A Sub-100 $\mu$ m-Range-Resolution Time-of-Flight Range Image Sensor With Three-Tap Lock-In Pixels, Non-Overlapping Gate Clock, and Reference Plane Sampling

Abstract

This paper aims to propose a time-of-flight (TOF) range imager with high range resolution in the sub-100 $\mu \text{m}$ range. The range imager employs a TOF measurement technique that uses an impulse photocurrent response and a three-tap lock-in pixel based on the lateral electric field modulation. To reduce the clock skew of the gating clocks, a column-parallel digital delay-locked loop (DLL) with a dual clock tree is implemented with a short calibration time of approximately 42 $\mu \text{s}$ . A non-overlapping gate clock generation included in the skew calibration circuit effectively suppresses the photogenerated-charge partitioning for reduced nonlinearity for distance measurements. Reference plane sampling (RPS) is proposed to reduce low-frequency jitter that limits the range resolution in the light-pulse trigger. In this technique, a reference pixel array is embedded in the same focal plane of the main pixel array to sample and cancel the correlated jitter in the light trigger. The prototype range imager with $192\,\,\times \,\,4$ effective pixels is implemented in a 0.11- $\mu \text{m}$ CMOS image sensor technology. Using the RPS, a range resolution of 64 $\mu \text{m}_{\mathrm {rms}}$ has been achieved, corresponding to a 430-fs time resolution with a 25-mm range. The range resolution has a large column-to-column deviation due to the jitter induced by the column-parallel gating buffers. In noisy columns, jitter that acts as random telegraph noise (RTN) is observed.