Jiaguo Zhang

Investigation of X-ray induced radiation damage at the Si-SiO2 interface of silicon sensors for the European XFEL

Experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors which can withstand X-ray doses up to 1 GGy. For the investigation of X-ray radiation damage up to these high doses, MOS capacitors and gate-controlled diodes built on high resistivity n-doped silicon with crystal orientations and produced by two vendors, CiS and Hamamatsu, have been irradiated with 12 keV X-rays at the DESY DORIS III synchrotron light source. Using capacitance/conductance-voltage, current-voltage and thermal dielectric relaxation current measurements, the surface densities of oxide charges and interface traps at the Si-SiO2 interface, and the surface-current densities have been determined as function of dose. Results indicate that the dose dependence of the surface density of oxide charges and the surface-current density depend on the crystal orientation and producer. In addition, the influence of the voltage applied to the gates of the MOS capacitor and the gate-controlled diode during X-ray irradiation on the surface density of oxide charges and the surface-current density has been investigated at doses of 100 kGy and 100 MGy. It is found that both strongly depend on the gate voltage if the electric field in the oxide points from the surface of the SiO2 to the Si-SiO2 interface. Finally, annealing studies have been performed at 60°C and 80°C on MOS capacitors and gate-controlled diodes irradiated to 5 MGy and the annealing kinetics of oxide charges and surface current determined.

Study of radiation damage induced by 12 keV X-rays in MOS structures built on high-resistivity n-type silicon.

Imaging experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors with extraordinary performance specifications: doses of up to 1 GGy of 12 keV photons, up to 10(5) 12 keV photons per 200 µm × 200 µm pixel arriving within less than 100 fs, and a time interval between XFEL pulses of 220 ns. To address these challenges, in particular the question of radiation damage, the properties of the SiO(2) layer and of the Si-SiO(2) interface, using MOS (metal-oxide-semiconductor) capacitors manufactured on high-resistivity n-type silicon irradiated to X-ray doses between 10 kGy and 1 GGy, have been studied. Measurements of capacitance/conductance-voltage (C/G-V) at different frequencies, as well as of thermal dielectric relaxation current (TDRC), have been performed. The data can be described by a dose-dependent oxide charge density and three dominant radiation-induced interface states with Gaussian-like energy distributions in the silicon band gap. It is found that the densities of the fixed oxide charges and of the three interface states increase up to dose values of approximately 10 MGy and then saturate or even decrease. The shapes and the frequency dependences of the C/G-V measurements can be quantitatively described by a simple model using the parameters extracted from the TDRC measurements.

Study of X-ray radiation damage in silicon sensors

The European X-ray Free Electron Laser (XFEL) will deliver 30,000 fully coherent, high brilliance X-ray pulses per second each with a duration below 100 fs. This will allow the recording of diffraction patterns of single complex molecules and the study of ultra-fast processes. Silicon pixel sensors will be used to record the diffraction images. In 3 years of operation the sensors will be exposed to doses of up to 1 GGy of 12 keV X-rays. At this X-ray energy no bulk damage in silicon is expected. However fixed oxide charges in the insulating layer covering the silicon and interface traps at the Si-SiO2 interface will be introduced by the irradiation and build up over time. We have investigated the microscopic defects in test structures and the macroscopic electrical properties of segmented detectors as a function of the X-ray dose. From the test structures we determine the oxide charge density and the densities of interface traps as a function of dose. We find that both saturate (and even decrease) for doses between 10 and 100 MGy. For segmented sensors the defects introduced by the X-rays increase the full depletion voltage, the surface leakage current and the inter-pixel capacitance. We observe that an electron accumulation layer forms at the Si-SiO2 interface. Its width increases with dose and decreases with applied bias voltage. Using TCAD simulations with the dose dependent parameters obtained from the test structures, we are able to reproduce the observed results. This allows us to optimize the sensor design for the XFEL requirements. In addition the Si-SiO2 interface region has been studied with time resolved signals induced by sub-nanosecond 660 nm laser light, which has a penetration of about 3 μm in silicon. Depending on the biasing history, humidity and irradiation dose, losses of either electrons or holes or no charge losses are observed. The relevance of these results for the sensor stability and performance is under investigation.

AGIPD, a high dynamic range fast detector for the European XFEL

AGIPD—(Adaptive Gain Integrating Pixel Detector) is a hybrid pixel X-ray detector developed by a collaboration between Deutsches Elektronen-Synchrotron (DESY), Paul-Scherrer-Institut (PSI), University of Hamburg and the University of Bonn. The detector is designed to comply with the requirements of the European XFEL. The radiation tolerant Application Specific Integrated Circuit (ASIC) is designed with the following highlights: high dynamic range, spanning from single photon sensitivity up to 104 12.5keV photons, achieved by the use of the dynamic gain switching technique using 3 possible gains of the charge sensitive preamplifier. In order to store the image data, the ASIC incorporates 352 analog memory cells per pixel, allowing also to store 3 voltage levels corresponding to the selected gain. It is operated in random-access mode at 4.5MHz frame rate. The data acquisition is done during the 99.4ms between the bunch trains. The AGIPD has a pixel area of 200× 200 μ m2 and a 500μ m thick silicon sensor is used. The architecture principles were proven in different experiments and the ASIC characterization was done with a series of development prototypes. The mechanical concept was developed in the close contact with the XFEL beamline scientists and is now being manufactured. A first single module system was successfully tested at APS.

Challenges for silicon pixel sensors at the European XFEL

Abstract A systematic experimental study of the main challenges for silicon-pixel sensors at the European XFEL is presented. The high instantaneous density of X-rays and the high repetition rate of the XFEL pulses result in signal distortions due to the plasma effect and in severe radiation damage. The main parameters of X-ray-radiation damage have been determined and their impact on p + n sensors is investigated. These studies form the basis of the optimized design of a pixel-sensor for experimentation at the European XFEL.

