Fully digital, phase-domain ΔΣ 3D range image sensor in 130nm CMOS imaging technology
Walker, Richard John
Three-Dimensional (3D) optical range-imaging is a field experiencing rapid growth, expanding into a wide variety of machine vision applications, most recently including consumer gaming. Time of Flight (ToF) cameras, akin to RADAR with light, sense distance by measuring the round trip time of modulated Infra-Red (IR) illumination light projected into the scene and reflected back to the camera. Such systems generate 'depth maps' without requiring the complex processing utilised by other 3D imaging techniques such as stereo vision and structured light. Existing range-imaging solutions within the ToF category either perform demodulation in the analogue domain, and are therefore susceptible to noise and non-uniformities, or by digitally detecting individual photons using a Single Photon Avalanche Diode (SPAD), generating large volumes of raw data. In both cases, external processing is required in order to calculate a distance estimate from this raw information. To address these limitations, this thesis explores alternative system architectures for ToF range imaging. Specifically, a new pixel concept is presented, coupling a SPAD for accurate detection of the arrival time of photons to an all-digital Phase- Domain Delta-Sigma (PDΔΣ) loop for the first time. This processes the SPAD pulses locally, converging to estimate the mean phase of the incoming photons with respect to the outgoing illumination light. A 128×96 pixel sensor was created to demonstrate this principle. By incorporating all of the steps in the range-imaging process – from time resolved photon detection with SPADs, through phase extraction with the in-pixel phase-domain ΔΣ loop, to depth map creation with on-chip decimation filters – this sensor is the first fully integrated 3D camera-on-achip to be published. It is implemented in a 130nm CMOS imaging process, the most modern technology used in 3D imaging work presented to date, enabled by the recent availability of a very low noise SPAD structure in this process. Excellent linearity of ±5mm is obtained, although the 1σ repeatability error was limited to 160mm by a number of factors. While the dimensions of the current pixel prevent the implementation of very high resolution arrays, the all-digital nature of this technique will scale well if manufactured in a more advanced CMOS imaging process such as the 90nm or 65nm nodes. Repartitioning of the logic could enhance fill factor further. The presented characterisation results nevertheless serve as first validation of a new concept in 3D range-imaging, while proposals for its future refinement are presented.