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

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Effects in Pinned Photodiode CMOS Image Sensors: Pixel Performance Degradation Due to Total Ionizing Dose Open Archive Toulouse Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID: 8355 To link to this article: DOI: 10.1109/TNS.2012.2222927 URL: http://dx.doi.org/10.1109/TNS.2012.2222927 To cite this version: Goiffon, Vincent and Estribeau, Magali and Marcelot, Olivier and Cervantes, Paola and Magnan, Pierre and Gaillardin, Marc and Virmontois, Cédric and Martin-Gonthier, Philippe and Molina, Romain and Corbière, Franck and Girard, Sylvain and Paillet, Philippe and Marcandella, Claude Radiation Effects in Pinned Photodiode CMOS Image Sensors: Pixel Performance Degradation Due to Total Ionizing Dose. (2012) IEEE Transactions on Nuclear Science, vol. 59 (n° 6). pp. 2878-2887. ISSN 0018-9499 Any correspondence concerning this service should be sent to the repository administrator: [email protected] Radiation Effects in Pinned Photodiode CMOS Image Sensors: Pixel Performance Degradation Due to Total Ionizing Dose Vincent Goiffon, Member, IEEE, Magali Estribeau, Member, IEEE, Olivier Marcelot, Member, IEEE, Paola Cervantes, Pierre Magnan, Member, IEEE, Marc Gaillardin, Member, IEEE, Cédric Virmontois, Student Member, IEEE, Philippe Martin-Gonthier, Member, IEEE, Romain Molina, Franck Corbière, Sylvain Girard, Senior Member, IEEE, Philippe Paillet, Senior Member, IEEE, and Claude Marcandella Abstract—Several Pinned Photodiode (PPD) CMOS Image Sen- medical and military instruments). However, whereas some ra- sors (CIS) are designed, manufactured, characterized and exposed diation-hardening-by-design initiatives can be found in the lit- biased to ionizing radiationupto10kGy(SiO ). In addition to erature [2], [3], the behavior of the pinned photodiode and its the usually reported dark current increase and quantum efficiency drop at short wavelengths, several original radiation effects are associated Transfer Gate (TG) after exposure to ionizing radia- shown: an increase of the pinning voltage, a decrease of the buried tion is not fully understood today. Indeed, the physical process photodiode full well capacity, a large change in charge transfer effi- at the origin of radiation induced dark current in pinned photo- ciency, the creation of a large number of Total Ionizing Dose (TID) diode has not been completely clarified and only a few other pa- inducedDarkCurrent Random Telegraph Signal (DC-RTS) cen- rameters or characteristics have been studied on irradiated PPD ters active in the photodiode (even when the Transfer Gate (TG) is accumulated) and the complete depletion of the Pre-Metal Di- CIS (e.g., External Quantum Efficiency (EQE) in [4] and noise electric (PMD) interface at the highest TID leading to a large dark in [5]–[7]). Moreover, most of the previous work has been per- current and the loss of control of the TG on the dark current. The formed on devices grounded during irradiation, which may hide proposed mechanisms at the origin of these degradations are dis- some effects that could be enhanced by the local electric field cussed. It is also demonstrated that biasing (i.e., operating) the PPD during exposure. CIS during irradiation does not enhance the degradations com- pared to sensors grounded during irradiation. The aim of this work is to provide an overview of the Total Ionizing Dose (TID) effects that may occur in PPD CIS operated Index Terms—4T pixel, active pixel sensor, APS, buried photo- during irradiation (not to evaluate precisely the radiation hard- diode, charge transfer, CMOS image sensor (CIS), dark current, deep submicron process, DSM, interface states, ionizing radiation, ness of the tested sensors at a given TID for a specificenviron- MAPS, monolithic active pixel sensor, photon transfer curve ment). To do so, we study the main parameters and characteris- (PTC), pinned photodiode (PPD), pinning voltage, pre-metal di- tics of six 4T-PPD-CIS manufactured in two widely used com- electrics (PMD), quantum efficiency, radiation hardening, RHBD, mercially available CIS foundries. Several original radiation ef- shallow trench isolation (STI), total ionizing dose (TID), trapped fects on PPD CIS are reported, and all these results seem to indi- charge. cate that the electrostatic structure of the buried photodiode (i.e., the space charge region thickness and location) is changed, even I. INTRODUCTION at low TID, leading to complex degradation mechanisms. After the description of the experimental details, the results of INNED photodiode (PPD) CMOS Image Sensors (CIS) the measurements performed on the irradiated PPD CIS are pre- P [1] are the most promising photodetectors for a growing sented and discussed in Section III. More detailed discussions number of high performance imaging applications where ion- on the overall degradation mechanism and the effect of biasing izing radiation can be an issue (such as space, scientific, nuclear, during irradiation are finally presented in Section IV. II. EXPERIMENTAL DETAILS Two unhardened CIS designs based on PPD pixels (see Fig. 1 for a cross sectional view and Fig. 2 for a simplified layout il- lustration), with four transistors (the most simple PPD pixel ar- chitecture) and without micro-lens or color filter, were manu- factured using two different CMOS foundries. Both processes are widely used commercially available 0.18 m CIS technolo- gies. The image sensor details are summarized in Table I. As discussedindetailin[8], represents the voltage applied on the TG when it is turned off. The value shown in the table is the optimal value selected for this study. Fig. 3. 