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1

Abstract— Complementary metal oxide semiconductor

(CMOS) active pixel sensor (APS) prototypes made using 0.18

µm technology process have been developed for the Extreme

Ultraviolet Imager of the Solar Orbiter mission. Backside

illuminated CMOS APS devices i.e., 256 x 256 pixels area, 10 µm

pixel pitch with different pixel designs have been fully

characterized in the visible and in EUV wavelengths. A set of

irradiation tests were carried out to investigate the degradation

of the devices expected in the space environment conditions of

Solar Orbiter. Total ionizing dose effects from grounded

measurements are presented up to 150 krad[SiO2]. The prototype

sensors show the immunity to single-event latch up at linear

energy transfer’s of 67.7 MeV cm2/mg but were observed to

suffer from strong degradations after proton irradiation test

(with a cumulated fluence up to 4x1011 protons/cm2) and from

single event functional interrupt.

Index Terms— Image sensors, Radiation effects, Space

applications.

I. INTRODUCTION

he Extreme Ultraviolet Imager (EUI) onboard Solar

Orbiter consists of a suite of two high-resolution imagers

centered at wavelengths of 17.4 nm (HRI17.4) and 121.6 nm

(HRILy) and one dual-band full Sun imager (FSI) telescope at

17.4 nm and 30.4 nm [1], [2].

For the EUI instruments, the APSOLUTE project for “APS

Optimized for Low-noise and Ultraviolet Tests and

Manuscript received November 15, 2012. The APSOLUTE development

project was supported by the Belgian Federal Science Policy Office

(BELSPO), which is part of the EUI project (ref: EUI PEA C90343).

A. BenMoussa, B. Giordanengo and S. Gissot are with the Solar Terrestrial Center of Excellence (STCE), Royal Observatory of Belgium, Circular 3, B-

1180 Brussels, Belgium (corresponding author: 32 2 373 02 76; fax: 32 2 374

98 22; e-mail: ali.benmoussa@stce.be). G. Meynants, X. Wang, B. Wolfs and J. Bogaerts are with the CMOSIS nv,

Coveliersstraat 15, 2600 Antwerpen, Belgium.

U. Schühle is with the Max-Planck-Institut für Sonnensystemforschung, 37191, Katlenburg-Lindau, Germany.

G. Berger is with the Catholic University of Louvain-la-Neuve, Chemin du

cyclotron 2, B-1348 Louvain la Neuve, Belgium. A. Gottwald, C. Laubis, U. Kroth and F. Scholze are with the Physikalisch-

Technische Bundesanstalt (PTB), Abbestr. 2-12, D-10587 Berlin, Germany.

A. Soltani is with the Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN) F-59652, Villeneuve d'Ascq, France.

T. Saito is with the department of Environment & Energy, Tohoku Institute

of Technology, Sendai, Miyagi 982-8577, Japan.

Experiments” has been initiated to develop a complementary

metal oxide semiconductor (CMOS) active pixel sensor (APS)

with 10 μm pixel pitch. To be sensitive to the extreme

ultraviolet (EUV) range i.e., for the HRI17.4 and FSI

telescopes, a backside illumination (BSI) approach is proposed

[3], [4].

In this paper, we report on the irradiation damage tests

performed on the BSI-APSOLUTE prototypes. We

characterize their dark current and responsivity in the visible

and EUV i.e., at around 17 nm spectral range, before and after

irradiation tests, conducted with:

- gamma rays to simulate a total ionization dose (TID), using

Cobalt-60 (60

Co) sources up to 150 krad[SiO2],

- protons for displacement damages (DD) with a cumulated

fluence up to 4x1011

particles/cm2,

- heavy ions for a single event effect (SEE) with ions at linear

energy transfer (LET) values up to 67.7 MeV cm2/mg.

We conclude with a summary of the various results and

offer recommendations to reduce expected degradation for the

fabrication of the EUI flight model (FM) detectors.

II. EXPERIMENTAL DETAILS

A. APSOLUTE Structure

APSOLUTE, 256 x 256 pixels sensors (rolling shutter)

containing 16 pixel designs, organized in blocks of 64 x 64

pixels, have been fabricated in a 0.18 µm CMOS process

technology (see Fig. 1(a)). Each pixel structure is based on the

pinned photodiode (PPD) where their in-pixel transistors differ

in shape, size and number. One of the main advantages of the

PPD is that the thin (70 nm) and heavily doped i.e., 2x1018

cm-

3 analyzed by secondary ion mass spectrometry, p

+ layer can

significantly reduce the dark current.

