I. INTRODUCTION

P INNED photodiodes (PPD) CMOS image sensors (CIS) are today the main solid-state image sensor technology and they are widely used for the development of various scientific applications, thanks to their high performances, high integration capabilities, and low-power consumption [START_REF] Fossum | A review of the pinned photodiode for CCD and CMOS image sensors[END_REF]. Nowadays, the demand for these sensors is growing, notably for applications in harsh environments, such as space remote sensing and medical imaging where the total-ionizing Digital Object Identifier 10.1109/TNS.2017.2778760 dose (TID) can goes up to several kGy, but also such as viewing applications in nuclear environments (e.g., waste repository and ITER) for which the TID reach the MGy range.

The main ionizing radiation effect that degrades the PPD performances is the generation of different defects (i.e., positive-trapped charges and interface states) in the oxides within the pixel structure (see Fig. 1). Several TID degradation source locations, presented in Fig. 1(b), are proposed in the literature: the premetal dielectric (PMD) on top of the PPD (source A) [START_REF] Goiffon | Radiation effects in pinned photodiode CMOS image sensors: Pixel performance degradation due to total ionizing dose[END_REF], the PPD-transfer gate (TG) edge (PMD/TG spacer), source B, [START_REF] Goiffon | Influence of transfer gate design and bias on the radiation hardness of pinned photodiode CMOS image sensors[END_REF], the gate oxide (source C) [START_REF] Goiffon | Radiation effects in pinned photodiode CMOS image sensors: Pixel performance degradation due to total ionizing dose[END_REF], [START_REF] Place | Radiation effects on CMOS image sensors with sub-2 μm pinned photodiodes[END_REF], and the shallow trench isolation (STI) around the PPD [START_REF] Place | Radiation effects on CMOS image sensors with sub-2 μm pinned photodiodes[END_REF], [START_REF] Rao | Degradation of CMOS image sensors in deep-submicron technology due to γ -irradiation[END_REF], and the TG (source D). During ionizing irradiation, the most degraded parameter in CIS is the dark current which increases because of the generated interface states at the Si/SiO 2 interfaces [START_REF] Goiffon | Radiation effects in pinned photodiode CMOS image sensors: Pixel performance degradation due to total ionizing dose[END_REF], [START_REF] Goiffon | Influence of transfer gate design and bias on the radiation hardness of pinned photodiode CMOS image sensors[END_REF]. However, other parameters deteriorate under irradiation exposure. The charge transfer in PPD CIS can be enhanced or degraded (depending on the limiting lag causes in the nonirradiated device) by the TID-induced-trapped positive charges in the TG spacer vicinity [START_REF] Goiffon | Influence of transfer gate design and bias on the radiation hardness of pinned photodiode CMOS image sensors[END_REF], [START_REF] Liu | Simulation and measurement of total ionizing dose radiation induced image lag increase in pinned photodiode CMOS image sensors[END_REF]. Moreover, the positive-trapped charges and interface states in the PMD lead to an increase of the pinning voltage, whereas the TIDinduced-trapped charges in the TG STI sidewalls generate a TG subthreshold leakage leading to full well capacity (FWC) reduction [START_REF] Goiffon | Influence of transfer gate design and bias on the radiation hardness of pinned photodiode CMOS image sensors[END_REF]. In order to achieve good performances for the different applications in harsh environments it is then mandatory to improve our understanding of the TID-induced In this paper, the radiation-induced effects on the charge transfer process are investigated in 4T-PPD pixels up to 20 kGy(SiO 2 ) (2 Mrad). The response of different pixels which differ from their layouts (i.e., PPD and TG dimensions) as well as the influence of the TG channel doping is investigated in order to explain the contribution of the diverse radiation degradation sources in the charge transfer. The aim is to enlighten the PPD behavior under radiation and to identify possible radiation hardened by design (RHBD) solutions.

II. MATERIALS AND METHODS

The investigated CIS, constituted of 256 × 256 -7-μmpitch 4T-PPD pixels [see cross section in Fig. 1(b)], has been designed and manufactured by using a mature 0.18-μm CIS technology. The sensor is divided in different subarrays each one being constituted of about 2500 identical pixels.

Each pixel is composed by three main components illustrated in Fig. 1: the PPD where the charges are collected, the TG transistor, which allows the electrons transfer, and the floating diffusion (FD), the N+ doped region where the electrons are transferred by the TG and, then, readout. Three other transistors are also present in each pixel to reset the FD, amplify the signal and select the pixel [START_REF] Fossum | CMOS image sensors: Electronic camera-on-a-chip[END_REF]. The studied subarrays are composed by pixel variants differing from the reference one (which design is reported in Fig. 1) by their doping profiles under the TG and by design layouts, i.e., different TG lengths, TG widths, or PPD sizes. A summary of the pixel variants is reported in Table I.

As observed in [START_REF] Goiffon | Radiation effects in pinned photodiode CMOS image sensors: Pixel performance degradation due to total ionizing dose[END_REF] to irradiate with bias applied does not lead to enhanced radiation degradations when dealing with CIS performances (dark current, pinning voltage changes, and image lag degradation) for the studied technologies. Since the CIS studied in this paper is fabricated with the same technology as one of those in [START_REF] Goiffon | Radiation effects in pinned photodiode CMOS image sensors: Pixel performance degradation due to total ionizing dose[END_REF], the circuits were exposed grounded up to TIDs from 3 Gy(SiO 2 ) to 20 kGy(SiO 2 ). The dose rate was 1 Gy(SiO 2 )/s for TID from 3 Gy(SiO 2 ) to 1 kGy(SiO2), whereas the dose rate was increased up to 10 Gy(SiO 2 )/s for TID above 1 kGy(SiO 2 ). The 10-keV X-ray irradiations were performed at CEA, DAM, DIF (Arpajon, France), with the ARACOR facility. All the measurements were performed after three weeks of annealing at room temperature.

Image lag measurements were performed at 22 °C using an LED source at 540 nm to obtain bright-to-dark transition conditions.

For our measurements, the nominal high and low TG bias conditions are V HITG = 3.3 V and V LOTG = -0.5 V, with a transfer time, i.e., the time when TG is in its ON state, of 1 μs. The reset supply voltage, V DDRST , was nominally set at 3.3 V. The image lag results reported along this paper are displayed at a charge level corresponding to 25% of the FWC of each pixel in order to compare the pixels response at the same PPD filling level.