AGIPD - The adaptive gain integrating pixel detector for the European XFEL development and status

The European XFEL [1] will provide fully coherent, 100 fs X-ray pulses, with up to 1012 photons at 12 keV. The high intensity per pulse will allow recording diffraction patterns of single particles or small crystals in a single shot. Consequently 2D-detectors have to cope with a large dynamic range: detection from single photon to > 104 photons/pixel in the same image. An additional challenge is the European XFEL machine: an Electron bunch train with 10 Hz repetition rate, consisting of up to 2,700 bunches with a 220 ns spacing. Recorded images have to be stored inside the pixel during the bunch trains and readout in between. To meet these requirements, the European XFEL has launched 3 detector development projects. The AGIPD project is a collaboration between DESY, PSI and the Universities of Bonn and Hamburg. The goal is a 1024 × 1024 pixel detector, with 200 µm pixel size and a central hole for the primary beam. The ASIC operates in charge integration mode: the output of each pixels preamplifier is proportional to the charge from the sensor generated by the X-rays. The input stage of the pixel cells uses dynamically adjustable gains. The output signal is stored in an analogue memory, which has to be a compromise between noise performance and the number of images. This is operated in random access mode, providing means to overwrite bad frames for optimal use of the 352 memory cells per pixel, which have to be readout and digitized in the 99.4ms bunch gap. The detector will be built of 8 × 2 fully depleted monolithic silicon sensors with a 8 × 2 array of CMOS readout chips bump-bonded to these. Several prototypes of the readout ASIC have been produced. The results presented originate from the 16 × 16 pixel matrices AGIPD 0.2, which was bump-bonded to a pixel sensor, and AGIPD 0.3, which includes the intended control algorithm and a fast differential interface to the off-chip world.

Time dependence of charge losses at the Si-SiO2 interface in p + n-silicon strip sensors

The collection of charge carriers generated in p + n-strip sensors close to the Si-SiO2 interface before and after 1 MGy of X-ray irradiation has been investigated using the transient current technique with sub-nanosecond focused light pulses of 660 nm wavelength, which has an absorption length of 3.5 m in silicon at room temperature. The paper describes the measurement and analysis techniques used to determine the number of electrons and holes collected. Depending on biasing history, humidity and irradiation, incomplete collection of either electrons or holes is observed. The charge losses change with time. The time constants are dierent for electrons and holes and increase by two orders of magnitude when reducing the relative humidity from about 80 % to less than 1 %. An attempt to interpret these results is presented.

Study of X-ray radiation damage in the AGIPD sensor for the European XFEL

The European X-ray Free Electron Laser (XFEL), currently being constructed in Hamburg and planned to be operational in 2017 for users, will deliver 27,000 fully coherent, high brilliance X-ray pulses per second with duration less than 100 fs. The unique features of the X-ray beam pose major challenges for silicon detectors used at the European XFEL for imaging experiments, in particular a radiation tolerance of silicon sensors for doses up to 1 GGy for 3 years of operation at an operating voltage above 500 V. One of the detectors under development at the European XFEL is the Adaptive Gain Integrating Pixel Detector (AGIPD), which is a hybrid detector system with ASICs bump-bonded to p+n silicon pixel sensors. We have designed the silicon sensors for the AGIPD, which have been fabricated by SINTEF and delivered in the beginning of February 2013. To demonstrate the performance of the AGIPD sensor with regard to radiation hardness, mini-sensors with the same pixel and guard-ring designs as the AGIPD together with test structures have been irradiated at the beamline P11 of PETRA III with 8 keV and 12 keV monoenergetic X-rays to dose values up to 10 MGy. The radiation hardness of the AGIPD sensor has been proven and all electrical properties are within specification before and after irradiation. In addition, the oxide-charge density and surface-current density from test structures have been characterized as function of the X-ray dose and compared to previous measurements for test structures produced by four vendors.

Towards AGIPD1.0: optimization of the dynamic range and investigation of a pixel input protection

AGIPD is a charge integrating, hybrid pixel readout ASIC, which is under development for the European XFEL [1,2]. A dynamic gain switching logic at the output of the preamplifier (preamp) is used to provide single photon resolution as well as covering a dynamic range of at least 104·12.4 keV photons [3,4]. Moreover, at each point of the dynamic range the electronics noise should be lower than the Poisson fluctuations, which is especially challenging at the points of gain switching. This paper reports on the progress of the chip design on the way to the first full-scale chip AGIPD1.0, focusing on the optimization of the dynamic range and the implementation of protection circuits at the preamplifier input to avoid pixel destruction due to high intense spots.

Optimization of the noise performance of the AGIPD prototype chips

The charge integrating readout electronics AGIPD (adaptive gain integrating pixel detector) is a hybrid detector system developed for the European XFEL. It features a threefold dynamic gain switching to be able to resolve single photons and to cover a dynamic range of 104·12.4 keV photons. As a result of dynamic gain switching, single photon resolution will be achieved in the high gain stage, while the maximum dynamic range will be reached in the low gain stage. The specification to resolve single photons requires a signal-over-noise ratio of at least 10 for a single incoming photon with an energy of 12.4 keV. When using a silicon sensor, that translates to an equivalent noise charge of less than 343 e-. Several AGIPD prototype chips have been designed and characterized, particularly focusing on the noise performance. During the testing phase, the dominant noise sources were identified and the corresponding circuit blocks were improved in the subsequent ASICs. This paper reports on the procedures to identify the dominating noise sources, the optimization process of the circuit blocks and discusses the effect of the optimization on the noise performance.© 2013 IOP Publishing Ltd and Sissa Medialab srl.