Wolfgang Treberspurg, Robert Andritschke, Alexander Bähr, Davide Bianchi, Anna Koch, Norbert Meidinger, Johannes Müller-Seidlitz, Sabine Ott, Matteo Porro
KEYWORDS: Field effect transistors, Sensors, Imaging systems, Staring arrays, Field effect transistors, Sensors, Electronics, X-rays, Photons, Analog electronics, Space operations, Data processing
The Wide Field Imager of the Athena telescope will combine an excellent spectroscopic performance and high count rate capability with a large field of view. For these purposes, its focal plane consists of two complementary detectors, using DEPFET active pixel sensors. One is the high count rate detector with a small field of view, which has to be operated with a readout speed of 80 μs per frame. In contrast, the large area detector will cover a large field of view and has to be read out with a frame rate ≤ 5 ms. Its sensitive area is covered by four identical active pixel arrays, consisting of 512 x 512 pixels, each. Since a column parallel readout will be used, 512 pixels are connected to one single channel of a readout ASIC. The readout will be accomplished by either sensing a voltage step on the source node or a change of the transistor drain current. The former so-called source follower mode requires long settling times - proportional to the load capacitances - but can cope with local inhomogeneities. Alternatively, the latter so-called drain current mode provides a fast readout - independent to the load capacitance - but implicates a higher sensitivity on local variations of the DEPFETs bias currents. Both modes are implemented in the VERITAS 2.1 readout ASIC and were studied with 64 x 64 pixels arrays. Drain current devices could be operated with significantly smaller settling times but suffer from a slightly increased noise at similar shaping times in comparison to the source follower ones. By using an optimized timing with dedicated settling and shaping times, the devices of both modes feature a comparable spectral performance.
The Advanced Telescope for High Energy Astrophysics (Athena) has been selected for ESA’s L2 mission, scheduled for launch in 2028. It will provide the necessary capabilities to achieve the ambitious goals of the science theme “The Hot and Energetic Universe.” Athena’s x-ray mirrors will be based on silicon pore optics technology with a 12-m focal length. Two complementary focal plane camera systems are foreseen, which can be moved interchangeably to the focus of the mirror system: the actively shielded micro-calorimeter spectrometer X-IFU and the wide field imager (WFI). The WFI camera will provide an unprecedented survey power through its large field of view of 40 arc min with a high count-rate capability (∼1 Crab). It permits a state-of-the-art energy resolution in the energy band of 0.1 to 15 keV during the entire mission lifetime (e.g., full width at half maximum ≤150 eV at 6 keV). This performance is accomplished by a set of depleted P-channel field effect transistor (DEPFET) active pixel sensor matrices with a pixel size well suited to the angular resolution of 5 arc sec (on-axis) of the mirror system. Each DEPFET pixel is a combined detector-amplifier structure with a MOSFET integrated onto a fully depleted 450-μm-thick silicon bulk. This manuscript will summarize the current instrument concept and design, the status of the technology development, and the envisaged baseline performance.
KEYWORDS: Field effect transistors, Camera shutters, Sensors, Signal processing, Switching, Electrons, Active sensors, Image resolution, Measurement devices, Pulsed laser operation
The WFI instrument of ATHENA will provide large field of view in combination with high count-rate capability to address key questions of modern astrophysics. It will utilize a DEPFET based active pixel sensor as focal plane detector. To achieve fastest timings, these sensors can be operated by addressing a region of interest. While this window mode operation enhances time resolution, the probability to collect events during signal processing will become non negligible. Due to the incomplete signal evaluation, these so called misfit events cause an additional background contribution, which will be dominant at very fast timings as required for ATHENA. To sustain the spectral performance a built-in electronic shutter and an intermediate storage can be implemented into each pixel. While the shutter is capable to effectively suppress misfit collection and thus maintains the spectral performance, the implementation of a storage region diminishes possible dead times and improves throughput. We will present measurements on prototype devices demonstrating the benefit of a fast built-in shutter for DEPFET devices operated at high frame rates. Furthermore we will show results of first measurements on structures that combine a built-in shutter with an intermediate storage, obviating dead times and simultaneously improving the spectral response.
