Point diffraction interferometer (PDI) is a new method for measurement of spherical or aspherical surface in extreme
ultraviolet lithography(EUVL) with hypo-nano accuracy. In PDI method, a nearly ideal spherical wavefront with
spherical wavefront error less than λ/105 rms, can be acquired through diffraction of a very small pinhole which diameter
is only a few microns. This diffraction spherical wavefront can replace conventional real standard lens, and be used as
reference spherical wavefront in PDI. The shape and quality of diffraction wavefront are key influence factors of
measurement range and accuracy, these must be considered strictly in designing.
In this paper, theory model of point diffraction is discussed based on theory of scalar diffraction theory. Relationships
between size of pinhole, range of diffraction and NA of measurable are obtained and diffraction wavefront error factors
such as pinhole size, roundness error are numerically analyzed. The analysis results show that a high accuracy diffraction
wavefront can be obtained through comprehensive optimizing of pinhole size, NA range, light intensity, shape error of
pinhole and so on. A pinhole with diameter no more than 2μm and radius errorΔr≤0.1r is suitable in designing and its
residual wavefront error can reach λ/105 in certain diffraction range. The analysis can provide reliable theory and
reference data for the actual design of point diffraction interferometer.