Pinning voltage characteristics were obtained by using the charge injection technique procedure implemented in [START_REF] Goiffon | Pixel level characterization of pinned photodiode and transfer gate physical parameters in CMOS image sensors[END_REF] at the nominal bias conditions (e.g., V HITG = 3.3 V, V LOTG = -0.5 V, and V DDRST = 3.3 V). These measurements give useful information about the potential diagram of the PPD-TG-FD structure allowing investigating on the TID-induced modification. Moreover, it has been possible to evaluate the charge partition phenomenon [START_REF] Goiffon | Pixel level characterization of pinned photodiode and transfer gate physical parameters in CMOS image sensors[END_REF] in the studied devices. This phenomenon, observed by the pinning voltage characteristic measurements, is very important since it is linked to the spillback phenomenon, responsible of a very bad charge transfer (as explained in Section III) which is observable in the normal readout CIS configuration.

III. IMAGE LAG SOURCES

In a 4T-pixel (see Fig. 1), the readout of the stored charge in the PPD is done thanks to the OFF-ON switch of the TG which allows the charge transfer of the photo-generated electrons from the PPD to the FD node. During the integration phase, Fig. 2(a), the photo-generated charges are collected in the PPD which is isolated from the FD by turning the TG OFF. At the end of the integration, the TG is turned ON and the charges can be transferred to the FD [Fig. 2(b)]. In an ideal situation, all the accumulated electrons in the PPD should be transferred and converted into the voltage signal which is readout. However, this is not the real situation since different mechanisms and/or nonidealities in the transfer path can cause that a fraction of the stored charges remains in the PPD after the transfer, phenomenon known as image lag. Fig. 2(b) illustrates the different causes of the image lag than can occur in a 4T-pixel, which are linked to the different parts of the pixel, i.e., PPD, TG, and FD.

1) Thermal diffusion of the charges inside the photodiode is considered as the limiting mechanism in the charge transfer. The electrons stored in the PPD, once allowed to the transfer, diffuse along the PPD before arriving at the TG edge [START_REF] Carnes | Free charge transfer in charge-coupled devices[END_REF]. This can cause, if the PPD is not correctly sized or the transfer time is not long enough, that some electrons, far from the PPD-TG edge, remain in the PPD. 2) Potential nonidealities, such as barriers or pocket, occur because of the doping profile in the transfer channel and/or because of the manufacturing process. They slow down the transfer speed preventing the charges to reach the FD [START_REF] Chen | Measurement of charge transfer potential barrier in pinned photodiode CMOS image sensors[END_REF]. 3) Charges trapping by fast interface states can occur when the charges pass through the transfer channel [START_REF] Bonjour | Experimental analysis of lag sources in pinned photodiodes[END_REF]. 4) Spillback, i.e., the charges coming back to the PPD from the FD, happens when a too large amount of charges is transferred to the FD (e.g., if it is not correctly sized [START_REF] Marcelot | Pinned photodiode CMOS image sensor TCAD simulation: In-depth analysis of inpixel pinning voltage measurement for a diagnostic tool[END_REF]) which means that the FD potential is lowered down to the TG potential and then that the charges under the TG can go back to the FD once the TG switch in its OFF state. Because of all these image lag sources, the determination of the charge transfer efficiency in a 4T-pixel is an important and, at the same time, a tricky point, especially when dealing also with radiation degradation such as the dark current increase. The output signal as a function of consecutive images during a bright-to-dark transition [Fig. 2(c)] shows that, when a pixel has a very strong image lag (red curve), the evolution of the signal with time takes more time (about 15 images here) to go back to the dark level because of the remaining electrons in the photodiode. Conversely, in presence of good transfer performances, the dark condition is reached in the image right after the pulse (black curve).

In order to clarify on the possible lag sources in the studied design, Fig. 3 shows the reference pixel potential diagram along the X-X cross section of Fig. 1(b) before irradiation. The presence of a PPD-TG P-doped region (P PPD-TG ) and a TG-FD P-doped region (P TG-FD ) in the TG channel generates a potential step in the potential diagram, due to differences in the doping concentration of the two implants, whereas a potential barrier arises in the P PPD-TG /P TG-FD overlap. This potential configuration allows high transfer performances thanks to the presence of the step in the middle of the TG which prevents the spillback of transferred electrons [START_REF] Cao | Design of pixel for high speed CMOS image sensors[END_REF]. During the charge transfer from PPD-to-FD, most of signal electrons-the small barrier induced by the overlap may retain a few carriers-do not remain in the first half of the TG channel since a higher electrostatic potential exists in the second half of the TG (near the FD).

Then, when the TG is switched in its OFF state, the potential difference between P PPD-TG and P TG-FD levels directs the electrons that are in the right half of the TG channel toward the FD and prevent them to return back to the PPD. The image lag is, then, mostly due to the barrier in the middle of the TG, which avoids the transfer of the few carriers retained by it. A large number of samples different from each other by the layout of the transfer channel doping profile allow investigating the radiation-induced effects on charge transfer.

The image lag is usually [START_REF] Janesick | Fundamental performance differences of CMOS and CCD imagers: Part V[END_REF] determined in terms of charge transfer efficiency (CTE) or charge transfer inefficiency (CTI = 1 -CTE). As explained in Section II, the image lag was measured during a bright-to-dark transition. The CTI is then given by the ratio of the readout signal in the image after the pulse (e.g., dark), Q N+1 , to the one recorded in the frame in which the pulse is temporally confined (e.g., bright), Q N . For our purpose, and taking in account that with the increase of the TID the degradation mechanisms can influence and/or misrepresent the information about the charge transfer only the photo generated charges (i.e., without the dark charges, Q dark ) are used to calculate the CTI

CTI = Q N+1 -Q dark Q N -Q dark . ( 1
)

IV. RADIATION EFFECTS ON IMAGE LAG

In this section, the experimental data are presented and discussed. Four sections, each one focusing on a particular radiation effect location (summarized in Fig. 1), will be developed in order to clarify the TID impact on the image lag and the influence of the generated defects with respect to the TID range considered.

