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 depleted p-channel field effect transistor is the chosen sensor type for the Wide Field Imager of the Athena mission. It will be used in two types of cameras. One will enable observations of a field of view of 40′ × 40′ by using an array of four 512 × 512 pixel sensors in a 2 × 2 configuration. A second, small one is designed to investigate bright, point-like sources with a time resolution of up to 40 μs. Sensors of final size, layout, and technology were fabricated, assembled and characterised. Also, first results from the flight production are available and confirm the excellent performance. In order to be able to estimate the future performance of degraded detectors, a simulation was developed that takes into account the non-analytical threshold effects on the basis of measurement results. We present the measurement analysis and the comparison of simulated and measured values as well as first attempts to use the Monte Carlo simulation to predict performance results based on noise measurements.
We present a silicon drift detector (SDD) system for the spectroscopy focusing array (SFA) of the enhanced X-ray timing and polarimetry (eXTP) mission. The SFA focuses on fast timing (time resolution below 10 μs) and good spectroscopy capabilities (energy resolution better than 180 eV @ 6 keV). The sensor, consisting of 19 hexagonally shaped pixels with a total sensitive area of 5.05 cm2, is connected to three high time resolution spectroscopy (HTRS) ASICs, allowing a fast readout of the detector signals. The detector works in a Charge-Sensitive Amplifier configuration. We assembled a prototype detector module and present here its mechanical design, describe the used sensor, and report about its performance.
The wide field imager for the Athena x-ray telescope is composed of two back side illuminated detectors using DEPFET sensors operated in rolling shutter readout mode: a large detector array featuring four sensors with 512×512 pixels each and a small detector that facilitates the high count rate capability of the WFI for the investigation of bright, point-like sources. Both sensors were fabricated in full size featuring the pixel layout, fabrication technology and readout mode chosen in a preceding prototyping phase. We present the spectroscopic performance of these flight-like detectors for different photon energies in the relevant part of the targeted energy range from 0.2 keV to 15 keV with respect to the timing requirements of the instrument. For 5.9 keV photons generated by an 55Fe source the spectral performance expressed as full width at half maximum of the emission peak in the spectrum is 126.0 eV for the large detector and 129.1 eV for the fast detector. A preliminary analysis of the camera’s signal chain also allows for a first prediction of the performance in space at the end of the nominal operation phase.
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 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: 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.
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.
KEYWORDS: Sensors, X-rays, Analog electronics, Electronics, Monte Carlo methods, Aluminum, Silicon, Staring arrays, Field programmable gate arrays, Quantum efficiency
The High Time Resolution Spectrometer (HTRS) is one of the five focal plane instruments of the International
X-ray Observatory (IXO). The HTRS is the only instrument matching the top level mission requirement of
handling a one Crab X-ray source with an efficiency greater than 10%. It will provide IXO with the capability
of observing the brightest X-ray sources of the sky, with sub-millisecond time resolution, low deadtime, low
pile-up (less than 2% at 1 Crab), and CCD type energy resolution (goal of 150 eV FWHM at 6 keV). The HTRS
is a non-imaging instrument, based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a
circular envelope and a X-ray sensitive volume of 4.5 cm2 x 450 μm. As part of the assessment study carried
out by ESA on IXO, the HTRS is currently undergoing a phase A study, led by CNES and CESR. In this
paper, we present the current mechanical, thermal and electrical design of the HTRS, and describe the expected
performance assessed through Monte Carlo simulations.
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.
The High Time Resolution Spectrometer (HTRS) is one of six scientific payload instruments of the International X-ray
Observatory (IXO). HTRS is dedicated to the physics of matter at extreme density and gravity and will observe the
X-rays generated in the inner accretion flows around the most compact massive objects, i.e. black holes and neutron
stars. The study of their timing signature and in addition the simultaneous spectroscopy of the gravitationally shifted and
broadened iron line allows for probing general relativity in the strong field regime and understanding the inner structure
of neutron stars. As the sources to be observed by HTRS are the brightest in the X-ray sky and the studies require good
photon statistics the instrument design is driven by the capability to operate at extremely high count rates.
The HTRS instrument is based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a circular
envelope and a sensitive volume of 4.5 cm2 × 450 μm. The SDD principle uses fast signal charge collection on an
integrated amplifier by a focusing internal electrical field. It combines a large sensitive area and a small capacitance, thus
facilitating good energy resolution and high count rate capability. The HTRS is specified to provide energy spectra with
a resolution of 150 eV (FWHM at 6 keV) at high time resolution of 10 μsec and with high count rate capability up to a
goal of 2·106 counts per second, corresponding to a 12 Crab equivalent source. As the HTRS is a non-imaging instrument
and will target only point sources it is placed on axis but out of focus so that the spot is spread over the array of 31 SDD
cells. The SDD array is logically organized in four independent 'quadrants', a dedicated 8-channel quadrant readout chip
is in development.