A particularity of the APSOLUTE sensor is that it applies a

so-called ‘dual-transfer’ scheme to achieve a high dynamic

range and minimize the noise contribution from the analog

chain. The approach reads out individual pixels through a high

gain (HG) and low gain (LG) path. The associated floating

diffusion nodes (FDHG=2.7 fF and FDLG=10 fF) determine the

sensitivity of the pixel path. For efficient charge transfer from

the PPD to the FD node, the transfer gate (TX) transistor

shape and size were optimized by adapting the gate width (W).

As an example, pixel designs #9 to #12 contain 6 transistors

Irradiation damage tests on backside-illuminated

CMOS APS prototypes for the Extreme

Ultraviolet Imager on-board Solar Orbiter

A. BenMoussa, S. Gissot, B. Giordanengo, G. Meynants, X. Wang, B. Wolfs, J. Bogaerts, U. Schühle

G. Berger, A. Gottwald, C. Laubis, U. Kroth , F. Scholze, A. Soltani, and T. Saito

T

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(T) per pixel with 2 transfer gates (TXHG and TXLG) as shown

in Fig. 1(b). Pixel design #9 and #12 have the same regular

TXHG (WHG=2.2 µm) but with an extra p+ implant for the latter

on the FDHG that increases the full well capacity (FWC). The

goal of this implant is to reduce leakage between the PPD and

the FD during the exposure time. Pixel design #10 has a

smaller TXHG (WHG=1.2 µm) and pixel design #11 has a ‘U-

shaped trench-gated’ [5] TXHG (WHG=2.2 µm). For pixel

design #9 to #12, TXLG has the same design i.e., WLG=2.7 µm

and LLG=0.6 µm and the source follower (SF) transistors width

and length are identical i.e., 0.8 µm.

Fig. 1. (a) Pixel arrangement of the 16 pixel designs (256x256 pixels format),

organized in blocks of 64x64 pixels. (b) Schematic representation of the dual TX 6-T PPD pixel design (BSI approach). It should be noted that pixel

designs #8 and #13-16 using a dual source follower (SFHG, SFLG) transistors

could not meet the desired specification showing a limited FWC (<30 ke-) and were not fully characterized.

The EUV sensitivity of the APSOLUTE prototypes is

achieved with BSI on a silicon-on-insulator (SOI) material (p-

type, Si epilayer resistivity of 30 Ω cm). The epitaxial Si layer

is thinned down to either 2.75 or 2.6 µm depending on the

prototypes to gain further sensitivity in the EUV range.

Fig. 2 shows the basic analog output sensor architecture. Each

pixel contains 512 analog data samples, two for each pixel due

to the dual-transfer scheme. 512 column gain stages (up to

16x) are implemented. The output stage converts the signal to

a fully differential signal, which is sent to an off-chip analog-

to-digital converter. The APSOLUTE sensor architecture

details and operation are reported elsewhere [3], [4].

Fig. 2. Analog output sensor architecture of APSOLUTE (256x256 pixel format). The top right panel shows the simplified pixel analog path

architecture.

B. Irradiation Conditions

Three APSOLUTE prototypes containing all pixel designs

structures (denoted APT-BSI-642-0xx in the following) were

characterized before, during, and after each irradiation test and

annealing step. Although the bias applied to the test devices

shall be worst-case conditions to produce the greatest

radiation-induced damage [6], the prototypes were grounded

during 60

Co -rays and proton irradiation tests and during the

post-annealing. However it should be noted that during the

Solar Orbiter mission phase, the duty cycle of EUI will be less

than 20% with long interruptions between solar observation

periods. Moreover during the cruise phase of Solar Orbiter

(approximately 3 years), the EUI detectors will remain almost

all the time unbiased.

The 60

Co irradiations were carried out at the Cyclotron

Research Center (CRC) facility at Louvain-La-Neuve

(Belgium) [7]. Three devices were irradiated with a dose level

up to 150 krad[SiO2] corresponding to the TID in SiO2

expected over the Solar Orbiter mission lifetime (under the

worst case of 1 mm thick aluminum shielding) [8]. In space,

detectors are usually exposed to a low and continuous dose

rate. However, for this accelerated test, 1 krad[SiO2]/h was

recommended by the Evaluation Testing Method (ESCC

22900) [6].

The proton irradiations were performed at the Institut de

Physique Nucléaire et Atomique et de Spectroscopie at the

University of Liège (Belgium). Three APSOLUTE detectors

were tested in a vacuum chamber at the available energies of

10 and 15 MeV, with three different fluence levels up to

4x1011

p/cm2.

The single event effect (SEE) tests were carried out at the

Heavy Ion Facility (HIF) CYCLONE of the CRC. The

radiation sources were high-LET i.e., low penetration

cocktails of ions capable of delivering the required fluence of

107 ions/cm

2 at 5 different LETs up to 67.7 MeVcm

2/mg (cf.