A. Pre Metal Dielectric Oxide Effect

First of all the influence of PMD dielectric in the charge transfer process is studied, by testing three pixels differing by their PPD lengths (Pixel 1-Pixel 3 in Table I).

Before irradiation these pixels have a potential diagram as the one of Fig. 3. Fig. 4 shows that before irradiation, when the PPD is too long the charge transfer is strongly limited by the thermal diffusion of the electrons [START_REF] Carnes | Free charge transfer in charge-coupled devices[END_REF]. In the medium and small photodiodes, the limiting factor is rather the barrier in the middle of the TG since our measurements have shown that increasing the transfer time above 1 μs does not reduce the image lag. Up to 30 Gy(SiO 2 ), the CTI remains almost unchanged in the three pixels, whereas it suddenly changes with the increase of the TID above 0.5 kGy(SiO 2 ).

In Fig. 5, the evolution of the pinning voltage with the dose is reported.

It is observed that V PIN monotonically increases with the dose at TID > 0.5 kGy(SiO 2 ) when L PPD = 0.3 μm, whereas it remains constant up to 10 kGy(SiO 2 ) for the other two L PPD . The results reported in Figs. 4 and5 have shown that PPD with different L PPD are differently affected by the radiation above 0.1 kGy(SiO 2 ) as regards both lag performances and V PIN characteristics. A summary of the radiation-induced changes is reported in Fig. 6.

For L PPD = 2 μm [Fig. 6(a)], the transfer before irradiation is limited by the barrier in the middle of the TG and the pixel exhibits very good lag performances when nonirradiated (CTI ∼ 0.05%). After TID of 0.1 kGy(SiO 2 ), the CTI increase is mostly due to the modification of the potential in the PPD-TG interface where a potential pocket is created (simulation of this effect is reported in [START_REF] Liu | Simulation and measurement of total ionizing dose radiation induced image lag increase in pinned photodiode CMOS image sensors[END_REF]) as illustrated by the blue line in Fig. 6(a). Since V PIN is unchanged up to 5 kGy(SiO 2 ), it is possible to conclude that the potential modification for 0.1 kGy(SiO 2 ) < TID < 5 kGy(SiO 2 ) is only due to the positive-trapped charges and interface states in the TG vicinity and/or in the spacer. In this region the pinning layer is, indeed, thinner in order to allow the PPD-to-FD electrons transfer. V PIN starts to increase at TID > 5 kGy(SiO 2 ) thus suggesting that at these doses the whole PPD potential is influenced by radiation. A possible explanation is that, in this TID range, the interface states and positive-trapped charges generated by radiation in the PMD start impacting the pinning voltage of the PPD.

When the L PPD is reduced to 0.3 μm [Fig. 6(b)], the effect of the induced defects at the PPD-TG interface becomes too important and affects the whole PPD potential. It is observed that V PIN starts to increase at 0.5 kGy(SiO 2 ). This also explains the difference in CTI value between medium and short PPD observed in Fig. 4 since for the short PPD, the potential increase creates a bigger potential pocket that affects more the charge transfer performances in the short PPD than in the medium one.

The long PPD CTI case is displayed in Fig. 6(c). As already observed, in this pixel, the preirradiation charge transfer is strongly affected by the thermal diffusion of the electrons in the PPD that have to reach the TG. When irradiated, the potential modification induced by the generated defects in the PPD-TG edge improves the lag performance up to TID of 1 kGy(SiO 2 ). As shown in Fig. 6(c), at first the creation of a potential pocket near the PPD-TG edge lets the electrons to diffuse in this region during the integration phase thus reducing the thermal diffusion mechanisms. The potential pocket has rather the function of a speed-up implant in the PPD [START_REF] Janesick | Fundamental performance differences of CMOS and CCD imagers: Part V[END_REF]. At doses higher than 5 kGy(SiO 2 ) the potential pocket becomes too big and the retained charges deteriorate again the electron transfer increasing the CTI. It can also be observed that in the long PPD an increase of V PIN occurs at TID > 5 kGy(SiO 2 ) due to the PMD interface states and positive transfer charges that influence the whole PPD potential. It is worth noting that the effect of interface states and positive-trapped charges present in the PMD does not affect the image lag since CTI has the same trend with radiation in both medium and short PPD. If present, PMD defects effect should affect more the medium length PPD than the smallest one thus revealing a different CTI behavior between the two pixel types.

B. PPD-TG Edge Effect

In this section, the influence of the PPD-TG edge is investigated. As already explained in Section IV-A and suggested in [START_REF] Goiffon | Influence of transfer gate design and bias on the radiation hardness of pinned photodiode CMOS image sensors[END_REF] with the increase of the dose above 0.1 kGy(SiO 2 ), the radiation-induced defects in the PMD/TG spacer can modify the PPD-TG edge potential. This is mainly due to the fact that in this region the P + pinning layer is thinner to let the PPD reaching the TG channel for the charge transfer. Then, the generation of defects in this region more strongly influences the pixel operation. The way the PMD/TG spacer influences the charge transfer has been investigated thanks to pixels differing by their doping profiles in the TG channel which potential diagrams before irradiation are reported in Fig. 7(a)-(d) together with the main lag sources for each pixel before irradiation. From the results shown in Fig. 8, it can be noted that the image lag is strongly influenced by the TG channel doping profile.

In pixels with the antispillback step [see Fig. 7(a) and (c)], the CTI increases with the dose. The positive-trapped charges and interface states created by the ionizing radiation in the TG vicinity and in the spacer induce a modification of the potential in this region since the P + pinning layer is thinner to let the PPD reaching the TG channel for the charge transfer [START_REF] Goiffon | Influence of transfer gate design and bias on the radiation hardness of pinned photodiode CMOS image sensors[END_REF]. For these two pixel types, the CTI increase is then due to a creation of a potential pocket in the PPD-TG interface which retains the electrons thus degrading the image lag performances [see Fig. 6(a)].