KEYWORDS: Field effect transistors, Sensors, Electrons, Analog electronics, Electronics, Prototyping, Spectroscopy, Signal detection, Capacitance, Signal processing
VERITAS 2.0 is a multi-channel readout ASIC for pnCCDs and DEPFET arrays. The main chip application is the readout of the DEPFET pixel arrays of the Wide Field Imager for the Athena mission. Every readout channel implements a trapezoidal weighting function and it is based on a fully differential architecture. VERITAS 2.0 is the first ASIC able to readout the DEPFETs both in source follower mode and in drain current mode. The drain readout should make it possible to achieve a processing time of about 2-3 μs/line with an electronics noise ≤ 5 electrons r.m.s.. The main concept and first measurements are presented.
KEYWORDS: Sensors, Field effect transistors, Optical filters, Cameras, X-rays, Signal processing, Mirrors, Analog electronics, Control systems, Imaging systems
The "Hot and Energetic Universe" has been selected as the science theme for ESA's L2 mission, scheduled for launch in 2028. The proposed Athena X-ray observatory provides the necessary capabilities to achieve the ambitious goals of the science theme. The X-ray mirrors are based on silicon pore optics technology and will have a 12 m focal length. Two complementary camera systems are foreseen which can be moved in and out of the focal plane by an interchange mechanism. These instruments are the actively shielded micro-calorimeter spectrometer X-IFU and the Wide Field Imager (WFI). The WFI will combine an unprecedented survey power through its large field of view of 40 arcmin with a high countrate capability (approx. 1 Crab). It permits a state-of-the-art energy resolution in the energy band of 0.1 keV to 15 keV during the entire mission lifetime (e.g. FWHM ≤ 150 eV at 6 keV). This performance is accomplished by a set of DEPFET active pixel sensor matrices with a pixel size matching the angular resolution of 5 arcsec (on-axis) of the mirror system. Each DEPFET pixel is a combined detector-amplifier structure with a MOSFET integrated onto a fully depleted 450 micron thick silicon bulk. The signal electrons generated by an X-ray photon are collected in a so-called internal gate below the transistor channel. The resulting change of the conductivity of the transistor channel is proportional to the number of electrons and thus a measure for the photon energy. DEPFETs have already been developed for the "Mercury Imaging X-ray Spectrometer" on-board of ESA’s BepiColombo mission. For Athena we develop enhanced sensors with integrated electronic shutter and an additional analog storage area in each pixel. These features improve the peak-to-background ratio of the spectra and minimize dead time. The sensor will be read out with a new, fast, low-noise multi-channel analog signal processor with integrated sequencer and serial analog output. The architecture of sensor and readout ASIC allows readout in full frame mode and window mode as well by addressing selectively arbitrary sub-areas of the sensor allowing time resolution in the order of 10 μs. The further detector electronics has mainly the following tasks: digitization, pre-processing and telemetry of event data as well as supply and control of the detector system. Although the sensor will already be equipped with an on-chip light blocking filter, a filter wheel is necessary to provide an additional external filter, an on-board calibration source, an open position for outgassing, and a closed position for protection of the sensor. The sensor concept provides high quantum efficiency over the entire energy band and we intend to keep the instrumental background as low as possible by designing a graded Z-shield around the sensor. All these properties make the WFI a very powerful survey instrument, significantly surpassing currently existing observatories and in addition allow high-time resolution of the brightest X-ray sources with low pile-up and high efficiency. This manuscript will summarize the current instrument concept and design, the status of the technology development, and the envisaged baseline performance.
Petra Majewski, Florian Aschauer, Alexander Bähr, Giulio de Vita, Bettina Günther, Kathrin Hermenau, Sven Herrmann, Martin Hilchenbach, Thomas Lauf, Peter Lechner, Gerhard Lutz, Danilo Miessner, Matteo Porro, Jonas Reiffers, Gerhard Schaller, Florian Schopper, Heike Soltau, Alexander Stefanescu, Rafael Strecker, Lothar Strüder, Johannes Treis
The Mercury Imaging X-ray Spectrometer (MIXS) is an instrument on board of the 5th ESA cornerstone mission
BepiColombo. This Spectrometer comprises two instruments for imaging x-ray spectroscopy of the Mercury
surface. The detector plane arrays (DPA) for the energy and spatial resolved detection of x-rays are based on
DEPFET (Depleted P-channel FET) macropixel detectors with 64×64 pixel each and 300×300 μm2 pixel
size. The MIXS target energy band is from 0.5 to 7 keV with an energy resolution better than 200 eV at 1 keV
at mission end. This allows to access the Fe-L line at about 0.7 keV, which was not accessible to previous
instruments, and to separate the x-ray lines of the elements of interest.
