Methods for the determination of center of focus and process control for a
lithographic tool. Diffraction signatures are obtained from a plurality
of diffraction structures located within multiple different focus setting
fields. Variability of diffraction signatures with each field are
determined, by direct analysis or comparison to a library. The variation
or uniformity may be represented by any measure, including the standard
deviation or the range of values of a chosen feature of a library of
theoretical diffraction structures or the variability or uniformity of
the diffraction signatures themselves, such as by RMS difference or
intensity range. The methods may be used for process control and
monitoring of focus drift by determining intra-field variation of
diffraction signatures of multiple diffraction structures in a series of
wafers.