Impact of low-dose electron irradiation on n+p silicon strip sensors

Adam, W. and Bergauer, T. and Dragicevic, M. and Fried, M. and Fruehwirth, R. and Hajdu, Csaba (2015) Impact of low-dose electron irradiation on n+p silicon strip sensors. NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 803. pp. 100-112. ISSN 0168-9002

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Abstract

The response of n +p silicon strip sensors to electrons from a 90Sr source was measured using a multi-channel read-out system with 25 ns sampling time. The measurements were performed over a period of several weeks, during which the operating conditions were varied. The sensors were fabricated by Hamamatsu Photonics K. K. on 200 µm thick float-zone and magnetic-Czochralski silicon. Their pitch was 80 µm, and both p-stop and p-spray isolation of the n + strips were studied. The electrons from the 90Sr source were collimated to a spot with a full-width-at-half-maximum of 2 mm at the sensor surface, and the dose rate in the SiO2 at the maximum was about 50 Gy/d. After only a few hours of making measurements, significant changes in charge collection and charge sharing were observed. Annealing studies, with temperatures up to 80◦C and annealing times of 18 hours, showed that the changes can only be partially annealed. The observations can be qualitatively explained by the increase of the positive oxide-charge density due to the ionization of the SiO2 by the radiation from the β source. TCAD simulations of the electric field in the sensor for different oxide-charge densities and different boundary conditions at the sensor surface support this explanation. The relevance of the measurements for the design of p +n strip sensors is discussed.

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