M. Schnecke

Design and technology of DEPFET pixel sensors for linear collider applications

The performance requirements of vertex detectors for future linear collider experiments is very challenging especially for the detectors innermost sensor layers. The DEPleted Field Effect Transistor (DEPFET) combining detector and amplifier operation is capable to meet these requirements. A silicon technology is presented which allows production of large sensor arrays consisting of linear DEPFET detector structures. The envisaged pixel array offers a low noise and low power operation. To ensure a high radiation length a thinning technology based on direct wafer bonding is proposed.

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.

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

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.

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

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.

New simulation and measurement results on gateable DEPFET devices

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.

DEPFET macropixel arrays as focal plane instrumentation for SIMBOL-X and MIXS on BepiColombo

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.

The DEPFET-based focal plane detectors for MIXS on BepiColombo

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.

DEPFET Active Pixel Sensors

DEPFET pixels offer a unique possibility for a high resolution pixel vertex detector at a future linear collider (ILC) experiment. The key idea of DEPFET sensors is the integration of amplifying transistors into a fully depleted bulk. The excellent noise performance obtained through the low input capacitance in combination with the full signal from the depleted bulk leads to a large S/N ratio. The sensor itself can therefore be made very thin (50μm) without loss of efficiency. In this article the progress of the DEPFET development towards an ILC vertex detector is presented. Properties of prototype matrices and dedicated ASIC electronics have been characterized in various laboratory and test beam measurements. In particular a point resolution of less than 2 μm has been demonstrated (using 450 μm thick sensors). Based on these results larger matrices, improved readout and control electronics have been designed which are presently in production. In parallel software was developed to simulate the performance of a DEPFET based vertex detector in an ILC experiment.

The Wide Field Imager of the European X-Ray Observatory

The European X-ray Observatory (XRO) also denoted as X-ray Evolving Universe Spectroscopy (XEUS) mission is currently under assessment by the European Space Agency as a follow-up to XMM-Newton and Chandra with an intended launch date around 2015. XEUS/XRO aims to place an X-ray telescope in space with an aperture comparable to the largest ground based observatories. The large collecting area sets new frontiers for the focal plane instrumentation in terms of sensitive area, spectral and spatial resolution and readout speed. To cope with the challenging requirements of the XEUS/XRO Wide Field Imager a new type of Active Pixel Sensor based on the integrated amplifier DEPFET (DEpleted P-channel Field Effect Transistor) has been developed and characterized. This concept combines excellent energy resolution, high speed readout and low power consumption with the attractive features of readout on demand and random accessibility of pixels. A first series of prototype devices produced and characterized at the MPI semiconductor laboratory in Munich/Germany already meets most of the XEUS/XRO specifications. A second generation of devices with substantially larger formats is currently in production. Other device concepts and operation modes derived from the DEPFET principle are introduced.

DEPFET-based instrumentation for the MIXS focal plane on BepiColombo

BepiColombo, ESAs fifth cornerstone mission, is a planetary exploration mission to Mercury. On board of BepiColombos Mercury Planetary Orbiter (MPO), the MIXS instrument will perform a complete X-ray fluorescence analysis of Mercurys 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.