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.
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.
Silicon Drift Detectors (SDDs) are used as low-capacitance photon detectors for the optical light emitted by scintillators.
The scintillator crystal is directly coupled to the SDD entrance window. The entrance window's transmittance can be
optimized for the scintillator characteristic by deposition of a wavelength-selective anti-reflective coating. Compared to
conventional photomultiplier tubes the SDD readout offers improved energy resolution and avoids the practical
problems of incompatibility with magnetic fields, instrument volume and requirement of high voltage.
A compact imaging spectrometer for hard X-rays and γ-rays has been developed by coupling a large area (29 × 26 mm2)
monolithic SDD array with 77 hexagonal cells to a single
non-structured CsI-scintillator of equal size. The scintillation
light generated by the absorption of an energetic photon is seen by a number of detector cells and the position of the
photon interaction is reconstructed by the centroid method. The measured spatial resolution of the system (≤ 500 μm) is
considerably smaller than the SDD cell size (3.2 mm) and in the order required at the focal plane of high energy
missions. The energy information is obtained by summing the detector cell signals.
Compared to direct converting pixelated detectors, e.g. CdTe with equal position resolution the scintillator-SDD
combination requires a considerably lower number of readout channels. In addition it has the advantages of
comprehensive material experience, existing technologies, proven long term stability, and practically unlimited
availability of high quality material.
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.
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.
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.
The Simbol-X mission, currently undergoing a joint CNES-ASI phase A, is essentially a classical X-ray telescope having an exceptional large focal length obtained by formation flying technics. One satellite houses the Wolter I optics to focus, for the first time in space, X-rays above ~10 keV, onto the focal plane in the second satellite. This leads to improved angular resolution and sensitivity which are two orders of magnitude better than those obtained so far with non-focusing techniques. Tailored to the 12 arcmin field of view and ~15 arcsec angular resolution of the optics, the ~8x8 cm2 detection area of the spectro-imager has ~ 500x500 μm2 pixels, and covers the full energy range of Simbol-X, from ~0.5 to ~80 keV, with a good energy resolution at both low and high energy. Its design leads to a very low residual background in order to reach the required sensitivity. The focal plane ensemble is made of two superposed spectro-imaging detectors: a DEPFET-SDD active pixel sensor on top of an array of pixelated Cd(Zn)Te crystals, surrounded by an appropriate combination of active and passive shielding. Besides the overall concept and structure of the focal plane including the anti-coincidence and shielding, this paper also emphasizes the promising results obtained with the active pixel sensors and the Cd(Zn)Te crystals combined with their custom IDeF-X ASICs.
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.
X-ray timing with musec time resolution can be used to probe strong
gravity fields around collapsed objects and to constrain the equation
of state of dense matter in neutron stars. With its large collecting
area, XEUS will be ideally suited for very high signal to noise
studies of such objects. An instrument dedicated to X-ray timing of
bright Galactic sources has thus been foreseen as part of the XEUS
instrumentation. In this contribution we present numerical simulations
for silicon based detectors (silicon drift detectors and pixel based
sensors) for a variety of astrophysical sources such as neutron star
power spectra (including kHz quasi-periodic oscillations) and black
hole lightcurves to illustrate the expected scientific capabilities of
the fast timing mode instrument.
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.
Fast X-ray timing observations are a method to probe strong gravity fields around compact massive objects. The European X-ray Evolving Universe Spectroscopy (XEUS) mission with its large collecting area telescope will be able to deliver the required extremely good photon statistics for these studies. The fast timing detector in the focal plane must be able to operate at up to 107 incoming photons from the brightest X-ray objects in the sky with an energy resolution of 200 eV FWHM at 5.9 keV at a dead time not exceeding 1% and a time resolution of 10 μsec. Silicon Drift Detectors (SDDs) with their extremely small value of the readout capacitance have proved that they can handle high count rates with simultaneous good energy resolution. For the XEUS fast timing detector it is proposed to operate a multi-channel SDD at an out of focus position to distribute the flux of photons over a number of detector cells.
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.