Before a detector chip is integrated into a detector module, it is electrically pre-characterized in order to select
only the best chips for the complex and time-consuming integration. The high degree of complexity of the
integration process comes from the need to thermally decouple the detector chip from its readout and steering
ASICs by a sophisticated mechanical structure, due to the limited amount of cooling power available for the
instrument. After the spectroscopic characterization of the detector modules, the flight and flight spare detectors
were calibrated at the PTB (Physikalisch-Technische Bundesanstalt) beamlines at the BESSY-II synchrotron.
We report on the pre-characterization, integration, qualification and calibration of MIXS flight and flight spare
detectors, which is now successfully completed.
To improve the signal to noise level, devices for optical and x-ray astronomy use techniques to suppress background
events. Well known examples are e.g. shutters or frame-store Charge Coupled Devices (CCDs). Based
on the DEpleted P-channel Field Effect Transistor (DEPFET) principle a so-called Gatebale DEPFET detector
can be built. Those devices combine the DEPFET principle with a fast built-in electronic shutter usable for
optical and x-ray applications. The DEPFET itself is the basic cell of an active pixel sensor build on a fully
depleted bulk. It combines internal amplification, readout on demand, analog storage of the signal charge and a
low readout noise with full sensitivity over the whole bulk thickness. A Gatebale DEPFET has all these benefits
and obviates the need for an external shutter.
Two concepts of Gatebale DEPFET layouts providing a built-in shutter will be introduced. Furthermore proof
of principle measurements for both concepts are presented. Using recently produced prototypes a shielding of
the collection anode up to 1 • 10−4 was achieved. Predicted by simulations, an optimized geometry should result
in values of 1 • 10−5 and better. With the switching electronic currently in use a timing evaluation of the shutter
opening and closing resulted in rise and fall times of 100ns.
The Wide Field Imager (WFI) of the International X-ray Observatory (IXO) is an X-ray imaging spectrometer based on a
large monolithic DePFET (Depleted P-channel Field Effect Transistor) Active Pixel Sensor. Filling an area of
10 x 10 cm2 with a format of 1024 x 1024 pixels it will cover a field of view of 18 arcmin. The pixel size of
100 x 100 μm2 corresponds to a fivefold oversampling of the telescope's expected 5 arcsec point spread function. The
WFI's basic DePFET structure combines the functionalities of sensor and integrated amplifier with nearly Fano-limited
energy resolution and high efficiency from 100 eV to 15 keV. The development of dedicated control and amplifier
ASICs allows for high frame rates up to 1 kHz and flexible readout modes. Results obtained with representative
prototypes with a format of 256 x 256 pixels are presented.
NHXM, under study by ASI (Agenzia Spaziale Italiana), is an X-ray observatory in the energy band between 0.5 and
80 keV and will have 3 telescopes dedicated to X-ray imaging with a field of view diameter of 12 arcmin and a focal
length of 10 m. We report on the development of high-speed and low-noise readout of a monolithic array of DEPFET
detector. The DEPFET based detectors, thanks to an intrinsic low anode capacitance, are suitable as low-energy
detectors (from 0.5 to 10 keV) of the new NHXM telescope.
The challenging requirements of the NHXM cameras regard the necessity to obtain images and spectra with
nearly Fano-limited energy resolution with an absolute time resolution of about 100 μs. In order to exploit the speed
capability of the DEPFET array, it has been developed a readout architecture based on the VELA circuit: a drain
current readout configuration to implement an extremely fast readout (2 μs/row) and preserve the excellent noise
performance of the detector.
In the paper the foreseen maximum achievable frame-rate and the best energy resolution will be presented in
order to assert the VELA suitability for X-ray imaging and spectroscopy.
