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Current limitations of SEM and AFM metrology for the characterization of 3D nanostructures

This contribution discusses the developed approaches as well as the limitations of scanning electron microscopy (SEM) and atomic force microscopy (AFM) to measure topographic details of structures on lithographic masks and wafers aiming at uncertainties in the nanometre range or even below. In the first approximation, the SEM signal, that is the change of secondary electron (SE) yield, is interpreted as being due to the surface tilt contrast and SE diffusion from the sidewall. The signal is biased by the width of the initial electron beam and, in the case of smaller penetration depths of the initial electron beam, also by the corner rounding of the specimen. Oscillation mode AFMs have been operated such that steep edge regions could be resolved better than with standard scanning methods. A single-point probing strategy enables the instrument to approach the surface from any direction and to leave the probe in the interaction region for a minimum of time maintaining the tip geometry. The ambiguity of the AFM measurement due to the changing response behaviour of the oscillating probe at differing geometries is discussed.