ccd lecture1

8/14/2019 ccd lecture1

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CCDs : Current Developments

Part 1 : Deep Depletion CCDsImproving the red response of CCDs.

Part 2 : Low Light Level CCDs (LLLCCD)A new idea from Marconi (EEV) to reduce or eliminate CCD read-out noise.


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Part 1 : Deep Depletion CCDs

Improving the red response of CCDs.


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pixel

boun

dary

Charge packetp-type silicon

n-type silicon

SiO2 Insulating layer

Electrode Structure

pixe

l

bou

ndary

incoming

photons

Charge Collection in a CCD.

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


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Electricpotential

Potential along this line

shown in graph above.

Electricpotential

Cross section through a thick frontside illuminated CCD

Deep Depletion CCDs 1.

The electric field structure in a CCD defines to a large degree its Quantum Efficiency (QE). Consider

first a thick frontside illuminated CCD, which has a poor QE.

In this region the electric potential gradient

is fairly low i.e. the electric field is low.

Any photo-electrons created in the region of low electric field stand a much higher chance of

recombination and loss. There is only a weak external field to sweep apart the photo-electron

and the hole it leaves behind.


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Electricpotential

Electricpotential

Cross section through a thinned CCD


In a thinned CCD , the field free region is simply etched away.

There is now a high electric field throughout the

full depth of the CCD.

Photo-electrons created anywhere throughout the depth of the device will now be detected.

Photons no longer have to pass through the electrode structure to reach active silicon.

This volume is

etched away

during manufacture

Problem : Thinned CCDs may have good blue

response but they become transparent

at longer wavelengths; the red response

suffers.

Red photons can now pass

right through the CCD.


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Electricpotential

Electricpotential

Cross section through a Deep Depletion CCD


Ideally we require all the benefits of a thinned CCD plus an improved red response. The solution is to use a

CCD with an intermediate thickness of about 40m constructed from Hi-Resistivity silicon. The increased

thickness makes the device opaque to red photons. The use of Hi-Resistivity silicon means that there are no field

free regions despite the greater thickness.

There is now a high electric field throughout the full depth of the CCD. CCDs manufactured in this way

are known as Deep depletion CCDs. The name implies that the region of high electric field, also known as

the depletion zone extends deeply into the device.

Red photons are now absorbed inthe thicker bulk of the device.

Problem :Hi resistivity silicon contains much lower

impurity levels than normal. Very few wafer

fabrication factories commonly use this

material and deep depletion CCDs have to

be designed and made to order.


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QE Improvements with Deep Depletion CCDs

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000

nm

QE

%

CC1D20 MBE singleAR @320nm

CC1D20 BIV BroadBand AR

EEV12 (StandardThinned)

Marconi DeepDepletion (broad

Band AR)


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Thinned Marconi CCD (Current ISIS Blue)

Fringing will also be reduced

CCID20 Deep Depletion CCD

Images illuminated by 900nm filter with 2nm bandpass

Test data courtesy of ESO


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Part 2 : Low Light Level CCDs (LLLCCDs)

A new idea from Marconi that creates internal electron gain

in a CCD and reduces read-noise to sub-electron levels.


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RAIN (PHOTONS)

BUCKETS (PIXELS)

VERTICAL

CONVEYORBELTS

(CCD COLUMNS)

HORIZONTAL

CONVEYOR BELT

(SERIAL REGISTER)

MEASURING

CYLINDER

(OUTPUT

AMPLIFIER)

CCD Analogy


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Edgeof

Silicon

Image Area

Serial Register

Read Out Amplifier

Buswires

Photomicrograph of a corner of an EEV CCD.


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pixel

boundary

Charge packetp-type silicon

n-type silicon

SiO2 Insulating layer

Electrode Structure

pixel

bou

ndary

incoming

phot

ons

Charge Collection in a CCD.

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.


