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

干涉显微镜能够快速、非接触地测量表面形貌，并提供地形测量数据。在轴向上，实现了个位数纳米范围的分辨率。然而，由于衍射限制，横向分辨率在物理上被限制在光波长的大约一半。在这个项目的第一阶段，我们研究了在测量对象表面放置微球的近场支撑在多大程度上可以提高干涉显微镜的横向分辨率。表面近场中的球体影响成像过程，从而使低于显微镜衍射极限的光栅周期变得可见。根据最先进的技术，这仅针对低和中等数值孔径的光学系统进行了演示。因此，在项目的第一阶段，我们有意识地使用了自制的高数值孔径(NA=0.9,100倍放大倍率)的Linnik干涉仪进行实验研究，理论横向分辨率约为250 nm。在微球的辅助下，横向分辨率可提高约10%。为了解释实验结果，建立了微球增强分辨率的模型。该模型通过严格的模拟计算了电磁场在微球近场区域的位相传播，并在三维空间频域中利用传递函数分析了光学成像过程。为了更显著地提高横向分辨率能力，申请的项目旨在通过使用光纤耦合干涉传感器而不是干涉显微镜的微型光学元件继续采用近场辅助成像的方法。因此，干涉测量原理将直接与与表面相互作用的光子纳米射流相结合。纳米射流现象发生在微球的背面，微球受到平面波的照射。微光学元件将通过双光子光刻技术制造，并连接到光纤耦合探头上，在可见光谱范围内产生直径小于300 nm的光子纳米射流。这保证了150纳米以下的横向分辨率。传感器和物体之间的距离的周期性调制导致相位调制的干涉信号，并使轴向分辨率在个位数纳米范围内。如果这种传感器以较小的工作距离横向扫描测量对象，则可以获得横向分辨率低于阿贝衍射极限的表面形貌。因此，要研究的测量方法介于作为远场技术的显微成像和近场光学显微镜之间。

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.