Table I). The flux was around 104 ions/cm

2/s (dosimetry error

bars are within 10%) and was incident normal to the detector

surface.

1(5T, single TX)

2(5T, single TX)

3(5T, single TX)

4(5T, single TX)

5(5T, single TX)

6(5T, single TX)

7(5T, single TX)

8(7T, single TX,

dual SF)

9(6T, dual TX)TXHG (regular)W/L=2.2/0.6

10(6T, dual TX)TXHG (regular)W/L=1.2/0.6

11(6T, dual TX)

TXHG (U-shaped):W/L=2.2/0.6

12(6T, dual TX)TXHG (regular)W/L=2.2/0.6

(FDHG p+ implant)

13(8T, dual TXand dual SF)

14(8T, dual TXand dual SF)

15(8T, dual TXand dual SF)

16(8T, dual TXand dual SF)

64 pixels

64

pix

els

a)

native SiO2

p-Si (epitaxial 2.6 or 2.75 µm)

Backside illumination

Shallow TrenchIsolation (STI)

Si handle wafer (750 µm)

pinned photodiodeFDHG (n+)

SCR

SiO2-SiO2 bonding

TXHG Vout

Ibias

Columni

SFrow select

reset

Vdd (3.3V)

p-well

b)

Field-free region

S (switch)

FDLG

TXLG

p+

(n)

Pixel Array

256 x 256

512 Column Gain Stage

Column Multiplexer (512:1)

Output Stage

SP

I r

eg

iste

rP

ixel C

on

tro

l

Column

Stage Control

MUX Clock

and Sync.

Analog Data Output

External

Pixel Control

Sensor Settings

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TABLE I PARAMETERS OF THE IRRADIATION TEST CAMPAIGN

Gamma TID krad[SiO2]

1.173 & 1.332 MeV up to 150

Proton Fluence (cm-2) DD (TeV/g)

10 MeV 1x1011 915 60 2x1011 1830 120

4x1011 3660 240

15 MeV 5x1010 390 20 1x1011 780 40

4x1011 3120 170

Heavy Ions LET (MeV.cm2/mg)

N 3.3 0.5 Ne 6.4 1.0

Ar 15.9 2.5

Kr 40.4 6.5 Xe 67.7 10

C. Optical Setup

The optical measurements in the 450-1000 nm spectral

range were carried out at the Detector Measurement

Laboratory (DeMeLab, STCE) using a collimated and tunable

monochromatic light beam emitted by a 100 W tungsten light

source combined with a monochromator and integrating

sphere. The distance between the integrating sphere and the

detectors was kept at a large enough distance for the beam to

be considered approximately normal to the detector surface.

The responsivity in the EUV spectral range was measured

in the laboratory of the Physikalisch-Technische

Bundesanstalt (PTB) at the electron storage ring BESSY II

(Berliner Elektronenspeichering Gesellschaft für

Synchrotronstrahlung) of the Helmhotz-Centre Berlin. The

detector calibration chamber was under ultra-high vacuum

conditions, and manipulation stages allowed us to scan the

sample area and to toggle between test and calibrated

reference detectors [9]. The detector characterizations before

irradiation are reported elsewhere [4].

III. RESULTS AND DISCUSSION

A. Dark current

The dark current (DC), expressed in e-/s/pix, is the signal

measured in the absence of incident photons and is a limiting

factor for long-time exposures, where DC shot noise forms a

dominant source of noise.

The DC measurements between two irradiation doses were

performed in air inside an optical black box. The detector was

held in place by a holder, which was electrically isolated from

but thermally connected to a thermoelectric cooling system. A

calibrated resistance thermometer (Pt100) was added close to

the detector surface to better estimate the absolute temperature

error, which is estimated to be around 1K. The variation of the

DC (cf. inset of Fig. 3b) was measured over the temperature

range with the detector mounted inside a stainless steel

vacuum chamber. The refrigerated/heating circulator system

(model Huber Unistat 405) was used to thermalize the detector

through a copper cold finger. This setup allows us to stabilize

the temperature with ± 0.05 K.

The mean DC is derived from the linear fit of the integrated

signal versus the integration time (IT) from 0% up to 50% of

the saturation level [4]. In case of a non-linearity is observed

(typically at low IT during the irradiation tests), the best-fit

straight line is performed on the slope region up to 80% of the

saturation level. Note that during integration time, the TX low

level is 0V (high level is 3.3 V), which still provides anti-

blooming during exposure.

