Spatial frequency resolved measurement of surface micro-topographies using interference microscopy with wavelength selective pupil illumination at high numerical aperture

• Applicant: Professor Dr.-Ing. Peter Lehmann
• Subject Area: Measurement Systems
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 437311458

White-light interferometry has established as a standard tool for the measurement of surface micro-topography. The measuring process comprises a continuous path-length change in the measurement arm of the interferometer, while a camera captures interference images equidistantly. In order to obtain a height value for each camera pixel the position of maximum interference contrast of the corresponding path-length dependent interference signal will be determined. If the phase of the interference signal is analyzed additionally, a height resolution in the nanometer range is reached, since the period of the measured signal equals half of the mean effective wavelength of the contributing light. The measurement of lateral sub-micrometer dimensions requires interference microscopes with high numerical aperture (NA). For these, the resulting interference signals show additional low-frequency components, which are related to the oblique angles of incidence of the light.Consequently, the well-known NA-effect occurs, which leads to an increasing period of the interference signal and thus an increasing mean effective wavelength caused by the increased NA. Investigations carried out by the applicant turned out that as a further result of this effect, the choice of the evaluation wavelength used for phase analysis systematically influences the lateral resolution of the measurement. The intention of this proposal is to analyze the underlying mechanisms by theoretical and experimental investigations. Therefore, ring-shaped apertures of different diameter providing different angles of incidence of light on the measured surface will be introduced in a high-NA Linnik interferometer. The resulting interference signals are to be analyzed with respect to their characteristic parameters, in particular their spectrally resolved phases. As a final result we expect to achieve an improved accuracy of topography measurement using WLI in many cases of practical application.

• DFG Programme: Research Grants