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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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