Electron multiplying CCD (EMCCD) technology has found important initial applications in low light surveillance and photon starved scientific instrumentation. This paper provides a description of the technology and discusses the attributes of the EMCCD which may make it useful for other detectors, particularly those which are photon starve

D J Burt

M.S R T Bell

P A Jerram

P J Pool

Robbins

CiteSeerX

1
 SNIC Symposium, Stanford, California – 3-6 April 2006  
 
 
 
 
Electron Multiplying CCDs 
P.A.Jerram, P. J. Pool, D. J. Burt, R. T. Bell, M.S.Robbins 
e2v technologies ltd, 106, Waterhouse Lane, Chelmsford, Essex, UK CM1 2QU 
Electron multiplying CCD (EMCCD) technology has found important initial applications in low light surveillance and photon 
starved scientific instrumentation. This paper provides a description of the technology and discusses the attributes of the 
EMCCD which may make it useful for other detectors, particularly those which are photon starved 
 
 
1. INTRODUCTION 
The electron multiplying charge coupled device  
(EMCCD), such as the e2v L3CCD, uses impact 
ionization in silicon to provide gain in the charge domain 
[1].  This enables performance with an input equivalent 
noise of less than 1 rms electron at pixel rates up to and 
beyond those required for TV imaging applications. The 
structure and mode of operation of the EMCCD is well 
known [2,3] and so only an overview will be given here.  
 
Since EMCCDs were first produced at e2v 
technologies in the late 1990s they have found use in 
many applications where either the signal level is very 
low or where high frame rate imaging is required. These 
include surveillance, astronomy (in Lucky Imaging [3] 
and as a wave-front sensor) for LIDAR and in other 
scientific imaging applications including very low level 
bioluminescence for drug discovery and genetic 
engineering applications. A number of these application 
will be descried briefly later in this paper. 
 
 
 
2. PRINCIPLE OF EMCCD OPERATION 
The architecture of the EMCCD is shown in figure 1.  
 
 
 
 
Figure1: EMCCD Schematic 
 
 
The image, store and read-out register are of 
conventional design. An extended section of 
multiplication register is placed between the normal 
serial register and the charge to voltage conversion 
circuitry. Each section of the multiplication register 
comprises four gates, three of which are clocked. This is 
shown schematically in figure 2. 
 
 
 
 
Figure 2: Charge transfer through a multiplication 
element 
 
 
Two of the gates (Ø1 and Ø3) are clocked with normal 
amplitude drive pulses (~10V) and the same pulses can 
be applied to both sections of the readout register. The 
pulses applied to Ø2 of the multiplication register have a 
higher amplitude, typically 40-45V. A gate is placed 
prior to Ø2, which is held at a low dc voltage. The 
potential difference between the dc gate and the high 
level of Ø2 can be set sufficiently high so that signal 
electrons can undergo impact ionization processes as 
they are transferred from Ø1 to Ø2 during the normal 
clocking sequence. Thus the number of electrons in the 
charge packet increases as it passes through a 
multiplication element. Although the probability of 
impact ionization and thus the mean gain per stage, g is 
very low, the number of stages, N can be high. The total 
gain of the cascaded multiplication elements is given by 
M =gN. For 600 elements a gain of 1.5% per stage will 
give a total gain in the charge domain of over 7,500. 
 
 
 
0019
2
 SNIC Symposium, Stanford, California – 3-6 April 2006 2 
 
 
 
3. PERFORMANCE REQUIREMENTS 
3.1. Noise 
 Robbins and Hadwen [4] have shown that the 
multiplication process increases the input referred noise by 
a factor √2 above the ideal input shot noise level 
(sometimes interpreted as an effective halving of quantum 
efficiency).   
 
The noise referred to the image area, effσ , is given by: 
 
( ) ⎟⎟
⎠
⎞
⎜
⎜
⎝
⎛
++=
2
2
2
M
SSF readoutdarkeff
σ
σ . 
 
Where  readoutσ  is the readout noise, S is the signal, 
Sdark is the dark signal. M is the multiplication gain and F 
is the noise factor. 
 
It can be seen that setting the multiplication gain to a 
sufficiently high value virtually eliminates the effect of 
readout noise. Although the readout noise increases as a 
function of frame rate as in a conventional CCD the 
multiplication gain is independent of readout rate and so 
the gain can be set sufficiently high to ensure that readout 
noise is still eliminated at very high frame rates. 
 
