Towards a Carbon-contamination-tolerant EUV power sensor

A reproducible measurement of in-band EUV power over time is essential in EUV lithography, e.g. for dose control, for monitoring the transmission of (parts of) the optical path and for detecting changes in EUV source performance. However, all currently available sensors suffer from sensitivity degradation over time due to photon-induced contamination and/or structural degradation. For instance, a growing carbon layer on a sensor surface, as may be deposited during exposure to EUV, inhibits detection of an increasingly significant fraction of the EUV power. To minimize a change of sensor response over time, TNO is developing a carbon-contamination-tolerant EUV power sensor that can operate under typical EUV vacuum conditions. The sensor uses the photo-electric effect to distinguish between in- and out-of-band EUV. The sensor concept has been tested in the EUV beam line at TNO using a Xe discharge produced plasma (DPP) EUV source. Here, we present the latest experimental results, showing the time-resolved response of the sensor to the EUV pulse. A spectral purity filter was used to compare the sensor response to the full source spectrum output and to typical Xe DPP EUV emission lines (wavelength range 10-20 nm) only. The EUV signature in the sensor signal was clearly recognizable in both cases. The normalized observed EUV pulse power reproducibility was σ/μ = 23%, reflecting the typical EUVemission power fluctuations of this Xe DPP EUV source.