To minimize the influence of the temporal noise,

measurements were performed by taking 100 frames for each

IT steps. To convert the signal from digital number (DN) to e-,

two different methods were used to extract the detector

conversion gain ( ): the Photon Transfer Curve [10] and the

mean-variance analysis [11]. For both methods, the

measurements were performed with fixed and high-intensity

light sources and short exposure times to negate the DC

contribution [4].

1) Total Ionization Dose-induced DC

Fig. 3(a) shows the mean DC (and distribution) measured at

room temperature as a function of the TID. The DC shows a

relatively small increase i.e., 1 to 2 e-/s/pix/krad up to 110

krad[SiO2]. The variation of the mean DC below 110

krad[SiO2] is not fully understood and could be related to the

non-stabilization of the detector temperature. Note that the

maximum duration for the characterization of the three

APSOLUTE detectors between two -ray doses was only two

hours to avoid any annealing effect [6].

Different mechanisms could contribute to the DC increase

related to the ionization effect in oxides (e.g., SiO2) [12]-[16].

Intensive studies have recently been performed to understand

and mitigate the TID-induced effect on 4T PPD image sensors

[17]-[19]. They all report a linear DC increase as a function of

-rays exposure level in contrast to our measurements. Thus

the small TID-induced DC below 110krad[SiO2] might be

related to a combination of factors, such as the irradiation

condition (e.g., dose rate [20], applied bias condition [21]) and

the pixel design (e.g., geometry, doping concentration, BSI

approach).

Since the scaling of the process technology (here 0.18 m) has

already demonstrated the hardness of MOSFET gate oxides up

to 1 Mrad[SiO2] [22], major sources of leakage current are

due to radiation-induced defects located in the Shallow Trench

Isolation (STI) [23]-[25]. As in any PPD CMOS image sensor

process, the STIs are protected by a p-well, which has proved

to be robust against STI-induced leakage [26]. For

APSOLUTE, the space charge region (SCR) ends inside the p-

well region and depending on the concentrations, the SCR is

about 30 to 50 nm away from the STI (cf. Fig. 1b).

Above 110 krad[SiO2], the DC sharply increases up to at least

a factor two at 150 krad[SiO2]. The sudden increase is not yet

understood. During the TID irradiation, we could observed a

decrease in the FWCLG as reported in [18],[27] but after

annealing at room temperature, the FWCLG increases back to

its pre-irradiation level (within 2% error bar). For APSOLUTE

prototypes, the FWC is dependent on the TX size and shape,

and it is limited by the TX sub-threshold current and not only

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by the size (area) of the photodiode [4], [28]. Ongoing

investigations have already shown that the TX leakage could

be reduced significantly with an additional implant in the FD

region. This approach strongly reduces the dependence of the

FWC as a function of the photon flux and is ready to be

implemented for the EUI FM detectors.

Fig. 3. (a) Mean DC at room temperature as a function of the TID (pixel

design #12) and (b) after irradiation, as a function of elapsed time during

annealing (in hours). The inset of a) shows the DC distribution as a function of the TID and the inset of b) shows the mean DC as a function of 1000/T for

APT-BSI-642-019 (irradiated up to 150 krad[SiO2]). The detectors were

grounded during the irradiation and post annealing.

After irradiation (see Fig. 3(b)), the DC continues to increase

during the first 12 hours showing a reverse annealing which is

generally attributed to an increase of new interface states with

time created after irradiation in the MOS oxides (see page 176

in [12]). After 24 hours the DC recovers at an exponential

decay with time during two weeks at room temperature. After

a two-week period, the DC levels off and remains almost

constant, however it did not completely return to its pre-

irradiated value (~ +40%).

The inset of Fig. 3(b) shows the mean DC Arrhenius plots

after the 60

Co -rays irradiation tests and annealing. At low

temperature and depending of the pixel design, the extracted

activation energy (Ea) is very small (e.g., 0.05 eV for pixel

design #12). Before irradiation, the same DC behavior has

been observed [4], which does not correspond to the

Shockley-Read-Hall (SRH) generation mechanism. This

leakage current, which is dominant at low temperature, is still

unclear and our current hypothesis is that process-like direct

(band-to-band) tunneling under the TX transistor (with 7 nm

thick SiO2 gate oxide) could explain this unusual behavior.

The dark signal (of individual pixel) shows also temporal

fluctuations after TID irradiation due to the recombination-

generation centers at the Si-SiO2 interfaces [29], [30]. As

shown in Fig. 4, this effect is integration time (IT) and

temperature dependent, and therefore, it is referred as the dark

current random telegraph signal (DC-RTS). The DC-RTS

should not be confused with the MOSFET or SF-RTS [31].

