CCD detectors for spectroscopy and imaging of x-rays with the eROSITA space telescope
ABSTRACT A special type of CCD, the so-called PNCCD, was originally developed for the focal plane camera of the XMMNewton space telescope. After the satellite launch in 1999, the MPI Halbleiterlabor continued the detector development for various ground-based applications. Finally, a new X-ray PNCCD was designed again for a space telescope named eROSITA. The space telescope will be equipped with an array of seven parallel oriented X-ray mirror systems of Wolter-I type and seven cameras, placed in their foci. This instrumentation will permit the exploration of the X-ray universe in the energy band from 0.3 keV up to 10 keV with a time resolution of 50 ms for a full image comprising 384 x 384 pixels. eROSITA will be accommodated on the new Russian Spectrum-RG satellite. The mission was already approved by the responsible German and Russian space agencies. The detector development is focussed to fulfil the scientific specifications for detector performance under the constraints of all the mechanical, power, thermal and radiation hardness issues for space instrumentation. This considers also the recent change of the satellite's orbit. The Lagrange point L2 was decided as new destination of the satellite instead of a low-Earth orbit (LEO). We present a detailed description of the detector system and the current development status. The most recent test results are reported here. Essential steps for completion of the seven focal plane detectors until satellite launch in 2012 will be itemized.
- SourceAvailable from: Robert Hartmann[show abstract] [hide abstract]
ABSTRACT: A special type of charge-coupled device, the pnCCD, has been developed in the nineties as focal-plane detector for the X-ray astronomy mission XMM-Newton of the European Space Agency. The pnCCD detector has been in operation since the satellite launch in 1999. It is performing up to date spectroscopy of X-rays in combination with imaging and high time resolution. The excellent performance of the flight camera is still maintained; in particular, the energy resolution has been nearly constant since launch. In order to satisfy the requirements of future X-ray astronomy missions as well as those of ground-based experiments, a new type of pnCCD has been developed. The 'frame store pnCCD' shows various optimizations in device design and fabrication process. Devices with up to 256 Â 512 pixels have been fabricated in 2004 and recently tested. Simultaneously, a programmable analog signal processor for the readout of the CCD signals, the DUO CAMEX, has been developed. The readout noise of the new detector has a value of 2 electrons ENC which is less than half of the figure of the XMM-Newton pnCCD. We measured an energy resolution that is close to the theoretical limit given by the Fano noise. In particular the low-energy response of the new devices was substantially improved. The quantum efficiency for X-rays is at least 90% in the entire energy band from 0.3 keV up to 11 keV. This is due to the ultra-thin photon entrance window as well as the full depletion of the 450 mm thick back-illuminated pnCCD. The position resolution is better than the pixel sizes of 75 mm Â 75 mm or 51 mm Â 51 mm because the signal charge is spread over up to four pixels which allows a more accurate event position determination. 'Out of time' events are substantially reduced to the order of 0.1% by operating the pnCCD in frame store mode. Higher operating temperatures, e.g. À20 1C, are possible due to the smaller thermally generated dark-current level of the new devices and the operation at higher frame rates. Low power consumption applications like for the ROSITA X-ray astronomy mission with low frame rates of, e.g. 20 images/s, as well as high frame rate applications, e.g. 200 images/s, are possible with the same device.Nuclear Instruments and Methods in Physics Research A30.Kv. 01/2006; 568(29).
CCD detectors for spectroscopy and imaging of X-rays
with the eROSITA space telescope
N. Meidinger∗,a,c, R. Andritschkea,c, S. Ebermayera,c, J. Elbsa,c, O. Hälkera,c, R. Hartmannb,c,
S. Herrmanna,c, N. Kimmela,c, P. Predehla, G. Schächnera,c, H. Soltaub,c, L. Strüdera,c, and
aMax-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany;
bPNSensor GmbH, Römerstrasse 28, 80803 München, Germany;
cMPI Halbleiterlabor, Otto-Hahn-Ring 6, 81739 München, Germany
A special type of CCD, the so-called PNCCD, was originally developed for the focal plane camera of the XMM-
Newton space telescope. After the satellite launch in 1999, the MPI Halbleiterlabor continued the detector development
for various ground-based applications. Finally, a new X-ray PNCCD was designed again for a space telescope named
eROSITA. The space telescope will be equipped with an array of seven parallel oriented X-ray mirror systems of
Wolter-I type and seven cameras, placed in their foci. This instrumentation will permit the exploration of the X-ray
universe in the energy band from 0.3 keV up to 10 keV with a time resolution of 50 ms for a full image comprising
384 x 384 pixels. eROSITA will be accommodated on the new Russian Spectrum-RG satellite. The mission was already
approved by the responsible German and Russian space agencies. The detector development is focussed to fulfil the
scientific specifications for detector performance under the constraints of all the mechanical, power, thermal and
radiation hardness issues for space instrumentation. This considers also the recent change of the satellite’s orbit. The
Lagrange point L2 was decided as new destination of the satellite instead of a low-Earth orbit (LEO). We present a
detailed description of the detector system and the current development status. The most recent test results are reported
here. Essential steps for completion of the seven focal plane detectors until satellite launch in 2012 will be itemized.
