Research And Design Of Time-of-flight Detector Pixel Based On SPAD

Time-of-flight(TOF)technology has been widely applied in 3D imaging,laser ranging and space exploration due to its high sensitivity and large dynamic range.TOF detector achieved by silicon-based single photon avalanche diode(SPAD)is characterized by high gain,low cost and fast response.However,limitions of existing SPAD detectors arise from low measurement accuracy and vulnerability to noise.Furthermore,low detection efficiency at near-infrared short-wave is not capable of guaranteeing eye saftey.In this thesis,noise monitor method and pipeline technology are studied and applied.A SPAD-based pixel is designed which has good performances of high accuracy,low noise and high detection efficiency at near-infrared short-wave.The research contents and results are as follows:(1)Modeling and research on SPAD temporal response.A simple analytical modeling method is proposed to directly calculate and predict timing jitter.The current variation in the depletion layer is researched instead of direct simulation on impact ionization which is difficult to be implemented.According to the statistical distribution of avalanche buildup time at different positions,the jitter error of the depletion layer is calculated.Considering that minority carriers drift,diffuse and recombine in a weak electric field,the exponential tail of the neutral layer is investigated.By means of the accurate parameters provided by TCAD simulation,the jitter error and exponential tail of traditional p+/n-well SPAD device are calculated.The computation results agreeing with the measurement indicate the accuracy and feasibility of this modeling method.In summary,this modeling method is capable of accurately predicting timing jitter for different SPAD devices,requiring short operation time and low computational power consumption.(2)Research and design on SPAD device structure.Two new SPAD device structures are designed based on dielectric layer process and BCD(Bipolar-CMOS-DMOS)process to address insufficient photon detection efficiency at near-infrared short-wave.The diffraction windows are manufactured in the dielectric layer of p-well/deep n-well SPAD,which increases the photon generation rate in the avalanche region.A detection efficiency gain is obtained owing to the total enhancement of quantum efficiency,meanwhile,dark noise increasement is avoided.Another way is the design of medium voltage p-well/n+ buried layer SPAD device fabricated in BCD technology.A deep PN junction formed between the n+ buried layer and the medium voltage p-well is used as avalanche multiplication region.Therefore,the photon detection efficiency ranging from 800 nm to 900 nm wavelengths is obviously enhanced.In addition,a lightly and uniformly doped p-type epitaxial layer as virtual protection ring allows for device dimension and dark noise reduction.(3)Research and design on pixel and circuits.Photon event discriminator and two-stage pipeline TDC(Time-to-Digital Converter)are shared between four SPADs to suppress noise and achieve accurate detection in each pixel.The responses triggered by noise can be coincidentally discriminated and promptly stopped by employing photon event discriminator.This discriminator is applied with a variable threshold adapting to different background and dark noies.A time amplifier is designed in two-stage pipeline TDC to amplify the time residue from the former coarse measurement.The further fine measurement provides a high accuracy for TOF detection.The simulation results indicate that noise can be discriminated and suppressed by the photon event discriminator during TOF detection.Crucially,a high detection accuracy achieved by pipeline TDC attributes to 42.3 ps time resolution.