KEYWORDS: Electrons, Sensors, Quantum reading, Silicon, Active sensors, Signal processing, Quantum gates, Charge-coupled devices, Quantum sensors, Signal to noise ratio
Sensors with repetitive non-destructive readout, which achieve a deep sub-electron noise have been established for high precision applications. The Depleted P-channel Field-Effect Transistor with Repetitive Non-Destructive Readout - so-called RNDR-DEPFET - provides an active pixel sensor on a fully depleted silicon bulk with the capability to collect, store and read out charge carriers within each pixel. The readout process takes place by shifting the collected electrons between two readout nodes within one pixel in order to enable statistically independent measurements. In a conventional mode like the rolling shutter operation, the collected electrons are removed after the desired number of readings has been reached. However, the active pixel concept enables a continuous or incremental sampling of the signal during charge collection in combination with a high level of parallelization, as well. In this mode, the charge collection and readout takes places simultaneously and electrons are just removed before the storing capacity of the readout node has been exceeded. After the working principle of RNDR-DEPFET detectors has been demonstrated on a 64×64 pixel sensor, a incremental readout mode with a high time resolution of single electron events is studied. A time resolution in the order of 300 μs for single electron detection is demonstrated, which significantly improves the capabilities for background rejection and detection of rare signals. The paper concludes with an evaluation of applications for light dark matter searches and astrophysical applications.
The combined storage and amplifier structure of a Depleted P-channel Field-Effect Transistor provides the capability to collect, store and read out charge carriers. In combination with an efficient charge transfer between two storage regions, this enables a statistically independent repetitive non-destructive readout of active pixels integrated on a fully depleted, high purity silicon bulk. Averaging the repetitions allows for deep sub-electron noise levels. After the working principle of those sensors was demonstrated on single pixel devices, a 64×64 pixel detector has been operated for the first time. The sensor achieved a single electron sensitivity by recording the spectrum of a light emitting diode. A mean sub-electron noise below 0.2 e−ENC at a readout time below 230 ms/frame are demonstrated.
KEYWORDS: Field effect transistors, Signal to noise ratio, Sensors, Ionization, Active sensors, Signal attenuation, Electrons, Modulation, Transistors, TCAD
The DePFET is an active pixel sensor utilized or suggested for several experiments. For future applications, it is desirable to improve the signal to noise ratio of the DePFET. The DePFET is essentially a pMOSFET built on a high resistive, fully depleted bulk. A deep-n implant beneath the MOS-gate forms a positive potential. Electrons collected in this internal gate modulate the transistors conductivity. This charge gain is influenced by the geometry and operational parameters of the DePFET. To study all influences, we utilized Sentaurus TCAD based simulations to investigate the DePFETs characteristics and compared the results from simulations and measurements. Using the simulations, we studied possible improvements of the DePFET, optimizing its charge gain and noise. From these simulations, it becomes evident, that especially impact ionization poses a problem. We developed an optimized technology that can overcome this limitation.
The Sunrise observatory consists of a one-meter solar telescope operated in the gondola of a stratospheric balloon. The first two science flights of Sunrise have shown the unreached imaging quality at lower costs than satellitebased missions, as well as a general problem of balloon missions: Micro-vibrations have occurred during parts of the observation time and made the determination of the point spread function difficult. This paper introduces an adaption of deconvolution from wave-front sensing (DWFS) as a possible solution. The case of vibrations in the common path is verified in simulations. The utilization of high-cadence spectro-polarimeters is approached in order to extend DWFS to non-common path errors at the scientific camera.
The Wide Field Imager (WFI) of ESA’s next X-ray observatory Athena will combine a high count rate capability with a large field of view, both with state-of-the-art spectroscopic performance. To meet these demands, specific DEPFET active pixel detectors have been developed and operated. Due to the intrinsic amplification of detected signals they are best suited to achieve a high speed and low noise performance. Different fabrication technologies and transistor geometries have been implemented on a dedicated prototype production in the course of the development of the DEPFET sensors. The main modifications between the sensors concern the shape of the transistor gate – regarding the layout – and the thickness of the gate oxide – regarding the technology. To facilitate the fabrication and testing of the resulting variety of sensors the presented studies were carried out with 64×64 pixel detectors. The detector comprises a control ASIC (Switcher-A), a readout ASIC (VERITAS- 2) and the sensor. In this paper we give an overview on the evaluation of different prototype sensors. The most important results, which have been decisive for the identification of the optimal fabrication technology and transistor layout for subsequent sensor productions are summarized. It will be shown that the developments result in an excellent performance of spectroscopic X-ray DEPFETs with typical noise values below 2.5 ENC at 2.5 μs/row.