Fig. 4. Dark signal RTS (e-) at 20°C of one pixel (pixel design #9 and #12)

of APT-BSI-642-019 at three different integration time after TID tests and

annealing. The inset shows the RTS of one pixel (pixel design #10) as a function of the temperature between +20°C and +40°C.

2) Displacement Damage-induced DC

For the available proton energies i.e., 10 and 15 MeV,

Coulomb and elastic nuclear interactions are mainly

responsible for displacement damage [32]. The Non-Ionizing

Energy Loss (NIEL, expressed in MeV cm2/g) describes the

rate of the particle energy loss due to atomic displacements

inside the material through nuclear elastic/inelastic reactions

[33]-[36]. The product of the NIEL and the particle fluence

gives the non-ionizing (displacement damage) dose.

As an example, Fig. 5 shows the DC distribution as a function

of the proton fluence (at 15 MeV proton) and after 1 week

post-annealing at room temperature. The mean DC, measured

just after the proton irradiations, increases almost linearly with

the displacement damage (DD) dose. The high DC value

observed during proton irradiation (see histogram at 4

1011

p/cm2

in Fig. 5) is partially attributed to the collisions of

the incident protons with the Si nuclei leading to the creation

of Frenkel defects, such as the creation of vacancy-interstitial

pair atoms [12]. After 1 week post-annealing, the mean DC

decreases by a factor 3.

The inset of Fig. 5 shows the mean DC Arrhenius plots after

the proton irradiation tests and annealing. The extracted Ea is

located close to the Si mid-gap, i.e., 0.70 eV at high

temperature and 0.62 eV at lower temperature, which suggests

0 20 40 60 80 100 120 140 1600.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

103

100

101

102

103

104

pre-irradiation

150 krad[SiO2]

130 krad[SiO2]110 krad[SiO

2]

Dark Current (e-/s/pix)

Pix

el C

ou

nt

60 krad[SiO2]

APT-BSI-642-019

a)

1.5 to 2 e-/s/pix/kradMea

n D

ark

Cu

rren

t (k

e-/s

/pix

)

Pixel design #12

APT-BSI-642-011

APT-BSI-642-012

APT-BSI-642-019

Total Ionisation Dose (krad[SiO2])

TIDthreshold

=110 krad[SiO2]

0 100 200 300 400 500 600 7000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.210

0

101

102

103

104

105

106

107

Ea2

= 0.0125 eV

Ea2

= 0.05 eV

Mea

n D

ark c

urr

ent

(e-/

s/p

ix)

1000/T (K-1)

APT-BSI-642-019

150 krad[SiO2] + annealing

Pixel design #10

Pixel design #12

Ea1

= 1.12 eV

reverse annealing

b)

M

ean

Dar

k C

urr

ent

(ke-

/s/p

ix)

RT: Room Temperature

at 100°C

Annealing Time (h)

RT

0 20 40 60 80 100 120 140 160 180 200

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

0 20 40 60 80 100 120 140 160 180

1000

1500

2000

+40°C

+30°C

+20°C

Dar

k S

ignal

(a.

u)

Time (min)

one pixel (design #10), IT=0.59s

pixel design #9

pixel design #12

IT=0.59s

IT=1.77s

IT=1.18s

Dar

k S

ign

al (

e-)

at 2

0°C

Time (min)

APT-BSI-642-019

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5

that the SRH generation emission is the dominant mechanism

over the whole temperature range i.e., between +100°C and -

35°C. However to learn more about the exact nature of these

defects that exhibit distinctive DC generation rates, specific

measurements, characterization and annealing are required

that fall beyond the scope of this study.

Fig. 5. Dark current distribution at room temperature of APT-BSI-642-017

(pixel design #12) as a function of the proton fluence at 15 MeV. The inset shows the mean DC as a function of 1000/T. Note that the detector was

grounded during the irradiation and post annealing.

Fig. 6 shows the thermal generation rate increase i.e., DC

increase normalized to the pixel depletion volume with the

universal dark current damage factor (Kdark)[37]. The large

deviation between the experimental results and the Kdark curve

(underestimated by a factor 3) cannot be dominantly attributed

only to the ionization damage contribution induced by protons.

Fig. 6. Mean increase of the thermal generation rate of APT-BSI-642-017 as a

function of the displacement damage dose. The red line corresponds to the universal damage factor Kdark at 300K after 1 week post-annealing [37]. The

inset shows a close view of the dark signal RTS (IT=0.345s) at 40°C of one pixel (pixel design #12) after the proton irradiation tests and annealing.

