SPAD Pixel Design For Tof-3D Image Sensor

Tof image sensor has already become a hot research topic for its broad application prospects in such field as laser three-dimensional imaging,automatic driving,robot vision,gesture recognition and so on.Compared with Tof image sensor based on indirect time difference ranging method,Tof image sensor based on direct time difference ranging method has the advantages,such as fast imaging,a little influence by photon noise and so on.This method is wildly applied in fast motion scene imaging and remote detection,and its accuracy is mainly determined by single photon avalanche diode(SPAD)detector performance and the circuit accuracy of time digital conversion(TDC)which is also the development difficulty of the technology and this paper has taken an in-depth study on this problem.First,SPAD of traditional technology has the problem of large area covered by common devices and high avalanche breakdown voltage.In this paper,based on the SMIC 0.13?m CIS process,the SPAD structure with low avalanche breakdown voltage and smaller device area is optimized.And by way of the simulation feature of Silvaco TCAD process device,meeting the requirement of low edge electric field of this SPAD structure has been proved.At the same time,11 SPAD versions in different size are designed and will be practically testified according to video test results.Second,design of SPAD Quenching Circuit.According to the design requirements,this paper adopts the improved active quenching circuit,through electric current controlling mode to improve the response speed of the circuit,and achieves a good performance of the active quenching circuit by using low-voltage CMOS transistor.The overall dead time is controlled in 5ns,SPAD Quenching Circuit has the advantages of rapid quenching,simple structure,small layout area and high integration,and a precise delay circuit is also designed by the timing requirements,which realizes the reset controlling on reset switch.Third,design of high accuracy TDC circuit.In this paper,TDC circuit composed of coarse precision measurement and fine precision measurement is designed via direct counting method and the cursor method.The fine precision measurement is the design focus,and it isrealized by the two-step delay chain of the cursor method.At last,the time resolution of ps is realized.In this paper,the optimal designed SPAD has a lower avalanche breakdown voltage,the Quenching Circuit has a very fast quenching speed and the overall dead time is about 5ns which achieved the best performance reported in the current literature.At the same time,the area of the whole pixel structure is very small,so it has a high degree of integration.At present,the research of Tof imaging sensor at abroad is in the leading position.Compared with the research result and theoretical level at abroad,the domestic still has a long way to go and is still in the level of software simulation.After finishing the design of pixel unit,the design will take an actual verification by way of the SMIC 0.13 ?m CIS process.The research on this aspect will play a significant role in promoting our research level in this field.