Ionizing Radiation Effects on CMOS Imagers Manufactured in Deep Submicron Process

Ionizing Radiation Eﬀects on CMOS Imagers Manufactured in Deep Submicron Process

Vincent Goiffona, Pierre Magnana, Frédéric Bernardb, Guy Rollandb, Olivier Saint-Péc, Nicolas Hugera and Franck Corbièrea aUniversitéde Toulouse, ISAE, 10 avenue E. Belin, 31055, Toulouse, France; bCNES, 18 avenue E. Belin, 31401, Toulouse, France; cEADS Astrium, 31 avenue des cosmonautes, 31402, Toulouse, France

ABSTRACT We present here a study on both CMOS sensors and elementary structures (photodiodes and in-pixel MOSFETs) manufactured in a deep submicron process dedicated to imaging. We designed a test chip made of one 128×128- 3T-pixel array with 10 µm pitch and more than 120 isolated test structures including photodiodes and MOSFETs with various implants and diﬀerent sizes. All these devices were exposed to ionizing radiation up to 100 krad and their responses were correlated to identify the CMOS sensor weaknesses. Characterizations in darkness and under illumination demonstrated that dark current increase is the major sensor degradation. Shallow trench isolation was identiﬁed to be responsible for this degradation as it increases the number of generation centers in photodiode depletion regions. Consequences on hardness assurance and hardening-by-design are discussed. Keywords: CMOS image sensor, CIS, APS, deep submicron technology, ionizing radiation, total dose, dark current, STI, hardening by design, RHDB

1. INTRODUCTION Ionizing radiation effects on CMOS image sensors for space and scientific applications have been studied1–6 for several years. However, the use of deep submicron technologies (DSM) has brought new behaviors7 such as enhanced gate oxide hardness or radiation induced narrow channel effect.8 These effects have been initially observed and explained on deep submicron low voltage MOS transistors dedicated to digital logic applications. Nevertheless, these results can not be transposed directly to image sensors nowadays manufactured in DSM CMOS technologies dedicated to imaging. Indeed, these technologies make use of additional implants and process steps in order to maximize photo-electron collection and minimize dark current through dedicated doping profiles. Moreover, CMOS imagers often use “thick gate oxide” MOSFETs (3.3 V or 2.5 V) to ensure a large dynamic range. This can significantly impact the radiation hardness of “in-pixel” devices which are rarely studied and whose sensitivity to ionizing radiation is not totally quantified. The aim of our work is to identify the weaknesses of deep submicron irradiated CMOS sensors in order to choose the hardening method to use in the future. Figure 1 presents a schematic 3-transistor (3T) active pixel architecture. This simple architecture is composed by three N-MOSFETs and one photodiode. All these devices are separated thanks to shallow trench isolations (STI). The photodiode is reverse biased and reset thanks to the reset transistor. The source follower transistor isolates the collection node from the rest of the readout chain while the SelY FET is used to select the pixel. All these three transistor dimensions are close to minimum sizes to optimize the fill-factor. As it can be inferred from the Fig. 1, ionizing radiation can degrade CMOS sensor pixel performances by changing the characteristics of the “in-pixel” MOSFETs, the photodiode and/or the STI oxide. In fact ionizing radiation is known to generate trapped charge and interface states in MOS oxides. These can lead to voltage shifts and current leakages in MOSFETs, dark current increase in photodiodes and inversion layer creation below the shallow trench isolations. In some cases, these effects on elementary devices have been seen to induce complex effects on Further author information: V.G.: E-mail: vincent.goiff[email protected], Telephone: +33(0)5 61 33 82 51 STI Collection node

N VDDrst V DDrst RST V RSTDD SelY VDD SelY (a) (b) Figure 1: 3T active pixel (a) simpliﬁed layout and (b) schematic.

