Romain Molina

Radiation Effects in Pinned Photodiode CMOS Image Sensors: Pixel Performance Degradation Due to Total Ionizing Dose

Several Pinned Photodiode (PPD) CMOS Image Sensors (CIS) are designed, manufactured, characterized and exposed biased to ionizing radiation up to 10 kGy(SiO2 ). In addition to the usually reported dark current increase and quantum efficiency drop at short wavelengths, several original radiation effects are shown: an increase of the pinning voltage, a decrease of the buried photodiode full well capacity, a large change in charge transfer efficiency, the creation of a large number of Total Ionizing Dose (TID) induced Dark Current Random Telegraph Signal (DC-RTS) centers active in the photodiode (even when the Transfer Gate (TG) is accumulated) and the complete depletion of the Pre-Metal Dielectric (PMD) interface at the highest TID leading to a large dark current and the loss of control of the TG on the dark current. The proposed mechanisms at the origin of these degradations are discussed. It is also demonstrated that biasing (i.e., operating) the PPD CIS during irradiation does not enhance the degradations compared to sensors grounded during irradiation.

Influence of Transfer Gate Design and Bias on the Radiation Hardness of Pinned Photodiode CMOS Image Sensors

The effects of Cobalt 60 gamma-ray irradiation on pinned photodiode (PPD) CMOS image sensors (CIS) are investigated by comparing the total ionizing dose (TID) response of several transfer gate (TG) and PPD designs manufactured using a 180 nm CIS process. The TID induced variations of charge transfer efficiency (CTE), pinning voltage, equilibrium full well capacity (EFWC), full well capacity (FWC) and dark current measured on the different pixel designs lead to the conclusion that only three degradation sources are responsible for all the observed radiation effects: the pre-metal dielectric (PMD) positive trapped charge, the TG sidewall spacer positive trapped charge and, with less influence, the TG channel shallow trench isolation (STI) trapped charge. The different FWC evolutions with TID presented here are in very good agreement with a recently proposed analytical model. This work also demonstrates that the peripheral STI is not responsible for the observed degradations and thus that the enclosed layout TG design does not improve the radiation hardness of PPD CIS. The results of this study also lead to the conclusion that the TG OFF voltage bias during irradiation has no influence on the radiation effects. Alternative design and process solutions to improve the radiation hardness of PPD CIS are discussed.

Study of CCD Transport on CMOS Imaging Technology: Comparison Between SCCD and BCCD, and Ramp Effect on the CTI

This paper presents measurements performed on charge-coupled device (CCD) structures manufactured on a deep micrometer CMOS imaging technology, in surface channel CCD and in buried channel CCD mode. The charge transfer inefficiency is evaluated for both CCD modes with regard to the injected charge, and the influence of the rising and falling time effect is explored. Controlling the ramp and especially reducing its abruptness allows to get much lower charge transfer inefficiency in buried CCD mode. On the contrary, we did not observe any effect of the ramp on surface channel CCD mode because of the presence of interface traps at the silicon-oxide interface.

Multi-MGy Radiation Hard CMOS Image Sensor: Design, Characterization and X/Gamma Rays Total Ionizing Dose Tests

A Radiation Hard CMOS Active Pixel Image Sensor has been designed, manufactured and exposed to X and 60Co γ-ray sources up to several MGy of Total Ionizing Dose (TID). It is demonstrated that a Radiation-Hardened-By-Design (RHBD) CMOS Image Sensor (CIS) can still provide useful images after 10 MGy(SiO2) (i.e. 1 Grad). This paper also presents the first detailed characterizations of CIS opto-electrical performances (i.e. dark current, quantum efficiency, gain, noise, transfer functions, etc.) in the MGy range. These results show that it is possible to design a CIS with good performances even after having absorbed several MGy. Four different RHBD photodiode designs are compared: a standard photodiode design, two well known RHBD layouts and a proposed improvement of the gated photodiode design. The proposed layout exhibits the best performances over the entire studied TID range and further optimizations are discussed. Several original MGy radiation effects are presented and discussed at the device and circuit levels and mitigation techniques are proposed to improve further the radiation hardness of future Rad-Hard CIS developments for extreme TID applications (e.g. for nuclear power plant monitoring/dismantling, experimental reactors (e.g. ITER) or next generation particle physics experiments (e.g. CERN)).

