SOLID STATE IMAGING - TECHNICAL NOTE ON
DEEP DEPLETION OF CHARGE COUPLED DEVICES
.
Charge Coupled Devices (CCDs) requiring enhanced performance in the Near Infrared (NIR) region of the
spectrum or at higher X-ray energies can benefit from being constructed on specially selected silicon with
an optimised thickness. Such a CCD, where the electron generation is within this thick silicon, is known as
a Deep Depletion Device (High Resistivity).
Backside illuminated devices
Introduction
'Deep depletion' devices are variants of certain standard
e2v technologies products fabricated on different silicon
material to increase the quantum efficiency at near-IR
wavelengths and with higher energy X-rays, but without
loss of spatial resolution.
In this case, the total thickness of silicon is thinned to
typically <20 µm and longer wavelengths can pass
through this thickness. This means that long
wavelengths (e.g. >800 nm) are not efficiently absorbed.
The introduction of deep depletion silicon
Basic principles
The depth over which photons generate signal charge in
the silicon beneath the CCD electrode structure increases
with increasing wavelength, the consequences of which are
shown schematically in Fig. 1. Front and backside
illuminated devices are shown. Fig. 2 also shows the
variation of absorption coefficient with wavelength.
Frontside illuminated devices
At visible wavelengths the charge is largely generated
within the depletion region beneath the electrodes and the
probability of charge being collected in the 'correct' pixel
(i.e. the pixel on which the photon was incident) is high.
This is because there is an electric field in the depletion
region normal to the silicon surface that causes the charge
to move directly to the nearest potential well in the buried
channel for subsequent storage. However, at longer
wavelengths much of the generation can be deep in the
field-free silicon below the depletion region and charge can
diffuse (in a random manner) away from the point of
generation before being attracted to the depletion region of
a pixel for storage. The probability of being collected in the
correct pixel is therefore reduced, and this loss of spatial
resolution is usually defined in terms of increased pixel
cross-talk or reduced Modulation Transfer Function. Similar
effects are observed with high energy X-rays.
In order to minimise this effect most devices are now made
on 'epitaxial' silicon. This material comprises a heavily
doped substrate on which is deposited a more lightly doped
surface layer, the parameters of which can be controlled in
manufacture. The heavy doping in the substrate
considerably reduces the distance any photo-generated
charge can diffuse before it recombines, with the result that
almost none of it is collected as signal. The substrate is
therefore effectively 'dead', and device performance is
dependent on the properties of the 'active' epitaxial layer.
Detailed analysis suggests that, for most applications, a
reasonable compromise between quantum efficiency and
resolution is for the thickness to be comparable to the pixel
pitch and 20 to 25 µm is typically used for e2v technologies
devices of standard manufacture.
A few specialised applications do however require
highest possible quantum efficiency at NIR wavelengths
or higher X-ray energies and, for a given device
structure, this can only be achieved by increasing the
active thickness, as now described.
So far the analysis has not considered the depth of
depletion in the silicon below the electrodes. Although
device performance would obviously benefit if this could
be increased, various practical constraints exist to limit
the depth to no more than about 10 µm (which is
adequate for 20 to 25 µm total layer thickness)
The actual depth of depletion is proportional to (V/NA),
where V is the channel potential and NA is the doping
concentration of the silicon below the buried channel. V
is generally set by the range of normal operating
voltages, and NA cannot easily be reduced below a
certain minimum value through dopant out-diffusion from
the underlying heavily-doped substrate, hence the
current limit. However, with careful optimisation of the
deposition conditions, epitaxial silicon can be obtained
with considerably reduced doping concentrations, thus
permitting the total layer thickness to be increased to
nominally 50 µm with a depth of depletion of at least
30 µm. These high resistivity devices fabricated on such
material are designated 'deep depletion'.
Frontside and backside illuminated devices are
manufactured with deep depletion silicon. Both devices
benefit from improved red response, whilst the backside
illuminated devices can deliver improved overall
quantum efficiency and better blue response.
Performance
Figure 3 shows measurements made on frontside
CCD15-11 devices to illustrate the improvement in nearIR quantum efficiency possible with deep depletion
devices over that available with standard products. The
spatial resolution of the two variants is similar, as are the
operating voltages and all other parameters, e.g. full well
capacity, output responsivity, noise etc.
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© e2v technologies limited 2003
A1A-CCDTN101 Issue 6, June 2003
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Figure 4 indicates performance of backside-illuminated devices. One curve indicates the QE of a normal silicon ‘astro
broadband’device (as used on many large area astronomy sensors). The other curve indicates QE of deep depletion
silicon, process with the standard midband AR coating.
Note that the spatial resolution of deep depletion devices can degrade when fast optics are used if the cone of light
extending into the silicon from a focused spot generates charge directly beneath adjacent pixels. This is simply a
consequence of active depth being greater than the pixel pitch, and standard devices may be preferable for such
applications.
Also, since we consider that cosmetic quality of deep depletion silicon is somewhat poorer, a differing cosmetic
specification may be applied for devices of this material.
UV light
< 400nm
Visible
400-700nm
IR
> 700nm
Frontside electrodes
Depleted silicon
Undepleted silicon
Figure 1a.
Photon absorption in frontside illuminated CCD; an undepleted region is also shown.
UV light
< 400nm
Visible
400-700nm
IR
> 700nm
Backside surface
Depleted silicon
Frontside electrodes
Figure 1b.
Photon absorption in backside illuminated CCD
© e2v technologies limited 2003
A1A-CCDTN101 Issue 6, June 2003
Optical Absorption Coefficient
in Silicon
1E6
1E5
Alpha (/mm)
1E4
1E3
1E2
1E1
1E0
1E-1
1E-2
0
200
400
600
800
1000
Wavelength (nm)
Marconi Applied Technologies
Figure 2. Silicon absorption coefficient
Typical front illuminated quantum efficiency
60
Standard
Deep depleted
Quantum efficiency (%) at 20C
50
40
30
20
10
0
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 3. Frontside illuminated CCD- comparison of standard and deep-depletion silicon response.
© e2v technologies limited 2003
A1A-CCDTN101 Issue 6, June 2003
Typical back illuminated quantum efficiency
100%
D-D Si Std mid
Std Si Astro BB
Quantum efficiency at -100C
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 4. Backside illuminated CCD- comparison of standard and deep depletion silicon
(note that different AR coatings have been applied in the two cases).
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and also reserves the right to change the specification of goods without notice. e2v technologies accepts no liability beyond that set out in its standard conditions of sale in
respect of infringement of third party patents arising from the use of tubes or other devices in accordance with information contained herein.
© e2v technologies limited 2003
A1A-CCDTN101 Issue 6, June 2003