Figure 2: 5×5 mm2 integrated circuit manufactured in a 0.18 µm technology dedicated to imaging. The 128×128- pixel array is located at the bottom left corner of the die. The rest of the circuit is dedicated to test structures. all the CMOS sensor performances such as dark current, conversion gain, photoresponse, PRNU and/or DSNU possibly leading to general malfunction. Therefore, we decided to study simultaneously an array of 3T pixel and “in-pixel” elementary structures, such as photodiodes and FETs to identify the degradations and their causes. In order to perform this work, the test chip presented on Fig. 2 was manufactured in a commercial 0.18 µm technology dedicated to imaging. It is constituted by one 128 × 128-3T-pixel array with 10µm-pitch and more than 120 isolated test structures such as photodiodes, gated diodes and MOSFETs with various implants and different sizes. This paper focuses on pixel behavior, hence we only present here results regarding “in-pixel” structures: photodiodes and N-channel MOSFETs. First we present the degradations observed on photodiodes and “in-pixel” field-effect transistors (FET) irradiated up to 100 krad(Si). In a second part we address total ionizing dose effects on the CMOS pixel array we designed. Finally, results and their consequences are discussed to identify the imager weaknesses and to suggest direction for future work.

2. IONIZING RADIATION EFFECTS ON ELEMENTARY DEVICES 2.1 Test structure description The selected 0.18 µm technology dedicated to imaging allows the use of devices with 1.8 V or 3.3 V maximum operating voltages. The latest have thicker gate oxides and lower doping densities to avoid high electric field effects such as gate oxide breakdown or hot carrier generation. In addition to these classical features, devices meant to be placed inside the pixel array differ from ones located outside the photo-generation area. Especially, ILDs ILDs P P STI N CIS NN CISCIS

P- P- (a) (b) Figure 3: Schematic drawing of (a) a CIS photodiode and (b) a recessed ﬁeld oxide photodiode with a P surface implant. The RFD diode is the same than the latest without the surface P implant. ILD stand for inter-layer dielectric.

activepolysilicon region gatew/o implant active area w/o implant active region w/o implantP+ active region w/o implant STI STI STI

+ N+ N+ N+ N+ N+ N+ N

(a) (b) (c) (d) Figure 4: Layout of the studied MOSFETs. (a) Standard, (b) recessed field oxide, (c) square bracket shaped STI and (d) enclosed layout transistor. substrate doping profiles are modified to improve the collection of photo-generated charges. These devices will be referred to as “in-pixel” devices, in opposition to “regular” ones which are located outside the pixel area. This is also true for photodiodes. Actually, this technology offers optimized photodiodes dedicated to CMOS sensor design. These devices will be referred to as “CIS photodiodes”. Since we are interested in pixel behavior, our study focuses on 3.3 V “in-pixel” transistors and CIS photodiodes. The designed CIS photodiodes are presented on Fig. 3. These photodiodes use CIS implants with dedicated doping profiles to improve the sensitivity and reduce dark current generation. Several aspect ratios where drawn for the CIS diode : 5 × 2000, 500 × 20, 100 × 100 and 10 × 10 µm2. This allows discriminating between perimeter dependent effects and area dependent effects. The recessed field oxide diodes (RFD) are simply CIS diodes with recessed field oxide. A surface P implant (Fig. 3b) is added to the recessed field oxide diode with P implant (RFD-P). Figure 4 presents the main MOSFETs used for this study. All the MOSTs are 3.3 V N type “in-pixel” devices. Several W/L ratios were used for helping the localization of probable degradation. Three types of edgeless transistors were manufactured and tested. The shallow trench isolation of recessed field oxide transistors (RFT) has been recessed from the channel to prevent edge leakage. Square bracket shaped STI (SBS) transistors are also “edgeless” since the surrounding field oxide is discontinued below the polysilicon gate. Finally, the enclosed layout transistor (ELT) simply does not have any edge. ELT aspect ratios were computed thanks to CERN work.9 Polysilicon gate was deposited on some STI oxides separating two N+ regions in order to form a field oxide FET (FOXFET). They were drawn with the following dimensions W = 300 µm and L = 0.84. µm to ensure a sufficient transconductance and were used to study the behavior of shallow trench oxides separating “in-pixel” devices.

