Optimization of the radiation hardness of silicon pixel sensors for high x-ray doses using TCAD simulations

Abstract

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 × 200 μm2. The particular requirements for the detector are a high dynamic range (0, 1 up to 105 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 teststructures and the macroscopic electrical properties of segmentedsensors have been investigated. From the test structures the oxidecharge density, the density of interface traps and their properties asfunction of dose have been determined. It is found that both saturate(and even decrease) for doses above a few MGy. For segmented sensorssurface damage introduced by the X-rays increases the full depletionvoltage, the surface leakage current and the inter-pixelcapacitance. In addition an electron accumulation layer forms at theSi-SiO2 interface which increases with dose and decreases withapplied voltage. Using TCAD simulations with the dose dependentdamage parameters obtained from the test structures the results of themeasurements can be reproduced. This allows the optimization of thesensor design for the XFEL requirements.