Characterizing a scatterfield optical platform for semiconductor metrology

Scatterfield microscopy is the union of a high-magnification imaging platform and the angular and/or wavelength control of scatterometry at the sample surface. Scatterfield microscopy uses Kohler illumination, where each point on the source translates to a particular angle of illumination yet also yields spatial illumination homogeneity. To apply scatterfield microscopy to quantitative metrology, several aspects of the optical column must be well understood. Characterizations are presented of the illumination intensity, angle, polarization, and measured glare as functions of the position of an aperture in the conjugate to the back focal plane (CBFP) of the objective lens. The characterization of a reference sample is shown to be as important as the inspection of other optical elements in the optical column. Reflectivity can be derived for line arrays lacking diffractive orders by measuring such a reference and deriving a 'tool function' to account for the current state of the optical platform. Examples from defect, critical dimension, and overlay metrologies are presented to demonstrate the necessity of characterization for scatterfield microscopy.