H. Perrey

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.

Study of the radiation hardness of silicon sensors for the XFEL

Imaging experiments at the XFEL pose unprecedented requirements to the detectors in terms of radiation tolerance: Fluxes of up to 1016(12 keVphotons/cm2) corresponding to approximately 109 Gy in silicon, are expected. An irradiation station has been set up in the DORIS beam line F4, MOS test structures have been irradiated, and first results on the dose dependence of the C/V-characteristics, surface current density, and interface trap density have been obtained.

The adaptive gain integrating pixel detector (AGIPD): A detector for the European XFEL. development and status

The European XFEL under construction in Hamburg will provide fully coherent, 100 fs long X-ray pulses, with 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. As a consequence the 2D detectors have to cope with a large dynamic range in the images (one to 104 photons/pixel). An additional challenge is the European XFEL machine: an Electron bunch train with 10 Hz repetition rate, consisting of up to 3,000 bunches with a 200 ns spacing. This means that recorded images have to be stored inside the pixel during the bunch trains and read out between bunch trains. In order 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 1000 ? 1000 pixel detector, with 200 ?m pixel size and a central hole for the primary beam. The ASIC operates in charge integration mode: the output of each pixel preamplifier is proportional to the charge from the sensor generated by the X-rays. The input stage of the pixel cells will have dynamically adjustable gains. The output signal is stored in an analogue pipeline, which has to be a compromise between noise performance and the number of images. 200 to 400 images have to be readout and digitized in the 99.4 ms long bunch gap. The detector will be built of 2 ? 8 fully depleted monolithic silicon sensors with a 2 ? 8 array of CMOS readout chips bump-bonded to these. The interface electronics is designed to process 400 images in 99 msec, without compromising single photon sensitivity or the full dynamic range. Since the ASIC is the linchpin of the project, several MPW runs to test the different aspects in terms of radiation hardness, noise and adaptive switching are submitted. We will give a general overview and report on the current status.