Radiation-Induced Leakage Current and Electric Field Enhancement in CMOS Image Sensor Sense Node Floating Diffusions

Abstract

This paper investigates the leakage currents as well as the leakage current Random Telegraph Signals (RTSs) sources in sense node floating diffusions (FDs) and their consequences on imaging performances specifically after exposure to high-energy particle radiation. Atomic displacement damage and ionization effects are separately studied thanks to neutron and X-ray irradiations. Proton irradiations have been performed to simultaneously study displacement damage dose (DDD) and total ionizing dose (TID) effects while being more representative of the space environment. The studied DDD ranges from 500 TeV • g −1 to 40 GeV • g −1 , and the TID ranges from 24 krad(SiO 2) to 72 krad(SiO 2). High-magnitude electric field effects, such as transfer-gate-induced leakage current, are investigated to further understand the phenomena involved in FDs while giving new insights into the Electric Field Enhancement of the charge generation mechanisms. This paper shows that FDs are very sensitive to ionizing radiation because of the presence of depleted Si/SiO 2 interface with high-magnitude electric fields around the junction. On the other hand, displacement damage in the FDs is a major source of high amplitude leakage current RTSs and leakage current nonuniformity. Such radiation-induced degradations can prevent the use of CMOS image sensor with long FD retention time (e.g., global shutter operating mode or burst imagers) in radiation environments.

Radiation-Induced Variable Retention Time in Dynamic Random Access Memories

“…Such mechanism seems possible but disagrees with some experimental observations such as the fact that increasing the electric field magnitude enhances the RTS center generation rate but not its time constant (which depends on the modulator generation rate that should also be enhanced by the electric field) as can be seen in [29] (Fig. 15), in [37] (Fig. 13 and 14) and in [38] (Fig.…”

“…DC-RTS in solid state image sensor is caused by generation centers in the depletion region of reverse biased photodiodes that exhibit meta-stable generation current levels [31]. Even if electric field enhancement has been proposed in the past to justify the large DC-RTS amplitudes in early work on CCDs [15] and active pixel sensor technologies [28], recent results on state-of-the-art CIS technologies tend to indicate that the electric field does not play a significant role in the generation rate of DC-RTS centers [17], [23], [37], [41] except if an electric field hot spot is created by design [42] or if DC-RTS is studied in non-optimized PN junction with much higher electric fields than the photodiode [37]. Hence, the physical generation mechanism behind photodiode DC-RTS current in mature silicon image sensor technologies is considered to be a classical SRH generation current without EFE.…”

Random Telegraph Noises from the Source Follower, the Photodiode Dark Current, and the Gate-Induced Sense Node Leakage in CMOS Image Sensors

“…It is generally known that ionizing radiation can degrade the performance of CIS in terms of increasing DC, SN leakage, RN, and RTN [25][26][27][28][29][30][31][32]. To study the radiation damage effects, several Chip-A samples are irradiated, while grounded, by 10 keV X-ray at CEA-DAM facility at room temperature, with 0 rad, 10 krad, 100 krad, 500 krad, 1 Mrad, 2 Mrad, 5 Mrad, 10 Mrad, and 20 Mrad (SiO 2 ) total ionizing dose (TID), respectively [23].…”

“…The general effects of radiation damage on CIS and the design of radiation-hard CIS are outside the scope of this work [25][26][27][28][29][30][31][32]. Instead, X-ray irradiation is utilized as a tool to alter the composition of RTN types and to increase the number of RTN pixels for investigation.…”

Leakage Current Non-Uniformity and Random Telegraph Signals in CMOS Image Sensor Floating Diffusions Used for In-Pixel Charge Storage

“…Indeed, in the 3T photodiode structure, the doping profile of the photodiode n-implant is optimized (usually with p-well implantations) to locate the depleted region at the bottom of the STIs avoiding the STI sidewalls and its corners which is not the case in FD. When depleted, the STI sidewalls are known to be a high defect concentration region and have been identified as the dominant leakage current source in [11]. The results reported in Figure 6 suggest the existence of a multitude of generation centers located at the Si/SiO 2 interfaces like the TG oxide and the shallow trench isolation (STI) sidewalls which are directly in contact with the FD depleted regions.…”