The Wide Field Imager (WFI) of the International X-ray Observatory (IXO) is an X-ray imaging spectrometer based on a
large monolithic DePFET (Depleted P-channel Field Effect Transistor) Active Pixel Sensor. Filling an area of
10 × 10 cm² with a format of 1024 × 1024 pixels it will cover a field of view of 18 arcmin. The pixel size of
100 × 100 μm² corresponds to a fivefold oversampling of the telescope's expected 5 arcsec point spread function. The
WFI's basic DePFET structure combines the functionalities of sensor and integrated amplifier with nearly Fano-limited
energy resolution and high efficiency from 100 eV to 15 keV. The development of dedicated control and amplifier
ASICs allows for high frame rates up to 1 kHz and flexible readout modes. Results obtained with representative
prototypes with a format of 256 × 256 pixels are presented.
J. Treis, L. Andricek, F. Aschauer, K. Heinzinger, S. Herrmann, T. Lauf, P. Lechner, G. Lutz, P. Majewski, M. Porro, J. Reiffers, R. Richter, G. Schaller, M. Schnecke, F. Schopper, H. Soltau, A. Stefanescu, L. Strüder, G. de Vita
X-ray detectors based on arrays of DEPFET macropixels, which consist of a silicon drift detector combined with
a detector/amplifier structure DEPFET as readout node, provide a convenient and flexible way to adapt the pixel
size of a focal plane detector to the resolving power of any given X-ray optical system. Macropixels combine the
traditional benefits of an SDD, like scalability, arbitrary geometry and excellent QE even in the low energy range,
with the advantages of DEPFET structures: Charge storage capability, near Fano-limited energy resolution, low
power consumption and high speed readout. Being part of the scientific payload of ESA's BepiColombo mission,
the MIXS instrument will be the first instrument to make use of DEPFET macropixel based FPA detectors in
space. MIXS will perform a complete planetary X-ray fluorescence analysis of Mercury's crust with high spectral
and spatial resolution. MIXS will contain two focal plane detectors consisting of a 64 × 64 macropixel matrix
with 300 × 300 μm2 pixel size. The main challenges for the instrument are the difficult radiation and thermal
environment around Mercury, requiring high speed readout and sophisticated thermal management to reduce
the impact of thermally generated leakage current within an irradiated detector. Dedicated VLSI integrated
readout electronics has been developed for MIXS: a fast, radiation hard, low power, high voltage switch circuit
to control the device, and a low noise, high speed amplifier/shaper IC. Detector assemblies have been built,
electrical screening tests for the flight models and spectroscopical qualification tests are in progress.
J. Treis, L. Andricek, F. Aschauer, S. Herrmann, K. Heinzinger, M. Hilchenbach, T. Lauf, P. Lechner, G. Lutz, P. Majewski, M. Porro, R. Richter, G. Schaller, M. Schnecke, F. Schopper, H. Soltau, A. Stefanescu, L. Strüder, G. de Vita
BepiColombo, ESA's fifth cornerstone mission, is a planetary exploration mission to Mercury. On board of
BepiColombo's Mercury Planetary Orbiter (MPO), the MIXS instrument will perform a complete X-ray fluorescence
analysis of Mercury's crust with unprecedented spectral and spatial resolution. This is achieved by
using a lightweight X-ray mirror system and by using of DEPFET based Macropixel devices as X-ray detectors.
DEPFET based Macropixel detectors combine the advantages of the DEPFETs, like flexible readout modes,
Fano-limited energy resolution and low power consumption, with the properties of the drift detectors, like arbitrary
scalable pixel size and geometry. In addition, the excellent properties of the entrance window, like good
QE even in the low energy range and 100% fill factor, are preserved. An energy resolution better than 200 eV
FWHM @ 1 keV and an energy range from 0.5 keV to 10 keV, for a pixel size of 300 x 300 square micron, is
required. To be sensitive to the Iron-L energy, the quantum efficiency at 0.5 keV is required to be larger than
80%. Main challenges for the instrument are the difficult radiation and thermal environment in the mercury
orbit. The production of the first batch of flight devices has been finished at the MPI semiconductor laboratory,
and first laboratory modules have been built. The properties of the sensors have been evaluated at the BESSY
facility, and the devices have been used for XRF measurements at the ELETTRA synchrotron facility in Trieste.
