MIXS on BepiColombo and its DEPFET based focal ... - MPG HLL

Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in
Physics Research A
journal homepage: www.elsevier.com/locate/nima
<strong>MIXS</strong> on BepiColombo and its DEPFET based focal plane instrumentation
J. Treis a,b, , L. Andricek a,e , F. Aschauer a,c , K. Heinzinger a,d , S. Herrmann a,c , M. Hilchenbach b , T. Lauf a,c ,
P. Lechner a,d , G. Lutz a,d , P. Majewski a,d , M. Porro a,c , R.H. Richter a,e , G. Schaller a,c , M. Schnecke a,e ,
F. Schopper a,c , H. Soltau a,d , A. Stefanescu a,c , L. Strüder a,c , G. de Vita a,c
a MPI Semiconductor Laboratory, Otto-Hahn-Ring 6, 81739 Munich, Germany
b MPI for Solar System Research, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
c MPI for Extraterrestrial Physics, Giessenbachstrasse, 85748 Garching, Germany
d PNSensor GmbH, Römerstraße 28, 80803 Munich, Germany
e MPI for Physics, Föhringer Ring 6, 80805 Munich, Germany
article info
Available online 24 April 2010
Keywords:
<strong>MIXS</strong>
BepiColombo
IXO
DEPFET
X-ray detectors
Planetary science
Planetary XRF
Spectroscopy
Active pixel sensor
Imaging
abstract
Focal plane instrumentation based on DEPFET Macropixel devices, being a combination of the Detector–
Amplifier structure DEPFET with a silicon drift chamber (SDD), has been proposed for the <strong>MIXS</strong>
(Mercury Imaging X-ray Spectrometer) instrument on ESA’s Mercury exploration mission BepiColombo.
<strong>MIXS</strong> images X-ray fluorescent radiation from the Mercury surface with a lightweight X-ray mirror
system on the focal plane detector to measure the spatially resolved element abundance in Mercury’s
crust. The sensor needs to have an energy resolution better than 200 eV FWHM at 1 keV and is required
to cover an energy range from 0.5 to 10 keV, for a pixel size of 300 300 mm 2 . Main challenges for the
instrument are radiation damage and the difficult thermal environment in the mercury orbit. The
production of the first batch of flight devices has been finished at the MPI semiconductor laboratory.
Prototype modules have been assembled to verify the electrical properties of the devices; selected
results are presented here. The prototype devices, Macropixel prototypes for the SIMBOL-X focal plane,
are electrically fully compatible, but have a pixel size of 0.5 0.5 mm 2 . Excellent homogeneity and near
Fano-limited energy resolution at high readout speeds have been observed on these devices.
& 2010 Elsevier B.V. All rights reserved.
1. Introduction
Named after the Italian engineer and mathematician Giuseppe
‘‘Bepi’’ Colombo, ESA’s fifth cornerstone mission is a planetary
exploration mission to Mercury [1,2]. As Mercury represents an
extreme example of planetary formation, knowledge of its history
and formation is essential to understand the genesis and
evolution of the inner solar system as a whole.
Upon arrival at Mercury in 2019, the BepiColombo spacecraft
will deploy two independent probes, the Mercury Magnetospheric
Orbiter (MMO) and the Mercury Planetary Orbiter (MPO) in their
individual polar orbits around the planet. Each orbiter carries an
individual set of instrumentation tailored to its specific task.
Hereby, main focus of the MPO is the comprehensive observation
of Mercury’s crust. A large part of its instrumentation is made to
do multi-wavelength scans of the surface. One of the instruments
on board the MPO is the so-called Mercury Imaging X-ray
Spectrometer (<strong>MIXS</strong>) [3] instrument, which will perform a
Corresponding author at: MPI Semiconductor Laboratory, Otto-Hahn-Ring 6,
81739 Munich, Germany. Tel.: +49 89 83940045; fax: +49 89 83940013.
E-mail address: jft@hll.mpg.de (J. Treis).
planetary XRF analysis of Mercury’s crust. For planetary XRF, the
X-ray fluorescence response from the surface of the planet
generated by the coronar X-ray radiation from the sun is imaged
with an energy-dispersive X-ray detector. In this way, the
elemental abundance on the surface can be determined. In case
of Mercury, especially the concentration of the important tracer
elements Magnesium, Silicon, Aluminum, and Iron in particular,
within the crust are of interest.
