Topography measurements of high NA aspherical microlenses by digital holographic microscopy with spherical illumination
2017
In this contribution, we propose a method of digital holographic microscopy (DHM) that enables measurement of high
numerical aperture spherical and aspherical microstructures of both concave and convex shapes. The proposed method
utilizes reflection of the spherical illumination beam from the object surface and the interference with a spherical
reference beam of the similar curvature. In this case, the NA of DHM is fully utilized for illumination and imaging of the
reflected object beam. Thus, the system allows capturing the phase coming from larger areas of the quasi-spherical
object and, therefore, offers possibility of high accuracy characterization of its surface even in the areas of high
inclination. The proposed measurement procedure allows determining all parameters required for the accurate shape
recovery: the location of the object focus point and the positions of the illumination and reference point sources. The
utility of the method is demonstrated with characterization of surface of high NA focusing objects. The accuracy is
firstly verified by characterization of a known reference sphere with low error of sphericity. Then, the method is applied
for shape measurement of spherical and aspheric microlenses. The results provide a full-field reconstruction of high NA
topography with resolution in the nanometer range. The surface sphericity is evaluated by the deviation from the best
fitted sphere or asphere, and the important parameters of the measured microlens: e.g.: radius of curvature and conic
constant.
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