ccds : current developments part 1 : deep depletion ccds improving the red response of ccds. part 2...

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.

Part 1 : Deep Depletion CCDs

Improving the red response of CCDs.

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Charge packetp-type silicon

n-type silicon

SiO2 Insulating layer

Electrode Structure

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oton

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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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Potential along this line shown in graph above.

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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). Considerfirst 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-electronand the hole it leaves behind.

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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 responsesuffers.

Red photons can now passright through the CCD.

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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 in the thicker bulk of the device.

Problem :Hi resistivity silicon contains much lower impurity levels than normal. Very few waferfabrication factories commonly use thismaterial and deep depletion CCDs have to be designed and made to order.

QE Improvements with Deep Depletion CCDs

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300 400 500 600 700 800 900 1000

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QE

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CC1D20 MBE singleAR @320nm

CC1D20 BIV BroadBand AR

EEV12 (StandardThinned)

Marconi DeepDepletion (broadBand AR)


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

ING Deep Depletion Camera

Destined for ISIS RED sometime this Summer

Part 2 : Low Light Level CCDs (LLLCCDs)

A new idea from Marconi that creates internal electron gainin a CCD and reduces read-noise to sub-electron levels.

RAIN (PHOTONS)

BUCKETS (PIXELS)

VERTICALCONVEYORBELTS(CCD COLUMNS)

HORIZONTALCONVEYOR BELT

(SERIAL REGISTER)

MEASURING CYLINDER(OUTPUT AMPLIFIER)

CCD Analogy

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e of

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Image Area

Serial Register

Read Out Amplifier

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ires

Photomicrograph of a corner of an EEV CCD.

pixe

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Charge packetp-type silicon

n-type silicon

SiO2 Insulating layer

Electrode Structure

pixe

l bo

unda

ry

inco

min

gph

oton

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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

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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

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yMultiplication Clocking 1

Gain electrode

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

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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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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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Clock High Voltage

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Readout Noise of CCD65

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Clock High Voltage

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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

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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

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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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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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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

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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:

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

ccds : current developments part 1 : deep depletion ccds improving the red response of ccds. part 2...

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