4T PPD timing diagram showing the optional injection or dump phase. Fig. 1. Cross sectional view (not to scale) of a 4T Pinned PhotoDiode (PPD) MOSFET supply voltage is kept at its nominal value pixel. Insets show the TG depletion region for two bias cases: (a) V: the TG depletion region merges with the photodiode depletion region. (b) , the buried photodiode is emptied a second time by V: the gate is accumulated and the photodiode depletion region does activating the TG and the RST MOSFET simultaneously. This not touch any oxide interface anymore. The typical depth of Shallow Trench dump phase is used in most of commercial products since it re- Isolation (STI) in the studied technology is about 400 nm. duces the image lag (but without improving the charge transfer efficiency) and provides additional benefits such as a better be- havior in case of blooming. It is interesting to notice that if is lowered during this phase, charges can be injected into the photodiode (this is discussed in detail in Section III.D). We have voluntarily decided not to use this dump/injection phase (except for pinning voltage estimation). We use the most elementary 4T-PPD timing diagram with no dump/injection phase for two main reasons: 1) the dump phase artificially improves the charge transfer efficiency and so, hides its degra- dation and 2) a simpler timing diagram is supposed to ease the radiation effects analysis. Fig. 2. Simplified top-view illustration of the studied 4T pinned photodiode pixels. For each tested CIS foundry, two devices were exposed biased—or more precisely biased and sequenced continuously TABLE I thanks to an FPGA board as in standard operating condi- KEY FEATURES OF THE STUDIED CMOS IMAGE SENSORS tions—and one was exposed grounded (for a total of six studied pixel arrays) to 10 X-rays at CEA-DIF with no lid or cover glass on top of the sensors. The absorbed TID ranges from 0.5 kGy(SiO )to10kGy(SiO) and the dose rate was about 1Gy(SiO )/s. III. RESULTS A. Unchanged Parameters Before discussing the main results, it is necessary to verify that the sensors are still working well after the highest TID. Fig. 4 shows four raw frames taken by two irradiated sensors The pixels have been designed for testing purpose. It means after 3 and 10 kGy(SiO ). It can be seen that both sensors that the pixel layout has been kept simple (e.g., rectangular pho- are still functional after 10 kGy(SiO ), confirming once again todiode) to ease the interpretation and that these sensors are [9] that the integrated digital circuits (address decoders and not optimized (e.g., fill factor, TG shape, Maximum Output switches) are not significantly degraded. Indeed, in order to get Voltage Swing (MOVS), dynamic range ) for a dedicated high an image, the logic gates of the row and column decoders must performance application. It is worth mentioning that only the operate properly to activate, in the right order, every pixel. If functions necessary to the imagers are on-chip for easier anal- a single digital MOSFET was not working after irradiation, it ysis: the pixels, the address decoders and the analog sampling/ would lead to image artifacts and the original pattern would readout chains. The sequencer and the analog-to-digital conver- not be reconstituted properly. Supply current and electrical sion chain are located on the test boards (and were not exposed transfer function (from the Floating Diffusion (FD) to the to ionizing radiation) to ensure that the observed degradations sensor output) were measured after each step and no change come only from the sensor. was observed, even after the highest TID. The mean-variance All the measurements were performed using the timing dia- characteristics (i.e., output voltage variance as a function of gram illustrated in Fig. 3, but without the optional dump/injec- mean output voltage) after each TID did not show any sig- tion phase (if not stated otherwise). During this phase, if the RST nificant conversion factor (CVF) degradation. These results Fig. 5. Sensor A (left) and B (right) PTC before and after irradiation. Fig. 4. Raw images of the 1951 USAF resolution test chart captured with sensor A (a) after 3 kGy and (b) after 10 kGy.
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