2.2 Experimental details A dedicated test bench was designed to reduce leakage currents. It is composed by: a semiconductor parameter analyzer (Keithley 4200) with preamplifiers, a switch matrix with low current switching cards (Keithley 707A with 7174 cards), a custom shielded box and a custom test board using guard techniques10 to prevent leakage currents and a 16 pin TO8 metallic package with guard potential applied on the can. The noise/leakage limit of the system is limited by the switching matrix to 10 fA on the chosen voltage range: ±3.3 V. This setup was used for the measurement of photodiode dark currents, field oxide (FOX) leakage and gate leakage currents. The same system was used for MOSFET characterization except that most of the circuits were assembled and tested in PGA208 packages. This allowed performing a greater number of automated measurements but these packages suffered from high parasitic leakage current. Indeed, since no guard potential can be applied on the ceramic, the measurement limit was then increase to 1 pA. During measurements, temperature was regulated to 23◦C and relative humidity kept below 25% thanks to a nitrogen sweep. Chips were irradiated at room temperature at ONERA-DESP facility thanks to a 60Co γ-ray source. Dose rate was set to 300 rad(Si)/h, and the irradiation steps were fixed to 14, 28, 50 and 100 krad(Si). An additional step at 72 krad was performed on some devices. Then, each 100 krad(Si) irradiated sample was baked one week at 100◦C for accelerated annealing tests. Since worst case biases are well know11 for MOSFET and have recently been confirmed12 on technology below the 0.25 µm node, we decided to apply the following biases during irradiation and annealing: gate to 3.3 V and all other terminals grounded for NMOST and FOXFET. Four circuits were tested, one per irradiation step. All the threshold voltages were extracted thanks to the linear extrapolation technique13 at |VDS| = 50 mV while the subthreshold characteristics were measured at |VDS| = 500 mV. Concerning the photodiodes, worst case conditions are not well defined and can be technology and design dependent. Actually, fringing electric fields can increase the yield of electron/hole pair in the surrounding oxide but can also move the generated charge away from the sensitive area. We then decided to use two circuits, one biased during irradiation (and annealing) and one unbiased to estimate the influence of diode biasing. After each step, the irradiation was stopped in order to characterize these two circuits.

2.3 Results 2.3.1 Photodiodes As a first approximation and for reverse voltage greater than a few kT/q, reverse current of an ideal abrupt P-N junction14 can be expressed∗ Irev(Vrev) = Isat + Igr(Vrev) . (1) The first term of Eq. 1 corresponds to diffusion current from quasi neutral region, while the other term refers to generation currents from the depletion region volume and interface with SiO2. Considering only mid-gap states and equal electron and hole capture cross sections, these currents can be approximated by

2 Isat = qni AJK , and Igr = qniσvth [AJNtW (Vrev) + PJntWint(Vrev)] , (2) where K is a constant determined by minority carrier diﬀusion parameters and doping density, ni the intrinsic carrier concentration, AJ the junction area, PJ the junction perimeter, Vrev the reverse voltage, W the depletion region width, Wint the depletion region width at the Si-SiO2 interface, σ the electron and hole capture cross section, vth the thermal velocity, Nt the generation/recombination center density and nt the interface generation/recombination center area density. Reverse voltage-current characteristics of the 2000 × 5 µm2 CIS photodiode are plotted for several total doses on Fig. 5. Reverse bias current, often called dark current in imaging application, is increasing with dose and reverse voltage. This last growth is attributed to depletion region extension. As a matter of fact, the junction space charge region is known to extend with reverse voltage, and Eq. 2 shows that generation current is proportional to this extension†. This indicates that pre-irradiation and post-irradiation reverse current are dominated by generation process. This was veriﬁed thanks to the Arrhenius plot presented on Fig. 16 which shows that post-irradiation reverse current behaves like ni. This temperature dependence was also expected thanks to Eq. 2. Figure 6 presents the dark current of the four CIS diodes biased at 2.4 V before and just after the 100 krad irradiation. These currents are plotted versus the junction perimeter. It shows a very linear variation with ∗Tunneling and impact ionization are assumed negligible at room temperature in the studied lightly doped devices. †Assuming a uniform trap density. −12 x 10 60 6 7 Before irradiation After 100krad Before irradiation 5 After 14krad 50 6 After 29krad 4 After 50krad 5 After 100krad 40 3 After 168h at 100°C 2 4 30 1 3 20 0

