Low Light Level CCDs (LLLCCD)A new idea from Marconi (EEV) to reduce or eliminate CCD read-out noise.
RAIN (PHOTONS)
BUCKETS (PIXELS)
VERTICALCONVEYORBELTS(CCD COLUMNS)
HORIZONTALCONVEYOR BELT
(SERIAL REGISTER)
MEASURING CYLINDER(OUTPUT AMPLIFIER)
CCD Analogy
Edg
e of
Sil
icon
Image Area
Serial Register
Read Out Amplifier
Bu
s w
ires
Photomicrograph of a corner of an EEV CCD.
pixe
l bo
unda
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Charge packetp-type silicon
n-type silicon
SiO2 Insulating layer
Electrode Structure
pixe
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unda
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inco
min
gph
oton
s
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.
Pot
enti
al E
nerg
yConventional Clocking 1
Surface electrodesCharge packet (photo-electrons)
P-type siliconN-type silicon
Insulating layer
Charge packets occupy potential minimums
Pot
enti
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yConventional Clocking 2
Pot
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Pot
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nerg
yConventional Clocking 4
Pot
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nerg
yConventional Clocking 5
Pot
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nerg
yConventional Clocking 6
Pot
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nerg
yConventional Clocking 7
Pot
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Pot
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yConventional Clocking 9
Pot
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Charge packets have moved one pixel to the right
Image Area Image Area(Architecture unchanged)
Serial register Serial register{Gain register
On-ChipAmplifier
On-ChipAmplifier
The Gain Register can be added to any existing design
LLLCCD Gain Register Architecture
Conventional CCD LLLCCD
Pot
enti
al E
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yMultiplication Clocking 1
Gain electrode
In this diagram we see a small section of the gain register
Pot
enti
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yMultiplication Clocking 2
Pot
enti
al E
nerg
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
Pot
enti
al E
nerg
yMultiplication Clocking 3
Pot
enti
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y
Clocking continues but each time the charge packets pass through the gain electrode, furtheramplification is produced. Gain per stage is low, <1.015, however the number of stages is high so the total gain can easily exceed 10,000
Gain Sensitivity of CCD65
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10000
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Clock High Voltage
Ga
in
Readout Noise of CCD65
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20 25 30 35 40
Clock High Voltage
Eq
uiv
ale
nt
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lec
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RM
S
The Multiplication Register has a gain strongly dependant on the clock voltage
Multiplication Clocking 4
SNR = Q.I.t.[Q.t.( I +BSKY) +Nr2 ] -0.5
Q = Quantum Efficiency I = Photons per pixel per second
t = Integration time in seconds BSKY = Sky background in photons per pixel per second Nr = 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 readoutsignificantly. At TV frame rates, noise > 50e
Trade-off between readout speed and readout noise
Noise Equations 2.
SNR = Q.I.t.Fn.[Q.t.Fn.( I +BSKY) +(Nr/G)2 ] -0.5
G = Gain of the Gain RegisterFn = 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 atTV frame rates.
Unfortunately, the problem of multiplication noise is introduced
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
Pro
babi
lity
Histogram broadenedby multiplication noise
M=1.4
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-speed regimes.
Signal Level
SN
R
Conventional CCD
LLLCCD
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-electron
No photo-electron
Co-incidence losshere
CCD Video waveform
Approx 100ns
Photon Counting 1.
SNR = Q.I.t.[Q.t.( I +BSKY)] -0.5
Noiseless Detector
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 Losses
Photo-electrongeneration rate Non-Linearity
(electrons per pixel per frame) %0.02 10.033 1.60.1 5
In the case of a hypothetical 1K x 1K photon counting CCD, the maximum frame ratewould be approximately 10Hz. If we can only accept 5% non-linearity then the maximumillumination would be approximately 1 photo-electron per pixel per second.
The three operational regimes of LLLCCDs
1) Unity Gain Mode.
The CCD operates normally with the SNR dictated by the photon shot noise added inquadrature 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 therewill 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 triggerof the comparator is then treated as a single photo-electron of equal weight. Multiplicationnoise is thus eliminated. Risk of coincidence losses at higher illumination levels.
Summary.
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 (0.1”per pixel), 25Hz frame rate
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17 18 19 20 21 22Mv
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Normal CCD
L3CS (LLLCCD)
theoretical limit
Zero-noise image tube
Possible Application 2.Acquisition Cameras
As for the previous slide but instead the exposure time is increased to 10s
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17 18 19 20 21 22Mv
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Cryocam (standard CCD)
L3CS (LLLCCD)
theoretical limit
Zero-noise image tube
QE=70%Amplifier Noise =5eBackground =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
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Exposure Time Seconds
So
urc
e in
ten
sity
at
the
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ecto
r (p
ho
ton
s p
er p
ixel
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on
d)
Thinned LLLCCD with Gain=1000
Thinned LLLCCD +Photon Counting
Conventional CCD
Possible Application 4.Wave Front Sensors
Amplifier Noise=5eQE= 70%
Algorithm used on the current NAOMI WFS produces reliable centroiddata when total signal per sub-aperture exceeds about 60 photons.
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Photons per pixel per WFS frame
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Current NAOMI WFS
Thinned LLLCCD With Gain=1000
shot noise limit
CCD65Aimed at TV applicationsas a substitute for imagetube sensors. 576 x 288 pixels. Thick frontside illuminated, peak QE of 35%. 20 x 30um pixels
CCD 60128x 128 pixel, thinned, has been builtbut still underdevelopment. For possible application to Wavefront Sensing.
CCD 79,86,87Proposed future devices up to 1K square,> 10 frames per second readout atsub-electron noise levels.
Marconi LLLCCD Products 1.
Camera systems based on thischip available winter 2001
As above
Low Priority for Marconi withoutencouragement from the astronomicalcommunity
Would subtend 51” x 39” at WHT CASS
L3CSPackaged camera containing TE cooled CCD65frontside illuminated20ms-100sec integration times2e per pix per sec dark currentBinning and Windowing availableFirewire Interface +video output Available towards end of 2001 (£25K)
L3CAPackaged camera containing TE cooled CCD65frontside illuminated20ms-100sec integration times<1e per pix per sec dark currentBinning availablevideo output
Marconi LLLCCD Products 2.
Lecture slides available on the ING web: