Direct time-of-flight SPAD image sensors for light detection and ranging

Depth sensing is an increasingly important feature in many applications of consumer, automotive, augmented/virtual reality (AR-VR), space and bio-medical imaging. Long range, high depth resolution, high spatial resolution, and high frame rates are often conflicting requirements and difficult to be simultaneously achieved due to extreme operating conditions. Direct time-of-flight (DTOF) has evolved to becoming a powerful technique to perform light detection and ranging (LiDAR). Thanks to advances in low-jitter optical detectors, such as single-photon avalanche diodes (SPADs), and accurate chronometers like time-to-digital converters (TDCs), picosecond timing resolution is possible, thus enabling millimetric depth resolutions.

High ambient light is an inevitable challenge in LiDAR applications, whose levels may exceed up to 100 klux on a bright sunny day, making it particularly challenging to detect a target submerged within an overwhelming noise floor. High ambient light operation can be accommodated by means of optical filtering, a higher laser power or temporal filtering techniques. Optical filtering is often restricted to a narrow, 10-50 nm bandwidth, insufficient at high ambient light levels. Higher laser power is not always possible, due to eye safety regulations and power constraints. Temporal filtering such as time gating and coincidence detection can thus be powerful tools to cope with high ambient light.

This thesis focuses on the design of DTOF sensors for LiDAR. To that end, two SPAD-based DTOF sensors are designed. The first sensor is designed in a 3D-stacked 45/65 nm CMOS technology, thus, enabling a modular architecture where the module itself comprises of 8x16 pixels. With a 60 ps-resolution TDC at its core, the sensor provides centimetric accuracy up to 300 m range in free space. The second sensor, named Jatayu, advances the previous design by hosting 256x128 pixels, thereby, significantly improving on its spatial resolution. While retaining its modularity, Jatayu also enables multi-level coincidence detection and progressive time-gating to suppress background light. To the best of the author’s knowledge, progressive gating has been implemented in a LiDAR for the first time in this thesis. Designed in a 3D-stacked 45/22 nm CMOS technology, the sensor achieves under 7 cm accuracy over 100 m ranging and 10 klux background light. With its capability of acquiring 128£128, 3D depth maps of high dynamic range scenes, Jatayu is highly suitable for a variety of imaging applications in many different scenarios.