Near-field assisted low-coherence interference microscopy for 3D measurement of sub-micrometer structures

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Interference microscopy enables fast and contactless measurement of surface topography and provides topographical measurement data. In the axial direction, a resolution in the single-digit nanometer range is achieved. However, due to the diffraction limit the lateral resolution is physically limited to approximately half the wavelength of light. In the first period of this project we investigated to what extent near-field support by microspheres put onto the surface of the measurement object can improve the lateral resolution of an interference microscope. The spheres in the near-field of the surface influence the imaging process such that grating periods below the diffraction limit of the microscope become visible. According to the state-of-the-art this was demonstrated solely for optical systems of low and medium numerical aperture. Therefore, in the first period of the project we consciously used a home-build Linnik interferometer of high numerical aperture (NA = 0.9, 100x magnification) with a theoretical lateral resolution of approximately 250 nm for our experimental investigations. With microsphere assistance an improvement of the lateral resolution of approximately 10 % could be achieved. In order to explain the experimental results a model for the resolution enhancement by microspheres was developed. This model calculates the phase propagation of the electromagnetic field in the near-field region of the microsphere via rigorous simulations and analyses the optical imaging process using transfer functions in the 3D spatial frequency domain. To reach a more significant improvement of the lateral resolution capabilities, the project applied for aims to continue the approach of near-field assisted imaging by means of a micro-optical element using a fiber-coupled interferometric sensor instead of an interference microscope. Hence, an interferometric measuring principle will be combined directly with a photonic nanojet interacting with the surface. The nanojet phenomenon occurs on the backside of a microsphere, which is illuminated by a plane wave. The micro-optical element will be fabricated via two-photon-lithography and attached to a fiber coupled probe head to generate photonic nanojets with diameters below 300 nm in the visible spectral range. This promises a lateral resolution below 150 nm. A periodic modulation of the distance between the sensor and the object leads to phase-modulated interference signals and enables an axial resolution in the single-digit nanometer range. If such a sensor scans the measurement object laterally with a small working distance the surface topography will be obtained with a lateral resolution below Abbe’s diffraction limit. Hence, the measuring method to be investigated ranges between microscopic imaging as a far-field technique and near-field optical microscopy.