A CMOS SPAD Imager with Collision Detection and 128 Dynamically Reallocating TDCs for Single-Photon Counting and 3D Time-of-Flight Imaging

Abstract

Per-pixel time-to-digital converter (TDC) architectures have been exploited by single-photon avalanche diode (SPAD) sensors to achieve high photon throughput, but at the expense of fill factor, pixel pitch and readout efficiency. In contrast, TDC sharing architecture usually features high fill factor at small pixel pitch and energy efficient event-driven readout. While the photon throughput is not necessarily lower than that of per-pixel TDC architectures, since the throughput is not only decided by the TDC number but also the readout bandwidth. In this paper, a SPAD sensor with 32 × 32 pixels fabricated with a 180 nm CMOS image sensor technology is presented, where dynamically reallocating TDCs were implemented to achieve the same photon throughput as that of per-pixel TDCs. Each 4 TDCs are shared by 32 pixels via a collision detection bus, which enables a fill factor of 28% with a pixel pitch of 28.5 μm. The TDCs were characterized, obtaining the peak-to-peak differential and integral non-linearity of -0.07/+0.08 LSB and -0.38/+0.75 LSB, respectively. The sensor was demonstrated in a scanning light-detection-and-ranging (LiDAR) system equipped with an ultra-low power laser, achieving depth imaging up to 10 m at 6 frames/s with a resolution of 64 × 64 with 50 lux background light.

Keywords: LiDAR; SPAD; collision detection bus; dynamic reallocation; image sensor; light detection and ranging; single-photon avalanche diode; time-of-flight; time-to-digital converter.

Figure 1

Image sensor architecture.

Figure 2

Pixel schematic with cascaded quenching.

Figure 3

(a) Poisson distribution and (b) cumulative distribution of P_N(k) at column activity of 0.17, 0.34, 0.69 and 1.39.

Figure 4

ALTDC daisy chain block diagram.

Figure 5

ALT schematic with the main functionality circuit and interface to the readout block.

Figure 6

ALTDC timing diagram associated with photon detection by ALTDC<1>.

Figure 7

TDC schematic based on a four-stage differential ring oscillator.

Figure 8

Chip microphotograph.

Figure 9

(a) DCR map and (b) DCR cumulative proportion of the whole array with 5 V excess bias voltage at 20 °C.

Figure 10

PDP measurement at excess bias voltage of 3 V, 5 V and 7 V.

Figure 11

LSB distribution of the 128 TDCs shows a standard deviation of 0.48 ps.

Figure 12

TDC (a) DNL and (b) INL measurement with different STOP frequency.

Figure 13

Peak-to-peak (a) DNL and (b) INL cumulative distribution with different STOP frequency.

Figure 14

(a) Single shot SPAD-TDC timing jitter measurement with a minimum FWHM of 2.28 LSB (b) jitter distribution of all the pixels at each TDC measurement, leading to the average and standard deviation of 2.68 LSB and 0.15 LSB, respectively.

Figure 15

Flash 3D imaging of a human subject at distance of 0.7 m with 2D intensity image inset.

Figure 16

Jitter measurement before and after calibration, where the average jitter is reduced from 10.63 LSB to 5.87 LSB.

Figure 17

(a) Measured distance up to 50 m as a function of the actual distance; (b) The maximum non-linearity and (c) worst-case precision were achieved at 6.9 cm and 0.62 cm respectively.

Figure 18

Block diagram of the LiDAR system.

Figure 19

Scan imaging of a mannequin at distance of 1.3 m with a resolution of 128 × 128, where both the depth and intensity images were obtained at the same time.

Figure 20

(a) Nine consecutive frames were recorded at 6 frames/s with resolution of 64 × 64 at 10 m, where a human subject was waving his right hand and turning around; (b) image captured with a commercial camera.

Figure 21

Proposed sensor architecture with coincidence event detection among 32 pixels, based on collision detection bus.