2 −1 Reverse current (A) 10 −2 1

0 −3 Reverse current after 100krad (pA) Reverse current before irradiation (fA) 0 0 500 1000 1500 2000 2500 3000 3500 4000 0 0.5 1 1.5 2 2.5 3 Junction perimeter (µm) Reverse voltage (V) Figure 6: Dark current of the CIS photodiodes as a Figure 5: Reverse voltage current characteristics of the function of diode perimeter at 23◦C before and after 2000 × 5 µm2 CIS diode at 23◦C. 100 krad. The reverse voltage was fixed to 2.4 V. perimeter about 7 aA/µm before exposition and 0.4 fA/µm after. It clearly demonstrates that these generation currents are only due to generation centers located on the perimeter of the photodiodes. This agrees with previous work15, 16 which emphasized the important role of junction perimeter in LOCOS (local oxidation of silicon) and STI technologies. It also strongly suggests that ionizing radiation enhances this generation process. This was confirmed by forward characteristics which presented an increase of forward current at low forward voltage with dose. In addition to the general reverse current augmentation, it is interesting to notice on Fig. 5 that 60Co radiation greatly enhances the voltage dependence of dark current. Indeed, the slope of the reverse I-V characteristics is dramatically rising with ionizing dose. Equation 2 shows this can either be caused by a radiation induced increase of generation center density, by a radiation induced depletion with extension or by both. The first hypothesis is a well know phenomenon due to the buildup of interface states. The second would be caused by a positive charge buildup in the oxide. This is also a well known effect of ionizing radiation. Figure 7 illustrates how radiation induced positive trapped charge could lead to an extension of surface depletion width. This was qualitatively confirmed by Sentaurus physical simulation. In order to discriminate the contribution of the two kinds of defect, a standard 168 h 100◦C annealing step was performed on 100 krad irradiated photodiodes (Fig. 5). Dark current recovers more than 80% of its post- irradiation value. Most of interface states encountered in last decade CMOS technologies are known to anneal at high temperature well above 100◦C while trapped charge are known to disappear after this annealing step.17 This suggests that this dark current degradation was mainly caused by depletion region extension due to trapped charges. The remaining post-anneal dark current would so be caused by the interface states generated during irradiation. However, recent work18 has shown that interface states generated in STI oxides can anneal below 100◦C. This study was done on a comparable technology. If the technology studied here behaves the same way, the previous post-anneal interpretation is no longer valid, and the discrimination can not be achieved with our data. Further study of field-oxide FET should be realized in the future to clarify this point. Figure 8 shows the evolution of 2000 × 5 µm2 photodiode dark currents with irradiation. One can see that the four dark currents exhibit the same trend but with different amplitudes. Both structures, whether they were biased or not during irradiation, behave the same. This suggests that photodiode biasing does not play a significant role in the observed degradation. Concerning the recessed field oxide diodes, as expected, the surface doping level plays an important role. In fact, the RFD dark current is much more radiation and voltage dependent than the RFD-P one. This is clearly due to a strong reduction of the depletion region width by the use of the additional P implant. Supplementary measurements on gated diode structures have shown that without this P implant, the depletion region extends to the trench sidewalls for very low reverse voltage even before exposure to 60Co. Therefore, RFD space charge region extends downward, along the trench sidewalls while the RFD with P implant is thought to extend laterally, along the above SiO2 layer. From the radiation hardness point of view, −10 10 CIS diode + + + + CIS diode unbiased during irradiation RFD diode −11 N + + N + + 10 RFD−P diode

−12 10 P- P-

−13 (a) (b) 10

Figure 7: Radiation induced depletion region exten- Reverse current at 2.4V (A) sion. (a) Before irradiation. (b) After irradiation, −14 10 positive trapped charge generated in the trench ox- 0 20 40 60 80 100 After annealing120 Total ionizing dose (krad) ide helps the depletion region to extend further in the Figure 8: Photodiode (2000 × 5 µm2) reverse currents P doped region. evolution with total ionizing dose. Vrev = 2.4 V. the RFD-P is obviously better than the other structures even if the CIS diodes exhibit a dark current four times lower before irradiation. Whether this hardness is due to a reduced depletion extension or a weaker interface state generation is still under investigation. The effect of γ-ray irradiation on CIS diode photoresponse was also investigated. This was achieved with a two- step method. A normalized spectral response was measured before and after exposure thanks to an automated scanning monochromator with a 2 nm bandwidth. In order to compare the two responses, the absolute effective quantum efficiency was measured at 650 nm with a 10 nm filter and the spectral responses were corrected by this absolute measurement. The effective quantum efficiency is define here as the number of collected electrons per photon reaching the detector surface. Reflection and transmission losses in the passivation layers and inter- layer dielectrics (ILD) are then included in this efficiency factor. Figure 9 shows the resulting effective quantum efficiency before and after irradiation. No significant degradation was observed after 100 krad. This indicates