Indeed to allow comparison with the 60

Co -rays DC results,

the proton TID level has been estimated (not shown). As an

example, at 40 krad[SiO2] there is more than two orders of

magnitude between the mean DC of APT-BSI-642-017 i.e.,

41.35 ke-/s/pix estimated from the deviation of the Kdark curve,

and the mean DC measured during the 60

Co -rays irradiation

on APT-BSI-642-019 i.e., 319 e-/s/pix. Note the intra-

columnar recombination [16] is neglected, which is an

important parameter to be accounted for when comparing the

effect of the two different radiation sources.

The inset in Fig. 6, show the DC-RTS of pixel design #9 and

#12. In contrast with the RTS observed after the 60

Co -rays

irradiation tests (cf. Fig. 4), we observed more discrete RTS

levels of the dark signal due to meta-stable generation centers

located in the SCR [29]. The DC-RTS induced by ionization

and by displacement damage processes will be studied in more

detail in future works since it may impact the low-light

sensitivity of the EUI instrument.

B. External Quantum Efficiency (EQE)

The external quantum efficiency (EQE) is defined as the ratio

of the number of detected photons per second, , to

the number of incident photons, , measured by the

calibrated reference detector. is computed by

summing over the beam region in DN and dividing by the

effective conversion gain ( ) as follow:

[ ⁄ ] ∑

with,

[ ⁄ ] [ ⁄ ] [ ⁄ ]

where is the DN value of the ith

pixel corrected from the

detector offset in the region of interest (ROI), is the

conversion gain measured in the visible range and is the

external quantum yield defined, at a given wavelength, as the

ratio of the number of collected photoelectrons to the number

of incident photons. In the EUV range, is higher than

one as shown in the inset of Fig. 7(b). The EQE was measured

by estimating at each wavelength and to reduce the

random error, 100 frames were taken per IT at regular

intervals. The same measurements and analysis were repeated

under ‘dark’ conditions to remove the DC.

1) TID-induced EQE

As an example, Fig. 7(a) shows the EQE of APT-BSI-642-

012 between 450 and 1000 nm. The EQE was measured

several weeks after the 60

Co -ray irradiation and it decreases

by 2% at 500 nm, 24% at 600 nm and around 18% between

650 and 900 nm. A different wavelength dependency with a

stronger decrease towards the UV range has been reported in

similar image sensor technology [18],[26]. However, as shown

in Fig. 7(a), the EQE below 500 nm does not seem to be

affected by the 60

Co photons. If the measurements between

450 and 500 nm are correct, it implies that the TID-induced

defects do not impact the APT-BSI-642-012 EQE.

Note that APSOLUTE detectors are not fully depleted (cf.

Fig. 1(b)). For APT-BSI-642-012, the field-free region

thickness is approximately 950 nm (Siepi = 2.75 m). The inset

of Fig. 7(a) shows APT-BSI-642-019, which has a thinner

102

103

104

105

106

10-3

10-2

10-1

100

101

102

103

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.210

0

101

102

103

104

105

106

107

Ea2

= 0.62 eV

Ea1

= 0.70 eV

Mea

n D

ark

cu

rren

t (e

-/s/

pix

)

1000/T (K-1)

4x1011

p/cm2 (15 MeV) + annealing

Pixel design #12

pre-irradiation

5x1010

p/cm2

1x1011

p/cm2

4x1011

p/cm2

Norm

aliz

ed P

ixel

Co

un

t

Dark Current (e-/s/pix)

APT-BSI-642-017 (15 MeV)

1 week annealing (RT)

102

103

104

101

102

103

104

105

106

30 40 50 601000

1100

1200

Dar

k S

ign

al (

DN

)

Time (min)

IT=0.345s, +40°C

Ther

mal

Gen

erat

ion R

ate

(e-/

s/

m3)

data (APT-BSI-642-017, Pixel design #12)

Kdark

= 0.19 e-m

-3s

-1(TeV/g)

-1

Non-ionizing absorbed dose (TeV/g)

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

6

field-free region of 800 nm (Siepi= 2.6 m) and as a

consequence, starts to show a stronger decrease of its EQE

(e.g., 12% at 500 nm) between 450 and 500 nm following the 60

Co -rays irradiation test.

An important remark is that APSOLUTE detectors do not

have a passivation layer, only a SiO2 native oxide layer on top

of the illuminated surface. Moreover in the BSI approach, the

p+ implant layer which could reduce the surface generation

DC and improve the near UV responsivity is at the bottom of

the Si epilayer i.e., facing the SiO2-SiO2 wafer bond and the

handle wafer (see Fig. 1(b)).

Fig. 7(b) shows the absolute EQE of APT-BSI-642-012 in

the EUV range. The EQE decreases by almost 38 % at 17.4

nm after the 60

Co -ray irradiation test. At this wavelength, the

absorption length i.e., the inverse of the absorption coefficient

in Si is about 520 nm [38] and is equal to that of the visible at

around 490 nm.

