G. De Vita

ASTEROID: A new 64 channel ASIC for source follower based readout of the MIXS DEPFET arrays on BepiColombo

ASTEROID is a new 64 channel ASIC developed to read out a DEPFET macropixel array. This sensor will be part of the MIXS instrument that will be used in the ESAs BepiColombo Mercury exploration mission. The detector will operate in an environment with high density of solar proton flux that will cause severe radiation damage and an increase of leakage current during mission lifetime. Given the relatively high value of the leakage current, a high speed readout and a proper cooling of the detector is needed in order to achieve the required energy resolution. The contribution of the readout electronics to the noise should be smaller than 10 electrons r.m.s. The foreseen 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 - most of all - 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, which represents the time-limited optimum filter for white series noise. This is the major electronics noise source at the foreseen readout speed. Measurements on a first prototype of the ASIC have shown the very satisfactory resolution of 147 eV on Mn-Kalphapeak of 55Fe with a readout time of approximately 1.2 mus, that would lead, considering the settling time of the DEPFET, to a total processing time of 4.2 mus. We present the experimental results of a first prototype that includes 4 analog channels and a new digital section for the generation of the timing signals of the circuit. This digital section is based on radiation- hardened dual-port memory cells and registers and has already been implemented to serve all the 64 channels foreseen for the final version of the ASIC.

DEPFET active pixel sensor with non-linear amplification

One of the X-ray imaging detectors in development for the European XFEL (X-ray Free Electron Laser) is the DSSC (DEPFET Sensor with Signal Compression). The DSSC sensor array will have a format of 1024 × 1024 pixels with a pixel size in the order of 200 µm. It is optimized for the detection of low-energy X-ray photons and designed to operate at frame rates up to 4.5 MHz with a dynamic range of several thousand photons per pixel and frame and single photon resolution at small photon numbers. The DSSC systems core component is an active pixel sensor based on the DEPFET (DEpleted P-channel Field Effect Transistor) structure. A DEPFET is an integrated detector-amplifier combining internal amplification, full sensitivity over the whole bulk thickness, analog data storage, readout on demand, low serial noise, and absence of reset noise. For the DSSC-adaptation of the DEPFET principle the new feature of signal compression has been added, i.e. the device has a high gain at small signals and a reduced gain at large signals. With this strongly non-linear response the DSSC-DEPFET provides the required high charge handling capacity with simultaneous single photon resolution for low-energy X-rays. This paper presents the concept of the DSSC-DEPFET. Its functionality is demonstrated by measurements of a first series of prototypes.

VERITAS: A 128-Channel ASIC for the Readout of pnCCDs and DEPFET Arrays for X-Ray Imaging, Spectroscopy and XFEL Applications

VERITAS (VErsatile Readout based on Integrated Trapezoidal Analog Shapers) is the first 128-channel ASIC developed to read out both the pnCCDs and the DEPFET arrays produced at the MPI-Halbleiterlabor in Munich. These detectors are used in a large variety of scientific applications, ranging from high-speed optical astronomy and X-ray astronomy to the new X-ray Free Electron Laser sources. The main concept of VERITAS is to provide a flexible readout chip able to cope not only with different kinds of detectors, but also with a large set of operating conditions that may require very different noise thresholds and input dynamic ranges. These can vary by more than two orders of magnitude. Every analog channel of VERITAS provides a trapezoidal weighting function. This filtering strategy had never been applied to the pnCCD before. The very first measurements obtained coupling VERITAS with a 128 × 256 pnCCD are shown. With a readout time of 4 μs/line a noise of 3.9 electrons has been measured in the highest gain mode. The resolution obtained on the Mn-Kα peak of a 55Fe source is 136 eV for single events. A noise of 30 electrons has been achieved in the lowest gain mode at a speed of 6.4 μs/line. In this low gain setting an input charge up to 2 5 × 105 electrons can be processed. These striking results fulfill the requirements of the main foreseen applications of large-size pnCCDs. In order to further improve the performance and the flexibility of the ASIC, a second version based on a fully differential architecture has been designed. The new topology allows one also to switch with the same ASIC between the source follower and drain current readout of the DEPFET sensors and to reach a processing time of about 2-3 μs/line with an electronics noise ≤10 el. For this reason the second version of VERITAS is very attractive for the proposed ESA X-ray astronomy mission ATHENA.

