This paper gives an overview of the possible methods for a three-dimensional surface acquisition in the micrometer scale. It is pointed out that Scanning Electron Microscopy is a capable method for measurement tasks of this kind; therefore, it presents possible ways for implementing this technique in a three-dimensional surface reconstruction. The improved photometric method promises the best performance; its further implementation is developed and explained. Therefore, some modifications of the employed Scanning Electron Microscope (SEM) are described, for instance, the integration of two supplemental detectors, a modified collector grid and a gun shielding. All modifications were evaluated using FEM-Simulations before their implementation. A signal mixing is introduced in order to still be able to use the improved photometric method with four detectors in spite of the fact that it was designed for a two-detector system. For verification purposes, a sphere normal is measured by means of the modified system. It can be seen that the maximal detectable slope angle could be increased compared to the old photometric method. In addition, we introduce an electron trap consisting of nano structured titanium. The structure is tested regarding its ability to catch electrons of different energies and compared to non-structured titanium. The trap can later be implemented on the bottom of the electron gun to catch unwanted backscattered electron (BSE) emission which could otherwise affect the three-dimensional reconstruction.
KEYWORDS: Sensors, Signal detection, Scanning electron microscopy, Electron microscopes, Optical spheres, Reconstruction algorithms, Mathematical modeling, Finite element methods, Scintillators, Absorption
Due to the emerging degree of miniaturization in microstructures, Scanning-Electron-Microscopes (SEM) have become
important instruments in the quality assurance of chip manufacturing. With a two- or multiple detector system for
secondary electrons, a SEM can be used for the reconstruction of three dimensional surface profiles. Although there are
several projects dealing with the reconstruction of three dimensional surfaces using electron microscopes with multiple
Everhart-Thornley detectors (ETD), there is no profound knowledge of the behaviour of emitted electrons. Hence,
several values, which are used for reconstruction algorithms, such as the photometric method, are only estimates; for
instance, the exact collection efficiency of the ETD, which is still unknown. This paper deals with the simulation of
electron trajectories in a one-, two- and four-detector system with varying working distances and varying grid currents.
For each detector, the collection efficiency is determined by taking the working distance and grid current into account.
Based on the gathered information, a new collection grid, which provides a homogenous emission signal for each
detector of a multiple detector system, is developed. Finally, the results of the preceding tests are utilized for a
reconstruction of a three dimensional surface using the photometric method with a non-lambert intensity distribution.
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