ToF: Sony iToF and CAPD Analysis
1. Sony’s iToF and CAPD
CAPD pixel technology achieves high-efficiency signal output
SoftKinetic’s CAPD method reads signals at differing phases utilizing light source flight time, multiple times, and then outputs the signal ratio to convert to the distance. Generally, in order to increase the depth map’s signal-to-noise ratio (SNR), it is necessary to place multiple readout circuits inside the pixel to read multiple signals at different phases accurately so that the delay of reflected light can be calculated precisely, as well as to improve the usage efficiency of the reflected light signal. Depth measurement accuracy also depends on the capacity to use a faster modulation frequency.
To satisfy all of these requirements, CAPD dynamically creates a potential gradient (slope for electron transfer) and performs high-speed transfer via internal pixel drift current.*3 This design ensures efficient, high-speed transfer of electrons converted from reflected light in the photo detector between two collectors, making it possible to obtain signals of different phases more accurately.
- *The depth accuracy is governed by the relationship shown in the following equation.
- σdepth:Variation of measured distance, Fmod: Modulation frequency, Cmod: Signal distribution contrast ratio
2. CPAD KPI
Figure 5 summarizes the main ToF metrics determining the device ToF performance, showing state-of-the-art ToF performance for 10um pixel pitch. The table also includes a figure of merit describing the ToF performance not taking into account pixel and system noise floor. First part of the Figure of Merit (FoM) describes how well the device can convert photons into electrons using the Fill-Factor and responsivity values reported around 850nm. The second part of the FoM revolves around the ToF performance of the device, focusing on the modulation contract (MC) at the reported modulation frequency (Fmod). In the active light shot noise limit case, the first part contributes with the square-root to the system performance, while the second ToF part contributes in a linear way, hence the FoM used in this work. For the comparison table, only performance metrics from imagers with QVGA or higher resolution are used. 
3. The Pros and Cons of Sony CAPD
Different from the working principle of Pinned photodiode modulation device that uses MIS modulation gate structure to modulate electric field to realize the separation of charge signals, Sony CAPD uses the device structure shown in Figure below, DETA and DETB are two collection electrodes, and VmixA and VmixB are modulation electrodes. . When the voltage applied between VmixA and VmixB is zero, DETA and DETB will collect the same amount of light to generate electrons when light is irradiated on the device. If a voltage is applied between VmixA and VmixB, a hole current will be generated between the two modulation electrodes, the ohmic drop is generated through the p-type epitaxial layer, and the generated electric field guides the electrons generated by the light to be collected by the two collection nodes of DETA and DETB.
The modulation electrode of this structure is directly in contact with Si. Compared with traditional MIS modulation gate devices, the modulation electric field can penetrate the silicon substrate, so it has good sensitivity and demodulation contrast. In addition, at the two collection electrodes, the charge integration method can be used for signal readout like a traditional photodiode pixel sensor, thereby reducing the circuit complexity of the pixel. The important thing about the CAPD structure is that it is a non-surface device. The body region photocurrent generated by the photoelectric process can also be quickly transferred to the external integration node through the body region longitudinal modulation electric field of the structure. It is a device form with high modulation contrast. But the main disadvantage of this structure is that it is a modulating device for resistive load, which makes its heat loss and driving load pressure larger, and thermal noise will also accompany the entire modulation process. Therefore, this structure is not suitable for high resolution and large load conditions. The modulation is used under. It is believed that Sony will also introduce new device structures in future I-TOF pixel units.
As shown in Figure above, in the FSI process, the front-illuminated process structure adopted by SONY can achieve a demodulation contrast of 40% at 100 MHz. This parameter is already considered the best among the same generation of processes and products. However, as the modulation frequency of the FSI process CAPD pixel increases, its modulation contrast is still significantly reduced.
With the maturity of the BSI and stacking process, the latest generation of SONY has gradually adopted BSI CAPD pixels, making it possible to achieve higher modulation contrast at higher modulation frequencies, thereby further optimizing the performance parameters of the device, making it at 100MHz The demodulation contrast can reach 85% under the modulation frequency, and the ranging accuracy can reach 5.9mm@1m, which is more satisfied with the high resolution and high ranging accuracy requirements of I-TOF applications.