For pixels where the image lag before radiation is caused by the spillback of the charges under the TG [Fig. 7(b) and(d)], a CTI decrease for 0.1 < TID < 5 kGy(SiO 2 ) is observed, as shown in Fig. 8. The radiation defects increase the potential under in the PPD-TG region thus reducing the spillback. A similar spillback reduction has been already observed in nonirradiated CIS [START_REF] Goiffon | Pixel level characterization of pinned photodiode and transfer gate physical parameters in CMOS image sensors[END_REF] and it was shown that increasing V LOTG leads to a reduction of the charge partition phenomenon and then the spillback. This hypothesis has been verified in the studied designs by the comparison of CTI before irradiation in accumulation and depletion regimes. The results reported in Fig. 9 clearly show that, in the pixels which have high lag before irradiation, more the TG OFF barrier is lowered better is the CTI at the same charge level in the PPD (see red curves), whereas for the low lag pixels there is no influence of V LOTG in the charge transfer performances. This result allows then concluding that the TID-induced potential increase leads to the reduction of the spillback thus improving the image lag for the pixel exhibiting a high image lag before irradiation. On the other hand, when the TID increases above 5 kGy(SiO 2 ) the CTI for these pixels starts to increase (Fig. 8) since the positive effects of the TG OFF barrier decrease are of less importance in comparison to the potential increase during the transfer phase (TG ON). In this case, as discussed above for the low lag pixels, positive-trapped charges create a potential pocket in the PPT-TG interface which retains part of the electrons.

C. TG Gate Oxide Effect

As regards the gate oxide, the effects on charge transfer by the investigation of pixels with different TG lengths (L TG ) have been studied; the results reported in Fig. 10 show that the CTI does not depend on the L TG in the whole tested range. This suggests that the charge trapping at the (Si/SiO 2 ) TG interface does not occur in the studied pixels.

D. STI Oxide Effect

The radiation-induced STI degradation contribution, source D in Fig. 1, on the charge transfer process degradation is investigated thanks to pixels which differ by their TG width, W TG (Pixel 1 and Pixel 4 in Table I).

The results, reported in Fig. 11, show that before irradiation and up to 1 kGy(SiO 2 ) the two pixels have almost the same CTI since the charge transfer is limited by the barrier in the middle of the TG. However, when the dose increases, the narrowest W TG pixel exhibits the lowest CTI. As discussed in Section IV-B, the radiation-induced defects in the TG channel increase the P doping concentration thus influencing the image lag via a reduction of the spillback (lowering of TG OFF barrier). This effect can be seen in Fig. 12 where the pinning voltage characteristic for the two pixels is reported at different doses. The inset better highlights the charge partition plateau. At low doses, the characteristics are equals in the two pixels, whereas the charge partition plateau is clearly lower at high doses for pixel with reduced W TG . It is worth mentioning that the spillback reduction is more effective in the narrowest TG channel effect since it can be expected that the generated defects in the STI sidewalls induced higher changes with respect to the wider TG.

V. TID DEGRADATION RECAP

The results and discussion reported in Section IV have allowed identifying the role played by the different oxides in the charge transfer degradation with the irradiation. A summary on the main-induced degradation discussed in this paper is reported in Fig. 13.

It is worth noting the TID induces the generation of different defects which are located in the oxides within the pixel structure (i.e., PMD, TG, spacer, and STI). These defects, i.e., the positive-trapped charges and the Si/SiO 2 interface states, have, however, a similar effect on the CIS performances degradation and the single contribution is difficult to discriminate.

A. Degradation Mechanisms

It has been shown that the TID-induced effect depends on the investigated dose range [START_REF] Goiffon | Radiation effects on CMOS active pixel image sensors[END_REF], and that at each dose range, the defects can be generated in the oxides (PMD, TG, spacers, and STI), thus modifying one or more pixel characteristics.

1) At TID < 0.1 kGy(SiO 2 ) image lag performances remain unchanged for all the considered pixel designs, as well as the pinning voltage characteristics. It is possible to assume that, up to these doses, the radiationinduced defect creation in the oxides, such as interface states and positive charge traps, is not enough to modify the potential of the PPD-to-FD transfer path in the considered accumulation regime. 2) In the 0.1 kGy(SiO 2 ) < TID < 5 kGy(SiO 2 ) range, the CTI changes because of the increase of the potential near the PPD-TG edge (see Fig. 13). Image lag is degraded for the pixels where the charge transfer before irradiation is limited by the potential barrier in the middle of the TG. This degradation is mostly due to a creation of a potential pocket formed in this region since the P + pinning layer is thinner to let the PPD reaching the TG channel for the charge transfer and which may retain part of the electrons. If the charge transfer before irradiation is instead limited by the thermal diffusion in the PPD, the increase of the potential in the vicinity of the TG improves the lag performance acting as a speed-up implant for the electron transfer. For spillback transfer limited pixels, it has been observed that in this dose range, the CTI decreases because of radiationinduced positive-trapped charge leading to an increase of the potential in the PPD-TG region reducing the spillback.

3) For TID>5 kGy(SiO 2 ), the CTI increases in all the pixels because of the increase in the potential pocket amplitude which becomes the main cause of the image lag. Moreover, a pinning voltage increase has been observed in this TID range. The V PIN increase has been associated with the radiation-induced defects in the PMD which influence the potential of the whole PPD.

B. Improvement Solutions

The analysis of the main degradation mechanisms induced on 4T-pixels up to TID levels of 20 kGy(SiO 2 ) has shown that variations in the pixel design, i.e., PPD length and TG width and TG channel doping profile, can be made in order to achieve good charge transfer performances in low to moderate TID range, i.e., up to 5 kGy(SiO 2 ). Conversely, when the TID level increase above 5 kGy(SiO 2 ) the image lag degradation affects all the studied pixels design due to the deep potential pocket in the PPD-TG edge. In this case 4T PPD pixels are not suitable and 3T-pixels with conventional photodiodes [START_REF] Hancock | Total dose testing of a CMOS charged particle spectrometer[END_REF]- [START_REF] Goiffon | Total ionizing dose effects on a radiation hardened CMOS image sensor demonstrator for ITER remote handling[END_REF] have to be preferred. Indeed, they allow the use of RHBD solutions such as drawing a ring of polysilicon (or P+ doping) around the photodiode.

