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8/8/2019 2004 CCDs Introduction
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Introduction to CCDs
Claudio CumaniOptical Detector Team - European Southern Observatory
for ITMNR-5
Fifth International Topical Meeting on Neutron Radiography
Technische Universitt Mnchen, Garching, July 26, 2004
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CCDs - Introduction
Charge Coupled Devices (CCDs) were inventedin October 19, 1969, by William S. Boyle andGeorge E. Smith at Bell Telephone Laboratories
(A new semiconductor device concept has been
devised which shows promise of having wideapplication, article on Bell System TechnicalJournal, 49, 587-593 (April 1970).
CCDs are electronic devices, which work byconverting light into electronic charge in a siliconchip (integrated circuit). This charge is digitisedand stored as an image file on a computer.
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Bucket brigade analogy
RAIN (PHOTONS)
BUCKETS (PIXELS)
VERTICALCONVEYOR
BELTS
(CCD COLUMNS)
HORIZONTAL
CONVEYOR BELT
(SERIAL REGISTER)
METERING
STATION
(OUTPUT
AMPLIFIER)
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Exposure finished, buckets now contain samples of rain.
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Conveyor belt starts turning and transfers buckets.Rain collected on the vertical conveyor is tipped into buckets on the horizontal conveyor.
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Vertical conveyor stops.
Horizontal conveyor starts up and tips each bucket in turn into the metering station.
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`
After each bucket has been measured, the metering station is emptied, ready for the nextbucket load.
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A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.
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CCD structure
A CCD is a two-dimensional array of metal-oxide-semiconductor (MOS) capacitors
The charges are stored in the depletion region of theMOS capacitors
Charges are moved in the CCD circuit by manipulatingthe voltages on the gates of the capacitors so as to allowthe charge to spill from one capacitor to the next (thusthe name charge-coupled device)
A charge detection amplifier detects the presence of thecharge packet, providing an output voltage that can beprocessed
The CCD is a serial device where charge packets areread one at a time.
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CCD structure - 1
Charge motion
Chargem
otion
Serial (horizontal) register
Parallel (vertical) registers
Pixel
Image area
(exposed to light)
Output amplifier
masked area
(not exposed to light)
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CCD structure - 2
One pixel
Channel stops to define the columns of the image
Transparent
horizontal electrodes
to define the pixels
vertically. Also
used to transfer the
charge during readout
Plan View
Cross section
ElectrodeInsulating oxide
n-type silicon
p-type silicon
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Photomicrograph of a corner of an EEV CCD
Edge
of
Silic
on
160mm
Image Area
Serial Register
Read Out Amplifier
Buswires
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Full-Frame CCD
Charge motion
Charge motion
Image area = parallel registers
Masked area = serial register
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Frame-Transfer CCD
Image areaStorage (masked) area
Serial registerCharge motion
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Interline-Transfer CCD
Image areaStorage (masked) area
Serial register
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Basic CCD functions
Charge generation
photoelectric effect
Charge collection
potential well
Charge transfer
potential well
Charge detection
sense node capacitance
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Photoelectric Effect - 1
Atoms in a silicon crystal have electronsarranged in discrete energy bands: Valence Band
Conduction Band
Increasi
ngenergy
Valence Band
Conduction Band
1.12 eV
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Photoelectric Effect - 2
The electrons in the valence band can be
excited into the conduction band by
heating or by the absorption of a photon
ph
oton p
hoton
Hole Electron
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During integration of the image, one of the electrodes in each pixel is held at a positive potential. This
further increases the potential in the silicon below that electrode and it is here that the photoelectrons
are accumulated. The neighboring electrodes, with their lower potentials, act as potential barriers that
define the vertical boundaries of the pixel. The horizontal boundaries are defined by the channel
stops.
n p
Electricp o
tential
Region of maximum
potential
Potential Well - 2
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pixel
boundary
Charge packetp-type silicon
n-type silicon
SiO2 Insulating layer
Electrode Structure
pix
el
bo
undary
incoming
pho
tons
Photons entering the CCD create electron-hole pairs. The electrons are then attracted
towards the most positive potential in the device where they create charge packets.