Keywords: CAMEX, eROSITA, PNCCD, Spectrum-RG, X-ray imaging, X-ray spectroscopy, X-ray telescope.
The excellent energy resolution in combination with a quantum efficiency close to 100% over the eROSITA energy
band (0.3 keV – 10 keV), provides an essential performance characteristic of the PNCCD detector. A further important
feature of the eROSITA telescope with respect to the all-sky survey is its very high grasp, i.e. the product of effective
area and the field of view (FoV). The large FoV of 1.0° in diameter is obtained with a PNCCD detector image area of
nearly 3 cm x 3 cm in the focal plane. A CCD format of 384 x 384 pixels provides the required spatial resolution, which
is determined by the angular resolution of the mirror system. The pixel size of 75 µm x 75 µm is exactly as large as
specified. The readout time of an image is minimized to 10 ms by the parallel signal transfer and processing architecture
of the PNCCD and its readout ASIC, called CAMEX. As a result, we achieve a high time resolution along with low-
noise performance of 2 electrons rms read noise.
Each of these features gives an essential contribution to the scientific power of the eROSITA telescope. After the
introduction of the motivation for the PNCCD detector, we describe in the next section briefly the concept.
∗ firstname.lastname@example.org, phone: ++49 89 83940022, fax: ++49 89 83940011, hll.mpg.de, mpe.mpg.de
UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XVI,
edited by Oswald H. Siegmund, Proc. of SPIE Vol. 7435, 743502 · © 2009 SPIE
CCC code: 0277-786X/09/$18 · doi: 10.1117/12.825234
Proc. of SPIE Vol. 7435 743502-1
2. DETECTOR CONCEPT
The development of the early PNCCDs got a great impulse when this detector type was proposed and selected for one of
the three focal plane cameras for the XMM-Newton X-ray telescope of ESA. After successful launch of the satellite in
1999 and successful commissioning of the PNCCD camera, the further development of the PNCCD detector for future
space applications was mainly driven by the following three topics:
a) Optimization of energy resolution: For this purpose we reduced the detector read noise as well as the charge
b) Improvement of detector response to low-energy X-ray photons. The improvement of the photon entrance
window was achieved by optimization of the fabrication process.
c) Suppression of image smearing due to out-of-time events. We solved the problem by adding a frame store to
the chip and operating the PNCCD in frame transfer mode.
The most important conceptual features of the present PNCCD detector are itemized subsequently. The chip thickness
of 450 µm is fully depleted by use of the principle of sideward depletion and thus sensitive. X-ray photons enter the
CCD from the back side which is equipped with an ultra-thin unstructured pn-diode. As a result, we obtain high
quantum efficiency for a broad band of X-ray photon energies. The values are at least 90% at photon energies from
0.3 keV up to 11 keV. The shift registers also realized as pn-diodes are implemented on the front side. Signal charge
collection and transfer is done in a depth of about 7 µm, which allows for relatively large pixel sizes. In frame transfer
mode (alias frame store mode), the signal charge of all pixels of the image area is transferred rapidly (≈ 0.2 ms per
eROSITA image) into the adjacent frame store section (see Figure 1). Since the frame store is shielded against X-rays,
only photons entering the image area during this short period of charge transfer, become out-of-time events, i.e. show a
wrong position in transfer direction. This applies for only 0.4% of the photons in the case of eROSITA. No serial charge
transfer is necessary because each transfer channel is equipped with an anode and a n-channel JFET for signal
amplification. All PNCCDs are thereby fully column parallel CCDs. A multi-channel analog signal processor chip, the
CAMEX, completes the parallel detector architecture. Each CCD channel is connected to a dedicated CAMEX channel.