VI. CONCLUSION

In this paper, we have investigated the radiation-induced degradation on the charge transfer in PPD CIS. Thanks to different pixel designs we were able to isolate the major radiation-induced effects proposed in the literature and to study how they influence the image lag. It has been pointed out that the main cause of the charge transfer degradation is the radiation-induced defects generation in the PMD/TG spacer which modifies the potential diagram at the PPD-TG edge. It has been shown that the two TID regimes are identified. For 0.1 kGy(SiO 2 ) < TID < 5 kGy(SiO 2 ), CTI changes due to increasing of the potential at the PPD-TG edge. Moreover, the results have pointed out that the TID effect on the image lag depends on the pixel design. At TID > 5 kGy(SiO 2 ) CTI increase due to the creation of a deep potential pocket at the PPD-TG edge inducing too important degradations which badly affect image lag performances. Pinning voltage study has pointed out that TID degradation depends on the geometry of the PPD since an increase of V PIN has been observed for the shortest PPD at 1 kGy(SiO 2 ), whereas for the other pixels the pinning voltage starts to increase for TID > 5 kGy(SiO 2 ). The pinning voltage increase has been attributed to induced defects in the PMD.

The reported results allow to conclude that PPD CIS will be able to be integrated in the future space missions (from low to moderate dose range) suggesting ways to obtain very good charge transfer performances (i.e., the optimization of the pixel characteristics such as PPD, TG and FD size, and TG channel doping profile). when for high TID level, the PPD-TG edge degradation is too high whatever the pixel design, preventing the use PPD CIS in such environments.

  Total-Ionizing Dose Effects on Charge Transfer Efficiency and Image Lag in Pinned Photodiode CMOS Image Sensors Serena Rizzolo , Member, IEEE, Vincent Goiffon , Member, IEEE, Magali Estribeau, Member, IEEE, Philippe Paillet, Senior Member, IEEE, Claude Marcandella, Clémentine Durnez , Student Member, IEEE, and Pierre Magnan, Member, IEEE Abstract-The total-ionizing dose (TID) effects on image lag in pinned photodiode CMOS image sensors are investigated thanks to various device variants in order to isolate the major radiationinduced effects on the charge transfer. It is shown that the main cause of the charge transfer degradation is the radiation-induced defects generation in the premetal dielectric (PMD) and in the transfer gate (TG) spacer vicinity which modifies the potential diagram at the photodiode/TG interface by the creation of a potential pocket retaining the electrons that are not transferred. For 0.1 kGy(SiO 2 ) < TID < 5 kGy(SiO 2 ), the potential pocket degrades the pixel which exhibits very good lag performances, whereas it improves the high lag pixels by creating a speedup implant enhancing the electron transfer or by reducing the spillback. For TID > 5 kGy(SiO 2 ), the defects generated in the PMD influence the whole photodiode potential inducing a pinning voltage increase and degrading the charge transfer by enlarging the potential pocket effect which becomes the main image lag source. The reported results clarify the impact of ionizing radiation on the charge transfer, suggesting radiation hardened by design solutions for future space or nuclear applications. Index Terms-Charge transfer, CMOS image sensors (CIS), image lag, pinned photodiode (PPD), radiation effects, total-ionizing dose (TID), transfer gate (TG).

Fig. 1 .

 1 Fig. 1. (a) Top view and (b) X-X cross-sectional view of a 4T-PPD together with the possible TID degradation sources. PPD = photodiode; TG = transfer gate; FD = floating diffusion; STI = shallow trench isolation; PMD = premetal dielectric; P PPD-TG = PPD-TG P-doped region; and P TG-FD = TG-FD P-doped region.

Fig. 2 .

 2 Fig. 2. (a) Schematic illustrations of the charge collection and (b) charge transfer and image lag sources. (c) Output signal as a function of the consecutive images. The pulse, used to generate the bright-to-dark transition, is synchronized in such a way that it is entirely temporal localized in a single frame (i.e., the first in the graph).

Fig. 3 .

 3 Fig. 3. Potential diagram of the reference pixel reported in Fig. 1.

Fig. 4 .

 4 Fig. 4. (a) Schematic of PMD oxide TID effect. PPD = photodiode; TG = transfer gate; FD = floating diffusion; STI = shallow trench isolation; PMD = premetal dielectric; P PPD-TG = PPD-TG P-doped region; and P TG-FD = TG-FD P-doped region. (b) CTI as a function of the TID at Q PPD = FWC/4 for pixels with different PPD lengths.

Fig. 5 .

 5 Fig. 5. V PIN as a function of the TID for pixels with different PPD lengths.

Fig. 6 .

 6 Fig.6. Schematic potential diagrams of (a) medium, (b) short, and (c) long PPD pixels before irradiation (black line), after TID up to 1 kGy(SiO 2 ) (light blue line), and for TID above 5 kGy(SiO 2 ) (red dotted lines).

Fig. 7 .

 7 Fig. 7. Potential diagrams to illustrate the charge transfer (TG ON) limiting or improving mechanisms for pixels (a) with PPPD-TG and PTG-FD, (b) without PPPD-TG, (c) without PTG-FD, and (d) pixel D without PPPD-TG and PTG-FD.

Fig. 8 .

 8 Fig. 8. (a) Schematic of TG PPD/TG TID effect is reported in the inset. PPD = photodiode; TG = transfer gate; FD = floating diffusion; STI = shallow trench isolation; PMD = premetal dielectric; P PPD-TG = PPD-TG P-doped region; and P TG-FD = TG-FD P-doped region. (b) CTI as a function of the TID at Q PPD = FWC/4 for pixels with different TG doping profile; the inset shows the schematic degradation mechanism representation.

Fig. 9 .

 9 Fig. 9. CTI as a function of the photodiode charge level for a low lag pixel (black curves) and a high lag pixel (red curves) with accumulated TG (solid points) and depleted TG (open points) before irradiation.

Fig. 10 .

 10 Fig. 10. (a) Schematic of TG gate oxide TID effect. PPD = photodiode; TG = transfer gate; FD = floating diffusion; STI = shallow trench isolation; PMD = premetal dielectric; P PPD-TG = PPD-TG P doped region; and P TG-FD = TG-FD P-doped region. (b) CTI as a function of the TID at Q PPD = FWC/4 for pixels with different TG lengths.

Fig. 11 .

 11 Fig. 11. (a) Schematic of STI oxide TID effect. TG = transfer gate. (b) CTI as a function of the TID at Q PPD = FWC/4 for pixels with different TG widths.

Fig. 12 .