100 Before irradiation 90 After 100krad 80

10 Effective quantum efficiency (%) 0 400 500 600 700 800 900 Wavelength (nm) Figure 9: Effective quantum efficiency of a 800 × 800 CIS diode before and after 60Co exposure. that up to this total dose, ionizing radiation has negligible effect on optical transmission, charge diffusion and charge collection.

2.3.2 In-pixel transistors Figure 10a presents the effect of ionizing radiation on minimum size (W = 0.24 µm and L = 0.34 µm) “in-pixel” NMOS transistors. The only noticeable effect is an off-state current increase. As expected7 for these thin gate oxides, no radiation induced threshold voltage (Vt) shift was observed. Hence no radiation induced trapped hole was detected in the gate oxide. There is not any change in the subthreshold slope either, thus radiation induced interface state generation seems negligible too. −4 −8 10 10 In−pixel 0.24/0.34 In−pixel 10/0.34 −6 10 −9 In−pixel 0.5/0.5 10 In−pixel 0.24/10

−8 In−pixel 10/10 10 −10 10

−10 10

−11 Before irradiation 10 −12 10 After 14krad

Drain current (A) After 29krad

After 50krad Off state current (A) −12 −14 10 10 After 72krad After 100krad After 168h at 100°C −16 −13 10 10 −2 −1 0 1 2 3 0 20 40 60 80 100 After annealing120 Gate to source voltage (V) Total ionizing dose (krad) (a) (b) Figure 10: Evolution of NMOST characteristics with total dose. (a) Subthreshold characteristics of a minimum size “in-pixel” NMOST (W = 0.24 µm, L = 0.34 µm and VDS = 0.5 V). (b) Oﬀ-state current for several aspect ratio (VGS =0V, VDS = 0.5 V). Regarding the W/L ratios, the dimensions are given in micrometer.

Figure 10b gives further insight on the off-state current increase. It presents this increase as a function of ionizing dose for every transistor dimension.‡ One can see that this effect is strongly attenuated when the length of the transistor increase from 0.34 to 10 µm while the width is of less importance when increasing from 0.24 to 10 µm. This indicates a length dependent effect, most likely an edge effect. This edge effect is confirmed by Fig. 11 which shows the same plot for edgeless transistors. One can see that whatever the design used, edgeless transistors are not sensitive to ionizing dose up to 100 krad. This indicates that radiation induced off-state current is due to lateral parasitic field oxide transistors. The phenomenon is well known for LOCOS and STI processes.19 A fraction of holes generated in the oxide trenches are trapped in the STI and induce a positive charge increasing with irradiation. This charge is able to form a lateral leakage path between NMOST drain and source by activating these parasitic field oxide FETs. As expected for trapped charge related defects, this off state parasitic current disappears after the 100◦C annealing step (Fig. 10). + In addition to this drain to source leakage, drain N /Psub junction reverse current is also increasing with total dose. This is caused by the process already mentioned in sec. 2.3.1. At 2.4 V reverse voltage, 2000 × 5 µm2 N+ diode measurements showed a current increase from 0.1 pA to 10 pA after 100 krad exposure. This effect can not be seen on Fig. 10 since this current stays below the measurement limit even on the largest transistors (W = 10 µm). Gate leakage was monitored on all the standard MOSFETs. Moreover, a 100 × 100 µm2 MOS capacitor was also tested. Between −3.3 V and +3.3 V, gate currents were always below measurement limits (below 10 fA). Therefore, gate leakage current density stays below 1 aA/µm2 before and after irradiation. Field oxide transistor drain current is presented on Fig. 12. The drain current offset is clearly increasing with + total dose. This is caused by the drain N /Psub junction reverse current augmentation mentioned previously in this section. Since this particular device was not able to stand more than 8 V between its gate and the substrate, the subthreshold slope is not visible at 50 krad and below. However, 72 krad and 100 krad plots show an important threshold voltage shift, which confirm a large buildup of trapped charges. This shift let us see the beginning of the FOXFET subthreshold slope. Nevertheless, nothing can be inferred about the interface state density evolution since the main part of the characteristics is out of the measurement window. Note that some FOXFETs were also irradiated unbiased. These devices exhibit only the drain current offset increase showing that less trapped charges are generated when no electric field is applied on the STI.

