Saturation effects in solid-state photodiodes and impact on EUVL pulse energy measurements

With the advent of extreme ultraviolet lithography (EUVL) it is becoming increasingly important to understand the response of solid-state photodiodes to pulsed radiation where the peak power incident on the device may be quite high even when the average power is modest. The high peak power causes the photodiode signal to saturate and can create errors in the measurement of pulse energy unless the saturation effect is corrected. We have used 532 nm radiation from a frequency-doubled, Q-switched Nd : YAG laser as a proxy for the 13.5 nm radiation used in EUVL and have studied the saturation behaviour of Si photodiodes at peak power densities from less than 0.1 W mm−2 to over 2 × 106 W mm−2. Here we test a previously reported model of saturation at pulse lengths from 8 ns to 1 µs and examine the effect of power density (illumination area) on the saturation behaviour. The saturation parameters measured at 532 nm are used as inputs to the model, and the calibration of a filter radiometer in pulsed 13.5 nm radiation is corrected for saturation effects. The corrected result agrees with the value expected from individual component calibrations, but the uncorrected data differ by more than the combined uncertainty. This result supports the hypothesis that 532 nm radiation is a valid proxy for 13.5 nm radiation when studying saturation behaviour. Finally, we derive quantitative limits imposed by the saturation effects on the use of Si photodiodes in EUVL applications.