5.2 A 1200×900 6µm 450fps Geiger-Mode Vertical Avalanche Photodiodes CMOS Image Sensor for a 250m Time-of-Flight Ranging System Using Direct-Indirect-Mixed Frame Synthesis with Configurable-Depth-Resolution Down to 10cm

Long-range (~250m) measurement systems with 3D resolution are highly anticipated for various applications such as automotive, surveillance and robotics systems. Direct time-of-flight (ToF) systems based on CMOS image sensors (CIS) with avalanche photodiodes with the Geiger-mode or single-photon avalanche diodes (SPAD) have been developed for these purposes. One difficulty is to meet practical requirements of long-range (~250m) measurement capability together with both lateral and depth resolution. Although indirect-ToF systems based on modulation and phase sensitive detection have fine depth resolution of the order of sub-cm, their full range is limited only to ~4m with standard Si-photodiodes [1] and ~20m with the gain assist of APD [2]. Direct-ToF systems based on SPAD pixels with time-to-digital converters (TDC) have long range measurement capability but have not reached megapixel resolution [3], [4]. Another direct-ToF of sub-range syntheses (SRS) system, which detects single photons returned from an object located in each sub-range by synchronous gating, was demonstrated to have a long range (250m) measurement capability with high lateral resolution of 10cm. However, it has a depth resolution limited by the width of optical pulse source [5]. In order to circumvent this issue, in this work, we develop an SRS-ToF system based on a 6μm pitch, 1200×900pixels, Geiger-mode operated vertical avalanche photodiodes (VAPD) CIS that enables one to configure depth resolution down to 10cm for short-distance (<; 20m) ranging by building an indirect-ToF system on the previous one [5], where a capability for long-distance (250m) ranging with a lateral resolution of 10cm is preserved. The full range comprising 15 subranges is measured in real time (30fps) with a 450fps speed of each subrange capture. Charge packets of Geiger-mode pulses accumulated on an in-pixel charge accumulator (ICA) are converted to a precise photon-count-image by a single-slope analog-to-digital converter (SSADC). In addition, by phase differentiating accumulated Geiger-mode charge on the ICA, resolving the near side of a sub-range down to 10cm is demonstrated. Thus, a Geiger-mode ToF system capable of imaging a frame with direct-indirect mixed subranges by a single CIS chip is achieved.