Diffraction wavefront analysis of point diffraction interferometer for measurement of aspherical surface

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.