SPIE Astronomical Instrumentation June 26, 2008 Physics of reverse annealing in high- resistivity ACIS Chandra CCDs Catherine E. Grant (MIT) Bev LaMarr,

SPIE Astronomical Instrumentation June 26, 2008

Physics of reverse annealing in high-resistivity ACIS Chandra CCDs

Catherine E. Grant (MIT)

Bev LaMarr, Gregory Prigozhin, Steve Kissel, Stephen Brown, Mark Bautz

Catherine Grant (MIT) June 26, 2008

Talk Outline

• (Brief) Description of ACIS CCDs• Flight experience with irradiation/annealing• First ground experiment in 2002• Experimental setup in 2005• Data analysis results

– Sources of systematic errors

• Summary & future plans


ACIS CCDs

• Framestore-transfer• High-resistivity float-zone

silicon• Depletion depth: 50-75 m• 24 m pixels• 40 sec/pix image-to-

framestore transfer rate• Four output nodes 105 pix/s• 3.2 sec nominal frame time


1999 flight experience with irradiation and annealing

• Displacement damage in imaging array• Charge transfer inefficiency (CTI) ~ 1-2 x 10-4 at 6 keV• No damage in framestore and serial-transfer arrays• No damage to back-illuminated CCDs• Believed to be due to soft protons (~200 keV) scattered by

mirror during radiation belt passages• After focal plane was warmed from –100°C to +30°C for 8

hours, CTI increased by 34%


Laboratory experiment 2002

• Designed to duplicate flight experience• Low-temperature irradiation

– CCD at –100°C; 120 keV protons

• 8-hour +30°C annealing cycle• CTI increased by 150%

– Much larger than flight increase (34%)

• Possible causes: variations between CCDs, different irradiating particle spectrum, ?

• See Bautz, et al. 2005, IEEE Trans. Nucl. Sci, 52(2), 519

CXC

ACIS Page 6

Proposed “Model” for CTI Increase from AnnealingOne possible model

Reverse annealing of carbon impurities causes CTI increase during bakeout.

Expect chip-to-chip variations in carbon concentration to cause variations in CTI increase.

Measurements of carbon concentration show much smaller variation than required by differences between 2002 laboratory & flight results.

Schematic of silicon lattice changes during irradiation & bakeSi-Si- C -Si-Si| | | | |Si-Si-Si- P -Si| | | | |Si-Si-Si-Si-Si

-Si-Si-Si-Si| C | | |Si-Si-Si- P -Si| | | | |Si-Si-Si-Si-Si

-Si-Si-Si-Si| | | | |Si-Si-Si-CP-Si| | | | |Si-Si-Si-Si-Si

Pre-irradiation:• P impurites intentionally

implanted• C impurities benign• CTI perfect

Post-irradiation, pre-bake:• Si displaced (some CTI increase)• C impurites displaced to benign

locations; don’t affect CTI• C can’t migrate at low temp.• CTI somewhat greater

Post-irradiation, post-bake:• C migrates during bake &

bonds to P (or C) causing additional traps

• CTI increases due to bake


Laboratory experiment 2005• Designed to better explore parameter space and understand

why flight and ground experience differed• Six front-illuminated CCDs

– 5 from ACIS backup focal plane– 1 from 2002 experiment (only 2 quadrants were used)

• Four proton energies• Three types of annealing cycle

– 8-hour +30C (like flight and 2002)– Long duration +30C anneal (over 100 hrs)– Multi-T isochronal (1-hr at 0°C, +10°C, +20°C, +30°C)


Experiment Details: Irradiation

• 2 MeV van de Graaff accelerator at GSFC radiation lab

• Proton energies of 100, 120, 180 and 400 keV• Dosimetry via surface barrier detector between

accelerator and CCDs• Dose chosen to cause (pre-anneal) CTI ~ 10-4

• CCDs irradiated cold (–100°C) and powered off


Experimental Details: Camera• Camera holds two CCDs

side by side• Framestore shielded• Slit-shaped baffle

confined beam (3.5mm x 12 mm)


Experimental Details: Beamline

CCD chamber

Accelerator

55Fe source

Gate valve

Dosimeter

Flexible bellows


Irradiation Pattern

• Pivoting CCD chamber• Beam confined by slit• Beam fits within one

readout quadrant• CCD aligned by

directly imaging low-flux proton beam


Experimental Procedure

• Cool CCD to –100°C; measure CTI• Irradiate CCD quadrant (x 4)

– Select energy; align proton beam with low-flux CCD images

– Irradiate quadrant (CCD power off) periodically monitoring flux w/ beam monitor

• Measure CTI• Perform annealing cycle (one of three types)• Cool CCD to –100°C; measure CTI


CTI Measurements

• Fractional CTI increase due to annealing:– R = CTIannealing/CTIirradiation

• Two (known) sources of systematic error:– Temperature variations– Post-irradiation CTI relaxation

• Correct where possible; increase error budget to compensate


Anomalous Annealing Results

• Averaged over all CCD quadrants• Weak dependence on proton energy• Result similar to flight, 2002 experiment is highly discrepant


Long Duration Annealing

• Anneal at +30°C for increasingly longer intervals• Maximum CTI increase after 8 hours


Multi-T Isochronal Annealing

• Test sensitivity of annealing CTI increase to temperature• 1-hour each at 0°C, +10°C, +20°C, +30°C• CTI only increases after Tanneal reaches +30°C• A puzzle! - CTI initially decreases


Why was 2002 different from 2005?

• Cannot be due to:– Proton energies (dependence is too weak)– CCD variations

• one CCD was irradiated/annealed in both ground experiments - no significant difference from other five CCDs in 2005

• Temperature differences? Possibly– Camera setups, temperature sensor position different– CTI measurements indicate CCD was ~5°C warmer in

2002 than 2005– If CTI temperature dependence is different pre- and

post-annealing, R is also dependent on temperature


Summary & Future Plans

• Six CCDs irradiated cold by soft protons• Room temperature annealing increases CTI• Fractional increase due to annealing ~ 0.2• Time constant for annealing CTI increase less than 8 hours• CTI increase requires Tanneal ≥ +30°C

• Plan to study charge in trailing pixels – May help explain isochronal annealing CTI decrease– May better validate physical model

Documents

SPIE Astronomical Instrumentation June 26, 2008 Physics of reverse annealing in high- resistivity ACIS Chandra CCDs Catherine E. Grant (MIT) Bev LaMarr,