Owing to outstanding advantages such as small size, low power consumption, and high time resolution, direct time of flight (dTOF) detectors have attracted great interest in numerous fields, including automotive driving, facial recognition, augmented and virtual reality (AR/VR), and 3D imaging. Such systems exploit a fast on-chip time-to-digital converter (TDC) in conjunction with single-photon avalanche diode (SPAD) to measure the TOF value, which can achieve high interference immunity and wide dynamic range. Currently, they are rapidly developing towards the direction of low-cost and high integration density compatible with the silicon-based process. However, some problems still exist in practical applications, such as the low safety threshold of human eyes and mutual restriction between resolution and dynamic range. To this end, we implement a near-infrared single-photon dTOF detector with high sensitivity, high time resolution, and wide dynamic range based on 0.18 μm Bipolar-CMOS-DMOS (BCD) technology.

The detector is mainly composed of a TDC circuit, 16 SPADs, and an analog front-end (AFE) circuit that is coupled with each SPAD (Fig. 3). The integrated SPAD device (Fig. 1) adopts a new structure with a deep high-voltage p well (HVPW)/buried n⁺ junction as avalanche multiplication region to significantly improve the near-infrared photon detection probability (PDP). A certain distance is set aside between the high-voltage n well (HVNW) and HVPW, and is the guard ring width (d_GRW). The virtual guard ring formed by the low doping p-type epitaxial layer helps lower the dark count noise. The proposed SPAD devices with d_GRW is 0.5, 1.5, 2.5, and 3.5 μm respectively are simulated by technology computer-aided design (TCAD) based on 0.18 μm BCD technology to study the influence of d_GRW on device performance (Fig. 2). The simulation results show that the device can work normally at d_GRW=2.5 μm, and the high avalanche field can be obtained to ensure a higher detection probability for near-infrared photons. Meanwhile, the low field in the guard ring region can avoid the dark count noise caused by the carriers generated at the shallow trench isolation (STI) interface. In the readout circuit, the AFE circuits are directly connected with the SPADs to cut off avalanche current and generate narrow pulses. These narrow avalanche pulses are combined by OR tree into one signal which is fed into TDC as a stop signal. Furthermore, a three-step hybrid TDC consisting of the coarse counter, fine counter, interpolator, and phase-locked loop (PLL) is designed to obtain high time resolution and wide dynamic range (Fig. 4). The PLL is a third-order type-Ⅱ loop and the voltage-controlled oscillator (VCO) is composed of a four-stage ring oscillator to offer four-channel multi-phase clocks (P1, P2, P3, and P4) with low jitter, low phase noise, and uniform phase distribution. The fine counter adopting an asynchronous counter can not only count the clock number of P1 but also generate a lower frequency clock to drive the coarse counter. The coarse counter driven by a lower frequency clock adopts a synchronous counter with a linear feedback shift register structure, which can easily expand the dynamic range by increasing the number of counter bits. The start/stop interpolator employs D flip-flop and transmission gate (TG) to latch the state of four-channel multi-phase clocks when the rising edge of the start and stop signal arrives, achieving a high resolution which is 1/8 period of the clock P1.

The proposed dTOF detector is fabricated in the 0.18 μm BCD technology and its electrical and optical properties are verified. The I?V characteristic of the SPAD is firstly measured with avalanche breakdown voltage of around 42.5 V, which matches well the TCAD simulation results (Fig. 7). DCR measurement results (Fig. 8) show that the DCR variation with temperature is not obvious and the overall level is lower than 1 kHz when the temperature is below 60 ℃. More importantly, the data demonstrates excellent performance of 200 Hz at 24 ℃ and 5 V excess bias voltage (V_ex). The PDP measurements (Fig. 9) reveal that the PDP reaches a peak of 43.3% (600 nm) at V_ex=5 V. Additionally, due to the deep avalanche region, there is a higher response sensitivity for near-infrared photons (780-940 nm), and a PDP of 7.6% is obtained at 905 nm. The measurement is performed by the external triggering to evaluate the dTOF readout circuit performance. Driven by a 30 MHz clock, the dTOF readout circuit can achieve a high resolution of 130 ps and a dynamic range of 258 ns (Fig. 11), with a differential nonlinearity (DNL) and integral nonlinearity (INL) less than ±1 LSB (1 LSB=130 ps) respectively (Fig. 12). In addition, the precision of the proposed detector has also been evaluated by carrying out almost 1000 consecutive single-shot measurement for different fixed TOF values. The measured results show that the statistic histogram of the fixed TOF (80 ns) presents Gaussian distribution and the peak histogram data matches well with the actual TOF (Fig. 13).

A near-infrared single-photon dTOF detector with high sensitivity, high time resolution, and wide dynamic range is implemented by the 0.18 μm BCD technology. The test results show that at V_ex=5 V, the PDP peak of the SPAD reaches 45%, the near-infrared PDP at the 905 nm wavelength is larger than 7.6 %, and the dark count rate (DCR) is as low as 200 Hz. Furthermore, the TDC circuit driven by multi-phase clocks with low jitter, low phase noise, and uniform phase distribution achieves a high resolution of 130 ps and a dynamic range of 258 ns with excellent linearity. The proposed dTOF detector features a high safety threshold for human eyes, high sensitivity, low noise, and high linearity, which is suitable for the application of low-cost and high-precision lidar systems.