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X-ray CCDs
Andrew HollandI
Abstract
In recent years, the silicon charge coupled device (CCD) has found use in many
applications in space science providing imaging solutions to a variety of instrument
needs. Whilst the majority of imaging applications using CCDs are in the visible
region, the ability of silicon to perform efficient detection of X-rays in the 0.1 keV
to 10 keV band enables their use in, for example, X-ray astronomy. Since the early
1990s several instruments have been flown using X-ray-optimised CCDs as both
non-imaging and imaging detectors at the focal planes of X-ray telescopes. This
chapter reviews the use of the silicon CCD for direct detection in the X-ray region
and describes some successful applications, together with radiation damage effects
when used in the space environment.
X-ray interactions in silicon CCDs
High-energy photons in the X-ray band can be detected and imaged using silicon
CCDs. Figure 24.1 depicts the X-ray absorption coefficient σI as a function of
photon energy Eγ . Since the detection thicknesses are measured in ≈ 10 µm
or ≈ 30 µm, silicon can efficiently stop X-rays in the energy range 100 eV to
10 keV, where the photoelectric effect dominates. At these energies, the photon
interacts with bound electrons in the silicon atom and ejects an energetic photoelectron. The excited atom then returns to its ground state through a series of Auger
and fluorescence processes, whilst the energetic photo-electron releases energy by
ionisation (Fraser et al 1994). The whole process results in the creation of an
electron cloud around the initial X-ray absorption site, with the mean number
of free electrons generated equal to Ne = Eγ /ω, where Eγ is the photon energy
and ω is the mean energy required to liberate one electron-hole pair. In silicon
at room temperature, ω = 3.65 eV. Thus a 6 keV photon would create typically
1640 electrons per X-ray interaction (as compared to only one electron per optical
photon interaction).
I Planetary and Space Science Research Institute, Open University, Walton Hall, Milton
Keynes, UK
409
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24. X-ray CCDs
Figure 24.1: Absorption coefficient vs. photon energy in silicon. The dominant
energy range for direct absorption in silicon is between 100 eV and 10 keV where
the photoelectric effect is important.
The X-rays are penetrating and can be absorbed throughout the active structure
in the CCD. Figure 24.2 depicts the various layers which affect signal generation
in a CCD, and shows the two illumination methods of front-illumination and backillumination. As described in Chapter 23 (Waltham 2010), the CCD is initially
fabricated on a high-purity epitaxial silicon layer which is grown on top of a substrate. The electrodes plus gate oxide and nitride layers are electrically inactive
and form a “dead” layer. In the front-illuminated structure, X-rays impinge on the
CCD from the front/top as indicated in Figure 24.2.
The X-rays may be absorbed in either the depletion layer, the field-free layer, or
the substrate. As described earlier, some devices are back-illuminated, where the
substrate and some of the field-free layer are removed through a combination of
mechanical plus chemical means. In most back-illuminated devices, e.g., for optical
applications, a finite field-free layer is retained (around 5 µm to 10 µm thick). For Xray applications, however, the device can be thinned into the depletion layer, which
results in a fully-depleted structure. Figure 24.3 shows how the signal, generated
in each of the depletion and field-free regions in the front-illuminated structure,
spreads prior to being stored at the pixel potential minimum just below the electrode surface. In general, the signal arising from X-ray interactions in the depletion
region is rapidly collected and is stored in single pixels, while that from the fieldfree region undergoes lateral diffusion during collection, producing larger “event”
sizes, and is collected over several pixels. The image in Figure 24.3 shows discrete
X-ray photon interactions in a CCD in what is referred to as “photon counting
mode”. In this image the single and multi-pixel events are clearly identified.
The detection efficiency of the X-ray CCD is therefore built up from several
discrete components. For low-energy photons, absorption occurs in the dead layers
above the X-ray sensitive volume, while the limit to high-energy detection is affected
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Figure 24.2: Detection and “dead” layers both for both, front and back-illuminated
structures.
a)
b)
Figure 24.3: a) Sketch showing charge spreading from the point interactions.
b) Image of discrete photon detections from these layers demonstrating the effect
of charge spreading.
by the absorption of photons in the active layer. In the front-illuminated device
this can be typically equivalent to ≈ 1 µm of silicon, while in the back-illuminated
device the real dead layer can be as little as 50 nm. Standard CCDs have depletion
depths of between 3 µm and 10 µm, and for X-ray applications “deep depletion”
devices constructed on high-purity silicon can provide depletion depths of 30 µm to
300 µm, depending upon silicon type and CCD design and bias. Care must therefore
be exercised when specifying CCDs for specific applications to ensure that the best
detection efficiency and CCD type (front/back-illumination) is obtained.