‡Note that only 0.24/0.34 and 0.5/0.5 MOSTs are representative of devices usually designed in a pixel. Other devices were designed for study purpose only. −8 −8 10 10 In−pixel ELT 3.3/0.34 Before irradiation In−pixel RFT 0.4/0.34 After 14krad −9 −9 In−pixel SBS 0.24/0.68 10 After 29krad 10 After 50krad

−10 After 72krad 10 After 100krad −10 10

−11 10

−11 10 −12 10 Drain current (A)

Off state current (A) −12 10 −13 10

−13 −14 10 10 0 20 40 60 80 100 After annealing120 −4 −2 0 2 4 6 8 Total ionizing dose (krad) Gate to source voltage (V) Figure 11: Edgeless transistor oﬀ-state current evolu- Figure 12: Field oxide subthreshold I-V characteris- tion with irradiation. tics. VDS = 3.3 V, W = 300 µm and L = 0.84 µm

3. IONIZING RADIATION EFFECTS ON CMOS IMAGE SENSORS In the ﬁrst part of the paper we have presented the responses of the elementary structures which are utilized for designing CMOS sensor pixels. In the following part, we study the eﬀect of ionizing radiation on CMOS sensors knowing how the elementary structures behave under irradiation.

3.1 Design and experimental The photosensitive arrays are composed by 128×128 3T active pixels with a 10 µm pitch. These pixels contained three “in-pixel” NMOS transistors and one CIS photodiode. All these devices correspond to structures studied in the previous sections. Two pixel arrays were irradiated with the same condition than the diodes and FETs presented in sec. 2.2. The first, sensor A, was operated during exposure while the second, sensor B, was grounded. Dark current measurements were performed just after 14, 28, 50, 72 and 100 krad(Si). Then both circuits underwent a one week annealing step at 100 ◦C. Photoresponse, conversion factor, and electrical readout chain gain measurements were performed prior to exposure and just after 100 krad(Si). In order to control the photodiode cathode voltage, all the measurements were performed in “hard reset” mode by setting the reset transistor drain voltage to 2.4 V. Integration times were kept short enough to ensure a small cathode voltage variation. This was done for reducing the influence of non linear effects.

3.2 Results It is important to notice that no change was observed on the linear part of the readout chain electrical transfer function. Figure 13 shows the well known “mean-variance” plot20 of sensor A output voltage. The slope of this plot allows conversion gain estimation, which is the photodiode cathode voltage variation induced by one collected electron. One can see that this slope is the same before and after irradiation. Therefore, total ionizing dose did not change the conversion gain of the presented sensors. Note that this gain was about 7 µV/e−. More- over, as expected from photodiode results, Fig. 14 shows no significant change in effective quantum efficiency with irradiation. The photoresponse non-uniformity (PRNU) also stayed unchanged after exposure to ionizing radiation. However, both sensors underwent a large degradation. As it can be seen on Fig. 15, mean dark current dramatically increased with total dose for both operated and grounded devices. Unbiased device seems more affected by irradiation than sensor A. It is quite hard to say if it is really a bias effect or if it is just due to discrepancies between the two circuits. Like for the photodiodes, this current decreases after the annealing step. Another similitude with photodiode dark current behavior is presented on Fig. 16. One can notice on this plot that sensor dark current behaves like a generation current before and after being irradiated. In agreement with 3.5 Before irradiation 100 )

2 After 100krad 3 90

80 2.5 70

2 60

1.5 50 40 1 30

0.5 20 Output voltage variance (mV 10 Before irradiation

0 Effective quantum efficiency (%) After 100krad 100 200 300 400 500 600 0 Mean output voltage (mV) 400 450 500 550 600 650 700 750 800 850 Wavelength (nm) Figure 13: Sensor A mean-variance plot before and Figure 14: Sensor A effective quantum efficiency be- after irradiation. Integration time was fixed. Output fore and after irradiation. The monochromatic light voltage level was swept thanks to an increase of in- was obtained thanks to 10 nm interferometric filters. coming light flux.