KEYWORDS: Sensors, Mirrors, Field effect transistors, Spatial resolution, Electrons, Space telescopes, Space operations, Telescopes, Hard x-rays, X-ray telescopes
SIMBOL-X is a hard X-ray mission, operating in the 0.5-70 keV range, which is proposed by a consortium of European laboratories for a launch around 2010. Relying on two spacecraft in a formation flying configuration, SIMBOL-X uses a 30 m focal length X-ray mirror to achieve an unprecedented angular resolution (30 arcsec HEW) and sensitivity (100 times better than INTEGRAL below 50 keV) in the hard X-ray range. SIMBOL-X will allow to elucidate fundamental questions in high energy astrophysics, such as the physics of accretion onto Black Holes, of acceleration in quasar jets and in supernovae remnants, or the nature of the hard X-ray diffuse emission. The scientific objectives and the baseline concepts of the mission and hardware design are presented.
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.
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.
We describe a proposal for an added capability of fast timing to the European x-ray astronomy mission XEUS. The scientific value of fast timing observations for the investigation of compact objects is recognized and has been demonstrated through observations by the Rossi x-ray Timing Explorer. We propose to make use of the huge collecting area of XEUS for timing studies with unparalleled photon statistics and time resolution. We describe the sceintifc motivation, e.g. to probe strong gravity fields around collapsed objects and to constrain the equation of state of dense matter in neutron stars. We discuss options for the implementation of detectors which coudl be small silicon drift detectors out of focus.
Based on the operational experience with the EPIC pn-CCD system on board of XMM-Newton, new imaging X-ray spectroscopic detector systems for future X-ray missions will be introduced in terms of energy, position and time resolving detectors. As the readout speed requirement in the case of single photon coating detectors increases drastically with the collecting area and improved angular resolution, but noise figures have to be on the lowest possible level, new detector schemes must be developed: Active pixel sensors (APS) for X-ray detection have the capability to randomly select areas of interest and to operate at noise levels below 1 electron (rms). About 1000 frames per second can be read out with a relatively low level of electric power with the proposed DEPFET arrays. One prominent candidate for the use of an APS is ESA's XEUS 0 the X-ray Evolving Universe Spectroscopy mission. It represents a potential follow-on mission to the cornerstone XMM-Newton, currently in orbit. The XEUS mission is considered as part of ESA's Horizon 2000+ program within the context of the International Space Station (ISS).
Silicon Drift Detectors (SDDs) with integrated readout transistors combine a large sensitive area with a small total readnode capacitance and are therefore well suited for high resolution, high count rate X-ray spectroscopy. The low leakage current level obtained by elaborated process technology makes it possible to operate them at room temperature or with moderate thermo-electric cooling. The monolithic combination of several SDDs to a multichannel drift detector solves the limited of size and allows for the realization of new physics experiments and systems. Up to 3 cm2 large SDDs for spectroscopic applications were fabricated and tested. Position sensitive X-ray systems are introduced. The description of the device principle is followed by the introduction of the multichannel drift detector concept. Layout, performance and examples of current and future applications are presented.
Fully depleted silicon pn-CCDs with an active thickness of 300 micrometers exhibit a quantum efficiency of 90 percent at a wavelength of 1 micrometers in the near IR. The multi-parallel readout architecture allows for a frame time shorter than 2 ms for a device having a format of 256 by 256 pixel. It can be operated in a full frame mode and in a frame store mode. The pixel size is 50 micrometers by 50 micrometers . The active area is then 12.8 by 12.8 mm2. Cooled down to -90 degrees C the electronic noise floor is below 5 electrons at 50 Megapixel per second. Quantum efficiency measurements will be shown as well as the physical models of the radiation entrance window. A camera system with comparable specifications - except for the pixel size - was already fabricated for ESA's XMM-NEWTON satellite mission, which was launched in early December 1999. The active size of the detector is 60 by 60 mm2 with a readout time of 4 ms. Future work includes the extension of the active area to 1000 by 1000 pixel, monolithically fabricated on a high resistivity 6 inch silicon wafer. The main driver for this development is ESA's planned XEUS mission, to be launched at the end of the next decade.
Silicon Drift Detectors (SDDs) have been recently employed as scintillation detectors for (gamma) -ray spectroscopy and imaging applications. With respect to conventional PMTs, these devices offer the higher quantum efficiency to the scintillation light, typical of a silicon detector. Moreover, thanks to the low value of output capacitance, a SDD is characterized by a lower electronics noise with respect to a conventional silicon photodiode. This feature allows a detector based on the CsI(Tl)-SDD architecture to reach high energy and position resolution in gamma detection. In this work we present the results obtained in the development of a first prototype of gamma detector for 1D position measurements and of a first prototype of small gamma camera for 2D position measurements, both detectors based on a single scintillator coupled to an array of SDDs.
