T. Lauf

DEPFET based focal plane instrumentation for X-ray imaging spectroscopy in space

The combined Detector-Amplifier structure DEPFET (Depleted P-channel FET) is a promising new building block for large area silicon detector devices, e.g. in X-ray astronomy and high energy physics. The DEPFET structure combines excellent energy resolution, high speed readout and low power consumption with the attractive features of random accessibility of pixels and on-demand readout. In addition, it features all advantages of a sideways depleted device in terms of fill factor and quantum efficiency. Finally, the newly introduced combination of a DEPFET structure and a silicon drift diode (SDD) like drift ring structure to form a so-called macropixel device allows for large flexibility in terms of pixel size. Presently, focal plane instrumentation for X-ray imaging spectroscopy based on DEPFET arrays is being developed for a variety of space experiments with very different requirements. The next European X-ray Observatory XEUS is going to have a wide field imager covering the full FOV, which consists of a large-area DEPFET array. The concept for the French-Italian X-ray Astronomy mission SIMBOL-X includes a focal plane array based on DEPFET macropixels, and, finally, the MIXS (Mercury Imaging X-ray Spectrometer) instrument on the European Mercury exploration mission BepiColombo also contains two DEPFET macropixel based focal plane arrays. While for XEUS and SIMBOL-X excellent energy resolution and quantum efficiency in the low energy range are mandatory, radiation hardness is imperative for MIXS. A first production of DEPFET prototype arrays showed very promising results. More sophisticated prototype devices for SIMBOL-X and XEUS with a large sensitive area as well as flight grade devices for the MIXS instrument have been produced at the MPI semiconductor laboratory in Munich/Germany. The strategies to meet the respective requirements by an appropriate design of the focal plane instrumentation are shown as well as first results of the new production.

Performance of ASTEROID: A 64 channel ASIC for source follower readout of DEPFET matrices for X-ray astronomy

We present the complete 64 channel version of the ASTEROID ASIC, developed to readout DEPFET Macro-Pixel Arrays operated in source follower mode. These sensors have been designed for the X-Ray Astronomy applications Simbol-X and BepiColombo. Both are satellite based missions that require a detector system with high speed readout, high energy resolution and radiation hardness properties. The foreseen baseline pixel arrays are optimized for source follower readout. In this operating mode of the DEPFETs, the readout electronics is AC-coupled. This allows easy coping with non homogeneity of the pixel matrix and an easy compensation of threshold voltage shifts of the DEPFET devices due to radiation damage. In order to achieve the low noise value required at short processing time, ASTEROID implements a trapezoidal weighting function. This filtering scheme substitutes the Multi-Correlated-Double-Sampling used so far by CAMEX ASICs and represents the time-limited optimum filter for white series noise, which is dominant at the foreseen readout speed. Measurements on single pixels had shown that an electronics noise as low as of about 8 electrons r.m.s. is achievable at room temperature with a total processing time of 4µs. In this work, preliminary measurements on the complete 64 channel readout ASIC coupled with a 64×64 DEPFET matrix have confirmed this noise performance. This result fits the requirements both of BepiColombo and Simbol-X. Also the other parameters of the ASIC have been tested on the 64 channel version. In order to fully operate the 64 channels in parallel, the ASIC contains a digital section that generates the timing signals for the analog circuits. This digital section is based on SEU-immune dual port memory cells. The outputs of the 64 analog channels are multiplexed to one serial output with a speed up to 20MHz. Thanks to the new multiplexer architecture adopted, ASTEROID will be the only ASIC that allows window-mode readout of the pixel matrices, i.e. that allows to address selectively arbitrary sub-areas of the pixel array or even to readout different sub-areas at different speeds.

Development and Characterization of New 256

DEPFET detectors are silicon (Si) active pixel sensors designed and manufactured in the Max-Planck-Institut semiconductor lab. Their high spatial resolution and high energy resolution in X-rays make them attractive for particle tracking in colliders and for X-ray astronomy. This technology is foreseen for the Wide Field Imager of the International X-ray Observatory currently in study with ESA, NASA, and JAXA. New DEPFET matrixes with 256 × 256 pixels of 75-μm pitch have been produced, mounted on ceramic boards with dedicated front-end electronics and integrated in a new setup able to acquire large-format images and spectra. Excellent homogeneity has been observed. Energy resolution as low as 127 eV FWHM at 5.9 keV has been obtained including all single events of the matrix back illuminated at -45<;°C and read out at a 300-frames/s rate. This paper presents experimental methods and results.

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Pixelated semiconductor detectors for X-ray imaging spectroscopy are foreseen as key components of the payload of various future space missions exploring the x-ray sky. Located on the platform of the new Spectrum-Roentgen-Gamma satellite, the eROSITA (extended Roentgen Survey with an Imaging Telescope Array) instrument will perform an imaging all-sky survey up to an X-ray energy of 10 keV with unprecedented spectral and angular resolution. The instrument will consist of seven parallel oriented mirror modules each having its own pnCCD camera in the focus. The satellite born X-ray observatory SIMBOL-X will be the first mission to use formation-flying techniques to implement an X-ray telescope with an unprecedented focal length of around 20 m. The detector instrumentation consists of separate high- and low energy detectors, a monolithic 128 × 128 DEPFET macropixel array and a pixellated CdZTe detector respectively, making energy band between 0.5 to 80 keV accessible. A similar concept is proposed for the next generation X-ray observatory IXO. Finally, the MIXS (Mercury Imaging X-ray Spectrometer) instrument on the European Mercury exploration mission BepiColombo will use DEPFET macropixel arrays together with a small X-ray telescope to perform a spatially resolved planetary XRF analysis of Mercurys crust. Here, the mission concepts and their scientific targets are briefly discussed, and the resulting requirements on the detector devices together with the implementation strategies are shown.

