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Scatterometry is a common technique for the characterization of nano-structured surfaces. The goal is to establish scatterometry as a traceable and absolute metrological method for dimensional measurements. Scatterometry is an indirect measuring method. There are three aspects to be investigated for a proper estimation of the measurement uncertainties. Firstly, the measured optical properties will have measurement uncertainties. The optimization theory provides well-proven algorithms to propagate the uncertainties of the input parameters to the output, using covariance or Monte Carlo methods. Secondly, it must be assured that the model represents the measurement object sufficiently well. Due to the large number of parameters it takes to model a real object, it is not possible to simultaneously determine all of them from the measured data. Therefore, a priori knowledge about reasonable values for certain model parameters and their associated variances is needed. Their impact on the uncertainties of the reconstructed parameters describing the nano-structured surface has to be considered too. Thirdly, there are assumptions in the model which are inherent properties of the mathematical methods used. The assumption that the structures are periodic over a sufficiently large area, i.e. the measurement field, is of the most importance. Line-edge or line-width roughness are obviously effects disturbing the periodicity and the corresponding uncertainty contribution can be derived by an analytical approach to estimate their influence on the diffracted intensities. In our example the latter contribution is the dominant one.

We report on an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3–20-μm wavelength range with pulse durations as short as 50 fs at 5 μm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-infrared signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS_2, two-photon absorption in InSb, and cross-correlation measurements with near-infrared pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-infrared pulses.