Fig. 7. EQE of APT-BSI-642-012 (pixel design #10) before and after 60Co γ-

ray irradiation tests a) between 450 and 1000 nm and b) between 15 and 20 nm. The inset of a) shows the APT-BSI-642-019 (pixel design #10) EQE

between 450 and 1000 nm, and inset of b) shows the theoretical and

experimental between 13 and 20 nm.

In such a situation where the absorption length is shallower

than the junction depth, the probability of the surface

recombination is governed by the absorption coefficient of the

material [39] since the density of the photogenerated electrons

and holes becomes higher as the photon penetration depth

becomes shorter. Based on this theory, the stronger decrease

of the EQE in the EUV range cannot be explained by the

surface recombination loss. One of the possible explanations

for the cause of the large change in EQE over the EUV range

may be because of a change in surface layer thickness

(oxidation) or surface contamination since the oxide is

transparent in the visible but absorptive in the EUV and

therefore a slight change in thickness or contamination can

result in a large change in EUV transmittance.

2) Displacement Damage-induced EQE

The EQE was also measured after the proton irradiation tests

(see Fig. 8) where it is found to decrease by approximately

65% at 17.4 nm. Here, the decrease can be explained by the

fact that the proton irradiation has strongly affected the charge

collection within the pixel i.e., SCR due to bulk displacement

damage, increasing the probability of recombination of the

photo-generated carriers. Although precaution was taken, a

surface contamination should not be excluded.

Fig. 8. Absolute EQE of APT-BSI-642-018 (pixel design #12) between 5 and 20 nm wavelength before and after proton tests at 10 MeV (4x1011 p/cm2) and

1 week annealing at RT. The inset shows the normalized FWCLG and GLG of

APT-BSI-642-017 (pixel design #12) as a function of the fluence at 15 MeV proton energy. The filled symbols are the values after 1000 h annealing at RT.

As shown in the inset of Fig. 8, the FWC decreases by

almost 17% at high displacement damage dose, suggesting a

reduction of the doping concentration in the PPD [40] or a

shift in the TX transistor threshold voltage [28] though

ionization effect. After 1000h annealing at room temperature

the FWC increases, but this is still around 10% below the pre-

irradiation level. Note that we did not observe (within the 5%

error bar) any change in the mean value of the detector

conversion gain ( ) after 60

Co -rays (see inset of Fig. 6(b)) or

after the proton irradiation tests (see inset of Fig. 8). The fact

that is unchanged suggests that the FD capacitances and the

CMOS electronics i.e. the column readout circuitry, have not

been affected by the irradiation tests.

500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0

500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0APT-BSI-64

2-019 (Si

epi=2.6 m)

pre-irradiation

150 krad[SiO2] + annealing

EQ

E

Wavelength (nm)

a)

APT-BSI-642-012 (Si

epi=2.75 m)

pre-irradiation

150 krad[SiO2] and annealing

EQ

E

Wavelength (nm)

5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

1.2

13 14 15 16 17 18 19 200

10

20

30

Theoretical QYext

(Si)

Qu

antu

m Y

ield

(e-

/ph)

pre-irradiation

after irradiation / annealing

Wavelength (nm)

EQ

E

APT-BSI-642-012 (Si

epi= 2.75 m)

pre-irradiation (EQE17.4nm

=52%)

150 krad[SiO2] and annealing (EQE

17.4nm=32%)

Wavelength (nm)

b)5 10 15 20

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 1x1011

2x1011

3x1011

4x1011

0.8

0.9

1.0

1.1

1.2

GLG

FWCLG

Fluence (p+/cm

2)

No

rmal

ized

F

WC

, G

APT-BSI-642-017 (Pixel design #12)

FWCLG

1000 h annealing

GLG

1000 h annealing

EQ

E

APT-BSI-642-018 (Si

epi= 2.6 m)

pre-irradiation (EQE17.4nm

= 62 %)

4x1011

p/cm2 (10 MeV) and annealing (EQE

17.4nm= 22 %)

Wavelength (nm)

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

7

C. Heavy Ion Single Event Effects (SEE)

The SEE is of increasing concern to the space community

due to damage of cosmic rays and solar energetic particles.

