5.3 An Up-to-1400nm 500MHz Demodulated Time-of-Flight Image Sensor on a Ge-on-Si Platform

Abstract

Indirect time-of-flight (iToF) image sensors on silicon platforms [1]–[3] are attracting rising attention because of their better immunity to background light and potential to achieve higher pixel resolution. Shifting the laser wavelength to 940nm [3] may further improve the outdoor performance but at a cost of reduced responsivity and lower modulation speed. As shown in red in Fig. 5.3.1, the solar irradiance at 940nm is only one-fourth of the case at 850nm, and it keeps going down with increasing wavelength and even vanishes at ~1350nm. To further increase the system SNR, using larger laser power is a feasible choice, but this introduces a laser safety issue [4]. The maximum permissible exposure (MPE) at the cornea versus laser wavelength with 0.1s exposure time is plotted in blue in Fig. 5.3.1 as well. Obviously, the exposure limit is quite stringent at 940nm, but much relaxed at a longer near-infrared (NIR) wavelength. Both curves in Fig. 5.3.1 indicate that a better SNR can be achieved with lasers emitting at a longer NIR wavelength, while the system can still satisfy the constraint of MPE. Unfortunately, traditional silicon solutions fail to operate at a wavelength beyond 1000nm because of the 1.1eV indirect bandgap. However, it is shown that this can be solved by using germanium-on-silicon (Ge-on-Si) as the absorption material [5]. Since Ge has a direct bandgap of 0.8eV, Ge-on-Si technology can extend the absorption wavelength up to nearly 1550nm.