The results of the first tests will be presented here.
KEYWORDS: Sensors, Field effect transistors, X-rays, Analog electronics, Field programmable gate arrays, Silicon, X-ray imaging, Prototyping, Space operations, Quantum efficiency
The large collecting area of the X-ray optics on the International X-ray Observatory (IXO), their good angular
resolution, the wide bandwidth of X-ray energies and the high radiation tolerance required for the X-ray detectors
in the focal plane have stimulated a new development of devices which unify all those science driven specifications
in one single detector. The concept of a monolithic, back-illuminated silicon active pixel sensor (APS) based on
the DEPFET structure is proposed for the IXO mission, being a fully depleted, back-illuminated 450 μm thick
detector with a physical size of about 10 × 10 cm2 corresponding to the 18 arcmin field of view. The backside
will be covered with an integrated optical light and UV-filter. Corresponding to the 5 arcsec angular resolution
of the X-ray optics, 100 x 100 cm2 large pixels in a format of approximately 1024 x 1024 are envisaged, matching
the point spread function of approximately 500 μm HEW of the optics. The energy range from 100 eV to 15 keV
is achieved by an ultra thin radiation entrance window for the low energies and 450 μm depleted silicon thickness
for higher energies. The fast readout of 1.000 full frames per second is realized by a dedicated analog CMOS
front end amplifier IC. The detector device is intrinsically radiation hard. The leakage current from the bulk
damage is controlled through the operation temperature around -60 °C and by the high readout speed. Results
of various prototype measurements will be shown.
DEPFET Macropixel detectors, based on the fusion of the combined Detector-Amplifier structure DEPFET with
a silicon drift chamber (SDD) like drift ring structure, combine the excellent properties of the DEPFETs with
the advantages of the drift detectors. As both device concepts rely on the principle of sideways depletion, a
device entrance window with excellent properties is obtained at full depletion of the detector volume.
DEPFET based focal plane arrays have been proposed for the Focal Plane Detectors for the MIXS (Mercury
Imaging X-ray Spectrometer) instrument on BepiColombo, ESAs fifth cornerstone mission, with destination
Mercury. MIXS uses a lightweight Wolter Type 1 mirror system to focus fluorescent radiation from the Mercury
surface on the FPA detector, which yields the spatially resolved relative element abundance in Mercurys crust.
In combination with the reference information from the Solar Intensity X-ray Spectrometer (SIXS), the element
abundance can be measured quantitatively as well. The FPA needs to have an energy resolution better than
200 eV FWHM @ 1 keV and is required to cover an energy range from 0.5 keV to 10 keV, for a pixel size of
300 x 300 μm2. Main challenges for the instrument are the increase in leakage current due to a high level of
radiation damage, and the limited cooling resources due to the difficult thermal environment in the mercury
orbit. By applying an advanced cooling concept, using all available cooling power for the detector itself, and
very high speed readout, the energy resolution requirement can be kept during the entire mission lifetime up to
an end-of-life dose of ~ 3 × 1010 10 MeV p / cm2. The production of the first batch of flight devices has been
finished at the MPI semiconductor laboratory, and first prototype modules have been built. The results of the
first tests will be presented here.
Two new DEPFET concepts are presented motivated by potential applications in adaptive optics and
in synchrotron radiation experiments at the future Free Electron
X-ray Laser (XFEL) in Hamburg.
The gatable DEPFET structure allows the selection of signal charges arriving in a predefined time
interval. Charges produced outside this gate interval are lead to a sink electrode while charge
collected already is protected and kept for later delayed readout.
In synchrotron radiation experiments one faces the challenge of being sensitive enough for single X-ray
photons in some parts of the detector while on other regions a very large charge due to the
superposition of many X-rays has to be measured. A DEPFET with strongly non-linear
characteristics combines naturally excellent energy resolution with high dynamic range, large charge
handling capability and high read out speed.