 12 Fig.[START_REF] Marcelot | Pinned photodiode CMOS image sensor TCAD simulation: In-depth analysis of inpixel pinning voltage measurement for a diagnostic tool[END_REF]. V PIN characteristic is reported for pixels with different TG widths. The inset shows a magnification of the characteristics to highlight the charge partition and thermionic emission regimes.

Fig. 13 .

 13 Fig. 13. Summary and localization of the TID-induced degradation sources with the TG in accumulation regime. The TID range in which each degradation source becomes important is also

TABLE I SUMMARY

 I OF THE PIXEL DESIGNS INVESTIGATED damages with the aim to develop radiation resistant CIS able to withstand the environmental constraints.


Jérôme Boudet

Marc C Jacob

Joëlle Caro

Emmanuel Jondeau

Bo Li

Wavelet Analysis of a Blade Tip-Leakage Flow

I. Introduction

I N TURBOMACHINES (shrouded fans, compressors, turbines, etc.), clearances are located between vane or blade tips and end walls (hub or casing) in order to allow a relative motion. Such gaps result in a strong leakage flow, driven by the pressure difference and affected by the relative wall motion. When it interacts with the main flow, the leakage jet produces a streamwise vortex, referred to as the tip-leakage vortex (TLV) and possibly other vortices (e.g., counterrotating vortices) [START_REF] Lakshminarayana | Fluid Dynamics and Heat Transfer of Turbomachinery[END_REF]. This is sketched in Fig. 1. At a high Reynolds number, turbulence is transported and produced in this vortical flow, and broadband noise is generated. Similar flows also develop on aircraft wings, around the clearances located between the flap sides and the enfolding static airframe components.

Detailed experimental characterizations of the tip-leakage flow in a blade cascade have been carried out by Muthanna and Devenport [START_REF] Muthanna | Wake of a Compressor Cascade with Tip Gap, Part 1: Mean Flow and Turbulence Structure[END_REF] and Wang and Devenport [START_REF] Wang | Wake of a Compressor Cascade with Tip Gap, Part 2: Effects of Endwall Motion[END_REF], with stationary and moving end walls. This extensive work has been used as reference for a large-eddy simulation (LES) performed by You et al. [START_REF] You | Large-Eddy Simulation Analysis of Mechanisms for Viscous Losses in a Turbomachinery Tip-Clearance Flow[END_REF]. LES uses a direct description of the largest and most energetic turbulent eddies, which enables a detailed analysis of the turbulent dynamics. More recently, Pogorelov et al. [START_REF] Pogorelov | Cut-Cell Method Based Large-Eddy Simulation of Tip-Leakage Flow[END_REF] presented LES simulations of a five-blade rotor, with a particular attention paid to the tip-leakage flow.

In the present paper, a rather simple configuration is considered: a single airfoil, between two end plates, with a clearance at the lower end. This simplicity enabled Jacob et al. [START_REF] Jacob | Time-Resolved PIV Measurements of a Tip Leakage Flow[END_REF][START_REF] Jacob | Tip-Leakage Flow: Advanced Measurements and Analysis[END_REF] to carry out a detailed experimental characterization, on both aerodynamics and acoustics. A zonal large-eddy simulation was also performed on the same configuration and compared very favorably to the experiment, as shown by Boudet et al. [START_REF] Boudet | Zonal Large-Eddy Simulation of a Tip Leakage Flow[END_REF][START_REF] Boudet | Tip-Leakage Flow: A Detailed Simulation with a Zonal Approach[END_REF] on mean flow, Reynolds stresses, and spectra. In both the experiment and the simulation, a broad hump was observed on pressure spectra at tip, around 1.3 kHz, within the frequency range in which the tip leakage has a significant noise contribution (0.7-7 kHz). The objective of the present paper is to exploit the high-fidelity simulation for a detailed analysis of this unsteadiness in the tip-leakage flow, around 1.3 kHz.

II. Configuration and Numerical Parameters

A. Experimental Configuration

The experimental configuration is constituted by a single airfoil set in the potential core of a jet, between two end plates. As sketched on Fig. 2, a clearance is arranged between the airfoil and the lower (casing) plate. The chord length is c 0.2 m, the upstream velocity is U 0 70 m∕s, which yields a Reynolds number Re c 9.3 × 10 5 and a Mach number M 0.2. The clearance height is h 0.01 m, and the freestream rms velocity fluctuation at inflow is 0.5% U 0 . In a previous version of the experiment [START_REF] Jacob | Aeroacoustic Investigation of a Single Airfoil Tip Leakage Flow[END_REF], the angle of attack was 15 deg (0.5 deg), but the experimental results used in the present paper have been obtained during a more recent campaign, in which the angle of attack had to be set to 16.5 deg (0.5 deg) in order to recover the same airfoil loading as the original experiment. The details of the experimental configuration and an analysis of the experimental results are presented by Jacob et al. [START_REF] Jacob | Time-Resolved PIV Measurements of a Tip Leakage Flow[END_REF][START_REF] Jacob | Tip-Leakage Flow: Advanced Measurements and Analysis[END_REF].