Silicon Drift Detectors (SDDs) with integrated readout transistor combine a large sensitive area with a small value of the output capacitance and are therefore well suited for high resolution, high count rate X-ray spectroscopy. The low leakage current level obtained by the elaborated process technology makes it possible to operate them at room temperature or with moderate cooling. The monolithic combination of a number of SDDs to a Multichannel Drift Detector solves the limitation in size of the single device and allows the realization of new physics experiments and systems. The description of the device principle is followed by the introduction of the Multichannel Drift Detector concept. Layout, performance, and examples of current and future applications are given.
Large format arrays covering a wide bandwidth from 1 eV to 25 keV will be used in the focal plane of X-ray telescopes as well as in adaptive optics systems. As the readout speed requirements increase drastically with the collecting area, but noise figures have to be on the lowest possible level, CCD-type detectors do not seem to be able to fulfill the experiment expectations. Active pixel sensors (APS) have the capability to randomly select areas of interest and to operate at noise levels below 1 electron (rms). One prominent candidate for the use of an APS is XEUS: The X-ray Evolving Universe Spectroscopy mission. It represents a potential follow-on mission to the ESA cornerstone XMM currently in orbit. The XEUS mission was considered as part of ESA's Horizon 2000+ within the context of the International Space Station.
The main features of silicon drift detector modules currently produced by KETEK GmbH and MPI Halbleiterlabor, Munich will be summarized, giving an overview over state of the art and future possible applications.
ESA's future x-ray mission, the x-ray evolving ESA's future x-ray mission, the x-ray evolving universe spectroscopy mission (XEUS) is actually under study as a potential successor of the XMM satellite. Its collecting area for x- rays form 100 eV up to 20 eV will be about 200 times larger than compared to XMM. The angular resolution will be improved by a factor of five. The field of view will eventually be as large as 10 arcmin. Novel wide field images are needed to overcome the limitations by the state-of-the- art CCD type detectors, limited due to the high number of x- rays focused into the focal plane. To face the problem of high count rate and large formats with simultaneous good energy resolution and high quantum efficiency we have proposed two new focal plane x-ray detectors: (a) a back illuminated active pixel sensor and (b) a 200 readout channel back illuminated frame store pn-CCD. Both will be fabricated at the MPI semiconductor lab on 500 micrometers high resistivity silicon.
The concept and performance of the fully depleted pn- junction CCD system, developed for the European XMM- and the German ABRIXAS-satellite missions for soft x-ray imaging and spectroscopy in the 0.1 keV to 15 keV photon range, is presented. The 58 mm X 60 mm large pn-CCD array uses pn- junctions for registers and for the backside instead of MOS registers. This concept naturally allows to fully deplete the detector volume to make it an efficient detector to photons with energies up to 15 keV. For high detection efficiency in the soft x-ray region down to 100 eV, an ultrathin pn-CCD backside deadlayer has been realized. Each pn-CCD-channel is equipped with an on-chip JFET amplifier which, in combination with the CAMEX-amplifier and multiplexing chip, facilitates parallel readout with a pixel read rate of 3 MHz and an electronic noise floor of ENC < e-. With the complete parallel readout, very fast pn-CCD readout modi can be implemented in the system which allow for high resolution photon spectroscopy of even the brightest x-ray sources in the sky.
Monolithic arrays of 12 CCDs, 3 by 1 cm2 each, have been developed and produced for the focal plane instrumentation of the European photon imaging camera (EPIC) on XMM and the German ABRIXAS x-ray satellite mission. The design parameters have been optimized to match the properties of the x-ray imaging optics as well as the x-ray intensity, energy bandwidth and characteristic time constants of the objects to observe. The pixel size is 150 by 150 micrometer2; readout is performed in parallel; low noise, spectroscopic performance is realized by on-chip integrated JFET electronics; highohmic, ultrapure bulk material allows full depletion and enhances the efficiency for higher energy x-ray detection. The fabrication process, the layout topology and the operating conditions guarantee for a ten year operation in space without performance degradation.
PN-CCDs are being developed as focal plane detectors for ESA's X-ray Multi-Mirror satellite mission (XMM), to be launched at the end of this century. As a part of the European Photon Imaging Camera (EPIC) the pn-CCDs will convert the incoming X-ray radiation with high quantum efficiency, low readout noise, excellent background rejection, timing in the microsec regime, radiation tolerance up to several hundreds of krads and a position resolution tailored according to the angular resolution of the telescope. The goal of our laboratorial efforts for this mission is to fabricate a monolithic pn-CCD of an active area of 6 x 6 sq cm having 768 on-chip JFET amplifiers located at the end of each CCD line. It is the aim of this contribution to report on the ongoing work of the pn-CCD system. This article focuses on the position resolution capabilities of fully depleted pn-CCDs, some recent results in the noise analysis and preliminary results on 10 MeV proton damage.
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