DEPMOSFET based Active Pixel Sensor (APS) matrices are a new detector concept for X-ray imaging spectroscopy missions. They can cope with the challenging requirements of the XEUS Wide Field Imager and combine excellent energy resolution, high speed readout and low power consumption with the attractive feature of random accessibility of pixels. From the evaluation of first prototypes, new concepts have been developed to overcome the minor drawbacks and problems encountered for the older devices. The new devices will have a pixel size of 75 μm × 75 μm. Besides 64 × 64 pixel arrays, prototypes with a sizes of 256 × 256 pixels and 128 × 512 pixels and
an active area of about 3.6 cm2 will be produced, a milestone on the way towards the fully grown XEUS WFI device. The production of these improved devices is currently on the way. At the same time, the development of the next generation of front-end electronics has been started, which will permit to operate the sensor devices with the readout speed required by XEUS. Here, a summary of the DEPFET capabilities, the concept of the sensors of the next generation and the new front-end electronics will be given. Additionally, prospects of new device developments using the DEPFET as a sensitive element are shown, e.g. so-called RNDR-pixels, which feature repetitive non-destructive readout to lower the readout noise below the 1 e- ENC limit.
DEPMOSFET based Active Pixel Sensor (APS) matrix devices, originally
developed to cope with the challenging requirements of the XEUS Wide
Field Imager, have proven to be a promising new imager concept for a
variety of future X-ray imaging and spectroscopy missions like Simbol-X. The devices combine excellent energy resolution, high speed readout and low power consumption with the attractive feature of random accessibility of pixels. A production of sensor prototypes with 64 x 64 pixels with a size of 75 μm x 75 μm each has recently been finished at the MPI semiconductor laboratory in Munich. The devices are built for row-wise readout and require dedicated control and signal processing electronics of the CAMEX type, which is integrated together with the sensor onto a readout hybrid. A number of hybrids incorporating the most promising sensor design variants has been built, and their performance has been studied in detail. A spectroscopic resolution of 131 eV has been measured, the readout noise is as low as 3.5 e- ENC. Here, the dependence of readout noise and spectroscopic resolution on the device temperature is presented.
KEYWORDS: Field effect transistors, Sensors, Prototyping, Transistors, Signal processing, Analog electronics, Electrons, Cadmium sulfide, Field programmable gate arrays, Imaging systems
The concept of an Active Pixel Sensor (APS) based on the
integrated detector/amplifier structure DEPFET (DEpleted P-channel
Field Effect Transistor) has been developed to cope with the
challenging requirements of the XEUS Wide Field Imager. The
DEPFET-APS combines high energy resolution, fast readout, and random accessible pixels allowing the application of flexible readout modes. First prototypes of DEPFET-based Active Pixel Sensors with a 64 x 64 pixel format and 75 μm x 75 μm pixel area have been produced at the MPI semiconductor laboratory. The APS is read out row by row, i.e. the pixel signals of one row are processed in parallel by a 64 channel CMOS amplifier/multiplexer chip of the CAMEX type. The addressing of one row of pixels for readout and reset is done by two control chips of the SWITCHER type fabricated in a high-voltage CMOS technology. The processing time for one row is of the order of a few micro-seconds. APS operation, the control and data acquisition system are described, and first experimental results are presented.
A new type of Active Pixel Sensor is proposed which will be capable to meet the requirements of the wide field imager of ESA's future X-ray mission XEUS: the simultaneous energy and position resolved detection of X-rays at high count rate on a large format sensor. The Active Pixel Sensor is based on the integrated detector-amplifier structure DEpleted P-channel Field Effect Transistor (DEPFET). The device operates on a fully depleted bulk and provides internal signal amplification at the position of the charge generation. A very low value of the overall output capacitance leads to extremely low read noise. In the matrix arrangement of an Active Pixel Sensor the single DEPFET pixels can be randomly accessed for readout, and various flexible readout modes are possible. In contrast to CCDs the DEPFET-based Active Pixel Sensor avoids the transfer of signal charges over long distances within the detector bulk, and related problems of transfer loss or out-of-time-events cannot occur. An interesting feature is the non-destructive nature of the DEPFET readout which can be used for the reduction of the low-frequency noise contribution by repetitive readings of the signal information. The device principle of the DEPFET based pixel sensor is explained. First results of single DEPFET measurements are presented.
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