一种集成近红外SPAD阵列的激光雷达测距系统

LiDAR Ranging System with Integrated Near-Infrared SPAD Array

基于0.18μm BCD(Bipolar-CMOS-DMOS)工艺实现了一种高探测效率、低暗计数噪声、低误码率、高分辨率、大动态范围的近红外激光雷达测距系统。该系统集成了一个4×4单光子雪崩二极管(SPAD)阵列和一个基于延迟锁相环(DLL)的两段式时间数字转换器(TDC)。SPAD阵列采用一种新型高压p阱/高压n+埋层的深雪崩区结构,显著提高了对近红外光子的探测概率(PDP)。TDC采用双链计数器以及相位插值法,降低了段间误码概率并获得了高分辨率和大动态范围。测试结果表明,SPAD器件在5 V过偏压下的暗计数率(DCR)低至162 s^(-1),在905 nm近红外波长处的PDP大于8.5%,可工作在人眼安全阈值较高的近红外波段。TDC实现了208 ps的高时间分辨率和1.28μs的动态范围,微分非线性度(DNL)和积分非线性度(INL)分别在±0.1L_(SB)(L_(SB)代表最低有效位)和±0.6L_(SB)范围内,在50~75 MHz输入时钟下的直接光子飞行时间(dTOF)测量误差为0.37 ns。

Objective The laser ranging technology based on single-photon avalanche diode(SPAD)has been widely applied to unmanned driving,intelligent robots,and 3D imaging due to its long detection distance,high resolution,and strong antiinterference ability.The laser ranging methods mainly include direct time-of-flight(dTOF)and indirect time-of-flight(iTOF)measurement techniques.dTOF has higher anti-interference capability and a wider dynamic range than iTOF.Currently,the SPAD-based dTOF laser ranging technology is rapidly developing towards low cost and high integration with silicon-based processes.However,there are still problems such as low near-infrared light detection efficiency,poor ranging stability,and mutual constraints between time resolution and measurement error.To this end,we propose a nearinfrared laser ranging system based on a 0.18μm bipolar-CMOS-DMOS(BCD)process,which has high detection efficiency,low dark count noise,low bit error rate,high resolution,and large dynamic range.Methods The laser ranging system primarily consists of a pulse laser driver,an optical lens,a 4×4 SPAD array,quenching circuits,and a dTOF readout circuit(Fig.1).The integrated SPAD device employs a high-voltage p-well(HVPW)/high-voltage n+buried layer(HVBN)structure as the avalanche multiplication region(Fig.2).By utilizing the HVPW and HVBN,a thicker and deeply buried avalanche multiplication region is formed to enhance the absorption of near-infrared shortwave photons and improve quantum efficiency.Simultaneously,a low-voltage p-well is injected into the HVPW to increase the net doping concentration in the neutral photon collection region of the HVPW.This widens the effective photon collection region,and facilitates the transfer of optically generated electrons from the HVPW to the avalanche multiplication region,thereby triggering the avalanche effect and further improving the detection probability of near-infrared photons.Meanwhile,combined with an embedded deep-junction avalanche multiplication region,a lowdoped p-type epitaxial layer(p-Epi)is employed to form a virtual guard ring and thus reduce dark counts.This addresses the high dark count rate(DCR)in traditional p-well guard ring or deep n-well virtual guard ring structures.The dTOF readout circuit(Fig.4)mainly consists of a delay-locked loop(DLL),a phase interpolation circuit,and a counter.An offchip crystal oscillator provides a clock of 50-75 MHz,which is multiplied by four to provide a high-frequency reference clock to the circuit.The timing start signal"Start"of the time-to-digital converter(TDC)is synchronized with the laser emission signal,while the timing stop signal"Stop"is generated by narrow pulse signals produced by the quenching circuit from a detection array composed of 16 SPADs and fed into OR gates.The dTOF readout circuit achieves timing by a twostage process of an 8-bit coarse TDC followed by an 8-bit fine TDC.The coarse TDC calculates the clock cycle number of the DLL output clock Clk<0>,which is a dual-chain counter,while the fine TDC determines the start initial phase interpolation and stop final phase interpolation using the 16-phase divided clocks Clk<0-15>.Finally,this system achieves a minimum time resolution of 208 ps/312 ps and a dynamic range of 852 ns/1.28μs.Results and Discussions The proposed SPAD array and dTOF readout circuit are fabricated by the 0.18μm BCD process,and their optical and electrical characteristics are tested.Firstly,the avalanche breakdown voltage,DCR,and photon detection probability(PDP)of the SPAD devices are tested.The results(Fig.11)show that the avalanche breakdown voltage is 42.4 V in both light and dark conditions.The DCR gradually increases with the bias voltage and temperature.At a bias voltage of 5 V,the DCR is only 162 s^(-1),and it does not exceed 1000 s^(-1)at 60℃.The SPAD device exhibits a strong response in the wide spectral range of 400-940 nm.At a bias voltage of 5 V,the PDP peak at 650 nm reaches over 39%.Due to the deep avalanche multiplication region,the device also demonstrates enhanced sensitivity to near-infrared photons in the range of 780-940 nm,with a PDP of 8.5%at 905 nm.The dTOF readout circuit is tested by inputting an external 50 MHz/75 MHz reference clock.The 0th and 16th phases of the voltagecontrolled delay line(VCDL)in the DLL are introduced by four frequency dividers onto the PAD for output testing.The 0th and 16th phase signals almost completely overlap(Fig.13),indicating minimal phase error.The time accuracy of the TDC is tested by generating adjustable time interval pulses using a digital delay generator,which is then adopted as the Start and Stop signals input into the TDC circuit.The test results(Fig.14)show that the TDC linearity reaches 99.9%under 312 ps and 208 ps time resolution measurements,with measurement errors smaller than L_(SB)(L_(SB)represents least significant bit).The differential nonlinearity(DNL)and integral nonlinearity(INL)are within±0.1L_(SB)and±0.6L_(SB)respectively(Fig.15).Furthermore,the performance of the laser ranging system was tested,as shown in Fig.16.The measured values of the TOF vary linearly with the actual photon flight time,with a maximum error of 0.37 ns.Nearly 1000 consecutive single measurements are performed to evaluate the accuracy of the proposed detector,and the measurement results are concentrated around 20 ns.With a resolution of 208 ps,the RMS is only 255 ps,indicating high linearity and stability of the designed ranging system.Conclusions A near-infrared laser ranging system with high detection efficiency,low dark count noise,low error rate,high resolution,and large dynamic range is realized based on the 0.18μm BCD process.Test results show that the SPAD device achieves a DCR as low as 162 s^(-1)under 5 V excess bias voltage,and the PDP at 905 nm near-infrared wavelength exceeds 8.5%.The system can operate in the near-infrared bands with a higher eye-safe threshold.The TDC achieves a high time resolution of 208 ps and a dynamic range of 1.28μs under 50 MHz/75 MHz input clock.The DNL and INL are within±0.1L_(SB)and±0.6L_(SB)respectively.The measurement error of dTOF for photon flight time is 0.37 ns.The proposed laser ranging system features high eye-safe threshold,high sensitivity,low noise,and high linearity,providing references for low-cost and high-precision laser ranging applications.