Radiation effects on the silicon semiconductor detectors for the ASTRO-H mission

Katsuhiro Hayashi (Hiroshima University)

Park InChun, Kyohei Dotsu, Sho Nishino, Masayuki Matsuoka, Hajimu Yasuda, Yasushi Fukazawa, Takashi Ohsugi, Tsunufumi Mizuno, Hiromitsu Takahashi, Masanori Ohno, Satoru Endo (Hiroshima University), Takaaki Tanaka (KIPAC/Stanford University), Hiroyasu Tajima (Nagoya University), Motohide Kokubun, Shin Watanabe, Tadayuki Takahashi (ISAS/JAXA), Kazuhiro Nakazawa (The University of Tokyo), Hisashi Kitamuara, Yukio Uchihori (NIRS)

The ASTRO-H mission

ASTRO-H

• 6th Japanese X-ray satellite• Scheduled to be launched in 2014• 4 observation systems

HXI

SGD

HXT

2

HXI/SGD• Multi layers of strip-type sensors• Hybrid structure of Si (5—30 keV) and CdTe (30—80 keV)

• Narrow field of view Si/CdTe Compton camera• Si: good scatterer to determine the photon direction by the Compton kinematics

SGD

Surrounded by BGO crystal lower the background level

HXI Si

CdTe

hard X-ray

2

2

21

2

1cosEcm

EEcm ee

3

HXI/SGD• Multi layer of strip-type sensors• Hybrid structure of Si (5—30 keV) and CdTe (30—80 keV)

• Si/CdTe Compton camera• Narrow field of view• Si: good scatterer to determine the photon direction by the Compton kinematics

SGD

Surrounded by BGO crystal lower the BGD level

HXI Si

CdTe

hard X-ray

2

2

21

2

1cosEcm

EEcm ee

4

RequirementEnergy Energy resolution

HXI 5—80 keV < 2 keV @ 60keV (FWHM)

SGD 10—600keV < 2 keV @ 100keV (FWHM)

Flux

(ph/

s/ke

V/cm

2 )

detectable sensitivity

4 Energy (keV)

Suzaku/HXD

HXISGD

Specifications

Size 3.4 x 3.4 cm2

Strip pitch 250 μm

Strip width 150 μm

Thickness 500 μm

Number of strips 128

Strip capacitance <10pF

Leakage current <70pA/strip@-15deg

Bias voltage 300V

Energy resolution <2 keV @60keV (FWHM)

DSSD (for HXI)

• Utilized for HXI • DC-coupled electrode

5

n-strip is surrounded by atoll-shaped p-stop for localization of each strip

Specifications

Size 3.4 x 3.4 cm2

Strip pitch 250 μm

Strip width 150 μm

Thickness 500 μm

Number of strips 128

Strip capacitance <10pF

Leakage current <70pA/strip@-15deg

Bias voltage 300V

Energy resolution <2 keV @60keV (FWHM)

DSSD (for HXI)

• Utilized for HXI • DC-coupled electrode

6

• n-strip is surrounded atoll-shaped p-stop for individualization of each strip

Improved configurationDC coupled Al electrode is attached on the p-stop to reduce the Johnson noise by the resistance

Si-pad (for SGD)

Specifications

Size 5.4 x 5.4 cm2

Pixel size 3.14 mm2

Thickness 600 μm

Number of pixels 256 (16x16)

Capacitance <10pF/pad

Leakage current <30 pA/pad @-20deg

Full depletion voltage 250 V

Energy resolution <2 keV@60keV(FWHM)

• Pixel-array-type Si sensor• DC-coupled electrode• Signal is brought out by a readout electrode on the sensor• Improved configuration

Increase the thickness of SiO2 layer (1um1.5um)

SiO2

Si

readout electrode

readout electrode

7

Basic performance of DSSD

60keV

TestPulse

• Leakage current vs bias voltage

• 241Am spectrum (-15 deg) • Energy resolution @60 keV (FWHM)

8

measurement requirement

20 pA/strip @-15deg <70 pA/stirp @-15deg

measurement requirement

Pside 1.2(+/-0.04, stat) keV < 2 keV

Nside 1.8 (+/-0.1, stat) keV

Basic performance of Si-pad• Leakage current vs bias voltage

• 241Am spectrum (-15 deg)• Energy resolution @60keV (FWHM)

Bias voltage(V)

Leak

age

curr

ent (

pA) 20℃

0℃

-20℃

9

measurement requirement

10 pA/pad @-20deg <30 pA/pad @-20deg

measurement requirement

1.4(+/-0.05,stat) keV < 2keV

10

Proton and 60Co γ-ray irradiation tests were performed for the DSSD and Si-pad with some improved structures

