Aberration characterisation in coherent Fourier scatterometry
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Abstract
Coherent Fourier scatterometry (CFS) is a very sensitive optical metrology technique that has been applied for detection and characterisation of nanostructures. It is a scanning-based technique where the samplie is illuminated with a focused light spot. However, in practical CFS systems, residual optical aberrations can distort the focused spot and degrade the signal-to-noise ratio during measurements. Here, we present a systematic study of the influence of low-order aberrations: defocus, spherical, astigmatism, oblique astigmatism, and coma on the differential split-detector CFS signal. Controlled amounts of each aberration, described by Zernike polynomials, were introduced into the Fourier plane via a spatial light modulator. Two-dimensional differential scattering maps were recorded on a reference sample of 425 nm diameter, 150 nm deep pits etched in silicon, and the peak-to-peak differential signal was quantified as a function of peak-valley (PV) wavefront error. We find that defocus has the strongest impact, halving the signal at just 0.27λ PV, followed by spherical (0.32λ) and coma (0.40λ), whereas astigmatism and oblique astigmatism require larger wavefront errors (> 0.6λ) to produce comparable signal loss. These results define quantitative aberration tolerances for CFS systems. The insights gained here can guide the design and optimisation of different CFS implementations for in-line process control and nanostructure metrology.