Smart CMOS image sensor for lightning detection and imaging

We present a CMOS image sensor dedicated to lightning detection and imaging. The detector has been designed to evaluate the potentiality of an on-chip lightning detection solution based on a smart sensor. This evaluation is performed in the frame of the predevelopment phase of the lightning detector that will be implemented in the Meteosat Third Generation Imager satellite for the European Space Agency. The lightning detection process is performed by a smart detector combining an in-pixel frame-to-frame difference comparison with an adjustable threshold and on-chip digital processing allowing an efficient localization of a faint lightning pulse on the entire large format array at a frequency of 1 kHz. A CMOS prototype sensor with a 256×256 pixel array and a 60 μm pixel pitch has been fabricated using a 0.35 μm 2P 5M technology and tested to validate the selected detection approach.

Dark Current Spectroscopy in Neutron, Proton and Ion Irradiated CMOS Image Sensors: From Point Defects to Clusters

Dark current spectroscopy is tested on twenty CMOS image sensors irradiated with protons, neutrons and various ions at different energies. The aim of this work is to differentiate the effect of coulomb and nuclear interactions on the radiation-induced dark current distribution and to identify the main radiation-induced defects responsible for the dark current increase for each type of interaction. For low-energy protons and low-energy light ions (which produce well-separated low energy coulomb interactions), we find that most of the pixels belong to a quantized dark current spectrum at low dark current. In these pixels, the dark current increase seems mainly dominated by specific point defects such as the divacancy and the vacancy-phosphorus complex. Thus, these simple defects seem to form when the displacement damage is rather low and sparse. On the contrary, for nuclear interactions (with neutrons or high-energy protons) producing high coulomb NIEL silicon PKAs or for low energy heavy ions (also having high coulomb NIEL), the DCS spectrum is not visible and all the pixels belong to an exponential hot pixel tail which extends to very high dark current. In these pixels, the dark current increase is mainly dominated by defects with close-to-midgap energy levels. These defects seem more complex than point defects because they can have many different generation rates (explaining the smooth hot pixel tail) and because they tend to form when the displacement damage is high and dense.

Exploring the 3D integration technology for CMOS image sensors

3D fabrication technologies allow microelectronic circuits such as processors or memories to achieve very high integration densities. These technologies applied to CMOS image sensors, make possible the implementation of specific processing architectures without damaging key parameters of CMOS imagers. This paper illustrates these benefits with an implementation of a 3D image sensor integrating at the pixel level a low noise circuit coupled to an analog to digital converter.

Analysis and optimization of noise response for low-noise CMOS image sensors

CMOS image sensors are nowadays widely used in imaging applications and particularly in low light flux applications. This is really possible thanks to a reduction of noise obtained, among others, by the use of pinned photodiode associated with a Correlated Double Sampling readout. It reveals new noise sources which become the major contributors. This paper presents noise measurements on low-noise CMOS image sensor. Image sensor noise is analyzed and optimization is done in order to reach an input referred noise of 1 electron rms by column gain amplifier insertion and dark current noise optimization. Pixel array noise histograms are analyzed to determine noise impact of dark current and column gain amplifier insertion. Transfer noise impact, due to the use of pinned photodiode (4T photodiode), is also measured and analyzed by a specific readout sequence.

Radiation Effects in CCD on CMOS Devices: First Analysis of TID and DDD Effects

As CMOS image sensors become more and more attractive and with high performances, it becomes possible to use CCD on CMOS devices with reasonable lengths. However, no study has been done on the radiation hardness of such CCD on CMOS devices. Therefore, we propose in this paper a first study of Charge Transfer Inefficiency (CTI) and dark current degradation under TID and DDD irradiations. To do so, test chips have been processed in conventional deep submicron CMOS imaging technologies, and characterized before and after irradiations.