NEWRAD 2005: Characterization of detectors for extreme UV radiation

Photodiodes are used as easy-to-operate detectors in the extreme ultraviolet (EUV) spectral range. The Physikalisch-Technische Bundesanstalt calibrates photodiodes in the spectral range from 1 nm to 30 nm with an uncertainty of the spectral responsivity of 0.3% or better. For the dissemination of these high-accuracy calibrations, we investigated the stability and linearity of silicon n-on-p junction photodiodes under intense EUV irradiation in ultra high vacuum. We used quasi-monochromatic direct undulator radiation or focused radiation from a bending magnet to achieve high radiant power. The maximum current in linear operation (1% relative saturation) ranges from about 3 mA for 6 mm photon beam diameter to 0.2 mA for a 0.25 mm diameter spot. The corresponding irradiance increases from 30 mW/cm2 for 6 mm photon beam size to about 2 W/cm2 for a 0.25 mm beam. Diodes with diamond-like carbon as well as TiSiN top layer proved to be stable up to a radiant exposure of about 100 kJ/cm. The observed changes of the responsivity could be explained as the result of carbon contamination of the surface and changes in the electrical behaviour at the interface between top layer and sensitive silicon region. Under UHV conditions, no indication of oxidation of the surface was found.