Low-noise filtering of the signals, when they are transferred to the anodes, is accomplished by 8-fold correlated double
sampling. While the next CCD row is processed, the shaped analog signals are multiplexed to the output buffer of the
CAMEX. From there they are fed into a fast 14-bit ADC for digitization. Processing of the digital signals starts with a
subtraction of the individual pixel pedestals (offset values). The parallel processing allows for a common mode
correction of the signals of each row. Finally, the slightly different gains of the channels are normalized and their charge
transfer losses are corrected to obtain an optimum energy resolution.
For the development of a very compact camera for space, we shortened the pixel size to 51 µm x 75 µm in the frame
store section in order to minimize the area of the CCD chip. Additionally, a thin light filter is directly deposited on the
photon entrance window, which supersedes an external filter in front of the CCD detector.
3. EROSITA DETECTOR DESIGN
The 37 mm x 56 mm large PNCCD chip contains an image area of 28.8 mm x 28.8 mm, a frame store section with the
same number of pixels, an anode and amplifying transistor per channel, charge clear structures, an inject electrode for
test purposes and temperature diodes (Figure 1). The CCD is glued on a six-layer detector hybrid board together with
the CAMEX ASICs (Figure 2).
A five-layer rigid-flex printed board with 133 lines connects the CCD module with the further supply, control and data
acquisition electronics outside the focal plane. Since the detector is cooled to a temperature of about -80°C, its power
consumption has to be minimized and the detector board must be thermally decoupled from the warm electronics, which
is more power consuming and thereby heat dissipating. The power consumption on the detector board is reduced by
switching the CAMEX ASICs into standby mode after the readout of an image has been finished. The ASIC is powered
again when the next frame should to be processed. We obtain by increasing the integration time to 50 ms a total active
heat load of only 0.7 W per detector (at the expense of time resolution).
Proc. of SPIE Vol. 7435 743502-2
Thermal decoupling of the detector board from the warm electronics is achieved by an appropriate design of the rigid-
flex printed board. The wire cross section was therefore minimized to less than 2 mm2 and the length enlarged to about
25 cm. As a result, the total heat load in the focal plane including the power consumption amounts to only 2 W per
detector. It will be dissipated on the satellite via heat pipes to the radiators. The cooling concept of eROSITA is
described in detail by Fürmetz et al.
Figure 1: Diagram of one eROSITA PNCCD detector showing
the dimensions and illustrating the structure. Every 50 ms a
signal image is transferred rapidly to the frame store area of the
CCD. The 384 CCD channels are read out simultaneously by
three 128-channel VLSI CAMEX chips. During signal
processing of the next row, the signals of the previous row are
serialized to the CAMEX output buffer, which feeds the analog
signals into an ADC for digitization. The seven eROSITA
detectors are identical. The total camera array is thus equipped
with seven PNCCDs, 21 CAMEX ASICs and 21 ADCs.
The recent decision to fly the eROSITA telescope not in a low-Earth orbit (600 km altitude, 30° inclination), but at
Lagrange point L2, means a substantial change of the radiation environment. Calculations with “The Space
Environment Information System” (SPENVIS) showed for the planned mission time from 2012 to 2017 an ionization
dose between 20 krad and 60 krad (depending on the model) behind a shielding of 1 mm aluminum. The eROSITA
PNCCD detector needs a thicker shielding because it collects, stores, transfers and amplifies analog signals consisting of
only several ten up to a few thousand electrons. It is therefore relatively susceptible to radiation damage. Protons are the
most critical radiation component in space because of their high flux and high ionizing and non-ionizing energy loss. A
proton shield around the detector was designed with the specification to shield the protons up to an energy of
160 MeV. An estimate of the proton fluence to the detector behind the shielding gives under these assumptions a
number of 2 x 109 protons per cm2 and five year mission time.
