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Proton Radiation Damage in High-Resistivity n-Type Silicon CCDs

C. Bebek, D. Groom, S. Holland, A. Karcher, W. Kolbe, J. Lee, M. Levi, N. Palaio, B. Turko,

M. Uslenghi, M. Wagner, G. Wang

Lawrence Berkeley National Laboratory

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Outline

Description of LBNL developed CCDs Radiation damage study and measurement

of charge transfer efficiency (CTE) Identification of radiation induced traps

using pocket pumping techniques Fitting of CTE data to trap densities Summary of results

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LBNL CCD Technology

LBNL CCDs differ from conventional devices in that they are made from high resistivity, n-type silicon which can be fully depleted

They use buried p-channels (instead of n-channels) for charge storage and transport. Carriers are holes.

Large photosensitive volume results in a greatly enhanced near IR response.

They can be operated back illuminated without the need for thinning, resulting in a significant cost savings.

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Radiation Testing of CCDs

Plans to employ LBNL CCDs in long term space based missions require them to have high radiation tolerance.

We performed room temperature irradiation study using 12 MeV proton doses of 5x109, 1x1010, 5x1010 and 1x1011 p/cm2.

Characterized the devices by measuring their CTE and dark current.

Measurements demonstrated excellent radiation tolerance superior to n-channel CCDs.

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Charge Transfer Efficiency Calculation

•CTE measured by exposing CCD to 55Fe x-rays which deposit 1620 e- per pixel•X-ray peak heights are plotted vs row or column number transferred.•Slope of the line fitted to the clustered single pixel events is measure of the CTE

CCD irradiation dose 1x1010 p/cm2 measured at 125K

X-ray stacking plot for analysis

Parallel CTE =0.99997

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Measured CTE vs Radiation Dose

CTE vs Radiation Dose

0.999550.999600.999650.999700.99975

0.999800.999850.999900.999951.00000

0 2 4 6 8 10Dose (1010Protons/cm2)

CT

E

Parallel CTE

Serial CTE

•CTE vs radiation dose measured at 128K

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CTE vs Temperature at 1x1011 p/cm2

CTE vs Temperature

0.99955

0.99960

0.99965

0.99970

0.99975

0.99980

0.99985

0.99990

0.99995

1.00000

100 120 140 160 180 200 220

Temperature (K)

CT

E

serial CTE

parallel CTE

Both serial and parallel CTE exhibit significant temperature dependence due to interactions with radiation induced trapping centers.

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Pocket Pumping Illustrated

•Pre-radiation measurement

•Trap density 0.0021 traps/pixel

Pocket pumping:

5 shifts, 60000 cycles

Dose 1x1010 protons/cm2

Trap density 0.096 traps/pixel

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Flatfield

level

Pocket pumping

peak

Depletion

peak

Saturation

Pixel Value (adu)

Histogram to Measure Trap Efficiency

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Measured Trap Effectiveness

•Clock overlap period = 19.5 s

•Peaks at 160 and 190K.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

100 150 200 250

Temperature (K)

Tra

p D

ep

th (

e- )

.

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Hole Traps Found in n-Type Si

Trap Name Cross Section Energy Level[cm2] [eV]

VV(+/0) 8.0x10-16 0.200Ci

(+/0) 4.28x10-15 0.284CiOi

(+/0) 2.45x10-15 0.360CiCs

(+/0) 5.26x10-14 0.084

Sii

V V + V VV

Ci

CiOi

CiCs

Proton Irradiation

Trap parameters measured using DLTS

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Charge Transfer Efficiency Model

][)(

e

Pz

e

TNTT

e

t

e

t een

NFn

NCTI

00 2

1

CTECTI 1where

•The effectiveness factor, F determines the temperature range over which the trap will significantly reduce the CTE.•F should also predict the shape, amplitude and location of each trap as measured in the pocket pumping experiment.•We use an improved model which includes asymmetrical clocking and finite trap capture time.

Nt and ne are the density of traps and electrons (holes) per pixel

T0 and Tp are the clock overlap and period

Nz distance between x-rays and e the trap emission time

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Trap Effectiveness vs Temperature

0

0.1

0.2

0.3

0.4

0.5

100 150 200 250 300

Temperature (K)

Tra

p E

ffe

ctiv

en

es

sV-V

Ci

C-O

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Trap Effectiveness vs Temperature

0

0.1

0.2

0.3

0.4

0.5

100 150 200 250 300

Temperature (K)

Tra

p E

ffe

ctiv

en

es

s

•Excellent matching of calculated and pocket pumping measurement of trap effectiveness for clock overlap time of 19.5 s.

•Confirms the existence of the proposed traps.

V-VCi

C-O

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Fitting Trap Concentrations to CTI

Dose Trap concentration (pixel-1)(p/cm2)

TimeV-V C C-O

1x1011 9 days 0.680±0.045 -0.079±0.040 0.283±0.0325x1010 13 days 0.268±0.020 -0.024±0.018 0.156±0.015

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Fitted Trap Density

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Summary

4 LBNL p-channel CCDs proton irradiated at 12 MeV and doses up to 1x1011 p/cm2.

Measurements show they are significantly more radiation resistant than n-channel CCDs.

Pocket pumping experiments identified primary trapping centers impacting CTE.

Fit of CTE to trap concentrations shows good agreement with radiation dose.

Results will help us to predict and perhaps enhance long term performance in radiation environment.

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