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
Toru Okino, Shota Yamada, Yusuke Sakata, Shigetaka Kasuga, Masato Takemoto, Yugo Nose, Hiroshi Koshida, Masaki Tamaru, Yuki Sugiura, Shigeru Saito, Shinzo Koyama, Mitsuyoshi Mori, Yutaka Hirose, Masayuki Sawada, Akihiro Odagawa, Tsuyoshi Tanaka
Abstract
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.