KEYWORDS: Sensors, Active sensors, Camera shutters, Sensors, Field effect transistors, Transistors, Spectroscopy, Active sensors, Prototyping, Electroluminescent displays, Image resolution, Doping
The focal plane of the WFI of Athena consists of two sensors. One features a large field of view of 40' X 40' and one is forseen to be used for bright point like sources. Both parts base on DEPFET active pixel sensors. To fulfil the count rate requirement for the smaller sensor of less than 1% pile-up for a one Crab source it has to have a sufficient high frame rate. Since therefore the readout becomes a large fraction of the total photon integration time, the probability of measurements with incomplete signals increases. A shutter would solve the problem of these so called misfits but is not in agreement with the required high throughput of more than 80%. The Infinipix design has implemented a storage in addition to separate the collection and the readout of the charges without discarding them. Its working principle was successfully shown by Bähr et al.1 on single pixel level. For the further development three layout variants were tested on a 32 X 32 pixel array scale. The measurements of the spectroscopic performance show very promising results even for the intended readout speed for the Athena WFI of 2:5 μs per sensor row. Although, there are still layout and technology improvements necessary to ensure the reliability needed for space missions. In this paper we present the measurement results on the comparison of the three prototype layout variants.
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: Field effect transistors, Electrons, Sensors, Transistors, Signal processing, X-ray imaging, Imaging systems, Analog electronics, Active sensors, Imaging spectroscopy
Since many years DEPFETs have been developed for space and ground based X-ray imaging and spectroscopy experiments. Prototypes have been successfully tested and qualified. Over the past years, the DEPFET technology was improved and additional features of DEPFETs were developed: increase of dynamic range, improvement of radiation hardness, implementation of electronic shutters, integration of an analog storage, reduction of readout noise and improvement of the low energy performance. This paper will present two novel DEPFET concepts which are able to fulfill the demanding requirements of the proposed ATHENA Wide Field Imager. It will summarize the most important DEPFET characteristics on the basis of measurements and device simulations, taking into account the given boundary conditions of the mission.
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.
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.
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 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.
We report on the first results from a new setup for electrical qualification measurements of DEPFET pixel detector
matrices. In order to measure the transistor properties of all pixels, the DEPFET device is placed into
a benchtest setup and electrically contacted via a probecard. Using a switch matrix, each pixel of the detector
array can be addressed individually for characterization.
These measurements facilitate to pre-select the best DEPFET matrices as detector device prior to the mounting
of the matrix and allow to investigate topics like the homogeneity of transistor parameters on device, wafer
and batch level in order to learn about the stability and reproducibility of the production process. Especially
with regard to the detector development for the IXO Wide Field Imager (WFI), this yield learning will be an
important tool.
The first electrical qualification measurements with this setup were done on DEPFET macropixel detector flight
hardware, which will form the FPAs of the Mercury Imaging X-ray Spectrometer (MIXS) on board of the 5th
ESA cornerstone mission BepiColombo. The DEPFET array consists of 64×64 macropixel for which the transfer,
output and clear characteristics were measured.
Like the International X-ray Observatory (IXO) mission, the Simbol-X mission is a projected X-ray space telescope
with spectral and imaging capabilities covering the energy range from 500 eV up to 80 keV. To detect photons
within this wide range of energies, a silicon based "Depleted P-channel Field Effect Transistor" (DePFET)-
matrix is used as the Low Energy Detector (LED) on top of an array of CdTe-Caliste modules, which act as the
High Energy Detector (HED).
A Science Verification Model (SVM) consisting of one LED quadrant in front of one Caliste module will be set
up at our institute (IAAT) and operated under laboratory conditions that approximate the expected environment
in space. As a first step we use the SVM to test and optimize the performance of the LED operation and data
acquisition chain, consisting of an ADC, an event-preprocessor, a sequencer, and an interface controller. All
these components have been developed at our institute with the objective to handle the high readout rate of
approximately 8000 frames per second. The second step is to study the behaviour and the interactions of LED
and HED operating as a combined detector system.
We report on the development status of the SVM and its associated electronics and present first results of
the currently achieved spectral performance.
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.
We report progress in the design of the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). This instrument
consists of two modules; a Wolter I soft X-ray telescope based on radially packed microchannel plate
optics (MIXS-T) and a profiled collimator which uses a square pore square packed microchannel plate array to
restrict its field of view (MIXS-C). Both instrument modules have identical focal planes (DEPFET macropixel
array) providing an energy resolution of better than 200 eV FWHM throughout the mission.
The primary science goal of MIXS is to perform X-ray fluorescence spectroscopy of the Hermean surface with
unprecedented spatial and energy resolution. This allows discrimination between different regolith types, and
by combining with data from other instruments, between competing models of crustal evolution and planetary
formation. MIXS will also probe the complex coupling between the planet's surface, exosphere and magnetosphere
by observing Particle Induced X-ray Emission (PIXE).
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.
Simbol-X is a French-Italian-German hard energy X-ray mission with a projected launch in 2014. Being sensitive in the
energy range from 500 eV to 80 keV it will cover the sensitivity gap beyond the energy interval of today's telescopes
XMM-Newton and Chandra. Simbol-X will use an imaging telescope of nested Wolter-I mirrors. To provide a focal
length of 20 m it will be the first mission of two independent mirror and detector spacecrafts in autonomous formation
flight.