During the heavy ion irradiation tests, three APSOLUTE

detectors were exposed and biased with the dedicated

electronic board allowing video recording inside the vacuum

chamber. The image sensor was running on a mains power

supply (VDD) of 3.3V and the pixel array was controlled by

the pixel control block (see in Fig. 2). As a particular case of

SEE, single event latch up (SEL) is a serious occurrence, since

it is long-lived and potentially destructive. During the

irradiation no SEL has been detected on the three APSOLUTE

detectors up to a LET of 67.7 MeVcm2/mg (which is assumed

to be constant within the detector thickness). However single

event failure interrupt (SEFI) and single event transient (SET)

were detected as shown in the inset of Fig. 9 (bottom panel)

but without causing any long-term effect. Indeed, SEFI is a

type of anomaly (soft latch) caused by a single ion strike

leading to a non-functionality of the sensor that fails to

respond to control signals (block condition) and this state may

last as long as the power is maintained [12]. Cycling of the

power supply voltage was sufficient to restore normal

operation. SEFI is most likely to be linked to the serial

peripheral interface (SPI) bus setting being corrupted by heavy

ions and thus different sensor functionality errors may appear.

For instance, gain, offset or biasing settings may be drastically

changed.

The APSOLUTE sensitivity (occurrence yield) is shown in

Fig. 9 where the heavy ions cross section curve is defined as

the number of SEFI events divided by the particle fluence

(ion/cm2). To determine the saturation cross section (σsat), data

are fitted using a Weibull distribution [42].

Fig. 9. Cross section curve of APT-BSI-642-020 (pixel design #9 to #12) for SEFI as a function of the particle LET at +35°C. The inset shows successive

dark frames (rolling shutter) for pixel design #9 to #12 (4x64x64 pixels)

before (top) and after (bottom) a SEFI leading to a block condition (white screen). The ion is Kr with LET = 40.4 MeV.cm2/mg and irradiations were

performed at normal incidence.

The inset of Fig. 9 shows also the presence of hot pixels

created before the irradiation tests [4]. It has been confirmed

that the hot pixels are introduced by metal contamination

during the SOI CMOS processing, whereby the contaminated

particles are stuck in the pixel (around the STI sidewall) and

therefore contribute significantly to the DC. The EUI FM

detectors should not suffer from the hot pixel problem due to

improved solutions including a different pixel design and

CMOS processing which has already been investigated with

promising results [41].

IV. CONCLUSIONS AND RECOMMENDATIONS

Backside illuminated CMOS APS prototypes with different

pixel designs have been fully characterized regarding dark

current and responsivity in the visible and EUV spectral

ranges before and after grounded irradiation tests. The

radiation hardness has been investigated by exposure to 60

Co

-rays, protons and heavy ions.

Even if the physical degradation mechanisms are not yet

fully understood, for EUV applications the minimization of

the detector surface defects and the recombination losses is

crucial. In order to prevent the growth of a native oxide, a

backside surface treatment including a buried backside p+

implant should be applied to the EUI FM detectors to reduce

the DC and improve the detector optical performance. In

addition to minimize the DC, the leakage current observed

before irradiation and probably from tunneling effect should

be cancelled or significantly reduced.

DC-RTS was not investigated in detail in this work but is

strongly recommended for future studies of the EUI FM

detectors. DC-RTS might become a serious limitation during

low-light level solar observations.

For APSOLUTE, the sensors were grounded during the

Co60 -rays and proton irradiation tests because the processing

board was assumed to be not radiation tolerant. However

future irradiation tests and post annealing shall be performed

preferentially under operating conditions i.e., the worst-case

bias condition. In addition, for displacement damage tests,

higher proton energies (e.g., 30 and 60 MeV) shall be

performed to assess also the nuclear interactions contribution.

Although robustness to single event latch-up has been

demonstrated up to 67.7 MeVcm2/mg, SEFI effects must be

properly analyzed. It is recommended to implement a triple

redundancy concept for the EUI FM detectors, which will

largely reduce SPI setting errors caused by single event upset.

In addition, during EUI solar science operations, it is

recommended to frequently update the SPI setting after

acquiring a certain amount of frames to ensure the corrupted

data is removed and corrected from time to time.

Finally the irradiation test campaigns of the APSOLUTE

developments give promising results for EUI detector

fabrication which impose the use of large, thinned arrays for

back-side illumination, namely formats of 2048 x 2048 pixels

for the two high resolution imagers and 3072 x 3072 pixels for

the full Sun imager, all with a 10 µm pixel pitch.

ACKNOWLEDGMENT

We would like to thank the EUI detector working group

members. A special thank is addressed to Dr Matthew West

0 10 20 30 40 50 60 7010

-4

10-3

10-2

#9 #10 #11 #12

#9 #10 #11 #12

LET (MeVcm2/mg)

Cro

ss S

ecti

on (

cm2)

APT-BSI-642-020 (+35°C)

SEFI

fit (sat

= 0.96 mm2, LET

th=15.9 MeV.cm

2/mg)

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

8

(ROB) for reviewing the paper.

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