The <strong>MIXS</strong> instrument, the setup of which is shown in Fig. 1,
will provide unprecedented spatial and spectral resolution. The
excellent spatial resolution is achieved by two complementary
channels, <strong>MIXS</strong>-C and <strong>MIXS</strong>-T, equipped with low mass and high
efficiency Microchannel Plate (MCP) X-ray optical elements [4].
<strong>MIXS</strong>-C uses a slumped collimator providing a footprint on the
scale of 70–270 km. <strong>MIXS</strong>-T uses an imaging X-ray telescope,
which offers spatial resolutions at the order of below 1 km during
times of intensive solar X-ray flux, providing access to individual
landforms such as craters and their internal structures.
The excellent spectral resolution is achieved by the use of
so-called DEpleted P-channel Field-Effect Transistor (DEPFET)
Macropixel X-ray detectors. Both <strong>MIXS</strong> channels are equipped with
identical Focal Plane Assemblies (FPAs) with one DEPFET detector
each. The DEPFET detectors will measure X-rays in the energy band
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2010.03.173

J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547 541
<strong>MIXS</strong>-T
FPAs
Panel not shown
<strong>MIXS</strong>-C
Heat pipes coming
from below
MEB
Fig. 1. Sketch of the <strong>MIXS</strong> instrument on BepiColombo. Two channels, one equipped with a high resolution X-ray telescope and one with a collimator optics, equipped with
the same focal plane array, are located on a common panel (not shown), and their optics’ apertures facing the planet’s surface. They are controlled with a common readout
electronics located in the <strong>MIXS</strong> electronics box (MEB). <strong>MIXS</strong>-C, the collimator instrument, serves for large scale observations of a large footprint in case of low solar
X-ray intensity. <strong>MIXS</strong>-T, which requires higher solar X-ray fluxes, is used for imaging the fluorescence radiation of the surface, capable of resolving features in the subkilometer
range.
of 0.5–7.5 keV with a spectral resolution 5100 eV FWHM at 1 keV
at the start of operations and less than 200 eV at the end of the
mission lifetime, after two years in the Mercury orbit. This
performance allows the separation of X-ray line emission from
elements of interest and provides access to lines not accessible to
previous instruments, including the Fe-L emission lines.
2. The DEPFET device
The combined detector-amplifier structure DEpleted P-channel
MOSFET (DEPMOSFET) [5,6] consists of a conventional p-channel
MOS-field effect transistor integrated on the surface of a high
resistivity n-type silicon bulk. Using the principle of sidewards
depletion [7], the bulk can be fully depleted. By applying
appropriate bias potentials to the front- and backside, a potential
minimum for electrons can be generated near the surface of the
bulk, and with an additional n-doped region this minimum can be
enforced and confined to the area below the transistor channel.
Bulk generated electron–hole pairs are separated by the electric
field and the electrons are collected in the potential minimum
below the transistor channel. Here, their presence modulates the
charge carrier density in the channel, and the channel conductivity
becomes a function of the charge in the potential minimum, which
is therefore also referred to as internal gate. As the potential
minimum is persistent regardless from the presence of a transistor
current, the transistor can be turned off during signal integration
and only turned on for readout. The accumulated charge can be
removed by applying a positive voltage pulse to an n + -doped region
close to the internal gate, the so-called clear-contact. The type of
DEPMOSFET structures used for X-ray spectroscopy have circular
transistor geometry. The clear region here consists of an additional
N-MOSFET composed of the clear contact and a surrounding MOS
structure, the so-called cleargate. The clear contact is separated from
the detector bulk and the internal gate by an additional deep-p
implantation, to prevent charge loss to the clear. Fig. 2 shows a
cutaway of a circular DEPMOSFET structure.
During matrix operation, the signal is evaluated by sampling
the signal levels before and after the clear and calculating the
difference, a technique usually referred to as correlated double
sampling (CDS). As the clear is complete, the DEPFET is a kTCnoise
free device. In large matrix devices, a pixel array is placed
on a common bulk. As shown in Fig. 3, the pixels are interconnected
for row-wise, column-parallel readout in a rolling
shutter mode.