150 Pixel before irradiation −10 Operated 10 Pixel after 100krad Unbiased Photodiode after 100krad

−12 100 10 kn → i

−14 10

50 Dark current (A) k′n2 →

Dark current (fA) i −16 10

34 35 36 37 38 39 40 41 42 43 0 0 20 40 60 80 100 After annealing120 q/kT (eV−1) Total ionizing dose Figure 16: Dark current Arrhenius plot. Pixel dark Figure 15: Tested pixel array dark current increase current is plotted before and after 100 krad. The post with total dose. The measurements were performed irradiation current of the 2000 × 5 µm2 CIS was mea- at 23 ◦C. sured at 2.4 V reverse bias. previous work, dark signal non uniformity follows the dark current evolution with total dose. Dark current standard deviation increased from 0.07 fA to 4 fA after 100 krad and then decreased to 1 fA after annealing. It should be emphasized that the unique total dose eﬀect observed on this devices is this dark current augmentation, which seems related to photodiode generation current increase. The following section discusses the likely cause of this eﬀect.

4. DISCUSSION 4.1 Dark current sources in 3T CMOS sensors § We have seen in sec. 2.3.2 that in worst case condition MOSFET leakage current at VGS = 0 V can grow up to 1 nA after 100 krad. This current is negligible compared to currents that ﬂow through the source follower and the other conducting transistors during sampling. Taking into account switch MOST size and biasing, radiation induced leakage could only lead to 1 mV variation of the voltage stored on a sampling capacitor according to a worst case scenario. Moreover, this leakage current is also negligible for digital circuit used for addressing. Since gate leakage has not been observed and since threshold voltage shift was also negligible, it can be said that all

§Minimum size NMOST and worst case bias during irradiation. the electronics located after the source follower is naturally hardened against ionizing radiation at least up to 100 krad. Before the charge to voltage conversion, leakage currents are critical. Indeed photodiode dark current is about 0.2 fA before irradiation. Hence, any collection node parasitic current will dramatically change the output voltage. Figure 17 shows the parasitic current sources connected to the photodiode cathode. Source follower gate leakage is neglected accordingly to previous results. The total photodiode capacitance discharge current is then given by: IDC = IrevCIS + IrevN+ − IRST ± IFOXFET . (3)

IrevCIS and IrevN+ are respectively the photodiode and drain junction reverse currents while IRST and IFOXFET represent the reset transistor and field oxide transistor off state currents. Since unbiased FOXFET irradiation has shown no subthreshold conduction up to 100 krad its leakage current can be neglected in front of photodiode junction reverse current. Note that reset MOSFET can reduce the pixel dark signal by constantly resetting the photodiode with its leakage drain to source current (IRST). However, this parasitic current is unlikely to increase much with total dose in irradiated sensors. In fact, gate to source and drain to source voltages of these devices are negative¶ during integration. In the operated sensors, reset MOSFETs were active less than 1% of the frame readout time under irradiation. As a consequence, reset transistors were mainly irradiated with the following bias conditions: 12 VGS < 0, VGD < 0 and VGSub = 0. These conditions are known to consequently reduce trapped charge effects. In sensors grounded during irradiation, trapped charge effects on MOSFETs are also thought to be very small. Moreover, after irradiation, these negative biases, in addition to substrate effect, are limiting off-state conduction during integration. Therefore, we will suppose that this recharge current is also negligible. Hence, pixel total dose degradation can be approximated by the dark current increase of the CIS and N+ diodes alone: ∆IDC = ∆IrevCIS + ∆IrevN+ . (4) Using the fact that these current variations are proportional to the junction perimeter allows estimation of pixel dark current augmentation by multiplying the test diode currents by the ratio of pixel junction perimeters over the test diode perimeters: ∆IDC = PCIS∆jrevCIS + PN+ ∆jrevN+ . (5)