If the signal level is sufficiently low the single electrons 
may be detected, however it must be borne in mind that 
the gain statistics of a multiplication register do not allow 
the unambiguous discrimination between integral numbers 
of input electrons (i.e. by means of measuring signal levels 
at the output), so photon counting is only possible when 
the event density is low enough to be confident that all 
events can be counted as single electrons [5, 6] and the 
gain is high enough to discriminate the full range of 
consequent output signals from read noise.  If conditions 
allow operation in a photon counting regime, the excess 
noise referred to above is, of course, eliminated.  
 
3.2. Dark noise 
As can be seen from the formula above if the gain is 
sufficiently high the remaining noise sources are noise on 
the dark signal Sdark and noise on the signal, S. Thus it is 
clear that devices should be operated under conditions in 
which the dark charge/pixel does not contribute 
significantly to the noise. Dark charge can result from 
thermal generation and also from the clocking process, as 
clock induced charge (CIC).    
 
In low signal scientific applications it is preferable, if 
possible, to control dark current generation rate by 
appropriate cooling of the device.  Fig 3 shows that 
suitably low rates can be achieved.  The alternative is to 
use multi phase pinned (MPP) operation to neutralize 
surface dark current, which allows operation at higher 
temperature. This is advantageous for some applications  
but results in significant hole multiplication during charge 
transfer and higher temperature-independent CIC.  Since 
the charge magnitude increases exponentially with the  
clock amplitude, operation with the lowest-possible clock 
levels is necessary to minimize this contribution. 
Figure 3: Dark signal in e2v CCDs 
 
Note that, if using the device with MPP operation, as a 
gate is taken high (out of pinning) during transfer, holes 
are released from surface traps with a characteristic, 
temperature dependant, time constant.  These holes are 
subject to multiplication as they release, so lowest CIC is 
observed with fast clocking which minimizes the clock 
high time [7]. 
3.3. Quantum efficiency 
To make best use of the low read noise, EMCCDs 
benefit from the high quantum efficiency (QE) available 
from back-illumination.  Choice of an appropriate process 
can provide useful QE anywhere in the wavelength range 
1 Å to 1 μm. Figures 4 shows typical QE from 
backthinned CCDs with and without anti-reflection (AR) 
coating. 
Figure 4: Typical QE curves 
 
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E0
1E1
1E2
1E3
El
ec
tr
on
s/
se
c/
sq
ua
re
 m
ic
ro
n
-120 -100 -80 -60 -40 -20 0 20 40 
Temperature, °C
Dark Current of E2V CCDs
MAXIMUM VALUES
Back illuminated
MPP
Standard
El
ec
tr
on
s/
se
c/
sq
ua
re
 m
ic
ro
n
0019
3
 SNIC Symposium, Stanford, California – 3-6 April 2006  
 
 
 
 
3.4. Linearity 
Provided the output circuit is not saturated, EMCCD 
gain registers have been shown to be linear to 3% up to 
~75% of register capacity [8].  Such performance requires 
the high voltage gate to be operated with a flat-topped 
pulse, rather than a sinusoidal pulse, which is often used in 
surveillance applications, where control of power 
dissipation is more important than linearity 
 
4. EXAMPLE APPLICATIONS 
4.1. Surveillance 
With its variable gain and resistance to overload the 
EMCCD is ideal for surveillance and most of the first 
designs have been intended for surveillance. When 
compared to conventional technology using an image 
intensifier coupled to a CCD the EMCCD gives 
significantly improved performance at high light levels as 
the extra gain is turned off and the device is operated as a 
standard CCD, whereas the image intensifier performance 
at high light levels is relatively poor.  
 