The DEPFET concept allows for more flexible interconnection
techniques, even pixel-individual addressing and readout are
feasible in case sufficient routing resources are available. Small
DEPMOSFET matrix prototypes of matrices of 64 64 pixel with
75 75 mm 2 size each have successfully been operated and tested
[8,9]. The DEPFET’s sensitivity is expressed as change in drainsource
current as a function of the charge in the internal gate;
g Q ¼ dI DS =dQ IG , which is a function of device geometry. The
structures showing the best performance have a nominal gate
width by design of 5 mm, the circular transistor gate has a
circumference of 47:5 mm; g Q values of 350–500 pA/e have been
measured, depending on the biasing conditions. As the source
follower conversion gain is given by dV S ¼ g Q =g m Q IG , gain
values of 3:525 mV=e can be achieved. Operation of a matrix
device requires dedicated VLSI integrated control and readout
electronics. The control-ICs used are the so-called SWITCHER II
ICs [10], 64-channel dual high voltage multiplexer ICs manufactured
in an AMS high voltage CMOS process. The Switchers drive
the Gate, Cleargate and Clear voltages during operation. The
readout amplifier IC used is the ASTEROID 64 [11], a 64 channel
shaper/amplifier circuit with a trapezoidal filtering stage, internal
sample and hold, serial analog readout and integrated shaping
sequencer allowing for flexible application of different filter
timing.
3. DEPFET Macropixels
When imaging X-ray wavelengths, the size of the PSF of the
optics is typically in the range around 1 mm. Considering that
charge sharing between pixels generally deteriorates the integral
energy resolution due to recombination noise, the ideal pixel size
is as small as necessary for reasonable oversampling of the PSF,
but as large as possible to reduce charge sharing and power
consumption by reducing the number of required readout
channels. For an X-ray optical system with PSF around 1mm
size, pixel sizes at the order of several hundreds of microns are
desirable. DEPFET Macropixels [12] offer a convenient way to
adapt the pixel size to the requirement without sacrificing the
excellent noise and energy resolution of the standard DEPFET
structure. By surrounding the DEPFET cell with an SDD-like
drift ring structure [7] as shown in Fig. 4, the pixel size can be
adapted to the requirements by simply changing number, size and
geometry of the drift electrodes. The resulting device, henceforth
referred to as Macropixel, has the combined properties of DEPFET
and SDD:
By changing number, geometry and size of the drift rings, any
given pixel size can be implemented.

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J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547
75μm
p+ drain
polysilicon
gate
p+ source
polysilicon
cleargate
n+ clear
Fig. 2. Cutaway of a circular DEPMOSFET pixel device and the equivalent circuit schematics: A MOSFET with two gates: External and Internal Gate, while the latter can be
contacted via the n-channel Clear-FET. A photo of a DEPFET structure inside a matrix is shown at the right.
Fig. 3. Schematic interconnection of DEPMOSFET matrix pixels for row-wise readout. Row-wise connection of Gate- and Clear-voltages provides for row addressing, and
column-wise connection of the readout nodes provides for column-parallel readout.
Placing an array of square-shaped Macropixel devices on a
common bulk yields a large area FPA.
Array readout in rolling shutter mode is possible due to the
DEPFET’s charge storage capability.
A Macropixel matrix is very power efficient, as the DEPFET is
powered only in case it is actually read out.
Like for any sideways depleted detector, the backside serves as
radiation entrance window and can be made completely

J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547 543
Fig. 4. Cross-cut through a Macropixel device. By replacing the readout anode of a conventional SDD with a DEPFET, charge storage capability can be added to an SDD
array. In this way, large integrating imaging detectors can be built from an array of Macropixels.
homogeneous and unobstructed by any structure, yielding
100% fill factor and extremely good quantum efficiency.
Due to the low input capacitance of the DEPFETs, the devices
can be read out very fast at low noise; readout noise values of
4e ENC at a readout speed of o6 ms have been achieved,
The radiation hardness of the DEPFET technology has already
been proven experimentally [13,14].
These properties make DEPFET Macropixel matrix devices
suitable for a large number of experiments. Single Macropixel
prototypes from an earlier prototyping run have successfully
tested and showed outstanding performance in spite of the large
pixel area [12].
4. The <strong>MIXS</strong> focal plane detector
Table 1 shows the key specifications on the <strong>MIXS</strong> FPA sensors.
The sensitive area is made to match the mirror system field of
view (FOV). The pixel size is achieved adopting a layout with three
drift rings around a standard circular DEPFET structure. The
quantum efficiency has been achieved with the standard radiation
entrance window configuration for the DEPFET technology. The
time resolution is mostly driven by the spacecraft movement over
ground, the pixel size of 300 300 mm 2 is designed to allow for
3 fold oversampling of the optic’s 1 mm PSF. The energy range
is selected to cover both the iron-K line (6.4 keV) and the iron-L
lines ( 0:71 keV). Being sensitive permits to detect iron even in
case of a quiet sun state, where both intensity and energy of the
coronar X-rays are too low to generate a significant flux of Iron-K
photons. As a result, the iron sensitivity is drastically increased.
