Study of X-ray radiation damage in silicon sensors

Abstract

The European X-ray Free Electron Laser (XFEL) will deliver 30,000fully coherent, high brilliance X-ray pulses per second each with a durationbelow 100 fs. This will allow the recording of diffraction patterns of single complexmolecules and the study of ultra-fast processes. Silicon pixel sensors willbe used to record the diffraction images. In 3 years of operationthe 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. Howeverfixed 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 microscopicdefects in test structures and the macroscopic electrical propertiesof segmented detectors as a function of the X-ray dose. From the test structures we determine the oxidecharge 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 100MGy. For segmented sensors the defects introduced by the X-rays increasethe full depletion voltage, the surface leakage current and the inter-pixelcapacitance. We observe that an electron accumulation layer forms at theSi-SiO2 interface. Its width increases with dose and decreaseswith applied bias voltage. Using TCAD simulations with the dose dependent parametersobtained from the test structures, we are able to reproduce the observedresults. This allows us to optimize the sensor design for the XFEL requirements.In addition the Si-SiO2 interface region has been studied withtime resolved signals induced by sub-nanosecond 660 nm laser light, which has a penetrationof about 3 μm in silicon. Depending on the biasing history, humidityand irradiation dose, losses of either electrons or holes or no chargelosses are observed. The relevance of these results for the sensor stabilityand performance is under investigation.