A Short-Pulse Indirect ToF Imager Using Six-Tap Pixel with a Backside-Illuminated Structure for High-Speed Demodulation

This paper evaluates the effectiveness of a backside illumination (BSI) structure in a short-pulse indirect time-of-flight (SP-iToF) sensor employing 6-tap pixels. Impulse response measurements comparing 6-tap iToF pixels fabricated with both front-side illumination (FSI) and BSI structures demonstrate that the BSI configuration yields a significantly faster response to near-infrared (NIR) light at 850 nm. Specifically, the time constants near the response peak and tail are 0.35 ns and 0.93 ns, respectively-approximately half those observed in the FSI counterpart. Demodulation contrast (DC) measurements further highlight the advantages of the BSI structure. The BSI pixel achieves a DC of 99.5% with a gating pulse width (PW) of 10 ns, which decreases only slightly to 95.3% at a PW of 3 ns. In contrast, the FSI pixel shows a DC of 97.0% at 10 ns, but drops markedly to 80.0% at 3 ns. These improvements are primarily attributed to the thinner substrate used in the BSI sensor. The implemented 6-tap ToF sensor exhibits excellent depth linearity (<±0.8% full scale) and high resolution (<2%) across an indoor measurement range of 3 to 28 m.