Figure 24.4 gives the quantum efficiency (QE) as a function of photon energy for
several types of CCD construction, ranging from a standard commercial TV-type
sensor, to a special fully-depleted, 300 µm thick back-illuminated type. Factors
such as price, yield and availability might be traded against detection efficiency
and energy range.
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24. X-ray CCDs
Figure 24.4: Comparison of different types of CCD construction and their resulting
quantum efficiency.
Noise sources and X-ray energy resolution
The detection sensitivity and energy resolution are dependent upon the system
noise, leakage current, etc. At high readout frequencies, in the absence of other
√
degrading factors, the on-chip amplifier white noise σw dominates and σw ∝ f ,
where f is the readout frequency (or more accurately the electronic bandwidth of
the system). As the readout speed decreases, the noise profile settles at the output
node’s 1/f noise floor, where readout noise is independent of readout speed. This
1/f noise floor for scientific type CCDs can be between two to three electrons
RMS. However, for low readout frequencies, the integrated leakage current can
dominate, and Figure 24.5a shows the additional noise contributions arising from
leakage current at a number of temperatures, for both normal and inverted, or MPP
(multi-pinned phase, cf., Chapter 23, Waltham 2010), mode operation. For many
scientific space applications where noise is important, the devices are operated at
temperatures around –100 ◦ C to ensure that the contribution from leakage current
is negligible.
In X-ray photon counting spectroscopy applications, whilst the number of photogenerated electrons is Eγ /ω, the variance on this electron number is not given by
the normal Poisson statistics, but is smaller due to the correlation between the
energy-recovery processes (Fraser et al 1994). In this instance the variance on the
signal generated is given by F Eγ /ω, where F = 0.115 is the Fano factor in silicon
(Fraser et al 1994). The total X-ray energy resolution as a function of energy may
be expressed as
∆EFWHM = 2.35 ω
r
σ2 + F
Eγ
ω
.
(24.1)
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Figure 24.5: a) Total system noise as a function of readout speed for different
operational temperatures. b) X-ray FWHM resolution vs. energy for several readout
noise values.
This relationship is given in Figure 24.5b as a function of X-ray energy for a
number of different readout-noise values. Since the efficient detection of photons
starts at energies above ≈ 200 eV to 300 eV, system noise of three to five electrons RMS is satisfactory for most applications of this type (e.g., XMM-Newton
EPIC MOS cameras operate with ≈ 4.5 electrons RMS). The figure also demonstrates that an energy resolution of only 200 eV is required to resolve the elemental
lines from the elements heavier than oxygen, which is an important parameter for
instruments for planetary science investigating elemental composition.
Figure 24.6: X-ray spectrum from a variety of elemental fluorescence lines between
oxygen at 0.5 keV and copper at 8 keV and 9 keV, demonstrating the energy
resolving power of the CCD in X-ray photon counting mode.
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24. X-ray CCDs
Figure 24.7: CCD22 from e2v designed for the EPIC cameras on XMM-Newton
together with one of the flight focal plane arrays which consists of an array of
seven such detectors.
Figure 24.8: Schematic showing the SCD layout, together with a photograph of the
SCD module with four detectors for the D-CIXS instrument.
X-ray CCD arrays
Figure 24.7 shows a single flight CCD developed by e2v technologies and Leicester
University for the EPIC instrument on ESA’s XMM-Newton spacecraft (Turner et
al 2001). The two EPIC MOS CCD cameras, at the focal surfaces of X-ray grazingincidence optics, comprise arrays of seven such CCDs in a focal plane array. The
devices are optimised for single photon X-ray spectroscopy in the 3 eV to 10 keV
band, and are of the “open electrode” format for enhanced soft X-ray detection
efficiency (Holland 1996). Due to the large depth of focus of the X-ray optic, the
CCDs can be overlapped to minimise the dead space in the array.
