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形貌测量数据。在轴向方向上，实现了个位数纳米范围内的分辨率。然而，由于衍射极限，横向分辨率在物理上被限制为大约光波长的一半。在本项目的第一阶段，我们研究了在测量对象的表面上放置微球的近场支持可以在多大程度上提高干涉显微镜的横向分辨率。在表面的近场中的球体影响成像过程，使得低于显微镜的衍射极限的光栅周期变得可见。根据国家的最先进的，这是证明只有低和中等数值孔径的光学系统。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倍放大率）的Linnik干涉仪，理论横向分辨率约为250 nm，用于我们的实验研究。在微球辅助下，可以实现大约10%的横向分辨率的改进。为了解释实验结果的微球的分辨率增强模型的发展。该模型通过严格的仿真计算了电磁场在微球近场区域的相位传播，并使用三维空间频域中的传递函数分析了光学成像过程。为了实现横向分辨率能力的更大改进，该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光学元件来继续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喷射相结合。纳米喷射现象发生在微球的背面，其由平面波照射。微光学元件将通过双光子光刻制造，并连接到光纤耦合探针头，以在可见光谱范围内产生直径低于300 nm的光子纳米射流。这保证了低于150 nm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干涉信号，并实现个位数纳米范围内的轴向分辨率。如果这种传感器以小的工作距离横向扫描测量对象，则将以低于阿贝衍射极限的横向分辨率获得表面形貌。因此，要研究的测量方法的范围之间的显微成像作为远场技术和近场光学显微镜。

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.