The detector spacecraft's payload is composed of an imaging silicon low energy detector in front of a pixelated
cadmium-telluride hard energy detector. Both have a sensitive area of 8 × 8 cm2 to cover a 12 arcmin field of view and a pixel size of 625 × 625 μm2 adapted to the telescope's resolution of 20 arcsec. The additional LED specifications are:
high energy resolution, high quantum efficiency, fast readout and optional window mode, monolithic device with 100 %
fill factor and suspension mounting, and operation at warm temperature.
To match these requirements the low energy detector is composed of 'active macro pixels', combining the large, scalable
area of a Silicon Drift Detector and the low-noise, on-demand readout of an integrated DEPFET amplifier. Flight
representative prototypes have been processed at the MPI semiconductor laboratory, and the prototype's measured
performance demonstrates the technology readiness.
This paper describes the conceptual thermo-mechanical design of the MIXS (Mercury Imaging X-ray Spectrometer)
Focal Plane Assembly (FPA). This design is mainly driven by thermal requirements: The Detector is required to operate
below -45 ºC, while the Detector and proximity electronics dissipate more than 2 W, which the passive cooling system
can not handle at the required temperature.
In order to get rid of this cross-constraint, the Detector was separated from the Proximity electronics board, which in turn
has introduced a new dimension of mechanical requirements, as the 370+ bond wires that interconnect both are
extremely delicate and have a high thermal conductivity.
The DEPMOSFET (Depleted p-channel MOSFET) is an Active Pixel Sensor (APS) for the XEUS Wide Field
Imager (WFI), which is developed and produced by the MPI semiconductor laboratory in Munich (HLL). The
current prototype detector consists of a hybrid where a 64 x 64 pixel matrix with 75 μm x 75 μm pixel size each
is mounted together with CMOS SWITCHER II ICs for row-selection and a CAMEX 64 ASIC for readout.
First measurements for this device have shown the high energy resolution and quantum efficiency as well as the
potential for fast readout. For fast timing studies on XEUS an instrument is needed which is able to deal with
count rates up to 106 photons s-1 with 10 μs time resolution. At the Institut fuer Astronomie und Astrophysik,
we have built a setup to investigate the timing performance of the current prototype detector and to study
the capability of the DEPMOSFET detector to handle high count rates. In this paper we present the Data
Acquisition System and the future plans for this setup.
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.
KEYWORDS: Sensors, Field effect transistors, Mirrors, Monte Carlo methods, Photons, Point spread functions, Spectroscopy, Imaging systems, Detector arrays, Imaging spectroscopy
With its large collecting area XEUS will be ideally suited to probe strong gravity fields around collapsed objects and to constrain the equation of state of dense matter in neutron stars. For these studies, detectors are needed which can measure 106 events/sec with high time resolution (10 μsec) and good energy resolution (ΔE = 200 - 300 eV FWHM) combined with an energy and flux independent dead time. The current baseline for a dedicated fast timing detector on XEUS is an array of 19 silicon drift detectors (SDD) operated as single photon detectors. Optionally we have studied an array of 40 x 20 SDD/DEPFET macro pixel detectors read out at a constant frame rate of 105/sec. Alternatively to these two dedicated detectors, a high time resolution mode of the Wide Field Imager (1024 x 1024 DEPFET array with 78μm x 78μm pixels) is considered here. We have simulated the expected timing performance of these detector options based on results from laboratory measurements. We have performed Monte Carlo simulations using the latest available XEUS mirror response files for Crab like sources and intensities ranging from 102 up to 4x106 events/sec. Our results are discussed in the light of the scientific requirements for fast timing as expressed in the ESA Cosmic Vision 2015-2025 plan.
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.
The pn-CCD camera on board of XMM-Newton as well as the Wide Field Imager (WFI) currently being designed for the XEUS mission can be read out in special fast timing modes, providing spectroscopy at very high time resolution. The two fast modes, Timing and Burst mode, of the pn-CCD camera on board XMM-Newton provide a time resolution of 30 μs, respectively. However, this fast timing is only possible at the expense of spatial resolution in readout direction. In contrast, the current baseline design of the WFI for XEUS will provide 25 μs timing at full spatial resolution. We describe the basic principles of the fast readout schemes of the pn-CCD and the SFI, discuss the potential of XEUS for high time resolution spectroscopy and present first results of pulse phase resolved spectroscopy of the Crab pulsar with the pn-CCD in Timing mode.
Active Pixel Sensors (APS) offer high-resolution imaging in combination with a fast and flexible readout. The MPI Halbleiterlabor develops and produces DEPFET (Depleted Field Effect Transistor) based APS devices. They are additionally characterized by enhanced sensitivity for X-ray photons in the range from 0.1 keV to 25 keV, spectroscopic energy resolution (below 1 electron r.m.s.) and radiation hardness. Moreover, the production process on high-ohmic silicon allows incorporating additional high-speed spectrometers based on silicon drift detectors. Such a detector system is proposed as a wide field imager for the XEUS (X-ray Evolving Universe Spectroscopy) mission. XEUS is a planned project within the European Space Agency's Horizon 2000+ program. We will present a focal plane concept for XEUS and measurement results from DEPFET-APS prototypes and high speed drift detectors.
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