The swept charge device (SCD)
A variant detector type based on CCD technology has been generated which
dispenses with the spatial (imaging) information and which is designed as a photon
counting X-ray spectrometer. The “swept charge device” (SCD) is a relatively
simple CCD, having ≈ 10 interconnections, compared to between 20 to 30 for
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Figure 24.9: Upper: Dispersed X-ray spectrometer system used on ESA’s XMMNewton where gratings are used to disperse X-rays for readout by an array of
CCDs.
Lower: First-light results from the RGS instrument (courtesy of ESA).
a normal CCD. The device is continuously read out and is similar in operation
to a linear CCD though with large area. The continuous readout maintains a low
leakage current by “dither mode clocking”, and X-ray spectroscopy can be obtained
at much higher temperatures than normal. The original SCD, which possessed
an area of ≈ 1 cm2 , could produce Fano-limited spectroscopy at temperatures as
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24. X-ray CCDs
Figure 24.10: Increase in parallel CTI for e2v’s standard CCD62 as a function of
10 MeV proton fluence, together with results for an experimental narrow channel
CCD for improved hardness.
high as –10 ◦ C, while newer generations of the device can yield good spectroscopy
even at +10 ◦ C for detection areas of 4 cm2 . The figure also shows a module
containing four original-type SCD detectors which was used for lunar elemental Xray fluorescence mapping in the D-CIXS instrument on ESA’s Smart-1 spacecraft
(Grande 2003), and has been launched in October 2008 on the C1XS spectrometer
of India’s Chandrayaan-1 lunar orbiter (Howe et al 2009).
Dispersed X-ray spectroscopy
An alternative method for X-ray spectroscopy is through the use of an energydispersive medium such as a Bragg crystal or diffraction grating. Figure 24.9 shows
the configuration adopted for the reflection grating spectrometer (RGS) instrument
on XMM-Newton, where a grating is used behind the X-ray optic to disperse the
X-rays (Brinkman 2001). In this approach the spatial information is essentially
lost, and the technique is mainly intended for the brighter sources. The dispersed
X-rays come to a focus on the Rowland circle of the grating, and a linear array of
nine CCDs is used to read out the spectrum, where linear position equates to Xray energy. The original “first light” results from the instrument are also shown in
the figure. These demonstrate the improved energy resolution that can be achieved
using the dispersed spectrum method.
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Space radiation damage
The space environment and its impact on CCDs was discussed in Chapter 23
(Waltham 2010). For a thorough description of radiation effects see Holmes-Siedle
(2002). In the X-ray photon-counting applications, the most degrading effect is that
of the high-energy protons, originating from the Sun or from the trapped radiation
belts and South Atlantic Anomaly (SAA) (Holland 1992). These protons create
trap sites which capture electrons from the X-ray signals and can release the charge
when the X-ray signal has been removed by readout. The release-time constants
are dependent upon operating temperature and trap species, but the signal can be
released in a range between nanoseconds to minutes after trapping. In the X-ray
spectroscopy application, where the energy resolution is well defined through the
system noise and Fano statistical variation, such carrier removal through trapping
can significantly degrade the FWHM energy resolution of the CCD.
The hardness to radiation damage can be improved by restricting the charge
storage and transport within the CCD during readout. Figure 24.10 shows the
increase in CTI (charge transfer inefficiency) for a CCD against fluence for 10 MeV
protons, showing results for standard off-the-shelf CCDs together with experimental
devices intended to improve the radiation-transport properties.
Future developments
Current and future developments in X-ray direct-detection CCDs are targeted
at improving the readout speed, and hence the X-ray throughput of the systems, to
enable their use with X-ray optics having larger effective area. In addition, the size
of CCDs can now be much larger, e.g., up to 6 cm × 6 cm so either single detectors
can replace earlier arrays, or the focal-plane array size can be much larger. The highenergy detection efficiency arises through the use of high-purity bulk silicon, such
that detection of photons out to 20 keV can be achieved with a relative detection
efficiency of 90 % at 10 keV. Finally the CCD technology is being increasingly
combined with application specific integrated circuit (ASIC) technology so that
much more of the drive and readout function of the electronics is included at the
CCD module level. This is especially important for large mosaic arrays intended for
future telescopes (both optical and X-ray). The improvement of radiation hardness
in CCDs is an ongoing development, and measures such as thin-gate dielectrics,
narrow-channel technology and p-channel technology are being used to improve
the performance of these sensors for use in space.
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