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The development of ever higher resolution surface 2,5D reconstruction systems is required for validating microsystem machining processes. Finding an easy, non-contact, quantitative method of characterization with a wide field, is still an issue for the semiconductor industry. Optical profilometry, based on white light interferometry, is one of the relevant methods that can provide such solutions. One of its limitations though is the lateral resolution, which is generally worse than the diffraction limit due to system influences. In 2016, several papers from Wang, Montgomery and Kassamakov showed that by adding a dielectric microsphere of a few micrometers in diameter between the interferometer and the sample, the lateral resolution could be increased by a factor of around 4, up to 100 nm line width. Several interferometric configurations can be used. The Linnik configuration, with a complete reference arm, and a second objective lens and microsphere, is probably the most powerful one due to its ability to compensate all the phase delays and aberrations of the imaging system. Conversely, the Mirau configuration is one of the simpler methods, since the reference arm is located directly within the microscope objective assembly. However, due to the virtual image made by the microsphere and the phase shift introduced in the object arm by the sphere, by default, a mismatch occurs between the imaging plane and the coherence plane. This may affect the profile reconstruction. The selection of the adapted microsphere for this kind of interference objective lens will be discussed both experimentally and theoretically. We will show how this mismatch can be minimized and how the coherence length of the source and the depth of field of the microsphere provide a certain tolerance, leading to a more practical nano-characterization tool.
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