20 May 2004 Linearity of silicon photodiodes for EUV radiation
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Abstract
Photodiodes are used as easy-to-operate detectors in the extreme ultraviolet spectral range. The Physikalisch-Technische Bundesanstalt calibrates photodiodes with an 0.3% or better relative uncertainty for the spectral responsivity. These calibrations are based on the comparison of the photodiodes to a primary detector standard using monochromatized synchrotron radiation with a rather low radiant power of about 1 μW. At the customer’s, these diodes may be used for strongly pulsed radiation and very different radiant powers. The linearity of the photodiode signal with incident radiant power was studied with EUV radiation. We used quasi-monochromatic direct undulator radiation to achieve high radiant power. 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 with the photon beam spot size is shown. The maximum current in linear operation (less than 1% relative saturation) decreased from about 3 mA for 6 mm photon beam diameter to 0.2 mA for 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 Ohm for a 6 mm and 540 Ohm for a 0.25 mm beam. The effect can be explained by the finite conductivity of the thin front contact layer which carries the current to the electrode.
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Frank Scholze, Roman Markus Klein, Ralph Mueller, "Linearity of silicon photodiodes for EUV radiation", Proc. SPIE 5374, Emerging Lithographic Technologies VIII, (20 May 2004); doi: 10.1117/12.532151; https://doi.org/10.1117/12.532151
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