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PotentialEnerg

y

Conventional Clocking 1

Surface electrodesCharge packet (photo-electrons)

P-type siliconN-type silicon

Insulating layer

Charge packets occupy potential minimums


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PotentialEnerg

y



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PotentialEnerg

y



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PotentialEnerg

y



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PotentialEnerg

y



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PotentialEnerg

y



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PotentialEnerg

y



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PotentialEnerg

y



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PotentialEnerg

y


Charge packets have moved one pixel to the right


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Image Area Image Area(Architecture unchanged)

Serial register Serial register{Gain register

On-Chip

Amplifier

On-Chip

Amplifier

The Gain Register can be added to any existing design

LLLCCD Gain Register Architecture

Conventional CCD LLLCCD


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PotentialEnerg

y

Multiplication Clocking 1

Gain electrode

In this diagram we see a small section of the gain register


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PotentialEnerg

y


PotentialEnerg

y

Gain electrode energised. Charge packets accelerated strongly into deep potential well.

Energetic electrons loose energy through creation of more charge carriers (analogous tomultiplication effects in the dynodes of a photo-multiplier) .

Gain electrode


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PotentialEnerg

y


PotentialEnerg

y

Clocking continues but each time the charge packets pass through the gain electrode, further

amplification is produced. Gain per stage is low,


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Gain Sensitivity of CCD65

1

10

100

1000

10000

20 25 30 35 40

Clock High Voltage

Gain

Readout Noise of CCD65

0.01

0.1

1

10

100

20 25 30 35 40

Clock High Voltage

Equivalentnoise

electr

onsRMS

The Multiplication Register has a gain strongly dependant on the clock voltage



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SNR = Q.I.t.[Q.t.( I +BSKY) +Nr2 ]-0.5

Q = Quantum Efficiency

I = Photons per pixel per second

t = Integration time in secondsBSKY = Sky background in photons per pixel per secondNr = Amplifier (read-out) noise in electrons RMS

Conventional CCD SNR Equation

Noise Equations 1.

Very hard to get Nr < 3e, and then only by slowing down the readout

significantly. At TV frame rates, noise > 50e

Trade-off between readout speed and readout noise


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Noise Equations 2.

SNR = Q.I.t.Fn.[Q.t.Fn.( I +BSKY) +(Nr/G)2 ]-0.5

G = Gain of the Gain Register

Fn = Multiplication Noise factor = 0.5

LLLCCD SNR Equation

Readout speed and readout noise are decoupled

With G set sufficiently high,

this term goes to zero, even at

TV frame rates.

Unfortunately, the problem of multiplication noise is introduced


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Ideal Histogram, StdDev=Gain x (Mean Illumination in electrons )0.5

Actual Histogram, StdDev=Gain x (Mean Illumination in electrons )0.5 x M

Multiplication Noise 1.

In this example, A flat field image is read out through the multiplication register.

Mean illumination is 16e/pixel. Multiplication register gain =100

Electrons per pixel at output of multiplication register

Probability

Histogram broadened

by multiplication noise

M=1.4


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Multiplication Noise 2.

Multiplication noise has the same effect as a reduction of QE by a factor of two.

In high signal environments , LLLCCDs will generally perform worse than

conventional CCDs. They come into their own, however, in low signal, high-speedregimes.

Signal Level

SNR

Conventional CCD

LLLCCD


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Offers a way of removing multiplication noise.

Photo-electron

detection threshold

Fast comparator

Photo-electron detection pulses

One

photo-electron

One

photo-electron

Two

photo-electrons

CCD

No

photo-electron

No

photo-electronNo

photo-electron

Co-incidence loss

here

CCD Video waveform

Approx 100ns

Photon Counting 1.

SNR = Q.I.t.[Q.t.( I +BSKY)]-0.5

Noiseless Detector


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Photon Counting 2.

If exposure levels are too high, multi-electron events will be counted as single-electron

events, leading to co-incidence losses . This limits the linearity and reduces the effective

QE of the system.