384 x 384 Pixel
75µm x 75µm
frame store area
384 x 384 Pixel
75µm x 51µm
(shielded against X-rays)
Proc. of SPIE Vol. 7435 743502-3
Figure 2: Detector hybrid board produced in thick film technology on an Al2O3 ceramic substrate (shown without PNCCD and
CAMEX chips). The board is equipped with RC-filters in SMD technology for the voltage supplies. The three CAMEX chips will be
mounted in the central part of the board, adjacent to the large-area PNCCD. In the upper part of the picture, we see the cut-out in the
board where the double-sided processed PNCCD will be placed, only touching the board at the edges. At the bottom, we see the
interface for a rigid-flex printed board connecting the detector board to the further electronics outside the focal plane.
4. DEVELOPMENT STATUS
The layout and fabrication process sequence is based on that of the PNCCD devices developed for the DUO project
having a format of 256 x 256 pixels in the image area. The CCD performance was tested and the detectors are
meanwhile used in various ground-based applications. For eROSITA the format was enlarged to 384 x 384 pixels while
keeping the layout of the pixels. As a result of our tests with the DUO CCDs, we optimized the charge clear of the
anodes for the eROSITA devices. If solar or cosmic protons penetrate the CCD, they generate by orders of magnitude
higher signal charge in the device than the X-rays focussed through the telescope. A fast drain of the charge, as soon as
it is transferred to the anode and sampled, is necessary for an accurate measurement of the charge amounts of
subsequent X-ray photon signals. The first CCD wafers were produced and the eROSITA CCDs studied in laboratory
tests (see Figure 3). We measured a read noise of the detector system between 2.0 and 2.5 electrons rms. The energy
resolution parameterized as full width at half maximum (FWHM) of relevant spectral lines is presented in Table 1. The
measurements were carried out at a temperature of -80°C and with a rate of 20 images / s, i.e. under conditions as
planned for the space telescope. After verification of the eROSITA CCD function and quality, a second batch of
Proc. of SPIE Vol. 7435 743502-4
eROSITA wafers was produced in the MPI Halbleiterlabor and is presently finished. It will basically provide the flight
CCDs for the space project. The performance of each produced CCD will be tested in order to select the seven best
CCDs for the camera array. For this purpose the so-called cold-chuck probe station was upgraded. It was actually
developed for testing and selection of the XMM-Newton PNCCDs. The set-up permits full operation and spectroscopic
test of the CCDs just by contact of probe needles without mounting the chips on boards. The CCDs are cooled and
tested by use of a Fe55 source. First tests with eROSITA CCDs proved that the important performance parameters can
be determined by use of the probe station.
Figure 3: eROSITA wafer with four of the large-area CCDs in the centre. The outer devices serve for test purposes.
Table 1: Energy resolution of spectral lines measured with eROSITA CCDs in frame transfer mode. The analysis of the spectra
comprises all event pattern types, single events as well as split events where the signal electrons are collected in up to four pixels.
Concurrently with the PNCCD development, its analog signal processor CAMEX (see Figure 4) was optimized. All
measurement results were achieved using this mixed signal ASIC. A current source in the input stage of each channel
biases the PNCCD on-chip n-channel JFET for operation in source follower mode. Eight-fold correlated double
sampling is applied for signal filtering. This yields low noise and allows, due to parallel processing in the 128 channels,
fast signal readout. A digital sequencer implemented in the CAMEX permits a compact design. Programming is done
via a serial LVDS interface. Slight deviations among the three CAMEX chips per eROSITA detector can be adjusted by
appropriate programming of the CAMEX bias DACs. The prototype eROSITA CAMEX was tested and the flight
Proc. of SPIE Vol. 7435 743502-5
eROSITA CAMEX is presently produced in a 5 V process with JFET-CMOS technology, the same as used for the
Figure 4: CAMEX chip developed for the readout of the eROSITA PNCCD signals. The signals of 128 CCD channels are
simultaneously processed in the mixed signal ASIC and finally multiplexed to one differential output buffer (see also Figure 1).
4.3. Detector board
For performance testing of the eROSITA PNCCDs and CAMEX ASICs in the laboratory, we developed an appropriate
detector board. It was used for verification of the performance of the first eROSITA CCDs. Based on the same
process technology and schematic diagram, the detector board for the eROSITA cameras was designed. This flight
design had to meet a few more conditions. The layout was made even more compact in order to minimize the total
weight of the camera with its proton shield. The layout is matched to the size of the qualified SMD-components. An
interface to the rigid-flex printed board is provided as well as to the support and cooling interface made of ceramics and
titanium. The support structure is equipped with a graded-Z shield in the area of the PNCCD frame store. The inner
most layer of the graded-Z shield next to the PNCCD is made of boroncarbide (B4C) featuring the required small atomic
numbers Z of 5 and 6. The adjacent image area of the CCD is as a matter of course not obstructed. The entire detector
structure has to resist the loads caused by thermal cycling during detector testing and by vibrations during satellite
First boards are produced and equipped with SMD components. They are ready for integration and will soon be tested
with CAMEX and CCD.
