,low power consumption,fast response,etc.It has been widely studied by domestic and foreign scholars in many fields,such as high-resolution 3D imaging,molecular imaging,spectroscopy,and quantum communication.Specially,in the laser ranging imaging,the realization of a SPAD detector with high time resolution,long detection distance,low power consumption and small area has become a research hotspot at home and abroad.This thesis presents a SPAD laser ranging pixel based on Direct Time of Flight(D-TOF)method.The specific research content is as follows:First of all,the structure of SPAD device is designed and modeled.This thesis proposes a new structure of SPAD based on silicon-based CMOS process,which solves the problem of low detection efficiency for near-infrared photons of SPAD devices.The structure has a primary avalanche zone and two secondary avalanche regions.The primary avalanche zone is formed between deep Nwell and P-epitaxial layer,which enhances the detection efficiency of near-infrared short-wave photons.Simultaneously,two symmetric annular secondary regions are formed in the deep Nwell,extending the spectral response range.TCAD simulation results show the new structure achieves high photon detection efficiency(PDE)of 19.9% at the 850 nm near-infrared short wavelength band,which is about 5 times over P+/Nwell strcture.It also gets better response in the spectral range of 300nm-1000 nm.Moreover,the new structure is slightly affected by the band-band tunneling effect(BTBT),so the dark count rate(DCR)changes very little with the excess bias voltage,and the DCR is also lower than that of the P+/Nwell structure at temperatures below 20 °C.Secondly,the TOF ranging circuit in SPAD pixel is deeply studied.In this work,a TDC(Time to Digital Converter)laser ranging circuit with simple structure and large dynamic range is designed,including a 6 bit coarse counter,a 4 bit fine interpolator,and a storage and readout circuit.Circuit simulation,layout and verification of the TDC has been finished by SMIC 0.18?m CMOS process.The pitch of the SPAD pixel layout is about 70?m,and the SPAD fill factor is up to 4.3%.The fine interpolator adopts a direct delay line structure.The time resolution of the whole pixel is 208 ps,and the full-scale-range time is 193.8ns,and the Differential Nonlinearity(DNL)and Integral Nonlinearity(INL)of fine interpolator is 0.225/-0.14 LSB and 0.225/-0.151 LSB separately,and the average non-uniformity is less than 257.76 ps.Overall,the SPAD pixel designed in this work has the following advantages:(1).The SPAD device has high detection efficiency at near-infrared short wave band,and also at visible light band.Manufactured by a standard silicon-based CMOS process,the fabrication cost can be reduced significantly.(2).The 10-bit TDC ranging circuit has low power consumption,high temporal resolution,low nonlinearity and large scale time.(3).The TDC ranging circuit has a small layout area and a high fill factor.This work is an attempt in the field of silicon-based CMOS SPAD laser ranging,providing a feasible solution for SPAD 3D imaging pixel.