Sensors, Vol. 25, Pages 7581: A Short-Pulse Indirect ToF Imager Using Six-Tap Pixel with a Backside-Illuminated Structure for High-Speed Demodulation

Sensors doi: 10.3390/s25247581

Authors:
Tomohiro Okuyama
Haruya Sugimura
Gabriel Alcade
Seiya Ageishi
Hyeun Woo Kwen
De Xing Lioe
Kamel Mars
Keita Yasutomi
Keiichiro Kagawa
Shoji Kawahito

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.

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Tomohiro Okuyama www.mdpi.com