Non-Linearity from Photon-Counting Coincidence Loss

Photo-electron

generation rate Non-Linearity(electrons per pixel per frame) %

0.02 1

0.033 1.6

0.1 5

In the case of a hypothetical 1K x 1K photon counting CCD, the maximum frame rate

would be approximately 10Hz. If we can only accept 5% non-linearity then the maximum

illumination would be approximately 1 photo-electron per pixel per second.


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The three operational regimes of LLLCCDs

1) Unity Gain Mode.The CCD operates normally with the SNR dictated by the photon shot noise added in

quadrature with the amplifier read noise. In general a slow readout is required (300KPix/second)

to obtain low read noise (4 electrons would be typical). Higher readout speeds possible but there

will be a trade-off with the read-noise.

2) High Gain Mode.Gain set sufficiently high to make noise in the readout amplifier of the CCD negligible.The drawback is the introduction of Multiplication Noise that reduces the SNR

by a factor of 1.4. Read noise is de-coupled from read-out speed. Very high speed readout

possible, up to 11MPixels per second, although in practice the frame rate will probably be

limited by factors external to the CCD.

3) Photon Counting Mode.Gain is again set high but the video waveform is passed through a comparator. Each trigger

of the comparator is then treated as a single photo-electron of equal weight. Multiplication

noise is thus eliminated. Risk of coincidence losses at higher illumination levels.

Summary.


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Possible Application 1.Acquisition Cameras

Performance at CASS of WHT analysed below. The calculated SNR is for a single TV frame (40ms).

It is assumed that the seeing disc of the target star evenly illuminates 28 pixels

(0.6 seeing, 0.1/pixel plate scale). SNR calculated for each pixel of the image.

Assumptions: CCD QE=85%, LLLCCD QE=30%, Image Tube QE =11%

dark of moon, seeing 0.6, 24um pixels (0.1per pixel), 25Hz frame rate

0

0.5

1

1.5

2

2.5

3

3.5

17 18 19 20 21 22

Mv

SNR

Normal CCD

L3CS (LLLCCD)

theoretical limit

Zero-noise image tube


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Possible Application 2.Acquisition Cameras

As for the previous slide but instead the exposure time is increased to 10s

0

1

2

3

4

5

6

7

8

9

10

17 18 19 20 21 22

Mv

SNR

Cryocam (standard CCD)

L3CS (LLLCCD)

theoretical limitZero-noise image tube


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QE=70%

Amplifier Noise =5e

Background =0.001 photons per pixel per second

Possible Application 3.Photon Counting Faint Object Spectroscopy

LLLCCDs operating in photon counting mode would seem to offer some promise.

The graph below shows the time taken to reach a SNR=3 for various source intensities

0.01

0.1

1

10

0 200 400 600 800 1000

Exposure Time Seconds

Sourceintensityatthedetector

(photonsperpixelp

ersecond)

Thinned LLLCCD with Gain=1000

Thinned LLLCCD +Photon Counting

Conventional CCD


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Possible Application 4.Wave Front Sensors

Amplifier Noise=5e

QE= 70%

Algorithm used on the current NAOMI WFS produces reliable centroid

data when totalsignal per sub-aperture exceeds about 60 photons.

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100

Photons per pixe l per WFS frame

SNR

Current NAOMI WFS

Thinned LLLCCD With Gain=1000

shot noise limit


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CCD65Aimed at TV applicationsas a substitute for image

tube sensors. 576 x 288 pixels.

Thick frontside illuminated,

peak QE of 35%.

20 x 30um pixels

CCD 60128x 128 pixel, thinned, has been built

but still under

development. For possible

application to Wavefront Sensing.

CCD 79,86,87Proposed future devices up to 1K square,

> 10 frames per second readout at

sub-electron noise levels.

Marconi LLLCCD Products 1.

Camera systems based on this

chip available winter 2001

As above

Low Priority for Marconi without

encouragement from the astronomical

community

Would subtend 51 x 39 at WHT CASS


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Lecture slides available on the ING web:

http://www.ing.iac.es/~smt/LLLCCD/lllccd.htm

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