The current linear density increase, ∆jrevCIS and ∆jrevN+ are retrieved from the CIS photodiode measurements presented in sec. 2.3.1 and the N+ diode measurements presented in sec. 2.3.2. The CIS junction perimeter, PCIS, and the reset MOST source perimeter PN+ are given by the pixel layout. Figure 18 presents the result of this simple approach on sensor A dark current. As expected by previous results, pixel dark current and model dark current behave the same. Taking into account the simplifying hypothesis and device mismatching, the model ﬁt quite well the data set. This is strong evidence that the photodiode and the reset MOST source PN junctions are the main sources of irradiated sensor degradations. As already mentioned in sec. 2.3.1, generation process around these diodes is the main cause of this eﬀect. This obviously agrees with the similar behaviors of test photodiode and sensor pixel dark currents discussed in the previous section.

4.2 Hardness assurance and hardening perspectives The fact that total dose effects in these pixel arrays are dominated by photodiode generation current increase can have an impact on hardness assurance for CMOS sensor qualification. Actually, total dose effect standard test methods∗ were mainly designed for MOSFET devices and systems. Since, MOSFETs degradation does not have any effect on the sensor response up to 100 krad, results of qualification procedures on these imagers could lead to misinterpretations. Especially regarding worst case biasing and annealing step. Indeed we have seen previously that worst case biasing are not well defined for these photodiode generation currents and that the one week 100◦C step can reduce both interface state and trapped charge densities, while interface state density in MOS oxides could increase after irradiation if kept at room temperature.21 As interface state density plays

¶between −1 and −2.4V ∗MIL-STD-883 Test Method 1019.7, and ESNSCC Basic Speciﬁcation No. 22900 Collection 100 Measurement node 90 Model Irev CIS 80 I 70 FOXFET 60 IRST 50 40 V DDrst 30

20 Dark current increase (fA) RST 10 Irev N + V 0 DD SelY 0 20 40 60 80 100 After annealing120 Figure 17: 3T active pixel schematic with parasitic Total ionizing dose Figure 18: Comparison between theoretical and mea- current sources. sured radiation induced dark current increase. a significant role in the degradation, real mission behavior could be worse than the worst case given by the standard MOSFET test methods. Hence, further study of photodiode and shallow trench isolation behaviors under irradiation should be done to correctly use this qualification techniques on CMOS sensors manufactured in deep sub-micron process. From the hardening-by-design point of view, MOSFETs hardening does not seem necessary for total dose up to 100 krad. Design efforts should focus on reducing the ionizing radiation effects on the collection node. Using an enclosed layout transistor for resetting the diode is still a very good solution, since it will considerably reduce reset MOST source generation current (IrevN+ ). This is true only if the source is enclosed by the gate. Regarding the photodiode itself, a recessed field oxide design should reduce this dark current by a factor five, according to Fig. 8. A more efficient solution, which could completely eliminate this parasitic effect, could be the use of a gated photodiode, also called surround-gate pixel by Pain et al. . In fact, to surround the photodiode by a gate will allow the control of peripheral surface potential, hence to control the surface depletion region extension. Moreover, MOSFETs results have shown that no interface states are created in the gate oxides up to 100 krad. Further work will focus on implementing efficiently such a solution in the pixel.

5. CONLUSION Pixel arrays and elementary devices, such as photodiodes and MOSFETs, were total dose tested to 100 krad. Ionizing radiation caused off-state edge leakage current increases on minimum size N channel transistors and a large dark current increase on photodiodes and CMOS sensors. On one hand, we have shown that MOSFET degradations have negligible effects on sensor performances. On the other hand, the work presented here shows that the generation process enhancement in the perimeter of the photodiode is responsible for the sensor dark current growth. In order to improve the radiation hardness of these imagers, designers should focus on reducing the impact of total dose on photodiode generation current. Worst case bias and worst case accelerated annealing temperature should be carefully determined to ensure that qualification tests will lead to photodiode worst case responses.

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