Confocal Linnik white-light interferometer for micro- and nanostructure measurement with high lateral resolution

• Applicant: Professor Dr.-Ing. Peter Lehmann
• Subject Area: Measurement Systems
• Term: from 2011 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 198145256

White-light interference microscopy is known as a well-established method for three dimensional measurement of surface microstructure with height resolution in the nanometre range. Although diffraction limits the lateral resolution in microscopic imaging, no generally accepted definition of the lateral resolution of 3D microscopes exists. In addition, even if the structures are laterally well-resolved systematic discrepancies between measured and real microstructures occur. In the first period of the project the transfer characteristics of white-light interferometers measuring micrometer and sub-micrometer structures with high lateral resolution were studied. Instrumental modifications as well as modified signal processing algorithms were investigated. The experimental arrangement based on a Linnik interferometer was extended and optimized successively. Several topics of technological improvement were identified, namely the dependence of measured height profiles on the centre wavelength and the state of polarization of the used light, the introduction of confocal and structured illumination, as well as the dependence of the measurement results on the evaluation wavelength in case of phase evaluation of the measured interference signals.In the second period of the project the knowledge obtained in the first period shall be systematically validated, generalized, and precisely extended. For this purpose the existing setup shall be equipped with a low coherent light source module with adjustable centre wavelength and a laterally highly resolving low-noise camera. The project aims to minimize systematic measurement artifacts known as batwings and to improve the lateral resolution even below the Abbe resolution limit. Both, experimental as well as theoretical investigations and simulations shall result in improved interferometric measurement systems and strategies for reliable measurement of micrometer and even sub-micrometer structures. These novel systems are characterized by better lateral resolution and significantly smaller systematic deviations compared to todays instruments.

• DFG Programme: Research Grants