The energy resolution limit is very moderate, as well as the limit
on the electronic noise. This is as the main task of the detector is
to directly observe and separate the unaltered X-ray fluorescence
spectral lines from the most important tracer elements on a
background from bremsstrahlung and scattered radiation, in
contrast to applications in X-ray astronomy, where both
strongly red- and blue-shifted emission lines have to be
detected and separated from the background, a task which
imposes much more demanding requirements on the energy
resolution. The biggest challenge for the <strong>MIXS</strong> detector is the
radiation hardness. The main radiation damage is caused by low
energy protons from the solar wind. The expected threshold
Table 1
Requirements on the focal plane arrays of the <strong>MIXS</strong> instruments. The requirement
on the FWHM at 1 keV allows for a much higher ENC, but the main source of
energy resolution deterioration is due to leakage current increase.
Parameter
Requirement
Sensitive area 1.92 1.92 cm 2
Quantum efficiency
Z80% at 0:5 keV
Time resolution
o1ms
Pixel size 300 300 mm 2
Array dimension
64 64 pixels
Energy resolution
200 eV FWHM at 1 keV
Energy range
0:5 keVZEZ10 keV
Electronic noise (without o10e ENC
leakage current)
Leakage current induced o20e ENC
noise contribution
Radiation hardness
Z3 10 10 eq: 10 MeV p=cm 2 r20 krad TID
voltage shift in the DEPFET gates [13] of below 0.7 V from the total
ionizing dose (TID) of o20 krad can easily be compensated by
adapting the operating conditions. The expected increase of
leakage current due to proton-induced bulk damage (nonionizing
energy loss, NIEL), however, must be compensated by a
faster readout speed and a correspondingly lower operating
temperature, although the specifications allow for a relatively
large leakage current induced noise contribution. In a space
environment, applying annealing to the detector is an uncommon
procedure due to the involved risks. Therefore, the detector will
be mostly kept in the cooled state, and radiation damage tests
have been made to verify the damage constant in cold conditions
without self-annealing [14] and to estimate the effect of
annealing vs. no annealing. Fig. 5 shows the extrapolated
tradeoff between energy resolution after receiving the full
mission lifetime dose, readout speed and detector temperature,
assuming an annealing scenario of 80 min at 60 1C (left), and
without applying any annealing at all (right). A frame integration
time of 192 ms corresponds to 6 ms readout time per row, which is
the standard readout speed with the ASTEROID IC. For this
integration time, an operating temperature of 43 1C is required
to keep the energy resolution requirement. If the readout speed is
increased to 128 ms, a temperature below 391C is sufficient, at
the cost of higher initial readout noise due to the shorter shaping

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J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547
Integration time (µs)
350
300
250
200
150
100
-43°C
-39°C
Energy resolution
FWHM @ 1 keV
60.00
100.0
140.0
180.0
220.0
260.0
300.0
White line:
200 eV requirement
Integration time (µs)
350
300
250
200
150
100
-47°C
-50°C
Energy resolution
FWHM @ 1 keV
60.00
100.0
140.0
180.0
220.0
260.0
300.0
White line:
200 eV requirement
-30 -35 -40 -45 -50 -55 -60
Temperature (°C)
-30 -35 -40 -45 -50 -55 -60
Temperature (°C)
Fig. 5. Radiation damage tradeoff for the <strong>MIXS</strong> focal plane detectors applying an annealing cycle of 80 min at 60 1C (left) and without applying any annealing (right). In
case an annealing scenario is applied, an operation temperature of, depending on the readout speed, 39243 3 C is required to fulfill the energy resolution requirement
having received the full proton dose at the end of the mission lifetime. Without annealing, the required operating temperatures are significantly lower.
