Single Photon Imaging Using a CCD and Electron Multiplication … · Slide 3 EMCCD Concept ÂFor...

Single Photon Imaging Using a CCD and Electron Multiplication

Dr Mark RobbinsPredevelopment Technology Manager, Imaging Division

Invited TalkFrontiers in Electronic Imaging 15th –16th June 2009

Introduction

CCD have been around for 40 yearsSignificant advances in manufacture, performance and architecturesElectron Multiplying CCDs (EMCCDs) commercially available for 10 years

EMCCDs are currently the technology of choice for many photon starved applications demanding

Sub electron readout noiseLow noise factor Achievable at high data/frame rates

High quantum efficiency and high fill factor (up to 100%)

This talk presents the EMCCD and associated technology together with some novel application areas

EMCCD Concept

For imaging in a photon starved environment sources of noise must be minimised

Shot noise on input signalShot noise on dark signalNoise from charge to voltage conversionSpurious noise from video chain

Shot noise on input minimised by efficiently converting photons to signal electrons.

Shot noise on dark signal minimised by reducing dark signal

EMCCD technology reduces the effect of charge to voltage conversion noise and noise from the video chain

Amplifier Noise

Noise introduced by the charge to voltage conversion process

Resetting the node introduces “kTC” noiseEliminated by the use of correlated double sampling (CDS)

Left with noise of the source follower FETs. Noise Equivalent Signal (NES) given approximately by

NES (e rms) ≈ 0.5 (1 + f/f0)1/2 Cn1/2

f = pixel rate, f0 ~ 150 kHz and Cn = node capacitance (fF)

readout register

Reset FET

Output FET

outputnode

φR RD OD

readout register

Reset FET

Output FET

outputnode

φR RD OD

Amplifier Noise

To get low noise from a conventional device need to run in slow scan mode.

Can non-destructively sample the output many times (skipper)

Can use column parallel approach to increase frame rate.

Difficult to reduce noise for single photon imaging.

0

5

10

15

10 100 1,000 10,000

Readout Rate (kHz)

inpu

t ref

erre

d no

ise

(ele

ctro

ns) CCD30 Spectroscopic - Measurement

CCD30 Spectroscopic - Theory

CCD44 Astronomy - Measurement

CCD44 Astronomy - Theory

Conventional CCD

Iφ1Iφ2Iφ3

image/store areas

Rφ1 Rφ2 Rφ3

readout register

charge to voltageconversion

EMCCD CCD

Rφ1 Rφ2 Rφ3

Iφ1Iφ2Iφ3

multiplicationregister

Rφ2HV

Single Photon Imaging: EMCCD technology

Signal clocked through many gain elements, each with a probability of inducing impact ionisation, α.

( ) elements gain of number the is 1 NG Nα+=

φ2-φDC (Volts)

40.5 41.0 41.5 42.0 42.5

Mul

tiplic

atio

n G

ain

0

200

400

600

800

1000

1200

10 oC 25 oC

CCD97536 Multiplication Elements

If N = 536 1000x gain reached with an α of 1.3%

α is clock amplitude and temperature dependant


Single carrier impact ionisation is a very low noise processcan “noiselessly” increase the signal level above the amplifier noise, σamp

effective noise given by

( ) 2

22

GF amp

dsσ

μμσ ++=

µs = mean signal

µd = mean dark signal

G = multiplication gain

F = “Noise Factor”

0.01

0.1

1

10

38 39 40 41 42 43 44Rφ2HV-RφDC (Volts)

NES

(rm

s e)

10

100

1000

10000

Mul

tiplic

atio

n G

ain


Noise from a BI CCD97 operating at 35 frames per second

Data courtesy of Andor Technology

EMCCD technology: The F factor

The Noise Factor, Fcaused by statistical fluctuations in the gainmuch lower than conventional APDs or intensifiers etc.

( ) ( )( )

GGG

GG

F

NN 112

1121

/1

2

+−=

⎟⎠⎞

⎜⎝⎛

+−+

=

+−

αα


The effect of the noise factor

Increases the width of the ideal shot noise distribution by 1.4

For very low photon fluxes can be eliminated by “photon counting” (counting output when above threshold)

Noise from the EMCCD will be less than a CCD if Signal < (Amp Noise)2

Not always the whole story as no account taken of noise on background or object visibility.

Output Signal (electrons)0 2000 4000 6000 8000 10000

Prob

abili

ty D

ensi

ty (e

-1)

0.0000

0.0001

0.0002

0.0003

0.0004

5e 4e 3e 2e 1e

1000x gain, F2 = 2Mean Input

Output Signal (electrons)0 2000 4000 6000 8000 10000

Pro

babi

lity

0.0

0.1

0.2

0.3

0.4

5e 4e 3e 2e 1e

1000x gain, F2 = 1Mean Input


Rose criterion used to quantify visibility of a feature in a noisy image

Assumes noise on background = noise in featureRose criterion must be modified to compare emccd with conventional ccd

probability

ΔSfeature

Sbackground

5 ifcertainty 100% withvisible >Δ

= RSNRbackground

feature

σ

Figure of Merit for uniform feature visibility is proposed as

N is the number of pixels in a feature and accounts for the “averaging” performed by the eye.

A more sophisticated approach would take into account different noise distributions visibility due to noise difference.