• Basic performance of DSSD and Si-pad is satisfied with the requirements• In the orbit of ASTRO-H, the sensors suffers radiation damage about 1krad/year

Proton Irradiation

Si-pad

Proton beamDSSD

11

Value Note

Energy 150 MeV Typical energy in the ASTRO-H orbit

Total particle number 3x109 cm-2 • 0.2 krad• 10 years irradiation level in the ASTRO-H orbit with the BGO crystals surrounding

Beam size Φ ~1cm

Bias voltage 300V (DSSD)250V (Si-pad)

Full depletion voltage

Leakage current (-15 )℃

• Leakage current on the spot area increased by ~10 pA/strip (DSSD) and ~40 pA/pad (Si-pad)

• Low current increase of DSSD limited spot region (φ~1cm) for the irradiated strip

• α = 2.2x10-8(nA/cm) (Si-pad)almost consistent with other Si sensor measurements (e.g; Ohsugi et al. 1988)

DSSD Si-pad

beforeafter

before

after

DSSD

Si-pad

spot size

α=current/fluence(damage constant)

12

Φ~1cm

60Co γ-ray irradiationLeakage current after irradiation (Si-pad)

Si-pad DSSD

60Co

13

Value Note

Total dose 1, 2(3), 5, 10 krad

Bias voltage 300V (DSSD)250V (Si-pad)

Full depletion voltage

Annealing -15 deg Annealing effect were seen after 5 and10 krad irradiations

Dose vs. Leakage current (-15 ) ℃

• Leakage current became larger as the dose level increased. ~90 pA/strip (DSSD-Pside) and ~40 pA/pad (Si-pad) (10 krad)

• Increase of the leakage current is 0.13nA/cm2/krad (DSSD-Pside)consistent with that of other measurements of the micro-strip sensors for GLAST (0.11nA/cm2/krad)

DSSD Si-pad

14

Defect of trap level (gamma-ray irradiation)

Defect of trap level (Et-Ei)

DSSD Si-pad

0 krad 0.049 +/- 0.006 eV 0.197+/-0.007 eV

10 krad 0.023 +/- 0.005 eV 0.099+/- 0.006 eV

)/)(exp()/)exp(()2/exp(2

kTEEkTEEkTE

CTJitit

ggen

DSSD Si-pad

Ei Eg

Et (0krad)

Et-Ei lowered new trap level is created by the irradiation and becomes dominant source of the leakage current 15

Et (10krad)

Effect on the noise level

2/1

22 2144pfpsBn

p

BRMS CCDCTRkCBI

RTkAE

meets the requirement of < 2 keV and is not significantly change

• Leakage current affects the noise performance as the shot noise • Noise level (-15deg)

• Noise level is due to the leakage current, capacity and resistance of the detector and electronic circuit

DSSD Si-pad

Shot noise Before irradiation 0.37 keV 0.20 keV

10 krad 0.68 keV 0.41 keV

Sum of noise level due to the other components

Did not change by irradiation

1.1 keV 1.4 keV

Total Before irradiation 1.2 keV 1.4 keV

10 krad 1.3 keV 1.5 keV

16

Shot noise

Summary

We have developed Si sensors (DSSD and Si-pad) utilized for the X-ray observatory ASTRO-H

• Basic performance of the Si sensors meets the requirements for the mission• Proton and gamma-ray irradiation test for the Si sensors were performed in order to evaluate the irradiation damage• Leakage current level became larger by 40--90 pA/strip(pad), which is almost consistent with other measurements of Si sensor• From the noise level, radiation effects are not significant under the operation temperature for 10 years orbits

17

Back up

18

Improvement configurations

19

• Noise level lowered by attached Al electrode on the p-stop for n-strip of DSSD

beforeafter

readout electrode length (cm)

capa

cita

nce

(pF)

• Capacitance of dependency of readout electrode length is lowered by increase of the thickness of SiO2 layer for Si-pad

Radiation effect on the Si sensor

• In the orbit of ASTRO-H, the sensors suffers radiation damage mainly by the cosmic-ray protons with the level of ~1 krad per year Surface damage

- Holes generated in the surface SiO2 layer is trapped in the defects close to the interface Accumulation of positive charge in the SiO2 layer

Bulk damageLattice atoms are displaced from original lattice position by interaction with high energy particles

These defects create energy levels in the forbidden band and leakage current increases noise level increase• Carry out irradiation test to evaluate effect on the DSSD and Si-pad with some improvement configurations

SiO2

hSi+

defect crystal

Gamma-ray, Particle

High energyparticle

h h

Si

Si

hSi+Si+

h

20