Linearity tests of silicon photodiodes for EUV radiation

Photodiodes are used as easy-to-operate detectors in the extreme ultraviolet (EUV) spectral range. The Physikalisch-Technische Bundesanstalt (PTB) calibrates photodiodes with a 0.3% or better relative uncertainty for the spectral responsivity. These calibrations are based on the comparison of the photodiodes to a cryogenic electrical substitution radiometer as primary detector standard using monochromatized synchrotron radiation. The linearity of the diode signal with incident radiant power was studied with EUV radiation. The linearity of the photodiodes was tested with quasi-dc illumination for different photon beam spot sizes. A systematic and significant variation of the maximum external photocurrent of the diodes with the photon beam spot size was found. The maximum current in linear operation without bias voltage (less than 1% relative saturation) decreased from about 3 mA for a 6 mm photon beam diameter to 0.2 mA for a 0.25 mm diameter. The corresponding irradiance increased from 30 mW/cm2 for the 6 mm aperture to about 2 W/cm2 for the 0.25 mm aperture. This behaviour is attributed to a change in the effective serial resistance with the photon beam size. The values derived from the saturation measurement vary between 65 Ω for a 6 mm and 540 Ω for a 0.25 mm diameter beam. For spot sizes much smaller than the diode's active area, the serial resistance scales logarithmically with the spot size.