At the lowest light levels the EMCCD still gives 
superior performance because the higher inherent quantum 
efficiency of silicon as compared to GaAs. EMCCDs 
provide usable images under overcast starlight conditions 
with less than one electron per pixel per frame where all 
alternative technologies fail to give useful images. 
4.2. LIDAR [9,10] 
Laser power available for LIDAR (LIght Detection And 
Ranging) applications is limited by available power and, 
more importantly, by eye safety considerations. In 
addition, the atmospheric backscatter coefficient is low 
and the diameter of the telescope limited, so the returning 
photon flux is very low and also needs to be sampled at 
high frequency to achieve depth resolution.  These 
requirements combine to give an expected signal in the 
range 1 to 4,000 photons/μsec in the sampling window.  
For this application a single pixel CCD is being designed 
that will operate at up to 3MHz, which can be 
implemented with a scientific, low noise output circuit, so 
requires only moderate multiplication gain (~x80) to 
discriminate the signal to an adequate level.  The high 
sampling rate from a small area of silicon results in very 
little dark current in each sample. In combination with the 
moderate gain, this allows the sensor to be operated at 
around 0 -10°C.  
4.3. Adaptive optics [11,12] 
CCDs have been used for adaptive optics for astronomy 
for many years. Adaptive optics requires a combination of 
very high frame rates and low noise. Hence sensors for 
this application have relatively small image areas and a 
large number of outputs. For example e2v’s CCD50  has 
16 separate outputs and an image format of only 128 x 128 
pixels. This enables operation at frame rates of over 500Hz 
with a readout noise of approximately 6 electrons. 
EMCCD technology is being used to design an adaptive 
optics CCD for ESO (the European Southern Observatory) 
with an increased image area of 240 x 240 pixels that will 
operate at over 1kHz with 8 outputs with less than one 
electron of read noise. Figure 5 shows a schematic image 
of this CCD. 
 
   
      
Half store section
Half image section
Half store section
Half image section
Gain registers and
output circuits
Gain registers and
output circuits
240 x 120 
pixels
240 x 120 pixels
240 x 120 pixels
    
              
 
Figure 5: Adaptive optics EMCCD 
4.4.  Ground scanning 
EMCCDs have been considered for ground scanning 
applications.  However, the usual ground image has a high 
background signal with a small, superimposed, 
modulation.  Reference to Fig 6 shows that, for such 
signals, the EMCCD excess noise, referred to in the 
introduction, would degrade the performance below that of 
a conventional output. 
 
Noise vs Signal
0
5
10
15
20
25
0 50 100 150 200
Signal  (electrons)
N
oi
se
  (
rm
s e
le
ct
ro
ns
)
EMCCD
7e-
5e-
3e-
 
 
Figure 6: Noise as a function of signal 
 
0019
4
 SNIC Symposium, Stanford, California – 3-6 April 2006 4 
 
 
 
The main reason that EMCCD is unsuitable for ground 
scanning is that such instruments are normally operated 
over a sun-illuminated scene.  If there were requirements 
to ground scan the dark side of the planet, an EMCCD 
solution would clearly be beneficial.  
 
5. OTHER POTENTIAL DETECTOR 
APPLICATIONS 
5.1. Particle and Synchrotron radiation 
detection 
5.1.1. Indirect detection  
Indirect detection typically consists of detecting the light 
output from scintillating fibres. Detectors used are either 
photodiodes or photomultiplier tubes (PMTs), when the 
signal level is low. For these low signal applications the 
EMCCD would give very significant advantages in 
detection efficiency and system complexity and cost as 
compared to PMTs. 
 
For applications where a relatively low frame rate can 
be used a number of fibres could be bundled together onto 
an EMCCD operating at a standard frame rate of 50Hz. 
For higher detection rates an adaptive optics device could 
be used at up to 1kHz or for very high frame rates a 
LIDAR EMCCD would allow detection at up to 3MHz. 
Several LIDAR devices could be fabricated monolithically 
to provide a well-defined pitch for coupling of the ends of 
the optical fibres. 
 
5.1.2. Direct detection 
CCDs have been used for direct particle detection for 
example the VXD2 and VXD3 vertex detectors at SLAC. 
However in general for direct detection the signal level is 
relatively high and so the electron multiplying output is 
not expected to give significant advantages over a 
conventional CCD. 
 
One of the main requirements for direct detection is 
radiation hardness. It has been shown [13,14,15] that the 
addition of the multiplication register in the EMCCD has 
no effect of the radiation hardness of the device. Hence if 
there are applications where an EMCCD would give 
advantages it could be used in the same environments as a 
conventional CCD. 
5.2. Astronomy and Space instrumentation 
Apart from the lack of atmospheric distortion, 
astronomical imaging from space is performed against a 
very low background, compared with that presented by 
sky glow, which can appear as around 21st 
magnitude/arcsec at a good terrestrial site.  Though useful 
for spectroscopy, lucky imaging and adaptive optics, the 
presence of sky glow limits the application of EMCCD in 
ground-based imaging astronomy.  The very low 
background in space raises the possibility of imaging very 
faint objects.  In addition, such objects may be imaged in 
the presence of much brighter objects by operating the 
imager at high frame rate and integrating a wide dynamic 
range image in a frame store.  In this case, photon flux 
from the faintest objects may be low enough to assume 
that all events are single photons, giving best possible 
signal/noise ratio. 
 