Each packet corresponds to one pixel
Charge collection in a CCD - 1
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1
23
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
Time-slice shown in diagram
1
2
3
Charge transfer in a CCD
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1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
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1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
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1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
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1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
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1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
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Photon absorption length
c
: beyond this wavelength
CCDs become insensitive.
Semiconductor T (K) (ECond EVal ) (eV) c (nm)
CdS 295 2.4 500
CdSe 295 1.8 700
GaAs 295 1.35 920
Si 295 1.12 1110
Ge 295 0.67 1850
PbS 295 0.42 2950
InSb 295 0.18 6900
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(Thin) back-side illuminated CCDs
Silicon chemically etched and polished down to a thickness of about 15microns.
Light enters from the rear and so the electrodes do not obstruct the photons. The QE can
approach 100% . Become transparent to near infra-red light and poor red response
Response can be boosted by the application of anti-reflective coating on the thinned rear-side
Expensive to produce
n-type silicon
p-type silicon
Silicon dioxide insulating layer
Polysilicon electrodes
Incomingp
hotons
Anti-reflective (AR) coating
15 m
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Front vs. Back side CCD QE
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CCD QE and neutron detectors - 1
Phosphor/Scintillators from Applied Scintillation Technologies data sheets (www.appscintech.com)
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CCD QE and neutron detectors - 2
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Dark current
Thermally generated electrons are indistinguishable from photo-generated electrons : Dark Current (noise)
Cool the CCD down!!!
1
10
100
1000
10000
-110 -100 -90 -80 -70 -60 -50 -40
Temperature Centigrade
Elec
trons
perpixelperhour
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Full well - 1
Blooming
pixel
boundary
Photons
Pho
tonsOverflowing
charge packet
Spillage Spillage
pixel
boundary
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Full well - 2
Blooming
Bloomed star images
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CTE - 1
Percentage of charge which is really
transferred.
n 9s: five 9s = 99,99999%
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CTE - 2
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Read-Out Noise
Mainly caused by thermally induced motions of electrons in the output amplifier. These causesmall noise voltages to appear on the output. This noise source, known as Johnson Noise, can be
reduced by cooling the output amplifier or by decreasing its electronic bandwidth. Decreasing the
bandwidth means that we must take longer to measure the charge in each pixel, so there is
always a trade-off between low noise performance and speed of readout.
The graph below shows the trade-off between noise and readout speed for an EEV4280 CCD.
0
2
4
6
8
10
12
14
2 3 4 5 6
Time s pent me asuring each pixel (microseconds)
Read
Noise(electronsRMS)
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CCD defects - 2
Dark columns: caused by traps
that block the vertical transfer of
charge during image readout.
Traps can be caused by crystalboundaries in the silicon of the
CCD or by manufacturing defects.
Although they spoil the chip
cosmetically, dark columns are nota big problem (removed by
calibration).
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CCD defects - 3
Dark column
Hot spots and bright columns
Bright first image row caused byincorrect operation of signal
processing electronics.
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CCDs:
- small, compact, rugged, stable, low-power devices- excellent, near-perfect sensitivity over a wide range in wavelengths
- wide dynamic range (from low to high light levels)
- no image distortion (pixel fixed by construction)
- easily connected to computer
The CCD is an almost perfect detector
Ian S. McLean - Craig Mackay
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The only uniform CCD is a dead CCD
Craig Mackay
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CCD Calibration - 1
Bias: exposure time = 0, no lightshows variations in electronic response across the CCD
Flat Field: exposure time 0, uniform lightshows variations in the sensitivity of the pixels across the CCD
Dark Frame: exposure time 0, no lightshows variations in dark current generation across the CCD
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CCD calibration - 2
Dark Frame Flat Field
Dark frame shows a number of bright defects on the chipFlat field shows a pattern on the chip created during manufacture and a slight loss of sensitivity in
two corners of the image
Some dust spots are also visible
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CCD calibration - 3
Flat Field Image
Bias Image
Flat
-Dark
-Bias
Science
-Dark
-Bias Output Image
Flat-Dark-Bias
Sc-Dark-Bias
Dark Frame
Science Frame
If there is significant dark current present:
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CCD Calibration - 4If negligible dark current
Flat Field Image
Bias Image
Flat
-Bias
Science
-BiasOutput Image
Flat-Bias
Science -Bias
Science Frame
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