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A New Generation of Red-Sensitive CCDs€¦ · 1 Jessamyn Fairfield Lawrence Berkeley National...
Transcript of A New Generation of Red-Sensitive CCDs€¦ · 1 Jessamyn Fairfield Lawrence Berkeley National...
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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
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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!