Applications have been proposed in which the projected 
photon flux from discrete objects would be ~10-4 
photons/pixel/sec.  Such requirements raise interesting 
questions.  Operation at low temperature and with low 
clock swing should ensure that signal electrons are not lost 
in dark signal and CIC, and cosmic ray events can be 
discriminated by the magnitude of deposited charge.  
However, there's going to be a growing problem of how to 
understand the signal since, in the steady state, the average 
signal will equal the input signal, but we have no way of 
knowing which electrons have been cycled through traps 
and therefore lost their spatial/temporal identity.   In space, 
after the collection of proton damage, the device will 
accumulate an increasing number of single electron traps, 
which will further complicate the analysis.  Further 
analysis of the dynamics of single electron trapping is 
essential, leading to recommendations for an optimum 
operating regime.  Only then do we have a chance of 
predicting the lower limits of object intensity that can be 
imaged with such devices. 
 
6. ACKNOWLEDGMENTS 
 
The content of this paper has benefited from the 
contributions of many people within the CCD group at e2v 
technologies. 
 
References 
[1] S.K. Madan, B. Bhaumik, J. M. Vasi, Experimental 
observation of avalanche multiplication in charge 
coupled devices,  IEEE Trans. Electron Devices, 
vol. ED-30, pp. 694-699,  June 1983 
[2] P.Jerram et al. The LLLCCD: Low light imaging 
without the need foran intensifier, Proc. SPIE vol. 
4306 pp. 178-186, 2001. 
[3] R. N. Tubbs, J. E. Baldwin, C. D. Mackay, and G. 
C. Cox, Diffraction limited CCD imaging with faint 
reference stars, Astronomy Astrophys., vol. 387, 
pp. L21-L24, 2002 
[4] M. S. Robbins, B. J. Hadwen, The noise 
performance of electron multiplying charge-
coupled devices, IEEE Trans Electron Devices, Vol. 
50,  pp. 1227-2003, May 2003 
[5] A. G. Basden, D. F. Buscher, C. A. Haniff, C. D. 
Mackay, Low readout noise CCDs in optical 
interferometry, SPIE 4835-145, 2002 
0019
5
 SNIC Symposium, Stanford, California – 3-6 April 2006  
 
 
 
 
[6] A.G.Basden, C. A. Haniff, C. D. Mackay, Photon 
counting strategies with low-light-level CCDs,  
Mon. Not. Astron. Soc., 345, pp. 985-991, 2003 
[7] David Burt, Private communication.  
[8] Sandy Denney, e2v internal technical note. 
[9] J.Bézy, W. Leibrandt, A. Hélière, P. Silvestrin, C. 
Lin, P. Ingmann, K. Kimura, The ESA EarthCARE 
mission,  SPIE, 5882-17, 2005 
[10] ATLID in 
http://esamultimedia.esa.int/docs/EEUCM/EarthCA
RE_TPA.pdf 
[11] S. Tulloch, Application of L3 Technology to 
Wavefront Sensing, SPIE-5490-1167 2004 
[12] M. Downing et al, A dedicated L3CCD for 
Adaptive Optics applications, Scientific Detector 
Workshop 2005 (to be published by Kluwer) 
[13] B. J. Hadwen, M. Alcon Camas, M. S. Robbins, 
The Effects of Co60 Gamma Radiation on Electron 
Multiplying Charge-Coupled Devices, IEEE Trans. 
Nuclear Science, Vol. 51, October 2004 
[14] D. R. Smith, A. D. Holland, Proton irradiation of 
e2v technologies L3Vision devices, Nuc. Inst. 
Meth., vol. A546, (2005), pp. 544-52. 
[15] D. R. Smith, R. Ingley, A. D. Holland, e2v 
technologies L3Vision CCD97Irradiation Testing, 
submitted to IEEE Trans. Elec. Devices, 2005
0019


Electron Multiplying CCDs

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=84AC84D54E7360CF1645E76EB63CBE75?doi=10.1.1.1043.6305&rep=rep1&type=pdf

Electron Multiplying CCDs

Abstract

Similar works

Full text

Available Versions

CiteSeerX