Method and apparatus for determining a feature-forming variant of a lithographic system

A method of determining an optimum for one of a focus and an exposure of a system of interest comprising,

• forming a series of exposures for a system of interest, each exposure differing with respect to at least one of focus and exposure,
• for each exposure, forming a Fourier power spectrum having axes of maximum optical power, each Fourier power spectrum having an origin,
• from each Fourier power spectrum, extracting an angular power function, including quantifying power along at least portions of lines originating at various angles from the origin of the Fourier power spectrum, the lines including first lines that are coincident with the axes of maximum optical power and including second lines at angles to the axes of maximum optical power,
• for each angular power function, data in comparing power along the first lines to power along the second lines, including determining a figure of merit based upon the comparing of power,
• selecting a preferred exposure of the series of exposures based upon comparing the figures of merit, and
• selecting a setting for the system of interest based upon the setting for the system during the forming of the preferred exposure of the series of exposures.

An apparatus for use in determining an optimum of a feature-forming variant of fabricating a pattern on a substrate comprising,

• means for fabricating a pattern on a substrate,
• means for receiving an image of the pattern,
• transforming means for providing a two-dimensional Fourier power spectrum of the image, the Fourier power spectrum having axes of maximum power which intersect at an origin,
• means for extracting an angular power function from the Fourier power spectrum, including memory for retaining power values for lines extending at varying angles from said origin of said Fourier power spectrum, and
• means for determining an optimum feature-forming variant of said means for fabricating in response to a comparison of the power values.