256 Pixel DEPFET Detectors for X-Ray Astronomy

The Mercury imaging X-ray spectrometer (MIXS) on board of ESAs fifth cornerstone mission BepiColombo will be the first space instrument using DEpleted P-channel FET (DEPFET) based detectors. The MIXS spectrometer comprises two channels with identical focal plane detectors and is dedicated to energy resolved imaging of X-ray fluorescence from the mercurial surface. We report on the characterization, integration, and spectroscopic qualification of MIXS flight detectors. Detector chips were precharacterized at die level in order to select the best dies for integration and to do homogeneity and yield studies. Then, the detector chips were integrated to MIXS Detector Plane Arrays (DPAs), a complicated process due to the sophisticated mechanical structure, which allows the thermal decoupling of the detector from its readout and control chips. After integration, spectroscopic qualification measurements were done in order to analyze the detector performance and to prove the excellent spectroscopic performance of the DEPFET Macropixel detectors over a wide temperature range. The integration and spectroscopic qualification of all flight grade modules is now successfully completed.

Pixel detectors for x-ray imaging spectroscopy in space

An experiment was performed to measure the current related damage rate of silicon soon after a 10 MeV proton irradiation at −50° C, in a condition in which the effect of the leakage current annealing is negligible. This measurement is fundamental to predict the spectroscopic performance of the macropixel detectors which will be mounted on the Simbol-X and BepiColombo space missions. Macropixel detectors consist on matrices of Silicon Drift Detectors with an integrated DEPFET readout node on each pixel and offer an optimal solution when a large pixel area is needed but the noise should be kept at levels allowing X-ray spectroscopy. The most critical aspect of the operation of these detectors, in particular of the one which will be used in the BepiColombo mission, is whether the leakage current increase due to the proton irradiation would still allow the required energy resolution. This leakage current increase cannot be predicted with the available models because, during the whole mission, the sensor will be kept at temperatures below −40° C, and the existing empirical parameterizations are valid only at higher annealing temperatures. The irradiation was performed with diodes at the tandem accelerator of the Meier-Leibnitz Laboratorium in Garching with 10-MeV protons and fluences below 1011 protons/cm2, the interesting range for the missions. The diodes were cooled at a temperature of −50° C during the experiment and biased and read out with a charge sensitive preamplifier to perform a dosimetry based on proton counting. The leakage current was measured at the end of every exposure, before warming up and replacing the samples. Its time evolution after several steps of annealing at 60° C was then studied in the laboratory to check the agreement with the NIEL hypothesis predictions and thus, to validate the experiment. The experimental setup, the measurements of current induced damage rate at −50° C and its annealing are discussed in detail. The consequences on the Simbol-X and BepiColombo experiments are also examined.

DEPFET Macropixel Detectors for MIXS: Integration and Qualification of the Flight Detectors

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 telescopes expected 5 arcsec point spread function. The WFIs 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.

Measurement of the current related damage rate at −50° C and consequences on macropixel detector operation in space experiments

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.

The wide-field imager for IXO: status and future activities

The new advancements in low-noise X-Ray detection pave the way to new space missions for deep space investigation like IXO. The concept of APS (Active Pixel Sensor) devices based on DEPFET (Depleted P-channel FET) structure has been developed to cope with the emerging requirements of low noise and high readout speed. Although the use of DEPFET makes achievable the required noise performance, it is necessary to develop a suitable front-end ASIC to fulfill the challenging speed requirements. We have developed VELA, a new fast multi-channel ASIC for the readout of a DEPFET matrix at high frame rate. The implemented circuit operates the DEPFET pixels in a drain current configuration to achieve a readout speed of 2 ¿s per line or even faster. The current version of VELA integrates 64 analog channels to readout a 64 × 64 DEPFET matrix up to 7800 frames per second. In the paper, the circuit and the working principle are presented; the VELA performance and the first measurement with a 64 × 64 DEPFET matrix are also reported.

DEPFET based X-ray detectors for the MIXS focal plane on BepiColombo

Future x-ray observatories, such as the proposed ATHENA+ mission, will investigate bright and rapidly evolving radiation sources. To reach the scientific goals, high speed, spatial resolving sensors with excellent spectroscopic performance are mandatory. Well suited for this task are matrices of Depleted P-channel Field Effect Transistors (DEPFETs). DEPFETs provide intrinsic signal amplification, 100 percent fill factor, charge storage capability and a low read noise. Previous studies of DEPFET matrices of 256 × 256 pixels demonstrated an excellent energy resolution of 126 eV FWHM at 5.9 keV (compared to the theoretical Fano limit 120 eV). Usually these matrices are read out on demand, using e.g. the ASTEROID ASIC. Because the DEPFET is always sensitive, charge collected during the readout, causes so called misfits, which increase the background. For low frame rates this can be neglected. However, for fast timings, as suggested for ATHENA+, this effect reduces the spectral performance. We will present measurements on DEPFET macropixel structures, read out using a semi-Gaussian shaper, which demonstrate the excellent spectroscopic performance of these devices. Furthermore we will investigate the effect of misfits on the spectral background of DEPFET devices read out on demand. These measurements show the necessity to suppress misfits when the devices are operated for fast timing modes. As will be shown this can be done using so called gateable DEPFETs. The general advantage of gateable DEPFETs at fast timings, in terms of peak-to-background ratio will be demonstrated.