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Jessamyn FairfieldLawrence Berkeley National Laboratory

For the SNAP Collaboration

A New Generation of Red-Sensitive CCDs

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Overview

• How does a charge-coupled device work?

• What do we need to improve?

• How do we know it's better?

CCDs: A primer

• Ideal imaging: want to record every photon that strikes detector 100% quantum efficiency

• QE of film: ~2%• Use photoelectric effect• Visible light: 2-3 eV• How to use this to create an image?

CCDs: A primer

Charge generation

Charge collection

Charge transfer

Charge measurement

Readout

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Clocking Mechanism

• Three-phase clocking• “Red” electrode, “blue” electrode, “green” electrode

Scientific versus commercial

• Want robustness, low cost• Serial readout problems

—Multiple transistors—Charge Transfer Efficiency—Speed

• Temperature, dark current• Backside vs. frontside illumination

—Depletion—Thickness

• Additional traits?—Radiation tolerance—Infrared sensitivity

The new idea

• 4-10 kΩ/cm• Simplified fabrication• Depleted externally• Greatly reduced fringing• Enhanced NIR sensitivity

High-resistivity Silicon!

What to look for?

• Traditional parameters—Charge transfer efficiency—Quantum efficiency

• Science parameters—Radiation tolerance—Infrared sensitivity

• Important parameters—Charge diffusion—Edge effects

CTE/Dark current

• Use Fe55 X-rays for energy resolution• Long dark exposures to measure dark current

Results courtesy Bill Kolbe

Quantum Efficiency: Setup

• Compare light to calibrated photodiode

Quantum Efficiency: Results

Results courtesy Max Fabricius

Quantum Efficiency: Hubble WFC

From WFPC2 Handbook

Radiation Tolerance

• Traps created through Non-Ionizing Energy Loss• SNAP radiation estimate: 5 year mission, 2014 launch

—5*10^7 MeV/g(Si)‏• Irradiated with 55 MeV, 12.5 MeV protons at 88”

cyclotron

Results courtesy Kyle Dawson

Diffusion: The Problem

• Undepleted: Diffusion equals undepleted thickness

• Depleted: Diffusion can be slightly lowered with bias voltage

200-

300

µm

Diffusion: Measurement Technique

• Virtual knife edge technique• Point spread function• Gaussian beam profile

Diffusion: Setup

• Pinhole projector on 2-axis motorized translation stage.

• Spot size: 1 μm

• Pixel size: 10.5 μm

Diffusion: Voltage Dependence

VSUB = 5V 20V 115V

Diffusion: Results

Edge Effects

• Electric field deformation at edges• Changes in spot shape, level, position

Edge Effects: Results

• Re-use pinhole projector• Where does distortion occur,

in number of pixels from edge?

122Serial Register

512Mystery Structure

20-22Side edge

Spot position distortion (0.5 px) ‏

Spot level distortion (20%) ‏

Spot shape distortion (20%) ‏

SERIAL REGISTER

PAR

ALL

EL

REG

ISTE

RS

What can we conclude?

CTE, QE, diffusion, edge effects, radiation tolerance, NIR sensitivity

These are excellent scientific detectors!

Thanks

Special thanks to Natalie Roe and the SNAP collaboration!