CCDs : Current Developments

1
CCDs Current Developments
Part 1 Deep Depletion CCDs Improving 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.
2
Part 1 Deep Depletion CCDs
Improving the red response of CCDs.
3
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
pixel boundary
pixel boundary
incoming photons
Electrode Structure
Charge packet
SiO2 Insulating layer
4
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.
Cross section through a thick frontside
illuminated CCD
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.
5
Deep Depletion CCDs 2.
In a thinned CCD , the field free region is
simply etched away.
Cross section through a thinned CCD
Electric potential
Electric potential
There is now a high electric field throughout the
full depth of the CCD.
Problem
Thinned CCDs may have good blue response but
they become transparent at longer wavelengths
the red response suffers.
This volume is etched away during manufacture
Red photons can now pass right through 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.
6
Deep Depletion CCDs 3.
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 40mm 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.
Cross section through a Deep Depletion CCD
Electric potential
Electric potential
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.
Red photons are now absorbed in the thicker bulk
of the device.
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.
7
(No Transcript)
8
Deep Depletion CCDs 4.
Fringing will also be reduced
Images illuminated by 900nm filter with 2nm
bandpass
Thinned Marconi CCD (Current ISIS Blue)
CCID20 Deep Depletion CCD
Test data courtesy of ESO
9
ING Deep Depletion Camera
Destined for ISIS RED sometime this Summer
10
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.
11
CCD Analogy
VERTICAL CONVEYOR BELTS (CCD COLUMNS)
RAIN (PHOTONS)
BUCKETS (PIXELS)
MEASURING CYLINDER (OUTPUT AMPLIFIER)
HORIZONTAL CONVEYOR BELT (SERIAL REGISTER)
12
(No Transcript)
13
Photomicrograph of a corner of an EEV CCD.
Image Area
Serial Register
Bus wires
Edge of Silicon
Read Out Amplifier
14
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.
pixel boundary
pixel boundary
incoming photons
Electrode Structure
Charge packet
SiO2 Insulating layer
15
Conventional Clocking 1
Insulating layer
Surface electrodes
Charge packet (photo-electrons)
N-type silicon
P-type silicon
Potential Energy
Charge packets occupy potential minimums
16
Conventional Clocking 2
Potential Energy
17
Conventional Clocking 3
Potential Energy
18
Conventional Clocking 4
Potential Energy
19
Conventional Clocking 5
Potential Energy
20
Conventional Clocking 6
Potential Energy
21
Conventional Clocking 7
Potential Energy
22
Conventional Clocking 8
Potential Energy
23
Conventional Clocking 9
Potential Energy
24
Conventional Clocking 10
Charge packets have moved one pixel to the right
Potential Energy
25
LLLCCD Gain Register Architecture
Conventional CCD
LLLCCD
Image Area (Architecture unchanged)
Image Area
On-Chip Amplifier
On-Chip Amplifier

Serial register
Serial register
Gain register
The Gain Register can be added to any existing
design
26
Multiplication Clocking 1
In this diagram we see a small section of the
gain register
Gain electrode
Potential Energy
27
Multiplication Clocking 2
Gain electrode energised. Charge packets
accelerated strongly into deep potential
well. Energetic electrons loose energy through
creation of more charge carriers (analogous
to multiplication effects in the dynodes of a
photo-multiplier) .
Gain electrode
Potential Energy
Potential Energy
28
Multiplication Clocking 3
Clocking continues but each time the charge
packets pass through the gain electrode,
further amplification is produced. Gain per stage
is low, high so the total gain can easily exceed 10,000
Potential Energy
Potential Energy
29
Multiplication Clocking 4
The Multiplication Register has a gain strongly
dependant on the clock voltage
30
Noise Equations 1.
Very hard to get Nr slowing down the readout significantly. At TV
frame rates, noise 50e
Trade-off between readout speed and readout noise
31
Noise Equations 2.
With G set sufficiently high, this term goes to
zero, even at TV frame rates.
Unfortunately, the problem of multiplication
noise is introduced
Readout speed and readout noise are decoupled
32
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
Histogram broadened by multiplication noise
M1.4
33
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.
34
Photon Counting 1.
Offers a way of removing multiplication noise.
Photo-electron detection threshold
CCD Video waveform
No photo-electron
One photo-electron
One photo-electron
Two photo-electrons
No photo-electron
No photo-electron
Co-incidence loss here
Photo-electron detection pulses
Fast comparator
CCD
Approx 100ns
SNR Q.I.t.Q.t.( I BSKY) -0.5
Noiseless Detector
35
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.
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.
36
Summary.
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.
37
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 QE85, LLLCCD QE30, Image
Tube QE 11 dark of moon, seeing 0.6, 24um
pixels (0.1per pixel), 25Hz frame rate
38
Possible Application 2. Acquisition Cameras As
for the previous slide but instead the exposure
time is increased to 10s
39
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 SNR3 for various
source intensities
QE70 Amplifier Noise 5e Background 0.001
photons per pixel per second
40
Possible Application 4. Wave Front Sensors
Algorithm used on the current NAOMI WFS produces
reliable centroid data when total signal per
sub-aperture exceeds about 60 photons.
Amplifier Noise5e QE 70
41
Marconi LLLCCD Products 1.
CCD65 Aimed at TV applications as a substitute
for image tube sensors. 576 x 288 pixels. Thick
frontside illuminated, peak QE of 35. 20 x
30um pixels CCD 60 128x 128 pixel, thinned, has
been built but still under development. For
possible application to Wavefront Sensing. CCD
79,86,87 Proposed future devices up to 1K
square, 10 frames per second readout
at sub-electron noise levels.
Camera systems based on this chip available
winter 2001
Would subtend 51 x 39 at WHT CASS
Low Priority for Marconi without encouragement
from the astronomical community
As above
42
Marconi LLLCCD Products 2.
L3CS Packaged camera containing TE cooled
CCD65 frontside illuminated 20ms-100sec
integration times 2e per pix per sec dark
current Binning and Windowing available Firewire
Interface video output Available towards end of
2001 (25K) L3CA Packaged camera containing TE
cooled CCD65 frontside illuminated 20ms-100sec
integration times current Binning available video output
43
Lecture slides available on the ING web
http//www.ing.iac.es/smt/LLLCCD/lllccd.htm