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. 2023 Apr 14;23(8):3987.
doi: 10.3390/s23083987.

A Time-of-Flight Image Sensor Using 8-Tap P-N Junction Demodulator Pixels

Ryosuke Miyazawa  1 , Yuya Shirakawa  2 , Kamel Mars  2 , Keita Yasutomi  2 , Keiichiro Kagawa  2 , Satoshi Aoyama  3 , Shoji Kawahito  2   3
Affiliations

A Time-of-Flight Image Sensor Using 8-Tap P-N Junction Demodulator Pixels

Ryosuke Miyazawa et al. Sensors (Basel). .

Abstract

This paper presents a time-of-flight image sensor based on 8-Tap P-N junction demodulator (PND) pixels, which is designed for hybrid-type short-pulse (SP)-based ToF measurements under strong ambient light. The 8-tap demodulator implemented with multiple p-n junctions used for modulating the electric potential to transfer photoelectrons to eight charge-sensing nodes and charge drains has an advantage of high-speed demodulation in large photosensitive areas. The ToF image sensor implemented using 0.11 µm CIS technology, consisting of an 120 (H) × 60 (V) image array of the 8-tap PND pixels, successfully works with eight consecutive time-gating windows with the gating width of 10 ns and demonstrates for the first time that long-range (>10 m) ToF measurements under high ambient light are realized using single-frame signals only, which is essential for motion-artifact-free ToF measurements. This paper also presents an improved depth-adaptive time-gating-number assignment (DATA) technique for extending the depth range while having ambient-light canceling capability and a nonlinearity error correction technique. By applying these techniques to the implemented image sensor chip, hybrid-type single-frame ToF measurements with depth precision of maximally 16.4 cm (1.4% of the maximum range) and the maximum non-linearity error of 0.6% for the full-scale depth range of 1.0-11.5 m and operations under direct-sunlight-level ambient light (80 klux) have been realized. The depth linearity achieved in this work is 2.5 times better than that of the state-of-the-art 4-tap hybrid-type ToF image sensor.

Keywords: PN-junction demodulator (PND); depth precision; depth-adaptive time-gating-number assignment (DATA); image sensor; time-of-flight (ToF).

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Structure and principle of the two-tap p-n junction demodulator (PND): (a) Top view; (b) Cross-sectional view (X1–X1’); (c) Cross-sectional view (X2–X2’); (d) Potential diagram at the channel (X1–X1’); (e) Potential diagram at Si surface (X2–X2’).
Figure 2
Figure 2
8-tap demodulation pixel and the operations: (a) Top view of the 8-tap PND; (b) equivalent pixel readout circuits.
Figure 3
Figure 3
3D device simulation results of the 8-tap PND: (a) X-Y 2D potential plot and carrier traces to transfer to G6; (b) X-Y 2D potential plot and carrier traces to transfer to GD; (c) demodulator top view; (d) 1D potential plot (A–A’) for carrier transfer to floating diffusions, FD6 and FD2; (e) 1D potential plot (B–B’) for carrier transferring to a drain through GD only (red line) and that for carrier transferring to a drain through GD and GDO (black line).
Figure 4
Figure 4
Gate timing and its correspondence to the depth range to be measured: (a) Gate timing when all the gates are activated in every cycle and its correspondence to the distance profile of the back-reflected light intensity; (b) Gate timing when G4–G8 are activated for signal light sampling and G1–G3 are activated for ambient light sampling.
Figure 5
Figure 5
Example of the modified DATA timing diagram for cancelling ambient light.
Figure 6
Figure 6
Chip micrograph.
Figure 7
Figure 7
Response of the 8-tap outputs to the light pulse delay. (a) Response to Short Pulse (940 nm, T0 = 10 ns). (b) Response to Short Pulse (T0 = 10 ns, Normalized). (c) Response to Very Short Pulse (FWHM = 69 ps, 851 nm, Normalized). (d) Time Derivative of (c) by The Delay Time (Normalized). (e) FWHM of The Pixel Response to Very Short Pulse (FWHM = 69 ps) Measured with (d).
Figure 8
Figure 8
Experimental setup for ToF measurements under ambient-light illumination.
Figure 9
Figure 9
Measurement distance linearity and nonlinearity errors under background light conditions: (a) measured distance and non-linearity error (without error corrections); (b) measured distance and non-linearity error (with error corrections).
Figure 10
Figure 10
Measured and theoretical depth precision under ambient light illumination and dark for the distance range of 1.0 m to 11.5 m.
Figure 11
Figure 11
Depth image (1.0 m to 11.5 m) while moving a reflector board.
See this image and copyright information in PMC

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