Clear
Switchers
North
ASTEROID
Gate/
Cleargate
Switchers
Northwest
Switcher
Northern
hemisphere
Northeast
Switcher
Fanouts
Southern
hemisphere
Southwest
Switcher
Southeast
Switcher
W
N
E
South
ASTEROID
S
Fig. 6. Readout scheme of the FPA detectors for <strong>MIXS</strong> (left). The sensor is divided in two equally sized hemispheres of 32 64 pixels each, which are controlled and read out
by an individual set of front-end electronics. Photograph of a laboratory module for <strong>MIXS</strong> (mechanical sample) during integration (right). The overall sensor sensitive area
is 1.92 1.92 cm 2 , the envelope size of the central region of the hybrid is 4 in. 4 in.
time in the ASTEROID IC. If no annealing is provided, the operating
temperatures have to be 8 3 C lower. As the cooling resources on
board of the BepiColombo spacecraft are very limited and even
the less cold temperatures are hard to be achieved, annealing
scenarios are being considered in spite of the risk involved.
Fig. 6 shows the structure and readout scheme for the <strong>MIXS</strong>
FPA sensors and a photo of a laboratory hybrid. The sensor is
divided in two individual hemispheres of 64 32 pixels each,
which are controlled and read out by their hemisphere-individual
front end electronics. For this configuration, two rows can be read
at the same time, which increases the achievable framerate by a
factor of two, and one degree of redundancy is added to the
system. Every hemisphere is read and controlled by its
hemisphere-individual ASTEROID and Switcher circuits.
The production of the flight detectors for <strong>MIXS</strong> has recently
been finished at the MPI semiconductor laboratory in Munich. The
devices will undergo a test program on die level to select flight
devices without cosmetic defects or inefficiencies prior to their
integration into a laboratory or flight hybrid.
5. Prototype tests
To evaluate the prototypes and to learn about the properties of
a Macropixel matrix device, even larger Macropixel detector
prototypes with 64 64 pixels of a pixel size of 500 500 mm 2
and an overall sensitive area of 3.2 3.2 cm 2 , being about three
times the size of the <strong>MIXS</strong> Macropixel devices, have been

J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547 545
Fig. 7. Photos of front- (left) and backside (top right) of a Macropixel matrix prototype, being an representative sample for the <strong>MIXS</strong> detectors. The overall sensor sensitive
area is 3.2 3.2 cm 2 , the envelope size of the hybrid is 4 in. 4 in. The pixel array dimension is 64 64 and the pixels (bottom right) have five drift rings each and a size of
500 500 mm 2 . The pixel structure and the control- and read-out electronics on the frontside is clearly visible, as well as the completely homogeneous entrance window on
the backside.
60
Offset (ADU)
50
2600
Row number
60
50
40
30
20
Noise
(ADURMS)
10.00
11.00
12.00
13.00
14.00
15.00
16.00
17.00
18.00
19.00
20.00
Row number
40
30
20
10
60
50
10 20 30 40 50 60
Columnnumber
2800
3000
3200
3400
3600
Residual
Offset (A DU)
10
10 20 30 40 50 60
Column number
Row number
40
30
20
-500.0
-300.0
-100.0
100.0
300.0
10
10 20 30 40 50 60
Columnnumber
Fig. 8. Offset (right) and noise (left) map of the tested Macropixel device. The offset map entries are the mean values of typically 1000 offset values per pixel, the noise
map values are their variances. The column-like structure on the offset map (top right) is due to the ASTEROID’s channel-to-channel offset variation, the sensor itself shows
in the residual offset map (bottom right) no bright pixels at all, and no systematic variation, except for a slightly higher offset values at the edges, which is due to thermal
reasons. The noise map is also very homogeneous, the noise dispersion is at the order of 7%. The mean value of the noise map entries translates in an ENC value of
4.1e rms. The slightly higher noise at the bottom sensor rim is due to additional thermal load from the front end ICs.
successfully taken into operation and have been tested. These
devices, originally being manufactured as prototypes for the
SIMBOL-X focal plane [15], have the identical pixel structure and
array dimension compared to the <strong>MIXS</strong> detectors, but the pixels
have five drift rings instead of three to cover the larger active area.
In addition, the sensor is not divided into hemispheres. As the

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J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547
scaling behavior of the drift ring structure is very well known,
the prototypes are being considered as representative samples for
<strong>MIXS</strong>. The sensors also possess the same entrance window
configuration. Fig. 7 shows photographs of the sensors frontand
backside, and of the pixel structure itself. The completely
homogeneous aluminized entrance window can be recognized, as
well as the pixel structure on the frontside, together with the two
control-ICs and the ASTEROID IC. The overall hybrid size is
4 in. 4 in. A total number of five hybrids of that type was built.
Three devices were homogeneous and defect-free, one had only
one defective row, and only one structure had a larger number of
defective pixels. Given the larger area of the devices compared to
the <strong>MIXS</strong> flight detectors, a production yield of around 70% can be
expected.
