7.4 A 413×240-pixel sub-centimeter resolution Time-of-Flight CMOS image sensor with in-pixel background canceling using lateral-electric-field charge modulators

Abstract

Time-of-Flight (ToF) range imagers have a wide range of applications, such as 3D mice, gesture-based remote controllers, amusement, robots, security systems, and automobiles. Numerous ToF range imager developments have been reported [1][2][3][4]. Recent developments are often based on CMOS image sensor technology with pinned photodiode options [5][6][7], which are suitable for cost-effective mass production. Reported CMOS ToF range imagers use singletap or two-tap lock-in pixels; to cancel the influence of background light, two or four sub-frames are used to produce a background-canceled range image. These architectures, however, have difficulty with precise range measurements of moving objects, because background light cancelation is not guaranteed for moving objects. Lock-in pixels without any charge-draining gate suffer from background light during the readout time of the operation. Another important issue with CMOS ToF range imagers for high range resolution is the speed of lock-in pixels, which must be improved to use high-modulation-frequency light or short-duration light pulses.To address these problems and requirements, this paper presents a CMOS ToF range imager using pinned-photodiode high-speed lock-in pixels with background light-canceling capability. The lock-in pixel structure, which uses lateral electric field control, is suitable for implementing a multiple-tap charge modulator while achieving high-speed charge transfer for high time resolution. Figure 7.4.1 shows the charge modulator using lateral electric field control with three-tap outputs and a drain. In this lateral electric field modulator (LEFM), 4 sets of gates (G 1 , G 2 , G 3 and G D ) are used for applying a lateral electric field in the channel region created in a pinned diode. The gates are not used for transferring photo charge under the gates, but for controlling the electric field in the Y-Y` and X 1 -X 1` directions in Fig. 7.4.1. To do this, a small positive voltage (HIGH=1.8V) and negative voltage (Low=-0.8V) are used for the operation. As shown in the cross-section and potential profile of X 2 -X 2` in Fig. 7.4.1, the depleted potential of the pinned diode can be modulated by applying a negative or small positive voltage to the gates while maintaining the potential barrier to the gate region [8]. For example, when G 1 is HIGH and the others are LOW, the potential of the LEFM (see profiles of Y-Y` and X 1 -X 1`) attracts photo electrons generated in the aperture region to be transferred to a floating diffusion of the terminal T 1 . Similarly, photo electrons can be transferred to the terminal T 2 , terminal T 3 or the drain by applying a HIGH level to G 2 , G 3 , G D , respectively, and LOW levels to the others. During signal readout, a HIGH level is applied to G D , and LOW levels to the other gates for preventing the influence of background light. To realize a large potential modulation in the pinned diode, the doping concentration of the p-type epitaxial layer and surface p+ layer for hole pinning are optimized. Figure 7.4.2...

A Time-of-Flight Range Image Sensor With Background Canceling Lock-in Pixels Based on Lateral Electric Field Charge Modulation

“…Fig. 1 shows the structure of two-tap photo-charge modulator proposed in this paper [9], [13]. In this modulator called the lateral electric field modulator (LEFM), two sets of gates (G 1 and G 2 ) for applying lateral electric field are used for charge transportation in the depleted pinned diodes.…”

“…To address these problems and requirements, this paper presents a CMOS ToF range image sensor using high-speed lock-in pixels with background light cancelling capability [9]. The proposed lock-in pixel structure uses a pinned photodiode and lateral electric field control for photo-charge modulation.…”

“…The peak depleted potential in the pinned diode is modulated to be 0.55V for the difference of the gate voltage of 2.6V (= 1.8 − (−0.8)V). The potential barrier to the gate of 0.29V is still remained when the G 1 gates are set to High, which is sufficient to prevent the leakage current from channel to gate[9]. Fig.6shows plots of equipotential lines and carrier transportations for 4 cases of applying High to G 1 , G 2 , G 3 or G D ,…”

A Time-of-Flight Range Sensor Using Four-Tap Lock-In Pixels with High near Infrared Sensitivity for LiDAR Applications

“…On the other hand, current indirect TOF sensors based on lock-in pixels are useful devices for 3D depth imaging of a few meters or up to several meters [10][11][12]. As a 1Mpixel indirect TOF image sensor has demonstrated [13], indirect types are good for having high spatial resolution.…”

A Back-Illuminated Time-of-Flight Image Sensor with SOI-Based Fully Depleted Detector Technology for LiDAR Application

“…For this reason, the system requires two-dimensional scanning, leading to higher cost and limited frame rate. The other candidate is indirect ToF sensors based on lock-in pixels [3][4][5], which is suitable for increasing the pixel count, and plenty of ToF sensors have been reported. Using indirect ToF sensors, the scanning system can be simplified to one dimension, and the resulting speed and spatial resolution can be increased.…”