B. Zonal Large-Eddy Simulation

The simulation uses a zonal approach. It enables defining a region of interest (the tip clearance region in the present case), in which large-eddy simulation is used for a direct description of the most energetic turbulent eddies. In the other regions, a Reynolds-averaged Navier-Stokes (RANS) approach is used in order to reduce the computational cost. The zonal decomposition of the domain is defined by the user, through a weighting function of the LES and RANS eddy viscosities. The lateral boundaries of the LES zones need to be approximately positioned along mean stream surfaces. The formulation of the zonal approach is explained in details by Boudet et al. [START_REF] Boudet | Zonal Large-Eddy Simulation of a Fan Tip-Clearance Flow, with Evidence of Vortex Wandering[END_REF]. In Fig. 3, instantaneous contours of velocity are shown on a plane just above the clearance, together with the zonal decomposition of the computational domain for the present simulation. The region of higher velocities corresponds to the tip clearance jet, and the velocity fluctuations downstream of the suction side indicate the TLV location. The main jet is deviated by the airfoil, and this deviation has to be taken into account to reproduce the airfoil loading, as discussed by Moreau et al. [START_REF] Moreau | Analysis of Flow Conditions in Freejet Experiments for Studying Airfoil Self-Noise[END_REF]. At this spanwise position, near the tip, the LES zone corresponds approximately to the interior of the jet. Along the span, it extends over 5h above the lower end plate. This zonal decomposition has been defined based upon a preliminary RANS computation. Most of the jet shear layers and the outer region at rest are simulated with RANS in order to alleviate the computational cost. On the lateral boundaries of the LES zone, where the flow is roughly tangent to the surface, no specific treatment is done; eddy viscosity evolves smoothly from the LES value to the RANS value, and vice versa, controlled by the weighting function. However, turbulent quantities need to be imposed at the inflow of the LES zone; a flatplate boundary layer is simulated with LES over a limited width and duplicated laterally to feed the LES zone. The duplication is sketched by the arrows in Fig. 3. The width of the incoming boundary layer domain is 1.4h, corresponding to 12 times the momentum thickness at 0.5c upstream of the blade leading edge. At the inflow, turbulence outside of the boundary layer is neglected. Concerning the models, LES uses the shear-improved Smagorinsky model from Lévêque et al. [START_REF] Lévêque | Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows[END_REF], and RANS uses the Wilcox k -ω model [START_REF] Wilcox | Reassessment of the Scale-Determining Equation for Advanced Turbulence Models[END_REF].

The solver Turb'Flow is an in-house finite-volume code for multiblock structured grids. The inviscid flux interpolation uses a four-point centered scheme. In the LES zone, a fourth-order artificial viscosity is used (definition in Boudet et al. [START_REF] Boudet | Implementation of a Roughness Element to Trip Transition in Large-Eddy Simulation[END_REF]), with a coefficient evolving smoothly from 0.003 at the lower end-wall, up to 0.02 above. Increased numerical viscosity is used in peripheral regions. The viscous flux interpolation uses a two-point centered scheme. Time marching is explicit, with a three-step Runge-Kutta scheme and a time step of 5.6 × 10 -6 c∕U 0 . Because the simulation was initiated before the second test campaign, the original angle of attack (15 deg) was chosen. After convergence, results were stored every 3000 iterations over more than 10 ⋅ c∕U 0 (instead of 6 ⋅ c∕U 0 in [START_REF] Boudet | Zonal Large-Eddy Simulation of a Tip Leakage Flow[END_REF]), and flow statistics were computed on the fly.

In the LES zone, the grid resolution is Δx < 80 (streamwise), Δy < 1.5 (wall normal), and Δz < 30 (cross-stream), for a full LES resolution of the boundary layers. The computational domain extends over 29c axially, 37c laterally, and 1c spanwise, with the end plates extending over the whole domain. The total number of grid points is about 150 × 10 6 , distributed over 524 structured blocks for parallel computing. More details about the computation and its validation against experimental data can be found in [START_REF] Boudet | Zonal Large-Eddy Simulation of a Tip Leakage Flow[END_REF][START_REF] Boudet | Tip-Leakage Flow: A Detailed Simulation with a Zonal Approach[END_REF].

An instantaneous three-dimensional view of the simulated flowfield is shown in Fig. 4. The turbulent eddies in the incoming boundary layer are shown to interact with the leakage flow. The TLV vortex also appears on the figure and is characterized by higher velocities.

III. Wavelet Analysis: Methodology

In both the experiment [START_REF] Jacob | Time-Resolved PIV Measurements of a Tip Leakage Flow[END_REF][START_REF] Jacob | Tip-Leakage Flow: Advanced Measurements and Analysis[END_REF] and the simulation [START_REF] Boudet | Zonal Large-Eddy Simulation of a Tip Leakage Flow[END_REF][START_REF] Boudet | Tip-Leakage Flow: A Detailed Simulation with a Zonal Approach[END_REF], a broad hump was observed in near-field spectra at the airfoil tip, around 1.3 kHz. Moreover, its frequency range, which extends over more than two octave bands, is located within that of the tip-leakage noise (0.7-7 kHz), which makes it particularly interesting. In the present section, a methodology is set up to identify the flow mechanism at the origin of this unsteadiness.

A time trace of the pressure felt by probe 46 is plotted in Fig. 5. Probe 46 is located on the pressure side at 77.5%c, 1.5 mm above the tip edge. Large oscillations are observed, with a time scale of order 10 -3 s, which is consistent with the frequency 1.3 kHz. The objective is to detect these events and identify the flow features that are correlated with them. Since these events are unevenly spaced and show variations in their duration, wavelet analysis appears to be promising if not more appropriate than classical Fourier analysis to identify such intermittent events. The detection of flow events with wavelets is a well-known topic of research and has already been applied to the present configuration by Camussi et al. [START_REF] Camussi | Experimental Study of a Tip Leakage Flow: Wavelet Analysis of Pressure Fluctuations[END_REF] on the experimental results of the first campaign. The present paper is not focused on the mathematics of the wavelet technique but merely relies on an elementary wavelet decomposition, with the objective to identify the unsteady flow phenomena in the tip region. Compared to the paper of Camussi et al., the novelty of the present investigation lies in the use of validated numerical results, which provide a threedimensional high-frequency description of the flow, particularly of interest for the detection of local flow phenomena.

The classical wavelet known as Mexican hat is used to decompose the pressure signal from probe 46. The continuous wavelet transform of the full-length signal is carried out with MATLAB® (R2012b), and the resulting scalogram is plotted in Fig. 6. This figure shows the time evolution of the energy content associated with the main unsteady flow events. These events are not identified through a classical Fourier time-frequency analysis but by the time scales of the wavelets they are associated with. A number of energetic events are observed around the wavelet scale 2 ⋅ 10 -4 s. Given the shape of the Mexican hat wavelet (for which the scale corresponds to the abscissa distance between the maximum and the x intercept), this scale value can be associated with a period of 8 ⋅ 10 -4 s, i.e., a frequency of ∼1.3 kHz. These events correspond to the large oscillations observed in Fig. 5 and to the spectrum hump around 1.3 kHz. In the scalogram (Fig. 6), the maxima that satisfy the following rules are marked with symbols: the frequency between 0.5 × 1.3 and 2 × 1.3 kHz, and the energy superior to 20% of the scalogram maximum. These maxima of energy correspond to either local pressure maxima or local pressure minima. The crosses indicate the local pressure maxima (positive decomposition coefficient), and the circles indicate the local pressure minima (negative decomposition coefficient). These extrema are used to locate the events. Considering their distribution in the scalogram, the following comments can be drawn. The events do not occur regularly in time but by packets of variable duration, during which minima and maxima of pressure alternate. The scale of the events (and the associated frequency) is also variable, which can be related to the broad shape of the hump in the spectrum.