Figure 5: Rigid-flex printed board (left hand) connected to the detector board (right hand).
Proc. of SPIE Vol. 7435 743502-6
4.4. Rigid-flex printed board
The rigid-flex printed board is connected to the detector board by wedge bonds. The other terminal is equipped with a
connector as interface to the electronics outside the focal plane. The concept is the same as already tested for the DUO
detectors with a CCD format of 256 x256 pixels. A prototype rigid-flex printed board for eROSITA has been
designed, produced and is awaiting integration (see Figure 5).
4.5. Front-end board
The rigid-flex printed board is directly connected to the frontend-board which carries performance susceptible
electronics such as the drivers generating the analog pulses for charge transfer. A prototype of this board was designed
and produced (Figure 6). It is used to verify the circuit design and for a first test of the flight detector system.
Figure 6: Prototype of the front-end board (top) connected with rigid-flex printed board (bottom).
A first verification of the flight detector concept was obtained by various tests of DUO modules. They were tested at
the MPI Halbleiterlabor and the Max-Planck-Institut für extraterrestrische Physik and applied at the BESSY synchrotron
and the FLASH VUV free electron laser (FEL).,
Function and performance of the eROSITA PNCCD and CAMEX were successfully tested with lab modules and
We have to test the radiation hardness of the eROSITA CCDs because the results will presumably differ from those
obtained with the XMM-Newton PNCCD. The reason for this is based on the fact that although the PNCCD concept is
the same, the wafer starting material, process technology, CCD layout and operating conditions were changed. In a first
radiation hardness test, we exposed a lab detector module to a proton fluence, equivalent to that after five years in
L2 orbit. No malfunction of the lab detector module or any single event upset occurred. The degradation of energy
resolution due to dark current increase and charge transfer efficiency decrease is under study.
Presently, the integration of flight-type modules for electrical and first environmental tests (thermal cycling and
vibration tests) is in preparation. These tests will be finally completed by ESD (electrostatic discharge) and EMC
(electromagnetic compatibility) tests.
The following detector models present the milestones in the eROSITA camera development: Our detector design will be
tested with an engineering model (EM) and then qualified by a so-called qualification model (QM). The seven flight
detectors (FM1 – FM7) will be integrated successively. Integration, test and calibration of the seven detectors however
will overlap because of time constraints.
Proc. of SPIE Vol. 7435 743502-7
5. SUMMARY AND OUTLOOK
After the conceptual design had been determined for the eROSITA detector, we prepared a detailed design and produced
the components of the detector system: the eROSITA PNCCD, its analog signal processor CAMEX for readout of the
CCD signals, the detector board, the rigid-flex printed board and the front-end board. The key components, PNCCD and
CAMEX, were already tested with respect to performance in a lab setup. The results, an improved read noise of nearly
2 electrons rms and an energy resolution of 134 eV FWHM at 5.9 keV energy and 53 eV FWHM at 277 eV energy
respectively, provide the building blocks for excellent flight cameras. The other detector components are presently
prepared for integration.
Recently the decision was made to fly eROSITA on the Spectrum-RG satellite in a L2 orbit. We have started to test the
effects of this radiation environment on our custom-made PNCCD and CAMEX devices.
Based on the test results achieved with the upcoming engineering module, the final design will be determined next year.
Until satellite launch, which is scheduled for 2012, the final design has to be verified in tests as well as the seven flight
detectors set up, tested, calibrated and integrated into the eROSITA telescope.
The authors are grateful to all colleagues who supported the detector development, fabrication and tests, in particular the
staff of the MPI Halbleiterlabor, the Max-Planck-Institut für extraterrestrische Physik, the Max-Planck-Institut für
Physik, PNSensor, the Ingenieurbüro Werner Buttler and bonerz engineering. This work was also supported by the DLR
and the Heidenhain foundation.
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