  The European X-ray Free Electron Laser (XFEL) will deliver 27000 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
molecules and the study of ultra-fast processes. One of the detector systems
under development for the XFEL is the Adaptive Gain Integrating Pixel Detector
(AGIPD), which consists of a pixel array with readout ASICs bump-bonded to a
silicon sensor with pixels of 200 {\mu}m \times 200 {\mu}m. The particular
requirements for the detector are a high dynamic range (0, 1 up to 10E5 12 keV
photons/XFEL-pulse), a fast read-out and radiation tolerance up to doses of 1
GGy of 12 keV X-rays for 3 years of operation. At this X-ray energy no bulk
damage in silicon is expected. However fixed oxide charges in the SiO2 layer
and interface traps at the Si-SiO2 interface will build up. As function of the
12 keV X-ray dose the microscopic defects in test structures and the macro-
scopic electrical properties of segmented sensors have been investigated. From
the test structures the oxide charge density, the density of interface traps
and their properties as function of dose have been determined. It is found that
both saturate (and even decrease) for doses above a few MGy. For segmented
sensors surface damage introduced by the X-rays increases the full depletion
voltage, the surface leakage current and the inter-pixel capacitance. In
addition an electron accumulation layer forms at the Si-SiO2 interface which
increases with dose and decreases with applied voltage. Using TCAD simulations
with the dose dependent damage parameters obtained from the test struc- tures
the results of the measurements can be reproduced. This allows the optimization
of the sensor design for the XFEL requirements.