To isolate the flow phenomenon responsible for these events, a conditional average of the three-dimensional flow is carried out, triggered by the events marked in the scalogram of probe 46. This is expected to smooth out most of the turbulent fluctuations and bring out the unsteady phenomenon. The operation is first carried out on pressure probe 46 itself. Practically, we consider all the pressure minima in Fig. 6, we define time windows over one period on each side of the minima, and we average the different windows with each other. This yields the time trace of the mean event, as shown in Fig. 7. Two methods are tested for the average: the period is either assumed fixed (equal to 1∕1300 ≈ 8 ⋅ 10 -4 s) or calculated from the local scale value of the event (4 × scale). In the first method (standard averaging), the time scale of the windows is not altered, whereas in the second method (rescaled-events averaging), the instantaneous windows are rescaled to the reference period (equal to 1∕1300 ≈ 8 ⋅ 10 -4 s) before the average. The amplitude of the mean events in Fig. 7 is significant (≈250 Pa peak to peak) and comparable to that of the instantaneous events observed in Fig. 5. But the shape of the mean events is much smoother. These observations indicate there is a good repetition of the event shape in the instantaneous signal, and the averaging is effective. The rescaled-events averaging is slightly more effective, with a slightly larger amplitude of the mean event. The difference is moderate, but the methods are of similar complexity, and consequently the rescaled-events averaging is used for the rest of the study. To conclude this presentation of the conditional averaging procedure, one must notice the choice of the pressure minima as event centers is arbitrary. The whole procedure could be performed likewise by centering the events on the pressure maxima. Furthermore, Fig. 6 shows a good alternation of the minima and maxima. Consequently, even if the events are centered on the minima, the maxima can be detected by the lateral maxima of the wavelet, at an equivalent 180 deg phase shift.

IV. Analysis of Unsteadiness at Tip

The conditional average controlled by the wavelet analysis of the probe signal is now applied to the whole three-dimensional (3D) unsteady flowfield. This process is expected to preserve the unsteady phenomenon associated with the events detected by probe 46, while smoothing out the other fluctuations. The 3D flow event is decomposed into 32 phases, from phase -180 deg (time - cutting plane at constant x positioned through the maximum of VFI. The most intense fluctuations occur in the aft part of the airfoil (constant x plane at 79%c), near the probe that has been used to detect and define the event (probe 46 is at 77.5%c). The VFI is the most intense around the suction-side/tip corner, in the shear zone between the leakage flow and the main flow. Fluctuations are also observed in the TLV, but with a smaller intensity.

To characterize the flow phenomenon associated with this event, profiles of the conditionally averaged y-velocity component v CA ∕U 0 are plotted in Fig. 9 on a line at the clearance outlet (position shown in Fig. 8). These profiles are characterized by the intense leakage flow at low Δz∕h (v CA ∕U 0 ≈ 0.8) and backflow above (v CA ∕U 0 < 0). The backflow corresponds to a tip separation. Interestingly, this separation is shown to oscillate, in correlation with the event phase: the separation is thick at θ -180 deg, it progressively reduces until θ 0 deg, and it thickens again for positive θ. These results indicate the event observed on probe 46, associated with the spectrum hump around 1.3 kHz, corresponds to an unsteady tip separation. The corresponding Strouhal number, calculated with U 0 and the tip-gap height (of the same order as the airfoil thickness in this region), is St 0.19. This order of magnitude is compatible with a vortex shedding process.

V. Conclusions

The unsteadiness of a simplified tip-leakage flow has been analyzed in a zonal LES simulation, previously validated against experimental results. Attention has been focused around a specific frequency (1.3 kHz, or St 0.19), at which a hump was previously observed in spectra at the tip.

A methodology has been proposed to identify the unsteady phenomenon related to this frequency. It uses wavelets to 1) detect the irregular occurrences of the events and 2) define a conditional average of the 3D flowfield. Fluctuations in the time scales of the events have been captured by the methodology and relate to the broad nature of the spectrum hump. Finally, the analysis of the conditionally averaged flow allowed to locate the origin of the events and to characterize the physical phenomenon as an unsteady separation at the blade tip.

The present methodology can be used to locate and identify intermittent flow phenomena around a given frequency. In the present configuration, it allowed discerning a blade tip separation that contributes to the far-field noise.

Fig. 1

 1 Fig. 1 Sketch of a tip-clearance flow.

Fig. 2

 2 Fig. 2 Experimental configuration. The origin of the coordinate system is on the blade tip/trailing-edge corner, with z ≤ 0 in the clearance.

Fig. 3

 3 Fig. 3 Instantaneous contours of the velocity magnitude around the airfoil at z 0.5h, with indication of the LES and RANS zones (the separation between the zones is marked by a thick gray line).

Fig. 4 Fig. 5

 45 Fig. 4 3D instantaneous view of the zonal LES simulation. Isosurface of Q criterion, colored by the velocity magnitude.

2 ×Fig. 6

 26 Fig. 6 Scalogram of the fluctuating pressure signal collected from probe 46. Crosses (x) indicate local pressure maxima and circles (o) indicate local pressure minima.

Fig. 7

 7 Fig. 7 Time trace of the mean event on probe 46.

Fig. 8 Fig. 9

 89 Fig.8VFI of the conditional average on a constant x plane at 79%c. The thin white line at the clearance outlet corresponds to the position of the velocity profiles in Fig.9.

Acknowledgments

This work has been carried out in the frame of the Sino-French project AXIOOM, funded by Agence Nationale de la Recherche (ANR) and National Natural Science Foundation of China (NSFC). It was granted access to the high-performance computing (HPC) resources of Centre informatique national de l'enseignement supérieur (CINES) under the allocation c20152a5039 made by Grand Equipement National de Calcul Intensif. Bo Li was supported by the China Scholarship Council during his Ph.D.