Characterization of the Flip Capacitor Filter for the XFEL-DSSC Project

The European X-ray Free Electron Laser (XFEL) under construction in Hamburg (Germany) will be able to deliver high intensity X-ray pulses of few tens of femto seconds of duration, at 5 MHz pulse repetition rate. Thanks to its characteristics, this instrument will open up new research opportunities for experiments in femto chemistry, structural biology and material research. A high speed focal plane detector system has been presented, based on a novel non-linear DEPFET as a detector. DEPFETs have been chosen since they can provide excellent energy resolution and high speed readout, a fundamental property to cope with the very demanding pulse time structure of XFEL. Full parallel readout is required, with every channel comprising analog filtering, data conversion and memory storage. In this paper we show results for the first prototype of the analog front end that implements the current readout of the DEPFET pixel. The circuit is based on the Flip Capacitor Filter technique, here briefly presented, and has been designed in 130 nm 1.2 V CMOS technology from IBM. Careful characterization of the individual circuit alone and also connected to a single pixel linear detector has been carried out and the measured performance is summarized.

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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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.

256 Pixel DEPFET Detectors for X-Ray Astronomy

We present a new CCD detector system for single optical photon and low light sources imaging. PnCCDs produced in the double sided silicon technology of the Max-Planck-Institut Halbleiterlabor have been operating for long in the field of X-ray imaging and have shown outstanding performance with respect to quantum efficiency, charge transfer efficiency, speed and spectroscopic resolution. With an anti-reflecting coating deposited on the ultra-thin and homogeneous photon entrance window it is possible to achieve a quantum efficiency close to 100% for near infrared and optical photons. However the electronics noise of 2 electrons r.m.s. obtained so far with X-ray pnCCD would hardly allow to detect signals generated by less than 10 optical photons. So, in order to perform single optical photon counting, a RNDR-DEFET amplifier has been implemented in the anode region of each pnCDD channel. This device allows measuring the signal charge coming from the pnCCD detector non destructively and arbitrary often. It has already theoretically and experimentally verified that this procedure makes it possible to reduce the readout noise down to a value below 0.3 electrons r.m.s. In order to readout a complete pnCCD a low noise multichannel readout time-variant shaping amplifier is mandatory. First results obtained coupling a RNDR device together with a custom designed ASIC implementing a trapezoidal weighting function will be presented. They show a readout noise of 0.25 electrons r.m.s. operating the pixel at -50degC.

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

The combined Detector/Amplifier structure DePFET (Depleted P-channel Field Effect Transistor) features excellent energy resolution, low noise readout at high speed and low power consumption. This is combined with the possibility of random acessibility of pixels and on-demand readout. In addition it possesses all advantages of a sideways depleted device, i.e. 100% fill factor and very good quantum efficiency. Combining the DePFET structure with a silicon drift detector (SDD) like drift ring structure forms a so-called macropixel device which allows for large flexibility in terms of pixel size. All this makes DePFET macropixels a promising new building block for large area silicon radiation detector devices. In this paper, first results of qualifying measurements performed with a macropixel prototype are presented. The detector shows good linearity over the investigated energy range and no significant charge loss within the pixel area.

A new silicon detector system for optical and low light imaging, based on DEPFET RNDR

Focal plane instrumentation based on the combined Detector-Amplifier structure DEPFET (Depleted P-channel FET) are being considered for the application in a large variety of experiments in X-ray astronomy and high energy physics. DEPFET based detectors show excellent energy resolution and can be read out at high speed. In addition, they combine low power consumption with the attractive features of random accessibility of pixels and on-demand readout. They feature all advantages of a sideways depleted device in terms of fill factor and quantum efficiency, and so-called macropixel devices, being a combination of a DEPFET with a drift ring structure, allow for large flexibility in terms of pixel size. Presently, DEPFET based focal plane instrumentation for X-ray imaging spectroscopy is being developed for a variety of space experiments with very different requirements. The payload of the French-Italian X-ray Astronomy mission SIMBOL-X includes a focal plane array based on DEPFET macropixels, and one of the instruments on board the European Mercury exploration mission BepiColombo, the so-called MIXS (Mercury Imaging X-ray Spectrometer) instrument, also uses DEPFET macropixel based focal plane arrays for the detection of X-rays. In both cases, the performance of the respective X-ray optics demands the use of macropixel matrices due to the large required pixel sizes. The MPI semiconductor laboratory in Munich has produced prototype devices for SIMBOL-X with a large sensitive area as well as the first flight grade devices for the MIXS instrument; and the first large-area detectors have been tested. The devices show excellent performance in terms of energy resolution, peak-to-background ratio and homogeneity. Here, the requirements of the respective experiments on their detectors are outlined, together with the strategies to comply with the requirements by choosing a suitable readout strategy and an appropriate design of the focal plane instrumentation. Finally, the first test results of the large area macropixel prototype devices are shown.

Performance and spectroscopic behaviour of DePFET macropixels

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 ESAs 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 Mercurys 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.