Extreme ultraviolet lithography (EUVL) sources emit a broad spectrum of wavelengths ranging from EUV to DUV and
beyond. If the deep ultraviolet (DUV) reaches the wafer it will affect imaging performance by exposing the photoresist.
Hence it is critical to determine the amount of DUV out of band (OoB) present in a EUVL tool, as well as its effect on
the printed features on the wafer.
In this study we investigate the effect of DUV OoB in EUVL. A model is developed in order to be able to quantify the
DUV/EUV ratio at wafer level and all the required input parameters are estimated in the range from 140 to 400nm, as
well as for the EUV at 13.5nm. The transmission of the optical system was estimated based on the optical design and
reflectivity measurements of the mirrors. The mask reflectivity for multilayer (ML) and absorber was measured at
wavelengths down to 140 nm and for EUV. The sensitivity to EUV and DUV for a variety of resist platforms was
measured at 13.5 nm, 157 nm, 193 nm, 248 nm and 365 nm. The source spectra were also measured. By using these
inputs, it was possible to estimate the DUV/EUV ratio for two different ASML tool configurations, the EUV Alpha
Demo Tool and the NXE:3100. Both NXE:3100 with LPP (laser produced plasma) source and Alpha Demo Tool with
DPP (discharge produced plasma) source show less than 1% DUV/EUV ratio in resist.
The modeling predictions were compared to experimental results. A methodology is introduced to measure the
DUV/EUV ratio at wafer level in situ. With this aim, an aluminum coated mask was fabricated and its reflectivity was
qualified in both EUV and DUV wavelength range. By comparing the dose to clear exposures of a reflective blank and
of the aluminum mask, it is possible to quantify the DUV/EUV ratio. The experimental results are in order of magnitude
agreement with modeling predictions. The proposed experimental approach can be used to benchmark the DUV
sensitivity of different resist platforms and may be used to monitor DUV OoB.