However, useful for a comparison.


backgroundfeature

featureSNVσσ +Δ

=2


For a conventional device with no gain we have

For an emccd where gain >> amp noise

The visibilities, Vccd and Vemccd, are equal when

( ) ( )22

2

ampbackgroundampbackgroundfeature

featureccd

SSS

SNVσσ ++++Δ

Δ=

( ) backgroundbackgroundfeature

featureemccd

SSSSNV

222

++Δ

Δ=

2

2

83

22

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

++Δ

Δ−

+Δ=

backgroundbackgroundfeature

featurebackgroundfeatureamp

SSSSSS

σ

0 when8 2 ==Δ backgroundampfeature SS σ


Amplifier noise versus mean signal in feature for equal “visibilities”

ΔSfeature (electrons)

0.01 0.1 1 10 100 1000

Am

plifi

er N

oise

(r.m

.s. e

lect

rons

)

0.01

0.1

1

10

100

zero background0.01e background0.1e background1e background

Conventional CCD Beneficial

EMCCD Beneficial


Mean Signal in spots = 1 electron

No background signal

emccd 1000x gain ccd σ = 1.5 e ccd σ = 1.0 e

ccd σ = 0.7 e ccd σ = 0.35 e ccd σ = 0.2 e

Visibility equal

S = σ2


Mean Signal in spots = 0.1 electrons

No background signal

emccd 1000x gain ccd σ = 0.5 e ccd σ = 0.3 e

ccd σ = 0.2 e ccd σ = 0.1 e ccd σ = zero

Visibility equal

S = σ2

Optimising Quantum Efficiency

Electrode structure for normal front face devices limits QE to about 40%

Apertures can be created as in “Open Electrode” or virtual phasedevices. QE increases to around 60%.

For ultimate sensitivity back illumination is required. Removes absorbing layersAllows anti reflection coatings to be appliedQE over 95% can be achieved


QE as a function of wavelength (midband AR coated CCD97)


CCD216 back thinned 2/3” format device in L3C216 Camera

Overcast Starlight Illumination.

Lens f1.4. ~10 µLux faceplate illumination

Spatial (Gaussian 3x3 window) and k4 Temporal Filtered.

40ms integration


QE in the NIR can be increased further by using thicker silicon

standard thickness is ~16 µm.Our “Deep Depletion” devices are

40 µm thick.depletion depth must increase else

MTF will be degraded.1500 Ωcm silicon used for good

depletion.

Electron multiplying deep depletion CCDs have been demonstrated

Wavelength (nm)200 400 600 800 1000 1200

Abs

orpt

ion

Coe

ffici

ent, α

(µm

-1)

10-4

10-3

10-2

10-1

100

101

102

103

Abso

rptio

n Le

ngth

(µm

)

10-3

10-2

10-1

100

101

102

103

104

Absorption CoefficientAbsorption Length


Even thicker (100 to 300 µm) 8 kΩcm Si used for specially designed “HiRho” devices.

“Over depleted” with high negative substrate bias (>70V) to maximise the MTF.

Amplifier structures biased with local substrates to increase design options and minimise noise

low substrate potential high substrate potential


CCD217 100µm thick HiRho device (NIR antireflection coating)

Wavelength (nm)

400 600 800 1000

Qua

ntum

Effi

cien

cy (%

)

0

20

40

60

80

100

Temperature = +20oCTemperature = -100oC

measurements

Example EMCCD Applications

EMCCD technology + back illumination employed where very high sensitivity with high frame rate is required.e.g. high end surveillance, special forces etc.Life sciences

Single molecular imagingFluourescence microscopy (TIRF, Confocal etc)

COTs sensors have been available for several yearsCustom sensors now being developed

no gain 1000x gain

Example EMCCD Applications: LIDAR

Light Detection and Ranging (LIDAR)A pulsed laser beam is transmitted from the satellite (100 Hz rep. freq.).light will be backscattered from aerosols and clouds and detected at the

focal plane.


e2v LIDAR sensor


Measurements from simulated laser return and ground echo.

Time (µs)

0 50 100 150 200 250

Inpu

t Ref

erre

d Si

gnal

(ele

ctro

ns)

0.1

1

10

100

Mean of 16 Atmospheric ReturnsSingle Artificial Atmospheric Return

GR

OU

ND

EC

HO

0.2e

0.9e

2.5e

6e12e

31e

69e

Example EMCCD Applications: Adaptive Optics

The atmosphere limits the performance of ground-based telescopes. Adaptive optics can correct for the effects of atmospheric turbulence. A real or artificial guide star is focused on a sensor to sample the wavefront. A spot is focused onto different parts of the image sensor. The wavefront error is calculated and a deformable mirror adjusted

significantly greater sampling in both spatial and temporal domains required for next gen 8-10m telescopes.

Greater sampling implies fewer photons per pixel. A read noise << 1 electron for centroiding to the required accuracy. 240x240 pixels at a frame rate of 1.5 kHz have to be sampled

Application ideally suited to EMCCD technology.


e2v funded by The European Southern Observatory and JRA2 OPTICON to develop sensor

8 Outputs for high frame rate. BI and deep depleted silicon for high QE.Novel integral shutter being evaluated


Packaged with a 2-stage peltier pack for cooling and reduction of dark signal.


Test results operating at 1300 fps, 1000x multiplication gain.

Mean of 2000 frames3.5 electrons peak white

Single frame3.5 electrons peak white



Conclusion

EMCCDs utilise established CCD technology

Addition of new readout register structure provides 1000+ gainApplies very low noise gain before the charge to voltage conversionAmplifies signal above the CCD readout and video chain noise

Amplification can be applied at high pixel rateNoise equivalent signals < 1e possible up to video rates and above

The combination with back thinning provides an image sensor ideal for many applications requiring the ultimate sensitivity.

Single Photon Imaging Using a CCD and Electron Multiplication … · Slide 3 EMCCD Concept ÂFor...

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