A CCD that can stand up to cosmic rays

A single instrument aboard SNAP, the proposed SuperNova/Acceleration
Probe, will make it unique among satellites: its billion-pixel
astronomical camera, the GigaCAM. Built from an array of
revolutionary Berkeley Lab CCDs developed by Stephen Holland and
his colleagues in the Lab’s Engineering and Physics Divisions, the
GigaCAM will be the largest and most sensitive astronomical CCD
imager ever constructed.

Standard astronomical CCDs are fragile affairs, and their ability
to obtain high-quality images degrades quickly in the hostile
radiation environment of space — one reason why astronauts have
already replaced all of the Hubble Space Telescope’s original
imaging instruments.

"The Hubble and many other satellites were designed to be
maintenance friendly, but SNAP is going to be placed in an
unreachable orbit," says Engineering’s William Kolbe, noting
that once the GigaCAM is carried aloft on its five-year-plus
mission, it can’t afford to fail.

At 300 microns (millionths of a meter) thick, the Berkeley Lab
"high-resistivity, p-channel" CCDs are much more rugged than
conventional astronomical CCDs measuring only a few tens of
microns thick. In recent months Kolbe and Armin Karcher have
been conducting tests at the 88-Inch Cyclotron to see just how
well the new CCD can stand up to radiation damage.

Outer space in the lab

In space the culprits are cosmic rays, high-velocity particles
packing a tremendous energetic punch that destroys pixels,
increases "dark current" (a source of background noise), and
worst of all degrades the efficiency of charge transfer from
the pixels to the amplifiers at the edges of the chip. A few
cosmic rays are massive atomic nuclei like iron, nickel, or
zinc, but the majority are protons and electrons.

The 88-Inch Cyclotron simulates the cosmic ray environment with
both heavy ions and protons, to test spacecraft components
ranging from memory chips to transistors to entire computer
systems, and to calibrate detectors used in compiling space
"weather" reports. CCD chips and solar cells are particularly
prone to degradation from large numbers of protons generated
during high solar activity.

"At the Cyclotron, protons are delivered to target components at
the Light Ion Irradiation Station located in Cave 3," says Peggy
McMahan, research coordinator for the 88-Inch Cyclotron. "We owe
this station to a group from the Life Sciences and Engineering
Divisions, who worked with our operations staff to develop it
in order to maintain a small radiation biology program here
after the closure of the Bevalac in 1993."

In the station the dose is measured and the beam is "blown up"
to four inches in diameter to uniformly irradiate silicon wafers
(and the Petrie dishes used in Life Sciences experiments too).
The Materials Sciences, Chemical Sciences, and Advanced Light
Source divisions have also used the irradiation facility when
the cyclotron is not busy with nuclear science experiments, its
primary mission. Companies who have tested components for space
applications on a fee basis include Eastman Kodak, Aerospace
Corporation, Lucent Technologies, and Mitsubishi Electronics.

Test CCDs for the SNAP project are bombarded with beams of
protons ranging in energy from 10 to 55 MeV (million electron
volts); by testing several wafers, each at a different dose —
from a few billion to a hundred billion protons per square
centimeter of surface — dosage can be scaled to equal what
CCDs would receive after several years in orbit.

Performance check

Before irradiation, Kolbe and Karcher assess the test wafers in
their laboratory for dark current, charge transfer efficiency,
and "cosmetic" defects. Computer processing and other electronic
tricks can compensate for cosmetic flaws like a few damaged
("hot") pixels, endemic to all CCDs, but dark current and charge
transfer efficiency pose more serious challenges.

Dark current is electronic noise caused by thermal motion of the
atoms that make up the chip; the colder the chip, the less the
dark current. The Berkeley Lab CCD is much thicker than ordinary
astronomical chips so there is more material in which dark
current can be generated, but its high purity, negative
"doping," and low operating temperature work to suppress dark
current. In space, SNAP’s GigaCAM will operate at about 140
degrees Kelvin (by comparison, nitrogen under normal atmospheric
pressure liquefies at 77 deg K).

Typically the most serious radiation damage to CCDs is a steady
reduction in charge transfer efficiency. Photons from distant
objects like stars are focused on pixels in the CCD, and the
brighter the object, the more photons are converted to charge.
Negative electrons or positive "holes" are collected and
transferred to the edge of the chip along specific channels,
like buckets of water in a bucket brigade. The chip’s electronics
reconstruct the image of a star by associating the precise amount
of charge and the precise location of the pixels that generated
it.

"When you irradiate a CCD with protons, silicon nuclei are
knocked out of their lattice position, and what was fairly
perfect material develops defects," Kolbe says. "These form
electron or hole ‘traps’ that can grab charge that’s being
transferred, hold onto it for a time, then let it go later."

Armin Karcher notes that these inefficiencies in charge transfer
"can affect the apparent brightness of objects in the sky and
the interpretation of their spectra." The success of the SNAP
satellite will greatly depend on its ability to measure
supernova spectra with extreme accuracy.

To characterize the test CCDs, Kolbe and Karcher create star
fields by exposing them to a small x-ray source. "Each x-ray
photon deposits an artificial ‘star’ every 50 to 70 pixels,
generating a cloud of charge," says Karcher. "We know how many
electrons it takes to represent each of these, so when we read
out the data we know what the reading should be."

And the winner is …

After irradiation at the 88-Inch Cyclotron, Kolbe and Karcher
take the test wafers back into the laboratory to measure
radiation effects from different doses. In the three batches
tested so far they found that, while radiation dosage
increased dark current, the effect was important only at high
temperatures — and the impact of radiation on charge transfer
efficiency was remarkably small. Available studies indicate
that the charge transfer efficiency of conventional CCDs falls
off rapidly with increased radiation, while the Berkeley Lab
CCDs are little affected even at very high doses.

These results were not unexpected; after all, the Berkeley
Lab CCD descends from a long line of detectors designed to
withstand radiation from colliding beams of particles in giant
research accelerators — "much more hostile than outer space,"
Kolbe remarks.

"The high-resistivity p-channel CCDs exhibit extremely low
dark current at the operating temperature," the researchers
concluded in their latest report. "Radiation damage proved to
be much less detrimental than in conventional CCDs. … Their
potential lifetime in space is measured in decades, not years."

Kolbe and Karcher have devised new instruments to test larger
CCD wafers, to measure the efficiency of their response at
all wavelengths, and to investigate what effect pixel size,
different levels of doping, and other manufacturing variables
may have on their performance after radiation exposure.

"We’re expanding our capabilities constantly," Karcher says.
"We plan to keep going till SNAP flies."

Additional information:

* More on the Berkeley Lab CCD
http://www.lbl.gov/Science-Articles/Archive/ccd-infrared.html

* CCD technical details
http://design.lbl.gov/ccd/

* More on the SNAP satellite
http://snap.lbl.gov/

* More on Berkeley Lab’s 88-Inch Cyclotron
http://www-nsd.lbl.gov/LBL-Programs/nsd/user88/

* And for the latest space weather report
http://www.spaceweather.com/

[NOTE: Images supporting this release are available at
http://www.lbl.gov/Science-Articles/Archive/CCD-Kolbe-Karcher-cosmic.html ]