The sensors have been tested in a vacuum environment
at a relatively low sensor temperature around 90 1C, with a
18000
16000
10000
14000
12000
1000
Pattern multiplicity
Singles
Doubles
Triples
Quadruples
Counts
10000
8000
6000
4000
Counts
100
2000
0
5600 5800 6000 6200 6400 6600 6800 7000
Energy(eV)
14000
12000
10
10000
1
1000 2000 3000 4000 5000 6000 7000 8000
Energy(eV)
Counts
8000
6000
4000
2000
0
5600 5800 6000 6200 6400 6600 6800 7000
Energy(eV)
Fig. 9. Sample spectra taken with the Macropixel array using an 55 Fe source. No inefficient pixels were found. The left figure shows the spectra for the various multiplicities
(gray coded), the right figures show the peak region for the singles spectrum and the integral spectrum taking into account all multiplicities. The energy resolution
measured was 126 eV FWHM at 5.9 keV taking only singles, i.e. events without charge sharing, and 128 eV FWHM taking all valid event patterns, which is in agreement
with the expectations. The peak to background ratio of 3000 shows the suitability of the devices for spectroscopic measurements.
60
60
Row number
50
40
30
20
10.00
133.8
257.5
381.3
505.0
628.8
752.5
876.3
1000
Row number
50
40
30
20
10.00
133.8
257.5
381.3
505.0
628.8
752.5
876.3
1000
10
10
10 20 30 40 50 60
Column number
10 20 30 40 50 60
Column number
Fig. 10. Imaging test using an silicon baffle of 0.45 mm thickness. The smallest feature size is 0.4 mm. The left image shows an intensity plot (photons per pixel), the right
image an extrapolated contour plot.

J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547 547
framerate of around 2.5 kHz. For <strong>MIXS</strong>, the framerate will be by a
factor of two higher, the integration time will be r200 ms. As
can be seen from Fig. 8, the sensor shows excellent homogeneity
and noise. The visible structures in the offset and noise maps
are due to the ASTEROID channel-to-channel offset. The noise
distribution is very flat, no noisy pixels have been observed.
The noise dispersion of 7% is due to pixels close to the sensor rim
at the bottom having a slightly higher noise. This is caused by the
proximity of the ASTEROID IC inducing additional heat load,
probably in form of infrared radiation.
The sensors have been exposed to an 55 Fe source. The devices
show excellent spectral resolution. The resulting spectra are
shown in Fig. 9. The energy resolution is with 126 eV FWHM at
5.9 keV close to the Fano limit, and the slightly worse energy
resolution of the integral spectra taking into account also splits, is
with 128 eV within the expectations. The overlay of the spectra of
the various multiplicities gives an impression of the quality of the
gathered spectra. 76.9% of the recorded patterns are singles, 20.6%
are doubles and the remaining 2.5% distribute evenly among
triples and quadruples, a split behavior which is in agreement
with the simulations. The peak-to background ratio of C3000
qualifies the devices as suitable for spectroscopy applications and
is a proof of the good quality of the entrance window.
Imaging tests have been done using a 450 mm thick silicon
baffle. The resulting images are shown in Fig. 10. the smallest
feature size for this baffle was 0.4 mm. Although a quantitative
analysis using MTF structures still need to be done, these images
give an impression of the homogeneity and excellent imaging
properties of the sensors.
6. Summary and outlook
DEPFET based Macropixel detectors have been designed
and produced for the focal plane instrumentation of the <strong>MIXS</strong>
spectrometer on board BepiColombo’s MPO. The devices are in the
process of being die-tested for flight qualification. Prototypes of
even larger area and pixel sizes have been successfully taken into
operation. The tested devices are homogeneous and defect free
and show excellent imaging and spectroscopic properties.
As soon as the die testing is finished, device qualification will
continue with quantitative testing of the imaging properties, a
quantitative analysis of the entrance window quantum efficiency
and the radiation hardness of the device to confirm the results of
the diode test irradiation. The radiation qualification of the
ASTEROID and SWITCHER ICs is also a matter of concern.
In addition, prototype devices have been taken to the BESSY II
beamline at the PTB facility in Berlin, Germany, for calibration
purposes. The calibrated sensors are going to be used for X-ray
fluorescence analysis measurements on Iron samples at the
ELETTRA synchrotron facility in Trieste, Italy.
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